Science.gov

Sample records for grenville orogenic cycle

  1. The Grenville orogenic cycle of southern Laurentia: Unraveling sutures, rifts, and shear zones as potential piercing points for Amazonia

    NASA Astrophysics Data System (ADS)

    Bartholomew, Mervin J.; Hatcher, Robert D., Jr.

    2010-01-01

    The magnetic anomaly map of North America serves as a useful base from which to attempt palinspastic reconstruction of terranes accreted during the Elzevirian orogeny (1250-1200 Ma); the Shawinigan (1200-1150 Ma), Ottawan (1080-1020 Ma), and Rigolet (1020-1000 Ma) phases of the Grenvillian orogeny; and post-Grenvillian magmatism (760-600 Ma) and deformation prior to Iapetan rifting at 565 Ma. Accreted terranes had unique histories prior to amalgamation and share common tectonic events afterwards. Comparisons with magnetic signatures of the Paleozoic craton-craton suture, sutures of accreted terranes, and the Jurassic rifted-margin for the southern-central Appalachians provide a basis for discriminating among alternative Grenvillian sutures beneath the Appalachian orogen. The Elzevirian suture is partially preserved beneath the Appalachians where it separates the Reading Prong terrane from Laurentia (i.e., Adirondacks and composite-arc terrane and Canadian Grenville Province). The Shawinigan suture is partially preserved in the Llano area (Texas), but separated the now-fragmented and allochthonous Amazonian (as indicated from Pb-isotope data) blocks of the outboard Blue Ridge terrane from the Reading Prong terrane in the Appalachians. Isolated blocks of the Sauratown Mountains terrane are interpreted as outboard of the Blue Ridge terrane, but were also accreted during the Shawinigan phase. Within present-day Laurentia, the only fragment of a terrane believed to have been accreted during the main Ottawan phase is the Mars Hill terrane (North Carolina-Tennessee). This suggests that the outboard Ottawan suture may have served as the locus of Iapetan rifting along much of Laurentia. The Rigolet phase (1020-1000 Ma) is characterized by widespread "Basin and Range" type extension (NW-SE) associated with sinistral or dextral movement on the NY-AL lineament, mobilization of core-complexes (Adirondack Highlands), and AMCG magmatism along the outboard flank of the extensional

  2. Suturing and extensional reactivation in the Grenville orogen, Canada

    NASA Astrophysics Data System (ADS)

    Busch, Jay P.; Mezger, Klaus; van der Pluijm, Ben A.

    1997-06-01

    Sutures are zones of weakness within orogenic belts that have the potential to become reactivated during orogenic evolution. The Robertson Lake shear zone marks a major tectonic boundary in the southeastern Grenville orogen of Canada that has been intermittently active for at least 130 m.y. The shear zone played a major role in the compressional stage of the orogenic cycle as well as during postorogenic collapse. The zone separates the Elzevir terrane to the west and the Frontenac terrane to the east. Sphene ages (U-Pb) indicate that these two terranes have distinct tectonothermal histories and that the shear zone represents a “cryptic suture.” In its current state, the shear zone is a low angle (30°ESE dip) plastic to brittle extensional shear zone that separates the Mazinaw (footwall) and Sharbot Lake (hanging wall) domains. Integration of structural, metamorphic, and chronologic data leads to a model that describes the complete evolution of this fundamental tectonic boundary that evolved from an early compressional zone (ca. 1030 Ma) to a late extensional zone (until at least 900 Ma).

  3. Evolution of the Late Mesoproterozoic Cordilleran-Type (not COLLISIONAL!) Sveconorwegian (grenville) Orogen in SW Norway

    NASA Astrophysics Data System (ADS)

    Slagstad, T.; Sauer, S.; Roberts, N. M.; Marker, M. K.; Røhr, T. S.; Schiellerup, H.

    2011-12-01

    The Late Mesoproterozoic Sveconorwegian orogen in SW Baltica is traditionally interpreted as the eastward continuation of the Grenville orogen in Canada, resulting from collision with Amazonia and forming a central part in the assembly of the Rodinia supercontinent. We challenge this conventional view based on results from recent work in SW Norway that demonstrate voluminous subduction-related magmatism from ca. 1070 to 990 Ma, encompassing the period when collision is purported to have taken place. The rocks form a >100 km long and 50 km wide belt referred to as the Sirdal Magmatic Belt, intruding ca. 1500 Ma crust. Coeval granitoid magmatism up to 200 km to the east into the orogen has transitional calc-alkaline/A-type to A-type compositions, which we interpret to reflect continental back-arc magmatism. High-grade metamorphism in the region started at 1035 Ma with a peak at 1000 Ma, indicating that metamorphism may have resulted from plutonic activity, but not the other way around as would be expected in a collision setting. Following Sveconorwegian orogenesis, widespread A-type granite magmatism between 970 and 930 Ma (Hornblende-Biotite Granite Suite), and anorthosite-mangerite-charnockite magmatism (Rogaland Igneous Complex) between 930 and 915 Ma, is recorded in the Sveconorwegian Province. We interpret this evolution to suggest that the cessation of subduction-related magmatism at ca. 990 Ma did not involve continent-continent collision, but instead reflects a period of horizontal subduction between 990 and 930 Ma (or ca. 950 Ma), during which hydration of the overlying lower continental crust induced widespread crustal melting and A-type magmatism. Subsequent foundering of this underthrust oceanic crust and delamination of the arc's mantle root between 950 and 930 Ma may have given rise to the Rogaland Igneous Complex, the youngest massif-type anorthosite complex in the Grenville-Sveconorwegian orogen. Isotope data on the Sirdal Magmatic Belt indicate a

  4. Sampling Laurentia: Detrital zircon geochronology offers evidence for an extensive Neoproterozoic river system originating from the Grenville orogen

    NASA Astrophysics Data System (ADS)

    Rainbird, Robert H.; Hearnan, Larry M.; Young, Grant

    1992-04-01

    Neoproterozoic fluvial quartzarenites from the Shaler Group on Victoria Island in the western Canadian Arctic yield a diverse suite of detrital zircons. U-Pb ages from single zircons fall into three modes: Archean (3.01-2.62 Ga), Paleoproterozoic (1.97-1.84 Ga), and Meso proterozoic (1.64-1.03 Ga). Particularly intriguing is the unusually high proportion (50%) of the Mesoproterozoic mode, as the nearest exposed source is the Grenville structural province, ˜3000 km southeast of Victoria Island. Unimodal northwest paleocurrents, obtained from cross-bedding, are compatible with a southeasterly provenance. These data, supported by sedimentological evidence from the Shaler Group and from correlative strata in the northwestern Canadian Cordillera, imply the existence of a large, perennial river system flowing north-westward from the Grenville orogen. An analogy with the Llanos drainage basin in Venezuela and Colombia is proposed on the basis of similar geography, climate, and sedimentation. These data also provide a maximum age for the Shaler Group (basal Reynolds Point Formation) of about 1.11 Ga.

  5. Crustal thickness and VP/ VS variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functions

    NASA Astrophysics Data System (ADS)

    Eaton, David W.; Dineva, Savka; Mereu, Robert

    2006-06-01

    We have developed a simple semblance-weighted stacking technique to estimate crustal thickness and average VP/ VS ratio using teleseismic receiver functions. We have applied our method to data from 32 broadband seismograph stations that cover a 700 × 400 km 2 region of the Grenville orogen, a 1.2-0.98 Ga Himalayan-scale collisional belt in eastern North America. Our seismograph network partly overlaps with L ITHOPROBE and other crustal refraction surveys. In 8 out of 9 cases where a crustal-refraction profile passes within 30 km of a seismograph station, the two independent crustal thickness estimates agree to within 7%. Our regional crustal-thickness model, constructed using both teleseismic and refraction observations, ranges between 34.0 and 52.4 km. Crustal-thickness trends show a strong correlation with geological belts, but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium. The thickest crust (52.4 ± 1.7 km) was found at a station located within the 1.1 Ga mid-continent (failed) rift. The Central Gneiss Belt, which contains rocks exhumed from deep levels of the crust, is characterized by VP/ VS ranging from 1.78 to 1.85. In other parts of the Grenville orogen, VP/ VS is found to be generally less than 1.80. The thinnest crust (34.5-37.0 km) occurs northeast of the 0.7 Ga Ottawa-Bonnechere graben and correlates with areas of high intraplate seismicity.

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

    NASA Astrophysics Data System (ADS)

    Cosca, Michael A.; Sutter, John F.; Essene, Eric J.

    1991-10-01

    Stepwise 40Ar/39Ar degassing experiments of 57 mineral separates of hornblende, muscovite, biotite, and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the Grenville Orogen in Ontario. The 40Ar/39Ar data support an interpretation of slow, nearly uniform cooling (1°-4°C/m.y.) from temperatures of ˜500°C to below ˜150°C. In the Central Gneiss Belt (CGB) hornblendes cooled through Ar closure between 930 and 1025 Ma, whereas in the Central Metasedimentary Belt (CMB) hornblendes record the following range in 40Ar/39Ar cooling ages: 1104 Ma in the Frontenac terrane, 1007-1067 Ma in the Sharbot Lake terrane, 919-1026 Ma in the Elzevir terrane, and 972 Ma in the Central Metasedimentary Belt Boundary Zone. Regional uplift/erosion rates of 0.03-0.14 km/m.y. have been estimated for the Grenville Orogen in Ontario based on the 40Ar/39Ar data, a model retrograde P-T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambrian and Ordovician sediments. These erosion rates are consistent with rates estimated for other Proterozoic or Archean granulite terranes but are an order of magnitude slower than active orogens such as the Alps and Himalayas. A regular variation in hornblende 40Ar/39Ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the CMB. The pattern of 40Ar/39Ar ages is interpreted to reflect late-tectonic extension, consistent with field observations in the Central Metasedimentary Belt Boundary Zone and elsewhere in the CMB. Up to 13 km of vertical displacement is inferred for some rocks in the CMB between the time they cooled below closure to argon diffusion in hornblende (˜500°C) and their exposure at the surface (˜25°C).

  7. A Reconsideration of Pan African Orogenic Cycle in the Anti-Atlas Mountains, Morocco

    NASA Astrophysics Data System (ADS)

    Hefferan, K. P.; Soulaimani, A.; Samson, S. D.; Admou, H.; Inglis, J.; Saquaque, A.; Heywood, N. C.

    2013-12-01

    The term 'Pan African' orogeny was first proposed in 1964 for a tectonothermal event in Africa ~ 500+/- 50 Ma. Over the past 50 years, the Pan African orogeny has been extended to as much as ~1050-450 Ma and recognized in other Gondwanan continents where regional names such as Brasiliano (South America), Adelaidean (Australian) and Bearmore (Antarctica) have been applied. The Pan African time span of ~500 million years is much longer than any Phanerozoic orogeny. However, it does correlate with time ranges of well defined Phanerozoic orogenic cycles such as the Appalachian cycle, extending from ~1,100 to 250 Ma, and the Cordilleran cycle of ~350 Ma to the present. A significant difference of course is that the Appalachian orogenic cycle has long been recognized as consisting of separate Grenville, Taconic, Acadian and Alleghenian orogenies. Similarly, the Mesozoic-Cenozoic Cordilleran orogenic cycle consists of distinct Antler, Sonoma, Nevadan, Sevier, Laramide and ongoing Cascadian-Andean orogenies. Until recently, the absence of precise geochronology in West Africa has prevented a more refined analysis of individual orogenic events within the Pan-African orogenic cycle. Since 2000, precision geochronologic dating by various researchers in the Anti-Atlas Mountains, Morocco, has provided critical data by which it is now appropriate to designate a Pan African orogenic cycle consisting of three separate orogenic events. We herein propose the following distinct orogenic events in the Anti-Atlas Mountains of Morocco: Iriri-Tichibanine orogeny (750-700 Ma), Bou Azzer orogeny (660-640 Ma) and the WACadomian orogeny (620-580 Ma). Generalized tectonic map of the Anti-Atlas Mountain inliers (Adapted from Ennih and Liégeois, 2008). Geodynamic model of the Pan African orogenic cycle in the Anti-Atlas Mountains. Modified from Walsh et al. (2012) and incorporating ideas from Thomas et al. (2002) and El Hadi et al. (2012).

  8. The fate of olivine in the lower crust: Pseudomorphs after olivine in coronitic metagabbro from the Grenville Orogen, Ontario

    NASA Astrophysics Data System (ADS)

    Kendrick, J. L.; Jamieson, R. A.

    2016-09-01

    Orthopyroxene-oxide symplectites after olivine are among the most enigmatic features of corona assemblages in metagabbros. Two coronitic metagabbro bodies from the Algonquin suite in the Grenville Orogen, Ontario, contain exceptionally well preserved orthopyroxene + Fe-Ti oxide symplectite formed during prograde Ottawan (ca. 1060 Ma) granulite-facies metamorphism. Based on textural evidence, we propose a new hypothesis for the formation of these symplectites. Under oxidising conditions associated with fluid infiltration, magmatic olivine and ilmenite underwent a coupled reaction whereby magnetite produced by oxidation of olivine replaced adjacent igneous ilmenite. Ilmenite was re-precipitated as a fine-grained intergrowth with orthopyroxene and some magnetite in the former olivine sites. This hypothesis is supported by textural evidence showing partial replacement of magmatic ilmenite by magnetite and a close spatial association between magmatic oxides and orthopyroxene + Fe-Ti oxide symplectite, which locally radiates from ilmenite into olivine. Measured orthopyroxene/oxide ratios in the symplectite (20-35% oxides) agree with the ratio predicted from the proposed reaction (ca. 30%). Coronas and pseudomorphs formed during high-grade metamorphism, with increasing fO2 interpreted to result from fluid infiltration at near-peak conditions of ca. 13 kbar, 800 °C. The same samples contain red-brown fine-grained aggregates interpreted as iddingsite pseudomorphs after olivine. Raman spectroscopy suggests that the iddingsite consists largely of amorphous silica and Fe-hydroxide; textural evidence indicates that it formed by late-stage oxidation and hydration of olivine that survived earlier metamorphism. The unusual co-occurrence of granulite-facies pseudomorphs after olivine with an alteration product formed at near-surface conditions indicates that some olivine may survive protracted high-grade metamorphism in environments where fluid access is limited.

  9. Comparison of garnet-biotite, calcite-graphite, and calcite-dolomite thermometry in the Grenville Orogen; Ontario, Canada

    NASA Astrophysics Data System (ADS)

    Rathmell, Mark A.; Streepey, Margaret M.; Essene, Eric J.; van der Pluijm, Ben A.

    The Elzevir Terrane of the Grenville Orogen in southern Ontario contains metapelites and abundant graphitic marbles that were regionally metamorphosed from the upper greenschist to upper amphibolite facies. Comparative thermometry was undertaken with widely used calibrations for the systems garnet-biotite, calcite-dolomite, and calcite-graphite. Temperatures that are obtained from matrix biotites paired with prograde garnet near-rim analyses are usually consistent with those determined using calcite-graphite thermometry. However, calcite-graphite thermometry occasionally yields low temperatures due to lack of equilibration of anomalously light graphite. Application of calcite-graphite and garnet-biotite systems may yield temperatures up to 70°C higher than calcite-dolomite in amphibolite facies rocks. Calcite-dolomite temperatures most closely approach those from calcite-graphite and garnet-biotite when the samples contain a single generation of dolomite and calcite grains contain no visible dolomite exsolution lamellae. However, some of these samples yield temperatures considerably lower than temperatures calculated from calcite-graphite and garnet-biotite thermometry, indicating that the calcite-dolomite thermometer may have been partially reset during retrogression. Estimated peak metamorphic temperatures of regional metamorphism between Madoc (upper greenschist facies) and Bancroft (upper amphibolite facies) range from 500 to 650°C. These results place the chlorite-staurolite isograd at 540°C, the kyanite-sillimanite isograd at 590°C, and the sillimanite-K-feldspar isograd at 650°C. Although each thermometer may have an absolute uncertainty of as much as +/-50°C, the 50 to 60°C temperature differences between the isograds are probably accurate to 10 to 20°C. An incomplete picture of the thermal gradients can result from the application of only one thermometer in a given area. Simultaneous application of several systems allows one to recognize and

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

    SciTech Connect

    Cosca, M.A.

    1989-01-01

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

  11. Reconstructing deep crustal dynamics in a large, hot orogen: Application of integrated zircon petrochronology and petrological modeling to the Canadian Grenville Province

    NASA Astrophysics Data System (ADS)

    Marsh, Jeffrey; Culshaw, Nicholas

    2014-05-01

    The timing and conditions of high pressure (HP) metamorphism, crustal anatexis, and shear zone formation are of primary importance in understanding lithosphere-scale dynamics of collisional orogens. Within the western Grenville Province, Ontario, Canada, a number of structural and metamorphic relationships are preserved that represent specific orogenic stages or processes. Lower allochthonous domains contain variably retrogressed mafic complexes that typically retain vestages of earlier HP metamorphic assemblages (e.g. garnet porphyroblasts, kyanite, and rutile) within a matrix dominated by incomplete decompression reactions (e.g. pseudomorphous diopside + sodic plagioclase intergrowths after omphacite and concentric coronas of aluminous minerals surrounding kyanite). Zircon from these samples yield U-Pb ages between 1085-1097 Ma, and exhibit REE characteristics consistent with crystallization in an eclogite-facies (garnet-rich, plagioclase-poor) mineral assemblage. REE partitioning between zircon and garnet suggests zircon growth coincided with the latter stages of garnet growth. Titanium concentrations in zircon constrain crystallization temperatures between ~678-736 °C, whereas Zr concentration in rutile yield crystallization temperatures of ~705-740 °C (for rutile inclusions in garnet) and 742-764 °C (for rutile in the matrix). Intersection of zircon and rutile crystallization temperature with the calculated stability field for the HP assemblage (Grt+Cpx+Ky+Rt+Zrn±Hbl) yields minimum pressures of ~15 kbar. Thus, HP metamorphism apparently occurred at ca. 1090 Ma across the orogen, at minimum depths of ~53 km and T ~700±50 °C, yielding a geothermal gradient of < 15 °C/km. Widespread high-T, medium-P metamorphism and migmatization in the surrounding gneisses apparently began around 1090-1080 Ma, suggesting a rapidly evolving thermal field synchronous with the exhumation of HP rocks to mid-crustal depths. Zircon U-Pb ages and REE patterns from a large

  12. Sensitive high resolution ion microprobe (SHRIMP) detrital zircon geochronology provides new evidence for a hidden neoproterozoic foreland basin to the Grenville Orogen in the eastern Midwest, U.S.A

    USGS Publications Warehouse

    Santos, J.O.S.; Hartmann, L.A.; McNaughton, N.J.; Easton, R. M.; Rea, R.G.; Potter, P.E.

    2002-01-01

    A sensitive high resolution ion microprobe (SHRIMP) was used in combination with backscattered electron (BSE) and cathodoluminescence (CL) images to determine the age of detrital zircons from sandstones in the Neoproterozoic Middle Run Formation of the eastern Midwest, United States. Eleven samples from seven drill cores of the upper part of the Middle Run Formation contain detrital zircons ranging in age from 1030 to 1982 Ma (84 analyses), with six distinctive modes at 1.96, 1.63, 1.47, 1.34, 1.15, and 1.08 Ga. This indicates that most, but not all, of the zircon at the top of the Middle Run Formation was derived from the Grenville Orogen. The youngest concordant detrital zircon yields a maximum age of 1048 ?? 22 Ma for the Middle Run Formation, indicating that the formation is younger than ca. 1026 Ma minus the added extra time needed for later uplift, denudation, thrusting, erosion, and transport to southwestern Ohio. Thus, as judged by proximity, composition, thickness, and geochronology, it is a North American equivalent to other Neoproterozoic Grenvillian-derived basins, such as the Torridon Group of Scotland and the Palmeiral Formation of South America. An alternate possibility, although much less likely in our opinion, is that it could be much younger, any time between 1048 ?? 22 Ma and the deposition of the Middle Cambrian Mount Simon Sandstone at about 510 Ma, and still virtually almost all derived from rocks of the Grenville Orogen.

  13. The Impact of Partial Melting in the Orogenic Cycle

    NASA Astrophysics Data System (ADS)

    Rey, P. F.; Teyssier, C.; Whitney, D. L.

    2010-12-01

    Open source, community driven numerical codes available at geodynamics.org allow geologists to model orogenic processes including partial melting and its consequences during orogenic cycles. Here we explore the role of partial melting during continental subduction and its impact on the evolution of orogenic plateaux and that of migmatite-cored metamorphic core complexes. Continental subduction and orogenic plateaux: Numerical experiments show that when continental slabs buried into the mantle meet their solidus, crustal melt is confined to the slab during its ascent and ponds at the Moho (Fig. 1a). The displaced overlying crust is extruded horizontally into the weak lower crust of the continent, resulting in Earth’s surface uplift to form an orogenic plateau, and Moho downward motion to accommodate the influx of material into the lower crust. This model suggests a link between continental subduction, melting and the build up of orogenic plateaux, and show that partial melting may be a significant process in exhumation of ultrahigh-pressure (UHP) rocks. Model results are consistent with the common association of UHP rocks and migmatite. Growth and destruction of orogenic plateaux: The lateral growth of orogenic plateaux is often attributed to the flow of the plateau weak partially melted lower crust into its foreland in some cases over a distance > 1500 km in 15 myr. Using pre-thickening temperatures compatible with Tibet’s uplift history, we show that mass redistribution processes are dynamically coupled, and that CFE velocities are limited to less than 1 cm.yr-1 (~150 km in 15 myr) by cooling and crystallization of the melted channel in the foreland and by any upward deviation into metamorphic domes of the melted channel by extension in the plateau (Fig. 1b). Gravitational collapse and metamorphic core complexes: Localization of extension in the upper crust triggers convergent flow in the partially molten deep crust channel. This convergent flow leads to the

  14. Juvenile Middle Proterozoic crust in the Adirondack Highlands, Grenville province, northeastern North America

    SciTech Connect

    Daly, J.S. ); McLelland, J.M. )

    1991-02-01

    Nd isotope data indicate that minimal amounts of significantly older crust have contributed to the genesis of the oldest (ca. 1.3-13.5 Ga) plutons in the Adirondack Highlands. These are magmatic arc tonalites with positive initial {epsilon}{sub Nd} values and Sm-Nd depleted mantle model ages (t{sub DM}) that are within 70 m.y. of the time of their crystallization. Granitoids of the anorthosite-mangerite-charnockite-granite suite, dated at 1,156-1,134 Ma, as well as the 1,100-1,050 Ma plutons, associated with the Ottawan phase of the Grenvillian orogenic cycle, also have positive initial {epsilon}{sub Nd} values and t{sub DM} ages similar to the tonalites. Derivation of both groups of granitoids by crustal melting of the magmatic arc is consistent with the available isotopic and geochemical data. Juvenile late Middle Proterozoic crust that formed during or just prior to the Grenville cycle appears to dominate the southwestern Grenville province as well as the Grenville inliers to the south. In contrast, most of the contiguous Grenville province in Canada comprises largely reworked older crust.

  15. Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids.

    PubMed

    Camacho, Alfredo; Lee, James K W; Hensen, Bastiaan J; Braun, Jean

    2005-06-30

    Collision tectonics and the associated transformation of continental crust to high-pressure rocks (eclogites) are generally well-understood processes, but important contradictions remain between tectonothermal models and petrological-isotopic data obtained from such rocks. Here we use 40Ar-39Ar data coupled with a thermal model to constrain the time-integrated duration of an orogenic cycle (the burial and exhumation of a particular segment of the crust) to be less than 13 Myr. We also determine the total duration of associated metamorphic events to be approximately 20 kyr, and of individual heat pulses experienced by the rocks to be as short as 10 years. Such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events (earthquakes). Such events triggered transient heat advection by hot fluid along deformation (shear) zones, which cut relatively cool and dry subducted crust. In contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust, our non-steady-state cold-crust model satisfactorily explains several otherwise contradictory geological observations.

  16. The Midcontinent Rift and Grenville connection

    SciTech Connect

    Cambray, F.W.; Fujita, K. . Dept. of Geological Sciences)

    1994-04-01

    The Mid-Proterozoic, Midcontinent Rift System (MRS) is delineated by an inverted U shaped gravity and magnetic anomaly. It terminates in southeast Michigan but a less continuous series of anomalies and sediments, the Eastcontinent Rift occur on a north-south line through Ohio and Kentucky. The geometry allows for a north-south opening, the Lake Superior section being orthogonal to opening, the western arm transtensional and the north-south trending eastern arm a transform boundary offset by pull-apart basins. The opening and closing of the rift overlaps in time with the Grenville Orogeny. Grenville age rocks can also be found in the Llano uplift of Texas. The authors propose a model to explain the temporal and geographic association of the opening and closing of the MRS with the Grenville Orogeny that involves irregular suturing between two continental masses. Initiation of Grenville suturing, associated with south dipping subduction, in the northeast and in the Llano area of Texas would leave portion of unclosed ocean in between. Tensional stresses in the continental crust adjacent to the oceanic remnant could lead to its fragmentation and the formation of the MRS. The remaining oceanic lithosphere would eventually subduct, limiting the opening of the MRS. Continued convergence of the plates would induce compressional stresses thus accounting for the deformation of the MRS. An analogy is made with more recent opening of the Red Sea, Gulf of Aden Rift System in association with irregular collision along the Zagros-Bitlis Sutures.

  17. Linking collisional and accretionary orogens during Rodinia assembly and breakup: Implications for models of supercontinent cycles

    NASA Astrophysics Data System (ADS)

    Cawood, Peter A.; Strachan, Robin A.; Pisarevsky, Sergei A.; Gladkochub, Dmitry P.; Murphy, J. Brendan

    2016-09-01

    Periodic assembly and dispersal of continental fragments has been a characteristic of the solid Earth for much of its history. Geodynamic drivers of this cyclic activity are inferred to be either top-down processes related to near surface lithospheric stresses at plate boundaries or bottom-up processes related to mantle convection and, in particular, mantle plumes, or some combination of the two. Analysis of the geological history of Rodinian crustal blocks suggests that internal rifting and breakup of the supercontinent were linked to the initiation of subduction and development of accretionary orogens around its periphery. Thus, breakup was a top-down instigated process. The locus of convergence was initially around north-eastern and northern Laurentia in the early Neoproterozoic before extending to outboard of Amazonia and Africa, including Avalonia-Cadomia, and arcs outboard of Siberia and eastern to northern Baltica in the mid-Neoproterozoic (∼760 Ma). The duration of subduction around the periphery of Rodinia coincides with the interval of lithospheric extension within the supercontinent, including the opening of the proto-Pacific at ca. 760 Ma and the commencement of rifting in east Laurentia. Final development of passive margin successions around Laurentia, Baltica and Siberia was not completed until the late Neoproterozoic to early Paleozoic (ca. 570-530 Ma), which corresponds with the termination of convergent plate interactions that gave rise to Gondwana and the consequent relocation of subduction zones to the periphery of this supercontinent. The temporal link between external subduction and internal extension suggests that breakup was initiated by a top-down process driven by accretionary tectonics along the periphery of the supercontinent. Plume-related magmatism may be present at specific times and in specific places during breakup but is not the prime driving force. Comparison of the Rodinia record of continental assembly and dispersal with that

  18. Accretionary orogens: definition, character, significance

    NASA Astrophysics Data System (ADS)

    Cawood, P. A.; Kroener, A.; Windley, B. F.

    2003-04-01

    Classic models of orogens involve a Wilson cycle of ocean opening and closing with orogenesis related to continent-continent collision. Such models fail to explain the geological history of a significant number of orogenic belts throughout the world in which deformation, metamorphism and crustal growth took place in an environment of on-going plate convergence. These belts are termed accretionary orogens but have also been refereed to as non-collisional orogens, Pacific-type orogens, Turkic-type and exterior orogens. Accretionary orogens evolve in generally curvilinear belts comprising dominantly mafic to silicic igneous rocks and their sedimentary products and accumulated largely in marine settings. They are variably deformed and metamorphosed by tectono-thermal events aligned parallel to, and punctuating, facies trends. Accretionary orogens form at sites of subduction of oceanic lithosphere and consist of magmatic arcs systems along with material accreted from the downgoing plate and eroded from the upper plate. Deformational features include structures formed in extension and compressive environments during steady-state convergence (arc/backarc vs. accretionary prism) that are overprinted by short regional compressive orogenic events. Orogenesis takes place through coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat slab subduction, and rapid absolute upper plate motion over-riding the downgoing plate. The Circum-Pacific region provides outstanding examples of accretionary orogens. The Pacific formed during breakup of Rodinia in the Neoproterozoic and has never subsequently closed, resulting in a series of overall ocean-ward younging orogenic systems that have always faced an open ocean, yet have been the sites of repeated tectono-thermal events and

  19. Structural investigation of the Grenville Province by radar and other imaging and nonimaging sensors

    NASA Technical Reports Server (NTRS)

    Lowman, P. D., Jr.; Blodget, H. W.; Webster, W. J., Jr.; Paia, S.; Singhroy, V. H.; Slaney, V. R.

    1984-01-01

    The structural investigation of the Canadian Shield by orbital radar and LANDSAT, is outlined. The area includes parts of the central metasedimentary belt and the Ontario gneiss belt, and major structures as well-expressed topographically. The primary objective is to apply SIR-B data to the mapping of this key part of the Grenville orogen, specifically ductile fold structures and associated features, and igneous, metamorphic, and sedimentary rock (including glacial and recent sediments). Secondary objectives are to support the Canadian RADARSAT project by evaluating the baseline parameters of a Canadian imaging radar satellite planned for late in the decade. The baseline parameters include optimum incidence and azimuth angles. The experiment is to develop techniques for the use of multiple data sets.

  20. A regional paleomagnetic study of lithotectonic domains in the Central Gneiss Belt, Grenville Province, Ontario

    NASA Astrophysics Data System (ADS)

    Constanzo Alvarez, Vincenzo; Dunlop, David J.

    1998-04-01

    We have made a regional paleomagnetic study of lithotectonic domains in the Central Gneiss Belt of the Grenville Province in Ontario along ten N-S and E-W traverses up to 200 km in length. Although originally intended to clarify the tectonic mechanism by which these exotic terranes were assembled and welded to the Archean Superior craton during the ˜1150-Ma Grenvillian orogeny, we actually learned much more about the timing of post-orogenic uplift of the various domains around 1000-900 Ma as they stabilized and became part of the Rodinia supercontinent. The normal (N) and reverse (R) natural remanent magnetizations (NRMs) of all domains, as well as those of reactivated regions flanking the Grenville Front (GF, the Superior-Grenville boundary) to the north and south, have paleomagnetic poles falling on the 980-920-Ma ( 40Ar/ 39Ar calibrated) portion of the Grenville apparent polar wander track for Laurentia. There is a general tendency for paleopoles to young with increasing distance of domains from the GF, implying that more southerly domains were uplifted and magnetized later, but two of the domains do not fit this pattern. Previously reported younging trends away from the GF, based on K/Ar thermochron maps and paleomagnetic 'zone poles', are untrustworthy because of hydrothermal alteration, which causes chemical remagnetization and anomalously old K/Ar ages near the GF. Another trend in our data is a regular increase in the R/N ratio with increasing distance south of the GF. In the reactivated zones flanking the GF, NRMs are overwhelmingly of N polarity, whereas well away from the GF, R/N is close to 50:50. Also, NRM intensities and susceptibility values increase 100-fold away from the GF, peaking ≈10 km south of the front, with a pulse-like pattern similar to that documented in anomalously high 40Ar/ 39Ar dates in the same region. Both the magnetic and Ar/Ar results are likely due to a 'wave' of hydrothermal alteration and remagnetization during which fluids

  1. Grenville-era Crustal Architecture of Central Australia, and its Importance in Constraining Rodinia Models.

    NASA Astrophysics Data System (ADS)

    Aitken, A. R.; Betts, P. G.

    2007-12-01

    continuous and coherent northeast trending orogenic belt connecting the Albany Fraser, Musgrave and Warumpi provinces. The geometry and extent of this orogenic belt precludes a direct connection between the Musgrave Province and contemporaneous orogens in Laurentia. Any model of Australian orogenic activity during the Grenvillian era, must take account of the NE oriented architecture, and intracontinental termination of the orogenic belt. Continental reconfiguration within Australia via the rotation of the South Australian Craton can adequately explain the Grenville-aged architecture of Australia.

  2. Acadian orogen Which Acadian orogen

    SciTech Connect

    Ludman, A. )

    1993-03-01

    The Acadian orogeny is widely viewed as the climactic event in much of the Northern Appalachians, resulting from the closing of one or more ocean basins and the accretion of the Avalonian continent to ancestral North America. Unconformities have traditionally dated the Acadian orogeny as Middle Devonian in the east and Late Middle to Late Devonian in the western part of the orogen. The recent recognition that several post-Acadian'' plutons are actually Late Silurian suggests that the tectonic models are too simplistic; that accretion previously attributed to a Devonian (Acadian) event may have been a two-stage process: Late Silurian (Salinic ) and Acadian sensu stricto; that orogenic activity may have been continuous from Taconian through Acadian'' times. Differences between the records of the Taconic (Ordovician) and Acadian orogenies suggest different plate interactions: near-orthogonal subduction for the former vs transcurrent faulting for the latter as the dominant mechanisms. Distribution of Salinic'' unconformities across the Northern Appalachian orogen in Maine suggests that accretion of Avalon may have been completed by that Silurian event, but faunal provincialities displayed by fossils as young as Early Devonian in terranes bordering the Fredericton Trough argue for later (Devonian) accretion. A further complication is the imprecise knowledge of the extent to which post-Devonian tectonism is responsible for the current terrane configuration in the Northern Appalachians. Alleghenian folding and thrusting is well-established in some areas where post-Acadian (Carboniferous) strata are present, and is now being documented in older rocks in eastern Maine as well: transcurrent faulting as young as post-Mesozoic( ) along the Norumbega Fault Zone; east-directed thrusting involving the Early Devonian Center Pond pluton.

  3. Grenville foreland thrust belt hidden beneath the eastern US midcontinent

    SciTech Connect

    Hauser, E.C. )

    1993-01-01

    Grenville foreland thrust structures are observed beneath the eastern US midcontinent on COCORP (Consortium for Continental Reflection Profiling) line OH-1 and a short seismic line in southwest Ohio. These structures represent the first evidence for a significant Grenville foreland thrust belt preserved in eastern North America. On the COCORP lines, the structures include a thrust ramp anticline and an associated asymmetric syncline. The Grenville front tectonic zone appears to truncate these foreland structures, indicating a later, second phase expressed as a deeply penetrating, out-of-sequence thrust zone associated with the main uplift of the Grenville province on the east. A short, shallow seismic line in southwestern Ohio reveals an east-dipping sequence of prominently layered rocks that may lie above a footwall ramp to a deeper Grenville thrust fault. A drill hole into the less reflective top of this dipping sequence encountered unmetamorphosed sedimentary rocks like those increasingly reported from other drill holes in southwestern Ohio and adjacent states. Although possibly part of a late Precambrian (Keweenawan ) rift, these clastic sedimentary rocks may instead preserve evidence of a heretofore unrecognized Grenville foreland basin in eastern North America. Alternatively these Precambrian sedimentary rocks together with an underlying, but yet undrilled, strongly layered sequence may correlate with similarly layered rocks observed on COCORP and industrial seismic lines within the Middle Proterozoic granite-rhyolite province to the west in Indiana and Illinois and indicate that unmetamorphosed sedimentary material is an important constituent of the granite-rhyolite province. 25 refs., 6 figs.

  4. The Grenville Front Tectonic Zone: Results from the 1986 Great Lakes Onshore Seismic Wide-Angle Reflection and Refraction Experiment

    NASA Astrophysics Data System (ADS)

    Epili, Duryodhan; Mereu, Robert F.

    1991-09-01

    The Grenville Front, which marks the orogenic boundary between the Archean Superior Structural Province and the much younger Grenville Province to the southeast, is one of the major tectonic features of the Canadian Shield. Within Canada, it is approximately 1900 km in length extending from the north shore of Lake Huron across Ontario and Quebec to Labrador. In 1986, a major coincident onship near-vertical reflection and onshore wide-angle reflection/refraction experiment (GLIMPCE-Great Lakes International Multidisciplinary Program on Crustal Evolution) was conducted along a series of lines across the Great lakes. One of the lines, line J, ran across Georgian Bay and Lake Huron for a distance of 350 km and crossed the Grenville Front Tectonic Zone (GFTZ). The seismic signals from the air gun array source were well recorded by the onshore stations up to distances of 250 km with a seismic trace spacing of 50-62.5 m. The GFTZ had a profound effect on the nature of the reflector patterns observed on the onshore seismic sections. Data recorded by the stations on the east end of the line indicate that the crustal P phases are very complex and form a "shinglelike" pattern of reflected waves. Data recorded by stations at the center and at the western end of the line show that the Pg phases are normal and lack the shinglelike appearance. This character of arrivals was also observed on the corresponding S wave sections. A combined P and S wave forward modeling analysis shows that the GFTZ is composed of bands of reflectors dipping at angles of 20°-35° extending to the lower crust. These reflectors were also well imaged on the coincident near-vertical reflection data. Reflectors under the Britt domain to the east of the GFTZ have a shallower dip than those along the zone. The structure of the crust under the Manitoulin terrane to the west of the GFTZ is laterally homogeneous with a major intracrustal reflector at a depth of 17-20 km below the surface. Poisson's ratio is

  5. A Mantle Xenolith Window Into the Grenville Orogeny of Southern Laurentia

    NASA Astrophysics Data System (ADS)

    Young, H. P.; Lee, C. A.

    2008-12-01

    The creation and isolation of the craton, or stable SCLM, is intimately connected to orogenesis. However, the nature of the lithospheric mantle beneath orogenic belts is incompletely understood due to the general lack of mantle xenolith-bearing basaltic magmas in such regions. One such place where we are afforded the opportunity to study the deep lithosphere beneath an orogenic belt is in central Texas, United States. Mantle xenoliths occur in late Cretaceous alkali basaltic magmas erupted through the remnants of the Appalachian - Ouachita structural belt of eastern and southern Laurentia. The Appalachian - Ouachita structural belt, which is buried beneath most of the Gulf Plain, represents two dissimilar cycles of orogenesis. The earlier cycle was the culmination of a long period of Proterozoic juvenile crust formation along Laurentia's southern and eastern margin. The more recent (Paleozoic) cycle created the fold and thrust belts currently exposed in the Appalachian and Ouachita Mountains, but involved mainly thin-skin tectonics and accretion of terranes, rather than continental suprasubduction settings. We are interested in identifying the process which emplaced mantle lithosphere beneath this ancient orogenic belt, and whether the original lithospheric mantle has been preserved there. Here, we show that the xenoliths beneath west-central Texas are of continental origin. These samples also have geochemical signatures suggestive of a suprasubduction zone setting in the form of enrichments in fluid-mobile trace (e.g. La) elements over fluid-immobile trace elements (e.g. Nb). These observations imply that the original continental lithosphere created in the Proterozoic suprasubduction zone setting was likely preserved during continent-continent collision and did not undergo wholesale delamination over a billion year period. During this period, the mantle convectively resisted two episodes of supercontinent rifting. However, the lithospheric mantle may have been

  6. The Central Metasedimentary Belt (Grenville Province) as a failed back-arc rift zone: Nd isotope evidence

    NASA Astrophysics Data System (ADS)

    Dickin, A. P.; McNutt, R. H.

    2007-07-01

    Nd isotope data are presented for granitoid orthogneisses from the Central Metasedimentary Belt (CMB) of the Grenville Province in order to map the extent of juvenile Grenvillian-age crust within this orogenic belt that is composed mostly of older crustal terranes. The data reveal a 150 km-wide belt of juvenile crust in Ontario, but this belt contains a block of pre-Grenvillian crust (containing the Elzevir pluton) which yields an estimated crustal formation age of 1.5 Ga. The recognition of an older block within the CMB has profound implications for its structure and tectonic evolution, because it implies that juvenile Grenvillian crust, apparently forming a wide NE-SW belt, is in fact distributed in two narrower segments with approximately N-S strike. We suggest that the CMB comprises an en echelon series of ensimatic rift segments, created by back-arc spreading behind a continental margin arc. These rift segments extend southwards (in the subsurface) into the northeastern Unites States. The rift segments contain abundant marble outcrops, consistent with marine incursion into the rift zone, and these deposits also continue northwards into a 'Marble domain' of the CMB in Quebec. However, crustal formation ages in the latter domain are largely pre-Grenvillian, implying that the Quebec rift segment was ensialic. Hence, we interpret the CMB in Ontario and Quebec as the northern termination of a failed back-arc rift zone.

  7. Geophysical constraints on the presence of mafic material within the Scranton rift and its influence on later orogenic cycles of the Eastern North American Margin

    NASA Astrophysics Data System (ADS)

    Benoit, M. H.; Ebinger, C. J.; Crampton, M.

    2012-12-01

    The transition zone between the modern-day northern and southern Appalachian mountains is located in Pennsylvania, where the there is a pronounced change in the orientation of structural features. The Appalachian orogenic belt steps eastward and wraps around the Scranton Bouguer gravity high, corresponding to a sector of the Taconic foreland basin. Vintage wide-angle crustal seismic studies suggest that crust beneath the thick foreland basin fill was thickened and heavily intruded in a failed Neoproterozoic rift (Hawman et al., 1992). Here, we investigate the role of this ancient crustal heterogeneity on the structure of the Appalachian orogen and subsequent Mesozoic rift structures through analyses of receiver function analysis from temporary and permanent stations throughout the region. Forward and inverse models of Bouguer gravity data constrain the geometry and depth extent of the high density zone, and seismic data constrain thickness and structure of the crust. Receiver functions were calculated using time-domain deconvolution, and then bulk crustal thickness and Vp/Vs values were found using Hk stacking analysis. A simple grid-search inversion which solved for layer thickness, Vs, and Poisson's ratio was applied to a subset of stations to identify mid-crustal discontinuities,. Results from the receiver function analysis suggest that the crust is ~47- 49 km thick beneath the gravity high, and that stations located within the high contain a 6-14 km thick layer in the lower crust with a Vs ~ 4.1 km/s and Vp 7.4 km/s. Seismic velocities of the upper crust are also higher than average for the eastern US. Based on the results of a Parker-Oldenberg inversion of Bouguer gravity data, the gravity high corresponds to a 400 kg m-3 density contrast in the lower crust. The top of the high density body at ~35 km depth strongly correlates to the depth to the high velocity layer found in the models produced from receiver function analysis. Furthermore, results from Euler

  8. The Grenville-age basement of the Andes

    NASA Astrophysics Data System (ADS)

    Ramos, Victor A.

    2010-01-01

    The analysis of the basement of the Andes shows the strong Grenville affinities of most of the inliers exposed in the different terranes from Colombia to Patagonia. The terranes have different histories, but most of them participated in the Rodinia supercontinent amalgamation during the Mesoproterozoic between 1200 and 1000 Ma. After Rodinia break-up some terranes were left in the Laurentian side such as Cuyania and Chilenia, while others stayed in the Gondwanan side. Some of the terranes once collided with the Amazon craton remained attached, experiencing diverse rifting episodes all along the Phanerozoic, as the Arequipa and Pampia terranes. Some other basement inliers were detached in the Neoproterozoic and amalgamated again to Gondwana in the Early Cambrian, Middle Ordovician or Permian times. A few basement inliers with Permian metamorphic ages were transferred to Gondwana after Pangea break-up from the Laurentian side. Some of them were part of the present Middle America terrane. An exceptional case is the Oaxaquia terrane that was detached from the Gondwana margin after the Early Ordovician and is now one of the main Mexican terranes that collided with Laurentia. These displacements, detachments, and amalgamations indicate a complex terrane transfer between Laurentia and Gondwana during Paleozoic times, following plate reorganizations and changes in the absolute motion of Gondwana.

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

    NASA Astrophysics Data System (ADS)

    Cosca, Michael A.; Essene, Eric J.; Kunk, Michael J.; Sutter, John F.

    1992-04-01

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

  10. The Precambrian of Transangaria, Yenisei Ridge (Siberia): Neoproterozoic microcontinent, Grenville-age orogen, or reworked margin of the Siberian craton?

    NASA Astrophysics Data System (ADS)

    Kuzmichev, Alexander B.; Sklyarov, Eugene V.

    2016-01-01

    The Yenisei Ridge was traditionally perceived as an uplifted segment of the western Siberian craton affected by Neoproterozoic collision events. However, the suggestions for Archaean or Palaeoproterozoic ('Siberian') basement in Transangaria have not been confirmed by reliable geochronological data. A new view regards most of the Ridge, namely, its Transangarian segment, to be an exotic Neoproterozoic terrane that collided with Siberia in the late Neoproterozoic. This paper presents new U-Pb SHRIMP zircon ages demonstrating that Archaean rocks (2611 ± 12 Ma) actually exist in this territory. We also provide a review of published U-Pb zircon ages for igneous and metamorphic rocks of Transangaria together with our new age data. This geochronological dataset clarifies the geology of the Yenisei Ridge and leads to new conclusions, as follows. (1) It is likely that Transangaria was originally underlain by an Archaean-Palaeoproterozoic basement, similar to that of the Siberian craton. (2) Geochronological data do not confirm the idea of widespread "Greenvillian age" granitoides in Transangaria. (3) The Neoproterozoic evolution of the Yenisei Ridge segment of the Siberian craton margin includes the following events. (i) Collision of an unidentified terrane with the western margin (in recent coordinates) of the Siberian craton during 900-855 Ma. The colliding terrane is no longer present in the current structure. (ii) Dextral shearing during 830-800 Ma may have been caused by counter-clockwise rotation of the Siberian craton. (iii) Extensional conditions prevailed during 800-700 Ma. The Isakovka oceanic basin formed at this time interval. (iv) Thrusting of the Isakovka island arc and accretionary prism onto the Siberian margin occurred during the late Neoproterozoic (650-630 Ma) and caused high-pressure metamorphism.

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

    USGS Publications Warehouse

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

    1992-01-01

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

  12. Timing and conditions of high-pressure metamorphism in the western Grenville Province: Constraints from accessory mineral composition and phase equilibrium modeling

    NASA Astrophysics Data System (ADS)

    Marsh, Jeffrey H.; Culshaw, Nicholas G.

    2014-07-01

    Previous geochronological analyses of high pressure (HP) metamorphic rocks in the western Grenville Province, Ontario, Canada have yielded precise U-Pb zircon ages; however, uncertainty has remained as to whether these ages represent the timing of HP metamorphism or the granulite/amphibolite facies overprint accompanying exhumation to a hot middle orogenic crust. Detailed study of these HP rocks, involving garnet, rutile, and zircon trace element analysis, phase equilibrium modeling, and zircon U-Pb geochronology, has yielded much improved constraints on the timing and conditions of HP metamorphism. Zircon from five of the six HP samples yield anchored discordia upper intercept and 207Pb/206Pb weighted average ages between 1097 and 1085 Ma, and typically have trace element compositions consistent with growth in a garnet-rich, plagioclase-poor eclogite-type assemblage (i.e. no negative Eu anomaly and flat HREE trends). Titanium-in-zircon and Zr-in-rutile thermometry indicates that the range of zircon crystallization temperatures for most samples (643-767 °C) is close to that of rutile inclusions in garnet (668-753 °C) and matrix rutile (690-772 °C). Phase relations in a pseudosection calculated for the sample that best preserves the HP assemblage indicate that: (1) the stability field for the inclusions observed in garnet and kyanite is between 11.5 < P < 14 kbar and 600 < T < 700 °C, and (2) zircon and rutile crystallization temperatures intersect the inferred HP assemblage field (Grt + Cpx + Ky + Rt + Hbl + Qtz) and garnet and kyanite modal isopleths at P > ~ 15 kbar, indicating that the ca. 1090 Ma zircon ages date metamorphism at eclogite facies conditions. Thus, the deep burial of mafic lower crust that resulted in HP metamorphism in the western CGB occurred just prior to the main "Ottawan" phase of continental collision in the western Grenville Province (ca. 1080-1040 Ma).

  13. Reworked pre-Grenville crust and timing of Grenville orogenesis in the southeastern Llano Uplift, Texas: Results from U-Pb geochronometry

    SciTech Connect

    Reese, J.F.; Roback, R.C.; Walker, N.W. . Geological Sciences)

    1992-01-01

    Zircon geochronometry of metaigneous units in the southeastern Llano Uplift (1) show that 1355-1215 Ma rocks were tectonically reworked during the Grenville orogeny, (2) further constrain the timing of Grenville orogenesis along the southern margin of North America, and (3) necessitate revision of previously proposed stratigraphic and structural relations within the Llano Supergroup. Multigrain, abraded zircon fractions from a sample of quartzofeldspathic Valley Spring Gneiss (VSG) define a discordia trajectory with an upper intercept of 1355 [plus minus] 3 Ma, interpreted as the igneous crystallization age of the protolith. This age is the oldest thus far documented for Llano Supergroup protoliths and is substantially older than a recently reported age (1232 [plus minus]4 Ma) from a different part of the VSG. The lithologic and chronometric similarities of the old VSG to rocks to the Mid-continent Granite-Rhyolite terrane raise the possibility of genetic relations between these units. Thus components of the VSG are tentatively interpreted to represent a deformed and metamorphosed part of the southernmost Mid-Continent Granite-Rhyolite terrane. Alternatively, this rock could be part of an ensialic arc built upon the margin of southern North America and later deformed during the Grenville orogeny. Analyses from four abraded zircon fractions from a quartzofeldspathic gneiss in the structurally overlying Packsaddle Schist yield an upper intercept of 1215 [plus minus] 3 Ma, interpreted as the igneous protolith crystallization age. This age is the youngest yet determined for a polydeformed metamorphic rock in the Llano Uplift and more tightly brackets the timing of Grenville deformation in the southeastern Llano Uplift to post-1215 Ma and pre-1098 Ma.

  14. Seismic images of a tectonic subdivision of the Greenville Orogen beneath lakes Ontario and Erie

    USGS Publications Warehouse

    Forsyth, D. A.; Milkereit, B.; Davidson, A.; Hanmer, S.; Hutchinson, Deborah R.; Hinze, W. J.; Mereu, R. F.

    1994-01-01

    New seismic data from marine air-gun and Vibroseis profiles in Lake Ontario and Lake Erie provide images of subhorizontal Phanerozoic sediments underlain by a remarkable series of easterly dipping reflections that extends from the crystalline basement to the lower crust. These reflections are interpreted as structural features of crustal-scale subdivisions within the Grenville Orogen. Broadly deformed, imbricated, and overlapping thrust sheets within the western Central Metasedimentary Belt are succeeded to the west by a complex zone of easterly dipping, apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the Central Metasedimentary Belt and the Central Gneiss Belt. The interpreted Central Metasedimentary Belt boundary zone has a characteristic magnetic anomaly that provides a link from the adjacent ends of lakes Ontario and Erie to structures exposed 150 km to the north. Less reflective, west-dipping events are interpreted as structures within the eastern Central Gneiss Belt. The seismic interpretation augments current tectonic models that suggest the exposed ductile structures formed at depth as a result of crustal shortening along northwest-verging thrust faults. Relatively shallow reflections across the boundary region suggest local, Late Proterozoic extensional troughs containing post-Grenville sediments, preserved possibly as a result of pre-Paleozoic reactivation of basement structures.

  15. Deep crustal structure and seismic expression of the central Appalachian orogenic belt

    SciTech Connect

    Herman, G.C. Rutgers Univ., New Brunswick, NJ )

    1992-03-01

    Deep-seismic reflection profiles across parts of the Central Appalachian orogenic belt indicate that the crust here includes an exposed Paleozoic fold-and-thrust belt that is mainly soled in Proterozoic Grenville basement. Translation strain in the foreland and Highlands resulting from Paleozoic orogenesis is as much as 25 km, exclusive of layer-parallel-shortening penetrative strains. This estimate is limited by the slight structural relief beneath the Pocono plateau and by the lack of extensive Cambrian-Ordovician cover beneath the sole thrust. Earlier estimates of translation strain are much higher. To the southwest, this parautochthonous region is separated by an unrecognized structural transition from the more allochthonous parts of the central and southern Appalachian overthrust belt, marked by significantly larger translation strains. Crustal architecture in the hinterland part of the region is poorly understood due to data gaps.

  16. Oral Histories in Meteoritics and Planetary Science--XVI: Grenville Turner

    NASA Astrophysics Data System (ADS)

    Sears, Derek W. G.

    2012-03-01

    In this interview, Grenville Turner (Fig. 1) recounts how he became interested in meteorites during postdoctoral research with John Reynolds at the University of California, Berkeley, after completing a DPhil with Ken Mayne at the University of Oxford. At Berkeley, he worked on xenon isotopes with fellow students Bob Pepin and Craig Merrihue, but Reynolds' insistence that they analyze all the inert gases in their samples meant that they also made important contributions to Ne isotope studies and potassium-argon dating leading to the Ar-Ar technique. In 1964, Grenville obtained a teaching position at the University of Sheffield where he developed his own laboratory for inert gas isotope measurements. After the return of samples from the Moon by the Apollo program, he became involved in determining the chronology of volcanism and major impacts on the Moon. In 1988, Grenville and his team moved to the University of Manchester as part of a national reorganization of earth science departments. During the post Apollo years, Grenville's interest turned to the development of new instrumentation (resonance ionization mass spectrometry and the ion microprobe), and to problems in terrestrial isotope geochemistry, particularly the source of inert gases in fluid inclusions. He received the Leonard Medal of the Meteoritical Society in 1999, and he has also received awards from the Royal Society, the European Association of Geochemistry, and the Royal Astronomical Society.

  17. The palaeomagnetism of (Mesoproterozoic) Eriksfjord Group red beds, South Greenland: multiphase remagnetization during the Gardar and Grenville episodes

    NASA Astrophysics Data System (ADS)

    Piper, J. D. A.; Thomas, D. N.; Share, S.; Rui, Zhang Qi

    1999-03-01

    ' magnetization (D/I = 305/34°, α95 = 4.3°, 57 samples, palaeopole at 202.1°E, 32.4°N, dp/dm = 2.8/4.9° ) resident in haematite. The pole position does not correspond with any part of the Gardar Track, but does correlate with the return Keweenawan Track at ca. 1090 Ma, close to the time of Grenville orogenesis along the bordering southeastern margin of the Laurentian Shield. This remanence is attributed to diagenesis during extensional tectonism linked to the collapse of the Grenville Orogen formerly sited 100-200 km to the south.

  18. U-Pb zircon dates of morin anorthosite suite rocks, Grenville Province, Quebec

    SciTech Connect

    Doig, R. )

    1991-09-01

    U-Pb zircon ages of samples of anorthosite, pyroxene monzodiorite (jotunite), and pyroxene quartz monzonite (quartz mangerite) of the Morin anorthosite complex, Grenville Province, Quebec, are 1155 {plus minus} 3, 1146 {plus minus} 4 and 1135 {plus minus} 3 Ma, respectively. These dates are very similar to available dates for equivalent units of the Lac St-Jean and Adirondack anorthosite suite occurrences and slightly predate estimates of 1075-1100 Ma for peak metamorphic conditions during the Grenville orogeny in this region. {sup 87}Sr/{sup 86}Sr initial ratios of 0.7048-0.7051 for the three Morin units sampled permit a comagmatic origin if the different emplacement or cooling times can be reconciled. The mangerite sampled cannot have been formed by fusion of the upper crust, but may have been derived from a relatively juvenile crust at depth. All three units have interacted with continental crust, given the likely depleted nature of the mantle in this region.

  19. River Valley pluton, Ontario - A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    NASA Technical Reports Server (NTRS)

    Ashwal, Lewis D.; Wooden, Joseph L.

    1989-01-01

    This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.

  20. Accretionary orogens through Earth history

    USGS Publications Warehouse

    Cawood, Peter A.; Kroner, A.; Collins, W.J.; Kusky, T.M.; Mooney, W.D.; Windley, B.F.

    2009-01-01

    Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero. ?? The Geological Society of London 2009.

  1. Detrital mineral chronology of the Uinta Mountain Group: Implications for the Grenville flood in southwestern Laurentia

    USGS Publications Warehouse

    Mueller, P.A.; Foster, D.A.; Mogk, D.W.; Wooden, J.L.; Kamenov, George D.; Vogl, J.J.

    2007-01-01

    Numerous studies have shown that large quantities of Grenville-age detritus dominate Neo-proterozoic to Cambrian arenites in southwest Laurentia (southwestern United States). U-Pb ages and Hf isotopic compositions of zircons and 40Ar/39Ar ages of white mica from clastic sedimentary rocks of the Neoproterozoic Uinta Mountain Group also indicate significant Mesoproterozoic detritus mixed with a variably abundant Archean component. Zircons with ages representative of the Paleoproterozoic basement in the eastern Uinta Mountains or the younger Paleoproterozoic rocks of the adjacent Yavapai-Mazatzal terranes were not observed. A limited range of initial ??Hf (???90% between -3 and +3) for Mesoproterozoic zircons suggests derivation from a source region (or regions) characterized by mixing between juvenile and reworked older crust during Grenville orogenesis. The enriched Grenville-age basement proposed to underlie much of southeastern North America may be this source based on similarities of Hf isotopic data from Mesoproterozoic zircons in Mississippi River sand and available paleocurrent data. If so, then disruption of this supply in the Cambrian may be related to Iapetan rifting and, perhaps, the separation of the Precordillera terrane from Laurentia. ?? 2007 The Geological Society of America.

  2. Petrologic, geochemical, and geochronologic constraints on the tectonic evolution of the southern Appalachian orogen, Blue Ridge Province of western North Carolina

    NASA Astrophysics Data System (ADS)

    Anderson, Eric Douglas

    2011-07-01

    The Blue Ridge Province of western North Carolina contains a wide variety of metamorphosed igneous and sedimentary rocks that record the tectonic effects of Precambrian and Paleozoic orogenic cycles. Tectonic interpretations of the events that led to the present configuration are varied and often conflicting. This investigation examines metamorphosed mafic rocks that are widely interpreted to have formed during the closure of ocean basins. Metabasites, and specifically eclogites, have a tendency to mark tectonic sutures and frequently preserve pressure (P), temperature (T), and age data (t) that can be gleaned from mineral equilibria and U-Pb isotopic compositions. As such, the examination of the metabasites is considered the key to understanding the orogenic history of the southern Blue Ridge where these metabasites occur. Chapter 2 is an investigation of the retrograde reactions related to the decompression of sodic pyroxenes that react to form diopside-plagioclase-hornblende-quartz symplectites as stability fields are overstepped during isothermal decompression. In Chapter 3 metabasites from the central and eastern Blue Ridge are re-examined and P-T pathways of these lithologies are determined. The argument is made that the Taconic orogeny of the Blue Ridge is the result of a continent-continent collision event that culminated in a mega-melange that coincides with the Cullowhee terrane and the eastern Blue Ridge melange of western North Carolina. Chapter 4 contains the results of a geochronological investigation of the Precambrian basement complex of the eastern Great Smoky Mountains. Chapter 5 is a whole rock geochemical study of the same basement complex. In Chapter 6, a potential lithologic correlation between the southern Blue Ridge basement and the Arequipa-Antofalla block of Peru is discussed. The geologic history of western South America from the Mesoproterozoic through Cambrian is summarized, a potential isotope-based lithologic correlation is proposed

  3. A Paleomagnetic Traverse of the Frontenac, Sharbot Lake, Mazinaw and Elsevir Terranes, Grenville Province, Ontario

    NASA Astrophysics Data System (ADS)

    Brett, J. S.; Dunlop, D. J.

    2004-05-01

    A paleomagnetic traverse has been carried out across the Ontario section of the Central Metasedimentary Belt (CMB) of the Precambrian Grenville Province, between the St. Lawrence and Ottawa Rivers in the east and extending west as far as the Bancroft Terrane, where it links with an earlier traverse of the Central Gneiss Belt (CGB) and the CMB Boundary Zone (CMBBZ). 54 sites were sampled, 11 in the Frontenac (F) Terrane, 12 in the Sharbot Lake (SL) Terrane, 12 in the Mazinaw (M) Terrane, 3 in the Bancroft (B) Terrane, and 16 in the Elsevir Terrane, mainly in the Belmont Domain. Ar/Ar thermochronometric work across the CMB by Cosca et al. (Tectonics 10, 959-77, 1991; Contrib. Min. Petr. 110, 211-25, 1992) documented differential unroofing of different terranes and slow cooling (1.0-1.5 oC /Ma) along parallel T-t paths offset by 50-100 Ma for the Frontenac, the western and eastern Elsevir (Ew, Ee) and the CMBBZ. Around 1000 Ma, inferred burial depths were 20 km for the CMBBZ, 15 km for Ew, >20 km for Ee (or M), 10 km for F, and transitional (12-16 km) in SL between F and Ee/M. The very different hornblende, phlogopite, muscovite and biotite Ar/Ar ages between neighboring terranes should have a counterpart in the paleomagnetic results of the terranes because the interval 1100-850 Ma was a time of rapid paleolatitude shift. However our results and previous data from B, CMBBZ and the adjacent CGB (Costanzo-Alvarez & Dunlop, Earth Planet. Sci. Lett. 157, 89-103, 1998) do not bear this out. F, SL and M have very similar R-polarity poles in the central part of the Grenville Track, while F and SL N-polarity poles are typical 980 Ma Grenville A poles. Cooling through 500oC occurred around 1100 Ma for F but the F poles do not resemble Keweenawan poles of this age. Cooling through 500oC occurred around 950 Ma for Ee/M but the M pole does not match this age. Our Ew mean pole, like that of the Tudor Gabbro, falls off the Grenville Track on a possible pre-Grenvillian track.

  4. 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.

  5. Coeval emplacement and orogen-parallel transport of gold in oblique convergent orogens

    NASA Astrophysics Data System (ADS)

    Upton, Phaedra; Craw, Dave

    2016-12-01

    Varying amounts of gold mineralisation is occurring in all young and active collisional mountain belts. Concurrently, these syn-orogenic hydrothermal deposits are being eroded and transported to form placer deposits. Local extension occurs in convergent orogens, especially oblique orogens, and facilitates emplacement of syn-orogenic gold-bearing deposits with or without associated magmatism. Numerical modelling has shown that extension results from directional variations in movement rates along the rock transport trajectory during convergence, and is most pronounced for highly oblique convergence with strong crustal rheology. On-going uplift during orogenesis exposes gold deposits to erosion, transport, and localised placer concentration. Drainage patterns in variably oblique convergent orogenic belts typically have an orogen-parallel or sub-parallel component; the details of which varies with convergence obliquity and the vagaries of underlying geological controls. This leads to lateral transport of eroded syn-orogenic gold on a range of scales, up to > 100 km. The presence of inherited crustal blocks with contrasting rheology in oblique orogenic collision zones can cause perturbations in drainage patterns, but numerical modelling suggests that orogen-parallel drainage is still a persistent and robust feature. The presence of an inherited block of weak crust enhances the orogen-parallel drainage by imposition of localised subsidence zones elongated along a plate boundary. Evolution and reorientation of orogen-parallel drainage can sever links between gold placer deposits and their syn-orogenic sources. Many of these modelled features of syn-orogenic gold emplacement and varying amounts of orogen-parallel detrital gold transport can be recognised in the Miocene to Recent New Zealand oblique convergent orogen. These processes contribute little gold to major placer goldfields, which require more long-term recycling and placer gold concentration. Most eroded syn-orogenic

  6. Lead isotope study of orogenic lherzolite massifs

    NASA Astrophysics Data System (ADS)

    Hamelin, Bruno; Allègre, Claude J.

    1988-12-01

    Orogenic lherzolites allow for almost "in-situ" observation of mantle isotopic heterogeneities on a restricted geographical scale, in contrast to basalts for which melting processes have averaged original mantle compositions over uncertain scales. Pb isotopes from whole rocks and clinopyroxenes from the massifs of Lherz (Pyrenees), Lanzo (Alps), Beni Bousera (Morocco) and Zabargad (Red Sea) show internal heterogeneities that encompass the entire range of variation observed in oceanic basalts. Some depleted lherzolites have a very unradiogenic composition similar to that of the most depleted ridge tholeiites. Pyroxenites from mafic layers generally have more radiogenic compositions, some of them comparable to the most radiogenic oceanic island results. The isotopic differences between lherzolites and pyroxenites vanish where layers are very closely spaced ( < 2 cm). In this case, the lherzolites may have equilibrated with the more Pb-rich pyroxenites through solid-state diffusion under mantle conditions. These results directly illustrate the smallest scales at which Pb isotopic heterogeneity may survive within the mantle. The genesis of these heterogeneities are discussed within the framework of the "marble cake" mantle model [1], where lherzolites are residues left over after oceanic crust extraction, whereas pyroxenites represent either basaltic or cumulate portions of the oceanic crust, reinjected by subduction and stretched by solid-state mixing during mantle convection. The Pb isotope data suggest that each massif was involved in several cycles of convective overturn, segregation and reinjection of the oceanic crust, during periods well over 1 Ga. If the upper mantle is made of interlayered radiogenic and unradiogenic layers, basalt heterogeneities may result from preferential melt-extraction from different layers depending on the degree of melting, as well as from large-scale, plume-related mantle heterogeneities. Orogenic lherzolites therefore allow direct

  7. Ages and petrogenetic significance of igneous mangerite-charnockite suites associated with massif anorthosites, Grenville Province

    SciTech Connect

    Emslie, R.F.; Hunt, P.A. )

    1990-03-01

    U-Pb ages of zircon fractions of major anorthosite-mangerite-charnockite-granite (AMCG) igneous suites imply that this magmatism inaugurated what is widely regarded as the Grenvillian event between about 1.16 and 1.12 Ga ago over about two-thirds of the Grenville Province east, northeast, and southeast of the Central Metasedimentary Belt. Pre-Grenvillian AMCG suites about 1.36 and 1.64 Ga old have much more restricted distribution. An apparent time lag of about 0.05 to 0.10 Ga is indicated between culmination of AMCG magmatism and the widely recognized Grenvillian metamorphic peak (about 1.10 to 1.03 Ga), perhaps the most distinctive hallmark of the Grenville event. The time lag is consistent with conductive heating of thick subcontinental lithosphere that began with initiation of AMCG magmatism and continued until geotherms rose sufficiently to produce granulites in much of the lower to middle crust. Tectonic crustal thickening did not likely occur until later in the sequence of events, perhaps after some cooling from the metamorphic peak. Compressive forces were externally applied, possibly at a distant plate margin, while the continental lithosphere was still thermally weakened from preceding magmatic-metamorphic culminations.

  8. Baddeleyite-zircon relationships in coronitic metagabbro, Grenville Province, Ontario: implications for geochronology

    NASA Astrophysics Data System (ADS)

    Davidson, A.; van Breemen, O.

    1988-11-01

    In the high grade gneiss terranes of the Grenville Province in Ontario, metagabbro with silicate coronas around primary olivine and Fe-Ti oxide contains trace amounts of both baddeleyite and zircon. Single baddeleyite crystals with radiating columnar coronas of polycrystalline zircon, in places with outer rims of garnet, are analogous to the coronas around olivine and ilmenite. Single crystals of zircon and baddeleyite without zircon coronas also occur in the same rocks. Both zirconium minerals have been isotopically dated at three widely separated localities. In each case baddeleyite records an igneous crystallization age of ca.1170 Ma, 125 Ma older than the ca.1045 Ma age of the zircons, interpreted to be the age of metamorphism. These data agree well with U-Pb ages obtained from other rocks in the same region and refute the alternative interpretation that silicate coronas in these metagabbros formed during cooling following magmatic crystallization. The age does not support direct correlation with diabase dykes of either the Sudbury or Abitibi swarms northwest of the Grenville Front.

  9. Composition of the crust in the Grenville and Appalachian Provinces of North America inferred from VP/VS ratios

    USGS Publications Warehouse

    Musacchio, G.; Mooney, W.D.; Luetgert, J.H.; Christensen, N.I.

    1997-01-01

    We use the ratios between P and S wave velocities (VP/VS), derived from seismic refraction data, to infer the composition of the crust in the Grenville and the Appalachian Provinces of North America. The crust exhibits VP/VS increasing with depth from 1.64 to 1.84; there is a clear distinction between the Grenville Province (average VP/VS=1.81) and the Appalachian Province (average VP/VS=1.73) which persists at all depths. The boundary between these provinces is east dipping extending for 100 km east of the Champlain thrust. In the Appalachian Province the increase in VP/VS ratios with depth from 1.67 to 1.74 ?? 0.02 may reflect a normal decrease of silica content in the continental crust. In the Grenville Province beneath the Central Granulite Terrane, an anomalous VP/VS ratio of 1.82 ?? 0.02 is observed extending to a depth of 10 km; this correlates with the abundance of Ca-plagioclase in the Marcy Anorthosite. At greater depth (15-20 km), where seismic lamination and high electrical conductivity is observed, VP/VS is 1-84 ?? 0.02 and correlates with the Tahawus Complex, a layered mafic intrusion. Within the 25-km-thick lower crust of the Grenville Province the VP/VS is 1-84 ?? 0.02 and P-velocity is 7.0 ?? 0.1 km/s, which are typical for plagioclase-bearing rocks (gabbro-norite). The high VP/VS ratio in the Grenville Province has not been reported in crust of any other age. Since the Grenville Province contains 75% of the world's known anorthosites, high VP/VS ratio is related to high plagioclase. We suggest that the composition of the Grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid-Proterozoic. Copyright 1997 by the American Geophysical Union.

  10. Continental deformation and the mid-lithospheric discontinuity along the Grenville Front

    NASA Astrophysics Data System (ADS)

    Abrahams, L.; Long, M. D.; Ford, H. A.; Wirth, E. A.

    2015-12-01

    The existence of a mid-lithospheric discontinuity (MLD) within the stable continental mantle lithosphere of North America has been well established, but its interpretation remains difficult. Recent work with Ps receiver functions has found evidence for anisotropic structure at MLD depths within the western portion of the Granite-Rhyolite Province, suggesting that the MLD is the result of deformation associated with the formation of the continent. The last significant deformation to occur within the province was approximately 1.3-0.9 Ga and impacted the lithosphere east of the Grenville Front. In this study we analyzed six stations east of the front using Ps receiver functions in order to characterize anisotropy associated with the MLD in the region. Transverse and radial component Ps receiver functions were calculated for six stations (ACSO, BINY, ERPA, MCWV, SSPA, TZTN) using a multi-taper correlation technique and binned as a function of back azimuth and of epicentral distance. All six stations analyzed displayed significant positive phase energy on the radial component at ~6 seconds, which was interpreted as the Moho. At four of the six stations (ACSO, MCWV, SSPA, TZTN) the Moho showed moderate to significant complexity. At stations MCWV, SSPA, TZTN, all located along the Appalachian margin, there was significant transverse component energy at crustal depths with both two- and four-lobed anisotropy patterns observed. While ACSO, BINY, and ERPA displayed evidence of isotropic and/ or anisotropic crustal structure, a coherent pattern in back azimuth could not be established. The radial component receiver functions also exhibited negative phase energy, interpreted as the MLD, between 7.5 and 12.5 seconds (or ~80 to 120 km), at five of the six stations, with the exception of BINY, where no negative phase was observed. The transverse component receiver functions at stations ACSO, ERPA, SSPA and TZTN, also displayed a two-lobed pattern in back azimuth at MLD depths

  11. River Valley pluton, Ontario: A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    USGS Publications Warehouse

    Ashwal, L.D.; Wooden, J.L.

    1989-01-01

    The River Valley pluton is a ca. 100 km2 body of anorthositic and gabbroic rocks located about 50 km northeast of Sudbury, Ontario. The pluton is situated entirely within the Grenville Province, but its western margin is a series of imbricate thrust faults associated with the Grenville Front Tectonic Zone. It is dominated by coarse leuconorite and leucogabbro, with lesser anorthosite, gabbro, and rare ultramafics. Igneous textured rocks are abundant and consist of plagioclase (An60-70) charged with Fe-Ti oxide inclusions, low Ca pyroxene (orthopyroxene and/or inverted pigeonite) and augite. The most unfractionated rocks are minor olivine gabbros with Fo70-80. A variety of deformed and recrystallized equivalents of the igneous-textured rocks is also present, and these are composed largely of calcic plagioclase and hornblende. Ten samples, including both igneous and deformed lithologies give a Pb-Pb whole-rock isochron of 2560??155Ma, which is our best estimate of the time of primary crystallization. The River Valley pluton is thus the oldest anorthositic intrusive yet reported from the Grenville Province, but is more calcic and augitic than typical massifs, and lacks their characteristic Fe-Ti oxide ore deposits. The River Valley body may be more akin to similar gabbro-anorthosite bodies situated at the boundary between the Archean Superior Province and Huronian supracrustal belt of the Southern Province west of the Grenville Front. An Sm-Nd isochron from 3 igneous-textured leucogabbros and an augite mineral separate gives 2377 ?? 68 Ma, implying slight disturbance of the Sm-Nd whole-rock-mineral system during later metamorphism. The Rb-Sr system has been substantially disturbed, giving an age of 2185 ?? 105 Ma, which is similar to internal Pb-Pb isochron ages of 2165 ?? 130 Ma and 2100 ?? 35 Ma for two igneous-textured rocks. It is uncertain whether these ages correspond to a discrete event at this time or represent a partial resetting of the Rb-Sr and Pb

  12. River Valley pluton, Ontario: A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    SciTech Connect

    Ashwal, L.D. ); Wooden, J.L. )

    1989-03-01

    The River Valley pluton is a ca. 100 km{sup 2} body of anorthositic and gabbroic rocks located about 50 km northeast of Sudbury, Ontario. The pluton is situated entirely within the Grenville Province, but its western margin is a series of imbricate thrust faults associated with the Grenville Front Tectonic Zone. It is dominated by coarse leuconorite and leucogabbro, with lesser anorthosite, gabbro, and rare ultramafics. Igneous textured rocks are abundant and consist of plagioclase (An{sub 60-70}) charged with Fe-Ti oxide inclusions, low Ca pyroxene (orthopyroxene and/or inverted pigeonite) and augite. The most unfractionated rocks are minor olivine gabbros with Fo{sub 70-80}. A variety of deformed and recrystallized equivalents of the igneous-textured rocks is also present, and these are composed largely of calcic plagioclase and hornblende. An Sm-Nd isochron from 3 igneous-textured leucogabbros and an augite mineral separate gives 2,377 {plus minus} 68 Ma, implying slight disturbance of the Sm-Nd whole-rock-mineral system during later metamorphism. The Rb-Sr system has been substantially disturbed, giving an age of 2,185 {plus minus} 105 Ma, which is similar to internal Pb-Pb isochron ages of 2,165 {plus minus} 130 Ma and 2,100 {plus minus} 35 Ma for two igneous-textured rocks. Initial isotopic ratios for the River Valley pluton correspond to single-stage model parameters of {mu} = 8.06, {epsilon}{sub Nd} = O to {minus}3, and I{sub Sr} = 0.7015 to 0.7021. Collectively, these suggest either an enriched mantle source or crustal contamination of a mantle-derived magma. The crustal component involved must have been older and more radiogenic than the majority of rocks exposed at the surface in the nearby Superior Province.

  13. Pb isotopic composition of Paleozoic sediments derived from the Appalachian orogen

    SciTech Connect

    Krogstad, E.J. . Dept. of Geology)

    1993-03-01

    Differences in [sup 207]Pb/[sup 204]Pb at restricted ranges of [sup 206]Pb/[sup 204]Pb are robust indicators of differences in the earliest history of crust or mantle reservoirs, surviving later changes in U/Pb that may be due to melting, metamorphism, or sedimentary reworking. Ayuso and Bevier (1991) have used the [sup 207]Pb/[sup 204]Pb differences between Late Paleozoic granites in the N. Appalachians to trace their sources in either Laurentian (Grenville) lithosphere, or docked (Avalonian) lithosphere. If the Pb isotopic composition of Avalonian lithosphere is unique to that source among all lithospheric reservoirs in the Appalachian orogeny, the sediments shed off the orogen should record the first appearance of rocks with this extraneous Pb isotopic composition as they become accreted. The high [sup 207]Pb/[sup 204]Pb at similar [sup 206]Pb/[sup 204]Pb that may be indicative of all outboard terranes occurs in sedimentary rocks younger than middle Ordovician in New York and Maine, and younger than Ordovician in Virginia. Older sediments (Hadrynian, Cambrian), as well as autochthonous basement and paraautochonous basement slices, have lower [sup 207]Pb/[sup 204]Pb at similar [sup 206]Pb/[sup 204]Pb. The low [sup 207]Pb/[sup 204]Pb at similar [sup 206]Pb/[sup 204]Pb shown by these rocks may be a locally diagnostic signature of Late Proterozoic Laurentian lithosphere. The high [sup 207]Pb/[sup 204]Pb at similar [sup 206]Pb/[sup 204]Pb may be a locally diagnostic signature of Late Proterozoic accreted terranes. Rocks with accreted terrane Pb isotopic composition became dominant in the provenance of sediments along the strike of the Appalachian orogen by middle Ordovician time.

  14. Newly discovered eclogite in the southern Appalachian orogen, northwestern North Carolina

    NASA Astrophysics Data System (ADS)

    Willard, R. A.; Adams, Mark G.

    1994-05-01

    Recent detailed geologic mapping in the Blue Ridge belt of northwestern North Carolina has revealed the first reported eclogite ( sensu stricto) in the southern Appalachian orogen. The eclogite occurs at the base of the Ashe Metamorphic Suite within a shear zone that separates the Spruce Pine thrust sheet from the underlying Grenville-age basement rocks. This contact juxtaposes rocks of apparent oceanic affinities (predominantly metapelite, metabasite, and meta-ultramafic rocks) with rocks of the ancient North American craton. The primary metamorphic assemblage in the eclogite is garnet + omphacite + quartz + rutile. Hornblende and plagioclase occur only as retrograde amphibolite facies phases in the eclogite. Pressure-temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 13-17 kbar at 625-790°C, while the retrograde amphibolite facies assemblage suggests conditions of 8.5-12 kbar and 650-730°C. Ultramafic blocks also occur within the shear zone containing the eclogite. The presence of high-pressure metamorphic rocks associated with ultramafic rocks and pelitic schists within a shear zone separating continental rocks from oceanic rocks is strong evidence for an ancient subduction event and indicates that at least part of the Ashe Metamorphic Suite is a subduction-related, accretionary mélange.

  15. The Arequipa Massif of Peru: New SHRIMP and isotope constraints on a Paleoproterozoic inlier in the Grenvillian orogen

    NASA Astrophysics Data System (ADS)

    Casquet, C.; Fanning, C. M.; Galindo, C.; Pankhurst, R. J.; Rapela, C. W.; Torres, P.

