Crustal structure and continental dynamics of Central China: A receiver function study and implications for ultrahigh-pressure metamorphism

NASA Astrophysics Data System (ADS)

He, Chuansong; Dong, Shuwen; Chen, Xuanhua; Santosh, M.; Li, Qiusheng

2014-01-01

The Qinling-Tongbai-Hong'an-Dabie-Sulu orogenic belt records the tectonic history of Paleozoic convergence between the South China and North China Blocks. In this study, the distribution of crustal thickness and P- and S-wave velocity ratio (Vp/Vs) is obtained by using the H-k stacking technique from the Dabie-Sulu belt in central China. Our results show marked differences in the crustal structure between the Dabie and Sulu segments of the ultrahigh-pressure (UHP) orogen. The lower crust in the Dabie orogenic belt is dominantly of felsic-intermediate composition, whereas the crust beneath the Sulu segment is largely intermediate-mafic. The crust of the Dabie orogenic belt is thicker by ca. 3-5 km as compared to that of the surrounding region with the presence of an ‘orogenic root’. The crustal thickness is nearly uniform in the Dabie orogenic belt with a generally smooth crust-mantle boundary. A symmetrically thickened crust in the absence of any deep-structural features similar to that of the Yangtze block suggests no supportive evidence for the proposed northward subduction of the Yangtze continental block beneath the North China Block. We propose that the collision between the Yangtze and North China Blocks and extrusion caused crustal shortening and thickening, as well as delamination of the lower crust, resulting in asthenospheric upwelling and lower crustal UHP metamorphism along the Dabie Orogen. Our results also reveal the presence of a SE to NW dipping Moho in the North China Block (beneath the Tran-North China Orogen and Eastern Block), suggesting the fossil architecture of the northwestward subduction of the Kula plate.

Spatial Relationship Between Crustal Structure and Mantle Seismicity in the Vrancea Seismogenic Zone of Romania

NASA Astrophysics Data System (ADS)

Knapp, C. C.; Enciu, D. M.; Knapp, J. H.

2007-12-01

Active crustal deformation and subsidence in the Southeast Carpathian foreland has previously been attributed to active foundering of thickened continental lithosphere beneath the Carpathian bend region (Knapp et al, 2005). The present study involves integration of active and passive-source seismic data in order to place constraints on the duration, timing, and scale of crustal deformation in the Carpathian foreland, and in particular to assess the genetic relationship with the Vrancea intermediate-depth seismogenic zone (VSZ). Relocated crustal earthquakes and focal mechanisms were correlated with four deep industry seismic profiles, the reprocessed DACIA PLAN deep seismic profile, and the DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) II and III profiles. Projection of foreland crustal hypocenters onto the deep seismic lines correlates well with previously identified crustal faults such as the Trotus and Sinaia, as well as the newly identified Ialomita Fault. Specifically, results of this study (1) image the full crustal and uppermost mantle structure of the Focsani Basin in the close proximity of the VSZ, (2) show evidence for a sub-horizontal, slightly east-dipping Moho in the vicinity of the VSZ and thinning of the crust towards the Carpathian orogen, (3) illustrate the conspicuous absence of west-dipping fabrics or structures in the crust and across the Moho, (4) present evidence that the Trotus Fault is a crustal-scale active fault with a dextral sense of motion, (5) suggest that the Paleozoic age Peceneaga-Camena and Capidava-Ovidiu Faults have not been active in post-Paleozoic time, and (6) show evidence for a new active crustal scale sinistral fault, named the Ialomita fault. Both the seismogenic Vrancea body and deformation in the Focsani Basin appear to be concentrically bound by the Trotus Fault in the north and east and the Sinaia-Ialomita Fault in the south, suggesting a coupled deformation between the VSZ and the foreland deformation, possibly accommodated on these two major fault systems. These results contradict both the "subduction-in-place" and "slab- break-off" hypotheses as feasible explanations for VSZ intermediate-depth seismicity, and lend additional support to a lithospheric delamination model to explain both the origin of the VSZ and the crustal architecture of the Southeast Carpathian foreland.

United States crustal thickness

NASA Technical Reports Server (NTRS)

Allenby, R. J.; Schnetzler, C. C.

1983-01-01

The thickness of the crust, the thickness of the basal (intermediate or lower) crustal layer, and the average velocity at the top of the mantle have been mapped using all available deep-penetrating seismic-refraction profiles in the conterminous United States and surrounding border areas. These profiles are indexed to their literature data sources. The more significant long wavelength anomalies on the three maps are briefly discussed and analyzed. An attempt to use Bouguer gravity to validate mantle structure was inconclusive.

Micro-seismicity in the Gulf of Cadiz: Is there a link between micro-seismicity, high magnitude earthquakes and active faults?

NASA Astrophysics Data System (ADS)

Silva, Sónia; Terrinha, Pedro; Matias, Luis; Duarte, João C.; Roque, Cristina; Ranero, César R.; Geissler, Wolfram H.; Zitellini, Nevio

2017-10-01

The Gulf of Cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes, occasionally punctuated by high magnitude events such as the M 8.7 1755 Great Lisbon earthquake and the M = 7.9 event of February 28th, 1969. Micro-seismicity was recorded during 11 months by a temporary network of 25 ocean bottom seismometers (OBSs) in an area of high seismic activity, encompassing the potential source areas of the mentioned large magnitude earthquakes. We combined micro-seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the Gulf of Cadiz area. Three controlling mechanisms are explored: i) active tectonic structures, ii) transitions between different lithospheric domains and inherited Mesozoic structures, and iii) fault weakening mechanisms. Our results show that micro-seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric/rheological domains. Even though the crustal structure is well imaged in the seismic profiles and in the bathymetry, crustal faults show low to negligible seismic activity. A possible explanation for this is that the crustal thrusts are thin-skinned structures rooting in relatively shallow sub-horizontal décollements associated with (aseismic) serpentinization levels at the top of the lithospheric mantle. Therefore, co-seismic slip along crustal thrusts may only occur during large magnitude events, while for most of the inter-seismic cycle these thrusts remain locked, or slip aseismically. We further speculate that high magnitude earthquake's ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers.

Lead-isotopic evidence for distinct source of granite and for distinct basement in the northern Appalachians, Maine.

USGS Publications Warehouse

Ayuso, R.A.

1986-01-01

Lead-isotopic compositions of feldspars in high-level Devonian granitic plutons across the northern Appalachians were measured. The presence of three fundamentally different sources of granites was indicated by three distinct lead-isotope groups. Plutons in the coastal lithotectonic block are the most radiogenic (206Pb/204Pb) 18.25-19.25; 207Pb/204Pb 15.59-15.67; 208Pb/204Pb 38.00-38.60); plutons in northern Maine are the least radiogenic (206Pb/204Pb 18.00-18.50; 207Pb/204Pb 15.51-15.55; 208Pb/204Pb 37.80-38.38). Intermediate lead-isotope values characterize the plutons in central Maine. All plutons show relatively radiogenic lead values for their ages and suggest the imprint of continental crustal sources, particularly in the coastal block. These plutons were formed in different crustal fragments in a continental environment, that were juxtaposed after emplacement of the granites.-L.C.H.

Magmatic arc structure around Mount Rainier, WA, from the joint inversion of receiver functions and surface wave dispersion

NASA Astrophysics Data System (ADS)

Obrebski, Mathias; Abers, Geoffrey A.; Foster, Anna

2015-01-01

The deep magmatic processes in volcanic arcs are often poorly understood. We analyze the shear wave velocity (VS) distribution in the crust and uppermost mantle below Mount Rainier, in the Cascades arc, resolving the main velocity contrasts based on converted phases within P coda via source normalization or receiver function (RF) analysis. To alleviate the trade-off between depth and velocity, we use long period phase velocities (25-100 s) obtained from earthquake surface waves, and at shorter period (7-21 s) we use seismic noise cross correlograms. We use a transdimensional Bayesian scheme to explore the model space (VS in each layer, number of interfaces and their respective depths, level of noise on data). We apply this tool to 15 broadband stations from permanent and Earthscope temporary stations. Most results fall into two groups with distinctive properties. Stations east of the arc (Group I) have comparatively slower middle-to-lower crust (VS = 3.4-3.8 km/s at 25 km depth), a sharp Moho and faster uppermost mantle (VS = 4.2-4.4 km/s). Stations in the arc (Group II) have a faster lower crust (VS = 3.7-4 km/s) overlying a slower uppermost mantle (VS = 4.0-4.3 km/s), yielding a weak Moho. Lower crustal velocities east of the arc (Group I) most likely represent ancient subduction mélanges mapped nearby. The lower crust for Group II ranges from intermediate to felsic. We propose that intermediate-felsic to felsic rocks represent the prearc basement, while intermediate composition indicates the mushy andesitic crustal magmatic system plus solidified intrusion along the volcanic conduits. We interpret the slow upper mantle as partial melt.

Magma interaction in the root of an arc batholith

NASA Astrophysics Data System (ADS)

Chapman, T.; Robbins, V.; Clarke, G. L.; Daczko, N. R.; Piazolo, S.

2016-12-01

Fiordland, New Zealand, preserves extensive Cretaceous arc plutons, emplaced into parts of the Delamerian/Ross Orogen. Dioritic to gabbroic material emplaced at mid to lower crustal levels are exposed in the Malaspina Pluton (c. 1.2 GPa) and the Breaksea Orthogneiss (c. 1.8 GPa). Distinct magmatic pulses can be mapped in both of these plutons consistent with cycles of melt advection. Relationships are consistent with predictions from lower crustal processing zones (MASH and hot zones) considered important in the formation of Cordilleran margins. Metamorphic garnet growth is enhanced along magmatic contacts, such as where hornblende gabbronorite is cut by garnet-clinopyroxene-bearing diorite. Such features are consistent with cycles of incremental emplacement, younger magma having induced localised garnet granulite metamorphism in wall rock of older material. Temperature estimates and microstructures preserved in garnet granulite are consistent with sub-solidus, water-poor conditions in both the Malaspina and Breaksea Orthogneiss. The extent and conditions of the metamorphism implies conditions and duration was incapable of partially melting older wall rock material. The nature of interactions in intermediate to basic compositions are assessed in terms of magma genesis in the Cretaceous batholith. Most of the upper crustal felsic I-type magmatism along the margin being controlled by high-pressure garnet-clinopyroxene fractionation.

Evolution of Continental Lower Crust Recorded By an Exhumed Deep Crustal Intracontinental Shear Zone

NASA Astrophysics Data System (ADS)

Dumond, G.; Mahan, K. H.; Regan, S. P.; Williams, M. L.; Goncalves, P.; Wood, V. R.

2014-12-01

Exposures of deep crustal shear zones are fundamental records of strain localization and the temporal evolution of ductile to brittle behavior as these tectonites were exhumed to the surface. We present results from a decade of field-based research on a deeply exhumed (~35 km-paleodepths) strike-slip shear zone in the western Churchill province of the Canadian Shield. The Grease River shear zone is a >400 km-long and 7 km-thick structure that cuts the Athabasca granulite terrane, North America's largest exposure of continental lower crust (>20,000 km2). The shear zone is dominated by granulite- to amphibolite-grade L-S and L>S tectonites characterized by penetrative NE-striking steeply-dipping foliations with gently-plunging to sub-horizontal stretching and intersection lineations. These fabrics are locally overprinted by pseudotachylyte and narrow (<500 m-thick) greenschist-grade zones of cataclasite. Dextral kinematics are defined by deflected foliation trajectories, C' shear bands, and well-developed σ- and δ-type porphyroclasts of Kfs + Pl + Opx + Grt + Hb in felsic to intermediate granulite paragneisses and orthogneisses. Data collected along a well-exposed, nearly 150 km-long segment of the shear zone documents a >100 m.y. episodic record of transpressive to strike-slip intracontinental strain accumulation that coincided with two oppositely convergent orogenies: the east-vergent arc-continent collision of the 1.94-1.90 Ga Taltson orogen and the west-vergent continent-continent collision of the 1.9-1.8 Ga Trans-Hudson orogen. Deformation mechanisms evolved from distributed ductile dynamic recrystallization and grain-size reduction to localized pseudotachylyte development, cataclastic flow, and brittle faulting. Lower crustal behavior during strain localization was dynamic. Melt-weakened mono-cyclic crust was juxtaposed against strong isobarically-cooled poly-cyclic crust along the shear zone at 1.92-1.90 Ga. Brittle-ductile reactivation of the structure during exhumation to middle crustal levels was coincident with fluid-mediated retrograde reactions that facilitated crustal-scale segmentation and transpressive uplift of lower crustal granulites at 1.85 Ga. This study illustrates that lower crustal rheology is spatially and temporally heterogeneous.

Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

USGS Publications Warehouse

McBride, J.H.; England, R.W.

1999-01-01

Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an "acoustic window" into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse. The appearance of two levels of uneven and diffractive (i.e., corrugated) reflectivity in the lower crust, best developed on east-west-oriented profiles, is characteristic of the East Shetland platform. However, a north-south-oriented profile reveals an interpreted south-vergent folded and imbricated thrust structure in the lower crust that appears to be tied to the two levels of corrugated reflectivity on the east-west profiles. A thrust-belt origin for lower crustal reflectivity would explain its corrugated appearance. Regional seismic velocity models derived from refraction data suggest that this reflectivity correlates with a continuous lower crustal layer that has an intermediate seismic velocity. The lower crustal reflectivity is determined to be older than Mesozoic age by the bending down and truncation of the two reflectivity levels at the western margin of the North Sea Viking graben by a major mantle reflector inferred to be associated with Mesozoic rifting. The results of this study are thus in contrast with orthodox interpretations of the reflective layered lower crust as being caused by mantle-derived igneous intrusion or by deformation fabrics associated with stretching in response to continental rifting.

Crustal structure of China from deep seismic sounding profiles

USGS Publications Warehouse

Li, S.; Mooney, W.D.

1998-01-01

More than 36,000 km of Deep Seismic Sounding (DSS) profiles have been collected in China since 1958. However, the results of these profiles are not well known in the West due to the language barrier. In this paper, we summarize the crustal structure of China with a new contour map of crustal thickness, nine representative crustal columns, and maps showing profile locations, average crustal velocity, and Pn velocity. The most remarkable aspect of the crustal structure of China is the well known 70+ km thickness of the crust of the Tibetan Plateau. The thick (45-70 km) crust of western China is separated from the thinner (30-45 km) crust of eastern China by the north-south trending seismic belt (105??E). The average crustal velocity of China ranges from 6.15 to 6.45 km/s, indicating a felsic-to-intermediate bulk crustal composition. Upper mantle (Pn) velocities are 8.0 ?? 0.2 km/s, equal to the global continental average. We interpret these results in terms of the most recent thermo-tectonic events that have modified the crust. In much of eastern China, Cenoxoic crustal extension has produced a thin crust with a low average crustal velocity, similar to western Europe and the Basin and Range Province, western USA. In western China, Mesozoic and Cenoxoic arc-continent and continent-continent collisions have led to crustal growth and thickening. Inferences on the process of crustal thickening are provided by the deep crustal velocity structure as determined by DSS profiles and other seismological studies. A high velocity (7.0-7.4 km/s) lower-crustal layer has been reported in western China only beneath the southernmost Tibetan Plateau. We identity this high-velocity layer as the cold lower crust of the subducting Indian plate. As the Indian crust is injected northward into the Tibetan lower crust, it heats and assimilates by partial melting, a process that results in a reduction in the seismic velocity of the lower crust in the central and northern Tibetan Plateau. ?? 1998 Elsevier Science B.V. All rights reserved.

High-K granites of the Rum Jungle Complex, N-Australia: Insights into the Late Archean crustal evolution of the North Australian Craton

NASA Astrophysics Data System (ADS)

Drüppel, K.; McCready, A. J.; Stumpfl, E. F.

2009-08-01

The Late Archean (c. 2.54-2.52 Ga) high-K granitoids of the Rum Jungle Complex, Northern Australia, display the igneous mineral assemblage of K-feldspar, quartz, plagioclase, biotite, and magnetite, and accessories such as zircon, monazite, titanite, allanite, apatite, and ilmenite. The granites underwent a variably severe greenschist facies alteration and associated deformation during the Barramundi Orogeny (1.88-1.85 Ga). The K-rich granitoids have variable compositions, mainly comprising syenogranite and quartz-monzonite. They can be subdivided into two major groups, (1) felsic granites and (2) intermediate to felsic granites, quartz-monzonites, and diorite. The felsic group (69-76 wt.% SiO 2) shares many features with typical Late Archean potassic granites. They are K- and LILE-rich and show marked depletion in Sr and Eu and the high field strength elements (HFSE), particularly Nb and Ti, relative to LILE and LREE. Compared to the average upper crust they have anomalously high Th (up to 123 ppm) and U (up to 40 ppm). The intermediate to felsic group (56-69 wt.% SiO 2) differs from the felsic group in having weakly lower Th and U but higher Mg#, Ti, Ba, Sr, Ni, Cr and REE, with a less pronounced negative Eu anomaly. This group displays well-defined trends in Harker diagrams, involving a negative correlation of Si with Sr, Ca, Na, and P whereas K, Rb, and Ba increase in the same direction, suggesting fractional crystallization of feldspar was more prominent than in the felsic suite. The mineralogical and geochemical characteristics of the felsic group are consistent with granite formation by intracrustal melting of plagioclase-rich igneous protoliths, probably of tonaltic to granodioritic composition, at moderate crustal levels. The intermediate to felsic granites, on the other hand, appear to be the products of mantle-crust interaction, possibly by melting of or mixing with more mafic igneous rocks. As evidenced by the presence of older inherited zircons crustal recycling of a pre-greenstone crust of the North Australian Craton of > 3.5 Ga played an important role in the formation of the Late Archean granites of the Rum Jungle Complex.

Abrupt shift in δ18O values at Medicine Lake volcano (California, USA)

USGS Publications Warehouse

Donnelly-Nolan, J. M.

1998-01-01

 Oxygen-isotope analyses of lavas from Medicine Lake volcano (MLV), in the southern Cascade Range, indicate a significant change in δ18O in Holocene time. In the Pleistocene, basaltic lavas with <52% SiO2 averaged +5.9‰, intermediate lavas averaged +5.7‰, and silicic lavas (≥63.0%SiO2) averaged +5.6‰. No analyzed Pleistocene rhyolites or dacites have values greater than +6.3‰. In post-glacial time, basalts were similar at +5.7‰ to those erupted in the Pleistocene, but intermediate lavas average +6.8‰ and silicic lavas +7.4‰ with some values as high as +8.5‰. The results indicate a change in the magmatic system supplying the volcano. During the Pleistocene, silicic lavas resulted either from melting of low-18O crust or from fractionation combined with assimilation of very-low-18O crustal material such as hydrothermally altered rocks similar to those found in drill holes under the center of the volcano. By contrast, Holocene silicic lavas were produced by assimilation and/or wholesale melting of high-18O crustal material such as that represented by inclusions of granite in lavas on the upper flanks of MLV. This sudden shift in assimilant indicates a fundamental change in the magmatic system. Magmas are apparently ponding in the crust at a very different level than in Pleistocene time.

Time-dependent changes in magmatic and hydrothermal activity at the Costa Rica Rift recorded by variations in oceanic crustal structure

NASA Astrophysics Data System (ADS)

Wilson, D. J.; Peirce, C.; Hobbs, R. W.; Gregory, E. P. M.; Zhang, L.

2016-12-01

Geophysical studies of crustal structure at a diverse range of ridges have provided evidence that the balance between spreading rate and magma supply determines whether spreading predominantly occurs by magmatic accretion of new oceanic crust or through tectonic stretching of the whole lithosphere. Asymmetric spreading, patterns of on- and off-axis volcanism, the evolution of oceanic core complexes and the distribution of hydrothermal systems all indicate that the process of spreading is not constant over geologically short timescales. The structure of the resulting crust reflects this complexity in origin. Studies along flow-lines across ridges spreading at intermediate rates suggest variations in topographic style and crustal structure have periodically occurred, controlled by the interplay between magmatic accretion and tectonic stretching, and coupled to the degree of hydrothermal activity. Seismic reflection images and tomographic models derived from wide-angle seismic data have enabled a detailed examination of the oceanic crust that formed at the fast-to-intermediate-spreading (36 mm yr-1) Costa Rica Rift over the last 6 Ma, to look for any temporal variation in basement topography, upper crust (layer 2) P-wave velocity/density structure and crustal thickness. Coincident marine gravity and magnetic data not only allow us to test the validity of the final velocity-density model but also review variability in half-spreading rate, respectively. Collectively our analyses allow us to investigate the timescale and cyclicity of crustal structure variations and, having determined the spreading rate over time, consider how this may reflect changes in magma supply and/or hydrothermal activity at the Costa Rica Rift, using borehole 504B as the ground-truth. This research is part of a major, interdisciplinary NERC-funded collaboration entitled: Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge (OSCAR).

Timescale of Petrogenetic Processes Recorded in the Mount Perkins Magma System, Northern Colorado River Extension Corridor, Arizona

NASA Technical Reports Server (NTRS)

Danielson, Lisa R.; Metcalf, Rodney V.; Miller, Calvin F.; Rhodes Gregory T.; Wooden, J. L.

2013-01-01

The Miocene Mt. Perkins Pluton is a small composite intrusive body emplaced in the shallow crust as four separate phases during the earliest stages of crustal extension. Phase 1 (oldest) consists of isotropic hornblende gabbro and a layered cumulate sequence. Phase 2 consists of quartz monzonite to quartz monzodiorite hosting mafic microgranitoid enclaves. Phase 3 is composed of quartz monzonite and is subdivided into mafic enclave-rich zones and enclave-free zones. Phase 4 consists of aphanitic dikes of mafic, intermediate and felsic compositions hosting mafic enclaves. Phases 2-4 enclaves record significant isotopic disequilibrium with surrounding granitoid host rocks, but collectively enclaves and host rocks form a cogenetic suite exhibiting systematic variations in Nd-Sr-Pb isotopes that correlate with major and trace elements. Phases 2-4 record multiple episodes of magma mingling among cogenetic hybrid magmas that formed via magma mixing and fractional crystallization at a deeper crustal. The mafic end-member was alkali basalt similar to nearby 6-4 Ma basalt with enriched OIB-like trace elements and Nd-Sr-Pb isotopes. The felsic end-member was a subalkaline crustal-derived magma. Phase 1 isotropic gabbro exhibits elemental and isotopic compositional variations at relatively constant SiO2, suggesting generation of isotropic gabbro by an open-system process involving two mafic end-members. One end-member is similar in composition to the OIB-like mafic end-member for phases 2-4; the second is similar to nearby 11-8 Ma tholeiite basalt exhibiting low epsilon (sub Nd), and depleted incompatible trace elements. Phase 1 cumulates record in situ fractional crystallization of an OIB-like mafic magma with isotopic evidence of crustal contamination by partial melts generated in adjacent Proterozoic gneiss. The Mt Perkins pluton records a complex history in a lithospheric scale magma system involving two distinct mantle-derived mafic magmas and felsic magma sourced in the crust. Mixing and fractional crystallization of these magmas at various levels in the crust generated a suite of intermediate composition magmas. U-Pb zircon SHRIMP ages of phase 1 (15.7 +/- 0.2 Ma), phase 3 (15.8 +/- 0.2 Ma) and phase 4 (15.4 +/- 0.3 Ma) document a 100-300k year timescale for petrogenetic processes recorded in the Mt Perkins magma system.

Crustal structure beneath northeast India inferred from receiver function modeling

NASA Astrophysics Data System (ADS)

Borah, Kajaljyoti; Bora, Dipok K.; Goyal, Ayush; Kumar, Raju

2016-09-01

We estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast India, by using H-Vp/Vs stacking method and a non-linear direct search approach, Neighbourhood Algorithm (NA) technique followed by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data. Results show significant variations of thickness, shear velocities (Vs) and Vp/Vs ratio in the crust of the study region. The inverted shear wave velocity models show crustal thickness variations of 32-36 km in Shillong Plateau (North), 36-40 in Assam Valley and ∼44 km in Lesser Himalaya (South). Average Vp/Vs ratio in Shillong Plateau is less (1.73-1.77) compared to Assam Valley and Lesser Himalaya (∼1.80). Average crustal shear velocity beneath the study region varies from 3.4 to 3.5 km/s. Sediment structure beneath Shillong Plateau and Assam Valley shows 1-2 km thick sediment layer with low Vs (2.5-2.9 km/s) and high Vp/Vs ratio (1.8-2.1), while it is observed to be of greater thickness (4 km) with similar Vs and high Vp/Vs (∼2.5) in RUP (Lesser Himalaya). Both Shillong Plateau and Assam Valley show thick upper and middle crust (10-20 km), and thin (4-9 km) lower crust. Average Vp/Vs ratio in Assam Valley and Shillong Plateau suggest that the crust is felsic-to-intermediate and intermediate-to-mafic beneath Shillong Plateau and Assam Valley, respectively. Results show that lower crust rocks beneath the Shillong Plateau and Assam Valley lies between mafic granulite and mafic garnet granulite.

Origin of hybrid ferrolatite lavas from Magic Reservoir eruptive center, Snake River Plain, Idaho

NASA Astrophysics Data System (ADS)

Honjo, Norio; Leeman, William P.

1987-06-01

The mineralogy and geochemical characteristics of intermediate composition ferrolatites and related lavas from the Magic Reservoir eruptive center (central Snake River Plain) have been investigated to evaluate the origin and petrologic significance of these hybrid lavas. The ferrolatites are chemically uniform, but contain a disequilibrium phenocryst/xenocryst assemblage derived in part from mixed rhyolitic and basaltic magmas that are closely represented by extrusive units in the area. The hybrid lavas also contain xenoliths of Archean granulites and have high 87Sr/ 86Sr and low 143Nd/144Nd ratios, all of which suggest significant magma-crust interaction. Quantitative models including magma mixing, minor crystal fractionation, and crustal contamination very closely reproduce the observed compositions of these ferrolatites; closed system fractionation and (or) simple bulk contamination models are not as successful and can be ruled out. It appears that preexisting mafic and silicic magmas from distinct sources (e.g., mantle and crust) encounter one another in crustal-level magma chambers under conditions where intimate mixing may occur despite wide differences in the physical properties of these liquids.

Formation of continental crust in a temporally linked arc magma system from 5 to 30 km depth: ~ 90 Ma plutonism in the Cascades Crystalline Core composite arc section

NASA Astrophysics Data System (ADS)

Ratschbacher, B. C.; Miller, J. S.; Kent, A. J.; Miller, R. B.; Anderson, J. L.; Paterson, S. R.

2015-12-01

Continental crust has an andesitic bulk composition with a mafic lower crust and a granodioritic upper crust. The formation of stratified continental crust in general and the vertical extent of processes active in arc crustal columns leading to the differentiation of primitive, mantle-derived melts entering the lower crust are highly debated. To investigate where in the crustal column magma mixing, fractionation, assimilation and crystal growth occur and to what extent, we study the ~ 90 Ma magmatic flare-up event of the Cascades arc, a magma plumbing system from ~ 5 to 30 km depth. We focus on three intrusive complexes, emplaced at different depths during major regional shortening in an exceptionally thick crust (≥ 55 km1) but which are temporally related: the upper crustal Black Peak intrusion (1-3 kbar at 3.7 to 11 km; ~ 86.8 to 91.7 Ma2), the mid-crustal Mt. Stuart intrusion (3.5-4.0 kbar at 13 to 15 km; 90.8 and 96.3 Ma3) and the deep crustal Tenpeak intrusion (7 to 10 kbar at 25 to 37 km; 89.7 to 92.3 Ma4). These intrusive complexes are well characterized by geochronology showing that they have been constructed incrementally by multiple magma batches over their lifespans and thus allow the monitoring and comparison of geochemical parameters over time at different depths. We use a combination of whole rock major and trace element data and isotopes combined with detailed investigation of amphibole, which has been recognized to be important in the generation of calc-alkaline rocks in arcs to test the following hypotheses: (a) compositional bimodality is produced in the lower crust, whereas upper crustal levels are dominated by mixing to form intermediate compositions, or (b) differentiation occurs throughout the crustal column with different crystallizing phases and their compositions controlling the bulk chemistry. 1. Miller et al. 2009: GSA Special Paper 456, p. 125-149 2. Shea 2014: PhD thesis, Massachusetts Institute of Technology 3. Anderson et al. 2012: International Geology Review, v. 54, no. 5, p. 491-508 4. Matzel et al. 2006: GSA Bulletin, v. 118, no. 11-12, p. 1412-1430

Crustal structure of the Kaapvaal craton and its significance for early crustal evolution

NASA Astrophysics Data System (ADS)

James, David E.; Niu, Fenglin; Rokosky, Juliana

2003-12-01

High-quality seismic data obtained from a dense broadband array near Kimberley, South Africa, exhibit crustal reverberations of remarkable clarity that provide well-resolved constraints on the structure of the lowermost crust and Moho. Receiver function analysis of Moho conversions and crustal multiples beneath the Kimberley array shows that the crust is 35 km thick with an average Poisson's ratio of 0.25. The density contrast across the Moho is ˜15%, indicating a crustal density about 2.86 gm/cc just above the Moho, appropriate for felsic to intermediate rock compositions. Analysis of waveform broadening of the crustal reverberation phases suggests that the Moho transition can be no more than 0.5 km thick and the total variation in crustal thickness over the 2400 km 2 footprint of the array no more than 1 km. Waveform and travel time analysis of a large earthquake triggered by deep gold mining operations (the Welkom mine event) some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the Kimberley array and the event. P- and S-wave velocities for the lowermost crust are modeled to be 6.75 and 3.90 km/s, respectively, with uppermost mantle velocities of 8.2 and 4.79 km/s, respectively. Seismograms from the Welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the Moho. Correlation between observed and synthetic waveforms and phase amplitudes of the Moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the Moho transition zone is everywhere less than about 2 km thick. While the extremely flat Moho, sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation, a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.

Crustal and Uppermost Mantle Structure across the Tibet-Qinling Transition Zone in NE Tibet: Implications for Material Extrusion of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Ye, Z.; Li, J.; Gao*, R.; Song, X.; Li, Q.; Li, Y.; Huang, X.; Xiong, X.; Li, W.; WANG, Y.

2017-12-01

Based on a dense linear seismic array traversing across the eastern margin of the Tibetan plateau to the Qinling belt, we conducted joint inversion of receiver functions and surface wave dispersions under constraints of P-wave velocity and derived a crustal and uppermost mantle Vs profile simultaneously with a Vp/Vs ratio profile. Our observations indicate that the Qinling belt, which shows ratio Vp/Vs<1.8 indicative of intermediate-to-felsic components in the lower crust, is currently not acting as a channel accommodating extrusion of the mid-lower crustal flow; and extrusion of Tibet's ductile mantle flow through the Qinling belt as a channel would only be feasible in the sub-lithosphere depth (asthenosphere). Our results suggest that ductile material extrusion of the mid-lower crustal flow accompanied with fault-related tectonics and gravitational buoyancy resulted from lithospheric detachment (triggered by the asthenospheric flow) may jointly work on the plateau uplift and expansion in this Tibet-Qinling transition zone. Corresponding Author: R.Gao, ruigao126@126.com

Crustal structure between Lake Mead, Nevada, and Mono Lake, California

USGS Publications Warehouse

Johnson, Lane R.

1964-01-01

Interpretation of a reversed seismic-refraction profile between Lake Mead, Nevada, and Mono Lake, California, indicates velocities of 6.15 km/sec for the upper layer of the crust, 7.10 km/sec for an intermediate layer, and 7.80 km/sec for the uppermost mantle. Phases interpreted to be reflections from the top of the intermediate layer and the Mohorovicic discontinuity were used with the refraction data to calculate depths. The depth to the Moho increases from about 30 km near Lake Mead to about 40 km near Mono Lake. Variations in arrival times provide evidence for fairly sharp flexures in the Moho. Offsets in the Moho of 4 km at one point and 2 1/2 km at another correspond to large faults at the surface, and it is suggested that fracture zones in the upper crust may displace the Moho and extend into the upper mantle. The phase P appears to be an extension of the reflection from the top of the intermediate layer beyond the critical angle. Bouguer gravity, computed for the seismic model of the crust, is in good agreement with the measured Bouguer gravity. Thus a model of the crustal structure is presented which is consistent with three semi-independent sources of geophysical data: seismic-refraction, seismic-reflection, and gravity.

Faulting and hydration of the Juan de Fuca plate system

NASA Astrophysics Data System (ADS)

Nedimović, Mladen R.; Bohnenstiehl, DelWayne R.; Carbotte, Suzanne M.; Pablo Canales, J.; Dziak, Robert P.

2009-06-01

Multichannel seismic observations provide the first direct images of crustal scale normal faults within the Juan de Fuca plate system and indicate that brittle deformation extends up to ~ 200 km seaward of the Cascadia trench. Within the sedimentary layering steeply dipping faults are identified by stratigraphic offsets, with maximum throws of 110 ± 10 m found near the trench. Fault throws diminish both upsection and seaward from the trench. Long-term throw rates are estimated to be 13 ± 2 mm/kyr. Faulted offsets within the sedimentary layering are typically linked to larger offset scarps in the basement topography, suggesting reactivation of the normal fault systems formed at the spreading center. Imaged reflections within the gabbroic igneous crust indicate swallowing fault dips at depth. These reflections require local alteration to produce an impedance contrast, indicating that the imaged fault structures provide pathways for fluid transport and hydration. As the depth extent of imaged faulting within this young and sediment insulated oceanic plate is primarily limited to approximately Moho depths, fault-controlled hydration appears to be largely restricted to crustal levels. If dehydration embrittlement is an important mechanism for triggering intermediate-depth earthquakes within the subducting slab, then the limited occurrence rate and magnitude of intraslab seismicity at the Cascadia margin may in part be explained by the limited amount of water imbedded into the uppermost oceanic mantle prior to subduction. The distribution of submarine earthquakes within the Juan de Fuca plate system indicates that propagator wake areas are likely to be more faulted and therefore more hydrated than other parts of this plate system. However, being largely restricted to crustal levels, this localized increase in hydration generally does not appear to have a measurable effect on the intraslab seismicity along most of the subducted propagator wakes at the Cascadia margin.

Looking through the Zircon Kaleidoscope: Durations, Rates, and Fluxes in Silicic Magmatic System

NASA Astrophysics Data System (ADS)

Schaltegger, U.; Wotzlaw, J. F.

2014-12-01

The crystallization rate of zircon in a cooling magma depends on the cooling rate through the saturation interval in addition to compositional and kinetic factors. Repeated influx of hot magma over 10-20 ka leads to short-amplitude temperature oscillations, which are recorded by resorption/crystallization cycles of zircon. Plotting the number of dated zircons versus their high-precision U-Pb date results in curves that qualitatively relate to the evolution of magma temperature over time [1], [2]. The trace elemental, O and Hf isotopic composition of zircon gives indications about the degree of magma homogenization and thermal evolution. Zircons from systems with small volumes and magma fluxes record non-systematic chemical and Hf isotopic heterogeneity, suggesting crystallization in non-homogenized ephemeral magma batches. Such systems typically lead to small, mid-upper crustal plutons [3]. Zircons from large-volume crystal-poor rhyolites record initial heterogeneities and rapid amalgamation of smaller magma batches over 10 ka [4], while zircons from monotonous intermediates record magma evolution over several 100 ka with coherent fractionation trends suggesting homogenization and a coherent thermal evolution [2]. In both cases, volumes and flux rates were sufficient to produce large volumes of eruptible magma on very contrasting time scales. Zircon is therefore recording cyclic crystallization-rejuvenation processes during temperature fluctuations in intermediate to upper crustal magma reservoirs but may not relate to the physical pluton emplacement or eruption. We can quantify volumes, rates of magma influx, rates of cooling and crystallization, and the degree of convective homogenization from zircon data, and infer reservoir assembly and eruption trigger mechanisms. These parameters largely control the evolution of long-lived, low-flux silicic magmatic system typical for mid-to-upper crustal plutons, monotonous intermediates are characterized by intermediate durations and fluxes while short-lived, high-flux systems preferentially produce crystal-poor rhyolites. References: [1] Caricchi et al. (2014) Nature 511, 457-461; [2] Wotzlaw et al. (2013) Geology 41, 867-870; [3] Broderick (2013) PhD thesis, Univ. of Geneva; [3] Wotzlaw et al. (2014) Geology, doi:10.1130/G35979.1

Crustal structure along the geosciences transect from Altay to Altun Tagh

USGS Publications Warehouse

Wang, Y.-X.; Han, G.-H.; Jiang, M.; Yuan, X.-C.; Mooney, W.D.; Coleman, R.G.

2004-01-01

Based upon the P- and S-wave data acquired along the geoscience transect from Altay to Altun Tagh in Northwest China, the crustal structures of velocities and Poisson's ratio are determined. The crustal velocity structure features an obvious three-layer structure with velocities of 6. 0 ??? 6. 3km/s, 6. 3 ??? 6. 6km/s and 6.9 ??? 7. Okm/s from surface to depth, respectively. The crustal thickness along the. entire profile is mostly 50km with the thickest crust (56km) beneath the Altay and the thinnest (46km) beneath the Junggar basin. The velocities underlying Moho are 7.7 to 7.8km/s between the Tianshan and the Junggar basin, and 7.9 to 8.0km/s below the Altay Mountains and eastern margin of the Tarim basin. The southern half of the profile, including the eastern Tianshan Mountains and eastern margin of the Tarim basin, shows low P-wave velocities and ?? = 0. 25 to a depth, of 30km, which suggests a quartz-rich, granitic upper crustal composition. The northern half of the profile below the Altay Mountains and Junggar Accretional Belt has a higher Poisson's ratio of ?? = 0.26 ??? 0.27 to a depth of 30km, indicative of an intermediate crustal composition, The entire profile is underlain by a 15 to 30km thick high-velocity (6.9 ??? 7.0km/s; ?? = 0. 26 - 0.28) lower crustal layer that we interpret to have a bulk composition of mafic granulite. At the southern end of the profile a 5km-thick midcrustal low-velocity layer ( Vp, = 5.9km/s, ?? = 0.25) underlies the Tianshan and the region to the south, and may be indicative of granitic intrusive in Late Paleozoic.

Structure of the crust and upper mantle in the western United States

USGS Publications Warehouse

Pakiser, L.C.

1963-01-01

Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2,000 stations in the western conterminous United States as a part of the VELA UNIFORM program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The speed of compressional waves in the upper-mantle rocks ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province. In general, the speed of compressional waves in the upper-mantle rocks tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The crust tends to be thick in regions where the speed of compressional waves in the upper-mantle rocks (and presumably the density) is high, and tends to be relatively thin where the speed of compressional waves in the upper-mantle rocks (and density) is lower. With in the Basin and Range province, crustal thickness seems to vary directly with regional altitude above sea level. Evidence that a layer of intermediate compressional-wave speed exists in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper- mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic time. Regions of thinner crust and low-density upper mantle have had a Cenozoic history of intense diastrophism and silicic volcanism.

Neogene-Quaternary Volcanic forms in the Carpathian-Pannonian Region: a review

NASA Astrophysics Data System (ADS)

Lexa, Jaroslav; Seghedi, Ioan; Németh, Karoly; Szakács, Alexandru; Koneĉny, Vlastimil; Pécskay, Zoltan; Fülöp, Alexandrina; Kovacs, Marinel

2010-09-01

Neogene to Quaternary volcanic/magmatic activity in the Carpathian-Pannonian Region (CPR) occurred between 21 and 0.1 Ma with a distinct migration in time from west to east. It shows a diverse compositional variation in response to a complex interplay of subduction with rollback, back-arc extension, collision, slab break-off, delamination, strike-slip tectonics and microplate rotations, as well as in response to further evolution of magmas in the crustal environment by processes of differentiation, crustal contamination, anatexis and magma mixing. Since most of the primary volcanic forms have been affected by erosion, especially in areas of post-volcanic uplift, based on the level of erosion we distinguish: (1) areas eroded to the basement level, where paleovolcanic reconstruction is not possible; (2) deeply eroded volcanic forms with secondary morphology and possible paleovolcanic reconstruction; (3) eroded volcanic forms with remnants of original morphology preserved; and (4) the least eroded volcanic forms with original morphology quite well preserved. The large variety of volcanic forms present in the area can be grouped in a) monogenetic volcanoes and b) polygenetic volcanoes and their subsurface/intrusive counterparts that belong to various rock series found in the CPR such as calc-alkaline magmatic rock-types (felsic, intermediate and mafic varieties) and alkalic types including K-alkalic, shoshonitic, ultrapotassic and Na-alkalic. The following volcanic/subvolcanic forms have been identified: (i) domes, shield volcanoes, effusive cones, pyroclastic cones, stratovolcanoes and calderas with associated intrusive bodies for intermediate and basic calclkaline volcanism; (ii) domes, calderas and ignimbrite/ash-flow fields for felsic calc-alkaline volcanism and (iii) dome flows, shield volcanoes, maars, tuffcone/tuff-rings, scoria-cones with or without related lava flow/field and their erosional or subsurface forms (necks/ plugs, dykes, shallow intrusions, diatreme, lava lake) for various types of K- and Na-alkalic and ultra-potassic magmatism. Finally, we provide a summary of the eruptive history and distribution of volcanic forms in the CPR using several sub-region schemes.

Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data

NASA Astrophysics Data System (ADS)

Bécel, Anne; Shillington, Donna J.; Nedimović, Mladen R.; Webb, Spahr C.; Kuehn, Harold

2015-08-01

Multi-channel seismic (MCS) reflection profiles across the Pacific Plate south of the Alaska Peninsula reveal the internal structure of mature oceanic crust (48-56 Ma) formed at fast to intermediate spreading rates during and after a major plate re-organization. Oceanic crust formed at fast spreading rates (half spreading rate ∼ 74 mm /yr) has smoother basement topography, thinner sediment cover with less faulting, and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates (half spreading rate ∼ 28- 34 mm /yr). MCS data across fast-spreading oceanic crust formed during plate re-organization contain abundant bright reflections, mostly confined to the lower crust above a highly reflective Moho transition zone, which has a reflection coefficient (RC) of ∼0.1. The lower crustal events dip predominantly toward the paleo-ridge axis at ∼10-30°. Reflections are also imaged in the uppermost mantle, which primarily dip away from the ridge at ∼10-25°, the opposite direction to those observed in the lower crust. Dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re-organization, where a Moho reflection is weak or absent. Our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt. The abundance and reflection amplitude strength of these events (RC ∼ 0.15) can be explained by a combination of solidified melt that was segregated within the shear structures, mylonitization of the shear zones, and crystal alignment, all of which can result in anisotropy and constructive signal interference. Formation of shear zones with this geometry requires differential motion between the crust and upper mantle, where the upper mantle moves away from the ridge faster than the crust. Active asthenospheric upwelling is one possible explanation for these conditions. The other possible interpretation is that lower crustal reflections are caused by magmatic (mafic/ultramafic) layering associated with accretion from a central mid-crustal magma chamber. Considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re-organizations associated with ridge jumps or rift propagation, we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle, and therefore to shearing in the ductile lower crust or, alternatively, that plate reorganization could produce magmatic pulses which may lead to mafic/ultramafic banding.

Regional implications of geochemistry and style of emplacement of Miocene I-type diorite and granite, Delos, Cyclades, Greece

NASA Astrophysics Data System (ADS)

Pe-Piper, Georgia; Piper, David J. W.; Matarangas, Dionysis

2002-01-01

The Miocene plutons of the Cyclades were emplaced in a subduction setting during regional back-arc extension of continental crust, that led to flat-lying mid-crustal detachment faulting. Mapping of the island of Delos shows that quartz diorite and tonalite were emplaced as dykes in country rock of schist and marble within shear zones parallel to the extension direction. Mafic magmas were followed by numerous small batches of felsic magma, with magmatic and ductile deformation synchronous with magma emplacement. Late granite dykes occupy brittle fractures in the more deformed rocks. Mafic and intermediate rocks show a bimodal distribution of incompatible trace elements, with one group of broadly tholeiitic character and the other with substantial enrichment in Sr, Nb, and HFSE, but low Th and Ba. These differences appear to be inherited from two distinct mafic sources that are different from the mafic source for the plutons of the eastern Cyclades. Voluminous granodiorite results from these mafic magmas fractionating and/or mixing with felsic crustal material, some of which was derived by anatexis of a sedimentary protolith, indicated by high B and Mn. Some late granites appear derived from partial melting of Hercynian paragneiss. Regionally, the shear zones appear to be feeders to more extensive granitic plutons located at space produced at ramps in detachment fault zones. The shear zones parallel the Mid-Cycladic Lineament, a broad zone of displacement between two crustal blocks rotating in opposing directions as rollback took place at the Hellenic subduction zone. Distinctive geochemical features in Miocene igneous rocks suggests that these two blocks had quite different geological histories. The localisation of plutonism and core complexes near the Mid Cycladic Lineament suggests that this crustal-scale shear played a role in bringing subduction-derived magmas to mid-crustal levels. The heat supplied by the mafic magmas promoted ductile deformation high in the crust, where extension was concentrated, leading to the formation of core complexes. The regional extension resulted in progressive shallowing of the position of the granite solidus within the crust, leading to welding of the Mid-Cycladic Lineament, which is no longer seismically active.

Crustal accretion at fast spreading ridges and implications for hydrothermal circulation

NASA Astrophysics Data System (ADS)

Theissen-Krah, S.; Rupke, L.; Hasenclever, J.

2015-12-01

Oceanic crust is continuously created at mid-ocean ridges, but the location of lower crust crystallization continues to be debated since the proposal of the gabbro glacier and many sills end-member models. Geophysical and geochemical studies find evidence for either of the models. The crust is cooled by a combination of heat diffusion and advection, and hydrothermal circulation is thought to play a key role in distinguishing between both models. We use our numerical model for joint modeling of crustal accretion and hydrothermal circulation1 to test different accretion and hydrothermal cooling scenarios. The results match the seismic and structural observations from the East Pacific Rise2 and the Oman Ophiolite3, with a shallow melt lens at the correct location overlaying a narrow volume of partially molten rocks. Our results show that no more than 25-50% of the lower crust crystallizes in situ and that deep circulation is likely to occur at fast and intermediate spreading ridges. The occurrence of deep hydrothermal cooling however does not rule out that a major portion of the lower crust is formed in the shallow melt lens; our simulations rather suggest that it is necessary independent of where in the lower crust crystallization takes place. 1 Theissen-Krah, S., Iyer, K., Rupke, L. H. & Morgan, J. P. Coupled mechanical and hydrothermal modeling of crustal accretion at intermediate to fast spreading ridges. Earth and Planetary Science Letters 311, 275-286, doi:10.1016/j.epsl.2011.09.018 (2011). 2 Dunn, R. A., Toomey, D. R. & Solomon, S. C. Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9 degrees 30'N. Journal of Geophysical Research-Solid Earth 105, 23537-23555 (2000). 3 Nicolas, A. & Boudier, F. Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise. Terra Nova 27, 77-96, doi:10.1111/ter.12137 (2015).

The intermediate wavelength magnetic anomaly field of the north Pacific and possible source distributions

NASA Technical Reports Server (NTRS)

Labrecque, J. L.; Cande, S. C.; Jarrard, R. D. (Principal Investigator)

1983-01-01

A technique that eliminates external field sources and the effects of strike aliasing was used to extract from marine survey data the intermediate wavelength magnetic anomaly field for (B) in the North Pacific. A strong correlation exists between this field and the MAGSAT field although a directional sensitivity in the MAGSAT field can be detected. The intermediate wavelength field is correlated to tectonic features. Island arcs appear as positive anomalies of induced origin likely due to variations in crustal thickness. Seamount chains and oceanic plateaus also are manifested by strong anomalies. The primary contribution to many of these anomalies appears to be due to a remanent magnetization. The source parameters for the remainder of these features are presently unidentified ambiguous. Results indicate that the sea surface field is a valuable source of information for secular variation analysis and the resolution of intermediate wavelength source parameters.

Seismicity and active tectonics in the Alboran Sea, Western Mediterranean: Constraints from an offshore-onshore seismological network and swath bathymetry data

NASA Astrophysics Data System (ADS)

Grevemeyer, Ingo; Gràcia, Eulàlia; Villaseñor, Antonio; Leuchters, Wiebke; Watts, Anthony B.

2015-12-01

Seismicity and tectonic structure of the Alboran Sea were derived from a large amphibious seismological network deployed in the offshore basins and onshore in Spain and Morocco, an area where the convergence between the African and Eurasian plates causes distributed deformation. Crustal structure derived from local earthquake data suggests that the Alboran Sea is underlain by thinned continental crust with a mean thickness of about 20 km. During the 5 months of offshore network operation, a total of 229 local earthquakes were located within the Alboran Sea and neighboring areas. Earthquakes were generally crustal events, and in the offshore domain, most of them occurred at crustal levels of 2 to 15 km depth. Earthquakes in the Alboran Sea are poorly related to large-scale tectonic features and form a 20 to 40 km wide NNE-SSW trending belt of seismicity between Adra (Spain) and Al Hoceima (Morocco), supporting the case for a major left-lateral shear zone across the Alboran Sea. Such a shear zone is in accord with high-resolution bathymetric data and seismic reflection imaging, indicating a number of small active fault zones, some of which offset the seafloor, rather than supporting a well-defined discrete plate boundary fault. Moreover, a number of large faults known to be active as evidenced from bathymetry, seismic reflection, and paleoseismic data such as the Yusuf and Carboneras faults were seismically inactive. Earthquakes below the Western Alboran Basin occurred at 70 to 110 km depth and hence reflected intermediate depth seismicity related to subducted lithosphere.

Petrogenesis of the granitic Donkerhuk batholith in the Damara Belt of Namibia: protracted, syntectonic, short-range, crustal magma transfer

NASA Astrophysics Data System (ADS)

Clemens, J. D.; Buick, I. S.; Kisters, A. F. M.; Frei, D.

2017-07-01

The areally extensive (>5000 km2), syn-tectonic, ca. 520 Ma, mainly S-type Donkerhuk batholith was constructed through injection of thousands of mainly sheet-like magma pulses over 20-25 Myr. It intruded schists of the Southern Zone accretionary prism in the Damara Belt of Namibia. Each magma pulse had at least partly crystallised prior to the arrival of the following batch. However, much of the batholith may have remained partially molten for long periods, close to the H2O-saturated granite solidus. The batholith shows extreme variation in chemistry, while having limited mineralogical variation, and seems to be the world's most heterogeneous granitic mass. The Nd model ages of 2 Ga suggest that Eburnean rocks of the former magmatic arc, structurally overlain by the accretionary wedge, are the most probable magma sources. Crustal melting was initiated by mantle heat flux, probably introduced by thermal diffusion rather than magma advection. The granitic magmas were transferred from source to sink, with minimal intermediate storage; the whole process having occurred in the middle crust, resulting in feeble crustal differentiation despite the huge volume of silicic magma generated. Source heterogeneity controlled variation in the magmas and neither mixing nor fractionation was prominent. However, due to the transpressional emplacement régime, local filter pressing formed highly silicic liquids, as well as felsic cumulate rocks. The case of the Donkerhuk batholith demonstrates that emplacement-level tectonics can significantly influence compositional evolution of very large syn-tectonic magma bodies.

Continental lithospheric subduction and intermediate-depth seismicity: Constraints from S-wave velocity structures in the Pamir and Hindu Kush

NASA Astrophysics Data System (ADS)

Li, Wei; Chen, Yun; Yuan, Xiaohui; Schurr, Bernd; Mechie, James; Oimahmadov, Ilhomjon; Fu, Bihong

2018-01-01

The Pamir has experienced more intense deformation and shortening than Tibet, although it has a similar history of terrane accretion. Subduction as a primary way to accommodate lithospheric shortening beneath the Pamir has induced the intermediate-depth seismicity, which is rare in Tibet. Here we construct a 3D S-wave velocity model of the lithosphere beneath the Pamir by surface wave tomography using data of the TIPAGE (Tien Shan-Pamir Geodynamic program) and other seismic networks in the area. We imaged a large-scale low velocity anomaly in the crust at 20-50 km depth in the Pamir overlain by a high velocity anomaly at a depth shallower than 15 km. The high velocity anomalies colocate with exposed gneiss domes, which may imply a similar history of crustal deformation, partial melting and exhumation in the hinterland, as has occurred in the Himalaya/Tibet system. At mantle depths, where the intermediate-depth earthquakes are located, a low velocity zone is clearly observed extending to about 180 km and 150 km depth in the Hindu Kush and eastern Pamir, respectively. Moreover, the geometry of the low-velocity anomaly suggests that lower crustal material has been pulled down into the mantle by the subducting Asian and Indian lithospheric mantle beneath the Pamir and Hindu Kush, respectively. Metamorphic processes in the subducting lower crust may cause the intermediate-depth seismicity down to 150-180 km depth beneath the Pamir and Hindu Kush. We inverted focal mechanisms in the seismic zone for the stress field. Differences in the stress field between the upper and lower parts of the Indian slab imply that subduction and detachment of the Indian lithosphere might cause intense seismicity associated with the thermal shear instability in the deep Hindu Kush.

Revised crustal architecture of the southeastern Carpathian foreland from active and passive seismic data

NASA Astrophysics Data System (ADS)

Enciu, Dana M.; Knapp, Camelia C.; Knapp, James H.

2009-08-01

Integration of active and passive source seismic data is employed in order to study the nature of the relationships between crustal seismicity and geologic structures in the southeastern (SE) Carpathian foreland of Romania and the possible connection with the Vrancea Seismogenic Zone (VSZ) of intermediate-depth seismicity, one of the most active earthquake-prone areas in Europe. Crustal epicenters and focal mechanisms are correlated with four deep industry seismic profiles, the reprocessed Danube and Carpathian Integrated Action on Process in the Lithosphere and Neotectonics (DACIA PLAN) profile and the Deep Reflection Acquisition Constraining Unusual Lithospheric Activity II and III (DRACULA) profiles in order to understand the link between neotectonic foreland deformation and Vrancea mantle seismicity. Projection of crustal foreland hypocenters onto deep seismic profiles identifies several active crustal faults in the SE Carpathian foreland and suggests a mechanical coupling between the mantle located VSZ and the overlying foreland crust. The coupled associated deformation appears to take place on the Trotus Fault, the Sinaia Fault, and the newly detected Ialomita Fault. Seismic reflection imaging reveals the absence of west dipping reflectors in the crystalline crust and a slightly east dipping to horizontal Moho in the proximity of the Vrancea area. These findings argue against previously purported mechanisms to generate mantle seismicity in the VSZ including oceanic lithosphere subduction in place and oceanic slab break off, furthermore suggesting that the Vrancea seismogenic body is undetached from the overlying crust in the foreland.

Crustal Thickness on the South East Indian Ridge from OBH data

NASA Astrophysics Data System (ADS)

Tolstoy, M.; Cochran, J. R.; Carbotte, S. M.; Floyd, J. S.

2002-12-01

Seismic reflection and refraction data were collected on the intermediate-rate spreading South East Indian Ridge during December 2001 and January 2002 aboard the RV Ewing. A total of six lines of Ocean Bottom Hydrophone (OBH) refraction data were collected along four segments with contrasting ridge axis morphology. All lines were shot ridge parallel, with four lines on-axis, and two lines approximately 20 km off-axis. Each line used four OBHs and the line lengths varied between 102 km and 124 km, depending on the length of each ridge segment. For the two western most segments, an axial magma chamber is observed with crustal arrivals disappearing or being significantly delayed in the 15-20 km range. This indicates a magma chamber deeper than those observed on the faster spreading East Pacific Rise. Off-axis in this area clear crustal arrivals are seen out to 40-50 km. This indicates relatively thick crust in this most inflated of the sections studied, consistent with a higher magma supply. The two eastern most segments have on-axis lines only, and both of these indicate relatively thin crust. This is consistent with the more magma starved character of the bathymetry in these areas. Data will be presented, along with preliminary crustal velocity and thickness models.

Sr, Nd, and Pb Isotopic Geochemistry of Rhyolites from the Eastern Rhodopes, Bulgaria

NASA Astrophysics Data System (ADS)

Ivanova, R.; Kamenov, G. D.; Yanev, Y.

2002-12-01

Paleogene Eastern Rhodopes Volcanic Area (ERVA) is part of a more than 2000 km long magmatic belt in SE Europe extending from the Inner Dinarids (West Bosnia-Herzegovina) to Western Anatolia (European Turkey). Volcanic activity occurred during the Late Eocene-Early Oligocene and was spatially related to extensional Paleogene shallow marine basins underlain by a high-grade metamorphic basement. The volcanism is bimodal in character, with minor mafic (basalts) and major intermediate (mainly andesites) to acid (mainly rhyolites) volcanics present in similar volumes. This work focuses on Maritsa volcanic group (36-32 Ma) located in the NE part of the ERVA, S Bulgaria. The volcanic group comprises Lozen volcano composed of dacites, rhyodacites, and rhyolites, St Marina rhyolite dome, and Sheinovets rhyolite dome-cluster located within a caldera with the same name. Measured present day 87Sr/86Sr of the rhyolites range from 0.7075 to 0.7180, however on a plot 87Rb/86Sr vs 87Sr/86Sr the data form an errorchron (MSWD=21) with 30.5 +/-3.6Ma age and 87Sr/86Sr initial equal to 0.7074. Pb isotopic compositions in all of the volcanoes show similar values ranging from 18.712 to 18.768 in 206Pb/204Pb, 15.643 to 15.687 in 207Pb/204Pb, and 38.790 to 38.922 in 208Pb/204Pb. Nd isotopes show also little variations with 143Nd/144Nd ranging from 0.51242 to 0.51249. The similarity in the isotopic compositions between the volcanoes suggests common, homogeneous magmatic source. Crustal origin of the rhyolites as a result of melting of the metamorphic basement is not plausible because the rhyolites have different Sr and Nd isotopic compositions from the gneisses in the ERVA. Sr and Nd isotopic data for the rhyolites differ also from the basalts (i.e. possible mantle melts) in the Eastern Rhodopes region. Rhyolites have higher 87Sr/86Sr and lower 143Nd/144Nd ratios compared to the basalts, thus suggesting involvement of crustal component in the magma generation, most probably the metamorphic basement that underlies the volcanoes. On 143Nd/144Nd vs 87Sr/86Sr diagram the rhyolites plot on a mixing curve between the basalts and the gneisses from the metamorphic basement indicating about 70% mafic and 30% basement component involved in their genesis. Lead concentrations in the mantle is relatively low compared to the upper crust and the Pb isotopic signature in lavas generated in continental volcanic arcs is often completely dominated by crustal components. The rhyolites have more radiogenic 207Pb/204Pb and 208Pb/204Pb isotopic compositions than the basalts and plot entirely within the field defined by the metamorphic rocks in the ERVA suggesting that any mantle derived Pb was completely swamped by crustal Pb. Our isotopic results provide evidence for extensive crustal contamination of the felsic magmas in the region. Mafic melts, generated either during subduction or delamination ascended in the mantle wedge until they become stalled at a level of neutral buoyancy. Crustal melting and assimilation occurred at that level accompanied by homogenisation and crystal fractionation in large magma chambers until the evolving magmas reached the required density to re-establish buoyant ascent. The observed isotopic homogeneity in the felsic volcanoes in the region suggests that large zone of MASH at crustal levels existed during the Paleogene beneath the ERVA.

Making mushy magma chambers in the lower continental crust: Cold storage and compositional bimodality

NASA Astrophysics Data System (ADS)

Jackson, Matthew; Blundy, Jon; Sparks, Steve

2017-04-01

Increasing geological and geophysical evidence suggests that crustal magma reservoirs are normally low melt fraction 'mushes' rather than high melt fraction 'magma chambers'. Yet high melt fractions must form within these mush reservoirs to explain the observed flow and eruption of low crystallinity magmas. In many models, crystallinity is linked directly to temperature, with higher temperature corresponding to lower crystallinity (higher melt fraction). However, increasing temperature yields less evolved (silicic) melt composition for a given starting material. If mobile, low crystallinity magmas require high temperature, it is difficult to explain how they can have evolved composition. Here we use numerical modelling to show that reactive melt flow in a porous and permeable mush reservoir formed by the intrusion of numerous basaltic sills into the lower continental crust produces magma in high melt fraction (> 0.5) layers akin to conventional magma chambers. These magma-chamber-like layers contain evolved (silicic) melt compositions and form at low (close to solidus) temperatures near the top of the mush reservoir. Evolved magma is therefore kept in 'cold storage' at low temperature, but also at low crystallinity so the magma is mobile and can leave the mush reservoir. Buoyancy-driven reactive flow and accumulation of melt in the mush reservoir controls the temperature and composition of magma that can leave the reservoir. The modelling also shows that processes in lower crustal mush reservoirs produce mobile magmas that contain melt of either silicic or mafic composition. Intermediate melt compositions are present but are not within mobile magmas. Silicic melt compositions are found at high melt fraction within the magma-chamber like layers near the top of the mush reservoir. Mafic melt compositions are found at high melt fraction within the cooling sills. Melt elsewhere in the reservoir has intermediate composition, but remains trapped in the reservoir because the local melt fraction is too low to form a mobile magma. The model results are consistent with geochemical data suggesting that lower crustal magma reservoirs supply silicic and mafic melts to arc volcanoes, but intermediate magmas are formed by mixing in shallower reservoirs. We suggest here that lower crustal magma chambers primarily form in response to changes in bulk composition caused by melt migration and chemical reaction in a mush reservoir. This process is different to the conventional and widely applied models of magma chamber formation. Similar processes are likely to operate in shallow mush reservoirs, but will likely be further complicated by the presence of volatile phases, and mixing of different melt compositions sourced from deeper mush reservoirs.

The Imaging and Evolution of Seismic Layer 2A Thickness from a 0-70 Ma Oceanic Crustal Transect in the South Atlantic

NASA Astrophysics Data System (ADS)

Estep, J. D.; Reece, R.; Kardell, D. A.; Christeson, G. L.; Carlson, R. L.

2017-12-01

Seismic layer 2A, the uppermost igneous portion of oceanic crust, is commonly used to refer to the seismic velocities of upper crust that are bounded below by a steep vertical velocity gradient. Layer 2A velocities are known to increase with crustal age, from 2.5 km/s in crust <1 Ma to 4.5-5 km/s in crust >15 Ma. Thickness of layer 2A has been shown to increase by a factor of 2 within 1 Ma at fast spreading ridges and then remain relatively constant, while layer 2A maintains a fairly consistent thickness, irrespective of age, at slow-intermediate spreading ridges. Layer 2A thickness and velocity evolution studies to date have been largely focused on young oceanic crust very proximal to a spreading center with little investigation of changes (or lack thereof) that occur at crustal ages >10 Ma. We utilize a multichannel seismic dataset collected at 30° S in the western South Atlantic that continuously images 0 - 70 Ma oceanic crust along a single flowline generated at the slow-intermediate spreading Mid-Atlantic Ridge. We follow the methods of previous studies by processing the data to image the layer 2A event, which is then used for calculating thickness. 1D travel time forward modeling at regularly spaced age intervals across the transect provides for the conversion of time to depth thickness, and for determining the evolution of velocities with age. Our results show layer 2A in 20 Ma crust is roughly double the thickness of that in crust 0-5 Ma (830 vs. 440 m), but thickness does not appear to change beyond 20 Ma. The layer 2A event is readily observable in crust 0-50 Ma, is nearly completely absent in crust 50-65 Ma, and then reappears with anomalously high amplitude and lateral continuity in crust 65-70 Ma. Our results suggest that layer 2A thickens with age at the slow-intermediate spreading southern Mid-Atlantic Ridge, and that layer 2A either continues to evolve at the older crustal ages, well beyond the expected 10-15 Ma "mature age", or that external factors have altered the crust at 65-70 Ma. The proximity and thermal influence of the Rio Grande Rise might explain the anomalous appearance of the layer 2A event at the older ages of crust for the study.

Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts

NASA Astrophysics Data System (ADS)

Kay, S. Mahlburg; Kay, Robert W.

1985-07-01

Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40 50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.

Geochemical and Sr-Nd-Pb-Li isotopic characteristics of volcanic rocks from the Okinawa Trough: Implications for the influence of subduction components and the contamination of crustal materials

NASA Astrophysics Data System (ADS)

Guo, Kun; Zhai, Shikui; Yu, Zenghui; Wang, Shujie; Zhang, Xia; Wang, Xiaoyuan

2018-04-01

The Okinawa Trough is an infant back-arc basin developed along the Ryukyu arc. This paper provides new major and trace element and Sr-Nd-Pb-Li isotope data of volcanic rocks in the Okinawa Trough and combines the published geochemical data to discuss the composition of magma source, the influence of subduction component, and the contamination of crustal materials, and calculate the contribution between subduction sediment and altered oceanic crust in the subduction component. The results showed that there are 97% DM and 3% EMI component in the mantle source in middle trough (MS), which have been influenced by subduction sediment. The Li-Nd isotopes indicate that the contribution of subduction sediment and altered oceanic crust in subduction component are 4 and 96%, respectively. The intermediate-acidic rocks suffer from contamination of continental crust material in shallow magma chamber during fractional crystallization. The acidic rocks in south trough have experienced more contamination of crustal material than those from the middle and north trough segments.

Tomographic imaging of the transition from asthenospheric to lithospheric melt migration processes: 3-D structure of the topmost mantle and crust at the Endeavour Segment, Juan de Fuca Ridge

NASA Astrophysics Data System (ADS)

Arnoux, G. M.; Toomey, D. R.; Hooft, E. E. E.; Wilcock, W. S. D.

2017-12-01

We present tomographic images of the intermediate-spreading Endeavour Segment that constrain the nature of an axial magmatic system as it transitions from asthenospheric- to lithospheric-dominated rheologies. We use seismic energy from 5500 air gun shots refracted through the crust (Pg), reflected off the Moho (PmP), and refracted below the Moho (Pn)—as recorded by 64 OBSs from the Endeavour tomography experiment—to image the isotropic and anisotropic P-wave velocity structure of the topmost mantle and crust, as well as crustal thickness. At crustal depths, results reveal a low-velocity zone (LVZ)—inferred to be the axial magmatic system—that: (i) is continuous along the entire Endeavour Segment at depths of 2-3 km below seafloor and closely follows the axis of spreading, (ii) broadens and becomes more discontinuous at lower crustal depths, and (iii) has its largest amplitude from the mid- to lower-crust at the segment center. The ridge-tracking trend of the mid-crustal LVZ is in contrast to the crustal thickness pattern; in particular, a swath of thin crust is skewed with respect to both the ridge axis and the mid-crustal magmatic system and connects two overlapping spreading centers bounding the segment. The trend of thinner crust, however, is aligned with the mantle LVZ, which constrains the thermal structure and distribution of melt within the topmost mantle. The systematic depth variation of the map-view orientation and structure of the magmatic system indicates a distinct transition from a broad, cross-axis regime in the topmost asthenosphere governed by a regional, north-south trending thermal structure, to a narrow, cross-axis regime in the mid- to upper-crust governed by lithospheric rifting, magma injection, and hydrothermal processes. The lower-crustal magmatic system connects these two regimes. We also postulate that accumulation and differentiation of magma immediately beneath the crust-mantle boundary increases temperatures and suppresses plagioclase crystallization, thereby reducing the depth of lower crustal accretion and resulting in the observed north-south trending swath of thinner crust.

Shallow-level magma-sediment interaction and explosive behaviour at Anak Krakatau (Invited)

NASA Astrophysics Data System (ADS)

Troll, V. R.; Jolis, E. M.; Dahren, B.; Deegan, F. M.; Blythe, L. S.; Harris, C.; Berg, S. E.; Hilton, D. R.; Freda, C.

2013-12-01

Crustal contamination of ascending arc magmas is generally thought to be a significant process which occurs at lower- to mid-crustal magma storage levels where magmas inherit their chemical and isotopic character by blending, assimilation and differentiation [1]. Anak Krakatau, like many other volcanoes, erupts shallow-level crustal xenoliths [2], indicating a potential role for upper crustal modification and hence late-stage changes to magma rheology and thus potential eruptive behaviour. Distinguishing deep vs. shallow crustal contamination processes at Krakatau, and elsewhere, is therefore crucial to understand and assess pre-eruptive magmatic conditions and their associated hazard potential. Here we report on a multi-disciplinary approach to unravel the crustal plumbing system of the persistently-active and dominantly explosive Anak Krakatau volcano [2, 3], employing rock-, mineral- and gas-isotope geochemistry and link these results with seismic tomography [4]. We show that pyroxene crystals formed at mid- and lower-crustal levels (9-11 km) and carry almost mantle-like isotope signatures (O, Sr, Nd, He), while feldspar crystals formed dominantly at shallow levels (< 5km) and display unequivocal isotopic evidence for late stage contamination (O, Sr, Nd). This obeservation places a significant element of magma-crust interaction into the uppermost, sediment-rich crust beneath the volcano. Magma storage in the uppermost crust can thus offer a possible explanation for the compositional modifications of primitive Krakatau magmas, and likely provides extra impetus to increased explosivity at Anak Krakatau. [1] Annen, et al., 2006. J. Petrol. 47, 505-539. [2] Gardner, et al., 2013. J. Petrol. 54, 149-182. [3] Dahren, et al., 2012. Contrib. Mineral. Petrol. 163, 631-651. [4] Jaxybulatov, et al., 2011. J. Volcanol. Geoth. Res. 206, 96-105.

Precise leveling, space geodesy and geodynamics

NASA Technical Reports Server (NTRS)

Reilinger, R.

1981-01-01

The implications of currently available leveling data on understanding the crustal dynamics of the continental United States are investigated. Neotectonic deformation, near surface movements, systematic errors in releveling measurements, and the implications of this information for earthquake prediction are described. Vertical crustal movements in the vicinity of the 1931 Valentine, Texas, earthquake which may represent coseismic deformation are investigated. The detection of vertical fault displacements by precise leveling in western Kentucky is reported. An empirical basis for defining releveling anomalies and its implications for crustal deformation in southern California is presented. Releveling measurements in the eastern United States and their meaning in the context of possible crustal deformation, including uplift of the Appalachian Mountains, eastward tilting of the Atlantic Coastal Plain, and apparent movements associated with a number of structural features along the east coast, are reported.

Waveform tomography of crustal structure in the south San Francisco Bay region

USGS Publications Warehouse

Pollitz, F.F.; Fletcher, J.P.

2005-01-01

We utilize a scattering-based seismic tomography technique to constrain crustal tructure around the southern San Francisco Bay region (SFBR). This technique is based on coupled traveling wave scattering theory, which has usually been applied to the interpretation of surface waves in large regional-scale studies. Using fully three-dimensional kernels, this technique is here applied to observed P, S, and surface waves of intermediate period (3-4 s dominant period) observed following eight selected regional events. We use a total of 73 seismograms recorded by a U.S. Geological Survey short-period seismic array in the western Santa Clara Valley, the Berkeley Digital Seismic Network, and the Northern California Seismic Network. Modifications of observed waveforms due to scattering from crustal structure include (positive or negative) amplification, delay, and generation of coda waves. The derived crustal structure explains many of the observed signals which cannot be explained with a simple layered structure. There is sufficient sensitivity to both deep and shallow crustal structure that even with the few sources employed in the present study, we obtain shallow velocity structure which is reasonably consistent with previous P wave tomography results. We find a depth-dependent lateral velocity contrast across the San Andreas fault (SAF), with higher velocities southwest of the SAF in the shallow crust and higher velocities northeast of the SAF in the midcrust. The method does not have the resolution to identify very slow sediment velocities in the upper approximately 3 km since the tomographic models are smooth at a vertical scale of about 5 km. Copyright 2005 by the American Geophysical Union.

Mercury's lithospheric thickness and crustal density, as inferred from MESSENGER observations

NASA Astrophysics Data System (ADS)

James, P. B.; Mazarico, E.; Genova, A.; Smith, D. E.; Neumann, G. A.; Solomon, S. C.

2015-12-01

The gravity field and topography of Mercury measured by the MESSENGER spacecraft have provided insights into the thickness of the planet's elastic lithosphere, Te. We localized the HgM006 free-air gravity anomaly and gtmes_125v03 shape datasets to search for theoretical elastic thickness solutions that best fit a variety of localized coherence spectra between Bouguer gravity anomaly and topography. We adopted a crustal density of ρcrust =2700 kg m-3 for the Bouguer gravity correction, but density uncertainty did not markedly affect the elastic thickness estimates. A best-fit solution in the northern smooth plains (NSP) gives an elastic thickness of Te =30-60 km at the time of formation of topography for a range of ratios of top to bottom loading from 1 to 5. For a mechanical lithosphere with a thickness of ~2Te and a temperature of 1600 °C at the base, this solution is consistent with a geothermal gradient of 9-18 K km-1. A similar coherence analysis exterior to the NSP produces an elastic thickness estimate of Te =20-50 km, albeit with a poorer fit. Coherence in the northern hemisphere as a whole does not approach zero at any wavelength, because of the presence of variations in crustal thickness that are unassociated with elastic loading. The ratios and correlations of gravity and topography at intermediate wavelengths (harmonic degree l between 30 and 50) also constrain regional crustal densities. We localized gravity and topography with a moving Slepian taper and calculated regionally averaged crustal densities with the approximation ρcrust=Zl/(2πG), where Zl is the localized admittance and G is the gravitational constant. The only regional density estimates greater than 2000 kg m-3 for l=30 correspond to the NSP. Density estimates outside of the NSP were unreasonably low, even for highly porous crust. We attribute these low densities to the confounding effects of crustal thickness variations and Kaula filtering of the gravity dataset at the highest harmonic degrees, both of which tend to introduce a downward bias to crustal density estimation. An alternative analysis—which corrected for crustal thickness variability and was restricted to regions with gravity/topography coherence greater than 0.6—yielded an aggregate crustal density of ρcrust=2602 ± 470 kg m-3 for Mercury's high northern latitudes.

Effects of crustal thickness on magmatic differentiation in subduction zone volcanism: A global study

NASA Astrophysics Data System (ADS)

Farner, Michael J.; Lee, Cin-Ty A.

2017-07-01

The majority of arc magmas are highly evolved due to differentiation within the lithosphere or crust. Some studies have suggested a relationship between crustal thickness and magmatic differentiation, but the exact nature of this relationship is unclear. Here, we examine the interplay of crustal thickness and magmatic differentiation using a global geochemical dataset compiled from active volcanic arcs and elevation as a proxy for crustal thickness. With increasing crustal thickness, average arc magma compositions become more silicic (andesitic) and enriched in incompatible elements, indicating that on average, arc magmas in thick crust are more evolved, which can be easily explained by the longer transit and cooling times of magmas traversing thick arc lithosphere and crust. As crustal thickness increases, arc magmas show higher degrees of iron depletion at a given MgO content, indicating that arc magmas saturate earlier in magnetite when traversing thick crust. This suggests that differentiation within thick crust occurs under more oxidizing conditions and that the origin of oxidation is due to intracrustal processes (contamination or recharge) or the role of thick crust in modulating melting degree in the mantle wedge. We also show that although arc magmas are on average more silicic in thick crust, the most silicic magmas (>70 wt.% SiO2) are paradoxically found in thin crust settings, where average compositions are low in silica (basaltic). We suggest that extreme residual magmas, such as those exceeding 70 wt.% SiO2, are preferentially extracted from shallow crustal magma bodies than from deep-seated magma bodies, the latter more commonly found in regions of thick crust. We suggest that this may be because the convective lifespan of crustal magma bodies is limited by conductive cooling through the overlying crustal lid and that magma bodies in thick crust cool more slowly than in thin crust. When the crust is thin, cooling is rapid, preventing residual magmas from being extracted; in the rare case that residual magmas can be extracted, they represent the very last melt fractions, which are highly silicic. When the crust is thick, cooling is slow, so intermediate melt fractions can readily segregate and erupt to the surface, where they cool and crystallize before highly silicic residual melts can be generated.

Models for extracting vertical crustal movements from leveling data

NASA Technical Reports Server (NTRS)

Holdahl, S. H.

1978-01-01

Various adjustment strategies are being used in North America to obtain vertical crustal movements from repeated leveling. The more successful models utilize polynomials or multiquadric analysis to describe elevation change with a velocity surface. Other features permit determination of nonlinear motions, motions associated with earthquakes or episodes, and vertical motions of blocks where boundaries are prespecified. The preferred models for estimating crustal motions permit the use of detached segments of releveling to govern the shape of a velocity surface and allow for input from nonleveling sources such as tide gages and paired lake gages. Some models for extracting vertical crustal movements from releveling data are also excellent for adjusting leveling networks, and permit mixing old and new data in areas exhibiting vertical motion. The new adjustment techniques are more general than older static models and will undoubtedly be used routinely in the future as the constitution of level networks becomes mainly relevelings.

Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field.

PubMed

Singh, Satish C; Crawford, Wayne C; Carton, Hélène; Seher, Tim; Combier, Violaine; Cannat, Mathilde; Pablo Canales, Juan; Düsünür, Doga; Escartin, Javier; Miranda, J Miguel

2006-08-31

Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3-4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.

Crustal thickness control on Sr/Y signatures of recent arc magmas: an Earth scale perspective

PubMed Central

Chiaradia, Massimo

2015-01-01

Arc magmas originate in subduction zones as partial melts of the mantle, induced by aqueous fluids/melts liberated by the subducted slab. Subsequently, they rise through and evolve within the overriding plate crust. Aside from broadly similar features that distinguish them from magmas of other geodynamic settings (e.g., mid-ocean ridges, intraplate), arc magmas display variably high Sr/Y values. Elucidating the debated origin of high Sr/Y signatures in arc magmas, whether due to mantle-source, slab melting or intracrustal processes, is instrumental for models of crustal growth and ore genesis. Here, using a statistical treatment of >23000 whole rock geochemical data, I show that average Sr/Y values and degree of maturation (MgO depletion at peak Sr/Y values) of 19 out of 22 Pliocene-Quaternary arcs correlate positively with arc thickness. This suggests that crustal thickness exerts a first order control on the Sr/Y variability of arc magmas through the stabilization or destabilization of mineral phases that fractionate Sr (plagioclase) and Y (amphibole ± garnet). In fact, the stability of these mineral phases is function of the pressure at which magma evolves, which depends on crustal thickness. The data presented show also that high Sr/Y Pliocene-Quaternary intermediate-felsic arc rocks have a distinct origin from their Archean counterparts. PMID:25631193

Geophysical characteristics and crustal structure of greenstone terranes: Canadian Shield

NASA Technical Reports Server (NTRS)

Thomas, M. D.; Losier, L.; Thurston, P. C.; Gupta, V. K.; Gibb, R. A.; Grieve, R. A. F.

1986-01-01

Geophysical studies in the Canadian Shield have provided some insights into the tectonic setting of greenstone belts. Greenstone belts are not rooted in deep crustal structures. Geophysical techniques consistently indicate that greenstones are restricted to the uppermost 10 km or so of crust and are underlain by geophysically normal crust. Gravity models suggest that granitic elements are similarly restricted, although magnetic modelling suggests possible downward extension to the intermediate discontinuity around approx. 18 km. Seismic evidence demonstrates that steeply-dipping structure, which can be associated with the belts in the upper crust, is not present in the lower crust. Horizontal intermediate discontinuities mapped under adjacent greenstone and granitic components are not noticeably disrupted in the boundary zone. Geophysical evidence points to the presence of discontinuities between greenhouse-granite and adjacent metasedimentary erranes. Measured stratigraphic thicknesses of greenstone belts are often twice or more the vertical thicknesses determined from gravity modelling. Explantations advanced for the discrepancy include stratigraphy repeated by thrust faulting and/or listric normal faulting, mechanisms which are consistent with certain aspects of conceptual models of greenstone development. Where repetition is not a factor the gravity evidence points to removal of the root zones of greenstone belts. For one region, this has been attributed to magmatic stopping during resurgent caldera activity.

Reconstructing the plumbing system of Krakatau volcano

NASA Astrophysics Data System (ADS)

Troll, Valentin R.; Dahrén, Börje; Deegan, Frances M.; Jolis, Ester M.; Blythe, Lara S.; Harris, Chris; Berg, Sylvia E.; Hilton, David R.; Freda, Carmela

2014-05-01

Crustal contamination of ascending arc magmas is generally thought to be significant at lower- to mid-crustal magma storage levels where magmas inherit their chemical and isotopic character by blending, assimilation and differentiation [1]. Anak Krakatau, like many other volcanoes, erupts shallow-level crustal xenoliths [2], indicating a potential role for upper crustal modification and hence late-stage changes to magma rheology and thus eruptive behaviour. Distinguishing deep vs. shallow crustal assimilation processes at Krakatau, and elsewhere, is therefore crucial to understand and assess pre-eruptive magmatic conditions and their associated hazard potential. Here we report on a multi-disciplinary approach to unravel the crustal plumbing system of the persistently-active and dominantly explosive Anak Krakatau volcano [2, 3]. We employ rock-, mineral- and gas-isotope geochemistry and link these results with seismic tomography [4]. We show that pyroxene crystals formed at mid- and lower-crustal levels (9-11 km) and carry almost mantle-like isotope signatures (O, Sr, Nd, He), while feldspar crystals formed dominantly at shallow levels (< 5km) and display unequivocal isotopic evidence for late stage contamination (O, Sr, Nd). Coupled with tomographic evidence, the petrological and geochemical data place a significant element of magma-crust interaction (and hence magma storage) into the uppermost, sediment-rich crust beneath the volcano. Magma - sediment interaction in the uppermost crust offers a likely explanation for the compositional variations in recent Krakatau magmas and most probably provides extra impetus to increased explosivity at Anak Krakatau. [1] Annen, et al., 2006. J. Petrol. 47, 505-539. [2] Gardner, et al., 2013. J. Petrol. 54, 149-182. [3] Dahren, et al., 2012. Contrib. Mineral. Petrol. 163, 631-651. [4] Jaxybulatov, et al., 2011. J. Volcanol. Geoth. Res. 206, 96-105.

Mid-Ocean Ridge Melt Supply and Glacial Cycles: A 3D EPR Study of Crustal Thickness, Layer 2A, and Bathymetry

NASA Astrophysics Data System (ADS)

Boulahanis, B.; Aghaei, O.; Carbotte, S. M.; Huybers, P. J.; Langmuir, C. H.; Nedimovic, M. R.; Carton, H. D.; Canales, J. P.

2017-12-01

Recent studies suggest that eustatic sea level fluctuations induced by glacial cycles in the Pleistocene may influence mantle-melting and volcanic eruptions at mid-ocean ridges (MOR), with models predicting variation in oceanic crustal thickness linked to sea level change. Previous analyses of seafloor bathymetry as a proxy for crustal thickness show significant spectral energy at frequencies linked to Milankovitch cycles of 1/23, 1/41, and 1/100 ky-1, however the effects of faulting in seafloor relief and its spectral characteristics are difficult to separate from climatic signals. Here we investigate the hypothesis of climate driven periodicity in MOR magmatism through spectral analysis, time series comparisons, and statistical characterization of bathymetry data, seismic layer 2A thickness (as a proxy for extrusive volcanism), and seafloor-to-Moho thickness (as a proxy for total magma production). We utilize information from a three-dimensional multichannel seismic study of the East Pacific Rise and its flanks from 9°36`N to 9°57`N. We compare these datasets to the paleoclimate "LR04" benthic δ18O stack. The seismic dataset covers 770 km2 and provides resolution of Moho for 92% of the imaged region. This is the only existing high-resolution 3-D image across oceanic crust, making it ideal for assessing the possibility that glacial cycles modulate magma supply at fast spreading MORs. The layer 2A grid extends 9 km (170 ky) from the ridge axis, while Moho imaging extends to a maximum of 16 km (310 ky). Initial results from the East Pacific Rise show a relationship between sea level and both crustal thickness and sea floor depth, consistent with the hypothesis that magma supply to MORs may be modulated by glacial cycles. Analysis of crustal thickness and bathymetry data reveals spectral peaks at Milankovitch frequencies of 1/100 ky-1 and 1/41 ky-1 where datasets extend sufficiently far from the ridge. The layer 2A grid does not extend sufficiently far from the ridge to be conclusive. Correlations between sea level and crustal thickness suggest a lag of 65 ky between sea level forcing and crustal thickness response. A further lag of 25 ky is observed between crustal thickness variations and seafloor depth change, which we attribute to the finite width of the crustal formation zone.

Structure and Evolution of the Forearc-Arc Crust Along the Tonga-Kermadec Subduction System from Integrated Geophysical Data

NASA Astrophysics Data System (ADS)

Funnell, M.; Peirce, C.; Robinson, A. H.; Watts, A. B.; Grevemeyer, I.

2016-12-01

Variations in tectonic forces and inputs to subduction systems generate, alter, and deform overriding crustal material. Although these processes are recorded in the crustal structure of volcanic arcs and their backarcs, the continuous nature of plate convergence superimposes subsequent episodes of crustal evolution on older features. Seismic imaging at modern subduction zones enhances our understanding of forearc development and variations in present-day deformation caused by inherited structures. In 2011 a set of multichannel and wide-angle seismic profiles imaged the forearc-arc crust and upper mantle structure along the 2700 km-long NNE-SSW trending Tonga-Kermadec subduction zone. The Tonga forearc region exhibits an 100 km-wide, 2 km high bathymetric elevation, with a 3 km-thick upper and mid-crust (Vp <6 km s-1), and a lower-crustal ridge 30 km wide comprising velocities up to 7.4 km s-1 that characterize an extinct Eocene ( 50 Ma) arc. By contrast, the active arc is <10 km wide and exhibits lower-crustal velocities below 7.0 km s-1, most likely representing intermediate compositions. This structural change suggests significant evolution, alteration, and modification of the overriding crust since the onset of subduction at this margin. Gravity anomaly modelling suggests that the extinct arc within the Tonga forearc region comprises relatively dense mafic-ultrabasic material that extends south beneath the Kermadec forearc and terminates at 32°S. The apparent southern termination of the extinct arc coincides with the partitioning of morphological features at 32°S, including a 10-km westward-step of the active arc and a 1.5 km deeper backarc to the south. We propose that tectonic partitioning about the 32°S boundary is the result of variations in the inherited crustal structure, which is divided by the presence and absence, to the north and south respectively, of the extinct volcanic arc.

Segmentation Control on Crustal Accretion: Insights From the Chile Ridge

NASA Astrophysics Data System (ADS)

Martinez, F.; Karsten, J. L.; Milman, M. S.; Klein, E. M.

2002-12-01

Controls on crustal accretion at mid-ocean ridges include spreading rate and mantle temperature and composition. Less studied is the effect of the segmentation geometry, although it has been known for some time that large offset transforms have significant effects on the extent of melting and lava compositions produced by ridges in their vicinity. The PANORAMA 4 expedition surveyed the Chile Ridge between 36°-43°S in order to examine the effects of ridge segmentation on crustal accretion. This section of the ridge is spreading uniformly at intermediate rates (~53 mm/yr) and rock sampling and regional data indicate a largely uniform mantle composition with no systematic changes in mantle thermal structure. Thus the segmentation geometry is the primary crustal accretion variable. The survey mapped and sampled 19 first order ridge segments and their transform offsets. The ridges range from 130 to 10 km in length with mapped transform offsets from 168 to 19 km. The segments primarily have axial valley morphology, with segments longer than ~65 km typically displaying central highs deepening toward segment ends. Mantle Bouguer anomalies (MBAs) show that these segments also have bulls eye lows associated with the central highs indicating thicker crust than at segment ends. Overall the mapped segments displays a trend of increasing depth and MBA, implying diminishing crustal production, with decreasing segment length and increasing transform offset. We examine the cause of this trend by modeling the mantle flow pattern generated by finite length ridge segments using the Phipps-Morgan and Forsyth (1988) algorithm. The results indicate that at a constant spreading rate mantle upwelling rates are greatest and extend deeper near the segment center, and that for segments that are significantly offset, upwelling rates decrease overall with decreasing segment length. The modeling implies that segmentation itself, even without cooling and lithospheric relief at transforms has a strong influence on mantle advection and therefore on crustal production.

Isotopic and chemical evidence concerning the genesis and contamination of basaltic and rhyolitic magma beneath the Yellowstone Plateau Volcanic Field

USGS Publications Warehouse

Hildreth, W.; Halliday, A.N.; Christiansen, R.L.

1991-01-01

Since 2.2 Ma, the Yellowstone Plateau Volcanic Field has produced ~6000 km3 of rhyolite tuffs and lavas in >60 separate eruptions, as well as ~100 km3 of tholeiitic basalt from >50 vents peripheral to the silicic focus. Intermediate eruptive products are absent. Early postcollapse rhyolites show large shifts in Nd, Sr, Pb, and O isotopic composition caused by assimilation of roof rocks and hydrothermal brines during collapse and resurgence. Younger intracaldera rhyolite lavas record partial isotopic recovery toward precaldera ratios. Thirteen extracaldera rhyolites show none of these effects and have sources independent of the subcaldera magma system. Contributions from the Archaean crust have extreme values and wide ranges of Nd-, Sr, and Pb-isotope ratios, but Yellowstone rhyolites have moderate values and limited ranges. This requires their deep-crustal sources to have been pervasively hybridized by distributed intrusion of Cenozoic basalt, most of which was probably contemporaneous with the Pliocene and Quaternary volcanism. Most Yellowstone basalts had undergone cryptic clinopyroxene fractionation in the lower crust or crust-mantle transition zone and, having also ascended through or adjacent to crustal zones of silicic-magma generation, most underwent some crustal contamination. -from Authors

Seismotectonics and Crustal Thickness of Northwest Mindoro, Philippines

NASA Astrophysics Data System (ADS)

Chen, P. F.; Olavere, E. A.; Lee, K. M.; Bautista, B.; Solidum, R., Jr.; Huang, B. S.

2015-12-01

Mindoro Island locates where the Palawan Continental Block (PCB) indented into the Philippine Mobile Belt (PMB) during the Early Miocene and where the Manila Trench terminates, having ceased convergence due to collision. On the transition from subduction to collision, Northwest Mindoro exhibits vigorous seismic activity and has been debated about its affiliation being PCB or PMB. Here, we use data from both the EHB and Global Centroid Moment Tensor catalogues to study the regional seismotectonics. We also deployed five broadband stations to probe the crustal thickness beneath NW Mindoro using receiver function analysis. Results show that, following the southeasterly reduction of convergence rates at the southern termination of the Manila Trench, the slab dipping angles steepen, were initiated at depth (~200 km) and propagate upwards. The horizontal distances of the trench and slab, as measured from the Wadati-Benioff zone at 200 km depth, also reduce in a southeasterly direction. Observations of intermediate-depth earthquakes that exhibit predominantly down-dip extensional stress patterns attest that the steepening of slab dipping angles is due to the negative buoyancy of the slab. Preliminary results of receiver function analysis suggest that the crustal thickness beneath NW Mindoro is about 40 km and is probably PCB affiliated.

The crustal structure from the Altai Mountains to the Altyn Tagh fault, northwest China

USGS Publications Warehouse

Wang, Y.; Mooney, W.D.; Yuan, X.; Coleman, R.G.

2003-01-01

We present a new crustal section across northwest China based on a seismic refraction profile and geologic mapping. The 1100-km-long section crosses the southern margin of the Chinese Altai Mountains, Junggar Accretional Belt and eastern Junggar basin, easternmost Tianshan Mountains, and easternmost Tarim basin. The crustal velocity structure and Poisson's ratio (??), which provide a constraint on crustal composition, were determined from P and S wave data. Despite the complex geology, the crustal thickness along the entire profile is nearly uniform at 50 km. The thickest crust (56 km) occurs at the northern end of the profile beneath the Altai Mountains and the thinnest (46 km) crust is beneath the Junggar basin. Beneath surficial sediments, the crust is found to have three layers with P wave velocities (Vp) of 6.0-6.3, 6.3-6.6, and 6.9-7.0 km/s, respectively. The southern half of the profile, including the eastern Tianshan Mountains and eastern margin of the Tarim basin, shows low P wave velocities and ?? = 0.25 to a depth of 30 km, which suggests a quartz-rich, granitic upper crustal composition. The northern half of the profile below the Altai Mountains and Junggar Accretional Belt has a higher Poisson's ratio of ?? = 0.26-0.27 to a depth of 30 km, indicative of an intermediate crustal composition. The entire 1100-km-long profile is underlain by a 15-30 km thick high velocity (6.9-7.0 km/s; ?? = 0.26-0.28) lower-crustal layer that we interpret to have a bulk composition of mafic granulite. At the southern end of the profile, a 5-km-thick midcrustal low-velocity layer (Vp = 5.9 km/s, ?? = 0.25) underlies the Tianshan and the region to the south, and may be indicative of a near-horizontal detachment interface. Pn velocities are ???7.7-7.8 km/s between the Tianshan and the Junggar basin, and ???7.9-8.0 km/s below the Altai Mountains and eastern margin of the Tarim basin. We interpret the consistent three-layer stratification of the crust to indicate that the crust has undergone partial melting and differentiation after Paleozoic terrane accretion. The thickness (50 km) of the crust appears to be related to compression resulting from the Indo-Asian collision.

Seafloor Spreading in the Lau-Havre Backarc Basins: From Fast to Ultra Slow

NASA Astrophysics Data System (ADS)

Martinez, F.; Dunn, R. A.; Sleeper, J. D.

2013-12-01

Seafloor spreading in the Lau Basin occurs along the well-organized Eastern Lau Spreading Center (ELSC) and Valu Fa Ridges (VFR) opening at 97-39 mm/yr. The ELSC/VFR produce two distinct crustal types sub-parallel to the ridge as a function of their separation from the arc volcanic front. Arc-proximal spreading forms a shallow, thick crust with arc-like lavas that abruptly changes to a deeper, thinner crust with backarc basin basalt (BABB)-like lavas as the ridges separate from the arc volcanic front. Southward in the Havre Trough opening rates decrease to 15 mm/yr and a well-organized spreading axis is largely absent. Instead, active volcanism appears to be distributed across a broad zone located asymmetrically near the arc side of the basin. Further, crustal accretion appears to have two distinct styles forming a shallower terrain floored by arc-like lavas and deeper rifted basins floored by more BABB-like lavas [Wysoczanski et al., 2010, G-cubed]. Although these crustal terrains broadly resemble those flanking the ELSC/VFR, in the Havre Trough they are organized into bands that trend across the basin with the shallower arc-like terrains typically trailing from Kermadec arc front volcanoes. We hypothesize that the variation in style of crustal accretion along the Lau-Havre backarc system is controlled by the southward decreasing rate of plate extension superimposed on a compositionally variable mantle wedge. Distinct hydrous and less-hydrous mantle domains have been proposed for the mantle wedge [Martinez & Taylor, 2002; Dunn & Martinez, 2011; Nature]. Within the hydrous domain (< about 50 km from the arc volcanic front) further compositional 'fingers' trailing basinward from arc front volcanoes have been interpreted in the Lau Basin based on ridge axis morphology and chemistry [Sleeper & Martinez, submitted]. In the Lau Basin, intermediate to fast spreading rates impose a 2D plate-driven advective regime in the mantle wedge constraining volcanic accretion to the 2D narrow ridge axis. Effects of the cross trending compositional 'fingers' are minimized and only expressed as second-order geological and geochemical features at the ridge. As opening rates decrease to ultra-slow in the Havre Trough, 2D plate-driven components of mantle advection and melting are minimized. The inherent buoyancy of melts dominate advection and volcanic emplacement allowing a clearer expression of intrinsic 3D compositional and melt generation patterns in the mantle wedge. These observations suggest that mantle wedge structure fundamentally consists of arc-like mantle source compositional fingers trailing basinward from arc front volcanoes within a hydrous but more MORB source-like mantle. Spreading rate controls the degree of expression of these compositional fingers in back-arc volcanic crustal accretion. Fast to intermediate rate spreading imposes a 2D ridge-parallel distribution to crustal domains whereas slow to ultra slow spreading rates allow 3D mantle wedge compositional and melt generation patterns to be expressed.

Thin and layered subcontinental crust of the great Basin western north America inherited from Paleozoic marginal ocean basins?

USGS Publications Warehouse

Churkin, M.; McKee, E.H.

1974-01-01

The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton. ?? 1974.

Plutonism at Different Crustal Levels of an Arc: Insights From the 5 to 40 km (Paleodepth) North Cascades Crustal Section, Washington

NASA Astrophysics Data System (ADS)

Miller, R. B.; Paterson, S. R.; Matzel, J. P.

