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  • 1
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    Simultaneous miocene extension and shortening in the himalayan orogen (1992)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1992-11-27
    Description: The South Tibetan detachment system separates the high-grade metamorphic core of the Himalayan orogen from its weakly metamorphosed suprastructure. It is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front. Geochronologic data from the Rongbuk Valley, north of Qomolangma (Mount Everest) in southern Tibet, demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago, an interval characterized by large-scale crustal thickening at lower structural levels. These data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of Himalayan tectonics in Miocene time.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Hodges, K V -- Parrish, R R -- Housh, T B -- Lux, D R -- Burchfiel, B C -- Royden, L H -- Chen, Z -- New York, N.Y. -- Science. 1992 Nov 27;258(5087):1466-70.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17755108" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 2
    Electronic Resource
    Electronic Resource
    Exhumation of the Main Central Thrust from Lower Crustal Depths, Eastern Bhutan Himalaya (2003)
    Oxford, UK : Blackwell Science Inc
    Journal of metamorphic geology 21 (2003), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35–45 km. Garnet-bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650–675 °C and 9–13 kbar. Staurolite-kyanite schists, about 30 m above the MCT, yield P-T conditions near 650 °C, 8–10 kbar. Kyanite-bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10–14 kbar at 750–800 °C. Top-to-the-south shearing is synchronous with, and postdates peak metamorphic mineral growth.Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U/Pb ages of 20–18 Ma. Staurolite-kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde metamorphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U/Pb crystallization ages of monazite and xenotime in a deformed kyanite-bearing leucogranite and kyanite-garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18–16 Ma. Higher in the hanging wall, south-verging shear bands filled with leucogranite and pegmatite yield U/Pb crystallization ages for monazite and xenotime of 14–15 Ma, and a 1–2 km thick leucogranite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS continued until at least 13 Ma. P-T conditions at this time are estimated to be 500–600 °C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3–9 mm year−1. This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1525-1314.2003.00445.x
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  • 3
    Electronic Resource
    Electronic Resource
    Zircon U/Pb and Pb/Pb geochronology of the Rastenberg granodiorite, South Bohemian Massif, Austria (1996)
    Springer
    Mineralogy and petrology 58 (1996), S. 197-214 
    Details
    ISSN: 1438-1168
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Description / Table of Contents: Zusammenfassung Zirkon-typologische Untersuchungen, Einzelzirkon Pb-Pb-Evaporations- und konventionelle U-Pb-Altersbestimmungen an Gesteinen des spät- bis postkinematischen Rastenberger Granodiorits geben folgende Resultate: Typologisch lassen sich zwei Zirkonpopulationen unterscheiden: Typ 1: S24 Subtyp vonPupin, farblos bis leicht rosa, klar bis getrübt, häufig mit Kernen, wenige bis viele Einschlüsse, lang prismatisch; Typ 2: S4 Subtyp vonPupin, farblos bis rötlich oder leicht rosa, klar bis leicht getrübt, keine sichtbaren Kerne, viele Einschlüsse, flachtafeliger Habitus, kurzprismatisch. Mindestens 4 unterschiedliche Altersgruppen lassen sich unterscheiden: Ererbte Kerne von Typ 1 Zirkonen mit Altern um 623±22Ma und einzelnen Altern 〉 1206Ma sprechen für die Aufarbeitung von Gesteinen, die von cadomischen und proterozoischen bis archaischen Gesteinen im Krustenbereich des Südböhmischen Plutons abstammen könnten. Alter um 353±9Ma von Typ 1 Zirkonen werden einer ersten magmatischen Phase oder dem Beginn der Krustenaufschmelzung im Zuge der Bildung der variszischen Plutonite zugeordnet. Die eigentliche Intrusion des Granodiorits um 338±2Ma wird mit Typ 2 und mit Randpartien von Typ 1 Zirkonen erfaßt. In den großen K-Feldspat-Phänokristallen finden sich nur Zirkone vom Typ 1. Dies deutet darauf hin, daß die Phänokristalle vor dem 338Ma Ereignis gebildet wurden, eventuell also bis 353Ma alt sein können.
