Isotopic perspectives on the western Himalayan syntaxis

NASA Astrophysics Data System (ADS)

Argles, T. W.; Foster, G. L.; Whittington, A. G.; George, M. T.

2003-04-01

The western syntaxis has been characterised as a structural and metamorphic anomaly within the Himalaya, resulting from extreme Neogene exhumation and associated partial melting. However, an integration of detailed fieldwork with whole-rock isotopic data indicates that all the major tectonic units observed along the arc of the orogen also occur in the syntaxis. Most of the rocks exposed by the extreme exhumation have very different characteristics to their correlatives in the rest of the Himalayan mountain belt, because they represent very different crustal levels. The generally higher metamorphic grade of most syntaxial units obscures their affinities, while high strain throughout the syntaxis also conspires to mask the major tectonic faults that form boundaries to the units in the rest of the orogen. The Lesser Himalayan affinity of the gneissic core of the Nanga Parbat massif has been revealed previously using Nd isotopes. This study confirms the distinction between Lesser (E(Nd) = -20 to -29) and High (E(Nd) = -12 to -19) Himalayan rocks, but further subdivides those units with a High Himalayan Nd signature using Sr isotopic data. Some low-grade schists within the syntaxis have a relatively low 87Sr/86Sr ratio (<0.720) that distinguishes them from the High Himalayan rocks, and suggests they are metamorphic equivalents of the Tethyan sediments exposed in the main Himalayan orogen. The tectonic contact between the Lesser and High Himalayan units in the central Himalaya is the Main Central Thrust, a zone characterised by inverted metamorphism and high strain, but in the uniformly high-strain syntaxis this thrust is difficult to locate except by isotopic signatures. Extensive thermobarometric studies in the syntaxis, however, show two things. The first is the varying intensity of Neogene metamorphic overprint, whose strength is closely related to the degree of deformation (and rheology). The second is a zone of distinctly lower temperature mineral assemblages

Geochemical evidence for Paleozoic crustal growth and tectonic conversion in the Northern Beishan Orogenic Belt, southern Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Yuan, Yu; Zong, Keqing; He, Zhenyu; Klemd, Reiner; Jiang, Hongying; Zhang, Wen; Liu, Yongsheng; Hu, Zhaochu; Zhang, Zeming

2018-03-01

The Beishan Orogenic Belt is located in the central southernmost part of the Central Asian Orogenic Belt (CAOB), which plays a key role in understanding the formation and evolution of the CAOB. Granitoids are the documents of crustal and tectonic evolution in orogenic belts. However, little is known regarding the petrogenesis and geodynamic setting of the widely distributed Paleozoic granitoids in the Northern Beishan Orogenic Belt (NBOB). The present study reveals significant differences concerning the petrogenesis and tectonic setting of early and late Paleozoic granitoids from the NBOB. The early Paleozoic granitoids from the 446-430 Ma Hongliuxia granite complex of the Mazongshan unit and the 466-428 Ma Shibanjing complex of the Hanshan unit show classic I-type granite affinities as revealed by the relative enrichment of LILEs and LREEs, pronounced depletions of Nb, Ta and Ti and the abundant presence of hornblende. Furthermore, they are characterized by strongly variable zircon εHf(t) values between - 16.7 and + 12.8 and evolved plagioclase Sr isotopic compositions of 0.7145-0.7253, indicating the involvement of both juvenile and ancient continental crust in the magma source. Thus, we propose that the early Paleozoic granitoids in the NBOB were generated in a subduction-related continental arc setting. In contrast, the late Paleozoic 330-281 Ma granitoids from the Shuangjingzi complex of the Hanshan unit exhibit positive zircon εHf(t) values between + 5.8 and + 13.2 and relatively depleted plagioclase Sr isotopic compositions of 0.7037-0.7072, indicating that they were mainly formed by remelting of juvenile crust. Thus, an intra-plate extensional setting is proposed to have occurred during formation of the late Paleozoic granitoids. Therefore, between the early and late Paleozoic, the magma sources of the NBOB granitoids converted from the reworking of both juvenile and ancient crusts during a subduction-induced compressional setting to the remelting of

The final pulse of the Early Cenozoic adakitic activity in the Eastern Pontides Orogenic Belt (NE Turkey): An integrated study on the nature of transition from adakitic to non-adakitic magmatism in a slab window setting

NASA Astrophysics Data System (ADS)

Eyuboglu, Yener; Dudas, Francis O.; Santosh, M.; Eroğlu-Gümrük, Tuğba; Akbulut, Kübra; Yi, Keewook; Chatterjee, Nilanjan

2018-05-01

The Eastern Pontides Orogenic Belt, one of the best examples of a fossil continental arc in the Alpine-Himalayan system, is characterized by adakitic magmatism during the Early Cenozoic. Popular models correlate the adakitic magmatism to syn- or post-collisional processes occurring after the collision between the Eastern Pontides Orogenic Belt and the Tauride Platform at the end of Late Mesozoic and/or beginning of the Cenozoic. We present new geological, petrological and chronological data from andesites and felsic tuffs exposed in the Bayburt area, in the southern part of the Eastern Pontides Orogenic Belt, and discuss the nature of the transition from adakitic to non-adakitic activities in a continental arc. Major, trace and rare earth element concentrations of both andesites and felsic tuffs clearly suggest that they are related to arc magmatism in a continental arc with adakitic composition. The isotopic compositions are permissive of mixing between a component similar to depleted mantle and a second component that is either mafic lower crust or subducted oceanic crust. 39Ar/40Ar hornblende and U/Pb zircon dating indicate that this adakitic magmatism in the Bayburt area ended by about 47 Ma, and transformed into non-adakitic, granitoid arc magmatism in the area immediately north of Bayburt in the Lutetian (∼46 Ma). Based on our new results in conjunction with available data, we propose that the beginning of northward rollback of a south-directed subducting slab, and simultaneous opening of a slab window related to ridge subduction, triggered both adakitic magmatism for approximately a 10 Myr period between 57.6 and 47 Ma and arc-parallel extension that caused the opening of the Early Cenozoic sedimentary basins. We also suggest that the shallow marine environment, in which Nummulite-bearing sandy limestones accumulated in the Early Cenozoic, was transformed into a saline-lake environment during the pyroclastic activity that produced the studied felsic tuffs

Timing and nature of Holocene glacier advances at the northwestern end of the Himalayan-Tibetan orogen

NASA Astrophysics Data System (ADS)

Saha, Sourav; Owen, Lewis A.; Orr, Elizabeth N.; Caffee, Marc W.

2018-05-01

Holocene glacial chronostratigraphies are developed for four glaciated valleys at the northwestern end of the Himalayan-Tibetan orogen using geomorphic mapping and cosmogenic 10Be surface exposure dating. The study areas include the Hamtah valley in the Lahul Himalaya, and the Karzok, Lato and upper Stok valleys in Zanskar. Five local glacial stages are dated to ∼10.4, ∼6.1-3.3, ∼2.1-0.9, ∼0.7-0.4, and ∼0.3-0.2 ka based on 49 new moraine boulder ages. Large age dispersions are evident for each of the local glacial stages. This is especially the case for ∼6.1-3.3 and ∼2.1-0.9 ka, which is likely a result of prior and/or incomplete exposures in very young moraine boulders. An additional compilation of 187 published 10Be moraine boulder ages help define seven Himalayan Holocene regional glacial stages (HHs) for the northwestern end of the Himalayan-Tibetan orogen. These HHs date to ∼10.9-9.3, ∼8.2-7.4, ∼6.9-4.3, ∼4.5-2.8, ∼2.7-1.8, ∼1.8-0.9, and <1 ka. Early Holocene glacier advances were generally more extensive and had larger equilibrium-line altitude depressions (ΔELA = ∼425 ± 229 m) than glacier advances during the mid-Holocene (ΔELA = ∼141 ± 106) and late Holocene (ΔELA = ∼124 ± 121 m). The early Holocene glacier advances likely correspond to orbitally-forced northerly migration of the Intertropical Convergence Zone and enhanced summer monsoon. The timing of the majority of HHs during mid- and late Holocene corresponds well with the North Atlantic cooling that is likely teleconnected via mid-latitude westerlies, particularly during ∼8 ka and after ∼5 ka. These chronostratigraphies suggest that Holocene glaciation in the northwestern part of the Himalayan-Tibetan orogen is largely influenced by long-term orbital forcing amplified by large-scale migration of the Earth's thermal equator and the associated hemispheric oceanic-atmospheric systems.

The eastern Central Asian Orogenic Belt: formation and evolution

NASA Astrophysics Data System (ADS)

Xu, Bei; Xu, Wenliang

2017-08-01

The Central Asian Orogenic Belt (CAOB) extends from the northern Eurasian continent in the west via Mongolia, Inner Mongolia and northeast part of China to the Russia Far East in the east. It is characterized by complex trench-arc-basin subduction system, exotic terrane (microcontinents) accretion, massive generation of juvenile crust during the Neoproterozoic-Phanerozoic (e.g., Jahn et al., 2000, 2004; Sengör et al., 1993). A lot of papers about formation and evolution of the CAOB have been published and new field observations and geochemical data for key areas of the CAOB challenge to previous assessments. Several areas previously defined as juvenile are now shown to have mixed crustal compositions. For example, Kröner et al. (2014, in press) estimated that the distribution of various crustal provinces is truly juvenile crustal material ca. 20%, mixed crust ca. 30%, old crust ca. 50%，respectively, in the CAOB, similar to those in other accretionary orogens through Earth history. A two-stage model for the evolution of the CAOB has been suggested based on recent data from the Eastern Tianshan and Beishan (Gao et al., 2011; Su et al., 2011; Chen et al., 2016; Wang et al., 2017), which suggests the process of the formation and evolution of the CAOB includes closure of the Paleo Asian ocean (PAO), formation of orogenic belt before the late Paleozoic and crustal extension and magmatism resulted from plume upon the young orogenic belt after the late Paleozoic. This new model changes previous concept that the CAOB developed through the Paleozoic and is supported by recent researches on the eastern CAOB.

Reconciling late fracturing over the entire Alpine belt: from structural analysis to geochronological constrains

NASA Astrophysics Data System (ADS)

Audrey, Bertrand; Sue, Christian

2016-04-01

Brittle deformations allow assessing the late stage of deformation of an orogenic chain. We reappraised the meaning of the late fracturing over the entire Alps in a global geodynamic context. The closure temperature of ZFT corresponds to the brittle-ductile transition in quartz. Therefore, ZFT ages are a proxy for the maximal age of brittle deformation. Combined analyses of ZFT ages with paleostresses data allow the comparison of the brittle deformations over the belt. In the Western Alps, paleostress indicate a major occurrence of orogen-paralell extension and associated strike-slip regimes (Champagnac et al. 2006; Sue et al. 2007 ; Sue and Tricart, 2003). Indeed, paleostress data show a rotation of the main σ3 stress axes along the arc. Those structures are of Miocene age and are related to the propagation of the Alpine front toward the external zone. In the Central Alps, Paleostress fields are dominated by orogen-parrallel extensional regimes both in the Bergell area (Ciancaleoni and Marquer 2008) and the Lepontie dome (Allanic, 2012). In the Eastern Alps, the only area where ZFT ages are of Tertiary ages is the Tauern Window. The brittle deformation is here dominated by orogen-parallel extension at the eastern and western borders of the dome and by strike-slip faulting in the central parts (Bertrand et al., 2015), and inferred to be driven by the combined collapse and lateral escape of the orogenic wedge, due to indentation on the Adriatic indenter (Ratschbacher et al., 1991). Major orogen-parallel extensional signal is closely linked with transcurrent deformation's component. It appears extremely stable all over the Alps and coeval with the propagation of the alpine front top the W-NW. Looking deeper, SKS splitting over the Alps [Qorbani et al., 2015] roughly indicates an orogen-parallel anisotropy pattern in the upper mantle. Indeed, the scheme of the SKS is very comparable with faulting data along-strike of the Alps. In this frame, we can compare both kinds

Petrogenesis and U-Pb zircon chronology of felsic tuffs interbedded with turbidites (Eastern Pontides Orogenic Belt, NE Turkey): Implications for Mesozoic geodynamic evolution of the eastern Mediterranean region and accumulation rates of turbidite sequences

NASA Astrophysics Data System (ADS)

Eyuboglu, Yener

2015-01-01

The Meso-Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt, which is one of the key areas of the Alpine-Himalayan system, is still controversial due to lack of systematic geological, geophysical, geochemical and chronological data. The prevailing interpretation is that this belt represents the southern margin of Eurasia during the Mesozoic and its geodynamic evolution is related to northward subduction of oceanic lithosphere. This paper reports the first detailed geological, geochemical and chronological data from felsic tuffs interbedded with late Cretaceous turbidites in the Southern Zone of the Eastern Pontides Orogenic Belt. Individual tuff layers are thin, mostly < 2 m in thickness, implying that these are dominantly air-fall tuffs. Petrographic data indicate that the felsic tuffs, which exhibit various degrees of alteration, can be classified as crystal-rich and crystal-poor tuffs. The crystal-poor tuffs consist mainly of 45-65% devitrified glass shards and 10-20% broken quartz crystals, whereas the crystal-rich tuffs consist of > 50% crystals. The zircon U-Pb data show three statistically distinct ages at 84, 81 and 77 Ma, with uncertainties of about 1 Ma, suggesting that tuff-forming late Cretaceous magmatism started about 84 Ma ago and was episodically active over a minimum of 7 Ma. The age data also indicate that the average accumulation rate of the turbiditic sequence that hosts the felsic tuffs remained constant between 36 and 40 cm/10 ky. Their enrichment in LIL and LRE elements relative to HFS and HRE elements, and also strongly negative Nb, Ta and Ti anomalies, are consistent with those of magmas generated by subduction-related processes. The tuffs have relatively low initial ratios of 143Nd/144Nd (0.512296-0.512484; εNd: - 2.1 and - 7.2) and 87Sr/86Sr (0.704896-0.706159). Their initial Pb isotopic compositions range from 18.604 to 18.646 for 206Pb/204Pb, from 15.644 to 15.654 for 207Pb/206Pb and from 38.712 to 38.763 for 208Pb/204

High-resolution surface velocity and strain rate mapping across the Alpine-Himalayan belt using InSAR and GNSS data

NASA Astrophysics Data System (ADS)

Weiss, J. R.; Walters, R. J.; Wright, T. J.; Hussain, E.; González, P. J.; Hooper, A. J.

2017-12-01

Accurate and high-resolution measurements of interseismic crustal velocity and the strain-rate fields derived from these measurements are an important input for the assessment of earthquake hazard. However, most strain-rate estimation methods and associated seismicity forecasts rely heavily on Global Navigation Satellite System (GNSS) networks with sparse and heterogeneous spatial coverage, limiting both accuracy and resolution. Interferometric Synthetic Aperture Radar (InSAR) provides remotely-sensed observations of surface motion, with accuracy comparable to GNSS data, and with a spatial resolution of a few tens of meters. The recently launched Sentinel-1 (S1) radar satellites can measure deformation at the tectonic-plate scale and across slowly straining regions where earthquake hazard is poorly characterised. We are producing large-scale crustal velocity and strain-rate fields for the Alpine-Himalayan belt (AHB) by augmenting global GNSS data compilations with InSAR-derived surface velocities. We are also systematically processing S1 interferograms for the AHB and these products are freely available to the geoscience community. We focus on the Anatolian microplate, where we have used both Envisat and S1 data to measure crustal velocity. We address some of the challenges associated with merging the complementary geodetic datasets including reference-frame issues, treatment of uncertainties, and comparison of different velocity/strain-rate inversion methods. We use synthetic displacement fields to illustrate how inclusion of InSAR can aid in identifying features such as unmapped active faults and fault segments that are creeping. From our preliminary results for Anatolia, we investigate the spatial distribution of strain and variation of strain rates during the seismic cycle.

Late Mesozoic deformations of the Verkhoyansk-Kolyma orogenic belt, Northeast Russia

NASA Astrophysics Data System (ADS)

Fridovsky, Valery

2016-04-01

The Verkhoyansk-Kolyma orogenic belt marks the boundary between the Kolyma-Omolon superterrane (microcontinent) and the submerged eastern margin of the North Asian craton. The orogenic system is remark able for its large number of economically viable gold deposits (Natalka, Pavlik, Rodionovskoe, Drazhnoe, Bazovskoe, Badran, Malo-Tarynskoe, etc.). The Verkhoyansk - Kolyma orogenic belt is subdivided into Kular-Nera and the Polousny-Debin terranes. The Kular-Nera terrane is mainly composed of the Upper Permian, Triassic, and Lower Jurassic black shales that are metamorphosed at lower greenschist facies conditions. The Charky-Indigirka and the Chai-Yureya faults separate the Kular-Nera from the Polousny-Debin terrane that is predominantly composed of the Jurassic flyschoi dturbidites. The deformation structure of the region evolved in association with several late Mesozoic tectonic events that took place in the north-eastern part ofthe Paleo-Pacific. In Late Jurassic-Early Cretaceous several generations of fold and thrust systems were formed due to frontal accretion of the Kolyma-Omolon superterrane to the eastern margin of the North Asian craton.Thrusting and folding was accompanied by granitic magmatism, metamorphic reworking of the Late Paleozoic and the Early Mesozoic sedimentary rocks, and formation of Au-Sn-W mineralization. Three stages of deformation related to frontal accretion can be distinguished. First stage D1 has developed in the north-eastern part of the Verkhoyansk - Kolyma orogenic belt. Early tight and isoclinal folds F1 and assosiated thrusts are characteristic of D1. Major thrusts, linear concentric folds F2 and cleavage were formed during D2. The main ore-controlling structures are thrust faults forming imbricate fan systems. Frontal and oblique ramps and systems of bedding and cross thrusts forming duplexes are common. It is notable that mineralized tectonized zones commonly develop along thrusts at the contacts of rocks of contrasting competence

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.

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.

Geometry and kinematics of the fold-thrust belt and structural evolution of the major Himalayan fault zones in the Darjeeling -- Sikkim Himalaya, India

NASA Astrophysics Data System (ADS)

Bhattacharyya, Kathakali

a total minimum shortening of ˜502 km (˜82%) south of the South Tibetan Detachment system (STDS). Based on this shortening, the average long-term shortening rate is estimated to be ˜22mm/yr in this region. The available shortening estimates from different parts of the Himalayan arc show significant variations in shortening, but based on the present available data, it is difficult to evaluate the primary cause for this variation. The shortening in the Himalayan fold-thrust belt (FTB) is highest in the middle of the Himalayan arc (western Nepal) and progressively decreases towards the two syntaxes. Although the width of the Lesser Himalayan belt decreases in the eastern Himalaya, the Lesser Himalayan shortening percentage remains approximately similar to that in the Nepal Himalaya. In addition, the shortening accommodated within the Lesser Himalayan duplex progressively increases from the western to the eastern Himalaya where it accommodates nearly half of the total shortening. The regional restorations suggest that the width of the original Lesser Himalayan basin may have played an important role in partitioning the shortening in the Himalayan FTB. In addition, the retrodeformed cross section in the Darjeeling-Sikkim Himalaya provides insights into the palinspastic reconstruction of the Gondwana basin of Peninsular India, suggesting that this basin extended ˜150 km northward of its present northernmost exposure in this region. The balanced cross section suggests that each of the MCT sheets has undergone translation of ≥100km in this region. Although a regional scale flat-on-flat relationship is seen in the MCT sheets, there is a significant variation in overburden from the trailing portion to the leading edge of the MCT due to the geometry of the tapered crystalline orogenic wedge. Microstructural studies from three segments of the MCT2 fault zone suggest that the MCT2 zone has undergone strain softening by different mechanisms along different portions of its

Growth of the Zagros Fold-Thrust Belt and Foreland Basin, Northern Iraq, Kurdistan

NASA Astrophysics Data System (ADS)

Koshnaw, Renas; Horton, Brian; Stockli, Daniel; Barber, Douglas; Ghalib, Hafidh; Dara, Rebwar

2016-04-01

The Zagros orogenic belt in the Middle Eastern segment of the Alpine-Himalayan system is among the youngest seismically active continental collision zones on Earth. However, due to diachronous and incremental collision, the precise ages and kinematics of shortening and deposition remain poorly understood. The Kurdistan region of the Zagros fold-thrust belt and foreland basin contains well-preserved Neogene wedge-top and foredeep deposits that include clastic nonmarine fill of the Upper Fars, Lower Bakhtiari, and Upper Bakhtiari Formations. These deposits record significant information about orogenic growth, fold-thrust dynamics, and advance of the deformation front. Thermochronologic and geochronologic data from thrust sheets and stratigraphic archives combined with local earthquake data provide a unique opportunity to address the linkages between surface and subsurface geologic relationships. This research seeks to constrain the timing and geometry of exhumation and deformation by addressing two key questions: (1) Did the northwestern Zagros fold-thrust belt evolve from initial thin-skinned shortening to later thick-skinned deformation or vice-versa? (2) Did the fold-thrust belt advance steadily under critical/supercritical wedge conditions involving in-sequence thrusting or propagate intermittently under subcritical conditions with out-of-sequence deformation? From north to south, apatite (U-Th)/He ages from the Main Zagros Thrust, the Mountain Front Flexure (MFF), and additional frontal thrusts suggest rapid exhumation by ~10 Ma, ~5 Ma, and ~8 Ma respectively. Field observations and seismic sections indicate progressive tilting and development of growth strata within the Lower Bakhtiari Formation adjacent to the frontal thrusts and within the Upper Bakhtiari Formation near the MFF. In the Kurdistan region of Iraq, a regional balanced cross section constrained by new thermochronometric results, proprietary seismic reflection profiles, and earthquake hypocenters

Accretionary and collisional orogenesis in the south domain of the western Central Asian Orogenic Belt (CAOB)

NASA Astrophysics Data System (ADS)

Cai, Keda; Long, Xiaoping; Chen, Huayong; Sun, Min; Xiao, Wenjiao

2018-03-01

The Central Asian Orogenic Belt (CAOB) was the result of long-lived multi-stage tectonic evolution, including Proterozoic to Paleozoic accretion and collision, Mesozoic intracontinental modification, and Cenozoic rapid deformation and uplift. The accretionary and collisional orogenesis of its early history generated a huge orogenic collage consisting of diverse tectonic units including island arcs, ophiolites, accretionary prisms, seamounts, oceanic plateaus and micro-continents. These incorporated orogenic components preserved valuable detailed information on orogenic process and continental crust growth, which make the CAOB a key region to understanding of continental evolution, mantle-crust interaction and associated mineralization. The western CAOB refers to the west region in North Xinjiang of China and circum-Balkash of Kazakhstan, with occurrences of the spectacular Kazakhstan orocline and its surrounding mountain belts. Because orogenic fabrics of this part mostly preserve their original features caused by the interactions among the southern Siberian active margin in the north and the Tarim Craton in the south, the western CAOB can be regarded as an ideal region to study the processes of the accretionary and collisional orogenesis and associated mineralization. Since a large number of researchers have been working on this region, research advances bloom strikingly in a short-time period. Therefore, we, in this special issue, focus on these new study advances on the south domain of the western CAOB, including the Kazakhstan collage system, Tianshan orogenic belt and Beishan region, and it is anticipated that this issue can draw more attention from the international research groups to be interested in the studies on orogenesis of the CAOB.

The pre-Devonian tectonic framework of Xing'an-Mongolia orogenic belt (XMOB) in north China

NASA Astrophysics Data System (ADS)

Xu, Bei; Zhao, Pan; Wang, Yanyang; Liao, Wen; Luo, Zhiwen; Bao, Qingzhong; Zhou, Yongheng

2015-01-01

A new tectonic division of the Xing'an-Mongolia orogenic belt (XMOB) in north China has been presented according to our research and a lot of new data of tectonics, geochronology and geochemistry. Four blocks and four sutures have been recognized in the XMOB, including the Erguna (EB), Xing'an-Airgin Sum (XAB), Songliao-Hunshandake (SHB), and Jiamusi (JB), and Xinlin-Xiguitu (XXS), Xilinhot-Heihe (XHS), Mudanjiang (MS) and Ondor Sum-Yongji sutures (OYS). The framework of the XMOB is characterized by a tectonic collage of the blocks and orogenic belts between them. Different Precambrian basements have been found in the blocks, including the Neoproterozoic metamorphic rocks and plutons in the EB, the Neoproterozoic metamorphic rocks in western and eastern of segments of the XAB, Mesoproterozoic and Neoproterozoic metamorphic rocks in middle segments of the XAB, respectively, the Neoproterozoic metamorphic rocks and Mesoproterozoic volcanic rocks and plutons in the SHB, and Neoproterozoic metamorphic rocks in the JB. The XXS resulted from a northwestward subduction of the XAB beneath the EB during the Cambrian, which was followed by the forming of the XHS and OYS in the northwest and south margins of the SHB in the Silurian, respectively. The MS was caused by a westward subduction of the JB beneath the east margin of the SHB during the middle Devonian. The three Cambrian, Silurian and middle Devonian events indicate that the XMOB belongs to a pre-middle Devonian multiple orogenic belt in the Central Asian Orogenic Belt (CAOB). Forming of the XMOB suggests that the southeast part of the Paleo Asian Ocean closed before the middle Devonian.

Oligo-Miocene Alpine Sediment Routing from Integrated Analysis of Seismic-Reflection Data and Detrital Zircon U-Pb Geochronology

NASA Astrophysics Data System (ADS)

Hubbard, S. M.; Sharman, G.; Covault, J. A.

2014-12-01

We integrate detrital zircon geochronology and 3D seismic-reflection data to reconstruct Oligo-Miocene paleogeography and sediment routing from the Alpine hinterland to Austrian Molasse foreland basin. Three-dimensional seismic-reflection data image a network of deepwater tributaries and a long-lived (>8 Ma) foredeep-axial channel belt through which predominantly southerly and westerly turbidity currents are interpreted to have transported Alpine detritus >100 km. We analyzed 793 detrital zircon grains from ten sandstone samples collected from the seismically mapped network of channel fill. Grain age populations correspond with major Alpine orogenic cycles: the Cadomian (750-530 Ma), the Caledonian (500-400 Ma), and the Variscan orogenies (350-250 Ma). Additional age populations correspond with Eocene-Oligocene Periadriatic magmatism (40-30 Ma) and pre-Alpine, Precambrian sources >750 Ma. The abundances of these age populations vary between samples. Sediment that entered the foredeep-axial channel belt from the west (freshwater Molasse) and southwest (Inntal fault zone) is characterized by statistically indistinguishable, well-distributed detrital zircon ages. Sandstone from a shallow marine unit that was deposited proximal to the northern basin margin consists of >75% Variscan (350-300 Ma) zircon, which is believed to have originated from the Bohemian Massif to the north. Mixing calculations based on the Kolmogorov-Smirnoff statistic suggest that the Alpine fold-thrust belt was an important source of detritus to the deepwater Molasse basin. We document east-to-west provenance dilution within the axial channel belt via one or more southern tributaries. Our results have important implications for sediment dispersal patterns within continental-scale orogens, including the relative role of longitudinal versus transverse sediment delivery in peripheral foreland basins.

The relationship between continental collision process and metamorphic pattern in the Himalayan collision belts

NASA Astrophysics Data System (ADS)

Oh, Chang-Whan

2015-04-01

Both UHP and HP eclogites are reported from the Kaghan Valley and Tso Morari Massif in the western part of the Himalayan collision belt (Ghazanfar and Chaudhry, 1987; Thakur, 1983). UHP eclogites in the Kaghan record peak metamorphic conditions of 770 °C and 30 kbar (O'Brien et al., 2001) and was retrograded into the epidote-amphibolite or blueschist (580-610 °C, 10-13 kbar; Lombardo and Rolfo, 2000). Sensitive high-resolution ion microprobe dating of zircon reveals that the UHP eclogite formed at ca. 46 Ma (Kaneko et al., 2003; Parrish et al., 2006). The Tso Morari UHP eclogite had formed at 750 °C, > 39 kbar (Mukheerjee et al., 2003; Bundy, 1980) and underwent amphibolite facies retro-grade metamorphism (580 °C, 11 kbar) during uplift (Guillot et al., 2008). Peak metamorphism of the Tso Morari Massif was dated at ca. 53-55 Ma (Leech et al., 2005). Only HP eclogites have been reported from the mid-eastern part of the Himalayan collision belt (Lombardo and Rolfo, 2000; Corrie et al., 2010). The HP eclogite in the mid-eastern part may have formed at ca. > 780 °C and 20 kbar and was overprinted by high-pressure granulite facies metamorphism (780-750°C, 12-10 kbar) at ca. 30 Ma (Groppo et al. 2007; Corrie et al., 2010). HP granulite (890 °C, 17-18 kbar) is reported from the NBS, at the eastern terminus of the Himalayan collision belt; the granulite was subjected to retrograde metamorphism to produce lower-pressure granulite (875-850°C, 10-5 kbar), representing near-isothermal decompression (Liu and Zhong, 1997). The HP granulite metamorphism may have occurred at ca. 22-25 Ma. Along the Himalayan collision belt, peak metamorphism changes eastward from UHP eclogite facies through HP eclogite facies to high-pressure granulite facies, indicating a progressive eastwards decrease in the depth of subduction of continental crust and an eastwards increase in the geothermal gradient. The peak metamorphic ages also decrease from 53-46 Ma in the west to 22-25 Ma in the

Was Himalayan normal faulting triggered by initiation of the Ramgarh-Munsiari Thrust?

USGS Publications Warehouse

Robinson, Delores M.; Pearson, Ofori N.

2013-01-01

The Ramgarh–Munsiari thrust is a major orogen-scale fault that extends for more than 1,500 km along strike in the Himalayan fold-thrust belt. The fault can be traced along the Himalayan arc from Himachal Pradesh, India, in the west to eastern Bhutan. The fault is located within the Lesser Himalayan tectonostratigraphic zone, and it translated Paleoproterozoic Lesser Himalayan rocks more than 100 km toward the foreland. The Ramgarh–Munsiari thrust is always located in the proximal footwall of the Main Central thrust. Northern exposures (toward the hinterland) of the thrust sheet occur in the footwall of the Main Central thrust at the base of the high Himalaya, and southern exposures (toward the foreland) occur between the Main Boundary thrust and Greater Himalayan klippen. Although the metamorphic grade of rocks within the Ramgarh–Munsiari thrust sheet is not significantly different from that of Greater Himalayan rock in the hanging wall of the overlying Main Central thrust sheet, the tectonostratigraphic origin of the two different thrust sheets is markedly different. The Ramgarh–Munsiari thrust became active in early Miocene time and acted as the roof thrust for a duplex system within Lesser Himalayan rocks. The process of slip transfer from the Main Central thrust to the Ramgarh–Munsiari thrust in early Miocene time and subsequent development of the Lesser Himalayan duplex may have played a role in triggering normal faulting along the South Tibetan Detachment system.

Stacking and metamorphism of continuous segments of subducted lithosphere in a high-pressure wedge: The example of Alpine Corsica (France)

NASA Astrophysics Data System (ADS)

Vitale Brovarone, Alberto; Beyssac, Olivier; Malavieille, Jacques; Molli, Giancarlo; Beltrando, Marco; Compagnoni, Roberto

2013-01-01

Alpine Corsica consists of a stack of variably metamorphosed units of continental and Tethys-derived rocks. It represents an excellent example of high-pressure (HP) orogenic belt, such as the Western Alps, exposed over a small and accessible area. Compared to the Western Alps, the geology of Alpine Corsica is poorly unraveled. During the 1970s-80s, based on either lithostratigraphic or metamorphic field observations, various classifications of the belt have been proposed, but these classifications have been rarely matched together. Furthermore, through time, the internal complexity of large domains has been progressively left aside in the frame of large-scale geodynamic reconstructions. As a consequence, major open questions on the internal structure of the belt have remained unsolved. Apart from a few local studies, Alpine Corsica has not benefited of modern developments in petrology and basin research. This feature results in several uncertainties when combining lithostratigraphic and metamorphic patterns and, consequently, in the definition of an exhaustive architecture of the belt. In this paper we provide a review on the geology of Alpine Corsica, paying particular attention to the available lithostratigraphic and metamorphic classifications of the metamorphic terranes. These data are completed by a new and exhaustive metamorphic dataset obtained by means of thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM). This technique provides reliable insights on the peak temperature of the metamorphic history for CM-bearing metasediments. A detailed metamorphic characterization of metasediments, which have been previously largely ignored due to retrogression or to the lack of diagnostic mineralogy, is thus obtained and fruitfully coupled with the available lithostratigraphic data. Nine main tectono-metamorphic units are defined, from subgreenschist (ca. 280-300 °C) to the lawsonite-eclogite-facies (ca. 500-550 °C) condition. These units are

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

A Paleozoic Japan-type subduction-accretion system in the Beishan orogenic collage, southern Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Song, Dongfang; Xiao, Wenjiao; Windley, Brian F.; Han, Chunming; Tian, Zhonghua

2015-05-01

Magmatic arcs ascribed to oceanic lithosphere subduction played a dominant role in the construction of the accretionary Central Asian Orogenic Belt (CAOB). The Beishan orogenic collage, situated between the Tianshan Orogen to the west and the Inner Mongolia Orogen to the east, is a key area to understanding the subduction and accretionary processes of the southern CAOB. However, the nature of magmatic arcs in the Beishan and the correlation among different tectonic units along the southern CAOB are highly ambiguous. In order to investigate the subduction-accretion history of the Beishan and put a better spatial and temporal relationship among the tectonic belts along the southern CAOB, we carried out detailed field-based structural geology and LA-ICP-MS zircon U-Pb geochronological as well as geochemical studies along four cross-sections across crucial litho-tectonic units in the central segment of the Beishan, mainly focusing on the metamorphic assemblages and associated plutons and volcanic rocks. The results show that both the plutonic and volcanic rocks have geochemical characteristics similar to those of subduction-related rocks, which favors a volcanic arc setting. Zircons from all the plutonic rocks yield Phanerozoic ages and the plutons have crystallization ages ranging from 464 ± 2 Ma to 398 ± 3 Ma. Two volcanic-sedimentary rocks yield zircons with a wide age range from Phanerozoic to Precambrian with the youngest age peaks at 441 Ma and 446 Ma, estimated to be the time of formation of the volcanic rocks. These new results, combined with published data on ophiolitic mélanges from the central segment of the Beishan, favor a Japan-type subduction-accretion system in the Cambrian to Carboniferous in this part of the Paleo-Asian Ocean. The Xichangjing-Niujuanzi ophiolite probably represents a major suture zone separating different tectonic units across the Beishan orogenic collage, while the Xiaohuangshan-Jijitaizi ophiolitic mélange may represent a

Relationship Between Earthquake b-Values and Crustal Stresses in a Young Orogenic Belt

NASA Astrophysics Data System (ADS)

Wu, Yih-Min; Chen, Sean Kuanhsiang; Huang, Ting-Chung; Huang, Hsin-Hua; Chao, Wei-An; Koulakov, Ivan

2018-02-01

It has been reported that earthquake b-values decrease linearly with the differential stresses in the continental crust and subduction zones. Here we report a regression-derived relation between earthquake b-values and crustal stresses using the Anderson fault parameter (Aϕ) in a young orogenic belt of Taiwan. This regression relation is well established by using a large and complete earthquake catalog for Taiwan. The data set consists of b-values and Aϕ values derived from relocated earthquakes and focal mechanisms, respectively. Our results show that b-values decrease linearly with the Aϕ values at crustal depths with a high correlation coefficient of -0.9. Thus, b-values could be used as stress indicators for orogenic belts. However, the state of stress is relatively well correlated with the surface geological setting with respect to earthquake b-values in Taiwan. Temporal variations in the b-value could constitute one of the main reasons for the spatial heterogeneity of b-values. We therefore suggest that b-values could be highly sensitive to temporal stress variations.

Geochemical and Sm-Nd isotope-geochemical patterns of metavolcanic rocks, diabase, and metagabbroids on the northeastern flank of the South Mongolian-Khingan orogenic belt

NASA Astrophysics Data System (ADS)

Smirnov, Yu. V.; Sorokin, A. A.

2017-05-01

The first results of geochemical and Sm-Nd isotope-geochemical studies of metavolcanic rocks, metagabbroids, and diabase of the Nora-Sukhotino terrane, the least studied part of the South Mongolian-Khingan orogenic belt in the system of the Central Asian orogenic belt are reported. It is established that the basic rocks composing this terrane include varieties comparable with E-MORB, tholeiitic, and calc-alkaline basalt of island arc, calc-alkaline gabbro-diabase, and gabbroids of island arcs. Most likely, these formations should be correlated with metabasalt and associated Late Ordovician gabbro-amphibolite of the Sukdulkin "block" of the South Mongolian-Khingan orogenic belt, which are similar to tholeiite of intraplate island arcs by their geochemical characteristics.

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.

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.

Seismic evidence for multiple-stage exhumation of high/ultrahigh pressure metamorphic rocks in the eastern Dabie orogenic belt

NASA Astrophysics Data System (ADS)

Luo, Yinhe; Zhao, Kaifeng; Tang, Chi-Chia; Xu, Yixian

2018-05-01

The Dabie-Sulu orogenic belt in China contains one of the largest exposures of high and ultrahigh pressure (HP and UHP) metamorphic rocks in the world. The origin of HP/UHP metamorphic rocks and their exhumation to the surface in this belt have attracted great interest in the geologic community because the study of exhumation history of HP/UHP rocks helps to understand the process of continental-continental collision and the tectonic evolution of post-collision. However, the exhumation mechanism of the HP-UHP rocks to the surface is still contentious. In this study, by deploying 28 broadband seismic stations in the eastern Dabie orogenic belt and combining seismic data from 40 stations of the China National Seismic Network (CNSN), we image the high-resolution crustal isotropic shear velocity and radial anisotropy structure using ambient noise tomography. Our high-resolution 3D models provide new information about the exhumation mechanism of HP/UHP rocks and the origin of two dome structures.

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

Palinspastic reconstruction of the Alpine thrust belt at the Alpine-Carpathian transition - A geological Sudoku

NASA Astrophysics Data System (ADS)

Beidinger, A.; Decker, K.; Zamolyi, A.; Hölzel, M.; Hoprich, M.; Strauss, P.

2009-04-01

The palinspastic reconstruction of the Austroalpine thrust belt is part of the project Karpatian Tectonics, which is funded by OMV Austria. The objective is to reconstruct the evolution of the thrust belt through the Early to Middle Miocene in order to obtain information on the palaeogeographic position of the Northern Calcareous Alps (NCA) in the region of the present Vienna Basin. A particular goal of the study is to constrain the position of reservoir rocks within the Rhenodanubic Flysch units and the NCA with respect to the autochthonous Malmian source rocks overlying the European basement below the Alpine-Carpathian thrust wedge, and to constrain the burial history of these source rocks. Reconstruction uses regional 2D seismic lines crossing from the European foreland into the fold-thrust belt, 3D seismic data covering the external thrust sheets, and lithostratigraphic data from a total of 51 selected wells, which were drilled and provided by OMV Austria. The main criterion, whether a well was suitable for palinspastic reconstruction or not, was its penetration of Alpine thrust sheets down to the Autochthonous Molasse of the foreland. Additional wells, which do not penetrate the entire Alpine thrust complex but include the Allochthonous Molasse or the external Alpine-Carpathian nappes (Waschberg and Roseldorf thrust unit, Rhenodanubic Flysch nappes) in their well path, were also taken into account. The well data in particular comprise stratigraphic information on the youngest overthrust sediments of the different thrust units and the underlying Autochthonous foreland Molasse. These data allow constraining the timing of thrust events in the allochthonous thrust units and overthrusting of the Autochthonous Molasse. In the particular case of overthrust Autochthonous Molasse, additionally to the timing of overthrusting, which can be derived from the youngest overthrust sediments, the palaeogeographic position of the Alpine Carpathian thrust front could directly be

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.

Geometry, kinematics and tectonic models of the Kazakhstan Orocline, Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Li, Pengfei; Sun, Min; Rosenbaum, Gideon; Yuan, Chao; Safonova, Inna; Cai, Keda; Jiang, Yingde; Zhang, Yunying

2018-03-01

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens on Earth and is characterized by the occurrence of tight oroclines (Kazakhstan and Tuva-Mongolian oroclines). The origin of these large-scale orogenic curvatures is not quite understood, but is fundamentally important for understanding crustal growth and tectonic evolution of the CAOB. Here we provide an outline of available geological and paleomagnetic data around the Kazakhstan Orocline, with an aim of clarifying the geometry, kinematics and geodynamic origin of the orocline. The Kazakhstan Orocline is evident in a total magmatic image, and can be traced by the continuation of high magnetic anomalies associated with the Devonian Volcanic Belt and the Late Devonian to Carboniferous Balkhash-Yili arc. Paleomagnetic data show ∼112-126° clockwise rotation of the northern limb relative to the southern limb in the Late Devonian to Early Carboniferous, as well as ∼15-28° clockwise rotation of the northern limb and ∼39-40° anticlockwise rotation of the southern limb relative to the hinge of the orocline during the Late Carboniferous to Permian. We argue that the Kazakhstan Orocline experienced two-stage bending with the early stage of bending (Late Devonian to Early Carboniferous; ∼112-126°) driven by slab rollback, and the later stage (Late Carboniferous to Permian; 54-68°) possibly associated with the amalgamation of the Siberian, Tarim and Baltic cratons. This new tectonic model is compatible with the occurrence of rift basins, the spatial migration of magmatic arc, and the development of large-scale strike-slip fault systems during oroclinal bending.

Hissar-Alai and the Pamirs: Junction and Position in the System of Mobile Belts of Central Asia

NASA Astrophysics Data System (ADS)

Leonov, M. G.; Rybin, A. K.; Batalev, V. Yu.; Matyukov, V. E.; Shchelochkov, G. G.

2018-01-01

The position of the Pamirs and the Hissar-Alai mountainous system in the structure of Central Asia and features of their junction are considered. It is shown that their outer contours and tectonic infrastructure are significantly distinct in the planar pattern: latitudinally linear and arched for the Hissar-Alai and the Pamirs, respectively. These structures logically match those of the Central Asian and Alpine-Himalayan belts, respectively. The Pamir orogen is a relatively autonomous structural element of the crust, which is located discordantly relative to the country lithospheric blocks. Most of the Pamirs (at least, the Northern and Central) probably form a giant allochthon on the ancient basement of the Tarim and Afghan-Tajik blocks. The junction zone of these two "hard" crustal segments is reflected in the transverse Transpamir threshold, which is expressed in the relief, deep structure, and seismicity. The specific geological structure of the junction zone of the Pamirs and Hissar-Alai (systems of the Tarim, Alai, and Afghan-Tajik troughs) is shown. It suggested that this zone is a damper, which significantly neutralizes the dynamic influence of the Pamir and the southernmost elements of the Pamir-Punjab syntax on Hissar-Alai structures.

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

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

Features of the Caucasian segment of the Alpine-Himalayan-Indonesian Convergence Zone

NASA Astrophysics Data System (ADS)

Sharkov, E.

2012-04-01

The Caucasus Mountain System is a part of the Cenozoic Alpine-Himalayan-Indonesian Convergence Zone (AHICZ) which lasted throughout Eurasia from Western Mediterranean to Western Pacific. This belt has been formed after closure of the Mesozoic Tethys and is marked by mountains building processes, appearance of riftogenic structures, numerous late Cenozoic basaltic plateaus, and chain of often within-continental andesite-latite volcanic arcs, which trace suture zones of the continental plates collision. Caucasus Mountains are located in eastern part of the proper Alpine Zone in zone of Arabian-Eurasian syntaxis and appeared as a result of submeridional pressure which generated by oncoming moving of these plates. The Great Caucasus is represent the south border of the Eurasian plate, uplifted along the Main Caucasian Fault (Thrust). The latter is a part of super-regional deep-seated fault ranged from the Kopetdag through Caspian Sea, Caucasus and Crimea; very likely, that its further continuation is Tornquist-Teisseyre Zone. This superfault separates areas of Alpine convergence from Eurasian plate sensu stricto. The Caucasus occurred between Black and Caspian seas with passive margins and oceanic crust, covered by sediments of 10-15 km thick. Depressions of the seas form large "downfall", or caldrons which cut off pre-Pliocene structures of Caucasus and Kopetdag. These seas are, probably, small remnants of the Tethys which gradually shallowing in the Miocene (Zonenshain, Le Pichon, 1986). New essential deepening of the Black Sea and South-Caspian deep began in the Pliocene- Quaternary; it occurred simultaneously with uprising of Crimea and Caucasus, which were not marked in relief before (Grachev, 2000). Large positive isostatic anomaly beneath the Trans-Caucasian Transverse Uplift (TCTU) of the Great Caucasus and Lesser Caucasus, which stretch out to Arabian plate, occurred between "subsides" Black and especially Caspian seas with neutral to negative isostatic

Cenozoic structural evolution, thermal history, and erosion of the Ukrainian Carpathians fold-thrust belt

NASA Astrophysics Data System (ADS)

Nakapelyukh, Mykhaylo; Bubniak, Ihor; Bubniak, Andriy; Jonckheere, Raymond; Ratschbacher, Lothar

2018-01-01

The Carpathians are part of the Alpine-Carpathian-Dinaridic orogen surrounding the Pannonian basin. Their Ukrainian part constitutes an ancient subduction-accretion complex that evolved into a foreland fold-thrust belt with a shortening history that was perpendicular to the orogenic strike. Herein, we constrain the evolution of the Ukrainian part of the Carpathian fold-thrust belt by apatite fission-track dating of sedimentary and volcanic samples and cross-section balancing and restoration. The apatite fission-track ages are uniform in the inner―southwestern part of the fold-thrust belt, implying post-shortening erosion since 12-10 Ma. The ages in the leading and trailing edges record provenance, i.e., sources in the Trans-European suture zone and the Inner Carpathians, respectively, and show that these parts of the fold-thrust were not heated to more than 100 °C. Syn-orogenic strata show sediment recycling: in the interior of the fold-thrust belt―the most thickened and most deeply eroded nappes―the apatite ages were reset, eroded, and redeposited in the syn-orogenic strata closer to the fore- and hinterland; the lag times are only a few million years. Two balanced cross sections, one constructed for this study and based on field and subsurface data, reveal an architecture characterized by nappe stacks separated by high-displacement thrusts; they record 340-390 km shortening. A kinematic forward model highlights the fold-thrust belt evolution from the pre-contractional configuration over the intermediate geometries during folding and thrusting and the post-shortening, erosional-unloading configuration at 12-10 Ma to the present-day geometry. Average shortening rates between 32-20 Ma and 20-12 Ma amounted to 13 and 21 km/Ma, respectively, implying a two-phased deformation of the Ukrainian fold-thrust belt.

Nature and provenance of the Beishan Complex, southernmost Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Zheng, Rongguo; Li, Jinyi; Xiao, Wenjiao; Zhang, Jin

2018-03-01

The ages and origins of metasedimentary rocks, which were previously mapped as Precambrian, are critical in rebuilding the orogenic process and better understanding the Phanerozoic continental growth in the Central Asian Orogenic Belt (CAOB). The Beishan Complex was widely distributed in the southern Beishan Orogenic Collage, southernmost CAOB, and their ages and tectonic affinities are still in controversy. The Beishan Complex was previously proposed as fragments drifted from the Tarim Craton, Neoproterozoic Block or Phanerozoic accretionary complex. In this study, we employ detrital zircon age spectra to constrain ages and provenances of metasedimentary sequences of the Beishan Complex in the Chuanshanxun area. The metasedimentary rocks here are dominated by zircons with Paleoproterozoic-Mesoproterozoic age ( 1160-2070 Ma), and yield two peak ages at 1454 and 1760 Ma. One sample yielded a middle Permian peak age (269 Ma), which suggests that the metasedimentary sequences were deposited in the late Paleozoic. The granitoid and dioritic dykes, intruding into the metasedimentary sequences, exhibit zircon U-Pb ages of 268 and 261 Ma, respectively, which constrain the minimum deposit age of the metasedimentary sequences. Zircon U-Pb ages of amphibolite (274 and 216 Ma) indicate that they might be affected by multi-stage metamorphic events. The Beishan Complex was not a fragment drifted from the Tarim Block or Dunhuang Block, and none of cratons or blocks surrounding Beishan Orogenic Collage was the sole material source of the Beishan Complex due to obviously different age spectra. Instead, 1.4 Ga marginal accretionary zones of the Columbia supercontinent might have existed in the southern CAOB, and may provide the main source materials for the sedimentary sequences in the Beishan Complex.

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

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

Melting of subducted continental crust: Geochemical evidence from Mesozoic granitoids in the Dabie-Sulu orogenic belt, east-central China

NASA Astrophysics Data System (ADS)

Zhao, Zi-Fu; Liu, Zhi-Bin; Chen, Qi

2017-09-01

Syn-collisional and postcollisional granitoids are common in collisional orogens, and they were primarily produced by partial melting of subducted continental crust. This is exemplified by Mesozoic granitoids from the Dabie-Sulu orogenic belt in east-central China. These granitoids were emplaced in small volumes in the Late Triassic (200-206 Ma) and the Late Jurassic (146-167 Ma) but massively in the Early Cretaceous (111-143 Ma). Nevertheless, all of them exhibit arc-like trace element distribution patterns and are enriched in Sr-Nd-Hf isotope compositions, indicating their origination from the ancient continental crust. They commonly contain relict zircons with Neoproterozoic and Triassic U-Pb ages, respectively, consistent with the protolith and metamorphic ages for ultrahigh-pressure (UHP) metaigneous rocks in the Dabie-Sulu orogenic belt. Some granitoids show low zircon δ18O values, and SIMS in-situ O isotope analysis reveals that the relict zircons with Neoproterozoic and Triassic U-Pb ages also commonly exhibit low δ18O values. Neoproterozoic U-Pb ages and low δ18O values are the two diagnostic features that distinguish the subducted South China Block from the obducted North China Block. Thus, the magma source of these Mesozoic granitoids has a genetic link to the subducted continental crust of the South China Block. On the other hand, these granitoids contain relict zircons with Paleoproterozoic and Archean U-Pb ages, which are present in both the South and North China Blocks. Taken together, the Mesozoic granitoids in the Dabie-Sulu orogenic belt and its hanging wall have their magma sources that are predominated by the continental crust of the South China Block with minor contributions from the continental crust of the North China Block. The Triassic continental collision between the South and North China Blocks brought the continental crust into the thickened orogen, where they underwent the three episodes of partial melting in the Late Triassic, Late

Continental origin of the Gubaoquan eclogite and implications for evolution of the Beishan Orogen, Central Asian Orogenic Belt, NW China

NASA Astrophysics Data System (ADS)

Saktura, Wanchese M.; Buckman, Solomon; Nutman, Allen P.; Belousova, Elena A.; Yan, Zhen; Aitchison, Jonathan C.

2017-12-01

The Gubaoquan eclogite occurs in the Paleozoic Beishan Orogen of NW China. Previously it has been interpreted as a fragment of subducted oceanic crust that was emplaced as a mélange within continental rocks. Contrary to this, we demonstrate that the Gubaoquan eclogite protolith was a Neoproterozoic basic dyke/sill which intruded into Proterozoic continental rocks. The SHRIMP Usbnd Pb zircon dating of the metamorphic rims of the Gubaoquan eclogite yields an age 466 ± 27 Ma. Subdued heavy rare earth element abundances and lack of negative Eu anomalies of the metamorphic zircon domains confirm that this age represents eclogite facies metamorphism. The host augen orthogneiss has a Usbnd Pb zircon age of 920 ± 14 Ma, representing the timing of crystallization of the granitic protolith. A leucogranitic vein which intrudes the eclogite has a Usbnd Pb zircon age of 424 ± 8.6 Ma. This granitic vein marks the end of high-grade metamorphism in this area. The overcomplication of tectonic history of the Beishan Orogen is partially caused by inconsistent classifications and nomenclature of the same rock units and arbitrary subdivisions of Precambrian blocks as individual microcontinents. In an attempt to resolve this, we propose a simpler model that involves the partial subduction of the northern passive margin of the Dunhuang Block beneath the active continental margin developing on the Mazongshan-Hanshan Block to the north. Ocean closure and continental collision during the Late Ordovician resulted in continental thickening and eclogite facies metamorphism recorded by the mafic dykes/sills (now the Gubaoquan eclogite). In the light of the new data, the tectonothermal evolution of the Beishan Orogen is reviewed and integrated with the evolution of the Central Asian Orogenic Belt.

A Tale of Two Orogens: Comparing Crustal Processes in the Proterozoic Trans-Hudson and Grenville Orogens, Eastern Canada

NASA Astrophysics Data System (ADS)

Darbyshire, F. A.; Bastow, I. D.; Gilligan, A.; Petrescu, L.

2016-12-01

The Precambrian core of North America is an assemblage of Archean cratons and Proterozoic orogenic belts, preserving over 3 billion years of Earth history. Here we focus on two of the largest collisional orogens, using recent and ongoing seismological studies to probe their present-day structure and tectonic history. The 1.8 Ga collision between the Western Churchill and Superior cratons, along with microcontinental and island arc terranes, formed the Trans-Hudson Orogen (THO), a collisional belt similar in scale and shape to the present-day Himalaya-Karakoram-Tibet Orogen (HKTO). In the Mesoproterozoic, a series of collisions reworked the SE margin of the Superior craton and added new material over a period of several hundred Ma, culminating in the Grenvillian orogeny and the assembly of the supercontinent Rodinia. The Grenville Orogen is thought to have been a large, hot, long-lived plateau which subsequently underwent orogenic collapse. While similar in spatial scale, the Trans-Hudson and Grenville Orogens have significantly different tectonic histories, notably in terms of longevity and tectonic evolution. Comparison of these collisional belts with each other, and with the HTKO, provide valuable insights into plate-tectonic history. Recently a number of broadband seismograph installations have allowed a detailed study of present-day crustal structure beneath the THO and the Grenville. Receiver-function and surface wave studies provide information on crustal thickness variations, bulk crustal composition and crustal heterogeneity. The crust beneath the orogens is generally thicker, more mafic and more heterogeneous than that beneath neighbouring Archean and Phanerozoic domains, with significant along-strike variability and Moho complexity. We review and interpret the new crustal structure information in the context of the tectonic processes affecting the two contrasting orogens.

Fault-related fold styles and progressions in fold-thrust belts: Insights from sandbox modeling

NASA Astrophysics Data System (ADS)

Yan, Dan-Ping; Xu, Yan-Bo; Dong, Zhou-Bin; Qiu, Liang; Zhang, Sen; Wells, Michael

2016-03-01

Fault-related folds of variable structural styles and assemblages commonly coexist in orogenic belts with competent-incompetent interlayered sequences. Despite their commonality, the kinematic evolution of these structural styles and assemblages are often loosely constrained because multiple solutions exist in their structural progression during tectonic restoration. We use a sandbox modeling instrument with a particle image velocimetry monitor to test four designed sandbox models with multilayer competent-incompetent materials. Test results reveal that decollement folds initiate along selected incompetent layers with decreasing velocity difference and constant vorticity difference between the hanging wall and footwall of the initial fault tips. The decollement folds are progressively converted to fault-propagation folds and fault-bend folds through development of fault ramps breaking across competent layers and are followed by propagation into fault flats within an upper incompetent layer. Thick-skinned thrust is produced by initiating a decollement fault within the metamorphic basement. Progressive thrusting and uplifting of the thick-skinned thrust trigger initiation of the uppermost incompetent decollement with formation of a decollement fold and subsequent converting to fault-propagation and fault-bend folds, which combine together to form imbricate thrust. Breakouts at the base of the early formed fault ramps along the lowest incompetent layers, which may correspond to basement-cover contacts, domes the upmost decollement and imbricate thrusts to form passive roof duplexes and constitute the thin-skinned thrust belt. Structural styles and assemblages in each of tectonic stages are similar to that in the representative orogenic belts in the South China, Southern Appalachians, and Alpine orogenic belts.

Thermobaric structure of the Himalayan Metamorphic Belt in Kaghan Valley, Pakistan

NASA Astrophysics Data System (ADS)

Rehman, Hafiz Ur; Yamamoto, Hiroshi; Kaneko, Yoshiyuki; Kausar, Allah Bakhsh; Murata, Mamoru; Ozawa, Hiroaki

2007-02-01

The thermobaric structure of the Himalayan Metamorphic Belt (HMB) has been constructed along the Kaghan Valley transect, Pakistan. The HMB in this valley represents mainly the Lesser Himalayan Sequence (LHS) and Higher Himalayan Crystallines (HHC). Mineral parageneses of 474 samples, from an approximately, 80-km traverse from southwest to northeast, were examined. Microprobe analyses were carried out to quantify the mineral composition. To determine the pressure-temperature (P-T) conditions, 65 thin sections (7 pelites from LHS and 25 pelites, 9 mafic rocks/amphibolites and 19 eclogites from HHC) were selected. Based on field observations and mineral paragenesis, low-grade to high-grade metapelites, show Barrovian-type progressive metamorphic sequence, with chlorite, biotite, garnet and staurolite zones in LHS and staurolite, kyanite and sillimanite zones in HHC. By using well-calibrated geothermobarometers, P-T conditions for pelitic and mafic rocks are estimated. P-T estimates for pelitic rocks from the garnet zone indicate a condition of 534 ± 17 °C at 7.6 ± 1.2 kbar. P-T estimates for rocks from the staurolite and kyanite zones indicate average conditions of 526 ± 17 °C at 9.4 ± 1.2 kbar and 657 ± 54 °C at 10 ± 1.6 kbar, respectively. P-T conditions for mafic rocks (amphibolites) and eclogites from HHC are estimated as 645 ± 54 °C at 10.3 ± 2 kbar and 746 ± 59 °C at 15.5 ± 2.1 kbar, respectively. The coesite-bearing ultrahigh-pressure (UHP) eclogites record a peak P-T condition of 757-786 °C at 28.6 ± 0.4 kbar and retrograde P-T conditions of 825 ± 59 °C at 18.1 ± 1.7 kbar. These results suggest that HMB show a gradual increase in metamorphic grade from southwest to northeast. The P-T conditions from Pelitic and adjacent mafic rocks having identical peak conditions in the same metamorphic zone, while the structural middle in HHC reached the highest P-T condition upto the UHP grade.

Crustal structure of the Dabie orogenic belt (eastern China) inferred from gravity and magnetic data

NASA Astrophysics Data System (ADS)

Yang, Yu-shan; Li, Yuan-yuan

2018-01-01

In order to better characterize the crustal structure of the Dabie orogen and its tectonic history, we present a crustal structure along a 500 km long profile across the Dabie orogenic belt using various data processing and interpretation of the gravity and magnetic data. Source depth estimations from the spectral analysis by continuous wavelet transform (CWT) provide better constraints for constructing the initial density model. The calculated gravity effects from the initial model show great discrepancy with the observed data, especially at the center of the profile. More practical factors are then incorporated into the gravity modeling. First, we add a high density body right beneath the high pressure metamorphic (HPM) and ultrahigh pressure metamorphic (UHPM) belt considering the exposed HPM and UHPM rocks in the mid of our profile. Then, the anomalous bodies A, B, and C inferred from the CWT-based spectral analysis results are fixed in the model geometry. In the final crustal density structure, two anomalous bodies B and C with high density and low magnetization could possibly be attributed to metasomatised mantle materials by SiO2-rich melt derived from the foundering subducted mafic lower crust. Under the extensional environment in the early Cretaceous, the upwelling metasomatised mantle was partially melted to produce the parental magma of the post-collisional mafic-ultramafic intrusive rocks. As for the low density body A with strong magnetization located in the lower crust right beneath the HP and UHP metamorphic belt, it is more likely to be composed of serpentinized mantle peridotite (SMP). This serpentinized mantle peridotite body (SMPB) represents the emplacement of mantle-derived peridotites in the crust, accompanying the exhumation of the UHP metamorphic rocks.

What can hafnium isotope ratios arrays tell us about orogenic processes? An insight into geodynamic processes operating in the Alpine/Mediterranean region

NASA Astrophysics Data System (ADS)

Henderson, B.; Murphy, J.; Collins, W. J.; Hand, M. P.

2013-12-01

Over the last decade, technological advances in laser-ablation sampling techniques have resulted in an increase in the number of combined U-Pb-Hf zircon isotope studies used to investigate crustal evolution on a local, regional and global scale. Hafnium isotope arrays over large time scales (>500 myr) have been interpreted to track evolving plate tectonic configurations, and the geological outputs associated with changing plate boundaries. We use the Alpine-Mediterranean region as an example of how hafnium isotope arrays record the geodynamic processes associated with the complex geological evolution of a region. The geology of Alpine-Mediterranean region preserves a complex, semi-continuous tectonic history that extends from the Neoproterozoic to the present day. Major components of the Variscan and Alpine orogens are microcontinental ribbons derived from the northern Gondwanan margin, which were transferred to the Eurasian plate during the opening and closing of the Rheic and Paleo-Tethys Oceans. Convergence of the Eurasian and African plates commenced in the Mid-Late Cretaceous, following the destruction of the Alpine-Tethys Ocean during the terminal breakup of Pangea. In general, convergence occurred slowly and is characterised by northward accretion of Gondwanan fragments, interspersed with subduction of African lithosphere and intermittent roll-back events. A consequence of this geodynamic scenario was periods of granite-dominated magmatism in an arc-backarc setting. New Hf isotope data from the peri-Gondwanan terranes (Iberia, Meguma and Avalonia) and a compilation of existing Phanerozoic data from the Alpine-Mediterranean region, indicate ~500 myr (Cambrian-Recent) of reworking of peri-Gondwanan crust. The eHf array follows a typical crustal evolution pattern (Lu/Hf=0.015) and is considered to reflect reworking of juvenile peri-Gondwanan (Neoproterozoic) crust variably mixed with an older (~1.8-2.0 Ga) source component, probably Eburnian crust from the West

Discovery of Latest Cretaceous OIB-type alkaline gabbros in the Eastern Pontides Orogenic Belt, NE Turkey: Evidence for tectonic emplacement of seamounts

NASA Astrophysics Data System (ADS)

Eyuboglu, Yener; Dudas, Francis O.; Chatterjee, Nilanjan; Liu, Ze; Yılmaz-Değerli, Sedanur

2018-06-01

The Meso-Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt, a mountain chain extending parallel to the southeastern margin of the Black Sea, has been controversial for the last forty years. Here we present data for a newly discovered alkaline gabbro body and its surrounding basaltic rocks in the northern part of the Eastern Pontides Orogenic Belt. We also provide a comprehensive assessment of the Late Mesozoic-Cenozoic geodynamic evolution of the Eastern Mediterranean region. The gabbroic body is bounded by reverse faults along its northern and southern borders and is surrounded by vesicular, pillow-fragment breccias and pillow basalts. Mineral compositions suggest that crystallization of the gabbros began at about 1170 °C, and the lowest preserved crystallization T is near 1000 °C. Estimated pressure at the beginning of crystallization is 5.7-7.4 kb. The 40Ar/39Ar dating of kaersutite and plagioclase and Usbnd Pb dating of titanite indicated that the Hayrat gabbro crystallized at 67 Ma (Late Maastrichtian). Whole rock major-trace-rare earth element and Sr-Nd-Pb isotope data indicate that the gabbros and basalts have different origins. The gabbros are alkaline and exhibit the geochemical features of OIB, whereas the basalts are tholeiitic and reveal depletions of HFSE that are similar to those of arc rocks. The gabbros are strongly fractionated, and derive from an enriched, lithospheric mantle source, with partial melting occurring in a garnet-stable environment. The basalts are less fractionated, and probably derive from a shallower source in which spinel peridotite was the predominant lithology. Considering all new and old geological, geochemical, geochronological and geophysical data from the Black Sea Basin and the Eastern Pontides-Lesser Caucasus-Alborz Orogenic Belt, we suggest that the alkaline Hayrat gabbro formed in an oceanic intraplate setting, and was accreted to the forearc region of the Eastern Pontides Orogenic Belt during

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.

Paleozoic intrusive rocks from the Dunhuang tectonic belt, NW China: Constraints on the tectonic evolution of the southernmost Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Zhao, Yan; Sun, Yong; Diwu, Chunrong; Zhu, Tao; Ao, Wenhao; Zhang, Hong; Yan, Jianghao

2017-05-01

The Dunhuang tectonic belt (DTB) is of great importance for understanding the tectonic evolution of the southernmost Central Asian Orogenic Belt (CAOB). In this study, the temporal-spatial distribution, petrogenesis and tectonic setting of the Paleozoic representative intrusive rocks from the DTB were systematically investigated to discuss crustal evolution history and tectonic regime of the DTB during Paleozoic. Our results reveal that the Paleozoic magmatism within the DTB can be broadly divided into two distinct episodes of early Paleozoic and late Paleozoic. The early Paleozoic intrusive rocks, represented by a suite metaluminous-slight peraluminous and medium- to high-K calc-alkaline I-type granitoids crystallized at Silurian (ca. 430-410 Ma), are predominantly distributed along the northern part of the DTB. They were probably produced with mineral assemblage of eclogite or garnet + amphibole + rutile in the residue, and were derived from magma mixing source of depleted mantle materials with various proportions of Archean-Mesoproterozoic continental crust. The late Paleozoic intrusive rocks can be further subdivided into two stages of late Devonian stage (ca. 370-360 Ma) and middle Carboniferous stage (ca. 335-315 Ma). The former stage is predominated by metaluminous to slight peraluminous and low-K tholeiite to high-K calc-alkaline I-type granitic rocks distributed in the central part of the DTB. They were also generated with mineral assemblage of amphibolite- to eclogite-facies in the residue, and originated from magma source of depleted mantle materials mixed with different degrees of old continental crust. The later stage is represented by adakite and alkali-rich granite exposed in the southern part of the DTB. The alkali-rich granites studied in this paper were possibly produced with mineral assemblage of granulite-facies in the residue and were generated by partial melting of thickened lower continental crust. Zircon Hf isotopes and field distribution of

Geochemistry, 40Ar/39Ar geochronology, and geodynamic implications of Early Cretaceous basalts from the western Qinling orogenic belt, China

NASA Astrophysics Data System (ADS)

Zhang, Feifei; Wang, Yuejun; Cawood, Peter A.; Dong, Yunpeng

2018-01-01

The Qinling-Dabie orogenic belt was formed by the collision of the North and South China Cratons during the Early Mesozoic and subsequently developed into an intracontinental tectonic process during late Mesozoic. Field investigations identified the presence of late Mesozoic basalts in the Duofutun and Hongqiang areas in the western Qinling orogenic belt. The petrogenesis of these basalts provides an important constraint on the late Mesozoic geodynamics of the orogen. The representative basaltic samples yield the 40Ar/39Ar plateau age of about 112 Ma. These samples belong to the alkaline series and have SiO2 ranging from 44.98 wt.% to 48.19 wt.%, Na2O + K2O from 3.44 wt% to 5.44 wt%, and MgO from 7.25 wt.% to 12.19 wt.%. They demonstrate the right-sloping chondrite-normalized REE patterns with negligible Eu anomalies (1.00-1.10) and PM-normalized patterns enriched in light rare earth element, large ion lithophile element and high field strength element, similar to those of OIB rocks. These samples additionally show an OIB-like Sr-Nd isotopic signature with εNd(t) values ranging from +6.13 to +10.15 and initial 87Sr/86Sr ratios from 0.7028 to 0.7039, respectively. These samples are geochemically subdivided into two groups. Group 1 is characterized by low Al2O3 and high TiO2 and P2O5 contents, as well as high La/Yb ratios (>20), being the product of the high-pressure garnet fractionation from the OIB-derived magma. Group 2 shows higher Al2O3 but lower P2O5 contents and La/Yb ratios (<20) than Group 1, originating from asthenospheric mantle with input of delaminated lithospheric component. In combination with available data, it is proposed for the petrogenetic model of the Early Cretaceous thickened lithospheric delamination in response to the asthenospheric upwelling along the western Qinling orogenic belt.

Evaluation of the Orogenic Belt Hypothesis for the Formation of Thaumasia, Mars

NASA Astrophysics Data System (ADS)

Nahm, A. L.; Schultz, R. A.

2008-12-01

The Thaumasia Highlands (TH) and Solis Planum are two of the best-known examples of compressional tectonics on Mars. The TH is a region of high topography located in the southern portion of the Tharsis Province, Mars. Solis Planum is located in eastern Thaumasia. Two hypotheses for the formation of this region have been suggested: sliding on a weak horizon or thrusting analogous to orogenic wedges on Earth. Both hypotheses require a shallowly dipping to sub-horizontal weak horizon below Thaumasia. Wrinkle ridges in Solis Planum are also inferred to sole into a décollement. If Thaumasia formed by thrusting related to sliding on a décollement, then certain conditions must be met as in critical taper wedge mechanics (CTWM) theory. If the angle between the surface slope and the basal décollement is less than predicted by the critical taper equation, the 'subcritical' wedge will deform internally until critical taper is achieved. Once the critical taper has been achieved, internal deformation ceases and the wedge will slide along its base. Formation of orogenic belts on Earth (such as the Central Mountains in Taiwan) can be described using CTWM. This method is applied here to the Thaumasia region on Mars. The surface slope (alpha) was measured in three locations: Syria Planum-Thaumasia margin, Solis Planum, and the TH. Topographic slopes were compared to the results from the critical taper equation. Because the dip of the basal décollement (beta) cannot be measured directly as on Earth, the dip angle was varied at 0 - 10 degrees; these values span the range of likely values based on terrestrial wedges. Pore fluid pressure (lambda) was varied between 0 (dry) and 0.9 (overpressured); these values span the full range of this important unknown parameter. Material properties, such as the coefficients of internal friction and of the basal décollement, were varied using reasonable values. Preliminary results show that for both reasonable (such as lambda = 0, mu b = 0

Molecular Phylogenies indicate a Paleo-Tibetan Origin of Himalayan Lazy Toads (Scutiger).

