Modern and Past Rock Exhumation in the St. Elias Mountains Revealed by Onshore and Offshore Detrital Thermochronology

NASA Astrophysics Data System (ADS)

Enkelmann, E.; Dunn, C. A.; Ridgway, K.; Allen, W. K.

2016-12-01

The St. Elias Mountains in southeastern Alaska provide a natural laboratory to study the interacting processes of tectonics and climate. Because of the high-latitude, coastal geography the surface processes in the St. Elias Mountains are dominated by glacial erosion that varied during late Cenozoic climate shifts. Sediment eroded from the St. Elias Mountains are transported into the Gulf of Alaska by large tidewater glaciers and rivers, where they are deposited on the shelf and in large deep-sea submarine fans. Surface uplift and erosion jointly results in exhumation of deep crustal rocks that has been quantified by thermochronology, revealing spatial variations in exhumation rates across the St. Elias Mountains. We present new thermochronology data (apatite and zircon fission-track ages) from offshore strata derived from boreholes drilled by IODP Expedition 341 in the Gulf of Alaska. This offshore record provides an integrated signal of rock exhumation from the St. Elias and southeast Alaska since 10 Ma. Integrating the offshore data with the existing onshore thermochronology reveals that very rapid exhumation (>2 km/Myr) from 8-10 km depths has been occurring in southeastern Alaska since 11­-10 Ma and thus prior to the onset of glaciation. The majority of our offshore data record the past 1 Myr of deposition related to the Bagley-Bering Glacier and allows assessing the long-standing question of the western limit of extreme exhumation observed at the St. Elias syntaxis in the Seward and Hubbard glacial drainages. The zircon fission-track ages from the Bering sediment reveal small age populations that peak between 15­-9 Ma, suggesting much slower exhumation beneath the Bagley Ice Valley than farther east underneath the Seward and Hubbard glacier. Our results point out the first-order role of tectonics in providing sustained uplift and crustal weakening in the upper plate that than serve as areas for focused exhumation due to secondary erosional/climate processes.

Ar-Ar Thermochronlogy of Apollo 12 Impact-Melt Breccia 12033,638-1

NASA Technical Reports Server (NTRS)

Crow, C. A.; Cassata, W. S.; Jolliff, B. L.; Ziegler, R. A.; Borg, L. E.; Shearer, C. K.

2017-01-01

We have undertaken an Ar-Ar thermochronology investigation as part of a coordinated multichronometer analysis of a single Apollo 12 impact- melt breccia to demonstrate the wide range of information that can be obtained for a single complex rock. This has implications for the age of formation, component makeup, and subsequent impact/shock and exposure history of the sample. This study also serves as a capabilities demonstration for the proposed MoonRise Mission [1]. The goal of this investigation is to elucidate the history of this sample through coordinated 40Ar*/39Ar, Sm-Nd, Rb-Sr and zircon 207Pb-206Pb ages along with geochemical and petrographic context on a relatively small (approximately 450 mg) sample. Here, we report preliminary results of the Ar-Ar thermochronology.

Comment on 'A reporting protocol for thermochronologic modeling illustrated with data from the Grand Canyon' by Flowers, Farley and Ketcham

NASA Astrophysics Data System (ADS)

Gallagher, Kerry

2016-05-01

Flowers et al. (2015) propose a framework for reporting modeling results for thermochronological data problems, particularly when using inversion approaches. In the final paragraph, they state 'we hope that the suggested reporting table template will stimulate additional community discussion about modeling philosophies and reporting formats'. In this spirit the purpose of this comment is to suggest that they have underplayed the importance of presenting a comparison of the model predictions with the observations. An inversion-based modeling approach aims to identify those models which makes predictions consistent, perhaps to varying degrees, with the observed data. The concluding section includes the phrase 'clear documentation of the model inputs and outputs', but their example from the Grand Canyon shows only the observed data.

Sensitivity analysis of a variability in rock thermal conductivity concerning implications on the thermal evolution of the Brazilian South Atlantic passive continental margin

NASA Astrophysics Data System (ADS)

Stippich, Christian; Krob, Florian; Glasmacher, Ulrich Anton; Hackspacher, Peter Christian

2017-04-01

The aim of the research is to quantify the long-term evolution of the western South Atlantic passive continental margin (SAPCM) in SE-Brazil. Excellent onshore outcrop conditions and extensive pre-rift to post-rift archives between São Paulo and Laguna allow a high precision quantification of exhumation, and rock uplift rates, influencing physical parameters, long-term acting forces, and process-response systems. The research integrates published (Karl et al., 2013) and partly published thermochronological data from Brazil, and test lately published new concepts on causes of long-term landscape and lithospheric evolution in southern Brazil. Six distinct lithospheric blocks (Laguna, Florianópolis, Curitiba, Ilha Comprida, Peruibe and Santos), which are separated by fracture zones (Karl et al., 2013) are characterized by individual thermochronological age spectra. Furthermore, the thermal evolution derived by numerical modeling indicates variable post-rift exhumation histories of these blocks. In this context, we will provide information on the causes for the complex exhumation history of the Florianópolis, and adjacent blocks. Following up on our latest publication (Braun et al., 2016) regarding the effect of variability in rock thermal conductivity on exhumation rate estimates we performed a sensitivity analysis to quantify the effect of a differentiated lithospheric crust on the thermal evolution of the Florianópolis block versus exhumation rates estimated from modelling a lithospheric uniform crustal block. The long-term landscape evolution models with process rates were computed with the software code PECUBE (Braun, 2003; Braun et al., 2012). Testing model solutions obtained for a multidimensional parameter space against the real thermochronological and geomorphological data set, the most likely combinations of parameters, values, and rates can be constrained. References Braun, J., 2003. Pecube: A new finite element code to solve the 3D heat transport equation including the effects of a time-varying, finite amplitude surface topography. Computers and Geosciences, v.29, pp.787-794. Braun, J., Stippich, C., Glasmacher, U. A., 2016. The effect of variability in rock thermal conductivity on exhumation rate estimates from thermochronological data. Tectonophysics, v.690, pp.288-297 Braun, J., van der Beek, P., Valla, P., Robert, X., Herman, F., Goltzbacj, C., Pedersen, V., Perry, C., Simon-Labric, T., Prigent, C., 2012. Quantifying rates of landscape evolution and tectonic processes by thermochronology and numerical modeling of crustal heat transport using PECUBE. Tectonophysics, v.524-525, pp.1-28. Karl, M., Glasmacher, U.A., Kollenz, S., Franco-Magalhaes, A.O.B., Stockli, D.F., Hackspacher, P., 2013. Evolution of the South Atlantic passive continental margin in southern Brazil derived from zircon and apatite (U-Th-Sm)/He and fission-track data. Tectonophysics, Volume 604, Pages 224-244.

New insights on the geological evolution of the continental margin of Southeastern Brazil derived from zircon and apatite (U-Th-Sm)/He and fission-track data

NASA Astrophysics Data System (ADS)

Krob, Florian; Stippich, Christian; Glasmacher, Ulrich A.; Hackspacher, Peter

2017-04-01

New insights on the geological evolution of the continental margin of Southeastern Brazil derived from zircon and apatite (U-Th-Sm)/He and fission-track data Krob, F.C.1, Stippich, C. 1, Glasmacher, U.A.1, Hackspacher, P.C.2 (1) Institute of Earth Sciences, Research Group Thermochronology and Archaeometry, Heidelberg University, INF 234, 69120, Heidelberg, Germany (2) Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Av. 24-A, 1515 Rio Claro, SP, 13506-900, Brazil Passive continental margins are important geoarchives related to mantle dynamics, the breakup of continents, lithospheric dynamics, and other processes. The main concern yields the quantifying long-term lithospheric evolution of the continental margin between São Paulo and Laguna in southeastern Brazil since the Neoproterozoic. We put special emphasis on the reactivation of old fracture zones running into the continent and their constrains on the landscape evolution. In this contribution, we represent already consisting thermochronological data attained by fission-track and (U-Th-Sm)/He analysis on apatites and zircons. The zircon fission-track ages range between 108.4 (15.0) and 539.9 (68.4) Ma, the zircon (U-Th-Sm)/He ages between 72.9 (5.8) and 427.6 (1.8) Ma whereas the apatite fission-track ages range between 40.0 (5.3) and 134.7 (8.0) Ma, and the apatite (U-Th-Sm)/He ages between 32.1 (1.52) and 92.0 (1.86) Ma. These thermochronological ages from metamorphic, sedimentary and intrusive rocks show six distinct blocks (Laguna, Florianópolis, Curitiba, Ilha Comprida, Peruibe and Santos) with different evolution cut by old fracture zones. Furthermore, models of time-temperature evolution illustrate the differences in Pre- to post-rift exhumation histories of these blocks. The presented data will provide an insight into the complex exhumation history of the continental margin based on the existing literature data on the evolution of the Paraná basin in Brazil and the latest thermochronological data. We used the geological model of the Paraná basin supersequences (Rio Ivaí, Paraná, Gondwana I-III and Bauru) to remodel the subsidence and exhumation history of our consisting thermochronological sample data. First indications include a fast exhumation during the early Paleozoic, a slow shallow (northern blocks) to fast and deep (Laguna block) subduction from middle Paleozoic to Mesozoic time and a extremely fast exhumation during the opening of the South Atlantic (Cretaceous time). This enables a possible interpretation of the southeastern Brazilian margin being an outer part of the Paraná basin and even the possible source area for the Ordovician to Carboniferous sediments. Further on, we try to research the newly gained exhumation history models for indications on the evolution and movement of the lithosphere of the southeastern Brazilian mantle.

Southwest USA Exhumation History Recorded Below the Great Unconformity

NASA Astrophysics Data System (ADS)

Heizler, M. T.; Karlstrom, K. E.

2002-05-01

The Southwestern USA Precambrian terranes preserve a long and variable exhumation history that can be tracked using thermochronological methods. This exhumation history is controlled on two interrelated scales. At first order, it is recognized that 1.7 to 1.4 Ga mid-crustal (10 km, 2-4 kbar) rocks were ultimately exhumed and reside below unconformities of variable age. In Arizona, Mesoproterozoic Apache Group and Neoproterozoic Supergroup sedimentary rocks lie directly on basement and thus indicate exhumation of some regions relatively soon following the 1.4 Ga events. In the Rocky Mountains of Colorado and the Rio Grande rift uplifts of New Mexico, basement is generally overlain by Cambrian to Mississippian strata. The unconformities are useful markers of net exhumation; however do not reveal a time-integrated path. Using published, and hundreds of new 40Ar/39Ar analyses of hornblende, muscovite, biotite and K-feldspar, and a growing U/Pb accessory mineral thermochronology database, we are extracting exhumation information with great detail. The thermochronological data continue to support the claim that relatively low net exhumation occurred following 1.7 to 1.6 Ga accretion of volcanic arc terranes to the southern margin of Laurentia. Mid-crustal (2-4 kbar) rocks stabilized soon after accretion, whereas in some regions like the Upper Granite Gorge, Grand Canyon deeper (6 kbar) metamorphic terranes decompressed to 3 kbar before stabilization. The cooling history of these mid-crustal rocks post 1.65 Ga remains somewhat unknown. Overall slow-cooling models (550 \\deg C to 300 \\deg C from 1.7 to 1.4 Ga) require high geothermal gradients in order to maintain 10 km deep rocks at high temperatures for 100's of Ma. Alternatively, isobaric cooling models to more normal geothermal gradients (i.e. 25 \\deg C/km) at ca. 1.65 Ga require later (1.4 Ga) thermal pulses and/or Mesoproterozoic vertical displacements to explain highly discordant thermochronological data. Either model supports relatively normal crustal thickness during 1.8-1.6 Ga arc accretion. In the Grand Canyon, cooling and exhumation are recorded at 1.4 Ga and 1.25 Ga. 1.4 Ga tectonism is shown by a sharp contrast in mica ages (1.4 vs. 1.6 Ga) across the 96-mile shear zone. The 1.25 Ga denudation is required by 1.25-1.30 Ga K-feldspar argon ages from basement that is unconformably overlain by a 1253 Ma volcanic ash horizon in the Unkar Group sediments. Arizona transition zone rocks of similar metamorphic pressure yield highly variable argon ages and indicate that small differences in exposed paleodepth can yield significantly different data. This is supported by cooling ages that systematically decrease with increase in paleodepth in the Gold Butte block, NV. Combined, the thermochronological data from the SW USA record a complex exhumation history that is characterized by discrete block uplift superimposed on an overall billion year erosional history that brings mid-crustal rocks to the surface a variable times.

Thermochronologic constraints on the tectonic evolution of the western Antarctic Peninsula in late Mesozoic and Cenozoic times

USGS Publications Warehouse

Brix, M.R.; Faundez, V.; Hervé, F.; Solari, M.; Fernandez, J.; Carter, A.; Stöckhert, B.

2007-01-01

West of the Antarctic Peninsula, oceanic lithosphere of the Phoenix plate has been subducted below the Antarctic plate. Subduction has ceased successively from south to north over the last 65 Myr. An influence of this evolution on the segmentation of the crust in the Antarctic plate is disputed. Opposing scenarios consider effects of ridge crest – trench interactions with the subduction zone or differences in slip along a basal detachment in the overriding plate. Fission track (FT) analyses on apatites and zircons may detect thermochronologic patterns to test these hypotheses. While existing data concentrate on accretionary processes in Palmer Land, new data extend information to the northern part of the Antarctic Peninsula. Zircons from different geological units over wide areas of the Antarctic Peninsula yield fission track ages between 90 and 80 Ma, indicating a uniform regional cooling episode. Apatite FT ages obtained so far show considerable regional variability

The Sidi Ifni transect across the rifted margin of Morocco (Central Atlantic): Vertical movements constrained by low-temperature thermochronology

NASA Astrophysics Data System (ADS)

Charton, Rémi; Bertotti, Giovanni; Arantegui, Angel; Bulot, Luc

2018-05-01

The occurrence of km-scale exhumations during syn- and post-rift stages has been documented along Atlantic continental margins, which are also characterised by basins undergoing substantial subsidence. The relationship between the exhuming and subsiding domains is poorly understood. In this study, we reconstruct the evolution of a 50 km long transect across the Moroccan rifted margin from the western Anti-Atlas to the Atlantic basin offshore the city of Sidi Ifni. Low-temperature thermochronology data from the Sidi Ifni area document a ca. 8 km exhumation between the Permian and the Early/Middle Jurassic. The related erosion fed sediments to the subsiding Mesozoic basin to the NW. Basement rocks along the transect were subsequently buried by 1-2 km between the Late Jurassic and the Early Cretaceous. From late Early/Late Cretaceous onwards, rocks present along the transect were exhumed to their present-day position.

New insight into the thermal-tectonic history of the southern Antarctic Peninsula: (Uranium-Thorium-Samarium)/Helium and fission-track thermochronologic results from northwest Palmer Land and Alexander Island

NASA Astrophysics Data System (ADS)

Savrda, Amanda Marie

2011-12-01

This study examines the thermal history of the southern Antarctic Peninsula through the application of thermochronometry, and presents the first high-resolution thermochronologic dataset for arc rocks of northwest Palmer Land. I present 19 new thermochronologic ages obtained via (U-Th-Sm)/He and fission-track analyses of apatite and zircon from arc granitoids of northwest Palmer Land and fore-arc rocks of the LeMay and Fossil Bluff Groups of Alexander Island. These data were modeled via Monte Carlo simulations to generate time-temperature pathways. Thermal models generated for arc granitoids of northwest Palmer Land reveal a Late Cretaceous-Early Cenozoic episode of accelerated cooling from ca. 78--55 Ma not previously recognized in the southern Antarctic Peninsula. Here, faster cooling at an average rate of ˜15°C/Myr is bracketed by slower cooling at rates <3°C/Myr. Modeled thermal histories of metamorphosed fore-arc sedimentary rocks of Alexander Island reveal rapid cooling throughout the Eocene at an average rate of ˜13°C/Myr, preceded and followed by slower rates of cooling on the order of <3°C/Myr. The spatial and temporal distribution of the observed cooling trends may reflect localized variations in the thermal regime due to regional changes in plate kinematics, subduction dynamics, and related magmatism, but the cooling rates are also within range of those typical of exhumational processes such as normal faulting, ductile thinning, and erosion.

Applying Low Temperature Thermochronology to Constrain Exhumation Patterns along the Eastern Denali Fault Corner, Alaska

NASA Astrophysics Data System (ADS)

Warfel, T. S.; Fitzgerald, P. G.; Benowitz, J.; Ridgway, K.; Allen, W. K.

2017-12-01

The Denali Fault (DF) constitutes a long ( 2000 km), arcuate, dextrally transpressive intracontinental fault system sketching across south-central Alaska. Strain-partitioning along the DF is accommodated as slip on the fault and fault-normal motion on a series of thrusts located north and south of the fault itself. High topography in the central and eastern Alaska Range, also locations of the greatest exhumation along the fault, are associated with restraining bends in those regions. East of the Richardson Highway, along the eastern Denali fault corner (or east-central segment of the DF), thrust faults south of the DF, including the McCallum thrust have accommodated the fault-normal component of motion along the DF. The aim of this project is to better understand what controls exhumation along large strike-slip faults, in particular the DF. Previous work along the DF in the central and eastern Alaska Range (to the west of this region) indicate the importance of fault geometry and rheological contrasts between terranes that have been juxtaposed against one another in controlling the location of exhumation. Our area of interest is a largely unstudied section along the Denali Fault (eastern DF corner) located between the DF/Hines Creek fault intersection and the Totschunda/DF intersection. We are applying a combination of apatite fission track thermochronology and apatite (U-Th)/He dating to samples collected north and south of the DF, and across thrust faults south of the DF. Thermochronology is being applied to bedrock samples, collected in vertical profiles and/or hanging wall - footwall pairs. Cobbles were also collected within a stratigraphic framework (constrained by tephras), from Miocene sediments in inverted basins south of the DF. Thermochronologic data from these cobbles; using lag-time analyses and inverse thermal models, will constrain the exhumation history of the hinterland. Assuming modern rates for slip along the DF will allow constraints to be placed on spatial and temporal patterns of exhumation and hence, help constrain the underlying control on exhumation patterns. Preliminary results indicate older AFT and AHe ages (up to 50 Ma) away from the DF but yield apatite (U-Th)/He ages as young as 2 Ma for a sample from the footwall of the closest thrust south of the DF.

Reconciling Conflicting Geologic and Thermochronologic Interpretations Via Multiple Apatite Thermochronometers (AHe, AFT, and 4He/3He): 6 Ma Incision of the Westernmost Grand Canyon

NASA Astrophysics Data System (ADS)

Winn, C.; Karlstrom, K. E.; Shuster, D. L.; Kelley, S.; Fox, M.

2017-12-01

The application of low-temperature apatite thermochronology to the incision history of the Grand Canyon has led to conflicting hypotheses of either a 70 Ma ("old") or <6 Ma ("young") Grand Canyon. This controversy is best captured in the westernmost segment of the Grand Canyon, where several lines of evidence favor a "young" Canyon: 1) North-derived Paleocene Hindu Fanglomerate was deposited across the present track of the Canyon; 2) The Separation Point basalt (19 Ma) is stranded between high relief tributaries and the main stem of the Colorado River; 3) Relief generation in tributaries and on plateaus adjacent to the Canyon took place after 17 Ma; and 4) The late Miocene-Pliocene Muddy Creek Formation shows that no far-traveled materials entered the Grand Wash Trough until after 6 Ma. Some interpretations of apatite thermochronology data conflict with these lines of evidence and indicate a much older ( 70 Ma) westernmost Grand Canyon. We reconcile this conflict by applying apatite (U-Th)/He ages (AHe), 4He/3He thermochronometry, and apatite fission track ages and lengths (AFT) to the same sample at a key location. Using HeFTy, t-T paths that predict these data show cooling from ˜100 °C to 40-60 °C at 70-50 Ma, long-term residence at 40-60 °C from 50-10 Ma, and cooling to surface temperatures after 10 Ma, indicating young incision. New AFT (5) and AHe (3) datasets are also presented here. When datasets are examined separately, AHe data show t-T paths that cool to surface temperatures during the Laramide, consistent with an "old" Canyon. When multiple methods are applied, t-T paths instead show young incision. This inconsistency demonstrates the age of the Grand Canyon controversy. Here we reconcile the difference in t-T paths by adjusting model parameters to account for uncertainty in the rate of radiation damage annealing in apatite during burial heating and the resulting variations in He retentivity. In this area, peak burial conditions during the Laramide were likely insufficient to fully anneal radiation damage that accumulated during prolonged near-surface residence prior to burial. We conclude that application of multiple thermochronometers from common rocks reconciles conflicting thermochronologic interpretations and these data are best explained by a "young" westernmost Grand Canyon.

Crustal-scale alpine tectonic evolution of the western Pyrenees - eastern Cantabrian Mountains (N Spain) from integration of structural data, low-T thermochronology and seismic constraint

NASA Astrophysics Data System (ADS)

DeFelipe, I.; Pedreira, D.; Pulgar, J. A.; Van der Beek, P.; Bernet, M.; Pik, R.

2017-12-01

The Pyrenean-Cantabrian Mountain belt extends in an E-W direction along the northern border of Spain and resulted from the convergence between the Iberian and European plates from the Late Cretaceous to the Miocene, in the context of the Alpine orogeny. The main aim of this work is to characterize the tectonic evolution at a crustal-scale of the transition zone from the Pyrenees to the Cantabrian Mountains, in the eastern Basque-Cantabrian Basin (BCB). We integrate structural work, thermochronology (apatite fission track and zircon (U-Th)/He) and geophysical information (shallow seismic reflection profiles, deep seismic refraction/wide-angle reflection profiles and seismicity distribution) to propose an evolutionary model since the Jurassic to the present. During the Albian, hyperextension related to the opening of the Bay of Biscay yielded to mantle unroofing to the base of the BCB. This process was favored by a detachment fault that connected the mantle in its footwall with the base of a deep basin in its hanging wall. During this process, the basin experienced HT metamorphism and fluid circulation caused the serpentinization of the upper part of the mantle. There is no evidence of seafloor mantle exhumation before the onset of the Alpine orogeny. The thermochronological study points to a N-vergent phase of contractional deformation in the late Eocene represented by the thin-skinned Leiza fault system followed in the early Oligocene by the S-vergent, thick-skinned, Ollín thrust. Exhumation rates for the late Eocene-early Oligocene are of 0.2-0.7 km/Myr. After that period, deformation continues southwards until the Miocene. The crustal-scale structure resultant of the Alpine orogeny consists of an Iberian plate that subducts below the European plate. The crust is segmented into four blocks separated by three S-vergent crustal faults inherited from the Cretaceous extensional period. The P-wave velocities in this transect show anomalous values (7.4 km/s) in the deepest part of the Iberian crust that may correspond to serpentinized mantle formed during the Cretaceous and later subducted. The Alpine shortening in this transect is estimated in ca. 90 km. Integration of structural, geophysical and thermochronological data, allows a more precise reconstruction of the crustal-scale Alpine cycle in the eastern BCB.

QUANTIFICATION OF GLACIAL EROSION IN THE ALPS USING VERY LOW-TEMPERATURE THERMOCHRONOLOGY (OSL & AHe)

NASA Astrophysics Data System (ADS)

Champagnac, J.; Herman, F.; Rhodes, E. J.; Fellin, M.; Jaiswal, M.; Schwenninger, J.; Reverman, R. L.

2009-12-01

The impact of glaciations on the topography of the Alps is still unclear: Long-term denudation rate determined by low-T thermochronology are in the range of 0.2 to 1 mm/yr, and increased during the Plio-Quaternary by 3 fold (Vernon et al., 2008). Such an increase is also documented by peri-alpine sediment budget (Kuhleman, 2000), with a similar increase in sediment yields since 5-3 Ma. This increase was considered as evidence of a climatically-driven surface process change, attributed to increased precipitation (Cederbom et al., 2004) and erosion by glacial processes (Champagnac et al., 2007). The timing of the onset of intense glacial erosion as well as its rates are still ambiguous. The glacial erosion seems to have accelerated around 0.9 Ma as suggested by the ten fold increase of incision rates of a valley in the Central Alps (Häuselmann et al., 2007), and by information about vegetation and sedimentologic changes (Muttoni et al., 2003). There is however no direct quantification of topographic change during the Plio-Quaternary. We present here how we use OSL-thermochronology, a new thermochronometer of exceptionally low closure temperature (about 30°-40°C) (Herman et al subm.), new {U-Th}/He on apatites data, and a glacial erosion model (Herman and Braun 2008) to estimate topographic changes in the Alps in response to glaciations. Because of their low closure temperature, OSL and AHe thermochronology enables quantification of events of less than 1 Ma at very small wavelength of the topography. We collected two vertical profiles, one in the Zermatt Valley (Valais) and one in Maurienne Valley (Savoy). We infer from these results changes in topography, date and quantify relief creation under glacial-interglacial cycles. Cederbom, C.E, et al., Climate induced rebound and exhumation of the European Alps. Geology 32, 709-712 (2000). Champagnac, J.-D., et al., Quaternary erosion-induced isostatic rebound in the western Alps. Geology 35, 195-198 (2007). Haüselmann P., et al.,et al. Abrupt glacial valley incision at 0.8 Ma dated from cave deposits in Switzerland. Geology 35, 33-42 (2007). Herman F. and Braun J. Evolution of the glacial landscape of the Southern Alps of New Zealand: Insights from a glacial erosion model, J. Geophys. Res., 113, F02009, doi:10.1029/2007JF000807 (2008). Herman F., Rhodes E.J. and Braun J. A new thermochronometer reveals steady state relief and exhumation in a small active orogen during the last glacial cycle, submitted. Kuhlemann J., et al., Quantifying tectonic versus erosive denudation by the sediment budget: the Miocene core complexes of the Alps, Tectonophysics 330, 1-23 (2000). Muttoni G., et al., Onset of major Pleistocene glaciations in the Alps. Geology 31, 989-992 (2003). Vernon, A.J., et al., Increase in late Neogene denudation of the European Alps confirmed by analysis of a fission-track thermochronology database. Earth and Planetary Science Letters, 270 (3-4), pp. 316-329 (2008).

Cooling and inferred uplift/erosion history of the Grenville Orogen, Ontario: Constraints from sup 40 Ar/ sup 39 Ar thermochronology

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cosca, M.A.

1989-01-01

Thermochronological ({sup 40}Ar/{sup 39}Ar) data are presented from 76 mineral separates of hornblende, muscovite, biotite, phlogopite, and K-feldspar. Samples were selected from regionally metamorphosed gneiss, amphibolite, metasediment, marble, metagabbro and pegmatite across the two major metamorphic belts of the Grenville Province, the Central Metasedimentary Belt (CMB) and the Central Gneiss Belt (CGB). When combined with published temperature estimates for closure to argon diffusion in the phases analyzed, cooling rates from {approximately}500 C to {approximately}120 C of 1-4 C/MA are calculated across the entire Grenville Province of Ontario. Regional uplift/erosion rates for the Grenville Orogen of Ontario have been estimated frommore » the {sup 40}Ar/{sup 39}Ar data, a retrograde P-T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambro-Ordovician sediments. Twenty-two of the hornblendes used for thermochronology have been quantitatively analyzed for major elements by microprobe, Fe{sup 2+}/Fe{sup 3+} by wet chemistry, and for H{sub 2}O by manometric measurement. Water activities calculated from hornblende equilibria are typically low (<0.01) because of the exponential dilutions in hornblende (tremolite) activity required by present activity-composition models. An oxyamphibole component of 25% further reduces any amphibole component and the H{sub 2}O activity by as much as 50% below that calculated with simplifying assumption. These findings indicate that different amphibole normalization schemes have a marked effect on the activity calculated for a specific amphibole or H{sub 2}O, and should be carefully evaluated.« less

Episodic exhumation and relief growth in the Mont Blanc massif, Western Alps from numerical modelling of thermochronology data

NASA Astrophysics Data System (ADS)

Glotzbach, C.; van der Beek, P. A.; Spiegel, C.

2011-04-01

The Pliocene-Quaternary exhumational and topographic evolution of the European Alps and its potential climatic and tectonic controls remain a subject of controversy. Here, we apply inverse numerical thermal-kinematic modelling to a spatially dense thermochronological dataset (apatite fission-track and (U-Th)/He) of both tunnel and surface samples across the Mont Blanc massif in the Western Alps, complemented by new zircon fission-track data, in order to better quantify its Neogene exhumation and relief history. Age-elevation relationships and modelling results show that an episodic exhumation scenario best fits the data. Initiation of exhumation in the Mont Blanc massif at 22 ± 2 Ma with a rate of 0.8 ± 0.15 km/Myr is probably related to NW-directed thrusting during nappe emplacement. Exhumation rates decrease at 6 ± 2 Ma to values of 0.15 ± 0.65 km/Myr, which we interpret to be the result of a general decrease in convergence rates and/or extensive exposure of less erodible crystalline basement rocks from below more easily erodible Mesozoic sediments. Finally, local exhumation rates increase up to 2.0 ± 0.6 km/Myr at 1.7 ± 0.8 Ma. Modelling shows that this recent increase in local exhumation can be explained by valley incision and the associated increase in relief at 0.9 ± 0.8 Ma, leading to erosional unloading, isostatic rebound and additional rock uplift and exhumation. Given the lack of tectonic activity as evidenced by constant thermochronological ages along the tunnel transect, we suggest that the final increase in exhumation and relief in the Mont Blanc massif is the result of climate change, with the initiation of mid-Pleistocene glaciations leading to rapid valley incision and related local exhumation.

Geochronology and thermochronology of the gneisses of the Brasiliano/Pan-African Araguaia Belt: Records of exhumation of West Gondwana and Pangea break up

NASA Astrophysics Data System (ADS)

Dias, A. N. C.; Moura, C. A. V.; Milhomem Neto, J. M.; Chemale, F., Jr.; Girelli, T. J.; Masuyama, K. M.

2017-12-01

The 1200 km long Araguaia Belt, central-north Brazil, was formed as a result of the Neoproterozoic (Ediacaran) to Early Cambrian collage of West Gondwana and has several dome structures in whose nuclei crop out basement inliers of Archean and Paleoproterozoic orthogneisses. Thus, the orthogneisses from Lontra, Cocalândia, Colméia and Cantão dome structures were selected for fission-track thermochronology and U-Pb double dating. Three Archean magmatic episodes ranging from 2830.9 ± 6.9 Ma (Colméia dome) to 2905 ± 5.1 Ma (Lontra dome) and one Paleoproterozoic magmatic event at 1835 ± 31 Ma (Cantão dome) are recognized by U-Pb LA-MC-ICP-MS zircon dating. Fission-track thermochronology in zircon permitted to identify three well-defined age populations, which correspond to major regional geological events. The older population (489 ± 15 Ma to 498±8Ma) records the exhumation process and the orogenetic collapse of the Araguaia Belt. The intermediate population (331 ± 8 Ma to 345 ± 13 Ma) is related to the Gondwanides orogeny which affected the western margin of Gondwana and therefore the interior of South American. Finally, the younger fission track ages (197 ± 3 Ma, 197 ± 4 Ma, 198 ± 2 Ma and 208 ± 10 Ma) reflect crustal heating around 200 Ma, which is associated with the Lower Jurassic Mosquito tholeiitic magmatism and extended the influence of the Central Atlantic Magmatic Province to the Araguaia Belt. This mafic magmatism has been recognized in different regions of the South American and was an important source of heat for hydrocarbon generation in some Paleozoic intracratonic basins.

Thermochronologic constraints on Jurassic rift flank denudation in the Thiel Mountains, Antarctica

USGS Publications Warehouse

Fitzgerald, P.G.; Baldwin, S.L.

2007-01-01

The Thiel Mountains are part of the Transantarctic Mountains (TAM) and occupy a strategic position close to the East-West Antarctic boundary. They occur in a region of relatively subdued topography distal from high topography and high relief of most of the TAM adjacent to the West Antarctic rift system. Low-temperature thermochronology on samples collected from the Reed Ridge granite on the north flank of the Thiel Mountains constrain the thermal and hence tectonic history. Apatite fission track data plus thermal models indicate cooling from ca. 165-150 Ma. In conjunction with 40Ar/39Ar K-feldspar data, the results indicate cooling was due to relatively slow erosional denudation, and not thermal relaxation following Jurassic tholeiitic magmatism. Denudation was most likely associated with the formation of the Jurassic rift system across Antarctica that marked the initial breakup of Gondwana. This is the oldest episode of denudation associated with formation of the present day TAM

Conodont (U Th)/He thermochronology: Initial results, potential, and problems

NASA Astrophysics Data System (ADS)

Peppe, Daniel J.; Reiners, Peter W.

2007-06-01

We performed He diffusion experiments and (U-Th)/He age determinations on conodonts from a variety of locations to explore the potential of conodont (U-Th)/He thermochronology to constrain thermal and exhumation histories of some sedimentary-rock dominated terrains. Based on two diffusion experiments and age results from some specimens, He diffusion in conodont elements appears to be similar to that in Durango apatite fragments of similar size, and closure temperatures are approximately 60-70 °C (for cooling rates of ˜ 10 °C/m.y.). (U-Th)/He ages of conodonts from some locations yield reproducible ages consistent with regional thermal history constraints and, in at least two cases, require a closure temperature lower than ˜ 80 °C. Other samples however, yield irreproducible ages, and in one case yield ages much younger than expected based on regional geologic considerations. These irreproducible samples show inverse correlations between parent nuclides and age consistent with late-stage open-system U-Th behavior.

Tectono-thermal evolution of north Kuqa Depression and South Tian Shan: constraints from apatite (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Xiao, Y.; Qiu, N.; Chang, J.

2017-12-01

The Kuqa Depression, located between Tarim Basin and South Tian Shan Orogen, is considered the most promising site for study the tectono-thermal evolution of the Central Asia Orogenic Belt (CAOB). (U-Th)/He is a new dating method and apatite He ages can record the cooling histories at low temperature between 40 ° 75°1. At present, the low temperature thermochronological data, especially the (U-Th)/He, from north Kuqa Depression are scarce, resulting in controversial issues regarding the cooling history of the upper crust and the latest uplift of South Tian Shan. We present new apatite (U-Th)/He (AHe) thermochronologic data from kuqa depression, aiming to provide insights into exhumation and thermal history of the north kuqa depression. In this study, we firstly present 43 apatite (U-Th)/He ages of 9 samples in north rim of Kuqa Depression. The (U-Th)/He ages range from 1.4±1.0Ma to 17.9±1.2Ma, which are obviously both younger than both the depositional ages and the corresponding AFT ages. To better understand the deformation evolution and thermal history of north Tarim Basin, we use these thermochronological data as inputs for HeFTy software implemented with radiation damage accumulation and annealing model (RDAAM) for time-temperature reconstruction1,2. The relationship between AHe or AFT ages and the relative stratigraphic ages shows that the AHe ages are young and almost about 10 Ma.Thermal modeling using apatite (U-Th)/He ages and geological background information in the same area allow us to obtain a cooling history. The sample T-03 suffered a cooling between 170 and 130Ma, they were re-heated at around 100° between 100 and 40Ma then rapidly cooled and exhumed to reach the surface temperature at around 5Ma. The thermal modeling results indicated the north Kuqa Depression experienced a Miocene rapid cooling event initialed at 12Ma and continued to 5Ma, resulting from far field effect of India-Asia collision. References1. Ketcham, R.A. Forward and reverse Modeling of Low-Temperature Thermochronometry Data. Reviews in Mineralogy and Geochemistry. 58, 275-314(2005). 2. Flowers, R.M. Ketcham, R.A., Shuster, D.l. & Farley, K.A. Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model. Geochimica et Cosmochimica Acta. 73, 2347-2365 (2009).

Cenozoic erosion of the Transantarctic Mountains: A source-to-sink thermochronological study

NASA Astrophysics Data System (ADS)

Zattin, M.; Pace, D.; Andreucci, B.; Rossetti, F.; Talarico, F. M.

2014-09-01

The formation of the Transantarctic Mountains (TAM) is strictly related to the evolution of the West Antarctic Rift system, but the timing of their exhumation is still not fully assessed. In this work, we provide new apatite fission-track data collected on the region between the Royal Society Range and the Britannia Range. Cooling ages are late Eocene-Oligocene in the center of the region but they get older both northwards and southwards. We infer that exhumation was strictly controlled by TAM-parallel fault strands that were active after the Oligocene. The Royal Society Range and the Britannia Range represent transition zones corresponding to transverse structures, probably inherited from early basement crustal discontinuities and reactivated as transfer regions during rift propagation. The exhumation of the investigated region has been then modeled and predicted thermochronological ages have been compared with detrital data from the Miocene sedimentary succession drilled in the Victoria Land Basin. Results indicate that this sector of the TAM is the most probable candidate for the source of sediments and that during the Neogene 3 km (but up to 5 km) of rocks was exhumed.

Exhumation of the Cordillera de Domeyko: Implications for Andean retroarc evolution between the Late Cretaceous and the Oligocene

NASA Astrophysics Data System (ADS)

Henriquez, S.; Carrapa, B.; DeCelles, P. G.

2017-12-01

In Cordilleran-type orogens, exhumation of the thrust belt records the kinematic history of the orogenic system. In the Central Andes, the widest and thickest part of this orogen, several authors have documented the exhumation of the thrust belt in the modern forearc (Chile) and retroarc region (Bolivia and Argentina) showing an overall eastward propagation of deformation since the late Eocene. However, the exhumation of earlier Andean retroarc tectonic events remains poorly documented. In the forearc, the Cordillera de Domeyko and Salar de Atacama basin exhibit multiple pieces of evidence for earlier Andean orogenesis. The goal of this study is to document the thermal record of Late Cretaceous to Eocene retroarc deformation. To this end, this study investigates the cooling history of the easternmost basement uplift of the Cordillera de Domeyko. We couple this record with detrital thermochronology from cobbles in the Late Cretaceous to Miocene sedimentary units from the Salar de Atacama basin which records the unroofing history of this uplift. We employed a multi-dating approach combining apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) thermochronology to constrain the timing and amount of exhumation in the early Andean retroarc region. Our results show episodic cooling ca. 90-80, 65-60 and 45-40 Ma. This new data provides a thermochronologic record of Late Cretaceous and Paleocene deformation in the retroarc region as well as of the widely recognized Eocene deformation event. The cooling signal is interpreted to reflect exhumation controlled by uplift and erosion in the retroarc region. These exhumation events reflect episodes of internal deformation, crustal thickening, and roughly similar amounts of local erosion. Exhumation in this region decreased by the late Oligocene; by this time the orogenic front was established to the east, in the Eastern Cordillera.

Role of Neogene Exhumation and Sedimentation on Critical-Wedge Kinematics in the Zagros Orogenic Belt, Northeastern Iraq, Kurdistan

NASA Astrophysics Data System (ADS)

Koshnaw, R. I.; Horton, B. K.; Stockli, D. F.; Barber, D. E.; Tamar-Agha, M. Y.; Kendall, J. J.

2014-12-01

The Zagros orogenic belt and foreland basin formed during the Cenozoic Arabia-Eurasia collision, but the precise histories of shortening and sediment accumulation remain ambiguous, especially at the NW extent of the fold-thrust belt in Iraqi Kurdistan. This region is characterized by well-preserved successions of Cenozoic clastic foreland-basin fill and deformed Paleozoic-Mesozoic hinterland bedrock. The study area provides an excellent opportunity to investigate the linkage between orogenic wedge behavior and surface processes of erosion and deposition. The aim of this research is to test whether the Zagros orogenic wedge advanced steadily under critical to supercritical wedge conditions involving in-sequence thrusting with minimal erosion or propagated intermittently under subcritical condition involving out-of-sequence deformation with intense erosion. These endmember modes of mountain building can be assessed by integrating geo/thermochronologic and basin analyses techniques, including apatite (U-Th)/He thermochronology, detrital zircon U-Pb geochronology, stratigraphic synthesis, and seismic interpretations. Preliminary apatite (U-Th)/He data indicate activation of the Main Zagros Fault (MZF) at ~10 Ma with frontal thrusts initiating at ~8 Ma. However, thermochronometric results from the intervening Mountain Front Flexure (MFF), located between the MZF and the frontal thrusts, suggest rapid exhumation at ~6 Ma. These results suggest that the MFF, represented by the thrust-cored Qaradagh anticline, represents a major episode of out-of-sequence deformation. Detrital zircon U-Pb analyses from the Neogene foreland-basin deposits show continuous sediment derivation from sources to the NNE in Iraq and western Iran, suggesting that out-of-sequence thrusting did not significantly alter sedimentary provenance. Rather, intense hinterland erosion and recycling of older foreland-basin fill dominated sediment delivery to the basin. The irregular distribution of thermochronologic ages, hinterland growth, extensive erosion, and recycled sediment in the Neogene foreland basin imply that the Zagros orogenic wedge in the Iraqi Kurdistan region largely developed under subcritical wedge conditions.

Thermal and exhumation history of the central Tianshan (NW China): Constraints by U-Pb geochronology and Ar-Ar and (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Yin, J.; Chen, W.; Hodges, K. V.; Xiao, W.; Van Soest, M. C.; Cai, K.; Zhang, B.; Mercer, C. M.; Yuan, C.

2015-12-01

Geochronology and thermochronology using multiple mineral-isotopic chronometers reveals the thermo-tectonic history of the central Tianshan (NW China) from emplacement to exhumation. Granites from the central Tianshan, which are associated with the southward subduction of the northern Tianshan Ocean, have been dated at 362-354 Ma using the LA-ICP-MS Zircon U-Pb method. A younger diorite sample (282 ± 1 Ma, Zircon U-Pb method by LA-ICP-MS) from northern Tianshan formed during the final closure of the Northern Tianshan Ocean when the Junggar Block collided with the Yili-Central Tianshan Block. 40Ar/39Ar step-wise heating plateau dates (biotite Ar/Ar: 312-293 Ma; Plagioclase Ar/Ar: 270-229 Ma) from the Central Tianshan show rapid post-magmatic cooling during the Late Carboniferous-Early Permian followed by a more modest rate of cooling from the middle Permian to the middle Jurassic. The northern Tianshan diorite (biotite Ar/Ar: 240 ± 1 Ma) also reveals a middle Jurassic cooling. Apatite (U-Th )/He dates from the central Tianshan samples range from ca. 130 Ma to ca. 116 Ma. The Apatite (U-Th )/He date for the northern Tianshan sample is ca. 27 Ma. Previous studies also reported Apatite (U-Th)/He ages of ca. 44 Ma-11 Ma in the Baluntai area of the southern Central Tianshan[1]. Two episodes of cooling are distinguished by thermal history modelling: (1) Mesozoic cooling occurred as the result of the exhumation and tectonic reactivation of the central Tianshan; and (2) The Tianshan orogenic belt has been rapidly exhumed since the Middle Cenozoic. References [1] Lü, H.H., Chang, Y., Wang, W., Zhou, Z.Y., 2013. Rapid exhumation of the Tianshan Mountains since the early Miocene: Evidence from combined apatite fission track and (U-Th)/He thermochronology. Science China: Earth Sciences, 43(12): 1964-1974 (in Chinese).

(U-Th-Sm)/He thermochronological age distribution in a slowly cooled plutonic complex (Ploumanac'h intrusion, France): insights into helium diffusion processes.

NASA Astrophysics Data System (ADS)

Recanati, A. C.; Gautheron, C.; Barbarand, J.; Tassan-Got, L.; Missenard, Y.; Pinna-Jamme, R.

2015-12-01

(U-Th-Sm)/He thermochronology is widely used to determine the thermal histories of mountain ranges and sedimentary basins. Apatite crystals retain helium at low temperatures, thus giving an insight into upper crustal evolution (e.g. exhumation, subsidence, erosion). Advanced models predict He production and diffusion rates in apatite crystals, thereby allowing determination of helium dates by integration over time/temperature paths (e.g. Gautheron et al., 2009). However, scattered dates and discordance between predicted and measured dates suggest that other parameters than time or temperature may also influence helium contents in apatite. The present study determines the variables that affect He diffusion in apatite over long timescales. We report the (U-Th-Sm)/He date distribution within a slowly cooled intrusion, along with AFT data, as well as extensive petrological and chemical characterization. The Ploumanac'h site (Brittany, France) was chosen because it includes small-scale spatial variations in petrology and chemistry (6-9106 tracks/cm2). GAUTHERON C., TASSAN-GOT L., BARBARAND J., PAGEL M., 2009. Effect of alpha-damage annealing on apatite (U-Th)/He thermochronology. Chem. Geol. 266, 166-179.

From ocean depths to mountain tops: uplift of the Troodos Massif (Cyprus) constrained by (U-Th)/He thermochronology and geomorphic analysis

NASA Astrophysics Data System (ADS)

Morag, N.; Haviv, I.; Katzir, Y.

2013-12-01

The Troodos Massif of Cyprus, rising to nearly 2000 meters above sea level, encompasses one of the world's classic ophiolites. Following its formation at a seafloor spreading center in Late Cretaceous times, this slice of the NeoTethyan oceanic lithosphere was uplifted and eventually exposed on mountain tops during the Neogene. The final uplift and exhumation of the Troodos was previously assigned to Pleistocene age by observations in the circum-Troodos sedimentary strata. However, quantitative thermochronological and geomorphological data from the Massif itself were not available. Here we use apatite (U-Th)/He low-temperature thermochronology complemented by zircon (U-Th)/He and apatite fission track data, and combined with geomorphic analysis to constrain the exhumation and uplift history of the Troodos ophiolite. Apatite (U-Th)/He ages vary with depth from ~ 22 Ma at the top of the Gabbro sequence to ~ 6 Ma at the bottom of the sequence. The deepest sample from a Gabbro pegmatitic dyke intruding the ultramafic sequence yielded an age of ~ 3 Ma. Thermal modeling of apatite (U-Th)/He and fission track data delineates Plio - Pleistocene initiation of rapid uplift and exhumation of the Troodos ophiolite. The estimated cumulative exhumation since its initiation is 2-3 km. No evidence was found for significant uplift of the central Troodos area prior to that time. The geomorphic analysis delineates a bull's-eye zone at the center of the Troodos Massif, where local relief and channel steepness index are highest. The boundaries of this zone roughly correspond with the Mt. Olympus mantle outcrop and suggest recent, differential uplift of this zone relative to its surroundings. The most likely mechanism, which could drive such a focused bull's-eye uplift pattern is hydration of ultramafic rocks (serpentinization) leading to a decrease in rock density and subsequent diapiric uplift of the serpentinized lithospheric mantle.

Radiation damage-He diffusivity models applied to deep-time thermochronology: Zircon and titanite (U-Th)/He datasets from cratonic settings

NASA Astrophysics Data System (ADS)

Guenthner, W.; DeLucia, M. S.; Marshak, S.; Reiners, P. W.; Drake, H.; Thomson, S.; Ault, A. K.; Tillberg, M.

2017-12-01

Advances in understanding the effects of radiation damage on He diffusion in uranium-bearing accessory minerals have shown the utility of damage-diffusivity models for interpreting datasets from geologic settings with long-term, low-temperature thermal histories. Craton interiors preserve a billion-year record of long-term, long-wavelength vertical motions of the lithosphere. Prior thermochronologic work in these settings has focused on radiation damage models used in conjunction with apatite (U-Th)/He dates to constrain Phanerozoic thermal histories. Owing to the more complex damage-diffusivity relationship in zircon, the zircon (U-Th)/He system yields both higher and, in some cases, lower temperature sensitivities than the apatite system, and this greater range in turn allows researchers to access deeper time (i.e., Proterozoic) segments of craton time-temperature histories. Here, we show two examples of this approach by focusing on zircon (U-Th)/He datasets from 1.8 Ga granitoids of the Fennoscandian Shield in southeastern Sweden, and 1.4 Ga granites and rhyolites of the Ozark Plateau in southeastern Missouri. In the Ozark dataset, the zircon (U-Th)/He data, combined with a damage-diffusivity model, predict negative correlations between date and effective uranium (eU) concentration (a measurement proportional to radiation damage) from thermal histories that include an episode of Proterozoic cooling (interpreted as exhumation) following reheating (interpreted as burial) to temperature of 260°C at 850-680 Ma. In the Fennoscandian Shield, a similar damage model-based approach yields time-temperature constraints with burial to 217°C between 944 Ma and 851 Ma, followed by exhumation from 850 to 500 Ma, and burial to 154°C between 366 Ma and 224 Ma. Our Fennoscandian Shield samples also include titanite (U-Th)/He dates that span a wide range (945-160 Ma) and are negatively correlated with eU concentration, analogous to our zircon He dataset. These results support the initial findings of Baughman et al. (2017, Tectonics), and suggest that further research into the radiation damage effect on He diffusion in titanite could yield a comprehensive damage-diffusivity model for the titanite (U-Th)/He thermochronometer.

Preliminary constraints on the kinetics of OSL thermochronology

NASA Astrophysics Data System (ADS)

Guralnik, B.; Herman, F.; Lowick, S.; Preusser, F.; Rhodes, E. J.

2010-12-01

Utilizing the fact that temperature exerts a strong control on the retention of radiogenic products at their production sites (and hence their rate of accumulation), thermochronology is an extension of geochronology used to elucidate the cooling histories (t-T trajectories) of rocks over time (Dodson, 1973). Here we present a potential thermochronometer based on the optically stimulated luminescence (OSL) signal of bedrock quartz, with an estimated “closure” temperature of ~25-40°C (-dT/dt=10°C/Ma). In this method, the radiogenic “products” measured are electrons trapped at the naturally occurring lattice defects (electrically unbalanced sites). These electrons can be subsequently released back into the conduction band by exposure to heat and/or light. The mean lifetime of an electron in a particular trapping site is well described by an Arrhenius relationship of the form τ=s-1exp(-E/kT), where E and s are activation energy and frequency factors, respectively, and k is Boltzmann’s constant. Reported lifetimes of the major trap types at 20°C are well beyond Ma-timescale, but for typical natural radioactivity dose rates, these traps saturate well before ~0.5 Ma. In the current work, we analyzed the OSL signals of bedrock quartz from the KTB-VB drillhole (Southern Germany), where long-term stable temperature conditions provide a natural isothermal holding experiment at a Ma-timescale. Eleven core samples from a depth range of 560 m - 2335 m (22°C - 71°C, respectively), were processed to obtain an unbleached quartz fraction of 180-250 µm. Natural and regenerated luminescence signals were measured by stimulation with infrared and then blue light using a single-aliquot regeneration (SAR) protocol. As a function of sample depth, the measured OSL signal varies from a saturated signal at shallow depths to reset signals below a depth of 1 km (i.e., ambient rock temperature higher than 40°C). Several unstable trapping sites with potentially different closure temperatures are suggested by comparison of the natural to regenerated signals.

Tectonic and Climatic Implications of Late Miocene to Pleistocene (5.8-1.8 Ma) Paleo-Erosion Rates from the Rio Iruya Canyon, Northwest Argentina (23°S)

NASA Astrophysics Data System (ADS)

Fisher, G. B.; Amidon, W. H.; Luna, L. V.; Burbank, D. W.

2015-12-01

One fundamental hypothesis that underpins tectonic geomorphology is that climate can modify the pattern and magnitude of erosion in orogenic landscapes and in turn control deformation. While conceptually appealing, empirical evidence is often ambiguous owing to the inherent spatial coupling between present-day tectonic and precipitation maxima and/or the long-term blurring of climate signals by thermochronologic techniques. Although cosmogenic nuclides provide considerable insight into centennial to millennial scale tectonic-erosion-climate linkages, extracting long-term records of erosion from older sedimentary deposits has proved challenging. If successful, such records have the potential to reveal long-term relationships between erosion, uplift, and climate, which should integrate over time to match long term exhumation rates obtained from low temperature thermochronology. Here we utilize a unique field setting along a 100-m deep, young canyon (~100 years old) along the Rio Iruya in northwestern Argentina to create a high-resolution (~100 kyr) terrestrial record of paleo-erosion rates in the eastern Cordillera spanning the late Miocene to Pleistocene (5.8-1.8 Mya). In total, 49 cosmogenic 10Be samples were analyzed along with detailed magnetostratigraphy, U-Pb tephra ages, detrital zircon, and quartz trace elements to yield a detailed paleo-erosion rate, chronology, and provenance record for the Rio Iruya section. Apparent erosion rates occur in three different regimes: from 5.8-4.0 Ma rates are high with little variability, from 4.0- 2.3 Ma rates oscillate by a factor of 5 on a ~400 kyr timescale, and from 2.3-1.8 Ma they are again high without clear oscillations. These three regimes correspond to changes in provenance recorded by detrital zircons and quartz chemistry, and suggest that during the late Pliocene the eastern Cordillera was responding strongly to the 400 kyr eccentricity paced orbital frequency. This unique finding is both perplexing and encouraging as it argues for a coupling of sediment flux to broad-scale climate teleconnections and may evidence a frequency dependent response of the Andean orogen to climate oscillations, consistent with recent numerical and theoretical models.

Thermochronology in southeast Alaska and southwest Yukon: Implications for North American Plate response to terrane accretion

NASA Astrophysics Data System (ADS)

Enkelmann, Eva; Piestrzeniewicz, Adam; Falkowski, Sarah; Stübner, Konstanze; Ehlers, Todd A.

2017-01-01

This study presents the first comprehensive dataset of low-temperature thermochronology from 43 bedrock samples collected north of the active Yakutat-North American plate boundary. Our apatite and zircon (U-Th)/He and fission-track data reveal the cooling history of the inboard Wrangellia Composite Terrane that is dominated by rapid cooling after Late Jurassic to Early Cretaceous arc magmatism followed by very little cooling and exhumation until today. Deformation resulting in rock exhumation due to the collision of the Yakutat microplate is spatially very limited (20-30 km) and is concentrated mainly in the Chugach-Prince William Terrane and rocks near the Border Ranges Fault. Focused exhumation from greater depths of ca. 10 km with very high rates (>5 km/Myr) is localized at the syntaxis region, starting ca. 10 Ma and shifted south through time. The rapid exhumation rates are explained by the development of strong feedbacks between tectonically driven surface uplift and erosion, which started already before glaciation of the area. The shift in the location towards the south is a consequence of continuous readjusting between tectonics and climate, which is changing on local and global scales since the Late Miocene.

Cenozoic topographic build-up of the Iranien plateau: first constraints from low-temperature thermochronology

NASA Astrophysics Data System (ADS)

François, Thomas; Agard, Philippe; Meyer, Bertrand; Zarrinkoub, Mohammad; Chung, Sun-Lin; Bernet, Matthias; Burov, Evgueni

2013-04-01

The Iranian plateau is a smooth topographic high at the rear of the Zagros mountains, with average elevation of c. 1.5 km. Its formation is thought to result from the collision between the Arabian and Eurasian plates since ~35 Myrs, following a long-standing subduction, and represents an interesting analogue to the so far better documented Tibetan plateau. Yet, while the Zagros orogeny was reappraised by numerous authors over the past few years, the topographic build-up of both the Zagros and the Iranian plateau remains ill-constrained. We herein present (U-Th)/He and fission track (FT) thermochronology results to reconstruct the Cenozoic tectonic evolution of the Iranien plateau and quantify the age and amount of vertical movements. Apatite and zircon single grain cooling age data were collected on plutonic rocks (for which crystallization ages were already available: Chiu et al., 2010) from the internal domains of Sanandaj-Sirjan Zone (SSZ), Urumieh-Doktar magmatic arc (UDMA), Central Iran and, for comparison, Kopet Dagh. We stress that an important milestone for topographic build-up is the presence of the marine Qom formation (coeval with the external Asmari formation) in the UDMA and part of the SSZ, indicating that the plateau was at or near sea level at 20 Ma. Temperature time paths inferred from low temperature thermochronology suggest a spatial and temporal separation of exhumation processes. The results show that the SSZ was exhumed very early in the collision process (essentially before 20 Ma), with a likely acceleration around the Oligocene (i.e., at the onset of continental collision) from 0.05 to 0.3 mm/yr. Post-collision cooling along the UDMA is marked by an average, constant exhumation rate of 0.3-0.4 mm/yr, which suggests that no significant increase or decrease of erosion occurred since continental collision. In Central Iran, the overlap (within error) of ZrFT, AFT and AHe ages from gneissic samples points to their rapid cooling during the upper Eocene (~42°C/Ma). These results are consistent with the reported formation of several small metamorphic core-complexes in Central Iran towards the end of oceanic subduction, possibly associated with slab rollback. Our thermochronological data allow to locate major topographic and erosional changes during the Cenozoic. Topographic build-up occurred in the SSZ during oceanic subduction and onset of collision (35 Ma) and shifted to the UDMA during continental collision (20 Ma), while progressing towards the external parts of the belt during the Mio-Pliocene (5-10 Ma). Most importantly, we conclude that the uplift of the Iranian plateau was a constant, steady process over the last 20 Ma, at least as a first approximation, as inferred from the combination of constant exhumation in the UDMA and sedimentary records of the central Iranian basin.

South Virgin-White Hills detachment fault system of SE Nevada and NW Arizona: Applying apatite fission track thermochronology to constrain the tectonic evolution of a major continental detachment fault

NASA Astrophysics Data System (ADS)

Fitzgerald, Paul G.; Duebendorfer, Ernest M.; Faulds, James E.; O'Sullivan, Paul

2009-04-01

The South Virgin-White Hills detachment (SVWHD) in the central Basin and Range province with an along-strike extent of ˜60 km is a major continental detachment fault system. Displacement on the SVWHD decreases north to south from ˜17 to <6 km. This is accompanied by a change in fault and footwall rock type from mylonite overprinted by cataclasite to chlorite cataclasite and then fault breccia reflecting decreasing fault displacement and footwall exhumation. Apatite fission track (AFT) thermochronology was applied both along-strike and across-strike to assess this displacement gradient. The overall thermal history reflects Laramide cooling (˜75 Ma) and then rapid cooling beginning in the late early Miocene. Age patterns reflect some complexity but extension along the SVWHD appears synchronous with rapid cooling initiated at ˜17 Ma due to tectonic exhumation. Slip rate is more rapid (˜8.6 km/Ma) in the north compared to ˜1 km/Ma in the south. The displacement gradient results from penecontemporaneous along-strike motion and formation of the SVWHD by linkage of originally separate fault segments that have differential displacements and hence differential slip rates. East-west transverse structures likely play a role in linkage of different fault segments. The preextension paleogeothermal gradient is well constrained in the Gold Butte block as 18-20°C/km. We present a new thermochronologic approach to constrain fault dip during slip, treating the vertical exhumation rate and the slip as vectors, with the angle between them used to constrain fault dip during slip through the closure temperature of a particular thermochronometer. AFT data from the western rim of the Colorado Plateau constrain the initiation of timing of cooling associated with the Laramide Orogeny at ˜75 Ma, and a reheating event in the late Eocene/early Oligocene associated with burial by sediments ("rim gravels") most likely shed from the Kingman High to the west of the plateau.

Deciphering the post-cratonization history of the Kaapvaal craton, South Africa from titanite and zircon (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Baughman, J. S.; Flowers, R. M.

2017-12-01

Cratons are the most stable portions of continents, but the degree to which they are affected by post-cratonization tectonic and magmatic processes is unclear. Complete time-temperature (t-T) histories are necessary to understand the timing, extent, and characteristics of post-cratonization events that disrupted these regions. However, deciphering extended cratonic t-T records is difficult owing to the incomplete stratigraphic records of continental interior settings, and the challenge of accessing the appropriate thermal history range with conventional thermochronometers. The Kaapvaal craton in South Africa is an archetypal craton that initially stabilized in the Archean and was subsequently affected by magmatic and marginal accretionary events. Here we exploit titanite and zircon (U-Th)/He (THe, ZHe) thermochronology to better decipher the somewhat cryptic Proterozoic through early Paleozoic history of the craton. Radiation damage effects on the He diffusivity of these two minerals provides the potential to access a wide temperature window from 200°C to near surface conditions. Existing low-temperature apatite (U-Th)/He and fission-track results constrain Late Paleozoic to Mesozoic burial of the Karoo basin and subsequent Cretaceous unroofing, while 40Ar/39Ar and Rb-Sr data document cooling through temperatures of 300°C by 2 Ga. We obtained THe and ZHe dates from across the northern Kaapvaal craton to fill in the thermal history gap between these constraints. THe and ZHe dates range from 1200 to 200 Ma, and 1000 to 30 Ma, respectively. Both sets of dates are negatively correlated with effective uranium concentration (eU), manifesting the effect of radiation damage on the He retentivity, and therefore closure temperature, of these minerals. The results allow us to assess the Mesoproterozoic through present day thermal history of the northern Kaapvaal craton. The THe data suggest that Mesoproterozoic exhumation and large-scale reheating associated with Namaqua-Natal orogenesis affected the region. While, the ZHe data constrain maximum burial temperatures during Mesozoic Karoo burial. Together the results show the power of exploiting thermochronologic advances to access new information about long-term continental evolution.

Late Eocene Inversion and Exhumation of the Sivas Basin (Central Anatolia) Based On Low-Temperature Thermochronometry: Implications for Diachronous Initiation of Arabia-Eurasia Collision

NASA Astrophysics Data System (ADS)

Darin, M. H.; Umhoefer, P. J.; Thomson, S. N.; Schleiffarth, W. K.

2017-12-01

The timing of initial Arabia-Eurasia collision along the Bitlis-Zagros suture is controversial, with widely varying estimates from middle Eocene to late Miocene ( 45-10 Ma). The Cenozoic Sivas Basin (central Anatolia) preserves a detailed record of the initial stages of Arabia collision directly north of the suture in the Eurasian foreland. New apatite fission track and (U-Th)/He thermochronology data from Late Cretaceous to Paleogene units indicate rapid basin inversion and initiation of the north-vergent Southern Sivas Fold and Thrust Belt (SSFTB) during the late Eocene to early Oligocene ( 40-30 Ma), consistent with the age of a basin-wide unconformity and switch from marine to nonmarine sedimentation. We interpret late Eocene exhumation and the predominantly north-vergent kinematics of the SSFTB to reflect northward propagation of contraction into the Sivas retro-foreland basin due to initial collision of the Arabian passive margin with the Anatolide-Tauride block along the southern Eurasian margin during the late middle Eocene. We test this hypothesis by comparing our new results with regional-scale compilations of both published thermochronology and geochronology data from the entire Arabia-Eurasia collision zone. Low-temperature thermochronology data from eastern Anatolia, the Caucasus, Zagros, and Alborz demonstrate that rapid cooling and intraplate deformation occurred across much of the Eurasian foreland during the middle Eocene to early Oligocene ( 45-30 Ma). Our regional compilation of published geochronology data from central and eastern Anatolia reveals a distinct magmatic lull during the latest Eocene, Oligocene, and earliest Miocene (ca. 38-20 Ma), slightly earlier than a diachronous magmatic lull initiating at 25-5 Ma from northwest to southeast in Iran (Chiu et al., 2013). These results support a tectonic model for diachronous collision in which initial collision of the Arabia promontory occurred in central-eastern Anatolia during the middle-late Eocene ( 45-35 Ma) followed by magmatic quiescence due to subduction termination. Collisional strain transferred into the Caucasus, Alborz, and northwestern Zagros by the late Eocene to early Oligocene and became progressively younger towards the southeast along the Zagros suture from the late Oligocene to late Miocene.

Apatite U-Pb thermochronolgy applied to complex geological settings - insights from geo/thermochronology and geochemistry

NASA Astrophysics Data System (ADS)

Paul, Andre; Spikings, Richard; Ulyanov, Alexey; Chew, David

2016-04-01

Application of high temperature (>350oC) thermochronology is limited to the U-Pb system of accessory minerals, such as apatite, under the assumption that radiogenic lead is lost to thermally activated volume diffusion into an infinite reservoir. Cochrane et al. (2015) have demonstrated a working example from the northern Andes of South America. Predictions from volume diffusion theory were compared with measured single grain U-Pb date correlated to shortest diffusion radius and in-situ profiles measured by LA-ICP-MS. Results from both techniques were found to be in agreement with predictions from thermally activated, volume diffusion. However, outliers from the ID-TIMS data suggested some complexity, as grains were found to be too young relative to their diffusion radius. Interaction of multiple processes can be responsible for the alteration of apatite U-Pb dates such as: (1) metamorphic (over)growth, (2) fluid aided alteration/recrystallization and (3) metamictization and fracturing of the grain. Further, predictions from volume diffusion rely on the input parameters: (a) diffusivity, (b) activation energy and (c) shortest diffusion radius. Diffusivity and activation energy are potentially influenced by the chemical composition and subsequent changes in crystal structure. Currently there is one value for diffusion parameter and activation energy established for (Durango) apatite (Cherniak et al., 1991). Correlation between diffusivity/activation energy and composition has not been established. We investigate if correlations exist between diffusivity/activation energy and composition by obtaining single grain apatite U-Pb date and chemical compostion and correlating these to their diffusion radius. We test the consistency of apatite closure temperature, by comparing the apatite U-Pb dates with lower temperature thermochronometers such as white mica and K-feldspar Ar/Ar and by petrographic observations. We test if chemical information can be a proxy to identify metamorphic (over)growth and fluid aided alteration/recrystallization. We seek to evaluate if apatite U-Pb thermochronology can be applied to a broad range of rock types and geological environments or if limitations must be drawn.

Low-Temperature Thermochronology Investigation in Uruguay and Southernmost Brazil: Apatite (U-Th)/He Results

NASA Astrophysics Data System (ADS)

Machado, J. P.; Bicca, M. M.

2017-12-01

Low-temperature thermochronology has successfully allowed one to reveal exhumation histories of many orogenic belts across the Earth, and lately these techniques have been applied in cratonic regions. The present study aims to constrain thermal history and exhumation of the South Atlantic Passive Margin, between Uruguay and Southernmost Brazil, a region scarce of thermochronological data. This location has become relatively stable after the Neoproterozoic Brasiliano Orogeny, being more intensely disturbed by tectonics during the Gondwana Breakup and consequent opening of the Atlantic Ocean (Jurassic - Cretaceous). Both apatite and zircon (U-Th)/He methods are being applied on basement rocks, and since those have a long cooling history, radiation damage is expected to play an important role in crystal ages. A total of 25 samples were analyzed, and preliminary apatite (U-Th)/He results reveals unweighted sample ages ranging from Permian to Lower-Cretaceous in Southernmost Brazil, with a couple of outliers with Devonian - Carboniferous ages. In Uruguay results can be grouped in two different clusters: one of samples with Jurassic to Lower-Cretaceous ages, and another of Devonian to Permian ages. This wide range of results can be assign to variations in the uranium content of the grains, due the presence of inclusions, compositional zonation or substantial radiation damage of the crystalline lattice. In most cases, ages tend to increase with crystal eU content. No clear relationship between ages and tectonic terranes has been found so far, neither between ages and elevation, since the region does not have significant topographic variations. Younger ages are commonly found closer to the coastal region, possibly in response to the rift shoulders uplift during Gondwana breakup and further exhumation and denudation at higher rates. An anomalous concentration of older ages in the southern region of the Pelotas Batholith, close to the Brazil - Uruguay border, suggests a geotectonic stable area near the margin. A NW-trend of younger ages throughout southernmost Brazilian Shield may reflect cooling triggered by the structural reactivation of Precambrian basement fabrics. Hopefully, future work will permit a better comprehension of the regional exhumation patterns during the Phanerozoic.

Characterization of Stream Channel Evolution Due to Extensional Tectonics Along the Western Margin of North Boulder Basin (Bull Mountain), SW Montana with the Use of Geologic Mapping and Thermochronologic (U-Th/He) Dating.

NASA Astrophysics Data System (ADS)

Cataldo, K.; Douglas, B. J.; Yanites, B.

2017-12-01

Landscape response to active tectonics, such as fault motion or regional uplift, can be recorded in river profiles as changes in slope (i.e. knickpoints) or topography. North Boulder basin region (SW Montana), experienced two separate phases of extension, from 45 - 35 Ma and again beginning 14 Ma to the present, producing basin-and-range style fault-blocks. Focusing on the Bull Mountain region, located on the western margin of the North Boulder basin, data is collected to test the hypothesis that Bull Mountain is located on the hanging wall of a half-graben. Our objective is to elucidate the active tectonics of the study area within a regional context by utilizing river profile analysis and thermochronometric data. High-resolution (< 5cm) river profile data is obtained from five of the main tributaries of Bull Mountain. Comprehensive geologic mapping along the main tributaries and topographic highs of the region allowed for the identification and measurement of knickpoints, composition of detailed lithologic descriptions, and analysis of key structural features. The absence of knickpoints within the four tributaries mapped on east Bull Mountain are consistent with a lack of tectonic activity. In contrast, Dearborn Creek, on western Bull Mountain, is located along an active normal fault and presents several knickpoints. Geologic mapping confirms that the primary lithologies of the region belong to the Elkhorn Mountain Volcanics. At lower elevations, there are massive plutonic intrusions of Quartz Monzonite and Diorite, both constituents of the Boulder batholith. These lithologies contain minerals suited for low-temperature thermochronology (U-Th/He) to constrain the timing of tectonic activity (i.e. uplift and exhumation) and erosion rates in the region. High-resolution stream profiles and a 10m DEM are used to delineate watersheds and produce steepness and concavity maps of major tributaries to investigate changes in slope or topography. The effects of extensional tectonic events can reshape drainage patterns of streams and their distribution of water, which is an important commodity in SW Montana for ranchers and farmers. Thus, the ability to discern the probability of recurring tectonic events and the effects on the regional watersheds, could help facilitate solutions before these events take place.

Quantitative Identification of the Annealing Degree of Apatite Fission Tracks Using Terahertz Time Domain Spectroscopy (THz-TDS).

PubMed

Wu, Hang; Wu, Shixiang; Qiu, Nansheng; Chang, Jian; Bao, Rima; Zhang, Xin; Liu, Nian; Liu, Shuai

2018-01-01

Apatite fission-track (AFT) analysis, a widely used low-temperature thermochronology method, can provide details of the hydrocarbon generation history of source rocks for use in hydrocarbon exploration. The AFT method is based on the annealing behavior of fission tracks generated by 238 U fission in apatite particles during geological history. Due to the cumbersome experimental steps and high expense, it is imperative to find an efficient and inexpensive technique to determinate the annealing degree of AFT. In this study, on the basis of the ellipsoid configuration of tracks, the track volume fraction model (TVFM) is established and the fission-track volume index is proposed. Furthermore, terahertz time domain spectroscopy (THz-TDS) is used for the first time to identify the variation of the AFT annealing degree of Durango apatite particles heated at 20, 275, 300, 325, 450, and 500 ℃ for 10 h. The THz absorbance of the sample increases with the degree of annealing. In addition, the THz absorption index is exponentially related to annealing temperature and can be used to characterize the fission-track volume index. Terahertz time domain spectroscopy can be an ancillary technique for AFT thermochronological research. More work is urgently needed to extrapolate experimental data to geological conditions.

The asymmetric evolution of the Colombian Eastern Cordillera. Tectonic inheritance or climatic forcing? New evidence from thermochronology and sedimentology

NASA Astrophysics Data System (ADS)

Ramirez-Arias, Juan Carlos; Mora, Andrés; Rubiano, Jorge; Duddy, Ian; Parra, Mauricio; Moreno, Nestor; Stockli, Daniel; Casallas, Wilson

2012-11-01

New thermochronological data, facies, paleocurrents and provenance allow us to refine the chronology of deformation in the central segment of the Colombian Eastern Cordillera. Based on a new extensive AFT dataset, we document the spatial evolution of active deformation, from the axial zone of the Eastern Cordillera at about 50 Ma in to active growth of the frontal thin skinned structures in Late Miocene time. Paleocurrents allow us to push backwards into the Middle to Early Late-Miocene the emergence of the easternmost frontal thrust; whereas careful assessment of exposure gates tied to AFT data enable to refine the unroofing history for Eocene to Miocene times. Based on that, we produced a kinematically restored cross section with higher resolution than previous assessments. Using these datasets, we compare the evolution of the central segment of the Eastern Cordillera in this region with the evolution of adjacent areas in the context of climatic forcing of orogenic evolution. We find that in this region and, in the Eastern Cordillera in general, tectonic inheritance and transpression exert an initial dominant control on the initial orogen asymmetry, which is later enhanced due to an orographically-focused erosion. We therefore suggest that it is not climate alone the factor controlling orogenic asymmetry in the Eastern Cordillera of Colombia.

Mass spectrometry in Earth sciences: the precise and accurate measurement of time.

PubMed

Schaltegger, Urs; Wotzlaw, Jörn-Frederik; Ovtcharova, Maria; Chiaradia, Massimo; Spikings, Richard

2014-01-01

Precise determinations of the isotopic compositions of a variety of elements is a widely applied tool in Earth sciences. Isotope ratios are used to quantify rates of geological processes that occurred during the previous 4.5 billion years, and also at the present time. An outstanding application is geochronology, which utilizes the production of radiogenic daughter isotopes by the radioactive decay of parent isotopes. Geochronological tools, involving isotopic analysis of selected elements from smallest volumes of minerals by thermal ionization mass spectrometry, provide precise and accurate measurements of time throughout the geological history of our planet over nine orders of magnitude, from the accretion of the proto-planetary disk, to the timing of the last glaciation. This article summarizes the recent efforts of the Isotope Geochemistry, Geochronology and Thermochronology research group at the University of Geneva to advance the U-Pb geochronological tool to achieve unprecedented precision and accuracy, and presents two examples of its application to two significant open questions in Earth sciences: what are the triggers and timescales of volcanic supereruptions, and what were the causes of mass extinctions in the geological past, driven by global climatic and environmental deterioration?

Long-term evolution of the western South Atlantic passive continental margin in a key area of SE Brazil revealed by thermokinematic numerical modeling using the software code Pecube

NASA Astrophysics Data System (ADS)

Stippich, Christian; Krob, Florian; Glasmacher, Ulrich A.; Hackspacher, Peter C.

2016-04-01

The aim of the research is to quantify the long-term evolution of the western South Atlantic passive continental margin (SAPCM) in SE-Brazil. Excellent onshore outcrop conditions and extensive pre-rift to post-rift archives between São Paulo and Laguna allow a high precision quantification of exhumation, and rock uplift rates, influencing physical parameters, long-term acting forces, and process-response systems. Research will integrate published1 and partly published thermochronological data from Brazil, and test lately published new concepts on causes of long-term landscape and lithospheric evolution in southern Brazil. Six distinct lithospheric blocks (Laguna, Florianópolis, Curitiba, Ilha Comprida, Peruibe and Santos), which are separated by fracture zones1 are characterized by individual thermochronological age spectra. Furthermore, the thermal evolution derived by numerical modeling indicates variable post-rift exhumation histories of these blocks. In this context, we will provide information on the causes for the complex exhumation history of the Florianópolis, and adjacent blocks. The climate-continental margin-mantle coupled process-response system is caused by the interaction between endogenous and exogenous forces, which are related to the mantle-process driven rift - drift - passive continental margin evolution of the South Atlantic, and the climate change since the Early/Late Cretaceous climate maximum. Special emphasis will be given to the influence of long-living transform faults such as the Florianopolis Fracture Zone (FFZ) on the long-term topography evolution of the SAPCM's. A long-term landscape evolution model with process rates will be achieved by thermo-kinematic 3-D modeling (software code PECUBE2,3 and FastScape4). Testing model solutions obtained for a multidimensional parameter space against the real thermochronological and geomorphological data set, the most likely combinations of parameter rates, and values can be constrained. The data and models will allow separating the exogenous and endogenous forces and their process rates. References 1. Karl, M., Glasmacher, U.A., Kollenz, S., Franco-Magalhaes, A.O.B., Stockli, D.F., Hackspacher, P., 2013. Evolution of the South Atlantic passive continental margin in southern Brazil derived from zircon and apatite (U-Th-Sm)/He and fission-track data. Tectonophysics, Volume 604, Pages 224-244. 2. Braun, J., 2003. Pecube: A new finite element code to solve the 3D heat transport equation including the effects of a time-varying, finite amplitude surface topography. Computers and Geosciences, v.29, pp.787-794. 3. Braun, J., van der Beek, P., Valla, P., Robert, X., Herman, F., Goltzbacj, C., Pedersen, V., Perry, C., Simon-Labric, T., Prigent, C. 2012. Quantifying rates of landscape evolution and tectonic processes by thermochronology and numerical modeling of crustal heat transport using PECUBE. Tectonophysics, v.524-525, pp.1-28. 4. Braun, J. and Willett, S.D., 2013. A very efficient, O(n), implicit and parallel method to solve the basic stream power law equation governing fluvial incision and landscape evolution. Geomorphology, v.180-181, 170-179.

Mirror denudation pattern on both sides of the Central Atlantic - a trace of the Pangea break-up?

NASA Astrophysics Data System (ADS)

Ruiz, G.; Saddiqi, O.; Sebti, S.; Negro, F.; Stuart, F.; Foeken, J.; Frizon de Lamotte, D.; Kramers, J.

2009-04-01

The target area of this project is the WSW- ENE oriented intra-continental Atlas chain in Morocco located between the West Africa Craton and the Betic-Rif system. It is a key natural laboratory because it 1) is the southernmost expression of Alpine deformation in Africa, and 2) encompasses Pre-Cambrian to recent evolution of the region. The presence of high surface elevations today in the High-Atlas (>4000m) and Anti-Atlas (>2500m) to the south is subject to discussions because there is little quantitative data available at present. Phases of uplift are thus ill constrained as places where the associated erosion products were accumulated. To better constrain the most recent orogenic growth of the Atlas chain, we selected a section located to the SW of Morocco, and investigated the time-Temperature paths from the different morpho-structural domains using low-temperature thermochronology analyses. These are Fission-Track analysis on Apatite (120-60°C), Zircon (270-210°C) and U-Th/He analysis still on Apatite (80-45°C) and Zircon (200-170°C) minerals. Results are much contrasted from one domain to the other: Pre-Cambrian bedrocks from the Anti-Atlas domain yield old thermochronological Fission-Track ages on zircon (380-300 Ma) and apatite (180-120 Ma) minerals that are associated with slightly younger (U-Th)/He ages on apatite (150-110 Ma). U-Th/He ages on apatite from the High-Atlas are much younger (~35-5 Ma) with a clear alpine signature. Apatite Fission-Track ages from the Meseta region further north are also relatively old ranging between 200 and 140 Ma. We here concentrate on the interpretation of old thermochronological ages from the Meseta and Anti-Atlas regions leaving the Alpine signal for another contribution. There are two direct, possibly inter fingering, interpretations for the preservation of such old thermal record in the Anti-Atlas and Meseta regions. First they remained "stable" being unaffected by 'Alpine' deformation that took place in the High-Atlas. Second, they are being affected "now" but no level with such record is yet exposed. Thermal modelling was performed to decipher between the 2 scenarios using our new thermochronological and available geological constraints. Models suggest that the first scenario is most likely with a clear Triassic to Late Jurassic phase of heating until ~100-90°C that was followed by a phase of cooling until the Middle Cretaceous. These results suggest that the Meseta was buried by a 2-3 kilometres thick sedimentary pile until ~180 Ma and as a result that the concept of a topographic high limiting the Tethysian from the Atlantic has to be reconsidered. Further, denudation patterns from the mirror image of the Atlas system on the other side of the Atlantic ocean are almost identical (Grist and Zentilli, 2003) suggesting that the patterns we constrained for the Triassic until Middle Cretaceous in SW Morocco have to be related to the break-up of the Pangea and oceanization in the Central Atlantic from ~180 Ma.

Control of insects and mites in grain using a high temperature/short time (HTST) technique.

PubMed

Mourier; Poulsen

2000-07-01

Wheat infested with grain mites (Acari) and Sitophilus granarius, and maize infested with Prostephanus truncatus, were exposed to hot air in a CIMBRIA HTST Microline toaster((R)). Inlet temperatures of the hot air were in the range of 150-750 degrees C decreasing to outlet temperatures in the range of 100-300 degrees C during the exposure period. A rotating drum, connected to a natural-gas burner was fed with grain which was in constant movement along the drum and thereby mixed thoroughly during the process. The capacity of the toaster was 1000 kg per hour.Complete control of grain mites and adult S. granarius in wheat was obtained with an inlet temperature of 300-350 degrees C and an average residence time in the drum of 6 s. More than 99% mortality was obtained for all stages of S. granarius with an inlet temperature of 300-350 degrees C and an average exposure period of 40 s. For control of P. truncatus in maize, an inlet temperature of 700 degrees C resulted in a complete disinfestation when the exposure time was 19 s.The reduction in grain moisture content was 0.5-1% at treatments giving 100% control. Germination tests indicate that it is possible to choose a combination of inlet temperatures and exposure periods which effectively kills mites and insects in small grains, without harming the functional properties of the grain.Economy of the method was considered to be competitive with fumigation using phosphine.

Low temperature thermochronology in the Easter Alps. New data, interpretations and perspectives.

NASA Astrophysics Data System (ADS)

Wölfler, Andreas

2015-04-01

The aim of this study is to evaluate new and published low temperature thermochronological data of the Eastern Alps in terms of its Mesozoic and Cenozoic tectonic evolution and the possible connection with deep seated lithospheric processes. In the Eastern Alps, the tectonic units that originate from the Penninic domain are buried beneath the Austroalpine nappe stack. Overthrusting of the Austroalpine nappes over the Penninic units occurred throughout the Cretaceous and lasted until the Eocene. During lateral tectonic extrusion in Oligocene to Miocene times the footwall penninic units exposed in the Tauern Window, were tectonically exhumed from below the Austroalpine hanging wall. This is well documented by Miocene to Pliocene zircon- and apatite fission track (ZFT, AFT) and (U-Th)/He data. However, the Austroalpine hanging wall shows a more complex age pattern. Late Cretaceous ZFT data reflect post-metamorphic exhumational cooling after Eo-Alpine metamorphism that goes along with an extensional phase that affected large parts of the Eastern Alps. Paleogene AFT and apatite (U-Th)/He data of the Austroalpine units to the east of the Tauern Window reflect exhumation of this area that supplied clastic material, the so-called Augenstein formation. Exhumation and erosion of the area left a probably hilly surface in Early Miocene times that was only moderately uplifted since then. These areas are well known for paleosurfaces exposed in the Gurktal- Kor- and Seckauer Alps to the east of the Tauern Window and in the central and eastern Northern Calcareous Alps. However, distinct parts of the Austroalpine hanging wall experienced substantial exhumation and surface uplift in the Miocene, contemporaneous to the exhumation of Penninic units and lateral extrusion of the Eastern Alps. These areas are restricted to the south and northeast of the Tauern Window and are characterized by steep and rugged reliefs that contrast the hilly and moderately shaped reliefs of the paleosurfaces. To summarize, low temperature thermochronological data of the Eastern Alps display at least three different exhumation scenarios during Cretaceous, Paleogene and Neogene times. Recent studies suggest that these time frames mark substantial changes in the lithosphere beneath the European Alps. Therefore exhumation in the Eastern Alps may reflect processes like lithsopheric thinning, changes in slab polarities and the formation of slab gaps.

Meso-Cenozoic morphological evolution of NW Africa, the case of the Tuareg swell.

NASA Astrophysics Data System (ADS)

Rougier, S.; Gautheron, C.; Barbarand, J.; Missenard, Y.; Zeyen, H.; Pinna, R.; Bonin, B.; Liégeois, J.-P.; Ouabadi, A.; Frizon de Lamotte, D.

2012-04-01

The continental crust of Africa, largely built during the Pan-African orogeny (late Neoproterozoic) has acquired in its northern part, during Paleozoic times, an arch and basin morphology. Meso-Cenozoic large scale topographic anomalies, associated to Cenozoic intraplate volcanism, such as Hoggar, Tibesti or Darfur domes, are superimposed to these structures. Precise ages of swells, as well as their relations with Paleozoic arch and basin morphology of the area, remain controversial. The aim of this study, focussed on the Hoggar dome, in southern Algeria, is to produce new constraints on the Post-Paleozoic evolution of this region. The Tuareg shield, from which Hoggar is the main central part and Aïr a SE extension, forms a topographic high reaching an altitude >2900m (Mt Tahat, Atakor district), exposing Precambrian rocks over 500000km2. While presumed Cretaceous sedimentary remnants suggest a possible stage of slightly positive topography during the Mesozoic, current high topography is emphasized by Cenozoic volcanic formations, mostly basaltic in composition. We present new low-temperature thermochronology data, with apatite fission track and (U-Th)/He ages on Hoggar and Aïr substratum. We combine these results with thermal, gravimetric and isostatic two-dimensional lithosphere-scale geophysical models, following the method of Zeyen & Fernandez (1994). Preliminary thermochronological results present ages from 99+-6 to 166+-10 Myr for AFT, and AHe from 10 to 300 Myr. Thermal simulations of these data suggest that currently outcropping Precambrian Hoggar basement could have experienced temperatures of approximately 80°C between Upper Cretaceous and Eocene. We propose that these elevated temperatures are related to burial beneath a 1 to 3 km thick sedimentary cover, depending on thermal gradient. The base of this sedimentary cover could correspond to the poorly described Upper Cretaceous remnants, currently uplifted up to 1450 m. These results are in agreement with geophysical calculations showing that, when eliminating the topographical effect of lithosphere heating related to recent volcanism and assuming an Upper Cretaceous to Late Eocene thermally unperturbed lithosphere, a sedimentary basin may have existed. Up to 2 km of Cretaceous sediments could have been deposited on the Hoggar, confirming the thermochronological results. Ref: Zeyen, H. and M. Fernàndez (1994): Integrated lithospheric modeling combining thermal, gravity and local isostasy analysis: application to the NE Spanish Geotransect. J. Geophys. Res. 99: 18089-18102.

Uplift Sequence of the Main Morphoestructural Units of the South Central Andes at 30°S: Insights from a Multidisciplinary Approach

NASA Astrophysics Data System (ADS)

Lossada, A. C.; Mardónez, D.; Suriano, J.; Hoke, G. D.; Fitzgerald, P. G.; Mahoney, J. B.; Giambiagi, L.; Aragon, E.

2015-12-01

The South Central Andes at 30°S represent a key area to understand the Andes geodynamics as it corresponds to the flat slab segment, characterized by a subhorizontal subduction angle, absence of Neogene magmatism and a highly compressive tectonic regime. Under these settings, crustal shortening is believed to be the principal mechanism responsible for the rise of the Andes. However, the sequence of uplift of the different morphoestructural units composing the orogen is not fully understood; neither do the location and time of activity of intracrustal detachments and their connection with shallower structures. We attempt to develop a multidisciplinary analysis that allow us to characterize the timing, magnitude and activity of the principal structures involved in the construction of the Andes at 30°S trough the Coastal Range, the Frontal Cordillera and the Precordillera. The main phase of deformation in the Coastal Range occurred between 60 and 40Ma, based on published thermochronological and structural data. Our structural analyses constrain the Frontal Cordillera uplift between 18 and 13Ma. In the Precordillera area, in turn, we carried out structural, sedimentological and U-Pb provenience studies. Provenience studies and the 12 and 9Ma ages obtained for the youngest zircons indicated that the main thrusts uplifting the western sector of the Precordillera thrust system were activated since 13Ma at this latitude, and not before that time as it was previously suggested. In order to better constrain the exhumation and thermal history of Frontal Cordillera, a thermochronological study is being conducted. Twenty samples for apatite fission tracks (AFT) and apatite (U-Th)/He (AHe) were collected from two vertical profiles located at western sector (Guanta granitoid) and eastern sector (Colanguil granitoid) of the Frontal Cordillera system. Samples are currently being processed, and they are partially reseted, which will allow us to obtain a cooling age. The aim is to combine this new thermochronologic data with field observations of the geological structures and kinematic analysis of the principal features, and develop an exhumation/uplift model for the Frontal Cordillera at 30°S, which will improve our understanding of the structural evolution of this area.

Thermal history from both sides of the South Atlantic passive margin - A comparison: Argentinean pampa vs. South African escarpement.

NASA Astrophysics Data System (ADS)

Kollenz, Sebastian; Glasmacher, Ulrich A.

2014-05-01

The eastern Argentina South Atlantic passive continental margin is distinguished by a very flat topography. Out of the so called Pampean flat two mountain ranges are arising. These mountain ranges, the Sierras Australes and the Sierras Septentrionales, are located in the State of Buenos Aires south of the capital Buenos Aires. In existing literature the Sierras Australes are correlated with the South African cape fold belt (Torsvik 2009; Lopez Gamundi & Rossello 1998). Existing thermochronological data shows different post-breakup cooling histories for both areas and different AFT-ages. Published thermochronological ages (e.g. Raab et al. 2002, 2005, Gallagher et al et al. 1998)from the south African escarpement vary around 150 and 100 Ma (Gallagher et al. 1998). Only some spots in the eastern part of South Africa towards the pacific margin show older ages of 250 Ma and older than 350 Ma (Gallagher et al. 1998). New thermochronological data (AHe, AFT and ZHe) from the Sierras Australes indicate a different cooling history by revealing a range of varying ages due to younger tectonic activity. By comparing the data sets from both areas it is getting clear that the post-rift evolution of both continents is differing very strong. Gallagher, K., Brown, R. and Johnson, C. 1998. Fission track analysis and its application to geological problems. Annual review of Earth and Planetary Science, 26, 519-572. Lopez Gamundi, O.R., Rossello, E.A. (1998): Basin fill evolution and paleotectonic patterns along the Samfrau geosyncline: the Sauce Grande basin-Ventana foldbelt (Argentina) and Karoo basin-Cape foldbelt (South Africa) revisited. Geol Rundsch 86 :819-834. Raab, M.J., Brown, R.W., Gallagher, K., Carter, A. and Webber, K. 2002. late Cretaceous reactivation of major crustal shear zones in northern Namibia: constraints from apatite fission track analysis. Tectonophysics. 349, 75-92. Raab, M.J., Brown, R.W., Gallagher, K., Webber, K. and Gleadow, A.J.W. 2005. denudational and thermal history of the Early Cretaceous Brandberg and Okenyenya igneous complexes on Namibia's passive margin. Tectonics. 24, TC3006, doi:10.1029/2004TC001688 Torsvik, T.H., Rousse, S., Labails, C., Smethurst, M. A. (2009): A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin. Geophys. J. Int. 177, 1315-1333.

Revealing the long-term landscape evolution of the South Atlantic passive continental margin, Brazil and Namibia, by thermokinematic numerical modeling using the software code Pecube.

NASA Astrophysics Data System (ADS)

Stippich, Christian; Glasmacher, Ulrich Anton; Hackspacher, Peter

2015-04-01

The aim of the research is to quantify the long-term landscape evolution of the South Atlantic passive continental margin (SAPCM) in SE-Brazil and NW-Namibia. Excellent onshore outcrop conditions and complete rift to post-rift archives between Sao Paulo and Porto Alegre and in the transition from Namibia to Angola (onshore Walvis ridge) allow a high precision quantification of exhumation, and uplift rates, influencing physical parameters, long-term acting forces, and process-response systems. Research will integrate the published and partly published thermochronological data from Brazil and Namibia, and test lately published new concepts on causes of long-term landscape evolution at rifted margins. The climate-continental margin-mantle coupled process-response system is caused by the interaction between endogenous and exogenous forces, which are related to the mantle-process driven rift - drift - passive continental margin evolution of the South Atlantic, and the climate change since the Early/Late Cretaceous climate maximum. Special emphasis will be given to the influence of long-living transform faults such as the Florianopolis Fracture Zone (FFZ) on the long-term topography evolution of the SAPCM's. A long-term landscape evolution model with process rates will be achieved by thermo-kinematic 3-D modeling (software code PECUBE1,2 and FastScape3). Testing model solutions obtained for a multidimensional parameter space against the real thermochronological and geomorphological data set, the most likely combinations of parameter rates, and values can be constrained. The data and models will allow separating the exogenous and endogenous forces and their process rates. References 1. Braun, J., 2003. Pecube: A new finite element code to solve the 3D heat transport equation including the effects of a time-varying, finite amplitude surface topography. Computers and Geosciences, v.29, pp.787-794. 2. Braun, J., van der Beek, P., Valla, P., Robert, X., Herman, F., Goltzbacj, C., Pedersen, V., Perry, C., Simon-Labric, T., Prigent, C. 2012. Quantifying rates of landscape evolution and tectonic processes by thermochronology and numerical modeling of crustal heat transport using PECUBE. Tectonophysics, v.524-525, pp.1-28. 3. Braun, J. and Willett, S.D., 2013. A very efficient, O(n), implicit and parallel method to solve the basic stream power law equation governing fluvial incision and landscape evolution. Geomorphology, v.180-181, 170-179.

Linking time-Temperature history of the Aquitaine basin with post-orogenic evolution of the Pyrenees : new insights from borehole thermochronology

NASA Astrophysics Data System (ADS)

Fillon, Charlotte; Calassou, Sylvain; Mouthereau, Frédéric; Pik, Raphaël; Bellahsen, Nicolas; Gautheron, Cécile

2017-04-01

Within their sedimentary record, foreland basins document vertical movements of the lithosphere, climatic changes, paleogeograhic evolution but also history of exhumation of the adjacent mountain belt. Comparing vertical movements in a range and in its foreland is key to identify processes involved in growth and destruction of mountain belts. The Aquitaine basin, geomorphologically stable since the early Pyrenean orogenesis has the potential to help understanding the driving mechanisms during the late to post-orogenic phases, but the lack of outcrops makes the studies particularly difficult to achieve. To bring a new point of vue on the processes involved in the Cenozoic exhumation of this range, we present new low-Temperature thermochronology data from boreholes of the Aquitaine basin. With the objectives to study rift-related to post-orogenic processes, numerous low-T thermochronological ages ( 300 across the range) have been published, documenting pre-, syn- , and post-orogenic exhumation in the Pyrenees. Using thermal modeling of a new low-T database in the western Axial Zone, we show that a late Miocene (around 10 Ma) uplift occured in the western Pyrenees, which generalizes the post-orogenic signal already detected in the south central Pyrenees. In previous studies, we linked the post-orogenic exhumation in the Southern Pyrenees to the excavation of the foreland valleys caused by the opening of the endorheic Ebro basin towards the Mediterranean Sea. To the West, the tectonic out-of sequence reactivation of the Gavarnie thrust has been invoked to explain the late Miocene AHe ages in the Bielsa massif. These new data might lead us to re-think the causes for such an exhumation signal during "post-orogenic" times. We thus summarize all evidences for the post-orogenic phase and attempt to provide explanation for it: is exhumation driven by Aquitaine foreland basin evolution? Does it reflect a tectonic reactivation of the Pyrenees? or is the signature of a regional/global climate change conditions ? To answer these questions, we present a new dataset of ZHe and AFT ages from borehole samples in three localities of the Aquitaine basin. We use these new data to link the late Miocene exhumation history with the vertical movements in the Aquitaine basin. This study is part of the Orogen projet, an academic-industrial collaboration (CNRS-Total-BRGM)

Structure and thermochronology of the metamorphic core of the Brooks Range, Alaska

NASA Astrophysics Data System (ADS)

Toro, Jaime

1999-11-01

Detailed field studies were undertaken in two key areas of the Central Belt of the Brooks Range: (1) the north flank of Mt. Igikpak in the Survey Pass Quadrangle and (2) in the Shishakshinovik Pass area in the eastern Ambler River Quadrangle. In both areas structural, stratigraphic, petrologic, 40Ar/39Ar, apatite fission-track and U-Pb data were used to constrain the kinematic and thermal history of metamorphic rocks of those areas. North of the Mt. Igikpak massif a crustal section ˜15 km thick is exposed. There are upper greenschist facies rocks in the deeper portions, and very low grade metamorphic rocks at higher structural levels. Two foliations are found: a higher grade relict S1 fabric and a lower grade S 2 fabric that controls the metamorphic layering. 40Ar/ 39Ar analyses from S1 white mica in the low-grade rocks at the northern end of the transect indicate that peak M1 metamorphism occurred before ˜112 Ma. We ascribe M1 to shortening that occurred during collision of an island arc against the Arctic Alaska margin. S 2 involved the retrogression of earlier assemblages. Kinematic indicators on S2 are top-to-the-north. A rapid cooling event from 500 +/- 50°C to 300 +/- 50°C took place between ˜98 and ˜90 Ma. The driving mechanism for ductile deformation during S2, and for rapid cooling documented by our thermochronologic data, was probably the gravitational collapse of the core of the orogen, over-thickened during the preceding collision. At Shishakshinovik Pass there are Mississippian Lisburne Group strata surrounded by metamorphic rocks typical of the Central Belt of the Brooks Range. All the rocks at Shishakshinovik Pass are intensely deformed, so that one cannot distinguish between an autochthonous and an allochthonous sequence. Furthermore the Mississippian rocks, instead of being attached to the underlying basement, are in the hanging wall of a northwest dipping shear zone. Based on the variations in metamorphic grade and the 40Ar/ 39Ar thermochronology, we argue that this shear zone was an extensional structure active during the mid-Cretaceous orogenic collapse of the Brooks Range. A consequence of this structural interpretation is that the Endicott Mountains allochthon need not be restored south of the Shishakshinovik orthogneiss.

Tectonic Evolution of an Intracratonic Plateau: The Ups and Downs of the Ozark Dome, Midcontinent United States

NASA Astrophysics Data System (ADS)

DeLucia, M. S.; Marshak, S.; Guenthner, W.

2017-12-01

Though intracratonic platforms have been affected both by epeirogenic movements (producing regional-scale basins and domes) and by local faulting, they typically have low relief. The Ozark Plateau (OP) of Missouri, a region underlain by the structural Ozark Dome (OD), is an exception for it rises to elevations of up to 0.7 km above the surface of the adjacent Illinois Basin (IB). Structural and geomorphic analysis, and low-temperature thermochronology, provide insight into vertical movements of the OD relative to the IB. The basement top of both the IB and OP exposes 1.47 Ga extrusive rhyolite. Therefore, basement of both the IB and OD sat at the Earth's surface then. Rifting after emplacement established a rectilinear array of steep faults, which delineated what would become the OD, and set the stage for IB subsidence. Zircon (U-Th)/He thermochronology indicates that exhumation of the Midcontinent to form the Great Unconformity (GU) happened from 0.85 to 0.68 Ga. To reset the zircon system, the region must have been buried >4 km prior to 0.85 Ga, perhaps by Grenville foreland deposits. Deposition began on the GU at 0.5 Ga, burying paleotopography of the OD and IB. Differential vertical motion between these regions initiated in the Paleozoic. Strata thin towards the apex of the dome, emphasizing that the OD remained high during IB subsidence. Eventually, as much as 7.5 km of structural relief accumulated across the boundary between the OD and IB. Some of the movement was accommodated by fault slip that was coeval with Appalachian orogenies, emphasizing that orogenic stress penetrated into the continental interior. Faulting contributed to tilting the OD crustal block. Apatite (U-Th)/He and fission-track thermochronology suggests that Mesozoic exhumation removed post-Pennsylvanian cover. Both Proterozoic and Mesozoic exhumation events took place just before supercontinent breakup, suggesting a link between mantle phenomena and intracratonic elevation. Contemporary river incision of OP bedrock and ongoing seismicity along the OP's borders suggest that OP uplift continues, though a lack of knickpoints suggests the movement is very slow. The persistence of the OP through geologic time implies that Precambrian modification of continental-interior lithosphere continues to influence its geodynamic response today.

Low-Temperature Thermochronology of Laramide Ranges in Montana and Wyoming Provides Information on Exhumation and Tectonics Associated with Flat-Slab Subduction

NASA Astrophysics Data System (ADS)

Armenta, M.; Carrapa, B.; DeCelles, P. G.

2014-12-01

Timing of exhumation of Laramide basement uplifts can be used as a proxy for tectonic processes associated with thick-skinned deformation resulting from flat-slab subduction. Despite its significance, the timing and pattern of Laramide deformation remains poorly constrained in Montana. Thermochronological data from Wyoming indicate exhumation of Laramide ranges during the late Cretaceous and Paleogene. Whereas a few data exist for the Bearthooth Range in Montana; the exhumation history of most of the Montana ranges remains unexplored preventing testing of current tectonic models. We report apatite fission track thermochronologic (AFT) data from modern river sands derived from Laramide ranges, bedrock basement samples, and synorogenic conglomerate clasts to determine the regional exhumation history of the Beartooth, Gravelly, Tobacco Root, Ruby, the Highland Mountains, and the Wind River Range. AFT permits reconstruction of thermal histories and rates of erosion of the upper few kilometers of the crust. In particular detrital AFT of river sands provides information on regional exhumation of the drainage area. AFT detrital ages derived from the southern end of the Beartooth Range are dominated by a 60-80 Ma signal, consistent with ages reported for bedrock basement samples in the Beartooth Range. A Cenozoic synorogenic conglomerate clast was obtained from the Highland Mountains, AFT results show a 69.56 +/- 5.45 Ma cooling age. In the Wind River Range, Wyoming AFT data from a Cenozoic synorogenic conglomerate clast from the Wind River Formation indicates a 59.32 +/- 4.83 Ma cooling age. This age is consistent with AFT ages from Gannett Peak indicating rapid cooling at ~60 Ma and ~50 Ma (Fan and Carrapa, 2014). Overall, samples from the easternmost ranges, the Beartooth and Bighorn, clearly preserve a Cretaceous signal; samples from Wind River Range and the rest of southwest Montana mainly record a Cenozoic signal. This suggests deeper and younger exhumation to the west than to the east. These results combined with thermal modeling provide additional constraints on the tectono-thermal history of Laramide ranges. In addition, these results allow for a temporal-spatial comparison between cooling and exhumation in the Montana and Wyoming Laramide regions and help test current models of the Laramide Orogeny.

Why natural monazite never becomes amorphous: Experimental evidence for alpha self-healing

DOE PAGES

Seydoux-Guillaume, Anne -Magali; Deschanels, Xavier; Baumier, Cedric; ...

2018-05-01

Monazite, a common accessory rare-earth orthophosphate mineral in the continental crust widely used in U-Pb geochronology, holds promise for (U-Th)/He thermochronology and for the immobilization of Pu and minor actinides (MA) coming from spent nuclear fuel reprocessing. Previous results obtained on natural and plutonium-doped monazite have demonstrated the ability of this structure to maintain a crystalline state despite high radiation damage levels. However, the low critical temperature (180 °C), above which amorphization cannot be achieved in natural monazite under ion irradiation, does not explain this old and unsolved paradox: why do natural monazites, independent of their geological history, remain crystallinemore » even when they did not experience any thermal event that could heal the defects? This is what the present study aims to address. Synthetic polycrystals of LaPO 4-monazite were irradiated sequentially and simultaneously with α particles (He) and gold (Au) ions. Here, our results demonstrate experimentally for the first time in monazite, the existence of the defect recovery mechanism, called α-healing, acting in this structure due to electronic energy loss of α particles, which explains the absence of amorphization in natural monazite samples. This mechanism is critically important for monazite geo- and thermochronology and to design and predictively model the long-term behavior of ceramic matrices for nuclear waste conditioning.« less

Paleogeographic constraints on continental-scale source-to-sink systems: Northern South America and its Atlantic margins

NASA Astrophysics Data System (ADS)

Bajolet, Flora; Chardon, Dominique; Rouby, Delphine; Dall'Asta, Massimo; Roig, Jean-Yves; Loparev, Artiom; Coueffe, Renaud

2017-04-01

Our work aims at setting the evolving boundary conditions of erosion and sediments transfer, transit, and onshore-offshore accumulations on northern South America and along its Atlantic margins. Since the Early Mesozoic, the source-to-sink system evolved under the interplay of four main processes, which are (i) volcanism and arc building along the proto-Andes, (ii) long-term dynamics of the Amazon incratonic basin, (iii) rifting, relaxation and rejuvenation of the Atlantic margins and (iv) building of the Andes. We compiled information available from geological maps and the literature regarding tectonics, plate kinematics, magmatism, stratigraphy, sedimentology (including paleoenvironments and currents) and thermochronology to produce a series of paleogeographic maps showing the tectonic and kinematic framework of continental areas under erosion (sources), by-pass and accumulation (sinks) over the Amazonian craton, its adjacent regions and along its Atlantic margins. The maps also allow assessing the relative impact of (i) ongoing Pacific subduction, (ii) Atlantic rifting and its aftermath, and (iii) Atlantic slab retreat from under the Caribbean domain on the distribution and activity of onshore/offshore sedimentary basins. Stratigraphic and thermochronology data are also used to assess denudation / vertical motions due to sediment transfers and lithosphere-asthenosphere interactions. This study ultimately aims at linking the sediment routing system to long-wavelength deformation of northern South America under the influence of mountain building, intracratonic geodynamics, divergent margin systems and mantle dynamics.

Why natural monazite never becomes amorphous: Experimental evidence for alpha self-healing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seydoux-Guillaume, Anne -Magali; Deschanels, Xavier; Baumier, Cedric

Monazite, a common accessory rare-earth orthophosphate mineral in the continental crust widely used in U-Pb geochronology, holds promise for (U-Th)/He thermochronology and for the immobilization of Pu and minor actinides (MA) coming from spent nuclear fuel reprocessing. Previous results obtained on natural and plutonium-doped monazite have demonstrated the ability of this structure to maintain a crystalline state despite high radiation damage levels. However, the low critical temperature (180 °C), above which amorphization cannot be achieved in natural monazite under ion irradiation, does not explain this old and unsolved paradox: why do natural monazites, independent of their geological history, remain crystallinemore » even when they did not experience any thermal event that could heal the defects? This is what the present study aims to address. Synthetic polycrystals of LaPO 4-monazite were irradiated sequentially and simultaneously with α particles (He) and gold (Au) ions. Here, our results demonstrate experimentally for the first time in monazite, the existence of the defect recovery mechanism, called α-healing, acting in this structure due to electronic energy loss of α particles, which explains the absence of amorphization in natural monazite samples. This mechanism is critically important for monazite geo- and thermochronology and to design and predictively model the long-term behavior of ceramic matrices for nuclear waste conditioning.« less

Low-Temperature Thermochronology Of Rio Grande Rise, South Atlantic Ocean

NASA Astrophysics Data System (ADS)

Doranti Tiritan, Carolina; Hackspacher, Peter Christian; Carina Siqueira Ribeiro, Marli

2017-04-01

The Rio Grande Rise (RGR) is a submerse plateau 1500 km distant from Brazilian coast, on Ocidental South Atlantic Ocean. It is interpreted as a great igneous province connected in the past with the Walvis Ridge on the Oriental South Atlantic. First hypothesis about their genesis rely on the great magmatic activity associated with the Tristao Cunha-Gough plume. However, recently, more evidences suggest that the RGR is a fragment of continental crust that could have been separated from the São Paulo Plateau during the South Atlantic opening process. New seismic data and in situ observation have reinforced this interpretation, which has been created more significantly questioning about the from the continental break has occurred, as well on how was the role and genesis of the hot spots that were manifested through the plume. This work will be presenting thermochronological data from RGR rocks collected by Brazilian Geological Service (CPRM) and Bremen IODP repository. We intend to have more data that will allow to reconstruct the processes of subsidence and rock uplift that could have occurred during Cretaceous and Eocene. It will be possible calculate exhumation rates and correlate them or not to the formation of the oceanic crust, time as an island and if the velocity of the subsidence. This work is part of a major project called "Elevação do Alto Rio Grande, Atlantico Sul Ocidental", financed by IODP/CAPES.

In situ TEM observation of alpha-particle induced annealing of radiation damage in Durango apatite.

PubMed

Li, Weixing; Shen, Yahui; Zhou, Yueqing; Nan, Shuai; Chen, Chien-Hung; Ewing, Rodney C

2017-10-26

A major issue in thermochronology and U-Th-Pb dating is the effect of radiation damage, created by α-recoils from α-decay events, on the diffusion of radiogenic elements (e.g., He and Pb) in host mineral. Up until now, thermal events have been considered as the only source of energy for the recovery of radiation-damage. However, irradiation, such as from the α-particle of the α-decay event, can itself induce damage recovery. Quantification of radiation-induced recovery caused by α-particles during α-decay events has not been possible, as the recovery process at the atomic-scale has been difficult to observe. Here we present details of the dynamics of the amorphous-to-crystalline transition process during α-particle irradiations using in situ transmission electron microscopy (TEM) and consecutive ion-irradiations: 1 MeV Kr 2+ (simulating α-recoil damage), followed by 400 keV He + (simulating α-particle annealing). Upon the He + irradiation, partial recrystallization of the original, fully-amorphous Durango apatite was clearly evident and quantified based on the gradual appearance of new crystalline domains in TEM images and new diffraction maxima in selected area electron diffraction patterns. Thus, α-particle induced annealing occurs and must be considered in models of α-decay event damage and its effect on the diffusion of radiogenic elements in geochronology and thermochronology.

Thermochronological Record of a Jurassic Heating-Cooling Cycle Within a Distal Rifted Margin (Calizzano Massif, Ligurian Alps)

NASA Astrophysics Data System (ADS)

Seno, S.; Decarlis, A.; Fellin, M. G.; Maino, M.; Beltrando, M.; Ferrando, S.; Manatschal, G.; Gaggero, L.; Stuart, F. M.

2017-12-01

The aim of the present study is to analyse, through thermochronological investigations, the thermal evolution of a fossil distal margin owing to the Alpine Tethys rifting system. The studied distal margin section consists of a polymetamorphic basement (Calizzano basement) and of a well-developed Mesozoic sedimentary cover (Case Tuberto unit) of the Ligurian Alps (NW Italy). The incomplete reset of zircon (U-Th)/He ages and the non-reset of the zircon fission track ages during the Alpine metamorphism indicate that during the subduction and the orogenic stages these rocks were subjected to temperatures lower than 200 ºC. Thus, the Alpine metamorphic overprint occurred during a short-lived, low temperature pulse. The lack of a pervasive orogenic reset, allowed the preservation of an older heating-cooling event that occurred during Alpine Tethys rifting. Zircon fission-track data indicate, in fact, that the Calizzano basement records a cooling under 240 °C, at 156 Ma (early Upper Jurassic). This cooling followed a Middle Jurassic syn-rift heating at temperatures of about 300-350°C, typical of greenschist facies conditions occurred at few kilometres depth, as indicated by stratigraphic and petrologic constraints. Thus, in our interpretation, major crustal thinning likely promoted high geothermal gradients ( 60-90°C/km) triggering the circulation of hot, deep-seated fluids along brittle faults, causing the observed thermal anomaly at shallow crustal level.

Late Cenozoic cooling history of the central Menderes Massif and the contribution of erosion to rock exhumation during active continental extension

NASA Astrophysics Data System (ADS)

Nilius, Nils-Peter; Wölfler, Andreas; Heineke, Caroline; Glotzbach, Christoph; Hetzel, Ralf; Hampel, Andrea; Akal, Cüneyt; Dunkl, István

2017-04-01

The Menderes Massif constitutes the western part of the Anatolide belt in western Turkey and experienced a prolonged history of post-orogenic extension. A large amount of the extension was accommodated by the two oppositely dipping Gediz and Büyük Menderes detachment faults, which led to the exhumation of the central Menderes Massif (Gessner et al., 2013). Previous studies proposed a synchronous, bivergent exhumation of the central Menderes Massif since the Miocene (Gessner et al., 2001), although only the evolution of the north-dipping Gediz detachment is well constrained (Buscher et al., 2013). Detailed structural and thermochronological investigations from the south-dipping Büyük Menderes detachment have still been missing. Here we present results from different thermochronometers, which constrain the cooling and exhumation history of footwall and hanging wall rocks of the Büyük Menderes detachment. Our new zircon and apatite (U-Th)/He and fission track ages of footwall rocks from the Büyük Menderes detachment document two phases of increased cooling and exhumation (Wölfler et al., in revision). The first episode of increased footwall exhumation ( 0.9 km/Myr) occurred during the middle Miocene, followed by a second phase during latest Miocene and Pliocene ( 1.0 km/Myr). Apatite fission track ages yield a slip rate for the Pliocene movement along the Büyük Menderes detachment of 3.0 (+1.1/-0.6) km/Myr. Thermochronological data of hanging wall units reflect a slow phase of exhumation ( 0.2 km/Myr) in the late Oligocene and an increased exhumation rate of 1.0 km/Myr during the early to middle Miocene, when hanging wall units cooled below 80 °C. In comparison with the Gediz detachment, our thermochronological data from the Büyük Menderes detachment confirms the concurrent activity of both detachments during the late Miocene and Pliocene. With respect to the relative importance of normal faulting and erosion to rock exhumation, a comparison with 10Be erosion rates from catchments in the exposed footwall of the Büyük Menderes detachment indicates that erosion has contributed 10-40% to the exhumation of metamorphic rocks beneath the detachment. Our finding underlines that the contribution of erosion to rock exhumation cannot be neglected in regions of active continental extension. References Buscher, J.T., Hampel, A., Hetzel, R., Dunkl, I, Glotzbach, C., Struffert, A., Akal, C., Rätz, M. 2013. Quantifying rates of detachment faulting and erosion in the central Menderes Massif (western Turkey) by thermochronology and cosmogenic 10Be. J. Geol. Soc. London. 170, 669-683. Gessner, K., Ring, U., Johnson, C., Hetzel, R., Passchier, C.W., Güngör, T., 2001. An active bivergent rolling-hinge detachment system: Central Menderes metamorphic core complex in western Turkey. Geology 29, 611-614. Gessner, K., Gallardo, L.A., Markwitz, V., Ring, U., Thomson, S.N., 2013. What caused the denudation of the Menderes Massif: Review of crustal evolution, lithosphere structure, and dynamic topography in southwest Turkey. Gondwana Research 24, 243-274. Wölfler, A., Glotzbach, C., Heineke, C., Nilius, N.P., Hetzel, R., Hampel, A., Akal, C., Dunkl, I., Christl, M. (manuscript in revision for Tectonophysics). Late Cenozoic cooling history of the central Menderes Massif: timing and slip rate of the Büyük Menderes detachment and the relative contribution of normal faulting and erosion to rock exhumation.

Linking craton stability and deep earth processes using thermochronology; a case study in the Superior Province of the Canadian Shield.

NASA Astrophysics Data System (ADS)

Sturrock, C. P.; Flowers, R. M.; Zhong, S.; Metcalf, J. R.; Kohn, B. P.

2017-12-01

Ancient, cratonic continental interiors are often presumed to be stable in the long term, neither accumulating nor shedding significant amounts of overlying sediment. However, recent low-temperature thermochronologic work suggests that such long term stability is an overly simplistic view and that forces besides plate tectonics, such as dynamic topography, may play a significant role. New apatite (U-Th)/He (AHe) and apatite fission track (AFT) data from Archean-Proterozoic basement rocks along a 1400km NW-SE transect in the Superior Province of the Canadian Shield record a spatially variable thermal history for the craton in Paleozoic through the end of Mesozoic time. Dates range from 600­­­­­­±60 Ma (AHe) and 529­±48 Ma (AFT) in the west to 184±14 Ma (AHe) and 174±9 Ma (AFT) in the east. Tectonic activity within the Superior Province ceased by 1.8 Ga, with the latest activity at the margins ending at 1 Ga. Widespread resetting of both AHe and AFT systems post 1 Ga is most likely due to regional scale burial at one or more times since the Cambrian. The temperature sensitivity of the AHe and AFT systems (30-90°C and 60-120°C, respectively) require at least a few km of burial across the craton that has since been stripped away. Preliminary inverse thermal history models, utilizing geologic constraints and radiation damage effects on He diffusion in apatite, indicate significant reheating in the Paleozoic-early Mesozoic (37 to >120°C) and a possible lesser reheating event since the mid Mesozoic (<100°C). Making the simplified assumption of a 25°C/km geothermal gradient and 0°C surface temperature, burial in some areas must have been at least 2-5km in the Paleozoic and was <4km in the Mesozoic. These burial and denudation patterns do not correlate with global sea level changes, making dynamic topography a good candidate for a driving mechanism. New AHe data from kimberlites emplaced in the early to mid-Jurassic will provide an important new constraint on the post-Jurassic thermal history of the Superior Province and result in better temperature/burial estimates for the earlier history. Ongoing work will compare these histories with dynamic topography predictions from geodynamic models back into the Paleozoic.