    2010-01-01

    The enigmatic Arequipa Massif of southwestern Peru is an outcrop of Andean basement that underwent Grenville-age metamorphism, and as such it is important for the better constraint of Laurentia-Amazonia ties in Rodinia reconstruction models. U-Pb SHRIMP zircon dating has yielded new evidence on the evolution of the Massif between Middle Paleoproterozoic and Early Paleozoic. The oldest rock-forming events occurred in major orogenic events between ca. 1.79 and 2.1 Ga (Orosirian to Rhyacian), involving early magmatism (1.89-2.1 Ga, presumably emplaced through partly Archaean continental crust), sedimentation of a thick sequence of terrigenous sediments, UHT metamorphism at ca. 1.87 Ga, and late felsic magmatism at ca. 1.79 Ga. The Atico sedimentary basin developed in the Late-Mesoproterozoic and detrital zircons were fed from a source area similar to the high-grade Paleoproterozoic basement, but also from an unknown source that provided Mesoproterozoic zircons of 1200-1600 Ma. The Grenville-age metamorphism was of low- P type; it both reworked the Paleoproterozoic rocks and also affected the Atico sedimentary rocks. Metamorphism was diachronous: ca. 1040 Ma in the Quilca and Camaná areas and in the San Juán Marcona domain, 940 ± 6 Ma in the Mollendo area, and between 1000 and 850 Ma in the Atico domain. These metamorphic domains are probably tectonically juxtaposed. Comparison with coeval Grenvillian processes in Laurentia and in southern Amazonia raises the possibility that Grenvillian metamorphism in the Arequipa Massif resulted from extension and not from collision. The Arequipa Massif experienced Ordovician-Silurian magmatism at ca. 465 Ma, including anorthosites formerly considered to be Grenvillian, and high-T metamorphism deep within the magmatic arc. Focused retrogression along shear zones or unconformities took place between 430 and 440 Ma.

  16. Convergence rate controls seismicity styles in collision orogens

    NASA Astrophysics Data System (ADS)

    Dal Zilio, Luca; van Dinther, Ylona; Gerya, Taras

    2016-04-01

    The 25 April 2015 Mw 7.8 Gorkha earthquake in Nepal resulted from the unzipping of the previously locked Main Himalayan Thrust (MHT) fault, along which the Himalayan wedge is thrust over India. Strong ground shaking caused the collapse of more than half a million homes, killing more than 8500 people. Can such a large magnitude event also occur within the populated European Alps? Or is there a distinctly different seismicity pattern in different orogens? We show that their long-term seismicity patterns are indeed different and that their differences can be explained by a single parameter: their convergence rate. To do so we present the first self-consistent seismic cycle model for continental collisional margins. We use the viscoelastoplastic continuum Seismo-Thermo-Mechanical model (STM) validated for seismic cycle applications against a laboratory model (van Dinther et al., 2013a) and natural observations (van Dinther et al., 2013b), which includes Drucker-Prager plasticity and spontaneous rupture events governed by strongly rate-dependent friction. The 2-D model setup consists of two continental plates separated by an oceanic plate, in which the incipient subduction phase is followed by collisional orogeny. Results show the physically consistent spontaneous emergence of complex rupture paths, both on and off the main frontal thrust. These off-main frontal thrust events within the upper and lower plate complement the main frontal thrust seismicity leading to a Gutenberg-Richter frequency-magnitude distribution. This is a key observational feature of seismicity, which is typically not reproduced in seismic cycle models. The range of simulated b-values agrees with natural ranges, as we observe values from 0.97 up to 1.25 for convergence rates decreasing from 5 to 1 cm/yr. Decreasing convergence rates thus lead to relatively larger amounts of smaller earthquakes (increasing b-value) and lower maximum magnitudes. This change in b-value also observed to corresponds to

  17. The Western Sierras Pampeanas: Protracted Grenville-age history (1330-1030 Ma) of intra-oceanic arcs, subduction-accretion at continental-edge and AMCG intraplate magmatism

    NASA Astrophysics Data System (ADS)

    Rapela, C. W.; Pankhurst, R. J.; Casquet, C.; Baldo, E.; Galindo, C.; Fanning, C. M.; Dahlquist, J. M.

    2010-01-01

    basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent.

  18. Orogenic, Ophiolitic, and Abyssal Peridotites

    NASA Astrophysics Data System (ADS)

    Bodinier, J.-L.; Godard, M.

    2003-12-01

    "Tectonically emplaced" mantle rocks include subcontinental, suboceanic, and subarc mantle rocks that were tectonically exhumed from the upper mantle and occur:(i) as dispersed ultramafic bodies, a few meters to kilometers in size, in suture zones and mountain belts (i.e., the "alpine," or "orogenic" peridotite massifs - De Roever (1957), Thayer (1960), Den Tex (1969));(ii) as the lower ultramafic section of large (tens of kilometers) ophiolite or island arc complexes, obducted on continental margins (e.g., the Oman Ophiolite and the Kohistan Arc Complex - Coleman (1971), Boudier and Coleman (1981), Burg et al. (1998));(iii) exhumed above the sea level in ocean basins (e.g., Zabargad Island in the Red Sea, St. Paul's islets in the Atlantic and Macquarie Island in the southwestern Pacific - Tilley (1947), Melson et al. (1967), Varne and Rubenach (1972), Bonatti et al. (1981)).The "abyssal peridotites" are samples from the oceanic mantle that were dredged on the ocean floor, or recovered from drill cores (e.g., Bonatti et al., 1974; Prinz et al., 1976; Hamlyn and Bonatti, 1980).Altogether, tectonically emplaced and abyssal mantle rocks provide insights into upper mantle compositions and processes that are complementary to the information conveyed by mantle xenoliths (See Chapter 2.05). They provide coverage to vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths, particularly in the ocean basins and beneath passive continental margins, back-arc basins, and oceanic island arcs.Compared with mantle xenoliths, a disadvantage of some tectonically emplaced mantle rocks for representing mantle compositions is that their original geodynamic setting is not exactly known and their significance is sometimes a subject of speculation. For instance, the provenance of orogenic lherzolite massifs (subcontinental lithosphere versus upwelling asthenosphere) is still debated (Menzies and Dupuy, 1991, and references herein), as is the original setting

  19. 1.1 Ga K-rich alkaline plutonism in the SW Grenville Province

    NASA Astrophysics Data System (ADS)

    Corriveau, Louise; Heaman, Larry M.; Marcantonio, Franco; van Breemen, Otto

    1990-09-01

    U-Pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic, potassic alkaline and shoshonitic magmatism with island-arc affinities in the Central Metasedimentary Belt (CMB) of the southwestern Grenville Province, Canada took place between 1089 and 1076 Ma, along a 400-km-long, northeast-trending plutonic belt. These ages indicate that ultrapotassic rocks with arc affinities are not unique to the Phanerozoic. West to east emplacement ages along a northern and southern cross-section of this belt range from 1083±2 Ma (Kensington), through 1081±2 Ma (Lac Rouge) to 1076{-1/+3}Ma (Loranger) in the north, and from 1089{-3/+4}Ma (loon Lake) and 1088±2 Ma (Calabogie), to 1076±2 Ma (Westport) in the south. Although closely spaced in time, in detail these ages suggest a slight younging of this magmatic activity to the southeast. Integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the K-rich alkaline suite is distinct from the nepheline-syenite belt of the Bancroft terrane and from the syenite-monzonite suite of the Frontenac terrane of the CMB, and it is considered to be a magmatic episode unique to the Elzevir terrane and its Gatineau segment. The timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite-facies metamorphism of the country rock (precise age unknown) is older than 1089 Ma throughout the entire Elzevir terrane. The potassic magmatism is interpreted as the initiation of the 1090 1050 Ma Ottawan Orogeny in the Elzevir terrane; thus, the regional metamorphism in this terrane, previously assigned to the Ottawan Orogeny, is an earlier event. The contemporaneous emplacement of this postmetamorphic plutonic belt with Keweenawan volcanism is at variance with current tectonic models which consider the Keweenawan rift to be formed at the same time as regional metamorphism in the CMB.

  20. 13C 12C exchange between calcite and graphite: A possible thermometer in Grenville marbles

    USGS Publications Warehouse

    Valley, J.W.; O'Neil, J.R.

    1981-01-01

    The fractionation of 13C between calcite and graphite, ??(Cc-Gr). is consistently small (2.6-4.8 permil) in 34 assemblages from upper amphibolite- and granulite-facies marbles of the Grenville Province. In 25 samples from the Adirondack Mountains, New York, it decreases regularly with increasing metamorphic temperature. The fractionations are independent of absolute ??13C values of calcite (-2.9 to +5.0). For T = 600-800??C, the Adirondack data are described by ??(Cc-Gr) = -0.00748T (??C) + 8.68. This good correlation between ?? and T suggests that carbon isotope equilibrium was attained in these high-grade marbles and that the theoretical calculations of this fractionation by Bottinga are approximately 2 permil too large in this temperature range. Because of the relatively high temperature sensitivity suggested by these results and by Bottinga's calculations, and the pressure independence of isotope fractionation, ??(Cc-Gr) may provide a very good thermometer for high-grade marbles. Comparison of this field calibration for ??(Cc-Gr) vs temperature with results from other terranes supports the utility of ??(Cc-Gr) for geothermometry and suggests that graphite is much more sluggish to exchange than calcite, that exchange between calcite and graphite occurs at temperatures as low as 300??C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500-600??C. Because 13C exchange is an unavoidable metamorphic process at temperatures above 300??C, high values of ??13C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite. ?? 1981.

  1. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution

    NASA Astrophysics Data System (ADS)

    Fritz, H.; Abdelsalam, M.; Ali, K. A.; Bingen, B.; Collins, A. S.; Fowler, A. R.; Ghebreab, W.; Hauzenberger, C. A.; Johnson, P. R.; Kusky, T. M.; Macey, P.; Muhongo, S.; Stern, R. J.; Viola, G.

    2013-10-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world´s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of

  2. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution☆

    PubMed Central

    Fritz, H.; Abdelsalam, M.; Ali, K.A.; Bingen, B.; Collins, A.S.; Fowler, A.R.; Ghebreab, W.; Hauzenberger, C.A.; Johnson, P.R.; Kusky, T.M.; Macey, P.; Muhongo, S.; Stern, R.J.; Viola, G.

    2013-01-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara–Congo–Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian–Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite–Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650–620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo–Tanzania–Bangweulu Cratons and the Zimbabwe–Kalahari Craton. They closed during the ∼600–500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600–550 Ma extension is recorded in the Arabian–Nubian Shield and the Eastern Granulite–Cabo Delgado Nappe Complex. Later ∼550–480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings

  3. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution.

    PubMed

    Fritz, H; Abdelsalam, M; Ali, K A; Bingen, B; Collins, A S; Fowler, A R; Ghebreab, W; Hauzenberger, C A; Johnson, P R; Kusky, T M; Macey, P; Muhongo, S; Stern, R J; Viola, G

    2013-10-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution

  4. The anatomy of a deep intracontinental orogen

    NASA Astrophysics Data System (ADS)

    Raimondo, Tom; Collins, Alan S.; Hand, Martin; Walker-Hallam, Althea; Smithies, R. Hugh; Evins, Paul M.; Howard, Heather M.

    2010-08-01

    The crustal architecture of central Australia has been profoundly affected by protracted periods of intracontinental deformation. In the northwestern Musgrave Block, the Ediacaran-Cambrian (600-530 Ma) Petermann Orogeny resulted in pervasive mylonitic reworking of Mesoproterozoic granites and granitic gneisses at deep crustal levels (P = 10-14 kbar and T = 700-800°C). SHRIMP and LA-ICPMS dating of zircon indicate that peak metamorphic conditions were attained at circa 570 Ma, followed by slow cooling to ˜600-660°C at circa 540 Ma driven by exhumation along the Woodroffe Thrust. Strong links between regional kinematic partitioning, pervasive high shear strains and partial melting in the orogenic core, and an anomalous lobate thrust trace geometry suggest that north vergent shortening was accompanied by the gravitational collapse and lateral escape of a broad thrust sheet. Like the present-day Himalayan-Tibetan system, the macroscopic structural, metamorphic, and kinematic architecture of the Petermann Orogen appears to be dominantly shaped by large-scale ductile flow of lower crustal material. We thus argue that the anatomy of this deep intracontinental orogen is comparable to collisional orogens, suggesting that the deformational response of continental crust is remarkably similar in different tectonic settings.

  5. Tectonics of Atlantic Canada

    USGS Publications Warehouse

    Williams, H.; Dehler, S.A.; Grant, A.C.; Oakey, G.N.

    1999-01-01

    The tectonic history of Atlantic Canada is summarized according to a model of multiple ocean opening-closing cycles. The modern North Atlantic Ocean is in the opening phase of its cycle. It was preceded by an early Paleozoic lapetus Ocean whose cycle led to formation of the Appalachian Orogen. lapetus was preceded by the Neoproterozoic Uranus Ocean whose cycle led to formation of the Grenville Orogen. The phenomenon of coincident, or almost coincident orogens and modern continental margins that relate to repeated ocean opening-closing cycles is called the Accordion Effect. An understanding of the North Atlantic Ocean and its continental margins provides insights into the nature of lapetus and the evolution of the Appalachian Orogen. Likewise, an understanding of lapetus and the Appalachian Orogen raises questions about Uranus and the development of the Grenville Orogen. Modern tectonic patterns in the North Atlantic may have been determined by events that began before 1000 m.y.

  6. High-pressure metamorphism in the southern New England Orogen: Implications for long-lived accretionary orogenesis in eastern Australia

    NASA Astrophysics Data System (ADS)

    Phillips, G.; Offler, R.; Rubatto, D.; Phillips, D.

    2015-09-01

    New geochemical, metamorphic, and isotopic data are presented from high-pressure metamorphic rocks in the southern New England Orogen (eastern Australia). Conventional and optimal thermobarometry are augmented by U-Pb zircon and 40Ar/39Ar phengite dating to define pressure-temperature-time (P-T-t) histories for the rocks. The P-T-t histories are compared with competing geodynamic models for the Tasmanides, which can be summarized as (i) a retreating orogen model, the Tasmanides formed above a continuous, west dipping, and eastward retreating subduction zone, and (ii) a punctuated orogen model, the Tasmanides formed by several arc accretion, subduction flip, and/or transference events. Whereas both scenarios are potentially supported by the new data, an overlap between the timing of metamorphic recrystallization and key stages of Tasmanides evolution favors a relationship between a single, long-lived subduction zone and the formation, exhumation, and exposure of the high-pressure rocks. By comparison with the retreating orogen model, the following links with the P-T-t histories emerge: (i) exhumation and underplating of oceanic eclogite during the Delamerian Orogeny, (ii) recrystallization of underplated and exhuming high-pressure rocks at amphibolite facies conditions coeval with a period of rollback, and (iii) selective recrystallization of high-pressure rocks at blueschist facies conditions, reflecting metamorphism in a cooled subduction zone. The retreating orogen model can also account for the anomalous location of the Cambrian-Ordovician high-pressure rocks in the Devonian-Carboniferous New England Orogen, where sequential rollback cycles detached and translated parts of the leading edge of the overriding plate to the next, younger orogenic cycle.

  7. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    NASA Astrophysics Data System (ADS)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

  8. GEOPHYSICS. Layered deformation in the Taiwan orogen.

    PubMed

    Huang, T-Y; Gung, Y; Kuo, B-Y; Chiao, L-Y; Chen, Y-N

    2015-08-14

    The underthrusting of continental crust during mountain building is an issue of debate for orogens at convergent continental margins. We report three-dimensional seismic anisotropic tomography of Taiwan that shows a nearly 90° rotation of anisotropic fabrics across a 10- to 20-kilometer depth, consistent with the presence of two layers of deformation. The upper crust is dominated by collision-related compressional deformation, whereas the lower crust of Taiwan, mostly the crust of the subducted Eurasian plate, is dominated by convergence-parallel shear deformation. We interpret this lower crustal shearing as driven by the continuous sinking of the Eurasian mantle lithosphere when the surface of the subducted plate is coupled with the orogen. The two-layer deformation clearly defines the role of subduction in the formation of the Taiwan mountain belt.

  9. Compressional intracontinental orogens: Ancient and modern perspectives

    NASA Astrophysics Data System (ADS)

    Raimondo, Tom; Hand, Martin; Collins, William J.

    2014-03-01

    Compressional intracontinental orogens are major zones of crustal thickening produced at large distances from active plate boundaries. Consequently, any account of their initiation and subsequent evolution must be framed outside conventional plate tectonics theory, which can only explain the proximal effects of convergent plate-margin interactions. This review considers a range of hypotheses regarding the origins and transmission of compressive stresses in intraplate settings. Both plate-boundary and intraplate stress sources are investigated as potential driving forces, and their relationship to rheological models of the lithosphere is addressed. The controls on strain localisation are then evaluated, focusing on the response of the lithosphere to the weakening effects of structural, thermal and fluid processes. With reference to the characteristic features of intracontinental orogens in central Asia (the Tien Shan) and central Australia (the Petermann and Alice Springs Orogens), it is argued that their formation is largely driven by in-plane stresses generated at plate boundaries, with the lithosphere acting as an effective stress guide. This implies a strong lithospheric mantle rheology, in order to account for far-field stress propagation through the discontinuous upper crust and to enable the support of thick uplifted crustal wedges. Alternative models of intraplate stress generation, primarily involving mantle downwelling, are rejected on the grounds that their predicted temporal and spatial scales for orogenesis are inconsistent with the observed records of deformation. Finally, inherited mechanical weaknesses, thick sedimentary blanketing over a strongly heat-producing crust, and pervasive reaction softening of deep fault networks are identified as important and interrelated controls on the ability of the lithosphere to accommodate rather than transmit stress. These effects ultimately produce orogenic zones with architectural features and evolutionary

  10. Lu-Hf and Sm-Nd geochronology of garnet gneisses in the central Appalachians, U.S.: Implications for the timing and duration of Grenville Orogeny

    NASA Astrophysics Data System (ADS)

    Vervoort, Jeff; Ramsey, Molly; Mulcahy, Sean; Aleinikoff, John; Southworth, Scott

    2014-05-01

    The Grenville orogeny is one of the most significant geological events in Earth's history with remnants of this event prominent on virtually every continent. Constraining its timing and duration is important not only for understanding the tectonics of the Grenville itself, but also for understanding supercontinent cycles and other questions of Earth's evolution. In order to provide better constraints on the timing of Grenvillian metamorphism, we analyzed garnet-bearing Mesoproterozoic ortho and paragneisses, collected along a 150 km transect in the northern Blue Ridge Province, using combined Lu-Hf and Sm-Nd geochronology. The orthogneisses have U-Pb zircon crystallization ages of ~1140 and 1100 Ma. The paragneisses have maximum depositional ages ~1050 to 1020 Ma, based on the youngest detrital zircon populations. Zircon overgrowths and monazite ages suggest metamorphic events between ~1050 and 960 Ma. The Lu-Hf and Sm-Nd data for these samples both yield robust garnet ages with large spread of parent/daughter ratios, low age uncertainties, and low MSWD values. Lu-Hf ages define a narrow time span (1043±12 Ma to 1016±4 Ma; wtd. mean, 1024±7 Ma, 2σ). The Sm-Nd ages, determined on the same solutions as Lu-Hf, also define a narrow time range but are systematically younger (974±11 Ma to 932±5 Ma; wtd. mean, 957±10 Ma). The average difference between Lu-Hf and Sm-Nd ages is 67 Ma; the oldest Sm-Nd age is 40 Ma younger than the youngest Lu-Hf age. These large systematic differences in the ages are enigmatic. While Sm-Nd ages younger than Lu-Hf are not uncommon, these differences are typically small. There are, however, potential explanations for these differences. (1) Lu partitions strongly into garnet during growth resulting in high Lu/Hf ratios in the core and yielding ages weighted toward the beginning of growth (e.g., Skora, 2006); no similar partitioning exists in Sm/Nd and these ages reflect mean garnet growth. (2) Lu diffuses much faster than Hf at elevated

  11. SVM-based base-metal prospectivity modeling of the Aravalli Orogen, Northwestern India

    NASA Astrophysics Data System (ADS)

    Porwal, Alok; Yu, Le

    2010-05-01

    The Proterozoic Aravalli orogen in the state of Rajasthan, northwestern India, constitutes the most important metallogenic province for base-metal deposits in India and hosts the entire economically viable lead-zinc resource-base of the country. The orogen evolved through near-orderly Wilson cycles of repeated extensional and compressional tectonics resulting in sequential opening and closing of intracratonic rifts and amalgamation of crustal domains during a circa 1.0-Ga geological history from 2.2 Ga to 1.0 Ga. This study develops a conceptual tectonostratigraphic model of the orogen based on a synthesis of the available geological, geophysical and geochronological data followed by deep-seismic-reflectivity-constrained 2-D forward gravity modeling, and links it to the Proterozoic base-metal metallogeny in the orogen in order to identify key geological controls on the base-metal mineralization. These controls are translated into exploration criteria for base-metal deposits, validated using empirical spatial analysis, and used to derive input spatial variables for model-based base-metal prospectivity mapping of the orogen. A support vector machine (SVM) algorithm augmented by incorporating a feature selection procedure is used in a GIS environment to implement the prospectivity mapping. A comparison of the SVM-derived prospectivity map with the ones derived using other established models such as neural-networks, logistic regression, and Bayesian weights-of-evidence indicates that the SVM outperforms other models, which is attributed to the capability of the SVM to return robust classification based on small training datasets.

  12. Petrogenesis of the Elzevir batholith and related trondhjemitic intrusions in the grenville province of eastern Ontario, Canada

    NASA Astrophysics Data System (ADS)

    Pride, C.; Moore, J. M.

    1983-06-01

    The Elzevir batholith belongs to a suite of trondhjemitic intrusions emplaced at ca. 1,240 Ma in the Grenville Province of eastern Ontario. New major and trace element data, including REE, combined with isotopic and petrographic data indicate that: 1) the batholith has calc-alkalic affinities; 2) the Elzevir parental magma is very similar to that of dacites in the nearby, coeval metavolcanic rocks; the magma formed by partial melting of crustal material at granulite grade; 3) chemical differences between the plutonic and volcanic rocks can be best explained by accumulation of plagioclase in the plutonic environment; 4) fractionation was dominated by plagioclase and quartz, with lesser biotite and epidote, and minor zircon and apatite. It is suggested that melting of sialic crust took place during the ‘docking’ of a partly-evolved, originally ensimatic arc system against the main cratonic mass to the northwest.

  13. SW U. S. diabase province: A 1. 1-Ga intrusion event of middle Grenville and middle Keweenawan age

    SciTech Connect

    Conway, C.M.; Elston, D.P. ); Wrucke, C.T. )

    1993-02-01

    Diabase in the southwestern US intrudes Middle Proterozoic stratified rocks as sills and Early and Middle Proterozoic crystalline rocks as subhorizontal sheets and subvertical dikes. It is discontinuous in a broad belt extending from western Texas to southeastern California. The best known intrusions are sills in Middle Proterozoic strata in Death Valley, Grand Canyon, and central Arizona. Sparse to rare dikes in some of these strata trend mostly north but range from north-northeast to west-northwest. Diabase dikes widespread in crystalline rocks in western Arizona and adjacent parts of southeastern California strike from north to west-northwest, but are predominantly northwesterly. Dikes and sheets are also present in crystalline rocks in the southern Pinaleno Mountains, southeastern Arizona, where dikes strike west-northwest. The northwest trend of the diabase province and prevalent northwesterly trend of dikes in crystalline rocks suggest that intrusion was controlled by an approximately horizontal least compressive stress field roughly parallel to the Grenville Front. Radiometric ages of Arizona and California diabase indicate emplacement at [approximately]1,100 Ma. Paleomagnetic poles from diabase sills and enclosing stratified rocks in Arizona correlate with poles reported from middle and early-late Keweenawan rocks of Lake Superior. Emplacement of the diabase coincides with: (1) the middle Keweenawan eruptive and intrusive episode of the Midcontinent Rift System; (2) a major episode of (middle) Grenville thrusting and deformation documented in the Van Horn area; and (3) a time of abrupt reversal in North American apparent polar wander. These interrelated manifestations presumably arose in response to a major episode of plate interaction and collision between North American and a plate that encroached from the southeast.

  14. Flow of ultra-hot Precambrian orogens and the making of crustal layering in Phanerozoic orogenic plateaux

    NASA Astrophysics Data System (ADS)

    Chardon, Dominique; Gapais, Denis; Cagnard, Florence; Jayananda, Mudlappa; Peucat, Jean-Jacques

    2010-05-01

    Reassessment of structural / metamorphic properties of ultra-hot Precambrian orogens and shortening of model weak lithospheres support a syn-convergence flow mode on an orogen scale, with a large component of horizontal finite elongation parallel to the orogen. This orogen-scale flow mode combines distributed shortening, gravity-driven flow, lateral escape, and three-dimensional mass redistribution of buried supracrustal rocks, magmas and migmatites in a thick fluid lower crust. This combination preserves a nearly flat surface and Moho. The upper crust maintains a nearly constant thickness by real-time erosion and near-field clastic sedimentation and by ablation at its base by burial of pop-downs into the lower crust. Steady state regime of these orogens is allowed by activation of an attachment layer that maintains kinematic compatibility between the thin and dominantly plastic upper crust and a thick "water bed" of lower crust. Because very thin lithospheres of orogenic plateaux and Precambrian hot orogens have similar thermomechanical structures, bulk orogenic flow comparable to that governing Precambrian hot orogens should actually operate through today's orogenic plateaux as well. Thus, syn-convergence flow fabrics documented on exposed crustal sections of ancient hot orogens that have not undergone collapse may be used to infer the nature of flow fabrics that are imaged by geophysical techniques beneath orogenic plateaux. We provide a detailed geological perspective on syn-convergence crustal flow in relation to magma emplacement and partial melting on a wide oblique crustal transition of the Neoarchean ultra-hot orogen of Southern India. We document sub-horizontal bulk longitudinal flow of the partially molten lower crust over a protracted period of 60 Ma. Bulk flow results from the interplay of (1) pervasive longitudinal transtensional flow of the partially molten crust, (2) longitudinal coaxial flow on flat fabrics in early plutons, (3) distributed, orogen

  15. Metamorphic complexes in accretionary orogens: Insights from the Beishan collage, southern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Song, Dongfang; Xiao, Wenjiao; Windley, Brian F.; Han, Chunming; Yang, Lei

    2016-10-01

    The sources of ancient zircons and the tectonic attributions and origins of metamorphic complexes in Phanerozoic accretionary orogens have long been difficult issues. Situated between the Tianshan and Inner Mongolia orogens, the Beishan orogenic collage (BOC) plays a pivotal role in understanding the accretionary processes of the southern Central Asian Orogenic Belt (CAOB), particularly the extensive metamorphic and high-strained complexes on the southern margin. Despite their importance in understanding the basic architecture of the southern CAOB, little consensus has been reached on their ages and origins. Our new structural, LA-ICP-MS zircon U-Pb and Hf isotopic data from the Baidunzi, Shibandun, Qiaowan and Wutongjing metamorphic complexes resolve current controversial relations. The metamorphic complexes have varied lithologies and structures. Detrital zircons from five para-metamorphic rocks yield predominantly Phanerozoic ages with single major peaks at ca. 276 Ma, 286 Ma, 427 Ma, 428 Ma and 461 Ma. Two orthogneisses have weighted mean ages of 294 ± 2 Ma and 304 ± 2 Ma with no Precambrian inherited zircons. Most Phanerozoic zircons show positive εHf(t) values indicating significant crustal growth in the Ordovician, Silurian and Permian. The imbricated fold-thrust deformation style combined with diagnostic zircon U-Pb-Hf isotopic data demonstrate that the metamorphic rocks developed in a subduction-accretion setting on an arc or active continental margin. This setting and conclusion are supported by the nearby occurrence of Ordovician-Silurian adakites, Nb-rich basalts, Carboniferous-Permian ophiolitic mélanges, and trench-type turbidites. Current data do not support the presence of a widespread Precambrian basement in the evolution of the BOC; the accretionary processes may have continued to the early Permian in this part of the CAOB. These relationships have meaningful implications for the interpretation of the tectonic attributions and origins of other

  16. Neoproterozoic, Paleozoic, and Mesozoic granitoid magmatism in the Qinling Orogen, China: Constraints on orogenic process

    NASA Astrophysics Data System (ADS)

    Wang, Xiaoxia; Wang, Tao; Zhang, Chengli

    2013-08-01

    The Qinling Orogen is one of the main orogenic belts in Asia and is characterized by multi-stage orogenic processes and the development of voluminous magmatic intrusions. The results of zircon U-Pb dating indicate that granitoid magmatism in the Qinling Orogen mainly occurred in four distinct periods: the Neoproterozoic (979-711 Ma), Paleozoic (507-400 Ma), and Early (252-185 Ma) and Late (158-100 Ma) Mesozoic. The Neoproterozoic granitic magmatism in the Qinling Orogen is represented by strongly deformed S-type granites emplaced at 979-911 Ma, weakly deformed I-type granites at 894-815 Ma, and A-type granites at 759-711 Ma. They can be interpreted as the products of respectively syn-collisional, post-collisional and extensional setting, in response to the assembly and breakup of the Rodinia supercontinent. The Paleozoic magmatism can be temporally classified into three stages of 507-470 Ma, 460-422 Ma and ˜415-400 Ma. They were genetically related to the subduction of the Shangdan Ocean and subsequent collision of the southern North China Block and the South Qinling Belt. The 507-470 Ma magmatism is spatially and temporally related to ultrahigh-pressure metamorphism in the studied area. The 460-422 Ma magmatism with an extensive development in the North Qinling Belt is characterized by I-type granitoids and originated from the lower crust with the involvement of mantle-derived magma in a collisional setting. The magmatism with the formation age of ˜415-400 Ma only occurred in the middle part of the North Qinling Belt and is dominated by I-type granitoid intrusions, and probably formed in the late-stage of a collisional setting. Early Mesozoic magmatism in the study area occurred between 252 and 185 Ma, with the cluster in 225-200 Ma. It took place predominantly in the western part of the South Qinling Belt. The 250-240 Ma I-type granitoids are of small volume and show high Sr/Y ratios, and may have been formed in a continental arc setting related to subduction

  17. Mantle refertilization and magmatism in old orogenic regions: The role of late-orogenic pyroxenites

    NASA Astrophysics Data System (ADS)

    France, Lydéric; Chazot, Gilles; Kornprobst, Jacques; Dallai, Luigi; Vannucci, Riccardo; Grégoire, Michel; Bertrand, Hervé; Boivin, Pierre

    2015-09-01

    Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr-Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1-2 GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions. The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.

  18. 1.45 Ga granulites in the southwestern Grenville province: Geologic setting, P-T conditions, and U-Pb geochronology

    NASA Astrophysics Data System (ADS)

    Ketchum, J. W. F.; Jamieson, R. A.; Heaman, L. M.; Culshaw, N. G.; Krogh, T. E.

    1994-03-01

    In the southwestern Grenville province, the parautochthonous Britt domain includes a variety of pre-Grenvillian metamorphic and plutonic rocks that were reworked at upper amphibolite facies during the Grenvillian orogeny. Near Pointe-au-Baril, Ontario, a crustal block containing pre-Grenvillian granulite facies mineral assemblages and pre-Grenvillian to early Grenvillian tectonic fabrics has been identified. The block is bounded on the northwest and southeast by extensional shear zones that may have isolated it from regional late Gren- villian deformation. Multiequilibria pressure-temperature (P-T) calculations for orthopyroxene-bearing mafic rocks suggest conditions of 625-700 °C and 7.2-8.4 kbar for the pre-Grenvillian metamorphism. The granulite facies assemblages were locally overprinted during higher pressure Grenvillian metamorphism, which peaked at 720-775 °C and 10.8-11.5 kbar. U-Pb zircon data from migmatitic, mafic supracrustal gneiss indicate metamorphism and leucosome development at ca. 1450-1430 Ma, in agreement with other pre-Grenvillian metamorphic ages for the Central gneiss belt and Grenville Front tectonic zone. An expanding data base on pre-Grenvillian events in the southwestern Grenville province indicates that high-grade metamorphism at ca. 1450-1430 Ma affected a large region of crust and was coeval with widespread felsic to intermediate plutonism.

  19. Ambient tremors in a collisional orogenic belt

    USGS Publications Warehouse

    Chuang, Lindsay Yuling; Chen, Kate Huihsuan; Wech, Aaron G.; Byrne, Timothy; Peng, Wei

    2014-01-01

    Deep-seated tectonic tremors have been regarded as an observation tied to interconnected fluids at depth, which have been well documented in worldwide subduction zones and transform faults but not in a collisional mountain belt. In this study we explore the general features of collisional tremors in Taiwan and discuss the possible generation mechanism. In the 4 year data, we find 231 ambient tremor episodes with durations ranging from 5 to 30 min. In addition to a coseismic slip-induced stress change from nearby major earthquake, increased tremor rate is also highly correlated with the active, normal faulting earthquake swarms at the shallower depth. Both the tremor and earthquake swarm activities are confined in a small, area where the high attenuation, high thermal anomaly, the boundary between high and low resistivity, and localized veins on the surfaces distributed, suggesting the involvement of fluids from metamorphic dehydration within the orogen.

  20. Orogenic float of the Venezuelan Andes

    NASA Astrophysics Data System (ADS)

    Monod, Bernard; Dhont, Damien; Hervouët, Yves

    2010-07-01

    The Venezuelan (or Mérida) Andes are a NE-trending intracontinental orogen that started to rise from the Middle Miocene due to the E-W far field convergence between the Maracaibo block to the northwest and the Guyana shield to the southeast. Oblique convergence is responsible for strain partitioning with thrusting along both foreland basins and right-lateral strike-slip faulting along the NE-SW Boconó fault cutting the Venezuelan Andes along-strike. The central part of the belt is also cut by the N-S left-lateral strike-slip Valera fault that connects the Boconó fault, both faults bounding the Trujillo block that escapes towards the NNE. Even though the regional geology of belt is well known, its structure at depth remains a matter of debate. Our work, based on the integration of geological and geophysical data aims to better constrain the deep geometry of faults and the tectonic evolution of the mountain belt. We used the orogenic float model to construct two NW-SE trans-Andean crustal scale balanced sections. The Late Neogene-Quaternary shortening varies from 40 km in the south to 30 km in the north across the Trujillo block, indicating that a quarter of the deformation seems to be absorbed by the tectonic escape process. More importantly, a major reorganization in the crust took place in the Early Pliocene. It is characterized by the imbrication of the Maracaibo crust into the Guyana crust. This resulted in the subduction of the Guyana lower crust and the formation of a NW-vergent basement thrust propagating upwards and surfacing along the Las Virtudes thrust. Rapid uplift of the northern flank of the belt subsequently occurred together with massive deposition of the Plio-Quaternary coarse grained Betijoque formation in the northwestern foreland basin.

  1. Proterozoic subduction and terrane amalgamation in the southwestern Grenville province, Canada: Evidence from ultrapotassic to shoshonitic plutonism

    NASA Astrophysics Data System (ADS)

    Corriveau, Louise

    1990-07-01

    A late Grenvillian (1089-1076 Ma) subduction regime followed by terrane amalgamation is postulated as the paleoenvironment of a 400-km-long belt of potassium-rich alkaline and shoshonitic plutons in the Central metasedimentary belt of the southwestern Grenville province, Canada. Emplacement of the plutons postdates the regional metamorphism in the country rock, but predates major shear zones that form a structural boundary for the plutonic belt. The extent, timing, and magmatic affinities of the suite delineate the Gatineau domain within the current Mont-Laurier terrane of Quebec and are compelling evidence for the allochthonous nature of the Elzevir terrane and its extension eastward to Rideau Lake in Ontario and northward into the Gatineau domain. The belt trends northeast, the emplacement ages are younger to the southeast, and the magmatic affinities are those of island-arc ultrapotassic to shoshonitic rocks. This is interpreted to reflect the existence of a southeast-dipping, northeast-trending subduction zone beneath the combined Elzevir-Gatineau terrane between 1089 and 1076 Ma. Subsequent to subduction, amalgamation of the Elzevir terrane to the other terranes formed the Central metasedimentary belt, which then accreted and collided with the allochthonous polycyclic belt. These events provide evidence that the Ottawan orogeny commenced at ca. 1090 Ma in the Central metasedimentary belt. A modern analogue of this plutonism and its tectonic setting may have been the magmatism and arc-continent collision and subduction setting of the Sunda are, Indonesia.

  2. A 40Ar/ 39Ar study of post-tectonic cooling in the Britt domain of the Grenville Province, Ontario

    NASA Astrophysics Data System (ADS)

    Culshaw, N.; Reynolds, P. H.; Check, G.

    1991-08-01

    We report the results of a 40Ar/ 39Ar study of rocks from the Britt domain, northwestern Grenville Province. Most of the age spectra obtained for micas and amphiboles have well defined age plateaus. Typically, K-feldspars yielded spectra with age gradients and Arrhenius plots with kinks or offsets. Hence, some of the data were interpreted in terms of a multi-diffusion domain-size model. This 40Ar/ 39Ar data base (5 amphiboles, 9 micas, 4 K-feldspar), along with some recently acquired U&z.sbnd;Pb data, provide reasonable constraints on the post-peak metamorphic cooling history which began at depth in the upper amphibolite facies. Comparison of the data with exhumation paths generated by thermal models suggest that cooling, taking place over ˜ 400 Ma, was for the most part due to erosionally controlled exhumation. Although the decompression may have initiated with a short period of rapid (extensional?) uplift, the entire cooling history may have been erosionally controlled. There is an apparent difference in cooling ages between north and south in the 40Ar/ 39Ar mica and K-feldspar data for the Britt domain, which may be due to somewhat higher exhumation rates in the north and/or an elevated geotherm in the south.

  3. Mesoproterozoic syntectonic garnet within Belt Supergroup metamorphic tectonites: Evidence of Grenville-age metamorphism and deformation along northwest Laurentia

    USGS Publications Warehouse

    Nesheim, T.O.; Vervoort, J.D.; McClelland, W.C.; Gilotti, J.A.; Lang, H.M.

    2012-01-01

    Northern Idaho contains Belt-Purcell Supergroup equivalent metamorphic tectonites that underwent two regional deformational and metamorphic events during the Mesoproterozoic. Garnet-bearing pelitic schists from the Snow Peak area of northern Idaho yield Lu-Hf garnet-whole rock ages of 1085??2. Ma, 1198??79. Ma, 1207??8. Ma, 1255??28. Ma, and 1314??2. Ma. Garnet from one sample, collected from the Clarkia area, was micro-drilled to obtain separate core and rim material that produced ages of 1347??10. Ma and 1102??47. Ma. The core versus rim ages from the micro-drilled sample along with the textural and spatial evidence of the other Lu-Hf garnet ages indicate two metamorphic garnet growth events at ~. 1330. Ma (M1) and ~. 1080. Ma (M2) with the intermediate ages representing mixed ages. Some garnet likely nucleated and grew M1 garnet cores that were later overgrown by younger M2 garnet rims. Most garnet throughout the Clarkia and Snow Peak areas are syntectonic with a regional penetrative deformational fabric, preserved as a strong preferred orientation of metamorphic matrix minerals (e.g., muscovite and biotite). The syntectonic garnets are interpreted to represent one regional, coeval metamorphic and deformation event at ~. 1080. Ma, which overlaps in time with the Grenville Orogeny. The older ~. 1330. Ma ages may represent an extension of the East Kootenay Orogeny described in western Canada. These deformational and metamorphic events indicate that western Laurentia (North America) was tectonically active in the Mesoproterozoic and during the assembly of the supercontinent Rodinia. ?? 2011 Elsevier B.V.

  4. Paleozoic orogens in New England, USA

    USGS Publications Warehouse

    Robinson, P.; Tucker, R.D.; Bradley, D.; Berry, H.N.; Osberg, P.H.

    1998-01-01

    Stratigraphy and isotope geochronology in the crystalline core of the Appalachians suggest revised interpretations of the extent, nature and timing of Paleozoic orogens in New England. Five major episodes of magmatism, deformation, and high-grade regional metamorphism are recognized: Taconian (455-442 Ma), Acadian (423-385 Ma), Neo-Acadian (366-350 Ma), Late Pennsylvanian (300-290 Ma) and Alleghanian (280-260 Ma). In the Taconian, the passive margin of Laurentia was subducted below a complex magmatic arc lasting from 480 to 442 Ma, founded in part on continental crust of a Medial New England terrane with possible affinities with Amazonia. Questions about Medial New England involve its coherence as a single plate, and the nature of its underlying crust. The Acadian began in Late Silurian as a collision between the amalgamated Laurentia-Medial New England and outer belts of Composite Avalon along a cryptic suture in coastal Maine, and progressed northwestward to the Connecticut Valley basin by mid-Devonian. Tonalitic-granitic magmatism and up to granulite-facies metamorphism culminated in Early Devonian, possibly tied to lithospheric detachment below the subducting northwestern plate and consequent asthenosphere upwelling. Newly discovered Neo-Acadian Late Devonian to Early Mississippian tonalitic-granitic magmatism, up to granulite-facies metamorphism, and severe deformation in central Massachusetts took place in a plate context poorly understood. Late Pennsylvanian effects include magmatism, metamorphism, and deformation near south New England gneiss domes and the Sebago batholith, and development of the right-lateral Norumbega fault system. Permian Alleghanian effects include penetrative deformation, granitic intrusions and up to sillimanite-grade metamorphism of Pennsylvanian beds in southeastern New England. These last two episodes relate to the arrival of Africa.

  5. Tectonometamorphic evolution of the Rhodope orogen

    NASA Astrophysics Data System (ADS)

    Krenn, Kurt; Bauer, Christoph; Proyer, Alexander; KlöTzli, Urs; Hoinkes, Georg

    2010-08-01

    This study combines new data on tectonostratigraphy, macrostructures and microstructures, petrology, and geochronology to propose a comprehensive model for the tectonometamorphic evolution of the Rhodope orogen from the Jurassic to the early Paleogene. Rocks from two study areas in the central and eastern Greek Rhodope represent a continental suture zone (Rhodope Suture Zone), with the included material most likely forming an extensional allochthon south of the European continent during Permo/Triassic times that was subsequently subducted beneath Europe in the Early Jurassic (≥180 Ma). On the basis of comparable metamorphic ages and coherent structures but differences in metamorphic conditions and lithologies, the rocks of the Rhodope Suture Zone are subdivided into an upper and a lower part. The prograde history is linked with subduction-related structures in the lower part (uniaxial stretching, deformation stage D1). In metapelites, the earliest stage of metamorphism recorded at circa 180 Ma occurred at least under ultrahigh-pressure metamorphic conditions. The rocks of the upper part experienced isothermal decompression with partial anatexis. Exhumation paths of both parts differ in temperature because of the relative tectonic position within the exhuming wedge. Exhumation was forced by the Nestos Shear Zone that controlled the early phase of normal displacement by SW shearing at the base (lower part) and NE shearing on top (upper part) from the Late Jurassic to the Late Cretaceous (deformation stage D2). An intervening stage of mineral recrystallization and thermal reequilibration in the upper part was followed by a common exhumation history of both parts at pressures lower than about 12 kbar (35-40 km depth). During this stage, exhumation was controlled by southwest directed shearing and folding (deformation stage D3). Slab retreat to the south led to subsequent extension (deformation stage D4) and final exhumation coeval with the formation of basement domes

  6. Stalled Orogen Linked to East Antarctic Craton Assembly

    NASA Astrophysics Data System (ADS)

    Martos, Y. M.; Ferraccioli, F.; Finn, C.; Bell, R. E.; Jordan, T. A.; Damaske, D.

    2015-12-01

    The interior of East Antarctica is often regarded as a coherent Archean craton surrounded by Paleo to Neoproterozoic orogenic belts. Here we use recent aerogeophysical, satellite magnetic, satellite gravity and passive seismic results in central East Antarctica to challenge this view. Firstly, anomalously thick crust (compared to most other cratons) has been imaged in East Antarctica by both passive seismic and gravity modelling with values up to 60 km (Ferraccioli et al., 2011, Nature; An et al., 2015, JGR). The thick crust underlies both the Gamburtsev Subglacial Mountains and an elevated region between Lake Vostok and Dronning Maud Land, referred to as the East Antarctic Mountain Ranges plateau (An et al., 2015). Second, satellite magnetic data reveal that the Gamburtev Province lies in between the Ruker/Princess Elizabeth Land, Vostok, Nimrod/South Pole and Recovery provinces. The Nimrod/ South Pole province is a Paleo to Meso Proterozoic igneous province formed along the edges of the Archean/Paleoproterozoic Mawson continent (e.g. Goodge and Fanning, 2010 JGR). Our aerogeophysical and sediment provenance data interpretations suggest that the Gamburtsev Province represents a distinct Grenvillian-age orogenic belt. A stalled orogen with thick crust (i.e. an orogen where widespread orogenic collapse and root delamination has not occurred)- is preserved in the interior of East Antarctica resembling e.g. the Paleoproterozoic Trans Hudson Orogen and segments of Grenvillian orogens in Laurentia. The stalled orogen may relate to widespread accretionary and collisional events within Rodinia. However, passive seismic interpretations (An et al., 2015) favour linking crustal thickening to the Pan-African age assembly of Greater India, East Antarctica and Australia within Gondwana (e.g. Aitken et al., 2014 GRL). Further aerogeophysical observations over Princess Elizabeth Land are timely to enable more robust correlations with geological observations and to help dating the

  7. Linking magmatism with collision in an accretionary orogen

    PubMed Central

    Li, Shan; Chung, Sun-Lin; Wilde, Simon A.; Wang, Tao; Xiao, Wen-Jiao; Guo, Qian-Qian

    2016-01-01

    A compilation of U-Pb age, geochemical and isotopic data for granitoid plutons in the southern Central Asian Orogenic Belt (CAOB), enables evaluation of the interaction between magmatism and orogenesis in the context of Paleo-Asian oceanic closure and continental amalgamation. These constraints, in conjunction with other geological evidence, indicate that following consumption of the ocean, collision-related calc-alkaline granitoid and mafic magmatism occurred from 255 ± 2 Ma to 251 ± 2 Ma along the Solonker-Xar Moron suture zone. The linear or belt distribution of end-Permian magmatism is interpreted to have taken place in a setting of final orogenic contraction and weak crustal thickening, probably as a result of slab break-off. Crustal anatexis slightly post-dated the early phase of collision, producing adakite-like granitoids with some S-type granites during the Early-Middle Triassic (ca. 251–245 Ma). Between 235 and 220 Ma, the local tectonic regime switched from compression to extension, most likely caused by regional lithospheric extension and orogenic collapse. Collision-related magmatism from the southern CAOB is thus a prime example of the minor, yet tell-tale linking of magmatism with orogenic contraction and collision in an archipelago-type accretionary orogen. PMID:27167207

  8. Linking magmatism with collision in an accretionary orogen.

    PubMed

    Li, Shan; Chung, Sun-Lin; Wilde, Simon A; Wang, Tao; Xiao, Wen-Jiao; Guo, Qian-Qian

    2016-05-11

    A compilation of U-Pb age, geochemical and isotopic data for granitoid plutons in the southern Central Asian Orogenic Belt (CAOB), enables evaluation of the interaction between magmatism and orogenesis in the context of Paleo-Asian oceanic closure and continental amalgamation. These constraints, in conjunction with other geological evidence, indicate that following consumption of the ocean, collision-related calc-alkaline granitoid and mafic magmatism occurred from 255 ± 2 Ma to 251 ± 2 Ma along the Solonker-Xar Moron suture zone. The linear or belt distribution of end-Permian magmatism is interpreted to have taken place in a setting of final orogenic contraction and weak crustal thickening, probably as a result of slab break-off. Crustal anatexis slightly post-dated the early phase of collision, producing adakite-like granitoids with some S-type granites during the Early-Middle Triassic (ca. 251-245 Ma). Between 235 and 220 Ma, the local tectonic regime switched from compression to extension, most likely caused by regional lithospheric extension and orogenic collapse. Collision-related magmatism from the southern CAOB is thus a prime example of the minor, yet tell-tale linking of magmatism with orogenic contraction and collision in an archipelago-type accretionary orogen.

  9. Rotund versus skinny orogens: Well-nourished or malnourished gold?

    USGS Publications Warehouse

    Goldfarb, R.J.; Groves, D.I.; Gardoll, S.

    2001-01-01

    Orogenic gold vein deposits require a particular conjunction of processes to form and be preserved, and their global distribution can be related to broad-scale, evolving tectonic processes throughout Earth history. A heterogeneous distribution of formation ages for these mineral deposits is marked by two major Precambrian peaks (2800-2555 Ma and 2100-1800 Ma), a singular lack of deposits for 1200 m.y. (1800-600 Ma), and relatively continuous formation since then (after 600 Ma). The older parts of the distribution relate to major episodes of continental growth, perhaps controlled by plume-influenced mantle overturn events, in the hotter early Earth (ca. 1800 Ma or earlier). This worldwide process allowed preservation of gold deposits in cratons, roughly equidimensional, large masses of buoyant continental crust. Evolution to a less episodic, more continuous, modern-style plate tectonic regime led to the accretion of volcano-sedimentary complexes as progressively younger linear orogenic belts sorrounding the margins of the more buoyant cratons. The susceptibility of these linear belts to uplift and erosion can explain the overall lack of orogenic gold deposits at 1800-600 Ma, their exposure in 600-50 Ma orogens, the increasing importance of placer deposits back through the Phanerozoic since ca. 100 Ma, and the absence of gold deposits in orogenic belts younger than ca. 50 Ma.

  10. A geophysical model of the Variscan orogenic root (Bohemian Massif): Implications for modern collisional orogens

    NASA Astrophysics Data System (ADS)

    Guy, Alexandra; Edel, Jean-Bernard; Schulmann, Karel; Tomek, Čestmir; Lexa, Ondrej

    2011-05-01

    surface geology are interpreted as a result of Carboniferous partial overturn of low density lower crust and high and intermediate density crust in the area of central root and by viscous extrusion of low density orogenic lower crust over the high density Bruno-Vistulian continent. Comparison of these data with geophysical profiling of the Andean and the Tibetan plateaus suggests that modern orogenic systems reveal comparable deep crustal geophysical pattern. Based on these similarities we propose that the Variscan root represents a deep crustal section of above mentioned plateaus, which may have develop by the same orogenic process.

  11. Grenville age of basement rocks in Cape May NJ well: New evidence for Laurentian crust in U.S. Atlantic Coastal Plain basement Chesapeake terrane

    USGS Publications Warehouse

    Sheridan, R.E.; Maguire, T.J.; Feigenson, M.D.; Patino, L.C.; Volkert, R.A.

    1999-01-01

    The Chesapeake terrane of the U.S. mid-Atlantic Coastal Plain basement is bounded on the northwest by the Salisbury positive gravity and magnetic anomaly and extends to the southeast as far as the Atlantic coast. It underlies the Coastal Plain of Virginia, Maryland, Delaware and southern New Jersey. Rubidium/Strontium dating of the Chesapeake terrane basement yields an age of 1.025 ?? 0.036 Ga. This age is typical of Grenville province rocks of the Middle to Late Proterozoic Laurentian continent. The basement lithologies are similar to some exposed Grenville-age rocks of the Appalachians. The TiO2 and Zr/P2O5 composition of the metagabbro from the Chesapeake terrane basement is overlapped by those of the Proterozoic mafic dikes in the New Jersey Highlands. These new findings support the interpretation that Laurentian basement extends southeast as far as the continental shelf in the U.S. mid-Atlantic region. The subcrop of Laurentian crust under the mid-Atlantic Coastal Plain implies unroofing by erosion of the younger Carolina (Avalon) supracrustal terrane. Dextral-transpression fault duplexes may have caused excessive uplift in the Salisbury Embayment area during the Alleghanian orogeny. This extra uplift in the Salisbury area may have caused the subsequent greater subsidence of the Coastal Plain basement in the embayment.

  12. The Relation of the Grenville/Appalachian Boundary to the April 20, 2002 Au Sable Forks, New York and the April 29, 2003 Fort Payne, Alabama Earthquakes

    NASA Astrophysics Data System (ADS)

    Detweiler, S. T.; Mooney, W. D.

    2003-12-01

    On April 20, 2002, an M=5.1 earthquake occurred near Au Sable Forks, New York at 10:50AM UTC (6:50AM locally). The significance of this event is twofold: first, such moderate earthquakes are relatively rare in this part of the North American continent, and second, the structure of the crust is well-known here because a major seismic investigation [Musacchio et al., 1997] took place before the Au Sable Forks event. That seismic survey derived detailed information about the deep subsurface, including the Champlain thrust fault, which was possibly the causative fault for this earthquake. The Champlain fault marks the local boundary between the Grenville and the Appalachian Provinces. On April 29, 2003 another event (M=4.6) occurred along the province boundary, this time in Fort Payne, Alabama. This second event is believed to have occurred on a nearly-vertical strike-slip fault. These two earthquakes are examples of the kind of temblors that the Grenville/Appalachian boundary is capable of producing, and we explore these events as they relate to seismic zones and background seismicity.

  13. The Queensborough mafic-ultramafic complex: a fragment of a Meso-Proterozoic ophiolite? Grenville Province, Canada

    NASA Astrophysics Data System (ADS)

    Smith, T. E.; Harris, M. J.

    1996-11-01

    The Queensborough mafic-ultramafic complex occurs as a fault-bounded block, up to 10 km wide and having an area > 220 km 2. It lies in the Grimsthorpe Domain of the Bancroft-Elzevir-Mazinaw-Sharbot Lake Terrane in the Central Metasedimentary Belt of the Grenville Province. It has been suggested, without adequate supporting data, that the complex may represent oceanic crust, a fragment of an ophiolite, or even a metavolcanic sequence made up of basaltic and komatiitic flows. The geological and tectonic significance of the complex is assessed using field relationships, petrography and geochemistry. Structurally the lowest part of the complex comprises a series of ultramafic rocks characterized by metre-scale compositional layering, represented by several different metamorphic assemblages of talc, chlorite, carbonate, anthophyllite, and actinolite-tremolite. These assemblages indicate that the original rocks were cumulate peridotites and pyroxenites. The ultramafic rocks are overlain structurally by a series of mafic rocks, predominantly massive to highly sheared gabbros. The gabbros are penetrated by a series of mafic dykes and include a few small enclaves of pillowed mafic volcanics. Major- and trace-element chemistry shows that the mafic rocks represent a fractionally crystallized sequence of tholeiitic gabbros, lavas, and mafic dykes and that the ultramafic cumulates are co-genetic. The regional geological setting, and the trace-element signatures of the mafic rocks suggest that they were formed in a back-arc basin. Comparison of the Queensborough Complex with Proterozoic and Phanerozoic igneous complexes suggests that it represents a partially preserved crustal section of a Mesoproterozoic ophiolite. In addition, the rocks of the Queensborough Complex are petrographically and geochemically similar to those of the Vavilov Basin which occurs in the deepest part of the Tyrrhenian Sea. By analogy with this Neogene back-arc basin we suggest that the Central

  14. The Capricorn Orogen Passive source Array (COPA) in Western Australia

    NASA Astrophysics Data System (ADS)

    Gessner, K.; Yuan, H.; Murdie, R.; Dentith, M. C.; Johnson, S.; Brett, J.

    2015-12-01

    COPA is the passive source component of a multi-method geophysical program aimed at assessing the mineral deposits potential of the Proterozoic Capricorn Orogen. Previous results from the active source surveys, receiver functions and magnetotelluric studies show reworked orogenic crust in the orogen that contrasts with more simple crust in the neighbouring Archean cratons, suggesting progressive and punctuated collisional processes during the final amalgamation of the Western Australian craton. Previous seismic studies are all based on line deployment or single station analyses; therefore it is essential to develop 3D seismic images to test whether these observations are representative for the whole orogen. With a careful design that takes advantage of previous passive source surveys, the current long-term and short-term deployments span an area of approximately 500 x 500 km. The 36-month total deployment can guarantee enough data recording for 3D structure imaging using body wave tomography, ambient noise surface wave tomography and P- and S-wave receiver function Common Conversion Point (CCP) stacking techniques. A successive instrument loan from the ANSIR national instrument pool, provided 34 broadband seismometers that have been deployed in the western half of the orogen since March 2014. We expect approximately 40-km lateral resolution near the surface for the techniques we propose, which due to low frequency nature of earthquake waves will degrade to about 100 km near the base of the cratonic lithosphere, which is expected at depths between 200 to 250 km. Preliminary results from the first half of the COPA deployment will be presented in the light of the hypotheses that 1) distinct crustal blocks can be detected continuously throughout the orogen (using ambient noise/body wave tomography); 2) distinct lithologies are present in the crust and upper mantle across the orogen (using receiver function CCP images); and 3) crustal and lithosphere deformation along

  15. Neoproterozoic to Paleozoic Geological Evolution of Mongolia: Constraints on Modes of "Crustal Growth" in the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Macdonald, F. A.; Bold, U.; Smith, E.; Olin, P. H.; Crowley, J. L.; Schmitz, M. D.

    2012-12-01

    The Central Asian Orogenic Belt (CAOB) is widely considered the largest area of Phanerozoic juvenile crustal growth on Earth. However, the timing and nature of the orogenic events in the core of the CAOB in Mongolia has remained poorly constrained due to a dearth of detailed geological and geochronological studies. To bridge this gap and test models of crustal growth, here we refine the sequencing of geological events by focusing on the formation and destruction of Neoproterozoic and Paleozoic tectonic basins. Mongolia's basins record a complete Neoproterozoic to Cambrian Wilson cycle with rifting of the Mongolian continent at ca. 700 Ma, the development of a Cryogenian to Ediacaran thermally subsiding passive margin, an arc-continent collision at ca. 520 Ma, and a continent-arc-continent collision at ca. 500 Ma. During this collisional orogeny, that is the Cambrian Altaids, crustal growth occurred largely through the obduction of ophiolites. Rifting of the southern margin occurred during the Ordovician Period, with the development of a Silurian passive margin. Oblique northwest-dipping subduction was initiated during the Devonian and resulted in a transpressional accretionary orogen. The CAOB culminated with a continent-arc-continent collision and the accretion of the North China and Tarim Blocks in the latest Permian. The Devonian to early Permian accretionary orogen is associated not only with voluminous plutonism, but also, major translational structures oblique to the margin resulting in the appearance of many accreted terranes. These data are consistent with existing coarse Hf and Nd isotopic data, but also provide a framework for future detailed studies. Although our geological constraints suggest distinct periods of apparent crustal growth through either collisional or accretionary orogenies, net crustal growth after accounting for recycling is equivocal.

  16. Sr, Nd and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador: Implications for crustal contamination and basement mapping

    USGS Publications Warehouse

    Ashwal, L.D.; Wooden, J.L.; Emslie, R.F.

    1986-01-01

    We report Sr, Nd and Pb isotopic compositions of mid-Proterozoic anorthosites and related rocks (1.45-1.65 Ga) and of younger olivine diabase dikes (1.4 Ga) from two complexes on either side of the Grenville Front in Labrador. Anorthositic or diabasic samples from the Mealy Mountains (Grenville Province) and Harp Lake (Nain-Churchill Provinces) complexes have very similar major, minor and trace element compositions, but distinctly different isotopic signatures. All Mealy Mountains samples have ISr = 0.7025-0.7033, ??{lunate}Nd = +0.6 to +5.6 and Pb isotopic compositions consistent with derivation from a mantle source depleted with respect to Nd/Sm and Rb/Sr. Pb isotopic compositions for the Mealy Mountains samples are slightly more radiogenic than model mantle compositions. All Harp Lake samples have ISr = 0.7032-0.7066, ??{lunate}Nd = -0.3 to -4.4 and variable, but generally unradiogenic 207Pb 204Pb and 206Pb 204Pb compared to model mantle, suggesting mixing between a mantle-derived component and a U-depleted crustal contaminant. Crustal contaminants are probably a variety of Archean high-grade quartzofeldspathic gneisses with low U/Pb ratios and include a component that must be isotopically similar to the early Archean (>3.6 Ga) Uivak gneisses of Labrador or the Amitsoq gneisses of west Greenland. This would imply that the ancient gneiss complex of coastal Labrador and Greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the Labrador Trough. If Harp Lake and Mealy Mountains samples were subjected to the same degree of contamination, as suggested by their chemical similarities, then the Mealy contaminants must be much younger, probably early or middle Proterozoic in age. The Labrador segment of the Grenville Front, therefore, appears to coincide with the southern margin of the Archean North Atlantic craton and may represent a pre mid-Proterozoic suture. ?? 1986.

  17. Field Vectors to Metamorphosed Ores: A Prelude to Finding Currently Concealed Volcano-Plutonic Arc Settings and Their Mineral Deposits in The Grenville Province

    NASA Astrophysics Data System (ADS)

    Corriveau, L.; Bonnet, A.; van Breemen, O.

    2004-05-01

    Recent mineral deposits synthesis highlights the largely barren nature of the high-grade metamorphic terrains of the Canadian Shield in terms of large mining camps. No where is the gap most startling than in the Grenville Province even though a lot of its Paleo- to Mesoproterozoic crust consists of magmatic arcs renown worldwide to host IOCG, VHMS and Porphyry Cu deposits. All these deposit types have significant alteration halos that can serve as vectors to ore. The use of such vectors forced a complete reinterpretation of the nature of the La Romaine domain in the eastern Grenville Province. Mapped in the 70's as being a metasedimentary basin with >500 km2 of meta-arkose and minor pelite, quartzite, conglomerate and marble, the domain is herein reassessed as a major 1.5 Ga Pinwarian continental magmatic arc fertile in Cu-sulphides and Fe-oxides mineralizing systems. The original markers used to prognosticate a sedimentary origin can now be demonstrated to be a series of rhyolitic to dacitic lapillistone, sericitized tuff with Al nodules and veins, Al gneiss locally with lapilli textures, garnetite, ironstones and calc-silicate rocks. The distribution, paragenesis and mode of the Al-, Fe- and Ca-rich units significantly depart from those of normal metasediments but are very diagnostic of metamorphosed hydrothermal alteration zones and meta-exhalites. Mapping alteration vectors provided clues to search for and find the volcanic rocks concealed among the composite granitic gneiss, the zones of hydrothermal leaching (e.g., sericitic, argillic and advanced argillic alterations) and discharge, the cap rocks, and the Cu mineralization. Spatial and stratigraphic relationships provided a means to compare their settings with ore deposit models. Roof pendants of Ba-rich meta-exhalite in surrounding 1.5 Ga granitic plutons and intrusion of 1495 Ma Qtz-Kfs porphyry across hydrothermally altered 1500 Ma tuffs attest to coeval hydrothermal activity and sub-volcanic plutons. The

  18. European Variscan orogenic evolution as an analogue of Tibetan-Himalayan orogen: Insights from petrology and numerical modeling

    NASA Astrophysics Data System (ADS)

    Maierová, P.; Schulmann, K.; Lexa, O.; Guillot, S.; Štípská, P.; Janoušek, V.; Čadek, O.

    2016-07-01

    The European Variscan orogeny can be compared to the Tibetan-Himalayan system for three main reasons: (1) The Variscan belt originated through progressive amalgamation of Gondwanan blocks that were subsequently squeezed between the Laurussia and Gondwana continents. Similarly, the Tibetan-Himalayan orogen results from amalgamated Gondwanan blocks squeezed between Asia and India. (2) The duration of the collisional period and the scale of the two orogens are comparable. (3) In both cases the collisional process resulted in formation of a thick crustal root and long lasting high-pressure granulite facies metamorphism. Recent petrological data allow a more detailed comparison pointing to similarities also in the midcrustal re-equilibration of the granulites and their association with specific (ultra)potassic magmatic rocks. In both orogens, the origin of the granulites was attributed to relamination and thermal maturation of lower crustal allochthon below upper plate crust. Subsequent evolution was explained by midcrustal flow eventually leading to extrusion of the high-grade rocks. We propose that the lower and middle crustal processes in hot orogens are connected by gravity overturns. Such laterally forced gravity-driven exchanges of material in the orogenic root were already documented in the Variscides, but the recent data from Tibet and Himalaya show that this process may have occurred also elsewhere. Using numerical models, we demonstrate that the exchange of the lower and middle crust can be efficient even for a minor density inversion and various characteristics of the crustal layers. The modeled pressure-temperature paths are compatible with two-stage metamorphism documented in Tibet and Himalaya.

  19. Structural inversion of the Tamworth Belt: Insights into the development of orogenic curvature in the southern New England Orogen, Australia

    NASA Astrophysics Data System (ADS)

    Phillips, G.; Robinson, J.; Glen, R.; Roberts, J.

    2016-05-01

    The middle to late Permian Hunter Bowen Event is credited with the development of orogenic curvature in the southern New England Orogen, yet contention surrounds the structural dynamics responsible for the development of this curvature. Debate is largely centred on the roles of orogen parallel strike-slip and orogen normal extension and contraction to explain the development of curvature. To evaluate the dynamic history of the Hunter Bowen Event, we present new kinematic reconstructions of the Tamworth Belt. The Tamworth Belt formed as a Carboniferous forearc basin and was subsequently inverted during the Hunter Bowen Event. Kinematic reconstructions of the Tamworth Belt are based on new maps and cross-sections built from a synthesis of best-available mapping, chronostratigraphic data and new interpretations of depth-converted seismic data. The following conclusions are made from our study: (i) the Hunter Bowen Event was dominantly driven by margin normal contraction (east-west shortening; present-day coordinates), and; (ii) variations in structural style along the strike of the Tamworth Belt can be explained by orthogonal vs. oblique inversion, which reflects the angular relationship between the principal shortening vector and continental-arc margin. Given these conclusions, we suggest that curvature around the controversial Manning Bend was influenced by the presence of primary curvature in the continental margin, and that the Hastings Block was translated along a sinistral strike-slip fault system that formed along this oblique (with respect to the regional east-west extension and convergence direction) part of the margin. Given the available temporal data, the translation of the Hastings Block took place in the Early Permian (Asselian) and therefore preceded the Hunter Bowen Event. Accordingly, we suggest that the Hunter Bowen Event was dominantly associated with enhancing curvature that was either primary in origin, or associated with fault block translation

  20. Sr, Nd, and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador - Implications for crustal contamination and basement mapping

    NASA Technical Reports Server (NTRS)

    Ashwal, L. D.; Wooden, J. L.; Emslie, R. F.

    1986-01-01

    Trace element and Pb, Sr, and Nd isotopic compositions of anorthosites and related rocks, and of younger mafic dikes from Harp Lake and Mealy Mountains in Labrador, Canada are estimated and compared. The effects of crustal component contaminants on the isotopic compositions of the dikes are investigated. The correlation between the isotopic data and the crustal contamination model is studied. It is observed that for Harp Lake the initial Sr ratios are higher and the Nd values are lower than Mealy samples, and the data do not correspond to the crustal contamination model; however, the Pb isotope data favor a crustal contamination model. It is noted that the Labrador segment of the Grenville Front appears to coincide with the southern margin of the Archean North Atlantic craton, and may represent a pre mid-Proterozoic suture.

  1. Asymmetric exhumation across the Pyrenean orogen: implications for the tectonic evolution of a collisional orogen

    NASA Astrophysics Data System (ADS)

    Fitzgerald, P. G.; Muñoz, J. A.; Coney, P. J.; Baldwin, S. L.

    1999-11-01

    The Pyrenees are a collisional mountain belt formed by convergence between the Afro-Iberian and European plates. Apatite fission track thermochronology from three vertical profiles along the ECORS seismic line constrain the exhumation history of the Pyrenean orogen and hence tectonic models for its formation. In the Eocene there is relatively uniform exhumation across the Pyrenees, but significantly more exhumation occurs on the southern flank of the axial zone in the Oligocene. The variation in exhumation patterns is controlled by a change in how convergence is accommodated within the Pyrenean double-wedge. Accommodation of thrusting on relict extensional features that leads to inversion dominated thrust stacking resulted in relatively slow exhumation in the Eocene. However, subsequent crustal wedging and internal deformation in the upper crust under the stacked duplex of antiformal nappes resulted in extremely rapid exhumation on the southern flank in the Oligocene. The Maladeta profile in the southern axial zone records extremely rapid Early Oligocene exhumation followed by dramatic slowing or cessation of exhumation in the middle Oligocene and the formation of an apatite partial annealing zone (PAZ). This PAZ has subsequently been exhumed 2-3 km since the Middle Miocene, supporting the observations of Coney et al. [J. Geol. Soc. London 153 (1996) 9-16] that the southern flank of the range was buried by ≤2-3 km of syntectonic conglomerates in the Oligocene and subsequently re-excavated from Late Miocene to Recent. The present-day topographic form of the Pyrenees is largely a relict of topography that formed in the Eocene and the Oligocene. Comparison with paleoclimatic records indicates that the Eocene-Oligocene exhumation patterns are controlled by tectonic forces rather than resulting from an orographic effect due to uplift of the Pyrenees.

  2. Paleozoic tectonics of the Ouachita Orogen through Nd isotopes

    SciTech Connect

    Gleason, J.D.; Patchett, P.J.; Dickinson, W.R.; Ruiz, J. . Dept. of Geosciences)

    1992-01-01

    A combined isotopic and trace-element study of the Late Paleozoic Ouachita Orogenic belt has the following goals: (1) define changing provenance of Ouachita sedimentary systems throughout the Paleozoic; (2) constrain sources feeding into the Ouachita flysch trough during the Late Paleozoic; (3) isolate the geochemical signature of proposed colliding terranes to the south; (4) build a data base to compare with possible Ouachita System equivalents in Mexico. The ultimate aim is to constrain the tectonic setting of the southern margin of North America during the Paleozoic, with particular emphasis on collisional events leading to the final suturing of Pangea. Nd isotopic data identify 3 distinct groups: (1) Ordovician passive margin sequence; (2) Carboniferous proto-flysch (Stanley Fm.), main flysch (Jackfork and Atoka Fms.) and molasse (foreland Atoka Fm.); (3) Mississippian ash-flow tuffs. The authors interpret the Ordovician signature to be essentially all craton-derived, whereas the Carboniferous signature reflects mixed sources from the craton plus orogenic sources to the east and possibly the south, including the evolving Appalachian Orogen. The proposed southern source is revealed by the tuffs to be too old and evolved to be a juvenile island arc terrane. They interpret the tuffs to have been erupted in a continental margin arc-type setting. Surprisingly, the foreland molasse sequence is indistinguishable from the main trough flysch sequence, suggesting the Ouachita trough and the craton were both inundated with sediment of a single homogenized isotopic signature during the Late Carboniferous. The possibility that Carboniferous-type sedimentary dispersal patterns began as early as the Silurian has important implications for the tectonics and paleogeography of the evolving Appalachian-Ouachita Orogenic System.

  3. Jurassic sedimentary basins in the Central Asian orogenic belt

    SciTech Connect

    Bebeshev, I.I.

    1995-05-01

    The principal stages of development of Jurassic sedimentary basins (from their origin to the end of their existence) in the Central Asian orogenic belt are considered. The interrelations of the basins with the surrounding paleorises are investigated. Paleogeographic maps are compiled representing the evolution of paleolandscapes and revealing their interrelations in space and time for each stage. Areas with the highest prospects for coal are found.

  4. Orogenic gold and geologic time: A global synthesis

    USGS Publications Warehouse

    Goldfarb, R.J.; Groves, D.I.; Gardoll, S.

    2001-01-01

    Orogenic gold deposits have formed over more than 3 billion years of Earth's history, episodically during the Middle Archean to younger Precambrian, and continuously throughout the Phanerozoic. This class of gold deposit is characteristically associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustal structures. A consistent spatial and temporal association with granitoids of a variety of compositions indicates that melts and fluids were both inherent products of thermal events during orogenesis. Including placer accumulations, which are commonly intimately associated with this mineral deposit type, recognized production and resources from economic Phanerozoic orogenic-gold deposits are estimated at just over one billion ounces gold. Exclusive of the still-controversial Witwatersrand ores, known Precambrian gold concentrations are about half this amount. The recent increased applicability of global paleo-reconstructions, coupled with improved geochronology from most of the world's major gold camps, allows for an improved understanding of the distribution pattern of orogenic gold in space and time.

  5. Influence of structural setting on sulphur isotopes in Archean orogenic gold deposits, Eastern Goldfields Province, Yilgarn, Western Australia

    NASA Astrophysics Data System (ADS)

    Hodkiewicz, P. F.; Groves, D. I.; Davidson, G. J.; Weinberg, R. F.; Hagemann, S. G.

    2009-02-01

    The published mean δ34S values of ore-related pyrites from orogenic gold deposits of the Eastern Goldfields Province, Yilgarn Craton lie between -4‰ and +4‰. As for orogenic gold deposits worldwide, most deposits have positive means and a restricted range of δ34S values, but some have negative means and wider ranges of δ34S values. Wall-rock carbonation and back-mixing of similar-source fluids with different fluid pathways can explain some of the more negative δ34S signatures. However, structural setting appears to be the most important factor controlling ore-fluid oxidation state and hence the distribution of δ34S values in gold-related pyrites. Shear-hosted deposits appear to have experienced fluid-dominated processes such as phase separation, whereas stockwork, vein-hosted or disseminated deposits formed under conditions of greater rock buffering. At Victory-Defiance, in particular, negative δ34S values are more common in gently dipping dilational structures, compared to more compressional steeply dipping structures. It appears most likely that fluid-pressure fluctuations during fault-valve cycles establish different fluid-flow regimes in structures with different orientations. Rapid fluid-pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore-fluid oxidation, leading to large, local fluctuations in oxidation state. It is thus not necessary to invoke mixing with oxidised magmatic fluids to explain δ34S signatures indicative of oxidation. In any case, available, robust geochronology in the Eastern Goldfields Province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis. Thus, negative mean δ34S values and large variations in δ34S values of ore-related pyrites in world-class orogenic gold deposits are interpreted to result from

  6. Orogenic structural inheritance and rifted passive margin formation

    NASA Astrophysics Data System (ADS)

    Salazar Mora, Claudio A.; Huismans, Ritske S.

    2016-04-01

    Structural inheritance is related to mechanical weaknesses in the lithosphere due to previous tectonic events, e.g. rifting, subduction and collision. The North and South Atlantic rifted passive margins that formed during the breakup of Western Gondwana, are parallel to the older Caledonide and the Brasiliano-Pan-African orogenic belts. In the South Atlantic, 'old' mantle lithospheric fabric resulting from crystallographic preferred orientation of olivine is suggested to play a role during rifted margin formation (Tommasi and Vauchez, 2001). Magnetometric and gravimetric mapping of onshore structures in the Camamu and Almada basins suggest that extensional faults are controlled by two different directions of inherited older Brasiliano structures in the upper lithosphere (Ferreira et al., 2009). In the South Atlantic Campos Basin, 3D seismic data indicate that inherited basement structures provide a first order control on basin structure (Fetter, 2009). Here we investigate the role of structural inheritance on the formation of rifted passive margins with high-resolution 2D thermo-mechanical numerical experiments. The numerical domain is 1200 km long and 600 km deep and represents the lithosphere and the sublithospheric mantle. Model experiments were carried out by creating self-consistent orogenic inheritance where a first phase of orogen formation is followed by extension. We focus in particular on the role of varying amount of orogenic shortening, crustal rheology, contrasting styles of orogen formation on rifted margin style, and the time delay between orogeny and subsequent rifted passive formation. Model results are compared to contrasting structural styles of rifted passive margin formation as observed in the South Atlantic. Ferreira, T.S., Caixeta, J.M., Lima, F.D., 2009. Basement control in Camamu and Almada rift basins. Boletim de Geociências da Petrobrás 17, 69-88. Fetter, M., 2009. The role of basement tectonic reactivation on the structural evolution

  7. Deformation during terrane accretion in the Saint Elias orogen, Alaska

    USGS Publications Warehouse

    Bruhn, R.L.; Pavlis, T.L.; Plafker, G.; Serpa, L.

    2004-01-01

    The Saint Elias orogen of southern Alaska and adjacent Canada is a complex belt of mountains formed by collision and accretion of the Yakutat terrane into the transition zone from transform faulting to subduction in the northeast Pacific. The orogen is an active analog for tectonic processes that formed much of the North American Cordillera, and is also an important site to study (1) the relationships between climate and tectonics, and (2) structures that generate large- to great-magnitude earthquakes. The Yakutat terrane is a fragment of the North American plate margin that is partly subducted beneath and partly accreted to the continental margin of southern Alaska. Interaction between the Yakutat terrane and the North American and Pacific plates causes significant differences in the style of deformation within the terrane. Deformation in the eastern part of the terrane is caused by strike-slip faulting along the Fairweather transform fault and by reverse faulting beneath the coastal mountains, but there is little deformation immediately offshore. The central part of the orogen is marked by thrusting of the Yakutat terrane beneath the North American plate along the Chugach-Saint Elias fault and development of a wide, thin-skinned fold-and-thrust belt. Strike-slip faulting in this segment may he localized in the hanging wall of the Chugach-Saint Elias fault, or dissipated by thrust faulting beneath a north-northeast-trending belt of active deformation that cuts obliquely across the eastern end of the fold-and-thrust belt. Superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen, where the sedimentary cover of the Yakutat terrane is accreted into the upper plate of the Aleutian subduction zone. These three structural segments are separated by transverse tectonic boundaries that cut across the Yakutat terrane and also coincide with the courses of piedmont glaciers that flow from

  8. Sand fairway mapping as a tool for tectonic restoration in orogenic belts

    NASA Astrophysics Data System (ADS)

    Butler, Rob

    2016-04-01

    geomorphology. The stratigraphic records are preserved in many parts of the Alpine-Mediterranean region. Examples are drawn from the Eo-Oligocene of the western Alps and the early Miocene of the Maghreb-Apennine system to illustrate how turbidite sedimentology, linked to studies of basin structure, can inform understanding of tectonic processes on regional and local scales. In both examples, sediment was delivered across deforming basin arrays containing contractional structures, sourced from beyond the immediate orogenic segments. The depositional systems show that multiple structures were active in parallel, rather than develop in any particular sequence. Both systems show that significant deformation occurs, emerging to the syn-orogenic surface ahead of the main orogenic wedge. The cycling of uplift and subsidence of "massifs" can be significantly more complex that the histories resolved from thermochronological data alone.

  9. Thermo-kinematic evolution of the Annapurna-Dhaulagiri Himalaya, central Nepal: The Composite Orogenic System

    NASA Astrophysics Data System (ADS)

    Parsons, A. J.; Law, R. D.; Lloyd, G. E.; Phillips, R. J.; Searle, M. P.

    2016-04-01

    The Himalayan orogen represents a "Composite Orogenic System" in which channel flow, wedge extrusion, and thrust stacking operate in separate "Orogenic Domains" with distinct rheologies and crustal positions. We analyze 104 samples from the metamorphic core (Greater Himalayan Sequence, GHS) and bounding units of the Annapurna-Dhaulagiri Himalaya, central Nepal. Optical microscopy and electron backscatter diffraction (EBSD) analyses provide a record of deformation microstructures and an indication of active crystal slip systems, strain geometries, and deformation temperatures. These data, combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the GHS. The profiles define a three-stage thermokinematic evolution from midcrustal channel flow (Stage 1, >700°C to 550-650°C), to rigid wedge extrusion (Stage 2, 400-600°C) and duplexing (Stage 3, <280-400°C). These tectonic processes are not mutually exclusive, but are confined to separate rheologically distinct Orogenic Domains that form the modular components of a Composite Orogenic System. These Orogenic Domains may be active at the same time at different depths/positions within the orogen. The thermokinematic evolution of the Annapurna-Dhaulagiri Himalaya describes the migration of the GHS through these Orogenic Domains and reflects the spatial and temporal variability in rheological boundary conditions that govern orogenic systems.