PubMed

Hofmann, Sylvia; Stöck, Matthias; Zheng, Yuchi; Ficetola, Francesco G; Li, Jia-Tang; Scheidt, Ulrich; Schmidt, Joachim

2017-06-12

The Himalaya presents an outstanding geologically active orogen and biodiversity hotspot. However, our understanding of the historical biogeography of its fauna is far from comprehensive. Many taxa are commonly assumed to have originated from China-Indochina and dispersed westward along the Himalayan chain. Alternatively, the "Tibetan-origin hypothesis" suggests primary diversification of lineages in Paleo-Tibet, and secondary diversification along the slopes of the later uplifted Greater Himalaya. We test these hypotheses in high-mountain megophryid anurans (Scutiger). Extensive sampling from High Asia, and analyses of mitochondrial (2839 bp) and nuclear DNA (2208 bp), using Bayesian and Maximum likelihood phylogenetics, suggest that the Himalayan species form a distinct clade, possibly older than those from the eastern Himalaya-Tibet orogen. While immigration from China-Indochina cannot be excluded, our data may indicate that Himalayan Scutiger originated to the north of the Himalaya by colonization from Paleo-Tibet and then date back to the Oligocene. High intraspecific diversity of Scutiger implies limited migration across mountains and drainages along the Himalaya. While our study strengthens support for a "Tibetan-origin hypothesis", current sampling (10/22 species; 1 revalidated: S. occidentalis) remains insufficient to draw final conclusions on Scutiger but urges comparative phylogeographers to test alternative, geologically supported hypotheses for a true future understanding of Himalayan biogeography.

Shaping mobile belts by small-scale convection.

PubMed

Faccenna, Claudio; Becker, Thorsten W

2010-06-03

Mobile belts are long-lived deformation zones composed of an ensemble of crustal fragments, distributed over hundreds of kilometres inside continental convergent margins. The Mediterranean represents a remarkable example of this tectonic setting: the region hosts a diffuse boundary between the Nubia and Eurasia plates comprised of a mosaic of microplates that move and deform independently from the overall plate convergence. Surface expressions of Mediterranean tectonics include deep, subsiding backarc basins, intraplate plateaux and uplifting orogenic belts. Although the kinematics of the area are now fairly well defined, the dynamical origins of many of these active features are controversial and usually attributed to crustal and lithospheric interactions. However, the effects of mantle convection, well established for continental interiors, should be particularly relevant in a mobile belt, and modelling may constrain important parameters such as slab coherence and lithospheric strength. Here we compute global mantle flow on the basis of recent, high-resolution seismic tomography to investigate the role of buoyancy-driven and plate-motion-induced mantle circulation for the Mediterranean. We show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region. More generally, vigorous small-scale convection in the uppermost mantle may also underpin other complex mobile belts such as the North American Cordillera or the Himalayan-Tibetan collision zone.

Why inputs matter: Selection of climatic variables for species distribution modelling in the Himalayan region

NASA Astrophysics Data System (ADS)

Bobrowski, Maria; Schickhoff, Udo

2017-04-01

Betula utilis is a major constituent of alpine treeline ecotones in the western and central Himalayan region. The objective of this study is to provide first time analysis of the potential distribution of Betula utilis in the subalpine and alpine belts of the Himalayan region using species distribution modelling. Using Generalized Linear Models (GLM) we aim at examining climatic factors controlling the species distribution under current climate conditions. Furthermore we evaluate the prediction ability of climate data derived from different statistical methods. GLMs were created using least correlated bioclimatic variables derived from two different climate models: 1) interpolated climate data (i.e. Worldclim, Hijmans et al., 2005) and 2) quasi-mechanistical statistical downscaling (i.e. Chelsa; Karger et al., 2016). Model accuracy was evaluated by the ability to predict the potential species distribution range. We found that models based on variables of Chelsa climate data had higher predictive power, whereas models using Worldclim climate data consistently overpredicted the potential suitable habitat for Betula utilis. Although climatic variables of Worldclim are widely used in modelling species distribution, our results suggest to treat them with caution when remote regions like the Himalayan mountains are in focus. Unmindful usage of climatic variables for species distribution models potentially cause misleading projections and may lead to wrong implications and recommendations for nature conservation. References: Hijmans, R.J., Cameron, S.E., Parra, J.L., Jones, P.G. & Jarvis, A. (2005) Very high resolution interpolated climate surfaces for global land areas. International Journal of Climatology, 25, 1965-1978. Karger, D.N., Conrad, O., Böhner, J., Kawohl, T., Kreft, H., Soria-Auza, R.W., Zimmermann, N., Linder, H.P. & Kessler, M. (2016) Climatologies at high resolution for the earth land surface areas. arXiv:1607.00217 [physics].

Flexural bending-induced plumelets and their seamounts in accretionary (Japanese-style) and collisional (Tethyan-style) orogenic belts

NASA Astrophysics Data System (ADS)

Hirano, N.; Dilek, Y.

2015-12-01

Seamounts and seamount chains are common in both the upper and lower plates of active subduction zones. Their OIB-type volcanic products are distinctly different from suprasubduction zone (arc, forearc and backarc) generated volcanic rocks in terms of their compositions and mantle sources. Tectonic accretion of such seamounts into the Japanese archipelago in the NW Pacific and into subduction-accretion complexes and active margins of continents/microcontinents within the Tethyan realm during the Cretaceous played a significant role in continental growth. Seamount assemblages comprise alkaline volcanic rocks intercalated with radiolarian and hemipelagic chert, and limestone, and may also include hypabyssal dolerite and gabbro intrusions. In the Tethyan orogenic belts these seamount rocks commonly occur as km-scale blocks in mélange units beneath the late Jurassic - Cretaceous ophiolites nappes, whereas on the Japanese islands they form discrete, narrow tectonic belts within the late Jurassic - Cretaceous accretionary prism complexes. We interpret some of these OIB occurrences in the Japanese and Tethyan mountain belts as asperities in downgoing oceanic plates that formed in <10 million years before their accretion. Their magmas were generated by decompressional melting of upwelling asthenosphere, without any significant mantle plume component, and were brought to the seafloor along deep-seated brittle fractures that developed in the flexed, downgoing lithosphere as it started bending near a trench. The modern occurrences of these "petit-spot volcanoes" are well established in the northwestern Pacific plate, off the coast of Japan. The proposed mechanism of the formation of these small seamounts better explains the lack of hotspot trails associated with their occurrence in the geological record. Magmatic outputs of such flexural bending-induced plumelets should be ubiquitious in the accretionary (Japanese-style) and collisional (Tethyan-style) orogenic belts.

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

Structure and Tectonics of the Saint Elias Orogen

NASA Astrophysics Data System (ADS)

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

2001-12-01

The Saint Elias orogen of western Canada and southern Alaska is a complex mountain belt formed by transform faulting and subduction between the Pacific and North American plates, and collision of the Yakutat terrane. The orogen is segmented into three regions of different structural style caused by lateral variations in transpression and processes of terrane accretion. Deformation is strain and displacement partitioned throughout the orogen; transcurrent motion is focused along discrete strike-slip faults, and shortening is distributed among reverse faults and folds with sub-horizontal axes. Plunging folds accommodate horizontal shortening and extension in the western part of the orogen. Segment boundaries extend across the Yakutat terrane where they coincide with the courses of huge piedmont glaciers that flow from the topographic backbone of the range onto the coastal plain. The eastern segment is marked by strike-slip faulting along the Fairweather transform fault and by a narrow belt of reverse faulting where the transpression ratio is 0.4:1 shortening to dextral shear. The transpression ratio is 1.7:1 in the central part of the orogen where a broad thin-skinned fold and thrust belt deforms the Yakutat terrane south of the Chugach-Saint Elias (CSE) suture. Dextral shearing is accommodated by strike-slip faulting beneath the Seward and Bagley glaciers in the hanging wall of the CSE suture, and partly by reverse faulting along a structural belt that cuts across the Yakutat terrane along the western edge of the Malaspina Glacier and links to the Pamplona fold and thrust belt offshore. Deformation along this segment boundary is probably also driven by vertical axis bending of the Yakutat microplate during collision. Subduction & accretion in the western segment of the orogen causes re-folding of previously formed structures when they are emplaced into the upper plate of the Alaska-Aleutian mega-thrust. Second phase folds plunge at moderate to steep angles and

Elemental and Sr-Nd isotopic geochemistry of Cretaceous to Early Paleogene granites and volcanic rocks in the Sikhote-Alin Orogenic Belt (Russian Far East): implications for the regional tectonic evolution

NASA Astrophysics Data System (ADS)

Zhao, Pan; Jahn, Bor-ming; Xu, Bei

2017-09-01

The Sikhote-Alin Orogenic Belt in Russian Far East is an important Late Mesozoic to Early Cenozoic accretionary orogen related to the subduction of the Paleo-Pacific Plate. This belt was generated by successive accretion of terranes made of accretionary prisms, turbidite basins and island arcs to the continental margin of northeastern Asia (represented by the Bureya-Jiamusi-Khanka Block) from Jurassic to Late Cretaceous. In order to study the tectonic and crustal evolution of this orogenic belt, we carried out zircon U-Pb dating, and whole-rock elemental and Sr-Nd isotopic analyses on granites and volcanic rocks from the Primorye region of southern Sikhote-Alin. Zircon dating revealed three episodes of granitoid emplacement: Permian, Early Cretaceous and Late Cretaceous to Early Paleogene. Felsic volcanic rocks (mainly rhyolite, dacite and ignimbrite) that overlay all tectonostratigraphic terranes were erupted during 80-57 Ma, postdating the accretionary process in the Sikhote-Alin belt. The Cretaceous-Paleogene magmatism represents the most intense tectonothermal event in the Sikhote-Alin belt. Whole-rock major and trace elemental data show arc-like affinity for granitoids and volcanic rocks, indicating that they were likely generated in a supra-subduction setting. Their initial 87Sr/86Sr ratios range from 0.7048 to 0.7114, and εNd(t) values vary from +1.7 to -3.8 (mostly < 0). Thus, the elemental and Sr-Nd isotopic data suggest that the felsic magmas were generated by partial melting of source rocks comprising mantle-derived juvenile component and recycled crustal component. In addition to the occurrence in the Sikhote-Alin orogenic belt, Cretaceous to Early Paleogene magmatic rocks are also widespread in NE China, southern Korean peninsula, Japanese islands and other areas of Russian Far East, particularly along the coastal regions of the Okhotsk and Bering Seas. These rocks constitute an extended magmatic belt along the continental margin of NE Asia. The

Reworked crustal of early Paleozoic WuYi Orogen revealed by receiver function data

NASA Astrophysics Data System (ADS)

Wei, Y.; Duan, Y.; Tian, X.; Zhao, Y.

2017-12-01

Intraplate orogenic belt, which occurs at the rigid and undeformable plate interiors, is a distinct new type of orogen rather than an interplate or plate marginal orogenic belt, whose deformation occurs exclusively at plate margins. Therefore, intraplate orogenic belts are the most obvious exception to the plate-tectonic paradigm, they are uncommon in Earth's history. The early Paleozoic Wuyi orogen in South China is one of the few examples of intraplate orogen, and is a key to understanding the process of intraplate orogenesis and global early Paleozoic geodynamics. In this study, we select teleseismic records from 45 mobile linear seismic stations deployed in Wuyi Mountain and 58 permanent stations setting in Jiangxi and Fujian provinces, from January 2011 to December 2012, and calculate the crustal thickness and average crustal Vp/Vs ratio using the H-κ stacking method. The main results include the following: 1) the crustal average Poission's ratio shows an increase tendency from land to sea, the interior of Wuyi orogen belt with an low ration less than 0.23, and the coastline with high ration which is up to 0.28, which indicate a very heterogeneous crustal structure and composition in Wuyi orogen and coast belt. 2) the crustal thickness ranges 28-34 km and shows a tendency of thinning from inland to coast in the region of SE China margin, which maight mean the eastern Eurasia lithospheric is extension and thinning induced by the subducted paleo-Pacific slab. To conclusion, we assume that Wuyi orogen experienced upper crustal thickening, lower crust and lithosphere delamination during the early Paleozoic orogeny, and lithosphere extension in Mesozoic. This research is founded by the Natural Science Foundation of China (41174052 and 41604048).

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

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

Exhumation at orogenic indentor corners under long-term glacial conditions: Example of the St. Elias orogen, Southern Alaska

NASA Astrophysics Data System (ADS)

Spotila, James A.; Berger, Aaron L.

2010-07-01

Syntaxial bends in convergent plate boundaries, or indentor corners, display some of the most intriguing deformation patterns on Earth and are type localities for "aneurysms" of coupled erosion, thermal weakening, and strain. The St. Elias orogen in Alaska is a small, young convergent system that has been dominated by a glacial climate for much of its history and exhibits two prominent indentor corners that are not well understood. We have added 40 new apatite (U-Th)/He ages to the already extensive dataset for the low-temperature cooling history of this orogen to constrain the pattern of exhumation in these indentor corners. Ages from the western syntaxis show minor variation across the structural hinge, suggesting that the bend has little effect on the pattern of exhumation and that structures, including the Bagley fault, connect smoothly from the orogen core to the subduction zone to the southwest. Rock uplift on the north flank of the range appears to increase steadily towards the eastern syntaxis, which represents the apex in the right-angle bend between a transform fault in the south and the collision zone in the west. Based on age-elevation relationships, zones of relative rock uplift can be defined in which the Mt. Logan massif, or the area just north of the eastern syntaxis, experienced ˜ 4.8 km greater rock uplift than background levels northwest of the western syntaxis. A bulge in relative rock uplift is symmetric about the hinge in the eastern indentor corner. However, rates of denudation in this bulge are not as rapid as the core of the fold and thrust belt and are lower than those implied by detrital cooling ages from beneath the Seward Glacier. This implies that a large bull's eye of ultra-rapid (˜ 5 mm/yr) exhumation does not occur and that the subpopulation of young detrital ages may be sourced from a narrow transpressional zone along the Fairweather fault. Unlike the Himalayan syntaxes, it thus appears that an aneurysm of coupled erosion

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

Links Between Earthquake Characteristics and Subducting Plate Heterogeneity in the 2016 Pedernales Ecuador Earthquake Rupture Zone

NASA Astrophysics Data System (ADS)

Bai, L.; Mori, J. J.

2016-12-01

The collision between the Indian and Eurasian plates formed the Himalayas, the largest orogenic belt on the Earth. The entire region accommodates shallow earthquakes, while intermediate-depth earthquakes are concentrated at the eastern and western Himalayan syntaxis. Here we investigate the focal depths, fault plane solutions, and source rupture process for three earthquake sequences, which are located at the western, central and eastern regions of the Himalayan orogenic belt. The Pamir-Hindu Kush region is located at the western Himalayan syntaxis and is characterized by extreme shortening of the upper crust and strong interaction of various layers of the lithosphere. Many shallow earthquakes occur on the Main Pamir Thrust at focal depths shallower than 20 km, while intermediate-deep earthquakes are mostly located below 75 km. Large intermediate-depth earthquakes occur frequently at the western Himalayan syntaxis about every 10 years on average. The 2015 Nepal earthquake is located in the central Himalayas. It is a typical megathrust earthquake that occurred on the shallow portion of the Main Himalayan Thrust (MHT). Many of the aftershocks are located above the MHT and illuminate faulting structures in the hanging wall with dip angles that are steeper than the MHT. These observations provide new constraints on the collision and uplift processes for the Himalaya orogenic belt. The Indo-Burma region is located south of the eastern Himalayan syntaxis, where the strike of the plate boundary suddenly changes from nearly east-west at the Himalayas to nearly north-south at the Burma Arc. The Burma arc subduction zone is a typical oblique plate convergence zone. The eastern boundary is the north-south striking dextral Sagaing fault, which hosts many shallow earthquakes with focal depth less than 25 km. In contrast, intermediate-depth earthquakes along the subduction zone reflect east-west trending reverse faulting.

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

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.

The AlpArray Seismic Network: A Large-Scale European Experiment to Image the Alpine Orogen

NASA Astrophysics Data System (ADS)

Hetényi, György; Molinari, Irene; Clinton, John; Bokelmann, Götz; Bondár, István; Crawford, Wayne C.; Dessa, Jean-Xavier; Doubre, Cécile; Friederich, Wolfgang; Fuchs, Florian; Giardini, Domenico; Gráczer, Zoltán; Handy, Mark R.; Herak, Marijan; Jia, Yan; Kissling, Edi; Kopp, Heidrun; Korn, Michael; Margheriti, Lucia; Meier, Thomas; Mucciarelli, Marco; Paul, Anne; Pesaresi, Damiano; Piromallo, Claudia; Plenefisch, Thomas; Plomerová, Jaroslava; Ritter, Joachim; Rümpker, Georg; Šipka, Vesna; Spallarossa, Daniele; Thomas, Christine; Tilmann, Frederik; Wassermann, Joachim; Weber, Michael; Wéber, Zoltán; Wesztergom, Viktor; Živčić, Mladen

2018-04-01

The AlpArray programme is a multinational, European consortium to advance our understanding of orogenesis and its relationship to mantle dynamics, plate reorganizations, surface processes and seismic hazard in the Alps-Apennines-Carpathians-Dinarides orogenic system. The AlpArray Seismic Network has been deployed with contributions from 36 institutions from 11 countries to map physical properties of the lithosphere and asthenosphere in 3D and thus to obtain new, high-resolution geophysical images of structures from the surface down to the base of the mantle transition zone. With over 600 broadband stations operated for 2 years, this seismic experiment is one of the largest simultaneously operated seismological networks in the academic domain, employing hexagonal coverage with station spacing at less than 52 km. This dense and regularly spaced experiment is made possible by the coordinated coeval deployment of temporary stations from numerous national pools, including ocean-bottom seismometers, which were funded by different national agencies. They combine with permanent networks, which also required the cooperation of many different operators. Together these stations ultimately fill coverage gaps. Following a short overview of previous large-scale seismological experiments in the Alpine region, we here present the goals, construction, deployment, characteristics and data management of the AlpArray Seismic Network, which will provide data that is expected to be unprecedented in quality to image the complex Alpine mountains at depth.

Neogene-Recent Reactivation of Cretaceous-age Faults in Southern Vietnam, with Implications for the Himalayan-Tibetan Orogen

NASA Astrophysics Data System (ADS)

Burberry, C. M.; Elkins, L. J.; Hoang, N.; Anh, L. D.; Dinh, S. Q.

2017-12-01

The tectonic activity and ongoing diffuse volcanic activity of the Central Highlands of Vietnam have, to date, been challenging to explain using accepted plate tectonics principles. The various hypotheses invoked to explain the voluminous magmatism include extrusion related to the Himalayan-Tibetan orogen, extension related to the South China Sea, and plume activity beneath Hainan. We present a combined remote sensing and field study, focused on fault orientation and age relative to lava flows in order to discriminate between these models. Landsat ETM+ and SPOT data were processed to highlight variations in lithology and to remove vegetation, and lineaments were interpreted from these images. The lineament data were compared to existing geologic maps, and to regions of known flow age. Key locations were visited in the field, where fault orientations and relative age were recorded. At many locations, the slip direction could be measured using trend and plunge of mineral lineations. The remote data reveal a complex pattern of lineaments, with prominent N-S, NE-SW and NW-SE directions. Lineaments are observed to cut lava flows with ages of 2.2+/- 0.1 Ma and younger. In the field, NE-SW oriented faults were identified in Jurassic-Cretaceous sedimentary rocks with two phases of movement; a dip-slip phase and a younger, dominantly strike-slip phase. Strike-slip faults were identified in lava flows of approx. 3.2 Ma, also oriented NE-SW. These results indicate that there has been fault activity since the Pliocene, and that this fault activity includes reactivation of dip-slip faults as strike-slip. This is consistent with the movement vector of the southern Indochina Block SE with respect to the Sunda block, and with microplate rotation due to asthenospheric extrusion. These results therefore suggest that ongoing Himalayan-Tibetan collision is still being accommodated, in part, by active lithospheric extrusion of the Indo-China block.

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.

Late Triassic granitic rocks of the Central Qiangtang Orogenic Belt, northern Tibet: tracing crustal thickening through post-collisional silicic magmatism

NASA Astrophysics Data System (ADS)

Wu, H.; Chen, J.

2017-12-01

The Central Qiangtang Orogenic Belt (CQOB) was formed through Triassic continental collision between the Southern and Northern Qiangtang terranes. Numerous granitic intrusions occur along the CQOB, forming a Late Triassic granitic belt that stretches 1000 km from west to east. This Central Qiangtang granitic belt was believed to constitute most of the CQOB. Therefore, the CQOB thus provides a typical composite orogen for the study of relationships between granitoid magmatism and orogenic processes. Recently, many studies have been carried out, and the close relationship of the magmatic belt with the evolutionary history of the CQOB is well established. Late Triassic intrusive rocks are widely exposed in the Riwanchaka area of Central Qiangtang, northern Tibet. In this study, new U-Pb zircon ages reveal that Late Triassic magmatism in Riwanchaka took place at ca 225-205 Ma, coeval with exhumation of the metamorphic rocks in Central Qiangtang. Our new and previously published data enable us to correlate the subduction-related volcanic arc rocks in the Riwanchaka area to a post-collisional extension setting related to slab break-off during northward subduction of the Paleo-Tethys Ocean seafloor. Geochemical characteristics suggested that the samples from CQOB can be divided into low-Sr/Y granitoids (LSG) and high-Sr/Y granitoids (HSG). The LSG are normal calc-alkaline I-type granitoids, characterized by varying major and trace element contents indicative of partial melting of ancient mafic lower crust. The HSG are characterized by high Sr/Y ratios and (La/Yb)N (chondrite-normalized) ratios. These signatures indicate that the HSG were derived by partial melting of garnet-bearing thickened lower crust. The crustal structure and evolution of the CQOB are considered on the basis of available data and variations in Sr/Y, La/Yb, and Hf isotopic ratios. Temporal geochemical and Hf isotopic changes, diagnostic of crustal thickening, indicate that the CQOB was greatly

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.

First thermochronological constraints on the Cenozoic extension along the Balkan fold-thrust belt (Central Stara Planina Mountains, Bulgaria)

NASA Astrophysics Data System (ADS)

Kounov, Alexandre; Gerdjikov, Ianko; Vangelov, Dian; Balkanska, Eleonora; Lazarova, Anna; Georgiev, Stoyan; Blunt, Edward; Stockli, Daniel

2017-11-01

The Balkan fold-thrust belt, exposed in Bulgaria and north-east Serbia, is part of the north-east vergent segment of the bi-vergent Eastern Mediterranean Alpine orogen. It was formed during two distinct compressional stages; the first one lasted from the Middle Jurassic to the Early Cretaceous and the second from Late Cretaceous to the Paleogene. Although the compressional tectonic evolution of the Balkan fold-thrust belt since the Middle Jurassic and during most of the Mesozoic is relatively well studied, the final exhumation of the rocks of the belt during the Cenozoic has remained poorly understood. Here, we present the first thermochronological constraints, based on fission-track and [U-Th-(Sm)]/He analysis, showing that along the central part of the belt syn- to post-orogenic extension could have started as early as the middle Eocene. Low-temperature thermochronological analysis of samples collected from three areas reveals at least two phases of increased cooling and exhumation during the Cenozoic. The first exhumation phase took place between 44 and 30 Ma and appears to be related to the syn- to post-orogenic collapse coeval with the earliest Cenozoic extensional stage observed across the southern Balkan Peninsula. A period of relative quiescence (between 30 and 25 Ma) is followed by the next cooling stage, between 25 and 20 Ma, which appears to be related to late Oligocene to early Miocene crustal extension across the Balkan Peninsula. Extension accommodated by the late Miocene to Recent age Sub-Balkan Graben System does not appear to have produced exhumation of rocks from beneath 2-4 km depth, as it was not detected by the low-temperature thermochronological methods applied in this study.

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

Petrogenesis and tectonic implications of the Early Paleozoic granites in the western segment of the North Qilian orogenic belt, China

NASA Astrophysics Data System (ADS)

Wang, Nan; Wu, Cai-Lai; Lei, Min; Chen, Hong-Jie

2018-07-01

Early Paleozoic granitic magmatism in the North Qilian orogenic belt records a complete Wilson cycle and provides critical geological clues for unraveling the regional tectonic history. In this study, we report the results of zircon U-Pb ages, Hf isotopic analysis and systematic whole-rock geochemical data for the Late Ordovician Hongliuhe granite and Early Silurian Qingshan monzogranite in the western segment of the North Qilian orogenic belt to constrain their emplacement ages, petrogenesis, and regional evolution history. U-Pb dating reveals that the Hongliuhe granite was emplaced around 453-452 Ma, and the Qingshan monzogranite was emplaced about 440-438 Ma. A geochemical study shows that the two granites belong to the calc-alkaline to high-K calc-alkaline series. The Hongliuhe granite shows adakitic and peraluminous features, while the Qingshan monzogranite belongs to metaluminous to weak peraluminous granites. Zircons in the Hongliuhe granite show εHf(t) values ranging from -15.1 to +11.7 with two-stage Hf model ages (tDM2) of 687-2398 Ma, whereas zircons in the Qingshan monzogranite show εHf(t) values ranging from +5.7 to +11.0 with two-stage Hf model ages from 814 to 1057 Ma. The geochemical characteristics indicate that the Hongliuhe granite was a transitional I/S-type granite and was generated from a thickened lower crust with the addition of minor Paleo- to Mesoproterozoic crustal materials, which left a rutile + garnet + pyroxene ± plagioclase residue. The Qingshan monzogranite formed from the partial melting of mafic crust with minor mantle-derived materials, and the fractionation of Ti-bearing phases, apatite and pyroxene occurred during the magma's evolution, which left an amphibole and plagioclase residue. We infer that the Hongliuhe granite formed during the northward subduction of the North Qilian Ocean, while the Qingshan monzogranite was generated during the post-collision stage between the Qilian and Alxa blocks. This observation indicates

Seismic profile analysis of the Kangra and Dehradun re-entrant of NW Himalayan Foreland thrust belt, India: A new approach to delineate subsurface geometry

NASA Astrophysics Data System (ADS)

Dey, Joyjit; Perumal, R. Jayangonda; Sarkar, Subham; Bhowmik, Anamitra

2017-08-01

In the NW Sub-Himalayan frontal thrust belt in India, seismic interpretation of subsurface geometry of the Kangra and Dehradun re-entrant mismatch with the previously proposed models. These procedures lack direct quantitative measurement on the seismic profile required for subsurface structural architecture. Here we use a predictive angular function for establishing quantitative geometric relationships between fault and fold shapes with `Distance-displacement method' (D-d method). It is a prognostic straightforward mechanism to probe the possible structural network from a seismic profile. Two seismic profiles Kangra-2 and Kangra-4 of Kangra re-entrant, Himachal Pradesh (India), are investigated for the fault-related folds associated with the Balh and Paror anticlines. For Paror anticline, the final cut-off angle β =35{°} was obtained by transforming the seismic time profile into depth profile to corroborate the interpreted structures. Also, the estimated shortening along the Jawalamukhi Thrust and Jhor Fault, lying between the Himalayan Frontal Thrust (HFT) and the Main Boundary Thrust (MBT) in the frontal fold-thrust belt, were found to be 6.06 and 0.25 km, respectively. Lastly, the geometric method of fold-fault relationship has been exercised to document the existence of a fault-bend fold above the Himalayan Frontal Thrust (HFT). Measurement of shortening along the fault plane is employed as an ancillary tool to prove the multi-bending geometry of the blind thrust of the Dehradun re-entrant.

Porphyry copper assessment of the Central Asian Orogenic Belt and eastern Tethysides: China, Mongolia, Russia, Pakistan, Kazakhstan, Tajikistan, and India: Chapter X in Global mineral resource assessment

USGS Publications Warehouse

Mihalasky, Mark J.; Ludington, Stephen; Hammarstrom, Jane M.; Alexeiev, Dmitriy V.; Frost, Thomas P.; Light, Thomas D.; Robinson, Gilpin R.; Briggs, Deborah A.; Wallis, John C.; Miller, Robert J.; Bookstrom, Arthur A.; Panteleyev, Andre; Chitalin, Andre; Seltmann, Reimar; Guangsheng, Yan; Changyun, Lian; Jingwen, Mao; Jinyi, Li; Keyan, Xiao; Ruizhao, Qiu; Jianbao, Shao; Gangyi, Shai; Yuliang, Du

2015-01-01

The U.S. Geological Survey collaborated with international colleagues to assess undiscovered resources in porphyry copper deposits in the Central Asian Orogenic Belt and eastern Tethysides. These areas host 20 known porphyry copper deposits, including the world class Oyu Tolgoi deposit in Mongolia that was discovered in the late 1990s. The study area covers major parts of the world’s largest orogenic systems. The Central Asian Orogenic Belt is a collage of amalgamated Precambrian through Mesozoic terranes that extends from the Ural Mountains in the west nearly to the Pacific Coast of Asia in the east and records the evolution and final closure of the Paleo-Asian Ocean in Permian time. The eastern Tethysides, the orogenic belt to the south of the Central Asian Orogenic Belt, records the evolution of another ancient ocean system, the Tethys Ocean. The evolution of these orogenic belts involved magmatism associated with a variety of geologic settings appropriate for formation of porphyry copper deposits, including subduction-related island arcs, continental arcs, and collisional and postconvergent settings. The original settings are difficult to trace because the arcs have been complexly deformed and dismembered by younger tectonic events. Twelve mineral resource assessment tracts were delineated to be permissive for the occurrence of porphyry copper deposits based on mapped and inferred subsurface distributions of igneous rocks of specific age ranges and compositions. These include (1) nine Paleozoic tracts in the Central Asian Orogenic Belt, which range in area from about 60,000 to 800,000 square kilometers (km2); (2) a complex area of about 400,000 km2 on the northern margin of the Tethysides, the Qinling-Dabie tract, which spans central China and areas to the west, encompassing Paleozoic through Triassic igneous rocks that formed in diverse settings; and (3) assemblages of late Paleozoic and Mesozoic rocks that define two other tracts in the Tethysides, the 100

The Hulan Group: Its role in the evolution of the Central Asian Orogenic Belt of NE China

NASA Astrophysics Data System (ADS)

Wu, Fu-Yuan; Zhao, Guo-Chun; Sun, De-You; Wilde, Simon A.; Yang, Jin-Hui

2007-05-01

The Hulan Group of central Jilin Province, northeastern (NE) China, consists of felsic volcanic and sedimentary rocks that underwent sub-greenschist to amphibolite facies metamorphism and multiple phases of deformation during formation of the Central Asian Orogenic Belt. Young Nd model ages of ˜1100 Ma obtained in this study do not support earlier models that the Hulan Group was either rifted from the North China Craton to the south or from the Jiamusi Block to the north, since both these blocks have much older Nd model ages. Detrital zircon U-Pb ages from a meta-sedimentary rock of the Hulan Group define a maximum age of deposition of 287 ± 6 Ma, not Early Palaeozoic as previously believed. Rb-Sr mineral isochron data indicate that metamorphism of the Hulan Group occurred at ˜250 Ma. This conclusion is further supported by an age of 248 ± 4 Ma for the Dayushan granite, which intruded the Hulan Group. These new isotopic data, together with evaluation of the regional geology, indicate that final oceanic closure in the Chinese segment of the Central Asian Orogenic Belt took place in the Late Permian. The Hulan area is located along the Solonker-Xra Moron-Changchun suture that extends from Inner Mongolia across to central Jilin Province. It is this zone that marks the final ocean closure, resulting in the amalgamation of the North China and Siberian cratons.

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

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

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

The Evolution of Eastern Himalayan Syntaxis of Tibetan Plateau

NASA Astrophysics Data System (ADS)

Zhang, S.; Wu, T.; Li, M.; Zhang, Y.; Hua, Y.; Zhang, B.

2017-12-01

Indian plate has been colliding with Eurasian plate since 50Ma years ago, resulting in the Tethys extinction, crust shortening and Tibetan plateau uplift. But it is still a debate how the Tibetan Plateau material escaped. This study tries to invert the distributions of dispersion phase velocity and anisotropy in Eastern Himalayan Syntaxis (EHS) based on the seismic data. We focused on the seven sub-blocks around EHS region. Sub-block "EHS" represents EHS corner with high velocity anomalies, significantly compressed in the axle and strike directions. Sub-blocks "LSD", "QTB" and "SP-GZB" are located at its northern areas with compressions also, and connected with low-velocity anomalies in both crustal and upper mantle rocks. Sub-block "ICB" is located at its southern area with low velocity anomaly, and connected with Tengchong volcano. Sub-blocks "SYDB" and "YZB" are located at its eastern areas with high velocity anomalies in both crustal and upper mantle rocks. Our results demonstrated that significant azimuthal anisotropy of crust (t£30s) and upper mantle (30s£t£60s). Crustal anisotropy indicates the orogenic belt matched well with the direction of fast propagation, and upper mantle anisotropy represents the lattic-preferred orientation (LPO) of mantle minerals (e.g. olivine and basalt), indicating the features of subducting Indian plate. Besides, Red River fault is a dextral strike fault, controlling the crustal and mantle migration. There is a narrow zone to be the channel flow of Tibetan crustal materials escaping toward Yunnan area. The evolution of EHS seems constrained by gravity isostatic mechanism. Keywords: Tibetan Plateau; Eastern Himalayan Syntaxis; Red River fault; crustal flow; surface wave; anisotropy

Moho depth model for the Central Asian Orogenic Belt from satellite gravity gradients

NASA Astrophysics Data System (ADS)

Guy, Alexandra; Holzrichter, Nils; Ebbing, Jörg

2017-09-01

The main purpose of this study is to construct a new 3-D model of the Central Asian Orogenic Belt (CAOB) crust, which can be used as a starting point for future lithospheric studies. The CAOB is a Paleozoic accretionary orogen surrounded by the Siberian Craton to the north and the North China and Tarim Cratons to the south. This area is of great interest due to its enigmatic and still not completely understood geodynamic evolution. First, we estimate an initial crustal thickness by inversion of the vertical gravity component of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and DTU10 models. Second, 3-D forward modeling of the GOCE gravity gradients is performed, which determines the topography of the Moho, the geometry, and the density distribution of the deeper parts of the CAOB and its surroundings, taking into account the lateral and vertical density variations of the crust. The model is constrained by seismic refraction, reflection, and receiver function studies and geological studies. In addition, we discuss the isostatic implications of the differences between the seismic Moho and the resulting 3-D gravity Moho, complemented by the analysis of the lithostatic load distribution at the upper mantle level. Finally, the correlation between the contrasting tectonic domains and the thickness of the crust reveals the inheritance of Paleozoic and Mesozoic geodynamics, particularly the magmatic provinces and the orocline which preserve their crustal features.

Himalayan Strain Accumulation 100 ka Timescales

NASA Astrophysics Data System (ADS)

Cannon, J. M.; Murphy, M. A.; Liu, Y.

2015-12-01

Crustal scale fault systems and tectonostratigraphic units in the Himalaya can be traced for 2500 km along strike. However regional studies have shown that there is variability in the location and rate of strain accumulation which appears to be driven by Main Himalayan Thrust (MHT) geometry and convergence obliquity. GPS illuminates the modern interseismic strain rate and the historical record of great earthquakes elucidates variations in strain accumulation over 103 years. To connect these patterns with the 106 year structural and thermochronometric geologic record we examine normalized river channel steepness (ksn), a proxy for rock uplift rate, which develops over 104 - 105 years. Here we present a ksn map of the Himalaya and compare it with bedrock geology, precipitation, the historic earthquake record, GPS, seismicity, and seismotectonic models. Our map shows significant along strike changes in the magnitude of channel steepness, the areal extent of swaths of high ksn channels, and their location with respect to the range front. Differences include the juxtaposition of two narrow (30 - 40 km) range parallel belts of high ksn in west Nepal and Bhutan coincident with MHT duplexes and belts of microseismcity, with a single broad (70 km) swath of high ksn and microseismicity in central and eastern Nepal. Separating west and central Nepal a band of low ksn crosses the range coincident with the West Nepal Fault (WNF) and the lowest rate of microseismicity in Nepal. To the west the orogen is obliquely convergent and has less high ksn channels, while the orthogonally convergent region to the east contains the highest concentration of oversteepened channels in the Himalaya supporting the idea that the WNF is a strain partitioning boundary. The syntaxes are characterized by locally high channel steepness surrounded by low to moderate ksn channels consistent with the hypothesis that rapid exhumation within the syntaxes is sustained by an influx of lower crust.

First-order control of syntectonic sedimentation on crustal-scale structure of mountain belts

NASA Astrophysics Data System (ADS)

Erdős, Zoltán.; Huismans, Ritske S.; van der Beek, Peter

2015-07-01

The first-order characteristics of collisional mountain belts and the potential feedback with surface processes are predicted by critical taper theory. While the feedback between erosion and mountain belt structure has been fairly extensively studied, less attention has been given to the potential role of synorogenic deposition. For thin-skinned fold-and-thrust belts, recent studies indicate a strong control of syntectonic deposition on structure, as sedimentation tends to stabilize the thin-skinned wedge. However, the factors controlling basement deformation below fold-and-thrust belts, as evident, for example, in the Zagros Mountains or in the Swiss Alps, remain largely unknown. Previous work has suggested that such variations in orogenic structure may be explained by the thermotectonic "age" of the deforming lithosphere and hence its rheology. Here we demonstrate that sediment loading of the foreland basin area provides an additional control and may explain the variable basement involvement in orogenic belts. When examining the role of sedimentation, we identify two end-members: (1) sediment-starved orogenic systems with thick-skinned basement deformation in an axial orogenic core and thin-skinned deformation in the bordering forelands and (2) sediment-loaded orogens with thick packages of synorogenic deposits, derived from the axial basement zone, deposited on the surrounding foreland fold-and-thrust belts, and characterized by basement deformation below the foreland. Using high-resolution thermomechanical models, we demonstrate a strong feedback between deposition and crustal-scale thick-skinned deformation. Our results show that the loading effects of syntectonic sediments lead to long crustal-scale thrust sheets beneath the orogenic foreland and explain the contrasting characteristics of sediment-starved and sediment-loaded orogens, showing for the first time how both thin- and thick-skinned crustal deformations are linked to sediment deposition in these

> Exploring the Scandinavian Mountain Belt by Deep Drilling (COSC)

NASA Astrophysics Data System (ADS)

Juhlin, C.; Gee, D. G.; Lorenz, H.; Pascal, C.; Pedersen, K.; Tsang, C.-F.

2012-04-01

The Collisional Orogeny in the Scandinavian Caledonides (COSC) project proposes to drill two fully cored scientific boreholes, both to c. 2.5 km depth, in the Swedish Caledonides, one near the town of Åre (COSC 1) and the other further east (COSC 2). Together they will provide a c. 5 km deep high-resolution mid-crustal section through this major mid-Palaeozoic orogen. Main project objectives include (i) improved understanding of mountain building processes (orogeny), (ii) investigation of the geothermal gradient and its response to palaeoclimatic influences, (iii) the hydrogeological-hydrochemical state of the mountain belt, (iv) the deep biosphere in the metamorphic rocks and crystalline basement, and (v) calibration of surface geophysics and geology. The Caledonide Orogen is comparable in size and many other respects to today's Himalayan mountain belt. Silurian collision with underthrusting of the paleo-continent Baltica below Laurentia resulted in widespread formation of eclogite. Major allochthons were transported many hundreds of kilometers onto the Baltoscandian Platform, including high-grade metamorphic rocks and migmatites which were generated during continental margin subduction and emplaced ductilely at mid-crustal levels. COSC will provide detailed insight into mid-Palaeozoic mountain building processes and further our understanding of past, present and future orogen dynamics. Located in a key-area for Caledonian geology, it is close to a major geophysical transect across the mountain belt which has been complemented recently with high-resolution reflection seismics and aerogeophysics for site-selection. The COSC research program is being developed by five working groups, geology, geophysics, geothermics, hydrogeology and microbiology. It has direct relevance for society by improving our understanding of mountain building processes, hydrological-hydrochemical regimes in mountain areas and Precambrian shields, deep subsurface conditions for underground

Detrital fingerprints of fossil continental-subduction zones (Axial Belt Provenance, European Alps)

NASA Astrophysics Data System (ADS)

Resentini, Alberto; Garzanti, Eduardo; Vezzoli, Giovanni; Andò, Sergio; Malusà, Marco G.; Padoan, Marta; Paparella, Paolo

2010-05-01

Alpine-type collision orogens are generated by attempted subduction of thinned continental margins. Because of complex tectonic structure, orogenic detritus is characterized by a range of detrital signatures, making its recognition an arduous task (Dickinson and Suczek, 1979). Among the various orogenic sub-provenances, Axial Belt Provenance, derived from the erosion of the neometamorphic axial pile, can be regarded as the most typifying signature of collision orogens (Garzanti et al., 2007). In the Austroalpine Cretaceous and Penninic Eocene axial belts of the Alps, we ideally distinguish three structural levels, each characterized by diagnostic detrital fingerprints. The shallow level chiefly consists of offscraped remnant-ocean turbidites and unmetamorphosed continental-margin sediments, and mostly produces lithic to lithoquartzose sedimentaclastic sands yielding very-poor heavy-mineral suites including ultrastable minerals. The intermediate level includes low-grade metasediments and polymetamorphic basements, and sheds lithoquartzose to quartzolithofeldspathic metamorphiclastic sands yielding moderately-rich epidote- amphibole suites with chloritoid or garnet. The deep level contains eclogitic remnants of continent- ocean transitions, and supplies quartzofeldspathic to quartzolithic high-rank metamorphiclastic to lithic ultramaficlastic sands yielding rich to extremely-rich suites dominated by garnet, hornblende, or epidote depending on protoliths (continental vs. oceanic) and pressure/temperature paths followed during exhumation. Although widely overprinted under greenschist-facies or amphibolite-facies conditions, occurrence of ultradense eclogite in source areas is readily revealed by the Heavy Mineral Concentration (HMC) index, which mirrors the average density of source rocks in the absence of hydraulic-sorting effects (Garzanti and Andò 2007). The Metamorphic Index (MI, Garzanti and Vezzoli, 2003) and Hornblende Colour Index (HCI) reflect peak

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

of distinctly different orogen styles. The Arabian-Nubian Shield is an accretion-type orogen comprising a stack of thin-skinned nappes resulting from the oblique convergence of bounding plates. The Eastern Granulite-Cabo Delgado Nappe Complex is interpreted as a hot- to ultra-hot orogen that evolved from a formerly extended crust. Low viscosity lower crust resisted one-sided subduction, instead a sagduction-type orogen developed. The regions of Tanzania and Madagascar affected by the Kuungan Orogeny are considered a Himalayan-type orogen composed of partly doubly thickened crust.

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

distinctly different orogen styles. The Arabian-Nubian Shield is an accretion-type orogen comprising a stack of thin-skinned nappes resulting from the oblique convergence of bounding plates. The Eastern Granulite-Cabo Delgado Nappe Complex is interpreted as a hot- to ultra-hot orogen that evolved from a formerly extended crust. Low viscosity lower crust resisted one-sided subduction, instead a sagduction-type orogen developed. The regions of Tanzania and Madagascar affected by the Kuungan Orogeny are considered a Himalayan-type orogen composed of partly doubly thickened crust.

Gis-Based Accessibility Analysis of Urban Emergency Shelters: the Case of Adana City

NASA Astrophysics Data System (ADS)

Unal, M.; Uslu, C.

2016-10-01

Accessibility analysis of urban emergency shelters can help support urban disaster prevention planning. Pre-disaster emergency evacuation zoning has become a significant topic on disaster prevention and mitigation research. In this study, we assessed the level of serviceability of urban emergency shelters within maximum capacity, usability, sufficiency and a certain walking time limit by employing spatial analysis techniques of GIS-Network Analyst. The methodology included the following aspects: the distribution analysis of emergency evacuation demands, the calculation of shelter space accessibility and the optimization of evacuation destinations. This methodology was applied to Adana, a city in Turkey, which is located within the Alpine-Himalayan orogenic system, the second major earthquake belt after the Pacific-Belt. It was found that the proposed methodology could be useful in aiding to understand the spatial distribution of urban emergency shelters more accurately and establish effective future urban disaster prevention planning. Additionally, this research provided a feasible way for supporting emergency management in terms of shelter construction, pre-disaster evacuation drills and rescue operations.

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

Duplex thrusting in the South Dabashan arcuate belt, central China

NASA Astrophysics Data System (ADS)

Li, Wangpeng; Liu, Shaofeng; Wang, Yi; Qian, Tao; Gao, Tangjun

2017-10-01

Due to later tectonic superpositioning and reworking, the South Dabashan arcuate belt extending NW to SE has experienced several episodes of deformation. The earlier deformational style and formation mechanism of this belt remain controversial. Seismic interpretations and fieldwork show that the curved orogen can be divided into three sub-belts perpendicular to the strike of the orogen, the imbricate thrust fault belt, the detachment fold belt and the frontal belt from NE to SW. The imbricate thrust fault belt is characterized by a series of SW-directed thrust faults and nappes. Two regional detachment layers at different depths have been recognized in the detachment fold and frontal belts, and these detachment layers divide the sub-belts into three structural layers: the lower, middle, and upper structural layers. The middle structural layer is characterized by a passive roof duplex structure, which is composed of a roof thrust at the top of the Sinian units, a floor thrust in the upper Lower Triassic units, and horses in between. Apatite fission track dating results and regional structural analyses indicate that the imbricate thrust fault belt may have formed during the latest Early Cretaceous to earliest Paleogene and that the detachment fold belt may have formed during the latest Late Cretaceous to earliest Neogene. Our findings provide important reference values for researching intra-continental orogenic and deformation mechanisms in foreland fold-thrust belts.

[Maples at the sub-Alpine vegetation belt: a long history].

PubMed

David, F; Barbero, M

2001-02-01

Pollen analysis was carried out on lacustrine sediment of a small hollow (15 m x 25 m) at the treeless sub-Alpine belt (202 m) of the inner Maurienne valley in the northern French Alps. A 2,500-year-long maple settlement was demonstrared. Three AMS dates of terrestrial plant macroremains support the chronology. First, Betula and Salix spread prior to 9,000 C14 BP. The first pollen grains of Acer, Abies and Pinus cembra are quoted at 8,600 C14 BP. High frequencies of Alnus glutinosa/incana (20%) and Acer (10%) show that mixed communities of Acer and Alnus persisted above the mountainous Abies forest between 7,490 and 5,850 C14 BP. After 5,850 C14 BP, the decrease in Acer stands could be attributed to fire as suggested by the strong increase in Betula and by the delayed expansion of Pinus cembra.

Non-cylindrical fold growth in the Zagros fold and thrust belt (Kurdistan, NE-Iraq)

NASA Astrophysics Data System (ADS)

Bartl, Nikolaus; Bretis, Bernhard; Grasemann, Bernhard; Lockhart, Duncan

2010-05-01

The Zagros mountains extends over 1800 km from Kurdistan in N-Iraq to the Strait of Hormuz in Iran and is one of the world most promising regions for the future hydrocarbon exploration. The Zagros Mountains started to form as a result of the collision between the Eurasian and Arabian Plates, whose convergence began in the Late Cretaceous as part of the Alpine-Himalayan orogenic system. Geodetic and seismological data document that both plates are still converging and that the fold and thrust belt of the Zagros is actively growing. Extensive hydrocarbon exploration mainly focuses on the antiforms of this fold and thrust belt and therefore the growth history of the folds is of great importance. This work investigates by means of structural field work and quantitative geomorphological techniques the progressive fold growth of the Permam, Bana Bawi- and Safeen- Anticlines located in the NE of the city of Erbil in the Kurdistan region of Northern Iraq. This part of the Zagros fold and thrust belt belongs to the so-called Simply Folded Belt, which is dominated by gentle to open folding. Faults or fault related folds have only minor importance. The mechanical anisotropy of the formations consisting of a succession of relatively competent (massive dolomite and limestone) and incompetent (claystone and siltstone) sediments essentially controls the deformation pattern with open to gentle parallel folding of the competent layers and flexural flow folding of the incompetent layers. The characteristic wavelength of the fold trains is around 10 km. Due to faster erosion of the softer rock layers in the folded sequence, the more competent lithologies form sharp ridges with steeply sloping sides along the eroded flanks of the anticlines. Using an ASTER digital elevation model in combination with geological field data we quantified 250 drainage basins along the different limbs of the subcylindrical Permam, Bana Bawi- and Safeen- Anticlines. Geomorphological indices of the drainage

Kinematic development of the Tibetan Plateau's northern margin: A traverse across the Qilian Shan-Nan Shan thrust belt

NASA Astrophysics Data System (ADS)

Zuza, A. V.; Levy, D. A.; Wang, Z.; Xiong, X.; Chen, X.

2017-12-01

The active Cenozoic Qilian Shan-Nan Shan thrust belt defines the northern margin of the Tibetan Plateau. The kinematic development of this thrust belt has implications models of plateau growth and Himalayan-Tibetan orogen strain accommodation. We present new field observations and analytical data from a traverse across the 350-km-wide doubly vergent Qilian Shan, which is bound by the south-dipping North Qilian thrust system in the north and the north-dipping range-bounding Qinghai Nanshan-Dulan Shan thrust system in the south. These faults, and several other major thrusts within the thrust-belt interior, disrupt relatively thick Oligocene-Miocene basin deposits. Of note, many of the thrust faults across the width of the Qilian Shan have Quaternary fault scarps, indicating that active deformation is distributed and not only concentrated along the northern frontal faults. By integrating our detailed structural traverse with new geophysical observations and thermochronology data across the northern plateau margin, we construct a kinematic model for the development of the Tibetan Plateau's northern margin. Deformation initiated in the Eocene-Oligocene along the north-dipping Qinghai Nanshan-Dulan Shan and south-dipping Tuolai Nan Shan thrusts, the latter of which then defined the northern boundary of the Tibetan Plateau. This early deformation was focused along preexisting early Paleozoic structures. A 200-km-wide basin formed between these ranges, and from the Miocene to present, new thrust- and strike-slip-fault-bounded ranges developed, including the north-directed North Qilian and the south-directed Tuolai Nan thrusts. Thus, our observations do not support northward propagating thrust-belt expansion. Instead, we envision that the initial thrust-belt development generated a wide Oligocene-Miocene north-plateau basin that was subsequently disintegrated by later Miocene to present thrusting and strike-slip faulting. Ultimately, the Qilian Shan-Nan Shan thrust belt

A model for the origin of Himalayan anatexis and inverted metamorphism

NASA Astrophysics Data System (ADS)

Harrison, T. Mark; Grove, Marty; Lovera, Oscar M.; Catlos, E. J.

1998-11-01

The origin of the paired granite belts and inverted metamorphic sequences of the Himalaya has generally been ascribed to development of the Main Central Thrust (MCT). Although a variety of models have been proposed that link early Miocene anatexis with inverted metamorphism, recent dating studies indicate that recrystallization of elements of the MCT footwall occurred in the central Himalaya as recently as ˜6 Ma. The recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the High Himalayan leucogranites and North Himalayan granites, which differ in age, petrogenesis, and emplacement style. We suggest that their origin is linked to shear heating on a continuously active thrust that cuts through Indian supracrustal rocks that had previously experienced low degrees of partial melting. Numerical simulations assuming a shear stress of 30 MPa indicate that continuous slip on the Himalayan decollement beginning at 25 Ma could trigger partial melting reactions leading to formation of the High Himalayan granite chain between 25 and 20 Ma and the North Himalayan belt between 17 and 8 Ma. The ramp-flat geometry we apply to model the Himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle Miocene. Late Miocene, out-of-sequence thrusting within the broad shear zone beneath the MCT provides a mechanism to bring these rocks to the surface in their present location (i.e., well to the north of the present tectonic front) and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the MCT. We envision that formation of the MCT Zone involved successive accretion of tectonic slivers of the Lesser Himalayan Formations to the hanging wall and incorporate these effects into the model. The model

From a collage of microplates to stable continental crust - an example from Precambrian Europe

NASA Astrophysics Data System (ADS)

Korja, Annakaisa

2013-04-01

Svecofennian orogen (2.0-1.7 Ga) comprises the oldest undispersed orogenic belt on Baltica and Eurasian plate. Svecofennian orogenic belt evolved from a series of short-lived terrane accretions around Baltica's Archean nucleus during the formation of the Precambrian Nuna supercontinent. Geological and geophysical datasets indicate W-SW growth of Baltica with NE-ward dipping subduction zones. The data suggest a long-lived retreating subduction system in the southwestern parts whereas in the northern and central parts the northeasterly transport of continental fragments or microplates towards the continental nucleus is also documented. The geotectonic environment resembles that of the early stages of the Alpine-Himalayan or Indonesian orogenic system, in which dispersed continental fragments, arcs and microplates have been attached to the Eurasian plate margin. Thus the Svecofennian orogeny can be viewed as proxy for the initial stages of an internal orogenic system. Svecofennian orogeny is a Paleoproterozoic analogue of an evolved orogenic system where terrane accretion is followed by lateral spreading or collapse induced by change in the plate architecture. The exposed parts are composed of granitoid intrusions as well as highly deformed supracrustal units. Supracrustal rocks have been metamorphosed in LP-HT conditions in either paleo-lower-upper crust or paleo-upper-middle crust. Large scale seismic reflection profiles (BABEL and FIRE) across Baltica image the crust as a collage of terranes suggesting that the bedrock has been formed and thickened in sequential accretions. The profiles also image three fold layering of the thickened crust (>55 km) to transect old terrane boundaries, suggesting that the over-thickened bedrock structures have been rearranged in post-collisional spreading and/or collapse processes. The middle crust displays typical large scale flow structures: herringbone and anticlinal ramps, rooted onto large scale listric surfaces also suggestive

Paleozoic tectonics in the eastern part of Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Liu, Yongjiang; Li, Weimin; Feng, Zhiqiang; Neubauer, Franz

2017-04-01

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogen in the world, which is responsible for considerable Phanerozoic juvenile crustal growth. The NE China and its adjacent areas compose the eastern segment of the CAOB, which is a key area for providing important evidence of the CAOB evolution and understanding the NE Asian tectonics. The eastern segment of the CAOB is composed tectonically of four micro-blocks and four sutures, i.e. Erguna block (EB), Xing'an block (XB), Songliao-Xilinhot block (SXB), Jiamusi block (JB), Xinlin-Xiguitu suture (XXS), Heihe-Hegenshan suture (HHS), Mudanjiang-Yilan suture (MYS) and Solonker-Xar Moron-Changchun-Yanji suture (SXCYS). The EB and XB were amalgamated by westward subduction, oceanic island accretions and final collision in ca. 500 Ma. The XB and SXB were amalgamated by subduction-related Early Paleozoic marginal arc, Late Paleozoic marginal arc and final collision in the late Early Carboniferous to early Late Carboniferous. The JB probably had been attached to the SXB in the Early Paleozoic, but broken apart from the SXB in the Triassic and collided back in the Jurassic. The closure of Paleo-Asian Ocean had experienced a long continue/episodic subduction-accretion processes on margins of the NCC to the south and the SXB to the north from the Early to Late Paleozoic. The final closure happened along the SXCYS, from west Solonker, Sonid Youqi, Kedanshan (Keshenketengqi), Xar Moron River through Songliao Basin via Kailu, Tongliao, Horqin Zuoyizhongqi, Changchun, to the east Panshi, Huadian, Dunhua, Yanji, with a scissors style closure in time from the Late Permian-Early Triassic in the west to the Late Permian-Middle Triassic in the east. The amalgamated blocks should compose a united micro-continent, named as Jiamusi-Mongolia Block (JMB) after Early Carboniferous, which bounded by Mongo-Okhotsk suture to the northwest, Solonker-Xar Moron-Changchun suture to the south and the eastern margin of JB to the

Imaging the slab structure in the Alpine region by high-resolution P-wave tomography

NASA Astrophysics Data System (ADS)

Guillot, Stéphane; Zhao, Liang; Paul, Anne; Malusà, Marco G.; Xu, Xiaobing; Zheng, Tianyu; Solarino, stefano; Schwartz, Stéphane; Dumont, Thierry; Salimbeni, Simone; Aubert, Coralie; Pondrelli, Silvia; Wang, Qingchen; Zhu, Rixiang

2017-04-01

Based upon a finite-frequency inversion of traveltimes, we computed a new high-resolution tomography model using P-wave data from 527 broadband seismic stations, both from permanent networks and temporary experiments (Zhao et al., 2016). This model provides an improved image of the slab structure in the Alpine region, and fundamental pin-points for the analysis of Cenozoic magmatism, (U)HP metamorphism and Alpine topography. Our results document the lateral continuity of the European slab from the Western to the Central Alps, and the down-dip slab continuity beneath the Central Alps, ruling out the hypothesis of slab breakoff to explain Cenozoic Alpine magmatism. A low velocity anomaly is observed in the upper mantle beneath the core of the Western Alps, pointing to dynamic topography effects (Malusà et al., this meeting). A NE-dipping Adriatic slab, consistent with Dinaric subduction, is possibly observed beneath the Eastern Alps, whereas the laterally continuous Adriatic slab of the Northern Apennines shows major gaps at the boundary with the Southern Apennines, and becomes near vertical in the Alps-Apennines transition zone. Tear faults accommodating opposite-dipping subductions during Alpine convergence may represent reactivated lithospheric faults inherited from Tethyan extension. Our results suggest that the interpretations of previous tomography results that include successive slab breakoffs along the Alpine-Zagros-Himalaya orogenic belt might be proficiently reconsidered. Malusà M.G. et alii (2017) On the potential asthenospheric linkage between Apenninic slab rollback and Alpine topographic uplift: insights from P wave tomography and seismic anisotropy analysis. EGU 2017. Zhao L. et alii (2016), Continuity of the Alpine slab unraveled by high-resolution P wave tomography. J. Geophys. Res., doi:10.1002/2016JB013310.

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

resulted in the evolution of distinctly different orogen styles. The Arabian–Nubian Shield is an accretion-type orogen comprising a stack of thin-skinned nappes resulting from the oblique convergence of bounding plates. The Eastern Granulite–Cabo Delgado Nappe Complex is interpreted as a hot- to ultra-hot orogen that evolved from a formerly extended crust. Low viscosity lower crust resisted one-sided subduction, instead a sagduction-type orogen developed. The regions of Tanzania and Madagascar affected by the Kuungan Orogeny are considered a Himalayan-type orogen composed of partly doubly thickened crust. PMID:27065752

Land Clearing on the Himalayan Lower Slopes, Nepal

NASA Technical Reports Server (NTRS)

1989-01-01

This view of the Himalayan Mountain Range, Nepal (28.5N, 82.5E) shows a portion of the mountains and lower valleys filled with smoke from fires set among the subtropical temperate rainforests and alpine slope forests. Some valleys are entirely filled with smoke palls and over fifty individual point sources of burning can be delineated in this scene. These pristine forests are threatened by intensive agriculture and land clearing processes.

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.

Paleo- and Neo-Tethyan ophiolites of Iran: a progress report

NASA Astrophysics Data System (ADS)

Ghazi, M.; Hassanipak, A.; Babaie, H.

2003-04-01

The Bitlis-Zagros and Alborz stuture zones of Iran mark two collisional plate boundaries in the Alpine-Himalayan orogenic belt. The ophiolites of these zones together with the ophiolites of Makran accretionary prism and Central Iran form discontinuous linear belts of Tethyan oceanic fragments, which form a bridge between the Mediterranean and Himalayan ophiolites. Based on age alone these ophiolites have been divided into less abundant Paleozoic and much more abundant Mesozoic ophiolites. The Paleozoic ophiolites are located along the Alborz orogenic belt [i.e., Rasht and Mashhad ophiolites (297 Ma and 268 Ma)] and near Anarak in Central Iran. which are the remnants of the Paleo-Tethys ocean crust emplaced as result of closure of the Paleo-Tethys between the Turan and the Central Iranian Microplates (CIM). The Mesezoic ophiolites of Iran are more abundant and include the Zagros ophiolites (i.e., Neyriz and the Kermanshah ophiolites which appear to be coeval with the Oman ophiolite obducted onto the Arabian plate (˜96-92 Ma). The Khoy ophiolite in NW Iran which has formation age of Middle to Late Jurrasic (˜159-155 Ma), and emplacement age Albian ages (˜ 109-104 Ma) has a different tectonics than other Zagros ophiolites. Unfragmented ophiolites of the Makran accretionary prism which are located to the south of the Sanandaj-Sirjan microcontinental block, including complexes such as Band-e-Zeyarat/Dar Anar, Ganj and Remeshk/Mokhtarabad (˜140-98 Ma) are similar in age to the Masirah ophiolite (i.e., ˜150-120 Ma). The ophiolites of the Central Iran include those inside of the Sanandaj-Sirjan microcontinental block, such as Shahr-e-Babak (120 Ma), Naien (100 Ma), Baft, Sabzevar in north central Iran (98-70 Ma) and Tchehel Kureh on the eastern boundary of CIM.Geochemically, these ophiolites are quite diverse and show a significant variations in rock composition, representing a wide range of tectonic environment of formation. In terms of radiogenic isotopic data

Pliocene episodic exhumation and the significance of the Munsiari thrust in the northwestern Himalaya

NASA Astrophysics Data System (ADS)

Stübner, Konstanze; Grujic, Djordje; Dunkl, István; Thiede, Rasmus; Eugster, Patricia

2018-01-01

The Himalayan thrust belt comprises three in-sequence foreland-propagating orogen-scale faults, the Main Central thrust, the Main Boundary thrust, and the Main Frontal thrust. Recently, the Munsiari-Ramgarh-Shumar thrust system has been recognized as an additional, potentially orogen-scale shear zone in the proximal footwall of the Main Central thrust. The timing of the Munsiari, Ramgarh, and Shumar thrusts and their role in Himalayan tectonics are disputed. We present 31 new zircon (U-Th)/He ages from a profile across the central Himachal Himalaya in the Beas River area. Within a ∼40 km wide belt northeast of the Kullu-Larji-Rampur window, ages ranging from 2.4 ± 0.4 Ma to 5.4 ± 0.9 Ma constrain a distinct episode of rapid Pliocene to Present exhumation; north and south of this belt, zircon (U-Th)/He ages are older (7.0 ± 0.7 Ma to 42.2 ± 2.1 Ma). We attribute the Pliocene rapid exhumation episode to basal accretion to the Himalayan thrust belt and duplex formation in the Lesser Himalayan sequence including initiation of the Munsiari thrust. Pecube thermokinematic modelling suggests exhumation rates of ∼2-3 mm/yr from 4-7 to 0 Ma above the duplex contrasting with lower (<0.3 mm/yr) middle-late Miocene exhumation rates. The Munsiari thrust terminates laterally in central Himachal Pradesh. In the NW Indian Himalaya, the Main Central thrust zone comprises the sheared basal sections of the Greater Himalayan sequence and the mylonitic 'Bajaura nappe' of Lesser Himalayan affinity. We correlate the Bajaura unit with the Ramgarh thrust sheet in Nepal based on similar lithologies and the middle Miocene age of deformation. The Munsiari thrust in the central Himachal Himalaya is several Myr younger than deformation in the Bajaura and Ramgarh thrust sheets. Our results illustrate the complex and segmented nature of the Munsiari-Ramgarh-Shumar thrust system.

The Tintina Gold Belt - A global perspective

USGS Publications Warehouse

Goldfarb, Richard J.; Hart, Craig J.R.; Miller, Marti L.; Miller, Lance D.; Farmer, G. Lang; Groves, David I.; Tucker, Terry L.; Smith, Moira T.

2000-01-01

The so-called Tintina Gold Belt extends for more than 1000 km along the length of the northern North American Cordillera. Middle to Late Cretaceous Au deposits within the belt have various similar characteristics, among which are a spatial and temporal association with magmatism; Bi-W-Te signatures in deposits hosted by granitod stocks and As-Sb signatures where hosted by sedimentary rocks and dyke systems; and δ180 values consistently > 12 per mil for Au-bearing quartz. Nevertheless significant differences in structural styles, levels of deposit emplacement, ore-fluid chemistry, and Au grades suggest that the characteristics represent a broad range of deposit types. Many of these are best classified as orogenic Au deposits in the Yukon-Tanana terrane, as epithermal and porphyry-style Au deposits in the Kuskokwim region, and as Au-bearing, granite-related veins and stockworks, replacements, and skarns, as well as associated polymetallic lodes, in central Yukon. The diverse types of Au deposits and associated plutons of the Tintina Gold Belt collectively define a 45-m.y.-long period of arc magmatism that migrated northwesterly, for about 1000 km, across the active collisional margin of Cretaceous northwestern North America. The initiation of fluid flow and plutonism in Albian time seems to correlate with the onset of oblique subduction and dextral strike-slip on the Denali-Farewell, Tintina-Kaltag, and related fault systems. Initial Au-vein formation and subduction-related magmatism at about 115-110 Ma (e.g., including the Goodpaster and Fortymile districts), within the seaward side of the Yukon-Tanana terrane, correlate with the arrival of the Wrangellia superterrane off the continental margin. Dextral translation of the allochthonous Wrangellia block was associated with the migration of the thermal pulse to the northwest at about 95-90 Ma. Orogenic (or so­ called mesotherrnal) and granitoid-related Au deposits formed across the width of the Yukon

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

Timescale dependent deformation of orogenic belts?

NASA Astrophysics Data System (ADS)

Hoth, S.; Friedrich, A. M.; Vietor, T.; Hoffmann-Rothe, A.; Kukowski, N.; Oncken, O.

2004-12-01

deformation front advance cycle requires the longest timescale. Thus, despite known and constant boundary conditions during the simulations, we found only one regular temporal pattern of deformation in a steady active bivergent-wedge. We therefore propose that the structural inventory of an orogenic belt is hierarchically ordered with respect to accumulated slip, in analogy to the discharge pattern in a drainage network. The deformation front would have the highest, a branching splay the lowest order. Since kinematic boundary conditions control deformation front advance, its timing and the related maximum magnitude of finite strain, i.e. throw on the frontal thrust are predictable. However, the number of controlling factors, such as the degree of strain softening, the orientation of faults or fluid flow and resulting cementation of faults, responsible for the reactivation of faults increases with increasing distance from the deformation front. Since it is rarely possible to determine the complete network of forces within a wedge, the reactivation of lower order structures is not predictable in time and space. Two implications for field studies may emerge: A change of the propagation of deformation can only be determined, if at least two accretion cycles are sampled. The link between geodetic, paleoseismologic and geologic fault slip estimates can only be successfully derived if the position of the investigated fault within the hierarchical order has not changed over the time interval of interest.

3D crustal structure of the Alpine belt and foreland basins as imaged by ambient-noise surface wave

NASA Astrophysics Data System (ADS)

Molinari, Irene; Morelli, Andrea; Cardi, Riccardo; Boschi, Lapo; Poli, Piero; Kissling, Edi

2016-04-01

We derive a 3-D crustal structure (S wave velocity) underneath northern Italy and the wider Alpine region, from an extensive data set of measurements of Rayleigh-wave phase- and group-velocities from ambient noise correlation among all seismographic stations available to date in the region, via a constrained tomographic inversion made to honor detailed active source reflection/refraction profiles and other geological information. We first derive a regional-scale surface wave tomography from ambient-noise-based phase- and group- surface wave velocity observations (Verbeke et al., 2012). Our regional 3D model (Molinari et al., 2015) shows the low velocity area beneath the Po Plain and the Molasse basin; the contrast between the low-velocity crust of the Adriatic domain and the high-velocity crust of the Tyrrhenian domain is clearly seen, as well as an almost uniform crystalline crust beneath the Alpine belt. However, higher frequency data can be exploited to achieve higher resolution images of the Po Plain and Alpine foreland 3D crustal structure. We collected and analyze one year of noise records (2011) of ~100 North Italy seismic broadband stations, we derive the Green functions between each couple of stations and we measure the phase- and group-Rayleigh wave velocity. We conduct a suite of linear least squares inversion of both phase- and group-velocity data, resulting in 2-D maps of Rayleigh-wave phase and group velocity at periods between 3 and 40s with a resolution of 0.1x0.1 degrees. The maps are then inverted to get the 3D structure with unprecedented details. We present here our results, we compare them with other studies, and we discuss geological/geodynamical implications. We believe that such a model stands for the most up-to-date seismological information on the crustal structure of the Alpine belt and foreland basins, and it can represent a reliable reference for further, more detailed, studies to come, based on the high seismograph station density

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.

An analysis of the daily precipitation variability in the Himalayan orogen using a statistical parameterisation and its potential in driving landscape evolution models with stochastic climatic forcing

NASA Astrophysics Data System (ADS)

Deal, Eric; Braun, Jean

2015-04-01

A current challenge in landscape evolution modelling is to integrate realistic precipitation patterns and behaviour into longterm fluvial erosion models. The effect of precipitation on fluvial erosion can be subtle as well as nonlinear, implying that changes in climate (e.g. precipitation magnitude or storminess) may have unexpected outcomes in terms of erosion rates. For example Tucker and Bras (2000) show theoretically that changes in the variability of precipitation (storminess) alone can influence erosion rate across a landscape. To complicate the situation further, topography, ultimately driven by tectonic uplift but shaped by erosion, has a major influence on the distribution and style of precipitation. Therefore, in order to untangle the coupling between climate, erosion and tectonics in an actively uplifting orogen where fluvial erosion is dominant it is important to understand how the 'rain dial' used in a landscape evolution model (LEM) corresponds to real precipitation patterns. One issue with the parameterisation of rainfall for use in an LEM is the difference between the timescales for precipitation (≤ 1 year) and landscape evolution (> 103 years). As a result, precipitation patterns must be upscaled before being integrated into a model. The relevant question then becomes: What is the most appropriate measure of precipitation on a millennial timescale? Previous work (Tucker and Bras, 2000; Lague, 2005) has shown that precipitation can be properly upscaled by taking into account its variable nature, along with its average magnitude. This captures the relative size and frequency of extreme events, ensuring a more accurate characterisation of the integrated effects of precipitation on erosion over long periods of time. In light of this work, we present a statistical parameterisation that accurately models the mean and daily variability of ground based (APHRODITE) and remotely sensed (TRMM) precipitation data in the Himalayan orogen with only a few

Geochemical Characteristics of Granitoids in southwest Tianshan: Four Stages for Geodynamic Evolution of the Southwest Tianshan Orogenic Belt

NASA Astrophysics Data System (ADS)

Zhu, Y.