2008-12-01

The crystalline core of the North Cascades preserves a Cretaceous crustal section that facilitates evaluation of pluton construction, emplacement, geometry, composition, and deformation at widely variable crustal levels (~5 to 40 km paleodepth) in a thick (> 55 km) continental magmatic arc. The oldest and largest pulse of plutonism was focused between 96-89 Ma when fluxes were a minimum of 3.9x10-6km3/yr/km of arc length, but the coincidence with regional crustal thickening and underthrusting of a cool outboard terrane resulted in relatively low mid- to deep-crustal temperatures for an arc. A second, smaller peak of magmatism at 78-71 Ma (minimum of 8.2x10-7km3/yr/km of arc length) occurred during regional transpression. Tonalite dominates at all levels of the section. Intrusions range from large plutons to thin (< 50 m) dispersed sheets encased in metamorphic rocks that record less focused magmatism. The percentage of igneous rocks increases systematically from shallow to middle to deep levels; from approximately 37% to 55% to 65% of the total rock volume. Unfocused magmas comprise much higher percentages (approximately 19%) of the total plutonic rock at deep- and mid-crustal depths, but only 1% at shallower levels, whereas the largest intrusions were emplaced into shallow crust. Plutons have a range of shapes, including: asymmetric wedges to funnels; subhorizontal tabular sheets; steep-sided, blade-shaped bodies with high aspect ratios in map view; and steep-sided, vertically extensive (> 8 km) bodies shaped like thick disks and/or hockey pucks. Sheeted intrusions and gently dipping tabular bodies are more common with depth. Some of these plutons fit the model that most intrusions are subhorizontal and tabular, but many do not, reflecting the complex changes in lithology and rheology in arc crust undergoing regional shortening. The steep sheeted plutons partly represent magma transfer zones that fed the large shallow plutons, which were sites of intermittent magma accumulation for up to 5.5 m.y. Downward movement of host rocks by multiple processes occurred at all crustal levels during pluton emplacement. Ductile flow and accompanying rigid rotation were the dominant processes; stoping played an important secondary role, and magma wedging and regional deformation also aided emplacement. Overall, there are some striking changes with increasing depth, but many features and processes in the arc are similar throughout the crustal section, probably reflecting the relatively small differences in peak temperatures between the mid- and deep crust. Such patterns may be representative of thick continental magmatic arcs constructed during regional shortening.

Lower crustal xenoliths, Chinese Peak lava flow, central Sierra Nevada.

USGS Publications Warehouse

Dodge, F.C.W.; Calk, L.C.; Kistler, R.W.

1986-01-01

This assemblage of pyroxenite, peridotite and mafic granulite xenoliths in the toe of a 10 m.y. trachybasalt flow remnant overlying late Cretaceous granitic rocks, indicates the presence of a mafic-ultramafic complex beneath this part of central California; orthopyroxenites, websterites and clinopyroxenites are dominant. A few of the xenoliths contain ovoid opaque patches that are apparently pseudomorphs after garnet and have pyralspite garnet compositions; using a garnet-orthopyroxene geobarometer, they indicate a lower crustal depth of approx 40 km. Abundant mafic granulites can be subdivided into those with Al2O3 = or 15% and showing considerable scatter on oxide variation diagrams. The high-alumina granulite xenoliths have relatively low 87Rb/86Sr but high 87Sr/86Sr, whereas the low-alumina and ultramafic xenoliths have a wide range of 87Rb/86Sr, but lower 87Sr/86Sr; the isotopic data indicate roughly the same age as that of overlying granitic plutons (approx 100 m.y.). However, the granitic rocks have initial 87Sr/86Sr ratios intermediate between those of the high-alumina and ultramafic xenoliths, suggesting that they result from the mixing of basaltic magma (represented by the ultramafic rocks) and crustal materials, with subsequent crystal fractionation.-R.A.H.

Intrusive rocks of the Wadi Hamad Area, North Eastern Desert, Egypt: Change of magma composition with maturity of Neoproterozoic continental island arc and the role of collisional plutonism in the differentiation of arc crust

NASA Astrophysics Data System (ADS)

Basta, Fawzy F.; Maurice, Ayman E.; Bakhit, Bottros R.; Azer, Mokhles K.; El-Sobky, Atef F.

2017-09-01

The igneous rocks of the Wadi Hamad area are exposed in the northernmost segment of the Arabian-Nubian Shield (ANS). These rocks represent part of crustal section of Neoproterozoic continental island arc which is intruded by late to post-collisional alkali feldspar granites. The subduction-related intrusives comprise earlier gabbro-diorites and later granodiorites-granites. Subduction setting of these intrusives is indicated by medium- to high-K calc-alkaline affinity, Ta-Nb troughs on the spider diagrams and pyroxene and biotite compositions similar to those crystallized from arc magmas. The collisional alkali feldspar granites have high-K highly fractionated calc-alkaline nature and their spider diagrams almost devoid of Ta-Nb troughs. The earlier subduction gabbro-diorites have lower alkalis, LREE, Nb, Zr and Hf values compared with the later subduction granodiorites-granites, which display more LILE-enriched spider diagrams with shallower Ta-Nb troughs, reflecting variation of magma composition with arc evolution. The later subduction granitoids were generated by lower degree of partial melting of mantle wedge and contain higher arc crustal component compared with the earlier subduction gabbro-diorites. The highly silicic alkali feldspar granites represent extensively evolved melts derived from partial melting of intermediate arc crustal sources during the collisional stage. Re-melting of arc crustal sources during the collisional stage results in geochemical differentiation of the continental arc crust and the silicic collisional plutonism drives the composition of its upper part towards that of mature continental crust.

Raising the Gangdese Mountains in southern Tibet

NASA Astrophysics Data System (ADS)

Zhu, Di-Cheng; Wang, Qing; Cawood, Peter A.; Zhao, Zhi-Dan; Mo, Xuan-Xue

2017-01-01

The surface uplift of mountain belts is in large part controlled by the effects of crustal thickening and mantle dynamic processes (e.g., lithospheric delamination or slab breakoff). Understanding the history and driving mechanism of uplift of the southern Tibetan Plateau requires accurate knowledge on crustal thickening over time. Here we determine spatial and temporal variations in crustal thickness using whole-rock La/Yb ratios of intermediate intrusive rocks from the Gangdese arc. Our results show that the crust was likely of normal thickness prior to approximately 70 Ma ( 37 km) but began to thicken locally at approximately 70-60 Ma. The crust reached (58-50) ± 10 km at 55-45 Ma extending over 400 km along the strike of the arc. This thickening was likely due to magmatic underplating as a consequence of rollback and then breakoff of the subducting Neo-Tethyan slab. The crust attained a thickness of 68 ± 12 km at approximately 20-10 Ma, as a consequence of underthrusting of India and associated thrust faulting. The Gangdese Mountains in southern Tibet broadly attained an elevation of >4000 m at approximately 55-45 Ma as a result of isostatic surface uplift driven by crustal thickening and slab breakoff and reached their present-day elevation by 20-10 Ma. Our paleoelevation estimates are consistent not only with the C-O isotope-based paleoaltimetry but also with the carbonate-clumped isotope paleothermometer, exemplifying the promise of reconstructing paleoelevation in time and space for ancient orogens through a combination of magmatic composition and Airy isostatic compensation.

New constraints on the crustal structure beneath northern Tyrrhenian Sea

NASA Astrophysics Data System (ADS)

Levin, V. L.; Park, J. J.

2009-12-01

We present new seismological data on the seismic structure beneath the Tyrrhenian Sea between Corsica and the coast of Italy. Teleseismic receiver functions from two Tyrrhenian islands (Elba and Gorgona) identify clear P-to-S mode-converted waves from two distinct interfaces, at ~20 and ~45 km depth. Both interfaces are characterized by an increase of seismic wavespeed with depth. Using a summation of direct and multiply-reflected body waves within the P wave coda we estimate the mean ratio of compressional and shear wave speeds above the 45 km interface to be 1.75-1.80. Using reflectivity computations in 1D layered models we develop a model of seismic wavespeed distribution that yields synthetic seismograms very similar to those observed. We apply a Ps-multiple summation procedure to the synthetic waveforms to further verify the match between observed and predicted wavefields. The lower layer of our model, between 20 and 45 km, has Vp ~ 7.5 km/sec, a value that can be ascribed to either very fast crustal rocks or very slow upper mantle rocks. The Vp/Vs ratio is ~1.8 in this intermediate layer. On the basis of a well-constrained downward increase in seismic wave speed beneath this second layer, we interpret it as the magmatically reworked lower crust, a lithology that has been proposed to explain high-Vp layers in the crustal roots of island-arc terranes and volcanically altered continental margins, as well as lower-crustal high-Vp features sometimes seen beneath continental rifts. The presence of a thick layer of high-Vp, but crustal, lithology beneath the Tyrrhenian Sea differs considerably from previous estimates that interpreted the interface at ~20 km as the Moho. Our new interpretation obviates a need for a crustal thickness change of over 20 km at the crest of the Apennines orogen. We propose an alteration in the properties of the lower crust instead. We argue that ongoing convergent subduction of the Adriatic lithospehre is not required beneath northern Apennines, and that a delamination or vertical "drip" of detached lithosphere would fit the observations well.

Kinematic and rheological model of exhumation of high pressure granulites in the Variscan orogenic root: example of the Blanský les granulite, Bohemian Massif, Czech Republic

NASA Astrophysics Data System (ADS)

Franěk, J.; Schulmann, K.; Lexa, O.

2006-03-01

A large-scale relict domain of granulite facies deformation fabrics has been identified within the Blanský les granulite body. The granulite facies mylonitic fabric is discordant to the dominant amphibolite facies structures of the surrounding retrograde granulite. The complex geometry of retrograde amphibolite facies fabric indicates a large-scale fold-like structure, which is interpreted to be a result of either crustal-scale buckling of an already exhumed granulite sheet or active rotation of a rigid granulite facies ellipsoidal domain in kinematic continuity with the regional amphibolite facies deformation. We argue that both concepts allow similar restoration of the original granulite facies fabrics prior to the amphibolite facies deformation and “folding”. The geometry of the granulite facies foliations coincides with the earliest fabrics in the nearby mid-crustal units suggesting complete mechanical coupling between the deep lower crust and the mid-crustal levels during the vertical movements of crustal materials. Microstructures indicate grain-size sensitive flow enhanced by the presence of silicate melts at deep crustal levels and a beginning of an exhumation process of low viscosity granulites through a vertical channel. The amphibolite facies fabrics developed at middle crustal levels and their microstructures indicate significant hardening of feldspar-made rigid skeleton of the retrograde granulite. Increase in the strength of the granulite allowed an active buckling or a rigid body rotation of the granulite sheet, which acted as a strong layer inside the weaker metasediments.

Crustal-scale magmatism and its control on the longevity of magmatic systems

NASA Astrophysics Data System (ADS)

Karakas, Ozge; Degruyter, Wim; Bachmann, Olivier; Dufek, Josef

2017-04-01

Constraining the duration and evolution of crustal magma reservoirs is crucial to our understanding of the eruptive potential of magmatic systems, as well as the volcanic:plutonic ratios in the crust, but estimates of such parameters vary widely in the current literature. Although no consensus has been reached on the lifetime of magma reservoirs, recent studies have revealed about the presence, location, and melt fraction of multi-level (polybaric) storage zones in the crust. If magma accumulates at different crustal levels, it must redistribute significant enthalpy within the crustal column and therefore must influence the lifetime of magma plumbing systems. However, an evaluation of the mass and heat budget of the entire crustal column is lacking. Here, we use a two-dimensional thermal model to determine the thermal conditions under which both lower and upper crustal magma bodies form. We find that large lower crustal mush zones supply heat to the upper crust and reduce the amount of thermal energy necessary to form subvolcanic reservoirs. This indicates that the crust is thermally viable to sustain partially molten magma reservoirs over long timescales (>10^5-106 yr) for a range of magma fluxes (10^-4 to 10^-2 km^3/yr). Our results reconcile physical models of crustal magma evolution and field-based estimates of intrusion rates in numerous magmatic provinces (which include both volcanic and plutonic lithologies). We also show that young magmatic provinces (< 105 yr old) are unlikely to support large upper crustal reservoirs, whereas longer-lived systems (> 106 yr) can accumulate magma and build reservoirs capable of triggering supereruptions, even with intrusion rates as low as ≤10^-2 km^3/yr. Hence, the total duration of magmatism is critical in determining the size of the magma reservoirs, and should be combined with the magma intrusions rates to assess the capability of volcanic systems to form the largest eruptions on Earth.

Visualization and dissemination of global crustal models on virtual globes

NASA Astrophysics Data System (ADS)

Zhu, Liang-feng; Pan, Xin; Sun, Jian-zhong

2016-05-01

Global crustal models, such as CRUST 5.1 and its descendants, are very useful in a broad range of geoscience applications. The current method for representing the existing global crustal models relies heavily on dedicated computer programs to read and work with those models. Therefore, it is not suited to visualize and disseminate global crustal information to non-geological users. This shortcoming is becoming obvious as more and more people from both academic and non-academic institutions are interested in understanding the structure and composition of the crust. There is a pressing need to provide a modern, universal and user-friendly method to represent and visualize the existing global crustal models. In this paper, we present a systematic framework to easily visualize and disseminate the global crustal structure on virtual globes. Based on crustal information exported from the existing global crustal models, we first create a variety of KML-formatted crustal models with different levels of detail (LODs). And then the KML-formatted models can be loaded into a virtual globe for 3D visualization and model dissemination. A Keyhole Markup Language (KML) generator (Crust2KML) is developed to automatically convert crustal information obtained from the CRUST 1.0 model into KML-formatted global crustal models, and a web application (VisualCrust) is designed to disseminate and visualize those models over the Internet. The presented framework and associated implementations can be conveniently exported to other applications to support visualizing and analyzing the Earth's internal structure on both regional and global scales in a 3D virtual-globe environment.

Magmatic infiltration and melting in the lower crust and upper mantle beneath the Cima volcanic field, California

USGS Publications Warehouse

Wilshire, H.G.; McGuire, A.V.

1996-01-01

Xenoliths of lower crustal and upper mantle rocks from the Cima volcanic field (CVF) commonly contain glass pockets, veins, and planar trains of glass and/or fluid inclusions in primary minerals. Glass pockets occupy spaces formerly occupied by primary minerals of the host rocks, but there is a general lack of correspondence between the composition of the glass and that of the replaced primary minerals. The melting is considered to have been induced by infiltration of basaltic magma and differentiates of basaltic magma from complex conduits formed by hydraulic fracturing of the mantle and crustal rocks, and to have occurred during the episode of CVF magmatism between ???7.5 Ma and present. Variable compositions of quenched melts resulted from mixing of introduced melts and products of melting of primary minerals, reaction with primary minerals, partial crystallization, and fractionation resulting from melt and volatile expulsion upon entrainment of the xenoliths. High silica melts (> ??? 60% SiO2) may result by mixing introduced melts with siliceous melts produced by reaction of orthopyroxene. Other quenched melt compositions range from those comparable to the host basalts to those with intermediate Si compositions and elevated Al, alkalis, Ti, P, and S; groundmass compositions of CVF basalts are consistent with infiltration of fractionates of those basalts, but near-solidus melting may also contribute to formation of glass with intermediate silica contents with infiltration only of volatile constituents.

Petrogenesis of metamorphosed Paleoproterozoic, arc-related tonalites, granodiorites and coeval basic to intermediate rocks from southernmost Brazil, based on elemental and isotope geochemistry

NASA Astrophysics Data System (ADS)

Gregory, Tiago Rafael; Bitencourt, Maria de Fátima; Nardi, Lauro Valentim Stoll; Florisbal, Luana Moreira

2017-04-01

In southern Brazil, three associations of metamorphosed tonalites and granodiorites that are compositionally similar to tonalite-trondhjemite-granodiorite (TTG) or adakitic associations have been identified in the Arroio dos Ratos Complex (ARC) Paleoproterozoic magmatism. The metatonalites of Association 1 (A1; 2148 ± 33 Ma) have a well-developed fabric, compatible with strong solid-state deformation. The metatonalites and metagranodiorites of Association 2 (A2; 2150 ± 28 Ma) are intrusive in A1 and have a similar composition, but are less deformed, and their primary structures are partly preserved. Both associations display contemporaneity relations with basic to intermediate magmas. Association 3 (A3; 2077 ± 13 Ma) is represented by tonalitic to granodioritic gneisses, without any associated basic to intermediate magmatism, and its main characteristic is the banding that resulted from strong solid-state deformation. Partial melting features are locally present in A3. The geochemical compositions of the three associations are similar and indicate sources related to a continental magmatic arc environment. The 87Sr/86Sr(i) ratios (between 0.701 and 0.703), positive ƐNd(t) values (+ 1.45 to + 5.19), and TDM ages close to the crystallization ages indicate juvenile sources for the A1 and A2 associations. The A3 rocks have a 87Sr/86Sr(i) ratio of 0.715, an ƐNd(t) value of + 0.47 and a TDM age that is close to the crystallization age, indicating a source composition different from those of the other associations. The Pb isotope ratios of A1 and A2 are similar and compatible with the evolution of mantle and orogen (208Pb/204Pb = 37.3-37.6; 207Pb/204Pb = 15.62-15.65; 206Pb/204Pb = 18.0-18.2). The Pb isotope ratios of A3 differ from A1 and A2, indicating a more Th-poor source (208Pb/204Pb = 37.1; 207Pb/204Pb = 15.64; 206Pb/204Pb = 18.5). The geochemistry of associations A1 and A2 suggests a juvenile source with contamination by crustal material. However, the Sr-Nd-Pb isotope signature of this contaminant is similar to that of the source material that originated these associations. This may be the crust generated in the magmatic arc, which is compatible with the geochronological results. The dataset points to the occurrence of self-cannibalism processes in the generation of the ARC rocks. The similar chemical composition and ƐNd(t) values of A3 relative to A1 and A2 indicate that the A3 source is similar to the one that generated the tonalitic and granodioritic rocks of A1 and A2. However, the slightly lower 208Pb/204Pb, and higher 206Pb/204Pb and 87Sr/86Sr(i) ratios indicate that the A3 association has also the addition of a distinct crustal source. The A3 association high values of the parameter 87Sr/86Sr(i) and its Pb isotope signature indicate a source with high Rb and U, and low Th contents. Such features, and moreover the depletion of HREE combined with TDM values near the igneous age, suggest that the source for A3 may be juvenile arc sediments. The greater degree of crustal contribution, the lack of associated basic to intermediate rocks, and the younger age possibly mark the more mature or late stages of the arc. The major and trace elements, as well as the isotope data obtained in this study suggest that melting of a metasomatized mantle wedge can be the process that generated the ARC basic to intermediate rocks (A1 and A2). The generation of tonalitic and granodioritic rocks with adakitic characteristics (i.e., the depletion of heavy rare earth elements in tonalitic and granodioritic rocks of A1, A2 and A3) may have arisen from the melting of a garnet-rich source or from fractional crystallization of ARC basic to intermediate magmas, which in time increased crustal assimilation under high-pressure conditions. The crustal garnet-rich source could be the basic rocks newly placed at the base of the crust, derived from partial melting of metasomatized mantle. The remobilization of this material by partial melting may have generated tonalitic and granodioritic liquids depleted in heavy REE due to the presence of garnet in the residue. The three associations display microstructures that indicate two episodes of recrystallization, one of a higher temperature and one of a lower temperature. The age of the high-temperature structure remains under discussion and may be attributed to a Paleoproterozoic metamorphic event. The low-temperature event is compatible with the temperature conditions observed in adjacent areas, in the host rocks of the Neoproterozoic post-collisional granitoids that have been emplaced along the Southern Brazilian Shear Belt (SBSB). Zircon crystals with Paleoproterozoic igneous cores exhibit a metamorphic overgrowth at 635 ± 6 Ma, compatible with the crystallization ages of the SBSB granitoids.

Microstructural and fabric characterization of brittle-ductile transitional deformation of middle crustal rocks along the Jinzhou detachment fault zone, Northeast China

NASA Astrophysics Data System (ADS)

Zhang, Juyi; Jiang, Hao; Liu, Junlai

2017-04-01

Detachment fault zones (DFZs) of metamorphic core complexes generally root into the middle crust. Exhumed DFZs therefore generally demonstrate structural, microstructural and fabric features characteristic of middle to upper crustal deformation. The Jinzhou detachment fault zone from the Liaonan metamorphic core complex is characterized by the occurrence of a sequence of fault rocks due to progressive shearing along the fault zone during exhumation of the lower plate. From the exhumed fabric zonation, cataclastic rocks formed in the upper crust occur near the Jinzhou master detachment fault, and toward the lower plate gradually changed to mylonites, mylonitic gneisses and migmatitic gneisses. Correspondingly, these fault rocks have various structural, microstructural and fabric characteristics that were formed by different deformation and recrystallization mechanisms from middle to upper crustal levels. At the meanwhile, various structural styles for strain localization were formed in the DFZ. As strain localization occurs, rapid changes in deformation mechanisms are attributed to increases in strain rates or involvement of fluid phases during the brittle-ductile shearing. Optical microscopic studies reveal that deformed quartz aggregates in the lower part of the detachment fault zone are characterized by generation of dynamically recrystallized grains via SGR and BLG recrystallization. Quartz rocks from the upper part of the DFZ have quartz porphyroclasts in a matrix of very fine recrystallized grains. The porphyroclasts have mantles of sub-grains and margins grain boundary bulges. Electron backscattered diffraction technique (EBSD) quartz c-axis fabric analysis suggests that quartz grain aggregates from different parts of the DFZ possess distinct fabric complexities. The c-axis fabrics of deformed quartz aggregates from mylonitic rocks in the lower part of the detachment fault zone preserve Y-maxima which are ascribed to intermediate temperature deformation (500-630˚ C), whereas complicated fabric patterns (e.g. asymmetric single girdles) are formed in fault rocks from the upper part of the DFZ. The increasing fabric complexity is here interpreted as the result of progressive superposition of fault rocks by shearing either at relatively shallow levels or high rate of strain, during exhumation of the lower plate and shear zone rocks. The above observations and interpretations imply that dislocation creep processes contribute to the dynamic recrystallization of quartz in the middle crustal brittle-ductile transition. Progressive shearing as a consequence of exhumation of the lower plate of the MCC contributed to the obvious structural, microstructural and fabric superpositions. Strain localization occurs as the progressive shearing proceeded. Transition of mechanisms of deformation and dynamic recrystallization during strain localization may be resulted from changes in temperature conditions, in strain rates or addition of minor amount water.

Polybaric evolution of phonolite, trachyte, and rhyolite volcanoes in Eastern Marie Byrd Land, Antarctica: Controls on peralkalinity and silica saturation

USGS Publications Warehouse

LeMasurier, W.E.; Futa, K.; Hole, M.; Kawachi, Y.

2003-01-01

In the Marie Byrd Land volcanic province, peralkaline and metaluminous trachytes, phonolites, and rhyolites occur in 18 large shield volcanoes that are closely associated in time and space. They are arrayed radially across an 800 km wide structural dome, with the oldest at the crest and the youngest around the flanks. Several lines of evidence suggest that these rocks evolved via open-system, polybaric fractionation. We have used mass balance modeling of major elements together with trace-element data and mineral chemistry to help explain the evolution of this diverse suite of felsic rocks, which appear to have been generated coevally in isolated magma chambers, and erupted close to each other in patterns related to tectonic uplift and extension within the West Antarctic rift system. Isotopic and trace-element data indicate that this occurred with only minimal crustal contamination. We focus on volcanoes of the Executive Committee Range and Mount Murphy, where we find good representation of basalts and felsic rocks within a small area. Our results suggest that the felsic rocks were derived from basaltic magmas that differentiated at multiple levels during their passage to the surface: first to ferrogabbroic compositions near the base of the lithosphere, then to intermediate compositions near the base of the crust, and finally to felsic compositions in mid- to upper crustal reservoirs. The high-pressure history has been largely masked by low-pressure processes. The best indications of a high-pressure history are the mineral phases in cumulate nodules and their correlation with modeling results, with REE anomalies, and with the composition of an unusual gabbroic intrusion. Silica saturation characteristics are believed to have originated in magma chambers near the base of the crust, via fractionation of variable proportions of kaersutite and plagioclase. Development of peralkalinity in felsic rocks took place in upper crustal reservoirs by fractionating a high ratio of plagioclase to clinopyroxene under conditions of low pH2O. With increasing pH2O, the ratio plagioclase/clinopyroxene in the fractionated assemblage decreases and metaluminous liquids resulted. Crustal contamination seems to have had a role in suppressing peralkalinity, and was probably a factor in the origin of high-silica metaluminous rhyolite, but metaluminous rocks are uncommon. The volume and diversity of felsic rocks were probably enhanced by the structure of the lithosphere, the persistence of plume activity, and the immobility of the Antarctic plate. Mechanical boundaries at the base of the lithosphere and crust, and within the crust, appear to have acted as filters, trapping magmas at multiple levels, and prolonging the fractionation process. Final volumes would have been further enhanced by repeated refluxing of the same magma chambers, controlled by plume activity and plate immobility.

Mid-Neoproterozoic intraplate magmatism in the northern margin of the Southern Granulite Terrane, India: Constraints from geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

NASA Astrophysics Data System (ADS)

Deeju, T. R.; Santosh, M.; Yang, Qiong-Yan; Pradeepkumar, A. P.; Shaji, E.

2016-11-01

The northern margin of the Southern Granulite Terrane in India hosts a number of mafic, felsic and alkaline magmatic suites proximal to major shear/paleo-suture zones and mostly represents magmatism in rift-settings. Here we investigate a suite of gabbros and granite together with intermediate (dioritic) units generated through mixing and mingling of a bimodal magmatic suite. The massive gabbro exposures represent the cumulate fraction of a basic magma whereas the granitoids represent the product of crystallization in felsic magma chambers generated through crustal melting. Diorites and dioritic gabbros mostly occur as enclaves and lenses within host granitoids resembling mafic magmatic enclaves. Geochemistry of the felsic units shows volcanic arc granite and syn-collisional granite affinity. The gabbro samples show mixed E-MORB signature and the magma might have been generated in a rift setting. The trace and REE features of the rocks show variable features of subduction zone enrichment, crustal contamination and within plate enrichment, typical of intraplate magmatism involving the melting of source components derived from both depleted mantle sources and crustal components derived from older subduction events. The zircons in all the rock types show magmatic crystallization features and high Th/U values. Their U-Pb data are concordant with no major Pb loss. The gabbroic suite yields 206Pb/238U weighted mean ages in the range of 715 ± 4-832.5 ± 5 Ma marking a major phase of mid Neoproterozoic magmatism. The diorites crystallized during 206Pb/238U weighted mean age of 724 ± 6-830 ± 2 Ma. Zircons in the granite yield 206Pb/238U weighted mean age of 823 ± 4 Ma. The age data show broadly similar age ranges for the mafic, intermediate and felsic rocks and indicate a major phase of bi-modal magmatism during mid Neoproterozoic. The zircons studied show both positive and negative εHf(t) values for the gabbros (-6.4 to 12.4), and negative values for the diorites (-7.8 to -16.7) and granite (-16.6 to -6.7). Together with the Hf depleted model ages and crustal model ages, we infer that the magma sources involved both juvenile depleted mantle and reworked Mesoproterozoic, Paleoproterozoic and Neoarchean components. The mid Neoproterozoic intraplate magmatism is considered to be a response to mantle upwelling in an aborted rift 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.

The Glacial BuzzSaw, Isostasy, and Global Crustal Models

NASA Astrophysics Data System (ADS)

Levander, A.; Oncken, O.; Niu, F.

2015-12-01

The glacial buzzsaw hypothesis predicts that maximum elevations in orogens at high latitudes are depressed relative to temperate latitudes, as maximum elevation and hypsography of glaciated orogens are functions of the glacial equilibrium line altitude (ELA) and the modern and last glacial maximum (LGM) snowlines. As a consequence crustal thickness, density, or both must change with increasing latitude to maintain isostatic balance. For Airy compensation crustal thickness should decrease toward polar latitudes, whereas for Pratt compensation crustal densities should increase. For similar convergence rates, higher latitude orogens should have higher grade, and presumably higher density rocks in the crustal column due to more efficient glacial erosion. We have examined a number of global and regional crustal models to see if these predictions appear in the models. Crustal thickness is straightforward to examine, crustal density less so. The different crustal models generally agree with one another, but do show some major differences. We used a standard tectonic classification scheme of the crust for data selection. The globally averaged orogens show crustal thicknesses that decrease toward high latitudes, almost reflecting topography, in both the individual crustal models and the models averaged together. The most convincing is the western hemisphere cordillera, where elevations and crustal thicknesses decrease toward the poles, and also toward lower latitudes (the equatorial minimum is at ~12oN). The elevation differences and Airy prediction of crustal thickness changes are in reasonable agreement in the North American Cordillera, but in South America the observed crustal thickness change is larger than the Airy prediction. The Alpine-Himalayan chain shows similar trends, however the strike of the chain makes interpretation ambiguous. We also examined cratons with ice sheets during the last glacial period to see if continental glaciation also thins the crust toward higher latitudes. The glaciated North American and European cratons show a trend of modest thinning (~3km), and glaciated western Asia minor thinning (~1.5 km). These values are at the level of model uncertainties, but we note that cratons without ice sheets during the last glacial period show substantially different patterns.

The size distribution of chemical elements of atmospheric aerosol at a semi-rural coastal site in Venice (Italy). The role of atmospheric circulation.

PubMed

Masiol, Mauro; Squizzato, Stefania; Ceccato, Daniele; Pavoni, Bruno

2015-01-01

The concentrations of selected elemental tracers were determined in the aerosol of a semi-rural coastal site near Venice (Italy). Size-segregated aerosol samples were collected using an 8-stage cascade impactor set at 15m above ground, during the cold season (late autumn and winter), when high levels of many pollutants are known to cause risks for human health. From the experimental data, information was extracted on potential pollutant sources by investigating the relationships between elements in the different size fractions. Moreover, an approach to highlight the importance of local atmospheric circulation and air mass origin in influencing the PM composition and fractional distribution is proposed. Anthropogenic elements are strongly inter-correlated in the submicrometric (<1 μm) (S, K, Mn, Cu, Fe and Zn) and intermediate mode (1-4 μm) (Mn, Cu, Zn, Ni) and their relationships highlight the presence of several sources (combustions, secondary aerosol, road traffic). In the intermediate mode, associations having geochemical significance exist between marine (Na, Cl and Mg) and crustal (Si, Mg, Ca, Al, Ti and K) elements. In the coarse mode (>4 μm) Fe and Zn are well correlated and are probably linked to tire and brake wear emissions. Regarding atmospheric circulation, results show increasing levels of elements related to pollution sources (S, K, Mn, Ni, Cu, Zn) when air masses come from Central and Eastern Europe direction and on the ground wind blows from NWN-N-NE (from mainland Venice). Low wind speed and high percentage of wind calm hours favor element accumulation in the submicrometric and intermediate modes. Furthermore, strong winds favor the formation of sea-spray and the increase of Si in the coarse mode due to the resuspension of sand fine particles. Copyright © 2014 Elsevier Ltd. All rights reserved.

The source of the intermediate wavelength component of the Earth's magnetic field

NASA Technical Reports Server (NTRS)

Harrison, C. G. A.

1985-01-01

The intermediate wavelength component of the Earth's magnetic field has been well documented by observations made by MAGSAT. It has been shown that some significant fraction of this component is likely to be caused within the core of the Earth. Evidence for this comes from analysis of the intermediate wavelength component revealed by spherical harmonics between degrees 14 and 23, in which it is shown that it is unlikely that all of this signal is crustal. Firstly, there is no difference between average continental source strength and average oceanic source strength, which is unlikely to be the case if the anomalies reside within the crust, taking into account the very different nature and thickness of continental and oceanic crust. Secondly, there is almost no latitudinal variation in the source strength, which is puzzling if the sources are within the crust and have been formed by present or past magnetic fields with a factor of two difference in intensity between the equator and the poles. If however most of the sources for this field reside within the core, then these observations are not very surprising.

Chemical composition of the continental crust: Insights from a quantitative interpretation of the Vp/Vs ratio

NASA Astrophysics Data System (ADS)

Guerri, M.; Youssof, M.; Fullea, J.

2017-12-01

The processes driving continental crust formation are not yet fully understood. One of the fundamental keys necessary to investigate the enigma is represented by crustal composition. The Vp/Vs ratio from seismic receiver functions or tomography studies is a powerful tool to constrain the crustal composition. However, to date only qualitative relationships between Vp/Vs and composition have been proposed. We present a quantitative interpretation of the Vp/Vs in terms of major oxide components, based on thermo-elastic constrained modelling of rock phase equilibria and physical properties. The geophysical-petrological approach is implemented in the new release of the software package LitMod, which now allows for integrated and self-consistent modeling of the entire lithosphere (crust + lithospheric mantle) and upper mantle. Forward modelling of the Vp/Vs, based on petrology and thermodynamics, reveals that, as expected, mafic compositions have higher Vp/Vs than felsic ones. However, in high temperature settings (surface heat flow > 75 mW/m2), the quartz alpha / quartz beta transition strongly increases Vp, leaving Vs almost unaltered, leading to SiO2-rich compositions displaying Vp/Vs values higher than those associated with mafic compositions. Additionally, we highlight the importance of H2O, the presence of which stabilizes amphibole (in place of pyroxene), characterized by a relatively low Vp/Vs. If H2O is present, mafic compositions show Vp/Vs ratios that are comparable to those produced by anhydrous SiO2-rich compositions. The destabilization of amphibole (in favour of pyroxene) generates a sharp seismic discontinuity, potentially detectable by, for example, seismic refraction and receiver function investigations. We invert the Vp/Vs ratio for composition and hydrous state of the crust in the Southern African cratons. Our results show that the Kaapvaal craton, Archean in age, has an intermediate (SiO2 60 wt%) composition. The finding has implications on our understanding of the mechanisms responsible for crustal formation as an intermediate bulk composition is potentially generated by subduction-related processes. We conclude that, already before the Meso-Archean (3.2 Ga), subduction related crust-forming processes were active in the area and potentially also in other cratons.

Evidence from xenoliths for a dynamic lower crust, eastern Mojave Desert, California

USGS Publications Warehouse

Hanchar, John M.; Miller, Calvin F.; Wooden, Joseph L.; Bennett, Victoria C.; Staude, John-Mark G.

1994-01-01

Garnet-rich xenoliths in a Tertiary dike in the eastern Mojave Desert, California, preserve information about the nature and history of the lower crust. These xenoliths record pressures of ∼ 10–12 kbar and temperatures of ∼ 750–800°C. Approximately 25% have mafic compositions and bear hornblende + plagioclase + clinopyroxene + quartz in addition to garnet. The remainder, all of which contain quartz, include quartzose, quartzofeldspathic, and aluminous (kyanite±sillimanite-bearing) varieties. Most xenoliths have identifiable protoliths—mafic from intermediate or mafic igneous rocks, quartzose from quartz-rich sedimentary rocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathic from feldspathic sediments and/or intermediate to felsic igneous rocks. However, many have unusual chemical compositions characterized by high FeO(t), FeO(t)/MgO, Al2O3, and Al2O3/CaO, which correspond to high garnet abundance. The mineralogy and major-and trace-element compositions are consistent with the interpretation that the xenoliths are the garnet-rich residues of high-pressure crustal melting, from which granitic melt was extracted. High 87Sr/86Sr and low 143Nd/144Nd, together with highly discordant zircons from a single sample with Pb/Pb ages of ∼ 1.7 Ga, demonstrate that the crustal material represented by the xenoliths is at least as old as Early Proterozoic. This supracrustal-bearing lithologic assemblage may have been emplaced in the lower crust during either Proterozoic or Mesozoic orogenesis, but Sr and Nd model ages> 4 Ga require late Phanerozoic modification of parent/daughter ratios, presumably during the anatectic event. Pressures of equilibration indicate that peak metamorphism and melting occurred before the Mojave crust had thinned to its current thickness of <30 km. The compositions of the xenoliths suggest that the lower crust here is grossly similar to estimated world-wide lower-crustal compositions in terms of silica and mafic content; however, it is considerably more peraluminous, has a lower mg-number, and is distinctive in some trace element concentrations, reflecting its strong metasedimentary and restitic heritage.

Paleo-Productivity across the Paleocene-Eocene Thermal Maximum, Walvis Ridge Transect (ODP Sites 1262, 1263, and 1266)

NASA Astrophysics Data System (ADS)

Chun, C. O.; Delaney, M. L.; Zachos, J. C.

2005-12-01

Walvis Ridge transect (Ocean Drilling Program (ODP) Leg 208) provides the first high-resolution depth-transect of deep-sea sediments recovered from the south Atlantic across the P/E boundary. A geographically restricted depth transect (~ 2.2 km, water depths between 2500 and 4770 m) allows us to constrain the surface waters by assuming marine productivity conditions in the overlying water column are similar across all sites. The sediment record will reveal variations for processes that are water-depth dependent. We use the geochemical tracers; biogenic barium, phosphorus, calcium carbonate, and the redox sensitive trace elements manganese and uranium, to reconstruct nutrient burial, paleoproductivity, and bottom water redox chemistry across the Paleocene-Eocene Thermal Maximum (PETM). We calculate our concentrations on a calcium carbonate-free basis to account for dilution by non-carbonate sediments. Trace metal enrichment factors (EFs) are calculated relative to bulk crustal averages. We chose three sites from the depth transect: the shallowest (Site 1263, 2717 m water depth), an intermediate site (Site 1266, 3798 m water depth), and the deepest site (Site 1262, 4755 m water depth). We sampled each site at a sample resolution of ~ 1-2 kyr for 5 m.y. centered at 55 Ma. Uranium EFs at the shallow site exhibits values ~ 5 pre-event and drop to values near crustal averages during and after the carbon isotope excursion (CIE). No dramatic changes in U EFs across the P/E boundary are recorded at the deep and intermediate sites. Mn EFs range between 2.9 -8.6 prior to the event across all three sites, suggesting an oxygenated depositional environment. At the boundary, Mn EFs drop to crustal averages at all sites, then gradually return to pre-event values, indicating more reducing environments during the CIE, a possible explanation for the benthic extinction event (BEE) observed across this transect. Ba excess and reactive phosphorus exhibit decreased concentrations during the CIE with gradual return to pre-event values at the shallowest and deepest sites. We will compare the paleo-productivity and redox chemistry response at the Walvis Ridge sites across the PETM.

Magmatism at different crustal levels in the ancient North Cascades magmatic arc

NASA Astrophysics Data System (ADS)

Shea, E. K.; Bowring, S. A.; Miller, R. B.; Miller, J. S.

2013-12-01

The mechanisms of magma ascent and emplacement inferred from study of intrusive complexes have long been the subject of intense debate. Current models favor incremental construction based on integration of field, geochemical, geochronologic, and modeling studies. Much of this work has been focused on a single crustal level. However, study of magmatism throughout the crust is critical for understanding how magma ascends through and intrudes surrounding crustal material. Here, we present new geochronologic and geochemical work from intrusive complexes emplaced at a range of crustal depths in the Cretaceous North Cascades magmatic arc. These complexes were intruded between 92 and 87 Ma at depths of at ≤5 -10 km, ~20 km, and ~25 km during this time. U-Pb CA-TIMS geochronology in zircon can resolve <0.1% differences in zircon dates and when combined with detailed field relationships allow new insights into how magmatic systems are assembled. We can demonstrate highly variable rates of intrusion at different crustal levels: the shallow-crustal (5-10 km) Black Peak intrusive complex was assembled semi-continuously over ~5 My, while the deep-crustal (25-30 km) Tenpeak intrusive complex was assembled in brief, high-flux events over ~2.6 My. Between these bodies is the Seven-Fingered Jack-Entiat intrusive complex, a highly elongate amalgamation of intrusions recording two episodes of magmatism between~92-88 Ma and ~80-77 Ma. Each of these complexes provides a window into crustal processes that occur at different depths. Our data suggest assembly of the Black Peak intrusive complex occurred via a series of small (0.5-2 km2) magmatic increments from ~92 Ma to ~87 Ma. Field relations and zircon trace element geochemistry indicate each of these increments were emplaced and crystallized as closed systems-we find no evidence for mixing between magmas in the complex. However, zircon inheritance becomes more common in younger intrusions, indicating assimilation of older plutonic material, possibly during magma production or transport. The Seven-Fingered Jack intrusive complex, emplaced around 15-20 km, preserves a much more discontinuous record of intrusion than the Black Peak. Our data indicate major magmatism in the complex occurred between ~92.1-91.1 Ma. Inheritance in the Seven-Fingered Jack is common, particularly along contacts between intrusions. The Tenpeak intrusive complex, assembled between ~92 Ma and 89 Ma, represents one of the deepest exhumed complexes in the North Cascades. Our geochronology indicates that plutons comprising the complex were intruded rapidly (<200 ka) and followed by periods of magmatic quiescence. Contact relations between contemporaneous intrusions are often mixed, further supporting rapid assembly. Zircon systematics in the Tenpeak are relatively simple, showing no evidence for inheritance from the surrounding host rock or from earlier intrusions. However, zircon oxygen isotope data indicates many magmas contain significant crustal input. The Black Peak, Seven-Fingered Jack, and Tenpeak intrusions illustrate the complicated nature of magmatism at different crustal levels in the 92-87 Ma North Cascades magmatic arc. Our data support incremental assembly of these complexes, but show that many features, such as style of emplacement, zircon chemical and temporal systematics, and magma composition vary between these intrusions.

Testing The Magmatic Underplating Hypothesis: An Example From The Uk and Ireland

NASA Astrophysics Data System (ADS)

Al-Kindi, S.; White, N.; Sinha, M.; England, R.

Magmatic underplating associated with mantle plume activity is an important mech- anism for driving regional surface uplift and denudation of large portion of the continents. Here we present quantitative and predictive models linking the surface- measured uplift and denudation with deep crustal structure across the British Isles. The crustal model was derived from re-interpreting the 1982 wide-angle Caledonian Suture Seismic Project and it's Irish extension (CSSP&ICSSP) data sets. A joint CSSP/ICSSP velocity model was obtained for the first time by inverting for six main travel-time phases comprising more than 3000 picks having picking uncertainty ranges of 50-100 ms and average of 82 ms with best picks for first breaks at close offsets. Two indepen- dent tomographic codes namely RAYINVR (Zelt and Smith, 1992) and Jive3D (Ho- bro, 2000) were used to model the picked travel-times adopting 'interpreter-guided' and 'pure tomographic' approaches, respectively. The codes represent natural end- member approaches to travel-time tomography where the former seeks an irregular grid, minimum-parameter velocity model, whereas the later seeks minimum-structure velocity model. The final outcome of the two methods are remarkably similar which has greatly boosted confidence in the interpretation. Complementary resolution and uncertainty tests were preformed. The most striking feature of the outcome of the inversion processes is the emergence of a discrete high-velocity (7.0-7.5 km/s) intermediate layer above the Moho. The top interface of this layer is sampled by lower crustal reflections, whereas the layer velocity is sampled by refracted rays. The base of the layer is bounded by the Moho interface roughly at 33 km, constrained by upper mantle diving rays. Some Moho reflections were observed on some record sections, but the majority are believed to be masked by the early arriving, highly-reflective coda generated by resonance of seismic waves within the intermediate layer. The layer has maximum thickness of 8 + 1.6 km roughly half-way across the East Irish Sea and thins out towards the edges. The minimum width of this layer is well constrained by the strong lower crustal reflections to be approximately 550 km. The maximum width, could extend outside the ray coverage of this experiment with maximum layer thickness of 2 km at the edge of the model. This is roughly of the thickness of the smallest resolvable structure 1 at a depth of 30 km using a 5 Hz signal. Filtered gravity data was used to model a density model derived by converting the final preferred velocity model using an appropriate P-wave velocity-to-density conversion. The profile is characterised by a high positive gravity anomaly of about 30 mGal over the East Irish Sea. An excess of denser material in the lower crust (+0.20 Mgm-3) was essential to account for this gravity high, which is consistent with the wide-angle velocity model. Synthetic denudation values were calculated along the 2D crustal model assuming Airly isostasy for different elastic thickness, and were compared to real estimates pro- vided by Rowely, 1998. The two data sets show good correlation within the uncertain- ties of the estimates, which has encouraged futher analysis. An attempt at extending the two-dimensional results into the third dimension was carried out based on a sta- tistical correlation of 800 pairs of modelled underplating thickness values and 150 km high-pass gravity samples along the 2D line. The analysis has shown a high posi- tive correlation with R2=0.72 with a significant linear regression at the 95% confident level. This relationship was then computed to predict underplate thickness from the filtered gravity map and then compared with the available denudation maps. This anal- ysis has highlighted specific areas where underplating is postulated to derive surface uplift. 2

Glacial cycles drive variations in the production of oceanic crust

NASA Astrophysics Data System (ADS)

Crowley, J. W.; Katz, R. F.; Langmuir, C. H.; Huybers, P. J.

2013-12-01

Changes in sea level accompanying glacial cycles affect the static pressure within the asthenosphere; these variations could modulate melting rates beneath the mid-ocean ridge system as well as crustal thickness. These effects can be investigated and quantified using models of ridges based on conservation of mass, momentum, energy, and composition for two phases (magma & mantle) and two thermodynamic components (enriched & depleted). The models predict that the sensitivity of crustal thickness to oscillations in sea-level depends on the period of oscillation, the spreading rate of the ridge, and the assumed permeability scale of the melting regime. In contrast to previous studies (Huybers & Langmuir, 2009 and Lund & Asimow, 2011), the new results indicate that effects are larger for ridges with faster spreading rates. They also show that the dominant period of variations in crustal thickness changes with spreading rate and permeability. Sea-level variations with periods in the range of 10 ky - 100 ky can result in significant changes in crustal thickness that are orders of magnitude larger than the sea-level variations that drive them. Accurately modelling this process requires the inclusion of two previously unaccounted for processes: (1) determining the volume of the melting regime that is consistent with the ridge spreading rate and (2) properly treating the transport of melt. These enable us to capture the non-linear dependencies on spreading rate and other model parameters. Spectral analysis of bathymetry at two ridge segments that have a symmetric bathymetric signal and hence are undisturbed by off-axis volcanism or ridge jumps reveals the presence of variability at frequencies associated with precession, obliquity, and the 100 ky glacial/inter-glacial variability. Furthermore, the faster spreading ridge has larger amplitude responses to changes in sea level and shows a proportionately greater response at higher frequencies. These observations reinforce the possible links among climate cycles at the surface, mantle melting at depth and the crustal fabric of the sea floor.

Mars - Crustal structure inferred from Bouguer gravity anomalies.

NASA Technical Reports Server (NTRS)

Phillips, R. J.; Saunders, R. S.; Conel, J. E.

1973-01-01

Bouguer gravity has been computed for the equatorial region of Mars by differencing free air gravity and the gravity predicted from topographic variations. The free air gravity was generated from an eighth-order set of spherical harmonic coefficients. The gravity from topographic variations was generated by integrating a two-dimensional Green's function over each contour level. The Bouguer gravity indicates crustal inhomogeneities on Mars that are postulated to be variations in crustal thickness. The Tharsis ridge is a region of thick continental type crust. The gravity data, structural patterns, topography, and surface geology of this region lead to the interpretation of the Tharsis topographic high as a broad crustal upwarp possibly associated with local formation of lower-density crustal material and subsequent rise of a thicker crust. The Amazonis region is one of several basins of relatively thin crust, analogous to terrestrial ocean basins. The Libya and Hellas basins, which are probable impact features, are also underlain by thin crust and are possible regions of mantle upwelling.

Magma traps and driving pressure: consequences for pluton shape and emplacement in an extensional regime

NASA Astrophysics Data System (ADS)

Hogan, John P.; Price, Jonathan D.; Gilbert, M. Charles

1998-09-01

The level of emplacement and final form of felsic and mafic igneous rocks of the Wichita Mountains Igneous Province, southwestern Oklahoma, U.S.A. are discussed in light of magma driving pressure, lithostatic load, and crustal magma traps. Deposition of voluminous A-type rhyolites upon an eroded gabbroic substrate formed a subhorizontal strength anisotropy that acted as a crustal magma trap for subsequent rising felsic and mafic magma. Intruded along this crustal magma trap are the A-type sheet granites (length/thickness 100:1) of the Wichita Granite Group, of which the Mount Scott Granite sheet is typical, and smaller plutons of biotite bearing Roosevelt Gabbro. In marked contrast to the subhorizontal granite sheets, the gabbro plutons form more equant stocks with flat roofs and steep side walls. Late Diabase dikes cross-cut all other units, but accompanying basaltic flows are extremely rare in the volcanic pile. Based on magmastatic calculations, we draw the following conclusions concerning the level of emplacement and the shape of these intrusions. (1) Magma can rise to a depth at which the magma driving pressure becomes negligible. Magma that maintains a positive driving pressure at the surface has the potential to erupt. (2) Magma ascent may be arrested at a deeper level in the crust by a subhorizontal strength anisotropy (i.e. crustal magma trap) if the magma driving pressure is greater than or equal to the lithostatic load at the depth of the subhorizontal strength anisotropy. (3) Subhorizontal sheet-intrusions form along crustal magma traps when the magma driving pressure greatly exceeds the lithostatic load. Under such conditions, the magma driving pressure is sufficent to lift the overburden to create the necessary space for the intrusion. (4) Thicker steep-sided stocks or batholiths, with flat roofs, form at crustal magma traps when the magma driving pressure approximates that of the lithostatic load. Under these conditions, the necessary space for the intrusion must be created by other mechanisms (e.g. stoping). (5) Subvertical sheets (i.e. dikes) form when the magma driving pressure is less than the lithostatic load at the level of emplacement.

Geochemical Relationships between Middle- to Upper-Crustal Exposures of the Alisitos Oceanic Arc (Baja California, Mexico): An Outstanding Field Analog to Active Extensional Oceanic Arcs

NASA Astrophysics Data System (ADS)

Morris, R.; DeBari, S. M.; Busby, C.; Medynski, S.

2016-12-01

The southern volcano-bounded basin of the Rosario segment of the Cretaceous Alisitos oceanic arc provides outstanding 3-D exposures of an extensional arc, where crustal generation processes are recorded in the upper-crustal volcanic units and underlying middle-crustal plutonic rocks. Geochemical linkages between exposed crustal levels provide an analog for extensional arc systems such as the Izu-Bonin-Mariana (IBM) Arc. Upper-crustal units comprise a 3-5 km thick volcanic-volcaniclastic stratigraphy with hypabyssal intrusions. Deep-seated plutonic rocks intrude these units over a transition of <500m, where rafted volcanic blocks and evidence of magma mingling are exposed. Thermobarometry suggests <6 km emplacement depths. Compositional ranges (basalt to rhyolite) and mineral assemblages are similar in both middle-crustal and upper-crustal units, with striking compositional overlap. The most mafic compositions occur in upper-crustal hypabyssal units, and as amphibole cumulates in the plutonic rocks ( 51% SiO2). The most felsic compositions occur in welded ignimbrites and a tonalite pluton ( 71% SiO2). All units are low K with flat REE patterns, and show LILE enrichment and HFSE depletion. Trace element ratios show limited variation throughout the crustal section. Zr/Y and Nb/Y ratios are similar to the Izu active ( 3 Ma to present) zone of extension immediately behind the arc front, suggesting comparable mantle melt % during extension. Th/Zr ratios are more enriched in Alisitos compared to Izu, suggesting greater subducted sediment input. The Alisitos crustal section shows a limited range in ɛNd (5.7-7.1), but a wider range in 87Sr/86Sr (0.7035-0.7055) and 206Pb/204Pb (18.12-19.12); the latter is likely alteration effects. Arc magmas were derived from a subduction-modified MORB mantle source, less depleted than Izu arc front and less enriched than the rear arc, but is a good match with the zone of extension that lies between. Differentiation occurred in a closed system (i.e., fractional crystallization/self-melting with back mixing), producing the entire crustal section in <3 Ma.

Seafloor Age-Stacking Reveals No Evidence for Milankovitch Cycle Influence on Abyssal Hills at Intermediate, Fast and Super-Fast Spreading Rates

NASA Astrophysics Data System (ADS)

Goff, J.; Zahirovic, S.; Müller, D.

2017-12-01

Recently published spectral analyses of seafloor bathymetry concluded that abyssal hills, highly linear ridges that are formed along seafloor spreading centers, exhibit periodicities that correspond to Milankovitch cycles - variations in Earth's orbit that affect climate on periods of 23, 41 and 100 thousand years. These studies argue that this correspondence could be explained by modulation of volcanic output at the mid-ocean ridge due to lithostatic pressure variations associated with rising and falling sea level. If true, then the implications are substantial: mapping the topography of the seafloor with sonar could be used as a way to investigate past climate change. This "Milankovitch cycle" hypothesis predicts that the rise and fall of abyssal hills will be correlated to crustal age, which can be tested by stacking, or averaging, bathymetry as a function of age; stacking will enhance any age-dependent signal while suppressing random components, such as fault-generated topography. We apply age-stacking to data flanking the Southeast Indian Ridge ( 3.6 cm/yr half rate), northern East Pacific Rise ( 5.4 cm/yr half rate) and southern East Pacific Rise ( 7.8 cm/yr half rate), where multibeam bathymetric coverage is extensive on the ridge flanks. At the greatest precision possible given magnetic anomaly data coverage, we have revised digital crustal age models in these regions with updated axis and magnetic anomaly traces. We also utilize known 2nd-order spatial statistical properties of abyssal hills to predict the variability of the age-stack under the null hypothesis that abyssal hills are entirely random with respect to crustal age; the age-stacked profile is significantly different from zero only if it exceeds this expected variability by a large margin. Our results indicate, however, that the null hypothesis satisfactorily explains the age-stacking results in all three regions of study, thus providing no support for the Milankovitch cycle hypothesis. The random nature of abyssal hills is consistent with a primarily faulted origin. .

Venus: Preliminary geologic mapping of northern Atla Regio

NASA Technical Reports Server (NTRS)

Nikishin, A. M.; Burba, G. A.

1992-01-01

A preliminary geologic map of C1 sheet 15N197 was compiled according to Magellan data. Northern Atla Regio is dominantly a volcanic plain with numerous volcanic features: radar-bright and -dark flows and spots, shield volcanos, volcanic domes and hills with varied morphology, and coronalike constructions. Tesserae are the oldest terrains semiflooded by plain materials. There are many lineated terrains on this territory. They are interpreted as old, partly buried ridge belts. Lineated terrains have intermediate age between young plains and old tesserae. Ozza Mons and Sapas Mons are the high shield volcanos. The prominent structure of northern Atla Regio is Ganis Chasma rift. The rift dissected the volcanic plain and evolved nearly contemporaneously with Ozza Mons shield volcano. Ganis Chasma rift valley is highly fractured and bounded by fault scarps. There are a few relatively young volcanic features in the rift valley. The rift originated due to 5-10 percent crustal extension and crustal subsidence according to analysis of fracturing and rift valley geometry. Ganis Chasma is characterized by rift shoulder uplifts. Geological structures of Alta Regio and Beta Regio are very similar as assumed earlier.

Moho geometry along a north-south passive seismic transect through Central Australia

NASA Astrophysics Data System (ADS)

Sippl, Christian

2016-04-01

Receiver functions from a temporary deployment of 25 broadband stations along a north-south transect through Central Australia are used to retrieve crustal and uppermost mantle structural constraints from a combination of different methods. Using H-K stacking as well as receiver function inversion, overall thick crust with significant thickness variation along the profile (40 to ≥ 55 km) is found. Bulk crustal vp/vs values are largely in the felsic to intermediate range, with the southernmost stations on the Gawler Craton exhibiting higher values in excess of 1.8. A common conversion point (CCP) stacking profile shows three major discontinuities of the crust-mantle boundary: (1) a two-sided Moho downwarp beneath the Musgrave Province, which has previously been associated with the Neoproterozoic to early Cambrian Petermann Orogeny, (2) a Moho offset along the Redbank Shear Zone further north attributed to the Middle to Late Paleozoic Alice Springs Orogeny, and (3) another Moho offset further north, located at the boundary between the Davenport and Warramunga Provinces, which has not been imaged before. In all cases, the difference in crustal thickness between the two sides of the offset is > 8-10 km. Unlike the two southern Moho offsets, the northernmost one does not coincide with a prominent gravity anomaly. Its location and the absence of known reactivation events in the region make it likely that it belongs to a Proterozoic suture zone that marks a previously unknown block boundary within the North Australian Craton.

A multiple fault rupture model of the November 13 2016, M 7.8 Kaikoura earthquake, New Zealand

NASA Astrophysics Data System (ADS)

Benites, R. A.; Francois-Holden, C.; Langridge, R. M.; Kaneko, Y.; Fry, B.; Kaiser, A. E.; Caldwell, T. G.

2017-12-01

The rupture-history of the November 13 2016 MW7.8 Kaikoura earthquake recorded by near- and intermediate-field strong-motion seismometers and 2 high-rate GPS stations reveals a complex cascade of multiple crustal fault rupture. In spite of such complexity, we show that the rupture history of each fault is well approximated by simple kinematic model with uniform slip and rupture velocity. Using 9 faults embedded in a crustal layer 19 km thick, each with a prescribed slip vector and rupture velocity, this model accurately reproduces the displacement waveforms recorded at the near-field strong-motion and GPS stations. This model includes the `Papatea Fault' with a mixed thrust and strike-slip mechanism based on in-situ geological observations with up to 8 m of uplift observed. Although the kinematic model fits the ground-motion at the nearest strong station, it doesn not reproduce the one sided nature of the static deformation field observed geodetically. This suggests a dislocation based approach does not completely capture the mechanical response of the Papatea Fault. The fault system as a whole extends for approximately 150 km along the eastern side of the Marlborough fault system in the South Island of New Zealand. The total duration of the rupture was 74 seconds. The timing and location of each fault's rupture suggests fault interaction and triggering resulting in a northward cascade crustal ruptures. Our model does not require rupture of the underlying subduction interface to explain the data.

Mineralogy of the Martian Surface: Crustal Composition to Surface Processes

NASA Technical Reports Server (NTRS)

Mustard, John F.

1997-01-01

The main results have been published in the refereed literature, and thus this report serves mainly to summarize the main findings and indicate where the detailed papers may be found. Reflectance spectroscopy has been an important tool for determining the mineralogic makeup of the near surface materials on Mars. Analysis of the spectral properties of the surface have demonstrated that these attributes are heterogeneous from the coarse spatial but high spectral resolution spectra obtained with telescopes to the high spatial but coarse spectral resolution Viking data (e.g. Arvidson et al., 1989; McEwen et al., 1989). Low albedo materials show strong evidence for the presence of igneous rock forming minerals while bright materials are generally interpreted as representing heavily altered crustal material. How these materials are physically and genetically related has important implications for understanding martian surface properties and processes, weathering histories and paths, and crustal composition. The goal of this research is to characterize the physical and chemical properties of low albedo materials on Mars and the relationship to intermediate and high albedo materials. Fundamental science questions to be pursued include: (1) the observed distributions of soil, rock, and dust a function of physical processes or weathering and (2) different stages of chemical and physical alteration fresh rock identified. These objectives will be addressed through detailed analyses and modelling of the ISM data from the Phobos-2 mission with corroborating evidence of surface composition and properties provided by data from the Viking mission.

Magmatic Processes at Kilauea Volcano Revealed by the Puu Oo Eruption

NASA Astrophysics Data System (ADS)

Garcia, M. O.; Marske, J. P.; Pietruszka, A. P.; Rhodes, J. M.; Norman, M. D.; Eiler, J.

2008-12-01

The ongoing Puu Oo eruption (1983 to present) provides an unprecedented opportunity to probe the crustal and mantle magmatic processes beneath Kilauea volcano. Here we present Pb, Sr, Nd and O isotope ratios, major- and trace-element abundances, olivine compositions, and petrography data for Puu Oo lavas an compare them to the Kilauea historical record. Crustal processes are dominated by olivine fractionation and accumulation with minor clinopyroxene fractionation, and to a lesser extent and only periodically when eruption rates decrease, by crustal contamination. Systematic variations in Sr isotope ratios, incompatible trace element ratios, and MgO-normalized major elements document remarkable changes in parental magma compositions delivered to Puu Oo. Inflections in some trends correlate broadly with increasing intermediate depth earthquakes under the Kilauea's summit and to changes in eruption rate. Thus, volcanic events are influenced by melting and transport processes. One surprising feature is the systematic trend of Puu Oo rock compositions away from and beyond typical historical Kilauea compositions towards those of lavas from neighboring Mauna Loa volcano. The source for this component in Puu Oo lavas is a hybrid with about equal mixtures of historical Kilauea and Mauna Loa end members. The Puu Oo lava trend continues the cyclic pattern of compositional variation that extends back over 1000 years. Similar trends are also recorded on a coarser scale in HSDP lavas. These patterns of cyclic compositional variation are important for understanding melting processes in Hawaiian and other volcanoes.

Uplifting the Stable Crust of the Colorado Plateau through Crustal Hydration and Warming

NASA Astrophysics Data System (ADS)

Porter, R. C.; Holt, W. E.

2016-12-01

The Colorado Plateau (CP) is a high ( 2 km above sea level), low-relief, orogenic plateau located within the interior of the southwestern United States that presents several outstanding geologic questions, most notably about the timing and mechanism(s) for uplift. The CP was located below sea level during the Cretaceous and was uplifted to its modern elevation with little crustal shortening, making the cause of uplift enigmatic. Numerous mechanisms have been hypothesized to explain the uplift of this stable block and include delamination, mantle heating/phase changes, mantle convection, volatile addition, and various combinations of these. In order to better understand the crustal contribution to uplift, we utilize data from the EarthScope Transportable Array network to image the CP lithosphere and inform thermodynamic models of CP lower crustal composition. Rayleigh wave phase velocities calculated using ambient noise tomography and surface wave gradiometry were inverted for shear velocity resulting in a high-resolution velocity model of the CP crust and upper mantle. In order to provide greater context to these results, the thermodynamic modeling code Perple_X was utilized to forward model crustal densities, seismic velocities, and water content based on psuedosections calculated using published major element chemistry. Our seismic and modeling results show that uplift of the plateau is partially driven by hydration and extension of the lower crust, both of which reduce its density. Hydration of the CP crust likely occurred due to dewatering of the Farallon slab during flat-slab subduction and reduced lower crustal density by 70 kg/m3. Warming and extension further reduced the lower crustal density by 90 kg/m3 at the CP margins. Though these processes played a role in the uplift of the CP, additional mechanisms, likely due to mantle processes, are required to fully explain its high elevation. Additionally, hydration and subsequent dehydration may play an important role in the recent encroachment of deformation and volcanism into the interior of the CP.

A deterministic and stochastic velocity model for the Salton Trough/Basin and Range transition zone and constraints on magmatism during rifting

NASA Astrophysics Data System (ADS)

Larkin, Steven P.; Levander, Alan; Okaya, David; Goff, John A.

1996-12-01

As a high resolution addition to the 1992 Pacific to Arizona Crustal Experiment (PACE), a 45-km-long deep crustal seismic reflection profile was acquired across the Chocolate Mountains in southeastern California to illuminate crustal structure in the transition between the Salton Trough and the Basin and Range province. The complex seismic data are analyzed for both large-scale (deterministic) and fine-scale (stochastic) crustal features. A low-fold near-offset common-midpoint (CMP) stacked section shows the northeastward lateral extent of a high-velocity lower crustal body which is centered beneath the Salton Trough. Off-end shots record a high-amplitude diffraction from the point where the high velocity lower crust pinches out at the Moho. Above the high-velocity lower crust, moderate-amplitude reflections occur at midcrustal levels. These reflections display the coherency and frequency characteristics of reflections backscattered from a heterogeneous velocity field, which we model as horizontal intrusions with a von Kármán (fractal) distribution. The effects of upper crustal scattering are included by combining the mapped surface geology and laboratory measurements of exposed rocks within the Chocolate Mountains to reproduce the upper crustal velocity heterogeneity in our crustal velocity model. Viscoelastic finite difference simulations indicate that the volume of mafic material within the reflective zone necessary to produce the observed backscatter is about 5%. The presence of wavelength-scale heterogeneity within the near-surface, upper, and middle crust also produces a 0.5-s-thick zone of discontinuous reflections from a crust-mantle interface which is actually a first-order discontinuity.

Evaluating Crustal Contamination Effects on the Lithophile Trace Element Budget of Shergottites

NASA Technical Reports Server (NTRS)

Brandon, A. D.; Ferdous, J.; Peslier, A. H.

2017-01-01

The origin of the incompatible trace element (ITE) enriched compositions of shergottites has been a point of contention for decades [1-2]. Two scenarios have been proposed, the first is that enriched shergottite compositions reflect an ITE-enriched mantle source, whereas in the second, the ITE enrichment reflects crustal contamination of mantle-derived parent magmas. Evidence supporting the first scenario is that the ITE-enriched shergottite compositions are consistent with the outcomes of magma ocean crystallization [3], and that Os-Nd isotope relationships for shergottites cannot be explained by realistic crustal contamination models [4]. In contrast, Cl and S isotopes are consistent with shergottite magmas interacting with Mars crust [5,6], and ITE-enriched olivine-hosted melt inclusions and interstitial glass are found in depleted shergottite Yamato 980459 [7]. These findings indicate that some level of crustal interaction occurred but the question of whether ITE-enrichments in some bulk shergottites reflect crustal contamination remains open. Recently, a Mars crustal breccia meteorite has been found, NWA 7034 and its paired stones, that is our best analogue to an average of Mars ancient crust [8-10]. This allows for better constraints on crustal contamination of shergottite magmas. We modeled magma-crust mixing and assimilation-fractional crystallization (AFC) using ITE-depleted shergottite compositions and bulk NWA 7034 and its clasts as end-members. The results of these models indicate that crustal contamination can only explain the ITE-enriched compositions of some bulk shergottites under unusual circumstances. It is thus likely that the shergottite range of compositions reflects primarily mantle sources.

Performance of Irikura's Recipe Rupture Model Generator in Earthquake Ground Motion Simulations as Implemented in the Graves and Pitarka Hybrid Approach.

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

Pitarka, A.

We analyzed the performance of the Irikura and Miyake (2011) (IM2011) asperity-­ based kinematic rupture model generator, as implemented in the hybrid broadband ground-­motion simulation methodology of Graves and Pitarka (2010), for simulating ground motion from crustal earthquakes of intermediate size. The primary objective of our study is to investigate the transportability of IM2011 into the framework used by the Southern California Earthquake Center broadband simulation platform. In our analysis, we performed broadband (0 -­ 20Hz) ground motion simulations for a suite of M6.7 crustal scenario earthquakes in a hard rock seismic velocity structure using rupture models produced with bothmore » IM2011 and the rupture generation method of Graves and Pitarka (2016) (GP2016). The level of simulated ground motions for the two approaches compare favorably with median estimates obtained from the 2014 Next Generation Attenuation-­West2 Project (NGA-­West2) ground-­motion prediction equations (GMPEs) over the frequency band 0.1–10 Hz and for distances out to 22 km from the fault. We also found that, compared to GP2016, IM2011 generates ground motion with larger variability, particularly at near-­fault distances (<12km) and at long periods (>1s). For this specific scenario, the largest systematic difference in ground motion level for the two approaches occurs in the period band 1 – 3 sec where the IM2011 motions are about 20 – 30% lower than those for GP2016. We found that increasing the rupture speed by 20% on the asperities in IM2011 produced ground motions in the 1 – 3 second bandwidth that are in much closer agreement with the GMPE medians and similar to those obtained with GP2016. The potential implications of this modification for other rupture mechanisms and magnitudes are not yet fully understood, and this topic is the subject of ongoing study.« less

The connection between crustal reworking and petrological diversity in the deep crust: clues from migmatites

NASA Astrophysics Data System (ADS)

Carvalho, Bruna B.; Sawyer, Edward W.; de Assis Janasi, Valdecir

2016-04-01

The deep levels of the continental crust have been extensively reworked as result of crustal differentiation. Migmatites are widespread in these high-grade metamorphic terrains, and provide valuable information on how processes such as partial melting, segregation of the melt from the residue and subsequent chemical exchanges lead to the petrological diversity found in the deep crust. This study investigates processes that transformed a largely uniform, metagranodiorite protolith into a very complex migmatite that contains three varieties of diatexites (grey, schlieren and homogenous diatexites) and several types of leucosomes. The Kinawa Migmatite is part of the Archean TTG crust in the São Francisco Craton (Brazil), which has been reworked in a shear zone environment at upper amphibolite facies conditions (<730°C and 5-6 kbar); thus it may be typical of crustal reworking in the interior of old cratons [1]. Grey diatexites are residual rocks formed by the extraction of a water-fluxed melt created via the reaction Pl + Kfs + Qz + H2O = melt. Diversity within the grey diatexites arises from different degrees of melt segregation (maximum ~40% melt). Schlieren diatexites are very heterogeneous rocks in which residuum-rich domains alternate with leucocratic quartzo-feldspathic domains where melt accumulated. Homogeneous diatexites are coarse-grained leucocratic rocks and represent larger bodies of anatectic melt with minor amounts (<20%) of entrained residuum. Leucosomes display a wide range of compositions from tonalitic to alkali-feldspar granite. Leucosomes, homogeneous diatexites and the quartzo-feldspathic domains in the schlieren diatexites all show a sequence of microstructural stages from plagioclase-dominated to K-feldspar-dominated frameworks many of which show evidence for tectonic compaction. Thus, further segregation of melt from solids occurred during crystallization. Minor amphibolite dykes in the metagranodiorite did not melt. They occur as angular to rounded fragments (schollen or rafts) in the diatexites and show strong evidence for mechanical and chemical interaction with their melt rich hosts. Typically, the diatexites and the leucosomes around the schollen contain higher proportion of amphibole and/or biotite than that farther away; a number of features suggest that this is due to disaggregation that contaminated the melt rich rocks. Our data indicates that in the deep levels of the crust petrological diversity is produced by melt segregation, both during partial melting and crystallization, and by interaction of the anatectic melt with unmelted material in the source. During melting, segregation produced residuum plus anatectic melt and all intermediate stages, whereas during crystallization it resulted in crystal fractionation and generated diverse plagioclase-rich rocks and fractionated melts. Finally, crystals disaggregated from the amphibolites entrained and interact with anatectic melt producing leucosomes and diatexites with the compositional signature of contamination. [1] Carvalho, B.B; Sawyer, E.W.; Janasi, V.A. (2016). Crustal reworking in a shear zone: transformation of metagranite to migmatite. Journal of Metamorphic Geology DOI: 10.1111/jmg.12180

Crustal structure of the Colorado Plateau, Arizona: Application of new long-offset seismic data analysis techniques

USGS Publications Warehouse

Parsons, T.; McCarthy, J.; Kohler, W.M.; Ammon, C.J.; Benz, H.M.; Hole, J.A.; Criley, E.E.

1996-01-01

The Colorado Plateau is a large crustal block in the southwestern United States that has been raised intact nearly 2 km above sea level since Cretaceous marine sediments were deposited on its surface. Controversy exists concerning the thickness of the plateau crust and the source of its buoyancy. Interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick. A thick crust could support the observed topography of the Colorado Plateau isostatically, while a thinner crust would indicate the presence of an underlying low-density mantle. This paper reports results on long-offset seismic data collected during the 1989 segment of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that extended from the Transition Zone into the Colorado Plateau in northwest Arizona. We apply two new methods to analyze long-offset data that employ finite difference travel time calculations: (1) a first-arrival time inverter to find upper crustal velocity structure and (2) a forward-modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals. We find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern Basin and Range province to about 42 km beneath the northern Transition Zone and southern Colorado Plateau margin. We observe some crustal thinning (to ???37 km thick) and slightly higher lower crustal velocities farther inboard; beneath the Kaibab uplift on the north rim of the Grand Canyon the crust thickens to a maximum of 48 km. We observe a nonuniform crustal thickness beneath the Colorado Plateau that varies by ???15% and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations. Crustal compositions (as inferred from seismic velocities) appear to be the same beneath the Colorado Plateau as those in the Basin and Range province to the southwest, implying that the plateau crust represents an unextended version of the Basin and Range. Some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the Proterozoic Era.

Why Does Sea Level Change? Crustal Evolution Education Project. Teacher's Guide [and] Student Investigation.

ERIC Educational Resources Information Center

Stoever, Edward C., Jr.

Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

Sea-level Fingerprinting, Vertical Crustal Motion from GIA, and Projections of Relative Sea-level Change in the Canadian Arctic

NASA Astrophysics Data System (ADS)

James, Thomas; Simon, Karen; Forbes, Donald; Dyke, Arthur; Mazzotti, Stephane

2010-05-01

We present projections of relative sea-level rise in the 21st century for communities in the Canadian Arctic. First, for selected communities, we determine the sea-level fingerprinting response from Antarctica, Greenland, and mountain glaciers and ice caps. Then, for various published projections of global sea-level change in the 21st century, we determine the local amount of "absolute" sea-level change. We next determine the vertical land motion arising from glacial isostatic adjustment (GIA) and incorporate this into the estimates of absolute sea-level change to obtain projections of relative sea-level change. The sea-level fingerprinting effect is especially important in the Canadian Arctic owing to proximity to Arctic ice caps and especially to the Greenland ice sheet. Its effect is to reduce the range of projected relative sea-level change compared to the range of global sea-level projections. Vertical crustal motion is assessed through empirically derived regional isobases, the Earth's predicted response to ice-sheet loading and unloading by the ICE-5G ice sheet reconstruction, and Global Positioning System vertical velocities. Owing to the large rates of crustal uplift from glacial isostatic adjustment across a large region of central Arctic Canada, many communities are projected to experience relative sea-level fall despite projections of global sea-level rise. Where uplift rates are smaller, such as eastern Baffin Island and the western Canadian Arctic, sea-level is projected to rise.

Constraints on crustal hydration below the Colorado plateau from Vp measurements on crustal xenoliths

NASA Astrophysics Data System (ADS)

Padovani, Elaine R.; Hall, Jeremy; Simmons, Gene

1982-04-01

Seismic velocities have been measured as a function of confining pressure to 8 kbar for crustal xenoliths from the Moses Rock Dike and Mule Ear Diatreme, two kimberlite pipes on the Colorado Plateau. Rock types measured include rhyolite, granite, diorite, metasedimentary schists and gneisses, mafic amphibolites and granulites. Many of our samples have been hydrothermally altered to greenschist facies mineral assemblages during transport to the earth's surface. The velocity of compressional waves measured on altered amphibolites and granulites are too low by 0.1-0.3 km/s for such rock types to be characteristic of deep crustal levels. A direct correlation exists between progressive alteration and the presence of microcracks extending into the xenoliths from the kimberlitic host rock. Velocities of pristine samples are compatible with existing velocity profiles for the Colorado Plateau and we conclude that the crust at depths greater than 15 km has probably not undergone a greenschist facies metamorphic event. The xenolith suite reflects a crustal profile similar to that exposed in the Ivrea-Verbano and Strona-Ceneri zones in northern Italy.

Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes

NASA Astrophysics Data System (ADS)

Rudnick, R. L.; Liu, X. M.

2012-04-01

The continental crust has an "intermediate" bulk composition that is distinct from primary melts of peridotitic mantle (basalt or picrite). This mismatch between the "building blocks" and the "edifice" that is the continental crust points to the operation of processes that preferentially remove mafic to ultramafic material from the continents. Such processes include lower crustal recycling (via density foundering or lower crustal subduction - e.g., relamination, Hacker et al., 2011, EPSL), generation of evolved melts via slab melting, and/or chemical weathering. Stable isotope systems point to the influence of chemical weathering on the bulk crust composition: the oxygen isotope composition of the bulk crust is distinctly heavier than that of primary, mantle-derived melts (Simon and Lecuyer, 2005, G-cubed) and the Li isotopic composition of the bulk crust is distinctly lighter than that of mantle-derive melts (Teng et al., 2004, GCA; 2008, Chem. Geol.). Both signatures mark the imprint of chemical weathering on the bulk crust composition. Here, we use a simple mass balance model for lithium inputs and outputs from the continental crust to quantify the mass lost due to chemical weathering. We find that a minimum of 15%, a maximum of 60%, and a best estimate of ~40% of the original juvenile rock mass may have been lost via chemical weathering. The accumulated percentage of mass loss due to chemical weathering leads to an average global chemical weathering rate (CWR) of ~ 1×10^10 to 2×10^10 t/yr since 3.5 Ga, which is about an order of magnitude higher than the minimum estimates based on modern rivers (Gaillardet et al., 1999, Chem. Geol.). While we cannot constrain the exact portion of crustal mass loss via chemical weathering, given the uncertainties of the calculation, we can demonstrate that the weathering flux is non-zero. Therefore, chemical weathering must play a role in the evolution of the composition and mass of the continental crust.

Crustal structure of the northeastern margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin

USGS Publications Warehouse

Liu, M.; Mooney, W.D.; Li, S.; Okaya, N.; Detweiler, S.

2006-01-01

The 1000-km-long Darlag-Lanzhou-Jingbian seismic refraction profile is located in the NE margin of the Tibetan plateau. This profile crosses the northern Songpan-Ganzi terrane, the Qinling-Qilian fold system, the Haiyuan arcuate tectonic region, and the stable Ordos basin. The P-wave and S-wave velocity structure and Poisson's ratios reveal many significant characteristics in the profile. The crustal thickness increases from northeast to southwest. The average crustal thickness observed increases from 42??km in the Ordos basin to 63??km in the Songpan-Ganzi terrane. The crust becomes obviously thicker south of the Haiyuan fault and beneath the West-Qinlin Shan. The crustal velocities have significant variations along the profile. The average P-wave velocities for the crystalline crust vary between 6.3 and 6.4??km/s. Beneath the Songpan-Ganzi terrane, West-Qinling Shan, and Haiyuan arcuate tectonic region P-wave velocities of 6.3??km/s are 0.15??km/s lower than the worldwide average of 6.45??km/s. North of the Kunlun fault, with exclusion of the Haiyuan arcuate tectonic region, the average P-wave velocity is 6.4??km/s and only 0.5??km/s lower than the worldwide average. A combination of the P-wave velocity and Poisson's ratio suggests that the crust is dominantly felsic in composition with an intermediate composition at the base. A mafic lower crust is absent in the NE margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin. There are low velocity zones in the West-Qinling Shan and the Haiyuan arcuate tectonic region. The low velocity zones have low S-wave velocities and high Poisson's ratios, so it is possible these zones are due to partial melting. The crust is divided into two layers, the upper and the lower crust, with crustal thickening mainly in the lower crust as the NE Tibetan plateau is approached. The results in the study show that the thickness of the lower crust increases from 22 to 38??km as the crustal thickness increases from 42??km in the Ordos basin to 63??km in the Songpan-Ganzi terrane south of the Kunlun fault. Both the Conrad discontinuity and Moho in the West-Qinling Shan and in the Haiyuan arcuate tectonic region are laminated interfaces, implying intense tectonic activity. The arcuate faults and large earthquakes in the Haiyuan arcuate tectonic region are the result of interaction between the Tibetan plateau and the Sino-Korean and Gobi Ala Shan platforms. ?? 2006.

Magnetic fabric in granitoid plutons emplaced during Variscan orogeny and timing of their intrusions: Eastern Variscan front

NASA Astrophysics Data System (ADS)

Hrouda, F.; Schulmann, K.; Chlupacova, M.; Aichler, J.; Mixa, P.; Pecina, V.; Zacek, V.; Kroener, A.

2003-04-01

The eastern Variscan front at the Czech and Polish border is characterised by oblique underthrusting of Neo-Proterozoic continental margin below thickened crustal root. The underthrust plate is subsequently imbricated and forms obliquely convergent crustal wedge which was further thrust over the foreland. Several granitic plutons of arc geochemical affinity are intruded during different stages of crustal thickening and exhumation. Analysis of anisotropy of magnetic susceptibility was carried out to study the relationships between host rock deformation and magma emplacement fabrics in different crustal levels and geographical positions with respect to crustal wedge and westerly orogenic root. Deep seated granodiorite sheets (Javornik intrusion 348 Ma, and Stare Mesto sill 340 Ma) are emplaced in the deepest and more internal high grade parts of the orogen along the margin of thickened crustal root. They show AMS fabrics entirely concordant with surrounding high grade gneisses and were emplaced during contractional (transpressive) regime.The Sumperk granodiorite is a more shallow intrusion emplaced in the central part of the crustal wedge. This sheet-like intrusion shows its AMS fabrics conformable to transpressional fabrics of surrounding mylonitised barovian schists and gneisses. The Zulova Pluton 330 Ma, representing the shallowest intrusion, intrudes the most external part of the crustal wedge. It shows the magnetic fabrics virtually perpendicular to compressional structures in the neighbouring areas. In addition, these fabrics are clearly concordant with large-scale detachment zone along which the Devonian meta-sedimentary cover slided to the west. The AMS fabrics of granitoids thus testify the progressive oblique convergence prograding to the east followed by collapse of external part of orogenic wedge. The AMS fabric data allow us to evaluate the mechanical role of arc magmas syntectonically emplaced during oblique convergence and finally during normal shearing perpendicular to the orogen.

Stress pattern of the Shanxi rift system, North China, inferred from the inversion of new focal mechanisms

NASA Astrophysics Data System (ADS)

Li, Bin; Atakan, Kuvvet; Sørensen, Mathilde Bøttger; Havskov, Jens

2015-05-01

Earthquake focal mechanisms of the Shanxi rift system, North China, are investigated for the time period 1965-April 2014. A total of 143 focal mechanisms of ML ≥ 3.0 earthquakes were compiled. Among them, 105 solutions are newly determined in this study by combining the P-wave first motions and full waveform inversion, and 38 solutions are from available published data. Stress tensor inversion was then performed based on the new database. The results show that most solutions in the Shanxi rift system exhibit normal or strike-slip faulting, and the regional stress field is transtensional and dominated by NNW-SSE extension. This correlates well with results from GPS data, geological field observations and levelling measurements across the faults. Heterogeneity exists in the regional stress field, as indicated by individual stress tensor inversions conducted for five subzones. While the minimum stress axis (σ3) appears to be consistent and stable, the orientations, especially the plunges, of the maximum and intermediate stresses (σ1 and σ2) vary significantly along the strike of the different subzones. Based on our results and combining multidisciplinary observations from geological surveys, GPS and cross-fault monitoring, a kinematic model is proposed for the Shanxi rift system, in which the rift is situated between two opposite rotating crustal blocks, exhibiting a transtensional stress regimes. This model illustrates the present-day stress field and its correlation to the regional tectonics, as well as the current crustal deformation of the Shanxi rift system. Results obtained in this study, may help to understand the geodynamics, neotectonic activity, active seismicity and potential seismic hazard in this region.

A-type granites and related rocks: Evolution of a concept, problems and prospects

NASA Astrophysics Data System (ADS)

Bonin, Bernard

2007-08-01

Although A-type granites have long been recognized as a distinct group of granites, the term A-type was coined first less than thirty years ago. A-type suites occur in geodynamic contexts ranging from within-plate settings to plate boundaries, locations and times of emplacement are not random. Rare in the lower crust, as some charnockite suites, they are fairly common at shallower depths, especially at the subvolcanic level where they form ring complexes rooting caldera volcanoes. Characteristic features include hypersolvus to transsolvus to subsolvus alkali feldspar textures, iron-rich mafic mineralogy, bulk-rock compositions yielding ferroan, alkali-calcic to alkaline affinities, high LILE+HFSE abundances, and pronounced anomalies due to high degrees of mineral fractionation. Isotopic features evidence sources containing a large mantle input. Experimental data show that A-type magmas contain dissolved OH F-bearing fluids, crystallised under reduced and oxidized conditions, and yield high-temperature liquidus, favouring early crystallisation of anhydrous iron minerals, such as fayalite. Though many petrogenetic models imply solely crustal derivation, no convincing A-type liquids were produced experimentally from crustal materials, nor have any leucosomes of A-type composition been detected within migmatitic terranes. As it occurs in association with mafic igneous rocks in continents as well as on the ocean floor, A-type granite is likely to come from mantle-derived transitional to alkaline mafic to intermediate magmas. Rare felsic materials found in the meteoritic and lunar record yield dominantly A-type features. Contrary to the more common types of granite, A-type granite is, therefore, not typical of Earth and was produced in planetary environments differing from those prevailing on Earth.

Tectonic Setting and Bimodal Magmatic Evolution of Eocene Volcanic Rocks of the Bijgerd-Kuh-e Kharchin area, Uromieh-Dokhtar Zone, Iran

NASA Astrophysics Data System (ADS)

Davarpanah, A.; Khalatbari-Jafari, M.; Babaie, H. A.; Krogstad, E. J.; Mobasher, K.; La Tour, T. E.; Deocampo, D. M.

2008-12-01

Geochemical composition and texture of the Middle and Late Eocene volcanic, volcaniclastic, and volcanic- sedimentary rocks in the Bijgerd-Kuh-e Kharchin area, northwest of Saveh, provide significant geochemical and geological clues for the tectonic and magmatic evolution of the Uromieh-Dokhtar volcanic-plutonic zone of Iran. The Middle Eocene volcanic rocks have an intermediate composition and include green tuff and tuffaceous sandstone with intercalated sandstone, sandy tuff, and shale. The shale has lenses of nummulite- bearing limestone with a Middle Eocene detrital age. The time between the Middle and Late Eocene volcanic activities in this area is marked by the presence of andesite and rhyolitic tuff. The Late Eocene succession is distinguished by the presence of four alternating levels (horizons) of intermediate lava and ignimbrite which we designate as Eig. The ignimbrites of the Eig sequence have a rhyolitic composition and include ignimbrite- breccia, ignimbrite-tuff, and ignimbrite-lava pairs. The volume of the felsic volcanic rocks in this sequence far exceeds that of the intermediate rocks, which makes it unlikely that they evolved through the magmatic differentiation of a basaltic magma. The presence of the nummulite-bearing limestone lenses, and sandstone and conglomerate interbeds between the ignimbrites, suggests a shallow marine environment for the pyroclastic deposition and probably the eruptions. The tuff and siltstone of the Est unit that sits above the first ignimbrite may represent deep water, Late Eocene deposit. Oligo-Miocene limestone of the Qom Formation unconformably overlies the uppermost Late Eocene ignimbrite. Washings from red marls give microfossils with Late Eocene age for the Eig sequence, which is synchronous with other paleontological evidence that puts the peak volcanic activity as Late Eocene in the Bijgerd-Kuh-e Kharchin area. Field and petrographic evidence for magma mixing/mingling is given by the presence of mafic- intermediate enclaves in the ignimbrite, hybrid breccias with felsic and mafic clasts, felsic pseudo-flames filled with intermediate lava, heterogeneity in the ignimbrite texture, and sieve texture and oscillatory zoning of plagioclase and opacitization of olivine in the intermediate lava. Geochemical analyses of the major and trace elements (including the REE) and rock texture and assemblages indicate the bimodal magmatic characteristics of the mafic-intermediate lavas and ignimbrites. The tuff and the breccia show a hybrid elemental distribution between those of rhyolite and basalt. The ignimbrites show more enriched compositions than those of the mafic and intermediate rocks on the chondrite-normalized trace element distribution diagram. The higher enrichment of the LREE in the ignimbrites may be due to a crustal contribution. The primitive mantle-normalized elemental distributions show a distinct depletion of Nb and Ti, which suggests a subduction-related volcanism during Eocene.

Contemporary crustal deformations in the Izu peninsula, Honshu, Japan and acceleration of plate bending in the northernmost part of the Philippine Sea plate

NASA Astrophysics Data System (ADS)

Fujii, Yoichiro

1991-07-01

Since the beginning of the anomalous vertical crustal movement in the Izu peninsul, Honshu, Japan, many repeated precise levellings have been carried out by the Geographical Survey Institute. Trilaterations covering the entire Izu peninsula have also been carried out by the Geographical Survey Institute. A new technique is developed to adjust the results of levellings, because they had been carried out for different epochs along each levelling route and because of rapid vertical crustal movements. In conventional least-squares adjustment of levelling network, only corrections to the approximate height are assumed to be unknown, while in the present analysis a special model in which rates of vertical deformation at any bench marks are also assumed to be unknown, is adopted. In addition, tidal stations along the coast of the Izu peninsula yield the rate of vertical crustal movement from analysis of tidal data independent of levelling data. We select several special bench marks in which rates of vertical movement are determined by tidal analysis, thereafter special adjustment is applied according to the type of network. The results show that the peninsula is inclined to the south-west. Uplift in the northeastern part of the peninsula is accompanied by remarkable subsidence in the southwest. The rate of contemporary inclination is many times higher than the rate during the period from 1929 to 1972. The deformation is concentrated in the area where Nakamura (1979, 1980) pointed out the bending of the Philippine Sea plate. The mode and rate of the detected crustal deformation suggest the accelerated bending of the peninsula. There are some local “uplift” that deviate from the general pattern of deformation. The most remarkable land uplift was observed near Ito, a city within the peninsula, and the focus of this uplift migrated with time. The accelerated plate bending will produce an extension at the earth's surface and contraction in the deeper part of the subcrustal layer, additionally it triggered the intrusion of magma from the deeper part to the shallower. Moreover, the accelerated plate bending also triggered seismic swarms and destructive, earthquakes in and around the peninsula.

Time evolution of a rifted continental arc: Integrated ID-TIMS and LA-ICPMS study of magmatic zircons from the Eastern Srednogorie, Bulgaria

NASA Astrophysics Data System (ADS)

Georgiev, S.; von Quadt, A.; Heinrich, C. A.; Peytcheva, I.; Marchev, P.

2012-12-01

Eastern Srednogorie in Bulgaria is the widest segment of an extensive magmatic arc that formed by convergence of Africa and Europe during Mesozoic to Tertiary times. Northward subduction of the Tethys Ocean beneath Europe in the Late Cretaceous gave rise to a broad range of basaltic to more evolved magmas with locally associated Cu-Au mineralization along this arc. We used U-Pb geochronology of single zircons to constrain the temporal evolution of the Upper Cretaceous magmatism and the age of basement rocks through which the magmas were emplaced in this arc segment. High precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) was combined with laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) for spatial resolution within single zircon grains. Three tectono-magmatic regions are distinguished from north to south within Eastern Srednogorie: East Balkan, Yambol-Burgas and Strandzha. Late Cretaceous magmatic activity started at ~ 92 Ma in the northernmost East Balkan region, based on stratigraphic evidence and limited geochronology, with the emplacement of minor shallow intrusions and volcanic rocks onto pre-Cretaceous basement. In the southernmost Strandzha region, magmatism was initiated at ~ 86 Ma with emplacement of gabbroic to dioritic intrusions and related dikes into metamorphic basement rocks that have previously been overprinted by Jurassic-Lower Cretaceous metamorphism. The Yambol-Burgas region is an extensional basin between the East Balkan and the Strandzha regions, which broadens and deepens toward the Black Sea further east and is filled with a thick pile of marine sediments and submarine extrusive volcanic rocks accompanied by coeval intrusions. This dominantly mafic magmatism in the intermediate Yambol-Burgas region commenced at ~ 81 Ma and produced large volumes of potassium-rich magma until ~ 78 Ma. These shoshonitic to ultrapotassic basaltic to intermediate magmas formed by differentiation of ankaramitic (high Ca) parental melts, produced from partial remelting of amphibole clinopyroxenites upon interaction with subduction-modified mantle wedge melts, according to earlier petrological studies. This peak of dominantly extrusive activity in the Yambol-Burgas region extended into the Strandzha region further south, in the form of numerous tholeiitic, calc-alkaline and high-K intrusions emplaced in the same time period between 81 and 78 Ma. Granitic rocks from exposed basement of Eastern Srednogorie zone are dated as Permian/Carboniferous (~ 275-300 Ma). Zircons with similar ages occur in Upper Cretaceous rocks from the East Balkan and Strandzha regions, indicating local incorporation as xenocrysts. In contrast, magmatic rocks from the intermediate Yambol-Burgas region contain mostly Ordovician (~ 460 Ma) or older inherited zircons, suggesting a either a different basement history or, more likely, a different level of magma storage and crustal assimilation. Integrating these geochronological results with a synthesis of the regional geology, we propose a two-stage geodynamic evolution for the Eastern Srednogorie segment of the Tethyan arc. The earlier stage of normal arc magmatism was driven by a southward slab retreat, which formed the ~ 92 Ma calc-alkaline to high-K shallow intrusions and volcanics in the north (East Balkan), 87-86 Ma old tholeiitic and calc-alkaline intrusions in the south (Strandzha), and the voluminous 81-78 Ma old gabbroic to granitic intrusions with predominantly calc-alkaline to high-K composition throughout the Strandzha region. This stage continued westward into the Central Srednogorie zone, where the southward younging of calc-alkaline magmatism correlates well with an increased input of primitive mantle melts, indicating asthenospheric incursion into a widening mantle wedge as a result of slab roll-back. The second stage proceeded in the Eastern Srednogorie zone only, where more extreme extension associated with the opening of the Black Sea back-arc basin led to the formation of an intra-arc rift in the Yambol-Burgas region, which now separates the East Balkan region from the Strandzha region. In this extensional environment, crustal thinning lead to decompression and increased heat flow, facilitating large-scale melting of lower crustal rocks and the formation of 81-78 Ma magmas. The unusual calcic composition of the parent magmas, their isotopic character and distinct xenocrystic population are consistent with a component of re-melting of hydrous lower-crustal cumulates, which probably formed in part during the first stage of the evolving arc.

Lithium isotope constraints on crust-mantle interactions and surface processes on Mars

NASA Astrophysics Data System (ADS)

Magna, Tomáš; Day, James M. D.; Mezger, Klaus; Fehr, Manuela A.; Dohmen, Ralf; Aoudjehane, Hasnaa Chennaoui; Agee, Carl B.

2015-08-01

Lithium abundances and isotope compositions are reported for a suite of martian meteorites that span the range of petrological and geochemical types recognized to date for Mars. Samples include twenty-one bulk-rock enriched, intermediate and depleted shergottites, six nakhlites, two chassignites, the orthopyroxenite Allan Hills (ALH) 84001 and the polymict breccia Northwest Africa (NWA) 7034. Shergottites unaffected by terrestrial weathering exhibit a range in δ7Li from 2.1 to 6.2‰, similar to that reported for pristine terrestrial peridotites and unaltered mid-ocean ridge and ocean island basalts. Two chassignites have δ7Li values (4.0‰) intermediate to the shergottite range, and combined, these meteorites provide the most robust current constraints on δ7Li of the martian mantle. The polymict breccia NWA 7034 has the lowest δ7Li (-0.2‰) of all terrestrially unaltered martian meteorites measured to date and may represent an isotopically light surface end-member. The new data for NWA 7034 imply that martian crustal surface materials had both a lighter Li isotope composition and elevated Li abundance compared with their associated mantle. These findings are supported by Li data for olivine-phyric shergotitte NWA 1068, a black glass phase isolated from the Tissint meteorite fall, and some nakhlites, which all show evidence for assimilation of a low-δ7Li crustal component. The range in δ7Li for nakhlites (1.8 to 5.2‰), and co-variations with chlorine abundance, suggests crustal contamination by Cl-rich brines. The differences in Li isotope composition and abundance between the martian mantle and estimated crust are not as large as the fractionations observed for terrestrial continental crust and mantle, suggesting a difference in the styles of alteration and weathering between water-dominated processes on Earth versus possibly Cl-S-rich brines on Mars. Using high-MgO shergottites (>15 wt.% MgO) it is possible to estimate the δ7Li of Bulk Silicate Mars (BSM) to be 4.2 ± 0.9‰ (2σ). This value is at the higher end of estimates for the Bulk Silicate Earth (BSE; 3.5 ± 1.0‰, 2σ), but overlaps within uncertainty.

Numerical modeling of forceful pluton emplacement and associated deformation at different crustal levels - instantaneous, continuous or episodic intrusion?

NASA Astrophysics Data System (ADS)

Chen, Y.; Nabelek, P. I.

2015-12-01

The Papoose Flat pluton in the White-Inyo Range, California, is one of the best examples of forceful magma emplacement at mid-crustal levels that is revealed by a highly strained aureole. A thermo-rheological 2-D model of the pluton and its aureole is proposed. We explored how the frequency of magma input, from instantaneous to continuous to the bottom of the laccolith, affects the ductile width of the aureole and the crystallinity of the pluton, which has implications for eruption of magma. We modeled these aspects at mid- and upper-crustal levels. The pluton was assumed to be 5 km thick in the middle and 13 km wide. Except for instantaneous growth, pluton was assumed to grow over 5 m.y. The aureole was assumed to have power-law rheology of quartz with dependence on H2O fugacity, which was calculated using the CORK equation (Holland & Powell, 1991) Our result shows that the bottom of the Papoose Flat pluton was emplaced at the brittle-ductile transition zone of the crust. The crustal rheology profile assisted the softening of rocks around the pluton. The simulated temperature and strength profiles confirm that ductile deformation was related to thermal weakening (Saint-Blanquat et al., 2001). Results of incremental growth calculations show that the pluton remains hot and only partially crystalline for millions of years when it grows by frequent input of small batches of liquid. At the mid-crustal level, the ductile region around the pluton is much wider and exists longer than at the shallow crustal level. Brittle rheology is dominant during the late stage growth at the shallow depth. When the pluton grows instantly or by only few episodes of large batches of input, the mobile part of the pluton is thin and the ductile aureole is narrower. High-frequency incremental growth by smaller magma batches produces a large volume of mobile magma that has the potential to induce internal magmatic layering that may be reflected in aligned acquired magnetic susceptibility (AMS) in upper parts of the pluton. By examing the chamber overpressure generated by injection of magma (Jellinek and DePaolo, 2003) and the overpressure related to magma buoyancy (Karlstrom et al, 2010), eruption during high-frequency magma input may be promoted by the magma buoyancy while an eruption during low-frequency input may be caused by injection of a large magma batch.

The dehydration, rehydration and tectonic setting of greenstone belts in a portion of the northern Kaapvaal Craton, South Africa

NASA Technical Reports Server (NTRS)

Vanreenen, D. D.; Barton, J. M., Jr.; Roering, C.; Vanschalkwyk, J. C.; Smit, C. A.; Debeer, J. D.; Stettler, E. H.