Sand dunes on the central Delmarva Peninsula, Maryland and Delaware

USGS Publications Warehouse

Denny, Charles Storrow; Owens, James Patrick

1979-01-01

Inconspicuous ancient sand dunes are present in parts of the central Delmarva Peninsula, Maryland and Delaware. Many dunes are roughly V-shaped, built by northwest winds, especially on the east sides of some of the large rivers. On the uplands, the form and spacing of the dunes are variable. A surficial blanket composed mainly of medium and fine-grained sand-the Parsonsburg Sand-forms both the ancient dunes and the broad plains between the dunes. The sand that forms the dunes is massive and intensely burrowed in the upper part; traces of horizontal or slightly inclined bedding appear near the base. Quartz is the dominant mineral constituent of the sand. Microline is abundant in the very fine to fine sand fraction. The heavy-mineral assemblages (high zircon, tourmaline, rutile) are more mature than in most of the possible source rocks. The most abundant minerals in the clay-sized fraction are dioctahedral vermiculite, kaolinite, illite, montmorillonite, and gibbsite. The first four minerals are common in deposits of late Wisconsin and Holocene age. The gibbsite may be detrital, coming from weathered rocks of Tertiary age. The soil profile in the dune sand is weakly to moderately developed. At or near the base of the Parsonsburg Sand are peaty beds that range in age from about 30,000 to about 13,000 radiocarbon years B.P. Microfloral assemblages in the peaty beds suggest that the dunes on the uplands formed in a spruce parkland during the late Wisconsin glacial maximum. The river dunes may also be of late Wisconsin age, but could be Holocene.

Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

USGS Publications Warehouse

Benowitz, J.A.; Layer, P.W.; Armstrong, P.; Perry, S.E.; Haeussler, Peter J.; Fitzgerald, P.G.; VanLaningham, S.

2011-01-01

40Ar/39Ar, apatite fission-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from ~33 Ma to ~18 Ma for 40Ar/39Ar biotite, ~18 Ma to ~6 Ma for K-feldspar minimum closure ages, and ~15 Ma to ~1 Ma for apatite fission-track ages, and apatite (U-Th)/He cooling ages range from ~4 Ma to ~1 Ma. There has been at least ~11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite 40Ar/39Ar thermochronology. Variations in exhumation history along and across the strike of the fault are influenced by both far-field effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least ~22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (~past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.

Source to sink evaluation of sediment routing in the Gulf of Alaska and Southeast Alaska: A thermochronometric perspective

NASA Astrophysics Data System (ADS)

Dunn, Catherine A.; Enkelmann, Eva; Ridgway, Kenneth D.; Allen, Wai K.

2017-03-01

In this study, we present a source to sink evaluation of sediment routing at the glaciated convergent margin in Southeast Alaska. We investigate the efficacy of thermochronology to record spatial and temporal exhumation patterns in synorogenic sediment using Neogene strata drilled by Integrated Ocean Drilling Program Expedition 341 in the Gulf of Alaska. We present 1641 and 529 new detrital zircon and apatite fission track ages, respectively, from strata deposited on the continental shelf, slope, and deep-sea fans. These data are compared to results from the proposed source terrains, including the St. Elias Mountains and new data from the Alsek River. We find that the offshore Bagley-Bering sediment contains grains recording cooling ages much older (80-35 Ma) than those reported from the St. Elias syntaxis (3-2 Ma), indicating that extreme rapid exhumation does not extend west of the Seward-Bagley divide. Data from the sediment on the continental shelf, slope, and proximal deep sea all yield similar results, suggesting the same general source region since 1.2 Ma and limited sediment mixing along this glaciated margin. Data from sediment in the distal deep sea show that extreme, rapid, and deep-seated exhumation was ongoing at 11-8 Ma. Overall, this study demonstrates the strengths and limitations of using detrital fission track thermochronology to understand sediment routing on a glaciated convergent margin and to record changes in exhumation rates over geologic time scales.

Novel calibration for LA-ICP-MS-based fission-track thermochronology

NASA Astrophysics Data System (ADS)

Soares, C. J.; Guedes, S.; Hadler, J. C.; Mertz-Kraus, R.; Zack, T.; Iunes, P. J.

2014-01-01

We present a novel age-equation calibration for fission-track age determinations by laser ablation inductively coupled plasma mass spectrometry. This new calibration incorporates the efficiency factor of an internal surface, [ ηq]is, which is obtained by measuring the projected fission-track length, allowing the determination of FT ages directly using the recommended spontaneous fission decay constant. Also, the uranium concentrations in apatite samples are determined using a Durango (Dur-2, 7.44 μg/g U) crystal and a Mud Tank (MT-7, 6.88 μg/g U) crystal as uranium reference materials. The use of matrix-matched reference materials allows a reduction in the uncertainty of the uranium measurements to those related to counting statistics, which are ca. 1 % taking into account that no extra source of uncertainty has to be considered. The equations as well as the matrix-matched reference materials are evaluated using well-dated samples from Durango, Fish Canyon Tuff, and Limberg as unknown samples. The results compare well with their respective published ages determined through other dating methods. Additionally, the results agree with traditional fission-track ages using both the zeta approach and the absolute approach, suggesting that the calibration presented in this work can be robustly applied in geological context. Furthermore, considering that fission-track ages can be determined without an age standard sample, the fission-track thermochronology approach presented here is assumed to be a valuable dating tool.

Long-lived Control of Sierras Pampeanas Ranges on Andean Foreland Basin Evolution Revealed by Coupled Low-temperature Thermochronology and Sedimentology

NASA Astrophysics Data System (ADS)

Stevens Goddard, A.; Carrapa, B.; Larrovere, M.; Aciar, R. H.

2017-12-01

The Sierras Pampeanas ranges of west-central Argentina (28º- 31ºS) are a classic example of thick-skinned style basement block uplifts. The style and timing of uplift in these mountain ranges has widely been attributed to the onset of flat-slab subduction in the middle to late Miocene. However, the majority of low-temperature thermochronometers in the Sierras Pampeanas have much older cooling dates. Thermal modeling derived from new low-temperature thermochronometers in Sierra de Velasco, one of the highest relief (> 4 km) mountains in the Sierras Pampeanas, suggest that the rocks in these ranges have been at near-surface temperatures (< 50ºC) since the Paleozoic. Reheating to temperatures between 80ºC and 100ºC occurred during late Cretaceous rifting and may be partially attributed to a temporary elevation of the regional geothermal gradient. Cooling attributed to late Miocene exhumation, and coincident with estimates of the onset of flat-slab subduction, contributed to modern relief, but cannot explain all of the modern topography. We compare the results from low-temperature thermochronology with the regional sedimentary basin record to confirm that paleorelief plausibly controlled sedimentation patterns throughout the development of the Cenozoic Andean foreland basin at these latitudes. We propose that the history of long-lived topography illustrated in Sierra de Velasco can be expanded to other ranges in the Sierras Pampeanas by integrating multiple data sets.

Initiation of the Bukadaban Feng Normal Fault and Implications for the Topographic Evolution of Northern Tibet

NASA Astrophysics Data System (ADS)

Niemi, N. A.; Chang, H.; Li, L.; Molnar, P. H.

2017-12-01

The Bukadaban Feng massif in northern Tibet forms the footwall of an east-west trending graben that is kinematically linked to the Kunlun fault. Extension across this graben accommodates left-lateral slip on the Kunlun fault, as evidenced by the 2001 Kunlun earthquake rupture. New geochronologic and thermochronologic data from Bukadaban Feng provide insight into the evolution of this normal fault system. The Bukadaban Feng massif is composed of two plutonic units, an eastern unit of dacitic composition and a western unit of rhyolitic composition. Sixty-five LA-ICP-MS zircon U-Pb age determinations on the rhyolitic unit reveal a range of ages from 873 - 6.3 Ma. CA-TIMS U-Pb zircon geochronology on the nine youngest of these zircons yields an emplacement age of 6.8 Ma. Twenty-seven LA-ICP-MS zircon U-Pb ages on the dacite range from 208 to 7.9 Ma. No coherent population of young zircons was observed, and CA-TIMS analysis was not performed. Zircon (U-Th)/He analysis on the dacite and rhyolite yield ages of 3.9 and 5.0 Ma, respectively, while apatite (U-Th-Sm)/He thermochronology on 5 samples collected from both units along the trace of the normal fault yield ages ranging from 1.4 - 2.6 Ma. The emplacement ages and compositions of plutonic rocks at Bukadaban Feng are consistent with the eruptive timing and geochemistry of silicic volcanic rocks in the graben (Zhang et al., 2012). Silicic magmatism is often associated with the onset of crustal extension, and the combination of plutonism and correlative silicic volcanism provides an indirect constraint on the initiation of this graben at 7 Ma. The distinct zircon (U-Pb) and (U-Th)/He ages indicates that the rocks presently exposed at Bukadaban Feng were emplaced at ambient temperatures in excess of 180°C. The zircon and apatite thermochronologic data require exhumation at rates of 1-2 mm/yr since the late Miocene. A 7 Ma initiation age for the Bukadaban Feng normal fault is consistent with both published estimates of total offset across the Kunlun fault ( 70 km; Kidd and Molnar, 1988) and recent fault slip rates ( 10 mm/yr; van der Woerd et al., 2002). To the extent that the onset of extension and strike-slip faulting can be related to the attainment of high topography, these new data imply that northernmost Tibet may have reached maximum elevations as recently as the late Miocene.

New constraints on the thermochronologic evolution at the boundary between the Eastern and Western Alps - Vorarlberg, Austria

NASA Astrophysics Data System (ADS)

Bertrand, Audrey; Pomella, Hannah; Fügenschuh, Bernhard; Zerlauth, Michael; Ortner, Hugo

2013-04-01

The study area in the westernmost part of Austria is marked by the limit between the Western and the Eastern Alps that takes place along the Rhine Valley, south of the Lake Constance. The area is composed, form the north to the south and from lowermost to uppermost structural position, by the European basement together with its autochthonous Mesozoic cover, autochthonous Molasse, subalpine Molasse, the Helvetic and Ultra-Helvetic, the Penninic and the Austroalpine nappes. These units are stacked in a succession of nappes separated by large south-trending overthrusts. This study presenting new apatite and zircon fission track ages, together with a crustal-scale cross-section (Pomella et al., this session) addresses the thermotectonic evolution of this nappe stack. In comparison with similar studies from eastern Switzerland the boundary between Western and Eastern Alps should be enlightened. Zircon fissions track ages from the lower freshwater Molasse reveal different age populations. Since all zircon fission track ages are older than the stratigraphic age this clearly indicates that post-depositional temperatures were well below the zircon partial annealing zone (i.e. below 200 °C) and the different age populations can be attributed to different source areas derived from the coevally forming and eroding alpine chain. Preliminary fission track results on apatite from the lower freshwater Molasse indicate a strong dependence of apatite fission track single-grain ages on their annealing kinetics as inferred from Dpar analyses (Gleadow and Duddy, 1981). F-rich apatites systematically yielded younger ages compared to the Cl-rich grains. The younger ages derived from the F-rich apatites are consistently younger than the stratigraphic age and thus fully annealed while Cl-rich apatites display older ages than the stratigraphic one. The difference in annealing temperatures between Cl- and F-rich apatites (Ravenhurst and Donelick, 1992) thus constrains the maximum temperature to < 100 °C, most likely reached between 20 Ma and 14 Ma by combined sediment and tectonic overburden. References Gleadow, A.J.W., and Duddy, I.R., 1981, A natural long-term annealing experiment for apatite. Nuclear Tracks Radiation Experiments, 5, 169-174. Pomella et al., this session. Alpine nappe stack in western Austria: A crustal-scale cross-section. Ravenhurst, C.E., and Donelick, R.A., 1992. Fission track thermochronology. In Short Course. Handbook on Low Temperature Thermochronology, ed. M. Zentilli & P.H. Reynolds. pp.21-42.

Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by 40Ar/39Ar thermochronology

NASA Astrophysics Data System (ADS)

Krol, Michael A.; Zeitler, Peter K.; Poupeau, GéRard; Pecher, Arnaud

1996-04-01

The 40Ar/39Ar thermochronology of the Late Cretaceous Hunza plutonic complex reveals an episodic cooling and denudation history for this regional-scale pluton. The 40Ar/39Ar analyses of biotites from a vertical relief section of >3200 m reveal a pulse of rapid cooling at ˜20 Ma. In the interval of 110-27 Ma, age-elevation distributions suggest denudation rates of the order of 0.02 ± 0.003 mm/yr. At ˜20 Ma, denudation rates increased significantly to 2.7 ± 0.7 mm/yr, then returned to much slower rates until 12 Ma. A second pulse of rapid cooling beginning at 12 Ma is revealed by inverse numerical modeling of multidiffusion domain alkali feldspars from a vertical section of 1700 m. Decreasing in elevation, these samples record the onset of rapid cooling at 12, 9, and 7 Ma, respectively. All of the alkali feldspars record a period of nearly isothermal conditions prior to the onset of rapid cooling when rates increased to 30°C/m.y. Assuming a geothermal gradient of 30°C/km, these cooling rates translate into denudation rates of 1.0 mm/yr. Apatite fission track analysis indicates denudation rates of 0.7 ± 0.1 mm/yr over the interval of 6.6 Ma to 2.4 Ma in agreement with the alkali feldspar data. These data suggest denudation of 2.9 ± 0.4 km since the Plio-Pleistocene. Together, the alkali feldspar and apatite data indicate that a minimum of 10 km of overburden has been removed since the mid-late Miocene. An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system).(Paper 95TC02424, Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by 40Ar/39Ar thermochronology, M.A. Krol, P.K. Zeitler, G. Poupeau, and A. Pecher). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.

Controls on Cenozoic exhumation of the Tethyan Himalaya from fission-track thermochronology and detrital zircon U-Pb geochronology in the Gyirong basin area, southern Tibet

NASA Astrophysics Data System (ADS)

Shen, Tianyi; Wang, Guocan; Leloup, Philippe Hervé; van der Beek, Peter; Bernet, Matthias; Cao, Kai; Wang, An; Liu, Chao; Zhang, Kexin

2016-07-01

The Gyirong basin, southern Tibet, contains the record of Miocene-Pliocene exhumation, drainage development, and sedimentation along the northern flank of the Himalaya. The tectonic controls on basin formation and their potential link to the South Tibetan Detachment System (STDS) are not well understood. We use detrital zircon (ZFT) and apatite (AFT) fission-track analysis, together with detrital zircon U-Pb dating to decipher the provenance of Gyirong basin sediments and the exhumation history of the source areas. Results are presented for nine detrital samples of Gyirong basin sediments (AFT, ZFT, and U-Pb), two modern river-sediment samples (ZFT and AFT), and six bedrock samples (ZFT) from transect across the Gyirong fault bounding the basin to the east. The combination of detrital zircon U-Pb and fission-track data demonstrates that the Gyirong basin sediments were sourced locally from the Tethyan Sedimentary Sequence. This provenance pattern indicates that deposition was controlled by the Gyirong fault, active since 10 Ma, whose vertical throw was probably < 5000 m, rather than being controlled by normal faults associated with the STDS. The detrital thermochronology data contain two prominent age groups at 37-41 and 15-18 Ma, suggesting rapid exhumation at these times. A 15-18 Ma phase of rapid exhumation has been recorded widely in both southern Tibet and the Himalaya. A possible interpretation for such a major regional exhumation event might be detachment of the subducting Indian plate slab during the middle Miocene, inducing dynamic uplift of the Indian plate overriding its own slab.

How to build a mid-crustal intrusive suite: geologic mapping, U-Pb geo-/thermochronology, and thermal modeling of the Bergell Intrusion, Central Alps

NASA Astrophysics Data System (ADS)

Samperton, K. M.; Schoene, B.; Annen, C.

2015-12-01

Insights into the characteristic rates and processes of crustal magmatic systems can best be made through the integration of observational, analytical and modeling perspectives. We present such an approach in reconstructing the emplacement, differentiation and cooling history of the Bergell Intrusion (N Italy/SE Switzerland), a normally-zoned pluton preserving a ~10 km mid-crustal transect. U-Pb zircon, titanite and allanite geo-/thermochronology of Bergell granitoids provide key empirical constraints for informing numerical simulations of pulse-wise, incremental assembly. Protracted zircon crystallization histories, representing the time between magma zircon saturation and cooling to the solidus, provide a direct petrologic link to forward models of magma emplacement, both of which can be used to derive quantitative magmatic cooling rates for the middle crust. Titanite and allanite dates provide additional constraints on the timing of solidification. Geochronology and modeling are performed in the context of detailed field and structural observations, including those previously interpreted as evidence of upward, pluton-scale melt migration via floor convergence/roof ballooning. Combined Bergell data and modeling demonstrate that pulsed assembly can lead to the formation of substantial melt reservoirs in the middle crust: this finding is largely in contrast to similar models of shallow crustal plutons, highlighting the importance of factors such as ambient country rock temperature in affecting melt residence timescales. This work emphasizes the importance of implementing joint data/modeling studies to intrusive rocks across the full range of spatial scales, emplacement levels and tectonic settings observed on Earth.

Thermochronologic Evidence for Late Eocene Andean Mountain Building at 30°S

NASA Astrophysics Data System (ADS)

Lossada, Ana C.; Giambiagi, Laura; Hoke, Gregory D.; Fitzgerald, Paul G.; Creixell, Christian; Murillo, Ismael; Mardonez, Diego; Velásquez, Ricardo; Suriano, Julieta

2017-11-01

The Andes between 28° and 30°S represent a transition between the Puna-Altiplano Plateau and the Frontal/Principal Cordillera fold-and-thrust belts to the south. While significant early Cenozoic deformation documented in the Andean Plateau, deciphering the early episodes of deformation during Andean mountain building in the transition area is largely unstudied. Apatite fission track (AFT) and (U-Th-Sm)/He (AHe) thermochronology from a vertical and a horizontal transect reveal the exhumation history of the High Andes at 30°S, an area at the heart of this major transition. Interpretation of the age-elevation profile, combined with inverse thermal modeling, indicates that the onset of rapid cooling was underway by 35 Ma, followed by a significant decrease in cooling rate at 30-25 Ma. AFT thermal models also reveal a second episode of rapid cooling in the early Miocene ( 18 Ma) related to rock exhumation to its present position. Low exhumation between the rapid cooling events allowed for the development of a partial annealing zone. We interpret the observed Eocene rapid exhumation as the product of a previously unrecognized compressive event in this part of the Andes that reflects a southern extension of Eocene orogenesis recognized in the Puna/Altiplano. Renewed early-Miocene exhumation indicates that the late Cenozoic compressional stresses responsible for the main phase of uplift of the South Central Andes also impacted the core of the range in this transitional sector. The major episode of Eocene exhumation suggests the creation of significant topographic relief in the High Andes earlier than previously thought.

Cenozoic tectono-thermal history of the Tordrillo Mountains, Alaska: Paleocene-Eocene ridge subduction, decreasing relief, and late Neogene faulting

USGS Publications Warehouse

Benowitz, Jeff A.; Haeussler, Peter J.; Layer, Paul W.; O'Sullivan, Paul B.; Wallace, Wes K.; Gillis, Robert J.

2012-01-01

Topographic development inboard of the continental margin is a predicted response to ridge subduction. New thermochronology results from the western Alaska Range document ridge subduction related orogenesis. K-feldspar thermochronology (KFAT) of bedrock samples from the Tordrillo Mountains in the western Alaska Range complement existing U-Pb, 40Ar/39Ar and AFT (apatite fission track) data to provide constraints on Paleocene pluton emplacement, and cooling as well as Late Eocene to Miocene vertical movements and exhumation along fault-bounded blocks. Based on the KFAT analysis we infer rapid exhumation-related cooling during the Eocene in the Tordrillo Mountains. Our KFAT cooling ages are coeval with deposition of clastic sediments in the Cook Inlet, Matanuska Valley and Tanana basins, which reflect high-energy depositional environments. The Tordrillo Mountains KFAT cooling ages are also the same as cooling ages in the Iliamna Lake region, the Kichatna Mountains of the western Alaska Range, and Mt. Logan in the Wrangell-St. Elias Mountains, thus rapid cooling at this time encompasses a broad region inboard of, and parallel to, the continental margin extending for several hundred kilometers. We infer these cooling events and deposition of clastic rocks are related to thermal effects that track the eastward passage of a slab window in Paleocene-Eocene time related to the subduction of the proposed Resurrection-Kula spreading ridge. In addition, we conclude that the reconstructed KFATmax negative age-elevation relationship is likely related to a long period of decreasing relief in the Tordrillo Mountains.

[Evaluation of long-term occupational exposure to dust and its effect on health during production of ceramic tiles].

PubMed

Gielec, L; Izycki, J; Woźniak, H

1992-01-01

A medical examination has been carried out of 500 workers (290 men and 210 women) of a ceramic plates plant. Also, the measurements of dust concentrations were made at some standard work places . In the materials used for manufacturing the plates crystalline phases and the content of free crystalline silica were determined using the X-ray diffraction method. In the animal experiments the fibrogenic activity of all materials used in the plant was examined and compared to the fibrogenic activity of standard quartz. As a result of the medical examination 64 cases of pneumoconiosis were diagnosed (13% of the subjects). The incidence rate of pneumoconiosis was similar for men and women. The radiological changes characteristic of pneumoconiosis took approximately 24 years of the workers tenure to develop. Type q changes were most frequent (69%), types p and r were observed in 14% of workers (mostly women). In 31% of workers tuberous changes of size B were observed. In 43.8% of the subjects restrictive disorders of ventilation were found. In 30% of workers chronic bronchitis was diagnosed. Dust concentrations at 11 work places were measured using the individual dosimetry method. Total dust concentrations ranged from 0.6 mg/m3 at the electricians posts to 60.1 mg/m3 at the workposts where the furnace truck restorers worked. Dust concentrations exceeded the MACs at 7 workposts. The respirable fraction concentrations ranged from 0.1 mg/m3 to 8.4 mg/m3. During the replacement of asbestos ropes and asbestos board used for insulating the furnace trucks mineral fibres (0.1-0.5 fibre/cm3) were found in the air. The following crystalline phases were determined in the materials: kaolinite, illite, quartz, orthoclase and microline.(ABSTRACT TRUNCATED AT 250 WORDS)

Sediment underthrusting within a continental magmatic arc: Coast Mountains batholith, British Columbia

NASA Astrophysics Data System (ADS)

Pearson, David M.; MacLeod, Douglas R.; Ducea, Mihai N.; Gehrels, George E.; Jonathan Patchett, P.

2017-10-01

Though continental magmatic arcs are factories for new continental crust, a significant proportion of continental arc magmas are recycled from supracrustal material. To evaluate the relative contributions of retroarc underthrusting and trench side partial sediment subduction for introducing supracrustal rocks to the middle and lower crust of continental magmatic arcs, we present results from the deeply exposed country rocks of the Coast Mountains batholith of western British Columbia. Prior work demonstrates that these rocks underwent widespread partial melting that contributed to the Coast Mountains batholith. We utilize U-Pb zircon geochronology, Sm-Nd thermochronology, and field-based studies to document the protoliths and early burial history of amphibolite and granulite-facies metasedimentary rocks in the Central Gneiss Complex. U-Pb detrital zircon data from the structurally highest sample localities yielded 190 Ma unimodal age peaks and suggest that retroarc rocks of the Stikine terrane constitute a substantial portion of the Central Gneiss Complex. These supracrustal rocks underwent thrust-related burial and metamorphism at >25 km depths prior to 80 Ma. These rocks may also be underlain at the deepest exposed structural levels by Upper Cretaceous metasedimentary rocks, which may have been emplaced as a result of trench side underplating or intraarc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.

Linking orogen and peripheral foreland basin: conceptual model and application to the Southalpine-Dinaric (Friuli) orocline

NASA Astrophysics Data System (ADS)

Heberer, Bianca; Neubauer, Franz

2010-05-01

Surface uplift and rock exhumation within an orogen are generally a consequence of convergence, and can often be linked with subsidence in a peripheral foreland. Since vertical loads act on the entire lithosphere, these processes can, therefore, be considered as plate-scale processes. Here, we propose a conceptual model for this linkage for the Friuli orocline and its surrounding units. The Friuli orocline stretches from the ENE-trending Southern Alps to the SE-trending Dinarides. There, two Neogene stages of convergence and associated deformation can be differentiated: (1) a Mid-Late Miocene phase of increased surface uplift and intra-orogenic subsidence of sedimentary basins reflecting intra-orogenic crustal-scale folding. Depocentres are e.g. the flexural Belluno, Ljubljana and Klagenfurt basins. (2) A second stage of convergence during Late Pliocene-Pleistocene times led to overall surface uplift in the orogen and contemporaneous pronounced subsidence in the peripheral foreland basin (Venetian platform and the northern Adriatic Sea). We propose, that the spatially variable extent of subsidence originates in variably strong orogen-basin coupling, i.e. weak coupling during stage 1 vs. strong coupling during stage 2. This interpretation is based on the apatite fission track age pattern, the distribution of intra-orogenic Neogene sediment basins and subsidence analyses in the foreland basin (Barbieri et al., 2007). Available low-temperature thermochronological data for the Southern Alps and the NW Dinarides are sparse, in contrast to a dense network of primarily apatite fission track ages north of the Periadriatic lineament (e.g. summarized by Luth & Willingshofer, 2008). AFT ages adjacent to the eastern Periadriatic Lineament mainly range from 15 to 25 Ma (Hejl, 1997; Fodor et al., 2008). Detrital studies on Oligocene to Miocene sediments from the Venetian foreland basin yielded dominant age groups clustering roughly around 20 and 30 Ma (Stefani et al., 2008). Bedrock ages from the vicinity of the Valsugana thrust indicate an important exhumational event at about 10 Ma (Zattin et al., 2006). The existing data already hint at decreasing rates of thermal overprint towards the foreland. Basement uplifts partly display AFT ages contemporaneous to subsidence in intra-orogenic basins. Consequently, existing AFT data and their relationships to intervening Neogene basins suggest a Neogene large-wavelength crustal-scale fold structure between the Klagenfurt basin and the Adriatic Sea. The main stage of subsidence in the Venetian-Adriatic foreland is younger and of Late Pliocene-Pleistocene age reflecting the final, still ongoing stage of shortening (Barbieri et al., 2007). In order to further test these observations, we aim at collecting more structural and low-T thermochronological data from the region. First results from the recently started project "AlDi-Adria" will be presented. References Barbieri, C. et al. 2007: Natural subsidence of the Venice area during the last 60 Myr. Basin Res., 19, 105-123. Fodor, L. et al. 2008: Miocene emplacement and rapid cooling of the Pohorje pluton at the Alpine-Pannonian-Dinaric junction: a geochronological and structural study. Swiss J. Geosci., 101 Suppl. 1, S255-S271. Hejl, E. 1997: 'Cold spots' during the Cenozoic evolution of the Eastern Alps: thermochronological interpretation of apatite fission-track data. Tectonophysics, 272, 159-172. Luth S. W. & Willingshofer, E. 2008: Mapping of the Post-Collisional Cooling History of the Eastern Alps. Swiss J. Geosci., 101, 207-223. Stefani, C. 2008: Provenance and Paleogeographic Evolution in a Multi-Source Foreland: The Cenozoic Venetian-Friulian Basin (NE Italy). J. Sediment. Res., 77, 867-887. Zattin, M. et al. 2006: From Middle Jurassic heating to Neogene cooling: The thermochronological evolution of the southern Alps. Tectonophysics, 414, 191-202.

Thermochronological modeling of the age of Vologda crystalline basement of the Russian platform

NASA Astrophysics Data System (ADS)

Gerasimov, V. Yu.; Petrov, D. B.; Lebedev, V. A.

2010-05-01

The results of the complex petrological and isotope-geochronological study of the crystalline rock from the deep drilling hall of the south of Vologda segment are presented in this work. The crystalline basement of the platform in Vologda region lie in a depth 2.5 km and represented by high alumina mica schist. The thick sedimentary cover consists of vendian and phanerozoic sediments. Upper level covered by quaternary glacial deposits up to 50 m. A core sample from the borehole of Fedotovo village was obtained from the depth 2600 m. It is fine-medium grained metamorphic mica schist with sillimanite. The mineral assemblage represented by association: Pl-Bt-Ms-Sil-Qtz-Mag +Zrn +Mnz. The metamorphic schist of the crystalline basement contains several radio isotope sensors. There are two rock forming potassium reach mica, - biotite (Bt) and muscovite (Ms) and accessories monazite (Mnz), - the phosphate of REE enriched by Th and U. It was a reason why traditional K-Ar isotope dating method in the combination with electron microprobe U-Th-Pb dating method CHIME [Suzuki et al. 1991] was used for Vologda metapelite rocks dating. In addition to geochronology, the detailed petrological investigation using electron microprobe allowed also to determine thermodynamic parameters of metamorphic system with a help of the mineral thermobarometry and finally estimate the age of the metamorphic thermal event using experimental diffusion data of Ar and Pb in minerals [Gerasimov et al. 2004]. The temperature of the regional metamorphism was estimated using Bt+Mag+Qtz and Bt+Ms geothermometers [Glassley 1983, Hoisch 1989]. Taking into account the field of the sillimanite P-T stability it is possible to conclude that the peak of metamorphism was reached at temperature about ТоС=550+/-30° C and pressure Р=4+/-1 kbar. Isotope thermochronology of the sample demonstrate nearly Svecofenian age 1.7-1.8 Ga of Vologda crystalline basement. K-Ar isotope dating of black and white mica demonstrates regular progression of ages in a concordance with closure temperature of each mineral. The apparent Bt age is about 1670 Ma and Ms age is 1710 Ma. The estimation of closure temperature (Tc) for each of the minerals using Dodsn's theory [Dodson 1973] and DCT computer program with concordance procedure of cooling rate simulation for the two coexisting minerals demonstrate value 340° C and и 460° C for Bt and Ms respectively. The rate of cooling in this temperature range is about 3° C/Ma and time span between closure temperatures of the two micas is about 40 Ma. The value of the cooling rate is a very typical for regional metamorphism conditions. The linear extrapolation of the simulated time-temperature trend to the thermal peak of the regional metamorphism (estimated by mineral equilibriums at 550° C) demonstrates that cooling of the metamorphic system from the thermal peak to 460° C(closure temperature of Ms) takes about 30-40 Ma. It is a value of systematic thermochronological correction to the muscovite apparent K-Ar age which has to be added to estimate the age of regional metamorphism, after that we finally receive the age 1750+/-40 Ma. U-Th-Pb system of monazite grains was tested by electron microprobe JEOL 8200 in IGEM RAS using CHIME method. The analysis of 8 grains demonstrated averaged value of age 1790+/-55 Ma. It is in a very good agreement with K-Ar isotope dating results. Moreover, the estimation of monazite closure temperature using experimental data of Pb diffusion [Smith & Giletti 1997] shows the value Tc=540-560° C which almost exactly corresponds to the peak temperature of regional metamorphism. It is also an explanation of the very close results of dating in different isotope systems (conservative U-Th-Pb system of monazite and flexible K-Ar system) in the condition of slow cooling and demonstrates the thermochronological modeling effectiveness.

In Situ UV-Laser Ablation 40Ar/39Ar Muscovite Thermochronology Reveals Excess Argon and 405-399 Ma Age for the Nordfjord-Sogn Detachment Zone, Hornelen region, Norway

NASA Astrophysics Data System (ADS)

Johnston, S. M.; Hacker, B. R.; Eide, E. A.; Hendriks, B.

2006-12-01

The Nordfjord-Sogn Detachment Zone (NSDZ) is a 2-6 km thick, normal-sense shear zone that juxtaposes the eclogite-bearing Western Gneiss Complex in its footwall with Devonian sediments in its hangingwall, and is widely cited as the primary structure responsible for the exhumation of the Norwegian ultrahigh-pressure (UHP) rocks. Critical to our understanding of this crustal-scale detachment, detailed thermochronology places essential constraints on the rates of exhumation and documents spatial variability in the timing of exhumation- related cooling. Regional thermochronology has shown that 40Ar/39Ar muscovite ages increase gradually from 405-390 Ma across the Western Gneiss Complex and into the lower part of the NSDZ, but at higher structural levels within the NSDZ, these ~400 Ma ages are mixed with an older population at 420-418 Ma. Although neither excess argon nor incomplete resetting of argon in these older samples has been ruled out, these results have been interpreted to suggest that extension and exhumation within the NSDZ may have initiated as early as 420 Ma. Here, we investigate these older 40Ar/39Ar muscovite ages by comparing resistance furnace (RF) ages from bulk muscovite separates with in situ UV-laser ablation ages from a suite of 6 samples that transect the NSDZ. RF spectra from 5 of the 6 analyzed samples yield plateau ages that range from 405-399 Ma; in situ ages of these same 5 samples reveal no age zonation within large grains and no differences in ages among texturally different grains. In contrast, the structurally highest sample yields a RF plateau age of 425±6 Ma, and in situ ages of 474-417 Ma in grain cores and c. 400 Ma in grain rims and small grains. On an inverse isochron diagram, these in situ ages define two distinct populations, with grain cores distributed along a line between an excess argon component of 40Ar/36Ar = 660±60 and an age of 404±7 Ma. Analyses from rims and small grains plot in triangle defined by these two apices and an atmospheric 40Ar/36Ar ratio. These results are consistent with the interpretation that the older 40Ar/39Ar muscovite ages from the NSDZ are the result of excess argon. These findings refute the possibility of extension and exhumation as early as 420 Ma along the NSDZ, and suggest that the entire package of NSDZ mylonites was cooled through muscovite closure to argon between 405-399 Ma immediately following UHP metamorphism.

U-Pb thermochronology of the lower crust: producing a long-term record of craton thermal evolution

NASA Astrophysics Data System (ADS)

Blackburn, T.; Bowring, S. A.; Mahan, K. H.; Perron, T.; Schoene, B.; Dudas, F. O.

2010-12-01

The EarthScope initiative is focused on providing an enhanced view of the North American lithosphere and the present day stress field of the North American continent. Of key interest is the interaction between convecting asthenosphere and the conducting lithospheric mantle that underlie the continents, especially the cold ‘keels’ that underlie Archean domains. Cratonic regions are in general characterized by minimal erosion and or sediment accumulation. The Integration of seismic tomography, and mantle xenolith studies reveal a keel of seismically fast and relatively buoyant and viscous mantle; physical properties that are intimately linked with the long-term stability and topographic expression of the region. Missing from this model of the continental lithosphere is the 4th dimension--time--and along with it our understanding of the long-term evolution of these stable continental interiors. Here we present a thermal record from the North American craton using U-Pb thermochronology of lower crustal xenoliths. The use of temperature sensitive dates on lower crustal samples can produce a unique time-temperature record for a well-insulated and slowly cooling lithosphere. The base of the crust is insulated enough to remain unperturbed by any plausible changes to surface topography, yet unlike the subadjacent lithospheric mantle, contains accessory phases amenable to U-Pb dating (rutile, apatite, titanite). With near steady state temperatures in the lower crust between 400-600 °C, U-Pb thermochronometers with similar average closure temperatures for Pb are perfectly suited to record the long-term cooling of the lithosphere. Xenoliths from multiple depths, and across the craton yield time-temperature paths produced from U-Pb thermochronometers that record extremely slow cooling (<0.25 °C/Ma) over time scales of billions of years. Combining these data with numerical thermal modeling allow constraints to be placed on the dominant heat transfer mechanisms operating within the lithosphere including exhumation, conduction, decay of heat producing elements and thickness of crustal layers/lithospheric mantle. The thermal histories produced from this numerical model can in turn be used to calculate model U-Pb thermochronometric data using a numerical solution to the diffusion/production equation. Integration of thermal and volume diffusion models for the U-Pb system suggests that the extreme slow cooling recorded by U-Pb thermochronology is consistent with low integrated exhumation rates (<0.005 km/Ma). This exhumation rate is integrated over time-scales of hundreds of million to a billion years and does not preclude the possibility for rapid or short-wave length uplift/subsidence. This long-term record of continental lithosphere stability and apparent neutral buoyancy of the craton within a cooling mantle may be further used to refine our estimates of secular cooling within the mantle.

Middle Miocene Hotspot-Related Uplift, Exhumation, and Extension north of the Snake River Plain: Evidence from Apatite (U-Th)/He Thermochronology

NASA Astrophysics Data System (ADS)

Foster, D. A.; Vogl, J.; Min, K. K.; Bricker, A.; Gelato, P. W.

2013-12-01

Passage of North America over the Yellowstone hotspot has had a profound influence on the topography of the northern Rocky Mountains. One of the most prominent topographic features is the Yellowstone crescent of high topography, which comprises two elevated shoulders bounding the eastern Snake River Plain (SRP) and converging at a topographic swell centered on the Yellowstone region. Kilometer-scale erosion has occurred locally within the topographic crescent, but it is unclear if rock exhumation is due to surface uplift surrounding the propagating hot spot, subsidence of the Snake River Plain after passage of the hot spot, or relief initiated by extension in the Northern Basin and Range Province. We have applied (U-Th/He) apatite (AHe) thermochronology to the Pioneer-Boulder Mountains (PBM) on the northern flank of the SRP, and the southern Beartooth Mountains (BM) directly north of the modern Yellowstone caldera, to constrain the timing, rates, and spatial distribution of exhumation. AHe ages from the PBM indicate that >2-3 km of exhumation occurred in the core of this topographic culmination since ~11 Ma. Age-elevation relationships suggest an exhumation rate of ~0.3 mm/yr between ~11 and 8 Ma. Eocene Challis volcanic rocks are extensively preserved and Eocene topographic highs are locally preserved to the north and south of the PBM, indicating minimal erosion adjacent to the PBM culmination. Spatial patterns of both exhumation and topography indicate that faulting was not the primary control on uplift and exhumation. Regional exhumation at 11-8 Ma was synchronous with silicic eruptions from the ~10.3 Ma Picabo volcanic field located immediately to the south and with S-tilting of the southern flank of the PBM that is likely the result of loading of the ESRP by mid-crustal mafic intrusions. AHe data from Archean rocks of the southern BM reveal Miocene-Pliocene cooling ages and include samples as young as ~2-6 Ma. Discordant single grain ages in samples with Miocene mean ages suggest that exhumation is now reaching to depths of the Miocene He partial retention zone. Miocene-Pliocene erosional exhumation of the South Snowy block is partly attributed to integration of the Yellowstone River drainage system and incision of the Yellowstone Canyon. The thermochronology of these two locations shows that localized uplift, exhumation and incision occurred progressively as NA moved over the hot spot, but that exhumation is not uniform and not always controlled by Neogene basin-bounding faults. This suggests a causal relationship between hotspot processes and exhumation through potential contributions of flexure and mantle dynamics to uplift, and changes in drainage networks and base-level associate with uplift and/or extension.

Tracing the Origins of Coarse Sediment in Steep Mountain Catchments

NASA Astrophysics Data System (ADS)

Lukens, C. E.; Riebe, C. S.; Shuster, D. L.; Sklar, L. S.; Beyeler, J. D.

2011-12-01

Where does coarse sediment come from? How long does it persist in channels? What can the origins of sediment tell us about erosional processes and particle comminution in hillslope soils and mountain streams? To address these questions, we present new apatite-helium (AHe) ages from coarse sediment in steep streams of the Sierra Nevada, California. The evolution of grain size in sediment reflects both the physical and chemical breakdown of particles as they travel downstream. It also should reflect the dominant mechanisms of landscape evolution within a watershed. Previous studies have exploited detrital thermochronology in tracing the origins of sand-sized particles; the approach uses AHe age distributions in the sand as a geochemical fingerprint that can be compared with age-elevation relationships in bedrock as an indicator of provenance. In steep catchments, however, sand-sized particles comprise only a fraction of the sediment on the bed, and therefore tell only part of the erosional story. Much can be learned by examining age distributions of coarser grain sizes. Source elevations of coarse particles, for instance, may help reveal the relative importance of erosional mechanisms. For example, if boulders are sourced at high elevations, rock fall and debris flows likely dominate their transport. Conversely, if boulders are sourced only at lower elevations (nearer the sample location), they are more likely produced locally, and thus break down in place. We show how hypotheses such as these can be tested using detrital thermochronology on coarse sediment. We show how our analysis of detrital apatite can be coupled with a numerical model of the evolution of grain-size distributions by particle breakdown and input from slopes. We elaborate on how this approach can shed new quantitative light on processes of sediment production, transport, and breakdown in mountainous settings.

Constraints on the magnitude and rate of CO2 dissolution at Bravo Dome natural gas field

PubMed Central

Sathaye, Kiran J.; Hesse, Marc A.; Cassidy, Martin; Stockli, Daniel F.

2014-01-01

The injection of carbon dioxide (CO2) captured at large point sources into deep saline aquifers can significantly reduce anthropogenic CO2 emissions from fossil fuels. Dissolution of the injected CO2 into the formation brine is a trapping mechanism that helps to ensure the long-term security of geological CO2 storage. We use thermochronology to estimate the timing of CO2 emplacement at Bravo Dome, a large natural CO2 field at a depth of 700 m in New Mexico. Together with estimates of the total mass loss from the field we present, to our knowledge, the first constraints on the magnitude, mechanisms, and rates of CO2 dissolution on millennial timescales. Apatite (U-Th)/He thermochronology records heating of the Bravo Dome reservoir due to the emplacement of hot volcanic gases 1.2–1.5 Ma. The CO2 accumulation is therefore significantly older than previous estimates of 10 ka, which demonstrates that safe long-term geological CO2 storage is possible. Integrating geophysical and geochemical data, we estimate that 1.3 Gt CO2 are currently stored at Bravo Dome, but that only 22% of the emplaced CO2 has dissolved into the brine over 1.2 My. Roughly 40% of the dissolution occurred during the emplacement. The CO2 dissolved after emplacement exceeds the amount expected from diffusion and provides field evidence for convective dissolution with a rate of 0.1 g/(m2y). The similarity between Bravo Dome and major US saline aquifers suggests that significant amounts of CO2 are likely to dissolve during injection at US storage sites, but that convective dissolution is unlikely to trap all injected CO2 on the 10-ky timescale typically considered for storage projects. PMID:25313084

Mountain building processes during continent continent collision in the Uralides

NASA Astrophysics Data System (ADS)

Brown, D.; Juhlin, C.; Ayala, C.; Tryggvason, A.; Bea, F.; Alvarez-Marron, J.; Carbonell, R.; Seward, D.; Glasmacher, U.; Puchkov, V.; Perez-Estaun, A.

2008-08-01

Since the early 1990's the Paleozoic Uralide Orogen of Russia has been the target of a significant research initiative as part of EUROPROBE and GEODE, both European Science Foundation programmes. One of the main objectives of these research programmes was the determination of the tectonic processes that went into the formation of the orogen. In this review paper we focus on the Late Paleozoic continent-continent collision that took place between Laurussia and Kazakhstania. Research in the Uralides was concentrated around two deep seismic profiles crossing the orogen. These were accompanied by geological, geophysical, geochronological, geochemical, and low-temperature thermochronological studies. The seismic profiles demonstrate that the Uralides has an overall bivergent structural architecture, but with significantly different reflectivity characteristics from one tectonic zone to another. The integration of other types of data sets with the seismic data allows us to interpret what tectonic processes where responsible for the formation of the structural architecture, and when they were active. On the basis of these data, we suggest that the changes in the crustal-scale structural architecture indicate that there was significant partitioning of tectonothermal conditions and deformation from zone to zone across major fault systems, and between the lower and upper crust. Also, a number of the structural features revealed in the bivergent architecture of the orogen formed either in the Neoproterozoic or in the Paleozoic, prior to continent-continent collision. From the end of continent-continent collision to the present, low-temperature thermochronology suggests that the evolution of the Uralides has been dominated by erosion and slow exhumation. Despite some evidence for more recent topographic uplift, it has so far proven difficult to quantify it.

Postrift history of the eastern central Atlantic passive margin: Insights from the Saharan region of South Morocco

NASA Astrophysics Data System (ADS)

Leprêtre, Rémi; Missenard, Yves; Barbarand, Jocelyn; Gautheron, Cécile; Saddiqi, Omar; Pinna-Jamme, Rosella

2015-06-01

The passive margin of South Morocco is a low-elevated passive margin. It constitutes one of the oldest margins of the Atlantic Ocean, with an Early Jurassic breakup, and little geological data are available concerning its postrift reactivation so far. We investigated the postrift thermal history of the onshore part of the margin with low-temperature thermochronology on apatite crystals. Fission track and (U-Th-Sm)/He ages we obtained are significantly younger than the breakup ( 190 Ma). Fission track ages range from 107 ± 8 to 175 ± 16 Ma, with mean track lengths from 10.7 ± 0.3 to 12.5 ± 0.2 µm. (U-Th-Sm)/He ages range from 14 ± 1 to 185 ± 15 Ma. Using inverse modeling of low-temperature thermochronological data, we demonstrate that the South Moroccan continental margin underwent a complex postrift history with at least two burial and exhumation phases. The first exhumation event occurred during Late Jurassic/Early Cretaceous, and we attribute this to mantle dynamics rather than to intrinsic rifting-related processes such as flexural rebound. The second event, from Late Cretaceous to early Paleogene, might record the onset of Africa/Europe convergence. We show a remarkably common behavior of the whole Moroccan passive margin during its early postrift evolution. The present-day differences result from a segmentation of the margin domains due to the Africa/Europe convergence. Finally we propose that varying retained strengths during rifting and also the specific crustal/lithospheric geometry of stretching explain the difference between the topographical expressions on the continental African margin compared to its American counterpart.

Late Cenozoic thrusting of major faults along the central segment of Longmen Shan, eastern Tibet: Evidence from low-temperature thermochronology

NASA Astrophysics Data System (ADS)

Tan, Xi-Bin; Xu, Xi-Wei; Lee, Yuan-Hsi; Lu, Ren-Qi; Liu, Yiduo; Xu, Chong; Li, Kang; Yu, Gui-Hua; Kang, Wen-Jun

2017-08-01

The Cenozoic orogenic process of the Longmen Shan (LMS) and the kinematics of major faults along the LMS are crucial for understanding the growth history and mechanism of the eastern Tibetan Plateau. Three major faults, from west to east, are present in the central segment of the LMS: the Wenchuan-Maoxian Fault (WMF), the Beichuan-Yingxiu Fault (BYF), and the Jiangyou-Guanxian Fault (JGF). Previous researchers have placed great impetus on the Pengguan Massif, between the WMF and BYF. However, limited low-temperature thermochronology data coverage in other areas prevents us from fully delineating the tectonic history of the LMS. In this study, we collect 22 samples from vertical profiles in the Xuelongbao Massif and the range frontal area located at the hanging walls of the WMF and JGF respectively, and conduct apatite and zircon fission track analyses. New fission track data reveal that the Xuelongbao Massif has been undergoing rapid exhumation with an average rate of 0.7-0.9 mm/yr since 11 Ma, and the range frontal area began rapid exhumation at 7.5 Ma with total exhumation of 2.5-4.5 km. The exhumation histories indicate that the three major faults (WMF, BYF and JGF) in the central LMS are all reverse faults, and show a basinward in-sequence propagation from middle Miocene to present-day. Such a pattern further implies that upper crustal shortening is the primary driver for the LMS' uplift during the Late Cenozoic. Nevertheless, middle-lower crustal deformation is difficult to be constrained by the exhumation histories, and its contribution to LMS' uplift cannot be ruled out.

Middle Miocene Displacement Along the Rand Detachment Fault, Rand Mountains

NASA Astrophysics Data System (ADS)

Shulaker, D. Z.; Grove, M. J.

2015-12-01

Laramide flat-slab subduction extinguished Sierra Nevada pluton emplacement in southern California by ca. 85 Ma as trench-derived sediments were underthrust and accreted beneath arc basement. These relationships are well illustrated in the Rand Mountains, situated just south of the Garlock fault in the northwestern Mojave Desert. Here, accreted rocks within the Rand Mountains are referred to as Rand Schist. The Rand Detachment fault juxtaposes Rand Schist beneath 87 Ma Sierran granitoids. New zircon (U-Th)/He age results from schist and basement juxtaposed across the Rand Detachment fault are 15 ± 3 Ma and 30 ± 5 Ma, respectively. When considered within the context of previously reported thermochronology from the Rand Mountains, our data shows that the Rand Detachment fault in the Rand Mountains is a middle Miocene fault that facilitated extension of the northwest Mojave Desert. This timing is in temporal and spatial agreement with regional extension throughout the Mojave triggered by northern migration of the slab window after collision of the Mendocino Triple Junction with the southern California margin. Further evidence of slab-window-related magmatism in the easternmost Rand Mountains is provided by the 19 Ma Yellow Aster pluton and 19 Ma rhyolite porphyry. It is possible that Miocene extension re-activated an older structure within the Rand Mountains. For example, a similar low-angle fault juxtaposing schist and basement present in the San Emigdio Mountains is believed to have accommodated large scale Late Cretaceous displacement, exhuming Rand Schist and overlying deepest Sierran basement to shallow crustal levels by 77 Ma [1]. However, 68-72 Ma phengite cooling ages and other thermochronology from the Rand Mountains indicates that any pre-Miocene extension in this area must postdate that in the San Emigdio Mountains. [1] Chapman et al., 2012. Geosphere, 8, 314-341.

Thermochronology of the Sulu ultrahigh-pressure metamorphic terrane: Implications for continental collision and lithospheric thinning

NASA Astrophysics Data System (ADS)

Liu, Li-Ping; Li, Zheng-Xiang; Danišík, Martin; Li, Sanzhong; Evans, Noreen; Jourdan, Fred; Tao, Ni

2017-08-01

The thermal history of the Dabie-Sulu orogenic belt provides important constraints on the collision process between the South China and North China blocks during the Mesozoic, and possible lithospheric thinning event(s) in the eastern North China Block. This study reports on the thermal evolution of the Sulu ultrahigh-pressure metamorphic (UHP) terrane using zircon U-Pb geochronology and multiple thermochronology methods such as mica and hornblende 40Ar/39Ar, zircon and apatite fission track, and zircon and apatite (U-Th)/He dating. 40Ar/39Ar and zircon (U-Th)/He data show that the UHP terrane experienced accelerated cooling during 180-160 Ma. This cooling event could be interpreted to have resulted from extensional unroofing of an earlier southward thrusting nappe, or, more likely, an episode of northward thrusting of the UHP rocks as a hanging wall. A subsequent episode of exhumation took place between ca. 125 Ma and 90 Ma as recorded by zircon (U-Th)/He data. This event was more pronounced in the northwest section of the UHP terrane, whereas in the southeast section, the zircon (U-Th)/He system retained Jurassic cooling ages of ca. 180-160 Ma. The mid-Cretaceous episode of exhumation is interpreted to have resulted from crustal extension due to the removal of thickened, enriched mantle. A younger episode of exhumation was recorded by apatite fission track and apatite (U-Th)/He ages at ca. 65-40 Ma. Both latter events were linked to episodic thinning of lithosphere along the Sulu UHP terrane in an extensional environment, likely caused by the roll-back of the Western Pacific subduction system.

Applications of detrital geochronology and thermochronology from glacial deposits to the Paleozoic and Mesozoic thermal history of the Ross Embayment, Antarctica

NASA Astrophysics Data System (ADS)

Welke, Bethany; Licht, Kathy; Hennessy, Andrea; Hemming, Sidney; Pierce Davis, Elizabeth; Kassab, Christine

2016-07-01

Till from moraines at the heads of six major outlet glaciers in the Transantarctic Mountains (TAM) and from till beneath three West Antarctic ice streams have a ubiquitous zircon U-Pb age population spanning the time of the Ross/Pan-African Orogenies (610-475 Ma). Geochronology and thermochronology of detrital minerals in these Antarctic glacial tills reveal two different thermal histories for the central and southern TAM. Double-dating of the zircons reveals a geographically widespread (U-Th)/He (ZHe) population of 180-130 Ma in most of the till samples. Sandstone outcrops at Shackleton Glacier, and three Beacon Supergroup sandstone clasts from three moraines, have ZHe ages that fall entirely within this range. The similar population and proximity of many of the till samples to Beacon outcrops lead us to suggest that this extensive ZHe population in the tills is derived from Beacon Supergroup rocks and reflects the thermal response of the Beacon Basin to the breakup of Gondwana. A second population of older (>200 Ma) ZHe ages in tills at the head of Byrd, Nimrod, and Reedy Glaciers. For the tills at the head of the Nimrod and Byrd Glaciers, integrating the double-dated zircon results with 40Ar/39Ar of hornblende, muscovite and biotite, and U-Pb and (U-Th-Sm)/He double-dates on apatite yields a typical pattern of early rapid orogenic cooling (˜4-10°C/Myr) 590-475 Ma after the emplacement of the Granite Harbour Intrusives. Low temperature thermochronometers at these sites yield variable but quite old ages (ZHe 480-70 Ma and AHe 200-70 Ma) that require a long history at low temperature.

Predicting and testing continental vertical motion histories since the Paleozoic

NASA Astrophysics Data System (ADS)

Zhang, Nan; Zhong, Shijie; Flowers, Rebecca M.

2012-02-01

Dynamic topography at the Earth's surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry, sea-level change, continental flooding, sedimentation and erosion. These observations provide important constraints on and test of mantle dynamic models. Based on global mantle convection models coupled with the surface plate motion history, we compute dynamic topography and its history for the last 400 Ma associated with Pangea assembly and breakup, with particular focus on cratonic regions. We propose that burial-unroofing histories of cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models. Our models show that there are currently two broad dynamic topography highs in the Pacific and Africa for the present-day Earth that are associated with the broad, warm structures (i.e., superplumes) in the deep mantle, consistent with previous proposals of dynamical support for the Pacific and African superswells. Our models reveal that Pangea assembly and breakup, by affecting subduction and mantle upwelling processes, have significant effects on continental vertical motions. Our models predict that the Slave craton in North America subsides before Pangea assembly at 330 Ma but uplifts significantly from 330 Ma to 240 Ma in response to pre-Pangea subduction and post-assembly mantle warming. The Kaapvaal craton of Africa is predicted to undergo uplift from ~180 Ma to 90 Ma after Pangea breakup, but its dynamic topography remains stable for the last 90 Ma. The predicted histories of elevation change for the Slave and Kaapvaal cratons compare well with the burial-unroofing histories inferred from thermochronology studies, thus supporting our dynamic models including the development of the African superplume mantle structure. The vertical motion histories for other cratons can provide further tests of and constraints on our mantle dynamic models.

Predicting and testing continental vertical motion histories since the Paleozoic

NASA Astrophysics Data System (ADS)

Zhang, N.; Zhong, S.; Flowers, R. M.

2011-12-01

Dynamic topography at the Earth's surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry, sea-level change, continental flooding, sedimentation and erosion. These observations provide important constraints on and test of mantle dynamic models. Based on global mantle convection models coupled with the surface plate motion history, we compute dynamic topography and its history for the last 400 Ma associated with Pangea assembly and breakup, with particular focus on continental cratonic regions. We propose that burial-unroofing histories of continental cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models. Our models show that there are currently two broad dynamic topography highs in the Pacific and Africa for the present-day Earth that are associated with the broad, warm structures (i.e., superplumes) in the deep mantle, consistent with previous proposals of dynamical support for the Pacific and African superswells. Our models reveal that Pangea assembly and breakup, by affecting subduction and mantle upwelling processes, have significant effects on continental vertical motions. Our models predict that the Slave craton in North America subsides before Pangea assembly at 330 Ma but uplifts significantly from 330 Ma to 240 Ma in response to pre-Pangea subduction and post-assembly mantle warming. The Kaapvaal craton of Africa is predicted to undergo uplift from ~180 Ma to 90 Ma after Pangea breakup, but its dynamic topography remains stable for the last 90 Ma. The predicted histories of elevation change for the Slave and Kaapvaal cratons compare well with the burial-unroofing histories inferred from thermochronology studies, thus supporting our dynamic models including the development of the African superplume mantle structure. The vertical motion histories for other cratons can provide further tests and constraints on our mantle dynamic models.

Role of erosion and isostasy in the Cordillera Blanca uplift: insights from Low-T thermochronology and landscape evolution modeling (northern Peru, Andes)

NASA Astrophysics Data System (ADS)

Margirier, A.; Robert, X.; Braun, J.; Laurence, A.

2017-12-01

The uplift and exhumation of the highest Peruvian peaks seems closely linked to the Cordillera Blanca normal fault that delimits and shape the western flank of the Cordillera Blanca. Two models have been previously proposed to explain the occurrence of extension and the presence of this active normal fault in a compression setting but the Cordillera Blanca normal fault and the uplift and exhumation of the Cordillera Blanca remain enigmatic. Recent studies suggested an increase of exhumation rates during the Quaternary in the Cordillera Blanca and related this increase to a change in climate and erosion process (glacial erosion vs. fluvial erosion). The Cordillera Blanca granite has been significantly eroded since its emplacement (12-5 Ma) indicating a significant mass of rocks removal. Whereas it has been demonstrated recently that the effect of eroding denser rocks can contribute to an increase of uplift rate, the impact of erosion and isostasy on the increase of the Cordillera Blanca uplift rates has never been explored. Based on numerical modeling of landscape evolution we address the role of erosion and isostasy in the uplift and exhumation of the Cordillera Blanca. We performed inversions of the present-day topography, total exhumation and thermochronological data using a landscape evolution model (FastScape). Our results evidence the contribution of erosion and associated flexural rebound to the uplift of the Cordillera Blanca. Our models suggest that the erosion of the Cordillera Blanca dense intrusion since 3 Ma could also explain the Quaternary exhumation rate increase in this area. Finally, our results allow to question the previous models proposed for the formation of the Cordillera Blanca normal fault.

Two-phase Neogene extension in the northwestern basin and range recorded in a single thermochronology sample

USGS Publications Warehouse

Colgan, J.P.; Shuster, D.L.; Reiners, P.W.

2008-01-01

We use a combination of apatite 4He/3He, (U-Th)/ He, and fission-track thermochronology to date slip on the Surprise Valley fault in northeastern California by analyzing a single sample from the Warner Range in the footwall of the fault. This sample, a granitic clast from a conglomerate, yielded a fission-track age of 11.6 ?? 2.8 Ma and a (U-Th)/He age of 3.02 ?? 0.52 Ma. Geologic relationships indicate that this sample was buried to a depth of ???3.3 km prior to exhumation during slip on the Surprise Valley fault. Fission-track age and length data indicate that the sample was fully reset (>120 ??C) prior to exhumation, which began sometime after 14 Ma. A single aliquot of nine apatite grains was step-heated for 4He/3He analysis; modeling of the resulting 4He distribution indicates that cooling from <80 ??C to ???20 ??C occurred between 3 and 1 Ma. Interconsistent time-temperature (t-T) solutions to the combined 4He/3He, (U-Th)/He, and fission-track data require two distinct periods of cooling, consistent with non-continuous slip on the Surprise Valley fault. Early cooling and fault slip took place between 14 and 8 Ma, followed by more recent fault slip ca. 3 Ma. This timing is consistent with both local geologic relationships and with the regional timing of faulting along the western margin of the Basin and Range Province. These data demonstrate the resolving power of combined fission-track, (U-Th)/He, and 4He/3He thermochronometric data to extract low-temperature t-T information from a single sample close to Earth's surface. ?? 2008 The Geological Society of America.

Post-breakup tectonics in southeast Brazil from thermochronological data and combined inverse-forward thermal history modeling

NASA Astrophysics Data System (ADS)

Cogné, Nathan; Gallagher, Kerry; Cobbold, Peter R.; Riccomini, Claudio; Gautheron, Cecile

2012-11-01

The continental margin of southeast Brazil is elevated. Onshore Tertiary basins and Late Cretaceous/Paleogene intrusions are good evidence for post breakup tectono-magmatic activity. To constrain the impact of post-rift reactivation on the geological history of the area, we carried out a new thermochronological study. Apatite fission track ages range from 60.7 ± 1.9 Ma to 129.3 ± 4.3 Ma, mean track lengths from 11.41 ± 0.23 μm to 14.31 ± 0.24 μm and a subset of the (U-Th)/He ages range from 45.1 ± 1.5 to 122.4 ± 2.5 Ma. Results of inverse thermal history modeling generally support the conclusions from an earlier study for a Late Cretaceous phase of cooling. Around the onshore Taubaté Basin, for a limited number of samples, the first detectable period of cooling occurred during the Early Tertiary. The inferred thermal histories for many samples also imply subsequent reheating followed by Neogene cooling. Given the uncertainty of the inversion results, we did deterministic forward modeling to assess the range of possibilities of this Tertiary part of the thermal history. The evidence for reheating seems to be robust around the Taubaté Basin, but elsewhere the data cannot discriminate between this and a less complex thermal history. However, forward modeling results and geological information support the conclusion that the whole area underwent cooling during the Neogene. The synchronicity of the cooling phases with Andean tectonics and those in NE Brazil leads us to assume a plate-wide compressional stress that reactivated inherited structures. The present-day topographic relief of the margin reflects a contribution from post-breakup reactivation and uplift.

He, U, and Th Depth Profiling of Apatite and Zircon Using Laser Ablation Noble Gas Mass Spectrometry and SIMS

NASA Astrophysics Data System (ADS)

Monteleone, B. D.; van Soest, M. C.; Hodges, K. V.; Hervig, R.; Boyce, J. W.

2008-12-01

Conventional (U-Th)/He thermochronology utilizes single or multiple grain analyses of U- and Th-bearing minerals such as apatite and zircon and does not allow for assessment of spatial variation in concentration of He, U, or Th within individual crystals. As such, age calculation and interpretation require assumptions regarding 4He loss through alpha ejection, diffusive redistribution of 4He, and U and Th distribution as an initial condition for these processes. Although models have been developed to predict 4He diffusion parameters, correct for the effect of alpha ejection on calculated cooling ages, and account for the effect of U and Th zonation within apatite and zircon, measurements of 4He, U, and Th distribution have not been combined within a single crystal. We apply ArF excimer laser ablation, combined with noble gas mass spectrometry, to obtain depth profiles within apatite and zircon crystals in order to assess variations in 4He concentration with depth. Our initial results from pre-cut, pre-heated slabs of Durango apatite, each subjected to different T-t schedules, suggest a general agreement of 4He profiles with those predicted by theoretical diffusion models (Farley, 2000). Depth profiles through unpolished grains give reproducible alpha ejection profiles in Durango apatite that deviate from alpha ejection profiles predicted for ideal, homogenous crystals. SIMS depth profiling utilizes an O2 primary beam capable of sputtering tens of microns and measuring sub-micron resolution variation in [U], [Th], and [Sm]. Preliminary results suggest that sufficient [U] and [Th] zonation is present in Durango apatite to influence the form of the 4He alpha ejection profile. Future work will assess the influence of measured [U] and [Th] zonation on previously measured 4He depth profiles. Farley, K.A., 2000. Helium diffusion from apatite; general behavior as illustrated by Durango fluorapatite. J. Geophys. Res., B Solid Earth Planets 105 (2), 2903-2914.

Final « pop-up » structural reactivation of the internal part of an orogenic wedge: west-central Pyrenees

NASA Astrophysics Data System (ADS)

Meresse, F.; Jolivet, M.; Labaume, P.; Teixell, A.

2009-04-01

Université Montpellier 2, INSU-CNRS, Laboratoire Géosciences Montpellier, cc060, 34095 Montpellier Cedex 5, France florian.meresse@gm.univ-montp2.fr Tectonics-sedimentation relationships are often used to describe the tectonic evolution of orogenic wedges. However, does the sedimentary record associated to the build-up of the wedge recall the entire tectonic history? Numerous studies based on tectono-stratigraphic and thermochronological data, as well as numerical modeling, have demonstrated that on the large scale the growth of the Pyrenees is characterized by a southward propagation of the deformation (e.g., Muñoz, 1992; Morris et al., 1998; Fitzgerald et al., 1999; Beaumont et al., 2000). However, in the west-central Pyrenees, recent thermochronological data have suggested that the in-sequence propagation of the basement thrust system was followed by out-of-sequence (re)activation of hinterland structures after the South-Pyrenean Frontal Thrust had been sealed (Jolivet et al., 2007). To better describe the structural evolution of the Pyrenean prism, we focused our work on a NNE-SSW transect from the northern piedmont (Bagnères-de-Bigorre), through the Axial Zone and down to the Jaca basin where tectonics-sedimentation relationships have been extensively described (e.g., Teixell, 1996). A crustal scale cross-section combined with detailed apatite fission track analysis are used as a case study to unravel in detail the deformation history. Apatite fission track data from the Bagnères-de-Bigorre Paleozoic massif (central ages: 41-42 Ma) and the Lesponne Hercynian granite (central age: 31 Ma) located in the North-Pyrenean Zone and in the north of the Axial Zone, respectively, reveal Middle Eocene-Early Oligocene denudation ages of the northern part of the wedge. Immediately to the south, central ages around 24-20 Ma attest to a Latest Oligocene-Early Miocene denudation ages of the Chiroulet granite. According to the structural context, these results suggest a late exhumation stage associated with the tectonic (re)activation of north-vergent thrusts in the northern part of the Axial Zone. Similarly, results from the southern flank of the Axial Zone and the northern part of the Jaca basin suggest a denudation age around 18 Ma (Meresse et al., this volume), which may be linked to out-of-sequence tectonic movements on a south-vergent basement thrust (Bielsa thrust, Jolivet et al., 2007). In conclusion, thermochronological data reveal an Early Miocene "pop-up" exhumation of the internal parts of the Pyrenean wedge, which also shows that the Pyrenean compressional deformation ended later than the generally accepted Aquitanian age deduced from tectonics-sedimentation relationships. This late exhumation was achieved through out-of-sequence (re)activation of hinterland structures linked to a final internal thickening stage in the orogenic prism.

Deciphering Past and Present Tectonics of the Rio Grande Rift in New Mexico Utilizing Apatite Fission Track Thermochronology, Geochronology, Quaternary Faulting, and Cross-Section Restoration

NASA Astrophysics Data System (ADS)

Ricketts, J. W.; Karlstrom, K. E.; Kelley, S. A.; Priewisch, A.; Crossey, L. J.; Asmerom, Y.; Polyak, V.; Selmi, M.

2011-12-01

The Rio Grande rift provides an excellent laboratory for understanding styles and processes of extensional tectonics, and their driving forces. We apply apatite fission track (AFT) thermochronology, geochronology, fracture analysis, and cross-section restoration to decipher past and present tectonics of the Rio Grande rift. AFT data has been compiled from rift flank uplifts along the Rio Grande rift in an attempt to recognize long wavelength spatial and temporal patterns. AFT ages record time of cooling of rocks below ~110°C and, when cooling is due to exhumation, age elevation traverses can record upward advection of rocks through paleo 110°C isotherms. The relatively passive sides of half-grabens (e.g. Manzanos and Santa Fe Range) preserve Laramide AFT ages ranging from 45-70 Ma, indicating they were cooled during the Laramide Orogeny and have remained cooler than 110°C since then. Rift flanks on the tectonically active sides of half-grabens, (e.g. Sierra Ladrones, Sandias, Taos Range, and Sierra Blanca) have AFT ages that range from 35 Ma to <10 Ma, and record cooling that initiated with the Oligocene ignimbrite flare-up and continues through the Neogene. Our analysis tracks the approximate elevation of paleo 110°C isotherms in 10 Ma intervals from the Laramide to the present and shows that reconstructed paleoisotherms have been differentially uplifted, warped, and faulted since their time of formation, and hence serve as markers of uplift history and its mechanisms. AFT data at Ladron Peak, an active rift flank along the western margin of the Rio Grande rift in central New Mexico, indicates that it was rapidly unroofed between 20-10 Ma. Preliminary apatite helium data gives a similar age vs. elevation trend, but apatites have highly radiogenically damaged lattices and hence have corrected closure temperatures tens of degrees higher than AFT ages. The style of faulting at Ladron Peak is unusual because it is bounded by the anomalously low-angle (~15°) Jeter fault. In order to understand the evolution of faulting in this region, a balanced cross-section was constructed and restored to its pre-rift geometry. Our working hypothesis is that the low angle of the Jeter fault is most adequately explained by a rolling hinge model, where isostatic uplift causes progressive rotation of an initially steep (~60°) normal fault to shallower dips. Thirty km north of Ladron along the west side of the rift, Quaternary extensional faulting is evident in large travertine deposits at the Belen Quarry. Extensional fractures and cm-scale displacement normal faults at 4 locations give average paleostress orientations of 087, 112, 116, 127. A U-series age of 312 ka on faulted upper layers in one quarry indicates post-312 ka slip that we interpret to reflect surface manifestations of microseismicity above the Socorro magma body.

Uranium-lead Isotope Evidence in the Shelyabinsk LL5 Chondrite Meteorite for Ancient and Recent Thermal Events

NASA Technical Reports Server (NTRS)

Lapen, T. J.; Kring, D. A.; Zolensky, M. E.; Andreasen, R.; Righter, M.; Swindle, T. D.; Beard, S. P.; Swindle, T. D.

2014-01-01

The impact histories on chondrite parent bodies can be deduced from thermochronologic analyses of materials and isotope systems with distinct apparent closure temperatures. It is especially critical to better understand the geological histories and physical properties of potenally hazardous near-Earth asteroids. Chelyabinsk is an LL5 chondrite meteorite that was dispersed over a wide area tens of kilometers south of the town of Chelyabinsk, Russia by an explosion at an altitude of 27 km at 3:22 UT on 15 Feb 2013 [1,2]. The explosion resulted in significant damage to surrounding areas and over 1500 injuries along with meteorite fragments being spread over a wide area [1].

Low temperature thermochronology in the Eastern Alps: Implications for structural and topographic evolution

PubMed Central

Wölfler, Andreas; Stüwe, Kurt; Danišík, Martin; Evans, Noreen J.

2012-01-01

According to new apatite fission track, zircon- and apatite (U–Th)/He data, we constrain the near-surface history of the southeastern Tauern Window and adjacent Austrolapine units. The multi-system thermochronological data demonstrate that age-elevation correlations may lead to false implications about exhumation and cooling in the upper crust. We suggest that isothermal warping in the Penninic units that are in the position of a footwall, is due to uplift, erosion and the buildup of topography. Additionally we propose that exhumation rates in the Penninic units did not increase during the Middle Miocene, thus during the time of lateral extrusion. In contrast, exhumation rates of the Austroalpine hangingwall did increase from the Paleogene to the Neogene and the isotherms in this unit were not warped. The new zircon (U–Th)/He ages as well as zircon fission track ages from the literature document a Middle Miocene exhumation pulse which correlates with a period of enhanced sediment accumulation during that time. However, enhanced sedimentation- and exhumation rates at the Miocene/Pliocene boundary, as observed in the Western- and Central Alps, cannot be observed in the Eastern Alps. This contradicts a climatic trigger for surface uplift, and makes a tectonic trigger and/or deep-seated mechanism more obvious to explain surface uplift in the Eastern Alps. In combination with already published geochronological ages, our new data demonstrate Oligocene to Late Miocene fault activity along the Möll valley fault that constitutes a major shear zone in the Eastern Alps. In this context we suggest a geometrical and temporal relationship of the Katschberg-, Polinik–Möll valley- and Mur–Mürz faults that define the extruding wedge in the eastern part of the Eastern Alps. Equal deformation- and fission track cooling ages along the Katschberg–Brenner- and Simplon normal faults demonstrate overall Middle Miocene extension in the whole alpine arc. PMID:27065501

The Thermochronologist's Progress

NASA Astrophysics Data System (ADS)

Zeitler, P. K.

2011-12-01

We owe our current understanding of thermochronology less to a series of revolutionary insights than to a somewhat uneven intellectual pilgrimage that over fifty years has progressed in fits and starts. Though hampered at times by overenthusiasm, oversimplification, and misunderstandings, on balance the field advanced thanks to a blend of curiosity-driven research, tool-building motivated by new ideas about Earth science, and improvements in technology. But now that we've exploited most radiogenic systems and the major minerals that host them, and now that our models can devour CPU time along with the best of them, are we done? Have we reached peak thermochron? The answer of course is no, and papers in this session will demonstrate what new technologies and techniques might have to offer in the coming years. However, I will argue that the discipline as a whole has matured to a point where if thermochronology is to remain a mainstream tool as opposed to a weekend sport, we need to get serious about several challenges. The most fundamental challenge is that current geodynamic models (and even more complex models we can envision coding) have outpaced our meagre stockpile of kinetic calibrations, our understanding of detailed isotope systematics, and our ability to generate data with sufficient throughput. These issues will not be addressed adequately through the business-as-usual approach that brought us to our current knowledge, and some community effort will probably be needed to coordinate the hard work that will be required. But any serious attempt to answer important questions with accurate thermal histories that have low and well-defined uncertainties will require that we actually know the kinetics for the specific samples we are analyzing, that we fully understand scatter in the data, that we work with the large sample numbers that are required for some problems like landscape evolution, and that inversion tools fully explore the important aspects of both the geochemical and geodynamical parameter spaces.

Differential unroofing within the central metasedimentary Belt of the Grenville Orogen: constraints from 40Ar/39Ar thermochronology

USGS Publications Warehouse

Cosca, M.A.; Essene, E.J.; Kunk, Michael J.; Sutter, J.F.

1992-01-01

An 40Ar/39Ar thermochronological investigation of upper greenschist to granulite facies gneiss, amphibolite and marble was conducted in the Central Metasedimentary Belt (CMB), Ontario, to constrain its cooling history. Incremental 40Ar/39Ar release spectra indicate that substantial differential unroofing occurred in the CMB between ??? 1000 and ??? 600 Ma. A consistent pattern of significantly older hornblende and phlogopite 40Ar/3Ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation. Variations in hornblende 40Ar/39Ar age plateaus exceeding 200 Ma occur over distances less than 50 km with major age discontinuities occurring across the Robertson Lake shear zone and the Sharbot Lake mylonite zone which separate the Sharbot Lake terrane from the Elzevir and Frontenac terranes. Extensional displacements of up to 14 km are inferred between the Frontenac and Elzevir terranes of the CMB. No evidence for significant post argon-closure vertical displacement is indicated in the vicinity of the Perth Road mylonite within the Frontenac terrane. Variations of nearly 100 Ma in phlogopite 40Ar/39Ar plateau ages occur in undeformed marble on either side of the Bancroft Shear Zone. Phlogopites from sheared and mylonitized marble within the shear zone yield 40Ar/39Ar diffusional loss profiles, but have older geologically meaningless ages thought to reflect incorporation of excess argon. By ??? 900 Ma, southeast directed extension was occurring throughout the CMB, possibly initiated along previous zones of compressional shearing. An easterly migration of active zones of extension is inferred, possibly related to an earlier, overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone. The duration of extensional shearing is not well constrained, but must have ceased before ??? 600 Ma as required by the deposition of overlying undeformed Cambrian and/or Ordovician sedimentary rocks. ?? 1992 Springer-Verlag.

Asymmetric Exhumation Patterns Revealed through Apatite-Zircon Thermochronology of the Santa Lucia Range, Central California

NASA Astrophysics Data System (ADS)

Mere, A.; Steely, A.; Hourigan, J. K.

2016-12-01

Previous thermochronological analyses of crystalline bedrock in the central Santa Lucia range have yielded surprisingly rapid rates of surface uplift and bedrock extrusion despite lack of modern seismicity along nearby faults. We use 8 new apatite and zircon (U-Th)/He dates in order to better constrain the history of bedrock extrusion in response to the transpressional North American-Pacific plate boundary. Granitic samples were collected along coastal fault blocks bounded by the Palo Colorado (PCF), Sur-Nacimiento (SNF), and San Gregorio Hosgri faults (SGHF); as well as one sample from Salinian Basement >25km NE of the SGHF. Helium was extracted and analyzed using a quadrupole equipped multi-sample laser microfurnace and U/Th content was measured using high precision isotope-dilution ICP mass spectrometry. Rapid late Cretaceous unroofing is captured in Salinian basement zircon and apatite by the respective 67Ma and 63Ma dates. Zircon along coastal silvers proximal to PCF and SNF record 28-31Ma dates while zircon in close proximity to SGHF record ages as young as 6.5Ma. Apatite ages proximal to PCF and SNF range between 6-9Ma and are as young as 1.5Ma directly NE of the SGHF. These data reflect increased exhumation beginning as recently as the Miocene and additionally indicate rates of modern (<2Ma) uplift exceeding 1.3 mm/yr. These results indicate that stresses caused by the active plate boundary are accommodated by the SGHF and associated faults as vertical deformation despite low rates of modern seismicity. We suggest that the SGHF and nearby faults are more active, or behave differently, that previously acknowledged. The pattern of focused exhumation within narrow fault blocks appears to be related to underplating of low strength schist that is thought to be synchronous with late Cretaceous unroofing of Salinian basement.

Tertiary structural evolution of the Gangdese thrust system southeastern Tibet

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yin, An; Harrison, M.; Ryerson, F.J.

1994-09-10

Structural and thermochronological investigations of southern Tibet (Xizang) suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost Tibetan plateau since late Oligocene. Two thrust systems are documented in this study: the north dipping Gangdese system (GTS) and the younger south dipping Renbu-Zedong system (RZT). West of Lhasa, the Gangdese thrust juxtaposes the Late Cretaceous forearc basin deposits of the Lhasa Block (the Xigaze Group) over the Tethyan sedimentary rocks of the Indian plate, whereas east of Lhasa, the fault juxtaposes the Late Cretaceous-Eocene, Andean-type arc (the Gangdese batholith) over Tethyan sedimentary rocks.more » Near Zedong, 150 km southeast of Lhasa, the Gangdese thrust is marked by a >200-m-thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks. A major south dipping backthrust in the hanging wall of the Gangdese thrust puts the Xigaze Group over Tertiary conglomerates and the Gangdese plutonics north of Xigaze and west of Lhasa. A lower age bound for the Gangdese thrust of 18.3{+-}0.5 Ma is given by crosscutting relationships. The timing of slip on the Gangdese thrust is estimate to be 27-23 Ma from {sup 40}Ar/{sup 39}Ar thermochronology, and a displacement of at least 46{+-}9 km is indicated near Zedong. The age of the Gangdese thrust (GT) is consistent with an upper age limit of {approximately}24 Ma for the initiation of movement on the Main Central thrust. In places, the younger Renbu-Zedong fault is thrust over the trace of the GT, obscuring its exposure. The RZT appears to have been active at circa 18 Ma but had ceased movement by 8{+-}1 Ma. The suture between India and Asia has been complexely modified by development of the GTS, RZT, and, locally, strike-slip and normal fault systems. 64 refs., 14 figs., 2 tabs.« less

Decoding Dynamic Topography: Geologic and Thermochronologic Constraints From Madagascar

NASA Astrophysics Data System (ADS)

Stephenson, S.; White, N.

2017-12-01

Madagascar's topography is characterized by flights of low relief peneplains separated by escarpments. Remarkably, nearly 50% of the landscape is higher than 500 m despite being surrounded by passive margins. Eocene marine limestones crop out at elevations of 400-800 m, staircases of Pleistocene marine terraces fringe the coastline and longitudinal river profiles are disequilibrated. Together, these observations suggest that Madagascar has experienced Neogene epeirogenic uplift. Positive oceanic residual depth anomalies surrounding the island, long wavelength free-air gravity anomalies, Neogene basaltic volcanism and slow sub-plate shear wave velocities show that Neogene uplift is generated by convective circulation within the upper mantle. However, the landscape's erosional response to long wavelength uplift is poorly known. Here, we present 18 apatite fission track and apatite He analyses of granitoid samples from sub-vertical transects in central and northern Madagascar. Apatite fission track ages are 200-250 Ma with mean track lengths of 12 μm. Apatite He ages are highly dispersed in samples from the highlands (i.e. AHe age > 150 Ma) but a narrower, younger range of 30-60 Ma is found on the coastal lowlands. Joint inverse modeling was carried out using the QTQt transdimensional reversible jump Markov Chain Monte Carlo (MCMC) algorithm to determine time-temperature histories. Results show that the coastal lowlands experienced up to 1 km of exhumation during the Neogene Period, whilst the central highlands experienced either very slow or negligible exhumation. This spatial distribution is expected when kinematic waves of incision propagate through a fluvially eroding landscape from coast to interior. Inverse modeling of suites of river profiles and forward landscape simulations support this interpretation. Our results show that the landscape response to modest (i.e. 1 km) regional uplift is diachronous and that thermochronologic observations can be used to detect spatial patterns of denudation. These combined observations help to constrain the fluid dynamical evolution of the upper mantle beneath Madagascar.

Intracratonic exhumation and uplift: Insight from low-temperature thermochronology and geomorphic analysis of the Ozark Plateau, Missouri

NASA Astrophysics Data System (ADS)

DeLucia, M. S.; Marshak, S.; Guenthner, W.; Anders, A. M.; Thomson, S. N.

2016-12-01

The Ozark Plateau is an uplift in the cratonic platform of Midcontinent United States. In the northeast corner of the plateau (the St. Francois Mountains), Precambrian basement of 1.47 Ga granite and rhyolite crops out. These rocks are overlain, at the Great Unconformity, by Paleozoic strata, defining the map pattern of the Ozark Dome. Strata thicken substantially eastward into the Illinois Basin, so that there is over 7 km of structural relief across the boundary between the Illinois Basin and the Ozark Dome at the level of the Great Unconformity. Multiple unconformities in the Paleozoic section indicates that the crest of the Ozark Dome was at or above sea level several times during the Paleozoic. Key questions about the Plateau remain. For example: (1) Did the 1.47 Ga basement remain at upper-crustal depths since its formation, or was it buried deeply and later exhumed? (2) Has the plateau remained high since the Paleozoic or has it undergone post-Paleozoic uplift? Results from new zircon (U-Th)/He thermochronology indicate that the 1.47 Ga granites were exhumed significantly in the Neoproterozoic (about 750Ma), after the Rodinia supercontinent assembly. Fission-track dates (Brown, 2005) and (U-Th)/He apatite dates (Flowers and Kelley, 2011; Zhang et al., 2012; and new results) hint that some post-Paleozoic exhumation has occurred. Analysis of a high-resolution DEM of the Ozark Plateau supports this proposal; bedrock-incising streams occur throughout the plateau (locally producing incised meanders), and strath terraces can be identified. The rate of uplift, however, must be relatively slow, for drainages do not display knick points, and drainage networks display mature profiles. Given these constraints, we propose that the lithospheric architecture that distinguished the Ozark Dome from the Illinois Basin became established in the Neoproterozoic, and that the Ozark Plateau has been maintained isostatically by subsequent slow exhumation.

Tectono-thermal evolution of the southwestern Alxa Tectonic Belt, NW China: Constrained by apatite U-Pb and fission track thermochronology

NASA Astrophysics Data System (ADS)

Song, Dongfang; Glorie, Stijn; Xiao, Wenjiao; Collins, Alan S.; Gillespie, Jack; Jepson, Gilby; Li, Yongchen

2018-01-01

The Central Asian Orogenic Belt (CAOB) is regarded to have undergone multiple phases of intracontinental deformation during the Meso-Cenozoic. Located in a key position along the southern CAOB, the Alxa Tectonic Belt (ATB) connects the northernmost Tibetan Plateau with the Mongolian Plateau. In this paper we apply apatite U-Pb and fission track thermochronological studies on varieties of samples from the southwestern ATB, in order to constrain its thermal evolution. Precambrian bedrock samples yield late Ordovician-early Silurian ( 430-450 Ma) and late Permian ( 257 Ma) apatite U-Pb ages; the late Paleozoic magmatic-sedimentary samples yield relatively consistent early Permian ages from 276 to 290 Ma. These data reveal that the ATB experienced multiple Paleozoic tectono-thermal events, as the samples passed through the apatite U-Pb closure temperature ( 350-550 °C). We interpret these tectonic events to record the long-lived subduction-accretion processes of the Paleo-Asian Ocean during the formation of the southern CAOB, with possible thermal influence of the Permian Tarim mantle plume. Apatite fission track (AFT) data and thermal history modelling reveal discrete low-temperature thermal events for the ATB, inducing cooling/reheating through the AFT partial annealing zone ( 120-60 °C). During the Permian, the samples underwent rapid cooling via exhumation or denudation from deep crustal levels to temperatures < 200 °C. Subsequent thermal events in the Triassic were thought to be associated with the final amalgamation of the CAOB or the closure of the Paleotethys. During the Jurassic-Cretaceous the study area experienced heating by burial, followed by renewed cooling, which may be related with the construction and subsequent collapse of the Mongol-Okhotsk Orogeny, or the Lhasa-Eurasia collision and subsequent slab break-off. These results indicate that the ATB may have been stable after late Cretaceous in contrast to the Qilian Shan and Tianshan. Finally, our results indicate differential exhumation scenario occurred across the southwestern ATB during the Cretaceous.

Insights into the Geographic Sequence of Deglaciation in the Weddell Sea Embayment by Provenance of Ice-Rafted Debris

NASA Astrophysics Data System (ADS)

Williams, T.; Hemming, S. R.; Licht, K.; Agrios, L.; Brachfeld, S. A.; van de Flierdt, T.; Hillenbrand, C. D.; Ehrmann, W. U.; Zhai, X.; Cai, Y.; Corley, A. D.; Kuhn, G.

2017-12-01

The geochemical and geochronological fingerprint of rock debris eroded and carried by ice streams may be used to identify the provenance of iceberg-rafted debris (IRD) in the marine sediment record. During ice retreat following glacial maxima, it has been shown that there is an increase in IRD accumulation in marine sediments underlying the western limb of the Weddell Gyre. Here we present IRD provenance records from sediment core PS1571-1 in the NW Weddell Sea, and interpret these records in terms of the geographic sequence of ice sheet retreat in the Weddell Sea embayment during the most recent deglaciation. We first characterize the source areas of eroded debris around the Weddell Sea Embayment, using published mapping of the embayment and new material from: 1. Till in modern moraines at the edges of ice streams, including the Foundation Ice Stream, the Academy Glacier, and the Recovery Glacier; and 2. Subglacial till and proximal glaciomarine sediment from existing cores located along the front of the Filchner and Ronne Ice Shelves, collected on past expeditions of the RV Polarstern. The analyses on these samples include 40Ar/39Ar hornblende and biotite thermochronology and U-Pb zircon geochronology on individual mineral grains, and K-Ar thermochronology, Nd isotopes, and clay mineralogy on the clay grain size fraction. Results so far indicate that samples along the front of the Filchner and Ronne Ice Shelves record the geochemical and geochronological fingerprint that would be expected from tracing ice flow lines back to the bedrock terranes. The Ronne (west), Hughes (central), and Filchner (east) sectors have distinguishable provenance source signatures, and further subdivision is possible. In core PS1571-1, downcore IRD provenance changes reflect iceberg output and ice sheet retreat from the different sectors of the embayment through the last deglaciation. The detrital provenance method of interpreting the geographic sequence of ice retreat can equally be applied to previous deglaciations of the Weddell Sea Embayment.

Tectonic evolution and provenance of the Santa Bárbara Group, Camaquã Mines region, Rio Grande do Sul, Brazil

NASA Astrophysics Data System (ADS)

Bicca, Marcos Müller; Chemale, Farid; Jelinek, Andrea Ritter; de Oliveira, Christie Helouise Engelmann; Guadagnin, Felipe; Armstrong, Richard

2013-12-01

Cu- and Pb-Zn-hosting sedimentary units of the upper part of the Camaquã Basin (Ediacaran-Lower Ordovician) in the Dom Feliciano Belt of southernmost Brazil were formed during the late stages of the West Gondwana amalgamation and were controlled by large left-handed strike-slip shear zones. Integration of structural geology, stratigraphy and thermochronology allow recognition of five structural events (D1, Ediacaran-Lower Cambrian, through D5, Cretaceous). D1 structures are related to a N30E-trending, sinistral strike-slip shear zone that controlled the deposition of the mineralized sedimentary unit and its overlying units, the Santa Barbara and Guaritas Groups, respectively, in a transtensional setting. Based on U-Pb in situ methods, it is possible (a) to establish a maximum depositional age of 566 ± 6.9 Ma for the basal section of the Santa Barbara Group and, therefore, a minimum age of ca. 566 Ma for D1, and (b) to recognize two main zircon populations, Neoproterozoic and Paleoproterozoic, with sources from the eastern and southern parts of the Dom Feliciano Belt and reworking of older units of the Camaquã Basin. The D2 structures are mainly N-trending shear zones that developed after the deposition of the Guaritas Group during the Cambrian. During the Phanerozoic (post-Cambrian), the recognized structures were connected to compressional and extensional events that affected West Gondwana and the South America Platform. Thermochronological fission track analyses on apatite revealed four main age populations. The first three are interpreted to have formed during tectonic processes at the Gondwana Margin, namely the Famatinian and Gondwanides orogenies, and can be related to the D3 and D4 tectonic events in the basin. The last age population formed from thermal heating by the Upper Cretaceous continental flood basalts, which are represented in the area by volcanic intrusions, that were related to the separation of Africa and South America.

Cryptic microtextures and geological histories of K-rich alkali feldspars revealed by charge contrast imaging

NASA Astrophysics Data System (ADS)

Flude, Stephanie; Lee, Martin R.; Sherlock, Sarah C.; Kelley, Simon P.

2012-06-01

Charge contrast imaging in the scanning electron microscope can provide new insights into the scale and composition of alkali feldspar microtextures, and such information helps considerably with the interpretation of their geological histories and results of argon isotope thermochronological analyses. The effectiveness of this technique has been illustrated using potassium-rich alkali feldspars from the Dartmoor granite (UK). These feldspars contain strain-controlled lamellar crypto- and microperthites that are cross-cut by strain-free deuteric microperthites. The constituent albite- and orthoclase-rich phases of both microperthite generations can be readily distinguished by atomic number contrast imaging. The charge contrast results additionally show that sub-micrometre-sized albite `platelets' are commonplace between coarser exsolution lamellae and occur together to make cryptoperthites. Furthermore, charge contrast imaging reveals that the orthoclase-rich feldspar is an intergrowth of two phases, one that is featureless with uniform contrast and another that occurs as cross-cutting veins and grains with the {110} adularia habit. Transmission electron microscopy shows that the featureless feldspar is tweed orthoclase, whereas the veins and euhedral grains are composed of irregular microcline that has formed from orthoclase by `unzipping' during deuteric or hydrothermal alteration. The charge contrast imaging results are especially important in demonstrating that deuteric perthites are far more abundant in alkali feldspars than would be concluded from investigations using conventional microscopy techniques. The unexpected presence of such a high volume of replacement products has significant implications for understanding the origins and geological histories of crustal rocks and the use of alkali feldspars in geo- and thermochronology. Whilst the precise properties of feldspars that generate contrast remain unclear, the similarity between charge contrast images and corresponding cathodoluminescence images of deuteric microperthites indicates that trace element chemistry and possibly also elastic strain within the crystal play a major role.

Thermochronological evolution of an intra-plate magmatic event inferred from an integrated modeling approach: A case study in the Westerwald, Germany

NASA Astrophysics Data System (ADS)

Tirone, M.; Rokitta, K.; Schreiber, U.

2016-09-01

A lava sample from the Tertiary Westerwald volcanic field was selected for a detailed study using various analytical techniques in combination with petrological, thermodynamic and diffusion modeling to extract information related to the thermochronological evolution of a magmatic event before eruption. The lava sample contains large olivine phenocrysts which are compositionally zoned and two coexisting but chemically distinct melts, a host melt with basaltic composition and small spherical pockets of a less abundant trachytic melt (globules). The sample was analyzed by electron microprobe, x-ray fluorescence (XRF) X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The primary melt of the host lava was determined using the program PRIMELT2.XLS. Partial fractional crystallization of olivine was modeled using the program alphaMELTS. Timescale and cooling rate were retrieved by fitting the measured Fe-Mg zoning along two directions in four olivine grains from the host lava using a 3-D numerical diffusion model. The measured variation of Ca is also consistent with a chemical diffusion process, while a numerical growth model applied to the same olivines does not appear to explain the Fe-Mg zoning. Chemical zoning of major elements in the melt globules were reproduced with a multicomponent diffusion model. The results of this study show that the host magma fractionated about 9% of olivine in a first stage, then the crystallization proceeded without further separation of mineral phases. Modeling of diffusion in the olivine crystals suggests that this second stage lasted at least 5 yrs and the temperature of the melt decreased from 1120-1150 °C to 1090 °C during this time. According to the results of the multicomponent diffusion model applied to the melt globules, the coexistence of the two melts was extremely short (less than few hours), possibly recording the assimilation of the globules during eruption or cooling of the whole system on the surface.

Multiple episodes of fast exhumation since Late Cretaceous in the Three Rivers region, SE Tibetan Plateau, revealed by low-temperature thermochronology

NASA Astrophysics Data System (ADS)

Liu, J.; Zhang, J.; McPhillips, D.; Reiners, P. W.; Wang, W.; Pik, R.; Zeng, L.; Hoke, G. D.; Xie, K.; Xiao, P.; Zheng, D.; Ge, Y.

2017-12-01

The Three Rivers region in southeast Tibet is characterized by deeply incised river valleys separated by a perched low relief landscape that gently descends from the high Tibetan plateau towards the southeast. When and how this unique landscape formed is debated. The onset of increased river incision is often interpreted as a proxy for the timing of surface uplift. Here,apatite and zircon U-Th/He and apatite fission track thermochronology are employed to map the spatial and temporal pattern of exhumation in the region. Vertical profiles of granitic rocks were collected near Deqin ( 28.5°N) and Weixi ( 27.5°N). The two transects share a similar exhumation history, with two episodes of relatively fast exhumation ( 100-300 m/Myr): during the Paleocene to Eocene (60-40 Ma) and Miocene to present (20-0 Ma), separated by an intervening period of slow exhumation. A pulse of moderate to high exhumation (70-300 m/Myr) during the mid- to late-Cretaceous (120-80 Ma) is also present in the data. Overall, the rate and total amount of exhumation near Deqin is larger than at Weixi and is especially pronounced in the interval between 20 Ma to present. This difference is likely related to whether a profile's is exposed adjacent to the more erosive trunk stream (Deqin) or contained within tributary (Weixi). In addition, the Paleocene to Eocene episode of fast exhumation in the Three Rivers region occurred prior to that observed on the eastern plateau margin. This Paleocene to Eocene period of rapid exhumation is likely due to early Cenozoic deformation related to the transpressional collision of the Indian plate with this region. Pre-Miocene episodes of fast exhumation corroborate with recent paleoaltimetry studies, which shows that the Three Rivers region was elevated prior to the Oligocene.

Apatite fission track evidence for Miocene denudation history in the Gangdese conglomerate belt and Yarlung Tsangpo River: Implications for the evolution of Southern Tibet

NASA Astrophysics Data System (ADS)

Song, Shiyu; Cao, Daiyong; Zhang, QingChao; Wang, Anming; Peng, Yangwen

2018-07-01

Low-temperature thermochronology is used widely in the Tibet plateau uplift. Some researches, however, have defined the time of rapid denudation as simply rock uplift and have neglected the fact that the rock denudation recorded by fission track (FT) data was controlled by both surface incision and rock uplift. The incision of the Yarlung Zangbo River had a significant influence on uplift history inversion in Southern Tibet. This paper simulated the bedrock denudation and river incision histories using apatite fission track (AFT) data sampled from the Gangdese conglomerate belt, which is located in the middle of Southern Tibet, and analyzed the geological meaning of the AFT age of each sample. The results showed the following: (1) In the early Miocene (22-16 Ma), both the value of the denudation rate and the incision rate were high (0.56 mm/yr and 0.24 mm/yr). (2) In the middle-late Miocene, the incision rate (0.12 mm/yr) was similar to the denudation rate (0.09-0.11 mm/yr). (3) The historical model between river incision and bedrock denudation revealed a significant difference in the denudation rate during the period ca. 8-6 Ma. Combining these data with previously published thermochronological ages and synthesizing these ages with regional geological, we arrived at the following conclusions: (1) In the early Miocene, the denudation event probably was caused by a combined result of Indian plate rollback and the incision of the Yarlung Zangbo River. (2) In the middle-late Miocene, the denudation rate was consistent with the incision rate, which suggested that the denudation episode was caused by climate change associated with Asian monsoon intensification. (3) After 8 Ma, the stable and slow incision rate indicated that regional drastic uplift had ceased. The paleo-elevation of the research area had approached, and even exceeded, the present-day elevation in the late Miocene.

The Thermal Evolution of the Southeast Baffin Island Continental Margin: An Integrated Apatite Fission Track and Apatite (U-Th)/He Study

NASA Astrophysics Data System (ADS)

Jess, S.; Stephenson, R.; Brown, R. W.

2017-12-01

The elevated continental margins of the North Atlantic continue to be a focus of considerable geological and geomorphological debate, as the timing of major tectonic events and the age of topographic relief remain controversial. The West Greenland margin, on the eastern flank of Baffin Bay, is believed by some authors to have experienced tectonic rejuvenation and uplift during the Neogene. However, the opposing flank, Baffin Island, is considered to have experienced a protracted erosional regime with little tectonic activity since the Cretaceous. This work examines the thermal evolution of the Cumberland Peninsula, SE Baffin Island, using published apatite fission track (AFT) data with the addition of 103 apatite (U-Th)/He (AHe) ages. This expansion of available thermochronological data introduces a higher resolution of thermal modelling, whilst the application of the newly developed `Broken Crystals' technique provides a greater number of thermal constraints for an area dominated by AHe age dispersion. Results of joint thermal modelling of the AFT and AHe data exhibit two significant periods of cooling across the Cumberland Peninsula: Devonian/Carboniferous to the Triassic and Late Cretaceous to present. The earliest phase of cooling is interpreted as the result of major fluvial systems present throughout the Paleozoic that flowed across the Canadian Shield to basins in the north and south. The later stage of cooling is believed to result from rift controlled fluvial systems that flowed into Baffin Bay during the Mesozoic and Cenozoic during the early stages and culmination of rifting along the Labrador-Baffin margins. Glaciation in the Late Cenozoic has likely overprinted these later river systems creating a complex fjordal distribution that has shaped the modern elevated topography. This work demonstrates how surface processes, and not tectonism, can explain the formation of elevated continental margins and that recent methodological developments in the field of low temperature thermochronology are improving our understanding of onshore passive margin development.

Thermal history and differential exhumation across the Eastern Musgrave Province, South Australia: Insights from low-temperature thermochronology

NASA Astrophysics Data System (ADS)

Glorie, Stijn; Agostino, Kate; Dutch, Rian; Pawley, Mark; Hall, James; Danišík, Martin; Evans, Noreen J.; Collins, Alan S.

2017-04-01

Multi-method geo- and thermochronological data obtained for Palaeo- and Mesoproterozoic granitoids traversing the main structural architecture of the eastern Musgrave Province within South Australia reveal multiphase cooling histories. Apatite U-Pb dating on six samples yield consistent ages of 1075-1025 Ma, suggesting a thermal reset coinciding with mantle-derived magmatism of the greater Warakurna Large Igneous Province ( 1080-1040 Ma). Apatite fission track (AFT) analysis indicate that four discrete thermal events affected the study area, inducing cooling through the AFT partial annealing zone ( 60-120 °C), supported by apatite and zircon (U-Th-Sm)/He data. Late Neoproterozoic cooling from deep crustal levels to temperatures < 200 °C was discerned, which is thought to be related to exhumation and denudation during the Petermann Orogeny. Subsequent cooling events at 450-400 Ma (Silurian-Devonian) and 310-290 Ma (Late Carboniferous) are interpreted to represent exhumation associated with the Alice Springs Orogeny. The latter event exhumed the sampled plutons to shallow crustal depths. An additional Triassic - early Jurassic thermal event, likely recording elevated geothermal gradients at that time, was observed throughout the study area, however, more data is needed to further support this interpretation. The high sample density across the structural architecture of the study area furthermore reveals patterns of fault reactivation and resulting differential exhumation, indicating shallower exhumation levels in the centre and deeper exhumation towards the margins of the sampled transect. The observed differential exhumation patterns match with existing seismic data and fit a model of an inverted graben system for the Phanerozoic evolution of the eastern Musgraves. The results highlight a complex Phanerozoic thermal history for the eastern Musgraves and help to elucidate the poorly appreciated tectonic evolution of inland Australia. This study further demonstrates how high-density sample transects across structural architecture can assess the relative crustal level and associated preservation of the thermal history record within fault-reactivated terranes.

Detrital Geochemical Fingerprints of Rivers Along Southern Tibet and Nepal: Implications for Erosion of the Indus-Yarlung Suture Zone and the Himalayas

NASA Astrophysics Data System (ADS)

Hassim, M. F. B.; Carrapa, B.; DeCelles, P. G.; Kapp, P. A.; Gehrels, G. E.

2014-12-01

Our detrital geochemical study of modern sand collected from tributaries of the Yarlung River in southern Tibet and the Kali Gandaki River and its tributaries in Nepal shed light on the ages and exhumation histories of source rocks within the Indus-Yarlung Suture (IYS) zone and the Himalayas. Seven sand samples from rivers along the suture zone in southern Tibet between Xigatze to the east and Mt. Kailas to the west were collected for detrital zircon U-Pb geochronologic and Apatite Fission Track (AFT) thermochronologic analyses. Zircon U-Pb ages for all rivers range between 15 and 3568 Ma. Rivers draining the northern side of the suture zone mainly yield ages between 40 and 60 Ma, similar to the age of the Gangdese magmatic arc. Samples from rivers draining the southern side of the suture zone record a Tethyan Himalayan signal characterized by age clusters at 500 Ma and 1050 Ma. Our results indicate that the ages and proportion of U-Pb zircons ages of downstream samples from tributaries of the Yarlung River directly reflect source area ages and relative area of source rock exposure in the catchment basin. Significant age components at 37 - 40 Ma, 47 - 50 Ma, 55 - 58 Ma and 94 - 97 Ma reflect episodicity in Gangdese arc magmatism. Our AFT ages show two main signals at 23-18 Ma and 12 Ma, which are in agreement with accelerated exhumation of the Gangdese batholith during these time intervals. The 23 - 18 Ma signal partly overlaps with deposition of the Kailas Formation along the suture zone and may be related to exhumation due to upper plate extension in southern Tibet in response to Indian slab rollback and/or break-off events. Detrital thermochronology of four sand samples from the Kali Gandaki River and some of its tributaries in Nepal is underway and will provide constraints on the timing of erosion of the central Nepal Himalaya.

Dating of major normal fault systems using thermochronology: An example from the Raft River detachment, Basin and Range, western United States

USGS Publications Warehouse

Wells, M.L.; Snee, L.W.; Blythe, A.E.

2000-01-01

Application of thermochronological techniques to major normal fault systems can resolve the timing of initiation and duration of extension, rates of motion on detachment faults, timing of ductile mylonite formation and passage of rocks through the crystal-plastic to brittle transition, and multiple events of extensional unroofing. Here we determine the above for the top-to-the-east Raft River detachment fault and shear zone by study of spatial gradients in 40Ar/39Ar and fission track cooling ages of footwall rocks and cooling histories and by comparison of cooling histories with deformation temperatures. Mica 40Ar/39Ar cooling ages indicate that extension-related cooling began at ???25-20 Ma, and apatite fission track ages show that motion on the Raft River detachment proceeded until ???7.4 Ma. Collective cooling curves show acceleration of cooling rates during extension, from 5-10??C/m.y. to rates in excess of 70-100??C/m.y. The apparent slip rate along the Raft River detachment, recorded in spatial gradients of apatite fission track ages, is 7 mm/yr between 13.5 and 7.4 Ma and is interpreted to record the rate of migration of a rolling hinge. Microstructural study of footwall mylonite indicates that deformation conditions were no higher than middle greenschist facies and that deformation occurred during cooling to cataclastic conditions. These data show that the shear zone and detachment fault represent a continuum produced by progressive exhumation and shearing during Miocene extension and preclude the possibility of a Mesozoic age for the ductile shear zone. Moderately rapid cooling in middle Eocene time likely records exhumation resulting from an older, oppositely rooted, extensional shear zone along the west side of the Grouse Creek, Raft River, and Albion Mountains. Copyright 2000 by the American Geophysical Union.

New thermochronological constraints on the timing of shear from the Khlong Marui and Ranong faults, Peninsular Thailand: implications for Himalayan lateral extrusion.

NASA Astrophysics Data System (ADS)

Watkinson, I.; Elders, C.; Hall, R.

2009-04-01

New Ar-Ar data from the strike-slip faults of Peninsular Thailand indicate rapid uplift of mid-crustal ductile shear zones during the Eocene. The cooling ages are consistent with a northwards younging pattern of Ar-Ar cooling ages from the NW-trending Three Pagodas and Mae Ping faults in Northern Thailand, to the Ailao Shan-Red River fault in Vietnam and Yunnan, taken to reflect the northwards movement of India during the Cenozoic. The peninsular structures: the Khlong Marui fault (KMF) and Ranong fault (RF), are major NNE trending strike-slip faults of respectively 220 km and 420 km length. Exposed mylonitic rocks bear consistently dextral kinematic indicators, unlike the sinistral mylonites of the NW-trending structures to the north. Brittle strike-slip and dip-slip faults overprint all the shear zones. Rocks ranging from low grade mylonites to syn-kinematic amphibolite facies migmatites from the RF and KMF yield similar biotite Ar-Ar cooling ages, suggesting that uplift from all depths in the shear zone was rapid. Retrograde shear fabrics in places show that dextral shear may have continued during uplift. While the new thermochronological data show that the peninsular mylonites cooled during the Eocene, constraint from pre- and post-kinematic granitoids strongly suggests that ductile shear occurred during the Late-Cretaceous to Paleocene. Since this is well before the onset of India-Eurasia collision, much of the ductile shear must pre-date that orogeny, and therefore cannot be related to Himalayan lateral extrusion, as has been speculated. The regional cooling pattern, however, shows that Indian indentation may have triggered progressive northward exhumation of mylonitic rocks. If the model of the peninsular faults is applied to the NW-trending faults in northern Thailand, then a pre-Himalayan history may also be recorded by those mylonites, rather than a simple, lateral extrusion-related history.

Reconstructing the Thermo-tectonic history of the Rwenzori Mountains, D. R. Congo

NASA Astrophysics Data System (ADS)

Mansour, S.; Bauer, F.; Glasmacher, P. D. U. A. A.; Grobe, R. W.; Starz, M.

2014-12-01

The Albertine Rift forms the northern section of the western Rift of the East African Rift System (EARS). The Rwenzori Mtns evolved along the eastern rift shoulder of the Albertine Rift, rising up to form a striking feature within the rift valley with elevations reaching 5109 m a.s.l. While, the scarcity of volcanic activity in the Western Rift has raised questions about the Rwenzori Mtns origin and how this fits into the general evolution of the Albertine Rift and the EARS. Detailed thermochronologic study of Bauer et al., (2013) on the eastern side on Rwenzori Mtns, differentiated it into northern and southern blocks. The northern block cooled faster to ~120 °C in Carboniferous to Permian times. The second cooling event to ~70 °C occurred in Mesozoic time. The third cooling event to surface temperature occurred in the Neogene. While, the southern block shows an earlier onset of cooling at >400 Ma. Temperatures of about 70 °C were reached in Silurian to Devonian times. During this study, 33 samples were collected from the western side of central Rwenzori. Zircon and apatite fission track and (U/Th)-He techniques were applied on these samples. The apatite fission track data could be divided into three age groups; ~45±11, ~25±5, ~12±2 Ma. These results reveal the difference in thermo-tectonic history between the eastern and western flanks of Rwenzori Mtns and support the tilt uplift geometry hypotheses (e.g. Pickford et al., 1993). ReferencesBauer, F.U., Glasmacher, U.A., Ring, U., Karl, M., Schumann, A., Nagudi, B., 2013. Tracing the exhumation history of the Rwenzori Mountains, Albertine Rift, Uganda, using low-temperature thermochronology, Tectonophysics, 599, 8-28. http://dx.doi.org/10.1016/j.tecto.2013.03.032. Pickford, M., Senut, B., Hadoto, D., 1993. Geology and Palaeobiology of the Albertine Rift Valley Uganda-Zaire, vol. 1. Geology. CIFEG Occas, Orleans. Publication, vol. 24, pp. 1-190.

Constraints on the magnitude and rate of CO 2 dissolution at Bravo Dome natural gas field

DOE PAGES

Sathaye, Kiran J.; Hesse, Marc A.; Cassidy, M.; ...

2014-10-13

The injection of carbon dioxide (CO 2) captured at large point sources into deep saline aquifers can significantly reduce anthropogenic CO 2 emissions from fossil fuels. Dissolution of the injected CO 2 into the formation brine is a trapping mechanism that helps to ensure the long-term security of geological CO 2 storage. We use thermochronology to estimate the timing of CO 2 emplacement at Bravo Dome, a large natural CO 2 field at a depth of 700 m in New Mexico. Together with estimates of the total mass loss from the field we present, to our knowledge, the first constraintsmore » on the magnitude, mechanisms, and rates of CO 2 dissolution on millennial timescales. Apatite (U-Th)/He thermochronology records heating of the Bravo Dome reservoir due to the emplacement of hot volcanic gases 1.2–1.5 Ma. The CO 2 accumulation is therefore significantly older than previous estimates of 10 ka, which demonstrates that safe long-term geological CO 2 storage is possible. Here, integrating geophysical and geochemical data, we estimate that 1.3 Gt CO 2 are currently stored at Bravo Dome, but that only 22% of the emplaced CO 2 has dissolved into the brine over 1.2 My. Roughly 40% of the dissolution occurred during the emplacement. The CO 2 dissolved after emplacement exceeds the amount expected from diffusion and provides field evidence for convective dissolution with a rate of 0.1 g/(m 2y). Finally, the similarity between Bravo Dome and major US saline aquifers suggests that significant amounts of CO 2 are likely to dissolve during injection at US storage sites, but that convective dissolution is unlikely to trap all injected CO 2 on the 10-ky timescale typically considered for storage projects.« less

Thermochronology of mineral grains in the Red and Mekong Rivers, Vietnam: Provenance and exhumation implications for Southeast Asia

NASA Astrophysics Data System (ADS)

Clift, Peter D.; Carter, Andrew; Campbell, Ian H.; Pringle, Malcolm S.; van Lap, Nguyen; Allen, Charlotte M.; Hodges, Kip V.; Tan, Mai Thanh

2006-10-01

Sand samples from the mouths of the Red and Mekong Rivers were analyzed to determine the provenance and exhumation history of their source regions. U-Pb dating of detrital zircon grains shows that the main sources comprise crust formed within the Yangtze Craton and during the Triassic Indosinian Orogeny. Indosinian grains in the Mekong are younger (210-240 Ma) than those in the Red River (230-290 Ma), suggesting preferential erosion of the Qiangtang Block of Tibet into the Mekong. The Red River has a higher proportion of 700-800 Ma grains originally derived from the Yangtze Craton. 40Ar/39Ar dating of muscovite grains demonstrates that rocks cooled during the Indosinian Orogeny are dominant in both rivers, although the Mekong also shows a grain population cooling at 150-200 Ma that is not seen in the Red River and which is probably of original Qiangtang Block origin. Conversely, the Red River contains a significant mica population (350-500 Ma) eroded from the Yangtze Craton. High-grade metamorphic rocks exposed in the Cenozoic shear zones of southeast Tibet-Yunnan are minority sources to the rivers. However, apatite and zircon fission track ages show evidence for the dominant sources, especially in the Red River, only being exhumed through the shallowest 5-3 km of the crust since ˜25 Ma. The thermochronology data are consistent with erosion of recycled sediment from the inverted Simao and Chuxiong Basins, from gorges that incise the eastern flank of the plateau. Average Neogene exhumation rates are 104-191 m/Myr in the Red River basin, which is within error of the 178 ± 35 m/Myr estimated from Pleistocene sediment volumes. Sparse fission track data from the Mekong River support the Ar-Ar and U-Pb ages in favoring tectonically driven rock uplift and gorge incision as the dominant control on erosion, with precipitation being an important secondary influence.

Thermochronological constraints on the Cambrian to recent geological evolution of the Argentina passive continental margin

NASA Astrophysics Data System (ADS)

Kollenz, Sebastian; Glasmacher, Ulrich A.; Rossello, Eduardo A.; Stockli, Daniel F.; Schad, Sabrina; Pereyra, Ricardo E.

2017-10-01

Passive continental margins are geo-archives that store information from the interplay of endogenous and exogenous forces related to continental rifting, post-breakup history, and climate changes. The recent South Atlantic passive continental margins (SAPCMs) in Brazil, Namibia, and South Africa are partly high-elevated margins ( 2000 m a.s.l.), and the recent N-S-trending SAPCM in Argentina and Uruguay is of low elevation. In Argentina, an exception in elevation is arising from the higher topography (> 1000 m a.s.l.) of the two NW-SE-trending mountain ranges Sierras Septentrionales and Sierras Australes. Precambrian metamorphic and intrusive rocks, and siliciclastic rocks of Ordovician to Permian age represent the geological evolution of both areas. The Sierras Australes have been deformed and metamorphosed (incipient - greenschist) during the Gondwanides Orogeny. The low-temperature thermochronological (LTT) data (< 240 °C) indicated that the Upper Jurassic to Lower Cretaceous opening of the South Atlantic has not completely thermally reset the surface rocks. The LTT archives apatite and zircon still revealed information on the pre- to post-orogenic history of the Gondwanides and the Mesozoic and Cenozoic South Atlantic geological evolution. Upper Carboniferous zircon (U-Th/He)-ages (ZHe) indicate the earliest cooling below 180 °C/1 Ma. Most of the ZHe-ages are of Upper Triassic to Jurassic age. The apatite fission-track ages (AFT) of Sierras Septentrionales and the eastern part of Sierras Australes indicate the South Atlantic rifting and, thereafter. AFT-ages of Middle to Upper Triassic on the western side of the Sierras Australes are in contrast, indicating a Triassic exhumation caused by the eastward thrusting along the Sauce Grande wrench. The corresponding t-T models report a complex subsidence and exhumation history with variable rates since the Ordovician. Based on the LTT-data and the numerical modelling we assume that the NW-SE-trending mountain ranges received their geographic NW-SE orientation during the syn- to post-orogenic history of the Gondwanides.

Mountain building, strike-slip faulting, and landscape evolution in the Marlborough Fault System, NZ: Insights from new low-temperature thermochronology and modeling

NASA Astrophysics Data System (ADS)

Duvall, A. R.; Collett, C.; Flowers, R. M.; Tucker, G. E.; Upton, P.

2016-12-01

The 150 km wide Marlborough Fault System (MFS) and adjacent dextral-reverse Alpine Fault accommodate oblique convergence of the Australian and Pacific plates in a broad transform boundary that extends for much of the South Island New Zealand. Understanding the deformation history of the Marlborough region offers the opportunity to study topographic evolution in a strike-slip setting and a fuller picture of the evolving New Zealand plate boundary as the MFS lies at the transition from oceanic Pacific plate subduction to oblique continental collision. Here we present low-temperature thermochronology from the MFS to place new limits on the timing and style of mountain building. We sampled a range of elevations spanning 2 km within and adjacent to the Kaikoura Mountains, which stand high as topographic anomalies above active strike-slip faults. Young apatite (U-Th)/He ages ( 2-5 Ma) on both sides of range-bounding faults are consistent with regional distributed deformation since the Pliocene initiation of strike-slip faulting. However, large differences in both zircon helium and apatite fission track ages, from Paleogene/Neogene ages within hanging walls to unreset >100 Ma ages in footwalls, indicate an early phase of fault-related vertical exhumation. Thermal modeling using the QTQt program reveals two phases of exhumation within the Kaikoura Ranges: rapid cooling at 15-12 Ma localized to hanging wall rocks and regional rapid cooling reflected in all samples starting at 4-5 Ma. These results and landscape evolution models suggest that, despite the presence of active mountain front faults, much of the topographic relief in this region may predate the onset of strike-slip faulting and that portions of the Marlborough Faults are re-activated thrusts that coincide with the early development of the transpressive plate boundary. Regional exhumation after 5 Ma likely reflects increased proximity to the migrating Pacific plate subduction zone and the buoyant Chatham Rise.