2016-12-01

Paleozoic intrusive rocks widely exposed in the west Tianshan orogenic belt provides key to understand the geodynamic evolution of the central Asian orogenic belt. A synthesis involving the data for Chinese Yili-central Tianshan and southwest Tianshan and comparison of Kyrgyz Tianshan with a broader dataset including zircon U-Pb ages, zircon Hf isotopic composition, major and trace elements for Paleozoic intrusions are presented to classify the Paleozoic intrusive rocks in four categories which corresponding to subduction of the Terskey Ocean, initial subduction stage of South Tianshan Ocean (STO), major subduction stage of the STO, and collisional to post-collisional stages. The subduction of the Terskey Oceanic crust finally caused the closure of the Terskey Ocean and the opening of the South Tianshan back-arc basin. The development of the Southwest Tianshan back-arc basin formed the STO, which subducted under the Yili-central Tianshan during early Silurian to early Carboniferous, and consequently formed huge arc magmatic rocks. Both the Silurian and early Carboniferous intrusions showing arc geochemical characteristics were derived from partial melting of juvenile arc-derived rocks with involvement of old continental crust. The STO finally closed by the end of early Carboniferous. Afterwards, geodynamic setting changed from convergence to extensional during late Carboniferous to early Permian periods. There is a significant geodynamic change from convergence to extension during late Carboniferous to early Permian, which may be resulted from breakoff of the subducted slab (Fig. 1). Such processes caused upwelling of asthenosphere and triggered partial melting of continental crust, as evidenced by emplacement of voluminous granitic rocks. References: An F, et al, 2013. Journal of Asian Earth Sciences, 78: 100-113; Zhu YF, 2011. Ore Geology Reviews, 40: 108-121; Zhu YF, et al, 2009. Geological Society, London, 166: 1085-1099; Zhu YF et al, 2016. Journal of Earth

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

The main features of the Uralian Paleozoic magmatism and the epioceanic nature of the orogen

NASA Astrophysics Data System (ADS)

Fershtater, G. B.

2013-02-01

The 2000 km Uralian Paleozoic orogen is situated on the western flank of the Uralo-Mongolian folded belt. It is characterized by an abundant variety of magmatic rocks and related ore deposits. Uralian Paleozoic magmatism is entirely subduction-related. It is proposed that the Uralian orogen represents a cold mobile belt in which the mantle temperature was 200 to 500 °C cooler than in the adjacent areas; a situation which is similar to the modern West Pacific Triangle Zone including Indonesia, the Philippine Islands, and southern Asia. During the course of the geological evolution of the Uralian orogen, the nature of the magmatism has changed from basic rocks of indisputable mantle origin (460-390 Ma) to mantle-crust gabbro-granitic complexes (370-315 Ma) followed by pure crustal granite magmatism (290-250 Ma). This order in rock type and age reflects the evolution of Paleozoic magmatic complexes from the beginning of subduction to the final stages of the orogen development.

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

-normal shortening, (4) emplacement of late prolate shape plutons in the direction of flow, and (5) late, conjugate strike-slip shearing. The macroscopic- to regional scale tectonoplutonic pattern produced by longitudinal flow forms a flat composite anisotropy throughout the lower crust. In the light of GPS data, these results suggest that bulk longitudinal flow accounts for observed deformation of the Tibetan plateau as well as for its seismic structure. This flow mode may be preferred to lateral, east-directed channel flow because it combines both lateral gravity-driven thinning and distributed, orogen-normal shortening of the crust. These results further suggest that lower crustal seismic reflectivity in orogenic belts may not necessarily images fabrics produced by extensional tectonics, as commonly thought, but crustal layering produced by syn-convergence lateral flow.

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.

87Sr/86Sr ratios in some eugeosynclinal sedimentary rocks and their bearing on the origin of granitic magma in orogenic belts

USGS Publications Warehouse

Peterman, Z.E.; Hedge, C.E.; Coleman, R.G.; Snavely, P.D.

1967-01-01

Rb and Sr contents and 87Sr/86Sr values were determined for samples of eugeosynclinal sedimentary rocks, mostly graywackes, from Oregon and California. These data are compatible with the theory of anataxis of eugeosynclinal sedimentary rocks in orogenic belts to produce granitic magmas provided that the melting occurs within several hundreds of m.y. after sedimentation. The low (87Sr/86Sr)0 values of the eugeosynclinal sedimentary rocks are related to the significant amounts of volcanogenic detritus present which probably were originally derived from the mantle. ?? 1967.

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

Orogenic plateau growth: Expansion of the Turkish-Iranian Plateau across the Zagros fold-and-thrust belt

NASA Astrophysics Data System (ADS)

Allen, M. B.; Saville, C.; Blanc, E. J.-P.; Talebian, M.; Nissen, E.

2013-03-01

This paper shows how the Turkish-Iranian Plateau grows laterally by incrementally incorporating adjacent parts of the Zagros fold-and-thrust belt. The limit of significant, seismogenic, thrusting in the Zagros (Mw > 5) occurs close to the regional 1250 m elevation contour. The seismicity cutoff is not a significant bedrock geology boundary. Elevations increase northward, toward regional plateau elevations of 2 km, implying that another process produced the extra elevation. Between the seismogenic limit of thrusting and the suture, this process is a plausibly ductile thickening of the basement, suggesting depth-dependent strain during compression. Similar depth-dependant crustal strain may explain why the Tibetan plateau has regional elevations 1500 m greater than the elevation limit of seismogenic thrusting at its margins. We estimate 68 km shortening across the Zagros Simply Folded Belt in the Fars region, and 120 km total shortening of the Arabian plate. The Dezful Embayment is a low strain zone in the western Zagros. Deformation is more intense to its northeast, in the Bakhtyari Culmination. The orogenic taper (across strike topographic gradient) across the Dezful Embayment is 0.0004, and across the Bakhtyari Culmination, 0.022. Lateral plateau growth is more pronounced farther east (Fars), where a more uniform structure has a taper of 0.010 up to elevations of 1750 m. A >100 km wide region of the Zagros further northeast has a taper of 0.002 and is effectively part of the Turkish-Iranian Plateau. Internal drainage enhances plateau development but is not a pre-requisite. Aspects of the seismicity, structure, and geomorphology of the Zagros do not support critical taper models for fold-and-thrust belts.

Alpine thermal events in the central Serbo-Macedonian Massif (southeastern Serbia)

NASA Astrophysics Data System (ADS)

Antić, Milorad D.; Kounov, Alexandre; Trivić, Branislav; Wetzel, Andreas; Peytcheva, Irena; von Quadt, Albrecht

2016-07-01

The Serbo-Macedonian Massif (SMM) represents a crystalline belt situated between the two diverging branches of the Eastern Mediterranean Alpine orogenic system, the northeast-vergent Carpatho-Balkanides and the southwest-vergent Dinarides and the Hellenides. We have applied fission-track analysis on apatites and zircons, coupled with structural field observations in order to reveal the low-temperature evolution of the SMM. Additionally, the age and geochemistry of the Palaeogene igneous rocks (i.e. Surdulica granodiorite and dacitic volcanic rocks) were determined by the LA-ICPMS U-Pb geochronology of zircons and geochemical analysis of main and trace elements in whole-rock samples. Three major cooling stages have been distinguished from the late Early Cretaceous to the Oligocene. The first stage represents rapid cooling through the partial annealing zones of zircon and apatite (300-60 °C) during the late Early to early Late Cretaceous (ca. 110-ca. 90 Ma). It is related to a post-orogenic extension following the regional nappe-stacking event in the Early Cretaceous. Middle to late Eocene (ca. 48-ca. 39 Ma) cooling is related to the formation of the Crnook-Osogovo-Lisets extensional dome and its exhumation along low-angle normal faults. The third event is related to regional cooling following the late Eocene magmatic pulse. During this pulse, the areas surrounding the Surdulica granodiorite (36 ± 1 Ma) and the slightly younger volcanic bodies (ca. 35 Ma) have reached temperatures higher than the apatite closure temperature (120 °C) but lower than ca. 250 °C. The geochemistry of the igneous samples reveals late- to post-orogenic tectonic setting during magma generation.

Earthquake hazard assessment in the Zagros Orogenic Belt of Iran using a fuzzy rule-based model

NASA Astrophysics Data System (ADS)

Farahi Ghasre Aboonasr, Sedigheh; Zamani, Ahmad; Razavipour, Fatemeh; Boostani, Reza

2017-08-01

Producing accurate seismic hazard map and predicting hazardous areas is necessary for risk mitigation strategies. In this paper, a fuzzy logic inference system is utilized to estimate the earthquake potential and seismic zoning of Zagros Orogenic Belt. In addition to the interpretability, fuzzy predictors can capture both nonlinearity and chaotic behavior of data, where the number of data is limited. In this paper, earthquake pattern in the Zagros has been assessed for the intervals of 10 and 50 years using fuzzy rule-based model. The Molchan statistical procedure has been used to show that our forecasting model is reliable. The earthquake hazard maps for this area reveal some remarkable features that cannot be observed on the conventional maps. Regarding our achievements, some areas in the southern (Bandar Abbas), southwestern (Bandar Kangan) and western (Kermanshah) parts of Iran display high earthquake severity even though they are geographically far apart.

Ginger and the beetle: Evidence of primitive pollination system in a Himalayan endemic alpine ginger (Roscoea alpina, Zingiberaceae)

PubMed Central

Shrestha, Mani; Dyer, Adrian G.; Li, Qing-Jun

2017-01-01

The Himalayan endemic alpine genus Roscoea, like other members of ginger family, exhibits the combination of floral traits that would fit pollination by long distant foragers such as bees, birds or flies. We studied the pollination biology of Roscoea alpina, observed potential floral visitors and determined their foraging behaviour, visitation frequency and pollination efficiency, to seek evidence in support of the pollination syndrome hypothesis. We also measured the floral spectra of R. alpina flowers to evaluate if signals fit with the currently known framework for observed floral visitors. We found that R. alpina have autonomous selfing and pollinator-mediated crossing, but lack apomixis. We observed that a beetle (Mylabris sp.), and a moth (Macroglossum nycteris) visit the flowers of R. alpina for pollen and nectar feeding respectively. Our field observations, the stigmatic pollen count and fruit set data indicated that the visit by the beetle was legitimate, while that of the moth was illegitimate. Emasculated flowers visited by beetles set as many fruits and seeds/fruit as auto-selfed and naturally pollinated flowers, while emasculated flowers excluded from beetle visits did not set fruit and seed; indicating that a single visit of a beetle to the flowers of R. alpina can facilitate pollination. We found that flower spectral signal of R. alpina does not fit typical spectra previously reported for beetle or bee-visited flowers. Our results suggest that, to ensure reproductive success in alpine habitat, R. alpina has evolved autonomous selfing as a predominant mode of reproduction, while beetle pollination would promote genetic diversity of this plant species. The visitation of beetles to the flowers of R. alpina, despite floral signal mismatch with the classically associated beetle vision, suggests that a different visual processing may operate in this plant-pollinator interaction at high altitudes. PMID:28723912

Ginger and the beetle: Evidence of primitive pollination system in a Himalayan endemic alpine ginger (Roscoea alpina, Zingiberaceae).

PubMed

Paudel, Babu Ram; Shrestha, Mani; Dyer, Adrian G; Li, Qing-Jun

2017-01-01

The Himalayan endemic alpine genus Roscoea, like other members of ginger family, exhibits the combination of floral traits that would fit pollination by long distant foragers such as bees, birds or flies. We studied the pollination biology of Roscoea alpina, observed potential floral visitors and determined their foraging behaviour, visitation frequency and pollination efficiency, to seek evidence in support of the pollination syndrome hypothesis. We also measured the floral spectra of R. alpina flowers to evaluate if signals fit with the currently known framework for observed floral visitors. We found that R. alpina have autonomous selfing and pollinator-mediated crossing, but lack apomixis. We observed that a beetle (Mylabris sp.), and a moth (Macroglossum nycteris) visit the flowers of R. alpina for pollen and nectar feeding respectively. Our field observations, the stigmatic pollen count and fruit set data indicated that the visit by the beetle was legitimate, while that of the moth was illegitimate. Emasculated flowers visited by beetles set as many fruits and seeds/fruit as auto-selfed and naturally pollinated flowers, while emasculated flowers excluded from beetle visits did not set fruit and seed; indicating that a single visit of a beetle to the flowers of R. alpina can facilitate pollination. We found that flower spectral signal of R. alpina does not fit typical spectra previously reported for beetle or bee-visited flowers. Our results suggest that, to ensure reproductive success in alpine habitat, R. alpina has evolved autonomous selfing as a predominant mode of reproduction, while beetle pollination would promote genetic diversity of this plant species. The visitation of beetles to the flowers of R. alpina, despite floral signal mismatch with the classically associated beetle vision, suggests that a different visual processing may operate in this plant-pollinator interaction at high altitudes.

The Wulonggou metaluminous A2-type granites in the Eastern Kunlun Orogenic Belt, NW China: Rejuvenation of subduction-related felsic crust and implications for post-collision extension

NASA Astrophysics Data System (ADS)

Xin, Wei; Sun, Feng-Yue; Li, Liang; Yan, Jia-Ming; Zhang, Yu-Ting; Wang, Ying-Chao; Shen, Ting-Shuo; Yang, Yi-Jun

2018-07-01

The Wulonggou Pluton is located in Wulonggou area, eastern segment of the Eastern Kunlun Orogenic Belt, NW China, and consists of mainly alkali-feldspar granites covering an area of about 150 km2. Petrogenesis of these granitoids has been investigated through an integrated study of petrography, zircon Usbnd Pb ages, whole-rock geochemistry, and Hfsbnd Nd isotopic compositions. Usbnd Pb dating of magmatic zircons indicated these granites crystallized during 426-424 Ma in the middle Silurian. The granites display high SiO2 (75.26-77.55 wt%), K2O + Na2O (7.98-9.03 wt%), extremely low MgO (0.04-0.19 wt%), CaO (0.28-0.61 wt%), and TiO2 (0.05-0.09 wt%) contents showing metaluminous, calcic-alkali and ferroan features; enrichment in Rb and some HFSEs (Zr, U, Nb, Ta, and Y), depletion in Sr, Ba, P, and Ti, mostly right-inclined REE curve, flat HREE patterns, high 10,000 ∗ Ga/Al and intensively negative Eu anomalies, exhibiting an A2-type granite affinity with Y/Nb > 1.2 mostly. The primitive magma of these large quantities of granites was generated under a high temperature, low pressure, reduced and anhydrous environment indicating intense upwelling of asthenosphere. Combining with the positive uniform zircon εHf(t) values of -0.2 to +3.8 and decoupled εNd(t) values of -4.9 to -2.1 at t = 424 Ma, it can be concluded that subduction-related juvenile materials, probably calc-alkaline granitoids, are the source of these A-type granites. Geochemical studies of Wulonggou granites, spatial and temporal distributions of regional magmatism, metamorphism, and sedimentary records throughout the Eastern Kunlun Orogen Belt jointly indicate that the whole orogenic belt was in a typical post-collision extension setting and experienced an isostatic uplift during the middle Silurian triggered by delamination after the convergence of the northeastern margin of Gondwana.

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

New Ar/Ar single grain mineral ages from Korean orogenic belts with implications for the Triassic cooling and exhumation history

NASA Astrophysics Data System (ADS)

de Jong, Koenraad; Ruffet, Gilles; Han, Seokyoung

2013-04-01

The Korean peninsula is located in the eastern margin of the Eurasian continent where major late Palaeozoic to early Mesozoic continental collision zones, like the Central Asian Orogenic Belt and the Qinling-Dabie-Sulu Belt, merge with circum-Pacific subduction-accretion systems. Deciphering the tectonic evolution of Korea is thus crucial for the understanding of the amalgamation of East Asia. Classically, research in Korea has focused on the search for (ultra)high-pressure metamorphic rocks and their isotopic dating, most recently applying SHRIMP on Th- and U-bearing accessory minerals, in order to substantiate links with the Qinling-Dabie-Sulu Belt across the Yellow Sea in China. Instead of trying to date peak pressure conditions we focused on 40Ar/39Ar laser-probe step-heating dating of single grains of the fabric-forming minerals muscovite, biotite and amphibole, formed during retrograde recrystallisation and exhumation. This is a big advantage as their growth can be straightforwardly correlated to major phases of the tectono-metamorphic evolution of rocks. This approach helps to meet the major geochronological challenge of obtaining age estimates for the timing of specific tectono-metamorphic events in the Korean orogenic belts. The Korean peninsula comprises a number of Palaeoproterozoic high-grade gneiss terranes; only one of which has been affected by Permo-Triassic metamorphism: the Gyeonggi Massif. We concentrated on the uppermost Gyeonggi Massif and the overlying Imjingang Belt, to the North, and the ill-defined Hongseong zone to the West, both constituted by younger metamorphic rocks. Both belts contain rare lenses of mafic rocks with relics of high-pressure metamorphism. Hornblende from a corona-textured amphibolite from the lowermost part of the Imjingang Belt yielded a U-shaped age spectrum, the base of which is formed by four concordant steps with a weighted mean age of 242.8 ± 2.4 Ma (15% 39Ar release). Muscovites from strongly retrogressed and

West-directed thrusting south of the eastern Himalayan syntaxis indicates clockwise crustal flow at the indenter corner during the India-Asia collision

NASA Astrophysics Data System (ADS)

Haproff, Peter J.; Zuza, Andrew V.; Yin, An

2018-01-01

Whether continental deformation is accommodated by microplate motion or continuum flow is a central issue regarding the nature of Cenozoic deformation surrounding the eastern Himalayan syntaxis. The microplate model predicts southeastward extrusion of rigid blocks along widely-spaced strike-slip faults, whereas the crustal-flow model requires clockwise crustal rotation along closely-spaced, semi-circular right-slip faults around the eastern Himalayan syntaxis. Although global positioning system (GPS) data support the crustal-flow model, the surface velocity field provides no information on the evolution of the India-Asia orogenic system at million-year scales. In this work, we present the results of systematic geologic mapping across the northernmost segment of the Indo-Burma Ranges, located directly southeast of the eastern Himalayan syntaxis. Early research inferred the area to have experienced either right-slip faulting accommodating northward indentation of India or thrusting due to the eastward continuation of the Himalayan orogen in the Cenozoic. Our mapping supports the presence of dip-slip thrust faults, rather than strike-slip faults. Specifically, the northern Indo-Burma Ranges exposes south- to west-directed ductile thrust shear zones in the hinterland and brittle fault zones in the foreland. The trends of ductile stretching lineations within thrust shear zones and thrust sheets rotate clockwise from the northeast direction in the northern part of the study area to the east direction in the southern part of the study area. This clockwise deflection pattern of lineations around the eastern Himalayan syntaxis mirrors the clockwise crustal-rotation pattern as suggested by the crustal-flow model and contemporary GPS velocity field. However, our finding is inconsistent with discrete strike-slip deformation in the area and the microplate model.

The Sanfengshan copper deposit and early Carboniferous volcanogenic massive sulfide mineralization in the Beishan orogenic belt, Northwestern China

NASA Astrophysics Data System (ADS)

Wang, Jialin; Gu, Xuexiang; Zhang, Yongmei; Zhou, Chao; He, Ge; Liu, Ruiping

2018-03-01

The Sanfengshan copper deposit, located in the Beishan orogenic belt, Northwestern China, is hosted in the lower member of the Hongliuyuan Formation, an early Carboniferous metavolcanic-sedimentary sequence. Mineralization occurs as stratiform, stratiform-like and lenticular orebodies, and comprises of laminated, brecciated, banded, massive, and disseminated ores. The mineralogy is dominated by pyrite, chalcopyrite and sphalerite. Fe-Mn chert is widely distributed and generally occurs as massive, laminated, bands or lenses, which are consistent with the orebody. Alteration at Sanfengshan displays a clear concentric zoning pattern and the footwall alteration is more intense and somewhat thicker than the hanging-wall alteration. Systematic geochemical investigation on the volcanic rocks in this area shows that the basalts of the Hongliuyuan Formation (HLY) are predominantly tholeiites with nearly flat rare earth element (REE) pattern, insignificant negative anomalies of high field strength elements (HFSEs), and low Ti/V and Th/Nb ratios. They were most likely derived from partial melting of depleted asthenospheric mantle and formed in a fore-arc setting during initiation of the southward subduction of the Paleo-Asian Ocean. The basalts of the Maotoushan Formation (MTS) display a calc-alkaline nature and are enriched in large ion lithophile elements (LILEs) and depleted in HFSEs, suggesting an active continental margin setting. Sulfur isotope (δ34S) values of the sulfide and sulfate minerals vary between 0‰ and 5.4‰, which are consistent with sulfur derivation from leaching of the host volcanic rocks, although a direct magmatic contribution cannot be ruled out. The Re-Os isotope data of pyrite yield an isochron age of 353 ± 35 Ma, consistent with the age of the host HLY basalts. Thus, a syngenetic (volcanogenic massive sulfide) model is proposed and it is concluded that the Sanfengshan copper deposit is a typical Cyprus-type VMS deposit that formed in an early

Post-orogenic subsidence and uplift of the Carpathian belt: An integrated approach

NASA Astrophysics Data System (ADS)

Bertotti, G.; Matenco, L.; Drijkonigen, G.; Krijgsman, W.; Tarapoanca, M.; Panea, I.; Vasiliev, I.; Milea, M.; Cloetingh, S.

2003-04-01

Several hundred metres thick Pliocene to Quaternary sequences outcropping along the Carpathian front steeply dip away from the mountain belt towards the Carpathian foredeep. They overly the Carpathian fold-and-thrust belt and document that, following the main contractional stages, the orogenic wedge first subsided and was then uplifted. Uplift occurred coeval with substantial subsidence in the basin adjacent to the E, the Focsani Depression. To define the precise kinematics of such movements and thereby constrain these vertical movements taking place in the "wrong" place and in the "wrong" time, the Netherlands Research Center for Integrated Solid Earth Science has launched a large campaign of geological and geophysical investigation. The main components of the project are as follows: 1) acquisition of nearly 100km of seismic data designed to image the uppermost hundred metres of the Earth's crust and thereby making a precise connection between features visible in Industry lines and at the surface 2) paleomagnetic investigations in order to constrain the age of the poorly dated continental to lacustrine sediments 3) A seismic experiment designed to detect 3-D effects on 2-D acquisition 4) Structural work to determine the stress/strain conditions during subsidence and subsequent uplift At a larger scale, these activities are embedded in the effort made by ISES and connected groups to precisely constrain the kinematics of the Pannonian-Carpathian system. Seismic acquisition has been performed during the summer 2002 and has been technically very successful thanks also to the effort of the prospecting company Prospectiunii SA. Lines have been processed and are currently being interpreted. The most apparent feature is the lack of localized deformation demonstrating that subsidence and tilting affected areas of several tens of kilometers and are not related to single faults. Sampling for paleomagnetic studies has been carried out in 2002 along the same section where

Erosional Reduction of an Orogenic Wedge: Structural Response to Neogene Climate Change within the St. Elias Orogen, Alaska

NASA Astrophysics Data System (ADS)

Berger, A. L.; Spotila, J. A.; Chapman, J. B.; Pavlis, T. L.; Enkelmann, E.; Buscher, J. T.

2007-12-01

The kinematics and architecture of orogenic systems may be heavily influenced by climate, but little research has focused on the long term effects of glacial erosion on orogenesis. Apatite and zircon (U-Th)/He thermochronometry on >75 bedrock samples across the St. Elias orogen, one of the best examples of a glaciated orogenic wedge, is the basis for a new kinematic model and demonstrates an association between glacial denudation and orogenic architecture. The spatial pattern of low temperature cooling indicates that exhumation and deformation are focused within a thin-skinned fold and thrust belt on the windward flank, whereas the leeward flank of the orogen functions as a deformational backstop. A previously unrecognized structure beneath the Bagley ice field must separate these domains with south-side-up motion. We propose this structure is a backthrust making the orogen doubly-vergent. Suggestive of accelerated backthrust motion in response to climate change, cooling rates within the hanging wall block and across the entire windward flank of the orogen accelerated ten-fold coeval with enhanced glaciation. As backthrust motion increased, glacial unroofing also coincided with a regional shift in deformation away from prominent forethrusts including the North American-Yakutat terrane suture (Chugach St. Elias fault) and the seaward deformation front (Pamplona zone). Across the windward flank of the orogen, exhumation, at rates of up to 5 mm/yr, is focused within a narrow zone, where the glacial equilibrium line altitude (ELA) intersects the orogenic wedge. This zone of rapid exhumation, not present prior to the onset of enhanced glaciation, cuts across the structural trend of the orogen and is more narrowly focused than orographic precipitation. Accelerated denudation at the ELA thus appears to have redistributed strain along a series of forethrusts that lie at the zone of heaviest glacial flux, while the backthrust progressively truncates the southward

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

NASA Astrophysics Data System (ADS)

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

2004-09-01

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

High diversity of protistan plankton communities in remote high mountain lakes in the European Alps and the Himalayan mountains

PubMed Central

Kammerlander, Barbara; Breiner, Hans-Werner; Filker, Sabine; Sommaruga, Ruben; Sonntag, Bettina; Stoeck, Thorsten

2015-01-01

We analyzed the genetic diversity (V4 region of the 18S rRNA) of planktonic microbial eukaryotes in four high mountain lakes including two remote biogeographic regions (the Himalayan mountains and the European Alps) and distinct habitat types (clear and glacier-fed turbid lakes). The recorded high genetic diversity in these lakes was far beyond of what is described from high mountain lake plankton. In total, we detected representatives from 66 families with the main taxon groups being Alveolata (55.0% OTUs97%, operational taxonomic units), Stramenopiles (34.0% OTUs97%), Cryptophyta (4.0% OTUs97%), Chloroplastida (3.6% OTUs97%) and Fungi (1.7% OTUs97%). Centrohelida, Choanomonada, Rhizaria, Katablepharidae and Telonema were represented by <1% OTUs97%. Himalayan lakes harbored a higher plankton diversity compared to the Alpine lakes (Shannon index). Community structures were significantly different between lake types and biogeographic regions (Fisher exact test, P < 0.01). Network analysis revealed that more families of the Chloroplastida (10 vs 5) and the Stramenopiles (14 vs 8) were found in the Himalayan lakes than in the Alpine lakes and none of the fungal families was shared between them. Biogeographic aspects as well as ecological factors such as water turbidity may structure the microbial eukaryote plankton communities in such remote lakes. PMID:25764458

High diversity of protistan plankton communities in remote high mountain lakes in the European Alps and the Himalayan mountains.

PubMed

Kammerlander, Barbara; Breiner, Hans-Werner; Filker, Sabine; Sommaruga, Ruben; Sonntag, Bettina; Stoeck, Thorsten

2015-04-01

We analyzed the genetic diversity (V4 region of the 18S rRNA) of planktonic microbial eukaryotes in four high mountain lakes including two remote biogeographic regions (the Himalayan mountains and the European Alps) and distinct habitat types (clear and glacier-fed turbid lakes). The recorded high genetic diversity in these lakes was far beyond of what is described from high mountain lake plankton. In total, we detected representatives from 66 families with the main taxon groups being Alveolata (55.0% OTUs 97%, operational taxonomic units), Stramenopiles (34.0% OTUs 97%), Cryptophyta (4.0% OTUs 97%), Chloroplastida (3.6% OTUs 97%) and Fungi (1.7% OTUs 97%). Centrohelida, Choanomonada, Rhizaria, Katablepharidae and Telonema were represented by <1% OTUs 97%. Himalayan lakes harbored a higher plankton diversity compared to the Alpine lakes (Shannon index). Community structures were significantly different between lake types and biogeographic regions (Fisher exact test, P < 0.01). Network analysis revealed that more families of the Chloroplastida (10 vs 5) and the Stramenopiles (14 vs 8) were found in the Himalayan lakes than in the Alpine lakes and none of the fungal families was shared between them. Biogeographic aspects as well as ecological factors such as water turbidity may structure the microbial eukaryote plankton communities in such remote lakes. © FEMS 2015.

Architecture, kinematics, and exhumation of a convergent orogenic wedge: A thermochronological investigation of tectonic-climatic interactions within the central St. Elias orogen, Alaska

NASA Astrophysics Data System (ADS)

Berger, Aaron L.; Spotila, James A.; Chapman, James B.; Pavlis, Terry L.; Enkelmann, Eva; Ruppert, Natalia A.; Buscher, Jamie T.

2008-06-01

The kinematics and architecture of orogenic systems along the leading edges of accreting terranes may be heavily influenced by climate, but little research has been devoted to the long-term effects of glacial erosion on orogenesis. Here we use low-temperature apatite and zircon (U-Th)/He and fission-track thermochronometry, along with subsidiary structural relationships and seismicity, to develop a new architectural model of the St. Elias orogen in southern Alaska, which is one of the best examples of a glaciated orogenic wedge worldwide. These data illustrate that the orogen consists of a deformational backstop on the leeward flank and a rapidly deforming and eroding, thin-skinned fold and thrust belt on the windward flank. A structure beneath the Bagley ice field separates these distinct deformational domains, which we propose is a backthrust that makes the orogen doubly-vergent. Thermochronometry within the orogenic wedge suggests that denudation and deformation are strongly influenced by glacial erosion. Long-term exhumation, at rates of up to 4 mm/yr, is concentrated within a narrow zone along the windward flank, where glacier equilibrium lines intersect the orogenic wedge. The onset of enhanced glaciation also coincided with a marked acceleration in exhumation across the orogenic wedge, accelerated backthrust motion, and a major shift in deformation away from the North American-Yakutat terrane suture (Chugach St. Elias fault). We propose that accelerated glacial erosion forced the redistribution of strain along the backthrust and an en echelon array of forethrusts that lie beneath the zone of heaviest glaciation, which in turn are systematically truncated by the backthrust. This focusing of deformation matches predictions from analytical models of orogenic wedges and implies a high degree of coupling between climate and tectonics in this glacially-dominated orogen.

Late Alpine to recent thick-skinned tectonics of the central Swiss Molasse Basin, Canton of Bern, Switzerland

NASA Astrophysics Data System (ADS)

Mock, Samuel; Allenbach, Robin; Wehrens, Philip; Reynolds, Lance; Kurmann-Matzenauer, Eva; Michael, Salomè; Herwegh, Marco

2017-04-01

The Swiss Molasse Basin (SMB) forms part of the North Alpine Foreland Basin. It is a typical peripheral foreland basin, which developed in Paleogene and Neogene times in response to flexural bending of the European lithosphere induced by the orogenic loading of the advancing Alpine thrust wedge. The tectonics of the SMB and the role of Paleozoic and Mesozoic structures are still poorly understood. It is widely accepted that during the main deformation phase of the Jura fold-and-thrust belt, the SMB was riding piggy-back above a major detachment horizon situated within Triassic evaporites. In recent years it has been observed that the Jura fold-and-thrust belt is today deforming in a thick-skinned tectonic style. As for the western and central SMB, most authors still argue in favor of a classical foreland type, thin-skinned style of deformation. Based on the geological 3D modeling of seismic interpretations, we present new insights into the structural configuration of the central SMB. Revised and new interpretations of 2D reflection seismic data from the 1960s to the 1980s reveal a major strike-slip fault zone affecting not only the Mesozoic and Cenozoic cover, but also the crystalline basement beneath. The fault zone reactivated late Paleozoic synsedimentary normal faults bounding a Permo-Carboniferous trough. Basement-involved thrusting observed in the southern part of the SMB seems to be controlled by the presence of slightly inverted Permo-Carboniferous troughs as well. These observations, combined with a compiled structural map and the distribution of recent earthquake hypocenters suggest a late stage, NNW-SSE directed, compressional thick-skinned and strike-slip dominated tectonic activity of the central SMB, post-dating the main deformation phase of the Jura fold-and-thrust belt. This still ongoing deformation might be related to the slab rollback of the European plate and the associated lower crustal delamination as recently suggested by Singer et al. (2014

Sporotrichosis in Sub-Himalayan India

PubMed Central

Verma, Santwana; Verma, Ghanshyam K.; Singh, Gagandeep; Kanga, Anil; Shanker, Vinay; Singh, Digvijay; Gupta, Poonam; Mokta, Kiran; Sharma, Vinita

2012-01-01

Sporotrichosis is endemic in the Sub-Himalayan belt, which ranges from the northern to the north-eastern Indian subcontinent. Similar to many parts of the developing world, sporotrichosis is commonly recognized clinically in this region however consolidated epidemiological data is lacking. We report epidemiological, clinical and microbiological data from a hundred culture positive cases of sporotrichosis. Out of 305 clinically suspicious cases of sporotrichosis, a total of 100 isolates were identified as Sporothrix schenckii species complex (S. schenckii) on culture. Out of the culture proven cases 71% of the cases presented with lymphocutaneous type of lesions while 28% had fixed localized type and 1% had disseminated sporotrichosis. Presentation with lesions on hands was most frequently seen in 32% with arm (23%) and face (21%) in that sequence. The male to female ratio was 1∶1.27. Age ranged from 1 ½ years to 88 years. Mean age was 43.25 years. Disease was predominantly seen in the fourth to sixth decade of life with 58% cases between 31 and 60 years of age. Since the first report from the region there has been a steady rise in the number of cases of sporotrichosis. Seasonal trends reveal that most of the patients visited for consultation in the beginning of the year between March and April. This is the first study, from the most endemic region of the Sub-Himalayan belt, to delve into epidemiological and clinical details of such a large number of culture proven cases over a period of more than eighteen years which would help in the understanding of the local disease pattern of sporotrichosis. PMID:22720100

Sporotrichosis in sub-himalayan India.

PubMed

Verma, Santwana; Verma, Ghanshyam K; Singh, Gagandeep; Kanga, Anil; Shanker, Vinay; Singh, Digvijay; Gupta, Poonam; Mokta, Kiran; Sharma, Vinita

2012-01-01

Sporotrichosis is endemic in the Sub-Himalayan belt, which ranges from the northern to the north-eastern Indian subcontinent. Similar to many parts of the developing world, sporotrichosis is commonly recognized clinically in this region however consolidated epidemiological data is lacking. We report epidemiological, clinical and microbiological data from a hundred culture positive cases of sporotrichosis. Out of 305 clinically suspicious cases of sporotrichosis, a total of 100 isolates were identified as Sporothrix schenckii species complex (S. schenckii) on culture. Out of the culture proven cases 71% of the cases presented with lymphocutaneous type of lesions while 28% had fixed localized type and 1% had disseminated sporotrichosis. Presentation with lesions on hands was most frequently seen in 32% with arm (23%) and face (21%) in that sequence. The male to female ratio was 1∶1.27. Age ranged from 1 ½ years to 88 years. Mean age was 43.25 years. Disease was predominantly seen in the fourth to sixth decade of life with 58% cases between 31 and 60 years of age. Since the first report from the region there has been a steady rise in the number of cases of sporotrichosis. Seasonal trends reveal that most of the patients visited for consultation in the beginning of the year between March and April. This is the first study, from the most endemic region of the Sub-Himalayan belt, to delve into epidemiological and clinical details of such a large number of culture proven cases over a period of more than eighteen years which would help in the understanding of the local disease pattern of sporotrichosis.

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

Constraints of C-O-S isotope compositions and the origin of the Ünlüpınar volcanic-hosted epithermal Pb-Zn ± Au deposit, Gümüşhane, NE Turkey

NASA Astrophysics Data System (ADS)

Akaryali, Enver; Akbulut, Kübra

2016-03-01

The Eastern Pontide Orogenic Belt (EPOB) constitutes one of the best examples of the metallogenic provinces in on the Alpine-Himalayan belt. This study focuses on the genesis of the Ünlüpınar Pb-Zn ± Au deposit in the southern part of the Eastern Pontide Orogenic Belt. The main lithological units in the study area are the Early Carboniferous Kurtoğlu Metamorphic Complex the Late Carboniferous Köse Granitoid and the Early-Middle Jurassic Şenköy Formation. The studied deposit is hosted by the Şenköy Formation, which consists predominantly of basaltic-andesitic rocks and associated pyroclastic rocks that are calc-alkaline in composition. Silicic, sulfidic, argillic, chloritic, hematitic, carbonate and limonite are the most obvious alteration types observed in the deposit site. Ore microscopy studies exhibit that the mineral paragenesis in deposits includes pyrite, chalcopyrite, sphalerite, galena, gold, quartz and calcite. Electron microprobe analyses conducted on sphalerite indicate that the Zn/Cd ratio varies between 84 and 204, and these ratios point at a hydrothermal deposit related to granitic magmas. Fluid inclusion studies in calcite and quartz show that the homogenization temperature of the studied deposit ranges between 90-160 °C and 120-330 °C respectively. The values of sulfur isotope analysis of pyrite, sphalerite and galena minerals vary between 1.6‰ and 5.7‰, and the results of oxygen and carbon isotope analysis range between 8.4‰ and 18‰ and -5‰ and -3.6‰, respectively. The average formation temperature of the ore was calculated as 264 °C with a sulfur isotope geothermometer. All of the data indicate that the Ünlüpınar deposit is an epithermal vein-type mineralization that was formed depending on the granitic magmatism.

Collisional Tectonics in the St. Elias Orogen, Alaska Observed by GPS

NASA Astrophysics Data System (ADS)

Elliott, J.; Freymueller, J. T.; Larsen, C. F.

2008-12-01

The rugged topography of the St. Elias orogen of southern Alaska and the adjacent region of Canada is the result of the on-going collision of the Yakutat block with southern Alaska. Nearly 45 mm/yr of NW-SE directed convergence from the collision is currently accommodated within the St. Elias orogen. A key to understanding this complex collisional boundary is knowing the locations of the structures taking up the convergence. GPS provides a snapshot of the present-day strain field and helps to delineate active structures. As part of the St. Elias Erosion/Tectonics Project (STEEP), we re-surveyed 70 campaign GPS sites across the St. Elias orogen during the summer of 2008. Strain rates derived from our GPS data highlight several areas within the St. Elias orogen. The highest strain rates occur across Icy Bay and the western edge of the Malaspina Glacier. Rates there approach -1 microstrain/yr, a value higher than that observed in the Himalaya. Lower, but still significant, strain rates of about -0.2 microstrain/yr extend north from Icy Bay to the region surrounding Mt. St. Elias. The second major focus of compressive strain in the orogen is centered over the Yakataga fold-and-thrust belt. Strain rates there are in the range of -0.40 to -0.50 microstrain/yr. Little significant strain is seen across the Bagley icefield or to the north of that feature. These results suggest that most of the convergence across the St. Elias orogen is currently accommodated on structures located south of the Bagely icefield, specifically in the Icy Bay, upper Malaspina/Mt. St. Elias, and Yakataga fold-and-thrust belt regions. We use block modeling techniques to describe the tectonic elements of the St. Elias orogen and connect them with the tectonic regime in southeast Alaska. Our preliminary results indicate that a single thrust fault through Icy Bay cannot explain the data there; multiple NW and N directed thrust faults through Icy Bay, along the western edge of the Malaspina Glacier

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

Intermediate P/T-type regional metamorphism of the Isua Supracrustal Belt, southern west Greenland: The oldest Pacific-type orogenic belt?

NASA Astrophysics Data System (ADS)

Arai, Tatsuyuki; Omori, Soichi; Komiya, Tsuyoshi; Maruyama, Shigenori

2015-11-01

The 3.7-3.8 Ga Isua Supracrustal Belt (ISB), southwest Greenland, might be the oldest accretionary complex on Earth. Regional metamorphism of the ISB has a potential to constrain the tectonothermal history of the Earth during the Eoarchean. Chemical and modal analyses of metabasite in the study area (i.e., the northeast part of the ISB) show that the metamorphic grade increases from greenschist facies in the northern part of the study area to amphibolite facies in the southern part. To determine the precise metamorphic P-T ranges, isochemical phase diagrams of minerals of metabasite were made using Perple_X. A synthesis of the estimated metamorphic P-T ranges of the ISB indicates that both the metamorphic pressure and temperature increase systematically to the south in the study area from 3 kbar and 380 °C to 6 kbar and 560 °C. The monotonous metamorphic P-T change suggests that the northeast part of the ISB preserves regional metamorphism resulting from the subduction of an accretionary complex although the ISB experienced metamorphic overprints during the Neoarchean. Both the presence of the regional metamorphism and an accretionary complex having originating at subduction zone suggest that the ISB may be the oldest Pacific-type orogenic belt. The progressive metamorphism can be considered as a record of intermediate-P/T type geothermal gradient at the subduction zone in the Eoarchean. Intermediate-P/T type geothermal gradient is typical at the current zones of subducting young oceanic crust, such as in the case of the Philippine Sea Plate in the southwest part of Japan. Considering the fact that almost all metamorphisms in the Archean are greenschist-amphibolite facies, the intermediate-P/T type geothermal gradient at the ISB might have been worldwide in the Archean. This would indicate that the subduction of young micro-plates was common because of the vigorous convection of hot mantle in the Archean.

Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps

NASA Astrophysics Data System (ADS)

Roda, Manuel; Zucali, Michele; Li, Zheng-Xiang; Spalla, Maria Iole; Yao, Weihua

2018-06-01

In the Southern Steep Belt, Italian Central Alps, relicts of the pre-Alpine continental crust are preserved. Between Valtellina and Val Camonica, a poly-metamorphic rock association occurs, which belongs to the Austroalpine units and includes two classically subdivided units: the Languard-Campo nappe (LCN) and the Tonale Series (TS). The outcropping rocks are low to medium grade muscovite, biotite and minor staurolite-bearing gneisses and micaschists, which include interlayered garnet- and biotite-bearing amphibolites, marbles, quartzites and pegmatites, as well as sillimanite-bearing gneisses and micaschists. Permian intrusives (granitoids, diorites and minor gabbros) emplaced in the metamorphic rocks. We performed a detailed structural, petrological and geochronological analysis focusing on the two main lithotypes, namely, staurolite-bearing micaschists and sillimanite-bearing paragneisses, to reconstruct the Variscan and Permian-Triassic history of this crustal section. The reconstruction of the tectono-metamorphic evolution allows for the distinction between two different tectono-metamorphic units during the early pre-Alpine evolution (D1) and predates the Permian intrusives, which comprise rocks from both TS and LCN. In the staurolite-bearing micaschists, D1 developed under amphibolite facies conditions (P = 0.7-1.1 GPa, T = 580-660 °C), while in the sillimanite-bearing paragneisses formed under granulite facies conditions (P = 0.6-1.0 GPa, T> 780 °C). The two tectono-metamorphic units coupled together during the second pre-Alpine stage (D2) under granulite-amphibolite facies conditions at a lower pressure (P = 0.4-0.6 GPa, T = 620-750 °C) forming a single tectono-metamorphic unit (Languard-Tonale Tectono-Metamorphic Unit), which comprised the previously distinguished LCN and TS. Geochronological analyses on zircon rims indicate ages ranging between 250 and 275 Ma for D2, contemporaneous with the emplacement of Permian intrusives. This event developed under

Imaging of the Main Himalayan Thrust and Moho beneath Satluj Valley, Northwest Himalaya

NASA Astrophysics Data System (ADS)

Wadhawan, M.; Hazarika, D.; Paul, A.; Kumar, N.

2016-12-01

The ongoing continental collision between India and Eurasia gave rise to the formation of the great Himalayan fold-thrust belt. Satluj valley is found to be well exposed from foreland to Higher Himalayan Crystalline series along the Satluj River. Receiver function method has been utilized to image crustal features using Common Conversion Point (CCP) stacking beneath Satluj valley recorded by a seismological array of 18 broadband seismometers. The seismological stations cover the geotectonic units starting from the Himalayan Frontal Thrust (HFT) in the south to the Tethyan Himalaya (TH) to the north. The study inferred gentle northward dipping nature of the Main Himalayan Thrust (MHT) between Sub Himalaya (SH) and Higher Himalaya (HH) in the study area rather than flat-ramp-flat geometry as reported in Nepal Himalaya and Garhwal Himalaya. The depth of the MHT obtained from CCP image and inversion of receiver functions shows that it varies from 16 km in the SH to 27 km near the STD which further increases up to 38 km beneath the TH. The absence of both large and moderate magnitude earthquakes in the Himalayan Seismic Belt (HSB) straddling northern Lesser Himalaya and southern Higher Himalaya in Satluj valley is correlated with absence of ramp structure in this part of HSB. The CCP image has mapped the Moho discontinuity at 44 km depth near the HFT which has increased to 62 km beneath the TH. An extremely low shear wave velocity ranging between 0.8 and 1.8 km s-1 is estimated at stations near the HFT, in the upper most 3-4 km of the crust which indicates the effect of sedimentary column of Indo-Gangetic plains. An intra crustal low velocity layer (IC-LVL) is observed beneath the study profile and inferred as partial melt and/or aqueous fluid at mid-crustal depth beneath the TH. The H-K stacking is applied and average Poisson's ratio is observed to be higher in the TH as compared to the stations to the south of STD.

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

Age, tectonic setting, and metallogenic implication of Phanerozoic granitic magmatism at the eastern margin of the Xing'an-Mongolian Orogenic Belt, NE China

NASA Astrophysics Data System (ADS)

Chen, Cong; Ren, Yunsheng; Zhao, Hualei; Yang, Qun; Shang, Qingqing

2017-08-01

The eastern margin of the Xing'an-Mongolian Orogenic Belt is characterised by widespread Phanerozoic granitic magmatism, some of which is closely related to significant ore mineralisation. This paper presents new geochronological, petrogenetic, and tectonic data for selected intrusions. Zircon U-Pb geochronology for five granitoid plutons indicates they were emplaced during the middle-late Permian (264-255 Ma) and Cretaceous (106-94 Ma), and thus granitic magmatism occurred throughout the Phanerozoic, Permian (268-252 Ma), Early-Middle Triassic (248-240 Ma), Early Jurassic (183 Ma), and Cretaceous (112-94 Ma). The Permian granitoids consist of monzogranite, granodiorite, tonalite, and quartz diorite, characterised by enrichment in Na2O (3.60-4.72 wt.%), depletion in K2O (0.97-2.66 wt.%), and a negative correlation between P2O5 and SiO2. Together with the presence of hornblende, these geochemical features are indicative of an I-type affinity. The Permian granitic magmatism is associated with quartz-vein-type tungsten deposits (252 Ma; unpublished Sm-Nd isochron age), which formed in an active continental margin setting related to subduction of the Palaeo-Asian Ocean. The Cretaceous quartz diorites have an adakitic affinity, having relatively high Sr (374-502 ppm), low Yb (0.51-0.67 ppm) and Y (8.7-10.7 ppm), and high Sr/Y (39.4-46.8) and (La/Yb)N values (16.2-34.7), suggesting that they were related to the partial melting of subducted oceanic crust. In addition, they are associated with porphyry Au-Cu deposits. We conclude that the Cretaceous granitic rocks and associated porphyry Au-Cu mineralisation occurred in an extensional tectonic setting related to the subduction of the Palaeo-Pacific Plate beneath the Eurasian Plate. In addition, the large-scale Early-Middle Triassic syn-collisional granite belt at the eastern margin of the Xing'an-Mongolian Orogenic Belt extends from the middle of Jilin Province to the Wangqing-Hunchun region, constraining the timing of the

Granulite belts of Central India with special reference to the Bhopalpatnam Granulite Belt: Significance in crustal evolution and implications for Columbia supercontinent

NASA Astrophysics Data System (ADS)

Vansutre, Sandeep; Hari, K. R.

2010-11-01

The Central Indian collage incorporates the following major granulite belts: (1) the Balaghat-Bhandara Granulite Belt (BBG), (2) the Ramakona-Katangi Granulite Belt (RKG), (3) the Chhatuabhavna Granulite (CBG) of Bilaspur-Raigarh Belt, (4) the Makrohar Granulite Belt (MGB) of Mahakoshal supracrustals, (5) the Kondagaon Granulite Belt (KGGB), (6) the Bhopalpatnam Granulite Belt (BGB), (7) the Konta Granulite Belt (KTGB) and (8) the Karimnagar Granulite Belt (KNGB) of the East Dharwar Craton (EDC). We briefly synthesize the general geologic, petrologic and geochronologic features of these belts and explain the Precambrian crustal evolution in Central India. On the basis of the available data, a collisional relationship between Bastar craton and the EDC during the Paleo-Mesoproterozoic is reiterated as proposed by the earlier workers. The tectonic evolution of only few of the orogenic belts (BGB in particular) of Central India is related to Columbia.

Shoshonites and Associated Calc-Alkaline Rocks from the Eastern Sayan, Central Asian Orogenic Belt: Geochemistry and Tectonic Setting

NASA Astrophysics Data System (ADS)

Vernikovskaya, A. E.; Romanov, M. I.; Kadilnikov, P. I.; Matushkin, N. Y.; Romanova, I.

2017-12-01

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens in the world, which formation started in the Neoproterozoic giving rise to numerous assemblages of island arcs, ophiolites, continental fragments and sedimentary basins. The Eastern Sayan, located at the southwestern margin of the Siberian craton, is the key area in understanding the initiation of orogenic processes in the CAOB. Widely distributed mafic igneous rocks (dolerites, gabbro etc.) in the Eastern Sayan were previously considered as part of the Nersa igneous complex of the Neoproterozoic age, whereas tectonic setting of these rocks remained highly debatable. New geochemical and mineralogical data from igneous mafic rocks within the Eastern Sayan show presence of rocks with shoshonitic and high- and low-K calc-alkaline affinities and allowed us to refine the tectonic context of their formation in the southwestern margin of the Siberian craton.All studied intrusive and volcanic rocks in the Eastern Sayan showing OIB-like geochemical signatures. The high-K rocks contain orthoclase, olivine, diopside, augite, anorthite, various amphiboles, including edenite, cataphorite, Mg-cataphorite, anthophyllite-gedrite, Mg-Fe hornblende, biotites of the siderophyllite-eastonite-annite series, as well as zircon, baddeleyite, apatite, magnetite, ilmenite and Cr-spinel. The high-K rock type is characterised by high K2O contents (up to 9.2 wt. %), K2O/Na2O ratios over 90, lowered TiO2 and MgO and moderate FeO contents and negative P and Sr anomalies. In contrast, low-K rocks, characterised by moderate and increased TiO2 and MgO contents, contain augite, pigeonite, olivine, andesine and accessory minerals, such as rutile, titanite, ilmenite and apatite. Both rock types vary considerably in Nb and Ta concentrations, from OIB-like to E-MORB. Such geochemical signatures of calc-alkaline and shoshonitic igneous rocks are indicative of an active continental margin setting. Presence of the active

Lithologically controlled detachment strength and steady state since 10 Ma: Exploring the Alpine wedge using critical taper analysis combined with low-temperature thermochronometry

NASA Astrophysics Data System (ADS)

von Hagke, Christoph; Oncken, Onno; Ortner, Hugo; Cederbom, Charlotte

2014-05-01

Although evidence for weak detachments underlying foreland thrust belts exists, very little is known about the lateral variations in effective strength, as well as the geological nature of such variations. Using critical taper analysis, we show that a detailed and systematic measurement of surface slope of the Central European Alps reveals variations in strength along the detachment, based on the argument that the Alps are close to the critical state. We show that the basal detachment is very weak near the deformation front but strengthens towards the hinterland. These changes in detachment strength coincide with changes of detachment lithology in the hangingwall and footwall respectively, emphasizing the dominant role of weak shales. The very low strength values we find in shales in the frontal part of the alpine sole detachment are caused partly by slightly elevated pore pressures but may also require additional mechanisms of dynamic weakening. Using the constraints on the present day alpine taper, we investigate the change in taper through time. To this end, we produced new apatite fission track and apatite (U-Th)/He data from the Alpine orogenic front, the Austrian Subalpine Molasse. We combine this data set with existing thermochronometry, and reconstruct the Central Alpine pro-wedge geometry at 10 Ma. We show that the taper of the Central Alps has not changed significantly and presumably remained close to kinematic and mass flux steady state since then. This indicates a feedback between ongoing shortening and erosion at low rates during the Late Neogene to present.

New geochronological constraints on the thermal and exhumation history of the Lesser and Higher Himalayan Crystalline Units in the Kullu–Kinnaur area of Himachal Pradesh (India)

PubMed Central

Thöni, M.; Miller, C.; Hager, C.; Grasemann, B.; Horschinegg, M.

2012-01-01

New geochronological, petrological and structural data from the Beas–Sutlej area of Himachal Pradesh (India) are used to reconstruct the tectonothermal and exhumation history of this part of the Himalayan orogen. Sm–Nd garnet ages at 40.5 ± 1.3 Ma obtained on a pegmatoid from the inverse metamorphic High Himalayan Crystalline (HHC) in the Malana–Parbati area probably mark local melting during initial decompression. Ongoing exhumation in ductilely deformed leuco-gneiss is constrained by Sm–Nd garnet ages at 29 ± 1 Ma and white mica Rb–Sr ages around 24–20 Ma, while Bt Rb–Sr ages indicate a drop of regional metamorphic temperatures below 300 °C between 15 and 12 Ma. The deep Sutlej gorge exposes medium-grade paragneisses and Proterozoic orthogneisses of the Lesser Himalayan Crystalline (LHC), overthrust by the HHC along the Main Central Thrust (MCT). Mica cooling ages in the HHC are in the range of 14–11 Ma. Above the extruded wedge of the HHC, the Leo Pargil leucogranite and associated dykes intrude the Haimanta Unit (HU) below the weakly metamorphic Palaeo-Mesozoic sediments of the Tethyan Himalayas (TH). The Leo Pargil leucogranite yielded a mean Sm–Nd garnet age of 19 ± 1 Ma and Rb–Sr muscovite and biotite cooling ages between 16.4 and 11.6 Ma. Marked young extrusion of LHC units resulted in differentiated exhumation/cooling of more frontal parts of the orogen. Very young ductile deformation of LHC gneisses near Wangtu is constrained by late-kinematic pegmatite intrusions crosscutting the main mylonitic foliation. Sm–Nd garnet and Rb–Sr muscovite ages of these pegmatites range between 7.9 ± 0.9 and 5.5 ± 0.1 Ma. Published apatite FT ages down to 0.6 Ma also document accelerated diachronous sub-recent exhumation of different parts of the orogen. Together with geochronological data from the literature, the new results demonstrate that the HHC and the HU were deformed by shortening and crustal thickening during the

Ages and origin of felsic rocks from the Eastern Erenhot ophiolitic complex, southeastern Central Asian Orogenic Belt, Inner Mongolia China

NASA Astrophysics Data System (ADS)

Yang, Jinfu; Zhang, Zhicheng; Chen, Yan; Yu, Haifei; Qian, Xiaoyan

2017-08-01

The Central Asian Orogenic Belt (CAOB) is known for its massive Phanerozoic generation of juvenile crust. The tectonic evolution of the CAOB during the late Paleozoic era is still debated. The Eastern Erenhot ophiolite complex (EOC) has been recognized as one of the numerous late Paleozoic ophiolitic blocks in the southeastern part of the CAOB. Zircon U-Pb dating on rhyolite and plagiogranite from the EOC yielded a tight range of ages from 360 to 348 Ma, indicating that the complex formed in the early Carboniferous. The primitive mantle-normalized spider diagram of rhyolites (εNd(t) values of +6.8 and +7) and basalts almost overlaps. Such rhyolites may have been derived from partial melting of juvenile basaltic rocks during the initial opening of the Erenhot-Hegenshan oceanic basin. All of the plagiogranites exhibit similar trace element behaviours of High Field-Strength Elements, such as U, Zr and Hf, and Large Ion Lithophile Elements, such as Ba and Rb, to these of gabbros. These plagiogranites were considered products of episodes of partial melting of hydrous gabbros during ocean floor spreading. We conclude that the northern subduction of the Paleo-Asian Ocean stopped before 360 Ma and the southeastern CAOB experienced extension during the late Paleozoic era. The Erenhot-Hegenshan Ocean, which is comparable to the present Red Sea, originated from syn-collisional crustal thickening, subsequent lithosphere extension, and upwelling of the asthenosphere during orogenic quiescence with an age of 20 Ma.

Paleozoic Orogens of Mexico and the Laurentia-Gondwana Connections: an Update

NASA Astrophysics Data System (ADS)

Ortega-Gutierrez, F.

2009-05-01

The present position of Mexico in North America and the fixist tectonic models that prevailed prior to the seventies of the past century, have considered the main Paleozoic tectonic systems of Mexico as natural extensions of the orogens that fringed the eastern and southern sides of the Laurentian craton. Well known examples of pre-Mesozoic orogens in Mexico are the Oaxacan, Acatlan, and Chiapas polymetamorphic terranes, which have been correlated respectively with the Grenville and Appalachian-Ouachitan orogens of eastern North America. Nonetheless, several studies conducted during the last decade in these Mexican orogenic belts, have questioned their Laurentian connections, regarding northwestern Gondwana instead as the most plausible place for their birth and further tectonic evolution. This work pretends to approach the problem by briefly integrating the massive amount of new geological information, commonly generated through powerful dating methods such as LA-ICPM-MS on detrital zircon of sedimentary and metasedimentary units in the Paleozoic crustal blocks, which are widely exposed in southern and southeastern Mexico. The Acatlan Complex bears the closest relationships to the Appalachian orogenic system because it shows thermotectonic evidence for opening and closure of the two main oceans involved in building the Appalachian mountains in eastern Laurentia, whereas two other Paleozoic terranes in NW and SE Mexico, until recently rather geologically unknown, may constitute fundamental links between the Americas for the last-stage suturing and consolidation of western Pangea. The buried basement of the Yucatan platform (400,000 squared km) on the other hand, remains as one of the most relevant problems of tectonostratigraphic correlations across the Americas, because basement clasts from the Chicxulub impact ejecta reveal absolute and Nd-model ages that suggest close Gondwanan affinities. Major changes in the comprehension of the Paleozoic orogens in Mexico

Structural profile reconstructions and thermal metamorphic evolution in the slate belt of southern Hsuehshan Range in the active Taiwan mountain belt

NASA Astrophysics Data System (ADS)

Wu, Yu; Chen, Chih-Tung; Lee, Jian-Cheng; Shyu, J. Bruce H.

2017-04-01

The fate of passive continental margin in collisional orogens is crucial in understanding tectonic evolution of mountain belts. The active arc-continent collision of Taiwan is considered as a model case in studying mountain building processes, and largely consists of deformed margin basement and cover series. Among the whole orogeny belt, the slate belt of the Hsuehshan Range (HR) is a prominent large-scale pop-up structural on the prowedge part of the orogen, and is composed of metamorphosed Eocene to Miocene sediments which experienced only the Neogene Taiwan orogeny after diagenesis in margin graben. Characterizing the metamorphic history of the HR is essential for reconstructing its geological evolution during the mountain building processes. However, previous studies were mostly focused on northern and central HR, structural investigation coupled with metamorphic documentation in the southern part of HR, which is the most active part of the orogeny belt, is therefore targeted in this work. Since carbonaceous material is common in pelitic protolith of HR slates, the Raman spectrum of carbonaceous material (RSCM) measuring the rock peak temperature is chosen for quantitative thermal metamorphic documentation. In this study, we reconstruct a geological structural profile in western central Taiwan across the prowedge part of the mountain belt containing the southern HR by combining the surface geological data, well log records and published seismic reflection profiles. Although most of the existing data are concentrated in the fold-and-thrust belt, they are now reinforced by new field structural measurements and RSCM samples in the southern HR. In total 27 RSCM samples were collected along 2 transects perpendicular to the average strike with a dense interval about 2 km. The results allow us to map peak temperature distribution across southern HR, and provide new constraints for structural profile reconstruction and reappraisal of the structural evolution of the HR

U-Pb Dating and Lu-Hf Isotopes of Detrital Zircons From the Southern Sikhote-Alin Orogenic Belt, Russian Far East: Tectonic Implications for the Early Cretaceous Evolution of the Northwest Pacific Margin

NASA Astrophysics Data System (ADS)

Liu, Kai; Zhang, Jinjiang; Wilde, Simon A.; Liu, Shiran; Guo, Feng; Kasatkin, Sergey A.; Golozoubov, Vladimir V.; Ge, Maohui; Wang, Meng; Wang, Jiamin

2017-11-01

The Sikhote-Alin orogenic belt in Russian Far East is comprised of several N-S trending belts, including the Late Jurassic to Early Cretaceous accretionary prisms and turbidite basin which are now separated by thrusts and strike-slip faults. The origin and collage of the belts have been studied for decades. However, the provenance of the belts remains unclear. Six sandstone samples were collected along a 200 km long east-west traverse across the major belts in the southern Sikhote-Alin for U-Pb dating and Lu-Hf isotope analysis to constrain the provenance and evaluate the evolution of the northwest Pacific margin at this time. The result reveals that the sediments from the main Samarka belt was mainly from the adjacent Bureya-Jiamusi-Khanka Block (BJKB); the eastern Samarka belt and the Zhuravlevka turbidite basin were supplied by detritus from both the North China Craton (NCC) and the BJKB; the Taukha belt was mainly fed by sediments from the NCC; whereas the data from the Sergeevka nappes are insufficient to resolve their provenance. In the Late Jurassic to Early Cretaceous, collision and subduction was important in the initial collage of most belts in Sikhote-Alin. However, merely E-W trending collage cannot explain the increasing importance of the NCC provenance from west to east. It is proposed that the main Samarka belt was located adjacent to the BJKB when deposited, whereas the other belts were farther south to accept the materials from the NCC. Sinistral strike-slip faulting transported the eastern belts northward after their initial collage by thrusting.

Sink to survive: The persistence of ancient mountain belts through crustal density changes

NASA Astrophysics Data System (ADS)

Blackburn, T. J.; Ferrier, K.; Perron, J.

2012-12-01

Mountain belts form when collisions between continents thicken the Earth's crust, which buoyantly rises to remain in isostatic equilibrium with the underlying asthenosphere. Just as isostasy leads to the birth of mountain belts, it contributes to their destruction by responding to erosion with rock uplift, which in turn promotes further erosion. If the continental crust consisted of a single layer of constant density, erosion and isostatic rebound would continue thinning the crust until it was completely eroded. Such total destruction evidently does not happen, however, as the roots of Earth's oldest mountains have persisted for billions of years. One explanation for this preservation is that an orogen's isostatic response to erosion decreases over time as the crust increases in density as the lower crust undergoes metamorphic phase changes that accompany lithosphere cooling. The implication of this hypothesis is that erosion rates in mountain belts are linked to the thermal and density evolution of the lithosphere. We test this hypothesis with a global compilation of exhumation rates and erosion rates determined from published apatite fission track and cosmogenic 10Be measurements in collisional orogens ranging in formation age from 0 to 2 billion years. We compare these data to a numerical model of the thermal, density and erosional evolution of a decaying mountain belt. Measured and modeled data indicate that erosion is fastest in young, hot, low-density, and topographically high mountain belts, and that erosion rates decrease dramatically after 200-300 million years (My). This 200-300 My timescale is consistent with titanite U-Pb thermochronologic data from lower crustal xenoliths, which record cooling to temperatures consistent with garnet growth and crustal densification (~650 °C) within 200-300 My after orogenesis. For the same orogens, Sm-Nd and/or Lu-Hf garnet-whole rock isochron dates constrains lower crustal garnet growth and a corresponding crustal

Peering into the deep: Illuminating the crustal evolution of the Eucla basement and its relationship to the Albany-Fraser Orogen of southwest Australia.

NASA Astrophysics Data System (ADS)

Hartnady, Michael; Kirkland, Chris; Clark, Chris; Spaggiari, Catherine; Smithies, Hugh

2017-04-01

The Albany-Fraser Orogen is a 1200 km long east to northeasterly trending Palaeoproterozoic to Mesoproterozoic orogenic belt that defines the southern to southeastern margin of the West Australian Craton (WAC). The belt records a long and complex geological history spanning the break-up of Nuna between 2000 and 1700 Ma and amalgamation of Rodinia between 1300 and 1000 Ma. Recent geochronological, geochemical and isotopic work has shown that the Albany-Fraser Orogen formed through a protracted period of reworking of the margin of the Archean Yilgarn Craton (part of the WAC) with various additions of mantle-derived material. The Cretaceous Bight and Cenozoic Eucla Basins partially overlie the northeastern part of the Albany-Fraser Orogen and completely cover 1000 km of crystalline basement (the Eucla basement) that separates the belt from the South Australian Craton. This basement constitutes the glue between the major building blocks of Proterozoic Australia, yet, its geological history is poorly understood. New drill cores penetrating the basement have intersected interlayered granitic and gabbroic rocks that yield U-Pb zircon dates that are dissimilar to any magmatic ages from units within the adjoining Albany-Fraser Orogen, with the exception of the youngest, 1190-1125 Ma magmatic suite. In addition, mantle-like hafnium and neodymium isotopic signatures indicate that the rocks of the Eucla basement are dominated by new juvenile addition, and may represent an allochthonous terrane of oceanic heritage. New ɛHf contour maps for the Albany-Fraser Orogen and Eucla basement highlight this difference. Time-slicing the isotopic dataset reveals a pattern of Palaeoproterozoic juvenile magmatism sub-perpendicular to the present day structural grain in the belt. If this marks the presence of an older lithospheric structure then it demonstrates the power that time-constrained isotopic mapping provides for illuminating lithospheric architecture through time. This may be

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

The Zagros hinterland fold-and-thrust belt in-sequence thrusting, Iran

NASA Astrophysics Data System (ADS)

Sarkarinejad, Khalil; Ghanbarian, Mohammad Ali

2014-05-01

The collision of the Iranian microcontinent with the Afro-Arabian continent resulted in the deformation of the Zagros orogenic belt. The foreland of this belt in the Persian Gulf and Arabian platform has been investigated for its petroleum and gas resource potentials, but the Zagros hinterland is poorly investigated and our knowledge about its deformation is much less than other parts of this orogen. Therefore, this work presents a new geological map, stratigraphic column and two detailed geological cross sections. This study indicates the presence of a hinterland fold-and-thrust belt on northeastern side of the Zagros orogenic core that consists of in-sequence thrusting and basement involvement in this important part of the Zagros hinterland. The in-sequence thrusting resulted in first- and second-order duplex systems, Mode I fault-bend folding, fault-propagation folding and asymmetric detachment folding which indicate close relationships between folding and thrusting. Study of fault-bend folds shows that layer-parallel simple shear has the same role in the southeastern and northwestern parts of the study area (αe = 23.4 ± 9.1°). A major lateral ramp in the basement beneath the Talaee plain with about one kilometer of vertical offset formed parallel to the SW movement direction and perpendicular to the major folding and thrusting.

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

NASA Astrophysics Data System (ADS)

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

2014-10-01

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.

Structural and metamorphic evolution of serpentinites and rodingites recycled in the Alpine subduction wedge

NASA Astrophysics Data System (ADS)

Zanoni, D.; Rebay, G.; Spalla, M. I.

2015-12-01

Hydration-dehydration of mantle rocks affects the viscosity of the mantle wedge and plays a prominent role in subduction zone tectonics, facilitating marble cake-type instead of large-slice dynamics. An accurate structural and petrologic investigation of serpentinites from orogenic belts, supported by their long-lived structural memory, can help to recognize pressure-sensitive mineral assemblages for deciphering their P-prograde and -retrograde tectonic trajectories. The European Alps preserve large volumes of the hydrated upper part of the oceanic lithosphere that represents the main water carrier into the Alpine subduction zone. Therefore, it is important to understand what happens during subduction when these rocks reach P-T conditions proximal to those that trigger the break-down of serpentine, formed during oceanic metamorphism, to produce olivine and clinopyroxene. Rodingites associated with serpentinites are usually derived from metasomatic ocean floor processes but rodingitization can also happen in subduction environments. Multiscale structural and petrologic analyses of serpentinites and enclosed rodingites have been combined to define the HP mineral assemblages in the Zermatt-Saas ophiolites. They record 3 syn-metamorphic stages of ductile deformation during the Alpine cycle, following the ocean floor history that is testified by structural and metamorphic relics in both rock types. D1 and D2 developed under HP to UHP conditions and D3 under lower P conditions. Syn-D2 assemblages in serpentinites and rodingites indicate conditions of 2.5 ± 0.3 GPa and 600 ± 20°C. This interdisciplinary approach shows that the dominant structural and metamorphic imprint of the Zermatt-Saas eclogitized serpentinites and rodingites developed during the Alpine subduction and that subduction-related serpentinite de-hydration occurred exclusively at Pmax conditions, during D2 deformation. In contrast, in the favourable rodingite bulk composition (Ca-rich), hydrated minerals

A Silurian-early Devonian slab window in the southern Central Asian Orogenic Belt: Evidence from high-Mg diorites, adakites and granitoids in the western Central Beishan region, NW China

NASA Astrophysics Data System (ADS)

Zheng, Rongguo; Xiao, Wenjiao; Li, Jinyi; Wu, Tairan; Zhang, Wen

2018-03-01

The Beishan orogenic belt is a key region for deciphering the accretionary processes of the southern Central Asian Orogenic Belt. Here in this paper we present new zircon U-Pb ages, bulk-rock major and trace element, and zircon Hf isotopic data for the Baitoushan, and Bagelengtai plutons in the western Central Beishan region to address the accretionary processes. The Baitoushan pluton consists of quartz diorites, monzonites and K-feldspar granites, with zircon LA-ICP-MS U-Pb ages of 435 Ma, 421 Ma and 401 Ma, respectively. The Baitoushan quartz diorites and quartz monzonites exhibit relatively high MgO contents and Mg# values (63-72), display enrichments in LILEs and LREEs, and exhibit high Ba (585-1415 ppm), Sr (416-570 ppm) and compatible element (such as Cr and Ni) abundances, which make them akin to typical high-Mg andesites. The Baitoushan quartz diorites and quartz monzonites were probably generated by the interaction of subducted oceanic sediment-derived melts and mantle peridotites. The Baitoushan K-feldspar granites are ascribed to fractionated I-type granites with peraluminous and high-K calc-alkaline characteristics. They exhibit positive εHf(t) values (2.43-7.63) and Mesoproterozoic-Neoproterozoic zircon Hf model ages (0.92-1.60 Ga). Those early Devonian granites, including Baitoushan K-feldspar granite and Gongpoquan leucogranites (402 Ma), are derived from melting of the mafic lower crust and/or sediments by upwelling of hot asthenospheric mantle. The Bagelengtai granodiorites exhibit similar geochemical signatures with that of typical adakites, with a zircon SHRIMP U-Pb age of 435 Ma. They exhibit relatively high Sr (502-628 ppm) and Al2O3 (16.40-17.40 wt.%) contents, and low MgO (1.02-1.29 wt.%), Y (3.37-6.94 ppm) and HREEs contents, with relatively high Sr/Y and (La/Yb)N ratios. The Bagelengtai granodiorites were derived from partial melting of subducted young oceanic crust, with significant contributions of subducted sediments, subsequently

Origins of two types of serpentinites from the Qinling orogenic belt, central China and associated fluid/melt-rock interactions

NASA Astrophysics Data System (ADS)

Wu, Kai; Ding, Xing; Ling, Ming-Xing; Sun, Wei-dong; Zhang, Li-Peng; Hu, Yong-Bin; Huang, Rui-Fang

2018-03-01

Serpentinites are important volatile and fluid mobile element repositories in oceanic lithosphere and subduction zones, and thus provide significant constraints on global geochemical cycles and tectonic evolution at convergent margins. In this contribution, two types of serpentinites from the Mianlue suture zone in the Qinling orogenic belt, central China, are identified on the basis of detailed mineralogical and geochemical study. Serpentinites from the Jianchaling region (Group 1) are composed of lizardite/chrysotile + magnesite + magnetite. Most of these serpentinites (Group 1a), consist of pseudomorphic orthopyroxene and olivine, and are characterized by low Al2O3/SiO2, high MgO/SiO2 and Ir-type PGEs to Pt ratios, suggesting a residual mantle origin. Meanwhile, the U-shape REE pattern and positive Eu, Sr and Ba anomalies of these serpentinites indicate that serpentinization fluids have interacted with gabbroic cumulates at moderately high temperatures or associate with the chlorinity and redox conditions of the fluid. Considering the limited mobility of U in the hydrating fluids for the Group 1a serpentinites, hydrating fluids for these serpentinites are most likely derived from the dehydrated slab, and have been in equilibrium with subducting sediments. There are also some serpentinites with low-grade metamorphic recrystallization from the Jianchaling region (Group 1b), represented by recrystallized serpentine minerals (antigorite). The trace element compositions of these Group 1b serpentinites suggest that partial dehydration of serpentinites associated with the transformation from lizardite to antigorite in subduction zone is also likely to affect the geochemistry of serpentinites. Serpentinites from the Liangyazi region (Group 2) are composed of antigorite + dolomite + spinel + magnetite. The high Cr number (0.65-0.80) and low Ti concentrations of spinels in Group 2 serpentinites indicate a refractory mantle wedge origin. Fertile major element compositions

Chemical evolution of Himalayan leucogranites based on an O, U-Pb and Hf study of zircon

NASA Astrophysics Data System (ADS)

Hopkinson, Thomas N.; Warren, Clare J.; Harris, Nigel B. W.; Hammond, Samantha J.; Parrish, Randall R.