1986-01-01

High-grade gneiss terranes and low-grade granite-greenstone terranes are well known in several Archaean domains. The geological relationship between these different crustal regions, however, is still controversial. One school of thought favors fundamental genetic differences between high-grade and low-grade terranes while others argue for a depth-controlled crustal evolution. The detailed examination of well-exposed Archaean terranes at different metamorphic grades, therefore, is not only an important source of information about the crustal levels exposed, but also is critical to the understanding of the possible tectonic and metamorphic evolution of greenstone belts with time. Three South African greenstone belts are compared.

Oblique reactivation of lithosphere-scale lineaments controls rift physiography - the upper-crustal expression of the Sorgenfrei-Tornquist Zone, offshore southern Norway

NASA Astrophysics Data System (ADS)

Phillips, Thomas B.; Jackson, Christopher A.-L.; Bell, Rebecca E.; Duffy, Oliver B.

2018-04-01

Pre-existing structures within sub-crustal lithosphere may localise stresses during subsequent tectonic events, resulting in complex fault systems at upper-crustal levels. As these sub-crustal structures are difficult to resolve at great depths, the evolution of kinematically and perhaps geometrically linked upper-crustal fault populations can offer insights into their deformation history, including when and how they reactivate and accommodate stresses during later tectonic events. In this study, we use borehole-constrained 2-D and 3-D seismic reflection data to investigate the structural development of the Farsund Basin, offshore southern Norway. We use throw-length (T-x) analysis and fault displacement backstripping techniques to determine the geometric and kinematic evolution of N-S- and E-W-striking upper-crustal fault populations during the multiphase evolution of the Farsund Basin. N-S-striking faults were active during the Triassic, prior to a period of sinistral strike-slip activity along E-W-striking faults during the Early Jurassic, which represented a hitherto undocumented phase of activity in this area. These E-W-striking upper-crustal faults are later obliquely reactivated under a dextral stress regime during the Early Cretaceous, with new faults also propagating away from pre-existing ones, representing a switch to a predominantly dextral sense of motion. The E-W faults within the Farsund Basin are interpreted to extend through the crust to the Moho and link with the Sorgenfrei-Tornquist Zone, a lithosphere-scale lineament, identified within the sub-crustal lithosphere, that extends > 1000 km across central Europe. Based on this geometric linkage, we infer that the E-W-striking faults represent the upper-crustal component of the Sorgenfrei-Tornquist Zone and that the Sorgenfrei-Tornquist Zone represents a long-lived lithosphere-scale lineament that is periodically reactivated throughout its protracted geological history. The upper-crustal component of the lineament is reactivated in a range of tectonic styles, including both sinistral and dextral strike-slip motions, with the geometry and kinematics of these faults often inconsistent with what may otherwise be inferred from regional tectonics alone. Understanding these different styles of reactivation not only allows us to better understand the influence of sub-crustal lithospheric structure on rifting but also offers insights into the prevailing stress field during regional tectonic events.

Axial crustal structure of the Costa Rica Rift: Implications for along-axis hydrothermal circulation

NASA Astrophysics Data System (ADS)

Zhang, L.; Tong, V.; Hobbs, R. W.; Peirce, C.; Lowell, R. P.; Haughton, G.; Murton, B. J.; Morales Maqueda, M. A.; Harris, R. N.; Robinson, A. H.

2017-12-01

In 2015, a multidisciplinary geophysical cruise surveyed the Costa Rica Rift (CRR) in the Panama Basin of the equatorial East Pacific, acquiring a grid of multichannel seismic and wide-angle profiles to determine the mode of oceanic crustal accretion at intermediate-spreading ridges, and how the crustal structure may be influenced by hydrothermal fluid flow. Analysis of 69,000 P-wave first arrivals recorded by 25 ocean-bottom seismographs deployed over a 20 × 20 km area that straddles the ridge axis, reveals a 3D velocity-depth model of upper crustal structure. In particular, the model shows a low velocity anomaly that extends to 2 km below seabed centred on a small-offset non-transform discontinuity (NTD), and a pattern of increasing velocity with distance off-axis that may reflect changes in porosity and permeability in layer 2 of the crust. Assuming the upper crustal velocity anomalies are linked with porosity and hence represent the ability of fluid to flow, comparison of the tomographic model with the volcanic seabed morphology suggests that the broad low velocity zone beneath the NTD may be a region of extensive fracturing. Hence, we infer that this region may provide a primary pathway for the recharge of seawater into the crust. Further west along the axis, beneath the bathymetric dome, which is the shallowest portion along the axis, the low-velocity anomaly is less pronounced, suggesting that fractures are less open and that fluid-rock interaction has encouraged mineral precipitation and alteration, as a result of a longer established hydrothermal fluid flow driven by the axial magma lens observed beneath it. This interpretation is supported by the presence of a plume from an active hydrothermal vent system. Hence, we infer that the variable velocity structure of the upper crust of the CRR is a proxy that reflects the primary porosity, faulting and fracturing related to phases of magma-driven accretion and/or ridge geometry re-adjustment, and that there is along-axis hydrothermal circulation transferring heat and impacting the properties of newly accreted oceanic crust. This research is part of a major, interdisciplinary NERC-funded collaboration entitled: Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge (OSCAR).

Geology of the Ulugh Muztagh area, northern Tibet

USGS Publications Warehouse

Burchfiel, B.C.; Molnar, P.; Zhao, Ziyun; Liang, K'uangyi; Wang, Shuji; Huang, Minmin; Sutter, J.

1989-01-01

Within the Ulugh Muztagh area, north central Tibet, an east-west-trending ophiolitic melange marks a suture that apparently was formed during a late Triassic or slightly younger collision between a continental fragment to the south and the rest of Asia. The southern continental fragment carries a thick sequence of upper Triassic sandstone, but the contact between the sandstone and the ophiolitic melange is covered by a younger redbed sequence of unknown age. A suite of 2-mica, tourmaline-bearing leucogranite plutons and dikes intruded the Triassic sandstone at shallow crustal levels 10.5 to 8.4 Ma. These rocks range from granite to tonalite in composition, are geochemically very similar to slightly older High Himalayan leucogranite and are interpreted to have been derived by the partial melting of crustal material. We interpret this to mean that crustal thickening began in this part of the Tibetan plateau at least by 10.5 Ma. Welded rhyolitic tuff rests on a conglomerate that consists of abundant debris from the Ulugh Muztagh intrusive rocks and has yielded Ar Ar ages of about 4 Ma. The tuffs are geochemically identical to the intrusive rocks suggesting that crustal thickening may have continued to 4 Ma. Crustal thickening probably occurred by distributed crustal shortening similar to shortening now occurring north of Ulugh Muztagh along the northern margin of the Tibetan Plateau. ?? 1989.

Sea level data and techniques for detecting vertical crustal movements

NASA Technical Reports Server (NTRS)

Lennon, G. W.

1978-01-01

An attempt is made to survey problems, requirements, and the outlook for the future in the study of sea level time series so as to determine the relative movement of land and sea levels. The basic aim is to eliminate from the record the contributions from whatever marine dynamic phenomena respond to treatment, allowing the secular element to be identified with optimum clarity. Nevertheless the concept of sea level perturbation varies according to regional experience. The recent work of the Permanent Service for Mean Sea Level helps to eliminate geodetic noise from the series and makes it possible, perhaps, to treat the global mean sea level data bank so as to define eustatic changes in ocean volume which, in the present context, may be regarded as the final goal, allowing the identification of vertical crustal motion itself.

Heat and extension at mid- and lower crustal levels of the Rio Grande rift

NASA Technical Reports Server (NTRS)

Olsen, K. H.; Baldridge, W. S.; Callender, J. F.

1985-01-01

The process by which large amounts (50 to 200 percent) of crustal extension are produced was concisely described by W. Hamilton in 1982 and 1983. More recently, England, Sawyer, P. Morgan and others have moved toward quantifying models of lithospheric thinning by incorporating laboratory and theoretical data on rock rheology as a function of composition, temperature, and strain rate. Hamilton's description identifies three main crustal layers, each with a distinctive mechanical behavior; brittle fracturing and rotation in the upper crust, discontinuous ductile flow in the middle crust and laminar ductile flow in the lower crust. The temperature and composition dependent brittle-ductile transition essentially defines the diffuse boundary between upper and middle crust. It was concluded that the heat responsible for the highly ductile nature of the lower crust and the lensoidal and magma body structures at mid-crustal depths in the rift was infused into the crust by relatively modest ( 10 percent by mass) magmatic upwelling (feeder dikes) from Moho levels. Seismic velocity-versus-depth data, supported by gravity modeling and the fact that volumes of rift related volcanics are relatively modest ( 6000 cubic km) for the Rio Grande system, all imply velocities and densities too small to be consistent with a massive, composite, mafic intrusion in the lower crust.

On the state of stress in the near-surface of the earth's crust

USGS Publications Warehouse

Savage, W.Z.; Swolfs, H.S.; Amadei, B.

1992-01-01

Five models for near-surface crustal stresses induced by gravity and horizontal deformation and the influence of rock property contrasts, rock strength, and stress relaxation on these stresses are presented. Three of the models-the lateral constraint model, the model for crustal stresses caused by horizontal deformation, and the model for the effects of anisotropy-are linearly elastic. The other two models assume that crustal rocks are brittle or viscoelastic in order to account for the effects of rock strength and time on near-surface stresses. It is shown that the lateral constraint model is simply a special case of the combined gravity-and deformation-induced stress field when horizontal strains vanish and that the inclusion of the effect of rock anisotropy in the solution for crustal stresses caused by gravity and horizontal deformation broadens the range for predicted stresses. It is also shown that when stress levels in the crust reach the limits of brittle rock strength, these stresses become independent of strain rates and that stress relaxation in ductile crustal rocks subject to constant horizontal strain rates causes horizontal stresses to become independent of time in the long term. ?? 1992 Birkha??user Verlag.

Fragmentation of wall rock garnets during deep crustal earthquakes

PubMed Central

Austrheim, Håkon; Dunkel, Kristina G.; Plümper, Oliver; Ildefonse, Benoit; Liu, Yang; Jamtveit, Bjørn

2017-01-01

Fractures and faults riddle the Earth’s crust on all scales, and the deformation associated with them is presumed to have had significant effects on its petrological and structural evolution. However, despite the abundance of directly observable earthquake activity, unequivocal evidence for seismic slip rates along ancient faults is rare and usually related to frictional melting and the formation of pseudotachylites. We report novel microstructures from garnet crystals in the immediate vicinity of seismic slip planes that transected lower crustal granulites during intermediate-depth earthquakes in the Bergen Arcs area, western Norway, some 420 million years ago. Seismic loading caused massive dislocation formations and fragmentation of wall rock garnets. Microfracturing and the injection of sulfide melts occurred during an early stage of loading. Subsequent dilation caused pervasive transport of fluids into the garnets along a network of microfractures, dislocations, and subgrain and grain boundaries, leading to the growth of abundant mineral inclusions inside the fragmented garnets. Recrystallization by grain boundary migration closed most of the pores and fractures generated by the seismic event. This wall rock alteration represents the initial stages of an earthquake-triggered metamorphic transformation process that ultimately led to reworking of the lower crust on a regional scale. PMID:28261660

Signatures of present and past melt distribution along fast and intermediate spreading centers

NASA Astrophysics Data System (ADS)

Marjanovic, Milena

The work presented in this dissertation depicts past and present signatures of melt distribution at fast and intermediate spreading centers. The primary goal of the studies included in this thesis is to provide better understanding of melt distribution and variation in melt physical properties within and at the base of oceanic crust formed at these spreading centers. Furthermore, this work examines effects that melt presence might have on formation and structural characteristics of oceanic crust. To explore the above we use geophysical data collected during two expeditions conducted along the Juan de Fuca Ridge (intermediate) and the East Pacific Rise (fast). The major part of the thesis is based on the work conducted on high resolution reflection seismic data that investigate present day intracrustal melt distribution along the East Pacific Rise (EPR) axis extending between 8°20' and 10°10'N. Here, the character of the melt reservoir is examined from different aspects and by using different seismic data analysis methods. By systematic analysis of the seismic reflection data, we show that the axial melt lens (AML) is segmented at different segment scales. Locations of the mapped disruptions in the AML correspond to previously identified tectonic discontinuities well expressed in the seafloor bathymetry. The above result corroborates genetic relationship between tectonic and magmatic segmentation. To examine melt distribution along the EPR, here for the first time we use amplitude variation with angle of incidence (AVA) crossplotting technique that was developed by oil and gas industry experts to look for presence of hydrocarbons. Further data examination for the first time for the mid-ocean ridges show presence of deeper lenses (lenses that are present below the AML). Presence of gaps in these sub-events and their collocation with what is believed to be the location of origin of the last documented eruption occurred in 2005--06, may shed light on the mechanisms behind the mid-ocean ridges volcanic processes. To explore variation in crustal structure and melt distribution at present day along the Juan de Fuca Ridge and relicts of past melt presence near ridge propagators wakes, a combination of gravity and multi-channel seismic data was used. Gravity modeling, constrained by seismic data, showed that robust topography (shallow axial depth and wide axial high) and thicker crust observed for the southern portion of this ridge system originate from enhanced melt supply at the base of the crust. In addition, prominent crustal thickening on the younger crust side of the inner propagators wakes (now on the ridge flanks) is brought into relationship with collocated frozen magma lenses imaged at the base of the crust. Spatial relationship of the two argues for their causal relationship at the time of the crustal formation on the axis. Our study suggests that these frozen lenses represent the record of once molten reservoir that most probably actively participated in the formation of the thicker crust.

Earthquakes and sea level - Space and terrestrial metrology on a changing planet

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

Bilham, R.

1991-02-01

A review is presented of the stability and scale of crustal deformation metrology which has particular relevance to monitoring deformation associated with sea level and earthquakes. Developments in space geodesy and crustal deformation metrology in the last two decades have the potential to acquire a homogeneous global data set for monitoring relative horizontal and vertical motions of the earth's surface to within several millimeters. New tools discussed for forecasting sea level rise and damaging earthquakes include: very long baseline interferometry, satellite laser ranging, the principles of GPS geodesy, and new sea level sensors. Space geodesy permits a unified global basismore » for future metrology of the earth, and the continued availability of the GPS is currently fundamental to this unification.« less

A Measurement of Long-Term Tilt in Colorado and Wyoming.

DTIC Science & Technology

1980-06-01

aligned with the axes of the tiltmeters . (It is exactly parallel to the sensitive axis of one sensor and hence is perpendicular to the sensitive axis of...Borehole tiltmeters aeconceptuallyatrciefrmnoigln-pid crustal deformation and the spatial variations of tidal tilt response due to crustal inhomogeneities...against a stainless steel casing section at the bottom of a hole cased with standard steel pipe. The capsule contains two tilt sensors on a leveling

Geochronological, geochemical, and Sr-Nd-Hf isotopic characteristics of Cretaceous monzonitic plutons in western Zhejiang Province, Southeast China: New insights into the petrogenesis of intermediate rocks

NASA Astrophysics Data System (ADS)

Liu, Liang; Qiu, Jian-Sheng; Zhao, Jiao-Long; Yang, Ze-Li

2014-05-01

We present comprehensive petrological, geochemical, and Sr-Nd-Hf isotopic data for the Matou and Dalai plutons in western Zhejiang Province, Southeast China, with the aim of constraining the petrogenesis of monzonites and to offer new insights into the deep processes of interaction between crustal- and mantle-derived magmas beneath SE China. The Matou pluton comprises quartz monzonite, whereas the Dalai pluton consists of quartz monzodiorite. Zircon U-Pb ages obtained by laser ablation-inductively coupled plasma-mass spectrometry show that both plutons were emplaced at 99-101 Ma. Rocks of both plutons are intermediate to silicic, metaluminous to weakly peraluminous, subalkaline, and K-rich in composition. Samples of the plutons are enriched in large ion lithophile (e.g., Rb, K, and Pb) and light rare earth elements, depleted in high-field strength elements (e.g., Nb, Ta, and Ti), and have small negative or no Eu anomalies. In addition, the rocks have high Mg# values (up to 53.9), high zircon ɛHf(t) values (up to - 1.4), and low Nb/U and Ta/U ratios. Geochemical evidence suggests that both depleted asthenospheric and metasomatically enriched mantle components were involved in the formation of these monzonitic rocks. The presence of inherited zircons with Palaeoproterozoic ages and zircons with unusually low ɛHf(t) values (- 12.9) in the Matou quartz monzonites indicates that ancient crustal materials were also involved in their petrogenesis. In combination with the presence of abundant mafic microgranular enclaves (MMEs) with spheroidal to ellipsoidal-ovoidal shapes and xenocrysts within the more diffused enclaves, and the results of trace element modelling, we suggest that the Matou quartz monzonites were generated by mixing between mantle-derived mafic magmas and crustally derived silicic magmas. The Dalai pluton is relatively homogeneous and contains fewer MMEs than the Matou pluton. Zircons from the Dalai pluton show no inherited components, indicating that crustal materials have played a limited role in the petrogenesis of the quartz monzodiorites. The Dalai quartz monzodiorites have lower SiO2 contents, higher Mg# values, and considerably higher and variable Cr, Co, and Ni concentrations than the Matou quartz monzonites. Zircon Hf isotopic compositions of the Dalai pluton are relatively homogeneous (ɛHf(t) = - 5.2 to - 3.2). The combined petrological, geochemical, and isotopic features indicate that the Dalai monzodiorites were generated by olivine- and pyroxene-dominated fractional crystallisation from basaltic magmas, which were in turn produced by mixing between melts from depleted asthenosphere and subduction-enriched mantle. Our interpretation implies that Late Mesozoic monzonitic rocks in Southeast China require a significant input of mantle melts, and some may have been generated solely by fractionation of basaltic magmas. This petrogenetic model may be applicable to other monzonitic rocks in Southeast China, and to similar tectonic settings and sites of monzonitic magma generation worldwide.

New insights on regional tectonics and crustal composition of the eastern Sierras Pampeanas in the Andean back arc region, Argentina (31-32ºS)

NASA Astrophysics Data System (ADS)

Ammirati, J. B.; Venerdini, A. L.; Alvarado, P. M.; Gilbert, H. J.

2017-12-01

Within the flat slab region of the south central Andes, the eastern Sierras Pampeanas (ESP) are the easternmost expression of a series of foreland uplifts affecting the Argentine back arc region ( 31-32ºS). This important crustal deformation has been related to the subduction of the Juan Fernández Ridge (JFR) under the South American plate. Geological observations suggest that the regional crustal structure is inherited from the accretion of different terranes during the Ordovician and later reactivated since the Miocene during the Andean compression. Geophysical experiments allowed to image how the structure observed at the surface behave in depth as décollement levels that accommodate regional crustal shortening. In order to get new insights on the mechanisms that control crustal regional tectonics, we computed teleseismic receiver functions (RF) and jointly invert them with Rayleigh-wave phase velocity dispersion curves. RFs allow resolving crustal thickness and intra crustal velocity variations with a good vertical resolution whereas surface wave information helps to constrain absolute seismic wave velocities. Our results show how the crustal thickness is increasing to the west with an important step in Moho depth. We observe that this step presents a NW-SE orientation, parallel to the trace at the surface of the Valle Fértil - La Huerta (SVF-LH) fault which suggest that this Moho step marks the transition in depth between the Pampia terrane (east) and the Cuyania terrane (west). Our images also reveal the presence of a high wave velocity lower crust west of this Moho step, beneath the eastern Sierras Pampeanas. This observation suggests that the SVF-LH fault is underthrusting the Cuyania lower crust under the Pampia terrane. Finally, our seismic images show very localized low velocity zones located at 10 km beneath late Cenozoic volcanic fields. We believe that these low velocity zones correspond to old magma chambers associated to the recent flat slab-related volcanism in the ESP.

Major Crustal Fault Zone Trends and Their Relation to Mineral Belts in the North-Central Great Basin, Nevada

USGS Publications Warehouse

Rodriguez, Brian D.; Sampson, Jay A.; Williams, Jackie M.

2007-01-01

The Great Basin physiographic province covers a large part of the western United States and contains one of the world's leading gold-producing areas, the Carlin Trend. In the Great Basin, many sedimentary-rock-hosted disseminated gold deposits occur along such linear mineral-occurrence trends. The distribution and genesis of these deposits is not fully understood, but most models indicate that regional tectonic structures play an important role in their spatial distribution. Over 100 magnetotelluric (MT) soundings were acquired between 1994 and 2001 by the U.S. Geological Survey to investigate crustal structures that may underlie the linear trends in north-central Nevada. MT sounding data were used to map changes in electrical resistivity as a function of depth that are related to subsurface lithologic and structural variations. Two-dimensional (2-D) resistivity modeling of the MT data reveals primarily northerly and northeasterly trending narrow 2-D conductors (1 to 30 ohm-m) extending to mid-crustal depths (5-20 km) that are interpreted to be major crustal fault zones. There are also a few westerly and northwesterly trending 2-D conductors. However, the great majority of the inferred crustal fault zones mapped using MT are perpendicular or oblique to the generally accepted trends. The correlation of strike of three crustal fault zones with the strike of the Carlin and Getchell trends and the Alligator Ridge district suggests they may have been the root fluid flow pathways that fed faults and fracture networks at shallower levels where gold precipitated in favorable host rocks. The abundant northeasterly crustal structures that do not correlate with the major trends may be structures that are open to fluid flow at the present time.

Hidden Earthquake Potential in Plate Boundary Transition Zones

NASA Astrophysics Data System (ADS)

Furlong, Kevin P.; Herman, Matthew; Govers, Rob

2017-04-01

Plate boundaries can exhibit spatially abrupt changes in their long-term tectonic deformation (and associated kinematics) at triple junctions and other sites of changes in plate boundary structure. How earthquake behavior responds to these abrupt tectonic changes is unclear. The situation may be additionally obscured by the effects of superimposed deformational signals - juxtaposed short-term (earthquake cycle) kinematics may combine to produce a net deformational signal that does not reflect intuition about the actual strain accumulation in the region. Two examples of this effect are in the vicinity of the Mendocino triple junction (MTJ) along the west coast of North America, and at the southern end of the Hikurangi subduction zone, New Zealand. In the region immediately north of the MTJ, GPS-based observed crustal displacements (relative to North America (NAm)) are intermediate between Pacific and Juan de Fuca (JdF) motions. With distance north, these displacements rotate to become more aligned with JdF - NAm displacements, i.e. to motions expected along a coupled subduction interface. The deviation of GPS motions from the coupled subduction interface signal near the MTJ has been previously interpreted to reflect clock-wise rotation of a coastal, crustal block and/or reduced coupling at the southern Cascadia margin. The geologic record of crustal deformation near the MTJ reflects the combined effects of northward crustal shortening (on geologic time scales) associated with the MTJ Crustal Conveyor (Furlong and Govers, 1999) overprinted onto the subduction earthquake cycle signal. With this interpretation, the Cascadia subduction margin appears to be well-coupled along its entire length, consistent with paleo-seismic records of large earthquake ruptures extending to its southern limit. At the Hikurangi to Alpine Fault transition in New Zealand, plate interactions switch from subduction to oblique translation as a consequence of changes in lithospheric structure of the Pacific plate (without a triple junction). Here, the short-term, earthquake-cycle signal recorded by GPS shows a reduction in plate motion-directed displacements, which has been interpreted to reflect reduced coupling along the southernmost segment. However, this signal records both the subduction interface coupling effects related to the megathrust earthquake cycle and the shear deformation produced by the extensive right-lateral shear of the Marlborough Fault system (MFS). This superposition of deformation signals combine to mask a strongly coupled interface. The relevance of this effect is seen in the recent (November 2016) Kaikoura earthquake ,which appears to have both ruptured the megathrust interface and produced strike slip displacements on upper-plate crustal faults. These effects seen at these locations and elsewhere may cause misinterpretations of short-term deformation signals in terms of the longer term tectonic behavior of the plate boundary, missing a significant component of the earthquake potential.

The Source of Proterozoic Anorthosites: Bringing It All Back Home

NASA Astrophysics Data System (ADS)

Scoates, J. S.

2004-05-01

Proterozoic anorthosites are coarse-grained cumulate igneous rocks dominated by plagioclase of intermediate composition (An70-35) that occur in spatial and temporal association with both intrusions of troctolite and Fe-enriched rocks (ferrodiorite, monzonite) and with predominantly crustally-derived granitic batholiths. Given the relatively limited range of plagioclase compositions within individual intrusions, differences in plagioclase anorthite content between intrusions likely reflects primarily differences in pressures of segregation of plagioclase-rich magma bodies (An content of plag decreases with increasing pressures of crystallization). More importantly, Proterozoic anorthosite plutonic suites formed over an extended interval of time (1.2 byr) during the Middle Proterozoic from 2.1-0.9 Ga and thus are recording fundamental relationships between plate tectonics, mantle temperatures, and crust-mantle interactions over 1/4 of Earth history. Experimental work on opx-normative gabbroic/dioritic rocks from Harp Lake and Rogaland appears to show that some proposed anorthosite parental liquids lie across the trace of the plag+2-px cotectic from 1-1.3 GPa and that they straddle the thermal divide on the plag+px liquidus surface, thus apparently requiring a mafic source region (i.e. lower continental crust). It is unlikely that small amounts of dry partial melting of lower crustal granulite will produce melt compositions that are strongly plag-saturated nor will it yield the large quantities of melt (and corresponding cumulates) required by mass balance constraints. In addition, noritic-gabbronoritic lower crust is opx-normative and cannot be responsible for producing the olivine-bearing anorthosites or troctolites typical of the largest Proterozoic anorthosites. A compilation of high-Al,Fe basaltic magmas from Proterozoic anorthosite plutonic suites worldwide shows them to have compositions that are significantly less silica-rich than the opx-normative rocks that plot along the plag+2-px cotectic at high pressures. The important thermal divide for the petrogenesis of Proterozoic anorthosites is the plag+olivine+cpx divide as it separates opx-absent from opx-present fractionation trends at mid-crustal pressures. The least fractionated ol-normative compositions project into the region of mantle-derived melts at relatively high pressures (1-1.3 GPa). Radiogenic isotopic studies (Pb, Nd, Sr, Os) are particularly useful for constraining crustal input to anorthosite and have successfully traced out different-aged crustal reservoirs beneath them, especially when the underlying crust is 1 byr or more older than the anorthosites (e.g. Nain). Os isotopic studies do not effectively constrain the source of Proterozoic anorthosites, but rather yield important information about additions of crustal sulfur to ascending and slowly-cooling anorthosite bodies. Although a lower crustal tongue melting origin for Proterozoic anorthosites is clearly untenable, it is likely that no magma associated with Proterozoic anorthosites escaped contamination during ascent through the crust. The lower crust may have acted as a highly effective near-solidus "reactive filter" capable of stabilizing plagioclase as a liquidus phase for the duration of these long-lived (tens of millions of years for the largest suites), low magma flux magmatic systems. Combined low magma productivity and flux are consistent with only small amounts of crustal extension implicating the compositionally heterogeneous continental lithospheric mantle as the dominant source component for Proterozoic anorthosites.

Unraveling the diversity in arc volcanic eruption styles: Examples from the Aleutian volcanic arc, Alaska

NASA Astrophysics Data System (ADS)

Larsen, Jessica F.

2016-11-01

The magmatic systems feeding arc volcanoes are complex, leading to a rich diversity in eruptive products and eruption styles. This review focuses on examples from the Aleutian subduction zone, encompassed within the state of Alaska, USA because it exhibits a rich diversity in arc structure and tectonics, sediment and volatile influx feeding primary magma generation, crustal magma differentiation processes, with the resulting outcome the production of a complete range in eruption styles from its diverse volcanic centers. Recent and ongoing investigations along the arc reveal controls on magma production that result in diversity of eruptive products, from crystal-rich intermediate andesites to phenocryst-poor, melt-rich silicic and mafic magmas and a spectrum in between. Thus, deep to shallow crustal "processing" of arc magmas likely greatly influences the physical and chemical character of the magmas as they accumulate in the shallow crust, the flow physics of the magmas as they rise in the conduit, and eruption style through differences in degassing kinetics of the bubbly magmas. The broad spectrum of resulting eruption styles thus depends on the bulk magma composition, melt phase composition, and the bubble and crystal content (phenocrysts and/or microlites) of the magma. Those fundamental magma characteristics are in turn largely determined by the crustal differentiation pathway traversed by the magma as a function of tectonic location in the arc, and/or the water content and composition of the primary magmas. The physical and chemical character of the magma, set by the arc differentiation pathway, as it ascends towards eruption determines the kinetic efficiency of degassing versus the increasing internal gas bubble overpressure. The balance between degassing rate and the rate at which gas bubble overpressure builds then determines the conditions of fragmentation, and ultimately eruption intensity.

Dipping Magnetic Reversal Boundaries at Endeavor Deep: Implications for Crustal Accretion

NASA Astrophysics Data System (ADS)

Pockalny, R. A.; Shields, A. C.; Larson, R. L.; Popham, C.

2005-12-01

Endeavor Deep, created by ongoing rifting along the northeastern boundary of the Juan Fernandez Microplate, provides a generous 75-km long view of the upper 1-3 km of oceanic crust created ~3 Ma at a fast-spreading ridge (~80 km/Myr, half-rate). Recent near-bottom surveys with the ROV Jason collected high-resolution video, rock samples, and 3-component magnetometer data along a 5 km-wide section of the southern wall of the deep. The video and rock samples define a crustal section with 300-500 m of primarily pillows and flows overlying a 400-500 m transition zone of extrusives and dykes. Forward modeling of the total magnetic intensity calculated from the 3-component magnetometer data identifies a magnetic polarity reversal that corresponds to a reversal boundary within magnetic anomaly 2a (C2An.2r - C2AN.3n , ~3.33 Ma). The location of the modeled polarity transition suggests the reversal boundary dips downward toward the original ridge axis with shallow dips (15 degrees) in the extrusive layer becoming increasingly steeper (25 degrees) in the deeper transition zone. The dipping character of the reversal boundary has also been observed along the walls of the Blanco Fracture Zone and is consistent with evolving crustal accretion models for seafloor created at intermediate- and fast-spreading rates, which predicts the rotation of the upper extrusive layer back toward the ridge axis. As a consequence of this rotation, originally horizontal flow boundaries will dip back toward the ridge axis and the magnitude of the dip will increase with depth into the crustal section. A small reversed magnetic polarity is also observed deeper within normally magnetized C2AN.3n chron, but with a very shallow dip (3-5 degrees). We doubt this is another normal-reverse-normal polarity transition, since the anomaly suspiciously coincides with the transition from dykes to extrusives. Therefore, we believe this anomaly is either the result of an edge-effect created by the different magnetic properties of the dykes and extrusives or evidence off-axis volcanism that occurred during a more recent period of normal magnetization.

Geophysical and geochemical constraints on the geodynamic origin of the Vrancea Seismogenic Zone Romania

NASA Astrophysics Data System (ADS)

Fillerup, Melvin A.

The Vrancea Seismogenic Zone (VSZ) of Romania is a steeply NW-dipping volume (30 x 70 x 200 km) of intermediate-depth seismicity in the upper mantle beneath the bend zone of the Eastern Carpathians. The majority of tectonic models lean heavily on subduction processes to explain the Vrancea mantle seismicity and the presence of a Miocene age calc-alkaline volcanic arc in the East Carpathian hinterland. However, recent deep seismic reflection data collected over the Eastern Carpathian bend zone image an orogen lacking (1) a crustal root and (2) dipping crustal-scale fabrics routinely imaged in modern and ancient subduction zones. The DRACULA I and DACIA-PLAN deep seismic reflection profiles show that the East Carpathian orogen is supported by crust only 30-33 km thick while the Focsani basin (foreland) and Transylvanian basin (hinterland) crust is 42 km and 46 km thick respectively. Here the VSZ is interpreted as the former Eastern Carpathian orogenic root which was removed as a result of continental lithospheric delamination and is seismically foundering beneath the East Carpathian bend zone. Because large volumes of calc-alkaline volcanism are typically associated with subduction settings existing geochemical analyses from the Calimani, Gurghiu, and Harghita Mountains (CGH) have been reinterpreted in light of the seismic data which does not advocate the subduction of oceanic lithosphere. CGH rocks exhibit a compositional range from basalt to rhyolite, many with high-Mg# (Mg/Mg+Fe > 0.60), high-Sr (>1000 ppm), and elevated delta-O18 values (6-8.7 /) typical of arc lavas, and are consistent with mixing of mantle-derived melts with a crustal component. The 143Nd/144Nd (0.5123-0.5129) and 87Sr/86Sr (0.7040-0.7103) ratios similarly suggest mixing of mantle and crustal end members to obtain the observed isotopic compositions. A new geochemical model is presented whereby delamination initiates a geodynamic process like subduction but with the distinct absence of subducted oceanic lithosphere to produce the CGH lavas. The origin of the VSZ presented here suggests that the delamination of continental lithosphere is a process capable of producing mantle earthquakes and calc-alkaline volcanism without subduction tectonics.

A satellite-based digital data system for low-frequency geophysical data

USGS Publications Warehouse

Silverman, S.; Mortensen, C.; Johnston, M.

1989-01-01

A reliable method for collection, display, and analysis of low-frequency geophysical data from isolated sites, which can be throughout North and South America and the Pacific Rim, has been developed for use with the Geostationary Operational Environmental Satellite (GEOS) system. This system provides real-time monitoring of crustal deformation parameters such as tilt, strain, fault displacement, local magnetic field, crustal geochemistry, and water levels, as well as meteorological and other parameters, along faults in California and Alsaka, and in volcanic regions in the western United States, Rabaul, and other locations in the New Britain region of the South pacific. Various mathematical, statistical, and graphical algorithms process the incoming data to detect changes in crustal deformation and fault slip that may indicate the first stages of catastrophic fault failure. -from Authors

Paleokarst on the top of the Maokou Formation: Further evidence for domal crustal uplift prior to the Emeishan flood volcanism

NASA Astrophysics Data System (ADS)

He, Bin; Xu, Yi-Gang; Guan, Jun-Peng; Zhong, Yu-Ting

2010-09-01

The ~ 260 Ma Emeishan Large Igneous Province (ELIP) in southwest China has previously been demonstrated to provide compelling evidence for pre-volcanic crustal doming in support of the mantle plume hypothesis. However this has been questioned by Ukstins-Peate and Bryan (2008) by showing hydrothermal magmatic activity at the Daqiao section. To solve this argument, a detailed characterization of the contact between the Emeishan basalts and the Maokou Formation was carried out. The contact is shown to be an unconformity, which is characterized by paleokarst on top of the Maokou Formation, including paleokarst relief, sinkholes, caves, tower karst and its corresponding rocks (such as kaolinite, bauxite and ferruginous duricrust and collapsed breccias, etc.). This paleokarst unconformity was in turn covered or infilled by the Emeishan basalts and tuffs, suggesting that uplift and erosion occurred prior to the eruption of the ELIP. The extent of erosion of the Maokou Formation indicates the ELIP can be divided into three roughly concentric zones: the inner, intermediate, and outer zones. The paleokarst features on the top of Maokou Formation vary across the ELIP. In the inner zone, a likely sinkhole and an incision valley with 450 m relief in height are found. In the intermediate zone, various paleokarst landforms such as karst relief, sinkholes and tower karsts are well developed. Some sinkholes that developed in the Qixia Formation below the Maokou Formation imply that the paleorelief is more than 350 m in height. In the outer zone, the paleokarstic surface is a paleo-weathering layer with minor karstification and development of caves at 10-50 m. This spatial variation of the paleokarst reflects variation of uplift height across the ELIP. The extent of minimal uplift is estimated to be at least 450 m in the inner zone, 350 m in the intermediate zone, whereas uplift is minor (tens-50 m) in the outer zone. The magnitude and shape of the uplift is roughly consistent with that predicted by mantle plume models. The paleokarst was formed after the deposition of the Maokou Formation and the eruption of the Emeishan basalts at the end-Guadalupian and indicates a short duration of uplift. Thus this study lends further support to domal uplift prior to the Emeishan flood volcanism, but also to the mantle plume initiation model for the generation of the ELIP.

The Compensation State of Intermediate Size Lunar Craters

NASA Astrophysics Data System (ADS)

Reindler, Lucas; Arkani-Hamed, Jafar

2001-09-01

The compensation state of 49 intermediate size (120 to 600 km diameter) lunar craters are investigated using the most recent spherical harmonic models of the lunar topography and gravity, truncated at degree n=110. The total mass anomalies per unit area (i.e., the lateral variations of the vertically integrated density perturbations per unit area) within an otherwise uniform crust of 60 km thickness are determined such that, together with the surface topography, give rise to the model gravity anomalies. Crustal thicknesses of 40 and 80 km are also considered, but the general results of this study are not significantly affected. Excess mass anomalies are obtained by subtracting from the total mass anomalies the mass anomalies that are required for the isostatic compensation of the surface topography. The excess mass anomaly of a crater denotes its particular state of compensation. Dependencies of the excess mass anomalies on crater location, size, and age are investigated, but in general few discernable trends are evident. Although the vast majority of craters indicate some compensation, no correlation exists between age or size and the state of compensation. Roughly 16% of the craters show no compensation, and in some cases have mass deficiencies most likely due to the shock fractured bedrock: the breccia lens of lower density. The crust in these regions was likely cold and rigid enough at the time of impact to rigidly support the stress caused by crater excavation. These features are seen throughout different geological periods, demonstrating that the lunar crust cooled quickly and strengthened soon after formation. A comparison of the compensation state of craters Apollo, Korolev, and Hertzsprung suggests that the thermal and mechanical properties of the crust prior to impact had an appreciable effect on the compensation, and that crustal thickness may be the single most important factor controlling the compensation of intermediate size craters. The characteristics of the excess mass a nomaly profiles of the eight well-known near side mascon basins are used to identify new mascon-like craters. Ten newly found mascons are confirmed: Humboldtianum, Moscoviense, Mendel-Rydberg, Lorentz, Hertzsprung, Korolev, Schrodinger, Freundlich-Sharonov, Coulomb-Sarton, and Schiller-Zucchius, while two more, Deslandres and Dirichlet-Jackson, are very plausible. These results show that mare flow is not necessarily required to produce mascon-like characteristics.

Zircon oxygen isotopes reveal Ivrea-Verbano Zone source characteristics of the Sesia Valley Caldera

NASA Astrophysics Data System (ADS)

Economos, R. C.; Quick, J. E.; Sinigoi, S.; de Silva, S. L.

2013-12-01

The Sesia Valley, in the Italian Alpine foothills, contains >14 km diameter caldera adjacent to and structurally shallower than the famous Ivrea-Verbano Zone deep crustal section. The caldera and its associated eruptive sequence presents opportunity to explore volcanic magmatism in light of exposed and well characterized source candidates, namely lower crustal gabbros and the mid-crustal metasedimentary Kinzigite formation. Original geochemical characteristics of volcanic units have been obscured by the effects of subsequent hydrothermal alteration. The resistance of the mineral zircon to fluid alteration makes it a prime candidate for the preservation and exploration of these geochemical signals, such as O isotopes. Lower crustal gabbros in the Ivrea-Verbano Zone have broadly monotonic whole-rock δ18O values between +8 and +9‰VSMOW (Sinigoi et al., 1994). Kinzigites preserve a much higher and more heterogeneous δ18O values, typically ranging from +10‰ up to +15‰ (Baker, 1990). Zircons from the caldera-forming rhyolitic eruption units and a pre-caldera rhyodacitic unit were analyzed by ion microprobe at UCLA for in-situ oxygen isotope ratios. External reproducibility of within-mount standard R33 grains range from 0.27 to 0.36‰. Rhyolites from the caldera-forming eruption yield a range of δ18O(zircon) values from 6.3‰ to 8.3‰. This range displays rough correlation with CL activity - CL active grains have lower δ18O(zircon) values while CL dark grains have higher δ18O(zircon) values. This variation may correlate with U contents, which are notoriously low in zircons from Ivrea-Verbano Zone gabbros. We argue that the range in O isotope values suggests zircons are a good fit for magmas influenced by gabbro and Kinzigite sources. However, these zircons do not appear to be inherited directly from either the gabbro or Kinzigite sources as their O isotope signatures are typically intermediate between the two. The pre-caldera rhyodacite sample displays a much broader range of δ18O(zircon) values, from +6 to +10‰. These values, when corrected for melt-zircon isotopic fractionation, are an excellent match for mafic and felsic sources in the Ivrea-Verbano Zone. Thus, volcanic rocks of the Sesia Valley share spatial, temporal, and geochemical affinities for Ivrea-Verbano Zone sources, strengthening the body of evidence that the Sesia Valley Caldera represents the upper crustal portions of a complete crustal section contiguous with these mid- and lower-crustal Alpine exposures. These data demonstrate a difference in extent of hybridization of source signals in the rhyodacite (little homogenization) compared to the caldera-forming eruption (more homogenization). This suggests a record of variation in magmatic processes for precursor and climactic eruptions that is potentially related to the thermal maturation of the volcanic system and warrants additional study. Additional work on trace element concentrations, including Ti thermometry, on these grains will further elucidate these processes and their relationship to known zircon-bearing sources in the mid- to deep-crust of the Ivrea-Verbano Zone.

Sulphur-Rich Melt At Upper Crustal Levels At Cerro La Torta, Central Andes: Evidence From Melt Inclusions Assemblages.

NASA Astrophysics Data System (ADS)

Cannatelli, C.; Godoy, B.; Alvear, B.; Moncada, D.

2016-12-01

Central Andes present some of the biggest and most important porphyry copper ore deposits in the world. Porphyry copper ore formation is related to precipitation of ore minerals from sulphur and chlorine-rich fluids. Genesis of these deposits occurred 4 km below surface, while mineralized fluids are released by magmatic melts located between 5 and 15 km depth (Sillitoe, 2010). Cerro La Torta is part of a cluster of <105 ka rhyodacitic domes related to the waning stage of the Altiplano-Puna Volcanic Complex at Central Andes (Tierney et al., 2016). These domes reflect a crystal-rich mush layer at the upper crust - named Altiplano-Puna Magma Body (APMB) - which is proposed to be a voluminous partially molten body locate at shallow depth (4-25 km), with a thickness up to 11 km (Ward et al., 2014). Cerro La Torta is a crystal-rich ( 40% vol.) dacitic flow with plagioclase, amphibole, biotite, and quartz phenocrysts on a glassy (up to 50% vol.) groundmass. During detailed petrographic observation, two types of Melt Inclusions Assemblages (MIAs) were observed in the plagioclase. Group I is found in the core of crystals, and contains sulphide, pyrite ± bubbles. Group II of bubble-bearing MIAs is observed at the rim of the phenocrysts, with no associated sulphide mineral present. Melt Inclusions size ranges from 10-40 µm, suggesting an intermediate cooling rate (Roedder 1979). Out hypothesis is that during cooling, Group I MIA is trapped as result of a metal sulfur-rich event, leading to the suggestion that sulphide-bearing MIAs from Cerro La Torta are the evidence of mineralized magmas ponding at shallow crustal levels. Furthermore, the presence of MIAs in the mush-type magmas related to the APMB implies that such systems are suitable to porphyry copper ore generation. Roedder, 1979. The Evolution of the Igneous Rocks. 15-57 Sillitoe, 2010. Econ. Geol. 105:3-41 Tierney et al., 2016. Geology 44:683-686. doi:10.1130/G37968.1Ward et al., 2014. Earth Planet Sci Letters 404:43-54

Geologic Evolution of North America: Geologic features suggest that the continent has grown and differentiated through geologic time.

PubMed

Engel, A E

1963-04-12

The oldest decipherable rock complexes within continents (more than 2.5 billion years old) are largely basaltic volcanics and graywacke. Recent and modern analogs are the island arcs formed along and adjacent to the unstable interface of continental and oceanic crusts. The major interfacial reactions (orogenies) incorporate pre-existing sial, oceanic crust, and mantle into crust of a more continental type. Incipient stages of continental evolution, more than 3 billion years ago, remain obscure. They may involve either a cataclysmic granite-forming event or a succession of volcanic-sedimentary and granite-forming cycles. Intermediate and recent stages of continental evolution, as indicated by data for North America, involve accretion of numerous crustal interfaces with fragments of adjacent continental crust and their partial melting, reinjection, elevation, unroofing, and stabilization. Areas of relict provinces defined by ages of granites suggest that continental growth is approximately linear. But the advanced differentiation found in many provinces and the known overlaps permit wide deviation from linearity in the direction of a more explosive early or intermediate growth.

Ancient crustal components in the Fra Mauro breccias

NASA Technical Reports Server (NTRS)

Shervais, J. W.; Taylor, L. A.; Laul, J. C.

1983-01-01

Texturally pristine clasts preserve primary petrographic relationships and mineral compositions, yielding insights into igneous processes of the early lunar crust that cannot be gained from highly shocked and brecciated 'chemically pristine' samples. The use of texture as a prime criterion allows for expansion of the data base derived solely from chemical criteria, and provides complementary data. Texturally pristine clasts from the Apollo 14 site studied here include anorthosite, troctolites, gabbronorites, and basalts. Alkali anorthosites are plagioclase orthocumulates and may form by flotation in Mg-suite plutons. Ferroan anorthosite was cataclastically deformed and metamorphosed to granulite facies. Troctolites include both 01 + Plg and 01 + En + Plg cumulates. Major and trace element analyses of two troctolites reveal 'eastern' geochemical affinities that contrast other 'western' troctolites. Gabbronorites are Pig + Plg + or - Sp cumulates whose parent magmas may range from high-Al to intermediate-Ti mare basalt. At least three varieties of mare basalt are found at Apollo 14: high-Al, low-Ti; low-Al, intermediate-Ti; and low-Al, Ti VHK basalt. VHK (Very High Potassium) basalt is a new variety indigenous to Apollo 14.

Moho depth across the Trans-European Suture Zone from P- and S-receiver functions

NASA Astrophysics Data System (ADS)

Knapmeyer-Endrun, Brigitte; Krüger, Frank; Passeq Working Group

2014-05-01

The Mohorovičić discontinuity, Moho for short, which marks the boundary between crust and mantle, is the main first-order structure within the lithosphere. Geodynamics and tectonic evolution determine its depth level and properties. Here, we present a map of the Moho in central Europe across the Teisseyre-Tornquist Zone, a region for which a number of previous studies are available. Our results are based on homogeneous and consistent processing of P- and S-receiver functions for the largest passive seismological data set in this region yet, consisting of more than 40 000 receiver functions from almost 500 station. Besides, we also provide new results for the crustal vP/vS ratio for the whole area. Our results are in good agreement with previous, more localized receiver function studies, as well as with the interpretation of seismic profiles, while at the same time resolving a higher level of detail than previous maps covering the area, for example regarding the Eifel Plume region, Rhine Graben and northern Alps. The close correspondence with the seismic data regarding crustal structure also increases confidence in use of the data in crustal corrections and the imaging of deeper structure, for which no independent seismic information is available. In addition to the pronounced, stepwise transition from crustal thicknesses of 30 km in Phanerozoic Europe to more than 45 beneath the East European Craton, we can distinguish other terrane boundaries based on Moho depth as well as average crustal vP/vS ratio and Moho phase amplitudes. The terranes with distinct crustal properties span a wide range of ages, from Palaeoproterozoic in Lithuania to Cenozoic in the Alps, reflecting the complex tectonic history of Europe. Crustal thickness and properties in the study area are also markedly influenced by tectonic overprinting, for example the formation of the Central European Basin System, and the European Cenozoic Rift System. In the areas affected by Cenozoic rifting and volcanism, thinning of the crust corresponds to lithospheric updoming reported in recent surface wave and S-receiver function studies, as expected for thermally induced deformation. The same correlation applies for crustal thickening, not only across the Trans-European Suture Zone, but also within the southern part of the Bohemian Massif. A high Poisson's ratio of 0.27 is obtained for the craton, which is consistent with a thick mafic lower crust. In contrast, we typically find Poisson's ratios around 0.25 for Phanerozoic Europe outside of deep sedimentary basins. Mapping of the thickness of the shallowest crustal layer, that is low-velocity sediments or weathered rock, indicates values in excess of 6 km for the most pronounced basins in the study area, while thicknesses of less than 4 km are found within the craton, central Germany and most of the Czech Republic.

Present-day crustal deformation and strain transfer in northeastern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Li, Yuhang; Liu, Mian; Wang, Qingliang; Cui, Duxin

2018-04-01

The three-dimensional present-day crustal deformation and strain partitioning in northeastern Tibetan Plateau are analyzed using available GPS and precise leveling data. We used the multi-scale wavelet method to analyze strain rates, and the elastic block model to estimate slip rates on the major faults and internal strain within each block. Our results show that shear strain is strongly localized along major strike-slip faults, as expected in the tectonic extrusion model. However, extrusion ends and transfers to crustal contraction near the eastern margin of the Tibetan Plateau. The strain transfer is abrupt along the Haiyuan Fault and diffusive along the East Kunlun Fault. Crustal contraction is spatially correlated with active uplifting. The present-day strain is concentrated along major fault zones; however, within many terranes bounded by these faults, intra-block strain is detectable. Terranes having high intra-block strain rates also show strong seismicity. On average the Ordos and Sichuan blocks show no intra-block strain, but localized strain on the southwestern corner of the Ordos block indicates tectonic encroachment.

Evidence for a Battle Mountain-Eureka crustal fault zone, north-central Nevada, and its relation to Neoproterozoic-Early Paleozoic continental breakup

USGS Publications Warehouse

Grauch, V.J.S.; Rodriguez, B.D.; Bankey, V.; Wooden, J.L.

2003-01-01

Combined evidence from gravity, radiogenic isotope, and magnetotelluric (MT) data indicates a crustal fault zone that coincides with the northwest-trending Battle Mountain-Eureka (BME) mineral trend in north-central Nevada, USA. The BME crustal fault zone likely originated during Neoproterozoic-Early Paleozoic rifting of the continent and had a large influence on subsequent tectonic events, such as emplacement of allochthons and episodic deformation, magmatism, and mineralization throughout the Phanerozoic. MT models show the fault zone is about 10 km wide, 130-km long, and extends from 1 to 5 km below the surface to deep crustal levels. Isotope data and gravity models imply the fault zone separates crust of fundamentally different character. Geophysical evidence for such a long-lived structure, likely inherited from continental breakup, defies conventional wisdom that structures this old have been destroyed by Cenozoic extensional processes. Moreover, the coincidence with the alignment of mineral deposits supports the assertion by many economic geologists that these alignments are indicators of buried regional structures.

Continents as lithological icebergs: The importance of buoyant lithospheric roots

USGS Publications Warehouse

Abbott, D.H.; Drury, R.; Mooney, W.D.

1997-01-01

An understanding of the formation of new continental crust provides an important guide to locating the oldest terrestrial rocks and minerals. We evaluated the crustal thicknesses of the thinnest stable continental crust and of an unsubductable oceanic plateau and used the resulting data to estimate the amount of mantle melting which produces permanent continental crust. The lithospheric mantle is sufficiently depleted to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25-27 km. These unsubductable oceanic plateaus and hotspot island chains are important sources of new continental crust. The newest continental crust (e.g., the Ontong Java plateau) has a basaltic composition, not a granitic one. The observed structure and geochemistry of continents are the result of convergent margin magmatism and metamorphism which modify the nascent basaltic crust into a lowermost basaltic layer overlain by a more silicic upper crust. The definition of a continent should imply only that the lithosphere is unsubductable over ??? 0.25 Ga time periods. Therefore, the search for the oldest crustal rocks should include rocks from lower to mid-crustal levels.

Geology, mineralization, and fluid inclusion characteristics of the Skrytoe reduced-type W skarn and stockwork deposit, Sikhote-Alin, Russia

NASA Astrophysics Data System (ADS)

Soloviev, Serguei G.; Kryazhev, Sergey G.

2017-08-01

The Skrytoe deposit (>145 Kt WO3, average grade 0.449% WO3) in the Sikhote-Alin orogenic system (Eastern Russia) is situated in a metallogenic belt of W, Sn-W, Au, and Au-W deposits formed in a late to post-collisional tectonic environment after cessation of active subduction. It is localized within a mineralized district of reduced-type skarn W and veined Au (±W) deposits and occurrences related to the Early Cretaceous ilmenite-series plutonic suite. The deposit incorporates large stockworks of scheelite-bearing veinlets related to propylitic (amphibole, chlorite, quartz) and phyllic (quartz, sericite, albite, apatite, and carbonate) hydrothermal alteration. The stockwork cuts flat-lying mafic volcanic rocks and limestone partially replaced by pyroxene skarn that host the major W orebodies. Scheelite is associated with pyrrhotite and/or arsenopyrite, with minor chalcopyrite and other sulfide minerals; the late phyllic stage assemblages hosts Bi and Au mineralization. The fluid evolution included low-salinity moderate-temperature, moderate-pressure (˜370-390 °C, ˜800 bars) methane-dominated carbonic-aqueous fluids that formed post-skarn propylitic alteration assemblages. Then, at the phyllic stage, there has been an evolution from methane-dominated, moderate-temperature (330-360 °C), low-salinity (<12.3 wt% NaCl equiv.) fluids forming the early quartz-sericite-albite-arsenopyrite assemblage, through lower temperature (290-330 °C) methane-dominated, low-salinity (˜9-10 wt% NaCl equiv.) fluids forming the intermediate quartz-sericite-albite-scheelite-pyrrhotite assemblage, to yet lower temperature (245-320 °C) CO2-dominated carbonic-aqueous low-salinity (˜1-7 wt% NaCl equiv.) fluids forming the late quartz-sericite-sulfide-Bi-Au assemblage. Recurrent fluid immiscibility (phase separation) and cooling probably affected W solubility and promoted scheelite deposition. The stable isotope data support a sedimentary source of carbon (δ13Cfluid = ˜-21 to -10 ‰), a magmatic source for water (δ18OH2O = +7.4 to +7.7 ‰), and dominantly crustal-derived source of sulfur (δ34S = -4.6 to -2.9 ‰) in the hydrothermal fluids. This is consistent with the development of larger, longer crystallizing crustal intermediate to felsic magma chambers in the late to post-collisional tectonic environment, with their protracted magmatic evolution advancing magmatic differentiation and partitioning of W into magmatic-hydrothermal fluid. The dominating role of the crustal-derived magmatic water, sulfur, and carbon appears to be an important feature of reduced W skarn deposits related to ilmenite-series granitoids.

Crustal Strain Patterns in Magmatic and Amagmatic Early Stage Rifts: Border Faults, Magma Intrusion, and Volatiles

NASA Astrophysics Data System (ADS)

Ebinger, C. J.; Keir, D.; Roecker, S. W.; Tiberi, C.; Aman, M.; Weinstein, A.; Lambert, C.; Drooff, C.; Oliva, S. J. C.; Peterson, K.; Bourke, J. R.; Rodzianko, A.; Gallacher, R. J.; Lavayssiere, A.; Shillington, D. J.; Khalfan, M.; Mulibo, G. D.; Ferdinand-Wambura, R.; Palardy, A.; Albaric, J.; Gautier, S.; Muirhead, J.; Lee, H.

2015-12-01

Rift initiation in thick, strong continental lithosphere challenges current models of continental lithospheric deformation, in part owing to gaps in our knowledge of strain patterns in the lower crust. New geophysical, geochemical, and structural data sets from youthful magmatic (Magadi-Natron, Kivu), weakly magmatic (Malawi, Manyara), and amagmatic (Tanganyika) sectors of the cratonic East African rift system provide new insights into the distribution of brittle strain, magma intrusion and storage, and time-averaged deformation. We compare and contrast time-space relations, seismogenic layer thickness variations, and fault kinematics using earthquakes recorded on local arrays and teleseisms in sectors of the Western and Eastern rifts, including the Natron-Manyara basins that developed in Archaean lithosphere. Lower crustal seismicity occurs in both the Western and Eastern rifts, including sectors on and off craton, and those with and without central rift volcanoes. In amagmatic sectors, lower crustal strain is accommodated by slip along relatively steep border faults, with oblique-slip faults linking opposing border faults that penetrate to different crustal levels. In magmatic sectors, seismicity spans surface to lower crust beneath both border faults and eruptive centers, with earthquake swarms around magma bodies. Our focal mechanisms and Global CMTs from a 2007 fault-dike episode show a local rotation from ~E-W extension to NE-SE extension in this linkage zone, consistent with time-averaged strain recorded in vent and eruptive chain alignments. These patterns suggest that strain localization via widespread magma intrusion can occur during the first 5 My of rifting in originally thick lithosphere. Lower crustal seismicity in magmatic sectors may be caused by high gas pressures and volatile migration from active metasomatism and magma degassing, consistent with high CO2 flux along fault zones, and widespread metasomatism of xenoliths. Volatile release and migration may be critical to strength reduction of initially cold, strong cratonic lithosphere. Our comparisons suggest that large offset border faults that develop very early in rift history create fluid pathways that maintain the initial along-axis segmentation until magma (if available), reaches mid-crustal levels.

Crust and Upper Mantle Structure of Antarctica from Rayleigh Wave Tomography

NASA Astrophysics Data System (ADS)

Wiens, D. A.; Heeszel, D. S.; Sun, X.; Chaput, J. A.; Aster, R. C.; Nyblade, A.; Anandakrishnan, S.; Wilson, T. J.; Huerta, A. D.

2012-12-01

We combine data from three temporary arrays of seismometers (AGAP/GAMSEIS 2007-2010, ANET/POLENET 2007-2012, TAMSEIS 2001-2003) deployed across Antarctica, along with permanent stations in the region, to produce a large scale shear velocity model of the continent extending from the Gamburtsev Subglacial Mountains (GSM) in East Antarctica, across the Transantarctic Mountains (TAM) and West Antarctic Rift System (WARS) to Marie Byrd Land (MBL) in West Antarctica. Our combined dataset consists of Rayleigh wave phase and amplitude measurements from 112 stations across the study region. We first invert for 2-D Rayleigh wave phase velocities using the two-plane wave method. These results are then inverted for shear velocity structure using crustal thicknesses derived from ambient noise tomography and teleseismic receiver functions. We refine our shear velocity model by performing a Monte Carlo simulation that explores the tradeoff between crustal thickness and upper mantle seismic velocities. The resulting model is higher resolution than previous studies (~150 km resolution length) and highlights significant differences in crustal and uppermost mantle structure between East and West Antarctica in greater detail than previously possible. East Antarctica is underlain by thick crust (reaching ~55 km beneath the GSM) and fast, cratonic lithosphere. West Antarctica is defined by thinner crust and slow upper mantle velocities indicative of its more recent tectonic activity. The observed boundary in crustal thickness closely follows the TAM front. MBL is underlain by a thicker lithosphere than that observed beneath the WARS, but slow mantle velocities persist to depths greater than 200 km, indicating a 'deep seated' (i.e. deeper than the deepest resolvable features of our model) thermal source for volcanism in the region. The slowest seismic velocities at shallow depths are observed in the Terror Rift region of the Ross Sea along an arc following the TAM front, where the most recent extension has occurred, and in another region of active volcanism. The Ellsworth-Whitmore Mountains are underlain by relatively thick crust and an intermediate thickness lithosphere, consistent with its hypothesized origin as a remnant Precambrian crustal block. We also produce upper mantle viscosity models for the study region using a temperature-dependent rheology, assuming that mantle seismic anomalies are dominated by temperature variations. Initial results closely correlate with the velocity model, with viscosities beneath West Antarctica inferred to be orders of magnitude lower than beneath East Antarctica. These viscosity results have important implications for our understanding of glacial isostatic adjustment, which is of particular interest in producing models of past and future changes in the Antarctic Ice Sheets.

Radiogenic Ingrowth of 40CA from Decay of 40K Provides a Powerful Tracer for Understanding the Origins of Felsic Magmas

NASA Technical Reports Server (NTRS)

Mills, Ryan D.; Simon, Justin I.; Depaolo, Donald J.; Bachmann, Olivier

2013-01-01

Over time high K/Ca continental crust produces a unique Ca isotopic reservoir, with measurable 40Ca excesses compared to Earth's mantle (?Ca=0). Thus, values of ?Cai > 1 indicate a significant crustal contribution to a magma. Values of ?Cai (<1) indistinguishable from mantle Ca indicate that the Ca in those magmas is either directly from the mantle, or is from partial melting of newly formed crust. So, whereas 40Ca excesses clearly define crustal contributions, mantle-like 40Ca/44Ca ratios are not as definitive. Here we present Ca isotopic measurements of intermediate to felsic igneous rocks from the western United States, and two crustal xenoliths found within the Fish Canyon Tuff (FCT). The two crustal xenoliths found within the 28.2 Ma FCT of the southern Rocky Mountain volcanic field (SRMVF) yield ?Ca values of 4 and 7.5, respectively. The 40Ca excesses of these possible source rocks are due to long-term in situ 40K decay and suggest that they are Precambrian in age. However, the FCT (?Cai 0.3) is within uncertainty of the mantle 40Ca/44Ca. Together, these data indicate that little Precambrian crust was involved in the petrogenesis of the FCT. Nd isotopic analyses of the FCT imply that it was generated from 10- 75% of an enriched component, and the Ca isotopic data appear to restrict that component to newly formed lower crust, or enriched mantle. However, the Ca isotopic data do permit assimilation of some crust with low Ca/Nd; decreasing the 143Nd/144Nd without adding much excess 40Ca to the FCT. Several other large tuffs from the SRMVF and from Yellowstone have ?Cai indistinguishable from the mantle. However, a few large tuffs from the SRMVF show significant 40Ca excesses. These tuffs (Wall Mountain, Blue Mesa, and Grizzly Peak) are likely sourced from near, or within the Colorado Mineral Belt. New isotopic measurements of Mesozoic and Tertiary granites from across the northern Great Basin show a range of ?Cai from 0 to 3. In these samples ?Cai is generally correlated with ?Sri and is broadly negatively correlated with ?Ndi. However, for granites with similar ?Ndi at a given general location ?Cai can vary significantly (1 to 2 epsilon units). In rocks where low ?Ndi could also be due to melting from enriched reservoirs in the mantle lithosphere, the combination of high ?Cai with low ?Ndi clearly identifies crustal melts.

S-type granitic magmas—petrogenetic issues, models and evidence

NASA Astrophysics Data System (ADS)

Clemens, J. D.

2003-04-01

Despite a perception that it represents a perverse divergence, it is perfectly possible to believe in the existence of S- and I-type granites (and the implications for the nature of their protoliths), and to disbelieve in the applicability of the restite-unmixing model for chemical variation in granitic magmas. White and Chappell erected the S-I classification with impeccable validity. The isotopic evidence demands contrasting source reservoirs for S- and I-type granitic magmas. However, the major advance was not the classification, but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas. The restite-unmixing model is commonly seen as a companion to the S-I classification, but it is really a separate issue. This model implies that the compositions of granites 'image' those of their source rocks in a simple way. However, there are other equally valid models that can explain the data, and none of them represents a unique solution. The most cogent explanation for the high-grade metasedimentary enclaves in most S-type granites is that they represent mid-crustal xenoliths; restitic enclaves are either rare or absent. Inherited zircons in S-type rocks are certainly restitic. However, the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock. Zircon and zirconium behaviours are controlled by disequilibrium and kinetics, and Zr contents of granitic rocks can rarely be used to infer magma temperatures. Since the dominant ages among inherited zircons in Lachlan Fold Belt (LFB) S-type granites are Ordovician and Proterozoic, it seems likely that crust of this age, but geochemically different from the exposed rocks, not only underlies much of the LFB but also forms a component in the granite magma sources. The evidence is overwhelming that the dark, microgranular enclaves that occur in both S- and I-type granites are igneous in origin. They represent globules of quenched, more mafic magma mingled and modified by exchange with the host granitic magma. However, magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas. Likewise, the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and, in some cases, violate constraints from isotopic studies. S- and I-type magmas commonly retain their distinct identities. This suggests limited source mixing, limited magma mixing and limited wall-rock assimilation. Though intermediate types certainly exist, they are probably relatively minor in volume. Crystal fractionation probably plays the major role in the differentiation of very many granitic magmas, including most S-types, especially those emplaced at high crustal levels or in the volcanic environment. Minor mechanisms include magma mixing, wall-rock assimilation and restite unmixing. Isotopic variations within plutons and in granite suites could be caused by source heterogeneities, magma mixing, assimilation and even by isotopic disequilibrium. However, source heterogeneity, coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity. Normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust, and crustal thickening likewise fails to provide sufficient heat. Generally, the mantle must be the major heat source. This might be provided through mantle upwelling and crustal thinning, and possibly through the intra- and underplating of mafic magmas. Upper crustal extension seems to have been common in regions undergoing granitic magmatism. Migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid-present reactions. Granitic magmas are mostly formed by fluid-absent processes. Where we do see rare evidence for arrested fluid-absent partial melting, the melt fraction is invariably concentrated into small shear zones, veinlets and small dykes. Thus, it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels. However, one major missing link in the chain is the mechanism by which melt fractions, in small-scale segregations occurring over a wide area, can be gathered and focused to efficiently feed much wider-spaced major magma conduits. Answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones. Self-organization almost certainly plays a role.

Magmatic evolution of a Cordilleran flare-up and its role in the creation of silicic crust.

PubMed

Ward, Kevin M; Delph, Jonathan R; Zandt, George; Beck, Susan L; Ducea, Mihai N

2017-08-22

The role of magmatic processes as a significant mechanism for the generation of voluminous silicic crust and the development of Cordilleran plateaus remains a lingering question in part because of the inherent difficulty in quantifying plutonic volumes. Despite this difficulty, a growing body of independently measured plutonic-to-volcanic ratios suggests the volume of plutonic material in the crust related to Cordilleran magmatic systems is much larger than is previously expected. To better examine the role of crustal magmatic processes and its relationship to erupted material in Cordilleran systems, we present a continuous high-resolution crustal seismic velocity model for an ~800 km section of the active South American Cordillera (Puna Plateau). Although the plutonic-to-volcanic ratios we estimate vary along the length of the Puna Plateau, all ratios are larger than those previously reported (~30:1 compared to 5:1) implying that a significant volume of intermediate to silicic plutonic material is generated in the crust of the central South American Cordillera. Furthermore, as Cordilleran-type margins have been common since the onset of modern plate tectonics, our findings suggest that similar processes may have played a significant role in generating and/or modifying large volumes of continental crust, as observed in the continents today.

Microdiamonds from the European Variscan Orogenic Belt

NASA Astrophysics Data System (ADS)

Kotkova, J.; Jakubova, P.; Whitehouse, M.; Fedortchouk, Y.

2014-12-01

Diamond, along with coesite, has been discovered recently in the continental crustal rocks of the European Variscan orogenic belt, namely the Bohemian Massif (BM). In addition to the garnet-phengite gneiss in Germany, western BM, microdiamond occurs in major rock forming minerals - garnet, kyanite - and in zircon in ultrahigh-pressure rocks overprinted under high-pressure granulite facies conditions (c. 16-20 kbar, c. 1000°C) in the northern and eastern BM. Well-preserved 10-30 μm-sized microdiamonds from northern BM exhibit diverse morphologies (SEM data) depending upon the host rock type. Octahedral diamond occurs in felsic garnet-kyanite-feldspar-quartz rock (metasediment), whereas intermediate garnet-clinopyroxene-feldspar-quartz rock contains a cubo-octahedral variety. Diamond morphology can be thus controlled by solid impurities available in the medium of crystallization (K- vs. Ca-bearing fluids or melts), as shown by experiments. Pointed-bottom negatively oriented trigonal etch pits on the octahedral diamond faces developed due to diamond resorption at CO2-dominated environment (less than 50 wt % of H2O, experimental data), possibly by action of a residual fluid. SIMS determined δ13C values range from -22 to -21 ‰ for the felsic rock and from - 26 to - 33 for the intermediate one, corresponding to the typical range of organic carbon δ13C and inconsistent with a significant mantle carbon (δ13C ~ - 5 ‰) input. Diamond-bearing domains in zircon, also analysed by SIMS, yielded a Variscan U-Pb age of c. 340 Ma. The present stage of knowledge allows us to conclude that (i) metamorphic diamonds in the BM occur in lithologies of metasedimentary character, and their carbon source was organic; (ii) crustal-derived CO2-rich fluids with impurities played an important role in diamond formation and dissolution; (iii) diamonds formed during the Variscan orogenic cycle and (iv) diamonds are best preserved in the external domain of the Variscan orogenic belt.

Origin of the high plateau in the Central Andes, Bolivia, South America

NASA Astrophysics Data System (ADS)

Lamb, Simon; Hoke, Leonore

1997-08-01

The Bolivian Altiplano, in the Central Andes of South America, is part of the second largest high plateau on Earth. It is an elongate region of subdued relief, ˜1.2 × 105 km2 and ˜4 km above sea level, bounded by the Eastern Cordillera and volcanic arc (Western Cordillera). Here the crust is up to ˜75 km thick. We describe the Cenozoic geological evolution of this region, using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure. Crustal shortening and magmatic addition and, locally, sedimentation are the main mechanisms of Cenozoic crustal thickening, leading to nearly 4 km of surface uplift since the Paleocene. Addition of mafic melts appears to be a first-order mechanism of Cenozoic crustal growth, contributing ˜40% of the crustal thickening beneath the volcanic arc. Removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind-arc mafic volcanism, at ˜23 Ma and 0 - ˜5 Ma, contributing to the surface uplift. The Altiplano originated as a sedimentary basin, several hundred kilometers wide, between the proto-Western Cordillera and a narrow zone of uplift (proto-Eastern Cordillera) farther east. The latter zone formed by inversion of the center of a wide lacustrine or marine Cretaceous - Paleocene basin close to sea-level at ˜45 Ma. A thickness of 2-4 km of Paleogene continental elastics accumulated in the proto-Altiplano basin. Subsequently, in the Oligocene, we estimate that this region and the western margin of the Eastern Cordillera were technically shortened ˜22% (˜65 km), resulting in ˜9 km of average crustal thickening. The Altiplano basin was rejuvenated at ˜25 Ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting Eastern and Western Cordilleras. Between ˜25 and 5 Ma, folding and thrusting in the western margin of the Eastern Cordillera migrated westward into the center of the Altiplano basin, essentially terminating deposition, except in local subbasins, and accommodating ˜13% (˜30 km) of shortening and an estimated ˜7 km of average crustal thickening. Subsequently, there has been strike-slip deformation and limited local thrusting (< 5 km of shortening). Geomorphological and geochronological evidence for 1.5-2 km of surface uplift of this region since the Late Miocene suggests ˜14 km of lower crustal thickening beneath an essentially rigid "lid", and can be explained by ˜100-150 km of underthrusting of the Brazilian shield and adjacent regions beneath the eastern margin of the Central Andes. The present subdued relief in the Altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin.

Validity of ambient levels of fine particles as surrogate for personal exposure to outdoor air pollution--results of the European EXPOLIS-EAS Study (Swiss Center Basel).

PubMed

Oglesby, L; Künzli, N; Röösli, M; Braun-Fahrländer, C; Mathys, P; Stern, W; Jantunen, M; Kousa, A

2000-07-01

To evaluate the validity of fixed-site fine particle levels as exposure surrogates in air pollution epidemiology, we considered four indicator groups: (1) PM2.5 total mass concentrations, (2) sulfur and potassium for regional air pollution, (3) lead and bromine for traffic-related particles, and (4) calcium for crustal particles. Using data from the European EXPOLIS (Air Pollution Exposure Distribution within Adult Urban Populations in Europe) study, we assessed the associations between 48-hr personal exposures and home outdoor levels of the indicators. Furthermore, within-city variability of fine particle levels was evaluated. Personal exposures to PM2.5 mass were not correlated to corresponding home outdoor levels (n = 44, rSpearman (Sp) = 0.07). In the group reporting neither relevant indoor sources nor relevant activities, personal exposures and home outdoor levels of sulfur were highly correlated (n = 40, rSp = 0.85). In contrast, the associations were weaker for traffic (Pb: n = 44, rSp = 0.53; Br: n = 44, rSp = 0.21) and crustal (Ca: n = 44, rSp = 0.12) indicators. This contrast is consistent with spatially homogeneous regional pollution and higher spatial variability of traffic and crustal indicators observed in Basel, Switzerland. We conclude that for regional air pollution, fixed-site fine particle levels are valid exposure surrogates. For source-specific exposures, however, fixed-site data are probably not the optimal measure. Still, in air pollution epidemiology, ambient PM2.5 levels may be more appropriate exposure estimates than total personal PM2.5 exposure, since the latter reflects a mixture of indoor and outdoor sources.

Validity of Ambient Levels of Fine Particles as Surrogate for Personal Exposure to Outdoor Air Pollution-Results of the European EXPOLIS-EAS Study (Swiss Center Basel).

PubMed

Oglesby, Lucy; Künzli, Nino; Röösli, Martin; Braun-Fahrländer, Charlotte; Mathys, Patrick; Stern, Willem; Jantunen, Matti; Kousa, Anu

2000-07-01

To evaluate the validity of fixed-site fine particle levels as exposure surrogates in air pollution epidemiology, we considered four indicator groups: (1) PM 25 total mass concentrations, (2) sulfur and potassium for regional air pollution, (3) lead and bromine for traffic-related particles, and (4) calcium for crustal particles. Using data from the European EXPOLIS (Air Pollution Exposure Distribution within Adult Urban Populations in Europe) study, we assessed the associations between 48-hr personal exposures and home outdoor levels of the indicators. Furthermore, within-city variability of fine particle levels was evaluated. Personal exposures to PM 2.5 mass were not correlated to corresponding home outdoor levels (n = 44, r S (S) =r o v ' Spearman (Sp) 0.07). In the group reporting neither relevant indoor sources nor relevant activities, personal exposures and home outdoor levels of sulfur were highly correlated (n = 40, r Sp = 0.85). In contrast, the associations were weaker for traffic (Pb: n = 44, r Sp = 0.53; Br: n = 44, r Sp = 0.21) and crustal (Ca: n = 44, r Sp = 0.12) indicators. This contrast is consistent with spatially homogeneous regional pollution and higher spatial variability of traffic and crustal indicators observed in Basel, Switzerland. We conclude that for regional air pollution, fixed-site fine particle levels are valid exposure surrogates. For source-specific exposures, however, fixed-site data are probably not the optimal measure. Still, in air pollution epidemiology, ambient PM 2.5 levels may be more appropriate exposure estimates than total personal PM 2.5 exposure, since the latter reflects a mixture of indoor and outdoor sources.

Developing a Crustal and Upper Mantle Velocity Model for the Brazilian Northeast

NASA Astrophysics Data System (ADS)

Julia, J.; Nascimento, R.

2013-05-01

Development of 3D models for the earth's crust and upper mantle is important for accurately predicting travel times for regional phases and to improve seismic event location. The Brazilian Northeast is a tectonically active area within stable South America and displays one of the highest levels of seismicity in Brazil, with earthquake swarms containing events up to mb 5.2. Since 2011, seismic activity is routinely monitored through the Rede Sismográfica do Nordeste (RSisNE), a permanent network supported by the national oil company PETROBRAS and consisting of 15 broadband stations with an average spacing of ~200 km. Accurate event locations are required to correctly characterize and identify seismogenic areas in the region and assess seismic hazard. Yet, no 3D model of crustal thickness and crustal and upper mantle velocity variation exists. The first step in developing such models is to refine crustal thickness and depths to major seismic velocity boundaries in the crust and improve on seismic velocity estimates for the upper mantle and crustal layers. We present recent results in crustal and uppermost mantle structure in NE Brazil that will contribute to the development of a 3D model of velocity variation. Our approach has consisted of: (i) computing receiver functions to obtain point estimates of crustal thickness and Vp/Vs ratio and (ii) jointly inverting receiver functions and surface-wave dispersion velocities from an independent tomography study to obtain S-velocity profiles at each station. This approach has been used at all the broadband stations of the monitoring network plus 15 temporary, short-period stations that reduced the inter-station spacing to ~100 km. We expect our contributions will provide the basis to produce full 3D velocity models for the Brazilian Northeast and help determine accurate locations for seismic events in the region.

Thermal and petrologic constraints on the lower crustal melt accumulation in the Salton Sea Geothermal Field

NASA Astrophysics Data System (ADS)

Karakas, O.; Dufek, J.; Mangan, M.; Wright, H. M. N.

2014-12-01

Heat transfer in active volcanic areas is governed by complex coupling between tectonic and magmatic processes. These two processes provide unique imprints on the petrologic and thermal evolution of magma by controlling the geometry, depth, longevity, composition, and fraction of melt in the crust. The active volcanism, tectonic extension, and significantly high surface heat flow in Salton Sea Geothermal Field, CA, provides information about the dynamic heat transfer processes in its crust. The volcanism in the area is associated with tectonic extension over the last 500 ka, followed by subsidence and sedimentation at the surface level and dike emplacement in the lower crust. Although significant progress has been made describing the tectonic evolution and petrology of the erupted products of the Salton Buttes, their coupled control on the crustal heat transfer and feedback on the melt evolution remain unclear. To address these concepts, we develop a two-dimensional finite volume model and investigate the compositional and thermal evolution of the melt and crust in the Salton Sea Geothermal Field through a one-way coupled thermal model that accounts for tectonic extension, lower crustal magma emplacement, sedimentation, and subsidence. Through our simulations, we give quantitative estimates to the thermal and compositional evolution and longevity of the lower crustal melt source in the crustal section. We further compare the model results with petrologic constraints. Our thermal balance equations show that crustal melting is limited and the melt is dominated by mantle-derived material. Similarly, petrologic work on δ18O isotope ratios suggests fractional crystallization of basalt with minor crustal assimilation. In addition, we suggest scenarios for the melt fraction, composition, enthalpy release, geometry and depth of magma reservoirs, their temporal evolution, and the timescales of magmatic storage and evolution processes. These parameters provide the source conditions for the dynamics of surface volcanism and the presence of a geothermal system, which modify the thermal and mechanical structure of the crust.

High-resolution and Deep Crustal Imaging Across The North Sicily Continental Margin (southern Tyrrhenian Sea)

NASA Astrophysics Data System (ADS)

Agate, M.; Bertotti, G.; Catalano, R.; Pepe, F.; Sulli, A.

Three multichannel seismic reflection profiles across the North Sicily continental mar- gin have been reprocessed and interpreted. Data consist of an unpublished high pene- tration seismic profile (deep crust Italian CROP Project) and a high-resolution seismic line. These lines run in the NNE-SSW direction, from the Sicilian continental shelf to the Tyrrhenian abyssal plain (Marsili area), and are tied by a third, high penetration seismic line MS104 crossing the Sisifo High. The North Sicily continental margin represents the inner sector of the Sicilian-Maghrebian chain that is collapsed as con- sequence of extensional tectonics. The chain is formed by a tectonic wedge (12-15 km thick. It includes basinal Meso-Cenozoic carbonate units overthrusting carbonate platform rock units (Catalano et al., 2000). Presently, main culmination (e.g. Monte Solunto) and a number of tectonic depressions (e.g. Cefalù basin), filled by >1000 m thick Plio-Pleistocene sedimentary wedge, are observed along the investigated tran- sect. Seismic attributes and reflector pattern depicts a complex crustal structure. Be- tween the coast and the M. Solunto high, a transparent to diffractive band (assigned to the upper crust) is recognised above low frequency reflective layers (occurring be- tween 9 and 11 s/TWT) that dips towards the North. Their bottom can be correlated to the seismological (African?) Moho discontinuity which is (26 km deep in the Sicilian shelf (Scarascia et al., 1994). Beneath the Monte Solunto ridge, strongly deformed re- flectors occurring between 8 to 9.5 s/TWT (European lower crust?) overly the African (?) lower crust. The resulting geometry suggests underplating of the African crust respect to the European crust (?). The already deformed crustal edifice is dissected by a number of N-dipping normal faults that open extensional basins and are associ- ated with crustal thinning. The Plio-Pleistocene fill of the Cefalù basin can be subdi- vided into three subunits by well-developed unconformities. The stratal pattern of the lower subunit (Early Pliocene?) points out thrust-top basin. The intermediate subunit (Middle-Late Pliocene?) shows a wide sedimentary lateral accretion with syntectonic growth geometries. Upper Pliocene layers are overlain by well-stratified sediments of supposedly Pleistocene to Recent age, which drape and smooth underlying features (Pepe et al., 2000). Crustal thinning is (2 in the Cefalù basin and reach (3.54 north of Sisifo volcano, where crustal separation occurs and oceanic crust emplaced (Marsili 1 basin). In this area the Moho is located at (8 s/TWT, corresponding to 10-km depth. References Catalano R., Franchino A., Merlini S. e Sulli A., 2000. Mem. Soc. Geol. It., 55, 5-16. Pepe F., Bertotti G., Cella F. Marsella E., 2000. Tectonics, 19, 241-257. Scarascia S., Lozej A. Cassinis R., 1994. Boll. Geof. Teor. Appl., 36 (141-144), 5-19. 2

Risk-targeted maps for Romania

NASA Astrophysics Data System (ADS)

Vacareanu, Radu; Pavel, Florin; Craciun, Ionut; Coliba, Veronica; Arion, Cristian; Aldea, Alexandru; Neagu, Cristian

2018-03-01

Romania has one of the highest seismic hazard levels in Europe. The seismic hazard is due to a combination of local crustal seismic sources, situated mainly in the western part of the country and the Vrancea intermediate-depth seismic source, which can be found at the bend of the Carpathian Mountains. Recent seismic hazard studies have shown that there are consistent differences between the slopes of the seismic hazard curves for sites situated in the fore-arc and back-arc of the Carpathian Mountains. Consequently, in this study we extend this finding to the evaluation of the probability of collapse of buildings and finally to the development of uniform risk-targeted maps. The main advantage of uniform risk approach is that the target probability of collapse will be uniform throughout the country. Finally, the results obtained are discussed in the light of a recent study with the same focus performed at European level using the hazard data from SHARE project. The analyses performed in this study have pointed out to a dominant influence of the quantile of peak ground acceleration used for anchoring the fragility function. This parameter basically alters the shape of the risk-targeted maps shifting the areas which have higher collapse probabilities from eastern Romania to western Romania, as its exceedance probability increases. Consequently, a uniform procedure for deriving risk-targeted maps appears as more than necessary.

Crustal tomographic imaging of a transitional continental rift: the Ethiopian rift

NASA Astrophysics Data System (ADS)

Daly, E.; Keir, D.; Ebinger, C. J.; Stuart, G. W.; Bastow, I. D.; Ayele, A.

2008-03-01

In this study we image crustal structure beneath a magmatic continental rift to understand the interplay between crustal stretching and magmatism during the late stages of continental rifting: the Main Ethiopian Rift (MER). The northern sector of this region marks the transition from continental rifting in the East African Rift to incipient seafloor spreading in the southern Red Sea and western Gulf of Aden. Our local tomographic inversion exploits 172 broad-band instruments covering an area of 250 × 350 km of the rift and adjacent plateaux. The instruments recorded a total of 2139 local earthquakes over a 16-month period. Several synthetic tests show that resolution is good between 12 and 25 km depth (below sea level), but some horizontal velocity smearing is evident along the axis of the Main Ethiopian Rift below 16 km. We present a 3-D P-wave velocity model of the mid-crust and present the first 3-D Vp/Vs model of the region. Our models show high P-wave velocities (6.5 km s-1) beneath the axis of the rift at a depth of 12-25 km. The presence of high Vp/Vs ratios (1.81-1.84) at the same depth range suggest that they are cooled mafic intrusions. The high Vp/Vs values, along with other geophysical evidence, suggest that dyking is pervasive beneath the axis of the rift from the mid-crustal depths to the surface and that some portion of partial melt may exist at lower crustal depths. Although the crustal stretching factor across the Main Ethiopian Rift is ~1.7, our results indicate that magma intrusion in narrow zones accommodates a large proportion of extensional strain, with similarities to slow-spreading mid-ocean ridge processes.

Strain transformation between tectonic extrusion and crustal thickening in the growth of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Liu, M.; Li, Y.; Sun, Y.; Shen, X.

2017-12-01

The Indo-Eurasian continental collision since 50 Ma has thickened the crust to raise the Himalayan-Tibetan Plateau and driven lateral extrusion of Asian lithospheric blocks to affect Cenozoic tectonics in central and east Asia. The relative roles of crustal thickening and tectonic extrusion, and the strain partitioning between them over time and space, remain controversial. We have analyzed the strain rates using GPS velocities, and correlated the results with vertical motion derived from precise leveling. We found that tectonic extrusion largely transforms to crustal thickening near the margins of the Tibetan Plateau. Near the NW margin of the Tibetan Plateau, the shear stain transforms to compressive strain, consistent with neotectonic studies that indicate crustal shortening and uplift. Around the SE margin, shear stain largely terminates in the southern Yunnan province of China. The present-day crustal motion in SE Tibetan Plateau can be well explained by gravitational spreading without invoking plate-edge push as envisioned in the tectonic extrusion model. Using data collected from local seismic arrays, we derived receiver functions to image the lithospheric structures across the Tibetan Plateau and the Alashan block to its north and the Ordos block to its east. Our results indicate that the mantle lithosphere of these bounding Asian blocks has not been reworked by Tibetan tectonics; instead they have acted as restrictive walls to the growing Tibetan Plateau. Our finite element modeling shows that crustal deformation along the margins of the Tibetan Plateau are consistent with the notion that the east- and southeastward extrusion of the Tibetan lithosphere is largely confined to the Tibetan Plateau because of the restrictive bounding blocks of the Asian lithosphere. Thus the tectonic impact of the Indo-Eurasian collision on the Cenozoic Asian tectonics may not be as extensive as previously thought.

Geochemistry of the 1989-1990 eruption of redoubt volcano: Part I. Whole-rock major- and trace-element chemistry

USGS Publications Warehouse

Nye, C.J.; Swanson, S.E.; Avery, V.F.; Miller, T.P.

1994-01-01

The 1989-1990 eruption of Redoubt Volcano produced medium-K calc-alkaline andesite and dacite of limited compositional range (58.2-63.4% SiO2) and entrained quenched andesitic inclusions (55% SiO2) which bear chemical similarities to the rest of the ejecta. The earliest (December 15) magmas are pumiceous, often compositionally banded, and the majority is relatively mafic ( 60 wt.%). They have Mg, Cr, Ni, Sc, and V contents higher than the andesites, but lower than Redoubt basalts and basaltic andesites. Thus, they may be crystallization products of andesites, but do not represent the cumulate residue of basalt fractionation. The xenoliths were probably derived from a shallow or intermediate crustal chamber. ?? 1994.

Crustal deformations in the Central Mediterranean derived from the WHAT A CAT GPS project.

NASA Astrophysics Data System (ADS)

Kaniuth, K.; Drewes, H.; Stuber, K.; Tremel, H.; Kahler, H.-G.; Peter, Y.; Zerbini, S.; Tonti, G.; Veis, G.; Fagard, H.

1999-03-01

The West Hellenic Arc Tectonics and Calabrian Arc Tectonics (WHAT A CAT) project aimes at monitoring crustal deformations in the Central Mediterranean by repeated GPS campaigns. The data set acquired so far is rather heterogeneous in terms of availability of GPS satellites, performance of the involved receiver systems and quality of the satellites' orbits. The paper presents the velocity estimates achieved using a modified version of the Bernese GPS software. Main characteristic of the solution strategy is the definition of station velocity parameters already on theobservation equation level.

The nature of crustal reflectivity at the southwest Iberian margin

NASA Astrophysics Data System (ADS)

Buffett, G. G.; Torne, M.; Carbonell, R.; Melchiorre, M.; Vergés, J.; Fernàndez, M.

2017-11-01

Reprocessing of multi-channel seismic reflection data acquired over the northern margin of the Gulf of Cádiz (SW Iberian margin) places new constraints on the upper crustal structure of the Guadalquivir-Portimão Bank. The data presented have been processed with optimized stacking and interval velocity models, a better approach to multiple attenuation, preserved amplitude information to derive the nature of seismic reflectivity, and accurate time-to-depth conversion after migration. The reprocessed data reveal a bright upper crustal reflector just underneath the Paleozoic basement that spatially coincides with the local positive free-air gravity high called the Gulf of Cádiz Gravity High. To investigate the nature of this reflector and to decipher whether it could be associated with pieces of mantle material emplaced at upper crustal levels, we calculated its reflection coefficient and compared it to a buried high-density ultramafic body (serpentinized peridotite) at the Gorringe Bank. Its reflection coefficient ratio with respect to the sea floor differs by only 4.6% with that calculated for the high-density ultramafic body of the Gorringe Bank, while it differs by 35.8% compared to a drilled Miocene limestone unconformity. This means that the Gulf of Cádiz reflector has a velocity and/or density contrast similar to the peridotite at the Gorringe Bank. However, considering the depth at which it is found (between 2.0 and 4.0 km) and the available geological information, it seems unlikely that the estimated shortening from the Oligocene to present is sufficient to emplace pieces of mantle material at these shallow levels. Therefore, and despite the similarity in its reflection coefficient with the peridotites of the Gorringe Bank, our preferred interpretation is that the upper crustal Gulf of Cádiz reflector represents the seismic response of high-density intracrustal magmatic intrusions that may partially contribute to the Gulf of Cádiz Gravity High.

Giant magmatic water reservoirs at mid-crustal depth inferred from electrical conductivity and the growth of the continental crust

NASA Astrophysics Data System (ADS)

Laumonier, Mickael; Gaillard, Fabrice; Muir, Duncan; Blundy, Jon; Unsworth, Martyn

2017-01-01

The formation of the continental crust at subduction zones involves the differentiation of hydrous mantle-derived magmas through a combination of crystallization and crustal melting. However, understanding the mechanisms by which differentiation occurs at depth is hampered by the inaccessibility of the deep crust in active continental arcs. Here we report new high-pressure electrical conductivity and petrological experiments on hydrated andesitic melt from Uturuncu volcano on the Bolivian Altiplano. By applying our results to regional magnetotelluric data, we show that giant conductive anomalies at mid-crustal levels in several arcs are characterized by relatively low amounts of intergranular andesitic partial melts with unusually high dissolved water contents (≥8 wt.% H2O). Below Uturuncu, the Altiplano-Puna Magma Body (APMB) displays an electrical conductivity that requires high water content (up to 10 wt.%) dissolved in the melt based on crystal-liquid equilibria and melt H2O solubility experiments. Such a super-hydrous andesitic melt must constitute about 10% of the APMB, the remaining 90% being a combination of magmatic cumulates and older crustal rocks. The crustal ponding level of these andesites at around 6 kbar pressure implies that on ascent through the crust hydrous magmas reach their water saturation pressure in the mid-crust, resulting in decompression-induced crystallization that increases magma viscosity and in turn leads to preferential stalling and differentiation. Similar high conductivity features are observed beneath the Cascades volcanic arc and Taupo Volcanic Zone. This suggests that large amounts of water in super-hydrous andesitic magmas could be a common feature of active continental arcs and may illustrate a key step in the structure and growth of the continental crust. One Sentence Summary: Geophysical, laboratory conductivity and petrological experiments reveal that deep electrical conductivity anomalies beneath the Central Andes, Cascades and Taupo Volcanic Zone image the ponding of super-hydrous andesitic melts which contributes to the growth of continental crust.

Miocene volcanism in the Oaş-Gutâi Volcanic Zone, Eastern Carpathians, Romania: Relationship to geodynamic processes in the Transcarpathian Basin

NASA Astrophysics Data System (ADS)

Kovacs, Marinel; Seghedi, Ioan; Yamamoto, Masatsugu; Fülöp, Alexandrina; Pécskay, Zoltán; Jurje, Maria

2017-12-01

We present the first comprehensive study of Miocene volcanic rocks of the Oaş-Gutâi Volcanic Zone (OGVZ), Romania, which are exposed in the eastern Transcarpathian Basin (TB), within the Eastern Alpine-Western Carpathian-Northern Pannonian (ALCAPA) block. Collision between the ALCAPA block and Europe at 18-16 Ma produced the Carpathian fold-and-thrust belt. This was followed by clockwise rotation and an extensional regime forming core complexes of the separated TB fragment. Based on petrographic and geochemical data, including Srsbnd Nd isotopic compositions and Ksbnd Ar ages, we distinguish three types of volcanic activity in the OGVZ: (1) early Miocene felsic volcanism that produced caldera-related ignimbrites in the Gutâi Mountains (15.4-14.8 Ma); (2) widespread middle-late Miocene intermediate/andesitic volcanism (13.4-7.0 Ma); and (3) minor late Miocene andesitic/rhyolitic volcanism comprising the Oraşu Nou rhyolitic volcano and several andesitic-dacitic domes in the Oaş Mountains (11.3-9.5 Ma). We show that magma evolution in the OGVZ was controlled by assimilation-fractional crystallization and magma-mixing processes within an interconnected multi-level crustal magmatic reservoir. The evolution of volcanic activity within the OGVZ was controlled by the geodynamics of the Transcarpathian Basin. The early felsic and late intermediate Miocene magmas were emplaced in a post-collisional setting and were derived from a mantle source region that was modified by subduction components (dominantly sediment melts) and lower crust. The style of volcanism within the eastern TB system exhibits spatial variations, with andesitic composite volcanoes (Gutâi Mountains) observed at the margins, and isolated andesitic-rhyolitic monogenetic volcanoes (Oaş Mountains) in the center of the basin.

Boron isotope fractionation in magma via crustal carbonate dissolution

PubMed Central

Deegan, Frances M.; Troll, Valentin R.; Whitehouse, Martin J.; Jolis, Ester M.; Freda, Carmela

2016-01-01

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ11B values down to −41.5‰, reflecting preferential partitioning of 10B into the assimilating melt. Loss of 11B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports 11B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ11B melt values in arc magmas could flag shallow-level additions to the subduction cycle. PMID:27488228

Boron isotope fractionation in magma via crustal carbonate dissolution

NASA Astrophysics Data System (ADS)

Deegan, Frances M.; Troll, Valentin R.; Whitehouse, Martin J.; Jolis, Ester M.; Freda, Carmela

2016-08-01

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ11B values down to -41.5‰, reflecting preferential partitioning of 10B into the assimilating melt. Loss of 11B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports 11B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ11B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

Boron isotope fractionation in magma via crustal carbonate dissolution.

PubMed

Deegan, Frances M; Troll, Valentin R; Whitehouse, Martin J; Jolis, Ester M; Freda, Carmela

2016-08-04

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ(11)B values down to -41.5‰, reflecting preferential partitioning of (10)B into the assimilating melt. Loss of (11)B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports (11)B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ(11)B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

Subduction of lower continental crust beneath the Pamir imaged by receiver functions from the seismological TIPAGE network

NASA Astrophysics Data System (ADS)

Schneider, F. M.; Yuan, X.; Schurr, B.; Mechie, J.; Sippl, C.; Kufner, S.; Haberland, C. A.; Minaev, V.; Oimahmadov, I.; Gadoev, M.; Abdybachaev, U.; Orunbaev, S.

2013-12-01

As the northwestern promontory of the Tibetan Plateau, the Pamir forms an outstanding part of the India-Asia convergence zone. The Pamir plateau has an average elevation of more than 4000 m surrounded by peaks exceeding 7000 m at its northern, eastern and southern borders. The Pamir is thought to consist of the same collage of continental terranes as Tibet. However, in this region the Indian-Asian continental collision presents an extreme situation since, compared to Tibet, in the Pamir a similar amount of north-south convergence has been accommodated within a much smaller distance. The Pamir hosts a zone of intermediate depth earthquakes being the seismic imprint of Earth's most spectacular active intra-continental subduction zone. We present receiver function (RF) images from the TIPAGE seismic profile giving evidence that the intermediate depth seismicity is situated within a subducted layer of lower continental crust: We observe a southerly dipping 10-15 km thick low-velocity zone (LVZ), that starts from the base of the crust and extends to a depth of more than 150 km enveloping the intermediate depth earthquakes that have been located with high precision from our local network records. In a second northwest to southeast cross section we observe that towards the western Pamir the dip direction of the LVZ bends to the southeast following the geometry of the intermediate depth seismic zone. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust. These observations provide important implications for the geodynamics of continental collision: First, it shows that under extreme conditions lower crust can be brought to mantle depths despite its buoyancy, a fact that is also testified by the exhumation of ultra-high pressure metamorphic rocks. Recent results from teleseismic tomography show a signal of Asian mantle lithosphere down to 600 km depth, implying a great amount of mantle lithosphere to be involved in the subduction, which possibly transmits pull forces to the lower crust to overcome its buoyancy. Secondly, the observation that earthquakes occur within the subducted crust implies that similar to oceanic subduction, metamorphic processes within the lower continental crust can cause or enable earthquakes at depths, where the high pressure and temperature conditions would normally not allow brittle failure of rocks. For imaging of the dipping LVZ, cross sections of Q- and T-component RFs are generated using a migration technique that accounts for the inclination of the conversion layers. Furthermore we present a Moho map of the Pamir, showing crustal thickness in most places of the Pamir ranging between 65 km and 75 km, while the greatest Moho depths of around 80 km are observed at the upper end of the LVZ. The surrounding areas namely the Tajik Depression, and the Ferghana and Tarim Basins show Moho depths of around 40 to 45 km giving an estimate of the pre-collisional crustal thickness of the former Basin area that was overthrust by the Pamir.