  10. Late-orogenic, post-orogenic, and anorogenic granites: Distinction by major-element and trace-element chemistry and possible origins

    SciTech Connect

    Rogers, J.J.W.; Greenberg, J.K. )

    1990-05-01

    Granites classified into four categories based solely on tectonics of occurrence and associated rock types also have compositional characteristics that are consistent within groups and different among groups. Orogenically related granites include late-orogenic varieties (LO) associated with calc-alkaline batholiths, and post-orogenic varieties (PO), which occur in broad zones of isolated diapiric plutons in recently deformed orogenic belts. Inclined REE patterns, moderate Sr contents, and K{sub 2}O-SiO{sub 2} relationships show that late-orogenic granites formed by fractionation of plagioclase, clinopyroxene, and amphibole from calcalkaline magmas. Flatter REE patterns and K{sub 2}O contents near 5%, plus the absence of associated magmatic rocks, indicate that the post-orogenic granites developed by partial melting of subduction-produced mafic/intermediate magmatic rocks. Both the late- and post-orogenic granites can be part of material newly added to continental crust as a result of orogeny. Anorogenic granites in anorthosite/rapakivi complexes (AR) or alkaline ring complexes (RC) have LIL contents too high to have been equilibrated with a mafic mineral assemblage. These anorogenic rocks probably formed by partial melting of preexisting sialic crust and do not represent new crustal increment.

  11. Plume-orogenic lithosphere interaction recorded in the Haladala layered intrusion in the Southwest Tianshan Orogen, NW China

    NASA Astrophysics Data System (ADS)

    He, Peng-Li; Huang, Xiao-Long; Xu, Yi-Gang; Li, Hong-Yan; Wang, Xue; Li, Wu-Xian

    2016-03-01

    The plume-orogenic lithosphere interaction may be common and important for the generation of large igneous provinces. The information regarding such a process is recorded by the Haladala gabbroic intrusion (~300 Ma), the largest layered ultramafic-mafic intrusion hosting V-Ti magnetite deposits in the Southwest Tianshan Orogen, NW China. The Haladala gabbros exhibit unfractionated chondrite-normalized rare earth element patterns with negative Nb and Ta anomalies and positive Pb anomaly on the primitive mantle-normalized multielement variation diagram. They are characterized by low initial Sr isotopes, slightly decoupled but high positive bulk rock ɛNd(t) and ɛHf(t), and high 207Pb/204Pb and 208Pb/204Pb relative to 206Pb/204Pb, delineating a DUPAL signature in the sources. The Haladala gabbros cannot be arc or postcollisional magmatism, given the lack of hydrous minerals and low K contents, respectively. This is further supported by the relatively low oxygen fugacity required for the gradual enrichment of V-Ti magnetite during the magma fractionation and by an overall anhydrous mantle source suggested by troctolite mineral assemblage (olivine + plagioclase). The emplacement age of the Haladala gabbros is identical to that of the Wajilitag kimberlites in the Tarim's interior, which have been interpreted as the first magmatic expression of the Tarim mantle plume. We thus propose that the Haladala gabbroic intrusion was generated in a hybrid geodynamic setting in which the Southwest Tianshan Orogen was impacted by an upwelling mantle plume. In this sense, the Haladala layered gabbroic intrusion records the early phase of magmatism of the Tarim plume, which was preferentially emplaced in a lithospheric weak zone.

  12. Orogen-scale L tectonite domain in the Tongbai orogenic belt, central China: Geological setting and origin

    NASA Astrophysics Data System (ADS)

    Liu, Huan; Lin, Shoufa; Song, Chuanzhong

    2017-01-01

    L tectonite is well developed and widely distributed in the Tongbai orogenic belt in central China. The orogenic belt as a whole has an antiformal geometry and the hinge of the antiform is subhorizontal and trends NW-SE. The L tectonite occurs in the core of the antiform, in a zone that is 10-30 km wide and over 100 km long. Lineations in the L tectonite are sub-horizontal, parallel to the hinge of the antiform. Sheath folds are also well developed associated with the L tectonite, with the hinges parallel to the lineations. Migmatite occurs in the core and structurally below the L tectonite and has a gradational relationship with the L tectonite. The domain of L tectonite is bounded by three ductile shear zones, on the north, at the top and on the south, respectively. Well-developed shear sense indicators indicate that the southern, the overlying and the northern shear zones have a dextral, top-to-NW and sinistral sense of shear, respectively. These geometrical and kinematic data indicate that the three shear zones are likely part of a single shear zone that wraps around the L-tectonite domain. The L-tectonite zone in the core moves southeast relative to the hanging wall. The development of the tectonite is interpreted to be a result of this special geometry and kinematics and reflects a post-collisional orogen-parallel extension synchronous with migmatization and the continuing convergence between the Yangtze Block and the North China Block in the Early Cretaceous.

  13. Regional and local controls on mineralization and pluton emplacement in the Bondy gneiss complex, Grenville Province, Canada interpreted from aeromagnetic and gravity data

    NASA Astrophysics Data System (ADS)

    Dufréchou, G.; Harris, L. B.; Corriveau, L.; Antonoff, V.

    2015-05-01

    The Bondy gneiss complex in the Grenville Province of Southwest Quebec hosts a mineralized iron oxide- and copper-rich hydrothermal system. The northern part of the complex overlies the lithospheric-scale Mont-Laurier lineament and is cut by the regional Mont-Laurier South shear zone interpreted from Bouguer gravity. A sinistral 6 km wide strike-slip corridor defined by several second-order shears (the Mont-Laurier South shear zone) in the complex was identified from geophysical data, including a new high-resolution airborne magnetic survey, and field observations. The spatial association of a metamorphosed alteration system, several pre- to post-metamorphic mineralized zones and mafic intrusions within the Mont-Laurier South shear zone suggests that (i) underlying basement structures controlled hydrothermal fluid migration during the formation of epithermal-IOCG mineralization and associated alteration system before ca. 1.2 Ga high-grade metamorphism and penetrative ductile deformation in the complex; (ii) post-metamorphic reactivation allowed magma ascent and pluton emplacement in the complex and adjacent supracrustal rocks within dilatational sites; and (iii) brittle-ductile shears that postdate high-grade metamorphism provided channel ways for fluid migration associated with magnetite-related mineralization. Although the complex does not host an economic mineral deposit, the role between structures at different levels and the combination of gravity and aeromagnetics at different scales provides an example of an approach for mineral exploration in similar high grade gneiss terrains.

  14. Field evidence of hydraulic connections between bedrock aquifers and overlying granular aquifers: examples from the Grenville Province of the Canadian Shield

    NASA Astrophysics Data System (ADS)

    Richard, Sandra K.; Chesnaux, Romain; Rouleau, Alain; Morin, Roger; Walter, Julien; Rafini, Silvain

    2014-12-01

    Field evidence of hydraulic connections between a bedrock aquifer and an overlying granular aquifer in the Canadian Shield (Grenville Province) is presented. This issue is rarely considered and investigated despite its important hydraulic and chemical consequences and its widespread occurrence worldwide. The methodology employed is based on complementary field tests conducted at specific experimental sites instrumented both in the rock and in the overlying deposits. One of the bedrock sites revealed a natural hydraulic connection with the overlying granular aquifer caused by the weathered surface of the uppermost bedrock. Another site revealed an artificial hydraulic connection between the bedrock and the granular aquifer created by an improperly sealed casing. A regional study showed that hydraulic connections yield an erroneous interpretation of the true hydraulic properties of the tested aquifer. The detection of hydraulic connections is therefore essential to properly define well-capture areas and contamination conditions. It is recommended to practitioners that pumping tests be performed as well as hydrochemical comparisons of each existing aquifer unit. Falling-head permeability tests are also helpful in verifying the quality of the seal at the bedrock-casing contact. More effective procedural controls and better well-construction practices are necessary to reduce the risks of cross-contamination induced by defective seals.

  15. Post-orogenic thermal evolution of newborn Archean continents

    NASA Astrophysics Data System (ADS)

    Jaupart, C.; Mareschal, J.-C.

    2015-12-01

    The post-orogenic thermal evolution of newborn cratons in the Archean is marked by high-temperature metamorphism and plutonic activity that lag accretion by several tens of million years. The source of the heat that is required remains controversial. Here, we show that such late activity is consistent with the thermal evolution of new continental crust that adjusts to heat released by radioactive decay. Quantitative results depend on the total amount of radioactive elements in the newborn crust. Using heat flow and heat production data from the Archean Superior Province of the Canadian Shield, we show that temperatures ≈800-900 °C were reached in the lower crust a few tens of million years after the final accretion event. The timing of post-orogenic metamorphism is sensitive to the thermal structure acquired at the end of accretion. For the Superior Province, the relatively short time-lag between the end of accretion and metamorphism suggests that the lithosphere was thin or had been heated up by sustained magma percolation.

  16. Continuation of the New England Orogen, Australia, beneath the Queensland Plateau and Lord Howe rise

    USGS Publications Warehouse

    Mortimer, N.; Hauff, F.; Calvert, A.T.

    2008-01-01

    Greywacke, argillite, greyschist and hypabyssal igneous rocks have been obtained from an Ocean Drilling Program core on the Queensland Plateau and from xenoliths in a volcanic breccia dredged from the crest of the Lord Howe Rise. Low to intermediate detrital quartz contents, 260-240 Ma K-Ar ages, and only moderately radiogenic Sr and Nd isotope compositions, suggest a correlation with the New England Orogen of eastern Australia, rather than with Australia's Lachlan Orogen or other adjacent geological provinces. Our results indicate that the New England Orogen terranes continue towards New Zealand at least as far as the southern Lord Howe Rise. The projected offshore boundaries of the major east Australian orogens are now known with more confidence, and do not appear to require any major cross-orogen offsets.

  17. The Andes as a peripheral orogen of the breaking-up Pangea

    NASA Astrophysics Data System (ADS)

    Lomize, M. G.

    2008-05-01

    Formation conditions of the peripheral orogen are expressed most fully in the Central Andes, a mountain system almost not yielding in height to the Himalayan-Tibetan system but formed at the margin of ocean without any relations to intercontinental collision. The marine transgression and rejuvenation of subduction in the Early Jurassic during the origination of foldbelt at the margin of Pangea marked the transition to a new supercontinental cycle, and the overall further evolution began and continues now in the frame of the first half of this cycle. The marginal position of this belt above the subduction zone, the rate and orientation of convergence of the lithospheric plates, the age of “absolute” movement of the continental plate, variation in slab velocity, and subduction of heterogeneities of the oceanic crust were the crucial factors that controlled the evolution of the marginal foldbelt. At the stage of initial subsidence (Jurassic-Mid-Cretaceous), during extension of the crust having a moderate thickness (30-35 km), the Andean continental margin comprises the full structural elements of an ensialic island arc that resembled the present-day Sunda system. These conditions changed with the separation and onset of the western drift of the South American continent. Being anchored in the mantle and relatively young, the slab of the Andean subduction zone served as a stop that brought about compression that controlled the subsequent evolution. Due to the contribution of deep magma sources along with marine sediments and products of tectonic erosion removed to a depth, the growth of crust above the subduction zone was favorable for heating of the crust. By the middle Eocene, when compression enhanced owing to the acceleration of subduction, the thermal evolution of the crust had already prepared the transition to the orogenic stage of evolution, i.e., to the progressive viscoplastic shortening and swelling of the mechanically weakened lower crust and the

  18. Crustal Development in the Northeast Asian Orogenic Belt and its comparison with the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Jahn, Bor-ming

    2016-04-01

    The Northeast Asian Orogenic Belt is a Mesozoic-Cenozoic accretionary orogenic collage, and it constitutes the northern and principal part of the "Nipponides" (Sengor and Natal'in, 1996). The tectonic framework was formed in Mesozoic and Cenozoic, and it continues to evolve along the modern Pacific arc-trench systems. Generally, a oceanward younging of tectonic units may be discerned, but such a simple pattern is disrupted in many places by extensive strike-slip faulting, most of which is left-lateral. In this talk, the issue of crustal development in the sector of Sikhote-Alin and Japanese Islands will be discussed based on the geochemical and isotopic analyses of granitoids that intruded in various tectonostratigraphic terrains. The majority of granitoids in the NE Asian Orogenic Belt formed from Jurassic to late Cenozoic, with Cretaceous as the dominant period of granitic magmatism and tectonothermal events. A few Early Paleozoic granitic rocks (500 to 450 Ma) have been identified in SW Japan (Kurosegawa Belt) as well as in NE Japan (Kitakami Belt), among them the ca. 500 Ma diorites and tonalites of southern Kitakami are the oldest rocks in Japan and interpreted as the first TTG crust of proto-Japan (Isozaki et al., 2015). Cretaceous granitoids are widespread in Sikhote-Alin and in NE and SW Japan. However, granitoids were emplaced only in the Cenozoic in Sakhalin (ca. 44 - 42 Ma) and Hokkaido (45, 37 and 18 Ma). Most granitoids from Sikhote-Alin are of I-type and have ISr = 0.7040 to 0.7083, and ɛNd(T) = +3.0 to -6.0 (mostly 0 to -5). The Sr-Nd isotopic data fall within the range of granitoids from SW Japan (0.704 to 0.712; +5.0 to -13.0), and the data of Cretaceous granitoids from Sikhote-Alin and SW Japan overlap almost completely. The Cenozoic granitoids of Hokkaido are characterized by ISr = 0.7044 - 0.7061, ɛNd(T) = +1.0 to +4.7, and Sm-Nd model-1 ages = 400-1000 Ma. This is remarkably similar to the Sakhalin granitoids with ISr = 0.7047 - 0.7050,

  19. Stress Dynamics of Magma Activity during Orogenic Evolution: An Example from Kinmen Island, SE China

    NASA Astrophysics Data System (ADS)

    Chen, Ping-Chuan; Yeh, En-Chao; Lin, Jian-Wei; Lee, Chi-Yu; Chen, Rou-Fei; Lin, Wayne; Hsieh, Pei-Shan; Lin, Cheng-Kuo; Iizuka, Yoshiyuki

    2016-04-01

    During orogeny, a mountain belt experienced different orogenic stages with various conditions of temperature, pressure, stress and fluid pressure. The speculation, that the orogenic stresses evolve from reverse faulting via strike-slip faulting to normal faulting stress regimes corresponding to syn-orogenic, post-orogenic and an-orogenic stage respectively, has been proposed but has not been proved yet. Here we report the study of dikes from Kinmen Island can shed light on understanding the stress evolution of orogeny. The Kinmen Island, located in the southeastern continental margin of Mainland China, cropped out the middle to lower crust of NE structural grain, which was experienced deformation and metamorphism during Late Yenshanian Orogeny(LYO). Based on previous studies of geochemistry, geochronology, and P-T conditions, various types of dike have been identified. They are syn-orognic dikes of amphibolite (130-110Ma), post-orogenic dikes of pegmatite and aplite (110-100Ma), and an-orogenic dike of gabbro (94-76Ma). During syn-orogenic stage of LYO, dike intrusion appeared as low-angle dip, which reflected that reverse faulting regime and horizontal maximum stress direction in E-W orientation. In post-orogenic stage, stress would be divided into two sub-stages. The early one was strike-slip faulting regime and horizontal maximum stress was in NW-SE orientation. The late one was normal faulting regime and horizontal maximum stress direction returned to E-W orientation. Finally, an-orogenic dike intrusion striked NE-SW with vertical dip, which displayed that normal faulting regime and NE-SW horizontal maximum stress direction. Our observation is consistent with the expected stress evolution during orogeny. Deviatoric stresses from new findings were decreased at early post-orogenic stage but increased after late post-orogenic stage, indicating the change of fluid pressure ratio with time. The spatiotemporal variation of stress field might be suggested the change

  20. Orogen parallel britlle extension in the Inner Northwestern Alps.

    NASA Astrophysics Data System (ADS)

    Champagnac, J. D.; Sue, C.; Delacou, B.; Burkhard, M.

    2003-04-01

    Rising attention has been recently paid to the latest extensional structures in the Alps, which took place under increasingly brittle conditions (e.g. Bistacchi, et al. 2000, Sue and Tricart 2002). Some of these structures seems to be still active . This brittle extension mainly occurs in the core of the arc, in the internal zones and could be linked to the ongoing extension observed under the light of seismotectonics studies (Sue, et al. 1999). Here we analyze the brittle deformation in the internal zone of the Northwestren Alps, from the Simplon fault zone (to the Northeast) to the Vanoise area (to the South). Our analysis is based on systematic mapping of the fault patterns, using remote sensing analysis combined with field work. The fault pattern we studied crosscuts all the ductile compression-related structures and piles of nappes. Thus, this brittle deformation postdates the ductile deformations, and is associated to one of the latter tectonic event in the belt during the recent-alpine history (Neogene times). The determination of paleostress field(s), based on the inversion of a large database of fault/stria measurements constrain the behaviour of this fault pattern: The results indicate a major large scale extensional paleostress field, in the NE-SW direction. In the South Valais, this orogen-parallel extension could be linked to the normal/dextral Simplon-Rhône fault zone dynamics. Further to the South, the paleostress fields also locally indicate NE-SW extension, but the driving forces remains a matter of debate. The orogen-parallel extension observed in this study have to be linked to the orogen-radial extension observed by (Sue and Tricart 2002) in the Briançonnais area, further to the South. We must also determine the relationship between the large scale brittle extension and the seismotectonics radial-to-the-arc extension. The geographic and/or temporal variations of the (paleo)stress fields remain to be constrain to precise the recent geodynamics

  1. Collision-Orogen provenance: Modern sands from big Himalayan rivers

    NASA Astrophysics Data System (ADS)

    Garzanti, E.; Vezzoli, G.; Andò, S.; France-Lanord, C.; Singh, S. K.; Clift, P.

    2003-04-01

    The Himalayan orogen represents the most important source of terrigenous detritus on Earth. The Ganga-Brahmaputra river system ranks first in terms of sediment load, and together with the Indus carried to the Indian Ocean ca 2 billion tons of sediments annually. Stored in the Bengal and Indus fans, the world's largest turbiditic cones by far, are ca 15*10^6km^3 of detritus derived from the Himalayas since the Paleogene. Nevertheless, petrographic and mineralogic composition of sediments transported by big Himalayan rivers has been poorly documented so far. This high-resolution actualistic study provides a key to interpret detrital modes of Tertiary foreland basin strata, and sheds light on diagnostic features of collision-orogen provenance. Composition of Himalayan-derived sands indicates dominant contribution from amphibolite-facies rocks exposed both south and north of the Indus-Tsangpo suture, reflecting extreme uplift and widespread exhumation of the deep roots of the orogen. Hornblende-dominated dense-mineral suites of both Indus and Brahmaputra sands are largely derived from Asian active-margin plutons. The Indus sands in particular reflect major supply from arc batholiths, widely exposed in Ladakh, Kohistan, and along the Karakorum and Hindukush belts. The Ganga sands are instead chiefly derived from High-Himalayan nappes with Tertiary metamorphism up to sillimanite-grade. Intermediate composition characterizes the Brahmaputra sands, shed largely from High Himalayan crystalline rocks subject to very rapid erosion around the Namche-Barwa syntaxis, and subordinately from Gangdese batholiths in the Tibetan tract and from plutonic rocks of the Mishmi hills farther downstream. Supply from sedimentary covers and recycling of accreted foreland-basin strata are significant, whereas volcanic and ophiolitic detritus is volumetrically negligible. Carbonate grains, common in the Indus sands and present in the Ganga sands, are negligible in the Brahmaputra sands

  2. Neoproterozoic and Paleozoic accretionary orogens exposed at different crustal levels

    NASA Astrophysics Data System (ADS)

    Kroener, A.

    2002-12-01

    Accretionary orogens in the upper crust are dominated by trench and forearc deposits, obducted ophiolite fragments, exotic terranes and well defined structural boundaries such as major shear zones. The Neoproterozoic Arabian-Nubian shield (ANS) of western Arabia and NE Africa, the huge terrain of the Neoproterozoic to Palaeozoic Central Asian mobile belt (CAMB) and the present Indonesian Archipelago are prime examples of such orogens. In the ANS and CAMB, field relationships, rock associations, differences in structural style and metamorphic grade, and geochronology have led to the recognition of terrane assemblages that are related to processes of lateral accretion as now observed in the southwest Pacific and lasting for several hundred my. In the ANS, ocean crust and arc formation began about 900 Ma ago, and terrane accretion was completed by ~600 Ma, whereas in the CAMB the oldest oceanic crust formed some 1000 Ma ago, and terrane accretion continued into the late Palaeozoic. Typical rock associations are trench and forearc sediments, island-arc volcanics, calc-alkaline granitoids, dismembered ophiolite suites and gneissic rocks (microcontinents?) constituting exotic terranes and mostly of distinctly older age and more complex tectono-metamorphic history than the surrounding lower grade rocks. Shear zones frequently separate the terranes and in the ANS also constitue seismic discontinuities extending to the lower crust. The middle to lower crustal high grade assemblages of the Neoproterozoic Mozambique belt (MB) of East Africa, Madagascar, southernmost India, Sri Lanka and East Antarctica are considered to be a deep crustal analogue to the upper crustal accretionary belts described above. Typical characteristics are (1) voluminous calc-alkaline granitoid suites, now layered gneisses, and interpreted as root zones of arc terranes, (2) tectonic interdigitation of Archaean to Palaeoproterozoic gneisses with Neoproterozoic rocks, probably brought about during

  3. Terrestrial cosmogenic nuclide surface exposure dating of the oldest glacial successions in the Himalayan orogen: Ladakh Range, northern India

    USGS Publications Warehouse

    Owen, L.A.; Caffee, M.W.; Bovard, K.R.; Finkel, R.C.; Sharma, M.C.

    2006-01-01

    Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya. The glacial stages that have been identified are: the Indus Valley glacial stage, dated at older than 430 ka; the Leh glacial stage occurring in the penultimate glacial cycle or older; the Karglacial stage, occurring during the early part of the last glacial cycle; the Bazgo glacial stage, at its maximum during the middle of the last glacial cycle; and the early Holocene Khalling glacial stage. The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation. A possible explanation is that uplift of Himalayan ranges to the south and/or of the Karakoram Mountains to the west of the region may have effectively blocked moisture supply by the south Asian summer monsoon and mid-latitude westerlies, respectively. Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene. ?? 2006 Geological Society of America.

  4. Complexities of Lu-Hf geochronology in convergent orogens

    NASA Astrophysics Data System (ADS)

    Mulcahy, S. R.; Vervoort, J. D.

    2015-12-01

    Subduction, terrane accretion, and arc magmatism leave a complex and sometimes incomplete record of metamorphism and deformation. The range of metamorphic temperatures and assemblages produced throughout the tectonic evolution of a single orogen often requires multiple isotopic systems to date distinct events. Lu-Hf geochronology, notably, has proven successful for dating metamorphism from a variety of bulk compositions spanning temperatures <350-850 C. We review the success of applying Lu-Hf geochronology in combination with other isotopic systems to date metamorphism from range of metamorphic conditions within convergent margins. We then discuss some complexities of Lu-Hf geochronology when dating samples with complicated metamorphic histories. Garnet and lawsonite isochrons often exhibit excess scatter (high MSWD's) that can be attributed to a number of factors: secondary mineral inclusions, prolonged garnet growth durations, disequilibrium at low temperatures, and polyphase metamorphic histories. Samples with high-Hf inclusions in isotopic equilibrium host phases can lead to decreased precision, but still produce meaningful ages. At high temperatures Lu-Hf ages may date peak metamorphism, cooling from peak temperatures, or result in spurious ages because of preferential retention of 176Hf over 176Lu in garnet. Despite these complexities, and perhaps because of them, new aspects of the metamorphic history may be revealed that are not readily recorded by other isotopic systems. Minerals other than garnet and lawsonite, particularly apatite, and perhaps epidote, offer exciting new possibilities for Lu-Hf geochronology. Careful fieldwork, detailed petrology and geochemistry, and collaborative efforts using multiple isotopic systems offer the best approach to solving tectonic problems in convergent orogens.

  5. Young upper crustal chemical composition of the orogenic Japan Arc

    NASA Astrophysics Data System (ADS)

    Togashi, Shigeko; Imai, Noboru; Okuyama-Kusunose, Yasuko; Tanaka, Tsuyoshi; Okai, Takashi; Koma, Takeshi; Murata, Yasuaki

    2000-11-01

    A new geochemical estimate of the young (mainly Paleozoic age to present) upper crust of the Japan Arc shows a dacitic composition in contrast to the idea that andesite is predominant in active orogenic arcs. Temporal changes in composition are not significant from the Paleozoic age to the present for the Japan Arc. The major element composition is similar to previous models of old cratonic upper crusts. The coincidence in the major elements between young and old crusts indicates that essential mechanisms during crust formation have not changed from the Archean era to the present. In trace element compositions the average young upper crust of the Japan Arc has higher Sb and As concentrations and lower concentrations of alkaline, light rare earth, and high field strength elements with respect to previous models of continental upper crusts. The large degree of constancy of trace element composition in marine sedimentary rocks is in contrast to the large variety in igneous rocks. However, the averages for both accretionary and nonaccretionary sedimentary rocks are almost identical to the average for the igneous rocks of the Japan Arc, with the exceptions of high Sb and As concentrations in unmetamorphosed sedimentary rocks. The compositional homogeneity among different types of rocks on an arc scale implies that recycling processes mechanically mix the arc-derived igneous materials to homogenize the chemical composition during erosion, transportation, sedimentation, accretion, and uplifting. Since the contribution of oceanic crust to the composition of arc crust is small, the recycling processes have not changed the bulk upper crustal composition of the active continental margin except increase the Sb and As from sediments. Instead, the influx of differentiated acidic rocks from depth is essential to characterize the orogenic crust formation of the young Japan Arc. The characteristically low incompatible element content of the Japanese upper arc crust appears

  6. Vorticity analysis in the Zagros orogen, Shiraz area, Iran

    NASA Astrophysics Data System (ADS)

    Sarkarinejad, Khalil; Heibati, Zahra

    2016-10-01

    Quantitative vorticity analyses in orogenic belts are essential for studying the kinematics of deformation and can be performed using a range of methods. The combination of microstructural analysis for vorticity with other methods creates a more rigorous analysis. In order to determine the degree of non-coaxiality and spatial pattern of vorticity during deformation in the Zagros Orogenic Belt, a study area containing the boundary of the Zagros Folded Belt and the Zagros Fold-and-Thrust Belt is selected. The study area is situated in the Shiraz region of E-Zagros in Iran. The kinematic vorticity analysis is carried out using 4 methods based on: (1) the degree of asymmetry of the calcite c-axis fabric, (2) the assumption that the orientation of the long axes of calcite within an oblique stylolite foliation delineates the direction of the instantaneous stretching axis, (3) the assumption that the tension gash tips determine the direction of the instantaneous stretching axis and (4) stylolite teeth determine the direction of the instantaneous stretching axis. C-axis data from calcite give a kinematic vorticity number between 0.68 and 0.83, and the orientation of the long axes of calcite grains yields a range between 0.5 and 0.84. Stylolites provide a kinematic vorticity number between 0.5 and 0.79, and tension gashes provide a kinematic vorticity number between 0.56 and 0.81. This range of vorticity numbers confirms the contributions of both simple (33-59%) and pure shear (41-67%). Twining of calcite also reveals that the last stage of deformation occurred at a temperature of 170-200 °C. Spatial analysis reveals an increase in the simple shear component from the SW of the Zagros Folded Belt to the NE of the Zagros Fold-and-Thrust Belt.

  7. Seismic anisotropy and mantle creep in young orogens

    USGS Publications Warehouse

    Meissner, R.; Mooney, W.D.; Artemieva, I.

    2002-01-01

    Seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere. We discuss the origin of anisotropy at various depths, and relate it to tectonic stress, geotherms and rheology. The anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine, and may result from ductile deformation, dynamic recrystallization or annealing. Anisotropy beneath young orogens has been measured for the seismic phase Pn that propagates in the uppermost mantle. This anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event, and thus provides information on the process of mountain building. Whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts, Pn anisotropy is aligned parallel to the structural axis. We interpret this to indicate mountain-parallel ductile (i.e. creeping) deformation in the uppermost mantle that is a consequence of mountain-perpendicular compressive stresses. The preferred orientation of the fast axes of some anisotropic minerals, such as olivine, is known to be in the creep direction, a consequence of the anisotropy of strength and viscosity of orientated minerals. In order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal 'escape' (or 'extrusion') tectonics to the uppermost mantle. We present rheological model calculations to support this hypothesis. Mountain-perpendicular horizontal stress (determined in the upper crust) and mountain-parallel seismic anisotropy (in the uppermost mantle) require a zone of ductile decoupling in the middle or lower crust of young mountain belts. Examples for stress and mountain-parallel Pn anisotropy are given for Tibet, the Alpine chains, and young mountain ranges in the Americas. Finally, we suggest a simple model for initiating mountain parallel creep.

  8. Origin of magmatic sulfides in a Proterozoic island arc—an example from the Portneuf-Mauricie Domain, Grenville Province, Canada

    NASA Astrophysics Data System (ADS)

    Sappin, Anne-Aurélie; Constantin, Marc; Clark, Thomas

    2011-03-01

    The Portneuf-Mauricie Domain (PMD), located in the south-central part of the Grenville province, contains Mesoproterozoic Ni-Cu ± platinum-group element (PGE) prospects hosted in a variety of plutonic intrusions (layered, with simple structures, or zoned) and emplaced in a mature island arc setting. A two-stage model is envisaged to explain the formation of magmatic sulfides. An early loss of a small amount of sulfides in the conduits of primitive, hydrous mantle-derived melts under high fO2, resulted in depletion of the magmas in chalcophile and precious metals (Cu/Pd ratios vary from initial mantle values up to 1.6 × 106). Then, nearer the mineralized zones, the magmas interacted with sulfide-bearing country rocks, resulting in felsification of the magmas, assimilation of crustal sulfur ( δ 34S values up to +5.5‰), and the formation of an immiscible sulfide liquid. Liquid-sulfide formation was followed by variable interactions between the silicate and sulfide magmas, which were responsible for the enrichment of sulfides in Ni, Cu, and, locally, PGE. Indeed, low R factors are found for prospects hosted in intrusions with a simple internal structure and in layered intrusions whereas high R factors are found for prospects hosted in zoned intrusions. Finally, sulfide melt may have been partly incorporated into later pulses of magma and injected into shallow magma chambers to form the PMD prospects. The PMD prospects share common characteristics with other well-known deposits (Aguablanca, Vammala, Stormyrplunen, and deposits in Alaskan/Ural-type intrusions), attesting to the Ni, Cu, and PGE potential of deposits associated with subduction-zone settings.

  9. Magmatism in the Carolina terrane: Isotopic evidence for a Grenville-age source for Late Proterozoic volcanics and a mantle source for Silurian Concord syenite

    SciTech Connect

    Kozuch, M.; Heatherington, A.L.; Mueller, P.A. . Dept. of Geology); Offield, T.W.; Koeppen, R.P.; Klein, T.L. )

    1992-01-01

    Rhyolitic to andesitic volcanic rocks from the central portion of the Carolina slate belt in North Carolina were analyzed for Sr and Nd isotopic composition and dated by U-Pb zircon geochronology. Samples were from the greenschist-facies Late Proterozoic Albemarle Group, Uwharrie Formation, and the informal Virginia sequence. A rhyolite from the Cid Formation of the Albemarle Group dated by U-Pb zircon geochronology yielded a Pb-207/Pb-206 age of 575 [+-] 7.6 Ma, consistent with its position below strata containing the Late Proterozoic trace fossil Pteridinium and above rocks previously dated at 586 [+-] 10 Ma. Rb-Sr isotopic analyses of late Proterozoic rocks showed average initial Sr-87/Sr-86 ratios of approximately 0.704, indicating a moderately depleted source for these samples. E[sub ND] values at 600 Ma are moderately positive (+0.7 [minus] +2.3) and T(DM) values range from 1.19--1.04 Ga. These isotopic data, along with major and trace element data, suggest that andesites and rhyolites of the Carolina slate belt may have formed by partial melting of attenuated, Grenville-aged continental lithosphere during a 600 Ma episode of arc volcanism. In contrast, Sr and Nd data for the younger ([approximately]400 Ma) Concord pluton indicate it was derived from a depleted mantle source (Sr-87/Sr-86 = 0.7021 and E[sub ND] = +0.4 at 400 Ma) without significant involvement of older lithosphere (T(DM) = 370 Ma).

  10. The Grenville orogeny in the Llano Uplift, Texas: A record of collision and contraction along the southern margin of North America

    SciTech Connect

    Reese, J.F. . Dept. of Geological Sciences)

    1993-02-01

    Precambrian metamorphic rocks in the SE Llano Uplift record NE-directed ductile thrusting and regional-scale polyphase folding. This deformation is in response to Grenville-age shortening and crustal thickening associated with the collision of a south-lying tectonic block with the southern margin of North America. In the SE Llano Uplift, the most intense and pervasive deformational event, D2, is characterized in the Packsaddle Schist (PS) and Valley Spring Gneiss (VSG) by SSE-plunging, NE-verging isoclinal folds (F2) with an associated SW-dipping axial planar metamorphic layering (S2), and SW-dipping mylonite zones with kinematic indicators showing top-to-the-NE motion. In the Red Mountain and augen-bearing Big Branch gneisses, D2 structures are SW-dipping mylonite zones parallel to S2, and a SW-plunging stretching lineation. Taken together, this suite of structures indicates tectonic transport was to the NE, perpendicular to the NW trending regional structural and metamorphic grain. D2 structures were reoriented by at least two later phases of folding. Timing of all ductile deformation in the SE Llano Uplift is constrained from post-1,215 Ma (deformed PS) to pre-1,098 Ma (undeformed melarhyolite dike). From south to north, metatonalitic, arc-derived Big Branch Gneiss ([approximately] 1,303 Ma) and older mafic schist country rock, previously interpreted as possible ophiolitic melange, structurally overlie much younger, lithologically heterogeneous PS units (1,248-1,215 Ma), previously considered as arc flank deposits. In turn, the PS has been tectonically emplaced above the predominantly felsic VSG (1,270-1,232 Ma). The presence of older zircons in the VSG, of similar age ([approximately]1360 Ma) to Western Granite-Rhyolite Terrane rocks to the north, suggests that the VSG formed in a settling proximal to North America.

  11. Granulite-Facies High-sulfidation VHMS-like Hydrothermal System in the La Romaine Area, Eastern Grenville Province, Quebec: a Metamorphic and Geochemical Overview

    NASA Astrophysics Data System (ADS)

    Bonnet, A.; Corriveau, L.; Laflèche, M. R.

    2004-05-01

    An extensive Cu-mineralized hydrothermal system has been recognized among the 1.5 Ga La Romaine volcano-sedimentary belt, in the eastern Grenville Province. This high-grade metamorphosed supracrustal belt occurs as a narrow basin within coeval granitoids. Granulite-facies alteration halos, encompassing rocks diagnostic of advanced argillic alteration (sillimanite-garnet-cordierite gneiss), silicification (quartz-cordierite gneiss) and sericite alteration (quartz-muscovite-sillimanite-iron oxide nodules or veins), are mapped among rhyolitic to dacitic tuffs and lapillistones. Some of these altered rocks have preserved primary lapilli textures. Amphibolite units of uncertain volcanic or intrusive origin overly the felsic pyroclastics and form a structurally coherent, east-west oriented unit. A narrow zone of ironstones (magnetite-rich amphibolite and biotite gneiss), carbonated zones (epidote-, diopside-, anorthite-, Ca garnet-, and/or calcite-rich calc-silicate rocks) and disseminated Cu sulphides, is found across its trend and testify of focused fluid discharge and mineralization. Structural and petrographic data suggest that mineralization and alteration are controlled by synvolcanic faults, with the amphibolite unit serving as a cap rock. Despite high-grade metamorphism, the volcanic and granitic rocks preserve a reproducible signature of calc-alkaline affinity. Element ratios analysis indicates that these rocks have not experienced significant LILE depletion and that metamorphism was, for the most part, isochemical. Element mobility of altered rocks is thus interpreted as produced by hydrothermal activity. AFM, ACF and AKF ternary plots of altered rocks and their protolith, define diagnostic alteration vectors, which reflect major elements mobility for the various alteration facies. Strong silica mobility is revealed by mass-balance calculations for altered pyroclastics. REE patterns of these rocks also show the mobility of heavy REE, in particular Tb, Dy and Ho

  12. Mechanisms of doubly-vergent vs. single-sided orogens: insights from numerical modelling

    NASA Astrophysics Data System (ADS)

    Vogt, K.; Matenco, L. C.; Gerya, T.; Cloetingh, S.

    2014-12-01

    Zones of continent collision form mountain ranges with high topographies and complex geometries. Compressional stresses during ongoing convergence result in crustal thickening and localized deformation, where crustal material is transported and redistributed within the orogen. We use numerical high-resolution thermo-mechanical models to investigate the physical processes of continent collision zones and its implications on rock exhumation. We demonstrate that compression of two continental blocks, separated by a rheologically weak suture zone can result in (i) doubly-vergent (Fig. A) or (ii) single-sided orogens (Fig. B), with distinct geometries, deformation and exhumation patterns. The transition between these different modes of collision is strongly controlled by the rheology of the continental lithosphere and therefore its temperature distribution. Doubly-vergent orogens form at relatively high thermal gradients, while single-sided orogens are typical for lower ones. Doubly-vergent orogens (Fig. A) are formed in response to the gradual accretion of crustal material to the upper plate along retro-shears. In these models continental subduction results in upper plate deformation and nested exhumation against retro-shears. Typical examples include the collision recorded by the Swiss Alps and the Pyrenees. In contrast, single-sided orogens are characterized by large-scale lower plate deformation and are accompanied by the subduction of lower crustal material (Fig. B). Modeling infers that shortening and associated exhumation will gradually propagate towards the foreland. In this situation, no significant retro-shear formation is observed, which is in agreement with recent physical modelling studies on deformation of the continental lithosphere [Willingshofer et al., 2013]. Natural examples of such single sided orogens are common in the Mediterranean (Carpathians, Dinarides, Apennines, Betics) or the SE Asia subduction zones. We conclude that deformation and

  13. How Orogen-scale Exhumed Strike-slip Faults Initiate

    NASA Astrophysics Data System (ADS)

    Cao, S.; Neubauer, F.

    2015-12-01

    Orogen-scale strike-slip faults present one the most important geodynamic processes affecting the lithosphere-asthenosphere system. In specific subtypes, faulting is virtually initiated along hot-to-cool boundaries, e.g. at such of hot granite intrusions or metamorphic core complexes to cool country rocks. Such fault zones are often subparallel to mountain ranges and expose a wide variety of mylonitic, cataclastic and non-cohesive fault rocks, which were formed at different structural levels of the crust and are stacked within each other ("telescoping"). Exhumation of rocks is, therefore, a common feature of such strike-slip faults implying major transtensive and/or transpressive processes accompanying pure strike-slip motion. The hot-to-cool thermal structure across the fault zone significantly influences the physical fault rock properties. One major question is how and where a major strike-slip initiates and further development. Here, we propose a model in which major continental exhumed strike-slip faults potentially evolve along rheologically weak zones such as plutons or margins of metamorphic complexes. As an example, we propose a model for the Ailao Shan-Red River (ASRR) fault, SE Asia, which initiated along the edge of a plutonic belt and evolved in response to India-Asia collision with four tectonic phases.

  14. Petrology of mafic and ultramafic intrusions from the Portneuf-Mauricie Domain, Grenville Province, Canada: Implications for plutonic complexes in a Proterozoic island arc

    NASA Astrophysics Data System (ADS)

    Sappin, A.-A.; Constantin, M.; Clark, T.

    2012-12-01

    The Portneuf-Mauricie Domain (PMD), located in the south-central part of the Grenville Province, comprises several mafic and ultramafic intrusions hosting Ni-Cu ± platinum-group element (PGE) prospects and a former small mining operation (Lac Édouard mine). These meter- to kilometer-scale, sulfide-bearing intrusions display diverse forms, such as layered and tabular bodies with no particular internal structure, and zoned plutons. They were injected ~ 1.40 Ga into a mature oceanic arc, before and during accretion of the arc to the Laurentian margin. The pressure-temperature conditions of the magmas at the beginning of their emplacement were 3 kbar and 1319-1200 °C (according to the petrologic modeling results from this study). The PMD mineralized intrusions are interpreted to represent former magma chambers or magma conduits in the roots of the oceanic arc. The parent magmas of the mineralized intrusions resulted mainly from the partial melting of a mantle source composed of spinel-bearing lherzolite. Petrologic modeling and the occurrence of primary amphibole in the plutonic rocks indicate that these parent melts were basaltic and hydrous. In addition, fractional crystallization modeling and Mg/Fe ratios suggest that most of the intrusions may have formed from evolved magmas, with Mg# = 60, resulting from the fractionation of more primitive magmas (primary magmas, with Mg# = 68). Petrologic modeling demonstrates that 30% fractional crystallization resulted in the primitive to evolved characteristics of the studied intrusive rocks (as indicated by the crystallization sequences and mineral chemistry). Exceptions are the Réservoir Blanc, Boivin, and Rochette West parent magmas, which may have undergone more extensive fractional crystallization, since these intrusions contain pyroxenes that are more iron rich and have lower Mg numbers than pyroxenes in the other PMD intrusions. The PMD mafic and ultramafic intrusions were intruded into an island arc located

  15. Tectono-metamorphic evolution of the internal zone of the Pan-African Lufilian orogenic belt (Zambia): Implications for crustal reworking and syn-orogenic uranium mineralizations

    NASA Astrophysics Data System (ADS)

    Eglinger, Aurélien; Vanderhaeghe, Olivier; André-Mayer, Anne-Sylvie; Goncalves, Philippe; Zeh, Armin; Durand, Cyril; Deloule, Etienne

    2016-01-01

    The internal zone of the Pan-African Lufilian orogenic belt (Zambia) hosts a dozen uranium occurrences mostly located within kyanite micaschists in a shear zone marking the contact between metasedimentary rocks attributed to the Katanga Neoproterozoic sedimentary sequence and migmatites coring domes developed dominantly at the expense of the pre-Neoproterozoic basement. The P-T-t-d paths reconstructed for these rocks combining field observations, microstructural analysis, metamorphic petrology and thermobarometry and geochronology indicate that they have recorded burial and exhumation during the Pan-African orogeny. Both units of the Katanga metasedimentary sequence and pre-Katanga migmatitic basement have underwent minimum peak P-T conditions of 9-11 kbar and 640-660 °C, dated at ca. 530 Ma by garnet-whole rock Lu-Hf isochrons. This suggests that this entire continental segment has been buried up to a depth of 40-50 km with geothermal gradients of 15-20 °C.km- 1 during the Pan-African orogeny and the formation of the West Gondwana supercontinent. Syn-orogenic exhumation of the partially molten root of the Lufilian belt is attested by isothermal decompression under P-T conditions of 6-8 kbar at ca. 530-500 Ma, witnessing an increase of the geothermal gradients to 25-30 °C·km- 1. Uranium mineralizations that consist of uraninite and brannerite took place at temperatures ranging from 600 to 700 °C, and have been dated at ca. 540-530 Ma by U-Pb ages on uraninite. The main uranium deposition thus occurred at the transition from the syn-orogenic burial to the syn-orogenic exhumation stages and has been then partially transposed and locally remobilized during the post-orogenic exhumation accommodated by activation of low-angle extensional detachment.

  16. The nature of orogenic crust in the central Andes

    NASA Astrophysics Data System (ADS)

    Beck, Susan L.; Zandt, George

    2002-10-01

    The central Andes (16°-22°S) are part of an active continental margin mountain belt and the result of shortening of the weak western edge of South America between the strong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield. We have combined receiver function and surface wave dispersion results from the BANJO-SEDA project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure. The major results are as follows: (1) The crust supporting the high elevations is thick and has a felsic to intermediate bulk composition. (2) The relatively strong Brazilian lithosphere is underthrusting as far west (65.5°W) as the high elevations of the western part of the Eastern Cordillera (EC) but does not underthrust the entire Altiplano. (3) The subcrustal lithosphere is delaminating piecemeal under the Altiplano-EC boundary but is not completely removed beneath the central Altiplano. The Altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation, leading to lower crustal flow and flat topography. In contrast, in the high-relief, inland-sloping regions of the EC and sub-Andean zone, the upper crust is still strongly coupled across the basal thrust of the fold-thrust belt to the underthrusting Brazilian Shield lithosphere. Subcrustal shortening between the Altiplano and Brazilian lithosphere appears to be accommodated by delamination near the Altiplano-EC boundary. Our study suggests that orogenic reworking may be an important part of the "felsification" of continental crust.

  17. Propagation tectonics and multiple accretionary processes of the Qinling Orogen

    NASA Astrophysics Data System (ADS)

    Dong, Yunpeng; Zhang, Xiaoning; Liu, Xiaoming; Li, Wei; Chen, Qing; Zhang, Guowei; Zhang, Hongfu; Yang, Zhao; Sun, Shengsi; Zhang, Feifei

    2015-05-01

    The Qinling Orogen was built through collision between the North China and South China Blocks. Previous detailed geological, geochemical and geochronological investigations revealed that the mountain range can be divided into four tectonic units with distinct tectono-lithostratigraphy, which are, from north to south, the southern sector of the North China Block, North Qinling Belt, South Qinling Belt and northern sector of the South China Block, separated by the Kuanping, Shangdan and Mianlue sutures. According to the petrology, geochemistry and geochronology of ophiolitic mélanges and related magmatic rocks, as well as the features of sedimentary units, we think that the North China Block, the North Qinling Belt and the South China Block were originally independent continental units while the South Qinling Belt had been the northern part of the South China Block. These units experienced three episodes of accretionary tectonic processes and amalgamation from south to north. The Neoproterozoic accretion took place along the Luonan-Luanchuan Fault and Kuanping ophiolitic mélange belt as a result of southward subduction and subsequent collision between the North Qinling and North China Blocks during ca. 1.0-0.8 Ga related to the formation of the supercontinent of Rodinia. The Paleozoic accretion occurred along the Shangdan suture resulted from northward subduction of oceanic lithosphere in the Early Paleozoic and subsequent continental subduction in the Late Paleozoic. Late Triassic accretion took place along the Mianlue suture between the South Qinling and South China Blocks due to northward subduction of the Mianlue oceanic lithosphere during the Permian-Early Triassic and subsequent collision in the Late Triassic. After the Late Triassic collision along the Mianlue suture the whole Qinling Mountain range entered the phase of intense intracontinental deformation.

  18. Conflicting Lu-Hf and Sm-Nd ages in the garnet gneisses of the Central Appalachians, Eastern U.S.: Implications for the timing and duration of Grenville Orogeny and a case for using coupled Lu-Hf and Sm-Nd geochronology

    NASA Astrophysics Data System (ADS)

    Vervoort, J. D.; Ramsey, M.; Aleinikoff, J. N.; Southworth, S.

    2013-12-01

    The Grenville orogeny is one of the most significant geological events in Earth's history with remnants of this event prominent on virtually every continent. Constraining its timing and duration is important not only for understanding the tectonics of the Grenville itself, but also for understanding supercontinent cycles and other questions of Earth's evolution. In order to provide better constraints on the timing of Grenvillian metamorphism, we analyzed garnet-bearing Mesoproterozoic ortho- and paragneisses, collected along a 150 km transect in the northern Blue Ridge Province, using combined Lu-Hf and Sm-Nd geochronology. The orthogneisses have U-Pb zircon crystallization ages of ~1140 and 1100 Ma. The paragneisses have maximum depositional ages ~1050-1020 Ma, based on the youngest detrital zircon populations. Zircon overgrowths and monazite suggest metamorphic events between ~1050 and 960 Ma. The Lu-Hf and Sm-Nd data for these samples both yield robust garnet ages with large spread of parent/daughter ratios, low age uncertainties, and low MSWD values. Lu-Hf ages define a narrow time span (1043×12 Ma to 1016×4 Ma; wtd. mean, 1024×7 Ma, 2σ). The Sm-Nd ages, determined on the same solutions as Lu-Hf, also define a narrow time range but are systematically younger (974×11 Ma to 932×5 Ma; wtd. mean, 957×10 Ma). The average difference between Lu-Hf and Sm-Nd ages is 67 Ma; the oldest Sm-Nd age is 40 Ma younger than the youngest Lu-Hf age. These large systematic differences in the ages are enigmatic. While Sm-Nd ages younger than Lu-Hf are not uncommon, these differences are typically small; nothing of this magnitude has been previously reported. There are, however, potential explanations for these differences. (1) Lu partitions strongly into garnet during growth yielding ages weighted toward the beginning of growth (e.g., Skora, 2006); no similar partitioning exists in Sm-Nd and these ages reflect mean garnet growth. (2) Lu diffuses much faster than Hf at

  19. Crustal Architecture along BABEL and FIRE profiles - Insight in the Growth of the Svecofennian Orogen

    NASA Astrophysics Data System (ADS)

    Korja, Annakaisa

    2016-04-01

    The Precambrian Svecofennian orogen is characterized by LP- HT metamorphism and voluminous granitoid magmatism that usually develop in transitional to plateau stages of a collisional orogeny. Deep seismic reflection profiles BABEL and FIRE have been interpreted using PURC concepts: prowedge, retrowedg, uplifted plug, subduction conduit and elevated plateau. BABEL profiles image a transitional orogen with several nuclei displaying prowedge-uplifted plug-retrowedge architecture above paleo-subduction conduits. Prowedge and -continent are on the south-southwestern side and retrowedge and -continent on the north-northwestern side. This implies a long-lived southwesterly retreating convergent margin, where transitional accretionary orogens have developed. FIRE1-3 profiles images a hot orogen with a pronounced super-infra structure, typical of an elevated plateau stage, below the Central Finland Granitoid Complex. Large volumes of granitoid intrusions suggest large scale melting of the middle and/or lower crust. Reflection structures, analogue and numerical modeling suggest midcrustal flow. The plateau is flanked by prowedges that are characterized by HT-LP migmatite belts. The Svecofennian orogeny has progressed to an elevated plateau stage in the thickest core of the orogen, west of the arc-continent collision zone.

  20. Variations in erosional efficiency modulate orogenic growth of the Alborz Mountains (N Iran)

    NASA Astrophysics Data System (ADS)

    Ballato, Paolo; Landgraf, Angela; Stockli, Daniel; Ghasemi, Mohammad; Strecker, Manfred; Kirby, Eric

    2014-05-01

    The recognition that redistribution of mass by erosion governs orogenic evolution has radically changed our perspective on the coupling between climate and mountain building processes. Climate modulates the efficiency of surface processes, which modifies crustal stresses and this is expected to produce the cessation of shortening at the orogenic front, onset of out-of-sequence thrusting, and increased rates of rock -uplift and sediment supply. Unambiguous characterization of these multiple responses through field-based studies, however, has remained challenging. Here, we show that coordinated changes in the rates and patterns of exhumation and deformation during the development of the Alborz Mountains (N Iran) were driven by abrupt, large magnitude (0.6 to 1.5 km) fluctuations in base level in the adjacent Caspian Sea. We argue that sustained regression of the paleoshoreline from ~6 to 3.2 Ma enhanced erosional efficiency of fluvial systems and increased exhumation within the axial orogenic zone and along the northern range flank which, in turn, drove coordinated retreat of the deformation fronts. When base level rose again at 3.2 Ma, exhumation in the orogen interior slowed and range-bounding faults were reactivated. This was associated with the progressive establishment of positive feedbacks loop between orographically-induced precipitation, focused erosion, exhumation, and rock uplift. Overall, these coordinated changes offer compelling evidence that enhanced erosion can indeed trigger a structural reorganization within an actively deforming orogen.

  1. Magnetotelluric imaging beneath the Taiwan orogen: An arc-continent collision

    NASA Astrophysics Data System (ADS)

    Bertrand, Edward A.; Unsworth, Martyn J.; Chiang, Chih-Wen; Chen, Chow-Son; Chen, Chien-Chih; Wu, Francis T.; TürkoǧLu, Ersan; Hsu, Han-Lun; Hill, Graham J.

    2012-01-01

    The Taiwan orogen has formed since the late Miocene by oblique collision between the Luzon Volcanic Arc on the Philippine Sea Plate, and the Eurasian continental margin. This oblique collision has produced an orogen that decreases in age from north to south, and permits study of the temporal evolution of an arc-continent collision. These factors make Taiwan a favorable location to study the process of arc-continent collision. The first long-period magnetotelluric (MT) measurements were recorded in Taiwan as part of the Taiwan Integrated Geodynamics Research (TAIGER) project in 2006-7. Measurements were made at 82 sites on three transects across south, central and north Taiwan, that span the breadth of the orogen and cross all major tectonic boundaries. Robust, remote reference processing of the MT time series data resulted in high-quality soundings that were modeled in both 2 and 3-dimensions. These MT models support predictions of lithospheric deformation (i.e., thick-skinned tectonics) beneath the Central Ranges in south and central Taiwan, but are inconsistent with predictions of orogen-scale thin-skinned models. The MT resistivity model for northern Taiwan is consistent with dewatering of the subducting Philippine slab, and with deformation described by the subducting-indenter tectonic model. Modeling the TAIGER MT data has definitively shown that conductive, and seismically active crustal structures, exist to 30+ km beneath the orogen. These conductive regions, interpreted as interconnected fluid, map pervasive zones of collisional deformation that are lithospheric in scale.

  2. Early Cretaceous extensional reworking of the Triassic HP-UHP metamorphic orogen in Eastern China

    NASA Astrophysics Data System (ADS)

    Lin, W.; Ji, W.; Faure, M.; Wu, L.; Li, Q. L.; Shi, Y.; Scharer, U.; Wang, F.; Wang, Q.

    2015-12-01

    Corresponding to the Early Mesozoic continental subduction between the North China Block (NCB) and the South China Block (SCB), the Tongbaishan-Hong'an-Dabieshan-Sulu massifs are famous for their HP-UHP metamorphism. More than 50% of the HP-UHP Orogenic Belt was significantly reworked by Early Cretaceous extensional tectonics. This Early Cretaceous event with a fast cooling period, at 130-120 Ma, superimposed on the Early Mesozoic HP-UHP orogenic belt and intensively changed the architecture of this orogen. Each individual segment documents different Early Cretaceous extensional structures, namely the central Tongbaishan domain is a metamorphic core complex (MCC) represented by an A-type non-cylindrical antiform; the central Dabieshan domain is a typical Cordilleran-type migmatite-cored MCC; the Southern Sulu UHP domain is a "wedge-shaped" structure exhumed by a simple detachment fault. These late stage extensional structures expose the previous HP-UHP orogenic belt as fragments along the NCB-SCB boundary. The geodynamic setting of this Early Cretaceous extensional tectonics along the HP-UHP orogen is a part of a 1000 km-scale crustal extension belt that is widespread in eastern Eurasia continent from Trans-Baikal to the central part of the South China Block. Convective erosion or delamination of the mantle lithosphere might be considered as a possible mechanism for mantle removal.

  3. Early Cretaceous extensional reworking of the Triassic HP-UHP metamorphic orogen in Eastern China

    NASA Astrophysics Data System (ADS)

    Lin, Wei; Ji, Wenbin; Faure, Michel; Wu, Lin; Li, Qiuli; Shi, Yonghong; Scharer, Urs; Wang, Fei; Wang, Qingchen

    2015-11-01

    Corresponding to the Early Mesozoic continental subduction between the North China Block (NCB) and the South China Block (SCB), the Tongbaishan-Hong'an-Dabieshan-Sulu massifs are famous for their HP-UHP metamorphism. More than 50% of the HP-UHP Orogenic Belt was significantly reworked by Early Cretaceous extensional tectonics. This Early Cretaceous event with a fast cooling period, at 130-120 Ma, superimposed on the Early Mesozoic HP-UHP orogenic belt and intensively changed the architecture of this orogen. Each individual segment documents different Early Cretaceous extensional structures, namely the central Tongbaishan domain is a metamorphic core complex (MCC) represented by an A-type non-cylindrical antiform; the central Dabieshan domain is a typical Cordilleran-type migmatite-cored MCC; the Southern Sulu UHP domain is a "wedge-shaped" structure exhumed by a simple detachment fault. These late stage extensional structures expose the previous HP-UHP orogenic belt as fragments along the NCB-SCB boundary. The geodynamic setting of this Early Cretaceous extensional tectonics along the HP-UHP orogen is a part of a 1000 km-scale crustal extension belt that is widespread in eastern Eurasia continent from Trans-Baikal to the central part of SCB. Convective erosion or delamination of the mantle lithosphere might be considered as a possible mechanism for mantle removal.

  4. Orogen-perpendicular structures in the central Tasmanides and implications for the Paleozoic tectonic evolution of eastern Australia

    NASA Astrophysics Data System (ADS)

    Abdullah, Rashed; Rosenbaum, Gideon

    2017-01-01

    The curvilinear E-W structures of the southern Thomson Orogen are approximately orthogonal to the general N-S structural trend of the Tasmanides of eastern Australia. The origin of these orogen-perpendicular structures and their implications to tectonic reconstructions of eastern Gondwana are not fully understood. Here we use geophysical data to unravel the geometry, kinematics and possible timing of major structures along the boundary between the Thomson Orogen and the southern Tasmanides (Delamerian and Lachlan orogens). Aeromagnetic data from the southern Thomson Orogen show WNW, E-W and/or ENE trending structural grains, corresponding to relatively long wavelength linear geophysical anomalies. Kinematic analyses indicate strike-slip and transpressional deformation along these geophysically defined faults. Structural relationships indicate that faulting took place during the Benambran (Late Ordovician to Middle Silurian) and Tabberabberan (late Early to Middle Devonian) orogenies. However, some of the described crustal-scale structures may have developed in the Cambrian during the Delamerian Orogeny. Interpretation of deep seismic data shows that the crust of the southern Thomson Orogen is substantially thicker than the Lachlan Orogen crust, which is separated from the Thomson Orogen by the north-dipping Olepoloko Fault. A major lithospheric-scale change across this boundary is also indicated by a contrast in seismic velocities. Together with evidence for the occurrence of Delamerian deformation in both the Koonenberry Belt and northeastern Thomson Orogen, and a significant contrast in the width of the northern Tasmanides versus the southern Tasmanides, it appears that the southern Thomson Orogen may represent the locus of orogen-perpendicular segmentation, which may have occurred in response to along-strike plate boundary variations.

  5. Redefinition of the Lac-St.-Jean Anorthosite, Central Grenville Province, Québec, Based on Compositional, Structural, Geochronological, and Mineral Deposit Features

    NASA Astrophysics Data System (ADS)

    Hebert, C.; van Breemen, O.

    2004-05-01

    The Lac-St.-Jean Anorthosite was previously recognized as one of the biggest anorthositic masses in the world (20,000 km2). Recent field and geochronological work has demonstrated that this anorthositic mass is, in fact, an assemblage resulting from four temporally distinct magmatic episodes, which took place between 1327 and 1012 Ma. This discovery has required the redefinition of the Lac-St.-Jean Anorthosite in terms of its component units. The 1327 ± 16 Ma De La Blache Mafic Plutonic Suite occupies the northeastern limb of the former Lac-St.-Jean Anorthosite. This suite is characterized by labradorite-type anorthosite, a large volume of olivine-bearing anorthositic rocks, a wide gabbronoritic fringe, and Fe-Ti and Ni-Cu occurrences. This magmatic body is constrained by a paired, dextral and sinistral strike-slip fault system. The Lac-St.-Jean Anorthositic Suite represents that part of the former Lac St.-Jean Anorthosite that was emplaced between 1160 and 1140 Ma. The suite still occupies the largest area of any anorthositic mass in the Central Grenville Province. It is characterized by labradorite- and andesine-type anorthositic rocks, is bordered by gabbronoritic fringes along its northern and southeastern margins, and contains sizeable Ti-Fe-P and Ni-Cu occurrences. The western part of this huge anorthositic mass is undeformed, whereas the eastern part was pervasively affected by a system of thrust and strike-slip faults, which resulted in a high degree of recrystallization of the anorthositic rocks. The 1180-1160 Ma Vanel Anorthosite occupies a large part of the eastern margin of the former Lac-St.-Jean-Anorthosite. This unit is characterized by labradorite- and andesine-type anorthositic rocks containing typically pink-colored and almost ubiquitously recristallized plagioclase. It includes abundant coronitic leuconorite, orthopyroxene-bearing leucotroctolite, and norite, and contains a few Ti-Fe-P occurrences. The 1012 +6/-4 Ma Mattawa Anorthosite is an

  6. Architecture of orogenic belts and convergent zones in Western Ishtar Terra, Venus

    NASA Technical Reports Server (NTRS)

    Head, James W.; Vorderbruegge, R. W.; Crumpler, L. S.

    1989-01-01

    Linear mountain belts in Ishtar Terra were recognized from Pioneer-Venus topography, and later Arecibo images showed banded terrain interpreted to represent folds. Subsequent analyses showed that the mountains represented orogenic belts, and that each had somewhat different features and characteristics. Orogenic belts are regions of focused shortening and compressional deformation and thus provide evidence for the nature of such deformation, processes of crustal thickening (brittle, ductile), and processes of crustal loss. Such information is important in understanding the nature of convergent zones on Venus (underthrusting, imbrication, subduction), the implications for rates of crustal recycling, and the nature of environments of melting and petrogenesis. The basic elements of four convergent zones and orogenic belts in western Ishtar Terra are identified and examined, and then assess the architecture of these zones (the manner in which the elements are arrayed), and their relationships. The basic nomenclature of the convergent zones is shown.

  7. Orogenic plateau magmatism of the Arabia-Eurasia collision zone

    NASA Astrophysics Data System (ADS)

    Allen, M. B.; Neill, I.; Kheirkhah, M.; van Hunen, J.; Davidson, J. P.; Meliksetian, Kh.; Emami, M. H.

    2012-04-01

    generally applicable as melt triggers. Enigmatic lavas are erupted over the thick lithosphere of Kurdistan Province, Iran. These alkali basalts and basanites have the chemical characteristics of small degree (<1%) melts in the garnet stability field. Most possess supra-subduction zone chemistry (La/Nb = 1-3), but this signature is highly variable. Similar La/Nb variability occurs in the basic lavas of Damavand volcano in the Alborz Mountains of northern Iran. Modelling suggests the depletion of residual amphibole during the progression of partial melting can explain the observed La/Nb range. This melting may occur as the result of lithospheric thickening. At depths of ~90 km, amphibole-bearing peridotite crosses an experimentally-determined "backbend" in its solidus. Melting can continue while the source remains hydrated. Such "compression" melting may apply to parts of other orogenic plateaux, including Tibet.

  8. Relating shortening, erosion, and exhumation to orogen width during Alpine collision

    NASA Astrophysics Data System (ADS)

    Rosenberg, Claudio; Berger, Alfons; Bellahsen, Nicolas; Bousquet, Romain

    2014-05-01

    The width of orogens may change through time depending on the amount of shortening, on the efficiency of erosion, on the strength and thickness of the plates, or on the occurrence of pre-existing and newly formed weaknesses within the plates. The effect of erosion rates on the width of the Alps was controversially discussed, based on estimates of paleo-erosion rates and paleo-widths of the orogen. However, both parameters are difficult to reconstruct. In this contribution we investigate the causes of present-day, along-strike changes of width of the Eastern and the Central Alps to understand its width changes through time. Based on a series of 6 orogen-scale cross-sections and their retro-deformation we set the width of the thickened accreted lower plate in relation to the amount of collisional shortening and exhumation. We conclude that higher amounts of shortening systematically coincide with smaller widths of the thickened, accreted lower plate, i.e. the width of the mountain chain north of the South-Alpine indenters. Changes of width by a factor 2 along orogen segments of less than 200 km length cannot result from long-term climatic differences and sedimentary or paleontological evidences suggesting such differences are lacking. Therefore, erosional processes did not directly control the width of the orogen, which did not behave as a critical taper. Higher amounts of shortening coincide with larger amplitudes of orogen-scale, upright folds, with larger amounts of exhumation, and with higher exhumation rates. Hence, erosion did play a major role in reducing by up to 35 km the vertical crustal thickness in order to accommodate and allow shortening by folding, but along-strike changes of erosion rates were governed by different amounts of shortening, not by different climate.

  9. La Escalerilla pluton, San Luis Argentina: The orogenic and post-orogenic magmatic evolution of the famatinian cycle at Sierras de San Luis

    NASA Astrophysics Data System (ADS)

    Morosini, Augusto Francisco; Ortiz Suárez, Ariel Emilio; Otamendi, Juan Enrique; Pagano, Diego Sebastián; Ramos, Gabriel Alejandro

    2017-01-01

    Field relationships, geochemical analysis and two new absolute ages (LA-MC-ICP-MS U/Pb-zircon) allow the division of the La Escalerilla pluton (previously considered to be a single granitic body) into two different plutons: a new La Escalerilla pluton (s.s.), dated at 476.7 ± 9.6 Ma, that represents the northern portion, and the El Volcán pluton, dated at 404.5 ± 8.5 Ma, located in the southern sector. The La Escalerilla pluton is composed of three facies: (1) biotite-bearing granodiorite, (2) porphyritic biotite-bearing granite, and (3) porphyritic two micas-bearing leucogranite, being the presence of late-magmatic dykes in these facies common. The El Volcán pluton is composed of two main facies: 1) porphyritic biotite-bearing granite, and 2) two micas-bearing leucogranite, but amphibole-bearing monzodioritic and tonalititic mega-enclaves are also common, as well as some dykes of amphibole and clinopyroxene-bearing syenites. A peculiarity between the two plutons is that their most representative facies (porphyritic biotite-bearing granites) have, apart from different absolute ages, distinctive geochemical characteristics in their concentrations of trace elements; the La Escalerilla granite is comparatively poorer in Ba, Sr, Nb, La, Ce, P, and richer in Rb, Tb, Y, Tm and Yb. The El Volcán granite is notably enriched in Sr and depleted in Y, resulting in high Sr/Y ratios (12.67-39.08) compared to the La Escalerilla granite (1.11-2.41). These contrasts indicate that the separation from their sources occurred at different depths: below 25 km for the La Escalerilla, and above 30 km for the El Volcán. Moreover, the contrasts allow us to interpret a thin crust linked to an environment of pre-collisional subduction for the first case, and a thickened crust of post-collisional environment for the second, respectively.

  10. Trondhjemitic, 1.35-1.31 Ga gneisses of the Mount Holly Complex of Vermont: evidence for an Elzevirian event in the Grenville Basement of the United States Appalachians

    USGS Publications Warehouse

    Ratcliffe, N.M.; Aleinikoff, J.N.; Burton, W.C.; Karabinos, P.

    1991-01-01

    A newly recognized suite of trondhjemite-tonalite and dacitic gneiss forms a 10 km wide belt of rocks within the Mount Holly Complex in the central part of the Green Mountain massif. Field relationships and chemistry indicate that these gneisses are calc-alkaline, volcanic, and hypabyssal plutonic rocks older than the Middle Proterozoic regional deformation that affected the Mount Holly Complex. U-Pb zircon dates indicate ages as great as 1.35 Ga for crystallization of the volcanic protoliths and for intrusion of crossing trondhjemite. Tonalitic plutonism continued until 1.31 Ga. The Mount Holly intrusives and volcanics may have formed during 1.35-1.31 Ga ensialic volcanic-arc activity, contemporaneous with ensimatic arc activity during the early part of the Elzevirian phase of the Grenville orogeny. -from Authors

  11. Basin-mountain structures and hydrocarbon exploration potential of west Junggar orogen in China

    NASA Astrophysics Data System (ADS)

    Wu, X.; Qi, X.; Zheng, M.

    2015-12-01

    Situated in northern Xinjiang, China, in NE-SW trend, West Junggar Orogen is adjacent to Altai fold belt on the north with the Ertix Fault as the boundary, North Tianshan fold belt on the south with the Ebinur Lake Strike-slip Fault as the boundary, and the Junggar Basin on the southeast with Zaire-Genghis Khan-Hala'alat fold belt as the boundary. Covering an area of about 10×104 km2 in China, there are medium and small intermontane basins, Burqin-Fuhai, Tacheng, Hefeng and Hoxtolgay, distributing inside the orogen. Tectonically West Junggar Orogen lies in the middle section of the Palaeo-Asian tectonic domain where the Siberia, Kazakhstan and Tarim Plates converge, and is the only orogen trending NE-SW in the Palaeo-Asian tectonic domain. Since the Paleozoic, the orogen experienced pre-Permian plate tectonic evolution and post-Permian intra-plate basin evolution. Complex tectonic evolution and multi-stage structural superimposition not only give rise to long term controversial over the basin basement property but also complex basin-mountain coupling relations, structures and basin superimposition modes. According to analysis of several kinds of geological and geophysical data, the orogen was dominated by compressive folding and thrust napping from the Siberia plate in the north since the Late Paleozoic. Compressive stress weakened from north to south, corresponding to subdued vertical movement and enhanced horizontal movement of crustal surface from north to south, and finally faded in the overthrust-nappe belt at the northwest margin of the Junggar Basin. The variation in compressive stress is consistent with the surface relief of the orogen, which is high in the north and low in the south. There are two kinds of basin-mountain coupling relationships, i.e. high angle thrusting and overthrusting and napping, and two kinds of basin superimposition modes, i.e. inherited and progressive, and migrating and convulsionary modes. West Junggar orogen has rich oil and gas

  12. Trans-Hudson Orogen of North America and Himalaya-Karakoram-Tibetan Orogen of Asia: Structural and thermal characteristics of the lower and upper plates

    NASA Astrophysics Data System (ADS)

    St-Onge, Marc R.; Searle, Michael P.; Wodicka, Natasha

    2006-08-01

    The Trans-Hudson Orogen (THO) of North America and the Himalaya-Karakoram-Tibetan Orogen (HKTO) of Asia preserve a Paleoproterozoic and Cenozoic record, respectively, of continent-continent collision that is notably similar in scale, duration and character. In THO, the tectonothermal evolution of the lower plate involves (1) early thin-skinned thrusting and Barrovian metamorphism, (2) out-of-sequence thrusting and high-T metamorphism, and (3) fluid-localized reequilibration, anatexis, and leucogranite formation. The crustal evolution of the Indian lower plate in HKTO involves (1) early subduction of continental crust to ultrahigh pressure (UHP) eclogite depths, (2) regional Barrovian metamorphism, and (3) widespread high-T metamorphism, anatexis, and leucogranite formation. The shallow depths of the high-T metamorphism in HKTO are consistent with early to mid-Miocene ductile flow of an Indian lower plate midcrustal channel, from beneath the southern Tibetan Plateau to the Greater Himalaya. Melt weakening of the lower plate in THO is not observed at a similar scale probably due to the paucity of pelitic lithologies. Tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks, emplacement of Andean-type plutonic suites, and high-T metamorphism. Syncollisional to postcollisional events include emplacement of garnet-biotite-muscovite leucogranites, anatectic granites, and sporadic metamorphism (up to 90 Myr following the onset of collision in THO). Comparing the type and duration of tectonothermal events for THO and HKTO supports the notion of tectonic uniformitarianism for at least the later half of dated Earth history and highlights the complementary nature of the rock record in an older "exhumed" orogen compared to one undergoing present-day orogenesis.

  13. Sulfur- and lead-isotope signatures of orogenic gold mineralisation associated with the Hill End Trough, Lachlan Orogen, New South Wales, Australia

    NASA Astrophysics Data System (ADS)

    Downes, P. M.; Seccombe, P. K.; Carr, G. R.

    2008-11-01

    The Hill End Trough (HET) is a deformed middle Silurian to Early Devonian sediment-dominated rift within the northeastern Lachlan Orogen. The HET hosts the Hill End, Hargraves, Napoleon Reefs, Stuart Town and Windeyer low-sulfide orogenic gold deposits. Adjacent to the HET are the Bodangora and Gulgong gold deposits. In this study we present 91 new sulfur- and 18 new lead-isotope analyses and collate a further 25 sulfur- and 32 lead-isotopes analyses from unpublished sources for these deposits. Larger gold deposits in the HET have near 0 δ34S‰ values indicating that sulfur in these systems was sourced from a magmatic reservoir. The dominant lead isotope signature for HET-hosted deposits reflects a crustal source however some mantle-derived lead has been introduced into the HET. Sulfur- and lead-isotopic results suggest that gold was sourced from mantle-derived magmatic units beneath the HET. The study supports earlier studies at Hill End by concluding that the majority of orogenic gold mineralisation in and adjacent to the HET formed during the Early Carboniferous period.

  14. Crustal structures from the Wuyi-Yunkai orogen to the Taiwan orogen: The onshore-offshore wide-angle seismic experiments of the TAIGER and ATSEE projects

    NASA Astrophysics Data System (ADS)

    Kuo, Yao-Wen; Wang, Chien-Ying; Kuo-Chen, Hao; Jin, Xin; Cai, Hui-Teng; Lin, Jing-Yi; Wu, Francis T.; Yen, Horng-Yuan; Huang, Bor-Shouh; Liang, Wen-Tzong; Okaya, David; Brown, Larry

    2016-12-01

    Knowledge of the crustal structure is important for understanding the tectonic framework and geological evolution of southeastern China and adjacent areas. In this study, we integrated the datasets from the TAIGER (TAiwan Integrated GEodynamic Research) and ATSEE (Across Taiwan Strait Explosion Experiment) projects to resolve onshore-offshore deep crustal seismic profiles from the Wuyi-Yunkai orogen to the Taiwan orogen in southeastern China. Three seismic profiles were resolved, and the longest profile was 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. In total, data from 40 shots, 2 earthquakes, and approximately 1,950 stations were used; 15,612 arrivals were selected among three transects. Using these data, we determined the complex crustal evolution since the Paleozoic era , involving the closed Paleozoic rift basin in central Fujian, the Cenozoic extension due to the South China Sea opening beneath the coastline of southern Fujian, and the on-going collision of the Taiwan orogen. The shape of the Moho, which also reflects the crustal evolution, can be summarized as follows: 30 km deep to the west of Fujian, deepening toward central Fujian ( 35 km), becoming shallower toward the Taiwan Strait ( 28 km), deepening again toward the mountain belt of Taiwan ( 42 km), and becoming shallower toward the Pacific Ocean ( 10 km).

  15. How does the mid-crust accommodate deformation in large, hot collisional orogens? A review of recent research in the Himalayan orogen

    NASA Astrophysics Data System (ADS)

    Cottle, John M.; Larson, Kyle P.; Kellett, Dawn A.

    2015-09-01

    The presence of hot, weak crust is a central component of recent hypotheses that seek to explain the evolution of continent-continent collisions, and in particular may play an important role in accommodating the >3000 km of convergence within the Himalaya-Tibetan collision over the last ∼55 Myr. Models that implicate flow of semi-viscous midcrustal rocks south toward the front of the Himalayan orogen, 'channel flow', are able to account for many geologic observations in the Himalaya, while alternative models of collision, particularly 'thrust-wedge taper', demonstrate that much of the observed geology could have formed in the absence of a low-viscosity mid-crustal layer. Several recent studies, synthesized here, have prompted a shift from initial assumptions that channel flow and thrust-wedge taper processes are by definition mutually exclusive. These new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel- and wedge-type processes operate in spatially and temporally distinct domains within the orogen, and further, that the system may migrate back and forth between these types of behavior. This continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the Himalayan orogen and, hence, for understanding the evolution of Earth's many continent-continent collision zones.

  16. A study of models and controls for basin formation during continental collision: (1) Australian lithosphere along Banda orogen (Indonesia) and (2) Alboran Sea basin (western Mediterranean)

    NASA Astrophysics Data System (ADS)

    Tandon, Kush

    Mechanisms for the formation of a foreland basin at the beginning of continental collision (Pliocene-Recent Australian continental foreland along the Banda orogen) and a post-orogenic, continental, Miocene extensional basin (Alboran Sea) are studied. Such a study investigates the controls on the basin formation during the start of the Wilson cycle and later during the break up of a thickened continent in a collisional environment. Effective Elastic Thickness (EET) of the Australian continental lithosphere from Roti to the Kai Plateau ({˜}121sp°{-}137sp°E longitude) are estimated using an elastic-half beam model to match the sea floor bathymetry and the Bouguer gravity anomalies. Range of constant EET values from 27-75 km across the shelf of Australian lithosphere shows a variation of 64% with the highest value in the vicinity of central Timor where the collision is most advanced. Downdip on the Australian continental lithosphere from shelf to beneath the Banda orogen, the reduction in EET is from ˜90 km-˜30 km (66%). Variations in EET can be explained by inelastic yielding (brittle and plastic failure, crust-mantle decoupling in the lower crust and brittle-ductile decoupling in the upper-middle crust) in the Australian lithosphere. Change in EET occurred at the start of continental subduction due to change in curvature, both in map and cross-sectional view. Oroclinal bending of the continental Australian lithosphere increased the inelastic failure in the eastern end. Different mechanisms of basin formation at a site of post-orogenic collapse are studied by constraining the timing of rifting in the western, eastern, and northern parts of the Alboran Sea basin on seismic reflectors via synthetic seismograms using ODP Leg 161 and Andalucia A-1 data. Regions of adjacent coeval compression and extension are found in the Alboran Sea basin. Normal faulting continues in parts of the eastern Alboran Sea basin later than in the western Alboran Sea basin. The development

  17. 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

    -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).

  18. An Early Cretaceous garnet pressure-temperature path recording synconvergent burial and exhumation from the hinterland of the Sevier orogenic belt, Albion Mountains, Idaho

    NASA Astrophysics Data System (ADS)

    Kelly, Eric D.; Hoisch, Thomas D.; Wells, Michael L.; Vervoort, Jeffrey D.; Beyene, Mengesha A.

    2015-08-01

    Rocks may undergo complex pressure-temperature ( P- T) histories during orogenesis in response to alternating episodes of synconvergent burial and exhumation. In this study, chemical zoning in garnets combined with textural and chemical evidence from the schist of Willow Creek in the Albion Mountains of south-central Idaho (USA), reveals a complex P- T path during the early stages of Sevier orogenesis. The distribution of quartz inclusions combined with internal resorption features establishes a hiatus in garnet growth. Chemical zoning was simulated using a G-minimization approach to yield a P- T path consisting of three distinct pressure changes during increasing temperature, defining an "N" shape. Lu-Hf isochron ages from multiple garnet fractions and whole-rock analyses in two samples are 132.1 ± 2.4 and 138.7 ± 3.5 Ma. The samples were collected from the hanging wall of the Basin-Elba thrust fault and yielded results similar to those previously obtained from the footwall. This leads to several conclusions: (1) Both the hanging wall and footwall experienced the same metamorphic event, (2) the paths document a previously unrecognized crustal thickening and synorogenic extension cycle that fills an important time gap in the shortening history of the Sevier retroarc, suggesting progressive eastward growth of the orogen rather than a two-stage history, and (3) episodes of extensional exhumation during protracted convergent orogenesis are increasingly well recognized and highlight the dynamic behavior of orogenic belts.

  19. The role of radiogenic heat in prolonged intraplate reworking: The Capricorn Orogen explained?

    NASA Astrophysics Data System (ADS)

    Korhonen, Fawna J.; Johnson, Simon P.

    2015-10-01

    Links between intraplate crustal deformation and the thermal regime of the lithosphere have been investigated using numerical models, but these conceptual models must be tested using well-constrained natural systems. The Proterozoic Capricorn Orogen of Western Australia is ideally suited for such a study; after arc accretion, it records a protracted history of intracrustal differentiation and over one billion years of subsequent tectonic reworking. The early addition of juvenile arc material initially reduced the heat production of the crust, whereas subsequent recycling of the deep crust ultimately produced radiogenic granites in the shallow crust. Using whole rock and magmatic zircon compositions, we show that the shallow crust of the Capricorn Orogen has been progressively enriched in heat-producing elements with time. The high heat production is mainly controlled by thorium, concentrated in LREE-enriched phosphates that grew during the late stages of crystallization of crustally-derived granites. Using a numerical model, we investigate the thermal effects of this evolved crustal stratification and compare them with quantitative data on the pressure-temperature-time evolution of a non-magmatic Mesoproterozoic reworking event. The results provide new information concerning the process by which the thermal conditions necessary for the reworking of orogenic crust are generated and sustained for prolonged timescales (>110 Ma). Furthermore, we demonstrate that perturbations to the radiogenic crust lead to warmer geotherms and consequent lithospheric weakening, which has a fundamental control on intraplate reworking of long-lived orogens.