2015-04-01

Crustal melting is a characteristic process at convergent plate margins, where crustal rocks are heated and deformed. Miocene leucogranite sheets and plutons are found intruded into the high-grade metasedimentary core (the Greater Himalayan Sequence, GHS) across the Himalayan orogen. Previously-published Himalayan whole-rock data suggest that these leucogranites formed from a purely meta-sedimentary source, isotopically similar to those into which they now intrude. Bulk rock analyses carry inherent uncertainties, however: they may hide contributions from different contributing sources, and post-crystallization processes such as fluid interaction may significantly alter the original chemistry. In contrast, zircon is more able to retain precise information of the contributing sources of the melt from which it crystallises whilst its resistant nature is impervious to post-magmatic processes. This multi-isotope study of Oligocene-Miocene leucogranite zircons from the Bhutan Himalaya, seeks to differentiate between various geochemical processes that contribute to granite formation. Hf and O isotopes are used to detect discrete changes in melt source while U-Pb isotopes provide the timing of zircon crystallisation. Our data show that zircon rims of Himalayan age yield Hf-O signatures that lie within the previously reported whole-rock GHS field, confirming the absence of a discernible mantle contribution to the leucogranite source. Importantly, we document a decrease in the minimum ɛHf values during Himalayan orogenesis through time, correlating to a change in Hf model age from 1.4 Ga to 2.4 Ga. Nd model ages for the older Lesser Himalayan metasediments (LHS) that underthrust the GHS are significantly older than those for the GHS (2.4-2.9 Ga compared with 1.4-2.2 Ga), and as such even minor contributions of LHS material incorporated into a melt would significantly increase the resulting Hf model age. Hence our leucogranite data suggest either a change of source within

Geosphere - Cryosphere Interactions in the Saint Elias orogen, Alaska and Yukon (Invited)

NASA Astrophysics Data System (ADS)

Bruhn, R. L.; Sauber, J. M.; Forster, R. R.; Cotton, M. M.

2009-12-01

North America's largest alpine and piedmont glaciers occur in the Saint Elias orogen, where microplate collision together with the transition from transform faulting to subduction along the North American plate boundary, create extreme topographic relief, unusually high annual precipitation by orographic lift, and crustal displacements induced by both tectonic and glacio-isostatic deformation. Lithosphere-scale structure dominates the spatial pattern of glaciation; the piedmont Bering and Agassiz-Malaspina glaciers lay along deeply eroded troughs where reverse faults rise from the underlying Aleutian megathrust. The alpine Seward and Bagley Ice Valley glaciers flow along an early Tertiary plate boundary that has been reactivated by reverse faulting, and also by dextral shearing at the NW end of the Fairweather transform fault. Folding above a crustal-scale fault ramp near Icy Bay localizes orographic uplift of air masses, creating alpine glaciers that spill off the highlands into large ice falls, and rapidly dissect evolving structure by erosion. The rate and orientation of ice surface velocities, and the location of crevassing and folding partly reflect changes in basal topography of the glaciers caused by differential erosion of strata, and juxtaposition of variably oriented structures across faults. The effects of basal topography on ice flow are investigated using remote sensing measurements and analog models of glacier flow over uneven topography. Deformation of the ice in turn affects englacial hydrology and sub-ice fluvial systems, potentially impacting ice mass balance, on-set of surging, and loci of glacier quakes. The glaciers impact tectonics by localizing uplift and exhumation within the orogen, and modulating tectonic stress fields as ice masses wax and wane. This is particularly evident in crustal seismicity rates at annual to decadal time scales, while stratigraphy of coastal terraces record both earthquake deformation and glacial isostasy over

Petrogenesis and tectonic implications of Early Cretaceous volcanic rocks from Lingshan Island in the Sulu Orogenic Belt

NASA Astrophysics Data System (ADS)

Meng, Yuanku; Santosh, M.; Li, Rihui; Xu, Yang; Hou, Fanghui

2018-07-01

The Dabie-Sulu orogenic belt in eastern China marks the boundary between the Yangtze Block and the North China Block. Here we investigate a suite of volcanic rocks from Lingshan Island in the Sulu belt comprising rhyolite, trachyte, trachyandesite and basaltic trachyandesite. We present petrological, geochemical and zircon Usbnd Pb ages and Hfsbnd O isotope data with a view to gain insights on the petrogenesis and tectonic implications. SHRIMP II analyses of zircon grains from the rhyolite yield 206Pb/238U age of 127.6 ± 1.3 Ma and LA-MC-ICP-MS dating show 126.3 ± 1.2 Ma and 127.3 ± 1.1 Ma, together constraining the eruption time as Early Cretaceous. LA-MC-ICP-MS analyses of zircon grains from the andesitic rocks yield 206Pb/238U ages of 129.0 ± 1.6 Ma, 129.8 ± 1.5 Ma and 130.9 ± 1.0 Ma. Geochemically, the rhyolite shows shoshonitic features with low MgO and Cr, but high Na2O + K2O. The zircon grains from these rocks yield negative εHf(t) values and low δ18O values, and these together with the presence of Neoproterozoic inherited zircons suggest that the magma source involved melting of the Yangtze crust. The andesitic rocks, including basaltic trachyandesite, trachyandesite and trachyte, show a wide range of SiO2, Mg# values, and Cr, enriched in LILE and LREE, depleted in HFSE (Nb, Ta and Ti), and have significantly negative zircon εHf(t) values, suggesting derivation from subcontinental lithosphere mantle that was metasomatized by felsic melts. Our results, integrated with those from previous studies suggest heterogeneous magma involving the mixing of mantle and crustal sources within an extensional setting in the Early Cretaceous.

Imprints of a Pan-African transpressional orogen superimposed on an inferred Grenvillian accretionary belt in central East Antarctica

NASA Astrophysics Data System (ADS)

Ferraccioli, Fausto; Seddon, Samuel; Finn, Carol; Bell, Robin; Wu, Guochao; Jordan, Tom

2017-04-01

as revealing a right-lateral predominantly transpressional fault system roughly parallel to the previously proposed Gamburtsev Suture flanking the northern domain. Simple magnetic modelling provides support for the existence of potential positive flower structures and basement push ups. An analogy with well-known modern strike-slip fault systems in New Zealand also supports our magnetic interpretation for a transpressional orogenic belt within the composite Gamburtsev Province. We propose that large-scale Pan-African age transpression in interior East Antarctica is kinematically linked with collision of Greater India and a mosaic of distinct lithospheric provinces in East Antarctica. Pan-African transpression likely reactivated pre-existing fault systems that may have formed during Grenvillian-age accretion of arc terranes, as recently hypothesised in the interior of Eastern Dronning Maud Land, in the so called Tonian Ocean Arc Superterrane (Jacobs et al., 2015). By compiling aeromagnetic, airborne gravity, and satellite magnetic and satellite gravity data over the Gamburtserv Province and Eastern Dronning Maud Land we test whether these two areas could potentially be linked together during the inferred Grenvillian and Pan-African accretionary and collisional stages of tectonic evolution.

Tectonic stress evolution in the Pan-African Lufilian Arc and its foreland (Katanga, DRC): orogenic bending, late orogenic extensional collapse and transition to rifting

NASA Astrophysics Data System (ADS)

Kipata, M. L.; Delvaux, D.; Sebagenzi, M. N.; Cailteux, J.; Sintubin, M.

2012-04-01

Between the paroxysm of the Lufilian orogeny at ~ 550 Ma and the late Neogene to Quaternary development of the south-western branch of the East African rift system, the tectonic evolution of the Lufilian Arc and Kundelungu foreland in the Katanga region of the Democratic Republic of Congo remains poorly unknown although it caused important Cu-dominated mineral remobilizations leading to world-class ore deposits. This long period is essentially characterized by brittle tectonic deformations that have been investigated by field studies in open mines spread over the entire arc and foreland. Paleostress tensors were computed for a database of 1450 fault-slip data by interactive stress tensor inversion and data subset separation, and the relative succession of 8 brittle deformation events established. The oldest brittle structures observed are related to the Lufilian brittle compressional climax (stage 1). They have been re-oriented during the orogenic bending that led to the arcuate shape of the belt. Unfolding the stress directions from the first stage allows to reconstruct a consistent NE-SW direction of compression for this stage. Constrictional deformation occurred in the central part of the arc, probably during orogenic bending (Stage 2). After the orogenic bending, a sequence of 3 deformation stages marks the progressive onset of late-orogenic extension: strike-slip deformations (stages 3-4) and late-orogenic arc-parallel extension (stage 5). It is proposed that these 3 stages correspond to orogenic collapse. In early Mesozoic, NW-SE compression was induced by a transpressional inversion, interpreted as induced by far-field stresses generated at the southern active margin of Gondwana (stage 6). Since then, this region was affected by rift-related extension, successively in a NE-SW direction (stage 7, Tanganyika trend) and NW-SE direction (stage 8, Moero trend).

Biogeography of thermophilic phototrophic bacteria belonging to Roseiflexus genus.

PubMed

Gaisin, Vasil A; Grouzdev, Denis S; Namsaraev, Zorigto B; Sukhacheva, Marina V; Gorlenko, Vladimir M; Kuznetsov, Boris B

2016-03-01

Isolated environments such as hot springs are particularly interesting for studying the microbial biogeography. These environments create an 'island effect' leading to genetic divergence. We studied the phylogeographic pattern of thermophilic anoxygenic phototrophic bacteria, belonging to the Roseiflexus genus. The main characteristic of the observed pattern was geographic and geochronologic fidelity to the hot springs within Circum-Pacific and Alpine-Himalayan-Indonesian orogenic belts. Mantel test revealed a correlation between genetic divergence and geographic distance among the phylotypes. Cluster analysis revealed a regional differentiation of the global phylogenetic pattern. The phylogeographic pattern is in correlation with geochronologic events during the break up of Pangaea that led to the modern configuration of continents. To our knowledge this is the first geochronological scenario of intercontinental prokaryotic taxon divergence. The existence of the modern phylogeographic pattern contradicts with the existence of the ancient evolutionary history of the Roseiflexus group proposed on the basis of its deep-branching phylogenetic position. These facts indicate that evolutionary rates in Roseiflexus varied over a wide range. © FEMS 2016. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Pollinator scarcity drives the shift to delayed selfing in Himalayan mayapple Podophyllum hexandrum (Berberidaceae)

PubMed Central

Xiong, Ying-Ze; Fang, Qiang; Huang, Shuang-Quan

2013-01-01

Recent molecular phylogenetics have indicated that American mayapple (mainly self-incompatible, SI) and Himalayan mayapple, which was considered to be self-compatible (SC), are sister species with disjunct distribution between eastern Asia and eastern North America. We test a hypothesis that the persistence of this early spring flowering herb in the Himalayan region is attributable to the transition from SI to SC, the capacity for selfing in an unpredictable pollination environment. Pollinator observations were conducted in an alpine meadow with hundreds of Himalayan mayapple (Podophyllum hexandrum Royle) individuals over 2 years. To examine autogamy, seed set under different pollination treatments was compared. To clarify whether automatic self-pollination is achieved by movement of the pistil as a previous study suggested, we measured incline angles of the pistil and observed flower movement during anthesis using video. Floral visitors to the nectarless flowers were very rare, but solitary bees and honeybees could be potential pollinators. Seed set of bagged flowers was not significantly different from that of open-pollinated, self- or cross-pollinated flowers. However, removal of petals or stamens lowered seed yield. The angles of inclination of pistils did not change during the process of pollination. Automatic self-pollination was facilitated by petals closing and stamens moving simultaneously to contact the stigma. Stigmatic pollen load increased little during the day time, in contrast to a sharp increase when the flowers closed during the night-time. These observations indicated that Himalayan mayapple was SC and delayed self-pollination was facilitated by the movement of petals rather than the pistil. Compared with SI American mayapple, no obvious inbreeding depression in SC Himalayan mayapple may contribute its existence in the uplifting zone. A scarcity of pollinators may have driven the shift to delayed selfing in P. hexandrum.

Nitrogen and carbon source-sink relationships in trees at the Himalayan treelines compared with lower elevations.

PubMed

Li, Mai-He; Xiao, Wen-Fa; Shi, Peili; Wang, San-Gen; Zhong, Yong-De; Liu, Xing-Liang; Wang, Xiao-Dan; Cai, Xiao-Hu; Shi, Zuo-Min

2008-10-01

No single hypothesis or theory has been widely accepted for explaining the functional mechanism of global alpine/arctic treeline formation. The present study tested whether the alpine treeline is determined by (1) the needle nitrogen content associated with photosynthesis (carbon gain); (2) a sufficient source-sink ratio of carbon; or (3) a sufficient C-N ratio. Nitrogen does not limit the growth and development of trees studied at the Himalayan treelines. Levels of non-structural carbohydrates (NSC) in trees were species-specific and site-dependent; therefore, the treeline cases studied did not show consistent evidence of source/carbon limitation or sink/growth limitation in treeline trees. However, results of the combined three treelines showed that the treeline trees may suffer from a winter carbon shortage. The source capacity and the sink capacity of a tree influence its tissue NSC concentrations and the carbon balance; therefore, we suggest that the persistence and development of treeline trees in a harsh alpine environment may require a minimum level of the total NSC concentration, a sufficiently high sugar:starch ratio, and a balanced carbon source-sink relationship.

The genus Hebeloma in the alpine belt of the Carpathians including two new species.

PubMed

Eberhardt, Ursula; Ronikier, Anna; Schütz, Nicole; Beker, Henry J

2015-01-01

Between 2002 and 2012 regular visits to the Carpathians were made and a number of Hebeloma spp. were collected from the alpine area. In total 44 collections were made that represent 11 species, two of which, Hebeloma grandisporum and H. oreophilum, are described here as new. Of the 11 species, four (H. alpinum, H. marginatulum and the two species described as new) are known only from alpine or Arctic habitats. Hebeloma dunense and H. mesophaeum are commonly found in, but not restricted to, alpine habitats. The other five species (H. aanenii, H. laterinum, H. naviculosporum, H. vaccinum, H. velutipes) are usually found in lowland or boreal habitats. Hebeloma naviculosporum is reported for the first time from the alpine zone and H. alpinum for the first time as growing with Helianthemum. All but two species (H. alpinum, H. marginatulum) are reported for the first time from the Carpathian alpine zone. In this paper we discuss the habitat, the 11 recorded species and give detailed descriptions of the two new species, both morphologically and molecularly. A key for Hebeloma species from sect. Hebeloma occurring in Arctic-alpine habitats is provided. © 2015 by The Mycological Society of America.

Linkages Between Critical Wedges and Crustal Channels Using 2-D Coupled Thermomechanical Finite Element Models: Implications for Himalayan Orogenic Evolution

NASA Astrophysics Data System (ADS)

Sparks, S. A.; Thigpen, J. R.

2017-12-01

In continental tectonics, questions remain regarding the dominant mechanisms of shortening accommodation during orogen evolution. Two quantitatively-supported models, critical wedge and channel flow, have been applied to the Himalaya and proposed for other large collisional systems. These two models represent fundamentally distinct mechanisms for accommodating shortening in collisional systems and until recently have been viewed as mutually exclusive. While there remains support for these mechanisms being incompatible end-members, in more recent studies it has been proposed that either: (1) both geodynamic mechanisms may operate simultaneously yet in spatially distinct parts of the larger composite orogenic system or (2) both mechanisms are present yet they operate at temporally distinct intervals, wherein the orogen progressively develops through stages dominated by mid-crustal channel flow followed by shallow thrust stacking and duplex development. In both scenarios, the mechanism active at each stage in orogen evolution is presumably dependent upon local to regional scale rheological conditions (as a function of orogen dynamic and thermal evolution) that are likely to be transient in both space and time. However, questions regarding the dynamic, mechanical, and thermal-kinematic relationships of such a system remain. Also, while field observations and deformation records derived from analyses of transects within the Himalaya can be interpreted in such a way to be consistent with a unified model, numerical models that predict the behavior of interactions between the end-member models have - until now - not existed. Here, we present results from 2-D coupled thermomechanical finite-element numerical experiments that examine the necessary conditions for mechanical compatibility between the channel and critical wedge by focusing on the role of rheology. These model results will eventually allow us to make preliminary comparisons between model-derived stress

High-Resolution Seismic Velocity and Attenuation Models of the Caucasus-Caspian Region

DTIC Science & Technology

2009-09-30

region is part of the Alpine-Himalayan collision belt and is an area of complex structure accompanied by large variations in seismic wave velocities...velocity structure is developed using teleseismic receiver functions and surface waves. Joint inversion of surface wave group dispersion curves...Caucasus and the thinnest is in the Arabian Plate. Thin crust is also observed near the Caspian. The lithospheric mantle in the Greater Caucasus and the

An evolving tectonic environment of Late Carboniferous to Early Permian granitic plutons in the Chinese Altai and Eastern Junggar terranes, Central Asian Orogenic Belt, NW China

NASA Astrophysics Data System (ADS)

Zhang, Chen; Liu, Dongdong; Luo, Qun; Liu, Luofu; Zhang, Yunzhao; Zhu, Deyu; Wang, Pengfei; Dai, Quanqi

2018-06-01

The Central Asian Orogenic Belt (CAOB) represents one of the most important sites of juvenile crustal growth during the Phanerozoic. Located in the central part of the CAOB, the Chinese Altai and Eastern Junggar terranes record the collisional processes between the peri-Siberian and Kazakhstan orogenic systems. However, the precise timing of collision between the two terranes remains controversial. The Wukuli and Kadelat plutons in the Chinese Altai belt are dated at ∼305 and ∼280 Ma respectively, whereas the Aketas pluton in the Eastern Junggar terrane is dated at ∼308 Ma. Granites from the Wukuli and Kadelat plutons are strongly peraluminous (A/CNK > 1.1), and are characterized by low Al2O3, Na2O, MnO, MgO, CaO and heavy rare earth element (HREE) contents, but with high SiO2, K2O and Rb contents as well as high Rb/Sr ratios. Granites from the Wukuli pluton have low εNd(t) and εHf(t) values of -3.7 to -3.4 and -9.7 to +4.9, whereas those from the Kadelat pluton have values of -3.6 to -3.4 and -8.0 to +2.6. These features suggest S-type affinity for the Wukuli and Kadelat plutons with magma derivation through partial melting of Mesoproterozoic metasediments. The Aketas pluton is composed of weakly peraluminous quartz monzonites that have A/CNK values ranging from 0.92 to 1.08, with high Na2O, Sr, and Sr/Y, and low Y, Yb, Nb, and Ta. These rocks display positive εNd(t) (+4.8 to +6.4) and εHf(t) (+9.7 to +14.6) values, and low initial 87Sr/86Sr ratios (0.703357-0.703868), similar to modern adakites, suggesting that the quartz monzonites were derived from the partial melting of lower crustal material. The geochemical characteristics suggest that the Aketas pluton was formed in a subduction-related setting, the Wukuli pluton in a syn-collisional setting, and the Kadelat pluton in the subsequent post-orogenic strike-slip-related setting. In combination with data from other granitoids in these two terranes, the Aketas pluton represents the youngest record of

Wave disturbances induced by crustal earthquakes: Case study of two strong earthquakes in the Caucasian-Anatolian sector of the Alpine Mediterranean mobile belt

NASA Astrophysics Data System (ADS)

Rogozhin, E. A.; Sobisevich, L. E.

2014-03-01

The display conditions of strong earthquakes in the Caucasian-Anatolian sector of the Mediterranean mobile belt are analyzed with allowance for the instrumental observations by titlmeters and magnetic variometers at the North Caucasian geophysical observatory of Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (NCGO IPE RAS) (Elbrus volcanic region) and at the magnetic stations operated by the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN). It is demonstrated that the geophysical information obtained during the preparatory stages of the earthquakes on March 8, 2010 and January 19, 2011 in Turkey and the North Caucasus, respectively, reflect the fine structure of the anomalous wave processes of the preparation and evolution of the crustal earthquakes. The results provide a general idea of the geomagnetic activity and some characteristic features of the induced anomalous disturbances in the Caucasian-Anatolian sector of the Alpine Mediterranean mobile belt, which precede and accompany strong seismic events.

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

Looking at the roots of the highest mountains: the lithospheric structure of the Himalaya-Tibet and the Zagros orogens. Results from a geophysical-petrological study

NASA Astrophysics Data System (ADS)

Tunini, L.; Jimenez-Munt, I.; Fernandez, M.; Villasenor, A.; Afonso, J. C.; Verges, J.

2013-12-01

The Himalaya-Tibet and Zagros orogens are the two most prominent mountain belts built by continental collision. They are part of a huge belt of Cenozoic age which runs from the Pyrenees to Burma. In its central sector, the collision with the southern margin of the Eurasian plate has resulted not only in the building of mountain ranges over the north-eastern edges of the Arabian and Indian plates but also in widespread deformation 1000-3000 km from the suture zones. Zagros and Himalaya-Tibet orogens share many geodynamic processes but at different rates, amount of convergence and stage of development. The study of their present-day structures provides new insights into their quasi coeval collisional event pointing out differences and similarities in the mountain building processes. We present 2D crust and upper mantle cross-sections down to 400 km depth, along four SW-NE trending profiles. Two profiles cross the Zagros Mountains, running from the Mesopotamian Foreland Basin up to the Alborz and Central Iran. Two other profiles run through the Himalaya-Tibetan orogen: the western transect crosses the western Himalaya, Tarim Basin, Tian Shan Mountains and Junggar Basin; the eastern transect runs from the Indian shield to the Beishan Basin, crossing the eastern Himalaya, Tibetan Plateau, Qaidam Basin and Qilian Mountains. We apply the LitMod-2D code which integrates potential fields (gravity and geoid), isostasy (elevation) and thermal (heat flow and temperature distribution) equations, and mantle petrology. The resulting crust and upper mantle structure is constrained by available data on elevation, Bouguer anomaly, geoid height, surface heat flow and seismic data including P- and S-wave tomography models. Our results show distinct deformation patterns between the crust and the lithospheric mantle beneath the Zagros and Himalaya-Tibetan orogens, indicating a strong strain partitioning in both areas. At crustal level, we found a thickening beneath the Zagros and the

Metabasalts as sources of metals in orogenic gold deposits

NASA Astrophysics Data System (ADS)

Pitcairn, Iain K.; Craw, Dave; Teagle, Damon A. H.

2015-03-01

Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9 ± 0.5 ppb) and upper greenschist facies (1.0 ± 0.5 ppb), but decrease significantly in amphibolite facies samples (0.21 ± 0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.

Juxtaposition of Neoproterozoic units along the Baruda - Tulu Dimtu shear-belt in the East African Orogen of western Ethiopia

USGS Publications Warehouse

Braathen, A.; Grenne, Tor; Selassie, M.G.; Worku, T.

2001-01-01

Amalgamation of East and West Gondwanaland during the Neoproterozoic East African Orogen is recorded by several shear-belts or 'suture zones', some of which are associated with ultramafic and mafic complexes that have been interpreted as ophiolite fragments. The Baruda shear-belt is a major structure of this type that belongs to the N-S trending Barka - Tulu Dimtu zone. The significance of this zone has been studied within a transect in western Ethiopia which covers a variety of metasedimentary and metavolcanic sequences, ultramafic rocks and synkinematic intrusive complexes. All rocks participated in the regional D1 event as reflected in a penetrative steep foliation in supracrustal rocks and marginal parts of the intrusions. Highly strained rocks contain a stretching lineation that plunge to the east. The several-km thick Baruda shear-belt, comprising mylonitic supracrustal and plutonic rocks including mafic-ultramafic mega-lenses, is the most prominent expression of this event. Shear-sense indicators demonstrate top-to-the-west shear. Subsequent D2 deformation is recorded in 2-300 m wide, N-S striking, subvertical shear-zones with subhorizontal stretching lineation relatable to sinistral transcurrent movements. Our data indicate that rock units on either side of the Baruda shear-belt are related, rather than being exotic to each other as implied in suture zone models, since there is no major lithologic or metamorphic difference, geochemical data on metavolcanic rocks and pre-tectonic intrusions suggest a paleotectonic link, and style and extent of deformation is similar across the shear-belt. A tentative model for the transect suggests an arc and back-arc setting which experienced later continental collision and tectonic shortening. The initial setting was that of a shallow marine platform characterised by carbonates and sandstones, which covered extensive areas prior to break-up of a pre-existing supercontinent. Continental convergence is first recorded in high

Partially Melted UHP Eclogite in the Sulu Orogenic Belt, China and its rheological significance to deep continental subduction: Micro- to Macro-scale Evidence

NASA Astrophysics Data System (ADS)

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

2015-04-01

Partially Melted UHP Eclogite in the Sulu Orogenic Belt, China and its rheological significance to deep continental subduction: Micro- to Macro-scale Evidence Numerous studies have described partial melting processes in low-high pressure meta-sedimentary rocks, some of which may generate melts that coalesce to form plutons. However, migmatized ultrahigh pressure (UHP) eclogite has never been clearly described from the microscale to macroscale, though experimental studies prove dehydration partial melting of eclogite at high pressure condition1 and low degrees of partially melted eclogite have been reported from the Qaidam UHP orogenic belt in NW China2,3 or inferred from multiphase solid (MS) inclusions within eclogite4 in the Sulu UHP belt. We present field-based documentation of decompression partial melting of UHP eclogite from Yangkou and General's Hill, Sulu Orogen. Migmatized eclogite shows successive stages of anatexis, initially starting from intragranular and grain boundary melt droplets, which grow into a 3D interconnected intergranular network, then segregate and accumulate in pressure shadow areas, and finally merge to form melt channels and dikes that transport melts to upper lithospheric levels. In-situ phengite breakdown-induced partial melting is directly identified by MS inclusions of Kfs+ barium-bearing Kfs + Pl in garnet, connected by 4-10 μm wide veinlets consisting of Bt + Kfs + Pl next to the phengite. Intergranular veinlets of plagioclase + K-feldspar first form isolated beads of melt along grain boundaries and triple junctions of quartz, and with higher degrees of melting, eventually form interconnected 3D networks along grain boundaries in the leucosome, allowing melt to escape from the intergranular realm and collect in low-stress areas. U-Pb (zircon) dating and petrological analyses on residue and leucocratic rocks shows that partial melting occurred at 228-219 Ma, shortly after peak UHP metamorphism (~230 Ma), and at depths of 30-90 km

Limited climate control of the Chugach/St. Elias thrust wedge in southern Alaska demonstrated by orogenic widening during Pliocene to Quaternary climate change

NASA Astrophysics Data System (ADS)

Meigs, Andrew

2014-05-01

Critical taper wedge theory is the gold standard by which climate control of convergent orogenic belts is inferred. The theory predicts (and models reproduce) that an orogenic belt narrows if erosion increases in erosion in the face of a constant tectonic influx. Numerous papers now argue on the basis of thermochronologic data that the Chugach/ St. Elias Range (CSE) of southern Alaska narrowed as a direct response to Quaternary climate change because glaciers dominated erosion of the orogenic belt. The CSE formed in response to collision of a microplate with North America and is notable because glacial erosion has dominated the CSE for the past 5 to 6 Ma. An increase in sediment accumulation rates in the foreland basin over that time suggests that glacial erosion become more efficient. If correct, it is possible that glacial erosion outpaced rock influx thereby inducing a climatically controlled narrowing of the orogenic wedge during the Quaternary. Growth strata preserved within the wedge provide a test of that interpretation because they demonstrate the spatial and temporal pattern of deformation during the Pliocene to Quaternary climate transition. A thrust front established between 6 and 5 Ma jumped towards the foreland by 30 and 15 km at 1.8 and 0.25 Ma, respectively. Distributed deformation within the thrust belt accompanied the thrust front relocations. Continuous exhumation recorded by low-temperature thermochronometers occurred contemporaneously with the shortening, parallel the structural not the topographic grain, and ages become younger towards the foreland as well. Interpreted in terms of critical wedge theory, continuous distributed deformation reflects a sub-critical wedge taper resulting from the combined effects of persistent exhumation and incremental accretion and orogenic widening via thrust front jumps into the undeformed foreland. Taper angle varies according to published cross-sections and ranges from 3 to 9 degrees. If the wedge oscillated

Cambro-Ordovician post-collisional granites of the Ribeira belt, SE-Brazil: A case of terminal magmatism of a hot orogen

NASA Astrophysics Data System (ADS)

Valeriano, Claudio de Morisson; Mendes, Julio Cezar; Tupinambá, Miguel; Bongiolo, Everton; Heilbron, Monica; Junho, Maria do Carmo Bustamante

2016-07-01

This work presents an overview of the geology and chemical composition of the Cambrian-Ordovician post-collisional (COPC) granites and associated rocks of Ribeira belt, SE-Brazil. These COPC granites make up some of the most picturesque and highest (>2000 m) rocky peaks and cliffs of Rio de Janeiro state, an accessible case of post-orogenic granitic magmatism associated with the terminal stages of a hot Ediacaran-Cambrian (Brasiliano-Panafrican) orogen. The COPC magmatism intruded tonalitic to granitic orthogneisses of the Rio Negro arc (∼790-600 Ma) and associated paragneisses of the São Fidelis Group. Post-collisional magmatism started ∼10 m.y. after the latest collisional event, the Buzios Orogeny, lasting discontinuously from ∼510 Ma until ∼470 Ma. The 15 largest intrusive bodies in Rio de Janeiro State are referred to in the literature as the Parati/Mangaratiba, Vila Dois Rios, Pedra Branca, Suruí, Silva Jardim, Favela, Andorinha, Teresópolis, Frade, Nova Friburgo, Conselheiro Paulino, São José do Ribeirão, Sana and Itaoca granites. They crop out as rounded/elliptical stocks or gently-dipping sheets, always with sharp contacts with the country rocks, along with pegmatite and aplitic veins and dykes. COPC granites are grey and pink undeformed medium-grained biotite monzogranites with (K-feldspar) porphyritic, mega-crystic, equigranular and serial textures. Magmatic flow foliation is frequently observed. Peripheric xenolith zones are common as well as isolated xenoliths from the country rocks. In a compilation of more than 100 chemical compositions, SiO2 contents display a major mode at 71wt%. The COPC magmatism generated high-K calc-alkaline granites and quartz monzonites with predominantly metaluminous granites. Meso to melanocratic gabbroic and dioritic enclaves also have calc-alkaline affinity and likely represent more resistant mafic xenoliths from the Rio Negro Arc.

Nutrient status in soil of Ski runs in the sub-alpine belt of Uludag Mountain, Bursa, Turkey.

PubMed

Guleryuz, Gurcan; Kirmizi, Serap; Arslan, Hulya

2010-01-01

Large areas of land are disturbed in sensitive bio-diverse mountain environments by Skiruns. Restoration of vegetation on such disturbed mountain sites may be hampered by soil degradation but the severity and nature of the constraints is not well understood. This study was designed to compare the water holding and nutritional status of soil in three Ski runs which had different construction dates and disturbance levels, and the adjacent undisturbed site in the Abies bommuelleriana forest community in the sub-alpine belt of Uludag Mountain (Bithynian Olympus). The values of soil parameters were depressed in proportion to the disturbance level. Water holding capacity (WHC), total nitrogen (N), organic carbon (C) and calcium (Ca2+), magnesium (Mg2+) and potassium (K+) contents (mg kg(-1) dry weight) of soils in the Ski run which had the highest disturbance level were lower than that of the undisturbed adjacent sites. However the results indicated that the soil parameters were less degraded when secondary vegetation was growing on the disturbed areas.

Two-stage formation model of the Junggar basin basement: Constraints to the growth style of Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

He, Dengfa

2016-04-01

Junggar Basin is located in the central part of the Central Asian Orogenic Belt (CAOB). Its basement nature is a highly controversial scientific topic, involving the basic style and processes of crustal growth. Some researchers considered the basement of the Junggar Basin as a Precambrian continental crust, which is not consistent with the petrological compositions of the adjacent orogenic belts and the crust isotopic compositions revealed by the volcanic rocks in the basin. Others, on the contrary, proposed an oceanic crust basement model that does not match with the crustal thickness and geophysical characteristics of the Junggar area. Additionally, there are several viewponits, such as the duplex basement with the underlying Precambrian crystalline rocks and the overlying pre-Carboniferous folded basement, and the collaged basement by the Precambrian micro-continent block in the central part and the Hercynian accretionary folded belts circling it. Anyway, it is necessary to explain the property of basement rock, its strong inhomogeneous compositions as well as the geophysical features. In this paper, based on the borehole data from more than 300 industry wells drilled into the Carboniferous System, together with the high-resolution gravity and magnetic data (in a scale of 1:50,000), we made a detailed analysis of the basement structure, formation timing and processes and its later evolution on a basis of core geochemical and isotopic analysis. Firstly, we defined the Mahu Pre-Cambrian micro-continental block in the juvenile crust of Junggar Basin according to the Hf isotopic analysis of the Carboniferous volcanic rocks. Secondly, the results of the tectonic setting and basin analysis suggest that the Junggar area incorporates three approximately E-W trending island arc belts (from north to south: Yemaquan- Wulungu-Chingiz, Jiangjunmiao-Luliang-Darbut and Zhongguai-Mosuowan- Baijiahai-Qitai island arcs respectively) and intervened three approximately E-W trending

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt, NW China: Constraints on the initiation of a magmatic arc in the southern Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Du, Long; Long, Xiaoping; Yuan, Chao; Zhang, Yunying; Huang, Zongying; Sun, Min; Zhao, Guochun; Xiao, Wenjiao

2018-03-01

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt (ETOB) have been studied in order to constraint the initiation of a magmatic arc formed in this region. Zircon U-Pb dating indicates that two dioritic plutons in the northern ETOB were generated in the Late Ordovician (452 ± 4 Ma) and the Early Silurian (442 ± 3 Ma), respectively. Diorites from the two plutons are characterized by enrichments in large ion lithophile elements (LILE) and highly incompatible elements, with depletions in high field strength elements (HSFE) displaying typical geochemical features of a subduction-related origin. They have positive εNd(t) values (+5.08-+6.58), relatively young Nd model ages (TDM = 0.71-1.08 Ga), with Ta/Yb (0.05-0.09) and Nb/Ta ratios (12.06-15.19) similar to those of depleted mantle, suggesting a juvenile mantle origin. Their high Ba/La (13.3-35.9), low Th/Yb (0.72-2.02), and relatively low Ce/Th (4.57-14.7) and Ba/Th (47.8-235) ratios indicate that these diorites were probably produced by partial melting of a depleted mantle wedge metasomatized by both subducted sediment-derived melts and slab-derived aqueous fluids. Zircon U-Pb dating of a granitic pluton in the northern ETOB yielded a Late Ordovician intrusion age of 447 ± 5 Ma. Granites from this pluton show calc-alkaline compositions with geochemical characteristics of I-type granites. They also show positive εNd(t) values (+6.49-+6.95) and young Nd model ages (TDM = 0.69-0.87 Ga), indicating that the granites were most likely derived from juvenile lower crust. Our new dating results on the dioritic and granitic plutons suggest that arc-type magmatism in the northern ETOB began prior to or at the Late Ordovician (452-442 Ma). In addition, north-dipping subduction of the Kangguertage oceanic lithosphere may account for the arc-type magmatism and the geodynamic process of the ETOB in the Early Paleozoic.

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.

Eclogites and garnet clinopyroxenites in the Anrakhai complex, Central Asian Orogenic Belt, Southern Kazakhstan: P-T evolution, protoliths and some geodynamic implications

NASA Astrophysics Data System (ADS)

Pilitsyna, Anfisa V.; Tretyakov, Andrey A.; Degtyarev, Kirill E.; Cuthbert, Simon J.; Batanova, Valentina G.; Kovalchuk, Elena V.

2018-03-01

The Anrakhai Metamorphic Complex (AMC), located in the SE part of the Chu-Ili Mountains of Southern Kazakhstan in the western part of Central Asian Orogenic Belt, exhibits occurrences of HP metamorphic rocks in the form of eclogites and garnet clinopyroxenites with peak metamorphic conditions of 750-850° and 15-19 kbar estimated with both conventional geothermobarometric methods and phase diagram modeling. P-T estimates as well as intimate field relations evidently imply a common metamorphic history for eclogites and garnet clinopyroxenites of the AMC. These high-pressure, medium temperature eclogite facies P-T conditions are indicative of a collision or subduction tectonic setting. Major and trace element geochemistry suggests that they probably had a common magmatic origin as part of a suite of differentiated tholeiitic intrusions. Furthermore, distinctive mineral and chemical compositions of these eclogites and garnet clinopyroxenites correspond to the Fe-Ti type of ultramafic rocks suggesting that they may have been derivatives of intraplate tholeiitic melts, introduced into continental crust before HP metamorphism.

Tracking the multi-stage exhumation history of the western Chinese Tianshan by Apatite Fission Track (AFT) dating - Implications for the preservation of epithermal deposits in ancient orogenic belt

NASA Astrophysics Data System (ADS)

Wang, Yannan; Cai, Keda

2017-04-01

The western Chinese Tianshan, located in the southern domain of the Central Asian Orogenic Belt (CAOB), was originally constructed by multiple accretion-collision processes in the Paleozoic, and was superimposed by complex intracontinental tectonic evolution in the Mesozoic-Cenozoic. Understanding the timing and mechanism of the latter geological processes is critical to unravel the preservation conditions of the epithermal deposits in the western Chinese Tianshan. This work presents new apatite fission track (AFT) data for three mountain ranges of the western Chinese Tianshan to track their exhumation history. Our AFT data gave a wide range of ages from 76.8 ± 5.5 Ma to 182.3 ± 9.9 Ma, and the mean confined fission track lengths are between 9.8 ± 0.5 μm and 12.3 ± 0.2 μm. The new data, in combination with the thermal history modeling，enable us to attribute the exhumation history to three primary stages, including Early Permian (300-280 Ma), Late Triassic-Early Cretaceous (230-130 Ma), and Late Oligocene-Early Miocene (30-20 Ma). The first stage may be caused by the terrane accretion-collision in the late Paleozoic. The second stage was likely related to the closure of the Mongol-Okhotsk Ocean during the Mesozoic. The last one is regarded as the result of the collision between the Indian Plate and the Eurasia Plate in the Cenozoic. The extraordinary exhumation processes of these three major mountain ranges might have been responsible for sediment supply to the corresponding intra-mountain basins in the western Chinese Tianshan, and the particularly mountain-basin coupling evolution is ascribed to an essential condition for the preservation of epithermal deposits in ancient orogenic belt.

Seismological Constraints on Lithospheric Evolution in the Appalachian Orogen

NASA Astrophysics Data System (ADS)

Fischer, K. M.; Hopper, E.; Hawman, R. B.; Wagner, L. S.

2017-12-01

Crust and mantle structures beneath the Appalachian orogen, recently resolved by seismic data from the EarthScope SESAME Flexible Array and Transportable Array, provide new constraints on the scale and style of the Appalachian collision and subsequent lithospheric evolution. In the southern Appalachians, imaging with Sp and Ps phases reveals the final (Alleghanian) suture between the crusts of Laurentia and the Gondwanan Suwannee terrane as a low angle (<15°) southward-dipping interface that soles into a flat-lying mid-crustal detachment. The suture location near the top of the crust coincides closely with the northern limit of the Suwannee terrane reconstructed from its lower Paleozoic shelf strata (Boote and Knapp, 2016). The observed suture geometry implies over 300 km of head-on shortening across a plate boundary structure similar in scale to the Himalayan mid-crustal detachment. While the suture and other structures from the Alleghanian collision are preserved in the upper and mid-crust, the lower crust and mantle lithosphere beneath this region have been significantly modified by later processes. Ps receiver functions, wavefield migration and SsPmp modeling reveal that crustal thickness reaches a maximum of 58 km (beneath high elevations in the Blue Ridge terrane) and decreases to 29-35 km (beneath lower elevations in the Carolina and Suwannee terranes). Given metamorphic estimates of unroofing (Duff and Kellogg, 2017) isostatic arguments indicate crustal thicknesses were 15-25 km larger at the end of the orogeny, indicating a thick crustal root across the region. The present-day residual crustal root beneath the Blue Ridge mountains is estimated to have a density contrast with the mantle of only 104±20 kg/m3. This value is comparable to other old orogens but lower than values typical of young or active orogens, indicating a loss of lower crustal buoyancy over time. At mantle depths, the negative shear velocity gradient that marks the transition from

The Western Carpathians fold and thrust belt and its relationships with the inner zone of the orogen: constraints from sequentially restored, balanced cross-sections integrated with low-temperature thermochronometry

NASA Astrophysics Data System (ADS)

Mazzoli, Stefano; Castelluccio, Ada; Andreucci, Benedetta; Jankowski, Leszek; Ketcham, Richard A.; Szaniawski, Rafal; Zattin, Massimiliano

2017-04-01

The Western Carpathians are the northernmost, W-E-trending branch of a more than 1500 km long, curved orogen. Traditionally, the Western Carpathians have been divided into two distinct parts, namely the Inner Carpathians (including basement nappes) and the Outer Carpathians fold and thrust belt. These two major domains are separated by the so-called 'Pieniny Klippen Belt', a narrow zone of intensely deformed and sheared Mesozoic to Palaeogene rocks. In this contribution, a new interpretation for the tectonic evolution of the Western Carpathians is provided based on: (i) the analysis of the stratigraphy of the Mesozoic-Tertiary successions across the different orogenic domains; (ii) the construction of a series of balanced and restored cross-sections, validated by 2D forward modeling; and (iii) the integration of a large thermochronometric dataset (apatite fission tracks and apatite and zircon (U-Th-(Sm))/He ages). The latter work included thermo-kinematic modeling using FetKin, a finite element solver that takes as input a series of balanced cross-sections. The software solves the heat flow equations in 2D together with the predicted thermochronometric ages, which can be compared with the measured data. Moreover, the spatial distribution of burial depths, cooling ages and the rate of exhumation were correlated with heat flow, topographic relief, crustal and lithospheric thickness. This process allowed us to obtain the cooling history along each section and test the response of low-temperature thermochronometers to the changes in the thrust belt geometry produced by fault activity and topography evolution. Our sequentially restored, balanced cross-sections, showing a mix of thin-skinned thrusting and thick-skinned tectonic inversion involving the reactivation of pre-existing basement normal faults, effectively unravel the tectonic evolution of the thrust belt-foreland basin system. Our analysis provides a robust correlation of the stratigraphy from the Outer to the

Cumberland batholith, Trans-Hudson Orogen, Canada: Petrogenesis and implications for Paleoproterozoic crustal and orogenic processes

NASA Astrophysics Data System (ADS)

Whalen, Joseph B.; Wodicka, Natasha; Taylor, Bruce E.; Jackson, Garth D.

2010-06-01

Large volume, plutonic belts, such as the ˜ 221,000 km 2, ca. 1.865-1.845 Ga Cumberland batholith (CB) of the Trans-Hudson Orogen in Canada, are major components of Paleoproterozoic orogenic belts. In many cases, they have been interpreted as continental arc batholiths. The petrogenesis and tectonic context of the CB and implications for crustal growth and recycling are interpreted herein based on a 900 km geochemical-isotopic (Nd-O) transect across it and into granitoid plutons within bounding Archean cratons in central and southern Baffin Island. The mainly granulite grade CB, emplaced over an age span of between 14 and 24 Ma, consists mainly of high-K to shoshonitic monzogranite and granodiorite, but also includes low- and medium-K granitoid rocks. Metaluminous to slightly peraluminous compositions and δ 18O (VSMOW) values (+ 6 to + 10‰) indicate derivation from infracrustal (I-type) sources. ɛ Nd 1.85 Ga signatures (- 12 to - 2) of both mafic and felsic units suggest a dominance of evolved sources. Isotopic signatures in the interior of the CB (- 2 to - 7) are more radiogenic than those within Archean domains in central (- 8 to - 15) and southern (- 5 to - 19) Baffin Island. The isotopic transect is interpreted as 'imaging' an accreted microcontinental block (Meta Incognita) and bounding Archean cratons. The CB includes granites of arc, within-plate (A-type) and post-collisional affinity and volumetrically minor mafic rocks with both arc and non-arc features. (La/Yb) CN and Sr/Y values range from < 1 to 225 and < 1 to 611, respectively. In these respects, some CB granitoid rocks resemble Paleozoic adakitic granites, interpreted as partial melts of greatly thickened crust within post-collisional settings, such as Tibet. Thus, the CB likely encompasses various non-consanguineous magmatic suites generated at deep- to mid-crustal depths. Although CB granitoid rocks undoubtedly had important crustal sources, it is hard to assess the relative contribution of

Cambrian ophiolite complexes in the Beishan area, China, southern margin of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Shi, Yuruo; Zhang, Wei; Kröner, Alfred; Li, Linlin; Jian, Ping

2018-03-01

We present zircon ages and geochemical data for Cambrian ophiolite complexes exposed in the Beishan area at the southern margin of the Central Asian Orogenic Belt (CAOB). The complexes consist of the Xichangjing-Xiaohuangshan and Hongliuhe-Yushishan ophiolites, which both exhibit complete ophiolite stratigraphy: chert, basalt, sheeted dikes, gabbro, mafic and ultramafic cumulates and serpentinized mantle peridotites. Zircon grains of gabbro samples yielded 206Pb/238U ages of 516 ± 8, 521 ± 4, 528 ± 3 and 535 ± 6 Ma that reflect the timing of gabbro emplacement. The geochemical data of the basaltic rocks show enrichment in large-ion lithophile elements and depletion in the high field strength elements relative to normal mid-oceanic ridge basalt (NMORB) in response to aqueous fluids or melts expelled from the subducting slab. The gabbro samples have higher whole-rock initial 87Sr/86Sr ratios and lower positive εNd(t) values than NMORB. These geochemical signatures resulted from processes or conditions that are unique to subduction zones, and the ophiolites are therefore likely to have formed within a supra-subduction zone (SSZ) environment. We suggest that the Cambrian ophiolite complexes in the Beishan area formed within a SSZ setting, reflecting an early Paleozoic subduction of components of the Paleo-Central Asian Ocean and recording an early Paleozoic southward subduction event in the southern CAOB along the northern margin of the Tarim and North China Cratons.

Titanite-bearing calc-silicate rocks constrain timing, duration and magnitude of metamorphic CO2 degassing in the Himalayan belt

NASA Astrophysics Data System (ADS)

Rapa, Giulia; Groppo, Chiara; Rolfo, Franco; Petrelli, Maurizio; Mosca, Pietro; Perugini, Diego

2017-11-01

The pressure, temperature, and timing (P-T-t) conditions at which CO2 was produced during the Himalayan prograde metamorphism have been constrained, focusing on the most abundant calc-silicate rock type in the Himalaya. A detailed petrological modeling of a clinopyroxene + scapolite + K-feldspar + plagioclase + quartz ± calcite calc-silicate rock allowed the identification and full characterization - for the first time - of different metamorphic reactions leading to the simultaneous growth of titanite and CO2 production. The results of thermometric determinations (Zr-in-Ttn thermometry) and U-Pb geochronological analyses suggest that, in the studied lithology, most titanite grains grew during two nearly consecutive episodes of titanite formation: a near-peak event at 730-740 °C, 10 kbar, 30-26 Ma, and a peak event at 740-765 °C, 10.5 kbar, 25-20 Ma. Both episodes of titanite growth are correlated with specific CO2-producing reactions and constrain the timing, duration and P-T conditions of the main CO2-producing events, as well as the amounts of CO2 produced (1.4-1.8 wt% of CO2). A first-order extrapolation of such CO2 amounts to the orogen scale provides metamorphic CO2 fluxes ranging between 1.4 and 19.4 Mt/yr; these values are of the same order of magnitude as the present-day CO2 fluxes degassed from spring waters located along the Main Central Thrust. We suggest that these metamorphic CO2 fluxes should be considered in any future attempts of estimating the global budget of non-volcanic carbon fluxes from the lithosphere.

Transient Landscape Evolution is Characteristic of Post-Orogenic Decay: An Example from the Southern Appalachians, U.S.A.

NASA Astrophysics Data System (ADS)

Gallen, S. F.

2016-12-01

Long-term landscape evolution in post-orogenic settings remains an outstanding question in the geosciences. Despite conventional wisdom that topography in dead orogens will slowly and steadily decay through time, observations from around the globe show that dynamic, unsteady (e.g. transient) landscape evolution is the norm. Unraveling the mechanisms that drive unsteadiness in dead orogens is paramount to understanding the stratigraphic record of offshore basins and the geologic factors that contribute to the high biodiversity common in these settings. Here we address the enigma of unsteady post-orogenic landscape evolution with a study of the geomorphology of southern Appalachians, U.S.A. We focus on the 58,000 km2 Upper Tennessee River Basin that covers portions of the fold-and-thrust belt (Valley and Ridge), foreland basin (Appalachian Plateau), and a deeply exhumed thrust sheet (Blue Ridge) of this dead orogen. Using published millennial-scale erosion rates and quantitative analysis of fluvial topography, we show that this region is in a transient state of adjustment to 400 m of base level fall. Ongoing adjustment to base level drop is observed as a zone of high erosion rates, steep river channels and numerous knickpoints located upstream of and surrounding the contact between the Valley and Ridge and adjacent lithotectonic units. We argue that the association of adjusting landscapes and the Valley and Ridge contact is due to the rapid response time of rivers incising soft Valley and Ridge rocks, relative to the harder metamorphic rocks in the Blue Ridge and resistant capstone in the Appalachian Plateau. We propose that base level fall was triggered by incision through the Appalachian Plateau capstone into underlying weaker rocks that set off a wave of transient adjustment, drainage reorganization and ultimately capture of the paleo-Upper Tennessee Basin. Our results indicate that transient landscape evolution is characteristic of post-orogenic settings, as

Paleozoic tectonics of the Ouachita Orogen through Nd isotopes

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

Gleason, J.D.; Patchett, P.J.; Dickinson, W.R.

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. Ndmore » 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.« less

Zircon (U-Th)/He Thermochronometric Constraints on Himalayan Thrust Belt Exhumation, Bedrock Weathering, and Cenozoic Seawater Chemistry

NASA Astrophysics Data System (ADS)

Colleps, Cody L.; McKenzie, N. Ryan; Stockli, Daniel F.; Hughes, Nigel C.; Singh, Birendra P.; Webb, A. Alexander G.; Myrow, Paul M.; Planavsky, Noah J.; Horton, Brian K.

2018-01-01

Shifts in global seawater 187Os/188Os and 87Sr/86Sr are often utilized as proxies to track global weathering processes responsible for CO2 fluctuations in Earth history, particularly climatic cooling during the Cenozoic. It has been proposed, however, that these isotopic records instead reflect the weathering of chemically distinctive Himalayan lithologies exposed at the surface. We present new zircon (U-Th)/He thermochronometric and detrital zircon U-Pb geochronologic evidence from the Himalaya of northwest India to explore these contrasting interpretations concerning the driving mechanisms responsible for these seawater records. Our data demonstrate in-sequence southward thrust propagation with rapid exhumation of Lesser Himalayan strata enriched in labile 187Os and relatively less in radiogenic 87Sr at ˜16 Ma, which directly corresponds with coeval shifts in seawater 187Os/188Os and 87Sr/86Sr. Results presented here provide substantial evidence that the onset of exhumation of 187Os-enriched Lesser Himalayan strata could have significantly impacted the marine 187Os/188Os record at 16 Ma. These results support the hypothesis that regional weathering of isotopically unique source rocks can drive seawater records independently from shifts in global-scale weathering rates, hindering the utility of these records as reliable proxies to track global weathering processes and climate in deep geologic time.

Proterozoic orogens in southern Peninsular India: Contiguities and complexities

NASA Astrophysics Data System (ADS)

Chetty, T. R. K.; Santosh, M.

2013-12-01

The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents - Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ˜400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P-T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale "flower structures", transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a

Accessing The Fourth Dimension In Orogenic Reconstruction Using Granitoid Thermobarometry

NASA Astrophysics Data System (ADS)

Alexander, E.; Wielicki, M. M.; Harrison, M.; Lovera, O. M.; DePaolo, D.

2016-12-01

Tectonic models for the Tibetan-Himalayan orogen predict very different crustal thickness histories, providing a possible test of these various hypotheses. However, reconstructing the evolution of the Tibetan-Himalayan crust is a four-dimensional problem. Knowing the 2D distribution of U-Pb zircon dated samples permits insights into changes in magmatic style, but understanding of N-S thickening history requires depth information. In S. Tibet, voluminous granitoids emplaced between 200-20 Ma provides a spatiotemporal window into the tectonic evolution of the Tibetan crust. A thermoisotopic model utilizing systematic N-S ɛNd variations as a crustal thickness proxy indicates that, at the time of collision, the crust beneath the Indus-Tsangpo suture was relatively thin ( 20 km), increasing to >45 km 100 km to the N. Given evidence of little post-50 Ma upper-crustal shortening, the Tibetan crust appears to have reached its present 85 km thickness via 20 km of tectonic accretion, 15 km of juvenile magma input, as well as the underthrusted Indian crust. Since zircon ɛHf correlates well with whole rock ɛNd, using it together with in situ δ18O, Ti-thermometry and trace element analyses, has provided a refined picture of the relative roles of assimilation and recharge in granitoid formation. To understand the spatiotemporal progression of thickening, we utilize two thermobarometers to reconstruct crystallization depths: Al-in-hornblende and Ti-in-quartz. While Al-in-hornblende has been more widely tested on igneous systems, our data show that it may be sensitive to closure effects during high T storage and hydrothermal alteration. In contrast, Ti-in-quartz preserves magmatic Ti signatures, and quartz's exclusive structure and resistance to alteration provides a more robust proxy for emplacement conditions. Using these thermobarometers in conjunction with Ti-in-zircon thermometry can establish emplacement depth of syn-collisional Lhasa block granites providing the missing

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

Transition From Archean Plume-Arc Orogens to Phanerozoic Style Convergent Margin Orogens, and Changing Mantle Lithosphere

NASA Astrophysics Data System (ADS)

Kerrich, R.; Jia, Y.; Wyman, D.

2001-12-01

Mantle plume activity was more intense in the Archean and komatiite-basalt volcanic sequences are a major component of many Archean greenstone belts. Tholeiitic basalts compositionally resemble Phanerozoic and Recent ocean plateau basalts, such as those of Ontong Java and Iceland. However, komatiite-basalt sequences are tectonically imbricated with bimodal arc lavas and associated trench turbidites. Interfingering of komatiite flows with boninite series flows, and primitive to evolved arc basalts has recently been identified in the 2.7 Ga Abitibi greenstone belt, demonstrating spatially and temporally associated plume and arc magmatism. These observations are consistent with an intra-oceanic arc migrating and capturing an ocean plateau, where the plateau jams the arc and imbricated plateau-arc crust forms a greenstone belt orogen. Melting of shallowly subducted plateau basalt crust (high Ba, Th, LREE) accounts for the areally extensive and voluminous syntectonic tonalite batholiths. In contrast, the adakite-Mg-andesite-Niobium enriched basalt association found in Archean greenstone belts and Cenozoic arcs are melts of LREE depleted MORB slab. Buoyant residue from anomalously hot mantle plume melting at > 100km rises to couple with the composite plume-arc crust to form the distinctively thick and refractory Archean continental lithospheric mantle. New geochemical data for structurally hosted ultramafic units along the N. American Cordillera, from S. California to the Yukon, show that these are obducted slices of sub-arc lithospheric mantle. Negatively fractionated HREE with high Al2O3/TiO2 ratios signify prior melt extraction, and variably enriched Th and LREE with negative Nb anomalies a subduction component in a convergent margin. A secular decrease of mantle plume activity and temperature results in plume-arc dominated geodynamics in the Archean with shallow subduction and thick CLM, whereas Phanerozoic convergent margins are dominated by arc-continent, arc

Comments on "The Cenozoic fold-and-thrust belt of Eastern Sardinia: Evidences from the integration of field data with numerically balanced geological cross section" by Arragoni et al., 2016

NASA Astrophysics Data System (ADS)

Berra, F.; Lanfranchi, A.; Jadoul, F.

2017-02-01

Arragoni et al. (2016) suggest in their paper published on tectonics that the carbonate succession of Eastern Sardinia represents a Cenozoic fold-and-thrust belt, related to the Alpine orogenesis. According to these authors, this supposed fold-and-thrust belt represents the southward continuation of the Alpine Corsica collisional chain and the missing link between the Alpine Chain and the Calabria-Peloritani domain. Field evidence and the published literature document instead that all the surfaces that Arragoni et al. interpret as thrust are actually stratigraphic contacts. The balanced geological section of Arragoni represents thus a geometric exercise missing the basic data needed to nurse the proposed model, and it does not reflect the geology of Eastern Sardinia. The data provided by Arragoni et al. (2016) do not support the presence of an Alpine thrust-and-fold belt in Eastern Sardinia, and this paper may suggest to the geological community a misleading interpretation of the geodynamic evolution of the Alpine and Mediterranean area.

Surface Wave Tomography across the Alpine-Mediterranean Mobile Belt

NASA Astrophysics Data System (ADS)

El-Sharkawy, A. M. M. E.; Meier, T. M.; Lebedev, S.; Weidle, C.; Cristiano, L.

2017-12-01

The Alpine-Mediterranean mobile belt is, tectonically, one of the most complicated and active regions in the world. Since the Mesozoic, collisions between Gondwana-derived continental blocks and Eurasia, due to the closure of a number of rather small ocean basins, have shaped the Mediterranean geology. Despite the numerous studies that have attempted to characterize the lithosphere-asthenosphere structure in that area, details of the lithospheric structure and dynamics, as well as flow in the asthenosphere are, however, poorly known. The purpose of this study is to better define the 3D shear-wave velocity structure of the lithosphere-asthenosphere system in the Mediterranean using new tomographic images obtained from surface wave tomography. An automated algorithm for inter-station phase velocity measurements is applied here to obtain Rayleigh fundamental mode phase velocities. We utilize a database consisting of more than 4000 seismic events recorded by more than 3000 broadband seismic stations within the area (WebDc/EIDA, IRIS). Moreover, for the first time, data from the Egyptian National Seismological Network (ENSN), recorded by up to 25 broad band seismic stations, are also included in the analysis. For each station pair, approximately located on the same great circle path, the recorded waveforms are cross correlated and the dispersion curves of fundamental modes are calculated from the phase of the cross correlation functions weighted in the time-frequency plane. Path average dispersion curves are obtained by averaging the smooth parts of single-event dispersion curves. We calculate maps of Rayleigh phase velocity at more than 100 different periods. The phase-velocity maps provide the local phase-velocity dispersion curve for each geographical grid node of the map. Each of these local dispersion curves is inverted individually for 1D shear wave velocity model using a newly implemented Particle Swarm Optimization (PSO) algorithm. The resulted 1D velocity

Do Himalayan treelines respond to recent climate change? An evaluation of sensitivity indicators

NASA Astrophysics Data System (ADS)

Schickhoff, U.; Bobrowski, M.; Bohner, J.; Burzle, B.; Chaudhary, R. P.; Gerlitz, L.; Heyken, H.; Lange, J.; Muller, M.; Scholten, T.; Schwab, N.; Wedegartner, R.

2015-05-01

Climate warming is expected to induce treelines to advance to higher elevations. Empirical studies in diverse mountain ranges, however, give evidence of both advancing alpine treelines and rather insignificant responses. The inconsistency of findings suggests distinct differences in the sensitivity of global treelines to recent climate change. It is still unclear where Himalayan treeline ecotones are located along the response gradient from rapid dynamics to apparently complete inertia. This paper reviews the current state of knowledge regarding sensitivity and response of Himalayan treelines to climate warming, based on extensive field observations, published results in the widely scattered literature, and novel data from ongoing research of the present authors. Several sensitivity indicators such as treeline type, treeline form, seed-based regeneration, and growth patterns are evaluated. Since most Himalayan treelines are anthropogenically depressed, observed advances are largely the result of land use change. Near-natural treelines are usually krummholz treelines, which are relatively unresponsive to climate change. Nevertheless, intense recruitment of treeline trees suggests a great potential for future treeline advance. Competitive abilities of seedlings within krummholz thickets and dwarf scrub heaths will be a major source of variation in treeline dynamics. Tree growth-climate relationships show mature treeline trees to be responsive to temperature change, in particular in winter and pre-monsoon seasons. High pre-monsoon temperature trends will most likely drive tree growth performance in the western and central Himalaya. Ecological niche modelling suggests that bioclimatic conditions for a range expansion of treeline trees will be created during coming decades.

Active Tectonics of Himalayan Faults/Thrusts System in Northern India on the basis of recent & Paleo earthquake Studies

NASA Astrophysics Data System (ADS)

Kumar, S.; Biswal, S.; Parija, M. P.

2016-12-01

The Himalaya overrides the Indian plate along a decollement fault, referred as the Main Himalayan Thrust (MHT). The 2400 km long Himalayan mountain arc in the northern boundary of the Indian sub-continent is one of the most seismically active regions of the world. The Himalayan Frontal Thrust (HFT) is characterized by an abrupt physiographic and tectonic break between the Himalayan front and the Indo-Gangetic plain. The HFT represents the southern surface expression of the MHT on the Himalayan front. The tectonic zone between the Main Boundary Thrust (MBT) and the HFT encompasses the Himalayan Frontal Fault System (HFFS). The zone indicates late Quaternary-Holocene active deformation. Late Quaternary intramontane basin of Dehradun flanked to the south by the Mohand anticline lies between the MBT and the HFT in Garhwal Sub Himalaya. Slip rate 13-15 mm/yr has been estimated on the HFT based on uplifted strath terrace on the Himalyan front (Wesnousky et al. 2006). An out of sequence active fault, Bhauwala Thrust (BT), is observed between the HFT and the MBT. The Himalayan Frontal Fault System includes MBT, BT, HFT and PF active fault structures (Thakur, 2013). The HFFS structures were developed analogous to proto-thrusts in subduction zone, suggesting that the plate boundary is not a single structure, but series of structures across strike. Seismicity recorded by WIHG shows a concentrated belt of seismic events located in the Main Central Thrust Zone and the physiographic transition zone between the Higher and Lesser Himalaya. However, there is quiescence in the Himalayan frontal zone where surface rupture and active faults are reported. GPS measurements indicate the segment between the southern extent of microseismicity zone and the HFT is locked. The great earthquake originating in the locked segment rupture the plate boundary fault and propagate to the Himalaya front and are registered as surface rupture reactivating the fault in the HFFS.

A hidden Late Cretaceous arc and subsequent magmatic events in the Caucasus-Iran-Anatolia (CIA) orogenic belt: Detrital zircon U-Pb and Hf isotopic constraints

NASA Astrophysics Data System (ADS)

Tien, C. Y.; Lin, Y. C.; Chu, M. F.; Chung, S. L.; Bi˙ngöl, A. F.

2017-12-01

The Caucasus-Iran-Anatolia (CIA) orogenic belt formed by "Turkic-type orogeny" consists mainly of subduction-accretion complexes following the collision between Eurasia and Arabia and the closure of Neotethy. This study reports U-Pb and Hf isotopic data of detrital zircon separates from five Eocene to mid-Miocene sandstone samples from Divrigi and Duranlar in the west to the Mus basin in the east, all locating in the northern part of the Bitlis-Zagros suture zone. The U-Pb age data suggest four main magmatic episodes: (1) 100-70 Ma, (2) 60-40 Ma, (3) 30 Ma, and (4) 15 Ma. The Late Cretaceous zircons recovered mainly from the Mus basin are marked by a significant Hf isotopic variation over time, with ɛHf(T) values dropping from +15 to -10. Zircons from the second and third episodes show spatial variations in isotopic compositions, with positive ɛHf(T) values (+10 to +5) in the Mus basin and heterogeneous ɛHf(T) values (+10 to -10) in the west. The fourth and youngest episode of zircons, mainly from Duranlar area, shows uniform ɛHf(T) values around +5. We attribute the Late Cretaceous episode of zircons to the broadly coeval Elazig arc magmatism that, according to our counterpart study, occurred as a short-lived, intra-oceanic arc system by subduction initiation after the formation of Neotethyan ophiolites in the region. Moreover, we argue that this Late Cretaceous arc system may have existed more widely within the southern branch of Neothethys than that suggested by present-day outcrops. The dramatic change in Hf isotopic composition from 100 to 70 Ma, also observed in the rock record by our counterpart study, may be interpreted as a result of subduction to accretion processes. The remaining three episodes of zircons are related to younger stages of magmatism within or around the suture zone that remains poorly studied. Our results indicate that detrital zircon is a useful tool to uncover "hidden" magmatic records in the CIA and other "Turkic-type" orogenic

Quantifying present and future glacier melt-water contribution to runoff in a Central Himalayan river basin

NASA Astrophysics Data System (ADS)

Prasch, M.; Mauser, W.; Weber, M.

2012-10-01

Water supply of most lowland cultures heavily depends on rain and melt-water from the upstream mountains. Especially melt-water release of alpine mountain ranges is usually attributed a pivotal role for the water supply of large downstream regions. Water scarcity is assumed as consequence of glacier shrinkage and possible disappearance due to Global Climate Change, particular for large parts of Central and South East Asia. In this paper, the application and validation of a coupled modeling approach with Regional Climate Model outputs and a process-oriented glacier and hydrological model is presented for a Central Himalayan river basin despite scarce data availability. Current and possible future contributions of ice-melt to runoff along the river network are spatially explicitly shown. Its role among the other water balance components is presented. Although glaciers have retreated and will continue to retreat according to the chosen climate scenarios, water availability is and will be primarily determined by monsoon precipitation and snow-melt. Ice-melt from glaciers is and will be a minor runoff component in summer monsoon-dominated Himalayan river basins.

Quantitative Comparison of Mountain Belt Topographic Profiles on Earth and Venus

NASA Astrophysics Data System (ADS)

Stoddard, P. R.; Jurdy, D. M.

2016-12-01

Earth's mountain belts result from interactions between tectonic plates. Several styles of belts reflect the differing nature of those interactions: The narrow spine of the Andes results from subduction of the oceanic Nazca plate under the continental South American plate, the soaring Himalayas from the collision of India and Asia, the broad Rockies and Alaskan cordillera from multiple collisions, and the gentle Appalachians and Urals are remnants from ancient collisions. Venus' mountain chains - Maxwell, Freyja, Akna and Danu - surround Lakshmi Planum, a highland with an elevation of 4 km. These make up Ishtar Terra. Maxwell Montes ascends to over 11 km, the highest elevation on the planet. Freyja rises just over 7 km and Akna to about 6 km. The arcuate Danu belt on Ishtar's western boundary comes up to only 1.5 km over the planum. No other mountain belts exist on Venus. The origins of these venusian orogenic belts remain unknown. Earliest explanations invoked subduction around Lakshmi Planum; subsequent models included either up- or down-welling of the mantle, horizontal convergence, or crustal thickening. We quantitatively compare topography of Venus' mountain chains with Earth's for similarities and differences. Patterns may provide clues to the dynamics forming venusian orogenic belts. To do this, we find topographic profiles across the various chains, determine average profiles for each, and then correlate averages to establish the degree of similarity. From this correlation we construct a covariance matrix, diagonalized for eigenvalues, or principal components. These can be displayed as profiles. Correlations and principal components allow us to assess the degree of similarity and variability of the shapes of the average profiles. These analyses thus offer independent and objective modes of comparison; for example, with respect to terrestrial mid-ocean ridges, some Venus chasmata were shown to most closely resemble the ultra-slow Arctic spreading center.

Trans-Himalayan water contributions to river discharge

NASA Astrophysics Data System (ADS)

Andermann, Christoff; Stieglitz, Thomas; Schuessler, Jan A.; Parajouli, Binod

2017-04-01

-defined diurnal cycles in water temperature, stage level and water chemistry. These diurnal cycles have a profound impact on the chemical concentrations and need to be corrected for to estimate representative geochemical fluxes for the full river and end member mixing modeling. Radon and trace element data indicate that groundwater contributions are primarily associated with the main tectonic structures of the Himalayan range, but also concentrate on the steep southern mountain front, and that groundwater outflow from the Lesser Himalayas is limited during baseflow season. Over the seasons the chemical dilution signature across the Himalayan range is persistent. However, specific elements have temporally distinct dilution signatures highlighting the alternating contribution of different hydrological compartments over the annual hydrological cycle. Our analysis allows to decipher the hydrological contribution of different water reservoirs to the surface water discharge in rivers, along a major Himalayan stream. Our results highlight the volumetric importance of a high mountain deep-groundwater storage compartment across the Himalayan mountain belt and provides first order quantification of groundwater contribution to stream flow.