Geophysical Imprints of the Geodynamic Evolution of Moesia Following the Black Sea Opening

NASA Astrophysics Data System (ADS)

Besutiu, Lucian

2014-05-01

Genesis of the two types of the Moesia basement (the so called Walachian, and Dobrogean sectors) along with the complex fault system affecting its cover and basement are still debated issues. Besides, there are two other intriguing aspects raised by the seismicity map of Romania: the sub-crustal events in the bending zone of East Carpathians, and the crust seismicity of the eastern Moesian Plate (MoP). Both the intermediate-depth earthquakes within full intra-continental environment and the intense craton seismicity are unusual aspects, and their apparent association difficult to explain. The paper proposes an integrated geodynamic model of MoP able to justify its current tectonics and both the crustal events in front of Carpathians, and the intermediate-depth earthquakes in the Vrancea zone within the frame of a unique geodynamic process. It starts from the idea that tectonic and geodynamic evolution of the E MoP and the bending zone of East Carpathians has been strongly affected by the opening of the W Black Sea basin, and is currently maintained by active rifting in SW Arabian Plate. The model is supported by geophysical and geodetic evidence. Unlike some previous geology-based models assuming that Black Sea opened during a singular geodynamic event (northward subduction of the Neo-Tethys Ocean floor), the pattern of the gravity and geomagnetic field, along with off-shore seismics bring convincing evidence on the distinct timing of the W and E Black Sea basins opening. Fingerprints of the lithosphere expelled by the W Black Sea rifting in the NW inland may be seen in the distribution of compression (P) wave velocity. In-depth development of NW striking major faults (splitting MoP into numerous vertical compartments) is also well revealed by seismic tomography (e.g. Peceneaga-Camena Fault, as the limit between MoP and East European Plate (EEP), still separates two distinct P wave velocity domains at 150 km depth). A second major fault system was created by the downward bending of MoP pushed towards vertical edge of Intra-Alpine Plate. It seems that W Black Sea opening also created the necessary environment for a FFT unstable triple junction within the bending zone of East Carpathians (VTJ), to which intermediate-depth earthquakes should be associated through thermo-baric accommodation phenomena occurring within the lithosphere sunken into the upper mantle. The triangle-shape and in-depth increase of the lateral extension of the VTJ high velocity seismic body are revealed by the high accuracy P wave tomography performed within Vrancea zone. Current geodetic and geophysical monitoring in the area has suggested a close link between crust and intermediate-depth seismic events. The intensification in tectonic forces may firstly led to the intensification of crust seismicity in the Carpathians foreland (by provoking slips between the MoP vertical compartments), followed, after a time-span depending on the force intensity and upper mantle viscosity, by VTJ sinking and consequent intermediate-depth seismic events in the Vrancea zone.

Orphan Basin crustal structure from a dense wide-angle seismic profile - Tomographic inversion

NASA Astrophysics Data System (ADS)

Watremez, Louise; Lau, K. W. Helen; Nedimović, Mladen R.; Louden, Keith E.; Karner, Garry D.

2014-05-01

Orphan Basin is located on the eastern margin of Canada, offshore of Newfoundland and East of Flemish Cap. It is an aborted continental rift formed by multiple episodes of rifting. The crustal structure across the basin has been determined by an earlier refraction study using 15 instruments on a 550 km long line. It shows that the continental crust was extended over an unusually wide region but did not break apart. The crustal structure of the basin thus documents stages in the formation of a magma-poor rifted margin up to crustal breakup. The OBWAVE (Orphan Basin Wide-Angle Velocity Experiment) survey was carried out to image crustal structures across the basin and better understand the processes of formation of this margin. The spacing of the 89 recording stations varies from 3 to 5 km along this 500-km-long line, which was acquired along a pre-existing reflection line. The highest resolution section corresponds to the part of the profile where the crust was expected to be the thinnest. We present the results from a joint tomography inversion of first and Moho reflected arrival times. The high data density allows us to define crustal structures with greater detail than for typical studies and to improve the understanding of the processes leading to the extreme stretching of continental crust. The final model was computed following a detailed parametric study to determine the optimal parameters controlling the ray-tracing and the inversion processes. The final model shows very good resolution. In particular, Monte Carlo standard deviations of crustal velocities and Moho depths are generally < 50 m/s and within 1 km, respectively. In comparison to the velocity models of typical seismic refraction profiles, results from the OBWAVE study show a notable improvement in the resolution of the velocity model and in the level of detail observed using the least a priori information possible. The final model allows us to determine the crustal thinning and variable structures across the basin. In particular, we observe (1) a zone of extreme thinning, where the crust is thinner than 7 km; (2) basement highs and lows highlighting the blocks that accommodate the crustal thinning; (3) a central block that is thicker compared to the rest of the basin; (4) lower crustal thinning that is highly variable, which suggests a ductile deformation in the lower crust and an extensional discrepancy between the upper and lower crust (DDS); and (5) no evidence for upper-mantle serpentinization under the ultra-thinned crust. Furthermore, we show the importance of structural inheritance in rifting of the Avalon crust. Thus, we suggest that Orphan Basin is the result of rifting of a non-homogeneous Avalon terrane where the lower crust is primarily ductile.

Juvenile crustal recycling in an accretionary orogen: Insights from contrasting Early Permian granites from central Inner Mongolia, North China

NASA Astrophysics Data System (ADS)

Yuan, Lingling; Zhang, Xiaohui; Xue, Fuhong; Liu, Fulin

2016-11-01

Coeval high-K calc-alkaline to alkaline granites constitute important components of post-collisional to post-orogenic igneous suites in most orogenic belts of various ages on Earth and their genesis harbors a key to ascertaining critical geodynamic controls on continental crustal formation and differentiation. This zircon U-Pb dating and geochemical study documents three contrasting Early Permian granites from Erenhot of central Inner Mongolia, eastern Central Asian Orogenic Belt (CAOB) and reveals concurrent high-K calc-alkaline to alkaline granite association derived from successive partial melting of distinct protoliths. The ca. 280 Ma Gancihuduge (GCG) pluton shows a calc-alkaline I-type character, with initial 87Sr/86Sr ratios of 0.7035 to 0.7039, εNd(t) of + 1.87 to + 4.70, zircon εHf(t) of + 8.0 to + 13.2 and δ18O from 7.4 to 8.7‰. The ca. 276 Ma Cailiwusu (CLS) pluton is magnesian and peraluminous, with initial 87Sr/86Sr ratios of 0.7036 to 0.7040, εNd(t) of + 1.9 to + 2.4, zircon εHf(t) of + 6.5 to + 12.1 and δ18O from 9.7 to 10.9‰. These features are consistent with partial melts of mixed sources composed of newly underplated meta-basaltic to -andesitic protoliths and variable supracrustal components, with distinctively higher proportion of the latter in the CLS pluton. By contrast, the ca. 279 Ma Kunduleng (KDL) suite exhibits an A-type magmatic affinity, with typical enrichment in alkalis, Ga, Zr, Nb and Y, εNd(t) of + 2.39 to + 3.55, zircon εHf(t) from + 8.3 to + 12.3 and δ18O values from 6.8 to 7.5‰. These features suggest that they stem from high-temperature fusion of dehydrated K-rich mafic to intermediate protoliths. Besides presenting a snapshot into a stratified crustal architecture in δ18O, these contrasting granites could not only serve as a temporal marker for monitoring post-collisional extension in the aftermath of a retreating subduction zone, but also present spatial magmatic proxy for tracing crustal formation and differentiation within back-arc basin environments in the CAOB.

Normal block faulting in the Airport Graben, Managua pull-apart rift, Nicaragua: gravity and magnetic constraints

NASA Astrophysics Data System (ADS)

Campos-Enriquez, J. O.; Zambrana Arias, X.; Keppie, D.; Ramón Márquez, V.

2012-12-01

Regional scale models have been proposed for the Nicaraguan depression: 1) parallel rifting of the depression (and volcanic front) due to roll back of the underlying subducted Cocos plate; 2) right-lateral strike-slip faulting parallel to the depression and locally offset by pull-apart basins; 3) right-lateral strike-slip faulting parallel to the depression and offset by left-lateral transverse or bookshelf faults. At an intermediate scale, Funk et al. (2011) interpret the depression as half graben type structures. The E-W Airport graben lies in the southeastern part of the Managua graben (Nicaragua), across which the active Central American volcanic arc is dextrally offset, possibly the result of a subducted transform fault where the subduction angle changes. The Managua graben lies within the late Quaternary Nicaragua depression produced by backarc rifting during roll back of the Middle American Trench. The Managua graben formed as a pull-apart rift associated with dextral bookshelf faulting during dextral shear between the forearc and arc and is the locus of two historical, large earthquakes that destroyed the city of Managua. In order to asses future earthquake risk, four E-W gravity and magnetic profiles were undertaken to determine its structure across the Airport graben, which is bounded by the Cofradia and Airport fault zones, to the east and west, respectively. These data indicated the presence of a series of normal faults bounding down-thrown and up-thrown fault blocks and a listric normal fault, Sabana Grande Fault. The models imply that this area has been subjected to tectonic extension. These faults appear to be part of the bookshelf suite and will probably be the locus of future earthquakes, which could destroy the airport and surrounding part of Managua. Three regional SW-NE gravity profiles running from the Pacific Ocean up to the Caribbean See indicate a change in crustal structure: from north to south the crust thins. According to these regional crustal models the offset observed in the Volcanic Front around the Nicaragua Lake is associated with a weakness zone related with: 1) this N-S change in crustal structure, 2) to the subduction angle of the Cocos plate, and 3) to the distance to the Middle America Trench (i.e. the location of the mantle wedge). As mentioned above a subducted transform fault might have given rise to this crustal discontinuity.

Interpreting the paleo-redox record: Mn enrichment factors

NASA Astrophysics Data System (ADS)

Chun, C. O.; Delaney, M. L.

2006-12-01

Redox-sensitive metal enrichment factors (EF), have the ability to describe the redox chemistry of the overlying water and marine sediments at time of burial. Manganese (Mn) precipitates as Mn-rich oxyhydroxides in oxic environments, leading to sedimentary EF > 1 calculated relative to average continental crust as the presumed detrital source. Mn EF can also occur from source changes that are unrelated to redox changes. We compared bulk sediment digestions to sample splits treated with a reductive cleaning step prior to sediment digestion, to test whether the Mn EF are from oxyhydroxides. We measured sedimentary Mn EF for the past 30 m.y. for a Nazca Ridge site in the southeast Pacific (ODP Site 1237). The site is marked by a pronounced color change at 162 mcd, within an interval dominated by calcareous-rich lithology, prompting questions of source versus paleo-redox changes. Mn EF were measured across the Paleocene-Eocene Thermal Maximum (PETM) at three sites on Walvis Ridge in the southeast Atlantic (ODP Sites 1262, 1266, and 1263). The PETM global warming event leads to questions of redox changes. At Nazca Ridge Mn EF range from 10-70 prior to the change with decrease to crustal averages after the boundary. After two reductive cleanings on sediments exhibiting Mn EF >1, Mn EF were at crustal averages. Mn EF prior to the color change are oxyhydroxides and not a major input of detrital material. We suggest the color change represents a paleo-redox boundary, more oxygenated depositional setting prior to the change and more reducing depositional setting afterwards. Walvis Ridge PETM sections exhibit Mn EF values ranging between 4 and 12 prior to the warming, values at crustal averages during the warming, return to pre-event values in the recovery period. After the reductive cleaning procedure the deep (1262) and intermediate (1266) sites with Mn EF >1 before and after the warming event reduced to crustal averages with no change to Mn EF during the event. Bottom waters at those two sites were most likely oxygenated prior to the event, reducing at the onset of the warming, and returned to pre-event conditions in the recovery. Future studies of Mn EF as a paleo-redox indicator should include the reductive cleaning procedure to verify Mn-oxyhydroxides.

Intermediate-consumer identity and resources alter a food web with omnivory.

PubMed

Kneitel, Jamie M

2007-07-01

1. Omnivory is an important interaction that has been the centre of numerous theoretical and empirical studies in recent years. Most of these studies examine the conditions necessary for coexistence between an omnivore and an intermediate consumer. Trait variation in ecological interactions (competition and predator tolerance) among intermediate consumers has not been considered in previous empirical studies despite the evidence that variation in species-specific traits can have important community-level effects. 2. I conducted a multifactorial microcosm experiment using species from the Sarracenia purpurea phytotelmata community, organisms that inhabit the water collected within its modified leaves. The basal trophic level consisted of bacterial decomposers, the second trophic level (intermediate consumers) consisted of protozoa and rotifers, and the third trophic level (omnivore) were larvae of the pitcher plant mosquito Wyeomyia smithii. Trophic level number (1, 2 and 3), resources (low and high), omnivore density (low and high) and intermediate consumer (monoculture of five protozoa and rotifers) identity were manipulated. Abundance of the basal trophic level, intermediate consumers, and growth of the omnivore were measured, as well as time to extinction (intermediate consumers) and time to pupation (mosquito larvae). 3. The presence of different intermediate consumers affected both bacteria abundance and omnivore growth. At high resource levels, Poteriochromonas, Colpidium and Habrotrocha rosa reduced bacteria densities greater than omnivore reduction of bacteria. Mosquito larvae did not pupate at low resource levels except when Poteriochromonas and Colopoda were present as intermediate consumers. Communities with H. rosa were the only ones consistent with the prediction that omnivores should exclude intermediate consumers at high resources. 4. These results had mixed support for predictions from omnivory food web theory. Intermediate consumers responded and affected this community differently under different community structures and resource levels. Consequently, variation in species-specific traits can have important population- and community-level effects and needs to be considered in food webs with omnivory.

A reconnaissance space sensing investigation of crustal structure for a strip from the eastern Sierra Nevada to the Colorado Plateau

NASA Technical Reports Server (NTRS)

Bechtold, I. C. (Principal Investigator)

1973-01-01

The author has identified the following significant results. Analysis of ERTS-1 MSS imagery over the sourthern Basin-Range Province of California, Nevada, and Arizona has led to recognition of regional tectonic control of volcanism, plutonism, mineralization, and fault patterns. This conclusion is the result of geologic reconnaissance of anomalies observed in ERTS-1 and Apollo-9 data, guided by intermediate scale U-2 photography, SLAR, and relevant geologic literature. In addition to regional tectonic studies, the ERTS-1 imagery provides a basis for detailed research of relatively small geologic features. Interpretation of ERTS-1 and Apollo-9 space imagery and intermediate scale X-15 and U-2 photography indicates the presence of a major fault zone along the California-Nevada state line, here named the Pahrump fault zone. Field mapping confirms previously unreported evidence of fault breaks in bedrock, along range fronts and in Quaternary alluvium and lake sediments. Regional gravity lows and fault traces within the Pahrump fault zone from a general left stepping en echelon pattern. The trend and postulated diplacement for this fault are similar to other major strike slip fault zones in the southern Basin-Range Province.

Tectonic deformation of the Andes and the configuration of the subducted slab in central Peru: Results from a micro-seismic experiment

NASA Technical Reports Server (NTRS)

Suarez, G.; Gagnepain, J. J.; Cisternas, A.; Hatzfeld, D.; Molnar, P.; Ocola, L.; Roecker, S. W.; Viode, J. P.

1983-01-01

The vast majority of the microearthquakes recorded occurred to the east: on the Huaytapallana fault in the Eastern Cordillera or in the western margin of the sub-Andes. The sub-Andes appear to be the physiographic province subjected to the most intense seismic deformation. Focal depths for the crustal events here are as deep as 50 km, and the fault plane solutions, show thrust faulting on steep planes oriented roughly north-south. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a northeast-southwest trend that agrees with the fault scarp and the east dipping nodal plane of two large earthquakes that occurred on this fault in 1969. The recorded microearthquakes of intermediate depth show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with the suggestion that beneath Peru the slab first dips at an angle of 30 deg to a depth of 100 km and then flattens following a quasi-horizontal trajectory. Fault plane solutions of intermediate depth microearthquakes have horizontal T axes oriented east-west.

A geodynamic constraint on Archean continental geotherms

NASA Astrophysics Data System (ADS)

Bailey, R. C.

2003-04-01

Dewey (1988) observed that gravitational collapse appears to currently limit the altitudes of large plateaus on Earth to about 3 to 5 km above sea level. Arndt (1999) summarized the evidence for the failure of large parts of the continental crust to reach even sea-level during the Archean. If this property of Archean continental elevations was also enforced by gravitational collapse, it permits an estimation of the geothermal gradient in Archean continental crust. If extensional (collapse) tectonics is primarily a balance between gravitational power and the power consumed by extensional (normal) faulting in the upper brittle crust, as analysed by Bailey (1999), then it occurs when continental elevations above ocean bottoms exceed about 0.4 times the thickness of the brittle crust (Bailey, 2000). Assuming an Archean oceanic depth of about 5 km, it follows that that the typical thickness of Archean continental brittle crustal must have been less than about 12 km. Assuming the brittle-ductile transition to occur at about 350 degrees Celsius, this suggests a steep geothermal gradient of at least 30 degrees Celsius per kilometer for Archean continents, during that part of the Archean when continents were primarily submarine. This result does not help resolve the Archean thermal paradox (England and Bickle, 1984) whereby the high global heat flow of the Archean conflicts with the rather shallow crustal Archean geotherms inferred from geobarometry. In fact, the low elevation of Archean continental platforms raises another paradox, a barometric one: that continents were significantly below sea-level implies, by isostasy, that continental crustal thicknesses were significantly less than 30 km, yet the geobarometric data utilized by England and Bickle indicated burial pressures of Archean continental material of up to 10 kb. One resolution of both paradoxes (as discussed by England and Bickle) would be to interpret such deep burials as transient crustal thickening events of duration less than the crustal thermal equilibriation time (about 10 to 30 Ma). Temporary entrainment in the wake of basal eclogite ``sinkers'' might provide such transient burial. Vlaar's (1994) modelling of this eclogite delamination process (tectonically elaborated by Zegers and van Keken (2001)) indicates such sinker events would be significantly shorter than 10 Ma. The topographic re-equilibriation of a hot moho above such a process would be similarly short (Kaufmann and Royden, 1994).

Archean crustal evolution of the Narryer Gneiss Terrane, Western Australia, as revealed by the U-Pb age and Hf-isotope compositions of zircon from the granitic gneisses

NASA Astrophysics Data System (ADS)

Sylvester, P.; Souders, K.; Crowley, J. L.; Myers, J.

2011-12-01

The Narryer Gneiss Terrane of the Yilgarn Craton, Western Australia, is an important area for studies of early crustal evolution because of the preservation of (1) detrital zircons of Hadean to Archean age in the Jack Hills and Mt. Narryer metasedimentary belts, and (2) several widespread units of granitic gneisses emplaced between ca. 3.7 and 2.6 Ga. We have analyzed the U-Pb geochronology and Hf-isotope geochemistry of magmatic zircons from 38 samples of the granitic gneisses using laser ablation - (multicollector) - ICPMS. The sample suite is dominated by the Meeberrie gneiss, a banded quartz-microcline-oligoclase-biotite gneiss of monzogranite to granodiorite composition, and the Dugel gneiss, a leucocratic, pegmatite-layered syenogranite gneiss, but gneisses of dioritic to tonalitic composition, as well as less deformed granite sheets, are also represented. Magmatic zircons were identified on the basis of the preservation of oscillatory zoning in BSE and CL images, igneous Th/U ratios (>0.2), and concordant U-Pb isotopic systematics with low common Pb contents. The results indicate many of the gneisses are composed of the products of multiple magmatic events, as has been reported previously for samples of the Meeberrie gneiss (Kinny & Nutman, 1996, Precambrian Res. 78, 165-178). Major ages of magmatism preserved in the gneisses occurred at ca. 3685-3665 Ma, 3620-3565 Ma, 3495-3440 Ma, 3375-3330 Ma, and 3300-3260 Ma. The late granite sheets crystallized at 2710-2645 Ma. Hf-isotope compositions of the zircons trend to less radiogenic values with decreasing age, with ɛHf values of ca. 0 to -5 for 3.7-3.4 Ga gneisses, ca. -1 to -9 for 3.4-3.2 Ga gneisses and ca. -5 to -20 for the late granite sheets. The array of the Hf isotopic compositions with time for the entire sample set are fit well by a regression indicating a source reservoir with a 176Lu/177Hf of 0.022 extracted from the depleted mantle at 3.9 Ga. This suggests that the Narryer gneisses and late granite sheets were derived by repeated partial melting of an Eoarchean mafic to intermediate crustal source that persisted throughout the Archean. The ɛHf evolution trend for the Narryer gneisses plots distinctly above the ɛHf trend of the Hadean mafic crustal reservoir inferred for the source of the Hadean detrital zircons from the Jack Hills. The implication is that the inferred Hadean crustal reservoir was not an important source of the Narryer gneisses, either because it was largely isolated from zones of melting in the crust of the Narryer Gneiss Terrane during the Archean, or because it was simply of limited volume.

Correlated Geophysical, Geochemical and Volcanological Manifestations of Plume-Ridge Interaction Along the Galápagos Spreading Center, 90.5-98° W

NASA Astrophysics Data System (ADS)

Sinton, J. M.; Detrick, R. S.; Canales, J. P.; Ito, G.; Behn, M.; Blacic, T.; Cushman, B.; Dixon, J.

2001-12-01

As the Galápagos plume is approached from the west along the Galápagos Spreading Center there are systematic increases in crustal thickness, and K/Ti and H2O content of recovered lavas. These increases correlate with progressive transitions from axial deep to axial high morphology along with decreases in axial depth, residual mantle Bouguer gravity anomaly (MBA), average swell depth, average lava Mg # (atomic MgO/(MgO+FeO)), and the frequency of isolated axial seamounts. Although K/Ti, H2O and Nb/Zr (likely indicators of plume source enrichment) show step-wise increases across the 95.5° W propagating offset, trends in crustal thickness, axial bathymetry, MBA, swell depth, and seamount frequency generally show either no effect or only local perturbations to regional trends. East of ~92.7° W, sharp increases in K/Ti, Nb/Zr, H2O, and Na8 (Na2O corrected for fractionation to 8 wt % MgO) coincide with the transition to axial high morphology, a rapid shoaling of axial magma chamber (AMC) seismic reflectors, and thinning of seismic layer 2A. Maximum values in K/Ti (>0.4), Nb/Zr (>0.10), H2O (>1.0 wt %), Na8 ( ~3.2) and crustal thickness (7.9 km), and minima in axial depth (<1700 m), Mg # (<40), and Ca8/Al8 (<0.7) all occur between 91.25° W and 92° W, whereas the minimum MBA (-25 mGal) and AMC depth ( ~0.5 sec 2-way travel time) are found near 92.25° W. These general correlations can be modeled by the combined effects of changes in source composition and melt generation processes on the thickness, composition and structure of the oceanic crust. Key elements of this model include: (1) compensation of the swell is partitioned between crustal thickening (2.3 km) between 98° W and 90.5° W [Ito et al., this meeting] and thermal and compositional buoyancy of the mantle [Canales et al., this meeting]; (2) increased melt production near the hotspot is associated with lower mean extents of melting from a larger region of an increasingly hydrous, and other incompatible element-enriched mantle [Cushman et al., this meeting]; and (3) higher magma supply results in stabilization of axial magma chambers at increasingly shallow crustal depths [Blacic et al., this meeting] and the dominance of fissure-fed rather than point-source volcanism. The hotspot-related effect of increased magma supply on axial morphology, AMC depth and volcanic style along this intermediate-spreading ridge is similar to that between slow and faster spreading mid-ocean ridges.

Large-scale removal of lithosphere underneath the North China Craton in the Early Cretaceous: Geochemical constraints from volcanic lavas in the Bohai Bay Basin

NASA Astrophysics Data System (ADS)

Sun, Jing; Liu, Zheng; Zhang, Shuai; Li, Xiaoguang; Qi, Jiafu

2017-11-01

Cratons are generally considered as the most stable tectonic units on the Earth. Rare magmatism, seismic activity, and intracrustal ductile deformation occur in them. However, several cratons experienced entirely different fates, including the North China Craton (NCC), and were subsequently destroyed. Geodynamic mechanisms and timing of the cratonic destruction are strongly debated. In this paper, we investigate a suite of Mesozoic intermediate to felsic volcanic rocks which are collected from boreholes in the Liaohe Depression of the Bohai Bay Basin the eastern NCC. These volcanic rocks have Precambrian basement-like Sr-Nd isotopic characteristics, consistent with derivation from the lower continental crust underneath the NCC. The Late Jurassic ( 165 Ma) intermediate volcanic rocks don't exhibit markedly negative Eu anomalies, which require a source beyond the plagioclase stability field. And the low heavy rare earth elements (HREEs) contents of these samples indicate that their source has garnet as residue. The Early Cretaceous ( 122 Ma) felsic volcanic rocks are depleted in HREEs but with remarkable Eu anomalies, suggesting that their source have both garnet and plagioclase. The crust thicknesses, estimated from the geochemistry of the intermediate and felsic rocks, are ≥ 50 km at 165 Ma and 30-50 km at 122 Ma, respectively. The crustal thinning is attributed to lithospheric delamination beneath the NCC. Our results combined with previous studies imply that the large-scale lithospheric removal occurred in the Early Cretaceous, between 140 and 120 Ma.

Structural variation of the oceanic Moho in the Pacific plate revealed by active-source seismic data

NASA Astrophysics Data System (ADS)

Ohira, Akane; Kodaira, Shuichi; Nakamura, Yasuyuki; Fujie, Gou; Arai, Ryuta; Miura, Seiichi

2017-10-01

The characteristics of the oceanic Moho are known to depend on various factors, such as seafloor spreading rate, crustal age, and accretionary processes at a ridge. However, the effect of local magmatic activities on the seismic signature of the Moho is poorly understood. Here an active-source reflection and refraction survey is used to investigate crustal structure and Moho characteristics along a >1000-km-long profile southeast of the Shatsky Rise in a Pacific Ocean basin formed from the Late Jurassic to Early Cretaceous and spanning the onset of Shatsky Rise volcanism. Although the seismic velocity structure estimated from the refraction data showed typical characteristics of the oceanic crust of the old Pacific plate, the appearance of the Moho reflections was spatially variable. We observed clear Moho reflections such as those to be expected where the spreading rate is fast to intermediate only at the southwestern end of the profile, whereas Moho reflections were diffuse, weak, or absent along other parts of the profile. The poor Moho reflections can be explained by the presence of a thick crust-mantle transition layer, which is temporally coincident with the formation of the Shatsky Rise. We inferred that the crust-mantle transition layer was formed by changes in on-axis accretion process or modification of the primary Moho by off-axis magmatism, induced by magmatic activity of the Shatsky Rise.

Intra-arc Seismicity: Geometry and Kinematic Constraints of Active Faulting along Northern Liquiñe-Ofqui and Andean Transverse Fault Systems [38º and 40ºS, Southern Andes

NASA Astrophysics Data System (ADS)

Sielfeld, G.; Lange, D.; Cembrano, J. M.

2017-12-01

Intra-arc crustal seismicity documents the schizosphere tectonic state along active magmatic arcs. At oblique-convergent margins, a significant portion of bulk transpressional deformation is accommodated in intra-arc regions, as a consequence of stress and strain partitioning. Simultaneously, crustal fluid migration mechanisms may be controlled by the geometry and kinematics of crustal high strain domains. In such domains shallow earthquakes have been associated with either margin-parallel strike-slip faults or to volcano-tectonic activity. However, very little is known on the nature and kinematics of Southern Andes intra-arc crustal seismicity and its relation with crustal faults. Here we present results of a passive seismicity study based on 16 months of data collected from 33 seismometers deployed along the intra-arc region of Southern Andes between 38˚S and 40˚S. This region is characterized by a long-lived interplay among margin-parallel strike-slip faults (Liquiñe-Ofqui Fault System, LOFS), second order Andean-transverse-faults (ATF), volcanism and hydrothermal activity. Seismic signals recorded by our network document small magnitude (0.2P and 2,796 S phase arrival times have been located with NonLinLoc. First arrival polarities and amplitude ratios of well-constrained events, were used for focal mechanism inversion. Local seismicity occurs at shallow levels down to depth of ca. 16 km, associated either with stratovolcanoes or to master, N10˚E, and subsidiary, NE to ENE, striking branches of the LOFS. Strike-slip focal mechanisms are consistent with the long-term kinematics documented by field structural-geology studies. Unexpected, well-defined NW-SE elongated clusters are also reported. In particular, a 72-hour-long, N60˚W-oriented seismicity swarm took place at Caburgua Lake area, describing a ca. 36x12x1km3 faulting crustal volume. Results imply a unique snapshot on shallow crustal tectonics, contributing to the understanding of faulting processes in volcanic arcs and ultimately, providing useful knowledge to improve the quality of hazard assessment communities emplaced in Southern Andes volcanic arc.

Deep Crustal Melting and the Survival of Continental Crust

NASA Astrophysics Data System (ADS)

Whitney, D.; Teyssier, C. P.; Rey, P. F.; Korchinski, M.

2017-12-01

Plate convergence involving continental lithosphere leads to crustal melting, which ultimately stabilizes the crust because it drives rapid upward flow of hot deep crust, followed by rapid cooling at shallow levels. Collision drives partial melting during crustal thickening (at 40-75 km) and/or continental subduction (at 75-100 km). These depths are not typically exceeded by crustal rocks that are exhumed in each setting because partial melting significantly decreases viscosity, facilitating upward flow of deep crust. Results from numerical models and nature indicate that deep crust moves laterally and then vertically, crystallizing at depths as shallow as 2 km. Deep crust flows en masse, without significant segregation of melt into magmatic bodies, over 10s of kms of vertical transport. This is a major mechanism by which deep crust is exhumed and is therefore a significant process of heat and mass transfer in continental evolution. The result of vertical flow of deep, partially molten crust is a migmatite dome. When lithosphere is under extension or transtension, the deep crust is solicited by faulting of the brittle upper crust, and the flow of deep crust in migmatite domes traverses nearly the entire thickness of orogenic crust in <10 million years. This cycle of burial, partial melting, rapid ascent, and crystallization/cooling preserves the continents from being recycled into the mantle by convergent tectonic processes over geologic time. Migmatite domes commonly preserve a record of high-T - low-P metamorphism. Domes may also contain rocks or minerals that record high-T - high-P conditions, including high-P metamorphism broadly coeval with host migmatite, evidence for the deep crustal origin of migmatite. There exists a spectrum of domes, from entirely deep-sourced to mixtures of deep and shallow sources. Controlling factors in deep vs. shallow sources are relative densities of crustal layers and rate of extension: fast extension (cm/yr) promotes efficient ascent of deep crust, whereas slow extension (mm/yr) produces significantly less exhumation. Recognition of the importance of migmatite (gneiss) domes as archives of orogenic deep crust is applicable to determining the chemical and physical properties of continental crust, as well as mechanisms and timescales of crustal differentiation.

Quantifying the Variation in Shear Zone Character with Depth: a Case Study from the Simplon Shear Zone, Central Alps

NASA Astrophysics Data System (ADS)

Cawood, T. K.; Platt, J. P.

2017-12-01

A widely-accepted model for the rheology of crustal-scale shear zones states that they comprise distributed strain at depth, in wide, high-temperature shear zones, which narrow to more localized, high-strain zones at lower temperature and shallower crustal levels. We test and quantify this model by investigating how the width, stress, temperature and deformation mechanisms change with depth in the Simplon Shear Zone (SSZ). The SSZ marks a major tectonic boundary in the central Alps, where normal-sense motion and rapid exhumation of the footwall have preserved evidence of older, deeper deformation in rocks progressively further into the currently-exposed footwall. As such, microstructures further from the brittle fault (which represents the most localized, most recently-active part of the SSZ) represent earlier, higher- temperature deformation from deeper crustal levels, while rocks closer to the fault have been overprinted by successively later, cooler deformation at shallower depths. This study uses field mapping and microstructural studies to identify zones representing deformation at various crustal levels, and characterize each in terms of zone width (representing width of the shear zone at that time and depth) and dominant deformation mechanism. In addition, quartz- (by Electron Backscatter Diffraction, EBSD) and feldspar grain size (measured optically) piezometry are used to calculate the flow stress for each zone, while the Ti-in-quartz thermometer (TitaniQ) is used to calculate the corresponding temperature of deformation. We document the presence of a broad zone in which quartz is recrystallized by the Grain Boundary Migration (GBM) mechanism and feldspar by Subgrain Rotation (SGR), which represents the broad, deep zone of deformation occurring at relatively high temperatures and low stresses. In map view, this transitions to successively narrower zones, respectively characterized by quartz SGR and feldspar Bulge Nucleation (BLG); quartz BLG and brittle deformation of feldspar; and finally, a zone of generally brittle deformation. These zones represent deformation in progressively narrower regions at shallower depths, under lower temperatures and higher stresses.

How tectonics controlled post-collisional magmatism within the Dinarides: Inferences based on study of tectono-magmatic events in the Kopaonik Mts. (Southern Serbia)

NASA Astrophysics Data System (ADS)

Mladenović, Ana; Trivić, Branislav; Cvetković, Vladica

2015-04-01

In this study, we report evidence about coupling between tectonic and magmatic processes in a complex orogenic system. The study focuses on the Kopaonik Mts. situated between the Dinarides and the Carpatho-Balkanides (Southern Serbia), and a perfect area for investigating tectono-magmatic evolution. We combine a new data set on tectonic paleostress tensors with the existing information on Cenozoic magmatic rocks in the wider Kopaonik Mts. area. The paleostress study revealed the presence of four brittle deformational phases. The established link between fault mechanism and igneous processes suggests that two large tectono-magmatic events occurred in this area. The Late Eocene-Early Miocene tectono-magmatic event was generally characterized by transpressional tectonics that provided conditions for formation of basaltic underplating and subsequent lower crustal melting and generation of I-type magmas. Due to predominant compression in the first half of this event, these magmas could not reach the upper crustal levels. Later on, limited extensional pulses that occurred before the end of this event opened pathways for newly formed mantle melts to reach shallower crustal levels and mix with the evolving I-type magmas. The second event is Middle-Late Miocene in age. It was first associated with clear extensional conditions that caused advancing of basaltic melts to mid-crustal levels. This, in turn, induced the elevation of geotherms, melting of shallow crust and S-type granite formation. This event terminated with transpression that produced small volumes of basaltic melts and finally closed the igneous scene in this part of the Balkan Peninsula. Although we agree that the growth of igneous bodies is usually internally controlled and can be independent from the ambient structural pattern, we have strong reasons to believe that the integration of regional scale observations of fault kinematics with crucial petrogenetic information can be used for establishing spatial-temporal relationships between brittle tectonics and magmatism.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

NASA Astrophysics Data System (ADS)

Tetreault, J. L.; Buiter, S. J. H.

2014-07-01

Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate during subduction by accretionary processes. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and have 3 distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. In addition many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. And other times we find evidence of collision leaving behind accreted terranes 25 to 40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to accrete and others to accrete. In many modern FATs on the ocean floor, a sub-crustal layer of high seismic velocities, interpreted as ultramafic material, could serve as a detachment or delaminate during subduction.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

NASA Astrophysics Data System (ADS)

Tetreault, J. L.; Buiter, S. J. H.

2014-12-01

Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and three distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane-continent collision leaving behind accreted terranes 25-40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to accrete and others to subduct. In many modern FATs on the ocean floor, a sub-crustal layer of high seismic velocities, interpreted as ultramafic material, could serve as a detachment or delaminate during subduction.

Understanding Intermediate-Level Speakers' Strengths and Weaknesses: An Examination of OPIc Tests from Korean Learners of English

ERIC Educational Resources Information Center

Cox, Troy L.

2017-01-01

This study profiled Intermediate-level learners in terms of their linguistic characteristics and performance on different proficiency tasks. A stratified random sample of 300 Korean learners of English with holistic ratings of Intermediate Low (IL), Intermediate Mid (IM), and Intermediate High (IH) on Oral Proficiency Interviews-computerized…

Petrogenesis of Cretaceous (133-84 Ma) intermediate dykes and host granites in southeastern China: Implications for lithospheric extension, continental crustal growth, and geodynamics of Palaeo-Pacific subduction

NASA Astrophysics Data System (ADS)

Yang, Jinbao; Zhao, Zhidan; Hou, Qingye; Niu, Yaoling; Mo, Xuanxue; Sheng, Dan; Wang, Lili

2018-01-01

This paper presents U-Pb zircon geochronology, petrology, and major and trace element, Sr-Nd and zircon Hf isotopic geochemistry of Cretaceous granites and intermediate dykes in the Quanzhou and Xiamen regions of southeastern China. These data are used to investigate igneous petrogenesis and Cretaceous tectonic evolution, and interpret the geodynamics of Palaeo-Pacific slab subduction. Granites in Quanzhou and Xiamen range in age from 133 Ma to 87 Ma, have high SiO2 and K2O contents, low abundances in P2O5, and an A/CNK index that ranges from 0.97 to 1.09, indicating that they are high-K calc-alkaline metaluminous I-type rocks. Slightly negative ɛ Nd (t) values (- 1.2 to - 4.4), young Nd model ages (0.87 Ga to 1.20 Ga) and positive ɛ Hf (t) values (- 0.5 to + 9.9) of zircon grains indicate that the granites were derived from magmas that melted amphibolite in the middle-lower crust, and which may have assimilated country rocks during emplacement in shallow chambers. The intermediate dykes have no genetic link to the granites and magma mixing was negligible. Eight dyke samples have low SiO2 and high MgO, Ni and Cr contents. Negative ε Nd (t) values (- 1.5 to - 2.7) and positive ε Hf (t) values (2.7 to 7.6) suggest that the dykes were derived from residual basic lower crust after mafic-crystal accumulation. Two samples of adakite-like dykes are characterised by high Sr/Y ratios (89 to 100) and high SiO2, low K2O, Ni, Cr contents. In combination with slightly negative ε Nd (t) values (- 1.7 to - 1.8) and positive ε Hf (t) values (2.9 to 4.3), the adakite-like dykes were derived from cumulate basic lower crust which had a mixed source between depleted mantle- and crust-derived melts. Based on our data, combined with previously published work, we suggest that extension-induced middle-lower crustal melting and underplating of mantle-derived basaltic melts were the principal driving mechanisms for Cretaceous granitic magmatism in coastal Fujian Province. Extension was related to subduction retreat whereas steep slab subduction caused underplating of mantle-derived basaltic melts. These processes were coupled and mainly responsible for the tectonic transition during the Cretaceous from compression to extension in the coastal belt of the Cathaysia Plate.

Amphibole Fractional Crystallization and Delamination in Arc Roots: Implications for the `Missing' Nb Reservoir in the Earth

NASA Astrophysics Data System (ADS)

Galster, F.; Chatterjee, R. N.; Stockli, D. F.

2017-12-01

Most geologic processes should not fractionate Nb from Ta but Earth's major silicate reservoirs have subchondritic Nb/Ta values. Nb/Ta of >10000 basalts and basaltic andesites from different tectonic settings (GEOROC) cluster around 16, indistinguishable from upper mantle values. In contrast, Nb/Ta in more evolved arc volcanics have progressively lower values, reaching continental crust estimates, and correlate negatively with SiO2 (see figure) and positively with TiO2 and MgO. This global trend suggests that differentiation processes in magmatic arcs could explain bulk crustal Nb/Ta estimates. Understanding processes that govern fractionation of Nb from Ta in arcs can provide key insights on continental crust formation and help identify Earth's `missing' Nb reservoir. Ti-rich phases (rutile, titanite and ilmenite) have DNb/DTa <1, and therefore, their fractionation from mafic to intermediate liquids cannot explain the observed trend. Instead, fractionation of biotite and amphibole could lower Nb/Ta values in the evolved liquid. Lack of correlation between Nb/Ta and K2O in global volcanic rocks implies that biotite plays a minor role in fractionating Nb from Ta during differentiation. Experimental petrology and evidence from exposed arc sections indicate that amphibole fractionation and delamination of island arc roots can explain the andesitic composition of bulk continental crust. Experimental studies have shown that amphibole Mg# correlate with DNb/DTa and amphibole could effectively fractionate Nb from Ta. Preliminary data from lower to middle crustal amphiboles from preserved arcs show sub- to super-chondritic Nb/Ta up to >60. This suggests that delamination of amphibole-rich cumulates can be a viable mechanism for the preferential removal of Nb from the continental crust. Future examination of Nb/Ta ratios in lower crustal amphiboles from various preserved arcs will provide improved constraints on the Nb-Ta paradox of the silicate Earth.

Evaluating the influence of chemical weathering on the composition of the continental crust using lithium and its isotopes

NASA Astrophysics Data System (ADS)

Rudnick, R. L.; Liu, X.

2011-12-01

The continental crust has an "intermediate" bulk composition that is distinct from primary melts of peridotitic mantle (basalt or picrite). This mismatch between the "building blocks" and the "edifice" of the continental crust points to the operation of processes that preferentially remove mafic to ultramafic material from the continents. Such processes include lower crustal recycling (via density foundering or lower crustal subduction - e.g., relamination, Hacker et al., 2011, EPSL), generation of evolved melts via slab melting, and/or chemical weathering. Stable isotope systems document the influence of chemical weathering on the bulk crust composition: the oxygen isotope composition of the bulk crust is distinctly heavier than that of primary, mantle-derived melts (Simon and Lecuyer, 2005, G-cubed) and the Li isotopic composition of the bulk crust is distinctly lighter than that of mantle-derive melts (Teng et al., 2004, GCA; 2008, Chem. Geol.). Both signatures mark the imprint of chemical weathering on the bulk crust composition. Here, we use a simple mass balance model for lithium inputs and outputs from the continental crust to quantify the mass lost due to chemical weathering. We find that a minimum of 15%, a maximum of 60%, and a best estimate of ~40% of the original juvenile rock mass may have been lost via chemical weathering. The accumulated percentage of mass loss due to chemical weathering leads to an average global chemical weathering rate (CWR) of ~ 8×10^9 to 2×10^10 t/yr since 3.5 Ga, which is about an order of magnitude higher than the minimum estimates based on modern rivers (Gaillardet et al., 1999, Chem. Geol.). While we cannot constrain the exact portion of crustal mass loss via chemical weathering, given the uncertainties of the calculation, we can demonstrate that the weathering flux is non-zero. Therefore, chemical weathering must play a role in the evolution of the composition and mass of the continental crust.

A Geochemical Comparison of the Northern Peninsular Ranges Batholith in Southern California and the Coastal Batholith in Southern Peru

NASA Astrophysics Data System (ADS)

Clausen, B. L.; Martínez Ardila, A. M.; Morton, D. M.

2010-12-01

An extensive geochemical data set from the northern Peninsular Ranges Batholith (PRB) in southern California is compared and contrasted with the Arequipa segment of the Peruvian Coastal Batholith, including new granitoid samples recently collected near Ica (14°S, 76°W). The data include major and trace elements and Sr isotope ratios. This is part of an on-going study of subduction-related magmatism to refine a petrogenetic model of crust formation at plate boundaries, with a particular interest in the role of magma mixing. Research in the northern PRB suggests that continental crust is formed in several cycles: (1) mantle melting to give mafic volcanics and gabbroic intrusives, (2) basalt/gabbro melting to give felsic granitoids uncontaminated by continental crust and having low initial 87Sr/86Sr (Sri) values less than 0.704, and (3) crustal melting to give high Sri values greater than 0.704. Geochemical evidence was used to determine the extent of mixing between mafic and felsic magma that produced rocks of intermediate SiO2 composition. These differentiation cycles formed a west to east chronologic sequence and yielded granitoids of gabbro, tonalite, and granodiorite composition. Using principal component analysis on the northern PRB granitoids, the four factors affecting geochemical composition were categorized as differentiation, crustal contamination, depth of magma source, and conditions that yield a range from calcic to more alkaline granitoids. A similar major and trace element analysis is being done for a classic result of subduction in the Peruvian Coastal Batholith. The Peruvian samples recently collected include granitoids of the upper Cretaceous Coastal Batholith, as well as the associated volcanics of Cretaceous and Jurassic age. The Coastal Batholith samples include a range of granitoids from the early gabbros and from the four batholithic super-units (from west to east: Linga, Pampahuasi, Tiabaya, and Incahuasi) containing a combination of diorite, tonalite, monzonite, and granodiorite. In addition, samples were taken from the adjacent Precambrian basement and Paleozoic San Nicolas batholith in order to estimate the local geochemistry for continental crust contamination and to compare with published crustal averages. For this part of the Coastal Batholith, gabbroic plutons that formed during extension have little contamination from continental crust; the time-separated super-units form distinct assemblages; some of the super-units went through several differentiation cycles; parts of the Linga super-unit have low Sri ratios below 0.704; and magma mixing played an important role. The Coastal Batholith of western Peru (similar to the eastern PRB) has an intermediate calc-alkaline composition, in comparison to batholiths in Chile (similar to the western PRB) that are more calcic and those in eastern Peru (similar to the Sierra Nevadas) that are more alkaline.

Mantle downwelling and crustal convergence - A model for Ishtar Terra, Venus

NASA Technical Reports Server (NTRS)

Kiefer, Walter S.; Hager, Bradford H.

1991-01-01

Models of viscous crustal flow driven by gradients in topography are presented in order to explore quantitatively the implications of the hypothesis that Ishtar is a crustal convergence zone overlying a downwelling mantle. Assuming a free-slip surface boundary condition, it is found that, if the crustal convergence hypothesis is correct, then the crustal thickness in the plains surrounding Ishtar can be no more than about 25 km thick. If the geothermal gradient is larger or the rheology is weaker, the crust must be even thinner for net crustal convergence to be possible. This upper bound is in good agreement with the several independent estimates of crustal thickness of 15-30 km in the plains of Venus based on modeling of the spacing of tectonic features and of impact crater relaxation. Although Ishtar is treated as a crustal convergence zone, this crustal flow model shows that under some circumstances, near-surface material may actually flow away from Ishtar, providing a possible explanation for the grabenlike structures in Fortuna Tessera.

The Phillips pluton, Maine, USA: evidence of heterogeneous crustal sources and implications for granite ascent and emplacement mechanisms in convergent orogens

NASA Astrophysics Data System (ADS)

Pressley, Rachel A.; Brown, Michael

1999-03-01

The Phillips pluton (age of 403.8±1.3 Ma) was assembled at a crustal level below the contemporary brittle-plastic transition during regional dextral-reverse transpressive deformation. The pluton is composed dominantly of medium- to coarse-grained leucogranite sensu lato (s.l.), but within its bounds includes decametric massive outcrop of fine- to medium-grained granodiorite (s.l.). In places, the leucogranite contains centimetric enclaves apparently of the granodiorite. Granodiorite is host to more biotite than muscovite, and more calcic, oscillatory-zoned plagioclase, compared to the leucogranite. Pegmatitic granite and composite pegmatite-aplite occur as metric sheets within the pluton and as larger bodies outside the pluton to the SW. Magmatic fabrics, defined by biotite schlieren, occur locally in the leucogranite; the attitude of these fabrics and layering within the leucogranite are concordant with the NE-striking, steeply-dipping country rock foliation. K 2O contents, Rb/Sr ratios, Rb, Sr and Ba covariations, and chondrite-normalized rare earth element (REE) patterns of leucogranite are consistent with high-to-moderate a(H 2O) muscovite dehydration equilibrium eutectic melting of a predominantly pelite source similar to metasedimentary rocks of the surrounding central Maine belt (CMB). The REE patterns and Rb/Sr ratios of granodiorite also suggest derivation from a metasedimentary source, but more likely by moderate-to-low a(H 2O) (muscovite-) biotite dehydration equilibrium eutectic to non-eutectic (minimum) melting of a protolith dominated by greywacke in which garnet and plagioclase were residual phases. Both granite (s.l.) types have heterogeneous initial Nd isotope compositions. Samples of granodiorite define a range in ɛNd (404 Ma) of -1.8 to +0.1 (±0.3 2 σ uncertainty), and samples of leucogranite define a range in ɛNd (404 Ma) of -8.0 to -5.3 (±0.3 2 σ uncertainty). This bimodal distribution suggests that melts were derived from a minimum of two sources. The data are consistent with these sources being CMB metasedimentary rocks ( ɛNd (404 Ma)<-4) for the leucogranite, and Avalon-like (peri-Gondwanan) metasedimentary crust ( ɛNd (404 Ma)>-4) for the granodiorite. The range of Nd isotope compositions within each granite type most likely reflects isotopic heterogeneity inherited from the source. These data imply that the integrity of individual melt batches was maintained during ascent, and that extensive mixing of melt batches during emplacement at this level in the pluton did not occur, although centimetric enclaves have intermediate Nd isotope compositions consistent with small-scale interactions between magmas. We infer that the Phillips pluton represents the root of a larger pluton, and that what remains of this larger pluton is the feeder constructed from multiple melt batches arrested during waning flow of granite magma through a crustal-scale shear zone system.

Variations in the crustal structure beneath western Turkey

NASA Astrophysics Data System (ADS)

Saunders, Paul; Priestley, Keith; Taymaz, Tuncay

1998-08-01

We use teleseismic receiver functions to investigate the crustal structure at two locations in western Turkey using seismic data recorded on small arrays of temporary broad-band seismographs. The results from these analyses are compared with receiver function results from the GDSN station ANTO on the Anatolian Plateau in central Turkey. The crust is ~ 30 km thick in the region of western Turkey where active normal faulting reveals present-day extension in the upper crust and alkali-basaltic volcanism reveals recent extension within the subcrustal lithosphere The crust is ~ 34 km thick further east where crustal extension is still evident but less pronounced. In the Anatolian Plateau, which is not currently extending, the crust is ~ 38 km thick. The level of extension estimated from these measurements of crustal thickness implies a β -factor of ~ 1.2. This value agrees with the amount of extension estimated in the upper crust from the integrated seismic strain rate (β -factor of ~ 1.3), from surface faulting(β -factor of ~ 1.25) and from the amount of extension in the subcrustal lithosphere estimated from the volcanism (β -factor < 2), all indicating that the extension is approximately uniformly distributed vertically throughout the lithosphere. The Moho transition in this region appears to thin slightly as the degree of extension increases westwards.

The topography of a continental indenter: The interplay between crustal deformation, erosion, and base level changes in the eastern Southern Alps

PubMed Central

Heberer, B.; Prasicek, G.; Neubauer, F.; Hergarten, S.

2017-01-01

Abstract The topography of the eastern Southern Alps (ESA) reflects indenter tectonics causing crustal shortening, surface uplift, and erosional response. Fluvial drainages were perturbed by Pleistocene glaciations that locally excavated alpine valleys. The Late Miocene desiccation of the Mediterranean Sea and the uplift of the northern Molasse Basin led to significant base level changes in the far field of the ESA and the Eastern Alps (EA), respectively. Among this multitude of mechanisms, the processes that dominate the current topographic evolution of the ESA and the ESA‐EA drainage divide have not been identified. We demonstrate the expected topographic effects of each mechanism in a one‐dimensional model and compare them with observed channel metrics. We find that the normalized steepness index increases with uplift rate and declines from the indenter tip in the northwest to the foreland basin in the southeast. The number and amplitude of knickpoints and the distortion in longitudinal channel profiles similarly decrease toward the east. Changes in slope of χ‐transformed channel profiles coincide spatially with the Valsugana‐Fella fault linking crustal stacking and uplift induced by indenter tectonics with topographic evolution. Gradients in χ across the ESA‐EA drainage divide imply an ongoing, north directed shift of the Danube‐ESA watershed that is most likely driven by a base level rise in the northern Molasse basin. We conclude that the regional uplift pattern controls the geometry of ESA‐EA channels, while base level changes in the far field control the overall architecture of the orogen by drainage divide migration. PMID:28344912

The topography of a continental indenter: The interplay between crustal deformation, erosion, and base level changes in the eastern Southern Alps.

PubMed

Robl, J; Heberer, B; Prasicek, G; Neubauer, F; Hergarten, S

2017-01-01

The topography of the eastern Southern Alps (ESA) reflects indenter tectonics causing crustal shortening, surface uplift, and erosional response. Fluvial drainages were perturbed by Pleistocene glaciations that locally excavated alpine valleys. The Late Miocene desiccation of the Mediterranean Sea and the uplift of the northern Molasse Basin led to significant base level changes in the far field of the ESA and the Eastern Alps (EA), respectively. Among this multitude of mechanisms, the processes that dominate the current topographic evolution of the ESA and the ESA-EA drainage divide have not been identified. We demonstrate the expected topographic effects of each mechanism in a one-dimensional model and compare them with observed channel metrics. We find that the normalized steepness index increases with uplift rate and declines from the indenter tip in the northwest to the foreland basin in the southeast. The number and amplitude of knickpoints and the distortion in longitudinal channel profiles similarly decrease toward the east. Changes in slope of χ -transformed channel profiles coincide spatially with the Valsugana-Fella fault linking crustal stacking and uplift induced by indenter tectonics with topographic evolution. Gradients in χ across the ESA-EA drainage divide imply an ongoing, north directed shift of the Danube-ESA watershed that is most likely driven by a base level rise in the northern Molasse basin. We conclude that the regional uplift pattern controls the geometry of ESA-EA channels, while base level changes in the far field control the overall architecture of the orogen by drainage divide migration.

Crustal structure of Precambrian terranes in the southern African subcontinent with implications for secular variation in crustal genesis

NASA Astrophysics Data System (ADS)

Kachingwe, Marsella; Nyblade, Andrew; Julià, Jordi

2015-07-01

New estimates of crustal thickness, Poisson's ratio and crustal shear wave velocity have been obtained for 39 stations in Angola, Botswana, the Democratic Republic of Congo, Malawi, Mozambique, Namibia, Rwanda, Tanzania and Zambia by modelling P-wave receiver functions using the H-κ stacking method and jointly inverting the receiver functions with Rayleigh-wave phase and group velocities. These estimates, combined with similar results from previous studies, have been examined for secular trends in Precambrian crustal structure within the southern African subcontinent. In both Archean and Proterozoic terranes we find similar Moho depths [38-39 ± 3 km SD (standard deviation)], crustal Poisson's ratio (0.26 ± 0.01 SD), mean crustal shear wave velocity (3.7 ± 0.1 km s-1 SD), and amounts of heterogeneity in the thickness of the mafic lower crust, as defined by shear wave velocities ≥4.0 km s-1. In addition, the amount of variability in these crustal parameters is similar within each individual age grouping as between age groupings. Thus, the results provide little evidence for secular variation in Precambrian crustal structure, including between Meso- and Neoarchean crust. This finding suggests that (1) continental crustal has been generated by similar processes since the Mesoarchean or (2) plate tectonic processes have reworked and modified the crust through time, erasing variations in structure resulting from crustal genesis.

Influence of stretching and density contrasts on the chemical evolution of continental magmas: An example from the Ivrea-Verbano Zone

USGS Publications Warehouse

Sinigoi, S.; Quick, J.E.; Mayer, A.; Budahn, J.

1996-01-01

The southern Ivrea-Verbano Zone of the Italian Western Alps contains a huge mafic complex that intruded high-grade metamorphic rocks while they were resident in the lower crust. Geologic mapping and chemical variations of the igneous body were used to study the evolution of underplated crust. Slivers of crustal rocks (septa) interlayered with igneous mafic rocks are concentrated in a narrow zone deep in the complex (Paragneiss-bearing Belt) and show evidence of advanced degrees of partial melting. Variations of rare-earth-element patterns and Sr isotope composition of the igneous rocks across the sequence are consistent with increasing crustal contamination approaching the septa. Therefore, the Paragneiss-bearing Belt is considered representative of an "assimilation region" where in-situ interaction between mantle- and crust-derived magmas resulted in production of hybrid melts. Buoyancy caused upwards migration of the hybrid melts that incorporated the last septa and were stored at higher levels, feeding the Upper Mafic Complex. Synmagmatic stretching of the assimilation region facilitated mixing and homogenization of melts. Chemical variations of granitoids extracted from the septa show that deep septa are more depleted than shallow ones. This suggests that the first incorporated septa were denser than the later ones, as required by the high density of the first-injected mafic magmas. It is inferred that density contrasts between mafic melts and crustal rocks play a crucial role for the processes of contamination of continental magmas. In thick under- plated crust, the extraction of early felsic/hybrid melts from the lower crust may be required to increase the density of the lower crust and to allow the later mafic magmas to penetrate higher crustal levels.

Slab detachment during continental collision: Influence of crustal rheology and interaction with lithospheric delamination

NASA Astrophysics Data System (ADS)

Duretz, T.; Gerya, T. V.

2013-08-01

Collision between continents can lead to the subduction of continental material. If the crust remains coupled to the downgoing slab, a large buoyancy force is generated. This force slows down convergence and promotes slab detachment. If the crust resists to subduction, it may decouple from the downgoing slab and be subjected to buoyant extrusion. We employ two-dimensional thermo-mechanical modelling to study the importance of crustal rheology on the evolution of subduction-collision systems. We propose simple quantifications of the mechanical decoupling between lithospheric levels (σ*) and the potential for buoyant extrusion of the crust (ξ*). The modelling results indicate that a variable crustal rheological structure results in slab detachment, delamination, or the combination of both mechanisms. A strong crust provides coupling at the Moho (low σ*) and remains coherent during subduction (low ξ). It promotes deep subduction of the crust (180 km) and slab detachment. Exhumation occurs in coherent manners via eduction and thrusting. Slab detachment triggers the development of topography (> 4.5 km) close to the suture. A contrasting style of collision occurs using a weak crustal rheology. Mechanical decoupling at the Moho (high σ*) promotes the extrusion of the crust (high ξ), disabling slab detachment. Ongoing shortening leads to buckling of the crust and development of topography on the lower plate. Collisions involving rheologically layered crust allow decoupling at mid-crustal depths. This structure favours both the extrusion of upper crust and the subduction of the lower crust. Such collisions are successively affected by delamination and slab detachment. Topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions.

Crustal Structure of the Tengchong Intra-plate Volcanic Area

NASA Astrophysics Data System (ADS)

Qian, Rongyi; Tong, Vincent C. H.

2015-09-01

We here provide an overview of our current understanding of the crustal structure of Tengchong in southwest China, a key intra-plate volcanic area along the Himalayan geothermal belt. Given that there is hitherto a lack of information about the near-surface structure of intra-plate volcanic areas, we present the first seismic reflection and velocity constraints on the shallow crust between intra-plate volcanoes. Our near-surface seismic images reveal the existence of dome-shaped seismic reflectors (DSRs) in the shallow crust between intra-plate volcanic clusters in Tengchong. The two DSRs are both ~2 km wide, and the shallowest parts of the DSRs are found at the depth of 200-300 m. The velocity model shows that the shallow low-velocity layer (<4 km/s) is anomalously thick (~1 km) in the region where the DSRs are observed. The presence of DSRs indicates significant levels of intra-plate magmatism beneath the along-axis gap separating two volcano clusters. Along-axis gaps between volcano clusters are therefore not necessarily an indicator of lower levels of magmatism. The seismic images obtained in this technically challenging area for controlled-source seismology allow us to conclude that shallow crustal structures are crucial for understanding the along-axis variations of magmatism and hydrothermal activities in intra-plate volcanic areas.

Thinning Factors and Crustal Thicknesses at the Propagating Tip of Sea-floor Spreading in the Woodlark Basin

NASA Astrophysics Data System (ADS)

Gozzard, S. P.; Kusznir, N.; Goodliffe, A.; Manatschal, G.

2007-12-01

Understanding how the continental crust and lithosphere thins at the propagating tip of sea-floor spreading is the key to understanding the continental breakup process. The Woodlark Basin, a young ocean basin located in the Western Pacific to the east of Papua New Guinea, commenced formation at approximately 8.4Ma and is propagating westwards at a rate of approximately 140km/Myr. Immediately to the west of the most recent segment of sea-floor spreading propagation, in the vicinity of the Moresby Seamount, evidence from bathymetry, subsidence and seismic Moho depth suggests that continental lithosphere is being thinned. In this study we have determined lithosphere thinning in the vicinity of the Moresby Seamount at the level of the whole lithosphere, the whole crust and the upper crust. Whole lithosphere thinning factors have been determined from subsidence analysis; whole continental crustal thinning factors have been determined from gravity inversions and upper crustal thinning factors have been determined from fault analysis. Three 2D seismic profiles surrounding the Moresby Seamount have been flexurally backstripped to the base of the syn-rift sediments to determine the water loaded subsidence. Using the McKenzie lithosphere extension model, modified to include volcanic addition at high thinning factors, whole thinning factors for the lithosphere have been determined from the water loaded subsidence. Results show that thermal subsidence alone cannot account for the observed subsidence, and that an additional initial subsidence is needed. Whole lithosphere thinning factors increase from an average of 0.5 to 0.8 across the Moresby Seamount eastwards towards the propagating tip. A satellite gravity inversion incorporating a lithosphere thermal gravity anomaly correction has been used to determine Moho depth, crustal thickness and thinning factors for the propagating tip in the Woodlark Basin. Moho depths are consistent with depths obtained from receiver function analysis (Ferris et al. 2006). Crustal thickness estimates do not include a correction for sediment thickness and are upper bounds. Crustal thinning factors in the vicinity of the Moresby Seamount are similar to those observed for the whole lithosphere. Fault analysis of the three 2D profiles have been used to determine upper crustal thinning factors. Upper crustal thinning factors between 0.1 to 0.2 are observed for the vicinity of the Moresby Seamount, substantially lower than thinning factors predicted for the whole lithosphere and continental crust, suggesting depth-dependent lithosphere thinning. Crustal thicknesses predicted from gravity inversion immediately to the east of the Moresby Seamount are substantially greater than would be expected for oceanic lithosphere in this region, while highly thinned, has not completely ruptured.

Bouguer gravity trends and crustal structure of the Palmyride Mountain belt and surrounding northern Arabian platform in Syria

NASA Astrophysics Data System (ADS)

Best, John A.; Barazangi, Muawia; Al-Saad, Damen; Sawaf, Tarif; Gebran, Ali

1990-12-01

This study examines the crustal structure of the Palmyrides and the northern Arabian platform in Syria by two- and three-dimensional modeling of the Bouguer gravity anomalies. Results of the gravity modeling indicate that (1) western Syria is composed of at least two different crustal blocks, (2) the southern crustal block is penetrated by a series of crustal-scale, high-density intrusive complexes, and (3) short-wavelength gravity anomalies in the southwest part of the mountain belt are clearly related to basement structure. The crustal thickness in Syria, as modeled on the gravity profiles, is approximately 40 ±4 km, which is similar to crustal thicknesses interpreted from refraction data in Jordan and Saudi Arabia. The different crustal blocks and large-scale mafic intrusions are best explained, though not uniquely, by Proterozoic convergence and suturing and early Paleozoic rifting, as interpreted in the exposed rocks of the Arabian shield. These two processes, combined with documented Mesozoic rifting and Cenozoic transpression, compose the crustal evolution of the northern Arabian platform beneath Syria.

Trace element differences between Archean, Proterozoic and Phanerozoic crustal components: Implications for crustal growth processes

NASA Technical Reports Server (NTRS)

Tarney, J.; Wyborn, L. E. A.; Sheraton, J. W.; Wyborn, D.

1988-01-01

Critical to models for continental crust growth and recycling are the processes through which crustal growth takes place. In particular, it is important to know whether these processes have changed fundamentally with time in response to the earth's thermal evolution, and whether the crustal compositions generated are compatible with crustal remobilization, crustal recycling, or represent primary additions. There are some significant and consistent differences in the major and trace element compositions of crustal components with time which have important implications for crustal growth processes. These will be illustrated with reference to Archean rocks from a number of shield areas, Proterozoic granitoids from Australia and elsewhere, Palaeozoic granitoids from Australia and Scotland, and Mesozoic - recent granitoids from present continental margin belts. Surprisingly some rather simple and consistent patterns energy using this technique. There are then significant differences in compositions of granitoid crustal additions throughout geological time, with a particular type of granitoid apparently dominating a particular time period. This implies that the tectonic processes giving rise to granite generation have changed in response to the earth's thermal evolution.

Performance of Irikura recipe rupture model generator in earthquake ground motion simulations with Graves and Pitarka hybrid approach

USGS Publications Warehouse

Pitarka, Arben; Graves, Robert; Irikura, Kojiro; Miyake, Hiroe; Rodgers, Arthur

2017-01-01

We analyzed the performance of the Irikura and Miyake (Pure and Applied Geophysics 168(2011):85–104, 2011) (IM2011) asperity-based kinematic rupture model generator, as implemented in the hybrid broadband ground motion simulation methodology of Graves and Pitarka (Bulletin of the Seismological Society of America 100(5A):2095–2123, 2010), for simulating ground motion from crustal earthquakes of intermediate size. The primary objective of our study is to investigate the transportability of IM2011 into the framework used by the Southern California Earthquake Center broadband simulation platform. In our analysis, we performed broadband (0–20 Hz) ground motion simulations for a suite of M6.7 crustal scenario earthquakes in a hard rock seismic velocity structure using rupture models produced with both IM2011 and the rupture generation method of Graves and Pitarka (Bulletin of the Seismological Society of America, 2016) (GP2016). The level of simulated ground motions for the two approaches compare favorably with median estimates obtained from the 2014 Next Generation Attenuation-West2 Project (NGA-West2) ground motion prediction equations (GMPEs) over the frequency band 0.1–10 Hz and for distances out to 22 km from the fault. We also found that, compared to GP2016, IM2011 generates ground motion with larger variability, particularly at near-fault distances (<12 km) and at long periods (>1 s). For this specific scenario, the largest systematic difference in ground motion level for the two approaches occurs in the period band 1–3 s where the IM2011 motions are about 20–30% lower than those for GP2016. We found that increasing the rupture speed by 20% on the asperities in IM2011 produced ground motions in the 1–3 s bandwidth that are in much closer agreement with the GMPE medians and similar to those obtained with GP2016. The potential implications of this modification for other rupture mechanisms and magnitudes are not yet fully understood, and this topic is the subject of ongoing study. We concluded that IM2011 rupture generator performs well in ground motion simulations using Graves and Pitarka hybrid method. Therefore, we recommend it to be considered for inclusion into the framework used by the Southern California Earthquake Center broadband simulation platform.

Performance of Irikura Recipe Rupture Model Generator in Earthquake Ground Motion Simulations with Graves and Pitarka Hybrid Approach

NASA Astrophysics Data System (ADS)

Pitarka, Arben; Graves, Robert; Irikura, Kojiro; Miyake, Hiroe; Rodgers, Arthur

2017-09-01

We analyzed the performance of the Irikura and Miyake (Pure and Applied Geophysics 168(2011):85-104, 2011) (IM2011) asperity-based kinematic rupture model generator, as implemented in the hybrid broadband ground motion simulation methodology of Graves and Pitarka (Bulletin of the Seismological Society of America 100(5A):2095-2123, 2010), for simulating ground motion from crustal earthquakes of intermediate size. The primary objective of our study is to investigate the transportability of IM2011 into the framework used by the Southern California Earthquake Center broadband simulation platform. In our analysis, we performed broadband (0-20 Hz) ground motion simulations for a suite of M6.7 crustal scenario earthquakes in a hard rock seismic velocity structure using rupture models produced with both IM2011 and the rupture generation method of Graves and Pitarka (Bulletin of the Seismological Society of America, 2016) (GP2016). The level of simulated ground motions for the two approaches compare favorably with median estimates obtained from the 2014 Next Generation Attenuation-West2 Project (NGA-West2) ground motion prediction equations (GMPEs) over the frequency band 0.1-10 Hz and for distances out to 22 km from the fault. We also found that, compared to GP2016, IM2011 generates ground motion with larger variability, particularly at near-fault distances (<12 km) and at long periods (>1 s). For this specific scenario, the largest systematic difference in ground motion level for the two approaches occurs in the period band 1-3 s where the IM2011 motions are about 20-30% lower than those for GP2016. We found that increasing the rupture speed by 20% on the asperities in IM2011 produced ground motions in the 1-3 s bandwidth that are in much closer agreement with the GMPE medians and similar to those obtained with GP2016. The potential implications of this modification for other rupture mechanisms and magnitudes are not yet fully understood, and this topic is the subject of ongoing study. We concluded that IM2011 rupture generator performs well in ground motion simulations using Graves and Pitarka hybrid method. Therefore, we recommend it to be considered for inclusion into the framework used by the Southern California Earthquake Center broadband simulation platform.

The influence of regional extensional tectonic stress on the eruptive behaviour of subduction-zone volcanoes

NASA Astrophysics Data System (ADS)

Tost, M.; Cronin, S. J.

2015-12-01

Regional tectonic stress is considered a trigger mechanism for explosive volcanic activity, but the related mechanisms at depth are not well understood. The unique geological setting of Ruapehu, New Zealand, allows investigation on the effect of enhanced regional extensional crustal tension on the eruptive behaviour of subduction-zone volcanoes. The composite cone is located at the southwestern terminus of the Taupo Volcanic Zone, one of the most active silicic magma systems on Earth, which extends through the central part of New Zealand's North Island. Rhyolitic caldera eruptions are limited to its central part where crustal extension is highest, whereas lower extension and additional dextral shear dominate in the southwestern and northeastern segments characterized by andesitic volcanism. South of Ruapehu, the intra-arc rift zone traverses into a compressional geological setting with updoming marine sequences dissected by reverse and normal faults. The current eruptive behaviour of Ruapehu is dominated by small-scaled vulcanian eruptions, but our studies indicate that subplinian to plinian eruptions have frequently occurred since ≥340 ka and were usually preceded by major rhyolitic caldera unrest in the Taupo Volcanic Zone. Pre-existing structures related to the NNW-SSE trending subduction-zone setting are thought to extend at depth and create preferred pathways for the silicic magma bodies, which may facilitate the development of large (>100 km3) dyke-like upper-crustal storage systems prior to major caldera activity. This may cause enhanced extensional stress throughout the entire intra-arc setting, including the Ruapehu area. During periods of caldera dormancy, the thick crust underlying the volcano and the enhanced dextral share rate likely impede ascent of larger andesitic magma bodies, and storage of andesitic melts dominantly occurs within small-scaled magma bodies at middle- to lower-crustal levels. During episodes of major caldera unrest, ascent and storage of voluminous rhyolitic magma bodies at upper crustal levels may cause the extensional stress to supercede the dextral shear rate in the Ruapehu area, facilitating ascent of larger andesitic magma bodies at depth, and changing the volcano's eruptive behaviour from dominantly vulcanian to violently subplinian/plinian.

Transient radon signals driven by fluid pressure pulse, micro-crack closure, and failure during granite deformation experiments

NASA Astrophysics Data System (ADS)

Girault, Frédéric; Schubnel, Alexandre; Pili, Éric

2017-09-01

In seismically active fault zones, various crustal fluids including gases are released at the surface. Radon-222, a radioactive gas naturally produced in rocks, is used in volcanic and tectonic contexts to illuminate crustal deformation or earthquake mechanisms. At some locations, intriguing radon signals have been recorded before, during, or after tectonic events, but such observations remain controversial, mainly because physical characterization of potential radon anomalies from the upper crust is lacking. Here we conducted several month-long deformation experiments under controlled dry upper crustal conditions with a triaxial cell to continuously monitor radon emission from crustal rocks affected by three main effects: a fluid pressure pulse, micro-crack closure, and differential stress increase to macroscopic failure. We found that these effects are systematically associated with a variety of radon signals that can be explained using a first-order advective model of radon transport. First, connection to a source of deep fluid pressure (a fluid pressure pulse) is associated with a large transient radon emission increase (factor of 3-7) compared with the background level. We reason that peak amplitude is governed by the accumulation time and the radon source term, and that peak duration is controlled by radioactive decay, permeability, and advective losses of radon. Second, increasing isostatic compression is first accompanied by an increase in radon emission followed by a decrease beyond a critical pressure representing the depth below which crack closure hampers radon emission (150-250 MPa, ca. 5.5-9.5 km depth in our experiments). Third, the increase of differential stress, and associated shear and volumetric deformation, systematically triggers significant radon peaks (ca. 25-350% above background level) before macroscopic failure, by connecting isolated cracks, which dramatically enhances permeability. The detection of transient radon signals before rupture indicates that connection of initially isolated cracks in crustal rocks may occur before rupture and potentially lead to radon transients measurable at the surface in tectonically active regions. This study offers thus an experimental and physical basis for understanding predicted or reported radon anomalies.

Transient radon signals driven by fluid pressure pulse, micro-crack closure, and failure during granite deformation experiments

NASA Astrophysics Data System (ADS)

Schubnel, A.; Girault, F.; Pili, E.

2017-12-01

In seismically active fault zones, various crustal fluids including gases are released at the surface. Radon-222, a radioactive gas naturally produced in rocks, is used in volcanic and tectonic contexts to illuminate crustal deformation or earthquake mechanisms. At some locations, intriguing radon signals have been recorded before, during, or after tectonic events, but such observations remain controversial, mainly because physical characterization of potential radon anomalies from the upper crust is lacking. Here we conducted several month-long deformation experiments under controlled dry upper crustal conditions with a triaxial cell to continuously monitor radon emission from crustal rocks affected by three main effects: a fluid pressure pulse, micro-crack closure, and differential stress increase to macroscopic failure. We found that these effects are systematically associated with a variety of radon signals that can be explained using a first-order advective model of radon transport. First, connection to a source of deep fluid pressure (a fluid pressure pulse) is associated with a large transient radon emission increase (factor of 3-7) compared with the background level. We reason that peak amplitude is governed by the accumulation time and the radon source term, and that peak duration is controlled by radioactive decay, permeability, and advective losses of radon. Second, increasing isostatic compression is first accompanied by an increase in radon emission followed by a decrease beyond a critical pressure representing the depth below which crack closure hampers radon emission (150-250 MPa, ca. 5.5-9.5 km depth in our experiments). Third, the increase of differential stress, and associated shear and volumetric deformation, systematically triggers significant radon peaks (ca. 25-350% above background level) before macroscopic failure, by connecting isolated cracks, which dramatically enhances permeability. The detection of transient radon signals before rupture indicates that connection of initially isolated cracks in crustal rocks may occur before rupture and potentially lead to radon transients measurable at the surface in tectonically active regions. This study offers thus an experimental and physical basis for understanding predicted or reported radon anomalies.

Spectral Estimation of Seismic Moment, Corner Frequency and Radiated Energy for Earthquakes in the Lesser Antilles

NASA Astrophysics Data System (ADS)

Satriano, C.; Mejia Uquiche, A. R.; Saurel, J. M.

2016-12-01

The Lesser Antilles are situated at a convergent plate boundary where the North- and South-American plates subduct below the Caribbean Plate at a rate of about 2 cm/y. The subduction forms the volcanic arc of Lesser Antilles and generates three types of seismicity: subduction earthquakes at the plate interface, intermediate depth earthquakes within the subducting oceanic plates and crustal earthquakes associated with the deformation of the Caribbean Plate. Even if the seismicity rate is moderate, this zone has generated in the past major earthquakes, like the subduction event on February 8, 1843, estimated M 8.5 (Beauducel et Feuillet, 2012), the Mw 6.3 "Les Saintes" crustal earthquake of November 24, 2004 (Drouet et al., 2011), and the Mw 7.4 Martinique intermediate earthquake of November 29, 2007 (Bouin et al., 2010). The seismic catalogue produced by the Volcanological and Seismological Observatories of Guadeloupe and Martinique comprises about 1000 events per year, most of them of moderate magnitude (M < 5.0). The observation and characterization of this background seismicity has a fundamental role in understanding the processes of energy accumulation and liberation preparing major earthquakes. For this reason, the catalogue needs to be completed by information like seismic moment, corner frequency and radiated energy which give access to important fault properties like the rupture size, the static and the apparent stress drop. So far, this analysis has only been performed for the "Les Saintes" sequence (Drouet et al., 2011). Here we present a systematic study of the Lesser Antilles merged seismic catalogue (http://www.seismes-antilles.fr), between 2002 and 2013, using broadband data from the West Indies seismic network and recordings from the French Accelerometric Network. The analysis is aimed at determining, from the inversion of S-wave displacement spectra, source parameters like seismic moment, corner frequency and radiated energy, as well as the inelastic attenuation factor. The results are discussed, for each type of seismicity in terms of scaling of corner frequency and energy release with seismic moment. We further discuss the steps realized to implement spectral analysis as an automated processing routine at the observatories of Guadeloupe and Martinique.

Modelling guided waves in the Alaskan-Aleutian subduction zone

NASA Astrophysics Data System (ADS)

Coulson, Sophie; Garth, Thomas; Reitbrock, Andreas

2016-04-01

Subduction zone guided wave arrivals from intermediate depth earthquakes (70-300 km depth) have a huge potential to tell us about the velocity structure of the subducting oceanic crust as it dehydrates at these depths. We see guided waves as the oceanic crust has a slower seismic velocity than the surrounding material, and so high frequency energy is retained and delayed in the crustal material. Lower frequency energy is not retained in this crustal waveguide and so travels at faster velocities of the surrounding material. This gives a unique observation at the surface with low frequency energy arriving before the higher frequencies. We constrain this guided wave dispersion by comparing the waveforms recorded in real subduction zones with simulated waveforms, produced using finite difference full waveform modelling techniques. This method has been used to show that hydrated minerals in the oceanic crust persist to much greater depths than accepted thermal petrological subduction zone models would suggest in Northern Japan (Garth & Rietbrock, 2014a), and South America (Garth & Rietbrock, in prep). These observations also suggest that the subducting oceanic mantle may be highly hydrated at intermediate depth by dipping normal faults (Garth & Rietbrock 2014b). We use this guided wave analysis technique to constrain the velocity structure of the down going ~45 Ma Pacific plate beneath Alaska. Dispersion analysis is primarily carried out on guided wave arrivals recorded on the Alaskan regional seismic network. Earthquake locations from global earthquake catalogues (ISC and PDE) and regional earthquake locations from the AEIC (Alaskan Earthquake Information Centre) catalogue are used to constrain the slab geometry and to identify potentially dispersive events. Dispersed arrivals are seen at stations close to the trench, with high frequency (>2 Hz) arrivals delayed by 2 - 4 seconds. This dispersion is analysed to constrain the velocity and width of the proposed waveguide. The velocity structure of this relatively young subducting plate is compared to the velocity structure resolved in the older oceanic lithosphere subducted beneath Northern Japan. We also use guided wave observations to investigate the thickness and low velocity structure of the subducting Yakutat terrain. Additionally we discuss the dependence of the inferred slab geometry on the earthquake catalogues that are used.

The crust of the Moon as seen by GRAIL.

PubMed

Wieczorek, Mark A; Neumann, Gregory A; Nimmo, Francis; Kiefer, Walter S; Taylor, G Jeffrey; Melosh, H Jay; Phillips, Roger J; Solomon, Sean C; Andrews-Hanna, Jeffrey C; Asmar, Sami W; Konopliv, Alexander S; Lemoine, Frank G; Smith, David E; Watkins, Michael M; Williams, James G; Zuber, Maria T

2013-02-08

High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moon's highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth.

Layered Crustal Anisotropy in the NE Tibetan Plateau Inferred from Ambient Noise Tomography

NASA Astrophysics Data System (ADS)

Jiang, C.; Yang, Y.; Zheng, Y.

2016-12-01

The Tibetan Plateau is the highest and largest plateau in the world with an average elevation of 4-5 km and 60-70 km thick crust, about twice of the thickness of average continental crust. Two end-member models have bene invoked to explain the crustal thickening and the growth of the plateau: (1) continuous and uniform thickening of the whole crust and (2) mid/lower crustal channel flow. However, which mechanism dominates the crustal thickening and the growth of the plateau is still under hot debate. Seismic anisotropy can provide observational constraints on deformation mode, which would have distinguished pattern resulting from the two different thickening models. Thus, by studying seismic anisotropy, we can distinguish different models of crustal thickening and plateau growth. In this study, we employ an eikonal tomography method of ambient noise to investigate azimuthal anisotropy of Rayleigh waves in the NE Tibetan Plateau. Our tomography reveals significant anisotropy in the crust. In particular, stratification of crustal azimuthal anisotropy is observed: an upper crustal anisotropic layer characterized by a NE-SW fast direction and a mid/lower crustal anisotropic layer with a NNE-SSW fast direction. The dominantly NE-SW oriented anisotropy in the upper crust is likely caused by shape-preferred orientation (SPO) of faults and fractures in the shallow depths. The anisotropy in the mid/lower crust, however, is nearly orthogonal to that in the shallow crust, suggesting a different mechanism. The NNE-SSW fast direction coincides with the proposed flow direction by the crustal flow model in NE Tibetan Plateau, suggesting anisotropy in the mid/lower crust may be related to the crustal flow. The two-layered crustal stratigraphy observed in the NE Tibetan Plateau is contrary to the continuous thickening model, but favours the crustal flow model.

Zircon petrochronology reveals the temporal link between porphyry systems and the magmatic evolution of their hidden plutonic roots (the Eocene Coroccohuayco deposit, Peru)

NASA Astrophysics Data System (ADS)

Chelle-Michou, Cyril; Chiaradia, Massimo; Ovtcharova, Maria; Ulianov, Alexey; Wotzlaw, Jörn-Frederik

2014-06-01

We present zircon geochronologic (LA-ICPMS and ID-TIMS), trace element and Hf isotopic evidence for a complex evolution of the plutonic roots of the Eocene Coroccohuayco porphyry system, southern Peru. LA-ICPMS U-Pb dating has initially been carried out to optimize grain selection for subsequent high-precision ID-TIMS dating and to characterize crustal assimilation (xenocrystic cores). This combined in-situ and whole-grain U-Pb dating of the same grains has been further exploited to derive a robust temporal interpretation of the complex magmatic system associated with the Coroccohuayco porphyry-skarn deposit. Our data reveal that a heterogeneous gabbrodioritic complex was emplaced at ca. 40.4 Ma and was followed by a nearly 5 Ma-long magmatic lull until the emplacement of dacitic porphyry stocks and dykes associated with the mineralizing event at ca. 35.6 Ma. However, at the sample scale, zircons from the porphyries provide insight into a 2 Ma-long lived “hidden” magmatism (probably at 4-9 km paleodepth) prior to porphyry intrusion and mineralization for which no other evidence can be found on the surface today. These dates together with zircon trace element analysis and Hf isotopes argue for the development of a long-lived magmatic system dominated by amphibole fractionation with an increasing amount of crustal assimilation and the development of a large and sustained thermal anomaly. The system was probably rejuvenated at an increasing rate from 37.5 to 35.6 Ma with injection of fresh and oxidized magma from the lower crust, which caused cannibalism and remelting of proto-plutons. The porphyry intrusions at Coroccohuayco were emplaced at the peak thermal conditions of this upper crustal magma chamber, which subsequently cooled and expelled ore fluids. Zircon xenocrysts and Hf isotopes in the porphyritic rocks suggest that this large upper crustal system evolved at stratigraphic levels corresponding to Triassic sediments similar to the Mitu group that may be present below the district. Using the zircon Ce anomaly as a proxy for oxidation state of the magma through time, we show that the high oxidation state of the porphyries is not the result of upper-crustal processes but is rather controlled by magmatic processes occurring at deeper levels. A comparison of our data with available high-precision geochronologic data at other porphyry systems suggests that such deposits may form when injection rate, volume and heat of their long-lived upper crustal magmatic system reach their peaks. These features might be diagnostic of a productive deposit.

Crustal Magnetization Model of Maud Rise in the Southwest Indian Ocean

NASA Technical Reports Server (NTRS)

Kim, Hyung Rae; vanFrese, Ralph R. B.; Golynsky, Alexander V.; Taylor, Patrick T.; Kim, Jeong Woo

2004-01-01

We modeled the crustal magnetization for the Maud Rise in the south-west Indian Ocean off the coast of East Antarctica using magnetic observations from the Oersted satellite and near-surface surveys complied by the Antarctic Digital Magnetic Anomaly Project (ADMAP). A new inversion modeling scheme of the multi-altitude anomaly fields suggests that the magnetic effects due to crustal thickness variations and remanence involving the normal polarity Cretaceous Quiet Zone (KQZ) become increasingly dominant with altitude. The magnetic crustal thickness effects were modeled in the Oersted data using crustal thickness variations derived from satellite altitude gravity data. Remanent magnetization modeling of the residual Oersted and near-surface magnetic anomalies supports extending the KQZ eastwards to the Astrid Ridge. The remaining near-surface anomalies involve crustal features with relatively high frequency effects that are strongly attenuated at satellite altitudes. The crustal modeling can be extended by the satellite magnetic anomalies across the Indian Ocean Ridge for insight on the crustal properties of the conjugate Agulhas Plateau. The modeling supports the Jurassic reconstruction of Gondwana when the African Limpopo-Zambezi and East Antarctic Princess Astrid coasts were connected as part of a relatively demagnetized crustal block.

Evidence for Terrane Accretion, Localized Rifting and Magmatism from the Crustal Velocity Structure of the Southeastern United States

NASA Astrophysics Data System (ADS)

Marzen, R. E.; Shillington, D. J.; Lizarralde, D.; Harder, S. H.

2017-12-01

The crustal structure in the Southeastern United States records a rich tectonic history, including multiple terrane accretion events, the formation of the supercontinent Pangea, widespread magmatism from the Central Atlantic Magmatic Province (CAMP), and crustal thinning before the breakup of Pangea. We use wide-angle refraction seismic data from Lines 1 and 2 of the SUGAR (SUwannee suture and GeorgiA Rift basin) seismic experiment to constrain crustal structure in order to better understand these tectonic events. The 320 and 420 km lines extend from the northwest to the southeast, crossing the Mesozoic rift basins that record crustal thinning prior to the breakup of Pangea and multiple potential suture zones between accreted terranes. We model crustal P-wave velocity structure with reflection/refraction tomography based on refractions through the sediments, crust and mantle and reflections from the base of the sediments, within the crust and the Moho. To the north on Line 2, we observe high Vp and Vs within the Inner Piedmont and Carolina accreted terranes underlain by a low velocity zone at 5 km depth. These observations are consistent with metamorphosed terranes accreting onto the Laurentian margin along a low velocity region that represents meta-sedimentary rocks and/or an Appalachian detachment. Additionally, differences in the basin structure, lower crustal velocities, and crustal thickness between Lines 1 and 2 reflect varying extension and magmatism between the two Mesozoic rift segments. Line 1 has thicker and more laterally extensive syn-rift sediments and a more pronounced region of crustal thinning. In contrast, syn-rift sediments along Line 2 are thinner and limited to a couple of smaller basins, and the crust of Line 2 gradually thins towards the coast. The thinned crust beneath Line 1 is characterized by high velocities of >7.0 km/s, which we interpret as mafic intrusions related to rifting or CAMP; in contrast, no evidence of elevated lower crustal velocities is observed on Line 2. Because intrusions into the lower crust increase both lower crustal velocities and crustal thickness, the correspondence of high lower crustal velocities with regions of greater crustal thinning suggests that extension and magmatism were more localized than one would infer based only on variations in crustal thickness.

Elementary Theoretical Forms for the Spatial Power Spectrum of Earth's Crustal Magnetic Field

NASA Technical Reports Server (NTRS)

Voorhies, C.

1998-01-01

The magnetic field produced by magnetization in Earth's crust and lithosphere can be distinguished from the field produced by electric currents in Earth's core because the spatial magnetic power spectrum of the crustal field differs from that of the core field. Theoretical forms for the spectrum of the crustal field are derived by treating each magnetic domain in the crust as the point source of a dipole field. The geologic null-hypothesis that such moments are uncorrelated is used to obtain the magnetic spectrum expected from a randomly magnetized, or unstructured, spherical crust of negligible thickness. This simplest spectral form is modified to allow for uniform crustal thickness, ellipsoidality, and the polarization of domains by an periodically reversing, geocentric axial dipole field from Earth's core. Such spectra are intended to describe the background crustal field. Magnetic anomalies due to correlated magnetization within coherent geologic structures may well be superimposed upon this background; yet representing each such anomaly with a single point dipole may lead to similar spectral forms. Results from attempts to fit these forms to observational spectra, determined via spherical harmonic analysis of MAGSAT data, are summarized in terms of amplitude, source depth, and misfit. Each theoretical spectrum reduces to a source factor multiplied by the usual exponential function of spherical harmonic degree n due to geometric attenuation with attitude above the source layer. The source factors always vary with n and are approximately proportional to n(exp 3) for degrees 12 through 120. The theoretical spectra are therefore not directly proportional to an exponential function of spherical harmonic degree n. There is no radius at which these spectra are flat, level, or otherwise independent of n.

Crustally derived granites in Dali, SW China: new constraints on silicic magmatism of the Central Emeishan Large Igneous Province

NASA Astrophysics Data System (ADS)

Zhu, Bei; Peate, David W.; Guo, Zhaojie; Liu, Runchao; Du, Wei

2017-10-01

We have identified a new crustally derived granite pluton that is related to the Emeishan Large Igneous Province (ELIP). This pluton (the Wase pluton, near Dali) shows two distinct SHRIMP zircon U-Pb age groups ( 768 and 253 Ma). As it has an intrusive relationship with Devonian limestone, the younger age is interpreted as its formation, which is related to the ELIP event, whereas the 768 Ma Neoproterozoic-aged zircons were inherited from Precambrian crustal component of the Yangtze Block, implying the pluton has a crustally derived origin. This is consistent with its peraluminous nature, negative Nb-Ta anomaly, enrichment in light rare earth elements, high 87Sr/86Sr(i) ratio (0.7159-0.7183) and extremely negative ɛ(Nd)(i) values (-12.15 to -13.70), indicative of melts derived from upper crust materials. The Wase pluton-intruded Devonian strata lie stratigraphically below the Shangcang ELIP sequence, which is the thickest volcanic sequence ( 5400 m) in the whole ELIP. The uppermost level of the Shangcang sequence contains laterally restricted rhyolite. Although the rhyolite has the same age as the Wase pluton, its geochemical features demonstrate a different magma origin. The rhyolite displays moderate 87Sr/86Sr(i) (0.7053), slightly negative ɛ(Nd)(i) (-0.18) and depletions in Ba, Cs, Eu and Sr, implying derivation from differentiation of a mantle-derived mafic magma source. The coexistence of crustally and mantle-derived felsic systems, along with the robust development of dike swarms, vent proximal volcanics and thickest flood basalts piles in Dali, shows that the Dali area was probably where the most active Emeishan magmatism had once existed.

New paleomagnetic data from the Wadi Abyad crustal section and their implications for the rotation history of the Oman ophiolite

NASA Astrophysics Data System (ADS)

Meyer, Matthew; Morris, Antony; Anderson, Mark; MacLeod, Chris

2015-04-01

The Oman ophiolite is an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution. Previous paleomagnetic research in lavas of the northern ophiolitic blocks (Perrin et al., 2000) has demonstrated substantial clockwise intraoceanic tectonic rotations. Paleomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization, and have been used to infer that clockwise rotations seen in the north are internal to the ophiolite rather than regionally significant (Weiler, 2000). Here we demonstrate the importance and advantages of sampling crustal transects in the ophiolite in order to understand the nature and variability in magnetization directions. By systematically sampling the lower crustal sequence exposed in Wadi Abyad (Rustaq block) we resolve for the first time in a single section a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to the Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used by Weiler (2000) to infer lack of significant rotation of the southern blocks and to argue, therefore, that rotation of the northern blocks was internal to the ophiolite. In contrast, E/ENE-directed remanences in the uppermost levels of Wadi Abyad imply large, clockwise rotation of the Rustaq block, of a sense and magnitude consistent with intraoceanic rotations inferred from extrusive sections in the northern blocks. We conclude that without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

The eye of the field geologist and the mind of the tectonician: one view of dynamic crustal rheology in convergent orogens

NASA Astrophysics Data System (ADS)

Brown, M.

2004-05-01

Orogens record evidence of interaction between converging plates. However, the response of continental crust to tectonic and gravitational loads is dependent on rheology, which is influenced by composition, architecture, thermal profile and strain rate. Crustal rocks undergo melting in deeper parts of orogens. Greywackes and metapelites are the most fertile protoliths, generating 20-50 and 30 vol. % melt respectively at 1 GPa and 1173K; geophysical data suggest >6 but <20 vol. % interconnected melt in deep crust of active orogens. In numerical models of orogens, the transition from coupled doubly-vergent wedge structure to plateau formation and full basal decoupling requires a viscosity drop of 4 orders of magnitude, inferred to be melt weakening. Deformation experiments on granite indicate a dramatic drop in strength (to 100-200 MPa) as the melt wetting transition is approached at 7 vol. % melt, and a more gradual decrease to <1 MPa prior to the drop at the solid-to-liquid transition (RCMP). Important properties of melting systems are viscosity of the melt, rheology of the crystalline framework of grains and permeability of this framework to flow. Permeability is due to an intergranular network of connected pores, compositional layering/fabric and networks of deformation bands; melt distribution is heterogeneous on multiple length scales. The microstructure of anatectic rocks and the magnitude of weakening accompanying melting suggest a limited role for intracrystalline plasticity with increasing vol. % melt and dominance of melt-assisted diffusion creep or diffusion accommodated granular flow. The intrinsic weakness of melt-bearing intervals in the crust makes them ideal detachment horizons. Observations from metasedimentary migmatitic granulites show preservation of (i) early fabrics, suggesting that the strain field emergent under subsolidus conditions controlled initial distribution of melt produced by suprasolidus mica breakdown, and (ii) layering in melt-depleted rocks, implying that they were quasi-continuously drained. Studies of migmatitic granulites demonstrate that melt migrates from grain boundaries to mesoscale networks of structures (mm to m) to steeply-inclined conduits recorded by rod or tabular granite intrusions (m to dm). Melt loss from lower crust yields residual rocks composed of strong minerals (feldspar, pyroxene and garnet) with only minor melt on grain boundaries. Thus, weakening of lower crust due to melting is followed by its strengthening. Around the brittle-to-viscous transition zone granite accumulates in subhorizontal tabular plutons, which implies transient presence of significantly weaker layers in shallow orogenic crust; these are potential detachment horizons. Field studies of exhumed orogens suggest deformation commonly is laterally, transversely and vertically diachronous, reflecting the spatial and temporal variation in the weakening-to-strengthening cycle. There may be important sub-horizontal movement horizons, which allow (partial) decoupling of crustal layers. At upper-to-middle crustal levels rocks are metamorphosed in greenschist-amphibolite facies, with local enhancement by pluton-advected heat to amphibolite-granulite facies and thrust-style brittle-ductile deformation (e.g., Acadian, NH). Rocks from middle crustal levels are in amphibolite facies and have penetrative steep fabrics (e.g., Acadian, western ME) or exhibit a complex network of shallowly- and steeply-dipping fabrics (e.g., St. Malo, France). Rocks from lower crustal levels are in amphibolite-granulite facies and have shallow fabrics due to crustal flow, although these may be steepened by later deformation including core-complex formation (e.g., S. Brittany, France).

Probabilistic seismic hazard assessment for the two layer fault system of Antalya (SW Turkey) area

NASA Astrophysics Data System (ADS)

Dipova, Nihat; Cangir, Bülent

2017-09-01

Southwest Turkey, along Mediterranean coast, is prone to large earthquakes resulting from subduction of the African plate under the Eurasian plate and shallow crustal faults. Maximum observed magnitude of subduction earthquakes is Mw = 6.5 whereas that of crustal earthquakes is Mw = 6.6. Crustal earthquakes are sourced from faults which are related with Isparta Angle and Cyprus Arc tectonic structures. The primary goal of this study is to assess seismic hazard for Antalya area (SW Turkey) using a probabilistic approach. A new earthquake catalog for Antalya area, with unified moment magnitude scale, was prepared in the scope of the study. Seismicity of the area has been evaluated by the Gutenberg-Richter recurrence relationship. For hazard computation, CRISIS2007 software was used following the standard Cornell-McGuire methodology. Attenuation model developed by Youngs et al. Seismol Res Lett 68(1):58-73, (1997) was used for deep subduction earthquakes and Chiou and Youngs Earthq Spectra 24(1):173-215, (2008) model was used for shallow crustal earthquakes. A seismic hazard map was developed for peak ground acceleration and for rock ground with a hazard level of a 10% probability of exceedance in 50 years. Results of the study show that peak ground acceleration values on bedrock change between 0.215 and 0.23 g in the center of Antalya.

The Variscan evolution in the External massifs of the Alps and place in their Variscan framework

NASA Astrophysics Data System (ADS)

von Raumer, Jürgen F.; Bussy, François; Stampfli, Gérard M.

2009-02-01

In the general discussion on the Variscan evolution of central Europe the pre-Mesozoic basement of the Alps is, in many cases, only included with hesitation. Relatively well-preserved from Alpine metamorphism, the Alpine External massifs can serve as an excellent example of evolution of the Variscan basement, including the earliest Gondwana-derived microcontinents with Cadomian relics. Testifying to the evolution at the Gondwana margin, at least since the Cambrian, such pieces took part in the birth of the Rheic Ocean. After the separation of Avalonia, the remaining Gondwana border was continuously transformed through crustal extension with contemporaneous separation of continental blocks composing future Pangea, but the opening of Palaeotethys had only a reduced significance since the Devonian. The Variscan evolution in the External domain is characterised by an early HP-evolution with subsequent granulitic decompression melts. During Visean crustal shortening, the areas of future formation of migmatites and intrusion of monzodioritic magmas in a general strike-slip regime, were probably in a lower plate situation, whereas the so called monometamorphic areas may have been in an upper plate position of the nappe pile. During the Latest Carboniferous, the emplacement of the youngest granites was associated with the strike-slip faulting and crustal extension at lower crustal levels, whereas, at the surface, detrital sediments accumulated in intramontaneous transtensional basins on a strongly eroded surface.

Petrogenesis of Mount Rainier andesite: magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades

USGS Publications Warehouse

Sisson, Thomas W.; Salters, V.J.M.; Larson, P.B.