    Notes: Summary A combined zircon typology, zircon Pb-Pb evaporation, and conventional U-Pb study of the late- to post-tectonic Rastenberg granodiorite yields the following results: Typological investigations show two distinguishable zircon populations. Type l: subtype S24 ofPupin, colourless to slightly pink, clear to turbid, often with cores, few to abundant inclusions, long prismatic; type 2: subtype S4 ofPupin, colourless to reddish or slightly pink, clear to slightly turbid, no visible cores, abundant inclusions, tabular habit, short prismatic. At least 4 different zircon-forming events can be distinguished: Inherited cores with ages around 623±22Ma and single ages 〉 1206Ma from type 1 zircons imply the reworking of rocks derived from Cadomian and Proterozoic to Archean crust. Ages around 353±9Ma from type 1 zircons are interpreted as timing a first magma formation or the onset of a long-lasting magma-generating event during the Variscan plutonism in the South Bohemian pluton. The actual intrusion of the granodioritic magma into the middle crust took place around 338±2Ma (type 2 and rims of type 1 zircons). Only type 1 zircons are found as inclusions in large K-feldspar phenocrysts providing evidence that these phenocrysts have grown before the 338Ma event and may be as old as 353 Ma.[⇃]
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01172096
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  • 4
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    Locking of southward extrusion in favour of rapid crustal-scale buckling of the Greater Himalayan sequence, Nar valley, central Nepal (2007)
    In:  Geological Society Special Publication 268: 269-292.
    Details
    Publication Date: 2007-10-08
    Description: The South Tibetan detachment system (STDS) bounds the upper limit of the Greater Himalayan sequence (GHS), which consists of the exhumed middle crust of the Himalaya. In the Annapurna range of central Nepal, the GHS comprises a sequence of amphibolite-grade augen gneisses with a 3.5 km thick leucogranite at the higher structural levels (Manaslu granite). Two major low-angle normal-sense shear zones have been mapped. The Chame detachment has similar grade metamorphic rocks above and below and is interpreted as a ductile shear zone wholly within the GHS. The Phu detachment is a ductile-brittle normal fault which wraps around the top of the Manaslu leucogranite and defines the uppermost, youngest strand of the STDS, placing folded unmetamorphosed Palaeozoic rocks of the Tethyan sedimentary sequence above the GHS. Our data indicate that ductile flow and southward extrusion of the GHS terminated with cessation of movement on the brittle upper strand of the Phu detachment at c. 19 Ma, which was followed almost immediately by crustal-scale buckling. Argon thermochronology reveals that the bulk of the metamorphic rocks and lower portions of the Tethyan sedimentary sequence in the Nar valley cooled through the hornblende, biotite and muscovite closure temperatures at c. 16 Ma, suggesting very rapid cooling rates. The thermochronology results indicate that this cooling occurred 2-3 million years earlier than in the frontal part of the extruded GHS. Although the extrusion in the frontal part of the GHS must have locked at the same time as in the Nar valley, the exhumation there was slower, and most probably only assisted by erosion, rather than by rapid folding as is the case in the Nar valley. This buckling indicates a step northward in deformation within the Himalayan belt, which may be a response to a lower deforming taper geometry in the foreland.
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  • 5
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    Subduction zone polarity in the Oman Mountains: implications for ophiolite emplacement (2003)
    In:  Geological Society Special Publication 218: 467-480.