  20. Paleomagnetic data from the New England Orogen (eastern Australia) and implications for oroclinal bending

    NASA Astrophysics Data System (ADS)

    Shaanan, Uri; Rosenbaum, Gideon; Pisarevsky, Sergei; Speranza, Fabio

    2015-11-01

    Orogenic curvatures (oroclines) are common in modern and ancient orogens, but the geodynamic driving forces of many oroclines remain controversial. Here we focus on the New England oroclines of eastern Australia, the formation of which had been previously broadly constrained to the Early-Middle Permian. This time interval encompasses periods of both back-arc extension (at ~ 300-280 Ma) and subsequent contractional deformation (Hunter-Bowen Orogeny) that commenced at ~ 270 Ma along the paleo-Pacific and Gondwanan subduction plate boundary. We present new paleomagnetic data from volcanic rocks that were extruded during the transition from extension to contraction (at ~ 272 Ma), and we show that the oroclinal structure must have formed prior to the emplacement of the volcanic rocks. Our results thus indicate that oroclinal bending in the southernmost New England Orogen has been completed prior to the onset of Middle Permian contractional deformation. It is therefore concluded that the oroclines have likely formed during back-arc extension, and that a major contribution to the orogenic curvature was driven by trench retreat.

  1. Evidence of Early Cretaceous transpression in the Sulu orogenic belt, eastern China

    NASA Astrophysics Data System (ADS)

    Wang, Jun; Chang, Su-Chin; Lin, Peijun; Zhu, Xiaoqing; Fu, Yongtao; Zhang, Haichun

    2016-09-01

    Recent studies have documented marine turbidites with syn-sedimentary deformation features in the central Sulu orogenic belt of eastern China. These units preserve essential information on the Late Mesozoic evolution of the Sulu orogenic belt. Referred to as the Baxiandun Formation, the turbidites exhibit similar lithologic characteristics to nearby units such as the Lingshandao Formation that have been well studied even though precise geochronologic constraints are lacking for a more precise correlation. This study reports detrital zircon Usbnd Pb age data that correlate the Baxiandun Formation turbidites of the central Sulu orogenic belt to the Early Cretaceous Lingshandao Formation. We also report Al-in-hornblende emplacement depth estimates for granitic intrusions of the Sulu orogenic belt's Laoshan mountain. A sharp contact between the Laoshan granites and the marine Baxiandun Formation indicates that the Baxiandun basin rapidly subsided to the emplacement depth of the Laoshan plutons. Lateral correlation among the marine turbidites, the Lingshandao and Baxiandun Formations, combined with information established by previous studies indicates initiation of transpressional tectonics at 122-121 Ma. Transpression ceased with the emplacement of the Laoshan granites, whose A1-type composition indicates a return to extensional tectonics at ca. 111 Ma.

  2. Post-collisional magmatism in the central East African Orogen: The Maevarano Suite of north Madagascar

    USGS Publications Warehouse

    Goodenough, K.M.; Thomas, Ronald J.; De Waele, B.; Key, R.M.; Schofield, D.I.; Bauer, W.; Tucker, R.D.; Rafahatelo, J.-M.; Rabarimanana, M.; Ralison, A.V.; Randriamananjara, T.

    2010-01-01

    Late tectonic, post-collisional granite suites are a feature of many parts of the Late Neoproterozoic to Cambrian East African Orogen (EAO), where they are generally attributed to late extensional collapse of the orogen, accompanied by high heat flow and asthenospheric uprise. The Maevarano Suite comprises voluminous plutons which were emplaced in some of the tectonostratigraphic terranes of northern Madagascar, in the central part of the EAO, following collision and assembly during a major orogeny at ca. 550 Ma. The suite comprises three main magmatic phases: a minor early phase of foliated gabbros, quartz diorites, and granodiorites; a main phase of large batholiths of porphyritic granitoids and charnockites; and a late phase of small-scale plutons and sheets of monzonite, syenite, leucogranite and microgranite. The main phase intrusions tend to be massive, but with variably foliated margins. New U-Pb SHRIMP zircon data show that the whole suite was emplaced between ca. 537 and 522 Ma. Geochemically, all the rocks of the suite are enriched in the LILE, especially K, and the LREE, but are relatively depleted in Nb, Ta and the HREE. These characteristics are typical of post-collisional granitoids in the EAO and many other orogenic belts. It is proposed that the Maevarano Suite magmas were derived by melting of sub-continental lithospheric mantle that had been enriched in the LILE during earlier subduction events. The melting occurred during lithospheric delamination, which was associated with extensional collapse of the East African Orogen. ?? 2009 Natural Environment Research Council.

  3. Orogen-parallel deformation of the Himalayan midcrust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna-Dhaulagiri Himalaya, central Nepal

    NASA Astrophysics Data System (ADS)

    Parsons, A. J.; Ferré, E. C.; Law, R. D.; Lloyd, G. E.; Phillips, R. J.; Searle, M. P.

    2016-11-01

    The metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna-Dhaulagiri region, central Nepal, recorded orogen-parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field-based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate strain regime with associated formation of low-angle northward dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right-lateral orogen-parallel stretching. We propose that oblate strain within a midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the need for strain partitioning in the upper crust. Oblate flattening may have also promoted orogen-parallel melt migration and development of melt-depleted regions between km3 scale leucogranite culminations at 50-100 km intervals along orogen strike. Following the cessation of flow, continued oblique convergence led to upper crustal strain partitioning between orogen-perpendicular convergence on thrust faults and orogen-parallel extension on normal and strike-slip faults. In the Annapurna-Dhaulagiri Himalaya, orogen-parallel stretching lineations are interpreted as a record of transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel stretching. Our findings suggest that midcrustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen-wide transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel extension during the mid-Miocene. The 3-D nature of oblate strain and orogen-parallel stretching cannot be replicated by 2-D numerical simulations of the Himalayan orogen.

  4. Lithosphere structure of the west Qinling orogenic belt revealed by deep seismic reflection profile

    NASA Astrophysics Data System (ADS)

    Wang, H.

    2009-12-01

    The west Qinling orogen located in the northeastern margin of the Qinghai-Tibet plateau, is transformation zone between the N-S-trending and E-W-trending tectonics in the Chinese continent. Further study of the fine crust structure of the west Qinling orogen and its relationships with surrounding basins have very important significance for understanding tectonic response of the northeastern margin of the plateau about collision convergence of the Indian block and Asian block and learning formation and evolution of the plateau. In 2009, we reprocessed the data of the Tangke-Hezuo deep seismic reflection profiles collected in 2004 across the west Qinling orogen and the northern Songpan block. The new results show the lithosphere fine structure of the west Qinling orogen. Reflection features indicate that an interface at 6.0-7.0s (TWT) divided the crust into the upper and lower crust, whose structural style and deformation are totally different. Integrating geological data, we deduce that the interface at 6.0-7.0s (depth with 18-21 km) was the basement detachment, which made deformation decoupled of the upper and lower crust. The multi-layered reflections in the upper crust reveal the sedimentary covers of the west Qinling orogen, disclose the thickness of the various structure layer and deformation degree, and provide a basis for the prospective evaluation of a multi-metallic mineral and energy exploration. The north dipping strong reflection characteristics of the lower crust in the west Qinling orogen constituted imbricate structure, such imbricate structural features provide seismology evidence for researching the west Qinling thrusting toward the northern Songpan block, and have great significance for studying formation and evolution of the Songpan-Garze structure. Moho reflections are observed around 17.0-17.2s, characterized by nearly horizontal reflections, which implies the west Qinling orogen underwent an intense extension post orogeny caused the lithosphere

  5. Shear-zone systems and melts: feedback relations and self-organization in orogenic belts

    NASA Astrophysics Data System (ADS)

    Brown, Michael; Solar, Gary S.

    1998-03-01

    In orogenic belts, the common spatial and temporal association of granites with crustal-scale shear-zone systems suggests melt transfer from source to upper crust was the result of a feedback relation. In this relation, the presence of melt in the crust profoundly affects the rheology, and induces localization of strain within shear-zone systems. Consequently, melt is moved out of the source preferentially along high-strain zones, which helps the system to accommodate strain. Because actively deforming orogenic belts are non-equilibrium systems, they may generate dissipative structure by self-organization; we interpret crustal-scale shear-zone systems and their associated granites as the manifestation of this self-organization. The architecture and permeability structure are controlled by the type of shear-zone system (transcurrent, normal, reverse or oblique); this is the primary control on melt transfer in orogenic belts. During active deformation, movement of melt is by percolative flow and melt essentially is pumped through the system parallel to the maximum principal finite elongation direction. If a build-up of melt pressure occurs, melt-enhanced embrittlement enables tensile and dilatant shear fracturing, and transfer of melt is by channelized flow. We illustrate feedback relations between migmatites, crustal-scale shear-zone systems and granites using examples from the Cadomian belt of western France and the northern Appalachian orogen of the eastern U.S.A. In orogenic belts dominated by transcurrent shear, where the maximum principal finite elongation direction may have a shallow to subhorizontal plunge, granite arrested during ascent through the system commonly develops C- S fabrics. This suggests percolative flow is not effective in expelling melt from these systems; the resulting build-up of melt pressure enables fracturing and channelized transfer of melt, which crystallizes during persistent deformation (e.g. the St. Malo migmatite belt, Cadomian belt

  6. Lateral variations in lithospheric and landscape evolution at both ends of the Himalaya-Tibet orogen

    NASA Astrophysics Data System (ADS)

    Zeitler, P. K.; Schmidt, J. L.; Meltzer, A.

    2015-12-01

    At the broadest scale, like many orogens the Himalaya encompass a range of orogenic features that are remarkably similar along much of the length of the mountain belt and its neighboring terranes. At one scale of consideration, these similarities appear to be a signal that fundamental processes associated with lithospheric collision have been active. However, the vast size of the Himalaya and Tibet, the different climate regimes experienced by the orogen across time and space, and the along-strike variations in the continental and arc margins that faced one another before collision, make it at once remarkable that any similarities exist, and important to more critically evaluate their nature. The eastern and western Himalayan syntaxes confound any attempt to generalize too much about the Himalaya-Tibet orogen. By area these features occupy at least 25% of the orogenic belt, and compared to the "main" portions of the arc they show clear differences in their lithospheric structures, landscapes, and evolution. The boundary and initial conditions that shaped the eastern and western indentor corners were and are different, as is the nature and timing of erosional exhumation. Some of the most active geologic processes on Earth have recently been in play within the syntaxes, and the evolution of landscapes and fluvial systems, important in developing the sedimentary record of the Himalaya-Tibet system, has been complex and variable in space and time. Southeasternmost Tibet and the Lhasa Block in particular exemplify this complexity both in its complex topographic evolution linked to surface processes and climate, and in lateral variability in lithospheric structure. Taking a system viewpoint, an important question to debate is the degree to which there are features in the Himalaya-Tibet system that are robustly emergent, given the broad boundary conditions of the continental collision plus the suite of local and regional geodynamical processes that have operated during

  7. North-vergent thrust fault between Baltica and Laurentian affinity rocks in the frontal part of Romanzof orogen, NE Brooks Range, Alaska

    NASA Astrophysics Data System (ADS)

    Moore, T. E.; O'Sullivan, P. B.

    2012-12-01

    One of the most striking features of Arctic physiography is the long, linear Canadian Arctic margin of the Arctic Basin, which extends from the Lincoln Sea north of Greenland to the eastern Beaufort Sea and projects into northeastern Alaska. Among other ideas, this margin has been proposed to have developed by sinistral transform faulting in the Middle Devonian as a result of tectonic escape of terranes from the Caledonites (the so-called "Northwest Passage"). The faults on which the transform motion might have occurred, however, have not been recognized along the northern margin of North America. One candidate for such a fault is exposed at the southern boundary of the Sadlerochit Mountains province in the Tertiary frontal part of the NE Brooks Range. In the Plunge Creek area of the Arctic National Wildlife Refuge, a contact between rocks of Laurentian affinity and Baltica affinity is exposed on the back limb of a basement-involved map-scale thrust block formed by Brookian thrusting. The regional unconformity at the base of the Mississippian to Triassic Ellesmerian Sequence provides a near flat-lying datum that overlaps the contact between the pre-Mississippian tectonic units and demonstrates that it was not reactivated by Brookian thrusting. The Sadlerochit Mountains succession to the north of the contact consists of a Neoproterozoic and lower Paleozoic carbonate sequence that rests on metaclastic rocks that yield Grenville-Sveconorwegian (0.95-1.2 Ga) and other Mesoproterozoic detrital zircon U-Pb age populations similar to those reported from the northern parts of Baltica and eastern Greenland. In contrast, the Romanzof Mountains succession to the south consists of Neoproterozoic deep-marine clastic rocks (Neruokpuk Quartzite) and overlying lower Paleozoic chert and argillite. Detrital zircon U-Pb age populations from the Neruokpuk are very similar to those from Laurentian-derived clastic rocks in the Canadian margin of North America. Field relations show that

  8. Three Dimensional Spatial and Temporal Evolution of Rheology in Orogens: A Field Perspective

    NASA Astrophysics Data System (ADS)

    Brown, M.; Solar, G.

    2001-12-01

    To understand mountain building, we require knowledge of behavior and mechanics of deformation of lithosphere materials. Based on deformation experiments at high-strain-rates, including work by Mervyn Paterson, and data from field studies of rocks inferred to have been melt-bearing during deformation at natural strain rates, melt-bearing rocks generally are weaker than those at subsolidus conditions. Further, we require knowledge of how material behavior changes in 3-d during mountain building as geotherms evolve and rocks melt and melt migrates and ponds at shallower levels. Data to address this issue come from the field, where relations among melt flow (continuous vs. cyclic), rheological transitions [brittle-viscous transition zone (BVTZ) and anatectic front (AF)] and deformation (viscous vs. brittle) yield natural constraints for use in dynamic models of orogenesis. In the Acadian Orogen of New England, based on high-precision ages, Lower Paleozoic rocks were deformed synchronously with greenschist-granulite facies metamorphism and emplacement of plutons. Based on geochemistry, migmatites are residual and peraluminous leucogranite was derived from a similar source, implying a genetic relation. In middle crust, below the BVTZ [in part below the AF (in migmatite)] deformation involved non-coaxial non-plane strain flow in which the inclined vorticity vector was stretched along its length and deformation partitioned into steeply inclined S-L and L>>S tectonite zones. Metamorphic field gradient is of low dP/dT type, P-T paths are clockwise and thermal peak was late syntectonic, reflecting regionally elevated thermal gradients and pluton-driven thermal pulses. In migmatite, leucosome relations with tectonite fabrics and dilatant shear surfaces, and sheets and cylinders of cumulate-dominated granite, suggest cyclic failure and batch melt transport by shear surface and channel flow (conditional open-system behavior). Below the AF, differential stress is low and if melt

  9. Tectonic controls of Mississippi Valley-type lead-zinc mineralization in orogenic forelands

    USGS Publications Warehouse

    Bradley, D.C.; Leach, D.L.

    2003-01-01

    Most of the world's Mississippi Valley-type (MVT) zinc-lead deposits occur in orogenic forelands. We examine tectonic aspects of foreland evolution as part of a broader study of why some forelands are rich in MVT deposits, whereas others are barren. The type of orogenic foreland (collisional versus Andean-type versus inversion-type) is not a first-order control, because each has MVT deposits (e.g., Northern Arkansas, Pine Point, and Cevennes, respectively). In some MVT districts (e.g., Tri-State and Central Tennessee), mineralization took place atop an orogenic forebulge, a low-amplitude (a few hundred meters), long-wavelength (100-200 km) swell formed by vertical loading of the foreland plate. In the foreland of the active Banda Arc collision zone, a discontinuous forebulge reveals some of the physiographic and geologic complexities of the forebulge environment, and the importance of sea level in determining whether or not a forebulge will emerge and thus be subject to erosion. In addition to those on extant forebulges, some MVT deposits occur immediately below unconformities that originated at a forebulge, only to be subsequently carried toward the orogen by the plate-tectonic conveyor (e.g., Daniel's Harbour and East Tennessee). Likewise, some deposits are located along syn-collisional, flexure-induced normal and strike-slip faults in collisional forelands (e.g., Northern Arkansas, Daniel's Harbour, and Tri-State districts). These findings reveal the importance of lithospheric flexure, and suggest a conceptual tectonic model that accounts for an important subset of MVT deposits-those in the forelands of collisional orogens. The MVT deposits occur both in flat-lying and in thrust-faulted strata; in the latter group, mineralization postdated thrusting in some instances (e.g., Picos de Europa) but may have predated thrusting in other cases (e.g., East Tennessee).

  10. Early tectonic evolution of the Thomson Orogen in Queensland inferred from constrained magnetic and gravity data

    NASA Astrophysics Data System (ADS)

    Spampinato, Giovanni P. T.; Betts, Peter G.; Ailleres, Laurent; Armit, Robin J.

    2015-05-01

    The crustal architecture as well as the kinematic evolution of the Thomson Orogen in Queensland is poorly resolved because the region is concealed under thick Phanerozoic sedimentary basins and the basement geology is known from limited drill holes. Combined potential field and seismic interpretation indicates that the Thomson Orogen is characterized by prominent regional NE- and NW-trending structural grain defined by long wavelength and low amplitude geophysical anomalies. The 'smooth' magnetic signature is interpreted to reflect deeply buried source bodies in the mid- to lower crust. Short wavelength positive magnetic features that correlate with negative gravity anomalies are interpreted to represent shallower granitic intrusions. They appear to be focused along major fault zones that might have controlled the locus for magmatism. The eastern Thomson Orogen is characterized by a prominent NE structural grain and orthogonal faults and fold interference patterns resulting in a series of troughs and highs. The western Thomson Orogen consists of a series of NW-trending structures interpreted to reflect reverse faults. Sedimentation and basin development are interpreted to have initiated in the Neoproterozoic to Early Cambrian during E-W- to ENE-WSW extension, possibly related to the Rodinia break-up. This extensional event was followed by Late Cambrian shortening recorded in the Maneroo Platform and the Diamantina River Domain which possibly correlates with the Delamerian Orogeny. Renewed deposition and volcanism occurred during the Ordovician and may have continued until Late Silurian, resulting in thinned Proterozoic basement crust and extensive basin systems that formed in a distal continental back-arc environment. Our interpretation places the Thomson Orogen to the west of the Neoproterozoic passive margin preserved in the Anakie Inlier. The region is likely to represent the internal extensional architecture during the Rodinia break-up that has been

  11. Lateral thinking: 2-D interpretation of thermochronology in convergent orogenic settings

    NASA Astrophysics Data System (ADS)

    Batt, Geoffrey E.; Brandon, Mark T.

    2002-05-01

    Lateral motion of material relative to the regional thermal and kinematic frameworks is important in the interpretation of thermochronology in convergent orogens. Although cooling ages in denuded settings are commonly linked to exhumation, such data are not related to instantaneous behavior but rather to an integration of the exhumation rates experienced between the thermochronological 'closure' at depth and subsequent exposure at the surface. The short spatial wavelength variation of thermal structure and denudation rate typical of orogenic regions thus renders thermochronometers sensitive to lateral motion during exhumation. The significance of this lateral motion varies in proportion with closure temperature, which controls the depth at which isotopic closure occurs, and hence, the range of time and length scales over which such data integrate sample histories. Different chronometers thus vary in the fundamental aspects of the orogenic character to which they are sensitive. Isotopic systems with high closure temperature are more sensitive to exhumation paths and the variation in denudation and thermal structure across a region, while those of lower closure temperature constrain shorter-term behaviour and more local conditions. Discounting lateral motion through an orogenic region and interpreting cooling ages purely in terms of vertical exhumation can produce ambiguous results because variation in the cooling rate can result from either change in kinematics over time or the translation of samples through spatially varying conditions. Resolving this ambiguity requires explicit consideration of the physical and thermal framework experienced by samples during their exhumation. This can be best achieved through numerical simulations coupling kinematic deformation to thermal evolution. Such an approach allows the thermochronological implications of different kinematic scenarios to be tested, and thus provides an important means of assessing the contribution of

  12. Orogenic gold deposits: a proposed classification in the context of their crustal distribution and relationship to other gold deposit types

    USGS Publications Warehouse

    Groves, D.I.; Goldfarb, R.J.; Gebre-Mariam, M.; Hagemann, S.G.; Robert, F.

    1998-01-01

    The so-called 'mesothermal' gold deposits are associated with reginally metamorphosed terranes of all ages. Ores were formed during compressional to transpressional deformation processes at convergent plate margins in accretionary and collisional orogens. In both types of orogen, hydrated marine sedimentary and volcanic rocks have been added to continental margins during tens to some 100 million years of collision. Subduction-related thermal events, episodically raising geothermal gradients within the hydrated accretionary sequences, initiate and drive long-distance hydrothermal fluid migration. The resulting gold-bearing quartz veins are emplaced over a unique depth range for hydrothermal ore deposits, with gold deposition from 15-20 km to the near surface environment. On the basis of this broad depth range of formation, the term 'mesothermal' is not applicable to this deposit types as a whole. Instead, the unique temporal and spatial association of this deposit type with orogeny means that the vein systems are best termed orogenic gold deposits. Most ores are post-orogenic with respect to to tectonism of their immediate host rocks, but are simultaneously syn-orogenic with respect to ongoing deep-crustal, subduction-related thermal processes and the prefix orogenic satisfies both these conditions. On the basis of their depth of formation, the orogenic deposits are best subdivided into epizonal (12 km) classes.

  13. Recycling of crustal materials through study of ultrahigh-pressure minerals in collisional orogens, ophiolites, and mantle xenoliths: A review

    NASA Astrophysics Data System (ADS)

    Liou, Juhn G.; Tsujimori, Tatsuki; Yang, Jingsui; Zhang, R. Y.; Ernst, W. G.

    2014-12-01

    Newly recognized occurrences of ultrahigh-pressure (UHP) minerals including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mantle xenoliths suggest the recycling of crustal materials through deep subduction, mantle upwelling, and return to the Earth's surface. This circulation process is supported by crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths in kimberlites, and from chromitities of several Alpine-Himalayan and Polar Ural ophiolites; some of these minerals contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and inferred stishovite, a number of nanometric minerals have been identified as inclusions employing state-of-the-art analytical tools. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. For example, Tibetan chromites containing exsolution lamellae of coesite + diopside suggest that the original chromitites formed at P > 9-10 GPa at depths of >250-300 km. The precursor phase most likely had a Ca-ferrite or a Ca-titanite structure; both are polymorphs of chromite and (at 2000 °C) would have formed at minimum pressures of P > 12.5 or 20 GPa respectively. Some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin, including zircon, feldspars, garnet, kyanite, andalusite, quartz, and rutile; the zircons possess much older U-Pb ages than the time of ophiolite formation. These UHP mineral-bearing chromitite hosts evidently had a deep-seated evolution prior to extensional mantle upwelling and partial

  14. Laser-probe 40Ar/39Ar dating of strain fringes: Mid-Cretaceous synconvergent orogen-parallel extension in the interior of the Sevier orogen

    NASA Astrophysics Data System (ADS)

    Wells, Michael L.; Spell, Terry L.; Hoisch, Thomas D.; Arriola, Tonia; Zanetti, Kathleen A.

    2008-06-01

    UV and CO2 laser-probe 40Ar/39Ar in situ analyses of phlogopite and muscovite in fibrous strain fringes from greenschist-facies metamorphic rocks document mica growth ages at temperatures lower than their closure temperatures, and therefore directly date deformation. The new dates resolve the age of the earliest ductile fabric recorded in the Raft River-Albion-Grouse Creek metamorphic core complex of Utah and Idaho. Phlogopite was dated in quartz-calcite-phlogopite strain fringes around pyrite in Pennsylvanian-Permian rocks from the Grouse Creek Mountains (Utah) using both the UV and CO2 laser probe; muscovite was dated in quartz-muscovite strain fringes around pyrite in deformed Jurassic sills from the Black Pine Mountains (Idaho) using the CO2 laser probe. Phlogopite 40Ar/39Ar ages for individual strain fringes (Grouse Creek Mountains) range from 92 Ma to 110 Ma, with the most reliable ages ranging from 101 Ma to 110 Ma (mean age, 105.0 ± 5.8 Ma). Muscovite 40Ar/39Ar ages for individual strain fringes (Black Pine Mountains) range from 97 Ma to 112 Ma (mean age, 104.7 ± 5.8 Ma). Strain fringes are associated with a subhorizontal foliation and a generally N-trending elongation lineation exhibiting components of top-to-the-north simple shear and coaxial strain accommodating N-S extension and subvertical shortening. Midcrustal northward flow at 105 (±6) Ma within the interior of the Sevier orogen, coeval with east-directed shortening in the foreland and with plate convergence, records orogen-parallel synconvergent extension. We favor gravitational relaxation of structural culminations resulting from focused crustal shortening as a driving mechanism for orogen-parallel flow.

  15. Extensional orogenic collapse captured by strike-slip tectonics: Constraints from structural geology and Usbnd Pb geochronology of the Pinhel shear zone (Variscan orogen, Iberian Massif)

    NASA Astrophysics Data System (ADS)

    Fernández, Rubén Díez; Pereira, Manuel Francisco

    2016-11-01

    The late Paleozoic collision between Gondwana and Laurussia resulted in the polyphase deformation and magmatism that characterizes the Iberian Massif of the Variscan orogen. In the Central Iberian Zone, initial continental thickening (D1; folding and thrusting) was followed by extensional orogenic collapse (D2) responsible for the exhumation of high-grade rocks coeval to the emplacement of granitoids. This study presents a tectonometamorphic analysis of the Trancoso-Pinhel region (Central Iberian Zone) to explain the processes in place during the transition from an extension-dominated state (D2) to a compression-dominated one (D3). We reveal the existence of low-dipping D2 extensional structures later affected by several pulses of subhorizontal shortening, each of them typified by upright folds and strike-slip shearing (D3, D4 and D5, as identified by superimposition of structures). The D2 Pinhel extensional shear zone separates a low-grade domain from an underlying high-grade domain, and it contributed to the thermal reequilibration of the orogen by facilitating heat advection from lower parts of the crust, crustal thinning, decompression melting, and magma intrusion. Progressive lessening of the gravitational disequilibrium carried out by this D2 shear zone led to a switch from subhorizontal extension to compression and the eventual cessation and capture of the Pinhel shear zone by strike-slip tectonics during renewed crustal shortening. High-grade domains of the Pinhel shear zone were folded together with low-grade domains to define the current upright folded structure of the Trancoso-Pinhel region, the D3 Tamames-Marofa-Sátão synform. New dating of syn-orogenic granitoids (SHRIMP Usbnd Pb zircon dating) intruding the Pinhel shear zone, together with the already published ages of early extensional fabrics constrain the functioning of this shear zone to ca. 331-311 Ma, with maximum tectonomagmatic activity at ca. 321-317 Ma. The capture and apparent cessation

  16. Crustal Structure And Magmatism, Coast Mountains Orogen, Latitude 52-53 degrees North, British Columbia, Canada

    NASA Astrophysics Data System (ADS)

    Rusmore, M. E.; Gehrels, G.; Woodsworth, G. J.

    2007-12-01

    New geologic data and U-Pb ages reveal complex history of arc accretion, crustal thickening and migration of magmatic fronts during deformation. Plutonic ages define distinct western and eastern Jurassic - mid Cretaceous arcs that share a common history after ~90 Ma. Juxtaposition of these arcs occurred during mid- Cretaceous crustal shortening in a dominantly SW-vergent crustal-scale thrust belt. Significant crustal thickening buried 151 Ma granitic clasts to pressures > 6 kb, and mid-Cretaceous plutons were emplaced at this depth along the axis of the orogen. Thrusting continued after establishment of the 90 Ma arc; a regional SW-verging thrust emplaced high-grade metamorphic rocks of the Yukon-Tanana terrane and deep-seated plutons over low- grade rocks of the Alexander and Wrangellia terranes. The shear zone is coincident with the western boundary of 82-89 Ma plutons and a regionally extensive, late-kinematic, sill-like pluton. Dextral shear zones preserved on the flanks of the orogen suggest a component of Late Cretaceous transpression. By 75 Ma, metamorphism, deformation, and magmatism had migrated central portions of the orogen and there is no evidence of ductile deformation and syn-kinematic metarmorphism younger than ~70 - 65 along the western flank of the orogen. The Coast shear zone localized 62-58 Ma synkinematic plutons during NE-side up displacement, creating a sharp western magmatic front. Sparse cooling ages suggest plutons and metamorphic rocks adjacent to the CSZ cooled through 500-600 deg between 54-58 Ma during exhumation along the shear zone. Voluminous granitic plutons were emplaced from ~55-50 Ma, but significant crustal extension that affected the eastern side of the orogen farther north is not evident along this transect. This history supports previous models of crustal subcretion and the generation of arc magmas in thickened crust. Definition of two pre-90 Ma arcs negates models calling for simple Andean-style orogen prior to mid

  17. The deep structure of Alpine-type orogens: how important is rift-inheritance?

    NASA Astrophysics Data System (ADS)

    Tugend, Julie; Manatschal, Gianreto; Mohn, Geoffroy

    2016-04-01

    Collisional belts are commonly thought to result from the closure of oceanic basins and subsequent inversion of former rifted margins. The formation and evolution of collisional belts should therefore be closely interlinked with the initial architecture of former rifted margins. Reflection and refraction seismic data from present-day magma-poor rifted margins show the omnipresence of hyperextended domains (severely thinned continental crust (<10 km) and/or exhumed serpentinized mantle with minor magmatic additions) between unequivocal continental and oceanic domains. Integrating these new observations and exploring their impact on mountain building processes may result in alternative interpretations of the lithospheric structure of collisional orogens. We focus on the Pyrenees and Western to Central Alps, respectively resulting from the inversion of a Late Jurassic to Mid Cretaceous and an Early to Mid Jurassic rift system eventually floored by hyperextended crust, exhumed mantle or proto-oceanic crust. The rift-related pre-collisional architecture of the Pyrenees shows many similarities with that proposed for the Alps; although the width of the hyperextended and in particular of the proto-oceanic domains is little constrained. Contrasting with the Pyrenees, remnants of these domains are largely affected by orogeny-related deformation and show a HP-LT to HT-MP metamorphic overprint in the Alps. Nevertheless, in spite of the occurrence of these highly deformed and metamorphosed rocks constituting the internal parts of the Alps, the overall crustal and lithospheric structure looks surprisingly comparable. High resolution tomographic images across both orogens unravel the occurrence of a velocity anomaly dipping underneath the internal domains and progressively attenuated at depth that we interpret as former hyperextended domains subducted/underthrusted during collision. This interpretation contrasts with the classical assumption that the subducted material is made of

  18. Cyclicity and paleo-environmental dynamics of 1 1. 9 Ga passive-margin carbonate terrace, Wopmay Orogen, N. W. T

    SciTech Connect

    Grotzinger, J.P.

    1985-02-01

    The 1.90-1.89 billion year old Rocknest Formation in the Northwest Territories is a west-facing, passive-margin carbonate terrace in the foreland of Wopmay orogen. Initial outbuilding of an accretionary stromatolite rim over downslope facies was followed by upbuilding of the rim, local backstepping of the rim, and terminal subduction-related drowning of the entire shelf. The rim was flanked to the west by deep-water slope rhythmite and breccia, and on the east by a carbonate-shoal complex, separating the ocean from a broad (100-200 km wide) lagoon with a siliciclastic eastern shoreline. Concurrently, the shoal complex underwent repeated eastward expansion over the lagoon to form about 150 shoaling-upward cycles (1-25 m thick), consisting of carbonate tidal-flat tufa that overlies storm-dominated, mixed carbonate-siliciclastic lagoonal facies. Correlation of cycles for over 200 km parallel with and 100 km across depositional strike shows that cycle boundaries abut facies boundaries, indicating that complete shoaling of the lagoon to sea level was not required to induce the next submergence increment, suggesting an allocyclic rather than autocyclic mechanism. Radiometric constraints bracket cycle periodicity between 25,000-40,000 yr/cycle. These values are within the range of known earth orbital cycles (periods at 19,000, 23,000, 41,000, and 100,000 yr), the likely cause of Pleistocene glacio-eustatic sea level oscillations, and possibly Rocknest cyclicity. Rocknest cycles can be modeled using period and amplitude of sea level oscillation, and subsidence and sedimentation rates as variables. Resulting computer-generated cyclic stratigraphies are compared to actual Rocknest cyclic stratigraphy in order to constrain variables responsible for cycle development.

  19. Association of orogenic activity with the Ordovician radiation of marine life

    NASA Technical Reports Server (NTRS)

    Miller, A. I.; Mao, S.

    1995-01-01

    The Ordovician radiation of marine life was among the most substantial pulses of diversification in Earth history and coincided in time with a major increase in the global level of orogenic activity. To investigate a possible causal link between these two patterns, the geographic distributions of 6576 individual appearances of Ordovician vician genera around the world were evaluated with respect to their proximity to probable centers of orogeny (foreland basins). Results indicate that these genera, which belonged to an array of higher taxa that diversified in the Middle and Late Ordovician (trilobites, brachiopods, bivalves, gastropods, monoplacophorans), were far more diverse in, and adjacent to, foreland basins than they were in areas farther removed from orogenic activity (carbonate platforms). This suggests an association of orogeny with diversification at that time.

  20. Tectonic implications of U-Pb zircon ages of the himalayan orogenic belt in nepal

    PubMed

    DeCelles; Gehrels; Quade; LaReau; Spurlin

    2000-04-21

    Metasedimentary rocks of the Greater Himalaya are traditionally viewed as Indian shield basement that has been thrust southward onto Lesser Himalayan sedimentary rocks during the Cenozoic collision of India and Eurasia. Ages determined from radioactive decay of uranium to lead in zircon grains from Nepal suggest that Greater Himalayan protoliths were shed from the northern end of the East African orogen during the late Proterozoic pan-African orogenic event. These rocks were accreted onto northern Gondwana and intruded by crustal melts during Cambrian-Ordovician time. Our data suggest that the Main Central thrust may have a large amount of pre-Tertiary displacement, that structural restorations placing Greater Himalayan rocks below Lesser Himalayan rocks at the onset of Cenozoic orogenesis are flawed, and that some metamorphism of Greater Himalayan rocks may have occurred during early Paleozoic time.

  1. Axial Belt Provenance: modern river sands from the core of collision orogens

    NASA Astrophysics Data System (ADS)

    Resentini, A.; Vezzoli, G.; Paparella, P.; Padoan, M.; Andò, S.; Malusà, M.; Garzanti, E.

    2009-04-01

    Collision orogens have a complex structure, including diverse rock units assembled in various ways by geodynamic processes. Consequently, orogenic detritus embraces a varied range of signatures, and unravelling provenance of clastic wedges accumulated in adjacent foreland basins, foredeeps, or remnant-ocean basins is an arduous task. Dickinson and Suczek (1979) and Dickinson (1985) recognized the intrinsically composite nature of orogenic detritus, but did not attempt to establish clear conceptual and operational distinctions within their broad "Recycled Orogenic Provenance". In the Alpine and Himalayan belts, the bulk of the detritus is produced by focused erosion of the central backbone of the orogen, characterized by high topography and exhumation rates (Garzanti et al., 2004; Najman, 2006). Detritus derived from such axial nappe pile, including slivers of thinned continental-margin lithosphere metamorphosed at depth during early collisional stages, has diagnostic general features, which allows us to define an "Axial Belt Provenance" (Garzanti et al., 2007). In detail, "Axial Belt" detrital signatures are influenced by metamorphic grade of source rocks and relative abundance of continental versus oceanic protoliths, typifying distinct subprovenances. Metasedimentary cover nappes shed lithic to quartzolithic detritus, including metapelite, metapsammite, and metacarbonate grains of various ranks; only amphibolite-facies metasediments supply abundant heavy minerals (e.g., almandine garnet, staurolite, kyanite, sillimanite, diopsidic clinopyroxene). Continental-basement nappes shed hornblende-rich quartzofeldspathic detritus. Largely retrogressed blueschist to eclogite-facies metaophiolites supply albite, metabasite and foliated antigorite-serpentinite grains, along with abundant heavy minerals (epidote, zoisite, clinozoisite, lawsonite, actinolitic to barroisitic amphiboles, glaucophane, omphacitic clinopyroxene). Increasing metamorphic grade and deeper

  2. Basin-mountain structures and hydrocarbon exploration potential of west Junggar orogen in China

    NASA Astrophysics Data System (ADS)

    Wu, Xiaozhi; He, Dengfa; Qi, Xuefeng

    2016-04-01

    Situated in northern Xinjiang, China, in NE-SW trend, West Junggar Orogen is adjacent to Altai fold belt on the north with the Ertix Fault as the boundary, North Tianshan fold belt on the south with the Ebinur Lake Strike-slip Fault as the boundary, and the Junggar Basin on the southeast with Zaire-Genghis Khan-Hala'alat fold belt as the boundary. Covering an area of about 10×104 km2 in China, there are medium and small intermontane basins, Burqin-Fuhai, Tacheng, Hefeng and Hoxtolgay, distributing inside the orogen. Tectonically West Junggar Orogen lies in the middle section of the Palaeo-Asian tectonic domain where the Siberia, Kazakhstan and Tarim Plates converge, and is the only orogen trending NE-SW in the Palaeo-Asian tectonic domain. Since the Paleozoic, the orogen experienced pre-Permian plate tectonic evolution and post-Permian intra-plate basin evolution. Complex tectonic evolution and multi-stage structural superimposition not only give rise to long term controversial over the basin basement property but also complex basin-mountain coupling relations, structures and basin superimposition modes. According to analysis of several kinds of geological and geophysical data, the orogen was dominated by compressive folding and thrust napping from the Siberia plate in the north since the Late Paleozoic. Compressive stress weakened from north to south, corresponding to subdued vertical movement and enhanced horizontal movement of crustal surface from north to south, and finally faded in the overthrust-nappe belt at the northwest margin of the Junggar Basin. The variation in compressive stress is consistent with the surface relief of the orogen, which is high in the north and low in the south. There are two kinds of basin-mountain coupling relationships, i.e. high angle thrusting and overthrusting and napping, and two kinds of basin superimposition modes, i.e. inherited and progressive, and migrating and convulsionary modes. West Junggar orogen has rich oil and gas

  3. Geology of the Byrd Glacier Discontinuity (Ross Orogen): New survey data from the Britannia Range, Antarctica

    USGS Publications Warehouse

    Carosi, R.; Giacomini, F.; Talarico, F.; Stump, E.

    2007-01-01

    Field activities in the Britannia Range (Transantarctic Mountains, Antarctica) highlighted new geological features around the so-called Byrd Glacier discontinuity. Recent field surveys revealed the occurrence of significant amounts of medium- to high-grade metamorphic rocks, intruded by abundant coarse-grained porphyritic granitoids. Most of the granitoids are deformed, with foliation parallel to the regional foliation in the metamorphics. Two main episodes of deformation are observed. Tight to isoclinal folds and penetrative axial plane foliation are related to the D1 phase, open folds to the D2. The main foliation (D1) trends nearly E-W in agreement with the trend in the southern portion of the Byrd Glacier. In most outcrops, granitic dykes are folded and stretched by the D2 deformation, which shows similar characteristics with the D2 deformation south of the Byrd Glacier. This suggests the occurrence in the Ross orogen of an orogen-normal structure south and north of the Byrd Glacier.

  4. Miocene thrusting in the eastern Sila Massif: Implication for the evolution of the Calabria-Peloritani orogenic wedge (southern Italy)

    NASA Astrophysics Data System (ADS)

    Vignaroli, G.; Minelli, L.; Rossetti, F.; Balestrieri, M. L.; Faccenna, C.

    2012-05-01

    Alpine orogens in the central Mediterranean region have revealed the concomitance of crustal extension in back-arc domain and crustal shortening in frontal domain. Quantitative data of deformation in the frontal orogenic wedges are necessary to understand how the shortening-extension pair evolves in terms of structures, orogenic transport, timing, and exhumation rate. This paper deals with kinematics and ages of the frontal thrust systems of the Calabria-Peloritani Arc (Italy) exposed in the eastern Sila Massif. We first present structural fieldwork, onshore and offshore well log data, and new apatite fission-track (AFT) thermochronology. Then, we describe the structural architecture of the studied area as an ENE-verging stacking of thrust sheets involving basement units and syn-orogenic sediments. The AFT study documents that thrust sheets entered the partial annealing zone from 18 Ma to 13 Ma. This Early-Middle Miocene thrusting phase was coeval with exhumation of high-pressure/low temperature metamorphic rocks in the hinterland of the orogen (Coastal Chain area), mainly driven by top-to-the-W extensional tectonics. Opposite kinematic shear senses (contractional top-to-the-E and extensional top-to-the-W) and different exhumation rates (slow in the frontal, more rapid in the hinterland) are framed in a tectonic scenario of a critically tapered orogenic wedge during the eastward retreating of the Apennine slab.

  5. The structure, stratigraphy, tectonostratigraphy, and evolution of the southernmost part of the Appalachian Orogen

    SciTech Connect

    Higgins, M.W.; Atkins, R.L.; Crawford, T.J.; Crawford, R.F. III; Brooks, R.; Cook, R.B.

    1988-01-01

    The southernmost part of the Appalachian Orogen is composed of three stacks of folded thrust sheets, which preserve rocks formed in a variety of environments that virtually spanned the Iapetus Ocean. All of the metamorphism, deformation, and plutonism in the southernmost Appalachians can be related to the movement of the thrust sheets and stacks. Thrusting took place continuously from Early Ordovician through Carboniferous time. An account is given of the origins, assembly, transport, and arrival of the thrust sheets and stacks.

  6. Early proterozoic evolution of the saskatchewan craton and its allochthonous coyer, trans-hudson Orogen

    USGS Publications Warehouse

    Chiarenzelli, J.; Aspler, L.; Villeneuve, M.; Lewry, J.

    1998-01-01

    The composition, chronology, and structural relations of the Saskatchewan Craton and enveloping mylonitic rocks exposed in basement windows of the Glennie Domain, Trans-Hudson Orogen, have been determined by geochemical, geochronologic, and structural studies accompanying detailed field mapping. Basement windows lie along the hinge zone of a regional crustal culmination and consist mostly of 2.4-2.5 Ga felsic plutonic rocks enveloped by the Nistowiak Thrust. The Nistowiak Thrust is a folded, 1-2 km thick, upper amphibolite facie??s mylonite zone formed during emplacement of the Flin Flon-Glennie Complex across the Saskatchewan Craton. It is likely correlative to the Pelican Thrust, which envelops basement windows in the Hanson Lake Block -100 km to the east. An internal high strain zone within the overlying nappe pile, the Guncoat Thrust, is composed primarily of mylonitized porphyroclastic pelitic and psammitic migmatites. U-Pb geochronological results suggest calc-alkaline plutonism from 1889-1837 Ma, thrust stacking, peak metamorphism and associated anatexis between 1837 and 1809 Ma, isotopic closure of titanite at 1790-1772 Ma, and intrusion of late granitic rocks at 1770-1762 Ma. This is in agreement with ages from the Hanson Lake Block, and La Ronge, Kisseynew, and Flin-Flon domains in Saskatchewan and Manitoba, and from the Ungava-Baffin portion of Trans-Hudson Orogen, suggesting broadly synchronous thermotectonic processes along a strike length of 2000 km. We speculate that the Saskatchewan Craton, rather than representing an exotic continental fragment, rifted from the Superior and/or Hearne Provinces at ca. 2.1 Ga and that the Trans-Hudson Orogen is an internal orogen. In this scenario the Maniwekan Ocean, developed between the Rae-Hearne and Superior cratons, opened and closed about similar pole(s) of plate motion. ?? 1998 by The University of Chicago. All rights reserved.

  7. Forearc basin correlations from around the Texas Orocline, New England Orogen, east Australia

    NASA Astrophysics Data System (ADS)

    Hoy, Derek; Rosenbaum, Gideon; Shaanan, Uri; Wormald, Richard

    2014-05-01

    The late Paleozoic to early Mesozoic New England Orogen occupies much of the eastern seaboard of Australia. The orogen formed by west-dipping subduction (present-day coordinates) of the paleo-Pacific plate beneath eastern Gondwana. The southern part of the orogen exhibits a series of tight bends (oroclines) that are evident in the curvature of a Devonian-Carboniferous subduction complex, in particular the forearc basin and accretionary complex. The Emu Creek Block is thought to be part of the forearc basin that is exposed in the eastern limb of the Texas Orocline, but until now the tectonostratigraphic origin of the Emu Creek Block has only been inferred from limited geological data. Here we present detrital zircon geochronology (U/Pb ICP-MS ages), a new geological map of the block, and a revised stratigraphic section. Lithological investigation of strata within the block and the age distribution of detrital zircons indicate that the sediments in the Emu Creek Block were derived from a Carboniferous magmatic arc and were most likely deposited in a forearc basin. Our new geochronological constraints indicate deposition during the late Carboniferous. We therefore propose that rocks in the Emu Creek Block are arc-distal correlatives of the forearc basin in the opposing (western) limb of the Texas Orocline, specifically the Willuri and Currabubula formations. Extensive orocline-parallel structures in the forearc basin indicate that the eastern limb of the Texas Orocline was rotated in the course of oroclinal bending by approximately 135 degrees relative to the western limb. The correlation of the forearc basin blocks on opposite limbs of the Texas Orocline provides an independent constraint on its geometry and further improves our understanding of New England Orogen tectonostratigraphy and the crustal structure of eastern Australia.

  8. Topography of the Variscan orogen in Europe: failed-not collapsed

    NASA Astrophysics Data System (ADS)

    Franke, Wolfgang

    2014-07-01

    The Variscan orogenic collage consists of three subduction-collision systems (Rheno-Hercynian, Saxo-Thuringian and Massif Central-Moldanubian). Devonian to early Carboniferous marine strata are widespread not only in the individual foreland fold and thrust belts, but also in post-tectonic basins within these foreland belts and on the Cadomian crust of peri-Gondwanan microcontinental fragments, which represent the upper plates of the subduction/collision zones. These marine basins preclude high elevations in the respective areas and also in their neighbourhood. Widespread late Carboniferous intra-montane basins with their coal-bearing sequences are likewise incompatible with high and dry plateaus. While narrow belts with high elevations remain possible along active margins within the orogen, comparison of the Variscides with the Himalaya/Tibetan plateau is unfounded. Plausible reasons for the scarcity of high Variscan relief include subduction of oceanic and even continental crust, subduction erosion, orogen-parallel extension and—most important—lithospheric thinning accompanied by high heat flow and magmatism. In many areas, timing and areal array of magmatism and HT metamorphism are not compatible with a model of tectonic thickening and subsequent gravitational collapse. It is suggested, instead, that lithospheric thinning and heating are due to mantle activities caused by the Tethys rift. The lower and middle crust were thermally softened and rendered unfit for stacking and isostatic uplift: in terms of topography, the Variscides represent a failed orogen. The HT regime also explains the abundance of granitoids and HT/LP metamorphic rocks typical of the Variscides. Melting in the HT regime extracted mafic components from Variscan and Cadomian crust as well as from Cadomian metasomatized lithospheric mantle, thus mimicking subduction-related magmatism. The onset of the HT regime at c. 340 Ma may also have triggered the final ascent of HP/UHP felsic metamorphic

  9. Application of the Orogenic Float Model for the Structural Evolution of the Venezuelan Andes

    NASA Astrophysics Data System (ADS)

    Dhont, D.; Monod, B.; Hervouet, Y.; Klarica, S.

    2010-12-01

    The Venezuelan (or Mérida) Andes form a NE-SW-striking intracontinental orogen that started to uplift in the Middle Miocene due to E-W convergence between the Maracaibo block to the northwest and the Guyana shield to the southeast. Oblique collision resulted in strain partitioning accommodated by (1) transverse shortening along thrust faults bounding the belt on both flanks, (2) right-lateral slip along the Bocono fault running more or less along the chain axis and (3) tectonic escape of the Trujillo block moving towards the NE in between the Bocono and the N-S-striking sinistral Valera faults. Even though the surface geology of the Venezuelan Andes is well known, its structure at depth remains a matter of debate. Among the mechanisms that have proposed to account for the crustal architecture and evolution of the mountain belt, we develop the idea that the deformation process in this orogen is consistent with a model of orogenic float where the upper crust is decoupled from its underlying lithosphere above a large-scale mid-crustal detachment zone. According to this model, all the major faults involved in the strain partitioning sole into the detachment horizon and may therefore be considered as upper crustal faults. The integration of the orogenic float into a coherent evolutionary model provides further insight on both the crustal structure of the Venezuelan Andes and on the tectonic history of the region. A major reorganization in the crust occurred in the Early Pliocene when the Maracaibo block penetrated as a wedge into the Guyana crust. This event was accompanied by a rapid uplift of the Venezuelan Andes in association with the NE-ward crustal escape of the Trujillo block whose motion is accompanied by the lateral spreading of the upper crust.

  10. Topography and subduction geometry in the central Andes: Clues to the mechanics of a noncollisional orogen

    NASA Technical Reports Server (NTRS)

    Gephart, John W.

    1994-01-01

    The central Andeean orogen between 12 deg and 32 deg S latitude exhibits a high degree of spatial order: principally an extraordinary bilateral symmetry that is common to the Earth's surface, the underlying Wadati-Benioff zone, and the Nazca/South America plate kinematics, which has been stable since the mid-Tertiary. This spatial order must reflect the physical mechanisms of mountain building in this noncollisional orogen. The shapes of the topography and subduction zone can be reduced to symmetric and antisummeric components relative to any verical symmetry plane; the particular plaen which minimizes the antisymmetry (and maximizes the symmetry) is well resolved and is essentially coincident with the stable Euler equator of Nacza/South America relative motion since the mid-Tertiary. That the topography, subduction geometry, and persistent mid-Tertiary plate kinematics share common spatial and geometric elements suggests that he distribution of topography in this orogen depends strongly on the dynamics of subduction. Other factors that might affect the topography and underlying tectonics, such as climate and inherited strutura fabric, which have different spatial characterisitcs, must be of less significance at a continental scale. Furthermore, the small components of asymmetry among the various elements of the orogen appear to be mutually relate in a simple way; it is possible that this coupled asymmetry is associated with a late Teriary change in plate kinematics. These observations suggest that there is a close connection between plate tectonics and the form of the Earth's surface in this noncollisional setting. It follows hta the distribution of topography near convergent plate boundaries may provide a powerful constraing for understanding the dynamics of subduction.

  11. Anatexis of mafic and felsic lower crust: Geochemistry and Nd, Sr and Pb isotopes of late-orogenic granodiorites and leucogranites (Damara orogen, Namibia)

    NASA Astrophysics Data System (ADS)

    Osterhus, Lennart; Jung, Stefan

    2010-05-01

    The Damara orogen (Namibia) represents a well-exposed and deeply eroded orogenic mobile belt consisting of the north-south trending Kaoko belt and the northeast-southwest trending intracontinental Damara belt. The latter has been subdivided into a Northern, a Central and a Southern Zone based on stratigraphy, metamorphic grade, structure and geochronology. The late-orogenic granodioritic to leucogranitic Gawib pluton is a cross-cutting, pear-shaped post-tectonic stock within the southern Central Zone which is elsewhere dominated by basement rocks, high-grade metasedimentary rocks of the Tinkas Formation and syn-orogenic granites (Salem-type). The non-foliated granodiorites consist of plagioclase, quartz, microcline, hornblende and biotite whereas the leucogranites consist of microcline, quartz, plagioclase and biotite. Major element variation of the granodiorites show two distinct magma types were some samples have high TiO2, MgO and Fe2O3 and low Al2O3 and others have low TiO2, MgO and Fe2O3 and high Al2O3. Based on high REE, Nb, Zr and Y concentrations some granodiorites can be classified as A-type granitoids. Strontium concentrations are high in the granodiorites (up to 939 ppm) and decrease to < 200 ppm in the leucogranites. Rb/Sr ratios are low (1) in the leucogranites. Granodiorites have moderately radiogenic initial 87Sr/86Sr ratios (0.7088-0.7132), strongly negative initial ɛ Nd values (ca. -12) and comparatively unradiogenic Pb isotope data, the latter obtained on acid-leached feldspar separates. Leucogranites have more radiogenic initial 87Sr/86Sr ratios (0.7223-0.7336) and more negative initial ɛ Nd values (ca. -18). Pb isotopes tend to be less radiogenic than in the granodiorites. The mean crustal residence ages of the granodiorites, expressed as depleted mantle Nd model ages, are ca. 2.0 Ga but the leucogranites tend to have older Nd model ages (2.5 Ga). Therefore, a likely source for the granodiorites and leucogranites is a sequence of mafic to

  12. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    SciTech Connect

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.

  13. Orogen-parallel mass transport along the arcuate Himalayan front into Nanga Parbat and the western Himalayan syntaxis

    NASA Astrophysics Data System (ADS)

    Whipp, David; Beaumont, Christopher

    2016-04-01

    Along the length of the Himalayan arc, Quaternary rock exhumation rates are highest in the Himalayan syntaxes at the lateral ends of the arc. In the western Himalayan syntaxis, these rates may exceed 10 mm/a over the past 2 Ma, requiring an additional source of crustal mass into this region to maintain the high-elevation topography. We have previously demonstrated that strain partitioning of oblique convergence can produce a significant orogen-parallel mass flux into the syntaxis of a Himalaya-like orogen and balance the rapid rates of surface denudation. However, the magnitude of this orogen parallel mass flux and whether strain is partitioned across the Himalayan thrust front is affected by the strength of the material bounding and within the Himalayan orogenic wedge, the dip angle of the basal detachment and the convergence obliquity angle γ. Strain partitioning is expected for a finite-length Himalaya-like segmented linear orogen with an obliquity of γ = 30 - 40°, but the obliquity angle in the Himalayan arc varies from 0 at the center of the arc to ˜ 40° in the western Himalayan syntaxis region. Thus, the conditions in which strain partitioning will occur may not be met along much of the length of the arc. Though there is clear evidence of strain partitioning in the Himalaya, preliminary results from 3D numerical geodynamic models of an orogen with an arcuate geometry based on the Himalaya suggest strain partitioning does not occur for the same conditions observed in earlier models of segmented linear orogens or orogens with a smaller arc radius. In those models, the proportion of the orogen length with a high obliquity angle was greater, which favors strain partitioning. In numerical experiments of an arcuate Himalayan orogen with weak material (friction angle φ ≤ 5°) at the back of the orogenic wedge, strain partitioning is only observed in the toe of the orogenic wedge (10-15 km from the thrust front) at the western end of the arc, rather than for

  14. 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

  15. Deep structures and surface boundaries among Proto-Tethyan micro-blocks: Constraints from seismic tomography and aeromagnetic anomalies in the Central China Orogen

    NASA Astrophysics Data System (ADS)

    Sun, Wenjun; Li, Sanzhong; Liu, Xin; Santosh, M.; Zhao, Shujuan; Guo, Lingli; Cao, Huahua; Yu, Shan; Dai, Liming; Zhang, Yong

    2015-09-01

    The Qinling-Qilian orogen preserves the records of Early Paleozoic convergence among the Proto-Tethyan micro-blocks. In this study, we analyze the seismic velocity structure and the aeromagnetic anomalies in the Qinling-Qilian orogen and its adjacent areas, showing that the northernmost boundaries of these Proto-Tethyan micro-blocks are defined by the Guyuan-Longshoushan Fault in the Qilian orogen and by the Luonan-Luanchuan Fault in the Qinling orogen, respectively. The lithosphere north of the Qinling-Qilian orogen subducted southward under the Qinling-Qilian orogen. The boundaries of the micro-blocks in the Qinling-Qilian orogen of the Proto-Tethys domain are mostly EW- or NE-trending. Combined with Early Paleozoic geological records, our results indicate that the south-dipping low-velocity anomaly under the Alxa block and the south-dipping high-velocity ones under the North Qilian orogen and the North China block might indicate that the Alxa block, the North Qilian Oceanic slab and the North China block subducted southward. The south-dipping high-velocity anomaly under the North Qilian orogen is interpreted as the remnant of the Proto-Tethys Ocean in that area. However, the north-dipping low-velocity anomaly under the South Qinling and the North Qinling orogens, the Qaidam block and the north-dipping high-velocity one under the Yangtze and the Bikou blocks suggest that the South Qinling and the North Qinling orogens, the Qaidam, the Yangtze and the Bikou blocks subducted northward. The present spatial framework of the Qinling-Qilian orogen is related to Early Paleozoic convergence among the Proto-Tethyan micro-blocks.

  16. Late Paleozoic closure of the Ob-Zaisan Ocean along the Irtysh/Chara shear zone and implications for arc amalgamation and oroclinal bending in the western Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Li, Pengfei; Sun, Min; Rosenbaum, Gideon

    2016-04-01

    The Irtysh/Chara Shear Zone is one of the largest strike-slip systems in the Central Asian Orogenic Belt (CAOB). It records collisional processes of the peri-Siberian orogenic system with the West Junggar-Kazakhstan-Tianshan orogenic system following the closure of the Ob-Zaisan Ocean, but the exact timing of these events remains enigmatic. We conducted detailed structural analysis along the Irtysh Shear Zone (NW China), which together with new geochronological data allows us to reconstruct the tectonic evolution during the final closure of the Ob-Zaisan Ocean. Our results showed that subduction-accretion processes lasted at least until the Late Carboniferous in the Chinese Altai and the East/West Junggar. The subsequent arc amalgamation is characterized by a cycle of crustal thickening, orogenic collapse and transpressional thickening. On a larger scale, the West Junggar- Kazakhstan -Tianshan orogenic system defines a U-shape oroclinal structure (e.g. Xiao et al., 2010). A major phase of oroclinal bending that involved ~110° rotation may have occurred during the Late Devonian to Early Carboniferous (Levashova et al., 2012). Previous authors have linked oroclinal bending with the late Paleozoic amalgamation of the western CAOB, and proposed that a quasi-linear West Junggar- Kazakhstan -Tianshan orogenic system was buckled during the convergence of the Siberian and Tarim cratons following the closure of the Ob-Zaisan Ocean (in the north) and the South Tianshan Ocean (in the south) (e.g. Abrajevitch et al., 2008). This model, however, is not supported by our new data that constrain the closure of the Ob-Zaisan Ocean to the Late Carboniferous. Alternatively, we propose that oroclinal bending may have involved two phases of bending, with the ~110° rotation in the Late Devonian to Early Carboniferous possibly associated with trench retreat. Further tightening may have occurred in response to the convergence of the Siberian and Tarim cratons during the Late

  17. An accreted micro-continent in the north of the Dabie Orogen, East China: Evidence from detrital zircon dating

    NASA Astrophysics Data System (ADS)

    Zhu, Guang; Wang, Yongsheng; Wang, Wei; Zhang, Shuai; Liu, Cheng; Gu, Chengchuan; Li, Yunjian

    2017-02-01

    Continent-continent collision between the North China Block (NCB) and South China Block (SCB) took place along the Qinling-Tongbai-Hong'an-Dabie orogens during the Triassic. A micro-continent with Paleozoic arc magmatism has been recognized in the northern Qinling-Tongbai orogens; however, it remains unclear whether the micro-continent extended to the Dabie Orogen to form a ribbon-shaped micro-continent, due to later burial by the Hefei Basin in the north. To solve this problem, we conducted LA-ICP-MS Usbnd Pb dating of zircons from Silurian to Cretaceous sandstones and volcanic rocks from the southern margin of the basin. The age spectra of detrital zircons suggest that the Dabie Orogen and later basin cover were the sources of the analyzed sandstones. The detrital and inherited zircons indicate Neoproterozoic, early and late Paleozoic magmatism in the Beihuaiyang unit in the north of the Dabie Orogen. The zircon and previous geophysical data show that a micro-continent bounded by the Feizhong Suture in the north and the Xiaotian-Mozitang Suture in the south existed between the NCB and the Triassic Dabie Orogen, and its northern half is buried by the Jurassic-Paleogene Hefei Basin. The Beihuaiyang micro-continent experienced early Paleozoic arc magmatism caused by southward subduction of the Erlangping oceanic crust and late Paleozoic magmatism related to northward subduction of the Paleotethyan oceanic crust. The micro-continent was accreted to the southern edge of the NCB at the end of the Early Devonian (ca. 400 Ma) via arc-continent collision. Similarly to the Qinling-Tongbai orogens, the Dabie Orogen contains a Paleozoic accretionary system in the north and a Triassic collisional system in the south; thus, it is suggested that a ribbon-shaped micro-continent, > 900 km long and 50-100 km wide, was present along the entire Qinling-Tongbai-Hong'an-Dabie orogens prior to the middle Paleozoic. This micro-continent might have originated as a result of middle

  18. Rift inheritance in orogenes: a case study from the Western Pyrenees

    NASA Astrophysics Data System (ADS)

    Masini, E.; Manatschal, G.; Tugend, J.; Kusznir, N. J.; Flament, J.

    2012-12-01

    In plate tectonics, there is a general assumption that rifted margins represent most of the former material accreted into collisional orogenic prisms. In this regard, the former architecture, structures and composition of rifted margins, i.e. the pre-orogenic inheritances, play undoubtedly a major role during tectonic inversion. Studies have shown that rifted margins are more complex than a succession of tilted blocks. Indeed, the discovery of hyper-extended domains, where low-angle detachments replace high-angle normal faults and mantle material is exhumed to the seafloor implies a revision of the margin's template used in orogenic models. Because of overprint, the role of rift inheritance in orogenes remains often underestimated. The Pyrenees, located along the Iberian-European plate boundary, can be considered as one of the best places to study the reactivation of hyper-extended rifts. In this orogen, the Late Cretaceous and Tertiary convergence overprints a Latest Jurassic to Lower Cretaceous intracontinental rift linked with the opening of the North Atlantic. There, Albian hyper-extended rift basins developed where deep crustal and mantle rocks were exhumed along low-angle detachments to the seafloor. In this work we discuss the example of the Mauléon-Arzacq domain, which escaped from the most pervasive deformation because of its specific location between the western termination of the chain and the Bay of Biscay oceanic domain. Combining field study with subsurface geophysical and drillhole data, we show that the overall rift domain is asymmetric. The northern European upper plate is on the hangingwall of low-angle detachment systems affecting the southern Iberian Lower plate. The upper plate records depth-dependent crustal thinning and the development of a syn-rift sag basin. In contrast, the lower plate resulted from the hyper-extension of Iberian continental crust accommodated at the surface by two diachronous top-basement detachment systems. The first

  19. Discriminating fluid source regions in orogenic gold deposits using B-isotopes

    NASA Astrophysics Data System (ADS)

    Lambert-Smith, James S.; Rocholl, Alexander; Treloar, Peter J.; Lawrence, David M.

    2016-12-01

    The genesis of orogenic gold deposits is commonly linked to hydrothermal ore fluids derived from metamorphic devolatilization reactions. However, there is considerable debate as to the ultimate source of these fluids and the metals they transport. Tourmaline is a common gangue mineral in orogenic gold deposits. It is stable over a very wide P-T range, demonstrates limited volume diffusion of major and trace elements and is the main host of B in most rock types. We have used texturally resolved B-isotope analysis by secondary ion mass spectrometry (SIMS) to identify multiple fluid sources within a single orogenic gold ore district. The Loulo Mining District in Mali, West Africa hosts several large orogenic gold ore bodies with complex fluid chemistry, associated with widespread pre-ore Na- and multi-stage B-metasomatism. The Gara deposit, as well as several smaller satellites, formed through partial mixing between a dilute aqueous-carbonic fluid and a hypersaline brine. Hydrothermal tourmaline occurs as a pre-ore phase in the matrix of tourmalinite units, which host mineralization in several ore bodies. Clasts of these tourmalinites occur in mineralized breccias. Disseminated hydrothermal and vein hosted tourmaline occur in textural sites which suggest growth during and after ore formation. Tourmalines show a large range in δ11B values from -3.5 to 19.8‰, which record a change in fluid source between paragenetic stages of tourmaline growth. Pre-mineralization tourmaline crystals show heavy δ11B values (8-19.8‰) and high X-site occupancy (Na ± Ca; 0.69-1 apfu) suggesting a marine evaporite source for hydrothermal fluids. Syn-mineralization and replacement phases show lighter δ11B values (-3.5 to 15.1‰) and lower X-site occupancy (0.62-0.88 apfu), suggesting a subsequent influx of more dilute fluids derived from devolatilization of marine carbonates and clastic metasediments. The large, overlapping range in isotopic compositions and a skew toward the

  20. Spatially Concentrated Erosion Focuses Deformation Within the Himalayan Orogenic Wedge: Sutlej Valley, NW Himalaya, India

    NASA Astrophysics Data System (ADS)

    Thiede, R. C.; Arrowsmith, J.; Bookhagen, B.; McWilliams, M.; Sobel, E. R.; Strecker, M. R.

    2004-12-01

    Long-term erosion processes in the NW-Himalaya have not only shaped the distribution of topography and relief, but may also exert a regional control on the kinematic history of the Himalayan orogenic wedge. The topographic front of the orogenic wedge forms the southern margin of the High Himalaya and may be related to subsurface structures such as a crustal ramp or a blind thrust. Drastic along- and across-strike erosional gradients characterize the modern Himalaya and range from high-erosion regions along the southern High Himalayan front where monsoonal precipitation is able to penetrate far into the range, to low-erosion sectors across the moderately elevated Lesser Himalaya to the south and the high-elevation, arid sectors to the north. Published paleo-elevation estimates from the Thakkhola Graben (Nepal) suggest that by ~11 Ma the southern Tibetan Plateau and probably the High Himalaya had been uplifted to elevations comparable to the recent conditions. Thus, the presently observed pronounced erosional gradients have likely existed across the orogen since then. However, the cause of high rock-uplift and exhumation rates along distinct segments of the southern front of the High Himalaya are still a matter of debate. New apatite fission track (AFT) and 40Ar/39Ar data sampled along an orogen-perpendicular transect following the Sutlej Valley, approximately perpendicular to the Himalayan orogen, constrain the distribution patterns of rapid cooling related to rock uplift and exhumation. Combined with published thermochronologic data, this comprehensive AFT dataset from south of the High Himalaya mountain front to the interior of the Tethyan Himalaya allows us to derive a regional uplift and exhumation scenario. Our new 40Ar/39Ar ages ranging between 17 and 4 Ma reveal diachronous exhumation of two crystalline nappes (Higher and Lesser Himalayan crystalline) during Miocene-Pliocene time. In contrast, the AFT data ranging from 1.3 to 4.6 Ma indicate synchronous, fast

  1. Chemical remagnetization and paleomagnetic dating of fluid migration events: Testing the orogenic fluid hypothesis

    SciTech Connect

    Douglas, E.R. )

    1993-11-01

    Remagnetization, the acquisition of a secondary magnetization, is now recognized as a widespread phenomenon in sedimentary rocks. The recognition that many sedimentary rocks were remagnetized long after deposition has not only led to a reevaluation of the paleomagnetic database, but also has led to a new application of the paleomagnetic method to other areas of earth science. Many secondary magnetizations are tangible evidence of a chemical event caused by rock-fluid interactions. Recent studies demonstrate that isolation of a chemical remanent magnetization (CRM) and comparison of the corresponding pole position to the apparent polar wander path can provide essential constraints on the timing of a diagenetic event. Many CRMs can be spatially correlated with orogenic belts and temporally related to orogeny. In this talk several case studies will be presented where paleomagnetic and geochemical results are used to date diagenetic events and test the role of orogenic fluids as agents of remagnetization. The Ordovician Viola Limestone contains a pervasive Pennsylvanian CRM and a localized Permian CRM that occurs in a halo around veins mineralized by saline radiogenic fluids. The Permian CRM can be related to alteration by the basinal fluids. The pervasive CRM, which is similar to many other CRMs that have been related to orogenic fluids, occurs in relatively unaltered limestone. The acquisition of this CRM was caused by an as yet unidentified chemical mechanism that was not triggered by externally-derived fluids. Preliminary results, however, suggest a connection between the CRM and organic matter in the limestone. Radiogenic Cambrian-Ordovician dolomites near the Ouachita Mountains in southern Oklahoma that have been altered by basinal fluids contain a late Paleozoic CRM. Understanding the origin of these CRMs will require tests of mechanisms that do not rely on externally-derived fluids.

  2. Noble gases fingerprint a metasedimentary fluid source in the Macraes orogenic gold deposit, New Zealand

    NASA Astrophysics Data System (ADS)

    Goodwin, Nicholas R. J.; Burgess, Ray; Craw, Dave; Teagle, Damon A. H.; Ballentine, Chris J.

    2017-02-01

    The world-class Macraes orogenic gold deposit (˜10 Moz resource) formed during the late metamorphic uplift of a metasedimentary schist belt in southern New Zealand. Mineralising fluids, metals and metalloids were derived from within the metasedimentary host. Helium and argon extracted from fluid inclusions in sulphide mineral grains (three crush extractions from one sample) have crustal signatures, with no evidence for mantle input (R/Ra = 0.03). Xenon extracted from mineralised quartz samples provides evidence for extensive interaction between fluid and maturing organic material within the metasedimentary host rocks, with 132Xe/36Ar ratios up to 200 times greater than air. Similarly, I/Cl ratios for fluids extracted from mineralised quartz are similar to those of brines from marine sediments that have interacted with organic matter and are ten times higher than typical magmatic/mantle fluids. The Macraes mineralising fluids were compositionally variable, reflecting either mixing of two different crustal fluids in the metasedimentary pile or a single fluid type that has had varying degrees of interaction with the host metasediments. Evidence for additional input of meteoric water is equivocal, but minor meteoric incursion cannot be discounted. The Macraes deposit formed in a metasedimentary belt without associated coeval magmatism, and therefore represents a purely crustal metamorphogenic end member in a spectrum of orogenic hydrothermal processes that can include magmatic and/or mantle fluid input elsewhere in the world. There is no evidence for involvement of minor intercalated metabasic rocks in the Macraes mineralising system. Hydrothermal fluids that formed other, smaller, orogenic deposits in the same metamorphic belt have less pronounced noble gas and halogen evidence for crustal fluid-rock interaction than at Macraes, but these deposits also formed from broadly similar metamorphogenic processes.

  3. Feeding the "aneurysm": Orogen-parallel mass transport into Nanga Parbat and the western Himalayan syntaxis

    NASA Astrophysics Data System (ADS)

    Whipp, David M.; Beaumont, Christopher; Braun, Jean

    2014-06-01

    The Nanga Parbat-Haramosh massif (NPHM; western Himalayan syntaxis) requires an influx of mass exceeding that in the adjacent Himalayan arc to sustain high topography and rapid erosional exhumation rates. What supplies this mass flux and feeds this "tectonic aneurysm?" We show, using a simple 3-D model of oblique orogen convergence, that velocity/strain partitioning results in horizontal orogen-parallel (OP) crustal transport, and the same behavior is inferred for the Himalaya, with OP transport diverting converging crust toward the syntaxis. Model results also show that the OP flow rate decreases in the syntaxis, thereby thickening the crust and forming a structure like the NPHM. The additional crustal thickening, over and above that elsewhere in the Himalayan arc, sustains the rapid exhumation of this "aneurysm." Normally, velocity/strain partitioning would be minimal for the Himalayan arc where the convergence obliquity is no greater than ~40°. However, we show analytically that the Himalayan system can act both as a critical wedge and exhibit strain partitioning if both the detachment beneath the wedge and the bounding rear shear zone, which accommodates OP transport, are very weak. Corresponding numerical results confirm this requirement and demonstrate that a Nanga Parbat-type shortening structure can develop spontaneously if the orogenic wedge and bounding rear shear zone can strain rate soften while active. These results lead us to question whether the position of NPHM aneurysm is localized by river incision, as previously suggested, or by a priori focused tectonic shortening of the crust in the syntaxis region as demonstrated by our models.

  4. Erosion and Sediment Transport Across Pronounced Topographic and Climatic Gradients in the Himalayan Orogen

    NASA Astrophysics Data System (ADS)

    Strecker, M. R.; Bookhagen, B.

    2014-12-01

    The interaction between the NW-directed trajectories of moisture transport associated with the Indian Summer Monsoon circulation and the high topography of the Himalayan orogen results in one of the most efficient orographic barriers on Earth. The steep topographic gradients, the impact of focused rainfall along the southern flank of the range, and northward shifts of rainfall during frequent intensified Indian Summer Monsoons are responsible for an efficient erosional regime, with some of the highest known erosion rates. The spatiotemporal correlation between various topographic, tectonic, climatic, and exhumational phenomena in this region has resulted in the formulation of models of possible long-term erosional and tectonic feedback processes that drive the lateral expansion and vertical growth of the mountain belt. However, despite an increase in thermochronologic, cosmogenic radionuclide, and sedimentological datasets that help explain the underlying mechanisms, the true nature of these relationships is still unclear and controversies particularly exist concerning the importance of the different forcing factors that drive exhumation and, ultimately, deformation. Here, we synthesize and assess these controversies with observations from studies conducted perpendicular to and along strike of the orogen, and combine them with our new basin-wide erosion-rate data from the Sutlej Valley in the NW Himalaya. In our regional comparison we highlight the importance of large river systems and climate-controlled aspects of weathering concerning fluvial mass distribution as there appear to be positive feedbacks between tectonics and surface processes. In contrast, observations from smaller catchments along the orogenic front suggest a negative correlation. Similar to other environments with steep topographic and climatic gradients, our observations from the Sutlej catchments emphasize that erosional processes in the Himalayan realm are most efficient in geomorphic

  5. Porphyry molybdenum deposits in the Tianshan-Xingmeng orogenic belt, northern China

    NASA Astrophysics Data System (ADS)

    Zeng, Qingdong; Qin, Kezhang; Liu, Jianming; Li, Guangming; Zhai, Mingguo; Chu, Shaoxiong; Guo, Yunpeng

    2015-06-01

    Molybdenum (Mo) exploration activity in China has increased tremendously over the past decade, and China is now known to have the largest Mo reserves in the world. The Tianshan-Xingmeng orogenic belt, the second largest Mo metallogenic belt, possesses over 8.2 Mt of Mo reserves. Porphyry Mo deposits contain 99 % of the Mo reserves in the Tianshan-Xingmeng orogenic belt; other Mo deposits contain 1 % of the Mo reserves. Five subtypes of the porphyry Mo deposits can be distinguished by deposit associations, such as Mo, Mo-Cu, Mo-W, Mo-Pb-Zn-Ag, and Cu-Mo deposits. These porphyry Mo deposits are formed at different stages: during the Ordovician, Devonian, Carboniferous, Late Permian, Triassic, Jurassic, and Cretaceous Periods. The polystage porphyry Mo mineralizations indicate that polystage tectonic-magmatic activity occurred in the orogenic belt. The Ordovician-Carboniferous porphyry Cu-Mo deposits are formed in an island-arc setting; the Late Permian porphyry Mo deposits are formed in a syn-collisional tectonic setting; and the Triassic porphyry Mo deposits are formed in a syn-collisional to post-collisional tectonic setting. The Ordovician-Triassic porphyry deposits are related to the Paleo-Asian Ocean tectonic system. The Jurassic porphyry Mo deposits are formed at the eastern margin of the Asian continent and are associated with a Paleo-Pacific plate-subduction tectonic setting. Cretaceous porphyry Mo deposits are formed in a lithospheric thinning setting and are related to the rollback of the Paleo-Pacific subduction plate.

  6. Modification of the Himalayan Orogenic Wedge by Late Cenozoic Southeastward Flow of Tibet

    NASA Astrophysics Data System (ADS)

    Hodges, K. V.; Whipple, K. X.; Kirby, E.; Arrowsmith, R.; Shirzaei, M.

    2014-12-01

    Continental plateaus are reservoirs of excess gravitational energy that can influence the late-stage geodynamic evolution of adjacent orogenic wedges. In the central Himalaya (80-88˚E), most late Cenozoic deformation has involved roughly N-S shortening within the Himalayan orogenic wedge. Within this region, all 1976-2014 Mw 5 and larger earthquakes had thrust mechanisms associated with slip along major arc-parallel structures within or at the base of the orogenic wedge. In contrast, the segment of the wedge between 88˚E and 91˚E - including easternmost Nepal, the Sikkim region of India, and Bhutan - is characterized by a complex deformation field that includes thrusting on arc-parallel wedge structures but also transcurrent faulting at high angles to the Himalayan arc. In fact, over the same 1976-2014 period, all but one of the Mw 5 and larger earthquakes in this region had transcurrent fault mechanisms, mostly consistent with dextral strike-slip along NW-striking faults. We refer to this region as the central-eastern Himalayan transition zone. Although direct field evidence of the surface breaks of these faults has not been established, the orientations of nodal planes of large earthquakes, as well as alignments of microearthquake arrays, suggest that they may connect northward to a discontinuous family of arc-parallel faults - most showing evidence for oblique slip, with variable normal and dextral-transcurrent components - which we interpret as the long-term structural manifestation of the boundary between the distinctive modern strain fields of Tibet (E-W extension) and the central Himalaya (N-S shortening). In addition, transverse faults of the central-eastern Himalayan transition zone may project southward, beneath sediments near the Ganges and Bhramaputra confluence, as dextral tear faults linking the active thrust front of the central Himalaya to the active thrust front of the Shillong Plateau in northeastern India. We hypothesize that the broadening of

  7. Flake tectonics in the Sulu orogen in eastern China as revealed by seismic tomography

    NASA Astrophysics Data System (ADS)

    Xu, Peifen; Liu, Futian; Ye, Kai; Wang, Qingchen; Cong, Bolin; Chen, Hui

    2002-05-01

    Seismic tomographic image reveals a crocodile-like P-waves velocity structure beneath the Sulu orogenic belt, which marks the subduction/collision zone between the Sino-Korean block (SK) and Yangtze block (YZ) in eastern China. It may imply that the upper crust of the YZ was detached from its lower crust and thrust over the SK for a maximum of ~400 km in the Sulu region, whereas the remnant of the subducted Yangtze lithosphere was lay beneath the SK. This crustal detached structure (flake tectonics) might have occurred after the Triassic subduction/collision.

  8. Orogenic Float Model: an Explanation for the Dynamics of the Venezuelan Andes

    NASA Astrophysics Data System (ADS)

    Monod, B.; Dhont, D.; Hervouet, Y.