Inverted temperature sequences: role of deformation partitioning

NASA Astrophysics Data System (ADS)

Grujic, D.; Ashley, K. T.; Coble, M. A.; Coutand, I.; Kellett, D.; Whynot, N.

2015-12-01

The inverted metamorphism associated with the Main Central thrust zone in the Himalaya has been historically attributed to a number of tectonic processes. Here we show that there is actually a composite peak and deformation temperature sequence that formed in succession via different tectonic processes. The deformation partitioning seems to the have played a key role, and the magnitude of each process has varied along strike of the orogen. To explain the formation of the inverted metamorphic sequence across the Lesser Himalayan Sequence (LHS) in eastern Bhutan, we used Raman spectroscopy of carbonaceous material (RSCM) to determine the peak metamorphic temperatures and Ti-in-quartz thermobarometry to determine the deformation temperatures combined with thermochronology including published apatite and zircon U-Th/He and fission-track data and new 40Ar/39Ar dating of muscovite. The dataset was inverted using 3D-thermal-kinematic modeling to constrain the ranges of geological parameters such as fault geometry and slip rates, location and rates of localized basal accretion, and thermal properties of the crust. RSCM results indicate that there are two peak temperature sequences separated by a major thrust within the LHS. The internal temperature sequence shows an inverted peak temperature gradient of 12 °C/km; in the external (southern) sequence, the peak temperatures are constant across the structural sequence. Thermo-kinematic modeling suggest that the thermochronologic and thermobarometric data are compatible with a two-stage scenario: an Early-Middle Miocene phase of fast overthrusting of a hot hanging wall over a downgoing footwall and inversion of the synkinematic isotherms, followed by the formation of the external duplex developed by dominant underthrusting and basal accretion. To reconcile our observations with the experimental data, we suggest that pervasive ductile deformation within the upper LHS and along the Main Central thrust zone at its top stopped at ~11 Ma at which time the deformation shifted and focused within the external duplex and the Main Boundary Thrust.

Apatite (U-Th)/He thermochronology dataset interpretation: New insights from physical point of view

NASA Astrophysics Data System (ADS)

Gautheron, Cécile; Mbongo-Djimbi, Duval; Gerin, Chloé; Roques, Jérôme; Bachelet, Cyril; Oliviero, Erwan; Tassan-Got, Laurent

2015-04-01

The apatite (U-Th)/He (AHe) system has rapidly become a very popular thermochronometer to constrain burial and exhumation phases in a variety of geological contexts. However, the interpretation of AHe data depends on a precise knowledge of He diffusion in apatite. Several studies suggest that radiation damage generated by U and Th decay can create traps for He atoms, increasing He retention for irradiated minerals. The radiation damage also anneals with temperature and the amount of damage in an apatite crystal is at any time a balance between production and annealing, controlled by U-Th concentration, grain chemistry and thermal history (Flowers et al., 2009; Gautheron et al., 2009; 2013). However the models are not well constrained and do not fully explain the mechanism of He retention. In order to have a deeper insight on this issue, multidisciplinary studies on apatite combining diffusion experiments by Elastic Recoil Diffusion Analysis (ERDA) with a multi-scale theoretical diffusion calculation based on Density Functional Theory (DFT) and Kinetic Monte Carlo were performed. ERDA experiments were conducted on different macro-crystals, and we probed the shape of a He profile implanted into a planar and polished surface of the crystal. The helium profile evolves with temperature and allows quantifying the He diffusivity and damage impact. Additionally, DFT calculations of a damage-free crystal of apatite with different F and Cl compositions, in similar proportion as natural ones, have been run to find the favored paths of a helium atom between interstitial sites, leading to a computation of the activation energy and the diffusion coefficient. We show that damage free apatite crystals are characterized by low retention behavior and closure temperature range from 33-36°C for pure F-apatite to higher value for Cl riche apatite (up to 12°C higher), for typical grain size and cooling rate (Mbongo-Djimbi et al., in review). Using ERDA and DFT approaches, we demonstrate that in addition to grain chemistry, He diffusivity will be strongly influenced by damage and propose a new physical model. Finally, we propose a new way of interpreting AHe datasets and practical geological examples will be given. Flowers, R., Ketcham, R.A., Shuster, D., Farley, K.A., 2009. Apatite (U-Th)/He thermochronology using a radiation damage accumulation and annealing model. Geochimica et Cosmochimica Acta 73, 2347-2365. Gautheron, C., Tassan-got, L., Barbarand, J., Pagel, M., 2009. Effect of alpha-damage annealing on apatite (U-Th)/He thermochronology. Chemical Geology 266, 166-179. Gautheron, C., Barbarand, J., Ketcham R.A., Tassan-got, L., van der Beek, P., Pagel, M., Pinna-Jamme, R., Couffignal, F., Fialin, M., 2013. Chemical influence on α-recoil damage annealing in apatite: implications for (U-Th)/He dating. Chemical Geology 351, 257-267. Mbongo-Djimbi D., Gautheron C., Roques, J., Tassan-Got, L., Gerin, C., Simoni, E.. Apatite composition effect on (U-Th)/He thermochronometer: an atomistic point of view. In review at Geochimica Cosmochimica Acta.

Mesozoic fault reactivation along the St. Lawrence Rift System as constrained by (U-Th/He) thermochronology

NASA Astrophysics Data System (ADS)

Bouvier, L.; Pinti, D. L.; Tremblay, A.; Minarik, W. G.; Roden-Tice, M. K.; Pik, R.

2011-12-01

The Saint Lawrence Rift System (SLRS) is a half-graben, extending for 1000 km along St. Lawrence River valley. Late Proterozoic-Early Paleozoic faults of the graben form the contact with the metamorphic Grenvillian basement to the northwest and extend under the Paleozoic sedimentary sequences of the St. Lawrence Lowlands to the southeast. The SLRS is the second most seismically active area in Canada, but the causes of this activity remain unclear. Reactivation of the SLRS is believed to have occurred along Late Proterozoic to Early Paleozoic normal faults related to the opening of the Iapetus Ocean. The absence of strata younger than the Ordovician makes difficult to determine when the faults reactivated after the Ordovician. Field relations between the normal faults bordering the SLRS and those produced by the Charlevoix impact crater suggest a reactivation of the rift younger than the Devonian, the estimated age of the impact. Apatite (U-Th)/He thermochronology is an adequate tool to recognize thermal events related to fault movements. A thermochronology study was then started along three transects across the SLRS, from Québec up to Charlevoix. Apatites were extracted and separated from five granitic to charnockitic gneisses and an amphibolite of Grenvillian age. The samples were exposed on hanging wall and footwall of the Montmorency and Saint-Laurent faults at three different locations along the SLRS. For precision and accuracy, each of the six samples was analyzed for radiogenic 4He and U-Th contents at least twice. Apatite grains were isolated by heavy liquids and magnetic separation. For each sample, ten apatite grains were selected under optical microscope and inserted into Pt capsules. Particular care was taken to isolate apatite free of mineral and fluid inclusions. Indeed, SEM investigations showed that some inclusions are U-rich monazite, which is a supplementary source of 4He to be avoided. The 4He content was determined by using a static noble gas mass spectrometer in CRPG-Nancy and duplicates using a quadrupole mass spectrometer at GEOTOP-UQAM. 4He was measured against internal He gas standards and Durango apatite, with the reference U-Th/He age of 31.13 ± 1.01 Ma. U and Th contents were determined at CRPG-Nancy and duplicated at McGill University by ICP-MS. Preliminary results of U-Th/He on St.-Laurent fault yield an age of 137±12 Ma for the hanging wall, at Sault-au-Cochon and 118±10 Ma for a sample from the footwall, at Cap-aux-oies. Previous Apatite Fission Track (AFT) performed for the two locations gave expected older ages at 149±16 Ma and 196±19 Ma for the hanging wall and the footwall, respectively. These preliminary U-Th/He results are consistent with AFT ages of the area (i.e. as expected, U-Th/He ages are younger than AFT ages) but do not yet provide new constraints for the structural evolution of the St. Lawrence rift system. We are determining further U-Th/He ages and these ages will constrain an exhumation model of the region.

Cooling and exhumation of continents at billion-year time scales

NASA Astrophysics Data System (ADS)

Blackburn, T.; Bowring, S. A.; Perron, T.; Mahan, K. H.; Dudas, F. O.

2011-12-01

The oldest rocks on Earth are preserved within the continental lithosphere, where assembled fragments of ancient orogenic belts have survived erosion and destruction by plate tectonic and surface processes for billions of years. Though the rate of orogenic exhumation and erosion has been measured for segments of an orogenic history, it remains unclear how these exhumation rates have changed over the lifetime of any terrane. Because the exhumation of the lithospheric surface has a direct effect on the rate of heat loss within the lithosphere, a continuous record of lithosphere exhumation can be reconstructed through the use of thermochronology. Thermochronologic studies have typically employed systems sensitive to cooling at temperatures <300 °C, such as the (U-Th)/He and 40Ar/39Ar systems. This largely restricts their application to measuring cooling in rocks from the outer 10 km of the Earth's crust, resulting in a thermal history that is controlled by either upper crustal flexure and faulting and/or isotherm inflections related to surface topography. Combining these biases with the uplift, erosion and recycling of these shallow rocks results in a poor preservation potential of any long-term record. Here, an ancient and long-term record of lithosphere exhumation is constructed using U-Pb thermochronology, a geochronologic system sensitive to cooling at temperatures found at 20-50 km depth (400-650 °C). Lower crustal xenoliths provide material that resided at these depths for billions of years or more, recording a thermal history that is buried deep enough to remain insensitive to upper crustal deformation and instead is dominated by the vertical motions of the continents. We show how this temperature-sensitive system can produce a long-term integrated measure of continental exhumation and erosion. Preserved beneath Phanerozoic sedimentary rocks within Montana, USA, the Great Falls Tectonic Zone formed when two Archean cratons, the Wyoming Province and Medicine Hat Block collided at ~1.8 Ga. Rutile U-Pb data from multiple xenoliths, each exhumed from a different depth within the crustal column reveal a range of dates that varies as a function of xenolith residence depth. The shallowest mid- to lower crustal xenoliths (~25 km) cooled first, yielding the youngest dates and yet cooled at rates between 0.1-0.25 °C/Ma over 500 My or more. Deeper xenoliths record cooling at progressively younger times at similar rates and time-scales. From orogony to eruption of xenoliths onto the surface, the lithospheric thermal history constructed using this technique may exceed a billion years. Combining this cooling history with a lithosphere thermal model yields an estimate for the average integrated rate of craton erosion between 0.00-<0.0025 km/Ma across the orogen; a range far lower than the geologically recent to present day rates for continental erosion (<0.005-0.1 km/Ma). This marks the first ever determination of continental exhumation rates on time-scales that approach the age of the continents themselves and has implications for secular cooling of the asthenosphere.

Interpretation of thermochronological cooling ages using thermal modelling: an example from shallow magma intrusions from the Kerguelen archipelago

NASA Astrophysics Data System (ADS)

Ahadi, Floriane; Delpech, Guillaume; Gautheron, Cécile; Nomade, Sébastien; Zeyen, Hermann; Guillaume, Damien

2017-04-01

Low temperature thermochronology on plutonic rocks is traditionally used to calculate erosion rates over large time scale. However, this method requires a good knowledge of the local or regional geology and particularly the thermal structure and evolution of the crust. The Kerguelen Islands (48-50°S, 68/5-70.5°E, Indian Ocean) are the emerged part of a vast oceanic plateau and are mostly made up of Oligocene basaltic traps that are cross cut by a dense network of large and deep valleys. Numerous plutonic complexes of various age (20-4.5 Ma) locally intrude theses traps and cover about 15% of the main island's surface. The Rallier du Baty peninsula is the largest plutonic complex, it is mainly constituted of syenites and is divided into two adjacent circular plutonic complexes whose centres are distant of 15 km. The southern part has a laccolith structure with satellites plutons and was emplaced at shallow depth (about 1 to 3 km) between 13.7 ± 0.3 and 8.0 ± 0.2 Ma. The northern part was emplaced later between 7.8 ± 0.25 and 4.5 ± 0.1 Ma. The Kerguelen Islands are of particular interest to understand the impact of Cenozoïc climatic variations on the long-term geomorphological evolution of emerged reliefs at mid-latitudes. To understand the erosion of the area, we conducted the first study on the Kerguelen Islands using the biotite 40Ar/39Ar (BAr), apatite and zircon (U-Th)/He thermochronometers (AHe and ZHe). In the southern part, the BAr ages for the various intrusions of the complex range from 9.44 ± 0.13 Ma to 13.84 ± 0.07 Ma. These ages are identical to high-temperature crystallisation ages (U-Pb on zircon) indicating an extremely rapid cooling between ˜700 and ˜300°C. The mean ZHe ages range between 7.1 ± 2.3 and 8.8 ± 1.4 and the mean AHe ages range between 4.4 ± 0.3 Ma and 7.4 ± 0.7 Ma. The AHe ages of the southern complex are similar to the crystallization ages of the northern part of the complex. The mean AHe ages in the northern part are much younger and range from 1.4 ± 0.7 Ma to 0.8 ± 0.1 Ma. Combined with the thermochronological approach, the thermal structure of the crust beneath the Kerguelen Plateau was established by inverse modelling of gravity, geoid and topography data. The results suggest a mean current thermal gradient of ˜40°/km for the Kerguelen Plateau. Moreover, thermal modelling allows reconstructing heat diffusion in 1D after successive sill intrusions (vertically and horizontally) in order to confirm AHe data can be interpreted as exhumation ages in both complexes. In this case, the mean thermal gradient can be considered to convert the cooling rates in erosion rates.

Raising Tibet

DOE Office of Scientific and Technical Information (OSTI.GOV)

Harrison, T.M.; Yin, An; Copeland, P.

1992-03-27

Thermochronologic, sedimentologic, oceanographic, and paleoclimatic studies suggest that rapid uplift and unroofing of southern Tibet began about 20 million years ago and that the present elevation of much of the Tibetan plateau was attained by about 8 million years ago. Hypotheses advanced to explain the tectonic evolution of the India-Asia collision, which began about 40 to 50 million years ago, predict the timing and rates of crustal thickening of the southern margin of Asia. However, these models do not predict the prominently enhanced early Miocene denudation and uplift that are manifested in a variety of geological records. A model involvingmore » continental extrusion, development of a crustal-scale thrust ramp of the Main Central Thrust beneath the Gangdese belt, and lithospheric delamination provides a history consistent with these observations.« less

Using apatite fission track thermochronology to document the deformation sequence in an exhumed foreland basin: an example from the southern Pyrenees.

NASA Astrophysics Data System (ADS)

Meresse, F.; Labaume, P.; Jolivet, M.; Teixell, A.

2009-04-01

Université Montpellier 2, INSU-CNRS, Laboratoire Géosciences Montpellier, cc060, 34095 Montpellier Cedex 5, France florian.meresse@gm.univ-montp2.fr The study of foreland basins provides important constraints on the evolution of orogenic wedges. In particular, the study of tectonics-sedimentation relationships is essential to date the tectonic activity. However, processes linked to wedge growth are not always completely recorded by the tecto-sedimentary markers, and thermochronological study of the basin-fill can provide further insights. In this work, we have combined apatite fission track analysis (apatite FTA) with structural analysis to precise the timing of the deformation sequence and to characterise the coupling between thrust activity, burial and denudation in the south-Pyrenean foreland basin, a proximal foredeep of the Pyrenees that has been incorporated in the Pyrenean thrust wedge. We have focused the study on a NNE-SSW cross-section of the south-vergent thrust system from the southern flank of the Axial Zone to the South-Pyrenean Frontal Thrust (SPFT), in the west-central part of the belt. This section provides a complete transverse of the South-Pyrenean Zone, here corresponding to the Ainsa and Jaca basins. Apatite FTA provides important new constraints on the south-Pyrenean foreland basin evolution: (i) Data show the southward decrease of the fission track reset level, from a total reset (indicating heating at Tmax>110°C) in the Paleozoic of the Axial Zone, to a partial reset (110°C>Tmax>60°C) in the lower-middle Eocene Hecho Group turbidites in the northern part of the Jaca basin, and to the absence of reset (Tmax<60°C) in the middle Eocene-Oligocene continental sediments of the southern part of the Jaca basin. This indicates a decreasing amount of denudation going southwards, from more than 4.5 km in the north to less than 2.5 km in the south if we assume an average geothermal gradient around 25°/km. The structural setting of the Jaca basin attests that the burial of sediments was mainly due to sedimentary accumulation. (ii) Results in the Hecho Group turbidites bring evidence of exhumation around 18 Ma on the Oturia thrust in the middle of the Jaca basin, an age that is younger than the Middle Eocene to Aquitanian deformation registered by tecto-sedimentary relationships in the southernmost part of the basin (Guarga syncline and SPFT). These tectonic movements may be related to the exhumation, at the same time, of the southern flank of the Axial Zone by out-of-sequence thrusting on the Bielsa basement thrust (Jolivet et al., 2007*). Therefore, low-temperature thermochronology reveals an out-of-sequence episode of deformation in the interior of the south-Pyrenean thrust wedge that had remained unknown due to the lack of related sedimentary record. This late tectonic activity is younger than the generally admitted Aquitanian age for the end of the Pyrenean compression, and would be linked to an ultimate internal thickening stage in the orogenic wedge (Meresse et al., this volume). (*Tectonics, 2007, vol. 26, doi: 10.1029/2006TC002080)

Deformation record of 4-d accommodation of strain in the transition from transform to oblique convergent plate margin, southern Alaska (Invited)

NASA Astrophysics Data System (ADS)

Roeske, S.; Benowitz, J.; Enkelmann, E.; Pavlis, T. L.

2013-12-01

Crustal deformation at the transition from a dextral transform to subduction in the northern Cordillera is complicated by both the bend of the margin and the presence of low-angle subduction of an oceanic plateau, the Yakutat microplate, into the 'corner'. The dextral Denali Fault system located ~400 km inboard of the plate margin shows a similar transition from a dominantly strike-slip to transpressional regime as it curves to the west. Thermochronologic and structural studies in both areas indicate crustal response through the transition region is highly varied along and across strike. Previous thermochronology along the Fairweather fault SE of the St. Elias bend shows the most rapid exhumation occurs in close proximity to the fault, decreasing rapidly away from it. Enkelmann et al. (2010) and more recent detrital zircon FT (Falkowski et al., 2013 AGU abstract) show rapid and deep exhumation concentrated in the syntaxis, but over a fairly broad area continuing north beyond the Fairweather fault. Although the region is dominantly under ice, borders of the rapidly exhuming region appear to be previously identified major high-angle faults. This suggests that structures controlling the extreme exhumation may have significant oblique slip component, or, if flower structure, are reverse faults, and the region may be exhuming by transpression, with a significant component of pure shear. Southwest of the syntaxis, where convergence dominates over strike-slip, thin-skinned fold-and-thrust belts in the Yakutat microplate strata account for the shortening. The long-term record of convergence in this area is more cryptic due to sediment recycling through deep underplating and/or limited exhumation by upper crustal shortening, but a wide range of thermochronologic studies suggests that initial exhumation in the region began ~ 30 Ma and most rapid exhumation in the syntaxis began ~ 5 Ma. In the eastern Alaska Range a significant component of strike-slip, in addition to convergence, has been accommodated along the Denali Fault since E. Miocene. Southeast of the bend there is little evidence of convergence across the fault and Quaternary slip is ~12-13.5 mm/year. The eastern restraining bend of the Denali fault is much broader than the syntaxis and dextral slip continues at rates of ~10 mm/year, but the rock response to increasing obliquity is similar. Low and moderate-T cooling histories determined from a wide range of isotopic systems on minerals from bedrock show exhumation strongly localized on the north side of the high-angle Denali fault, south of the Hines Creek fault, since ~25 Ma. The structural record in ductilely deformed rocks from the most highly exhumed regions shows transpressive deformation over a few km wide region, but above the brittle-ductile transition strain becomes highly partitioned and is accommodated by thrust and normal faults on the north side of the bend. A connector fault between the Fairweather and Totschunda-Denali fault systems has been speculated on but it is not clear whether a single through-going fault is expressed at the surface. Any connector is likely a relatively young structure compared to the Fairweather and Denali systems' histories of long-lived oblique convergence. Overall, in both regions high-angle faults appear to be critical for controlling the location of major deep-seated and/or long-lived exhumation, and deformation at these geometrical complexities is dominated by transpression.

6 Ma age of carving Westernmost Grand Canyon: Reconciling geologic data with combined AFT, (U-Th)/He, and 4He/3He thermochronologic data

NASA Astrophysics Data System (ADS)

Winn, Carmen; Karlstrom, Karl E.; Shuster, David L.; Kelley, Shari; Fox, Matthew

2017-09-01

Conflicting hypotheses about the timing of carving of the Grand Canyon involve either a 70 Ma (;old;) or <6 Ma (;young;) Grand Canyon. This paper evaluates the controversial westernmost segment of the Grand Canyon where the following lines of published evidence firmly favor a ;young; Canyon. 1) North-derived Paleocene Hindu Fanglomerate was deposited across the present track of the westernmost Grand Canyon, which therefore was not present at ∼55 Ma. 2) The 19 Ma Separation Point basalt is stranded between high relief side canyons feeding the main stem of the Colorado River and was emplaced before these tributaries and the main canyon were incised. 3) Geomorphic constraints indicate that relief generation in tributaries and on plateaus adjacent to the westernmost Grand Canyon took place after 17 Ma. 4) The late Miocene-Pliocene Muddy Creek Formation constraint shows that no river carrying far-traveled materials exited at the mouth of the Grand Canyon until after 6 Ma. Interpretations of previously-published low-temperature thermochronologic data conflict with these lines of evidence, but are reconciled in this paper via the integration of three methods of analyses on the same sample: apatite (U-Th)/He ages (AHe), 4He/3He thermochronometry (4He/3He), and apatite fission-track ages and lengths (AFT). HeFTy software was used to generate time-temperature (t-T) paths that predict all new and published 4He/3He, AHe, and AFT data to within assumed uncertainties. These t-T paths show cooling from ∼100 °C to 40-60 °C in the Laramide (70-50 Ma), long-term residence at 40-60 °C in the mid-Tertiary (50-10 Ma), and cooling to near-surface temperatures after 10 Ma, and thus support young incision of the westernmost Grand Canyon. A subset of AHe data, when interpreted alone (i.e. without 4He/3He or AFT data), are better predicted by t-T paths that cool to surface temperatures during the Laramide, consistent with an ;old; Grand Canyon. However, the combined AFT, AHe, and 4He/3He analysis of a key sample from Separation Canyon can only be reconciled by a ;young; Canyon. Additional new AFT (5 samples) and AHe data (3 samples) in several locations along the canyon corridor also support a ;young; Canyon. This inconsistency, which mimics the overall controversy of the age of the Grand Canyon, is reconciled here by optimizing cooling paths so they are most consistent with multiple thermochronometers from the same rocks. To do this, we adjusted model parameters and uncertainties to account for uncertainty in the rate of radiation damage annealing in these apatites during sedimentary burial and the resulting variations in He retentivity. In westernmost Grand Canyon, peak burial conditions (temperature and duration) during the Laramide were likely insufficient to fully anneal radiation damage that accumulated during prolonged, near-surface residence since the Proterozoic. We conclude that application of multiple thermochronometers from common rocks reconciles conflicting thermochronologic interpretations and the data presented here are best explained by a ;young; westernmost Grand Canyon. Samples spread along the river corridor also suggest the possibility of variable mid-Tertiary thermal histories beneath north-retreating cliffs.

Early Mesozoic cooling from low temperature thermochronology in N Spain and N Africa

NASA Astrophysics Data System (ADS)

Grobe, R.; Alvarez-Marrón, J.; Glasmacher, U. A.; Menéndez-Duarte, R.

2009-04-01

In the western prolongation of the Pyrenees, the substratum of the Cantabrian Mountains consists of an E-W crustal section of the Gondwana continental margin involved in the Variscan collision. In Mesozoic times, the region was modified by rifting and the opening of the Atlantic and the Bay of Biscay, while in Paleogene-Neogene times it was affected by the convergence of the Iberian Plate with the Eurasian Plate resulting in the present mountains. Our thermochronological data and modelled time-temperature histories suggest an earlier, relative fast cooling period during Early Triassic to Early Jurassic. This cooling event coincides temporally with the process of rifting that caused Pangaea continental break-up and the opening of the North Atlantic. Other authors report similar cooling histories from Early Triassic to Middle Jurassic from other parts of the Iberian Peninsula (Juez-Larré, 2003; Barbero et al., 2005) as well as from the Moroccan Meseta, in N Africa (Ghorbal et al., 2008). Furthermore, the time span of this cooling event includes the period of main activity of the Central Atlantic Magmatic Province (CAMP) magmatism at around 200 Ma (Marzoli et al., 1999). Wilson (1997) postulates a relationship between this magmatic activity and upwelling of a large-scale mantle plume (super-plume) beneath the West African craton. Correlatives of this province have been identified as far as the southern Iberian Peninsula, Newfoundland, and possibly in Brittany, among other European areas (Pe-Piper et al., 1992; Jourdan et al., 2003). The current presentation aims to discuss possible African far-field effects on thermochronological data in the Cantabrian Mountains of NW Spain. References: Barbero, L.; Glasmacher, U. A.; Villaseca, C.; López García, J. A.; Martín-Romera, C. (2005). Long-term thermo-tectonic evolution of the Montes de Toledo area (Central Hercynian Belt, Spain): constraints from apatite fission-track analysis. International Journal of Earth Sciences , Volume 94, Issue 2, pp.193-203. Ghorbal, B.; Bertotti, G.; Foeken, J.; Andriessen, P. (2008). Unexpected Jurassic to Neogene vertical movements in ‘stable' parts of NW Africa revealed by low temperature geochronology. Terra Nova, Volume 20, Number 5, October 2008 , pp. 355-363(9). Jourdan, F.; Marzoli, A.; Bertrand, H.; Cosca, M.; Fontignie, D. (2003). The Northernmost CAMP: 40Ar/39Ar Age, petrology and Sr-Nd-Pb isotope geochemistry of the Kerforne Dike, Brittany, France. In: Hames, W.E., McHone, J.G., Renne, P.R., Ruppel, C. (Eds.), The Central Atlantic Magmatic Province: Insights From Fragments of Pangea. AGU, Geophys. Mon., vol. 136, pp. 209-226. Juez-Larré, J. (2003). Post Late Paleozoic tectonothermal evolution of the northeastern margin of Iberia, assessed by fission-track and (U-T)/He analysis: a case history from the Catalan Coastal Ranges. Ph.D. thesis, Free University of Amsterdam. 200 pp. Marzoli, A.; Renne, P.R.; Piccirillo, E.M.; Ernesto, M.; Bellieni, G.; De Min, A. (1999). Extensive 200-million-year-old continental food basalts of the Central Atlantic magmatic province. Science 284, 616-618. Pe-Piper, G.; Jansa, L.F.; Lambert, R.St.-J. (1992). Early Mesozoic magmatism of the Eastern Canadian margin. In: Puffer, J.H., Ragland, P.C. (Eds.), Eastern North American Mesozoic magmatism. Geol. Soc. Am., Spec. Paper, vol. 268, pp. 13-36. Wilson, M. (1997). Thermal evolution of the Central Atlantic passive margins: continental break-up above a Mesozoic super-plume. J. Geol. Soc. (Lond.) 154, 491-495.

Screening apatites for (U-Th)/He thermochronometry via continuous ramped heating: He age components and implications for age dispersion

NASA Astrophysics Data System (ADS)

McDannell, Kalin T.; Zeitler, Peter K.; Janes, Darwin G.; Idleman, Bruce D.; Fayon, Annia K.

2018-02-01

Old slowly-cooled apatites often yield dispersed (U-Th)/He ages for a variety of reasons, some well understood and some not. Analytical protocols like careful grain selection can reduce the impact of this dispersion but add costs in time and resources and too often have proven insufficient. We assess a new analytical protocol that utilizes static-gas measurement during continuous ramped heating (CRH) as a means to rapidly screen apatite samples. In about the time required for a conventional total-gas analysis, this method can discriminate between samples showing expected volume-diffusion behavior and those showing anomalous release patterns inconsistent with their direct use in thermochronologic applications. This method also appears able to discriminate between the radiogenic and extraneous 4He fractions released by a sample, potentially allowing ages to be corrected. Well-behaved examples such as the Durango standard and other apatites with good age reproducibility show the expected smooth, sigmoidal gas-release curves predicted for volume diffusion using typical apatite kinetics, with complete exhaustion by ∼900 °C for linear heating at 20 °C/min. Secondary factors such as U and Th zoning and alpha-loss distribution have a relatively minor impact on such profiles. In contrast, samples having greater age dispersion show significant He release in the form of outgassing spikes and He release deferred to higher temperatures. Screening results for a range of samples permit us to assess the degree to which CRH screening can identify misbehaving grains, give insight into the source of extraneous He, and suggest that in some cases it may be possible to correct ages for the presence of such components.

Testing Models of Modern Glacial Erosion of the St. Elias Mountains, Alaska Using Marine Sediment Provenance

NASA Astrophysics Data System (ADS)

Penkrot, M. L.; Jaeger, J. M.; Loss, D. P.; Bruand, E.

2015-12-01

The glaciated coastal St. Elias Range in Alaska is a primary site to examine climate-tectonic interactions. Work has primarily focused on the Bering-Bagley and Malaspina-Seward ice fields, utilizing detrital and bedrock zircon and apatite geochronology to examine local exhumation and glacial erosion (Berger et al., 2008; Enkelmann et al., 2009; Headly et al., 2013). These studies argue for specific regions of tectonically focused or climatically widespread glacial erosion. Analyzed zircon and apatite grains are sand size, however glacial erosion favors the production of finer-grained sediments. This study focuses on the geochemical provenance of the silt-size fraction (15-63μm) of surface sediments collected throughout the Gulf of Alaska (GOA) seaward of the Bering and Malaspina glaciers to test if the exhumation patterns observed in zircon and apatites are also applicable for the silt size fraction. Onshore bedrock Al-normalized elemental data were used to delineate sediment sources, and a subset of provenance-applicable elements was chosen. Detrital thermochronologic data suggest that sediment produced by the Bagley/Bering system is derived from bedrock on the windward side with input from the Chugach Metamorphic Complex (CMC) underlying the Bagley only during glacial surge events (Headly et al., 2013). Geochemical observations of GOA silt deposited during the 1994-95 surge event confirm input of CMC sediment (elevated in Cr, Ni, Sc, Sr, depleted in Hf, Pb and Rb relative to Kultieth and Poul Creek formations). We also observe a windward-side sediment source (Kultieth and Poul Creek). It is hypothesized that the sediment carried by the Malaspina is primarily from CMC rock underlying the Seward ice field mixed with Yakataga formation rock that underlies the Seward throat (Headly et al., 2013). Geochemical observations of GOA silt support this hypothesis.

Post-rift Tectonic History of the Songliao Basin, NE China: Cooling Events and Post-rift Unconformities Driven by Orogenic Pulses From Plate Boundaries

NASA Astrophysics Data System (ADS)

Song, Ying; Stepashko, Andrei; Liu, Keyu; He, Qingkun; Shen, Chuanbo; Shi, Bingjie; Ren, Jianye

2018-03-01

The classic lithosphere-stretching model predicts that the post-rift evolution of extensional basin should be exclusively controlled by decaying thermal subsidence. However, the stratigraphy of the Songliao Basin in northeastern China shows that the post-rift evolution was punctuated by multiple episodes of uplift and exhumation events, commonly attributed to the response to regional tectonic events, including the far-field compression from plate margins. Three prominent tectonostratigraphic post-rift unconformities are recognized in the Late Cretaceous strata of the basin: T11, T03, and T02. The subsequent Cenozoic history is less constrained due to the incomplete record of younger deposits. In this paper, we utilize detrital apatite fission track (AFT) thermochronology to unravel the enigmatic timing and origin of post-rift unconformities. Relating the AFT results to the unconformities and other geological data, we conclude that in the post-rift stage, the basin experienced a multiepisodic tectonic evolution with four distinct cooling and exhumation events. The thermal history and age pattern document the timing of the unconformities in the Cretaceous succession: the T11 unconformity at 88-86 Ma, the T03 unconformity at 79-75 Ma, and the T02 unconformity at 65-50 Ma. A previously unrecognized Oligocene unconformity is also defined by a 32-24 Ma cooling event. Tectonically, all the cooling episodes were regional, controlled by plate boundary stresses. We propose that Pacific dynamics influenced the wider part of eastern Asia during the Late Cretaceous until Cenozoic, whereas the far-field effects of the Neo-Tethys subduction and collision processes became another tectonic driver in the later Cenozoic.

Cenozoic deformation from the Yakutat-North American collision to the eastern margin of the Northern Canadian Cordillera

NASA Astrophysics Data System (ADS)

Enkelmann, E.

2017-12-01

The western margin of the Northern Cordillera of North America is dominated by transform motion of the Yakutat microplate along the Fairweather fault system. In southeast Alaska the transform boundary changes to convergence and the oblique collision of the buoyant Yakutat microplate formed the St. Elias Mountains. One of the outstanding questions in understanding the St. Elias orogeny is how stress from the plate boundary has been transferred inboard and distributed strain in the North American plate. The timing, amount, and spatial pattern of deformation and rock exhumation have been studied using multiple thermochronology methods. Together the data reveal that Late Cenozoic deformation inboard of the Fairweather Fault and the colliding Yakutat plate corner at the St. Elias syntaxis was spatially very limited, resulting in rock exhumation within a <30 km-wide corridor north and northeast of the plate boundary. The data from this inboard region, located in Yukon and northern British Columbia, record Late Cretaceous-Early Eocene cooling associated with Cordilleran deformation, and Paleocene-Eocene cooling due to spreading-ridge subduction. In contrast, the region west of the St. Elias syntaxis is dominated by convergence, which resulted in significant Cenozoic deformation in southeastern and southern Alaska. In the St. Elias orogen itself, most of the Late Cenozoic deformation and exhumation occurs within the Yakutat microplate and its Cenozoic sedimentary cover that composes the fold-thrust belt. The efficient interaction between tectonic uplift and glacial erosion resulted in rapid exhumation (>1 km/Myr) and extreme rates (4 km/Myr) that are localized at the syntaxis region and have shifted southward over the past 10 Myr. Far-field deformation reaches more than 500 km to the northwest of the convergent margin and caused mountain building in south-central Alaska. Deformation to the northeast is unclear. New thermochronology data from the eastern margin of the Northern Canadian Cordillera (Northwest Territory) reveal exhumation during the Oligocene to early Miocene. At this time, transform motion was already dominating the plate margin in the west. The post-Cordilleran deformation at the eastern front may thus be related to mantle convection and/or stresses associated with the North Atlantic opening.

Influence of inherited structures on the growth of basement-cored ranges, basin inversion and foreland basin development in the Central Andes, from apatite fission-track and apatite Helium thermochronology.

NASA Astrophysics Data System (ADS)

Zapata, S.; Sobel, E. R.; Del Papa, C.; Jelinek, A. R.; Muruaga, C.

2017-12-01

The Central Andes in NW of Argentina is part of a long-lived subduction zone, active since the Paleozoic. This region experienced several tectonic cycles; each of which created an unique set of structures and may have reactivated preexisting structures. These inherited structures may exert a first-order control over the different foreland deformational styles observed along the strike in the Central Andes. Our study area is located between 26°S and 28°S on the transition between the broken foreland (Santa Barbara system), which expresses a combination of thin-skin and thick-skin styles, and the Sierras Pampeanas, which is deform in a thick-skin style. The Cumbres Calchaquies range and the associated Choromoro Basin are located in the northern part of the study area, and are the southern expression of the Santa Barbara system. Published thermochronology data suggest that the rocks from the basement experienced Late Cretaceous and Late Miocene exhumation; the associated sedimentary rocks within the Choromoro basin experienced Paleogene and Late Miocene deformational phases. In contrast, the Sierra Aconquija range, located immediately south on the transition to the Sierras Pampeanas (thick skin) foreland basin, exhibit larger amounts of Miocene exhumation and lack of Cretaceous exhumation; the associated sedimentary rocks from the Tucuman basin have not been deformed since the Cretaceous. Our goal is to understand the evolution of the structural blocks and the structures responsible for the along strike changes in foreland basin deformational styles and their relation with inherited structures from previous tectonic cycles. We are obtaining new apatite U-Th/He and fission track data to reconstruct the thermal history of the basement, accompanied by U-Pb geochronology and stratigraphy to constrain the evolution of the associated sedimentary basins. Preliminary results combined with published data suggest that inherited structures within the study area have evolved through different tectonic cycles, controlling the thicknes and the geometry of the sediments within the Mesozoic rift basin, the Miocene amount of exhumation in the basement-cored ranges and the deformation style of the associated foreland basins.

Extensional unroofing of the metamorphic core of the southern Appalachian orogen prior to the breakup of Pangea: Insights from 40Ar/39Ar thermochronology

NASA Astrophysics Data System (ADS)

Ma, C.; Foster, D. A.; Hames, W. E.; Mueller, P. A.

2017-12-01

Orogenic collapse commonly occurs following the collisional phase of an orogeny and often leads to exhumation of deep crustal metamorphic rocks. The Alleghanian orogeny in the southern Appalachian orogen (SAO) occurred during final assembly of Pangea. 40Ar/39Ar data of hornblende, muscovite, and biotite from Alleghanian granitic plutons in Georgia, Alabama, and Florida of the SAO give cooling ages that progressively young toward the south-southeast prior to ca. 280 Ma and young locally toward the north-northwest after ca. 280 Ma. These cooling-age gradients, along with geometry of the Suwannee suture zone and timing/structures of the South Georgia basin, suggest that metamorphic rocks north of the Suwannee suture in the study area formed the lower plate of a metamorphic core complex. The faults of the Suwannee suture zone were reactivated to form a master extensional detachment fault with the Suwannee terrane comprising the upper plate. Thermochronologic data show that rapid extension of the metamorphic core complex footwall started at ca. 300-295 Ma and the extension continued to at least ca. 240 Ma. The maximum average extension rate is estimated to be 10.3 km/m.y. during ca. 300-280 Ma along the master detachment fault and 2.4 km/m.y. during ca. 280-240 Ma along a secondary detachment fault, reflecting differential extension over time. Main cooling rates of 10‒85˚C/m.y. and exhumation rates of 0.3‒2.8 km/m.y. are calculated for the Alleghanian granitic plutons studied. This work shows that, in the southernmost Appalachians, orogenic collapse resulted in metamorphic core complex-style extension between about 300 and 240 Ma. The horst-and-graben systems of the South Georgia basin formed within the upper plate in this tectonic setting. Metamorphic core complex-style extension, therefore, played a critical role in initial rifting that led to the eventual breakup of Pangea and formation of the Atlantic Ocean and the Gulf of Mexico.

Kinematic Modeling of Central Nepal: Thermochronometer Cooling Ages as a Constraint for Balanced Cross Sections

NASA Astrophysics Data System (ADS)

Olree, E.; Robinson, D. M.; McQuarrie, N.; Ghoshal, S.; Olsen, J.

2016-12-01

Using balanced cross sections, one can visualize a valid and admissible interpretation of the surface and subsurface data. Khanal (2014) and Cross (2014) produced two valid and admissible cross sections along the Marsyandi River in central Nepal. However, thermochronologic data adds another dimension that must be adhered to when producing valid and admissible balanced cross sections. Since the previous cross sections were produced, additional zircon-helium (ZHe) cooling ages along the Marsyandi River show ages of 1 Ma near the Main Central thrust in the hinterland to 4 Ma near the Main Boundary thrust closer to the foreland. This distribution of cooling ages requires recent uplift in the hinterland, which is not present in the cross sections. Although a restored version of the Khanal (2014) cross section is sequentially deformed using 2D Move, the kinematic sequence implied in the cross section is inconsistent with the ZHe age distribution. The hinterland dipping duplex proposed by Khanal would require cooling ages that are oldest near the Main Central thrust and young southwards toward the active ramp located 80 km north of the Main Frontal thrust. Instead, the 4 Ma age near the Main Boundary thrust and the increasingly younger ages to the north could be produced by either a foreland-dipping Lesser Himalayan duplex, which would keep active uplift in the north, or by translation of the hinterland dipping duplex southward over the ramp, moving the active thrust ramp northward. To address this problem, a new balanced cross section was produced using both new mapping through the region and the ZHe age distribution as additional constraints. The section was then restored and sequentially deformed in 2D Move. This study illustrates that multiple cross sections can be viable and admissible; however, they can still be incorrect. Thermochronology places additional constraints on the permissible geometries, and thus increases our ability to predict subsurface geometries. The next step of this project is to link the uplift and erosion implied by the kinematic sequence of the new cross section to the measured cooling history by importing the cross section kinematics into advection diffusion modeling software that predicts cooling ages.

Dating an actively exhuming metamorphic core complex, the Suckling Dayman Massif in SE Papua New Guinea

NASA Astrophysics Data System (ADS)

Oesterle, J.; Seward, D.; Little, T.; Stockli, D. F.; Mizera, M.

2016-12-01

Low-temperature thermochronology is a powerful tool for revealing the thermal and kinematic evolution of metamorphic core complexes (MCCs). Most globally studied MCCs are ancient, partially eroded, and have been modified by deformation events that postdate their origin. The Mai'iu Fault is a rapidly slipping active low-angle normal fault (LANF) in the Woodlark Rift in Papua New Guinea that has exhumed a >25 km-wide (in the slip direction), and over 3 km-high domal fault surface in its footwall called the Suckling-Dayman massif. Some knowledge of the present-day thermal structure in the adjacent Woodlark Rift, and the pristine nature of this active MCC make it an ideal candidate for thermochronological study of a high finite-slip LANF. To constrain the thermal and kinematic evolution of this MCC we apply the U/Pb, fission-track (FT) and (U-Th)/He methods. Zircon U/Pb analyses from the syn-extensional Suckling Granite that intrudes the footwall of the MCC yield an intrusion age of 3.3 Ma. Preliminary zircon FT ages from the same body indicate cooling below 300 °C at 2.7 Ma. Ages decrease to 2.0 Ma with increasing proximity to the Mai'iu Fault and imply cooling controlled by tectonic exhumation. Almost coincident zircon U/Pb and FT ages from the nearby syn-extensional Mai'iu Monzonite, on the other hand, record extremely rapid cooling from magmatic temperatures to 300 °C at 2 Ma. As apparent from the preliminary He extraction stage, these syn-extensional plutons have young zircon and apatite (U-Th)/He ages. These initial results suggest that the Mai'iu Fault was initiated as an extensional structure by 3.3 Ma. We infer that it reactivated an older ophiolitic suture that had emplaced the Papuan Ultramafic body in the Paleogene. Rapid cooling of the Mai'iu Monzonite indicates that it was intruded into a part of the MCC's footwall that was already shallow in the crust by 2 Ma. This inference is further supported by the mineral andalusite occurring in the contact aureole of the monzonite.

Quantifying the influence of sediment source area sampling on detrital thermochronometer data

NASA Astrophysics Data System (ADS)

Whipp, D. M., Jr.; Ehlers, T. A.; Coutand, I.; Bookhagen, B.

2014-12-01

Detrital thermochronology offers a unique advantage over traditional bedrock thermochronology because of its sensitivity to sediment production and transportation to sample sites. In mountainous regions, modern fluvial sediment is often collected and dated to determine the past (105 to >107 year) exhumation history of the upstream drainage area. Though potentially powerful, the interpretation of detrital thermochronometer data derived from modern fluvial sediment is challenging because of spatial and temporal variations in sediment production and transport, and target mineral concentrations. Thermochronometer age prediction models provide a quantitative basis for data interpretation, but it can be difficult to separate variations in catchment bedrock ages from the effects of variable basin denudation and sediment transport. We present two examples of quantitative data interpretation using detrital thermochronometer data from the Himalaya, focusing on the influence of spatial and temporal variations in basin denudation on predicted age distributions. We combine age predictions from the 3D thermokinematic numerical model Pecube with simple models for sediment sampling in the upstream drainage basin area to assess the influence of variations in sediment production by different geomorphic processes or scaled by topographic metrics. We first consider a small catchment from the central Himalaya where bedrock landsliding appears to have affected the observed muscovite 40Ar/39Ar age distributions. Using a simple model of random landsliding with a power-law landslide frequency-area relationship we find that the sediment residence time in the catchment has a major influence on predicted age distributions. In the second case, we compare observed detrital apatite fission-track age distributions from 16 catchments in the Bhutan Himalaya to ages predicted using Pecube and scaled by various topographic metrics. Preliminary results suggest that predicted age distributions scaled by the rock uplift rate in Pecube are statistically equivalent to the observed age distributions for ~75% of the catchments, but may improve when scaled by local relief or specific stream power weighted by satellite-derived precipitation. Ongoing work is exploring the effect of scaling by other topographic metrics.

Timing of Exhumation of the Mesozoic Blue Nile Rift, Ethiopia: A New Study from Apatite Fission Track Thermochronology

NASA Astrophysics Data System (ADS)

Gani, N. D.; Bowden, S. M.

2017-12-01

At present, tectonic features of Ethiopia are dominated by the 2.5 km high Ethiopian Plateau, and the NE-SW striking continental rift, the East African Rift System (EARS) that dissected the plateau into the northwest and southeast plateaus. The stress direction of the EARS is nearly perpendicular to the stress direction of the Mesozoic rifts of the Central African Rift System (CARS), located mostly in Sudan, Ethiopia and Kenya. During the Gondwana splitting in Mesozoic, active lithospheric extension within the CARS resulted in several NW-SE striking continental rifts including the Blue Nile, Muglad, Melut and Anza that are well documented in Sudan and Kenya, from a combination of geophysical and drill core analysis and field investigations. However, the timing and evolution of the poorly documented Blue Nile Rift in Ethiopia, now hidden in the subsurface of the Ethiopian Plateau and the EARS, is largely unknown. This study investigates, for the first time, the timing of tectono-thermal evolution of the Blue Nile Rift from cooling ages deduced from apatite fission track (AFT) thermochronology to understand the rift flank exhumation. Here, we report the AFT results from basement samples collected in a vertical transect from the Ethiopian Plateau. The fission track ages of the samples show a general trend of increasing cooling ages with elevations. The time-temperature simulations of the fission track ages illustrate that the cooling started at least 80 Ma ago with a significant amount of rapid cooling between 80 and 70 Ma, followed by a slow cooling after 70 Ma and then another accelerated cooling starting around 10 Ma. The Cretaceous rapid cooling event likely related to the flank uplift of the Blue Nile Rift and associated faulting, during which much of the exhumation occurred. Today, the Blue Nile Rift is buried under the thick cover of Mesozoic sedimentary rocks and Cenozoic volcanics. The late Neogene rapid cooling agrees well with our previous thermal model simulation from apatite (U-Th)/He ages that shows a rapid exhumation of the Ethiopian Plateau during late Neogene.

Initiation of continental accretion in the Betic-Rif domain

NASA Astrophysics Data System (ADS)

Maxime, Daudet; Frederic, Mouthereau; Stéphanie, Brichau; Ana, Crespo-Blanc; Arnaud, Vacherat

2017-04-01

The Betic - Rif cordillera in southern Spain and northern Morocco, respectively, form one of the tightest orogenic arc on Earth. The formation of this arcuate orogenic belt resulted from the westward migration of the Alboran crustal domain, constituted by the internal zone of the orogeny and the basement of the Alboran back-arc basin, that collided with the rifted margins of Iberia and Africa at least since the early Miocene. This collision is intimately linked to the post-35-30Ma regional slab roll-back and back-arc extension in the western Mediterranean region. The geodynamics of the Betic-Rif domain, which is of great importance for the paleogeographic reconstructions of the Tethys-Altantic and the Mediterranean sea, is still largely debated. Answers will come from a more detailed structural analyses, including refinement of the time-temperature paths and kinematics of the main structural units, which is one of the main objectives of the OROGEN research project, co-financed by BRGM, TOTAL & CNRS. In this study, we focus on the well-developed flysch-type sediments now accreted in the Betics-Rif but initially deposited in a basin, north of the african margin and on the iberian margin from the Early Cretaceous to the Early Miocene. Using low-temperature thermochronology (fission-track and (U-Th)/He analyses) combined with zircon U-Pb geochronology on the flyschs deposited on the most distal part of the margin, we aim to constrain the thermal history of both the source rocks and accreted thrust sheets at the earliest stages of continental accretion. Sample have been collected in flyschs series ranging from Mesozoic, Paleogene to Neogene ages. Additional samples have been collected in the Rif where Cretaceous series are more developed. Combined with a detailed structural analysis, LT thermochronological constraints will refine the kinematics of thrust units when continental accretion started before the final thrust emplacement occurred in the Early Miocene. Considering a selection of regional geological cross-sections from which a minimum amount of shortening will be derived, our results will be integrated in a tectonic reconstruction of the region.

Large Temporal and Spatial Variations in the Eastern Himalayan Syntaxis Exhumation From Detrital Thermochronology

NASA Astrophysics Data System (ADS)

Enkelmann, Eva; Ehlers, Todd A.

2010-05-01

The exhumation rates at the Eastern Himalayan Syntaxis, and in particular at the Namche Barwa anticline, are some of the highest in the Himalaya. At the core of the anticline high-grade metamorphic rocks of Indian crust were exhumed from ca. 40 km depth within <10 Ma. Bedrock thermochronology shows rapid exhumation since <4 Ma that has since migrated north and west of the anticline. The structural history of the anticline and its northern and western surroundings are relatively well studied. However, the geology and exhumation history of the southeastern flank is largely unknown. We present 1500 new detrital zircon fission track cooling ages from 15 modern river samples of the Tsangpo-Siang River and its tributaries. These samples provide cooling ages from drainage basins in the Namche Barwa anticline and the region north, west, and south of it (China and India). Results are as follows: (1) we find eight age populations (0.9 ± 0.2 Ma , 3.5 ± 1 Ma, 7 Ma, 11 ± 2 Ma, 18 ± 2 Ma, 24 ± 2 Ma, 35 ± 5 Ma and 50 - 55 Ma) that quantify temporal variations in the exhumation history of the eastern syntaxis since the Paleocene/Eocene. (2) Cooling ages are consistent with the development of the anticline structure since the Late Miocene. (3) Previous studies have suggested that ~ 50% of the Siang River sediments exiting the Himalaya today originate from only a small area of the Namche Barwa massif. However, our data reveal that the zone of localized rapid exhumation extends significantly (ca. 40 km) further to the southwest of the anticline than previously assumed. Finally, (4) spatial variations in cooling age populations suggest structural differences between the northwestern and southeastern flanks of the anticline. Northwest of the anticline rapid exhumation has migrated beyond the anticline and into the hangingwall (Asia). In the Southeast, the zone of exhumation is strictly limited to the anticline and bounded by a sharp structural boundary along by the Siang River. This spatial pattern suggests a northwestward migration of the syntaxial zone of exhumation in the last ca. 10 Ma.

Surface Evolution and Uplift History of the Central Apennines, Italy: New Constraints from Thermochronology and Paleoaltimetry

NASA Astrophysics Data System (ADS)

San José, M.; Faccenna, C.; Fellin, M. G.; Willett, S.; Funiciello, F.; Caves Rugenstein, J. K.

2017-12-01

The topography of mountain belts results from interactions between surface processes, lithospheric thickening, and mantle dynamics. However, the contributions of each mechanism have yet to be clearly quantified. The Apennines (Italy) provide a study area where all of these processes are at play. The central part of the Apennines is an orogenic wedge formed by the westward subducting Adriatic microplate during Miocene-Pliocene, and overlies an area of local slab detachment. Recent studies indicate anomalously high uplift rates in this area (Faccenna et al., 2015), as well as a simultaneous onset of post-orogenic extension across the Central Apennines at ̴ 2 Ma (Cosentino et al., 2017). These observations have been interpreted as an expression of dynamic topography due to the slab break-off and inherent mantle upwelling. In order to test this hypothesis and further constrain the topographic evolution of the orogen, we use low-temperature thermochronology to date the exhumation, and stable isotope paleoaltimetry (18O/16O on carbonates) to reconstruct paleoelevations. We couple this paleotopographic dataset with geomorphological analysis of the present day topography. Here we present a set of 30 new (U-Th)/He cooling ages on apatites (AHe) sampled from widespread syn-orogenic flysch basin deposits (including one high-resolution vertical profile), as well as preliminary isotopic measurements. Initial results show mean AHe ages ranging from 1.62 (± 0.38 ) Ma to 2.6 (±0.02) Ma, suggesting a regionally uniform exhumation during Pleistocene. Denudation rates inferred from our vertical profile are extremely high (>1 mm/year from 2 to 1 Ma). Some samples have not been thermally reset due to insufficient burial, and yield exhumation ages older than stratigraphic depositional ages (generally > 5 Ma). These results support the hypothesis that exhumation is mainly controlled by rapid regional scale uplift related to the opening of the Adriatic slab window at the end of the orogenic phase. However, denudation is not only driven by geodynamic processes, but also by climatic changes. Further research will consider climatic variations in the interpretation of our results.

Detrital thermochronology of Rhine, Elbe and Meuse river sediment (Central Europe): implications for provenance, erosion and mineral fertility

NASA Astrophysics Data System (ADS)

Glotzbach, C.; Busschers, F. S.; Winsemann, J.

2018-03-01

Here we present detrital apatite fission track (AFT), zircon fission track (ZFT) and a few apatite (U-Th)/He (AHe) data of Middle Pleistocene to modern Rhine, Meuse and Elbe river sediments in order to resolve processes that control detrital age distributions (provenance, erosion and mineral fertility). We used a modelling approach to compare observed with theoretically predicted age distributions from an interpolated in situ AFT and ZFT age map. In situ cooling ages do show large differences in the Rhine drainage basin, facilitating the differentiation between different source regions. Inconsistencies between observed and theoretical age distributions of the Meuse and Elbe samples can be explained by mixing and reworking of sediments with different provenances (Meuse Middle Pleistocene terrace sediment) and a yet unexplored source region with old AFT ages (Elbe samples). Overall, the results show that detrital thermochronology is capable of identifying the provenance of Middle Pleistocene to modern sediments. The AFT age distributions of Rhine sediments are dominated ( 70%) by AFT ages representing the Alps. A possible explanation is higher erosion rates in the Alps as compared to areas outside the Alps. A Late Pleistocene sample from the Upper Rhine Graben contains apatite grains from the Molasse and Hegau volcanics, which we explain with a shift of the headwaters of the Rhine to the north as a result of intense Middle Pleistocene Riss glaciation. Contrary to the observed dominance of Alpine-derived AFT ages in Rhine sediments, the relative contribution of zircon ages with sources in the Alps is lower and significantly decreases downstream, suggesting a major source of zircons outside the Alps. This can be explained by increased zircon fertility of sediments derived from the Rhenish massif. Therefore, we conclude that erosion and mineral fertility are the main processes controlling detrital AFT and ZFT age distributions of the sampled river sediment. In case of the Rhine samples, AFT age distributions are mainly controlled by differences in erosion rates, whereas this impact is completely balanced by differences in mineral fertility for the ZFT data.

Development of the EarthChem Geochronology and Thermochronology database: Collaboration of the EarthChem and EARTHTIME efforts

NASA Astrophysics Data System (ADS)

Walker, J. D.; Ash, J. M.; Bowring, J.; Bowring, S. A.; Deino, A. L.; Kislitsyn, R.; Koppers, A. A.

2009-12-01

One of the most onerous tasks in rigorous development of data reporting and databases for geochronological and thermochronological studies is to fully capture all of the metadata needed to completely document both the analytical work as well as the interpretation effort. This information is available in the data reduction programs used by researchers, but has proven difficult to harvest into either publications or databases. For this reason, the EarthChem and EARTHTIME efforts are collaborating to foster the next generation of data management and discovery for age information by integrating data reporting with data reduction. EarthChem is a community-driven effort to facilitate the discovery, access, and preservation of geochemical data of all types and to support research and enable new and better science. EARTHTIME is also a community-initiated project whose aim is to foster the next generation of high-precision geochronology and thermochoronology. In addition, collaboration with the CRONUS effort for cosmogenic radionuclides is in progress. EarthChem workers have met with groups working on the Ar-Ar, U-Pb, and (U-Th)/He systems to establish data reporting requirements as well as XML schemas to be used for transferring data from reduction programs to database. At present, we have prototype systems working for the U-Pb_Redux, ArArCalc, MassSpec, and Helios programs. In each program, the user can select to upload data and metadata to the GEOCHRON system hosted at EarthChem. There are two additional requirements for upload. The first is having a unique identifier (IGSN) obtained either manually or via web services contained within the reduction program from the SESAR system. The second is that the user selects whether the sample is to be available for discovery (public) or remain hidden (private). Search for data at the GEOCHRON portal can be done using age, method, mineral, or location parameters. Data can be downloaded in the full XML format for ingestion back into the reduction program or as abbreviated tables.

Beating the Heat: Magmatism in the Low-Temperature Thermochronologic Record

NASA Astrophysics Data System (ADS)

Murray, K. E.; Reiners, P. W.; Braun, J.; Karlstrom, L.; Morriss, M. C.

2017-12-01

The low-temperature thermochronology community was quick to recognize upper-crustal complexities in the geotherm that reflect landscape evolution, but the complex effects of crustal magmatism on thermochronometers can be difficult to independently document and remain underexplored. Because magmatism is common in many regions central to our understanding of tectonics, this is a significant gap in our ability to robustly interpret rock cooling. Here, we use several different numerical approaches to examine how local and regional crustal magmatism affects cooling age patterns and present examples from the western US that demonstrate the importance—and utility—of considering these effects. We modified the finite-element code Pecube to calculate how thermochronometers document the emplacement of simple hot bodies at different crustal levels. Results demonstrate the potential for mid-crustal plutons, emplaced at 10-15 km depth, to reset cooling ages in the overlying rocks at partial-retention depths at the time of magmatism. Permo-Triassic sandstones from the Colorado Plateau's Canyonlands region have apatite cooling ages that exemplify the resulting ambiguity: Oligocene rock cooling can be attributed to either 1 km of erosion or relaxation of a geothermal gradient transiently doubled by mid-crustal magmatism. Despite these complexities, there are compelling reasons to target rocks with magmatic histories. Shallowly emplaced plutons can usefully reset cooling ages in country rocks with protracted near-surface histories, as we have demonstrated in the Colorado Plateau's Henry Mountains. Cooling age patterns are also useful for quantifying magmatic processes themselves. In an ongoing project, we use the pattern of thermochronometer resetting around individual dikes that fed the Columbia River flood basalts, which are exposed in the Wallowa Mountains, to identify long-lived feeder dikes and model their thermal aureoles to further constrain eruptive dynamics. The pattern of resetting around dikes compliments higher-temperature constraints on the longevity of magma flow from phase equilibria in partially melted wall rocks. In principal, this technique should also resolve along-strike variability in flow localization, providing novel constraints on eruptive flux in large igneous provinces.

Building a viable decollement geometry for the Central Nepal Himalaya through integrating surface geology, thermochronology and data from the 2015 Gorkha Earthquake

NASA Astrophysics Data System (ADS)

Ghoshal, S.; McQuarrie, N.; Robinson, D. M.; Olree, E.; Valentino, C.; Olsen, J.

2017-12-01

Recent field mapping in the Central Himalaya revealed a marked change in the location and orientation of exposed Greater Himalayan rocks around the epicenter of the April 2015 Gorkha earthquake, arguing for a lateral structure in the Main Himalayan Thrust (MHT). The earthquake provided new insight into the geometry of the MHT, but left the position and depth of the mid-crustal ramp in dispute. Combining new field data with existing thermochronometric data from the region emphasizes that both the mapped geology and young cooling ages step abruptly southward from east to west, immediately adjacent to the earthquake epicenter. The distribution of cooling ages is strongly influenced by the location of ramps in the decollement surface, as the vertical component of uplift concentrates exhumation over the ramp, producing the youngest ages there. We propose that the existence and location of frontal and lateral ramps can be evaluated using the regional distribution of thermochronometric ages. Sequentially deformed cross-sections present a model of how structurally induced uplift varies in time and space, as well as a predicted geometry of the active, modern fault. We created new balanced cross-sections, constrained by surface geology and the proposed decollement geometries. For an accurate representation of the subsurface, the geometries must reproduce cooling ages measured at the surface. Each cross section was sequentially deformed, allowing for flexure and erosion. The resulting displacement field was used to predict cooling ages for muscovite 40Ar/39Ar, zircon (U-Th)/He, and apatite fission-track, using the thermokinematic model Pecube. The different closure temperatures for these systems allow them to represent different times and locations of exhumation driven by evolving fault geometries. The modeled cooling ages are the cumulative effect of the entire deformational sequence. However, the ages are particularly sensitive to the modern active decollement fault geometry, allowing us to evaluate the different proposed cross-section geometries, and identify the best match to the regional distribution of cooling ages. We argue that this final geometry is the most accurate representation of the subsurface, being constrained by surface geology, thermochronological ages, and data from the earthquake.

Post-orogenic evolution of mountain ranges and associated foreland basins: Initial investigation of the central Pyrenees

NASA Astrophysics Data System (ADS)

Bernard, Thomas; Sinclair, Hugh; Ford, Mary; Naylor, Mark

2017-04-01

Mountain topography, including surrounding foreland basins, results from the long-term competition between tectonic and surface processes linked to climate. Numerous studies on young active mountain ranges such as the Southern Alps, New Zealand and Taiwan, have investigated the interaction between tectonics, climate and erosion on the topographic landscape. However most of the mountain ranges in the world are in various stages of post-orogenic decay, such as the European Alps, Urals, Caledonides, Appalachians and Pyrenees. The landscape evolution of these decaying mountains, which involve relatively inactive tectonics, should appear simple with progressive and relatively uniform erosion resulting in a general lowering of both elevation and topographic relief. However, in a number of examples, post-orogenic systems suggest a complex dynamism and interactions with their associated foreland basins in term of spatio-temporal variations in erosion and sedimentary flux. The complexity and transition to post-orogenesis is a function of multiple processes. Underpinning the transition to a post-orogenic state is the competition between erosion and crustal thickening; the balance of these processes determines the timing and magnitude of isostatic rebound and hence subsidence versus uplift of the foreland basin. It is expected that any change in the parameters controlling the balance of erosion versus crustal thickening will impact the topographic evolution and sediment flux from the mountain range and foreland basin to the surrounding continental margin. This study will focus on the causes and origins of the processes that define post-orogenesis. This will involve analyses of low-temperature thermochronological and topographic data, geodynamical modelling and sedimentological analyses (grainsize distribution). The Pyrenees and its associated northern retro-foreland basin, the Aquitaine basin, will form the natural laboratory for the project as it is one of the best documented mountain range/foreland basin systems in the world. Initial results of a review of the low-temperature thermochronological data using inverse modelling, illustrates the asymmetric exhumation of the mountain range, and the diachronous timing of decelerated exhumation linked to the transition to post-orogenesis. This study is part of the Orogen project, an academic-industrial collaboration (CNRS-BRGM-TOTAL).

Impact of mineral fertility and bedrock erosion on single-mineral detrital studies: insights from trace-element and Nd-isotope systematics of detrital apatite from the Po River catchment

NASA Astrophysics Data System (ADS)

Malusa', Marco Giovanni; Wang, Jiangang; Garzanti, Eduardo; Villa, Igor M.; Wittman, Hella

2017-04-01

The detrital record provides an archive of mountain erosion that preserves key information for paleotectonic and paleoclimatic reconstructions. Detrital studies are often based on single-mineral analyses (e.g., geo/thermochronologic analyses on apatite and zircon). Their geologic interpretation can be challenging, because the impact of each eroding source on the detrital record is controlled by a range of factors including the rate of erosion and the fertility of chosen minerals in eroded bedrock. Here, we combine (i) a state-of-the art dataset of trace element and Nd isotope fingerprints of detrital apatite, (ii) a comprehensive dataset of apatite-fertility measurements (Malusà et al. 2016), (iii) fission-track data, and (iv) cosmogenic-derived erosion rates from the Po River catchment (Wittmann et al. 2016), to test the impact of mineral fertility and bedrock erosion on the single-mineral detrital signal preserved in the final sediment sink. Our results show that the information provided by accessory minerals, when complemented with accurate mineral fertility measurements, are fully consistent with information provided by the analysis of more abundant framework minerals. We found that trace element and Nd isotope analyses provide a reliable tool to disentangle the complex single-mineral record of orogenic erosion, and demonstrate that such a record is largely determined by high-fertility source rocks exposed within the drainage. Detrital thermochronology studies based on the lag-time approach should thus preferably include independent provenance discriminations and a full mineral fertility characterization of the potential source areas, in order to ensure a correct identification of the sediment sources and of the exogenic and endogenic processes monitored in the stratigraphic archive. Malusà M.G., Resentini A., Garzanti E., 2016. Hydraulic sorting and mineral fertility bias in detrital geochronology. Gondwana Res., 31, 1-19 Wittmann H., Malusà M.G., Resentini A., Garzanti E., Niedermann S., 2016. The cosmogenic record of mountain erosion transmitted across a foreland basin: Source-to-sink analysis of in situ 10Be, 26Al and 21Ne in sediment of the Po river catchment. Earth Planet. Sci. Lett. 452, 258-271

Geomorphic evidence for enhanced Pliocene-Quaternary faulting in the northwestern Basin and Range

USGS Publications Warehouse

Ellis, Magdalena A; Barnes Jason B,; Colgan, Joseph P.

2014-01-01

Mountains in the U.S. Basin and Range Province are similar in form, yet they have different histories of deformation and uplift. Unfortunately, chronicling fault slip with techniques like thermochronology and geodetics can still leave sizable, yet potentially important gaps at Pliocene–Quaternary (∼105–106 yr) time scales. Here, we combine existing geochronology with new geomorphic observations and approaches to investigate the Miocene to Quaternary slip history of active normal faults that are exhuming three footwall ranges in northwestern Nevada: the Pine Forest Range, the Jackson Mountains, and the Santa Rosa Range. We use the National Elevation Dataset (10 m) digital elevation model (DEM) to measure bedrock river profiles and hillslope gradients from these ranges. We observe a prominent suite of channel convexities (knickpoints) that segment the channels into upper reaches with low steepness (mean ksn = ∼182; θref = 0.51) and lower, fault-proximal reaches with high steepness (mean ksn = ∼361), with a concomitant increase in hillslope angles of ∼6°–9°. Geologic maps and field-based proxies for rock strength allow us to rule out static causes for the knickpoints and interpret them as transient features triggered by a drop in base level that created ∼20% of the existing relief (∼220 m of ∼1050 m total). We then constrain the timing of base-level change using paleochannel profile reconstructions, catchment-scale volumetric erosion fluxes, and a stream-power–based knickpoint celerity (migration) model. Low-temperature thermochronology data show that faulting began at ca. 11–12 Ma, yet our results estimate knickpoint initiation began in the last 5 Ma and possibly as recently as 0.1 Ma with reasonable migration rates of 0.5–2 mm/yr. We interpret the collective results to be evidence for enhanced Pliocene–Quaternary fault slip that may be related to tectonic reorganization in the American West, although we cannot rule out climate as a contributing mechanism. We propose that similar studies, which remain remarkably rare across the region, be used to further test how robust this Plio–Quaternary landscape signal may be throughout the Great Basin.

Thermochronologic constraints on post-Paleozoic tectonic evolution of the central Transantarctic Mountains, Antarctica

NASA Astrophysics Data System (ADS)

Fitzgerald, Paul G.

1994-08-01

Built upon the roots of a compressive orogenic belt of late Proterozoic-early Paleozoic age and once adjacent to North America, the present-day Transantarctic Mountains (TAM) represent a rift flank, resulting from episodic uplift in the Cretaceous and Cenozoic. Fault blocks are discernible in present-day topography and subglacial morphology. Fission track results give information on differential block movement (uplift and denudation) and are important in constraining models for the uplift of the range. Apatite fission track thermochronology on samples collected from the central TAM record a complex thermotectonic history for this region over the past 350 m.y. Apatite ages in the Miller Range vary from ˜250 to ˜350 Ma and are from an exhumed apatite partial annealing zone formed following cooling of Cambro-Ordovician granitoids. A period of Cretaceous denudation (≲2 km), beginning at ˜115 Ma, is recorded at Moody Nunatak on the inland side of the TAM. Near the coast, samples along the Beardmore Glacier record rapid cooling indicative of denudation initiated in the early Cenozoic (˜50 Ma). The amount of uplift ˜70 km inland of the coast in the Queen Alexandra Range since the early Cenozoic is ˜7 km, with the likelihood of an additional ˜3 km at the coast. Eastward facing topographic escarpments in the Queen Alexandra Range mark the likely position of steeply dipping normal faults, which offset the apatite ages. Apatite ages on the east side of the Beardmore Glacier mouth are generally younger (average 27 Ma) than on the west side (average 33 Ma), reflecting greater denudation. Assumptions made regarding the use of an assumed paleogeothermal gradient are tested with available geologic evidence. The fission track data neither conflict with nor confirm paleobotanical evidence from the Sirius Group in the central TAM which suggests significant surface uplift (2-3 km) of the TAM since the Pliocene. Results build upon the available fission track database along the TAM and emphasize the subtle variability of uplift along the TAM due to episodic uplift involving differential block movements.

Book review: Advances in 40Ar/39Ar dating: From archaeology to planetary sciences

USGS Publications Warehouse

Cosca, Michael A.

2015-01-01

The recently published book Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences is a collection of 24 chapters authored by international scientists on topics ranging from decay constants to 40Ar/39Ar dating of extraterrestrial objects. As stated by the editors in their introduction, these chapters were assembled with the goal of providing technique-specific examples highlighting recent advances in the field of 40Ar/39Ar dating. As this is the first book truly dedicated to 40Ar/39Ar dating since the second edition printing of the argon geochronologist’s handbook Geochronology and Thermochronology by the 40Ar/39Ar Method (McDougall and Harrison 1999), a new collection of chapters highlighting recent advances in 40Ar/39Ar geochronology offers much to the interested reader.

Drake Passage opening history: a synthesis of existing and new data from diverse proxies

NASA Astrophysics Data System (ADS)

Barbeau, D. L.; Scotia Project Team

2011-12-01

The tectonic opening of the Drake Passage marine gateway between the Antarctic Peninsula and southern South America enabled development of the Antarctic circumpolar current, which has been variably ascribed to establishment and/or expansion of Cenozoic ice caps on Antarctica. Despite the importance of this gateway, interpretations of its opening history vary greatly, and are widely debated. Using data collected using a variety of geologic, geochemical, geophysical, and paleobiologic techniques, I summarize existing and new constraints on the tectonic history of Drake Passage, and evaluate temporal and kinematic models for its opening. In particular, new data from sediment provenance and thermochronology of Drake Passage margins integrated with independent paleoclimate, eustasy and paleobiology studies suggest that this gateway may have begun opening at least as early as the Paleocene -- 15 Myr prior to the oldest previous estimates.

Argon thermochronology of mineral deposits; a review of analytical methods, formulations, and selected applications

USGS Publications Warehouse

Snee, Lawrence W.

2002-01-01

40Ar/39Ar geochronology is an experimentally robust and versatile method for constraining time and temperature in geologic processes. The argon method is the most broadly applied in mineral-deposit studies. Standard analytical methods and formulations exist, making the fundamentals of the method well defined. A variety of graphical representations exist for evaluating argon data. A broad range of minerals found in mineral deposits, alteration zones, and host rocks commonly is analyzed to provide age, temporal duration, and thermal conditions for mineralization events and processes. All are discussed in this report. The usefulness of and evolution of the applicability of the method are demonstrated in studies of the Panasqueira, Portugal, tin-tungsten deposit; the Cornubian batholith and associated mineral deposits, southwest England; the Red Mountain intrusive system and associated Urad-Henderson molybdenum deposits; and the Eastern Goldfields Province, Western Australia.

Late Paleozoic deformation and exhumation in the Sierras Pampeanas (Argentina): 40Ar/39Ar-feldspar dating constraints

NASA Astrophysics Data System (ADS)

Löbens, Stefan; Oriolo, Sebastián; Benowitz, Jeff; Wemmer, Klaus; Layer, Paul; Siegesmund, Siegfried

2017-09-01

Systematic 40Ar/39Ar feldspar data obtained from the Sierras Pampeanas are presented, filling the gap between available high- (> 300 °C) and low-temperature (< 150 °C) thermochronological data. Results show Silurian-Devonian exhumation related to the late stages of the Famatinian/Ocloyic Orogeny for the Sierra de Pocho and the Sierra de Pie de Palo regions, whereas the Sierras de San Luis and the Sierra de Comechingones regions record exhumation during the Carboniferous. Comparison between new and available data points to a Carboniferous tectonic event in the Sierras Pampeanas, which represents a key period to constrain the early evolution of the proto-Andean margin of Gondwana. This event was probably transtensional and played a major role during the evolution of the Paganzo Basin as well as during the emplacement of alkaline magmatism in the retroarc.

Timing and rates of long-term landscape evolution in Southern Argentina

NASA Astrophysics Data System (ADS)

Kollenz, S.; Glasmacher, P. A.

2013-12-01

The eastern Argentina South Atlantic passive continental margin is distinguished by a very flat topography. Out of the so called Pampean flat two mountain ranges are arising. These mountain ranges, the Sierras Australes and the Sierras Septentrionales, are located in the State of Buenos Aires south of the capital Buenos Aires. North of the Sierras Septentrionales the Salado basin is located. The Sierras Septentrionales and the Sierras Australes are also divided by a smaller intracratonic basin. Further in the South the Colorado basin is located. The Sierras Australes is a variscian fold belt originated by strong phases of metamorphosis, but till now it is unclear by how many tectonic phases the area was influenced (Tomezzoli & Vilas, 1999). It consists of Proterozoic to Paleozoic rocks. The Sierras Septentrionales consists mainly of Precambrian crystalline rocks. The Precambrian sequences are overlain by younger Sediments (Cingolani, 2010). The aim is to understand the long-term landscape evolution of the area by quantifiying erosion- and exhumation-rates and by dating ancient rock-uplift-events. Another goal is to find out how the opening of the south atlantic took effect on this region. To fulfill this goal, thermochronological techniques, such as fission-track dating and (U-Th-Sm)/He dating has been applied to samples from the region. Because there was no low- temperature thermochronology done in this area, both techniques were applied on apatites and zircons. Furthermore, numerical modeling of the cooling history has provided the data base for the quantification of the exhumation rates. The data-set shows clusters of different ages which can be linked to tectonic activities during late Paleozoic times. Also the thermokinematic modeling is leading to new insights of the evolution of both mountain ranges and shows patterns of ongoing tectonic processes in this region. Caltculated exhumation rates show also varying cooling historys and the influence of tectonics throughout the research area. References: Renata Nela Tomezzoli and Juan Francisco Vilas (1999): Palaeomagnetic constraints on the age of deformation of the Sierras Australes thrust and fold belt, Argentina. Geophys. J. Int. (1999) 138, 857-870 Carlos A. Cingolani (2010): The Tandilia System of Argentina as a southern extension of the Rio de la Plata craton: an overview, Int. J. Earth. Sci. (Geol. Rundsch.) (2011) 100, 221-242

Evidence for a dynamic East Antarctic ice sheet during the mid-Miocene climate transition

NASA Astrophysics Data System (ADS)

Pierce, Elizabeth L.; van de Flierdt, Tina; Williams, Trevor; Hemming, Sidney R.; Cook, Carys P.; Passchier, Sandra

2017-11-01

The East Antarctic ice sheet underwent a major expansion during the Mid-Miocene Climate Transition, around 14 Ma, lowering sea level by ∼60 m. However, direct or indirect evidence of where changes in the ice sheet occurred is limited. Here we present new insights on timing and locations of ice sheet change from two drill sites offshore East Antarctica. IODP Site U1356, Wilkes Land, and ODP Site 1165, Prydz Bay are located adjacent to two major ice drainage areas, the Wilkes Subglacial Basin and the Lambert Graben. Ice-rafted detritus (IRD), including dropstones, was deposited in concentrations far exceeding those known in the rest of the Miocene succession at both sites between 14.1 and 13.8 Ma, indicating that large amounts of IRD-bearing icebergs were calved from independent drainage basins during this relatively short interval. At Site U1356, the IRD was delivered in distinct pulses, suggesting that the overall ice advance was punctuated by short periods of ice retreat in the Wilkes Subglacial Basin. Provenance analysis of the mid-Miocene IRD and fine-grained sediments provides additional insights on the movement of the ice margin and subglacial geology. At Site U1356, the dominant 40Ar/39Ar thermochronological age of the ice-rafted hornblende grains is 1400-1550 Ma, differing from the majority of recent IRD in the area, from which we infer an inland source area of this thermochronological age extending along the eastern part of the Adélie Craton, which forms the western side of the Wilkes Subglacial Basin. Neodymium isotopic compositions from the terrigenous fine fraction at Site U1356 imply that the ice margin periodically expanded from high ground well into the Wilkes Subglacial Basin during periods of MMCT ice growth. At Site 1165, MMCT pebble-sized IRD are sourced from both the local Lambert Graben and the distant Aurora Subglacial Basin drainage area. Together, the occurrence and provenance of the IRD and glacially-eroded sediment at these two marine drill sites proximal to the Antarctic continent provide a previously undocumented record of dynamic ice margin change during the 14.1-13.8 Ma interval in three major East Antarctic drainage basins.

Exhumation history of the Anqing Orefield in the Lower Yangtze River Metallogenic Belt: Evidence from apatite fission-track thermochronology

NASA Astrophysics Data System (ADS)

Li, X.; Yang, X.

2017-12-01

The Lower Yangtze River Metallogenic Belt (LYRMB) is one of the most important Cu-Au-Fe polymetallic belts in China. These deposits along the Yangtze River region have been related to the Yanshanian intrusive rocks in the Mesozoic. The Anqing orefield is located in the northwestern Anqing, southwestern Anhui Province, eastern China, along the Lower Yangtze River Belt. Here, we report new apatite fission-track (AFT) thermochronology of the granitoids ( 130 Ma for zircon U-Pb age) associated with the Anqing orefields to reveal the exhumation history of the Anqing orefields in LYRMB, eastern China. AFT ages from 54.4±2.1 to 63.9±3.4 Ma with mean measured track lengths between 12.4±1.8 and 13.1±1.4 μm, were obtained for the granitoids sampled from the ore fields in the Anqing orefield, and AFT age of 36.3±1.3 Ma with mean track length of 12.3±2.3μm for the granitoids adjacent to the south Tan-Lu fault. A long, slow exhumation ( 60-15 Ma), and a short, rapid tectonic exhumation (15-0 Ma) have been identified in the study region based on the AFT data and modeling results for the samples from the ore fields. The results show that the granitoids underwent roughly similar cooling, and inferred exhumation pattern. Assuming a steady-state paleogeothermal gradient of 35ºC/km founded on geological setting, the exhumations of 570 m and 1140 m, were achieved in the Anqing orefield, during 60-15 Ma and 15-0 Ma, respectively. Further, the AFT age of the granitoids adjacent to the south Tan-Lu fault is less than the AFT ages of the granitoids associated with the ore fields, possibly owing to the activation of the Tan-Lu fault in the Cenozoic. The exhumation history of the Anqing orefields may be closely response to the Pacific Plate subduction in the Cenozoic, which could be implications for the preservation potential of ore bodies in the Anqing orefield. This work is supported by the National Natural Science Foundation of China (41372227), and the DREAM project of MOST China (2016YFC0600406).

Timing and Nature of Events Leading to the Formation of the Albion-Raft River-Grouse Creek (ARG) Metamorphic Core complex, Northern Great Basin, W. U.S.

NASA Astrophysics Data System (ADS)

Miller, E. L.; Konstantinou, A.; Sheu, D.; Strickland, A.; Grove, M.

2016-12-01

Interpretations of the geodynamic significance of metamorphic core complexes in the northern Basin and Range are intimately tied to a combination of P-T data, geochronology and mica thermochronology used to infer episodes of deformation and uplift related to syn-shortening gravitational collapse of the crust in the latest Cretaceous-early Cenozoic. The ARG is no exception and we bring new geologic mapping, microstructural analysis, geochronology and 40Ar/39Ar thermochronology to bear on these questions. The petrogenesis of Eocene-Miocene magmas, the structural fabrics and metamorphism developed in wall rocks of plutons and the history of flanking basins outline a three-part Cenozoic story of this complex: Part 1: Mantle-derived heat input into the crust in the Eocene (42-36 Ma), related to Farallon slab removal, produced volcanism, plutonism, but little regional extension. Part 2: Heat input led to increased crustal melting as surface volcanism ceased. Diapiric rise of granite-cored gneiss domes sheathed by high grade, high strain metamorphic fabrics and mylonites took place over a protracted time, 32-25 Ma, stalling at depths > 10 km. Transitions upward from penetrative stretching fabrics to brittle crust were complex damage zones of multiply deformed and faulted Paleozoic strata overlain by a more intact 7-8 km thick section of Late Paleozoic and Triassic. Extension was localized and no sedimentary basins formed during this time. Part 3: Metamorphic and igneous rocks were brought to near surface conditions during Miocene extension, between 14-8 Ma ago. Structures accommodating E-W extension are high-angle, rotational normal faults that currently bound both sides of the ARG complex with linked sedimentary basins in their hanging wall. New 40Ar/39Ar data show that country rocks near the Oligocene Almo pluton share the pluton's cooling history. Further from the pluton, where pre-Oligocene fabrics are variably preserved, white mica total gas and plateau ages increase up structural section. Ages in the 40-50 Ma range are likely the result of incomplete argon loss from Mesozoic (Jurassic and or Cretaceous?) micas in the Oligocene and/or partial retention zone residence prior to final Miocene uplift rather than the result of proposed latest Cretaceous-early Cenozoic tectonic or deformational events.

Regional uplift episodes along the NE Atlantic margin constrained by stratigraphic and thermochronologic data

NASA Astrophysics Data System (ADS)

Holford, S. P.; Green, P. F.; Hillis, R. R.; Duddy, I. R.; Turner, J. P.; Stoker, M. S.

2008-12-01

The magma-rich NE Atlantic passive margin provides a superb natural laboratory for studying vertical motions associated with continental rifting and the rift-drift transition. Here we present an extensive apatite fission-track analysis (AFTA) database from the British Isles which we combine with a detailed stratigraphic framework for the Cretaceous-Cenozoic sedimentary record of the NE Atlantic margin to constrain the uplift history along and inboard of this margin during the past 120 Myr. We show that the British Isles experienced a series of uplift episodes which began between 120 and 115 Ma, 65 and 55 Ma, 40 and 25 Ma and 20 and 15 Ma, respectively. Each episode is of regional extent (~100,000 sq km) and represents a major period of exhumation involving removal of up to 1 km or more of section. These uplift episodes can be correlated with a number of major tectonic unconformities recognised within the sedimentary succession of the NE Atlantic margin, suggesting that the margin was also affected by these uplift episodes. Anomalous syn- and post-rift uplift along this margin have been interpreted in terms of permanent and/or transient movements controlled by the Iceland plume, but neither the timing nor distribution of the uplift episodes, with the exception of the 65 to 55 Ma episode, supports a first-order control by plume activity on vertical motions. Each uplift episode correlates closely with key deformation events at adjacent plate boundaries, suggesting a causative link, and we examine the ways in which plate boundary forces can account for the observed uplift episodes. Similar km-scale uplift events are revealed by thermochronological studies in other magma-rich and magma-poor continental margins, e.g. SE Australia, South Africa, Brazil. The low angle unconformities which result from these regional episodes of km-scale burial and subsequent uplift are often incorrectly interpreted as representing periods of non-deposition and tectonic stability. Similar considerations have also led to an erroneous view of the post-rift stability of many continental margins. Our results indicate that km-scale regional uplift has affected many regions previously interpreted as areas of long-term stability, and that plate boundary deformation exerts the primary control on such episodes.

U-Pb thermochronology of rutile from Alpine Corsica: constraints on the thermal evolution of the European margin during Jurassic continental breakup

NASA Astrophysics Data System (ADS)

Ewing, T. A.; Beltrando, M.; Müntener, O.

2017-12-01

U-Pb thermochronology of rutile can provide valuable temporal constraints on the exhumation history of the lower crust, given its moderate closure temperature and the occurrence of rutile in appropriate lithologies. We present an example from Alpine Corsica, in which we investigate the thermal evolution of the distal European margin during Jurassic continental rifting that culminated in the opening of the Alpine Tethys ocean. The Belli Piani unit of the Santa Lucia nappe (Corsica) experienced minimal Alpine overprint and bears a striking resemblance to the renowned Ivrea Zone lower crustal section (Italy). At its base, a 2-4 km thick gabbroic complex contains slivers of granulite facies metapelites that represent Permian lower crust. Zr-in-rutile temperatures and U-Pb ages were determined for rutile from three metapelitic slivers from throughout the Mafic Complex. High Zr-in-rutile temperatures of 850-950 °C corroborate textural evidence for rutile formation during Permian granulite facies metamorphism. Lower Zr-in-rutile temperatures of 750-800 °C in a few grains are partly associated with elongate strings of rutile within quartz ribbons, which record recrystallisation of some rutile during high-temperature shearing. Zr thermometry documents that both crystallisation and re-crystallisation of rutile occurred above the closure temperature of Pb in rutile, such that the U-Pb system can be expected to record cooling ages uncomplicated by re-crystallisation. Our new high-precision single-spot LA-ICPMS U-Pb dates are highly consistent between and within samples. The three samples gave ages from 160 ± 1 Ma to 161 ± 2 Ma, with no other age populations detected. The new data indicate that the Santa Lucia lower crust last cooled through 550-650 °C at 160 Ma, coeval with the first formation of oceanic crust in the Tethys. The new data are compared to previous depth profiling rutile U-Pb data for the Belli Piani unit1, and exploited to cast light on the tectonothermal evolution of the Santa Lucia lower crust in the Jurassic. The new data integrated with published data from the Ivrea zone allow comparison of the thermal evolution of the opposing European (Santa Lucia) and Adriatic (Ivrea) continental margins created by rifting associated with the opening of the Tethys. 1Seymour NM et al., 2016, Tectonics 35, 2439-2466

Meso-Cenozoic evolution of the Tuareg Shield (Algeria, Sahara): insights from new thermochronological data

NASA Astrophysics Data System (ADS)

Rougier, Sylvain; Missenard, Yves; Gautheron, Cécile; Barbarand, Jocelyn; Zeyen, Hermann; Pinna, Rosella; Liégeois, Jean-Paul; Bonin, Bernard; Ouabadi, Aziouz; El-Messaoud Derder, Mohammed; Frizon de Lamotte, Dominique; Kettouche, Djouher

2013-04-01

In North Africa, Meso-Cenozoic large scale topographic swells, such as Hoggar, Tibesti or Darfur domes, are superimposed to a Paleozoic arch and basin morphology which characterizes this region. Although these topographic highs are associated to Cenozoic intraplate volcanism, their development remains poorly constrained, both from temporal and spatial points of view. This study is focused on the Tuareg Shield bulge, a topographic high where Precambrian rocks, exposed over 500000 km², can reach 2400 m above sea level (Atakor district, Hoggar, South Algeria). While presumed Cretaceous sedimentary remnants, resting unconformably over the basement, suggest a possible stage of weak topography during the Mesozoic, current high topography is emphasized by <35 Ma volcanic formations, mostly basaltic in composition. In this context, we present first apatite (U-Th)/He thermochronological data acquired across the whole swell (Rougier et al., Geology, in press). Mean ages range from 78 ± 22 Ma to 13 ± 3 Ma. These results demonstrate the existence of a widespread Eocene exhumation of the shield before volcanic activity began. In the northeastern part of the swell, Cretaceous sedimentary remnants unconformably lying on the basement close to our samples evidence that they were near the surface at that time. We show that basement rocks have thus suffered a subsequent heating stage at 60-80 °C. We also present new apatite fission track ages on same samples. Central ages range from 71 ± 6 to 285 ± 29 Ma. When track length measurements were possible, preliminary modelings of the time-temperature history were performed. As previously deduced from apatite (U-Th)/He analyzes, these modelings show that samples underwent a heating to at least 80°C before their Late Eocene exhumation. Moreover, they also indicate that samples underwent another cooling stage during Lower Cretaceous, prior to Upper Cretaceous/Paleogene heating. We interpret these results as an evidence of a large-scale subsidence stage after the Cretaceous and until the Eocene, which allowed the deposition of a 1.5 to 3 km thick sedimentary cover and a heating at ~80°C of the currently outcropping basement. During the Eocene, the establishment of a thermal anomaly beneath the Tuareg Shield lithosphere resulted in erosion of the major part of this cover and, since 35 Ma, the development of intraplate volcanism.

A novel approach to in-situ rutile petrochronology

NASA Astrophysics Data System (ADS)

Kooijman, Ellen; Smit, Matthijs; Kylander-Clark, Andrew

2017-04-01

Rutile petrochronology has become an increasingly important tool for deciphering the timing and conditions of petrological processes. Rutile provides a reliable single-mineral thermometer, capable of retaining temperature information during high and ultra-high temperature metamorphism. Its HFSE contents can be used to investigate the geochemical environment in which rutile crystallized. Most importantly, rutile strongly fractionates U/Pb and enables U-Pb thermochronology in the intermediate temperature range. Here we present a novel approach to using U-Pb thermochronology of rutile by exploring the use of Pb as a diffusive species in kinetics-based thermometry. We performed high spatial and analytical resolution micro-analysis of rutile by laser ablation multi-collector ICPMS to constrain Pb diffusion profiles in rutile from high-grade metamorphic rocks of the Western Gneiss Complex (WGC), Norway. The age and thermometric results from this analysis are used to constrain a full thermal history from single grains. Millimeter-sized single crystals of rutile from a rutile-rich phlogopitite vein in eclogite were mounted and polished to expose their geometric cores. The grains were analyzed in transects using rectangular spots (c. 15x45 μm). This ensures ablation of a significant volume while maintaining the required radial spatial resolution. The transects yielded well-defined Pb diffusion profiles, with U-Pb ages ranging from c. 415 Ma in the cores to c. 380 Ma in the outermost rims (±2%, 2σ on individual spots). Diffusion zoning length was used with well-established Pb diffusion parameters [1] to determine peak temperature conditions following the approach of [2]. The result, c. 810 ± 25 ˚ C, is consistent with 800 ± 25 ˚ C and c. 780 ˚ C estimated for the same sample using conventional and Zr-in-rutile thermometry, respectively. The cooling history that is reconstructed through age zoning analysis and diffusion modeling shows remarkable consistency with that established for the WGC through decades of 40Ar/39Ar dating. The data presented here demonstrate that in-situ rutile U-Pb analysis yields reliable and precise temperature and age information that can be combined to resolve full thermal histories from single crystals. This novel approach to the toolbox of rutile petrochronology has great potential for research into the tectonics and dynamics of the lithosphere. References [1] Cherniak (2000) Contrib. Mineral. Petrol. 139. 198-207. [2] Smit et al. (2013) J. Metamorph. Geol. 31. 339-358.

Sequence stratigraphy, geodynamics, and detrital geothermochronology of Cretaceous foreland basin deposits, western interior U.S.A

NASA Astrophysics Data System (ADS)

Painter, Clayton S.

Three studies on Cordilleran foreland basin deposits in the western U.S.A. constitute this dissertation. These studies differ in scale, time and discipline. The first two studies include basin analysis, flexural modeling and detailed stratigraphic analysis of Upper Cretaceous depocenters and strata in the western U.S.A. The third study consists of detrital zircon U-Pb analysis (DZ U-Pb) and thermochronology, both zircon (U-Th)/He and apatite fission track (AFT), of Upper Jurassic to Upper Cretaceous foreland-basin conglomerates and sandstones. Five electronic supplementary files are a part of this dissertation and are available online; these include 3 raw data files (Appendix_A_raw_isopach_data.txt, Appendix_C_DZ_Data.xls, Appendix_C_U-Pb_apatite.xls), 1 oversized stratigraphic cross section (Appendix_B_figure_5.pdf), and 1 figure containing apatite U-Pb concordia plots (Appendix_C_Concordia.pdf). Appendix A is a combination of detailed isopach maps of the Upper Cretaceous Western Interior, flexural modeling and a comparison to dynamic subsidence models as applied to the region. Using these new isopach maps and modeling, I place the previously recognized but poorly constrained shift from flexural to non-flexural subsidence at 81 Ma. Appendix B is a detailed stratigraphic study of the Upper Cretaceous, (Campanian, ~76 Ma) Sego Sandstone Member of the Mesaverde Group in northwestern Colorado, an area where little research has been done on this formation. Appendix C is a geo-thermochronologic study to measure the lag time of Upper Jurassic to Upper Cretaceous conglomerates and sandstones in the Cordilleran foreland basin. The maximum depositional ages using DZ U-Pb match existing biostratigraphic age controls. AFT is an effective thermochronometer for Lower to Upper Cretaceous foreland stratigraphy and indicates that source material was exhumed from >4--5 km depth in the Cordilleran orogenic belt between 118 and 66 Ma, and zircon (U-Th)/He suggests that it was exhumed from <8--9 km depth. Apatite U-Pb analyses indicate that volcanic contamination is a significant issue, without which, one cannot exclude the possibility that the youngest detrital AFT population is contaminated with significant amounts of volcanogenic apatite and does not represent source exhumation. AFT lag times are <5 Myr with relatively steady-state to slightly increasing exhumation rates. Lag time measurements indicate exhumation rates of ~0.9->>1 km/Myr.

Lesser Antillean Arc Initiation and Migration as a Proxy of Slab Dynamics: Geothermochronology, Thermobarometry and Structure of Saint Martin Granodiorites

NASA Astrophysics Data System (ADS)

Noury, M.; Münch, P.; Philippon, M. M.; Bernet, M.; Bruguier, O.; Balvay, M.

2017-12-01

In subduction zones, volcanic arc initiation, cessation, migration and associated upper plate deformation -i.e faulting and vertical motions- reflect large-scale slab dynamics. At the northeastern edge of the Caribbean plate, the Greater Caribbean subduction zone waned out during the Mid Eocene, following the subduction of the Bahamas bank. This arc cessation was contemporaneous with (i) a plate boundary re-organization (evolving from subduction to transform), (ii) upper plate deformation and (iii) arc initiation in the Lesser Antilles. As part of the GAARANTI project that aims at unraveling the relationships between the evolution of terrestrial Caribbean biodiversity and vertical motions resulting from the Lesser Antilles subduction zone dynamic, we study the Saint Martin granodiorites, one of the two Oligocene plutons outcropping in the Lesser Antillean forearc. We investigate the birth and evolution of the Lesser Antillean arc and its thermo-mechanical impact on the Caribbean upper plate. In order to characterize the P,T,t path of the pluton we performed several thermochronological analyses covering a wide range of temperature (U-Pb on zircon -Tc 850°C, Ar/Ar on amphibole -Tc 550°C- and biotite -Tc 325°C-, zircon and apatite fission-tracks -Tc 250 and 110°C, respectively as well as U-Th/He on apatite -Tc 60°C) coupled with in-situ thermobarometry analyses (Al in hornblendes) and structural data. Geochronology and thermobarometry reveal that the granodiorites emplaced at ca. 28 Ma, at a depth of 5 km. Based on the age difference between amphibole and biotite Ar/Ar ages, we show that the northern pluton cooled faster than the southern one. Preliminary thermochronological results show a fast cooling between 29 and 25 Ma and then a continuous and slow cooling since 25 Ma and inverse modeling points to a 10 Ma cooling event. Our investigations give insights on the thermo-mechanical evolution of the arc-forearc region of the Lesser Antilles subduction zone. Considering a mean high of 1200m for the volcanic edifice, the pluton emplaced at shallow depth (ca. 4 km) within the Caribbean plate. The pluton is bounded by N-S faults that could possibly be responsible for the 10 Ma exhumation event. This thermal event may be contemporaneous with the westward arc migration during Miocene times and may reflect slab flattening.

Middle to late Cenozoic basin evolution in the western Alborz Mountains: Implications for the onset of collisional deformation in northern Iran

NASA Astrophysics Data System (ADS)

Guest, Bernard; Horton, Brian K.; Axen, Gary J.; Hassanzadeh, Jamshid; McIntosh, William C.

2007-12-01

Oligocene-Miocene strata preserved in synclinal outcrop belts of the western Alborz Mountains record the onset of Arabia-Eurasia collision-related deformation in northern Iran. Two stratigraphic intervals, informally named the Gand Ab and Narijan units, represent a former basin system that existed in the Alborz. The Gand Ab unit is composed of marine lagoonal mudstones, fluvial and alluvial-fan clastic rocks, fossiliferous Rupelian to Burdigalian marine carbonates, and basalt flows yielding 40Ar/39Ar ages of 32.7 ± 0.3 and 32.9 ± 0.2 Ma. The Gand Ab unit is correlated with the Oligocene-lower Miocene Qom Formation of central Iran and is considered a product of thermal subsidence following Eocene extension. The Narijan unit unconformably overlies the Gand Ab unit and is composed of fluvial-lacustrine and alluvial fan sediments exhibiting contractional growth strata. We correlate the Narijan unit with the middle to upper Miocene Upper Red Formation of central Iran on the basis of lithofacies similarities, stratigraphic position, and an 8.74 ± 0.15 Ma microdiorite dike (40Ar/39Ar) that intruded the basal strata. Deformation timing is constrained by crosscutting relationships and independent thermochronological data. The Parachan thrust system along the eastern edge of the ancestral Taleghan-Alamut basin is cut by dikes dated at 8.74 ± 0.15 Ma to 6.68 ± 0.07 Ma (40Ar/39Ar). Subhorizontal gravels that unconformably overlie tightly folded and faulted Narijan strata are capped by 2.86 ± 0.83 Ma (40Ar/39Ar) andesitic lava flows. These relationships suggest that Alborz deformation had migrated southward into the Taleghan-Alamut basin by late Miocene time and shifted to its present location along the active range front by late Pliocene time. Data presented here demonstrate that shortening in the western Alborz Mountains had started by late middle Miocene time. This estimate is consistent with recent thermochronological results that place the onset of rapid exhumation in the western Alborz at ˜12 Ma. Moreover, nearly synchronous Miocene contraction in the Alborz, Zagros Mountains, Turkish-Iranian plateau, and Anatolia suggests that the Arabia-Eurasia collision affected a large region simultaneously, without a systematic outward progression of mountain building away from the collision zone.

In-Situ Apatite Laser Ablation U-Th-Sm/He Dating, Methods and Challenges

NASA Astrophysics Data System (ADS)

Pickering, J. E.; Matthews, W.; Guest, B.; Hamilton, B.; Sykes, C.

2015-12-01

In-situ, laser ablation U-Th-Sm/He dating is an emerging technique in thermochronology that has been proven as a means to date zircon and monzonite1-5. In-situ U-Th-Sm/He thermochronology eliminates many of the problems and inconveniences associated with traditional, whole grain methods, including; reducing bias in grain selection based on size, shape and clarity; allowing for the use of broken grains and grains with inclusions; avoiding bad neighbour effects; and eliminating safety hazards associated with dissolution. In-situ apatite laser ablation is challenging due to low concentrations of U and Th and thus a low abundance of radiogenic He. For apatite laser ablation to be effective the ultra-high-vacuum (UHV) line must have very low and consistent background levels of He. To reduce He background, samples are mounted in a UHV stable medium. Our mounting process uses a MicroHePP (Microscope Mounted Heated Platen Press) to press samples into FEP (fluorinated ethylene propylene) bonded to an aluminum backing plate. Samples are ablated using a Resonetics 193 nm excimer laser and liberated He is measured using a quadrupole mass spectrometer on the ASI Alphachron noble gas line; collectively this system is known as the Resochron. The ablated sites are imaged using a Zygo Zescope optical profilometer and ablated pit volume measured using PitVol, a custom MatLab algorithm developed to enable precise and unbiased measurement of the ablated pit geometry. We use the well-characterized Durango apatite to demonstrate the accuracy and precision of the method. He liberated from forty-two pits, having volumes between 1700 and 9000 um3, were measured using the Resochron. The ablated sites were imaged using a Zygo Zescope optical profilometer and ablated pit volume measured using PitVol. U, Th and Sm concentrations were measured by laser ablation and the U-Th-Sm/He age calculated by standard age equation. An age of 33.8±0.31 Ma was determined and compares well with conventional U-Th/He methods, whole grain degassing and dissolution, that produced an age of 32.73±0.47 Ma for chips of the same Durango crystal. Further dating of other well characterized apatite crystals will be used to test the robustness of the method.

Late Cenozoic thermochronology and exhumation history of central Anatolia: Implications for the timing and nature of transition from collision to escape tectonics

NASA Astrophysics Data System (ADS)

Thomson, S. N.; Lefebvre, C.; Umhoefer, P. J.; Darin, M. H.; Whitney, D.; Teyssier, C. P.

2016-12-01

The central part of the Anatolian microplate in Turkey forms a complex tectonic zone situated between ongoing convergence of the Arabian and Eurasian plates to the east, and lateral escape of the Anatolian microplate as a rigid block to the west facilitated by two major strike-slip faults (the North and East Anatolian fault zones) that transitions westward into an extensional tectonic regime in western Turkey and the Aegean Sea related to subduction retreat. However, the geodynamic processes behind the transition from collision to escape, and the timing and nature of this transition, are complex and remain poorly understood. To gain a better understanding of the timing and nature of this transition, including the debated timing of ca. 35-20 Ma onset of collision between Arabia and Eurasia, we have undertaken a comprehensive low-temperature thermochronologic study in central Turkey to provide a record of exhumation patterns. We have collected over 150 samples, focused on the Central Anatolian Crystalline Complex (CACC), the Central Anatolian fault zone (CAFZ - proposed as a major lithosphere-scale structure that may also be related to onset of tectonic escape), and Eocene to Neogene sedimentary basins. Results include 113 apatite fission track (FT) ages (62 bedrock ages and 51 detrital ages), 26 detrital zircon FT ages, 218 apatite (U-Th)/He (He) ages from 84 mostly bedrock samples, and 15 zircon He ages from 6 bedrock samples. Our most significant new finding is identification of an early Miocene (ca. 22-15 Ma) phase of rapid cooling seen in the CACC. These cooling ages are localized in the footwalls of several large high-angle NW-SE trending normal faults, and imply significant footwall uplift and exhumation at this time. This early Miocene exhumation is restricted to entirely west of the CAFZ, and supports this fault marking a major tectonic transition active at this time. East of the CAFZ, AFT ages in sedimentary rocks show Eocene and older detrital ages despite much higher elevations (up to 3000m) suggesting uplift of the fault block east of CAFZ occurred since the late Miocene. An earlier Eocene (40-35 Ma) phase of cooling and exhumation is identified in deformed Paleocene-Eocene sedimentary rocks either side of the CAFZ likely related to a regional episode of shortening during final closure of the inner Tauride suture.

Focused exhumation along megathrust splay faults in Prince William Sound, Alaska

USGS Publications Warehouse

Haeussler, Peter J.; Armstrong, Phillip A; Liberty, Lee M; Ferguson, Kelly M; Finn, Shaun P; Arkle, Jeannette C; Pratt, Thomas L.

2015-01-01

Megathrust splay faults are a common feature of accretionary prisms and can be important for generating tsunamis during some subduction zone earthquakes. Here we provide new evidence from Alaska that megathrust splay faults have been conduits for focused exhumation in the last 5 Ma. In most of central Prince William Sound, published and new low-temperature thermochronology data indicate little to no permanent rock uplift over tens of thousands of earthquake cycles. However, in southern Prince William Sound on Montague Island, apatite (U–Th)/He ages are as young as 1.1 Ma indicating focused and rapid rock uplift. Montague Island lies in the hanging wall of the Patton Bay megathrust splay fault system, which ruptured during the 1964 M9.2 earthquake and produced ∼9 m of vertical uplift. Recent geochronology and thermochronology studies show rapid exhumation within the last 5 Ma in a pattern similar to the coseismic uplift in the 1964 earthquake, demonstrating that splay fault slip is a long term (3–5 my) phenomena. The region of slower exhumation correlates with rocks that are older and metamorphosed and constitute a mechanically strong backstop. The region of rapid exhumation consists of much younger and weakly metamorphosed rocks, which we infer are mechanically weak. The region of rapid exhumation is separated from the region of slow exhumation by the newly identified Montague Strait Fault. New sparker high-resolution bathymetry, seismic reflection profiles, and a 2012 Mw4.8 earthquake show this feature as a 75-km-long high-angle active normal fault. There are numerous smaller active normal(?) faults in the region between the Montague Strait Fault and the splay faults. We interpret this hanging wall extension as developing between the rapidly uplifting sliver of younger and weaker rocks on Montague Island from the essentially fixed region to the north. Deep seismic reflection profiles show the splay faults root into the subduction megathrust where there is probable underplating. Thus the exhumation and extension in the hanging wall are likely driven by underplating along the megathrust décollement, thickening in the overriding plate and a change in rheology at the Montague Strait Fault to form a structural backstop. A comparison with other megathrust splay faults around the world shows they have significant variability in their characteristics, and the conditions for their formation are not particularly unique.

Controlling factors of spatial and temporal preservation of the geochronological signal in sediments during an orogenic cycle

NASA Astrophysics Data System (ADS)

Rat, Juliette; Mouthereau, Frédéric; Bernet, Matthias; Brichau, Stéphanie; Balvay, Mélanie; Garzanti, Eduardo; Ando, Sergio

2017-04-01

Detrital content of sediments preserved in basins provide constraints on the nature of source rocks, dynamics of sediment transport, and potentially on tectonics and climate changes. U-Pb dating method on detrital zircon is ideally suited for provenance studies due to the ability of U-Pb age data to resist several orogenic cycles. However, with the aim to track sediment source evolution over a single orogenic cycle and determine characteristic time and parameters controlling the geochronological signal preservation throughout the cycle from rifting, mountain building to post-collision evolution, low-temperature thermochronology combined with sediment petrography are more appropriate than the U-Pb dating approach taken alone. To better understanding processes at play in the long-term geochronological signal preservation we focus on the sediment record associated with the Iberia plate tectonic evolution, which is part of the OROGEN research project, co-financed by BRGM, TOTAL & CNRS. The Iberian plate recorded a period of extension in the Late Jurassic, followed during the Early Cretaceous (Aptian-Albian) by a major thinning event documented by thick syn-rift sediments in intraplate basins and plate-scale heating/cooling of the Iberia crust, as argued by published fission track ages. Paleogeographic reconstructions that are based on stratigraphic and lithofacies analyses in northern Iberia (Iberian Range, Pyrenees and Basque-Cantabrians Range), describe a large domain of continental/fluvial and shallow-marine siliciclastic deposition. The related detrital content was then recycled during the subsequent Pyrenean orogenic phase in the Ebro foreland basin, and eventually transfer to the Mediterranean realm during post-orogenic re-excavation of the Ebro basin. In this study, we complete the published time-temperature paths in the mesozoic syn-rift basins by providing new thermo-chronological analyses of well-dated syn-collision and post-collision stratigraphic sections of the Ebro basin to determine thermal control on preservation through burial and geothermal evolution. We combined this study with sediments petrography analyses to identify relative control of source petrography, hydraulic sorting, alteration and diagenesis processes on the signal preservation during sediment transfer. All these observations will ultimately be incorporated in a geodynamic reconstruction of Iberia, and compared with age predictions from a model coupling surface processes and thermal evolution.

Evidence of Variscan and Alpine tectonics in the structural and thermochronological record of the central Serbo-Macedonian Massif (south-eastern Serbia)

NASA Astrophysics Data System (ADS)

Antić, Milorad D.; Kounov, Alexandre; Trivić, Branislav; Spikings, Richard; Wetzel, Andreas

2017-07-01

The Serbo-Macedonian Massif (SMM) represents a composite crystalline belt within the Eastern European Alpine orogen, outcropping from the Pannonian basin in the north to the Aegean Sea in the south. The central parts of this massif (south-eastern Serbia) consist of the medium- to high-grade Lower Complex and the low-grade Vlasina Unit. Outcrop- and micro-scale ductile structures in this area document three major stages of ductile deformation. The earliest stage D1 is related to isoclinal folding, commonly preserved as up to decimetre-scale quartz-feldspar rootless fold hinges. D2 is associated with general south-eastward tectonic transport and refolding of earlier structures into recumbent metre- to kilometre-scale tight to isoclinal folds. Stages D1 and D2 could not be temporally separated and probably took place in close sequence. The age of these two ductile deformation stages was constrained to the Variscan orogeny based on indirect geological evidence (i.e. ca. 408-ca. 328). During this period, the SMM was involved in a transpressional amalgamation of the western and eastern parts of the Galatian super-terrane and subsequent collision with Laurussia. Outcrop-scale evidence of the final stage D3 is limited to spaced and crenulation cleavage, which are probably related to formation of large-scale open upright folds as reported previously. 40Ar/39Ar thermochronology was applied on hornblende, muscovite, and biotite samples in order to constrain the age of tectonothermal events and activity along major shear zones. These 40Ar/39Ar data reveal three major cooling episodes affecting the central SMM. Cooling below greenschist facies conditions in the western part of the Vlasina Unit took place in a post-orogenic setting (extensional or transtensional) in the early Permian (284 ± 1 Ma). The age of activity along the top-to-the-west shear zone formed within the orthogneiss in the Božica area of the Vlasina Unit was constrained to Middle Triassic (246 ± 1 Ma). This age coincides with widespread extension related to the opening of the Mesozoic Tethys. The greenschist facies retrogression in the Lower Complex probably occurred in the Early Jurassic (195 ± 1 Ma), and it was related to the thermal processes in the overriding plate above the subducting slab of the Mesozoic Tethys Ocean.

Detrital K-feldspar thermochronology of the Nanaimo Group: Characterization of Basement and Extraregional Basin Contributions

NASA Astrophysics Data System (ADS)

Isava, V.; Grove, M.; Mahoney, J. B.; Kimbrough, D. L.

2016-12-01

The Late Cretaceous-Early Paleogene Nanaimo Group covers the contact between Triassic basement Wrangellia terrane and the Jurassic-Cretaceous Coast Plutonic Complex (CPC) in southern British Columbia. Prior detrital zircon U-Pb and Hf studies indicate a change in sediment source for the Nanaimo basin, from the primitive CPC in Santonian-Early Campanian time to an isotopically evolved continental extraregional source during the late Campanian/Maastrictian. Two notably different areas have been proposed as potential source regions: (1) the Idaho/Boulder batholith and Belt Supergroup, and (2) the Mojave/Salinia segment of structurally disrupted late Cretaceous southern California margin. Single crystal 40Ar/39Ar laser fusion of ca. 100-200 grains apiece from seven detrital K-feldspar samples from Santonian-Maastrichtian strata of the northern Nanaimo Group constrain the history of the sediments' source regions. The two oldest samples, from the K-feldspar poor Comox and Extension Fms., display a monotonic increasing distribution of cooling ages 80-125 Ma that reflects shallow erosion of the CPC. In contrast, Late Campanian strata of the Cedar District and De Courcy Fms. exhibit a more pronounced cluster of cooling ages 80-95 Ma as well as a greater proportion of Jurassic ages that represent progressively deeper erosion of the CPC. Evidence for an extraregional sediment source appears abruptly in the Geoffrey Fm. by 72 Ma, matching the time of local-to-extraregional shift indicated in detrital zircon U-Pb studies. Over 90% of the detrital K-feldspars from these arkosic sandstones yield cooling ages of 70-80 Ma, with sparse older ages associated with the CPC. Samples from the successively younger Spray and Gabriola Fms. also yield >90% K-feldspar ages younger than 80 Ma and exhibit age maxima of 68 Ma and 65 Ma, respectively. These results are distinct from detrital zircon U-Pb and K-feldspar 40Ar/39Ar ages of the southern Sierra Nevada, Mojave/Salina, and northern Peninsular Ranges of southern California. Resemblance with the Idaho/Boulder batholith indicates the latter as a more likely source of extraregional detritus. Measurements are underway to couple detrital K-feldspar 40Ar/39Ar thermochronology with U-Pb isotopes on the same grains to further distinguish between these two possibilities.

Constraining Thermal Histories by Monte Carlo Simulation of Mg-Fe Isotopic Profiles in Olivine

NASA Astrophysics Data System (ADS)

Sio, C. K. I.; Dauphas, N.

2016-12-01

In thermochronology, random time-temperature (t-T) paths are generated and used as inputs to model fission track data. This random search method is used to identify a range of acceptable thermal histories that can describe the data. We have extended this modeling approach to magmatic systems. This approach utilizes both the chemical and stable isotope profiles measured in crystals as model constraints. Specifically, the isotopic profiles are used to determine the relative contribution of crystal growth vs. diffusion in generating chemical profiles, and to detect changes in melt composition. With this information, tighter constraints can be placed on the thermal evolution of magmatic bodies. We use an olivine phenocryst from the Kilauea Iki lava lake, HI, to demonstrate proof of concept. We treat this sample as one with little geologic context, then compare our modeling results to the known thermal history experienced by that sample. To complete forward modeling, we use MELTS to estimate the boundary condition, initial and quench temperatures. We also assume a simple relationship between crystal growth and cooling rate. Another important parameter is the isotopic effect for diffusion (i.e., the relative diffusivity of the light vs. heavy isotope of an element). The isotopic effects for Mg and Fe diffusion in olivine have been estimated based on natural samples; experiments to better constrain these parameters are underway. We find that 40% of the random t-T paths can be used to fit the Mg-Fe chemical profiles. However, only a few can be used to simultaneously fit the Mg-Fe isotopic profiles. These few t-T paths are close to the independently determined t-T history of the sample. This modeling approach can be further extended other igneous and metamorphic systems where data exist for diffusion rates, crystal growth rates, and isotopic effects for diffusion.

Exsolution lamellae as fast diffusion pathways in rutile: implications for U-Pb thermochronology and Zr thermometry

NASA Astrophysics Data System (ADS)

Smye, A.; Seman, S.; Roberts, N. M. W.; Condon, D. J.; Davis, B.

2017-12-01

Geophysical processes impart characteristic thermal signatures to the lithosphere. Near-continuous thermal histories can be obtained from inversion of intracrystalline U-Pb age profiles in rutile and apatite provided that it can be shown that profile formed in response to Fickian-type diffusion. Here, we present the results of a combined LA-ICPMS and ID-TIMS U-Pb study on rutile grains from two garnet-bearing granulite xenoliths from a kimberlite in the Archean Slave province. Interpreted using numerical models, we show that the rutile U-Pb isotope systematics are consistent with slow-cooling following crystallization at 1.2 Ga, contemporaneous with the Mackenzie dike swarm. However, inversion of rutile U-Pb age gradients is complicated by the ubiquitous presence of ilmenite exsolution lamellae. We show that these lamellae act as fast diffusion pathways for Pb and High Field Strength Elements, including Zr. This has important implications for the use of rutile as a U-Pb themochronometer and as a single-phase thermometer.

Analysis of the Exhumation Pathways Experienced in the Cascades Range

NASA Astrophysics Data System (ADS)

Giles, S. M.; Pesek, M.; Perez, N. D.

2017-12-01

The Cascades volcanic arc is the result of subduction of the Juan de Fuca plate beneath North America. The Cascades trend north to south and create a modern orographic precipitation gradient that focuses precipitation along the western flank of the range. However, the deformation style changes from shortening in the north to extension in the south. This experimental design is an ideal location to test how surface and tectonic processes contribute to rock uplift in orogens. In the Oregon Cascades, zircon U-Pb geochronology, and multiple thermochronologic techniques (apatite U-Pb, zircon U-Th/He) will be applied to an intrusive rock exposed along a west-flowing river to investigate the exhumation pathway. These intrusive rocks are capped by late Miocene basalt flows, constraining the timing of surface exposure. The results of this study will define a time-temperature pathway and be compared with existing exhumation constraints from the Washington Cascades to determine whether the exhumation pathways may correspond to the changing structural regimes or consistent climate patterns along strike.