2013-01-01

The dominant cause of magmatic evolution at Mount Rainier, however, is inferred to be a version of in situ crystallization-differentiation and mixing (Langmuir, 1989) wherein small magma batches stall as crustal intrusions and solidify extensively, yielding silicic residual liquids with trace element concentrations influenced by accessory mineral saturation. Subsequent magmas ascending through the intrusive plexus entrain and mix with the residual liquids and low-degree re-melts of those antecedent intrusions, producing hybrid andesites and dacites. Mount St. Helens volcanic rocks have geochemical similarities to those at Mount Rainier, and may also result from in situ differentiation and mixing due to low and intermittent long-term magma supply, accompanied by modest crustal assimilation. Andesites and dacites of Mount Adams isotopically overlap the least contaminated Mount Rainier magmas and derive from similar parental magma types, but have trace element variations more consistent with progressive crystallization-differentiation, probably due to higher magma fluxes leading to slower crystallization of large magma batches, allowing time for progressive separation of minerals from melt. Mount Adams also sits atop the southern projection of a regional anticlinorium, so Eocene sediments are absent, or are at shallow crustal levels, and so are cold and difficult to assimilate. Differences between southwest Washington stratovolcanoes highlight some ways that crustal geology and magma flux are primary factors in andesite generation.

Use of CAS in Secondary School: A Factor Influencing the Transition to University-Level Mathematics?

ERIC Educational Resources Information Center

Varsavsky, Cristina

2012-01-01

Australian secondary school systems offer three levels of senior (year 12) mathematics studies, none of them compulsory: elementary, intermediate and advanced. The intermediate and advanced studies prepare students for further mathematics studies at university level. In the state of Victoria, there are two versions of intermediate mathematics: one…

49 CFR Figure 2b to Subpart B of... - Example of an Intermediate Level Seating Area of a Multi-Level Car Complying With Window Location...

Code of Federal Regulations, 2010 CFR

2010-10-01

... 49 Transportation 4 2010-10-01 2010-10-01 false Example of an Intermediate Level Seating Area of a Multi-Level Car Complying With Window Location Requirements-§§ 238.113 and 238.114 2B Figure 2B to... Intermediate Level Seating Area of a Multi-Level Car Complying With Window Location Requirements—§§ 238.113 and...

49 CFR Figure 2a to Subpart B of... - Example of an Intermediate Level Seating Area of a Multi-Level Car Complying With Window Location...

Code of Federal Regulations, 2010 CFR

2010-10-01

... 49 Transportation 4 2010-10-01 2010-10-01 false Example of an Intermediate Level Seating Area of a Multi-Level Car Complying With Window Location Requirements-§§ 238.113 and 238.114 2A Figure 2A to... Intermediate Level Seating Area of a Multi-Level Car Complying With Window Location Requirements—§§ 238.113 and...

Radial anisotropy ambient noise tomography of volcanoes

NASA Astrophysics Data System (ADS)

Mordret, Aurélien; Rivet, Diane; Shapiro, Nikolai; Jaxybulatov, Kairly; Landès, Matthieu; Koulakov, Ivan; Sens-Schönfelder, Christoph

2016-04-01

The use of ambient seismic noise allows us to perform surface-wave tomography of targets which could hardly be imaged by other means. The frequencies involved (~ 0.5 - 20 s), somewhere in between active seismic and regular teleseismic frequency band, make possible the high resolution imaging of intermediate-size targets like volcanic edifices. Moreover, the joint inversion of Rayleigh and Love waves dispersion curves extracted from noise correlations allows us to invert for crustal radial anisotropy. We present here the two first studies of radial anisotropy on volcanoes by showing results from Lake Toba Caldera, a super-volcano in Indonesia, and from Piton de la Fournaise volcano, a hot-spot effusive volcano on the Réunion Island (Indian Ocean). We will see how radial anisotropy can be used to infer the main fabric within a magmatic system and, consequently, its dominant type of intrusion.

Modelling of physical influences in sea level records for vertical crustal movement detection

NASA Technical Reports Server (NTRS)

Anderson, E. G.

1978-01-01

Attempts to specify and evaluate such physical influences are reviewed with the intention of identifying problem areas and promising approaches. An example of linear modelling based on air/water temperatures, atmospheric pressure, river discharges, geostrophic and/or local wind velocities, and including forced period terms to allow for the long period tides and Chandlerian polar motion is evaluated and applied to monthly mean sea levels recorded in Atlantic Canada. Refinement of the model to admit phase lag in the response to some of the driving phenomena is demonstrated. Spectral analysis of the residuals is employed to assess the model performance. The results and associated statistical parameters are discussed with emphasis on elucidating the sensitivity of the technique for detection of local episodic and secular vertical crustal movements, the problem areas most critical to the type of approach, and possible further developments.

Pliocene-Quaternary crustal melting in central and northern Tibet and insights into crustal flow

PubMed Central

Wang, Qiang; Hawkesworth, Chris J.; Wyman, Derek; Chung, Sun-Lin; Wu, Fu-Yuan; Li, Xian-Hua; Li, Zheng-Xiang; Gou, Guo-Ning; Zhang, Xiu-Zheng; Tang, Gong-Jian; Dan, Wei; Ma, Lin; Dong, Yan-Hui

2016-01-01

There is considerable controversy over the nature of geophysically recognized low-velocity–high-conductivity zones (LV–HCZs) within the Tibetan crust, and their role in models for the development of the Tibetan Plateau. Here we report petrological and geochemical data on magmas erupted 4.7–0.3 Myr ago in central and northern Tibet, demonstrating that they were generated by partial melting of crustal rocks at temperatures of 700–1,050 °C and pressures of 0.5–1.5 GPa. Thus Pliocene-Quaternary melting of crustal rocks occurred at depths of 15–50 km in areas where the LV–HCZs have been recognized. This provides new petrological evidence that the LV–HCZs are sources of partial melt. It is inferred that crustal melting played a key role in triggering crustal weakening and outward crustal flow in the expansion of the Tibetan Plateau. PMID:27307135

Crustal gravitational potential energy change at the convergent plate boundary near Taiwan

NASA Astrophysics Data System (ADS)

Lo, C.; Hsu, S.

2003-12-01

The Taiwan orogen has formed due to the convergence between the Philippine Sea plate and Eurasian plate. Numerous earthquakes are occurring along the active convergent plate boundary in eastern Taiwan. To the northeast, the Philippine Sea plates is subducting northwards beneath the Ryukyu Arc. To the south, the Eurasian plate is subducting eastwards beneath the Luzon Arc. The plate interaction has caused crustal deformation and produced earthquakes. The earthquakes have caused radial permanent displacement of the crust and have altered the crustal gravitational potential energy. Here we use the earthquake source mechanisms, determined by the Broadband Array in Taiwan for Seismology (BATs) from 1995 to 2003, to calculate the crustal gravitational potential energy (GPE) change and discuss their tectonic implication along the convergent plate boundary. In Ilan Plain, the westernmost Okinawa Trough, it shows a crustal GPE loss. It is related to the crustal subsidence because of the backarc extension of the Okinawa Trough. In contrast, due to the Philippine Sea plate subucting northwards beneath Eurasian Plate, the Ryukyu convergent boundary shows systematic crustal GPE gain. Near Taiwan, the crustal GPE change is gained, indicating the collisional convergence of the Luzon Arc. To the south of Taiwan, along the Luzon Arc the crustal GPE is also gain, representing the initial uplifting of the Taiwan mountain belt.

Steeply-dipping extension fractures in the Newark basin, New Jersey

USGS Publications Warehouse

Herman, G.C.

2009-01-01

Late Triassic and Early Jurassic bedrock in the Newark basin is pervasively fractured as a result of Mesozoic rifting of the east-central North American continental margin. Tectonic rifting imparted systematic sets of steeply-dipping, en ??chelon, Mode I, extension fractures in basin strata including ordinary joints and veins. These fractures are arranged in transitional-tensional arrays resembling normal dip-slip shear zones. They contributed to crustal stretching, sagging, and eventual faulting of basin rift deposits. Extension fractures display progressive linkage and spatial clustering that probably controlled incipient fault growth. They cluster into three prominent strike groups correlated to early, intermediate, and late-stage tectonic events reflecting about 50- 60?? of counterclockwise rotation of incremental stretching directions. Finite strain analyses show that extension fractures allowed the stretching of basin strata by a few percent, and these fractures impart stratigraphic dips up to a few degrees in directions opposing fracture dips. Fracture groups display three-dimensional spatial variability but consistent geometric relations. Younger fractures locally cut across and terminate against older fractures having more complex vein-cement morphologies and bed-normal folds from stratigraphic compaction. A fourth, youngest group of extension fractures occur sporadically and strike about E-W in obliquely inverted crustal blocks. A geometric analysis of overlapping fracture sets shows how fracture groups result from incremental rotation of an extending tectonic plate, and that old fractures can reactivate with oblique slip components in the contemporary, compressive stress regime. ?? 2008 Elsevier Ltd. All rights reserved.

Diverse Seismic Imaging Created by the Seismic Explosion Experiment of the TAIGER Project

NASA Astrophysics Data System (ADS)

Wang, C.; Okaya, D.; Wu, F.; Yen, H.; Huang, B.; Liang, W.

2008-12-01

The TAIGER (TAiwan Integrated GEodynamics Research) project which examines the Taiwan orogeny includes five experiments: natural earthquake recording, man-made explosion recording, Magnetotelluic imaging, marine MCS and sea-land shooting, and deformation evolution modeling. During Feb-Mar 2008, the explosion experiment was carried out. Ten sources with 500~3000kg dynamite were detonated along two transects across northern and southern Taiwan. Over 600 PASSCAL Texans and 40 R-130 instruments record the signals over 100~300 km range. Additional arrays with 100 seismometers were deployed to collect north-south line and fan shoot data for 3D imaging. Furthermore, there are 9 ocean bottom seismometers (OBS) in the Taiwan Strait and two lines with 20 seismometers deployed on the mainland China side. A large volume of qualified data has been created. Except explosion signals, numerous local and regional earthquakes were also recorded even by the Texan instruments. The rich earthquake-explosion dataset now exists at the Institute of Earth Sciences, Academia Sinica operated by the Taiwan Earthquake Center (TEC). Preliminary examination of the data reveal crustal Pg, PmP, Pn and intermediate crustal reflection phases within the transect profiles and in the 3D cross-arrays. These data provide direct seismic imaging of the continental Moho under Taiwan and the sharp Moho root configuration associated with mountain building. Seismic tomography and raytrace methods reveal velocity structure consistent with convergence and vertical exhumation of the Central Ranges.

Three-dimensional models of P wave velocity and P-to-S velocity ratio in the southern central Andes by simultaneous inversion of local earthquake data

NASA Astrophysics Data System (ADS)

Graeber, Frank M.; Asch, Günter

1999-09-01

The PISCO'94 (Proyecto de Investigatión Sismológica de la Cordillera Occidental, 1994) seismological network of 31 digital broad band and short-period three-component seismometers was deployed in northern Chile between the Coastal Cordillera and the Western Cordillera. More than 5300 local seismic events were observed in a 100 day period. A subset of high-quality P and S arrival time data was used to invert simultaneously for hypocenters and velocity structure. Additional data from two other networks in the region could be included. The velocity models show a number of prominent anomalies, outlining an extremely thickened crust (about 70 km) beneath the forearc region, an anomalous crustal structure beneath the recent magmatic arc (Western Cordillera) characterized by very low velocities, and a high-velocity slab. A region of an increased Vp/Vs ratio has been found directly above the Wadati-Benioff zone, which might be caused by hydration processes. A zone of lower than average velocities and a high Vp/Vs ratio might correspond to the asthenospheric wedge. The upper edge of the Wadati-Benioff zone is sharply defined by intermediate depth hypocenters, while evidence for a double seismic zone can hardly be seen. Crustal events between the Precordillera and the Western Cordillera have been observed for the first time and are mainly located in the vicinity of the Salar de Atacama down to depths of about 40 km.

Subduction and collision processes in the Central Andes constrained by converted seismic phases.

PubMed

Yuan, X; Sobolev, S V; Kind, R; Oncken, O; Bock, G; Asch, G; Schurr, B; Graeber, F; Rudloff, A; Hanka, W; Wylegalla, K; Tibi, R; Haberland, C; Rietbrock, A; Giese, P; Wigger, P; Röwer, P; Zandt, G; Beck, S; Wallace, T; Pardo, M; Comte, D

The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovicić discontinuity--generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Recent advances in understanding the characteristics of seismogenic intraplate deformation in Australia, and the potential for using global analogues

NASA Astrophysics Data System (ADS)

Clark, Dan; McPherson, Andrew

2017-04-01

Continental intraplate Australia can be divided according to crustal type in terms of seismogenic potential and fault characteristics. Three 'superdomains' are recognized, representing cratonic, non-cratonic and extended crust. In the Australian context, cratonic crust is Archaean to Proterozoic in age and has not been significantly tectonically reactivated during the Phanerozoic Eon. Non-cratonic crust includes Phanerozoic accretionary terranes and older crust significantly deformed during Phanerozoic tectonic events. Extended crust includes any crustal type that has been significantly extended during the Mesozoic and Cenozoic, and often to a lesser degree in the Paleozoic. Aulacogens and passive margins fit into this category. Cratonic crust is characterized by the thickest lithosphere and has the lowest seismogenic potential, despite all eight documented historic surface ruptures in Australia having occurred within this category. Little strain accumulation is observed on individual faults and isolated single-rupture scarps are common. Where recurrence has been demonstrated, average slip rates of only a few metres per million years are indicated. In contrast, extended crust is associated with thinner lithosphere, better connection between faults, and strain localization on faults which can result in regional relief-building. The most active faults have accumulated several hundred metres of slip under the current crustal stress regime at rates of several tens of metres per million years. Non-cratonic crust is typically intermediate in lithospheric thickness and seismogenic character. The more active faults have accumulated tens to a couple of hundreds of metres of slip, at rates of a few to a few tens of metres per million years. Across all superdomains paleoseismological data suggest that the largest credible earthquakes are likely to exceed those experienced in historic times. In general, the concept of large earthquake recurrence might only be meaningful in relation to individual faults in non-cratonic and extended superdomains. However, large earthquake recurrence and slip are demonstrably not evenly distributed in time. Within the limitations of the sparse paleoseismological data, temporal clustering of large events appears to be a common (perhaps ubiquitous?) characteristic. Over the last few decades, permanent and campaign GPS studies have failed to detect a tectonic deformation signal from which a strain budget could be calculated. Recent studies have used these observations, amongst others, to propose an orders of magnitude difference in the timescales of strain accumulation and seismogenic strain release in intraplate environments - i.e. clusters of large events deplete long-lived pools of lithospheric strain. The recognition of a relationship between crustal type/lithospheric thickness and seismogenic potential in Australia provides a framework for assessing whether ergodic substitution (i.e. global analogue studies) might be warranted as a tool to better understand intraplate seismicity worldwide. Further research is required to assess how variation in crustal stress regime may influence faulting characteristics within different superdomains.

Generation of syntectonic calc-alkaline, magnesian granites through remelting of pre-tectonic igneous sources - U-Pb zircon ages and Sr, Nd and Pb isotope data from the Donkerhoek granite (southern Damara orogen, Namibia)

NASA Astrophysics Data System (ADS)

Schwark, L.; Jung, S.; Hauff, F.; Garbe-Schönberg, D.; Berndt, J.

2018-06-01

The 541 ± 4 Ma-old magnesian, weakly peraluminous, calc-alkalic Donkerhoek Onanis granite is part of the ca. 6000 km2 large Donkerhoek batholith in the Southern Zone of the Damara orogen of Namibia. Linear major and trace element variations and decreasing MgO, FeO, Al2O3, CaO, K2O, Na2O, Ba and Sr concentrations with increasing SiO2 indicate that this part of the batholith represent a coherent mass and underwent fractional crystallization processes. The Donkerhoek Onanis granites are isotopically evolved (initial εNd: -4.7 to -12.3, initial 87Sr/86Sr: 0.7099-0.7157) with moderately radiogenic Pb isotope ratios (206Pb/204Pb: 17.26-18.22; 207Pb/204Pb: 15.59-15.67; 208Pb/204Pb: 37.60-38.06). Beside heterogeneities imparted by the sources, an evaluation of LREE fractionation and Nd isotope data suggests that AFC processes also modified some samples. Based on the chemical and isotope data, the Donkerhoek Onanis granites cannot be derived by partial melting of Al- and Fe-rich metasedimentary rocks of the Kuiseb formation in which they intruded. Instead, melting of meta-igneous crustal sources with Proterozoic crustal residence ages is more likely. Three igneous to meta-igneous rock suites from the area (Matchless amphibolites, Proterozoic mafic to felsic gneisses from the southern Kalahari craton basement, syn-tectonic Salem granodiorites to granites) are potential sources. An evaluation of chemical and isotope data suggests that remelting of early syn-orogenic Salem-type granites is the most likely process which would also explain the existence of ca. 563 ± 4 Ma-old zircon in the Donkerhoek Onanis granites. Comparison of the Donkerhoek Onanis granites with experimentally derived melt compositions from an intermediate igneous parent indicates temperatures between 800 and 850 °C. It is suggested that the Pan-African igneous activity in this part of the Damara Belt was a moderate-temperature intra-crustal event. Although there are some compositional similarities with juvenile granites generated in subduction zones, unradiogenic Pb isotope ratios and moderately radiogenic Sr and unradiogenic Nd isotopes suggest that reprocessed crustal rocks are more likely sources. Previously obtained high δ18O values of the Donkerhoek Onanis granites ranging from 11.8 to 13.6‰, covering the range of δ18O values obtained on Salem-type granites from the area (12.5-13.3‰) confirm this view. In contrast to igneous processes along active continental margins that produce juvenile batholiths with calc-alkaline affinities, this igneous event was not a major crust-forming episode and the Donkerhoek Onanis granites represent reprocessed crustal material.

Crustal geometry of the northeastern Gulf of Aden passive margin: localization of the deformation inferred from receiver function analysis

NASA Astrophysics Data System (ADS)

Tiberi, C.; Leroy, S.; d'Acremont, E.; Bellahsen, N.; Ebinger, C.; Al-Lazki, A.; Pointu, A.

2007-03-01

Here we use receiver function analysis to retrieve crustal thickness and crustal composition along the 35-My-old passive margin of the eastern Gulf of Aden. Our aims are to use results from the 3-D seismic array to map crustal stretching across and along the Aden margin in southern Oman. The array recorded local and teleseismic events between 2003 March and 2004 March. Seventy-eight events were used in our joint inversions for Vp/Vs ratio and depth. The major results are: (1) Crustal thickness decreases from the uplifted rift flank of the margin towards the Sheba mid-ocean ridge. We found a crustal thickness of about 35 km beneath the northern rift flank. This value decreases sharply to 26 km beneath the post-rift subsidence zone on the Salalah coastal plain. This 10 km of crustal thinning occurs across a horizontal distance of less than 30 km showing a localization of the crustal thinning below the first known rifted block of the margin. (2) A second rift margin transect located about 50 km to the east shows no thinning from the coast to 50 km onshore. The lack of crustal thickness variation indicates that the maximum crustal stretching could be restricted to offshore regions. (3) The along-strike variations in crustal structure demonstrate the scale and longevity of the regular along-axis rift segmentation. (4) Extension is still observed north of the rifted domain, 70 km onshore from the coast, making the width of margin larger than first expected from geology. (5) The crust has a felsic to normal composition with a probably strong effect of the sedimentary layer on the Vp/Vs ratio (comprised between 1.67 and 1.91).

Magma Supply of Southwest Indian Ocean: Implication from Crustal Thickness Anomalies

NASA Astrophysics Data System (ADS)

Chiheng, L.; Jianghai, L.; Huatian, Z.; Qingkai, F.

2017-12-01

The Southwest Indian Ridge (SWIR) is one of the world's slowest spreading ridges with a full spreading rate of 14mm a-1, belonging to ultraslow spreading ridge, which are a novel class of spreading centers symbolized by non-uniform magma supply and crustal accretion. Therefore, the crustal thickness of Southwest Indian Ocean is a way to explore the magmatic and tectonic process of SWIR and the hotspots around it. Our paper uses Residual Mantle Bouguer Anomaly processed with the latest global public data to invert the relative crustal thickness and correct it according to seismic achievements. Gravity-derived crustal thickness model reveals a huge range of crustal thickness in Southwest Indian Ocean from 0.04km to 24km, 7.5km of average crustal thickness, and 3.5km of standard deviation. In addition, statistics data of crustal thickness reveal the frequency has a bimodal mixed skewed distribution, which indicates the crustal accretion by ridge and ridge-plume interaction. Base on the crustal thickness model, we divide three types of crustal thickness in Southwest Indian Ocean. About 20.31% of oceanic crust is <4.8km thick designated as thin crust, and 60.99% is 4.8-9.8km thick as normal crust. The remaining 18.70% is >9.8km thick as thick crust. Furthermore, Prominent thin crust anomalies are associated with the trend of most transform faults, but thick crust anomalies presents to northeast of Andrew Bain transform fault. Cold and depleted mantle are also the key factors to form the thin crust. The thick crust anomalies are constrained by hotspots, which provide abundant heat to the mantle beneath mid-ocean ridge or ocean basin. Finally, we roughly delineate the range of ridge-plume interaction and transform fault effect.

Influence of mid-crustal rheology on the deformation behavior of continental crust in the continental subduction zone

NASA Astrophysics Data System (ADS)

Li, Fucheng; Sun, Zhen; Zhang, Jiangyang

2018-06-01

Although the presence of low-viscosity middle crustal layer in the continental crust has been detected by both geophysical and geochemical studies, its influence on the deformation behavior of continental crust during subduction remains poorly investigated. To illustrate the crustal deformation associated with layered crust during continental subduction, we conducted a suite of 2-D thermo-mechanical numerical studies with visco-brittle/plastic rheology based on finite-differences and marker-in-cell techniques. In the experiments, we established a three-layer crustal model with a quartz-rich middle crustal layer embedded between the upper and lower continental crust. Results show that the middle crustal layer determines the amount of the accreted upper crust, maximum subduction depth, and exhumation path of the subducted upper crust. By varying the initial effective viscosity and thickness of the middle crustal layer, the further effects can be summarized as: (1) a rheologically weaker and/or thicker middle crustal layer results in a larger percentage of the upper crust detaching from the underlying slab and accreting at the trench zone, thereby leading to more serious crustal deformation. The rest of the upper crust only subducts into the depths of high pressure (HP) conditions, causing the absence of ultra-high pressure (UHP) metamorphic rocks; (2) a rheologically stronger and/or thinner middle crustal layer favors the stable subduction of the continental crust, dragging the upper crust to a maximum depth of ∼100 km and forming UHP rocks; (3) the middle crustal layer flows in a ductile way and acts as an exhumation channel for the HP-UHP rocks in both situations. In addition, the higher convergence velocity decreases the amount of subducted upper crust. A detailed comparison of our modeling results with the Himalayan collisional belt are conducted. Our work suggests that the presence of low-viscosity middle crustal layer may be another possible mechanism for absence of UHP rocks in the southern Tibet.

Constraints on the formation of the Martian crustal dichotomy from remnant crustal magnetism

NASA Astrophysics Data System (ADS)

Citron, Robert I.; Zhong, Shijie

2012-12-01

The Martian crustal dichotomy characterizing the topographic difference between the northern and southern hemispheres is one of the most important features on Mars. However, the formation mechanism for the dichotomy remains controversial with two competing proposals: exogenic (e.g., a giant impact) and endogenic (e.g., degree-1 mantle convection) mechanisms. Another important observation is the Martian crustal remnant magnetism, which shows a much stronger field in the southern hemisphere than in the northern hemisphere and also magnetic lineations. In this study, we examine how exogenic and endogenic mechanisms for the crustal dichotomy are constrained by the crustal remnant magnetism. Assuming that the dichotomy is caused by a giant impact in the northern hemisphere, we estimate that the average thickness of ejecta in the southern hemisphere is 20-25 km. While such a giant impact may cause crustal demagnetization in the northern hemisphere, we suggest that the impact could also demagnetize the southern hemisphere via ejecta thermal blanketing, impact demagnetization, and heat transfer from the hot layer of ejecta, thus posing a challenge for the giant impact model. We explore how the pattern of magnetic lineations relates to endogenic theories of dichotomy formation, specifically crustal production via degree-1 mantle convection. We observe that the pattern of lineations roughly corresponds to concentric circles about a single pole, and determine the pole for the concentric circles at 76.5° E and 84.5° S, which nearly overlaps with the centroid of the thickened crust in the southern hemisphere. We suggest that the crustal magnetization pattern, magnetic lineations, and crustal dichotomy (i.e., thickened crust in the highlands) can be explained by a simple endogenic process; one-plume convection causes melting and crustal production above the plume in the southern hemisphere, and strong crustal magnetization and magnetic lineations are formed in the southern hemisphere as crustal production fronts spread radially out from the plume center and as the newly created crust cools in the presence of a dynamo with polarity reversals.

Relative sea-level rise around East Antarctica during Oligocene glaciation

NASA Astrophysics Data System (ADS)

Stocchi, Paolo; Escutia, Carlota; Houben, Alexander J. P.; Vermeersen, Bert L. A.; Bijl, Peter K.; Brinkhuis, Henk; Deconto, Robert M.; Galeotti, Simone; Passchier, Sandra; Pollard, David; Brinkhuis, Henk; Escutia, Carlota; Klaus, Adam; Fehr, Annick; Williams, Trevor; Bendle, James A. P.; Bijl, Peter K.; Bohaty, Steven M.; Carr, Stephanie A.; Dunbar, Robert B.; Flores, Jose Abel; Gonzàlez, Jhon J.; Hayden, Travis G.; Iwai, Masao; Jimenez-Espejo, Francisco J.; Katsuki, Kota; Kong, Gee Soo; McKay, Robert M.; Nakai, Mutsumi; Olney, Matthew P.; Passchier, Sandra; Pekar, Stephen F.; Pross, Jörg; Riesselman, Christina; Röhl, Ursula; Sakai, Toyosaburo; Shrivastava, Prakash Kumar; Stickley, Catherine E.; Sugisaki, Saiko; Tauxe, Lisa; Tuo, Shouting; van de Flierdt, Tina; Welsh, Kevin; Yamane, Masako

2013-05-01

During the middle and late Eocene (~ 48-34Myr ago), the Earth's climate cooled and an ice sheet built up on Antarctica. The stepwise expansion of ice on Antarctica induced crustal deformation and gravitational perturbations around the continent. Close to the ice sheet, sea level rose despite an overall reduction in the mass of the ocean caused by the transfer of water to the ice sheet. Here we identify the crustal response to ice-sheet growth by forcing a glacial-hydro isostatic adjustment model with an Antarctic ice-sheet model. We find that the shelf areas around East Antarctica first shoaled as upper mantle material upwelled and a peripheral forebulge developed. The inner shelf subsequently subsided as lithosphere flexure extended outwards from the ice-sheet margins. Consequently the coasts experienced a progressive relative sea-level rise. Our analysis of sediment cores from the vicinity of the Antarctic ice sheet are in agreement with the spatial patterns of relative sea-level change indicated by our simulations. Our results are consistent with the suggestion that near-field processes such as local sea-level change influence the equilibrium state obtained by an ice-sheet grounding line.

Late Archean intermediate-felsic magmatism of the South Vygozersky and Kamennozersky greenstone structures of Central Karelia

NASA Astrophysics Data System (ADS)

Myskova, T. A.; Zhitnikova, I. A.; L'vov, P. A.

2015-07-01

The geochemistry and zircon geochronology (U-Pb, SHRIMP-II) of Late Archean intermediate-felsic dikes and plagiogranites of the Shilossky massif of the South Vygozersky and Kamennozersky greenstone belts of Central Karelia were studied. Subvolcanic rocks of the dike complex vary in composition from andesitobasalts to rhyolites, in structural-textural peculiarities, and in the formation age, from 2862 ± 8 to 2785 ± 15 Ma. Compositionally and geochronologically (2853 ± 11 Ma), plagiogranites of the Shilossky massif of the South Vygozersky greenstone belts are close to the most ancient dacite and granodiorite porphyry dikes. Dikes intruded synchronously with intrusion of plagiogranites over a period of at least 70 m.y. Geochronologically, subvolcanic rocks of the dike complex and plagiogranites of the Shilossky massif are similar to granitoids of the TTG assemblages of I- and M-type granites. The Sm-Nd model age of some dikes (2970-2880 Ma) is close to the age of rock crystallization, which is evidence in favor of juvenile origin of magma. Dikes with more ancient model age (3050 Ma) are presumed to contain crustal material. Variations in age and ɛNd (from -2.7 to +2.9) indicate the absence of a unified magmatic source.

Three-dimensional crustal structure of Long Valley caldera, California, and evidence for the migration of CO2 under Mammoth Mountain

USGS Publications Warehouse

Foulger, G.R.; Julian, B.R.; Pitt, A.M.; Hill, D.P.; Malin, P.E.; Shalev, E.

2003-01-01

A temporary network of 69 three-component seismic stations captured a major seismic sequence in Long Valley caldera in 1997. We performed a tomographic inversion for crustal structure beneath a 28 km ?? 16 km area encompassing part of the resurgent dome, the south moat, and Mammoth Mountain. Resolution of crustal structure beneath the center of the study volume was good down to ???3 km below sea level (???5 km below the surface). Relatively high wave speeds are associated with the Bishop Tuff and lower wave speeds characterize debris in the surrounding moat. A low-Vp/Vs anomaly extending from near the surface to ???1 km below sea level beneath Mammoth Mountain may represent a CO2 reservoir that is supplying CO2-rich springs, venting at the surface, and killing trees. We investigated temporal variations in structure beneath Mammoth Mountain by differencing our results with tomographic images obtained using data from 1989/1990. Significant changes in both Vp and Vs were consistent with the migration of CO2 into the upper 2 km or so beneath Mammoth Mountain and its depletion in peripheral volumes that correlate with surface venting areas. Repeat tomography is capable of detecting the migration of gas beneath active silicic volcanoes and may thus provide a useful volcano monitoring tool.

Heterogeneous brittle-ductile deformation at shallow crustal levels under high thermal conditions: The case of a synkinematic contact aureole in the inner northern Apennines, southeastern Elba Island, Italy

NASA Astrophysics Data System (ADS)

Papeschi, Samuele; Musumeci, Giovanni; Mazzarini, Francesco

2017-10-01

We present an example of interaction between magmatism and tectonics at shallow crustal levels. In the Late Miocene the metamorphic units of the eastern Elba Island (northern Apennines) were intruded at very shallow crustal levels by a large pluton (> 60 km2) with the development of an hectometre-sized contact aureole defined by growth of low-pressure/high-temperature mineral assemblages (Pmax < 0.2 GPa, Tmax 650 °C). Structural data show that the contact aureole is associated with a km-sized antiform of the foliation and by several metre- to decametre-thick high-strain domains consisting of strongly foliated rocks containing synkinematic HT/LP mineral assemblages and ductile shear zones of variable thickness. These shear zones are characterized by a mylonitic foliation variably overprinted by cataclasis. Quartz microfabrics indicate that the dynamic crystallization processes progressively changed from grain boundary migration, associated with the thermal peak of contact metamorphism, to subgrain rotation and bulging recrystallization, the latter mostly associated with the cataclastic overprint. These transitions of recrystallization mechanisms in quartz are related to a progressive decrease of temperature during deformation. Deformation accompanied the development and cooling of the contact aureole, which recorded the switch from high temperature ductile to low temperature brittle conditions. The geometry of the studied deformation structures is consistent with the constraints of the regional tectonic evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement of igneous rocks.

Workflow with pitfalls to derive a regional airborne magnetic compilation

NASA Astrophysics Data System (ADS)

Brönner, Marco; Baykiev, Eldar; Ebbing, Jörg

2017-04-01

Today, large scale magnetic maps are usually a patchwork of different airborne surveys from different size, different resolution and different years. Airborne magnetic acquisition is a fast and economic method to map and gain geological and tectonic information for large areas, onshore and offshore. Depending on the aim of a survey, acquisition parameters like altitude and profile distance are usually adjusted to match the purpose of investigation. The subsequent data processing commonly follows a standardized workflow comprising core-field subtraction and line leveling to yield a coherent crustal field magnetic grid for a survey area. The resulting data makes it possible to correlate with geological and tectonic features in the subsurface, which is of importance for e.g. oil and mineral exploration. Crustal scale magnetic interpretation and modeling demand regional compilation of magnetic data and the merger of adjacent magnetic surveys. These studies not only focus on shallower sources, reflected by short to intermediate magnetic wavelength anomalies, but also have a particular interest in the long wavelength deriving from deep seated sources. However, whilst the workflow to produce such a merger is supported by quite a few powerful routines, the resulting compilation contains several pitfalls and limitations, which were discussed before, but still are very little recognized. The maximum wavelength that can be resolved of each individual survey is directly related to the survey size and consequently a merger will contribute erroneous long-wavelength components in the magnetic data compilation. To minimize this problem and to homogenous the longer wavelengths, a first order approach is the combination of airborne and satellite magnetic data commonly combined with the compilation from airborne data, which is sufficient only under particular preconditions. A more advanced approach considers the gap in frequencies between airborne and satellite data, which motivated countries like Sweden and Australia (AWAGS) to collect high altitude- long distance airborne magnetic data for the entire country to homogenous the high-resolution magnetic data before the merger with satellite data. We present the compilation of a regional magnetic map for an area in northern Europe and discuss the problems and pitfalls for a common workflow applied.

The granite problem as exposed in the southern Snake Range, Nevada

USGS Publications Warehouse

Lee, D.E.; Christiansen, E.H.

1983-01-01

A geochemically and mineralogically diverse group of granitoids is present within an area of 900 km2 in the southern Snake Range of eastern Nevada. The granitoids exposed range in age from Jurassic through Cretaceous to Oligocene and include two calcic intrusions, two different types of two-mica granites, and aplites. The younger intrusions appear to have been emplaced at progressively more shallow depths. All of these granitoid types are represented elsewhere in the eastern Great Basin, but the southern Snake Range is distinguished by the grouping of all these types within a relatively small area. The Jurassic calcic pluton of the Snake Creek-Williams Canyon area displays large and systematic chemical and mineralogical zonation over a horizontal distance of five km. Although major element variations in the pluton compare closely with Daly's average andesite-dacite-rhyolite over an SiO2 range of 63 to 76 percent, trace element (Rb, Sr, Ba) variations show that the zonation is the result of in situ fractional crystallization, with the formation of relatively mafic cumulates on at least one wall of the magma chamber. Models of trace element and isotopic data indicate that relatively little assimilation took place at the level of crystallization. Nonetheless, an initial 87Sr/86Sr value of 0.7071 and ??18O values of 10.2 to 12.2 permil suggest a lower crustal magma that was contaminated by upper crustal clastic sedimentary rocks before crystallization. The involvement of mantle-derived magmas in its genesis is difficult to rule out. Two other Jurassic plutons show isotopic and chemical similarities to the Snake Creek-Williams Canyon pluton. Cretaceous granites from eastern Nevada that contain phenocrystic muscovite are strongly peraluminous, and have high initial Sr-isotope ratios and other features characteristic of S-type granitoids. They were probably derived from Proterozoic metasediments and granite gneisses that comprise the middle crust of this region. Another group of granitoids (including the Tertiary aplites) show chemical, mineralogic, and isotopic characteristics intermediate between the first two groups and may have been derived by contamination of magmas from the lower crust by the midcrustal metasediments. ?? 1983 Springer-Verlag.

Cytoplasmic peptidoglycan intermediate levels in Staphylococcus aureus.

PubMed

Vemula, Harika; Ayon, Navid J; Gutheil, William G

2016-02-01

Intracellular cytoplasmic peptidoglycan (PG) intermediate levels were determined in Staphylococcus aureus during log-phase growth in enriched media. Levels of UDP-linked intermediates were quantitatively determined using ion pairing LC-MS/MS in negative mode, and amine intermediates were quantitatively determined stereospecifically as their Marfey's reagent derivatives in positive mode. Levels of UDP-linked intermediates in S. aureus varied from 1.4 μM for UDP-GlcNAc-Enolpyruvyate to 1200 μM for UDP-MurNAc. Levels of amine intermediates (L-Ala, D-Ala, D-Ala-D-Ala, L-Glu, D-Glu, and L-Lys) varied over a range of from 860 μM for D-Ala-D-Ala to 30-260 mM for the others. Total PG was determined from the D-Glu content of isolated PG, and used to estimate the rate of PG synthesis (in terms of cytoplasmic metabolite flux) as 690 μM/min. The total UDP-linked intermediates pool (2490 μM) is therefore sufficient to sustain growth for 3.6 min. Comparison of UDP-linked metabolite levels with published pathway enzyme characteristics demonstrates that enzymes on the UDP-branch range from >80% saturation for MurA, Z, and C, to <5% saturation for MurB. Metabolite levels were compared with literature values for Escherichia coli, with the major difference in UDP-intermediates being the level of UDP-MurNAc, which was high in S. aureus (1200 μM) and low in E. coli (45 μM). Copyright © 2015 Elsevier B.V. and Société Française de Biochimie et Biologie Moléculaire (SFBBM). All rights reserved.

Localized crustal deformation in the Godavari failed rift, India

NASA Astrophysics Data System (ADS)

Mahesh, P.; Gahalaut, V. K.; Catherine, J. K.; Ambikapathy, A.; Kundu, Bhaskar; Bansal, Amit; Chadha, R. K.; Narsaiah, M.

2012-06-01

Six years of GPS measurements of crustal deformation in the Godavari failed rift (GFR) of stable India plate suggest very localized deformation. Elsewhere, all along the GFR the deformation is very low (<1.5 mm/yr). Localized deformation (up to 3.3±0.5 mm/yr) at least at two sites, implying compression on steep faults located on the southern margin of the GFR, is coincident with the region characterized by high level low-magnitude seismicity of past six years and implies strain accumulation for future moderate to strong magnitude earthquake in the region. The localized deformation is consistent with the view about deformation in such regions where seismicity migrates and deformation rate changes with time.

Simultaneous miocene extension and shortening in the himalayan orogen.

PubMed

Hodges, K V; Parrish, R R; Housh, T B; Lux, D R; Burchfiel, B C; Royden, L H; Chen, Z

1992-11-27

The South Tibetan detachment system separates the high-grade metamorphic core of the Himalayan orogen from its weakly metamorphosed suprastructure. It is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front. Geochronologic data from the Rongbuk Valley, north of Qomolangma (Mount Everest) in southern Tibet, demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago, an interval characterized by large-scale crustal thickening at lower structural levels. These data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of Himalayan tectonics in Miocene time.

Models of recurrent strike-slip earthquake cycles and the state of crustal stress

NASA Technical Reports Server (NTRS)

Lyzenga, Gregory A.; Raefsky, Arthur; Mulligan, Stephanie G.

1991-01-01

Numerical models of the strike-slip earthquake cycle, assuming a viscoelastic asthenosphere coupling model, are examined. The time-dependent simulations incorporate a stress-driven fault, which leads to tectonic stress fields and earthquake recurrence histories that are mutually consistent. Single-fault simulations with constant far-field plate motion lead to a nearly periodic earthquake cycle and a distinctive spatial distribution of crustal shear stress. The predicted stress distribution includes a local minimum in stress at depths less than typical seismogenic depths. The width of this stress 'trough' depends on the magnitude of crustal stress relative to asthenospheric drag stresses. The models further predict a local near-fault stress maximum at greater depths, sustained by the cyclic transfer of strain from the elastic crust to the ductile asthenosphere. Models incorporating both low-stress and high-stress fault strength assumptions are examined, under Newtonian and non-Newtonian rheology assumptions. Model results suggest a preference for low-stress (a shear stress level of about 10 MPa) fault models, in agreement with previous estimates based on heat flow measurements and other stress indicators.

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.

Paleozoic mafic-intermediate intrusions (320-287 Ma) in the Kalatongke area, southern Altai, NW China: Products of protracted magmatism in a convergent tectonic setting

NASA Astrophysics Data System (ADS)

Qian, Zhuangzhi; Duan, Jun; Li, Chusi; Xu, Gang; Feng, Yanqing; Ren, Meng

2018-06-01

Numerous small mafic-intermediate intrusions are present in the Kalatongke area in the southern part of the Paleozoic Altai Orogenic Belt, NW China. Previous studies reveal that most of these intrusions were emplaced at ∼287 Ma, broadly coeval with the eruption of alkaline flood basalts at ⩽282 Ma in the Tarim Craton. The similar ages have led some researchers to believe that the Tarim flood basalts and the Kalatongke mafic-intermediate intrusions are related to the same mantle plume. New and existing geochronological and geochemical data for the mafic-intermediate intrusions in the Kalatongke area together do not support such interpretation. Most of the intrusions in this small area (4.5 × 2 km) were emplaced at ∼287 Ma but our new data reveal that older intrusive rocks with zircon U-Pb ages from 290 ± 1.5 to 320 ± 2 Ma are also present. The temporal and spatial distribution of these intrusions does not show a hotspot track as expected by the mantle plume model. The intrusive rocks have similar Nd-Hf isotope compositions (εNd = 3-9, εHf = 15-19) and are all characterized by light REE enrichments relative to heavy REE, plus pronounced negative Nb-Ta anomalies. The oldest intrusion (∼320 Ma) also shows negative Zr-Hf anomalies, which are common in arc basalts but absent in continental flood basalts. The results of mixing calculations based on Sr-Nd-Hf isotopes and selected trace elements (Th, Nb and Yb) indicate that the parental magmas for these intrusions were all enriched in Th and depleted in Nb prior to crustal contamination, similar to the magmas generated in a convergent tectonic setting from subduction to post-subduction elsewhere in the world. The results from this study remind us that temporal correlation is not a reliable tool to determine the size of a mantle plume.

Sedimentary records on the subduction-accretion history of the Russian Altai, northwestern Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Chen, Ming; Sun, Min

2017-04-01

The Russian Altai, comprising the northern segment of the Altai-Mongolian terrane (AM) in the south, the Gorny Altai terrane (GA) in the north and the intervening Charysh-Terekta-Ulagan-Sayan suture zone, is a key area of the northwestern Central Asian Orogenic Belt (CAOB). A combined geochemical and detrital zircon study was conducted on the (meta-)sedimentary sequences from the Russian Altai to reveal the tectono-magmatic history of these two terranes and their amalgamation history, which in turn place constraints on the accretionary orogenesis and crustal growth in the CAOB. The Cambrian-Ordovician meta-sedimentary rocks from the northern AM are dominated by immature sediments possibly sourced from intermediate-felsic igneous rocks. Geochemical data show that the sediments were likely deposited in a continental arc-related setting. Zircons separated from these rocks are mainly 566-475 Ma and 1015-600 Ma old, comparable to the magmatic records of the Tuva-Mongolian terrane and surrounding island arcs in the western Mongolia. The similar source nature, provenance and depositional setting of these rocks to the counterparts from the Chinese Altai (i.e., the southern AM) imply that the whole AM possibly represents a coherent accretionary prism of the western Mongolia in the early Paleozoic rather than a Precambrian continental block with passive marginal deposition as previously thought. In contrast, the Cambrian to Silurian (meta-)sedimentary rocks from the GA are characterized by a unitary zircon population with ages of 640-470 Ma, which were potentially sourced from the Kuznetsk-Altai intra-oceanic island arc in the east of this terrane. The low abundance of 640-540 Ma zircons (5%) may attest that this arc was under a primitive stage in the late Neoproterozoic, when mafic igneous rocks dominated. However, the voluminous 530-470 Ma zircons (95%) suggest that this arc possibly evolved toward a mature one in the Cambrian to early Ordovician with increasing amount of intermediate-felsic igneous rocks, highlighting both crustal growth and recycling. Importantly, a significant amount of additional 2431-772 Ma zircons occur in the early Devonian sedimentary sequence of the GA. These detrital zircons possibly have the same source as their counterpart from the AM. This implies that the two terranes with countrary evolutionary history, i.e. the GA and AM, amalgamated before the early Devonian. To summary, the AM and GA represented two separated subduction-accretion systems in the early Paleozoic and subsequently amalgamated prior to the early Devonian, documenting complicated accretionary orogenesis and significant lateral crustal growth in the CAOB. Acknowledgement This study is financially supported by the Major Research Project of the Ministry of Science and Technology of China (2014CB44801 and 2014CB448000), Hong Kong Research Grant Council (HKU705313P and HKU17303415), National Science Foundation of China (41273048) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (162301132731).

The Crustal Structure of the Central Anatolia (Turkey) Using Receiver Functions

NASA Astrophysics Data System (ADS)

Yelkenci, S.; Benoit, M.; Kuleli, H.; Gurbuz, C.

2005-12-01

Central Anatolia lies in a transitional region between the extensional tectonics of western Anatolia and the complex transpressional tectonics of Eastern Anatolia, and has a complicated thermal and structural history. Few studies of the crustal structure of Anatolia have been performed, however, studies of the crustal structure of Eastern Anatolia showed that crustal thicknesses were thinner than previously thought. To further investigate the crustal structure in Central Anatolia, we present results from receiver function analysis using new data from broad-band instruments. The stations were equipped with 7 broadband three-component STS-2 and 13 short period three-component S-13 sensors. These stations operated for period of one and half months between the October and November, 2002, and yielded data for ~ 40 high quality receiver functions. Additionally, receiver functions were also computed using data from permanent stations MALT, ISP, and ANTO. We applied the hk-stacking technique of Zhu and Kanamori (2000) to receiver functions to obtain the crustal thickness and Vp/Vs ratios. Furthermore, we applied a waveform modeling technique to investigate mid-crustal discontinuties previously imaged in the region. Our results compare well with refraction-based crustal thicknesses in overlapped areas.

Preliminary crustal deformation model deduced from GPS and earthquakes’ data at Abu-Dabbab area, Eastern Desert, Egypt

NASA Astrophysics Data System (ADS)

Mohamed, Abdel-Monem S.; Hosny, A.; Abou-Aly, N.; Saleh, M.; Rayan, A.

2013-06-01

A local geodetic network consisting of eleven benchmarks has been established to study the recent crustal deformation in the Abu-Dabbab area. Seven campaigns of GPS measurements have been collected started from October 2008 and ended in March 2012. The collected data were processed using Bernese version 5.0, and the result values were adjusted to get the more accurate positions of the GPS stations. The magnitudes of horizontal displacements are variable from one epoch to another and in the range of 1-3 (±0.2) mm/yr. Due to the differences in rates of the horizontal displacement; the area is divided into two main blocks. The first one, moves to the east direction of about 3 mm/yr, while the second block, moves to the SW direction of about 6 mm/yr. According to the strain fields that were calculated for the different epochs of measurement, the main force is compression force and is taken the NW-SE to NWW-SEE direction. This force could be because of local and regional tectonic processes affecting on the study area. The maximum values of compression stress are found in the southern central and western part of study area. Estimated accumulation of this strain energy may be considered as an indicator of the possibility of earthquake occurrence. From the seismic tomography study, the 3D Vp and Vp/Vs crustal models indicate high Vp/Vs values forms an elongated anomaly, in the central part of the study area, that extends from a depth of 12 km to about 1-2 km of depth is obtained. By using this crustal model in relocations all seismicity informed that most of the seismicity strongly tend to occur in a cluster manner exactly above the southern part of the study area. Based on the conducted source mechanism study, it is noticed that shallow earthquakes are associated by a high CLVD ratio (up to 40%). Furthermore, initiation of a high level seismic activity, without a large seismic main shock is observed in the Abu-Dabbab area. The distribution of micro-earthquakes tends to align in an ENE-WSW direction marking a zone of activity verse the Red Sea. The nucleation of the seismic activity beneath the southern part of the Abu-Dabbab crust is more consistent with the obtained crustal deformation result by increasing the crustal movement in the south part than the northern part. Then, based on the obtained results of the above mentioned studies; seismic tomography; source mechanisms, and crustal deformation we conclude that these seismic activities that are associated by crustal deformation are owing to some magma activity beneath the crust of the Abu-Dabbab area.

[Stakeholder representations of the role of the intermediate level of the DRC health system].

PubMed

Mbeva, Jean Bosco Kahindo; Karemere, Hermès; Schirvel, Carole; Porignon, Denis

2014-01-01

Intermediate health care structures in the DRC were designed during the setting-up of primary health care in a perspective of health district support. This study was designed to describe stakeholder representations of the intermediate level of the DRC health system during the first 30 years of the primary health care system. This case study was based on inductive analysis of data from 27 key informant interviews.. The intermediate level of the health system, lacking sufficient expertise and funding during the 1980s, was confined to inspection and control functions, answering to the central level of the Ministry of health and provincial authorities. Since the 1990s, faced with the pressing demand for support from health district teams, whose self-management had to deal with humanitarian emergencies, the need to integrate vertical programmes, and cope with the logistics of many different actors, the intermediate heath system developed methods and tools to support heath districts. This resulted in a subsidiary model of the intermediate level, the perceived efficacy of which varies according to the province over recent years. The "subsidiary" model of the intermediary health system level seems a good alternative to the "control" model in DRC.

Deep crustal earthquakes associated with continental rifts

NASA Astrophysics Data System (ADS)

Doser, Diane I.; Yarwood, Dennis R.

1994-01-01

Deep (> 20 km) crustal earthquakes have occurred within or along the margins of at least four continental rift zones. The largest of these deep crustal earthquakes ( M ⩾ 5.0) have strike-slip or oblique-slip mechanisms with T-axes oriented similarly to those associated with shallow normal faulting within the rift zones. The majority of deep crustal earthquakes occur along the rift margins in regions that have cooler, thicker crust. Several deep crustal events, however, occur in regions of high heat flow. These regions also appear to be regions of high strain, a factor that could account for the observed depths. We believe the deep crustal earthquakes represent either the relative motion of rift zones with respect to adjacent stable regions or the propagation of rifting into stable regions.

Crustal structure of Yunnan province, People's Republic of China, from seismic refraction profiles

USGS Publications Warehouse

Kan, R.-J.; Hu, H.-X.; Zeng, R.-S.; Mooney, W.D.; McEvilly, T.V.

1986-01-01

Seismic refraction, profiles in Yunnan Province, southwestern China, define the crustal structure in an area of active tectonics, on the southern end of the Himalaya-Burma arc. The crustal thickness ranges from 38 to 46 kilometers, and the relatively low mean crustal velocity indicates a crustal composition compatible with normal continental crust and consisting mainly of meta-sedimentary and silicic intrusive rocks, with little mafic or ultramafic component. This composition suggests a crustal evolution involving sedimentary processes on the flank of the Yangtze platform rather than the accretion of oceanic island arcs, as has been proposed. An anomalously low upper-mantle velocity observed on one profile, but not on another at right angles to it may indicate active tectonic processes in the mantle or seismic anisotropy.

Estimating the Crustal Power Spectrum From Vector Magsat Data: Crustal Power Spectrum

NASA Technical Reports Server (NTRS)

Lowe, David A. J.; Parker, Robert L.; Purucker, Michael E.; Constable, Catherine G.

2000-01-01

The Earth's magnetic field can be subdivided into core and crustal components and we seek to characterize the crustal part through its spatial power spectrum (R(sub l)). We process vector Magsat data to isolate the crustal field and then invert power spectral densities of flight-local components along-track for R(sub l) following O'Brien et al. [1999]. Our model (LPPC) is accurate up to approximately degree 45 (lambda=900 km) - this is the resolution limit of our data and suggests that global crustal anomaly maps constructed from vector Magsat data should not contain features with wavelengths less than 900 km. We find continental power spectra to be greater than oceanic ones and attribute this to the relative thicknesses of continental and oceanic crust.

Crustal Structure of Yunnan Province, People's Republic of China, from Seismic Refraction Profiles.

PubMed

Kan, R J; Hu, H X; Zeng, R S; Mooney, W D; McEvilly, T V

1986-10-24

Seismic refraction, profiles in Yunnan Province, southwestern China, define the crustal structure in an area of active tectonics on the southern end of the Himalaya-Burma arc. The crustal thickness ranges from 38 to 46 kilometers, and the relatively low mean crustal velocity indicates a crustal composition compatible with normal continental crust and consisting mainly of meta-sedimentary and silicic intrusive rocks, with little mafic or ultramafic component. This composition suggests a crustal evolution involving sedimentary processes on the flank of the Yangtze platform rather than the accretion of oceanic island arcs, as has been proposed. An anomalously low upper-mantle velocity observed on one profile but not on another at right angles to it may indicate active tectonic processes in the mantle or seismic anisotropy.

Geochemistry of regional background aerosols in the Western Mediterranean

NASA Astrophysics Data System (ADS)

Pey, J.; Pérez, N.; Castillo, S.; Viana, M.; Moreno, T.; Pandolfi, M.; López-Sebastián, J. M.; Alastuey, A.; Querol, X.

2009-11-01

The chemical composition of regional background aerosols, and the time variability and sources in the Western Mediterranean are interpreted in this study. To this end 2002-2007 PM speciation data from an European Supersite for Atmospheric Aerosol Research (Montseny, MSY, located 40 km NNE of Barcelona in NE Spain) were evaluated, with these data being considered representative of regional background aerosols in the Western Mediterranean Basin. The mean PM 10, PM 2.5 and PM 1 levels at MSY during 2002-2007 were 16, 14 and 11 µg/m 3, respectively. After compiling data on regional background PM speciation from Europe to compare our data, it is evidenced that the Western Mediterranean aerosol is characterised by higher concentrations of crustal material but lower levels of OM + EC and ammonium nitrate than at central European sites. Relatively high PM 2.5 concentrations due to the transport of anthropogenic aerosols (mostly carbonaceous and sulphate) from populated coastal areas were recorded, especially during winter anticyclonic episodes and summer midday PM highs (the latter associated with the transport of the breeze and the expansion of the mixing layer). Source apportionment analyses indicated that the major contributors to PM 2.5 and PM 10 were secondary sulphate, secondary nitrate and crustal material, whereas the higher load of the anthropogenic component in PM 2.5 reflects the influence of regional (traffic and industrial) emissions. Levels of mineral, sulphate, sea spray and carbonaceous aerosols were higher in summer, whereas nitrate levels and Cl/Na were higher in winter. A considerably high OC/EC ratio (14 in summer, 10 in winter) was detected, which could be due to a combination of high biogenic emissions of secondary organic aerosol, SOA precursors, ozone levels and insolation, and intensive recirculation of aged air masses. Compared with more locally derived crustal geological dusts, African dust intrusions introduce relatively quartz-poor but clay mineral-rich silicate PM, with more kaolinitic clays from central North Africa in summer, and more smectitic clays from NW Africa in spring.

Geometry of a large-scale low-angle mid-crustal thrust (Woodroffe Thrust, Central Australia)

NASA Astrophysics Data System (ADS)

Wex, Sebastian; Mancktelow, Neil S.; Hawemann, Friedrich; Pennacchioni, Giorgio; Camacho, Alfredo

2015-04-01

Young orogens, such as the Alps, mainly expose the upper part of the continental crust and it is not possible to follow large-scale thrusts (e.g. the Glarus Thrust) to great depth in order to study their changing rheological behavior. This knowledge, however, is crucial for determining the overall kinematic and dynamic response during collision, as middle to lower crustal rocks represent the major part of the total crustal section. Information from deeper parts of the continental crust can only be obtained directly by investigating regions where these levels are now exhumed. The Musgrave Ranges in Central Australia is a very well exposed, semi-desert area, in which numerous large-scale shear zones developed during the Petermann Orogeny around 550 Ma. The most prominent structure is the ˜400 km long E-W trending Woodroffe Thrust, which placed ˜1.2 Ga granulites onto similarly-aged amphibolite and granulite facies gneisses along a generally south-dipping thrust plane with a top-to-north shear sense. Geothermobarometric calculations on the associated mylonites established that the structure developed under mid-crustal conditions (500-650°C, 0.8-1 GPa). Regional P/T variations in the direction of thrusting are small, but show trends consistent with the south-dipping orientation of the thrust plane, which predicts deeper levels and a higher metamorphic grade in the south than in the north. They imply a very low gradient of only around 3°C/km for a distance of some 30 km in the movement direction of the thrust. Combined with a geothermal gradient on the order of 20°C/km, calculated from four separate P/T estimates from the hanging wall and footwall, this regional gradient indicates that the Woodroffe Thrust was originally shallow-dipping at an average angle of only around 9°. This suggests that upper crustal brittle thrusts do not necessarily steepen into the middle to lower crust, but can define very shallow-dipping, large-scale planar features, with dimensions in the order of hundreds of kilometres. Such a geometry would require the rocks to be weak, but field observations (e.g. large volumes of syn-tectonic pseudotachylyte) argue for strong behaviour, involving alternating fast (seismic) fracturing and slow (aseismic) creep.

Investigating the magnitude of lower crustal flow and impact on surface deformation patterns in Tibet using 3-D geodynamic models

NASA Astrophysics Data System (ADS)

Bischoff, S. H.; Flesch, L. M.

2016-12-01

Differential flow in the lower crust of Tibet has been invoked to explain features in the region, including uniform plateau elevation, crustal thickness/topographic gradients, and uplift without observed shortening. Here, we use 3-D finite element modeling to test impacts of assumed lower crustal viscosities on deformation patterns in the India-Eurasia collision zone. We simulate instantaneous lithospheric deformation with Stokes flow using COMSOL Multiphysics (www.comsol.com). Our model geometry ranges eastward from the Pamir to Sichuan, northward from the southern tip of India to the Tien Shan, and vertically downward from the Earth's surface to 100 km below sea level. We divide model geometry into four domains: Indian lithosphere, Eurasian upper crust, lower crust, and upper mantle. Seismic and magnetotelluric study results guide inclusion of subducted Indian and Burma slabs along with our targeted weak lower crust. Within the larger Eurasian lower crust domain, weak lower crust is restricted to a zone bounded clockwise by the Himalayan Frontal Thrust, Karakorum, Altyn-Tagh, Kunlun, Longmen Shan, and onset of lower elevations along the plateau's southeastern margin. From top to bottom, vertical bounds of the zone are constrained by a constant 20 km below sea level and the shallower of either the top of the Indian slab or Moho. Strength is approximated via 3-D maps of effective viscosity constrained by the vertically-averaged lithospheric estimates of Flesch et al. [2001]. We forward model lower crust effective viscosities on the order of 1018 to 1022 Pa•s and inspect resulting horizontal and vertical deformation patterns. Results suggest that effective viscosities of less than 1020 Pa•s are required for both appreciable differential mass flux through lower crustal flow as well as decoupled lower crustal flow from the upper crust or mantle. Movement of the lower crust is partitioned within weaker fault zones. Effective viscosities of 1020 Pa•s or less produce pronounced patterns of surface subsidence in Qiangtang and uplift in eastern Lhasa and Longmen Shan inconsistent with observations. Solutions show lower crust strength impacts surface stress style with weaker strengths leading to regions of dominant extension separated by compression in the east central Tibetan Plateau.

Geochronologic Constraints on Duration of Magma Emplacement and Heat Transfer in the Deep Crust: new data from the Ivrea Zone, Western Alps, Italy

NASA Astrophysics Data System (ADS)

Peressini, G.; Quick, J. E.; Poller, U.; Todt, W.; Mayer, A.; Sinigoi, S.; Hofmann, A. W.

2002-12-01

The Mafic Complex (MC) of the Ivrea Zone is one of the largest gabbro bodies in the Alps (ca 8 km thick and 30 km long); it intruded the already metamorphic volcano-sedimentary sequence of the Kinzigite Formation (KF) at a depth of more than 20 km during the Late Paleozoic. New geochronologic data constrain the duration of the intrusion. The crustal section, uplifted, tilted and exposed in Alpine time, is tectonically bounded, but essentially undisturbed by Alpine tectonics; the internal structure of the MC and its relations with the KF are well preserved. Intrusion of the MC in extending continental crust is suggested by pre-Triassic, high-T, extensional shear zones in the Ivrea Zone, and is consistent with the internal arcuate structure of the MC, which is defined by high-T foliation and banding, that are discordant to the roof of the intrusion. Rocks in the roof of the MC attain upper-amphibolite grade and show evidence of partial melting within about 2 km of the MC. The country rock was melted as a consequence of heat released by the crystallizing mafic body; the chemistry of the mafic rocks was affected by up to 30% crustal contamination that occurred partially in situ, by mixing of the basaltic melts with anatectic melts derived from depleted crustal rocks. A thin (less than 20 m) seam of leucogranite crystallized from anatectic melt is present at the MC-KF contact. Syntectonic intrusion of leucogranite along shear zones within the KF demonstrates migration of anatectic melts to higher crustal levels. U/Pb SHRIMP ages on magmatic zircons range from 295+4 and 287+4 Ma for the MC, and 280+4 Ma for syntectonic leucogranites in the KF. Thus, intrusion of the MC may have taken as long as 10-15 Ma. Nd-Sm mineral isochrones for the MC range from 244 to 274, indicating that the Complex cooled to temperatures below 750C within about 40 Ma of final crystallization. The heat of crystallization of the MC was accommodated by anatexis and assimilation, and syntectonic migration of anatectic melts transferred heat to higher crustal levels.

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.

Oxygen and strontium isotopic studies of basaltic lavas from the Snake River plain, Idaho

USGS Publications Warehouse

Leeman, William P.; Whelan, Joseph F.

1983-01-01

The Snake Creek-Williams Canyon pluton of the southern Snake Range crops out over an area of about 30 km2, about 60 km southeast of Ely, Nev. This Jurassic intrusion displays large and systematic chemical and mineralogical zonation over a horizontal distance of 5 km. Major-element variations compare closely with Dalyls average andesite-dacite-rhyolite over an SiO2 range of 63 to 76 percent. For various reasons it was originally thought that assimilation played a dominant role in development of the Snake Creek-Williams Canyon pluton. However, based on modeling of more recently obtained trace element and isotopic data, we have concluded that the zonation is the result of in-situ fractional crystallization, with little assimilation at the level of crystallization. This report summarizes data available for each of the mineral species present in the zoned intrusion. Special attention has been paid to trends We present oxygen and strontium isotopic data for olivine tholeiites, evolved (that is, differentiated and (or) contaminated) lavas, rhyolites, and crustal- derived xenoliths from the Snake River Plain. These data show that the olivine tholeiites are fairly uniform in d80 (5.1 to 6.2) and 87Sr/86Sr (0.7056 to 0.7076) and reveal no correlation between these ratios. The tholeiites are considered representative of mantle-derived magmas that have not interacted significantly with crustal material or meteoric water. The evolved lavas display a wider range in d 80 (5.6 to 7.6) and 87Sr/86Sr (0.708 to 0.717) with positive correlations between these ratios in some suites but not in others. Crustal xenoliths have high and variable 8?Sr/86Sr (0.715 to 0.830) and d80 values that vary widely (6.7 to 9.2) and are a few permil greater than d80 values of the Snake River basalts. Thus, isotopic data for the evolved lavas are permissive of small degrees of contamination by crustal rocks similar to the most d80-depleted xenoliths. The d80 enrichments in some evolved lavas also are consistent with crystal fractionation processes and do not necessarily require bulk interaction with crustal rocks. Enrichment in d80 but not in 87Sr/86Sr in one suite of evolved lavas suggests that crustal contamination may not be essential to the petrogenesis of those lavas. Other suites of evolved lavas display large variations in 87Sr/86Sr that reflect at least some selective contamination with 87St. Bulk solid/liquid oxygen-isotope fractionation factors (a's) calculated for the evolved lavas from Craters of the Moon National Monument are comparatively large. These a's are dependent upon the nature and proportions of phases removed by crystal fractionation; basaltic lava a's differ from latitic lava a?s in accordance with different phenocryst assemblages in these rocks. Snake River Plain rhyolites are isotopically distinct from both the analyzed crustal xenoliths and olivine tholeiites. Their origin remains poorly understood, but crustal or sub-crustal sources may be viable. In the first case, they must be derived by anatexis of material distinct from the analyzed crustal xenoliths. In the second case, they must be derived from material unlike the source for tholeiites. No cogenetic relation with the tholeiites seems likely on the basis of available data. that might relate to the variation in the chemical petrology of the pluton.

Crustal growth and episodic reworking over one billion years in the Capricorn Orogen, Western Australia: evidence from Lu-Hf and O isotope data

NASA Astrophysics Data System (ADS)

Jahn, Inalee; Clark, Chris; Reddy, Steve; Taylor, Rich

2017-04-01

Fundamental to understanding the generation and evolution of a crustal block is knowledge of the relationship between additions of new material from the mantle, and the extent of crustal recycling [1]. Hafnium isotope ratios can be used to characterise relative contributions from mantle, crustal and recycled reservoirs within magmas. Oxygen isotopes can be used to constrain the extent of crustal interaction during magma emplacement. When used in conjunction, they can help unravel multiple crystallisation histories of a crustal block, and follow the source composition through magma evolution. The Capricorn Orogen records the Paleoproterozoic collision of the Yilgarn and Pilbara Cratons to form the West Australian Craton, and over one billion years of subsequent intracontinental crustal reworking. U-Pb zircon geochronology records three discrete tectono-magmatic events which resulted in voluminous granitic magmatism: the 2005-1975 Ma Glenburgh Orogeny, the 1820-1770 Ma Capricorn Orogeny, and the 1680-1620 Ma Durlacher Orogeny [2]. We present U-Pb, Lu-Hf and δ18O isotopic data from zircon from 50 samples of granites and granitoids from the Capricorn Orogen to provide constraints on the crustal evolution of the Paleoproterozoic crust. Our results confirm crustal growth by juvenile mantle input was limited to the Glenburgh Orogeny associated with the amalgamation of the West Australian Craton, while all subsequent Paleoproterozoic magmatism was primarily derived from significant reworking of the pre-existing crustal components. Time-sliced maps showing the variation in Hf and O isotopes can be used to image crustal evolution in space and time, and are particularly useful in constraining the spatial and temporal extent of juvenile magmatic additions to the crust. These maps suggest that crustal growth was concentrated along, or in the terranes adjacent to, the Yilgarn Craton margin. Our results are in agreement with previous isotopic studies [3], and provide additional constraints for the evolution of the Paleoproterozoic crust within the Capricorn Orogen. [1] Cawood et al. 2013. Geological Society of America Bulletin, 125(1-2), 14-32 [2] Sheppard et al. 2010. Geological Survey of Western Australia, Perth, Western Australia, 336 [3] Johnson et al. 2017. Lithos, 268, 76-86

Tracing crustal contamination along the Java segment of the Sunda Arc, Indonesia

NASA Astrophysics Data System (ADS)

Jolis, E. M.; Troll, V.; Deegan, F.; Blythe, L.; Harris, C.; Freda, C.; Hilton, D.; Chadwick, J.; Van Helden, M.

2012-04-01

Arc magmas typically display chemical and petrographic characteristics indicative of crustal input. Crustal contamination can take place either in the mantle source region or as magma traverses the upper crust (e.g. [1]). While source contamination is generally considered the dominant process (e.g. [2]), late-stage crustal contamination has been recognised at volcanic arcs too (e.g. [3]). In light of this, we aim to test the extent of upper crustal versus source contamination along the Java segment of the Sunda arc, which, due its variable upper crustal structure, is an exemplary natural laboratory. We present a detailed geochemical study of 7 volcanoes along a traverse from Anak-Krakatau in the Sunda strait through Java and Bali, to characterise the impact of the overlying crust on arc magma composition. Using rock and mineral elemental geochemistry, radiogenic (Sr, Nd and Pb) and, stable (O) isotopes, we show a correlation between upper crustal composition and the degree of upper crustal contamination. We find an increase in 87Sr/86Sr and δ18O values, and a decrease in 143Nd/144Nd values from Krakatau towards Merapi, indicating substantial crustal input from the thick continental basement present. Volcanoes to the east of Merapi and the Progo-Muria fault transition zone, where the upper crust is thinner, in turn, show considerably less crustal input in their isotopic signatures, indicating a stronger influence of the mantle source. Our new data represent a systematic and high-resolution arc-wide sampling effort that allows us to distinguish the effects of the upper crust on the compositional spectrum of individual volcanic systems along the Sunda arc. [1] Davidson, J.P, Hora, J.M, Garrison, J.M & Dungan, M.A 2005. Crustal Forensics in Arc Magmas. J. Geotherm. Res. 140, 157-170; [2] Debaille, V., Doucelance, R., Weis, D., & Schiano, P. 2005. Geochim. Cosmochim. Acta, 70,723-741; [3] Gasparon, M., Hilton, D.R., & Varne, R. 1994. Earth Planet. Sci. Lett., 126, 15-22.

Improved H-κ Method by Harmonic Analysis on Ps and Crustal Multiples in Receiver Functions with respect to Dipping Moho and Crustal Anisotropy

NASA Astrophysics Data System (ADS)

Li, J.; Song, X.; Wang, P.; Zhu, L.

2017-12-01

The H-κ method (Zhu and Kanamori, 2000) has been widely used to estimate the crustal thickness and Vp/Vs ratio with receiver functions. However, in regions where the crustal structure is complicated, the method may produce uncertain or even unrealistic results, arising particularly from dipping Moho and/or crustal anisotropy. Here, we propose an improved H-κ method, which corrects for these effects first before stacking. The effect of dipping Moho and crustal anisotropy on Ps receiver function has been well studied, but not as much on crustal multiples (PpPs and PpSs+PsPs). Synthetic tests show that the effect of crustal anisotropy on the multiples are similar to Ps, while the effect of dipping Moho on the multiples is 5 times that on Ps (same cosine trend but 5 times in time shift). A Harmonic Analysis (HA) method for dipping/anisotropy was developed by Wang et al. (2017) for crustal Ps receiver functions to extract parameters of dipping Moho and crustal azimuthal anisotropy. In real data, the crustal multiples are much more complicated than the Ps. Therefore, we use the HA method (Wang et al., 2017), but apply separately to Ps and the multiples. It shows that although complicated, the trend of multiples can still be reasonably well represented by the HA. We then perform separate azimuthal corrections for Ps and the multiples and stack to obtain a combined receiver function. Lastly, the traditional H-κ procedure is applied to the stacked receiver function. We apply the improved H-κ method on 40 CNDSN (Chinese National Digital Seismic Network) stations distributed in a variety of geological setting across the Chinese continent. The results show apparent improvement compared to the traditional H-κ method, with clearer traces of multiples and stronger stacking energy in the grid search, as well as more reliable H-κ values.

Receiver Function Study of the Crustal Structure Beneath the Northern Andes (colombia)

NASA Astrophysics Data System (ADS)

Poveda, E.; Monsalve, G.; Vargas-Jimenez, C. A.

2013-05-01

We have investigated crustal thickness beneath the Northern Andes with the teleseismic receiver function technique. We used teleseismic data recorded by an array of 18 broadband stations deployed by the Colombian Seismological Network, and operated by the Colombian Geological Survey. We used the primary P-to-S conversion and crustal reverberations to estimate crustal thickness and average Vp/Vs ratio; using Wadati diagrams, we also calculated the mean crustal Vp/Vs ratio around stations to further constrain the crustal thickness estimation. In northern Colombia, near the Caribbean coast, the estimated crustal thickness ranges from 25 to 30 km; in the Middle Magdalena Valley, crustal thickness is around 40 km; beneath the northern Central Cordillera, the Moho depth is nearly 40 km; at the Ecuador-Colombia border, beneath the western flank of the Andes, the estimated thickness is about 46 km. Receiver functions at a station at the craton in South East Colombia, near the foothills of the Eastern Cordillera, clearly indicate the presence of the Moho discontinuity at a depth near 36 km. The greatest values of crustal thickness occur beneath a plateau (Altiplano Cundiboyacense) on the Eastern Cordillera, near the location of Bogota, with values around 58 km. Receiver functions in the volcanic areas of the south-western Colombian Andes do not show a systematic signal from the Moho, indicating abrupt changes in Moho geometry. Signals at stations on the Eastern Cordillera near Bogota reveal a highly complex crustal structure, with a combination of sedimentary layers up to 9 km thick, dipping interfaces, low velocity layers, anisotropy and/or lateral heterogeneity that still remain to be evaluated. This complexity obeys to the location of these stations at a region of a highly deformed fold and thrust belt.

Late Cenozoic crustal extension and magmatism, southern Death Valley region, California

USGS Publications Warehouse

Calzia, J.P.; Rämö, O.T.

2000-01-01

The late Cenozoic geologic history of the southern Death Valley region is characterized by coeval crustal extension and magamatism. Crustal extension is accommodated by numerous listric and planar normal faults as well as right- and left-lateral strike slip faults. The normal faults sip 30°-50° near the surface and flatten and merge leozoic miogeoclinal rocks; the strike-slip faults act as tear faults between crustal blocks that have extended at different times and at different rates. Crustal extension began 13.4-13.1 Ma and migrated northwestward with time; undeformed basalt flows and lacustrine deposits suggest that extension stopped in this region (but continued north of the Death Valley graben) between 5 and 7 Ma. Estimates of crustal extension in this region vary from 30-50 percent to more than 100 percent. Magmatic rocks syntectonic with crustal extension in the southern Death Valley region include 12.4-6.4 Ma granitic rocks as well as bimodal 14.0-4.0 Ma volcanic rocks. Geochemical and isotopic evidence suggest that the granitic rocks get younger and less alkalic from south to north; the volcanic rocks become more mafic with less evidence of crustal interaction as they get younger. The close spatial and temporal relation between crustal extension and magmatism suggest a genetic and probably a dynamic relation between these geologic processes. We propose a rectonic-magmatic model that requires heat to be transported into the crust by mantle-derived mafic magmas. These magmas pond at lithologic or rheologic boundaries, begin the crystallize, and partially melt the surrounding crustal rocks. With time, the thermally weakened crust is extended (given a regional extensional stress field) concurrent with granitic magmatism and bimodal volcanism.

Lower crustal mush generation and evolution

NASA Astrophysics Data System (ADS)

Karakas, Ozge; Bachmann, Olivier; Dufek, Josef; Wright, Heather; Mangan, Margaret

2016-04-01

Recent seismic, field, and petrologic studies on several active and fossil volcanic settings provide important constraints on the time, volume, and melt fraction of their lower crustal magma bodies. However, these studies provide an incomplete picture of the time and length scales involved during their thermal and compositional evolution. What has been lacking is a thermal model that explains the temporal evolution and state of the lower crustal magma bodies during their growth. Here we use a two-dimensional thermal model and quantify the time and length scales involved in the long-term thermal and compositional evolution of the lower crustal mush regions underlying the Salton Sea Geothermal Field (USA), Mt St Helens (USA), and the Ivrea-Verbano Zone (North Italy). Although a number of seismic, tectonic, petrologic, and field studies explained the tectonic and magmatic evolution of these regions, controversy remains on their lower crustal heat sources, melt fraction, and origin of erupted magmas. Our thermal modeling results suggest that given a geologically reasonable range of basalt fluxes (~10^-3 to 10^-4 km3/yr), a long-lived (>105 yr) crystalline mush is formed in the lower crust. The state of the lower crustal mush is strongly influenced by the magma flux, crustal thickness, and water content of intruded basalt, giving an average melt fraction of <0.2 in thin crust with dry injections (Salton Sea Geothermal Field) and up to 0.4-0.5 in thicker crust with wet injections (Mt St Helens and Ivrea Zone). The melt in the lower crustal mush is mainly evolving through fractional crystallization of basalt with minor crustal assimilation in all regions, in agreement with isotopic studies. Quantification of the lower crustal mush regions is key to understanding the mass and heat balance in the crust, evolution of magma plumbing systems, and geothermal energy exploration.

Evaluation of six NEHRP B/C crustal amplification models proposed for use in western North America

USGS Publications Warehouse

Boore, David; Campbell, Kenneth W.

2016-01-01

We evaluate six crustal amplification models based on National Earthquake Hazards Reduction Program (NEHRP) B/C crustal profiles proposed for use in western North America (WNA) and often used in other active crustal regions where crustal properties are unknown. One of the models is based on an interpolation of generic rock velocity profiles previously proposed for WNA and central and eastern North America (CENA), in conjunction with material densities based on an updated velocity–density relationship. A second model is based on the velocity profile used to develop amplification factors for the Next Generation Attenuation (NGA)‐West2 project. A third model is based on a near‐surface velocity profile developed from the NGA‐West2 site database. A fourth model is based on velocity and density profiles originally proposed for use in CENA but recently used to represent crustal properties in California. We propose two alternatives to this latter model that more closely represent WNA crustal properties. We adopt a value of site attenuation (κ0) for each model that is either recommended by the author of the model or proposed by us. Stochastic simulation is used to evaluate the Fourier amplification factors and their impact on response spectra associated with each model. Based on this evaluation, we conclude that among the available models evaluated in this study the NEHRP B/C amplification model of Boore (2016) best represents median crustal amplification in WNA, although the amplification models based on the crustal profiles of Kamai et al. (2013, 2016, unpublished manuscript, see Data and Resources) and Yenier and Atkinson (2015), the latter adjusted to WNA crustal properties, can be used to represent epistemic uncertainty.

The Pikwitonei granulite domain: A lower crustal level along the Churchill-Superior boundary in central Manitoba

NASA Technical Reports Server (NTRS)

Weber, W.

1983-01-01

The greenschist to amphibolite facies tonalite-greenstone terrain of the Gods Lake subprovince grades - in a northwesterly direction - into the granulite facies Pikwitonei domain at the western margins of the Superior Province. The transition is the result of prograde metamorphism and takes place over 50 - 100 km without any structural or lithological breaks. Locally the orthopyroxene isograd is oblique to the structural grain and transects greenstone belts, e.g., the Cross Lake belt. The greenstone belts in the granulite facies and adjacent lower grade domain consist mainly of mafic and (minor) ultramafic metavolcanics, and clastic and chemical metasedimentary rocks. Typical for the greenstone belts crossed by the orthopyroxene isograd are anorthositic gabbros and anorthosites, and plagiophyric mafic flows. The Pikwitonei granulite domain has been interpreted as to represent a lower crustal level which was uplifted to the present level of erosion. On the basis of gravimetric data this uplift has been modelled as an obduction onto the Churchill Province during the Hudsonian orogeny, similar to the Ivrea Zone. The fault between the Churchill and Superior Province is described.

Crustal deformation mechanism in southeastern Tibetan Plateau: Insights from numerical modeling

NASA Astrophysics Data System (ADS)

Li, Y.; Liu, S.; Chen, L.

2017-12-01

The Indo-Asian collision developed the complicated crustal deformation around the southeastern Tibetan plateau. Numerous models have proposed to explain the crustal deformation, but the mechanism remains controversial, especially the increasing multi-geophysics data, which demonstrate the existence of lower velocity, lower resistivity and high conductivity, implying that lower crustal flow is responsible for the crustal deformation, arguing for the lower crust flow model. To address the relations between the crust flow and the surface deformation, we employ a three-dimensional viscoelastic finite model to investigate the possible influence on the surface deformation, and discuss the stress field distribution under the model. Our preliminary results suggest that lower crustal flow plays an important role in crustal deformation in southeastern Tibetan plateau. The best fitting is achieved when the flow velocity of the lower crust is approximately 10-11 mm/a faster than that of the upper crust. Crustal rheological properties affect regional crustal deformation, when the viscosity of the middle and lower crust in the South China block reaches 1022 and 1023 Pa.s, respectively; the predicted match observations well, especially for the magnitude within the South China block. The maximum principal stress field exhibits clear zoning, gradually shifting from an approximately east-west orientation in the northern Bayan Har block to southeast in the South China block, southwest in the western Yunnan block, and a radially divergent distribution in the Middle Yunnan and Southern Yunnan blocks.

Crustal structure of Mars from gravity and topography

NASA Technical Reports Server (NTRS)

Neumann, G. A.; Zuber, M. T.; Wieczorek, M. A.; McGovern, P. J.; Lemoine, F. G.; Smith, D. E.

2004-01-01

Mars Orbiter Laser Altimeter (MOLA) topography and gravity models from 5 years of Mars Global Surveyor (MGS) spacecraft tracking provide a window into the structure of the Martian crust and upper mantle. We apply a finite-amplitude terrain correction assuming uniform crustal density and additional corrections for the anomalous densities of the polar caps, the major volcanos, and the hydrostatic flattening of the core. A nonlinear inversion for Moho relief yields a crustal thickness model that obeys a plausible power law and resolves features as small as 300 km wavelength. On the basis of petrological and geophysical constraints, we invoke a mantle density contrast of 600 kg m-3; with this assumption, the Isidis and Hellas gravity anomalies constrain the global mean crustal thickness to be >45 km. The crust is characterized by a degree 1 structure that is several times larger than any higher degree harmonic component, representing the geophysical manifestation of the planet's hemispheric dichotomy. It corresponds to a distinction between modal crustal thicknesses of 32 km and 58 km in the northern and southern hemispheres, respectively. The Tharsis rise and Hellas annulus represent the strongest components in the degree 2 crustal thickness structure. A uniform highland crustal thickness suggests a single mechanism for its formation, with subsequent modification by the Hellas impact, erosion, and the volcanic construction of Tharsis. The largest surviving lowland impact, Utopia, post-dated formation of the crustal dichotomy. Its crustal structure is preserved, making it unlikely that the northern crust was subsequently thinned by internal processes.

Cervicovaginal cytokines, sialidase activity and bacterial load in reproductive-aged women with intermediate vaginal flora.

PubMed

Santos-Greatti, Mariana Morena de Vieira; da Silva, Márcia Guimarães; Ferreira, Carolina Sanitá Tafner; Marconi, Camila

2016-11-01

Studies have shown that not only bacterial vaginosis, but also intermediate vaginal flora has deleterious effects for women's reproductive health. However, literature still lacks information about microbiological and immunological aspects of intermediate flora. To characterize intermediate flora regarding levels of Interleukin (IL)-1beta, IL-6, IL-8, tumor necrosis factor-alpha, interleukin 1 receptor antagonist (IL-1ra), IL-10, sialidase; loads of Gardnerella vaginalis, total bacteria and to verify whether it is closer related to normal flora or bacterial vaginosis. This cross-sectional study enrolled 526 non-pregnant reproductive-aged women distributed in 3 groups according to pattern of vaginal flora using Nugent's system in normal, intermediate and bacterial vaginosis. Cervicovaginal levels of cytokines, sialidases, loads of G. vaginalis and total bacteria were assessed by ELISA, conversion of MUAN and quantitative real-time PCR, respectively. A principal component analysis(PCA) using all measured parameters was performed to compare the three different types of flora. Results showed that intermediate flora is associated with increased cervicovaginal IL-1beta in relation to normal flora(P<0.0001). When compared to bacterial vaginosis, intermediate flora has higher IL-8 and IL-10 levels(P<0.01). Sialidases were in significantly lower levels in normal and intermediate flora than bacterial vaginosis(P<0.0001). Loads of G. vaginalis and total bacterial differed among all groups(P<0.0001), being highest in bacterial vaginosis. PCA showed that normal and intermediate flora were closely scattered, while bacterial vaginosis were grouped separately. Although intermediate flora shows some differences in cytokines, sialidases and bacterial loads in relation to normal flora and bacterial vaginosis, when taken together, general microbiological and immunological pattern pattern of intermediate flora resembles the normal flora. Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.

Thermal and mass implications of magmatic evolution in the Lassen volcanic region, California, and minimum constraints on basalt influx to the lower crust

USGS Publications Warehouse

Guffanti, M.; Clynne, M.A.; Muffler, L.J.P.

1996-01-01

We have analyzed the heat and mass demands of a petrologic model of basaltdriven magmatic evolution in which variously fractionated mafic magmas mix with silicic partial melts of the lower crust. We have formulated steady state heat budgets for two volcanically distinct areas in the Lassen region: the large, late Quaternary, intermediate to silicic Lassen volcanic center and the nearby, coeval, less evolved Caribou volcanic field. At Caribou volcanic field, heat provided by cooling and fractional crystallization of 52 km3 of basalt is more than sufficient to produce 10 km3 of rhyolitic melt by partial melting of lower crust. Net heat added by basalt intrusion at Caribou volcanic field is equivalent to an increase in lower crustal heat flow of ???7 mW m-2, indicating that the field is not a major crustal thermal anomaly. Addition of cumulates from fractionation is offset by removal of erupted partial melts. A minimum basalt influx of 0.3 km3 (km2 Ma)-1 is needed to supply Caribou volcanic field. Our methodology does not fully account for an influx of basalt that remains in the crust as derivative intrusives. On the basis of comparison to deep heat flow, the input of basalt could be ???3 to 7 times the amount we calculate. At Lassen volcanic center, at least 203 km3 of mantle-derived basalt is needed to produce 141 km3 of partial melt and drive the volcanic system. Partial melting mobilizes lower crustal material, augmenting the magmatic volume available for eruption at Lassen volcanic center; thus the erupted volume of 215 km3 exceeds the calculated basalt input of 203 km3. The minimum basalt input of 1.6 km3 (km2 Ma)-1 is >5 times the minimum influx to the Caribou volcanic field. Basalt influx high enough to sustain considerable partial melting, coupled with locally high extension rate, is a crucial factor in development of Lassen volcanic center; in contrast. Caribou volcanic field has failed to develop into a large silicic center primarily because basalt supply there has been insufficient.

Seismic images of multiple magma sills beneath the East Pacific Rise

NASA Astrophysics Data System (ADS)

Marjanovic, M.; Carbotte, S. M.; Carton, H. D.; Mutter, J. C.; Nedimovic, M. R.; Canales, J.

2013-12-01

Along fast and intermediate spreading centers, thin and narrow axial magma lenses (AMLs) are detected beneath much of the ridge axis, and the notion that the AML is the primary melt reservoir for dike intrusions and volcanic eruptions that build the upper crust is commonly accepted. However the role of the AML in construction of the lower crust is still actively debated. Some models based on geochemistry and structural observations from ophiolites suggest that formation of the lower crustal gabbro section takes place in situ, from multiple small magma sills, with the AML being the shallowest of these. Here, we present new observations from multichannel seismic data collected in 2008 along the East Pacific Rise (EPR) for seismic reflectors below the AML or sub-axial magma lens (SAML). The most prominent SAML events are found between latitudes 9°20' and 9°56'N, where they appear as moderately bright, discontinuous reflectors, at ~ 50 to 300 ms (~ 200-600 m) below the AML. From an analysis of the characteristics of these events, we rule out possible 'artifact' origins for the SAML including, seafloor side scattering, out-of-plane imaging of the AML or other crustal horizons, internal multiples, and the presence of a P-to-S converted phase (PAMLS). We interpret these deep melt lenses to have a low crystalline component (i.e. they are mostly molten). Disruptions in the SAML reflector, represented by relatively abrupt steps in two-way travel time are collocated with small-scale discontinuities in the AML and further support the notion of crustal accretion through small magmatic units. In addition, within the area of documented volcanic eruptions in 1991-1992 and 2005-2006, two prominent gaps centered at 9°46' and 9°50.5' N in the SAML reflectors are identified. We hypothesize that magma from these deeper lenses have also contributed to the eruption, implying hydraulic connectivity between the AML and SAMLs during eruption events. We suggest that the SAMLs play an important role in eruption triggering and processes of magma lens replenishment and magma fractionation beneath this fast spreading ridge.

Lead isotopic evidence for mixed sources of Proterozoic granites and pegmatites, Black Hills, South Dakota, USA

NASA Astrophysics Data System (ADS)

Krogstad, Eirik J.; Walker, Richard J.; Nabelek, Peter I.; Russ-Nabelek, Carol

1993-10-01

The lead isotopic compositions of K-feldspars separated from the ca. 1700 Ma Harney Peak Granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources. It has been reported previously that the core of the Harney Peak Granite complex is dominated by relatively lower/ gd18O (avg. 11.5 %.) granites, whereas higher / gd18O (avg. 13.2%.) granites occur around the periphery of the complex. The higher δ 18O granites and one simple pegmatite have low values of 207Pb /204Pb for their 206Pb /204Pb Thus, they likely were derived from a source with a short crustal residence time. This source may have been the pelitic schists into which the Harney Peak Granite complex and pegmatites were intruded. Feldspars from granites with lower / gd18O values have significantly higher 207Pb /204Pb for their 206Pb /204Pb . The data define a linear array with a slope equivalent to an age of ca. 2.6 Ga with t 2 defined to be 1.7 Ga. Such a slope could represent a mixing array or a secondary isochron for the source. These low δ18O granites could have been derived from a source with a high U/ Pb and with a crustal residence beginning before the Proterozoic. The source (s) of these granites may have been a sediment derived from late Archean continental crust. The highly evolved Tin Mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups, suggesting either a mixed source or contamination. Two late Archean granites, the Little Elk Granite and the Bear Mountain Granite, had precursors with high U/Pb and low Th/U histories. The Th/U history of the Bear Mountain Granite is too low for this rock to have been an important component of the source of the Proterozoic granites. However, crustal rocks with lead isotopic compositions similar to those of the Little Elk Granite were an important source of lead for some of the Proterozoic granitic rocks.

Provenance and tectonic setting of siliciclastic rocks associated with the Neoproterozoic Dahongliutan BIF: Implications for the Precambrian crustal evolution of the Western Kunlun orogenic belt, NW China

NASA Astrophysics Data System (ADS)

Hu, Jun; Wang, He; Wang, Min

2017-10-01

The Late Neoproterozoic Dahongliutan BIF is associated with siliciclastic rocks in the Tianshuihai terrane of the Western Kunlun orogenic belt (WKO), NW China. The sedimentary rocks have various weathering indices (e.g., CIA = 57-87, PIA = 61-96 and Th/U = 4.85-12.45), indicative of varying degrees of weathering in the source area. The rocks have trace element ratios, such as Th/Sc = 0.60-1.21 and Co/Th = 0.29-1.67, and light rare earth element (LREE) enriched chondrite-normalized REE patterns, suggesting that they were mainly sourced from intermediate and felsic rocks. Available U-Pb ages of detrital zircon from these rocks reveal that the detrital sources may have been igneous and metamorphic rocks from the WKO and the Tarim Block. Our study suggests that the Dahongliutan BIF and hosting siliciclastic rocks may have deposited in a setting transitional from a passive to active continental margin, probably related to the Late Neoproterozoic-Early Cambrian seafloor spreading and subduction of the Proto-Tethys Ocean. U-Pb dating of 163 detrital zircons defines five major age populations at 2561-2329 Ma, 2076-1644 Ma, 1164-899 Ma, 869-722 Ma and 696-593 Ma. These age groups broadly correspond to the major stages of supercontinent assembly and breakup events widely accepted for Columbia, Rodinia and Gondwana. Some zircons have TDM2 model ages of 3.9-1.8 Ga and negative εHf(t) values, suggesting that the Archean to Paleoproterozoic (as old as Eoarchean) crustal materials were episodically reworked and incorporated into the late magmatic process in the WKO. Some Neoproterozoic zircons have TDM2 model ages of 1.47-1.07 Ga and 1.81-1.53 Ga and positive εHf(t) values, indicating juvenile crustal growth during the Mesoproterozoic. Our new results, combined with published data, imply that both the Tianshuihai terrane in the WKO and the Tarim Block share the same Precambrian tectonic evolution history.

Complex N-S variations in Moho depth and Vp/Vs ratio beneath the western Tibetan Plateau as revealed by receiver function analysis

NASA Astrophysics Data System (ADS)

Murodov, Davlatkhudzha; Zhao, Junmeng; Xu, Qiang; Liu, Hongbing; Pei, Shunping

2018-04-01

We present herein detailed images of the Moho depth and Vp/Vs ratio along ANTILOPE-1 profile beneath the western Tibetan Plateau derived from receiver function analysis. Along the ANTILOPE -1 profile, a rapidly northward dipping Moho extends from ˜50 km below the Himalaya to ˜80 km across the Indus-Yarlung suture (IYS), shallowing to ˜66 km under the central Lhasa terrane. The Moho depth shows a dramatic increase from ˜66 km north of the Bangong-Nujiang suture (BNS) to ˜93 km beneath central Qiangtang terrane where it reaches the maximum depth observed along this profile before steeply rising to ˜73 km. We interpret both the 15 km and 20 km offsets of Moho depth occurring beneath the central Lhasa and central Qiangtang terranes as being related to the northern frontiers of the decoupled underthrusting Indian lower crust and lithospheric mantle, respectively. The Moho remains at a depth of ˜70 km with a slight undulation beneath the northern Qiangtang and Songpan-Ganzi terranes, and then abruptly shallows to ˜45 km near the Altyn Tagh Fault. The ˜25 km Moho offset observed at the conjunction of the Tarim Basin and the Altyn Tagh mountain range suggests that the crustal shortening is achieved by pure shear thickening without much underthrusting. The average crustal Vp/Vs ratio changes from 1.66 to 1.80 beneath the Himalaya, the Lhasa terrane and the Tarim Basin indicating a felsic-to-intermediate composition. However, higher Vp/Vs ratios between 1.76 and 1.83 (except for a few outlying low values) are found beneath the Qiangtang and Songpan-Ganzi terranes, which could be attributed to the joint effects of the more mafic composition and partial melt within the crust. The Moho depth and Vp/Vs ratio exhibit complex N-S variations along this profile, which can be attributed to the joint effects of Indian lower crust underthrusting, the low velocity zone of the mid-upper crust, crustal shortening and thickening and other involved dynamic mechanisms.

Crustal Structure of the Flood Basalt Province of Ethiopia from Constrained 3-D Gravity Inversion

NASA Astrophysics Data System (ADS)

Mammo, Tilahun

2013-12-01

The Oligocene Afar mantle plume resulted in the eruption of a large volume of basaltic magma, including major sequences of rhyolitic ignimbrites, in a short span of time across Ethiopia. In order to assess the impact of these magmatic processes on the crust and to investigate the general crustal configuration beneath the Ethiopian plateau, northern part of the Main Ethiopian Rift and the Afar depression, analysis and modeling of the gravity field have been conducted. The Bouguer gravity map is dominated by long-wavelength anomalies that primarily arise from the isostatic compensation of the topography. Consequently, anomalies within the crust/upper mantle are masked and quantitative interpretation becomes difficult. The long-wavelength anomalies are approximated using admittance technique and subsequently removed from the Bouguer anomalies to obtain the residual isostatic anomalies. The residual map contains both short- and intermediate-wavelength anomalies related to geologic and tectonic features. The long-wavelength regional isostatic field is used to map the crust-mantle interface and the results are in good agreement with those determined by other geophysical methods. Seismic constrained gravity inversion was performed on the isostatic residual field and series of three-dimensional models have been constructed for the structures of the crust and upper mantle beneath the uplifted and rifted flood basalt province of northern Ethiopia. The inversion results have shown that the NW plateau has thick crust that rests on normal lithospheric mantle. Afar, On the other hand, is marked by thin stretched crust resting on a low-density upper mantle indicating a hotter thermal regime and partial melt. No lithospheric mantle is observed beneath Afar. The models further indicate the presence of an extensive sub-crustal thick (~12 km on average) and high-density (~3.06 gm/cc) mafic accreted igneous layer of fractionated cumulate (magmatic underplating) beneath the NW plateau. The study suggests that the underplate was fundamental to the accretion process and may have played a role in compensating most of the plateau uplift and in localizing stresses.

Gravity evidence for shaping of the crustal structure of the Ameca graben (Jalisco block northern limit). Western Mexico

NASA Astrophysics Data System (ADS)

Alatorre-Zamora, Miguel Angel; Campos-Enríquez, José Oscar; Fregoso-Becerra, Emilia; Quintanar-Robles, Luis; Toscano-Fletes, Roberto; Rosas-Elguera, José

2018-03-01

The Ameca tectonic depression (ATD) is located at the NE of the Jalisco Block along the southwestern fringe of the NW-SE trending Tepic-Zacoalco Rift, in the west-central part of the Trans-Mexican Volcanic Belt, western Mexico. To characterize its shallow crustal structure, we conducted a gravity survey based on nine N-S gravity profiles across the western half of the Ameca Valley. The Bouguer residual anomalies are featured by a central low between two zones of positive gravity values with marked gravity gradients. These anomalies have a general NW-SE trend similar to the Tepic-Zacoalco Rift general trend. Basement topography along these profiles was obtained by means of: 1) a Tsuboi's type inverse modeling, and 2) forward modeling. Approximately northward dipping 10° slopes are modeled in the southern half, with south tilted down faulted blocks of the Cretaceous granitic basement and its volcano-sedimentary cover along sub-vertical and intermediate normal faults, whereas southward dipping slopes of almost 15° are observed at the northern half. According to features of the obtained models, this depression corresponds to a slight asymmetric graben. The Ameca Fault is part of the master fault system along its northern limit. The quantitative interpretation shows an approximately 500 to 1100 m thick volcano-sedimentary infill capped by alluvial products. This study has several implications concerning the limit between the Jalisco Block and the Tepic-Zacoalco Rift. The established shallow crustal structure points to the existence of a major listric fault with its detachment surface beneath the Tepic-Zacoalco Rift. The Ameca Fault is interpreted as a secondary listric fault. The models indicate the presence of granitic bodies of the Jalisco Block beneath the TMVB volcanic products of the Tepic-Zacoalco rift. This implies that the limit between these two regional structures is not simple but involves a complex transition zone. A generic model suggests that the extension related normal faulting has been operating as a mechanism in the evolution of this rift. Analysis of seismicity affecting the study area and neighborhood indicates the inferred faults are active.

Three-dimensional thermal structure and seismogenesis in the Tohoku and Hokkaido subduction system

NASA Astrophysics Data System (ADS)

van Keken, P. E.; Kita, S.; Nakajima, J.; Bengtson, A. K.; Hacker, B. R.; Abers, G. A.