    Details
    Publication Date: 2003-01-01
    Description: Two end-member models have been proposed to account for the structure and metamorphism of rocks beneath the Semail ophiolite in the Oman mountains. Model A involves a single, continuous NE-directed subduction away from the continental margin during the late Cretaceous. The ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km SW onto the continental margin. Subduction of Triassic-Jurassic oceanic basalts to c. 10 kbar (c. 39 km depth) led to the accretion of amphibolite-facies rocks to the base of the ophiolite. Thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to c. 20 kbar (c. 78 km depth), choking the subduction zone. Buoyancy forces caused the rapid exhumation of eclogites, blueschists and carpholite-grade HP rocks along the NE margin of the continental plate. During the later phase of foreland-propagating thin-skinned thrusting in the SW, NE-facing backfolding and backthrusting occurred in the hinterland, with the final exhumation of the HP rocks. Model B follows recent suggestions that a nascent SW-dipping subduction zone, dipping beneath the continental margin, existed between 130 and 95 Ma, prior to formation and emplacement of the ophiolite. A major NE-facing fold-nappe structure in the pre-Permian basement rocks of Saih Hatat is interpreted as reflecting subduction beneath the margin. Two high-pressure metamorphic events have been suggested, the first predating ophiolite emplacement, the second caused by ophiolite loading. This model is untenable, being based on a misinterpretation of the NE-facing structures in northern Saih Hatat, and on some dubious older 40Ar/39Ar cooling ages from the eclogite-facies rocks of As Sifah. We conclude that all structures in northern Oman and all the reliable geochronology point to a single emplacement-obduction event lasting from Cenomanian-Turonian time (c. 95 Ma) when amphibolites were accreted along the metamorphic sole of the ophiolite, to Campanian time, when the continental margin was subducted to the NE producing blueschists and eclogites, to the final thin-skinned emplacement of all thrust sheets, which ended before the Late Maastrichtian, at c. 68 Ma.
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  • 6
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    Ar‐Ar and U‐Pb Geochronology of a Late Paleoproterozoic Rift Basin: Support for a Genetic Link with Hudsonian Orogenesis, Western Churchill Province, Nunavut, Canada (2006)
    University of Chicago Press
    Details
    Publication Date: 2006-01-01
    Print ISSN: 0022-1376
    Electronic ISSN: 1537-5269
    Topics: Geosciences
    Published by University of Chicago Press
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  • 7
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    Early hydrothermal carbon uptake by the upper oceanic crust: Insight from in situ U-Pb dating (2016)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2016-01-22
    Description: It is widely thought that continental chemical weathering provides the key feedback that prevents large fluctuations in atmospheric CO 2 , and hence surface temperature, on geological time scales. However, low-temperature alteration of the upper oceanic crust in off-axis hydrothermal systems provides an alternative feedback mechanism. Testing the latter hypothesis requires understanding the timing of carbonate mineral formation within the oceanic crust. Here we report the first radiometric age determinations for calcite formed in the upper oceanic crust in eight locations globally via in-situ U-Pb laser ablation–inductively coupled plasma–mass spectrometry analysis. Carbonate formation occurs soon after crustal accretion, indicating that changes in global environmental conditions will be recorded in changing alteration characteristics of the upper oceanic crust. This adds support to the interpretation that large differences between the hydrothermal carbonate content of late Mesozoic and late Cenozoic oceanic crust record changes in global environmental conditions. In turn, this supports a model in which alteration of the upper oceanic crust in off-axis hydrothermal systems plays an important role in controlling ocean chemistry and the long-term carbon cycle.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 8
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    The geology and tectonics of central Bhutan (2016)
    Geological Society of London
    Details
    Publication Date: 2016-03-02
    Description: Lithotectonic mapping, metamorphic observations and U–Pb zircon ages underpin a substantial revision of central Bhutan geology, notably a more extensive and continuous outcrop of the Tethyan Sedimentary Series (TSS) than previously mapped. Metamorphic grade in the TSS increases downward towards a basal north-vergent tectonic contact with the underlying Greater Himalayan Series (GHS), interpreted as a southward continuation of the South Tibetan Detachment (STD). Miocene ( c . 17–20 Ma) leucogranite sheets are associated with the STD in this region but appear to diminish southwards. Two leucogranite dykes that cross-cut TSS structures yield ages of 17.8 ± 0.2 and 17.9 ± 0.5 Ma. A 500 ± 4 Ma (U–Pb zircon) metamorphosed ash bed in the Pele La Group within the psammite-dominated lower TSS yields the first direct isotopic age for the TSS in the eastern Himalaya, confirming existing age constraints from detrital zircon and fossil studies. A continuation of the Paro metasedimentary unit underlying the GHS was mapped near Wangdue Phodrang. Our observations, notably the exposure of a wholly ductile STD so far south and the significance of large nappe-like structures in the TSS, prompt a major revision to the geological map of the Bhutan Himalaya and require a reassessment of tectonic interpretations of the Bhutan Himalaya. Supplementary materials: Zircon U–Pb geochronological data, sample locations and descriptions, features of analysed zircons, sample processing method and detailed analytical conditions are available at http://www.geolsoc.org.uk/SUP18876 .