Southern California Earthquake Center Geologic Vertical Motion Database

NASA Astrophysics Data System (ADS)

Niemi, Nathan A.; Oskin, Michael; Rockwell, Thomas K.

2008-07-01

The Southern California Earthquake Center Geologic Vertical Motion Database (VMDB) integrates disparate sources of geologic uplift and subsidence data at 104- to 106-year time scales into a single resource for investigations of crustal deformation in southern California. Over 1800 vertical deformation rate data points in southern California and northern Baja California populate the database. Four mature data sets are now represented: marine terraces, incised river terraces, thermochronologic ages, and stratigraphic surfaces. An innovative architecture and interface of the VMDB exposes distinct data sets and reference frames, permitting user exploration of this complex data set and allowing user control over the assumptions applied to convert geologic and geochronologic information into absolute uplift rates. Online exploration and download tools are available through all common web browsers, allowing the distribution of vertical motion results as HTML tables, tab-delimited GIS-compatible text files, or via a map interface through the Google Maps™ web service. The VMDB represents a mature product for research of fault activity and elastic deformation of southern California.

Mechanisms and Magnitude of Cenozoic Crustal Extension in the Vicinity of Lake Mead, Nevada and the Beaver Dam Mountains, Utah: Geochemical, Geochronological,Thermochronological and Geophysical Constraints

NASA Astrophysics Data System (ADS)

Almeida, Rafael V.

The central Basin and Range Province of Nevada and Utah was one of the first areas in which the existence of widespread low-angle normal faults or detachments was first recognized. The magnitude of associated crustal extension is estimated by some to be large, in places increasing original line lengths by as much as a factor of four. However, rock mechanics experiments and seismological data cast doubt on whether these structures slipped at low inclination in the manner generally assumed. In this dissertation, I review the evidence for the presence of detachment faults in the Lake Mead and Beaver Dam Mountains areas and place constraints on the amount of extension that has occurred there since the Miocene. Chapter 1 deals with the source-provenance relationship between Miocene breccias cropping out close to Las Vegas, Nevada and their interpreted source at Gold Butte, currently located 65 km to the east. Geochemical, geochronological and thermochronological data provide support for that long-accepted correlation, though with unexpected mismatches requiring modification of the original hypothesis. In Chapter 2, the same data are used to propose a refinement of the timing of ~1.45 Ga anorogenic magmatism, and the distribution of Proterozoic crustal boundaries. Chapter 3 uses geophysical methods to address the subsurface geometry of faults along the west flank of the Beaver Dam Mountains of southwestern Utah. The data suggest that the range is bounded by steeply inclined normal faults rather than a regional-scale detachment fault. Footwall folding formerly ascribed to Miocene deformation is reinterpreted as an expression of Cretaceous crustal shortening. Fission track data presented in Chapter 4 are consistent with mid-Miocene exhumation adjacent to high-angle normal faults. They also reveal a protracted history dating back to the Pennsylvanian-Permian time, with implications for the interpretation of other basement-cored uplifts in the region. A key finding of this dissertation is that the magnitude of crustal extension in this region has been overestimated. The pre-extensional width was increased by a factor of two across Lake Mead, through a combination of high-angle normal faulting and strike-slip deformation. Data from the transect across the Beaver Dam Mountains suggest substantially less extension, with the difference accommodated for the most part by displacement on the intervening Las Vegas Valley Shear Zone. The Colorado Plateau-Basin and Range transition zone may be a long-lived tectonic boundary where this assumption may be especially ill-suited.

Paleomagnetic study of the northern Ford Ranges, western Marie Byrd Land, West Antarctica: Motion between West and East Antarctica

USGS Publications Warehouse

Luyendyk, B.; Cisowski, S.; Smith, C.; Richard, S.; Kimbrough, D.

1996-01-01

A paleomagnetic study of Paleozoic and Mesozoic crystalline rocks in the northern Ford Ranges of Marie Byrd Land, West Antarctica, has determined a middle Cretaceous (circa 100 Ma) paleomagnetic pole and provided constraints on possible clockwise rotation of these ranges and on the rifting of east Gondwana. The 40Ar/39Ar thermochronology data from the Fosdick Mountains record a period of rapid cooling from ???700??C beginning at ???100 Ma. We relate this to extension, intrusion, and uplift associated with the beginning of rifting between Campbell Plateau and Marie Byrd Land. All rocks from the Fosdick and Chester Mountains are normally polarized. We interpret thermochronology and paleomagnetic data to infer that the region was extensively remagnetized in middle Cretaceous time. Inclinations in samples from the Chester Mountains are less steep than those from the Fosdick Mountains, which we interpret as ???25?? of south tilting of the Chesters. We interpret cooling age data for the time of magnetization to infer that the tilting began after 105 Ma and ended prior to 103 Ma. We further interpret this as constraining the beginning of extension between the Campbell Plateau and western Marie Byrd Land to the interval 105 to 103 Ma. Virtual geomagnetic poles from samples of Early Carboniferous age granodiorite from the western Phillips Mountains lie on the late Paleozoic apparent polar wander path for Australia transferred to Antarctica. Directions from 29 sites in the central and eastern Phillips and Fosdick Mountains give a Middle Cretaceous paleomagnetic pole at 222.3?? E, 70.5?? S (A95 6.1??, KAPPA 20.0). This pole is indistinguishable from other Middle Cretaceous poles for studies further east in Marie Byrd Land. Combining middle Cretaceous poles determined for three other studies of the Antarctic Peninsula. Thurston Island, and the Ruppert-Hobbs coasts with ours gives a Pacific West Antarctic pole at 215.2?? E, 73.5?? S (A95 4.0??, KAPPA 528.9). This pole is discordant by 5?? to 10?? from synthetic mid-Cretaceous East Antarctic reference poles, but the degree of discordance is very sensitive to the choice of the specific reference pole. The lack of native East Antarctic reference poles leaves this analysis inconclusive. Accepting 10?? of discordance, we favor an interpretation where Pacific West Antarctic crustal domains or microplates have rotated clockwise 40?? to 90?? and translated a few degrees away from East Antarctica during Late Cretaceous time.

Deciphering the driving forces of short-term erosion in glacially impacted landscapes, an example from the Western Alps

NASA Astrophysics Data System (ADS)

Glotzbach, Christoph; van der Beek, Peter; Carcaillet, Julien; Delunel, Romain

2013-04-01

Tectonic uplift is the main driver of long-term erosion, but climate changes can markedly affect the link between tectonics and erosion, causing transient variations in short-term erosion rate. Here we study the driving forces of short-term erosion rates in the French Western Alps as estimated from in-situ produced cosmogenic 10Be and detrital apatite fission-track thermochronology analysis of stream sediments. Short-term erosion rates from 10Be analyses vary between ~0.27 and ~1.33 mm/yr, similar to rates measured in adjacent areas of the Alps. Part of the data scales positively with elevation, while the full dataset shows a significant positive correlation with steepness index of streams and normalized geophysical relief. Mean long-term exhumation and short-term erosion rates are comparable in areas that are exhuming rapidly (>0.4 km/Myr), but short-term rates are on average two-three (and up to six) times higher than long-term rates in areas where the latter are slow (<0.4 km/Myr). These findings are supported by detrital apatite fission-track age distributions that appear to require similar variations in erosion rates. Major glaciations strongly impacted the external part of the Alps, increasing both long-term exhumation rates as well as relief (e.g. Glotzbach et al. 2011; Häuselmann et al. 2007; Valla et al.). Based on our data, it seems that glacial impact in the more slowly eroding internal part is mainly restricted to relief, which is reflected in high transient short-term erosion rates. The data further reveal that normalized steepness index and ridgeline geophysical relief are well correlated with (and could be used as proxies for) short-term erosion, in contrast to slope, corroborating studies in purely fluvial landscapes. Our study demonstrates that climate change, e.g. through occurrence of major glaciations, can markedly perturb landscapes short-term erosion patterns in regions of tectonically controlled long-term exhumation. Glotzbach C., P.A. van der Beek, C. Spiegel. (2011): Episodic exhumation and relief growth in the Mont Blanc massif, Western Alps from numerical modeling of thermochronology data. - Earth Planet. Sci. Lett. 304, 417-430. Häuselmann P., D.E. Granger, P.-Y. Jeanin, S.-E. Lauritzen (2007): Abrupt glacial valley incision at 0.8 Ma dated from cave deposits in Switzerland. - Geology 35, 143-146. Valla P.G., D.L. Shuster, P.A. van der Beek (2011): Significant increase in relief of the European Alps during mid-Pleistocene glaciations. - Nature Geosci. 4, 688-692.

Tectonic evolution of the Northern Pyrenees. Results of the PYRAMID project

NASA Astrophysics Data System (ADS)

Ford, Mary; Mouthereau, Fredéric; Christophoul, Fredéric; de Saint Blanquat, Michel; Espurt, Nicolas; Labaume, Pierre; Vergés, Jaume; Teixell, Antonio; Bellahsen, Nicolas; Vacharat, Arnaud; Pik, Raphael; Pironon, Jacques; Carpentier, Cédric; Angrand, Paul; Grool, Arjan; Salardon, Roland; Huismans, Ritske; Bader, Anne-Gaëlle; Baudin, Thierry; Aubourg, Charles

2017-04-01

The aims of the PYRAMID project funded by the Agence Nationale de la Recherche of France, were to investigate and constrain the 3D structural style and architecture of the North Pyrenean retrowedge and foreland basin, their evolution through time, to define the character and role of inherited crustal geometries, to investigate the interactions between deformation, fluids and thermicity in the different structural units, and to carry out source to sink studies In this talk we present a series of restored cross sections through the central and eastern Pyrenean retrowedge to illustrate structural style, amount and type of deformation and how it was accommodated within the upper crust along the orogen. The total amount of convergence appears to have been constant and the timing of onset of convergence was synchronous. However, in the retrowedge the complexity of the Cretaceous oblique rift system has led to high lateral structural variability. Inherited vertical late Variscan faults trending NE-SW to ENE-WSW segment the European crust and have strongly compartmentalised both retrowedge and foreland basin evolution along the orogen. Crustal scale restorations provide new evolutionary models for the geometry and style of inversion of the pre-orogenic hyper-extended rift system where mantle was exhumed in the most distal domain. Numerical models provide insight into retrowedge inversion. A new stratigraphic scheme has been developed for the eastern and central foreland. Subsidence analyses and foreland basin reconstructions document two pulses of convergence (Late Santonian to Early Paleocene and Eocene to Oligocene) separated by a quiet phase during the Paleocene. These phases can be linked to deformation in the North Pyrenean Zone thrust belt. The first phase was caused mainly by inversion and emplacement of the Metamorphic Internal Zone onto external zones associated with subduction of the exhumed mantle domain. Little or no relief was created during this phase although thermochronological data records the beginning of inversion in the eastern retrowedge. Full collision began in Early Eocene, distributed between the pro- and retro wedges, with only about 30% of convergence accommodated in the retrowedge. Low temperature thermochronology data records southward migrating exhumation of the axial zone while external basement massifs were being exhumed in the North Pyrenean Zone. The Cretaceous rift system was inverted by a combined thin-skinned-thick-skinned style with a decoupling level in the Keuper evaporites. The North Pyrenean Frontal thrust consists of a series of inverted Cretaceous rift margin faults, which in the east represent the main breakaway fault system.

Unraveling tectonics and climate forcing in the late-Neogene exhumation history of South Alaska

NASA Astrophysics Data System (ADS)

Valla, Pierre; Champagnac, Jean-Daniel; Shuster, David; Herman, Frédéric; Giuditta Fellin, Maria

2015-04-01

The southern Alaska range presents an ideal setting to study the complex interactions between tectonics, climate and surface processes in landscape evolution. It exhibits active tectonics with the ongoing subduction/collision between Pacific and North America, and major active seismogenic reverse and strike-slip faults. The alpine landscape, rugged topography and the important ice-coverage at present reveal a strong glacial imprint associated with high erosion and sediment transport rates. Therefore, the relative importance of climatically-driven glacial erosion and tectonics for the observed late-exhumation history appears to be quite complex to decipher. Here, we first perform a formal inversion of an extensive bedrock thermochronological dataset from the literature to quantify the large-scale 20-Myr exhumation history over the entire southern Alaska. We show that almost half of the variability within the thermochronological record can be explained by modern annual precipitations spatial distribution, the residuals clearly evidencing localized exhumation along major tectonic structures of the frontal fold and thrust belt. Our results confirm high exhumation rates in the St Elias "syntaxis" and frontal zones for the last 0-2 Myr, where major ice fields and high precipitation rates likely sustained high exhumation rates; however the impact of late Cenozoic glaciations is difficult to constrain because of the low resolution on the exhumation history older than ~2 Myr. On the contrary, our inversion outcomes highlight that north of the Bagley Icefield the long-term exhumation has remained quite slow and continuous over the last ~20 Myr, with no late-stage signal of exhumation change since the onset of glaciations despite a clear glacial imprint on the landscape. We thus focus on the Granite Range (Wrangell-St Elias National Park, Alaska), an area presenting a strong glacial imprint but minor tectonic activity with only localized brittle deformation. We sampled four elevation profiles over an East-West transect for low-temperature thermochrometry. Apatite (U-Th-Sm)/He dating provides ages between ~10 and 30 Ma, in agreement with published data, and shows apparent low long-term exhumation rates (~0.1 km/Myr). 4He/3He thermochronometry on a subset of samples reveals a more complex exhumation history, with a significant increase in exhumation since ~6-4 Ma that we relate to the early onset of glaciations and associated glacial erosion processes. Our results, in agreement with offshore sediment records, thus confirm an early glacial activity and associated erosion response in Alaska, well before the onset of Pliocene-Pleistocene Northern Hemisphere glaciations.

40Ar/39Ar thermochronologic constraints on the tectonothermal evolution of the Northern East Humboldt range metamorphic core complex, Nevada

USGS Publications Warehouse

McGrew, A.J.; Snee, L.W.

1994-01-01

The northern East Humboldt Range (NEHR) of northeastern Nevada exposes a suite of complexly deformed migmatitic, upper amphibolite-facies rocks in the footwall of the Ruby Mountains-East Humboldt Range (RM-EHR) detachment fault. New 40Ar/39Ar data on hornblende, muscovite, biotite, and potassium feldspar help constrain the kinematic and thermal evolution of this terrain during Tertiary extensional exhumation. Hornblende samples from relatively high structural levels yield discordant age spectra that suggest initial cooling during early Tertiary time (63-49 Ma). When coupled with petrological constraints indicating a strongly decompressional P-T-t path above 550??C, the hornblende data suggest that exhumation of the RM-EHR may have initiated in early Tertiary time, approximately coincident with the initial phases of unroofing in the Wood Hills immediately to the east and with the end of thrusting in the late Mesozoic to early Tertiary Sevier orogenic belt of eastern Nevada and western Utah. This temporal coincidence suggests that gravitational collapse of tectonically thickened crust in the internal zone of the Sevier belt could have driven the initial phases of unroofing. Thermal history during the final stage of exhumation of the NEHR is constrained by discordant hornblende cooling ages of 36-29 Ma from deep structural levels and biotite, muscovite, and potassium feldspar cooling ages of 27-21 Ma from a range of structural levels. Comparison of muscovite, biotite, and potassium feldspar cooling ages with previously published fission-track cooling ages implies very rapid cooling rates at temperatures below the closure temperature for muscovite (270??-350??C), but time gaps of > 7 m.y. between hornblende and mica cooling ages suggest that cooling at higher temperatures was more gradual. In addition, comparison of 40Ar 39Ar mica cooling ages with previously published fission-track apatite cooling ages suggests pronounced thermal gradients between the NEHR and adjacent areas during latest Oligocene to earliest Miocene time. Such thermal gradients could be readily explained if the RM-EHR detachment fault dipped > 30?? between the 300??C and 100??C isotherms. Finally, 40Ar 39Ar biotite cooling ages increase southward through the East Humboldt Range, compatible with northward extrapolation of a previously recognized pattern of WNW-younging biotite cooling ages from the Ruby Mountains. A simple model involving the propagation of footwall uplift in the direction of tectonic transport beneath an initially listric normal fault can explain the principle features of the Oligoce??ne to Miocene thermochronologic data set for the RM-EHR. ?? 1994.

Thermochronology of mid-Cretaceous dioritic granulites adjacent "Big Bend" in Australia-Pacific plate boundary, northern South Island, New Zealand

NASA Astrophysics Data System (ADS)

Sagar, M.; Seward, D.; Heizler, M. T.; Palin, J. M.; Toy, V. G.; Tulloch, A. J.

2012-12-01

The Western Fiordland Orthogneiss (WFO), situated south-east of the Australian-Pacific plate boundary (Alpine Fault), southern South Island, New Zealand is the largest suite of plutonic rocks intruded into the Pacific margin of Gondwana during the final stages of arc plutonism preceding break-up of the supercontinent in the Late Cretaceous. Dextral motion of c. 480 km along the Alpine Fault during the Cenozoic has offset originally contiguous Pacific Gondwana margin rocks in northern and southern South Island. The Glenroy Complex in northern South Island, west of the Alpine Fault is dominated by two-pyroxene+hornblende granulite facies monzodioritic gneisses. U-Pb zircon geochronological and geochemical data indicate the Glenroy Complex was emplaced between 128-122 Ma and is a correlative of the WFO. The Glenroy Complex forms the lower-most block bounded by an east-dipping set of imbricate thrusts that developed during the late Cenozoic to the west of the largest S-shaped restraining bend ("Big Bend") in the Alpine Fault. New 40Ar/39Ar and fission-track thermochronological data, combined with previous geological field-mapping, demonstrate that the Glenroy Complex cooled rapidly (c. 30° C/Ma) after emplacement and granulite facies metamorphism (c. 850°C) at c. 120 Ma, through c. 550 °C by c. 110-100 Ma. The average cooling rate during the Late Cretaceous-Cenozoic was relatively slow, and initial exposure in the late Early Miocene (c. 16 Ma) was followed by reburial to c. 3-4 km (c. 80-100 °C) before final exhumation post-Pliocene. This thermal history is similar to the WFO, which cooled rapidly through c. 350 °C during mid-Cretaceous continental extension, followed by slow cooling during the Late Cretaceous and Cenozoic until development of the Australian-Pacific boundary through New Zealand facilitated rapid, exhumation-related cooling from c. 240 °C at c. 20 Ma and final exhumation post-10 Ma (Davids, 1999). However, the Glenroy Complex cooled at a faster rate in the Paleogene-early Neogene and was at the surface (before reburial) at least 5 Ma earlier than the WFO. These differences are in part considered to reflect the influence of the Big Bend, which caused relatively early localised exhumation of the Glenroy Complex by local 'pop-up' mechanisms during a time when there was no significant component of overall convergence across the Pacific-Australian plate boundary and the Alpine Fault was dominantly strike-slip.

Thermochronological Evidence for Cenozoic Segmentation of Transantarctic Mountains

NASA Astrophysics Data System (ADS)

Zattin, M.; Pace, D.; Andreucci, B.; Rossetti, F.; Talarico, F.

2013-12-01

The Transantarctic Mountains (TAM) represent the boundary between the cratonic East Antarctica and the West Antarctica and are thus related to formation of the Western Antarctic Rift system (WARS). However, temporal relationships between timing of TAM uplift and evolution of the WARS are not clear. The large amount of existing thermochronological data indicate that exhumation of the TAM occurred at different times and extents, with main cooling events in the Early Cretaceous, Late Cretaceous, and early Cenozoic. Uplift of the different segments of the TAM was not recorded according to regular trends along the mountain chain, but instead appears diachronous and without a recognizable spatial pattern. Here we present apatite fission-track (AFT) data from 20 samples, collected from metamorphic and intrusive rocks from the region comprised between the Blue Glacier and the Byrd Glacier. AFT data show a large variety of ages, ranging from 28.0 to 88.8 Ma and without a clear correlation between age and elevation. As a whole, spatial variations suggest a decrease of ages from S to the region of the Koettlitz Glacier, where ages suddenly raise up to Cretaceous values. A marked increase of ages has been detected also south of Darwin Glacier, that is in correspondence of the Britannia Range. Thermal modelling shows that cooling paths are usually composite, with a main cooling event followed by slower cooling to present day temperatures. Time of main cooling event is late Cretaceous for samples from the Britannia Range whereas it is Eocene-Oligocene for samples from Koettlitz and Mulock areas. In any case, cooling rates are always quite low also during periods of enhanced uplift, with values not exceeding 5°C/Ma. These data support the idea of tectonic block segmentation of the TAM during the last phases of exhumation. Most of vertical displacements occurred during the Oligocene across transverse fault zones such as the Discovery Accommodation Zone to the north and the Britannia Range to the south. The region comprised between these major tectonic structures represented probably a main source for the sediments that filled the Victoria Land basin during the Neogene. In fact bedrock AFT ages match well with detrital grain age distributions detected on sedimentary successions drilled by ANDRILL and CRP projects. Modelling of detrital AFT and apatite U-Th/He ages reveal that most of the source region was exhumed of about 5 km during Cenozoic. This value largely exceeds the estimated ice-related erosion which has been supposed to locally reach about 3 km along overdeepened pre-existing river valleys and in localized areas below sea level.

Westernmost Tian Shan (Uzbekistan): Magmatism and Exhumation

NASA Astrophysics Data System (ADS)

Abdulhameed, Sanaa; Ratchbacher, Lothar; Gagala, Lukasz; Jonkheere, Raymond

2014-05-01

The westernmost segment of the Tian Shan comprises the Ghissar-Alai Range of Tajikistan and Uzbekistan; its southwestern promontory contains the Baysunta crystalline massif. This orocline extends northwest of the Pamir and southeast of the Turan platform and forms the (north) western margin of the intra-orogenic Afghan-Tajik basin that was inverted during the India-Asia collision, mainly as a result of the gravitational collapse of the Pamir Plateau. The area contains Paleozoic slope and shelf clastics mantling crystalline basement rocks, altogether intruded by massif granitoids; it hides the cryptic Late Paleozoic South Ghissar suture. In Uzbekistan, the crystalline basement rocks of the westernmost Tien Shan are involved in the folding and thrusting of the Jurassic to Neogene sediments of the Afghan-Tajik basin, spectacularly proving thick-skinned deformation and demonstrating basement involvement below the Jurassic evaporate décollement underneath the Afghan-Tajik basin. We sampled the crystalline basement rocks of the Tian Shan of Uzbekistan to constrain the formation of the enigmatic Baysunta block and date the crystallization and high-grade metamorphism of the granitoids and associated metamorphic rocks of the Ghissar range; we employed U-Pb zircon geochronology. To time the deformation and exhumation of the fold-and thrust belt of the westernmost Afghan-Tajik basin, we used apatite fission-track thermochronology. Concordant U-Pb crystallization ages of zircons in the orthogneiss and paragneiss comprise 620 to 300 Ma; the Neoproterozoic ages imply a correlation of the Baysunta block with the Garm crystalline massif of the central Ghissar-Alai range of northeastern Tajikistan. The youngest zircon crystallization ages from granitoids are ~220 Ma, revealing enigmatically young magmatism, post-dating the last known collision event by >50 Ma. Together with 270-240 Ma meta-basaltic dykes and stocks in Tajikistan, they may trace a regional post-orogenic delamination event. The apatite fission-track thermochronology suggests a two-phase exhumation history. Exhumation/cooling within the age range of ~17 and 4 Ma and clustering around 10 Ma date slip along the thick-skinned thrusts. This demonstrates the impact of the India-Asia collision on the edge of the Turan platform, far northwest of the western edge of the collision zone. Ages from the northwestern edge of the study area cover 197 to 69 Ma; they trace a fossil partial annealing zone. They show that the Tian Shan along the margin of the Turan platform was never covered by sedimentary rocks >3 km thick.

Eocene to Miocene Out-of-Sequence Deformation in the Eastern Tibetan Plateau: Insights From Shortening Structures in the Sichuan Basin

NASA Astrophysics Data System (ADS)

Tian, Yuntao; Kohn, Barry P.; Qiu, Nansheng; Yuan, Yusong; Hu, Shengbiao; Gleadow, Andrew J. W.; Zhang, Peizhen

2018-02-01

A distinctive NNE trending belt of shortening structures dominates the topography and deformation of the eastern Sichuan Basin, 300 km east of the Tibetan Plateau. Debate continues as to whether the structures resulted from Cenozoic eastward growth of the Tibetan Plateau. A low-temperature thermochronology (AFT and AHe) data set from four deep boreholes and adjacent outcrops intersecting a branch of the shortening structures indicates distinctive differential cooling at 35-28 Ma across the structure, where stratigraphy has been offset vertically by 0.8-1.3 km. This result forms the first quantitative evidence for the existence of a late Eocene-Oligocene phase of shortening in the eastern Sichuan Basin, synchronous with the early phase of eastward growth and extrusion of the Tibetan Plateau. Further, a compilation of regional Cenozoic structures reveals a Miocene retreat of deformation from the foreland basin to the hinterland areas. Such a tectonic reorganization indicates that Eocene to Miocene deformation in the eastern Tibetan Plateau is out-of-sequence and was probably triggered by enhanced erosion in the eastern Tibetan Plateau.

Characterization of helium diffusion behavior from continuous heating experiments: Sample screening and identification of multiple 4He components

NASA Astrophysics Data System (ADS)

McDannell, K. T.; Idleman, B. D.; Zeitler, P. K.

2015-12-01

Old, slowly cooled apatites often yield overdispersed helium ages due to factors such as parent zonation, He implantation, radiation damage, crystal defects, and fluid inclusions. Careful mineral selection and many replicate analyses can mitigate the impact of some of these effects. However, this approach adds unnecessary costs in time and resources when dating well-behaved apatites and is generally ineffective at identifying the root cause of age dispersion and providing suitable age corrections for poorly behaved samples. We assess a new technique utilizing static-gas measurement during continuous heating as a means to rapidly screen apatite samples. In about the time required for a conventional total-gas analysis, this method can discriminate between samples showing the volume-diffusion behavior expected for apatite and those showing anomalous release patterns, inconsistent with their use in thermochronologic applications. This method may also have the potential to quantify and discriminate between the radiogenic and extraneous 4He fractions released by a sample. Continuously heated samples that outgas by volume diffusion during a linear heating schedule should produce a characteristic sigmoidal 4He fractional loss profile, with the exact shape and position of these profiles (in loss vs. heating time space) controlled by sample kinetics, grain size, and heating rate. Secondary factors such as sample zoning and alpha-loss distribution have a relatively minor impact on such profiles. Well-behaved examples such as the Durango standard and other apatites with good age reproducibility show the expected smooth, sigmoidal gas release with complete exhaustion by temperatures predicted for volume diffusion using typical apatite kinetics (e.g., by ~900˚C for linear heating at 20˚C/minute). In contrast, "bad actor" samples that do not replicate well show significant degrees of helium release deferred to higher temperatures. We report on screening results for a range of samples including a suite of slowly cooled Cretaceous apatites from the Hangay Dome in central Mongolia, assessing the degree to which screening using cumulative heating can reliably identify bad-actor grains, and possibly, correct their ages.

Exsolution lamellae in volcanic pyroxene; Single phenocryst thermometry for long-lived magmatic reservoir

NASA Astrophysics Data System (ADS)

I Made, R.; Herrin, J. S.; Tay, Y. Y.; Costa Rodriguez, F.

2017-12-01

Comprehensive understanding of the relevant timescales of thermal and chemical evolution of magma below the active volcanoes can help us to better anticipate volcanic eruptions and their likely precursor signals. In recent years, several lines of thermochronological inquiry have converged on a realization that, within many volcanic systems, magmas experience prolonged periods of relatively low-temperature storage prior to eruption during short duration transient events. This prolonged storage at low magmatic temperatures can result in series of solid state phase transformations within minerals, producing a petrologic record of their thermal history. In this example, we observed pigeonite exsolution lamellae in augite phenocrysts from the 2011 eruption of Cordon Caulle volcano, Chile. The small size of these features ( 70nm width and <1µm spacing), however, presents an analytical challenge. Focused ion beam (FIB) sample preparation has enable us to prepare site and orientation specific samples suitable for transmission electron microscope (TEM) characterization. Compositions of augite hosts (Wo39En38Fs22) were determined by EPMA prior to FIB preparation, while the compositions of pigeonite lamellae (Wo8En51Fs40) were determined by EDS utilizing PENEPMA simulation software for data reduction. These compositions yielded augite-pigeonite equilibrium temperatures of 945°C, in reasonable agreement with augite-liquid and bulk rock-liquidus temperatures. Lamella formation temperature relative to the C2/c → P21/c transition temperature can also be determined from the angle of the exsolution boundary with respect to crystal orientation. Since the C2/c → P21/c transition temperature is compositionally-dependent, however, calibration of this temperature-structure relationship requires us to conduct heating experiments. Toward this aim, we have combined nano beam electron diffraction (NBED) technique with an in-situ TEM heating stage to measure the changing lattice parameters at magmatic temperatures. Through this work we hope to develop two independent temperature determinations from individual augite phenocrysts which bear exsolution textures and apply this knowledge to understanding the thermal conditions of magma storage in long-lived volcanic reservoirs.

Neogene Sediment Transport, Deposition, and Exhumation from the Southern Alaska Syntaxis to the Eastern Aleutian Subduction Zone

NASA Astrophysics Data System (ADS)

Ridgway, K. D.; Witmer, J. W.; Enkelmann, E.; Plafker, G.; Brennan, P. R.

2011-12-01

Over 5 km of Neogene sedimentary strata are well exposed in the Chugach-St. Elias Ranges within the southern Alaska syntaxis. This syntaxis forms where the Pacific-North America plate boundary changes from the northwest-trending Queen Charlotte-Fairweather transform system to the southwest-trending Alaska-Aleutian subduction zone. Active collision and subduction of the buoyant Yakutat microplate in the syntaxis results in a wide collisional zone defined by active mountain belts, extensive glaciation, and thick packages of synorogenic strata. New stratigraphic and U-Th/He thermochronologic data from Neogene synorogenic strata, named the Yakataga and Redwood Formations, provide insights on collisional tectonics, glacial erosion, and sediment transport, deposition, burial, and exhumation from the onshore Chugach and St. Elias Ranges to the exposed accretionary prism of the Aleutian trench. Stratigraphic analyses show that along the southeastern part of the syntaxis, Neogene strata are characterized by deposition in braid delta, shallow marine, and glaciomarine slope apron depositional systems that resulted in construction of a broad continental shelf. In the central part of the syntaxis, marine shelf and upper slope environments deposited thick-bedded sandstone and mudstone in a thrust belt/foreland basin system. Along the southwestern part of the syntaxis, Neogene strata were deposited in a regional submarine fan system that filled the easternmost part of the Aleutian trench. Geologic mapping of the contact between the Yakataga Formation and underlying strata along the syntaxis document an angular unconformity with maximum stratigraphic separation (> 5 km) in the central part of the syntaxis. Along strike, this unconformity becomes conformable along both the southwestern and southeastern parts of the syntaxis. The regional angular unconformity and facies transitions both point to the importance of the central part of the syntaxis in the generation and distribution of synorogenic sediment. Apatite and zircon U-Th/He thermochronologic data from granitoid and gneissic clasts in conglomerate suggest that Neogene sediments were buried no deeper than ~2 km in the central and southeastern parts of the syntaxis, and that burial temperatures did not exceed ~40-45°C. In contrast, Neogene sediment deposited by submarine fans in the Aleutian trench along the southwestern part of the syntaxis were buried at depths of 5 to 7.5 km and reached temperatures between ~120-160°C. These strata were subsequently exhumed as the trench fill was incorporated into the growing accretionary prism. Collectively, our data show that the first-order sediment pathway along a glaciated syntaxis is dynamically linked to tectonic uplift, focused glacial erosion, deposition of thick packages of glacial marine sediment, and rapid exhumation along thrust belts and accretionary prisms.

Plutons: Simmer between 350° and 500°C for 10 million years, then serve cold (Invited)

NASA Astrophysics Data System (ADS)

Coleman, D. S.; Davis, J.

2009-12-01

The growing recognition that continental plutons are assembled incrementally over millions of years requires reexamination of the thermal histories of intrusive rocks. With the exception of the suggestion that pluton magma chambers can be revitalized by mafic input at their deepest structural levels, most aspects of modern pluton petrology are built on the underlying assumption that silicic plutons intrude as discrete thermal packages that undergo subsequent monotonic decay back to a steady-state geothermal gradient. The recognition that homogeneous silicic plutons are constructed over timescales too great to be single events necessitates rethinking pluton intrusion mechanisms, textures, thermochronology, chemical evolution and links to volcanic rocks. Three-dimensional thermal modeling of sheeted (horizontal and vertical) incremental pluton assembly (using HEAT3D by Wohletz, 2007) yields several results that are largely independent of intrusive geometry and may help understand bothersome field and laboratory results from plutonic rocks. 1) All increments cool quickly below hornblende closure temperature. However, late increments are emplaced into walls warmed by earlier increments, and they cycle between hornblende and biotite closure temperatures, a range in which fluid-rich melts are likely to be present. These conditions persist until the increments are far from the region of new magma flux, or the addition of increments stops. These observations are supported by Ar thermochronology and may explain why heterogeneous early marginal intrusive phases often grade into younger homogeneous interior map units. 2) Early increments become the contact metamorphic wall rocks of later increments. This observation suggests that much of the contact metamorphism associated with a given volume of plutonic rock is “lost” via textural modification of early increments during intrusion of later increments. Johnson and Glazner (CMP, in press) argue that mappable variations in pluton texture can result from textural modification during thermal cycling associated with incremental assembly. 3) The thermal structure of the model pluton evolves toward roughly spheroidal isotherms even though the pluton is assembled from thin tabular sheets. The zone of melt-bearing rock and the shape of intrapluton contact metamorphic isograds bear little resemblance to the increments from which the pluton was built. Consequently, pluton contacts mapped by variations in texture that reflect the thermal cycling inherent to incremental assembly will inevitably be “blob” or diapir-like, but will yield little insight into magma intrusion geometry. 4) Although models yield large regions of melt-bearing rock, the melt fraction is low and the melt-bearing volume at any time is small compared to the total volume of the pluton. This observation raises doubts about the connections between zoned silicic plutons and large ignimbrite eruptions.

Impact of glaciations on the long-term erosion in Southern Patagonian Andes

NASA Astrophysics Data System (ADS)

Simon-Labric, Thibaud; Herman, Frederic; Baumgartner, Lukas; Shuster, David L.; Braun, Jean; Reiners, Pete W.; Valla, Pierre G.; Leuthold, Julien

2014-05-01

The Southern Patagonian Andes are an ideal setting to study the impact of Late-Cenozoic climate cooling and onset of glaciations impact on the erosional history of mountain belts. The lack of tectonic activity during the last ~12 Myr makes the denudation history mainly controlled by surface processes, not by tectonics. Moreover, the glaciations history of Patagonia shows the best-preserved records within the southern hemisphere (with the exception of Antarctica). Indeed, the dry climate on the leeward side of Patagonia and the presence of lava flows interbedded with glacial deposits has allowed an exceptional preservation of late Cenozoic moraines with precise dating using K-Ar analyses on lava flow. The chronology of moraines reveals a long history covering all the Quaternary, Pliocene, and up to the Upper Miocene. The early growth of large glaciers flowing on eastern foothills started at ~7-6 Myr, while the maximum ice-sheet extent dates from approximately 1.1 Myr. In order to quantify the erosion history of the Southern Patagonian Andes and compare it to the glaciations sediment record, we collected samples along an age-elevation profile for low-temperature thermochronology in the eastern side of the mountain belt (Torres del Paine massif). The (U-Th)/He age-elevation relationship shows a clear convex shape providing an apparent long-term exhumation rate of ~0.2 km/Myr followed by an exhumation rate increase at ~6 Myr. Preliminary results of 4He/3He thermochronometry for a subset of samples complete the erosion history for the Plio-Pleistocene epoch. We used inverse procedure predicting 4He distributions within an apatite grain using a radiation-damage and annealing model to quantify He-diffusion kinetics in apatite. The model also allows quantifying the impact of potential U-Th zonation throughout each apatite crystal. Inversion results reveal a denudation history composed by a pulse of denudation at ~6 Ma, as suggested by the age-elevation relationship, followed by a decrease in denudation rate to very low value (<0.1 km/Myr) and late-stage exhumation phase at ~1 km/Myr for the last ~2 Myr. Our (U-Th)/He and 4He/3He data demonstrate a tight connection between the glaciation history from moraines record and long-term erosion rates derived from low-temperature thermochronology. These results highlight the high sensitivity of the Southern Patagonian Andes to the progressive Late-Cenozoic climate cooling and the strong glacial imprint on erosion history and landscape evolution since the Late Miocene. Indeed, we interpret the observed increase in erosion at ~6 Myr as the landscape response to the onset of the Patagonian ice cap, while the inferred recent increase in erosion rates may reflect the intensification of the climate cooling since the Plio-Pleistocene.

Dating the emergence of the Africa Superswell: a window into mantle processes using combined (U-Th)/He and AFT thermochronology

NASA Astrophysics Data System (ADS)

Dobson, Katherine J.; McDonald, Rhona; Brown, Roderick W.; Gallagher, Kerry; Stuart, Finlay M.

2010-05-01

Southern Africa contains the second largest elevated plateau on Earth, however despite decades of study the evolution of the "African Superswell" remains poorly understood. The mantle anomaly beneath Southern Africa provides a mechanism that can account for both the distribution and the amount of uplift observed, however the timing of uplift cannot be constrained from models of mantle flow because of uncertainties in density and viscosity parameters in the convection models. In order to improve the models of topographic evolution in response to mantle convection, and improve our understanding of the coupling between mantle flow and dynamic topography at the surface we require better quantitative constraints on relatively modest (~1 km) long wavelength surface uplift. Efforts to provide the necessary temporal constraints from geomorphic and stratigraphic evidence in southern Africa have led to the development of three competing evolutionary models: A) the major phase of uplift occurred in the late Cretaceous [1], B) the major phase of uplift occurred at ~30 Ma [2], and C) that ~ 900m of the modern topography being generated rapidly 100m/Ma in the Plio-Pleistocene (c. 3 Ma) [3]. The aim of the current study is to provide better quantitative information in order to distinguish between these models. Apatite fission track thermochronology has been widely used to constrain the onset and evolution of the South African passive margin [e.g. 4, 5], but used alone it is relatively insensitive when trying to resolve the small amounts of uplift predicted for the onset of the African Superswell. Recent advances in the combined interpretation of fission track and (U-Th)/He data sets now enables us to provide preliminary quantitative constraints on the pattern of denudation through the Cenozoic. We present apatite fission track and (U-Th)/He data from a suite of deep boreholes from the high elevation plateau. When integrated with published fission track data and multi-chronometer modelling techniques we can constrain the temporal and spatial distribution of denudation across Southern Africa. Ultimately we aim to constrain both the timing and rate of the emergence of the African Superswell and to provide quantitative constraints on when the first-order topography of Africa was created. References: 1. Nyblade & Sleep, 2003.Geochem Geophys Geosys 4, DOI:1029/2003GC000573 2. Burke & Gunnell, 2008. Geol. Soc. of Am., Memoir 201, pp 66 3. Partridge and Maud, 1987. S Afr J Geol 90, 179-208 4. Brown et al. 2002. J Geophys Res 107, DOI: 101029/2001JB000745 5. Tinker et al. 2008. Tectonophysics, 455, 77-93

Hinterland tectonics and drainage evolution recorded by foreland basin archives: the Neogene Siwaliks of the Himalaya

NASA Astrophysics Data System (ADS)

Huyghe, Pascale; van der Beek, Peter; Matthias, Bernet; Catherine, Chauvel; Jean-Louis, Mugnier; Laurent, Husson; François, Chirouze

2014-05-01

Provenance analysis and detrital thermochronology of detrital synorogenic sediments, derived from erosion of mountain belts and deposited in surrounding sedimentary basins, are well-established methods to examine the exhumation history of convergent zones, tectonic activity and the associated evolution of the drainage network. We have conducted multidisciplinary studies on magnetostratigraphically dated sections throughout the Neogene Siwalik foreland basin of the Himalayan belt since more than 10 years. Sr, Nd and Hf isotopes are used as provenance indicators, providing information on the nature and size of catchment basins and their evolution through time in response to tectonics. Detrital zircon and apatite thermochronology provides constraints on exhumation rates in the hinterland of the Himalaya and the deformation of the Sub-Himalayan foreland basin. Throughout the Himalaya, detrital zircons from the Siwaliks generally show three age peaks: two static peaks (i.e., displaying constant peak ages through time), and a moving peak. The latter shows a constant lag time of ~4 m.y. corresponding to source-area exhumation rates on the order of 1.8 km/my, while the two static peaks respectively reveal a major 15-20 Ma exhumation event in the belt, the significance of which is still debated, and inheritance of pre-Himalayan ages that indicate recycling of Tethyan sediments. Therefore, our ZFT results suggest that the exhumation dynamics are broadly similar throughout the Himalaya since at least 13 m.y, as also shown by the Bengal Fan detrital sediment record. We relate this switch in tectonic regime to the destabilization of the Himalayan wedge that is rendered overcritical as a response to the transience of dynamic topography caused by the deforming underlying Indian slab. Nonetheless, in detail, the timing of thrusting in the Siwalik domain is delayed by about 1 my eastward as demonstrated by both structural and apatite fission-track data, suggesting overall eastward propagation of the main faults. The evolution of the sedimentary provenance can be explained by overall forward propagation of deformation in the Himalayan fold-thrust belt. In both the eastern and western syntaxes, it also shows stability of the major drainage systems of the Yarlung-Brahmaputra and Indus, respectively, suggesting that hinterland river incision kept pace with uplift of the syntaxes during the Neogene. Drainage reorganization may take place in the foreland basin because of thin-skinned tectonics but did not significantly affect sediment routing and the contribution of different sources of the upper catchment to the overall sediment budget. In contrast, major rivers in the Central Himalaya (such as the Kali Gandaki or the Karnali) could have been affected by changes in their upper catchment.

Constraining the near-surface response to lithospheric reorientation: Structural thermochronology along the TRANSALP geophysical transect

NASA Astrophysics Data System (ADS)

Glotzbach, Christoph; Büttner, Lukas; Ehlers, Todd

2017-04-01

Tomographic analyses of the lithosphere structure underneath the Alps suggest a complex geodynamic history (e.g. Lippitsch et al. 2003), indicating, among other things, switches in the direction of subduction. A subduction polarity switch is proposed to have occurred in Miocene times between the Central and Eastern Alps (e.g. Lippitsch et al. 2003; Handy et al. 2015). In the Western and Central Alps SE-directed subduction of European continental lithosphere occurs, whereas NW-directed subduction of Adriatic lithosphere occurs further east (e.g. Kissling et al. 2006). The subducted slab steepens at the transition to the Eastern Alps, roughly at the position of the TRANSALP geophysical profile (S. Germany to N. Italy). This lithospheric reorientation was pre-dated by slab breakoff and also involves the delamination of the lower lithosphere, both processes producing distinct long-wavelength deformation (e.g. Gerya et al. 2004). Thermochronological data can be used to study the surface response to such a long-wavelength deformation. We present new apatite and zircon (U-Th)/He ages of 23 samples collected along 210 km of the TRANSALP profile. The samples were collected along a balanced cross section the TRANSALP profile (e.g. Lüschen et al. 2004) across individual structures that can be tied to deeper, seismically imaged, structures. The thermochronometer ages provide a record of exhumation related to both crustal shortening and post deformation erosional exhumation. Interpretation of the data is in progress and being used to discriminate between competing kinematic/geometric models, and the timing of major fault activity. Variations in exhumation along the section will also unravel the timing and shape of possible long-wavelength rock uplift event(s). References Gerya, T.V., Yuen, D.A., Maresch, W.V. 2004. Thermomechanical modelling of slab detachment. Earth Planet. Sci. Lett. 226, 101-116. Handy, M.R., Ustaszewski, K., Kissling, E. 2015. Reconstructing the Alps-Carpathians-Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion. Int. J. Earth Sci. 104, 1-26. Kissling, E., Schmid, S.M., Lippitsch, R., Ansorge, J., Fügenschuh, B. 2006. Lithosphere structure and tectonic evolution of the Alpine arc: new evidence from high-resolution teleseismic tomography. In: Gee, D.G., Stephenson, R.A. (eds) European Lithosphere Dynamics. Geol. Soc. London Mem. 32, 129-145. Lippitsch, R., Kissling, E., Ansorge, J. 2003. Upper mantle structure beneath the Alpine orogen from high-resolution teleseismic tomography. J. Geophys. Res. 108, 2376, doi:10.1029/2002JB002016. Lüschen, E., Lammerer, B., Gebrande, H., Millahn, K., Nicolich, R., TRANSALP Working Group 2004. Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling. Tectonophysics 388, 85-102.

Structural evolution of the Rio Grande rift: Synchronous exhumation of rift flanks from 20-10 Ma, embryonic core complexes, and fluid-enhanced Quaternary extension

NASA Astrophysics Data System (ADS)

Ricketts, Jason William

The Rio Grande rift in Colorado and New Mexico is one of the well-exposed and well-studied continental rifts in the world. Interest in the rift is driven not only by pure scientific intrigue, but also by a desire and a necessity to quantify earthquake hazards in New Mexico as well as to assess various water related issues throughout the state. These motivating topics have thus far led to the publication of two Geological Society of America Special Publication volumes in 1994 and 2013. This dissertation aims at building on the wealth of previous knowledge about the rift, and is composed of three separate chapters that focus on the structural evolution of the Rio Grande rift at several different time and spatial scales. At the largest scale, apatite (U-Th)/He thermochronologic data suggest synchronous extension along the entire length of the Rio Grande rift in Colorado and New Mexico from 20-10 Ma, which is important for understanding and evaluating possible driving mechanisms which are responsible for the rift. Previous tectonic and magmatic events in western North America were highly influential in the formation of the Rio Grande rift, and the new thermochronologic data suggest that its formation may have been closely linked to foundering and removal of the underlying Farallon Plate. A fundamental result of rift development at these scales is a concentration of strain is some regions of the rift. In these regions of maximum extension, fault networks display a geometry involving both high- and low-angle fault networks. These geometries are similar to the early stages in the development of metamorphic core complexes, and thus these regions in the rift link incipient extensional environments to highly extended terranes. At shorter time scales, heterogeneous strain accumulation may be governed in part by fluids in fault zones. As an example, along the western edge of the Albuquerque basin, travertine deposits are cut by extensional veins that record anomalously high strain rates during the Quaternary at this location. The fluids that precipitated the travertine and calcite in veins also contain a small component of deeply-derived fluids such that surface extension in this part of the rift is coupled with processes at deeper levels. Together, these studies suggest that removal of the Farallon slab beneath Colorado and New Mexico may have been a primary mechanism establishing extension in the Rio Grande rift, while continued extension is heterogeneous in time and space and provides an important link between surface processes and processes that operate at mid-crustal levels.

An integrated study on microtectonics, geothermometry and thermochronology of the Çataldaǧ Core Complex (NW Turkey): Implications for cooling, deformation and uplift history

NASA Astrophysics Data System (ADS)

Kamaci, Omer; Altunkaynak, Safak

2017-04-01

We present an integrated study on structure, microstructure, geothermometry and thermochronology of the Çataldaǧ Core Complex (ÇCC) in NW Turkey in order to understand the cooling, deformation and uplift mechanisms. ÇCC is formed from an Eo-Oligocene granite-gneiss-migmatite complex (GGMC) and an Early Miocene I-type granodioritic body (ÇG: Çataldaǧ granodiorite) which were exhumed as a dome-shaped core complex in the footwall of a ring-shaped low-angle detachment zone (The Çataldaǧ Detachment Fault Zone; ÇDFZ) in the Early Miocene. New U-Pb zircon (LA-ICPMS) and monazite ages of GGMC yielded magmatic ages of 33.8 and 30.1 Ma (Latest Eocene-Early Oligocene). 40Ar/39Ar muscovite, biotite and K-feldspar from the GGMC yielded the deformation age span 21.38±0,05 Ma and 20.81±0.04 Ma, which is also the emplacement age (20.84±0.13 Ma and 21.6±0.04 Ma) of ÇG. ÇDFZ is responsible for mainly top-to-the-north sense kinematic processes. The microstructural features of quartz, feldspar and mica indicate that the ÇCC has undergone continuous deformations during its cooling, from submagmatic to cataclastic conditions. Five microstructural grades have been classified under ductile (DZ) and ductile-to-brittle shear zone (SZ), according to the estimated deformation temperature and intensity of the strain. Microcline twinning, marginally replacement myrmekite and flame-perthite are predominant features for feldspar while chessboard extinction, grain boundary migration and subgrain rotation recrystallization is common for quartz in the DZ which has a deformation temperature range of >600°C to 400°C. Grain size reduction is an important factor for the ductile to brittle shear zone (SZ). Feldspar is represented by bulging recrystallization (BLG), feldspar-fish and domino-type microfracture/microfaulting and quartz show more elongated structures such as ribbons with high aspect ratios. Mineral-fish (muscovite, biotite and feldspar) structures indicate a temperature range of 500°C to <250°C. The GGMC and ÇG, which were formed in different periods, suffered continuous ductile-to-brittle deformation and uplifted together along ÇDFZ during Early Miocene when the first major period of N-S extension began in the western Anatolia. Microstructural grades, two-feldspar geothermometry and geochronological data indicate that the GGMC and ÇG, which were formed in different periods, suffered continuous ductile-to-brittle deformation and uplifted together along ÇDFZ during Early Miocene when the first major period of N-S extension began in the western Anatolia.

Challenges to Progress in Studies of Climate-Tectonic-Erosion Interactions

NASA Astrophysics Data System (ADS)

Burbank, D. W.

2016-12-01

Attempts to unravel the relative importance of climate and tectonics in modulating topography and erosion should compare relevant data sets at comparable temporal and spatial scales. Given that such data are uncommonly available, how can we compare diverse data sets in a robust fashion? Many erosion-rate studies rely on detrital cosmogenic nuclides. What time scales can such data address, and what landscape conditions do they require to provide accurate representations of long-term erosion rates? To what extent do large-scale, but infrequent erosional events impact long-term rates? Commonly, long-term erosion rates are deduced from thermochronologic data. What types of data are needed to test for consistency of rates across a given interval or change in rates through time? Similarly, spatial and temporal variability in precipitation or tectonics requires averaging across appropriate scales. How are such data obtained in deforming mountain belts, and how do we assess their reliability? This study describes the character and temporal duration of key variables that are needed to examine climate-tectonic-erosion interactions, explores the strengths and weaknesses of several study areas, and suggests the types of data requirements that will underpin enlightening "tests" of hypotheses related to the mutual impacts of climate, tectonics, and erosion.

Introduction: CRevolution 2: origin and evolution of the Colorado River System II

USGS Publications Warehouse

Karlstrom, Karl E.; Beard, L. Sue; House, P. Kyle; Young, Richard A.; Aslan, Andres; Billingsley, George; Pederson, Joel

2012-01-01

A 2010 Colorado River symposium held in Flagstaff, Arizona, in May 2010, had 70 participants who engaged in intense debate about the origin and evolution of the Colorado River system. This symposium, built on two previous decadal scientific meetings, focused on forging scientific consensus where possible, while also articulating continued controversies regarding the Cenozoic evolution of the Colorado River System and the landscapes of the Colorado Plateau–Rocky Mountain region that it drains. New developments involved hypotheses that Neogene mantle flow is driving plateau tilting and differential uplift, with consensus that multidisciplinary studies involving differential incision studies and additional geochronology and thermochronology are needed to test the relative importance of tectonic and geomorphic forcings in shaping the spectacular landscapes of the Colorado Plateau region. In addition to the scientific goals, the meeting participants emphasized the iconic status of Grand Canyon for geosciences, and the importance of good communication between the research community, the geoscience education/interpretation community, the public, and the media. Building on a century-long tradition, this region still provides a globally important natural laboratory for studies of the interactions of erosion and tectonism in the shaping landscape of elevated plateaus.

The Pinto shear zone; a Laramide synconvergent extensional shear zone in the Mojave Desert region of the southwestern United States

USGS Publications Warehouse

Wells, M.L.; Beyene, M.A.; Spell, T.L.; Kula, J.L.; Miller, D.M.; Zanetti, K.A.

2005-01-01

The Pinto shear zone is one of several Late Cretaceous shear zones within the eastern fringe of the Mesozoic magmatic arc of the southwest Cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening. We demonstrate an extensional origin for the shear zone by describing the shear-zone geometry and kinematics, hanging wall deformation style, progressive changes in deformation temperature, and differences in hanging wall and footwall thermal histories. Deformation is constrained between ???74 and 68 Ma by 40Ar/39Ar thermochronology of the exhumed footwall, including multi-diffusion domain modeling of K-feldspar. We discount the interpretations, applied in other areas of the Mojave Desert region, that widespread Late Cretaceous cooling results from refrigeration due to subduction of a shallowly dipping Laramide slab or to erosional denudation, and suggest alternatively that post-intrusion cooling and exhumation by extensional structures are recorded. Widespread crustal melting and magmatism followed by extension and cooling in the Late Cretaceous are most consistent with production of a low-viscosity lower crust during anatexis and/or delamination of mantle lithosphere at the onset of Laramide shallow subduction. ?? 2005 Elsevier Ltd. All rights reserved.

Geochronologic evidence for Late Cretaceous and Miocene tectonism in northern New England

NASA Astrophysics Data System (ADS)

Amidon, W. H.; Barr, M.; Walcott, C.; Kylander-Clark, A. R.

2017-12-01

The persistence of mountainous relief in the northeastern U.S. suggests post-rift tectonic rejuvenation has occurred, although specific mechanisms and timing have been difficult to identify. Here we present direct evidence for significant tectonism in New Hampshire and Vermont during the Late-Cretaceous ( 85-65 Ma) and Miocene periods ( 20-5 Ma). Low temperature thermochronology from a drill core in the White Mountains of New Hampshire suggests 2-3 km of accelerated exhumation during the Late Cretaceous. This exhumation is synchronous with compressional thrusting and rapid exhumation on many other Atlantic margins and also with a change in spreading direction in the Atlantic from 85-65 Ma. Recently obtained U-Pb ages of vein calcite from faults and fractures in the Champlain Valley of New York and Vermont suggest significant brittle fracturing occurred during the Late Cretaceous and also during the Miocene. Although many questions remain, this evidence points to tectonic rejuvenation by lateral tectonic stresses in the latest Cretaceous and possibly in the Miocene. The Late Cretaceous seems to have been a particularly significant tectonic episode in northern New England and elsewhere in the circum-Atlantic region.

Exhumation history of the western Kyrgyz Tien Shan: Implications for intramontane basin formation

NASA Astrophysics Data System (ADS)

Bande, Alejandro; Sobel, Edward R.; Mikolaichuk, Alexander; Schmidt, Alexander; Stockli, Daniel F.

2017-01-01

The dextral Talas-Fergana Fault separates the western from the central Tien Shan. Recent work has shed light on the Cenozoic evolution of the eastern and central Tien Shan; much less attention has been paid to the western Tien Shan. In this contribution we present new thermochronological ages for the Fergana and Alai ranges that, combined with the available data set, constrain the Cenozoic exhumation history of the western Tien Shan. Following a tectonically quiet early Cenozoic period, we suggest an onset of exhumation at 25 Ma. This early onset was followed by a period of slower exhumation and in some areas minor reheating. A final, strong late Miocene rapid cooling event is well represented in the western Tien Shan as in other sectors of the range. The early onset of uplift of the western Tien Shan dissected the previously continuous westernmost Parathethyan Sea, progressively isolating basins (e.g., Fergana, Tarim, and Alai basins) in the central Asian hinterland. Moreover, the coeval timing of late Miocene uplift along the length of entire Tien Shan implies that neither the Pamir nor Tarim can be the sole driver for exhumation of the entire range.

Thermal and exhumation history of the central Rwenzori Mountains, Western Rift of the East African Rift System, Uganda

NASA Astrophysics Data System (ADS)

Bauer, F. U.; Glasmacher, U. A.; Ring, U.; Schumann, A.; Nagudi, B.

2010-10-01

The Rwenzori Mountains (Mtns) in west Uganda are the highest rift mountains on Earth and rise to more than 5,000 m. We apply low-temperature thermochronology (apatite fission-track (AFT) and apatite (U-Th-Sm)/He (AHe) analysis) for tracking the cooling history of the Rwenzori Mtns. Samples from the central and northern Rwenzoris reveal AFT ages between 195.0 (±8.4) Ma and 85.3 (±5.3) Ma, and AHe ages between 210.0 (±6.0) Ma to 24.9 (±0.5) Ma. Modelled time-temperature paths reflect a protracted cooling history with accelerated cooling in Permo-Triassic and Jurassic times, followed by a long period of constant and slow cooling, than succeeded by a renewed accelerated cooling in the Neogene. During the last 10 Ma, differentiated erosion and surface uplift affected the Rwenzori Mtns, with more pronounced uplift along the western flank. The final rock uplift of the Rwenzori Mtns that partly led to the formation of the recent topography must have been fast and in the near past (Pliocene to Pleistocene). Erosion could not compensate for the latest rock uplift, resulting in Oligocene to Miocene AHe ages.

Long-term landscape evolution of the Poços de Caldas Plateau revealed by thermokinematic numerical modeling using the software code Pecube, SE- Brazil

NASA Astrophysics Data System (ADS)

Doranti Tiritan, Carolina; Hackspacher, Peter C.; Glasmacher, Ulrich A.

2014-05-01

The Poços de Caldas Plateau in the southeastern Brazil, and it is characterized by a high relief topography supported by the pre-Cambrian crystalline rocks and by the Poços de Caldas Alkaline Massif (PCAM). Ulbrich et al (2002) determine that the ages for the predominant PCAM intermediate rocks were constrained ~83Ma. In addition, geologic observations indicates the phonolites, tinguaites and nepheline syenites were emplaced in a continuous and rapid sequence lasting between 1 to 2 Ma. The topography is characterized by dissected plateau with irregular topographic ridges and peaks with elevations between 900 and 1300m (a.s.l.) on the metamorphic basement and from 1300 to 1700m (a.s.l) on the PCAM region. Therefore, the aim of the work was quantify the main processes that were responsible for the evolution of the landscape by using methods as the low temperature thermochronology and the 3D thermokinematic modeling, for obtaining data of uplift and erosion rates and to correlate them with the thermal gradients of the region. The 3D thermokinematic modeling was obtained using the software code PECUBE (Braun 2003).

From orogenic buildup to extensional unroofing: the evolution of the Adria - Europe collisional zone in the Medvednica Mountains of Croatia

NASA Astrophysics Data System (ADS)

Beniest, Anouk; van Gelder, Inge; Matenco, Liviu; Willingshofer, Ernst; Gruic, Andrea; Tomljenovic, Bruno

2013-04-01

Quantifying the kinematics of the Miocene extension in the Pannonian Basin is of critical importance for understanding the evolution of Adria-Europe collision in particular in the transitional zone from the Alps (Adria the upper plate) to the Dinarides (Adria the lower plate). Recent studies have demonstrated that large-scale extensional unroofing and core-complex formation affected the Europe-Adria contact in the Dinarides during Miocene times. The relationship between this extensional exhumation of Adriatic units and the roughly coeval Miocene extension affecting the Alpine-derived units during their E-ward extrusion into the intra-Carpathians ALCAPA block and the formation of the Pannonian basin is still unknown. One key area situated in the transitional zone is the Medvednica Mountains of Croatia, an area that benefits from already existing and extensive petrological and structural studies. The area of the Medvednica Mountains has been targeted by the means of a field kinematic analysis complemented by low-temperature thermochronology, metamorphic petrology and sedimentological observations. The results demonstrate that two units, reflecting distinct Adriatic paleogeographical positions, make up the structural geometry of the mountains. The upper unit contains Paleozoic mostly fine clastic sequence metamorphosed in sub-greenschist facies, overlain by a proximal Adriatic facies consisting of Triassic shallow water carbonates. The lower unit is made up by a volcanic sequence overlain by gradual deepening Triassic carbonates metamorphosed in greenschist facies that bears a strong resemblance to the Triassic break-up volcanism and subsequent sedimentation affecting the distal Adriatic units observed elsewhere in the Jadar-Kopaonik unit of the Dinarides. The strong contrast between the Middle-Upper Triassic facies suggests large scale thrusting during Cretaceous nappe stacking. Subsequently, the studied area has been affected by significant extensional deformation creating the present-day turtleback geometry. This resulted in the formation of brittle normal faults in both units, locally tilted by the uplift of the mountain core, which indicate mostly NE-SW extension. The lower unit is affected by a pervasive deformation characterized by a wide mylonitic shear zone with stretching lineations indicating consistently top-NE to E sense of shear. The present-day structural geometry of the mountains was established during the Pliocene-Quaternary inversion. The exact ages of nappe-stacking and subsequent extensional exhumation will be clarified by the upcoming low-temperature thermochronology and absolute age dating study. However, available results demonstrate that the extensional geometry and sense of shear is typical for the Miocene extensional exhumation and basin formation that affected the Adria-Europe contact elsewhere in the Dinarids, e.g. Kozara-Prosara-Motajica and Fruska Gora extensional structures. By comparing similar extensional features observed in for instance the Rechnitz and Pohorje extensional structures, the combined study potentially demonstrates that the Miocene mechanism of extension and sense of shear is structurally coherent at the scale of the entire Dinaridic and Alpine margins.

New tools for integrative thermochronology, and their application to the Colombian Eastern Cordillera

NASA Astrophysics Data System (ADS)

Ketcham, R. A.; Mora, A.; Almendral, A.; Parra-Amezquita, M.; Casallas, W.; Robles, W.

2013-12-01

We present two new tools for interpreting thermochronometric data that facilitate the joint use of multiple samples to better constrain thermal history, and demonstrate their utilization in the Colombian Eastern Cordillera. The first, Fetkin, is a finite element solver that takes as input a series of detailed balanced cross sections created using dedicated software such as (2D)Move, and solves the heat flow equation in 2D along with predicted thermochronometric ages which can be compared against measured data. It also performs an independent analysis of the cross sections and flags aspects that are structurally out of balance. It is distinguished from similar tools in 2D and 3D principally by providing a level of detail that allows for investigation of samples in very specific and complex structural contexts, and a workflow that allows the interpreter to engage in successive refinements of the structural model using the inferences provided by thermochronometric data. The second tool is a new set of functionality in HeFTy for inverse modeling of thermochronometric data that allows for simultaneous modeling of samples down a well or borehole. This extension forces attention on issues that have previously been relatively neglected in such modeling, in particular that of multiple provenance. It is axiomatic that mineral grains in different strata may have come from different regions and have different inherited thermal histories. Interpreting such data in a realistic geological context thus requires allowing for different inherited populations within and between samples. The rewards in doing so include more robust modeling and interpretation and, in some cases, insights concerning the unroofing histories of the source rocks that contributed to a given sedimentary unit. Similarly, the mutual constraints imposed by modeling multiple samples with known or constrained depositional and structural context considerably amplifies the resolving power of thermochronometric data. The usefulness of these tools is demonstrated in our studies of the development and unroofing history of the Colombian Eastern Cordillera. Example insights gained using Fetkin include the degree of acceleration of thrust-induced unroofing along the eastern range-bounding faults, and the times at which potential petroleum source rocks were in the oil generation window. Multi-sample modeling in HeFTy results in considerably refined thermal reconstructions, and a possible division between quickly-unroofed and slowly-unroofed apatite populations contributing to different stratigraphic horizons.

Oligocene-Miocene burial and exhumation of the southernmost Gangdese mountains from sedimentary and thermochronological evidence

NASA Astrophysics Data System (ADS)

Ge, Yukui; Li, Yalin; Wang, Xiaonan; Qian, Xinyu; Zhang, Jiawei; Zhou, Aorigele; Liu-Zeng, Jing

2018-01-01

The Kailas conglomerates crop out ubiquitously along the southernmost boundary of the Gangdese batholith. They unconformably overlie the Gangdese batholith and are displaced by the Great Counter thrust (GCT) fault, forming a fault contact with the Xigaze forearc basin, the associated subduction complex and the Tethyan Himalayan sequence. These strata furnish a record of uplift and paleoenvironmental change in the Indus-Yarlung suture zone during the Oligocene-Miocene. Our new and previously published low-temperature thermochronometric data from the Gangdese batholith and the Kailas conglomerates indicate a period of rapid exhumation beginning approximately 17-15 Ma centered on the southern margin of the Gangdese batholith, whereas regional uplift commenced significantly earlier during the deposition of the Kailas conglomerates, based on the presence of an abrupt facies transition from deep-water lacustrine deposits to red alluvial fan or fluvial deposits. The period of rapid exhumation probably lagged behind the initiation of faster uplift, while the related changes in the depositional environment were most likely recorded immediately in the basin stratigraphy. Subsequently, the Kailas conglomerates were buried in association with the development of the north-directed Great Counter thrust, while rapid exhumation was facilitated by efficient incision by the paleo-Yarlung river at approximately 17-15 Ma.

Polyphased Inversions of an Intracontinental Rift: Case Study of the Marrakech High Atlas, Morocco

NASA Astrophysics Data System (ADS)

Leprêtre, R.; Missenard, Y.; Barbarand, J.; Gautheron, C.; Jouvie, I.; Saddiqi, O.

2018-03-01

The High and Middle Atlas intraplate belts in Morocco correspond to Mesozoic rifted basins inverted during the Cenozoic during Africa/Eurasia convergence. The Marrakech High Atlas lies at a key location between Atlantic and Tethyan influences during the Mesozoic rifting phase but represents today high reliefs. Age and style of deformation and the mechanisms underlying the Cenozoic inversion are nevertheless still debated. To solve this issue, we produced new low-temperature thermochronology data (fission track and [U-Th]/He on apatite). Two cross sections were investigated in the western and eastern Marrakech High Atlas. Results of inverse modeling allow recognizing five cooling events attributed to erosion since Early Jurassic. Apart from a first erosional event from Middle/Late Jurassic to Early Cretaceous, four stages can be related to the convergence processes between Africa and Europe since the Late Cretaceous. Our data and thermal modeling results suggest that the inversion processes are guided at first order by the fault network inherited from the rifting episodes. The sedimentary cover and the Neogene lithospheric thinning produced a significant thermal weakening that facilitated the inversion of this ancient rift. Our data show that the Marrakech High Atlas has been behaving as a giant pop-up since the beginning of Cenozoic inversion stages.

Late Permian rivers draining the uplifted Cape Fold Belt: magnetostratigraphy and detrital thermochronology of Karoo Basin sediments

NASA Astrophysics Data System (ADS)

Tohver, E.; Schmieder, M.; Arosio, R.; Lanci, L.; Jourdan, F.; Wilson, A.; Ratcliffe, K.; Payenberg, T.; Flint, S.

2017-12-01

The Cape Fold Belt and Karoo Basin of southern Africa formed during the Permian orogeny that affected the 13,000 km southern margin of the Gondwanan continent. In this report, we synthesize new and recent magnetostratigraphic and geochronologic data to establish a chronostratigraphic framework for Karoo Basin sedimentation for comparison with the thermal/exhumation history of the Cape Fold Belt. The source-sink model is evaluated using new data from detrital muscovite and zircon from 2 km composite section of fluvial sandstone and mudstones deposited at ca.275 - 260 Ma. Coherent age populations of detrital zircon grains indicate rapid incorporation of contemporary volcanic ashbeds into the sedimentary record. In contrast, cooling age distributions of detrital muscovite are typically ca. 5 - 10 Ma older than the age of deposition; similar lag times are observed from modern sediments in active mountain belts. Trace element geochemical signatures demonstrate a clear shift towards crustal recycling via headland erosion in the Beaufort Group relative to the underlying Ecca Group. These observations pinpoint the age of uplift for the Cape Fold Belt, which began to function as the major sediment source for the foreland Karoo Basin with the deposition of the uppermost Ecca Group and basal Beaufort Group.

Mesozoic to Eocene ductile deformation of western Central Iran: From Cimmerian collisional orogeny to Eocene exhumation

NASA Astrophysics Data System (ADS)

Kargaranbafghi, Fariba; Neubauer, Franz; Genser, Johann; Faghih, Ali; Kusky, Timothy

2012-09-01

To advance our understanding of the Mesozoic to Eocene tectonics and kinematics of basement units exposed in the south-western Central Iran plateau, this paper presents new structural and thermochronological data from the Chapedony metamorphic core complex and hangingwall units, particularly from the Posht-e-Badam complex. The overall Paleogene structural characteristics of the area are related to an oblique convergent zone. The Saghand area represents part of a deformation zone between the Arabian and Eurasian plates, and can be interpreted to result from the Central Iran intracontinental deformation acting as a weak zone during Mesozoic to Paleogene times. Field and microstructural evidence reveal that the metamorphic and igneous rocks suffered a ductile shear deformation including mylonitization at the hangingwall boundary of the Eocene Chapedony metamorphic core complex. Comparison of deformation features in the mylonites and other structural features within the footwall unit leads to the conclusion that the mylonites were formed in a subhorizontal shear zone by NE-SW stretching during Middle to Late Eocene extensional tectonics. The Chapedony metamorphic core complex is characterized by amphibolite-facies metamorphism and development of S and S-L tectonic fabrics. The Posht-e-Badam complex was deformed by two stages during Cimmerian tectonic processes forming the Paleo-Tethyan suture.

Cenozoic episodic uplift and kinematic evolution between the Pamir and Southwestern Tien Shan

NASA Astrophysics Data System (ADS)

Liu, Dongliang; Li, Haibing; Sun, Zhiming; Cao, Yong; Wang, Leizhen; Pan, Jiawei; Han, Liang; Ye, Xiaozhou

2017-08-01

The Pamir Salient and Southwestern Tien Shan belong to two different systems, which collided due to the continuous northward drift of the Indian Plate during the Cenozoic, resulting in a shortening of 300 km. The uplift history and kinematic evolution of the Pamir-Southwestern Tien Shan remain unclear. In this study, we chose the 2025 m-thick Pakabulake formation in the East Wuqia section, at the southern-most margin of the Southwestern Tien Shan system, to obtain a high-resolution magnetostratigraphic record spanning 16.61 Ma to 9.78 Ma. Based on its high sedimentation rate, stable ca. E-W paleocurrents and stable magnetic susceptibility values, the nearby Southwestern Tien Shan was inferred to have undergone stable uplift during this period of sedimentation. Combining our results with the previous low-temperature thermochronology, magnetostratigraphy and re-calculated block rotations, we conclude that four episodic uplift events occurred in the Pamir-Southwestern Tien Shan during the Cenozoic, at times of 50-40 Ma, 35-16 Ma, 11-7 Ma and < 5 Ma, and that the first episodic uplift only occurred in the Pamir Salient. In addition, the Pamir Salient underwent a tectonic transformation from entire- to a half-oroclinal bending rotation during the Miocene, caused by activity along the Karakorum Fault and Kashgar-Yecheng Transfer System.

Prospects for Practical Laser Ablation U/Pb and (U-Th)/He Double-Dating (LADD) of Detrital Apatite

NASA Astrophysics Data System (ADS)

Horne, A.; Hodges, K. V.; Van Soest, M. C.

2017-12-01

A laser ablation micro-analytical technique for (U-Th)/He dating has been shown to be an effective approach to the thermochronologic study of detrital zircons (Tripathy-Lang et al., J. Geophys. Res., 2013), while Evans et al. (J. Anal. At. Spectrom., 2015) and Horne et al. (Geochim. Cosmochim. Acta, 2016) demonstrated how the technique could be modified to enable laser ablation U/Pb and (U-Th)/He double-dating (LADD) of detrital zircon and titanite. These successes beg the question of whether or not LADD is viable for another commonly encountered detrital mineral: apatite. Exploratory LADD studies in Arizona State University's Group 18 Laboratories - using Durango fluorapatite, apatite from the Fish Canyon tuff, and detrital apatite from modern fluvial sediments in the eastern Sierra Nevada of California - illustrate that the method is indeed viable for detrital apatite. However, the method may not be appropriate for all detrital samples. For example, many apatite grains encountered in detrital samples from young orogenic settings have low concentrations of U and Th and small crystal sizes. This can lead to imprecise laser ablation (U-Th)/He dates, especially for very young grains potentially obscuring or inhibiting relevant interpretations of the data set.

Using slope-area and apatite fission track analysis to decipher the rock uplift pattern of the Yumu Shan: New insights into the growth of the NE Tibetan Plateau

NASA Astrophysics Data System (ADS)

Wang, Yizhou; Zheng, Dewen; Pang, Jianzhang; Zhang, Huiping; Wang, Weitao; Yu, Jingxing; Zhang, Zhuqi; Zheng, Wenjun; Zhang, Peizhen; Li, Youjuan

2018-05-01

Studies have shown that the growth of the Qilian Shan, the northeastern margin of the Tibetan Plateau, started 10 Ma ago. However, when and how it expanded northwards is still under debate. Here we focus on the rock uplift pattern of the Yumu Shan, an active fault-related fold in the Hexi Corridor north to the Qilian Shan. Normalized channel steepness achieved from the analysis of river longitudinal profiles shows a spatially variant rock uplift pattern, with higher rates in the middle part and lower rates towards the west and east tips. The compression of the mountain is typically accommodated by fault-fold related shortening and vertical thickening. Apatite fission track thermochronology reveals that the growth of the Yumu Shan started 4 Ma ago, similar to the work on active tectonics. Combining the onset ages of the growth of the Qilian Shan (10 Ma), Laojunmiao anticline (3-4 Ma), Baiyanghe anticline (3-4 Ma), Wenshu Shan (4.5 Ma) and Heli Shan (2 Ma), we draw an conclusion that the NE margin of the Tibetan Plateau initiated growth in the mid-Miocene and expanded to the Hexi Corridor and to the south of the Alxa block in the early Pleistocene.

Spatial and temporal uplift history of South America from calibrated drainage analysis

NASA Astrophysics Data System (ADS)

Rodríguez Tribaldos, V.; White, N. J.; Roberts, G. G.; Hoggard, M. J.

2017-06-01

A multidisciplinary approach is used to analyze the Cenozoic uplift history of South America. Residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present-day dynamic topography. Admittance analysis and crustal thickness measurements indicate that the elastic thickness of the Borborema and Altiplano regions is ≤10 km with evidence for sub-plate support at longer wavelengths. A drainage inventory of 1827 river profiles is assembled and used to investigate landscape development. Linear inverse modeling enables river profiles to be fitted as a function of the spatial and temporal history of regional uplift. Erosional parameters are calibrated using observations from the Borborema Plateau and tested against continent-wide stratigraphic and thermochronologic constraints. Our results predict that two phases of regional uplift of the Altiplano plateau occurred in Neogene times. Regional uplift of the southern Patagonian Andes also appears to have occurred in Early Miocene times. The consistency between observed and predicted histories for the Borborema, Altiplano, and Patagonian plateaux implies that drainage networks record coherent signals that are amenable to simple modeling strategies. Finally, the predicted pattern of incision across the Amazon catchment constrains solid sedimentary flux at the Foz do Amazonas. Observed and calculated flux estimates match, suggesting that erosion and deposition were triggered by regional Andean uplift during Miocene times.

Role of erosion and isostasy in the Cordillera Blanca uplift: Insights from landscape evolution modeling (northern Peru, Andes)

NASA Astrophysics Data System (ADS)

Margirier, Audrey; Braun, Jean; Robert, Xavier; Audin, Laurence

2018-03-01

The processes driving uplift and exhumation of the highest Peruvian peaks (the Cordillera Blanca) are not well understood. Uplift and exhumation seem closely linked to the formation and movement on the Cordillera Blanca normal fault (CBNF) that delimits and shapes the western flank of the Cordillera Blanca. Several models have been proposed to explain the presence of this major normal fault in a compressional setting, but the CBNF and the Cordillera Blanca recent rapid uplift remain enigmatic. Whereas the Cordillera Blanca morphology demonstrates important erosion and thus a significant mass of rocks removal, the impact of erosion and isostasy on the evolution of the Cordillera Blanca uplift rates has never been explored. We address the role of erosion and associated flexural rebound in the uplift and exhumation of the Cordillera Blanca with numerical modeling of landscape evolution. We perform inversions of the broad features of the present-day topography, total exhumation and thermochronological data using a landscape evolution model (FastScape) to provide constraints on the erosion efficiency factor, the uplift rate and the temperature gradient. Our results evidence the not negligible contribution of erosion and associated flexural rebound to the uplift of the Cordillera Blanca and allow us to question the models previously proposed for the formation of the CBNF.

The Taili-Yiwulüshan metamorphic core complex corridor: Diachronous exhumation and relationships to the adjacent basins based on new 40Ar/39Ar and (U-Th-Sm)/He mineral ages

NASA Astrophysics Data System (ADS)

Liang, Chenyue; Neubauer, Franz; Liu, Yongjiang; Genser, Johann; Dunkl, István; Heberer, Bianca; Jin, Wei; Zeng, Zuoxun; Li, Weimin; Wen, Quanbo; Li, Jing

2015-04-01

The Xingcheng-Taili ductile shear zone (western Liaoning Province in China) formed during latest Jurassic to Early Cretaceous crustal extension of the eastern North China craton, and exhumed low to medium metamorphic grade Archean, Upper Triassic and Upper Jurassic granitic rocks. The Mesozoic Yiwulüshan metamorphic core complex (Yiwulüshan MCC) is dominated by a NNE-SSW elongated dome with a left-lateral shear zone, which is located in the northeastern part of Xingcheng-Taili ductile shear zone, and combine as Taili-Yiwulüshan metamorphic core complex corridor. To the east, it is bounded by the NNE-trending Cretaceous to Eocene Liaohe basin (the northern extension of the Bohai Bay basin), and to the west by the Cretaceous-aged Fuxin-Yixian basin, which could potentially interpreted as supra-detachment basins. Here, we present results from a multi-method thermochronological study and coupled with structural investigations and sections of adjacent supra-detachment basins, which constrain the timing of regional deformation as well as the cooling history and exhumation processes of the low- to middle-grade metamorphic complex in the Taili-Yiwulüshan MCC corridor, in order to understand the mode of lithospheric scale reactivation, extension and thinning of the North China craton. The new40Ar/39Ar muscovite, biotite, K-feldspar and (U-Th)/He apatite ages from granitic rocks help constrain the thermal evolution during its exhumation. The thermochronologic studies have shown at least three stages of exhumation and cooling from late Jurassic to Eocene in Xingcheng-Taili shear zone should be distinguished, e.g., ~ 150-130 Ma, 130-115 Ma and 115-52 Ma, respectively. Diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history. The Yiwulüshan MCC has a similar exhumation history from 135 to 97 Ma with a similar cooling history. The development of Taili-Yiwulüshan MCC corridor is associated with synkinematic emplacement, exhumation, and volcanic-clastic deposition in the supra-detachment basins. Initiation of the unroofing history resulted from ductile left-lateral shearing since latest Jurassic times. Diachronous onset and subsequent cooling and exhumation characterize the early thermal history. The second and third stages of cooling started lasted until the recently active faulting. Start form the Early Cretaceous the detachment shear zone truncating by the later brittle normal fault. The (U-Th)/He age of 52.3 ± 4.7 Ma indicating final Eocene exhumation of the Taili area is consistent with normal faulting in the Bohai basin area in the east. Based on the present results and published information, that Cretaceous WNW-ESE extensional deformation and lithosphere thinning in the Taili-Yiwulüshan corridor and throughout the eastern North China craton, the synchroneity of cooling and exhumation of metamorphic core complexes, the formation of supra-detachment basins, and regional alkaline igneous activity reflects Early Cretaceous regional extensional tectonics , possibly resulting from roll-back of the subducted Pacific plate beneath North China Craton.

Thermo-tectonic history reconstruction of the Rwenzori Mountains, D. R. Congo

NASA Astrophysics Data System (ADS)

Mansour, Sherif; Bauer, Friederike; Glasmacher, Ulrich; Grobe, René; Starz, Matthias

2015-04-01

The Albertine Rift forms the northern section of the western Rift of the East African Rift System (EARS) which was developed during the Neogene time. The Rwenzori Mtns evolved along the eastern rift shoulder of the Albertine Rift, rising up to form a striking feature within the rift valley. The scarcity of volcanic activity in the Western Rift has raised questions about the Rwenzori Mtns origin and evolution and how this fits into the general evolution of the Albertine Rift and the EARS. The Rwenzori Mtns represent the horst block in the Albertine Rift, with elevations reaching 5109 m a.s.l. (Margherita Peak). The main lithologies of the Rwenzori Mtns are gneiss, schist and amphibolite, subordinate intrusive rocks with various metamorphic overprint, and quartzite of Precambrian age (Tanner, 1971). All these units are intensively truncated by N-S, NW-SE, NE-SW and E-W trending normal faults, locally with a significant strike-slip component (Ring, 2008). The slope Rwenzori Mtns western flank is much steeper than the eastern one. This asymmetry is most striking in the central part where the western flank rises from about 1000 m a.s.l. to more than 5000 m a.s.l. in less than 15 km while, the eastern flank plunges to 1000 m a.s.l. again along a distance of more than 30 km (Bauer et al., 2013). Detailed thermochronologic study on the eastern (Uganda) side of central Rwenzori differentiated it into northern and southern blocks (Bauer et al., 2013). Samples from the northern block cooled faster, with three cooling/exhumation events; to ~120 °C in Carboniferous to Permian times, to ~70 °C in Mesozoic times, and to surface temperature in the Neogene. While, the southern block shows an earlier onset of cooling/exhumation events (>400 Ma). On the other hand, 33 samples from the western (Congo) side of central Rwenzori were studied using fission track and (U/Th)-He techniques. The apatite fission track data gives much younger cooling ages. Which, distinguished in three cooling age groups with; Eocene, Oligo-Miocene, and Middle Miocene ages. These results reveal the difference in thermo-tectonic history between the eastern and western flanks of Rwenzori Mtns and support the tilt uplift geometry hypotheses (e.g. Pickford et al., 1993). References Bauer, F.U., Glasmacher, U.A., Ring, U., Karl, M., Schumann, A., Nagudi, B. (2013). Tracing the exhumation history of the Rwenzori Mountains, Albertine Rift, Uganda, using low-temperature thermochronology, Tectonophysics, 599, 8-28. http://dx.doi.org/10.1016/j.tecto.2013.03.032. Pickford, M., Senut, B., Hadoto, D., 1993. Geology and Palaeobiology of the Albertine Rift Valley Uganda-Zaire, vol. 1. Geology. CIFEG Occas, Orleans. Publication, vol. 24, pp. 1-190. Ring, U., 2008. Extreme uplift of the Rwenzori Mountains in the East African Rift, Uganda: structural framework and possible role of glaciations. Tectonics 27 (TC4018). http://dx.doi.org/10.1029/2007TC002176. Tanner, P.W.G., 1971. The Stanley Volcanics formation of Ruwenzori, Uganda. Fifteenth Annual Report of the Research Institute of African Geology. University of Leeds.

Fission track length distributions in multi-system thermochronology (Invited)

NASA Astrophysics Data System (ADS)

Gleadow, A. J.; Seiler, C.

2013-12-01

Fission track length distributions contain a unique record of past temperature variations and therefore play a key role in low-temperature thermochronology, for which there is no exact equivalent in any other method. Confined track lengths closely approximate the true etchable ranges of latent fission tracks [1] and are therefore favoured for fission track studies, but they still have a number of practical limitations. These include small numbers of suitable tracks, especially when only horizontal confined tracks are measured. Using only track-in-track events for measurement further limits the sample size. These restrictions become acute for low track-density samples, where length measurements may be impossible. Irradiating the surface with 252Cf tracks [2] can substantially increase the number of confined tracks, but many researchers do not have access to a Cf source. An even more significant issue has emerged from inter-laboratory comparison experiments that demonstrate a disturbingly poor reproducibility of length measurements between observers [3], a problem compounded by a lack of standardisation in measurement techniques. As a result, individual observers may measure different positions for the end of a track, contributing significantly to variability, and consequently blurring the thermal histories obtained. New digital microscopes open up important opportunities for improved track length measurements by reducing restrictions on sample size, and eliminating some sources of inter-observer bias. We have developed a track length measurement system that enables precise determination of vertical as well as horizontal track dimensions, allowing 3D lengths to be obtained. Lengths are measured on captured image stacks that can be analysed easily and may also be shared, for greater standardisation between laboratories. Length measurements are highly reproducible between different observers using this system, suggesting that at least one source of variability can be eliminated. The selection of lengths for imaging, however, still remains a source of potential bias between observers. The new measurement system also enables measurement of 3D lengths of surface-intersecting ';semi-tracks', the distributions of which have been well understood theoretically [1,4], but have not been used in practice because of difficulties of measuring vertical dimensions on older microscopes. Semi-track lengths are, of course, a degraded measure compared to confined tracks because they are randomly truncated. However, this is more than compensated by their very much greater abundance, by a factor of >60, compared to confined tracks. They are also more amenable to semi- or fully-automated measurement techniques than confined tracks. Moreover the distribution characteristics of semi-track lengths relative to confined track lengths are well understood so that in principle the two types could be used together in modelling thermal histories. The implementation of these new approaches for track length measurement should significantly improve the precision and standardisation of track length measurements at every stage of their utilisation, from annealing studies to thermal history modelling of unknowns. [1] Galbraith (2003) Statistics for FT Analysis, Chapman & Hall [2] Donelick et al. (2005) Rev Min Geochem 58, 49-94 [3] Ketcham et al. (2009) Ear Planet Sci Lett 284, 504-515 [4] Jonckheere & Van den haute (1999) Rad Meas 30, 155-179

Contrasting cooling and exhumation histories of tectonic blocks in the eastern part of the Eastern Alps and its possible relationships with lithospheric dynamics

NASA Astrophysics Data System (ADS)

Wölfler, Andreas; Glotzbach, Christoph

2017-04-01

In the European Alps two fundamental changes in the lithospheric structure controlled the geodynamic evolution of the orogen: (1) Late Eocene to Early Oligocene rupturing of the European slab and (2) Early Miocene change in subduction polarity between the Western and Eastern Alps. In particular, the eastern part of the Eastern Alps is a key area which will provide both, information about the surface response to the Eocene/Oligocene slab breakoff and the response to a proposed isostatic rebound due to slab detachment of the European lithosphere in the Miocene (Handy et al., 2015). The eastern part of the Eastern Alps is characterized by fault-bounded blocks with different thermochronological ages, mean elevations and hillslope angles, indicating a variable spatial and temporal evolution throughout the Cenozoic era. The topographic pattern of the study area can be described in terms of (1) high relief with a rugged surface and steep slopes in the Niedere Tauern and (2) lower relief and distinctly smoother topography in the Seckauer Tauern, the Gurktal, Saualpe and Koralpe blocks. Generally, tectonic blocks with higher relief and rugged topography display the youngest apatite fission track (AFT) and apatite (U-Th)/He data (AHe); the Niedere Tauern concurs with middle Miocene AFT and late Miocene/Pliocene AHe ages (Wölfler et al., 2016). The other tectonic blocks generally show Eocene to Oligocene AFT ages (Wölfler et al., 2016 and references therein) and Oligocene to early Miocene AHe ages (Legrain et al., 2014, own preliminary work). New AFT and AHe data from the Gurktal block and Seckauer Tauern indicate that substantial parts of the study area where at near-surface conditions since the Eocene and Oligocene. Thermal history models suggest fast cooling through the AFT and AHe closure temperatures in the Oligocene as a possible response to slab breakoff that was followed by thermal stagnation until the present. In contrast, the neighboring Niedere Tauern experienced enhanced cooling and exhumation in the middle Miocene and again at the late Miocene and Pliocene (Wölfler et al., 2016). Middle Miocene exhumation is interpreted as a result of tectonic escape and convergence that operated simultaneously during lateral extrusion of the Eastern Alps. As the higher late Miocene/Pliocene exhumation rates are restricted to a single tectonic block, namely the Niedere Tauern, we infer a tectonic trigger that is probably related to a change in the external stress field that affected the Alps during this time. References Legrain, N., Stüwe, K., Wölfler, A., 2014. Incised relict landscape in the eastern Alps. Geomorphology 221, 124-138. Handy, M.R., Ustaszewski, K., Kissling, E., 2015. Reconstructing the Alps-Carpathians-Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion. International Journal of Earth Sciences 104, 1-26. Wölfler, A., Kurz, W., Fritz, H., Glotzbach, C., Danišik, M., 2016. Late Miocene increasing exhumation rates in the eastern part of the Alps - implications from low temperature thermochronology. Terra Nova 28, 297-305.

Experience from a (U-Th)/He thermochronology CAREER grant

NASA Astrophysics Data System (ADS)

Reiners, P. W.

2011-12-01

An accomplished demigod of geosciences once offered advice on strategizing a research career: "Do a combination of things: something conventional and safe, something better than anyone else, and something weird and risky." Balancing precious effort along these lines may be an ideal strategy, but reality often makes it difficult at best. One of the greatest benefits of a CAREER grant is the opportunity to implement a strategy integrating a stable base of different approaches like this, especially the weird and risky. My CAREER grant started with realization that the then resurgent radiogenic He dating technique had potential in an enormous range of problems outside traditional cookie-cutter approaches. Exploring applications ranging well beyond conventional tectonic and detrital studies, to meteorite histories, magmatic ascent rates, biogenic phosphates, wildfire, and multi-method dating may not have revolutionized geosciences, but some of the unique insights that resulted are both conceptually and practically valuable and could not have been achieved through traditional funding avenues. Moreover, weird and risky adaptations, in this case of a versatile geochronologic tool, are well suited for student involvement as student side projects, which are scientifically and intellectually nourishing for both student and faculty. Some of these relatively short term explorations (e.g., dating turquoise, travertine, cassiterite, Neolithic pottery, pseudotachylite, mammoth teeth, fault gouge, impact craters, etc.) end up not leading far, or pointing to a route that requires much more work to explore fully. But there is value in cliffed-out side canyons, not the least of which is showing students that it is ok to eschew formulaic karaoke geology. The strategic career advice above accommodates outreach/education. In my experience, the more organic the synthesis between outreach and research, the more naturally functional, rewarding, and sustainable the efforts. One way to incorporate outreach is simply to open one's lab to the community to provide basic but valuable analyses. In my case this was routine (U-Th)/He dating for tectonic and geomorphic studies. This requires significant commitment to training and analytical reliability, but also provides broad scientific enrichment to the lab. In addition to a huge number of rocks-in-the-mailbox users, during the grant our lab hosted >50 visiting users from >35 institutions, generating thousands of analyses for outside users alone. But simply opening one's lab is not particularly distinctive outreach. A more important and consuming initiative was our summer workshops on thermochronology. These featured teaching and a fieldtrip but, most importantly, the chance for 8 or 9 visiting students to analyze as many (U-Th)/He (and recently, with help from colleagues, U/Pb and FT) dates as they could cram into a two-week period, interpret them, and present their results. The high-intensity workshops generated large amounts of data, many meeting abstracts, a few papers, valuable collaborations with advisors, and rewarding experiences with diverse students. They require a lot of time and effort, including stress of insuring continuous smooth analytical performance. One nugget of experience from these is that they are more rewarding when centered on a theme (e.g., Antarctichron 2011).

Thermochronometry using red TL of quartz - a feasibility study from in-situ drill-hole samples

NASA Astrophysics Data System (ADS)

Schmidt, Christoph; Zöller, Ludwig

2015-04-01

Thermochronometry - the revelation of the temperature history of rock related to subsidence or uplift and erosion - relies on methods with closure temperatures >40-70 °C, such as (U-Th-Sm)/He or fission track analysis on apatite. These methods are applicable to young and tectonically active mountain ranges, but results of calculated mean denudation rates are too imprecise for older orogens. Several studies attested the quartz luminescence signal (325 °C TL peak, OSL fast component) isothermal decay at ambient temperatures as low as 56 °C (Prokein and Wagner, 1994; Herman et al., 2010; de Sarkar et al., 2013). The so far determined closure temperatures of the quartz luminescence thermochronometry system vary between ~35 °C for the OSL fast component (Herman et al., 2010) and ~70 °C for red thermoluminescence (RTL; Tsuchiya and Fujino, 2000) and are dependent on the cooling rate and the charge trap parameters. Although featuring a favourably low closure temperature - thus allowing to study the geologically most recent temperature history -, especially quartz OSL suffers from low dose saturation, limiting the application to highly erosive orogens. Saturation doses of RTL exceed those of OSL by a factor of 10 or more (Fattahi and Stokes, 2000), what opens up new perspectives in low-temperature thermochronometry. We here present experimental results on the general suitability of RTL for thermochronometry, obtained for samples from a drilling hole in the granitic basement of the Variscan Fichtelgebirge (Franconia, Germany). The samples allowed studying the RTL signal saturation level in-situ at different ambient temperatures up to ~55 °C (at 1831 m depth). Measurements confirmed depletion of the 325 °C RTL peak for ambient temperatures >25 °C, most probably for even lower temperatures. Irradiation experiments showed that the RTL signal is not in saturation for ambient temperatures >25 °C, even for this 'old' mountain range. We could further demonstrate that the luminescence sensitivity of samples strongly increases with rising ambient temperature, opposite to the findings of Ypma and Hochman (1991) for samples from sedimentary basins. References Fattahi, M., Stokes, S., 2000. Extending the time range of luminescence dating using red TL (RTL) from volcanic quartz. Radiation Measurements 32, 479-485. Herman, F., Rhodes, E.J., Braun, J., Heiniger, L., 2010. Uniform erosion rates and relief amplitude during glacial cycles in the Southern Alps of New Zealand, as revealed from OSL-thermochronology. Earth and Planetary Science Letters 297, 183-189. Prokein, J., Wagner, G.A., 1994. Analysis of thermoluminescent glow peaks in quartz derived from the KTB-drill hole. Radiation Measurements 23, 85-94. de Sarkar, S., Mathew, G., Pande, K., Chauhan, N., Singhvi, A.K., 2013. Rapid denudation of Higher Himalaya during late Pleistocence, evidence from OSL thermochronology. Geochronometria 40, 304-310. Tsuchiya, N., Fujino, K., 2000. Evaluation of cooling history of the Quaternary Takidani pluton using thermoluminescence technique. Proceedings World Geothermal Congress, Kyushu-Tohoku, Japan. Ypma, P.J., Hochman, M.B., 1991. Thermoluminescence geothermometry - a case study of the Otway basin. APEA Journal, 312-324.

Analysis of Antarctic glacigenic sediment provenance through geochemical and petrologic applications

NASA Astrophysics Data System (ADS)

Licht, Kathy J.; Hemming, Sidney R.

2017-05-01

The number of provenance studies of glacigenic sediments in Antarctica has increased dramatically over the past decade, providing an enhanced understanding of ice sheet history and dynamics, along with the broader geologic history. Such data have been used to assess glacial erosion patterns at the catchment scale, flow path reconstructions over a wide range of scales, and ice sheet fluctuations indicated by iceberg rafted debris in circumantarctic glacial marine sediments. It is notable that even though most of the bedrock of the continent is ice covered and inaccessible, provenance data can provide such valuable information about Antarctic ice and can even be used to infer buried rock types along with their geo- and thermochronologic history. Glacigenic sediments provide a broader array of provenance analysis opportunities than any other sediment type because of their wide range of grain sizes, and in this paper we review methods and examples from all size fractions that have been applied to the Antarctic glacigenic sedimentary record. Interpretations of these records must take careful consideration of the choice of analytical methods, uneven patterns of erosion, and spatial variability in sediment transport and rock types, which all may lead to a preferential identification of different elements of sources in the provenance analyses. Because of this, we advocate a multi-proxy approach and highlight studies that demonstrate the value of selecting complementary provenance methods.

Thermal history of a metamorphic core complex

NASA Astrophysics Data System (ADS)

Dokka, R. K.; Mahaffie, M. J.; Snoke, A. W.

Fission track (FT) thermochronology studies of lower plate rocks of the Ruby Mountains-East Humbolt Range metamorphic core complex provide important constraints on the timing an nature of major middle Tertiary extension of northeast Nevada. Rocks analyzed include several varieties of mylonitic orthogneiss as well as amphibolitic orthognesses from the non-mylonitic infrastructural core. Oligocene-age porphyritic biotite granodiorite of the Harrison Pass pluton was also studied. The minerals dated include apatite, zircon, and sphene and were obtained from the same rocks that have been previously studied. FT ages are concordant and range in age from 26.4 Ma to 23.8 Ma, with all showing overlap at 1 sigma between 25.4 to 23.4 Ma. Concordancy of all FT ages from all structural levels indicates that the lower plate cooled rapidly from temperatures above approx. 285 C (assumed sphene closure temperature (2)) to below approx. 150 C (assumed apatite closure temperature) near the beginning of the Miocene. This suggests that the lower plate cooled at a rate of at least approx. 36 deg C/Ma during this event. Rapid cooling of the region is considered to reflect large-scale tectonic denudation (intracrustal thinning), the vertical complement to intense crustal extension. FT data firmly establish the upper limit on the timing of mylonitization during detachment faulting and also coincide with the age of extensive landscape disruption.

Differential exhumation at eastern margin of the Tibetan Plateau, from apatite fission-track thermochronology

NASA Astrophysics Data System (ADS)

Deng, Bin; Liu, Shu-gen; Li, Zhi-wu; Jansa, Luba F.; Liu, Shun; Wang, Guo-zhi; Sun, Wei

2013-04-01

New apatite fission-track (AFT) ages from Mesozoic sediments in the Sichuan basin, combined with previous fission-track data, demonstrate differential uplift and exhumation across the basin. Particularly significant change in exhumation (at least ~ 2000 m) was found across the Huaying Mts. Modeled temperature-time histories and the Boomerang plot of AFT dataset across the basin suggest rapid cooling and exhumation events during 120-80 Ma and at 20-10 Ma. They reflect the start of the basin-scale differential uplift and exhumation which effected the eastern growth of Tibetan Plateau. In particular, nested old-age center separated by Huaying Mts. was found in the center-to-northwest part of the Sichuan basin. A simplified one-dimensional, steady-state solution model was developed to calculate the mean exhumation rate, which is 0.05-0.2 mm/yr in most parts of the basin. It suggests a slow exhumation across much of the basin. The regional pattern of AFT age, length and erosion rate supports a progressive change from the nested old-age center towards the southwest. This pattern supports the idea of a prolonged, steady-state uplift and exhumation process across the basin, controlled by cratonic basin structure. The eastern growth of the Tibetan Plateau has exerted a significant effect on the rapid exhumation of the southwestern part of the Sichuan basin, but not on all of the basin during the Late Cenozoic.

The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

NASA Astrophysics Data System (ADS)

Cluzel, Dominique; Jourdan, Fred; Meffre, SéBastien; Maurizot, Pierre; Lesimple, StéPhane

2012-06-01

Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende (40Ar/39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ˜56 Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ˜0.5 GPa at ˜800-950°C. The thermal gradient inferred from the metamorphic assemblage (˜60°C km-1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ˜53 Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ˜56 Ma.

CRevolution 2—Origin and evolution of the Colorado River system, workshop abstracts

USGS Publications Warehouse

Beard, L. Sue; Karlstrom, Karl E.; Young, Richard A.; Billingsley, George H.

2011-01-01

A 2010 Colorado River symposium, held in Flagstaff, Arizona, involved 70 participants who engaged in intense debate about the origin and evolution of the Colorado River system. This symposium, built upon two previous decadal scientific meetings, focused on forging scientific consensus, where possible, while articulating continued controversies regarding the Cenozoic evolution of the Colorado River System and the landscapes of the Colorado Plateau-Rocky Mountain region that it drains. New developments involved hypotheses that Neogene mantle flow is driving plateau tilting and differential uplift and new and controversial hypotheses for the pre-6 Ma presence and evolution of ancestral rivers that may be important in the history and birth of the present Colorado River. There is a consensus that plateau tilt and uplift models must be tested with multidisciplinary studies involving differential incision studies and additional geochronology and thermochronology to determine the relative importance of tectonic and geomorphic forces that shape the spectacular landscapes of the Colorado Plateau, Arizona and region. In addition to the scientific goals, the meeting participants emphasized the iconic status of Grand Canyon for geosciences and the importance of good communication between the research community, the geoscience education/interpretation community, the public, and the media. Building on a century-long tradition, this region still provides a globally important natural laboratory for studies of the interactions of erosion and tectonism in shaping the landscape of elevated plateaus.

Thermal history of a metamorphic core complex

NASA Technical Reports Server (NTRS)

Dokka, R. K.; Mahaffie, M. J.; Snoke, A. W.

1985-01-01

Fission track (FT) thermochronology studies of lower plate rocks of the Ruby Mountains-East Humbolt Range metamorphic core complex provide important constraints on the timing an nature of major middle Tertiary extension of northeast Nevada. Rocks analyzed include several varieties of mylonitic orthogneiss as well as amphibolitic orthognesses from the non-mylonitic infrastructural core. Oligocene-age porphyritic biotite granodiorite of the Harrison Pass pluton was also studied. The minerals dated include apatite, zircon, and sphene and were obtained from the same rocks that have been previously studied. FT ages are concordant and range in age from 26.4 Ma to 23.8 Ma, with all showing overlap at 1 sigma between 25.4 to 23.4 Ma. Concordancy of all FT ages from all structural levels indicates that the lower plate cooled rapidly from temperatures above approx. 285 C (assumed sphene closure temperature (2)) to below approx. 150 C (assumed apatite closure temperature) near the beginning of the Miocene. This suggests that the lower plate cooled at a rate of at least approx. 36 deg C/Ma during this event. Rapid cooling of the region is considered to reflect large-scale tectonic denudation (intracrustal thinning), the vertical complement to intense crustal extension. FT data firmly establish the upper limit on the timing of mylonitization during detachment faulting and also coincide with the age of extensive landscape disruption.

Synchroneity of cratonic burial phases and gaps in the kimberlite record: Episodic magmatism or preservational bias?

NASA Astrophysics Data System (ADS)

Ault, Alexis K.; Flowers, Rebecca M.; Bowring, Samuel A.

2015-01-01

A variety of models are used to explain an apparent episodicity in kimberlite emplacement. Implicit in these models is the assumption that the preserved kimberlite record is largely complete. However, some cratons now mostly devoid of Phanerozoic cover underwent substantial Phanerozoic burial and erosion episodes that should be considered when evaluating models for global kimberlite distributions. Here we show a broad temporal coincidence between regional burial phases inferred from thermochronology and gaps in the kimberlite record in the Slave craton, Superior craton, and cratonic western Australia. A similar pattern exists in the Kaapvaal craton, although its magmatic, deposition, and erosion history differs in key ways from the other localities. One explanation for these observations is that there is a common cause of cratonic subsidence and suppression of kimberlite magmatism. Another possibility is that some apparent gaps in kimberlite magmatism are preservational artifacts. Even if kimberlites occurred during cratonic burial phases, the largest uppermost portions of the pipes would have been subsequently eroded along with the sedimentary rocks into which they were emplaced. In this model, kimberlite magmatism was more continuous than the preserved record suggests, implying that evidence for episodicity in kimberlite genesis should be carefully evaluated in light of potential preservational bias effects. Either way, the correlation between burial and kimberlite gaps suggests that cratonic surface histories are important for understanding global kimberlite patterns.

Thermochronological evidence for polyphase post-rift reactivation in SE Brazil

NASA Astrophysics Data System (ADS)

Cogné, N.; Gallagher, K.; Cobbold, P. R.; Riccomini, C.

2012-04-01

The continental margin of SE Brazil shows good evidence for tectonic activity well after the break-up of Western Gondwana (see Cobbold et al., 2001 for a review). Additionally, SE Brazil ranks as an HEPM (high elevation passive margin), summits reaching 2800 m. To constrain the onshore evolution of the margin, especially during the Tertiary, we did a new thermochronological and structural study. After an initial regional study, during which we found additional evidence for a major phase of exhumation during the Late Cretaceous to Early Tertiary (Cogné et al., 2011), we focussed on a region that was clearly subject to Tertiary tectonics. This region includes the Tertiary Taubaté basin and the adjacent Serra do Mar and Serra da Mantiqueira. We used two thermochronolgical methods on the same samples, apatite fission tracks (AFT) and U-Th/He on apatite (AHe). AFT ages range from 129.3±4.3 Ma to 60.7±1.9 Ma with mean track lengths (MTL) from 14.31±0.24 μm to 11.41±0.23 μm, whereas AHe ages range from 519.6±16.6 to 10.1±0.1 Ma. A subset of AHe ages, selected on the basis of data consistency and geological arguments, has a smaller range (122.4±2.5 to 45.1±1.5 Ma). We have combined inverse and forward modelling to assess the range of acceptable thermal histories. Results of inverse modelling confirm our earlier study by showing a Late Cretaceous phase of cooling. Around the onshore Taubaté Basin, for a limited number of samples, another period of cooling occurred during the Early Tertiary, around the time when the basin formed. The inferred thermal histories for most of the samples also imply a later reheating, followed by a Neogene cooling. According to forward modelling, the evidence for reheating seems to be robust around the margins of the Taubaté Basin, but elsewhere the data cannot really discriminate between this and a less complex thermal history. However forward modelling and geologically independent information support the conclusion that the whole area cooled and uplifted during the Neogene. The synchronicity of the cooling phases with tectonic pulses in the Andes and in NE Brazil, as well as the tectonic setting of the Tertiary basins (Cogné et al., submitted) lead us to attribute these phases to a plate-wide compressive stress, which reactivated inherited structures during the Late Cretaceous and Tertiary. The relief of the margin is therefore due, more to polyphase post-rift reactivation and uplift, than to rifting itself. - Cobbold, P.R., Meisling, K.E., Mount, V.S., 2001. Reactivation of an obliquely rifted margin, Campos and Santos Basins, Southeastern Brazil. AAPG Bulletin 85, 1925-1944. - Cogné, N., Gallagher, K., Cobbold, P.R., 2011. Post-rift reactivation of the onshore margin of southeast Brazil: Evidence from apatite (U-Th)/He and fission-track data. Earth and Planetary Science Letters 309, 118-130. - Cogné, N., Cobbold, P.R., Riccomini, C., Gallagher, K. Tectonic setting of the Taubaté basin (southeastern Brazil): insights from regional seismic profiles and outcrop data. Submitted to Journal of South American Earth Sciences.

Paleomagnetic study of the northern Ford Ranges, western Marie Byrd Land, West Antarctica: Motion between West and East Antarctica

NASA Astrophysics Data System (ADS)

Luyendyk, Bruce; Cisowski, Stan; Smith, Christine; Richard, Steve; Kimbrough, David

1996-02-01

A paleomagnetic study of Paleozoic and Mesozoic crystalline rocks in the northern Ford Ranges of Marie Byrd Land, West Antarctica, has determined a middle Cretaceous (circa 100 Ma) paleomagnetic pole and provided constraints on possible clockwise rotation of these ranges and on the rifting of east Gondwana. The 40Ar/39Ar thermochronology data from the Fosdick Mountains record a period of rapid cooling from ˜700°C beginning at ˜100 Ma. We relate this to extension, intrusion, and uplift associated with the beginning of rifting between Campbell Plateau and Marie Byrd Land. All rocks from the Fosdick and Chester Mountains are normally polarized. We interpret thermochronology and paleomagnetic data to infer that the region was extensively remagnetized in middle Cretaceous time. Inclinations in samples from the Chester Mountains are less steep than those from the Fosdick Mountains, which we interpret as ˜25° of south tilting of the Chesters. We interpret cooling age data for the time of magnetization to infer that the tilting began after 105 Ma and ended prior to 103 Ma. We further interpret this as constraining the beginning of extension between the Campbell Plateau and western Marie Byrd Land to the interval 105 to 103 Ma. Virtual geomagnetic poles from samples of Early Carboniferous age granodiorite from the western Phillips Mountains lie on the late Paleozoic apparent polar wander path for Australia transferred to Antarctica. Directions from 29 sites in the central and eastern Phillips and Fosdick Mountains give a Middle Cretaceous paleomagnetic pole at 222.3° E, 70.5° S (A95 6.1°, KAPPA 20.0). This pole is indistinguishable from other Middle Cretaceous poles for studies further east in Marie Byrd Land. Combining middle Cretaceous poles determined for three other studies of the Antarctic Peninsula, Thurston Island, and the Ruppert-Hobbs coasts with ours gives a Pacific West Antarctic pole at 215.2° E, 73.5° S (A95 4.0°, KAPPA 528.9). This pole is discordant by 5° to 10° from synthetic mid-Cretaceous East Antarctic reference poles, but the degree of discordance is very sensitive to the choice of the specific reference pole. The lack of native East Antarctic reference poles leaves this analysis inconclusive. Accepting 10° of discordance, we favor an interpretation where Pacific West Antarctic crustal domains or microplates have rotated clockwise 40° to 90° and translated a few degrees away from East Antarctica during Late Cretaceous time. An electronic supplement of this material may be obtained on a diskette or Anonymous FFP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the user name and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC02524, Paleomagnetic study of the northern Ford Ranges, western Marie Byrd Land, West Antarctica: Motion between West and East Antarctica, Bruce Luyendyk, Stan Cisowski, Christine Smith, Steve Richard, and David Kimbrough). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.

500 Myr of thermal history elucidated by multi-method detrital thermochronology of North Gondwana Cambrian sandstone (Eilat area, Israel)

NASA Astrophysics Data System (ADS)

Vermeesch, P.; Avigad, D.

2009-04-01

Following the Neoproterozoic Pan-African orogeny, the Arabian-Nubian Shield (ANS) of North Africa and Arabia was eroded and then covered by Cambrian sandstones that record the onset of platform sedimentation. We applied K-feldspar 40Ar/39Ar, zircon and apatite fission track and apatite (U-Th)/He thermochronology to detritus from Cambrian sandstones of southern Israel deposited at about 500 Ma. U-Pb detrital zircon ages from these sandstones predate deposition and record the earlier Neoproterozoic crustal evolution of the Pan-African orogens. 40Ar/39Ar ages from 50 single grains of K-feldspar yield a Cambrian mean of approximately 535 Ma. The 40Ar/39Ar age spectrum of a multi-grain K-feldspar aliquot displays diffusion behaviour compatible with >560 Ma cooling later affected by a heating event. Assuming that the high temperature domains of the K-feldspars have not been affected by subsequent (hydro)thermal events, and taking previously published K-Ar and Rb-Sr ages from other parts of the East African Orogen at face value, these ages apparently record Pan-African thermal resetting below a thick volcano-sedimentary pile similar to the Saramuj conglomerate in Jordan and/or the Hammamat in Egypt. Detrital zircon fission track (ZFT) ages cluster around 380 Ma, consistent with previous ZFT results from Neoproterozoic basement and sediments of the region, revealing that the Cambrian platform sequence experienced a middle Devonian thermal event and low-grade metamorphism. Regional correlation indicates that during Devonian time the sedimentary cover atop the Cambrian in Israel was never in excess of 2.5 km, requiring an abnormally steep geothermal gradient to explain the complete ZFT annealing. A basal Carboniferous unconformity can be traced from Syria to southern Saudi Arabia, suggesting that the observed Devonian ZFT ages represent a regional tectonothermal event. Similar Devonian ZFT ages were reported from ANS basement outcrops in the Eastern Desert, 500 km south of Eilat. The detrital apatites we studied all have extremely rounded cores suggestive of a distant provenance, but some grains also feature distinct euhedral U-rich apatite overgrowth rims. Authigenic apatite may have grown during the late Devonian thermal event we dated by ZFT, coinciding with existing Rb-Sr ages from authigenic clays in the same deposits and leading to the conclusion that the Devonian event was probably hydrothermal. Like the ZFT ages, the detrital apatite fission track (AFT) ages were also completely reset after deposition. Sixty single grain detrital apatite fission track (AFT) ages group at ~270 Ma with significant dispersion. Inverse modeling of the AFT data indicate extended and/or repeated residence in the AFT partial annealing zone, in turn suggesting an episodic burial-erosion history during the Mesozoic caused by low-amplitude vertical motions. Seven detrital apatite (U-Th)/He ages scatter between 33 and 77 Ma, possibly resulting from extreme compositional zonation associated with the authigenic U-rich overgrowths. The ~70 Ma (U-Th)/He ages are more likely to be accurate, setting 1-2 km as an upper limit (depending on the geothermal gradient) on the post-Cretaceous exhumation of the Cambrian sandstone and showing no evidence for substantial denudation related to Tertiary rifting of the Red Sea.

Orogen-scale anticline revealed in the Southern Alps of New Zealand by structural thermochronology

NASA Astrophysics Data System (ADS)

Zhou, Renjie; Brandon, Mark

2017-04-01

A dense set of cooling ages from the Southern Alps reveals an orogen-scale anticline of cooling-age isosurfaces (isochrones) and provides an interesting example of structural thermochronology, where isochrones are used as structural markers. The isochrone concept is an integral aspect of the age-elevation method, but the latter implicitly assumes that all isochrones are horizontal. Our experience in New Zeland and elsewhere is that isochrones are commonly tilted after formation. We use a more general approach that solves for orientation of the isochrone surfaces, and also the slope of the age-elevation trend, where "elevation" is measured normal to the isochrone surfaces. In New Zealand, collision and convergence between the Pacific and Australian plates have resulted in the formation and continuing growth of the Southern Alps, a prototypical orogenic wedge. In the western side, the Southern Alps is bounded by the Alpine fault, along with deeply exhumed rocks from depths up to 25 km. There are 150 apatite and 200 zircon fission-track (AFT, ZFT) ages that cover the vast region of the South Island of New Zealand from Lake Summer to Lake Wanaka. The AFT ages range from <0.5 to 140 Ma, and the ZFT ages, from <0.5 to 400 Ma. Our approach was initiated by McPhillips and Brandon (Earth and Planetary Science Letters, 2010, doi: 10.1016/j.epsl.2010.05.022). We use a least-squares method to solve for a best-fit sequence of dipping isochrone surfaces. The solution specifies the strike, dip and spacing of the parallel isochrones, the last of which indicates the velocity of the isochrones passing through the closure depth. We find that the calculation of the entire dataset failed to yield reasonable results, implying nonplanar structures at the regional scale. Using subsets of data, we observed three distinct zones of isochrones from E to W across the South Island. 1) The large area east of the Southern Alps in the central South Island contains ZFT isochrones that dip shallowly (< 1-5 degrees) to the east, with ages of 100 Ma and older. The spacing of the isochrones is narrow and indicates low ( 10-100 m/Ma) velocity crossing the closure depth of the samples. 2) In the eastern flank of the Southern Alps, the isochrone dip increases to 5-20 degrees with younger ages, 50 to < 5 Ma, and a much wide spacing of 1000-2000 m/Ma. 3) In the western flank of the Southern Alps around the Mount Cook, the isochrones have a similarly young age and spacing, but dip 10-30 degrees to the west. Collectively, these observations indicate an anticlinal structure across the Southern Alps. We have shown previously that cooling ages provide a kind of upside-down stratigraphy, with age surfaces (isochrones) formed at the closure isotherm and becoming older above that horizon. The isochrones first form as nearly horizontal surfaces and then serve as passive markers of deformation above the closure isotherm. We suggest that the isochrones exposed in the east flank of Southern Alps were formed and tilted to the east as they were advected through the large east-dipping retroshear zone that underlies the Southern Alps. In contrast, the west-dipping isochrones exposed in the west flank of the Southern Alps were formed within the retroshear zone, and their west dip is probably due to shear-induced rotation.

Effects of Caribbean oceanic plateau shallow subduction on topographic uplift and exhumation of the northwestern Maracaibo block, Colombia

NASA Astrophysics Data System (ADS)

Sanchez, J.; Mann, P.