2010-12-01

The Northern Japan arc is characterized by fast subduction of old oceanic lithosphere. The high density instrumentation and high seismicity make this an ideal natural laboratory to study the interplay between subduction zone dynamics, dehydration, migration of fluids, and seismogenesis. In this study we use high resolution finite element models to predict the thermal structure of the subduction slab below Tohoku (Northern Honshu) and Hokkaido. These models allow us to predict the pressure, temperature and mineralogy of the subducted crust and mantle. We use these models to predict the (p,T) conditions of earthquakes that are relocated with a precision of around 1 km by double difference techniques. Below Northern Hokkaido and Tohoku we find that the earthquake activity is strong in crust and the uppermost mantle for temperatures < 450 C. Above this temperature earthquakes occur more sporadically and have significantly reduced integrated seismic moment. The strongest 3D variations in this arc occur below southern Hokkaido. This 200 km wide region is characterized by a change in trench geometry, anomalously low heatflow and an anomalous velocity structure in the mantle wedge. Tomographic imaging suggest that continental crust is subducted to significant depth, thereby insulating the subducting slab from the hot mantle wedge at least at intermediate depths. The thermal insulation is also suggested by the deepening of the earthquakes in the slab (Kita et al., EPSL, 2010). This region may be characterized by active crustal erosion which would lead to a further blanketing of the crust by a sedimentary layer. Further modifications in thermal structure are possible due to the 3D wedge flow that is generated by the along-arc variations in trench geometry. We quantitatively verify the relative importance of these processes using 2D and 3D dynamical models. Without the seismically imaged crustal structure the earthquake temperatures are significantly elevated compared to the Tohoku and (northern) Hokkaido sections. If we take the modified crustal structure into account we find a (p,T) pattern that is quite similar to that in the other sections, suggesting that the processes that lead to earthquakes in crust and uppermost mantle of the downgoing slab are similar across the northern Japan arc.

Vertical Crustal Motion Derived from Satellite Altimetry and Tide Gauges, and Comparisons with DORIS Measurements

NASA Technical Reports Server (NTRS)

Ray, R. D.; Beckley, B. D.; Lemoine, F. G.

2010-01-01

A somewhat unorthodox method for determining vertical crustal motion at a tide-gauge location is to difference the sea level time series with an equivalent time series determined from satellite altimetry, To the extent that both instruments measure an identical ocean signal, the difference will be dominated by vertical land motion at the gauge. We revisit this technique by analyzing sea level signals at 28 tide gauges that are colocated with DORIS geodetic stations. Comparisons of altimeter-gauge vertical rates with DORIS rates yield a median difference of 1.8 mm/yr and a weighted root-mean-square difference of2.7 mm/yr. The latter suggests that our uncertainty estimates, which are primarily based on an assumed AR(l) noise process in all time series, underestimates the true errors. Several sources of additional error are discussed, including possible scale errors in the terrestrial reference frame to which altimeter-gauge rates are mostly insensitive, One of our stations, Male, Maldives, which has been the subject of some uninformed arguments about sea-level rise, is found to have almost no vertical motion, and thus is vulnerable to rising sea levels. Published by Elsevier Ltd. on behalf of COSPAR.

Sea Level, Land Motion, and the Anomalous Tide at Churchill, Hudson Bay

NASA Astrophysics Data System (ADS)

Ray, R. D.

2015-12-01

The importance of the tide gauge at Churchill, Manitoba, cannot be overstated. It is the only permanently operating tide gauge in the central Canadian Arctic, and it sits on a prime spot for monitoring the mantle's rebound from the Laurentide ice loss. Yet interpretation of the sea-level time series at Churchill has long been problematic, going back even to early work by Gutenberg in the 1940s. The long-term relative sea-level rates are inconsistent: approximately -4, -19, -5 ± 1 mm/y for the periods 1940-1970, 1970-1990, 1990-2014 respectively. Annual mean high water (MHW) and mean low water (MLW) reflect these trends until around 1990, after which MLW leveled off and is now nearly unchanging. Slightly later, around 2000, the semidiurnal tides became very anomalous, with falling amplitudes and slightly increasing phase lags. The amplitude of M2 was approximately 154 cm before 2000; it dropped to about 146 cm by 2010 and reached an all-time low of 142 cm in 2014. Satellite altimeter estimates of the tide in this region, although challenging because of seasonal ice cover, show no comparable M2 changes, so the tidal changes must be localized to the near vicinity of the gauge (or to the gauge itself if caused by a malfunction). On the other hand, altimetry confirms the post-1992 Churchill measurements of mean sea level, thanks to the long time series of land motion measurements obtained at GPS station CHUR, which gives a vertical uplift of 10.1 mm/y. Combining satellite altimeter data with the Churchill tide-gauge data gives an implied vertical crustal rate of about 9.0 ± 0.8 mm/y, in reasonable agreement with the GPS. In summary, we have still anomalous MSL measurements at the Churchill gauge for the intermediate 1970-1990 era, and very anomalous tidal measurements since 2000, but we have apparently quite reliable MSL rates since 1990.

A study of the hydrothermal alteration in Paleoproterozoic volcanic centers, São Félix do Xingu region, Amazonian Craton, Brazil, using short-wave infrared spectroscopy

NASA Astrophysics Data System (ADS)

da Cruz, Raquel Souza; Fernandes, Carlos Marcello Dias; Villas, Raimundo Netuno Nobre; Juliani, Caetano; Monteiro, Lena Virgínia Soares; de Almeida, Teodoro Isnard Ribeiro; Lagler, Bruno; de Carvalho Carneiro, Cleyton; Misas, Carlos Mario Echeverri

2015-10-01

Hypogene hydrothermal minerals have been identified by short-wave infrared spectroscopy in hydrothermally altered rocks from the Sobreiro and Santa Rosa formations, which belong to a Paleoproterozoic volcano-plutonic system in Amazonian craton. Three clay minerals are spectrally recognized: montmorillonite, kaolinite, and illite. The integration of these data with those available in the literature, including gold occurrences, suggests that those rocks are hydrothermal products of both volcanic thermal sources and later crustal intrusions, as evidenced by variable styles of propylitic, sericitic, potassic, and intermediate argillic alteration. The influence of meteoric fluids is emphasized. This low cost exploratory technique, which can be applied to hand samples, seems to be promising in the separation of hydrothermally altered volcano-plutonic centers in regions submitted to severe weathering conditions, in addition to aid elaborating models for prospecting mineral deposits.

Chemistry of Martian Soils from the Mars Exploration Rover APXS Instruments

NASA Technical Reports Server (NTRS)

Mittlefehldt, D. W.; Gellert, R.; Yen, A.

2007-01-01

The martian surface is covered with debris formed by several mechanisms and mobilized by various processes. Volcanism, impact, physical weathering and chemical alteration combine to produce particles of sizes from dust to boulders composed of primary mineral and rock fragments, partially altered primary materials, alteration minerals and shock-modified materials from all of these. Impacts and volcanism produce localized deposits. Winds transport roughly sand-sized material over intermediate distances, while periodic dust storms deposit a global dust layer of the finest fraction. The compositions of clastic sediments can be used to evaluate regional differences in crustal composition and/or weathering processes. Here we examine the growing body of chemical data on soils in Gusev crater and Meridiani Planum returned by the Alpha Particle X-ray Spectrometer (APXS) instruments on the rovers Spirit (MERA) and Opportunity (MERB), following on earlier results based on smaller data sets [1-4].

Crustal and uppermost mantle structures of the South China from joint analysis of receiver functions and Rayleigh wave dispersions

NASA Astrophysics Data System (ADS)

Guo, Zhi; Gao, Xing; Li, Tong; Wang, Wei

2018-05-01

We use P-wave receiver function H-k stacking and joint inversion of receiver functions and Rayleigh wave dispersions to investigate crustal and uppermost mantle structure beneath the South China. The obtained results reveal prominent crustal structure variations in the study area, Moho depth increases from ∼30 km in the Cathaysia Block to more than ∼60 km in the eastern Tibetan Plateau. A Moho undulation and Vp/Vs ratio variations can be observed from the Cathaysia Block to Yangtze Craton. These observations consistent with the crustal structures predict by the flat slab subduction model. We interpret these lateral crustal structure variations reflect the tectonic evolution of the Yangtze Craton and Cathaysia Block prior the Mesozoic and the post-orogenic magmatism due to the breaking up of the subducted flat slab and subsequent slab rollback in the South China. The observed variations of the crustal structures not only reveal the lateral crustal inhomogeneity, but also provide constraints on the geodynamic evolution of the South China.

Particle-In-Cell Simulations of the Solar Wind Interaction with Lunar Crustal Magnetic Anomalies: Magnetic Cusp Regions

NASA Technical Reports Server (NTRS)

Poppe, A. R.; Halekas, J. S.; Delory, G. T.; Farrell, W. M.

2012-01-01

As the solar wind is incident upon the lunar surface, it will occasionally encounter lunar crustal remanent magnetic fields. These magnetic fields are small-scale, highly non-dipolar, have strengths up to hundreds of nanotesla, and typically interact with the solar wind in a kinetic fashion. Simulations, theoretical analyses, and spacecraft observations have shown that crustal fields can reflect solar wind protons via a combination of magnetic and electrostatic reflection; however, analyses of surface properties have suggested that protons may still access the lunar surface in the cusp regions of crustal magnetic fields. In this first report from a planned series of studies, we use a 1 1/2-dimensional, electrostatic particle-in-cell code to model the self-consistent interaction between the solar wind, the cusp regions of lunar crustal remanent magnetic fields, and the lunar surface. We describe the self-consistent electrostatic environment within crustal cusp regions and discuss the implications of this work for the role that crustal fields may play regulating space weathering of the lunar surface via proton bombardment.

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

Fielding, E.J.; Barazangi, M.; Isacks, B.L.

Topography and heterogeneous crustal structure have major effects on the propagation of regional seismic phases. We are collecting topographical, geological, and geophysical datasets for Eurasia into an information system that can be accessed via Internet connections. Now available are digital topography, satellite imagery, and data on sedimentary basins and crustal structure thicknesses. New datasets for Eurasia include maps of depth to Moho beneath Europe and Scandinavia. We have created regularly spaced grids of the crustal thickness values from these maps that can be used to create profiles of crustal structure. These profiles can be compared by an analyst or anmore » automatic program with the crustal seismic phases received along the propagation path to better understand and predict the path effects on phase amplitudes, a key to estimating magnitudes and yields, and for understanding variations in travel-time delays for phases such as Pn, important for improving regional event locations. The gridded data could also be used to model propagation of crustal phases in three dimensions. Digital elevation models, Satellite imagery, Geographic information systems, Lg Propagation, Moho, Geology, Crustal structure, Topographic relief.« less

Evolution of the Median Tectonic Line fault zone, SW Japan, during exhumation

NASA Astrophysics Data System (ADS)

Shigematsu, Norio; Kametaka, Masao; Inada, Noriyuki; Miyawaki, Masahiro; Miyakawa, Ayumu; Kameda, Jun; Togo, Tetsuhiro; Fujimoto, Koichiro

2017-01-01

Like many crustal-scale fault zones, the Median Tectonic Line (MTL) fault zone in Japan preserves fault rocks that formed across a broad range of physical conditions. We examined the architecture of the MTL at a large new outcrop in order to understand fault behaviours under different crustal levels. The MTL here strikes almost E-W, dips to the north, and juxtaposes the Sanbagawa metamorphic rocks to the south against the Izumi Group sediments to the north. The fault core consists mainly of Sanbagawa-derived fault gouges. The fault zone can be divided into several structural units, including two slip zones (upper and lower slip zones), where the lower slip zone is more conspicuous. Crosscutting relationships among structures and kinematics indicate that the fault zone records four stages of deformation. Microstructures and powder X-ray diffraction (XRD) analyses indicate that the four stages of deformation occurred under different temperature conditions. The oldest deformation (stage 1) was widely distributed, and had a top-to-the-east (dextral) sense of slip at deep levels of the seismogenic zone. Deformation with the same sense of slip, then became localised in the lower slip zone (stage 2). Subsequently, the slip direction in the lower slip zone changed to top-to-the-west (sinistral-normal) (stage 3). The final stage of deformation (stage 4) involved top-to-the-north normal faulting along the two slip zones within the shallow crust (near the surface). The widely distributed stage 1 damage zone characterises the deeper part of the seismogenic zone, while the sets of localised principal slip zones and branching faults of stage 4 characterise shallow depths. The fault zone architecture described in this paper leads us to suggest that fault zones display different behaviours at different crustal levels.

Kinematics of syn- and post-exhumational shear zones at Lago di Cignana (Western Alps, Italy): constraints on the exhumation of Zermatt-Saas (ultra)high-pressure rocks and deformation along the Combin Fault and Dent Blanche Basal Thrust

NASA Astrophysics Data System (ADS)

Kirst, Frederik; Leiss, Bernd

2017-01-01

Kinematic analyses of shear zones at Lago di Cignana in the Italian Western Alps were used to constrain the structural evolution of units from the Piemont-Ligurian oceanic realm (Zermatt-Saas and Combin zones) and the Adriatic continental margin (Dent Blanche nappe) during Palaeogene syn- and post-exhumational deformation. Exhumation of Zermatt-Saas (U)HP rocks to approximately lower crustal levels at ca. 39 Ma occurred during normal-sense top-(S)E shearing under epidote-amphibolite-facies conditions. Juxtaposition with the overlying Combin zone along the Combin Fault at mid-crustal levels occurred during greenschist-facies normal-sense top-SE shearing at ca. 38 Ma. The scarcity of top-SE kinematic indicators in the hanging wall of the Combin Fault probably resulted from strain localization along the uppermost Zermatt-Saas zone and obliteration by subsequent deformation. A phase of dominant pure shear deformation around 35 Ma affected units in the direct footwall and hanging wall of the Combin Fault. It is interpreted to reflect NW-SE crustal elongation during updoming of the nappe stack as a result of underthrusting of European continental margin units and the onset of continental collision. This phase was partly accompanied and followed by ductile bulk top-NW shearing, especially at higher structural levels, which transitioned into semi-ductile to brittle normal-sense top-NW deformation due to Vanzone phase folding from ca. 32 Ma onwards. Our structural observations suggest that syn-exhumational deformation is partly preserved within units and shear zones exposed at Lago di Cignana but also that the Combin Fault and Dent Blanche Basal Thrust experienced significant post-exhumational deformation reworking and overprinting earlier structures.

Concentrations, sources and geochemistry of airborne particulate matter at a major European airport.

PubMed

Amato, Fulvio; Moreno, Teresa; Pandolfi, Marco; Querol, Xavier; Alastuey, Andrés; Delgado, Ana; Pedrero, Manuel; Cots, Nuria

2010-04-01

Monitoring of aerosol particle concentrations (PM(10), PM(2.5), PM(1)) and chemical analysis (PM(10)) was undertaken at a major European airport (El Prat, Barcelona) for a whole month during autumn 2007. Concentrations of airborne PM at the airport were close to those at road traffic hotspots in the nearby Barcelona city, with means measuring 48 microg PM(10)/m(3), 21 microg PM(2.5)/m(3) and 17 microg PM(1)/m(3). Meteorological controls on PM at El Prat are identified as cleansing daytime sea breezes with abundant coarse salt particles, alternating with nocturnal land-sourced winds which channel air polluted by industry and traffic (PM(1)/PM(10) ratios > 0.5) SE down the Llobregat Valley. Chemical analyses of the PM(10) samples show that crustal PM is dominant (38% of PM(10)), followed by total carbon (OC + EC, 25%), secondary inorganic aerosols (SIA, 20%), and sea salt (6%). Local construction work for a new airport terminal was an important contributor to PM(10) crustal levels. Source apportionment modelling PCA-MLRA identifies five factors: industrial/traffic, crustal, sea salt, SIA, and K(+) likely derived from agricultural biomass burning. Whereas most of the atmospheric contamination concerning ambient air PM(10) levels at El Prat is not attributable directly to aircraft movement, levels of carbon are unusually high (especially organic carbon), as are metals possibly sourced from tyre detritus/smoke in runway dust (Ba, Zn, Mo) and from brake dust in ambient PM(10) (Cu, Sb), especially when the airport is at its most busy. We identify microflakes of aluminous alloys in ambient PM(10) filters derived from corroded fuselage and wings as an unequivocal and highly distinctive tracer for aircraft movement.

Evidence for a Low Bulk Crustal Density for Mars from Gravity and Topography.

PubMed

Goossens, Sander; Sabaka, Terence J; Genova, Antonio; Mazarico, Erwan; Nicholas, Joseph B; Neumann, Gregory A

2017-08-16

Knowledge of the average density of the crust of a planet is important in determining its interior structure. The combination of high-resolution gravity and topography data has yielded a low density for the Moon's crust, yet for other terrestrial planets the resolution of the gravity field models has hampered reasonable estimates. By using well-chosen constraints derived from topography during gravity field model determination using satellite tracking data, we show that we can robustly and independently determine the average bulk crustal density directly from the tracking data, using the admittance between topography and imperfect gravity. We find a low average bulk crustal density for Mars, 2582 ± 209 kg m -3 . This bulk crustal density is lower than that assumed until now. Densities for volcanic complexes are higher, consistent with earlier estimates, implying large lateral variations in crustal density. In addition, we find indications that the crustal density increases with depth.

A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites.

PubMed

Reubi, Olivier; Blundy, Jon

2009-10-29

Andesites represent a large proportion of the magmas erupted at continental arc volcanoes and are regarded as a major component in the formation of continental crust. Andesite petrogenesis is therefore fundamental in terms of both volcanic hazard and differentiation of the Earth. Andesites typically contain a significant proportion of crystals showing disequilibrium petrographic characteristics indicative of mixing or mingling between silicic and mafic magmas, which fuels a long-standing debate regarding the significance of these processes in andesite petrogenesis and ultimately questions the abundance of true liquids with andesitic composition. Central to this debate is the distinction between liquids (or melts) and magmas, mixtures of liquids with crystals, which may or may not be co-genetic. With this distinction comes the realization that bulk-rock chemical analyses of petrologically complex andesites can lead to a blurred picture of the fundamental processes behind arc magmatism. Here we present an alternative view of andesite petrogenesis, based on a review of quenched glassy melt inclusions trapped in phenocrysts, whole-rock chemistry, and high-pressure and high-temperature experiments. We argue that true liquids of intermediate composition (59 to 66 wt% SiO(2)) are far less common in the sub-volcanic reservoirs of arc volcanoes than is suggested by the abundance of erupted magma within this compositional range. Effective mingling within upper crustal magmatic reservoirs obscures a compositional bimodality of melts ascending from the lower crust, and masks the fundamental role of silicic melts (>/=66 wt% SiO(2)) beneath intermediate arc volcanoes. This alternative view resolves several puzzling aspects of arc volcanism and provides important clues to the integration of plutonic and volcanic records.

Elementary Environmental Learning Packet Grades 4-6, Second Revised Edition. [Intermediate CEL Blocks. Teacher's Guide].

ERIC Educational Resources Information Center

Brevard County School Board, Cocoa, FL.

This environmental education program consists of two levels: primary and intermediate. The material in this publication encompasses the intermediate level. The learning materials are activity-based and incorporate process and subject area skills with knowledge and concern for the environment. The program is also interdisciplinary including…

Geophysical Investigations of Crustal and Upper Mantle Structure of Oceanic Intraplate Volcanoes (OIVs)

NASA Astrophysics Data System (ADS)

Robinson, A. H.; Peirce, C.; Funnell, M.; Watts, A. B.; Grevemeyer, I.

2016-12-01

Oceanic intraplate volcanoes (OIVs) represent a record of the modification of the oceanic crust by volcanism related to a range of processes including hot-spots, small scale mantle convection, and localised lithospheric extension. Geophysical studies of OIVs show a diversity in crustal and upper mantle structures, proposed to exist on a spectrum between two end-members where the main control is the age of the lithosphere at the time of volcanism. This hypothesis states that where the lithosphere is older, colder, and thicker it is more resistant to vertical magmatism than younger, hotter, thinner lithosphere. It is suggested that the Moho acts as a density filter, permitting relatively buoyant magma to vertically intrude the crust, but preventing denser magma from ascending to shallow levels. A key control may therefore be the melting depth, known to affect magma composition, and itself related to lithosphere age. Combined geophysical approaches allow us to develop robust models for OIV crustal structures with quantifiable resolution and uncertainty. As a case study, we present results from a multi-approach geophysical experiment at the Louisville Ridge Seamount Chain, believed to have formed on young (<10 Ma) lithosphere, which aimed at characterising the along-ridge crustal structure. The wide-angle seismic crustal model, generated by independent forward and inverse travel-time modelling of picked arrivals, is tested against reflection and gravity data. We compare our observations with studies of other OIVs to test whether lithospheric age controls OIV structure. Comparisons are limited by the temporal and spatial distribution of lithosphere and volcano ages, but suggest the hypothesis does not hold for all OIV features. While age may be the main control on OIV structure, as it determines lithosphere thermal and mechanical properties, other factors such as thermal rejuvenation, mechanical weakening, and volcano load size and distribution, may also come into play.

New Frontiers Science at Venus from Orbit plus Atmospheric Gas Sampling

NASA Astrophysics Data System (ADS)

Smrekar, Suzanne; Dyar, Melinda; Hensley, Scott; Helbert, Joern; VOX Science and Engineering Teams

2017-10-01

Venus remains the most Earth-like body in terms of size, composition, surface age, and insulation. Venus Origins Explorer (VOX) determines how Earth’s twin diverged, and enables breakthroughs in our understanding of rocky planet evolution and habitability. At the time of the Decadal Survey the ability to map mineralogy from orbit (Helbert et al.) and present-day radar techniques to detect active deformation were not fully appreciated. VOX leverages these methods and in-situ noble gases to answer New Frontiers science objectives:1. Atmospheric physics/chemistry: noble gases and isotopes to constrain atmospheric sources, escape processes, and integrated volcanic outgassing; global search for current volcanically outgassed water.2. Past hydrological cycles: global tessera composition to determine the role of volatiles in crustal formation.3. Crustal physics/chemistry: global crustal mineralogy/chemistry, tectonic processes, heat flow, resolve the catastrophic vs. equilibrium resurfacing debate, active geologic processes and possible crustal recycling.4. Crustal weathering: surface-atmosphere weathering reactions from redox state and the chemical equilibrium of the near-surface atmosphere.5. Atmospheric properties/winds: map cloud particle modes and their temporal variations, and track cloud-level winds in the polar vortices.6. Surface-atmosphere interactions: chemical reactions from mineralogy; weathering state between new, recent and older flows; possible volcanically outgassed water.VOX’s Atmosphere Sampling Vehicle (ASV) dips into and samples the well-mixed atmosphere, using Venus Original Constituents Experiment (VOCE) to measure noble gases. VOX’s orbiter carries the Venus Emissivity Mapper (VEM) and the Venus Interferometric Synthetic Aperture Radar (VISAR), and maps the gravity field using Ka-band tracking.VOX is the logical next mission to Venus because it delivers: 1) top priority atmosphere, surface, and interior science; 2) key global data for comparative planetology; 3) high-resolution topography, composition, and imaging to optimize future landers; 4) opportunities for revolutionary discoveries with a 3-year long mission, proven implementation and 44 Tb of data.

Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends and interrelationships

PubMed Central

Sorooshian, A.; Shingler, T.; Harpold, A.; Feagles, C. W.; Meixner, T.; Brooks, P. D.

2013-01-01

This study characterizes the spatial and temporal patterns of aerosol and precipitation composition at six sites across the United States Southwest between 1995 and 2010. Precipitation accumulation occurs mostly during the wintertime (December–February) and during the monsoon season (July–September). Rain and snow pH levels are usually between 5–6, with crustal-derived species playing a major role in acid neutralization. These species (Ca2+, Mg2+, K+, Na+) exhibit their highest concentrations between March and June in both PM2.5 and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with SO42−, NO3−, and Cl–, suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate SO42− show a statistically significant correlation with rain SO42− unlike snow SO42−, which may be related to some combination of the vertical distribution of SO42− (and precursors) and the varying degree to which SO42−-enriched particles act as cloud condensation nuclei versus ice nuclei in the region. The coarse : fine aerosol mass ratio was correlated with crustal species concentrations in snow unlike rain, suggestive of a preferential role of coarse particles (mainly dust) as ice nuclei in the region. Precipitation NO3− : SO42− ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM2.5; (ii) they exhibit the opposite annual cycle compared to particulate NO3− : SO42− ratios; and (iii) they are higher in snow relative to rain during the wintertime. Long-term trend analysis for the monsoon season shows that the NO3− : SO42− ratio in rain increased at the majority of sites due mostly to air pollution regulations of SO42− precursors. PMID:24432030

Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends and interrelationships.

PubMed

Sorooshian, A; Shingler, T; Harpold, A; Feagles, C W; Meixner, T; Brooks, P D

2013-08-01

This study characterizes the spatial and temporal patterns of aerosol and precipitation composition at six sites across the United States Southwest between 1995 and 2010. Precipitation accumulation occurs mostly during the wintertime (December-February) and during the monsoon season (July-September). Rain and snow pH levels are usually between 5-6, with crustal-derived species playing a major role in acid neutralization. These species (Ca 2+ , Mg 2+ , K + , Na + ) exhibit their highest concentrations between March and June in both PM 2.5 and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with [Formula: see text], [Formula: see text], and Cl - , suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate [Formula: see text] show a statistically significant correlation with rain [Formula: see text] unlike snow [Formula: see text], which may be related to some combination of the vertical distribution of [Formula: see text] (and precursors) and the varying degree to which [Formula: see text]-enriched particles act as cloud condensation nuclei versus ice nuclei in the region. The coarse : fine aerosol mass ratio was correlated with crustal species concentrations in snow unlike rain, suggestive of a preferential role of coarse particles (mainly dust) as ice nuclei in the region. Precipitation [Formula: see text] : [Formula: see text] ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM 2.5 ; (ii) they exhibit the opposite annual cycle compared to particulate [Formula: see text] : [Formula: see text] ratios; and (iii) they are higher in snow relative to rain during the wintertime. Long-term trend analysis for the monsoon season shows that the [Formula: see text] : [Formula: see text] ratio in rain increased at the majority of sites due mostly to air pollution regulations of [Formula: see text] precursors.

New Paleomagnetic Data from the Wadi Abyad Crustal Section and their Implications for the Rotation History of the Oman Ophiolite

NASA Astrophysics Data System (ADS)

Meyer, M.; Morris, A.; Anderson, M.; MacLeod, C. J.

2014-12-01

The Oman ophiolite is an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution. Previous paleomagnetic research in lavas of the northern ophiolitic blocks (Perrin et al., 2000, Mar. Geophys. Res.) has demonstrated substantial clockwise intraoceanic tectonic rotations. Paleomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization, and have been used to infer that clockwise rotations seen in the north are internal to the ophiolite rather than regionally significant (Weiler, 2000, Mar. Geophys. Res.). Here we demonstrate the importance and advantages of sampling crustal transects in the ophiolite in order to understand the nature and variability in magnetization directions. By systematically sampling the lower crustal sequence exposed in Wadi Abyad (Rustaq block) we resolve for the first time in a single section a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to the Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used by Weiler (2000, Mar. Geophys. Res.) to infer lack of significant rotation of the southern blocks and to argue, therefore, that rotation of the northern blocks was internal to the ophiolite. In contrast, E/ENE-directed remanences in the uppermost levels of Wadi Abyad imply a large, clockwise rotation of the Rustaq block, of a sense and magnitude consistent with intraoceanic rotations inferred from extrusive sections in the northern blocks. We conclude that without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

Workshop on Early Crustal Genesis: The World's Oldest Rocks

NASA Technical Reports Server (NTRS)

Ashwal, L. D. (Editor)

1986-01-01

Topics addressed include: a general review of Precambrain crustal evolution; geology and geochemistry of the Archean Craton in Greenland and Labrador; Precambrian crustal evolution in North and South America; and the field excursion to the Ameralik Fjord.

Comparison of orbital chemistry with crustal thickness and lunar sample chemistry

NASA Technical Reports Server (NTRS)

Schonfeld, E.

1977-01-01

A correlation between orbital chemistry (FeO, Al2O3, Mg/Al, MgO/FeO, Th) and the lunar crustal thickness is examined. The correlation suggests either lack of complete homogenization by lateral or vertical mixing, or lateral variation in the differentiation process. In addition, links between orbital chemistry and lunar sample chemistry are investigated. In regions with crustal thickness between 100 and 110 km, gabbroic anorthosites are very abundant, while in regions with crustal thickness of about 80 km anorthositic gabbros are frequent. Special attention is given to the distribution of low-potassium Fra Mauro basalt, found in high concentrations in regions with 50 to 60 km crustal thickness.

Viscous relaxation of impact crater relief on Venus - Constraints on crustal thickness and thermal gradient

NASA Technical Reports Server (NTRS)

Grimm, Robert E.; Solomon, Sean C.

1988-01-01

Models for the viscous relaxation of impact crater topography are used to constrain the crustal thickness (H) and the mean lithospheric thermal gradient beneath the craters on Venus. A general formulation for gravity-driven flow in a linearly viscous fluid has been obtained which incorporates the densities and temperature-dependent effective viscosities of distinct crust and mantle layers. An upper limit to the crustal volume of Venus of 10 to the 10th cu km is obtained which implies either that the average rate of crustal generation has been much smaller on Venus than on earth or that some form of crustal recycling has occurred on Venus.

Composition of island arcs and continental growth.

NASA Technical Reports Server (NTRS)

Jakes, P.; White, A. J. R.

1971-01-01

Island arc volcanism has contributed and is still contributing to continental growth, but the composition of island arcs differs from that of the upper continental crust in its lower abundance of Si, K, Rb, Ba, Sr and light rare earth elements. In their advanced stage of evolution, island arcs contain more than 80% of tholeiitic and 15% of ?island arc' calc-alkaline rocks with varied SiO2 contents. The larger proportion of tholeiitic rocks is in the lower crustal levels. The high stratigraphical levels of the island arcs are composed of tholeiitic plus calc-alkaline and/or high potash (shoshonitic) associations with higher abundances of K, Rb, Sr, and Ba. Stratification of the island arc crust is accentuated by another type of calc-alkaline volcanism (Andean type) originating at a late stage of arc evolution, probably by partial melting at the base of the crust. This causes enrichment of the upper crust in K, Rb, Ba and REE and accounts for upper crustal abundances of these elements as well as of SiO2.

The Sagatu Ridge dike swarm, Ethiopian rift margin. [tectonic evolution

NASA Technical Reports Server (NTRS)

Mohr, P. A.; Potter, E. C.

1976-01-01

A swarm of dikes forms the core of the Sagatu Ridge, a 70-km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to the north and south.

Plume-driven plumbing and crustal formation in Iceland

USGS Publications Warehouse

Allen, R.M.; Nolet, G.; Morgan, W.J.; Vogfjord, K.; Nettles, M.; Ekstrom, G.; Bergsson, B.H.; Erlendsson, P.; Foulger, G.R.; Jakobsdottir, S.; Julian, B.R.; Pritchard, M.; Ragnarsson, S.; Stefansson, R.

2002-01-01

Through combination of surface wave and body wave constraints we derive a three-dimensional (3-D) crustal S velocity model and Moho map for Iceland. It reveals a vast plumbing system feeding mantle plume melt into upper crustal magma chambers where crustal formation takes place. The method is based on the partitioned waveform inversion to which we add additional observations. Love waves from six local events recorded on the HOTSPOT-SIL networks are fitted, Sn travel times from the same events measured, previous observations of crustal thickness are added, and all three sets of constraints simultaneously inverted for our 3-D model. In the upper crust (0-15 km) an elongated low-velocity region extends along the length of the Northern, Eastern and Western Neovolcanic Zones. The lowest velocities (-7%) are found at 5-10 km below the two most active volcanic complexes: Hekla and Bardarbunga-Grimsvotn. In the lower crust (>15 km) the low-velocity region can be represented as a vertical cylinder beneath central Iceland. The low-velocity structure is interpreted as the thermal halo of pipe work which connects the region of melt generation in the uppermost mantle beneath central Iceland to active volcanoes along the neovolcanic zones. Crustal thickness in Iceland varies from 15-20 km beneath the Reykjanes Peninsula, Krafla and the extinct Snfellsnes rift zone, to 46 km beneath central Iceland. The average crustal thickness is 29 km. The variations in thickness can be explained in terms of the temporal variation in plume productivity over the last ~20 Myr, the Snfellsnes rift zone being active during a minimum in plume productivity. Variations in crustal thickness do not depart significantly from an isostatically predicted crustal thickness. The best fit linear isostatic relation implies an average density jump of 4% across the Moho. Rare earth element inversions of basalt compositions on Iceland suggest a melt thickness (i.e., crustal thickness) of 15-20 km, given passive upwelling. The observed crustal thickness of up to 46 km implies active fluxing of source material through the melt zone by the mantle plume at up to 3 times the passive rate.

A preliminary summary of a seismic-refraction survey in the vicinity of the Tonto Forest Observatory, Arizona

USGS Publications Warehouse

Roller, J.C.; Jackson, W.H.; Warren, D.H.; Healy, J.H.

1964-01-01

The U.S. Geological Survey complete d a seismic-refraction survey in the vicinity of the Tonto Forest Seismological Observatory (T.F.S.O.) in April and May 1964. More than 1200 km of reversed profiles were surveyed to determine the crustal structure and crustal and upper mantle velocities in this area. The purpose of this work was to provide information on wave-propagation paths of seismic events recorded at T.F.S.O. and to improve the performance of the Observatory in locating and identifying these events. First arrivals indicate that the Mohorovicic discontinuity dips to the northeast by as much as 6 degrees under T.F.S.O., and may even be displaced vertically by as much as 5 km immediately north of the Observatory near the boundary of the Basin and Range a n d t he Colorado Plateau Provinces. A preliminary examination of the first arrivals indicates that the crust at T.F.S.O. is at least 30 km thick and is made up of at least two seismic layers. A thin veneer at the surface with a velocity of approximately 4 km/sec is underlain by a layer with a velocity of approximately 5.9 km/sec to 6.1 km/sec. An intermediate layer with velocity of 6.6 to 7.0 km/sec is probably present in the lower crust, but is not revealed by first arrivals. The velocity of seismic waves in the upper mantle is about 7.9 km/sec.

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.

Pervasive Eclogitization Due to Brittle Deformation and Rehydration of Subducted Basement: Effects on Continental Recycling?

NASA Astrophysics Data System (ADS)

Engi, Martin; Giuntoli, Francesco; Lanari, Pierre; Burn, Marco; Kunz, Barbara; Bouvier, Anne-Sophie

2018-03-01

The buoyancy of continental crust opposes its subduction to mantle depths, except where mineral reactions substantially increase rock density. Sluggish kinetics limit such densification, especially in dry rocks, unless deformation and hydrous fluids intervene. Here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50-60 km depth through a dense network of probably seismically induced fractures. We combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction, with pulses of high-pressure (HP) fluid at 650-670°C, rehydrating the initially dry rocks to micaschists. SIMS oxygen isotopic data of quartz indicate fluids of crustal composition. HP growth rims in allanite and zircon show uniform U-Th-Pb ages of ˜65 Ma and indicate that hydration occurred during subduction, at eclogite facies conditions. Based on this case study in the Sesia Zone (Western Italian Alps), we conclude that continental crust, and in particular deep basement fragments, during subduction can behave as substantial fluid sinks, not sources. Density modeling indicates a bifurcation in continental recycling: Chiefly mafic crust, once it is eclogitized to >60%, are prone to end up in a subduction graveyard, such as is tomographically evident beneath the Alps at ˜550 km depth. By contrast, dominantly felsic HP fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it. Felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at HP.

Early Archaean collapse basins, a habitat for early bacterial life.

NASA Astrophysics Data System (ADS)

Nijman, W.

For a better definition of the sedimentary environment in which early life may have flourished during the early Archaean, understanding of the basin geometry in terms of shape, depth, and fill is a prerequisite. The basin fill is the easiest to approach, namely from the well exposed, low-grade metamorphic 3.4 - 3.5 Ga rock successions in the greenstone belts of the east Pilbara (Coppin Gap Greenstone Belt and North Pole Dome) in West Australia and of the Barberton Greenstone Belt (Buck Ridge volcano-sedimentary complex) in South Africa. They consist of mafic to ultramafic volcanic rocks, largely pillow basalts, with distinct intercalations of intermediate to felsic intrusive and volcanic rocks and of silicious sediments. The, partly volcaniclastic, silicious sediments of the Buck Ridge and North Pole volcano-sedimentary complexes form a regressive-transgressive sequence. They were deposited close to base level, and experienced occasional emersion. Both North Pole Chert and the chert of the Kittys Gap volcano-sedimentary complex in the Coppin Gap Greenstone Belt preserve the flat-and-channel architecture of a shallow tidal environment. Thickness and facies distribution appear to be genetically linked to systems, i.e. arrays, of syn-depositionally active, extensional faults. Structures at the rear, front and bottoms of these fault arrays, and the fault vergence from the basin margin towards the centre characterize the basins as due to surficial crustal collapse. Observations in the Pilbara craton point to a non-linear plan view and persistence for the basin-defining fault patterns over up to 50 Ma, during which several of these fault arrays became superposed. The faults linked high-crustal level felsic intrusions within the overall mafic rock suite via porphyry pipes, black chert veins and inferred hydrothermal circulations with the overlying felsic lavas, and more importantly, with the cherty sediments. Where such veins surfaced, high-energy breccias, and in the case of the North Pole Chert huge barite growths, are juxtaposed with the otherwise generally low-energy sediments. Such localities are interpreted as sites of hydrothermal vents. Within this large-scale geological context, many environments on the micro-scale were habitable for life, such as hydrothermal vents and their vicinities, volcanic rock surfaces, subsurface sediments and sediment surfaces. These early collapse basins, hosting this bacterial life, are only partially comparable to Earthly analogues. A resemblance with Venus' coronae and the chaos terranes on Mars is suggested. This study forms part of an international project on Earth's Earliest Sedimentary Basins, supported by the Dutch Foundation Dr. Schürmannfonds. 2

Crustal growth in subduction zones

NASA Astrophysics Data System (ADS)

Vogt, Katharina; Castro, Antonio; Gerya, Taras

2015-04-01

There is a broad interest in understanding the physical principles leading to arc magmatisim at active continental margins and different mechanisms have been proposed to account for the composition and evolution of the continental crust. It is widely accepted that water released from the subducting plate lowers the melting temperature of the overlying mantle allowing for "flux melting" of the hydrated mantle. However, relamination of subducted crustal material to the base of the continental crust has been recently suggested to account for the growth and composition of the continental crust. We use petrological-thermo-mechanical models of active subduction zones to demonstrate that subduction of crustal material to sublithospheric depth may result in the formation of a tectonic rock mélange composed of basalt, sediment and hydrated /serpentinized mantle. This rock mélange may evolve into a partially molten diapir at asthenospheric depth and rise through the mantle because of its intrinsic buoyancy prior to emplacement at crustal levels (relamination). This process can be episodic and long-lived, forming successive diapirs that represent multiple magma pulses. Recent laboratory experiments of Castro et al. (2013) have demonstrated that reactions between these crustal components (i.e. basalt and sediment) produce andesitic melt typical for rocks of the continental crust. However, melt derived from a composite diapir will inherit the geochemical characteristics of its source and show distinct temporal variations of radiogenic isotopes based on the proportions of basalt and sediment in the source (Vogt et al., 2013). Hence, partial melting of a composite diapir is expected to produce melt with a constant major element composition, but substantial changes in terms of radiogenic isotopes. However, crustal growth at active continental margins may also involve accretionary processes by which new material is added to the continental crust. Oceanic plateaus and other crustal units may collide with continental margins to form collisional orogens and accreted terranes in places where oceanic lithosphere is recycled back into the mantle. We use thermomechanical-petrological models of an oceanic-continental subduction zone to analyse the dynamics of terrane accretion and its implications to arc magmatisim. It is shown that terrane accretion may result in the rapid growth of continental crust, which is in accordance with geological data on some major segments of the continental crust. Direct consequences of terrane accretion may include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes and partial melting (Vogt and Gerya., 2014), forming complex suture zones of accreted and partially molten units. Castro, A., Vogt, K., Gerya, T., 2013. Generation of new continental crust by sublithospheric silicic-magma relamination in arcs: A test of Taylor's andesite model. Gondwana Research, 23, 1554-1566. Vogt, K., Castro, A., Gerya, T., 2013. Numerical modeling of geochemical variations caused by crustal relamination. Geochemistry, Geophysics, Geosystems, 14, 470-487. Vogt, K., Gerya, T., 2014. From oceanic plateaus to allochthonous terranes: Numerical Modelling. Gondwana Research, 25, 494-508

Structure and evolution of the NE Atlantic conjugate margins off Norway and Greenland (Invited)

NASA Astrophysics Data System (ADS)

Faleide, J.; Planke, S.; Theissen-Krah, S.; Abdelmalak, M.; Zastrozhnov, D.; Tsikalas, F.; Breivik, A. J.; Torsvik, T. H.; Gaina, C.; Schmid, D. W.; Myklebust, R.; Mjelde, R.

2013-12-01

The continental margins off Norway and NE Greenland evolved in response to the Cenozoic opening of the NE Atlantic. The margins exhibit a distinct along-margin segmentation reflecting structural inheritance extending back to a complex pre-breakup geological history. The sedimentary basins at the conjugate margins developed as a result of multiple phases of post-Caledonian rifting from Late Paleozoic time to final NE Atlantic breakup at the Paleocene-Eocene transition. The >200 million years of repeated extension caused comprehensive crustal thinning and formation of deep sedimentary basins. The main rift phases span the following time intervals: Late Permian, late Middle Jurassic-earliest Cretaceous, Early-mid Cretaceous and Late Cretaceous-Paleocene. The late Mesozoic-early Cenozoic rifting was related to the northward propagation of North Atlantic sea floor spreading, but also linked to important tectonic events in the Arctic. The pre-drift extension is quantified based on observed geometries of crustal thinning and stretching factors derived from tectonic modeling. The total (cumulative) pre-drift extension amounts to in the order of 300 km which correlates well with estimates from plate reconstructions based on paleomagnetic data. Final lithospheric breakup at the Paleocene-Eocene transition culminated in a 3-6 m.y. period of massive magmatic activity during breakup and onset of early sea-floor spreading, forming a part of the North Atlantic Volcanic Province. At the outer parts of the conjugate margins, the lavas form characteristic seaward dipping reflector sequences and lava deltas that drilling has demonstrated to be subaerially and/or neritically erupted basalts. The continent-ocean transition is usually well defined as a rapid increase of P-wave velocities at mid- to lower-crustal levels. Maximum igneous crustal thickness of about 18 km is found across the outer Vøring Plateau on the Norwegian Margin, and lower-crustal P-wave velocities of up to 7.3 km/s are found at the bottom of the igneous crust here. The igneous crust, including the characteristic 7+ km/s lower crustal body, is even thicker on the East Greenland Margin. During the main igneous episode, sills intruded into the thick Cretaceous successions throughout the NE Atlantic margins. Strong crustal reflections can be mapped widespread on both conjugate margins. In some areas they are associated with the top of the high-velocity lower crustal body, in other areas they may represent deeply buried sedimentary sequence boundaries or moho at the base of the crust. Following breakup, the subsiding margins experienced modest sedimentation until the late Pliocene when large wedges of glacial sediments prograded into the deep ocean from uplifted areas along the continental margins. The outbuilding was probably initiated in Miocene time indicating pre-glacial tectonic uplift of Greenland, Fennoscandia and the Barents Shelf. The NE Atlantic margins also reveal evidence of widespread Cenozoic compressional deformation.

Orogenic potassic mafic magmatism, a product of alkaline-peraluminous mixing ? Variscan 'calc-alkaline' rocks from the Central Iberian and Ossa Morena Zones, Central Spain.

NASA Astrophysics Data System (ADS)

Scarrow, Jane H.; Cambeses, Aitor; Bea, Fernando; Montero, Pilar; Molina, José F.; Moreno, Juan Antonio

2013-04-01

Orogenic magmatic rocks provide information about mantle and crust melt-generation and -interaction processes. In this context, minor potassic mafic stocks which are formed of enriched mantle and crustal components and are common as late-orogenic intrusions in granitic plutons give insight into the timing of new crust formation and crustal recycling. Potassic mafic stocks are prevalent, albeit low volume, constituents of granite batholiths all through the European Variscan (350-280 Ma). In the Central Iberia Zone, Spanish Central System, crustal-melt, S-type, granitoid plutons are intruded by minor concomitant ultramafic-intermediate appinitic-vaugneritic stocks. Notwithstanding their whole-rock calc-alkaline composition, the stocks apparently did not have a subduction-related origin. Recent studies have attributed their genesis to mixing of alkaline mantle and peraluminous crustal melts. Their primary alkaline character, as indicated by amphibole and biotite mineral chemistry data, points, rather, towards an extension-related genesis. In the Ossa Morena Zone, south of the Central Iberian Zone, the igneous rocks also have a whole-rock calc-alkaline composition which has been considered to be the result of northward subduction of the South Portuguese Zone. Nevertheless, identification of a 'sill' of significant volume of mafic magma in the middle crust, the ´IBERSEIS reflective body', in a seismic profile across the Ossa Morena and South Portuguese Zones has cast doubt upon the calc-alkaline magmatism-subduction model; leading, instead, to the magmatism being attributed to intra-orogenic extension related to a mantle plume active from 340 Ma to 330 Ma. The aim here, then, is to reinvestigate the petrogenesis and age of the calc-alkaline rocks of the Ossa Morena Zone to determine their tectonomagmatic context be it subduction-, plume- or extension-related, and establish what they may reveal about mantle-crust interactions. Focussing, initially, on the Valencia del Ventoso pluton, preliminary mineral chemistry, whole-rock and isotope data indicate that rather than a single-stage cogenetic calc-alkaline intrusion, the pluton is a multi-stage composite of compositionally diverse stocks. Including the metaluminous calc-alkaline Medina de las Torres gabbro and Cortijo del Pozuelo granite to the concentrically zone alkaline core though to calk-alkaline border of the main pluton. In addition, an associated older peraluminous La Jineta granite body and younger cross-cutting tholeiitic dykes crop out in the same region. Here we present new U-Pb single zircon IBERSIMS SHRIMP data which indicate that the compositionally diverse main pluton and associated stocks are contemporaneous at 334 ± 2 Ma. So, rather than reflecting reactivation of a zone of lithospheric weakness by successive magmatic events it appears that at Valencia del Ventoso diverse mantle and crustal sources were being tapped simultaneously. We suggest that this is linked to the generation of and thermal anomaly associated with the coeval ´IBERSEIS reflective body'. The question is, then, if other 'calc-alkaline' plutons have similar compositional, and so, possible tectonomagmatic complexity. To test this hypothesis studies are currently underway of the Ossa Morena Zone Burguillos del Cerro and Brovales plutons.

The Modulation of Crustal Magmatic Systems by Tectonic Forcing

NASA Astrophysics Data System (ADS)

Karakas, O.; Dufek, J.

2010-12-01

The amount, location and residence time of melt in the crust significantly impacts crustal structure and influences the composition, frequency, and volume of eruptive products. In this study, we develop a two dimensional model that simulates the response of the crust to prolonged mantle-derived intrusions in arc environments. The domain includes the entire crustal section and upper mantle and focuses on the evolving thermal structure due to intrusions and external tectonic forcing. Magmatic intrusion into the crust can be accommodated by extension or thickening of the crust or some combination of both mechanisms. Additionally, external tectonic forcing can generate thicker crustal sections, while tectonic extension can significantly thin the crust. We monitor the thermal response, melt fraction and surface heat flux for different tectonic conditions and melt flux from the mantle. The amount of crustal melt versus fractionated primary mantle melts present in the crustal column helps determine crustal structure and growth through time. We express the amount of crustal melting in terms of an efficiency; we define the melting efficiency as the ratio of the melted volume of crustal material to the volume of melt expected from a strict enthalpy balance as explained by Dufek and Bergantz (2005). Melting efficiencies are less than 1 in real systems because heat diffuses to sections of the crust that never melt. In general, thick crust and crust experiencing extended compressional regimes results in an increased melting efficiency; and thin crust and crust with high extension rates have lower efficiency. In most settings, maximum efficiencies are less than 0.05-0.10. We also observe that with a geophysically estimated flux, the mantle-derived magma bodies build up isolated magma pods that are distributed in the crust. One of the aspects of this work is to monitor the location and size of these magma chambers in the crustal column. We further investigate the rheological, stress and pre-existing structure control on the longevity of the individual magmatic systems.

Crustal anisotropy from Moho converted Ps wave splitting and geodynamic implications in Northeastern margin of Tibetan Plateau

NASA Astrophysics Data System (ADS)

Xie, Z.; Wu, Q.; Zhang, R.

2017-12-01

Collision between Indian and Eurasian result in intense deformation and crustal shortening in the Tibetan Plateau. NE margin of Tibetan Plateau experienced complex deformation between Qilian orogen and its adjacent blocks, Alxa Block in the north and Ordos Block in the east. We focus on if there any evidences exist in the NE margin of Tibetan Plateau, which can support crustal channel flow model. China Earthquake Administration had deployed temporary seismic array which is called ChinaArray Phase Ⅱ, dense seismic stations covered NE margin of Tibetan Plateau. Seismic data recorded by 81 seismic stations is applied in this research. We calculated receiver functions with time-domain deconvolution. We selected RFs which have clear Ps phase both in radial and transverse components to measure Ps splitting owing to crustal anisotropy, and 130 pairs of anisotropy parameters of 51 seismic stations were obtained. We would like to discuss about dynamic mechanism of this area using crustal anisotropy associated with the result of SKS-splitting and surface constrains like GPS velocity. The result can be summarized as follows. The large scale of delay time imply that the crustal anisotropy mainly derives from middle to lower crust rather than upper crust. In the southeastern part of the research area, crustal anisotropy is well agree with the result computed form SKS-splitting and GPS velocity directions trending NWW-SEE or E-W direction. This result imply a vertically coherent deformation in the area as the directions of crustal anisotropy trend to be perpendicular to the direction of normal stress. In the middle and north part of the research area, the fast polarization direction of crustal anisotropy is NEE-SWW or E-W direction, parallels with direction of GPS velocity, but differ to the direction of the result of SKS-splitting. This result may imply that decoupled deformation in this area associated with middle to lower crustal flow.

Determining the Extent of Hydrothermal Interaction on the Southern Costa Rica Rift Ridge Flank During the Past 8 Ma from Joint Inversion of Geophysical Data

NASA Astrophysics Data System (ADS)

Wilson, D. J.; Moorkamp, M.; Hobbs, R. W.; Peirce, C.; Harris, R. N.; Morgan, J. V.

2017-12-01

Advective hydrothermal systems preferentially develop in zones of high porosity and permeability, driven by a local heat source. Associated chemical reactions lead to changes in the bulk physical properties, so variations in velocity and density, and the relationship connecting them, may provide a record of alteration by hydrothermal fluids. Oceanic crust accreted at intermediate rate ridges displays a range of characteristics between those typical for fast and slow spreading rates so changes in crustal porosity and permeability are sensitive to the interplay between tectonic stretching, magmatic supply and plate motions. Hence, changes in spreading style and sediment cover will influence the extent of the hydrothermal interaction that occurs and the mode of heat loss as evidenced by heat flow measurements. Using a variety of geophysical data we determine where hydrothermal circulation has been active in young oceanic crust that was accreted at an intermediate spreading rate. Results from traveltime tomography along a 300 km profile across the southern flank of the Costa Rica Rift reveal several variations in the P-wave velocity structure of the upper crust (layer 2). Following an initial increase in P-wave velocity near the ridge axis there is a section of the model 80 km in length that has lower P-wave velocity (up to 0.5 km s-1) compared to adjacent crust. This section has shallower bathymetry, by up to 500 m, than predicted by the subsidence curve and the top basement surface is rougher with a greater amount of faulting and larger throws. This zone is preceded by crust with significantly faster P-wave velocities (up to 1.0 km s-1) that was sampled by DSDP/ODP 504B. We characterise these changes in the shallow crustal structure by jointly inverting travel-time data and gravity data with deeper control from coincident magnetotelluric data. Using a cross-gradient approach allows us to search for models with a structural match, thus determining the relationship between P-wave velocity and density models from which we infer the likely hydrothermal regimes at the time of formation at the spreading ridge. This research is part of a major, interdisciplinary NERC-funded collaboration entitled: Oceanographic and Seismic Characterisation of heat dissipation and alteration by hydrothermal fluids at an Axial Ridge (OSCAR).

Crystallization Temperatures of Lower Crustal Gabbros from the Oman Ophiolite and the Persistence of the 'Mush Zone' at Intermediate/Fast Spreading Ridges

NASA Astrophysics Data System (ADS)

VanTongeren, J. A.

2017-12-01

Oceanic crust is formed when mantle-derived magmas are emplaced at the ridge axis, a zone of intense rifting and extension. Magmas begin to cool and crystallize on-axis, forming what is termed the "Mush Zone", a region of partially molten rocks. Several attempts have been made to understand the nature of the Mush Zone at fast spreading mid-ocean ridges, specifically how much partial melt exists and how far off-axis the Mush Zone extends. Geophysical estimates of P-wave velocity perturbations at the East Pacific Rise show a region of low velocity approximately 1.5-2.5 km off-axis, which can be interpreted to be the result of higher temperature [e.g. Dunn et al., 2000, JGR] or the existence of partial melt. New petrological and geochemical data and methods allow for the calculation of the lateral extent of the Mush Zone in the lower oceanic crust on exposed sections collected from the Oman ophiolite, a paleo-fast/intermediate spreading center. I will present new data quantifying the crystallization temperatures of gabbros from the Wadi Khafifah section of lower oceanic gabbros from the Oman ophiolite. Crystallization temperatures are calculated with the newly developed plagioclase-pyroxene REE thermometer of Sun and Liang [2017, Contrib. Min. Pet.]. There does not appear to be any systematic change in the crystallization temperature of lower crustal gabbros with depth in the crust. In order to quantify the duration of crystallization and the lateral extent of the Mush Zone of the lower crust, crystallization temperatures are paired with estimates of the solidus temperature and cooling rate determined from the same sample, previously constrained by the Ca diffusion in olivine geothermometer/ geospeedometer [e.g. VanTongeren et al., 2008 EPSL]. There is no systematic variation in the closure temperature of Ca in olivine, or the cooling rate to the 800°C isotherm. These results show that gabbros throughout the lower crust of the Oman ophiolite remain in a partially molten state for an average of 10,000 years. Assuming a paleo-spreading rate similar to that of the East Pacific Rise, this translates to a "Mush Zone" of partially molten rock up to 1 km off-axis, slightly less than the low velocity zone observed geophysically on the East Pacific Rise.

Eclogitization of dry granulite triggers deep crustal seismicity in Southern Tibet

NASA Astrophysics Data System (ADS)

Shi, Feng; Wang, Yanbin; Yu, Tony; Zhu, Lupei; Gasc, Julien; Zhang, Junfeng; Jin, Zhenmin

2017-04-01

Intermediate-depth earthquakes (IDEQs) occur at focal depths from about 50 km to 300 km. Their physical mechanism has been enigmatic, because as pressure and temperature increase with depth, brittle failure should be suppressed, and rocks tend to flow plastically. IDEQs have been recorded down to depths of 80 - 100 km in Southern Tibet, where the lower crust is considered hot and dry [1]. It is questionable whether such seismicity can be produced by unassisted brittle shear fracture or frictional sliding. Pseudotachylytes that formed under conditions corresponding to the eclogitic facies are ubiquitously observed in western Norway [2], demonstrating that faulting took place in granulite, which is the main constituent of lower continental crust at pressures approaching 3 GPa. These observations suggest strongly that eclogitization is potentially involved in the seismicity in the deep continental crust. Here we conduct deformation experiments on natural and nominally dry granulite in a deformation-DIA (DDIA) apparatus and Griggs apparatus within the thermal stability fields of both granulite and eclogite, to investigate the mechanism of intermediate earthquake. The D-DIA, installed at the synchrotron beamline of GSECARS, is interfaced with an acoustic emission (AE) monitoring system, allowing in-situ detection of mechanical instability along with the progress of eclogitization based on x-ray diffraction. We found that granulite deformed within its own stability field (< 2 GPa and 1000°C) behaved in a ductile fashion without any AE activity. Unstable fault slip, on the other hand, occurred during deformation of metastable granulite in the eclogite field above 2 GPa. Numerous AE events were observed. Microstructural observation on recovered samples shows conjugated macroscopic faults. Strain is highly localized along the fault, and microcracks observed along grain boundary likely involve with eclogitization products. The fault zones consist of fine- grained (<< 1 μm) angular fragments likely to be eclogitic phase transformation products. However, no pseudotachylyte has been found in these samples so far. Therefore, we conclude that eclogitization of deforming metastable dry granulite can produce mechanical instability. No formation of pseudotachylytes is required for brittle failure. We suggest that syn-deformational eclogitization of dry granulite may be responsible to the lower crustal seismicity in Southern Tibet. References [1] Hacker, B, et al., Science, 287, 2463-2466 (2000). [2] Austrheim, H. & Boundy, T. M. Science 265, 82-83 (1994).

Use of CAS in secondary school: a factor influencing the transition to university-level mathematics?

NASA Astrophysics Data System (ADS)

Varsavsky, Cristina

2012-01-01

Australian secondary school systems offer three levels of senior (year 12) mathematics studies, none of them compulsory: elementary, intermediate and advanced. The intermediate and advanced studies prepare students for further mathematics studies at university level. In the state of Victoria, there are two versions of intermediate mathematics: one where students learn and are examined with a computer algebra system (CAS) and another where students can only use scientific calculators. This study compares the performance of 1240 students as they transitioned to traditional university-level mathematics and according to whether they learned intermediate mathematics with or without the assistance of a CAS. This study concludes that students without CAS show a slight advantage, but the most important factor affecting student performance is the uptake of advanced-level mathematics studies in secondary school.

MAVEN Observations of the Effects of Crustal Magnetic Fields on the Mars Ionosphere

NASA Astrophysics Data System (ADS)

Vogt, M. F.; Flynn, C. L.; Withers, P.; Andersson, L.; Girazian, Z.; Mitchell, D. L.; Xu, S.; Connerney, J. E. P.; Espley, J. R.

2017-12-01

Mars lacks a global intrinsic magnetic field but possesses regions of strong crustal magnetic field that influence the planetary interaction with the solar wind and affect the structure and dynamics of the ionosphere. Since entering Mars orbit in 2014, the MAVEN spacecraft has collected comprehensive measurements of the local plasma and magnetic field properties in the Martian dayside ionosphere. Here we discuss how crustal magnetic fields affect the structure, composition, and electrodynamics of the Martian ionosphere as seen by MAVEN. We present a survey of 17 months of MAVEN LPW measurements of the electron density and temperature in the dayside ionosphere and show that, above 200 km altitude, regions of strong crustal magnetic fields feature cooler electron temperatures and enhanced electron densities compared to regions with little or no crustal magnetic field. We also report on the influence of the magnetic field direction and topology on MAVEN electron density measurements in the southern crustal field areas, particularly in magnetic cusp regions. Finally, we discuss the effects of crustal magnetic fields on plasma boundaries like the ionopause, located at the top of the ionosphere and marked by a sharp and substantial gradient in the electron density.

Continental crustal growth and the supercontinental cycle: evidence from the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Hong, Dawei; Zhang, Jisheng; Wang, Tao; Wang, Shiguang; Xie, Xilin

2004-09-01

Studies of supercontinental cycle are mainly concentrated on the assembly, breakup and dispersal of supercontinents, and studies of continental crustal growth largely on the growth and loss (recycling) of the crust. These two problems have long been studied separately from each other. The Paleozoic-Mesozoic granites in the Central Asian Orogenic Belt have commonly positive ɛNd values, implying large-scale continental crustal growth in the Phanerozoic. They coincided temporally and spatially with the Phanerozoic Pangea supercontinental cycle, and overlapped in space with the P-wave high- V anomalies and calculated positions of subducted slabs for the last 180 Ma, all this suggests that the Phanerozoic Laurasia supercontinental assembly was accompanied by large-scale continental crustal growth in central Asia. Based on these observations, this paper proposes that there may be close and original correlations between a supercontinental cycle, continental crustal growth and catastrophic slab avalanches in the mantle. In this model we suggest that rapid continental crustal growth occurred during supercontinent assembly, whereas during supercontinental breakup and dispersal new additions of the crust were balanced by losses, resulting in a steady state system. Supercontinental cycle and continental crustal growth are both governed by changing patterns of mantle convection.

Sedimentary framework of Penobscot Bay, Maine

USGS Publications Warehouse

Knebel, Harley J.; Scanlon, Kathryn M.

1985-01-01

Analyses of seismic-reflection profiles, along with previously collected sediment samples and geologic information from surrounding coastal areas, outline the characteristics, distribution, and history of the strata that accumulated within Penobscot Bay, Maine, during the complex period of glaciation, crustal movement, and sea-level change since late Wisconsinan time. Sediments that overlie the rugged, glacially eroded surface of Paleozoic bedrock range in thickness from near zero to more than 50 m and consist of four distinct units.Massive to partly stratified, coarse-grained drift forms thin (< 15 m) isolated patches along the walls and floors of bedrock troughs and constitutes a thick (up to 30 m), hummocky end moraine in the central part of the bay. The drift was deposited by the last ice sheet between 12,700 and 13,500 years ago during deglaciation and coastal submergence (due to crustal depression).Well-stratified, fine-grained glaciomarine deposits are concentrated in bedrock depressions beneath the main passages of the bay. During the period of ice retreat and marine submergence, these sediments settled to the sea floor, draped the irregular underlying surface of bedrock or drift, and accumulated without disturbance by physical or biologic processes.Heterogeneous fluvial deposits fill ancestral channels of the Penobscot River beneath the head of the bay. The channels were incised during a −40 m postglacial low stand of sea level (due to crustal rebound) and later were filled as base level was increased during Holocene time.Muddy marine sediments, which are homogeneous to weakly stratified and rich in organic matter, blanket older deposits within bathymetric depressions in the middle and lower reaches of the bay and cover a pronounced, gently dipping, erosional unconformity in the upper reach. These sediments were deposited during the Holocene transgression as sea level approached its present position and the bay became deeper.Late Wisconsinan and Holocene sedimentation in Penobscot Bay has smoothed the sea floor, but it has not completely obscured the ice-sculptured bedrock topography.

Source Spectra and Site Response for Two Indonesian Earthquakes: the Tasikmalaya and Kerinci Events of 2009

NASA Astrophysics Data System (ADS)

Gunawan, I.; Cummins, P. R.; Ghasemi, H.; Suhardjono, S.

2012-12-01

Indonesia is very prone to natural disasters, especially earthquakes, due to its location in a tectonically active region. In September-October 2009 alone, intraslab and crustal earthquakes caused the deaths of thousands of people, severe infrastructure destruction and considerable economic loss. Thus, both intraslab and crustal earthquakes are important sources of earthquake hazard in Indonesia. Analysis of response spectra for these intraslab and crustal earthquakes are needed to yield more detail about earthquake properties. For both types of earthquakes, we have analysed available Indonesian seismic waveform data to constrain source and path parameters - i.e., low frequency spectral level, Q, and corner frequency - at reference stations that appear to be little influenced by site response.. We have considered these analyses for the main shocks as well as several aftershocks. We obtain corner frequencies that are reasonably consistent with the constant stress drop hypothesis. Using these results, we consider using them to extract information about site response form other stations form the Indonesian strong motion network that appear to be strongly affected by site response. Such site response data, as well as earthquake source parameters, are important for assessing earthquake hazard in Indonesia.

Crustal Dynamics Project: Catalogue of site information

NASA Technical Reports Server (NTRS)

1985-01-01

This document represents a catalogue of site information for the Crustal Dynamics Project. It contains information and descriptions of those sites used by the Project as observing stations for making the precise geodetic measurements useful for studies of the Earth's crustal movements and deformation.

Crustal evolution of the early earth: The role of major impacts

NASA Technical Reports Server (NTRS)

Frey, H.

1979-01-01

The role of major impact basins (such as those which formed on the moon before 4 billion years ago) is examined to determine the effects of such impacts on the early crustal evolution of the earth. Specifically addressed is the fundamental problem of what is the origin of the earth's fundamental crustal dichotomy of low density continental and high density oceanic crust and its relationship to the superficially similar highlands/maria crustal dichotomies of the moon, Mercury and Mars.

Using a New Crustal Thickness Model to Test Previous Candidate Lunar Basins and to Search for New Candidates

NASA Technical Reports Server (NTRS)

Meyer, H. M.; Frey, H. V.

2012-01-01

A new crustal thickness model was used to test the viability of 110 candidate large lunar basins previously identified using older topographic and crustal thickness data as well as photogeologic data. The new model was also used to search for new candidate lunar basins greater than 300 km in diameter. We eliminated 11 of 27 candidates previously identified in the older crustal thickness model, and found strong evidence for at least 8 new candidates.

Crustal and Upper Mantle Structure from Joint Inversion of Body Wave and Gravity Data

DTIC Science & Technology

2012-09-01

CRUSTAL AND UPPER MANTLE STRUCTURE FROM JOINT INVERSION OF BODY WAVE AND GRAVITY DATA Eric A. Bergman1, Charlotte Rowe2, and Monica Maceira2...for these events include many readings of direct crustal P and S phases, as well as regional (Pn and Sn) and teleseismic phases. These data have been...the usefulness of the gravity data, we apply high-pass filtering, yielding gravity anomalies that possess higher resolving power for crustal and

Investigation of the Crustal Structure in the Middle East from Body-Wave Analysis (POSTPRINT). Annual Report 2

DTIC Science & Technology

2012-06-05

Variations in the Zagros Fold and Trust Zone While crustal anisotropy may be indicative of tectonic stresses and alignments of faults and fracture zones...AFRL-RV-PS- AFRL-RV-PS- TP-2012-0042 TP-2012-0042 INVESTIGATION OF THE CRUSTAL STRUCTURE IN THE MIDDLE EAST FROM BODY-WAVE ANALYSIS...DATES COVERED (From - To) 01 Sep 2010 to 19 Mar 2012 4. TITLE AND SUBTITLE INVESTIGATION OF THE CRUSTAL STRUCTURE IN THE MIDDLE EAST FROM

Levels of Processing in Mild Disabilities.

ERIC Educational Resources Information Center

Al-Hilawani, Yasser A.; And Others

This study examined the effects of the second level (intermediate acoustical processing of rhyming words) and the third level (deep-semantic processing of words in sentences) of the "levels of processing" framework on memory performance of four types of intermediate-grade students (52 "normal" students, 50 students with…

Moho Depth and Bulk Crustal Properties in Northern Quebec and Labrador

NASA Astrophysics Data System (ADS)

Vervaet, F.; Darbyshire, F. A.

2016-12-01

Northern Quebec and Labrador lie at the heart of the Laurentian landmass and preserve over 3 billion years of continental evolution. In this region the Archean Superior and Nain cratons are surrounded by Paleoproterozoic orogens such as New-Quebec, Trans-Hudson and Torngat, as well as the younger Grenville orogen to the SE. Study of crustal structure in this region provides valuable information on the assembly of the North American continent. We use data from 8 seismic stations installed in summer 2011 as part of the QUiLLE (Quebec-Labrador Lithospheric Experiment) project to investigate crustal structure, using receiver function analysis. The data set covers 5 years (2011-2016) for most of the stations, comprising several hundred events of magnitude ≥5 and epicentral distance 30-90°. After initial data processing and quality control, several tens of events per station were used in an H-κ stacking analysis to estimate Moho depth and bulk crustal properties. Some stations show significant complexity in their receiver functions, leading to inconclusive H-κ results, but the majority show a consistent Moho signal from which crustal parameters are successfully extracted. Crustal thickness varies from 33 to 49 km, with the thickest crust associated with the Trans-Hudson orogen in the Ungava region of northernmost Quebec and the thinnest beneath the central Labrador coast. Vp/Vs ratios (κ) lie in the range 1.71-1.86, with the majority of values consistent with granite-gneiss-tonalite bulk crustal compositions. The receiver functions are combined with surface-wave group velocity data to model the crustal structures in more detail beneath each station, allowing us to investigate crustal layering, Moho complexity and lateral heterogeneity.

The crustal thickness of West Antarctica

NASA Astrophysics Data System (ADS)

Chaput, J.; Aster, R. C.; Huerta, A.; Sun, X.; Lloyd, A.; Wiens, D.; Nyblade, A.; Anandakrishnan, S.; Winberry, J. P.; Wilson, T.

2014-01-01

P-to-S receiver functions (PRFs) from the Polar Earth Observing Network (POLENET) GPS and seismic leg of POLENET spanning West Antarctica and the Transantarctic Mountains deployment of seismographic stations provide new estimates of crustal thickness across West Antarctica, including the West Antarctic Rift System (WARS), Marie Byrd Land (MBL) dome, and the Transantarctic Mountains (TAM) margin. We show that complications arising from ice sheet multiples can be effectively managed and further information concerning low-velocity subglacial sediment thickness may be determined, via top-down utilization of synthetic receiver function models. We combine shallow structure constraints with the response of deeper layers using a regularized Markov chain Monte Carlo methodology to constrain bulk crustal properties. Crustal thickness estimates range from 17.0±4 km at Fishtail Point in the western WARS to 45±5 km at Lonewolf Nunataks in the TAM. Symmetric regions of crustal thinning observed in a transect deployment across the West Antarctic Ice Sheet correlate with deep subice basins, consistent with pure shear crustal necking under past localized extension. Subglacial sediment deposit thicknesses generally correlate with trough/dome expectations, with the thickest inferred subice low-velocity sediment estimated as ˜0.4 km within the Bentley Subglacial Trench. Inverted PRFs from this study and other published crustal estimates are combined with ambient noise surface wave constraints to generate a crustal thickness map for West Antarctica south of 75°S. Observations are consistent with isostatic crustal compensation across the central WARS but indicate significant mantle compensation across the TAM, Ellsworth Block, MBL dome, and eastern and western sectors of thinnest WARS crust, consistent with low density and likely dynamic, low-viscosity high-temperature mantle.

Nd and Sr isotopic variations in acidic rocks from Japan: significance of upper-mantle heterogeneity

NASA Astrophysics Data System (ADS)

Terakado, Yasutaka; Nakamura, Noboru

1984-10-01

Initial Nd and Sr isotopic ratios have been measured for Cretaceous acidic and related intermediate rocks (24 volcanic and two plutonic rocks) from the Inner Zone of Southwest Japan (IZSWJ) to investigate the genesis of acidic magmas. The initial Nd and Sr isotopic ratios for these rocks show three interesting features: (1) ɛ Nd values for acidic rocks (+2 to -9) are negatively correlated with ɛ Sr values (+10 to +90) together with those for intermediate rocks ( ɛ Nd=+3 to -8; ɛ Sr=0 to +65). (2) The ɛ Nd values for silica rich rocks (>60% SiO2) correlate with the longitude of the sample locality, decreasing from west to east in a stepwise fashion: Four areas characterized by uniform ɛ Nd values are discriminated. (3) Low silica rocks (<60% SiO2) in a certain area have distinctly different ɛ Nd values from those of the high silica rocks in the same area. These results as well as those deduced from the additional samples collected, for comparison, from other provinces in Japan suggest that the acidic rocks can be formed neither by fractional crystallization processes from more basic magmas nor by crustal assimilation processes. The isotopic variations of the acidic rocks may reflect regional isotopic heterogeneity in the lower crust, and this heterogeneity may ultimately be attributed to the regional heterogeneity of the uppermost-mantle beneath the Japanese Islands.

Effects of Comic Strips on L2 Learners' Reading Comprehension

ERIC Educational Resources Information Center

Liu, Jun

2004-01-01

This article reports the results of an experiment investigating the role of comic strips on ESL learners' reading comprehension. The students' proficiency levels were estimated, and students were organized into a low intermediate-level proficiency group (low-level students) and a high intermediate-level proficiency group (high-level students).…

Trace elements in urban and suburban rainfall, Mersin, Northeastern Mediterranean

NASA Astrophysics Data System (ADS)

Özsoy, Türkan; Örnektekin, Sermin

2009-10-01

Spatial/temporal variabilities of rainwater constituents are examined based on soluble/insoluble trace elements, pH and electrical conductivity measurements in rainfall sampled during December 2003-May 2005 at two urban and two suburban sites in Mersin, an industrialized city of 850,000 inhabitants on the southern coast of Turkey. In the analyses, backward air mass trajectories for rainy days were used in addition to factor analyses, enrichment factors, phase distributions and correlations between trace elements. The pH varied from 4.8 to 8.5 with an average value of 6.2, reflecting a mainly alkaline regime. Mean concentrations of trace elements collected from urban and suburban sites are spatially variable. Based on the overall data, total concentrations of trace elements were ordered as Ca > Na > Fe > Al > Mg > K > Zn > Mn > Sr > Pb > Ni > Cr > Ba > Cu > Co > Cd. Mainly terrigeneous (Ca, Fe, Al) and, to a lesser extent, sea salt particles (Na, Mg) were shown to be the major source of trace elements. Excluding major cations, the solubilities of trace elements were found to be ordered as Sr > Zn > Ba > Mn > Cu > Ni > Cr > Fe > Al, confirming the lower solubility of crustal elements. Cd, Co and Pb were excluded from the above evaluation because of the low numbers of soluble samples allowing quantitative measurements. The solubilities of Al, Fe, Mn and particularly of Ni were found to be considerably lower than those reported for various sites around the world, most likely due to the effect of pH. During the entire sampling period, a total of 28 dust transport episodes associated with 31 red rain events were identified. Extremely high mean concentration ratios of Al (8.2), Fe (14.4) and Mn (13.1) were observed in red rain, compared to normal rain. The degree of this enhancement displayed a decrease from crustal to anthropogenic origin elements and the lowest enhancements were found for anthropogenic origin elements of Zn and Cd (both having a ratio of 1.1). Aerosol dust was found to be the main source of almost all analyzed elements in Mersin precipitation, regardless that they are crustal or anthropic derived elements. The magnitude of crustal source contribution to trace element budget of precipitation was at its highest levels for crustal originated elements, most probably due to much higher scavenging ratios of crustal elements compared to anthropogenic ones.

Influence of lithological heterogeneity, mechanical anisotropy, and magmatism on the rheology of an arc, North Cascades, Washington

NASA Astrophysics Data System (ADS)

Miller, Robert B.; Paterson, Scott R.

2001-12-01

Many aspects of crustal dynamics are dependent on changes in rheology and strength with depth in the lithosphere. Several of the controlling factors for rheology are difficult to study experimentally, particularly lithological heterogeneity, mechanical anisotropy, and magmatism, and we focus on these in a study of the deformation patterns in a thick crustal section (˜5- to 40-km paleodepth) through the Cretaceous Cascades core of the NW Cordillera. This magmatic arc consists of metamorphosed oceanic and arc terranes intruded by magmatic bodies ranging from <10-cm-thick sheets to large plutons. Heterogeneous brittle deformation marked by serpentinite melange characterizes the shallowest part of the crustal section, and the remainder of the section is characterized by heterogeneous, fold-dominated ductile deformation. Early tight to isoclinal recumbent folds and associated axial-planar fabrics are refolded by one or more cycles of nearly coaxial, open to isoclinal, upright to overturned folds. Layering played a mechanically active role during folding at all levels, as indicated by cleavage refraction, boudinaged layers, and kinematic indicators that record fold-related shear. Ductile deformation intensifies in the narrow structural aureoles of plutons, and SW-directed, reverse shear was partitioned into some of the aureoles. The poor strain memory of these magmatic bodies makes it difficult to determine if deformation was focused in the pluton magma chambers before they reached the solidus, as commonly predicted. All of the plutons have magmatic foliations that at least in part reflect regional strains, and these foliations are strong in the deeper plutons. The thinner sheets acted as competent bodies during folding and boudinage, after they reached the solidus, but generally did not cause marked strain gradients in their hosts. A relative strength profile constructed for the Cascades crustal section shows an overall decrease in strength with depth for the ductile part of the arc that fits idealized strength profiles. However, in more detail relative strengths are markedly variable. Units were able to accumulate large ductile strains, but even small variations in the physical properties of interlayered rock types exerted a strong influence on deformation patterns throughout the mid- to deep-crustal part of the profile. This profile thus emphasizes the complex vertical rheological stratification of arcs at the crustal to thin section scale, and should be applicable to many other magmatic arcs.

Crustal extension and magmatism during the mid-Cenozoic ignimbrite flare-up in the Guazapares Mining District and Cerocahui basin regions, northern Sierra Madre Occidental, western Chihuahua, Mexico

NASA Astrophysics Data System (ADS)

Murray, Bryan Patrick

Silicic large igneous provinces are significant in the geologic record, due to their unusually extensive areal coverage (>100,000 km2) and large volumes (>250,000 km3), and may be characteristic of continental regions undergoing broad lithospheric extension. The Sierra Madre Occidental of northwestern Mexico is the biggest and best-preserved silicic large igneous province of the Cenozoic and is considered part of the extensive mid-Cenozoic ignimbrite flare-up that affected much of the southwestern North American Cordillera. Despite its size and preservation, very little is known about the geology of the Sierra Madre Occidental, and the timing and spatial extent of ignimbrite flare-up volcanism in relation to crustal extension is relatively unknown. This study presents new geologic mapping, stratigraphy, zircon U-Pb laser ablation ICP-MS dating, modal analysis, and geochemical data from the Guazapares Mining District and Cerocahui basin regions, two adjacent areas of the northern Sierra Madre Occidental in western Chihuahua. The rock exposure and topographic relief in this previously unmapped ~450 km2 area make it ideal for studying the relationships between silicic large igneous province volcanism and crustal extension. Three informal formations are identified in the study area: (1) the ca. 27.5 Ma Parajes formation, a ~1-km-thick succession of primarily welded silicic outflow ignimbrite sheets erupted from sources within ~50--100 km of the study area that were active during the Early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up; (2) the ca. 27--24.5 Ma Temoris formation, composed primarily of locally erupted mafic-intermediate lavas and associated intrusions with interbedded alluvial deposits, likely related to rocks of the Southern Cordillera basaltic andesite province that were intermittently erupted across all of the northern Sierra Madre Occidental following the Early Oligocene ignimbrite pulse; and (3) the ca. 24.5--23 Ma Sierra Guazapares formation, composed of silicic vent to proximal facies ignimbrites, lavas, plugs, and reworked equivalents that record the initiation of explosive and effusive silicic fissure magmatism in the study area during the Early Miocene pulse of the mid-Cenozoic ignimbrite flare-up. The Guazapares Mining District and Cerocahui basin regions share this stratigraphy, but the rocks in the Cerocahui basin consist of a much higher proportion of alluvial deposits. The main geologic structures in the Guazapares Mining District and Cerocahui basin regions are NNW-trending normal faults, with an estimated minimum of 20% total horizontal extension. Many normal faults bound half-graben basins that show evidence of syndepositional extension. Normal faulting began by ca. 27.5 Ma during deposition of the youngest ignimbrites of the Parajes formation, concurrent with the end of the Early Oligocene silicic ignimbrite pulse of the ignimbrite flare-up to the east and before magmatism began in the study area. Preexisting normal faults localized mafic-intermediate volcanic vents of the Temoris formation and silicic vents of the Sierra Guazapares formation, and were active during deposition of these formations. In addition, the localization and timing of epithermal mineralization in the Guazapares Mining District appears to be favored where pre-to-synvolcanic extensional structures are in close association with Sierra Guazapares formation rhyolite plugs. The timing of extensional faulting and magmatism in the Guazapares Mining District and Cerocahui regions is consistent with regional-scale Middle Eocene to Early Miocene southwestward migration of active volcanism and extension in the northern Sierra Madre Occidental. Extension accompanied mafic-intermediate and silicic volcanism in the study area, and overlapped with the peak of mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental; this supports the interpretation that there is likely a relationship between lithospheric extension and silicic large igneous province magmatism.

Contrasted terrace systems of the lower Moulouya valley as indicator of crustal deformation in NE Morocco

NASA Astrophysics Data System (ADS)

Rixhon, Gilles; Bartz, Melanie; El Ouahabi, Meriam; Szemkus, Nina; Brueckner, Helmut

2016-04-01

The Moulouya river has the largest catchment in Morocco and drains an area which is characterized by active crustal deformation during the Late Cenozoic due to the convergence between the African and Eurasian plates. As yet, its Pleistocene terrace sequence remains poorly documented. Our study focuses on the lowermost reach of the river in NE Morocco, which drains the Triffa sedimentary basin directly upstream of the estuary. New field observations, measurements and sedimentological data reveal contrasted fluvial environments on either side of a newly identified thrust zone, which disrupts the whole sedimentary basin and is associated with N-S compressive shortening in this region (Barcos et al., 2014). Long-lasting fluvial aggradation, materialized by ≥37 m-thick stacked fill terraces, and the development of a well-preserved terrace staircase, with (at least) three Pleistocene terrace levels, occur in the footwall and the hanging wall of the thrust, respectively. Same as for the Pleistocene terrace sediments of the middle Moulouya, a recurrent sedimentary pattern, characterized by fining-upward sequences was observed in the studied terrace profiles. Assessing the rates of crustal deformation along this main thrust zone requires age estimations for these Pleistocene terrace deposits of the lower Moulouya on each side of the thrust. Samples for luminescence (OSL/IRSL), electron spin resonance (ESR, on quartz) and cosmogenic nuclide dating (26Al/10Be, burial dating) were collected in terrace deposits located both in the foot- and hanging walls. Sample preparation and analysis as well as age determination are in progress. The preliminary data mentioned above, soon to be completed by chronological data, agree well with morphometric indicators stating that the whole Moulouya catchment is at disequilibrium state (Barcos et al., 2014). This is confirmed by several knickpoints in its longitudinal profile. Late Cenozoic uplift associated with crustal shortening, which occurred in the lowermost reach of the river, may have both hindered profile rectification of the Moulouya and, at the same time, buffered the effects of long-term base-level changes due to eustatic sea-level variations. Reference: Barcos, L., Jabaloy, A., Azdimousa, A., Asebriy, L., Gómez-Ortiz, D., Rodríguez-Peces, M.J., Tejero, R., Pérez-Peña, J.V., 2014. Study of relief changes related to active doming in the eastern Moroccan Rif (Morocco) using geomorphological indices. J. African Earth Sci. 100, 493-509.

Extreme Mesozoic crustal thinning in the Eastern Iberia margin: The example of the Columbrets Basin (Valencia Trough)

NASA Astrophysics Data System (ADS)

Mohn, G.; Etheve, N.; Frizon de Lamotte, D.; Roca, E.; Tugend, J.; Gómez-Romeu, J.

2017-12-01

Eastern Iberia preserves a complex succession of Mesozoic rifts partly or completely inverted during the Late Cretaceous and Cenozoic in relation with Africa-Eurasia convergence. Notably, the Valencia Trough, classically viewed as part of the Cenozoic West Mediterranean basins, preserves in its southwestern part a thick Mesozoic succession (locally »10km thick) over a highly thinned continental basement (locally only »3,5km thick). This sub-basin referred to as the Columbrets Basin, represents a Late Jurassic-Early Cretaceous hyper-extended rift basin weakly overprinted by subsequent events. Its initial configuration is well preserved allowing us to unravel its 3D architecture and tectono-stratigraphic evolution in the frame of the Mesozoic evolution of eastern Iberia. The Columbrets Basin benefits from an extensive dataset combining high resolution reflection seismic profiles, drill holes, refraction seismic data and Expanding Spread Profiles. Its Mesozoic architecture is controlled by interactions between extensional deformation and halokinesis involving the Upper Triassic salt. The thick uppermost Triassic to Cretaceous succession describes a general synclinal shape, progressively stretched and dismembered towards the basin borders. The SE-border of the basin is characterized by a large extensional detachment fault acting at crustal scale and interacting locally with the Upper Triassic décollement. This extensional structure accommodates the exhumation of the continental basement and part of the crustal thinning. Eventually our results highlight the complex interaction between extreme crustal thinning and occurrence of a pre-rift salt level for the deformation style and tectono-stratigraphic evolution of hyper-extended rift basins.

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.

Crustal structure of Central Sicily

NASA Astrophysics Data System (ADS)

Giustiniani, Michela; Tinivella, Umberta; Nicolich, Rinaldo

2018-01-01

We processed crustal seismic profile SIRIPRO, acquired across Central Sicily. To improve the seismic image we utilized the wave equation datuming technique, a process of upward or downward continuation of the wave-field between two arbitrarily shaped surfaces. Wave equation datuming was applied to move shots and receivers to a given datum plane, removing time shifts related to topography and to near-surface velocity variations. The datuming procedure largely contributed to attenuate ground roll, enhance higher frequencies, increase resolution and improve the signal/noise ratio. Processed data allow recognizing geometries of crust structures differentiating seismic facies and offering a direct image of ongoing tectonic setting within variable lithologies characterizing the crust of Central Sicily. Migrated sections underline distinctive features of Hyblean Plateau foreland and above all a crustal thinning towards the Caltanissetta trough, to the contact with a likely deep Permo-Triassic rifted basin or rather a zone of a continent to oceanic transition. Inhomogeneity and fragmentation of Sicily crust, with a distinct separation of Central Sicily basin from western and eastern blocks, appear to have guided the tectonic transport inside the Caltanissetta crustal scale syncline and the accumulation of allochthonous terrains with south and north-verging thrusts. Major tectonic stack operated on the construction of a wide anticline of the Maghrebian chain in northern Sicily. Sequential south-verging imbrications of deep elements forming the anticline core denote a crust wedge indenting foreland structures. Deformation processes involved multiple detachment planes down to decoupling levels located near crust/mantle transition, supporting a presence of high-density lenses beneath the chain, interrelated to a southwards push of Tyrrhenian mantle and asthenosphere.

Crustal Dynamics Project: Catalogue of site information

NASA Technical Reports Server (NTRS)

Noll, Carey E. (Editor)

1988-01-01

This document represents a catalog of site information for the Crustal Dynamics Project. It contains information on and descriptions of those sites used by the Project as observing stations for making the precise geodetic measurements necessary for studies of the Earth's crustal movements and deformation.

Seismicity and active tectonics of the Andes and the origin of the Altiplano

NASA Technical Reports Server (NTRS)

Molnar, P.

1982-01-01

Large earthquakes and active deformation on the Andes were studied. Earthquakes on the east side of the Andes were generally found to reflect east-west crustal shortening. These earthquakes seem to occur throughout the crust and do not reflect a detachment and low angle thrusting of the sedimentary cover onto the Brazilian shield. Instead they imply deformation of the basement. The rate of shortening is compatible with construction of the Andes by crustal shortening since the late Cretaceous, and the surface geology, at least qualitatively, is considered to reflect this process. Andean margins are considered to be a result of crustal shortening. The crustal shortening in the sub-Andes occurs concurrently with normal faulting at high elevations in parts of the Andes. The normal faulting is associated with the buoyancy of the thick crust. Crustal shortening thickens the crust and work is done against gravity. When the crustal thickness and elevation reach limiting values, the range grows laterally by further thrusting on the margins.

Mars: Difference Between Lowland and Highland Basalts Confirms A Tendency Observed In Terrestrial and Lunar Basaltic Compositions

NASA Astrophysics Data System (ADS)

Kochemasov, G.

Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable 1 to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. 2 MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, 3 kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- 4 terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular 5 momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- 6 paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 7 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for 8 these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. 9 MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. Rock density is a very important factor for constructing tectonic blocks in celestial bodies (Theorem 4, [1]). Angular momenta regulation of different level tectonic blocks in rotating bodies is more effec- tively fulfilled at the crustal level as this level has the longest radius. Thus, composition of crustal basalts is very sensitive to hypsometric (tectonic0 position of certain plan- etary blocks. At Earth oceanic hollows are filled with Fe-rich tholeiites (the deepest Pacific depression is filled with the richest in Fe tholeiites), on continents prevail com- paratively Mg-rich continental basalts. Mare basalts of the Moon are predominantly Fe,Ti-rich. At higher crustal levels appear less dense feldspar-rich, KREEP basalts. This tendency for martian basalts became clear after TES experiment on MGS [2]. The TES data on mineralogy of low-albedo regions show that type1 spectra belong to less dense basic rocks (feldspar 50%, pyroxene 25%) than type2 spectra (feldspar 35%, pyroxene + glass 35%). It means that the highland basaltoids are less dense than the lowland ones. It is interesting that the type1 spectral shape is similar to a spec- trum of the Deccan Traps flood basalts [2]. These continental basalts of the low-lying Indostan subcontinent are known to be relatively Fe-rich and approach the oceanic tholeiites. Global gravity, magnetic, basaltic composition data, available upto now for these bodies: Earth, Moon, Mars, indicate that there is a regular planetology capable to make scientific predictions. References: [1] Kochemasov G.G. (1999) Theorems of wave planetary tectonics // Geophys. Res. Abstr., v. 1,# 3, 700; [2] Bandfield J.L., Hamilton V.E., Christensen Ph.R. (2000) A global view of martian surface composi- tions from MGS-TES // Science, v.287, # 5458, 1626-1630. MARS: DIFFERENCE BETWEEN LOWLAND AND HIGHLAND BASALTS CONFIRMS A TENDENCY OBSERVED IN TERRESTRIAL AND LUNAR BASALTIC COMPOSITIONS G. Kochemasov, IGEM RAS, 35 Staromonetny, Moscow 109017, Russia, 10 kochem@igem.ru, Fax: (007)(095) 230 21 79 Basalts are very widespread lithology on surfaces of terrestrial planets because their mantles, by general opinion, are predominantly basic in composition. Planetary sur- face unevennesses are often filled with this very fluid under high temperatures ma- terial. Basaltic compositions are however variable and this is helped by a wide iso- morphism of constituent minerals: Na-Ca feldspars and Fe-Mg dark minerals. Ratios between light and dark minerals as well as Fe/Mg ratios in dark minerals play an important role in regulation of basaltic densities. v 11

Precious metal enrichment at low-redox in terrestrial native Fe-bearing basalts investigated using laser-ablation ICP-MS

NASA Astrophysics Data System (ADS)

Howarth, Geoffrey H.; Day, James M. D.; Pernet-Fisher, John F.; Goodrich, Cyrena A.; Pearson, D. Graham; Luo, Yan; Ryabov, Viktor V.; Taylor, Lawrence A.

2017-04-01

Primary native Fe is a rare crystallizing phase from terrestrial basaltic magmas, requiring highly reducing conditions (fO2

Global Mapping of Oceanic and Continental Shelf Crustal Thickness and Ocean-Continent Transition Structure

NASA Astrophysics Data System (ADS)

Kusznir, Nick; Alvey, Andy; Roberts, Alan

2017-04-01

The 3D mapping of crustal thickness for continental shelves and oceanic crust, and the determination of ocean-continent transition (OCT) structure and continent-ocean boundary (COB) location, represents a substantial challenge. Geophysical inversion of satellite derived free-air gravity anomaly data incorporating a lithosphere thermal anomaly correction (Chappell & Kusznir, 2008) now provides a useful and reliable methodology for mapping crustal thickness in the marine domain. Using this we have produced the first comprehensive maps of global crustal thickness for oceanic and continental shelf regions. Maps of crustal thickness and continental lithosphere thinning factor from gravity inversion may be used to determine the distribution of oceanic lithosphere, micro-continents and oceanic plateaux including for the inaccessible polar regions (e.g. Arctic Ocean, Alvey et al.,2008). The gravity inversion method provides a prediction of continent-ocean boundary location which is independent of ocean magnetic anomaly and isochron interpretation. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we can improve the determination of pre-breakup rifted margin conjugacy and sea-floor spreading trajectory during ocean basin formation. By restoring crustal thickness & continental lithosphere thinning to their initial post-breakup configuration we show the geometry and segmentation of the rifted continental margins at their time of breakup, together with the location of highly-stretched failed breakup basins and rifted micro-continents. For detailed analysis to constrain OCT structure, margin type (i.e. magma poor, "normal" or magma rich) and COB location, a suite of quantitative analytical methods may be used which include: (i) Crustal cross-sections showing Moho depth and crustal basement thickness from gravity inversion. (ii) Residual depth anomaly (RDA) analysis which is used to investigate OCT bathymetric anomalies with respect to expected oceanic values. This includes flexural backstripping to produce bathymetry corrected for sediment loading. (iii) Subsidence analysis which is used to determine the distribution of continental lithosphere thinning. (iv) Joint inversion of time-domain deep seismic reflection and gravity anomaly data which is used to determine lateral variations in crustal basement density and velocity across the OCT, and to validate deep seismic reflection interpretations of Moho depth. The combined interpretation of these independent quantitative measurements is used to determine crustal thickness and composition across the ocean-continent-transition. This integrated approach has been validated on the Iberian margin where ODP drilling provides ground-truth of ocean-continent-transition crustal structure, continent-ocean-boundary location and magmatic type.

Magma plumbing in the Grímsvötn volcanic system, Iceland: an overview

NASA Astrophysics Data System (ADS)

Thordarson, T.

2016-12-01

The basaltic Grímsvötn volcanic system (GVS) consists of Grímsvötn central volcano (GCV) and an immature fissure swarm extending 70 km to the southwest from GCV. The GCV has the highest eruption frequency of all central volcanos in Iceland, or 7 events per 100 years. In contrast, the GVS fissure swarm has only featured two events in postglacial times, the 1783-4 Laki and the prehistoric Lambavatnsgígar fissure eruptions. These two events account for 25% of the total Holocene magma output from the GVS and 80% of the output in historic time (i.e. last 1100 years). Although GVS magma plumbing has been a topic of research for four decades, its general structure, extent and geometry is still deliberated. Is mantle-derived magma delivered straight up beneath the GCV to an upper crustal magma chamber and then vertically to eruptions at the GCV and laterally to eruption on the GVS fissure swarm? Or does the system feature two levels of crustal storage, one in the upper crust beneath GCV and another at mid-crustal depth? Or is the structure of the GVS plumbing more complex? The data that we have so far and is pertinent to GVS magma plumbing is summarised below: Geophysical measurements imply that shallowest magma storage beneath GCV is at 3-4 km. The Zr and Nb concentrations in the tephra from the 1998 and 2004 GCV plus Laki eruptions show that the parent magmas for each was produced by different degrees of partial melting of a similar mantle source. It also demonstrates transport to the surface via separate pathways and that neither magma can be derived by fractional crystallization from a Laki-like magma. Detailed petrological studies on the Laki tephra and lava indicate polybaric magma evolution within the mid-crust (at 6 to 15 km depth), with further evolution at shallower depths induced either by disequilibrium crystal growth during ascent of magma from the mid-crust storage or a brief residence at 3-6 km depths. The Laki magma contains significant abundances of polymineralic glomerocrysts, signifying that erupted magma interacted with preexisting crystal mushes. These data support the notion of a crustal plumbing system with multiple storage level involving polybaric magma evolution and are inconsistent with the idea that all of the magma erupted within the GVS is delivered from a single upper crustal magma chamber beneath GCV.

Experimental and Theoretical Studies of Long-Period Tilt of Earth’s Crust: Part I. Experimental.

DTIC Science & Technology

A three-element borehole tiltmeter system was developed and installed at Bedford, Massachusetts for the purpose of measurement and analysis of...crustal tilts. Each borehole unit contains two single-axis tiltmeters with diamagnetically suspended masses and leveling motors adapted for stepwise

Hidden Markov analysis of mechanosensitive ion channel gating.

PubMed

Khan, R Nazim; Martinac, Boris; Madsen, Barry W; Milne, Robin K; Yeo, Geoffrey F; Edeson, Robert O

2005-02-01

Patch clamp data from the large conductance mechanosensitive channel (MscL) in E. coli was studied with the aim of developing a strategy for statistical analysis based on hidden Markov models (HMMs) and determining the number of conductance levels of the channel, together with mean current, mean dwell time and equilibrium probability of occupancy for each level. The models incorporated state-dependent white noise and moving average adjustment for filtering, with maximum likelihood parameter estimates obtained using an EM (expectation-maximisation) based iteration. Adjustment for filtering was included as it could be expected that the electronic filter used in recording would have a major effect on obviously brief intermediate conductance level sojourns. Preliminary data analysis revealed that the brevity of intermediate level sojourns caused difficulties in assignment of data points to levels as a result of over-estimation of noise variances. When reasonable constraints were placed on these variances using the better determined noise variances for the closed and fully open levels, idealisation anomalies were eliminated. Nevertheless, simulations suggested that mean sojourn times for the intermediate levels were still considerably over-estimated, and that recording bandwidth was a major limitation; improved results were obtained with higher bandwidth data (10 kHz sampled at 25 kHz). The simplest model consistent with these data had four open conductance levels, intermediate levels being approximately 20%, 51% and 74% of fully open. The mean lifetime at the fully open level was about 1 ms; estimates for the three intermediate levels were 54-92 micros, probably still over-estimates.

Reports on block rotations, fault domains and crustal deformation

NASA Technical Reports Server (NTRS)

Nur, Amos

1990-01-01

Studies of block rotations, fault domains and crustal deformation in the western United States, Israel, and China are discussed. Topics include a three-dimensional model of crustal fracture by distributed fault sets, distributed deformation and block rotation in 3D, stress field rotation, and multiple strike slip fault sets.