Paleozoic magmatism and porphyry Cu-mineralization in an evolving tectonic setting in the North Qilian Orogenic Belt, NW China

NASA Astrophysics Data System (ADS)

Qiu, Kun-Feng; Deng, Jun; Taylor, Ryan D.; Song, Kai-Rui; Song, Yao-Hui; Li, Quan-Zhong; Goldfarb, Richard J.

2016-05-01

The NWW-striking North Qilian Orogenic Belt records the Paleozoic accretion-collision processes in NW China, and hosts Paleozoic Cu-Pb-Zn mineralization that was temporally and spatially related to the closure of the Paleo Qilian-Qinling Ocean. The Wangdian Cu deposit is located in the eastern part of the North Qilian Orogenic Belt, NW China. Copper mineralization is spatially associated with an altered early Paleozoic porphyritic granodiorite, which intruded tonalites and volcaniclastic rocks. Alteration zones surrounding the mineralization progress outward from a potassic to a feldspar-destructive phyllic assemblage. Mineralization consists mainly of quartz-sulfide stockworks and disseminated sulfides, with ore minerals chalcopyrite, pyrite, molybdenite, and minor galena and sphalerite. Gangue minerals include quartz, orthoclase, biotite, sericite, and K-feldspar. Zircon LA-ICPMS U-Pb dating of the ore-bearing porphyritic granodiorite yielded a mean 206Pb/238U age of 444.6 ± 7.8 Ma, with a group of inherited zircons yielding a mean U-Pb age of 485 ± 12 Ma, consistent with the emplacement age (485.3 ± 6.2 Ma) of the barren precursor tonalite. Rhenium and osmium analyses of molybdenite grains returned model ages of 442.9 ± 6.8 Ma and 443.3 ± 6.2 Ma, indicating mineralization was coeval with the emplacement of the host porphyritic granodiorite. Rhenium concentrations in molybdenite (208.9-213.2 ppm) suggest a mantle Re source. The tonalities are medium-K calc-alkaline. They are characterized by enrichment of light rare-earth elements (LREEs) and large-ion lithophile elements (LILEs), depletion of heavy rare-earth elements (HREEs) and high-field-strength elements (HFSEs), and minor negative Eu anomalies. They have εHf(t) values in the range of +3.6 to +11.1, with two-stage Hf model ages of 0.67-1.13 Ga, suggesting that the ca. 485 Ma barren tonalites were products of arc magmatism incorporating melts from the mantle wedge and the lithosphere. In contrast, the

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

NASA Astrophysics Data System (ADS)

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

2017-04-01

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

Dynamics and structure of the Alpine Fold Belt

NASA Technical Reports Server (NTRS)

Kahle, H. G.

1985-01-01

The structure and present-day dynamics of the Alps interms of geodesy and gravimetry are discusssed. A strong correlation of precise leveling and isostatic gravity along the central Alpine chain, especially in Canton Graubunden, East Switzerland are shown. It is assumed that the uplift is partly controlled by isostatic rebound effects. Field observations indicate that these phenomena are still active in the Alps. The study of the uplift processes by applying a number of geodetic and gravimetric measuring techniques, such as the determination of nonperiodic secular variations of gravity, of the deflections of the vertical and tilt changes monitored by hydrostatic leveling is proposed.

Exhumation history of the deepest central Himalayan rocks, Ama Drime range: Key pressure-temperature-deformation-time constraints on orogenic models

NASA Astrophysics Data System (ADS)

Kali, E.; Leloup, P. H.; Arnaud, N.; MahéO, G.; Liu, Dunyi; Boutonnet, E.; van der Woerd, J.; Liu, Xiaohan; Liu-Zeng, Jing; Li, Haibing

2010-04-01

The Ama Drime range located at the transition between the high Himalayan range and south Tibet is a N-S active horst that offsets the South Tibetan Detachment System (STDS). Within the horst, a paragneissic unit, possibly attributed to the upper Himalayan crystalline series, overly the lower Himalayan crystalline series Ama Drime orthogneissic unit containing large metabasite layers and pods that have experienced pressure ≥1.4 GPa. Combining structural analysis with new and published pressure-temperature (P-T) estimates as well as U-Th/Pb, 39Ar/40Ar and (U-Th)/He ages, the P-T-deformation-time (P-T-D-t) paths of the main units within and on both sides of the horst are reconstructed. They imply that N-S normal faults initiated prior to 11 Ma and have accounted for a total exhumation ≤0.6 GPa (22 km) that probably occurred in two phases: the first one until ˜9 Ma and the second one since 6 to 4 Ma at a rate of ˜1 mm/yr. In the Ama Drime unit, 1 to 1.3 GPa (37 to 48 km) of exhumation occurred after partial melting since ˜30 Ma until ˜13 Ma, above the Main Central Trust (MCT) and below the STDS when these two fault systems were active together. The switch from E-W (STDS) to N-S (Ama Drime horst) normal faulting between 13 and 12 Ma occurs at the time of propagation of thrusting from the MCT to the Main Boundary Thrust. These data are in favor of a wedge extrusion or thrust system rather than a crustal flow model for the building of the Himalaya. We propose that the kinematics of south Tibet Cenozoic extension phases is fundamentally driven by the direction and rate of India underthrusting.

Testing the effect of the Himalayan mountains as a physical barrier to gene flow in Hippophae tibetana Schlect. (Elaeagnaceae)

PubMed Central

Qiong, La; Zhang, Wenju; Wang, Hao; Zeng, Liyan; Birks, H. John B.; Zhong, Yang

2017-01-01

Hippophae tibetana is a small, dioecious wind-pollinated shrub endemic to the Tibetan-Qinghai Plateau. It is one of the shrubs that occur at very high elevations (5250 m a.s.l.). The Himalayan mountains provides a significant geographical barrier to the Qinghai-Tibetan Plateau, dividing the Himalayan area into two regions with Nepal to the south and Tibet to the north. There is no information on how the Himalayan mountains influence gene flow and population differentiation of alpine plants. In this study, we analyzed eight nuclear microsatellite markers and cpDNA trnT-trnF regions to test the role of the Himalayan mountains as a barrier to gene flow between populations of H. tibetana. We also examined the fine-scale genetic structure within a population of H. tibetana on the north slope of Mount (Mt.) Everest. For microsatellite analyses, a total of 241 individuals were sampled from seven populations in our study area (4 from Nepal, 3 from Tibet), including 121 individuals that were spatially mapped within a 100 m × 100 m plot. To test for seed flow, the cpDNA trnT-trnF regions of 100 individuals from 6 populations (4 from Nepal, 2 from Tibet) were also sequenced. Significant genetic differentiation was detected between the two regions by both microsatellite and cpDNA data analyses. These two datasets agree about southern and northern population differentiation, indicating that the Himalayan mountains represent a barrier to H. tibetana limiting gene flow between these two areas. At a fine scale, spatial autocorrelation analysis suggests significant genetic structure within a distance of less than 45 m, which may be attributed mainly to vegetative reproduction and habitat fragmentation, as well as limited gene flow. PMID:28489850

Testing the effect of the Himalayan mountains as a physical barrier to gene flow in Hippophae tibetana Schlect. (Elaeagnaceae).

PubMed

Qiong, La; Zhang, Wenju; Wang, Hao; Zeng, Liyan; Birks, H John B; Zhong, Yang

2017-01-01

Hippophae tibetana is a small, dioecious wind-pollinated shrub endemic to the Tibetan-Qinghai Plateau. It is one of the shrubs that occur at very high elevations (5250 m a.s.l.). The Himalayan mountains provides a significant geographical barrier to the Qinghai-Tibetan Plateau, dividing the Himalayan area into two regions with Nepal to the south and Tibet to the north. There is no information on how the Himalayan mountains influence gene flow and population differentiation of alpine plants. In this study, we analyzed eight nuclear microsatellite markers and cpDNA trnT-trnF regions to test the role of the Himalayan mountains as a barrier to gene flow between populations of H. tibetana. We also examined the fine-scale genetic structure within a population of H. tibetana on the north slope of Mount (Mt.) Everest. For microsatellite analyses, a total of 241 individuals were sampled from seven populations in our study area (4 from Nepal, 3 from Tibet), including 121 individuals that were spatially mapped within a 100 m × 100 m plot. To test for seed flow, the cpDNA trnT-trnF regions of 100 individuals from 6 populations (4 from Nepal, 2 from Tibet) were also sequenced. Significant genetic differentiation was detected between the two regions by both microsatellite and cpDNA data analyses. These two datasets agree about southern and northern population differentiation, indicating that the Himalayan mountains represent a barrier to H. tibetana limiting gene flow between these two areas. At a fine scale, spatial autocorrelation analysis suggests significant genetic structure within a distance of less than 45 m, which may be attributed mainly to vegetative reproduction and habitat fragmentation, as well as limited gene flow.

Geology, distribution, and classification of gold deposits in the western Qinling belt, central China

USGS Publications Warehouse

Mao, J.; Qiu, Yumin; Goldfarb, R.J.; Zhang, Z.; Garwin, S.; Fengshou, R.

2002-01-01

Gold deposits of the western Qinling belt occur within the western part of the Qinling-Dabie-Sulu orogen, which is located between the Precambrian North China and Yangtze cratons and east of the Songpan-Ganzi basin. The early Paleozoic to early Mesozoic orogen can be divided into northern, central, and southern zones, separated by the Shangdan and Lixian-Shanyang thrust fault systems. The northern zone consists of an early Paleozoic arc accreted to the North China craton by ca. 450 Ma. The central zone, which contains numerous orogenic gold deposits, is dominated by clastic rocks formed in a late Paleozoic basin between the converging cratonic blocks. The southern zone is characterized by the easternmost exposure of Triassic sedimentary rocks of the Songpan-Ganzi basin. These Early to Late Triassic turbidities, in part calcareous, of the immense Songpan-Ganzi basin also border the western Qinling belt to the west. Carlinlike gold deposits are abundant (1) along a westward extension of the southern zone defined by a window of early Paleozoic clastic rocks extending into the basin, and (2) within the easternmost margin of the basinal rocks to the south of the extension, and in adjacent cover rocks of the Yangtze craton. Triassic and Early Jurassic synkinematic granitoids are widespread across the western Qinling belt, as well as in the Songpan-Ganzi basin. Orogenic lode gold deposits along brittle-ductile shear zones occur within greenschist-facies, highly deformed, Devonian and younger clastic rocks of the central zone. Mainly coarse-grained gold, along with pyrite, pyrrhotite, arsenopyrite, and minor base metal sulfides, occur in networks of quartz veinlets, brecciated wall rock, and are dissminated in altered wall rock. Isotopic dates suggest that the deposits formed during the Late Triassic to Middle Jurassic as the leading edge of the Yangtze craton was thrust beneath rocks of the western Qinling belt. Many gold-bearing placers are distributed along the river

Metamorphozed Hercynian granitoids in the Alpine structures of the Central Rhodope, Bulgaria: geotectonic position and geochemistry

NASA Astrophysics Data System (ADS)

Cherneva, Zlatka; Georgieva, Milena

2005-05-01

Orthogneisses of late-Hercynian protolith age crop out in the Central Rhodope high-grade metamorphic complex, which is part of the Alpine orogen in south-eastern Europe. They compose a tectonic unit bordered by late-Alpine extensional shear zones. These rocks reflect Eocene amphibolite facies migmatization (<750 °C/0.9-0.5 GPa). The low-temperature melting favored zircon inheritance and disturbed mainly the LILE protolith compositions. Despite the intense Alpine metamorphic overprint, the major elements, HFSE and REE reflect the initial composition of the Hercynian protolith. A geochemical data set summarizing 200 whole rock analyses testifies to a calc-alkaline magma differentiation producing a compositional range of tonalite and/or granodiorite to granite and leucocratic granite. Geochemical compositions combined with published isotope and age data suggest dominant I-type protoliths and mixed magma sources including crustal and mantle material, and distinguish between older granitoids of volcanic-arc affinity and probably younger ones of late or post-collision origin.

Orogenic front propagation in the basement involved Malargüe fold and thrust belt, Neuquén Basin, (Argentina)

NASA Astrophysics Data System (ADS)

Branellec, Matthieu; Nivière, Bertrand; Callot, Jean-Paul; Ringenbach, Jean-Claude

2015-04-01

The Malargüe fold and thrust belt (MFTB) and the San Rafael Block (SRB) are located in the northern termination of the Neuquén basin in Argentina. This basin is a wide inverted intracratonic sag basin with polyphased evolution controlled at large scale by the dynamic of the Pacific subduction. By late Triassic times, narrow rift basins developed and evolved toward a sag basin from middle Jurassic to late Cretaceous. From that time on, compression at the trench resulted in various shortening pulses in the back-arc area. Here we aim to analyze the Andean system at 35°S by comparing the Miocene structuration in the MFTB and the current deformation along the oriental border or the San Rafael Block. The main structuration stage in the MFTB occurred by Miocene times (15 to 10 Ma) producing the principal uplift of the Andean Cordillera. As shown by new structural cross sections, Triassic-early Jurassic rift border faults localized the Miocene compressive tectonics. Deformation is compartmentalized and does not exhibit a classical propagation of homogeneous deformation sequence expected from the critical taper theory. Several intramontane basins in the hangingwall of the main thrusts progressively disconnected from the foreland. In addition, active tectonics has been described in the front of the MFTB attesting for the on-going compression in this area. 100 km farther to the east, The San Rafael Block, is separated from the MFTB by the Rio Grande basin. The SRB is mostly composed of Paleozoic terranes and Triassic rift-related rocks, overlain by late Miocene synorogenic deposits. The SRB is currently uplifted along its oriental border along several active faults. These faults have clear morphologic signatures in Quaternary alluvial terraces and folded Pleistocene lavas. As in the MFTB, the active deformation localization remains localized by structural inheritance. The Andean system is thus evolving as an atypical orogenic wedge partly by frontal accretion at the front

Geochemical budget of erosion in the Himalayan system

NASA Astrophysics Data System (ADS)

France-Lanord, C.; Lupker, M.; Lavé, J.

2011-12-01

Geochemistry of detrital sediment allow to constrain present and past processes of erosion and may be used to infer large scale budgets. The sediment flux exported by major rivers, corresponds to net physical erosion of a basin and, if combined with the river's dissolved flux, it allows in principle to quantify total erosion rates as well as the balance between chemical and physical erosion. This is nevertheless depending on our ability to quantify riverine fluxes, sediment composition and average source rock composition. Sediment composition results from weathering, attrition, mixing and sorting processes which determine their properties at a given location and time in the drainage basin. Particle sorting during transport exerts a first order control on bulk sediment composition as well as on trace and isotopic compositions. Repeated sampling of river sediments in the Ganga and Brahmaputra Rivers during the monsoon using depth sampling combined to river velocity profiling (ACDP) show the sensitivity of sediment composition to particle sorting effects that can be linked to hydrodynamic conditions. Applied to the Ganga in Bangladesh, and using a Rousean model, we estimate the average grain size and major element composition (Si, Al, Fe) of sediments exported by the Ganga (Lupker et al. 2011, JGR E.S. in press). This leads to a net sediment flux of ca. 380 million tons/yr which is comparable to fluxes reported from hydrological measurements. The average Al/Si ratio of the sediment is 0.23 which is well bellow values reported for the Upper Continental Crust (UCC≈0.27-0.29) and slightly above estimate of the Himalayan Silicate Crust (HSC) composition deduced from a Central Nepal geological sample collection. Deviations from UCC reflect the recycled nature of Himalayan formations having undergone several orogenic cycles and being enriched in quartz. The similarity with HSC suggest that either (1) there is no segregation due to floodplain sequestration, or (2) that the

The reproductive strategy of a pollinator-limited Himalayan plant, Incarvillea mairei (Bignoniaceae)

PubMed Central

2013-01-01

Background Plants may adapt to alpine habitats by specialization in the reproductive strategy and functional aspects of their flowers and pollination systems. Alpine habitats reduce the opportunities for cross-pollination in a relatively high proportion of alpine plant species, and self-pollination may be favored in these adverse conditions. Here, we investigated the mating system and pollination of Incarvillea mairei, a perennial Himalayan herb typically found at altitudes between 3000 and 4500 m. Results Analyses of floral morphology, observation of plant-pollinator interactions, and hand pollination experiments were conducted in three natural populations. Outcrossing rates and effective numbers of pollen donors were assessed in 45 open-pollinated families by using progeny analysis based on seven microsatellite markers. Incarvillea mairei displayed a set of apparently specialized floral traits, the stigma is sensitive to touch and close immediately and its reopening allows a second opportunity for the receipt of pollen. The species is fully self-compatible but employs a predominantly outcrossing mating system according to parentage analysis (tm > 0.9). Fruit set was low (26.3%), whereas seed set was high (67.2%), indicating that this species suffers pollinator limitation. Its main effective pollinator was Halictus sp., and visitation frequency was low. Conclusions Floral features such as having a sensitive stigma and anther-prongs, in conjunction with pollinator behavior, function together contributing to a set of unique reproductive adaptations that enhance outcrossing success. The increased floral longevity and high pollination efficiency operated as compensatory mechanisms to counteract low pollinator visitation frequency. PMID:24289097

Transpressional deformation, strain partitioning and fold superimposition in the southern Chinese Altai, Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Li, Pengfei; Sun, Min; Rosenbaum, Gideon; Cai, Keda; Chen, Ming; He, Yulin

2016-06-01

Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S1), is locally recognized in low strain areas, and is commonly isoclinally folded by F2 folds that have an axial plane orientation parallel to the dominant fabric (S2). S2 is associated with a shallowly plunging stretching lineation (L2), and defines ∼NW-SE tight-close upright macroscopic folds (F3) with the doubly plunging geometry. F3 folds are superimposed by ∼NNW-SSE gentle F4 folds. The F3 and F4 folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F3 fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F3) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F4 folds.

Distribution, microfabric, and geochemical characteristics of siliceous rocks in central orogenic belt, China: implications for a hydrothermal sedimentation model.

PubMed

Li, Hongzhong; Zhai, Mingguo; Zhang, Lianchang; Gao, Le; Yang, Zhijun; Zhou, Yongzhang; He, Junguo; Liang, Jin; Zhou, Liuyu; Voudouris, Panagiotis Ch

2014-01-01

Marine siliceous rocks are widely distributed in the central orogenic belt (COB) of China and have a close connection to the geological evolution and metallogenesis. They display periodic distributions from Mesoproterozoic to Jurassic with positive peaks in the Mesoproterozoic, Cambrian--Ordovician, and Carboniferous--Permian and their deposition is enhanced by the tensional geological settings. The compressional regimes during the Jinning, Caledonian, Hercynian, Indosinian, and Yanshanian orogenies resulted in sudden descent in their distribution. The siliceous rocks of the Bafangshan-Erlihe ore deposit include authigenic quartz, syn-depositional metal sulphides, and scattered carbonate minerals. Their SiO2 content (71.08-95.30%), Ba (42.45-503.0 ppm), and ΣREE (3.28-19.75 ppm) suggest a hydrothermal sedimentation origin. As evidenced by the Al/(Al + Fe + Mn), Sc/Th, (La/Yb) N, and (La/Ce) N ratios and δCe values, the studied siliceous rocks were deposited in a marginal sea basin of a limited ocean. We suggest that the Bafangshan-Erlihe area experienced high- and low-temperature stages of hydrothermal activities. The hydrothermal sediments of the former stage include metal sulphides and silica, while the latter was mainly composed of silica. Despite the hydrothermal sedimentation of the siliceous rocks, minor terrigenous input, magmatism, and biological activity partly contributed to geochemical features deviating from the typical hydrothermal characteristics.

Distribution, Microfabric, and Geochemical Characteristics of Siliceous Rocks in Central Orogenic Belt, China: Implications for a Hydrothermal Sedimentation Model

PubMed Central

Li, Hongzhong; Zhai, Mingguo; Zhang, Lianchang; Gao, Le; Yang, Zhijun; Zhou, Yongzhang; He, Junguo; Liang, Jin; Zhou, Liuyu; Voudouris, Panagiotis Ch.

2014-01-01

Marine siliceous rocks are widely distributed in the central orogenic belt (COB) of China and have a close connection to the geological evolution and metallogenesis. They display periodic distributions from Mesoproterozoic to Jurassic with positive peaks in the Mesoproterozoic, Cambrian—Ordovician, and Carboniferous—Permian and their deposition is enhanced by the tensional geological settings. The compressional regimes during the Jinning, Caledonian, Hercynian, Indosinian, and Yanshanian orogenies resulted in sudden descent in their distribution. The siliceous rocks of the Bafangshan-Erlihe ore deposit include authigenic quartz, syn-depositional metal sulphides, and scattered carbonate minerals. Their SiO2 content (71.08–95.30%), Ba (42.45–503.0 ppm), and ΣREE (3.28–19.75 ppm) suggest a hydrothermal sedimentation origin. As evidenced by the Al/(Al + Fe + Mn), Sc/Th, (La/Yb)N, and (La/Ce)N ratios and δCe values, the studied siliceous rocks were deposited in a marginal sea basin of a limited ocean. We suggest that the Bafangshan-Erlihe area experienced high- and low-temperature stages of hydrothermal activities. The hydrothermal sediments of the former stage include metal sulphides and silica, while the latter was mainly composed of silica. Despite the hydrothermal sedimentation of the siliceous rocks, minor terrigenous input, magmatism, and biological activity partly contributed to geochemical features deviating from the typical hydrothermal characteristics. PMID:25140349

U Pb zircon age, geochemical and Sr Nd Pb Hf isotopic constraints on age and origin of alkaline intrusions and associated mafic dikes from Sulu orogenic belt, Eastern China

NASA Astrophysics Data System (ADS)

Liu, Shen; Hu, Ruizhong; Gao, Shan; Feng, Caixia; Qi, Youqiang; Wang, Tao; Feng, Guangying; Coulson, Ian M.

2008-12-01

Post-orogenic alkaline intrusions and associated mafic dikes from the Sulu orogenic belt of eastern China consist of quartz monzonites, A-type granites and associated mafic dikes. We report here U-Pb zircon ages, geochemical data and Sr-Nd-Pb-Hf isotopic data for these rocks. The SHRIMP U-Pb zircon analyses yield consistent ages ranging from 120.3 ± 2.1 Ma to 126.9 ± 1.9 Ma for five samples from the felsic rocks, and two crystallization ages of 119.0 ± 1.7 Ma and 120.2 ± 1.9 Ma for the mafic dikes. The felsic rocks and mafic dikes are characterized by high ( 87Sr/ 86Sr) i ranging from 0.7079 to 0.7089, low ɛNd( t) values from - 15.3 to - 19.2, 206Pb/ 204Pb = 16.54-17.25, 207Pb/ 204Pb = 15.38-15.63, 208Pb/ 204Pb = 37.15-38.45, and relatively uniform ɛHf( t) values of between - 21.6 ± 0.6 and - 23.7 ± 1.0, for the magmatic zircons. The results suggest that they were derived from a common enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials before magma generation. Geochemical and isotopic characteristics imply that the primary magma to these rocks originated through partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dikes may have been generated by subsequent fractionation of clinopyroxene, whereas the felsic rocks resulted from fractionation of potassium feldspar, plagioclase and ilmenite or rutile. Both were not affected by crustal contamination. Combined with previous studies, these findings provide new evidence that the intense lithospheric thinning beneath the Sulu belt of eastern China occurred between 119 and 127 Ma, and that this was caused by the removal of the lower lithosphere (mantle and lower crust).

Pressure-temperature-fluid evolution of the Mongolian Altai in the Central Asian Orogenic Belt: evidence from mineral equilibrium modeling and fluid inclusion studies on amphibolite-facies rocks from western Mongolia

NASA Astrophysics Data System (ADS)

Zorigtkhuu, O.-E.

2012-04-01

The Central Asian Orogenic Belt (CAOB), also known as Altaids, located between the Archean Siberian Craton to the north and the Tarim and North China Cratons to the south, is regarded as one of the largest accretionary and collisional orogen in the world. Detailed petrological studies on the CAOB therefore provide useful information of pressure-temperature (P-T) history of the orogeny as well as the tectonic evolution of East Asia. This study reports detailed petrological data, particularly the results of phase equilibrium modeling and fluid inclusion analysis, of pelitic schists and amphibolites from Bodonch area, southwestern Mongolia, which occupies a significant part of the Paleozoic history of the Altai Orogen in the southwestern margin of the CAOB, and discuss pressure-temperature-fluid evolution of the area. The dominant mineral assemblages of pelitic schist in Bodonch area are garnet + kyanite + staurolite + biotite + plagioclase, garnet + biotite + staurolite + cordierite, and garnet + biotite + sillimanite + plagioclase with quartz and ilmenite, while amphibolite contains calcic amphibole + quartz + plagioclase + garnet + ilmenite assemblage. Application of conventional garnet-biotite and garnet-cordierite geothermometers as well as GASP geobarometer gave metamorphic conditions of 615-635°C/8.2-8.9 kbar from kyanite-bearing pelitic schist samples. Slightly higher P-T condition of 640-690°C/6.3-10.7 kbar was obtained by mineral equilibrium modeling of garnet-kyanite-staurolite and garnet-staurolite-cordierite assemblages using Theriak-Domino software. The calculation was made in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (NCKFMASH). We constructed a clockwise P-T path staring from high-pressure amphibolite facies condition within the stability field of kyanite (approximately 650°C/9 kbar) possibly through the stability field of sillimanite by post-peak decompressional cooling. Our petrographical observations of fluid inclusions in pelitic schists

The St. Elias orogen as an early stage in the development of indentor corners: Initial results from the STEEP project

NASA Astrophysics Data System (ADS)

Pavlis, T. L.; Bruhn, R. L.; Koons, P.; Berger, A.; Spotila, J.; Barker, A.; Chapman, J.; Doser, D.; Pavlis, G.

2006-12-01

The actively deforming St. Elias orogen of southern Alaska is developed at the transition between the Aleutian trench and the Queen Charlottes transform, and has produced a "one sided" indentor corner as the collided block has been carried obliquely into the subduction-transform transition. A distinctive feature generated within this evolving corner is a 90o bend in the suture at the western edge of the orogen that is associated with a refolding of earlier thin-skinned fold-thrust systems in the collided block. The refolding is recognizable in the area between the bent suture and a major glacial valley, the Bering Glacier, but is absent to the east. Bruhn et al. (2004, GSAB) used this observation to infer an active structure along the Bering Glacier, and speculated that the refolding may be active. New data collected as part of the St. Elias Erosion and tectonics Project (STEEP) provides support for this hypothesis from two observations: 1) clear evidence that many of the hundreds of surface ruptures in this region represent fault-scarps-- probably representing fold-related, bedding- plane slip during refolding; and 2) extremely young (locally <500Ka) U-Th-He apatite cooling ages just west of the Bering Glacier are consistent with the hypothesis of an active structure along the glacial valley. However, young cooling ages to the east of the Bering Glacier as well as geomorphology suggest that at least one of the fold-thrust belt structures is also active, indicating that the interplay of active faults is complex. Additional support for active deformation along the Bering Glacier comes from the spatial distribution of seismicity between the Bering Glacier and Bagley icefield and these observations should be further constrained as data are obtained from the new STEEP broadband seismic network in the area. Analog (sand) and numerical models suggest that this type of feature can originate as the indentor is driven into the corner through a variable vertical axis

Magmatism in the Shapinggou district of the Dabie orogen, China: Implications for the formation of porphyry Mo deposits in a collisional orogenic belt

NASA Astrophysics Data System (ADS)

Ren, Zhi; Zhou, Taofa; Hollings, Pete; White, Noel C.

2018-05-01

The Shapinggou molybdenum deposit is located in the Qinling-Dabie Orogen, which hosts the world's largest molybdenum belt. The igneous rocks at Shapinggou can be divided into two stages (136-127 Ma and 118-114 Ma), the early suite of felsic (136-127 Ma, SiO2 = 58.0 to 72.9 wt%) and mafic rocks (133-128 Ma, SiO2 = 45.2 to 57.0 wt%), and a later suite comprising syenite (117 Ma, SiO2 = 64.2 to 65.0 wt%), quartz syenite porphyry (116 Ma, 62.5 to 70.0 wt%), granite porphyry (112 Ma, SiO2 = 75.5 to 77.6 wt%) and diorite porphyry (111 Ma, SiO2 = 56.6 to 59.7 wt%). The early-stage felsic rocks display high SiO2, Al2O3, Na2O, K2O, Sr, LREE contents, and Sr/Y, (La/Yb)N ratios, initial Sr isotope ratios of 0.7076 to 0.7089, but low MgO, FeOT, Y, Yb contents and negative εNd(t) values, consistent with partial melting of the lower continental crust. The early-stage mafic rocks exhibit low SiO2, high MgO, Ni and Cr contents, consistent with an upper mantle source, but trace element and isotope data suggest a role for crustal contamination. The late-stage syenite and quartz syenite porphyry show high abundances of Na2O, K2O, Al2O3, HFSEs (e.g., Th, U, Zr, Hf) and significant negative Eu anomalies. The late-stage granite porphyry displays high SiO2 contents, and depletions in Ba, Sr, Eu and Ti. The geochemical features of the late-stage intrusions are similar to A-type granites. Crystal fractionation of plagioclase, K-feldspar, biotite/ muscovite, amphibole/ garnet and Fe-Ti oxides controlled the evolution of the magma. The geochemical and isotopic data suggest that the rocks at Shapinggou were likely derived from a mixed source of lithospheric mantle, subducted continental crust of the Yangtze Block (Kongling Group) and partial melts of the Dabie Complex. Early stage rocks represent melts of the source with a lower proportion of Dabie Complex materials, whereas late stage rocks were derived from a source with a higher proportion Dabie Complex component. The geochemical and

Phanerozoic strike-slip faulting in the continental interior platform of the United States: Examples from the Laramide Orogen, midcontinent, and Ancestral Rocky Mountains

USGS Publications Warehouse

Marshak, S.; Nelson, W.J.; McBride, J.H.

2003-01-01

Alpine-Himalayan collision, forcing crustal blocks to undergo significant lateral escape. The strong North American craton strained relatively little during collisional-convergent orogeny, so crustal blocks underwent relatively small displacements.

Origin and evolution of multi-stage felsic melts in eastern Gangdese belt: Constraints from U-Pb zircon dating and Hf isotopic composition

NASA Astrophysics Data System (ADS)

Guo, Liang; Zhang, Hong-Fei; Harris, Nigel; Pan, Fa-Bin; Xu, Wang-Chun

2011-11-01

This integrated study of whole rock geochemistry, zircon U-Pb dating and Hf isotope composition for seven felsic rocks from the Nyingchi Complex in eastern Himalayan syntaxis has revealed a complex magmatic history for the eastern Gangdese belt. This involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the Himalayan orogen. Our U-Pb zircon dating reveals five stages of magmatic or anatectic events: 165, 81, 61, 50 and 25 Ma. The Jurassic granitic gneiss (165 Ma) exhibits εHf(t) values of + 1.4 to + 3.5. The late Cretaceous granite (81 Ma) shows variable εHf(t) values from - 0.9 to + 6.2, indicating a binary mixing between juvenile and old crustal materials. The Paleocene granodioritic gneiss (61 Ma) has εHf(t) values of + 5.4 to + 8.0, suggesting that it originated from partial melting of a juvenile crustal material. The Eocene anatexis is recorded in the leucosome, which has Hf isotopic composition similar to that of the Jurassic granite, indicating that the leucosome could be derived from partial melting of the Jurassic granite. The late Oligocene biotite granite (25 Ma) shows adakitic geochemical characteristics, with Sr/Y = 49.3-56.6. The presence of a large number of inherited zircons and negative εHf(t) values suggest that it sourced from anatexis of crustal materials. In contrast to the Gangdese batholiths that are mainly derived from juvenile crustal source in central Tibet, the old crustal materials play an important role for the magma generation of the felsic rocks, suggesting the existence of a crustal basement in the eastern Gangdese belt. These correspond to specific magmatic evolution stages during the convergence between India and Asia. The middle Jurassic granitic gneiss resulted from the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous magmatism is probably related to the ocean ridge subduction. The Paleocene-Eocene magmatism, metamorphism and anatexis are

40 Ma of hydrothermal W mineralization during the Variscan orogenic evolution of the French Massif Central revealed by U-Pb dating of wolframite

NASA Astrophysics Data System (ADS)

Harlaux, Matthieu; Romer, Rolf L.; Mercadier, Julien; Morlot, Christophe; Marignac, Christian; Cuney, Michel

2018-01-01

We present U-Pb thermal ionization mass spectrometer (TIMS) ages of wolframite from several granite-related hydrothermal W±Sn deposits in the French Massif Central (FMC) located in the internal zone of the Variscan belt. The studied wolframite samples are characterized by variable U and Pb contents (typically <10 ppm) and show significant variations in their radiogenic Pb isotopic compositions. The obtained U-Pb ages define three distinct geochronological groups related to three contrasting geodynamic settings: (i) Visean to Namurian mineralization (333-327 Ma) coeval with syn-orogenic compression and emplacement of large peraluminous leucogranites (ca. 335-325 Ma), (ii) Namurian to Westphalian mineralization (317-315 Ma) synchronous with the onset of late-orogenic extension and emplacement of syn-tectonic granites (ca. 315-310 Ma) and (iii) Stephanian to Permian mineralization (298-274 Ma) formed during post-orogenic extension contemporaneous with the Permian volcanism in the entire Variscan belt. The youngest ages (276-274 Ma) likely reflect the reopening of the U-Pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration (e.g. ferberitization). Our results demonstrate that W(±Sn) mineralization in the FMC formed during at least three distinct hydrothermal events in different tectono-metamorphic settings over a time range of 40 Ma.

Paleozoic magmatism and porphyry Cu-mineralization in an evolving tectonic setting in the North Qilian Orogenic Belt, NW China

USGS Publications Warehouse

Qiu, Kun-Feng; Deng, Jun; Taylor, Ryan D.; Song, Kai-Rui; Song, Yao-Hui; Li, Quan-Zhong; Goldfarb, Richard J.

2016-01-01

The NWW-striking North Qilian Orogenic Belt records the Paleozoic accretion–collision processes in NW China, and hosts Paleozoic Cu–Pb–Zn mineralization that was temporally and spatially related to the closure of the Paleo Qilian-Qinling Ocean. The Wangdian Cu deposit is located in the eastern part of the North Qilian Orogenic Belt, NW China. Copper mineralization is spatially associated with an altered early Paleozoic porphyritic granodiorite, which intruded tonalites and volcaniclastic rocks. Alteration zones surrounding the mineralization progress outward from a potassic to a feldspar-destructive phyllic assemblage. Mineralization consists mainly of quartz-sulfide stockworks and disseminated sulfides, with ore minerals chalcopyrite, pyrite, molybdenite, and minor galena and sphalerite. Gangue minerals include quartz, orthoclase, biotite, sericite, and K-feldspar. Zircon LA-ICPMS U–Pb dating of the ore-bearing porphyritic granodiorite yielded a mean 206Pb/238U age of 444.6 ± 7.8 Ma, with a group of inherited zircons yielding a mean U–Pb age of 485 ± 12 Ma, consistent with the emplacement age (485.3 ± 6.2 Ma) of the barren precursor tonalite. Rhenium and osmium analyses of molybdenite grains returned model ages of 442.9 ± 6.8 Ma and 443.3 ± 6.2 Ma, indicating mineralization was coeval with the emplacement of the host porphyritic granodiorite. Rhenium concentrations in molybdenite (208.9–213.2 ppm) suggest a mantle Re source. The tonalities are medium-K calc-alkaline. They are characterized by enrichment of light rare-earth elements (LREEs) and large-ion lithophile elements (LILEs), depletion of heavy rare-earth elements (HREEs) and high-field-strength elements (HFSEs), and minor negative Eu anomalies. They have εHf(t) values in the range of +3.6 to +11.1, with two-stage Hf model ages of 0.67–1.13 Ga, suggesting that the ca. 485 Ma barren tonalites were products of arc magmatism incorporating melts from the mantle wedge and

Discussion Starter: The Case for Duplexing without Channel Flow During the Development and Emplacement of the Himalayan Middle Crust

NASA Astrophysics Data System (ADS)

Webb, A. G.; He, D.; Yu, H.

2015-12-01

This presentation and another presentation led by Dawn Kellett will preface a ten-minute open discussion on how the Himalayan middle crust was developed and emplaced. Current hypotheses are transitioning from a set including wedge extrusion, channel flow with focused denudation, and tectonic wedging to a revised dichotomy: models with intense upper plate out-of-sequence activity (i.e., tunneling of channel flow, and critical taper wedge behavior) versus models in which the upper plate mainly records basal accretion of horses (i.e., duplexing). Critical taper and duplexing offer a simple contrast that can be illustrated via food analogies. If a wedge is critical, it churns internally like a pile of CheeriosTM cereal pushed up an inclined plane. Stacking of a duplex acts like a deli meat-slicing machine: slice after slice is cut from the intact block to a stack of slices, but neither the block (~down-going plate) nor the stack (~upper plate) features much internal deformation. Thus critical taper and channel tunneling models predict much processing via out-of-sequence deformation, whereas duplexing predicts in-sequence thrusting. The two concepts may be considered end-members. Recent work shows that the Himalayan middle crust has been assembled along a series of mainly southwards-younging thrust faults. The thrust faults separate 1-5 km thick panels that experienced similar metamorphic cycles during different time periods. Out-of-sequence deformation is rare, with its apparent significance enhanced because of the high throw-to-heave ratio of out-of-sequence thrusting. Flattening fabrics developed prior to thrusting have been interpreted to record either (1) southwards channel tunneling across the upper plate, or (2) fabric development during metamorphism of the down-going plate. We will argue that the thrust faults dominantly represent in-sequence duplexing, and therefore conclude that the Himalaya and analogous hot orogens behave like other accretionary orogens.

What controls deformation in a bent three-dimensional orogen? An example from the Bolivian Andes

NASA Astrophysics Data System (ADS)

Kaislaniemi, L.; Whipp, D. M., Jr.

2017-12-01

The width of orogens is thought to be affected by both erosional intensity and strength of the rocks. Along-strike variation of the orogen width can be expected to reflect shifts in these factors. An example of such variation can be found around the Bolivian orocline, which is a change in the orientation of the central Andes, in central Bolivia, from N-S south of 18°S to roughly NW-SE in the north. This bend coincides with 50% reduction in the width of the orogen east of the Altiplano, an approximately eight-fold increase in the annual precipitation, and the presence of a basement arch that reduces the thickness of relatively weak Paleozoic sediments upon which the orogen detaches. This has led to uncertainty about whether the growth of the orogen is controlled primarily by climate (erosion) or tectonics (strength of the basal detachment). We study deformation in a segmented orogen using 3D geodynamic models to understand how along-strike variations in rainfall and basal detachment strength affect orogen deformation and growth of the frontal part of the Andean fold-and-thrust belt (FTB). We calculate the visco-plastic deformation in the retro-wedge of an Andean-style orogen using the finite element software DOUAR (Braun et al. 2008) coupled to the surface process model FastScape (Braun & Willett 2013). The model design includes the basement, the Altiplano, and the FTB east of the plateau. A weak basal detachment zone is prescribed. Strain softening allows development of new faults and free evolution of the detachment zone. The effects of varying rock strength and varying precipitation are considered to determine the primary control(s) on the geometry and evolution of curved orogens. Results show that both increased precipitation and stronger detachment zone can explain differences in the width of the FTB, as reflected in the topography. These factors, however, lead to different structural evolution of the orogen: Weak basal detachment zone promotes growth of the

The relationship between gold ore and geotectonic in West Qinling: Evidence from magnetotelluric soundings

NASA Astrophysics Data System (ADS)

Xu, Daili; Huang, Wei; Zhang, Letian

2017-03-01

The West Qinling belt is the westward extensioin of the Qinling-Dabie-Sulu orogen in central China. It links the Kunlun and Qilian orogens to the west and the Songpan-Ganze belt to the south, making it an important tectonics syntaxis in China. According to the collection and analysis of the data of West Qinling Mountains gold deposit metallogenic chronology, the gold deposits in this area can be divided into five groups by time: 225-245 Ma, 220-190 Ma, 170 Ma±, 135-110 Ma, 75-45 Ma. The formation time is related to the geological process at that time. The relationship between the formation of gold deposit and geological structure can be explored by the research on the present geological structures. According to the regional tectonic evolution and magmatic activity, the gold deposits are divided into four metallogenic epoches: from the end of the Indo-Chinese epoch to the early Yanshanian (245-225 Ma), is one of the initial stages of gold mineralization; the early and the middle Yanshanian (220-190 Ma, 170 Ma±), is the main forming time of the gold mineralization in the West Qinling; the late Yanshanian (135-110 Ma), is the superimposed mineralization stage; at last, the effect of Himalayan hydrothermal activity to the gold deposits. Under the support of three projects naemed " the research on the conductivity of the lithosphere of the Central Orogenic in China ", " the MT array observation of the intersection area of COC(Central Orogen in China) and the North-South seismic belt ", " the experiment research of MT standard network observation in the area of Qinghai-Tibetan Plateau and the North-China " (Sinoprobe-01-02), we collected massive magnetotellurics data for futher study of the present geological strusctures to discess the relationship between gold deposites and geotectonic in West Qinling.

Along-Strike Differences of the Main Himalayan Thrust and Deformation within the Indian Crust: Insights from Seismicity and Seismic Velocities in Bhutan and its Foreland

NASA Astrophysics Data System (ADS)

Diehl, T.; Singer, J.; Hetényi, G.; Kissling, E. H.; Clinton, J. F.

2015-12-01

The seismicity of Bhutan is characterized by the apparent lack of great earthquakes and a significantly lower activity compared to most other parts of the Himalayan arc. To better understand the underlying mechanisms of this anomalously low activity and to relate it with possible along-strike differences in the structure of the orogenic belt, a temporary network with up to 38 broadband seismometers was installed in Bhutan between January 2013 and November 2014. In this work we present a catalog of local and regional earthquakes detected and located with the GANSSER network complemented by regional stations in India, Bangladesh, and China. State-of-the-art data analysis and earthquake location procedures were applied to derive a high-precision earthquake catalog of Bhutan and surrounding regions. Focal mechanisms from regional moment tensor inversions and first-motion polarities complement the earthquake catalog. In the vicinity of the Shumar-Kuru Chu Spur in East Bhutan, seismicity forms a moderately dipping structure at about 12 km depth, which we associate with the Main Himalayan Thrust (MHT). North of 27.6°N the dip of the structure steepens, which can be interpreted as a ramp along the MHT. In West Bhutan seismicity occurs at depths of 20 to 40 km and receiver function images indicate that seismicity occurs in the underthrusting Indian crust rather than on the MHT. The highest seismic activity is clustered along the Goalpara Lineament, a dextral NE-SW striking shear zone in southwest Bhutan, which appears to connect to the western edge of the Shillong Plateau in the foreland. Focal depths indicate that this shear zone is located at depths of 20-30 km and therefore in the underthrusting Indian crust. Preliminary results of a 3D local earthquake tomography show substantial differences in the uppermost crust between east and west Bhutan. Consistent with our receiver function images, the results also indicate a thinning of the crustal root towards eastern Bhutan.

Climate-growth relationships of Abies spectabilis in a central Himalayan treeline ecotone

NASA Astrophysics Data System (ADS)

Schwab, Niels; Kaczka, Ryszard J.; Schickhoff, Udo

2017-04-01

Climate warming is expected to induce treelines to advance to higher elevations. Empirical studies in diverse mountain ranges, however, give evidence of both advancing alpine treelines as well as rather insignificant responses. The large spectrum of responses is not fully understood. In the framework of investigating the sensitivity and response of a near-natural treeline ecotone in Rolwaling Himal, Nepal, to climate warming we present results from dendroclimatological analyses of Abies spectabilis (Himalayan Fir) increment cores. Tree ring width was measured and cross-dated. After standardization, the chronology was correlated with temperature and precipitation variables. Preliminary results point to positive correlations with autumn temperature and precipitation. We will present improved climate-growth relationships. The resulting climate - tree growth relationships may be used as an indication of future growth patterns and treeline dynamics under climate change conditions.

The onset of alpine pastoral systems in the Eastern Alps

NASA Astrophysics Data System (ADS)

Oeggl, Klaus; Festi, Daniela; Putzer, Andreas

2015-04-01

Since the discovery of the Neolithic glacier mummy "Ötzi" in the nival belt of the main Alpine ridge, the onset of alpine pasture is matter of a highly controversial debate both in archaeology and in palaeo-ecology of the Eastern Alps. The implication is that his sojourn in the high-altitudes of the Alps is considered to be connected with pastoral nomadism. Regrettably any archaeological evidence for the existence of such Neolithic alpine pastoral systems is missing up to now and the assumption is based on palynological data only. However, also the palynological record is ambiguous, because pasture indicators in the alpine regions react positive on grazing as well as on fertilization induced by a higher runoff of precipitation. Thus alpine pasture indicators reflect both grazing pressure and climatic change. Anyhow, alpine pastoral systems are a common practice in Alpine animal husbandry, but from an economic point of view such a seasonal vertical transhumance is costly. There are three main reasons for its practice: i) climatic, ii) economic (mainly in connection with population pressure or mining activities), and iii) cultural ideology. In this study we tested the above mentioned reasons in an interdisciplinary study on the beginning of pastoral activities in high altitudes in the central part of the Eastern Alps. This is conducted by palynological analyses of peat deposits situated in the vicinity of the timberline (1600 - 2400 m a.s.l.) combined with archaeological surveys. The investigated sites are located in traditional Alpine transhumance regions and aligned on a transect through the central part of the Eastern Alps. The studies reveal that grazing pressure is reflected since the Bronze Age, which is corroborated by archaeological findings in the vicinity of the investigated sites.

40Ar-39Ar laser dating of ductile shear zones from central Corsica (France): Evidence of Alpine (middle to late Eocene) syn-burial shearing in Variscan granitoids

NASA Astrophysics Data System (ADS)

Di Vincenzo, Gianfranco; Grande, Antonietta; Prosser, Giacomo; Cavazza, William; DeCelles, Peter G.

2016-10-01

The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. The most debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40Ar-39Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P-T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike NNE-SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P-T conditions attained during shearing at 300 °C and minimum pressures of 0.6 GPa. In-situ 40Ar-39Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, 45-36 Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in-situ ages

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.

Ivrea mantle wedge and arc of the Western Alps (II): Kinematic evolution of the Alps-Apennines orogenic system

NASA Astrophysics Data System (ADS)

Schmid, Stefan; Kissling, Eduard; van Hinsbergen, Douwe J. J.; Molli, Giancarlo

2017-04-01

Integration of geological and geophysical data on the deep structure of the Alps (Kissling et al. 2017) reveals that the deep-seated Ivrea mantle played a crucial role during the formation of the arc of the Western Alps. Exhumation of the mantle beneath the Ivrea Zone to shallow crustal depths during Mesozoic rifting led to the formation of a strong Ivrea mantle wedge; its strength exceeds that of surrounding mostly quartz-bearing units, and consequently allows for indentation and wedging of a quasi-rigid Ivrea mantle wedge into the Western Alps during Alpine orogeny. A first early stage (pre-35 Ma) of the West-Alpine orogenic evolution is characterized by top-NNW thrusting in sinistral transpression causing at least some 260km displacement of internal Western Alps and E-W-striking Alps farther east, together with the Adria micro-plate, towards N to NNW with respect to stable Europe. It is during the second stage (35-25 Ma) that the Ivrea mantle wedge played a crucial role by accentuating the arc. This stage is associated with top-WNW thrusting in the external zones of the central portion of the arc and lateral indentation and wedging of the Ivrea mantle slice beneath the already existing nappe pile towards WNW by some 100-150km. The final stage of arc formation (25-0 Ma) is associated with orogeny in the Apennines leading to oroclinal bending in the southernmost Western Alps that by now became parts of the Apenninic orogen, in connection with the 50° counterclockwise rotation of the Corsica-Sardinia block and the Ligurian Alps. The lithological composition of some tectonic units originating from the Alpine Tethys (Piemont-Liguria Ocean) found in the Alps and the northern Apennines has much in common. The non-metamorphic parts of the Piemont-Liguria derived units form the upper plate of the Western Alps that is devoid of Austroalpine elements, while the lower plate includes highly metamorphic units derived from the same Piemont-Liguria Ocean. This points to a

Adverse impacts of pasture abandonment in Himalayan protected areas: Testing the efficiency of a Natural Resource Management Plan (NRMP)

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

Nautiyal, Sunil; Kaechele, Harald

2007-03-15

The high elevational areas in the Himalayas of India are dominated by forests and alpine pastures. There are many protected areas in the region, including Nanda Devi Biosphere Reserve (NDBR) and Valley of Flowers (VOF) where natural resource management plan (NRMP) has been implemented for the conservation of biodiversity. This has affected the traditional animal husbandry system, as well as the vegetation dynamics of alpine pastures. An integrated approach to studying the impact of NRMP in the region has been applied by us. First, a survey was conducted regarding livestock management, data pertaining the livestock husbandry, the role of animalmore » husbandry in economics of rural household, and socioeconomics. Second, field based study on phytosociology of some important alpine herbs was done to enumerate the density and species richness in different land mark of the region. Thereafter, satellite data and Geographic Information System (GIS) were used to develop a land cover map of the area and to note changes in the landscape over time after implementation of NRMP. From an economic point of view the implementation of such plan is a setback to the rural economy. However, the ecological perspective of such models is a threat to the diversity of alpine pastures. The invasion of bushes/thorny bushes/shrubs and weeds with their luxuriant growth is changing the vegetation index and dynamics. Consequently, the diversity of herbs in alpine pastures of the Himalayan Mountains is in jeopardy. Overall, the situation is leading to landscape change in the region. This study is helpful for generating useful outcomes and strategies considering the question or debate 'is grazing good or bad for pasture ecosystems in the Himalayas?'.« less

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

NASA Astrophysics Data System (ADS)

Goodenough, K. M.; Thomas, R. 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.

2010-04-01

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.

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

Triple junction orogeny: tectonic evolution of the Pan-African Northern Damara Belt, Namibia

NASA Astrophysics Data System (ADS)

Lehmann, Jérémie; Saalmann, Kerstin; Naydenov, Kalin V.; Milani, Lorenzo; Charlesworth, Eugene G.; Kinnaird, Judith A.; Frei, Dirk; Kramers, Jan D.; Zwingmann, Horst

2014-05-01

Trench-trench-trench triple junctions are generally geometrically and kinematically unstable and therefore can result at the latest stages in complicated collisional orogenic belts. In such geodynamic sites, mechanism and timescale of deformations that accommodate convergence and final assembly of the three colliding continental plates are poorly studied. In western Namibia, Pan-African convergence of three cratonic blocks led to pene-contemporaneous closure of two highly oblique oceanic domains and formation of the triple junction Damara Orogen where the NE-striking Damara Belt abuts to the west against the NNW-striking Kaoko-Gariep Belt. Detailed description of structures and microstructures associated with remote sensing analysis, and dating of individual deformation events by means of K-Ar, Ar-Ar (micas) and U-Pb (zircon) isotopic studies from the Northern Damara Belt provide robust constraints on the tectonic evolution of this palaeo-triple junction orogeny. There, passive margin sequences of the Neoproterozoic ocean were polydeformed and polymetamorphosed to the biotite zone of the greenschist facies to up to granulite facies and anatexis towards the southern migmatitic core of the Central Damara Belt. Subtle relict structures and fold pattern analyses reveal the existence of an early D1 N-S shortening event, tentatively dated between ~635 Ma and ~580 Ma using published data. D1 structures were almost obliterated by pervasive and major D2 E-W coaxial shortening, related to the closure of the Kaoko-Gariep oceanic domain and subsequent formation of the NNW-striking Kaoko-Gariep Belt to the west of the study area. Early, km-scale D1 E-W trending steep folds were refolded during this D2 event, producing either Type I or Type II fold interference patterns visible from space. The D2 E-W convergence could have lasted until ~533 Ma based on published and new U-Pb ages. The final D3 NW-SE convergence in the northernmost Damara Belt produced a NE-striking deformation

Age and composition of the UHP garnet peridotites in the Dabie orogenic belt (central China) record complex crust-mantle interaction in continental margin

NASA Astrophysics Data System (ADS)

Zhao, Y.; Zheng, J.; Wang, B.

2017-12-01

The Dabie-Sulu UHP belt was created by the collision between the North and South China cratons in Middle Triassic time (240-225 Ma). There are lots of garnet-bearing ultramafic body occurs as a lens in the belt. Age and composition of the Maowu garnet peridotites in the Dabie orogenic belt are reported. The garnet harzburgites are main moderately refractory (Mg#Ol=92) and minor fertile (Mg#Ol=88) with high Ni (2344-2603 ppm) and low Al2O3 (0.35-0.54 wt.%), CaO (0.76-2.19 wt.%) and TiO2 (˜0.01 wt.%). Zircons in the harzburgites mainly document metamorphism at 230 ± 2 Ma, 275 ± 5 Ma, 357 ± 4 Ma, and complex minor populations of ages including: 1.8 Ga, 1.3 Ga, and Neoproterozic-early Paleozoic ages (901-420 Ma). The early Meszosic and late Paleozoic zircons have similar trace-element patterns and ranges in ɛHf(t) (+0.6 to +3.4), Th/U ratio (0.2-0.7) and Hf depleted-mantle model ages (TDM ) mainly cluster in the interval 1.2-0.9 Ga. In contrast, the Paleo-Mesoproterozoic zircons have negative ɛHf(t) (-24.9 to -2.7) and oldest Hf TDM = 3.4Ga. Zircons of Neoproterozic-early Paleozoic have a wide range of Hf depleted-mantle model ages (2.4-0.7Ga) and ɛHf(t) (-15.3 to +9.5). Above of the all, we suggest that the Maowu garnet harzburgites are interpreted as a fragment of the metasomatized ancient lithospheric mantle beneath the southern margin of the North China Craton. They experienced the Proterozoic thermal event (1.9-1.8Ga), which is coeval with the assembly of the supercontinent Columbia. And then 1.3Ga mantle metasomatism with asthenospheric materials resulted in the final breakup of the Columbia supercontinent. Neoproterozic-early Paleozoic (901-420 Ma), deep parts of the south margin of the craton were metasomatized during the assembly and breakup of the Rodinia supercontinent. Then, the southern margin of the craton occurred oceanic crust subduction ( 357 Ma), subsequent continental deep subduction and final continent-continent collision in Triassic.

Brother is high Sr/Y two-mica granite and sister is leucogranite: twin granites in the Northern Himalayan Gneiss Domes, southern Tibet

NASA Astrophysics Data System (ADS)

Zeng, L.; Gao, L.; Xie, K.

2011-12-01

Leucogranites in the Himalayan orogen is widely considered as the type example of crustal melts, which provides a probe to investigate the interplay among high-grade metamorphism, crustal anatexis, and tectonic transition in large-scale collisional belts. Whether the leucogranite was a daughter product from a more primitive granitic melt is an interesting question that deserves careful examination to address the above issue. We report a new suite of two-mica granite (TMG) and leucogranite (LG) in the Yardoi gneiss dome (YGD) in the easternmost of the Northern Himalayan Gneiss Domes (NHGD), south of the Yarlung-Tsangpo suture. SHRIMP and LA-ICP-MS zircon U/Pb dating show that TMG and LG formed at ~17.7 Ma to ~20.0 Ma and at ~17.1 Ma, respectively. Both suites of granite have high Na/K (>1.30) ratios. The TMGs are characterized by (1) high Sr (>450 ppm), low Rb (<95 ppm) and Y (<6 ppm), and high Sr/Y (>86) ratios; (2) no Eu anomalies; and (3) low initial 87Sr/86Sr ratios (<0.7098) and higher ɛNd (>-8.5) values. In contrast, the LGs have (1) lower Sr (<130 ppm) and higher Rb (92-130 ppm); (2) pronounced negative Eu anomalies with Eu/Eu*<0.55; and (3) relatively higher Sr (87Sr/86Sr(t) =0.7136-0.7148) and unradiogenic Nd (ɛNd(t)=-7.7~-11.1). These data demonstrate that these Mid-Miocene granites have major and trace element and radiogenic isotope compositions similar to those of >35 Ma granites, but significantly different from those granites of similar ages in the High Himalaya as well as in the NHGD. High Sr/Y and relatively unradiogenic Sr isotope compositions in the TMGs could be derived from partial melting of mafic materials formed during previous compressional thickening event which was triggered by the input of juvenile heat and material associated with the Miocene E-W extension. An AFC process (plagioclase fractional crystallization and contamination by crustal materials) could be a primary factor leading to the formation of these LGs. Concurrence of high Sr

3D geodynamic models for the development of opposing continental subduction zones: The Hindu Kush-Pamir example

NASA Astrophysics Data System (ADS)

Liao, Jie; Gerya, Taras; Thielmann, Marcel; Webb, A. Alexander G.; Kufner, Sofia-Katerina; Yin, An

2017-12-01

The development of opposing continental subduction zones remains scantly explored in three dimensions. The Hindu Kush-Pamir orogenic system at the western end of the Himalayan orogen provides a rare example of continental collision linked to two opposing intra-continental subduction zones. The subducted plates feature a peculiar 3D geometry consisting of two distinct lithospheric fragments with different polarities, subduction angles and slab-curvatures beneath the Hindu Kush and Pamir, respectively. Using 3D geodynamic modeling, we simulate possible development of two opposing continental subduction zones to understand the dynamic evolution of the Hindu Kush-Pamir orogenic system. Our geodynamic model reproduces the major tectonic elements observed: (1) the deeper subduction depth, the steeper dip angle and the southward offset of the Hindu Kush subduction zone relative to the Pamir naturally occur if convergence direction of the subducting Indian plate and dip-direction of the Hindu Kush subduction zone match. (2) The formation of the highly asymmetrically curved Pamir region and the south-dipping subduction is promoted by the initial geometry of the indenting Indian lithosphere together with the existence of a major strike-slip fault on the eastern margin of the Pamir region. (3) Subduction of only the lower continental crust during continental collision can occur if the coupling between upper and lower crusts is weak enough to allow a separation of these two components, and that (4) the subduction of mainly lower crust then facilitates that conditions for intermediate-depth seismicity can be reached. (5) The secondary tectonic features modeled here such as strike-slip-fault growth, north-northwest striking extension zone, and lateral flow of the thickened ductile upper crust are comparable to the current tectonics of the region. (6) Model results are further compared to the potentially similar orogenic system, i.e., the Alpine orogen, in terms of the curved

Terrane-Scale Metastability in Subducted Himalayan Continental Crust as Revealed by Integrated Petrological and Geodynamic Modeling

NASA Astrophysics Data System (ADS)

Palin, R. M.; Reuber, G. S.; White, R. W.; Kaus, B. J. P.; Weller, O. M.

2017-12-01

The Tso Morari massif, northwest India, is one of only two regions in the Himalayan Range that exposes subduction-related ultrahigh-pressure (UHP) metamorphic rocks. The tectonic evolution of the massif is strongly debated, however, as reported pressure estimates for peak metamorphism range between 2.4 GPa and 4.8 GPa. Such ambiguity hinders effective lithospheric-scale modeling of the early stages of the orogen's evolution. We present the results of integrated petrological and geodynamic modeling (Palin et al., 2017, EPSL) that provide new quantitative constraints on the prograde-to-peak pressure-temperature-time (P-T-t) path, and predict the parageneses that felsic and mafic components of the massif crust should have formed under equilibrium conditions. Our model shows that peak P-T conditions of 2.6-2.8 GPa and 600-620 °C, representative of subduction to 90-100 km depth (assuming lithostatic pressure), were reached just 3 Myr after the onset of collision. These P-T-t constraints correlate well with those reported for similar UHP eclogite in the along-strike Kaghan Valley, Pakistan, suggesting that the northwest Himalaya contains dismembered remnants of a 400-km long UHP terrane comparable in size to the Western Gneiss Region, Norway, and the Dabie-Sulu belt, China. The extremely high pressures (up to 4.8 GPa) for peak metamorphism reported by some workers are likely to be unreliable due to thermobarometry having been performed on minerals that did not represent equilibrium assemblages. Furthermore, key high-P minerals predicted to form in subducted Tso Morari continental crust (e.g. jadeite, Mg-rich garnet) are absent from natural samples in the region, reflecting the widespread metastable preservation of lower-pressure protolith assemblages during subduction and exhumation. This result questions the reliability of geodynamic simulations of orogenesis that are commonly predicated on equilibrium metamorphism operating continuously throughout tectonic cycles.

Carboniferous Proto-type Basin Evolution of Junggar Basin in Northwest China: Implications for the Growth Models of Central Asia Orogenic Belt

NASA Astrophysics Data System (ADS)

He, D.

2016-12-01

The Junggar Basin locates in the central part of Paleo-Asian Ocean tectonic domain, and records the dynamic processes of the Central Asian Orogenic Belt from subduction-accretion-collision to later intracontinental deformations. Carboniferous is the key period from subduction to closure in the tectonic evolution of Paleo-Asian Ocean. Based on the borehole, outcrop, seismic and gravity and magnetic anomaly data, the paper made analysis of the Carboniferous basin evolution.Geo-chronological results for the borehole volcanic rocks suggest that the Junggar Basin and adjacent area had five periods of volcanic activities, including two periods in the Early Carboniferous (359-347Ma 347-331Ma and 331-324Ma) and three periods in the Late Carboniferous (323-307Ma and 307-300Ma). Regional unconformities divided the Carboniferous into two tectono-stratigraphic sequences: Lower Carboniferous and Upper Carboniferous. The former is characterized by compressional structures and involves massive calc-alkaline basalts, andesites, dacites and rhyolites, whereas the later is mainly controlled by extensional faults and dominated by intermediate-mafic volcanic rocks, with bimodal volcanic rocks in parts. The paper determined four Carboniferous arc-basin belts in the Junggar Basin and adjacent area from north to south: the Saur-Fuhai-Dulate, Heshituoluogai-Wulungu-Yemaquan, Darbut-Luliang-Karamaili, and Zhongguai-Mosuowan-Baijiahai-Qitai, and identified multi-type basins, such as fore-arc basin, retro-arc basin, intra-arc rift basin, foreland basin and passive continental margin basin,etc.. The Carboniferous proto-type basin evolution of the Junggar Basin can be divided into three phases such as, the early to middle Early Carboniferous subduction-related compressional phase, the late Early Carboniferous to middle Late Carboniferous subduction-related extensional phase and the late Late Carboniferous intra-continental fault-sag phase. The study discloses that the Junggar Basin is likely

The paradox of vertical σ2 in foreland fold and thrust belts

NASA Astrophysics Data System (ADS)

Tavani, Stefano

2014-05-01

Occurrence of aesthetically appealing thrust systems and associated large scale anticlines, in both active and fossil foreland fold and thrust belts, is commonly interpreted as an evidence for Andersonian compressional framework. Indeed, these structures would testify for a roughly vertical σ3. Such a correlation between thrusts occurrence and stress field orientation, however, frequently fails to explain denser observations at a smaller scale. The syn-orogenic deformation meso-structures hosted in exposed km-scale thrust-related folds, in fact, frequently and paradoxically witness for a syn-thrusting strike-slip stress configuration, with a near-vertical σ2 and a sub-horizontal σ3. This apparent widespread inconsistency between syn-orogenic meso-structures and stress field orientation is here named "the σ2 paradox". A possible explanation for such a paradox is provided by inherited extensional deformation structures commonly developed prior to thrusting, in the flexural foreland basins located ahead of fold and thrust belts. Thrust nucleation and propagation is facilitated and driven by the positive inversion of the extensional inheritances, and their subsequent linkage. This process eventually leads to the development of large reverse fault zones and can occur both in compressive and strike-slip stress configurations.

Seismic signature of the Alpine indentation, evidence from the Eastern Alps

PubMed Central

Bianchi, I.; Bokelmann, G.

2014-01-01

The type of collision between the European and the Adriatic plates in the easternmost Alps is one of the most interesting questions regarding the Alpine evolution. Tectonic processes such as compression, escape and uplift are interconnected and shape this area. We can understand these ongoing processes better, if we look for signs of the deformation within the Earth's deep crust of the region. By collecting records from permanent and temporary seismic networks, we assemble a receiver function dataset, and analyze it with the aim of giving new insights on the structure of the lower crust and of the shallow portion of the upper mantle, which are inaccessible to direct observation. Imaging is accomplished by performing common conversion depth stacks along three profiles that crosscut the Eastern Alpine orogen, and allow isolating features consistently persistent in the area. The study shows a moderately flat Moho underlying a seismically anisotropic middle-lower crust from the Southern Alps to the Austroalpine nappes. The spatial progression of anisotropic axes reflects the orientation of the relative motion and of the stress field detected at the surface. These observations suggest that distributed deformation is due to the effect of the Alpine indentation. In the shallow upper mantle right below the Moho interface, a further anisotropic layer is recognized, extended from the Bohemian Massif to the Northern Calcareous Alps. PMID:26525181

Seismic signature of the Alpine indentation, evidence from the Eastern Alps.

PubMed

Bianchi, I; Bokelmann, G

2014-12-01

The type of collision between the European and the Adriatic plates in the easternmost Alps is one of the most interesting questions regarding the Alpine evolution. Tectonic processes such as compression, escape and uplift are interconnected and shape this area. We can understand these ongoing processes better, if we look for signs of the deformation within the Earth's deep crust of the region. By collecting records from permanent and temporary seismic networks, we assemble a receiver function dataset, and analyze it with the aim of giving new insights on the structure of the lower crust and of the shallow portion of the upper mantle, which are inaccessible to direct observation. Imaging is accomplished by performing common conversion depth stacks along three profiles that crosscut the Eastern Alpine orogen, and allow isolating features consistently persistent in the area. The study shows a moderately flat Moho underlying a seismically anisotropic middle-lower crust from the Southern Alps to the Austroalpine nappes. The spatial progression of anisotropic axes reflects the orientation of the relative motion and of the stress field detected at the surface. These observations suggest that distributed deformation is due to the effect of the Alpine indentation. In the shallow upper mantle right below the Moho interface, a further anisotropic layer is recognized, extended from the Bohemian Massif to the Northern Calcareous Alps.

Synchronous partial melting, deformation, and magmatism: evidence from in an exhumed Proterozoic orogen

NASA Astrophysics Data System (ADS)

Levine, J. S. F.; Mosher, S.

2017-12-01

Older orogenic belts that now expose the middle and lower crust record interaction between partial melting, magmatism, and deformation. A field- and microstructural-based case study from the Wet Mountains of central Colorado, an exhumed section of Proterozoic rock, shows structures associated with anatexis and magmatism, from the grain- to the kilometer-scale, that indicate the interconnection between deformation, partial melting, and magmatism, and allow reconstructions of the processes occurring in hot active orogens. Metamorphic grade, along with the degree of deformation, partial melting, and magmatism increase from northwest to southeast. Deformation synchronous with this high-grade metamorphic event is localized into areas with greater quantities of former melt, and preferential melting occurs within high-strain locations. In the less deformed northwest, partial melting occurs dominantly via muscovite-dehydration melting, with a low abundance of partial melting, and an absence of granitic magmatism. The central Wet Mountains are characterized by biotite dehydration melting, abundant former melt and foliation-parallel inferred melt channels along grain boundaries, and the presence of a nearby granitic pluton. Rocks in the southern portion of the Wet Mountains are characterized by partial melting via both biotite dehydration and granitic wet melting, with widespread partial melting as evidenced by well-preserved former melt microstructures and evidence for back reaction between melt and the host rocks. The southern Wet Mountains has more intense deformation and widespread plutonism than other locations and two generations of dikes and sills. Recognition of textures and fabrics associated with partial melting in older orogens is paramount for interpreting the complex interplay of processes occurring in the cores of orogenic systems.

Timing of tectonic evolution of the East Kunlun Orogen, Northern Tibet Plateau

NASA Astrophysics Data System (ADS)

Dong, Yunpeng

2017-04-01

The East Kunlun Orogen, located at the northern Tibet Plateau, represents the western segment of the Central China Orogenic Belt which was formed by amalgamation of the North China blocks and South China blocks. It is a key to understanding the formation of Eastern Asian continent as well as the evolution of the Pangea supercontinent. Based on detailed geological mapping, geochemical and geochronological investigations, the orogen is divided into three main tectonic belts, from north to south, including the Northern Qimantagh, Central Kunlun and Southern Kunlun Belts by the Qimantagh suture, Central Kunlun suture and South Kunlun fault. The Qimantagh suture is marked by the Early Paleozoic ophiolites outcropped in the Yangziquan, Wutumeiren, and Tatuo areas, which consist mainly of peridotites, gabbros, diabases and basalts. Besides, the ophiolite in the Wutumeiren is characterized by occurring anorthosite while the ophiolite in the Tatuo occurring chert. The basalts and diabases from both Yaziquan and Tatuo areas display depletion of Nb, Ta, P and Ti, and enrichment of LILE, suggesting a subduction related tectonic setting. LA-ICP-MS zircon U-Pb age of 421 Ma for the diabase represents the formation age of the Yaziquan ophiolite, while the U-Pb ages of 490 Ma and 505 Ma for gabbro and anorthosite, respectively, constrain the formation age of the Tatuo ophiolite. The basaltic rocks in the Wutumeiren area display flat distribution of HFSEs (such as Nb, Ta, K, La, Ce, Pr, Nd, Zr, Sm, Eu, Ti, Dy, Y, Yb and Lu) and slightly enrichment in LREEs, while the peridotites showing depletion in MREEs. The LA-ICP-MS zircon U-Pb age of 431 Ma for the gabbro represents the formation age of the Wutumeiren ophiolite. Together with regional geology, we suggest herewith a back-arc basin tectonic setting during ca. 505-421 Ma at least for the Qimantagh suture. The Central Kunlun suture is represented by the ophiolite in the Wutuo area, which is characterized by depletion of Nb, Ta, P

Early Miocene rapid exhumation in southern Tibet: Insights from P-T-t-D-magmatism path of Yardoi dome

NASA Astrophysics Data System (ADS)

Wang, Jia-Min; Wu, Fu-Yuan; Rubatto, Daniela; Liu, Kai; Zhang, Jin-Jiang; Liu, Xiao-Chi

2018-04-01

Reconstructing the evolution of Gneiss domes within orogenic belts poses challenges because domes can form in a variety of geodynamic settings and by multiple doming mechanisms. For the North Himalayan gneiss domes (NHGD), it is debated whether they formed during shortening, extension or collapse of the plateau, and what is the spatial and temporal relationship of magmatism, metamorphism and deformation. This study investigates the Yardoi dome in southern Tibet using field mapping, petrography, phase equilibria modelling and new monazite ages. The resulting P-T-time-deformation-magmatism path for the first time reveals the spatial and temporal relationship of metamorphism, deformation and magmatism in the Yardoi dome: a) the dome mantle recorded prograde loading to kyanite-grade Barrovian metamorphic conditions of 650 ± 30 °C and 9 ± 1 kbar (M2) in the Early Miocene (18-17 Ma); b) the main top-to-the-north deformation fabric (D2) formed syn- to post-peak-metamorphism; c) the emplacement of leucorgranites related to doming is syn-metamorphism at 19-17 Ma. The link between the detachment shear zone in the Yardoi dome and the South Tibetan detachment system (STDS) is confirmed. By comparing with orogen-scale tectonic processes in the Himalaya, we suggest that north-south extension in a convergent geodynamic setting during Early Miocene accounts for formation of the Yardoi dome. In a wider tectonic context, the Early Miocene rapid exhumation of deep crustal rocks was contemporaneous with the rapid uplift of southern Tibet and the Himalayan orogen.