2013-12-01

The Maracaibo block in the northwestern South America is a triangular lithotectonic terrane bounded on its western edge by the Santa Marta-Bucaramanga left-lateral strike-slip fault and the Oca-Ancon right-lateral strike-slip fault on its northern edge. These faults bound two isolated Andean ranges within the Maracaibo block: the Sierra de Santa Marta massif (SSM) in the east whose highest point is 5700 m ASL, and the Serrania del Perija (SP) to the west, whose highest point is 3600 m ASL. The two ranges are separated by an elongate, NNE-trending sedimentary basin, the Cesar-Rancheria basin (CRB). Previous thermochronological studies in the region have shown three discrete exhumation pulses from Paleocene to Miocene that are attributed to various collisional and strike events along the Caribbean margin. However the tectonic origin of the late Neogene deformation that produced the isolated, high topography of the SSM and SP has remained unclear. To establish patterns of recent uplift and associate them with a specific tectonic mechanism affecting the area, we integrated the following results: 1) analysis of stream profiles using channel normalized steepness indices and identification of slope-break knickpoints as indicators of rock uplift; 2) integration of observations from geological maps; 3) interpretation of 2D onland seismic profiles; 4) analysis of published thermochronological data; and 5) analysis of 1D/2D basin model based on well subsidence analysis from the CRB. Our results from the extraction of 550 long stream profiles from different watersheds for the SSM and SP reveal: 1) undisturbed profiles at western flank of the northern SP characterized by a few vertical-step knickpoints associated with lithology changes; in contrast the eastern flank of the northern SP shows slope-break knickpoints and changes in steepness indices increasing by a factor of ~2, all indicative of active fault control affecting this area; 2) disturbed profiles in the elevated central SP show several slope-break knickpoints and changes in steepness indices by factor of ~1.9, all indicative of fault control in this area; 3) perturbations in stream profiles with change in steepness indices by a factor of ~1.6 occur in the southern SP are interpreted as the expression of the active Santa-Marta Bucaramanga fault system, 4) although most of the knickpoints in SSM profiles seem to be associated with changes in bedrock lithology, the alignments, slope-breaking character, and change in steepness indices by factor of ~1.8-2.2 indicate active ENE-WSW fault control along the south-southeastern edge of the SSM, 5) generally undisturbed profiles at northern SSM suggest current quiescence in rock uplift in this area; and 6) basin modeling of the CRB constrain the most recent uplift event as Mio-Pliocene at rates of 0.15-0.18 mm/yr. We propose that the most likely tectonic mechanism to explain widespread active fault activity and uplift within the SSM and SP is oblique, low-angle (20 degrees), southeastward subduction of the Caribbean plate beneath the area. The present rate of subduction and length of slab would have initiated uplift and fault activity at Early Miocene.

A multidisciplinary analysis to constrain exhumation and recent erosion history of the Tethyan Himalaya, based on apatite (U-Th-Sm)/He and cosmogenic nuclides dates from Central Nepal (Takkhola graben and the Mustang granite)

NASA Astrophysics Data System (ADS)

Rosenkranz, Ruben; Sahragard Sohi, Mohammad; Spiegel, Cornelia

2015-04-01

The exhumation of the Himalayan arc has been studied intensively throughout the last decades. For the Tethyan Himalaya, however, the youngest exhumation history is still unclear, mostly because of the lack of a significant apatite content in most Tethyan sediments (Crouzet et al. 2007). For this study we are using apatite (U-Th-Sm)/He thermochronology and cosmogenic nuclides for investigating exhumation and denudation of the Tethyan Himalaya back through time. Apatite (U-Th-Sm)/He thermochronology is sensitive to temperatures of ~40 to 85°C and thus to movements within the upper ~1.5 to 3 km of the earth's crust. During a recent field campaign, we sampled the Mustang granite, that intrudes the Tethyan marine sediments and the Takkhola-Graben. The graben can be seen as an inusual southern part of the normal faulting system affecting the whole Tibetan Plateau (Colchen, 1999). The timing of the activation of these faulting is still highly debated. The syntectonics filling of the Takkhola-Mustag graben consists of Mio-Pliocene fluvio-lacustrine deposits (Garzione et al. 2003). These were described as containing significant amounts of apatite, derived from the past erosion of the Mustang granitic body (Adhikari and Wagreich, 2011). Being only up to 1km thick, a post depositional thermal resetting of the apatite (U-Th-Sm)/He system is unlikely, so that the (U-Th-Sm)/He dates of the sediments are expected to have retained their information regarding source area exhumation. We took several sand samples from the Kali Gandaki River draining the present-day exposure of the Mustang granite. Furthermore, we sampled different stratigraphic levels of the Mio-Pliocene sedimentary rocks, i.e., from the Tetang and Takkahola formation deposited between 11 and 7 Ma. This sampling approach will not only provide information about the youngest denudation history of the Mustang granite /Tethyan Himalaya, but will also reveal insights into its past denudation and changes of denudation rates through time for the region north to the South Tibetan Detachment. It is our expectation to link the results to the tectonic behavior of the South Tibetan Detachment or to the newly recognized Western Nepal Fault System. Corroborating our results with other ages and diverse methods will provide a robust constraint on the exhumation and erosion history of the Tethyan Himalaya, as well as insights on the U-Th/He dating technique. References Adhikari, B.R. and Wagreich, M., 2011. Provenance evolution of collapse graben fill in the Himalaya-The Miocene to Quaternary Thakkhola-Mustang graben (Nepal). Sedimentary Geology, 233(1), pp.1-14. Blythe, A.E., Burbank, D.W., Carter, A., Schmidt, K., Putkonen, J., 2007. Plio-Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U-Th]/He analyses. Tectonics, 26(3). Crouzet, C. et al., 2007. Temperature and age constraints on the metamorphism of the Tethyan Himalaya in Central Nepal: A multidisciplinary approach. Journal of Asian Earth Sciences, 30(1), pp.113-130. Garzione, C.N, DeCelles, P.G., Hokinson, D.G., Ojha, T.P., Upreti, B.N., 2003. East-west extension and Miocene environmental change in the southern Tibetan plateau: Thakkhola graben, central Nepal. Geological Society of America Bulletin, 115(1), pp.3-20. Colchen, M., 1999. The Takkhola-Mustang graben in Nepal and the latee Cenozoic extension in the Higher Himalayas. Journal of Asian Earth Sciences, 17, pp. 683-702.

Tectonic/climatic control on sediment provenance in the Cape Roberts Project core record (southern Victoria Land, Antarctica): A pulsing late Oligocene/early Miocene signal from south revealed by detrital thermochronology

NASA Astrophysics Data System (ADS)

Olivetti, V.; Balestrieri, M. L.; Rossetti, F.; Talarico, F. M.

2012-04-01

The Mesozoic-Cenozoic West Antarctic Rift System (WARS) is one of the largest intracontinental rift on Earth. The Transantarctic Mountains (TAM) form its western shoulder, marking the boundary between the East and West Antarctica. The rifting evolution is commonly considered polyphase and involves an Early Cretaceous phase linked to the Gondwana break-up followed by a major Cenozoic one, starting at c. 50-40 Ma. This Cenozoic episode corresponds to the major uplift/denudation phase of the TAM, which occurred concurrently with transition from orthogonal to oblique rifting. The Cenozoic rift reorganization occurred concurrently with a major change in the global climate system and a global reorganization of plate motions. This area thus provide an outstanding natural laboratory for studying a range of geological problems that involve feedback relationships between tectonics and climate. A key to address the tectonic/climate feedback relations is to look on apparent synchronicity in erosion signal between different segments, and to compare these with well-dated regional and global climatic events. However, due to the paucity of Cenozoic rock sequences exposed along the TAM front, a few information is available about the neotectonics of the rift and rift-flank uplift system. The direct physical record of the tectonic/climate history of the WARS recovered by core drillings along the western margin of the Ross sea (DSDP, CIROS, Cape Roberts and ANDRILL projects) provides an invaluable tool to address this issue. Twenty-three samples distributed throughout the entire composite drill-cored stratigraphic succession of Cape Roberts were analyzed. Age probability plots of eighteen detrital samples with depositional ages between 34 Ma and the Pliocene were decomposed into statistically significant age populations or peaks using binomial peak-fitting. Moreover, three granitic pebbles, one dolerite clast and one sample of Beacon sandstones have been dated. From detrital samples, three peaks are detected reflecting different bedrock provenance areas. Two peaks older than 40 Ma (P2 and P3) are compatible with thermochronological data from TAM bedrock that underwent a stepwise denudation in Cretaceous times. A Peak younger than 40 Ma (P1) has been detected occasionally, recording the signal of a source area exhumed during late Oligocene /early Miocene with a constant denudation rate of 0.4 mm/yr (constant lag-time up-section), but absent in the onshore portion of the proximal TAM. Indeed, when compared with AFT data from ANDRILL cores, the relatively young P1 ages, suggest that part of sediments in the Cape Robert Rift basin have a provenance from source regions probably located far away in the south (i.e. Skelton-Byrd glaciers region) where bedrock experienced compatible thermal histories. This provenance would imply glacial systems with main flow patterns from south to the north, therefore orthogonal to the orientation of present-day drainage. We thus infer that the post-Eocene glacial and erosional history of the TAM front was significantly controlled by the N-S-trending transtensional regime that affected the western Ross Sea margin during transition from orthogonal to oblique rifting in the region. The appearance and disappearance of P1 along the drill-cored stratigraphic succession seems to be linked to the oscillation in the extent of the ice sheet.

Constraints for timing of extensional tectonics in the western margin of the Red Sea in Eritrea

NASA Astrophysics Data System (ADS)

Ghebreab, Woldai; Carter, Andrew; Hurford, Anthony J.; Talbot, Christopher J.

2002-06-01

Recent work on asthenosphere-lithosphere coupling reinforces past observations that active and passive rifting models do not adequately describe real rifts. There remains insufficient knowledge of fundamental controls on rift architecture. In the actively extending Red Sea margin of eastern Eritrea, which lies at the Red Sea/Danakil-Gulf of Aden and the East African rift triple junction zone, the geometry and kinematics of extension are complex and poorly defined due to large data gaps. Extension and sea-floor spreading in both the Red Sea and Gulf of Aden have influenced the Neogene tectonic development of Eritrea but many of the structures have Pan-African origins and do not follow normal plate opening geometries. To constrain the rifting history in eastern Eritrea, apatite fission-track thermochronologic data were measured for 22 Pan-African rock samples. Results identify late Oligocene-early Miocene cooling coincident with extension and erosion along the conjugate margin in Yemen. A younger age group, confined to Mt Ghedem, relates to an episode of fault reactivation and dyke injection that began ˜10 Ma coincident with rotation of the nearby Danakil block. Initially this was driven by onset of sea-floor spreading in the Gulf of Aden and later, in the Pliocene, aided by northward rifting in the Afar depression concomitant with spreading in the Red Sea. These different processes highlight the complex linkage between different extensional events and rift architecture.

Tracking the multiple-stage exhumation history and magmatic-hydrothermal events of the West Junggar region, NW China: Evidence from 40Ar/39Ar and (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Yin, Jiyuan; Chen, Wen; Xiao, Wenjiao; Long, Xiaoping; Tao, Ni; Liu, Li-Ping; Yuan, Chao; Sun, Min

2018-06-01

To decipher cooling events in the West Junggar region, biotite and K-feldspar 40Ar/39Ar, and zircon and apatite (U-Th)/He isotopic analyses of intrusive rocks were carried out. Previous U-Pb data showed that intrusive bodies in the Baogutu area were emplaced at 315-310 Ma. U-Pb and zircon (U-Th)/He dating results (313-241 Ma) suggest that a magmatic-hydrothermal event lasted for 72 Ma in the Baogutu area of the West Junggar region. Early-stage high temperature alteration (900-300 °C) lasted for 6-2 Ma and was followed by prolonged phyllic and argillic alteration lasting 67-63 Ma between 350 and 200 °C. Finally, slower cooling occurred between 200 and 70 °C, accompanied by post-mineralization uplift and erosion. In this study, three main episodes of relatively rapid cooling were distinguished in the West Junggar region, i.e. late Carboniferous-early Permian (307-277 Ma), middle Triassic (241-232 Ma) and early Cretaceous (145-120 Ma). The first rapid cooling during the late Carboniferous-early Permian was possibly associated with the release of magmatic heat. The middle Triassic and early Cretaceous cooling and exhumation are interpreted as a response to collision(s) between the Qiangtang and Kunlun-Qaidam or Lhasa blocks. The Cenozoic India-Eurasia collision, however, may have had little or no effect on modern tectonic reactivation of the West Junggar region.

Slab flattening and exhumation of the Eastern Cordillera of Colombia

NASA Astrophysics Data System (ADS)

Siravo, G.; Faccenna, C.; Fellin, M. G.; Herman, F.; Becker, T. W.; Molin, P.

2017-12-01

Mountain belt topography is shaped by processes acting at different time scales and depths, from the surface down to the crust and mantle. In particular, subduction dynamics is expected to strongly affect upper plate topography. Here, we present the case of the Eastern Cordillera (EC) in Colombia as a case history for dynamic mantle forcing from a subduction zone on the upper plate topography. The EC is an active double-vergent fold and thrust belt formed during the Cenozoic by the inversion of a Mesozoic rift, and topography there has grown up to 5000 m (Cocuy Sierra). The EC is located far ( 500 km) from the trench where the Nazca slab subducts below the South American plate. Tomography and seismicity show the presence of a flat slab subduction north of 5° N (Chiarabba et al., 2016). Slab flattening may have occurred transitionally from 10 to 5 Ma shutting down the arc volcanism (Wagner et al., 2017). We reconstruct the exhumation of the EC based on previously published and new thermochronologic data collected in the area of the Cocuy Sierra. Results of this analysis show notably fast exhumation rates since Late Miocene. We also analyze the likely contributions to topography and show that neither the present-day crustal thickness nor the cumulative shortening in the Cenozoic as extracted form balanced cross section can isostatically explain the present day topography. We conclude that fast EC exhumation and uplift are driven by mantle dynamics and likely occurred during the recent episode of slab flattening.

Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy

NASA Astrophysics Data System (ADS)

Fauquette, Séverine; Bernet, Matthias; Suc, Jean-Pierre; Grosjean, Anne-Sabine; Guillot, Stéphane; van der Beek, Peter; Jourdan, Sébastien; Popescu, Speranta-Maria; Jiménez-Moreno, Gonzalo; Bertini, Adele; Pittet, Bernard; Tricart, Pierre; Dumont, Thierry; Schwartz, Stéphane; Zheng, Zhuo; Roche, Emile; Pavia, Giulio; Gardien, Véronique

2015-02-01

We evaluate the topographic evolution of the southwestern Alps using Eocene to Pleistocene pollen data combined with existing sedimentological, petrographic and detrital geo- and thermochronological data. We report 32 new pollen analyses from 10 sites completed by an existing dataset of 83 samples from 14 localities situated across the southwestern Alps, including both the pro- and the retro-foreland basins. The presence of microthermic tree pollen (mainly Abies, Picea) indicates that this part of the mountain belt attained elevations over 1900 m as early as the Oligocene. Inferred rapid surface uplift during the mid-Oligocene coincided with a previously documented brief phase of rapid erosional exhumation, when maximum erosion rates may have reached values of up to 1.5-2 km/Myr. Slower long-term average exhumation rates of ∼0.3 km/Myr since the Late Oligocene helped maintaining the high Alpine topography of the southwestern Alps until today. The relative abundances of meso-microthermic tree pollen (Cathaya, Cedrus and Tsuga) and microthermic tree pollen (Abies, Picea) in the pro- and retro-foreland basin deposits, indicate that the present-day asymmetric topography, with a relatively gentle western flank and steeper eastern flank, was established early in the southwestern Alps, at least since the Early Miocene, and possibly since the Oligocene or Late Eocene. Therefore, the high topography and asymmetric morphology of this part of the Alps has been maintained throughout the past ∼30 Ma.

Surface Response to Regional Uplift of Madagascar Reveals Short Wavelength Dynamic Topography

NASA Astrophysics Data System (ADS)

Stephenson, S.; White, N.

2016-12-01

The physiography of Madagascar is characterized by high elevation but low relief topography with 42% of the landscape at an elevation grgeater than 500 m. Eocene marine limestones crop out at an elevation of 400 m, extensive low relief erosion surfaces capped by laterites occur at elevations of up to 2 km, and longitudinal river profiles are disequilibrated. Together, these observations suggest that Madagascar underwent regional uplift in Neogene times. Inverse modeling of drainage networks suggests that regional uplift is diachronous and has occurred on wavelengths of 1000 km. The existence of deeply incised river channels together with low-temperature thermochronologic measurements (i.e. AFT, AHe) implies that erosion occurred in response to regional Neogene uplift. Admittance analysis of long wavelength free-air gravity and topography shows that admittance, Z = 45 ± 5 mGal/km. The history of Neogene volcanism and a lack of significant tectonic shortening both suggest that uplift is dynamically supported. Here we present a suite of U-Th dates of emergent coral reef deposits from northern Madagascar, whose margins are sometimes considered `stable'. Elevation of these coeval coral reefs decreases from 7.2 m at the northern tip of Madagascar to sea level 100 km to the south. The existence of a spatial gradient suggests that differential vertical motions occurred during Late Quaternary times. These results raise significant questions about the reliability both of emergent coral reefs as global sea-level markers and the length-scale of variations in dynamic topography.

Geomorphic Response to Spatial and Temporal Tectonic uplift on the Kenya Rift of East African Rift System

NASA Astrophysics Data System (ADS)

Xue, L.; Abdelsalam, M. G.

2017-12-01

Tectonic uplifts of the shoulders of the East Africa Rift System (EARS) have significant impact on the geological record by reorganizing drainage systems, increasing sediment supply, and changing climate and biogeography. Recent studies in geochronology, geomorphology and geophysics have provided some understanding of the timing of tectonic uplift and its distribution pattern of the (EARS). We do not know how the vertical motion is localized along the rift axis and the relative roles of upwelling of magma and rift extensional processes play in tectonic uplift history. This work presents detailed morphometric study of the fluvial landscape response to the tectonic uplift and climate shifting of the Kenya Rift shoulders in order to reconstruct their incision history, with special attention to timing, location, and intensity of uplift episodes. This work compiles the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and Sentinel-2A data, summarized previous 39Ar-40Ar and thermochronology data, and calculates long-term incision rate and geomorphic proxies (normalized steepness and chi-integral) along the Kenya Rift. It also models the age of tectonic/climatic events by using knickpoint celerity model and R/SR integrative approach. It found that the maximum long-term incision rates of 300 mm/kyr to be at the central Kenya Rift, possibly related to the mantle-driven process and rapid tectonic uplift. The geomorphic proxies indicate southward decreasing pattern of the short-term incision rate, possibly related to the migration of the mantle plume.

Petrography and zircon U-Pb isotopic study of the Bayanwulashan Complex: Constrains on the Paleoproterozoic evolution of the Alxa Block, westernmost North China Craton

NASA Astrophysics Data System (ADS)

Wu, Sujuan; Hu, Jianmin; Ren, Minghua; Gong, Wangbin; Liu, Yang; Yan, Jiyuan

2014-11-01

The Bayanwulashan Metamorphic Complex (BMC) exposes along the eastern margin of the Alxa Block, the westernmost part of the North China Craton (NCC). BMC is principally composed of metamorphic rocks with amphibole plagiogneiss, biotite plagioclase gneiss and granitic gneiss. Our research has been focused on the petrography and zircon U-Pb geochronology of the BMC to better understand the evolution of the Alxa Block and its relationship with the NCC. Evidences from field geology, petrography, and mineral chemistry indicate that two distinct metamorphic assemblages, the amphibolite and greenschist facies, had overprinted the preexisting granitic gneiss and suggest that the BMC experienced retrograde metamorphic episodes. The LA-ICP-MS zircon U-Pb ages reveal that the primary magmatic activities of BMC were at ca. 2.30-2.24 Ga and the two metamorphic events were at ca. 1.95-1.91 Ga and ca. 1.88-1.85 Ga respectively. These ages indicate that BMC initially intruded during Paleoproterozoic, not as previously suggested at Archean period. The Early Paleoproterozoic metamorphic records and the magmatic thermochronological data in BMC exhibit different evolution paths between the Alxa Block and the NCC. The Alxa Block was most likely an independent Early Paleoproterozoic terrain. Following different amalgamation processes, The Alxa Block combined with Western Block at ca. 1.95 Ga and then united with NCC at ca. 1.85 Ga.

From nappe stacking to extensional detachments at the contact between the Carpathians and Dinarides - The Jastrebac Mountains of Central Serbia

NASA Astrophysics Data System (ADS)

Erak, Dalibor; Matenco, Liviu; Toljić, Marinko; Stojadinović, Uroš; Andriessen, Paul A. M.; Willingshofer, Ernst; Ducea, Mihai N.

2017-07-01

Reactivation of inherited nappe contacts is a common process in orogenic areas affected by back-arc extension. The amount of back-arc extension is often variable along the orogenic strike, owing to the evolution of arcuated mountain chains during stages of rapid slab retreat. This evolution creates low rates of extension near rotation poles, where kinematics and interplay with the pre-existing orogenic structure are less understood. The amount of Miocene extension recorded by the Pannonian Basin of Central Europe decreases SE-wards along the inherited Cretaceous - Paleogene contact between the Dinarides and Carpathian Mountains. Our study combines kinematic data obtained from field and micro-structural observations assisted with fission track thermochronological analysis and U-Pb zircon dating to demonstrate a complex poly-phase evolution in the key area of the Jastrebac Mountains of Serbia. A first event of Late Cretaceous exhumation was followed by latest Cretaceous - Eocene thrusting and magmatism related to a continental collision that sutured the accretionary wedge containing contractional trench turbidites. The suture zone was subsequently reactivated and exhumed by a newly observed Miocene extensional detachment that lasted longer in the Jastrebac Mountains when compared with similar structures situated elsewhere in the same structural position. Such extensional zones situated near the pole of extensional-driven rotation favour late stage truncations and migration of extension in a hanging-wall direction, while directions of tectonic transport show significant differences in short distances across the strike of major structures.

Topographic asymmetry of the South Atlantic from global models of mantle flow and lithospheric stretching

NASA Astrophysics Data System (ADS)

Flament, Nicolas; Gurnis, Michael; Williams, Simon; Seton, Maria; Skogseid, Jakob; Heine, Christian; Dietmar Müller, R.

2014-02-01

The relief of the South Atlantic is characterized by elevated passive continental margins along southern Africa and eastern Brazil, and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank. We investigate the origin of these topographic features in the present and over time since the Jurassic with a model of global mantle flow and lithospheric deformation. The model progressively assimilates plate kinematics, plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates, and predicts the evolution of mantle temperature, continental crustal thickness, long-wavelength dynamic topography, and isostatic topography. Mantle viscosity and the kinematics of the opening of the South Atlantic are adjustable parameters in thirteen model cases. Model predictions are compared to observables both for the present-day and in the past. Present-day predictions are compared to topography, mantle tomography, and an estimate of residual topography. Predictions for the past are compared to tectonic subsidence from backstripped borehole data along the South American passive margin, and to dynamic uplift as constrained by thermochronology in southern Africa. Comparison between model predictions and observations suggests that the first-order features of the topography of the South Atlantic are due to long-wavelength dynamic topography, rather than to asthenospheric processes. The uplift of southern Africa is best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle.

Topographic asymmetry of the South Atlantic from global models of mantle flow and lithospheric stretching

NASA Astrophysics Data System (ADS)

Flament, Nicolas; Gurnis, Michael; Williams, Simon; Seton, Maria; Skogseid, Jakob; Heine, Christian; Müller, Dietmar

2014-05-01

The relief of the South Atlantic is characterized by elevated passive continental margins along southern Africa and eastern Brazil, and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank. We investigate the origin of these topographic features in the present and over time since the Jurassic with a model of global mantle flow and lithospheric deformation. The model progressively assimilates plate kinematics, plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates, and predicts the evolution of mantle temperature, continental crustal thickness, long-wavelength dynamic topography, and isostatic topography. Mantle viscosity and the kinematics of the opening of the South Atlantic are adjustable parameters in multiple model cases. Model predictions are compared to observables both for the present-day and in the past. Present-day predictions are compared to topography, mantle tomography, and an estimate of residual topography. Predictions for the past are compared to tectonic subsidence from backstripped borehole data along the South American passive margin, and to dynamic uplift as constrained by thermochronology in southern Africa. Comparison between model predictions and observations suggests that the first-order features of the topography of the South Atlantic are due to long-wavelength dynamic topography, rather than to asthenospheric processes. We find the uplift of southern Africa to be best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle.

Tectonic reconstruction of the Panama arc using paleomagnetic, geochronology and thermochronologic data

NASA Astrophysics Data System (ADS)

Montes, C.; Bayona, G.; Cardona, A.; Pardo, A.; Nova, G.; Montano, P.

2013-05-01

A recent update of the geochronologic and mapping database of the Isthmus of Panama suggests that the Isthmus represents an arc that was left-laterally fragmented between 38 and 28 Ma, and then oroclinally bent. This was hypothesis was tested using paleomagnetic data (24 sites and 192 cores) that indicated large counterclockwise vertical-axis rotations (70.9°, ± 6.7°), and moderate clockwise rotations (between 40° ± 4.1° and 56.2° ± 11.1) on either side of an east-west trending fault at the apex of the Isthmus (Rio Gatun Fault), consistent with Isthmus curvature. Sampling for paleomagnetism was performed on Cretaceous basaltic rocks of the Panama arc, some of them probably correlative to the Caribbean large igneous province. Also, sampling took place in younger Cenozoic cover rocks, as well as in the younger arc rocks. This database is here complemented with 15 new pilot paleomagnetic sites taken in eastern, central, and western Panama, and 3 new sites from Miocene cover rocks of what is now considered to be the southeastern-most tip of the Central American arc. The latter record clockwise vertical-axis rotations between 12 and 40°, in agreement with oroclinal bending hypothesis for the formation of the Isthmus of Panama. These new results begin to fill a gap in the paleomagnetic vertical-axis rotation database for the Panama arc. These results also support the continuity of the Central America arc to the east, into what is now docked to western South America.

The Post-Eocene Evolution of the Doruneh Fault Region (Central Iran): The Intraplate Response to the Reorganization of the Arabia-Eurasia Collision Zone

NASA Astrophysics Data System (ADS)

Tadayon, Meisam; Rossetti, Federico; Zattin, Massimiliano; Nozaem, Reza; Calzolari, Gabriele; Madanipour, Saeed; Salvini, Francesco

2017-12-01

The Cenozoic deformation history of Central Iran has been dominantly accommodated by the activation of major intracontinental strike-slip fault zones, developed in the hinterland domain of the Arabia-Eurasia convergent margin. Few quantitative temporal and kinematic constraints are available from these strike-slip deformation zones, hampering a full assessment of the style and timing of intraplate deformation in Iran and the understanding of the possible linkage to the tectonic reorganization of the Zagros collisional zone. This study focuses on the region to the north of the active trace of the sinistral Doruneh Fault. By combing structural and low-temperature apatite fission track (AFT) and (U-Th)/He (AHe) thermochronology investigations, we provide new kinematic and temporal constraints to the deformation history of Central Iran. Our results document a post-Eocene polyphase tectonic evolution dominated by dextral strike-slip tectonics, whose activity is constrained since the early Miocene in response to an early, NW-SE oriented paleo-σ1 direction. A major phase of enhanced cooling/exhumation is constrained at the Miocene/Pliocene boundary, caused by a switch of the maximum paleo-σ1 direction to N-S. When integrated into the regional scenario, these data are framed into a new tectonic reconstruction for the Miocene-Quaternary time lapse, where strike-slip deformation in the intracontinental domain of Central Iran is interpreted as guided by the reorganization of the Zagros collisional zone in the transition from an immature to a mature stage of continental collision.

Multistage extensional evolution of the central East Greenland Caledonides

NASA Astrophysics Data System (ADS)

White, Arthur P.; Hodges, Kip V.

2002-10-01

Recent field investigations in the central East Greenland Caledonides (72°-74°N) resulted in the identification of an orogen-scale extensional fault system called the Fjord Region Detachment (FRD). Previous geochronologic constraints on this deformation indicated that the FRD was active circa 430-425 Ma, a time when the Baltica-Laurentia collision was thought to be occurring, and continued to be active for up to 80 million years. We present new 40Ar/39Ar thermochronologic data from an E-W transect that cuts across two splays of the FRD. Our data demonstrate that at least two distinct episodes of faulting were responsible for extension in the East Greenland Caledonides: an earlier phase (circa 425-423 Ma) that was synorogenic and penetrated to middle-crustal levels, followed by a post-Caledonian phase of reactivation (˜414 to 380 Ma) that affected even deeper structural levels. Furthermore, we present in situ UV laser 40Ar/39Ar data for pseudotachylite collected along the deepest splay of the FRD that indicate this fault was active again as recently as ˜357 Ma (coeval with Devonian basin formation). Altogether, our data suggest that rather than being active continuously for 80 million years, the FRD consisted of multiple splays that were active for shorter intervals over discrete time periods separated by as much as 60 million years. Finally, our data provide evidence that young extensional deformation associated with postorogenic collapse in East Greenland was not restricted to the formation of sedimentary basins in the far eastern part of the orogen, but also resulted in deformation of the Archean-Paleozoic crystalline basement.

Thermochronologic constraints on mylonite and detachment fault development, Kettle Highlands, northeastern Washington and southern British Columbia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Berger, B.R.; Snee, L.W.

1992-01-01

The Kettle dome, northeastern Washington and southern British Columbia, is one of several large metamorphic core complexes in the region. New Ar-40/Ar-39 cooling dates from the mylonite immediately beneath the Kettle River detachment fault at Barney's Junction, a cross-cutting mafic dike, and the youngest Eocene lavas in the Republic graben set constraints on kinematic models of the tectonic evolution of the dome and related grabens: Amphibolite--hornblende (59.0 [+-] 0.2); Pegmatite--muscovite (49.3 [+-] 0.2); Pegmatite--K-feldspar (49.2 [+-] 1); Augen gneiss--K-feldspar (48.0 [+-] 1); Mafic dike--hornblende (54.5 [+-] 0.1) and biotite (49.6 [+-] 0.1); Klondike Mt. Formation lava--feeder dike (48.8 [+-] 1).more » The authors interpret the dates to indicate that the tectonized amphibolite, part of a Cretaceous and older metamorphosed terrane, had formed and cooled to [approx] 500 C by Late Paleocene, the mylonite zone was being domed above the ductile zone by Early Eocene at the time of emplacement of the dike--temporally equivalent to the Keller Butte suite, Eocene Colville batholith--which crosscuts the mylonite, and incipient rifting was occurring in the Republic graben as evidenced by dike swarms. The mylonite complex reached 300 C by 49Ma coincident with the termination of Sanpoil volcanism, and then cooled rapidly to near or below 150 C by 48 Ma. At about this time, mafic Klondike Mt. lavas mark the termination of Republic graben rifting and possibly detachment faulting along the Kettle River fault.« less

The development of miocene extensional and short-lived basin in the Andean broken foreland: The Conglomerado Los Patos, Northwestern Argentina

NASA Astrophysics Data System (ADS)

del Papa, Cecilia E.; Petrinovic, Ivan A.

2017-01-01

The Conglomerado Los Patos is a coarse-grained clastic unit that crops out irregularly in the San Antonio de los Cobres Valley in the Puna, Northwestern Argentina. It covers different units of the Cretaceous-Paleogene Salta Group by means of an angular unconformity and, in turn, is overlaid in angular unconformity by the Viscachayoc Ignimbrite (13 ± 0.3 Ma) or by late Miocene tuffs. Three lithofacies have been identified in the Corte Blanco locality; 1) Bouldery matrix-supported conglomerate (Gmm); 2) Clast-supported conglomerate (Gch) and 3) Imbricated clast-supported conglomerate (Gci). The stratigraphic pattern displays a general fining upward trend. The sedimentary facies association suggests gravitational flow processes and sedimentation in alluvial fan settings, from proximal to medial fan positions, together with a slope decrease upsection. Provenance studies reveal sediments sourced from Precambrian to Ordovician units located to the southwest, except for volcanic clasts in the Gmm facies that shows U/Pb age of 14.5 ± 0.5 Ma. This new age represents the maximum depositional age for the Conglomerado Los Patos, and it documents that deposition took place simultaneously during a period of increased tectonic and volcanic activity in the area. The structural analysis of the San Antonio de los Cobres Valley and the available thermochronological ages, indicate active N-S main thrusts and NW-SE transpressive and locally normal faults during the middle Miocene. In this context, we interpret the Conglomerado Los Patos to represent sedimentation in a small, extensional and short-lived basin associated with the compressional Andean setting.

Inversion of the Erlian Basin (NE China) in the early Late Cretaceous: Implications for the collision of the Okhotomorsk Block with East Asia

NASA Astrophysics Data System (ADS)

Guo, Zhi-Xin; Shi, Yuan-Peng; Yang, Yong-Tai; Jiang, Shuan-Qi; Li, Lin-Bo; Zhao, Zhi-Gang

2018-04-01

A significant transition in tectonic regime from extension to compression occurred throughout East Asia during the mid-Cretaceous and has stimulated much attention. However, the timing and driving mechanisms of the transition remain disputed. The Erlian Basin, a giant late Mesozoic intracontinental petroliferous basin located in the Inner Mongolia, Northeast China, contains important sedimentary and structural records related to the mid-Cretaceous compressional event. The stratigraphical, sedimentological and structural analyses reveal that a NW-SE compressional inversion occurred in the Erlian Basin between the depositions of the Lower Cretaceous Saihan and Upper Cretaceous Erlian formations, causing intense folding of the Saihan Formation and underlying strata, and the northwestward migration of the depocenters of the Erlian Formation. Based on the newly obtained detrital zircon U-Pb data and previously published paleomagnetism- and fossil-based ages, the Saihan and Erlian formations are suggested as latest Aptian-Albian and post-early Cenomanian in age, respectively, implying that the inversion in the Erlian Basin occurred in the early Late Cretaceous (Cenomanian time). Apatite fission-track thermochronological data record an early Late Cretaceous cooling/exhuming event in the basin, corresponding well with the aforementioned sedimentary, structural and chronological analyses. Combining with the tectono-sedimentary evolutions of the neighboring basins of the Erlian Basin, we suggest that the early Late Cretaceous inversional event in the Erlian Basin and the large scale tectonic transition in East Asia shared the common driving mechanism, probably resulting from the Okhotomorsk Block-East Asia collisional event at about 100-89 Ma.

Low Pressure-High Temperature Metamorphism and the Advection of Heat to the Continental Crust: A Case Study from Northwest New Guinea

NASA Astrophysics Data System (ADS)

Jost, B.; Webb, M.; White, L. T.

2017-12-01

In northwest New Guinea, Palaeozoic basement rocks forming part of the northern margin of the Australian continent are exposed in a rugged mountain range. This remote and understudied region provides a unique window into the complex Palaeozoic evolution and tectonic history of this region, which we help unravel with new field, petrographic, geochemical, and geochronological data. The basement rocks consist of extensive meta-turbidites that were subject to low pressure-high temperature metamorphism along their eastern margin. They are cross-cut by predominantly acidic granitoids. U-Pb zircon dating reveals that these granitoids intruded in three episodes in the Devonian-Carboniferous, the Carboniferous, and the Triassic. The first episode has not previously been reported in the region. The granitoids are strongly peraluminous, suggesting that partial melting of the meta-sedimentary country rock contributed to their petrogenesis (S-type). The occurrence and character of country rock xenoliths and migmatites supports this interpretation. Equilibrium thermodynamic modelling of the metapelites and the migmatites indicates that a substantial amount of heat was added to the lower and middle crust to cause partial melting and regional metamorphism at relatively low pressure. We propose repeated intrusion of hot magma as the mechanism responsible for advecting the necessary heat from the mantle. This likely occurred in an active continental margin setting during the Devonian-Carboniferous and the Triassic, possibly separated by an interval of magmatic quiescence during most of the Permian. New biostratigraphic and low-temperature thermochronological data reveal very recent Pliocene-Pleistocene uplift and unroofing of these basement rocks.

Experimental evidence regarding the pressure dependence of fission track annealing in apatite

NASA Astrophysics Data System (ADS)

Schmidt, J. S.; Lelarge, M. L. M. V.; Conceicao, R. V.; Balzaretti, N. M.

2014-03-01

The main purposes of fission track thermochronology are unravelling the thermal histories of sedimentary basins, determining uplift and denudation rates, identifying the structural evolution of orogenic belts, determining sedimentary provenance, and dating volcanic rocks. The effect of temperature on fission tracks is well known and is used to determine the thermal history; however, the effect of pressure on the stability of tracks is still under debate. The present work aims to understand the role of pressure on the annealing kinetics of apatite fission tracks. The samples of Durango apatite used in our experiments were chosen for their international recognition as a calibration standard for geological dating. Neutron irradiation of the samples, after total annealing of their spontaneous tracks, produced induced tracks with homogeneous densities and lengths. The effect of pressure associated with temperature on fission track annealing was verified by experimental procedures using a hydraulic press of 1000 t with a toroidal chamber profile. The experiments consisted of a combination of applying 2 and 4 GPa with 20,150,190,235, and 290 °C for 1 and 10 h. The annealing rate was analysed by measuring the lengths of the fission tracks after each experiment using optical microscopy. The results demonstrate that the annealing of apatite fission tracks has a pressure dependence for samples subjected to 2 and 4 GPa. However, when extrapolated to pressures of ⩽150 MPa, compatible with the normal geological context in which apatite fission track methodology is broadly used, this dependence becomes insignificant compared to the temperature effect.

Stable isotopic constraints on the tectonic, topographic, and climatic evolution of the northern margin of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Kent-Corson, Malinda L.; Ritts, Bradley D.; Zhuang, Guangsheng; Bovet, Paul M.; Graham, Stephan A.; Page Chamberlain, C.

2009-05-01

This study presents oxygen and carbon isotopic records from lacustrine, paleosol, alluvial, and fluvial carbonate sampled in thirteen Cenozoic sedimentary sections that span the northern margin of the Tibetan Plateau. Isotopic analyses of 1475 carbonate samples yield δ18O values that range from 13.1 to 38.9‰ (SMOW), and δ13C values range from - 11.5 to 3.7‰ (PDB). Based on these analyses, we see two isotopic trends regionally. We interpret a Paleogene decrease in average oxygen isotope values to result from an increase in the mean elevation of the study region's drainages as they tapped waters from a growing Tibetan Plateau to the south of the study area. We interpret a Neogene increase in both carbon and oxygen isotope values to result from the initiation of local mountain building in the study area and the withdrawal of marine waters, which increased basin isolation, aridity, and temperatures. The timing of inferred initiation of uplift in this region is consistent with structural, thermochronological, and sedimentological evidence for a tectonic transition in the early and middle Miocene from accommodation of India-Eurasia convergence through extrusion along the Altyn Tagh fault to accommodation of convergence through active shortening and mountain building in this region. This study also supports tectonic models for the differential uplift of the Tibetan Plateau, in that mountain building in this study region significantly postdates estimates for the attainment of high elevation from isotopic studies of the central Tibetan Plateau.

Postcollisional cooling history of the Eastern and Southern Alps and its linkage to Adria indentation

NASA Astrophysics Data System (ADS)

Heberer, Bianca; Reverman, Rebecca Lee; Fellin, Maria Giuditta; Neubauer, Franz; Dunkl, István; Zattin, Massimiliano; Seward, Diane; Genser, Johann; Brack, Peter

2017-07-01

Indentation of rigid blocks into rheologically weak orogens is generally associated with spatiotemporally variable vertical and lateral block extrusion. The European Eastern and Southern Alps are a prime example of microplate indentation, where most of the deformation was accommodated north of the crustal indenter within the Tauern Window. However, outside of this window only the broad late-stage exhumation pattern of the indented units as well as of the indenter itself is known. In this study we refine the exhumational pattern with new (U-Th-Sm)/He and fission-track thermochronology data on apatite from the Karawanken Mountains adjacent to the eastern Periadriatic fault and from the central-eastern Southern Alps. Apatite (U-Th-Sm)/He ages from the Karawanken Mountains range between 12 and 5 Ma and indicate an episode of fault-related exhumation leading to the formation of a positive flower structure and an associated peripheral foreland basin. In the Southern Alps, apatite (U-Th-Sm)/He and fission-track data combined with previous data also indicate a pulse of mainly Late Miocene exhumation, which was maximized along thrust systems, with highly differential amounts of displacement along individual structures. Our data contribute to mounting evidence for widespread Late Miocene tectonic activity, which followed a phase of major exhumation during strain localization in the Tauern Window. We attribute this exhumational phase and more distributed deformation during Adriatic indentation to a major change in boundary conditions operating on the orogen, likely due to a shift from a decoupled to a coupled system, possibly enhanced by a shift in convergence direction.

Rapid cooling rates at an active mid-ocean ridge from zircon thermochronology

USGS Publications Warehouse

Schmitt, Axel K.; Perfit, Michael R.; Rubin, Kenneth H.; Stockli, Daniel F.; Smith, Matthew C.; Cotsonika, Laurie A.; Zellmer, Georg F.; Ridley, W. Ian

2011-01-01

Oceanic spreading ridges are Earth's most productive crust generating environment, but mechanisms and rates of crustal accretion and heat loss are debated. Existing observations on cooling rates are ambiguous regarding the prevalence of conductive vs. convective cooling of lower oceanic crust. Here, we report the discovery and dating of zircon in mid-ocean ridge dacite lavas that constrain magmatic differentiation and cooling rates at an active spreading center. Dacitic lavas erupted on the southern Cleft segment of the Juan de Fuca ridge, an intermediate-rate spreading center, near the intersection with the Blanco transform fault. Their U–Th zircon crystallization ages (29.3-4.6+4.8 ka; 1δ standard error s.e.) overlap with the (U–Th)/He zircon eruption age (32.7 ± 1.6 ka) within uncertainty. Based on similar 238U-230Th disequilibria between southern Cleft dacite glass separates and young mid-ocean ridge basalt (MORB) erupted nearby, differentiation must have occurred rapidly, within ~ 10–20 ka at most. Ti-in-zircon thermometry indicates crystallization at 850–900 °C and pressures > 70–150 MPa are calculated from H2O solubility models. These time-temperature constraints translate into a magma cooling rate of ~ 2 × 10-2 °C/a. This rate is at least one order-of-magnitude faster than those calculated for zircon-bearing plutonic rocks from slow spreading ridges. Such short intervals for differentiation and cooling can only be resolved through uranium-series (238U–230Th) decay in young lavas, and are best explained by dissipating heat convectively at high crustal permeability.

Late Eocene Uplift of the Al Hajar Mountains, Oman, Supported by Stratigraphy and Low-Temperature Thermochronology

NASA Astrophysics Data System (ADS)

Hansman, Reuben J.; Ring, Uwe; Thomson, Stuart N.; den Brok, Bas; Stübner, Konstanze

2017-12-01

Uplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test these hypotheses, the cooling of the central Al Hajar Mountains is constrained by 10 apatite (U-Th)/He (AHe), 15 fission track (AFT), and four zircon (U-Th)/He (ZHe) sample ages. These data show differential cooling between the two major structural culminations of the mountains. In the 3 km high Jabal Akhdar culmination AHe single-grain ages range between 39 ± 2 Ma and 10 ± 1 Ma (2σ errors), AFT ages range from 51 ± 8 Ma to 32 ± 4 Ma, and ZHe single-grain ages range from 62 ± 3 Ma to 39 ± 2 Ma. In the 2 km high Saih Hatat culmination AHe ages range from 26 ± 4 to 12 ± 4 Ma, AFT ages from 73 ± 19 Ma to 57 ± 8 Ma, and ZHe single-grain ages from 81 ± 4 Ma to 58 ± 3 Ma. Thermal modeling demonstrates that cooling associated with uplift and erosion initiated at 40 Ma, indicating that uplift occurred 30 Myr after ophiolite obduction and at least 10 Myr before the Zagros collision. Therefore, this uplift cannot be related to either event. We propose that crustal thickening supporting the topography of the Al Hajar Mountains was caused by a slowdown of Makran subduction and that north Oman took up the residual fraction of N-S convergence between Arabia and Eurasia.

Late Cenozoic cooling history of the central Menderes Massif: Timing of the Büyük Menderes detachment and the relative contribution of normal faulting and erosion to rock exhumation

NASA Astrophysics Data System (ADS)

Wölfler, Andreas; Glotzbach, Christoph; Heineke, Caroline; Nilius, Nils-Peter; Hetzel, Ralf; Hampel, Andrea; Akal, Cüneyt; Dunkl, István; Christl, Marcus

2017-10-01

Based on new thermochronological data and 10Be-derived erosion rates from the southern part of the central Menderes Massif (Aydın block) in western Turkey, we provide new insights into the tectonic evolution and landscape development of an area that undergoes active continental extension. Fission-track and (U-Th)/He data reveal that the footwall of the Büyük Menderes detachment experienced two episodes of enhanced cooling and exhumation. Assuming an elevated geothermal gradient of 50 °C/km, the first phase occurred with an average rate of 0.90 km/Myr in the middle Miocene and the second one in the latest Miocene and Pliocene with a rate of 0.43 km/Myr. The exhumation rates between these two phases were lower and range from 0.14 to 0.24 km/Myr, depending on the distance to the detachment. Cosmogenic nuclide-based erosion rates for catchments in the Aydın block range from 0.1 to 0.4 km/Myr. The similarity of the erosion rates on both sides of the Aydın block (northern and southern flank) indicate that a rather symmetric erosion pattern has prevailed during the Holocene. If these millennial erosion rates are representative on a million-year timescale they indicate that, apart from normal faulting, erosion in the hanging wall of the Büyük Menderes detachment fault did also contribute to the exhumation of the metamorphic rocks.

Exhumation of Basement-cored Uplifts: Example of the Kyrgyz Range Quantified with Apatite Fission-track Thermochronology

NASA Technical Reports Server (NTRS)

Sobel, Edward R.; Oskin, Michael; Burbank, Douglas; Mikolaichuk, Alexander

2005-01-01

The Kyrgyz Range, the northernmost portion of the Kyrgyzstan Tien Shan, displays topographic evidence for lateral propagation of surface uplift and exhumation. The highest and most deeply dissected segment lies in the center of the range. To the east, topography and relief decrease, and preserved remnants of a Cretaceous regional erosion surface imply minimal amounts of bedrock exhumation. The timing of exhumation of range segments defines the lateral propagation rate of the range-bounding reverse fault and quantifies the time and erosion depth needed to transform a mountain range from a juvenile to a mature morphology. New apatite fission-track (AFT) data from three transects from the eastern Kyrgyz Range, combined with published AFT data, demonstrate that the range has propagated over 110 km eastwards over the last 7-11 Myr. Based on the thermal and topographic evolutionary history, we present a model for a time-varying exhumation rate driven by rock uplift and changes in erodability and the time scale of geomorphic adjustment to surface uplift. Easily eroded, Cenozoic sedimentary rocks overlying resistant basement control early, rapid exhumation and slow surface upliftrates. As increasing amounts of resistant basement are exposed, exhumation rates decrease while surface uplift rates are sustained or increase, thereby growing topography. As the range becomes high enough to cause ice accumulation and develop steep river valleys, fluvial and glacial erosion become more powerful and exhumation rates once again increase. Independently determined range-noma1 shortening rates have also varied over time, suggesting a feedback between erosional efficiency and shortening rate.

Late-Quaternary exhumation of Namche Barwa constrained using low-temperature multi-OSL-thermochronometry

NASA Astrophysics Data System (ADS)

King, Georgina; Herman, Frederic

2016-04-01

The influence of climate on tectonic processes remains a controversial concept. Exhumation rates of >5 mm/yr make Namche Barwa, Tibet, one of the most rapidly exhuming places on earth, and two main hypotheses have developed to explain the very high rates of exhumation there. The tectonic aneurysm model (Zeitler et al., 2001) proposes that crustal weakening coupled with extremely active surface processes causes a spatial stationarity of exhumation. Alternatively, a northward plunging antiform that is progressively migrating north-eastward (Seward and Burg, 2008) may explain the concentration of extremely low cooling ages and rapid exhumation that characterise the Namche Barwa massif. Here we use multi-OSL-thermochronometry of feldspar, which comprises a series of different systems with closure temperatures ranging from 30 to 70 C, to quantify spatial and temporal changes in exhumation rates. We have applied this new technique to a suite of samples from the Namche Barwa massif and inverting our data enables us to precisely resolve cooling histories over 0.1 Ma timescales. Our data indicate propagation of a knick-point along the Parlung river, which can be explained by progressive north-eastward migration of a northward plunging antiform. We suggest that river incision does not therefore feedback onto tectonics, as proposed by the aneurysm model. Zeitler, P.K., Meltzer, A.S., Koons, P.O., et al., 2001. Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism. GSA Today 11, 4-9 Seward, D., Burg, J-P., 2008. Growth of the Namche Barwa Syntaxis and associated evolution of the Tsangpo Gorge: Constraints from structural and thermochronological data. Tectonophysics 451, 282-289.

High Tibetan Plateau: a Nature Reserve of C3 Flora

NASA Astrophysics Data System (ADS)

Zhuang, G.; Pagani, M.; Brandon, M. T.

2015-12-01

Uplift of the Tibetan Plateau and associated climate changes leading to enhanced seasonality in precipitation and drying, are argued to have induced a global ecological shift from C3-frorest to C4-grassland between 8 and 5 million years ago. However, both tectonic and climatic changes predate the timing of the C3-C4 transition, with paleoaltimetry studies pointing to a high Tibetan Plateau no later than 10 Ma and the existence of an intense monsoon since ca. 11 Ma. To better understand the role of the Tibetan Plateau, we present results of a paleoecology study based on the carbon isotope compositions of leaf-wax n-alkanes (δ13Calk) from Qaidam basin, an intermontane basin on the youngest and the northernmost portion of Tibetan Plateau. Samples were collected from a 5-km thick, fluvial-lacustrine sequence dated 15 Ma to 1.8 Ma. Organic geochemical indices and detrital thermochronological studies preclude post-depositional thermal alteration of the studied materials. Our results, which show persistently low δ13Calk values, are consistent with the dominance of C3 flora on the Tibetan Plateau, in contrast to the prominent C4 rise surrounding the Tibetan Plateau and around the globe. We argue that a high Tibetan Plateau, established prior to the global ecological C3-C4 shift, served as a shelter of C3 flora since the Miocene. High-elevation and cold environments provided a competitive advantage for C3 by damping/counteracting the physiological advantage of C4 which outperforms C3 plants under warm, high-irradiance, water-stress and low pCO2 conditions.

A view into crustal evolution at mantle depths

NASA Astrophysics Data System (ADS)

Kooijman, Ellen; Smit, Matthijs A.; Ratschbacher, Lothar; Kylander-Clark, Andrew R. C.

2017-05-01

Crustal foundering is an important mechanism in the differentiation and recycling of continental crust. Nevertheless, little is known about the dynamics of the lower crust, the temporal scale of foundering and its role in the dynamics of active margins and orogens. This particularly applies to active settings where the lower crust is typically still buried and direct access is not possible. Crustal xenoliths derived from mantle depth in the Pamir provide a unique exception to this. The rocks are well-preserved and comprise a diverse set of lithologies, many of which re-equilibrated at high-pressure conditions before being erupted in their ultrapotassic host lavas. In this study, we explore the petrological and chronological record of eclogite and felsic granulite xenoliths. We utilized accessory minerals - zircon, monazite and rutile - for coupled in-situ trace-element analysis and U-(Th-)Pb chronology by laser-ablation (split-stream) inductively coupled plasma mass spectrometry. Each integrated analysis was done on single mineral zones and was performed in-situ in thin section to maintain textural context and the ability to interpret the data in this framework. Rutile thermo-chronology exclusively reflects eruption (11.17 ± 0.06Ma), which demonstrates the reliability of the U-Pb rutile thermo-chronometer and its ability to date magmatic processes. Conversely, zircon and monazite reveal a series of discrete age clusters between 55-11 Ma, with the youngest being identical to the age of eruption. Matching age populations between samples, despite a lack of overlapping ages for different chronometers within samples, exhibit the effectiveness of our multi-mineral approach. The REE systematics and age data for zircon and monazite, and Ti-in-zircon data together track the history of the rocks at a million-year resolution. The data reveal that the rocks resided at 30-40 km depth along a stable continental geotherm at 720-750 °C until 24-20 Ma, and were subsequently melted, densified, and buried to 80-90 km depth - 20 km deeper than the present-day Moho - at 930 ± 35°C. The material descended rapidly, accelerating from 0.9-1.7 mm yr-1 to 4.7-5.8 mm yr-1 within 10-12 Myr, and continued descending after reaching mantle depth at 14-13 Ma. The data reflect the foundering of differentiated deep-crustal fragments (2.9-3.5 g cm-3) into a metasomatized and less dense mantle wedge. Through our new approach in constraining the burial history of rocks, we provided the first time-resolved record of this crustal-recycling process. Foundering introduced vestiges of old evolved crust into the mantle wedge over a relatively short period (c. 10 Myr). The recycling process could explain the variability in the degree of crustal contamination of mantle-derived magmatic rocks in the Pamir and neighboring Tibet during the Cenozoic without requiring a change in plate dynamics or source region.

Oxygen isotope geospeedometry by SIMS

NASA Astrophysics Data System (ADS)

Bonamici, C. E.; Valley, J. W.

2013-12-01

Geospeedometry, a discipline closely related and complimentary to thermochronology, exploits the phenomenon of diffusion in order to extract rate and duration information for segments of a rock's thermal history. Geospeedometry data, when anchored in absolute time by geochronologic data, allow for the construction of detailed temperature-time paths for specific terranes and geologic processes. We highlight the developing field of SIMS-based oxygen isotope geospeedometry with an application from granulites of the Adirondack Mountains (New York) and discuss potential future applications based on a recently updated and expanded modeling tool, the Fast Grain Boundary diffusion program (FGB; Eiler et al. 1994). Equilibrium oxygen isotope ratios in minerals are a function of temperature and bulk rock composition. In dynamic systems, intragrain oxygen isotope zoning can develop in response to geologic events that affect the thermal state of a rock and/or induce recrystallization, especially tectonic deformation and fluid infiltration. As an example, titanite grains from late-Grenville shear zones in the northwestern Adirondack Mountains exhibit a range of δ18O zoning patterns that record post-peak metamorphic cooling, episodic fluid infiltration, and deformation-facilitated recrystallization. Many titanite grains preserve smooth, core-to-rim decreasing, diffusional δ18O profiles, which are amenable to diffusion modeling. FGB models that best fit the measured δ18O profiles indicate cooling from ~700-500°C in just 2-5 m.y., a rapid thermal change signaling the final gravitational collapse of the late-Grenville orogen. Titanite can also be utilized as a U-Pb chronometer, and comparison of δ18O and U-Pb age zoning patterns within the Adirondack titanites pins the episode of rapid cooling inferred from the δ18O record to some time between 1054 and 1047 Ma. The expanded capabilities of FGB also allow for evaluation of a range of heating-cooling histories for the Adirondack granulites. Diffusional δ18O zoning profiles in titanite are best fit by complete re-equilibration at temperatures above 675 °C followed by rapid, monotonic cooling; FGB models that include only partial re-equilibration and/or episodes of reheating along the retrograde path do not fit the observed δ18O profiles. Beyond the Adirondack titanite example, FGB can be used as a predictive tool to target either specific minerals within a rock or specific rock types within a terrane for oxygen isotope geospeedometry and zoning studies. FGB generates predictions of δ18O zoning for all minerals in a rock of a given mineralogy and heating-cooling history. Different minerals within the same rock will record different segments of the thermal and fluid history based on their individual diffusivities, phase stabilities, and propensities for deformation-induced/facilitated recrystallization. It should therefore be possible to extract long thermal histories from a single sample by measuring oxygen isotope zoning profiles across several minerals with different partial retention zones for oxygen.

Polyphase vein mineralization in the Fennoscandian Shield at Åkerlandet, Järvsand, and Laisvall along the erosional front of the Caledonian orogen, Sweden

NASA Astrophysics Data System (ADS)

Saintilan, Nicolas J.; Stephens, Michael B.; Spikings, Richard; Schneider, Jens; Chiaradia, Massimo; Spangenberg, Jorge E.; Ulianov, Alexey; Fontboté, Lluís

2017-08-01

The Åkerlandet, Järvsand, and Laisvall deposits in Sweden are calcite-fluorite-sulfide vein deposits and occurrences located close to the current erosional front of the Caledonian orogen and hosted by crystalline basement rocks in the Fennoscandian Shield. At Laisvall, basement-hosted veinlets occur beneath Ediacaran to Cambrian sandstones that host a strata-bound Pb-Zn deposit. The mineralized fractures at Åkerlandet and Järvsand occur along fault systems oriented N-S to NNW-SSE. Veins or veinlets strike NNW-SSE and NW-SE at Åkerlandet, NNE-SSW at Järvsand, and NNW-SSE and NNE-SSW to NE-SW at Laisvall. At Åkerlandet and Järvsand, fractures acted as conduits for hydrothermal fluids of variable composition and formed during separate tectonic events. At Åkerlandet, the fault zone with NNW-SSE strike shows kinematic indicators consistent with NE-SW bulk horizontal extension. At Järvsand, the calcite-fluorite-galena veins formed along R-Riedel shears related to the host N-S to NNW-SSE fault system. The kinematic indicators are consistent with NW-SE bulk horizontal extension, similar to the extensional deformation during the later part of the Caledonian orogeny (Silurian to Devonian). At Åkerlandet, adularia-quartz deposition was followed by sphalerite ± galena and finally by precipitation of fluorite and calcite. 40Ar-39Ar thermochronology of a single adularia sample did not yield a well-defined plateau age but the gas released at higher temperatures suggests an early Tonian (980 to 950 Ma) crystallization age, i.e., during the later part of the Sveconorwegian orogeny, although the data do not exclude other less likely interpretations. Previous fluid inclusion microthermometry and geochronological studies and new petrographic and geochemical results suggest that sphalerite ± galena mineralization formed from saline, relatively oxidizing, moderate-temperature, and slightly acidic hydrothermal fluids, either during the Ediacaran or the Middle Ordovician. Metals and H2S were derived from local basement rocks. Based on petrographic evidence, rare earth element composition, and S, C, and O isotope data, fluorite and calcite precipitated under near neutral and relatively reducing conditions. Occurrence of solid bitumen in veins at Åkerlandet and C and O isotope data of calcite at Åkerlandet and in the Laisvall basement veinlets suggest that the precipitation of calcite and fluorite was triggered by interaction of hot and evolved hydrothermal fluids (87Sr/86Sr = 0.718-0.732) with organic matter. Structural, petrographic, and geochemical data at Laisvall suggest that the basement structures hosting calcite-fluorite ± pyrite veinlets were utilized in the Middle Ordovician as the plumbing system for the oxidizing, slightly acidic, metal-bearing brines that caused the economic Pb-Zn mineralization in the overlying sandstones.

Apatite fission-track evidence for regional exhumation in the subtropical Eocene, block faulting, and localized fluid flow in east-central Alaska

USGS Publications Warehouse

Dusel-Bacon, Cynthia; Bacon, Charles R.; O'Sullivan, Paul B.; Day, Warren C.

2016-01-01

The origin and antiquity of the subdued topography of the Yukon–Tanana Upland (YTU), the physiographic province between the Denali and Tintina faults, are unresolved questions in the geologic history of interior Alaska and adjacent Yukon. We present apatite fission-track (AFT) results for 33 samples from the 2300 km2 western Fortymile district in the YTU in Alaska and propose an exhumation model that is consistent with preservation of volcanic rocks in valleys that requires base level stability of several drainages since latest Cretaceous–Paleocene time. AFT thermochronology indicates widespread cooling below ∼110 °C at ∼56–47 Ma (early Eocene) and ∼44–36 Ma (middle Eocene). Samples with ∼33–27, ∼19, and ∼10 Ma AFT ages, obtained near a major northeast-trending fault zone, apparently reflect hydrothermal fluid flow. Uplift and erosion following ∼107 Ma magmatism exposed plutonic rocks to different extents in various crustal blocks by latest Cretaceous time. We interpret the Eocene AFT ages to suggest that higher elevations were eroded during the Paleogene subtropical climate of the subarctic, while base level remained essentially stable. Tertiary basins outboard of the YTU contain sediment that may account for the required >2 km of removed overburden that was not carried to the sea by the ancestral Yukon River system. We consider a climate driven explanation for the Eocene AFT ages to be most consistent with geologic constraints in concert with block faulting related to translation on the Denali and Tintina faults resulting from oblique subduction along the southern margin of Alaska.

Paleozoic to early Cenozoic cooling and exhumation of the basement underlying the eastern Puna plateau margin prior to plateau growth

NASA Astrophysics Data System (ADS)

Insel, N.; Grove, M.; Haschke, M.; Barnes, J. B.; Schmitt, A. K.; Strecker, M. R.

2012-12-01

Constraining the pre-Neogene history of the Puna plateau is crucial for establishing the initial conditions that attended the early stage evolution of the southern extent of the Andean plateau. We apply high- to low-temperature thermochronology data from plutonic rocks in northwestern Argentina to quantify the Paleozoic, Mesozoic and early Tertiary cooling history of the Andean crust. U-Pb crystallization ages of zircons indicate that pluton intrusion occurred during the early mid-Ordovician (490-470 Ma) and the late Jurassic (160-150 Ma). Lower-temperature cooling histories from 40Ar/39Ar analyses of K-feldspar vary substantially. Basement rocks underlying the western Puna resided at temperatures below 200°C (<6 km depth) since the Devonian (˜400 Ma). In contrast, basement rocks underlying the southeastern Puna were hotter (˜200-300°C) throughout the Paleozoic and Jurassic and cooled to temperatures of <200°C by ˜120 Ma. The southeastern Puna basement records a rapid cooling phase coeval with active extension of the Cretaceous Salta rift at ˜160-100 Ma that we associate with tectonic faulting and lithospheric thinning. The northeastern Puna experienced protracted cooling until the late Cretaceous with temperatures <200°C during the Paleocene. Higher cooling rates between 78 and 55 Ma are associated with thermal subsidence during the postrift stage of the Salta rift and/or shortening-related flexural subsidence. Accelerated cooling and deformation during the Eocene was focused within a narrow zone along the eastern Puna/Eastern Cordillera transition that coincides with Paleozoic/Mesozoic structural and thermal boundaries. Our results constrain regional erosion-induced cooling throughout the Cenozoic to have been less than ˜150°C, which implies total Cenozoic denudation of <6-4 km.

Sichuan Basin and beyond: Eastward foreland growth of the Tibetan Plateau from an integration of Late Cretaceous-Cenozoic fission track and (U-Th)/He ages of the eastern Tibetan Plateau, Qinling, and Daba Shan

NASA Astrophysics Data System (ADS)

Yang, Zhao; Shen, Chuanbo; Ratschbacher, Lothar; Enkelmann, Eva; Jonckheere, Raymond; Wauschkuhn, Bastian; Dong, Yunpeng

2017-06-01

Combining 121 new fission track and (U-Th)/He ages with published thermochronologic data, we investigate the Late Cretaceous-Cenozoic exhumation/cooling history of the eastern Tibetan Plateau, Qinling, Daba Shan, and Sichuan Basin of east central China. The Qinling orogen shows terminal southwestward foreland growth in the northern Daba Shan thrust belt at 100-90 Ma and in the southern Daba Shan fold belt at 85-70 Ma. The eastern margin of Tibetan Plateau experienced major exhumation phases at 70-40 Ma (exhumation rate 0.05-0.08 mm/yr), 25-15 Ma (≤1 mm/yr in the Pengguan Massif; 0.2 mm/yr in the imbricated western Sichuan Basin), and since 11-10 Ma along the Longmen Shan ( 0.80 mm/yr) and the interior of the eastern Tibetan Plateau (Dadu River gorge, Min Shan; 0.50 mm/yr). The Sichuan Basin records two basin-wide denudation phases, likely a result of the reorganization of the upper Yangtze River drainage system. The first phase commenced at 45 Ma and probably ended before the Miocene; >1 km of rocks were eroded from the central and eastern Sichuan Basin. The second phase commenced at 12 Ma and denudated the central Sichuan Basin, Longmen Shan, and southern Daba Shan; more than 2 km of rocks were eroded after the lower Yangtze River had cut through the Three Gorges and captured the Sichuan Basin drainage. In contrast to the East Qinling, which was weakly effected by late Cenozoic exhumation, the West Qinling and Daba Shan have experienced rapid exhumation/cooling since 15-13 Ma, a result of growth of the Tibetan Plateau beyond the Sichuan Basin.

U-Pb Data On Apatites With Common Lead Correction : Exemples From The Scottish Caledonides

NASA Astrophysics Data System (ADS)

Jewison, E.; Deloule, E.; Villeneuve, J.; Bellahsen, N.; Labrousse, L.; Rosenberg, C.; Pik, R.; Chew, D.

2017-12-01

Apatite is a widely used mineral in low-temperature thermochronology (U-Th/He and AFT). The use of apatite in U-Pb geochronology has a great potential, given its closure temperature around 450°C, for orogen thermostructural evolution studies. However, since apatite can accumulate significant amount of initial Pb in its structure, its use can be hindered by the lack of 204 Pb estimations. To work around this, two options are commonly used : either use a ploting sytem that does not require corrected ratios, or use a proxy to estimate 204Pb and use it to correct the ratios. In this study we use a SIMS to mesure 204Pb in order to compare Tera-Wasserburg diagram and corrected ages to examine the cooling pattern in the northern Highlands of Scotland. The Highlands is an extensively studied caledonian collision wedge which results from the closure of the Iapétus Ocean during the Orodivician-Silurian. Two orogenic events are related to this closing, the grampian event (480-460Ma) and the scandian event (435-415 Ma) that culminated in the stacking of major ductile thrusts. The thermal history of thoses nappes are hence complex and the cooling pattern poorly constrained. Corrected apatite U-Pb ages provide new constrains on ductile wedge building and improve our understanding of mid to lower-crustal deformation and orogenic exhumation. Thoses corrected ages yield equivalent errors and mean ages from the classic method. Those data suggest a global cooling younger than previously thought and a sequence departing from a simple forward sequence. We thus present a refined thermal evolution and conceptualize a model of ductile wedge evolution.

Meteorite Impact Structures as Outcrop-Scale Analogues for Mountain Building Events: Weaubleau and Decaturville, MO

NASA Astrophysics Data System (ADS)

Wu, S.; McKay, M.; Evans, K. R.

2017-12-01

Understanding the architecture of mountain belts is limited because studies are typically confined to surficial exposures with lesser amounts of subsurface data and active margins are prone to successive tectonism that obscures the rock record. In west-central Missouri, two Paleozoic meteorite impacts are exposed that contain a range of outcrop-scale structures. While the strain rate in a meteorite impact is an order of magnitude greater than that in orogeny-scale structures, the morphology and spatial relationships in these impact structures may provide insight into larger tectonic features. The entire crater could not be compared to an orogenic event because the amount of strain diffuses as distance increases from the impactor during an impacting event. The center of an impact crater could not be compared to an orogenic event because it has become too deformed. However, the crater rim and the immediate surrounding area could be used as a comparison because it has undergone the right amount of deformation to have recognizable structures. High-detail mapping and structural analyses of road cut exposures near Decaturville, MO reveals thrust fault sequences contain 1-2 m thick mixed carbonate and clastic sheets that include rollover anticlines, structural orphans, and lateral ramp features. Thrust faults dip away from the impact structure and represent gravitational collapse of the central uplift seconds after collision. Thrust sheet thickness, thrust fault spacing, ramp/flat morphology, and shortening of within these structures will be presented and assessed as an analogue for map-scale features in the Southern Appalachian fold and thrust belt. Because temperature controls rock mechanic properties, a thermal model based on thermochronology and thermobarometry for the section will also be presented and discussed in the context of orogenic thermomechanics.

Late Quaternary slip history of the Mill Creek strand of the San Andreas fault in San Gorgonio Pass, southern California: The role of a subsidiary left-lateral fault in strand switching

USGS Publications Warehouse

Kendrick, Katherine J.; Matti, Jonathan; Mahan, Shannon

2015-01-01

The fault history of the Mill Creek strand of the San Andreas fault (SAF) in the San Gorgonio Pass region, along with the reconstructed geomorphology surrounding this fault strand, reveals the important role of the left-lateral Pinto Mountain fault in the regional fault strand switching. The Mill Creek strand has 7.1–8.7 km total slip. Following this displacement, the Pinto Mountain fault offset the Mill Creek strand 1–1.25 km, as SAF slip transferred to the San Bernardino, Banning, and Garnet Hill strands. An alluvial complex within the Mission Creek watershed can be linked to palinspastic reconstruction of drainage segments to constrain slip history of the Mill Creek strand. We investigated surface remnants through detailed geologic mapping, morphometric and stratigraphic analysis, geochronology, and pedogenic analysis. The degree of soil development constrains the duration of surface stability when correlated to other regional, independently dated pedons. This correlation indicates that the oldest surfaces are significantly older than 500 ka. Luminescence dates of 106 ka and 95 ka from (respectively) 5 and 4 m beneath a younger fan surface are consistent with age estimates based on soil-profile development. Offset of the Mill Creek strand by the Pinto Mountain fault suggests a short-term slip rate of ∼10–12.5 mm/yr for the Pinto Mountain fault, and a lower long-term slip rate. Uplift of the Yucaipa Ridge block during the period of Mill Creek strand activity is consistent with thermochronologic modeled uplift estimates.

Control of rift asymmetry and segmentation on the thermal architecture of hyperextended rift systems: insights from Pyrenean field observations and numerical modelling

NASA Astrophysics Data System (ADS)

Lescoutre, Rodolphe; Tugend, Julie; Brune, Sascha; Manatschal, Gianreto

2017-04-01

Mid-Cretaceous rift basins are exposed in the Pyrenees providing key information on rifted domain formation that is not available at present-day rift system. Substantial paleotemperature and thermochronological data have been collected and published in numerous recent papers. These data show a strong heterogeneity in the distribution of peak temperatures within the Cretaceous rift basins. Locations that experienced relatively high or low temperatures appear to cluster in specific areas along strike. These areas have been interpreted as either reflecting hot and cold conditions during rifting, or alternatively, a change in the polarity of a strongly asymmetric rift systems. In this study, we test if the observed variability of peak temperatures can be explained by segmentation and a change in polarity of an asymmetrical upper/lower plate rift model. To this aim we restore the observed syn- to early post-rift peak temperatures to their paleo-location within sections across the evolving rift system. In the meantime, we conduct numerical models of rift migration leading to asymmetrical extension that are benchmarked with geological and geophysical observations from the Pyrenees. From the models, we extract thermal information at different stages of rifting that are finally compared to the thermal data from the Pyrenean Cretaceous rift basins. This work employs a novel approach by comparing thermal output from numerical modelling with the distribution of peak temperatures and thermal gradient from field data. As such, these results may have substantial implications to further understand the pre-orogenic thermal evolution of the Pyrenean rift system and the role of segmentation. More generally, the results of this work may unravel the role of rift asymmetry and segmentation on the thermal architecture of hyperextended rift basins and margins.

Spatial distribution of erosion in the Hangay Mountains of Mongolia and implications for the development of epeirogenic topography

NASA Astrophysics Data System (ADS)

West, A.; Fox, M.; Walker, R. T.; Carter, A.; Watts, A. B.; Gantulga, B.

2012-12-01

Potential feedbacks between climate-driven erosion and the development of intra-continental topography have received relatively little attention, particularly compared to the significant efforts to understand the interplay of climate, erosion, and uplift in orogenic settings. But such links may be vital for understanding the topographic evolution of epeirogenic topography and for making inferences about geodynamic processes based on associated sedimentary and geomorphic signals. In this study, we consider the role of orographically-driven climate variability in shaping continental topography by focusing on the Hangay mountain range, a uplifted dome in central Mongolia. The work presented here is based on results from a topographic analysis of the Hangay, making use of the flat-topped peaks that effectively represent preserved remnants of a pre-erosional surface. We have determined the scale and distribution of erosion by recreating this pre-erosional surface and subtracting the present-day, dissected topography. Our results show that the extent of erosion correlates with spatial variation in mean annual precipitation, but not with the extent of total surface uplift. The morphology of the range reflects the higher, climate-driven fluvial erosion rates by northern rivers that receive higher precipitation when compared to the southern rivers, which have steeper relief as a result of the asymmetric main drainage divide. Overall asymmetry in inferred isostatic response to erosional unloading is not mirrored in asymmetry of total surface uplift, hinting at interaction between surface erosion and the forces sustaining topography. This has important implications for understanding the geodynamics of epeirogenic uplift. In addition to these main outcomes from our topographic analysis, we will also present preliminary findings from detrital thermochronology and cosmogenic analyses that help to pinpoint the location of erosion and provide a basis for quantifying rates.

Empirical test of an illite/muscovite 40Ar/39Ar thermochronometer

NASA Astrophysics Data System (ADS)

Verdel, C.; van der Pluijm, B. A.; Niemi, N. A.; Hall, C. M.

2010-12-01

Minerals which both preserve age information and indicate metamorphic conditions are particularly useful in thermochronology. Variations in sub-greenschist facies metamorphism have traditionally been quantified in terms of the illite to muscovite transition, a transformation which involves the growth of crystallites of increasing thickness at higher metamorphic temperatures. Thickness variations may influence Ar retention within these K-rich minerals, both in nature and during neutron irradiation. Along a transect in the southwestern US from the Grand Canyon to Death Valley, metamorphic conditions of a stratigraphic interval (the Middle Cambrian Bright Angel Shale and laterally equivalent Carrara Fm.) range from zeolite facies in the east to greenschist facies in the west, as determined by estimating illite crystallite thickness with X-ray diffraction. 40Ar/39Ar step-heating experiments were conducted on illite/muscovite-rich, micron to submicron grain sizes of these shales that were encapsulated in quartz tubes prior to irradiation. The proportion of 39Ar expelled during irradiation decreases in these samples as both crystallite thickness and grain size increases. Spectra from the least metamorphosed samples (diagenetic zone) are staircase-shaped and reach maximum ages that appear to reflect the age of detrital muscovite. Spectra from the highest grade samples (epizone) display partial plateaus and yield much younger maximum ages. Based on these findings we conclude that Ar can escape from illite via two processes: loss from low retention sites on crystallite edges and c-axis perpendicular volume diffusion. Based on our empirical data, the closure temperature of illite appears to lie at or near the anchizone-epizone bounday, or roughly 200-300 °C. Illite/muscovite thickness and 40Ar/39Ar data may therefore be useful for studies of detrital muscovite geochronology in very low grade shales and as a thermochronometer for higher grade pelites.

Detrital Thermochronology Record of the Local-to-Extraregional Provenance Shift Recorded by the Northern Peninsular Ranges Forearc

NASA Astrophysics Data System (ADS)

Flores, M.; Shulaker, D. Z.

2016-12-01

Previously measured detrital zircon U-Pb age distributions have revealed that Late Cretaceous to Eocene forearc strata in the Santa Ana mountains region experienced a dramatic shift in sedimentary provenance from a 125-90 Ma northern Peninsular Ranges batholith (PRB) source region along the continental margin to a cratonal source area intruded by Late Cretaceous (85-75 Ma) plutons within the continental interior (western Sonora). To improve our understanding of the timing and magnitude of denudation prior to, and during this local to extraregional provenance shift, we have measured new detrital K-feldspar 40Ar/39Ar total fusion and zircon U-Pb age distributions from forearc sandstones. Our combined crystallization age and thermal history results confirm two pulses of rapid denudation of the PRB. These include a syn-batholith phase (Cenomanian) and a post-batholithic (Maastrichtian to Paleocene) phase attributed to shallow subduction. The new data require that significant (5-10 km) syn-batholithic erosional denudation of the northern PRB had already occurred by ca. 95 Ma and that post-emplacement denudation of the PRB accelerated again during the Maastrichtian with the eastern PRB providing the bulk of the detritus. Our new results demonstrate that deposition of the Paleocene Silverado Fm. occurred during a transitional period. Sand reaching the forearc during the Paleocene was derived from both the heavily denuded eastern PRB with additional input supplied either from Late Cretaceous plutons of northwestern Sonoran region and/or similar plutons emplaced in thrust sheets atop PRB basement within the Santa Rosa Mountains. In contrast, the Eocene Santiago Fm. was nearly entirely derived from extraregional sources that supplied abundant late Cretaceous (75-85 Ma) and Proterozoic (1.4 & 1.7 Ga) detritus with virtually no PRB-derived sediment detected.

Correlating rates of magmatic arc unroofing and sedimentation using detrital zircon U/Pb and (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Fosdick, J. C.

2017-12-01

Double and triple dating of minerals using multiple geo-thermochronometers has revolutionized efforts to evaluate complex thermotectonic histories of orogens, isolate unique sedimentary sources, and quantify basin burial reheating. A persisting challenge is to distinguish volcanic sources from rapidly exhumed sources, with the simplistic premise that coincident cooling dates among high- to low-temperature thermochronometers are diagnostic of volcanic sources. Coupled zircon U/Pb and (U-Th)/He geo-thermochronometry from the Miocene Bermejo foreland basin in the southern Central Andes reveals a high temporal resolution of unroofing signatures of the Choiyoi Group, a Permian-Triassic silicic volcanic and plutonic complex, and the Pennsylvanian-Permian Colangüil batholith. Both units are important sediment sources within the High Andes for the Cenozoic east-flowing sediment routing systems. Results show fluvial sourcing of Colangüil detrital zircons with progressively greater partial loss of He (<8% to 12-23% fractional loss from 9.5 Ma to 6 Ma), as indicated by upsection younging of zircon He dates for a given U/Pb age cluster. These findings suggest erosion of increasingly deeper levels of the Colangüil arc during late Miocene development of the High Andes. This progression of higher He loss and thus younger He dates during sedimentation for a given U/Pb age cluster is analogous to the magmatic arc unroofing trend revealed by undissected to dissected arc provenance fields in sandstone petrography. Multi-method thermochronometry of detrital minerals may reveal an added level of information regarding rates of cooling, unroofing, and thermal evolution of magmatic systems as preserved in the detrital record.

Contrasting accessory mineral behavior in minimum-temperature melts: Empirical constraints from the Himalayan metamorphic core

NASA Astrophysics Data System (ADS)

Cottle, John M.; Larson, Kyle P.; Yakymchuk, Chris

2018-07-01

Medium-grained leucogranite in the Tama Kosi region of the Nepalese Himalayan Metamorphic Core yields a relatively narrow range of monazite 208Pb/232Th dates with a dominant population at 21.0 Ma inferred to represent crystallization of an early plutonic phase. In contrast, the pegmatitic portion of the same intrusive complex, that cross-cuts the medium-grained leucogranite, contains zircon, monazite and xenotime that each display near-identical age spectra, recording semi-continuous (re-)crystallization from 27.5 Ma to 21.0 Ma, followed by a 2 m.y. hiatus then further (re-)crystallization between 19.4 and 18.6 Ma. The "gap" in pegmatite dates corresponds well to the crystallization age of the older leucogranite, whereas the end of accessory phase growth in the pegmatite coincides with the onset of regional-scale cooling. Detailed textural, trace element and thermochronologic data indicate that the range of zircon, monazite and xenotime dates recorded in the pegmatite reflect inherited components that underwent semi-continuous (re-)crystallization during metamorphism and/or anatexis in the source region(s), whereas dates younger than the hiatus indicate accessory phase recrystallization, related to both fluid influx and a concomitant increase in temperature. In contrast, the lack of an inherited component(s) in the medium-grained leucogranite phase is inferred to be a result of complete dissolution during partial melting. A model is proposed in which influx of heat and H2O-rich fluids associated with early leucogranite emplacement temporarily delayed zircon and monazite and xenotime crystallization, respectively. These data highlight the importance of measuring spatially resolved dates, trace elements and textural patterns from multiple accessory minerals combined with model constraints to better understand the often-complex crystallization history of anatectic melts in collisional orogens.

Neogene shortening and exhumation of the Zagros fold-thrust belt and foreland basin in the Kurdistan region of northern Iraq

NASA Astrophysics Data System (ADS)

Koshnaw, Renas I.; Horton, Brian K.; Stockli, Daniel F.; Barber, Douglas E.; Tamar-Agha, Mazin Y.; Kendall, Jerome J.

2017-01-01

The Zagros fold-thrust belt in the Kurdistan region of Iraq encroached southward toward a rapidly subsiding Neogene foreland basin and was later partitioned by out-of-sequence shortening focused along the Mountain Front Flexure (MFF), as defined by new low-temperature thermochronologic, stratigraphic, and provenance results. Apatite (U-Th)/He ages document rapid deformation advance from the Main Zagros Fault to southern frontal structures (Kirkuk, Shakal, and Qamar thrusts) at 10-8 Ma, followed by potential basement-involved out-of-sequence development of the MFF (Qaradagh anticline) by 5 Ma. Distinct shifts in detrital zircon U-Pb provenance signatures for Neogene foreland basin fill provide evidence for drainage reorganization during fold-thrust belt advance. U-Pb age spectra and petrologic data from the Injana (Upper Fars) Formation indicate derivation from a variety of Eurasian, Pan-African, ophiolitic and Mesozoic-Cenozoic volcanic terranes, whereas the Mukdadiya (Lower Bakhtiari) and Bai-Hasan (Upper Bakhtiari) Formations show nearly exclusive derivation from the Paleogene Walash-Naopurdan volcanic complex near the Iraq-Iran border. Such a sharp cutoff in Eurasian, Pan-African, and ophiolitic sources is likely associated with drainage reorganization and tectonic development of the geomorphic barrier formed by the MFF. As a result of Zagros crustal shortening, thickening and loading, the Neogene foreland basin developed and accommodated an abrupt influx of fluvial clastic sediment that contains growth stratal evidence of synkinematic accumulation. The apparent out-of-sequence pattern of upper crustal shortening in the hinterland to foreland zone of Iraqi Kurdistan suggests that structural inheritance and the effects of synorogenic erosion and accumulation are important factors influencing the irregular and episodic nature of orogenic growth in the Zagros.

Upper Cretaceous and Lower Jurassic strata in shallow cores on the Chukchi Shelf, Arctic Alaska: Chapter C in Studies by the U.S. Geological Survey in Alaska, vol. 15

USGS Publications Warehouse

Houseknecht, David W.; Craddock, William H.; Lease, Richard O.

2016-02-12

Shallow cores collected in the 1980s on the Chukchi Shelf of western Arctic Alaska sampled pre-Cenozoic strata whose presence, age, and character are poorly known across the region. Five cores from the Herald Arch foreland contain Cenomanian to Coniacian strata, as documented by biostratigraphy, geochronology, and thermochronology. Shallow seismic reflection data collected during the 1970s and 1980s show that these Upper Cretaceous strata are truncated near the seafloor by subtle angular unconformities, including the Paleogene mid-Brookian unconformity in one core and the Pliocene-Pleistocene unconformity in four cores. Sedimentary structures and lithofacies suggest that Upper Cretaceous strata were deposited in a low accommodation setting that ranged from low-lying coastal plain (nonmarine) to muddy, shallow-marine environments near shore. These observations, together with sparse evidence from the adjacent western North Slope, suggest that Upper Cretaceous strata likely were deposited across all of Arctic Alaska.A sixth core from the Herald Arch contains lower Toarcian marine strata, indicated by biostratigraphy, truncated by a Neogene or younger unconformity. These Lower Jurassic strata evidently were deposited south of the arch, buried structurally to high levels of thermal maturity during the Early Cretaceous, and uplifted on the Herald thrust-fault system during the mid to Late Cretaceous. These interpretations are based on regional stratigraphy and apatite fission-track data reported in a complementary report and are corroborated by the presence of recycled palynomorphs of Early Jurassic age and high thermal maturity found in Upper Cretaceous strata in two of the foreland cores. This dataset provides evidence that uplift and exhumation of the Herald thrust belt provided sediment to the foreland during the Late Cretaceous.

Late Cenozoic extension and crustal doming in the India-Eurasia collision zone: New thermochronologic constraints from the NE Chinese Pamir

NASA Astrophysics Data System (ADS)

Thiede, Rasmus C.; Sobel, Edward R.; Chen, Jie; Schoenbohm, Lindsay M.; Stockli, Daniel F.; Sudo, Masafumi; Strecker, Manfred R.

2013-06-01

northward motion of the Pamir indenter with respect to Eurasia has resulted in coeval thrusting, strike-slip faulting, and normal faulting. The eastern Pamir is currently deformed by east-west oriented extension, accompanied by uplift and exhumation of the Kongur Shan (7719 m) and Muztagh Ata (7546 m) gneiss domes. Both domes are an integral part of the footwall of the Kongur Shan extensional fault system (KES), a 250 km long, north-south oriented graben. Why active normal faulting within the Pamir is primarily localized along the KES and not distributed more widely throughout the orogen has remained unclear. In addition, relatively little is known about how deformation has evolved throughout the Cenozoic, despite refined estimates on present-day crustal deformation rates and microseismicity, which indicate where crustal deformation is presently being accommodated. To better constrain the spatiotemporal evolution of faulting along the KES, we present 39 new apatite fission track, zircon U-Th-Sm/He, and 40Ar/39Ar cooling ages from a series of footwall transects along the KES graben shoulder. Combining these data with present-day topographic relief, 1-D thermokinematic and exhumational modeling documents successive stages, rather than synchronous deformation and gneiss dome exhumation. While the exhumation of the Kongur Shan commenced during the late Miocene, extensional processes in the Muztagh Ata massif began earlier and have slowed down since the late Miocene. We present a new model of synorogenic extension suggesting that thermal and density effects associated with a lithospheric tear fault along the eastern margin of the subducting Alai slab localize extensional upper plate deformation along the KES and decouple crustal motion between the central/western Pamir and eastern Pamir/Tarim basin.

Late Cretaceous-recent tectonic assembly of diverse crustal blocks in Central America, the Nicaraguan Rise, the Colombian Basin and northern South America as seen on a 1600-km-long, geologic and structural transect

NASA Astrophysics Data System (ADS)

Sanchez, J.; Mann, P.

2015-12-01

We have constructed a 1600-km-long transect from northern Honduras to northern Colombia that crosses northeastward-striking crustal blocks using a combination of offshore seismic data, gravity and magnetic data, well subsidence information, nearby outcrop information, and results from previous thermochronological, geochronological, geochemical and paleostress studies. The transect defines three major crustal and structural provinces: 1) Precambrian-Paleozoic, Chortis continental block whose northern edge is defined by the North America-Caribbean plate boundary. Events in this ~20-25-km-thick province include two major unconformities at the top of the Cretaceous and Eocene, associated southeast-dipping thrust faults related to collision of the Great Arc of the Caribbean (GAC) and Caribbean Large Igneous Province (CLIP) with the Chortis continental block. A third event is Eocene to recent subsidence and transtensional basins formed during the opening of the Cayman trough; 2) Late Cretaceous GAC and CLIP of oceanic arc and plateau origin, whose northern, deformed edge corresponds to the mapped Siuna belt of northern Nicaragua. This crustal province has a ~15-20-km-thick crust and is largely undeformed and extends across the Lower Nicaraguan Rise, Hess fault, to the southern limit of the Colombian basin where about 300 km of this province has been subducted beneath the accretionary wedge of the South Caribbean deformed belt of northwestern South America; and 3) Eocene to recent accretionary prism and intramontane basins on continental crust of northern South America, where Miocene accelerated exhumation and erosion of Paleogene and Cretaceous rocks reflect either shallow subduction of the CLIP or the Panama collisional event to the southwest.

Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust

USGS Publications Warehouse

McCubbin, Francis M.; Boyce, Jeremy W.; Novak-Szabo, Timea; Santos, Alison; Tartese, Romain; Muttik, Nele; Domokos, Gabor; Vazquez, Jorge A.; Keller, Lindsay P.; Moser, Desmond E.; Jerolmack, Douglas J.; Shearer, Charles K.; Steele, Andrew; Elardo, Stephen M.; Rahman, Zia; Anand, Mahesh; Delhaye, Thomas; Agee, Carl B.

2016-01-01

The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali-rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K-Ar, U-Pb in apatite, and U-Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre-Amazonian high-temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification.

Quantifying the role of mantle forcing, crustal shortening and exogenic forcing on exhumation of the North Alpine Foreland Basin

NASA Astrophysics Data System (ADS)

von Hagke, C.; Luijendijk, E.; Hindle, D.

2017-12-01

In contrast to the internal zones of orogens, where the stacking of thrust sheets can overwhelm more subtle signals, foreland basins can record long-wavelength subsidence or uplift signals caused by mantle processes. We use a new and extensive compilation of geological and thermochronology data from the North Alpine Foreland Basin to understand the dynamics of foreland basins and their interaction with surface and geodynamic processes. We quantify cooling and exhumation rates in the basin by combining published and new vitrinite reflectance, apatite fission track and U-Th/He data with a new inverse burial and thermal history model, pybasin. No correlation is obvious between inferred cooling and exhumation rates and elevation, relief or tectonics. Uncertainty analysis shows that thermochronometers can be explained by cooling starting as early as the Miocene or as late as the Pleistocene. We compare derived temperature histories to exhumation estimates based on the retro-deformation of Molasse basin and the Jura mountains, and to exhumation caused by drainage reorganization and incision. Drainage reorganization can explain at most 25% of the observed cooling rates in the basin. Tectonic transport of the basin's sediments over the inclined basement of the alpine foreland as the Jura mountains shortened can explain part of the cooling signal in the western part of the basin. However, overall a substantial amount of cooling and exhumation remains unexplained by known tectonic and surface processes. Our results document basin wide exhumation that may be related to slab roll-back or other lithospheric processes. We suggest that new (U-Th)/He data from key areas close to the Alpine front may provide better constraints on the timing of exhumation.

Investigating links between climate and orography in the central Andes: Coupling erosion and precipitation using a physical-statistical model

NASA Astrophysics Data System (ADS)

Lowman, Lauren E. L.; Barros, Ana P.

2014-06-01

Prior studies evaluated the interplay between climate and orography by investigating the sensitivity of relief to precipitation using the stream power erosion law (SPEL) for specified erosion rates. Here we address the inverse problem, inferring realistic spatial distributions of erosion rates for present-day topography and contemporaneous climate forcing. In the central Andes, similarities in the altitudinal distribution and density of first-order stream outlets and precipitation suggest a direct link between climate and fluvial erosion. Erosion rates are estimated with a Bayesian physical-statistical model based on the SPEL applied at spatial scales that capture joint hydrogeomorphic and hydrometeorological patterns within five river basins and one intermontane basin in Peru and Bolivia. Topographic slope and area data were generated from a high-resolution (˜90 m) digital elevation map, and mean annual precipitation was derived from 14 years of Tropical Rainfall Measuring Mission 3B42v.7 product and adjusted with rain gauge data. Estimated decadal-scale erosion rates vary between 0.68 and 11.59 mm/yr, with basin averages of 2.1-8.5 mm/yr. Even accounting for uncertainty in precipitation and simplifying assumptions, these values are 1-2 orders of magnitude larger than most millennial and million year timescale estimates in the central Andes, using various geological dating techniques (e.g., thermochronology and cosmogenic nuclides), but they are consistent with other decadal-scale estimates using landslide mapping and sediment flux observations. The results also reveal a pattern of spatially dependent erosion consistent with basin hypsometry. The modeling framework provides a means of remotely estimating erosion rates and associated uncertainties under current climate conditions over large regions. 2014. American Geophysical Union. All Rights Reserved.

A Bayesian Hierarchical Modeling Scheme for Estimating Erosion Rates Under Current Climate Conditions

NASA Astrophysics Data System (ADS)

Lowman, L.; Barros, A. P.

2014-12-01

Computational modeling of surface erosion processes is inherently difficult because of the four-dimensional nature of the problem and the multiple temporal and spatial scales that govern individual mechanisms. Landscapes are modified via surface and fluvial erosion and exhumation, each of which takes place over a range of time scales. Traditional field measurements of erosion/exhumation rates are scale dependent, often valid for a single point-wise location or averaging over large aerial extents and periods with intense and mild erosion. We present a method of remotely estimating erosion rates using a Bayesian hierarchical model based upon the stream power erosion law (SPEL). A Bayesian approach allows for estimating erosion rates using the deterministic relationship given by the SPEL and data on channel slopes and precipitation at the basin and sub-basin scale. The spatial scale associated with this framework is the elevation class, where each class is characterized by distinct morphologic behavior observed through different modes in the distribution of basin outlet elevations. Interestingly, the distributions of first-order outlets are similar in shape and extent to the distribution of precipitation events (i.e. individual storms) over a 14-year period between 1998-2011. We demonstrate an application of the Bayesian hierarchical modeling framework for five basins and one intermontane basin located in the central Andes between 5S and 20S. Using remotely sensed data of current annual precipitation rates from the Tropical Rainfall Measuring Mission (TRMM) and topography from a high resolution (3 arc-seconds) digital elevation map (DEM), our erosion rate estimates are consistent with decadal-scale estimates based on landslide mapping and sediment flux observations and 1-2 orders of magnitude larger than most millennial and million year timescale estimates from thermochronology and cosmogenic nuclides.

Superimposed extension and shortening in the southern Salinas Basin and La Panza Range, California: A guide to Neogene deformation in the Salinian block of the central California Coast Ranges

USGS Publications Warehouse

Colgan, Joseph P.; McPhee, Darcy K.; McDougall, Kristin; Hourigan, Jeremy K.

2013-01-01

We synthesized data from geologic maps, wells, seismic-reflection profiles, potential-field interpretations, and low-temperature thermochronology to refine our understanding of late Cenozoic extension and shortening in the Salinian block of the central California Coast Ranges. Data from the La Panza Range and southern Salinas Basin document early to middle Miocene extension, followed by Pliocene and younger shortening after a period of little deformation in the late Miocene. Extension took place on high-angle normal faults that accommodated ∼2% strain at the scale of the ∼50-km-wide Salinian block (oriented perpendicular to the San Andreas fault). Shortening was accommodated by new reverse faults, reactivation of older normal faults, and strike-slip faulting that resulted in a map-view change in the width of the Salinian block. The overall magnitude of shortening was ∼10% strain, roughly 4–5 times greater than the amount of extension. The timing and magnitude of deformation in our study area are comparable to that documented in other Salinian block basins, and we suggest that the entire block deformed in a similar manner over a similar time span. The timing and relative magnitude of extension and shortening may be understood in the context of central Coast Range tectonic boundary conditions linked to rotation of the western Transverse Ranges at the south end of the Salinian block. Older models for Coast Range shortening based on balanced fault-bend fold-style cross sections are a poor approximation of Salinian block deformation, and may lead to mechanically improbable fault geometries that overestimate the amount of shortening.

Topographic Evolution and Climate Aridification during Continental Collision: Insights from Computer Simulations

PubMed Central

2015-01-01

How do the feedbacks between tectonics, sediment transport and climate work to shape the topographic evolution of the Earth? This question has been widely addressed via numerical models constrained with thermochronological and geomorphological data at scales ranging from local to orogenic. Here we present a novel numerical model that aims at reproducing the interaction between these processes at the continental scale. For this purpose, we combine in a single computer program: 1) a thin-sheet viscous model of continental deformation; 2) a stream-power surface-transport approach; 3) flexural isostasy allowing for the formation of large sedimentary foreland basins; and 4) an orographic precipitation model that reproduces basic climatic effects such as continentality and rain shadow. We quantify the feedbacks between these processes in a synthetic scenario inspired by the India-Asia collision and the growth of the Tibetan Plateau. We identify a feedback between erosion and crustal thickening leading locally to a <50% increase in deformation rates in places where orographic precipitation is concentrated. This climatically-enhanced deformation takes place preferentially at the upwind flank of the growing plateau, specially at the corners of the indenter (syntaxes). We hypothesize that this may provide clues for better understanding the mechanisms underlying the intriguing tectonic aneurisms documented in the Himalayas. At the continental scale, however, the overall distribution of topographic basins and ranges seems insensitive to climatic factors, despite these do have important, sometimes counterintuitive effects on the amount of sediments trapped within the continent. The dry climatic conditions that naturally develop in the interior of the continent, for example, trigger large intra-continental sediment trapping at basins similar to the Tarim Basin because they determine its endorheic/exorheic drainage. These complex climatic-drainage-tectonic interactions make the development of steady-state topography at the continental scale unlikely. PMID:26244662

40Ar/39Ar thermochronology of mesoproterozoic metamorphism in the Colorado Front Range

USGS Publications Warehouse

Shaw, C.A.; Snee, L.W.; Selverstone, J.; Reed, J.C.

1999-01-01

A low-pressure metamorphic episode in the Colorado Front Range has been identified by the presence of staurolite, andalusite, cordierite, and garnet porphyroblasts overprinting earlier assemblages. The overprinting assemblages and reaction textures are most consistent with porphyroblast growth on a prograde metamorphic path with peak temperatures exceeding ~525??C. Twenty-eight 40Ar/39Ar dates on hornblende, muscovite, biotite, and microcline were used to infer the age and thermal conditions of metamorphism. Muscovite and biotite 40Ar/39Ar ages fall mainly in the interval 1400-1340 Ma, consistent with cooling through the closure temperature interval of micas (~400??-300??C) after about 1400 Ma. In contrast, hornblende apparent ages (T(c)~500??-550??C) between 1600 and 1390 Ma reflect variable retention of radiogenic argon. Forward modeling of argon diffusion shows that the distribution of hornblende and mica ages is consistent with the partial resetting of argon systematics ca. 1400 Ma by a thermal pulse reaching maximum temperatures around 550??C and decaying within <20 m.yr. These temperatures match the conditions inferred from the overprinting assemblage; thus, muscovite and biotite ages are interpreted to date the cooling phase of this metamorphic event. This late metamorphism is broadly coeval with the intrusion of ca. 1400-Ma granitic plutons in the study area and throughout the southwestern United States. However, thermal effects are observed far from pluton margins, suggesting pervasive, regional crustal heating rather than restricted contact metamorphism. Our results suggest that ca. 1400-Ma metamorphism and plutonism are manifestations of a regional thermal episode that both partially melted the lower crust and pervasively metamorphosed middle crustal rocks.

Thermochronology and tectonics of the Leeward Antilles: Evolution of the southern Caribbean Plate boundary zone

USGS Publications Warehouse

van der Lelij, Roelant; Spikings, Richard A.; Kerr, Andrew C.; Kounov, Alexandre; Cosca, Michael; Chew, David; Villagomez, Diego

2010-01-01

Tectonic reconstructions of the Caribbean Plate are severely hampered by a paucity of geochronologic and exhumation constraints from anastomosed basement blocks along its southern margin. New U/Pb, 40Ar/39Ar, apatite fission track, and apatite (U-Th)/He data constrain quantitative thermal and exhumation histories, which have been used to propose a model for the tectonic evolution of the emergent parts of the Bonaire Block and the southern Caribbean Plate boundary zone. An east facing arc system intruded through an oceanic plateau during ~90 to ~87 Ma and crops out on Aruba. Subsequent structural displacements resulted in >80°C of cooling on Aruba during 70–60 Ma. In contrast, exhumation of the island arc sequence exposed on Bonaire occurred at 85–80 Ma and 55–45 Ma. Santonian exhumation on Bonaire occurred immediately subsequent to burial metamorphism and may have been driven by the collision of a west facing island arc with the Caribbean Plate. Island arc rocks intruded oceanic plateau rocks on Gran Roque at ~65 Ma and exhumed rapidly at 55–45 Ma. We attribute Maastrichtian-Danian exhumation on Aruba and early Eocene exhumation on Bonaire and Gran Roque to sequential diachronous accretion of their basement units to the South American Plate. Widespread unconformities indicate late Eocene subaerial exposure. Late Oligocene–early Miocene dextral transtension within the Bonaire Block drove subsidence and burial of crystalline basement rocks of the Leeward Antilles to ≤1 km. Late Miocene–recent transpression caused inversion and ≤1 km of exhumation, possibly as a result of the northward escape of the Maracaibo Block.

A prolonged Cenozoic erosional period in East Kunlun (Western China): Constraints of detrital apatite (U-Th)/He ages on the onset of mountain building along the northern margin of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Shi, Wenbei; Wang, Fei; Wu, Lin; Yang, Liekun; Zhang, Weibin; Wang, Yinzhi

2018-01-01

The timing of the development of the highland topography along the northern margin of the Tibetan Plateau is the key to understanding the manner of Tibetan growth and the far-field effects of the collision between India and Eurasia. Although various studies have investigated the exhumation and uplift of the basement rocks, thermochronological studies on the detritus from a continuous depositional sequence inan intermontane basin are lacking but might shed light on this issue. In this study, a set of detrital apatites from an upper Eocene to Pliocene depositional sequence in the Kumukol Basin, the largest intermontane basin in the East Kunlun, were dated by using the (U-Th)/He technique. The detrital apatites had consistent ages of ∼40 Ma for all deposit formations from bottom to top, which were older than the depositional ages. The ages and their consistency suggested that these detrital apatites were not reset and that the ages represented the original ages in the provenance region. These detrital apatite ages from the upper Eocene to Pliocene formations strongly suggested that the high mountains surrounding the Kumukol Basin formed at or before ∼40 Ma and have remained high since then. The process of mountain building resulted in a prolonged period of erosion from ∼40 to 2.6 Ma. The coupling of mountain and basin at ∼40 Ma was widespread along the East Kunlun. Our present results constrain the time of onset of the mountain building along the northern margin of the Tibetan Plateau, which may represent a rapid response to the India-Eurasian collision.

Changing exhumation patterns during Cenozoic growth and glaciation of the Alaska Range: Insights from detrital thermochronology and geochronology

USGS Publications Warehouse

Lease, Richard O.; Haeussler, Peter J.; O'Sullivan, Paul

2016-01-01

Cenozoic growth of the Alaska Range created the highest topography in North America, but the space-time pattern and drivers of exhumation are poorly constrained. We analyzed U/Pb and fission-track double dates of detrital zircon and apatite grains from 12 catchments that span a 450 km length of the Alaska Range to illuminate the timing and extent of exhumation during different periods. U/Pb ages indicate a dominant Late Cretaceous to Oligocene plutonic provenance for the detrital grains, with only a small percentage of grains recycled from the Mesozoic and Paleozoic sedimentary cover. Fission-track ages record exhumation during Alaska Range growth and incision and reveal three distinctive patterns. First, initial Oligocene exhumation was focused in the central Alaska Range at ~30 Ma and expanded outward along the entire length of the range until 18 Ma. Oligocene exhumation, coeval with initial Yakutat microplate collision >600 km to the southeast, suggests a far-field response to collision that was localized by the Denali Fault within a weak Mesozoic suture zone. Second, the variable timing of middle to late Miocene exhumation suggests independently evolving histories influenced by local structures. Time-transgressive cooling ages suggest successive rock uplift and erosion of Mounts Foraker (12 Ma) through Denali (6 Ma) as crust was advected through a restraining bend in the Denali Fault and indicate a long-term slip rate ~4 mm/yr. Third, Pliocene exhumation is synchronous (3.7–2.7 Ma) along the length of the Alaska Range but only occurs in high-relief, glacier-covered catchments. Pliocene exhumation may record an acceleration in glacial incision that was coincident with the onset of Northern Hemisphere glaciation.

Topographic Evolution and Climate Aridification during Continental Collision: Insights from Computer Simulations.

PubMed

Garcia-Castellanos, Daniel; Jiménez-Munt, Ivone

2015-01-01

How do the feedbacks between tectonics, sediment transport and climate work to shape the topographic evolution of the Earth? This question has been widely addressed via numerical models constrained with thermochronological and geomorphological data at scales ranging from local to orogenic. Here we present a novel numerical model that aims at reproducing the interaction between these processes at the continental scale. For this purpose, we combine in a single computer program: 1) a thin-sheet viscous model of continental deformation; 2) a stream-power surface-transport approach; 3) flexural isostasy allowing for the formation of large sedimentary foreland basins; and 4) an orographic precipitation model that reproduces basic climatic effects such as continentality and rain shadow. We quantify the feedbacks between these processes in a synthetic scenario inspired by the India-Asia collision and the growth of the Tibetan Plateau. We identify a feedback between erosion and crustal thickening leading locally to a <50% increase in deformation rates in places where orographic precipitation is concentrated. This climatically-enhanced deformation takes place preferentially at the upwind flank of the growing plateau, specially at the corners of the indenter (syntaxes). We hypothesize that this may provide clues for better understanding the mechanisms underlying the intriguing tectonic aneurisms documented in the Himalayas. At the continental scale, however, the overall distribution of topographic basins and ranges seems insensitive to climatic factors, despite these do have important, sometimes counterintuitive effects on the amount of sediments trapped within the continent. The dry climatic conditions that naturally develop in the interior of the continent, for example, trigger large intra-continental sediment trapping at basins similar to the Tarim Basin because they determine its endorheic/exorheic drainage. These complex climatic-drainage-tectonic interactions make the development of steady-state topography at the continental scale unlikely.

Multiple episodes of fast exhumation since Cretaceous in southeast Tibet, revealed by low-temperature thermochronology

NASA Astrophysics Data System (ADS)

Liu-Zeng, Jing; Zhang, Jinyu; McPhillips, Devin; Reiners, Peter; Wang, Wei; Pik, Raphael; Zeng, Lingsen; Hoke, Greg; Xie, Kejia; Xiao, Ping; Zheng, Dewen; Ge, Yukui

2018-05-01

The southeast margin of the Tibetan plateau is characterized by deeply incised river valleys separated by a perched low relief landscape that gently descends from the high Tibetan plateau towards the southeast. When and how this unique landscape formed is debated. The onset of increased river incision is often interpreted as a proxy for the timing of surface uplift. Here, apatite and zircon (U-Th)/He and apatite fission track thermochronometries are employed to map the spatial and temporal pattern of exhumation in the region. Vertical profiles of granitic rocks were collected near Deqin (∼28.5°N) and Weixi (∼27.5°N). The two transects share a similar exhumation history, with two episodes of relatively fast exhumation (∼100-300 m/Myr) in the Cenozoic: during the Paleocene to Eocene (60-40 Ma) and Miocene to present (20-0 Ma), separated by an intervening period of slow exhumation. A pulse of moderate to high exhumation (70-300 m/Myr) during the mid- to late-Cretaceous (120-80 Ma) is also present in the data. However, the rate and total amount of exhumation near Deqin is larger than at Weixi and is especially pronounced in the interval between 20 Ma to present. We interpret this difference as possibly related to differences in erosion rates between the Lancang (Deqin) and the Jinsha (Weixi) rivers. The Paleocene to Eocene episode of fast exhumation is likely in response to early Cenozoic deformation along tectonic boundary structures, related to the transpressional collision of the Indian plate with this region. Pre-Miocene episodes of fast exhumation corroborate recent paleoaltimetric studies, which show that the southeast margin of the Tibetan plateau was elevated prior to the Oligocene.

Tectono-thermal Evolution of a Distal Rifted Margin: Constraints From the Calizzano Massif (Prepiedmont-Briançonnais Domain, Ligurian Alps)

NASA Astrophysics Data System (ADS)

Decarlis, Alessandro; Fellin, Maria Giuditta; Maino, Matteo; Ferrando, Simona; Manatschal, Gianreto; Gaggero, Laura; Seno, Silvio; Stuart, Finlay M.; Beltrando, Marco

2017-12-01

The thermal evolution of distal domains along rifted margins is at present poorly constrained. In this study, we show that a thermal pulse, most likely triggered by lithospheric thinning and asthenospheric rise, is recorded at upper crustal levels and may also influence the diagenetic processes in the overlying sediments, thus representing a critical aspect for the evaluation of hydrocarbon systems. The thermal history of a distal sector of the Alpine Tethys rifted margin preserved in the Ligurian Alps (Case Tuberto-Calizzano unit) is investigated with thermochronological methods and petrologic observations. The studied unit is composed of a polymetamorphic basement and a sedimentary cover, providing a complete section through the prerift, synrift, and postrift system. Zircon fission track analyses on basement rocks samples suggest that temperatures exceeding 240 ± 25°C were reached before 150-160 Ma (Upper Jurassic) at few kilometer depth. Neoformation of green biotite, stable at temperatures of 350 to 450°C, was synkinematic with this event. The tectonic setting of the studied unit suggests that the heating-cooling cycle took place during the formation of the distal rifted margin and terminated during Late Jurassic (150-160 Ma). Major crustal and lithospheric thinning likely promoted high geothermal gradients ( 60-90°C/km) and triggered the circulation of hot, deep-seated fluids along brittle faults, causing the observed thermal anomaly. Our results suggest that rifting can generate thermal perturbations at relatively high temperatures (between 240 and 450°C) at less than 3 km depth in the distal domains during major crustal thinning preceding breakup and onset of seafloor spreading.

Late Mesozoic and Cenozoic thermotectonic evolution of the central Brooks Range and adjacent North Slope foreland basin, Alaska: Including fission track results from the Trans-Alaska Crustal Transect (TACT)

USGS Publications Warehouse

O'Sullivan, P. B.; Murphy, J.M.; Blythe, A.E.

1997-01-01

Apatite fission track data are used to evaluate the thermal and tectonic history of the central Brooks Range and the North Slope foreland basin in northern Alaska along the northern leg of the Trans-Alaska Crustal Transect (TACT). Fission track analyses of the detrital apatite grains in most sedimentary units resolve the timing of structures and denudation within the Brooks Range, ranging in scale from the entire mountain range to relatively small-scale folds and faults. Interpretation of the results indicates that rocks exposed within the central Brooks Range cooled rapidly from paleotemperatures 110?? to 50??C during discrete episodes at ???100??5 Ma, ???60??4 Ma, and ???24??3 Ma, probably in response to kilometer-scale denudation. North of the mountain front, rocks in the southern half of the foreland basin were exposed to maximum paleotemperatures 110??C in the Late Cretaceous to early Paleocene as a result of burial by Upper Jurassic and Cretaceous sedimentary rocks. Rapid cooling from these elevated paleotemperatures also occurred due to distinct episodes of kilometer-scale denudation at ???60??4 Ma, 46??3 Ma, 35??2 Ma, and ???24??3 Ma. Combined, the apatite analyses indicate that rocks exposed along the TACT line through the central Brooks Range and foreland basin experienced episodic rapid cooling throughout the Late Cretaceous and Cenozoic in response to at least three distinct kilometer-scale denudation events. Future models explaining orogenic events in northern Alaska must consider these new constraints from fission track thermochronology. Copyright 1997 by the American Geophysical Union.

Late-Quaternary Exhumation of Namche Barwa Constrained Using Low-temperature Multi-OSL-thermochronometry

NASA Astrophysics Data System (ADS)

King, G. E.; Herman, F.

2015-12-01

Exhumation rates >5 mm a-1 have been reported for Namche Barwa, making it one of the most rapidly exhuming places on earth. The driver of such high exhumation rates has been highly debated, and two principle hypotheses have evolved: first the aneurysm model (Zeitler et al., 2001) which proposes that a weakening of the crust coupled with extremely active surface processes causes a spatially stationary locus of exhumation. Secondly a northward plunging antiform that is progressively migrating north-eastward (Seward and Burg, 2008) may instead explain the concentration of extremely low cooling ages and rapid exhumation. Distinguishing the effects of tectonic and surface processes, as well as climate is complex, especially given that most existing thermochronometric systems are unable to resolve late-stage cooling histories. Here we present multi-OSL-thermochronometry which comprises a series of different systems with closure temperature ranging from 30 to 70 oC. We have applied this new technique to a suite of samples from the Namche Barwa massif and are able to resolve cooling histories over 0.1 Ma timescales. Our data indicate propagation of a knick-point along the Parlung river, which can be explained by progressive north-eastward migration of a northward plunging antiform. We suggest therefore that river incision does not feedback onto tectonics, as proposed by the aneurysm model. References Seward, D., Burg, J-P., 2008. Growth of the Namche Barwa Syntaxis and associated evolution of the Tsangpo Gorge: Constraints from structural and thermochronological data. Tectonophysics 451, 282-289. Zeitler, P.K., Meltzer, A.S., Koons, P.O., et al., 2001. Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism. GSA Today 11, 4-9.

Glaciation as a destructive and constructive control on mountain building.

PubMed

Thomson, Stuart N; Brandon, Mark T; Tomkin, Jonathan H; Reiners, Peter W; Vásquez, Cristián; Wilson, Nathaniel J

2010-09-16

Theoretical analysis predicts that enhanced erosion related to late Cenozoic global cooling can act as a first-order influence on the internal dynamics of mountain building, leading to a reduction in orogen width and height. The strongest response is predicted in orogens dominated by highly efficient alpine glacial erosion, producing a characteristic pattern of enhanced erosion on the windward flank of the orogen and maximum elevation controlled by glacier equilibrium line altitude, where long-term glacier mass gain equals mass loss. However, acquiring definitive field evidence of an active tectonic response to global climate cooling has been elusive. Here we present an extensive new low-temperature thermochronologic data set from the Patagonian Andes, a high-latitude active orogen with a well-documented late Cenozoic tectonic, climatic and glacial history. Data from 38° S to 49° S record a marked acceleration in erosion 7 to 5 Myr ago coeval with the onset of major Patagonian glaciation and retreat of deformation from the easternmost thrust front. The highest rates and magnitudes of erosion are restricted to the glacial equilibrium line altitude on the windward western flank of the orogen, as predicted in models of glaciated critical taper orogens where erosion rate is a function of ice sliding velocity. In contrast, towards higher latitudes (49° S to 56° S) a transition to older bedrock cooling ages signifies much reduced late Cenozoic erosion despite dominantly glacial conditions here since the latest Miocene. The increased height of the orogenic divide at these latitudes (well above the equilibrium line altitude) leads us to conclude that the southernmost Patagonian Andes represent the first recognized example of regional glacial protection of an active orogen from erosion, leading to constructive growth in orogen height and width.

Low-temperature thermochronology of the Laramide Ranges and eastward translation of shortening in the Sevier Belt, Wyoming, Utah and Montana

NASA Astrophysics Data System (ADS)

Peyton, Sara Lynn

This dissertation contains two studies that use very different techniques to investigate the Mesozoic and Cenozoic tectonics of the western USA. The first study investigates shortening in the Sevier thrust belt of northeast Utah and southwest Wyoming using cross sections and seismic reflection data. The second study investigates the low-temperature thermochronology of the Laramide Ranges using apatite (U-Th)/He dating. We used cross sections and seismic reflection data to investigate bed length discrepancies within the hanging wall of the Absaroka thrust in the Sevier thrust belt of northeast Utah and southwest Wyoming. Restoration of cross sections suggests that there was ˜8-14 km of pre-Absaroka-thrust shortening above the Jurassic Preuss salt detachment, but not below it, in the hanging wall of the Absaroka thrust. Reflection seismic data over the hanging wall of the Crawford thrust show that the Crawford thrust is not offset along the Preuss salt detachment, indicating that the additional shortening on the Absaroka plate was transferred east before main movement on the Crawford thrust. Although early displacement on the Crawford thrust cannot be ruled out as the cause of the extra shortening, surface and subsurface geology suggests slip from the western thrust system (Willard and Lost Creek thrusts) was transferred several tens of kilometers east along the Jurassic Preuss salt detachment between ˜102-90 Ma, to the future location of the Absaroka thrust hanging wall. The lack of deformation of the Crawford thrust on the seismic data, along with shortening and extension estimates from cross sections, also indicate that the magnitude of Paleocene and post-early Eocene shortening on the Medicine Butte thrust was essentially offset by subsequent extension on the middle Eocene to late Oligocene Almy-Acocks normal-fault system. For the second study in this dissertation, we dated 91 borehole and surface samples from Laramide-age, basement-cored uplifts of the Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges) and the Uncompahgre Uplift using the apatite (U-Th)/He system. Apatite (U-Th)/He ages generally decrease with increasing subsurface depth (decreasing elevation) and most samples show age dispersion ranging from tens to hundreds of Myr. Additionally, several samples show correlations between apatite (U-Th)/He age and effective U concentration (eU = [U] + 0.235[Th]) of the crystal, indicating that radiation damage has affected He diffusivity, and hence (U-Th)/ He age. Many surface and near-surface samples have apatite (U-Th)/He ages that are older than corresponding apatite fission-track ages. Forward modeling of Laramide-type thermal histories using a radiation damage diffusion model showed that (U-Th)/He ages may be widely dispersed, and may be older than corresponding apatite fission-track ages within a fossil He partial retention zone. Most of our samples, however, do not show the correlation between (U-Th)/He age and eU predicted by radiation damage diffusion models. We investigated the influence of both grain size and eU content and show that the effects of grain size can obscure (U-Th)/ He age-eU correlations and, similarly, the effect of eU variation can obscure (U-Th)/ He age-grain size correlations. (U-Th)/He ages that are older than fission-track ages from high peaks in the Wind River Range, and from some samples from the Beartooth Range, are most likely the result of He implantation from high eU phases. Best-fit thermal histories from the inversion of age-eU pairs were extrapolated to other elevations to create model age-elevation profiles for a range of eU concentrations. These model profiles approximate our real data. Inverse modeling of (U-Th)/He age data suggests that rapid exhumation within the Laramide province likely began earlier in the Bighorn Mountains (before ˜71 Ma) than the Beartooth Range (before ˜58 Ma), and that the borehole at the northern end of the Laramie Range penetrated a fault sliver at depth.