A Global 3D P-Velocity Model of the Earth’s Crust and Mantle for Improved Event Location -- SALSA3D

DTIC Science & Technology

2010-09-01

incorporates variable resolution in both the geographic and radial dimensions. For our starting model, we use a simplified two layer crustal model derived from... crustal model derived from the Crust 2.0 model over a uniform AK135 mantle. Sufficient damping is used to reduce velocity adjustments so that ray path...upper mantle, and a third tessellation with variable resolution to all crustal layers. The crustal tessellation (not shown) has 2° triangles in oceanic

Crustal deformation associated with crustal activities in the northern Izu-islands area during the summer, 2000

NASA Astrophysics Data System (ADS)

Kaidzu, M.; Nishimura, T.; Murakami, M.; Ozawa, S.; Sagiya, T.; Yarai, H.; Imakiire, T.

2000-08-01

In the end of June, 2000, intense crustal activity took place in Miyake-jima, Niijima, Kozu-shima and their vicinity. Here we report on the crustal deformation in the area during the period from June 24 to September 4, 2000, detected with the nationwide Global Positioning System (GPS) array operated by the Geographical Survey Institute. The deformation in this area during the above period is characterized by the deflation of Miyake-jima and the extension of the crust in the northeast-southwest direction over a wide area.

Estimating crustal thickness and Vp/Vs ratio with joint constraints of receiver function and gravity data

NASA Astrophysics Data System (ADS)

Shi, Lei; Guo, Lianghui; Ma, Yawei; Li, Yonghua; Wang, Weilai

2018-05-01

The technique of teleseismic receiver function H-κ stacking is popular for estimating the crustal thickness and Vp/Vs ratio. However, it has large uncertainty or ambiguity when the Moho multiples in receiver function are not easy to be identified. We present an improved technique to estimate the crustal thickness and Vp/Vs ratio by joint constraints of receiver function and gravity data. The complete Bouguer gravity anomalies, composed of the anomalies due to the relief of the Moho interface and the heterogeneous density distribution within the crust, are associated with the crustal thickness, density and Vp/Vs ratio. According to their relationship formulae presented by Lowry and Pérez-Gussinyé, we invert the complete Bouguer gravity anomalies by using a common algorithm of likelihood estimation to obtain the crustal thickness and Vp/Vs ratio, and then utilize them to constrain the receiver function H-κ stacking result. We verified the improved technique on three synthetic crustal models and evaluated the influence of selected parameters, the results of which demonstrated that the novel technique could reduce the ambiguity and enhance the accuracy of estimation. Real data test at two given stations in the NE margin of Tibetan Plateau illustrated that the improved technique provided reliable estimations of crustal thickness and Vp/Vs ratio.

Seismic evidence for widespread western-US deep-crustal deformation caused by extension

USGS Publications Warehouse

Moschetti, M.P.; Ritzwoller, M.H.; Lin, F.; Yang, Y.

2010-01-01

Laboratory experiments have established that many of the materials comprising the Earth are strongly anisotropic in terms of seismic-wave speeds. Observations of azimuthal and radial anisotropy in the upper mantle are attributed to the lattice-preferred orientation of olivine caused by the shear strains associated with deformation, and provide some of the most direct evidence for deformation and flow within the Earths interior. Although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes, large-scale observations have been limited to regions of particularly thick crust. Here we show that observations from ambient noise tomography in the western United States reveal strong deep (middle to lower)-crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the Cenozoic Era (since 65 Myr ago). The coincidence of crustal radial anisotropy with the extensional provinces of the western United States suggests that the radial anisotropy results from the lattice-preferred orientation of anisotropic crustal minerals caused by extensional deformation. These observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension. ?? 2010 Macmillan Publishers Limited. All rights reserved.

Crustal structure across the Altyn Tagh Range at the northern margin of the Tibetan Plateau and tectonic implications

USGS Publications Warehouse

Zhao, J.; Mooney, W.D.; Zhang, X.; Li, Z.; Jin, Z.; Okaya, N.

2006-01-01

We present new seismic refraction/wide-angle-reflection data across the Altyn Tagh Range and its adjacent basins. We find that the crustal velocity structure, and by inference, the composition of the crust changes abruptly beneath the Cherchen fault, i.e., ???100 km north of the northern margin of the Tibetan plateau. North of the Cherchen fault, beneath the Tarim basin, a platform-type crust is evident. In contrast, south the Cherchen fault the crust is characterized by a missing high-velocity lower-crustal layer. Our seismic model indicates that the high topography (???3 km) of the Altyn Tagh Range is supported by a wedge-shaped region with a seismic velocity of 7.6-7.8 km/s that we interpret as a zone of crust-mantle mix. We infer that the Altyn Tagh Range formed by crustal-scale strike-slip motion along the North Altyn Tagh fault and northeast-southwest contraction over the range. The contraction is accommodated by (1) crustal thickening via upper-crustal thrusting and lower-crustal flow (i.e., creep), and (2) slip-parallel (SW-directed) underthrusting of only the lower crust and mantle of the eastern Tarim basin beneath the Altyn Tagh Range. ?? 2005 Elsevier B.V. All rights reserved.

The crustal structure in the transition zone between the western and eastern Barents Sea

NASA Astrophysics Data System (ADS)

Shulgin, Alexey; Mjelde, Rolf; Faleide, Jan Inge; Høy, Tore; Flueh, Ernst; Thybo, Hans

2018-07-01

We present a crustal-scale seismic profile in the Barents Sea based on new data. Wide-angle seismic data were recorded along a 600 km long profile at 38 ocean bottom seismometer and 52 onshore station locations. The modelling uses the joint refraction/reflection tomography approach where co-located multichannel seismic reflection data constrain the sedimentary structure. Further, forward gravity modelling is based on the seismic model. We also calculate net regional erosion based on the calculated shallow velocity structure. Our model reveals a complex crustal structure of the Baltic Shield to Barents shelf transition zone, as well as strong structural variability on the shelf itself. We document large volumes of pre-Carboniferous sedimentary strata in the transition zone which reach a total thickness of 10 km. A high-velocity crustal domain found below the Varanger Peninsula likely represents an independent crustal block. Large lower crustal bodies with very high velocity and density below the Varanger Peninsula and the Fedynsky High are interpreted as underplated material that may have fed mafic dykes in the Devonian. We speculate that these lower crustal bodies are linked to the Devonian rifting processes in the East European Craton, or belonging to the integral part of the Timanides, as observed onshore in the Pechora Basin.

Crustal structure of the eastern Borborema Province, NE Brazil, from the joint inversion of receiver functions and surface wave dispersion: Implications for plateau uplift

NASA Astrophysics Data System (ADS)

Luz, Rosana M. N.; Julià, Jordi; do Nascimento, Aderson F.

2015-05-01

We investigate the crustal structure of the Borborema Province of NE Brazil by developing 44 S wave velocity-depth profiles from the joint inversion of receiver functions and fundamental mode, Rayleigh wave group velocities. The Borborema Province is located in the northeasternmost corner of the South American continent and represents a portion of a larger Neoproterozoic mobile belt that formed during the Brasiliano-Pan African orogeny. Extensional processes in the Mesozoic—eventually leading to the separation of Africa and South America—left a number of aborted rift basins in the continental interiors, and episodes of diffuse intraplate volcanism and uplift marked the evolution of the Province after continental breakup. Our velocity-depth profiles reveal the existence of two crustal types in the Province: (i) the thin crustal type, which consists of 30-32.5 km thick crust, with an upper layer of 3.4-3.6 km/s overlying a lower layer of 3.7-3.8 km/s and (ii) the thick crustal type, which consists of a 35-37.5 km thick crust, with velocities between 3.5 and 3.9 km/s down to ˜30 km depth and a gradational increase in velocity (VS≥4.0 km/s) down to upper mantle depths. The crustal types correlate well with topography, with the thick crustal type being mainly found in the high-standing southern Borborema Plateau and the thin crustal type being mostly found in the low-lying Sertaneja depression and coastal cuestas. Interestingly, the thin crustal type is also observed under the elevated topography of the northern Plateau. We argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes, as it is commonly believed. We propose that extensional processes in the Mesozoic stretched portions of the Brasiliano crust and formed the thin crustal type that is now observed in the regions of low-lying topography, leaving the rheologically strong thick crust of the southern Plateau at higher elevations. The crust making the northern Plateau would have thinned and subsided during Mesozoic extension as part of a greater Sertaneja depression, to then experience uplift in the Cenozoic and achieve its present elevation.

Contrasts in Seismicity Along the 1964 Great Alaska Earthquake Rupture Zone

NASA Astrophysics Data System (ADS)

Doser, D. I.; Veilleux, A. M.; Flores, C.; Brown, W. A.

2004-12-01

We have examined seismicity occurring over 35 years prior to and following the 1964 great Alaska earthquake. These studies indicate that the regions associated with the Prince William Sound (PWS) and Kodiak asperities have behaved very different seismically over time. Prior to 1964, the PWS region experienced an increase in crustal seismicity, especially in the region located down-dip of maximum slip in 1964. Since 1964 this same region has been aseismic at the M> 3 level. PWS intraslab seismicity has remained relatively constant, although down-dip migration of M > 4.5 events was observed following the 1964 mainshock. Low-level seismicity has occurred at the northeastern end of the Kodiak asperity throughout the past 70 years, but much of the central Kodiak asperity has been aseismic at the M > 5 level. In contrast, the southwestern edge of the Kodiak asperity has been associated with moderate to large (Mw 5.5 to 7) crustal, interplate, and intraslab events throughout the past 70 years. These changes in seismic behavior along the 1964 rupture zone are consistent with GPS/geodesy estimates of seismic coupling across the interface and with known changes in plate geometry.

Distribution of Hydrothermal Activity at the Lau ISS: Possible Controlling Parameters

NASA Astrophysics Data System (ADS)

Martinez, F.; Baker, E. T.; Resing, J. A.; Edwards, M. H.; Walker, S. L.; Buck, N.

2008-12-01

Seismic tomographic studies of intermediate to fast spreading rate mid-ocean ridges (MORs) interpret zones of rapid crustal cooling a few (3-4) km off axis surrounding the axial seismic low velocity zone (LVZ). These zones of rapid cooling also broadly correlate with the initiation and growth of large abyssal hill faults. The close association of both high thermal gradients and development of fault permeability at crustal scales suggests the hypothesis that these areas may be favorable locations for off-axis high temperature hydrothermal activity. In March-May 2008 on R/V Kilo Moana we conducted a near-bottom sidescan sonar and oceanographic survey along the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) in the Lau back-arc basin to map the distribution of hydrothermal activity within this region. The survey utilized the deep-towed DSL120A (IMI120) sonar, an array of miniature autonomous plume recorders (MAPRs) attached to the tow cable and tethered beneath the sonar's depressor weight, an in situ chemical scanner (VISA) and 23 CTD hydrocasts (see Baker et al., this session). At the ELSC the survey spanned ~100 x 10 km area encompassing the ABE, Tow Cam and Kilo Moana vent fields with ~ 1 km spaced lines overall and ~500 m spaced lines in the area of the ABE vent field. On the VFR the survey spanned a distance of ~100 km along axis by ~5 km across axis with 700 m spaced lines encompassing the Vai Lili, Mariner and Tui Malila vent sites. Initial results identified particle plumes, indicative of high temperature venting, only within about a km of the ridge axis at the ELSC and VFR with possible diffuse venting indicated by MAPR oxidation-reduction potential (ORP) measurements at flank sites at VFR. The expanded sonar coverage better defines the volcano-tectonic context of the hydrothermal signals and previously mapped vent sites. Initial results suggest, however, no high-T venting more than about 1 km from the ridge axis, an apparently negative test of the above hypothesis. This may suggest that hydrothermal fluids are efficiently channeled to the axis even if cooling off-axis crustal regions. Alternatively, at the back-arc VFR/ELSC subduction controls on magmatic productivity may play an important role in modifying the usual relationships observed at MORs among spreading rate, seismic low velocity zone width, faulting, sedimentation and hydrothermal activity. For example, the fast spreading ELSC is magmatically deficient for its spreading rate, forms a deep faulted axial floor and has no continuous axial magma lens seismic reflector. This may lead to a narrower LVZ at the ELSC relative to MORs spreading at the same rate. At the intermediate rate and arc- proximal VFR excess magmatic productivity and volatile-rich volcanics form a peaked shallow axis, few flanking faults and a sediment drape of volcaniclastics from the ridge axis (and nearby arc volcanoes). These effects may act to suppress fault-related permeability on ridge flanks or may rapidly blanket ridge-flanking hydrothermal systems. Some of these issues may be further clarified when planned seismic tomographic and moored hydrophone studies are carried out beginning next year at the ELSC/VFR.

Petrogenesis and tectonic significance of the calc-alkaline, bimodal Aztec Wash pluton, Eldorado Mountains, Colorado River extensional corridor

NASA Astrophysics Data System (ADS)

Falkner, Claudia M.; Miller, Calvin F.; Wooden, Joseph L.; Heizler, Matthew T.

1995-06-01

The 15.7 Ma Aztec Wash pluton is located in the central Eldorado Mountains of the Colorado River extensional corridor in southern Nevada, immediately south of the well-known imbricated volcanic sequence that has been widely cited in studies of extensional tectonism (e.g., Anderson, 1971). It is a shallow level (≤5 km), essentially bimodal complex, primarily made up of granite (˜72 wt % SiO2) and diabase and diorite (˜54 wt % SiO2), with minor amounts of more mafic, intermediate, and highly evolved rocks. The mafic and felsic magmas mingled extensively but mixed only to a limited extent. Late synplutonic mafic and felsic dikes represent continuing injection of the same bimodal magmas. The mafic rocks have high incompatible element concentrations (e.g., K2O ˜ 3 wt %, Ba ˜ 1600 ppm, light rare earth elements 350 × chondrite) and enriched isotopic compositions (ɛNd -7.5, 87Sr/86Sr 0.708); generation in ancient, enriched mantle lithosphere with limited subsequent crustal contamination is inferred. The granite is more potassic (˜5 wt %) than the mafic rocks, but it has comparable or lower concentrations of most incompatible elements; its isotopic composition (ɛNd -10, 87Sr/86Sr 0.710) is intermediate between those of the mafic rocks and local ancient crust. The granites thus indicate hybridization of the crust by mafic magma, but it is unclear whether this hybridization occurred at deeper levels in this magmatic system, or during an earlier mid-Tertiary or Mesozoic magmatic event. Emplacement of the Aztec Wash pluton preceded peak east west extension in the northern Eldorado Mountains (˜15.2 Ma (Gans et al., 1994)), but it coincided with at least modest extension as indicated by the uniform NS orientation of the late dikes and the mafic injections into the magma chamber. Total extension and tilting of the pluton after crystallization was minor, in contrast to the east tilted area to the north and west tilted area to the south. Timing and style of extension are thus consistent with the pluton's location within, and perhaps mechanical influence on, a major accommodation zone, as suggested by Faulds et al. (1990) and Faulds (1994).

The Impacts of 3-D Earth Structure on GIA-Induced Crustal Deformation and Future Sea-Level Change in the Antarctic

NASA Astrophysics Data System (ADS)

Powell, E. M.; Hay, C.; Latychev, K.; Gomez, N. A.; Mitrovica, J. X.

2016-12-01

Glacial Isostatic Adjustment (GIA) models used to constrain the extent of past ice sheets and viscoelastic Earth structure, or to correct geodetic and geological observables for ice age effects, generally only consider depth-dependent variations in Earth viscosity and lithospheric structure. A et al. [2013] argued that 3-D Earth structure could impact GIA observables in Antarctica, but concluded that the presence of such structure contributes less to GIA uncertainty than do differences in Antarctic deglaciation histories. New seismic and geological evidence, however, indicates the Antarctic is underlain by complex, high amplitude variability in viscoelastic structure, including a low viscosity zone (LVZ) under West Antarctica. Hay et al. [2016] showed that sea-level fingerprints of modern melting calculated using such Earth models differ from those based on elastic or 1-D viscoelastic Earth models within decades of melting. Our investigation is motivated by two questions: (1) How does 3-D Earth structure, especially this LVZ, impact observations of GIA-induced crustal deformation associated with the last deglaciation? (2) How will 3-D Earth structure affect predictions of future sea-level rise in Antarctica? We compute the gravitationally self-consistent sea level, uplift, and gravity changes using the finite volume treatment of Latychev et al. [2005]. We consider four viscoelastic Earth models: a global 1-D model; a regional, West Antarctic-like 1-D model; a 3-D model where the lithospheric thickness varies laterally; and a 3-D model where both viscosity and lithospheric thickness vary laterally. For our Last Glacial Maximum to present investigations we employ ICE6g [Peltier et al., 2015]. For our present-future investigations we consider a melt scenario consistent with GRACE satellite gravity derived solutions [Harig et al., 2015]. Our calculations indicate that predictions of crustal deformations due to both GIA and ongoing melting are strongly influenced by 3-D lithospheric thickness and viscosity structure. Future sea level change due to ongoing melting is primarily influenced by 3-D viscosity structure. We show that 1-D Earth models built using regional inferences of viscosity and lithospheric thickness do not accurately capture the variability introduced by 3-D Earth structure.

The Impacts of 3-D Earth Structure on GIA-Induced Crustal Deformation and Future Sea-Level Change in the Antarctic

NASA Astrophysics Data System (ADS)

Powell, E. M.; Hay, C.; Latychev, K.; Gomez, N. A.; Mitrovica, J. X.

2017-12-01

Glacial Isostatic Adjustment (GIA) models used to constrain the extent of past ice sheets and viscoelastic Earth structure, or to correct geodetic and geological observables for ice age effects, generally only consider depth-dependent variations in Earth viscosity and lithospheric structure. A et al. [2013] argued that 3-D Earth structure could impact GIA observables in Antarctica, but concluded that the presence of such structure contributes less to GIA uncertainty than do differences in Antarctic deglaciation histories. New seismic and geological evidence, however, indicates the Antarctic is underlain by complex, high amplitude variability in viscoelastic structure, including a low viscosity zone (LVZ) under West Antarctica. Hay et al. [2016] showed that sea-level fingerprints of modern melting calculated using such Earth models differ from those based on elastic or 1-D viscoelastic Earth models within decades of melting. Our investigation is motivated by two questions: (1) How does 3-D Earth structure, especially this LVZ, impact observations of GIA-induced crustal deformation associated with the last deglaciation? (2) How will 3-D Earth structure affect predictions of future sea-level rise in Antarctica? We compute the gravitationally self-consistent sea level, uplift, and gravity changes using the finite volume treatment of Latychev et al. [2005]. We consider four viscoelastic Earth models: a global 1-D model; a regional, West Antarctic-like 1-D model; a 3-D model where the lithospheric thickness varies laterally; and a 3-D model where both viscosity and lithospheric thickness vary laterally. For our Last Glacial Maximum to present investigations we employ ICE6g [Peltier et al., 2015]. For our present-future investigations we consider a melt scenario consistent with GRACE satellite gravity derived solutions [Harig et al., 2015]. Our calculations indicate that predictions of crustal deformations due to both GIA and ongoing melting are strongly influenced by 3-D lithospheric thickness and viscosity structure. Future sea level change due to ongoing melting is primarily influenced by 3-D viscosity structure. We show that 1-D Earth models built using regional inferences of viscosity and lithospheric thickness do not accurately capture the variability introduced by 3-D Earth structure.

Plagioclase-dominated Seismic Anisotropy in the Basin and Range Lower Crust

NASA Astrophysics Data System (ADS)

Bernard, R. E.; Behr, W. M.

2017-12-01

Observations of seismic anisotropy have the ability to provide important information on deformation and structures within the lithosphere. While the mechanisms controlling seismic anisotropy in the upper mantle are fairly well understood (i.e., olivine "lattice preferred orientation" or LPO), less is known about the minerals and structures controlling regional lower crustal anisotropy. We use lower crustal xenoliths from young cinder cones in the eastern Mojave/western Basin and Range to investigate mineral LPOs and their effect on seismic anisotropy. Lower crustal gabbros were collected from two areas roughly 80 km apart — the Cima and Deadman Lake Volcanic Fields. Lower crustal fabrics measured using EBSD are dominated by LPOs in plagioclase associated with both plastic deformation and magmatic flow. In all fabric types, plagioclase LPOs produce seismic fast axes oriented perpendicular to the foliation plane. This is in contrast to mantle peridotite xenoliths from the same locations, which preserve olivine LPOs with fast axes aligned parallel to the foliation plane. The orthogonal orientations of mantle and lower crustal fast axes relative to foliation implies that even where fabric development in both layers is coeval and kinematically compatible, their measured anisotropies can be perpendicular to each other, therefore appearing anti-correlated when measured seismically. Furthermore, our observation of plagioclase-dominated LPO and negligible concentrations of mica is at odds with the common assumption that lower crustal anisotropy is dominated by micaceous minerals, whose slow axes reliably align parallel to lineation or flow. In contrast, our data show that for plagioclase, fast axes align perpendicular to flow and the slow axes are variably aligned within the foliation plane. Therefore, for a crustal section dominated by plagioclase LPO with assumed horizontal foliation, there would be a vertical rather than a horizontal axis of symmetry, resulting in a lack of azimuthal anisotropy and minimal shear wave splitting for vertically propagating waves. Crustal seismic studies in this type of setting may only be able to identify crustal flow planes, but not flow directions. These findings may be generally applicable to regions of significant mafic volcanism and lower crustal magmatic underplating.

Geodynamic models for the post-orogenic exhumation of the lower crust

NASA Astrophysics Data System (ADS)

Bodur, O. F.; Gogus, O.; Karabulut, H.; Pysklywec, R. N.; Okay, A. I.

2015-12-01

Recent geodynamic modeling studies suggest that the exhumation of the high pressure and the very/ultra high-pressure crustal rocks may occur due to the slab detachment (break-off), slab roll-back (retreat) and the buoyancy-flow controlled subduction channel. We use convective removal (Rayleigh-Taylor, 'dripping' instability) mechanism to quantitatively investigate the burial and the exhumation pattern of the lower/middle crustal rocks from ocean subduction to post-collisional geodynamic configuration. In order to address the model evolution and track crustal particles for deciphering P-T-t variation, we conduct a series of thermo-mechanical numerical experiments with arbitrary Eularian-Lagrangian finite element code (SOPALE). We show how additional model parameters (e.g moho temperature, upper-middle crustal activation energy, density contrast between the lithosphere and the underlying mantle) can effectively influence the burial and exhumation depths, rate and the styles (e.g clockwise or counterclockwise). First series of experiments are designed to investigate the exhumation of crustal rocks at 32 km depth for only post-collisional tectonic setting -where pre-existing ocean subduction has not been implemented-. Model predictions show that a max. 8 km lower crustal burial occurs concurrent with the lower crustal convergence as a response to the mantle lithosphere dripping instability. The subsequent exhumation of these rocks up to -25 km- is predicted at an exhumation rate of 1.24 cm/year controlled by the removal of the underlying mantle lithosphere instability with crustal extension. At the second series of experiments, we tracked the burial and exhumation history of crustal particles at 22 and 31 km depths while pre-existing ocean subduction has been included before the continental collision. Model results show that burial depths down to 62 km occurs and nearly the 32 km of exhumation is predicted again by the removal of the mantle lithosphere after the dripping instability but the crustal rocks are buried deeper because of the downward forcing of the sinking ocean plate. We suggest that the first set of model results are comparable to the peak pressure calculations from the high pressure rocks of the Afyon Zone in western Turkey with a significant offset (175°C) in temperature values.

landforms evolution in collisional-dominated settings: the case of Northern Sicily (Central Mediterranean)

NASA Astrophysics Data System (ADS)

Nigro, Fabrizio; Renda, Pietro; Favara, Rocco

2010-05-01

In the young mountain chains underwent to emersion, the different crustal blocks which compose the belt may be subjected to differentiate tilting during uplift. The tilting process may be revealed both by the stratal pattern of the syn-uplifting deposits or deduced by the function altitude/area ratio. The prevailing of the uplift rate with respect to the tilting rate (and vice versa) result from the shape of this function. So, in young mountains the hypsometric analysis may results a useful tool for decipher how the crustal blocks are underwent to uplift. An integrate analysis based on stratigraphy, structural and morphometric data represents the correctly approach for characterise the landform evolution in regions underwent to active tectonics. In the aim to evaluate the recent tectonic history from topography in regions underwent to active deformations, by deducing the effect of tectonisms on landforms, the definition of the boundary conditions (regarding the crustal deformation) is fundamental for morphometric analysis. In fact, the morphologic style and the morphometric pattern in tectonically active settings are closely related to the dominance of rock masses exceeding for uplift (or failure for subsidence) with respect to the exogenous erosional processes. Collisional geodynamic processes induce crustal growth for faulting and folding. In this earth's sectors, the uplift of crustal blocks is a very common effect of compressional deformation. It reflects for example fold amplification and thrusting, but it is a very common process also in settings dominated by crustal thinning, where the viscoelastic properties of the lithosphere induce tilting and localised uplift of normal-faulted crustal blocks. The uplift rate is rarely uniform for wide areas within the orogens on the passive margins, but it changes from adjacent crustal blocks as the effect of space-variation of kinematics conditions or density. It also may change within a single block, as the effect of tilting, which induces synchronously mass elevation and subsidence. Not considering sea-level fluctuations and the climatic-lithologic parameters, the 2D distribution of uplift rate influences the landmass evolution in time. The tendency of rock masses to equilibrium resulting from concurrent tectonic building and denudation forces defines the geomorphic cycle. This evolution is checked by different stages, each characterised by a well-recognisable morphometric patterns. The dominance of uplift or erosion and concurrent block tilting induce characteristic a landform evolution tendency, which may be evaluated with the morphometric analysis. A lot of morphometric functions describe the equilibrium stage of landmasses, providing useful tools for deciphering how tectonics acts in typology (e.g. inducing uplift uniformly or with crustal block tilting) and resulting effects on landforms (magnitude of uplift rate vs tilting rate). We aim to contribute in the description of landforms evolution in Sicily (Central Mediterranean) under different morphoevolutive settings, where may prevails uplift, tilting or erosion, each characterised by different morphometric trends. The present-day elevation of Pliocene to upper Pleistocene deposits suggests that Northen Sicily underwent neotectonic uplift. The recent non-uniform uplift of Northern Sicily coastal sector is suggested by the different elevation of the Pliocene-Upper Pleistocene marine deposits. The maximum uplift rate characterise the NE Sicily and the minimum the NW Sicily. The overall westwards decreasing trend of uplift is in places broken in the sectors where are located a lot of morphostructures. Localised uplift rates higher than the adjacent coastal plains are suggested by the present-day elevation of the beachshore deposits of Tyrrhenian age. Northern Sicily may be divided into a lot of crustal blocks, underwent to different tilting and uplift rates. Accentuate tilting and uplift results from transtensional active faulting of the already emplaced chain units, as also suggested by seismicity and the focal plane solutions of recent strong earthquakes.

Crustal shear velocity structure in the Southern Lau Basin constrained by seafloor compliance

NASA Astrophysics Data System (ADS)

Zha, Yang; Webb, Spahr C.

2016-05-01

Seafloor morphology and crustal structure vary significantly in the Lau back-arc basin, which contains regions of island arc formation, rifting, and seafloor spreading. We analyze seafloor compliance: deformation under long period ocean wave forcing, at 30 ocean bottom seismometers to constrain crustal shear wave velocity structure along and across the Eastern Lau Spreading Center (ELSC). Velocity models obtained through Monte Carlo inversion of compliance data show systematic variation of crustal structure in the basin. Sediment thicknesses range from zero thickness at the ridge axis to 1400 m near the volcanic arc. Sediment thickness increases faster to the east than to the west of the ELSC, suggesting a more abundant source of sediment near the active arc volcanoes. Along the ELSC, upper crustal velocities increase from the south to the north where the ridge has migrated farther away from the volcanic arc front. Along the axial ELSC, compliance analysis did not detect a crustal low-velocity body, indicating less melt in the ELSC crustal accretion zone compared to the fast spreading East Pacific Rise. Average upper crust shear velocities for the older ELSC crust produced when the ridge was near the volcanic arc are 0.5-0.8 km/s slower than crust produced at the present-day northern ELSC, consistent with a more porous extrusive layer. Crust in the western Lau Basin, which although thought to have been produced through extension and rifting of old arc crust, is found to have upper crustal velocities similar to older oceanic crust produced at the ELSC.

From crustal thinning to mantle exhumation: what the Pyrenean breccia formations tell us.

NASA Astrophysics Data System (ADS)

Clerc, C.; Chauvet, A.; Lagabrielle, Y.; Reynaud, J.-Y.; Boulvais, P.; Bousquet, R.; Lahfid, A.; Vauchez, A.; Mahé, S.

2012-04-01

Several formations with various breccia types occur in Mesozoic basins disseminated along the North Pyrenean fault, on the northern flank of the French Pyrenees. Due to their location along the Iberia-Europa plate boundary, the North Pyrenean breccia formations represent complex archives documenting the tectonic and sedimentary evolution of the Pyrenean realm during the Aptian-Albian period. In particular, the North Pyrenean breccia formations have recorded the main stages of crustal thinning, continental break-up and mantle exhumation, which occurred along the North Pyrenean Zone (NPZ). We will review the main sedimentary, structural, metamorphic and geochemical characters of these breccias, based on new field investigations conducted in both the Western and Eastern Pyrenées (Agly, Aulus, Moncaup-St Béas and Urdach localities). Based on our new founding, we re-intrepret the significance of the breccia formations in the light of the most recent models developed for the pre-orogenic evolution of the Pyrenees. In several places and mostly close to the contact between Paleozoic basement and Mesozoic cover, we systematically recognized the following three types of breccias: i) Semi-ductile syn-metamorphic breccias resulting from the boudinage of silicic or dolomitic beddings in ductily deformed marbles. ii) Cataclastic breccias disturbing the neighbouring host rocks and displaying a relatively monogenetic character. These tectonic breccias result from the disruption of the Mesozoic metamorphic platform under cooling conditions. They are dominated by cataclastic levels mainly located in the Triassic and Liassic weaker levels, iii) Polymictic sedimentary breccias, which composition is dominated by clasts of Mesozoic metasediments. Locally, close to subcontinental mantle bodies, the sedimentary breccias include numerous clasts of ultramafic and/or crustal basement rocks. Such breccias are the witness of the disruption of the sedimentary cover of the North Pyrenean Zone massifs followed by clastic sedimentation in a context of hyper-extended crust and mantle exhumation. Improving the knowledge of the formation of the different types of breccia exposed all along the Northern Pyrenees brings important hints to decipher the tectonic history responsible for the formation of the metamorphic basins and the exhumation (and reworking) of deep crustal and mantle rocks in the NPZ.

Evidence for Crustal-Scale Imbrication and non-Equilibrium Topography in the Southern Central Range, Taiwan

NASA Astrophysics Data System (ADS)

Byrne, T. B.; Huang, C.; Ouimet, W. B.; Rau, R.; Hsieh, M.; Lee, Y.

2011-12-01

We integrate a suite of new and recently re-interpreted profiles of the 3-D crustal velocity structure from the southern Central Range of Taiwan with geomorphic data from the range and propose that the topography is supported by a crustal-scale, west-verging thrust. The extent and geometry of the thrust is indicated by contours of P-wave velocity that are progressively overturned from south to north, placing high Vp rocks above low Vp rocks. The interpreted thrust dips gently east (15-20 degrees) and carries pre-Tertiary metamorphic rocks and Eocene to Miocene rocks with a well-developed slaty cleavage in its hanging wall. The thrust is interpreted to cut up section to the west and link with the basal detachment of the fold-and-thrust belt. Leveling data1 along the South Cross-Island Highway also suggest that the thrust is active. Along-strike profiles suggest that the thrust is propagating southward, consistent with a progressive decrease in mean elevation and an increase in reset apatite fission track ages from north to south. The hanging wall of the propagating thrust also correlates with anomalous areas of low topographic relief that straddle the crest of the southern part of the range. The areas of low relief are fringed by stream channels with relatively high stream gradient indexes and do not appear related to weaker rock types, glacial erosion, or lower rock uplift rates along the range crest. We propose that the surfaces represent relict topography that formed prior to a recent acceleration in rock uplift rate, consistent with the presence of a propagating, crustal-scale thrust in the subsurface. Taken together, these results raise questions about the notion of steady state topography and critically tapered wedges in Taiwan. 1) Ching, Kuo-En, Hsieh, M.-L., Johnson, K. M., Chen, K-H., Rau, R.-J., Yang M., Modern vertical deformation rates and mountain building in Taiwan from precise leveling and continuous GPS observations, 2000-2008, in press, JGR.

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.

Thickening the outer margins of the Tibetan Plateau: The role of crustal shortening

NASA Astrophysics Data System (ADS)

Lease, R. O.; Burbank, D. W.

2012-12-01

One of the most direct consequences of the collision of two buoyant continents is large-scale crustal thickening that results in the upward and outward growth of high terrain. As the stronger Indian continent has collided with weaker Asia over at least the past 50 Myr, widespread crustal thickening has occurred over an area that is approximately 2.5 million km^2 at present. The resultant Tibetan crust is the thickest observed on Earth today with an average thickness of 65 km and a maximum that may reach 90 km in places. The mechanisms by which Tibetan crust has thickened, however, as well as the timing and distribution of these mechanisms across the plateau, remain debatable. Two of the most popular mechanisms for thickening the crust beneath the margins of the Tibetan Plateau are: 1) pure shear with faulting and folding in the upper crust and horizontal shortening below; and 2) flow and inflation of lower or middle crust without significant shortening of the upper crust. To help discriminate between the relative contributions of these two mechanisms, well-constrained estimates of upper crustal shortening are needed. Here we document the Cenozoic shortening budget across the northeastern Tibetan Plateau margin near 36°N 102.5°E with several 100- to 145-km-long balanced cross sections. Thermochronological and magnetostratigraphic data indicate that modest NNE-SSW shortening began in middle Eocene time, shortly after initial India-Asia collision. Accelerated east-west shortening, however, did not commence until ~35 Myr later. A five-fold acceleration in shortening rates in middle Miocene-to-Recent time accounts for more than half of the total Cenozoic crustal shortening and thickening in this region. Overall, the balanced cross sections indicate 11 ± 2 % east-west shortening since middle Miocene time, and ~9 ± 2 % NNE-SSW shortening between middle Eocene and middle Miocene times. Given the present-day crustal thickness of 56 ± 4 km in northeastern Tibet, crustal restorations that remove Cenozoic shortening suggest that the northeastern Tibetan crust was 45 ± 5 km thick prior to India-Asia continental collision. This pre-collision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the Tibetan plateau (44 ± 4 km) and globally (41 ± 6 km) and suggests that pure shear alone may account for Cenozoic crustal thickening in northeastern Tibet, obviating the need for lower crustal flow. Furthermore, a growing number of balanced cross sections across the margins of the Tibetan Plateau document Cenozoic shortening sufficient to generate modern crustal thicknesses: in northern Tibet [Yin et al., 2007; 2008a; 2008b], eastern Tibet [Hubbard et al., 2009; 2010], and northeastern Tibet [this work]. Collectively, these similar findings suggest that lower crustal flow is either unnecessary to account for Cenozoic crustal thickening beneath the outer margins of the Tibetan Plateau or, alternatively, has a more restricted role than originally proposed.

Microbial Life in Ridge Flank Crustal Fluids at Baby Bare Seamount, Juan de Fuca Ridge

NASA Astrophysics Data System (ADS)

Huber, J. A.; Johnson, H. P.; Butterfield, D. A.; Baross, J. A.

2005-12-01

To determine the microbial community diversity within old oceanic crust, a novel sampling strategy was used to collect crustal fluids at Baby Bare Seamount, a 3.5 Ma old outcrop located in the northeast Pacific Ocean on the eastern flank of the Juan de Fuca Ridge. Stainless steel probes were driven directly into the igneous ocean crust to obtain samples of ridge flank crustal fluids. Genetic signatures and enrichment cultures of microorganisms demonstrate that these crustal fluids host a microbial community composed of species indigenous to the subseafloor, including anaerobic thermophiles, and species from other deep-sea habitats, such as seawater and sediments. Evidence using molecular techniques indicates the presence of a relatively small but active microbial population, dominated by bacteria. The microbial community diversity found in the crustal fluids may indicate habitat variability in old oceanic crust, with inputs of nutrients from seawater, sediment pore-water fluids and possibly hydrothermal sources. This report further supports the presence of an indigenous microbial community in ridge flank crustal fluids and advances our understanding of the potential physiological and phylogenetic diversity of this community.

The crustal structures from Wuyi-Yunkai orogen to Taiwan orogen: the onshore-offshore wide-angle seismic experiment of TAIGER and ATSEE projects

NASA Astrophysics Data System (ADS)

Kuochen, H.; Kuo, N. Y. W.; Wang, C. Y.; Jin, X.; Cai, H. T.; Lin, J. Y.; Wu, F. T.; Yen, H. Y.; Huang, B. S.; Liang, W. T.; Okaya, D. A.; Brown, L. D.

2015-12-01

The crustal structure is key information for understanding the tectonic framework and geological evolution in the southeastern China and its adjacent area. In this study, we integrated the data sets from the TAIGER and ATSEE projects to resolve onshore-offshore deep crustal seismic profiles from the Wuyi-Yunkai orogen to the Taiwan orogen in southeastern China. Totally, there are three seismic profiles resolved and the longest profile is 850 km. Unlike 2D and 3D first arrival travel-time tomography from previous studies, we used both refracted and reflected phases (Pg, Pn, PcP, and PmP) to model the crustal structures and the crustal reflectors. 40 shots, 2 earthquakes, and about 1,950 stations were used and 15,319 arrivals were picked among three transects. As a result, the complex crustal evolution since Paleozoic era are shown, which involved the closed Paleozoic rifted basin in central Fujian, the Cenozoic extension due to South China sea opening beneath the coastline of southern Fujian, and the on-going collision of the Taiwan orogen.

Spatial relationships between crustal structures and mantle seismicity in the Vrancea Seismogenic Zone of Romania: Implications for geodynamic evolution

NASA Astrophysics Data System (ADS)

Enciu, Dana-Mihaela

Integration of active and passive-source seismic data is employed to study the relationships between crustal structures and seismicity in the SE Carpathian foreland of Romania, and the connection with the Vrancea Seismogenic Zone. Relocated crustal epicenters and focal mechanisms are correlated with industry seismic profiles Comanesti, Ramnicu Sarat, Braila and Buzau, the reprocessed DACIA PLAN profile and the DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) II and III profiles in order to understand the link between neo-tectonic foreland deformation and Vrancea mantle seismicity. Projection of crustal foreland hypocenters onto deep seismic profiles identified active crustal faults suggesting a mechanical coupling between sedimentary, crustal and upper mantle structures on the Trotus, Sinaia and newly observed Ialomita Faults. Seismic reflection imaging revealed the absence of west dipping reflectors in the crust and an east dipping to horizontal Moho in the proximity of the Vrancea area. These findings argue against both 'subduction-in-place' and 'slab break-off' as viable mechanisms for generating Vrancea mantle seismicity.

Continental crustal formation and recycling: Evidence from oceanic basalts

NASA Technical Reports Server (NTRS)

Saunders, A. D.; Tarney, J.; Norry, M. J.

1988-01-01

Despite the wealth of geochemical data for subduction-related magma types, and the clear importance of such magmas in the creation of continental crust, there is still no concensus about the relative magnitudes of crustal creation versus crustal destruction (i.e., recycling of crust into the mantle). The role of subducted sediment in the formation of the arc magmas is now well documented; but what proportion of sediment is taken into the deeper mantle? Integrated isotopic and trace element studies of magmas erupted far from presently active subduction zones, in particular basaltic rocks erupted in the ocean basins, are providing important information about the role of crustal recycling. By identifying potential chemical tracers, it is impossible to monitor the effects of crustal recycling, and produce models predicting the mass of material recycled into the mantle throughout long periods of geological time.

Estimating gravity changes caused by crustal strain: application to the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Yin, Zhi; Xu, Caijun

2017-08-01

Temporal gravimetry is an efficient tool for monitoring mass transfers, but distinguishing the contribution of each process to the measured signals is challenging. Few effective methods have been developed to estimate the changes in gravity caused by crustal strain for large-scale geophysical problems. To fill this research gap, we proposed a formula that describes a negative linear correlation between changes in gravity and crustal dilatational strain. Surface observations of gravity changes and dilatational strains were simulated using PSGRN/PSCMP, which is a numerical code used to calculate the surface response to fault dislocations, and the accuracy of the formula was quantitatively verified. Four parameters are required for this formula: the crustal dilatational strain, the crustal density, the Moho depth, and a coefficient that characterizes the degree of crust-mantle coupling. To illustrate the application of this new method to a natural case study, including specifying the values of the necessary parameters, the crustal strain-caused gravity changes (CSGCs) were calculated at 1° × 1° grid nodes over the Tibetan Plateau (TP). The CSGC model shows that most of the crust of the TP is undergoing extension, which generates negative gravity signals. The magnitude of the Tibetan CSGC model is approximately 0.2 μGal yr-1, which is similar to the results obtained from numerical modelling of the crustal tectonics of the Taiwanese Orogen. To evaluate the reliability of the Tibetan CSGC model, the uncertainties in the crustal dilatational strain, crustal density, Moho depth, and crust-mantle coupling factor were evaluated and then used to estimate the CSGC uncertainty by applying the error propagation law. The CSGC model was used to analyse the mass transfers of the TP. The results suggest that a significant mass accumulation process may be occurring beneath the crust of the northern TP.

Ductile crustal flow in Europe's lithosphere

NASA Astrophysics Data System (ADS)

Tesauro, Magdala; Burov, Evgene B.; Kaban, Mikhail K.; Cloetingh, Sierd A. P. L.

2011-12-01

Potential gravity theory (PGT) predicts the presence of significant gravity-induced horizontal stresses in the lithosphere associated with lateral variations in plate thickness and composition. New high resolution crustal thickness and density data provided by the EuCRUST-07 model are used to compute the associated lateral pressure gradients (LPG), which can drive horizontal ductile flow in the crust. Incorporation of these data in channel flow models allows us to use potential gravity theory to assess horizontal mass transfer and stress transmission within the European crust. We explore implications of the channel flow concept for a possible range of crustal strength, using end-member 'hard' and 'soft' crustal rheologies to estimate strain rates at the bottom of the ductile crustal layers. The models show that the effects of channel flow superimposed on the direct effects of plate tectonic forces might result in additional significant horizontal and vertical movements associated with zones of compression or extension. To investigate relationships between crustal and mantle lithospheric movements, we compare these results with the observed directions of mantle lithospheric anisotropy and GPS velocity vectors. We identify areas whose evolution could have been significantly affected by gravity-driven ductile crustal flow. Large values of the LPG are predicted perpendicular to the axes of European mountain belts, such as the Alps, Pyrenees-Cantabrian Mountains, Dinarides-Hellenic arc and Carpathians. In general, the crustal flow is directed away from orogens towards adjacent weaker areas. Gravitational forces directed from areas of high gravitational potential energy to subsiding basin areas can strongly reduce lithospheric extension in the latter, leading to a gradual late stage inversion of the entire system. Predicted pressure and strain rate gradients suggest that gravity driven flow may play an essential role in European intraplate tectonics. In particular, in a number of regions the predicted strain rates are comparable to tectonically induced strain rates. These results are also important for quantifying the thickness of the low viscosity zones in the lowermost part of the crustal layers.

Satellite measurements of the earth's crustal magnetic field

NASA Technical Reports Server (NTRS)

Schnetzler, C. C.

1989-01-01

The literature associated with the Magsat mission has evaluated the capabilities and limitations of satellite measurements of the earth's crustal magnetic field, and demonstrated that there exists a 300-3000 km magnetic field, related to major features in the earth's crust, which is primarily caused by induction. Due to its scale and sensitivity, satellite data have been useful in the development of models for such large crustal features as subduction zones, submarine platforms, continental accretion boundaries, and rifts. Attention is presently given to the lack of agreement between laboratory and satellite estimates of lower crustal magnetization.

The forgotten component of sub-glacial heat flow: Upper crustal heat production and resultant total heat flux on the Antarctic Peninsula

NASA Astrophysics Data System (ADS)

Burton-Johnson, Alex; Halpin, Jacqueline; Whittaker, Joanne; Watson, Sally

2017-04-01

Seismic and magnetic geophysical methods have both been employed to produce estimates of heat flux beneath the Antarctic ice sheet. However, both methods use a homogeneous upper crustal model despite the variable concentration of heat producing elements within its composite lithologies. Using geological and geochemical datasets from the Antarctic Peninsula we have developed a new methodology for incorporating upper crustal heat production in heat flux models and have shown the greater variability this introduces in to estimates of crustal heat flux, with implications for glaciological modelling.

Evidence for Thin Oceanic Crust on the Extinct Aegir Ridge, Norwegian Basin, N.E. Atlantic Derived from Satellite Gravity Inversion

NASA Astrophysics Data System (ADS)

Greenhalgh, E. E.; Kusznir, N. J.

2006-12-01

Satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the N.E. Atlantic. The inversion of gravity data to determine crustal thickness incorporates a lithosphere thermal gravity anomaly correction for both oceanic and continental margin lithosphere. Predicted crustal thicknesses in the Norwegian Basin are between 7 and 4 km on the extinct Aegir oceanic ridge which ceased sea-floor spreading in the Oligocene. Crustal thickness estimates do not include a correction for sediment thickness and are upper bounds. Crustal thicknesses determined by gravity inversion for the Aegir Ridge are consistent with recent estimates derived using refraction seismology by Breivik et al. (2006). Failure to incorporate a lithosphere thermal gravity anomaly correction produces an over-estimate of crustal thickness. Oceanic crustal thicknesses within the Norwegian Basin are predicted by the gravity inversion to increase to 9-10 km eastwards towards the Norwegian (Moere) and westwards towards the Jan Mayen micro-continent, consistent with volcanic margin continental breakup at the end of the Palaeocene. The observation (from gravity inversion and seismic refraction studies) of thin oceanic crust produced by the Aegir ocean ridge in the Oligocene has implications for the temporal evolution of asthenosphere temperature under the N.E. Atlantic during the Tertiary. Thin Oligocene oceanic crust may imply cool (normal) asthenosphere temperatures during the Oligocene in contrast to elevated asthenosphere temperatures in the Palaeocene and Miocene-Recent as indicated by volcanic margin formation and the formation of Iceland respectively. Gravity inversion also predicts a region of thin oceanic crust to the west of the northern part of the Jan Mayen micro-continent and to the east of the thicker oceanic crust currently being formed at the Kolbeinsey Ridge. Thicker crust (c.f. ocean basins) is predicted for the Jan Mayen micro- continent south of Jan Mayen Island, with crust of the order of 20 km thickness extending southwards to connect with both the Faroes-Iceland Ridge and N.E. Iceland. Predicted crustal thicknesses under the Faroes- Iceland Ridge are approximately 25 km. The lithosphere thermal model used to predict the lithosphere thermal gravity anomaly correction may be conditioned using magnetic isochron data to provide the age of oceanic lithosphere. The resulting crustal thickness determination and the location of ocean-continent transition (OCT) are however sensitive to errors in the magnetic isochron data. An alternative method of inverting satellite gravity to give crustal thickness, incorporating a lithosphere thermal correction, has been used which does not use magnetic isochron data and provides an independent prediction of crustal thickness and OCT location. The crustal thickness estimates and OCT locations detailed above are robust to these sensitivity tests.

The Crustal Structure of the North-South Earthquake Belt in China Revealed from Deep Seismic Soundings and Gravity Data

NASA Astrophysics Data System (ADS)

Zhao, Yang; Guo, Lianghui; Shi, Lei; Li, Yonghua

2018-01-01

The North-South earthquake belt (NSEB) is one of the major earthquake regions in China. The studies of crustal structure play a great role in understanding tectonic evolution and in evaluating earthquake hazards in this region. However, some fundamental crustal parameters, especially crustal interface structure, are not clear in this region. In this paper, we reconstructed the crustal interface structure around the NSEB based on both the deep seismic sounding (DSS) data and the gravity data. We firstly reconstructed the crustal structure of crystalline basement (interface G), interface between upper and lower crusts (interface C) and Moho in the study area by compiling the results of 38 DSS profiles published previously. Then, we forwardly calculated the gravity anomalies caused by the interfaces G and C, and then subtracted them from the complete Bouguer gravity anomalies, yielding the regional gravity anomalies mainly due to the Moho interface. We then utilized a lateral-variable density interface inversion technique with constraints of the DSS data to invert the regional anomalies for the Moho depth model in the study area. The reliability of our Moho depth model was evaluated by comparing with other Moho depth models derived from other gravity inversion technique and receiver function analysis. Based on our Moho depth model, we mapped the crustal apparent density distribution in the study area for better understanding the geodynamics around the NSEB.

Identification of Deep Earthquakes

DTIC Science & Technology

2010-09-01

discriminants that will reliably separate small, crustal earthquakes (magnitudes less than about 4 and depths less than about 40 to 50 km) from small...characteristics on discrimination plots designed to separate nuclear explosions from crustal earthquakes. Thus, reliably flagging these small, deep events is...Further, reliably identifying subcrustal earthquakes will allow us to eliminate deep events (previously misidentified as crustal earthquakes) from

Reconstruction of the pre-breakup crustal thickness in Australia/Antarctica

NASA Astrophysics Data System (ADS)

Goncharov, A.

2003-04-01

Some 140 million years ago, Australia and Antarctica were parts of a single continent Gondwana. Before it broke into parts there was a process of extensive crustal extension. Thinning of the crust during this process was accompanied by deposition of vast amounts of sedimentary rocks along Australia’s Southern Margin, where the total sediment thickness locally (e.g., Ceduna Sub-basin) reaches 15 km. These sedimentary rocks may have been involved in oil and gas formation. Knowledge of the pre-breakup crustal thickness in Australia/Antarctica is important because it provides additional constraints for plate tectonic reconstructions of the two continents and ultimately leads to a more accurate assessment of the petroleum potential of Australia’s Southern Margin. Most reliable estimates of crustal thickness come from refraction seismic measurements which define the depth to the Moho boundary, where seismic velocity increases to 8 km/s or more. Such measurements were used in this research for Australia. Unlike Australia, Antarctica has poor coverage of seismic measurements of crustal thickness. For Antarctica, seismic measurements were supplemented by values predicted by the regression between seismically defined crustal thickness and upwardly continued gravity. Upward continuation emphasizes the effects of variations in crustal thickness in the total gravity signal. After compilation and computation of crustal thickness was completed, data points located on Australian continent were reconstructed to their pre-breakup position. The most up-to-date finite rotation parameters defining the movement of Australia relative to Antarctica were used in this process. To ensure that pre-breakup extension and thinning of the crust (during the 140 to 95 Ma time interval) were accounted for, points with crustal thickness values less than 30 km on both Australian and Antarctic margins were excluded from subsequent gridding. Crust thinner than 30 km was taken to have been affected by pre-breakup extension. The resultant reconstructed pre-extensional crustal thickness may have existed in this part of Gondwana prior to the pre-breakup extension, assuming that geological processes on both continents (excluding margins) have not affected it significantly since then. Crustal thickness along the zone of subsequent Australia/Antarctica separation is clearly reduced and its width varies substantially. Thin crust is generally weaker than thick crust, so it is not surprising that the continents broke apart along this zone. A distinct zone of thick crust, which spans across Australia/Antarctica from the Eastern Highlands in Australia to the Transantarctic Mountains, is another obvious feature on the map of pre-extensional crustal thickness. This may explain why the break-up of the continents between Tasmania and Northern Victoria Land occurred as the last stage of the separation process. Thick crust in this region essentially served as a lock: only after this lock was broken did final separation occur. Clearly, thickest sediment has accumulated where the width of the zone of pre-extensional thin crust was minimal in the Ceduna Sub-basin. This may be due to the higher rate of subsidence in the zone with the steepest slope on the Moho. Rheology of the crust and sediment supply were also among the contributing factors; relative contributions of these factors will be studied in more detail in the future. Sedimentation in the Otway, Sorell, Bass and Gippsland basins to the north and west of Tasmania, unlike other basins on the Southern Margin, commenced in a thick crust environment: all four are located within the Eastern Highlands - Transantarctic Mountains zone. Although, crustal thickness immediately underneath the basins is not much different from the western part of the Margin, clearly there are two prominent (up to 45 km) Moho lows to the north and south of them. Onset of pre-breakup crustal extension within this zone was probably different from the western part of the Southern Margin: thicker crust is harder to break. Also, thicker crust generally means higher heat flow. These differences may have affected both the style of crustal extension and hydrocarbon maturation in deposited sediments. Non-uniform pre-extensional crustal thickness along Australian Southern and conjugate Antarctic margins, as well as implied differences in heat flow distribution, must be taken into consideration in modelling crustal extension and the formation of sedimentary basins.

Proceedings of the Geodesy/Solid Earth and Ocean Physics (GEOP) Research Conferences

NASA Technical Reports Server (NTRS)

Mueller, I. I. (Editor)

1975-01-01

Papers are presented dealing with interdisciplinary research in the fields of geodesy, solid earth and ocean physics. Topics discussed include: solid earth and ocean tides; the rotation of the earth and polar motion; vertical crustal motions; the geoid and ocean surface; earthquake mechanism; sea level changes; and lunar dynamics.

A deep structural ridge beneath central India

NASA Astrophysics Data System (ADS)

Agrawal, P. K.; Thakur, N. K.; Negi, J. G.

A joint-inversion of magnetic satellite (MAGSAT) and free air gravity data has been conducted to quantitatively investigate the cause for Bouguer gravity anomaly over Central Indian plateaus and possible fold consequences beside Himalayan zone in the Indian sub-continent due to collision between Indian and Eurasian plates. The appropriate inversion with 40 km crustal depth model has delineated after discriminating high density and magnetisation models, for the first time, about 1500 km long hidden ridge structure trending NW-SE. The structure is parallel to Himalayan fold axis and the Indian Ocean ridge in the Arabian Sea. A quantitative relief model across a representative anomaly profile confirms the ridge structure with its highest point nearly 6 km higher than the surrounding crustal level in peninsular India. The ridge structure finds visible support from the astro-geoidal contours.

Miocene rapakivi granites in the southern Death Valley region, California, USA

USGS Publications Warehouse

Calzia, J.P.; Ramo, O.T.

2005-01-01

Rapakivi granites in the southern Death Valley region, California, include the 12.4-Ma granite of Kingston Peak, the ca. 10.6-Ma Little Chief stock, and the 9.8-Ma Shoshone pluton. All of these granitic rocks are texturally zoned from a porphyritic rim facies, characterized by rapakivi textures and miarolitic cavities, to an equigranular aplite core. These granites crystallized from anhydrous and peraluminous to metaluminous magmas that were more oxidized and less alkalic than type rapakivi granites from southern Finland. Chemical and isotope (Nd-Sr-Pb) data suggest that rapakivi granites of the southern Death Valley region were derived by partial melting of lower crustal rocks (possibly including Mesozoic plutonic component) with some mantle input as well; they were emplaced at shallow crustal levels (4 km) in an actively extending orogen.

Miocene rapakivi granites in the southern Death Valley region, California, USA

USGS Publications Warehouse

Calzia, James P.; Ramo, O.T.

2005-01-01

Rapakivi granites in the southern Death Valley region, California, include the 12.4-Ma granite of Kingston Peak, the ca. 10.6-Ma Little Chief stock, and the 9.8-Ma Shoshone pluton. All of these granitic rocks are texturally zoned from a porphyritic rim facies, characterized by rapakivi textures and miarolitic cavities, to an equigranular aplite core. These granites crystallized from anhydrous and peraluminous to metaluminous magmas that were more oxidized and less alkalic than type rapakivi granites from southern Finland. Chemical and isotope (Nd–Sr–Pb) data suggest that rapakivi granites of the southern Death Valley region were derived by partial melting of lower crustal rocks (possibly including Mesozoic plutonic component) with some mantle input as well; they were emplaced at shallow crustal levels (4 km) in an actively extending orogen.

High-resolution receiver function imaging reveals Colorado Plateau lithospheric architecture and mantle-supported topography

USGS Publications Warehouse

Domingo, Dorothy L.; R. Aster,; S. Grand,; J Ni,; W.S. Baldridge,; David C. Wilson USGS,

2010-01-01

After maintaining elevations near sea level for over 500 million years, the Colorado Plateau (CP) has a present average elevation of 2 km. We compute new receiver function images from the first dense seismic transect to cross the plateau that reveal a central CP crustal thickness of 42–50 km thinning to 30–35 km at the CP margins. Isostatic calculations show that only approximately 20% of central CP elevations can be explained by thickened crust alone, with the CP edges requiring nearly total mantle compensation. We calculate an uplift budget showing that CP buoyancy arises from a combination of crustal thickening, heating and alteration of the lithospheric root, dynamic support from mantle upwelling, and significant buoyant edge effects produced by small-scale convecting asthenosphere at its margins.

Tectonic control of erosion in the southern Central Andes

NASA Astrophysics Data System (ADS)

Val, Pedro; Venerdini, Agostina L.; Ouimet, William; Alvarado, Patricia; Hoke, Gregory D.

2018-01-01

Landscape evolution modeling and global compilations of exhumation data indicate that a wetter climate, mainly through orographic rainfall, can govern the spatial distribution of erosion rates and crustal strain across an orogenic wedge. However, detecting this link is not straightforward since these relationships can be modulated by tectonic forcing and/or obscured by heavy-tailed frequencies of catchment discharge. This study combines new and published along-strike average rates of catchment erosion constrained by 10Be and river-gauge data in the Central Andes between 28°S and 36°S. These data reveal a nearly identical latitudinal pattern in erosion rates on both sides of the range, reaching a maximum of 0.27 mm/a near 34°S. Collectively, data on topographic and fluvial relief, variability of rainfall and discharge, and crustal seismicity suggest that the along-strike pattern of erosion rates in the southern Central Andes is largely independent of climate, but closely relates to the N-S distribution of shallow crustal seismicity and diachronous surface uplift. The consistently high erosion rates on either side of the orogen near 34°S imply that climate plays a secondary role in the mass flux through an orogenic wedge where the perturbation to base level is similar on both sides.

Evidence of Dynamic Crustal Deformation in Tohoku, Japan, From Time-Varying Receiver Functions

NASA Astrophysics Data System (ADS)

Porritt, R. W.; Yoshioka, S.

2017-10-01

Temporal variation of crustal structure is key to our understanding of Earth processes on human timescales. Often, we expect that the most significant structural variations are caused by strong ground shaking associated with large earthquakes, and recent studies seem to confirm this. Here we test the possibility of using P receiver functions (PRF) to isolate structural variations over time. Synthetic receiver function tests indicate that structural variation could produce PRF changes on the same order of magnitude as random noise or contamination by local earthquakes. Nonetheless, we find significant variability in observed receiver functions over time at several stations located in northeastern Honshu. Immediately following the Tohoku-oki earthquake, we observe high PRF variation clustering spatially, especially in two regions near the beginning and end of the rupture plane. Due to the depth sensitivity of PRF and the timescales over which this variability is observed, we infer this effect is primarily due to fluid migration in volcanic regions and shear stress/strength reorganization. While the noise levels in PRF are high for this type of analysis, by sampling small data sets, the computational cost is lower than other methods, such as ambient noise, thereby making PRF a useful tool for estimating temporal variations in crustal structure.

Continuous Spectrum of Crustal Structures and Spreading Processes from Volcanic Rifted Margins to Mid-Ocean Ridges

NASA Astrophysics Data System (ADS)

Karson, J. A.

2016-12-01

Structures generated by seafloor spreading in oceanic crust (and ophiolites) and thick oceanic crust of Iceland show a continuous spectrum of features that formed by similar mechanisms but at different scales. A high magma budget near the Iceland hotspot generates thick (40-25 km) mafic crust in a plate boundary zone about 50 km wide. The upper crust ( 10 km thick) is constructed by the subaxial subsidence and thickening of lavas fed by dense dike swarms over a hot, weak lower crust to produce structures analogous to seaward-dipping reflectors of volcanic rifted margins. Segmented rift zones propagate away from the hotspot creating migrating transform fault zones, microplate-like crustal blocks and rift-parallel strike-slip faults. These structures are decoupled from the underlying lower crustal gabbroic rocks that thin by along-axis flow that reduces the overall crustal thickness and smooths-out local crustal thickness variations. Spreading on mid-ocean ridges with high magma budgets have much thinner crust (10-5 km) generated at a much narrower (few km) plate boundary zone. Subaxial subsidence accommodates the thickening of the upper crust of inward-dipping lavas and outward-dipping dikes about 1-2 km thick over a hot weak lower crust. Along-axis (high-temperature ductile and magmatic) flow of lower crustal material may help account for the relatively uniform seismic thickness of oceanic crust worldwide. Spreading along even slow-spreading mid-ocean ridges near hotspots (e.g., the Reykjanes Ridge) probably have similar features that are transitional between these extremes. In all of these settings, upper crustal and lower crustal structures are decoupled near the plate boundary but eventually welded together as the crust ages and cools. Similar processes are likely to occur along volcanic rifted margins as spreading begins.

Inferring crustal viscosity from seismic velocity: Application to the lower crust of Southern California

NASA Astrophysics Data System (ADS)

Shinevar, William J.; Behn, Mark D.; Hirth, Greg; Jagoutz, Oliver

2018-07-01

We investigate the role of composition on the viscosity of the lower crust through a joint inversion of seismic P-wave (Vp) and S-wave (Vs) velocities. We determine the efficacy of using seismic velocity to constrain viscosity, extending previous research demonstrating robust relationships between seismic velocity and crustal composition, as well as crustal composition and viscosity. First, we calculate equilibrium mineral assemblages and seismic velocities for a global compilation of crustal rocks at relevant pressures and temperatures. Second, we use a rheological mixing model that incorporates single-phase flow laws for major crust-forming minerals to calculate aggregate viscosity from predicted mineral assemblages. We find a robust correlation between crustal viscosity and Vp together with Vs in the α-quartz regime. Using seismic data, geodetic surface strain rates, and heat flow measurements from Southern California, our method predicts that lower crustal viscosity varies regionally by four orders of magnitude, and lower crustal stress varies by three orders of magnitude at 25 km depth. At least half of the total variability in stress can be attributed to composition, implying that regional lithology has a significant effect on lower crustal geodynamics. Finally, we use our method to predict the depth of the brittle-ductile transition and compare this to regional variations of the seismic-aseismic transition. The variations in the seismic-aseismic transition are not explained by the variations in our model rheology inferred from the geophysical observations. Thus, we conclude that fabric development, in conjunction with compositional variations (i.e., quartz and mica content), is required to explain the regional changes in the seismic-aseismic transition.

Two-layer Crustal Structure of the Contiguous United States from Joint Inversion of USArray Receiver Functions and Gravity

NASA Astrophysics Data System (ADS)

Ma, X.; Lowry, A. R.

2015-12-01

The composition and thickness of crustal layering is fundamental to understanding the evolution and dynamics of continental lithosphere. Lowry and Pérez-Gussinyé (2011) found that the western Cordillera of the United States, characterized by active deformation and high heat flow, is strongly correlated with low bulk crustal seismic velocity ratio. They interpreted this observation as evidence that quartz controls continental tectonism and deformation. We will present new imaging of two-layer crustal composition and structure from cross-correlation of observed receiver functions and model synthetics. The cross-correlation coefficient of the two-layer model increases significantly relative to an assumed one-layer model, and the lower crustal thickness map from raw two-layer modeling (prior to Bayesian filtering with gravity models and Optimal Interpolation) clearly shows Colorado plateau and Appalachian boundaries, which are not apparent in upper crustal models, and also the high vP/vS fill the most of middle continental region while low vP/vS are on the west and east continental edge. In the presentation, we will show results of a new algorithm for joint Bayesian inversion of thickness and vP/vS of two-layer continental crustal structure. Recent thermodynamical modeling of geophysical models based on lab experiment data (Guerri et al., 2015) found that a large impedance contrast can be expected in the midcrust due to a phase transition that decreases plagioclase and increases clinopyroxene, without invoking any change in crustal chemistry. The depth of the transition depends on pressure, temperature and hydration, and in this presentation we will compare predictions of layer thicknesses and vP/vS predicted by mineral thermodynamics to those we observe in the USArray footprint.

Crustal scale detachment in the Himalayas: a reappraisal

NASA Astrophysics Data System (ADS)

Mukhopadhyay, S.; Sharma, J.

2010-11-01

According to the most popular tectonic model of the Himalayas proposed by a number of scientists the Indian crustal material underthrusts the Himalayas at a low angle and is relatively free of deformation compared to the overlying accreted material that makes up the Himalayan mountain chain. In this work we have carried out local earthquake tomography for the Garwhal-Kumaun Himalayas to estimate P- and S-wave velocity variations (Vp and Vs, respectively) and variation in their ratio (Vp/Vs) that would indicate the structure of the Himalayas and the underlying Indian crust in this part of the Himalayas. The results indicate that there is crustal level folding and faulting in this region indicating that the underlying Indian crustal material has also undergone deformation unlike what was postulated for the entire Himalayas by some workers before. By comparing our tomographic result with that for the eastern Nepal-southern Tibet region, it is concluded that there is variation in mode of deformation along the trend of the Himalayas. This observation matches well with the observed velocity variation in the upper mantle of these two regions reported by others. The area under investigation falls within a region where there is more oblique convergence between India and Eurasia compared to the Nepal Himalayas region. This may explain why such variation in mode of deformation is observed. The ratio Vp/Vs gets affected by strength of material. Presence or absence of fluid filled fractures or molten material affects it most strongly in the crustal region. The variation in Vp/Vs in the study area shows that almost the entire crust here have enough rheological strength such that it can store strain energy that can be released through earthquakes. A zone of low Vp/Vs beginning at the higher Himalayas and dipping towards SW is observed. This zone also has high Vp and Vs and is observed even when inversion is carried out with very high damping value. These observations do not suggest that seismic hazard will be less in the study area.

Deformation Processes In SE Tibet: How Coupled Are The Surface And The Deeper Lithosphere? (Invited)

NASA Astrophysics Data System (ADS)

Zeitler, P. K.; Meltzer, A.

2010-12-01

We all like to cite the Himalayan collision as a type example of continent-continent collision, and the region has been used as a natural laboratory by a considerable number of diverse investigations. Southeastern Tibet and the Lhasa Block provide an interesting case to consider in this context. Surrounding portions of the Himalayan-Tibet system have been and are being intensely deformed, whereas the Andean-arc lithosphere of the Lhasa Block has remained enigmatically unscathed. High elevations throughout much of the terrane are fairly uniform but the eastern and western portions of block have experienced very different degrees of exhumation. Regions that experienced more exhumation have thinner crustal thicknesses, with the results that that Moho is warped up with respect to the surface. Thicker, less-exhumed portions of the Lhasa Block currently are underlain by what is inferred to be eclogitized lower crust, but this eclogitization is not seen where exhumation is significant. Beneath SE Tibet, subduction of the Indian lithosphere has been complicated, with tomographic imaging showing variations in mantle structure that do not register with the strike of surface features. Adjacent to the Lhasa Block, the Namche Barwa-Gyala metamorphic massif demonstrates a strong coupling between shallower crustal flow and localized erosion that is significant for the evolution of the Lhasa Block in the way that this feature controls base level for the upper Tsangpo drainage and thus the erosional driver for the system. More broadly, a weak lower crust and lower-crustal flow have been invoked by many workers to explain aspects of the region’s deformation patterns and topography. Thus it would seem that in SE Tibet, mid-to-upper crustal, lower-crustal, and whole-lithosphere processes all have the potential to either impact Earth-surface dynamics or be impacted by them. This leads to a number of questions about the 4D nature and scale of compensation, controls on the evolution of topography, and the degree to which feedbacks might exist between the surface and the deeper lithosphere.

Noble Gas Signatures in Antrim Shale Gas in the Michigan Basin - Assessing Compositional Variability and Transport Processes

NASA Astrophysics Data System (ADS)

Wen, T.; Castro, M. C.; Ellis, B. R.; Hall, C. M.; Lohmann, K. C.; Bouvier, L.

2014-12-01

Recent studies in the Michigan Basin looked at the atmospheric and terrigenic noble gas signatures of deep brines to place constraints on the past thermal history of the basin and to assess the extent of vertical transport processes within this sedimentary system. In this contribution, we present noble gas data of shale gas samples from the Antrim shale formation in the Michigan Basin. The Antrim shale was one of the first economic shale-gas plays in the U.S. and has been actively developed since the 1980's. This study pioneers the use of noble gases in subsurface shale gas in the Michigan Basin to clarify the nature of vertical transport processes within the sedimentary sequence and to assess potential variability of noble gas signatures in shales. Antrim Shale gas samples were analyzed for all stable noble gases (He, Ne, Ar, Kr, Xe) from samples collected at depths between 300 and 500m. Preliminary results show R/Ra values (where R and Ra are the measured and atmospheric 3He/4He ratios, respectively) varying from 0.022 to 0.21. Although most samples fall within typical crustal R/Ra range values (~0.02-0.05), a few samples point to the presence of a mantle He component with higher R/Ra ratios. Samples with higher R/Ra values also display higher 20Ne/22Ne ratios, up to 10.4, and further point to the presence of mantle 20Ne. The presence of crustally produced nucleogenic 21Ne and radiogenic 40Ar is also apparent with 21Ne/22Ne ratios up to 0.033 and 40Ar/36Ar ratios up to 312. The presence of crustally produced 4He, 21Ne and 40Ar is not spatially homogeneous within the Antrim shale. Areas of higher crustal 4He production appear distinct to those of crustally produced 21Ne and 40Ar and are possibly related the presence of different production levels within the shale with varying concentrations of parent elements.

Isotope geochemistry of recent magmatism in the Aegean arc: Sr, Nd, Hf, and O isotopic ratios in the lavas of Milos and Santorini-geodynamic implications

USGS Publications Warehouse

Briqueu, L.; Javoy, M.; Lancelot, J.R.; Tatsumoto, M.

1986-01-01

In this comparative study of variations in the isotopic compositions (Sr, Nd, O and Hf) of the calc-alkaline magmas of the largest two volcanoes, Milos and Santorini, of the Aegean arc (eastern Mediterranean) we demonstrate the complexity of the processes governing the evolution of the magmas on the scale both of the arc and of each volcano. On Santorini, the crustal contamination processes have been limited, effecting the magma gradually during its differentiation. The most differentiated lavas (rhyodacite and pumice) are also the most contaminated. On Milos, by contrast, these processes are very extensive. They are expressed in the 143Nd/144Nd vs. 87Sr/86Sr diagram as a continuous mixing curve between a mantle and a crustal end member pole defined by schists and metavolcanic rocks outcropping on these volcanoes. In contrast with Santorini, the least differentiated lavas on Milos are the most contaminated. These isotopic singularities can be correlated with the geodynamic evolution of the Aegean subduction zone, consisting of alternating tectonic phases of distension and compression. The genesis of rhyolitic magmas can be linked to the two phases of distension, and the contamination of the calc-alkaline mantle-derived magmas with the intermediate compressive phase. The isotopic characteristics of uncontaminated calc-alkaline primitive magmas of Milos and Santorini are directly comparable to those of magmas generated in subduction zones for which a contribution of subducted sediments to partial melts from the mantle is suggested, such as in the Aleutian, Sunda, and lesser Antilles island arcs. However, in spite of the importance of the sediment pile in the eastern Mediterranen oceanic crust (6-10 km), the contribution of the subducted terrigenous materials remains of limited amplitude. ?? 1986.

Variability over time in the sources of South Portuguese Zone turbidites: evidence of denudation of different crustal blocks during the assembly of Pangaea

NASA Astrophysics Data System (ADS)

Pereira, M. F.; Ribeiro, C.; Vilallonga, F.; Chichorro, M.; Drost, K.; Silva, J. B.; Albardeiro, L.; Hofmann, M.; Linnemann, U.

2014-07-01

This study combines geochemical and geochronological data in order to decipher the provenance of Carboniferous turbidites from the South Portuguese Zone (SW Iberia). Major and trace elements of 25 samples of graywackes and mudstones from the Mértola (Visean), Mira (Serpukhovian), and Brejeira (Moscovian) Formations were analyzed, and 363 U-Pb ages were obtained on detrital zircons from five samples of graywackes from the Mira and Brejeira Formations using LA-ICPMS. The results indicate that turbiditic sedimentation during the Carboniferous was marked by variability in the sources, involving the denudation of different crustal blocks and a break in synorogenic volcanism. The Visean is characterized by the accumulation of immature turbidites (Mértola Formation and the base of the Mira Formation) inherited from a terrane with intermediate to mafic source rocks. These source rocks were probably formed in relation to Devonian magmatic arcs poorly influenced by sedimentary recycling, as indicated by the almost total absence of pre-Devonian zircons typical of the Gondwana and/or Laurussia basements. The presence of Carboniferous grains in Visean turbidites indicates that volcanism was active at this time. Later, Serpukhovian to Moscovian turbiditic sedimentation (Mira and Brejeira Formations) included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism. The abundance of Precambrian and Paleozoic zircons reveals strong recycling of the Gondwana and/or Laurussia basements. A peri-Gondwanan provenance is indicated by zircon populations with Neoproterozoic (Cadomian-Avalonian and Pan-African zircon-forming events), Paleoproterozoic, and Archean ages. The presence of late Ordovician and Silurian detrital zircons in Brejeira turbidites, which have no correspondence in the Gondwana basement of SW Iberia, indicates Laurussia as their most probable source.

Geochemistry, geochronology, and tectonic setting of Early Cretaceous volcanic rocks in the northern segment of the Tan-Lu Fault region, northeast China

NASA Astrophysics Data System (ADS)

Ling, Yi-Yun; Zhang, Jin-Jiang; Liu, Kai; Ge, Mao-Hui; Wang, Meng; Wang, Jia-Min

2017-08-01

We present new geochemical and geochronological data for volcanic and related rocks in the regions of the Jia-Yi and Dun-Mi faults, in order to constrain the late Mesozoic tectonic evolution of the northern segment of the Tan-Lu Fault. Zircon U-Pb dating shows that rhyolite and intermediate-mafic rocks along the southern part of the Jia-Yi Fault formed at 124 and 113 Ma, respectively, whereas the volcanic rocks along the northern parts of the Jia-Yi and Dun-Mi faults formed at 100 Ma. The rhyolite has an A-type granitoid affinity, with high alkalis, low MgO, Ti, and P contents, high rare earth element (REE) contents and Ga/Al ratios, enrichments in large-ion lithophile (LILEs; e.g., Rb, Th, and U) and high-field-strength element (HFSEs; e.g., Nb, Ta, Zr, and Y), and marked negative Eu anomalies. These features indicate that the rhyolites were derived from partial melting of crustal material in an extensional environment. The basaltic rocks are enriched in light REEs and LILEs (e.g., Rb, K, Th, and U), and depleted in heavy REEs, HFSEs (e.g., Nb, Ta, Ti, and P), and Sr. These geochemical characteristics indicate that these rocks are calc-alkaline basalts that formed in an intraplate extensional tectonic setting. The dacite is a medium- to high-K, calc-alkaline, I-type granite that was derived from a mixed source involving both crustal and mantle components in a magmatic arc. Therefore, the volcanic rocks along the Jia-Yi and Dun-Mi faults were formed in an extensional regime at 124-100 Ma (Early Cretaceous), and these faults were extensional strike-slip faults at this time.

A transitional volume beneath the Sannio-Irpinia border region (southern Apennines): Different tectonic styles at different depths

NASA Astrophysics Data System (ADS)

De Matteo, Ada; Massa, Bruno; Milano, Girolamo; D'Auria, Luca

2018-01-01

In this paper we investigate the border between the Sannio and Irpinia seismogenic regions, a sector of the southern Apennine chain considered among the most active seismic areas of the Italian peninsula, to shed further light on its complex seismotectonic setting. We integrated recent seismicity with literature data. A detailed analysis of the seismicity that occurred in the 2013-2016 time interval was performed. The events were relocated, after manual re-picking, using different approaches. To retrieve information about the stress field active in the area, inversion of Fault Plane Solutions was also carried out. Hypocentral distribution of the relocated events (ML ≤ 3.5), whose depth is included between 5 and 25 km with the deepest ones located in the NW sector of the study area, shows a different pattern between the northern sector and the southern one. The computed Fault Plane Solutions can be grouped in three depth ranges: < 12 km, dominated by normal dip-slip kinematics; 12-18 km, characterized by normal dip-slip and strike-slip kinematics; > 18 km, dominated by strike-slip kinematics. Stress field inversion across the whole area shows that we are dealing with an heterogeneous set of data, apparently governed by distinct stress fields. We built an upper crustal model profile through integration of geological data, well logs and seismic tomographic profiles. Our results suggest the co-existence of different tectonic styles at distinct crustal depths: the upper crust seems to be affected mostly by normal faulting, whereas strike-slip faulting prevails in the intermediate and lower crust. We infer about the existence of a transitional volume, located between 12 and 18 km depth, between the Sannio and Irpinia regions, acting as a vertical transfer zone.

Precambrian plate tectonic setting of Africa from multidimensional discrimination diagrams

NASA Astrophysics Data System (ADS)

Verma, Sanjeet K.

2017-01-01

New multi-dimensional discrimination diagrams have been used to identify plate tectonic setting of Precambrian terrains. For this work, nine sets of new discriminant-function based multi-dimensional discrimination diagrams were applied for thirteen case studies of Precambrian basic, intermediate and acid magmas from Africa to highlight the application of these diagrams and probability calculations. The applications of these diagrams indicated the following results: For northern Africa: to Wadi Ghadir ophiolite, Egypt indicated an arc setting for Neoproterozoic (746 ± 19 Ma). For South Africa: Zandspruit greenstone and Bulai pluton showed a collision and a transitional continental arc to collision setting at about Mesoarchaean and Neoarchaean (3114 ± 2.3 Ma and 2610-2577 Ma); Mesoproterozoic (1109 ± 0.6 Ma and 1100 Ma) ages for Espungabera and Umkondo sills were consistent with an island arc setting. For eastern Africa, Iramba-Sekenke greenstone belt and Suguti area, Tanzania showed an arc setting for Neoarchaean (2742 ± 27 Ma and 2755 ± 1 Ma). Chila, Bulbul-Kenticha domain, and Werri area indicated a continental arc setting at about Neoproterozoic (800-789 Ma); For western Africa, Sangmelima region and Ebolowa area, southern Cameroon indicated a collision and continental arc setting, respectively for Neoarchaean (∼2800-2900 Ma and 2687-2666 Ma); Finally, Paleoproterozoic (2232-2169 Ma) for Birimian supergroup, southern Ghana a continental arc setting; and Paleoproterozoic (2123-2108 Ma) for Katiola-Marabadiassa, Côte d'Ivoire a transitional continental arc to collision setting. Although there were some inconsistencies in the inferences, most cases showed consistent results of tectonic settings. These inconsistencies may be related to mixed ages, magma mixing, crustal contamination, degree of mantle melting, and mantle versus crustal origin.

Crustal volumes of the continents and of oceanic and continental submarine plateaus

NASA Technical Reports Server (NTRS)

Schubert, G.; Sandwell, D.

1989-01-01

Using global topographic data and the assumption of Airy isostasy, it is estimated that the crustal volume of the continents is 7182 X 10 to the 6th cu km. The crustal volumes of the oceanic and continental submarine plateaus are calculated at 369 X 10 to the 6th cu km and 242 X 10 to the 6th cu km, respectively. The total continental crustal volume is found to be 7581 X 10 to the 6th cu km, 3.2 percent of which is comprised of continental submarine plateaus on the seafloor. An upper bound on the contintental crust addition rate by the accretion of oceanic plateaus is set at 3.7 cu km/yr. Subduction of continental submarine plateaus with the oceanic lithosphere on a 100 Myr time scale yields an upper bound to the continental crustal subtraction rate of 2.4 cu km/yr.

The Precambrian crustal structure of East Africa

NASA Astrophysics Data System (ADS)

Young, A. J.; Tugume, F.; Nyblade, A.; Julia, J.; Mulibo, G.

2011-12-01

We present new results on crustal structure from East Africa from analyzing P wave receiver functions. The data for this study come from temporary AfricaArray broadband seismic stations deployed between 2007 and 2011 in Uganda, Tanzania and Zambia. Receiver functions have been computed using an iterative deconvolution method. Crustal structure has been imaged using the H-k stacking method and by jointly inverting the receiver functions and surface wave phase and group velocities. The results show remarkably uniform crust throughout the Archean and Proterozoic terrains that comprise the Precambrian tectonic framework of the region. Crustal thickness for most terrains is between 37 and 40 km, and Poisson's ratio is between 0.25 and 0.27. Results from the joint inversion yield average crustal Vs values of 3.6 to 3.7 km/s. For most terrains, a thin (1-5 km) thick high velocity (Vs>4.0 km/s) is found at the base of the crust.

A mechanism for crustal recycling on Venus

NASA Technical Reports Server (NTRS)

Lenardic, A.; Kaula, W. M.; Bindschadler, D. L.

1993-01-01

Entrainment of lower crust by convective mantle downflows is proposed as a crustal recycling mechanism on Venus. The mechanism is characterized by thin sheets of crust being pulled into the mantle by viscous flow stresses. Finite element models of crust/mantle interaction are used to explore tectonic conditions under which crustal entrainment may occur. The recycling scenarios suggested by the numerical models are analogous to previously studied problems for which analytic and experimental relationships assessing entrainment rates have been derived. We use these relationships to estimate crustal recycling rates on Venus. Estimated rates are largely determined by (1) strain rate at the crust/mantle interface (higher strain rate leads to greater entrainment); and (2) effective viscosity of the lower crust (viscosity closer to that of mantle lithosphere leads to greater entrainment). Reasonable geologic strain rates and available crustal flow laws suggest entrainment can recycle approximately equal 1 cu km of crust per year under favorable conditions.

Seismic structure and lithospheric rheology from deep crustal xenoliths, central Montana, USA

NASA Astrophysics Data System (ADS)

Mahan, K. H.; Schulte-Pelkum, V.; Blackburn, T. J.; Bowring, S. A.; Dudas, F. O.

2012-10-01

Improved resolution of lower crustal structure, composition, and physical properties enhances our understanding and ability to model tectonic processes. The cratonic core of Montana and Wyoming, USA, contains some of the most enigmatic lower crust known in North America, with a high seismic velocity layer contributing to as much as half of the crustal column. Petrological and physical property data for xenoliths in Eocene volcanic rocks from central Montana provide new insight into the nature of the lower crust in this region. Inherent heterogeneity in xenoliths derived from depths below ˜30 km support a composite origin for the deep layer. Possible intralayer velocity steps may complicate the seismic definition of the crust/mantle boundary and interpretations of crustal thickness, particularly when metasomatized upper mantle is considered. Mafic mineral-dominant crustal xenoliths and published descriptions of mica-bearing peridotite and pyroxenite xenoliths suggest a strong lower crust overlying a potentially weaker upper mantle.

The identification of selected vegetation types in Arizona through the photointerpretation of intermediate scale aerial photography. M.S. Thesis

NASA Technical Reports Server (NTRS)

Ross, G. F. (Principal Investigator)

1973-01-01

The author has identified the following significant results. Nine photography interpretation tests were performed with a total of 19 different interpreters. Three tests were conducted with black and white intermediate scale photography and six tests with color infrared intermediate scale photography. The black and white test results show that the interpretation of vegetation mapped at the association level of classification is reliable for all the classes used at 61%. The color infrared tests indicate that the association level of mapping is unsatisfactory for vegetation interpretation of classes 1 and 6. Students' t-test indicated that intermediate scale black and white photography is significantly better than this particular color infrared photography for the interpretation of southeastern Arizona vegetation mapped at the association level.

Tectonic History and Deep Structure of the Demerara Plateau from Combined Wide-Angle and Reflection Seismic Data and Plate Kinematic Reconstructions

NASA Astrophysics Data System (ADS)

Klingelhoefer, F.; Museur, T.; Roest, W. R.; Graindorge, D.; Chauvet, F.; Loncke, L.; Basile, C.; Poetisi, E.; Deverchere, J.; Lebrun, J. F.; Perrot, J.; Heuret, A.

2017-12-01

Many transform margins have associated intermediate depth marginal plateaus, which are commonly located between two oceanic basins. The Demerara plateau is located offshore Surinam and French Guiana. Plate kinematic reconstructions show that the plateau is located between the central and equatorial Atlantic in a position conjugate to the Guinean Plateau. In the fall of 2016, the MARGATS cruise acquired geophysical data along the 400 km wide Demerara plateau. The main objective of the cruise was to image the deep structure of the Demerara plateau and to study its tectonic history. A set of 4 combined wide-angle and reflection seismic profiles was acquired along the plateau, using 80 ocean-bottom seismometers, a 3 km long seismic streamer and a 8000 cu inch tuned airgun array. Forward modelling of the wide-angle seismic data on a profile, located in the eastern part of the plateau and oriented in a NE-SW direction, images the crustal structure of the plateau, the transition zone and the neighbouring crust of oceanic origin, up to a depth of 40 km. The plateau itself is characterised by a crust of 30 km thickness, subdivided into three distinct layers. However, the velocities and velocity gradients do not fit typical continental crust, with a lower crustal layer showing untypically high velocities and an upper layer having a steep velocity gradient. From this model we propose that the lowermost layer is probably formed from volcanic underplated material and that the upper crustal layer likely consists of the corresponding extrusive volcanic material, forming thick seaward-dipping reflector sequences on the plateau. A basement high is imaged at the foot of the slope and forms the ocean-continent transition zone. Further oceanward, a 5-6 km thick crust is imaged with velocities and velocity gradients corresponding to a thin oceanic crust. A compilation of magnetic data from the MARGATS and 3 previous cruises shows a high amplitude magnetic anomaly along the northern edge of the plateau thereby strengthening the hypothesis of an volcanic origin of at least part of the structure. We propose, that the plateau was formed by large-scale volcanism, possibly intruding into a thinner existing continental crust.

Geological development and Phanerozoic crustal accretion in the western segment of the southern Tien Shan (Kyrgyzstan, Uzbekistan and Tajikistan)

NASA Astrophysics Data System (ADS)

Brookfield, M. E.

2000-12-01

The Tien Shan form a high intracontinental mountain belt, lying north of the main India-Asia collision mountains, and consist of re-activated Paleozoic orogens. The western segment of the southern Tien Shan lies northwest of the Pamir and west of the Talas-Fergana fault. The stratigraphy, lithology, igneous and metamorphic petrology and geochemistry of this segment indicate that it was formed by the assembly of Lower Paleozoic arcs which developed into microcontinents with Upper Paleozoic mature shelf and slope clastic and carbonate sediments. Precambrian continental crust is confined to two small blocks along its southern margin. The bulk of the southern Tien Shan consists of ?Vendian to Silurian oceanic and slope clastic rocks, resting on oceanic lithosphere, and overlain by thick passive margin Devonian to mid-Carboniferous mature shelf clastics and carbonates. These are unconformably overlain by syn- and post-orogenic immature clastic sediments derived from mountains on the north formed by closure of a Carboniferus southern Tajik and a northern Vendian to Carboniferous Turkestan ocean with the southern Tien Shan microcontinent sandwiched between. Associated with these collisions are late Carboniferous to Permian intrusives, which form three south to north (though overlapping) suites; a southern calc-alkaline granodiorite-granite suite, an intermediate gabbro-monzodiorite-granite suite, and a northern alkaline monzodiorite-granite-alaskite suite. The gabbro-monzodiorite-granite suite forms the earliest subduction-related magmatism of the southern Tien Shan: rare earth element patterns are consistent with derivation from a primitive or slightly enriched mantle. The other suites show more crustal contamination. Rb and Sr vary with depth and degree of partial melting and are consistent with progressive involvement of crustal material in partial melts during collision. The gradual change in composition within each complex, lasting in some cases from 295 to 250 Ma (the entire Permian), may be explained by a consecutive shift in the melting sedimentary cover of the subducting plate from oceanic crust through transitional crust to marginal continental crust. Like the Central Asian orogenic belt (the main focus of IGCP 420), the Tien Shan represent a net addition of continental crust during the Phanerozoic. Very little of the belt has any Precambrian precursor.

Shallow crustal structure of eastern-central Trans-Mexican Volcanic Belt.

NASA Astrophysics Data System (ADS)

Campos-Enriquez, J. O.; Ramón, V. M.; Lermo-Samaniego, J.

2015-12-01

Central-eastern Trans-Mexican Volcanic Belt (TMVB) is featured by large basins (i.e., Toluca, Mexico, Puebla-Tlaxcala, Libres-Oriental). It has been supposed that major crustal faults limit these basins. Sierra de Las Cruces range separates the Toluca and Mexico basins. The Sierra Nevada range separates Mexico basin from the Puebla-Tlaxcala basin. Based in gravity and seismic data we inferred the Toluca basin is constituted by the Ixtlahuaca sub-basin, to the north, and the Toluca sub-basin to the south, which are separated by a relative structural high. The Toluca depression is more symmetric and bounded by sub-vertical faults. In particular its eastern master fault controlled the emplacement of Sierra de Las Cruces range. Easternmost Acambay graben constitutes the northern and deepest part of the Ixtlahuaca depression. The Toluca-Ixtlahuaca basin is inside the Taxco-San Miguel de Allende fault system, and limited to the west by the Guerrero terrane which continues beneath the TMVB up to the Acambay graben. Mexico basin basement occupies an intermediate position and featured by a relative structural high to the north-east, as established by previous studies. This relative structural high is limited to the west by the north-south Mixhuca trough, while to the south it is bounded by the east-west Copilco-Xochimilco-Chalco sub-basin. The Puebla-Tlaxcala basin basement is the shallowest of these 3 tectonic depressions. In general, features (i.e., depth) and relationship between these basins, from west to east, are controlled by the regional behavior of the Sierra Madre Oriental fold and thrust belt basement (i.e., Oaxaca Complex?). This study indicates that an active east-west regional fault system limits to the south the TMVB (from the Nevado de Toluca volcano through the Popocatepetl volcano and eastward along southern Puebla-Tlaxcala basin). The Tenango and La Pera fault systems constituting the western part of this regional fault system coincide with northern exposures of the Morelos platform to the west. The eastward extension of this system limits the northern Acatlan Complex exposures. Accordingly, eastern TMVB has been subjected to extension and associated faults are being activated at present. The basins act as independent crustal blocks. The Puebla-Tlaxcala and the Tehuacan basins merge to the east.

Counter-intuitive features of the dynamic topography unveiled by tectonically realistic 3D numerical models of mantle-lithosphere interactions

NASA Astrophysics Data System (ADS)

Burov, Evgueni; Gerya, Taras

2013-04-01

It has been long assumed that the dynamic topography associated with mantle-lithosphere interactions should be characterized by long-wavelength features (> 1000 km) correlating with morphology of mantle flow and expanding beyond the scale of tectonic processes. For example, debates on the existence of mantle plumes largely originate from interpretations of expected signatures of plume-induced topography that are compared to the predictions of analytical and numerical models of plume- or mantle-lithosphere interactions (MLI). Yet, most of the large-scale models treat the lithosphere as a homogeneous stagnant layer. We show that in continents, the dynamic topography is strongly affected by rheological properties and layered structure of the lithosphere. For that we reconcile mantle- and tectonic-scale models by introducing a tectonically realistic continental plate model in 3D large-scale plume-mantle-lithosphere interaction context. This model accounts for stratified structure of continental lithosphere, ductile and frictional (Mohr-Coulomb) plastic properties and thermodynamically consistent density variations. The experiments reveal a number of important differences from the predictions of the conventional models. In particular, plate bending, mechanical decoupling of crustal and mantle layers and intra-plate tension-compression instabilities result in transient topographic signatures such as alternating small-scale surface features that could be misinterpreted in terms of regional tectonics. Actually thick ductile lower crustal layer absorbs most of the "direct" dynamic topography and the features produced at surface are mostly controlled by the mechanical instabilities in the upper and intermediate crustal layers produced by MLI-induced shear and bending at Moho and LAB. Moreover, the 3D models predict anisotropic response of the lithosphere even in case of isotropic solicitations by axisymmetric mantle upwellings such as plumes. In particular, in presence of small (i.e. insufficient to produce solely any significant deformation) uniaxial extensional tectonic stress field, the plume-produced surface and LAB features have anisotropic linear shapes perpendicular to the far-field tectonic forces, typical for continental rifts. Compressional field results in singular sub-linear folds above the plume head, perpendicular to the direction of compression. Small bi-axial tectonic stress fields (compression in one direction and extension in the orthogonal direction) result in oblique, almost linear segmented normal or inverse faults with strike-slip components (or visa verse , strike-slip faults with normal or inverse components)

The Bashgumbaz "ophiolites" of SE Pamir (Tajikistan): time constraints for the South Pamir accretion to Eurasia

NASA Astrophysics Data System (ADS)

Zanchetta, Stefano; Worthington, James; Angiolini, Lucia; Zanchi, Andrea

2017-04-01

The Pamir belts, located NW of the Himalaya, formed in response to the Cenozoic collision and indentation of India with Eurasia. Several crustal blocks separated by complex and scarcely studied suture zones form the present day tectonic architecture of the Pamirs. The extreme crustal shortening resulted in strong deformation and bending of pre-existing Paleozoic to Mesozoic orogens. Such deformation hinders straightforward correlations between tectonic terranes of the Himalaya-Tibetan area with their potential continuations through the Pamirs into Afghanistan and Iran. Central and SE Pamir, together with Karakoram and Qiangtang blocks separated from the Gondwana margin in the Early Permian, drifted northward, following the closure of the Paleotethys and other minor oceanic branches, and diachronously collided with Northern Pamir and Tian Shan. The Central and South Pamir are now separated by the poorly known Rushan-Pshart Suture Zone. South Pamir is separated into two units: SE Pamir consists of a Permian to Cenozoic sedimentary succession; SW Pamir is instead made of gigantic basement domes that are in contact with the SE block along crustal scale detachments. In SE Pamir, intensively folded and faulted Permian and Triassic units are unconformably covered by lowermost Jurassic terrigeneous deposits, testifying for Middle to Lower Triassic tectonics related to the Cimmerian orogenic events. Along the contact between the SE and SW Pamir, a small (a few tens of km2 wide) magmatic-metasedimetary unit occurs, reported in literature as the Bashgumbaz ophiolites. They consist of a low-grade metamorphic association of serpentinized harzburgites and gabbros, minor bodies of diorite and plagiogranite, basalts and intermediate volcanic rocks, metasediments, and a flyschoid unit containing olistolithic blocks with Triassic faunas attributed to Central Pamir block. Petrographic and geochemical data suggest a supra-subduction zone affinity for the gabbroic complex. U-Th-Pb dating of zircons from a diorite provides a Carnian crystallization age. Deformation and metamorphism (up to greenschist facies) that affected the Bashgumbaz complex should therefore been placed in the Late Triassic. We suggest that the Bashgumbaz unit formed in a supra-subduction setting and was later underthrusted and then obducted onto the southern margin of the closing Rushan-Pshart ocean. The obduction of the Bashgumbaz ophiolites could be considered as a time-marker for the accretion of the South Pamir terrane to the Eurasian margin.

Variations in the degree of crustal extension during formation of a back-arc basin

NASA Astrophysics Data System (ADS)

de Wit, Maarten J.; Stern, Charles R.

1981-02-01

Ophiolite complexes in southern Chile represent the remnants of the mafic portion of the floor of a Cretaceous back-arc basin which widened markedly from north to south over a length of 600 km. Detailed field and geochemical studies of ophiolites in the northern (Sarmiento complex) and southern (Tortuga complex) extremities of the originally wedge-shaped back-arc basin floor, indicate significant north—south differences in the mode of emplacement of basaltic magmas into the pre-existing continental crust, during the formation of the basin. In the northern narrow extremity of the original basin, mafic melts intruded into the continental crust over a diffuse zone causing extensive remobilization and reconstitution of the sialic continental crust. In the southern wider part of the original basin, mafic magmas appear to have been emplaced at a localized oceanic-type spreading centre. The observed north—south variations resulted in formation of back-arc floor with crustal characteristics ranging from intermediate between continental and oceanic to typically oceanic. These variations are interpreted as representing different stages of evolution of a back-arc basin which formed due to a subtle interplay between subduction induced back-arc mantle convection and the release of stress across the convergent plate boundary, possibly related to ridge subduction. Prior to the release of stress, heat transferred from mantle diapirs to the base of crust caused widespread silicic volcanism in South America. With the release of stress, mantle derived melts erupted to the surface along structural pathways resulting in extensive basaltic volcanism in a linear belt behind the island arc and the cessation of silicic volcanism. Initially, basaltic magmas intruded the continental crust over a diffuse region causing reconstitution of sialic crustal rocks. Progressive localization of the zone of intrusion of mafic magmas from the mantle eventually resulted in the development of an oceanic-type spreading centre. Observations in southern Chile and elsewhere suggest that variability in horizontal stress across a convergent plate boundary may be the overriding factor in determining the regional response of continental crust to subduction induced back-arc convection, and hence the mechanism of emplacement into the crust of mafic mantle melts. The various lithologies observed in southern Chile could also be expected to form during the opening phase of major ocean basins and to currently underlie Atlantic-type continental margins.

A Statistical Investigation on a Seismic Transient Occurred in Italy Between the 17th and 20th Centuries

NASA Astrophysics Data System (ADS)

Bragato, P. L.