Formation of an Archean tectonic mélange in the Schreiber-Hemlo greenstone belt, Superior Province, Canada: Implications for Archean subduction-accretion process

NASA Astrophysics Data System (ADS)

Polat, Ali; Kerrich, Robert

1999-10-01

The late Archean (circa 2750-2670 Ma) Schreiber-Hemlo greenstone belt, Superior Province, Canada, is composed of tectonically juxtaposed fragments of oceanic plateaus (circa 2750-2700 Ma), oceanic island arcs (circa 2720-2695 Ma), and siliciclastic trench turbidites (circa 2705-2697 Ma). Following juxtaposition, these lithotectonic assemblages were collectively intruded by synkinematic tonalite-trondhjemite-granodiorite (TTG) plutons (circa 2720-2690 Ma) and ultramafic to felsic dikes and sills (circa 2690-2680 Ma), with subduction zone geochemical signatures. Overprinting relations between different sequences of structures suggest that the belt underwent at least three phases of deformation. During D1 (circa 2695-2685 Ma), oceanic plateau basalts and associated komatiites, arc-derived trench turbidites, and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex. Fragmentation of these sequences resulted in broken formations and a tectonic mélange in the Schreiber assemblage of the belt. D2 (circa 2685-2680 Ma) is consistent with an intra-arc, right-lateral transpressional deformation. Fragmentation and mixing of D2 synkinematic dikes and sills suggest that mélange formation continued during D2. The D1 to D2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating. The D2 intra-arc strike-slip faults may have provided conduits for uprising melts from the descending slab, and they may have induced decompressional partial melting in the subarc mantle wedge, to yield synkinematic ultramafic to felsic intrusions. A similar close relationship between orogen-parallel strike-slip faulting and magmatism has recently been recognized in several Phanerozoic transpressional orogenic belts, suggesting that as in Phanerozoic counterparts, orogen-parallel strike-slip faulting in the Schreiber-Hemlo greenstone belt played an important role in

Valemount strain zone: A dextral oblique-slip thrust system linking the Rocky Mountain and Omineca belts of the southeastern Canadian Cordillera

NASA Astrophysics Data System (ADS)

McDonough, Michael R.; Simony, Philip S.

1989-03-01

The Valemount strain zone (VSZ), a narrow zone of high orogen-parallel (OP) strain in pebble conglomerate of the Late Proterozoic Miette Group, is the footwall expression of a thrust fault on the western edge of the Rocky Mountain belt, marking the eastern limit of a wide zone of OP fabrics distributed through the Omineca crystalline and western Rocky Mountain belts of the southeastern Canadian Cordillera. Kinematic indicators from the VSZ and the adjacent Bear Foot thrust zone show that both thrust and dextral displacement are associated with folding and thrust motion in the Rocky Mountains, thereby linking the southern Rocky Mountain belt to the Omineca belt by an oblique-slip thrust regime that is tectonically unrelated to the Southern Rocky Mountain Trench. Transverse shortening of thrust sheets and subsequent distribution of OP shear are invoked to explain the parallelism of stretching lineations and fold axes. Strain and kinematic data and the thrust-belt geometry of the VSZ suggest that OP lineations are a product of a large amount of transverse shortening during slightly oblique A-type subduction. Thus, OP lineations are not representative of relative plate motions between North America and accreted terranes, but probably are a function of footwall buttressing of thrust sheets, a mechanism that may be widely applicable to the internal zones of collisional orogens.

Accreted seamounts in North Tianshan, NW China: Implications for the evolution of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Yang, Gaoxue; Li, Yongjun; Kerr, Andrew C.; Tong, Lili

2018-03-01

The Carboniferous Bayingou ophiolitic mélange is exposed in the North Tianshan accretionary complex in the southwestern part of the Central Asian Orogenic Belt (CAOB). The mélange is mainly composed of serpentinised ultramafic rocks (including harzburgite, lherzolite, pyroxenite, dunite and peridotite), pillowed and massive basalts, layered gabbros, radiolarian cherts, pelagic limestones, breccias and tuffs, and displays block-in-matrix structures. The blocks of ultramafic rocks, gabbros, basalts, cherts, and limestones are set in a matrix of serpentinised ultramafic rocks, massive basalts and tuffs. The basaltic rocks in the mélange show significant geochemical heterogeneity, and two compositional groups, one ocean island basalt-like, and the other mid-ocean ridge-like, can be distinguished on the basis of their isotopic compositions and immobile trace element contents (such as light rare earth element enrichment in the former, but depletion in the latter). The more-enriched basaltic rocks are interpreted as remnants/fragments of seamounts, derived from a deep mantle reservoir with low degrees (2-3%) of garnet lherzolite mantle melting. The depleted basalts most likely formed by melting of a shallower spinel lherzolite mantle source with ∼15% partial melting. It is probable that both groups owe their origin to melting of a mixture between plume and depleted MORB mantle. The results from this study, when integrated with previous work, indicate that the Junggar Ocean crust (comprising a significant number of seamounts) was likely to have been subducted southward beneath the Yili-Central Tianshan block in the Late Devonian-Early Carboniferous. The seamounts were scraped-off and accreted along with the oceanic crust in an accretionary wedge to form the Bayingou ophiolitic mélange. We present a model for the tectonomagmatic evolution of this portion of the CAOB involving prolonged intra-oceanic subduction with seamount accretion.

Interaction of the Siberian craton and Central Asian Orogenic Belt (CAOB) recorded by detrital zircons from Transbaikalia

NASA Astrophysics Data System (ADS)

Powerman, V.; Shatsillo, A.; Chumakov, N.; Kapitonov, I.; Hourigan, J. K.

2015-12-01

The goal of this study is to pinpoint the beginning of interaction of two gigantic crustal structures: the Siberian Craton and the Central Asian Orogenic Belt (CAOB). We hypothesize that the beginning of convergence should be recorded in the Neoproterozoic passive margin strata of Siberian Craton by the first appearance of extraregional Neoproterozoic zircons. In order to test this hypothesis, we have acquired U-Pb zircon age distributions from twelve Neoproterozoic clastic rocks from the Baikal-Patom margin of Siberia and one sample from the volcaniclastic Padrinsky Group that was deposited atop accreted CAOB crust. Stratigraphically lower strata from the Siberian margin yield Archean - Paleoproterozoic detrital zircon ages, which are similar to, and probably derived from the Siberian Precambrian craton. A few extra-regional Mesoproterozoic grains are also present. The provenance shift happens in the upper portion of the section and is marked by a strong influx of extra-regional Neoproterozoic sediments. The youngest grains of 610 Ma constrain the sedimentation age and confine the timing of interaction between CAOB and Siberia in this region. Neoproterozoic zircons also dominate the overlying sedimentary unit, suggesting the continuance of the convergence. The coeval volcanoclastic unit on the CAOB side has a similar U-Pb detrital age distribution, strengthening the provenance link. Analysis of the local tectonics suggests that the beginning of accretion might have started even before the first appearance of Neoproterozoic zircon: during the development of a regional unconformity, capped by 635 Ma (?) "Snowball Earth" tillites of Dzhemkukan Fm. The absence of Neoproterozoic zircons in Dzhemkukan Fm. is probably explained by a thin-skinned tectonics that did not result in massive orogenesis . Our data are in good correlation with other Neoproterozoic sedimentary basins of southern Siberian Craton, including Cisbaikalia and Bodaibo Synclinorium.

Geochemistry, geochronology, and Sr-Nd isotopic compositions of Permian volcanic rocks in the northern margin of the North China Block: implications for the tectonic setting of the southeastern Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Ji, Zejia; Zhang, Zhicheng; Chen, Yan; Li, Ke; Yang, Jinfu; Qian, Xiaoyan

2018-02-01

The southeastern part of the Central Asian Orogenic Belt (CAOB), which records the collision of the North China Block (NCB) with the South Mongolian microcontinent, is a key area for reconstructing the tectonic history of the CAOB. Controversy persists regarding the timing of the final structural amalgamation of the region; therefore, it remains unclear whether the Late Paleozoic thick volcanic successions were generated in a subduction or post-orogenic environment. Redefining the age of the formation and analyzing the geochemical compositions of these volcanic rocks can provide clues regarding the regional tectonic evolution during the Late Paleozoic and place constraints on the closure time of the Paleo-Asian Ocean. In this study, we present geochemical, geochronologic, and Sr-Nd isotopic data for 29 volcanic rock samples from the Elitu Formation in Xianghuangqi, central Inner Mongolia. The Elitu volcanic rocks have latest early-to-middle Permian ages between 272 and 268 Ma. Most of the mafic-intermediate and felsic rocks show K-normal and high-K calc-alkaline characteristics. Melting is considered to be due to large scale upwelling of the metasomatic lithospheric mantle and different degrees of melting of the thickened lower crust. The northern margin of the NCB, which represents the southeastern boundary of the CAOB, records transtensional and, subsequently, extensional tectonics associated with late Carboniferous to middle Permian volcanic activity.

Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana

NASA Astrophysics Data System (ADS)

Skipp, Betty

1987-03-01

The Clearwater orogenic zone in central Idaho and western Montana contains at least two major northeast-directed Cordilleran thrust plates of Early Proterozoic metasedimentary and metaigneous rocks that overrode previously folded Middle Proterozoic rocks of the Belt basin in Cretaceous time. The northeastward migration of the resultant thickened wedge of crustal material combined with Cretaceous subduction along the western continental margin produced a younger northern Bitterroot lobe of the Idaho batholith relative to an older southern Atlanta lobe. Eocene extensional unroofing and erosion of the Bitterroot lobe has exposed the roots of the thick Cordilleran thrust sheets.

Emplacement ages, geochemical and Sr-Nd-Hf isotopic characterization of Mesozoic to early Cenozoic granitoids of the Sikhote-Alin Orogenic Belt, Russian Far East: Crustal growth and regional tectonic evolution

NASA Astrophysics Data System (ADS)

Jahn, Bor-ming; Valui, Galina; Kruk, Nikolai; Gonevchuk, V.; Usuki, Masako; Wu, Jeremy T. J.

2015-11-01

The Sikhote-Alin Range of the Russian Far East is an important accretionary orogen of the Western Pacific Orogenic Belt. In order to study the formation and tectonic evolution of the orogen, we performed zircon U-Pb dating, as well as geochemical and Sr-Nd-Hf isotopic analyses on 24 granitoid samples from various massifs in the Primorye and Khabarovsk regions. The zircon dating revealed that the granitoids were emplaced from 131 to 56 Ma (Cretaceous to Paleogene). In the Primorye Region, granitoids in the coastal Sikhote-Alin intruded the Cretaceous Taukha Accretionary Terrane from ca. 90 to 56 Ma, whereas those along the Central Sikhote-Alin Fault zone intruded the Jurassic Samarka Accretionary Terrane during ca. 110-75 Ma. The "oldest" monzogranite (131 Ma) was emplaced in the Lermontovka area of the NW Primorye Region. Granitoid massifs along the Central Sikhote-Alin Fault zone in the Khabarovsk Region formed from 109 to 58 Ma. Thus, the most important tectonothermal events in the Sikhote-Alin orogen took place in the Cretaceous. Geochemical analysis indicates that most samples are I-type granitoids. They have initial 87Sr/86Sr ratios ranging from 0.7040 to 0.7083, and initial Nd isotopic ratios, expressed as εNd(t) values, from +3.0 to -5.0 (mostly 0 to -5). The data suggest that the granitoid magmas were generated by partial melting of sources with mixed lithologies, including the subducted accretionary complex ± hidden Paleozoic-Proterozoic basement rocks. Based on whole-rock Nd isotopic data, we estimated variable proportions (36-77%) of juvenile component (=mantle-derived basaltic rocks) in the generation of the granitic magmas. Furthermore, zircon Hf isotopic data (εHf(t) = 0 to +15) indicate that the zircon grains crystallized from melts of mixed sources and that crustal assimilation occurred during magmatic differentiation. The quasi-continuous magmatism in the Sikhote-Alin orogen suggests that the Paleo-Pacific plate subduction was very active in the

Detrital zircon fission track analysis reveals the thermotectonic history of ice-covered rocks of the Chugach-St. Elias orogen, SE-Alaska

NASA Astrophysics Data System (ADS)

Enkelmann, E.; Garver, J. I.; Pavlis, T. L.; Bruhn, R. L.; Chapman, J. B.

2007-12-01

Investigating the exhumation history of the Chugach-St. Elias orogen (SE Alaska) using low-temperature thermochronometers is challenged by significant ice cover. Assuming exhumation drove cooling, cooling ages increase with elevation in an orogenic belt, and as such the youngest ages occur in valley bottoms. Cooling and exhumation rates are expected to be very high in the Chugach-St. Elias orogen due to efficient glacial erosion and the most intense erosion occurs under the major ice fields. To study the cooling history of rapidly exhuming rocks underneath this ice cover, we analyzed detrital zircon fission track (DZFT) ages of Recent sand samples from modern rivers that drain the central Bagley Ice field and smaller glaciers draining north (Chitina valley) and south (Pacific) of the mountain range. A distinct advantage of DZFT is that it allows one to sample a landscape regardless of accessibility. The youngest ZFT component populations of samples north and south of the Bagley Ice field record a Late Miocene (5-13 Ma) cooling of the orogen. The pattern of cooling ages shows symmetry across the orogen predates the earliest record of the collision of the Yakutat terrane with Alaska. This result contrasts with the asymmetric cooling pattern displayed by low- temperature thermochronological ages (AFT and AHe) of the exposed bedrock within the range. Apatite FT and U- Th/He ages of bedrock samples south of the Bagley Ice field record the syn-collisional (<5 Ma) fast exhumation whereas apatite ages to the north reveal more heterogeneous exhumation and vary widely from Miocene to Eocene. The bedrock samples from throughout the orogenic belt thus display predominantly the effects of the recent climatic situation of the mountain range with very high precipitation on the south, seaward side versus a more arid north side. Our ZFT results from the northern drainages highlight the relative sense and timing of two important fault zones, both accommodate south-side-up exhumation

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

2017-10-01

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.

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

Evolving lithospheric flexure and paleotopography of the Pyrenean Orogen from 3D flexural modeling and basin analysis

NASA Astrophysics Data System (ADS)

Curry, M. E.; van der Beek, P.; Huismans, R. S.; Muñoz, J. A.

2017-12-01

The Pyrenees are an asymmetric, doubly-vergent orogen with retro- and pro- foreland basins that preserve a record of deformation since the Mesozoic. The extensive research and exploration efforts on the mountain belt and flanking foreland basins provide an exceptional dataset for investigating geodynamics and surface processes over large spatial and temporal scales in western Europe. We present the results of a numerical modeling study investigating the spatio-temporal variation in lithospheric flexure in response to the developing orogen. We employ a finite element method to model the 3D flexural deformation of the lithosphere beneath the Pyrenean orogen since the onset of convergence in the late Cretaceous. Using subsurface, geophysical, and structural data, we describe the evolving geometry of both the French Aquitaine and Spanish Ebro foreland basins at the present (post-orogenic), the mid-Eocene (peak orogenic), the Paleocene (early orogenic), and the end of the Cretaceous (pre- to early orogenic). The flexural modeling provides insight into how both the rigidity of the lithosphere and the paleotopographic load have varied over the course of orogenesis to shape the basin geometry. We find that the overriding European plate has higher rigidity than the subducting Iberian plate, with modern Effective Elastic Thickness (EET) values of 20 ± 2 and 12 ± 2 km, respectively. Modeling indicates that the modern rigidity of both plates decreases westward towards the Bay of Biscay. The lithospheric rigidity has increased by 50% since the Mesozoic with early Cenozoic EET values of 13 ± 2 and 8 ± 1 km for the European and Iberian plates, respectively. The topographic load began increasing with convergence in the late Cretaceous, reaching modern levels in the central and eastern Pyrenees by the Eocene. In contrast, the topographic load in the western Pyrenees was 70% of the modern value in the Eocene, and experienced topographic growth through the Oligo-Miocene. The

Geological evolution of the late Proterozoic ``Mozambique Belt'' of Kenya

NASA Astrophysics Data System (ADS)

Mosley, P. N.

1993-05-01

Within the "Mozambique Belt" of Kenya at least four distinct tectonothermal episodes are recognised on Rb-Sr isotopics. The dates are in broad agreement with those from surrounding countries; principal ages/age ranges being 830 - 800, ~ 760, 630 - 580 and 560 - 520 Ma. All except the last attained at least upper amphibolite/granulite grade (with local melts). The first event was responsible for the primary transformation of an essentially sedimentary sequence to paragneisses with an initial near-horizontal fabric parallel to the compositional layering. Associated with the later part of the first phase, and linked to the second, is the emplacement of allochthonous ophiolitic and volcanosedimentary "packages", coupled with thrusting and imbrication of the paragneiss groups. The subsequent phases record progressive shortening across the orogenic belt during collision between two major continental fragments (east and west Gondwana), involving extensive structural reorganisation and isotopic resetting. During the progressive 630 - 580 Ma event, regional N-S- to NNW-SSE-trending ductile shear zones (generally sinistral) were produced giving the dominant regional fabric (including a regional N-S-stretching lineation), and controlling the present gross distribution of gneiss groups. Cooling and uplift post a ~ 560 Ma thermal event has exposed high-grade gneisses with a distinct structural and metamorphic asymmetry across the orogen. The western part of the orogen shows clockwise P- T- t paths and involves overthrusting of, and imbrication with, the Tanzanian craton which probably obscures older (1900 and 1100 Ma) tectonothermal episodes. In contrast, the eastern part has anti-clockwise P- T- t paths, is characterised by extensive crustal melts, and retains the isotopic imprint of earlier Proterozoic events. The present level of uplift exposes tectonised high-grade gneisses of more than one age. Current evidence supports the suggestion that low-grade ophiolitic

Seismic behaviour of mountain belts controlled by plate convergence rate

NASA Astrophysics Data System (ADS)

Dal Zilio, Luca; van Dinther, Ylona; Gerya, Taras V.; Pranger, Casper C.

2018-01-01

The relative contribution of tectonic and kinematic processes to seismic behaviour of mountain belts is still controversial. To understand the partitioning between these processes we developed a model that simulates both tectonic and seismic processes in a continental collision setting. These 2D seismo-thermo-mechanical (STM) models obtain a Gutenberg-Richter frequency-magnitude distribution due to spontaneous events occurring throughout the orogen. Our simulations suggest that both the corresponding slope (b value) and maximum earthquake magnitude (MWmax) correlate linearly with plate convergence rate. By analyzing 1D rheological profiles and isotherm depths we demonstrate that plate convergence rate controls the brittle strength through a rheological feedback with temperature and strain rate. Faster convergence leads to cooler temperatures and also results in more larger seismogenic domains, thereby increasing both MWmax and the relative number of large earthquakes (decreasing b value). This mechanism also predicts a more seismogenic lower crust, which is confirmed by a transition from uni- to bi-modal hypocentre depth distributions in our models. This transition and a linear relation between convergence rate and b value and MWmax is supported by our comparison of earthquakes recorded across the Alps, Apennines, Zagros and Himalaya. These results imply that deformation in the Alps occurs in a more ductile manner compared to the Himalayas, thereby reducing its seismic hazard. Furthermore, a second set of experiments with higher temperature and different orogenic architecture shows the same linear relation with convergence rate, suggesting that large-scale tectonic structure plays a subordinate role. We thus propose that plate convergence rate, which also controls the average differential stress of the orogen and its linear relation to the b value, is the first-order parameter controlling seismic hazard of mountain belts.

Complex high-strain deformation in the Usagaran Orogen, Tanzania: structural setting of Palaeoproterozoic eclogites

NASA Astrophysics Data System (ADS)

Reddy, S. M.; Collins, A. S.; Mruma, A.

2003-11-01

The Palaeoproterozoic Usagaran Orogen of Tanzania contains the Earth's oldest reported examples of subduction-related eclogite facies rocks. Detailed field mapping of gneisses exposed in the high-grade, eclogite-bearing part of the orogen (the Isimani Suite) indicates a complex deformation and thermal history. Deformation in the Isimani Suite can be broadly subdivided into five events. The first of these (D 1), associated with formation of eclogite facies metamorphism, is strongly overprinted by a pervasive deformation (D 2) at amphibolite facies conditions, which resulted in the accumulation of high strains throughout all of the exposed Isimani rocks. The geometry of foliations and lineations developed during D 2 deformation are variable and have different shear directions that enable five D 2 domains to be identified. Analysis of these domains indicates a geometrical and kinematic pattern that is interpreted to have formed by strain and kinematic partitioning during sinistral transpression. U-Pb SHRIMP zircon ages from a post-D 2 granite and previously published geochronological data from the Usagaran eclogites indicate this deformation took place between 2000 ± 1 Ma and 1877 ± 7 Ma (at 1σ error). Subsequent greenschist facies deformation, localised as shear zones on boundaries separating D 2 domains, have both contractional and extensional geometries that indicate post-1877 Ma reactivation of the Isimani Suite. This reactivation may have taken place during Palaeoproterozoic exhumation of the Usagaran Orogen or may be the result of deformation associated with the Neoproterozoic East African Orogen. U-Th-Pb SHRIMP zircon ages from an Isimani gneiss sample and xenocrysts in a "post-tectonic" granite yield ˜2.7 Ga ages and are similar to published Nd model ages from both the Tanzanian Craton and gneiss exposed east of the Usagaran belt in the East African Orogen. These age data indicate that the Isimani Suite of the Usagaran Orogen reflects reworking of Archaean

Variable exhumation rates and variable displacement rates: Documenting recent slowing of Himalayan shortening in western Bhutan

USGS Publications Warehouse

McQuarrie, Nadine; Tobgay, Tobgay; Long, Sean P.; Reiners, Peter W.; Cosca, Michael A.

2014-01-01

We link exhumational variability in space and time to the evolving geometry of the Himalayan fold–thrust belt in western Bhutan. By combining new and published geochronologic and thermochronologic data we document the burial age, peak temperatures and complete cooling history from 20 Ma to the present over an across-strike distance of ∼125 km. These integrated cooling curves highlight windows of fast exhumation that vary spatially and temporally. We propose that pulses of fast exhumation are a result of structures that facilitate the vertical motion of material, illustrated in sequentially-restored cross sections. Due to a range of permissible geometries at depth, we explore and evaluate the impact of geometry on kinematics and rates of deformation. The linked cooling history and cross sections provide estimates of both magnitude and timing of thrust sheet displacement and highlight temporal variability in potential shortening rates. Structural and chronologic data illustrate a general north to south progression of Himalayan deformation, with emplacement of the Main Central thrust (MCT), Paro thrust and Shumar thrust by 12 to no later than 9 Ma. Two different geometries and kinematic scenarios for the Lesser Himalayan duplex are proposed. A north to south propagating duplex system requires that the southern portion of that system, south of the MCT, deformed and cooled by 9 Ma, leaving only the southernmost thrust sheets, including the Main Boundary and Main Frontal thrusts, to deform between 9 and 0 Ma. This limited post 9 Ma shortening would necessitate a marked slowdown in convergence accommodated on the Main Himalayan thrust. A two-tiered duplex system, which allows for the Paro window duplex and the southern Baxa duplex to form simultaneously, permits duplex formation and accompanying exhumation until 6 Ma. Limited cooling from ∼200 °C to the surface post 6 Ma suggests either a decrease in shortening rates from 6 to 0 Ma or that duplex formation and

Sources, Fluxes, and Effects of Fluids in the Alpine Fault Zone, South Island, New Zealand

NASA Astrophysics Data System (ADS)

Menzies, C. D.; Teagle, D. A. H.; Niedermann, S.; Cox, S.; Craw, D.; Zimmer, M.; Cooper, M. J.; Erzinger, J.

2015-12-01

Historic ruptures on some plate boundary faults occur episodically. Fluids play a key role in modifying the chemical and physical properties of fault zones, which may prime them for repeated rupture by the generation of high pore fluid pressures. Modelling of fluid loss rates from fault zones has led to estimates of fluid fluxes required to maintain overpressure (Faulkner and Rutter, 2001), but fluid sources and fluxes, and permeability evolution in fault zones remain poorly constrained. High mountains in orogenic belts can drive meteoric water to the middle crust, and metamorphic water is generated during rock dehydration. Additionally, fluids from the mantle are transported into the crust when fluid pathways are created by tectonism or volcanism. Here we use geochemical tracers to determine fluid flow budgets for meteoric, metamorphic and mantle fluids at a major compressional tectonic plate boundary. The Alpine Fault marks the transpressional Pacific-Australian plate boundary through South Island of New Zealand, it has historically produced large earthquakes (Mw ~8) and is late in its 329±68 year seismic cycle, having last ruptured in 1717. We present strontium isotope ratios of hot springs and hydrothermal minerals that trace fluid flow paths in and around the Alpine Fault to illustrate that the fluid flow regime is restricted by low cross-fault permeability. Fluid-rock interaction limits cross-fault fluid flow by the precipitating clays and calcite that infill pore spaces and fractures in the Alpine Fault alteration zone. In contrast, helium isotopes ratios measured in hot springs near to the fault (0.15-0.81 RA) indicate the fault acts as a conduit for mantle fluids from below. Mantle fluid fluxes are similar to the San Andreas Fault (<1x10-5 m3m-2/yr) and insufficient to promote fault weakening. The metamorphic fluid flux is of similar magnitude to the mantle flux. The dominant fluid throughout the seismogenic zone is meteoric in origin (secondary mineral �

Tectonic and kinematics of curved orogenic systems: insights from AMS analysis and paleomagnetism

NASA Astrophysics Data System (ADS)

Cifelli, Francesca; Mattei, Massimo

2016-04-01

During the past few years, paleomagnetism has been considered a unique tool for constraining kinematic models of curved orogenic systems, because of its great potential in quantifying vertical axis rotations and in discriminating between primary and secondary (orocline s.l.) arcs. In fact, based on the spatio-temporal relationships between deformation and vertical axis rotation, curved orogens can be subdivided as primary or secondary (oroclines s.l.), if they formed respectively in a self-similar manner without undergoing important variations in their original curved shape or if their curvature in map-view is the result of a bending about a vertical axis of rotation. In addition to the kinematics of the arc and the timing of its curvature, a crucial factor for understanding the origin of belts curvature is the knowledge of the geodynamic process governing arc formation. In this context, the detailed reconstruction of the rotational history is mainly based on paleomagnetic and structural analyses (fold axes, kinematic indicators), which include the magnetic fabric. In fact, in curved fold and thrust belts, assuming that the magnetic lineation is tectonically originated and formed during layer-parallel shortening (LPS) before vertical axis rotations, the orientation of the magnetic lineation often strictly follows the curvature of the orogeny. This assumption represents a fundamental prerequisite to fully understand the origin of orogenic arcs and to unravel the geodynamic processes responsible for their curvature. We present two case studies: the central Mediterranean arcs and the Alborz Mts in Iran. The Mediterranean area has represented an attractive region to apply paleomagnetic analysis, as it shows a large number of narrow arcs, whose present-day shape has been driven by the space-time evolution of the Mediterranean subduction system, which define a irregular and rather diffuse plate boundary. The Alborz Mts. form a sinuous range over 1,200 km long, defining

Carboniferous high-pressure metamorphism of Ordovician protoliths in the Argentera Massif (Italy), Southern European Variscan belt

NASA Astrophysics Data System (ADS)

Rubatto, Daniela; Ferrando, Simona; Compagnoni, Roberto; Lombardo, Bruno

2010-04-01

The age of high-pressure metamorphism is crucial to identify a suitable tectonic model for the vast Variscan orogeny. Banded H P granulites from the Gesso-Stura Terrain in the Argentera Massif, Italy, have been recently described (Ferrando et al., 2008) relicts of high-pressure metamorphism in the western part of the Variscan orogen. Bulk rock chemistry of representative lithologies reveals intermediate silica contents and calc-alkaline affinity of the various cumulate layers. Enrichment in incompatible elements denotes a significant crustal component in line with intrusion during Ordovician rifting. Magmatic zircon cores from a Pl-rich layer yield scattered ages indicating a minimum protolith age of 486 ± 7 Ma. Carboniferous zircons (340.7 ± 4.2 and 336.3 ± 4.1 Ma) are found in a Pl-rich and a Pl-poor layer, respectively. Their zoning, chemical composition (low Th/U, flat HREE pattern and Ti-in-zircon temperature) and deformation indicate that they formed during the high-pressure event before decompression and mylonitisation. The proposed age for high-pressure metamorphism in the Argentera Massif proves that subduction preceded anatexis by less than 20 Ma. The new data allow a first-order comparison with the Bohemian Massif, which is located at the eastern termination of the Variscan orogen. Similarities in evolution at either end of the orogen support a Himalayan-type tectonic model for the entire European Variscides.

Jurassic subduction initiation in the western and central Neo-Tethys and the origin of the Balkan ophiolites

NASA Astrophysics Data System (ADS)

Van Hinsbergen, D. J. J.; Maffione, M.

2017-12-01

Jurassic subduction initiation in the Neo-Tethys Ocean was the first, critical step of a long tectonic process that eventually led to the collision of the Adria-Africa and Eurasia plates and the formation of a 6000 km long Alpine orogenic belt spanning from the Balkan Peninsula to Iran. Investigating the process of subduction initiation in the Neo-Tethys during the Jurassic is crucial to (i) reconstruct the complex geological evolution of this orogen from its initial stages, and (ii) shed new lights over the enigmatic kinematics and driving mechanisms of subduction initiation. Records of the initial closure of the Neo-Tethys are today preserved in a fragmented belt of Middle Jurassic ophiolites (170-160 Ma) distributed above the Alpine orogen. In particular, the well-preserved and extensively studied ophiolites of the Balkan Peninsula offer a unique chance to study the mechanisms leading to the closure of the western domain of the Neo-Tethys. Here we provide the first quantitative constraints on the geometry of the Jurassic Neo-Tethyan subduction system using a net tectonic rotation analysis based on paleomagnetic and structural geological data from the sheeted dyke complexes of various ophiolites of Serbia (Maljen, Ibar) and Greece (Othris, Pindos, Vourinos, Guevgueli). Our results show that closure of the western Neo-Tethys was accommodated by two subduction zones, one intra-oceanic, formed at the N-S trending Neo-Tethyan ridge, the other initiated at the European passive margin and curving southward from a N-S to a NW-SE direction following the shape of the passive margin. We propose that these two subduction zones formed upon propagation of subduction(s) initiated in the central Neo-Tethys (modern Turkey) in the late Early Jurassic ( 185-180 Ma).

A revised subduction inception model to explain the Late Cretaceous, doubly vergent orogen in the pre-collisional western Tethys: evidences from the Northern Apennine

NASA Astrophysics Data System (ADS)

Meneghini, Francesca; Marroni, Michele; Pandolfi, Luca

2017-04-01

Orogenic processes are widely demonstrated to be strongly controlled by inherited structures. The paleogeography of the converging margins, and the tectonic processes responsible for their configuration, will influence the location of subduction initiation, the distribution of deformation between upper and lower plate, the shape of the accretionary prism and of the subsequent orogeny, through controlling the development of single or doubly-vergent orogens, and, as a corollary, the modality of exhumation of metamorphosed units. The "alpine age" collisional belts of the Mediterranean area are characterized by tangled architectures derived from the overlapping of several deformation events related to a multiphase, long history that comprises not only the collision of continental margins, but that can be regarded as an heritage of both the rifting-related configuration of the continental margins, and the subduction-related structures. The Northern Apennines is a segment of these collisional belts that originated by the Late Cretaceous-Middle Eocene closure of the northern branch of the western Tethys, and the subsequent Late Eocene-Early Oligocene continental collision between the Europe and Adria plates. Due to a different configuration of the paired Adria and Europe continental margins, inherited from a rifting phase dominated by asymmetric, simple-shear kinematics, the Northern Apennines expose a complex groups of units, referred to as Ligurian Units, that record the incorporation into the subduction factory of either fragments of the Ligure-Piemontese oceanic domain (i.e. Internal Ligurian Units), and various portions of the thinned Adria margin (i.e. External Ligurian Units), describable as an Ocean-Continent Transition Zone (OCTZ). The structural relationships between these groups of Units are crucial for the definition of the pre-collisional evolution of the belt and have been the subject of big debates in the literature, together with the location and

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.

Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana ( USA).

USGS Publications Warehouse

Skipp, B.

1987-01-01

The Clearwater orogenic zone in central Idaho and W Montana contains at least 2 major NE-directed Cordilleran thrust plates of Early Proterozoic metasedimentary and metaigneous rocks that overrode previously folded Middle Proterozoic rocks of the Belt basin in Cretaceous time. The northeastward migration of the resultant thickened wedge of crustal material combined with Cretaceous subduction along the W continental margin produced a younger N Bitterroot lobe of the Idaho batholith relative to an older S Atlanta lobe. Eocene extensional unroofing and erosion of the Bitterroot lobe has exposed the roots of the thick Cordilleran thrust sheets.-Author

Mantle wedge exhumation beneath the Dora-Maira (U)HP dome unravelled by local earthquake tomography (Western Alps)

NASA Astrophysics Data System (ADS)

Solarino, Stefano; Malusà, Marco G.; Eva, Elena; Guillot, Stéphane; Paul, Anne; Schwartz, Stéphane; Zhao, Liang; Aubert, Coralie; Dumont, Thierry; Pondrelli, Silvia; Salimbeni, Simone; Wang, Qingchen; Xu, Xiaobing; Zheng, Tianyu; Zhu, Rixiang

2018-01-01

In continental subduction zones, the behaviour of the mantle wedge during exhumation of (ultra)high-pressure [(U)HP] rocks provides a key to distinguish among competing exhumation mechanisms. However, in spite of the relevant implications for understanding orogenic evolution, a high-resolution image of the mantle wedge beneath the Western Alps is still lacking. In order to fill this gap, we perform a detailed analysis of the velocity structure of the Alpine belt beneath the Dora-Maira (U)HP dome, based on local earthquake tomography independently validated by receiver function analysis. Our results point to a composite structure of the mantle wedge above the subducted European lithosphere. We found that the Dora-Maira (U)HP dome lays directly above partly serpentinized peridotites (Vp 7.5 km/s; Vp/Vs = 1.70-1.72), documented from 10 km depth down to the top of the eclogitized lower crust of the European plate. These serpentinized peridotites, possibly formed by fluid release from the subducting European slab to the Alpine mantle wedge, are juxtaposed against dry mantle peridotites of the Adriatic upper plate along an active fault rooted in the lithospheric mantle. We propose that serpentinized mantle-wedge peridotites were exhumed at shallow crustal levels during late Eocene transtensional tectonics, also triggering the rapid exhumation of (U)HP rocks, and were subsequently indented under the Alpine metamorphic wedge in the early Oligocene. Our findings suggest that mantle-wedge exhumation may represent a major feature of the deep structure of exhumed continental subduction zones. The deep orogenic levels here imaged by seismic tomography may be exposed today in older (U)HP belts, where mantle-wedge serpentinites are commonly associated with coesite-bearing continental metamorphic rocks.

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.

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

A Comparative Study of the Electrical Structure of Circum Tibetan Plateau Orogenic Belts and its Tectonic Implications

NASA Astrophysics Data System (ADS)

Jin, Sheng; Zhang, Letian; Wei, Wenbo; Ye, Gaofeng; Jing, Jianen; Dong, Hao; Xie, Chengliang; Yin, Yaotian

2017-04-01

The Tibetan Plateau, as known as "roof of the world", was created through the on-going continent-continent collision between the Indian and Eurasian plates since 55 Ma. As the process continues, the plateau is growing both vertically and horizontally. The horizontal expansion of the plateau is blocked by the Yangtze block in the east, the Tarim block in the north, and the Ordos block in the northeast, and consequently lead to the formation of the circum Tibetan plateau orogenic belts. To better understand the mechanism behind this process, we conducted a comparative study by collecting 7 magnetotelluric (MT) profiles over the margins of the Tibetan plateau, namely, the INDEPTH 100, 700 and 800 lines in the southern Tibet, the INDEPTH 4000 and 5000 lines across the Altyn Tagh fault on the northern margin of the plateau, as well as other two profiles across the Haiyuan fault and the Longmenshan fault on the northeastern and eastern margins of the plateau deployed under the framework of project SinoProbe. The electrical features of the stable blocks surrounding the Tibetan plateau are generally resistive, while crustal conductive layers are found to be wide spread within the plateau. The southern margin of the Tibetan plateau is characterized by large scale underthrust of the Indian lithosphere beneath the plateau. This intense converging process created the thrust fault system distributed along the southern margin of the Tibetan plateau over 1000 km. Crustal conductive layers discovered in southern Tibet are generally associated with the southward crustal flow that originated from the lower crust within the plateau and exhumed along the thrust belts in the Himalayas. On the eastern margin of the Tibetan plateau, the electrical structures suggest that the Yangtze block wedged into the Tibetan lithosphere and caused decoupling between the crust and upper mantel. Large scale conductors discovered beneath the Songpan-Ganze block reflect that the eastward crustal flow was

The pre-Mesozoic tectonic unit division of the Xing-Meng orogenic belt (XMOB)

NASA Astrophysics Data System (ADS)

Xu, Bei; Zhao, Pan

2014-05-01

is between the SHB and JB, marked by the three phase granites of 485, 450 and 425Ma in the SHB. Tectonic units of the middle Devonian-Carboniferous tectonic stage include the middle-late Devonian continental basin, Carboniferious continental and epeiric sea basin, intrusive and irruptive igneous rock belt with ages from 300Ma to 330Ma containing granites, diorites, gabbros and biomodal volcanic rocks, and early Carboniferious ophiolites of 330-350Ma in Hegenshan and Erenhot. The Permian tectonic units can be divided into continental rift belt, ophiolite belt, alkaline rock belt and "red sea"-like ocean basin, which indicates an continuous extension environment during the Permian. The continental rift belt is composed of thick continental sedimentary rocks containing plant fossils, biomodal volcanic rocks (270-290Ma). The alkaline rocks can be divided into north and south belts by their distribution. The Solonker ophiolite is a thrust sheet that is inserted in a thrust stack containing the Upper Carboniferious epeiric sea clastic rocks and carbornates. The "red sea"-like ocean basin is characterized by basalt sequences with ages of 246-260Ma, which shows an affinity to E-MORB and a tendency towards OIB.

Detrital-zircon fission-track geochronology of the Lower Cenozoic sediments, NW Himalayan foreland basin: Clues for exhumation and denudation of the Himalaya during the India-Asia collision

NASA Astrophysics Data System (ADS)

Jain, A.; Lal, N.; Suelmani, B.; Awasthi, A. K.; Singh, S.; Kumar, R.

2007-12-01

evidences of the Pre-Himalayan Peaks in zircon FT ages and appear to be responsible for incoming of the Himalayan (HP) ~30.0 Ma and Young Himalayan Peaks (YHP) of ~15.0 Ma, respectively; the latter appears only between 13.0 and 11.0 Ma sedimentation of the Lower Siwalik Formation. Three distinct metamorphic events get recognised in source area of the Himalayan Metamorphic Belt. Though the Dagshai-Kasauli-Lower Siwalik sequence records uninterrupted fluvial sedimentation since 30 Ma, distinct breaks in zircon FT ages ~5.0-7.0 Myr at the beginning of each formation records pulsative exhumation of the source area in response to the collision between India and Asia.

Structural model of the eastern Achara-Trialeti fold and thrust belt using seismic reflection profiles

NASA Astrophysics Data System (ADS)

Alania, Victor; Chabukiani, Alexander; Enukidze, Onise; Razmadze, Alexander; Sosson, Marc; Tsereteli, Nino; Varazanashvili, Otar

2017-04-01

hangingwall structure with the syn-folding growth stratal sequence. Pre-growth Oligocene strata are overlain by Late (?) Quaternary alluvial fan gravels, sands and clays. Growth unconformity of back-limb showing flat clays unconformably on top of Oligocene sandstone and shale beds. The growth strata geometry of growth fold is related to the progressive limb-rotation model (Hardy & Poblet, 1994). References Alania, V., et al., 2016a. Structure of the eastern Achara-Trialeti fold and thrust belt using seismic reflection profiles: implication for tectonic model of the Lesser Caucasus orogen. 35TH International Geological Congress (IGC), 27 August - 4 September, 2016, Cape Town, South Africa. Alania, V., et al., 2016b. Growth structures, piggyback basins and growth strata of Georgian part of Kura foreland fold and thrust belt: implication for Late Alpine kinematic evolution. Geological Society, London, Special Publications no. 428, doi:10.1144/SP428.5. Hardy, S., and J. Poblet, 1994. Geometric and numerical model of progressive limb rotation in detachment folds: Geology, v. 22, p. 371-374. Shaw, J., Connors, C. & J. Suppe, 2005. Seismic interpretation of contractional fault-related folds. AAPG Studies in Geology 53, 156 pp. Sosson, M., et al., 2016. The Eastern Black Sea-Caucasus region during Cretaceous: new evidence to constrain its tectonic evolution. Compte-Rendus Geosciences, v. 348, Issue 1, p. 23-32.

Effect of footwall structures on kinematic evolution of dominant thrusts from hinterland of an orogenic wedge: Insights from Sikkim Himalayan FTB

NASA Astrophysics Data System (ADS)

Ghosh, Pritam; Bhattacharyya, Kathakali

2017-04-01

Deformation profile of a thrust sheet is generally characterized by a dominance of simple-shear toward the base and pure-shear higher up. In this study, we attempt to examine the effect of underlying footwall structure on the evolution of such a deformation profile with time. We focus on two dominant thrusts of the Sikkim Himalayan FTB, the northern most Main Central thrust (MCT) and its major footwall thrust, the Pelling thrust (PT). The MCT and the PT sheets are folded in an E-W trending antiform-synform pair by the growth of the underlying Lesser Himalayan duplex. The PT acts as the roof thrust of the duplex. The coarse-grained, quartzo-feldspathic gneissic protoliths transform into quartz-mica mylonite forming ˜1170m thick amphibolite facies MCT zone and ˜938m thick greenschist facies PT zone. Due to the forelandward progression of deformation front, the overlying MCT foliation is superposed by the underlying PT foliation. Within both the fault zones, quartz has undergone grain-size reduction dominantly by dislocation creep, and feldspar by fracturing mechanism. Interestingly, microfracturing is more dominant in MCT zone than in the PT zone. Additionally, pressure solution is significantly higher in the PT zone than in the MCT. Thus, there is a spatial variation in deformation mechanisms within the MCT and PT zones. Based on recrystallized quartz grain-sizes, we estimate deformation temperatures of ˜430˚ C-510˚ C and ˜400˚ C-430˚ C within the MCT and the PT, respectively. Both quartz and feldspar grains record a higher flattening strain in the MCT zone than in the PT zone. We infer fracturing and pressure solution accommodated a significant amount of strain, thereby under-representing the viscoplastic strain. Estimation of kinematic vorticity from two different incremental strain markers, namely oblique-fabric and subgrains, indicate both the MCT and the PT zones record a progressively higher pure-shear dominated deformation with time. The PT zone

Volcanogenic massive sulphide and orogenic gold deposits of northern southeast Alaska

USGS Publications Warehouse

Sack, Patrick J; Karl, Susan M.; Steeves, Nathan; Gemmell, J Bruce

2016-01-01

This five-day field trip visits the most significant mineral deposits in northern southeast Alaska. The trip begins and ends with regional transects in the interior Intermontane terranes around Whitehorse, Yukon, and the Insular terranes along the northern Chatham Strait region of southeast Alaska (Fig. A-1 and Fig. A-2; Plate-1). To put the deposits in a regional tectonic framework, the guidebook begins with an introduction to northern Cordilleran geology, tectonics and metallogeny. The foci of the deposit portion of the field trip are Late Triassic volcanogenic massive sulphide (VMS) deposits of the Alexander Triassic metallogenic belt and Paleogene orogenic gold deposits of the Juneau gold belt. Details of the local geology are further elaborated in each segment of the guide book (Days 1-5). The data that provide the basis for the VMS deposit interpretations come from a series of PhD and MSc studies by the Centre of Excellence in Ore Deposit Research (CODES) at the University of Tasmania and the University of Ottawa. These deposit-scale studies are complimented by a long history of regional mapping and research by the U.S. Geological Survey (USGS).

Mineral potential tracts for orogenic, Carlin-like, and epithermal gold deposits in the Islamic Republic of Mauritania, (phase V, deliverable 69): Chapter H in Second projet de renforcement institutionnel du secteur minier de la République Islamique de Mauritanie (PRISM-II)

USGS Publications Warehouse

Goldfarb, Richard J.; Marsh, Erin; Anderson, Eric D.; Horton, John D.; Finn, Carol A.; Beaudoin, Georges

2015-01-01

A preliminary evaluation of these gold data can be used to develop broad, firstorder tracts defining favorable and permissive areas for gold resources; detailed metamorphic and structural maps are required for more detailed future tract definition. Such a first-order assessment can, nonetheless, broadly identify four tracts of gold resource potential. Three of these are favorable for discovery of new orogenic gold deposits. One tract, although not favorable, is nevertheless permissive for discovery of epithermal gold deposits. Tract 1 is defined by favorable medium metamorphic grade greenstone belts within vast areas of unfavorable high metamorphic grade, Mesoarchean and Paleoproterozoic granite-gneiss basement of the Rgueïbat Shield. Faults >200 km in length following the general strike of the greenstone belts; lineament intersections with both exposed and buried parts of greenstone belts within 500 m of the surface, as defined by aeromagnetic data (Finn and Anderson, 2015); and areas of banded iron formation (BIF) in the belts are particularly favorable areas for hosting gold resources in orogenic gold deposits within and along the margins of the greenstone belts. Tracts 2 and 3, also for orogenic gold, reflect the favorable Proterozoic-Cambrian metamorphic rocks of the Northern and Southern Mauritanides, with >200-km-long faults following the general strike of the range, and areas underlain by ultramafic and BIF rocks being particularly favorable. Outcrops of Triassic-Jurassic igneous rocks along the margins of the Taoudeni Basin define tract 4, which is permissive for epithermal gold deposits. Although extensive data are lacking for the area, carbonate units along the northern side of the Taoudeni Basin could be considered permissive host rocks for Carlin-type mineralization, but the deep-water carbonate lithologies are typically not favorable for such. 

Cenozoic Deformation of the Tarim Basin (Xinjiang, China): a Record of the Deformation Propagation through the Asian Orogenic System

NASA Astrophysics Data System (ADS)

Laborde, A.; Barrier, L.; Simoes, M.; Li, H.

2016-12-01

During the Cenozoic, the ongoing India-Eurasia collision resulted in the formation of the Himalayan-Tibetan plateau and reactivated the Tian Shan and Altai ranges located thousands of kilometers further north. Despite numerous studies carried out on the geology and tectonics of this large convergent orogenic system, several mechanisms remain controversial such as the stress propagation through the Asia Continent or the strain partitioning between crustal thickening and lateral extruding of its lithosphere. Located between the Tibetan Plateau and the Tian Shan Range, the Tarim Basin and its several kilometres thick Cenozoic sediments derived from the surrounding mountain belts are key recorders to reconstruct the evolution of the latters. Moreover, this basin is often considered as a relatively rigid block, which behaved as a secondary ``indenter'' transmitting collisional stresses to the Tian Shan. However, due to the size of the Tarim and its thick Cenozoic sedimentary series hiding most of its structures, the constraints on the spatial distribution and timing of the its Cenozoic deformation remain fragmentary. Therefore, the main objective of our study was to produce a synthetic view of this deformation at the scale of the whole basin. Based on numerous surface and subsurface data (satellite images, field surveys, seismic profiles, and well data), we established a tectonic map of the Cenozoic structures in the region and built balanced geological cross-sections across the basin. Our surface and subsurface observations confirm that, contrary to what had been proposed, the Tarim block has also undergone a major deformation during the Cenozoic. The quantification and history of this deformation provide useful insights into the modalities of the crustal shortening in the area and the problems of stress propagation and strain partitioning following the Indo-Asian collision.

Unfolding the arc: The use of pre-orogenic constraints to assess the evolution of the Variscan belt in Western Europe

NASA Astrophysics Data System (ADS)

Casas, Josep M.; Brendan Murphy, J.

2018-06-01

We present a pre-orogenic, early Paleozoic, palinspastic reconstruction of the northern Gondwana margin that was subsequently involved in the Late Paleozoic Variscan orogeny in central and Western Europe. Our reconstruction is based on two pre-orogenic data sets, the age and distribution of Cambrian-Ordovician magmatism and the detrital zircon age signature of late Neoproterozoic-early Paleozoic clastic rocks. We obtain this reconstruction by unfolding the Ibero-Armorican arc and by restoring the movement of the large-scale dextral strike-slip faults that transect the different tectono-stratigraphic units. Our results favour an irregular shape for this part of the northern Gondwana margin with a N-S central segment linking two E-W oriented segments. The proposed reconstruction and the structural restoration of the main features of Variscan deformation is in accordance with some aspects of previously proposed structural models, such as the curved geometry of the Gondwanan margin required by the indentor model for continental collision, the role played by the large strike-slip faults in dispersing formerly juxtaposed units, and the regional-scale oroclinal folding of part of this margin during late Carboniferous-Early Permian times. The combined use of the pre-orogenic geological constraints and palinspastic restoration is a useful approach that may provide a foundation for continual refinement of reconstructions as more data become available.

The effect of flexural isostasy on the response time of orogenic systems

NASA Astrophysics Data System (ADS)

Braun, J.; Margirier, A.; Guerit, L.

2017-12-01

The concept of orogenic steady-state implies that mountain belts can reach a dynamic balance between uplift and erosion in order to maintain a quasi-constant shape. The final morphology of the mountain will be a function of the relative efficiency between uplift and erosion and is therefore likely to be modulated by climate. However, reaching such a steady-state cannot be instantaneous and there must exist a time lag between the onset of convergence and the full development of the mountain topography. Similarly, when an orogenic system is subject to a marked change in convergence rate or in climatic conditions, it takes a certain time for it to adapt to such a change and develop a new steady-state morphology. It is during these transient phases that the nature and efficiency of the interactions between tectonics and climate are most likely to be constrained by observations and understood. The duration of this transient stage remains, however, poorly constrained and understood. As shown by many authors (Whipple and Tucker, 1999, for example) the rate at which tectonic systems evolve to reach steady-state is likely controlled by climate and rock strength, which both determine the efficiency of erosional processes, and the rate of uplift. Here we show that isostasy also plays a very important role in determining the length of the transient phase and that, depending on the level of isostatic adjustment, which in turn depends on the flexural strength of the underlying lithosphere, isostasy can change the time it takes for an orogenic system to reach steady-state by an order of magnitude, i.,e. from a few millions to a few tens of millions of years. This has very important implications. It may explain why many young orogenic systems display an increase in uplift and erosion rate millions of years after the onset of collision and that, in these situations, such an increase does not require a steady change in tectonic and/or climate conditions/forcing. We also show that

Preserved organic matter in the Serpentinized Ocean-Continent Transition of Alpine Tethys

NASA Astrophysics Data System (ADS)

Mateeva, T.; Wolff, G. A.; Kusznir, N.; Manatschal, G.; Wheeler, J.

2017-12-01

Serpentinization occurs at slow-spreading ocean ridges and magma-poor rifted continental margins. At modern hydrothermal vents, serpentinization has been observed to support hydrogen-driven microbial environments including methanotrophic biosystems. An important question is: "Are such bio-systems locally restricted to hydrothermal vents or are they more pervasive, being linked with the exhumation of serpentinized mantle at the seafloor?" Fieldwork sampling of km scale exposures of orogenically exhumed serpentinized mantle in the Alps allows 3D mantle sampling that is not possible at ocean ridges and provides an opportunity to investigate the organic matter in an ophiolite sequence relative to the seafloor. Samples from the fossil Tethyan OCT, exhumed during Alpine collisional orogeny, have been examined for the presence or absence of biomarkers typical of methanotrophy within serpentinized exhumed mantle. Samples from the Totalp unit, Tasna nappe and Platta unit of the Eastern Swiss Alps and Chenaillet in the Western Alps from the Tethyan magma-poor OCT were selected for analysis because they have little Alpine deformation and underwent only low-grade Alpine metamorphism. Hand specimens and cores taken from these locations have been analysed to search for the presence or absence of biomarkers in the serpentinite and its overlying lithologies. Thin sections of samples from these OCT locations reveal multiple serpentinization events and calcification phases. All the lithologies sampled show the presence of hydrocarbons such as n-alkanes, low molecular weight polynuclear aromatic hydrocarbons (PAHs, of mixed petrogenic and pyrogenic source), hopanes, steranes (of marine origin), and branched alkanes (pristane and phytane, non-specific marine origin). The identifiable biomarkers and the isotopic data are consistent with organic matter of a marine origin and do not provide any evidence for a methanotrophic bio-system. It is noteworthy that basement mantle rocks still

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

NASA Astrophysics Data System (ADS)

Kusky, Timothy M.

1991-08-01

crush zones in the region of emergent thrusts in Keskarrah Bay. Depth differences can account for only half of the metamorphic gradient; thermal profiles which increased downwards in obducted greenstone belts and synthrusting plutonism explains other high metamorphic gradients. A tectonic model involving the collision of an accretionary prism with a continental margin best explains the structural and sedimentological evolution of the orogen.

Sedimentological and stratigraphic evolution of the southern part of the Barberton greenstone belt: A case of changing provenance and stability

NASA Technical Reports Server (NTRS)

Lowe, D. R.; Byerly, G. R.

1986-01-01

The sedimentological and stratigraphic evolution of the 3.5 to 3.3 Ga Barberton Greenstone Belt can be divided into three principal stages: (1) the volcanic platform stage during which at least 8 km of mafic and ultramafic volcanic rocks, minor felsic volcanic units, and thin sedimentary layers (Onverwacht Group) accumulated under generally anorogenic conditions; (2) a transitional stage of developing instability during which widespread dacitic volcanism and associated pyroclastic and volcaniclastic sedimentation was punctuated by the deposition of terrigenous debris derived by uplift and shallow erosion of the belt itself (Fig Tree Group); (3) an orogenic stage involving cessation of active volcanism, extensive thrust faulting, and widespread deposition of clastic sediments representing deep erosion of the greenstone belt sequence as well as sources outside of the belt (Moodies Group).

Shoshonitic- and adakitic magmatism of the Early Paleozoic age in the Western Kunlun orogenic belt, NW China: Implications for the early evolution of the northwestern Tibetan plateau

NASA Astrophysics Data System (ADS)

Wang, Jian; Hattori, Keiko; Liu, Jianguo; Song, Yue; Gao, Yongbao; Zhang, Han

2017-08-01

The Western Kunlun orogenic belt in the northwestern margin of the Tibetan plateau contains two magmatic belts; early Paleozoic belt in the northern part of Western Kunlun Terrane (WKT), and early Mesozoic belt in the southern part of WKT. Both formed from northward subduction of the Paleo-Tethys. The early Paleozoic belt contains large Datong and Qiukesu igneous complexes and many smaller plutons. The Datong complex is mainly composed of dark-colored porphyritic syenite and monzonite with minor light-colored dykes of granite and monzonite. The dark-colored rocks are characterized by moderate SiO2 (58.2-69.3 wt.%), and high Al2O3 (15.3-17.1 wt.%), total alkali (Na2O + K2O = 8.07-10.2 wt.%) and ratios of K2O/Na2O (0.77-1.83). They plot in "shoshonite" field, and show high abundances of LILE including LREE ((La/Yb)n = 15.4-26.2; mean 20.2) with pronounced negative anomalies of Nb-Ta-P-Ti in normalized trace elemental patterns and weak negative anomalies of Eu (δEu = 2Eun/(Smn + Gdn) = 0.68-0.80). The light-colored rocks contain slightly higher concentrations of SiO2 (60.3-72.0 wt.%), similar Al2O3 (14.7-17.6 wt.%), and slightly lower total alkalis (6.57-9.14 wt.%) than dark-colored rocks. They show adakitic geochemical signatures with low Y (5.80-17.2 ppm) and Yb (0.63-1.59 ppm), and high Sr/Y (> 40). U-Pb zircon dating indicates that shoshonitic rocks and adakitic dykes formed at 444 Ma to 443 Ma, and a separate small adakitic plug at 462 Ma. The mean εHf(t) values of zircon range from - 1.6 to - 0.94 (n = 14) with TDM2 of 1.5 Ga for shoshonitic rocks and εHf(t) values from - 1.8 to + 0.72 (n = 12) with TDM2 of 1.4 to 1.5 Ga for adakitic rocks. Shoshonitic rocks show initial 87Sr/86Sr and εNd(t) of 0.7092-0.7100 and - 3.9 to - 3.2, respectively, and adakitic rocks yield initial 87Sr/86Sr and εNd(t) of 0.7099-0.7134 and - 3.6 to - 3.1, respectively. Similar Sr, Nd, and Hf isotope compositions for the shoshonitic and adakitic rocks suggest similar ancient rocks

Mantle Response to Collision, Slab Breakoff & Lithospheric Tearing in Anatolian Orogenic Belts, and Cenozoic Geodynamics of the Aegean-Eastern Mediterranean Region

NASA Astrophysics Data System (ADS)

Dilek, Yildirim; Altunkaynak, Safak

2010-05-01

The geochemical and temporal evolution of the Cenozoic magmatism in the Aegean, Western Anatolian and peri-Arabian regions shows that plate tectonic events, mantle dynamics, and magmatism were closely linked in space and time. The mantle responded to collision-driven crustal thickening, slab breakoff, delamination, and lithospheric tearing swiftly, within geologically short time scales (few million years). This geodynamic continuum resulted in lateral mantle flow, whole-sale extension and accompanying magmatism that in turn caused the collapse of tectonically and magmatically weakened orogenic crust. Initial stages of post-collisional magmatism (~45 Ma) thermally weakened the orogenic crust in Tethyan continental collision zones, giving way into large-scale extension and lower crustal exhumation via core complex formation starting around 25-23 Ma. Slab breakoff was the most common driving force for the early stages of post-collisional magmatism in the Tethyan mountain belts in the eastern Mediterranean region. Magmatic rocks produced at this stage are represented by calc-alkaline-shoshonitic to transitional (in composition) igneous suites. Subsequent lithospheric delamination or partial convective removal of the sub-continental lithospheric mantle in collision-induced, overthickened orogenic lithosphere caused decompressional melting of the upwelling asthenosphere that in turn resulted in alkaline basaltic magmatism (<12 Ma). Attendant crustal extension and widespread thinning of the lithosphere facilitated rapid ascent of basaltic (OIB) magmas without much residence time in the crust and hence the eruption of relatively uncontaminated, asthenosphere-derived magmas at the surface (i.e. Kula lavas in SW Anatolia). Subduction of the Tethyan mantle lithosphere northward beneath Eurasia was nearly continuous since the latest Cretaceous, only temporarily punctuated by the collisional accretion of several ribbon continents (i.e. Pelagonia, Sakarya, Tauride-South Armenian

Permanent GPS network around the bend of the Jura Arc: preliminary results

NASA Astrophysics Data System (ADS)

Sue, Christian; Walpersdorf, Andrea; Sakic, Pierre; Rabin, Mickael; Champagnac, Jean daniel

2014-05-01

The Jura Mountain, the westernmost belt of the alpine orogeny, is one of the best-studied orogenic arcs in the world. The Jura arc is a typical fold-and-thrust belt, with a main décollement thrust localized in the Triasic evaporites under the Jurassic-Cretaceous series. It is directly linked to the alpine orogenic wedge, especially in term of critical taper. It is supposed to be still active in collision mode, which would rise up the issue of its relation with the Alps to the East, currently undergoing post-orogenic gravitational potential adjustment. Nevertheless, its current activity and recent deformation remain a matter of debate, few neotectonic-related data being available in this area. The Jura is crosscut by left-lateral strike-slip faults in a radial scheme with respect to the arc, and recent seismicity along one of them, the Vuache fault (Annecy earthquake Ml 5.3 1996), and at the northern front of the belt (Beaume-les-Dames earthquake, Ml 5.1, 2004), argues for ongoing active deformation across the Jura Mountain. Here we present preliminary results of permanent GPS network surrounding the Jura belt (RENAG and RPG data), which tend to show very slow, yet self-consistent strain pattern of the order of some tenth of mm/yr over 100 km-long typical baselines, with shortening perpendicular to the arc, and extension parallel to its axial trend. We also characterize a slow uplift in the same order of magnitude, which appears to be correlated to the current uplift observed in the Alps. Indeed, the uplift velocities are continuously decreasing from the core of the Alps (+2 mm/yr) to the westernmost part of the Jura (+0,4 mm/yr) and to the stable foreland (-0.1 mm/yr). Actually, from the Po plain to the Jura foreland, the GPS-related uplift velocities are well correlated to the topography, and the Jura arc appears connected to the Alps from this point of view. In order to better determine the deformation pattern in the Jura arc, we present a new regional GNSS

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

Influence of pre-existing basement faults on the structural evolution of the Zagros Simply Folded belt: 3D numerical modelling

NASA Astrophysics Data System (ADS)

Ruh, Jonas B.; Gerya, Taras

2015-04-01

The Simply Folded Belt of the Zagros orogen is characterized by elongated fold trains symptomatically defining the geomorphology along this mountain range. The Zagros orogen results from the collision of the Arabian and the Eurasian plates. The Simply Folded Belt is located southwest of the Zagros suture zone. An up to 2 km thick salt horizon below the sedimentary sequence enables mechanical and structural detachment from the underlying Arabian basement. Nevertheless, deformation within the basement influences the structural evolution of the Simply Folded Belt. It has been shown that thrusts in form of reactivated normal faults can trigger out-of-sequence deformation within the sedimentary stratigraphy. Furthermore, deeply rooted strike-slip faults, such as the Kazerun faults between the Fars zone in the southeast and the Dezful embayment and the Izeh zone, are largely dispersing into the overlying stratigraphy, strongly influencing the tectonic evolution and mechanical behaviour. The aim of this study is to reveal the influence of basement thrusts and strike-slip faults on the structural evolution of the Simply Folded Belt depending on the occurrence of intercrustal weak horizons (Hormuz salt) and the rheology and thermal structure of the basement. Therefore, we present high-resolution 3D thermo-mechnical models with pre-existing, inversively reactivated normal faults or strike-slip faults within the basement. Numerical models are based on finite difference, marker-in-cell technique with (power-law) visco-plastic rheology accounting for brittle deformation. Preliminary results show that deep tectonic structures present in the basement may have crucial effects on the morphology and evolution of a fold-and-thrust belt above a major detachment horizon.

Kyanite-bearing migmatites in the central Adirondack Mountains: Implications for late to post-orogenic metamorphism and melting in a collisional orogen

NASA Astrophysics Data System (ADS)

Reeder, J.; Metzger, E. P.; Bickford, M. E.; Leech, M. L.

2016-12-01

the Grenville Province and its Adirondacks outlier. Further analysis of age, geochemical, and petrographic data will help develop a better-defined P-T-t path and may lead to the development of a new tectonic model to be compared with other collisional orogens such as Himalaya or the Bohemian Massif of the Variscan orogenic belt.

Wrench tectonics control on Neogene-Quaternary sedimentation along the Mid-Hungarian Mobile Belt

NASA Astrophysics Data System (ADS)

Pogacsas, Gyorgy; Juhász, Györgyi; Mádl-Szőnyi, Judit; Simon, Szilvia; Lukács, Szilveszter; Csizmeg, János

2010-05-01

The Neogene Pannonian basin is underlain by a large orogenic collage which is built up by several tectonostratigraphic terrains. The basement of the Pannonian Basin became imbricate nappes during the Cretaceous Alpine collision. Nappes of Late Cretaceous in age have been proven below the Great Hungarian Plain (Grow et al 1994). The boundary of the two main terrains, the northwestern ALCAPA (Alpine-Carpathian-Pannonian) and the southeastern TISZA, is the Mid-Hungarian Mobile Belt. It is the most significant neotectonic zone of the Pannonian Basin. The structural analysis of the middle section of the Mid-Hungarian Mobile Belt was carried out on a 120km x 50km area, between the Danube and the Tisza river, on the basis of interpretation of seismic data. The structural analysis of the Neogene-Quaternary sediments was supported by sequence stratigraphic interpretation of seismic, well log and core-sample data. Regional seismic profiles were both oriented in the dip direction, which highlights sediment supply routes into the basin, and strike-oriented. The studied segment of the Mid-Hungarian Mobile Belt consists of several long (some ten kilometres long) strike slip fault zones. The offset lengths of the individual strike slipe faults varies between a few and a dozens of kilometres. Activity along the Mid-Hungarian Mobile Belt can be characterised by four periods, the size and shape of facies zones of each development period were controlled by tectonics: 1. During the early Miocene, the ALPACA moved eastward, bounded by sinistral strike-slipe system along its northern side and dextral strike-slipe fault system along its contact with the Southern Alps and the TISZA terrain. The largest movement took part during the Ottnangian-Karpatian (19-16.5 Ma). The TISZA unit moved northeastward over the remnant Carpathian Flysch Basin (Nemcok et al 2006). These terrains movements resulted in right lateral, convergent wide wrench along the Mid-Hungarian Mobile Belt. The ALPACA

>2500-km-Long Contemporaneous Deep Continental Subduction in the West Gondwana Orogen

NASA Astrophysics Data System (ADS)

Rubatto, D.; Ganade de Araujo, C. E.; Hermann, J.; Cordani, U. G.; Caby, R.; Basei, M. A. S.

2014-12-01

The 5000-km-long, deeply eroded West Gondwana Orogen (WGO) is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks (i.e. eclogites) along its strike. 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. We investigated the metamorphic conditions and age of high-pressure (HP) and ultrahigh-pressure (UHP) eclogites from Mali, Togo and NE-Brazil . P-T estimates confirm UHP to HP conditions for all these localities. The U-Pb age and trace element composition of metamorphic zircon domains demonstrate that continental subduction in the WGO occurred within 20 m.y. over at least 2500 km during the Ediacaran period (620-610 Ma). We consider this to be the first record of modern, large-scale deep-continental subduction and the consequent appearance of Himalayan-scale mountains in the geological record. The rise of such mountains in the Late Ediacaran is perfectly timed to deliver by erosion the sediments (nutrients) that have been deemed necessary for life sustainability in the following Earth evolution.

Alpine ecosystems

Treesearch

P.W. Rundel; C.I. Millar

2016-01-01

Alpine ecosystems are typically defined as those areas occurring above treeline, while recognizing that alpine ecosystems at a local scale may be found below this boundary for reasons including geology, geomorphology, and microclimate. The lower limit of the alpine ecosystems, the climatic treeline, varies with latitude across California, ranging from about 3500 m in...

Fault dating in the Canadian Rocky Mountains: Evidence for late Cretaceous and early Eocene orogenic pulses

USGS Publications Warehouse

van der Pluijm, B.A.; Vrolijk, P.J.; Pevear, D.R.; Hall, C.M.; Solum, J.

2006-01-01

Fault rocks from the classic Rocky Mountain foreland fold-and-thrust belt in south-western Canada were dated by Ar analysis of clay grain-size fractions. Using X-ray diffraction quantification of the detrital and authigenic component of each fraction, these determinations give ages for individual faults in the area (illite age analysis). The resulting ages cluster around 72 and 52 Ma (here called the Rundle and McConnell pulses, respectively), challenging the traditional view of gradual forward progression of faulting and thrust-belt history of the area. The recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges, which result in geologically reasonable strain rates for the area. In addition to regional considerations, this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper-crustal kinematics and regional tectonic analysis of ancient orogens. ?? 2006 Geological Society of America.

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

USGS Publications Warehouse

Lease, Richard O.; Ehlers, T.A.; Enkelmann, E.

2016-01-01

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

Lower Oligocene Alpine geodynamic change: tectonic and sedimentary evidences in the western arc

NASA Astrophysics Data System (ADS)

Dumont, T.; Rolland, Y.; Simon-Labric, T.

2009-04-01

propagation of syn-orogenic basins changed abruptly in earliest Oligocene times. In the southern Subalpine domain, previously SE-NW gradients (sedimentary transport, onlaps, thickness changes) shifted to westwards or southwestwards ones. This major syn-collisional change must have occurred in a short time interval bracketed between thrusting of the earliest, gravity-driven nappes over the Paleogene flexural basin, whose youngest sediments are dated as lowermost Oligocene, and eastwards underthrusting of the Pelvoux basement in the footwall of the Internal Alps indenter, having yielded Ar39/Ar40 ages of 31,2 ± 0,3 Ma to 33,7 ± 0,2 Ma (Simon-Labric et al. et al., in press). This 32 to 34 Ma old event can be traced all over the Alpine chain through its kinematic, structural, metamorphic and magmatic consequences. It played a key role in the generation of the modern, arcuate shape of the Western Alps. References: Dumont T., Champagnac J.D., Crouzet C. & Rochat P. (2008). Multistage shortening in the Dauphiné zone (French Alps): the record of Alpine collision and implications for pre-Alpine restoration. Swiss J. Geosci., 101, suppl. 1, p. 89-110. Jolivet L., Frizon De Lamothte D., Mascle A. & Séranne M. (1999). The Mediterranean Basins : Tertiary extension within the Alpine orogen - an introduction. In: Durand B., Jolivet L., Horwath F. & Séranne M. (eds.), Geological Soc. Spec. Publication, 156, p. 1-34. Simon-Labric T., Rolland Y., Dumont T., Heymes T., Authemayou C., Corsini M. & Fornari M. (in press). Ar39/Ar40 dating of Penninic Front tectonic displacement (W. Alps) during the Lower Oligocene (31-34 Ma). Terra Nova, in press. Rosenbaum G. & Lister G.S (2005). The Western Alps from the Jurassic to Oligocene: spatio-temporal constraints and evolutionary reconstructions. Earth-Sc. Rev., 69, p. 281-306.