40Ar* loss in experimentally deformed muscovite and biotite with implications for 40Ar/39Ar geochronology of naturally deformed rocks

USGS Publications Warehouse

Cosca, M.; Stunitz, H.; Bourgeix, A.-L.; Lee, J.P.

2011-01-01

The effects of deformation on radiogenic argon (40Ar*) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ???15mm in length and 6.25mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10kb and a temperature of 600??C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO2) heating of individual 1.5-2.5mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined 40Ar/39Ar plateau ages of 311??2Ma (2??). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40Ar* loss of 0-35% in muscovite and 2-3% 40Ar* loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation 40Ar/39Ar ages (??4-10%, 1??) of deformed muscovites range from 309??13 to 264??7Ma, consistent with 0-16% 40Ar* loss relative to the undeformed muscovite. The in situ UV laser ablation 40Ar/39Ar ages of deformed biotite vary from 301 to 217Ma, consistent with up to 32% 40Ar* loss. No spatial correlation is observed between in situ 40Ar/39Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed 40Ar* loss in the experimentally treated muscovite can be utilized to predict average 40Ar* diffusion dimensions. Maximum 40Ar/39Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for 40Ar* diffusion ???700??m. The UV laser ablation and IR laser incremental 40Ar/39Ar ages indicating 40Ar* loss of 16% and 35%, respectively, are consistent with an average diffusion radius ???100??m. These results support a hypothesis of grain-scale 40Ar* diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for 40Ar* in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both 40Ar* retention and intragrain distribution in deformed mica. Intragrain 40Ar/39Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas. ?? 2011.

In situ detrital zircon (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Tripathy, A.; Monteleone, B. D.; van Soest, M. C.; Hodges, K.; Hourigan, J. K.

2010-12-01

Detrital studies of both sand and rock are relevant to many problems, ranging from the climate and tectonics feedback debate to the long-term record of orogenic evolution. When applying the conventional (U-Th)/He technique to such studies, two important issues arise. Often, only euhedral grains are permissible for analysis in order to make simple geometric corrections for α-recoil. In detrital samples, this is problematic because euhedral grains can be scarce due to mechanical abrasion during transport, and potentially introduce bias in favour of more proximally sourced grains. Second, inherent to detrital studies is the need to date many grains (>100) per sample to ensure a representative sampling of the sediment source region, thus making robust conventional detrital studies both expensive and time-consuming. UV laser microprobes can improve this by permitting careful targeting of the grain interior away from the α-ejection zone, rendering the α-recoil correction unnecessary, thus eliminating bias toward euhedral grains. In the Noble Gas, Geochemistry, and Geochronology Laboratory at ASU, apatite and zircon have been successfully dated using in situ methods. For this study, the conventional and in situ techniques are compared by dating zircons from a modern river sand that drains a small catchment in the Mesozoic-Cenozoic Ladakh Batholith in NW India. This sample has a simple provenance, which allows us to demonstrate the robustness of the in situ method. Moreover, different microbeam techniques will be explored to establish the most efficient approach to obtain accurate and precise U-Th concentrations using synrock, which is our powdered, homogenized, and reconstituted zircon-rock standard. Without this, such in situ U-Th data would be difficult to obtain. 117 zircons were dated using the conventional (U-Th)/He method, revealing dates ranging from 9.70±0.35 to 106.6±3.5 Ma (2σ) with the major mode at 26 Ma. For comparison, 44 grains were dated using the in situ method, utilizing SIMS to determine the U-Th concentrations. The major mode is the same as the conventional dataset, with only single outliers missing from the in situ dataset. The in situ method is described as follows: Zircons are polished and imaged using cathodoluminesence to guide laser spot placement. Then, each grain is ablated with a 193nm ArF Excimer laser using 20-35µm beam sizes and depths of 10-17µm. After purification, 4He is analyzed on a ThermoScientific Helix Split Flight Tube mass spectrometer. U-Th concentrations are measured on the polished mount using a Cameca IMS 6f by placing a 60µm diameter beam directly over the He laser pit to cover the surface area contributing to the measured He. In theory, using LA-ICP-MS to obtain U-Th concentrations could be a faster, cheaper, but possibly less precise, technique. However, using laser ablation would allow pit sizes that average the U-Th concentration over the volume of material available that contributes to the measured He, thus accounting for intracrystalline α-recoil effects. Preliminary data on slabs of Sri Lankan zircon suggest this could be a viable method, but will require further exploration and refinement.

Phanerozoic polycyclic evolution of the southwestern Angola margin: New insights for apatite fission track and (U-Th)/He methodologies

NASA Astrophysics Data System (ADS)

Venancio da Silva, Bruno; Hackspacher, Peter; Carina Siqueira Ribeiro, Marli; Glasmacher, Ulrich Anton

2016-04-01

The low-temperature thermochronology has been an important tool to quantify geological process in passive continental margins. In this context, the Angolan margin shows evidence of a polycyclic post-rift evolution marked by different events of uplift, basin inversion and changes in sedimentation rates to the marginal basins, which have controlled the salt tectonics and the hydrocarbon deposits (1,2,3,4). To understand the post break-up evolution of the southwestern Angola margin, it were collected outcrop samples for apatite fission track (AFT) and (U-Th)/He analysis ranging in elevation from 79 m to 1675 m from the coast toward the interior plateau in a profile between Namibe and Lubango cities. The area lies on the edge of Central and Southern Atlantic segments a few kilometers northward the Walvis ridge and encompasses the Archean and Proterozoic basement rocks of the Congo craton. The AFT ages ranging from 120.6 ± 8.9 Ma to 328.8 ± 28.5 Ma and they show a trend of increasing age toward the Great Escarpment with some exceptions. The partial mean track lengths (MTLs) vary between 11.77 ± 1.82 μm to 12.34 ± 1.13 μm with unimodal track length distributions (TDLs). The partial (U-Th)/He ages ranging from 104.85 ± 3.15 Ma to 146.95 ± 4.41 Ma and show the same trend of increasing ages landward, little younger than the AFT ages, which could be interpreted as a fast exhumation episode in Late Jurassic - Early Cretaceous times. The thermal histories modelling has been constrained with the kinetic parameters Dpar (5) and c-axis angle (6) by the software Hefty (7). Both AFT and (U-Th)/He thermal histories modelling indicate three episodes of denudation/uplift driven cooling: (a) from Late Jurassic to Early Cretaceous, (b) a smallest one in the Late Cretaceous and (c) from Oligocene-Miocene to recent, which are compatible with geophysical data of the offshore Namibe basin that estimate the greater thickness of sediments formed in the first and third episodes, respectively (8,9). Our preliminary data suggest a polycyclic evolution of the southewestern Angola margin and support the importance of the Cenozoic event in the area which has been widely reported along the Angolan margin (2,4,10,11) but has not been evident in other regions of southern Africa where it has been documented mean Cretaceous events (12,13,14,15). Differences in magnitude of Late Cretaceous events between southern Angola and northern Namibia (16,17) suggest a likely basement control linked to different tectonic-denudation episodes, with the Neoproterozoic shear zones absorbing more deformation than the Congo craton during the shortening events of the margin during Late Cretaceous times. Acknowledgments: Capes /AULP 2012 (Proc. 28/13). Professor Antonio Olimpio Gonçalves, FCT/Univ. Agostinho Neto, Angola References 1. Giresse, P., Hoang, C. T., & Kouyoumontzakis, G., 1984. Analysis of vertical movements deduced from a geochronological study of marine Pleistocene deposits, southern coast of Angola. Journal of African Earth Sciences (1983), 2(2), 177-187. 2. Guiraud, M., Buta-Neto, A., & Quesne, D., 2010. Segmentation and differential post-rift uplift at the Angola margin as recorded by the transform-rifted Benguela and oblique-to-orthogonal-rifted Kwanza basins. Marine and Petroleum Geology, 27(5), 1040-1068. 3 Hudec, M. R., & Jackson, M. P., 2002. Structural segmentation, inversion, and salt tectonics on a passive margin: Evolution of the Inner Kwanza Basin, Angola. Geological Society of America Bulletin, 114(10), 1222-1244. 4. Jackson, M. P. A., Hudec, M. R., & Hegarty, K. A., 2005. The great West African Tertiary coastal uplift: Fact or fiction? A perspective from the Angolan divergent margin. Tectonics, 24(6). 5. Donelick, R. A., O'Sullivan, P. B., & Ketcham, R. A., 2005. Apatite fission-track analysis. Reviews in Mineralogy and Geochemistry, 58(1), 49-94. 6. Ketcham, R. A., 2003. Observations on the relationship between crystallographic orientation and biasing in apatite fission-track measurements. American Mineralogist, 88(5-6), 817-829. 7. Ketcham, R. A., 2013. HeFTy Version 1.8.0 User Manual. Department of Geological Sciences, The University of Texas Austin. p 3-10. 8. Maslanyj, M. P., Light, M. P. R., Greenwood, R. J., & Banks, N. L., 1992. Extension tectonics offshore Namibia and evidence for passive rifting in the South Atlantic. Marine and Petroleum Geology, 9(6), 590-601. 9. Maystrenko, Y. P., Scheck-Wenderoth, M., Hartwig, A., Anka, Z., Watts, A. B., Hirsch, K. K., & Fishwick, S., 2013. Structural features of the Southwest African continental margin according to results of lithosphere-scale 3D gravity and thermal modelling. Tectonophysics, 604, 104-121. 10. Green, P. F., & Machado, V., 2015. Pre-rift and synrift exhumation, post-rift subsidence and exhumation of the onshore Namibe Margin of Angola revealed from apatite fission track analysis. Geological Society, London, Special Publications, 438, SP438-2. 11. Rosante, K., 2013. Evolução Termocronológica do sudoeste de Angola e correlação com sudeste brasileiro: Termocronologia por traços de fissão em apatita. Master - Thesis Pós-Grad. Em Geol. Regional- IGCE/UNESP. 12. Wildman, M., Brown, R., Watkins, R., Carter, A., Gleadow, A., & Summerfield, M., 2015. Post break-up tectonic inversion across the southwestern cape of South Africa: new insights from apatite and zircon fission track thermochronometry. Tectonophysics. 654, 30-55. 13. Tinker, J., de Wit, M., & Brown, R., 2008. Mesozoic exhumation of the southern Cape, South Africa, quantified using apatite fission track thermochronology. Tectonophysics, 455(1), 77-93. 14. Brown, R. W., Summerfield, M. A., & Gleadow, A. J., 2002. Denudational history along a transect across the Drakensberg Escarpment of southern Africa derived from apatite fission track thermochronology. Journal of Geophysical Research: Solid Earth (1978-2012), 107(B12), ETG-10. 15. Flowers, R. M., & Schoene, B., 2010). (U-Th)/He thermochronometry constraints on unroofing of the eastern Kaapvaal craton and significance for uplift of the southern African Plateau. Geology, 38(9), 827-830. 16. Luft, F. F., 2004. Evolução tectono-termal das porções norte e central da Namíbia através da análise por traços de fissão em apatita. Dissertação em Geociências, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, 117p. 17. Menges, D., Karl, M., & Glasmacher, U. A., 2013. Thermal history and evolution of the South Atlantic passive continental margin in northern Namibia. Geophysical Research Abstracts Vol. 15, EGU.

Kinematic development of the Tibetan Plateau's northern margin: A traverse across the Qilian Shan-Nan Shan thrust belt

NASA Astrophysics Data System (ADS)

Zuza, A. V.; Levy, D. A.; Wang, Z.; Xiong, X.; Chen, X.

2017-12-01

The active Cenozoic Qilian Shan-Nan Shan thrust belt defines the northern margin of the Tibetan Plateau. The kinematic development of this thrust belt has implications models of plateau growth and Himalayan-Tibetan orogen strain accommodation. We present new field observations and analytical data from a traverse across the 350-km-wide doubly vergent Qilian Shan, which is bound by the south-dipping North Qilian thrust system in the north and the north-dipping range-bounding Qinghai Nanshan-Dulan Shan thrust system in the south. These faults, and several other major thrusts within the thrust-belt interior, disrupt relatively thick Oligocene-Miocene basin deposits. Of note, many of the thrust faults across the width of the Qilian Shan have Quaternary fault scarps, indicating that active deformation is distributed and not only concentrated along the northern frontal faults. By integrating our detailed structural traverse with new geophysical observations and thermochronology data across the northern plateau margin, we construct a kinematic model for the development of the Tibetan Plateau's northern margin. Deformation initiated in the Eocene-Oligocene along the north-dipping Qinghai Nanshan-Dulan Shan and south-dipping Tuolai Nan Shan thrusts, the latter of which then defined the northern boundary of the Tibetan Plateau. This early deformation was focused along preexisting early Paleozoic structures. A 200-km-wide basin formed between these ranges, and from the Miocene to present, new thrust- and strike-slip-fault-bounded ranges developed, including the north-directed North Qilian and the south-directed Tuolai Nan thrusts. Thus, our observations do not support northward propagating thrust-belt expansion. Instead, we envision that the initial thrust-belt development generated a wide Oligocene-Miocene north-plateau basin that was subsequently disintegrated by later Miocene to present thrusting and strike-slip faulting. Ultimately, the Qilian Shan-Nan Shan thrust belt differs from a typical orogenic thrust wedge, and active deformation is distributed across the range.

Simulation of tectonic evolution of the Kanto basin of Japan since 1 Ma due to subduction of the Pacific and Philippine sea plates and collision of the Izu-Bonin arc

NASA Astrophysics Data System (ADS)

Hashima, Akinori; Sato, Toshinori; Sato, Hiroshi; Asao, Kazumi; Furuya, Hiroshi; Yamamoto, Shuji; Kameo, Koji; Miyauchi, Takahiro; Ito, Tanio; Tsumura, Noriko; Kaneda, Heitaro

2015-04-01

The Kanto basin, the largest lowland in Japan, developed by flexure as a result of (1) the subduction of the Philippine Sea (PHS) and the Pacific (PAC) plates and (2) the collision of the Izu-Bonin arc with the Japanese island arc. Geomorphological, geological, and thermochronological data on long-term vertical movements over the last 1 My suggest that subsidence initially affected the entire Kanto basin after which the area of subsidence gradually narrowed until, finally, the basin began to experience uplift. In this study, we modelled the tectonic evolution of the Kanto basin following the method of Matsu'ura and Sato (1989) for a kinematic subduction model with dislocations, in order to quantitatively assess the effects of PHS and PAC subduction. We include the steady slip-rate deficit (permanent locking rate at the plate interface) in our model to account for collision process. We explore how the arc-arc collision process has been affected by a westerly shift in the PHS plate motion vector with respect to the Eurasian plate, thought to have occurred between 1.0-0.5 Ma, using long-term vertical deformation data to constrain extent of the locked zone on the plate interface. We evaluated the change in vertical deformation rate for two scenarios: (1) a synchronous shift in the orientation of the locked zone as PHS plate motion shifts and (2) a delayed shift in the orientation of the locked zone following a change in plate motion. Observed changes in the subsidence/uplift pattern are better explained by scenario (2), suggesting that recent (<1 My) deformation in the Kanto basin shows a lag in crustal response to the shift in plate motion. We also calculated recent stress accumulation rates and found a good match with observed earthquake mechanisms, which shows that intraplate earthquakes serve to release stress accumulated through long-term plate interactions.

Simulation of tectonic evolution of the Kanto Basin of Japan since 1 Ma due to subduction of the Pacific and Philippine Sea plates and the collision of the Izu-Bonin arc

NASA Astrophysics Data System (ADS)

Hashima, Akinori; Sato, Toshinori; Sato, Hiroshi; Asao, Kazumi; Furuya, Hiroshi; Yamamoto, Shuji; Kameo, Koji; Miyauchi, Takahiro; Ito, Tanio; Tsumura, Noriko; Kaneda, Heitaro

2016-06-01

The Kanto Basin, the largest lowland in Japan, developed by flexure as a result of (1) the subduction of the Philippine Sea (PHS) and the Pacific (PAC) plates and (2) the repeated collision of the Izu-Bonin arc fragments with the Japanese island arc. Geomorphological, geological, and thermochronological data on vertical movements over the last 1 My suggest that subsidence initially affected the entire basin after which the area of subsidence gradually narrowed until, finally, the basin began to experience uplift. In this study, we modeled the tectonic evolution of the Kanto Basin following the method of Matsu'ura and Sato (1989) for a kinematic subduction model with dislocations, in order to quantitatively assess the effects of PHS and PAC subduction. We include the steady slip-rate deficit (permanent locking rate at the plate interface) in our model to account for collision process. We explore how the latest collision of the Izu Peninsula block has been affected by a westerly shift in the PHS plate motion vector with respect to the Eurasian plate, thought to have occurred between 1.0-0.5 Ma, using long-term vertical deformation data to constrain extent of the locked zone on the plate interface. We evaluated the change in vertical deformation rate for two scenarios: (1) a synchronous shift in the orientation of the locked zone as PHS plate motion shifts and (2) a delayed shift in the orientation of the locked zone following the shift in plate motion. Observed changes in the uplift/subsidence pattern are better explained by scenario (2), suggesting that recent (< 1 My) deformation in the Kanto Basin shows a lag in crustal response to the plate motion shift. We also calculated stress accumulation rates and found a good match with observed earthquake mechanisms, which shows that intraplate earthquakes serve to release stress accumulated through long-term plate interactions.

Exhumation and stress history in the sedimentary cover during Laramide thick-skinned tectonics assessed by stylolite roughness analysis.

NASA Astrophysics Data System (ADS)

Beaudoin, Nicolas; Lacombe, Olivier; David, Marie-Eléonore; Koehn, Daniel; Coltier, Robin

2017-04-01

Basement-involvement in shortening in forelands has a strong impact on the overlying sedimentary cover. The basement influences namely the geometry of folds and structures, the stress evolution and the nature and pathways for fluid migrations. However, these influences are poorly documented in context where the basement/cover interface is shallow (<6 km). This contribution presents the reconstruction of paleostress and vertical burial history of the Palaeozoic sedimentary strata affected by the Sevier-Laramide deformation at the front of the Rocky Mountains, in the Bighorn Basin (Wyoming, USA). Stylolite populations have been considered as part of an extensive microstructure investigation including also fractures, striated microfaults and calcite twins in key major structures such as the Sheep Mountain Anticline, the Rattlesnake Mountain Anticline, and the Bighorn Mountains Arch. Stylolite recognized in the field are clearly related to successive stages of deformation of the sedimentary cover, including fold development. We further apply a newly developed roughness analysis of pressure-solution stylolites which grant access (1) to the magnitude of the vertical principal stress, hence the maximum burial depth of the strata based on sedimentary stylolites, (2) to the principal stress orientations and regimes based on tectonic stylolites and (3) ultimately to the complete stress tensor when sedimentary and tectonic stylolites can be considered coeval. This approach was then coupled to mechanical properties of main competent formations exposed in the basin. Results of stylolite paleopiezometry, compared and combined to existing paleostress estimates from calcite twins and to exhumation reconstruction from low-temperature thermochronology, unravel the potential of the method to refine the structural history at the structure- and basin-scale. On top of the advances this case study adds to the methodology, the quantified reconstruction of stress-exhumation evolution in such a broken-foreland context offers a unique opportunity to discuss how thick-skinned tectonics impacts stress distribution in the sedimentary cover.

Detrital provenance constraints from the Austral (Magallanes) Basin on dynamic changes in orogenic paleogeography during Cenozoic growth and denudation of the Patagonian Andes

NASA Astrophysics Data System (ADS)

Fosdick, J. C.; Leonard, J. S.; Bostelmann, J. E.; Ugalde, R.; Schwartz, T.

2015-12-01

The topographic development of the Patagonian Andes is influenced by crustal shortening, magmatism, asthenospheric mantle upwelling, climate, and erosion - yet knowledge of how these processes interact is hindered by an incomplete understanding of the timing and tempo of deformation and erosion. We report new detrital zircon U/Pb geochronology and sedimentology from the Cenozoic Austral (Magallanes) foreland basin in Argentina and Chile (near 51°S) that record changes in orogenic paleogeography during uplift of the Patagonian Andes. Near Cerro Castillo, Chile, zircons from deltaic and estuarine sandstones of the Cerro Dorotea Fm. indicate sedimentation ~60-61 Ma, revising the long-held Danian age assignment based on the foraminiferal content. Lower Eocene (47-46 Ma) zircons constrain the age of the overlying unit, the deltaic lower Río Turbio Fm., which shares sedimentological, paleontological, and provenance affinity with the northern Man Aike Fm. Deposition of the upper Río Turbio Fm. in Argentina occurred during the Eocene-Oligocene transition ~33-34 Ma and continued until ~26 Ma. Deposition of the Río Guillermo Fm. resumed ~23.5 Ma with the first occurrence of fluvial sedimentation that continued until the marine Patagonian transgression ~21-19 Ma at this location. Detrital zircon ages reveal upsection reduction in Late Jurassic and Paleozoic igneous sources, variable contributions of Late Cretaceous zircons, and younging of arc-derived zircons. Combined with published bedrock thermochronology and structural data, we suggest that early Miocene faulting and exhumation of the thrust-belt resulted in drainage reorganization and eastward shift in the drainage divide to the central domain, isolating the retroarc basin from the Jurassic Tobífera thrust sheets. Revised timing of sedimentation and changes in upland source areas during Paleocene-Miocene time reveals a complex relationship between basin evolution, Cenozoic climate, and phases of Andean tectonic activity.

Thermochronology, Uplift and Erosion at the Australian-Pacific Plate Boundary Alpine Fault restraining bend, New Zealand

NASA Astrophysics Data System (ADS)

Sagar, M. W.; Seward, D.; Norton, K. P.

2016-12-01

The 650 km-long Australian-Pacific plate boundary Alpine Fault is remarkably straight at a regional scale, except for a prominent S-shaped bend in the northern South Island. This is a restraining bend and has been referred to as the `Big Bend' due to similarities with the Transverse Ranges section of the San Andreas Fault. The Alpine Fault is the main source of seismic hazard in the South Island, yet there are no constraints on slip rates at the Big Bend. Furthermore, the timing of Big Bend development is poorly constrained to the Miocene. To address these issues we are using the fission-track (FT) and 40Ar/39Ar thermochronometers, together with basin-averaged cosmogenic nuclide 10Be concentrations to constrain the onset and rate of Neogene-Quaternary exhumation of the Australian and Pacific plates at the Big Bend. Exhumation rates at the Big Bend are expected to be greater than those for adjoining sections of the Alpine Fault due to locally enhanced shortening. Apatite FT ages and modelled thermal histories indicate that exhumation of the Australian Plate had begun by 13 Ma and 3 km of exhumation has occurred since that time, requiring a minimum exhumation rate of 0.2 mm/year. In contrast, on the Pacific Plate, zircon FT cooling ages suggest ≥7 km of exhumation in the past 2-3 Ma, corresponding to a minimum exhumation rate of 2 mm/year. Preliminary assessment of stream channel gradients either side of the Big Bend suggests equilibrium between uplift and erosion. The implication of this is that Quaternary erosion rates estimated from 10Be concentrations will approximate uplift rates. These uplift rates will help to better constrain the dip-slip rate of the Alpine Fault, which will allow the National Seismic Hazard Model to be updated.

Current strain accumulation in the hinterland of the northwest Himalaya constrained by landscape analyses, basin-wide denudation rates, and low temperature thermochronology

NASA Astrophysics Data System (ADS)

Morell, Kristin D.; Sandiford, Mike; Kohn, Barry; Codilean, Alexandru; Fülöp, Réka-H.; Ahmad, Talat

2017-11-01

Rupture associated with the 25 April 2015 Mw 7.8 Gorkha (Nepal) earthquake highlighted our incomplete understanding of the structural architecture and seismic cycle processes that lead to Himalayan mountain building in Central Nepal. In this paper we investigate the style and kinematics of active mountain building in the Himalayan hinterland of Northwest India, approximately 400 km to the west of the hypocenter of the Nepal earthquake, via a combination of landscape metrics and long- (Ma) and short-term (ka) erosion rate estimates (from low temperature thermochronometry and basin-wide denudation rate estimates from 10Be concentrations). We focus our analysis on the area straddling the PT2, the physiographic transition between the Lesser and High Himalaya that has yielded important insights into the nature of hinterland deformation across much of the Himalaya. Our results from Northwest India reveal a distinctive PT2 that separates a Lesser Himalaya region with moderate relief (∼1000 m) and relatively slow erosion (<1 mm/yr) from a High Himalaya with extreme relief (∼2500 m), steep channels, and erosion rates that approach or exceed 1 mm/yr. The close spatial similarity in relative rates of long- and short-term erosion suggests that the gradient in rock uplift rates inferred from the landscape metrics across the PT2 has persisted in the same relative position since at least the past 1.5 Ma. We interpret these observations to suggest that strain accumulation in this hinterland region throughout at least the past 1.5 Ma has been accomplished both by crustal thickening via duplexing and overthrusting along transient emergent faults. Despite the >400 km distance between them, similar spatiotemporal patterns of erosion and deformation observed in Northwest India and Central Nepal suggest both regions experience similar styles of active strain accumulation and both are susceptible to large seismic events.

Non-linear feedbacks drive strain partitioning within an active orogen, southern Alaska

NASA Astrophysics Data System (ADS)

Hooks, B.; Koons, P. O.; Upton, P.

2011-12-01

Temperature plays a very important role in the partitioning of deformation within an active orogen. Local variations in the thermal structure of actively uplifting areas can reinforce focused partitioning of strain locally, whereas regional variations can alter deformation patterns on a much broader scale resulting in the re-organization of an entire orogen. Within southern Alaska, the Yakutat micro-terrane has been subducting beneath North America over the previous ~10 Ma. Early deformation related to this event drove uplift of the Alaska Range, as evidenced by stratigraphic and thermochronologic datasets. This was followed by a southerly discontinuous spatial jump in the deformation front to the coastal St. Elias Range. Here we present 3D numerical models that simulate deformation of Earth materials given assigned applied velocity boundary conditions and mechanical and thermal constitutive relationships on a macro- (plate boundary) and meso-scale (<50-km). The goal is to reproduce first-order strain and uplift patterns within this evolving orogen. The macro-scale model undergoes a spatial and temporal reorganization of deformation as strain is progressively shifted to a trench-ward orogenic wedge, the inlet orogen. Subduction related cooling of the fore-arc (i.e. tectonic refrigeration) provides control on the location of the inlet orogen. This control is based upon the creation of a thin sliver of cold, strong material along the mega-thrust interface. The stronger mega-thrust facilitates more efficient transfer of strain, driving the formation of the inlet orogen and determining the location of its frontal toe. This toe is further stabilized by upward displacement of the upper crust over the refrigerated section. This upward motion causes thermal weakening of the upper crust as a tectonic aneurysm with the location controlled by the thermally strengthened lower crust. The net result is an ever weakening upper crust that focuses strain creating dramatic topography, extreme rates of erosion and uplift, and fast exhumation.

Mesozoic and Cenozoic exhumation history of the SW Iberian Variscides inferred from low-temperature thermochronology

NASA Astrophysics Data System (ADS)

Vázquez-Vílchez, Mercedes; Jabaloy-Sánchez, Antonio; Azor, Antonio; Stuart, Finlay; Persano, Cristina; Alonso-Chaves, Francisco M.; Martín-Parra, Luis Miguel; Matas, Jerónimo; García-Navarro, Encarnación

2015-11-01

The post-Paleozoic tectonothermal evolution of the SW Iberian Variscides is poorly known mainly due to the scarce low-temperature geochronological data available. We have obtained new apatite fission-tracks and apatite (U-Th)/He ages to constrain the Mesozoic and Cenozoic tectonic evolution of this portion of the Iberian Massif located just north of the Betic-Rif Alpine orogen. We have obtained nine apatite fission-track ages on samples from Variscan and pre-Variscan granitoids. These ages range from 174.4 (± 10.8) to 54.1 (± 4.9) Ma, with mean track lengths between 10.3 and 13.9 μm. We have also performed 5 (U-Th)/He datings on some of the same samples, obtaining ages between 74.6 (± 1.6) and 18.5 (± 1.4) Ma. Time-temperature path modeling of these low-temperature geochronological data leads us to envisage four post-Paleozoic tectonically controlled exhumation episodes in the SW Iberian Variscides. Three of these episodes occurred in Mesozoic times (Middle Triassic to Early Jurassic, Early Cretaceous, and Late Cretaceous) at rates of ≈ 1.1 to 2.5 °C Ma- 1, separated by periods with almost no cooling. We relate these Mesozoic cooling events to the formation of important marginal reliefs during the rifting and opening of the central and northern Atlantic realm. The fourth exhumation episode occurred in Cenozoic times at rates of ≈ 3.2 to 3.6 °C Ma- 1, being only recorded in samples next to faults with topographic escarpments. These samples cooled below 80 °C at ≈ 20 Ma at rates of 3-13 °C Ma- 1 due to roughly N-S oriented compressional stresses affecting the whole Iberian plate, which, in the particular case of SW Iberia, reactivated some of the previous Late Paleozoic thrusts.

Initiation of Extension in South China Continental Margin during the Active-Passive Margin Transition: Thermochronological and Kinematic Constraints

NASA Astrophysics Data System (ADS)

Zuo, X.; Chan, L. S.

2015-12-01

The South China continental margin is characterized by a widespread magmatic belt, prominent NE-striking faults and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from active to passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. Our zircon fission-track data in this region show two episodes of exhumation: The first episode, occurring during 170-120Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115-70Ma, including the Yunkai Terrane and the Nanling Range. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the fact that exhumation of the granite-dominant Nanling Range occurred earlier than that of the gneiss-dominant Yunkai Terrane. In addition to the difference in rock types, the heat from Jurassic-Early Cretaceous magmatism in Nanling may have softened the upper crust, causing the area to exhume more readily than Yunkai. Numerical modeling results also indicate that (1) high lithospheric geothermal gradient, high slab dip angle and low convergence velocity favor the reversal of crustal stress state from compression to extension in the upper continental plate; (2) late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension imply that the Late Cretaceous-early Paleogene red-bed basins in South China could have formed during the late stage of the subduction, accounting for the occurrence of volcanic events in some sedimentary basins. We propose that the rifting started as early as Late Cretaceous, probably before the cessation of subduction process.

Effects of Arabia-Eurasia Collision on Strike-slip Faults in Central Anatolia?

NASA Astrophysics Data System (ADS)

Whitney, D. L.; Lefebvre, C.; Thomson, S. N.; Idleman, L.; Cosca, M. A.; Kaymakci, N.; Teyssier, C. P.; Umhoefer, P. J.

2013-12-01

The North and East Anatolian faults accommodate much of the tectonic escape of Anatolia in response to Arabia-Eurasia collision and building of the Turkish-Iranian plateau, but these structures formed <10 m.y. ago, at least 25 m.y. after the onset of collision at ~35 Ma. Some of the major strike-slip fault zones located between the North and East Anatolian faults have had long and complex histories of displacement. These faults have deformed, and in some cases exhumed, metamorphic massifs located between fault strands. One example is the Nigde Massif, which was initially exhumed in the Late Cretaceous, then reburied and reheated, along with its overlying sedimentary basin, to a depth of ~10 km at 30 × 5 Ma. Final exhumation and cooling occurred by ~15-17 Ma (massif margin) to ~12 Ma (structurally deepest levels). This depth-temperature-time-deformation history is tracked by a combination of thermobarometric methods, structural and stratigraphic analysis, and geo/thermochronometry (U-Pb zircon, monazite; 40Ar/39Ar hornblende, muscovite, biotite, K-feldspar; zircon and apatite fission-track in metamorphic rocks and basin deposits; and apatite (U-Th)/He). Recent mapping shows the presence of at least two oblique-thrust slices; the structurally higher one accounts for the resetting of detrital apatite fission track and AHe ages in the basin rocks as well as metamorphic apatite near the margin of the massif. The structurally deeper one cuts through the metamorphic basement and explains why mineral lineations and metamorphic assemblages are different along the eastern margin relative to those in the core of the massif. Although the timing of displacement has not been dated directly, low-T thermochronology age and modeling results document a perturbation at ~30 Ma, consistent with the idea that the Ecemis Fault of the Central Anatolian Fault Zone, and probably other pre-existing strike-slip faults in central Anatolia, experienced Late Eocene-Oligocene displacement in response to Arabia-Eurasia collision to the south and SE.

Multidimensional Mantle Convection Models in Eastern Anatolia, the North Arabian Platform, and Caucasus Region

NASA Astrophysics Data System (ADS)

Sengul Uluocak, E.; Shahnas, H.; Pysklywec, R.; Gogus, O.; Eken, T.

2017-12-01

Eastern Anatolia, the North Arabian Platform, and Caucasus regions show many features of collisional tectonics with different convergence rates and shortening from south to north. The volcanism, sediment provenience, and thermochronological data suggest that the shortening and exhumation in the Greater Caucasus started during the Eocene-Oligocene synchronously with the collision between Arabia-Bitlis-Pötürge Massif in the south. Previous works indicate that the uplift (up to 2 km) in Eastern Anatolia related to upwelling mantle following the deformation of the Arabian oceanic lithosphere ( 11 Ma) during the ongoing Greater Caucasus closure is the dominant tectonic processes in the center of the region. However, there is no integrated geodynamic model that explains the deformation mechanisms of the region -and their possible interactions with each other -under the dynamic forces. In this study, we use multidimensional mantle-lithosphere convection/deformation models to quantify the geodynamic processes as constrained by the geological/geophysical observations in the region. For the models, seismic studies provide the high-resolution images of the upwelling mantle beneath Eastern Anatolia and the presence -and the locations- of the seismically fast structures associated with the relic/subducted slabs at varying depths such as the Bitlis slab in the south, and the Pontide and Kura slabs in the north. Fast polarization directions observed from splitting analyses exhibit an overall NE-SW oriented mantle anisotropy and a comparison between Pn and SKS derived fast wave azimuths indicates a crust-mantle coupling most likely implying vertically coherent deformation to the north of the study area. For the geodynamic models, we modify the mantle and lithosphere rheology as well as the thermal state. We interpret the estimated uplift and subsidence anomalies related to lithospheric variations (ranging from 54 km to 211 km) and subducting slab behavior with observed topographic anomalies. These interpretations are compatible with the free air admittance functions and surface observations such as high surface heat flows, young volcanism, and Curie point depths in the region.

Decoupling of long-term exhumation and short-term erosion rates in the Sikkim Himalaya

NASA Astrophysics Data System (ADS)

Abrahami, Rachel; van der Beek, Peter; Huyghe, Pascale; Hardwick, Elisabeth; Carcaillet, Julien

2016-01-01

Understanding the relative strengths of tectonic and climatic forcing on erosion at different spatial and temporal scales is important to understand the evolution of orogenic topography. To address this question, we quantified exhumation rates at geological timescales and erosion rates at millennial timescales in modern river sands from 10 sub-catchments of the Tista River drainage basin in the Sikkim Himalaya (northeast India) using detrital apatite fission-track thermochronology and cosmogenic 10Be analyses, respectively. We compare these rates to several potential geomorphic or climatic forcing parameters. Our results show that millennial erosion rates are generally higher and spatially more variable than long-term exhumation rates in Sikkim. They also show strongly contrasting spatial patterns, suggesting that the processes controlling these rates are decoupled. At geological timescales, exhumation rates decrease from south to north, with rates up to 1.2 ± 0.6 mm/yr recorded in southwest Sikkim and as low as 0.5 ± 0.2 mm/yr in the northernmost catchment. Long-term exhumation rates do not correlate with any geomorphic or climatic parameter. We suggest they are tectonically controlled: high rates in southwest Sikkim may be linked to the building of the Lesser Himalaya Rangit Duplex, whereas low rates in north Sikkim are consistent with cessation of extensional exhumation along the South Tibetan Detachment after 13 Ma. The highest apparent erosion rates recorded by cosmogenic nuclides (∼5 mm/yr) occur in catchments spanning the Main Central Thrust Zone, but these appear to be strongly influenced by recent landsliding. High millennial erosion rates (1-2 mm/yr) also occur in north Sikkim and may be climatically driven through strong glacial inheritance of the landscape, as attested by high channel-steepness values close to the maximum extent of glaciers during the Last Glacial Maximum. In contrast, variations in rainfall rate do not seem to strongly influence either millennial erosion or long-term exhumation rates in Sikkim.

Cenozoic Uplift, Erosion and Dynamic Support of Madagascar

NASA Astrophysics Data System (ADS)

Stephenson, Simon; White, Nicky

2016-04-01

The physiography of Madagascar is characterised by high-elevation but low-relief topography; 42% of the landscape is above 500 m in elevation. Eocene (marine) nummulitic (marine) limestones at elevations of ˜400 m above sea level and newly dated, emergent 125 ka coral reefs suggest that Madagascar has experienced differential vertical motions during Cenozoic times. Malagasy rivers are often deeply incised and contain steepened reaches, implying that they have responded to changes in regional uplift rate. However, low temperature thermochronology and 10Be derived erosion rates suggest that both Cenozoic and Recent average denudation rates have been low. Extensive laterite-capped, low-relief surfaces also suggest that there have been long periods of tectonic quiescence. In contrast, the modern landscape is characterised by erosional gullies (i.e. lavaka), with very high local erosion rates. To bridge the gap between this disparate evidence, we inverted 2566 longitudinal river profiles using a damped non-negative, least-squares linear inversion to determine the history of regional uplift. We used a simplified version of the stream power erosional law. River profiles were extracted from the 3 arc-second Shuttle Radar Topography Mission (SRTM) digital elevation model. Calibration of the stream power erosional law is based upon Cenozoic limestones and new radiometrically dated marine terraces. The residual misfit between observed and calculated river profiles is small. Results suggest that Malagasy topography grew diachronously by 1-2 km over the last 15-20 Ma. Calculated uplift and denudation are consistent with independent observations. Thus drainage networks contain coherent signals that record regional uplift. The resultant waves of incision are the principal trigger for modern erosional processes. Admittance calculations, the history of basaltic volcanism and nearby oceanic residual age-depth measurements all suggest that as much as 0.8 - 1.1 km of Cenozoic uplift in Madagascar is supported by mantle processes.

Integrated Laser Ablation U/Pb and (U-Th)/He Dating of Detrital Accessory Minerals from the Naryani River, Central Nepal

NASA Astrophysics Data System (ADS)

Horne, A.; Hodges, K. V.; Van Soest, M. C.

2015-12-01

The newly developed 'laser ablation double dating' (LADD) technique, an integrated laser microprobe U/Pb and (U-Th)/He dating method, could be an exceptionally valuable tool in detrital thermochronology for identifying sedimentary provenance and evaluating the exhumation history of a source region. A recent proof-of-concept study has used LADD to successfully date both zircon and titanite crystals from the well-characterized Fish Canyon tuff, but we also believe that another accessory mineral, rutile, could be amenable to dating via the LADD technique. To continue the development of the method, we present an application of LADD to detrital zircon, titanite, and rutile from a sample collected on the lower Naryani River of central Nepal. Preliminary analyses of the sample have yielded zircon U/Pb dates ranging from 31.4 to 2405 Ma; zircon (U-Th)/He from 1.8 to 15.4 Ma; titanite U/Pb between 18 and 110 Ma; titanite (U-Th)/He between 1 and 16 Ma; rutile U/Pb from 6 to 45 Ma; and rutile (U-Th)/He from 2 to 25 Ma. In addition to the initial data, we can use Ti-in-zircon, Zr-in-titanite, and Zr-in-rutile thermometers to determine the range of possible long-term cooling rates from grains with U/Pb ages younger than collision. Thus far our results from zircon analyses imply a cooling rate of approximately 15°C/Myr; titanite analyses imply between 10 and 67°C/Myr; and rutile between 9 and 267°C/Myr. This spread in potential cooling rates, especially in the order of magnitude differences of cooling rates calculated from the rutile grains, suggests that the hinterland source regions of the Naryani river experienced dramatically different exhumation histories during Himalayan orogenisis. Ongoing analyses will expand the dataset such that we can more adequately characterize the range of possibilities represented in the sample.

Heat flow study of the Emeishan large igneous province region: Implications for the geodynamics of the Emeishan mantle plume

NASA Astrophysics Data System (ADS)

Jiang, Qiang; Qiu, Nansheng; Zhu, Chuanqing

2018-01-01

The Emeishan large igneous province (ELIP) is widely considered to be a consequence of a mantle plume. The supporting evidence includes rapid emplacement, voluminous flood basalt eruptions, and high mantle potential temperature estimates. Several studies have suggested that there was surface uplift prior to the eruption of the Emeishan flood basalts. Additionally, the plume's lateral extent is hard to constrain and has been variously estimated to be 800-1400 km in diameter. In this study, we analyzed present-day heat flow data and reconstructed the Permian paleo-heat flow using vitrinite reflectance and zircon (U-Th)/He thermochronology data in the ELIP region and discussed implications for the geodynamics of the Emeishan mantle plume. The present-day heat flow is higher in the inner and intermediate zones than in the outer zone, with a decrease of average heat flow from 76 mW/m2 to 51 mW/m2. Thermal history modeling results show that an abnormal high paleo-heat flow of 90-110 mW/m2 was caused by the Emeishan mantle plume activity. Based on the present-day heat flow data, we can calculate that there is lithospheric thinning in the central ELIP region, which may be due to the destruction of the lithosphere by mantle plume upwelling and magmatic underplating. The Permian paleo-heat flow anomaly implies that there was a temperature anomaly in the mantle. The ascending high-temperature mantle plume and the thinned lithosphere may have induced the large-scale uplift in the ELIP region. According to the range of the surface heat flow anomaly, it can be estimated that the diameter of the flattened head of the Emeishan mantle plume could have reached 1600-1800 km. Our research provides new insights into the geodynamics of the Emeishan mantle plume through study of heat flow.

Architecture, kinematics, and exhumation of a convergent orogenic wedge: A thermochronological investigation of tectonic-climatic interactions within the central St. Elias orogen, Alaska

NASA Astrophysics Data System (ADS)

Berger, Aaron L.; Spotila, James A.; Chapman, James B.; Pavlis, Terry L.; Enkelmann, Eva; Ruppert, Natalia A.; Buscher, Jamie T.

2008-06-01

The kinematics and architecture of orogenic systems along the leading edges of accreting terranes may be heavily influenced by climate, but little research has been devoted to the long-term effects of glacial erosion on orogenesis. Here we use low-temperature apatite and zircon (U-Th)/He and fission-track thermochronometry, along with subsidiary structural relationships and seismicity, to develop a new architectural model of the St. Elias orogen in southern Alaska, which is one of the best examples of a glaciated orogenic wedge worldwide. These data illustrate that the orogen consists of a deformational backstop on the leeward flank and a rapidly deforming and eroding, thin-skinned fold and thrust belt on the windward flank. A structure beneath the Bagley ice field separates these distinct deformational domains, which we propose is a backthrust that makes the orogen doubly-vergent. Thermochronometry within the orogenic wedge suggests that denudation and deformation are strongly influenced by glacial erosion. Long-term exhumation, at rates of up to 4 mm/yr, is concentrated within a narrow zone along the windward flank, where glacier equilibrium lines intersect the orogenic wedge. The onset of enhanced glaciation also coincided with a marked acceleration in exhumation across the orogenic wedge, accelerated backthrust motion, and a major shift in deformation away from the North American-Yakutat terrane suture (Chugach St. Elias fault). We propose that accelerated glacial erosion forced the redistribution of strain along the backthrust and an en echelon array of forethrusts that lie beneath the zone of heaviest glaciation, which in turn are systematically truncated by the backthrust. This focusing of deformation matches predictions from analytical models of orogenic wedges and implies a high degree of coupling between climate and tectonics in this glacially-dominated orogen.

Interrogating the Effects of Radiation Damage Annealing on Helium Diffusion Kinetics in Apatite

NASA Astrophysics Data System (ADS)

Willett, C. D.; Fox, M.; Shuster, D. L.

2015-12-01

Apatite (U-Th)/He thermochronology is commonly used to study landscape evolution and potential links between climate, erosion and tectonics. The technique relies on a quantitative understanding of (i) helium diffusion kinetics in apatite, (ii) an evolving 4He concentration, (iii) accumulating damage to the crystal lattice caused by radioactive decay[1], and (iv) the thermal annealing of such damage[2],[3], which are each functions of both time and temperature. Uncertainty in existing models of helium diffusion kinetics has resulted in conflicting conclusions, especially in settings involving burial heating through geologic time. The effects of alpha recoil damage annealing are currently assumed to follow the kinetics of fission track annealing (e.g., reference [3]), although this assumption is difficult to fully validate. Here, we present results of modeling exercises and a suite of experiments designed to interrogate the effects of damage annealing on He diffusivity in apatite that are independent of empirical calibrations of fission track annealing. We use the existing experimental results for Durango apatite[2] to develop and calibrate a new function that predicts the effects of annealing temperature and duration on measured diffusivity. We also present a suite of experiments conducted on apatite from Sierra Nevada, CA granite to establish whether apatites with different chemical compositions have the same behavior as Durango apatite. Crystals were heated under vacuum to temperatures between 250 and 500°C for 1, 10, or 100 hours. The samples were then irradiated with ~220 MeV protons to produce spallogenic 3He, the diffusant then used in step-heating diffusion experiments. We compare the results of these experiments and model calibrations to existing models. Citations: [1]Shuster, D., Flowers R., and Farley K., (2006), EPSL 249(3-4), 148-161; [2]Shuster, D. and Farley, K., (2009), GCA 73 (1), 6183-6196; [3]Flowers, R., Ketcham, R., Shuster, D. and Farley, K., (2009), GCA 73, 2347-2365.

River Network Reorganization along the Upper Yangzte, Eastern Tibet: Insights from Thermochronology and Sedimentology.

NASA Astrophysics Data System (ADS)

Gourbet, L.; Yang, R.; Fellin, M. G.; Maden, C.; Gong, J.; Jean-Louis, P.

2017-12-01

The high relief and high elevation of the southeastern margin of the Tibetan Plateau are related to tectonic uplift and the fluvial incision of the Salween, Mekong, and Yangtze rivers. The upper Yangtze is the subject of numerous debates on the evolution of its drainage area, particularly in regards to the timing and geodynamic processes, and therefore has an impact on models of the Tibetan plateau evolution. Today, portions of the course of the Yangtze are controlled by active strike-slip faults. In order to study the evolution of the Cenozoic paleoriver network, we use low-temperature thermochronometry to estimate fluvial incision and palaeoenvironmental information derived from the detrital record. The Jianchuan basin, between the Yangtze and the Red River, contains late Eocene fluvial sediments that may correspond to an ancient connection between these rivers. Sediments located further north (DongWang formation, Yunnan-Sichuan boundary) consist of unsorted conglomerates and sandstones. They are exposed on the flanks of deep valleys. These sediments do not correspond to a large riverbed such as the Yangtze but rather indicate an episode of intense sedimentation with a significant contribution from talus, followed by a >1.2 km incision by a tributary of the upper Yangtze. In the same area, we performed apatite and zircon (U-Th)/He dating on a granitic pluton that is offset by an active sinistral strike-slip fault. Mean ZHe cooling ages range from 50 to 70 Ma. Samples located above 3870 m yield mean apatite (U-Th)/He ages ranging from 30 to 40 Ma. AHe ages for samples at lower elevation range from 8 to 15 Ma. Given the crystallization age of the pluton (83 Ma, U/Pb, zircon), cooling ages reflect exhumation, not post-intrusion cooling. Further research will use thermal modeling to infer incision rates and compare results with published data.

Earthquakes drive focused denudation along a tectonically active mountain front

NASA Astrophysics Data System (ADS)

Li, Gen; West, A. Joshua; Densmore, Alexander L.; Jin, Zhangdong; Zhang, Fei; Wang, Jin; Clark, Marin; Hilton, Robert G.

2017-08-01

Earthquakes cause widespread landslides that can increase erosional fluxes observed over years to decades. However, the impact of earthquakes on denudation over the longer timescales relevant to orogenic evolution remains elusive. Here we assess erosion associated with earthquake-triggered landslides in the Longmen Shan range at the eastern margin of the Tibetan Plateau. We use the Mw 7.9 2008 Wenchuan and Mw 6.6 2013 Lushan earthquakes to evaluate how seismicity contributes to the erosional budget from short timescales (annual to decadal, as recorded by sediment fluxes) to long timescales (kyr to Myr, from cosmogenic nuclides and low temperature thermochronology). Over this wide range of timescales, the highest rates of denudation in the Longmen Shan coincide spatially with the region of most intense landsliding during the Wenchuan earthquake. Across sixteen gauged river catchments, sediment flux-derived denudation rates following the Wenchuan earthquake are closely correlated with seismic ground motion and the associated volume of Wenchuan-triggered landslides (r2 > 0.6), and to a lesser extent with the frequency of high intensity runoff events (r2 = 0.36). To assess whether earthquake-induced landsliding can contribute importantly to denudation over longer timescales, we model the total volume of landslides triggered by earthquakes of various magnitudes over multiple earthquake cycles. We combine models that predict the volumes of landslides triggered by earthquakes, calibrated against the Wenchuan and Lushan events, with an earthquake magnitude-frequency distribution. The long-term, landslide-sustained "seismic erosion rate" is similar in magnitude to regional long-term denudation rates (∼0.5-1 mm yr-1). The similar magnitude and spatial coincidence suggest that earthquake-triggered landslides are a primary mechanism of long-term denudation in the frontal Longmen Shan. We propose that the location and intensity of seismogenic faulting can contribute to focused denudation along a high-relief plateau margin.

Tectonic evolution of the North Patagonian Andes (41°-44° S) through recognition of syntectonic strata

NASA Astrophysics Data System (ADS)

Echaurren, A.; Folguera, A.; Gianni, G.; Orts, D.; Tassara, A.; Encinas, A.; Giménez, M.; Valencia, V.

2016-05-01

The North Patagonian fold-thrust belt (41°-44° S) is characterized by a low topography, reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone. This particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment. Here, new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages, evaluating the prevailing crustal regime for the different mountain domains through time. Growth strata and progressive unconformities, controlled by extensional or compressive structures, were recognized in volcanic and sedimentary rocks from the cordilleran to the extra-Andean domain. These data were used to construct a balanced cross section, whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology. Our results indicate two main compressive stages, interrupted by an extensional relaxation period. The first contractional stage in the mid-Cretaceous inverted Jurassic-Lower Cretaceous half graben systems, reactivating the western Cañadón Asfalto rift border ~ 500 km away from the trench, at a time of arc foreland expansion. For this stage, available thermochronological data reveal forearc cooling episodes, and global tectonic reconstructions indicate mid-ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement. Widespread synextensional volcanism is recognized throughout the Paleogene during plate reorganization; retroarc Paleocene--Eocene flare up activity is interpreted as product of a slab rollback, and fore-to-retroarc Oligocene slab/asthenospheric derived products as an expression of enhanced extension. The second stage of mountain growth occurred in Miocene time associated with Nazca Plate subduction, reaching nearly the same amplitude than the first compressive stage. Extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation.

A Precise 6 Ma Start Date for Fluvial Incision of the Northeastern Colorado Plateau Canyonlands

NASA Astrophysics Data System (ADS)

Thomson, S. N.; Soreghan, G. S.; Reiners, P. W.; Peyton, S. L.; Murray, K. E.

2015-12-01

Outstanding questions regarding late Cenozoic Colorado Plateau landscape evolution include: (1) the relative roles of isostatic rebound as result Colorado River incision versus longer-term geodynamic processes in driving overall rock uplift of the plateau; and (2) whether incision was triggered by river integration or by a change in deep-seated mantle lithosphere dynamics. A key to answering these questions is to date more precisely the onset of incision to refine previous estimates of between 6 and 10 Ma. We present new low-temperature thermochronologic results from bedrock and deep borehole samples in the northeastern Colorado Plateau to show that rapid river incision began here at 6 Ma (5.93±0.66 Ma) with incision rates increasing from 15-50 m/Myr to 160-200 m/Myr. The onset time is constrained independently by both inverse time-temperature modeling and by the break-in-slope in fission track age-elevation relationships. This new time constraint has several important implications. First, the coincidence in time with 5.97-5.3 Ma integration of the lower Colorado River through the Grand Canyon to the Gulf of California strongly favors downstream river integration triggering carving of the canyonlands of the upper Colorado River system. Second, it implies integration of the entire Colorado River system in less than 2 million years. Third, rock uplift of the plateau driven by the flexural isostatic response to river incision is restricted to just the last 6 Ma, as is associated increased sediment budget. Fourth, incision starting at 6 Ma means that previous estimates of upper Colorado River incision rates based on 10-12 Ma basalt datum levels are too low. This also changes the dependency of measured time interval on incision rate from a non-steady-state negative power-law dependence (exponent of -0.24) to a near steady-state dependence (exponent of 0.07) meaning that long-term upper Colorado river incision rates can provide a reliable proxy for rock uplift rates.

Linking Quaternary Climate Changes to Mountain Building in Southeastern Alaska

NASA Astrophysics Data System (ADS)

Dunn, C. A.; Enkelmann, E.; Allen, W. K.; Ridgway, K.

2015-12-01

The Chugach-St. Elias Mountains in southeastern Alaska exhibit extreme topography as a result of the subduction and collision of the Yakutat microplate with the North American plate. Extensive glaciation in the area, coupled with the active tectonics, makes the Chugach-St. Elias Mountains the ideal site to investigate the resulting spatial and temporal variations in exhumation. The St. Elias Mountains have experienced three major climate changes including the start of regional glaciation at ~5.5 Ma, the onset of glaciation in the Northern Hemisphere during the Plio-Pleistocene Transition to colder temperatures at 2.5 Ma, and a change in global climate cyclicity during the mid-Pleistocene Transition from 1.2-0.7 Ma. Our goal is to determine whether these climate changes result in shifts in the rates and patterns of regional exhumation due to the intensification of glacial erosion. We investigate the late Cenozoic sediments from five boreholes that were drilled by IODP Expedition 341 along a transect crossing the Yakutat shelf (offshore Bering glacier) and into the deep-sea Surveyor Fan in the Gulf of Alaska. Magnetostratigraphy and biostratigraphy on these sediments establish an up to 10 Ma depositional history in the boreholes. To study rock exhumation of the sediment source over time, 17 detrital samples from various depths in the cores were dated using zircon fission track thermochronology. Single grain cooling ages range from 302.8 Ma to 0.2 Ma, and contain composite age populations with peaks coinciding with major regional tectonic events providing information on sediment provenance and exhumation. All single grains were also double dated using U-Pb dating to identify and eliminate volcanic grains from the sample. Common U-Pb age populations are 53, 62, 70, and 98 Ma with minor populations of 117, 154, and 170 Ma. These ages are being used to better constrain sediment provenance and reconstruct past glacial catchments.

Updated paleomagnetic pole from Cretaceous plutonic rocks of the Sierra Nevada, California: Tectonic displacement of the Sierra Nevada block

USGS Publications Warehouse

Hillhouse, John W.; Gromme, Sherman

2011-01-01

We report remanent magnetization measurements from 13 sites in Cretaceous plutonic rocks in the northern Sierra Nevada (38°N–39.5°N). By increasing the number of available paleomagnetic sites, the new data tighten constraints on the displacement history of the Sierra Nevada block and its pre-extensional position relative to interior North America. We collected samples in freshly exposed outcrops along four highway transects. The rocks include diorite, granodiorite, and tonalite with potassium-argon ages (hornblende) ranging from 100 Ma to 83 Ma. By combining our results with previous paleomagnetic determinations from the central and southern Sierra Nevada (excluding sites from the rotated southern tip east of the White Wolf–Kern Canyon fault system), we find a mean paleomagnetic pole of 70.5°N, 188.2°E, A95 = 2.6° (N = 26, Fisher concentration parameter, K = 118). Thermal demagnetization indicates that the characteristic remanence is generally unblocked in a narrow range within 35 °C of the Curie temperature of pure magnetite. Small apparent polar wander during the Cretaceous normal-polarity superchron, plus prolonged acquisition of remanence at the site level, may account for the low dispersion of virtual geomagnetic poles and relatively large K value. Tilt estimates based on overlapping sediments, stream gradients, and thermochronology of the Sierra Nevada plutons vary from 0° to 3° down to the southwest. Without tilt correction, the mean paleomagnetic pole for the Sierra Nevada is essentially coincident with the North American reference pole during the Cretaceous stillstand (125 Ma to 80 Ma). At 95% confidence, the apparent latitude shift is 1.1° ± 3.0° (positive northward), and the apparent rotation is negligible, 0.0° ± 4.7°. Correcting for each degree of tilt, which is limited to 3° on geologic evidence, increases the rotation anomaly 2.2° counterclockwise, while the apparent latitude shift remains unchanged.

The Exhumation of the Central Lhasa, Tibet: Evidence from the Low-temperature Thermochronology

NASA Astrophysics Data System (ADS)

Sun, G.; Xiumian, H.; Sinclair, H. D.; Stuart, F. M.

2017-12-01

The modern Tibetan Plateau has an average elevation of 4500 m above the sea level. But its early growth history still remains debate, despite its significance to the global climate system. In common, the early growth of the Tibetan Plateau has been attributed to the India-Asia collision in the early Paleocene. However, the structural reconstruction, Late Cretaceous sedimentation, and petrology studies, imply there would be a paleo-plateau or the high-elevation gain in the Lhasa terrane prior to the India-Asia collision. In order to examine this model, the zircon/apatite U-Th-He (ZHe and AHe) and apatite fission track (AFT) are employed to the mid-Cretaceous granites in Coqen area, central Lhasa. Eight samples from the plateau surface yield ZHe ages ranging from 88 to 54 Ma, while the AHe ages ranging from 70 to 45 Ma. Five samples from the above have been conducted the AFT analysis and yielded AFT ages ranging between 73 and 62 Ma, showing the similar age ranges with the corresponding AHe ages. Single-sample inverse thermal kinetic modeling reveal that these intrusive rocks have undergone the rapid cooling history since 85 Ma, after when, the relatively slow cooling process has been established at 45 Ma. Inverse thermal-kinetic modeling of these data, recorded in the context of the Late Cretaceous rapid cooling history, is best interpreted by the early plateau growth in the central Lhasa. In consideration of the substantial crustal thickening and shortening in the Lhasa terrane during the Cretaceous, this Late Cretaceous-Early Paleogene rapid cooling history reveal that the exhumation of the central Lhasa has initiated before the India-Asia collision. This scenario is consistent with a 30 Ma ( 90-60 Ma) sedimentation hiatus since the Late Cretaceous terrestrial conglomerate deposition in the central Lhasa terrane.

Faulting and erosion in the Argentine Precordillera during changes in subduction regime: Reconciling bedrock cooling and detrital records

NASA Astrophysics Data System (ADS)

Fosdick, Julie C.; Carrapa, Barbara; Ortíz, Gustavo

2015-12-01

The Argentine Precordillera is an archetypal retroarc fold-and-thrust belt that records tectonics associated with changing subduction regimes. The interactions between exhumation and faulting in the Precordillera were investigated using apatite and zircon (U-Th-Sm)/He and apatite fission track thermochronometry from the Precordillera and adjacent geologic domains. Inverse modeling of thermal histories constrains eastward in-sequence rock cooling associated with deformation and erosion from 18 to 2 Ma across the Central Precordillera tracking thrusting during this time. The youngest AHe ages (5-2 Ma) and highest erosion rates are located in the eastern and western extremities of the Precordillera and indicate that recent denudation is concentrated at its structural boundaries. Moreover, synchronous rapid Pliocene cooling of the Frontal Cordillera, Eastern Precordillera, and Sierra del Valle Fértil was coeval with initiation of basement-involved faulting in the foreland. Detrital zircon U-Pb geochronology from the ca. 16-8.1 Ma Bermejo foreland basin strata suggests fluvial connectivity westward beyond the Frontal Cordillera to the Main Cordillera and Coast Range followed by an important shift in sediment provenance at ca. 10 Ma. At this time, we suggest that a substantial decrease in Permo-Triassic igneous sources in the Frontal Cordillera and concurrent increase in recycled zircons signatures of Paleozoic strata are best explained by uplift and erosion of the Precordillera during widening of the thrust-belt. Bedrock thermochronology and modeling indicate a 2-6 Myr lag time between faulting-related cooling in the hinterland and the detrital record of deformation in the foreland basin, suggesting that for tectonically active semi-arid settings, bedrock cooling may be more sensitive to onset of faulting. We suggest that high erosion rates in the Frontal Cordillera and Eastern Precordillera are associated with increased interplate coupling during shallowing of the subducting Nazca plate that may concentrate stress along weak structural boundaries of the Precordillera.

Preliminary Depositional and Provenance Records of Mesozoic Basin Evolution and Cenozoic Shortening in the High Andes, La Ramada Fold-Thrust Belt, Southern-Central Andes (32-33°S)

NASA Astrophysics Data System (ADS)

Mackaman-Lofland, C.; Horton, B. K.; Fuentes, F.; Constenius, K. N.; McKenzie, R.; Alvarado, P. M.

2015-12-01

The Argentinian Andes define key examples of retroarc shortening and basin evolution above a zone of active subduction. The La Ramada fold-thrust belt (RFTB) in the High Andes provides insights into the relative influence and temporal records of diverse convergent margin processes (e.g. flat-slab subduction, convergent wedge dynamics, structural inversion). The RFTB contains Mesozoic extensional basin strata deformed by later Andean shortening. New detrital zircon U-Pb analyses of Mesozoic rift sediments reveal: (1) a dominant Permo-Triassic age signature (220-280 Ma) associated with proximal sources of effective basement (Choiyoi Group) during Triassic synrift deposition; (2) upsection younging of maximum depositional ages from Late Triassic through Early Cretaceous (230 to 100 Ma) with the increasing influence of western Andean arc sources; and (3) a significant Late Cretaceous influx of Paleozoic (~350-550 Ma) and Proterozoic (~650-1300 Ma) populations during the earliest shift from back-arc post-extensional subsidence to upper-plate shortening. The Cenozoic detrital record of the Manantiales foreland basin (between the Frontal Cordillera and Precordillera) records RFTB deformation prior to flat-slab subduction. A Permo-Triassic Choiyoi age signature dominates the Miocene succession, consistent with sources in the proximal Espinacito range. Subordinate Mesozoic (~80-250 Ma) to Proterozoic (~850-1800 Ma) U-Pb populations record exhumation of the Andean magmatic arc and recycling of different structural levels in the RFTB during thrusting/inversion of Mesozoic rift basin strata and subjacent Paleozoic units. Whereas maximum depositional ages of sampled Manantiales units cluster at 18-20 Ma, the Estancia Uspallata basin (~50 km to the south) shows consistent upsection younging of Cenozoic populations attributed to proximal volcanic centers. Ongoing work will apply low-temperature thermochronology to pinpoint basin accumulation histories and thrust timing.

Contrasting tectonothermal domains and faulting in the Potomac terrane, Virginia-Maryland - Discrimination by 40Ar/39Ar and fission-track thermochronology

USGS Publications Warehouse

Kunk, Michael J.; Wintsch, R.P.; Naeser, C.W.; Naeser, N.D.; Southworth, C.S.; Drake, Avery A.; Becker, J.L.

2005-01-01

New 40Ar/39Ar data reveal ages and thermal discontinuities that identify mapped and unmapped fault boundaries in the Potomac terrane in northern Virginia, thus confirming previous interpretations that it is a composite terrane. The rocks of the Potomac terrane were examined along the Potomac River, where it has been previously subdivided into three units: the Mather Gorge, Sykesville, and Laurel Formations. In the Mather Gorge Formation, at least two metamorphic thermal domains were identified, the Blockhouse Point and Bear Island domains, separated by a fault active in the late Devonian. Early Ordovician (ca. 475 Ma) cooling ages of amphibole in the Bear Island domain reflect cooling from Taconic metamorphism, whereas the Blockhouse Point domain was first metamorphosed in the Devonian. The 40Ar/39Ar data from muscovites in a third (eastern) domain within the Mather Gorge Formation, the Stubblefield Falls domain, record thrusting of the Sykesville Formation over the Mather Gorge Formation on the Plummers Island fault in the Devonian. The existence of two distinctly different thermal domains separated by a tectonic boundary within the Mather Gorge argues against its status as a formation. Hornblende cooling ages in the Sykesville Formation are Early Devonian (ca. 400 Ma), reflecting cooling from Taconic and Acadian metamorphism. The ages of retrograde and overprinting muscovite in phyllonites from domain-bounding faults are late Devonian (Acadian) and late Pennsylvanian (Alleghanian), marking the time of assembly of these domains and subsequent movement on the Plummers Island fault. Our data indicate that net vertical motion between the Bear Island domain of the Mather Gorge complex and the Sykesville Formation across the Plummers Island fault is east-side-up. Zircon fission-track cooling ages demonstrate thermal equillbrium across the Potomac terrane in the early Permian, and apatite fission-track cooling ages record tilting of the Potomac terrane in the Cretaceous or later with the west side up at least 1 km. ?? 2005 Geological Society of America.

Separation of the Guajira-Bonaire pair in the southern margin of the Caribbean: 65-50 Ma exhumation followed by 300 km right-lateral transtensional deformation

NASA Astrophysics Data System (ADS)

Zapata Henao, S.; Cardona, A.; Montes, C.; Valencia, V.; Vervoort, J. D.; Reiners, P. W.

2012-12-01

Middle to upper Eocene fluvial strata in the island of Bonaire contains detrital components that were tracked to the basement massifs of the Guajira Peninsula in northern Colombia. These detrital components confirm previous hypothesis that the Guajira-Bonaire pair constitute a tectonic piercing point along the southern Caribbean plate margin that was right-laterally displaced approximately 300 km after middle Eocene times. Other possible sources, the nearby Curacao and the far away Santa Marta massif, did not pass statistical similarity and overlap tests. U-Pb LA-ICP-MS from the metamorphic boulders of the Soebi Blanco Formation in Bonaire yield Grenvillian ages (1084 Ma, 1130 Ma and 1184 Ma), while the detrital zircons recovered from the sandy matrix of the conglomerates contains populations with peaks of 1000 Ma - 1200 Ma, 750 Ma - 950 Ma, and 200 Ma - 300 Ma. Overlap and Similarity tests run between these populations and published data from Guajira yield values of 0.750 and 0.680, which are significantly higher than the same comparison against the Santa Marta Massif (0.637 and 0.522), and the Curacao island (0.629 and 0.467). Thermochronological results from the metamorphic clasts yield Paleocene-middle Eocene ages (65 - 50 Ma) that confirm not only a regional-scale cooling event in this time period, but also help constrain the maximum depositional age (50 Ma) of the poorly dated Soebi Blanco Formation. Figure 6. U-Pb results from analyzed samples and other Caribbean provinces. (A), Detrital zircons from Soebi Blanco conglomerate matrix; (B), Zircon ages from metamorphic clasts (C), detrital zircons from late Cretaceous Etpana Formation in Guajira Peninsula (Weber et al., 2010); (D), detrital zircons from late Cretaceous Santa Marta San Lorenzo schists (Cardona et al., 2010a); (E), detrital zircons from late Cretaceous Knip Group (Wrigth and Wyld, 2010); (F), overlap and similarity values.

Postorogenic emplacement of the Santa Marta Batholith, northwestern flank of the Sierra Nevada de Santa Marta (SNSM).

NASA Astrophysics Data System (ADS)

Sanchez Sierra, Johan Miguel Sebastian; Kammer, Andreas

2017-04-01

The Santa Marta Batholith (BSM) belongs to a Paleogene intrusive suite of the Santa Marta massif, an exhumed triangular block at the southern Caribbean margin. Its Paleogene age precludes its association to an active margin, although its emplacement was controlled by the flexure of the down-bent Southamerican plate. Its internal structure is outlined by a mafic border facies and a felsic core, both having a petrologic affinity to a TTG-suite. According to existing age data, the BSM consolidated sequentially from SW to NE, with a first pulse having crystallized at 56 Ma in the southern domain and a final pulse in the northern domain at 52-50 Ma. Pressures varied between 5-7 kb, corresponding to depths of 14-19 km. This study combines structural, thermochronological and geochemical data with an analysis of Anisotropy and Magnetic Susceptibility (AMS) and paleomagnetism. The SNSM had a clockwise rotation of 30 ° and the ASM results help distinguish between two fault-bounded structural domains. The southern domain is characterized by a magnetic foliation concordant to the contact of the host rock that dips toward the hinterland. The northern domain, in contrast, displays a N-S trending magnetic foliation that is oblique to the regional structural northeastern trend. This divergence is supported by the orientation of mineral lineations, enclaves and dikes. In spite of its arc signature, anomalies like enrichment in Ti, depletion of Nb-Ta and Zr-Hf, as well as flat REE patterns can be associated to the accumulation of crystallized mafic minerals from less-fractionated magmas. These data evidence mingling. Asymmetric internal organization, as indicated by a hinterland-dipping roof pendent, the structural setting at the margin of a thickened continental margin and its geochemical signature favor a scenario of a magma generation at a mid-crustal level and its consequent extrusion along a channel, that connected to the crustal bend of the continental plate that was inherited from the Cretaceous subduction cycle.

Thrusting and back-thrusting as post-emplacement kinematics of the Almora klippe: Insights from Low-temperature thermochronology

NASA Astrophysics Data System (ADS)

Patel, R. C.; Singh, Paramjeet; Lal, Nand

2015-06-01

Crystalline klippen over the Lesser Himalayan Sequence (LHS) in the Kumaon and Garhwal regions of NW-Himalaya, are the representative of southern portion of the Main Central Thrust (MCT) hanging wall. These were tectonically transported over the juxtaposed thrust sheets (Berinag, Tons and Ramgarh) of the LHS zone along the MCT. These klippen comprise of NW-SE trending synformal folded thrust sheet bounded by thrusts in the south and north. In the present study, the exhumation histories of two well-known klippen namely Almora and Baijnath, and the Ramgarh thrust sheet, in the Kumaon and Garhwal regions vis-a-vis Himalayan orogeny have been investigated using Apatite Fission Track (AFT) ages. Along a ~ 60 km long orogen perpendicular transect across the Almora klippe and the Ramgarh thrust sheet, 16 AFT cooling ages from the Almora klippe and 2 from the Ramgarh thrust sheet have been found to range from 3.7 ± 0.8 to 13.2 ± 2.7 Ma, and 6.3 ± 0.8 to 7.2 ± 1.0 Ma respectively. From LHS meta-sedimentary rocks only a single AFT age of 3.6 ± 0.8 Ma could be obtained. Three AFT ages from the Baijnath klippe range between 4.7 ± 0.5 and 6.6 ± 0.8 Ma. AFT ages and exhumation rates of different klippen show a dynamic coupling between tectonic and erosion processes in the Kumaon and Garhwal regions of NW-Himalaya. However, the tectonic processes play a dominant role in controlling the exhumation. Thrusting and back thrusting within the Almora klippe and Ramgarh thrust sheet are the post-emplacement kinematics that controlled the exhumation of the Almora klippe. Combining these results with the already published AFT ages from the crystalline klippen and the Higher Himalayan Crystalline (HHC), the kinematics of emplacement of the klippen over the LHS and exhumation pattern across the MCT in the Kumaon and Garhwal regions of NW-Himalaya have been investigated.

40Ar ∗ loss in experimentally deformed muscovite and biotite with implications for 40Ar/ 39Ar geochronology of naturally deformed rocks

NASA Astrophysics Data System (ADS)

Cosca, Michael; Stunitz, Holger; Bourgeix, Anne-Lise; Lee, John P.

2011-12-01

The effects of deformation on radiogenic argon ( 40Ar ∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ˜15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas. Infrared (IR) laser (CO 2) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined 40Ar/ 39Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40Ar ∗ loss of 0-35% in muscovite and 2-3% 40Ar ∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation 40Ar/ 39Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16% 40Ar ∗ loss relative to the undeformed muscovite. The in situ UV laser ablation 40Ar/ 39Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% 40Ar ∗ loss. No spatial correlation is observed between in situ40Ar/ 39Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed 40Ar ∗ loss in the experimentally treated muscovite can be utilized to predict average 40Ar ∗ diffusion dimensions. Maximum 40Ar/ 39Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for 40Ar ∗ diffusion ⩾700 μm. The UV laser ablation and IR laser incremental 40Ar/ 39Ar ages indicating 40Ar ∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ≪100 μm. These results support a hypothesis of grain-scale 40Ar ∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for 40Ar ∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both 40Ar ∗ retention and intragrain distribution in deformed mica. Intragrain 40Ar/ 39Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas.

Markers for geodynamic stability of the Variscan basement: case study for the Montseny-Guilleries High (NE Iberia)

NASA Astrophysics Data System (ADS)

Parcerisa, David; Franke, Christine; Gómez-Gras, David; Thiry, Médard

2010-05-01

The Montseny-Guilleries High is a Miocene horst composed of Variscan basement rocks, situated in the northeastern part of the Catalan Coastal Ranges (NE Iberia). The Montseny-Guilleries High has an asymmetric profile with an abrupt faulted scarp at the southeastern margin and a smooth surface dipping to the Northwest; here Paleocene sediments of the Ebro basin margin are to some extend in onlap. The stratigraphic arrangement of the Mesozoic units in the Catalan Coastal Ranges indicate that the Montseny-Guilleries area was a relief during the Mesozoic, remaining exposed probably from the Permian to the Cretaceous [Anadón et al., 1979; Gómez-Gras, 1993]. The high subsequently has been faulted due to a rifting phase that took place during the Miocene [Anadón et al., 1979]. The geodynamic history (burial-exhumation processes and denudation rate) of the Montseny-Guilleries High can be deciphered from cooling markers, such as for example apatite fission tracks [Juez-Larré & Andriessen, 2006]. However, the cooling history of an area depends on many factors (i.e. denudation rate, variations of the geothermal gradient) that complicate interpretations [Juez-Larré & Ter Voorde, 2009]. Another solution is to search for datable paleoweathering records in order to obtain benchmarks for ancient surfaces of continental exposure. This is the case for the Permian-Triassic paleosurface, at which an extensive albitization-hematisation alteration occurred at shallow depth [Thiry et al., 2009]. Several paleoalterations have been identified in the Montsent-Guilleries High [Gómez-Gras & Ferrer, 1999]. These alterations are coupled to the smooth surface or peneplain of the northwestern margin of the high and form a paleoprofile with less altered rocks on the lower parts of the relief and more altered rocks located at the higher parts of the relief (i.e. on the peneplain). From base to top, the profile starts with week albitization-hematisation of the facies developed mainly in the fractures; the degree of albitization-hematisation progressively increases towards the top affecting the whole rock, which acquires a characteristic pink color. Finally, the top of the profile is formed by strongly altered to hematite and kaolinite rich reddish facies. These uppermost parts of the alteration profile are formed by a relatively soft rock and are therefore usually not preserved, but the intermediate albitized parts are more resistant to surface alteration than unaltered facies and protect the peneplain from weathering and erosion. The albitization-hematisation alterations observed in the Montseny-Guilleries peneplain are very similar to the Permian-Triassic paleoalteration profiles observed in other parts of Europe, affecting the Variscan basement [Ricordel et al., 2007; Parcerisa et al., 2009]. Dating these profiles using paleomagnetic methods will help us to identify the location of the Permian-Triassic surface in the area and deduce its geodynamic history during the Mesozoic and Tertiary periods. Anadón, P., Colombo, F., Esteban, M., Marzo, M., Robles, S., Santanach, P., Solé-Sugrañes, L., 1979, Evolución tectonostratigráfica de los Catalánides, Acta Geol. Hisp., 14: 242-270. Gómez-Gras, D., 1993, El Permotrias de la Cordillera Costero Catalana: facies y petrologia sedimentaria (Parte I), Bol. Geol. Min., 104 (2): 115-161. Gómez-Gras, D., Ferrer, C., 1999, Caracterización petrológica de perfiles de meteorización antiguos desarrollados en granitos tardihercínicos de la Cordillera Costero Catalana, Rev. Soc. Geol. Esp., 12(2): 281-299. Juez-Larre, J., Andriessen, P.A.M., 2006, Tectonothermal evolution of the northeastern margin of Iberia since the break-up of Pangea to present, revealed by low-temperature fission-track and (U-Th)/He thermochronology: A case history of the Catalan Coastal Ranges, Earth Planet. Sci. Let., 243 (1-2): 159-180. Juez-Larré, J., Ter Voorde, M., 2009, Thermal impact of the break-up of Pangea on the Iberian Peninsula, assessed by thermochronological dating and numerical modeling, Tectonophysics, v. 474, no. 1-2, p. 200-213. Parcerisa, D., Thiry, M., Schmitt, J.M., 2009, Albitisation related to the Triassic unconformity in igneous rocks of the Morvan Massif (France), Int. Jour. Earth Sci., DOI 10.1007/s00531-008-0405-1 Ricordel, C., Parcerisa, D., Thiry, M., Moreau, M.G., Gomez-Gras, D., 2007, Triassic magnetic overprints related to albitization in granites from the Morvan massif (France), Palaeogeogr. Palaeoclimatol. Palaeoecol., 251:268-282. Thiry, M., Parcerisa, D., Ricordel-Prognon, C., Schmitt, J.M., 2009, Sodium storage in deep paleoweathering profiles beneath the Paleozoic-Triassic unconformity, EGU General Assembly 2009, Vienna, Austria.

Resolving the chronology of the South African landscape through joint inverse modelling of AFT and apatite (U-Th)/He data

NASA Astrophysics Data System (ADS)

Wildman, Mark; Brown, Roderick; Beucher, Romain; Persano, Cristina; Stuart, Finlay

2013-04-01

Application of Low temperature thermochronometry (LTT) is a powerful method of constraining thermal history information on samples as they pass through isotherms in the upper crust. Inverse modelling of LTT data generates thermal history information which can then be correlated with independent datasets to infer geological processes that are responsible for producing the observed thermal history held in the thermochronometry record. A critical consideration when choosing which LTT method to use are the closure temperatures associated with each system. In order to generate more complete and robust thermal histories a single sample can be analysed using multiple low temperature thermochronometers that are sensitive over different but complimentary temperature ranges. The main focus of LTT work in South Africa has been on apatite fission track (AFT) analysis which is a world renowned method of constraining thermal history information between c. 60 and 110±10°C. The general conclusions that have been drawn from the South African AFT dataset is that the present day regional topography represents an eroded remnant of an elevated interior that experienced a significant uplift event with km-scale erosion in the Cretaceous following the break-up of Gondwana [1]. The exact nature of Cretaceous uplift and erosion varies both spatially and temporally, especially in south western Africa where at least two distinct denudation events are recorded at c. 130Ma and 90 Ma [2]. There are, however, alternative views suggesting significant epeirogenic-style uplift and subsequent erosion throughout the Cenozoic [3]. A key aspect of this debate which is yet to be fully resolved is the influence of mantle dynamics on the evolution of the overlying topography. To further investigate the timing and amount of Cenozoic uplift and erosion and to what degree this can be ascribed to dynamic topography, efforts have been made to complement the existing AFT record with Apatite (U-Th)/He analysis (AHe) (e.g. [4]). AHe is ideally suited to this task as it is sensitive over a lower temperature range (c. 40-75±5°C) and can therefore provide insights into erosion on the scale of 1-3 km. Correlating AFT and AHe datasets can be difficult due to variations in U and Th zonation, crystal geometry and structural defects (e.g. radiation damage) altering the closure temperature of the AHe system and resulting in significant dispersion of ages. It is therefore essential to fully understand the major cause of age dispersion and use thermal history modelling techniques that take these factors into account. Here we present a new suite of apatite (U-Th)/He data from samples along the Orange River alongside complimentary apatite fission track data from the same samples. Two samples with a large number of analysed grains, JN2 and GGO2 (23 and 21 grains, respectively) were modelled using HelFrag [5] which takes into account dispersion in AHe ages produced through analysis fragments of larger apatite grains. These modelled histories where compared with inverse modelling using QTQt [6] to extract thermal history information for each of the samples and provide new insights into the post-Cretaceous evolution of the SW African landscape and the behaviour of complex dispersion patterns commonly observed in AHe data sets. [1] Brown, R.W., Summerfield, M.A., and Gleadow, A.J.W., 2002, Denudational history along a transect across the Drakensberg Escarpment of southern Africa derived from apatite fission track thermochronology: Journal of Geophysical Research, v. 107, p. B12, 2350. [2] Kounov, A., Viola, G., de Wit, M., and Andreoli, M.A.G., 2009, Denudation along the Atlantic passive margin: new insights from apatite fission-track analysis on the western coast of South Africa: Geological Society, London, Special Publications, v. 324, p. 287-306. [3] Partridge, T.C., and Maud, R.R., 1987, Geomorphic evolution of southern Africa since the Mesozoic: South African Journal of Geology, v. 90, p. 179-208 [4] Flowers, R.M., and Schoene, B., 2010, (U-Th)/He thermochronometry constraints on unroofing of the eastern Kaapvaal craton and significance for uplift of the southern Africa Plateau: Geology, v. 38, p. 827-830. [5] Gallagher, K., 2012, Transdimensional inverse thermal history modeling for quantitative thermochronology: Journal of Geophysical Research, v. 117, p. 16. [6] Brown, R.W., Beucher, R., Roper, S., Gallagher, K., Persano, C. and Stuart, F., 2012, How to derive robust thermal histories from 'over-dispersed' single crystal apatite (U-Th)/He ages, Geophysical Research Abstracts, Vol. 14, EGU2012-11828, 2012, EGU General Assembly 2012

Timing, distribution, amount, and style of Cenozoic extension in the northern Great Basin

USGS Publications Warehouse

Henry, Christopher D.; McGrew, Allen J.; Colgan, Joseph P.; Snoke, Arthur W.; Brueseke, Matthew E.

2011-01-01

This field trip examines contrasting lines of evidence bearing on the timing and structural style of Cenozoic (and perhaps late Mesozoic) extensional deformation in northeastern Nevada. Studies of metamorphic core complexes in this region report extension beginning in the early Cenozoic or even Late Cretaceous, peaking in the Eocene and Oligocene, and being largely over before the onset of “modern” Basin and Range extension in the middle Miocene. In contrast, studies based on low-temperature thermochronology and geologic mapping of Eocene and Miocene volcanic and sedimentary deposits report only minor, localized extension in the Eocene, no extension at all in the Oligocene and early Miocene, and major, regional extension in the middle Miocene. A wealth of thermochronologic and thermobarometric data indicate that the Ruby Mountains–East Humboldt Range metamorphic core complex (RMEH) underwent ~170 °C of cooling and 4 kbar of decompression between ca. 85 and ca. 50 Ma, and another 450 °C cooling and 4–5 kbar decompression between ca. 50 and ca. 21 Ma. These data require ~30 km of exhumation in at least two episodes, accommodated at least in part by Eocene to early Miocene displacement on the major west-dipping mylonitic zone and detachment fault bounding the RMEH on the west (the mylonitic zone may also have been active during an earlier phase of crustal extension). Meanwhile, Eocene paleovalleys containing 45–40 Ma ash-flow tuffs drained eastward from northern Nevada to the Uinta Basin in Utah, and continuity of these paleovalleys and infilling tuffs across the region indicate little, if any deformation by faults during their deposition. Pre–45 Ma deformation is less constrained, but the absence of Cenozoic sedimentary deposits and mappable normal faults older than 45 Ma is also consistent with only minor (if any) brittle deformation. The presence of ≤1 km of late Eocene sedimentary—especially lacustrine—deposits and a low-angle angular unconformity between ca. 40 and 38 Ma rocks attest to an episode of normal faulting at ca. 40 Ma. Arguably the greatest conundrum is how much extension occurred between ca. 35 and 17 Ma. Major exhumation of the RMEH is interpreted to have taken place in the late Oligocene and early Miocene, but rocks of any kind deposited during this interval are scarce in northeastern Nevada and absent in the vicinity of the RMEH itself. In most places, no angular unconformity is present between late Eocene and middle Miocene rocks, indicating little or no tilting between the late Eocene and middle Miocene. Opinions among authors of this report differ, however, as to whether this indicates no extension during the same time interval. The one locality where Oligocene deposits have been documented is Copper Basin, where Oligocene (32.5–29.5 Ma) conglomerates are ~500 m thick. The contact between Oligocene and Eocene rocks in Copper Basin is conformable, and the rocks are uniformly tilted ~25° NW, opposite to a normal fault system dipping ~35° SE. Middle Miocene rhyolite (ca. 16 Ma) rests nonconformably on the metamorphosed lower plate of this fault system and appears to rest on the tilted upper-plate rocks with angular unconformity, but the contact is not physically exposed. Different authors of this report interpret geologic relations in Copper Basin to indicate either (1) significant episodes of extension in the Eocene, Oligocene, and middle Miocene or (2) minor extension in the Eocene, uncertainty about the Oligocene, and major extension in the middle Miocene. An episode of major middle Miocene extension beginning at ca. 16–17 Ma is indicated by thick (up to 5 km) accumulations of sedimentary deposits in half-graben basins over most of northern Nevada, tilting and fanning of dips in the synextensional sedimentary deposits, and apatite fission-track and (U-Th)/He data from the southern Ruby Mountains and other ranges that indicate rapid middle Miocene cooling through near-surface temperatures (~120–40 °C). Opinions among authors of this report differ as to whether this period of extension was merely the last step in a long history of extensional faulting dating back at least to the Eocene, or whether it accounts for most of the Cenozoic deformation in northeastern Nevada. Since 10–12 Ma, extension appears to have slowed greatly and been accommodated by high-angle, relatively wide-spaced normal faults that give topographic form to the modern ranges. Despite the low present-day rate of extension, normal faults are active and have generated damaging earthquakes as recently as 2008.

Dating exhumed peridotite with spinel (U-Th)/He chronometry

NASA Astrophysics Data System (ADS)

Cooperdock, Emily H. G.; Stockli, Daniel F.

2018-05-01

The timing of cooling and exhumation of mantle peridotites in oceanic and continental settings has been challenging to determine using traditional geo- and thermochronometric techniques. Hence, the timing of the exhumation of mantle rocks to the Earth's surface at mid-ocean ridges, rifted and passive continental margins, and within continental volcanic and orogenic systems has remained largely elusive or only loosely constrained by relative age bracketing. Magmatic spinel [(Mg, Fe)(Al,Cr)2O4] is a ubiquitous primary mineral phase in mantle peridotites and is often the only primary mineral phase to survive surface weathering and serpentinization. This work explores spinel (U-Th)/He thermochronology as a novel tool to directly date the exhumation and cooling history of spinel-bearing mantle peridotite. Samples were chosen from a range of tectonic and petrologic settings, including a mid-ocean ridge abyssal peridotite (ODP Leg 209), an orogenic tectonic sliver of sub-continental mantle (Lherz massif, France), and a volcanic-rock hosted mantle xenolith (Green Knobs, NM). Spinel grains were selected based on grain size and morphology, screened for internal homogeneity using X-ray computed tomography, and air abraded to eliminate effects of alpha ejection/implantation. These case studies yield spinel He age results that are reproducible and generally in good agreement with independent age constraints. For ODP Leg 209, a spinel He age of 1.1 ± 0.3 Ma (2 SE) (n = 8) is consistent with independent U-Pb and magnetic anomaly ages for the exhumation of oceanic crust by detachment faulting along this segment of the slow-spreading ridge. Spinel from the Lherz massif yield He ages from 60-70 Ma (n = 3), which correspond well with independent thermochronometric constraints for cooling associated with Pyrenean collisional exhumation. Spinel from a mantle xenolith within a previously undated kimberlite diatreme at Green Knobs, New Mexico, generate a reproducible mean He age of 11.7 ± 1.8 Ma (2 SE) (n = 6) that appears to record young volcanism in the area or age resetting by post-emplacement re-heating or alteration. The combined results of these case studies demonstrate the viability for spinel He thermochronometry to resolve cooling histories of peridotite exhumed through tectonic and volcanic processes.

SCANLIPS - A Study of Epirogenic Uplift of Scandinavia

NASA Astrophysics Data System (ADS)

England, R. W.; Ebbing, J.

2007-12-01

Thermochronology data and geomorphological interpretation indicate that parts of the Scandinavian mountains have risen by over 1 km since the Miocene. This permanent uplift, the cause of which is still disputed, varies across Norway, being greatest in southern and northern areas and least in the central region. To investigate this the SCANLIPS project employs passive seismology, coupled with modelling of potential field data to determine variations in crustal properties and structure across Norway and Sweden. Initially we intend to test whether lateral variations in crustal structure and properties are correlated with the uplift pattern. This would suggest that the cause of the differential uplift lies in a modification of the crust. If the test of this hypothesis is null we will use the data to investigate the present day upper mantle structure for the cause. Between April and October 2006 28 seismometers were deployed at sites along a c. 600 km long profile between Trondheim in Norway and Harnosand in Sweden to record teleseismic arrivals. Receiver Functions have been calculated for teleseismic events recorded at these stations and then modelled to determine crustal velocity structure, estimate Vp/Vs and depth to Moho. Preliminary results suggest that crustal thickness increases eastward beneath Norway and then remains deep beneath the lower topography of central Sweden. Along the profile a gradual eastward increase in seismic velocity, including a very high velocity lower crust beneath Sweden explains the compensation of shallow topography by thick crust. Forward density and isostatic modelling shows that the introduction of the high-density lower crust adjusts both the gravity field and the isostatic compensation. Beneath Norway the crust thins rapidly toward the continental margin at a rate that is faster than the topography decreases. This suggests that at least part of the topography is supported by the flexural strength of the crust in the footwall of the More-Trondelag fault zone. Recently published results of Svenningsen et al. (2007) show a similar thickening below the high topography of southern Norway, indicating Airy type compensation. Further work is required before a direct comparison can be made of the crustal properties between the two regions and a possible cause for the differential uplift of Scandinavia determined.

Cenozoic evolution of the Yakutat-North American collision zone and structural accommodation of St. Elias syntaxis exhumation, Alaska/Yukon

NASA Astrophysics Data System (ADS)

Falkowski, Sarah; Enkelmann, Eva; Ehlers, Todd

2016-04-01

Active convergent margins potentially pose multiple natural hazards to human life and infrastructure. Tectonic strain may be further focused where convergent margins are warped into broad syntaxes. However, the processes responsible for upper plate deformation in these settings are not well understood. The St. Elias syntaxis in southeast Alaska and southwest Yukon is located at the eastern corner of the Yakutat microplate, which indents into the North American Plate and subducts at a flat angle beneath Alaska. High rates of long-term glacial erosion and exhumation (>2 mm/yr) are found on the southern, coastal flanks of the St. Elias orogen, but the deepest and most rapid exhumation is focused at the St. Elias syntaxis. In this location, transform motion transitions into subduction of the wedge-shaped, oceanic plateau of the Yakutat microplate. In order to map the spatio-temporal pattern of exhumation in the Yakutat-North American collision zone, we conducted zircon and apatite fission-track analyses of predominantly detrital, sand-sized material and five bedrock samples from 47 different glacio-fluvial catchments covering an area of ~45,000 km2 around the St. Elias syntaxis. Integration of the new thermochronologic data with prior work and other geologic and geophysical observations yielded information on past terrane accretion events at the North American margin since the late Mesozoic and the evolution of exhumation at the St. Elias syntaxis in the context of the ongoing Yakutat-North American plate collision. Our results indicate a migrating focus of the most rapid exhumation from north to south and from the upper (North American Plate) to the lower (Yakutat microplate) plate in the syntaxis area over the past ~10 Myr. This migration occurred in response to a change in plate motions, increasingly thicker crust of the subducting Yakutat microplate, and changes in surface processes after glaciation began that resulted in modification of the rheology. We propose a positive, two-sided flower structure to have accommodated the rapid, and temporarily deep (~10 km), exhumation.

Beta Testing StraboSpot: Perspectives on mobile field mapping and data collection

NASA Astrophysics Data System (ADS)

Bunse, E.; Graham, K. A.; Rufledt, C.; Walker, J. D.; Müller, A.; Tikoff, B.

2017-12-01

Geologic field mapping has recently transitioned away from traditional techniques (e.g. field notebooks, paper mapping, Brunton compasses) and towards mobile `app' mapping technology. The StraboSpot system (Strabo) is an open-source solution for collection and storage for geologic field, microstructural, and lab-based data. Strabo's mission is to "enable recording and sharing data within the geoscience community, encourage interdisciplinary research, and facilitate the investigation of scientific questions that cannot currently be addressed" (Walker et al., 2015). Several mobile application beta tests of the system, on both Android and Apple iOS platforms using smartphones and tablets, began in Summer 2016. Students at the 2016 and 2017 University of Kansas Field Camps used Strabo in place of ArcGIS for Desktop on Panasonic Toughbooks, to field map two study areas. Strabo was also field tested by students of graduate and undergraduate level for both geo/thermochronologic sample collection and reconnaissance mapping associated with regional tectonic analysis in California. Throughout this period of testing, the app was geared toward structural and tectonic geologic data collection, but is versatile enough for other communities to currently use and is expanding to accommodate the sedimentology and petrology communities. Overall, users in each of the beta tests acclimated quickly to using Strabo for field data collection. Some key advantages to using Strabo over traditional mapping methods are: (1) Strabo allows for consolidation of materials in the field; (2) helps students track their position in the field with integrated GPS; and (3) Strabo data is in a uniform format making it simple for geologists to collaborate. While traditional field methods are not likely to go out of style in the near future, Strabo acts as a bridge between professional and novice geologists by providing a tool that is intuitive on all levels of geological and technological experience and allows for more effective collaboration in the field. Walker, J. Douglas, et al. (2015), Development of Structural Geology and Tectonics Data System with Field and Lab Interface, Abstract IN21E-04 presented at 2015 Fall Meeting, AGU, San Francisco, Calif., 14-18 Dec.

New constraints on the origin of the Sierra Madre de Chiapas (south Mexico) from sediment provenance and apatite thermochronometry

NASA Astrophysics Data System (ADS)

Witt, C.; Brichau, S.; Carter, A.

2012-12-01

The timing and source of deformation responsible for formation of the Sierra Madre de Chiapas (south Mexico) are unclear. To address this, apatite fission track and U-Th-He thermochronometry, combined with zircon U-Pb dating, were performed on bedrock and sedimentary samples of the Sierra Madre de Chiapas to discern timing of exhumation and identify sediment source areas. The U-Pb results show that Paleocene-Eocene terrigenous units outcropping at the northern section of the Sierra were mostly derived from Grenville (˜1 Ga) basement whereas the internal sections of the chain yield mainly Permian to Triassic ages (circa 270-230 Ma) typical of the Chiapas massif complex. Grenville-sourced sediments are most probably sourced by the Oaxacan block or the Guichicovi complex and were deposited to the north of the Sierra in a foreland setting related to a Laramide deformation front. Other possibly source areas may be related to metasedimentary units widely documented at the south Maya block such as the Baldi unit. The apatite fission track and U-Th-He data combined with previously published results record three main stages in exhumation history: (1) slow exhumation between 35 and 25 Ma affecting mainly the Chiapas massif complex; (2) fast exhumation between 16 and 9 Ma related to the onset of major strike-slip deformation affecting both the Chiapas massif complex and Chiapas fold-and-thrust belt; and (3) a 6 to 5 Ma period of rapid cooling that affected the Chiapas fold-and-thrust belt, coincident with the landward migration of the Caribbean-North America plate boundaries. These data suggest that most of the topographic growth of the Sierra Madre de Chiapas took place in the middle to late Miocene. The new thermochronological evidence combined with stratigraphic and kinematic information suggests that the left-lateral strike-slip faults bounding the Chiapas fold-and-thrust belt to the west may have accommodated most of the displacement between the North American and Caribbean plates during the last 6-5 Ma.

Accessory Mineral Depth-Profiling Applied to the Corsican Lower Crust: A Continuous Thermal History of Mesozoic Continental Rifting

NASA Astrophysics Data System (ADS)

Seymour, N. M.; Stockli, D. F.; Beltrando, M.; Smye, A.

2015-12-01

Despite advances in understanding the structural development of hyperextended magma-poor rift margins, the temporal and thermal evolution of lithospheric hyperextension during rifting remains only poorly understood. In contrast to classic pure-shear models, multi-stage rift models that include depth-dependent thinning predict significant lower-crustal reheating during the necking phase due to buoyant rise of the asthenosphere. The Santa Lucia nappe of NE Corsica is an ideal laboratory to test for lower-crustal reheating as it preserves Permian lower crust exhumed from granulitic conditions during Mesozoic Tethyan rifting. This study presents the first use of apatite U-Pb depth-profile thermochronology in conjunction with novel rutile U-Pb and zircon U-Pb thermo- and geochronology to reconstruct a continuous t-T path to constrain the syn-rift thermal evolution of this exposed lower-crustal section. LASS-ICP-MS depth-profile analyses of zircon reveal thin (<10 μm) ~210-180 Ma overgrowths on 300-270 Ma cores in lower-crustal lithologies, indicative of renewed thermal activity during Mesozoic rifting. Cooling due to rapid rift margin exhumation is recorded by the topology of rutile and apatite depth profiles caused by thermally-activated volume diffusion at T >400°C. Lower-crustal rutile reveal a rounded progression from core plateaus at ~170 Ma to 150-145 Ma at the outer 8-10 μm of grains while middle-crustal apatite records 170 Ma cores grading to 140-135 Ma rims. Inverse modeling of rutile profiles suggests the lower crust cooled from 700°C at 200 Ma to 425°C at 140 Ma. Middle-crustal apatite yield a two-stage history, with rapid cooling from 500°C at 200 Ma to 420°C at ~180 Ma followed by slow cooling to 400°C by 160 Ma. Combined with zircon overgrowth ages, these data indicate the Santa Lucia nappe underwent a thermal pulse in the late Triassic-early Jurassic associated with depth-dependent thinning and hyperextension of the Corsican margin.

Betwixt and Between: Structure and Evolution of Central Mongolia

NASA Astrophysics Data System (ADS)

Meltzer, A.; Ancuta, L. D.; Carlson, R. W.; Caves, J. K.; Chamberlain, C. P.; Gosse, J. C.; Idleman, B. D.; Ionov, D. A.; McDannell, K. T.; Tamra, M.; Mix, H.; Munkhuu, U.; Russo, R.; Sabaj-Perez, M.; Sahagian, D. L.; Sjostrom, D. J.; Smith, S. G.; Stachnik, J. C.; Tsagaan, B.; Wegmann, K. W.; Winnick, M. J.; Zeitler, P. K.; Prousevitch, A.

2015-12-01

Central Mongolia sits deep in the Asian continental interior between the Siberian craton to the north, the edge of the India-Asia collision to the south, and far-field subduction of the Pacific plate to the east. It has a complex geologic history comprising Archean to Early Proterozoic crystalline rocks modified by accretionary events in the Paleozoic, and Cenozoic deformation and basalt volcanism that continues today. Within central Mongolia, the broad domal Hangay upland is embedded in the greater Mongolian Plateau. Elevations within the dome average ~1.5 km above the regional trend and locally reach ~4000 m. This elevated landscape hosts a low-relief surface cut into crystalline basement, and a 30 Ma record of intermittent basalt magmatism. Here we integrate observations from geomorphology, geochronology, paleoaltimetry, biogeography, petrology, geochemistry, and seismology to document the timing, rate, and pattern of surface uplift in the Hangay and more broadly to understand the geodynamics of the Mongolian plateau. Results from mantle and crustal xenoliths, seismology, thermochronology, and basalt geochemistry are consistent with: a high geothermal gradient with temperatures reaching ~900°C at 60 km depth, intercepting the mantle adiabat at ~90 km depth; an uppermost mantle composed mostly of fertile peridotites; low-volume Cenozoic basaltic magmatism sourced below the lithosphere, with isotopic characteristics similar to much east-Asian Cenozoic mafic volcanism; a 42-57 km-thick crust of island-arc affinity formed during accretion of the Central Asia Orogenic Belt; elevations supported primarily by crustal isostasy; slow exhumation (30-100 m/My) over hundreds of millions of years; and long-term thermal stability of the upper crust and relief lowering since the Mesozoic. Results from geomorphology, paleoaltimetry, fish genetics, and basalt geochronology imply that drainage divides are stable since the mid-Miocene with modest surface uplift (up to 1 km) and topographic relief up to 800 m remaining largely unchanged since ~10 Ma. Surprisingly, this area of remarkable stability over significant time and space sits above a shallow convecting mantle and hosts some of the largest recorded intracontinental earthquakes.

Scaling of Viscous Shear Zones with Depth Dependent Viscosity and Power Law Stress-strain Rate Dependence

NASA Astrophysics Data System (ADS)

Meltzer, A.; Ancuta, L. D.; Carlson, R. W.; Caves, J. K.; Chamberlain, C. P.; Gosse, J. C.; Idleman, B. D.; Ionov, D. A.; McDannell, K. T.; Tamra, M.; Mix, H.; Munkhuu, U.; Russo, R.; Sabaj-Perez, M.; Sahagian, D. L.; Sjostrom, D. J.; Smith, S. G.; Stachnik, J. C.; Tsagaan, B.; Wegmann, K. W.; Winnick, M. J.; Zeitler, P. K.; Prousevitch, A.

2014-12-01

Central Mongolia sits deep in the Asian continental interior between the Siberian craton to the north, the edge of the India-Asia collision to the south, and far-field subduction of the Pacific plate to the east. It has a complex geologic history comprising Archean to Early Proterozoic crystalline rocks modified by accretionary events in the Paleozoic, and Cenozoic deformation and basalt volcanism that continues today. Within central Mongolia, the broad domal Hangay upland is embedded in the greater Mongolian Plateau. Elevations within the dome average ~1.5 km above the regional trend and locally reach ~4000 m. This elevated landscape hosts a low-relief surface cut into crystalline basement, and a 30 Ma record of intermittent basalt magmatism. Here we integrate observations from geomorphology, geochronology, paleoaltimetry, biogeography, petrology, geochemistry, and seismology to document the timing, rate, and pattern of surface uplift in the Hangay and more broadly to understand the geodynamics of the Mongolian plateau. Results from mantle and crustal xenoliths, seismology, thermochronology, and basalt geochemistry are consistent with: a high geothermal gradient with temperatures reaching ~900°C at 60 km depth, intercepting the mantle adiabat at ~90 km depth; an uppermost mantle composed mostly of fertile peridotites; low-volume Cenozoic basaltic magmatism sourced below the lithosphere, with isotopic characteristics similar to much east-Asian Cenozoic mafic volcanism; a 42-57 km-thick crust of island-arc affinity formed during accretion of the Central Asia Orogenic Belt; elevations supported primarily by crustal isostasy; slow exhumation (30-100 m/My) over hundreds of millions of years; and long-term thermal stability of the upper crust and relief lowering since the Mesozoic. Results from geomorphology, paleoaltimetry, fish genetics, and basalt geochronology imply that drainage divides are stable since the mid-Miocene with modest surface uplift (up to 1 km) and topographic relief up to 800 m remaining largely unchanged since ~10 Ma. Surprisingly, this area of remarkable stability over significant time and space sits above a shallow convecting mantle and hosts some of the largest recorded intracontinental earthquakes.

New Insights into Arctic Tectonics: Uranium-Lead, (Uranium-Thorium)/Helium, and Hafnium Isotopic Data from the Franklinian Basin, Canadian Arctic Islands

NASA Astrophysics Data System (ADS)

Anfinson, Owen Anthony

More than 2300 detrital zircon uranium-lead (U-Pb) ages, 32 176Hf/177Hf (eHf) isotopic values, 37 apatite helium (AHe) ages, and 72 zircon helium (ZHe) ages represent the first in-depth geochronologic and thermochronologic study of Franklinian Basin strata in the Canadian Arctic and provide new insight on >500 M.y. of geologic history along the northern Laurentian margin (modern orientation). Detrital zircon U-Pb age data demonstrate that the Franklinian Basin succession is composed of strata with three distinctly different provenance signatures. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland--northeast Canadian Shield. Lower Silurian to Middle Devonian strata are primarily derived from foreland basin strata of the East Greenland Caledonides (Caledonian orogen). Middle Devonian to Upper Devonian strata also contain detrital zircon populations interpreted as being primarily northerly derived from the continental landmass responsible for the Ellesmerian Orogen (often referred to as Crockerland). U-Pb age data from basal turbidites of the Middle to Upper Devonian clastic succession suggest Crockerland contributed sediment to the northern Laurentian margin by early-Middle Devonian time and that prior to the Ellesmerian Orogeny Crockerland had a comparable geologic history to the northern Baltica Craton. Detrital zircon U-Pb ages in Upper Devonian strata suggest Crockerland became the dominant source by the end of Franklinian Basin sedimentation. Mean eHf values from Paleozoic detrital zircon derived from Crockerland suggest the zircons were primarily formed in either an island arc or continental arc built on accreted oceanic crust setting. ZHe cooling ages from Middle and Upper Devonian strata were not buried deeper than 7 km since deposition and suggest Crockerland was partially exhumed during the Caledonian Orogen. AHe cooling ages are partially reset since deposition and experienced varying burial histories depending on stratigraphic and geographic location within the basin. AHe ages from Middle Devonian strata from the western margin of the basin indicate episodes of exhumation associated with clastic influxes of sediment into the Sverdrup Basin during the Late Jurassic-Early Cretaceous and Late Cretaceous.

Extracting information on the spatial variability in erosion rate stored in detrital cooling age distributions in river sands

NASA Astrophysics Data System (ADS)

Braun, Jean; Gemignani, Lorenzo; van der Beek, Peter

2018-03-01

One of the main purposes of detrital thermochronology is to provide constraints on the regional-scale exhumation rate and its spatial variability in actively eroding mountain ranges. Procedures that use cooling age distributions coupled with hypsometry and thermal models have been developed in order to extract quantitative estimates of erosion rate and its spatial distribution, assuming steady state between tectonic uplift and erosion. This hypothesis precludes the use of these procedures to assess the likely transient response of mountain belts to changes in tectonic or climatic forcing. Other methods are based on an a priori knowledge of the in situ distribution of ages to interpret the detrital age distributions. In this paper, we describe a simple method that, using the observed detrital mineral age distributions collected along a river, allows us to extract information about the relative distribution of erosion rates in an eroding catchment without relying on a steady-state assumption, the value of thermal parameters or an a priori knowledge of in situ age distributions. The model is based on a relatively low number of parameters describing lithological variability among the various sub-catchments and their sizes and only uses the raw ages. The method we propose is tested against synthetic age distributions to demonstrate its accuracy and the optimum conditions for it use. In order to illustrate the method, we invert age distributions collected along the main trunk of the Tsangpo-Siang-Brahmaputra river system in the eastern Himalaya. From the inversion of the cooling age distributions we predict present-day erosion rates of the catchments along the Tsangpo-Siang-Brahmaputra river system, as well as some of its tributaries. We show that detrital age distributions contain dual information about present-day erosion rate, i.e., from the predicted distribution of surface ages within each catchment and from the relative contribution of any given catchment to the river distribution. The method additionally allows comparing modern erosion rates to long-term exhumation rates. We provide a simple implementation of the method in Python code within a Jupyter Notebook that includes the data used in this paper for illustration purposes.

Oroclinal Bending and Mountain Uplift in the Central Andes

NASA Astrophysics Data System (ADS)

Mpodozis, C.; Arriagada, C.; Roperch, P.

2007-05-01

The large paleomagnetic database now available for the Central Andes permits a good understanding of the overall spatial and temporal variations of rotations. Mesozoic to Early Paleogene rocks along the forearc of northern Chile (23°-28°S) record significant clockwise rotations (>25°) [Arriagada et al., 2006, Tectonics, doi:10.1029/2005TC001923]. Along the forearc of southern Peru, counterclockwise rotations recorded within flat lying red-beds (Moquegua Formation) increase from about -30° at 17.5°S to - 45° at15.5°S and decrease through time from the late Eocene to the late Oligocene-early Miocene [Roperch et al., 2006, Tectonics, doi:10.1029/2005TC001882]. Recently published thermo-chronological studies show evidence for strong exhumation within Bolivian Eastern Cordillera and the Puna plateau starting in the Eocene while structural studies indicate that the majority of crustal shortening in the Eastern Cordillera occurred during the Eocene-Oligocene, although the final stages of deformation may have continued through the Early Miocene. Rotations in the Peruvian and north Chilean forearc thus occurred at the same time than deformation and exhumation/uplift within the Eastern Cordillera. In contrast Neogene forearc rocks in southern Peru and northern Chile do not show evidences of rotation but low magnitude (10°) counterclockwise rotations are usually found in mid to late Miocene rocks from the northern Altiplano. These Neogene rotations are concomitant with shortening in the Sub-Andean zone and sinistral strike-slip faulting along the eastern edge of the northern Altiplano. We interpret the rotation pattern along the southern Peru and north Chile forearc as a result of strong late Eocene- late Oligocene oroclinal bending of the Central Andes associated with shortening gradients along the Eastern Cordillera associated both with the Abancay deflection and the Arica bend. The amount and spatial distribution of pre-Neogene shortening needed to account for oroclinal bending is difficult to estimate as the rotations may be partly driven by transpression along strike slip shear zones. The large rotations strongly highlight the importance of the pre-Neogene tectonic history in the evolution of the Central Andes.

Eocene extensional exhumation of basement and arc rocks along southwesternmost Peru, Central Andes.

NASA Astrophysics Data System (ADS)

Noury, Mélanie; Bernet, Matthias; Sempéré, Thierry

2014-05-01

The overthickened crust of the current Central Andes is commonly viewed as the result of tectonic shortening. However, in the present-day terrestrial forearc and arc of southwesternmost Peru, crustal thickness increases from 30 km along the coastline to >60 km below the active arc, whereas the upper crust exhibits little to no evidence of crustal shortening and, in constrast, many extensional features. How (and when) crustal overthickness was acquired in this region is thus little understood. Because crustal overthickening often results in extensional collapse and/or significant erosion, here we address this issue through a regional-scale study of exhumation using fission-track thermochronology. The limited fission-track data previously available in the area suggested that exhumation began during the Mesozoic. In this study, we present new apatite and zircon fission-track data obtained along the current terrestrial forearc of southwesternmost Peru. This relatively restricted area presents the interest of providing extensive outcrops of Precambrian to Ordovician basement and Early Jurassic to Late Cretaceous arc plutons. In order to compare the chronology of exhumation of these units, we performed extensive sampling for fission-track dating, as well as structural mapping. Our results indicate that the basement rocks and Jurassic plutons that crop out in the Arequipa region, where the crust is now >50 km-thick, experienced a rapid cooling through the 240-110°C temperature range between ~65 and ~35 Ma. This period of rapid exhumation coincided in time with the accumulation of terrestrial forearc deposits (the Lower Moquegua Group), that exhibit many syn-sedimentary extensional features and are bounded by conspicuous normal faults, specifically along the region where intense activity of the main arc between ~90 and ~60 Ma had led to voluminous magma emplacement. This close succession of (1) intense magmatic activity and (2) regional-scale exhumation associated with extensional basins leads us to propose that arc magmatism between ~90 and ~60 Ma was productive enough to significantly thicken the crust, resulting in its subsequent extensional collapse between ~60 and ~35 Ma.

Upper-crustal Stress Field Variations During the Building of the Central Andes: Constrains on the Activation/deactivation of Megadetachments

NASA Astrophysics Data System (ADS)

Giambiagi, L.; Tassara, A.; Mescua, J.; Suriano, J.; Mahoney, J. B.; Hoke, G. D.; Spagnotto, S. L.; Lossada, A. C.; Mardónez, D.; Mazzitelli, M.; Barrionuevo, M.

2015-12-01

Nowadays, it is broadly accepted that the Central Andes resulted largely from crustal shortening in the last ~45 Ma, driven by horizontal forces as a consequence of subduction of the Nazca plate beneath South America. However, the way this shortening is achieved is still a matter a debate. Structural, seismological, thermochronological, isotopical and sedimentological studies of the Central Andes, together with thermomechanical modeling, suggest that different megadetachments located shallow in the upper crust were active during the construction of the Andes. Constrains on changes in the state of stress in the crust gleaned from more than 1,500 fault-slip data in the arc region provide insights into how and when these megadetachments get activated or deactivated. We used a forward modeling procedure to examine five transects across the Central Andes, at 21.5°, 24°, 30°, 34° and 35°S, with particular emphasis on the relationship between deep and shallow structures. Our kinematic-thermomechanical models show that most of the upper-middle crust has a brittle-elastic behavior particularly for the cold and rigid forearc and foreland regions, and a ductile behavior below the thermally weakened arc region. Our models assume a shallow, sub-horizontal megadetachment located at the shallowest brittle-ductile transition, which concentrates the majority of the horizontal crustal shortening between the fore-arc and the South American craton. During this horizontal shortening, the crust gets thick and topography rises due to buoyancy of the crustal root. The threshold of this thickening is achieved when the bouyancy force equals the horizontal force. At this point, the megadetachment deactives and the crustal root widens eastwards in concert with ductile deformation in the lower crust and the generation of a new megadetachment. By studying changes in the paleostress fields along the arc region, from compression to strike-slip, and strike-slip to extension, associated with σ3/σ2 and σ2/σ1 permutations respectively, together with the timing of uplift and exhumation of the morphostructural units across the transects, we can constrain the timing of activation/deactivation of the detachments responsible for the Andean deformation.

New constraints on the structure, thermochronology, and timing of the Ailao Shan-Red River shear zone, SE Asia

NASA Astrophysics Data System (ADS)

Leloup, P. H.; Arnaud, N.; Lacassin, R.; Kienast, J. R.; Harrison, T. M.; Trong, T. T. Phan; Replumaz, A.; Tapponnier, P.

2001-04-01

New structural, petrographic, and 40Ar/39Ar data constrain the kinematics of the ASRR (Ailao Shan-Red River shear zone). In the XueLong Shan (XLS), geochronological data reveal Triassic, Early Tertiary, and Oligo-Miocene thermal events. The latter event (33-26 Ma) corresponds to cooling during left-lateral shear. In the FanSiPan (FSP) range, thrusting of the SaPa nappe, linked to left-lateral deformation, and cooling of the FSP granite occurred at ≈35 Ma. Rapid cooling resumed at 25-29 Ma as a result of uplift within the transtensive ASRR. In the DayNuiConVoi (DNCV), foliation trends NW-SE, but is deflected near large-scale shear planes. Stretching lineation is nearly horizontal. On steep foliations, shear criteria indicate left-lateral shear sense. Zones with flatter foliations show compatible shear senses. Petrographic data indicate decompression from ≈6.5 kbar during left-lateral shear (temperatures >700°C). 40Ar/39Ar data imply rapid cooling from above 350°C to below 150°C between 25 and 22 Ma without diachronism along strike. Along the whole ASRR cooling histories show two main episodes: (1) rapid cooling from peak metamorphism during left-lateral shear; (2) rapid cooling from greenschist conditions during right-lateral reactivation of the ASRR. In the NW part of the ASRR (XLS, Diancang Shan), we link rapid cooling 1 to local denudations in a transpressive environment. In the SW part (Ailao Shan and DNCV), cooling 1 resulted from regional denudation by zipper-like tectonics in a transtensive regime. The induced cooling diachronism observed in the Ailao Shan suggests left-lateral rates of 4 to 5 cm/yr from 27 Ma until ≈17 Ma. DNCV rocks always stayed in a transtensive regime and do not show cooling diachronism. The similarities of deformation kinematics along the ASRR and in the South China Sea confirms the causal link between continental strike-slip faulting and marginal basin opening.