2017-03-01

According to the historical earthquake catalog of Italy, the country experienced a pulse of seismicity between the 17th century, when the rate of destructive events increased by more than 100%, and the 20th century, characterized by a symmetric decrease. In the present work, I performed a statistical analysis to verify the reliability of such transient, considering different sources of bias and uncertainty, such as completeness and declustering of the catalog, as well as errors on magnitude estimation. I also searched for a confirmation externally to the catalog, analyzing the correlation with the volcanic activity. The similarity is high for the eruptive history of Vesuvius, which agrees on both the main rate changes of the 17th and 20th centuries and on minor variations in the intermediate period. Of general interest, beyond the specific case of Italy, the observed rate changes suggest the existence of large-scale crustal processes taking place within decades and lasting for centuries, responsible for the synchronous activation/deactivation of remote, loosely connected faults in different tectonic domains. Although their origin is still unexplained (I discuss a possible link with the climate changes and the consequent variations of the sea level), their existence and long lasting is critical for seismic hazard computation. In fact, they introduce a hardly predictable time variability that undermines any hypothesis of regularity of the earthquake cycle on individual faults and systems of interconnected faults.

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.

The grand tour of the Ruby-East Humboldt metamorphic core complex, northeastern Nevada: Part 1 - Introduction & road log

USGS Publications Warehouse

Snoke, A.W.; Howard, K.A.; McGrew, A.J.; Burton, B.R.; Barnes, C.G.; Peters, M.T.; Wright, J.E.

1997-01-01

The purpose of this geological excursion is to provide an overview of the multiphase developmental history of the Ruby Mountains and East Humboldt Range, northeastern Nevada. Although these mountain ranges are commonly cited as a classic example of a Cordilleran metamorphic core complex developed through large-magnitude, mid-Tertiary crustal extension, a preceding polyphase Mesozoic contractional history is also well preserved in the ranges. An early phase of this history involved Late Jurassic two-mica granitic magmatism, high-temperature but relatively low-pressure metamorphism, and polyphase deformation in the central Ruby Mountains. In the northern Ruby Mountains and East Humboldt Range, a Late Cretaceous history of crustal shortening, metamorphism, and magmatism is manifested by fold-nappes (involving Archean basement rocks in the northern East Humboldt Range), widespread migmatization, injection of monzogranitic and leucogranitic magmas, all coupled with sillimanite-grade metamorphism. Following Late Cretaceous contraction, a protracted extensional deformation partially overprinted these areas during the Cenozoic. This extensional history may have begun as early as the Late Cretaceous or as late as the mid-Eocene. Late Eocene and Oligocene magmatism occurred at various levels in the crust yielding mafic to felsic orthogneisses in the deep crust, a composite granitic pluton in the upper crust, and volcanic rocks at the surface. Movement along a west-rooted, extensional shear zone in the Oligocene and early Miocene led to core-complex exhumation. The shear zone produced mylonitic rocks about 1 km thick at deep crustal levels, and an overprint of brittle detachment faulting at shallower levels as unroofing proceeded. Megabreccias and other synextensional sedimentary deposits are locally preserved in a tilted, upper Eocene through Miocene stratigraphic sequence. Neogene magmatism included the emplacement of basalt dikes and eruption of rhyolitic rocks. Subsequent Basin and Range normal faulting, as young as Holocene, records continued tectonic extension.

Crustal Movement: A Major Force in Evolution. Crustal Evolution Education Project. Teacher's Guide [and] Student Investigation.

ERIC Educational Resources Information Center

Stoever, Edward C., Jr.

Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

Surface Wave Dispersion Measurements and Tomography from Ambient Seismic Noise Correlation in China

DTIC Science & Technology

2010-03-15

R.A. Jamieson, M.H. Nguyen, et al., Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface...9 8. Crustal thickness map obtained in this study...not in the margin areas, consistent with the crustal channel flow model. 2. INTRODUCTION The overall objective of this project is to obtain

Mars' paleomagnetic field as the result of a single-hemisphere dynamo.

PubMed

Stanley, Sabine; Elkins-Tanton, Linda; Zuber, Maria T; Parmentier, E Marc

2008-09-26

Mars' crustal magnetic field was most likely generated by dynamo action in the planet's early history. Unexplained characteristics of the field include its strength, concentration in the southern hemisphere, and lack of correlation with any surface features except for the hemispheric crustal dichotomy. We used numerical dynamo modeling to demonstrate that the mechanisms proposed to explain crustal dichotomy formation can result in a single-hemisphere dynamo. This dynamo produces strong magnetic fields in only the southern hemisphere. This magnetic field morphology can explain why Mars' crustal magnetic field intensities are substantially stronger in the southern hemisphere without relying on any postdynamo mechanisms.

A thin, dense crust for Mercury

NASA Astrophysics Data System (ADS)

Sori, Michael M.

2018-05-01

Crustal thickness is a crucial geophysical parameter in understanding the geology and geochemistry of terrestrial planets. Recent development of mathematical techniques suggests that previous studies based on assumptions of isostasy overestimated crustal thickness on some of the solid bodies of the solar system, leading to a need to revisit those analyses. Here, I apply these techniques to Mercury. Using MESSENGER-derived elemental abundances, I calculate a map of grain density (average 2974 ± 89 kg/m3) which shows that Pratt isostasy is unlikely to be a major compensation mechanism of Mercury's topography. Assuming Airy isostasy, I find the best fit value for Mercury's mean crustal thickness is 26 ± 11 km, 25% lower than the most recently reported and previously thinnest number. Several geological implications follow from this relatively low value for crustal thickness, including showing that the largest impacts very likely excavated mantle material onto Mercury's surface. The new results also show that Mercury and the Moon have a similar proportion of their rocky silicates composing their crusts, and thus Mercury is not uniquely efficient at crustal production amongst terrestrial bodies. Higher resolution topography and gravity data, especially for the southern hemisphere, will be necessary to refine Mercury's crustal parameters further.

Crustal CO2 liberation during the 2006 eruption and earthquake events at Merapi volcano, Indonesia

NASA Astrophysics Data System (ADS)

Troll, Valentin R.; Hilton, David R.; Jolis, Ester M.; Chadwick, Jane P.; Blythe, Lara S.; Deegan, Frances M.; Schwarzkopf, Lothar M.; Zimmer, Martin

2012-06-01

High-temperature volcanic gas is widely considered to originate from ascending, mantle-derived magma. In volcanic arc systems, crustal inputs to magmatic gases mainly occur via subducted sediments in the mantle source region. Our data from Merapi volcano, Indonesia imply, however, that during the April-October 2006 eruption significant quantities of CO2 were added from shallow crustal sources. We show that prior to the 2006 events, summit fumarole gas δ13C(CO2) is virtually constant (δ13C1994-2005 = -4.1 ± 0.3‰), but during the 2006 eruption and after the shallow Yogyakarta earthquake of late May, 2006 (M6.4; hypocentres at 10-15 km depth), carbon isotope ratios increased to -2.4 ± 0.2‰. This rise in δ13C is consistent with considerable addition of crustal CO2 and coincided with an increase in eruptive intensity by a factor of ˜3 to 5. We postulate that this shallow crustal volatile input supplemented the mantle-derived volatile flux at Merapi, intensifying and sustaining the 2006 eruption. Late-stage volatile additions from crustal contamination may thus provide a trigger for explosive eruptions independently of conventional magmatic processes.

Isotopic constraints on crustal growth and recycling

NASA Technical Reports Server (NTRS)

Jacobsen, Stein B.

1988-01-01

The Sm-Nd isotopic data on clastic and chemical sediments are used with the present-day age distribution of continental crustal rocks to estimate the rates of crustal accretion, growth and recycling throughout earth's history. A new method for interpreting Nd model ages on both chemical and clastic sediments is proposed. A general relationship is derived between the mean crustal residence time of material recycled from the crust to the mantle (i.e., sediments), the mean age of the crust, and the crustal growth and recycling rates. This relationship takes into account the fact that the age distribution of material in the continental crust is generally different from the age distribution of material recycled into the mantle. The episodic nature of the present-day age distribution in crustal rocks results in similar episodicity in the accretion and recycling rates. The results suggest that by about 3.8 Ga ago, about 40 percent of the present continental volume was present. Recycling rates were extremely high 3-4 Ga ago and declined rapidly to an insignificant value of about 0.1 cu km/a during most of the Phanerozoic. The Nd model age pattern on sediments suggests a fairly high rate of growth during the Phanerozoic.

Seismological constraints on the crustal structures generated by continental rejuvenation in northeastern China

PubMed Central

Zheng, Tian-Yu; He, Yu-Mei; Yang, Jin-Hui; Zhao, Liang

2015-01-01

Crustal rejuvenation is a key process that has shaped the characteristics of current continental structures and components in tectonic active continental regions. Geological and geochemical observations have provided insights into crustal rejuvenation, although the crustal structural fabrics have not been well constrained. Here, we present a seismic image across the North China Craton (NCC) and Central Asian Orogenic Belt (CAOB) using a velocity structure imaging technique for receiver functions from a dense array. The crustal evolution of the eastern NCC was delineated during the Mesozoic by a dominant low seismic wave velocity with velocity inversion, a relatively shallow Moho discontinuity, and a Moho offset beneath the Tanlu Fault Zone. The imaged structures and geochemical evidence, including changes in the components and ages of continental crusts and significant continental crustal growth during the Mesozoic, provide insight into the rejuvenation processes of the evolving crust in the eastern NCC caused by structural, magmatic and metamorphic processes in an extensional setting. The fossil structural fabric of the convergent boundary in the eastern CAOB indicates that the back-arc action of the Paleo-Pacific Plate subduction did not reach the hinterland of Asia. PMID:26443323

Crustal structure of the southeastern Tibetan Plateau from gravity data: New evidence for clockwise movement of the Chuan-Dian rhombic block

NASA Astrophysics Data System (ADS)

Xuan, Songbai; Shen, Chongyang; Shen, Wenbin; Wang, Jiapei; Li, Jianguo

2018-06-01

The crustal deformation beneath the Chuan-Dian rhombic block (CDB) and surrounding regions has been studied in geological and geodetic methods, and provide important insights into the kinematics and dynamics about the clockwise movement of this tectonic block. In this work, we present images of the normalized full gradient (NFG) of the Bouguer gravity anomalies from five gravity profiles across the boundary faults of the CDB measured in recent years, and investigate the distribution characteristics of the crustal anomalous bodies along the profiles. Firstly, an anomalous body with eastward dipping exist beneath the Xianshuihe fault, suggesting that crustal mass move to east. Secondly, near the Xiaojiang fault, two anomalous bodies dip westward with depth increasing. The inferred movement direction of the north one is from west to east, and the south one is from east to west. Thirdly, anomalous bodies on the northeast and southwest sides of the Red River fault suggest the directions of crustal movement is from northeast to southwest. These results are also consistent with GPS solutions, and provide gravity evidence for crustal deformation of the CDB with clockwise rotation.

Gravity anomalies and associated tectonic features over the Indian Peninsular Shield and adjoining ocean basins

NASA Astrophysics Data System (ADS)

Mishra, D. C.; Arora, K.; Tiwari, V. M.

2004-02-01

A combined gravity map over the Indian Peninsular Shield (IPS) and adjoining oceans brings out well the inter-relationships between the older tectonic features of the continent and the adjoining younger oceanic features. The NW-SE, NE-SW and N-S Precambrian trends of the IPS are reflected in the structural trends of the Arabian Sea and the Bay of Bengal suggesting their probable reactivation. The Simple Bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins, which is attributed to isostatic compensation due to variations in the crustal thickness. A crustal density model computed along a profile across this region suggests a thick crust of 35-40 km under the continent, which reduces to 22/20-24 km under the Bay of Bengal with thick sediments of 8-10 km underlain by crustal layers of density 2720 and 2900/2840 kg/m 3. Large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the Bay of Bengal up to 150-200 km from the east coast. The crustal thickness under the Laxmi ridge and east of it in the Arabian Sea is 20 and 14 km, respectively, with 5-6 km thick Tertiary and Mesozoic sediments separated by a thin layer of Deccan Trap. Crustal layers of densities 2750 and 2950 kg/m 3 underlie sediments. The crustal density model in this part of the Arabian Sea (east of Laxmi ridge) and the structural trends similar to the Indian Peninsular Shield suggest a continent-ocean transitional crust (COTC). The COTC may represent down dropped and submerged parts of the Indian crust evolved at the time of break-up along the west coast of India and passage of Reunion hotspot over India during late Cretaceous. The crustal model under this part also shows an underplated lower crust and a low density upper mantle, extending over the continent across the west coast of India, which appears to be related to the Deccan volcanism. The crustal thickness under the western Arabian Sea (west of the Laxmi ridge) reduces to 8-9 km with crustal layers of densities 2650 and 2870 kg/m 3 representing an oceanic crust.

Crustal structure of the Transantarctic Mountains, Ellsworth Mountains and Marie Byrd Land, Antarctica: constraints on shear wave velocities, Poisson's ratios and Moho depths

NASA Astrophysics Data System (ADS)

Ramirez, C.; Nyblade, A.; Emry, E. L.; Julià, J.; Sun, X.; Anandakrishnan, S.; Wiens, D. A.; Aster, R. C.; Huerta, A. D.; Winberry, P.; Wilson, T.

2017-12-01

A uniform set of crustal parameters for seismic stations deployed on rock in West Antarctica and the Transantarctic Mountains (TAM) has been obtained to help elucidate similarities and differences in crustal structure within and between several tectonic blocks that make up these regions. P-wave receiver functions have been analysed using the H-κ stacking method to develop estimates of thickness and bulk Poisson's ratio for the crust, and jointly inverted with surface wave dispersion measurements to obtain depth-dependent shear wave velocity models for the crust and uppermost mantle. The results from 33 stations are reported, including three stations for which no previous results were available. The average crustal thickness is 30 ± 5 km along the TAM front, and 38 ± 2 km in the interior of the mountain range. The average Poisson's ratios for these two regions are 0.25 ± 0.03 and 0.26 ± 0.02, respectively, and they have similar average crustal Vs of 3.7 ± 0.1 km s-1. At multiple stations within the TAM, we observe evidence for mafic layering within or at the base of the crust, which may have resulted from the Ferrar magmatic event. The Ellsworth Mountains have an average crustal thickness of 37 ± 2 km, a Poisson's ratio of 0.27, and average crustal Vs of 3.7 ± 0.1 km s-1, similar to the TAM. This similarity is consistent with interpretations of the Ellsworth Mountains as a tectonically rotated TAM block. The Ross Island region has an average Moho depth of 25 ± 1 km, an average crustal Vs of 3.6 ± 0.1 km s-1 and Poisson's ratio of 0.30, consistent with the mafic Cenozoic volcanism found there and its proximity to the Terror Rift. Marie Byrd Land has an average crustal thickness of 30 ± 2 km, Poisson's ratio of 0.25 ± 0.04 and crustal Vs of 3.7 ± 0.1 km s-1. One station (SILY) in Marie Byrd Land is near an area of recent volcanism and deep (25-40 km) seismicity, and has a high Poisson's ratio, consistent with the presence of partial melt in the crust.

Developing Computer-Interactive Tape Exercises for Intermediate-Level Business French.

ERIC Educational Resources Information Center

Garnett, Mary Anne

One college language teacher developed computer-interactive audiotape exercises for an intermediate-level class in business French. The project was undertaken because of a need for appropriate materials at that level. The use of authoring software permitted development of a variety of activity types, including multiple-choice, fill-in-the-blank,…

Lithospheric buoyancy and continental intraplate stresses

USGS Publications Warehouse

Zoback, M.L.; Mooney, W.D.

2003-01-01

Lithospheric buoyancy, the product of lithospheric density and thickness, is an important physical property that influences both the long-term stability of continents and their state of stress. We have determined lithospheric buoyancy by applying the simple isostatic model of Lachenbruch and Morgan (1990). We determine the crustal portion of lithospheric buoyancy using the USGS global database of more than 1700 crustal structure determinations (Mooney et al., 2002), which demonstrates that a simple relationship between crustal thickness and surface elevation does not exist. In fact, major regions of the crust at or near sea level (0-200 m elevation) have crustal thicknesses that vary between 25 and 55 km. Predicted elevations due to the crustal component of buoyancy in the model exceed observed elevations in nearly all cases (97% of the data), consistent with the existence of a cool lithospheric mantle lid that is denser than the asthenosphere on which it floats. The difference between the observed and predicted crustal elevation is assumed to be equal to the decrease in elevation produced by the negative buoyancy of the mantle lid. Mantle lid thickness was first estimated from the mantle buoyancy and a mean lid density computed using a basal crust temperature determined from extrapolation of surface heat flow, assuming a linear thermal gradient in the mantle lid. The resulting values of total lithosphere thickness are in good agreement with thicknesses estimated from seismic data, except beneath cratonic regions where they are only 40-60% of the typical estimates (200-350 km) derived from seismic data. This inconsistency is compatible with petrologic data and tomography and geoid analyses that have suggested that cratonic mantle lids are ??? 1% less dense than mantle lids elsewhere. By lowering the thermally determined mean mantle lid density in cratons by 1%, our model reproduces the observed 200-350+ km cratonic lithospheric thickness. We then computed gravitational potential energy by taking a vertical integral over the computed lithosphere density. Our computed values suggest that the thick roots beneath cratons lead to strong negative potential energy differences relative to surrounding regions, and hence exert compressive stresses superimposed on the intraplate stresses derived from plate boundary forces. Forces related to this lithosphere structure thus may explain the dominance of reverse-faulting earthquakes in cratons. Areas of high elevation and a thin mantle lid (e.g., western U.S. Basin and Range, East African rift, and Baikal rift) are predicted to be in extension, consistent with the observed stress regime in these areas.

Seismic structure of the crust and uppermost mantle of north America and adjacent oceanic basins: A synthesis

USGS Publications Warehouse

Chulick, G.S.; Mooney, W.D.

2002-01-01

We present a new set of contour maps of the seismic structure of North America and the surrounding ocean basins. These maps include the crustal thickness, whole-crustal average P-wave and S-wave velocity, and seismic velocity of the uppermost mantle, that is, Pn and Sn. We found the following: (1) The average thickness of the crust under North America is 36.7 km (standard deviation [s.d.] ??8.4 km), which is 2.5 km thinner than the world average of 39.2 km (s.d. ?? 8.5) for continental crust; (2) Histograms of whole-crustal P- and S-wave velocities for the North American crust are bimodal, with the lower peak occurring for crust without a high-velocity (6.9-7.3 km/sec) lower crustal layer; (3) Regions with anomalously high average crustal P-wave velocities correlate with Precambrian and Paleozoic orogens; low average crustal velocities are correlated with modern extensional regimes; (4) The average Pn velocity beneath North America is 8.03 km/sec (s.d. ?? 0.19 km/sec); (5) the well-known thin crust beneath the western United States extends into northwest Canada; (6) the average P-wave velocity of layer 3 of oceanic crust is 6.61 km/ sec (s.d. ?? 0.47 km/sec). However, the average crustal P-wave velocity under the eastern Pacific seafloor is higher than the western Atlantic seafloor due to the thicker sediment layer on the older Atlantic seafloor.

Rocky Mountain evolution: Tying Continental Dynamics of the Rocky Mountains and Deep Probe seismic experiments with receiver functions

USGS Publications Warehouse

Rumpfhuber, E.-M.; Keller, Gordon R.; Sandvol, E.; Velasco, A.A.; Wilson, D.C.

2009-01-01

In this study, we have determined the crustal structure using three different receiver function methods using data collected from the northern transect of the Continental Dynamics of the Rocky Mountains (CD-ROM) experiment. The resulting migrated image and crustal thickness determinations confirm and refine prior crustal thickness measurements based on the CD-ROM and Deep Probe experiment data sets. The new results show a very distinct and thick lower crustal layer beneath the Archean Wyoming province. In addition, we are able to show its termination at 42??N latitude, which provides a seismic tie between the CD-ROM and Deep Probe seismic experiments and thus completes a continuous north-south transect extending from New Mexico into Alberta, Canada. This new tie is particularly important because it occurs close to a major tectonic boundary, the Cheyenne belt, between an Archean craton and a Proterozoic terrane. We used two different stacking techniques, based on a similar concept but using two different ways to estimate uncertainties. Furthermore, we used receiver function migration and common conversion point (CCP) stacking techniques. The combined interpretation of all our results shows (1) crustal thinning in southern Wyoming, (2) strong northward crustal thickening beginning in central Wyoming, (3) the presence of an unusually thick and high-velocity lower crust beneath the Wyoming province, and (4) the abrupt termination of this lower crustal layer north of the Cheyenne belt at 42??N latitude. Copyright 2009 by the American Geophysical Union.

Les premières phases d'édification du stratovolcan du Cantal (Massif central, France) entre 9,5 et 8,0Ma : géologie et géochimie du secteur de l'ÉlancèzeFirst stage of the Cantal stratovolcano (Massif Central, France) construction between 9.5 and 8.0Ma: geology and geochemistry of the Élancèze area

NASA Astrophysics Data System (ADS)

Legendre, Christelle; Caroff, Martial; Leyrit, Hervé; Nehlig, Pierre; Thièblemont, Denis

2001-05-01

Recent cartography of the Élancèze area resulted in the identification of a residual ancient topographic relief comprised of basaltic and intermediate silica-saturated lavas partly covered by brecciated formations, emplaced during the first stage of edification of the Cantal stratovolcano (9.5-8.0 Ma). The ancient relief has been the subject of a specified petrological study. Although the corresponding mineralogical and geochemical data are rather consistent with the assumption of a differentiation mainly governed by fractional crystallization, the study of some incompatible trace element diagrams suggests that this process cannot have occurred in closed-system. A model of crustal assimilation coupled with fractional crystallization involving an amphibole-bearing cumulate is proposed.

Evolution of the earth's crust: Evidence from comparative planetology

NASA Technical Reports Server (NTRS)

Lowman, P. D., Jr.

1973-01-01

Geochemical data and orbital photography from Apollo, Mariner, and Venera missions were combined with terrestrial geologic evidence to study the problem of why the earth has two contrasting types of crust (oceanic and continental). The following outline of terrestrial crustal evolution is proposed. A global crust of intermediate to acidic composition, high in aluminum, was formed by igneous processes early in the earth's history; portions survive in some shield areas as granitic and anorthositic gneisses. This crust was fractured by major impacts and tectonic processes, followed by basaltic eruptions analogous to the lunar maria and the smooth plains of the north hemisphere of Mars. Seafloor spreading and subduction ensued, during which portions of the early continental crust and sediments derived therefrom were thrust under the remaining continental crust. The process is exemplified today in regions such as the Andes/Peru-Chile trench system. Underplating may have been roughly concentric, and the higher radioactive element content of the underplated sialic material could thus eventually cause concentric zones of regional metamorphism and magmatism.

Airborne Gravity Gradiometry Resolves a Full Range of Gravity Frequencies

NASA Astrophysics Data System (ADS)

Mataragio, J.; Brewster, J.; Mims, J.

2007-12-01

Airborne Full Tensor Gradiometry (Air\\-FTGR) was flown at high altitude coincident with Airborne Gravity (AG) flown in 2003 in West Arnhem Land, Australia. A preliminary analysis of two data sets indicates that the Air\\-FTGR system has the capability of resolving intermediate to long wavelengths features that may be associated with relatively deeper geological structures. A comparison of frequency filtered slices and power spectral density (PSD) for both data sets using the short (> 5 km), intermediate (10 km) and long (20 km) wavelengths reveals that high altitude Air\\-FTGR data show greater response in high frequency anomalies than a conventional Airborne Gravity and matches well with the AG even at the longest wavelengths anomalies. The effect of line spacing and target resolution was examined between the two data sets. Reprocessed gradient and AG data at 2, 4 and 6 km line spacing suggest that Air\\-FTGR could be effectively flown at a comparatively wider line spacing to resolve similar targets the AG would resolve with tighter line spacing. Introduction Airborne Full Tensor Gradiometry (Air\\-FTGR) data have been available to the mining industry since 2002 and their use for geologic applications is well established. However, Air\\-FTGR data has been mostly considered and used in mapping and delineation of near surface geological targets. This is due to the fact that gravity gradiometer measurements are well suited to capture the high frequency signal associated with near\\-surface targets ( Li, 2001). This is possible because the gradiometer signal strength falls off with the cube of the distance to the target. Nonetheless, in recent years there has been an increasing demand from the mining, oil, and gas industry in utilizing Full Tensor Gravity Gradiometer as a mapping tool for both regional and prospect level surveys. Air\\-FTGR as a Regional Mapping Tool Several, relatively low altitude surveys have been successfully flown in Brazil, Canada and Australia mostly targeting large, regional\\- scale crustal structures as well as regional mapping of both lithology and regolith. Air\\-FTGR mapping is especially effective in areas of thick lateritic and/or clay cover where other geophysical methods such as airborne magnetics or electromagnetics become less effective. For instance, an Air\\-FTGR survey was successfully flown in Brazil in the Province of Minas Gerais, where several crustal\\-scale structures associated with iron oxide mineralization were identified ( Mataragio et. al., 2006). In addition, in 2006 Air\\-FTGR had good success in the regional mapping of structures associated with Iron Oxide Copper Gold (IOCG) and uranium mineralization in the Wernecke Mountains in the Yukon, and Northwest Territories, Canada. On the basis of these successful surveys, Bell Geospace has initiated a number of high altitude test surveys aiming at evaluating the performance of the Air\\-FTGR system in capturing low frequency signal that may be associated with regional\\-scale, deeper structures. One of the test surveys was conducted in December of 2006 in Australia, where the performance of Air\\-FTGR and the conventional Airborne Gravity were evaluated. Airborne gravity is currently considered well suited for capturing low frequency signal.

Study on the stress changes due to the regional groundwater exploitation based on a 3-D fully coupled poroelastic model: An example of the North China Plain

NASA Astrophysics Data System (ADS)

Cheng, H.; Zhang, H.; Pang, Y. J.; Shi, Y.

2017-12-01

With the quick urban development, over-exploitation of groundwater resources becomes more and more intense, which leads to not only widespread groundwater depression cones but also a series of harsh environmental and geological hazards. Among which, the most intuitive phenomenon is the ground subsidence in loose sediments. However, another direct consequence triggered by the groundwater depletion is the substantial crustal deformation and potential modulation of crustal stress underneath the groundwater over-pumping zones. In our previous 3-D viscoelastic finite element model, we found that continuous over-exploitation of groundwater resources in North China Plain during the past 60 years give rise to crustal-scale uplift reaching 4.9cm, with the Coulomb failure stress decreasing by up to 12 kPa, which may inhibit the nucleation of possible big earthquake events. Furthermore, according to the effective pressure principle and lab experiments, the pore pressure may also have changed due to the reduced water level. In order to quantitatively analyze the stress changes due to the regional groundwater exploitation in North China Plain, a three-dimensional fully coupled poroelastic finite element model is developed in this study. The high resolution topography, grounwater level fluctuation, fault parameters and etc, are taken into consideration. Further, the changes of Coulomb Failure Stress, in correspondence to elastic stress and pore pressure changes induced by fluid diffusion are calculated. Meanwhile, the elastic strain energy accumulation in region due to the regional groundwater exploitation is obtained. Finally, we try to analyze the seismic risk of major faults within North China Plain to further discuss the regional seismic activities.

Applications of Geodesy to Geodynamics, an International Symposium

NASA Technical Reports Server (NTRS)

Mueller, I. I. (Editor)

1978-01-01

Geodetic techniques in detecting and monitoring geodynamic phenomena are reviewed. Specific areas covered include: rotation of the earth and polar motion; tectonic plate movements and crustal deformations (space techniques); horizontal crustal movements (terrestrial techniques); vertical crustal movements (terrestrial techniques); gravity field, geoid, and ocean surface by space techniques; surface gravity and new techniques for the geophysical interpretation of gravity and geoid undulation; and earth tides and geodesy.

Fine-scale crustal structure of the Azores Islands from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Spieker, K.; Rondenay, S.; Ramalho, R. S.; Thomas, C.; Helffrich, G. R.

2016-12-01

The Azores plateau is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, most of which lie east of the MAR. Various methods including seismic reflection, gravity, and passive seismic imaging have been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 km and 30 km, but until now models of the fine-scale crustal structure have been lacking. A comparison of the crustal structure beneath the islands that lie west and east of the MAR might give further constraints on the evolution of the islands. For example, geochemical studies carried out across the region predict the existence of volcanic interfaces that should be detected seismically within the shallow crust of some of the islands. In this study, we use data from ten seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. We query our resulting receiver functions for signals associated with the volcanic edifice, the crust-mantle boundary, and potential underplated layers beneath the various islands. The islands west of the MAR have a crustal structure comprising two discontinuities - an upper one at 1-2 km depth marking the base of the volcanic edifice, and a lower one at 10 km depth that we interpret as crust-mantle boundary. The islands east of the MAR can be subdivided into two groups. The central islands that are closer to the MAR exhibit a crustal structure similar to that of the western islands, with a volcanic edifice reaching a depth of 2 km and an average crust-mantle boundary at around 12 km depth. The easternmost islands, located on the oldest lithosphere, exhibit a more complex crustal structure with evidence for a mid-crustal interface and an underplated layer, yielding an effective crust-mantle boundary at >15 km depth. The difference in structure between proximal and distal islands might be related to the age of the plate at the time of emplacement of the islands, with an older plate providing conditions that are more favourable for basaltic underplating.

Transect across the West Antarctic rift system in the Ross Sea, Antarctica

USGS Publications Warehouse

Trey, H.; Cooper, A. K.; Pellis, G.; Della, Vedova B.; Cochrane, G.; Brancolini, Giuliano; Makris, J.

1999-01-01

In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-long geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarctic rift system. Ray-trace modeling of P- and S-waves recorded on 47 ocean bottom seismograph (OBS) records, with strong seismic arrivals from airgun shots to distances of up to 120 km, show that crustal velocities and geometries vary significantly along the transect. The three major sedimentary basins (early-rift grabens), the Victoria Land Basin, the Central Trough and the Eastern Basin are underlain by highly extended crust and shallow mantle (minimum depth of about 16 km). Beneath the adjacent basement highs, Coulman High and Central High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between the upper and lower crust at a depth of about 10 to 12 km. Local zones of high velocities and inferred high densities are observed and modeled in the crust under the axes of the three major sedimentary basins. These zones, which are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differential crustal extension across the West Antarctic rift system, with greatest extension beneath the early-rift grabens. The large amount of crustal stretching below the major rift basins may reflect the existence of deep crustal suture zones which initiated in an early stage of the rifting, defined areas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occurred in the Ross Sea during late Mesozoic time, with relatively small extension, concentrated in the western half of the Ross Sea, during Cenozoic time.

Australian Seismological Reference Model (AuSREM): crustal component

NASA Astrophysics Data System (ADS)

Salmon, M.; Kennett, B. L. N.; Saygin, E.

2013-01-01

Although Australia has been the subject of a wide range of seismological studies, these have concentrated on specific features of the continent at crustal scales and on the broad scale features in the mantle. The Australian Seismological Reference Model (AuSREM) is designed to bring together the existing information, and provide a synthesis in the form of a 3-D model that can provide the basis for future refinement from more detailed studies. Extensive studies in the last few decades provide good coverage for much of the continent, and the crustal model builds on the various data sources to produce a representative model that captures the major features of the continental structure and provides a basis for a broad range of further studies. The model is grid based with a 0.5° sampling in latitude and longitude, and is designed to be fully interpolable, so that properties can be extracted at any point. The crustal structure is built from five-layer representations of refraction and receiver function studies and tomographic information. The AuSREM crustal model is available at 1 km intervals. The crustal component makes use of prior compilations of sediment thicknesses, with cross checks against recent reflection profiling, and provides P and S wavespeed distributions through the crust. The primary information for P wavespeed comes from refraction profiles, for S wavespeed from receiver function studies. We are also able to use the results of ambient noise tomography to link the point observations into national coverage. Density values are derived using results from gravity interpretations with an empirical relation between P wavespeed and density. AuSREM is able to build on a new map of depth to Moho, which has been created using all available information including Moho picks from over 12 000 km of full crustal profiling across the continent. The crustal component of AuSREM provides a representative model that should be useful for modelling of seismic wave propagation and calculation of crustal corrections for tomography. Other applications include gravity studies and dynamic topography at the continental scale.

An intermediate level of BMP signaling directly specifies cranial neural crest progenitor cells in zebrafish.

PubMed

Schumacher, Jennifer A; Hashiguchi, Megumi; Nguyen, Vu H; Mullins, Mary C

2011-01-01

The specification of the neural crest progenitor cell (NCPC) population in the early vertebrate embryo requires an elaborate network of signaling pathways, one of which is the Bone Morphogenetic Protein (BMP) pathway. Based on alterations in neural crest gene expression in zebrafish BMP pathway component mutants, we previously proposed a model in which the gastrula BMP morphogen gradient establishes an intermediate level of BMP activity establishing the future NCPC domain. Here, we tested this model and show that an intermediate level of BMP signaling acts directly to specify the NCPC. We quantified the effects of reducing BMP signaling on the number of neural crest cells and show that neural crest cells are significantly increased when BMP signaling is reduced and that this increase is not due to an increase in cell proliferation. In contrast, when BMP signaling is eliminated, NCPC fail to be specified. We modulated BMP signaling levels in BMP pathway mutants with expanded or no NCPCs to demonstrate that an intermediate level of BMP signaling specifies the NCPC. We further investigated the ability of Smad5 to act in a graded fashion by injecting smad5 antisense morpholinos and show that increasing doses first expand the NCPCs and then cause a loss of NCPCs, consistent with Smad5 acting directly in neural crest progenitor specification. Using Western blot analysis, we show that P-Smad5 levels are dose-dependently reduced in smad5 morphants, consistent with an intermediate level of BMP signaling acting through Smad5 to specify the neural crest progenitors. Finally, we performed chimeric analysis to demonstrate for the first time that BMP signal reception is required directly by NCPCs for their specification. Together these results add substantial evidence to a model in which graded BMP signaling acts as a morphogen to pattern the ectoderm, with an intermediate level acting in neural crest specification.

Quantitative PCR estimates Angiostrongylus cantonensis (rat lungworm) infection levels in semi-slugs (Parmarion martensi)

PubMed Central

Jarvi, Susan I.; Farias, Margaret E.M.; Howe, Kay; Jacquier, Steven; Hollingsworth, Robert; Pitt, William

2013-01-01

The life cycle of the nematode Angiostrongylus cantonensis involves rats as the definitive host and slugs and snails as intermediate hosts. Humans can become infected upon ingestion of intermediate or paratenic (passive carrier) hosts containing stage L3 A. cantonensis larvae. Here, we report a quantitative PCR (qPCR) assay that provides a reliable, relative measure of parasite load in intermediate hosts. Quantification of the levels of infection of intermediate hosts is critical for determining A. cantonensis intensity on the Island of Hawaii. The identification of high intensity infection ‘hotspots’ will allow for more effective targeted rat and slug control measures. qPCR appears more efficient and sensitive than microscopy and provides a new tool for quantification of larvae from intermediate hosts, and potentially from other sources as well. PMID:22902292

The effect of crustal anisotropy on SKS splitting analysis—synthetic models and real-data observations

NASA Astrophysics Data System (ADS)

Latifi, Koorosh; Kaviani, Ayoub; Rümpker, Georg; Mahmoodabadi, Meysam; Ghassemi, Mohammad R.; Sadidkhouy, Ahmad

2018-05-01

The contribution of crustal anisotropy to the observation of SKS splitting parameters is often assumed to be negligible. Based on synthetic models, we show that the impact of crustal anisotropy on the SKS splitting parameters can be significant even in the case of moderate to weak anisotropy within the crust. In addition, real-data examples reveal that significant azimuthal variations in SKS splitting parameters can be caused by crustal anisotropy. Ps-splitting analysis of receiver functions (RF) can be used to infer the anisotropic parameters of the crust. These crustal splitting parameters may then be used to constrain the inversion of SKS apparent splitting parameters to infer the anisotropy of the mantle. The observation of SKS splitting for different azimuths is indispensable to verify the presence or absence of multiple layers of anisotropy beneath a seismic station. By combining SKS and RF observations in different azimuths at a station, we are able to uniquely decipher the anisotropic parameters of crust and upper mantle.

The South Scandinavian crust: Structural complexities from seismic reflection and refraction profiling

NASA Astrophysics Data System (ADS)

Kinck, J. J.; Husebye, E. S.; Lund, C.-E.

1991-04-01

Pioneering work on mapping the Scandinavian crust commenced in the early 1960s and since then numerous profiling surveys have been undertaken, particularly as part of the on-going EUGENO-S project. However, the most significant contribution to mapping crustal structural details came from the M.V. Mobil Search cruises in the Skagerrak and off the West coast of Norway (16 s TWT reflection profiling). All past and present crustal profiling results have been integrated to produce detailed maps of Moho depths and crustal thicknesses for South Scandinavia. The thinnest crust is found in the North Sea and Skagerrak (approximately 20 km), while East-central Sweden features very thick crust (approximately 50 km). Other interesting features are the apparent correlation between crustal thinning and sedimentation/subsidence, magmatic activity, earthquake occurrences and the tectonic age of the crust. Moho depths and the crustal thicknesses clearly reflect the tectonic evolution and the present structural features of the region investigated.

A distinct and active bacterial community in cold oxygenated fluids circulating beneath the western flank of the Mid-Atlantic ridge

PubMed Central

Meyer, Julie L.; Jaekel, Ulrike; Tully, Benjamin J.; Glazer, Brian T.; Wheat, C. Geoffrey; Lin, Huei-Ting; Hsieh, Chih-Chiang; Cowen, James P.; Hulme, Samuel M.; Girguis, Peter R.; Huber, Julie A.

2016-01-01

The rock-hosted, oceanic crustal aquifer is one of the largest ecosystems on Earth, yet little is known about its indigenous microorganisms. Here we provide the first phylogenetic and functional description of an active microbial community residing in the cold oxic crustal aquifer. Using subseafloor observatories, we recovered crustal fluids and found that the geochemical composition is similar to bottom seawater, as are cell abundances. However, based on relative abundances and functional potential of key bacterial groups, the crustal fluid microbial community is heterogeneous and markedly distinct from seawater. Potential rates of autotrophy and heterotrophy in the crust exceeded those of seawater, especially at elevated temperatures (25 °C) and deeper in the crust. Together, these results reveal an active, distinct, and diverse bacterial community engaged in both heterotrophy and autotrophy in the oxygenated crustal aquifer, providing key insight into the role of microbial communities in the ubiquitous cold dark subseafloor biosphere. PMID:26935537

Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluric data

USGS Publications Warehouse

Unsworth, M.J.; Jones, A.G.; Wei, W.; Marquis, G.; Gokarn, S.G.; Spratt, J.E.; Bedrosian, P.; Booker, J.; Leshou, C.; Clarke, G.; Shenghui, L.; Chanhong, L.; Ming, D.; Sheng, J.; Solon, K.; Handong, T.; Ledo, J.; Roberts, B.

2005-01-01

The Cenozoic collision between the Indian and Asian continents formed the Tibetan plateau, beginning about 70 million years ago. Since this time, at least 1,400 km of convergence has been accommodated by a combination of underthrusting of Indian and Asian lithosphere, crustal shortening, horizontal extrusion and lithospheric delamination. Rocks exposed in the Himalaya show evidence of crustal melting and are thought to have been exhumed by rapid erosion and climatically forced crustal flow. Magnetotelluric data can be used to image subsurface electrical resistivity, a parameter sensitive to the presence of interconnected fluids in the host rock matrix, even at low volume fractions. Here we present magnetotelluric data from the Tibetan-Himalayan orogen from 77??E to 92??E, which show that low resistivity, interpreted as a partially molten layer, is present along at least 1,000 km of the southern margin of the Tibetan plateau. The inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in Southern Tibet. ?? 2005 Nature Publishing Group.

MAGSAT investigation of crustal magnetic anomalies in the eastern Indian Ocean

NASA Technical Reports Server (NTRS)

Sailor, R. V.; Lazarewicz, A. R.

1983-01-01

Crustal magnetic anomalies in a region of the eastern Indian Ocean were studied using data from NASA's MAGSAT mission. The investigation region (0 deg to 50 deg South, 75 to 125 deg East) contains several important tectonic features, including the Broken Ridge, Java Trench, Ninetyeast Ridge, and Southeast Indian Ridge. A large positive magnetic anomaly is associated with the Broken Ridge and smaller positive anomalies correlate with the Ninetyeast Ridge and western Australia. Individual profiles of scalar data (computed from vector components) were considered to determine the overall data quality and resolution capability. A set of MAGSAT ""Quiet-Time'' data was used to compute an equivalent source crustal magnetic anomaly map of the study region. Maps of crustal magnetization and magnetic susceptibility were computed from the equivalent source dipoles. Gravity data were used to help interpretation, and a map of the ratio of magnetization to density contrasts was computed using Poisson's relation. The results are consistent with the hypothesis of induced magnetization of a crustal layer having varying thickness and composition.

Satellite and surface geophysical expression of anomalous crustal structure in Kentucky and Tennessee

NASA Technical Reports Server (NTRS)

Mayhew, M. A.; Thomas, H. H.; Wasilewski, P. J.

1981-01-01

An equivalent layer magnetization model is discussed. Inversion of long wavelength satellite magnetic anomaly data indicates a very magnetic source region centered in south central Kentucky. Refraction profiles suggest that the source of the gravity anomaly is a large mass of rock occupying much of the crustal thickness. The outline of the source delineated by gravity contours is also discernible in aeromagnetic anomaly patterns. The mafic plutonic complex, and several lines of evidence are consistent with a rift association. The body is, however, clearly related to the inferred position of the Grenville Front. It is bounded on the north by the fault zones of the 38th Parallel Lineament. It is suggested that such magnetization levels are achieved with magnetic mineralogies produced by normal oxidation and metamorphic processes and enhanced by viscous build-up, especially in mafic rocks of alkaline character.

Crustal Cooling in the Neutron Star Low-Mass X-Ray Binary KS 1731-260

NASA Astrophysics Data System (ADS)

Merritt, Rachael L.

Neutron stars in binary systems can undergo periods of accretion (outburst), where in- falling material heats the crust of the star out of thermal equilibrium with the core. When accretion stops (quiescence), we can directly observe the thermal relaxation of the crust. Crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. The neutron star X-ray transient, KS 1731-260, was in outburst for 12.5 years before returning to quiescence in 2001. Here, we present a 150 ks Chandra observation of KS 1731-260 taken in August 2015, about 14.5 years into quiescence. We find that the neutron star surface temperature is consistent with the previous observation, suggesting the crust has reached thermal equilibrium with the core. Using a theoretical thermal evolution code, we fit the observed cooling curves and constrain the core temperature, composition, and the required level of extra shallow heating.

Seismic activity monitoring in the Izvorul Muntelui dam region

NASA Astrophysics Data System (ADS)

Borleanu, Felix; Otilia Placinta, Anca; Popa, Mihaela; Adelin Moldovan, Iren; Popescu, Emilia

2016-04-01

Earthquakes occurrences near the artificial water reservoirs are caused by stress variation due to the weight of water, weakness of fractures or faults and increasing of pore pressure in crustal rocks. In the present study we aim to investigate how Izvorul Muntelui dam, located in the Eastern Carpathians influences local seismicity. For this purpose we selected from the seismic bulletins computed within National Data Center of National Institute for Earth Physics, Romania, crustal events occurred between 984 and 2015 in a range of 0.3 deg around the artificial lake. Subsequently to improve the seismic monitoring of the region we applied a cross-correlation detector on the continuous recordings of Bicaz (BIZ) seismic stations. Besides the tectonic events we detected sources within this region that periodically generate artificial evens. We couldn't emphasize the existence of a direct correlation between the water level variations and natural seismicity of the investigated area.

Isostatic and dynamic support of high topography on a North Atlantic passive margin

NASA Astrophysics Data System (ADS)

Pedersen, Vivi K.; Huismans, Ritske S.; Moucha, Robert

2016-07-01

Substantial controversy surrounds the origin of high topography along passive continental margins. Here we focus on the well-documented elevated passive margin in southwestern Scandinavia, and quantify the relative contributions of crustal isostasy and dynamic topography in controlling the present topography. We find that majority of the topography is compensated by the crustal structure, suggesting a topographic age that is in accord with the 400 Myr old Caledonian orogenesis. In addition, we propose that dynamic uplift of ∼300 m has rejuvenated existing topography locally in the coastal region over the last 10 Myr. Such uplift, combined with a general sea level fall, can help explain a variety of observations that have traditionally been interpreted in favor of a peneplain uplift model. We conclude that high topography along the Scandinavian margin cannot represent remnants of a peneplain uplifted within the last 20 Myr. The topography must have been high since the Caledonian orogeny.

Rapid changes in the electrical state of the 1999 Izmit earthquake rupture zone

PubMed Central

Honkura, Yoshimori; Oshiman, Naoto; Matsushima, Masaki; Barış, Şerif; Kemal Tunçer, Mustafa; Bülent Tank, Sabri; Çelik, Cengiz; Çiftçi, Elif Tolak

2013-01-01

Crustal fluids exist near fault zones, but their relation to the processes that generate earthquakes, including slow-slip events, is unclear. Fault-zone fluids are characterized by low electrical resistivity. Here we investigate the time-dependent crustal resistivity in the rupture area of the 1999 Mw 7.6 Izmit earthquake using electromagnetic data acquired at four sites before and after the earthquake. Most estimates of apparent resistivity in the frequency range of 0.05 to 2.0 Hz show abrupt co-seismic decreases on the order of tens of per cent. Data acquired at two sites 1 month after the Izmit earthquake indicate that the resistivity had already returned to pre-seismic levels. We interpret such changes as the pressure-induced transition between isolated and interconnected fluids. Some data show pre-seismic changes and this suggests that the transition is associated with foreshocks and slow-slip events before large earthquakes. PMID:23820970

Research on the Crustal Deformation Characteristics in Beijing Using Insar and Gnss Technology

NASA Astrophysics Data System (ADS)

Hu, L.; Xing, C.; Dai, K.; Li, Y.; Li, Z.; Zhang, J.; Yan, R.; Xu, B.; Fan, Z.

2018-04-01

In this paper, we tried to reveal the characteristics of the crustal deformation in both the horizontal and vertical directions in Beijing using InSAR and GNSS observations. Regarding the serious land subsidence in Beijing plain, we also analysed the mechanism of the occurrence and development of the subsidence in combination with the tectonic settings. The GNSS results reveal that the crust in Beijing shows a significant left-lateral trend movement in the horizontal direction, while the vertical direction shows a gentle rise in the mountainous region and a significant subsidence in the plain area. The INSAR results shows a detailed subsidence area and the deformation characteristics were analyzed considering the fault activity. The foundation of geological structure dominates the subsiding in the Beijing Plain. The exploitation of groundwater exacerbates the level of subsidence and has new development. The active faults controlled the development of the subsiding in present days.

Geochemistry of lunar crustal rocks from breccia 67016 and the composition of the moon

NASA Technical Reports Server (NTRS)

Norman, Marc D.; Taylor, Stuart R.

1992-01-01

The geochemistry of anorthositic clasts from an Apollo 16 breccia 67016 is studied in order to investigate the role of these rock types in lunar crustal evolution. The samples have aluminous, alkali-poor compositions and varied FeO and MgO contents. Three compositional groups are recognized. One group is poor in mafic constituents with low abundances of lithophile trace elements typical of lunar anorthosites, while the other two groups are more mafic and are distinguished from each other by FeO/MgO ratios greater than one in the case of ferroan noritic and less than one in the case of magnesian troctolitic. These mafic-enriched varieties have considerably higher lithophile element concentrations, at levels similar to that of the bulk lunar crust. The ferroan noritic clasts may represent a fundamental type of igneous rock in the lunar crust which has not been widely recognized.

Archean high δ18O Mg-diorite: crustal-derived melt hybridized with enriched mafic accumulated rocks

NASA Astrophysics Data System (ADS)

Wang, Dan; Guo, Jing-Hui

2016-04-01

The genesis of Mg-diorite or sanukitoids has significances to understand the crustal growth and tectonic style in Archean. The chemical compositions of minerals and rocks, whole-rock Sm-Nd isotope, zircon SIMS U-Pb ages and Hf-O isotopes of Zhulagou (ZLG) Mg-diorite and their mafic enclaves (Yinshan Block, North China Craton) were studied to place constraints on their sources and genesis, and therefore provide information about dynamic processes. The ~2520 Ma ZLG diorites have intermediate SiO2 (59.4-65.5 wt.%), high Mg# (49-52), Cr (90.4-438 ppm), Ni (15.0-95.9 ppm), Sr (436-882 ppm) and Ba (237-1206 ppm) contents with fractionated rare earth elements (REE, LaN/YbN = 9.1-40.5) and depleted high field-strength element (HFSE, e.g. Nb, Ta and Ti). These geochemical signatures are similar to those Archean high-Mg diorites and sanukitoids. However, they are sodic with low K2O/Na2O (0.14-0.49) ratios, exhibiting an affinity with Archean trondhjemite-tonalite-granodiorite (TTG). Abundant coeval amphibole-bearing mafic enclaves (~2525 Ma) are enclosed within the ZLG diorites. They display low SiO2 (46.5-50.3 wt.%) contents but high concentrations of MgO (9.0-14.5 wt.%), Cr (647-1946 ppm) and Ni (197-280 ppm). They are enriched in K2O (0.64-3.43 wt.%) and large ion lithophile element (LILE), depleted in Nb, Ta and Ti. Combined with their concave REE patterns and prominent negative Eu anomaly, we suggest that they are cumulates of the melt which probably derived from subduction-related Archean metasomatized mantle source. Mineral trace element modelling results, similar ɛNd(t) (+0.6 to +2.3) and δ18O(Zrc) values (~8.6-9.0 ‰) of the diorites and mafic enclaves, strongly reflect that they had experienced intense interaction and hybridization. Evolved whole-rock Nd isotopes (TDM = 2.80-2.70 Ga), variable zircon ɛHf (t) (-1.6 to +6.0) and high δ18O (~9.0 ‰) values of the diorites indicate that they most likely originated from melting of an older continental crust (≥ 2.65 Ga). The identified Archean highest δ18O(Zrc) (~9.0 ‰) magmatism further demonstrates that supra-crustal sediments or fluids have been transferred into the lower continental crust. All our observations provide first evidence that Archean high-Mg rocks (sanukitoids) can also form by partial melting of lower crust that hybridized with enriched mafic rocks. The associated mantle-crust interaction, large-scale crustal anatexis and high-grade metamorphism were probably induced by rollback of oceanic slab in a subduction zone in the Yinshan Block of the NCC during 2.52-2.50 Ga. KEY WORDS: Archean lower crust; mafic cumulates; North China Craton; sanukitoids; zircon oxygen isotope

A Xenolith Perspective on the Composition and Age of the Northern Tanzanian Lithosphere (Invited)

NASA Astrophysics Data System (ADS)

Rudnick, R. L.; Aulbach, S.; Bellucci, J. J.; Blondes, M. S.; Chesley, J.; Lee, C.; Mansur, A. T.; Manya, S.; McDonough, W. F.

2009-12-01

Study of deep crustal and upper mantle xenoliths from rift volcanoes throughout northern Tanzania provides insights into the architecture of the Tanzanian lithosphere, as well as the interaction of this lithosphere with rift magmas. Like the upper crust, the lower crust and mantle lithosphere of the Tanzanian Craton (TC) and Mozambique Belt (MB) are Archean, but the lower crust of the MB has been thermally reactivated during the pan-African Orogeny, whereas that of the craton has not. In addition, both mantle sections have experienced interaction and heating associated with rift magmas. Cratonic lithospheric mantle is compositionally stratified, with highly refractory but strongly LREE-enriched peridotite comprising the bulk of the section (40-130 km depth), underlain by more fertile and deformed peridotites (130-150 km depth), which are also LREE-enriched. Lithospheric mantle of the MB is highly variable in thickness, ranging from a maximum of ˜150 km at Lashaine to <50 km within the Rift axis near the Kenyan border. Like the cratonic lithosphere, this mantle is also refractory and yields Archean Os model ages throughout. Mantle lithospheres of both the TC and MB have interacted with rift magmas, including carbonatites (at Olmani) and alkali basalts (s.l.), which, in some cases, precipitated veins containing phlogopite or amphibole. Late Pleistocene zircons in one of these veins testify to the youth of this interaction. Rift basalt precipitates that formed in the mantle (pyroxenites and glimmerites) and have, thus, never interacted with continental crust, have radiogenic Os isotopic compositions (γOs = +9), providing strong evidence for a plume source of the rift magmas. Sr and Nd isotopes in cpx from peridotites are highly variable: in some they are completely overprinted by rift magmas, whereas others contain Archean components. Granulite-facies xenoliths throughout northern Tanzania are generally mafic (including anorthositic compositions), with a few intermediate compositions; no granulite-facies metapelites have been found. Marbles, schists, quartzites and amphibolites from the MB likely derive from middle-crustal depths. All zircon U-Pb ages are Archean (≥ 2.6 Ga) and many of the samples fall along a 2.6 Ga Sm-Nd reference line. U-Pb thermochronology largely records slow cooling in the MB following the Pan-African Orogeny and is consistent with a present-day conductive geotherm of 47 mW/m2 in a crust with very low heat production (see Blondes et al., this meeting). Despite the fact that ɛNd varies from -4 to -32 in the lower crustal xenoliths, 87Sr/86Sr is much less variable and the data fall along a near-vertical trend in a Sr vs. Nd isotope plot, reflecting ancient Rb depletion relative to Sr. Similarly, the unradiogenic Pb in granulite feldspars from both TC and MB is consistent with ancient U depletion. Collectively, such distinctive radiogenic isotope characteristics can serve as a diagnostic signature of crustal assimilation in rift magmas from northern Tanzania.

Workshop on the Growth of Continental Crust

NASA Technical Reports Server (NTRS)

Ashwal, Lewis D. (Editor)

1988-01-01

Constraints and observations were discussed on a fundamental unsolved problem of global scale relating to the growth of planetary crusts. All of the terrestrial planets were considered, but emphasis was placed on the Earth's continental crust. The title of each session is: (1) Extraterrestrial crustal growth and destruction; (2) Constraints for observations and measurements of terrestrial rocks; (3) Models of crustal growth and destruction; and (4) Process of crustal growth and destruction.

Regional geology and geophysics of the Jemez Mountains

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

West, F.G.

1973-08-01

The western margin of the Rocky Mountain tectonic belt is the initial site for the Los Alamos Geothermal Project. lgneous activity in the area culminated with the formation of a collapsed volcanic caldera and the deposition of thick beds of tuff. Geophysical studies indicate that the region is one of relatively highterrestrial heat flow, low-crustal density, low-crustal seismic velocities, low-crustal magnetoelectric impedance, and thin crust. 34 references. (auth)

Evaluating Crustal Contamination Effects On The Lithophile Trace Element Budget Of Shergottites, NWA 856 As A Test Case

NASA Technical Reports Server (NTRS)

Brandon, A. D.; Ferdous, J.; Peslier, A. H.

2017-01-01

The issue of whether crustal contamination has affected the lithophile trace element budget of shergottites has been a point of contention for decades. The evaluation has focused on the enriched shergottite compositions as an outcome of crustal contamination of mantle-derived parent magmas or, alternatively, the compositions of these stones reflect an incompatible trace element (ITE) enriched mantle source.

A Regional Seismic Travel Time Model for North America

DTIC Science & Technology

2010-09-01

velocity at the Moho, the mantle velocity gradient, and the average crustal velocity. After tomography across Eurasia, rigorous tests find that Pn...velocity gradient, and the average crustal velocity. After tomography across Eurasia rigorous tests find that Pn travel time residuals are reduced...and S-wave velocity in the crustal layers and in the upper mantle. A good prior model is essential because the RSTT tomography inversion is invariably

Large-scale subduction of continental crust implied by India-Asia mass-balance calculation

NASA Astrophysics Data System (ADS)

Ingalls, Miquela; Rowley, David B.; Currie, Brian; Colman, Albert S.

2016-11-01

Continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle. When two continents collide, the mass balance for the continental crust is therefore assumed to be maintained. Here we use estimates of pre-collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the India-Asia collisional system. Using the current best estimates for the timing of the diachronous onset of collision between India and Eurasia, we find that about 50% of the pre-collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at Earth's surface today--represented by the mass preserved in the thickened crust that makes up the Himalaya, Tibet and much of adjacent Asia, as well as southeast Asian tectonic escape and exported eroded sediments. This implies large-scale subduction of continental crust during the collision, with a mass equivalent to about 15% of the total oceanic crustal subduction flux since 56 million years ago. We suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent-continent collisions is reflected in some ocean crust and ocean island basalt geochemistry. The subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry.

Upper crustal densities derived from sea floor gravity measurements: Northern Juan De Fuca Ridge

USGS Publications Warehouse

Holmes, Mark L.; Johnson, H. Paul

1993-01-01

A transect of sea floor gravity stations has been analyzed to determine upper crustal densities on the Endeavour segment of the northern Juan de Fuca Ridge. Data were obtained using ALVIN along a corridor perpendicular to the axis of spreading, over crustal ages from 0 to 800,000 years. Calculated elevation factors from the gravity data show an abrupt increase in density with age (distance) for the upper 200 m of crust. This density change is interpreted as a systematic reduction in bulk porosity of the upper crustal section, from 23% for the axial ridge to 10% for the off-axis flanking ridges. The porosity decrease is attributed to the collapse and filling of large-scale voids as the abyssal hills move out of the crustal formation zone. Forward modeling of a plausible density structure for the near-axis region agrees with the observed anomaly data only if the model includes narrow, along-strike, low-density regions adjacent to both inner and outer flanks of the abyssal hills. The required low density zones could be regions of systematic upper crustal fracturing and faulting that were mapped by submersible observers and side-scan sonar images, and whose presence was suggested by the distribution of heat flow data in the same area.

Local Wave Propagation and Crustal Structure Tomography in Northern Mississippi Embayment

NASA Astrophysics Data System (ADS)

Yang, Y.; Langston, C. A.

2016-12-01

Several datasets in the vicinity of the New Madrid Seismic Zone (NMSZ) are used to study local wave propagation and crustal structure in this region, including data collected for the Northern Embayment Lithosphere Experiment (NELE) project, Transportable Array, New Madrid Cooperative Network and Embayment Seismic Excitation Experiment (ESEE). Focal mechanisms and focal depths are determined with the help of synthetic seismograms for earthquakes with magnitude larger than 3. The thick unconsolidated sediment complicates waveforms inside the Mississippi Embayment by producing large converted PS, SP phases and reverberations that mask important near-source depth phases. Modeling events with well-constrained focal mechanisms using synthetic seismograms reveals a variety of waveguide propagation effects including P and S sediment reverberations as well as leaky mode P wave trains. Substantial differences in the travel time of the mid-crustal reflection are observed for waves traveling in different directions. The travel time of the mid-crustal reflection waves and direct waves are then used in a tomography for the crustal structure. The result reveals that there is a significant southwest dip to the top of the mid-crust in the vicinity of the NMSZ. Resulting image and the determined source parameters are essential for full waveform inversion to determine high-resolution crustal structure of the Northern Mississippi Embayment.

Contemporary crustal movement of southeastern Tibet: Constraints from dense GPS measurements

PubMed Central

Pan, Yuanjin; Shen, Wen-Bin

2017-01-01

The ongoing collision between the Indian plate and the Eurasian plate brings up N-S crustal shortening and thickening of the Tibet Plateau, but its dynamic mechanisms remain controversial yet. As one of the most tectonically active regions of the world, South-Eastern Tibet (SET) has been greatly paid attention to by many geoscientists. Here we present the latest three-dimensional GPS velocity field to constrain the present-day tectonic process of SET, which may highlight the complex vertical crustal deformation. Improved data processing strategies are adopted to enhance the strain patterns throughout SET. The crustal uplifting and subsidence are dominated by regional deep tectonic dynamic processes. Results show that the Gongga Shan is uplifting with 1–1.5 mm/yr. Nevertheless, an anomalous crustal uplifting of ~8.7 mm/yr and negative horizontal dilation rates of 40–50 nstrain/yr throughout the Longmenshan structure reveal that this structure is caused by the intracontinental subduction of the Yangtze Craton. The Xianshuihe-Xiaojiang fault is a major active sinistral strike-slip fault which strikes essentially and consistently with the maximum shear strain rates. These observations suggest that the upper crustal deformation is closely related with the regulation and coupling of deep material. PMID:28349926

New Crustal Thickness for Djibouti, Afar, Using Seismic Techniques

NASA Astrophysics Data System (ADS)

Dugda, Mulugeta; Bililign, Solomon

2008-10-01

Crustal thickness and Poisson's ratio for the seismic station ATD in Djibouti, Afar, has been investigated using two seismic techniques (H-κ stacking of receiver functions and a joint inversion of receiver functions and surface wave group velocities). Both techniques give consistent results of crustal thickness 23±1.5 km and Poisson's ratio 0.31±0.02. We also determined a mean P-wave velocity (Vp) of ˜6.2 km/s but ˜6.9-7.0 km/s below a 2 - 5 km thick low velocity layer at the surface. Previous studies of crustal structure for Djibouti reported that the crust is 6 to 11 km thick while our study shows that the crust beneath Djibouti is between 20 and 25 km. This study argues that the crustal thickness values reported for Djibouti for the last 3 decades were not consistent with the reports for the other neighboring region in central and eastern Afar. Our results for ATD in Djibouti, however, are consistent with the reports of crustal thickness in many other parts of central and eastern Afar. We attribute this difference to how the Moho (the crust-mantle discontinuity) is defined (an increase of Vp to 7.4 km/s in this study vs. 6.9 km/s in previous studies).

Lower crustal flow and the role of shear in basin subsidence: An example from the Dead Sea basin

USGS Publications Warehouse

Al-Zoubi, A.; ten Brink, Uri S.

2002-01-01

We interpret large-scale subsidence (5–6 km depth) with little attendant brittle deformation in the southern Dead Sea basin, a large pull-apart basin along the Dead Sea transform plate boundary, to indicate lower crustal thinning due to lower crustal flow. Along-axis flow within the lower crust could be induced by the reduction of overburden pressure in the central Dead Sea basin, where brittle extensional deformation is observed. Using a channel flow approximation, we estimate that lower crustal flow would occur within the time frame of basin subsidence if the viscosity is ≤7×1019–1×1021 Pa s, a value compatible with the normal heat flow in the region. Lower crustal viscosity due to the strain rate associated with basin extension is estimated to be similar to or smaller than the viscosity required for a channel flow. However, the viscosity under the basin may be reduced to 5×1017–5×1019 Pa s by the enhanced strain rate due to lateral shear along the transform plate boundary. Thus, lower crustal flow facilitated by shear may be a viable mechanism to enlarge basins and modify other topographic features even in the absence of underlying thermal anomalies.

GLIMPCE Seismic reflection evidence of deep-crustal and upper-mantle intrusions and magmatic underplating associated with the Midcontinent Rift system of North America

USGS Publications Warehouse

Behrendt, John C.; Hutchinson, D.R.; Lee, M.; Thornber, C.R.; Tréhu, A.; Cannon, W.; Green, A.

1990-01-01

Deep-crustal and Moho reflections, recorded on vertical incidence and wide angle ocean bottom Seismometer (OBS) data in the 1986 GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) experiment, provide evidence for magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension in the Midcontinent Rift system at 1100 Ma. The rift fill consists of 20-30 km (7-10 s) of basalt flows, secondary syn-rift volcaniclastic and post-basalt sedimentary rock. Moho reflections recorded in Lake Superior over the Midcontinent Rift system have times from 14-18 s (about 46 km to as great as 58 km) in contrast to times of about 11-13 s (about 36-42 km crustal thickness) beneath the surrounding Great Lakes. The Seismically complex deep-crust to mantle transition zone (30-60 km) in north-central Lake Superior, which is 100 km wider than the rift half-graben, reflects the complicated products of tectonic and magmatic interaction of lower-crustal and mantle components during evolution or shutdown of the aborted Midcontinent Rift. In effect, mantle was changed into crust by lowering Seismic velocity (through intrusion of lower density magmatic rocks) and increasing Moho (about 8.1 km s-1 depth. 

Evidence and models for lower crustal flow beneath the Galápagos platform

NASA Astrophysics Data System (ADS)

Orellana-Rovirosa, Felipe; Richards, Mark

2016-01-01

The volcanic Galápagos Islands are constructed upon a broad platform, with their active westernmost islands marking the current position of the hotspot. Built upon young oceanic lithosphere (<15 Ma), this platform exhibits unique morphologic features including a system of stepped terraces on the southwestern escarpment with 3 km relief, contrasting with gentle slopes off the eastern platform toward the Carnegie Ridge. Considering horizontal lithostatic pressure differences associated with this relief, along with high temperatures within the young, hotspot-affected lithosphere, it is likely that lower crustal flow contributes significantly to crustal deformation within the Galápagos platform. Using a 2-D, isostatic, thin-sheet approximation for the Stokes flow equation with (Newtonian) space-time-dependent viscosity, we suggest that the bathymetric rim along the eastern platform region (where gravimetry indicates Airy isostasy) near Española Island may be the expression of a mature lower crustal flow front developed over the last ˜3 Myr; horizontal mass displacements (˜50 km) associated with this crustal flow episode may have advected mantle plume geochemical signatures toward the southeast, and in directions not necessarily parallel to the hotspot track. Also, the stepped terraces along the southwestern platform may be explained by lower crustal flow-associated backward tilting of the bathymetric surface that, although resulting in small angular changes (˜0.1°), effectively hinders the horizontal flow of lava sheets. This backward-tilting process was likely restricted to the last ˜1 Ma or less, and may be a unique event involving extrusion of lavas from within the southwestward-marching lower-crustal flow front.

Crustal structure of north Peru from analysis of teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Condori, Cristobal; França, George S.; Tavera, Hernando J.; Albuquerque, Diogo F.; Bishop, Brandon T.; Beck, Susan L.

2017-07-01

In this study, we present results from teleseismic receiver functions, in order to investigate the crustal thickness and Vp/Vs ratio beneath northern Peru. A total number of 981 receiver functions were analyzed, from data recorded by 28 broadband seismic stations from the Peruvian permanent seismic network, the regional temporary SisNort network and one CTBTO station. The Moho depth and average crustal Vp/Vs ratio were determined at each station using the H-k stacking technique to identify the arrival times of primary P to S conversion and crustal reverberations (PpPms, PpSs + PsPms). The results show that the Moho depth correlates well with the surface topography and varies significantly from west to east, showing a shallow depth of around 25 km near the coast, a maximum depth of 55-60 km beneath the Andean Cordillera, and a depth of 35-40 km further to the east in the Amazonian Basin. The bulk crustal Vp/Vs ratio ranges between 1.60 and 1.88 with the mean of 1.75. Higher values between 1.75 and 1.88 are found beneath the Eastern and Western Cordilleras, consistent with a mafic composition in the lower crust. In contrast values vary from 1.60 to 1.75 in the extreme flanks of the Eastern and Western Cordillera indicating a felsic composition. We find a positive relationship between crustal thickness, Vp/Vs ratio, the Bouguer anomaly, and topography. These results are consistent with previous studies in other parts of Peru (central and southern regions) and provide the first crustal thickness estimates for the high cordillera in northern Peru.

Seismic crustal structure of the North China Craton and surrounding area: Synthesis and analysis

NASA Astrophysics Data System (ADS)

Xia, B.; Thybo, H.; Artemieva, I. M.

2017-07-01

We present a new digital model (NCcrust) of the seismic crustal structure of the Neoarchean North China Craton (NCC) and its surrounding Paleozoic-Mesozoic orogenic belts (30°-45°N, 100°-130°E). All available seismic profiles, complemented by receiver function interpretations of crustal thickness, are used to constrain a new comprehensive crustal model NCcrust. The model, presented on a 0.25° × 0.25°grid, includes the Moho depth and the internal structure (thickness and velocity) of the crust specified for four layers (the sedimentary cover, upper, middle, and lower crust) and the Pn velocity in the uppermost mantle. The crust is thin (30-32 km) in the east, while the Moho depth in the western part of the NCC is 38-44 km. The Moho depth of the Sulu-Dabie-Qinling-Qilian orogenic belt ranges from 31 km to 51 km, with a general westward increase in crustal thickness. The sedimentary cover is 2-5 km thick in most of the region, and typical thicknesses of the upper crust, middle crust, and lower crust are 16-24 km, 6-24 km, and 0-6 km, respectively. We document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence, the depth of the Moho. There is no systematic regional pattern in the average crustal Vp velocity and the Pn velocity. We examine correlation between the Moho depth and topography for seven tectonic provinces in the North China Craton and speculate on mechanisms of isostatic compensation.

Crustal thickness and Vp/Vs beneath the southeastern United States: Constraints from receiver function stacking

NASA Astrophysics Data System (ADS)

Yang, Q.; Gao, S. S.; Liu, K. H.

2017-12-01

To provide new constraints on crustal structure and evolution models beneath a collage of tectonic provinces in the southeastern United States, a total of 10,753 teleseismic receiver functions recorded by 125 USArray and other seismic stations are used to compute crustal thickness and Vp/Vs values. The resulting crustal thicknesses range from 25 km at the coast to 51 km beneath the peak of the southern Appalachians with an average of 36.2 km ± 5.5 km. The resulting crustal thicknesses correlate well with surface elevation and Bouguer gravity anomalies. Beneath the Atlantic Coastal Plain, the crustal thicknesses show a clear eastward thinning with a magnitude of 10 km, from about 40 km beneath the western margin to 30 km beneath the coast. The Vp/Vs values for the entire study area range from 1.71 to 1.90 with a mean value of 1.80 ± 0.04. The mean Vp/Vs value is 1.82±0.035 in the southern Appalachian Mountain. The slightly larger than normal crustal Vp/Vs for this area might be the result of significant erosion of the felsic upper crust over the past 300 million years. Alternatively, it could also suggest the existence of pervasive magmatic intrusion into the Appalachian crust. The Vp/Vs measurements in the Atlantic Coastal Plain increase toward the east, ranging from 1.75 to 1.82, probably indicating a gradual increase of mafic magmatic intrusion into thinner crust during the development of the passive continental margin.

Development of real-time mobile-buoy observation system for tsunami and crustal movement

NASA Astrophysics Data System (ADS)

Takahashi, N.; Ishihara, Y.; Fukuda, T.; Tahara, J.; Ochi, H.; Mori, T.; Deguchi, M.; Kido, M.; Ohta, Y.; Hino, R.; Mutoh, K.; Hashimoto, G.; Motohashi, O.; Kaneda, Y.

2014-12-01

We have developed real-time buoy system for tsunami and crustal movement since 2012. Our motivations are needs of the crustal movement data of not only for vertical component but horizontal, real-time data transmission for future prediction of the next large earthquake, and needs of relatively easily system comparing with the seafloor cable network system. Therefore, we are developing the above system using a buoy system, considering long term observation of approximately two years. Our system's characteristics are real-time observation, separation between tsunami and crustal movement, mobility, and environmental compatibility. Tsunami and crustal movement data are sent with intervals of an hour and a week respectively in real-time, and we can also get them on-demand via satellite transmission from the land station. We are going to observe tsunami using a pressure sensor and a PPP (precise point positioning) navigation system on the buoy, therefore, tsunami and vertical crustal deformation are separated in real-time. And the horizontal component of the crustal deformation is also measured by acoustic signals between the buoy and six seafloor transponders. Our system can be used under severe strong sea current with a speed of 5.5 knots due to adaption of slack mooring. Therefore, we can deploy it without consideration of sea current. In addition, the geometry including the size of the buoy, lengths of some ropes, and capacity of the electric battery and so on is tuned considering an environment of deployment location. Through twice sea trials, we are confirming each function. In this presentation, we introduce the outline and results of the sea trials.

Comparison of publically available Moho depth and crustal thickness grids with newly derived grids by 3D gravity inversion for the High Arctic region.

NASA Astrophysics Data System (ADS)

Lebedeva-Ivanova, Nina; Gaina, Carmen; Minakov, Alexander; Kashubin, Sergey

2016-04-01

We derived Moho depth and crustal thickness for the High Arctic region by 3D forward and inverse gravity modelling method in the spectral domain (Minakov et al. 2012) using lithosphere thermal gravity anomaly correction (Alvey et al., 2008); a vertical density variation for the sedimentary layer and lateral crustal variation density. Recently updated grids of bathymetry (Jakobsson et al., 2012), gravity anomaly (Gaina et al, 2011) and dynamic topography (Spasojevic & Gurnis, 2012) were used as input data for the algorithm. TeMAr sedimentary thickness grid (Petrov et al., 2013) was modified according to the most recently published seismic data, and was re-gridded and utilized as input data. Other input parameters for the algorithm were calibrated using seismic crustal scale profiles. The results are numerically compared with publically available grids of the Moho depth and crustal thickness for the High Arctic region (CRUST 1 and GEMMA global grids; the deep Arctic Ocean grids by Glebovsky et al., 2013) and seismic crustal scale profiles. The global grids provide coarser resolution of 0.5-1.0 geographic degrees and not focused on the High Arctic region. Our grids better capture all main features of the region and show smaller error in relation to the seismic crustal profiles compare to CRUST 1 and GEMMA grids. Results of 3D gravity modelling by Glebovsky et al. (2013) with separated geostructures approach show also good fit with seismic profiles; however these grids cover the deep part of the Arctic Ocean only. Alvey A, Gaina C, Kusznir NJ, Torsvik TH (2008). Integrated crustal thickness mapping and plate recon-structions for the high Arctic. Earth Planet Sci Lett 274:310-321. Gaina C, Werner SC, Saltus R, Maus S (2011). Circum-Arctic mapping project: new magnetic and gravity anomaly maps of the Arctic. Geol Soc Lond Mem 35, 39-48. Glebovsky V.Yu., Astafurova E.G., Chernykh A.A., Korneva M.A., Kaminsky V.D., Poselov V.A. (2013). Thickness of the Earth's crust in the deep Arctic Ocean: results of a 3D gravity modeling Russian Geology and Geophysics 54, 247-262. Jakobsson M, Mayer L, Coakley B, Dowdeswell JA, Forbes S, Fridman B, Hodnesdal H, Noormets R, Pedersen R, Rebesco M, Schenke HW, Zarayskaya Y, Accettella D, Armstrong A, Anderson RM, Bienhoff P, Camerlenghi A, Church I, Edwards M, Gardner JV, Hall JK, Hell B, Hestvik O, Krist-offersen Y, Marcussen C, Mohammad R, Mosher D, Nghiem SV, Pedrosa MT, Travaglini PG, Weatherall P (2012). The international bathymetric chart of the Arctic Ocean (IBCAO) version 3.0. Geophys Res Lett 39, L12609. Laske, G., Masters., G., Ma, Z. and Pasyanos, M. (2013). Update on CRUST1.0 - A 1-degree Global Model of Earth's Crust, Geophys. Res. Abstracts, 15, Abstract EGU2013-2658, 2013. Minakov A, Faleide JI, Glebovsky VY, Mjelde R (2012) Structure and evolution of the northern Barents-Kara Sea continental margin from integrated analysis of potential fields, bathymetry and sparse seismic data. Geophys J Int 188, 79-102. Petrov O., Smelror M., Shokalsky S., Morozov A., Kashubin S., Grikurov G., Sobolev N., Petrov E., (2013). A new international tectonic map of the Arctic (TeMAr) at 1:5 M scale and geodynamic evolution in the Arctic region. EGU2013-13481. Reguzzoni, M., & Sampietro, D. (2014). GEMMA: An Earth crustal model based on GOCE satellite data. International Journal of Applied Earth Observation and Geoinformation Spasojevic S. & Gurnis M., (2012). Sea level and vertical motion of continents from dynamic earth models since the late Cretaceous. American Association of Petroleum Geologists Bulletin, 96, pp. 2037-2064.

Insights on the seismotectonics of the western part of northern Calabria (southern Italy) by integrated geological and geophysical data: Coexistence of shallow extensional and deep strike-slip kinematics

NASA Astrophysics Data System (ADS)

Ferranti, L.; Milano, G.; Pierro, M.

2017-11-01

We assess the seismotectonics of the western part of the border area between the Southern Apennines and Calabrian Arc, centered on the Mercure extensional basin, by integrating recent seismicity with a reconstruction of the structural frame from surface to deep crust. The analysis of low-magnitude (ML ≤ 3.5) events occurred in the area during 2013-2017, when evaluated in the context of the structural model, has revealed an unexpected complexity of seismotectonics processes. Hypocentral distribution and kinematics allow separating these events into three groups. Focal mechanisms of the shallower (< 9 km) set of events show extensional kinematics. These results are consistent with the last kinematic event recorded on outcropping faults, and with the typical depth and kinematics of normal faulting earthquakes in the axial part of southern Italy. By contrast, intermediate ( 9-17 km) and deep ( 17-23 km) events have fault plane solutions characterized by strike- to reverse-oblique slip, but they differ from each other in the orientation of the principal axes. The intermediate events have P axes with a NE-SW trend, which is at odds with the NW-SE trend recorded by strike-slip earthquakes affecting the Apulia foreland plate in the eastern part of southern Italy. The intermediate events are interpreted to reflect reactivation of faults in the Apulia unit involved in thrust uplift, and appears aligned along an WNW-ESE trending deep crustal, possibly lithospheric boundary. Instead, deep events beneath the basin, which have P-axis with a NW-SE trend, hint to the activity of a deep overthrust of the Tyrrhenian back-arc basin crust over the continental crust of the Apulia margin, or alternatively, to a tear fault in the underthrust Apulia plate. Results of this work suggest that extensional faulting, as believed so far, does not solely characterizes the seismotectonics of the axial part of the Southern Apennines.

Mineral complexities as evidence for open-system processes in intermediate magmas of the Mount Baker volcanic field, northern Cascade arc

NASA Astrophysics Data System (ADS)

Escobar-Burciaga, R. D.; DeBari, S. M.

2015-12-01

The petrogenesis of intermediate magmas in arcs is a critical contribution to crustal growth. Andesites are commonly thought of as a hybrid product, the result of two endmember magmas mixing. At the Mount Baker volcanic field (MBVF), northern Cascade arc, andesites are the predominantly erupted lavas since 1 Ma and yet their origin is poorly constrained. Previous studies have suggested that open-system processes play a dominant role. However, the studies rely heavily on bulk rock compositions and overlook complex mineral textures and compositions. To better understand the complex processes at work at MBVF, we focus on establishing mineral and crystal clot populations in three andesitic flow units (55-59% SiO2). Petrographic and geochemical analyses suggest that variable-composition crystal clot and phenocryst populations in a single flow are related. We interpret the crystal clots to represent cumulates entrained in the erupting host magma and that related phenocrysts are disaggregates of crystal clots. The existence of common, multiple phenocryst and crystal clot populations in each flow of different age and SiO2 content provides strong evidence that intermediate magmas of MBVF are more than just the end product of mixing between two magmas. Furthermore, we suggest that most phenocrysts do not represent equilibrium products of their host liquid, evident from wide compositional ranges of ferromagnesian minerals (e.g. augite core Mg# 70-87). In fact, the most primitive phenocryst populations show the least amount of disequilibrium texture but represent assemblages expected to fractionate from basaltic to basaltic-andesitic liquids rather than equilibrium assemblages from their host bulk rock "liquid" composition. As a result, we interpret the variable SiO2 signature of the three andesitic flow units to have been obtained through the incorporation of cumulates/liquids as basaltic to basaltic-andesitic magma ascends.

Geology of the Payette National Forest and vicinity, west-central Idaho

USGS Publications Warehouse

Lund, Karen

2005-01-01

Before the Late Cretaceous, the eastern and western parts of the geologically complex Payette National Forest, as divided by the Salmon River suture, had fundamentally different geologic histories. The eastern part is underlain by Mesoproterozoic to Cambrian(?) rocks of the Laurentian (Precambrian North American) continent. Thick Mesopro-terozoic units, which are at least in part equivalent in age to the Belt Supergroup of northern Idaho and western Montana, underwent Mesoproterozoic metamorphic and deformational events, including intrusion of Mesoproterozoic plutons. Dur-ing the Neoproterozoic to early Paleozoic, the western edge of Laurentia was rifted. This event included magmatism and resulted in deposition of rift-related Neoproterozoic to Lower Cambrian(?) volcanic and sedimentary rocks above Mesopro-terozoic rocks. The western part of the forest is underlain by upper Paleozoic to lower Mesozoic island-arc volcanic and sedimentary rocks. These rocks comprise four recognized island-arc terranes that were amalgamated and intruded by intermediate-composition plutons, probably in the Late Juras-sic and Early Cretaceous, and then sutured to Laurentia along the Salmon River suture in the Late Cretaceous. The Salmon River suture formed as a right-lateral, transpressive fault. The metamorphic grade and structural complexity of the rocks increase toward the suture from both sides, and geochemical signatures in crosscutting plutonic rocks abruptly differ across the crustal boundary. Having been reactivated by younger structures, the Salmon River suture forms a north-trending topographic depression along Long Valley, through McCall, to the Goose Creek and French Creek drainages. During the last stages of metamorphism and deformation related to the suture event, voluminous plutons of the Idaho batholith were intruded east of the suture. An older plutonic series is intermediate in composition and preserved as elon-gated and deformed bodies near the suture and as parts of roof pendants to younger intrusions to the east. A younger magma series consists of undeformed, marginally peraluminous plu-tons that formed east of the suture after accretion. After suture-related compression, crustal extension resulted in voluminous volcanic and plutonic rocks of the Eocene Challis magmatic complex on the east side of the forest. Extension, from the Late Cretaceous to post-Miocene, uplifted the area of the Idaho batholith relative to the western part of the for-est and formed dominant highlands along the Snake River. Extensional basins also formed such that, in the Miocene, the Columbia River Basalt Group and related basaltic lavas flowed over most of the lower elevations on the western side of the forest and redirected erosional debris into north-trending, fault-controlled drainages and young sedimentary basins.

Geochemical and Nd isotopic constraints for the origin of Late Archean turbidites from the Yellowknife area, Northwest Territories, Canada

NASA Astrophysics Data System (ADS)

Yamashita, Katsuyuki; Creaser, Robert A.

1999-10-01

A detailed geochemical and isotopic study of Late Archean turbidites and volcanic rocks from the Yellowknife area, Slave province, was undertaken to constrain the nature of exposed crust at the time of 2.6 to 2.7 Ga crustal consolidation. The ɛNdT values of the volcanic rocks range from +1.7 to -4.4. This variation can be produced by assimilation of pre-2.8 Ga basement by a depleted mantle-derived magma, possibly followed by fractional crystallization. The turbidites are typically metamorphosed to greenschist to amphibolite facies, and where metamorphosed to greenschist facies, different units of Bouma sequence can be observed. The different units of Bouma sequence were sampled and analyzed separately to evaluate the possible differences in geochemical and isotopic signatures. The geochemical data presented here is in accord with the previously proposed model that argues for a mixture of 20% mafic-intermediate volcanic rocks, +55% felsic volcanic rocks, and +25% granitic rocks as a source of these turbidites. However, our revised calculation with the new data presented here argues for 1 to 2% input from an ultramafic source, as well as somewhat higher input from mafic-intermediate volcanic sources in the upper shale units compared to the lower sand units. The ɛNdT values of the turbidites generally are lower in the upper shale units compared to the lower sand units. Detailed inspection of trace-element data suggest that this is not an artifact of rare earth element-rich heavy minerals concentrating in the lower sand units of the turbidites, but rather is a result of “unmixing” of detritus with different ɛNdT values during sediment transportation and deposition. The upper shale units of the turbidites are isotopically compatible with a derivation mainly from crustally contaminated volcanic rocks, similar to those exposed in the Yellowknife area. The lower sand units contain a higher proportion of westerly derived plutonic rock detritus, characterized by higher ɛNdT, suggesting that there are area(s) west of Yellowknife not underlain by older (2.8-4.0 Ga) basement. The trace-element characteristics of these turbidites (i.e., Cr, Ni, La, Th, Sc, Eu/Eu∗, and GdN/YbN) are distinct from those of typical post-Archean turbidites. This observation is consistent with the models that predict that the chemical composition of the upper continental crust was slightly different in the Archean compared to post-Archean time.

Constraining the crustal root geometry beneath Northern Morocco

NASA Astrophysics Data System (ADS)

Díaz, J.; Gil, A.; Carbonell, R.; Gallart, J.; Harnafi, M.

2016-10-01

Consistent constraints of an over-thickened crust beneath the Rif Cordillera (N. Morocco) are inferred from analyses of recently acquired seismic datasets including controlled source wide-angle reflections and receiver functions from teleseismic events. Offline arrivals of Moho-reflected phases recorded in RIFSIS project provide estimations of the crustal thicknesses in 3D. Additional constraints on the onshore-offshore transition are inferred from shots in a coeval experiment in the Alboran Sea recorded at land stations in northern Morocco. A regional crustal thickness map is computed from all these results. In parallel, we use natural seismicity data collected throughout TopoIberia and PICASSO experiments, and from a new RIFSIS deployment, to obtain receiver functions and explore the crustal thickness variations with a H-κ grid-search approach. This larger dataset provides better resolution constraints and reveals a number of abrupt crustal changes. A gridded surface is built up by interpolating the Moho depths inferred for each seismic station, then compared with the map from controlled source experiments. A remarkably consistent image is observed in both maps, derived from completely independent data and methods. Both approaches document a large crustal root, exceeding 50 km depth in the central part of the Rif, in contrast with the rather small topographic elevations. This large crustal thickness, consistent with the available Bouguer anomaly data, favors models proposing that the high velocity slab imaged by seismic tomography beneath the Alboran Sea is still attached to the lithosphere beneath the Rif, hence pulling down the lithosphere and thickening the crust. The thickened area corresponds to a quiet seismic zone located between the western Morocco arcuate seismic zone, the deep seismicity area beneath western Alboran Sea and the superficial seismicity in Alhoceima area. Therefore, the presence of a crustal root seems to play also a major role in the seismicity distribution in northern Morocco.

Crustal structure of the Gulf of Aden southern margin: Evidence from receiver functions on Socotra Island (Yemen)

NASA Astrophysics Data System (ADS)

Ahmed, Abdulhakim; Leroy, Sylvie; Keir, Derek; Korostelev, Félicie; Khanbari, Khaled; Rolandone, Frédérique; Stuart, Graham; Obrebski, Mathias

2014-12-01

Breakup of continents in magma-poor setting occurs primarily by faulting and plate thinning. Spatial and temporal variations in these processes can be influenced by the pre-rift basement structure as well as by early syn-rift segmentation of the rift. In order to better understand crustal deformation and influence of pre-rift architecture on breakup we use receiver functions from teleseismic recordings from Socotra which is part of the subaerial Oligo-Miocene age southern margin of the Gulf of Aden. We determine variations in crustal thickness and elastic properties, from which we interpret the degree of extension related thinning and crustal composition. Our computed receiver functions show an average crustal thickness of ~ 28 km for central Socotra, which decreases westward along the margin to an average of ~ 21 km. In addition, the crust thins with proximity to the continent-ocean transition to ~ 16 km in the northwest. Assuming an initial pre-rift crustal thickness of 35 km (undeformed Arabian plate), we estimate a stretching factor in the range of ~ 2.1-2.4 beneath Socotra. Our results show considerable differences between the crustal structure of Socotra's eastern and western sides on either side of the Hadibo transfer zone; the east displays a clear intracrustal conversion phase and thick crust when compared with the western part. The majority of measurements across Socotra show Vp/Vs ratios of between 1.70 and 1.77 and are broadly consistent with the Vp/Vs values expected from the granitic and carbonate rock type exposed at the surface. Our results strongly suggest that intrusion of mafic rock is absent or minimal, providing evidence that mechanical thinning accommodated the majority of crustal extension. From our observations we interpret that the western part of Socotra corresponds to the necking zone of a classic magma-poor continental margin, while the eastern part corresponds to the proximal domain.

A Lower-Crust or Mantle Source for Mineralizing Fluids Beneath the Olympic Dam IOCG Deposit, Australia: New Evidence From Magnetotelluric Sounding

NASA Astrophysics Data System (ADS)

Heinson, G.

2005-12-01

The iron-oxide-copper-gold (IOCG) Olympic Dam (OD) deposit, situated along the margin of the Proterozoic Gawler Craton, South Australia, is the world's largest uranium deposit, and sixth largest copper deposit; it also contains significant reserves of gold, silver and rare-earth elements (REE). Gaining a better understanding of the mechanisms for genesis of the economic mineralisation is fundamental for defining exploration models in similar crustal-settings. To delineate crustal structures that may constrain mineral system fluid pathways, coincident deep crustal seismic and magnetotelluric (MT) transects were obtained along a 220 km section that crosses OD and the major crustal boundaries. We present results from 58 long-period (10-104 s) MT sites, with site spacing of 5 to 10 km. A 2D inversion of all MT data to a depth of 100 km shows four notable features: (a) sedimentary cover sequences with low resistivity (<20 Ω.m) thicken to 10 km towards the northern cover sequences of the Adelaide Rift Complex; (b) a northeast-dipping crustal boundary separates a highly resistive (>1000 Ω.m) Archaean crustal core, from a more conductive crust to the north (typically <500 Ω.m); (c) to the north of OD, the crust to about 20 km is quite resistive (~1000 Ω.m), but the lower crust is much more conductive (<100 Ω.m); and (d) beneath OD, we image a low-resistivity region (<100 Ω.m) throughout the crust, coincident with a seismically transparent region. We argue that the cause of the low-resistivity and low-reflectivity region beneath OD may be due to the upward movement of crustal-volatiles that have deposited conductive graphite mineralisation along grain boundaries, simultaneously annihilating acoustic impedance boundaries. The source of the volatiles may be from the mantle-degassing or retrograde metamorphism of the lower crust associated with Proterozoic crustal deformation.

Cratonic roots and lower crustal seismicity: Investigating the role of deep intrusion in the Western rift, Africa

NASA Astrophysics Data System (ADS)

Drooff, C.; Ebinger, C. J.; Lavayssiere, A.; Keir, D.; Oliva, S. J.; Tepp, G.; Gallacher, R. J.

2017-12-01

Improved seismic imaging beneath the African continent reveals lateral variations in lithospheric thickness, and crustal structure, complementing a growing crust and mantle xenolith data base. Border fault systems in the active cratonic rifts of East Africa are characterized by lower crustal seismicity, both in magmatic sectors and weakly magmatic sectors, providing constraints on crustal rheology and, in some areas, magmatic fluid migration. We report new seismicity data from magmatic and weakly magmatic sectors of the East African rift zone, and place the work in the context of independent geophysical and geochemical studies to models for strain localization during early rifting stages. Specifically, multidisciplinary studies in the Magadi Natron rift sectors reveal volumetrically large magmatic CO2 degassing along border faults with seismicity along projections of surface dips to the lower crust. The magmatic CO2 degassing and high Vp/Vs ratios and reflectivity of the lower crust implies that the border fault serves a conduit between the lower crustal underplating and the atmospheric. Crustal xenoliths in the Eastern rift sector indicate a granulitic lower crust, which is relatively weak in the presence of fluids, arguing against a strong lower crust. Within magmatic sectors, seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Within some weakly magmatic sectors, lower crustal earthquakes also occur along projections of border faults to the lower crust (>30 km), and they are prevalent in areas with high Vp/Vs in the lower crust. Within the southern Tanganyika rift, focal mechanisms are predominantly normal with steep nodal planes. Our comparative studies suggest that pervasive metasomatism above a mantle plume, and melt extraction in thin zones between cratonic roots, lead to high pore pressures that promote brittle failure in the lower crust, even in areas with no surface expression of magmatism.

Crustal structure of Baffin Bay from constrained 3-D gravity inversion and deformable plate tectonic models

NASA Astrophysics Data System (ADS)

Welford, J. Kim; Peace, Alexander L.; Geng, Meixia; Dehler, Sonya A.; Dickie, Kate

2018-05-01

Mesozoic to Cenozoic continental rifting, breakup, and spreading between North America and Greenland led to the opening, from south to north, of the Labrador Sea and eventually Baffin Bay between Baffin Island, northeast Canada, and northwest Greenland. Baffin Bay lies at the northern limit of this extinct rift, transform, and spreading system and remains largely underexplored. With the sparsity of existing crustal-scale geophysical investigations of Baffin Bay, regional potential field methods and quantitative deformation assessments based on plate reconstructions provide two means of examining Baffin Bay at the regional scale and drawing conclusions about its crustal structure, its rifting history, and the role of pre-existing structures in its evolution. Despite the identification of extinct spreading axes and fracture zones based on gravity data, insights into the nature and structure of the underlying crust have only been gleaned from limited deep seismic experiments, mostly concentrated in the north and east where the continental shelf is shallower and wider. Baffin Bay is partially underlain by oceanic crust with zones of variable width of extended continental crust along its margins. 3-D gravity inversions, constrained by bathymetric and depth to basement constraints, have generated a range of 3-D crustal density models that collectively reveal an asymmetric distribution of extended continental crust, approximately 25-30 km thick, along the margins of Baffin Bay, with a wider zone on the Greenland margin. A zone of 5 to 13 km thick crust lies at the centre of Baffin Bay, with the thinnest crust (5 km thick) clearly aligning with Eocene spreading centres. The resolved crustal thicknesses are generally in agreement with available seismic constraints, with discrepancies mostly corresponding to zones of higher density lower crust along the Greenland margin and Nares Strait. Deformation modelling from independent plate reconstructions using GPlates of the rifted margins of Baffin Bay was performed to gauge the influence of original crustal thickness and the width of the deformation zone on the crustal thicknesses obtained from the gravity inversions. These results show the best match with the results from the gravity inversions for an original unstretched crustal thickness of 34-36 km, consistent with present-day crustal thicknesses derived from teleseismic studies beyond the likely continentward limits of rifting around the margins of Baffin Bay. The width of the deformation zone has only a minimal influence on the modelled crustal thicknesses if the zone is of sufficient width that edge effects do not interfere with the main modelled domain.

Crustal Gravitational Potential Energy Change and Subduction Earthquakes

NASA Astrophysics Data System (ADS)

Zhu, P. P.

2017-05-01

Crustal gravitational potential energy (GPE) change induced by earthquakes is an important subject in geophysics and seismology. For the past forty years the research on this subject stayed in the stage of qualitative estimate. In recent few years the 3D dynamic faulting theory provided a quantitative solution of this subject. The theory deduced a quantitative calculating formula for the crustal GPE change using the mathematic method of tensor analysis under the principal stresses system. This formula contains only the vertical principal stress, rupture area, slip, dip, and rake; it does not include the horizontal principal stresses. It is just involved in simple mathematical operations and does not hold complicated surface or volume integrals. Moreover, the hanging wall vertical moving (up or down) height has a very simple expression containing only slip, dip, and rake. The above results are significant to investigate crustal GPE change. Commonly, the vertical principal stress is related to the gravitational field, substituting the relationship between the vertical principal stress and gravitational force into the above formula yields an alternative formula of crustal GPE change. The alternative formula indicates that even with lack of in situ borehole measured stress data, scientists can still quantitatively calculate crustal GPE change. The 3D dynamic faulting theory can be used for research on continental fault earthquakes; it also can be applied to investigate subduction earthquakes between oceanic and continental plates. Subduction earthquakes hold three types: (a) crust only on the vertical up side of the rupture area; (b) crust and seawater both on the vertical up side of the rupture area; (c) crust only on the vertical up side of the partial rupture area, and crust and seawater both on the vertical up side of the remaining rupture area. For each type we provide its quantitative formula of the crustal GPE change. We also establish a simplified model (called CRW Model) as follows: for Type B and Type C subduction earthquakes, if the seawater average depth on the vertical up side of the rupture area is less than a tenth of the hypocenter depth, then take the approximation that the seawater above the continental plate is replaced by the upper crustal material of the continental plate. The formula of quantitative calculating the crustal GPE change is also provided for this model. Finally, for 16 September 2015 Mw 8.3 Illapel Chile earthquake, we apply CRW Model and obtain the following results: the crustal GPE change is equal to 1.8 × 1019 J, and the hanging wall vertical moving-up height is 1.9 m with respect to the footwall. We believe this paper might be the first report on the quantitative solution of the crustal GPE change for this subduction earthquake; our results and related method will be helpful in research into the earthquakes in Peru-Chile subduction zone and the Andean orogeny. In short, this study expounds a new method for quantitative determining the crustal GPE change caused by subduction earthquakes, which is different from other existing methods.

Incoherence in the South African Labour Market for Intermediate Skills

ERIC Educational Resources Information Center

Kraak, Andre

2008-01-01

This article is concerned with the production and employment of technically skilled labour at the intermediate level in South Africa. Three differing labour market pathways to intermediate skilling are identified. These are: the traditional apprenticeship route, the new "Learnerships" pathway (similar to the "modern…

Concept for a research project in early crustal genesis

NASA Technical Reports Server (NTRS)

Phillips, R. J. (Compiler); Ashwal, L. (Compiler)

1983-01-01

Planetary volatiles, physical and chemical planetary evolution, surface processes, planetary formation, metallogenesis, crustal features and their development, tectonics, and paleobiology are discussed.

SPACE: Intermediate Level Modules.

ERIC Educational Resources Information Center

Indiana State Dept. of Education, Indianapolis. Center for School Improvement and Performance.

These modules were developed to assist teachers at the intermediate level to move away from extensive skill practice and toward more meaningful interdisciplinary learning. This packet, to be used by teachers in the summer Extended Learning Program, provides detailed thematic lesson plans matched to the Indiana Curriculum Proficiency Guide. The…