    2008-12-01

    The Venezuelan (or Merida) Andes are a NE-trending intracontinental orogen that started to rise from the late Miocene due to the E-W far field convergence between the Maracaibo block to the northwest and the Guyana shield to the southeast. Oblique convergence is responsible for strain partitioning with thrusting along both foreland basins and right-lateral strike-slip faulting along the NE-SW Bocono fault cutting the Venezuelan Andes along-strike. The central part of the belt is also cut by the N-S left-lateral strike-slip Valera fault that branches the Bocono fault in the triple junction geometry, favoring the crustal escape of the Trujillo triangular block towards the NE. Onset of strike-slip motion along major faults and their geometry at depth remains a matter of debate. Our work, based on the integration of geologic and geophysical data aims to better constrain both the geometry and the tectonic evolution of the major tectonic structures. We use the orogenic float model (Oldow et al., 1990) as a first hypothesis to construct two NW-SE trans-Andean crustal scale balanced sections. The late Neogene-Quaternary shortening varies from 40 km in the south to 30 km in the north across the Trujillo block, indicating that a quarter of the deformation seems to be absorbed by the tectonic escape process. The cross-sections served also as the basis for the building of a 3-D geologic model of the Venezuelan Andes, permitting to clearly understand the link and geometry of the faults at depth. The decollement level used for the orogenic float model, located at 20 km depth, is crucial for the motion of the Trujillo block. Both the Bocono and Valera faults have listric shapes connecting to the decollement level. The connexion of the two fault surfaces forms a hinge line dipping towards the north in a geometry favoring the escape of the Trujillo block and allowing the gravity forces to play an important role in the process. Oldow J. S., Bally A. W., Ave Lallemant H. G., 1990

  9. Structural development of an Archean Orogen, Western Point Lake, Northwest Territories

    NASA Astrophysics Data System (ADS)

    Kusky, Timothy M.

    1991-08-01

    The Point Lake orogen in the central Archean Slave Province of northwestern Canada preserves more than 10 km of structural relief through an eroded antiformal thrust stack and deeper anastomosing midcrustal mylonites. Fault restoration along a 25 km long transect requires a minimum of 69 km slip and 53 km horizontal shortening. In the western part of the orogen the basal decollement places mafic plutonic/volcanic rocks over an ancient tonalitic gneiss complex. Ten kilometers to the east in the Keskarrah Bay area, slices of gneiss unroofed on brittle thrusts shed molasse into several submerged basins. Conglomerates and associated thinly bedded sedimentary rocks are interpreted as channel, levee, and overbank facies of this thrust-related sedimentary fan system. The synorogenic erosion surface at the base of the conglomerate truncates premetamorphic or early metamorphic thrust faults formed during foreland propagation, while other thrusts related to hinterland-progressing imbrication displace this unconformity. Tightening of synorogenic depositional troughs resulted in the conglomerates' present localization in synclines to the west of associated thrust faults and steepening of structural dips. Eastern parts of the orogen consist of isoclinally folded graywackes composed largely of Mutti and Ricci-Lucchi turbidite facies B, C, and D, interpreted as submarine fan deposits eroded from a distant volcanic arc. Thrust faults in the metasedimentary terrane include highly disrupted slate horizons with meter-scale duplex structures, and recrystallized calcmylonites exhibiting sheath folds and boudin trains with very large interboudin distances. The sequence of fabric development and the overall geometry of this metasedimentary terrane strongly resembles younger forearc accretionary prisms. Conditions of deformation along the thrusts parallel the regional metamorphic zonation: amphibolite facies in the basal decollement through greenschist facies shear zones to cataclastic

  10. Carbonatitic metasomatism in orogenic dunites from Lijiatun in the Sulu UHP terrane, eastern China

    NASA Astrophysics Data System (ADS)

    Su, Bin; Chen, Yi; Guo, Shun; Chu, Zhu-Yin; Liu, Jing-Bo; Gao, Yi-Jie

    2016-10-01

    Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic ( 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic

  11. Application of electron spin resonance (ESR) dating to ductile shearing: Examples from the Qinling orogenic belt, China

    NASA Astrophysics Data System (ADS)

    Ren, Shenglian; Song, Chuanzhong; Li, Jiahao

    2016-04-01

    Shear zones are common structures in orogenic belts and elucidation of the tectonic evolution of these orogenic belts to a large degree depends on understanding the kinematics and timing of shear deformation. However, there is a lack of an accurate, fast and convenient way to determine the timing of deformation. In this paper, we apply the ESR (electron spin-resonance spectroscopy) dating method to three syntectonic quartz veins from the Funiushan tectonic belt in the Qinling orogenic belt in central China. The results agree well with the available ages of deformation in the area obtained through other dating methods. This demonstrates the accuracy and feasibility of using the ESR method to date quartz crystals formed during deformation. The method is fast and convenient, and satisfies the accuracy requirement. It is an effective means for determining the timing of deformation, especially in areas with intensive fluid activity during deformation.

  12. Zirconolite, zircon and monazite-(Ce) U-Th-Pb age constraints on the emplacement, deformation and alteration history of the Cummins Range Carbonatite Complex, Halls Creek Orogen, Kimberley region, Western Australia

    NASA Astrophysics Data System (ADS)

    Downes, Peter J.; Dunkley, Daniel J.; Fletcher, Ian R.; McNaughton, Neal J.; Rasmussen, Birger; Jaques, A. Lynton; Verrall, Michael; Sweetapple, Marcus T.

    2016-04-01

    In situ SHRIMP U-Pb dating of zirconolite in clinopyroxenite from the Cummins Range Carbonatite Complex, situated in the southern Halls Creek Orogen, Kimberley region, Western Australia, has provided a reliable 207Pb/206Pb age of emplacement of 1009 ± 16 Ma. Variably metamict and recrystallised zircons from co-magmatic carbonatites, including a megacryst ~1.5 cm long, gave a range of ages from ~1043-998 Ma, reflecting partial isotopic resetting during post-emplacement deformation and alteration. Monazite-(Ce) in a strongly foliated dolomite carbonatite produced U-Th-Pb dates ranging from ~900-590 Ma. Although the monazite-(Ce) data cannot give any definitive ages, they clearly reflect a long history of hydrothermal alteration/recrystallisation, over at least 300 million years. This is consistent with the apparent resetting of the Rb-Sr and K-Ar isotopic systems by a post-emplacement thermal event at ~900 Ma during the intracratonic Yampi Orogeny. The emplacement of the Cummins Range Carbonatite Complex probably resulted from the reactivation of a deep crustal structure within the Halls Creek Orogen during the amalgamation of Proterozoic Australia with Rodinia over the period ~1000-950 Ma. This may have allowed an alkaline carbonated silicate magma that was parental to the Cummins Range carbonatites, and generated by redox and/or decompression partial melting of the asthenospheric mantle, to ascend from the base of the continental lithosphere along the lithospheric discontinuity constituted by the southern edge of the Halls Creek Orogen. There is no evidence of a link between the emplacement of the Cummins Range Carbonatite Complex and mafic large igneous province magmatism indicative of mantle plume activity. Rather, patterns of Proterozoic alkaline magmatism in the Kimberley Craton may have been controlled by changing plate motions during the Nuna-Rodinia supercontinent cycles (~1200-800 Ma).

  13. A crustal model of the ultrahigh-pressure Dabie Shan orogenic belt, China, derived from deep seismic refraction profiling

    USGS Publications Warehouse

    Wang, Chun-Yong; Zeng, Rong-Sheng; Mooney, W.D.; Hacker, B.R.

    2000-01-01

    We present a new crustal cross section through the east-west trending ultrahigh-pressure (UHP) Dabie Shan orogenic belt, east central China, based on a 400-km-long seismic refraction profile. Data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35-km-thick crust consisting of three layers (upper, middle, and lower crust) with average seismic velocities of 6.0±0.2 km/s, 6.5±0.1 km/s, and 6.8±0.1 km/s. The crust reaches a maximum thickness of 41.5 km beneath the northern margin of the orogen, and thus the present-day root beneath the orogen is only 6.5 km thick. The upper mantle velocity is 8.0±0.1 km/s. Modeling of shear wave data indicate that Poisson's ratio increases from 0.24±0.02 in the upper crust to 0.27±0.03 in the lower crust. This result is consistent with a dominantly felsic upper crustal composition and a mafic lower crustal composition within the amphibolite or granulite metamorphic facies. Our seismic model indicates that eclogite, which is abundant in surface exposures within the orogen, is not a volumetrically significant component in the middle or lower crust. Much of the Triassic structure associated with the formation of the UHP rocks of the Dabie Shan has been obscured by post-Triassic igneous activity, extension and large-offset strike-slip faulting. Nevertheless, we can identify a high-velocity (6.3 km/s) zone in the upper (<5 km depth) crustal core of the orogen which we interpret as a zone of ultrahigh-pressure rocks, a north dipping suture, and an apparent Moho offset that marks a likely active strike-slip fault.

  14. Menstrual Cycle

    MedlinePlus

    ... receive Pregnancy email updates Enter email Submit The menstrual cycle Day 1 starts with the first day of ... drop around Day 25 . This signals the next menstrual cycle to begin. The egg will break apart and ...

  15. Devonian sedimentation in the Xiqingshan Mountains: Implications for paleogeographic reconstructions of the SW Qinling Orogen

    NASA Astrophysics Data System (ADS)

    Yan, Zhen; Aitchison, Jonathan C.; Fu, Changlei; Guo, Xianqing; Xia, Wenjing; Niu, Manlan

    2016-08-01

    The Qinling Orogen between the North China and Yangtze plates comprises various accreted and collisional terranes, recording the evolution of the Proto-Tethyan Ocean and the formation of east Asia. Knowledge of the provenance and tectonic setting of a thick succession of E-W striking Devonian sedimentary rocks sandwiched tectonically between the Shangdan and Mianlue ophiolitic suture zones in this belt is essential to understanding the evolution of the Qinling Orogen and the assembly of the North China and Yangtze plates during the Late Paleozoic. Systematic studies of the Devonian sedimentary facies in the Xiqingshan Mountains indicate the northern margin of the South Qinling belt was uplifted prior to the Early Devonian. Lower Devonian sandstones are dominated by feldspathic litharenite (Q12-31F18-37L40-65), indicative of a continental arc derivation. Detrital zircon U-Pb data demonstrate that the Lower and Middle Devonian sandstones were derived respectively from ca. 770 Ma and ca. 400 Ma felsic arc-related magmatic sources. Devonian sedimentation in the Xiqingshan Mountains is interpreted to have occurred in a foreland basin between the Yangtze plate and the North Qinling continental arc.

  16. Ediacaran 2,500-km-long synchronous deep continental subduction in the West Gondwana Orogen.

    PubMed

    Ganade de Araujo, Carlos E; Rubatto, Daniela; Hermann, Joerg; Cordani, Umberto G; Caby, Renaud; Basei, Miguel A S

    2014-10-16

    The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth.

  17. Shallow, old, and hydrologically insignificant fault zones in the Appalachian orogen

    NASA Astrophysics Data System (ADS)

    Malgrange, Juliette; Gleeson, Tom

    2014-01-01

    The permeability of fault zones impacts diverse geological processes such as hydrocarbon migration, hydrothermal fluid circulation, and regional groundwater flow, yet how fault zones affect groundwater flow at a regional scale (1-10 km) is highly uncertain. The objective of this work is to determine whether faults affect regional patterns of groundwater flow, by using radioactive radon and chloride to quantify groundwater discharge to lakes underlain by faults and not underlain by faults. We sampled lakes overlying the Paleozoic Appalachian fold and thrust belt in the Eastern Townships in Québec, and compared our results to a previous study in a crystalline watershed in the Canadian Shield. The field data was analyzed with an analytical geochemical mixing model. The uncertainties of model parameters were assessed in a sensitivity analysis using Monte Carlo simulation, and the difference between lakes tested with statistical analysis. While the model results indicate non-negligible groundwater discharge for most of the lakes in the Paleozoic orogen, the difference between the groundwater discharge rate into the lakes located on faults and the other lakes is not statistically significant. However, the groundwater discharge rate to lakes in the Paleozoic orogeny is significantly higher than lakes that overlay crystalline bedrock, which is consistent with independent estimates of permeability. The rate of groundwater discharge is not significantly enhanced or diminished around the thrust fault zones, suggesting that in a regional scale, permeability of fault zones is not significantly different from the bedrock permeability at shallow depth in this old, tectonically- inactive orogen.

  18. Origin of unusual HREE-Mo-rich carbonatites in the Qinling orogen, China

    PubMed Central

    Song, Wenlei; Xu, Cheng; Smith, Martin P.; Kynicky, Jindrich; Huang, Kangjun; Wei, Chunwan; Zhou, Li; Shu, Qihai

    2016-01-01

    Carbonatites, usually occurring within intra-continental rift-related settings, have strong light rare earth element (LREE) enrichment; they rarely contain economic heavy REE (HREE). Here, we report the identification of Late Triassic HREE-Mo-rich carbonatites in the northernmost Qinling orogen. The rocks contain abundant primary HREE minerals and molybdenite. Calcite-hosted fluid inclusions, inferred to represent a magmatic-derived aqueous fluid phase, contain significant concentrations of Mo (~17 ppm), reinforcing the inference that these carbonatitic magmas had high Mo concentrations. By contrast, Late Triassic carbonatites in southernmost Qinling have economic LREE concentrations, but are depleted in HREE and Mo. Both of these carbonatite types have low δ26Mg values (−1.89 to −1.07‰), similar to sedimentary carbonates, suggesting a recycled sediment contribution for REE enrichment in their mantle sources. We propose that the carbonatites in the Qinling orogen were formed, at least in part, by the melting of a subducted carbonate-bearing slab, and that 10 Ma younger carbonatite magmas in the northernmost Qinling metasomatized the thickened eclogitic lower crust to produce high levels of HREE and Mo. PMID:27857170

  19. Origin of unusual HREE-Mo-rich carbonatites in the Qinling orogen, China

    NASA Astrophysics Data System (ADS)

    Song, Wenlei; Xu, Cheng; Smith, Martin P.; Kynicky, Jindrich; Huang, Kangjun; Wei, Chunwan; Zhou, Li; Shu, Qihai

    2016-11-01

    Carbonatites, usually occurring within intra-continental rift-related settings, have strong light rare earth element (LREE) enrichment; they rarely contain economic heavy REE (HREE). Here, we report the identification of Late Triassic HREE-Mo-rich carbonatites in the northernmost Qinling orogen. The rocks contain abundant primary HREE minerals and molybdenite. Calcite-hosted fluid inclusions, inferred to represent a magmatic-derived aqueous fluid phase, contain significant concentrations of Mo (~17 ppm), reinforcing the inference that these carbonatitic magmas had high Mo concentrations. By contrast, Late Triassic carbonatites in southernmost Qinling have economic LREE concentrations, but are depleted in HREE and Mo. Both of these carbonatite types have low δ26Mg values (‑1.89 to ‑1.07‰), similar to sedimentary carbonates, suggesting a recycled sediment contribution for REE enrichment in their mantle sources. We propose that the carbonatites in the Qinling orogen were formed, at least in part, by the melting of a subducted carbonate-bearing slab, and that 10 Ma younger carbonatite magmas in the northernmost Qinling metasomatized the thickened eclogitic lower crust to produce high levels of HREE and Mo.

  20. Sequential kinetic modelling: A new tool decodes pulsed tectonic patterns in early hot orogens of Earth

    NASA Astrophysics Data System (ADS)

    Bhowmik, Santanu Kumar; Chakraborty, Sumit

    2017-02-01

    Tectonic styles in an early hot Earth were different from the present-day situation governed by plate tectonics. Processes in such hot settings remain poorly understood because they often occur on timescales that are below the resolution of conventional isotopic clocks, the rock records are fragmentary, and these have been superposed by later high-temperature events. We have developed a tool based on diffusion kinetics to overcome these difficulties and reconstruct sequences of short-lived episodes. Application of the method to a rock from the ultra-hot c.1.6 Ga orogenic domain of the Central Indian Tectonic Zone, where additional data are available to verify the results, shows that pulses of approach and roll-back of colliding plates preceded the final closure and collision. We demonstrate that cooling from ultra-high temperature metamorphic conditions in the orogen took place in multiple pulses that occurred with a periodicity of about 10 Myr at rates that vary between 100's to 10's °C/Myr, and burial-/exhumation-rates that vary between 30 and 2 km/Myr, respectively. Such details of tectonic processes in the Precambrian, with quantification of variable heating-, cooling-, burial-, and exhumation-rates of individual stages, have not been accessible until now. Application of this method to other regions would provide a means of exploring the thermal viability of the inferred long durations (>100 Myr) for some ultra-high temperature orogenies.

  1. Origin of unusual HREE-Mo-rich carbonatites in the Qinling orogen, China.

    PubMed

    Song, Wenlei; Xu, Cheng; Smith, Martin P; Kynicky, Jindrich; Huang, Kangjun; Wei, Chunwan; Zhou, Li; Shu, Qihai

    2016-11-18

    Carbonatites, usually occurring within intra-continental rift-related settings, have strong light rare earth element (LREE) enrichment; they rarely contain economic heavy REE (HREE). Here, we report the identification of Late Triassic HREE-Mo-rich carbonatites in the northernmost Qinling orogen. The rocks contain abundant primary HREE minerals and molybdenite. Calcite-hosted fluid inclusions, inferred to represent a magmatic-derived aqueous fluid phase, contain significant concentrations of Mo (~17 ppm), reinforcing the inference that these carbonatitic magmas had high Mo concentrations. By contrast, Late Triassic carbonatites in southernmost Qinling have economic LREE concentrations, but are depleted in HREE and Mo. Both of these carbonatite types have low δ(26)Mg values (-1.89 to -1.07‰), similar to sedimentary carbonates, suggesting a recycled sediment contribution for REE enrichment in their mantle sources. We propose that the carbonatites in the Qinling orogen were formed, at least in part, by the melting of a subducted carbonate-bearing slab, and that 10 Ma younger carbonatite magmas in the northernmost Qinling metasomatized the thickened eclogitic lower crust to produce high levels of HREE and Mo.

  2. Crustal structure of the alaska range orogen and denali fault along the richardson highway

    USGS Publications Warehouse

    Fisher, M.A.; Pellerin, L.; Nokleberg, W.J.; Ratchkovski, N.A.; Glen, J.M.G.

    2007-01-01

    A suite of geophysical data obtained along the Richardson Highway crosses the eastern Alaska Range and Denali fault and reveals the crustal structure of the orogen. Strong seismic reflections from within the orogen north of the Denali fault dip as steeply as 25?? north and extend downward to depths between 20 and 25 km. These reflections reveal what is probably a shear zone that transects most of the crust and is part of a crustal-scale duplex structure that probably formed during the Late Cretaceous. These structures, however, appear to be relict because over the past 20 years, they have produced little or no seismicity despite the nearby Mw = 7.9 Denali fault earthquake that struck in 2002. The Denali fault is nonreflective, but we interpret modeled magnetotelluric (MT), gravity, and magnetic data to propose that the fault dips steeply to vertically. Modeling of MT data shows that aftershocks of the 2002 Denali fault earthquake occurred above a rock body that has low electrical resistivity (>10 ohm-m), which might signify the presence of fluids in the middle and lower crust. Copyright ?? 2007 The Geological Society of America.

  3. Cenozoic landforms and post-orogenic landscape evolution of the Balkanide orogen: Evidence for alternatives to the tectonic denudation narrative in southern Bulgaria

    NASA Astrophysics Data System (ADS)

    Gunnell, Y.; Calvet, M.; Meyer, B.; Pinna-Jamme, R.; Bour, I.; Gautheron, C.; Carter, A.; Dimitrov, D.

    2017-01-01

    Continental denudation is the mass transfer of rock from source areas to sedimentary depocentres, and is typically the result of Earth surface processes. However, a process known as tectonic denudation is also understood to expose deep-seated rocks in short periods of geological time by displacing large masses of continental crust along shallow-angle faults, and without requiring major contributions from surface erosion. Some parts of the world, such as the Basin and Range in the USA or the Aegean province in Europe, have been showcased for their Cenozoic tectonic denudation features, commonly described as metamorphic core-complexes or as supradetachment faults. Based on 22 new apatite fission-track (AFT) and 21 helium (AHe) cooling ages among rock samples collected widely from plateau summits and their adjacent valley floors, and elaborating on inconsistencies between the regional stratigraphic, topographic and denudational records, this study frames a revised perspective on the prevailing tectonic denudation narrative for southern Bulgaria. We conclude that conspicuous landforms in this region, such as erosion surfaces on basement-cored mountain ranges, are not primarily the result of Paleogene to Neogene core-complex formation. They result instead from "ordinary" erosion-driven, subaerial denudation. Rock cooling, each time suggesting at least 2 km of crustal denudation, has exposed shallow Paleogene granitic plutons and documents a 3-stage wave of erosional denudation which progressed from north to south during the Middle Eocene, Oligocene, Early to Middle Miocene, and Late Miocene. Denudation initially prevailed during the Paleogene under a syn-orogenic compressional regime involving piggyback extensional basins (Phase 1), but subsequently migrated southward in response to post-orogenic upper-plate extension driven by trench rollback of the Hellenic subduction slab (Phase 2). Rare insight given by the denudation pattern indicates that trench rollback

  4. The early Cretaceous orogen-scale Dabieshan metamorphic core complex: implications for extensional collapse of the Triassic HP-UHP orogenic belt in east-central China

    NASA Astrophysics Data System (ADS)

    Ji, Wenbin; Lin, Wei; Faure, Michel; Shi, Yonghong; Wang, Qingchen

    2016-03-01

    The Dabieshan massif is famous as a portion of the world's largest HP-UHP metamorphic belt in east-central China that was built by the Triassic North-South China collision. The central domain of the Dabieshan massif is occupied by a huge migmatite-cored dome [i.e., the central Dabieshan dome (CDD)]. Origin of this domal structure remains controversial. Synthesizing previous and our new structural and geochronological data, we define the Cretaceous Dabieshan as an orogen-scale metamorphic core complex (MCC) with a multistage history. Onset of lithospheric extension in the Dabieshan area occurred as early as the commencement of crustal anatexis at the earliest Cretaceous (ca. 145 Ma), which was followed by primary (early-stage) detachment during 142-130 Ma. The central Dabieshan complex in the footwall and surrounding detachment faults recorded a consistently top-to-the-NW shearing. It is thus inferred that the primary detachment was initiated from a flat-lying detachment zone at the middle crust level. Removal of the orogenic root by delamination at ca. 130 Ma came into the extensional climax, and subsequently isostatic rebound resulted in rapid doming. Along with exhumation of the footwall, the mid-crustal detachment zone had been warped as shear zones around the CDD. After 120 Ma, the detachment system probably experienced a migration accommodated to the crustal adjustment, which led to secondary (late-stage) detachment with localized ductile shearing at ca. 110 Ma. The migmatite-gneiss with HP/UHP relicts in the CDD (i.e., the central Dabieshan complex) was product of the Cretaceous crustal anatexis that consumed the deep-seated part of the HP-UHP slices and the underlying para-autochthonous basement. Compared with the contemporaneous MCCs widely developed along the eastern margin of the Eurasian continent, we proposed that occurrence of the Dabieshan MCC shares the same tectonic setting as the "destruction of the North China craton". However, geodynamic trigger

  5. New constraints on fluid sources in orogenic gold deposits, Victoria, Australia

    NASA Astrophysics Data System (ADS)

    Fu, Bin; Kendrick, Mark A.; Fairmaid, Alison M.; Phillips, David; Wilson, Christopher J. L.; Mernagh, Terrence P.

    2012-03-01

    Fluid inclusion microthermometry, Raman spectroscopy and noble gas plus halogen geochemistry, complemented by published stable isotope data, have been used to assess the origin of gold-rich fluids in the Lachlan Fold Belt of central Victoria, south-eastern Australia. Victorian gold deposits vary from large turbidite-hosted `orogenic' lode and disseminated-stockwork gold-only deposits, formed close to the metamorphic peak, to smaller polymetallic gold deposits, temporally associated with later post-orogenic granite intrusions. Despite the differences in relative timing, metal association and the size of these deposits, fluid inclusion microthermometry indicates that all deposits are genetically associated with similar low-salinity aqueous, CO2-bearing fluids. The majority of these fluid inclusions also have similar 40Ar/36Ar values of less than 1500 and 36Ar concentrations of 2.6-58 ppb (by mass) that are equal to or much greater than air-saturation levels (1.3-2.7 ppb). Limited amounts of nitrogen-rich fluids are present at a local scale and have the highest measured 40Ar/36Ar values of up to 5,700, suggesting an external or distinct source compared to the aqueous fluids. The predominance of low-salinity aqueous-carbonic fluids with low 40Ar/36Ar values, in both `orogenic' and `intrusion-related' gold deposits, is attributed to fluid production from common basement volcano-sedimentary sequences and fluid interaction with sedimentary cover rocks (turbidites). Aqueous fluid inclusions in the Stawell-Magdala deposit of western Victoria (including those associated with N2) preserve mantle-like Br/Cl and I/Cl values. In contrast, fluid inclusions in deposits in the eastern structural zones, which contain more abundant shales, have elevated molar I/Cl ratios with maximum values of 5,170 × 10-6 in the Melbourne Zone. Br/I ratios in this zone range from 0.5 to 3.0 that are characteristic of fluid interaction with organic-rich sediments. The maximum I/Cl and characteristic

  6. Tectonic processes during oblique collision: Insights from the St. Elias orogen, northern North American Cordillera

    USGS Publications Warehouse

    Pavlis, T.L.; Picornell, C.; Serpa, L.; Bruhn, R.L.; Plafker, G.

    2004-01-01

    Oblique convergence in the St. Elias orogen of southern Alaska and northwestern Canada has constructed the world's highest coastal mountain range and is the principal driver constructing all of the high topography in northern North America. The orogen originated when the Yakutat terrane was excised from the Cordilleran margin and was transported along margin-parallel strike-slip faults into the subduction-transform transition at the eastern end of the Aleutian trench. We examine the last 3 m.y. of this collision through an analysis of Euler poles for motion of the Yakutat microplate with respect to North America and the Pacific. This analysis indicates a Yakutat-Pacific pole near the present southern triple junction of the microplate and' predicts convergence to dextral-oblique convergence across the offshore Transition fault, onland structures adjacent to the Yakutat foreland, or both, with plate speeds increasing from 10 to 30 mm/yr from southeast to northwest. Reconstructions based on these poles show that NNW transport of the collided block into the NE trending subduction zone forced contraction of EW line elements as the collided block was driven into the subduction-transform transition. This suggests the collided block was constricted as it was driven into the transition. Constriction provides an explanation for observed vertical axis refolding of both earlier formed fold-thrust systems and the collisional suture at the top of the fold-thrust stack. We also suggest that this motion was partially accommodated by lateral extrusion of the western portion of the orogen toward the Aleutian trench. Important questions remain regarding which structures accommodated parts of this motion. The Transition fault may have accommodated much of the Yakutat-Pacific convergence on the basis of our analysis and previous interpretations of GPS-based geodetic data. Nonetheless, it is locally overlapped by up to 800 m of undeformed sediment, yet elsewhere shows evidence of young

  7. Kinematics of post-orogenic extension and exhumation of the Taku Schist, NE Peninsular Malaysia

    NASA Astrophysics Data System (ADS)

    Md Ali, M. A.; Willingshofer, E.; Matenco, L.; Francois, T.; Daanen, T. P.; Ng, T. F.; Taib, N. I.; Shuib, M. K.

    2016-09-01

    Recent studies imply that the formation and evolution of many SE Asian basins was driven by extensional detachments or systems of low-angle normal faults that created significant crustal exhumation in their footwalls. In this context, the architecture of the Triassic Indosinian orogen presently exposed in Peninsular Malaysia is compatible with significant extension post-dating the orogenic event. In this study we performed a kinematic analysis based on fieldwork and microstructural observations in the Taku Schist, Kemahang granite and the surrounding Gua Musang sediments of northern Peninsular Malaysia in order to shed light on processes related to the build-up and subsequent demise of the Indosinian orogen. The first three phases of deformation were related to an overall period of E-W oriented contraction and burial metamorphism. These phases of deformation are characterized by isoclinal folding with flat lying axial plane cleavages (D1), asymmetrical folding, top-to-the-W-SW shearing (D2) and upright folding (D3). All are in general agreement with observations of the previously inferred Permo-Triassic Indosinian orogeny. During these times, the Taku Schist, a sequence of Paleozoic clastic sediments with mafic intercalations was metamorphosed to amphibolite facies. These rocks are most likely equivalent to the ones exposed in the Bentong-Raub suture zone. Structural relations suggest that the Triassic Kemahang pluton is syn-kinematic, which provides important constraints for the timing of these contractional events. We demonstrate that the overall shortening was followed by a hitherto undescribed extension in NW-SE direction resulting in the formation of a large-scale detachment, the Taku detachment, in northern Peninsular Malaysia. Extension probably reactivated the former subduction plane as a detachment and exhumed previously buried and metamorphosed rocks of similar lithological composition to the neighboring Bentong-Raub suture zone. Such a mechanism is

  8. Origin and structure of major orogen-scale exhumed strike-slip

    NASA Astrophysics Data System (ADS)

    Cao, Shuyun; Neubauer, Franz

    2016-04-01

    The formation of major exhumed strike-slip faults represents one of the most important dynamic processes affecting the evolution of the Earth's lithosphere and surface. Detailed models of the potential initiation and properties and architecture of orogen-scale exhumed strike-slip faults and how these relate to exhumation are rare. In this study, we deal with key properties controlling the development of major exhumed strike-slip fault systems, which are equivalent to the deep crustal sections of active across fault zones. We also propose two dominant processes for the initiation of orogen-scale exhumed strike-slip faults: (1) pluton-controlled and (2) metamorphic core complex-controlled strike-slip faults. In these tectonic settings, the initiation of faults occurs by rheological weakening along hot-to-cool contacts and guides the overall displacement and ultimate exhumation. These processes result in a specific thermal and structural architecture of such faults. These types of strike-slip dominated fault zones are often subparallel to mountain ranges and expose a wide variety of mylonitic, cataclastic and non-cohesive fault rocks, which were formed at different structural levels of the crust during various stages of faulting. The high variety of distinctive fault rocks is a potential evidence for recognition of these types of strike-slip faults. Exhumation of mylonitic rocks is, therefore, a common feature of such reverse oblique-slip strike-slip faults, implying major transtensive and/or transpressive processes accompanying pure strike-slip motion during exhumation. Some orogen-scale strike-slip faults nucleate and initiate along rheologically weak zones, e.g. at granite intrusions, zones of low-strength minerals, thermally weakened crust due to ascending fluids, and lateral borders of hot metamorphic core complexes. A further mechanism is the juxtaposition of mechanically strong mantle lithosphere to hot asthenosphere in continental transform faults (e.g., San

  9. Lawsonite Blueschists in Recycled Mélange Involved in K-Rich Orogenic Magmatism

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Prelevic, D.; Foley, S. F.; Buhre, S.; Galer, S. J. G.

    2014-12-01

    The origin of K-rich orogenic magmatism in the Alpine-Himalayan belt and its relationship to the large-scale elevations in several massifs of the orogen is controversial, particularly the significance of the widespread presence of a geochemical signal typical for recycled continental crust. Two competing scenarios invoke direct melting of continental crust during deep intercontinental subduction and removal of heavily metasomatised mantle lithosphere by delamination into the convecting mantle. Here we investigate the coupling of high Th/La ratio with crustal isotopic signatures in K-rich orogenic lavas that does not occur in volcanic rocks from other collisional environments to distinguish between these two models. High-pressure experimental results on a phyllite representing upper crustal composition and a detailed mineral and geochemical study of blueschists from Tavşanlı mélange, Turkey, indicate that this geochemical fingerprint originates by melting of subducted mélange. Melting of crust at the top of the subducted continental lithosphere cannot produce observed fingerprint, whereas lawsonites, especially those with terrigenous sediment origin from blueschists with high Th/La can. Lawsonites that grow in various components of a subduction mélange inherit the geochemical characteristics of either oceanic or continental protoliths. It is currently uncertain whether those carrying the high Th/La signature originate by direct melting of continental blocks in the mélange or by the introduction of supercritical fluids from lawsonite blueschist of continental origin that infiltrate oceanic sediment blocks. Either way, the high Th/La is later released into subsequently formed melts. This confirms the supposition that lawsonite is the main progenitor of the high Th/La and Sm/La ratio. However, lawsonite must break down completely to impart this unique feature to subsequent magmas. The source regions of the potassic volcanic rocks consist of blueschist facies m

  10. Alpine-type tectonics in the Paleoproterozoic Lapland-Kola Orogen

    NASA Astrophysics Data System (ADS)

    Mudruk, S. V.; Balagansky, V. V.; Gorbunov, I. A.; Raevsky, A. B.

    2013-07-01

    The Kola region in the northeastern Baltic Shield is characterized by diverse Paleoproterozoic collision processes. The Keivy Terrane is one of the major tectonic units in the northeastern foreland of the Paleoproterozoic Lapland-Kola Collisional Orogen, which markedly differs in a number of parameters from other tectonic units of the Kola region. The study of the Keivy Terrane allowed us to unravel one more basic difference: the large Paleoproterozoic sheath synform of the Serpovidny (Crescentic) Range localized in this terrane. Its core is occupied by volcanic and sedimentary rocks, which correlate with the fill of the Imandra-Varzuga Rift; the limbs are composed of metamorphosed mature sedimentary rocks known as Keivy paraschists of Neoarchean or Paleoproterozoic age. The lower limb of the Serpovidny Synform is strongly squeezed, whereas the upper limb consists of almost undeformed rocks. The deformed rocks underwent ductile flow under conditions of simple or general shear. In the degree of its asymmetry and main parameters, the Serpovidny Synform is similar to the plunging and recumbent anticlines in the Helvetic nappes of the Alps. It is concluded that the Paleoproterozoic core of the Serpovidny Sheath Synform, or plunging anticline, is a fragment of the almost completely eroded deep Serpovidny Nappe of the Helvetic type. During the collision related to the Lapland-Kola Orogeny (1.9-2.0 Ga), this nappe was pushed out northward from the Paleoproterozoic Imandra-Varzuga Rift, which is situated 50 km south of the Serpovidny structure, and thrust over the Keivy paraschists. The latter, together with underlying the Lebyazhka Gneiss, were folded in the process of thrusting and were involved in the structure of the Serpovidny Synform. The Keivy paraschists make up a para-autochthon or a separate nappe of the Pennine type. The Archean Lebyazhka metafelsic volcanics underlie the Keivy paraschists and overlie granitoids of the Archean basement that remained undeformed

  11. Lateral flow in the middle crust - Analogue experiments from the Svecofennian orogen

    NASA Astrophysics Data System (ADS)

    Nikkilä, Kaisa; Koyi, Hemin; Korja, Annakaisa; Eklund, Olav

    2013-04-01

    The exposed Svecofennian crust (50-65 km) has been suggested to have thickened in continental accretion between Archean and Paleoproterozoic terranes, probably at a high convergence rate. It is likely that this thickened orogen experienced lateral spreading during its final stages. This post-orogenic event has reshaped the collisional framework and modified its bulk appearance. In this study, we have used scaled analogue centrifuge modeling to simulate extensional lateral flow at the Archean- Paleoproterozoic boundary zone during final stages of the Svecofennian orogeny. The analogue models simulate both the evolution of a mechanical boundary between two rheologically different tectonic blocks, and the role of pre-existing weaknesses at moderate angles (representing the old stacking structures). In models the upper layer is brittle, the middle layer is ductile, and the lower layer is more viscous. The layers represent upper, middle and lower crust, respectively. The Proterozoic layers have lower viscosity values than the Archean layers at similar depths. The materials are based on the plastilina modelling putty, which is mixed with acid oil, silicone, sweetener and/or barium sulphate to get the appropriate composition for each layer. Both the Archean and the Paleoproterozoic blocks have a low-viscous middle crust. The three layered models are extended unilaterally. The model results show that during extension the rheologically different layers deform and spread at different rates during the tectonic collapse. This results in 1) vertical rotation of the Archean and Proterozoic boundary; 2) the pre-existed faults become listric and discontinuous; and 3) the upward flow of the low viscosity middle layer to fill the newly-formed gaps between the upper layer blocks. The experiments show geometrically similar crustal-scale structures to those observed in the deep seismic reflection profiles (FIRE). Thus it is possible that lateral flow has taken place in the core of the

  12. Origin of allanite in gneiss and granite in the Dabie orogenic belt, Central East China

    NASA Astrophysics Data System (ADS)

    Guo, Haihao; Xiao, Yilin; Xu, Lijuan; Sun, He; Huang, Jian; Hou, Zhenhui

    2017-03-01

    Allanite is a common accessory mineral phase, representing an important carrier of rare earth elements, Th, U, Sr and other trace elements in most continental rocks. As Th and U can be incorporated into the allanite lattice, the mineral is a good geochronological tool for constraining geological events. Moreover, the trace element features δEu, Th/U ratio and common lead content of allanite are indicators of the forming conditions. Allanite and coexisting epidote-group minerals are abundant in ultrahigh-pressure (UHP) metamorphic rocks from the Dabie-Sulu orogen in central East China. However, if these minerals formed in the Neoproterozoic as magmatic phases, or in the Triassic as metamorphic phases is a matter of long-standing controversy. We report major and trace element analyses of whole rocks, allanite and coexisting epidote-group minerals, together with U-Th-Pb isotopic compositions of allanite in UHP gneiss from the Dabie-Sulu orogen, and allanite in the adjacent Jingshan granite. The granite is emplaced along the southeastern margin of the North China Craton and considered a product of partial melting of the subducted Dabie-Sulu gneiss. Trace elements (low Th/U and La/Sm, high δEu and high Sr) and high common lead concentrations indicate a metamorphic origin of allanite-epidote in the UHP gneiss. On the other hand, coarse-grained allanite from the Jingshan granite shows a corrosion core and a magmatic rim with common 208Pb up to 70% in the core and less than 30% in the rim. The allanite cores are of peritectic and the rims of magmatic origin with ages of ∼160 Ma, consistent with the granite crystallization age. In combination with previous studies, we conclude that the allanite of the Jingshan granite has form from the subducted and remolten Dabie-Sulu gneiss. Allanite records Triassic UHP metamorphic ages as well as Jurassic peritectic-magmatic ages as a part of the evolution of the Dabie-Sulu orogen.

  13. Climate Variability and Surface Processes in Tectonically Active Orogens: Insights From the Southern Central Andes and the Northwest Himalaya

    NASA Astrophysics Data System (ADS)

    Strecker, M. R.; Bookhagen, B.

    2008-12-01

    The Southern Central Andes of NW Argentina and the NW Himalaya are important orographic barriers that intercept moisture-bearing winds associated with monsoonal circulation. Changes in both atmospheric circulation systems on decadal to millennial timescales fundamentally influence differences in the amount and location of rainfall in both orogens. In India, the eastern arm of the monsoonal circulation draws moisture from the Bay of Bengal and transports humid air masses along the southern Himalayan front to the northwest. There, at the end of the monsoonal conveyer belt, rainfall is diminished and moisture typically does not reach far into the orogen interior. Similar conditions apply to the NW Argentine Andes, which are located within the precipitation regime of the South American Monsoon. Here, pronounced local relief blocks humid air masses from the Amazon region, resulting in extreme gradients in rainfall that leave the orogen interior dry. However, during negative ENSO years (La Niña) and intensified Indian Summer Monsoon years, moisture penetrates farther into the Andean and Himalayan orogens, respectively. Structurally pre- conditioned valley systems may enhance this process and funnel moisture far into the orogen interior. The greater availability of moisture increases runoff, lateral scouring of mountin streams, and ultimately triggers intensified hillslope processes on decadal to centennial timescales. In both environments, the scenario of intensified present-day surface processes and rates is analogous to protracted episodes of enhanced mass removal from hillslopes via deep-seated landslides during the early Holocene and late Pleistocene. Apparently, these episodes were also associated with transient storage of voluminous conglomerates and lacustrine deposits in narrow intermontane basins. Subsequently, these deposits were incised, partly removed, and the fluvial systems adjusted themselves to the pre-depositional base levels through a readjustment and

  14. Hillslope response to knickpoint migration in the Southern Appalachians: Implications for the evolution of post-orogenic landscapes

    USGS Publications Warehouse

    Wegmann, S.F.G.; Franke, K.L.; Hughes, S.; Lewis, R.Q.; Lyons, N.; Paris, P.; Ross, K.; Bauer, J.B.; Witt, A.C.

    2011-01-01

    The southern Appalachians represent a landscape characterized by locally high topographic relief, steep slopes, and frequent mass movement in the absence of significant tectonic forcing for at least the last 200 Ma. The fundamental processes responsible for landscape evolution in a post-orogenic landscape remain enigmatic. The non-glaciated Cullasaja River basin of south-western North Carolina, with uniform lithology, frequent debris flows, and the availability of high-resolution airborne lidar DEMs, is an ideal natural setting to study landscape evolution in a post-orogenic landscape through the lens of hillslope-channel coupling. This investigation is limited to channels with upslope contributing areas >2.7 km2, a conservative estimate of the transition from fluvial to debris-flow dominated channel processes. Values of normalized hypsometry, hypsometric integral, and mean slope vs elevation are used for 14 tributary basins and the Cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration. Results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the Cullasaja basin and adjacent hillslopes. Metrics of topography (relief, slope gradient) and hillslope activity (landslide frequency) exhibit significant downstream increases below the current position of major knickpoints. The transient effect of knickpoint-driven channel incision on basin hillslopes is captured by measuring the relief, mean slope steepness, and mass movement frequency of tributary basins and comparing these results with the distance from major knickpoints along the Cullasaja River. A conceptual model of area-elevation and slope distributions is presented that may be representative of post-orogenic landscape evolution in analogous geologic settings. Importantly, the model explains how knickpoint migration and channel- hillslope coupling is an important factor in tectonically-inactive (i

  15. Paleozoic accretionary orogenesis in the eastern Beishan orogen: constraints from zircon U-Pb and 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Ao, Songjian; Xiao, Wenjiao; Windley, Brian; Mao, Qigui

    2016-04-01

    The continental growth mechanism of the Altaids in Central Asia is still in controversy between models of continuous subduction-accretion versus punctuated accretion by closure of multiple oceanic basins. The Beishan orogenic belt, located in the southern Altaids, is a natural laboratory to address this controversy. Key questions that are heavily debated are: the closure time and subduction polarity of former oceans, the emplacement time of ophiolites, and the styles of accretion and collision. This paper reports new structural data, zircon ages and Ar-Ar dates from the eastern Beishan Orogen that provide information on the accretion process and tectonic affiliation of various terranes. Our geochronological and structural results show that the younging direction of accretion was northwards and the subduction zone dipped southwards under the northern margin of the Shuangyingshan micro-continent. This long-lived and continuous accretion process formed the Hanshan accretionary prism. Our field investigations show that the emplacement of the Xiaohuangshan ophiolite was controlled by oceanic crust subduction beneath the forearc accretionary prism of the Shuangyingshan-Mazongshan composite arc to the south. Moreover, we address the age and terrane affiliation of lithologies in the eastern Beishan orogen through detrital zircon geochronology of meta-sedimentary rocks. We provide new information on the ages, subduction polarities, and affiliation of constituent structural units, as well as a new model of tectonic evolution of the eastern Beishan orogen. The accretionary processes and crustal growth of Central Asia were the result of multiple sequences of accretion and collision of manifold terranes. Reference: Ao, S.J., Xiao, W., Windley, B.F., Mao, Q., Han, C., Zhang, J.e., Yang, L., Geng, J., Paleozoic accretionary orogenesis in the eastern Beishan orogen: Constraints from zircon U-Pb and 40Ar/39Ar geochronology. Gondwana Research, doi: http://dx.doi.org/10.1016/j

  16. Carboniferous rifted arcs leading to an archipelago of multiple arcs in the Beishan-Tianshan orogenic collages (NW China)

    NASA Astrophysics Data System (ADS)

    Tian, Zhonghua; Xiao, Wenjiao; Windley, Brian F.; Zhang, Ji'en; Zhang, Zhiyong; Song, Dongfang

    2016-12-01

    The Beishan and East Tianshan Orogenic Collages in the southernmost Central Asian Orogenic Belt (CAOB) record the final stages of evolution of the Paleo-Asian Ocean. These collages and their constituent arcs have an important significance for resolving current controversies regarding their tectonic setting and age, consequent accretionary history of the southern CAOB, and the closure time of the Paleo-Asian Ocean. In this paper, we present our work on the southern Mazongshan arc and the northern Hongyanjing Basin in the Beishan Orogenic Collage (BOC), and our comparison with the Bogda arc and associated basins in the East Tianshan Orogenic Collage. Field relationships indicate that the Pochengshan fault defines the boundary between the arc and basin in the BOC. Volcanic rocks including basalts and rhyolites in the Mazongshan arc have bimodal calc-alkaline characteristics, an enrichment in large ion lithophile elements such as Rb, Ba, and Pb and depletion in high field-strength elements (e.g., Nb and Ta), which were probably developed in a subduction-related tectonic setting. We suggest that these bimodal calc-alkaline volcanic rocks formed in rifted arcs instead of post-orogenic rifts with mantle plume inputs. By making detailed geochemical comparisons between the Mazongshan arc and the Bogda arc to the west, we further propose that they are similar and both formed in arc rifts, and helped generate a Carboniferous archipelago of multiple arcs in the southern Paleo-Asian Ocean. These data and ideas enable us to postulate a new model for the tectonic evolution of the southern CAOB.

  17. Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway.

    PubMed

    Spengler, Dirk; van Roermund, Herman L M; Drury, Martyn R; Ottolini, Luisa; Mason, Paul R D; Davies, Gareth R

    2006-04-13

    The buoyancy and strength of sub-continental lithospheric mantle is thought to protect the oldest continental crust (cratons) from destruction by plate tectonic processes. The exact origin of the lithosphere below cratons is controversial, but seems clearly to be a residue remaining after the extraction of large amounts of melt. Models to explain highly melt-depleted but garnet-bearing rock compositions require multi-stage processes with garnet and clinopyroxene possibly of secondary origin. Here we report on orogenic peridotites (fragments of cratonic mantle incorporated into the crust during continent-continent plate collision) from Otrøy, western Norway. We show that the peridotites underwent extensive melting during upwelling from depths of 350 kilometres or more, forming a garnet-bearing cratonic root in a single melting event. These peridotites appear to be the residue after Archaean aluminium depleted komatiite magmatism.

  18. Drainage Pattern, Along-Strike Topography and Three-Dimensional Construction of the Himalayan orogen

    NASA Astrophysics Data System (ADS)

    Yin, A.

    2005-12-01

    Past studies on the Himalayan orogen have mostly emphasized its 2-D evolution in cross-section view. As a result, how the orogen has grown in 3-D remains poorly understood. For example, it is not clear if the >1500-km long Main Central Thrust (MCT) and Main Boundary Thrust (MBT) were initiated synchronously along the whole orogen or started at one segment and then propagated laterally during subsequent fault motion. To clarify this issue, I examined the Himalayan drainage pattern, along-strike topography, and geometry and kinematics of growing contractional structures across both the Himalayan front and the Shillong Plateau. The main observations may be summarized as follows. (1) The five rivers that cut across the Himalaya are arranged approximately symmetric with respect to the Himalayan-arc axis (~85° E): the Arun River (87° E) is in the middle with the Sutlej (77° E) and Indus (72° E) Rivers in the west and the Subansiri (93° E) and Yalu-Brahmaputra (96° E) Rivers in the east. (2) Between the eastern and western syntaxes, south-flowing drainages east of 85.5° E are consistently deflected to the east by east-growing anticlines and thrusts, whereas drainages west of 85.5°E are deflected systematically to the west by west-growing anticlines and thrusts along the Himalayan front. The only region where no drainage deflection is observed is the Bhutan Himalaya. There all rivers flow straight across the Himalayan front. (3) The deflected drainage pattern indicates that the Shillong Plateau south of the eastern Himalaya has been growing westward. (4) The along-crest Himalayan topographic profile concaves downward, starting from ~5200 m just inside the two syntaxes and reaching 8848 m at Mt. Everest (~87° E). (5) There are a total of 17 major growing contractional structures in the Main Frontal Thrust Zone (MFTZ), with 10 in the west and 7 in the east. Each structure has a length between ~20 km and >150 km, but the west-growing structures in the west are

  19. Active Tectonics in crossroads of an evolving orogen and morphological consequences: Anatolia

    NASA Astrophysics Data System (ADS)

    Koral, Hayrettin

    2016-04-01

    Anatolia lies in a curved setting of the active Alpine Mountain Range and is located in crossroads of the European and Asian terrains. It is one of the fastest deforming land in the world, manifested by seismicity, characteristic landforms and GPS measurements. Active tectonics in Anatolia provides not only a comparable geological model for the past orogens, but also a laboratory case for morphological consequences of an orogenic processes. Anatolia comprise different tectonic subsettings with its own characteristics. Northern part is influenced by tectonic characteristics of the Black Sea Basin, the Pontides and the Caucasian Range; northwestern part by the Balkanides; eastern-southeastern part by the Bitlis-Zagros suture; and south-southwestern part by the eastern Mediterranean subduction setting. Much of its present tectonic complexity was inherited from the convergence dominant plate tectonic setting of the platelets prior to the Middle-Neogene. Beginning about 11 Ma ago, the deformed and uplifted landmass unable to accommodate further deformation in Anatolia and ongoing tectonic activity gave rise to rearrangement of tectonic forces and westerly translational movements. Formation of major strike-slip faults in Anatolia including the North and East Anatolian Faults and a new platelet called the Anatolian Plate are the consequences of this episode. Such change in the tectonic regime has led to modification of previously-formed landscape, modification and sometimes termination of previously-formed basins. Evidence is present in the Plio-Quaternary stratigraphy, tectonic characteristics and morphology of the well-studied areas. This presentation will discuss active tectonic features of the northwestern, southwestern and eastern Anatolian subsettings and their influence on morphology that is closely related to sites of pre-historical human settlement.

  20. Self-consistent orogenic wedge formation and shear zone propagation due to thermal softening

    NASA Astrophysics Data System (ADS)

    Jaquet, Yoann; Duretz, Thibault; Schmalholz, Stefan M.

    2016-04-01

    We present two dimensional numerical simulations of orogenic wedge formation for a viscoelastoplastic lithosphere under compression. The thermo-mechanical model is based on the principle of energy conservation and includes temperature-dependent rheologies. With this approach, shear zones caused by thermal softening develop spontaneously in the absence of strain softening. The initial locus of shear localization is controlled by either lateral temperature variations (100°C) at the model base or by lateral variations in crustal thickness. The first episode of strain localization occurs after 15% bulk shortening. With ongoing strain, a series of shear zones arise and propagate towards the foreland leading to the self-consistent formation of an orogenic wedge. We investigate the impact of bulk shortening rates, erosion and rheology on the dynamics of wedge formation, the associated topography and uplift rates. The maximum topography reaches up to 10 km and the surface morphology evolves according to shear zone activation and deactivation. Uplift rates are transient and peak values are maintained only on very short time scales. A running average of the uplift rate versus time curves with a time-window of 4 My provides average uplift rates in the order of a few millimeters per year. Erosion is an important parameter for the formation and the evolution of the wedge (e.g. can control the spacing of shear zones by modifying crustal thickness). Rheological parameters, such as the friction angle or the upper crustal viscosity, control the occurrence of strain localization. Bulk shortening rates between 10-15 and 10-16 s-1 do not have a major impact on the resulting wedge structure.

  1. Iberseis: A Deep Seismic Reflection Image of A Variscan Transpresive Orogen

    NASA Astrophysics Data System (ADS)

    Carbonell, R.; Simancas, F.; Juhlin, C.; Ayarza, P.; Gonzalez-Loderio, F.; Perez-Estaun, A.; Plata, J.; Iberseis Group

    As part of EUROPROBE's SW Iberia project a 303 km long deep seismic reflection profile was acquired along one of the most complete transects of the Variscan Oro- gen providing key knowledge on fundamental questions about the dynamics of the southwestern European lithosphere. The seismic profile (IBERSEIS) crosses two su- tures that separate three main tectonic terranes which are: South Portuguese Zone, Ossa-Morena Zone and Central Iberian Zone. A prominent mid- lower crustal seismic fabric is observed along the involved terranes. The geometry of the most prominent surface sutures such as the Beja-Acebuches ophiolite and the Badajoz-Córdoba shear zone are clearly identified. A well defined horizontal Moho reveals an approximately constant crustal thikness of 30-33 km along the entire profile. The South Portuguese Zone features a typical geometry of a thrust and fold belt architecture. The high res- olution achieved by the relatively close receiver and shot spacing has imaged very detailed structures within this thin skinned tectonic unit, including thust staks and du- plexes which implying a realtively large crustal shortning. Good structural detail has also been obtained in the Ossa-Morena Zone, which is characterized by the presence of folded recumbent folds and thrusts. At mid-crutal level a high 2-3 s thick band of high reflectivity has been identified across the Ossa-Morena and Central Iberian Zones which is a unique feature among tanspresive orogens. This seismic image provably re- veals the strain partitioning during the oblique transpression tectonics that build up the orogen.

  2. Retrodeformable cross sections for 3-dimensional structural analysis, Ouachita orogen, Arkansas

    NASA Astrophysics Data System (ADS)

    Johnson, H. E.; Wiltschko, D. V.

    2010-12-01

    A fundamental tectonic problem is how deformation proceeds from hinterland to foreland in a fold and thrust belt (FTB). Wedge models explain many of the first-order observations found in most FTBs such as the internal deformation of material, thickening of hinterland, presence of a basal décollement, and an overall wedge shape that tapers to the foreland. These models currently have not been tested at the scale of the individual folds and faults. Moreover, most of the data available on, for instance, the sequence of events is best dated in the syntectonic sediments. Timing of uplift and motion of interior structures are not clear when using dates from these syntectonic sediments to some extent because an absolute connection between them is lacking. The purpose of this project is to develop a model for the evolution of the Ouachita orogen through the construction of a series of retrodeformable cross sections. A novel aspect of these cross sections is the combination of new and published thermal (i.e., illite ‘crystallinity’) and thermochronologic (i.e., zircon fission track) data collected at a variety of stratigraphic depths along the lines of section. These data will help to determine the cessation of thrust motion as well as the initial depth from which the thrust sheet emerged. An Ordovician Mazarn sample in the eastern exposed orogenic core has zircon grains with 55% reset fission track ages, whereas an overlying Ordovician Blakely sample about ~30 km to the southwest along strike has 15% being reset. Illite ‘crystallinity’ (IC) values indicate maximum burial metamorphism temperatures of anchizone (~250-350°C) coinciding with the location of the Ordovician Mazarn sample. Regionally, IC decreases from the culmination of the Benton Uplift and to the southwest along regional strike for samples that have similar stratigraphic age. These new timing and thermal constraints on an improved kinematic model are the necessary first steps in testing wedge models

  3. Sand petrology and focused erosion in collision orogens: the Brahmaputra case

    NASA Astrophysics Data System (ADS)

    Garzanti, Eduardo; Vezzoli, Giovanni; Andò, Sergio; France-Lanord, Christian; Singh, Sunil K.; Foster, Gavin

    2004-03-01

    The high-relief and tectonically active Himalayan range, characterized by markedly varying climate but relatively homogeneous geology along strike, is a unique natural laboratory in which to investigate several of the factors controlling the composition of orogenic sediments. Coupling of surface and tectonic processes is most evident in the eastern Namche Barwa syntaxis, where the Tsangpo-Siang-Brahmaputra River, draining a large elevated area in south Tibet, plunges down the deepest gorge on Earth. Here composition of river sands changes drastically from lithic to quartzofeldspathic. After confluence with the Lohit River, draining the Transhimalayan-equivalent Mishmi arc batholiths, sediment composition remains remarkably constant across Assam, indicating subordinate contributions from Himalayan tributaries. Independent calculations based on petrographical, mineralogical, and geochemical data indicate that the syntaxis, representing only ∼4% of total basin area, contributes 35±6% to the total Brahmaputra sediment flux, and ∼20% of total detritus reaching the Bay of Bengal. Such huge anomalies in erosion patterns have major effects on composition of orogenic sediments, which are recorded as far as the Bengal Fan. In the Brahmaputra basin, in spite of very fast erosion and detrital evacuation, chemical weathering is not negligible. Sand-sized carbonate grains are dissolved partially in mountain reaches and completely in monsoon-drenched Assam plains, where clinopyroxenes are selectively altered. Plagioclase, instead, is preferentially weathered only in detritus from the Shillong Plateau, which is markedly enriched in microcline. Most difficult to assess is the effect of hydraulic sorting in Bangladesh, where quartz, garnet and epidote tend to be sequestered in the bedload and trapped on the coastal plain, whereas cleavable feldspars and amphiboles are concentrated in the suspended load and eventually deposited in the deep sea. High-resolution petrographic and

  4. Seismological structure of the 1.8 Ga Trans-Hudson Orogen of North America

    NASA Astrophysics Data System (ADS)

    Gilligan, Amy; Bastow, Ian D.; Darbyshire, Fiona A.

    2016-06-01

    Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates—the ˜1.8 Ga Trans-Hudson Orogeny (THO)—and is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (˜39 km) and structurally simple, with a sharp Moho; upper-crustal wave speed variations are attributed to postformation events. However, the Quebec-Baffin segment of the THO has a deeper Moho (˜45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present-day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present-day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet.

  5. Asymmetric vs. symmetric deep lithospheric architecture of intra-plate continental orogens

    NASA Astrophysics Data System (ADS)

    Calignano, Elisa; Sokoutis, Dimitrios; Willingshofer, Ernst; Gueydan, Frédéric; Cloetingh, Sierd

    2015-08-01

    The initiation and subsequent evolution of intra-plate orogens, resulting from continental plate interior deformation due to transmission of stresses over large distances from the active plate boundaries, is controlled by lateral and vertical strength contrasts in the lithosphere. We present lithospheric-scale analogue models combining 1) lateral strength variations in the continental lithosphere, and 2) different vertical rheological stratifications. The experimental continental lithosphere has a four-layer brittle-ductile rheological stratification. Lateral heterogeneity is implemented in all models by increased crustal strength in a central narrow block. The main investigated parameters are strain rate and strength of the lithospheric mantle, both playing an important role in crust-mantle coupling. The experiments show that the presence of a strong crustal domain is effective in localizing deformation along its boundaries. After deformation is localized, the evolution of the orogenic system is governed by the mechanical properties of the lithosphere such that the final geometry of the intra-plate mountain depends on the interplay between crust-mantle coupling and folding versus fracturing of the lithospheric mantle. Underthrusting is the main deformation mode in case of high convergence velocity and/or thick brittle mantle with a final asymmetric architecture of the deep lithosphere. In contrast, lithospheric folding is dominant in case of low convergence velocity and low strength brittle mantle, leading to the development of a symmetric lithospheric root. The presented analogue modelling results provide novel insights for 1) strain localization and 2) the development of the asymmetric architecture of the Pyrenees.

  6. Southern Costa Rica: an Island-arc Segment That Behaves Like a Doubly Vergent Orogen

    NASA Astrophysics Data System (ADS)

    Brandes, C.; Winsemann, J.

    2007-12-01

    Southern Central America is a Mesosoic/Cenozoic island-arc that evolved from the subduction of the Farallón Plate below the Caribbean Plate. The southern Costa Rican land-bridge comprises deformed fore-arc and back-arc basins in the west and east, respectively, separated by the up to 3.8 km high Talamanca Range. The structure of the southern Central American island-arc is similar to doubly vergent and asymmetric orogens. The deformed fore-arc basin in the west and the Limon fold-and-thrust belt in the east can be interpreted as pro-wedge and retro-wedge, respectively. The Talamanca Range represents the uplifted block in between. The pro-wedge is wider and has a lower slope angle than the retro-wedge. The uplift of the Talamanca Range is probably related to a system of conjugate shear zones. Precipitation is unevenly distributed, with orographic effects concentrating precipitation in SW Costa Rica, which has caused pro-wedge denudation, leading to exhumation of granitic rocks at in the interior of the mountain range. The large-scale structure of the Central American island-arc in southern Costa Rica can be described using models of continental collision zones. Previous studies attributed the deformation and uplift pattern to the subduction/collision of the Cocos Ridge. Another reasonable driving mechanism for the evolution of such an orogen in an oceanic island-arc setting is the basal traction due to long-term subduction of the Cocos Plate at a very low angle.

  7. Some comparisons of the structure and evolution of the southern Appalachian Ouachita orogen and portions of the Trans-European Suture Zone region

    NASA Astrophysics Data System (ADS)

    Keller, G. Randy; Hatcher, Robert D.

    1999-12-01

    Many aspects of the tectonic history of the Appalachian-Ouachita orogen and the Trans-European suture region are similar. In a tectonic sense, they are geographically linked, because at the end of the Paleozoic, one could travel from Texas to Poland following a continuous orogenic belt. Much of each of these orogenic belts is, however, buried by younger strata, and thus geophysical studies are a key element of efforts to better understand them. Many more geophysical data are needed to provide a comprehensive picture of the deep structure, but in several areas new models of the lithospheric structure are emerging. In this paper, we review some of these models and introduce two new ones for the southern Appalachians. In addition, we explore the implications of these models for the evolution of these orogenic belts, drawing attention to similarities and differences in their structure and evolution. The Appalachian and Caledonian orogens represent temporally parallel development along irregular Late Proterozoic continental margins, but the crustal structure is different because of different accretionary elements and complications in the geometry of the original margins. Lateral transport via thrusting can confuse efforts to tie geologic structures identified on the surface with deep structures. For example, in the Appalachian-Ouachita orogen, the thrust-related transport distance for major features such as the Blue Ridge-Piedmont sheet and the Benton uplift is at least 200 km. On a crustal scale, deformation in the Appalachian-Ouachita orogen varies greatly in style and intensity. In the Ouachita orogen, the Late Proterozoic continental margin seems to be preserved, while in the northern Appalachians crustal-scale deformation is intense. Along the US Appalachians, the major crustal structure variation is the thin, dense crust of Avalonia abutting the relatively thick crust of Laurentia. Along the Ouachita orogen, several large crustal blocks can be identified that are

  8. The Tyrrhenian stage geodinamic evolution of Apenninic-Maghrebian orogen (Southern Apennines and Sicily)

    NASA Astrophysics Data System (ADS)

    Lentini, F.; Carbone, S.; Barreca, G.

    2009-04-01

    In the Central Mediterranean region the foreland domains are represented by two continental blocks, the Apulian Block to the north and the Pelagian Block to the south, respectively belonging to the Adria and to the Africa plates. They are separated since Permo-Triassic times by the oceanic crust of the Ionian Sea. The Apenninic-Maghrebian orogen is located between two oceanic crusts: the old Ionian crust, at present time subducting beneath the Calabrian Arc, and the new crust of the opening Tyrrhenian Sea. The orogenic belt is represented by a multilayer allochthonous edifice, composed of the Calabride Chain (CC) tectonically overlying the Apenninic-Maghrebian Chain (AMC), which in turn overthrust onto the Upper Miocene and Pliocene top-levels of a deep seated thrust system, originating by the deformation of the innermost carbonates of the Pelagian/Apulian blocks (External Thrust System: ETS). The AMC tectonic units derive from the orogenic transport during Oligo-Miocene times of sedimentary sequences deposited in palaeogeographical domains located between the Europe and the Afro-Adriatic plates. These units are composed of Meso-Cenozoic shallow-water carbonate successions detached from a continental type crust sector, the Panormide/Apenninic Block, recognizable by means of seismic lines shot in the Tyrrhenian offshore of Southern Apennines and Northern Sicily. The Meso-Cenozoic basinal units, that compose the AMC, can be distinguished into two main groups of sequences, originally located on oceanic crusts separated by the Panormide/Apenninic Block: the external ones (Ionides) related to an original basin belonging to branches of the Ionian Palaeobasin involved in the orogenesis, and the internal ones ascribed to the Alpine Tethys (Sicilide Units). The terrigenous deposits of the basinal sequences belonging to the Ionides are represented by Tertiary foreland/foredeep deposits, whose relationships with the substratum are occasionally preserved, although large

  9. Rivers, re-entrants, and 3D variations in orogenic wedge development: a case study of the NW Indian Himalaya

    NASA Astrophysics Data System (ADS)

    Webb, A. G.; Yu, H.; Hendershott, Z.

    2010-12-01

    Orogenic wedges are standard elements of collisional plate tectonics, from accretionary prisms to retro-arc basins. Recent study of orogenic wedge development has focused on links between mechanisms of internal deformation and surface processes. Models of orogenic wedges are commonly presented in the cross-section plane, which is generally effective as wedges largely develop via plane strain. The 3rd dimension can be utilized to explore effects of differences in controlling parameters on wedge evolution. We are investigating a stretch of the western Himalayan orogenic wedge that has two prominent changes in along-strike morphology: (1) a tectonic window (the Kullu Window) that appears to be strongly influenced by erosion along the 3rd largest river in the Himalayan system, the Sutlej River and (2) the Kangra Re-entrant, the largest re-entrant along the Himalayan arc. In addition to the along-strike heterogeneity, a key advantage of the proposed study area is its rich stratigraphy, with the most known diversity in the Himalayan arc. The stratigraphic wealth, combined with the along-strike heterogeneity in exposure level, offers a high resolution view of regional structural geometry. Our preliminary reconstructions suggest that the Sutlej River erosion increases the exposure depth and shortening budget across a narrow segment of the orogen, strongly warping the Kullu Window. Previous models have suggested that the out-of-sequence Munsiari thrust is the main structure associated with Kullu window formation, while our work suggests that most of this uplift and warping is accomplished by antiformal stacking of basement thrust horses. Late Miocene ages (U-Pb ages of zircons and Th-Pb ages of monazites) from a leucogranite in the core of the Kullu Window along the Sutlej River further suggests that this segment of the orogen represents a middle ground between plane strain orogenic wedge development and a tectonic aneurysm model. We have constructed a palinspastic

  10. Cycle Analysis

    SciTech Connect

    Wright, Steven A.

    2012-03-20

    1. The Cycle Analysis code is an Microsoft Excel code that performs many different types of thermodynamic cycle analysis for power producing systems. The code will calculate the temperature and pressure and all other thermodynamic properties at the inlet and outlet of each component. The code also calculates the power that is produced, the efficiency, and the heat transported in the heater, gas chiller and recuperators. The code provides a schematic of the loop and provides the temperature and pressure at each location in the loop. The code also provides a T-S (temperature-entropy) diagram of the loop and often it provides an pressure enthalpy plot as well. 2. This version of the code concentrates on supercritical CO2 power cycles, but by simply changing the name of the working fluid many other types of fluids can be analyzed. The Cycle Analysis code provided here contains 18 different types of power cycles. Each cycle is contained in one worksheet or tab that the user can select. The user can change the yellow highlighted regions to perform different thermodynamic cycle analysis.

  11. Cycling injuries.

    PubMed Central

    Cohen, G. C.

    1993-01-01

    Bicycle-related injuries have increased as cycling has become more popular. Most injuries to recreational riders are associated with overuse or improper fit of the bicycle. Injuries to racers often result from high speeds, which predispose riders to muscle strains, collisions, and falls. Cyclists contact bicycles at the pedals, seat, and handlebars. Each is associated with particular cycling injuries. Images Figure 1 Figure 3 Figure 4 Figure 5 PMID:8471908

  12. Kinematics of back-arc extension driven by the interference of subduction and/or collisional zones: application to a number of Mediterranean orogens

    NASA Astrophysics Data System (ADS)

    Matenco, L.; Cloetingh, S.

    2013-12-01

    The large number and distribution of roll-back systems in Mediterranean orogens infers the possibility of interacting extensional back-arc deformation driven by different slabs. Classical models of orogenic evolution assume that such back arc basins form in the hinterland of orogens, collapsing the upper plate above oceanic subduction zones. This is a common characteristic thought to apply to all low-topography orogens of Mediterranean type driven by the fast roll-back of genetically related slabs, or to other analogues such as the Miocene to recent evolution of the SE Asia subduction zones. This extension may take place far at the interior of the upper plate, as is the case in various segments of the Carpathians or in the core of the SE Asian domain, but in most cases of the Dinarides, Apennines or Hellenides it take place superposed or far into the foreland when compared with the position of oceanic suture zones. Mediterranean orogens often diverge from the typical scenario by widespread extensional deformation taking place during moments of continent-continent collision and by the interference of such deformation driven by different subduction zones. For instance, the formation of the Pannonian back-arc basin is generally related to the rapid Miocene roll-back of a slab attached to the European continent. The present-day extensional geometry of the Pannonian back-arc formed essentially during the Carpathians collision and was also driven by an additional Middle Miocene roll-back of a Dinaridic slab. In other orogenic systems, the back-arc extension takes place during continent-continent collision along major detachments that are located in the core of the orogen (Rif, Betics), in the accreted crustal material of the lower plate (Apennines, Dinarides) or even in a presumed former fore-arc (Aegean, Sunda-Banda arc). In all these subduction zones, collision has largely duplicated crustal blocks from the lower plate and has gradually shifted subduction zone far

  13. Deformation processes in orogenic wedges: New methods and application to Northwestern Washington State

    NASA Astrophysics Data System (ADS)

    Thissen, Christopher J.

    Permanent deformation records aspects of how material moves through a tectonic environment. The methods required to measure deformation vary based on rock type, deformation process, and the geological question of interest. In this thesis we develop two new methods for measuring permanent deformation in rocks. The first method uses the autocorrelation function to measure the anisotropy present in two-dimensional photomicrographs and three-dimensional X-ray tomograms of rocks. The method returns very precise estimates for the deformation parameters and works best for materials where the deformation is recorded as a shape change of distinct fabric elements, such as grains. Our method also includes error estimates. Image analysis techniques can focus the method on specific fabric elements, such as quartz grains. The second method develops a statistical technique for measuring the symmetry in a distribution of crystal orientations, called a lattice-preferred orientation (LPO). We show that in many cases the symmetry of the LPO directly constrains the symmetry of the deformation, such axial flattening vs. pure shear vs. simple shear. In addition to quantifying the symmetry, the method uses the full crystal orientation to estimate symmetry rather than pole figures. Pole figure symmetry can often be misleading. This method works best for crystal orientations measured in samples deformed by dislocation creep, but otherwise can be used on any mineral without requiring information about slip systems. In Chapter 4 we show how deformation measurements can be used to inform regional tectonic and orogenic models in the Pacific Northwestern United States. A suite of measurements from the Olympic Mountains shows that uplift and deformation of the range is consistent with an orogenic wedge model driven by subduction of the Juan de Fuca plate, and not northward forearc migration of the Oregon block. The deformation measurements also show that deformation within the Olympic Mountains

  14. Metamorphic evolution and geochronology of the Dunhuang orogenic belt in the Hongliuxia area, northwestern China

    NASA Astrophysics Data System (ADS)

    Wang, Hao Y. C.; Wang, Juan; Wang, Guo-Dong; Lu, Jun-Sheng; Chen, Hong-Xu; Peng, Tao; Zhang, Hui C. G.; Zhang, Qian W. L.; Xiao, Wen-Jiao; Hou, Quan-Lin; Yan, Quan-Ren; Zhang, Qing; Wu, Chun-Ming

    2017-03-01

    Garnet-bearing mafic granulites and amphibolites from the Hongliuxia area of the southern Dunhuang orogenic belt, northwestern China, commonly occur as lenses or boudinages enclosed within metapelite or marble, which represent the block-in-matrix feature typical of orogenic mélange. Three to four generations of metamorphic mineral assemblages are preserved in these rocks. In the high-pressure amphibolites, prograde mineral assemblages (M1) occur as inclusions (hornblende + plagioclase + quartz ± chlorite ± epidote ± ilmenite) preserved within garnet porphyroblasts, and formed at 550-590 °C and 7.7-9.2 kbar based on geothermobarometry. The metamorphic peak mineral assemblages (M2) are composed of garnet + hornblende + plagioclase + quartz + clinopyroxene, as well as titanite + zircon + rutile + apatite as accessory minerals in the matrix, and are estimated to have formed at 640-720 °C and 14.1-16.0 kbar. The first retrograde assemblages (M3) are characterized by "white-eye socket" symplectites (hornblende + plagioclase + quartz ± biotite ± epidote ± magnetite) rimming garnet porphyroblasts, which formed at the expense of the garnet rims and adjacent matrix minerals during the decompression stage under P-T conditions of 610-630 °C and 5.6-11.8 kbar. The second retrograde assemblages (M4) are intergrowths of actinolite and worm-like quartz produced by the breakdown of the matrix hornblendes, and formed under P-T conditions of ∼490 °C and ∼2.8 kbar. For the high-pressure mafic granulites, the prograde assemblages (M1) are represented by plagioclase + quartz preserved within the garnet porphyroblasts. The metamorphic peak assemblages (M2) are garnet + matrix minerals (clinopyroxene + plagioclase + quartz + hornblende + rutile + zircon) and were estimated to have formed at ∼680 °C and ∼15.4 kbar. The retrograde assemblages (M3) are characterized by fine-grained patches of hornblende + plagioclase + quartz rimming the garnet porphyroblasts, as well as

  15. Magma-assisted strain localization in an orogen-parallel transcurrent shear zone of southern Brazil

    NASA Astrophysics Data System (ADS)

    Tommasi, AndréA.; Vauchez, Alain; Femandes, Luis A. D.; Porcher, Carla C.

    1994-04-01

    In a lithospheric-scale, orogen-parallel transcurrent shear zone of the Pan-African Dom Feliciano belt of southern Brazil, two successive generations of magmas, an early calc-alkaline and a late peraluminous, have been emplaced during deformation. Microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions. Magmatic deformation is indicated by the coexistence of aligned K-feldspar, plagioclase, micas, and/or tourmaline with undeformed quartz. Submagmatic deformation is characterized by strain features, such as fractures, lattice bending, or replacement reactions affecting only the early crystallized phases. High-temperature solid-state deformation is characterized by extensive grain boundary migration in quartz, myrmekitic K-feldspar replacement, and dynamic recrystallization of both K-feldspar and plagioclase. Decreasing temperature during solid-state deformation is inferred from changes in quartz crystallographic fabrics, decrease in grain size of recrystallized feldspars, and lower Ti amount in recrystallized biotites. Final low-temperature deformation is characterized by feldspar replacement by micas. The geochemical evolution of the synkinematic magmatism, from calc-alkaline metaluminous granodiorites with intermediate 87Sr/86Sr initial ratio to peraluminous granites with very high 87Sr/86Sr initial ratio, suggests an early lower crustal source or a mixed mantle/crustal source, followed by a middle to upper crustal source for the melts. Shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle, but, whatever the process initiating partial melting, lithospheric transcurrent shear zones may collect melt at different depths. Because they enhance the vertical permeability of the crust, these zones may then act as heat conductors (by advection), promoting an upward propagation of partial melting in the crust

  16. Clinoform deposition across a boundary between orogenic front and foredeep - an example from the Lower Cretaceous in Arctic Alaska

    USGS Publications Warehouse

    Houseknecht, David W.; Wartes, Marwan A.

    2013-01-01

    The Lower Cretaceous Fortress Mountain Formation occupies a spatial and temporal niche between syntectonic deposits at the Brooks Range orogenic front and post-tectonic strata in the Colville foreland basin. The formation includes basin-floor fan, marine-slope and fan-delta facies that define a clinoform depositional profile. Texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge-front that included older turbidites and mélange. These new interpretations, based entirely on outcrop study, suggest that the Fortress Mountain Formation spans the boundary between orogenic wedge and foredeep, with proximal strata onlapping the tectonic wedge-front and distal strata downlapping the floor of the foreland basin. Our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load-induced flexural subsidence of the foreland basin.

  17. Mass flux into the Nanga Parbat-Haramosh massif: Orogen-parallel transport, lower crustal flow, or both?

    NASA Astrophysics Data System (ADS)

    Whipp, D. M.; Beaumont, C.; Braun, J.

    2011-12-01

    Relative to most of the Himalaya, the Nanga Parbat-Haramosh massif requires an additional mass flux into its base to balance extreme rates of surface denudation (>10 mm/a) over the last ~2-3 Ma. One proposed source is middle to lower crustal flow into the massif (e.g., Zeitler et al., 2001), which while likely inactive elsewhere along strike, may be sustainable by very rapid surface denudation, a high geothermal gradient, and thermal weakening of rocks beneath the syntaxes. An alternative source is orogen-parallel (OP) transport due to oblique convergence and strain partitioning along the Himalayan arc (e.g., Seeber and Pêcher, 1998). Several observations including (1) predominantly orogen-normal slip on the frontal thrusts deduced from seismic events, (2) OP extension accommodated on orogen-normal structures, and (3) distributed and segmented strike-slip faulting trending parallel to the arc are consistent with strain partitioning and OP mass transport. A key question is can this mechanism supply sufficient mass to the Nanga Parbat syntaxis, or is local channel flow required? We explore mass transport into the western Himalayan syntaxis region using lithospheric-scale 3D mechanical and coupled thermo-mechanical models of an arcuate orogen. The crust is either frictional plastic or power-law viscous, with a constant low viscosity lower crust present in some experiments. Applied velocity boundary conditions are transmitted to the base of the crust by a strong frictional plastic mantle lithosphere, and mantle detachment/subduction drives formation of a bivergent, arcuate orogen. To assess the magnitude of mass transport from strain partitioning, we first explore purely mechanical experiments featuring a 5-km-high Tibet-like plateau above a weak lower crust and with a frictional plastic decollement that dips at 10° beneath the incipient orogen, similar to the Himalayan basal detachment. Preliminary results show gravitational feedback from the plateau drives

  18. Orogenic gold mineralisation hosted by Archaean basement rocks at Sortekap, Kangerlussuaq area, East Greenland

    NASA Astrophysics Data System (ADS)

    Holwell, D. A.; Jenkin, G. R. T.; Butterworth, K. G.; Abraham-James, T.; Boyce, A. J.

    2013-04-01

    A gold-bearing quartz vein system has been identified in Archaean basement rocks at Sortekap in the Kangerlussuaq region of east Greenland, 35 km north-northeast of the Skaergaard Intrusion. This constitutes the first recorded occurrence of Au mineralisation in the metamorphic basement rocks of east Greenland. The mineralisation can be classified as orogenic style, quartz vein-hosted Au mineralisation. Two vein types have been identified based on their alteration styles and the presence of Au mineralisation. Mineralised type 1 veins occur within sheared supracrustal units and are hosted by garnet-bearing amphibolites, with associated felsic and ultramafic intrusions. Gold is present as native Au and Au-rich electrum together with arsenopyrite and minor pyrite and chalcopyrite in thin alteration selvages in the immediate wall rocks. The alteration assemblage of actinolite-clinozoisite-muscovite-titanite-scheelite-arsenopyrite-pyrite is considered to be a greenschist facies assemblage. The timing of mineralisation is therefore interpreted as being later and separate event to the peak amphibolite facies metamorphism of the host rocks. Type 2 quartz veins are barren of mineralisation, lack significant alteration of the wall rocks and are considered to be later stage. Fluid inclusion microthermometry of the quartz reveals three separate fluids, including a high temperature ( T h = 300-350 °C), H2O-CO2-CH4 fluid present only in type 1 veins that in interpreted to be responsible for the main stage of Au deposition and sulphidic wall rock alteration. It is likely that the carbonic fluids were actually trapped at temperatures closer to 400 °C. Two other fluids were identified within both vein types, which comprise low temperature (100-200 °C) brines, with salinities of 13-25 wt% eq. NaCl and at least one generation of low salinity aqueous fluids. The sources and timings of the secondary fluids are currently equivocal but they may be related to the emplacement of

  19. Syn-Uralian orogenic heavy mineral provenance analysis from southeastern Taimyr, Arctic Russia

    NASA Astrophysics Data System (ADS)

    Zhang, X.; Pease, V.; Scott, R. A.