Initiation of continental accretion: metamorphic conditions

NASA Astrophysics Data System (ADS)

Clement, Conand; Frederic, Mouthereau; Gianreto, Manatschal; Adbeltif, Lahfid

2017-04-01

The physical processes involved at the beginning of the continental collision are largely unknown because they are transient and therefore hardly identifiable from the rock record. Despite the importance of key parameters for understanding mountain building processes, especially the formation of deep mountain roots and their impacts on earthquakes nucleation, rock/fluid transfers and oil/gas resources in the continental crust, observations from the earliest collision stages remain fragmentary. Here, we focus on the example of Taiwan, a young and active mountain belt where the transition from oceanic subduction, accretion of the first continental margin to mature collision can be followed in space and time. We present preliminary results and provide key questions regarding the reconstruction of time-pressure-temperature paths of rocks & fluids to allow discriminating between rift-related thermal/rheological inheritance and burial/heating phases during convergence. Previous studies have focused on peak temperatures analyzed by Raman Spectrometry of Carbonaceous Matter from the deeper structural layers exposed in the Central Range of Taiwan. In the pre-rift sediments, these studies reported a positive gradient from West to Est, and values from <330 to 520 °C. We detect from our preliminary analyses of the Miocene "post-rift" sediments, a trend of increasing temperature from 170 to 340 °C northwards. These temperature data are discussed against key structural features recognized in the field and available low-temperature thermochronological constraints. We show that our RSCM temperatures cannot directly be interpreted in terms of syn-convergence nappe stacking only and must reflect a component of initial (pre-collisional) high-geothermal gradients (up to 60°C/km) known in the region, and higher temperature closer to the pre-rift units. Cross sections and maps with high resolution peak temperatures are in process as well as pressure estimations to determine how the sediments were metamorphosed. In addition to this work, we report a few inherited temperatures in the 390-570 °C range, indicating recycling of organic matter from metasediments that recorded HT events, likely originated from higher grade metamorphic units of mainland China, which have been eroded and deposited in the post-rift sediments.

Late Quaternary cooling rate constrained by multiple IRSL thermochronometers of potassium feldspars for granites from Kongur Shan, Chinese Pamir

NASA Astrophysics Data System (ADS)

Qin, Jintang; Chen, Jie; Valla, Pierre; Herman, Frédéric

2015-04-01

The Kongur Shan (East Pamir), located at the northwestern Tibetan Plateau, is one of the most active orogens on Earth, where both tectonic processes along major active faults and climatic forcing (extensive glaciers coverage) are contributing to the regional landscape evolution. The exhumation rates since late Miocene was constrained to be ~6.5 - 4.2 mm/yr. However, it is still debated whether the exhumation rate accelerated since the Quaternary, of which the climate was featured by the cyclic glaciations with periods of 100 ka and 40 ka. In this study, we tried to employ luminescence thermochronology, which is a still in developing method, to resolve the impact of glacial cycles on exhumation rate. Our study site is located ~10 km to the east of the active Kongur normal fault, along the major valley of Gez river. We sampled three granite rocks from a sub-horizontal tunnel across the granite massif; one was from the entrance of the tunnel, and other two samples were from inside of the tunnel, where the measured ambient temperature is as high as 60-70 ° C. The distances of these samples are within 2 km. Four types of IRSL signals extracted from potassium feldspars (K-feldspars) were measured for each individual sample, and the results of isothermal decay experiments indicated these signals were of different thermal stabilities. Therefore, they may serve as four thermochronometers with different closure temperature. We employ these multiple thermochronometers together for each single sample to constrain their cooling rates. Our preliminary results, which are based on the simplified luminescence model of K-feldspars, suggest that the averaged cooling rate of the last 200 ka is as high as 1.4 oC/ka, which corresponds to an exhumation rate of ~ 2.3 to 0.9 cm/yr with the geothermal gradient assumed to be 60 to 150 oC/km. It seems to imply that the glacial cycles during the Quaternary substantially accelerated the exhumation rate of granite massif of Kongur Shan.

GeoFrame Walker Lane: Overview, Rationale, and Objectives

NASA Astrophysics Data System (ADS)

Stockli, D. F.

2006-12-01

GeoFrame is an integrative geologic initiative that takes a multi-dimensional view of the building and modification of the North American continent by systematic integration of geologic and geochronometric investigations and the results from unprecedented geophysical imaging as part of the Earthscope Program. The GeoFrame effort envisions these focus site investigations to entail map-scale arrays of passive source seismic receivers and associated active source seismic studies and complementary geophysics in conjunction with geologic-based synthesis and targeted studies. One of these focus sites is the Walker Lane region in eastern California and western Nevada, situated between the Basin and Range province and the unextended Sierra Nevada block. This GeoFrame focus site workshop is particularly timely given the deployment schedule of the USArray "BigFoot" array. The Walker Lane intraplate deformation zone accommodates nearly ~25% of present-day relative motion between the Pacific and North American plates and might represent an incipient plate boundary. It provides a world-class example of the present modification of continental lithosphere by the process of transcurrent faulting and rifting and offers the opportunity to seamlessly integrate surface geology, structural geology, petrology, geo- and thermochronology, and the history of the continental lithosphere with ongoing processes in the Earth's mantle. It affords opportunities to address a number of questions posed within Earthscope such as: mechanisms of strain transfer, the role of lithospheric rheology in strain localization and seismic response, the nature and timescales of transient fault behavior, and the role of magmas and fluids in deforming lithosphere. Implicit in the design and implementation of Earthscope is the recognition that progress on issues such as these requires an integrative geophysical and geological investigation of the Walker Lane. As such, it will open new avenues of collaboration and identify new research needs and opportunities. We anticipate the integration of results and efforts with ongoing Earthscope projects, such as Sierra Nevada efforts of SNEP as well as the NSF Margins Rupturing of Continental Lithosphere (RCL) initiative in the Gulf of California by continuing the work onshore from the Gulf of California to the north into Nevada.

Did the Basement-Involved Main Caucasus Thrust Form during the Cenozoic Arabia-Eurasia Collision?

NASA Astrophysics Data System (ADS)

Vasey, D. A.; Cowgill, E.; Niemi, N. A.; Godoladze, T.; Javakhishvili, Z.; Skhirtladze, I.; Boichenko, G.

2017-12-01

The Greater Caucasus Mountains lie between the Black and Caspian Seas at the northern margin of the active Arabia-Eurasia collision zone. The north-dipping Main Caucasus Thrust (MCT) is commonly assumed to be a first-order structure within the range that places Paleozoic crystalline basement to the north over metasedimentary cover of inferred Mesozoic age to the south. Although most workers assume this juxtaposition occurred during Cenozoic growth of the range, the timing of ductile (quartz-plastic) shearing along the MCT remains to be established. Here, we present data to discriminate between two competing models of quartz-plastic deformation along the proposed MCT location at the basement-cover contact. In the first model, quartz-plastic deformation occurred during the Cenozoic Arabia-Eurasia collision, whereas in the second, this deformation took place during an older orogenic event, such as the Paleozoic Variscan orogeny. To test these models, we are combining field observations, microstructural investigations, and thermochronologic analyses on two 10 km-long traverses in the Republic of Georgia, separated by 200 km along strike, across the MCT. Our fieldwork and microstructural analyses along the basement-cover contact document north-dipping zones of high strain that are 100 m thick and show quartz-plastic deformation, top-to-the-south shear sense, and greenschist-facies metamorphism. Zircon (U-Th)/He (ZHe) analyses along the eastern traverse near Stepantsminda yielded latest Miocene ( 7-8 Ma) ages in both the MCT shear zone and 150 m structurally above the MCT. In contrast, along the western traverse near Nakra, a sample 300 m structurally below the MCT yielded an early Oligocene ( 31 Ma) ZHe age. These data require Cenozoic exhumation from temperatures >180° and are compatible with recent ductile shear along the MCT. However, results from biotite and muscovite 40Ar/39Ar analyses in progress are needed to confirm this hypothesis. The MCT appears to have exhumed rocks from crustal depths of at least 6 km since the Oligocene and may be a key first-order structure in the Arabia-Eurasia collision.

Linking erosion history and mantle processes in southern Africa

NASA Astrophysics Data System (ADS)

Stanley, J. R.; Braun, J.; Flowers, R. M.; Baby, G.; Wildman, M.; Guillocheau, F.; Robin, C.; Beucher, R.; Brown, R. W.

2017-12-01

The large, low relief, high elevation plateau of southern Africa has been the focus of many studies, but there is still considerable debate about how it formed. Lack of tectonic convergence and crustal thickening suggests mantle dynamics play an important role in the evolution of topography there, but the time and specific mechanisms of topographic development are still contested. Many mantle mechanisms of topographic support have been suggested including dynamic topography associated with either deep or shallow mantle thermal anomalies, thermochemical modification of the lithosphere, and plume tails related to Mesozoic magmatic activity. These mechanisms predict different timing and patterns of surface uplift such that better constraints on the uplift history have the potential to constrain the nature of the source of topographic support. Here we test several of these geodynamic hypotheses using a landscape evolution model that is used to predict the erosional response to surface uplift. Several recent studies have provided a clearer picture of the erosion history of the plateau surface and margins using low temperature thermochronology and the geometries of the surrounding offshore depositional systems. Model results are directly compared with these data. We use an inversion method (the Neighborhood Algorithm) to constrain the range in erosional and uplift parameters that can best reproduce the observed data. The combination of different types of geologic information including sedimentary flux, landscape shape, and thermochronolology is valuable for constraining many of these parameters. We show that both the characteristics of the geodynamic forcing as well as the physical characteristics of the eroding plateau have significant control on the plateau erosion patterns. Models that match the erosion history data well suggest uplift of the eastern margin in the Cretaceous ( 100 Ma) followed by uplift of the western margin 20 Myr later. The amplitude of this uplift is on the order of 1000 m. The data cannot resolve whether there was smaller amplitude phase of uplift in the Cenozoic. These results suggest that the scenario proposed by Braun et al. (2014) of uplift caused by the continent moving over the African superswell is viable. We are currently investigating the compatibility of other uplift geometries.

Exhumation of the Ladakh batholith revealed through the combined analysis of bedrock and detrital zircon (U-Th)/He data

NASA Astrophysics Data System (ADS)

Tripathy-Lang, A.; Fox, M.; Bohon, W.; Van Soest, M. C.; Hodges, K. V.; Dortch, J.

2013-12-01

Recent studies of the Ladakh batholith, in the northwestern Indian Himalaya, have yielded various hypotheses for its exhumation history and relationship with the evolution of the southwestern margin of the Tibetan Plateau, which is today bounded by the Karakoram fault. Different hypotheses are supported by various datasets with differing spatial and temporal resolution. First, low-temperature thermochronologic and thermobarometric data provide constraints on long term exhumation (10^6 - 10^7 yr) and suggest that the Ladakh batholith experienced multiple tilting events since ~40 Ma (Kirstein, Tectonophysics, 2011). Second, cosmogenic nuclide concentrations (CNCs), which provide evidence for erosion rates averaged over millennial timescales (10^2-10^4 yr), suggest that erosion rates increase toward the Karakoram fault (Dortch et al., Geomorphology, 2011). A third dataset comprises detrital zircon (U-Th)/He data obtained from the mouth of the Basgo catchment, on the southern flank of the Ladakh batholith (Tripathy-Lang et al., JGR-ES, 2013). This exceptionally large detrital dataset provides information about both the bedrock age distribution and recent erosion rates that sample different parts of the catchment. Interpreting this dataset requires an understanding of the erosion history at multiple timescales. To these already existing datasets, we add new bedrock zircon (U-Th)/He data from an age-elevation transect collected from the base to range crest of the Basgo catchment, which we use to verify models of bedrock age distribution. Through the combined analysis of the datasets, the resolution of both the long term exhumation rate and the spatial distribution of modern erosion rates can be greatly improved, thus advancing our understanding of this part of the Tibetan margin. With this aim, we use thermo-kinematic models to predict bedrock ages that we compare to our new bedrock data. We test different modern erosion rate distributions to generate synthetic detrital thermochronometric and CNC data. Through the comparison of predicted and measured data (both detrital thermochronometric data and CNC data) we infer long term exhumation histories and also modern erosion rate distribution.

Using Sediment Provenance to Study Ice Streams in the Weddell Sea Embayment of Antarctica

NASA Astrophysics Data System (ADS)

Hemming, S. R.; Williams, T.; Boswell, S.; Licht, K.; Agrios, L.; Brachfeld, S. A.; van de Flierdt, T.; Kuhn, G.; Hillenbrand, C. D.; Zhai, X.

2016-12-01

The geochemical and geochronological fingerprint of rock debris eroded and carried by ice streams may be used to identify the provenance of iceberg-rafted debris (IRD) in the marine sediment record. During deglacial times it has been shown that there is an increase in IRD accumulation in marine sediments underlying the western limb of the Weddell Gyre. We seek to find the provenance of this IRD, identify the ice streams contributing to the IRD load, and interpret the geographic sequence of ice sheet retreat in the Weddell Sea embayment for the last three deglaciations. In December 2014 we conducted fieldwork to collect samples of rock and sediment debris carried by three of the major ice streams draining the Weddell Sea embayment: the Foundation Ice Stream, the Academy Glacier, and the Recovery Glacier. We sampled both modern moraines at the edges of the ice streams and older till on hillsides next to the ice streams. In addition to rocks representing the geology of local outcrops, we found that each of the three ice streams carries a characteristic set of erratic lithologies from further upstream, giving clues to the geology hidden under the ice sheet. Downstream, subglacial till and proximal glaciomarine sediment from existing core sites located at the edge of the Filchner and Ronne Ice Shelves, collected on past expeditions of the RV Polarstern, characterize the geochemical and geochronological fingerprint along ice flow lines extending from the ice streams. Finally, two deep-water RV Polarstern sites contain a continuous record of IRD sourced from the set of Weddell embayment ice streams over the last few glacial cycles. Here we present new 40Ar/39Ar hornblende and biotite thermochronological data from individual mineral grains, K-Ar from the silt fraction, and U-Pb zircon geochronology from the onshore tills and offshore sediments. Using this data we will discuss provenance matching between the IRD and the ice streams, and the possibilities for using provenance to understand ice sheet dynamics over the course of glacial cycles.

Building the Pamir-Tibet Plateau: Eo-Oligocene Crustal Stacking and Orogen Parallel Evasion of Upper and Middle Crustal Material in the Pamir

NASA Astrophysics Data System (ADS)

Rutte, D.; Ratschbacher, L.; Stübner, K.; Schneider, S.

2015-12-01

The gneisses of the Central Pamir Domes and their cover document crustal stacking of a ~10 km thick Ediacaran-Paleogene succession to a thickness of >35 km and their exhumation along bi-vergent, top-to-N and top-to S, normal-sense shear zones. The giant South Pamir Shakhdara-Alichur gneiss-dome system formed similarly by N-S extension along bivergent detachments. Prograde amphibolite-facies metamorphism in the domes and low-grade metamorphism in their hanging wall is dated at ~40 Ma (Lu-Hf garnet, U-Pb titanite) [Smit et al., 2014; Stearns et al., 2015] and ~33 Ma (K/Ar sericite). Retrograde metamorphism―driven by crustal extension―started at ~21 Ma (multi-method thermochronology; Stearns et al.[2013]). These Gneiss Domes offer a unique window into the Eocene-Miocene state of the Asian middle crust of the Pamir-Tibet Plateau. Top-to-N thrust stacking accommodated thickening in the upper crust, with displacements of single thrust sheets of > 30 and > 19 km. At depth, ductile flow formed km-scale recumbent fold nappes. We reconstruct their geometry by structural mapping and U-Pb zircon dating, documenting repetition of metatuffite, and paragneiss layers. In the interior of the domes, amphibolite-facies deformation fabrics with prograde kyanite define an E-W stretching lineation. Associated microstructures indicate top-to-E and top-to-W shear senses. Chocolate tablet boudinage indicate vertical flattening during bulk crustal thickening. We suggest that prograde E-W stretching relates to an early orogen-parallel flow component in the middle crust, contemporaneous with crustal stacking during bulk top-to-N convergence prior to ~21 Ma. Material likely evaded laterally out of the Pamir, contributing to >60 km thick crust in the Hindu Kush, west of the India-Asia frontal collision. In the Neogene crust extruded laterally from the Pamir Plateau to the west by dextral wrenching and E-W extension; this component of deformation is accommodated by E-W shortening in the Afghan-Tajik Depression.

P- T- t constraints on the development of the Doi Inthanon metamorphic core complex domain and implications for the evolution of the western gneiss belt, northern Thailand

NASA Astrophysics Data System (ADS)

Macdonald, A. S.; Barr, S. M.; Miller, B. V.; Reynolds, P. H.; Rhodes, B. P.; Yokart, B.

2010-01-01

The western gneiss belt in northern Thailand is exposed within two overlapping Cenozoic structural domains: the extensional Doi Inthanon metamorphic core complex domain located west of the Chiang Mai basin, and the Mae Ping strike-slip fault domain located west of the Tak batholith. New P- T estimates and U-Pb and 40Ar/ 39Ar age determinations from the Doi Inthanon domain show that the gneiss there records a complex multi-stage history that can be represented by a clockwise P- T- t path. U-Pb zircon and titanite dating of mylonitic calc-silicate gneiss from the Mae Wang area of the complex indicates that the paragneissic sequence experienced high-grade, medium-pressure metamorphism (M1) in the Late Triassic - Early Jurassic (ca. 210 Ma), in good agreement with previously determined zircon ages from the underlying core orthogneiss exposed on Doi Inthanon. Late Cretaceous monazite ages of 84 and 72 Ma reported previously from the core orthogneiss are attributed to a thermal overprint (M2) to upper-amphibolite facies in the sillimanite field. U-Pb zircon and monazite dating of granitic mylonite from the Doi Suthep area of the complex provides an upper age limit of 40 Ma (Late Eocene) for the early stage(s) of development of the actual core complex, by initially ductile, low-angle extensional shearing under lower amphibolite-facies conditions (M3), accompanied by near-isothermal diapiric rise and decompression melting. 40Ar/ 39Ar laserprobe dating of muscovite from both Doi Suthep and Doi Inthanon provided Miocene ages of ca. 26-15 Ma, representing cooling through the ca. 350 °C isotherm and marking late-stage development of the core complex by detachment faulting of the cover rocks and isostatic uplift of the sheared core zone and mantling gneisses in the footwall. Similarities in the thermochronology of high-grade gneisses exposed in the core complex and shear zone domains in the western gneiss belt of northern Thailand (and also in northern Vietnam, Laos, Yunnan, and central Myanmar) suggest a complex regional response to indentation of Southeast Asia by India.

Discriminating Sediment Supply versus Accommodation Controls on Late Cretaceous Foreland Basin Stratigraphic Architecture in the Book Cliffs, Utah using Detrital Zircon Double Dating

NASA Astrophysics Data System (ADS)

Bartschi, N.; Saylor, J. E.

2016-12-01

Middle to late Campanian strata of the Book Cliffs, Utah record the Late Cretaceous deposition of three clastic wedges in the North American Cordilleran foreland basin east of the Sevier thrust-belt. Variations in wedge geometries provide an opportunity to evaluate the effects of sediment supply versus accommodation on foreland basin stratal architecture. There is a significant increase in eastward progradation rate from the Lower to the Upper Castlegate Sandstone. However, the progradation rate decreases in the overlying Bluecastle and Price River formations, as well as the laterally equivalent Farrer and Tuscher formations. Rapid progradation during Upper Castlegate deposition may be caused by increased sediment supply from either rapid exhumation of the Sevier thrust-belt or introduction of a new sediment source. Alternatively, reduced accommodation within the proximal foreland basin from uplifts associated with Laramide deformation, or a transition from flexural to dynamic subsidence, could produce the observed rapid wedge progradation. Changes in sediment provenance and source-area exhumation rate can be identified using a combination of detrital zircon U-Pb geochronology and (U-Th)/He thermochronology. Quantitative comparisons between collected samples and published provenance data indicates an upsection increase in a new sediment source, revealing a significant overall shift in provenance between wedge boundaries. This change in provenance is coupled by an upsection decrease in lag time between the Lower and Upper Castlegate, consistent with an increase in exhumation rate. Conversely, there is no change in lag time between the Upper Castlegate and overlying Price River Formation, suggesting a relatively constant exhumation rate. Near-zero lag times during the Upper Castlegate is consistent with rapid exhumation associated with increased thrusting of the Sevier thrust-belt. Therefore, progradation of the Upper Castlegate can be attributed to an increase in sediment supply due to both rapid exhumation of the Sevier thrust-belt and introduction of a new sediment source. However, the data do not rule out the potential influence of reduced accommodation associated with early Laramide deformation during Upper Castlegate deposition.

Provenance of Miocene Hinterland Basins in Ecuador: Implications for the Growth of Topographic Barriers in the Northern Andes

NASA Astrophysics Data System (ADS)

George, S. W. M.; Horton, B. K.; Vallejo, C.; Nogales, V.

2017-12-01

Establishment of the Eastern Cordillera of Ecuador as an Andean topographic barrier caused significant drainage reorganization, perhaps even as dramatic as the reversal of the Amazon River. Cenozoic growth of this barrier coincided with substantial increases in speciation rates in Andean and Amazonian environments. Situated in the Interandean Depression between the Eastern Cordillera and Western Cordillera of Ecuador, a series of well-preserved Miocene intermontane basins offer a unique opportunity to constrain the along-strike development of the flanking north-trending cordilleras as drainage divides in the Northern Andes. Here were provide detrital zircon U-Pb geochronological results for 17 samples from Ecuadorian hinterland basins (Cuenca, Giron-Santa Isabel, Nabón, Loja, and Vilcabamba), supplemented with measured sections in the Cuenca Basin, to provide insights on orogenic development of the cordilleras of Ecuador during the Miocene. In addition, we characterize the age distributions of basement units to more precisely determine sediment routing patterns through time. Detrital zircon geochronological data yields regional upsection trends throughout Miocene stratigraphic sections marked by: (1) middle Miocene deposits containing a strong syndepositional age peak, with a complementary Eocene-Oligocene peak in varying abundances, and subsidiary low-intensity Paleozoic-Proterozoic age peaks; and (2a) upper Miocene deposits maintaining similar trends to that of the middle Miocene, or (2b) upper Miocene deposits showing a dramatic shutoff of most Cenozoic populations and a switch to Paleozoic-Proterozoic sources, as seen in the Nabón and Loja basins. Syndepositional signatures reflect derivation from the magmatic arc, while varying inputs of Eocene-Oligocene zircons were derived from the Eocene-Oligocene volcanic rocks that comprise the effective basement of much of the Interandean Depression. The late Miocene shift to Paleozoic-Proterozoic sources observed in the Nabón and Loja basins reflects rapid exhumation of crystalline sources, and an eastward shift of the drainage divide, associated with exhumation of the Eastern Cordillera, which is supported by rapid cooling observed in thermochronological datasets and a switch to alluvial fan facies in the hinterland basins.

Modeling and Measuring the Effects of Radiation Damage Annealing on Helium Diffusion Kinetics in Apatite

NASA Astrophysics Data System (ADS)

Willett, C. D.; Fox, M.; Shuster, D. L.

2016-12-01

Understanding helium diffusion kinetics in apatite is critical for the accurate interpretation of (U-Th)/He thermochronometric data. This problem is complicated by the observation that helium diffusivity is not a simple function of temperature, but may evolve as a function of damage to the apatite crystal lattice resulting from alpha recoil. This `radiation damage' increases as a function of the amount of radiometric parent products, or effective uranium concentration, and time, but decreases due to thermal annealing of damage, necessitating a detailed understanding of radiation damage production and annealing in cases of burial heating over geologic timescales. Published observations [1,2] suggest that annealing rates of damage caused by alpha recoil and fission tracks in apatite differ. Existing models, however, assume the diffusion kinetics resulting from the two sources of damage are identical [3], demonstrating the need for further investigation of these damage sources. We present modeling and experimental work designed to interrogate the effects of radiation damage and its annealing on helium diffusion kinetics in apatite. Using previously published results [4] that investigated the effects of annealing temperature and duration on measured helium diffusivity, we fit a set of functions that are then integrated into a numerical model that tracks the evolution of radiation damage and apparent (U-Th)/He age. We compare the results of this model calibration to existing models [3]. In addition, we present data from two suites of diffusion experiments. The first suite, intended to test the published methodology and results, uses Durango apatite, while the second uses Sierran (CA) granite as a first test to determine if apatite of varying chemistry and age responds differently to the thermal annealing of radiation damage. Ultimately, the updated model and experimental results will benefit the interpretation of the effects of radiation damage accumulation and annealing in apatite and expand the range of geologic settings that can be studied using low-temperature thermochronology. References: [1] Fox, M., Shuster, D. (2014), EPSL 397, 174-183; [2] Gautheron, C. et al. (2013), Chem. Geol. 351, 257-267; [3] Flowers, R. et al. (2009), GCA 73, 2347-2365; [4] Shuster, D., Farley, K. (2009), GCA 73, 6183-6196.

Implications for the tectonic transition zone of active orogeny in Hoping drainage basin, by landscape evolution at the multi-temporal timescale

NASA Astrophysics Data System (ADS)

Chang, Q.; Chen, R. F.; Lin, W.; Hsieh, P. S.

2015-12-01

In an actively orogeny the landscape are transient state of disequilibrium in response to climatic and tectonic inputs. At the catchment scale, sensitivity of river systems plays an important role in landscape evolution. Hoping drainage basin is located at the tectonic transition zone in the north-eastern Taiwan, where the behavior of Philippine Sea plate switches from overriding above the east-dipping Eurasian Continental plate to northward subducting under the Ryukyu arc. However, extensive deep-seated landslides, debris flow, and numerous large alluvial terraces can be observed, suggesting strong surface processes in this watershed. This effect on regional climate fundamentally changed the landscape by reconfiguring drainage patterns and creating a vast influx of sediments into the basin. In this study we review the morphological evidence from multi-temporal timescale, including in-situ cosmogenic nuclides denudation rate and suspension load data, coupled with the analysis of the longitudinal profiles. The main goal of this study is to compare Holocene erosion rates with thermochronology and radiometric dating of river terraces to investigate the erosion history of Hoping area. The result shows that short-term erosion rate is around twice as large as the long-term denudation rate, which might due to the climate-driven erosion events such as typhoon-induced landslide. We've also mapped detail morphological features by using the high-resolution LiDAR image, which help us to identify not only the landslide but also tectonic features such as lineation, fault scarps, and fracture zones. The tectonic surface features and field investigation results show that the drainage basin is highly fractured, suggesting that even though the vertical tectonic activity rate is small, the horizontal shortening influenced by both southward opening of the back-arc Okinawa trough and the north-western collision in this area is significant. This might cause the reducing in rock strength and increase the hillslope erosion during heavy rainfall. By studying the erosion rate of Hoping River watershed we can understand more about surface processes in dynamic landscape, and more over, to establish a comprehensive understanding about the evolution of the ongoing Taiwan arc-continental collision process.

Mid-Cretaceous oblique rifting of West Antarctica: Emplacement and rapid cooling of the Fosdick Mountains migmatite-cored gneiss dome

USGS Publications Warehouse

McFadden, Rory; Teyssier, Christian; Siddoway, Christine; Cosca, Michael A.; Fanning, C. Mark

2015-01-01

In Marie Byrd Land, West Antarctica, the Fosdick Mountains migmatite-cored gneiss dome was exhumed from mid- to lower middle crustal depths during the incipient stage of the West Antarctic Rift system in the mid-Cretaceous. Prior to and during exhumation, major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle-derived diorite in dikes. A succession of melt- and magma-related structures formed at temperatures in excess of 665 ± 50 °C based on Ti-in-zircon thermometry. These record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique South Fosdick Detachment zone. Solid-state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels.To determine the duration of exhumation and cooling, we sampled granite and gneisses at high spatial resolution for U–Pb zircon geochronology and 40Ar/39Ar hornblende and biotite thermochronology. U–Pb zircon crystallization ages for the youngest granites are 102 Ma. Three hornblende ages are 103 to 100 Ma and 12 biotite ages are 101 to 99 Ma. All overlap within uncertainty. The coincidence of zircon crystallization ages with 40Ar/39Ar cooling ages indicates cooling rates > 100 °C/m.y. that, when considered together with overprinting structures, indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting. Orientations of structures and age-constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the Marie Byrd Land margin. The rotation may be a result of localized extension arising from unroofing and arching of the Fosdick dome, extensional opening within a pull-apart zone, or changes in plate boundary configuration.The rapid tectonic and temperature evolution of the Fosdick Mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension/transtension, and accords with numerous studies of migmatite-cored gneiss domes in transcurrent settings.

Burial and exhumation of the Eisenhower Range, Transantarctic Mountains, based on thermochronological, sedimentary rock maturity and petrographic constraints

NASA Astrophysics Data System (ADS)

Prenzel, J.; Lisker, F.; Elsner, M.; Schöner, R.; Balestrieri, M. L.; Läufer, A. L.; Berner, U.; Spiegel, C.

2014-09-01

The Eisenhower Range is a N-S trending mountain range in the Transantarctic Mountains (TAM) adjacent to the NW Ross Sea Embayment. New AFT and apatite (U-Th-Sm)/He (AHe) data from vertical basement profiles supplemented by paleotemperature and pressure estimates derived from Beacon sandstones provide new quantitative results on regional burial evolution and first regional constraints on basin inversion and exhumation processes. AFT ages between 32 ± 2 and 259 ± 18 Ma and AHe ages of 37 ± 3-173 ± 16 Ma correlate positively with sample elevations. Thermal history modeling of these data and complementary thermal indications detect heating of the paleosurface on the Eisenhower Range to temperatures ≥ 80 °C subsequent to Ferrar magmatism, and constrain Late Eocene rapid cooling. Regression of modeled paleotemperatures against sample elevations refers to a high Jurassic (~ 45 °C/km) and a moderate Cretaceous-Eocene (28 ± 8 °C/km) geothermal gradient. The texture of Beacon sandstones supports strong mechanical compaction that requires a higher overburden than preserved in the stratigraphic record. Modeled paleotemperatures and pressures suggest basement burial that increases from Late Jurassic (0.7-1.1 km) to Eocene (1.8-2.1 km). The overburden comprises 0.7-1.1 km cumulative Beacon/Ferrar rocks and 0.7-1.4 km of post-Ferrar sediments. Rapid cooling of the whole sample suite between ~ 35 and 30 Ma implies fast erosion of the post-Ferrar sediments and (re-) exposure of underlying magmatic rocks. Subsequent differential sample cooling to present-day surface temperature infers ongoing exhumation by glacial incision enhanced by isostatic response to basin inversion. Decreasing amounts of exhumation from the coast (> 3 km) toward the interior (1.5-2.2 km) point to backstepping incision along the fault controlled Priestley Glacier. Substantial exhumation of the Eisenhower Range since the Late Eocene is hence triggered by both tectonic and climatic factors, superimposed by considerable lithological influence during the initial exhumation stage.

Geological and Tectonic Evidence for the Formation and Extensional Collapse of the West Antarctic Plateau: Implications for the Formation of the West Antarctic Rift System and the Transantarctic Mountains

NASA Astrophysics Data System (ADS)

Fitzgerald, P. G.; Studinger, M.; Bialas, R. W.; Buck, W.

2007-12-01

The Transantarctic Mountains (TAM), the world's longest and highest non-contractional intracontinental mountain belt, define the western boundary of the West Antarctic rift system (WARS). The WARS is a broad region of extended continental lithosphere, ca. 750-1000 km wide, lying dominantly below sea-level. A new model (Bialas et al., 2007), proposes that a region of thickened continental crust and high-standing topography, the "West Antarctic Plateau", underwent extensional collapse to leave a remnant edge representing the proto-TAM. Tectonic and paleogeographic reconstructions indicate the plateau formed inboard of a continental arc along the paleo- Pacific margin of Antarctica, active throughout the Paleozoic until the late Mesozoic. This high-standing region was responsible for confining sediments (Beacon Supergroup) to elongate basins along the length of the TAM. Much of the present region of the WARS has been correlated with the Lachlan Fold belt of southeastern Australia. This belt formed from the Ordovician to Carboniferous during back-arc basin formation associated with slab roll- back with short periods of compression. Convergence along the paleo-Pacific margin, perhaps enhanced by subduction of more buoyant oceanic lithosphere as the Phoenix-Pacific ridge was obliquely subducted, resulted in crustal thickening and formation of high-standing terrain (the plateau). Extensional collapse of the plateau most likely began in the Jurassic during initial rifting between East and West Antarctica, but was mainly accomplished during distributed rifting in the Cretaceous (ca. 105-85) following subduction of the Phoenix-Pacific ridge and prior to the separation of New Zealand from Marie Byrd Land. Continued formation of the TAM continued in the Cenozoic concomitant with extension in the WARS that was localized along its western margin adjacent to the TAM. Glacial erosion in the Oligocene and early-Miocene enhanced peak height in the TAM. In this presentation we discuss the diverse geological, geophysical, thermochronological and tectonic evidence for the West Antarctic Plateau and the implications for the formation of the Transantarctic Mountains.

Timescales of Crustal Cooling of the Superior Craton near Attawapiskat, Ontario, Canada, and Implications for Extent of Keweenawan Plume Heating.

NASA Astrophysics Data System (ADS)

Edwards, G. H.; Blackburn, T.; Smit, K.

2017-12-01

The thermal history of the Superior Craton was punctuated by a period of mantle plume heating at 1.1 Ga associated with the Keweenawan Rift, though the plume's spatial extent, temperature, and duration of heating remain unresolved. Kimberlites of Mesoproterozoic and Jurassic age in the Attawapiskat area, Northern Ontario contain lithospheric mantle and lower crustal xenoliths that record the thermal history 600km to the north of exposed 1.1 Ga Keweenawan volcanics and the topographically and gravimetrically defined plume center. Previous work on Attawapiskat kimberlites identified two populations of diamonds with differing thermal histories, suggesting two distinct phases of diamond growth. Corresponding geothermobarometric data indicate geotherm relaxation and broadening of the diamond stability field between the Mesoproterozoic and Jurassic. These data, however, do not uniquely resolve whether the region experienced significant heating coincident with Keweenawan rifting ( 1.1 Ga) or prolonged, unperturbed cooling since amalgamation of the Superior Craton ( 2.6 Ga). To discern between these two possible histories, we use accessory phase U-Pb thermochronology to construct a continuous thermal record of the lower crust. Here we present a dataset of U-Pb ID-TIMS measurements of rutile and apatite from xenoliths (n=8) sourced from the Jurassic age Victor Kimberlite. The U/Pb and Pb isotopic compositions of rutile and apatite from shallow-residing amphibolite xenoliths exhibit Proterozoic dates with a high degree of U-Pb discordance, reflecting slow cooling of the middle crust prior to 1.1 Ga. Granulite and eclogite xenoliths record younger dates consistent with their deeper sample residence, but with a high degree of U-Pb concordance that is inconsistent with continuous cooling through the Proterozoic. Reproducing the measured trend with numerical models requires a reheating event at 1.1 Ga. Imposing a 60-70mW/m2 geotherm at 1.1 Ga is high enough to replicate the observed U-Pb data but low enough to permit cooling that satisfies diamond thermal data. This indicates that the Keweenawan plume head extended to at least the Attawapiskat area, where plume heating abutted, and likely extended beyond, the associated topographic and gravity anomalies of the Superior Region.

Thermochronologic constraints on the Miocene slip history of the South Tibetan detachment system in the Everest region, central Himalaya

NASA Astrophysics Data System (ADS)

Schultz, M.; Hodges, K. V.; Van Soest, M. C.; Wartho, J. A.

2014-12-01

North-dipping, low-angle normal faults of the South Tibetan detachment system (STDS) can be traced for a distance of more than 2000 km along strike and represent an important tectonic characteristic of the Miocene Himalayan-Tibetan orogenic system. Nowhere is the STDS better exposed than the N-S-trending Rongbuk Valley in southern Tibet, where it can be traced down dip from the summit of Everest for a distance of over 30 km before disappearing beneath the valley floor. This places a minimum constraint on Miocene displacement on the feature in this area, but some research groups have suggested ~200 km of displacement based on the difference in metamorphic pressures across the STDS and the very low (< 15˚) primary dip of the structure. We are exploring this issue further using developing (U-Th)/He and 40Ar/39Ar datasets from deformed footwall sillimanite gneisses and leucogranites. Data obtained thus far indicate relatively rapid cooling of the footwall after the intrusion of deformed leucogranites at ca. 16.7 Ma to muscovite 40Ar/39Ar closure temperatures (ca. 15.5-14.2 Ma) and zircon (U-Th)/He closure temperatures (ca. 14.5-11 Ma). We attribute this cooling to tectonic denudation related to ca. 16 Ma STDS slip. Although the (U-Th)/He systematics of apatites from these rocks is complex, our current interpretation of available data places cooling through the ca. 75˚C closure isotherm at ca. 8-9 Ma, which would suggest a significant reduction in cooling rate that is observed in our inverse model runs of the 1D program, HeFTy. Ongoing analyses of footwall samples from ~8 km to the north of our Rongbuk sample localities in the Ra Chu river valley will greatly strengthen our datasets. With the Ra Chu analyses, our datasets will constrain the cooling history of the footwall for more than 20 km perpendicular to the strike of the detachment. Our presentation will also incorporate results from the program Pecube that will contribute to our calculation of the slip rate by specifying the appropriate exhumation rate.

Measuring plume-related exhumation of the British Isles in Early Cenozoic times

NASA Astrophysics Data System (ADS)

Cogné, Nathan; Doepke, Daniel; Chew, David; Stuart, Finlay M.; Mark, Chris

2016-12-01

Mantle plumes have been proposed to exert a first-order control on the morphology of Earth's surface. However, there is little consensus on the lifespan of the convectively supported topography. Here, we focus on the Cenozoic uplift and exhumation history of the British Isles. While uplift in the absence of major regional tectonic activity has long been documented, the causative mechanism is highly controversial, and direct exhumation estimates are hindered by the near-complete absence of onshore post-Cretaceous sediments (outside Northern Ireland) and the truncated stratigraphic record of many offshore basins. Two main hypotheses have been developed by previous studies: epeirogenic exhumation driven by the proto-Iceland plume, or multiple phases of Cenozoic compression driven by far-field stresses. Here, we present a new thermochronological dataset comprising 43 apatite fission track (AFT) and 102 (U-Th-Sm)/He (AHe) dates from the onshore British Isles. Inverse modelling of vertical sample profiles allows us to define well-constrained regional cooling histories. Crucially, during the Paleocene, the thermal history models show that a rapid exhumation pulse (1-2.5 km) occurred, focused on the Irish Sea. Exhumation is greatest in the north of the Irish Sea region, and decreases in intensity to the south and west. The spatial pattern of Paleocene exhumation is in agreement with the extent of magmatic underplating inferred from geophysical studies, and the timing of uplift and exhumation is synchronous with emplacement of the plume-related British and Irish Paleogene Igneous Province (BIPIP). Prior to the Paleocene exhumation pulse, the Mesozoic onshore exhumation pulse is mainly linked to the uplift and erosion of the hinterland during the complex and long-lived rifting history of the neighbouring offshore basins. The extent of Neogene exhumation is difficult to constrain due to the poor sensitivity of the AHe and AFT systems at low temperatures. We conclude that the Cenozoic topographic evolution of the British Isles is the result of plume-driven uplift and exhumation, with inversion under compressive stress playing a secondary role.

Juxtaposition of contrasting structural regimes across a portion of the Norumbega fault system in the northern Casco Bay region of Maine

NASA Astrophysics Data System (ADS)

West, D. P., Jr.; Hussey, A. M., II

2015-12-01

It has long been recognized that Paleozoic stratified rocks in some regions of central New England are dominated by relatively flat structural features (e.g., recumbent folds, shallow dipping foliation) while other areas are dominated by near vertical upright structures. The northern Casco Bay region of coastal Maine (Brunswick 7.5' quadrangle and adjacent areas) provides an excellent venue for studying the relationships between these two structural regimes as they are in close proximity due to juxtaposition by high angle faulting associated with the Norumbega fault system. Stratified rocks exposed west of the Flying Point fault in northern Casco Bay are dominated by moderately east dipping foliation (ave. = 025o, 37o), moderate northeast plunging mineral lineations, and recumbent to gently inclined minor folds. In stark contrast, immediately east of the Flying Point fault, stratified rocks are dominated by steep east dipping foliation (ave. = 014o, 73o), subhorizontal mineral lineations, and upright to steeply inclined minor folds. The structural differences correspond directly to differences in the thermal histories preserved in these rocks as revealed by earlier thermochronological studies. Rocks in the zone of upright structures east of the Flying Point fault were last subjected to high grade metamorphic conditions and granitic plutonism in the Late Devonian and were relatively cold (<300oC) by Late Carboniferous time. In contrast, flat lying rocks west of the Flying Point fault were over 500oC in the Early Permian and Permian pegmatites are common. Geochronological studies north of the study area have revealed that the two distinctly different structural styles are not the product of strain partitioning during the same deformational episode, but rather they represent two temporally and kinematically distinct deformational events. Swanson (1999), originally suggested flat structures west of the Flying Point fault are consistent with an episode of northwest directed thrusting and our findings are consistent with this interpretation. However, this flat phase of deformation significantly post-dates the older upright structures preserved to the east and thus models for the structural evolution of the region must integrate both the kinematic and temporal differences in this deformation.

40Ar/39Ar geochronology of terrestrial pyroxene

NASA Astrophysics Data System (ADS)

Ware, Bryant; Jourdan, Fred

2018-06-01

Geochronological techniques such as U/Pb in zircon and baddeleyite and 40Ar/39Ar on a vast range of minerals, including sanidine, plagioclase, and biotite, provide means to date an array of different geologic processes. Many of these minerals, however, are not always present in a given rock, or can be altered by secondary processes (e.g. plagioclase in mafic rocks) limiting our ability to derive an isotopic age. Pyroxene is a primary rock forming mineral for both mafic and ultramafic rocks and is resistant to alteration process but attempts to date this phase with 40Ar/39Ar has been met with little success so far. In this study, we analyzed pyroxene crystals from two different Large Igneous Provinces using a multi-collector noble gas mass spectrometer (ARGUS VI) since those machines have been shown to significantly improve analytical precision compared to the previous single-collector instruments. We obtain geologically meaningful and relatively precise 40Ar/39Ar plateau ages ranging from 184.6 ± 3.9 to 182.4 ± 0.8 Ma (2σ uncertainties of ±1.8-0.4%) and 506.3 ± 3.4 Ma for Tasmanian and Kalkarindji dolerites, respectively. Those data are indistinguishable from new and/or published U-Pb and 40Ar/39Ar plagioclase ages showing that 40Ar/39Ar dating of pyroxene is a suitable geochronological tool. Scrutinizing the analytical results of the pyroxene analyses as well as comparing them to the analytical result from plagioclase of the same samples indicate pure pyroxene was dated. Numerical models of argon diffusion in plagioclase and pyroxene support these observations. However, we found that the viability of 40Ar/39Ar dating approach of pyroxene can be affected by irradiation-induced recoil redistribution between thin pyroxene exsolution lamellae and the main pyroxene crystal, hence requiring careful petrographic observations before analysis. Finally, diffusion modeling show that 40Ar/39Ar of pyroxene can be used as a powerful tool to date the formation age of mafic rocks affected by greenschist metamorphism and will likely play an important role in high temperature thermochronology.

Evolving lithospheric flexure and paleotopography of the Pyrenean Orogen from 3D flexural modeling and basin analysis

NASA Astrophysics Data System (ADS)

Curry, M. E.; van der Beek, P.; Huismans, R. S.; Muñoz, J. A.

2017-12-01

The Pyrenees are an asymmetric, doubly-vergent orogen with retro- and pro- foreland basins that preserve a record of deformation since the Mesozoic. The extensive research and exploration efforts on the mountain belt and flanking foreland basins provide an exceptional dataset for investigating geodynamics and surface processes over large spatial and temporal scales in western Europe. We present the results of a numerical modeling study investigating the spatio-temporal variation in lithospheric flexure in response to the developing orogen. We employ a finite element method to model the 3D flexural deformation of the lithosphere beneath the Pyrenean orogen since the onset of convergence in the late Cretaceous. Using subsurface, geophysical, and structural data, we describe the evolving geometry of both the French Aquitaine and Spanish Ebro foreland basins at the present (post-orogenic), the mid-Eocene (peak orogenic), the Paleocene (early orogenic), and the end of the Cretaceous (pre- to early orogenic). The flexural modeling provides insight into how both the rigidity of the lithosphere and the paleotopographic load have varied over the course of orogenesis to shape the basin geometry. We find that the overriding European plate has higher rigidity than the subducting Iberian plate, with modern Effective Elastic Thickness (EET) values of 20 ± 2 and 12 ± 2 km, respectively. Modeling indicates that the modern rigidity of both plates decreases westward towards the Bay of Biscay. The lithospheric rigidity has increased by 50% since the Mesozoic with early Cenozoic EET values of 13 ± 2 and 8 ± 1 km for the European and Iberian plates, respectively. The topographic load began increasing with convergence in the late Cretaceous, reaching modern levels in the central and eastern Pyrenees by the Eocene. In contrast, the topographic load in the western Pyrenees was 70% of the modern value in the Eocene, and experienced topographic growth through the Oligo-Miocene. The westward propagation of topographic growth and erosion is supported by subsidence analysis and low-temperature thermochronology data. These results have implications for surface processes and foreland basin development of the Pyrenean Orogen, inheritance of Hercynian crustal properties, and the geodynamic evolution of western Europe.

Uplift of the Eastern Cordillera, Colombia

NASA Astrophysics Data System (ADS)

Siravo, Gaia; Faccenna, Claudio; Giuditta Fellin, Maria; Molin, Paola; Bayona, German; Giachetta, Emanuele

2017-04-01

The Eastern Cordillera (EC) of Colombia is a double-vergent thrust and fold belt formed during the Cenozoic by the inversion of a Mesozoic rift. It is located 400 km far from the trench where the Nazca slab subducts below the South American plate. Tomography and seismicity highlight the presence of a flat slab subduction north of 5° N. Our goal is to explore the relation between surface and crustal processes in the topography growth of the EC north of 6° N. To reach this aim we integrate data derived from the fluvial network analysis with long-term erosion data derived from thermochronology. The northern EC is an asymmetric chain with a gentle flank on the western side and steep flank to the east. The two sides are separated by the maximum elevation of 5000 m reached at the Cocuy Sierra. Here the EC has the widest width ( 200 km) and changes northward to the Santander Massif Range. We obtained new low-T (U-Th)/He ages on zircons (ZHe) and apatites (AHe) from samples collected along two transects. Our ages combined with previous data indicate that along both transects exhumation occurred from shallow burial depths (≤ 6-8 km) during the Oligo-Miocene and locally continued until the Pliocene. Along the transect across the central part of the EC up to the Cocuy Sierra, the youngest Pliocene ages (AHe) are located at the highest elevations above 4000 m. Along the second transect the youngest ages are found close to and east of a main strike-slip structure (Bucaramanga fault). This fault has a significant dip-slip component evidenced by an offset of about 10 Ma among the AHe ages across the fault. The main rivers draining the northern EC flow parallel to the main tectonic structures and locally cut across them. The analysis of river longitudinal profiles reveal transient features such as knickpoint not related to lithological or structural contrast. Channel slope and chi analysis suggest a river capture event possibly influenced by the crustal structures. Wind gaps with matching Pleistocene fluvial deposits in addition to river elbows appear to confirm a drainage network reorganization phase given by fluvial piracy.

P-T-t-d History of the Lahul Valley, NW Indian Himalaya

NASA Astrophysics Data System (ADS)

Nieblas, A.; Leech, M. L.

2015-12-01

The Lahul Valley of NW India is located between the Zanskar Shear zone to the northwest and the Sangla detachment to the southeast. This region contains three east-trending, laterally-continuous tectonostratigraphic units separated by two major fault zones. To the south, low-grade metasediments of the Lesser Himalayan Sequence (LHS) are separated from high-grade crystalline rocks of the Greater Himalayan Sequence (GHS) by the north dipping Main Central Thrust (MCT). The northern extent of the GHS is separated from overlying low-grade sedimentary rocks of the Tethyan Himalayan Sequence (THS) along the north dipping South Tibetan Detachment System (STDS). There is controversy over the location and type of shear motion for the STDS in the ~50 km strip running through Lahul Valley where the STD is interpreted as a discrete fault, a dextral shear zone, and is unidentified in some areas along the trend of the STDS. This study focuses on understanding the pressure-temperature-time-deformation (P-T-t-d) evolution of THS and GHS rocks in Lahul Valley to better understand regional Cenozoic deformation and the location and role of the STDS in the extrusion of the GHS. Deformed granitics, migmatites, and leucogranites from the GHS contain a dominant mineralogy of Qz + Kfs + Pl + Bt + Ms ± Grt ± Ky ± St. Schists and phyllites from the THS contain a dominant mineralogy of Qz + Kfs + Pl + Bt + Ms ± Grt. Isochemical phase equilibria diagrams (pseudosections) are calculated in Perple_X using whole-rock chemistry data with solution models based on these mineral assemblages. Ti-in-quartz thermometry and the Fe-Mg exchange thermometry from garnet-biotite pairs used with mineral growth relationships constrain conditions during deformation and to establish P-T paths. U-Pb SHRIMP dating of zircon constrains peak metamorphic conditions and 40Ar/39Ar thermochronology of micas provide the cooling history along the valley and across the STDS. This multi-component approach to understand the metamorphic and deformational evolution of Lahul provides a holistic understanding of the GHS, THS, and STDS in a controversial area that can be used to draw comparisons, and build on tectonic models in the NW Himalaya.

The thermal history of the Karoo Moatize-Minjova Basin, Tete Province, Mozambique: An integrated vitrinite reflectance and apatite fission track thermochronology study

NASA Astrophysics Data System (ADS)

Fernandes, Paulo; Cogné, Nathan; Chew, David M.; Rodrigues, Bruno; Jorge, Raul C. G. S.; Marques, João; Jamal, Daud; Vasconcelos, Lopo

2015-12-01

The Moatize-Minjova Basin is a Karoo-aged rift basin located in the Tete Province of central Mozambique along the present-day Zambezi River valley. In this basin the Permian Moatize and Matinde formations consist of interbedded carbonaceous mudstones and sandstones with coal seams. The thermal history has been determined using rock samples from two coal exploration boreholes (ca. 500 m depth) to constrain the burial and exhumation history of the basin. Organic maturation levels were determined using vitrinite reflectance and spore fluorescence/colour. Ages and rates of tectonic uplift and denudation have been assessed by apatite fission track analysis. The thermal history was modelled by inverse modelling of the fission track and vitrinite reflectance data. The Moatize Formation attained a coal rank of bituminous coals with low to medium volatiles (1.3-1.7%Rr). Organic maturation levels increase in a linear fashion downhole in the two boreholes, indicating that burial was the main process controlling peak temperature maturation. Calculated palaeogeothermal gradients range from 59 °C/km to 40 °C/km. According to the models, peak burial temperatures were attained shortly (3-10 Ma) after deposition. Apatite fission track ages [146 to 84 Ma (Cretaceous)] are younger than the stratigraphic age. Thermal modelling indicates two episodes of cooling and exhumation: a first period of rapid cooling between 240 and 230 Ma (Middle - Upper Triassic boundary) implying 2500-3000 m of denudation; and a second period, also of rapid cooling, from 6 Ma (late Miocene) onwards implying 1000-1500 m of denudation. The first episode is related to the main compressional deformation event within the Cape Fold Belt in South Africa, which transferred stress northwards on pre-existing transtensional fault systems within the Karoo rift basins, causing tectonic inversion and uplift. During the Mesozoic and most of the Cenozoic the basin is characterized by very slow cooling. The second period of fast cooling and denudation during the Pliocene was likely related to the southward propagation of the East African Rift System into Mozambique.

Exhumation History Of Brasilian Highlands After Late Cretaceous Alcaline Magmatism

NASA Astrophysics Data System (ADS)

Doranti Tiritan, Carolina; Hackspacher, Peter Christian; Carina Siqueira Ribeiro, Marli; Glasmacher, Ulrich Anton; Françoso de Godoy, Daniel

2017-04-01

The southeast Brazilian margin recorded a long history of tectonic and magmatic events after the Gondwana continent break up. The drifting of the South American Platform over a thermal anomaly generated a series of alkaline intrusions that are distributed from the interior to the coast from west to east. Several exhumation events are recorded on the region and we are providing insights on the landscape evolution of the region since Late Cretaceous, comparing low temperature thermochronology results from two alkaline intrusions regions. Poços de Caldas Alkaline Massif (PCAM), is lied in the interior, 300km from the coastline, covering over 800km2 intruding the Precambrian basement around 83Ma, nepheline syenites, phonolites and tinguaites intruded in a continuous and rapid sequence lasting between 1 to 2 Ma. São Sebastião Island (SSI) on the other hand is located at the coast, 200 km southeast of São Paulo. It is characterized by an intrusion in Precambrian/Brazilian orogen and intruded by Early Cretaceous sub-alkaline basic and acid dykes, as well as by Late Cretaceous alkaline stocks (syenites) and dykes (basanite to phonolite). Will be presenting the apatite fission track (AFT) and (U-Th)/He results that shows the main difference between the areas is that PCAM region register older history then the coastal area of SSI, where thermal history starts register cooling event after the South Atlantic rifting process, while in the PCAM area register a previous history, since Carboniferous. The results are giving support to studies that indicate the development of the relief in Brazil being strongly influenced by the local and regional tectonic movements and the lithological and structural settings. The landscape at the Late Cretaceous was witness of heating process between 90 and 60Ma due the intense uplift of South American Platform. The elevation of the isotherms is associated with the mantellic plumes and the crustal thickness that caused thermal anomalies due the magma flow to subsurface. We are thankful to CAPES/PROBRAL/12809/13-6

Spatial and Temporal Strain Distribution Along the Central Red Sea Rift - A Study of the Hamd-Jizil Basin in Saudi Arabia

NASA Astrophysics Data System (ADS)

Szymanski, E.; Stockli, D.; Johnson, P.; Kattan, F. H.; Al Shamari, A.

2006-12-01

Numerous models exploring the rupturing modes and mechanisms of continental lithosphere are based on geological evidence from the Red Sea/Gulf of Suez rift system. Individually, the Red Sea basin is the prototype for many models of orthogonal continental rifting. Despite being a classic example of continental extension, many temporal and spatial strain distribution aspects, as well as the dynamic evolution of the rift architecture of the Red Sea, remain poorly constrained. Critical data come mostly from the Gulf of Suez and the Egyptian and Yemeni margins of the Red Sea; the rift flanks in Sudan and Saudi Arabia have remained largely unstudied, leaving a large information gap along the central portions of the rift system. Improving continental lithosphere rupture models requires an absolute understanding of the timing and magnitude of strain partitioning along the full rift flank. This study focuses on the development of extensional structures, syn- extensional sedimentary deposits, and rift-related Tertiary basaltic volcanism along the central flank of the rift system in Saudi Arabia. Geo- and thermochronometric techniques are used to elucidate the evolution of inboard and outboard strain markers manifested by structurally-controlled extensional basins that parallel the trend of the main Red Sea rift. Constraints on the dynamics of rift flank deformation are achieved through the collection of thermochronometric transects that traverse both the entire Arabian shield and individual normal faults that bound inland basins. Preliminary results show inland basins as asymmetric half-grabens filled by tilted Cenozoic sedimentary strata and separated by exhumed basement fault blocks. The most prominent extensional basin is the NW-trending Hamd-Jizil basin, located north of Madinah, measuring ~200 km along strike and up to 20 km in width. The Hamd-Jizil basin is structurally characterized by two half-grabens exposing a series of syn-rift siliciclastic sedimentary sections below Tertiary basalts. In certain areas, thick basalt sequences provide basin infill and appear faulted by a younger series of normal faults. Work continues on the production of further geo- and thermochronologic data for the Tertiary basalt sequences as well as the entire rift flank region.

Tectonic evolution of the Songpan Garzê and adjacent areas (NE Tibet) from Triassic to Present : a synthesis.

NASA Astrophysics Data System (ADS)

Roger, F.; Jolivet, M.; Malavieille, J.

2009-04-01

The 12th May 2008 Wenchuan earthquake in the Longmen Shan occurred on a large thrust fault largely inherited from an Indosinian structure itself probably controlled by an older structural heritage of the South China block continental margin. Within the whole northeast Tibet region, such a structural inheritance has had a major impact on the Tertiary deformation. It appears of primary importance to assess the pre-Tertiary tectonic evolution of the main blocks involved to understand the actual deformation in the eastern edge of Tibet. Over the past decades, the Proterozoic to Cenozoic tectonic, metamorphic and geochronologic history of the Longmen Shan and Songpan Garzê area have been largely studied. We present a synthesis of the tectonic evolution of the Songpan Garzê fold and thrust belt from Triassic to present. The Songpan-Garzê belt was formed during closure of a wide oceanic basin filled with a thick (5 to 15 km) sequence of Triassic flyschoid sediments [10]. Closure of the basin due to Triassic subduction involved strong shortening, intense folding and faulting of the Triassic series. A large-scale décollement, that presently outcrops along the eastern boundary of the belt (Danba area), allowed the growth of a wide and thick accretionary wedge [9]. It develops in the Paleozoic and Triassic series and separates the accretionary prism from an autochthonous crystalline basement [5, 12, 6] which shares many similarities with the basement of the Yangtze Craton (0.7-0.9 Ga). To the north and northwest, below the thickened Triassic series of the belt, the composition (oceanic or continental) of the basement remains unknown. During the Indosinian orogeny the emplacement of orogenic granites (220 - 150 Ma) was associated to crustal thickening [12, 13, 17, 15]. The isotopic composition of granitoids shows that their magma source were predominantly derived from melting of the proterozoic basement with varying degrees of sedimentary material and negligible mantle source contribution. In the Danba area, the décollement outcrops in a large tertiary antiform with a NNW-SSE axis [6, 12, 18]. It has been exhumed too in the hanging wall of the NE-SW faults of the Tertiary Longmen-Shan belt that marks the present day transition from the Tibetan plateau to the Sichuan basin. These faults have episodically absorbed significant shortening since the Late Triassic [3]. The amount and precise timing of post-triassic deformation are difficult to constrain especially because of the difficulty to isolate the tertiary thermochronological signal from the protracted late Triassic - Cretaceous thermal history (e.g. [14]). Nonetheless it is generally accepted that Jurassic - Cretaceous tectonism did not modified the general Triassic architecture of eastern Tibet contrarily to the Tertiary deformation (e.g. [2, 12, 5, 14]). The long-term cooling histories obtained on Mesozoic granites and on the metamorphic series of the Danba dome are very similar showing a very slow and regular cooling during Jurassic and Cretaceous, confirming the absence of major tectonic event between c.a. 150 and 30 Ma [16, 7, 6, 12, 18]. Low temperature thermochronology data indicate that final exhumation and cooling occurred in the Tertiary with an acceleration between 10 and 5 Ma along the major tectonic structures [11, 12, 1, 16, 7,14, 8]. Within the Longmen Shan range, a total denudation of 7 to 10 km is estimated for the late Cenozoic period [1, 7, 4]. Similar amounts of late Tertiary denudation have been estimated along an east-west section across the Xianshuihe fault [16]. 1 : Arne et al., (1997), Tectonophysics 280, 239-256. 2 : Burchfield et al. (1995), International Geology Review 37, 661-735. 3 : Chen and Wilson, (1996), Journal of Structural Geology 18, 413-440. 4 : Clark et al., (2005), Geology 33, 525-528. 5 : Harrowfield and Wilson, (2005), Journal of Structural Geology 27, 101-117. 6 : Huang et al., (2003), Journal of Metamorphic Geology 21(3), 223-240. 7 : Kirby et al., (2002), Tectonics 21(1), 10.1029/2000TC001246. 8 : Lai et al., (2007), Science in China Series D: Eath Sciences 50(2), 172-183. 9 : Mattauer et al., (1992), Comptes Rendus de l'Académie des Sciences Paris 314(6), 619-626. 10 : Nie et al., (1994), Geology 22, 999-1002. 12 : Roger et al., (1995), Earth and Planetary Science Letters 130, 201-216. 13 : Roger et al., (2004), Journal of Asian Earth Sciences 22, 465-481. 14 : Roger et al., (2008), Comptes Rendus Geoscience, Académie des sciences, Paris 340(2-3), 180-189. 15 : Wilson et al., (2006), Journal of Southeast Asian Earth Sciences 27, 341-357. 16 : Xiao et al., (2007), Lithos 96, 436-452. 17 : Xu and Kamp, (2000), Journal of Geophysical Research 105(B8), 19,231-19,251. 18 : Zhang et al., (2006), Journal of Asian Earth Sciences 27, 751-764. 19 : Zhou et al., (2008), Journal of Southeast Asian Earth Sciences 33, 414-427.

Mantle to Surface Dynamics Across Subduction-Collision Transitions in Space and Time: Results from the CD-CAT Project in Anatolia

NASA Astrophysics Data System (ADS)

Whitney, D. L.; Abgarmi, B.; Beck, S. L.; Brocard, G. Y.; Cosca, M. A.; Darin, M. H.; Delph, J. R.; Hui, H.; Kahraman, M.; Kaymakci, N.; Kuscu, G.; Meijers, M. J.; Mulch, A.; Özacar, A.; Portner, D. E.; Reid, M. R.; Rey, P. F.; Rojay, B.; Schlieffarth, W. K.; Sandvol, E. A.; Schoenbohm, L. M.; Tank, B.; Teoman, U.; Teyssier, C. P.; Thomson, S. N.; Turkelli, N.; Umhoefer, P. J.; Uslular, G.; Willenbring, J. K.

2017-12-01

From west to east, the southern plate boundary of Anatolia varies from subduction to continental collision; plate dynamics are influenced by the interaction of back-arc extension in the west (Aegean) and convergence in the east (Arabia-Eurasia). Prior to 40 Ma, the entire margin was a subduction zone. The NSF project "Continental Dynamics-Central Anatolian Tectonics (CD-CAT)" has contributed to understanding how the mantle, crust, and surface evolve in subduction-to-collision transitions in time and space. Differences are seen in changes in deformation style as collision proceeded; e.g. from distributed across a broad zone to highly localized on a series of oblique-slip faults, and from transpression to transtension (W of the Central Anatolian fault zone, CAFZ) or strike-slip (E of the CAFZ); age, composition, and sources of magmatism, including a magmatic lull from 40-20 Ma, followed by expansion of magmatism SE-ward in central Anatolia; properties and architecture of the lithosphere and sub-lithospheric mantle (e.g. significant and locally abrupt crustal thickness variations, including thick crust under the Tauride Mts; thin to absent lithospheric mantle; and a torn and disaggregating slab that varies from shallow to steep below central Anatolia); and a topographic gradient from a high eastern plateau (> 2 km) to a central plateau (1-1.5 km) bounded to the N and S by mountain ranges that rose > 2 km from the sea between 11-5 Ma, producing a rain shadow in the Anatolian interior. Thermochronologic and structural studies of exhumed mid-crust and associated basins and fault zones as well as geophysical data for Anatolia today show the extent to which inherited features (suture zones, faults) have affected the tectonic evolution of Anatolia, particularly in the vicinity of the CAFZ/East Anatolian Fault, and mantle properties. Results also show that the Miocene was a dynamic time in the thermal and mechanical evolution of the region, as early Miocene rollback/foundering of the subducted slab drove major volcanism, faulting, and landscape changes (uplift). Slab dynamics below Anatolia accelerated the late Miocene-Pliocene transition from distributed to highly localized deformation, including inception of the North and East Anatolia strike-slip faults that today accommodate tectonic escape of Anatolia.

Cenozoic landforms and post-orogenic landscape evolution of the Balkanide orogen: Evidence for alternatives to the tectonic denudation narrative in southern Bulgaria

NASA Astrophysics Data System (ADS)

Gunnell, Y.; Calvet, M.; Meyer, B.; Pinna-Jamme, R.; Bour, I.; Gautheron, C.; Carter, A.; Dimitrov, D.

2017-01-01

Continental denudation is the mass transfer of rock from source areas to sedimentary depocentres, and is typically the result of Earth surface processes. However, a process known as tectonic denudation is also understood to expose deep-seated rocks in short periods of geological time by displacing large masses of continental crust along shallow-angle faults, and without requiring major contributions from surface erosion. Some parts of the world, such as the Basin and Range in the USA or the Aegean province in Europe, have been showcased for their Cenozoic tectonic denudation features, commonly described as metamorphic core-complexes or as supradetachment faults. Based on 22 new apatite fission-track (AFT) and 21 helium (AHe) cooling ages among rock samples collected widely from plateau summits and their adjacent valley floors, and elaborating on inconsistencies between the regional stratigraphic, topographic and denudational records, this study frames a revised perspective on the prevailing tectonic denudation narrative for southern Bulgaria. We conclude that conspicuous landforms in this region, such as erosion surfaces on basement-cored mountain ranges, are not primarily the result of Paleogene to Neogene core-complex formation. They result instead from "ordinary" erosion-driven, subaerial denudation. Rock cooling, each time suggesting at least 2 km of crustal denudation, has exposed shallow Paleogene granitic plutons and documents a 3-stage wave of erosional denudation which progressed from north to south during the Middle Eocene, Oligocene, Early to Middle Miocene, and Late Miocene. Denudation initially prevailed during the Paleogene under a syn-orogenic compressional regime involving piggyback extensional basins (Phase 1), but subsequently migrated southward in response to post-orogenic upper-plate extension driven by trench rollback of the Hellenic subduction slab (Phase 2). Rare insight given by the denudation pattern indicates that trench rollback progressed at a mean velocity of 3 to 4 km/Ma. The Neogene horst-and-graben mosaic that defines the modern landscape (Phase 3) has completely overprinted the earlier fabrics of Phases 1 and 2, and has been the prime focus of tectonic geomorphologists working in the region. The new narrative proposed here for linking the geodynamic evolution of SE Europe with surface landform assemblages raises issues in favour of better documenting the regional sedimentary record of existing Paleogene basins, which constitute a poorly documented missing link to the thermochronological evidence presented here.

Climate-driven unsteady denudation and sediment flux in a high-relief unglaciated catchment-fan using 26Al and 10Be: Panamint Valley, California

NASA Astrophysics Data System (ADS)

Mason, Cody C.; Romans, Brian W.

2018-06-01

Environmental changes within erosional catchments of sediment routing systems are predicted to modulate sediment transfer dynamics. However, empirical and numerical models that predict such phenomena are difficult to test in natural systems over multi-millennial timescales. Tectonic boundary conditions and climate history in the Panamint Range, California, are relatively well-constrained by existing low-temperature thermochronology and regional multi-proxy paleoclimate studies, respectively. Catchment-fan systems present there minimize sediment storage and recycling, offering an excellent natural laboratory to test models of climate-sedimentary dynamics. We used stratigraphic characterization and cosmogenic radionuclides (CRNs; 26Al and 10Be) in the Pleasant Canyon complex (PCC), a linked catchment-fan system, to examine the effects of Pleistocene high-magnitude, high-frequency climate change on CRN-derived denudation rates and sediment flux in a high-relief, unglaciated catchment-fan system. Calculated 26Al/10Be burial ages from 13 samples collected in an ∼180 m thick outcropping stratigraphic succession range from ca. 1.55 ± 0.22 Ma in basal strata, to ca. 0.36 ± 0.18-0.52 ± 0.20 Ma within the uppermost part of the succession. The mean long-term CRN-derived paleodenudation rate, 36 ± 8 mm/kyr (1σ), is higher than the modern rate of 24 ± 0.6 mm/kyr from Pleasant Canyon, and paleodenudation rates during the middle Pleistocene display some high-frequency variability in the high end (up to 54 ± 10 mm/kyr). The highest CRN-derived denudation rates are associated with stratigraphic evidence for increased precipitation during glacial-pluvial events after the middle Pleistocene transition (post ca. 0.75 Ma), suggesting 100 kyr Milankovitch periodicity could drive the observed variability. We investigated the potential for non-equilibrium sedimentary processes, i.e. increased landslides or sediment storage/recycling, to influence apparent paleodenudation rates; end-member mixing models suggest that a mixture of >50% low-CRN-concentration sediment from landslides is required to produce the largest observed increase in paleodenudation rate. The overall pattern of CRN-derived burial ages, paleodenudation rates, and stratigraphic facies suggests Milankovitch timescale climate transitions drive variability in catchment denudation rates and sediment flux, or alternatively that climate transitions affect sedimentary process regimes that result in measurable variability of CRN concentrations in unglaciated catchment-fan systems.

Rio Grande rift evolution and accommodation mechanisms as revealed through low-temperature thermochronometry

NASA Astrophysics Data System (ADS)

Abbey, A. L.; Niemi, N. A.

2017-12-01

Low-temperature thermochronometry in the Rio Grande rift (RGR) in CO and NM, USA, allows for quantification of exhumation magnitudes and rates across the rift and reveals insights into rift basin segmentation and symmetry as well as the timing of extensional fault initiation and dominant mechanisms for rift accommodation. We combine new apatite helium (AHe) and zircon helium (ZHe) thermochronologic data with previously published AHe and apatite fission track (AFT) data to compile 17 vertical transects, each consisting of at least four samples, spanning more than >800 km along the RGR axis. Inverse thermal modeling (QTQt; Gallagher, 2012) of these vertical transects and compilation of bimodal rift related volcanism highlight transfer regions that separate several asymmetric basins with opposing fault dip directions. The Tularosa, Jornada and Albuquerque basins, in the southern RGR show extension initiation ca. 15 Ma with 3-4 km of exhumation accommodated on east dipping faults. Northward, the Española basin, a transfer zone of several strike slip, oblique-slip and smaller normal faults, does not record significant exhumation since the early Cenozoic. In the north-central part of the rift data from the San Luis Basin reveals 3-5 km of exhumation on west dipping faults began 20-15 Ma. East dipping faults in the upper Arkansas and Blue River grabens represent the northern extent of the rift and accommodate 3-5 km of exhumation beginning 15-10 Ma. RGR extension and magmatism initiation is commonly cited at 28 Ma (Tweto, 1979) however, our low-temperature thermochronometry modeling indicates that the majority of upper crustal extension initiated somewhat synchronously 15 Ma along the entire length of the rift. Rift related volcanism increased significantly in volume at 15 Ma, as well, but the locus of this volcanism is the Jemez lineament rather than the rift axis. As a result rifting within the RGR appears to be accommodated primarily by extensional faulting, with the exception of the central part of the rift (Española Basin) where the rift intersects the Jemez lineament. Widespread pre-rift thermochronometric ages in the Española Basin suggest that rifting in the central RGR is accommodated by, non-tectonic processes, most-likely magmatism.

Structural and exhumational response to oroclinal bending at the Eastern Alps - Western Carpathian transition

NASA Astrophysics Data System (ADS)

Heberer, Bianca; Neubauer, Franz

2017-04-01

Curvature is an intriguing feature within many mountain belts worldwide. Several proposals have been made for deciphering the origin of curvature, however, there is still significant debate about the bend-forming mechanisms, the consequences as well as on how bending is accommodated within the lithosphere. Only few of the worldwide oroclines have been studied in detail and a variety of alternative controlling factors, such as the role of inherited structures, the rheological coupling between lower and upper plates, the presence of a basement promontory in the foreland and its particular geometry, and lateral orogen-parallel extrusion are likely underestimated or not considered at all. This study focuses on oroclinal bending at the transition from the W-E trending Eastern Alps to the SW-NE oriented Western Carpathians. There, the orogenic front is concave towards the Alpine foreland and the greatest degree of curvature (ca. 55°) is found adjacent to the Bohemian massif. The oroclinal axis runs from the Bohemian promontory to the South Burgenland high. Various competing mechanisms occurred, i.e. rotation around a stiff foreland promontory and lateral extrusion induced by tectonic escape due to the indentation of a microplate and extensional collapse due to slab-rollback beneath the Carpathians. Little is known for such cases, where bending around as well as overriding of a promontory occurs, particularly on how it controls the exhumational and structural architecture within the orogen itself. Based on a synthesis of low-T thermochronology and structural data we find a significant impact of oroclinal bending on exhumation and structures: Highest amounts of erosion occur in the immediate vicinity of the Bohemian promontory and along its prolongation in the South Burgenland high, corroborating that shortening and exhumation are most pronounced there and should decrease along-strike of the orogenic front. In the outer bend strong Miocene extensional thinning parallel to the orogen occurred contemporaneously with sediment deposition to the east (Danube basin) and west (Styrian basin) of the oroclinal axis. We speculate that the location of extension at least from the Vienna and Danube basins are in part controlled by Jurassic extensional structures. The central sector along the oroclinal axis is largely overprinted by lateral block extrusion where the influence of oroclinal bending and a protruding promontory in the subsurface has yet to be demonstrated.

Geologic constraints on the setting and dynamics of subduction initiation

NASA Astrophysics Data System (ADS)

Encarnacion, John; Keenan, Timothy

2017-04-01

Understanding where and how subduction zones have and can initiate is important because, besides being a critical step in the plate tectonic system, it can provide insight into the complex interactions of crust and mantle rheology, forces acting on the plates, strain, metamorphic reactions, and erosional and depositional processes at the surface. Insight into the possibilities of where and how subduction zones start has been provided by numerical and analog modeling. All sites for subduction initiation are potential weak zones in the lithosphere and include the continent-ocean boundary, oceanic arc-oceanic crust boundary, oceanic transform faults and fracture zones, oceanic detachment faults, and active or recently extinct oceanic ridges/spreading centers. Within the constraints of modeling, it has also been shown that the forces involved in the initiation of subduction can be largely horizontal (induced by a collision, say, or through 'ridge push') or vertical (driven by density contrasts). The latter scenario is often referred to as "spontaneous" subduction initiation, whereas the former situation may be called "forced"or "induced" subduction initiation. It is prudent, however, not to assume that "what can happen, did happen." So, the challenge for geologists is to infer from the rock record, through structural mapping, thermochronology, thermobarometry, geochemistry, paleomagnetics, and sedimentological studies, how any given subduction zone began. Even with a complete data set, it is not always possible to fully constrain the specific geologic setting or dynamics involved in the initiation of a given subduction zone. One can, however, often rule out certain scenarios, increasing the probability of others. Part of the geologic record of subduction initiation preserved at some subduction zones are so-called "metamorphic soles," which include high-temperature (T) and high-pressure (P) metamorphosed oceanic crust that was underthrust to asthenospheric mantle depths, metamorphosed, and then preserved in the hanging wall of the eventual subduction zone. These metamorphic soles may preserve important information bearing on the timing of subduction initiation, the evolving P and T conditions during subduction initiation, and, importantly, the protolith age of the initially subducted crust. The latter parameter—the age of the initially subducted oceanic crust at the time of subduction initiation—is an important constraint that has been lacking in many previous geologic studies of subduction initiation. Recent work on metamorphic soles has provided new information on subduction initiation, including the possibility of rapidly converting oceanic divergent boundaries into subduction zones.

New Constraints for Tectono-Thermal Alpine Evolution of the Pyrenees: Combining Zircon Fission-Track and (U-Th)/He Analyses with Raman Spectrometry and In-Situ K-Ar Geochronology

NASA Astrophysics Data System (ADS)

Waldner, M.; Bellahsen, N.; Mouthereau, F.; Pik, R.; Bernet, M.; Scaillet, S.; Rosenberg, C.

2017-12-01

The pyrenean range was formed by the convergence of European and Iberian plates following the inversion of the Mesozoic rifting in the north of Pyrenees. In the Axial Zone, the collision caused an antiformal nappe-stacking of tectonic units. Recent studies pointed out the importance of pre-collision structural and thermal inheritance that may play a major role for orogeny such as: 1) Paleozoic Variscan inheritance; 2) Mesozoic rift-related high geothermal gradients, which are maintained during the onset of convergence in the North Pyrenean Zone. From a mineralogical point of view, pre-collision feldspars have been destabilized and influenced the development of alpine phyllonite in brittle-ductile conditions which suggests a weak crustal behavior during the formation of the orogenic wedge. Our aim is to get a better understanding of alpine deformation and exhumation by coupling different thermochronological, geochronological and thermometric methods. We document the thermal evolution of each tectonic unit by using low-temperature thermochronometers (Zircon Fission Tracks, U-Th/He on zircons including laser ablation profiles). Our data on vertical profiles combined to existing dataset on apatite allows to model alpine exhumation across the Axial zone. Structural observations through alpine thrusts coupled to geochronology (in situ K/Ar on phengites), Raman and chlorite-phengite thermo(baro)metry provide new key data to unravel the alpine evolution of the Pyrenees. According to preliminary ZFT results on granite massifs in the central part of Pyrenean Axial zone (near ECORS profile), exhumation ages potentially indicates a migration of exhumation towards the south. Exhumation ages of the northern massifs seems to have preserved the North Pyrenean Cretaceous rift evolution. Further south, the onset of exhumation is as old as Paleocene, which precedes the Eocene ages of the literature. The low burial estimated in the northern massifs may indicate a high thermal gradient. This dataset coupled to the above-cited other methods provide the most exhaustive and detailed image of the thermo-structural evolution of the Axial Zone that enables us to discuss the crustal rheology during collision. This study is part of the Orogen project, a partnership between academy and industry (Total, BRGM, CNRS)

Diffusion in the Muscovite 40K Decay System (Invited)

NASA Astrophysics Data System (ADS)

Harrison, T. M.

2010-12-01

The considerable potential of muscovite for thermochronological applications is beginning to be fully exploited following the belated publication of Ar kinetic data. Muscovite’s high potassium content, low solubility for excess 40Ar*, and ubiquitous presence in regionally metamorphosed terranes make it an important phase for 40Ar/39Ar thermochronometry, particularly in light of recognition that both age spectra and vacuum-step-heating-derived 39Ar Arrhenius plots reflect Ar release via the same volume diffusion mechanism. Thus instead of assuming a nominal closure temperature to estimate a single T-t datum, continuous and accurate thermal histories can be inferred in a similar fashion to that well-documented for K-feldspar using the multi-diffusion domain (MDD) model. The Arrhenius parameters for Ar diffusion in muscovite (E=64 kcal/mol, Do=4 cm2/s) correspond to an effective intragrain closure temperature range of ~500 to 300oC for ca. 100 μm grains cooling at ~10oC/Ma at 5 kbar. However, even greater exploitation of the 40K decay system remains possible as only one of every ten 40K atoms decay to 40Ar. The other 90% decay to 40Ca giving the 40K-40Ca branch, in principle, greater sensitivity for dating high K/Ca minerals such as muscovite. The advent of the ‘double-plus’ SIMS 40K++-40Ca++ dating method, which permits analysis of Ca isotopes at an MRP of ~4k rather than the ~25k required for full separation of 40K+ from 40Ca+, opens up the prospect of directly revealing 40K-40Ca closure profiles in muscovite (as opposed to their indirect inference from inversion of 40Ar/39Ar data through the MDD model) at a gain of enhanced precision and accuracy in thermal history reconstruction. We have used SIMS to observe K-Ca age variations in natural muscovites pressed into In. Translating this data into thermal history information, however, requires knowledge of the Arrhenius parameters for Ca tracer diffusion in muscovite. We are undertaking hydrothermal piston-cylinder experiments of natural muscovites to induce radiogenic 40Ca* diffusion gradients that can be measured with SIMS using a ~5 μm spot. Preliminary indications suggest that Ca diffusion is not substantially slower than Ar in muscovite suggesting a similar to somewhat elevated closure temperature range.

Exhumation of the Black Mountains in Death Valley, California, with new thermochronometric data from the Badwater Turtleback

NASA Astrophysics Data System (ADS)

Sizemore, T. M.; Cemen, I.; Wielicki, M. M.; Stockli, D. F.; Heizler, M. T.; Lutz, B. M.

2017-12-01

The Black Mountains, in Death Valley, California, are one of the key areas to better understand Basin and Range extension because they contain Cenozoic igneous and sedimentary rocks overlying mid- to deep-crustal, 1.74 Ga basement gneiss with abundant fault striations, large-scale extensional folds, and tectonite fabrics containing top-to-the-northwest shear-sense indicators. These rocks make up the footwall of three prominent, high-relief "turtleback" fault surfaces in the western flank of the Black Mountains, which are thought to have accommodated a significant amount of strain in the Death Valley area. It is unknown whether the missing Paleozoic and Mesozoic strata in the Black Mountains were removed in association with high-angle faulting, or along a continuous detachment surface with a rolling-hinge style of faulting as the hanging wall moved to the west, now forming the Panamint Range. The turtlebacks play an important role in resolving this question because they are commonly cited as containing conflicting evidence of both hypotheses. To provide insight into this problem, we are building an exhumation model across the Black Mountains using previously published thermochronometric data as well as new transect-based (U-Th)/He and Ar-Ar thermochronology and U-Pb geochronology for the Badwater turtleback. The model will provide a four-dimensional view of the exhumation history of the Black Mountains, to serve as evidence for either of the two previously mentioned hypotheses, or possibly some other style of exhumation. Additionally, we will compare the exhumation history of the Black Mountains to that of the Panamint Range using previously published data and interpretations. Our preliminary zircon U-Pb data suggest a crystallization age for the gneissic rocks on the Badwater turtleback of 1.74 Ga (207Pb/206Pb, 2σ error=31.8 Ma, n=6) with two younger populations at 1.46 Ga (207Pb/206Pb, 2σ error=51.8 Ma, n=3) and 79.6 Ma (206Pb/238U, 2σ error=10.0 Ma, n=2), possibly reflecting post-crystallization alteration. This is in agreement with previously published ages. The zircon 206Pb/238U ages of the Smith Mountain Granite, exposed at the top of the Badwater turtleback, range from 3.06-9.71 Ma (n=11), confirming Cenozoic intrusion of the pluton.

Present-day subglacial erosion efficiency inferred from sources and transport of glacial clasts in the North face of Mont Blanc

NASA Astrophysics Data System (ADS)

Mugnier, J. L.; Godon, C.; Buoncristiani, J. F.; Paquette, J. L.; Trouvé, E.

2012-04-01

The efficiency of erosional processes is classically considered from detrital composition at the outlet of a shed that reflects the rocks eroded within the shed. We adapt fluvial detrital thermochronology (DeCelles et al., 2004) and lithology (Attal and Lavé, 2006) methods to the subglacial streams of the north face of the Mont Blanc. The lithology of this area is composed by a ~303 Ma old granite intruded within an older poly metamorphic complex (orthogneisses). In this study,we use macroscopic criteria (~10 000 clasts) and Ur/Pb dating of zircons (~500 datings of sand grains) to determine the provenance of the sediment transported by the glacier and by the sub-glacial streams. Samples come from sediments collected around the glacier (above, below or laterally), from different bedrocks sources according to the surface flow lines and glacier characteristics (above or below the ELA; temperate or cold), and from different subglacial streams. A comparison between the proportion of granite and orthogneisses in these samples indicates that: 1) the supra load follows the flow lines of the glacier deduced from SAR images correlation and the displacement pattern excludes supra load mixing of the different sources; 2) the transport by the glacier does not mix the clasts issued from the sub-glacial erosion with the clasts issued from supraglacial deposition, except in the lower tongue where supraglacial streams and moulins move the supraglacial load from top to bottom; 3) the erosion rate beneath the glacier is very small: null beneath the cold ice but also very weak beneath the greatest part of the temperate glacier; the erosion increases significantly beneath the tongue, where supraglacial load incorporated at the base favors abrasion; 4) the glacial erosion rate beneath the tongue remains at least five time smaller than the erosion rate coming from non-glacial area. According to our results, we demonstrate that the glaciers of the Mont-Blanc north face protect the top of Europe from erosion. DeCelles et al., 2004, Earth and Planetary Science Letters, v. 227, p. 313-330. Attal and Lavé, 2006, Geol. Soc. Am. Spec. Publ. (S.D. Willett, N. Hovius, M.T. Brandon and D. Fisher, eds.), 398, p. 143-171.

Living on the Edge? Clumped Isotope and Oxygen Isotope Record of Early Cascade Topography (Eocene Chumstick Basin, WA, USA)

NASA Astrophysics Data System (ADS)

Methner, K.; Mulch, A.; Fiebig, J.; Wacker, U.; Umhoefer, P. J.; Chamberlain, C. P.

2014-12-01

The topographic evolution of the world's major orogens exerts a strong impact on atmospheric circulation and precipitation patterns and is a key element in reconstructing the interactions among tectonics, climate, and Earth surface processes. Using carbonate stable and clumped isotope data together with low-temperature thermochronology information from the terrestrial Eocene Chumstick Basin (Central Washington; USA), today located to the East of the Washington Cascades, we investigate the E-W extent of the western North American plateau region and the evolution of Cascade topography. Oxygen isotope measurements of Eocene (51 to 37 Ma) pedogenic carbonate concretions and calcic horizons yield low δ18Ocarbonate values of +9 to +13 ‰ (SMOW) despite the proximity of the Eocene Chumstick Basin to the Pacific moisture source and paleofloral data that indicate moderate elevations and montane rain forest conditions during a warm and rather wet, seasonal climate. This either suggests that 51-37 Ma ago Cascade-like topography characterized the western edge of the North American-Pacific plate margin to the West of the Chumstick Basin or that the δ18Ocarbonate data were variably reset or only formed during burial and diagenesis. Clumped isotope (Δ47) thermometry of pedogenic carbonate and carbonate concretions (n=11 samples) indicates spatially variable burial temperatures of 80 to 120 °C that correlate with vitrinite reflectance data in these sediments. In concordance with changes in depositional environment the youngest (<40 Ma) Chumstick sediments experienced a lesser degree of post-depositional burial and heating (ca. 70 - 80 °C) compared to the older Chumstick series (80 - 120 °C). Calculated δ18O values of the circulating fluids in the Chumstick basin sediments range from -6 ‰ (T ~100 °C at ca. 40-30 Ma) to -9 ‰ (T ~75 °C at ca. 25-15 Ma). These values suggest a low-altitude meteoric fluid source and as a consequence only moderate Cascade topography during the Eo-/Oligocene. The combined stable isotope and clumped isotope data, therefore support models of late Miocene Cascade uplift and provide insight into the topographic configuration of the western North American plate margin prior to the late Miocene.

A Stratigraphic Record from 16 Ma to Present of Compound-Specific Hydrogen Isotopes from northern Tibet: Implications for Paleoaltimetry and Paleoclimate of the Northern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Zhuang, G.; Brandon, M. T.; Pagani, M.

2012-12-01

The paleotopographic evolution of Tibet remains a key issue in testing models for the formation of orogenic plateaus. Stable isotopes from paleosols and lake carbonates provide the primary tool for estimating paleotopography. Unfortunately, the deposits are strongly controlled by evaporation, which means that the surface waters from which they formed were shifted towards heavier isotopic compositions relative to initial (pre-evaporation) meteoric compositions. As a result, estimates from these settings probably represent a lower bound for paleotopography. We report here on new analyses of compound-specific hydrogen stable isotopes, which were determined for n-alkanes extracts from 36 samples from Neogene strata in the northern Tibetan Plateau. N-alkanes represent long-chain hydrocarbons, commonly formed as leaf waxes in terrestrial high plants. The advantage of this record is that it is linked to times when moisture transport was high and evaporation low, as required to allow for the plants to thrive. Distributions of n-alkanes show maxima at C27, C29, and C31 with high odd-over-even preference values, suggesting excellent preservation of lipid biomarkers from terrestrial high plants. The deuterium values highly co-vary between three compounds. Application of an apparent fractionation factor based on modern ground waters and soil-derived lipid biomarkers suggest a multiple-phase evolution of paleometeoric waters consistent with well-constrained tectonic and climatic histories in the northern Tibetan Plateau. For example, a ~60‰ negative shift in δD between 16-10 Ma correlates well with sedimentological and thermochronologic evidence for rapid erosion at that time. The magnitude of this isotopic shift is equivalent to an increase in elevation of 2 to 3 km, assuming that the isotopic composition of the moisture source remained constant during this time. An abrupt positive δD shift at ~10 Ma is consistent with studies supporting intensified aridity in central Asia, whereas a negative δD shift at ~6.5 Ma potentially reflects a change to more moist conditions related to the onset/intensified East Asia Summer Monsoon. The attainment of high elevations in northern Tibetan Plateau is synchronous with the attainment of maximum elevation in the south-central Tibetan Plateau and with the transition from dominant tectonic extrusion to distributed crustal shortening in the northern Tibetan Plateau.

Initiation of extension in South China continental margin during the active-passive margin transition: kinematic and thermochronological constraints

NASA Astrophysics Data System (ADS)

ZUO, Xuran; CHAN, Lung

2015-04-01

The southern South China Block is characterized by a widespread magmatic belt, prominent NE-striking fault zones and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from an active to a passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. In this study, we used zircon fission-track dating (ZFT) and numerical modeling to examine the timing and kinematics of the active-passive margin transition. Our ZFT results on granitic plutons in the SW Cathaysia Block show two episodes of exhumation of the granitic plutons. The first episode, occurring during 170 Ma - 120 Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115 Ma - 70 Ma. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the observation based on ZFT data that exhumation of the granite-dominant Nanling Range occurred at an earlier time than the gneiss-dominant Yunkai Terrane. In addition to the difference in geology between Yunkai and Nanling, the heating from Jurassic-Early Cretaceous magmatism in the Nanling Range may have softened the upper crust, causing the area to exhume more readily. Numerical modeling results also indicate that (1) high slab dip angle, high geothermal gradient of lithosphere and low convergence velocity favor the subduction process and the reversal of crustal stress state from compression to extension in the upper plate; (2) the late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension has shed light on the geological condition producing the red bed basins during Late Cretaceous-early Paleogene in South China. It appears that the red bed basins could have formed during the late stage of the subduction process, accounting for the observations why concurrent volcanic rocks could be found in some sedimentary basin formation. We propose that the extensional events started as early as the Late Cretaceous, probably before the cessation of subduction process. (Funding from Total Company and matching support from UGC are gratefully acknowledged).

Growth of the Zagros Fold-Thrust Belt and Foreland Basin, Northern Iraq, Kurdistan

NASA Astrophysics Data System (ADS)

Koshnaw, Renas; Horton, Brian; Stockli, Daniel; Barber, Douglas; Ghalib, Hafidh; Dara, Rebwar

2016-04-01

The Zagros orogenic belt in the Middle Eastern segment of the Alpine-Himalayan system is among the youngest seismically active continental collision zones on Earth. However, due to diachronous and incremental collision, the precise ages and kinematics of shortening and deposition remain poorly understood. The Kurdistan region of the Zagros fold-thrust belt and foreland basin contains well-preserved Neogene wedge-top and foredeep deposits that include clastic nonmarine fill of the Upper Fars, Lower Bakhtiari, and Upper Bakhtiari Formations. These deposits record significant information about orogenic growth, fold-thrust dynamics, and advance of the deformation front. Thermochronologic and geochronologic data from thrust sheets and stratigraphic archives combined with local earthquake data provide a unique opportunity to address the linkages between surface and subsurface geologic relationships. This research seeks to constrain the timing and geometry of exhumation and deformation by addressing two key questions: (1) Did the northwestern Zagros fold-thrust belt evolve from initial thin-skinned shortening to later thick-skinned deformation or vice-versa? (2) Did the fold-thrust belt advance steadily under critical/supercritical wedge conditions involving in-sequence thrusting or propagate intermittently under subcritical conditions with out-of-sequence deformation? From north to south, apatite (U-Th)/He ages from the Main Zagros Thrust, the Mountain Front Flexure (MFF), and additional frontal thrusts suggest rapid exhumation by ~10 Ma, ~5 Ma, and ~8 Ma respectively. Field observations and seismic sections indicate progressive tilting and development of growth strata within the Lower Bakhtiari Formation adjacent to the frontal thrusts and within the Upper Bakhtiari Formation near the MFF. In the Kurdistan region of Iraq, a regional balanced cross section constrained by new thermochronometric results, proprietary seismic reflection profiles, and earthquake hypocenters suggest prolonged thin-skinned shortening in sequence from north to south followed by a thick-skinned out-of-sequence MFF deformation and intermittent hinterland uplift postdating initial collision. Magnetostratigraphic analyses of Dinarta wedge-top deposits and Kifri foredeep deposits constrain accumulation of the Upper Fars-Lower Bakhtiari synorogenic succession to 12.5-5 Ma. These findings suggest that temporal and spatial shifts in upper-crustal modes of deformation in the Kurdistan segment of the Zagros orogenic belt strongly influenced patterns of topographic growth, landscape development, and resulting foreland basin stratigraphy.

Geochronology and thermochronology of Cretaceous plutons and metamorphic country rocks, Anyui-Chukotka fold belt, North East Arctic Russia

NASA Astrophysics Data System (ADS)

Miller, E. L.; Katkov, S. M.; Strickland, A.; Toro, J.; Akinin, V. V.; Dumitru, T. A.

2009-09-01

U-Pb isotopic dating of seven granitoid plutons and associated intrusions from the Bilibino region (Arctic Chukotka, Russia) was carried out using the SHRIMP-RG. The crystallization ages of these granitoids, which range from approximately 116.9±2.5 to 108.5±2.7 Ma, bracket two regionally significant deformational events. The plutons cut folds, steep foliations and thrust-related structures related to sub-horizontal shortening at lower greenschist facies conditions (D1), believed to be the result of the collision of the Arctic Alaska-Chukotka microplate with Eurasia along the South Anyui Zone (SAZ). Deformation began in the Late Jurassic, based on fossil ages of syn-orogenic clastic strata, and involves strata as young as early Cretaceous (Valanginian) north of Bilibino and as young as Hauterivian-Barremian, in the SAZ. The second phase of deformation (D2) is developed across a broad region around and to the east of the Lupveem batholith of the Alarmaut massif and is interpreted to be coeval with magmatism. D2 formed gently-dipping, high-strain foliations (S2). Growth of biotite, muscovite and actinolite define S2 adjacent to the batholith, while chlorite and white mica define S2 away from the batholith. Sillimanite (± andalusite) at the southeastern edge the Lupveem batholith represent the highest grade metamorphic minerals associated with D2. D2 is interpreted to have developed during regional extension and crustal thinning. Extension directions as measured by stretching lineations, quartz veins, boudinaged quartz veins is NE-SW to NW-SE. Mapped dikes associated with the plutons trend mostly NW-SE and indicate NE-SW directed extension. 40Ar/39Ar ages from S2 micas range from 109.3±1.2 to 103.0±1.8 Ma and are interpreted as post-crystallization cooling ages following a protracted period of magmatism and high heat flow. Regional uplift and erosion of many kilometers of cover produced a subdued erosional surface prior to the eruption of volcanic rocks of the Okhotsk-Chukotka volcanic belt (OCVB) whose basal units (~87 Ma) overlie this profound regional unconformity. A single fission track age on apatite from granite in the Alarmaut massif yielded an age of 90±11 Ma, in good agreement with this inference.

In situ rutile petrochronology: texture-related T, Paleoproterozoic inheritance and a Pan-African overprint in the oldest subduction-related eclogites, Usagaran Orogen, Tanzania

NASA Astrophysics Data System (ADS)

Moeller, A.; Kraus, K.; Herms, P.; Appel, P.; Raase, P.

2014-12-01

Rutile U-Pb thermochronology is applied successfully by both TIMS and beam methods to date cooling events in mafic and metapelitic rocks, as well as in detrital studies. The Zr-in-rutile thermometer is very robust to thermal diffusion, and generally requires complete recrystallization to change recorded crystallization temperatures. Evidence for diffusion of HFSE elements in rutile is sparse; whereas U-Pb chronology generally records diffusion controlled cooling from the last event. This study follows conventional thermobarometry and U-Pb TIMS results on monazite, sphene and rutile of Möller et al. (1995) establishing a 2 Ga eclogite facies event from MORB-like metabasic, and metapelitic rocks in the Usagaran Orogen of Tanzania, interpreted to be the oldest outcrops of subduction-related eclogites. Rutile from both rock types were discordant near a ca. 500 Ma lower intercept, confirming a thermal overprint postulated on the basis of K-Ar and Rb-Sr mica ages by e.g. Wendt et al. (1972). The age of the eclogite-facies event was confirmed by U-Pb zircon dating of a 1991±2 Ma crosscutting pegmatite (Collins et al., 1999). We present in situ LA-ICP-MS rutile petrochronology on five metabasic and metapelitic eclogite facies samples with variable retrograde amphibolite-facies recrystallization. Thermometry confirms conventional Fe-Mg results, including higher peak temperatures in metabasites. Traverses on rutile inclusions in large garnet prophyroblasts in metapelites show increasing temperatures from cores outwards and a slight decrease towards outermost rims, with peak T coinciding with highest Mg# and highest grossular content, hence consistent with preservation of prograde zoning in the garnets and a brief eclogite facies event. Large rutiles (800μm) in recrystallized samples record temperature zoning profiles. U-Pb results show inheritance of near concordant 2 Ga domains, but dominantly confirm the ca. 490 Ma amphibolite facies overprint. The study is an excellent example of the potential of in-situ rutile petrochronology in complex, polymetamorphic rocks when meticulous attention is given to textural context. Möller et al., 1995, Geology, v. 23, p. 1067-1070. Collins et al., 2004, Earth Planet. Sci. Lett., v. 224, p. 175-192. Wendt et al., 1972, 24th Internat. Geol. Congr., Proc., p. 295-314.

The Use of Terrestrial Analogs in Preparing for Planetary Surface Exploration: Sampling and Radioisotopic Dating of Impactites and Deployment of In Situ Analytical Technologies

NASA Astrophysics Data System (ADS)

Young, Kelsey

Impact cratering has played a crucial role in the surface development of the inner planets. Constraining the timing of this bombardment history is important in understanding the origins of life and our planet's evolution. Plate tectonics, active volcanism, and vegetation hinder the preservation and identification of existing impact craters on Earth. Providing age constraints on these elusive structures will provide a deeper understanding of our planet's development. To do this, (U-Th)/He thermochronology and in situ 40 Ar/39Ar laser microprobe geochronology are used to provide ages for the Haughton and Mistastin Lake impact structures, both located in northern Canada. While terrestrial impact structures provide accessible laboratories for deciphering Earth's impact history, the ultimate goal for understanding the history of the reachable inner Solar System is to acquire robust, quantitative age constraints for the large lunar impact basins. The oldest of these is the South Pole-Aitken basin (SPA), located on the lunar farside. While it is known that this basin is stratigraphically the oldest on the Moon, its absolute age has yet to be determined. Several reports released in the last decade have highlighted sampling and dating SPA as a top priority for inner Solar System exploration. This is no easy task as the SPA structure has been modified by four billion subsequent years of impact events. Informed by studies at Mistastin---which has target lithologies analogous to those at SPA---sampling strategies are discussed that are designed to optimize the probability of a high science return with regard to robust geochronology of the SPA basin. Planetary surface missions, like one designed to explore and sample SPA, require the integration of engineering constraints with scientific goals and traverse planning. The inclusion of in situ geochemical technology, such as the handheld X-ray fluorescence spectrometer (hXRF), into these missions will provide human crews with the ability to gain a clearer contextual picture of the landing site and aid with sample high-grading. The introduction of hXRF technology could be of crucial importance in identifying SPA-derived melts. In addition to enhancing planetary field geology, hXRF deployment could also have real implications for enriching terrestrial field geology.

­­­Experimental Quantifications of Radiation Damage Annealing and Helium Diffusion Kinetics in Apatite

NASA Astrophysics Data System (ADS)

Willett, C. D.; Shuster, D. L.

2017-12-01

(U-Th)/He thermochronology in apatite requires a quantitative description of He diffusivity as a function of temperature and through geologic time. Although variability in diffusion kinetics across a range of natural apatite samples has revealed that higher concentrations of alpha-recoil radiation damage correlates with lower He diffusivity (i.e., at a given temperature, [1]), only one published study has experimentally quantified the effects of annealing for a single apatite specimen (Durango apatite, [2]). Although these effects have been incorporated into now widely applied numerical models, underlying assumptions in these models—in particular, that He diffusivity in all apatite crystals responds with the same rate of damage annealing—have been called into question, and further evaluation is warranted (e.g., [3], [4]). Here, we will describe a suite of experiments conducted on apatite from a single hand sample of granite from Sierra Nevada, CA as well as Durango apatite, to establish whether these two apatites with different chemical compositions and thermal pasts exhibit the same response to annealing conditions. Crystals from both samples were heated under vacuum to temperatures between 220 and 500 °C for 1, 10, 100 or 1000 hours. The samples were then irradiated with 220 MeV protons to produce spallation 3He, the diffusant used in subsequent step-heating degassing experiments. Our preliminary results indicate different minima in closure temperatures of 55 oC and 65 oC for the Durango and Sierra apatite, respectively, when exposed to sufficiently high temperatures (>350 oC) for durations > 1 hour, yet similar transitions from low diffusivities at T <200 oC (and higher activation energy, Ea) to higher diffusivity (lower Ea) across a range of experimental annealing temperatures and durations. We will interpret these results with a new model framework for describing the effects of annealing on diffusivity, and will discuss potential implications of our experimental results, the required assumptions in our analyses, and potential limitations of such empirical quantifications. References: [1] Shuster, D. et al. (2006), EPSL 294, 148-161; [2] Shuster, D., Farley, K. (2009), GCA 73 (1), 6183-6196; [3] Gautheron, C. et al. (2013), Chem. Geol. 351, 257-267; [4] Fox, M., Shuster, D. (2014), EPSL 397, 174-183.

Inverted Apatite (U-Th)/He and Fission-track Dates from the Rae craton, Baffin Island, Canada and Implications for Apatite Radiation Damage-He Diffusivity Models

NASA Astrophysics Data System (ADS)

Ault, A. K.; Reiners, P. W.; Thomson, S. N.; Miller, G. H.

2015-12-01

Coupled apatite (U-Th)/He and fission-track (AFT) thermochronology data from the same sample can be used to decipher complex low temperature thermal histories and evaluate compatibility between these two methods. Existing apatite He damage-diffusivity models parameterize radiation damage annealing as fission-track annealing and yield inverted apatite He and AFT dates for samples with prolonged residence in the He partial retention zone. Apatite chemistry also impacts radiation damage and fission-track annealing, temperature sensitivity, and dates in both systems. We present inverted apatite He and AFT dates from the Rae craton, Baffin Island, Canada, that cannot be explained by apatite chemistry or existing damage-diffusivity and fission track models. Apatite He dates from 34 individual analyses from 6 samples range from 237 ± 44 Ma to 511 ± 25 Ma and collectively define a positive date-eU relationship. AFT dates from these same samples are 238 ± 15 Ma to 350 ± 20 Ma. These dates and associated track length data are inversely correlated and define the left segment of a boomerang diagram. Three of the six samples with 20-90 ppm eU apatite grains yield apatite He and AFT dates inverted by 300 million years. These samples have average apatite Cl chemistry of ≤0.02 wt.%, with no correlation between Cl content and Dpar. Thermal history simulations using geologic constraints, an apatite He radiation damage accumulation and annealing model, apatite He dates with the range of eU values, and AFT date and track length data, do not yield any viable time-temperature paths. Apatite He and AFT data modeled separately predict thermal histories with Paleozoic-Mesozoic peaks reheating temperatures differing by ≥15 °C. By modifying the parameter controlling damage annealing (Rmr0) from the canonical 0.83 to 0.5-0.6, forward models reproduce the apatite He date-eU correlation and AFT dates with a common thermal history. Results imply apatite radiation damage anneals at higher temperatures than fission-track damage and the impact on coupled apatite He and AFT dates is magnified for protracted cooling histories. Further experimental and field-based tests are important for refining radiation damage and fission-track annealing parameters for accurate interpretation of apatite He- and AFT-derived thermal histories.

The disruption of H and L ordinary chondrite parent bodies at 60 Ma

NASA Astrophysics Data System (ADS)

Blackburn, T.; Alexander, C.; Carlson, R.; Elkins-Tanton, L. T.

2016-12-01

A working timeline for the history of ordinary chondrites (OCs) includes chondrule formation as early as 1-2 Ma after our Solar System's earliest forming solids (CAIs), followed by rapid accretion into undifferentiated planetesimals that were heated internally by 26Al decay and cooled over a period of tens of millions of years. There remains conflict, however, between metallographic cooling rates and radioisotopic thermochronometric data over the sizes and lifetimes of the chondrite parent bodies, as well as the timing of impact related disruption. The importance of establishing the timing of parent body disruption is heightened by the use of meteorites as recorders of asteroid belt wide disruption events as suggested by various dynamical models. Here we attempt to resolve these records with new Pb-phosphate dates for 9 previously unstudied OCs. These new results, along with previously published Pb-phosphate and metallographic data, are interpreted with a series of numerical models designed to simulate the thermal evolution for a chondrite parent body that is disrupted by impact prior to forming smaller unsorted "rubble piles". One model that could satisfy both the available thermochronologic and Ni-metal data takes into account subtle differences in closure temperatures for each system. It requires that disruption occur early enough such that the Ni-metal system can record the cooling rate associated with a rubble pile (<70 Ma), yet late enough that the Pb-phosphate system can record an onion shell structure (>30 Ma). For this 30-70 Ma timeline, reaccretion into smaller rubble piles will ensure that the originally deeply buried and hot Type 6 samples will always cool faster as a result of disruption, yielding nearly uniform ages that date parent body disruption. This is consistent with the available Pb-phosphate data, where all but one Type 6 chondrite (H, n=3; L, n=4) records a uniform cooling age (4508 ± 5 Ma). Our model results suggest that a disruption at this time is: 1) late enough so that shallow chondrite layers will have cooled and H5/L5s will always have older Pb-phosphate dates than deeper H6/L6s; 2) early enough to precede Ni-metal closure, replicating the lack of correlation between Ni-metal cooling rates and petrologic Type suggestive of cooling through Ni-metal closure in a reaccreted rubble pile.

A suture delta: the co-evolution of tectonics and sedimentology as a remnant ocean basin closes; the Indo Burman ranges, northeast India and Bangladesh

NASA Astrophysics Data System (ADS)

Sincavage, R.; Betka, P. M.; Seeber, L.; Steckler, M. S.; Zoramthara, C.

2017-12-01

The closure of an ocean basin involves the interplay of tectonics and sedimentology, whereby thick successions of fluvio-deltaic and shallow marine sediment accumulate in the closing gap between the subduction zone and passive margin. The transition from subduction to collision involves processes that are inherently time-transgressive and co-evolve to influence the nature of the developing tectonic wedge. The Indo-Burman Ranges (IBR) of eastern India present a unique opportunity to examine this scenario on a variety of spatial (10-2­­­-105 m2) and temporal (1 a-10 Ma) scales. Recent field mapping campaigns in the IBR have illuminated analogous depositional environments expressed in the Neogene outcrops of the IBR and the Holocene sediment archive of the Ganges-Brahmaputra-Meghna delta (GBMD). Six distinct lithofacies are present in shallow-marine to fluvial strata of the IBR, containing sedimentary structures that reflect depositional environments correlative with the modern delta. Cyclical alternations of fine sands and silts in packages on the order of 15-20 cm thick define part of the shallow-marine section (M2 facies) that we interpret to represent the foreset beds of the ancient subaqueous delta. The overall scale and sedimentary structures of M2 compare favorably with modern foreset deposits in the Bay of Bengal. Tan-orange medium-grained, well sorted fluvial sandstone that contain large scale (1-10 m) tabular and trough cross bedding represent large-river channel deposits (F2 facies) that overlie the shallow marine strata. F2 deposits bear a striking resemblance in scale and character to bar deposits along the modern Jamuna River. Preliminary grain size analyses on the F2 facies yield grain size distributions that are remarkably consistent with Brahmaputra-sourced mid-Holocene sediments from Sylhet basin within the GBMD. Current research on the GBMD has revealed quantifiable trends in bed thicknesses, downstream fining, and grain size within fluvial deposits. These data will be coupled with ongoing structural and geo- and thermochronology field studies of the IBR that aim to continue to reveal the structural and stratigraphic evolution of this geologically active and densely populated region.

Timing of mid-crustal ductile extension in the northern Snake Range metamorphic core complex, Nevada: Evidence from U/Pb zircon ages

NASA Astrophysics Data System (ADS)

Lee, J.; Blackburn, T.; Johnston, S. M.

2016-12-01

Metamorphic core complexes (Mccs) within the western U.S. record a history of Cenozoic ductile and brittle extensional deformation, metamorphism, and magmatism, and exhumation within the footwall of high-angle Basin and Range normal faults. Documenting these histories within Mccs have been topics of research for over 40 years, yet there remains disagreement about: 1) whether the detachment fault formed and moved at low angles or initiated at high angles and rotated to a low angle; 2) whether brittle and ductile extensional deformation were linked in space and time; and 3) the temporal relationship of both modes of extension to the development of the detachment fault. The northern Snake Range metamorphic core complex (NSR), Nevada has been central to this debate. To address these issues, we report new U/Pb dates from zircon in deformed and undeformed rhyolite dikes emplaced into ductilely thinned and horizontally stretched lower plate rocks that provide tight bounds on the timing of ductile extension at between 38.2 ± 0.3 Ma and 22.50 ± 0.36 Ma. The maximum age constraint is from the Northern dike swarm (NDS), which was emplaced in the northwest part of the range pre- to syn-tectonic with ductile extension. The minimum age constraint is from the Silver Creek dike swarm (SDS) that was emplaced in the southern part of the range post ductile extensional deformation. Our field observations, petrography, and U/Pb zircon ages on the dikes combined with published data on the geology and kinematics of extension, moderate and low temperature thermochronology on lower plate rocks, and age and faulting histories of Cenozoic sedimentary basins adjacent to the NSR are interpreted as recording an episode of localized upper crustal brittle extension during the Eocene that drove upward ductile extensional flow of hot middle crustal rocks from beneath the NSR detachment soon after, or simultaneous with, emplacement of the NDS. Exhumation of the lower plate continued in a rolling hinge/isostatic rebound style; the western part of the lower plate was exhumed first and the eastern part extended ductilely either continuously or episodically until the early Miocene when the post-tectonic SDS was emplaced. Major brittle slip along the eastern part of the NSR detachment and along high angle normal faults exhumed the lower plate during middle Miocene.

Topography of the Betics: crustal thickening, dynamic topography and relief inheritance

NASA Astrophysics Data System (ADS)

Janowski, Marianne; Loget, Nicolas; Bellahsen, Nicolas; Husson, Laurent; Le Pourhiet, Laetitia; Meyer, Bertrand

2017-04-01

The main mechanism that explains high orogenic topographies is the isostatic adjustment due to crustal thickening. However in the Betic Cordillera (South Spain), the present-day elevation and crustal thickness are not correlated. That is at odds with the general premise of isostasy and requires reappraising the question of the driving mechanisms leading to the current topography. The Betics are located at the western edge of the alpine Mediterranean belt. Its Cenozoic orogenic building was disrupted by a major crustal thinning event induced by a slab rollback in the internal zones (Alboran domain) during Neogene. Topography was largely levelled and flooded by the sea during Neogene extension, and then has been folded since the Late Tortonian inversion. The present-day topography shows flat summits still preserved from fluvial regression in the internal zones (central and eastern Betics). These low-relief surfaces may be inherited from the Neogene planation toward sea-level as rocks cooling histories inferred from low-temperature thermochronology seem to point it out. Post-Tortonian shortening estimated thanks to a crustal-scale N-S cross-section in the eastern Betics (at the Sierra Nevada longitude) does not exceed few kilometers which is much lower than the shortening required by isostatic equilibrium, and is thus insufficient to explain the post-Tortonian topography building. We tested the hypothesis that mantle dynamics could in fact be an important mechanism that explains the topography of the Betics. We first computed the residual topography (i.e. the non-isostatic component of the elevation) using the most recent published Moho mapping of the area. In the western Betics, our results show important negative residual topography (down to -3 km) possibly associated with the west-Alboran slab suction. In the eastern Betics however, positive residual topography is important (up to +3 km) and can be explained by the dynamic mantle support of the topography, possibly associated to tearing of the Iberian slab. Finally, the timing of topographic rise is approached thanks to an analysis of river profiles. It reveals a regional transient stage of the topography rather compatible with a large-scale uplift. We conclude that mantle dynamics contribute substantially to the Late Neogene building and evolution of the topography in the Betics.

Recognition of hyper-extended rifted margin remnants in the internal zone of the Alpine belt: A tribute to Marco Beltrando

NASA Astrophysics Data System (ADS)

Mohn, Geoffroy; Manatschal, Gianreto

2016-04-01

Marco Beltrando was part of the young generation of Alpine geologists who challenged the interpretation of the Western Alps by combining a classical field approach and modern techniques (e.g. 40Ar/39Ar and (U-Th)/He thermochronology). His work provides the foundation to re-interpret some of the classical sections through the Alpine belt and may impact the way of thinking about the nature and structure of internal parts of collisional orogens. This contribution will present the main outcomes of the work of Marco Beltrando and their implications for the understanding of Alpine type orogens. Since his PhD, Marco Beltrando focused most of his work on the study of the internal parts of the Western Alps. He investigated in great details the complex, multiphase structural and metamorphic evolution of the Penninic units in the Western Alps. He concluded that these units went through several cycles of shortening and extension during the Alpine orogeny, with major implications for the Alps but also other orogenic belts. After his PhD, he focused his research on the pre-orogenic evolution of the Alpine belt. He first worked on the Petit St. Bernard area, where he identified relics of the former hyper-extended Tethyan rifted margin. Thanks to his work and his amazing knowledge of the Western Alps, he understood the potential importance of rift-inheritance in controlling the architecture and evolution of the Alpine belt. In parallel to the study of the orogenic evolution, he developed a new methodology to recognize rift-related lithostratigraphic units in highly deformed and metamorphosed parts of the Alps. His innovative work allowed a re-assessment of several areas in the Western Alps and demonstrates the importance of rift inheritance. Recently, he started a new research project on the evolution of the Southern Alps highlighting the importance of heating and cooling cycles resulting from complex successions of rifting events. In spite of his young age, Marco Beltrando was at the forefront of new techniques, ideas and concepts that ultimately will improve our understanding of mountain belts and rift systems. Eventually more than anything, his motivation and enthusiasm were a constant source of inspiration.

A Model for Earth's Mantle Dynamic History for The Last 500 Ma and Its Implications for Continental Vertical Motions and Geomagnetism

NASA Astrophysics Data System (ADS)

Zhong, S.; Olson, P.; Zhang, N.