    Print ISSN: 0016-7649
    Topics: Geosciences
    Published by Geological Society of London
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  • 9
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    Geochronology and structural setting of Latest Devonian - Early Carboniferous magmatic rocks, Cape Kiber, northeast Russia (2015)
    National Research Council Canada (NRC)
    Details
    Publication Date: 2015-03-26
    Description: Cape Kiber on the Arctic coast of Chukotka, northeast Russia, consists of a granite intruding Devonian (and older?) strata in the core of a large southeast trending anticline. These strata are structurally overlain by Carboniferous and younger strata. A U–Pb age of 351.4 ± 5.6 (2) Ma shows that the granite is Early Carboniferous in age. A large granite cobble extracted from a Carboniferous conglomerate produced a Late Devonian or Early Carboniferous U–Pb age of ~355–361 Ma. Also, a deformed and altered granitic dyke yielded an age of 363.7 ± 5.7 (2) Ma. Major and trace elements suggest a syn-collisional (orogenic) setting. The granite’s (biotite) Ar release spectrum is reset. The granitic dyke also shows a disturbed Ar–Ar whole-rock spectrum implying an Early Cretaceous age (~122–130 Ma) for closure of the Ar system. We interpret this as due to widespread greenschist metamorphism accompanying regional deformation of the Anyuy–Chukotka Fold Belt that accompanied closure of the South Anyuy Ocean. Regionally, this event predates deposition of Aptian and Albian strata and the eruption of Okhotsk–Chukotsk magmatic rocks. An Ar–Ar (biotite) plateau age of 96.4 ± 1.0 (2) Ma from a mildly deformed, lamprophyre dyke reflects intrusion in a setting of regional extension. Its deformation reflects a younger tectonic event. The record of Devonian–Carboniferous magmatism and early Carboniferous unroofing is younger and less complex than that of Arctic Alaska. However, evidence for Early Devonian (Caledonian) or Late Devonian (Ellesmerian) deformation could have been masked by intense Mesozoic deformation. Outcrop data and geochronology support and refine regional interpretations of Early Cretaceous deformation and mineral growth accompanying accretion of Chukotka to north Asia, followed by regional extension and subsequent convergent deformation.
    Print ISSN: 0008-4077
    Electronic ISSN: 1480-3313
    Topics: Geosciences
    Published by National Research Council Canada (NRC)
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  • 10
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    Garnet-monazite rare earth element relationships in sub-solidus metapelites: a case study from Bhutan (2018)
    Geological Society of London
    Details
    Publication Date: 2018
    Description: 〈p〉A key aim of modern metamorphic geochronology is to constrain precise and accurate rates and timescales of tectonic processes. One promising approach in amphibolite and granulite-facies rocks links the geochronological information recorded in zoned accessory phases such as monazite to the pressure–temperature information recorded in zoned major rock-forming minerals such as garnet. Both phases incorporate rare earth elements (REE) as they crystallize and their equilibrium partitioning behaviour potentially provides a useful way of linking time to temperature. We report REE data from sub-solidus amphibolite-facies metapelites from Bhutan, where overlapping ages, inclusion relationships and Gd/Lu ratios suggest that garnet and monazite co-crystallized. The garnet–monazite REE relationships in these samples show a steeper pattern across the heavy (H)REE than previously reported. The difference between our dataset and the previously reported data may be due to a temperature-dependence on the partition coefficients, disequilibrium in either dataset, differences in monazite chemistry or the presence or absence of a third phase that competed for the available REE during growth. We urge caution against using empirically-derived partition coefficients from natural samples as evidence for, or against, equilibrium of REE-bearing phases until monazite–garnet partitioning behaviour is better constrained.〈/p〉 〈p〉〈b〉Supplementary material:〈/b〉 Trace element concentrations and data, detailed analytical information, field photographs, chemical maps and thin section information are available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4044323"〉https://doi.org/10.6084/m9.figshare.c.4044323〈/a〉〈/p〉
    Print ISSN: 0375-6440
    Electronic ISSN: 2041-4927
    Topics: Geosciences
    Published by Geological Society of London
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