    2012-12-01

    The Taimyr peninsula is on the northern margin of the Siberian craton and divides into the southern, central and northern NE - SW trending domains. The southern Taimyr domain represents the passive margin of Siberia and is dominated by a Paleozoic, extending into the early Mesozoic, succession. The central Taimyr domain accreted to the southern domain during Late Precambrian time, followed by collision with the northern Taimyr domain of Baltican affinity during the Late Paleozoic as part of Uralian orogenesis. The Carboniferous - Permian sedimentary succession, which was deposited during the later stage of Uralian Orogeny, can provide crucial information about the extent of contemporaneous Uralian orogensis and its influence on the tectonic evolution of southern Taimyr. Three Carboniferous - Permian samples from southeastern Taimyr were analyzed for petrography and heavy mineral analysis to define their sedimentary and provenance characteristics. The Upper Carboniferous to Lower Permian sample (VP10-25, Turuzovskya Formation, C2-P1tr), the Lower Permian sample (VP10-14, Sokolinskaya Formation, P1sk) and the Upper Permian sample (VP10-12, Baykurskaya Formation, P2bk)) classify as subarkose, lithic arkose and feldspathic litharenite, respectively- they record decreasing sediment maturity through time. While all the samples represent a 'recycled orogen' source based on QtFL plots, the C2-P1tr sample represents a recycled quartzite, while the P1sk sample plots within the mixed field, and the P2bk sample is transitional on QmFLt plots. According to the heavy mineral analysis results, the C2-P1tr sample and P1sk sample show great similarity in heavy mineral assemblage, dominated by zircon, apatite and rutile. The P2bk sample shows distinct differences, containing apatite, tourmaline, garnet and zircon. The prominent increase of garnet suggests a metamorphic source. These similarities and variations among the three samples are also shown in other discrimination diagrams

  20. Intraplate geodynamics and magmatism in the evolution of the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Yarmolyuk, V. V.; Kuzmin, M. I.; Ernst, R. E.

    2014-10-01

    The Central Asian Orogenic Belt (CAOB) was produced as a consequence of the successive closure of the Paleoasian Ocean and the accretion of structures formed within it (island arcs, oceanic islands, and backarc basins) to the Siberian continent. The belt started developing in the latest Late Neoproterozoic, and this process terminated in the latest Permian in response to the collision of the Siberian and North China continents that resulted in closure of the Paleoasian ocean (Metcalfe, 2006; Li et al., 2014; Liu et al., 2009; Xiao et al., 2010; Didenko et al., 2010). Throughout the whole evolutionary history of this Orogenic Belt, a leading role in its evolution was played by convergent processes. Along with these processes, an important contribution to the evolution of the composition and structure of the crust in the belt was made by deep geodynamic processes related to the activity of mantle plumes. Indicator complexes of the activity of mantle plumes are identified, and their major distribution patterns in CAOB structures are determined. A number of epochs and areas of intraplate magmatism are distinguished, including the Neoproterozoic one (Rodinia breakup and the origin of alkaline rock belt in the marginal part of the Siberian craton); Neoproterozoic-Early Cambrian (origin of oceanic islands in the Paleoasian Ocean); Late Cambrian-Early Ordovician (origin of LIP within the region of Early Caledonian structures in CAOB); Middle Paleozoic (origin of LIP in the Altai-Sayan rift system); Late Paleozoic-Early Mesozoic (origin of the Tarim flood-basalt province, Central Asian rift system, and a number of related zonal magmatic areas); Late Mesozoic-Cenozoic (origin of continental volcanic areas in Central Asia). Geochemical and isotopic characteristics are determined for magmatic complexes that are indicator complexes for areas of intraplate magmatism of various age, and their major evolutionary trends are discussed. Available data indicate that mantle plumes

  1. Orogenic superstructure behaviour and mid-crustal plastic flow in the central Nepal Himalaya

    NASA Astrophysics Data System (ADS)

    Godin, L.; Kellett, D. A.; Larson, K. P.

    2007-12-01

    In the central Nepal Himalaya, the Tethyan sedimentary sequence (TSS) forms the superstructure to mid-crustal infrastructure rocks of the Greater Himalayan sequence (GHS); the top-to-the-north South Tibetan detachment system (STDS) defines their contact. North-verging folds, opposite to the main orogenic vergence, structurally dominate the TSS. Although the absolute age of this folding is unknown, structural observations and 40Ar/39Ar thermochronology indicate that it formed between 50-23 Ma, predating the dominant Miocene motion on the STDS. The GHS records a two-stage post-collisional history, marked by ca. 35 Ma burial metamorphism, followed by high-T, low-P, ca. 22 Ma metamorphism. Dominant top-to-the-south shear fabrics developed at peak temperatures at ca. 22 Ma pervasively transpose linear and planar features within the GHS. Vorticity analyses yield kinematic vorticity numbers between 0.29 and 0.80 (81-41% pure shear), with a significant amount of stretch parallel to the flow plane (34-53%). 40Ar/39Ar thermochronological data indicate that southward extrusion of the GHS terminated with cessation of movement on the STDS at 19 Ma. Our data suggest that the orogenic superstructure actively influenced the behaviour of the infrastructure in the early stages of orogenesis through fold-thrust belt formation leading to prograde 35 Ma metamorphism in the GHS. Associated melt weakening in the infrastructure allowed the initiation of southward plastic flow of the GHS, locally modifying the vergence of superstructural folds towards the north. As melt weakening in the middle crust intensified and the rheological contrast between superstructure and infrastructure increased, the upper crust decoupled from the middle crust and deformation in the upper crust temporarily ceased. By 17 Ma the extruded mid-crustal rocks cooled sufficiently to require the upper, brittle component of the STDS to become active. As cooling continued (17-14 Ma), the superstructure and underlying

  2. A Late Cretaceous Orogen Triggering the Tertiary Rifting of the West Sunda Plate; Andaman Sea Region

    NASA Astrophysics Data System (ADS)

    Sautter, B.; Pubellier, M. F.; Menier, D.

    2015-12-01

    Rifted Basins often develop in internal zones of orogenic belts, although the latter may not be easy to unravel. We chose the example of the super-stretched Andaman sea region affected by several stages of rifting in the internal zone of a composite collage of allochthonous terranes. We made use of a set of geophysical, geochronological and structural data to analyze the rifting evolution and reconstruct the previous compressional structures. - Starting in the late Oligocene the East Andaman Basin opened as a back arc in a right-lateral pull- apart. The rifting propagated Westward to the central Andaman basin in the Middle Miocene, and to the oceanic spreading stage in the Pliocene. - An early extension occurred in the Paleogene, marked by widespread opening of isolated continental basins onshore Malay Peninsula and offshore Andaman Shelf and Malacca Straits. The rifting was accommodated by LANF's along preexisting weakness zones such as hinges of folds and granitic batholiths. Continuous extension connected the isolated basins offshore, whereas onshore, the grabens remained confined. There, AFT data show an uplift phase around 30Ma. In the Late Cretaceous, a major deformation occurred oblique to the pre-existing Indosinian basement fabrics. The convergence was partitioned into thrusting and uplift of the Cretaceous volcanic arc in Thailand and Myanmar, inversion of Mesozoic basins, and coeval wrenching responsible for large phacoid-shaped crustal slivers bounded by wide strike slip fault zones. The slivers share similar characteristics: a thick continental core of lower Paleozoic sedimentary basins units surrounded by Late Cretaceous granitoids. Radiometric data and fission tracks indicate a widespread thermal anomaly in all West Sunda Plate synchronous to a strong uplift. In the Latest Mesozoic, the Western Margin of Sunda plate was subjected to a major E-W compression, accommodated by oblique conjugate strike slip faults, leading to the formation of a large

  3. Magnetic signatures of the orogenic crust of the Patagonian Andes with implication for planetary exploration

    NASA Astrophysics Data System (ADS)

    Díaz Michelena, Marina; Kilian, Rolf

    2015-11-01

    The Patagonian Andes represent a good scenario of study because they have outcrops of diverse plutonic rocks representative of an orogenic crust on Earth and other planets. Furthermore, metamorphic surface rocks provide a window into deeper crustal lithologies. In such remote areas, satellite and aerial magnetic surveys could provide important geological information concerning exposed and not exposed rocks, but they integrate the magnetic anomalies in areas of kilometres. For the southernmost Andes long wavelength satellite data show clear positive magnetic anomalies (>+100 nT) for the Patagonian Batholith (PB), similar as parts of the older martian crust. This integrated signal covers regions with different ages and cooling histories during magnetic reversals apart from the variability of the rocks. To investigate the complex interplay of distinct magnetic signatures at short scale, we have analysed local magnetic anomalies across this orogen at representative sites by decimeter-scale magnetic ground surveys. As expected, the investigated sites have positive and negative local anomalies. They are related to surface and subsurface rocks, and their different formation and alternation processes including geomagnetic inversions, distinct Curie depths of the magnetic carriers, intracrustal deformation among other factors. Whole rock chemistry (ranging from 45 to >80 wt.% SiO2 and from 1 to 18 wt.% FeOtot.), magnetic characteristics (susceptibilities, magnetic remanence and Königsberger ratios) as well as the composition and texture of the magnetic carriers have been investigated for representative rocks. Rocks of an ultramafic to granodioritic intrusive suite of the western and central PB contain titanomagnetite as major magnetic carrier. Individual magnetic signatures of these plutonic rocks reflect their single versus multidomain status, complex exolution processes with ilmenite lamella formations and the stoichiometric proportions of Cr, Fe and Ti in the oxides. At

  4. Using large dynamite shots to image the structure of the Moho from deep seismic reflection experiment between the Sichuan basin and Qinling orogen

    NASA Astrophysics Data System (ADS)

    Li, Hongqiang; Gao, Rui; Wang, Haiyan; Li, Wenhui; Xiong, Xiaosong

    2016-12-01

    The Qinling orogen was formed as a result of the collision between the North and South China blocks. The Qinling orogen represents the location at which the southern and northern parts of the Chinese mainland collided, and it's also the intersection of the Central China orogen and the north-south tectonic belt. There is evidence of strong deformation in this orogen, and it has had a long and complex geological history. We investigated the structure of the Moho in the southern Qinling orogen using large dynamite shot imaging techniques. By integrating the analysis of the single-shot and the move-out corrections profile, we determined the structure of the Moho beneath the northern Dabashan thrust belt and the southern Qinling orogen, including the mantle suture beneath Fenghuang mountain. The Moho is divided into two parts by the mantle suture zone beneath Fenghuang mountain: (1) from Ziyang to Hanyin, the north-dipping Moho is at about 45-55 km depth and the depth increases rapidly; and (2) from Hanyin to Ningshan, the south-dipping Moho is at about 40-45 km depth and shallows slowly. The mantle suture is located beneath Fenghuang mountain, and the Moho overlaps at this location: the shallower Moho is connected to the northern part of China, and the deeper Moho is connected to the southern part. This may indicate that the lithosphere in the Sichuan basin subducts to the Qinling block and that the subduction frontier reaches at least as far as Fenghuang mountain.

  5. Mass elevation and lee effects markedly lift the elevational distribution of ground beetles in the Himalaya-Tibet orogen

    PubMed Central

    Schmidt, Joachim; Böhner, Jürgen; Brandl, Roland; Opgenoorth, Lars

    2017-01-01

    Mass elevation and lee effects markedly influence snow lines and tree lines in high mountain systems. However, their impact on other phenomena or groups of organisms has not yet been quantified. Here we quantitatively studied their influence in the Himalaya–Tibet orogen on the distribution of ground beetles as model organisms, specifically whether the ground beetle distribution increases from the outer to the inner parts of the orogen, against latitudinal effects. We also tested whether July temperature and solar radiation are predictors of the beetle’s elevational distribution ranges. Finally, we discussed the general importance of these effects for the distributional and evolutionary history of the biota of High Asia. We modelled spatially explicit estimates of variables characterizing temperature and solar radiation and correlated the variables with the respective lower elevational range of 118 species of ground beetles from 76 high-alpine locations. Both July temperature and solar radiation significantly positively correlated with the elevational ranges of high-alpine beetles. Against the latitudinal trend, the median elevation of the respective species distributions increased by 800 m from the Himalayan south face north to the Transhimalaya. Our results indicate that an increase in seasonal temperature due to mass elevation and lee effects substantially impact the regional distribution patterns of alpine ground beetles of the Himalaya–Tibet orogen and are likely to affect also other soil biota there and in mountain ranges worldwide. Since these effects must have changed during orogenesis, their potential impact must be considered when biogeographic scenarios based on geological models are derived. As this has not been the practice, we believe that large biases likely exist in many paleoecological and evolutionary studies dealing with the biota from the Himalaya-Tibet orogen and mountain ranges worldwide. PMID:28339461

  6. Early Mesozoic granitoid and rhyolite magmatism of the Bureya Terrane of the Central Asian Orogenic Belt: Age and geodynamic setting

    NASA Astrophysics Data System (ADS)

    Sorokin, A. A.; Kotov, A. B.; Kudryashov, N. M.; Kovach, V. P.

    2016-09-01

    Early Mesozoic granitoids and volcanic rocks are widespread throughout the structures of all of the continental massifs in the eastern part of the Central Asian Orogenic Belt, although its tectonic setting is not yet clear. Generally, they are associated with subduction and plume processes or rifting. Such uncertainty is mostly explained by the unequal investigation of Early Mesozoic magmatism. This paper presents the results of geochemical, Sm-Nd isotope, and U-Pb geochronologic (ID-TIMS) studies of "key-type" Early Mesozoic magmatic rock complexes of the Bureya Terrane. This is one of the largest continental massifs in the eastern Central Asian Orogenic Belt and knowledge of its geological structure is of fundamental importance in understanding the history of its formation. It has been established that the leucogranites of the Altakhtinsky Complex and the trachyrhyolites of the Talovsky Complex are practically coeval ( 209-208 Ma). The subalkaline leucogranites of the Kharinsky Complex have a slightly younger age of 199 Ma. These data correspond to the general stage of Early Mesozoic magmatic and metamorphic events (236-180 Ma) in most continental massifs in the eastern Central Asian Orogenic Belt. We believe that large-scale Early Mesozoic events were related to the amalgamation of the continental massifs of the eastern Central Asian Orogenic Belt into a single continental structure (the Amur superterrane or microcontinent Amuria) and collision with the North Asian Craton. It should be noted that the collision processes were followed by crustal thickening, thus creating the conditions for metamorphism and formation of magmatic rock complexes of various geochemical types.

  7. Orogeny processes of the western Jiangnan Orogen, South China:Insights from Neoproterozoic igneous rocks and a deep seismic profile

    NASA Astrophysics Data System (ADS)

    Su, Jinbao; Dong, Shuwen; Zhang, Yueqiao; Li, Yong; Chen, Xuanhua; Ma, Licheng; Chen, Jiansheng

    2017-01-01

    The Jiangnan Orogen is a collisional suture belt between the Yangtze and Cathaysia Blocks in South China, with many unanswered questions regarding its tectonic evolution. Using the basement structure of the Jiangnan Orogen, we investigate the granite and dacite exposed along the western Jiangnan Orogen and present new LA-ICP-MS zircon U-Pb ages, Hf isotopes, and whole rock geochemistry data. The results suggest that the granite plutons belong to the calc-alkaline series and are typical S-type granites. It yields a mean U-Pb age of 854 ± 2 Ma, which is determined from the core of zircon and possibly inherited from its source or wall rocks. The initial emplacement age of granite may be 826-805 Ma, whereas the dacites yield an age of 805 ± 1.6 Ma and belong to the shoshonite series. The initial Hf-isotope ratios (176Hf/177Hf) in the granite sample are mostly negative εHf(t), with a few of positive value with 1.38-1.6 Ga TDM and 1.67-2.06 Ga TDM2, whereas the dacite samples have mostly positive εHf(t), with a 0.78-1.6 Ga TDM and 0.83-2.2 Ga TDM2t. A comparison of the εHf(t) and TDM2t with the corresponding intruded strata, helps illustrate the origin of the magma and the finals stages ofcollision. Based on our results, we conclude that the western Jiangnan Orogen was a back-arc foreland basin that developed on the margin of the Yangtze continent and collided with the Cathaysia Block, forming a continent-arc-continent accretionary orogeny between 860 and 800 Ma.

  8. Large along-strike variations in the onset of Subandean exhumation: Implications for Central Andean orogenic growth

    NASA Astrophysics Data System (ADS)

    Lease, Richard O.; Ehlers, Todd A.; Enkelmann, Eva

    2016-10-01

    Plate tectonics drives mountain building in general, but the space-time pattern and style of deformation is influenced by how climate, geodynamics, and basement structure modify the orogenic wedge. Growth of the Subandean thrust belt, which lies at the boundary between the arid, high-elevation Central Andean Plateau and its humid, low-elevation eastern foreland, figures prominently into debates of orogenic wedge evolution. We integrate new apatite and zircon (U-Th)/He thermochronometer data with previously published apatite fission-track data from samples collected along four Subandean structural cross-sections in Bolivia between 15° and 20°S. We interpret cooling ages vs. structural depth to indicate the onset of Subandean exhumation and signify the forward propagation of deformation. We find that Subandean growth is diachronous south (11 ± 3 Ma) vs. north (6 ± 2 Ma) of the Bolivian orocline and that Subandean exhumation magnitudes vary by more than a factor of two. Similar north-south contrasts are present in foreland deposition, hinterland erosion, and paleoclimate; these observations both corroborate diachronous orogenic growth and illuminate potential propagation mechanisms. Of particular interest is an abrupt shift to cooler, more arid conditions in the Altiplano hinterland that is diachronous in southern Bolivia (16-13 Ma) vs. northern Bolivia (10-7 Ma) and precedes the timing of Subandean propagation in each region. Others have interpreted the paleoclimate shift to reflect either rapid surface uplift due to lithosphere removal or an abrupt change in climate dynamics once orographic threshold elevations were exceeded. These mechanisms are not mutually exclusive and both would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop, either through surface uplift or spatially variable erosion. In summary, we suggest that diachronous Subandean exhumation was driven by piecemeal hinterland uplift, orography, and the outward

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  10. Mass elevation and lee effects markedly lift the elevational distribution of ground beetles in the Himalaya-Tibet orogen.

    PubMed

    Schmidt, Joachim; Böhner, Jürgen; Brandl, Roland; Opgenoorth, Lars

    2017-01-01

    Mass elevation and lee effects markedly influence snow lines and tree lines in high mountain systems. However, their impact on other phenomena or groups of organisms has not yet been quantified. Here we quantitatively studied their influence in the Himalaya-Tibet orogen on the distribution of ground beetles as model organisms, specifically whether the ground beetle distribution increases from the outer to the inner parts of the orogen, against latitudinal effects. We also tested whether July temperature and solar radiation are predictors of the beetle's elevational distribution ranges. Finally, we discussed the general importance of these effects for the distributional and evolutionary history of the biota of High Asia. We modelled spatially explicit estimates of variables characterizing temperature and solar radiation and correlated the variables with the respective lower elevational range of 118 species of ground beetles from 76 high-alpine locations. Both July temperature and solar radiation significantly positively correlated with the elevational ranges of high-alpine beetles. Against the latitudinal trend, the median elevation of the respective species distributions increased by 800 m from the Himalayan south face north to the Transhimalaya. Our results indicate that an increase in seasonal temperature due to mass elevation and lee effects substantially impact the regional distribution patterns of alpine ground beetles of the Himalaya-Tibet orogen and are likely to affect also other soil biota there and in mountain ranges worldwide. Since these effects must have changed during orogenesis, their potential impact must be considered when biogeographic scenarios based on geological models are derived. As this has not been the practice, we believe that large biases likely exist in many paleoecological and evolutionary studies dealing with the biota from the Himalaya-Tibet orogen and mountain ranges worldwide.

  11. A Major Out of Sequence Fault in Central Range and Its Implication to Mountain Building Process of Taiwan Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Lee, Y. H.

    2015-12-01

    A Major Out of Sequence Fault in Central Range and Its Implication to Mountain Building Process of Taiwan Orogenic Belt Yuan-Hsi Lee1, Wei Lo2, Wei-Hau Wang1, Tim-Byrne 3, Ruey-Juen Rau 41. Department of Earth and Environmental Sciences, National Chung Cheng University, Taiwan, R.O.C. 2. Department of Materials and Mineral Resources Engineering, Taipei, National Taipei University of Technology, Taiwan, R.O.C. 3. Center for Integrative Geosciences, University of Connecticut, Storrs, CT, USA 4. Department of Earth Science, National Chen-Kung University, Taiwan, R.O.C. Taiwan mountain belt results from collision between Eurasia continental crust and Philippine Sea plate that result in exposing the metamorphic complex with high exhumation rate in eastern Central Range of Taiwan orogenic belt. In this study we combine with field survey, zircon fission track (ZFT), metamorphic grade, and tomography data to identify there exists a major out of sequence fault (MOSF) in eastern Central Range of Taiwan orogenic belt. This MOSF extends from north to south of eastern central Range with several segments and the total length is more than 250 km. The ZFT shows total annealing age of ca.1-3 Ma on the hanging wall and partial annealing ages on the foot wall. The seismicity data indicates the MOSF is still active from central to southern central Range. We consider that the MOSF is related with crustal channel flow in depth. To the western side of crustal flow it shows thrusting mechanism associated with MOSF and the normal faults (or normal shearing zone) develop in eastern side of the crustal channel flow. This crustal channel flow is also related with exposing the metamorphic complex in Central Range that is important mechanism for the mountain building process of Taiwan orogenic belt.

  12. Prolonged high relief in the northern Cordilleran orogenic front during middle and late Eocene extension based on stable isotope paleoaltimetry

    NASA Astrophysics Data System (ADS)

    Fan, Majie; Constenius, Kurt N.; Dettman, David L.

    2017-01-01

    The paleoelevation and size of the North America Cordilleran orogen during the late Cretaceous-Paleogene contractional and subsequent extensional tectonics remain enigmatic. We present new estimates of paleorelief of the northern Cordilleran orogenic front during the middle and late Eocene using oxygen isotope compositions of unaltered molluscan fossils and paleosol carbonates in the Kishenehn basin. Bounded by several mountains ranges to the east, the Kishenehn basin was a half graben developed during middle Eocene to early Miocene collapse of the Cordilleran orogen. These mollusk taxa include three sympatric groups with affinities to wet tropical, semi-arid subtropical, and temperate environments. Our reconstructed surface water δ18O values vary between -19.8‰ and -6.3‰ (VSMOW) during the middle and late Eocene. The large differences in paleoenvironments and surface water δ18O values suggest that the catchment of the Kishenehn basin was at variable elevation. The estimated paleorelief between the basin and the surrounding mountains, based on both Rayleigh condensation model and predictions of Eocene precipitation isotope values using an isotope-enabled global climate model, is ∼4 km, and the basin floor was <1.5 km high. This high topography and high relief paleogeography suggest that the Cordilleran orogenic front reached an elevation of at least 4 km, and the crust thickness may have reached more than 55 km before Eocene gravitational collapse. We attribute the maintenance of high Eocene topography to the combination of an inherited thick crust, thermal uplift caused by mantle upwelling, and isostatic uplift caused by removing lower lithosphere or oceanic slab.

  13. The contorted New England Orogen (eastern Australia): New evidence from U-Pb geochronology of early Permian granitoids

    NASA Astrophysics Data System (ADS)

    Rosenbaum, Gideon; Li, Pengfei; Rubatto, Daniela

    2012-02-01

    A series of sharp bends (oroclines) are recognized in the Paleozoic to early Mesozoic New England Orogen of eastern Australia. The exact geometry and origin of these bends is obscured by voluminous magmatism and is still debated. Here we present zircon U-Pb ages that confirm the lateral continuation of early Permian (296-288 Ma) granitoids and shed new light on the oroclinal structure. Orogenic curvature is defined by the alignment of early Permian granitoids parallel to the structural grain of the orogen, as well as the curved geometry of sub-vertical deformation fabrics, forearc basin terranes, and serpentinite outcrops. Alternative geometrical interpretations may involve two bends (Texas and Coffs Harbour Oroclines), three bends (+Manning Orocline), or even four bends (+Nambucca Orocline). We argue that the model involving four bends is most consistent with available data, although further kinematic constraints are required to confirm the existence of the Manning and Nambucca Oroclines. A subsequent phase of younger magmatism (<260 Ma) cuts across the curved structural grain, providing a minimum age constraint for orocline development. Assuming a structure of four oroclines, we suggest a tentative tectonic model that involves an early stage of subduction curvature during slab rollback at 300-285 Ma, followed by bending associated with dextral transpression. A final tightening of the curved structures was possibly obtained by E-W shortening during the late Permian to Triassic (265-230 Ma) Hunter-Bowen orogeny.

  14. Vapor Compression Cycle Design Program (CYCLE_D)

    National Institute of Standards and Technology Data Gateway

    SRD 49 NIST Vapor Compression Cycle Design Program (CYCLE_D) (PC database for purchase)   The CYCLE_D database package simulates the vapor compression refrigeration cycles. It is fully compatible with REFPROP 9.0 and covers the 62 single-compound refrigerants . Fluids can be used in mixtures comprising up to five components.

  15. Devonian magmatism in the Timan Range, Arctic Russia - subduction, post-orogenic extension, or rifting?

    NASA Astrophysics Data System (ADS)

    Pease, V.; Scarrow, J. H.; Silva, I. G. Nobre; Cambeses, A.

    2016-11-01

    Devonian mafic magmatism of the northern East European Craton (EEC) has been variously linked to Uralian subduction, post-orogenic extension associated with Caledonian collision, and rifting. New elemental and isotopic analyses of Devonian basalts from the Timan Range and Kanin Peninsula, Russia, in the northern EEC constrain magma genesis, mantle source(s) and the tectonic process(es) associated with this Devonian volcanism to a rift-related context. Two compositional groups of low-K2O tholeiitic basalts are recognized. On the basis of Th concentrations, LREE concentrations, and (LREE/HREE)N, the data suggest two distinct magma batches. Incompatible trace elements ratios (e.g., Th/Yb, Nb/Th, Nb/La) together with Nd and Pb isotopes indicate involvement of an NMORB to EMORB 'transitional' mantle component mixed with variable amounts of a continental component. The magmas were derived from a source that developed high (U,Th)/Pb, U/Th and Sm/Nd over time. The geochemistry of Timan-Kanin basalts supports the hypothesis that the genesis of Devonian basaltic magmatism in the region resulted from local melting of transitional mantle and lower crust during rifting of a mainly non-volcanic continental rifted margin.

  16. Contrasting Metamorphic Record of Heat Production Anomalies in the Penokean Orogen of Northern Michigan.

    PubMed

    Attoh

    2000-05-01

    It is proposed that the contrasting metamorphic mineral assemblages of the isolated amphibolite facies metamorphic highs in the Penokean orogen of northern Michigan may be caused by different heat production rates in the Archean basement. This hypothesis is based on concentrations of K, U, and Th in the Archean basement gneisses and Paleoproterozoic metasediments that indicate significant contribution of radiogenic heating during Penokean metamorphism. Heat production was anomalously high ( approximately 10.6 µWm-3) where andalusite-bearing mineral assemblages indicate that high temperatures were attained at shallow crustal levels ( approximately 550 degrees -600 degrees C at approximately 3 kbar). In contrast, where exposed metamorphic rocks indicate peak temperatures of 600 degrees -650 degrees C at 6-7 kbar, heat production in the Archean basement was lower ( approximately 3.7 µWm-3). The effect of heat production rates on the metamorphic pressure-temperature paths was tested with numerical thermal models. The calculations show (1) that if the heat production rate, where andalusite-bearing assemblages formed, was significantly <6.0 µWm-3, the estimated pressure at peak temperatures (PTmax) would be much higher and lie in the sillimanite or kyanite stability fields; and (2) differences between PTmax estimates for the metamorphic highs based on thermobarometry can be reproduced if thermal history involved significant crustal thickening as well as moderate unroofing rates.

  17. New seismic images of the crust in the central Trans-Hudson Orogen of Saskatchewan

    NASA Astrophysics Data System (ADS)

    Pandit, B. I.; Hajnal, Z.; Stauffer, M. R.; Lewry, J.; Ashton, K. E.

    1998-05-01

    A reprocessing program to enhance the correlation between the surface geology and the seismic data has been completed for seismic line 9 (eastern 100 km) and line 10 in the central region of the Trans-Hudson Orogen of Saskatchewan, Canada. The new seismic images through lateral continuity of reflectivity provide sufficient detail to resolve the discrepancy between the low-dipping, layer-parallel and dextral-reverse nature of the Sturgeon-Weir shear zone (line 9) observed in the field and its steeply dipping (apparent) normal displacement character interpreted on the basis of the initial processing. Furthermore, the new interpretation provides a strong confirmation of the role of Pelican Thrust as a major detachment zone — the main `sole thrust' — along which juvenile allochthons have been carried across the Archaean microcontinental block. The images are also refined enough to suggest: (a) a boundary within the Pelican Thrust between its internal and external suites; (b) a possible boundary separating a lower (older?) Archaean basement from its upper (younger?) counterpart; and (c) sub-Moho events (M2) which reveal possible involvement of the upper mantle in the collisional tectonic process in addition to the well defined Moho (M1) which probably represents the youngest of the post-collisional detachments.

  18. Partial melting of deeply subducted eclogite from the Sulu orogen in China.

    PubMed

    Wang, Lu; Kusky, Timothy M; Polat, Ali; Wang, Songjie; Jiang, Xingfu; Zong, Keqing; Wang, Junpeng; Deng, Hao; Fu, Jianmin

    2014-12-17

    We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U-Pb dating and petrological analyses, that partial melting occurred at 228-219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust.

  19. Morphogenetic Role of Rainsplash Transport in Hillslope Evolution in Post-Orogenic Landscapes

    NASA Astrophysics Data System (ADS)

    Dunne, K. B. J.; Dunne, T.; Malmon, D. V.

    2015-12-01

    The upper convex portions of hillslope profiles in soil-mantled landscapes have traditionally been interpreted as fundamentally different in origin from planar or concave hillslopes and have elicited different suggestions concerning their formation. In sub-humid landscapes, with sparse vegetation cover and little evidence for intense bioturbation, the upper convexity has usually been interpreted as the result of rainsplash. For the purpose of this study, we use a rainsplash transport equation developed from field and laboratory experiments and a model of ballistic trajectories to develop a method of predicting annual rainsplash transport rates under a range of environmental conditions, particularly related to climate reflected in mean annual rainfall, rainfall-intensity regime, and ground cover density. We derive diffusivity values of the kind frequently used in hillslope evolution models, and then examine the types of hillslope profiles that can be produced by this transport process. Under steady-state or decelerating uplift rates in post-orogenic environments in Africa, rainsplash transport is insufficient to create the observed convexities and lengths where erosion rates can be estimated. Additionally, the observed non-linear relationship between hillslope gradient and hillslope distance suggests additional erosive forces being in play, most likely that of sheetwash, despite this process usually being associated with concave hillslope profiles.

  20. IBERSEIS: A Seismic Reflection Image of the Variscan Orogen, SW Iberia

    NASA Astrophysics Data System (ADS)

    Carbonell, R.; Simancas, F.; Juhlin, C.; Ayarza, P.; Gonzalez-Lodeiro, F.; Pérez-Estaún, A.; Plata, J.

    2001-12-01

    The 303 km long IBERSEIS seismic profile was planned within the framework of the EUROPROBE program to study the deep structure of a transpressive part of the Variscan orogen and answer fundamental questions about the dynamics of the southwestern European lithosphere. Four 22 ton Vibroseis trucks generated an 8-80 Hz source signal which was recorded by a 7 km spread with stations located every 35 m. Shot spacing was 70 m. The close receiver and shot spacing resulted in high resolution images of the crust. Preliminary interpretations of the reflection data show that the highly reflective seismic fabric of the different units of the SW Iberian lithosphere correlate with the major tectonic terranes and contacts mapped at the surface. The two main sutures have been well imaged. The Moho can be identified along the entire transect at 10 to 11 s, indicating a 30-35 km average crustal thickness. The sub-horizontal geometry of the crust-mantle boundary that crosses Variscan sutures suggests a relatively young Moho. Diffractions at Moho depth beneath the southernmost suture zone indicate structure at the crust-mantle boundary.

  1. Polyphase Neoproterozoic orogenesis within the east Africa- Antarctica orogenic belt in central and northern Madagascar

    USGS Publications Warehouse

    Key, R.M.; Pitfield, P.E.J.; Thomas, Ronald J.; Goodenough, K.M.; Waele, D.; Schofield, D.I.; Bauer, W.; Horstwood, M.S.A.; Styles, M.T.; Conrad, J.; Encarnacion, J.; Lidke, D.J.; O'connor, E. A.; Potter, C.; Smith, R.A.; Walsh, G.J.; Ralison, A.V.; Randriamananjara, T.; Rafahatelo, J.-M.; Rabarimanana, M.

    2011-01-01

    Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa-Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820-760 Ma, 630-595 Ma and 560-520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid- Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global 'Terminal Pan-African' event (560-490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana. ?? The Geological Society of London 2011.

  2. Tip to midpoint observations on syntectonic veins, Ouachita orogen, Arkansas: Trading space for time

    NASA Astrophysics Data System (ADS)

    Cervantes, Pablo; Wiltschko, David V.

    2010-08-01

    By examining a vein from its tip to center, we have established the transition from a single filled fracture at the vein tip to typical 'crack-seal' textures observed in fibered, laminated veins. The vein is contained in the boudin neck of a sandstone layer within the Lower Ordovician Mazarn Formation, Benton Uplift, Ouachita orogen. The tip of the vein is composed of one or more isolated veinlets, defined as quartz-filled narrow (5-25 μm) fractures parallel to the larger vein's long dimension. Scanned SEM-based cathodoluminescence shows that quartz laminae of the same orientation and thickness are found throughout the vein. Wall-normal fibers first appear in the vein where detrital grains are cut by multiple veinlets, each veinlet mimicking the crystallographic orientation of the detrital grain, whereas later veinlets reflect the established crystallographic orientation of the fiber. Fibers throughout the vein retain evidence of having been formed by repeated fracturing and filling of a pre-existing grain (at the vein walls) or fiber. However, recrystallization later modified the fibers by obliterating some evidence of the veinlets and moving fiber walls. Boudin formation provided the extension site that localized fracturing and vein filling. The vein grows by the repeated addition of veinlets in the neck region. Recrystallization altered the shape of previously formed fibers.

  3. 3D Structural Analysis of the Benton Uplift, Ouachita orogen, Arkansas

    NASA Astrophysics Data System (ADS)

    Johnson, H. E.; Wiltschko, D. V.

    2011-12-01

    The date for the formation of the Benton Uplift, Ouachita orogeny, is bracketed by Carboniferous synorogenic sediments deposited to the north and Late Pennsylvanian to early Permian isotopic dates from the weakly metamorphosed rocks within the uplift. We address the largely unknown structural history between these two constraints by presenting an improved 3-dimensional kinematic model using better constrained retrodeformable sections. These new sections are based on all surface and subsurface data, new zircon fission track dates and thermal maturation data including new 'crystallinity' data to constrain the maximum burial depth. Zircon fission track ages range from 307 ± 18.8 Ma to 333.4 ± 38.9 Ma or from the Late Devonian to early Permian. Maximum 'crystallinity' of both illite and chlorite indicate these exposed rocks experienced a temperature of ~300°C across the eastern Benton Uplift. This temperature is consistent with reconstructed burial depths using cumulative stratigraphic thickness without having to call on structural thickening. Comparing coarse and fine clay fractions, computed temperature for the fine clay fraction is less by ~100°C than that of the coarse clay fraction. This difference is the same for all formations studied. This uniform difference in temperature may indicate cooling of the orogen as it deformed or more than one thermal event.

  4. Active shortening within the Himalayan orogenic wedge implied by the 2015 Gorkha earthquake

    NASA Astrophysics Data System (ADS)

    Whipple, Kelin X.; Shirzaei, Manoochehr; Hodges, Kip V.; Ramon Arrowsmith, J.

    2016-09-01

    Models of Himalayan neotectonics generally attribute active mountain building to slip on the Himalayan Sole Thrust, also termed the Main Himalayan Thrust, which accommodates underthrusting of the Indian Plate beneath Tibet. However, the geometry of the Himalayan Sole Thrust and thus how slip along it causes uplift of the High Himalaya are unclear. We show that the geodetic record of the 2015 Gorkha earthquake sequence significantly clarifies the architecture of the Himalayan Sole Thrust and suggests the need for revision of the canonical view of how the Himalaya grow. Inversion of Gorkha surface deformation reveals that the Himalayan Sole Thrust extends as a planar gently dipping fault surface at least 20-30 km north of the topographic front of the High Himalaya. This geometry implies that building of the high range cannot be attributed solely to slip along the Himalayan Sole Thrust over a steep ramp; instead, shortening within the Himalayan wedge is required to support the topography and maintain rapid rock uplift. Indeed, the earthquake sequence may have included a moderate rupture (Mw 6.9) on an out-of-sequence thrust fault at the foot of the High Himalaya. Such internal deformation is an expected response to sustained, focused rapid erosion, and may be common to most compressional orogens.

  5. Partial melting of deeply subducted eclogite from the Sulu orogen in China

    PubMed Central

    Wang, Lu; Kusky, Timothy M.; Polat, Ali; Wang, Songjie; Jiang, Xingfu; Zong, Keqing; Wang, Junpeng; Deng, Hao; Fu, Jianmin

    2014-01-01

    We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U–Pb dating and petrological analyses, that partial melting occurred at 228–219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust. PMID:25517619

  6. Extension of a double-wedged orogen potentially leads to the current South China Sea

    NASA Astrophysics Data System (ADS)

    Wu, G.; Lavier, L. L.

    2015-12-01

    The South China Sea (SCS) is surrounded by South China on the NW, Palawan and Reed Bank on the SE, as well as several microplates, resembling a jigsaw puzzle. In an attempt to better understand its evolution, we designed simplified thermomechanical models to simulate extension of a double-wedge-shaped orogen with highlands on both sides and lowland in the center to mimic the geological condition of the proto-SCS. We imposed constant extension rates on both sides and Gaussian-shaped thermal impulse in the center. We also varied the strength of lower crust but did not explicitly incorporate mid-ocean ridges and searfloor spreading mechanisms. We currently used symmetric double-wedge, but further tests are planed for asymmetric double-wedges. Our preliminary results show that the models produced many structures that resemble those of SCS, such as 1) a series of domino or conjugate faults sitting above a subsurface detachment (or décollement), 2) exhumed domes of middle-lower crust, 3) extreme thinning of both upper crust and lower crust, and 4) propagation of extension towards NW and SE margins. Our models suggest that superimposition of these modeled characteristics produced during several phases of extension of the SCS that may be due to thermal impulsion, magmatic events, and subduction related relaxation potentially produces high resemblance of the SCS.

  7. Tectonic setting of the low-grade metamorphic rocks of the Dabie Orogen, central eastern China

    NASA Astrophysics Data System (ADS)

    Xu, Shutong; Wu, Weiping; Lu, Yiqun; Wang, Dehua

    2012-04-01

    The tectonic setting on both the northern and southern sides of the Dabie Mountains reveals that low-grade metamorphic rocks are important constituents produced by the subduction of the oceanic crust prior to collision between the Sino-Korean and Yangtze cratons. The Zhangbaling Group/Mulanshan schist is a pre-Ordovician oceanic crust. The Sujiahe and Xinyang/Foziling Groups are trench sediments of the Ordovician-Devonian age, and constitute an accretionary prism associated with subduction. The Yangshan coal measures/Meishan Group was a forearc basin sediment of Carboniferous age, and was overthrust by the accretionary prism during collision. The Susong Group is composed of passive continental margin sediments of the Yangtze craton. Backarc basin sediments are postulated to be concealed by Mesozoic-Cenozoic sediments to the north of the Dabie Mountains. High-ultrahigh pressure terrains are exotic tectonic slices exhumed from depths, located between low-grade metamorphic rocks, and disturb the integrity of the earlier subduction orogen. Subduction occurred during the Ordovician to Devonian periods, and collision initiated at the beginning of the Permian.

  8. Influence of preexisting tectonic trends on geometries of Sevier orogenic belt and its foreland in Utah

    SciTech Connect

    Picha, F.; Gibson, R.I.

    1985-05-01

    The tectonic style of the late Mesozoic Sevier orogenic belt in Utah was greatly affected by preexisting structural trends that date from the late Precambrian rifting and fragmentation of the North American continent. The late Precambrian cratonic margin (Cordilleran hinge line) was marked by a system of prominent faults including the north-south-trending ancestral Wasatch and ancient Ephraim faults and the southwest-northeast-trending Leamington, Scipio, Cove Fort, and Paragonah faults. During the Paleozoic and Mesozoic, renewed activity on these faults affected the geometries of the late Paleozoic Paradox and Oquirrh basins, the boundaries of the Jurassic Arapien Formation, and the sedimentary pattern of the Cretaceous foreland basin. Many of these fault zones were reactivated as tectonic ramps (e.g., the ancient Ephraim fault) and tear faults (e.g., the Leamington fault) during the compressional Sevier tectonism. The Fillmore arch and some other structural highs situated along the edge of the late Precambrian craton caused ramping of the inner Keystone-Pavant-Canyon thrust sheets and telescoping of the frontal thrust sheets. Post-thrust uplift of basement highs led to tectonic denudation and to the development of low-angle, extensional faults, such as the Sevier detachment. Northeast-trending lineaments, such as the Cove Fort and Paragonah lineaments, were reactivated as right-lateral strike-slip faults. They also affected the extent of the Marysvale volcanic field.

  9. Strong Lg-wave attenuation in the Middle East continental collision orogenic belt

    NASA Astrophysics Data System (ADS)

    Zhao, Lian-Feng; Xie, Xiao-Bi

    2016-04-01

    Using Lg-wave Q tomography, we construct a broadband crustal attenuation model for the Middle East. The QLg images reveal a relationship between attenuation and geological structures. Strong attenuation is found in the continental collision orogenic belt that extends from the Turkish and Iranian plateau to the Pamir plateau. We investigate the frequency dependence of QLg in different geologic formations. The results illustrate that QLg values generally increase with increasing frequency but exhibit complex relationships both with frequency and between regions. An average QLg value between 0.2 and 2.0 Hz, QLg (0.2-2.0 Hz), may be a critical index for crustal attenuation and is used to infer the regional geology. Low-QLg anomalies are present in the eastern Turkish plateau and correlate well with low Pn-velocities and Cenozoic volcanic activity, thus indicating possible partial melting within the crust in this region. Very strong attenuation is also observed in central Iran, the Afghanistan block, and the southern Caspian Sea. This in line with the previously observed high crustal temperature, high-conductivity layers, and thick marine sediments in these areas, suggests the high Lg attenuation is caused by abnormally high tectonic and thermal activities.

  10. Cambro-Ordovician Granites in the Araçuaí Belt, in Brazil: snapshots from a late orogenic collapse

    NASA Astrophysics Data System (ADS)

    De Campos, Cristina P.; Mendes, Júlio Cesar; de Medeiros, Silvia Regina; Ludka, Isabel P.

    2014-05-01

    Along the Brazilian Coast, surrounding the São Francisco Craton and adjacent mobile belts, deep segments of a Neoproterozoic orogen (Araçuaí-West Congo) generated over 120 Ma of successive magmatic episodes of granitic magmatism. The c.630-585 Ma calc-alkaline magmatic arc consists of metatonalite to metagranodiorite, with metadioritic to noritic facies and enclaves. During the syn-collisional and crustal thickening stage (c. 585 to 560 Ma) S-type metagranites have been built by dehydration melting of a diverse package of sediments. Around 545-525 Ma late orogenic crustal remelting formed mostly non-foliated garnet-cordierite leucogranites. In the post-orogenic stage (c. 510-480 Ma) inversely zoned calc-alkaline to alkaline plutons intruded previous units. This work will focus on the youngest post-orogenic magmatism. It will present the state of the art by reviewing structural measurements, detailed mapping of flow patterns and additional geochemical and isotopic data. The architecture of around 10 plutons, ranging from c. 20 to 200 km2 in surface area, unravels deep mushroom- to funnel-like magma chambers and/or conduits. Available data point towards different compositional domains, which are interfingered in complex concentric layers, so that, each pluton depicts a unique internal flow pattern. In the silica-richer structures concentric fragmented or folded layers of granite, in a hybrid K-gabbroic/dioritic matrix, contrast with predominantly homogeneous K-basaltic to gabbroic regions. These may be separated by magmatic shear zones where mixing is enhanced, also resulting in hybrid compositions. Sharp and pillow-like contacts between granitic and K-basaltic rocks locally depict a frozen-in situation of different intrusive episodes. In the silica-poorer plutonic bodies gradational contacts are more frequent and may be the result of convection enhanced diffusion. For all plutons, however, mostly sub-vertical internal contacts between most- and least

  11. The evolution of a Gondwanan collisional orogen: A structural and geochronological appraisal from the Southern Granulite Terrane, South India

    NASA Astrophysics Data System (ADS)

    Plavsa, Diana; Collins, Alan S.; Foden, John D.; Clark, Chris

    2015-05-01

    Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (circa 600-500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat-Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system shows that the Moyar-Salem-Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740-550 Ma and is associated with dip-slip dextral transpression and north side-up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D1-D4), some of which are likely to be contemporaneous. The timing of high pressure-ultrahigh temperature metamorphism and deformation (D1-D3) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550-500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the

  12. Morphometric properties of the trans-Himalayan river catchments: Clues towards a relative chronology of orogen-wide drainage integration

    NASA Astrophysics Data System (ADS)

    Ghosh, Parthasarathi; Sinha, Sayan; Misra, Arindam

    2015-03-01

    The geomorphological evolution of the Himalayan mountain belt both in terms of crustal deformation and concomitant erosion by surface processes has been suggested to have a profound influence on a number of earth system processes and has been extensively researched through a number of different techniques. The huge catchments of the trans-Himalayan rivers are the product of long-term fluvial erosion of the landscape. This work attempts to understand their evolution through a study of drainage network, morphology, and internal organization of the smaller watersheds nested within each catchment. Using morphometric techniques applied to an orogen-wide digital elevation data grid, we characterized the drainage network structure and catchment of all the 18 trans-Himalayan rivers situated between the exits of the Indus and Brahmaputra rivers and constructed rectangular approximations of the catchment geometries. With the help of catchment dimensions measured transverse and parallel to the strike of the orogen, and by analyzing the dimension and spatial dispositions of the rectangular approximations, we demonstrate that the trans-Himalayan catchment shapes cannot be explained only as a product of the headward enlargement of drainage networks on a topographic slope, or orogenic taper. Within individual catchments we identified the existence of drainage components (watersheds) that are organized in a systematic manner with respect to the first-order physiographic features of the Himalayas, formed at different periods of geological time. Each of them shows distinct morphometric characteristics that are indicative of differences in processes and / or time scale involved in their formation. The hypsometric properties of the watersheds occupying the upper part of the catchments suggest that they are the remnants of pre-orogenic drainage that became confined to the leeward side of the Himalayas before the advent of monsoon circulation. The shape and organization of the

  13. Low-temperature thermochronology of the northern Thomson Orogen: Implications for exhumation of basement rocks in NE Australia

    NASA Astrophysics Data System (ADS)

    Verdel, Charles; Stockli, Daniel; Purdy, David

    2016-01-01

    The Tasmanides of eastern Australia record much of the Phanerozoic tectonic development of the retreating Pacific-Australia plate boundary and are an oft-cited example of an orogen that has undergone "tectonic mode switching." To begin to constrain the timing of exhumation of basement rocks that are now exposed in portions of the NE Tasmanides, we measured apatite and zircon (U-Th)/He ages from the Thomson Orogen and overlying Paleozoic strata in the back-arc of the New England Orogen in NE Australia. Zircon (U-Th)/He ages from basement samples (including those recovered from boreholes at depths of up to 1.1 km) are characterized by large inter- and intra-sample variability and range from approximately 180 Ma (Early Jurassic) to 375 Ma (Late Devonian). (U-Th)/He zircon ages from several individual samples are negatively correlated with effective uranium (eU), a pattern that is also true of the dataset as a whole, suggesting that variations in U and Th zoning and radiation damage are partially responsible for the age variability. The oldest zircon (U-Th)/He cooling ages coincide with the formation of regionally extensive Late Devonian-early Carboniferous back-arc basins, suggesting that Late Devonian extension played a significant role in exhumation of parts of the northern Thomson Orogen. Apatite (U-Th)/He ages from a basement sample and a late Permian sandstone in the overlying Bowen Basin, which are also marked by intra-sample variability and age-eU correlations, span from the Early Cretaceous through Oligocene, in general agreement with previous apatite fission track data. In conjunction with observations of key geologic relationships and prior K-Ar and 40Ar/39Ar data, our results suggest four overall phases in the thermal history of the northern Thomson Orogen: (1) Cambrian-early Silurian metamorphism during the Delamerian and Benambran Orogenies; (2) protracted cooling during the Late Devonian through mid-Permian that likely resulted from extensional

  14. Microstructural analysis of the Greater Himalayan Sequence, Annapurna-Dhaulagiri Himalaya, central Nepal: Channel Flow and Orogen-parallel deformation.

    NASA Astrophysics Data System (ADS)

    Parsons, A. J.; Phillips, R. J.; Lloyd, G. E.; Searle, M. P.; Law, R. D.

    2014-12-01

    Knowledge of deformation processes that occur in the lithosphere during orogenesis can be gained from microstructural analysis of exhumed terranes and shear zones. Here, we use Crystallographic Preferred Orientation (CPO) and Anisotropy of Magnetic Susceptibility (AMS) data to reveal the kinematic evolution of the metamorphic core of the Himalayan orogen, the Greater Himalayan Sequence (GHS). The Himalayan orogen is commonly explained with models of channel flow, which describe the GHS as a partially molten, rheologically weak mid crustal channel. Extrusion of the channel was facilitated by coeval reverse- and normal-sense shear zones, at the lower and upper channel margins respectively. Whilst many thermobarometric studies support the occurrence of channel flow, the spatial and temporal distribution of strain within the GHS is one aspect of the model that is yet to be fully resolved. We present a quantified strain proxy profile for the GHS in the Annapurna-Dhaulagiri region of central Nepal and compare our results with the kinematic predictions of the channel flow model. Samples were collected along a NS transect through the Kali Gandaki valley of central Nepal for CPO and AMS analysis. Variations in CPO strength are used as a proxy for relative strain magnitude, whilst AMS data provide a proxy for strain ellipsoid shape. Combining this information with field and microstructural observations and thermobarometric constraints reveals the kinematic evolution of the GHS in this region. Low volumes of leucogranite and sillimanite bearing rocks and evidence of reverse-sense overprinting normal-sense shearing at the top of the GHS suggest that channel flow was not as intense as model predictions. Additionally, observed EW mineral lineations and oblate strain ellipsoid proxies in the Upper GHS, indicative of three dimensional flattening and orogen parallel stretching, cannot be explained by current channel flow models. Whilst the results do not refute the occurrence of

  15. How the Extension-Rate of Rifting Influences an Alpine-Type Orogens: insights from 3D analog models.

    NASA Astrophysics Data System (ADS)

    Nestola, Y.; Storti, F.; Cavozzi, C.

    2015-12-01

    Alpine-type orogens are interpreted as result from the collision of former rifted margins. Recent studies showed that the rift-architecture inheritance could play a critical role in controlling the 4D evolution of Alpine-type orogens. In this framework, differences of inversion modes between the internal and external zones of the Western Alps can be related to the pre-orogenic rift-related domains. The external zone is affected by mild reactivation of the former proximal margin domain. On the other hand, the internal zone results from the reactivation of the former distal margin domain. This caused the stacking of a complex pile of pre- and syn-rift sequences against the 'necking zone', that is the locus where the lithosphere dramatically thins. The 'necking zone' separates the proximal and distal domains and acts as a buttress for shortening. Indeed, both rift architecture and shape of necking play a fundamental role in the building up of an Alpine-type orogen. In this study, we use analog modeling to investigate the role of extension-rate in rift-architecture. We simulated an ideal 4-layer lithosphere where brittle and ductile crustal layers rest on top of brittle and ductile mantle layers. The entire experimental lithosphere floats over a fluid analogue of the asthenosphere. Models were deformed pulling apart a mobile wall of the sandbox that confined the experimental lithosphere. We investigated three different extensional velocities, spanning one-order of magnitude. At the end of deformation, rift architectures show severe differences as a function of extension-rates, at both crustal and lithospheric scales. In particular, at lithospheric scales, localized necking occurred at low extension-rates, while a more distributed deformation happened with increasing the extensional velocity. At crustal scale, well-developed and localized necking zones formed for low and intermediate extension-rates, while tapering occurred over a wide cross-sectional length in high

  16. Restoration of Late Neoarchean-Early Cambrian tectonics in the Rengali orogen and its environs (eastern India): The Antarctic connection

    NASA Astrophysics Data System (ADS)

    Bhattacharya, A.; Das, H. H.; Bell, Elizabeth; Bhattacharya, Atreyee; Chatterjee, N.; Saha, L.; Dutt, A.

    2016-10-01

    Geological mapping and P-T path reconstructions are combined with monazite chemical age and Secondary Ion Mass Spectrometric (SIMS) U-Pb zircon age determinations to identify crustal domains with distinctive evolutionary histories in the Rengali orogen sandwiched between two Grenvillian-age metamorphic belts, i.e. the Eastern Ghats Granulite Belt (EGGB) in the south, and the amphibolite facies Gangpur Schist Belt (GSB) in the north, which in turn forms a collar along the NW/W margins of the Paleo/Mesoarchean Singhbhum Craton (SC) north of the Rengali orogen. Anatectic gneisses in the orogen core exhibit multi-phase Neoarchean/Paleoproterozoic deformation, metamorphic P-T histories and juvenile magma emplacement events. The high-grade belt is inferred to be a septum of the Bastar Craton (BC). The flanking supracrustal belt in the orogen - dominated by quartz-muscovite schists (± staurolite, kyanite, garnet pyrophyllite), inter-bedded with poorly-sorted and polymict meta-conglomerate, and meta-ultramafic/amphibolite bands - evolved along P-T paths characterized by sub-greenschist to amphibolite facies peak P-T conditions in closely-spaced samples. The supracrustal rocks and the anatectic gneisses of contrasting metamorphic P-T histories experienced D1, D2 and D3 fabric-forming events, but the high-angle obliquity between the steeply-plunging D3 folds in the anatectic gneisses and the gently-plunging D3 folds in the supracrustal unit suggests the two lithodemic units were tectonically accreted post-S2. The supracrustal belt is inferred to be a tectonic mélange formed in an accretionary wedge at the tri-junction of the Bastar Craton, the Eastern Ghats Granulite Belt and the Singhbhum Craton; the basin closure synchronous with the assembly of EGGB and the Singhbhum Craton-Gangpur Schist belt composite occurred between 510 and 610 Ma. Based on the available evidence across the facing coastlines of the Greater India landmass and the Australo-Antarctic blocks at ~ 500 Ma

  17. The Structure of The Lithosphere-asthenosphere System Beneath The Alpine Orogen Derived From High-resolution Teleseismic Tomography

    NASA Astrophysics Data System (ADS)

    Lippitsch, R.; Kissling, E.; Ansorge, J.; Transalp Working Group

    In the tectonically complex Alpine region, three different plates (European, Adriatic, and Ligurian) amalgamated when the orogen was formed. To understand the evolution of this orogen and the interactions between the three lithospheric blocks, knowledge of the actual structure of the lithosphere-asthenosphere system is of great importance. To illuminate the structure of the uppermost mantle we perform high-resolution tele- seismic tomography. Our data set consists of 4200 manually picked first P-arrivals from 220 teleseismic events with even azimuthal distribution recorded at permanent and temporary seismic networks in the greater Alpine area. In the first step of this study corrections are calculated for the contribution of the Alpine crust to travel-times of incoming wave fields that may account for up to 50% of the observed travel-time residuals. The 3D crustal model established from controlled-source seismology data represents the large-scale shallow Alpine structure which clearly reflects the effects of the Africa Europe plate collision. Tests with synthetic data document that the combi- nation of non-linear inversion, high-quality teleseismic data, and usage of an a priori 3D crustal model allows a reliable resolution of cells at 50km*50km*30km with a velocity variation in the order of +/- 3% in the upper mantle. Our tomographic images illuminate the structure of the uppermost mantle to depth of 400 km and reflect the cur- rent status of the complex processes that formed the Alpine orogen. Along strike of the Alps, the inversion reveals a fast, slab-like body beneath the orogen. We interpret this feature as the subducted mainly oceanic lithosphere, which is in many places still attached to continental European lower lithosphere. Down to 250 km depths, this slab seems to be rather thin (less than 80 km) and steeply dipping. It significantly broad- ens at greater depth. Our results are in general agreement with earlier tomographic studies. However, the increase

  18. Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study

    NASA Astrophysics Data System (ADS)

    Beltrando, Marco; Manatschal, Gianreto; Mohn, Geoffroy; Dal Piaz, Giorgio Vittorio; Vitale Brovarone, Alberto; Masini, Emmanuel

    2014-04-01

    Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-extended crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a pre-orogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments from the hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the

  19. Tectonic Evolution of Naxos (cyclades): A Record of The Thermal-mechanical Evolution of An Orogenic Wedge

    NASA Astrophysics Data System (ADS)

    Vanderhaeghe, O.; Duchêne, S.; Hibsch, C.; Malartre, F.; Aissa, R.; Martin, L.; Fotiades, .; de St Blanquat, M.; Habert, G.

    A synthesis of published work and preliminary results from a multidisciplinary study of the various crustal levels exposed in the island of Naxos in the Cyclades allows to propose a model for the evolution of the orogenic accretionary wedge in this region. Naxos is characterized by the juxtaposition of metamorphic rocks and Late Oligocene- Early Miocene to Upper Pliocene sedimentary basins along a low-angle detachment. Migmatites, dated at ca. 15 Ma (Keay et al., 2001), appear in the core of a structural dome and are surrounded by a sequence of metasedimentary rocks. From the edge of the island to the core of the dome, the metasedimentary sequence is affected by a metamorphism grading from blue schists to amphibolite facies (Jansen and Schuil- ing, 1976). Ar-Ar ages decrease from ca. 45 in the blue schists to ca. 10 Ma in the migmatites (Andriessen et al., 1979, Wijbrans and McDougall, 1986). The sedimen- tary basins are filled mainly by coarse silicoclastic deposits. Sedimentological and geomorphological features suggest a transition from a shallow marine to a continental environment during Miocene time. Data from the metamorphic rocks indicate that the dynamic evolution of the wedge is marked by a first stage of burial and subsequent exhumation of the metasedimen- tary rocks controlled by the interplay between deformation related to convergence and denudation (erosion and tectonic). The formation of the dome cored by migmatites is interpreted as the result of the development of a gravitational instability associated to the genesis of a partially molten layer at depth in the thickened orogenic wedge. This event is synchronous to the activity of the low-angle detachment and the per- vasive top-to-the-North non-coaxial deformation which affects all rock units above the migmatites. The nature of the link between diapiric upwelling of the dome and the transition from thickening to thinning of the orogenic wedge remains to be estab- lished. Progressive exhumation and

  20. Menu Cycles.

    ERIC Educational Resources Information Center

    Clayton, Alfred; Almony, John

    The curriculum guide for commercial foods instruction is designed to aid the teacher in communicating the importance of menu cycles in commercial food production. It also provides information about the necessary steps in getting food from the raw form to the finished product, and then to the consumer. In addition to providing information on how to…

  1. On protolith-, metamorphic overprint, microstructure and rheology of mineral assemblages in orogenic peridotites of the central Scandinavian Caledonides

    NASA Astrophysics Data System (ADS)

    Gilio, Mattia; Clos, Frediano; Van Roermund, Herman L. M.

    2013-04-01

    The Scandinavian Caledonides (SC) are a deeply eroded Alpine-type orogenic belt formed by closure of the Iapetus ocean and collision between Baltica and Laurentia (500-380 Ma). The SC consists of a stack of Nappe Complexes (from bottom to top called Lower, Middle, Upper and Uppermost Allochthons) thrusted to the east over the Baltic Shield (Brueckner and Van Roermund, 2004; Gee et al., 2008). Fossil lithospheric mantle fragments, called orogenic peridotites, have been found within the (upper part of) middle, upper and uppermost Allochthons, as well as in the reworked basement gneisses (a.o Western Gneiss Complex (WGC)) along the Norwegian west coast. They occur as isolated lenses that contain diverse mineral parageneses and/or bulk rock compositions. Crustal incorporation of orogenic peridotite is classically interpreted to be the result of plate collisional processes related to orogeny (Brueckner and Medaris, 2000). The WGC and parts of the upper part of the Middle Allochthon (a.o. Seve Nappe Complex (SNC) in N Jämtland/S Västerbotten, central Sweden), are well known for the occurrence of high (HP) and ultrahigh pressure (UHP) metamorphic terranes (of Caledonian age). The (U)HPM evidence clearly demonstrates the deep metamorphic origin of these rocks interpreted to be caused by continental subduction and/or collision. Other metamorphic rocks (of Caledonian age) exposed in allochthonous nappes are solely characterised by greenschist-, amphibolite- and/or MP granulite "facies" mineral assemblages that can be interpreted, in the absence of retrogression, to have formed in less deeply subducted (and/or metamorphic) environments. This duality in metamorphic "facies" allows for a discrimination (at least theoretically) between "deep" versus "shallow" rooted nappes (in central parts of the Scandinavian Caledonides). Conform this reasoning, this duality should also be present within the Caledonian mineral assemblages (= metamorphic overprint) of orogenic peridotites (in

  2. New assemly model of Jiangnan Orogen: insight from detrital zircon geochronology of pre-Cretaceous strata, South China

    NASA Astrophysics Data System (ADS)

    Su, J.; Dong, S.

    2013-12-01

    The Jiangnan Orogen separates the Yangtze and Cathaysian Blocks in South China and provokes a longstanding debate on the amalgamation history between the two Blocks. The assembly of the two Blocks is termed Sibao orogeny marked by undeformed Late Neoproterozoic strata (Banxi Group) overlying on the deformed Early Neoproterozoic strata (Lengjiaxi Group) in China. Detrital zircons can provide critical links in recognizing the source history of a deposit, sedimentary dispersal systems and tectonic reconstructions. Therefore, fifteen sandstone samples taken from pre-Cretaceous strata of Yangtze Block are analyzed to constrain the evolution of the South China Block (SCB), especially the assembly between Yangtze and Cathaysia Blocks. The results show that the detrital zircons from the Neoproterozoic Lengjiaxi Group (ca. 830 Ma) near the boundary of large detachment fault of Hengshan have similar age populations with that in the other sites of the Jiangnan Orogen, different from that of the Kunyang and Dahongshan Groups (>960 Ma) in the southwestern margin of the Yangtze Block. The detrital zircons from Paleozoic samples have similar age populations with that in the Cathaysia Block. We infer that they originate from the Cathaysia Block, together with paleogeography, paleocurrent and former research. The detrital zircons of middle-late Jurassic sandstones in southwestern and central Yangtze yield dominant populations at 2.0-1.7 Ga and subordinate Groups of 2.6-2.4Ga, 0.7-0.8Ga and 0.6-0.4Ga. The provenance of late Triassic strata may be derived from southern Yangtze and North China Block due to the collisions among the Indosina, South China and North China Blocks, whereas the Jurassic sediments may be partly derived from uplift erosion of Jiangnan Orogen due to intra-continental orogeny induced by pacific subduction towards Eurasia Plate. The tectothermal event occurred at ca. 1.1-0.8 Ga has long been attributed to the assembly or breakup of Yangtze and Cathaysia Blocks

  3. The role of inherited subduction asymmetries on the build-up of orogenic wedges: the Dinarides/Carpathians/Balkans and Eastern Alps/Dinarides connections

    NASA Astrophysics Data System (ADS)

    Matenco, L. C.; Cloetingh, S.

    2009-04-01

    The controlling mechanisms of orogenic shortening and collision govern the architecture of the coeval sedimentary infill of foreland and back-arc basins and their subsequent exhumation. Continental collision coincides with the onset of large-scale out-of-sequence deformation, modelling studies indicating that orogenic steady state largely involves exhumation in the core of the orogen, which is recorded in natural examples by hinterland exhumation associated with processes such as crustal-scale backthrusting. High convergence rates will apparently exhume large amounts of crustal material entering the collision zone along bi-vergent orogens defining a "retro-shear" type of collision. However, in "soft" collisional orogens (such as the Carpathians) the lower orogenic plate is not always a "conveyer belt", i.e. transferring and incorporating material into the upper plate. The low convergence rates couple this lower plate distributing crustal exhumation all across the orogen due to the gradual shift of the lower plate accretion, defining a "foreland-coupling" type of collision mechanics. The balance between the two types of collisions is lesss known. In the case of the Alps and the Hellenides-Balkanides systems, the "retro-shear" exhumation can also be alternatively defined as being in the direct continuation of pre-existing asymmetries from inherited subduction zones (such as the Dinarides or the Carpathians). These subduction zones evolved and were gradually closed/collided during Cretaceous and Eocene and are in the prolongation of the Southern Alps (and peri-Adriatic lineament) and the Balkans retro-wedges respectively. The activity of these retro-wedges is recorded during and/or immediately after the foreland-coupling collision of these neighbouring subduction zones with compatible kinematics. A causal relationship is therefore argued by means of structures, large scale profiling and quantitative reconstructions.

  4. Revisiting the Wilson Cycle in the North Atlantic: The role of inheritance

    NASA Astrophysics Data System (ADS)

    Chenin, Pauline; Manatschal, Gianreto; Müntener, Othmar; Picazo, Suzanne; Erratt, Duncan; Karner, Garry; Johnson, Christopher; Ulrich, Marc

    2016-04-01

    According to the Wilson Cycle, oceans open and close approximately parallel to ancient suture zones, suggesting a major control of inheritance in the extension and convergent process. While this paradigm is well illustrated in the northern North Atlantic where the rift follows largely the Iapetus suture between Norway and Greenland, this is not the case for the southern North Atlantic, where neither the westward, nor the northward propagating branch of the Central Atlantic rift, affected the Variscan sutures of Western Europe. These observations suggest that inheritance is not necessarily reactivated during subsequent rifting events and begs the question about what may truly control the localization and details of rift systems. One possible cause for the differing behavior of the North Atlantic rift with respect to the Caledonian and Variscan orogens may be their contrasting paleo-geographic settings. Indeed, the Scandinavian Caledonides resulted from the closure of one wide ocean between two cratonic shields, whereas the Variscides were built from the accretion of several terranes/micro-continents following the closure of a series of narrow oceans. The variability in the initial architecture of the intervening rift systems and in their subduction processes may have significantly controlled the subsequent orogenies. The aim of this presentation is twofold: first we investigate how the first-order structural and lithological characteristics of narrow/embryonic versus wide/mature oceans, as well as the processes associated with their subduction and collision, characterize orogens. Second, we study how this variable orogenic inheritance may impact subsequent rifting. Our results suggest that: (1) the margins from narrow/embryonic and wide/mature oceans are comparable, therefore the major difference between these end-members is the existence of a significant amount of normal oceanic crust; (2) subduction-induced processes significantly impact both the thermal state and

  5. Progressive orogenic deformation recorded in a mylonite sample from the Dent Blanche Basal Thrust (Pennine Alps)

    NASA Astrophysics Data System (ADS)

    Kirst, Frederik; Froitzheim, Nikolaus; Nagel, Thorsten

    2013-04-01

    The Dent Blanche Basal Thrust (DBBT) represents an Alpine reverse fault along which the continental Dent Blanche Nappe was thrust onto the ocean-derived Combin Zone. Ductile NW-directed shearing associated with Early Tertiary nappe emplacement was localized along this fault resulting in the formation of intensely sheared rocks. These basal mylonites have locally been overprinted by later deformation phases associated with post-nappe compression. In the following, microstructures of a mylonite sample from a locality near the village Ollomont in the Valpelline of Italy are described and correlated with orogen-scale deformational events. The penetrative mylonitic foliation (S1) dips to the E (88/23) and is defined by a distinct metamorphic layering consisting of alternating, sub-mm thin layers of quartz and fine-grained silicates, mainly white mica ± epidote ± feldspar. The mylonitic stretching lineation (L1) is an aggregate lineation that dips to the ESE (119/19). In some parts, a grain shape preferred orientation of quartz grains defines an oblique foliation which probably developed as a result of combined subgrain rotation and grain boundary migration during simple shear deformation (Passchier & Trouw, 2005). The orientation of elongated quartz grains indicates top-NW shearing and is interpreted to reflect the bulk shear sense during nappe emplacement (D1). This type of dynamically recrystallised quartz also indicates deformation at temperatures around 500°C (Stipp et al., 2002). In most parts of the Ollomont mylonite, a secondary foliation (S2) and lineation (L2) are produced by a SE-dipping crenulation cleavage with NE-dipping (47/17) axes. In some spots, this non-penetrative feature is only slightly developed and gradational whereas in others it is a discrete compressional crenulation cleavage with large fold amplitudes in microlithons. This secondary foliation is interpreted to have formed as a result of late-stage orogen-perpendicular (NW-SE) shortening (D2

  6. Crustal evolution in the East African Orogen: a neodymium isotopic perspective

    NASA Astrophysics Data System (ADS)

    Stern, Robert J.

    2002-05-01

    The East African Orogen (EAO) is one of Earth's great collision zones, where East and West Gondwana collided to form the supercontinent `Greater Gondwana' or `Pannotia' at the end of Neoproterozoic time. There is now sufficient Nd isotopic data for basement rocks of the EAO to yield a useful summary. A total of 449 samples were gleaned from the literature, recalculated to a common value for the La Jolla Nd standard, and entered in Excel spreadsheets. This data set was filtered to exclude samples with 147Sm/ 144Nd> 0.165, considered to yield unreliable model ages, leaving 413 suitable data. The crust of the Arabian-Nubian Shield, including Egypt east of the Nile, Sudan east of the Keraf suture, Sinai, Israel, Jordan, most of Arabia, Eritrea, and northern Ethiopia yields overwhelmingly Neoproterozoic model ages. Crust to the east, in the Afif terrane of Arabia, Yemen, Somalia, and Eastern Ethiopia yields much older model ages, averaging 2.1 Ga, demonstrating an abundance of reworked ancient crust. This provides an isotopic link with Madagascar (mean age of 2.4 Ga), which in pre-Jurassic reconstructions lies on the southern extension of this older, remobilized tract. Crust in the far southern extreme of the EAO in Tanzania also yields ancient model ages, averaging 2.3 Ga. The central EAO, in southern Ethiopia and Kenya, yields intermediate ages (mean 1.1-1.2 Ga), interpreted to indicate extensive mixing between Neoproterozoic mantle-derived melts and ancient crust. The Nd isotope data indicate that the northern EAO is composed of juvenile Neoproterozoic crust sandwiched between reworked older crust, whereas the EAO farther south is progressively dominated by ancient crust reworked during Neoproterozoic time. The distribution of juvenile and reworked ancient crust suggests that the most intense collision between East and West Gondwana occurred in the southern part of the EAO.

  7. Linking glacial erosion and low-relief landscapes in tropical orogens

    NASA Astrophysics Data System (ADS)

    Cunningham, M.; Stark, C. P.; Kaplan, M. R.; Schaefer, J. M.; Galewsky, J.; Yoo, J.

    2015-12-01

    One significant way that climate influences orogenic evolution is by modulating glacial erosion. At mid-latitudes it is hypothesized that this climate-tectonic interplay is so strong that a "glacial buzzsaw" acting throughout the Quaternary outpaced tectonic uplift in most mountain belts and concentrated topography in a zone defined by the bounds of ELA fluctuation. Less attention has been paid to how the buzzsaw might manifest itself at low latitudes, where many mountain belts are just high enough to have been glaciated at the LGM but today sit well below the ELA. We have focused on the glacial history of Costa Rica and Taiwan, where we find evidence of ice cap erosion coincident with low-relief landscapes near the LGM ELA. Previous attempts to understand the formation of these perched, low-relief landscapes has mostly concerned interactions between fluvial erosion and geodynamics. Our work aims instead to describe the role that glacial erosion played in the evolution of these landscapes, and how they fit in the buzzsaw paradigm. At Cerro Chirripó in Costa Rica we use 10-Be surface exposure age dating of moraine boulders and scoured bedrock, field mapping, and remote sensing to constrain the timing, areal extent, and pattern of glacial erosion. We made similar observations of ice extent at Nanhudashan in Taiwan, where surface exposure age dating has previously been applied to glacial landforms (e.g. Hebenstreit et al., 2011; Siame et al., 2007). In Costa Rica, our 10-Be dates from scoured bedrock near the highest peak and terminal/lateral moraines show signs of ice-cap erosion until 22 ka. Similar arguments for LGM ice cap erosion have been made for Nanhudashan. Regional climate simulations (WRF) further constrain the timing and spatial extent of glaciation in these places, and the combination of field data and climate modeling will inform estimates of the magnitude of glacial erosion on perched landscapes.

  8. Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling

    NASA Astrophysics Data System (ADS)

    Lüschen, Ewald; Lammerer, Bernd; Gebrande, Helmut; Millahn, Karl; Nicolich, Rinaldo; Transalp Working Group

    2004-09-01

    The TRANSALP Group, comprising of partner institutions from Italy, Austria and Germany, acquired data on a 340 km long deep seismic reflection line crossing the Eastern Alps between Munich and Venice. Although the field work was split into four campaigns, between fall 1998 and summer 2001, the project gathered for the first time a continuous profile across the Alps using consistent field acquisition and data processing parameters. These sections span the orogen itself, at its broadest width, as well as the editor Fred Davey and the two adjacent basins. Vibroseis and explosion data, complementary in their depth penetration and resolution characteristics, were obtained along with wide-angle and teleseismic data. The profile shows a bi-vergent asymmetric structure of the crust beneath the Alpine axis which reaches a maximum thickness of 55 km, and 80-100 km long transcrustal ramps, the southward dipping 'Sub-Tauern-Ramp' and the northward-dipping 'Sub-Dolomites-Ramp'. Strongly reflective patterns of these ramps can be traced towards the north to the Inn Valley and towards the south to the Valsugana thrust belt, both of which show enhanced seismicity in the brittle upper crust. The seismic sections do not reveal any direct evidence for the presence of the Periadriatic Fault system, the presumed equivalent to the Insubric Line in the Western Alps. According to our new evolutionary model, the Sub-Tauern-Ramp is linked at depth with remnants of the subducted Penninic Ocean. The 'crocodile'-type model describes an upper/lower crustal decoupling and wedging of both the European and the Adriatic-African continents.

  9. Multi-mineral geochronology: insights into crustal behaviour during exhumation of an orogenic root

    NASA Astrophysics Data System (ADS)

    LaFlamme, Crystal; McFarlane, Christopher R. M.; Fisher, Christopher M.; Kirkland, Christopher L.

    2017-03-01

    Under rare conditions, reworked cratons and their margins preserve the orogenic roots of ancient mountain-building events. However, based on the preservation of high-temperature ( 800 °C), middle and lower crustal metamorphic assemblages, present day exposure of these terrains is not simply a result of protracted denudation, but also must reflect a multifaceted exhumational history. In situ analysis within thin section preserves the textural setting of target minerals that can be used as thermochronometers such as U-Pb of zircon, monazite, titanite and apatite, and Sm-Nd of apatite. In situ analyses of these chronometers has the potential to provide critical timing constraints on exhumation processes related to decompression, melting and cooling across large metamorphic terrains. The Repulse Bay block of the Rae craton preserves a large composite amphibolite-granulite area (50,000 km2) of Archean orthogneiss, migmatite, and slivers of Proterozoic metasediments that underwent high-grade metamorphism, partial melting, ductile flow and finally exhumation during the Paleoproterozoic Trans-Hudson Orogeny. The granulite domain preserves dry granitoid assemblages, whereas the amphibolite domain is dominated by hydrated migmatites and orthogneiss. Metasediments occur in both domains and preserve mineral assemblages that are consistent with having undergone tectonometamorphic conditions of 9 kbar/800 °C during burial. U-Pb thermochronometers document identical cooling histories of the granulite and amphibolite domains through the U-Pb closure temperatures of titanite ( 650 °C) and apatite ( 450 °C). This suggests that melt-loss from the underlying granulite domain and melt-gain to the amphibolite domain prior to cooling through 650 °C are a controlling factor of the metamorphic assemblages across the composite granulite-amphibolite terrains such as the Repulse Bay block, rather than significant differences in burial history, cooling history, and/or reorganization of

  10. Thin visous sheet modelling of orogen scale deformation. The Eastern Alps in plan view

    NASA Astrophysics Data System (ADS)

    Robl, J.; Stuewe, K.

    2003-04-01

    We present first results of a new numerical model to describe the dynamic evolution of the eastern Alps in plan view on the orogen scale. We investigate the influence of boundary conditions, gravitational potential energy, rheology contrast of major tectonic units and internal structures on the deformation field. We aim at estimating the Argan number of the Eastern Alps and to calculate slip rates along big lineaments that represent the northern and southern border of the Austro-Alpine extrusion corridor. A further aim is to predict the position and the activity of major faults where they disappear below quartenary sediments. All calculation are perfomed with the mechanical finite element code BASIL that allows computation on a thin visous sheet. The starting conditons are controlled by the varying crustal thickness of the region and by the rheolgy of the Adriatic indenter in the south, the Bohemian massif in the north and the Eastern Alps inbetween. We assume that the Eastern Alps are fixed to the north and the west while the southern boundary moves northward at a rate of 6-8 mm /y. The geodynamic setting in the east changed over the last 5 my. While a roll back subduction zone beneath the Carparthian belt accompanied by extension, crustal thinning and basin formation controlled the deformation of the Eastern Alps until the Miocene. Later on subduction stopped and the overall stress field changed from extension to compression resulting in uplift of many basins and the lack sediments younger than Pliocene. This well known variation of plate tectonic scenarios over the last 5 my allows us to place tight constraints for the boundary conditions of our model. Although our work is only now in progress, we can report some promising results: some of the kinematics and mechaniscs predicted by our modelling are consistent with field observations of the structural geologists and geodeticists.

  11. Uplift mechanism of orogens inferred from GRACE temporal gravity changes - example of Qinghai-Tibet

    NASA Astrophysics Data System (ADS)

    Braitenberg, C. F.; Shum, C. K.

    2015-12-01

    Orogenic areas are subject to uplift and horizontal deformation as observed by present-day global positioning system and repeated leveling measurements. Crustal mass is conservative and less dense than the mantle, thus the horizontal shortening must be accompanied by crustal thickening and horizontal extrusion. According to the level of isostatic compensation, the thickening is partitioned into topographic uplift and Moho deepening. We investigate the mass change induced gravity signal and discuss whether this signal could be detected using terrestrial or satellite gravity observations. An example is the Qinghai-Tibet plateau, for which we model crustal thickening and calculate the expected gravity signal. The predictions are compared with present-day gravity changes observed by GRACE and with published in situ absolute gravity rates. It is found that the crustal thickening signal cannot be neglected and that it contributes significantly to the observed signal. Those studies with focus on the glacier and hydrologic mass fluxes should be aware that, if neglected, the crustal signal could introduce a significant bias. The GRACE observations give a positive gravity rate over central Tibetan Plateau, unexplained by the hydrologic or cryospheric signals, and a negative rate over the Himalayas and at its foothill, which is attributable to a prolonged hydrologic drought and/or depletion of groundwater. Our model shows that the positive gravity rate could be explained by elevation uplift, and a stable or upwelling Moho. The negative gravity change signal is due primarily to the strong elevation-gradient at the foothill of the Himalayas, and to an uplift accompanied by crustal thickening and Moho lowering. Finally, we place constraints and requirements on future gravity missions, for the feasibility to more accurately observe this signal, and to separate it from the background hydrologic and cryospheric processes.

  12. Magnetic signatures related to orogenic gold mineralization, Central Lapland Greenstone Belt, Finland

    NASA Astrophysics Data System (ADS)

    Airo, M.-L.; Mertanen, S.