The Cenozoic fold-and-thrust belt of Eastern Sardinia: Evidences from the integration of field data with numerically balanced geological cross section

NASA Astrophysics Data System (ADS)

Arragoni, S.; Maggi, M.; Cianfarra, P.; Salvini, F.

2016-06-01

Newly collected structural data in Eastern Sardinia (Italy) integrated with numerical techniques led to the reconstruction of a 2-D admissible and balanced model revealing the presence of a widespread Cenozoic fold-and-thrust belt. The model was achieved with the FORC software, obtaining a 3-D (2-D + time) numerical reconstruction of the continuous evolution of the structure through time. The Mesozoic carbonate units of Eastern Sardinia and their basement present a fold-and-thrust tectonic setting, with a westward direction of tectonic transport (referred to the present-day coordinates). The tectonic style of the upper levels is thin skinned, with flat sectors prevailing over ramps and younger-on-older thrusts. Three regional tectonic units are present, bounded by two regional thrusts. Strike-slip faults overprint the fold-and-thrust belt and developed during the Sardinia-Corsica Block rotation along the strike of the preexisting fault ramps, not affecting the numerical section balancing. This fold-and-thrust belt represents the southward prosecution of the Alpine Corsica collisional chain and the missing link between the Alpine Chain and the Calabria-Peloritani Block. Relative ages relate its evolution to the meso-Alpine event (Eocene-Oligocene times), prior to the opening of the Tyrrhenian Sea (Tortonian). Results fill a gap of information about the geodynamic evolution of the European margin in Central Mediterranean, between Corsica and the Calabria-Peloritani Block, and imply the presence of remnants of this double-verging belt, missing in the Southern Tyrrhenian basin, within the Southern Apennine chain. The used methodology proved effective for constraining balanced cross sections also for areas lacking exposures of the large-scale structures, as the case of Eastern Sardinia.

Petrogenesis and tectonic significance of the late Triassic mafic dikes and felsic volcanic rocks in the East Kunlun Orogenic Belt, Northern Tibet Plateau

NASA Astrophysics Data System (ADS)

Hu, Yan; Niu, Yaoling; Li, Jiyong; Ye, Lei; Kong, Juanjuan; Chen, Shuo; Zhang, Yu; Zhang, Guorui

2016-02-01

We present zircon U-Pb ages and geochemical data on the late Triassic mafic dikes (diabase) and felsic volcanic rocks (rhyolite and rhyolitic tuffs) in the East Kunlun Orogenic Belt (EKOB). These rocks give a small age window of 228-218 Ma. The mafic dikes represent evolved alkaline basaltic melts intruding ~ 8-9 Myrs older and volumetrically more abundant A-type granite batholith. Their rare earth element (REE) and multi-element patterns are similar to those of the present-day ocean island basalts (OIBs) except for a weak continental crustal signature (i.e., enrichment of Rb and Pb and weak depletion of Nb, Ta and Ti). Their trace element characteristics together with the high 87Sr/86Sr (0.7076-0.7104), low εNd(t) (- 2.18 to - 3.46), low εHf(t) (- 2.85 to - 4.59) and variable Pb isotopic ratios are consistent with melts derived from metasomatized subcontinental lithospheric mantle with crustal contamination. The felsic volcanic rocks are characterized by high LREE/HREE (e.g., [La/Yb]N of 5.71-17.00) with a negative Eu anomaly and strong depletion in Sr and P, resembling the model upper continental crust (UCC). Given the high 87Sr/86Sr (0.7213-0.7550) and less negative εNd(t) (- 3.83 to - 5.09) and εHf(t) (- 3.06 to - 3.83) than the UCC plus the overlapping isotopes with the mafic dikes and high Nb-Ta rhyolites, the felsic volcanic rocks are best interpreted as resulting from melting-induced mixing with 45-50% crustal materials and 50-55% mantle-derived mafic melts probably parental to the mafic dikes. Such mantle-derived melts underplated and intruded the deep crust as juvenile crustal materials. Partial melting of such juvenile crust produced felsic melts parental to the felsic volcanic rocks in the EKOB. We hypothesize that the late Triassic mafic dikes and felsic volcanic rocks are associated with post-collisional extension and related orogenic collapse. Such processes are probably significant in causing asthenospheric upwelling, decompression melting

Do along-strike tectonic variations in the Nepal Himalaya reflect different stages in the accretion cycle? Insights from numerical modeling

NASA Astrophysics Data System (ADS)

Mercier, Jonathan; Braun, Jean; van der Beek, Peter

2017-08-01

Whereas the large-scale morphology and dynamics of orogenic wedges are well explained by critical-taper theory, many questions remain unanswered regarding the details of how deformation is accommodated internally. Here, we investigate the dynamics of a collisional orogenic wedge bounded by an over-thickened continental plateau, using two-dimensional thermo-mechanical numerical models. These models, applied to the Himalayan orogen and compared with reference cross-sections, lead us to propose a new hypothesis to explain along-strike variations in tectonic style, topography and exhumation patterns observed along the Himalayan range by a combination of two mechanisms. First, numerical models produce a cycle of crustal ramp formation and advection toward the rear of the wedge. The asynchronous evolution of this cycle along different segments of the range may account for the well-documented lateral variations in the geometry of the Main Himalayan Thrust (MHT) and for the existence of a well-defined topographic transition in some segments of the range. Second, the models suggest that the formation of duplexes leading to the isolation of klippen along the range front may be controlled by rheological contrasts between the Tibetan plateau and/or the Greater Himalayan Sequence and the colliding Indian plate.

Active and fossil mantle flows in the western Alpine region unravelled by seismic anisotropy analysis and high-resolution P wave tomography

NASA Astrophysics Data System (ADS)

Salimbeni, Simone; Malusà, Marco G.; Zhao, Liang; Guillot, Stéphane; Pondrelli, Silvia; Margheriti, Lucia; Paul, Anne; Solarino, Stefano; Aubert, Coralie; Dumont, Thierry; Schwartz, Stéphane; Wang, Qingchen; Xu, Xiaobing; Zheng, Tianyu; Zhu, Rixiang

2018-04-01

The anisotropy of seismic velocities in the mantle, when integrated with high-resolution tomographic models and geologic information, can be used to detect active mantle flows in complex plate boundary areas, providing new insights on the impact of mantle processes on the topography of mountain belts. Here we use a densely spaced array of temporary broadband seismic stations to analyze the seismic anisotropy pattern of the western Alpine region, at the boundary between the Alpine and Apenninic slabs. Our results are supportive of a polyphase development of anisotropic mantle fabrics, possibly starting from the Jurassic to present. Geophysical data presented in this work, and geologic evidence taken from the literature, indicate that: (i) fossil fabrics formed during Tethyan rifting may be still preserved within the Alpine and Apenninic slabs; (ii) mantle deformation during Apenninic slab rollback is not compensated by a complete toroidal flow around the northern tip of the retreating slab; (iii) the previously observed continuous trend of anisotropy fast axes near-parallel to the western Alpine arc is confirmed. We observe that this arc-parallel trend of fast axes is located in correspondence to a low velocity anomaly in the European upper mantle, beneath regions of the Western and Ligurian Alps showing the highest uplift rates. We propose that the progressive rollback of the Apenninic slab, in the absence of a counterclockwise toroidal flow at its northern tip, induced a suction effect at the scale of the supraslab mantle. The resulting mantle flow pattern was characterized by an asthenospheric counterflow at the rear of the unbroken Western Alps slab and around its southern tip, and by an asthenospheric upwelling, mirrored by low P wave velocities, that would have favored the topographic uplift of the Alpine belt from the Mont Blanc to the Mediterranean sea.

Status, ecology, and conservation of the Himalayan griffon Gyps himalayensis (Aves, Accipitridae) in the Tibetan plateau.

PubMed

Lu, Xin; Ke, Dianhua; Zeng, Xianhai; Gong, Guohong; Ci, Ren

2009-05-01

The dramatic population crashes of 3 species of Gyps vulture have raised concerns about the status of their lesser-known congeners. Among these is the Himalayan griffon, G. himalayensis, an iconic vulture of the Tibetan plateau. The continued existence of this scavenger has not only ecological but also cultural implications because of their unique role in the centuries-old sky burial tradition that is followed by nearly 5 million Tibetan people. A lack of baseline information of the Himalayan griffon limits our ability to take conservation measures. The presented data, which were collected during 1996 and 2004 to 2007, indicate that this species is still widespread throughout the plateau and has not experienced a major population decline, likely as a result of protection by Tibetan Buddhism and limited disturbances from human activities largely due to the remoteness of the plateau. Both site and road counts showed that open meadow habitats had the highest griffon abundance, followed by alpine shrub and forest habitats. Estimates based on road transect counts showed that 229,339 Himalayan griffons (+/- 40,447) occupy the 2.5 million km2 Tibetan plateau. In contrast, the maximum carrying capacity of the plateau, on the basis of the total biomass of potential food resources, is 507,996 griffons, with meadow habitats accounting for about 76% of the total population. Griffons depend largely on livestock carcasses for food and forage in groups averaging 5.5 (range 1-100) individuals. Domestic yaks provide about 64% of the griffons' diet, while wild ungulates and human corpses provide 1% and 2%, respectively. Compared with its lowland congeners, this, the only high-elevation Gyps species, had both low population density and small group size, a likely response to the harsh environmental conditions. Although griffon abundance appears relatively stable in their fairly pristine environment, precautionary measures, including investigation of threats, monitoring of population

Metamorphic and tectonic evolution of the Greater Himalayan Crystalline Complex in Nyalam region, south Tibet

NASA Astrophysics Data System (ADS)

Wang, Jia-Min; Zhang, Jin-Jiang; Rubatto, Daniela

2016-04-01

timing along the South Tibetan detachment (27-16 Ma) but precedes that along the MCT (16-10 Ma). Comparison between the obtained P-T-t data and model predictions implies that a lateral crustal flow process dominated the exhumation of the high-grade upper GHC migmitites during 25-16 Ma, whereas a critical taper thrusting process dominated the exhumation of the MCT zone nonmigmatites and cooled migmatites in the lower GHC at 16-10 Ma. In other words, at different temporal and spatial scale, both propagating thrusting along large tectonic boundaries and a low-viscosity melting crust could contribute to the exhumation of high-grade metamorphic rocks in Himalaya-like large hot collisional orogens. KEY WORDS: Greater Himalayan Crystalline Complex; P-T path; U-Pb geochronology; channel flow; tectonic discontinuity References: Wang, J.M., Rubatto, D., Zhang, J.J., 2015a. Timing of partial melting and cooling across the Greater Himalayan Crystalline Complex (Nyalam, central Himalaya): in-sequence thrusting and its implications. Journal of Petrology, 56, 1677-1702. Wang, J.M., Zhang, J.J., Wei, C.J., Rai, S.M., Wang, M., Qian, J.H., 2015b. Characterizing the metamorphic discontinuity across the Main Central Thrust Zone of eastern-central Nepal. Journal of Asian Earth Sciences 101, 83-100. Wang, J.M., Zhang, J.J., Wang, X.X., 2013. Structural kinematics, metamorphic P-T profiles and zircon geochronology across the Greater Himalayan Crystalline Complex in south-central Tibet: implication for a revised channel flow. Journal of Metamorphic Geology 31, 607-628.

Natural selection among Kinnaura of the Himalayan highland: A comparative analysis with other Indian and Himalayan populations

PubMed Central

Gautam, Rajesh K.; Kapoor, Anup K.; Kshatriya, G. K.

2009-01-01

The present investigation on fertility and mortality differential among Kinnaura of the Himalayan highland is based on data collected from 160 post-menopausal women belonging to the middle and high altitude region of Kinnaur district of Himachal Pradesh (Indian Himalayas). Selection potential based on differential fertility and mortality was computed for middle-and high-altitude women. Irrespective of the methodology, the total index of selection was found to be highest among middle-altitude women (0.386) as compared with high-altitude (0.370) women, whereas for the total population it is estimated to be 0.384. It was found that the Kinnaura of the Himalayan highland showing moderate index of total selection and relative contribution of the mortality component (Im) to the index of total selection is higher than the corresponding fertility component (If). The analysis of embryonic and post-natal mortality components shows that the post-natal mortality components are higher in comparison with the embryonic mortality components among highlanders and needs special intervention and health care. The present findings are compared with other Indian tribes as well as non-tribes of the Himalayan region and other parts of the country. It reveals that this index among Kinnaura is moderate than the other population groups; among the Himalayan population, the highest was reported for Galong (It = 1.07) of Arunachal, whereas the lowest was reported from Ahom (It = 0.218) of Manipur. The correlation and regression analysis between total index of selection (It) and fertility (If) and mortality (Im) components for pooled data of populations of the Indian Himalayan states show that If and Im account for 21.6 and 29.1% variability, respectively. In Crow's total index of selection (It) along with strong association, which is significant at the 1% level, this indicates that mortality plays a greater role in natural selection in comparison with fertility among populations of the Indian

Shear zones of the Verkhoyansk fold-and-thrust belt, Northeast Russia

NASA Astrophysics Data System (ADS)

Fridovsky, Valery; Polufuntikova, Lena

2017-04-01

The Verkhoyansk fold-and-thrust belt is situated on the submerged eastern margin of the North Asian craton, and is largely composed of the Ediacaran - Middle Paleozoic carbonate and the Upper Paleozoic-Mesozoic terrigenous rocks. The Upper Carboniferous - Jurassic sediments constitute the Verkhoyansk terrigenous complex containing economically viable orogenic gold deposits. The structure of the belt is mainly controlled by thrusts and associated diagonal strike slips. Linear concentric folds are common all over the area of the belt. Shear zones with associated similar folds are confined to long narrow areas. Shear zones were formed during the early stages of the Oxfordian-Kimmeridgian collisional and accretionary events prior to the emplacement of large orogenic granitoid plutons. The main ore-controlling structures are shear zones associated with slaty cleavage, shear folds, mullion- and boudinage-structures, and transposition features. The shear zones are listric-type, and represent branches of a detachment structure, which is assumed to be present at the base of the Verkhoyansk fold-and-thrust belt. A vertical zonation of shear zones is correlated with the distance to the detachment. Changes in the dip angle of the shear zones (as indicated mainly by cleavage), structural paragenesis, the degree of microdeformation of the host rocks, and the type of ore-controlling structures can be clearly observed in the direction away from the detachment. Structural zoning is evidenced, among other things, by changing morphologic types of microstructures and by strain-indicators of the degree of rock metamorphism. Four morphologic types of microstructures are identified. The first platy-shear type is characterized by aggregate cleavage and the coefficient of deformation (Cd) of single grains from 1.0 to 2.0. Irregular angular fragments of variously oriented grains can be observed in thin sections. The second shear-cataclastic morphologic type (Cd from 2.0 to 3.0) exhibits

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.

Testing palaeotectonic models for the Internal Hellenides with sediment provenance

NASA Astrophysics Data System (ADS)

Meinhold, G.

2009-04-01

The Internal Hellenides of Greece are a result of the Alpine-Himalayan orogen. The relationships between different pre-Alpine crustal fragments of the Internal Hellenides are now masked by younger (Mesozoic to Cenozoic) complex structural and metamorphic events. This, together with the scarcity of biostratigraphic, geochronological and palaeomagnetic data, has given rise to equivocal palaeotectonic models and interpretations. However, the age and origin of pre-Alpine basement units in the Internal Hellenides has important implications for our in-depth understanding of the evolution of North Gondwana-derived terranes and consequently for alternative palaeotectonic reconstructions for the Palaeozoic and Mesozoic. A multidisciplinary sediment provenance study was undertaken since sedimentary rocks can provide information about rock lithologies in the source area, which have often been destroyed and recycled during ancient plate tectonic processes. Palaeozoic and Mesozoic sedimentary rocks from key areas of the Internal Hellenides were analysed using whole-rock major- and trace-element geochemistry (XRF, ICPMS), detrital chrome spinel, garnet, white mica and rutile chemistry (EMP), detrital zircon geochronology (SHRIMP, LA-ICPMS) and biostratigraphic analysis. These new data are used to constrain terrane accretion processes and the provenance of crustal sources for sediments during Palaeozoic and Mesozoic times and thus will test palaeotectonic models for the Internal Hellenides. This is expected to shed light on the Palaeo- and Neotethyan evolution in the Eastern Mediterranean.

Neotectonics and structure of the Himalayan deformation front in the Kashmir Himalaya, India: Implication in defining what controls a blind thrust front in an active fold-thrust belt

NASA Astrophysics Data System (ADS)

Gavillot, Y. G.; Meigs, A.; Yule, J. D.; Rittenour, T. M.; Malik, M. O. A.

2014-12-01

Active tectonics of a deformation front constrains the kinematic evolution and structural interaction between the fold-thrust belt and most-recently accreted foreland basin. In Kashmir, the Himalayan Frontal thrust (HFT) is blind, characterized by a broad fold, the Suruin-Mastargh anticline (SMA), and displays no emergent faults cutting either limb. A lack of knowledge of the rate of shortening and structural framework of the SMA hampers quantifying the earthquake potential for the deformation front. Our study utilized the geomorphic expression of dated deformed terraces on the Ujh River in Kashmir. Six terraces are recognized, and three yield OSL ages of 53 ka, 33 ka, and 0.4 ka. Vector fold restoration of long terrace profiles indicates a deformation pattern characterized by regional uplift across the anticlinal axis and back-limb, and by fold limb rotation on the forelimb. Differential uplift across the fold trace suggests localized deformation. Dip data and stratigraphic thicknesses suggest that a duplex structure is emplaced at depth along the basal décollement, folding the overlying roof thrust and Siwalik-Muree strata into a detachment-like fold. Localized faulting at the fold axis explains the asymmetrical fold geometry. Folding of the oldest dated terrace, suggest that rock uplift rates across the SMA range between 2.0-1.8 mm/yr. Assuming a 25° dipping ramp for the blind structure on the basis of dip data constraints, the shortening rate across the SMA ranges between 4.4-3.8 mm/yr since ~53 ka. Of that rate, ~1 mm/yr is likely absorbed by minor faulting in the near field of the fold axis. Given that Himalaya-India convergence is ~18.8-11 mm/yr, internal faults north of the deformation front, such as the Riasi thrust absorbs more of the Himalayan shortening than does the HFT in Kashmir. We attribute a non-emergent thrust at the deformation front to reflect deformation controlled by pre-existing basin architecture in Kashmir, in which the thick succession

Tectonic evolution and crustal-scale structure of Kyrgyz Central Asian Orogenic Belt: new insights from the Darius programme

NASA Astrophysics Data System (ADS)

Rolland, Yann; Loury, Chloé; Guillot, Stéphane; Mikolaichuk, Alexander

2014-05-01

Mechanisms and history of the Late Palaeozoic accretion followed by formation of trunscurrent strike-slip faults were studied in the southern segment of the Central Asian Orogenic Belt (CAOB) within Kyrgyz South Tianshan. 1. South Tianshan Suture: ending accretion process after docking of Tarim craton This study gives insights into the crustal-scale structure and Upper Paleozoic history of this mountain belt, currently intensely reactivated by the India-Asia collision. Structural, petrological and geochronological studies were carried out within South Tianshan suture east of the Talas-Ferghana Fault (TFF). New data highlight a south-dipping structure featured by a HP metamorphic core complex comprised of c. 320 Ma continental and oceanic eclogites exhumed by top-to-North motion. A large massif (10 x 50 km) of continental HP rocks in the Atbashi Range is comprised of hectometric boudins of eclogites embedded in metapelites and gneissesMetamorphic units exhibit blueschist to eclogite facies conditions, with oceanic (MORB) rocks in the blueschist facies representing the accretionary oceanic prism being thrusted by oceanic rocks and a continental unit in the eclogite facies (510 ± 50°C and 24 ± 2 kbar). Evidence for eclogite facies both in metasediments and mafic lithologies and geological structure are in agreement with a previously thinned continental margin. Subduction of this thinned COT (Continent-Ocean Transition) probably occurred by slab pull in a south-dipping subduction zone, while another north-dipping subduction was active below Middle Tianshan. Final stacking of Middle and South Tianshan occurred at 320-310 Ma. These opposite subduction zones are still reflected in the main structures of Tianshan. Reactivation of the South-dipping structures since 30-25 Ma is ascribed to explain the current Tianshan intra-continental subduction from seismology. 2. Talas-Ferghana Fault (TFF) activity & Basin formation After this accretionary episode, the South Tianshan

Structural analysis of the Elbow-Cranberry-Iskwasum lakes area: A multiply reactivated deformation corridor in the trans-Hudson orogen of Manitoba

NASA Astrophysics Data System (ADS)

Ryan, James Joseph

The Elbow-Cranberry-Iskwasum lakes area comprises a large portion of the eastern Amisk collage in the Palaeoproterozoic Flin Flon Belt (southern Trans-Hudson Orogen) of Manitoba, Canada. Deformation episodes recorded in the Flin Flon Belt are divided into pre-, early, late and post-Hudsonian orogeny, and are distinguished by the orientation of structures and changes in metamorphic conditions. Detailed structural analysis, petrography, geochemistry and U-Pb geochronology indicate a structural history spanning 180 m.y. in the Amisk collage. Accretion of the 1.92--1.88 Ga tectonostratigraphic assemblages that constitute the Amisk collage began prior to 1.868 Ga, the age of the oldest dyke to cross-cut the earliest mylonitic fabrics. The deformational history has been discerned, in which six generations of ductile structures F1 - F6 were followed by development of brittle-ductile and brittle structures F7 . Movements along the late structures may have continued until 1.690 Ga, during exhumation of the collage. The macroscopic structural grain in the central Flin Flon Belt is steeply dipping, generally trends north to north-northeast, and is dominated by two regionally pervasive foliations ( S2 and S5 ). Its grain contrasts strongly with the shallowly-dipping, east--west-trending grain in the adjacent Kisseynew domain. Foliations of different generations have been distinguished by their age relative to regional metamorphic mineral growth. Regional metamorphism in the Flin Flon Belt is interpreted as having culminated at moderate pressure and temperature, between 1.820 and 1.805 Ga. The development of S2 between 1.868 and 1.845 Ga was associated with east--west shortening of the successor magmatic arc that overprinted the Amisk collage. S3 and S4 were associated with shear zones, and are not regionally widespread. The S5 regional-scale Elbow Lake shear zone, and a pervasive crenulation cleavage in the wall rocks, developed during an episode of sinistral transpression

New Constraints for Tectono-Thermal Alpine Evolution of the Pyrenees: Combining Zircon Fission-Track and (U-Th)/He Analyses with Raman Spectrometry and In-Situ K-Ar Geochronology

NASA Astrophysics Data System (ADS)

Waldner, M.; Bellahsen, N.; Mouthereau, F.; Pik, R.; Bernet, M.; Scaillet, S.; Rosenberg, C.

2017-12-01

The pyrenean range was formed by the convergence of European and Iberian plates following the inversion of the Mesozoic rifting in the north of Pyrenees. In the Axial Zone, the collision caused an antiformal nappe-stacking of tectonic units. Recent studies pointed out the importance of pre-collision structural and thermal inheritance that may play a major role for orogeny such as: 1) Paleozoic Variscan inheritance; 2) Mesozoic rift-related high geothermal gradients, which are maintained during the onset of convergence in the North Pyrenean Zone. From a mineralogical point of view, pre-collision feldspars have been destabilized and influenced the development of alpine phyllonite in brittle-ductile conditions which suggests a weak crustal behavior during the formation of the orogenic wedge. Our aim is to get a better understanding of alpine deformation and exhumation by coupling different thermochronological, geochronological and thermometric methods. We document the thermal evolution of each tectonic unit by using low-temperature thermochronometers (Zircon Fission Tracks, U-Th/He on zircons including laser ablation profiles). Our data on vertical profiles combined to existing dataset on apatite allows to model alpine exhumation across the Axial zone. Structural observations through alpine thrusts coupled to geochronology (in situ K/Ar on phengites), Raman and chlorite-phengite thermo(baro)metry provide new key data to unravel the alpine evolution of the Pyrenees. According to preliminary ZFT results on granite massifs in the central part of Pyrenean Axial zone (near ECORS profile), exhumation ages potentially indicates a migration of exhumation towards the south. Exhumation ages of the northern massifs seems to have preserved the North Pyrenean Cretaceous rift evolution. Further south, the onset of exhumation is as old as Paleocene, which precedes the Eocene ages of the literature. The low burial estimated in the northern massifs may indicate a high thermal gradient

Role of the Kazerun fault system in active deformation of the Zagros fold-and-thrust belt (Iran)

NASA Astrophysics Data System (ADS)

Authemayou, Christine; Bellier, Olivier; Chardon, Dominique; Malekzade, Zaman; Abassi, Mohammad

2005-04-01

Field structural and SPOT image analyses document the kinematic framework enhancing transfer of strike-slip partitioned motion from along the backstop to the interior of the Zagros fold-and-thrust belt in a context of plate convergence slight obliquity. Transfer occurs by slip on the north-trending right-lateral Kazerun Fault System (KFS) that connects to the Main Recent Fault, a major northwest-trending dextral fault partitioning oblique convergence at the rear of the belt. The KFS formed by three fault zones ended by bent orogen-parallel thrusts allows slip from along the Main Recent Fault to become distributed by transfer to longitudinal thrusts and folds. To cite this article: C. Authemayou et al., C. R. Geoscience 337 (2005).

In situ U-Th-Pb ages of the Miaoya carbonatite complex in the South Qinling orogenic belt, central China

NASA Astrophysics Data System (ADS)

Ying, Yuancan; Chen, Wei; Lu, Jue; Jiang, Shao-Yong; Yang, Yueheng

2017-10-01

The Miaoya carbonatite complex in the South Qinling orogenic belt hosts one of the largest rare earth element (REE)-Nb deposits in China that is composed of carbonatite and syenite. The emplacement age of the complex and the geochronological relationship between the carbonatite and syenite have long been debated. In this study, in situ U-Th-Pb ages have been obtained for the constituent minerals zircon, monazite and columbite from carbonatite and syenite of the Miaoya complex, together with their chemical and isotopic compositions. In situ trace element compositions for zircon from carbonatite and syenite are highly variable. The zircon displays slightly heavy REE (HREE)-enriched chondrite-normalized patterns with no Eu anomaly and various light REE (LREE) contents. In situ Th-Pb dating for zircon from the Miaoya complex by laser ablation ICP-MS yields ages of 442.6 ± 4.0 Ma (n = 53) for syenite and 426.5 ± 8.0 Ma (n = 23) for carbonatite. Monazite from carbonatite and syenite shows similar chondrite-normalized REE patterns and yields a consistent Th-Pb age of 240 Ma. Based on petrographic and chemical composition, columbite from the carbonatite can be identified into two groups. The columbite dispersed within carbonatite is characterized by slightly LREE-enriched chondrite-normalized REE patterns, whereas columbite associated with apatite is characterized by LREE-depleted trends. Columbite has been further determined to have a weighted mean 206Pb/238U age of 232.8 ± 4.5 Ma (n = 9) using LA-ICP-MS. Detailed geochronological and chemical investigations suggest that there were two major episodes of magmatic/metasomatic activities in the formational history of the Miaoya carbonatite complex. The early alkaline magmatism emplaced in the Silurian was related to the opening of the Mianlue Ocean, whereas the late metasomatism or hydrothermal overprint occurred during the Triassic South Qinling orogeny. The latter serves as the major ore formation period for both REE (e

Air-Pollution-Mediated Changes in Alpine Ecosystems and Ecotones.

PubMed

Rusek, Josef

1993-08-01

Soil biological parameters (e.g., Collembola), soil types, soil chemical parameters (pH, humus substances), and plant communities were studied in different ecosystems and ecotones in alpine, subalpine, and spruce forest zones in the Tatra National Park, Slovak Republic. The preliminary, selected data, based on a long-term research program, showed a high sensitivity of some alpine ecotones and ecosystems to long-distance transported acid deposits. The changes in different ecosystem parameters since 1977 were more extensive in alpine grasslands on limestone than on granite. The greatest soil pH decrease was in the plant communities Festucetum versicoloris (-1.5 pH), Geranio-Alchemilletum crinitae (-1.32 pH), and Saxifragetum perdurantis (-1.25 pH), which are restricted to places with snow accumulation and water runoff gullies. In these ecosystems the greatest changes occurred in the leaching of humus substances. Some formerly less abundant and rare soil animals restricted to acid bedrock became dominant in some ecosystems on limestone as well as on granite; other formerly dominant species disappeared from the entire study area (e.g., Folsomia alpina). The aerial extent of some ecosystems changed substantially since 1977, and their surrounding ecotones moved into the space formerly occupied by one of the adjacent ecosystems. These changes are detectable by remote-sensing methods. In Central European mountains, strongly affected by global and regional industrial air pollution (e.g., Krusne Hory, Krkonose, Beskydy), spruce forests started to die back from higher to lower mountain elevations. The effects of air pollution on alpine and subalpine vegetation were not studied there. Strong alterations in alpine ecosystems and ecotones were detected by the author during long-term studies in the High Tatra Mountains, and I suggest that subalpine and mountain forest belts will be affected here in the near future as they were in the more polluted Central European mountains. The

Plants in alpine environments

USGS Publications Warehouse

Germino, Matthew J.

2014-01-01

Alpine and subalpine plant species are of special interest in ecology and ecophysiology because they represent life at the climate limit and changes in their relative abundances can be a bellwether for climate-change impacts. Perennial life forms dominate alpine plant communities, and their form and function reflect various avoidance, tolerance, or resistance strategies to interactions of cold temperature, radiation, wind, and desiccation stresses that prevail in the short growing seasons common (but not ubiquitous) in alpine areas. Plant microclimate is typically uncoupled from the harsh climate of the alpine, often leading to substantially warmer plant temperatures than air temperatures recorded by weather stations. Low atmospheric pressure is the most pervasive, fundamental, and unifying factor for alpine environments, but the resulting decrease in partial pressure of CO2 does not significantly limit carbon gain by alpine plants. Factors such as tree islands and topographic features create strong heterogeneous mosaics of microclimate and snow cover that are reflected in plant community composition. Factors affecting tree establishment and growth and formation of treeline are key to understanding alpine ecology. Carbohydrate and other carbon storage, rapid development in a short growing season, and physiological function at low temperature are prevailing attributes of alpine plants. A major contemporary research theme asks whether chilling at alpine-treeline affects the ability of trees to assimilate the growth resources and particularly carbon needed for growth or whether the growth itself is limited by the alpine environment. Alpine areas tend to be among the best conserved, globally, yet they are increasingly showing response to a range of anthropogenic impacts, such as atmospheric deposition.

Modelling the role of basement block rotation and strike-slip faulting on structural pattern in the cover units of fold-and-thrust belts

NASA Astrophysics Data System (ADS)

Koyi, Hemin; Nilfouroushan, Faramarz; Hessami, Khaled

2015-04-01

A series of scaled analogue models are run to study the degree of coupling between basement block kinematics and cover deformation. In these models, rigid basal blocks were rotated about vertical axis in a "bookshelf" fashion, which caused strike-slip faulting along the blocks and, to some degrees, in the overlying cover units of loose sand. Three different combinations of cover basement deformations are modeled; cover shortening prior to basement fault movement; basement fault movement prior to shortening of cover units; and simultaneous cover shortening with basement fault movement. Model results show that the effect of basement strike-slip faults depends on the timing of their reactivation during the orogenic process. Pre- and syn-orogen basement strike-slip faults have a significant impact on the structural pattern of the cover units, whereas post-orogenic basement strike-slip faults have less influence on the thickened hinterland of the overlying fold-and-thrust belt. The interaction of basement faulting and cover shortening results in formation of rhomb features. In models with pre- and syn-orogen basement strike-slip faults, rhomb-shaped cover blocks develop as a result of shortening of the overlying cover during basement strike-slip faulting. These rhombic blocks, which have resemblance to flower structures, differ in kinematics, genesis and structural extent. They are bounded by strike-slip faults on two opposite sides and thrusts on the other two sides. In the models, rhomb-shaped cover blocks develop as a result of shortening of the overlying cover during basement strke-slip faulting. Such rhomb features are recognized in the Alborz and Zagros fold-and-thrust belts where cover units are shortened simultaneously with strike-slip faulting in the basement. Model results are also compared with geodetic results obtained from combination of all available GPS velocities in the Zagros and Alborz FTBs. Geodetic results indicate domains of clockwise and

The Bossoroca Complex, São Gabriel Terrane, Dom Feliciano Belt, southernmost Brazil: Usbnd Pb geochronology and tectonic implications for the neoproterozoic São Gabriel Arc

NASA Astrophysics Data System (ADS)

Gubert, Mauricio Lemos; Philipp, Ruy Paulo; Stipp Basei, Miguel Angelo

2016-10-01

Usbnd Pb LA-ICPMS geochronological analyses were carried out on zircon grains from metavolcanic rocks of the Bossoroca Complex and for one ash tuff of the Acampamento Velho Formation of the Camaquã Basin, in order to understand the evolution of the Neoproterozoic São Gabriel magmatic arc. A total of 42 analyses of igneous zircon grains were performed in three samples. The results yielded Usbnd Pb ages of 767.2 ± 2.9 Ma for the metavolcanic agglomerate (BOS-02); 765 ± 10 Ma for the metacrystal tuff (BOS-03) and 565.8 ± 4.8 Ma for the ash tuff (BOS-04). The Orogenic Cycle in Brazil is characterized by a set of orogenic belts consisting of petrotectonic associations juxtaposed by two collisional events that occurred at the end of the Neoproterozoic. In southern Brazil this orogeny formed the Dom Feliciano Belt, a unit composed of associations of rocks developed during two major orogenic events called São Gabriel (900-680 Ma) and Dom Feliciano (650-540 Ma). The main São Gabriel associations are tectonically juxtaposed as elongated strips according to the N20-30°E direction, bounded by ductile shear zones. The Bossoroca Complex comprises predominantly metavolcano-sedimentary rocks, characterized by medium-K calc-alkaline association generated in a cordillera-type magmatic arc. The volcanism occurred in sub-aerial environment, developing deposits generated by flow, resurgence and fall, sporadically interrupted by subaqueous epiclastic deposits, suggesting an arc related basin. The São Gabriel Terrane contains the petrotectonic units that represent the closure of the Charrua Ocean associated to the subduction period of the Brasiliano Orogenic Cycle in the Sul-rio-grandense Shield.

Syn-orogenic magmatism over 100 m.y. in high crustal levels of the central Grenville Province: Characteristics, age and tectonic significance

NASA Astrophysics Data System (ADS)

Groulier, Pierre-Arthur; Indares, Aphrodite; Dunning, Gregory; Moukhsil, Abdelali; Jenner, George

2018-07-01

The Escoumins Supracrustal Belt (ESB) represents higher levels of the infrastructure of a large hot orogen, exposed in a broadly dome and basin pattern. It consists of remnants of a Pinwarian-age (1.52-1.46 Ga) oceanic arc and arc-rift sequence, preserved in the low-P Belt of the central Grenville Province, and was intruded by diverse Grenvillian-age plutons. The plutonic rocks range from quartz monzodiorite to granite and have intrusion ages covering a time interval of 100 My, that represents the entire range of the Grenvillian orogeny. Moreover, the ages, field relations and geochemical signatures of the different intrusions can be matched with different documented stages of the orogeny. The oldest pluton, the magnesian, biotite-bearing Bon-Désir granite (1086 ± 2 Ma), has positive εNd (+0.6), TDM = 1.52 Ga, and is attributed to melting of a juvenile Pinwarian crust as a result of slab break-off, at the onset of continental collision. The ferroan and Ba-Sr enriched, biotite-, amphibole- and clinopyroxene-bearing Michaud plutonic suite (1063 ± 3 Ma) and biotite-rich felsic sill (1045 ± 3 Ma) have εNd (-0.01 - +0.8) and TDM = 1.45-1.48 Ga. Their geochemistry is consistent with fractionation of a mafic magma derived from melting of a Geon 14 subduction-modified subcontinental lithospheric mantle. This magmatism is consistent with convective thinning of subcontinental lithosphere, potentially linked to tectonic extrusion and orogenic collapse. This collapse ultimately led to the juxtaposition of the low-P Belt with the high-T mid-P Belt in the hinterland of the Grenville Province and to amphibolite-facies metamorphism in the former, producing metamorphic zircon overgrowths at 1037 ± 10 Ma. Finally, 988 ± 5 Ma to 983 ± 5 Ma syn-kinematic peraluminous two-mica garnetiferous leucogranite bodies and pegmatites with inherited 1055 ± 2 Ma metamorphic monazite were derived from melting of previously metamorphosed deeper levels of the low-P Belt. This is consistent

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.

Exploration of the diversity and associated health benefits of traditional pickles from the Himalayan and adjacent hilly regions of Indian subcontinent.

PubMed

Chakraborty, Rakhi; Roy, Swarnendu

2018-05-01

The Himalayas have provided shelter to the various communities for thousands of years and have remained the cradle for the origin of diverse cultures and traditions. The Himalayan belt is rich in biodiversity and have ushered mankind with numerous gifts for survival and existence. The art of pickling is believed to have developed independently among the different communities of this region. In this region, the main meal is supplemented with pickles or achar that not only adds flavour, but also enhances the value of the meal and often comes with inherent health benefits. The prime objective of this article was to enlist the diverse pickles that are being prepared and consumed by the different tribes and communities, and at the same time analyse the science behind pickle preparation and health benefits and concerns associated with pickles. We have enlisted about hundred monotypic pickles, that are prepared from single fruit or vegetable; but sometimes fungi such as Agaricus and ferns like Diplazium and Pteridium have also been used. Also, fish and meat pickles are common mostly in the Eastern Himalayan region. Traditional pickles constitute the medicinal values of the ingredients and other beneficial properties conferred by the associated microorganisms.

Syn-collisional felsic magmatism and continental crust growth: A case study from the North Qilian Orogenic Belt at the northern margin of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Chen, Shuo; Niu, Yaoling; Xue, Qiqi

2018-05-01

The abundant syn-collisional granitoids produced and preserved at the northern Tibetan Plateau margin provide a prime case for studying the felsic magmatism as well as continental crust growth in response to continental collision. Here we present the results from a systematic study of the syn-collisional granitoids and their mafic magmatic enclaves (MMEs) in the Laohushan (LHS) and Machangshan (MCS) plutons from the North Qilian Orogenic Belt (NQOB). Two types of MMEs from the LHS pluton exhibit identical crystallization age ( 430 Ma) and bulk-rock isotopic compositions to their host granitoids, indicating their genetic link. The phase equilibrium constraints and pressure estimates for amphiboles from the LHS pluton together with the whole rock data suggest that the two types of MMEs represent two evolution products of the same hydrous andesitic magmas. In combination with the data on NQOB syn-collisional granitoids elsewhere, we suggest that the syn-collisional granitoids in the NQOB are material evidence of melting of ocean crust and sediment. The remarkable compositional similarity between the LHS granitoids and the model bulk continental crust in terms of major elements, trace elements, and some key element ratios indicates that the syn-collisional magmatism in the NQOB contributes to net continental crust growth, and that the way of continental crust growth in the Phanerozoic through syn-collisional felsic magmatism (production and preservation) is a straightforward process without the need of petrologically and physically complex processes.

Linkages and feedbacks in orogenic systems: An introduction

USGS Publications Warehouse

Thigpen, J. Ryan; Law, Richard D.; Merschat, Arthur J.; Stowell, Harold

2017-01-01

Orogenic processes operate at scales ranging from the lithosphere to grain-scale, and are inexorably linked. For example, in many orogens, fault and shear zone architecture controls distribution of heat advection along faults and also acts as the primary mechanism for redistribution of heat-producing material. This sets up the thermal structure of the orogen, which in turn controls lithospheric rheology, the nature and distribution of deformation and strain localization, and ultimately, through localized mechanical strengthening and weakening, the fundamental shape of the developing orogenic wedge (Fig. 1). Strain localization establishes shear zone and fault geometry, and it is the motion on these structures, in conjunction with climate, that often focuses erosional and exhumational processes. This climatic focusing effect can even drive development of asymmetry at the scale of the entire wedge (Willett et al., 1993).

From erosion to earthquakes: A geomorphic model for intraplate seismicity in post-orogenic settings

NASA Astrophysics Data System (ADS)

Gallen, S. F.; Thigpen, J. R.

2017-12-01

Intraplate seismicity does not conform to plate tectonics theory and its driving mechanisms remain uncertain, yet it is recognized as a relevant seismic hazard to populated regions, such as eastern North America. A variety of models, mostly geodynamic or tectonic in origin, have been proposed to explain this enigma, but conclusive supporting evidence remains elusive. In order to identify high hazard areas and derive predictive models, it is imperative to identify the underlying processes responsible for intraplate seismicity. Here we conduct an interdisciplinary study of the Eastern Tennessee Seismic Zone (ETSZ), the second most seismically active region east of the Rocky Mountains in the North American continent, to clarify the potential mechanisms driving intraplate seismicity in post-orogenic and passive margin settings. Previous studies document that the Upper Tennessee drainage basin, which lies directly above the ETSZ, is in a transient state of adjustment to 150 m of base level fall that was provoked by river capture in the Late Miocene. Using quantitative geomorphology, we demonstrate that base level fall enhanced erosion rates in a 75 km wide 400 km long corridor of highly erodible rocks in the late Paleozoic (Alleghanian orogen) fold-thrust belt. The total volume of rock preferentially removed above the ETSZ since 9 Ma is 3,600 ± 150 km3. Stress modeling indicates spatially focused erosion has of reduced clamping stresses on ancient basement normal faults beneath the Appalachian fold-thrust belt on the order of 3.5 MPa, with a time-averaged unclamping rate of 0.4 Pa yr-1. Under the assumption that the crust is critically stressed, we argue that the preferential erosion of less competent rock units reduced clamping stresses on relict faults such to induce seismic activity in the ambient stress field. This model for surface process-induced intraplate seismicity is generally transferable to other continental settings where complex geology and landscape

Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China

NASA Astrophysics Data System (ADS)

Jian, Ping; Kröner, Alfred; Shi, Yuruo; Zhang, Wei; Liu, Yaran; Windley, Brian F.; Jahn, Bor-ming; Zhang, Liqao; Liu, Dunyi

2016-06-01

We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99 Ma intraplate basaltic rock suite hosted in a subduction-accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431-99 Ma; n = 31; peak at 256 Ma), meta-granitoid-derived (ca. 449-113 Ma; n = 24; peak at 251 Ma) and gabbro-derived (436-242 Ma; n = 15; peaks at 264 and 244 Ma) grains. Each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation. The Precambrian population (2605-741 Ma; n = 40) exhibits a prominent age peak at 2520 Ma (granulite-facies metamorphism) and four small peaks at ca. 1900, 1600, and 800 Ma. Our new data, together with literature zircon ages, significantly constrain models of three seismically-determined deep crustal layers beneath the fossil subduction zone-forearc along the active northern margin of the North China Craton, namely: (1) an upper arc crust of early to mid-Paleozoic age, intruded by a major Permian-Triassic composite granitoid-gabbroic pluton (8-20 km depth); (2) a middle crust, predominantly consisting of mid-Meso- to Neoproterozoic felsic and mafic gneisses; and (3) a lower crust composed predominantly of late Archean granulite-facies rocks. We conclude that the Paleozoic orogenic crust is limited to the upper crustal level, and the middle to lower crust has a North China Craton affinity. Furthermore, integrating our data with surface geological, petrological and geochronological constraints, we present a new conceptual model of orogenic uplift, lithospheric delamination and crustal underthrusting for this key ocean-continent convergent margin.

Overview of radiometric ages in three allochthonous belts of Northern Venezuela: Old ones, new ones, and their impact on regional geology

USGS Publications Warehouse

Sisson, V.B.; Ave Lallemant, H.G.; Ostos, M.; Blythe, A.E.; Snee, L.W.; Copeland, Peter; Wright, J.E.; Donelick, R.A.; Guth, L.R.

2005-01-01

The margin of northern Venezuela is a complex zone representing the orogenic events from basement formation to subsequent subduction and exhumation during transpressional collision. This boundary zone has six east-west-trending belts that each record a different segment of its development. This geologic complexity requires radiometric ages to unravel, and we herein provide 48 new ages including U-Pb (4), Rb-Sr (2), 40Ar/39Ar (24), zircon and apatite fission-track (17), and 14C (1) ages to constrain the evolution of three of these belts. These three belts are the Cordillera de la Costa, Caucagua-El Tinaco, and Serran??a del Interior belts. In the Cordillera de la Costa belt, U-Pb geochronologic data indicate portions of the basement igneous and metaigneous rocks formed in the Cambro-Ordovician (513-471 Ma). New 40Ar/39Ar data from Margarita Island indicate that some of the subduction complex was rapidly cooled and exhumed, whereas other portions indicate slower cooling. This contrasts with new 40Ar/39Ar data from the Puerto Cabello. ?? 2005 Geological Society of America.

Palæo- and Neoproterozoic granitoids and rhyolites from the West Congolian Belt (Gabon, Congo, Cabinda, north Angola): chemical composition and geotectonic implications

NASA Astrophysics Data System (ADS)

Vicat, J.-P.; Pouclet, A.

2000-11-01

Various Palæo- and Neoproterozoic granitoid bodies and related rhyolites are located in the West Congolian Belt. The Palæoproterozoic granitoids, dated around 2 Ga, exhibit an Archæan-type tonalite-trondhjemite-granodiorite suite chemical signature and are related to the Eburnean tectnno-magmatic event. In contrast, Neoproterozoic granitoids and rhyolites, dated around 1 Ga, have chemical geotectonic signatures that range from orogenic to intraplate (Nb-negative anomaly, large ion lithophile element enrichment and high field strength element depletion). However, a late Kibaran orogenic event (1.35-1.00 Ga) is unknown in this area. The Neoproterozoic magmatism is interpreted as a consequence of the initiation of pre-Pan-African rifting, which implies the formation or the reactivation of major crustal strike-slip faults with asthenospheric upwelling and the generation of a thermal anomaly. This thermal anomaly could have been responsible for magmatic processes involving the lower crust, as encountered in post-orogenic environments.

What do the Variscan structures in the Central Pyrenees tell us about the Mesozoic Iberian margin and the Pyrenean orogenic prism?

NASA Astrophysics Data System (ADS)

Lemirre, Baptiste; Cochelin, Bryan; de Saint Blanquat, Michel; Denele, Yoann; Lahfid, Abdeltif; Duchene, Stephanie

2017-04-01

The formation of the Pyrenean mountain belt since late Variscan times is responsible of the exhumation of the basement in the central part of the belt. This basement is mainly made of Proterozoic to Paleozoic rocks involved in the Variscan orogeny. Following the publication of the ECORS deep seismic profile of the Central Pyrenees in 1989, it has been proposed that the Pyrenees are an asymmetrical double verging belt implying crustal nappe stacking resulting from the inversion of the Iberian margin. Such alpine deformation implies important Meso-Cenozoic bloc rotations and internal deformation, overprinting the earlier Variscan deformations that would define the basement. In order to constrain how the crust was affected by both Variscan and Alpine orogenies, we present a structural and petrological study along the trace of the ECORS profile in the axial zone. The section is composed of Precambrian to Carboniferous low-grade metasedimentary rocks intruded by large late-Variscan calc-alkaline plutons. We highlight a transpressional event which can be divided into three progressive stages: (1) a N-S folding, producing regional-scale open to southward verging anticlines and synclines, prior to the metamorphic peak; (2) a strong N-S horizontal shortening synchronous to the maximum temperature recorded which increases from 500 °C in the north, to 350 °C in the south (Raman Spectroscopy of Carbonaceous Materials geothermometry combined with a petrological study). This deformation induces vertical stretching, isoclinal folding and formation of a steep pervasive cleavage defined by biotite and chlorite; (3) a strain localization into retrogressive reverse mylonitic shear zones, responsible for limited vertical offset of the sedimentary pile and a maximum offset of the isotherms of 50 °C. The presence of undeformed and unconformable Permian deposits at the top of the pile underlines the Variscan age of, at least, the two first stages of pervasive deformation. The

Comparative analysis of geodynamic activity of the Caucasian and Eastern Mediterranean segments of the Alpine-Himalayan convergence zone

NASA Astrophysics Data System (ADS)

Chelidze, Tamaz; Eppelbaum, Lev

2013-04-01

The Alpine-Himalayan convergence zone (AHCZ) underwent recent transverse shortening under the effect of collisional compression. The process was accompanied by rotation of separate microplates. The Caucasian and Eastern Mediterranean regions are segments of the of the AHCZ and are characterized by intensive endogenous and exogenous geodynamic processes, which manifest themselves in occurrence of powerful (with magnitude of 8-9) earthquakes accompanied by development of secondary catastrophic processes. Large landslides, rock falls, avalanches, mud flows, etc. cause human deaths and great material losses. The development of the aforesaid endogenous processes is set forth by peculiarities of the deep structure of the region and an impact of deep geological processes. The Caucasus is divided into several main tectonic terranes: platform (sub-platform, quasi-platform) and fold-thrust units. Existing data enable to perform a division of the Caucasian region into two large-scale geological provinces: southern Tethyan and northern Tethyan located to the south of and to the north of the Lesser Caucasian ophiolite suture, respectively. The recent investigations show that the assessments of the seismic hazard in these regions are not quite correct - for example in the West Caucasus the seismic hazard can be significantly underestimated, which affects the corresponding risk assessments. Integrated analysis of gravity, magnetic, seismic and thermal data enables to refine the assessment of the seismic hazard of the region, taking into account real rates of the geodynamic movements. Important role play the last rheological constructions. According to Reilinger et al. (2006) tectonic scheme, the West flanking of the Arabian Plate manifests strike-slip motion, when the East Caucasian block is converging and shortening. The Eastern Mediterranean is a tectonically complex region located in the midst of the progressive Afro-Eurasian collision. The recent increasing geotectonic

Gold deposits in metamorphic belts: Overview of current understanding, outstanding problems, future research, and exploration significance

USGS Publications Warehouse

Groves, D.I.; Goldfarb, R.J.; Robert, F.; Hart, C.J.R.

2003-01-01

Metamorphic belts are complex regions where accretion or collision has added to, or thickened, continental crust. Gold-rich deposits can be formed at all stages of orogen evolution, so that evolving metamorphic belts contain diverse gold deposit types that may be juxtaposed or overprint each other. This partly explains the high level of controversy on the origin of some deposit types, particularly those formed or overprinted/remobilized during the major compressional orogeny that shaped the final geometry of the hosting metamorphic belts. These include gold-dominated orogenic and intrusion-related deposits, but also particularly controversial gold deposits with atypical metal associations. There are a number of outstanding problems for all types of gold deposits in metamorphc belts. These include the following: (1) definitive classifications, (2) unequivocal recognition of fluid and metal sources, (3) understanding of fluid migration and focusing at all scales, (4) resolution of the precise role of granitoid magmatism, (5) precise gold-depositional mechanisms, particularly those producing high gold grades, and (6) understanding of the release of CO2-rich fluids from subducting slabs and subcreted oceanic crust and granitoid magmas at different crustal levels. Research needs to be better coordinated and more integrated, such that detailed fluid-inclusion, trace-element, and isotopic studies of both gold deposits and potential source rocks, using cutting-edge technology, are embedded in a firm geological framework at terrane to deposit scales. Ultimately, four-dimensional models need to be developed, involving high-quality, three-dimensional geological data combined with integrated chemical and fluid-flow modeling, to understand the total history of the hydrothermal systems involved. Such research, particularly that which can predict superior targets visible in data sets available to exploration companies before discovery, has obvious spin-offs for global- to deposit

Subduction and exhumation of a continental margin in the Scandinavian Caledonides: Insights from ultrahigh pressure metamorphism, late orogenic basins and 3D numerical modelling

NASA Astrophysics Data System (ADS)

Cuthbert, Simon

2017-04-01

The Scandinavian Caledonides (SC) represents a plate collision zone of Himalayan style and scale. Three fundamental characteristics of this orogen are: (1) early foreland-directed, tectonic transport and stacking of nappes; (2) late, wholesale reversal of tectonic transport; (3) ultrahigh pressure metamorphism of felsic crust derived from the underthrusting plate at several levels in the orogenic wedge and below the main thrust surface, indicating subduction of continental crust into the mantle. The significance of this for crustal evolution is the profound remodeling of continental crust, direct geochemical interaction of such crust and the mantle and the opening of accommodation space trapping large volumes of clastic detritus within the orogen. The orogenic wedge of the SC was derived from the upper crust of the Baltica continental margin (a hyper-extended passive margin), plus terranes derived from an assemblage of outboard arcs and intra-oceanic basins and, at the highest structural level, elements of the Laurentian margin. Nappe emplacement was driven by Scandian ( 430Ma) collision of Baltica with Laurentia, but emerging Middle Ordovician ages for diamond-facies metamorphism for the most outboard (or rifted) elements of Baltica suggest prior collision with an arc or microcontinent. Nappes derived from Baltica continental crust were subducted, in some cases to depths sufficient to form diamond. These then detached from the upper part of the down-going plate along major thrust faults, at which time they ceased to descend and possibly rose along the subduction channel. Subduction of the remaining continental margin continued below these nappes, possibly driven by slab-pull of the previously subducted Iapetus oceanic lithosphere and metamorphic densification of subducted felsic continental margin. 3D numerical modelling based upon a Caledonide-like plate scenario shows that if a continental corner or promontory enters the subduction zone, the continental margin

From the plutonic root to the volcanic roof of a continental magmatic arc: a review of the Neoproterozoic Araçuaí orogen, southeastern Brazil

NASA Astrophysics Data System (ADS)

Gonçalves, Leonardo; Alkmim, Fernando F.; Pedrosa-Soares, Antônio; Gonçalves, Cristiane C.; Vieira, Valter

2018-01-01

The Araçuaí-West Congo orogen (AWCO) is one of the various components of the Brasiliano/Pan-African orogenic network generated during the amalgamation of West Gondwana. In the reconstructions of Gondwana, the AWCO, encompassing the Araçuaí orogen of South America and the West Congo belt of Southwestern Africa, appears as a tongue-shaped orogenic zone embraced by the São Francisco-Congo craton. Differing from the vast majority of the known orogens owing to its singular confined setting, the AWCO contains a large amount of orogenic igneous rocks emplaced in all stages of its tectonic evolution. We present new and revised information about the oldest Ediacaran granitic assemblage, the G1 Supersuite, which together with the Rio Doce Group defines the Rio Doce magmatic arc, and then we propose a new tectonic setting for the arc. Field relationships and mineralogical compositions of the G1 Supersuite allow us to characterize three lithofacies associations, Opx-bearing rocks, enclave-rich Tonalite-Granodiorite and enclave-poor Granite-Tonalite, suggesting different crustal levels are exposed in the central part of the Araçuaí orogen. The region is interpreted to represent a tilted crustal section, with deep arc roots now exposed along its western border. Chemically, these plutonic associations consist mostly of magnesian, metaluminous to slightly peraluminous, calc-alkaline to alkali-calcic and medium- to high-K acidic rocks. The dacitic and rhyolitic rocks of the Rio Doce Group are mainly magnesian, peraluminous, calcic to calc-alkaline, and medium- to high-K acidic rocks. Zircon U-Pb data constrain the crystallization of the granitoids between ca. 625 and 574 Ma, while the age of the metamorphosed volcanic rocks is around ca. 585 Ma. Thus, within errors, these rock associations likely belong to the same magmatic event and might represent the subduction-related, pre-collisional, evolution of the Araçuaí orogen. In addition, whole-rock Sm-Nd isotopic compositions

The Origin of the Chinese Central Tianshan Block in the Southern Central Asian Orogenic Belt: Evidence from Detrital Zircon Study

NASA Astrophysics Data System (ADS)

Huang, Z.; Long, X.; Yuan, C.

2016-12-01

The Chinese Central Tianshan Block (CTB) is one of the oldest continental fragments in the southern Central Asian Orogenic Belt (CAOB). Although it is vital for understanding the evolution of the CAOB, its origin has been poorly studied. The CTB was previously suggested to have been originated from the North China, the South China, the Tarim cratons or the East European Craton (Baltica). A total of 165 concordant U-Pb and Hf isotopic analyses of detrital zircon are obtained from three meta-sediments in the CTB, including one meta-sandstone from Xingxingxia formation and one meta-sandstone as well as one quartzite from Kawabulake formation. Detrital zircon grains from the Xingxingxia and Kawabulake formations are dominated by respective youngest age populations at 1002 Ma and 930-960 Ma, providing constraints on the maximum depositional ages for these two formations. Zircon grains from the meta-sediments have very similar age distributions, with two dominant peaks at 0.93-1.0 Ga and 1.0-1.6 Ga and a minor peak at 2.3-2.7 Ga. They have similar Hf isotopic signatures, suggesting that the meta-sediments in the CTB share similar sedimentary provenance. The early Neoproterozoic detrital zircon grains are mainly local-derived, whereas the Paleo-Mesoproterozoic grains are both autochthonous and allochthonous. The occurrence of these Mesoproterozoic and Neoproterozoic zircon grains are coincident with the Nuna breakup and the Rodinia assembly. This suggests that the CTB might experience the tectonic switching of the Nuna to the Rodinia. The distinct Meso-Neoproterozoic age patterns and Hf isotopic compositions of these detrital grains from the CTB and the surrounding blocks indicate that the CTB was not located close to the North China, the South China or the Tarim cratons in Precambrian. Our new data suggest that the CTB was most likely once a part of the East European Craton before the Neoproterozoic. This study was supported by National Basic Research Program of China

Influence of geology on groundwater-sediment interactions in arsenic enriched tectono-morphic aquifers of the Himalayan Brahmaputra river basin

NASA Astrophysics Data System (ADS)

Verma, Swati; Mukherjee, Abhijit; Mahanta, Chandan; Choudhury, Runti; Mitra, Kaushik

2016-09-01

The present study interprets the groundwater solute chemistry, hydrogeochemical evolution, arsenic (As) enrichment and aquifer characterization in Brahmaputra River Basin (BRB) involving three geologically and tectono-morphically distinct regions located in northeastern India. These study regions consist of the northwestern (NW) and the northern (N) region, both located along the western and eastern parts of Eastern Himalayas and the southern (S) region (near Indo-Burmese Range and Naga hills) of the Brahmaputra basin which show distinct tectonic settings and sediment provenances in the Himalayan orogenic belt. Stable isotopic composition (δ2H and δ18O) in groundwater suggests that some evaporation may have taken place through recharging of ground water in the study areas. The major-ion composition shows that groundwater composition of the NW and N parts are between Casbnd HCO3 and Casbnd Nasbnd HCO3 while the S-region is dominated by Nasbnd Casbnd HCO3 hydrochemical facies. The major mineralogical composition of aquifer sediments indicates the dominant presence of iron(Fe)-oxide and oxyhydroxides, mica (muscovite and biotite), feldspar, pyroxene, amphibole, abundance of quartz and clay minerals whereas clay is predominantly present in sediments of S-aquifers. These mafic minerals, aluminosilicates and clay minerals might offer available reactive surface for As-adsorption and co-precipitatation with amorphous Fe. These associated adsorbed and co-precipitated As might be released due to reductive dissolution of Fe-oxide and oxyhydroxides in groundwater. These minerals are assumed to be possible sources of As in groundwater. The stability diagrams of groundwater data suggest that solute might have been introduced into groundwater from weathering of K-feldspar, plagioclase, pyroxene of Himalayan rocks, the Siwalik Group and Eastern Syntaxes in NW and N-regions. However, basic cations might be derived from weathering of K-feldspar, plagioclase, pyroxene and olivine

Hydrocarbon concentrations at the Alpine mountain sites Jungfraujoch and Arosa

NASA Astrophysics Data System (ADS)

Li, Yingshi; Campana, Mike; Reimann, Stefan; Schaub, Daniel; Stemmler, Konrad; Staehelin, Johannes; Peter, Thomas

Volatile hydrocarbons have been measured for 1 yr at Arosa (2010 m asl) to determine the contribution of European emissions to the trace gas concentrations at this remote site. Results are compared to concurrent hydrocarbon concentrations at the high Alpine background site Jungfraujoch (3580 m asl). Hydrocarbon concentrations at Arosa are generally much higher than at Jungfraujoch. The influence of the Alpine boundary layer air was studied based on the diurnal variation of hydrocarbon concentrations, i.e. rising pollutant concentrations in the morning at Arosa and in the afternoon at Jungfraujoch. Different hydrocarbon emission sources of the uplifting air were found at the two sites. At Jungfraujoch, several transatlantic events were detected from October 2001 to January 2002 based on analysis of hydrocarbon ratios and air parcel trajectories. The OH concentration during the transatlantic transport was estimated to be around 5×10 5 cm -3, derived from simultaneous hydrocarbon oxidation and dilution in the free troposphere. These transatlantic transport events were tracked back to warm conveyor belts, characterized by uniform dynamics and relatively uniform surface sources. In addition, ozone production in the free tropospheric transport was also documented in these events.

Rock glaciers in crystalline catchments: Hidden permafrost-related threats to alpine headwater lakes.

PubMed

Ilyashuk, Boris P; Ilyashuk, Elena A; Psenner, Roland; Tessadri, Richard; Koinig, Karin A

2018-04-01

A global warming-induced transition from glacial to periglacial processes has been identified in mountainous regions around the world. Degrading permafrost in pristine periglacial environments can produce acid rock drainage (ARD) and cause severe ecological damage in areas underlain by sulfide-bearing bedrock. Limnological and paleolimnological approaches were used to assess and compare ARDs generated by rock glaciers, a typical landform of the mountain permafrost domain, and their effects on alpine headwater lakes with similar morphometric features and underlying bedrock geology, but characterized by different intensities of frost action in their catchments during the year. We argue that ARD and its effects on lakes are more severe in the alpine periglacial belt with mean annual air temperatures (MAAT) between -2°C and +3°C, where groundwater persists in the liquid phase for most of the year, in contrast to ARD in the periglacial belt where frost action dominates (MAAT < -2°C). The findings clearly suggest that the ambient air temperature is an important factor affecting the ARD production in alpine periglacial environments. Applying the paleoecological analysis of morphological abnormalities in chironomids through the past millennium, we tested and rejected the hypothesis that unfavorable conditions for aquatic life in the ARD-stressed lakes are largely related to the temperature increase over recent decades, responsible for the enhanced release of ARD contaminants. Our results indicate that the ARDs generated in the catchments are of a long-lasting nature and the frequency of chironomid morphological deformities was significantly higher during the Little Ice Age (LIA) than during pre- or post-LIA periods, suggesting that lower water temperatures may increase the adverse impacts of ARD on aquatic invertebrates. This highlights that temperature-mediated modulations of the metabolism and life cycle of aquatic organisms should be considered when reconstructing

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

Himalayan Foothills, Bangladesh

NASA Technical Reports Server (NTRS)

1992-01-01

This remarkably clear, pre-monsoon view of the Himalayan foothills of Bangladesh (26.0N, 89.5E) shows the deforestation of the lower slopes for agriculture and pasture lands. The cleared lower slopes are generally used for tea cultivation. The intensity of agricultural land use, mostly in the form of small, family subsistance farms on the Ganges Plain is evident over most of the scene. Note also, the aircraft contrail and Tista River.

The impact of Pleistocene climate change on an ancient arctic–alpine plant: multiple lineages of disparate history in Oxyria digyna

PubMed Central

Allen, Geraldine A; Marr, Kendrick L; McCormick, Laurie J; Hebda, Richard J

2012-01-01

The ranges of arctic–alpine species have shifted extensively with Pleistocene climate changes and glaciations. Using sequence data from the trnH-psbA and trnT-trnL chloroplast DNA spacer regions, we investigated the phylogeography of the widespread, ancient (>3 million years) arctic–alpine plant Oxyria digyna (Polygonaceae). We identified 45 haplotypes and six highly divergent major lineages; estimated ages of these lineages (time to most recent common ancestor, TMRCA) ranged from ∼0.5 to 2.5 million years. One lineage is widespread in the arctic, a second is restricted to the southern Rocky Mountains of the western United States, and a third was found only in the Himalayan and Altai regions of Asia. Three other lineages are widespread in western North America, where they overlap extensively. The high genetic diversity and the presence of divergent major cpDNA lineages within Oxyria digyna reflect its age and suggest that it was widespread during much of its history. The distributions of individual lineages indicate repeated spread of Oxyria digyna through North America over multiple glacial cycles. During the Last Glacial Maximum it persisted in multiple refugia in western North America, including Beringia, south of the continental ice, and within the northern limits of the Cordilleran ice sheet. Our data contribute to a growing body of evidence that arctic–alpine species have migrated from different source regions over multiple glacial cycles and that cryptic refugia contributed to persistence through the Last Glacial Maximum. PMID:22822441

Muscovite-Dehydration Melting: A Textural Study of a Key Reaction in Transforming Continental Margin Strata Into a Migmatitic Orogenic Core

NASA Astrophysics Data System (ADS)

Dyck, B. J.; St Onge, M. R.; Waters, D. J.; Searle, M. P.

2015-12-01

Metamorphosed continental margin sedimentary sequences, which comprise the dominant tectonostratigraphic assemblage exposed in orogenic hinterlands, are crucial to understanding the architecture and evolution of collisional mountain belts. This study explores the textural effect of anatexis in amphibolite-grade conditions and documents the mineral growth mechanisms that control nucleation and growth of K-feldspar, sillimanite and silicate melt. The constrained textural evolution follows four stages: 1) Nucleation - K-feldspar is documented to nucleate epitaxially on isomorphic plagioclase in quartzofeldspathic (psammitic) domains, whereas sillimanite nucleates in the Al-rich (pelitic) domain, initially on [001] mica planes. The first melt forms at the site of muscovite breakdown. 2) Chemically driven growth - In the quartzofeldspathic domain, K-feldspar progressively replaces plagioclase by a K+ - Na+ cation transfer reaction, driven by the freeing of muscovite-bound K+ during breakdown of the mica. Sillimanite forms intergrowths with the remaining hydrous melt components, contained initially in ovoid clots. 3) Merge and coarsening - With an increase in pressure, melt and sillimanite migrate away from clots along grain boundaries. A melt threshold is reached once the grain-boundary network is wetted by melt, increasing the length-scale of diffusion, resulting in grain boundary migration and grain-size coarsening. The melt threshold denotes the transition to an open-system on the lithology scale, where melt is a transient phase. 4) Residual melt crystallization - Residual melt crystallizes preferentially on existing peritectic grains as anatectic quartz, plagioclase, and K-feldspar. As the system cools and closes, grain growth forces melt into the intersections of grain-boundaries, recognized as irregular shaped melt films, or as intergrowths of the volatile-rich phases (i.e. Tur-Ms-Ap). In the Himalayan metamorphic core these processes result in the formation of

Geochronological and sedimentological evidences of Panyangshan foreland basin for tectonic control on the Late Paleozoic plate marginal orogenic belt along the northern margin of the North China Craton

NASA Astrophysics Data System (ADS)

Li, Jialiang; Zhou, Zhiguang; He, Yingfu; Wang, Guosheng; Wu, Chen; Liu, Changfeng; Yao, Guang; Xu, Wentao; Zhao, Xiaoqi; Dai, Pengfei

2017-08-01

Palaeo-uplift also was developed in the Early Permian to Middle Triassic (277-236 Ma), related to the final closure of the Paleo-Asian Ocean. Furthermore, we advocate that the tectonic setting of Inner Mongolia Palaeo-uplift probably belonged to the plate marginal orogenic belt during Early Permian-Middle Triassic.

Geochronological and sedimentological evidences of Panyangshan foreland basin for tectonic control on the Late Paleozoic plate marginal orogenic belt along the northern margin of the North China Craton

NASA Astrophysics Data System (ADS)

Li, Jialiang; Zhou, Zhiguang; He, Yingfu; Wang, Guosheng; Wu, Chen; Liu, Changfeng; Yao, Guang; Xu, Wentao; Zhao, Xiaoqi; Dai, Pengfei

2018-06-01

Palaeo-uplift also was developed in the Early Permian to Middle Triassic (277-236 Ma), related to the final closure of the Paleo-Asian Ocean. Furthermore, we advocate that the tectonic setting of Inner Mongolia Palaeo-uplift probably belonged to the plate marginal orogenic belt during Early Permian-Middle Triassic.

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

Continental collision with a sandwiched accreted terrane: Insights into Himalayan-Tibetan lithospheric mantle tectonics?

NASA Astrophysics Data System (ADS)

Kelly, Sean; Butler, Jared P.; Beaumont, Christopher

2016-12-01

Many collisional orogens contain exotic terranes that were accreted to either the subducting or overriding plate prior to terminal continent-continent collision. The ways in which the physical properties of these terranes influence collision remain poorly understood. We use 2D thermomechanical finite element models to examine the effects of prior 'soft' terrane accretion to a continental upper plate (retro-lithosphere) on the ensuing continent-continent collision. The experiments explore how the style of collision changes in response to variations in the density and viscosity of the accreted terrane lithospheric mantle, as well as the density of the pro-lithospheric mantle, which determines its propensity to subduct or compress the accreted terrane and retro-lithosphere. The models evolve self-consistently through several emergent phases: breakoff of subducted oceanic lithosphere; pro-continent subduction; shortening of the retro-lithosphere accreted terrane, sometimes accompanied by lithospheric delamination; and, terminal underthrusting of pro-lithospheric mantle beneath the accreted terrane crust or mantle. The modeled variations in the properties of the accreted terrane lithospheric mantle can be interpreted to reflect metasomatism during earlier oceanic subduction beneath the terrane. Strongly metasomatized (i.e., dense and weak) mantle is easily removed by delamination or entrainment by the subducting pro-lithosphere, and facilitates later flat-slab underthrusting. The models are a prototype representation of the Himalayan-Tibetan orogeny in which there is only one accreted terrane, representing the Lhasa terrane, but they nonetheless exhibit processes like those inferred for the more complex Himalayan-Tibetan system. Present-day underthrusting of the Tibetan Plateau crust by Indian mantle lithosphere requires that the Lhasa terrane lithospheric mantle has been removed. Some of the model results support previous conceptual interpretations that Tibetan

What Can Modern River Profiles Tell Us about Orogenic Processes and Orogen Evolution?