2012-12-01

Seismic tomography studies indicate that the Earth's mantle structure is characterized by African and Pacific seismically slow velocity anomalies (i.e., thermochemical piles) and circum Pacific seismically fast anomalies (i.e., degree 2) in the lower mantle. Mantle convection calculations including plate motion history for the last 120 Ma suggest that these degree 2 thermochemical structures result from plate subduction history (e.g., McNamara and Zhong, 2005). Given the important controls of mantle structure and dynamics on surface tectonics and volcanism and geodynamo in the core, an important question is the long-term evolution of mantle structures, for example, was the mantle structure in the past similar to the present-day's degree 2 structure, or significantly different from the present day? To address this question, we constructed a proxy model of plate motions for the African hemisphere for the last 450 Ma using the paleogeographic reconstruction of continents constrained by paleomagnetic and geological observations (e.g., Pangea assembly and breakup). Coupled with assumed oceanic plate motions for the Pacific hemisphere before 120 Ma, this proxy model for the plate motion history is used in three dimensional spherical models of mantle convection to study the evolution of mantle structure since the Early Paleozoic. Our model calculations reproduce well the present day degree 2 mantle structure including the African and Pacific thermochemical piles, and present-day surface heat flux, bathymetry and dynamic topography. Our results suggest that while the mantle in the African hemisphere before the assembly of Pangea is dominated by the cold downwelling structure resulting from plate convergence between Gondwana and Laurussia, it is unlikely that the bulk of the African superplume structure can be formed before ˜230 Ma. Particularly, the last 120 Ma plate motion plays an important role in generating the African thermochemical pile. We reconstruct temporal evolution of the surface and CMB heat fluxes and continental vertical motions since the Paleozoic. The predicted vertical motion histories for the Slave and Kaapvaal cratons are consistent with those inferred from thermochronology studies. The predicted CMB heat fluxes were used as time-dependent boundary conditions for geodynamo simulations. And the geodynamo modelling shows that the time-dependent CMB heat fluxes may explain to the first order the frequencies of geomagnetic polarity reversals (e.g., superchrons).

Preliminary global paleogeographic maps through the Greenhouse-Icehouse transition: forcing of the Drake Passage and Asian Monsoons.

NASA Astrophysics Data System (ADS)

Poblete, Fernando; Dupont-Nivet, Guillaume; Licht, Alexis; van Hinsbergen, Douwe; Roperch, Pierrick; Guillocheau, Francois; Baby, Guillaume; Baatsen, Michiel

2017-04-01

Paleogeographic maps are essential for understanding Earth dynamics. They provide the necessary boundary conditions for climate and geodynamic modeling, surface processes and biotic interactions. In particular, the opening and closing of ocean gateways and the growth of major mountain belts are major drivers of climate changes and biotic interchange. However, the timing and spatial extent of such events are highly controversial and regularly questioned by new data. As part of the ERC "MAGIC" project focusing on Asian Monsoons during the Icehouse to Greenhouse transition we thus produced a set of worldwide Cenozoic paleogeographic maps in the period time between 60 to 20 Ma, with a set of boundary conditions specific to the India-Asia collision zone and the Drake Passage. The creation of a paleogeographic map followed a rigorous and reproductively methodology that integrates paleobathymetric, paleoshoreline and paleotopographic data into a coherent plate tectonic model using the open source software GPlates. (1) We use the model provided by Seton et al. (2012) as a first order tectonic model modified to integrate the full restoration of five regions: the Andes, the Scotia Arc, Africa, The Mediterranean Sea and the Tibet-Himalayan collision zone. (2) The paleobathymetry was provided by Müller et al. (2008) using age-depth relationships and assuming symmetric ridge spreading. (3) Paleoshoreline maps were modified according to the fossil database from fossilworks.org and the geological record and were used to represent the boundary between terrestrial and marine paleo-environments. (4) To reconstruct paleoelevations, the most controversial task, we compiled a wide range of data including stable isotope, leaf physiognomy, and thermochronology combined with regional fossil and geological records (tectonic setting) and geomorphological data. Finally, we use the open source GMT software and a set of masks to modify the current Earth relief model (ETOPO) according to the estimated paleoelevation for specific region at each period of time. Our approach specifically takes into account the evolution of continental margins. Paleotopographic evolution is coupled with the evolving shape of continents. Considering the constant addition of new data and models, the value of this method is to generate a progressive paleorelief model of the Earth that can be easily compared and updated with new data.

Thrust Belt Architecture of the Central and Southern Western Foothills of Taiwan

NASA Astrophysics Data System (ADS)

Rodriguez, F.; Wiltschko, D.

2006-12-01

A structural model of the central and southern Western Foothills Fold and Thrust Belt (WFFTB) was constructed from serial balanced cross sections using available surface, drill, seismic and thermochronologic data. The WFFTB is composed of four main thrust sheets with minor splays. On the east, the Tulungwan fault, which separates the sedimentary rocks of the WFFTB from the low grade meta-sediments of the Slate Belt, evolves from a basement cored fold in the north (around 24°10' N) where the conformable contact between foothills sediments and meta-sediments from the Slate Belt on its western flank is present. At this point the tip of the fault is below the unconformity and the displacement amount is small. To the south this fault breaks the back limb of the fold and gains displacement, and continues gaining displacement to the south. The next thrust sheet to the west includes the Schuantung, Fenghuangchan, Luku, Tatou, Hopiya, and Pingchi faults. This fault system is interpreted as characterized by a long flat with small ramps along a Miocene detachment, not a series of imbricates, as it has been interpreted before. The next thrust sheet to the west is the Chulungupu-Chukou-Lunhou, this system appears to gain displacement to the south as the Schuantung fault system decreases in amount of displacement. The Chulungpu-Chukou-Lunhou fault system contains a wide monocline in the central foothills related with the Chulungpu fault and two wide synclines in the southern part, the Yuching and Tinpligling synclines. Modeling of these two last structures shows that both are uplifted with respect to the regional level above a wide and flat feature; the footwall of the Lunhou fault is a monocline. A geometric solution to lift the Lunhou system involves a major fault-bend-fold anticline with a long ramp and a detachment at ~13 km of depth. It explains, 1) the frontal monocline, which is the from limb of this fault-bend- fold, 2) the minor structures associated with minor back-thrusts and wedging, and 3) the uplift of the structures above the regional level over a wide anticlinal crest. The last thrust system toward the west shows a series of structures which closely associated with the Peikang high implying that the structures are either inversion structures or new thrust faults whose ramps are located in pre-existing normal faults.

Late Cretaceous Localized Crustal Thickening as a Primary Control on the 3-D Architecture and Exhumation Histories of Cordilleran Metamorphic Core Complexes

NASA Astrophysics Data System (ADS)

Gans, P. B.; Wong, M.

2014-12-01

The juxtaposition of mylonitic mid-crustal rocks and faulted supracrustal rocks in metamorphic core complexes (MMCs) is usually portrayed in 2 dimensions and attributed to a single event of large-scale slip ± isostatic doming along a low-angle "detachment fault"/ shear zone. This paradigm does not explain dramatic along strike (3-D) variations in slip magnitude, footwall architecture, and burial / exhumation histories of most MMCs. A fundamental question posed by MMCs is how did their earlier thickening and exhumation histories influence the geometric evolution and 3-D slip distribution on the subsequent detachment faults? New geologic mapping and 40Ar/39Ar thermochronology from the Snake Range-Kern Mts-Deep Creek Mts (SKDC) complex in eastern Nevada offer important insights into this question. Crustal shortening and thickening by large-scale non-cylindrical recumbent folds and associated thrust faults during the late Cretaceous (90-80 Ma) resulted in deep burial (650°C, 20-25 km) of the central part of the footwall, but metamorphic grade decreases dramatically to the N and S in concert with decreasing amplitude on the shortening structures. Subsequent Paleogene extensional exhumation by normal faulting and ESE-directed mylonitic shearing is greatest in areas of maximum earlier thickening and brought highest grade rocks back to depths of~10-12 km. After ≥15 Ma of quiescence, rapid E-directed slip initiated along the brittle Miocene Snake Range detachment at 20 Ma and reactivated the Eocene shear zone. The ≥200°C gradient across the footwall at this time implies that the Miocene slip surface originated as a moderately E-dipping normal fault. This Miocene slip surface can be tracked for more than 100 km along strike, but the greatest amount of Miocene slip also coincides with parts of the footwall that were most deeply buried in the Cretaceous. These relations indicate that not only is the SKDC MMC a composite feature, but that the crustal welt created by early thickening played a fundamental role in controlling the slip distribution on subsequent extensional structures and is still evident in the high modern surface elevations of the portions of the footwall what were most deeply buried.

Why do multi-attribute utility instruments produce different utilities: the relative importance of the descriptive systems, scale and 'micro-utility' effects.

PubMed

Richardson, Jeff; Iezzi, Angelo; Khan, Munir A

2015-08-01

Health state utilities measured by the major multi-attribute utility instruments differ. Understanding the reasons for this is important for the choice of instrument and for research designed to reconcile these differences. This paper investigates these reasons by explaining pairwise differences between utilities derived from six multi-attribute utility instruments in terms of (1) their implicit measurement scales; (2) the structure of their descriptive systems; and (3) 'micro-utility effects', scale-adjusted differences attributable to their utility formula. The EQ-5D-5L, SF-6D, HUI 3, 15D and AQoL-8D were administered to 8,019 individuals. Utilities and unweighted values were calculated using each instrument. Scale effects were determined by the linear relationship between utilities, the effect of the descriptive system by comparison of scale-adjusted values and 'micro-utility effects' by the unexplained difference between utilities and values. Overall, 66 % of the differences between utilities was attributable to the descriptive systems, 30.3 % to scale effects and 3.7 % to micro-utility effects. Results imply that the revision of utility algorithms will not reconcile differences between instruments. The dominating importance of the descriptive system highlights the need for researchers to select the instrument most capable of describing the health states relevant for a study. Reconciliation of inconsistent utilities produced by different instruments must focus primarily upon the content of the descriptive system. Utility weights primarily determine the measurement scale. Other differences, attributable to utility formula, are comparatively unimportant.

Scoping study of integrated resource planning needs in the public utility sector

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garrick, C J; Garrick, J M; Rue, D R

Integrated resource planning (IRP) is an approach to utility resource planning that integrates the evaluation of supply- and demand-site options for providing energy services at the least cost. Many utilities practice IRP; however, most studies about IRP focus on investor-owned utilities (IOUs). This scoping study investigates the IRP activities and needs of public utilities (not-for-profit utilities, including federal, state, municipal, and cooperative utilities). This study (1) profiles IRP-related characteristics of the public utility sector, (2) articulates the needs of public utilities in understanding and implementing IRP, and (3) identifies strategies to advance IRP principles in public utility planning.

Long-term landscape evolution of the South Atlantic passive continental margin along the Kaoko- and Damara Belts, NW-Namibia

NASA Astrophysics Data System (ADS)

Menges, Daniel; Glasmacher, Ulrich Anton; Hackspacher, Peter; Schneider, Gabriele; Salomon, Eric

2015-04-01

The Kaoko Belt in northwestern Namibia originates in the collision of the Rio de la Plata and Kongo Craton during the Pan-African Orogeny in the Neoproterozoic (1) and represents the northern arm of the Damara Orogen. NW-Namibias continental crust mainly consists of the NE-SW striking intracontinental branch of the Pan-African Damara mobile belt, which separates the Congo from the Kalahari craton. The Damara Orogen is divided into several tectonostratigraphic zones that are bounded by steeply dipping, ductile shear zones. These regional lineaments can be traced at least 150 km offshore (2). The lithostratigraphic units consist of Proterozoic and Cambrian metamorphosed rocks (534 (7) Ma - 481 (25) Ma (3) as well as Mesozoic sedimentary and igneous rocks. From Permo-Carboniferous to Mid Jurassic northern Namibia was affected by deep erosion of the Damara Orogen, Permo-Triassic collisional processes along the southern margin of Gondwana and eastern margin of Africa (4), and the deposition of the Nama Group sediments and the Karoo megasequence (5). Between the Otjihorongo and the Omaruru Lineament-Waterberg Thrust early Mesozoic tectonic activity is recorded by coarse clastic sediments deposited within NE trending half-graben structures. The Early Jurassic Karoo flood basalt lavas erupted rapidly at 183±1 Ma (6). The Early Cretaceous Paraná-Etendeka flood basalts (132±1 Ma) and mafic dike swarms mark the rift stage of the opening of the South Atlantic (7). Early Cretaceous alkaline intrusions (137-124 Ma) occur preferentially along Mesozoic half-graben structures and are called the Damaraland Igneous Province (8). Late Cretaceous alkaline intrusions and kimberlite pipes occur in northern Namibia. Post Early Paleocene siliciclastic sedimentation in Namibia was largely restricted to a 150 km wide zone (9) and is represented by the Tsondab Sandstone Formation (~ 300 m thickness). The oldest part has an age of early Paleocene and the upper part span from middle Miocene (~13 Ma) to Pliocene (~2 Ma) (10). Cenozoic alkaline intrusions and kimberlite pipes are also known from the region. The so-called "Great Escarpment" that reach elevation of up to 2350 m characterizes strongly the morphology of the passive continental margin in Namibia (11,12). In contrast to Brazil, the escarpment is more than 150 km inland of Namibia. Interesting enough the Brandenberg intrusive complex of ~130 Ma age clearly indicates the post-intrusion denudation of more than 4,000m (13). The Great Escarpment can be traced from central Angola to the eastern edge of South Africa. A considerable variation along its distribution reflects variations in tectonic history, in lithologies, and in the drainage system. In Namibia, the retreating model has dominated the genetic discussion (14,15,16). However, surface process modeling has suggested other possibilities11. In addition, apatite fission-track research, terrigenious cosmogenic nuclides (TCN) have been used on specific landscape elements to determine denudation rates. In the central Namib Desert, denudation rates calculated from 10Be and 26Al are in the range of ±5 m Ma-1 and might be representative for the last 103 - 106 a (17). The persistence of arid climatic conditions throughout the Cenozoic might even lead to such low denudation rates for the past 10-12 Ma. A low retreat rate of ~10 m Ma-1 representative for the last 1 Ma was determined for the Great Escarpment in central and southern Namibia. Considering all currently, available thermochronological data for the Namibian margin (18,19,20), the validity of the scarp retreat model is highly problematic. Apatite fission-track ages revealed so far range between 390.9±17.9 Ma and 80.8±6.0 Ma. The large spread in ages is partly related to significant changes of ages at the NW-SE trending Purros Lineament and at the Sesfontein thrust. In general, the AFT-ages are older northeast of the Purros Lineament. Furthermore, all basalt samples of Etendeka age display the same AFT-age range within error, between 103.5±4.9 and 108.0±5.6 Ma. The oldest ages are revealed from metamorphic rocks of the Damara Group as well as sandstones and glacial deposits of the Permo-Carboniferous Karoo series. References 1. Goscombe, B. D., Gray, D. R., 2008. Structure and strain variation at mid-crustal levels in a transpressional orogen: A review of Kaoko Belt structure and the character of west Gondwana amalgamation and dispersal. Gondwana Res. 13, 45-85. 2. Clemson, J., Cartwright, J., Booth, J., 1997. Structural segmentation and the influence of basement structure on the Namibian passive margin. J. Geol. Soc. London 154, 477-482. 3. Miller, R.M., 1983. Evolution of the Damara Orogen, Vol. 11, Geol. Soc., South Africa Spec. Pub.. 4. Coward, M.P., Daly, M.C., 1984. Crustal lineaments and shear zones in Africa: Their relationships to plate movements, Precambrian Research 24: 27-45. 5. Stollhofen, H., 1999. Karoo Synrift-Sedimentation und ihre tektonische Kontrolle am entstehenden Kontinentalrand Namibias, Z.dt.geol.Ges. 149: 519-632. 6. Duncan, R., Hooper, P., Rehacek, J., March, J., Duncan, A., 1997. The timing and duration of the Karoo igneous event, southern Gondwana, J. Geophy. Res. 102: 18127-18138. 7. Renne, P.R., Glen, J.M., Milner, S.C., Duncan, A.R., 1996. Age of Etendeka flood volcanism and associated intrusions in southwestern Africa, Geology 24 (7): 659- 662. 8. Watkins, R.T., McDougall, I., le Roex, A. P., 1994. K-Ar ages of the Brandberg and Okenenya igneous complexes, north-western Namibia, Geol. Rund. 83: 348-356. 9. Ward, J.D., 1988. Geology of the Tsondab Sandstone Formation, Journal of Sedimentary Geology 55: 143-162. 10. Senut, B., Pickford, M., 1995. Fossil eggs and Cenozoic continental biostratigraphy of Namibia, Pal. Afr.,32: 33-37. 11. Gilchrist, A.R., Kooi, H., Beaumont, C.,1994. Post Gondwana geomorphic evolution of southwestern Africa: Implications for the controls on landscape development from observations and numerical experiments, J. Geophy. Res. 99: 12211-12228. 12. Brown, R. W., Gallagher, K. and Gleadow, A. J. W., 2000. Morphotectonic evolution of the South Atlantic margins of Africa and South America, in M. A. Summerfield (ed.), Geomorphology and Global Tectonics, JohnWiley and Sons Ltd., Chichester, pp. 255-281. 13. Raab, M. J., Brown, R. W., Gallagher, K., Weber, K., Gleadow, A. J. W., 2005. Denudational and thermal history of the Early Cretaceous Brandberg and Okenyenya igneous complexes on Namibia's Atlantic passive margin Tectonics 24: 1-15. 14. Guillocheau, F., Rouby, D., Robin, C. Helm, C., Rolland, N., Le Carlier de Veslud, C., Braun, J., 2012. Quantification and causes of the terrigeneous sediment budget at the scale of a continent margin: a new method applied to the Namibia-South Africa Margin. BasinRes. 24, 3-30. 15. Dauteuil, O., Rouby, D., Braun, J., Guillocheau, F., Deschamps, F., 2013. Post-breakup evolution of the margin of Namibia: constraints from numerical approach. Tectonophysics 604, 122-138. 16. Rouby, D., Braun, J., Dauteuil, O., Deschamps, F., Robin, C., 2013. Long-term stratigraphic evolution of Atlantic-type passive margins: a numerical approach of interactions between surface processes, flexural isostasy and 3D thermal subsidence. Tectonophysics 604, 83-103. 17. Cockburn, H. A. P., Brown, R. W., Summerfield, M. A. and Seidl, M. A., 2000. Quantifying passive margin denudation and landscape development using a combined fission-track thermochronology and cosmogenic isotope analysis approach, EPSL 179: 429-435. 18. Brown, R. W., 1992. A fission track thermochronology study of the tectonic and geomorphic development of the sub-aerial continental margins of southern Africa., PhD thesis, La Trobe University, Bundoora, Australia. 19. Gallagher, K. and Brown, R. W., 1999. Denudation and uplift at passive margins: the record on the Atlantic Margin of southern Africa, Philosophical Transactions Royal Society London A 357: 835-859. 20. Raab, M. J., Brown, R. W., Gallagher, K., Carter, A., Weber, K., 2002. Late Cretaceous reactivation of major crustal shear zones in northern Namibia: constraints from apatite fission track analysis. Tectonophysics 349: 75-92.

24 CFR 245.416 - Initial submission of materials to HUD: Conversion from project-paid utilities to tenant-paid...

Code of Federal Regulations, 2010 CFR

2010-04-01

... HUD: Conversion from project-paid utilities to tenant-paid utilities or a reduction in tenant utility... Covered Action § 245.416 Initial submission of materials to HUD: Conversion from project-paid utilities to tenant-paid utilities or a reduction in tenant utility allowances. In the case of a conversion from...

Optimal Reward Functions in Distributed Reinforcement Learning

NASA Technical Reports Server (NTRS)

Wolpert, David H.; Tumer, Kagan

2000-01-01

We consider the design of multi-agent systems so as to optimize an overall world utility function when (1) those systems lack centralized communication and control, and (2) each agents runs a distinct Reinforcement Learning (RL) algorithm. A crucial issue in such design problems is to initialize/update each agent's private utility function, so as to induce best possible world utility. Traditional 'team game' solutions to this problem sidestep this issue and simply assign to each agent the world utility as its private utility function. In previous work we used the 'Collective Intelligence' framework to derive a better choice of private utility functions, one that results in world utility performance up to orders of magnitude superior to that ensuing from use of the team game utility. In this paper we extend these results. We derive the general class of private utility functions that both are easy for the individual agents to learn and that, if learned well, result in high world utility. We demonstrate experimentally that using these new utility functions can result in significantly improved performance over that of our previously proposed utility, over and above that previous utility's superiority to the conventional team game utility.

Impact of the residential conservation service program on natural gas and electric utilities. Appendix B

DOE Office of Scientific and Technical Information (OSTI.GOV)

Not Available

An assessment of the RCS program is provided from the perspective of eleven case study utilities. First, an introduction to the role and value of conservation in utility strategic planning is presented. The interplay of various utility system characteristics is shown to be the primary determinant of the value of conservation efforts from the utility's point of view. Simplified typologies of utilities are developed to aid in the identification of those combinations of the utility characteristics that will favor the adoption of the utility sponsored strategic conservation efforts. The integration of the RCS program with other utility conservation and loadmore » management programs is explored for the eleven case study utilities. Reference is made to the simplified typologies in order to show, through actual program experience, how the strategic position of the utility company affects its adoption of this federal program. Evaluative studies done by the eleven case study utilities of the RCS program are reviewed. Results are presented and the methodologies are critiqued. Conclusions regarding the RCS program from the utility perspective are presented.« less

Personal utility in genomic testing: is there such a thing?

PubMed

Bunnik, Eline M; Janssens, A Cecile J W; Schermer, Maartje H N

2015-04-01

In ethical and regulatory discussions on new applications of genomic testing technologies, the notion of 'personal utility' has been mentioned repeatedly. It has been used to justify direct access to commercially offered genomic testing or feedback of individual research results to research or biobank participants. Sometimes research participants or consumers claim a right to genomic information with an appeal to personal utility. As of yet, no systematic account of the umbrella notion of personal utility has been given. This paper offers a definition of personal utility that places it in the middle of the spectrum between clinical utility and personal perceptions of utility, and that acknowledges its normative charge. The paper discusses two perspectives on personal utility, the healthcare perspective and the consumer perspective, and argues that these are too narrow and too wide, respectively. Instead, it proposes a normative definition of personal utility that postulates information and potential use as necessary conditions of utility. This definition entails that perceived utility does not equal personal utility, and that expert judgment may be necessary to help determine whether a genomic test can have personal utility for someone. Two examples of genomic tests are presented to illustrate the discrepancies between perceived utility and our proposed definition of personal utility. The paper concludes that while there is room for the notion of personal utility in the ethical evaluation and regulation of genomic tests, the justificatory role of personal utility is not unlimited. For in the absence of clinical validity and reasonable potential use of information, there is no personal utility. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.

NNP-LANL Utilities - Condition Assessment and Project Approach

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stewart, Grant Lorenz

2017-11-21

This report is a presentation on LANL Utilities & Transportation Asset Management; Utility Assets Overview; Condition Assessment; Utilities Project Nominations & Ranking; and Utilities Project Execution.

Cpp Utility - Version 1.0

DOE Office of Scientific and Technical Information (OSTI.GOV)

III, FredOppel; Rigdon, J. Brian

2014-09-08

A collection of general Umbra modules that are reused by other Umbra libraries. These capabilities include line segments, file utilities, color utilities, string utilities (for std::string), list utilities (for std ::vector ), bounding box intersections, range limiters, simple filters, cubic roots solvers and a few other utilities.

Dopamine Reward Prediction Error Responses Reflect Marginal Utility

PubMed Central

Stauffer, William R.; Lak, Armin; Schultz, Wolfram

2014-01-01

Summary Background Optimal choices require an accurate neuronal representation of economic value. In economics, utility functions are mathematical representations of subjective value that can be constructed from choices under risk. Utility usually exhibits a nonlinear relationship to physical reward value that corresponds to risk attitudes and reflects the increasing or decreasing marginal utility obtained with each additional unit of reward. Accordingly, neuronal reward responses coding utility should robustly reflect this nonlinearity. Results In two monkeys, we measured utility as a function of physical reward value from meaningful choices under risk (that adhered to first- and second-order stochastic dominance). The resulting nonlinear utility functions predicted the certainty equivalents for new gambles, indicating that the functions’ shapes were meaningful. The monkeys were risk seeking (convex utility function) for low reward and risk avoiding (concave utility function) with higher amounts. Critically, the dopamine prediction error responses at the time of reward itself reflected the nonlinear utility functions measured at the time of choices. In particular, the reward response magnitude depended on the first derivative of the utility function and thus reflected the marginal utility. Furthermore, dopamine responses recorded outside of the task reflected the marginal utility of unpredicted reward. Accordingly, these responses were sufficient to train reinforcement learning models to predict the behaviorally defined expected utility of gambles. Conclusions These data suggest a neuronal manifestation of marginal utility in dopamine neurons and indicate a common neuronal basis for fundamental explanatory constructs in animal learning theory (prediction error) and economic decision theory (marginal utility). PMID:25283778

The predictive validity of prospect theory versus expected utility in health utility measurement.

PubMed

Abellan-Perpiñan, Jose Maria; Bleichrodt, Han; Pinto-Prades, Jose Luis

2009-12-01

Most health care evaluations today still assume expected utility even though the descriptive deficiencies of expected utility are well known. Prospect theory is the dominant descriptive alternative for expected utility. This paper tests whether prospect theory leads to better health evaluations than expected utility. The approach is purely descriptive: we explore how simple measurements together with prospect theory and expected utility predict choices and rankings between more complex stimuli. For decisions involving risk prospect theory is significantly more consistent with rankings and choices than expected utility. This conclusion no longer holds when we use prospect theory utilities and expected utilities to predict intertemporal decisions. The latter finding cautions against the common assumption in health economics that health state utilities are transferable across decision contexts. Our results suggest that the standard gamble and algorithms based on, should not be used to value health.

The impact of differences in EQ-5D and SF-6D utility scores on the acceptability of cost-utility ratios: results across five trial-based cost-utility studies.

PubMed

Joore, Manuela; Brunenberg, Danielle; Nelemans, Patricia; Wouters, Emiel; Kuijpers, Petra; Honig, Adriaan; Willems, Danielle; de Leeuw, Peter; Severens, Johan; Boonen, Annelies

2010-01-01

This article investigates whether differences in utility scores based on the EQ-5D and the SF-6D have impact on the incremental cost-utility ratios in five distinct patient groups. We used five empirical data sets of trial-based cost-utility studies that included patients with different disease conditions and severity (musculoskeletal disease, cardiovascular pulmonary disease, and psychological disorders) to calculate differences in quality-adjusted life-years (QALYs) based on EQ-5D and SF-6D utility scores. We compared incremental QALYs, incremental cost-utility ratios, and the probability that the incremental cost-utility ratio was acceptable within and across the data sets. We observed small differences in incremental QALYs, but large differences in the incremental cost-utility ratios and in the probability that these ratios were acceptable at a given threshold, in the majority of the presented cost-utility analyses. More specifically, in the patient groups with relatively mild health conditions the probability of acceptance of the incremental cost-utility ratio was considerably larger when using the EQ-5D to estimate utility. While in the patient groups with worse health conditions the probability of acceptance of the incremental cost-utility ratio was considerably larger when using the SF-6D to estimate utility. Much of the appeal in using QALYs as measure of effectiveness in economic evaluations is in the comparability across conditions and interventions. The incomparability of the results of cost-utility analyses using different instruments to estimate a single index value for health severely undermines this aspect and reduces the credibility of the use of incremental cost-utility ratios for decision-making.

Dopamine reward prediction error responses reflect marginal utility.

PubMed

Stauffer, William R; Lak, Armin; Schultz, Wolfram

2014-11-03

Optimal choices require an accurate neuronal representation of economic value. In economics, utility functions are mathematical representations of subjective value that can be constructed from choices under risk. Utility usually exhibits a nonlinear relationship to physical reward value that corresponds to risk attitudes and reflects the increasing or decreasing marginal utility obtained with each additional unit of reward. Accordingly, neuronal reward responses coding utility should robustly reflect this nonlinearity. In two monkeys, we measured utility as a function of physical reward value from meaningful choices under risk (that adhered to first- and second-order stochastic dominance). The resulting nonlinear utility functions predicted the certainty equivalents for new gambles, indicating that the functions' shapes were meaningful. The monkeys were risk seeking (convex utility function) for low reward and risk avoiding (concave utility function) with higher amounts. Critically, the dopamine prediction error responses at the time of reward itself reflected the nonlinear utility functions measured at the time of choices. In particular, the reward response magnitude depended on the first derivative of the utility function and thus reflected the marginal utility. Furthermore, dopamine responses recorded outside of the task reflected the marginal utility of unpredicted reward. Accordingly, these responses were sufficient to train reinforcement learning models to predict the behaviorally defined expected utility of gambles. These data suggest a neuronal manifestation of marginal utility in dopamine neurons and indicate a common neuronal basis for fundamental explanatory constructs in animal learning theory (prediction error) and economic decision theory (marginal utility). Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.

Utility usage forecasting

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hosking, Jonathan R. M.; Natarajan, Ramesh

The computer creates a utility demand forecast model for weather parameters by receiving a plurality of utility parameter values, wherein each received utility parameter value corresponds to a weather parameter value. Determining that a range of weather parameter values lacks a sufficient amount of corresponding received utility parameter values. Determining one or more utility parameter values that corresponds to the range of weather parameter values. Creating a model which correlates the received and the determined utility parameter values with the corresponding weather parameters values.

Health State Utility Values for Age-Related Macular Degeneration: Review and Advice.

PubMed

Butt, Thomas; Tufail, Adnan; Rubin, Gary

2017-02-01

Health state utility values are a major source of uncertainty in economic evaluations of interventions for age-related macular degeneration (AMD). This review identifies and critiques published utility values and methods for eliciting de novo utility values in AMD. We describe how utility values have been used in healthcare decision making and provide guidance on the choice of utility values for future economic evaluations for AMD. Literature was searched using PubMed, and health technology assessments (HTA) were searched using HTA agency websites to identify articles reporting utility values or approaches to derive utility values in AMD and articles applying utilities for use in healthcare decision making relating to treatments for AMD. A total of 70 studies qualified for data extraction, 22 of which were classified as containing utility values and/or elicitation methods, and 48 were classified as using utility values in decision making. A large number of studies have elicited utility values for AMD, although those applied to decision making have focused on a few of these. There is an appreciation of the challenges in the measurement and valuation of health states, with recent studies addressing challenges such as the insensitivity of generic health-related quality of life (HRQoL) questionnaires and utility in the worse-seeing eye. We would encourage careful consideration when choosing utility values in decision making and an explicit critique of their applicability to the decision problem.

17 CFR 250.41 - Exemption of public utility subsidiaries with respect to limited acquisition of utility assets.

Code of Federal Regulations, 2010 CFR

2010-04-01

... Exemption of public utility subsidiaries with respect to limited acquisition of utility assets. Any public... derived from its operations as a public-utility company during the preceding calendar year. (d... 17 Commodity and Securities Exchanges 3 2010-04-01 2010-04-01 false Exemption of public utility...

Characteristics of utility cyclists in Queensland, Australia: an examination of the associations between individual, social, and environmental factors and utility cycling.

PubMed

Sahlqvist, Shannon L; Heesch, Kristiann C

2012-08-01

Initiatives to promote utility cycling in countries like Australia and the US, which have low rates of utility cycling, may be more effective if they first target recreational cyclists. This study aimed to describe patterns of utility cycling and examine its correlates, among cyclists in Queensland, Australia. An online survey was administered to adult members of a state-based cycling community and advocacy group (n=1813). The survey asked about demographic characteristics and cycling behavior, motivators and constraints. Utility cycling patterns were described, and logistic regression modeling was used to examine associations between utility cycling and other variables. Forty-seven percent of respondents reported utility cycling: most did so to commute (86%). Most journeys (83%) were >5 km. Being male, younger, employed full-time, or university-educated increased the likelihood of utility cycling (P<.05). Perceiving cycling to be a cheap or a convenient form of transport was associated with utility cycling (P<.05). The moderate rate of utility cycling among recreational cyclists highlights a potential to promote utility cycling among this group. To increase utility cycling, strategies should target female and older recreational cyclists and focus on making cycling a cheap and convenient mode of transport.

Utility investigation best practices and effects on TxDOT highway improvement projects.

DOT National Transportation Integrated Search

2013-04-01

The lack of adequate information about the location and characteristics of utility facilities can result in a number of problems, including damages to utilities, disruptions to utility services and traffic, lost utility facilities as constructi...

Examining related influential factors for dental calculus scaling utilization among people with disabilities in Taiwan, a nationwide population-based study.

PubMed

Lai, Hsien-Tang; Kung, Pei-Tseng; Su, Hsun-Pi; Tsai, Wen-Chen

2014-09-01

Limited studies with large samples have been conducted on the utilization of dental calculus scaling among people with physical or mental disabilities. This study aimed to investigate the utilization of dental calculus scaling among the national disabled population. This study analyzed the utilization of dental calculus scaling among the disabled people, using the nationwide data between 2006 and 2008. Descriptive analysis and logistic regression were performed to analyze related influential factors for dental calculus scaling utilization. The dental calculus scaling utilization rate among people with physical or mental disabilities was 16.39%, and the annual utilization frequency was 0.2 times. Utilization rate was higher among the female and non-aboriginal samples. Utilization rate decreased with increased age and disability severity while utilization rate increased with income, education level, urbanization of residential area and number of chronic illnesses. Related influential factors for dental calculus scaling utilization rate were gender, age, ethnicity (aboriginal or non-aboriginal), education level, urbanization of residence area, income, catastrophic illnesses, chronic illnesses, disability types, and disability severity significantly influenced the dental calculus scaling utilization rate. Copyright © 2014 Elsevier Ltd. All rights reserved.

Decision Utility, Incentive Salience, and Cue-Triggered "Wanting"

PubMed

Berridge, Kent C; Aldridge, J Wayne

2009-01-01

This chapter examines brain mechanisms of reward utility operating at particular decision moments in life-moments such as when one encounters an image, sound, scent, or other cue associated in the past with a particular reward or perhaps just when one vividly imagines that cue. Such a cue can often trigger a sudden motivational urge to pursue its reward and sometimes a decision to do so. Drawing on a utility taxonomy that distinguishes among subtypes of reward utility-predicted utility, decision utility, experienced utility, and remembered utility-it is shown how cue-triggered cravings, such as an addict's surrender to relapse, can hang on special transformations by brain mesolimbic systems of one utility subtype, namely, decision utility. The chapter focuses on a particular form of decision utility called incentive salience, a type of "wanting" for rewards that is amplified by brain mesolimbic systems. Sudden peaks of intensity of incentive salience, caused by neurobiological mechanisms, can elevate the decision utility of a particular reward at the moment its cue occurs. An understanding of what happens at such moments leads to a better understanding of the mechanisms at work in decision making in general.

The utility score of epilepsy with partial seizure measured by TTO, VAS, and EQ-5D in the general Korean population.

PubMed

Kang, Hee-Jin; Kang, Eunjeong; Jo, Min-Woo; Park, Eun-Ja; Yoon, Seonyoung; Lee, Eui-Kyung

2014-07-01

This study aimed to measure utilities, which are quantitative terms incorporating preferences, for various health states of epilepsy with partial seizure in the general population in South Korea. It also aimed to find socio-demographic characteristics associated with the utility scores. Utility scores using Time Trade-Off (TTO), Visual Analog Scale (VAS), and EuroQol five Dimension (EQ-5D) were obtained from 300 people aged 16 and over by face-to-face interviews. We measured utilities for three hypothetical health states of epilepsy for which scenarios were defined based on the frequency of partial seizure: seizure-free, seizure reduction, and withdrawal. We compared utilities with varying seizure frequency using a repeated-measures ANOVA, and analyzed the association between utilities and socio-demographic characteristics using a generalized estimating equation (GEE). The mean utility scores for withdrawal state, seizure reduction state, and seizure-free state were 0.303, 0.493, and 0.899, respectively, when measured by TTO. VAS yielded the mean utility scores of 0.211, 0.424, and 0.752 for respective health states, and corresponding scores with EQ-5D were 0.261, 0.645, and 0.959. The utility scores for the three health states were statistically different in TTO, VAS, and EQ-5D. The withdrawal state had the lowest utility scores. There were differences in mean utilities for the three health states across the three methods. Utilities by EQ-5D tended to have higher values than those by TTO and VAS. Utilities by VAS had the lowest values. In GEE analysis, the severity of epilepsy and household income were significantly related to utility scores. The withdrawal state of epilepsy had the lowest utility value and the seizure-free state had the highest by all three techniques of utility measurement used. There were significant differences in utilities between one severity level of epilepsy and another. Utility was associated with household income and the severity of disease. Utility scores for distinct epilepsy states obtained in this study could facilitate health economic analyses of epilepsy treatments and thus help decision making in resource allocation. Copyright © 2014 Elsevier B.V. All rights reserved.

77 FR 46987 - Utility Allowances Submetering

Federal Register 2010, 2011, 2012, 2013, 2014

2012-08-07

... Utility Allowances Submetering AGENCY: Internal Revenue Service (IRS), Treasury. ACTION: Notice of... amend the utility allowance regulations concerning the low-income housing tax credit. The proposed regulations update the utility allowance regulations to clarify that utility costs paid by a tenant based on...

The causal structure of utility conditionals.

PubMed

Bonnefon, Jean-François; Sloman, Steven A

2013-01-01

The psychology of reasoning is increasingly considering agents' values and preferences, achieving greater integration with judgment and decision making, social cognition, and moral reasoning. Some of this research investigates utility conditionals, ''if p then q'' statements where the realization of p or q or both is valued by some agents. Various approaches to utility conditionals share the assumption that reasoners make inferences from utility conditionals based on the comparison between the utility of p and the expected utility of q. This article introduces a new parameter in this analysis, the underlying causal structure of the conditional. Four experiments showed that causal structure moderated utility-informed conditional reasoning. These inferences were strongly invited when the underlying structure of the conditional was causal, and significantly less so when the underlying structure of the conditional was diagnostic. This asymmetry was only observed for conditionals in which the utility of q was clear, and disappeared when the utility of q was unclear. Thus, an adequate account of utility-informed inferences conditional reasoning requires three components: utility, probability, and causal structure. Copyright © 2012 Cognitive Science Society, Inc.

Capital Improvement Program Environmental Assessment, Dyess Air Force Base, Texas

DTIC Science & Technology

2010-09-01

utility extension structures would be designed to avoid crossing wetlands to the extent feasible. If a project necessitates a wetlands utility... crossing , horizontal boring techniques would be used to route the utility under the wetlands. Because horizontal boring effectively avoids disturbance...utilized if the utility extension would involve crossing a roadway. • All new utility equipment would be polychlorinated biphenyl (PCB) free

Measuring change in health status of older adults at the population level: the transition probability model.

PubMed

Moineddin, Rahim; Nie, Jason X; Wang, Li; Tracy, C Shawn; Upshur, Ross E G

2010-11-09

The current demographic transition will lead to increasing demands on health services. However, debate exists as to the role age plays relative to co-morbidity in terms of health services utilization. While age has been identified as a critical factor in health services utilization, health services utilization is not simply an outcome of ill health, nor is it an inevitable outcome of aging. Most data on health service utilization studies assess utilization at one point in time, and does not examine transitions in health service utilization. We sought to measure health services utilization and to investigate patterns in the transition of levels of utilization and outcomes associated with different levels of utilization. We conducted a population-based retrospective cohort study of all Ontario residents aged 65+ eligible for public healthcare coverage from January 1998-December 2006. The main outcome measure was total number of utilization events. The total is computed by summing, on a per annum basis, the number of family physician visits, specialist visits, Emergency Department visits, drug claims, lab claims, X-rays, CT scans, MRI scans, and inpatient admissions. Three categories of utilization were created: low, moderate, and high. There is heterogeneity in health services utilization across the late lifespan. Utilization increased consistently in the 9-year study period. The probability of remaining at the high utilization category when the person was in the high category the previous year was more than 0.70 for both males and females and for all age groups. Overall healthcare utilization increases more rapidly among the high users compared to the low users. There was negligible probability for moving from high to low utilization category. Probability of death increased exponentially as age increased. Older adults in the low utilization category had the lowest probability of death. The number of male nonagenarians increased more rapidly than female nonagenarians. There are measurable and identifiable differences in the patterns of health services utilization among older adults. This data will permit clinicians and policy makers to tailor interventions appropriate to the risk class of patients.

Multiple utility constrained multi-objective programs using Bayesian theory

NASA Astrophysics Data System (ADS)

Abbasian, Pooneh; Mahdavi-Amiri, Nezam; Fazlollahtabar, Hamed

2018-03-01

A utility function is an important tool for representing a DM's preference. We adjoin utility functions to multi-objective optimization problems. In current studies, usually one utility function is used for each objective function. Situations may arise for a goal to have multiple utility functions. Here, we consider a constrained multi-objective problem with each objective having multiple utility functions. We induce the probability of the utilities for each objective function using Bayesian theory. Illustrative examples considering dependence and independence of variables are worked through to demonstrate the usefulness of the proposed model.

41 CFR 109-39.301 - Utilization guidelines.

Code of Federal Regulations, 2010 CFR

2010-07-01

... 41 Public Contracts and Property Management 3 2010-07-01 2010-07-01 false Utilization guidelines... Management System Vehicles § 109-39.301 Utilization guidelines. DOE activities utilizing GSA IFMS motor... meeting DOE utilization guidelines or established local use objectives, as appropriate. Those vehicles not...

The Tjellefonna fault system of Western Norway: Linking late-Caledonian extension, post-Caledonian normal faulting, and Tertiary rock column uplift with the landslide-generated tsunami event of 1756

NASA Astrophysics Data System (ADS)

Redfield, T. F.; Osmundsen, P. T.

2009-09-01

On February 22, 1756, approximately 15.7 million cubic meters of bedrock were catastrophically released as a giant rockslide into the Langfjorden. Subsequently, three ˜ 40 meter high tsunami waves overwhelmed the village of Tjelle and several other local communities. Inherited structures had isolated a compartment in the hanging wall damage zone of the fjord-dwelling Tjellefonna fault. Because the region is seismically active in oblique-normal mode, and in accordance with scant historical sources, we speculate that an earthquake on a nearby fault may have caused the already-weakened Tjelle hillside to fail. From interpretation of structural, geomorphic, and thermo-chronological data we suggest that today's escarpment topography of Møre og Trøndelag is controlled to a first order by post-rift reactivation of faults parallel to the Mesozoic passive margin. In turn, a number of these faults reactivated Late Caledonian or early post-Caledonian fabrics. Normal-sense reactivation of inherited structures along much of coastal Norway suggests that a structural link exists between the processes that destroy today's mountains and those that created them. The Paleozoic Møre-Trøndelag Fault Complex was reactivated as a normal fault during the Mesozoic and, probably, throughout the Cenozoic until the present day. Its NE-SW trending strands crop out between the coast and the base of a c. 1.7 km high NW-facing topographic 'Great Escarpment.' Well-preserved kinematic indicators and multiple generations of fault products are exposed along the Tjellefonna fault, a well-defined structural and topographic lineament parallel to both the Langfjorden and the Great Escarpment. The slope instability that was formerly present at Tjelle, and additional instabilities currently present throughout the region, may be viewed as the direct product of past and ongoing development of tectonic topography in Møre og Trøndelag county. In the Langfjorden region in particular, structural geometry suggests additional unreleased rock compartments may be isolated and under normal fault control. Although post-glacial rebound and topographically-derived horizontal spreading stresses might in part help drive present-day oblique normal seismicity, the normal-fault-controlled escarpments of Norway were at least partly erected in pre-glacial times. Cretaceous to Early Tertiary post-rift subsidence was interrupted by normal faulting at the innermost portion of the passive margin, imposing a strong tectonic empreinte on the developing landscape.

Drainage Analysis of the South American Landscape and its Tectonic Implications

NASA Astrophysics Data System (ADS)

Rodríguez Tribaldos, Verónica; White, Nicholas J.; Roberts, Gareth G.

2016-04-01

The majority of studies aimed at investigating topographic growth and landscape evolution have limited spatial coverage. Frequently, spot measurements of uplift and denudation are only available, which hampers spatial resolution of the growth of regional topographic features. This limitation can be overcome by quantitatively analysing substantial, continent-wide, drainage networks. The shapes of long wavelength longitudinal river profiles appear to be mainly controlled by regional uplift and moderated by erosional processes, both of which can vary as a function of space and time. By parametrizing erosional histories, it is feasible to develop inverse models that permit spatial and temporal patterns of regional uplift to be reliably retrieved. Here, a drainage inventory for South America consisting of 1827 rivers has been inverted. River profiles were extracted from the SRTM topographic dataset and modelled using a simplified version of the stream-power law, in which erosional processes are described using a linear advective formulation. The inverse problem is then solved by seeking smooth uplift rate histories that minimize the misfit between observed and calculated river profiles using a linearized, damped, non-negative, least squares algorithm. Calibration of erosional processes is achieved by inverting the complete drainage inventory and seeking a calculated uplift history that best honours independent geological observations from the Borborema Province of northeast Brazil. This province experienced regional Cenozoic uplift. Calculated uplift rate histories for South America suggest that the bulk of its topography developed during Cenozoic times. The model suggests, for instance, that the Andean mountain chain mostly arose in late Eocene-Oligocene (i.e. 40-28 Ma) times with an increase in elevation during Miocene times (i.e. the last 20 Ma). Uplift of the Central Andean Altiplano from an elevation of ~ 1 km to its present-day height of ~ 4 km occurred within the last 25-30 Ma. Our results are consistent with a wide range of independent geological observations across South America (e.g. elevated marine deposits, thermochronology, paleoelevation estimates from paleobotany, standard/clumped isotopes analyses). Finally, an important test of our thesis concerns offshore sedimentary flux. For example, our calculated uplift history can be used to predict the history of sedimentary flux into the Foz do Amazonas basin. This prediction agrees with offshore studies of the Amazon delta which suggest a rapid increase in clastic deposition since middle Miocene times. In summary, we propose that South American drainage contains useful information about spatial and temporal patterns of regional uplift which can help our understanding of regional topographic growth and landscape evolution.

Linking Anomalously Low Topographic Relief in the Taiwan Arc-Continent Collision to a Submarine Plateau South of the Collision.

NASA Astrophysics Data System (ADS)

Byrne, T. B.; Lin, A. T.; Su, C. C.; Lewis, J. C.; Liu, C. S.; Ouimet, W. B.; Chen, T. T.; Huang, C.

2015-12-01

Ouimet et al. (2014; submitted) have recently described broad areas of anomalously low relief and slope that straddle the topographic ridge crest in Taiwan. The areas are anomalous because Taiwan also preserves some of the steepest topography in the world, as expected from the high rates of convergence, exhumation and erosion. In addition, several age-elevation data sets (see Hsu et al. this meeting), using new and previously published ZrnFT, ZrnHe and ApFT ages, show exhumation cooling rates of 3 to 5 mm/yr since ~1.5-2.0 Ma, which is consistent with the exposure of reset zircons beneath the surfaces. Erosion rates of this magnitude, however, are expected to produce significant topographic relief, making it difficult to generate a topographic surface of low slope and low relief centered on the ridge crest. Very young ApFT and ZrnHe ages (e.g., < 0.5 Ma) also suggest a more recent acceleration in exhumation cooling. A relatively large area of low relief centered on the top of the submarine accretionary prism south of Taiwan may provide a solution for this conundrum. The area covers ca 1000 km2, has a mean elevation of ca 450 m below sea level and appears to be composed of hard, intact rocks rather than unlithified deep sea muds. Several attempts at gravity and piston coring failed to penetrate the substrate and samples retrieved in the core catcher include rounded clasts of quartz and siltstone. Geophysical studies also show high Vp velocities at relatively shallow depths beneath the sea floor, suggesting the removal of a significant volume of sediment and rock, presumably through erosion. We propose that this low relief plateau serves as a modern analog for the formation of the low relief surfaces that currently straddle the ridge crest in Taiwan. If this interpretation is correct, it suggests a tectonic model in which submarine erosion processes modulate vertical growth of the prism until collision of thick continental crust promotes uplift that outpaces erosion. This requires significant erosion rates (e.g., mm/yr) in the shallow water marine environment. Based on the young ApFT and ZrnHe ages, we propose that emergence of the prism occurred less than about a half a million years ago, and that this hypothesis could be tested by chronologic and thermochronologic data from cores retrieved through submarine drilling.

Sand fairway mapping as a tool for tectonic restoration in orogenic belts

NASA Astrophysics Data System (ADS)

Butler, Rob

2016-04-01

The interplay between regional subsidence mechanisms and local deformation associated with individual fold-thrust structures is commonly investigated in neotectonic subaerial systems using tectonic geomorphology. Taking these approaches back into the early evolution of mountain belts is difficult as much of the key evidence is lost through erosion. The challenge is to develop appropriate tools for investigating these early stages of orogenesis. However, many such systems developed under water. In these settings the connections between regional and local tectonics are manifest in complex bathymetry. Turbidity currents flowing between and across these structures will interact with their substrate and thus their deposits, tied to stratigraphic ages, can chart tectonic evolution. Understanding the depositional processes of the turbidity currents provides substantial further insight on confining seabed geometry and thus can establish significant control on the evolution of bathymetric gradients and continuity through basins. However, reading these records commonly demands working in structurally deformed terrains that hitherto have discouraged sedimentological study. This is now changing. Sand fairway mapping provides a key approach. Fairway maps chart connectivity between basins and hence their relative elevation through time. Larger-scale tectonic reconstructions may be tested by linking fairway maps to sand composition and other provenance data. More detailed turbidite sedimentology provides substantial further insight. In confined turbidite systems, it is the coarser sand component that accumulates in the deeper basin with fines fractionated onto the flanks. Flow bypass, evidenced by abrupt breaks in grading within individual event beds, can be used to predict sand fraction distribution down fairways. Integrating sedimentology into fairway maps can chart syntectonic slope evolution and thus provide high resolution tools equivalent to those in subaerial tectonic geomorphology. The stratigraphic records are preserved in many parts of the Alpine-Mediterranean region. Examples are drawn from the Eo-Oligocene of the western Alps and the early Miocene of the Maghreb-Apennine system to illustrate how turbidite sedimentology, linked to studies of basin structure, can inform understanding of tectonic processes on regional and local scales. In both examples, sediment was delivered across deforming basin arrays containing contractional structures, sourced from beyond the immediate orogenic segments. The depositional systems show that multiple structures were active in parallel, rather than develop in any particular sequence. Both systems show that significant deformation occurs, emerging to the syn-orogenic surface ahead of the main orogenic wedge. The cycling of uplift and subsidence of "massifs" can be significantly more complex that the histories resolved from thermochronological data alone.

Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard

NASA Astrophysics Data System (ADS)

Ader, Thomas; Avouac, Jean-Philippe; Liu-Zeng, Jing; Lyon-Caen, HéLèNe; Bollinger, Laurent; Galetzka, John; Genrich, Jeff; Thomas, Marion; Chanard, Kristel; Sapkota, Soma Nath; Rajaure, Sudhir; Shrestha, Prithvi; Ding, Lin; Flouzat, Mireille

2012-04-01

We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = - 1.34° ± 3.31°, latitude = 51.4° ± 0.3° with an angular velocity of Ω = 0.5029 ± 0.0072°/Myr. The pattern of coupling on the MHT is computed on a fault dipping 10° to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350°C. The convergence between India and South Tibet proceeds at a rate of 17.8 ± 0.5 mm/yr in central and eastern Nepal and 20.5 ± 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 ± 0.4 × 1019 Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 MW ≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 × 1019 Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT.

The Neogene tectonic evolution and climatic change of the Tianshui Basin, NE Tibetan Plateau

NASA Astrophysics Data System (ADS)

Peng, T.; Li, J.; Song, C.; Zhao, Z.; Zhang, J.; Wang, X.; Hui, Z.

2013-12-01

The Tianshui Basin, located at the conjunction of NE Tibetan Plateau and Chinese Loess Plateau, has received intensive attention recently. Fine-grained Miocene sediment was identified as loess in its north part and this pushes the onset of Asian aridification into 22 Ma. However, our sedimentological, biomarker, pollen, diatom and mammalian fossils evidence propose that these sediments were suggested to be mudflat/distal fan and floodplain deposit instead of eolian deposit. So detailed tectonic background and climate reconstruction may illustrate the controversy and shed light on the tectonic, climate and ecology interactions. Here we report our integrated studies on the tectonic evolution, climate change and paleoecology reconstruction in the Tianshui basin. Based on the magnetostratigraphy and fossil mammal ages, sedimentological and detrital fission-track thermochronologic (DFT) analysis reveals four episodic tectonic uplift events occurred at ~20 Ma, ~14 Ma, ~9.2-7.4 Ma and ~3.6 Ma along the basin and its adjacent mountains. The timing of these activities at Western Qinling have been documented at many segments of the Tibetan Plateau, so most likely they were the remote response to the ongoing India-Asia collision. Pollen, mammalian fossils and biomarker data permit us to illustrate the paleoenvironment in the Tianshui Basin. During the period of ~17-10 Ma, the climate was generally warm-humid revealed by the broad-leaved forest and low Average Chain Length (ACL) values, when the Paltybelodon and Gomphotherium were roaming near an extensive aquatic setting. In addition, the observed Middle Miocene Climatic Optimum and Middle Miocene Climatic Transition events may be a terrestrial response to global climate changes. During the interval of ~10-6 Ma, the climate was relatively arid characterized by the rapid development of steppe and appearance of the Hipparion fauna, consistent with the biomarker proxy. Although the NE Tibetan Plateau experienced a phase of active uplift around ~8 Ma, we mainly ascribe this arid interval to global change known as the C4 grass expansion, because the subsequent early Pliocene turned back to humid-warm climate again. Since ~4 Ma, it became obviously drier than the previous two arid intervals via the biomarker perspective. This dramatic dry trend may be related to the Tibetan Plateau uplift and/or global cooling, highlighting the importance and complexity of tectonic-climate interaction. Acknowledgements: This work was co-supported by the "Strategic Priority Research Program" of the CAS (XDB03020402), the (973) National Basic Research Program of China (2013CB956400) and the National Natural Science Foundation of China (41021091, 41101012).

Alpha-Recoil Damage Annealing Effecfs on Zircon Crystallinity and He Diffusivity: Improving Damage-Diffusivity Models

NASA Astrophysics Data System (ADS)

Thurston, O. G.; Guenthner, W.; Garver, J. I.

2017-12-01

The effects of radiation damage on He diffusion in zircon has been a major research focus in thermochronology over the past decade. In the zircon-He system, alpha-recoil damage effects He diffusivity in two ways: a decrease in He diffusivity at low radiation damage levels, and an increase in He diffusivity at high radiation damage levels. The radiation damage accumulation process within zircon is well understood; however, the kinetics of annealing of alpha-recoil damage at geologic timescales as they pertain to damage-diffusivity models, and for metamict zircon (i.e. transition from crystalline to amorphous glass via damage accumulation), has not been well constrained. This study aims to develop a more complete model that describes the annealing kinetics for zircon grains with a broad range of pre-annealing, alpha-induced radiation damage. A suite of zircon grains from the Lucerne pluton, ME were chosen for this study due to their simple thermal history (monotonic cooling), notable range of effective uranium (eU, eU = [U] +0.235*[Th]) (15 - 34,239 ppm eU), and large range of radiation damage as measured by Raman shift from crystalline (>1005 cm-1) to metamict (<1000 cm-1). The zircon grains selected represent the full range of eU and radiation damage present in the pluton. The zircon grains were first mapped for overall crystallinity using Raman spectroscopy, then annealed at different time-temperature (t-T) schedules from 1 hr to 24 hrs at temperatures ranging from 700-1100 °C, followed by remapping with Raman spectroscopy to track the total Raman shift for each t-T step. The temperature window selected is at the "roll-over" point established in prior studies (Zhang et al., 2000), at which most laboratory annealing occurs. Our data show that high radiation damage zircon grains show larger Raman shifts than low radiation damage zircon grains when exposed to the same t-T step. The high damage zircon grains typically show a Raman shift of 4 cm-1 toward crystalline, while low radiation damage grains show a shift of 2 cm-1. These shifts suggest that the annealing process occurs at a faster rate in high damage zircon grains, and slower rates in more crystalline grains. That is, the initial level of radiation damage prior to annealing must be considered in damage-diffusivity models that contain thermal histories from zircon-He dates.

Recognition and dynamics of syntectonic sediment routing systems, southern Pyrenees

NASA Astrophysics Data System (ADS)

Allen, P. A.; Duller, R.; Fordyce, S.; Smithells, R.; Springett, J.; Whitchurch, A.; Whittaker, A.; Carter, A.; Fedele, J.-J.

2009-04-01

The erosional, transportational and depositional aspects of the biogeochemical cycles involving particulate sediment and solutes are integrated in sediment routing systems. The component parts of these tectonic-geomorphic systems communicate with each other, especially in response to changes in external forcing mechanisms such as tectonic perturbations and climate change; that is, sediment routing systems are characterized by important teleconnections. We are only just beginning to understand how these teleconnections work, and what it means for the spatial and temporal scales of system behaviour. One strategy for investigating the dynamics of sediment routing systems is to link information on the denudation of upstream source regions with downstream patterns of deposition. This is most likely to be fruitful where upstream catchments are tectonically active. Sediment is released into basins whose long-term subsidence is also controlled by tectonic activity. The spatial distribution of subsidence and the magnitude of the sediment discharge from the catchment are critical factors in the dispersal of sediment of different grain size and composition away from a mountain front. We investigate the coarse clastic sediment routing systems of mid-late Eocene age (40-34 Ma) that were deposited in basins located at the boundary of the Axial Zone and the thrust belt of the South-Central Unit on the southern flank of the Pyrenees, Spain. Most of the fan deposits of interest are found in the Pobla Basin, situated north of Tremp, which benefits from outstanding exposure conditions and rigorous previous work on biostratigraphy, magnetostratigraphy and sedimentology (Mellere 1993; Beamud et al. 2003). Distinct fan depositional systems can be identified and mapped on the basis of their sediment composition, detrital thermochronology, facies and architectures, which can be related to correspondingly distinct catchment properties (size, location, exhumational history, lithologies). Downstream fining of clasts of variable composition in streamflood fanglomerates is interpreted in terms of abrasion, which is minor, and selective deposition, which dominates. The observed downstream trends in different fan systems are used as a test for the selective deposition model of Fedele & Paola (2007). Beamud, E., Garcés, M., Cabrera, L., Munoz, J.A. & Almar, Y., 2003, A new middle to late Eocene continental chronostratigraphy from NE Spain. Earth & Planetary Science Letters, 216, 501-514. Fedele, J.J., & Paola, C., 2007, Similarity solutions for fluvial sediment fining by selective deposition. Journal Geophysical Research-Earth Surface, 112, F02038, doi:10.1029/2005JF000409. Mellere, D., 1993, Thurst-generated, back-fill stacking of alluvial fan sequences, south-central Pyrenees, Spain (la Pobla de Segur Conglomerates). Special Publication International Association Sedimentologists, 20, 259-276.

Escarpment evolution at the Red Sea continental margin of southwestern Saudi Arabia

NASA Astrophysics Data System (ADS)

Turab, Syed Ali; Stüwe, Kurt; Stuart, Finlay M.; Cogné, Nathan; Chew, David M.

2017-04-01

Rifting of the Red Sea started around 30-32 Ma and resulted in the formation of one of the youngest and best developed escarpments of the world: the Great Escarpment of southwestern Saudi Arabia. The escarpment is perfectly developed over a length of more than 500 km and includes mountains up to 3000 m in elevation. To better understand the geodynamics of Red Sea rifting and to constrain a denudational model for the Great Escarpment, the results of apatite fission track and (U-Th-[Sm])/He thermochronologic techniques are combined with stream power analysis from the central part of this region. Pooled fission track ages (recording cooling through about 110 °C) range from 13.2 ± 1.7 to 352.1 ± 17.6 Ma (1σ) with all ages that are younger than about 50 Ma (and thus related to the rifting) being from elevations lower than about 500 m (i.e. towards the base of the escarpment). Apatite He ages range from 2.8 ± 0.3 to 264.5 ± 19.6 Ma with a similar age-elevation relationship. The base of the pre-uplift apatite partial annealing zone is interpreted to be lying at 200 m present-day elevation. Our fission-track data indicate that the amount of exhumation is insufficient to completely reset all the coastal plain samples, but exhumation along the escarpment appears to increase from south towards north. The highest amount of exhumation is confined to two separate regions, one in the north and one in the south, which are separated by a region of non-reset AFT ages and hence lower amounts of exhumation. This interpretation is also supported by stream power analysis from this region. The reset AFT ages indicate about 4.5 km of exhumation which may have started in the early Miocene, but the majority of this exhumation phase occurred after 13 Ma. This interpretation is consistent with a single isolated outcrop of Nubian sandstone at the summit of Saudi Arabia's highest peak. The distributions of AFT and AHe ages across the escarpment and coastal region supports the escarpment development by the established "down-wearing" or "plateau degradation" model of escarpment evolution, which implies that the present drainage divide may have been in its position already in the Miocene. Red Sea rifting may have resulted from the complex interaction of both passive and active rifting models as it did not follow the suggested sequence of events (i.e. rifting-uplift-volcanism or doming-volcanism-rifting) for either of the two rifting models.

Using U-Pb Detrital Zircon Geochronology to Study Ice Streams in the Weddell Sea Embayment, Antarctica

NASA Astrophysics Data System (ADS)

Agrios, L.; Licht, K.; Hemming, S. R.; Williams, T.

2016-12-01

Till from major ice streams of the Weddell Sea Embayment contain detrital zircons with distinct U-Pb age populations that can be used as a provenance tool to better understand ice stream dynamics. The ice streams in this study include the Foundation Ice Stream, and Academy, Slessor, and Recovery glaciers, all of which drain ice from the continent's interior into the Weddell Sea. Characterizing the U-Pb detrital zircon ages in till and rocks will (1) provide the zircon provenance signatures of the material carried by the ice stream - when these signatures are found in LGM and older deposits downstream they can enable interpretation of past ice flow history; and (2) constrain ice-covered upstream bedrock geology that supplies the till carried by ice streams and glaciers. U-Pb ages of detrital zircons were measured in 21 samples of onshore till, erratics, and bedrock of potential source rocks. Grains were analyzed by LA-ICPMS at the University of Arizona (n=300). Relative probability U-Pb age density plots of till in moraines along the Foundation Ice Stream and Academy Glacier show prominent peaks at 500-530 and 615-650 Ma, which overlap with the timing of the Ross and Pan-African orogenies. Zircon ages of 1000-1095 Ma are also present. Local bedrock in the Patuxent Range has the most prominent peak at 510 Ma, suggesting the till is predominantly derived from local Patuxent Formation. However, local bedrock also has fewer grains at 1030 Ma which suggests that this age population is carried in the till as well. Prominent peaks in U-Pb ages from till transported by the Recovery Glacier are 530, 635, 1610 and 1770 Ma. Bedrock of this area contains similar age peaks, with the exception of the 635 Ma peak, suggesting that this ice stream is carrying a signature from an unexposed source of this age completely buried by ice. The Slessor Glacier carries zircons with prominent populations at 1710 and 2260-2420 Ma, which overlap with a high-grade metamorphic event in the Shackleton Range between 1710-1680 Ma. In order to gain the offshore signature of ice streams, these data will be compared to 40Ar/39Ar hornblende and biotite thermochronological data, and U-Pb geochronology data from subglacial till and proximal glaciomarine sediment from existing core sites located at the edge of the Ronne-Filchner Ice Shelf.

Depositional and provenance record of the Paleogene transition from foreland to hinterland basin evolution during Andean orogenesis, northern Middle Magdalena Valley Basin, Colombia

NASA Astrophysics Data System (ADS)

Moreno, Christopher J.; Horton, Brian K.; Caballero, Victor; Mora, Andrés; Parra, Mauricio; Sierra, Jair

2011-10-01

The Central Cordillera and Eastern Cordillera of the northern Andes form the topographic flanks of the north-trending Magdalena Valley Basin. Constraining the growth of these ranges and intervening basin has implications for Andean shortening and the transformation from a foreland to hinterland basin configuration. We present sedimentological, paleocurrent, and sandstone petrographic results from Cenozoic type localities to provide insights into the tectonic history of the northern Middle Magdalena Valley Basin of Colombia. In the Nuevo Mundo Syncline, the mid-Paleocene transition from marine to nonmarine deposystems of the Lisama Formation corresponds with a paleocurrent shift from northward to eastward transport. These changes match detrital geochronological evidence for a contemporaneous shift from cratonic (Amazonian) to orogenic (Andean) provenance, suggesting initial shortening-related uplift of the Central Cordillera and foreland basin generation in the Magdalena Valley by mid-Paleocene time. Subsequent establishment of a meandering fluvial system is recorded in lower-middle Eocene strata of the lower La Paz Formation. Eastward paleocurrents in mid-Paleocene through uppermost Eocene fluvial deposits indicate a continuous influence of western sediment source areas. However, at the upper middle Eocene (˜40 Ma) boundary between the lower and upper La Paz Formation, sandstone compositions show a drastic decrease in lithic content, particularly lithic volcanic fragments. This change is accompanied by a facies shift from mixed channel and overbank facies to thick, amalgamated braided fluvial deposits of possible fluvial megafans, reflecting changes in both the composition and proximity of western sediment sources. We attribute these modifications to the growing influence of exhumed La Cira-Infantas paleohighs in the axial Magdalena Valley, features presently buried beneath upper Eocene-Quaternary basin fill along the western flank of the Nuevo Mundo Syncline. In uppermost Eocene strata of the lower Esmeraldas Formation, paleocurrents show a sharp reversal from eastward to dominantly westward transport that persisted into the Neogene. The Esmeraldas also records a change to more-distal, floodplain-dominated deposition of finer sediments. These adjustments are interpreted to reflect burial of the La Cira-Infantas highs and onset of Eastern Cordillera exhumation, resulting in a transition from foreland to hinterland basin conditions in the Magdalena Valley. The lack of significant variation in sandstone compositions suggests a bulk-rock compositional similarity between the La Cira-Infantas paleohighs (subsurface Magdalena Valley) and the Eastern Cordillera. Collectively, the data presented here refine previous thermochronologic and provenance studies and suggest that major uplift-induced exhumation in the Central Cordillera and Eastern Cordillera commenced by the mid-Paleocene and latest Eocene, respectively.

Thermal metamorphism of the Arunachal Himalaya, India: Raman thermometry and thermochronological constraints on the tectono-thermal evolution

NASA Astrophysics Data System (ADS)

Mathew, George; De Sarkar, Sharmistha; Pande, Kanchan; Dutta, Suryendu; Ali, Shakir; Rai, Apritam; Netrawali, Shilpa

2013-09-01

Determination of the peak thermal condition is vital in order to understand tectono-thermal evolution of the Himalayan belt. The Lesser Himalayan Sequence (LHS) in the Western Arunachal Pradesh, being rich in carbonaceous material (CM), facilitates the determination of peak metamorphic temperature based on Raman spectroscopy of carbonaceous material (RSCM). In this study, we have used RSCM method of Beyssac et al. (J Metamorph Geol 20:859-871, 2002a) and Rahl et al. (Earth Planet Sci Lett 240:339-354, 2005) to estimate the thermal history of LHS and Siwalik foreland from the western Arunachal Pradesh. The study indicates that the temperature of 700-800 °C in the Greater Himalayan Sequence (GHS) decreases to 650-700 °C in the main central thrust zone (MCTZ) and decreases further to <200 °C in the Mio-Pliocene sequence of Siwaliks. The work demonstrates greater reliability of Rahl et al.'s (Earth Planet Sci Lett 240:339-354, 2005) RSCM method for temperatures >600 and <340 °C. We show that the higher and lower zones of Bomdila Gneiss (BG) experienced temperature of ~600 °C and exhumed at different stages along the Bomdila Thrust (BT) and Upper Main Boundary Thrust (U.MBT). Pyrolysis analysis of the CM together with the Fission Track ages from upper Siwaliks corroborates the RSCM thermometry estimate of ~240 °C. The results indicate that the Permian sequence north of Lower MBT was deposited at greater depths (>12 km) than the upper Siwalik sediments to its south at depths <8 km before they were exhumed. The 40Ar/39Ar ages suggest that the upper zones of Se La evolved ~13-15 Ma. The middle zone exhumed at ~11 Ma and lower zone close to ~8 Ma indicating erosional unroofing of the MCT sheet. The footwall of MCTZ cooled between 6 and 8 Ma. Analyses of P-T path imply that LHS between MCT and U.MBT zone falls within the kyanite stability field with near isobaric condition. At higher structural level, the temperatures increase gradually with P-T conditions in the sillimanite stability field. The near isothermal (700-800 °C) condition in the GHS, isobaric condition in the MCTZ together with T-t path evidence of GHS that experienced relatively longer duration of near peak temperatures and rapid cooling towards MCTZ, compares the evolution of GHS and inverted metamorphic gradient closely to channel flow predictions.

Denudation rates derived from spatially-averaged cosmogenic nuclide analysis in Nelson catchments, South Island, New Zealand

NASA Astrophysics Data System (ADS)

Burdis, A.; Norton, K. P.; Ditchburn, B.; Zondervan, A.

2013-12-01

New Zealand's tectonically and climatically dynamic environment generates erosion rates that outstrip global averages by up to ten times in some locations. In order to assess recent changes in erosion rate, and also to predict future erosion dynamics, it is important to quantify long-term, background erosion. Current research on erosion in New Zealand predominantly covers short-term (100 yrs) erosion dynamics and Myr dynamics from thermochronological proxy data. Without medium-term denudation data for New Zealand, it is uncertain which variables (climate, anthropogenic disturbance of the landscape, tectonic uplift, lithological, or geomorphic characteristics) exert the dominant control on denudation in New Zealand. Spatially-averaged cosmogenic nuclide analysis can effectively offer this information by providing averaged rates of denudation on millennial timescales without the biases and limitations of short-term erosion methods. Basin-averaged denudation rates were obtained in the Nelson region, New Zealand, from analysis of concentrations of meteoric 10Be in clay and in-situ produced 10Be in quartz. The measured denudation rates integrate over ~8000 yrs (meteoric) and ~3000 yrs (in-situ). Not only do the 10Be records produce erosion rates that are remarkably consistent with each other, but they are also independent of topographic metrics. Denudation rates range from ~116 - 306 t km-2 yr-1, with the exception of one basin which is eroding at 789 t km-2 yr-1(derived from meteoric 10Be) and 644 t km-2 yr-1(derived from in-situ 10Be). The homogeneity of rates and absence of a significant correlation with geomorphic or lithological characteristics suggest another factor is exerting the dominant control on landscape denudation in the Nelson region. Storm variability is a likely driver of erosion in this setting. The background rates are higher than current short term rates (~50 - 200 t km-2 yr-1) due to the significant erosion caused by high magnitude, low frequency events. This is highlighted by sediment yields reaching ~2535 t km-2 yr-1 following storm events in the Nelson region (Basher et al., 2011). At least on the northern South Island of New Zealand, extreme climatic events appear to be the main driver of elevated denudation rates. Basher, L., Hicks, D., Clapp, B., & Hewitt, T. (2011). Sediment Yield Response to Large Storm Events and Forest Harvesting, Motueka River, New Zealand. New Zealand Journal ofMarine and Freshwater Research, Vol. 45, No. 3 , 333-356.

Early Neogene unroofing of the Sierra Nevada de Santa Marta along the Bucaramanga -Santa Marta Fault

NASA Astrophysics Data System (ADS)

Piraquive Bermúdez, Alejandro; Pinzón, Edna; Bernet, Matthias; Kammer, Andreas; Von Quadt, Albrecht; Sarmiento, Gustavo

2016-04-01

Plate interaction between Caribbean and Nazca plates with Southamerica gave rise to an intricate pattern of tectonic blocks in the Northandean realm. Among these microblocks the Sierra Nevada de Santa Marta (SNSM) represents a fault-bounded triangular massif composed of a representative crustal section of the Northandean margin, in which a Precambrian to Late Paleozoic metamorphic belt is overlain by a Triassic to Jurassic magmatic arc and collateral volcanic suites. Its western border fault belongs to the composite Bucaramanga - Santa Marta fault with a combined left lateral-normal displacement. SE of Santa Marta it exposes remnants of an Oligocene marginal basin, which attests to a first Cenoizoic activation of this crustal-scale lineament. The basin fill consists of a sequence of coarse-grained cobble-pebble conglomerates > 1000 m thick that unconformably overlay the Triassic-Jurassic magmatic arc. Its lower sequence is composed of interbedded siltstones; topwards the sequence becomes dominated by coarser fractions. These sedimentary sequences yields valuable information about exhumation and coeval sedimentation processes that affected the massif's western border since the Upper Eocene. In order to analyse uplifting processes associated with tectonics during early Neogene we performed detrital zircon U-Pb geochronology, detrital thermochronology of zircon and apatites coupled with the description of a stratigraphic section and its facies composition. We compared samples from the Aracataca basin with analog sequences found at an equivalent basin at the Oca Fault at the northern margin of the SNSM. Our results show that sediments of both basins were sourced from Precambrian gneisses, along with Mesozoic acid to intermediate plutons; sedimentation started in the Upper Eocene-Oligocene according to palynomorphs, subsequently in the Upper Oligocene a completion of Jurassic to Cretaceous sources was followed by an increase of Precambrian input that became the dominant source for sediments, this shift in provenance is related to an increase in exhumation and erosion rates. The instauration of such a highly erosive regime since the Upper Oligocene attests how the Santa Marta massif was subject to uplifting and erosion, our data shows how in the Upper Oligocene an exhaustion of Cretaceous to Permian sources was followed by an increase in Neo-Proterozoic to Meso-Proterozoic input that is related to the unroofing of the basement rocks, this accelerated exhumation is directly related to the reactivation of the Orihueca Fault as a NW verging thrust at the interior of the massif coeval with Bucaramanga-Santa Marta Fault trans-tensional tectonics in response to the fragmentation of the Farallon plate into the Nazca an Cocos Plates.

300 million years of basin evolution - the thermotectonic history of the Ukrainian Donbas Foldbelt

NASA Astrophysics Data System (ADS)

Spiegel, C.; Danisik, M.; Sachsenhofer, R.; Frisch, W.; Privalov, V.

2009-04-01

The Ukrainian-Russian Pripyat-Dniepr-Donets Basin is a large intracratonic rift structure formed during the Late Devonian. It is situated at the southern margin of the Precambrian East European Craton, adjacent to the Hercynian Tethyan belt in the Black Sea area and the Alpine Caucasus orogen. With a sediment thickness of more than 20 km, it is one of the deepest sedimentary basins on earth. The eastern part of the Pripyat-Dniepr-Donets Basin - called Donbas foldbelt - is strongly folded and inverted. Proposed models of basin evolution are often controversial and numerous issues are still a matter of speculation, particularly the erosion history and the timing of basin inversion. Basin inversion may have taken place during the Permian related to the Uralian orogeny, or in response to Alpine tectonics during the Late Cretaceous to Early Tertiary. We investigated the low-temperature thermal history of the Donbas Foldbelt and the adjacent Ukrainian shield by a combination of zircon fission track, apatite fission track and apatite (U-Th)/He thermochronology. Although apatite fission track ages of all sedimentary samples were reset shortly after deposition during the Carboniferous, we took advantage of the fact that samples contained kinetically variable apatites, which are sensitive to different temperatures. By using statistic-based component analysis incorporating physical properties of individual grains we identified several distinct age population, ranging from late Permian (~265 Ma) to the Late Cretaceous (70 Ma). We could thus constrain the thermal history of the Donbas Foldbelt and the adjacent basement during a ~300 Myr long time period. The Precambrian crystalline basement of the Ukrainian shield was affected by a Permo-Triassic thermal event associated with magmatic activity, which also strongly heated the sediments of the Donbas Foldbelt. The basement rocks cooled to near-surface conditions during the Early to Middle Triassic and since then was thermally stable. The basin margins started to cool during the Permo-Triassic whereas the central parts were residing or slowly cooling through the apatite partial annealing zone during the Jurassic and most of the Cretaceous and eventually cooled to near-surface conditions around the Cretaceous-Paleogene boundary. Our data show that Permian erosion was lower and Mesozoic erosion larger than generally assumed. Inversion and pop-up of the Donbas Foldbelt occurred in the Cretaceous and not in the Permian as previously thought. This is indicated by overall Cretaceous apatite fission track ages in the central parts of the basin.

Mobility of partially molten crust, heat and mass transfer, and the stabilization of continents

NASA Astrophysics Data System (ADS)

Teyssier, Christian; Whitney, Donna L.; Rey, Patrice F.

2017-04-01

The core of orogens typically consists of migmatite terrains and associated crustal-derived granite bodies (typically leucogranite) that represent former partially molten crust. Metamorphic investigations indicate that migmatites crystallize at low pressure (cordierite stability) but also contain inclusions of refractory material (mafic, aluminous) that preserve evidence of crystallization at high pressure (HP), including HP granulite and eclogite (1.0-1.5 GPa), and in some cases ultrahigh pressure (2.5-3.0 GPa) when the continental crust was subducted (i.e. Norwegian Caledonides). These observations indicate that the partially molten crust originates in the deep crust or at mantle depths, traverses the entire orogenic crust, and crystallizes at shallow depth, in some cases at the near-surface ( 2 km depth) based on low-T thermochronology. Metamorphic assemblages generally show that this nearly isothermal decompression is rapid based on disequilibrium textures (symplectites). Therefore, the mobility of partially molten crust results in one of the most significant heat and mass transfer mechanisms in orogens. Field relations also indicate that emplacement of partially molten crust is the youngest major event in orogeny, and tectonic activity essentially ceases after the partially molten crust is exhumed. This suggests that flow and emplacement of partially molten crust stabilize the orogenic crust and signal the end of orogeny. Numerical modeling (open source software Underworld; Moresi et al., 2007, PEPI 163) provides useful insight into the mechanisms of exhumation of partially molten crust. For example, extension of thickened crust with T-dependent viscosity shows that extension of the shallow crust initially drives the mobility of the lowest viscosity crust (T>700°C), which begins to flow in a channel toward the zone of extension. This convergent flow generates channel collision and the formation of a double-dome of foliation (two subdomes separated by a steep high strain zone). In turn, the rapid exhumation of low-viscosity deep crust within and between the two subdomes enhances localization of extension in the shallow crust; the positive feedback between exhumation of low-viscosity crust and localization of shallow crust extension explains the exhuming power of migmatite domes, the rapid isothermal decompression of dome rocks (order of 1.0-1.5 GPa), and the crystallization of melt at shallow depth followed by rapid cooling. Modeling results indicate that the mobility of low-viscosity (partially molten) crust is a major process for transferring heat and mass during the late stages of orogeny.

Source-to-Sink constraints on tectonic and sedimentary evolution of the Central Range, Cenderawasih Bay (Indonesia) and Gulf of Papua (Papua New Guinea)

NASA Astrophysics Data System (ADS)

Babault, J.; Viaplana-Muzas, M.; Legrand, X.; Van Den Driessche, J.; González-Quijano, M.; Mudd, S. M.; Kergaravat, C.; Ringenbach, J. C.; Callot, J. P.; Vetel, W.; Dhont, D.

2017-12-01

The island of Papua New Guinea is the result of continent-arc collision that began building the island's Central Range during the late Miocene. The tectono-sedimentary evolution of the Cenderawasih Bay, in the northwestern part of the island of Papua New Guinea (Indonesia), which links the Kepala Burung block to the Central Range is still poorly understood. Previous studies have shown that this bay contains a thick (> 8 km) sequence of undated sediments. Hypothesis claim that the embayment resulted from a 3 Ma aperture created by anticlockwise rotation of the Kepala Burung block with respect to the northern rim of the Australian plate, or from the southwest drift of a slice of volcanics/oceanic crust between 8 and 6 Ma. Using a source-to-sink approach, based on i) a geomorphologic analysis of the drainage network dynamics, ii) a reassessment of available thermochronological data, and iii) seismic lines interpretation, we suggest that sediments started to accumulate in the Cenderawasih Bay and onshore in the Waipoga Basin in the late Miocene since the beginning of the Central Range building at 12 Ma, resulting in sediment accumulation of up to 12200 m. At first order, we predict that infilling is mainly composed of siliciclastics sourced in the graphite-bearing Ruffaer Metamorphic Belt and its equivalent in the Weyland Overthrust. From the unroofing paths in the Central Range we deduce two rates of solid phase accumulation (SPAR) since 12 Ma, the first one at a mean SPAR ranging between 0.12-0.25 mm/a with a maximum SPAR of 0.23-0.58 mm/a, and the second during the last 3 Ma, at a mean SPAR ranging between 0.93-1.62 mm/a and with a maximum SPAR between 2.13-3.17 mm/a, i.e., 6700-10000 m of Plio-Pleistocene sediment accumulation. Local transtensional tectonics may explain these unusually high rates of sedimentation in an overall sinistral oblique convergence setting. We further extended this approach to the Gulf of Papua (Papua New Guinea), a foreland basin developed in the passive margin of the Coral Sea and fed by the Papuan fold-and-thrust belt and Aure fold-and-thrust belt. We compare these two source-to-sink systems to highlight the tectonic control on sedimentary flux, provenance and SPAR in the Cenderawasih Bay and Gulf of Papua.

Limited climate control of the Chugach/St. Elias thrust wedge in southern Alaska demonstrated by orogenic widening during Pliocene to Quaternary climate change

NASA Astrophysics Data System (ADS)

Meigs, Andrew

2014-05-01

Critical taper wedge theory is the gold standard by which climate control of convergent orogenic belts is inferred. The theory predicts (and models reproduce) that an orogenic belt narrows if erosion increases in erosion in the face of a constant tectonic influx. Numerous papers now argue on the basis of thermochronologic data that the Chugach/ St. Elias Range (CSE) of southern Alaska narrowed as a direct response to Quaternary climate change because glaciers dominated erosion of the orogenic belt. The CSE formed in response to collision of a microplate with North America and is notable because glacial erosion has dominated the CSE for the past 5 to 6 Ma. An increase in sediment accumulation rates in the foreland basin over that time suggests that glacial erosion become more efficient. If correct, it is possible that glacial erosion outpaced rock influx thereby inducing a climatically controlled narrowing of the orogenic wedge during the Quaternary. Growth strata preserved within the wedge provide a test of that interpretation because they demonstrate the spatial and temporal pattern of deformation during the Pliocene to Quaternary climate transition. A thrust front established between 6 and 5 Ma jumped towards the foreland by 30 and 15 km at 1.8 and 0.25 Ma, respectively. Distributed deformation within the thrust belt accompanied the thrust front relocations. Continuous exhumation recorded by low-temperature thermochronometers occurred contemporaneously with the shortening, parallel the structural not the topographic grain, and ages become younger towards the foreland as well. Interpreted in terms of critical wedge theory, continuous distributed deformation reflects a sub-critical wedge taper resulting from the combined effects of persistent exhumation and incremental accretion and orogenic widening via thrust front jumps into the undeformed foreland. Taper angle varies according to published cross-sections and ranges from 3 to 9 degrees. If the wedge oscillated about critical taper, a pore fluid ratio between 0.7 and 0.97 is suggested by range of taper angles. Thus, the thrust belt response to Pliocene to Quaternary climate change and a likely increase in glacial coverage is in fact the opposite of the expected response of a critical-taper wedge to an increase in hinterland erosion rate. The CSE hovered near critical taper throughout the Quaternary and the tectonic influx equaled or exceeded the erosional efflux, implying that glacial erosion was paced by, not independent of, tectonic rock uplift rate.

Source-to-sink constraints on tectonic and sedimentary evolution of the western Central Range and Cenderawasih Bay (Indonesia)

NASA Astrophysics Data System (ADS)

Babault, Julien; Viaplana-Muzas, Marc; Legrand, Xavier; Van Den Driessche, Jean; González-Quijano, Manuel; Mudd, Simon M.

2018-05-01

The island of New Guinea is the result of continent-arc collision that began building the island's Central Range during the late Miocene. Recent studies have shown that rapid subduction, uplift and exhumation events took place in response to rapid, oblique convergence between the Pacific and the Australian plates. The tectonic and sedimentary evolution of Cenderawasih Bay, in the northwestern part of the New Guinea Island is still poorly understood: this bay links a major structural block, the Kepala Burung block, to the island's Central Ranges. Previous studies have shown that Cenderawasih Bay contains a thick (>8 km) sequence of undated sediments. One hypothesis claims that the embayment resulted from a 3 Ma opening created by anticlockwise rotation of the Kepala Burung block with respect to the northern rim of the Australian plate. Alternatively, the current configuration of Cenderawasih Bay could have resulted from the southwest drift of a slice of volcanics and oceanic crust between 8 and 6 Ma. We test these hypotheses using (i) a geomorphologic analysis of the drainage network dynamics, (ii) a reassessment of available thermochronological data, and (iii) seismic lines interpretation. We suggest that sediments started to accumulate in Cenderawasih Bay and onshore in the Waipoga Basin in the late Miocene since the inception of growth of the Central Range, beginning at 12 Ma, resulting in sediment accumulation of up to 12,200 m. This evidence is more consistent with the second hypothesis, and the volume of sediment accumulated means it is unlikely that the embayment was the result of recent (2-3 Ma) rotation of structural blocks. At first order, we predict that infilling is mainly composed of siliciclastics sourced in the graphite-bearing Ruffaer Metamorphic Belt and its equivalent in the Weyland Overthrust. Ophiolites, volcanic arc rocks and diorites contribute minor proportions. From the unroofing paths in the Central Range we deduce two rates of solid phase accumulation (SPAR) since 12 Ma, the first one at a mean SPAR ranging between 0.12 and 0.25 mm/a with a maximum SPAR of 0.23-0.58 mm/a, and the second during the last 3 Ma, at a mean SPAR ranging between 0.93 and 1.62 mm/a and with a maximum SPAR between 2.13 and 3.17 mm/a, i.e., 6700-10,000 m of Plio-Pleistocene sediment accumulation. Local transtensional tectonics may explain these unusually high rates of sedimentation in an overall sinistral oblique convergence setting.

Instability of the southern Canadian Shield during the late Proterozoic

NASA Astrophysics Data System (ADS)

McDannell, Kalin T.; Zeitler, Peter K.; Schneider, David A.

2018-05-01

Cratons are generally considered to comprise lithosphere that has remained tectonically quiescent for billions of years. Direct evidence for stability is mainly founded in the Phanerozoic sedimentary record and low-temperature thermochronology, but for extensive parts of Canada, earlier stability has been inferred due to the lack of an extensive rock record in both time and space. We used 40Ar/39Ar multi-diffusion domain (MDD) analysis of K-feldspar to constrain cratonic thermal histories across an intermediate (∼150-350 °C) temperature range in an attempt to link published high-temperature geochronology that resolves the timing of orogenesis and metamorphism with lower-temperature data suited for upper-crustal burial and unroofing histories. This work is focused on understanding the transition from Archean-Paleoproterozoic crustal growth to later intervals of stability, and how uninterrupted that record is throughout Earth's Proterozoic "Middle Age." Intermediate-temperature thermal histories of cratonic rocks at well-constrained localities within the southern Canadian Shield of North America challenge the stability worldview because our data indicate that these rocks were at elevated temperatures in the Proterozoic. Feldspars from granitic rocks collected at the surface cooled at rates of <0.5 °C/Ma subsequent to orogenesis, seemingly characteristic of cratonic lithosphere, but modeled thermal histories suggest that at ca. 1.1-1.0 Ga these rocks were still near ∼200 °C - signaling either reheating, or prolonged residence at mid-crustal depths assuming a normal cratonic geothermal gradient. After 1.0 Ga, the regions we sampled then underwent further cooling such that they were at or near the surface (≪60 °C) in the early Paleozoic. Explaining mid-crustal residence at 1.0 Ga is challenging. A widespread, prolonged reheating history via burial is not supported by stratigraphic information, however assuming a purely monotonic cooling history requires at the very least 5 km of exhumation beginning at ca. 1.0 Ga. A possible explanation may be found in evidence of magmatic underplating that thickened the crust, driving uplift and erosion. The timing of this underplating coincides with Mid-Continent extension, Grenville orogenesis, and assembly of the supercontinent Rodinia. 40Ar/39Ar MDD data demonstrate that this technique can be successfully applied to older rocks and fill in a large observational gap. These data also raise questions about the evolution of cratons during the Proterozoic and the nature of cratonic stability across deep time.

Key influences on motivations for utility cycling (cycling for transport to and from places).

PubMed

Heesch, Kristiann C; Sahlqvist, Shannon

2013-12-01

Although increases in cycling in Brisbane are encouraging, bicycle mode share to work (the proportion of people travelling to work by bicycle) in the state of Queensland remains low. The aim of this qualitative study was to draw upon the lived experiences of Queensland cyclists to understand the main motivators for utility cycling (cycling as a means to get to and from places) and compare motivators between utility cyclists (those who cycle for utility as well as for recreation) and non-utility cyclists (those who cycle only for recreation). For an online survey, members of a bicycle group (831 utility cyclists and 931 non-utility cyclists, aged 18-90 years) were asked to describe, unprompted, what would motivate them to engage in utility cycling (more often). Responses were coded into themes within four levels of an ecological model. Within an ecological model, built environment influences on motivation were grouped according to whether they related to appeal (safety), convenience (accessibility) or attractiveness (more amenities) and included adequate infrastructure for short trips, bikeway connectivity, end-of-trip facilities at public locations and easy and safe bicycle access to destinations outside of cities. A key social-cultural influence related to improved interactions among different road users. The built and social-cultural environments need to be more supportive of utility cycling before even current utility and non-utility cyclists will be motivated to engage (more often) in utility cycling. SO WHAT?: Additional government strategies and more and better infrastructure that support utility cycling beyond commuter cycling may encourage a utility cycling culture.

Health State Utility Value in Chronic Obstructive Pulmonary Disease (COPD); The Challenge of Heterogeneity: A Systematic Review and Meta-Analysis.

PubMed

Moayeri, Foruhar; Hsueh, Ya-Seng Arthur; Clarke, Philip; Hua, Xinyang; Dunt, David

2016-06-01

Chronic obstructive pulmonary disease (COPD) has a considerable impact on quality of life and well-being of patients. Health state utility value (HSUV) is a recognized measure for health economic appraisals and is extensively used as an indicator for decision-making studies. This study is a systematic review of literature aimed to estimate mean utility value in COPD using meta-analysis and explore degree of heterogeneity in the utility values across a variety of clinical and study characteristic. The literature review covers studies that used EQ-5D to estimate utility value for patient level research in COPD. Studies that reported utility values elicited by EQ-5D in COPD patients were selected for random-effect meta-analysis addressing inter-study heterogeneity and subgroup analyses. Thirty-two studies were included in the general utility meta-analysis. The estimated general utility value was 0.673 (95% CI 0.653 to 0.693). Meta-analyses of COPD stages utility values showed influence of airway obstruction on utility value. The utility values ranged from 0.820 (95% CI 0.767 to 0.872) for stage I to 0.624 (95% CI 0.571 to 0.677) for stage IV. There was substantial heterogeneity in utility values: I(2) = 97.7%. A more accurate measurement of utility values in COPD is needed to refine valid and generalizable scores of HSUV. Given the limited success of the factors studied to reduce heterogeneity, an approach needs to be developed how best to use mean utility values for COPD in health economic evaluation.

26 CFR 1.42-10 - Utility allowances.

Code of Federal Regulations, 2012 CFR

2012-04-01

... into account, among other things, local utility rates, property type, climate and degree-day variables... which the building containing the units is located. (c) Changes in applicable utility allowance—(1) In...)), the applicable utility allowance for units changes, the new utility allowance must be used to compute...

26 CFR 1.42-10 - Utility allowances.

Code of Federal Regulations, 2014 CFR

2014-04-01

... into account, among other things, local utility rates, property type, climate and degree-day variables... which the building containing the units is located. (c) Changes in applicable utility allowance—(1) In...)), the applicable utility allowance for units changes, the new utility allowance must be used to compute...

26 CFR 1.42-10 - Utility allowances.

Code of Federal Regulations, 2013 CFR

2013-04-01

... into account, among other things, local utility rates, property type, climate and degree-day variables... which the building containing the units is located. (c) Changes in applicable utility allowance—(1) In...)), the applicable utility allowance for units changes, the new utility allowance must be used to compute...

26 CFR 1.42-10 - Utility allowances.

Code of Federal Regulations, 2011 CFR

2011-04-01

... into account, among other things, local utility rates, property type, climate and degree-day variables... which the building containing the units is located. (c) Changes in applicable utility allowance—(1) In...)), the applicable utility allowance for units changes, the new utility allowance must be used to compute...

26 CFR 1.42-10 - Utility allowances.

Code of Federal Regulations, 2010 CFR

2010-04-01

... into account, among other things, local utility rates, property type, climate and degree-day variables... which the building containing the units is located. (c) Changes in applicable utility allowance—(1) In...)), the applicable utility allowance for units changes, the new utility allowance must be used to compute...

42 CFR 456.200 - Scope.

Code of Federal Regulations, 2010 CFR

2010-10-01

... ASSISTANCE PROGRAMS UTILIZATION CONTROL Utilization Control: Mental Hospitals Utilization Review (ur) Plan... requirements for a written utilization review (UR) plan for each mental hospital providing Medicaid services...

75 FR 37883 - Transmission Planning and Cost Allocation by Transmission Owning and Operating Public Utilities

Federal Register 2010, 2011, 2012, 2013, 2014

2010-06-30

... Planning and Cost Allocation by Transmission Owning and Operating Public Utilities; Proposed Rule #0;#0...] Transmission Planning and Cost Allocation by Transmission Owning and Operating Public Utilities Issued June 17... Transmission Services by Public Utilities; Recovery of Stranded Costs by Public Utilities and Transmitting...

AHP for Risk Management Based on Expected Utility Theory

NASA Astrophysics Data System (ADS)

Azuma, Rumiko; Miyagi, Hayao

This paper presents a model of decision-making considering the risk assessment. The conventional evaluation in AHP is considered to be a kind of utility. When dealing with the risk, however, it is necessary to consider the probability of damage. In order to take risk into decision-making problem, we construct AHP based on expected utility. The risk is considered as a related element of criterion rather than criterion itself. The expected utility is integrated, considering that satisfaction is positive utility and damage by risk is negative utility. Then, evaluation in AHP is executed using the expected utility.

Assessment of distributed photovoltair electric-power systems

NASA Astrophysics Data System (ADS)

Neal, R. W.; Deduck, P. F.; Marshall, R. N.

1982-10-01

The development of a methodology to assess the potential impacts of distributed photovoltaic (PV) systems on electric utility systems, including subtransmission and distribution networks, and to apply that methodology to several illustrative examples was developed. The investigations focused upon five specific utilities. Impacts upon utility system operations and generation mix were assessed using accepted utility planning methods in combination with models that simulate PV system performance and life cycle economics. Impacts on the utility subtransmission and distribution systems were also investigated. The economic potential of distributed PV systems was investigated for ownership by the utility as well as by the individual utility customer.

Electric utility companies and geothermal power

NASA Technical Reports Server (NTRS)

Pivirotto, D. S.

1976-01-01

The requirements of the electric utility industry as the primary potential market for geothermal energy are analyzed, based on a series of structured interviews with utility companies and financial institution executives. The interviews were designed to determine what information and technologies would be required before utilities would make investment decisions in favor of geothermal energy, the time frame in which the information and technologies would have to be available, and the influence of the governmental politics. The paper describes the geothermal resources, electric utility industry, its structure, the forces influencing utility companies, and their relationship to geothermal energy. A strategy for federal stimulation of utility investment in geothermal energy is suggested. Possibilities are discussed for stimulating utility investment through financial incentives, amelioration of institutional barriers, and technological improvements.

FACTORS ASSOCIATED WITH HEALTHCARE UTILIZATION AMONG ARAB IMMIGRANTS AND REFUGEES

PubMed Central

2015-01-01

Background Arab migrants are exposed to pre- and post migration stressors that increase their risk for health problems. However, little is known regarding healthcare utilization rates or factors associated with healthcare utilization among Arab immigrants and refugees. Methods 590 participants were interviewed 1 year post-migration to the United States Factors associated with healthcare utilization including stress coping mechanisms were examined using binary logistic regressions. Results Compared to national healthcare utilization data, immigrants had significantly lower and refugees had significantly higher rates. Being a refugee, married, and having health insurance were significantly associated with medical service utilization. None of the immigrants in this study had utilized psychological services. Among refugees, the use of medications and having strategies for dealing with stress were inversely associated with utilization of psychological services. Discussion (Conclusion) Healthcare utilization was significantly higher among refugees, who also reported a greater need for services than immigrants. PMID:25331684

Team Formation and Communication Restrictions in Collectives

NASA Technical Reports Server (NTRS)

Agogino, Adrian K.; Turner, Kagan

2003-01-01

A collective of agents often needs to maximize a "world utility" function which rates the performance of an entire system, while subject to communication restrictions among the agents. Such communication restrictions make it difficult for agents which try to pursue their own "private" utilities to take actions that also help optimize the world utility. Team formation presents a solution to this problem, where by joining other agents, an agent can significantly increase its knowledge about the environment and improve its chances of both optimizing its own utility and that its doing so will contribute to the world utility. In this article we show how utilities that have been previously shown to be effective in collectives can be modified to be more effective in domains with moderate communication restrictions resulting in performance improvements of up to 75%. Additionally we show that even severe communication constraints can be overcome by forming teams where each agent of a team shares the same utility, increasing performance an additional 25%. We show that utilities and team sizes can be manipulated to form the best compromise between how "aligned" an agent s utility is with the world utility and how easily an agent can learn that utility.

The construction of standard gamble utilities.

PubMed

van Osch, Sylvie M C; Stiggelbout, Anne M

2008-01-01

Health effects for cost-effectiveness analysis are best measured in life years, with quality of life in each life year expressed in terms of utilities. The standard gamble (SG) has been the gold standard for utility measurement. However, the biases of probability weighting, loss aversion, and scale compatibility have an inconclusive effect on SG utilities. We determined their effect on SG utilities using qualitative data to assess the reference point and the focus of attention. While thinking aloud, 45 healthy respondents provided SG utilities for six rheumatoid arthritis health states. Reference points, goals, and focuses of attention were coded. To assess the effect of scale compatibility, correlations were assessed between focus of attention and mean utility. The certain outcome served most frequently as reference point, the SG was perceived as a mixed gamble. Goals were mostly mentioned with respect to this outcome. Scale compatibility led to a significant upward bias in utilities; attention lay relatively more on the low outcome and this was positively correlated with mean utility. SG utilities should be corrected for loss aversion and probability weighting with the mixed correction formula proposed by prospect theory. Scale compatibility will likely still bias SG utilities, calling for research on a correction. Copyright (c) 2007 John Wiley & Sons, Ltd.

Utility Computing: Reality and Beyond

NASA Astrophysics Data System (ADS)

Ivanov, Ivan I.

Utility Computing is not a new concept. It involves organizing and providing a wide range of computing-related services as public utilities. Much like water, gas, electricity and telecommunications, the concept of computing as public utility was announced in 1955. Utility Computing remained a concept for near 50 years. Now some models and forms of Utility Computing are emerging such as storage and server virtualization, grid computing, and automated provisioning. Recent trends in Utility Computing as a complex technology involve business procedures that could profoundly transform the nature of companies' IT services, organizational IT strategies and technology infrastructure, and business models. In the ultimate Utility Computing models, organizations will be able to acquire as much IT services as they need, whenever and wherever they need them. Based on networked businesses and new secure online applications, Utility Computing would facilitate "agility-integration" of IT resources and services within and between virtual companies. With the application of Utility Computing there could be concealment of the complexity of IT, reduction of operational expenses, and converting of IT costs to variable `on-demand' services. How far should technology, business and society go to adopt Utility Computing forms, modes and models?

The discrepancy between risky and riskless utilities: a matter of framing?

PubMed

Stalmeier, P F; Bezembinder, T G

1999-01-01

Utilities differ according to whether they are derived from risky (gamble) and riskless (visual analog scale, time-tradeoff) assessment methods. The discrepancies are usually explained by assuming that the utilities elicited by risky methods incorporate attitudes towards risk, whereas riskless utilities do not. In (cumulative) prospect theory, risk attitude is conceived as consisting of two components: a decision-weight function (attentiveness to changes in, or sensitivity towards, chance) and a utility function (sensitivity towards outcomes). The authors' data suggest that a framing effect is a hitherto unrecognized and important factor in causing discrepancies between risky and riskless utilities. They collected risky evaluations with the gamble method, and riskless evaluations with difference measurement. Risky utilities were derived using expected-utility theory and prospect theory. With the latter approach, sensitivity towards outcomes and sensitivity towards chance are modeled separately. When the hypothesis that risky utilities from prospect theory coincide with riskless utilities was tested, it was rejected (n = 8, F(1,7) = 132, p = 0.000), suggesting that a correction for sensitivity towards chance is not sufficient to resolve the difference between risky and riskless utilities. Next, it was assumed that different gain/loss frames are induced by risky and riskless elicitation methods. Indeed, identical utility functions were obtained when the gain/loss frames were made identical across methods (n = 7), suggesting that framing was operative. The results suggest that risky and riskless utilities are identical after corrections for sensitivity towards chance and framing.

Utility as a rationale for choosing observer performance assessment paradigms for detection tasks in medical imaging.

PubMed

Wunderlich, Adam; Abbey, Craig K

2013-11-01

Studies of lesion detectability are often carried out to evaluate medical imaging technology. For such studies, several approaches have been proposed to measure observer performance, such as the receiver operating characteristic (ROC), the localization ROC (LROC), the free-response ROC (FROC), the alternative free-response ROC (AFROC), and the exponentially transformed FROC (EFROC) paradigms. Therefore, an experimenter seeking to carry out such a study is confronted with an array of choices. Traditionally, arguments for different approaches have been made on the basis of practical considerations (statistical power, etc.) or the gross level of analysis (case-level or lesion-level). This article contends that a careful consideration of utility should form the rationale for matching the assessment paradigm to the clinical task of interest. In utility theory, task performance is commonly evaluated with total expected utility, which integrates the various event utilities against the probability of each event. To formalize the relationship between expected utility and the summary curve associated with each assessment paradigm, the concept of a "natural" utility structure is proposed. A natural utility structure is defined for a summary curve when the variables associated with the summary curve axes are sufficient for computing total expected utility, assuming that the disease prevalence is known. Natural utility structures for ROC, LROC, FROC, AFROC, and EFROC curves are introduced, clarifying how the utilities of correct and incorrect decisions are aggregated by summary curves. Further, conditions are given under which general utility structures for localization-based methodologies reduce to case-based assessment. Overall, the findings reveal how summary curves correspond to natural utility structures of diagnostic tasks, suggesting utility as a motivating principle for choosing an assessment paradigm.

A Healthcare Utilization Analysis Framework for Hot Spotting and Contextual Anomaly Detection

PubMed Central

Hu, Jianying; Wang, Fei; Sun, Jimeng; Sorrentino, Robert; Ebadollahi, Shahram

2012-01-01

Patient medical records today contain vast amount of information regarding patient conditions along with treatment and procedure records. Systematic healthcare resource utilization analysis leveraging such observational data can provide critical insights to guide resource planning and improve the quality of care delivery while reducing cost. Of particular interest to providers are hot spotting: the ability to identify in a timely manner heavy users of the systems and their patterns of utilization so that targeted intervention programs can be instituted, and anomaly detection: the ability to identify anomalous utilization cases where the patients incurred levels of utilization that are unexpected given their clinical characteristics which may require corrective actions. Past work on medical utilization pattern analysis has focused on disease specific studies. We present a framework for utilization analysis that can be easily applied to any patient population. The framework includes two main components: utilization profiling and hot spotting, where we use a vector space model to represent patient utilization profiles, and apply clustering techniques to identify utilization groups within a given population and isolate high utilizers of different types; and contextual anomaly detection for utilization, where models that map patient’s clinical characteristics to the utilization level are built in order to quantify the deviation between the expected and actual utilization levels and identify anomalies. We demonstrate the effectiveness of the framework using claims data collected from a population of 7667 diabetes patients. Our analysis demonstrates the usefulness of the proposed approaches in identifying clinically meaningful instances for both hot spotting and anomaly detection. In future work we plan to incorporate additional sources of observational data including EMRs and disease registries, and develop analytics models to leverage temporal relationships among medical encounters to provide more in-depth insights. PMID:23304306

A healthcare utilization analysis framework for hot spotting and contextual anomaly detection.

PubMed

Hu, Jianying; Wang, Fei; Sun, Jimeng; Sorrentino, Robert; Ebadollahi, Shahram

2012-01-01

Patient medical records today contain vast amount of information regarding patient conditions along with treatment and procedure records. Systematic healthcare resource utilization analysis leveraging such observational data can provide critical insights to guide resource planning and improve the quality of care delivery while reducing cost. Of particular interest to providers are hot spotting: the ability to identify in a timely manner heavy users of the systems and their patterns of utilization so that targeted intervention programs can be instituted, and anomaly detection: the ability to identify anomalous utilization cases where the patients incurred levels of utilization that are unexpected given their clinical characteristics which may require corrective actions. Past work on medical utilization pattern analysis has focused on disease specific studies. We present a framework for utilization analysis that can be easily applied to any patient population. The framework includes two main components: utilization profiling and hot spotting, where we use a vector space model to represent patient utilization profiles, and apply clustering techniques to identify utilization groups within a given population and isolate high utilizers of different types; and contextual anomaly detection for utilization, where models that map patient's clinical characteristics to the utilization level are built in order to quantify the deviation between the expected and actual utilization levels and identify anomalies. We demonstrate the effectiveness of the framework using claims data collected from a population of 7667 diabetes patients. Our analysis demonstrates the usefulness of the proposed approaches in identifying clinically meaningful instances for both hot spotting and anomaly detection. In future work we plan to incorporate additional sources of observational data including EMRs and disease registries, and develop analytics models to leverage temporal relationships among medical encounters to provide more in-depth insights.

Patient Portal Utilization Among Ethnically Diverse Low Income Older Adults: Observational Study

PubMed Central

Quandt, Sara A; Sandberg, Joanne C; Miller Jr, David P; Latulipe, Celine; Leng, Xiaoyan; Talton, Jenifer W; Melius, Kathryn P; Smith, Alden; Bertoni, Alain G

2017-01-01

Background Patient portals can improve patient communication with providers, provide patients with greater health information access, and help improve patient decision making, if they are used. Because research on factors facilitating and limiting patient portal utilization has not been conceptually based, no leverage points have been indicated for improving utilization. Objective The primary objective for this analysis was to use a conceptual framework to determine potentially modifiable factors affecting patient portal utilization by older adults (aged 55 years and older) who receive care at clinics that serve low income and ethnically diverse communities. The secondary objective was to delineate how patient portal utilization is associated with perceived usefulness and usability. Methods Patients from one urban and two rural clinics serving low income patients were recruited and completed interviewer-administered questionnaires on patient portal utilization. Results A total of 200 ethnically diverse patients completed questionnaires, of which 41 (20.5%) patients reported utilizing portals. Education, social support, and frequent Internet utilization improve the odds of patient portal utilization; receiving health care at a rural clinic decreases the odds of portal utilization. Conclusions Leverage points to address disparities in patient portal utilization include providing training for older adults in patient portal utilization, involving spouses or other care partners in this training, and making information technology access available at public places in rural and urban communities. PMID:29138129

Testing the effectiveness of deregulation in the electric utility industry: A market-based approach

NASA Astrophysics Data System (ADS)

Wang, Manfen

In this paper, I investigate one stated purpose of deregulation in the electric utility industry---to make utility operations more responsive to news releases, a proxy for market forces. My premise is that utilities providing electricity to highly deregulated states will be more responsive to market forces than those providing electricity to non-deregulated states. I employ intraday data from April to June 2001, the year after deregulation, and from 1994, the year before deregulation. I also employ the Brown-Forsythe-Modified Levene (BFL) test to determine the volatility differences between days with released news and days without released news. The results of BFL F tests for the year 2001 indicate that utilities headquartered in and serving states that have undergone substantial deregulation respond to news releases more strongly than those utilities headquartered in and serving states that are still regulated. The BFL F tests for utilities in 1994 confirm the premise that regulated utilities are less responsive to news releases. Finally, I conduct regression tests for utilities, the results of which support the findings from BFL tests---that all utilities serving highly deregulated states show pronounced responses to macroeconomic news releases. It appears that deregulation in the electric utility industry does, in fact, make utility operations more responsive to market forces and that deregulation is effective for states that implement a customer-choice model.

24 CFR 990.185 - Utilities expense level: Incentives for energy conservation/rate reduction.

Code of Federal Regulations, 2011 CFR

2011-04-01

...) Utility benchmarking. HUD will pursue benchmarking utility consumption at the project level as part of the... convene a meeting with representation of appropriate stakeholders to review utility benchmarking options so that HUD may determine whether or how to implement utility benchmarking to be effective in FY 2011...

24 CFR 990.185 - Utilities expense level: Incentives for energy conservation/rate reduction.

Code of Federal Regulations, 2010 CFR

2010-04-01

...) Utility benchmarking. HUD will pursue benchmarking utility consumption at the project level as part of the... convene a meeting with representation of appropriate stakeholders to review utility benchmarking options so that HUD may determine whether or how to implement utility benchmarking to be effective in FY 2011...

Overcoming Communication Restrictions in Collectives

NASA Technical Reports Server (NTRS)

Tumer, Kagan; Agogino, Adrian K.

2004-01-01

Many large distributed system are characterized by having a large number of components (eg., agents, neurons) whose actions and interactions determine a %orld utility which rates the performance of the overall system. Such collectives are often subject to communication restrictions, making it difficult for components which try to optimize their own private utilities, to take actions that also help optimize the world utility. In this article we address that coordination problem and derive four utility functions which present different compromises between how aligned a component s private utility is with the world utility and how readily that component can determine the actions that optimize its utility. The results show that the utility functions specifically derived to operate under communication restrictions outperform both traditional methods and previous collective-based methods by up to 75%.

Estimating the relative utility of screening mammography.

PubMed

Abbey, Craig K; Eckstein, Miguel P; Boone, John M

2013-05-01

The concept of diagnostic utility is a fundamental component of signal detection theory, going back to some of its earliest works. Attaching utility values to the various possible outcomes of a diagnostic test should, in principle, lead to meaningful approaches to evaluating and comparing such systems. However, in many areas of medical imaging, utility is not used because it is presumed to be unknown. In this work, we estimate relative utility (the utility benefit of a detection relative to that of a correct rejection) for screening mammography using its known relation to the slope of a receiver operating characteristic (ROC) curve at the optimal operating point. The approach assumes that the clinical operating point is optimal for the goal of maximizing expected utility and therefore the slope at this point implies a value of relative utility for the diagnostic task, for known disease prevalence. We examine utility estimation in the context of screening mammography using the Digital Mammographic Imaging Screening Trials (DMIST) data. We show how various conditions can influence the estimated relative utility, including characteristics of the rating scale, verification time, probability model, and scope of the ROC curve fit. Relative utility estimates range from 66 to 227. We argue for one particular set of conditions that results in a relative utility estimate of 162 (±14%). This is broadly consistent with values in screening mammography determined previously by other means. At the disease prevalence found in the DMIST study (0.59% at 365-day verification), optimal ROC slopes are near unity, suggesting that utility-based assessments of screening mammography will be similar to those found using Youden's index.

A study on utilization of oral contraceptives in the City of Zagreb (2008-2010).

PubMed

Zelić-Kerep, Ana; Stimac, Danijela; Ozić, Sanja; Zivković, Kresimir; Zivković, Nikica

2014-06-01

Main aim of this study is to quantify and analyze the utilization and utilization trends of oral hormonal contraceptives in the City of Zagreb, 2008-2010, and to propose potential interventions, if necessary. Data gathered from Zagreb pharmacies were assessed by Anatomical Therapeutic Chemical Classification of drugs and Daily Defined Dose methodology. An alarming decrease in total utilization of hormonal contraceptives by 76% from 2008-2009 was found as the main result of this study. A major decrease by 95.5% in utilization of G03AB04 subgroup, sequential combined oral contraceptives, was noted in the year 2009. The subgroup G03AC0, progesterone-only pill group, showed a stable trend, and it became the most utilized subgroup in 2010, due to the decrease in utilization of both fixed and sequential combined oral contraceptives. Utilization of oral contraceptives in Croatia is not regulated adequately, since such dynamics in utilization can occur unnoticed. Measures need to take place in order to improve this situation. Proposed measures include organized farmacovigilance, prescription based on guidelines, and strict screening for risk factors in women seeking oral contraception. More research is required in Croatia to understand the pattern of utilization of hormonal contraceptives and to find the true cause of decrease in utilization of oral contraceptives.

[Utilities: a solution of a decision problem?].

PubMed

Koller, Michael; Ohmann, Christian; Lorenz, Wilfried

2008-01-01

Utility is a concept that originates from utilitarianism, a highly influential philosophical school in the Anglo-American world. The cornerstone of utilitarianism is the principle of maximum happiness or utility. In the medical sciences, this utility approach has been adopted and developed within the field of medical decision making. On an operational level, utility is the evaluation of a health state or an outcome on a one-dimensional scale ranging from 0 (death) to 1 (perfect health). By adding the concept of expectancy, the graphic representation of both concepts in a decision tree results in the specification of expected utilities and helps to resolve complex medical decision problems. Criticism of the utility approach relates to the rational perspective on humans (which is rejected by a considerable fraction of research in psychology) and to the artificial methods used in the evaluation of utility, such as Standard Gamble or Time Trade Off. These may well be the reason why the utility approach has never been accepted in Germany. Nevertheless, innovative concepts for defining goals in health care are urgently required, as the current debate in Germany on "Nutzen" (interestingly translated as 'benefit' instead of as 'utility') and integrated outcome models indicates. It remains to be seen whether this discussion will lead to a re-evaluation of the utility approach.

[Characteristics of dry matter production and nitrogen accumulation in barley genotypes with high nitrogen utilization efficiency].

PubMed

Huang, Yi; Li, Ting-Xuan; Zhang, Xi-Zhou; Ji, Lin

2014-07-01

A pot experiment was conducted under low (125 mg x kg-1) and normal (250 mg x kg(-1)) nitrogen treatments. The nitrogen uptake and utilization efficiency of 22 barley cultivars were investigated, and the characteristics of dry matter production and nitrogen accumulation in barley were analyzed. The results showed that nitrogen uptake and utilization efficiency were different for barley under two nitrogen levels. The maximal values of grain yield, nitrogen utilization efficiency for grain and nitrogen harvest index were 2.87, 2.91 and 2.47 times as those of the lowest under the low nitrogen treatment. Grain yield and nitrogen utilization efficiency for grain and nitrogen harvest index of barley genotype with high nitrogen utilization efficiency were significantly greater than low nitrogen utilization efficiency, and the parameters of high nitrogen utilization efficiency genotype were 82.1%, 61.5% and 50.5% higher than low nitrogen utilization efficiency genotype under the low nitrogen treatment. Dry matter mass and nitrogen utilization of high nitrogen utilization efficiency was significantly higher than those of low nitrogen utilization efficiency. A peak of dry matter mass of high nitrogen utilization efficiency occurred during jointing to heading stage, while that of nitrogen accumulation appeared before jointing. Under the low nitrogen treatment, dry matter mass of DH61 and DH121+ was 34.4% and 38.3%, and nitrogen accumulation was 54. 8% and 58.0% higher than DH80, respectively. Dry matter mass and nitrogen accumulation seriously affected yield before jointing stage, and the contribution rates were 47.9% and 54.7% respectively under the low nitrogen treatment. The effect of dry matter and nitrogen accumulation on nitrogen utilization efficiency for grain was the largest during heading to mature stages, followed by sowing to jointing stages, with the contribution rate being 29.5% and 48.7%, 29.0% and 15.8%, respectively. In conclusion, barley genotype with high nitrogen utilization efficiency had a strong ability of dry matter production and nitrogen accumulation. It could synergistically improve yield and nitrogen utilization efficiency by enhancing the ability of nitrogen uptake and dry matter formation before jointing stage in barley.

18 CFR 35.13 - Filing of changes in rate schedules, tariffs or service agreements.

Code of Federal Regulations, 2014 CFR

2014-04-01

... be reconciled with the utility's most recent FERC Form 1. If the utility has not yet submitted Form 1 for the Test Period, the utility shall submit the relevant Form 1 pages in draft form. (D) The utility.... Balance sheets shall be constructed in accordance with the annual report form for electric utilities...