    2008-03-01

    A number of lode-gold occurrences are hosted by hydrothermally altered greenstones along the southern boundary of the Palaeoproterozoic Central Lapland Greenstone Belt. The hydrothermally altered and mineralised zones are related to a major thrust and shear zone system that extends much across northern Finland. Spatial correlation between mineralized zones, brittle structural features and chemical alteration was explored and identified from high-resolution aeromagnetic data, in combination with airborne electromagnetic and gamma-ray spectrometric data and coupled with petrophysical and palaeomagnetic studies. The most prominent magnetic, ductile signatures formed during the Svecofennian Orogeny (1900-1800 Ma), resulting in elastic, curved, continuous magnetic patterns. These elastic anomaly patterns were disturbed by tectonic stress from S-SW, resulting in parallel, regularly oriented fracture families and thrust faults normal to the main stress direction. According to aeromagnetic, palaeomagnetic and structural evidence, the thrust zone was active during the latest stage of the orogenic event, but was also reactivated at a later date. Airborne gamma-ray data reveals zones of potassic alteration in the ultramafic rock units in the vicinity of cross-sections of these two fault sets. Chemical and mineralogical changes during alteration and metamorphism strongly affected the mafic and ultramafic host rocks throughout the deformation zone. The strong potassium enrichment and coinciding destruction of magnetic minerals resulted in enhanced potassium concentration and reduction of magnetic anomaly amplitudes. Palaeomagnetic results indicate that the remanent magnetization for the altered ultramafic rocks along the thrust zone is of chemical origin (CRM) and was acquired at 1880-1840 Ma, which is presumed also to be the age of the chemical alteration related to gold mineralization.

  13. Understanding erosion rates in the Himalayan orogen: A case study from the Arun Valley

    NASA Astrophysics Data System (ADS)

    Olen, Stephanie M.; Bookhagen, Bodo; Hoffmann, Bernd; Sachse, Dirk; Adhikari, D. P.; Strecker, Manfred R.

    2015-10-01

    Understanding the rates and pattern of erosion is a key aspect of deciphering the impacts of climate and tectonics on landscape evolution. Denudation rates derived from terrestrial cosmogenic nuclides (TCNs) are commonly used to quantify erosion and bridge tectonic (Myr) and climatic (up to several kiloyears) time scales. However, how the processes of erosion in active orogens are ultimately reflected in 10Be TCN samples remains a topic of discussion. We investigate this problem in the Arun Valley of eastern Nepal with 34 new 10Be-derived catchment-mean denudation rates. The Arun Valley is characterized by steep north-south gradients in topography and climate. Locally, denudation rates increase northward, from <0.2 mm yr-1 to ~1.5 mm yr-1 in tributary samples, while main stem samples appear to increase downstream from ~0.2 mm yr-1 at the border with Tibet to 0.91 mm yr-1 in the foreland. Denudation rates most strongly correlate with normalized channel steepness (R2 = 0.67), which has been commonly interpreted to indicate tectonic activity. Significant downstream decrease of 10Be concentration in the main stem Arun suggests that upstream sediment grains are fining to the point that they are operationally excluded from the processed sample. This results in 10Be concentrations and denudation rates that do not uniformly represent the upstream catchment area. We observe strong impacts on 10Be concentrations from local, nonfluvial geomorphic processes, such as glaciation and landsliding coinciding with areas of peak rainfall rates, pointing toward climatic modulation of predominantly tectonically driven denudation rates.

  14. Vestiges of The Peri-rodinian Ocean: Sliced, Diced, Recycled But Preserved In Younger Orogens

    NASA Astrophysics Data System (ADS)

    Murphy, J. B.; Nance, R. D.; Keppie, J. D.; Dostal, J.

    Just as the amalgamation and dispersal of Pangea exerted a first-order influence on tectonothermal events in the Phanerozoic, Middle to late Proterozoic global scale tec- tonics were profoundly influenced by the amalgamation and subsequent breakup of the supercontinent Rodinia. Most tectonic studies of the evolution of Rodinia concentrate on the near-field effects of these events such as the collisional orogenies that result in its amalgamation and the sedimentary rift-drift record of its breakup. However, ves- tiges of the far field effects of these events are also preserved and their tectonothermal evolution can provide additional constraints on supercontinent configuration and the timing of breakup. During the time interval of Rodinian amalgamation, for exam- ple, ensimatic subduction and plume activity in the peri-Rodinian ocean resulted in the formation of oceanic crust with ca. 1.0 Ga Sm-Nd depleted mantle model (TDM) ages. Vestiges of this crust are now preserved in the terranes that subsequently accreted along the margins of the dispersing continents, including the peri-Gondwanan terranes (e.g. Avalonia, Carolina, Iberia) of eastern North America and Europe, the Tocantins orogenic belt of Brazil, and the terranes of the Arabian-Nubian Shield. The Neopro- terozoic tectonothermal evolution of these terranes was a far-field response to Rodinia breakup, in the same way that the Mesozoic and Cenozoic evolution of western North America was a far-field response to the breakup of Pangea. Hence, the Neoproterozoic histories of these terranes are geodynamically linked to that of Rodinia and so provide constraints on its configuration and the timing of its major tectonothermal events.

  15. New geomorphic data on the active Taiwan orogen: A multisource approach

    NASA Technical Reports Server (NTRS)

    Deffontaines, B.; Lee, J.-C.; Angelier, J.; Carvalho, J.; Rudant, J.-P.

    1994-01-01

    A multisource and multiscale approach of Taiwan morphotectonics combines different complementary geomorphic analyses based on a new elevation model (DEM), side-looking airborne radar (SLAR), and satellite (SPOT) imagery, aerial photographs, and control from independent field data. This analysis enables us not only to present an integrated geomorphic description of the Taiwan orogen but also to highlight some new geodynamic aspects. Well-known, major geological structures such as the Longitudinal Valley, Lishan, Pingtung, and the Foothills fault zones are of course clearly recognized, but numerous, previously unrecognized structures appear distributed within different regions of Taiwan. For instance, transfer fault zones within the Western Foothills and the Central Range are identified based on analyses of lineaments and general morphology. In many cases, the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently. In turn, the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic featues in other areas. Examples are the conjugate pattern of strike-slip faults within the Central Range and the oblique fold-and-thrust pattern of the Coastal Range. Furthermore, neotectonic and morphological analyses (drainage and erosional surfaces) has been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in Taiwan, such as for the Longitudinal Valley Fault. Next, at a more general scale, numerical processing of digital elevation models, resulting in average topography, summit level or base level maps, allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance. Finally, a preliminary morphotectonic sketch map of Taiwan, combining information from all the sources listed above, is presented.

  16. Upper mantle structure beneath the Alpine orogen from high-resolution teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Lippitsch, Regina; Kissling, Edi; Ansorge, JöRg

    2003-08-01

    To understand the evolution of the Alpine orogen, knowledge of the actual structure of the lithosphere-asthenosphere system is important. We perform high-resolution teleseismic tomography with manually picked P wave arrival times from seismograms recorded in the greater Alpine region. The resulting data set consists of 4199 relative P wave arrivals and 499 absolute P wave arrivals from 76 teleseismic events, corrected for the contribution of the Alpine crust to the travel times. The three-dimensional (3-D) crustal model established from controlled-source seismology data for that purpose represents the large-scale Alpine crustal structure. Absolute P wave arrival times are used to compute an initial reference model for the inversion. Tests with synthetic data document that the combination of nonlinear inversion, high-quality teleseismic data, and usage of an a priori 3-D crustal model allows a reliable resolution of cells at 50 km × 50 km × 30 km. Hence structures as small as two cells can be resolved in the upper mantle. Our tomographic images illuminate the structure of the uppermost mantle to depth of 400 km. Along strike of the Alps, the inversion reveals a high-velocity structure that dips toward the SE beneath the Adriatic microplate in the western and central Alps. In the eastern Alps we observe a northeastward dipping feature, subducting beneath the European plate. We interpret this feature in the western and central Alps as subducted, mainly continental European lower lithosphere. For the east, we propose that parts of the Vardar oceanic basin were subducted toward the NE, forcing continental Adriatic lower lithosphere to subduct northeastward beneath the European plate.

  17. Modeling Structural and Mechanical Responses to Localized Erosional Processes on a Bivergent Orogenic Wedge

    NASA Astrophysics Data System (ADS)

    Marzen, R.; Morgan, J. K.

    2014-12-01

    Critical Coulomb wedge theory established that orogenic and accretionary wedges should develop self-similarly and maintain a critical taper that reflects the balance of strength of the wedge material and a basal décollement. However, a variety of geological processes can perturb that balance, forcing readjustment of the wedge. For example, glacial erosion and landsliding can concentrate erosion on a localized portion of the wedge slope, leaving that portion of the wedge with an out-of-equilibrium slope that would need to re-develop for the wedge to resume self-similar growth. We use the discrete element method to analyze how growing bivergent wedges with different cohesive strengths respond structurally and mechanically to erosional events localized along upper, middle, and lower segments of the pro-wedge. Mechanically, pro-wedge erosion results in a sudden decrease followed by a quick recovery of the mean stress and maximum shear stress throughout the pro-wedge. However, when erosion is localized in the mid- to lower portions of the pro-wedge, a zone of increased mean stress develops where the wedge is concentrating deformation to recover its taper. In contrast, when erosion is localized in the upper axial zone, there is almost no recovery of the wedge taper, reflecting the fact that the material at the top of the wedge is being carried passively in a transition zone between the pro-wedge and retro-wedge. Structurally, wedges composed of lower cohesion material recover their critical taper almost immediately through distributed deformation, while wedges of higher-cohesion material recover more slowly, and incompletely, by concentrating deformation along existing fault surfaces. As a result, localized erosional episodes can have a lasting effect on the wedge morphology when the wedge is composed of higher cohesion material.

  18. Doming in compressional orogenic settings: New geochronological constraints from the NW Himalaya

    NASA Astrophysics Data System (ADS)

    Robyr, Martin; Hacker, Bradley R.; Mattinson, James M.

    2006-04-01

    In the central and southeastern parts of the Himalayas, the High Himalayan Crystalline (HHC) high-grade rocks are mainly exhumed in the frontal part of the range, as a consequence of a tectonic exhumation controlled by combined thrusting along the Main Central Thrust (MCT) and extension along the South Tibetan Detachment System (STDS). In the NW Himalaya, however, the hanging wall of the MCT in the frontal part of the range consists mainly of low- to medium-grade metasediments (Chamba zone), whereas most of the amphibolite facies to migmatitic gneisses of the HHC of Zanskar are exposed in a more internal part of the orogen as a large-scale dome structure referred to as the Gianbul dome. This Gianbul dome is cored by migmatitic paragneisses formed at peak conditions of 800°C and 8 kbar. This migmatitic core is symmetrically surrounded by rocks of the sillimanite, kyanite ± staurolite, garnet, biotite, and chlorite mineral zones. The structural data from the Miyar-Gianbul Valley section reveal that the Gianbul dome is bounded by two major converging thrust zones, the Miyar Thrust Zone and the Zanskar Thrust Zone, which were reactivated as ductile zones of extension referred to as the Khanjar Shear Zone (KSZ) and the Zanskar Shear Zone (ZSZ), respectively. Geochronological dating of monazites from various migmatites and leucogranite in the core of the Gianbul dome indicates ages between 26.6 and 19.8 Ma. These results likely reflect a high-temperature stage of the exhumation history of the HHC of Zanskar and consequently constrain the onset of extension along both the ZSZ and the KSZ to start shortly before 26.6 Ma. Several recent models interpret that ductile extrusion of the high-grade, low-viscosity migmatites of HHC reflects combined extension along the ZSZ and thrusting along the MCT. Hence our new data constrain the onset of the thrusting along the MCT to start shortly before 26.6 Ma.

  19. Differential decay of the East-African Antarctic Orogen : an integrated examination of Northeastern Mozambique

    NASA Astrophysics Data System (ADS)

    Ueda, K.; Jacobs, J.; Emmel, B.; Thomas, R. J.; Matola, R.

    2009-04-01

    In Northeastern Mozambique, the late Proterozoic - early Paleozoic East African-Antarctic Orogen can be subdivided into two major blocks that exhibit some relevant differences. The line of divide is represented by the Lurio Belt, a kinematically poorly constrained shear zone that also marks the conceptual northern limit of frequent late-tectonic granitoid intrusions. Moreover, far-travelled granulite-facies nappes cover a much larger area north of this belt (Viola et. al, 2008), giving rise to the assumption of different exhumation and present exposure levels. U/Pb data from previous surveys (e.g., Norconsult consortium, 2007) show coeval high-grade metamorphism in the whole region between c. 610 - 550 Ma, while the block south of the Lurio Belt also shows continuing metamorphism until c. 490 Ma that can be related to extension. Geothermobarometry for samples from within the Lurio Belt (Engvik et. al, 2007) indicates rapid exhumation after high-pressure granulite facies metamorphism and is consistant with the assumption of long tectonic activity. A possible model for the outlined pattern is the delamination of the orogenic root only in the southern part, followed by rapid mechanical thinning as well as by isostatic accommodation along the Lurio Belt. A valuable marker was identified in the metasedimentary Mecuburi group that overlies the southern basement. U/Pb analysis of detrital zircons have yielded a maximum deposition age of c. 600 Ma, while metamorphism is recorded until c. 505 Ma. Investigations of the relationship between metasediments and older basement show that the basal contact is a fairly preserved depositional contact, allowing to suppose a conjoint post-depositional evolution. It is notable that the timing of deposition shortly follows the onset of the main, widespread high-grade metamorphism. Relatively high but variable degrees of migmatisation in the Mecuburi Group require a phase of burial from surface to deep levels after 600 Ma, followed by

  20. A metamorphic mineral source for tungsten in the turbidite-hosted orogenic gold deposits of the Otago Schist, New Zealand

    NASA Astrophysics Data System (ADS)

    Cave, Ben J.; Pitcairn, Iain K.; Craw, Dave; Large, Ross R.; Thompson, Jay M.; Johnson, Sean C.

    2016-09-01

    The orogenic gold deposits of the Otago Schist, New Zealand, are enriched in a variety of trace elements including Au, As, Ag, Hg, W and Sb. We combine laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) traverses and images to show that detrital rutile is the most important host mineral for W in the subgreenschist facies rocks. Furthermore, the prograde metamorphic recrystallisation of detrital rutile to titanite releases significant amounts of W (potentially 0.41 g/tonne of rock). Scheelite development closely follows the progression of this W-liberating reaction. Scheelite micrograins form early within the fabric of the rock evolving to locally and regionally sourced scheelite-bearing veins. Scheelite from syn-metamorphic veins at Fiddlers Flat and Lake Hāwea shows distinct differences in composition compared with scheelite from late-metamorphic veins at the Macraes Mine, the latter of which is enriched in REEs, Y and Sr. We suggest that the scheelite at Macraes became enriched due to the liberation of these elements during alteration of the Ca-silicate minerals epidote and titanite by the ore-forming fluid. These results are supportive of recent models for orogenic gold mineralisation in the Otago Schist, whereby prograde metamorphic recrystallisation of diagenetic or detrital metal-rich mineral phases (pyrite to pyrrhotite: Au, As, Ag, Hg and Sb; rutile to titanite: W) releases significant amounts of metals into the concurrently developing metamorphic fluids that can be subsequently focussed into regional structures and form significant tungsten-bearing orogenic gold deposits.

  1. Partial melting of the South Qinling orogenic crust, China: Evidence from Triassic migmatites and diorites of the Foping dome

    NASA Astrophysics Data System (ADS)

    Zhang, He; Ye, Ri-Sheng; Liu, Bing-Xiang; Wang, Yan; Zhang, Yuan-Shuo; Siebel, Wolfgang; Chen, Fukun

    2016-09-01

    The Qinling orogen was ultimately formed by suturing of the South Qinling and Yangtze blocks, but the exact timing of the final amalgamation of the two blocks has not been well established so far. Partial melting of the Qinling orogenic continental crust resulted in the generation of migmatites, and such rocks may help to decipher the chronology of such event. In this paper, we report U-Pb ages, trace element, and Hf isotopic compositions of zircons from migmatites and diorite gneisses of the Foping dome, South Qinling. Zircons from migmatites form anhedral grains of variable sizes that are characterized by complex trace element compositions. Based on zircon U-Pb ages, the migmatites can be subdivided into two groups: Group 1 migmatites mainly retain Triassic zircons with U-Pb ages of 214-211 Ma and Hf model ages of 1.46 Ga in core and rim domains; zircons from Group 2 migmatites record both Triassic ( 210 Ma) and Neoproterozoic U-Pb ages, analogous to igneous rocks of the Wudang and Yaolinghe Groups exposed in South Qinling. Zircons from the diorite gneisses yield U-Pb ages of 216-210 Ma with Hf isotopic composition (TDM2 ages of 1.46 Ga) similar to the migmatites. Evidence from whole-rock Nd isotopic analyses also points to a similar genesis between migmatites and diorite gneisses. It is proposed that Group 1 migmatites were derived by melting of Triassic diorites, while Group 2 migmatites were derived from Neoproterozoic igneous rocks, a major basement lithology of South Qinling. Partial melting of the orogenic crust took place at 214-210 Ma, approximately consistent with the retrograde metamorphism of granulites exposed along the suture zone between the South Qinling and Yangtze blocks. We suggest that the collision of these two blocks occurred prior to 215 Ma and that the Foping dome resulted from rapid collapse of an overthickened crust followed by partial melting enhanced by asthenospheric influx.

  2. Signature of Cenozoic orogenic movements in combustion metamorphic rocks: mineralogy and geochronology (example of the Salair-Kuznetsk Basin transition)

    SciTech Connect

    Novikov, I.S.; Sokol, E.V.; Travin, A.V.; Novikova, S.A.

    2008-06-15

    Cenozoic combustion metamorphic (CM) complexes produced by fossil natural coal fires are widespread at range-basin junctions worldwide. Large-scale fires accompany the initial orogenic phases as fresh coal-bearing strata become drawn into the aeration zone as a result of crustal deformation. In combustion metamorphism, the protolith melts to different degrees either into ferrous basic paralava or in glassy clinker. The melt rocks have a phase composition favorable for Ar-40/Ar-39 dating of ignition coeval with the onset of each episode in Late Cenozoic orogenic events. We suggest an algorithm providing correct Ar-40/Ar-39 age determination of CM rocks followed by well-grounded geological interpretation and test the new approach on melt rocks from the Kuznetsk Coal Basin. Paralava samples were dated by Ar-40/Ar-39 incremental heating and the isotope ratios were corrected for Ca-, Cl-, and K-derived Ar isotopic interferences. The interpretation of age-spectrum results was checked against internal and external criteria. The former were plateau and isochrone ages and the latter included the so-called 'couple criterion' and conventional relative ages inferred from geological and stratigraphic evidence. As a result, we distinguished two groups of dates for combustion metamorphic events bracketed between 1.2 {+-} 0.4 and 0.2 {+-} 0.3 Ma. The older ages represent rocks in the western edge of the Prokopievsk-Kiselevsk block of the Salair zone and the younger dates correspond to those in its eastern edge. The reported dates record the time when the fault boundaries of the blocks were rejuvenated during recent activity and the block accreted to the Salair orogenic area as a submontane step. The suggested approach to the choice of objects, classification of rocks, and interpretation of Ar-40/Ar-39 spectra is universal and can be practiced in any area of combustion metamorphism.

  3. Distant effects in bivergent orogenic belts - How retro-wedge erosion triggers resource formation in pro-foreland basins

    NASA Astrophysics Data System (ADS)

    Hoth, Silvan; Kukowski, Nina; Oncken, Onno

    2008-08-01

    Timeseries derived from two-dimensional sandbox simulations involving surface erosion are taken for the first time to be implemented into flexure calculations of foreland basins. Based on our results we highlight that orogenic systems are a four component system, consisting of a pro-foreland basin, a pro-wedge, a retro-wedge, and a retro-foreland basin. These four components are mechanically coupled via the load dependence of tectonic faulting [Mandl, G., 1988. Mechanics of tectonic faulting, 1st Edition. Elsevier, Amsterdam.] and the finite flexural rigidity of lithospheric plates [Beaumont, C., 1981. Foreland basins. Geophys. J. R. Astron. Soc. 5 (2), 291-329.]. We further demonstrate that the impact of pro-wedge erosion is most pronounced within the pro-wedge but also modifies the shape and size of the retro-wedge, which in turn changes the geometry and propagation velocity of the retro-foreland basin and vice versa. This suggests that one out of the four components of an orogenic system cannot be fully understood without recognition of the other three components. Thus, spatial separation between processes or observations does not necessarily imply their physical independence. This conceptual model is applied in a case study to the Pyrenean orogenic wedge and its Ebro and Aquitaine foreland basins. Our analysis suggests that the Pyrenean pro- and retro-wedge are mechanically coupled and that this coupling manifests itself in the migration of depocentres in both foreland basins. We finally explore implications for the formation of Mississippi Valley Type deposits.

  4. Short episodes of crust generation during protracted accretionary processes: Evidence from Central Asian Orogenic Belt, NW China

    NASA Astrophysics Data System (ADS)

    Tang, Gong-Jian; Chung, Sun-Lin; Hawkesworth, Chris J.; Cawood, P. A.; Wang, Qiang; Wyman, Derek A.; Xu, Yi-Gang; Zhao, Zhen-Hua

    2017-04-01

    Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (∼540-270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ∼400 Ma and 280 Ma for the Altai segment, and ∼430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (∼20-40 Ma), and variations in magma volumes and in Nd-Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting. The Junggar segment is characterized by high and uniform Nd-Hf isotope ratios (εNd (t) = + 5 to + 8; zircon εHf (t) = + 10 to + 16) and it appears to have formed in an intra-oceanic arc system. In the Altai and Chinese Tianshan segments, the Nd-Hf isotope ratios (εNd (t) = - 7 to + 8; zircon εHf (t) = - 16 to + 16) are lower, although they increase with decreasing age of the rock units. The introduction of a juvenile component into the Chinese Tianshan and Altai granitic rocks appears to have occurred in continental arc settings and it reflects a progressive reduction in the contributions from old continental lower crust and lithospheric mantle. Within the long-lived convergent margin setting (over ∼200 Ma), higher volumes of magma, and greater contributions of juvenile material, were typically emplaced over short time intervals of ∼20-40 Ma. These intervals were associated with higher Nb/La ratios, coupled with lower La/Yb ratios, in both the mafic and granitic rocks, and these episodes of increased magmatism from intraplate-like sources are therefore thought to have been in response to lithospheric extension. The trace element and Nd-Hf isotope data, in combination with estimates of granitic magma volumes, highlight

  5. Laserprobe 40Ar/39Ar Dating Of Strain Fringes: An Example From The Interior Of The Sevier Orogen

    NASA Astrophysics Data System (ADS)

    Wells, M. L.; Spell, T. L.; Hoisch, T. D.; Arriola, T.; Zanetti, K. A.

    2008-12-01

    Determining the absolute ages of deformation fabrics in metamorphic rocks is critical to understanding the nature of orogenesis in deeply exhumed ancient mountain belts. In situ isotopic analysis of fibrous strain fringes is particularly advantageous because the relationships between the analyzed mineral, deformation fabric, and isotopic age can be established, and mineral growth can be linked to the progressive strain history. Furthermore, when isotopic closure temperatures (Tc) are higher than crystallization temperatures, isotopic ages reflect deformation ages. We have applied in situ 40Ar/39Ar UV and CO2 laser techniques to dating phlogopite in quartz-phlogopite-calcite and muscovite in quartz-muscovite fibrous strain fringes in greenschist-facies metamorphic rocks from the Grouse Creek Mountains (Utah) and Black Pine Mountains (Idaho). In both cases, syntectonic mica growth occurred below the minerals Tc. Phlogopite 40Ar/39Ar ages for individual strain fringes in the Grouse Creek Mountains range from 92 Ma to 110 Ma, with the most reliable ages ranging from 101 Ma to 110 Ma (mean age, 105.0 plus or minus 5.8 Ma). Muscovite 40Ar/39Ar ages for individual strain fringes in the Black Pine Mountains range from 97 Ma to 112 Ma (mean age, 104.7 plus or minus 5.8 Ma). Strain fringes are associated with a sub-horizontal foliation and a generally N-trending elongation lineation exhibiting components of top-to-the-north simple shear and coaxial strain accommodating N-S extension and sub-vertical shortening. The new dates confirm prior correlation of fabrics between these ranges and yield a mid-Cretaceous age for the earliest ductile fabric preserved in the Raft River-Albion-Grouse Creek metamorphic core complex, in the interior (hinterland) of the retroarc Sevier orogenic belt. Mid-crustal northward flow at 105 (plus or minus 6) Ma within the interior of the Sevier orogen, coeval with east-directed shortening in the foreland and with plate convergence, records orogen

  6. Mapping the Structure of the Lithosphere-Asthenosphere System Under the Alpine Orogen with High-Resolution Teleseismic Tomography

    NASA Astrophysics Data System (ADS)

    Lippitsch, R.; Kissling, E.; Ansorge, J.

    2001-12-01

    Understanding the evolution of the Alpine orogen and the interaction between different lithospheric blocks requires precise knowledge of the structure of the lithosphere-asthenosphere system. To assess the gross features of the uppermost mantel we perform high-resolution teleseismic tomography. The data base encompasses 5000 manually picked first P-arrivals from 220 teleseismic events with even azimuthal distribution recorded at permanent and temporary seismic networks in the greater Alpine area. The tomographic study consists of these components: (1) Corrections for the contribution of the Alpine crust to travel-times of incoming wave fields that may account for up to 50% of the observed travel-time residuals. The 3-D crustal model established from controlled-source seismology data represents the large-scale Alpine crustal structure which clearly reflects the effects of the African-European plate collision. (2) Tests with synthetic data document that the combination of non-linear inversions, high-quality teleseismic data, and usage of an a priori 3-D crustal model allows reliable resolution of cells at 50km*50km*30km with a velocity variation in the order of +/- 3% in the upper mantle. (3) Our tomographic images illuminate the structure of the uppermost mantle to depth of 400 km reflecting the complex processes that formed the Alpine orogen when three different plates were amalgamated (European, Adriatic, and Ligurian plates). In the western Alps, the inversion results show a steep W-E dipping high-velocity anomaly which we interpret as the subducting European plate. In the eastern Alps we find high-velocity anomalies in a depth range of 150 km to 300 km beneath the axis of the orogen. At present, the relation of this material with European or Adriatic lithosphere remains unclear. Our results are in general agreement with earlier lithospheric studies. However, the increase in resolution illuminates significantly more complex lithospheric slab geometries, which

  7. Geochronological data of igneous and metamorphic rocks from the Xing'an-Mongolia Orogenic Belt of the eastern Central Asian Orogenic Belt: implications for the final closure of the Paleo-Asian Ocean

    NASA Astrophysics Data System (ADS)

    Guosheng, Wang; Chen, Wu; Cheng, Chen; Zhiguang, Zhou; Changfeng, Liu; Tian, Jiang

    2017-03-01

    In order to better constrain the evolution of the Xing'an-Mongolia Orogenic Belt and the resulting closure of the Paleo-Asian Ocean, we conducted an integrated investigation involving U-Pb dating of igneous and detrital zircon and a synthesis of existing work across the Erdaojing and Ondor Sum complexes in the eastern segment of the Central Asian Orogenic Belt. Geochronological analysis revealed a different tectonic setting between the Ondor Sum complex (Tulinkai area) and the Erdaojing accretion complex, as detrital zircon U-Pb analysis identified different major age provenances: (1) the Precambrian basement of the North China Craton and Paleozoic Bainaimiao Arc along the northern margin of North China to the south are the provenance rocks of the schists in the Tulinkai area, and (2) the Precambrian basement of the Xilinhot complex and Paleozoic Baolidao arc to the north are the provenance rocks of the schists in the Erdaojing area. We infer that the existence of the ocean between the above two regions in the late of Early Paleozoic in accordance with the result of this study. Furthermore, a comparison of the youngest age population in the intruded-diorite rock and metasedimentary rocks with U-Pb ages implies that the final closure of the Paleo- Asian Ocean along the Solonker Suture Zone most likely occurred in the period between 239 and 222 Ma (Late Paleozoic-Triassic).

  8. COSC-1 - drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia

    NASA Astrophysics Data System (ADS)

    Lorenz, H.; Rosberg, J.-E.; Juhlin, C.; Bjelm, L.; Almqvist, B. S. G.; Berthet, T.; Conze, R.; Gee, D. G.; Klonowska, I.; Pascal, C.; Pedersen, K.; Roberts, N. M. W.; Tsang, C.-F.

    2015-05-01

    The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project focuses on mountain building processes in a major mid-Palaeozoic orogen in western Scandinavia and its comparison with modern analogues. The project investigates the subduction-generated Seve Nape Complex. These in part under ultra-high-pressure conditions metamorphosed outer continental margin and continent-ocean transition zone assemblages were emplaced onto the Baltoscandian platform and there influenced the underlying allochthons and the basement. COSC-1 is the first of two ca. 2.5 km deep, fully cored drill holes located in the vicinity of the abandoned Fröå mine, close to the town of Åre in Jämtland, central Sweden. It sampled a thick section of the lower part of the Seve Complex and was planned to penetrate its basal thrust zone into the underlying lower-grade metamorphosed allochthon. The drill hole reached a depth of 2495.8 m and nearly 100 % core recovery was achieved. Although planning was based on existing geological mapping and new high-resolution seismic surveys, the drilling resulted in some surprises: the Lower Seve Nappe proved to be composed of rather homogenous gneisses, with only subordinate mafic bodies, and its basal thrust zone was unexpectedly thick (> 800 m). The drill hole did not penetrate the bottom of the thrust zone. However, lower-grade metasedimentary rocks were encountered in the lowermost part of the drill hole together with garnetiferous mylonites tens of metres thick. The tectonostratigraphic position is still unclear, and geological and geophysical interpretations are under revision. The compact gneisses host only eight fluid conducting zones of limited transmissivity between 300 m and total depth. Downhole measurements suggest an uncorrected average geothermal gradient of ~ 20 °C km-1. This paper summarizes the operations and preliminary results from COSC-1 (ICDP 5054-1-A), drilled from early May to late August 2014, and is

  9. Late-orogenic basins in the Archaean Superior Province, Canada: characteristics and inferences

    NASA Astrophysics Data System (ADS)

    Mueller, W. U.; Corcoran, P. L.

    1998-09-01

    The late-orogenic Archaean Duparquet, Kirkland and Stormy basins of the Canadian Superior Province are characterized by bounding crustal-scale faults and abundant porphyry stock emplacement. Lava flows and pyroclastic deposits are restricted to the Kirkland and Stormy basins, and coarse clastic detritus characterizes the Duparquet basin. Seven distinct lithofacies are identified: (1) mafic volcanic, (2) felsic volcanic, (3) pyroclastic, (4) volcaniclastic, (5) conglomerate-sandstone, (6) sandstone-argillite (± conglomerate), and (7) argillite-sandstone (± tuffaceous sandstone). The mafic and felsic volcanic lithofacies represent effusive lava flows, the pyroclastic lithofacies is formed of subaerial surge and airfall deposits and the volcaniclastic lithofacies is composed of reworked volcanic debris. The conglomerate-sandstone lithofacies is interpreted as alluvial fan, fan delta or proximal braided stream deposits, whereas the sandstone-argillite lithofacies is consistent with sandy-dominated flood- or braidplain deposits. A dominantly shallow-water lacustrine setting is inferred for the argillite-sandstone lithofacies. These different lithofacies record the basin history and can be used to identify basin-forming processes. Lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation. Volcanism can also influence basin evolution and the delicate balance between erosion, sedimentation, and prevalent transport processes is affected by volcanic input. Catastrophic influx of pyroclastic material facilitated mass-wasting processes and formation of non-confined hyperconcentrated flood flow deposits account for local congestion of alluvial or fluvial dispersal patterns. Confined stream flow processes govern sedimentation during intravolcanic phases or prominent tectonic uplift. In addition, climate which controls the weathering processes

  10. Late Devonian Anoxia Events in the Central Asian Orogenic Belt: a Global Phenomenon

    NASA Astrophysics Data System (ADS)

    Carmichael, S. K.; Waters, J. A.; Suttner, T. J.; Kido, E.; DeReuil, A. A.; Moore, L. M.; Batchelor, C. J.

    2013-12-01

    Atmospheric CO2 values decreased dramatically during the Middle Devonian due to the rapid rise of land plants. These changing environmental conditions resulted in widespread anoxia and extinction events throughout the Late Devonian, including the critical Kellwasser and Hangenberg anoxia events, which are associated with major mass extinctions at both the beginning and end of the Famennian Stage of the Late Devonian. Fammenian sediments in northwestern Xinjiang Province, China, represent a highly fossiliferous shallow marine setting associated with a Devonian oceanic island arc complex. Analysis of multiple geochemical proxies (such as U/Th, Ba, normalized P2O5, V/Cr, Zr), magnetic susceptibility, and mineralogical data (biogenic apatite and pyrite framboids) indicates that these Famennian sequences record not only the Upper Kellwasser Anoxic Event at the Frasnian/Famennian (F/F) boundary but also the rebound from the F/F extinction event. Preliminary evidence suggests that the Hangenberg Anoxic Event can also be recognized in the same sequence, although our biostratigraphic control is less precise. Previous studies of the Kellwasser and Hangenberg Events have been performed on continental shelf environments of Laurussia, Gondwana, Siberia, and South China. The Devonian formations of northwest Xinjiang in this study, however, are part of the Central Asian Orogenic Belt (CAOB), which is thought to have formed as part of a complex amalgamation of intra-oceanic island arcs and continental fragments prior to the end of the latest Carboniferous. These results allow us to confirm the presence of the Kellwasser and Hangenberg Events in the open oceanic part of Paleotethys, indicating that both events were global in scope. The presence of an abundant diverse Famennian fauna between these anoxia/extinction events suggests that the shallow marine ecosystems in the CAOB were somewhat protected due to their tectonic location and relative isolation within an open ocean system

  11. Thermomechanical analogue modeling of the lateral spreading of the Svecofennian orogen

    NASA Astrophysics Data System (ADS)

    Nikkilä, K.; S. Roy Chowdhury, B.; Dietl, C.; Korja, A.

    2011-12-01

    The modeling exercise simulated the late Svecofennian orogen processes in the Central Finland area. The Svecofennian orogeny in the Northern Europe is characterized by low pressure and high temperature metamorphism (1.88 Ga). The environment was similar to that presently in Tibetian plateau. We assume the thickened continental crust were 80 kilometer thick after the continental collision event, based on the deep seismic reflection profiles BABEL and FIRE. The profiles prove that the thickness of the crust is still 65 kilometers, furthermore, we suppose the 15-20 uppermost kilometers are eroded. The FIRE sections indicate the crust has three different composition layers: the upper, the middle and the lower crust. With rising temperatures, the middle crust became ductile, while the lower crust is more viscous. Thus, the brittle upper crust moves along the flowing middle crust. A set of thermomechanical analogue experiments were used to simulate the lateral spreading of the Svecofennian orogeny in high temperature circumstances. Layered models were constructed with brittle upper crust (sand), flowing middle crust (low viscosity plasticine) and non-flowing lower crust (high viscosity plasticine). The miniature models with dimensions of 12 cm x 12 cm x 8 cm were used to represent 120 km x 120 km wide and 80 km deep Earth's crust, that is, one centimeter corresponds to 10 kilometers. The modeling results are similar to the deep seismic reflection profiles, and to the field observations. The surface structures of the analogue model show normal faulting and fine thrusting in the upper crust. The middle crust was flowing in the experiment, but the thinning of the crust is not as obvious as that observed in the FIRE profiles. However, the lower crust bulges upward and is mainly constant like in the deep seismic profiles. More surprising are the structures which developed subvertical in 45° angle to the extensional direction, and extend from the upper to the lower crust

  12. Orogenic-type copper-gold-arsenic-(bismuth) mineralization at Flatschach (Eastern Alps), Austria

    NASA Astrophysics Data System (ADS)

    Raith, Johann G.; Leitner, Thomas; Paar, Werner H.

    2015-10-01

    high Hg content (up to 11 mass %). The Cu-Au deposits in the Flatschach area show similarities with meso- to epizonal orogenic lode gold deposits regarding the geological setting, the structural control of mineralization, the type of alteration, the early (stage 1) sulfide assemblage and composition of gold. Unique about the Flatschach district is the lower-temperature overprint of copper arsenides (domeykite and koutekite) and copper sulfides (djurleite, yarrowite/spionkopite) on earlier formed sulfide mineralization. Based on mineralogical considerations temperature of stage 2 mineralization was between about 70 °C and 160 °C. Gold was locally mobilized during this low-temperature hydrothermal overprint as well as during stage 3 supergene oxidation and cementation processes.

  13. Pre-orogenic structural inheritances control on the Provence thrust system, SE France

    NASA Astrophysics Data System (ADS)

    Bestani, Lucie; Espurt, Nicolas; Lamarche, Juliette; Hollender, Fabrice; Bellier, Olivier

    2015-04-01

    The Provence thrust system located to the southeast of France corresponds to the foreland of the Pyrenean and Alps belts, characteristic of the late Cretaceous-Eocene and Miocene to present-day compressions respectively. Paleozoic-Mesozoic pre-orogenic substratum geometry still raise questions and the shortening of Provence and associated partitioning among these two compressional events, which are responsible for the present Provence foreland structure has never been precisely quantified. The Provence foreland is divided into two parts by the Middle-Durance and Aix-en-Provence faults system: the western and eastern Provence. The Middle Durance/Aix-en-Provence faults system is inherited from Paleozoic times and is responsible for strong thickness variations in the Mesozoic sedimentary pile ranging from 3 (easternpart) to ~10 km (western part). N-S balanced cross sections (~130 km) between the Baronnies to the north and the Mediterranean Sea to the south reveal that the Provence foreland structure results of 91% of the Pyrenean shortening. The Alpine compression led to minor reactivation of Pyrenean-Provence structures with a Miocene shortening of ~650 m (0.5%). Cross-section balancing shows an along-strike heterogeneous total horizontal shortening from ~7.6 km (6%) in the west to ~51 km (27%) in the east. These results show that during the Pyrenean-Provence compression, the Middle Durance/Aix-en-Provence faults system played the role of an oblique transfer zone between the western and eastern Provence domains and separated two contrasted structural domains. Eastward where the sedimentary pile is thin (<4 km), the inversion of deep-seated late Paleozoic-Triassic extensional structures induced a thick-skin style. On the contrary, westward where the sedimentary pile is thick (>7km), the reactivation of basement structures is not necessary involved in the accommodation of the shortening, leading to a thin-skinned tectonics above Triassic series. Paleozoic basement

  14. Nature and timing of large landslides within an active orogen, eastern Pamir, China

    NASA Astrophysics Data System (ADS)

    Yuan, Zhaode; Chen, Jie; Owen, Lewis A.; Hedrick, Kathryn A.; Caffee, Marc W.; Li, Wenqiao; Schoenbohm, Lindsay M.; Robinson, Alexander C.

    2013-01-01

    Large-scale landsliding (involving ≫ 106 m3 in volume) is important in landscape development in high mountains. To assess the importance of large landslides in high mountains, four large landslides (Bulunkou, Muztagh, Taheman, and Yimake) were mapped in the NE Chinese Pamir at the westernmost end of the Himalayan-Tibetan orogen and dated using 10Be terrestrial cosmogenic nuclides. The Bulunkou landslide at the southernmost end of Muji Valley is composed of ~ 1.7 × 107 m3 of landslide debris and has an age of 2.0 ± 0.1 ka. The Muztagh landslide, located on the SW side of the massif Muztagh Ata, is composed of ~ 4.7 × 108 m3 of debris, and has an age of 14.3 ± 0.8 ka. The Taheman landslide, located south of Muztagh Ata, is composed of ~ 2.6 × 108 m3 of landslide debris and has an age of 6.8 ± 0.2 ka. The Yimake landslide, on the northern frontal range of the Pamir at the southwestern end of the Tarim basin, is composed of ~ 1.4 × 109 m3 of landslide debris and has an age of 7.1 ± 0.6 ka. Two other large landslides are present in the region, the Aerpa Aigezi (on a tributary of the Gez River) and the Bile Jiyi (on the Yarkand River) landslides, and are composed of ~ 1.6 × 107 m3 and ~ 5.2 × 106 m3 of landslide debris, respectively. However, the Aerpa Aigezi and Bile Jiyi landslides were not studied in as much detail or dated because of their inaccessibility. Given the tectonically active nature of this region, with numerous active faults, and the morphology of the landslides, these landslides were likely triggered by earthquakes. However, other causes — including long-term increased precipitation and geologic bedrock structure — could be important contributing factors in their formation.

  15. 3D crustal and lithospheric structure of the Pyrenean orogenic wedge

    NASA Astrophysics Data System (ADS)

    Theunissen, Thomas; Chevrot, Sébastien; Sylvander, Matthieu; Monteiller, Vadim; Villasenor, Antonio; Benahmed, Sébastien; Calvet, Marie

    2013-04-01

    The Pyrenean orogenic wedge is the consequence of the collision between the Iberian microplate and the southwesternmost part of the Eurasian plate from 55 to 25 Ma (Eocene to Oligocene). The shortening began since Late Cretaceous, about 100 My ago, leading to about 75 km from the west to 125 km to the east of continental crustal shortening. Before this period of time, the region was characterized by a sedimentary basin associated with a very important thinning that later controlled the deformation during the shortening process. Mantle outcrops are therefore present along and north of the North Pyrenean Fault with a scattered localization to the west toward the Mauleon basin. Today, the horizontal deformation rate is very low and the recent seismicity, mainly normal faulting mechanisms, is certainly caused by coupling between erosion and isostatic readjustments. Images from seismic reflection, gravity modeling, local and teleseismic seismic waves inversions and magnetotellurics inversions are in agreement with the subduction of Iberia beneath Aquitania. The 3D crustal structure reveals the presence of an important thickening of the continental crust associated with the subduction of the Iberian lower crust through the north beneath Aquitania at the favor of a detachment. Lateral variations of the geometry (including that of the Moho) and the wave propagation properties are important. In order to better analyze waveforms from local, regional or teleseismic earthquakes and to better constrain the geodynamical evolution of the Pyrenean chain over the time, PYROPE and TOPO-IBERIA projects were born. Two temporary seismic arrays (using broadband seismometers), on the French and Spanish sides, have been deployed between 2010 and 2013. We present here preliminary results on 3D crustal structures (approximately in the window 40° N-45° N and -4° E and 5° E) from arrival-times of about 20000 earthquakes recorded at about 200 seismic stations between 1978 and 2012

  16. Horizontal motions, bedrock incision, and the structure of relief in growing folds and orogens

    NASA Astrophysics Data System (ADS)

    Seixas, G.; Alejandre, J.; Hilley, G. E.

    2013-12-01

    Topographic divide asymmetry may arise from gradients in rock erodibility and orographic precipitation across a mountain belt, and/or tectonic displacement fields that uplift and translate rock horizontally during orogenesis. While simple models exploring relief in active orogens typically consider tectonic motions that are uniform and vertical, the displacement field at Earth's surface generated by slip on finite-length faults is inherently inhomogeneous and contains vertical and horizontal components, which may play an important role in divide asymmetry. In this study, we consider channel profiles that evolve according to the shear stress rule for bedrock fluvial erosion and that experience inhomogeneous horizontal and vertical components of tectonic motion. Dimensional analysis of our revised shear stress rule reveals a dimensionless coefficient that relates bedrock erodibility and basin geometry to slip rate on the underlying fault. We implement our model in a series of 1D non-dimensional numerical experiments that calculate river profile geometry on either side of a topographic divide that is free to advect through the model domain in response to horizontal motion. We drive the models with displacements calculated over a dipping, buried edge dislocation, and examine non-dimensional relief and divide asymmetry resulting from variations in fault dip, non-dimensional fault tip location and non-dimensional model extent. We find that asymmetry results from the full displacement field and from the vertical displacement field alone. Fault dip plays a strong role in the magnitude and direction of divide asymmetry, both in models that include the horizontal motions and those that neglect them. The greatest divide asymmetry resulting from the full displacement field is achieved over dislocations with shallow non-dimensional upper tip depths and small dip angles, although steeply-dipping faults produce asymmetry in the opposite direction. By setting the horizontal

  17. Partial melting in amphibolites in a deep section of the Sveconorwegian Orogen, SW Sweden

    NASA Astrophysics Data System (ADS)

    Hansen, Edward; Johansson, Leif; Andersson, Jenny; LaBarge, Leah; Harlov, Daniel; Möller, Charlotte; Vincent, Stephanie

    2015-11-01

    Garnet amphibolite metataxites at the Steningekusten Nature Reserve in southwestern Sweden contain tonalitic patches and veins. Whole rock chemistry suggests that the protoliths were mafic igneous rocks with alkaline affinities. Orthopyroxene megacrysts are present in leucosome in parts of these garnet amphibolites but absent in others. Orthopyroxene megacrysts were formed by vapor-absent melting initiated by incongruent melting of biotite followed by the breakdown of hornblende. The net reaction was Bt + Hbl + Pl +/- Qtz ↔ Opx + Melt + Cpx + Gt. Melting occurred at pressures of approximately 1 GPa and temperatures which probably exceeded 800 °C. Pyroxenes are surrounded by hornblende-quartz symplectites, and hornblende in these coronas has distinctly lower concentrations of (Na + K) and Ti than that in adjacent mesosome. The hornblende rims formed upon cooling and reaction with crystallizing melt. This created a barrier to further reaction thus preserving the orthopyroxene megacrysts. Garnet amphibolite metatexites lacking pyroxene megacrysts have features characteristic of vapor-present melting including lack of peritectic phases predicted by vapor-absent melting reactions, larger amounts of leucosome (14 versus 7%), and less distinct melanosomes. The variation in these migmatites reflects open system behavior, either on a regional scale with the migration of aqueous fluids into the amphibolites or on a local scale with the migration of melt within the amphibolites. Zircons from all units have CL-dark core domains that are dated at 1415-1390 Ma. The core zones are cut and overgrown by CL-dark and CL-bright rims that are dated at 975-965 Ma. The zircon rims are thin in the mesosome but are thicker in the leucosome suggesting that they formed during migmatization. New growth of zircon associated with migmatization at ca. 970 Ma corresponds to the timing of crustal scale partial melting in the deep regions of the Sveconorwegian orogen, synchronous with east

  18. Two modes of orogenic collapse of the Pamir plateau recorded by titanite

    NASA Astrophysics Data System (ADS)

    Stearns, M. A.; Hacker, B. R.; Ratschbacher, L.; Rutte, D.; Kylander-Clark, A. R.

    2013-12-01

    Processes that operate in the mid- to lower crust during and following continent-continent collision are important for understanding how orogenic plateaux transition from thickening to collapse. In the central and southern Pamir, mid- to lower crustal rocks crop out in two belts of extensional domes. The central Pamir domes were exhumed by symmetrical N-S extension. In contrast, the southern Pamir domes were exhumed by asymmetrical top to the south (NNW-SSE) extension via a rolling-hinge detachment. To investigate the high-temperature exhumation history, titanites were dated using LASS (laser ablation split stream-ICP-MS). A multi-collector ICP was used to collect U-Pb isotopic ratios and a single collector ICP-MS was used to measure trace-element abundances. The data indicate that the central Pamir domes began exhumation synchronously at ~17 Ma. Titanite from the southern Pamir record two periods of protracted (re)crystallization: older metamorphic dates ranging from ~35-18 Ma and younger igneous and metamorphic dates from ~15-7 Ma. Samples with single populations of titanite dates are present throughout both groups. Samples with more-complex date populations typically have distinct trace-element (e.g., Sr, Y, Zr, and Nb) groups that can be used to distinguish different date populations (e.g., older dates may have higher Zr and younger dates lower Zr). The distinct early exhumation histories of the north and south Pamir require either a diachronous single process or two semi-independent processes. The N to S sequence of exhumation, ranges of dates, and overall extension directions may be related to two important plate-tectonic events inferred from seismic data: 1) breakoff of the northward subducting Indian slab around ~20 Ma, and 2) southward subduction and northwestward rollback of the Asian lithosphere between ~15-10 Ma based on geodetic convergence rates and Benioff zone length. We interpret these two lithospheric-detachment events to have driven the

  19. Extension of the Mid- to Lower Crust with Orogenic Inheritance: Examples from the Death Valley Region (Western US), and the Mauleon Basin (Southwestern France).

    NASA Astrophysics Data System (ADS)

    Lima, R. D.; Hayman, N. W.; Kelly, E. D.; Lavier, L. L.

    2015-12-01

    Continental margins exhibit a range of widths and symmetries defined by the strain patterns that arise during extension and rifting. An important pattern in this respect is the early localization of extension into necking zones. The rheology of the lower crust plays a large role in this localization, and can be affected by inherited orogenic structures, fabrics, and mineral assemblages. Here, we further evaluate the role of orogenic fabrics in continental extension using microstructural observations and thermodynamic modeling of geological sections exposed in the Funeral and Black Mountains of the Death Valley region, California, and from the Mauleon Basin, France. The Death Valley region sits within the Basin-and-Range region of broadly distributed Cenozoic extension, over a relatively flat and deep moho. In contrast, in the Mauleon basin, Cretaceous extension accommodated mantle exhumation, and was strongly localized in older Hercynian orogenic crust. In both areas, mid- to lower crustal rocks are characterized by inherited migmatitic fabrics overprinted by zones of localized, extensional fabrics. Mineral assemblages that formed over a P-T cooling path define the fabrics in each field area. The high-temperature fabrics record decompression-melting due to late- to post-orogenic collapse. Yet, the two field areas show contrasting retrograde assemblages, which are hypothesized to have resulted from changes in the local effective bulk composition produced by differences in melt segregation. At subsequent extensional stages, mid- to lower crustal deformation resulted in the transposition of the inherited post-orogenic fabrics, documented with quartz fabric analysis (including EBSD). The two contrasting regions show how the rheology of inherited orogenic lower crust responds to differences in melt-segregation and metamorphic histories, potentially controlling margin structural evolution.

  20. The long-term seismic cycle in collisional margins: insights from Seismo-Thermo-Mechanical models

    NASA Astrophysics Data System (ADS)

    Dal Zilio, L.; van Dinther, Y.; Gerya, T.

    2015-12-01

    The April 25, 2015, Mw 7.8 Gorkha earthquake is the largest one in the Nepal Himalaya since 1934. Since the foreland part of it is densely populated, these events represent a considerable seismic hazard. The restricted direct observations in time and space in combination with tectonic and rheological complexities, however, pose a difficult problem for both seismic hazard assessment and modeling efforts. In this study we for the first time simulate cycles of spontaneous earthquake-like ruptures on non a-priori defined faults within a generic continental collision zone. We use the Seismo-Thermo-Mechanical (STM) numerical modeling approach, which is based on a continuum, viscoelastoplastic code I2ELVIS and is validated for seismic cycle applications against a laboratory model and natural observations (van Dinther et al., 2013a, b). The 2-D model setup consists of two continental plates separated by an oceanic plate, in which the incipient subduction phase is followed by continent-continent collision. In different collisional stages, we evaluate a non-associative Drucker-Prager plasticity yield criterion with pressure dependent yield strength and a strongly rate-dependent friction formulation. Our results show physically consistent emergence of complex rupture paths, both on- and off-main frontal thrust. Assuming different physical properties of tectonic nappes, we find that ruptures propagate following rheological or tectonic discontinuities. Our findings suggest that the interseismic coupling of the main-thrust affects the seismic cycle of the entire orogenic belt. While thrust-faulting events mainly occur on the main frontal thrust, normal-faulting events spread throughout the orogenic belt as a consequence of gravitational extension. Results from an event detection algorithm shows events in the deeper portions of the orogenic belt, both within the oceanic slab and near the bottom of the wedge where material is squeezed between stiff lithospheric mantle portions.

  1. The tectonics and stages of the geological history of the Yenisei-Khatanga Basin and the conjugate Taimyr Orogen

    NASA Astrophysics Data System (ADS)

    Afanasenkov, A. P.; Nikishin, A. M.; Unger, A. V.; Bordunov, S. I.; Lugovaya, O. V.; Chikishev, A. A.; Yakovishina, E. V.

    2016-03-01

    A new interpretation of the seismic profile series for the Taimyr Orogen and the Yenisei-Khatanga Basin is given in terms of their tectonics and geological history. The tectonics and tectonostratigraphy of the Yenisei-Khatanga and the Khatanga-Lena basins are considered. In the Late Vendian and Early Paleozoic, a passive continental margin and postrift shelf basin existed in Taimyr and the Yenisei-Khatanga Basin. From the Early Carboniferous to the Mid-Permian, the North and Central Taimyr zones were involved in orogeny. The Late Paleozoic foredeep was formed in the contemporary South Taimyr Zone. In the Middle to Late Triassic, a new orogeny took place in the large territory of Taimyr and the Noril'sk district of the Siberian Platform. A synorogenic foredeep has been recognized for the first time close to the Yenisei-Khatanga Basin. In the Jurassic and Early Cretaceous, this basin was subsided under transpressional conditions. Thereby, anticlinal swells were formed from the Callovian to the Aptian. Their growth continued in the Cenozoic. The Taimyr Orogen underwent tectonic reactivation and apparently right-lateral transpression from Carboniferous to Cenozoic.

  2. Correlation of the Tonga Formation and the Chiwaukum Schist, North Cascades, Washington: Implications for Late Cretaceous orogenic mechanisms

    NASA Astrophysics Data System (ADS)

    Duggan, K. M.; Brown, E. H.

    1994-12-01

    The Tonga Formation of the North Cascades, Washington, consists of weakly deformed, bedded feldspathic graywacke and shale metamorphosed at chlorite to staurolite grade. Analysis of protolith lithology, Rb-Sr isotopic signature, and plutonic, metamorphic, and deformational evolution in the Tonga Formation suggests that this unit is correlative with the Chiwaukum Schist of the Cascades crystalline core. The Tonga Formation occurs on the flank of the Late Cretaceous orogen, whereas high-grade Chiwaukum Schist is exposed in the core of the orogen. Metamorphic pressure, as reflected by the distribution of metamorphic minerals and thermobarometry, increases continuously northward in the Tonga Formation from the chlorite zone (<4 kbar) to the staurolite zone (≈ 7 kbar) and then to the kyanite zone (≈ 7 kbar) in the Chiwaukum Schist. The findings reported herein do not support the previously advanced concept that the Cascades crystalline core represents the Easton blueschist terrane that was converted into higher-temperature metamorphic rock by thermal relaxation after cessation of subduction. The results of this study suggest a relatively simple two-stage metamorphic history for the Chiwaukum Schist represented by 90-93 Ma shallow contact metamorphism, overprinted by Harrovian metamorphism that increased sharply in pressure from south-west to northeast and reached peak conditions after 90 Ma, all events occurring in a plutonic/magmatic arc setting.

  3. Late orogenic mafic magmatism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion

    USGS Publications Warehouse

    Whitney, D.L.; Tepper, J.H.; Hirschmann, M.M.; Hurlow, H.A.

    2008-01-01

    The Skymo Complex in the North Cascades, Washington, is a layered mafic intrusion within the Ross Lake fault zone, a major orogen-parallel structure at the eastern margin of the Cascades crystalline core. The complex is composed dominantly of troctolite and gabbro, both with inclusions of primitive olivine gabbro. Low-pressure minerals in the metasedimentary contact aureole and early crystallization of olivine + plagioclase in the mafic rocks indicate the intrusion was emplaced at shallow depths (<12 km). The Skymo rocks have trace-element characteristics of arc magmas, but the association of Mg-rich olivine (Fo88-80) with relatively sodic plagioclase (An75-60) and the Al/Ti ratios of clinopyroxene are atypical of arc gabbros and more characteristic of rift-related gabbros. A Sm-Nd isochron indicates crystallization in the early Tertiary (ca. 50 Ma), coeval with the nearby Golden Horn alkaline granite. Mantle melting to produce Skymo magma likely occurred in a mantle wedge with a long history of arc magmatism. The Skymo mafic complex and the Golden Horn granite were emplaced during regional extension and collapse of the North Cascades orogen and represent the end of large-scale magmatism in the North Cascades continental arc. ?? 2008 Geological Society of America.

  4. Changes in basal dip and frictional properties controlling orogenic wedge propagation and frontal collapse: the External central Betics case

    NASA Astrophysics Data System (ADS)

    Jiménez-Bonilla, Alejandro; Torvela, Taija; Balanyá, Juan-Carlos; Díaz-Azpiroz, Manuel; Expósito, Inmaculada

    2016-04-01

    Orogenic wedges and their key component, thin-skinned fold-and-thrust belts (FTBs), have been extensively studied through both field examples and modelling. The overall dynamics of FTBs are, therefore, well understood. One of the less understood aspects is: what is the combined influence of across-strike changes in the detachment properties and the basement topography on the behaviour of an orogenic wedge, as the deformation progresses towards the foreland? In this study, we use field data combined with reflection seismic interpretation and well data from the External Zones of the Central Betics FTB, S Spain, to identify a basement "threshold" coinciding with a thinning out of a weak substrate (Triassic evaporites) in the wedge basal detachment. The basal changes influenced the tempo-spatial (4D) local wedge dynamics at ~Early Langhian times, leading to stagnation of FTB propagation, topographic build-up and subsequent collapse of the FTB front, which was enhanced by arc-parallel stretching. This development led to a formation of an important depocentre filled with a thick Langhian mélange unit and later sediments deposited in the NW-migrating foreland basin. This case study illustrates the importance of across-strike changes in wedge basal properties to the stability of the FTB front, especially in terms of the collapse/extensional structures.

  5. Comparative analysis of the geological evolution of the northern and southern Appalachian orogen: Late Ordovician-Permian

    USGS Publications Warehouse

    Hibbard, J.P.; van Staal, C.R.; Rankin, D.W.

    2010-01-01

    Review of the major post-Middle Ordovician lithotectonic elements of the Appalachian orogen indicates that the middle to late Paleozoic geologic evolution of the Appalachian margin was less uniform than that of the early Paleozoic. Evolutionary divergence betweenthe northern and southern segments of the orogen started in the Late Ordovician to Silurian with staggered accretion of the first peri-Gondwanan elements to reach the Laurentia margin, Carolinia in the south and Ganderia in the north. Divergence was amplified duringthe Silurian, specifi cally with respect to the nature of the Laurentian margin and the history of accretion. During this time frame, the northern margin was convergent, whereas the amagmatic southern margin may well have been a transform boundary. In terms of accretion, the Late Silurian-Early Devonian docking of Avalonia was restricted to the northern segment, whereas the southern Appalachians appear to have been largely quiescent during this interval. The evolutionary paths of the two segments of the margin converge on a commonhistory in the Late Devonian during the Famennian event; we suggest that this tectonism was related to the initial marginwide interaction of Laurentia with the peri-Gondwanan blocks of Meguma and Suwanee, providing a uniform tectonic template for margin evolution. The Laurentian-Gondwanan collision is marked by second-order divergences in history. Specifi cally, during the Carboniferous, the southern segment records a larger component of shortening than the northern Appalachians. ?? 2010 Geological Society of America.

  6. Pan-African granulites of central Dronning Maud Land and Mozambique: A comparison within the East-African-Antarctic orogen

    USGS Publications Warehouse

    Engvik, A.K.; Elevevold, S.; Jacobs, J.; Tveten, E.; de Azevedo, S.; Njange, F.

    2007-01-01

    Granulite-facies metamorphism is extensively reported in Late Neoproterozoic/Early Palaeozoic time during formation of the East-African-Antarctic orogen (EAAO). Metamorphic data acquired from the Pan-African orogen of central Dronning Maud Land (cDML) are compared with data from northern Mozambique. The metamorphic rocks of cDML are characterised by Opx±Grt-bearing gneisses and Sil+Kfs-bearing metapelites which indicate medium-P granulite-facies metamorphism. Peak conditions, which are estimated to 800-900ºC at pressures up to 1.0 GPa, were followed by near-isothermal decompression during late Pan-African extension and exhumation. Granulite-facies lithologies are widespread in northern Mozambique, and Grt+Cpx-bearing assemblages show that high-P granulite-facies conditions with PT reaching 1.55 GPa and 900ºC were reached during the Pan-African orogeny. Garnet is replaced by symplectites of Pl+Opx+Mag indicating isothermal decompression, and the subsequent formation of Pl+amphibole-coronas suggests cooling into amphibolite facies. It is concluded that high-T metamorphism was pervasive in EAAO in Late Neoproterozoic/Early Paleozoic time, strongly overprinting evidences of earlier metamorphic assemblages.

  7. The Ross Orogen and Lachlan Fold Belt in Marie Byrd Land, Northern Victoria Land and New Zealand: implication for the tectonic setting of the Lachlan Fold Belt in Antarctica

    USGS Publications Warehouse

    Bradshaw, J.D.

    2007-01-01

    Correlation of the Cambrian Delamerian Orogen of Australia and Ross Orogen of the Transantarctic Mountains widely accepted but the extension of the adjacent Lachlan Orogen into Antarctica is controversial. Outside the main Ross-Delamerian belt, evidence of this orogeny is preserved at Mt Murphy in Marie Byrd Land and the in Takaka Terrane of New Zealand. In all pre-break- configurations of the SW Pacific, these two areas are far removed from the Ross-Delamerian belt. Evidence from conglomerates in the Takaka Terrane, however, shows that in Late Cambrian times it was adjacent to the Ross Orogen. This indicates major tectonic displacements within Gondwana after the Cambrian and before break-up. The Lachlan Orogen formed in an extensional belt in a supra-subduction zone setting and the Cambrian rocks of Marie Byrd Land and New Zealand are interpreted as parts of a rifted continental ribbon on the outboard side of the Lachlan belt.

  8. The Kongsberg-Modum terrane of Southern Norway: a key toward a refined conceptual model of the Sveconorwegian orogen

    NASA Astrophysics Data System (ADS)

    Viola, Giulio; Bingen, Bernard; Henderson, Iain; Yi, Keewook; Ganerød, Morgan

    2013-04-01

    The Sveconorwegian orogen corresponds to the part of Scandinavia affected by the Sveconorwegian orogeny between ca. 1140 and 900 Ma. The orogen is generally interpreted as resulting from the collision between Fennoscandia and an unknown large continent, possibly Amazonia. Its first-order architecture is defined by the juxtaposition of a series of crustal blocks, separated by major, ca. N-S trending ductile deformation belts. Our current research focuses on the Kongsberg/Modum terrane (KMT), a small lithotectonic block located in the middle of the orogen in Southern Norway. With a still ongoing multidisciplinary study of the terrane, we aim at unravelling its geological evolution and constraining its role in the orogen build up. The study combines detailed lithological and structural mapping, carried out digitally, with airborne, high resolution potential field geophysics and new geochronological and geochemical studies. The KMT is characterised by an elongate N-S trending structural grain that corresponds to highly flattened and isoclinally folded metavolcanic, metaplutonic and metasedimentary sequences. The Modum sub-complex is a quartzite-rich metasedimentary belt, and it includes characteristic sillimanite-quartzite and anthophyllite-cordierite gneiss. A sheared sinistral transpressive contact separates it from the Kongsberg sub-complex, formed predominantly by metavolcanic and plutonic rocks. Steep amphibolite-facies shear zones within and between these lithological belts also invariably show a sinistral transpressive component. Structural data collected along the KMT-Telemark boundary in the west show that the KMT is thrusted westward over the leucocratic granitoids of the Telemark sector along the Sokna-Saggrenda shear zone (SSSZ). The SSSZ is a curved, narrow and laterally continuous (ca. 100 km long) lower amphibolite facies shear zone accommodating an overall top-to-the-west transport, although local kinematic variations exist due to strain partitioning

  9. Absorption Heat Pump Cycles

    NASA Astrophysics Data System (ADS)

    Kunugi, Yoshifumi; Kashiwagi, Takao

    Various advanced absorption cycles are studied, developed and invented. In this paper, their cycles are classified and arranged using the three categories: effect, stage and loop, then an outline of the cycles are explained on the Duehring diagram. Their cycles include high COP cycles for refrigerations and heat pumps, high temperature lift cycles for heat transformer, absorption-compression hybrid cycles and heat pump transformer cycle. The highest COPi is attained by the seven effect cycle. In addition, the cycles for low temperature are invented and explained. Furthermore the power generation • refrigeration cycles are illustrated.

  10. Tectonic Inheritance at Transform Faults in Successive Wilson Cycles (Invited)

    NASA Astrophysics Data System (ADS)

    Thomas, W. A.

    2010-12-01

    Preservation of the record of two complete Wilson cycles along the eastern (Iapetan/Atlantic) margin of Laurentia/North America provides an unusual opportunity to study tectonic inheritance at continental margins in successive Wilson cycles. At the largest scale of tectonic inheritance, during continental rifting, transform faults offset rift segments, framing promontories and embayments of the continental margin; a subsequent orogenic belt adapts to the trace of the continental margin, curving around recesses and salients during supercontinent assembly. From observations of the Iapetan and Atlantic Wilson cycles, inheritance is expressed during continental rifting and ocean opening most strongly along transform faults/margins, and during closing and supercontinent assembly primarily in adaptation to the trace of the pre-orogenic margin and in foreland subsidence. The Alabama-Oklahoma transform and Southern Oklahoma transform-parallel intracratonic fault system of the Iapetan margin are parallel with a Mesoproterozoic dike swarm that may be related to a transform fault of the pre-Rodinia margin of protoLaurentia. Although the Alabama recess and Ouachita salient of the Pangaean Appalachian-Ouachita orogenic belt curve around the transform-rift margin of the Iapetan Ouachita embayment, the Bahamas transform of Atlantic opening follows the trace of the Iapetan Alabama-Oklahoma transform. Similarly, the Atlantic South Grand Banks transform follows the Iapetan Sept Isles transform, oblique to the Pangaean Appalachian orogenic belt in the Quebec salient. In contrast, both the Atlantic Bahamas transform and East Coast rift system cut obliquely across the Pangaean Suwannee-Wiggins suture, which, although it is the continental suture between Laurentia and the African sector of Gondwana, was not reactivated as the younger rift margin. At a smaller scale, however, the Suwannee-Wiggins suture was reactivated as the South Georgia basin, an intracratonic graben parallel with

  11. Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie-Sulu orogen in China: implications for geodynamics and fluid regime

    NASA Astrophysics Data System (ADS)

    Zheng, Yong-Fei; Fu, Bin; Gong, Bing; Li, Long

    2003-07-01

    Discovery of coesite, diamond, and extreme 18O-depletion in eclogites from the Dabie-Sulu orogen in central-east China has contributed much to our understanding of subduction of continental crust to mantle depths and its subsequent exhumation. Hydrogen, oxygen, and carbon isotope distributions were systematically investigated in the past 8 years for ultrahigh pressure (UHP) eclogites, gneisses, granulites, marbles, and peridotites from this exciting region. The available data show the following characteristic features: (1) variable δ18O values of -11‰ to +10‰ for the eclogites and gneisses, with both equilibrium and disequilibrium fractionations of oxygen isotopes among minerals; (2) disequilibrium fractionation of hydrogen isotopes between mica and epidote from both eclogites and gneisses, with low δD values up to -127‰ to -100‰ for phengite; (3) negative δ13C values of -28‰ to -21‰ for apatite as well as host-eclogites and gneisses; (4) positive δ13C values of +1‰ to +6‰ for coesite-bearing marble associated with eclogites; (5) zircons from metamorphic rocks of different grades show a large variation in δ18O from -11‰ to +9‰, with U-Pb ages of 700 to 800 Ma for the timing of low- δ18O magma crystallization. It appears that the UHP metamorphic rocks exhibit ranges of δ18O values that are typical of potential precursor protolith rocks. Preservation of the oxygen isotope equilibrium fractionations among the minerals of the UHP eclogites and gneisses suggests that these rocks acquired the low δ18O values by meteoric-hydrothermal alteration before the UHP metamorphism. Thus, the UHP metamorphic rocks largely reflect the δ18O values of their premetamorphic igneous or sedimentary precursors. The stable isotope data demonstrate that basaltic, granitic, and sedimentary protoliths of the eclogites, orthogneiss, and paragneiss in the orogen were at or near the earth's surface, and subjected to varying degrees of water-rock interaction at some

  12. Hydrological cycle.

    PubMed

    Gonçalves, H C; Mercante, M A; Santos, E T

    2011-04-01

    The Pantanal hydrological cycle holds an important meaning in the Alto Paraguay Basin, comprising two areas with considerably diverse conditions regarding natural and water resources: the Plateau and the Plains. From the perspective of the ecosystem function, the hydrological flow in the relationship between plateau and plains is important for the creation of reproductive and feeding niches for the regional biodiversity. In general, river declivity in the plateau is 0.6 m/km while declivity on the plains varies from 0.1 to 0.3 m/km. The environment in the plains is characteristically seasonal and is home to an exuberant and abundant diversity of species, including some animals threatened with extinction. When the flat surface meets the plains there is a diminished water flow on the riverbeds and, during the rainy season the rivers overflow their banks, flooding the lowlands. Average annual precipitation in the Basin is 1,396 mm, ranging from 800 mm to 1,600 mm, and the heaviest rainfall occurs in the plateau region. The low drainage capacity of the rivers and lakes that shape the Pantanal, coupled with the climate in the region, produce very high evaporation: approximately 60% of all the waters coming from the plateau are lost through evaporation. The Alto Paraguay Basin, including the Pantanal, while boasting an abundant availability of water resources, also has some spots with water scarcity in some sub-basins, at different times of the year. Climate conditions alone are not enough to explain the differences observed in the Paraguay River regime and some of its tributaries. The complexity of the hydrologic regime of the Paraguay River is due to the low declivity of the lands that comprise the Mato Grosso plains and plateau (50 to 30 cm/km from east to west and 3 to 1.5 cm/km from north to south) as well as the area's dimension, which remains periodically flooded with a large volume of water.

  13. Structural evolution of the accretional continental margin of the Paleoproterozoic Svecofennian orogen in southern Sweden

    NASA Astrophysics Data System (ADS)

    Beunk, F. F.; Page, L. M.

    2001-09-01

    , left-lateral, NE-block-up slip suggests a W(SW)-E(NE) orientation of σ1. Peak metamorphism and partial melting in the supracrustal rocks was approximately coeval with ≥1800-1835 Ma old subduction-related magmatism in the Oskarshamn-Jönköping Belt (OJB), 100 km to the south-west. Seismic reflection profiling off the Baltic coast (BABEL line B) suggests northward polarity of subduction beneath the OJB and a back-arc environment of the Västervik and Valdemarsvik groups between the OJB arc and the margin of the >1850 Ma old Svecofennian orogen. We suggest that D 1-D 5 deformation was caused by intracontinental back-arc extension and subsequent closure of the back-arc basin by oblique accretion of the OJB marginal arc onto the northern continent. Accretion involved northward indentation by mid-crustal wedges, one of which coincides with and possibly caused the nucleation of the D 5 stage LLDZ. 40Ar/ 39Ar data from hornblende and white mica identify a prolonged cooling history of the area (1810-1490 Ma), influenced by the nearby, late- and postorogenic Transscandinavian Igneous Belt and, possibly, by largely hidden, c. 1530 Ma old anorogenic rapakivi-type intrusions.

  14. Rapid middle Miocene collapse of the Mesozoic orogenic plateau in north-central Nevada

    USGS Publications Warehouse

    Colgan, Joseph P.; Henry, Christopher D.

    2009-01-01

    The modern Sierra Nevada and Great Basin were likely the site of a high-elevation orogenic plateau well into Cenozoic time, supported by crust thickened during Mesozoic shortening. Although crustal thickening at this scale can lead to extension, the relationship between Mesozoic shortening and subsequent formation of the Basin and Range is difficult to unravel because it is unclear which of the many documented or interpreted extensional episodes was the most significant for net widening and crustal thinning. To address this problem, we integrate geologic and geochronologic data that bear on the timing and magnitude of Cenozoic extension along an ???200km east-west transect south of Winnemucca, Battle Mountain, and Elko, Nevada. Pre-Cenozoic rocks in this region record east-west Palaeozoic and Mesozoic compression that continued into the Cretaceous. Little to no tectonism and no deposition followed until intense magmatism began in the Eocene. Eocene and Oligocene ash-flow tuffs flowed as much as 200km down palaeovalleys cut as deeply as 1.5km into underlying Palaeozoic and Mesozoic rocks in a low-relief landscape. Eocene sedimentation was otherwise limited to shallow lacustrine basins in the Elko area; extensive, thick clastic deposits are absent. Minor surface extension related to magmatism locally accompanied intense Eocene magmatism, but external drainage and little or no surface deformation apparently persisted regionally until about 16-17Ma. Major upper crustal extension began across the region ca. 16-17Ma, as determined by cross-cutting relationships, low-temperature thermochronology, and widespread deposition of clastic basin fill. Middle Miocene extension was partitioned into high-strain (50-100%) domains separated by largely unextended crustal blocks, and ended by 10-12Ma. Bimodal volcanic rocks that erupted during middle Miocene extension are present across most of the study area, but are volumetrically minor outside the northern Nevada rift. The modern

  15. A review of the tectonic evolution of the Northern Pacific and adjacent Cordilleran Orogen

    NASA Astrophysics Data System (ADS)

    Jakob, Johannes; Gaina, Carmen; Johnston, Stephen T.

    2014-05-01

    Numerous plate kinematic models for the North Pacific realm have been developed since the advent of plate tectonics in the early seventies (e.g Atwater (1970), Mammerickx and Sharman (1988)). Although published kinematic models are consistent with the broad scale features of the North Pacific, the link between plate motions and the evolution of the North American Cordillera remains poorly understood. Part of the problem lies in conflicting interpretations of geological versus paleomagnetic data sets, with the result being a lack of consensus regarding: the paleolocation of key geological units; the paleogeography of terrane formation and amalgamation; the motion, boundaries and even existence of oceanic plates; and the character (e.g. trend of subduction) and position of plate boundaries within the northern Pacific basin. Remnants of the Farallon and Kula plates, and some short-lived microplates, demonstrate the complicated tectonic evolution of the oceanic realm west of the North American margin (e.g. Rea and Dixon (1983); McCrory and Wilson (2013); Shephard et al. (2013)). The creation and destruction of major tectonic plates and microplates has presumably left a record in the Cordilleran orogen of western North America. However, working backward from the geological relationships to plate reconstructions remains difficult. Here we investigate the relationship between the plate motions of the Pacific Ocean and the terrane movements in the North American Cordillera by revising the marine magnetic and gravity anomalies of the northern Pacific. In particular, we reevaluate plate boundaries at times of major changes in plate geometry of the Pacific, Kula, Chinook and Farallon plates from C34n onward. Our focus is also on the plate geometries of the Resurrection, Eshamy and Siletz-Crescent plates during the time between anomaly C26 and C12, and the links between plate interactions and on-shore tectonic events recorded in the geological record of Vancouver Island

  16. Palaeozoic - Mesozoic tectonics of the western Transbaikalian segment of the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Gladkochub, Dmitry; Donskaya, Tatiana; Mazukabzov, Anatoly

    2014-05-01

    The western Transbaikalian segment of the Central Asian Orogenic Belt (CAOB) is a key to understand a scenario of gradual growth of continental crust in this part of the Northern Eurasia. In general this growth was directly controlled by long-living processes related to interaction of the southern margin of the Siberian craton with different units of the Palaeo-Asian and Mongol-Okhotsk oceans. The Neoproterozoic activity within this area was related to opening and development of the Palaeo-Asian Ocean. The early Palaeozoic stage was characterized by collision of numerous terranes (microcontinents, relicts of island arcs and back-arc basins, etc.) with the southern flank of the Siberian craton. These events reflect an early stage of Palaeo-Asian Ocean closure and the beginning of CAOB building. Since the late Palaeozoic the development of the Transbaikalian segment of the CAOB was related to the evolution of the Mongol-Okhotsk Ocean. Late Silurian - middle Devonian clastic and carbonate sediments were deposited along a passive margin of the Mongol-Okhotsk Ocean. The low-angle subduction of oceanic lithosphere of the Mongol-Okhotsk Ocean beneath the Siberian continent started in the middle Devonian. In the early to late Carboniferous, a steeper dip of subduction led to tectonic switching from extension to compression. This period was characterized by compression, metamorphism, deformations and thickening of continental crust. Autochthonous biotite granites of the Angara-Vitim batholith (ca. 320-300 Ma) were emplaced during this phase. Destruction of the subducted slab and roll-back toward the ocean in the late Carboniferous - early Permian caused extension of continental lithosphere and magmatic input from the mantle into the continental crust. These processes were responsible for voluminous magmatism that is spectacularly represented by allochthonous granitoids of the Angara-Vitim batholith and magmatic rocks of the western Transbaikalian belt (ca. 300-270 Ma). In

  17. Kinematics of the Torcal Shear Zone: transpressional tectonics shaping orogenic curves in the northern Gibraltar Arc.

    NASA Astrophysics Data System (ADS)

    Barcos, Leticia; Balanyá, Juan Carlos; Díaz-Azpiroz, Manuel; Expósito, Inmaculada; Jiménez-Bonilla, Alejandro

    2014-05-01

    Structural trend line patterns of orogenic arcs depict diverse geometries resulting from multiple factors such as indenter geometry, thickness of pre-deformational sequences and rheology of major decollement surfaces. Within them, salient-recess transitions often result in transpressive deformation bands. The Gibraltar Arc results from the Neogene collision of a composite metamorphic terrane (Alboran Domain, acting as a relative backstop) against two foreland margins (Southiberian and Maghrebian Domains). Within it, the Western Gibraltar Arc (WGA) is a protruded salient, 200 km in length cord, closely coinciding with the apex zone of the major arc. The WGA terminates at two transpressional zones. The main structure in the northern (Betic) end zone is a 70 km long and 4-5 km wide brittle deformation band, the so-called Torcal Shear Zone (TSZ). The TSZ forms a W-E topographic alignment along which the kinematic data show an overall dextral transpression. Within the TSZ strain is highly partitioned into mainly shortening, extensional and strike-slip structures. The strain partitioning is heterogeneous along the band and, accordingly, four distinct sectors can be identified. i) The Peñarrubia-Almargen Transverse Zone (PATZ), located at the W-end of the TSZ presents WNW-ESE folds and dextral faults, together with normal faults that accommodate extension parallel to the dominant structural trend. WNW ESE dextral faults might be related with synthetic splays at the lateral end of the TSZ. ii) The Sierra del Valle de Abdalajís (SVA) is characterized by WSW-ENE trending folds and dextral-reverse faults dipping to SSE, and NW-SE normal faults. The southern boundary of the SVA is a dextral fault zone. iii) The Torcal de Antequera Massif (TAM) presents two types of structural domains. Two outer domains located at both margins characterized by E-W trending, dextral strike-slip structures, and an inner domain, characterized by en echelon SE-vergent open folds and reverse shear

  18. Geological and geochemical implications of the genesis of the Qolqoleh orogenic gold mineralisation, Kurdistan Province (Iran)

    NASA Astrophysics Data System (ADS)

    Taghipour, Batoul; Ahmadnejad, Farhad

    2015-03-01

    The Qolqoleh gold deposit is located in the northwestern part of the Sanandaj-Sirjan Zone (SSZ), within the NE-SW trending Qolqoleh shear zone. Oligocene granitoids, Cretaceous meta-limestones, schists and metavolcanics are the main lithological units. Chondrite-normalised REE patterns of the ore-hosting metavolcanics indicate REE enrichment relative to hanging wall (chlorite-sericite schist) and footwall (meta-limestone) rocks. The pattern also reflects an enrichment in LREE relative to HREE. It seems that the LREE enrichment is related to the circulation of SO42- and CO2-bearing fluids and regional metamorphism in the Qolqoleh shear zone. Both positive and negative Eu anomalies are observed in shear-zone metavolcanics. These anomalies are related to the degree of plagioclase alteration during gold mineralisation and hydrothermal alteration. In progressing from a metavolcanic protomylonite to an ultramylonite, significant changes occurred in the major/trace element and REE concentration. Utilising an Al-Fe-Ti isocon for the ore-hosting metavolcanics shows that Sc, Y, K, U, P, and M-HREE (except Eu) are relatively unchanged; S, As, Ag, Au, Ca, LOI, Rb and LREE are enriched, and Sr, Ba, Eu, Cr, Co and Ni decrease with an increasing degree of deformation. Based on geochemical features and comparison with other well-known shear zones in the world, the study area is best classified as an Isovolume-Gain (IVG) type shear zone and orogenic type gold mineralisation. Based on the number of phases observed at room temperature and their microthermometric behaviour, three fluid inclusion types have been recognised in quartz-sulphide and quartz-calcite veins: Type I monophase aqueous inclusions, Type II two-phase liquid-vapour (L-V) inclusions which are subdivided into two groups based on the homogenisation temperature (Th): a) L-V inclusions with Th from 205 to 255°C and melting temperature of last ice (Tm) from -3 to -9°C. b) L-V inclusions with higher Th from 335 to 385

  19. Seismic Tomography Reveals Breaking Crust and Lithosphere Beneath a Classic Orogen

    NASA Astrophysics Data System (ADS)

    Byrne, T. B.; Rau, R.; Kuo-Chen, H.; Lee, Y.; Ouimet, W. B.; Van Soest, M. C.; Huang, C.; Wu, F. T.

    2013-12-01

    The orogenic system in Taiwan is often considered a classic example of an accretionary prism that has grown to a steady-state size and shape above an also steady subduction zone. A new study of vertical and horizontal sections of a tomographic velocity model created by Kuo-Chen et al. (2012) show, however, both a well-developed crack in the subducted crust beneath southern Taiwan and a discontinuous lithosphere beneath northern Taiwan, suggesting that slab breakoff is actively occurring beneath Taiwan. The transition from slab breakoff to cracking crust in southern Taiwan also suggests that slab breakoff is propagating southward, consistent an oblique collision. The crack in the subducting crust is revealed by progressively deeper horizontal sections of the local-scale tomographic model. The sections show an ellipsoidal-shaped area of high velocity that plunges southeast, oblique to all of the regional trends. Taking into account the dip of the slab, however, the area of high velocity is nearly parallel to previously recognized fracture zone in the Eurasian continental margin. We interpret the area of high velocity to be a crack in the Eurasian crust that is filled high velocity Eurasian mantle. Support for this interpretation comes from: 1) new exhumation cooling data from Mt Yu, the highest peak in Taiwan; 2) a recent leveling survey along the South Cross-Island Highway that shows unusually high rates of surface uplift (up to 15 mm/yr; Ching et al., 2011); 3) Vp attenuation studies that suggest anomalously high temperatures and/or the presence of fluids; 4) earthquake focal mechanisms in the core of the southern Central Range that are dominated by NE-SW extension; and finally, 5) the core of the southern Central Range preserves anomalous areas of low topographic relief that straddle the crest 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

  20. Thrust-related, diapiric, and extensional doming in a frontal orogenic wedge: example of the Montagne Noire, Southern French Hercynian Belt

    NASA Astrophysics Data System (ADS)

    Soula, Jean-Claude; Debat, Pierre; Brusset, Stéphane; Bessière, Gilbert; Christophoul, Frédéric; Déramond, Joachim

    2001-11-01

    The Montagne Noire, which is situated at the toe of the orogenic wedge of the French Massif Central South European Variscides, appears to be a well-suited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins. The data reported in the present paper show that the Montagne Noire dome is a particular type of basement-involved frontal culmination in an orogenic wedge and foreland basin system. This frontal culmination is characterized by a syn-contractional HT decompression recorded by clockwise PTt paths and widespread strata overturning in thrust and fold structures, which controlled the sedimentation in the adjacent foreland basin. These unusual characteristics are interpreted to be a result of the succession of thrusting, diapirism and extensional collapse. Antiformal stacking of syn-metamorphic thrust sheets controlled the first stages of the foreland basin development. Diapirism was essentially responsible for the HT decompression and widespread strata overturning. Extensional doming was a result of late- to post-metamorphic collapse acting on the pre-existing high-amplitude dome. Diapirism and associated isothermal decompression metamorphism, which constitute the essential difference between the Montagne Noire and 'ordinary' frontal ridges in orogenic wedges, were probably enhanced by a local partial melting of the upper to middle crust. It is suggested that the occurrence of these phenomena in front of an orogenic wedge was related to local over-thickening due to the superposition of an upper crustal antiformal stack on top of a lower crustal ramp anticline.

  1. Crustal structure in the junction of Qinling Orogen, Yangtze Craton and Tibetan Plateau: implications for the formation of the Dabashan Orocline and the growth of Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Jiang, Chengxin; Yang, Yingjie; Zheng, Yong

    2016-06-01

    The crust at the junction of Qinling Orogen, Yangtze Craton and NE Tibetan Plateau bears imprints of the Triassic collision and later intracontinental orogeny between the Qinling Orogen and the Yangtze Craton, and the Cenozoic growth of Tibetan Plateau. Investigating detailed crustal structures in this region helps to better understand these tectonic processes. In this study, we construct a 3-D crustal Vs model using seismic ambient noise data recorded at 321 seismic stations. Ambient noise tomography is performed to generate Rayleigh wave phase velocity maps at 8-50 s periods, which are then inverted for a 3-D isotropic Vs model using a Bayesian Monte Carlo method. Our 3-D model reveals deep-rooted high velocities beneath the Hannan-Micang and Shennong-Huangling Domes, which are located on the west and east sides of the Dabashan Orocline. Similar high velocities are observed in the upper/mid crust of the western Qinling Orogen. We suggest the crustal-scale bodies with high velocity beneath the two domes and the western Qinling Orogen may represent mechanically strong rocks, which not only assisted the formation of the major Dabashan Orocline during late Mesozoic intracontinental orogeny, but also have impeded the northeastward expansion of the Tibetan Plateau during the Cenozoic era.

  2. A geophysical model of lower crustal structure of the Palaezoic crustal root (Bohemian Massif): implications for modern collisional orogens

    NASA Astrophysics Data System (ADS)

    Guy, Alexandra; Edel, Jean Bernard; Schulmann, Karel; Tomek, Cestmir; Lexa, Ondrej

    2010-05-01

    overturn of low density lower crust and high and intermediate density crust in the area of central root. The crustal structure in the east is interpreted as a result of viscous extrusion of low density orogenic lower crust over the high density Brunia continent. Comparison of these data with geophysical profiling of the Andean and the Tibetan plateaus suggests that modern orogenic systems reveal comparable but thicker deep crustal pattern. Based on these similarities we propose that the Variscan root represents a deep crustal section of above mentioned plateaus which may have develop by the same orogenic process.

  3. 990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico: roots of an orogen

    NASA Astrophysics Data System (ADS)

    Solari, L. A.; Keppie, J. D.; Ortega-Gutiérrez, F.; Cameron, K. L.; Lopez, R.; Hames, W. E.

    2003-04-01

    Inliers of ˜1.0-1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012±12 Ma anorthosite-mangerite-charnockite-granite (AMCG) suite; (2) El Catrı´n unit: ≥1350 Ma orthogneiss migmatized at 1106±6 Ma; and (3) El Marquez unit: ≥1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U-Pb isotopic analyses of zircon: (a) the 1106±6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004-978±3 Ma Zapotecan event produced at least two sets of structures: (Z1) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal WNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 °C at 977±12 Ma is documented by 40Ar/ 39Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential-longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of ˜25-30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modern analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events

  4. 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

  5. Active Folding In The Puli Basin, Constraints On Strain Across A Sub-Critical Region Of The Taiwanese Orogen

    NASA Astrophysics Data System (ADS)

    Wilcox, T.; Mueller, K.; Chen, Y.

    2006-12-01

    The Puli topographic embayment in central western Taiwan is interpreted as a region of sub-critical taper (assuming uniform decollement geometry) within the otherwise critically tapered wedge of the Taiwanese orogen. This sub-critical portion of the thrust belt drives the current orogenic architecture and reorganization of strain in and around Puli. Age dating and mapping of fold axis' that deform Late Quaternary terrace deposits in the Puli Basin suggest that at least 380 meters of fault slip (or a minimum of about 350 meters of horizontal shortening) has occurred over the last 50-60 Ka, yielding a minimum slip rate of 8mm/yr for the Shuilikeng blind thrust and fault-related fold. Given our existing age constraints, which only define the oldest Late Quaternary strata in the Puli Basin, the slip rate on this same structure could approach 13mm/yr (i.e. we use strain in younger deposits as a starting point for measuring shortening). Geodetic convergence rates (with respect to Asia) in the northern half of the island average ~10mm/yr or less, while convergence rates in the southwestern part of the orogen average ~40mm/yr. Previous maps of geodetic strain suggest the Puli Basin, corresponds with strain gradients of about 15mm/yr. Identifiable kink bends in terrace deposits were used to define the kinematics of actively growing folds, indicating the sense and magnitude of slip on blind thrusts that currently accommodate strain. This implies that active contraction occurs over a broader area across Puli than in other parts of Taiwan, but within the Basin contraction appears most likely limited to slip on a single fault (in addition to rapidly slipping thrusts in the foreland). Backstepping of thrusts at Puli is due to the first-order decrease in mass of the wedge at this latitude. While decreased mass in the wedge can be initially estimated by its thickness (as measured by the distance between the decollement as mapped at depth by microseismicity and average topography

  6. Modern Strain and Structural Architecture of the Central Taiwanese Orogen - Evidence for Active Backstepping in Response to Erosion?

    NASA Astrophysics Data System (ADS)

    Mueller, K.; Chen, Y.; Powell, L.

    2002-12-01

    The topography and tectonic geomorphology of west-central Taiwan is interpreted to suggest that increased erosion in a region of relatively lower relief is accommodated by backstepping of active thrust faults and folds. Average topography is 1.0-2.0 km lower in the west-central part of the orogen and is defined by a broad, 80 km-wide depression located between the Chelungpu thrust (to the west) and the main drainage divide of the Island (to the east). The area of lower relief is floored by a piggyback basin (the Puli Basin) and a region defined by radially-oriented river channels incised deeply into the slate belt further east. Evidence for stalling of frontal folds and active backstepping of more hindward thrust sheets includes (from west to east): low rates of shortening across the frontal Pakuashan anticline, latest Pleistocene reactivation and rapid slip across the Chelungpu thrust, rapidly uplifted fault-bend folds in the hangingwalls of the Shuangtung and Shuichanglin thrusts, and growth of nascent blind thrusts and folds in the piggyback Puli Basin further east. We argue the region underlain by the Puli Basin has always been an area of relatively lower relief and not been deeply eroded because it is located above a regional decollement which translates but does not uplift rocks in this region as much as in adjacent parts of the fold belt. Restorations of thrust sheets exposed in river valleys incised through the Puli Basin also do not require significant erosion of folds developed in their hangingwalls. Finally, the critical taper of the orogen appears to be maintained by a more steeply-dipping decollement that marks the base of the thin-skinned wedge beneath the Puli Basin. The region incised by radially-oriented river valleys in the slate belt has been more greatly exhumed, based on published thermochronologic data, the amount of river incision and metamorphic grade. Capture of longitudinal valleys incised into the slate belt by west-flowing rivers

  7. Disclosing the Paleoarchean to Ediacaran history of the São Francisco craton basement: The Porteirinha domain (northern Araçuaí orogen, Brazil)

    NASA Astrophysics Data System (ADS)

    Silva, Luiz Carlos da; Pedrosa-Soares, Antonio Carlos; Armstrong, Richard; Pinto, Claiton Piva; Magalhães, Joana Tiago Reis; Pinheiro, Marco Aurélio Piacentini; Santos, Gabriella Galliac

    2016-07-01

    This geochronological and isotopic study focuses on one of the Archean-Paleoproterozoic basement domains of the São Francisco craton reworked in the Araçuaí orogen, the Porteirinha domain, Brazil. It also includes a thorough compilation of the U-Pb geochronological data related to the adjacent Archean and Rhyacian terranes from the São Francisco craton and Araçuaí orogen. The main target of this study is the TTG gneisses of the Porteirinha complex (Sample 1). The gneiss dated at 3371 ± 6 Ma unraveled a polycyclic evolution characterized by two metamorphic overprinting episodes, dated at 3146 ± 24 Ma (M1) and ca. 600 Ma (M2). The former (M1) is so far the most reliable evidence of the oldest metamorphic episode ever dated in Brazil. The latter (M2), in turn, is endemic in most of the exposed eastern cratonic margin within the Araçuaí orogen. Whole-rock Sm-Nd analysis from the gneiss provided a slightly negative εNd(t3370) = - 0.78 value, and a depleted mantle model (TDM) age of 3.5 Ga, indicating derivation mainly from the melting of a ca. 3.5 Ga tholeiitic source. Sample 2, a K-rich leuco-orthogneiss from the Rio Itacambiriçu Complex, was dated at 2657 ± 25 Ma and also presents a ca. 600 Ma M2 overprinting M2 age. The other two analyses were obtained from Rhyacian granitoids. Sample 3 is syn-collisional, peraluminous leucogranite from the Tingui granitic complex, showing a crystallization age of 2140 ± 14 Ma and strong post-crystallization Pb*-loss, also ascribed to the Ediacaran overprinting. Accordingly, it is interpreted as a correlative of the late Rhyacian (ca. 2150-2050 Ma) collisional stage of the Mantiqueira orogenic system/belt (ca. 2220-2000 Ma), overprinted by the Ediacaran collage. Sample 4 is a Rhyacian post-orogenic (post-collisional), mixed-source, peralkaline, A1-type suite, with a crystallization age of 2050 ± 10 Ma, presenting an important post-crystallization Pb*-loss related to Ediacaran collision. The focused region records some

  8. Geodynamic control on orogenic and anorogenic magmatic phases in Sardinia and Southern Spain: Inferences for the Cenozoic evolution of the western Mediterranean

    NASA Astrophysics Data System (ADS)

    Beccaluva, L.; Bianchini, G.; Natali, C.; Siena, F.

    2011-04-01

    The genetic relationships between orogenic (i.e. subduction related) and anorogenic (i.e. intra-plate) Cenozoic igneous phases have been investigated in two regions of the western Mediterranean area: Sardinia and Southern Spain. In Sardinia the 'orogenic' magmatism (38-12 Ma) is followed by the 'anorogenic' volcanism, mostly since about 6 Ma, whereas in Southern Spain the 'anorogenic' volcanism follows the 'orogenic' phase (34-6 Ma) after a gap of 0-4 Ma in the Betic-Calatrava districts. The older orogenic magmatism (tholeiitic, calcalkaline and more potassic products) of both areas is related to the subduction of the Ionian oceanic lithosphere which developed beneath the Paleo-European-Iberian continental margin probably since Middle-Late Eocene. This subduction system migrated southeastwards with time up to its present position in the Eolian-Calabrian Arc and the Betic-Alboran regions along the Apennine-Maghrebide belt. Relics of subducted lithosphere are geophysically recorded as nearly-vertical bodies down to 500-600 km, flattening for several hundreds of kilometres under the Tyrrhenian-Sardinia and Betic-Calatrava areas, respectively. These relics of subducted slabs, which pond over large areas of the mantle transition zone, appear to play a significant role also in the genesis of the younger anorogenic magmas, whose major volcanic fields lie above the frontal part of the subducted slab where convective instabilities and upward mantle flow components are geophysically supported by laboratory and 3D numerical models. This dynamic response to subduction, involving localised mantle upwellings and remobilization of pre-existing mantle components, may have been a fundamental factor in the generation of anorogenic magmas. Due to slab roll-back and inter-arc extension in both Eolian-Tyrrhenian and Betic-Alboran regions, the magma sources of the previous orogenic phases can be completely replaced by "fresh" mantle diapirs from which anorogenic magmas will be generated

  9. Craton Development and Stabilization: Insights from SE Canada using P and S Wave Tomography

    NASA Astrophysics Data System (ADS)

    Boyce, A.; Bastow, I. D.; Darbyshire, F. A.; Gilligan, A.; Ellwood, A.; Levin, V. L.; Menke, W. H.

    2015-12-01

    Cratons, the ancient cores of the continents, are the longest-lived parts of Earth's surface that have survived thermal and mechanical erosion during multiple Wilson cycles. They are visible in tomographic images due to their thick (>200km), seismically fast keels or roots. The Laurentian keel beneath North America is intriguing since its root is thought to extend beneath both the Archean Superior craton and the Proterozoic Grenville province thus implying that keel formation may not have been restricted to Archean times. In order to address this issue we present a P and S wave relative arrival-time tomographic study using data from seismograph networks in SE Canada and the NE US, stretching from the southern tip of Hudson Bay within the Superior craton to the coastal Phanerozoic Appalachian terranes. The tomographic images display three broad zones of increasing mantle wavespeed from globally "slow" in the Appalachian terranes, to a "fast" Grenville Province and "extremely fast" Superior craton. We observe a linear low-velocity feature resulting from modification of the Laurentian keel by the passage of the Great Meteor hotspot. This feature is progressively offset southwestward with depth, potentially due to viscous coupling with mantle flow. No major plate-scale underthrusting during the Grenville Orogeny is apparent, which contradicts the inferred results from crustal seismic reflection and refraction studies. Our results therefore may have fundamental implications for the nature of the Grenville orogenic collision and cratonic stabilization of North America. The results also support the developing consensus that keels form in two stages: a chemically depleted core of Archean age followed by a thermally developed, less-depleted lithosphere during Proterozoic times, highlighted by an abrupt wavespeed contrast in the tomographic images.

  10. Paleo-Mesoproterozoic arc-accretion along the southwestern margin of the Amazonian craton: The Juruena accretionary orogen and possible implications for Columbia supercontinent

    NASA Astrophysics Data System (ADS)

    Scandolara, J. E.; Correa, R. T.; Fuck, R. A.; Souza, V. S.; Rodrigues, J. B.; Ribeiro, P. S. E.; Frasca, A. A. S.; Saboia, A. M.; Lacerda Filho, J. V.

    2017-01-01

    The southwestern portion of the Amazonian craton, between the Ventuari-Tapajós province and the Andean chain, has been ascribed to a succession of orogenic events from 1.81 to 0.95 Ga, culminating with widespread anorogenic magmatism. Southwestward of the Serra do Cachimbo graben occurs the Juruena accretionary orogenic belt (ca. 1.81-1.51 Ga), previously included in the Rio Negro-Juruena and Rondonian/San Ignácio geocronological provinces or Rondônia-Juruena geologic province. The Juruena orogen proposed here includes the Jamari and Juruena tectonostratigraphic terranes, products of convergence which culminated in the soft collision of the Paraguá protocraton and the Tapajós-Parima arc system (Tapajós Province) ca. 1.69-1.63 Ga ago. Geophysical, geochemical, petrological and geochronological data and systematic geological mapping suggest that the convergent event resulted in a single orogenic system with two continental margin arcs, namely the Jamari and Juruena arcs. Modern geological and tectonic approaches, combined with aerogeophysics data, enable to interpreting this wide region of the Amazonian craton as a Paleoproterozoic orogen with well defined petrotectonic units and tectonoestructural framework. The Juruena orogen is an E-W trending belt, about 1100 km long and 350 km wide, inflecting to NW-SE, in Mato Grosso, Amazonas and Rondonia, Brazil. The general direction of the belt, its inflections and internal geometric and kinematic aspects of its macrostructures do not corroborate the general NW-SE trend of the originally proposed geocronological provinces. The Juruena accretionary orogen has been the site of repeated reactivation with renewed basin formation, magmatism and orogeny during the Mesoproterozoic and the early Neoproterozoic. U-Pb and whole-rock Sm-Nd ages, Ar-Ar and Rb-Sr mineral ages suggest that the older high grade tectonometamorphic events in the Juruena accretionary orogen took place between 1.69 and 1.63 Ga, defining the metamorphic

  11. Lithospheric flexure and composite tectonic loads in the foreland of the Marathon orogenic belt: Permian Basin, west Texas and southern New Mexico

    SciTech Connect

    Yang, Kenn Ming; Dorobek, S. . Dept. of Geology)

    1992-01-01

    Lithospheric flexure caused by loading of orogenic belts is regarded as the main process that produces subsidence in foreland basins. However in some foreland areas, subsidence may be affected by synorogenic foreland uplifts that act as additional loads. The Permian Basin is located in the foreland area of the late Paleozoic Marathon orogenic belt (Mob). The Permian Basin consists of several sub-basins that are separated by several structurally complex uplifts. Uplift of the Central Basin Platform (CBP) and subsidence in adjacent basins were coeval with final stages of deformation in the Marathon orogen. The CBP is oriented at high angles to the Marathon orogen and consists of several blocks arranged in an en echelon pattern. Data suggest that uplift of the CBP was affected by clockwise rotation of crustal blocks between NNW-SSE trending boundary faults. Although both the Delaware Basin (DB) and Val Verde Basin (VVB) are adjacent to the Mob, the synorogenic geometries of these basins are different. The VVB has a typical flexural profile that apparently is due to loading of the Marathon orogen. However, the flexural profile becomes narrower and deeper toward the western end of the VVB where the basin is bordered by the southernmost block of the CBP. In contrast, synorogenic DB profiles have composite wavelengths which show maximum deflection next to the Mob and toward the uplifted blocks of the CBP. This suggests that synorogenic subsidence of the DB was affected by loading of the CBP. In addition, the loading geometry across the uplifted CBP is asymmetric, with greater uplift and basement shortening on the western side of the CBP and less uplift and basement shortening on the eastern side. This may explain greater synorogenic subsidence in the DB than the Midland Basin.

  12. Orogen-Parallel and -Normal Extension at the Dinarides-Hellenides Junction during Clockwise Rotation and Radial Expansion of the Retreating Hellenic Arc-Trench System

    NASA Astrophysics Data System (ADS)

    Handy, M.; Fügenschuh, B.; Giese, J.; Le Breton, E.; Muceku, B.; Onuzi, K.; Pleuger, J.; Schmid, S. M.; Ustaszewski, K. M.

    2015-12-01

    Normal faults at the junction of the Dinarides and Hellenides in northern Albania trend both perpendicular and parallel to thrusts and fold axes. Most prominent is the SSE-dipping Skutari-Pec Normal Fault (SPNF), one of a system of normal faults that accommodate NW-SE-directed, orogen-parallel extension. Extensional throw along the SPNF increases from zero in northwestern Albania to at least 2 km near the Albania-Kosovo border, near where the brittle arm of the SPNF bounds the Mio-Pliocene Kosovo Basin. This differential orogen-parallel extension along the SPNF defines a vertical rotational axis in northwestern Albania that is interpreted to have accommodated 20-30° of clockwise motion of internal units on the southeastern (Hellenic) side of the fault with respect to the northwestern (Dinaric) side. Such rotation is in agreement with previously published paleomagnetic data and plate motion studies. The footwall of the SPNF exposes lower units of the Dinaric nappe stack that underwent vertical shortening and non-coaxial shearing during extensional exhumation. In the hangingwall of the SPNF, a system of orogen-parallel trending normal faults accommodate orogen-normal displacement of up to several km. Both orogen-parallel and -perpendicular systems of normal faults cut Late Cretaceous to Oligocene thrusts and folds, and are associated with pronounced Miocene paleo-relief. Most of this normal faulting is Miocene to Pliocene based on syn-extensional deposits in the Kosovo Basins and on thermal modelling of ZHe, AFT and AHe data, which suggest accelerated cooling at 18 Ma and between 4-6 Ma. Both fault systems also cut Holocene deposits, indicating ongoing extension. This corroborates published GPS data and earthquake focal mechanisms indicating active extension of the crust south and southeast of the SPNF. It is proposed that extension and clockwise rotation in this area were coeval and accommodated southwestward retreat and radial expansion of the Hellenic arc during

  13. Sedimentology of plio-pleistocene late orogenic deposits associated with intraplate subduction—the Upper Siwalik Subgroup of a part of Panjab Sub-Himalaya, India

    NASA Astrophysics Data System (ADS)

    Kumar, R.; Tandon, S. K.

    1985-01-01

    This study provides a detailed sedimentologic documentation of the lithofacies, cyclicity, sandbody geometry, palaeoflow variability and depositional models of the youngest late orogenic deposits of the Siwalik molasse. These deposits represent one of the important examples of sedimentation in the zone of intraplate subduction of the Himalayan collision belt. Lithofacies analysis has resulted in the recognition of four conglomerate facies (massive, matrix-supported conglomerate; crudely stratified conglomerate; trough cross-stratified conglomerate; planar cross-stratified conglomerate), five sandstone facies (erosional scour and fill; pebbly sandstone; medium- to coarse-grained sandstone; fine- to medium-grained ripple drift laminated sandstone; bioturbated calcareous fine-grained sandstone), and six mudstone facies (parallel-laminated bioturbated mudstone; reddish brown massive mudstone; variegated mudstone; bluish-grey to violet mudstone; buff-coloured mudstone; pebbly mudstone). In addition, hard calcareous layers have been recognised as a separate facies state. The deduced cyclic sequences of the Tatrot and Pinjor Formations of the Upper Siwalik Subgroup show fining-upward characters from CS (coarse sandstone) to MS (mudstone) through FS (fine sandstone) and SS (alternations of sandstone and siltstone). The sandbodies of the Tatrot and Pinjor Formations reveal a sheet-like geometry and an internal dominance of inter-storey scour surfaces. The presence of intraformational debris, mud drapes, reactivation surfaces and a general fining upward of grain size indicates fluctuating hydrodynamic conditions with frequent channel abandonment and re-occupation, resulting in net vertical aggradation. The dominant style of sedimentation both in the channel and overbank facies is through vertical aggradation. Data on the palaeoflow variability indicates that the Upper Siwalik Subgroup can be subdivided into four and three sediment-vector sequences in the Khetpurali and

  14. The odyssey of the Cache Creek terrane, Canadian Cordillera: Implications for accretionary orogens, tectonic setting of Panthalassa, the Pacific superwell, and break-up of Pangea

    NASA Astrophysics Data System (ADS)

    Johnston, S. T.; Borel, G. D.

    2007-01-01

    The Cache Creek terrane (CCT) of the Canadian Cordillera consists of accreted seamounts that originated adjacent to the Tethys Ocean in the Permian. We utilize Potential Translation Path plots to place quantitative constraints on the location of the CCT seamounts through time, including limiting the regions within which accretion events occurred. We assume a starting point for the CCT seamounts in the easternmost Tethys at 280 Ma. Using reasonable translation rates (11 cm/a), accretion to the Stikinia-Quesnellia oceanic arc, which occurred at about 230 Ma, took place in western Panthalassa, consistent with the mixed Tethyan fauna of the arc. Subsequent collision with a continental terrane, which occurred at about 180 Ma, took place in central Panthalassa, > 4000 km west of North America yielding a composite ribbon continent. Westward subduction of oceanic lithosphere continuous with the North American continent from 180 to 150 Ma facilitated docking of the ribbon continent with the North American plate. The paleogeographic constraints provided by the CCT indicate that much of the Canadian Cordilleran accretionary orogen is exotic. The accreting crustal block, a composite ribbon continent, grew through repeated collisional events within Panthalassa prior to docking with the North American plate. CCT's odyssey requires the presence of subduction zones within Panthalassa and indicates that the tectonic setting of the Panthalassa superocean differed substantially from the current Pacific basin, with its central spreading ridge and marginal outward dipping subduction zones. A substantial volume of oceanic lithosphere was subducted during CCT's transit of Panthalassa. Blanketing of the core by these cold oceanic slabs enhanced heat transfer out of the core into the lowermost mantle, and may have been responsible for the Cretaceous Normal Superchron, the coeval Pacific-centred mid-Cretaceous superplume event, and its lingering progeny, the Pacific Superswell. Far field

  15. Himalayan-style escape tectonics vs late-orogenic extension in the Variscan Belt of Western Europe

    NASA Astrophysics Data System (ADS)

    Gebelin, A.; Brunel, M.; Ferre, E. C.

    2005-12-01

    The Variscan Orogenic Belt in Western Europe was formed mainly during the Carboniferous (Mississipian-Pennsylvanian) continental collision between the Gondwana and the Laurentia lithospheric plates. This major orogen, now deeply eroded, shows numerous geodynamic similarities with the Himalayan orogen, such as lateral continental-scale ductile wrench zones. New structural, geophysical and geochronological data on the NW Massif Central (France) provide new constrains on the timing and tectonic setting of late-Variscan granite magmatism during this orogeny. Previous investigations have emphasized the role of late-orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new dataset supports ascent and intrusion in a strike-slip regime. The Variscan granite plutons in the Limousin region are emplaced in a transpressive setting. They are spatially associated with major synmagmatic strike-slip shear zones that merge to the northwest with the South Armorican Shear Zone (SASZ). A continuous band of orthogneisses and mylonites attests of the right-lateral strike-slip deformation that affected plutons over a width of 1 to 5 km. The shear zones are characterized by steeply dipping foliation and shallowly plunging stretching lineations. A few granite plutons exhibit steep lineations suggesting local northward reverse movement. The Anisotropy of Magnetic Susceptibility (AMS) was used to delineate magmatic fabrics. The Millevaches pluton, a representative example of Limousin syntectonic granites, shows a sigmoidal map pattern of magnetic foliations and lineations consistent with right-lateral shearing. Magnetic lineations rotate gradually from NNW-SSE in the center of the shear zone to NW-SE outside. Magnetic foliations are steep in the center of the shear zone and gradually become sub-horizontal outside. New 40Ar/39Ar data shows that in Limousin, wrench tectonics started around 350 Ma and ended around 300 Ma. New U/Pb and microstructural

  16. Upper Triassic pyritized bivalve mollusks from the Sentachan orogenic gold-antimony deposit, eastern Yakutia: Mineralogy and sulfur isotopic composition

    NASA Astrophysics Data System (ADS)

    Pal'yanova, G. A.; Sobolev, E. S.; Reutsky, V. N.; Bortnikov, N. S.

    2016-11-01

    Pyritized bivalve mollusks have been revealed for the first time in upper Norian sedimentary rocks at the large orogenic Sentachan gold-antimony deposit in eastern Yakutia. It has been established that they are related to species of the genus Monotis ( M. ex gr. jacutica (Teller) (specimen no. 2077/1, CSGM) and M. pachypleura (Teller) (specimen no. 2077/2, CSGM), which existed during the Monotis densistriata phase of the Norian Age of the Triassic Epoch presumably 211.5-214 Ma ago. The occurrence of two pyrite generations differing in morphology, sulfur impurity, and isotopic compositions corroborates the suggestion that pyritization of fauna was a discrete process. It has been shown that sulfur that formed owing to bacterial sulfate reduction and interaction with organic matter was involved in the pyritization of bivalve mollusks, whereas mixing of magmatic sulfur and sulfur derived from host sedimentary rocks participated in deposition of Au-Sb ore.

  17. Late Paleozoic tectonic evolution and concentrated mineralization in Balkhash and West Junggar, western part of the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Dong, Shuwen; Chen, Xuanhua; Chen, Zhengle

    2016-04-01

    The Central Asia Orogenic Belt (CAOB) is an important area with significant growth of the crust and metallogeny in the Late Paleozoic. The Balkhash-Junggar tectono-metallogenic belt consists of the Balkhash, the West Junggar, and the East Junggar tectono-metallogenic belts in western part of the Central Asian Orogenic Belt (CAOB). According to the structural geological relationship, the East Junggar, the West Junggar, and the Balkhash belts are considered to be once a continuous E-W-trending tectono-metallogenic belt in Late Carboniferous. The West Junggar belt is featured with NE-trending left-lateral strike-slip faulting tectonic system (WJTS), while the left-lateral strike-slip faults are E-W-trending in the Balkhash belt. The WJTS consists of the Darabut, the Mayile, and the Baerluke faults, and the blocks among them. All these left-lateral strike-slip faults are forming due to the transition of tectonic settings from syn-collisional orogeny to post-collisional extension during the closure of the ocean (the Junggar Sea) in Late Carboniferous, with significant intrusion of batholiths and crust growth occurred in this period. These faults are truncated by the right-lateral strike-slip faults, such as the Chingiz-Junggar fault, and the Central Balkhash fault in Mesozoic. The Balkhash-Junggar tectono-metallogenic belt is important for the occurrence of many well-known super-large and large porphyry Cu-Mo deposits (such as the Kounrad, the Aktogai, the Borly, and the Baogutu deposits), large skarn Cu deposits (in the Sayak ore-filed), large rare metal deposits (such as the East Kounrad, the Zhanet, and the Akshatau deposits), and large gold deposits (such as the Hatu deposit). Zircon U-Pb ages, Re-Os isotopic dating of molybdenites, 40Ar/39Ar thermochronology of hornblendes, muscovites, biotites, and K-feldspars, and zircon and apatite fission track (FT) and (U-Th)/He dating and thermal history modeling, provide a multidisciplinary approach to constrain the whole

  18. Thermal evolution of a main detachment zone during late orogenic collapse: the pilat unit case (Variscan chain).

    NASA Astrophysics Data System (ADS)

    Gardien, V.; Allemand, P.

    2003-04-01

    The high temperature uplift of deep seated nappes in different tectonic settings is a major question in orogenic domains. Detailed petrological and structural study were performed in the Pilat Unit (Eastern french Massif Central: Variscan chain) in order to discuss the thermal evolution of a main shear zone during late orogenic extension. The Pilat Unit is a 20 km long x 5 km large metamorphic unit and as a strain gradient within it perpandicular to the long axe of the unit. The mylonitic fabrics, in the strained zone are notherly directed. Geometrical and geochronological data indicate that these fabrics are associated with extesional regime that was a post-thickening event. Thermobarometrical determination supported by microstructural analyses indicate the develpoment of contrasting P-T-time evolution in the Pilat Unit. In the strongly mylonitized southern domain in which foliation and streching lineation show a top to the north normal slip, the calculated P-T conditions indicate an increase in temperature during decompression illustrated by the following mineral assemblages:1) Grt + Stau + Kya + Rut, 2) Bio + Sill + Cord, 3) And + Musco + K-feld and dated at 322 Ma. In the northern and less deformed domain, the calculated P-T conditions correspond to a cooling event during decompression illustrated by the following assemblages: 1) Grt + Stau + Kya + Rut, 2) Bio + Musco + Ilm, 3) And + Chlo + Pyro + Tour, dated at 313 Ma. The difference in T° between the southern and the northern domain is 200-250°C and Δ t is 10Ma. Based on these data, we have elaborated 1D numerical model simulating the thermal evolution of the continental crust during thinning associated or not with magma underplating.

  19. Crustal Fluid Evolution and Changes in Deformation Conditions during Regional Syn- to Post-Orogenic Exhumation: Southeastern Piedmont, Southern Appalachians

    SciTech Connect

    Evans, M.A.

    2000-07-05

    Fluid inclusion microthermometric data from veins in the southeastern Piedmont province record the changes in fluid composition and deformation conditions during regional exhumation and cooling related to Late Paleozoic syn- to post-orogenic processes and early Mesozoic rifting. In general, the composition of post-metamorphic fluids that were trapped late during the Alleghanian orogeny and during post-orogenic exhumation are remarkably consistent across the southeastern Piedmont, indicating regional fracture connectivity. The first fluids were trapped in veins that formed during the last phases of the Alleghanian. These syn-deformational fluids are CO2-saturated low salinity brines (salinities of 2.6 to 5.7 wt. percent NaCl equivalent) with homogenization temperatures in the range of 200 degrees to 365 degrees C. They were trapped under lithostatic pressures between 240 and 280 MPa, indicating burial depths of 11.2 to 12.7 km. These depths are similar to emplacement depths of post-kinematic plutons, suggesting a period of rapid isobaric cooling. Low-salinity H2O inclusions and rare CO2-rich inclusions are evidence for Early Mesozoic regional decompression as fracturing above the brittle-to-ductile transition allowed regional pore-fluid pressure to drop to hydrostatic levels. Convective circulation of meteoric water resulted in the dilution of 'in-situ' fluids, and ultimately to a system saturated with meteoric water. These fluids continued to be trapped in vein minerals through much of the Mesozoic as rift basins formed during the opening of the Atlantic Ocean. Late Paleozoic through Mesozoic exhumation rates for the eastern Piedmont province average 0.063 km m.y.-1 and cooling rates average approximately 1.9 degrees C m.y.-1. These low rates may be directly related to thinned crust and lithosphere resulting from delamination processes during the late Alleghanian orogeny.

  20. Geochemical and Nd-Sr-Pb isotope characteristics of synorogenic lower crust-derived granodiorites (Central Damara orogen, Namibia)

    NASA Astrophysics Data System (ADS)

    Simon, I.; Jung, S.; Romer, R. L.; Garbe-Schönberg, D.; Berndt, J.

    2017-03-01

    The 547 ± 7 Ma old Achas intrusion (Damara orogen, Namibia) includes magnesian, metaluminous to slightly peraluminous, calcic to calc-alkalic granodiorites and ferroan, metaluminous to slightly peraluminous, calc-alkalic to alkali-calcic leucogranites. For the granodiorites, major and trace element variations show weak if any evidence for fractional crystallization whereas some leucogranites are highly fractionated. Both, granodiorites and leucogranites are isotopically evolved (granodiorites: εNdinit: - 12.4 to - 20.5; TDM: 2.4-1.9; leucogranites: εNdinit: - 12.1 to - 20.6, TDM: 2.5-2.0), show similar Pb isotopic compositions, and may be derived from late Archean to Paleoproterozoic crustal source rocks. Comparison with melting experiments and simple partial melting modeling indicate that the granodiorites may be derived by extensive melting (> 40%) at 900-950 °C under water-undersaturated conditions (< 5 wt.% H2O) of felsic gneisses. Al-Ti and zircon saturation thermometry of the most primitive granodiorite sample yielded temperatures of ca. 930 °C and ca. 800 °C. In contrast to other lower crust-derived granodiorites and granites of the Central Damara orogen, the composition of the magma source is considered the first-order cause of the compositional diversity of the Achas granite. Second-order processes such as fractional crystallization at least for the granodiorites were minor and evidence for coupled assimilation-fractional crystallization processes is lacking. The most likely petrogenetic model involves high temperature partial melting of a Paleoproterozoic felsic source in the lower crust ca. 10-20 Ma before the first peak of regional high-temperature metamorphism. Underplating of the lower crust by magmas derived from the lithospheric mantle may have provided the heat for melting of the basement to produce anhydrous granodioritic melts.

  1. Mantle Recycling of Crustal Materials through Study of Ultrahigh-Pressure Minerals in Collisional Orogens, Ophiolites, and Xenoliths

    NASA Astrophysics Data System (ADS)

    Liou, J. G.; Tsujimori, T.; Yang, J.; Zhang, R. Y.; Ernst, W. G.

    2014-12-01

    Newly recognized ultrahigh-pressure (UHP) mineral occurrences including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mafic/ultramafic xenoliths suggest the recycling of crustal materials through profound subduction, mantle upwelling, and return to the Earth's surface. Recycling is supported by unambiguously crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths, and from ultramafic bodies of several Alpine-Himalayan and Polar Ural ophiolites; some such phases contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and stishovite, a wide variety of nanometric minerals have been identified as inclusions employing state-of-the-art analysis. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. Some podiform chromitites and associated peridotites contain rare minerals of undoubted crustal origin, including Zrn, corundum, Fls, Grt, Ky, Sil, Qtz, and Rtl; the zircons possess much older U-Pb ages than the formation age of the host ophiolites. These UHP mineral-bearing chromitites had a deep-seated evolution prior to extensional mantle upwelling and its partial melting at shallow depths to form the overlying ophiolite complexes. These new findings plus stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere, recycling of biogenic carbon into the lower mantle, and ascent to the Earth's surface through deep mantle ascent.

  2. Unsteady rock uplift and erosion in a decaying orogen in response to surface and dynamic mantle processes (Invited)

    NASA Astrophysics Data System (ADS)

    Pazzaglia, F. J.; Zeitler, P. K.; McKeon, R. E.; Idleman, B. D.; Berti, C.

    2010-12-01

    The Appalachian Mountains in the eastern United States are constructed on a lithosphere that was assembled and modified during a long period of Paleozoic collisional tectonics, and then thinned by erosion and stretching with the opening of the Atlantic Ocean. Geologic, geomorphic, and geophysical data describe the long term erosion, persistence of mountainous topography, and coupling between surface and tectonic, isostatic, or eperiogenic processes of this ancient, decaying orogen. There are several key observables that must be reconciled to explain Appalachian post-orogenic (post-Triassic) evolution. These include subsidence of the former metamorphic core of the range and topographic inversion of the foreland, a long-term record of unsteady erosion preserved in the sediments of the Atlantic margin shelf-slope basins, the steepening of the longitudinal profiles of Atlantic slope rivers forming a zone of bedrock rapids as they approach the Atlantic, river knickpoints in the foreland with a common elevation that have no apparent relation to rock-type or structure, and evidence that the divide between Atlantic slope and Ohio drainages is not a static feature. The long term rate of Appalachian rock uplift (base level fall) and erosion is ~20-30 m/my., a result consistent with AHe thermochronology and the average sediment flux to the BCT, but misleading in terms of the unsteadiness demonstrated by river incision, growth of the Atlantic slope drainage, and short-term sediment flux to the BCT. This unsteadiness is likely influenced by several factors including climate change, sediment storage in the landscape and the Coastal Plain, and eustasy; however, we propose that the most important factor has been unsteady rock uplift as the result of a lithospheric flexural response to surface loads and sub-lithospheric mantle flow driven by the foundering Farallon slab.

  3. Early Jurassic tectonism occurred within the Basu metamorphic complex, eastern central Tibet: Implications for an archipelago-accretion orogenic model

    NASA Astrophysics Data System (ADS)

    Li, Hua-Qi; Xu, Zhi-Qin; Webb, A. Alexander G.; Li, Tian-Fu; Ma, Shi-Wei; Huang, Xue-Meng

    2017-04-01

    The Basu metamorphic complex, surrounded by ophiolitic melanges and intruded by a large volume of undeformed granitoid rocks along the eastern segment of the Bangong-Nujiang suture, holds one of the keys to understanding the pre-Cenozoic tectonic evolution of central Tibet. Zircon U-Pb dating of rocks from the Basu metamorphic complex reveals that meta-igneous rocks yield Early Paleozoic crystallization ages of 500-492 Ma and an Early Jurassic metamorphic age of 173 Ma, and that undeformed granitoid rocks yield crystallization ages of approximately 186-174 Ma. Whole rock geochemical and zircon Lu-Hf isotopic data indicate that the undeformed granitoid rocks originated mainly from partial melting of ancient crustal sources, which may reflect a collisional orogenic setting. 40Ar/39Ar dating of biotite from a sillimanite-garnet-biotite paragneiss shows cooling to 300 ± 50 °C at 165 Ma. These data indicate significant Early Jurassic tectonism, during which most of the Basu metamorphic complex was formed. Furthermore, the age data resemble those of the Amdo metamorphic complex located approximately 500 km to the west along the Bangong-Nujiang suture. Together, these complexes may represent a ;destroyed or unrecognized; block, i.e., the Amdo-Tongka block, which may be the eastern extension of the South Qiangtang terrane. Based on the tectonic outlines of the multiple ophiolitic zones and magmatic belts, we suggest a new archipelago-accretion model that attributes the Early Jurassic tectonism to an arc-continent/micro-continent collision. This model further enables the reconstruction of the eastern Tethyan Ocean and the orogenic processes of central Tibet during the Mesozoic.

  4. Non-volcanic tremors, low-frequency earthquake swarms, and their association with orogenic fluid flow in Taiwan

    NASA Astrophysics Data System (ADS)

    Chen, K. H.; Kim, A.; Chuang, Y. L.; Peng, W.; Leu, T.

    2012-12-01

    Taiwan is a young transpressive orogen exhibiting high uplift and exhumation. Under the southern flank of Central Range, two types of seismic activities typically observed in volcanic region are found to occur closely in space. Swarms of earthquakes showing vertical planar features occur frequently at a depth range of 0-20 km, whereas some shallow events (< 7 km) reveal the characteristics similar to volcanic low-frequency earthquakes: (1) dominant by ~ 2 Hz (2) lack of S phases (3) long coda. Below the swarm events, the deep-seated, ambient tremors are also observed. Spatial correlation between deeper tremor and shallow low-frequency swarm events indicates a common generation mechanism. These two seismic activities are confined in a small area where the localized veining, anomalous magnetic and thermal anomalies are distributed, suggesting the involvement of fluid-pressure processes within the orogen. Here we pay special attention to develop an automatic detection scheme for Taiwan tremors. Using this method we find 87 tremor episodes with duration ranging from 5 to 30 minutes. The tremors activity appears to have increased at the time of local M6 earthquakes. In March 2008, an active swarm composed of 632 events (M0.4-5.2) occurred only two days apart of the preceding tremor episode (duration 60 minutes). The possible spatial and temporal relationship between ambient tremors and earthquake swarms provides a rare opportunity for the understanding of tremor source model. Here we examine if the swarms are induced by shear stress increases due to slip events along the decollement underneath Central Range. Other than slip model, fluid-filled cracks model is also examined. Moment tensor inversion will be used to determine focal mechanism of the low-frequency earthquake swarms, to investigate the fluids related volume changes.

  5. Broad band and long period magnetotellurics for imaging the onshore portion of Santos basin and orogenic belts of southeast Brazil

    NASA Astrophysics Data System (ADS)

    Solon, F. F.; Fontes, S. L.; Miquelutti, L. G.; La Terra, E. F.

    2012-12-01

    Between October 2011 and April 2012, we carried out 81 broad band and 40 long period magnetotelluric soundings covering the frequency range 1000 Hz - 0.0001 Hz. These soundings are distributed into two parallel 210km long profiles, approximately 50 km apart, spaced 5 km with EM field components oriented to the magnetic north and east directions. Both ADU07 (Metronix) and LEMI 417 (Lviv) systems were used. Typically, three to four MT soundings were measured simultaneously for this study. Both profiles initiate at the coast and continues to the continent direction. These profiles crossed two major Neoproterozoic Orogenic Belts, Brasilia and Ribeira, as well as several geosutures in the basement of the marginal basins of southeastern Brazil. The geophysical survey associated with the available geological information brought important contributions to the understanding of the main geosutures presents in the area. One located at the southeast end of the MT profiles, separating the tectonic domains of Ribeira Belt from the Pre-cambrian lands of Santos Basin. The other one situated in the extreme northwest of the MT profiles, limiting the Ribeira and Brasilia Belts. Data were processed using a robust remote-reference technique. Strike and dimensionality analysis along with G-B decomposition support a general 2-D regional character of the conductivity distribution in the area, allowing us to create a 2-D inversion model. Good misfit can be observed between the measured and calculated transfer functions projected onto the profile direction. Very preliminary results imaged the two main suture zones between the orogenics belts and allow inferences about the geographic position of their lateral limits.

  6. The Permian Dongfanghong island-arc gabbro of the Wandashan Orogen, NE China: Implications for Paleo-Pacific subduction

    NASA Astrophysics Data System (ADS)

    Sun, Ming-Dao; Xu, Yi-Gang; Wilde, Simon A.; Chen, Han-Lin; Yang, Shu-Feng

    2015-09-01

    The Dongfanghong hornblende gabbro is located in the western part of the Wandashan Orogen and to the east of the Jiamusi Block in NE China. It was emplaced into Early Paleozoic oceanic crust (i.e. Dongfanghong ophiolite) at ~ 275 Ma and both later collided with the eastern margin of the Jiamusi Block. The Dongfanghong gabbro is sub-alkaline with high Na2O contents and is characterized by enrichment in light rare earth elements (LREE), large ion lithosphile elements (LILE), Sr, Eu, and Ba, and depletion in high field strength elements (HFSE). The enriched isotopic signatures (87Sr/86Sri = ~ 0.7065, εNd(t) = ~- 0.5, 208Pb/204Pbi = ~ 38.05, 207Pb/204Pbi = ~ 15.56, 206Pb/204Pbi = ~ 18.20 and zircon εHf(t) = ~+ 5.8) indicate an enriched mantle (EM2) source, with some addition of continental material. It has arc geochemical affinities similar to Permian arc igneous rocks in the eastern margin of the Jiamusi Block, the Yakuno Ophiolite in SW Japan, arc rocks along the western margin of the North America Craton, and also the Gympie Group in eastern Australia. All these features, together with information from tectonic discrimination diagrams, suggest that the Dongfanghong gabbro formed in an immature island arc. The spatial configuration of ~ 290 Ma immature continental arc rocks in the eastern part of the Jiamusi Block and the ~ 275 Ma immature island arc Dongfanghong gabbro in the Wandashan Orogen to the east is best explained by eastward arc retreat and slab roll-back of the Paleo-Pacific Plate. This model is also supported by the Carboniferous-Permian stratigraphic transition in the Jiamusi Block from oceanic carbonate rocks to coal-bearing terrestrial clastic rocks and andesites. We thus suggest that both Paleo-Pacific subduction and roll-back occurred in the Early Permian along the eastern margin of Asia.

  7. Out-of-sequence deformation and expansion of the Himalayan orogenic wedge: insight from the Changgo culmination, south central Tibet

    NASA Astrophysics Data System (ADS)

    Larson, Kyle P.; Godin, Laurent; Davis, William J.; Davis, Don W.

    2010-08-01

    The Changgo culmination, one of the North Himalayan domes in south central Tibet, consists of a multiphase granite core surrounded by a deformed metasedimentary carapace. The granitic core records general non-coaxial shear with a top-to-the-south sense shear component. The contact between the core and the carapace is a shear zone, characterized by general non-coaxial shear with a top-to-the-north shear sense, interpreted to be the northern continuation of the South Tibetan detachment system (STDS). The shear zone contains lenses of leucogranite dated at 35.4 Ma. This is interpreted to reflect Eocene crustal thickening, coeval with the earliest shortening event recorded in the carapace. The main phase of the Changgo granite crystallized at 23.5 Ma, while undeformed aplite dikes, the youngest phase observed in the granite, were intruded at 22.1 Ma. Aplite dikes crosscut the main deformation fabric within the Changgo granite; therefore, that deformation and associated south directed shearing must have ended between 23.5 Ma and 22.1 Ma. The dikes are strained within the STDS, indicating that final displacement along the STDS must post-date 22 Ma, yet be older than 18.4 Ma, the cooling age of muscovite in the shear zone. It is proposed that the exhumation of the Changgo culmination is related to tectonically driven erosion in response to crustal thickening and rebuilding of the orogenic critical taper wedge. Subsequently, deformation in the wedge migrated toward the foreland, expanding the orogenic wedge laterally and moving the locus of displacement from the Main Central thrust structurally downward to the Main Boundary thrust.

  8. Bipolar mood cycles and lunar tidal cycles.

    PubMed

    Wehr, T A

    2017-01-24

    In 17 patients with rapid cycling bipolar disorder, time-series analyses detected synchronies between mood cycles and three lunar cycles that modulate the amplitude of the moon's semi-diurnal gravimetric tides: the 14.8-day spring-neap cycle, the 13.7-day declination cycle and the 206-day cycle of perigee-syzygies ('supermoons'). The analyses also revealed shifts among 1:2, 1:3, 2:3 and other modes of coupling of mood cycles to the two bi-weekly lunar cycles. These shifts appear to be responses to the conflicting demands of the mood cycles' being entrained simultaneously to two different bi-weekly lunar cycles with slightly different periods. Measurements of circadian rhythms in body temperature suggest a biological mechanism through which transits of one of the moon's semi-diurnal gravimetric tides might have driven the patients' bipolar cycles, by periodically entraining the circadian pacemaker to its 24.84-h rhythm and altering the pacemaker's phase-relationship to sleep in a manner that is known to cause switches from depression to mania.Molecular Psychiatry advance online publication, 24 January 2017; doi:10.1038/mp.2016.263.

  9. How is Silurian-Early Devonian faulting in the North America continental interior related to orogenic processes at plate boundaries? A working hypothesis from the Canadian North

    NASA Astrophysics Data System (ADS)

    Pinet, Nicolas

    2015-04-01

    The Paleozoic Appalachian/Franklinian orogen that rims the North America continent on its eastern and northern sides is comparable in size with Tethyan orogenic belts. However, the far-field effects in the continental interior of the multiple Ordovician to Carboniferous deformation phases that built the orogen were relatively minor if compared with those associated with the Himalayas and Alps, a characteristics related to the high integrated strength of the North American craton. Despite the generally little deformation of the continental interior, two regional-scale tectonic features preserved evidence of significant Paleozoic tectonism: the fault bounded Hudson Bay Central High (HBCH) and the Boothia uplift/Cornwallis fold belt (BUCF) in the Canadian Arctic. In the Hudson Bay intracratonic basin, the lower part of the sedimentary succession (Upper Ordovician to Lower Devonian) is cut by high-angle faults and overlain by a saucer-shape, essentially underformed sedimentary package (Middle to Upper Devonian). The main structural feature is the NNW-trending HBCH that extends for a minimum length of 500 km with normal faults characterized by throws up to 500 m that were mainly active during the Silurian - Early Devonian period. The >700-km long, N-trending BUCF is nearly perpendicular to the deformation front of the Franklinian mobile belt. In its southern segment (Boothia uplift), its western side is characterized by an east-dipping reverse fault zone that puts Precambrian rocks over Paleozoic strata. In its northern segment (Cornwallis fold belt), the Paleozoic succession is involved in open folds and cuts by steeply dipping reverse faults. Syn-tectonic clastic sediments constrain the age of structures to the latest Silurian-Early Devonian. Comparison of the HBCH and BUCF indicates that they are grossly parallel, partly contemporaneous but with different kinematics. This kinematic variability may be explained if they are genetically linked with different segments of

  10. Syn- and post-orogenic alkaline magmatism in a continental arc: Along-strike variations in the composition, source, and timing of igneous activity in the Ross Orogen, Antarctica

    NASA Astrophysics Data System (ADS)

    Hagen-Peter, G.; Cottle, J. M.

    2013-12-01

    Neoproterozoic-Paleozoic convergence and subduction along the margin of East Gondwana (Australia, New Zealand, Antarctica) resulted in a belt of deformed and metamorphosed sedimentary rocks and batholith-scale igneous intrusions comparable in size to the present day Andes. Mid-crustal levels of this belt, known as the Ross Orogen in Antarctica, are exposed in the basement of the Cenozoic Transantarctic Mountains, providing snapshots of the intrusive magma system of a major continental arc. Whole rock major- and trace-element geochemistry, Hf isotopes in zircon, and U-Pb geochronology have identified along-strike variations in the composition, source, and timing of magmatism along ~200 km of the southern Victoria Land segment of the orogen. There is an apparent younging of the igneous activity from south to north. New U-Pb ages for intrusive rocks from the Koettlitz Glacier Alkaline Province (KGAP) reveal that igneous activity spanned ca. 565-500 Ma (~30 m.y. longer than previously recognized), while immediately to the north in the Dry Valleys area most igneous activity was confined to a relatively short period (ca. 515-495 Ma). Alkaline and subalkaline igneous rocks occur in both the Dry Valleys area and the KGAP, but alkaline rocks in the Dry Valleys are restricted to the latest phase of magmatism. Na-alkaline rocks in the KGAP, including nepheline syenites, carbonatites, and A-type granites, range in age from ca. 545-500 Ma and overlap in age with more typical subduction/collision-related I- and S-type granites elsewhere in southern Victoria Land. Strong enrichments in the LILE and LREE and high LILE/HFSE and LREE/HREE of samples from the KGAP reveal a source enriched in aqueous-mobile elements, potentially a strongly metasomatized mantle wedge beneath the arc. In the Dry Valleys area, rocks with alkali-calcic composition constitute only the youngest intrusions (505-495 Ma), apparently reflecting a shift to post-orogenic magmatism. Zircons from Dry Valleys

  11. Evolution of the Dharwar Craton: A terrain of early Archean crustal stability, long term orogenic cycles and large scale palaeobiological activity

    NASA Technical Reports Server (NTRS)

    Srinivasan, R.; Naqvi, S. M.

    1988-01-01

    Traceable history of the Dharwar Craton goes back to approximately 3400 m.y. old tonalitic to trondhjemitic fundamental gneiss whose REE composition indicates its derivation from a preexisting basalt which apparently had very short time of crustal residence. The fundamental gneiss is preserved only as rare relicts in the vast gneissic complex of the Indian Peninsula (the Peninsular Gneiss), and as pebbles in the conglomerates of the Archean Dharwar sequence. Study of these relicts, shows evidence of a deformation episode prior to the deposition of the high- and low-grade Dharwar supracrustal sequence. The Dharwar supracrustal sequence is briefly described. Geochemistry of the volcanic and sedimentary rocks of the Dharwar supracrustal belts are examined.

  12. Proto-Pacific-margin source for the Ordovician turbidite submarine fan, Lachlan Orogen, southeast Australia: Geochemical constraints

    NASA Astrophysics Data System (ADS)

    Offler, R.; Fergusson, C. L.

    2016-04-01

    The Early Palaeozoic proto-Pacific Pacific margin of Gondwana was characterised by a huge turbidite submarine fan with abundant clastic detritus derived from unknown sources within Gondwana. These deposits are widespread in the Lachlan Orogen of southeast Australia and include the Ordovician Adaminaby Group. Here we show that the mudstones and sandstones of the Adaminaby Group have chemical compositions that indicate the detritus in them was derived from a felsic, continental source similar in composition to Post Archean Australian Shales (PAAS). Chondrite normalised REE patterns showing LREE enrichment, flat PAAS normalised patterns and elemental ratios La/Sc, Cr/Th, Cr/V, Th/Sc and Th/U, have been used to support this interpretation. The dominance of quartz, and to a lesser degree plagioclase and biotite in the sandstones, suggests that the source was mainly granodioritic to tonalitic in composition. Th/Yb and Ta/Yb ratios indicate that the source was probably calc-alkaline, continental and shoshonitic. In addition, the presence of detrital muscovite, low-grade metamorphic and felsic volcanic clasts, demonstrates that a low-grade metamorphic terrane and volcanic arc contributed to the detritus observed in the samples. The presence of well-rounded zircons and tourmalines, very high Zr contents, high Zr/Sc and higher Cr/V ratios in some samples particularly in the Shoalhaven River area, indicate that some of the detritus was recycled. SiO2 versus (Al2O3 + K2O + Na2O) plots suggest the source areas experienced conditions varying from humid/semi-humid to semi-arid. Textural features and weathering trends of samples from all locations follow a curved pathway on Al2O3 - (CaO* + Na2O) - K2O (ACNK) diagrams, and indicate that the clays formed from weathering had been K-metasomatised prior to penetrative deformation. Chemical indices of alteration (CIA) reveal that even the freshest sandstones are altered and others are moderately to strongly altered. Discrimination

  13. Possible genetic link between I-type granite and orogenic gold deposits in Egypt (metamorphic-magmatic interaction?)

    NASA Astrophysics Data System (ADS)

    Abd El Monsef, Mohamed

    2015-04-01

    The orogenic gold deposits are a distinctive type of deposits that revealed unique temporal and spatial association with an orogeny. Where, the system of gold veins and related ore minerals was confined to hydrothermal solutions formed during compressional to transpressional deformation processes at convergent plate margins in accretionary and collisional orogens, with the respect to ongoing deep-crustal, subduction-related thermal processes. In Egypt, most of vein-type and dyke-type gold mineralization are restricted to granitic rocks or at least near of granitic intrusion that seems to have had an important influence on gold mineralization. Shear zone-related, mesothermal gold deposits of Fatira and Gidami mines in the northern Eastern Desert of Egypt are found within granitic bodies or at the contact between granites and metavolcanic rocks. The hosting-granitic rocks in Fatira and Gidami areas are mainly of granodioritic composition (I-Type granite) which is related to calc-alkaline magmatic series. However, Fatira granitoids were developed within island arc tectonic settings related to mature island arc system (Late-orogenic stage), at relatively low temperature (around 660° C) and medium pressure between (5 - 10 Kbar). On the other hand, Gidami granitoids were developed during the collision stage in continental arc regime related to active continental margin (Syn-orogeny), which were crystallized at relatively high temperature (700-720° C) and low pressure (around 0.1 Kbar). The ore mineralogy includes pyrite, chalcopyrite, sphalerite, covellite, ilmenite, goethite ± pyrrhotite ± pentlandite ± galena ± molybdenite. Native gold is detected only in Gidami mineralization as small inclusions within pyrite and goethite or as tiny grains scattered within quartz vein (in close proximity to the sulfides). In Fatira deposits, it is detected only by microprobe analysis within the crystal lattice of pyrite and jarosite. Fluid inclusions study for the mineralized

  14. From microscope to mountain belt: 150 years of petrology and its contribution to understanding geodynamics, particularly the tectonics of orogens

    NASA Astrophysics Data System (ADS)

    Brown, M.

    2001-09-01

    Thirty-five years ago the introduction of the plate tectonics paradigm led to a new understanding of orogeny. Subsequently, the development of advanced instruments for remote collection of information and for analysis of elemental and isotopic composition of materials, and the increases in computing power have enabled an unprecedented number of high-precision data about the Earth to be collected, analyzed, modelled and displayed. Within this revolution in global tectonics, the metamorphic petrologist has developed methods to unravel the depth, thermal, temporal and deformational history of orogens using detailed observations at map, hand sample and thin-section scales in combination with elemental and isotope data, and using inverse and forward modelling. Two exciting new directions in metamorphic petrology in relation to geodynamics concern the kinship between earthquakes and metamorphic reactions in subduction zones, and the petrology of the Earth's mantle. Evidence of the changes in pressure ( P) and temperature ( T) in the Earth's crust and upper mantle during the break up, movement, and collision of pieces of the continental lithosphere is sporadically recorded by the mineralogy and microstructures preserved in rocks exhumed to the surface. Better calibration of phase equilibria, the use of internally-consistent thermodynamic data sets and the development of techniques to retrieve close-to-peak P-T conditions from metamorphic rocks have yielded more precise P-T data that enhance our ability to characterize the path followed by individual rocks in P-T space. An improved ability to date segments of the P-T path, and to separate the length of time associated with the prograde (increasing T) evolution from the age of close-to-peak P-T conditions has enabled better understanding of the rates and processes involved in lithosphere thickening. At the same time, better constraints on the retrograde thermal history have contributed to our knowledge of the several

  15. Modes of orogen-parallel stretching and extensional exhumation in response to microplate indentation and roll-back subduction (Tauern Window, Eastern Alps)

    NASA Astrophysics Data System (ADS)

    Scharf, A.; Handy, M. R.; Favaro, S.; Schmid, S. M.; Bertrand, A.

    2013-09-01

    The Tauern Window exposes a Paleogene nappe stack consisting of highly metamorphosed oceanic (Alpine Tethys) and continental (distal European margin) thrust sheets. In the eastern part of this window, this nappe stack (Eastern Tauern Subdome, ETD) is bounded by a Neogene system of shear (the Katschberg Shear Zone System, KSZS) that accommodated orogen-parallel stretching, orogen-normal shortening, and exhumation with respect to the structurally overlying Austroalpine units (Adriatic margin). The KSZS comprises a ≤5-km-thick belt of retrograde mylonite, the central segment of which is a southeast-dipping, low-angle extensional shear zone with a brittle overprint (Katschberg Normal Fault, KNF). At the northern and southern ends of this central segment, the KSZS loses its brittle overprint and swings around both corners of the ETD to become subvertical, dextral, and sinistral strike-slip faults. The latter represent stretching faults whose displacements decrease westward to near zero. The kinematic continuity of top-east to top-southeast ductile shearing along the central, low-angle extensional part of the KSZS with strike-slip shearing along its steep ends, combined with maximum tectonic omission of nappes of the ETD in the footwall of the KNF, indicates that north-south shortening, orogen-parallel stretching, and normal faulting were coeval. Stratigraphic and radiometric ages constrain exhumation of the folded nappe complex in the footwall of the KSZS to have begun at 23-21 Ma, leading to rapid cooling between 21 and 16 Ma. This exhumation involved a combination of tectonic unroofing by extensional shearing, upright folding, and erosional denudation. The contribution of tectonic unroofing is greatest along the central segment of the KSZS and decreases westward to the central part of the Tauern Window. The KSZS formed in response to the indentation of wedge-shaped blocks of semi-rigid Austroalpine basement located in front of the South-Alpine indenter that was part

  16. The Kharapeh orogenic gold deposit: Geological, structural, and geochemical controls on epizonal ore formation in West Azerbaijan Province, Northwestern Iran

    USGS Publications Warehouse

    Niroomand, Shojaeddin; Goldfarb, Richard J.; Moore, Farib; Mohajjel, Mohammad; Marsh, Erin E.

    2011-01-01

    The Kharapeh gold deposit is located along the northwestern margin of the Sanandaj–Sirjan Zone (SSZ) in the West Azerbaijan province, Iran. It is an epizonal orogenic gold deposit formed within the deformed zone between central Iran and the Arabian plate during the Cretaceous–Tertiary Zagros orogeny. The deposit area is underlain by Cretaceous schist and marble, as well as altered andesite and dacite dikes. Structural analysis indicates that the rocks underwent tight to isoclinal recumbent folding and were subsequently co-axially refolded to upright open folds during a second deformation. Late- to post-tectonic Cenozoic granites and granodiorites occur northeast of the deposit area. Mineralization mainly is recognized within NW-trending extensional structures as veins and breccia zones. Normal faults, intermediate dikes, and quartz veins, oriented subparallel to the axial surface of the Kharapeh antiform, indicate synchronous extension perpendicular to the fold axis during the second folding event. The gold-bearing quartz veins are >1 km in length and average about 6 m in width; breccia zones are 10–50 m in length and ≤1 m in width. Hydrothermal alteration mainly consists of silicification, sulfidation, chloritization, sericitization, and carbonatization. Paragenetic relationships indicate three distinct stages—replacement and silicification, brecciation and fracture filling, and cataclastic brecciation—with the latter two being gold-rich. Fluid inclusion data suggest mineral deposition at temperatures of at least 220–255°C and depths of at least 1.4–1.8 km, from a H2O–CO2±CH4 fluid of relatively high salinity (12–14 wt.% NaCl equiv.), which may reflect metamorphism of passive margin carbonate sequences. Ore fluid δ18O values between about 7‰ and 9‰ suggest no significant meteoric water input, despite gold deposition in a relatively shallow epizonal environment. Similarities to other deposits in the SSZ suggest that the deposit formed as

  17. Dyke Swarms in Southeastern British Columbia: Mineralogical and Geochemical Evidence for Emplacement of Multiple Magma Types During Orogenic Collapse

    NASA Astrophysics Data System (ADS)

    Freeman, M.; Owen, J. P.; Hoskin, P. W.

    2009-05-01

    Eocene dyke swarms in southeastern British Columbia provide an important record of the tectonic and magmatic history of the Cordillera following orogenic collapse. New field mapping, petrographic, and geochemical data is presented for a swarm of more than thirty dykes located near the mining town of Trail, B.C. Detailed field mapping revealed that individual dykes are highly diverse, both in composition and morphology. As a group, the dykes trend northwest (average strike of 338 degrees) and dip steeply to the southwest. Their average thickness is approximately 1.5m, with a range from 4.5m to less than 1cm. Three sub-parallel dykes were mapped for a length of 2km, and exhibit irregularities in their form such as branching and offshoots that follow fractures in the country rock. Thin-section analysis shows a wide variety of rock types within the swarm, including: micro-quartz syenite, micro-syenite, micro-monzonite, latite, basalt, basaltic andesite, and lamprophyre. Texturally, these samples are consistently porphyritic and partially altered to chlorite and sericite. This alteration commonly occurs in concentric rims around phenocrysts. The samples are typically intergranular, although some display trachytic texture. Whole-rock geochemistry shows that the dykes have a wide range in composition, with SiO2 between 76.45 wt.% and 45.15 wt.% and MgO between 0.13 wt.% and 13.16 wt.%. The results also revealed that one dyke has very high values of Ni (430 ppm), Cr (1420 ppm), and Co (50 ppm), giving it a fairly primitive composition. Harker diagrams and trace element plots show three distinct groups: mafic calc-alkaline dykes, felsic calc- alkaline dykes, and minette lamprophyres. The felsic dykes are characterized by negative Eu and Sr anomalies suggesting fractionation of plagioclase feldspar, as well as pronounced negative P and Ti anomalies. The minettes are enriched in LILE and depleted in HSFE relative to the mafic dykes. The three groups do not appear to be

  18. Temporal evolution of granitic magmas in the Luanchuan metallogenic belt, east Qinling Orogen, central China: Implications for Mo metallogenesis

    NASA Astrophysics Data System (ADS)

    Li, Dong; Han, Jiangwei; Zhang, Shouting; Yan, Changhai; Cao, Huawen; Song, Yaowu

    2015-11-01

    The Luanchuan metallogenic belt, located within the eastern part of the Qinling Orogen, central China, hosts a number of world-class Mo deposits that are closely related to small late Mesozoic granitic plutons. Zircon U-Pb dating of distinct plutons in the Luanchuan metallogenic belt has yielded ages of 153 ± 1, 154 ± 2, 152 ± 2, and 148 ± 1 Ma. Molybdenite Re-Os isotopic compositions of Yuku ore district in the southern part of Luanchuan metallogenic belt has yielded an isochron age of 146 ± 1 Ma, which is consistent with the large-scale mineralization ages in the northern part of the Luanchuan metallogenic belt. A combination of previous studies and new geochronological and isotopic data show a concordant temporal and genetic link between granitic magmatism and Mo mineralization in the Luanchuan metallogenic belt, suggesting that this mineralization episode formed the most extensive Mo mineralization belt in the east Qinling Orogen. Zircon grains from Mo-related granitic plutons show similar trace element distributions. High-precision Multi Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS) Pb isotope analysis of K-feldspar megacrysts from mineralization-related granites suggest that they were derived from the lower crust. Similarly, the Pb isotopic compositions of pyrite coprecipitated with molybdenite also suggest that the metals were derived form the lower crust, with probably minor mantle contribution. A continuum mineralization model that describes the sourcing of Mo from an evolving granitic magma over successive differentiation events, possibly in separate but connected magma chambers, could explain the remarkable Mo enrichment in the Luanchuan metallogenic belt. The volatile- and Mo-bearing granitic magmas ascended as diapirs from the deep crust, and were emplaced as dikes in the upper crust. Lithological differences between these Mo-bearing granites may relate to different stages in the evolution of individual magmas. Finally, ore

  19. Hercynian post-collisional magmatism in the context of Paleozoic magmatic evolution of the Tien Shan orogenic belt

    NASA Astrophysics Data System (ADS)

    Seltmann, Reimar; Konopelko, Dmitry; Biske, Georgy; Divaev, Farid; Sergeev, Sergei

    2011-10-01

    The Hercynian Tien Shan (Tianshan) orogen formed during Late Palaeozoic collision between the Karakum-Tarim and the Kazakhstan paleo-continents. In order to constrain timing of Hercynian post-collisional magmatism, 27 intrusions were sampled for U-Pb zircon dating along a ca. 2000 km - long profile in Uzbekistan and Kyrgyzstan. The samples were dated utilizing sensitive high resolution ion microprobe (SHRIMP-II). The obtained ages, together with previously published age data, allowed the timing of Hercynian post-collisional magmatism to be constrained and interpreted in the context of the Paleozoic magmatic evolution of the region. Apart from Hercynian post-collisional magmatism, two older magmatic episodes have been recognized, and the following sequence of events has been established: (1) approximately 10 Ma after cessation of continuous Caledonian magmatism a number of Late Silurian-Early Devonian intrusions were emplaced in the Middle and Northern Tien Shan terranes between 420 and 390 Ma. The intrusions probably formed in an extensional back arc setting during coeval subduction under the margins of Caledonian Paleo-Kazakhstan continent; (2) the next relatively short Late Carboniferous episode of subduction under Paleo-Kazakhstan was registered in the Kurama range of the Middle Tien Shan. Calc-alkaline volcanics and granitoids with ages 315-300 Ma have distinct metallogenic affinities typical for subduction-related rocks and are not found anywhere outside the Middle Tien Shan terrane west of the Talas-Farghona fault; (3) the Early Permian Hercynian post-collisional magmatism culminated after the closure of the Paleo-Turkestan ocean and affected the whole region across terrane boundaries. The post-collisional intrusions formed within a relatively short time span between 295 and 280 Ma. The model for Hercynian post-collisional evolution suggests that after collision the Tien Shan was affected by trans-crustal strike-slip motions which provided suitable conduits

  20. Syn-Convergent Orogen-Parallel Low-Angle Normal-Sense Faults: One Process or Several?

    NASA Astrophysics Data System (ADS)

    Kellett, D. A.; Grujic, D.

    2010-12-01

    Low-angle normal-sense faults (LANFs) form in both extensional and contractional settings. Recent field and experimental studies have demonstrated that exhumed footwall tectonites in extensional LANFs are not necessarily down-dip equivalents of the surface detachment, instead they may deform during distributed flow in the mid- or lower crust. We propose that syn-contractional LANFs also need not result from a single, progressive deformation. The South Tibetan detachment system (STDS) is a network of low-angle normal sense faults and shear zones (LANFs) that formed in the Miocene coevally with and parallel to contractional structures during lithospheric shortening accompanying development of the Himalayan orogen. In the eastern Himalaya, the STDS is exposed multiple times across orogen strike, and each exposure is characterized by differences in: metamorphic grade between the footwall and hanging wall rocks; numbers of individual structures; amounts of ductile versus brittle deformation, and; times of activity. Using new and published geochronologic, geothermometric and structural data, we characterize and distinguish three different types of LANFs within the STDS: 1. Mid-crustal flow LANFs, formed by ductile flow of melt-weakened mid-crust between stronger upper and lower crust that produced a sub-horizontal diffuse, ductile shear zone separating high metamorphic grade mid-crustal anatectite from low metamorphic grade upper-crustal rocks. Such structures do not cut down-section and locally may preserve a reversal in shear sense. 2. Extrusion LANFs, formed during extrusion of a mid-crust slab along focused opposing-sense bounding shear zones. The extruding slab is roofed by discrete ductile-brittle to brittle faults that post-date and partly excise a mid-crustal flow LANF. 3. Destabilization LANFs, formed by local extension of the upper crust accompanying destabilization and doming of the mid-crust, and characterized by single or conjugate ductile-brittle to brittle

  1. The Solar Cycle.

    PubMed

    Hathaway, David H

    The solar cycle is reviewed. The 11-year cycle of solar activity is characterized by the rise and fall in the numbers and surface area of sunspots. A number of other solar activity indicators also vary in association with the sunspots including; the 10.7 cm radio flux, the total solar irradiance, the magnetic field, flares and coronal mass ejections, geomagnetic activity, galactic cosmic ray fluxes, and radioisotopes in tree rings and ice cores. Individual solar cycles are characterized by their maxima and minima, cycle periods and amplitudes, cycle shape, the equatorward drift of the active latitudes, hemispheric asymmetries, and active longitudes. Cycle-to-cycle variability includes the Maunder Minimum, the Gleissberg Cycle, and the Gnevyshev-Ohl (even-odd) Rule. Short-term variability includes the 154-day periodicity, quasi-biennial variations, and double-peaked maxima. We conclude with an examination of prediction techniques for the solar cycle and a closer look at cycles 23 and 24.

  2. The Solar Cycle

    NASA Astrophysics Data System (ADS)

    Hathaway, David H.

    2015-12-01

    The solar cycle is reviewed. The 11-year cycle of solar activity is characterized by the rise and fall in the numbers and surface area of sunspots. A number of other solar activity indicators also vary in association with the sunspots including; the 10.7 cm radio flux, the total solar irradiance, the magnetic field, flares and coronal mass ejections, geomagnetic activity, galactic cosmic ray fluxes, and radioisotopes in tree rings and ice cores. Individual solar cycles are characterized by their maxima and minima, cycle periods and amplitudes, cycle shape, the equatorward drift of the active latitudes, hemispheric asymmetries, and active longitudes. Cycle-to-cycle variability includes the Maunder Minimum, the Gleissberg Cycle, and the Gnevyshev-Ohl (even-odd) Rule. Short-term variability includes the 154-day periodicity, quasi-biennial variations, and double-peaked maxima. We conclude with an examination of prediction techniques for the solar cycle and a closer look at cycles 23 and 24.

  3. The distribution, geochronology and geochemistry of early Paleozoic granitoid plutons in the North Altun orogenic belt, NW China: Implications for the petrogenesis and tectonic evolution

    NASA Astrophysics Data System (ADS)

    Meng, Ling-Tong; Chen, Bai-Lin; Zhao, Ni-Na; Wu, Yu; Zhang, Wen-Gao; He, Jiang-Tao; Wang, Bin; Han, Mei-Mei

    2017-01-01

    Abundant early Paleozoic granitoid plutons are widely distributed in the North Altun orogenic belt. These rocks provide clues to the tectonic evolution of the North Altun orogenic belt and adjacent areas. In this paper, we report an integrated study of petrological features, U-Pb zircon dating, in situ zircon Hf isotope and whole-rock geochemical compositions for the Abei, 4337 Highland and Kaladawan Plutons from north to south in the North Altun orogenic belt. The dating yielded magma crystallization ages of 514 Ma for the Abei Pluton, 494 Ma for the 4337 Highland Pluton and 480-460 Ma for the Kaladawan Pluton, suggesting that they are all products of oceanic slab subduction because of the age constraint. The Abei monzogranites derived from the recycle of Paleoproterozoic continental crust under low-pressure and high-temperature conditions are products of subduction initiation. The 4337 Highland granodiorites have some adakitic geochemical signatures and are sourced from partial melting of thickened mafic lower continental crust. The Kaladawan quartz diorites are produced by partial melting of mantle wedge according to the positive εHf(t) values, and the Kaladawan monzogranite-syenogranite are derived from partial melting of Neoproterozoic continental crust mixing the juvenile underplated mafic material from the depleted mantle. These results, together with existing data, provide significant information about the evolution history of oceanic crust subduction during the 520-460 Ma. The initiation of subduction occurred during 520-500 Ma with formation of Abei Pluton; subsequent transition from steep-angle to flat-slab subduction at ca.500 Ma due to the arrival of buoyant oceanic plateaus, which induces the formation of 4337 Highland Pluton. With ongoing subduction, the steep-angle subduction system is reestablished to cause the formation of 480-460 Ma Kaladawan Pluton. Meanwhile, it is this model that account for the temporal-spatial distribution of these early

  4. High-temperature metamorphism of the Yushugou ophiolitic slice: Late Devonian subduction of seamount and mid-oceanic ridge in the South Tianshan orogen

    NASA Astrophysics Data System (ADS)

    Zhang, Li; Jin, Zhenmin

    2016-12-01

    The South Tianshan Orogenic Belt (STOB), representing the southern segment of the Central Asian Orogenic Belt (CAOB), underwent a long-lived and subduction-related accretionary orogenic process. Revealing the petrogenesis of high-pressure (HP) metamorphic ophiolitic slices within this orogen is of crucial importance to understanding the geodynamic evolution of the STOB. In this study, we carry out a p