NASA Astrophysics Data System (ADS)

Whipple, K. X.

2008-12-01

Numerous lines of evidence from theory, numerical simulations, and physical experiments suggest that orogen evolution is strongly coupled to atmospheric processes through the interrelationships among climate, topography, and erosion rate. In terms of orogenic processes and orogen evolution, these relationships are most important at the regional scale (mean topographic gradient, mean relief above surrounding plains) largely because crustal deformation is most sensitive to erosional unloading averaged over sufficiently long wavelengths. For this reason, and because above moderate erosion rates (> 0.2 mm/yr) hillslope form becomes decoupled from erosion rate, attention has focused on the river network, and even on particularly large rivers. We now have data that demonstrates a monotonic relationship between erosion rate and the channel steepness index (slope normalized for differences in drainage area) in a variety of field settings. Consequently, study of modern river profiles can yield useful information on recent and on-going patterns of rock uplift. It is not yet possible, however, to quantitatively isolate expected climatic and lithologic influences on this relationship. A combination of field studies and theoretical analyses are beginning to reveal the timescale of landscape response, and thus the topographic memory of past conditions. At orogen scale, river profile response to a change in rock uplift rate is on the order of 1-10 Myr. Because of these long response times, the modern profiles of large rivers and their major tributaries can potentially preserve an interpretable record of rock uplift rates since the Miocene and are insensitive to short-term climatic fluctuations. Only significant increases in rock uplift rate, however, are likely to leave a clear topographic signature. Strategies have been developed to differentiate between temporal and spatial (tectonic, climatic, or lithologic) influences on channel profile form, especially where spatially

Origin of the Uinta recess, Sevier fold thrust belt, Utah: influence of basin architecture on fold thrust belt geometry

NASA Astrophysics Data System (ADS)

Paulsen, Timothy; Marshak, Stephen

1999-11-01

Structural trends in the Sevier fold-thrust belt define a pronounced concave-to-the-foreland map-view curve, the Uinta recess, in north-central Utah. This recess separates two convex-to-the-foreland curves, the Provo salient on the south and the Wyoming salient on the north. The two limbs of the recess comprise transverse zones (fault zones at a high-angle to the regional trend of the orogen) that border the flanks of the east-west-trending Uinta/Cottonwood arch. Our structural analysis indicates that the transverse zones formed during the Sevier orogeny, and that they differ markedly from each other in structural style. The Charleston transverse zone (CTZ), on the south side of the arch, initiated as a complex sinistral strike-slip fault system that defines the abrupt northern boundary of the Provo salient. The Mount Raymond transverse zone (MRTZ), on the north side of the arch, represents the region in which the southeast-verging southern limb of the gently curving Wyoming salient was tilted northwards during the Laramide phase of uplift of the Uinta/Cottonwood arch. In effect, the MRTZ represents an oblique cross section through a thrust belt. The contrasting architecture of these transverse zones demonstrates how pre-deformation basin geometry influences the geometry of a fold-thrust belt. Analysis of isopach maps indicates that, at the time the Sevier fold-thrust belt formed, the area just north of the present site of the Uinta/Cottonwood arch was a basement high, with a gently dipping north flank, and a steeply dipping south flank. Thus, predeformational sediment thickened abruptly to the south of the high and thickened gradually to the north of the high. As illustrated by sandbox models, the distance that a fold-thrust belt propagates into the foreland depends on the thickness of the sedimentary layer being deformed, so the shape of the salient mimics the longitudinal cross-sectional shape of the sedimentary basin. Where basins taper gradually along strike

Seismological Structure of the 1.8Ga Trans-Hudson Orogen of North America and its affinity to present-day Tibet

NASA Astrophysics Data System (ADS)

Gilligan, A.; Bastow, I. D.; Darbyshire, F. A.

2015-12-01

How tectonic processes operated and changed through the Precambrian is debated: what was the nature and scale of orogenic events and were they different on the younger, hotter, more ductile Earth? The geology of northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates: the 1.8Ga Trans-Hudson Orogeny (THO) and is thus an ideal study locale to address this issue. It has been suggested, primarily on the strength of traditional field geology, that the THO was comparable in scale and style to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, understanding of the deep crustal architecture of the THO, and how it compares to the evolving HKTO is presently lacking. Through joint inversion of teleseismic receiver functions and surface wave data, we obtain new Moho depth estimates and shear velocity models for the crust and upper mantle. Archean crust in the Rae, Hearne and Churchill domains is thin and structurally simple, with a sharp Moho; upper crustal wavespeed variations are readily attributed to post-formation events. However, the Paleoproterozoic Quebec-Baffin segment of the THO has a deeper Moho and more complex crustal structure. Our observations are strikingly similar to recent models, computed using the same methods, of the HKTO lithosphere, where deformation also extends >400km beyond the collision front. On the strength of Moho character, present-day crustal thickness, and metamorphic grade, we thus propose that southern Baffin experienced uplift of a similar magnitude and spatial extent to the Himalayas during the Paleoproterozoic Trans-Hudson Orogeny.

Uncoupled vs. coupled thrust belt-foreland deformation: a model for northern Patagonia inferred from U-Th/He and apatite fission track dating

NASA Astrophysics Data System (ADS)

Savignano, Elisa; Mazzoli, Stefano; Zattin, Massimiliano; Gautheron, Cécile; Franchini, Marta

2017-04-01

The study of the Cretaceous - Cenozoic evolution of the Patagonian Andes represents a great opportunity to investigate the effects of coupling between deep lithospheric processes and near-surface deformation. Low-temperature thermochronological systems are ideally suited for detecting events involving rocks in the uppermost part of the crust because they record time and rates of cooling related to exhumation of the top few kilometers of the crust. The Patagonia region, although characterized by a general continuity of the Andean orogen along its strike, shows an appreciable internal tectonic segmentation (marked by a variable position of the magmatic arc and of the deformation front in the retroarc area) at various latitudes. This complex structural architecture has been interpreted as the result of different processes acting since the Late Cretaceous. The present-day configuration of the southern Andes is interpreted to have been controlled by alternating stages of flat- and steep-slab subduction, which produced shortening and upper plate extension episodes,, respectively. Furthermore, the deformation in this whole retroarc sector varied not only in time (i.e. with major 'cycles' of mountain building and orogenic collapse), but also in space, due to the variable transmission of horizontal compressive stress away from the orogen, that produced an irregular unroofing pattern. In this study, we have integrated field structural observations with new apatite (U-Th)/He data (AHe) and apatite fission-track (AFT) ages in the north Patagonia region (at latitudes between 40° and 44°S) in order to analyse and compare the exhumation patterns from the frontal part of the orogen and from the adjacent foreland sector, as well as to gain new insights into the timing and modes of coupling vs. uncoupling of the deformation between the northern Patagonian fold and thrust belt and its foreland. The obtained data indicate a markedly different unroofing pattern between the 'broken

Himalayan uplift shaped biomes in Miocene temperate Asia: evidence from leguminous Caragana.

PubMed

Zhang, Ming-Li; Xiang, Xiao-Guo; Xue, Juan-Juan; Sanderson, Stewart C; Fritsch, Peter W

2016-11-09

Caragana, with distinctive variation in leaf and rachis characters, exhibits three centers of geographic distribution, i.e., Central Asia, the Qinghai-Tibetan Plateau (QTP), and East Asia, corresponding to distinct biomes. Because Caragana species are often ecologically dominant components of the vegetation in these regions, it is regarded as a key taxon for the study of floristic evolution in the dry regions of temperate Asia. Based on an expanded data set of taxa and gene regions from those previously generated, we employed molecular clock and biogeographical analyses to infer the evolutionary history of Caragana and link it to floristic patterns, paleovegetation, and paleoclimate. Results indicate that Caragana is of arid origin from the Junggar steppe. Diversification of crown group Caragana, dated to the early Miocene ca. 18 Ma and onwards, can be linked to the Himalayan Motion stage of QTP uplift. Diversification of the major clades in the genus corresponding to taxonomic sections and morphological variation is inferred to have been driven by the uplift, as well as Asian interior aridification and East Asian monsoon formation, in the middle to late Miocene ca. 12~6 Ma. These findings demonstrate a synchronous evolution among floristics, vegetation and climate change in arid Central Asia, cold arid alpine QTP, and mesophytic East Asia.

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

NASA Astrophysics Data System (ADS)

Heberer, Bianca; Neubauer, Franz

2010-05-01

). Bedrock ages from the vicinity of the Valsugana thrust indicate an important exhumational event at about 10 Ma (Zattin et al., 2006). The existing data already hint at decreasing rates of thermal overprint towards the foreland. Basement uplifts partly display AFT ages contemporaneous to subsidence in intra-orogenic basins. Consequently, existing AFT data and their relationships to intervening Neogene basins suggest a Neogene large-wavelength crustal-scale fold structure between the Klagenfurt basin and the Adriatic Sea. The main stage of subsidence in the Venetian-Adriatic foreland is younger and of Late Pliocene-Pleistocene age reflecting the final, still ongoing stage of shortening (Barbieri et al., 2007). In order to further test these observations, we aim at collecting more structural and low-T thermochronological data from the region. First results from the recently started project "AlDi-Adria" will be presented. References Barbieri, C. et al. 2007: Natural subsidence of the Venice area during the last 60 Myr. Basin Res., 19, 105-123. Fodor, L. et al. 2008: Miocene emplacement and rapid cooling of the Pohorje pluton at the Alpine-Pannonian-Dinaric junction: a geochronological and structural study. Swiss J. Geosci., 101 Suppl. 1, S255-S271. Hejl, E. 1997: 'Cold spots' during the Cenozoic evolution of the Eastern Alps: thermochronological interpretation of apatite fission-track data. Tectonophysics, 272, 159-172. Luth S. W. & Willingshofer, E. 2008: Mapping of the Post-Collisional Cooling History of the Eastern Alps. Swiss J. Geosci., 101, 207-223. Stefani, C. 2008: Provenance and Paleogeographic Evolution in a Multi-Source Foreland: The Cenozoic Venetian-Friulian Basin (NE Italy). J. Sediment. Res., 77, 867-887. Zattin, M. et al. 2006: From Middle Jurassic heating to Neogene cooling: The thermochronological evolution of the southern Alps. Tectonophysics, 414, 191-202.

Investigating Environmental Tectonics in Northern Alpine Foreland of Europe

NASA Astrophysics Data System (ADS)

Cloetingh, Sierd; Ziegler, Peter; Cornu, Tristan; Ustaszewski, K.; Schmid, S.; Dezes, P.; Hinsch, R.; Decker, K.; Lopes Cardozo, G.; Granet, M.; Bertrand, G.; Behrmann, J.; Michon, L.; Pagnier, H.; van Wees, J. D.; Rozsa, S.; Heck, B.; Verdun, J.; Kahle, H. G.; Fracassi, U.; Winter, T.; Burov, E.

Until now, research on neotectonics and related seismicity has mostly focused on active plate boundaries characterized by a generally high level of earthquake activity. Current seismic hazard estimates for intraplate areas are commonly based on probabilistic analyses of historical and instrumental earthquake data. The accuracy of these hazard estimates is limited by the nature of the data (e.g., ambiguous historical sources), and by the restriction of available earthquake catalogues to time scales of only few hundred years. Both of these are geologically insignificant and unsuitable for describing tectonic processes causing earthquakes. This is especially relevant to intraplate regions, where faults show low slip rates resulting in long average recurrence times for large earthquakes (103 to 106 yrs), such as the devastating Basel earthquake of 1356, with an estimated magnitude of 6.5. The Alpine orogen and the intraplate sedimentary basins and rifts of its northern foreland are associated with a much higher level of neotectonic activity than hitherto assumed. Seismicity and stress indicator data, combined with geodetic and geomorphologic observations, demonstrate that the Northern Alpine foreland is being actively deformed [Cloetingh, 2000; Ziegler et al., 2002; Behrmann et al., 2003]. This has major implications for the assessment of their natural hazards and environmental degradation. The northwest European lithosphere has undergone a polyphase evolution, in which the interplay between upper mantle thermal perturbations [Goes et al., 2000; Ritter et al., 2001] and stress-induced intraplate deformation [Muller et al., 1992; Ziegler et al., 2002] played an important role. A number of recent findings point to an important role of lithospheric folding in thermally weakened lithosphere of the northwestern European foreland [Cloetingh et al., 1999].

Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model

NASA Astrophysics Data System (ADS)

Yakubchuk, Alexander

2004-09-01

of the Paleo-Pacific Ocean. Several world-class Cu-(Mo)-porphyry, Cu-Pb-Zn VMS and intrusion-related Au mineral camps, which formed in the Altaids at this stage, coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs. Major Pb-Zn and Cu sedimentary rock-hosted deposits of Kazakhstan and Central Asia formed in backarc rifts, which developed on the earlier amalgamated fragments. Major orogenic gold deposits are intrusion-related deposits, often occurring within black shale-bearing sutured backarc basins with oceanic crust. After amalgamation of the western Altaids, this part of the collage and adjacent cratons were affected by the Siberian superplume, which ascended at the Permian-Triassic transition. This plume-related magmatism produced various deposits, such as famous Ni-Cu-PGE deposits of Norilsk in the northwest of the Siberian craton. In the early Mesozoic, the eastern Altaids were oroclinally bent together with the overlapping Transbaikal magmatic arc in response to the northward migration and anti-clockwise rotation of the North China craton. The following collision of the eastern portion of the Altaid collage with the Siberian craton formed the Mongol-Okhotsk suture zone, which still links the accretionary wedges of central Mongolia and Circum-Pacific belts. In the late Mesozoic, a system of continent-scale conjugate northwest-trending and northeast-trending strike-slip faults developed in response to the southward propagation of the Siberian craton with subsequent post-mineral offset of some metallogenic belts for as much as 70-400 km, possibly in response to spreading in the Canadian basin. India-Asia collision rejuvenated some of these faults and generated a system of impact rifts.

The Togo-Benin-Nigeria Shield: evidence of crustal aggregation in the Pan-African belt

NASA Astrophysics Data System (ADS)

Ajibade, A. C.; Wright, J. B.

1989-08-01

The importance of "suspect" or "exotic" (i.e. allochthonous) terranes as a major element in collisional orogenic belts is becoming well established. We propose that the southern Pan-African domain in West Africa is an aggregation or " mosaic" of island arcs, interarc (ensimatic) basins and continental fragments. A fracture zone in northwestern Nigeria, already identified as a possible crustal suture, is shown to separate two contrasted basement terranes. Elsewhere in the shield are flat-lying structures characteristic of those associated with crustal convergence, lying within or near to major fractures. Many small ultramafic/mafic bodies occur in the shield and some of them may be remnants of ophiolites caught up in suture zones. An aggregation of allochthonous terranes (island arcs, sedimentary basins and continental blocks) would help to explain: (a) the great width of the Pan-African belt; (b) the spread of ages within the "Pan-African" range (c. 750-450 Ma), also the relict Liberian and Eburnian ages (c. 2700 and 2000 Ma respectively); and the enigmatic Kibaran "event" (c. 1100 Ma); (c) the contrasted volcano-sedimentary characteristics of the different supracrustal belts.

Geophysical Evidence for the Tectonic Evolution of the Inverted Belt-Purcell Basin, Northwestern Montana

NASA Astrophysics Data System (ADS)

Rutherford, B. S.; Speece, M. A.; Constenius, K. N.

2015-12-01

The geometry of the Precambrian Belt-Purcell basin and subsequent allochthon, that dominates the geology of northwestern Montana, played a critical role in the development of compressional structures during orogenesis and their ensuing reactivation during the later phase of extensional collapse. Five reprocessed seismic reflection profiles provide images in the Swan Range and adjacent valleys that we have correlated to published seismic data north into Canada. Reflections from syndepositional sills encased within Lower Belt rocks offer clues to the configuration of the basin prior to its tectonic inversion. Thick basinal facies of the Lewis salient are contrasted by thin shelfal facies found in hanging wall rocks of frontal Belt carrying thrusts south of the salient. The along strike change in hanging wall rocks reflects the original configuration of the Belt basin margin. Rocks of the Lewis salient were deposited in an embayment on the northeastern margin of the Belt basin. Shelfal accumlations of the embayment comprise an autochthonous wedge that has remained in the footwall of the Lewis thrust system. South of the embayment and related salient, nearly the entire Belt basin was detached from pre-Belt crystalline rocks and inverted at the latitude of the Sawtooth Range. Deeply exhumed Phanerozoic rocks of the Sawtooth Range are a direct consequence of the thin wedge geometry of the detached basin south of the Lewis salient that required growth of a substantial orogenic wedge to obtain critical taper values. We offer an alternate interpretation of a >10 km high, west facing décollement ramp that coincides with the Belt-Purcell basin margin. Previous interpretations in Montana have inferred the location of the basin margin ramp to approximate the trace of the Purcell Anticlinorium. Seismic data and cross-section balancing suggest the Rocky Mountain Trench as a more accurate location. Based on our proposed position of the basin margin the Belt-Purcell allocthon

Two novel bocaparvovirus species identified in wild Himalayan marmots.

PubMed

Ao, Yuanyun; Li, Xiaoyue; Li, Lili; Xie, Xiaolu; Jin, Dong; Yu, Jiemei; Lu, Shan; Duan, Zhaojun

2017-12-01

Bocaparvovirus (BOV) is a genetically diverse group of DNA viruses and a possible cause of respiratory, enteric, and neurological diseases in humans and animals. Here, two highly divergent BOVs (tentatively named as Himalayan marmot BOV, HMBOV1 and HMBOV2) were identified in the livers and feces of wild Himalayan marmots in China, by viral metagenomic analysis. Five of 300 liver samples from Himalayan marmots were positive for HMBOV1 and five of 99 fecal samples from these animals for HMBOV2. Their nearly complete genome sequences are 4,672 and 4,887 nucleotides long, respectively, with a standard genomic organization and containing protein-coding motifs typical for BOVs. Based on their NS1, NP1, and VP1, HMBOV1 and HMBOV2 are most closely related to porcine BOV SX/1-2 (approximately 77.0%/50.0%, 50.0%/53.0%, and 79.0%/54.0% amino acid identity, respectively). Phylogenetic analysis of these three proteins showed that HMBOV1 and HMBOV2 formed two distinctly independent branches in BOVs. According to these results, HMBOV1 and HMBOV2 are two different novel species in the Bocaparvovirus genus. Their identification expands our knowledge of the genetic diversity and evolution of BOVs. Further studies are needed to investigate their potential pathogenicity and their impact on Himalayan marmots and humans.

Incorporation of New and Old Tectonics Concepts Into a Modern Course in Tectonics.

ERIC Educational Resources Information Center

Hatcher, Robert D., Jr.

1983-01-01

Describes a graduate-level tectonics course which includes the historical basis for modern tectonics concepts and an in-depth review of pros/cons of plate tectonics. Tectonic features discussed include: ocean basins; volcanic arcs; continental margins; continents; orogenic belts; foreland fold and thrust belts; volcanic/plutonic belts of orogens;…

Late Carboniferous high-pressure metamorphism of the Kassan Metamorphic Complex (Kyrgyz Tianshan) and assembly of the SW Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Mühlberg, M.; Hegner, E.; Klemd, R.; Pfänder, J. A.; Kaliwoda, M.; Biske, Y. S.

2016-11-01

High-pressure (HP) metamorphism of the Kassan Metamorphic Complex (KMC) in the western Kyrgyz Tianshan has been related to either late Ordovician or late Carboniferous-Permian subduction processes. We report Sm-Nd ages for retrogressed eclogite samples and 40Ar/39Ar cooling ages for enclosing garnet-muscovite samples from the KMC as new age constraints on HP metamorphism and rock exhumation. These data will be used for an upgraded paleogeographic model for late Paleozoic crustal consolidation in the southwestern Central Asian Orogenic Belt. The retrogressed eclogite samples have transitional alkaline to tholeiitic affinity and trace-element patterns consistent with protoliths derived from garnet-bearing mantle sources at rifting plate margins. Geothermobarometric data for a retrogressed eclogite sample indicate peak-metamorphic conditions of 540 ± 30 °C at 1.6 ± 0.1 GPa. Samples from different lithotectonic units of the KMC provide coherent Sm-Nd garnet-whole rock ages of 317 ± 4 Ma and 316 ± 3 Ma (2σ). The prograde major-element zoning in the mm-sized garnets in combination with the moderate peak-metamorphic temperature, support our interpretation of the Sm-Nd garnet ages as unambiguous evidence for late Carboniferous HP metamorphism. The Sm-Nd garnet growth ages overlap within-error with the 40Ar/39Ar mica cooling ages of 314 ± 2 Ma and 313 ± 2 Ma (2σ) indicating rapid uplift of the subduction complex after peak metamorphism. The ca. 317-313 Ma HP-exhumation event of the KMC is contemporaneous with those of the Atbashi and Akeyazi (ca. 500 km east in NW China) HP complexes and implies similar collision histories at the South Tianshan Suture to the east and west of the Talas-Fergana Fault (TFF). The exhumation of the KMC and Atbashi HP complexes overlaps with the initiation of the TFF (Rolland et al., 2013) suggesting incipient separation of the Chatkal and Atbashi complexes during rock exhumation and early plate collision.

What can the Cretaceous-to-present latitude history of the Lhasa terrane tell us about plate-scale deformation in the Tibetan-Himalayan orogen? (Invited)

NASA Astrophysics Data System (ADS)

Lippert, P. C.; Van Hinsbergen, D. J.; Dupont-Nivet, G.; Huang, W.

2013-12-01

Published paleomagnetic data from well-dated sedimentary and volcanic rocks from the Lhasa terrane have been re-evaluated in a statistically consistent framework to assess the latitude history of southern Tibet from ~110 Ma to the present. We apply a methodology similar to the one used by the Time-Averaged geomagnetic Field Initiative to each paleomagnetic data set to establish coherency within and between paleomagnetic data from Tibet (see Session T023 for more details). Moreover, we use only sedimentary data that have been evaluated for and, where necessary, corrected for sedimentary inclination shallowing. The resulting apparent polar wander path (APWP) shows that the southern margin of the Lhasa terrane at the longitudes of Nepal remained at 20×4°N latitude from ~110 to at least 50 Ma and subsequently drifted northward to its present latitude of 29°N. This latitude history provides a paleomagnetically-determined collision age between the Tibetan Himalaya and the southern margin of Asia that is 49.5×4.5 Ma at 21×4° N latitude. The paleomagnetic age and latitude of this collision may be a few millions of years earlier and ~2° lower if estimates for shortening within the suture zone are considered. When compared to the global APWP of Torsvik et al. (2012) in Eurasian coordinates, the Lhasa APWP indicates that at most 1100×560 km of post-50 Ma India-Asia convergence was partitioned into Asian lithosphere. The lower bound of these paleomagnetic estimates is consistent with the magnitude of upper crustal shortening within Asia calculated from orogen-scale geological reconstructions. An implication is that 1700×560 km or more post-50 Ma India-Asia convergence was partitioned into Greater India. Paleomagnetic data from the Tibetan Himalaya are consistent with >2000 km of extension of Greater Indian lithosphere after break-up from Gondwana but prior to collision with the southern margin of Asia. Cenozoic subduction of this Cretaceous extensional basin following

Crustal investigations of the earthquake-prone Vrancea region in Romania - Part 2: Novel deep seismic reflection experiment in the southeastern Carpathian belt and its foreland basin - survey target, design, and first results

NASA Astrophysics Data System (ADS)

Mocanu, V. I.; Stephenson, R. A.; Diaconescu, C. C.; Knapp, J. H.; Matenco, L.; Dinu, C.; Harder, S.; Prodehl, C.; Hauser, F.; Raileanu, V.; Cloetingh, S. A.; Leever, K.

2001-12-01

Seismic studies of the outer Carpathian Orogen and its foreland (Focsani Basin) in the vicinity of the Vrancea Zone and Danube Delta (Romania) forms one component of a new multidisciplinary initiative of ISES (Netherlands Centre for Integrated Solid Earth Sciences) called DACIA PLAN ("Danube and Carpathian Integrated Action on Processes in the Lithosphere and Neotectonics"). The study area, at the margin of the European craton, constitutes one of the most active seismic zones in Europe, yet has remained a geological and geodynamic enigma within the Alpine-Himalayan orogenic system. Intermediate depth (50-220 km) mantle earthquakes of significant magnitude occur in a geographically restricted area in the south-east Carpathians bend. The adjacent, foreland Focsani Basin appears to exhibit recent extensional deformation in what is otherwise understood to be a zone of convergence. The deep seismic reflection component of DACIA PLAN comprises a ~140-km near-vertical profile across the Vrancea Zone and Focsani Basin. Data acquisition took place in August-September 2001, as part of the integrated refraction/reflection seismic field programme "Vrancea-2001" co-ordinated at Karlsruhe University (cf. Abstract, Part 1), utilising 640 independently deployed recorders provided by UTEP and IRIS/PASSCAL ("Texans"). Station spacing was every 100-m with shots every 1-km. These data are to be integrated with industry seismic as well as planned new medium-high resolution seismic reflection profiling across key neotectonically active structures in the Focsani Basin. Particular goals of DACIA PLAN include: (1) the architecture of the Tertiary/Quaternary basins developed within and adjacent to this zone, including the foreland Focsani Basin; (2) the presence and geometry of structural detachment(s) in relation with foreland basin development, including constraints for balanced cross-sections and geodynamic modelling of basin origin and evolution; (3) the relationship between crustal

3.3 Ga SHRIMP U-Pb zircon age of a felsic metavolcanic rock from the Mundo Novo greenstone belt in the São Francisco craton, Bahia (NE Brazil)

NASA Astrophysics Data System (ADS)

Peucat, J. J.; Mascarenhas, J. F.; Barbosa, J. S. F.; de Souza, S. L.; Marinho, M. M.; Fanning, C. M.; Leite, C. M. M.

2002-07-01

Felsic metavolcanics associated with supracrustal rocks provide U-Pb zircon and Sm-Nd TDM ages of approximately 3.3 Ga, which establish an Archean age of the Mundo Novo greenstone belt. A granodioritic gneiss from the Mairi complex, located on the eastern boundary of the Mundo Novo greenstone belt, exhibits a zircon evaporation minimum age of 3.04 Ga and a Nd model age of 3.2 Ga. These results constrain the occurrence of at least three major geological units in this area: the Archean Mundo Novo greenstone belt, the Archean Mairi gneisses, and the adjoining Paleoproterozoic (<2.1 Ga) Jacobina sedimentary basin. The Jacobina basin follows the same trend as the Archean structure, extending southward to the Contendas-Mirante belt, in which a similar Archean-Paleoproterozoic association appears. We postulate that during the Paleoproterozoic in the eastern margin of the Gavião block, these Archean greenstone belts constituted a zone of weakness along which a late-stage orogenic sedimentary basin developed.

Cenozoic intracontinental deformation of the Kopeh Dagh Belt, Northeastern Iran

NASA Astrophysics Data System (ADS)

Chu, Yang; Wan, Bo; Chen, Ling; Talebian, Morteza

2016-04-01

Compressional intracontinental orogens represent large tectonic zones far from plate boundaries. Since intracontinental mountain belts cannot be framed in the conventional plate tectonics theory, several hypotheses have been proposed to account for the formations of these mountain belts. The far-field effect of collision/subduction at plate margins is now well accepted for the origin and evolution of the intracontinental crust thickening, as exemplified by the Miocene tectonics of central Asia. In northern Iran, the Binalud-Alborz mountain belt witnessed the Triassic tectonothermal events (Cimmerian orogeny), which are interpreted as the result of the Paleotethys Ocean closure between the Eurasia and Central Iran blocks. The Kopeh Dagh Belt, located to the north of the Binalud-Alborz Belt, has experienced two significant tectonic phases: (1) Jurassic to Eocene rifting with more than 7 km of sediments; and (2) Late Eocene-Early Oligocene to Quaternary continuous compression. Due to the high seismicity, deformation associated with earthquakes has received more and more attention; however, the deformation pattern and architecture of this range remain poorly understood. Detailed field observations on the Cenozoic deformation indicate that the Kopeh Dagh Belt can be divided into a western zone and an eastern zone, separated by a series of dextral strike-slip faults, i.e. the Bakharden-Quchan Fault System. The eastern zone characterized by km-scale box-fold structures, associated with southwest-dipping reverse faults and top-to-the NE kinematics. In contrast, the western zone shows top-to-the SW kinematics, and the deformation intensifies from NE to SW. In the northern part of this zone, large-scale asymmetrical anticlines exhibit SW-directed vergence with subordinate thrusts and folds, whereas symmetrical anticlines are observed in the southern part. In regard to its tectonic feature, the Kopeh Dagh Belt is a typical Cenozoic intracontinental belt without ophiolites or

Pre-Alpine evolution of the Seckau Complex (Austroalpine basement/Eastern Alps): Constraints from in-situ LA-ICP-MS Usbnd Pb zircon geochronology

NASA Astrophysics Data System (ADS)

Mandl, Magdalena; Kurz, Walter; Hauzenberger, Christoph; Fritz, Harald; Klötzli, Urs; Schuster, Ralf

2018-01-01

The Variscan European Belt is a complex orogen with its southern margin partly obscured by Alpine tectonics and metamorphism. We present a study of one of the units, the Seckau Complex, that constitute the southern part of the Variscan European Belt in the Eastern Alps in order to clarify its origin, age and lithostratigraphy. The magmatic and geochronological evolution of this Complex in the northwestern part of the Seckau Nappe (as part of the Austroalpine Silvretta-Seckau Nappe System) was investigated by zircon Usbnd Pb dating of paragneisses and metagranitoids coupled with petrological and geochemical data. This reveals the distinction of three newly defined lithostratigraphic/lithodemic sub-units: (1) Glaneck Metamorphic Suite, (2) Hochreichart Plutonic Suite and (3) Hintertal Plutonic Suite. The Glaneck Metamorphic Suite is mainly composed of fine-grained paragneisses that yield Usbnd Pb zircon ages in the range between 2.7 Ga and 2.0 Ga, as well as concordia ages from 572 ± 7 Ma to 559 ± 11 Ma. All of these ages are interpreted as detrital zircon ages originating from an igneous source. The paragneisses are the host rock for the large volumes of metagranitoids of the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite. The Hochreichart Plutonic Suite comprises highly fractionated melts with mainly S-type characteristics and late Cambrian to Early Ordovician Usbnd Pb zircon ages (508 ± 9 Ma to 486 ± 9 Ma), interpreted as magmatic protolith ages. The Hintertal Plutonic Suite is composed of metagranitoids with Late Devonian to early Carboniferous (365 ± 11 Ma and 331 ± 10 Ma) protolith ages, that intruded during an early phase of the Variscan tectonometamorphic event. The metagranitoids of the Hintertal Plutonic Suites define a magmatic fractionation trend, seen in variable Rb/Sr ratios. On this base they can be further subdivided into (a) the Griessstein Pluton characterized by S-type metagranitoids and (b) the Pletzen Pluton distinguished by

Geophysical study of the structure and processes of the continental convergence zones: Alpine-Himalayan belt

NASA Technical Reports Server (NTRS)

Toksoz, M. N.; Molnar, P.

1983-01-01

Studies of the structure of the continental collision zones using seismic and body waves, theoretical modelling of the thermal regime of the convergence processes, and studies of earthquake mechanisms and deformation aspects of the model are covered.

Geophysical study of the structure and processes of the continental convergence zones: Alpine-Himalayan Belt

NASA Technical Reports Server (NTRS)

Toksoz, M. Nafi; Molnar, Peter

1988-01-01

Intracontinental deformation occurrence and the processes and physical parameters that control the rates and styles of deformation were examined. Studies addressing specific mechanical aspects of deformation were reviewed and the studies of deformation and of the structure of specific areas were studied considering the strength of the material and the gravitational effect.

Geophysical study of the structure and processes of the continental convergence zones: Alpine-Himalayan Belt

NASA Technical Reports Server (NTRS)

Toksoez, M. N.

1981-01-01

The seismic wave velocity structure in the crust and upper mantle region beneath the Tibetan plateau was studied in detail. Also, a preliminary study of the uppermost mantle P wave velocity beneath Iran and Turkey was carried out, and the results are compared with those for the Tibetan plateau. These two studies compose the bulk of the efforts on the observational aspects of continental collision zones in addition to satellite derived data. On the theoretical aspects the thermal evolution of converging plate boundaries was explored using a finite difference scheme.

Invasive alien plant species dynamics in the Himalayan region under climate change.

PubMed

Lamsal, Pramod; Kumar, Lalit; Aryal, Achyut; Atreya, Kishor

2018-01-25

Climate change will impact the dynamics of invasive alien plant species (IAPS). However, the ability of IAPS under changing climate to invade mountain ecosystems, particularly the Himalayan region, is less known. This study investigates the current and future habitat of five IAPS of the Himalayan region using MaxEnt and two representative concentration pathways (RCPs). Two invasive species, Ageratum conyzoides and Parthenium hysterophorus, will lose overall suitable area by 2070, while Ageratina adenophora, Chromolaena odorata and Lantana camara will gain suitable areas and all of them will retain most of the current habitat as stable. The southern Himalayan foothills will mostly conserve species ecological niches, while suitability of all the five species will decrease with increasing elevation. Such invasion dynamics in the Himalayan region could have impacts on numerous ecosystems and their biota, ecosystem services and human well-being. Trans-boundary response strategies suitable to the local context of the region could buffer some of the likely invasion impacts.

The Australasian frog family Ceratobatrachidae in China, Myanmar and Thailand: discovery of a new Himalayan forest frog clade

PubMed Central

YAN, Fang; JIANG, Ke; WANG, Kai; JIN, Jie-Qiong; SUWANNAPOOM, Chatmongkon; LI, Cheng; Jens, V. VINDUM; Rafe, M. BROWN; CHE, Jing

2016-01-01

In an effort to study the systematic affinities and specieslevel phylogenetic relationships of the enigmatic anurans variably assigned to the genera Ingerana or Limnonectes (family Dicroglossidae), we collected new molecular sequence data for five species including four Himalayan taxa, Limnonectes xizangensis, Lim. medogensis, Lim. alpine, Ingerana borealis and one southeast Asian species, I. tasanae, and analyzed these together with data from previous studies involving other ostensibly related taxa. Our surprising results demonstrate unequivocally that Lim. xizangensis, Lim. medogensis and Lim. alpine form a strongly supported clade, the sister-group of the family Australasian forest frog family Ceratobatrachidae. This discovery requires an expansion of the definition of Ceratobatrachidae and represents the first record of this family in China. These three species are distinguished from the species of Ingerana and Limnonectes by the: (1) absence of interdigital webbing of the foot, (2) absence of terminal discs on fingers and toes, (3) absence of circumarginal grooves on the fingers and toes, and (4) absence of tarsal folds. Given their phylogenetic and morphological distinctiveness, we assign them to the oldest available generic name for this clade, Liurana Dubois 1987, and transfer Liurana from Dicroglossidae to the family Ceratobatrachidae. In contrast, Ingerana tasanae was found to be clustered with strong support with the recently described genus Alcalus (Ceratobatrachidae), a small clade of otherwise Sundaic species; this constitutes a new record of the family Ceratobatrachidae for Myanmar and Thailand. Finally, Ingerana borealis clustered with the "true" Ingerana (family Dicroglossidae), for which the type species is I. tenasserimensis. PMID:26828029

Toward Broadband Source Modeling for the Himalayan Collision Zone

NASA Astrophysics Data System (ADS)

Miyake, H.; Koketsu, K.; Kobayashi, H.; Sharma, B.; Mishra, O. P.; Yokoi, T.; Hayashida, T.; Bhattarai, M.; Sapkota, S. N.

2017-12-01

The Himalayan collision zone is characterized by the significant tectonic setting. There are earthquakes with low-angle thrust faulting as well as continental outerrise earthquakes. Recently several historical earthquakes have been identified by active fault surveys [e.g., Sapkota et al., 2013]. We here investigate source scaling for the Himalayan collision zone as a fundamental factor to construct source models toward seismic hazard assessment. As for the source scaling for collision zones, Yen and Ma [2011] reported the subduction-zone source scaling in Taiwan, and pointed out the non-self-similar scaling due to the finite crustal thickness. On the other hand, current global analyses of stress drop do not show abnormal values for the continental collision zones [e.g., Allmann and Shearer, 2009]. Based on the compile profiling of finite thickness of the curst and dip angle variations, we discuss whether the bending exists for the Himalayan source scaling and implications on stress drop that will control strong ground motions. Due to quite low-angle dip faulting, recent earthquakes in the Himalayan collision zone showed the upper bound of the current source scaling of rupture area vs. seismic moment (< Mw 8.0), and does not show significant bending of the source scaling. Toward broadband source modeling for ground motion prediction, we perform empirical Green's function simulations for the 2009 Butan and 2015 Gorkha earthquake sequence to quantify both long- and short-period source spectral levels.

Detrital Record of Phanerozoic Tectonics in Iran: Evidence From U-Pb Zircon Geochronology

NASA Astrophysics Data System (ADS)

Horton, B. K.; Gillis, R. J.; Stockli, D. F.; Hassanzadeh, J.; Axen, G. J.; Grove, M.

2004-12-01

Ion-microprobe U-Pb ages of 91 detrital zircon grains supplement ongoing investigations of the tectonic history of Iran, a critical region bridging the gap between the Alpine and Himalayan orogenic belts. These data improve understanding of the distribution of continental blocks during a complex history of Late Proterozoic (Pan-African) crustal growth, Paleozoic passive-margin sedimentation, early Mesozoic collision with Eurasia, and Cenozoic collision with Arabia. U-Pb analyses of detrital zircon grains from four sandstone samples (two Lower Cambrian, one uppermost Triassic-Lower Jurassic, one Neogene) collected from the Alborz mountains of northern Iran reveal a spectrum of ages ranging from 50 to 2900 Ma. Most analyses yield concordant to moderately discordant ages. The Lower Cambrian Lalun and Barut sandstones yield age distribution peaks at approximately 550-650, 1000, and 2500 Ma, consistent with a Gondwanan source area presently to the south and west in parts of Iran and the Arabian-Nubian shield (Saudi Arabia and northwestern Africa). The uppermost Triassic-Lower Jurassic Shemshak Formation exhibits a broad range of U-Pb ages, including peaks of approximately 200-260, 330, 430, 600, and 1900 Ma, requiring a Eurasian source area presently to the north and east in the Turan plate (Turkmenistan and southwestern Asia). Neogene strata display both the youngest and oldest ages (approximately 50 and 2900 Ma) of any samples, a result of substantial sedimentary recycling of older Phanerozoic cover rocks. Because the youngest zircon ages for three of the four samples are indistinguishable from their stratigraphic (depositional) ages, these data suggest rapid exhumation and help constrain the termination age of Late Proterozoic-Early Cambrian (Pan-African) orogenesis and the timing of the Iran-Eurasia collision.

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

Active deformation processes of the Northern Caucasus deduced from the GPS observations

NASA Astrophysics Data System (ADS)

Milyukov, Vadim; Mironov, Alexey; Rogozhin, Eugeny; Steblov, Grigory; Gabsatarov, Yury

2015-04-01

The Northern Caucasus, as a part of the Alpine-Himalayan mobile belt, is a zone of complex tectonics associated with the interaction of the two major tectonic plates, Arabian and Eurasian. The first GPS study of the contemporary geodynamics of the Caucasus mountain system were launched in the early 1990s in the framework of the Russia-US joint project. Since 2005 observations of the modern tectonic motion of the Northern Caucasus are carried out using the continuous GPS network. This network encompasses the territory of three Northern Caucasian Republics of the Russian Federation: Karachay-Cherkessia, Kabardino-Balkaria, and North Ossetia. In the Ossetian part of the Northern Caucasus the network of GPS survey-mode sites has been deployed as well. The GPS velocities confirm weak general compression of the Northern Caucasus with at the rate of about 1-2 mm/year. This horizontal motion at the boundary of the Northern Caucasus with respect to the Eurasian plate causes the higher seismic and tectonic activity of this transition zone. This result confirms that the source of deformation of the Northern Caucasus is the sub-meridional drift of the Arabian plate towards the adjacent boundary of the Eastern European part of the Eurasian lithospheric plate. The concept of such convergence implies that the Caucasian segment of the Alpine-Himalayan mobile belt is under compression, the layers of sedimentary and volcanic rocks are folded, the basement blocks are subject to shifts in various directions, and the upper crust layers are ruptured by reverse faults and thrusts. Weak deviation of observed velocities from the pattern corresponding to homogeneous compression can also be revealed, and numerical modeling of deformations of major regional tectonic structures, such as the Main Caucasus Ridge, can explain this. The deformation tensor deduced from the velocity field also exhibits the sub-meridional direction of the major compressional axes which coincides with the direction of

History of India-Asia Suturing in Tibet: Constraints and Questions

NASA Astrophysics Data System (ADS)

Kapp, P. A.; Ding, L.

2011-12-01

The India-Asia collision zone is widely pointed to as the type Cenozoic example of continental suturing and collision, yet there remains considerable controversy about its geological and geodynamical evolution. This in part may reflect the richness and complexity of the geological records exposed across the collision zone and how much remains to be extracted from them. Separating the formerly Andean-style continental margin of southern Asia (Gangdese arc and forearc of the Lhasa terrane) in the north, from Indian-affinity strata deformed in the Tethyan Himalayan thrust belt to the south, is the Indus-Yarlung suture zone (IYSZ). In Tibet, ophiolitic rocks along the IYSZ crystallized and were obducted in a suprasubduction zone setting during Early Cretaceous time. The ophiolitic rocks are of the appropriate age to have formed the basement upon which Gangdese forearc strata accumulated. Alternatively, they may represent remnants of an intra-oceanic subduction system that persisted in the Tethys, far from Asia, until Greater India collided with it during the latest Cretaceous to Paleocene. There has been no documentation, however, of ophiolitic or arc fragments younger than Early Cretaceous within the IYSZ. Distinguishing between these two end-member scenarios is important for interpreting detrital records of orogenesis and seismic tomographic images of the mantle. A preponderance of evidence suggests that collision between the Tethyan Himalaya and Asia initiated by 52 Ma. Initial collision led abruptly to profound and far-field changes in paleogeography and tectonism such that by 45 Ma, major shortening and potassic volcanism was ongoing in northern Tibet, plateau-like conditions were established in central Tibet, Tethyan Himalayan crust was undergoing anatexis, and Eo-Himalayan prograde metamorphism was underway. Additional constraints on the shortening history of the Tethyan Himalayan thrust belt will be key to assessing when and how much Greater Indian lithosphere

Thick-skinned tectonics within the intracontinental easternmost Atlas foreland-and-thrust belt (Tunisia): Meso-Cenozoic kinematics and implications for regional geodynamics

NASA Astrophysics Data System (ADS)

Belkhiria, W.; Boussiga, H.; Inoubli, M. H.

2017-05-01

The transition zone between western and central Mediterranean domains presents a key area to investigate kinematic interactions within the adjacent orogen systems such as the easternmost Atlas foreland-and-thrust belt. Gravity and seismic data revealed a highly structured basement, characterizing a series of structural highs and lows delimited by high-angle N-S, E-W, and NW-SE extensional faults. This basement architecture is inherited from successive extensional events related to the openings of the Triassic-Early Cretaceous Tethys oceans (i.e., Alpine Tethys, Ligurian Tethys, and Mesogea). Throughout this period, this mosaic of continental blocks significantly controlled the thickness and facies distributions. Early stages of diapirism took place along these basement faults and allowed maximum subsidence in minibasins revealed by the development of growth strata. In response to the Late Cretaceous-Eocene shortenings, these extensional faults have been reactivated as trasnpressional shear zones, giving rise to narrow pop-up structures. In addition, gravity modeling indicates crustal thinning and deep-rooted faults affecting the crust south of the Zaghouan Thrust and along E-W transfer zones. From the late Miocene, a drastic change in the stress regime is attributed to the effect of the adjacent Sicily channel on the study area. This promotes crustal thinning, basin subsidence, and channeling up of mantle-derived helium along lithospheric-scale weak zones. Our results give rise to new insights into the reactivation of inherited weakness zones of southern Tethys margin in response to the complex interaction between African and Eurasian plates accommodated by subduction, rollback, collision, and slab segmentation.

Thermochronometry Across the Austroalpine-Pennine Boundary, Central Alps, Switzerland: Orogen-Perpendicular Normal Fault Slip on a Major "Overthrust" and Its Implications for Orogenesis

NASA Astrophysics Data System (ADS)

Price, Jason B.; Wernicke, Brian P.; Cosca, Michael A.; Farley, Kenneth A.

2018-03-01

Fifty-one new and 309 published thermochronometric ages (nine systems with closure temperatures ranging from 450 to 70°C) from the Graubünden region of the Central Alps demonstrate that a pronounced thermal mismatch between the Austroalpine allochthon (Alpine "orogenic lid") and the Pennine zone persisted until at least 29 Ma and, allowably, until circa 18 Ma. The observed mismatch supports previous suggestions that the famous "overthrust" between the Austroalpine allochthon and the Pennine zone, historically regarded as primarily an Eocene top-north thrust fault, is in fact primarily an Oligocene-Miocene normal fault that has a minimum of 60 km of displacement with top-south or top-southeast sense of shear. Two hallmarks of Alpine geology, deposition of the foredeep Molasse and emplacement of the Helvetic nappes, appear to be coeval, peripheral manifestations of crustal thickening via the interposition of the Pennine zone as a northward intruding wedge between the Austroalpine "lid" and the European cratonic margin, with the Helvetic system (European margin) acting as the "floor" of the wedge. We presume the Penninic wedge is driven by the buoyant rise of subducted crust no longer able to remain attached to the descending slab. If so, emplacement of the Pennine wedge could have occurred mainly after Adria was juxtaposed against cratonic Europe.

Dynamics of thin-skinned fold and thrust belts with a tilted detachment

NASA Astrophysics Data System (ADS)

Fernandez, Naiara; Kaus, Boris J. P.; Epard, Jean-Luc

2014-05-01

The formation of the Jura fold and thrust belt is linked to the Alpine orogeny. However, it is still a matter of debate why the Jura was formed tens of kilometres far away from the active deformation front while the Molasse basin that lies in between remained mostly undeformed. Progressive thickening of the Molasse basin due to its infill with sediments, and the existence of a tilted potential detachment level at the Triassic evaporitic units, have been pushed forward as the main causes for the detachment of the Molasse basin and the consequent jump of the deformation front from the Alpine front to the position of the Jura at around 22 Ma or later (e.g Willett and Schlunegger, 2010). In order to better understand the dynamics of a thin-skinned fold and thrust belt with a tilted detachment we have performed systematic forward numerical simulations with the 2D thermo-mechanical finite element code MILAMIN_VEP. The modelled setup consists of a tilted detachment, overlain by a sedimentary cover of constant thickness and a wedge shaped basin infill that makes the initial surface slope of the system to be zero. In this study we have tested the importance of the following factors in the dynamics of such a fold and thrust belt evolution: 1) the applied boundary conditions 2) the angle of a uniformly tilted detachment 3) the end displacement of a curved detachment with a flexural foreland basin profile. The implications of the studied factors are discussed for the case of the Jura-Molasse system. Acknowledgements Funding was provided by the European Research Council under the European Community's Seventh Framework program (FP7/2007-2013) ERC Grant agreement #258830. References Willett, S.D. and Schlunegger, F. 2010, The last phase of deposition in the Swiss Molasse Basin: from foredeep to negative-alpha basin. Basin Research 22, 623-639, doi: 10.1111/j.1365-2117.2009.00435.x

The Pico do Itapeva Formation: A record of gravitational flow deposits in an Ediacaran intracontinental basin, southern Brasília Orogen, SE Brazil

NASA Astrophysics Data System (ADS)

Caputo Neto, V.; Ribeiro, A.; Nepomuceno, F. O.; Dussin, I. A.; Trouw, R. A. J.

2018-07-01

The Pico do Itapeva Formation is a key metasedimentary unit to the understanding of the extensional events that occurred between the late stage of the southern Brasília Orogen collision and the main collision in the central Ribeira Orogen. The formation crops out in a 20 km long NE-trending narrow belt in the Mantiqueira mountain range in eastern São Paulo State, Brazil. It is located in the interference zone of the southern Brasília and the central Ribeira orogens and records deformation and greenschist facies metamorphism (biotite zone) related to the Brasiliano orogeny. The Pico do Itapeva Formation rests unconformably on a metaigneous substratum of the Socorro-Guaxupé Nappe/Embu Terrane and, on the southern side, is truncated by a steep SE-dipping dextral reverse shear zone. It consists of a coarsening- and thickening-upward succession, with minimum thickness of 800 m, composed of lutite, arkose and conglomerate. These rocks constitute three distinct lithofacies associations: LAI- arkose, arkose-lutite composite beds, lutite and fine conglomerate beds; LAII- arkose, pebbly arkose and scarce lutite and; LAIII- conglomerate and pebbly arkose. Most of the beds are massive; graded beds, dish and convolute structures occur locally. Bed thickness varies from thin to very thick and amalgamated bodies constitute up to 30m thick strata. Three mappable units at scale 1:20,000 were recognized based on different proportions of the three lithofacies associations. The deposits are interpreted as the record of mass flows and associated processes in a fan delta setting developed in an intermontane rift basin. U-Pb LA-ICP-MS detrital zircon ages suggest the maximum depositional age at ca. 611 Ma and the basin evolution is interpreted in the range between 611 and 580 Ma during an inter-orogenic stage between the Brasília and Ribeira orogenies.

Paired Magmatic-Metallogenic Belts in Myanmar - an Andean Analogue?

NASA Astrophysics Data System (ADS)

Gardiner, Nicholas; Robb, Laurence; Searle, Michael; Morley, Christopher

2015-04-01

Myanmar (Burma) is richly endowed in precious and base metals, having one of the most diverse collections of natural resources in SE Asia. Its geological history is dominated by the staged closing of Tethys and the suturing of Gondwana-derived continental fragments onto the South China craton during the Mesozoic-Cenozoic. The country is located at a crucial geologic juncture where the main convergent Tethyan collision zone swings south around the Namche Barwa Eastern Himalayan syntaxis. However, despite recent work, the geological and geodynamic history of Myanmar remains enigmatic. Plate margin processes, magmatism, metasomatism and the genesis of mineral deposits are intricately linked, and there has long been recognized a relationship between the distribution of certain mineral deposit types, and the tectonic settings which favour their genesis. A better knowledge of the regional tectonic evolution of a potential exploration jurisdiction is therefore crucial to understanding its minerals prospectivity. This strong association between tectonics and mineralization can equally be applied in reverse. By mapping out the spatial, and temporal, distribution of presumed co-genetic mineral deposits, coupled with an understanding of their collective metallogenetic origin, a better appreciation of the tectonic evolution of a terrane may be elucidated. Identification and categorization of metallotects within a geodynamically-evolving terrane thus provides a complimentary tool to other methodologies (e.g. geochemical, geochronological, structural, geophysical, stratigraphical), for determining the tectonic history and inferred geodynamic setting of that terrane through time. Myanmar is one such study area where this approach can be undertaken. Here are found two near-parallel magmatic belts, which together contain a significant proportion of that country's mineral wealth of tin, tungsten, copper, gold and silver. Although only a few 100 km's apart, these belts exhibit a

Lithospheric controls on the formation of provinces hosting giant orogenic gold deposits

USGS Publications Warehouse

Bierlein, F.P.; Groves, D.I.; Goldfarb, R.J.; Dube, B.

2006-01-01

Ages of giant gold systems (>500 t gold) cluster within well-defined periods of lithospheric growth at continental margins, and it is the orogen-scale processes during these mainly Late Archaean, Palaeoproterozoic and Phanerozoic times that ultimately determine gold endowment of a province in an orogen. A critical factor for giant orogenic gold provinces appears to be thickness of the subcontinental lithospheric mantle (SCLM) beneath a province at the time of gold mineralisation, as giant gold deposits are much more likely to develop in orogens with subducted oceanic or thin continental lithosphere. A proxy for the latter is a short pre-mineralisation crustal history such that thick SCLM was not developed before gold deposition. In constrast, orogens with protracted pre-mineralisation crustal histories are more likely to be characterised by a thick SCLM that is difficult to delaminate, and hence, such provinces will normally be poorly endowed. The nature of the lithosphere also influences the intrinsic gold concentrations of potential source rocks, with back-arc basalts, transitional basalts and basanites enriched in gold relative to other rock sequences. Thus, segments of orogens with thin lithosphere may enjoy the conjunction of giant-scale fluid flux through gold-enriched sequences. Although the nature of the lithosphere plays the crucial role in dictating which orogenic gold provinces will contain one or more giant deposits, the precise siting of those giants depends on the critical conjunction of a number of province-scale factors. Such features control plumbing systems, traps and seals in tectonically and lithospherically suitable terranes within orogens. ?? Springer-Verlag 2006.

Crustal nature and origin of the Russian Altai: Implications for the continental evolution and growth of the Central Asian Orogenic Belt (CAOB)

NASA Astrophysics Data System (ADS)

Cai, Keda; Sun, Min; Buslov, M. M.; Jahn, Bor-ming; Xiao, Wenjiao; Long, Xiaoping; Chen, Huayong; Wan, Bo; Chen, Ming; Rubanova, E. S.; Kulikova, A. V.; Voytishek, E. E.

2016-04-01

The Central Asian Orogenic Belt is a gigantic tectonic collage of numerous accreted terranes. However, its geodynamic evolution has been hotly debated primarily due to incomplete knowledge on the nature of these enigmatic terranes. This work presents new detrital zircon U-Pb and Hf isotopic data to constrain the crustal nature and origin of the Russian Altai, a critical segment of Altai-Mongolian terrane. The youngest zircon 206Pb/238U ages of 470 Ma constrain that the Terekta Formation, previously envisaged as Precambrian basement, was actually deposited after the Middle Ordovician. As for the three more sedimentary sequences above the Terekta Formation, they have youngest zircon 206Pb/238U ages of 425 Ma, 440 Ma and 380 Ma, respectively, indicating their depositions likely in the Late Silurian to Devonian. From all analyses, it is noted that many zircon U-Pb ages cluster at ca. 520 Ma and ca. 800 Ma, and these zircons display oscillatory zoning and have subhedral to euhedral morphology, which, collectively, suggests that adjacent Neoproterozoic to Paleozoic igneous rocks were possibly dominant in the sedimentary provenance. Additionally, a few rounded Archean to Mesoproterozoic zircon grains are characterized by complex texture, which are interpreted as recycling materials probably derived from the Tuva-Mongolian microcontinent. Precambrian rocks have not been identified in the Russian Altai, Chinese Altai and Mongolian Altai so far, therefore, Precambrian basement may not exist in the Altai-Mongolian terrane, but this terrane probably represents a large subduction-accretion complex built on the margin of the Tuva-Mongolian microcontinent in the Early Paleozoic. Multiple episodes of ridge-trench interaction may have caused inputs of mantle-derived magmas to trigger partial melting of the newly accreted crustal materials, which contributed to the accretionary complex. During accretionary orogenesis of the CAOB, formation of such subduction-accretion complex is

Microseismicity, tectonics and seismic potential in the Western Himalayan segment, NW Himalaya, India

NASA Astrophysics Data System (ADS)

Parija, Mahesh Prasad; Kumar, Sushil; Tiwari, V. M.; Rao, N. Purnachandra; Kumar, Narendra; Biswal, Shubhasmita; Singh, Ishwar

2018-06-01

The tectonics and seismic potential of the western Himalayan segment (30-33°N; 76-80°E) of the NW Himalayan (India) region have been determined in this study. 423 earthquakes were located in the NW Himalaya between 2004 and 2013 using more than 4495 P and 4453 S differential travel times to determine the moment tensors for 8 (Mw ≥ 4.0) of these earthquakes using their broadband regional waveforms. The geometry of the Main Himalayan Thrust (MHT) plane which varies along the strike of the Himalaya in flat and ramp segments with a dip ranging between ∼2.5 to ∼4° to ∼19° below the Himalayan Frontal Thrust (HFT) in the south to the South Tibetan Detachment (STD) in the north has also been deduced in this study. Two crustal ramps were reported in this study with a depth variance below the Main Central Thrust (MCT) and to the South Tibetan Detachment (STD) between 12 to 22 km and 28 to 40 km depth respectively. The estimated earthquake potential prevailing in the western Himalayan seismic gap lying between the epicentral zone of the 1905 Kangra earthquake and the 1975 Kinnaur earthquake reveals that the total amount of energy released since the last great event is only a fraction (3-5%) of the accommodated energy i.e.1.1E+28 dyne-cm/yr. This suggests that if an earthquake hits this NW Himalayan segment in the future, its magnitude might be around Mw ≥ 8.0.

Deformation of the Pacific Plate above the Alpine fault ramp and its relationship to expulsion of metamorphic fluids: An array of backshears

NASA Astrophysics Data System (ADS)

Wightman, Ruth H.; Little, Timothy A.

A ˜2 km-wide array of near-vertical backshears in the central Southern Alps, New Zealand, is interpreted to have slipped in an escalator-like way to up-ramp the Pacific Plate onto the Alpine Fault ramp, and to play an important role in channelling metamorphic fluids upward through this active orogen. The oblique-slip backshears formed in the lower crust, are evenly spaced (˜30 cm), and have an average offset of 14 cm that is brittle to ductile and extend over 500 m in vertical length. Cumulative vertical displacements suggest that the causative ramp-step in the Alpine Fault at depth had an angle of 22±8°. Microscale shearing between the backshears probably accomplished additional crustal tilting to ˜45°. We infer this shearing was focused above the basal ramp-step, was transient, and aseismic. Focal mechanisms of earthquakes in the Southern Alps suggest that similar backshearing may be accumulating at depth today, where it is linked to seismic-slip on upper crustal faults. Fluid was integral to the formation and accumulation of shear along the backshears. Near-lithostatic fluid pressures triggered deep, brittle shear failure (>20 km). The steep, dilative backshears allowed these fluids to escape upwards through low permeability (1 × 10-18m2) schist. Fluid expulsion may thus have accomplished a devolatilisation and rheological strengthening along the Alpine mylonite source region at depth, while also causing a hydrolytic weakening of the fluid-invaded rocks (especially quartz veins) in the Pacific Plate. These coupled strength changes may have enhanced the local partitioning of deformation onto steep planes in the Alpine Fault hangingwall.

Counter-intuitive influence of Himalayan river morphodynamics on Indus Civilisation urban settlements.

PubMed

Singh, Ajit; Thomsen, Kristina J; Sinha, Rajiv; Buylaert, Jan-Pieter; Carter, Andrew; Mark, Darren F; Mason, Philippa J; Densmore, Alexander L; Murray, Andrew S; Jain, Mayank; Paul, Debajyoti; Gupta, Sanjeev

2017-11-28

Urbanism in the Bronze-age Indus Civilisation (~4.6-3.9 thousand years before the present, ka) has been linked to water resources provided by large Himalayan river systems, although the largest concentrations of urban-scale Indus settlements are located far from extant Himalayan rivers. Here we analyse the sedimentary architecture, chronology and provenance of a major palaeochannel associated with many of these settlements. We show that the palaeochannel is a former course of the Sutlej River, the third largest of the present-day Himalayan rivers. Using optically stimulated luminescence dating of sand grains, we demonstrate that flow of the Sutlej in this course terminated considerably earlier than Indus occupation, with diversion to its present course complete shortly after ~8 ka. Indus urban settlements thus developed along an abandoned river valley rather than an active Himalayan river. Confinement of the Sutlej to its present incised course after ~8 ka likely reduced its propensity to re-route frequently thus enabling long-term stability for Indus settlements sited along the relict palaeochannel.

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

NASA Astrophysics Data System (ADS)

Chen, Ming; Sun, Min

2017-04-01

The Russian Altai, comprising the northern segment of the Altai-Mongolian terrane (AM) in the south, the Gorny Altai terrane (GA) in the north and the intervening Charysh-Terekta-Ulagan-Sayan suture zone, is a key area of the northwestern Central Asian Orogenic Belt (CAOB). A combined geochemical and detrital zircon study was conducted on the (meta-)sedimentary sequences from the Russian Altai to reveal the tectono-magmatic history of these two terranes and their amalgamation history, which in turn place constraints on the accretionary orogenesis and crustal growth in the CAOB. The Cambrian-Ordovician meta-sedimentary rocks from the northern AM are dominated by immature sediments possibly sourced from intermediate-felsic igneous rocks. Geochemical data show that the sediments were likely deposited in a continental arc-related setting. Zircons separated from these rocks are mainly 566-475 Ma and 1015-600 Ma old, comparable to the magmatic records of the Tuva-Mongolian terrane and surrounding island arcs in the western Mongolia. The similar source nature, provenance and depositional setting of these rocks to the counterparts from the Chinese Altai (i.e., the southern AM) imply that the whole AM possibly represents a coherent accretionary prism of the western Mongolia in the early Paleozoic rather than a Precambrian continental block with passive marginal deposition as previously thought. In contrast, the Cambrian to Silurian (meta-)sedimentary rocks from the GA are characterized by a unitary zircon population with ages of 640-470 Ma, which were potentially sourced from the Kuznetsk-Altai intra-oceanic island arc in the east of this terrane. The low abundance of 640-540 Ma zircons (5%) may attest that this arc was under a primitive stage in the late Neoproterozoic, when mafic igneous rocks dominated. However, the voluminous 530-470 Ma zircons (95%) suggest that this arc possibly evolved toward a mature one in the Cambrian to early Ordovician with increasing amount of

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

NASA Astrophysics Data System (ADS)

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

2014-01-01

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

Clues on active differential uplift across the Giudicarie belt (Central-Eastern Alps, Italy) by means of PSInSAR data

NASA Astrophysics Data System (ADS)

Massironi, Matteo; Zampieri, Dario; Schiavo, Alessio; Bianchi, Marco; Franceschini, Andrea

2010-05-01

The Permanent Scatterers Synthetic Aperture Radar INterferometry (PSInSAR) methodology provides high resolution assessment of surface deformations (precision ranging from 0.8 to 0.1 mm/year) over long periods of observation. Hence, it is particularly suitable to analyze surface motion over wide regions associated to a weak tectonic activity. For this reason we have adopted the PSInSAR technique to study regional movement across the Giudicarie belt, a NNE-trending trust belt oblique to the Southern Alpine chain and presently characterized by a low to moderate seismicity. Over 11,000 PS velocities along the satellite Line Of Sight (LOS) were calculated using images acquired in descending orbit during the 1992-1996 time span. The PSInSAR data show a differential uplift of around 1.4-1.7 mm/year across the most external WNW-dipping thrusts of the Giudicarie belt (Mt. Baldo, Mt. Stivo and Mt. Grattacul thrusts alignment). This corresponds to a horizontal contraction across the external part of the Giudicarie belt of about 1.3-1.5 mm/year.

Alpine Palaeogeography: new constraints from detrital zircon geochronology

NASA Astrophysics Data System (ADS)

Galster, Federico; Stockli, Daniel

2017-04-01

Schardt's (1898) discovery of the "allochtony" of the Préalpes Médianes and its exotic character, provided Alpine geologist with a first picture of Alpine palaeogeography: a Middle Jurassic sea divided in two branches by the rise of an emerged island. Later on, Schardt's island had been recognized at the scale of the Alpine belt and took the name of Briançonnais "geoanticline". In many Alpine palaeogeographic reconstructions, the Briançonnais and its exotic character have played a crucial role (e.g. Stampfli 1993; Manatschal et al., 2006;). In particular some of them explained the exotic character of the Briançonnais facies by proposing a pre-Cretaceous position located far from the Helvetic domain. In this view, the Briançonnais terrain was related to the Iberian plate and entered the Central Alpine system only after a Lower Cretaceous eastward drift associated with anticlockwise rotation of Iberia, opening of the northern Atlantic and closure of the Vardar ocean. In the Central Alps, the remnants of the northern Jurassic margin of the Alpine Tethys (sensu Stampfli) are contained in the Helvetic (s.l.) and Lower Penninic units. The basements and original substrate of these nappes are exposed in the crystalline external massifs and in the gneissic Lepontine dome. The highest, more internal, gneissic units within this dome are the Monte Leone, the Maggia and the Adula nappe. Theses units, as well as the autochthonous basement of the European margin, are characterized by large "Variscan" granitoids with ages between 290 and 330 Ma. The "ophiolite-bearing" units thrust on top of the Adula nappe are composed of Cretaceous and younger sedimentary rocks, with thin soles of Triassic and Jurassic strata. In addition to Variscan, Cambro-Ordovician and Proterozoic ages, detrital zircons in these soles show a peak at 260-280 Ma accompanied by a cluster of ca. 230 Ma zircons, similarly to what is observed in the Schams and Préalpes Médianes nappes (Briançonnais s

Himalayan gneiss dome formation in the middle crust and exhumation by normal faulting: New geochronology of Gianbul dome, northwestern India

USGS Publications Warehouse

Horton, Forrest; Lee, Jeffrey; Hacker, Bradley; Bowman-Kamaha'o, Meilani; Cosca, Michael A.

2015-01-01

A general lack of consensus about the origin of Himalayan gneiss domes hinders accurate thermomechanical modeling of the orogen. To test whether doming resulted from tectonic contraction (e.g., thrust duplex formation, antiformal bending above a thrust ramp, etc.), channel flow, or via the buoyant rise of anatectic melts, this study investigates the depth and timing of doming processes for Gianbul dome in the western Himalaya. The dome is composed of Greater Himalayan Sequence migmatite, Paleozoic orthogneiss, and metasedimentary rock cut by multiple generations of leucogranite dikes. These rocks record a major penetrative D2 deformational event characterized by a domed foliation and associated NE-SW–trending stretching lineation, and they are flanked by the top-down-to-the-SW (normal-sense) Khanjar shear zone and the top-down-to-the-NE (normal sense) Zanskar shear zone (the western equivalent of the South Tibetan detachment system). Monazite U/Th-Pb geochronology records (1) Paleozoic emplacement of the Kade orthogneiss and associated granite dikes; (2) prograde Barrovian metamorphism from 37 to 33 Ma; (3) doming driven by upper-crustal extension and positive buoyancy of decompression melts between 26 and 22 Ma; and (4) the injection of anatectic melts into the upper levels of the dome—neutralizing the effects of melt buoyancy and potentially adding strength to the host rock—by ca. 22.6 Ma on the southwestern flank and ca. 21 Ma on the northeastern flank. As shown by a northeastward decrease in 40Ar/39Ar muscovite dates from 22.4 to 20.2 Ma, ductile normal-sense displacement within the Zanskar shear zone ended by ca. 22 Ma, after which the Gianbul dome was exhumed as part of a rigid footwall block below the brittle Zanskar normal fault, tilting an estimated 5°–10°SW into its present orientation.

Origin of the Bashierxi monzogranite, Qiman Tagh, East Kunlun Orogen, NW China: A magmatic response to the evolution of the Proto-Tethys Ocean

NASA Astrophysics Data System (ADS)

Zheng, Zhen; Chen, Yan-Jing; Deng, Xiao-Hua; Yue, Su-Wei; Chen, Hong-Jin; Wang, Qing-Fei

2018-01-01

The Qiman Tagh of the East Kunlun Orogen, NW China, lies within the Tethysides and hosts a large W-Sn belt associated with the Bashierxi monzogranite. To constrain the origin of the granitic magmatism and its relationship with W-Sn mineralization and the tectonic evolution of the East Kunlun Orogen and the Tethys, we present zircon U-Pb ages and Hf isotopic data, and whole-rock compositional and Sr-Nd-Pb isotopic data of the Bashierxi monzogranite. The granite comprises quartz, K-feldspar, plagioclase, and minor muscovite, tourmaline, biotite, and garnet. It contains high concentrations of SiO2, K2O, and Al2O3, and low concentrations of TiO2 and MgO, indicating a peraluminous high-K calc-alkaline affinity. The rocks are enriched in Rb, U, Pb, and light rare earth elements, and relatively depleted in Eu, Ba, Nb, Sr, P, and Ti, and are classified as S-type granites. Twenty zircon grains yield a weighted mean 238U/206Pb age of 432 ± 2.6 Ma (mean square weighted deviation = 1.3), indicating the occurrence of a middle Silurian magmatic event in the region. Magmatic zircons yield εHf(t) values of -6.7 to 0.7 and corresponding two-stage Hf model ages of 1663-1250 Ma, suggesting that the granite was derived from Mesoproterozoic crust, as also indicated by 207Pb/206Pb ages of 1621-1609 Ma obtained from inherited zircon cores. The inherited zircon cores yield εHf(t) values of 8.3-9.6, which indicate the generation of juvenile crust in the late Paleoproterozoic. Samples of the Bashierxi granite yield high initial 87Sr/86Sr ratios and radiogenic Pb concentrations, and negative εNd(t) values. Isotopic data from the Bashierxi granite indicate that it was derived from partial melting of ancient (early Paleozoic to Mesoproterozoic) sediments, possibly representing recycled Proterozoic juvenile crust. Middle Silurian granitic magmatism resulted from continental collision following closure of the Proto-Tethys Ocean. The Qiman Tagh represents a Caledonian orogenic belt containing

Tectonic and metamorphic discontinuities in the Greater Himalayan Sequence in Central Himalaya: in-sequence shearing by accretion from the Indian plate

NASA Astrophysics Data System (ADS)

Carosi, Rodolfo

2016-04-01

The Greater Himalayan Sequence (GHS) is the main metamorphic unit of the Himalayas, stretching for over 2400 km, bounded to the South by the Main Central Thrust (MCT) and to the North by the South Tibetan Detachment (STD) whose contemporanous activity controlled its exhumation between 23 and 17 Ma (Godin et al., 2006). Several shear zones and/or faults have been recognized within the GHS, usually regarded as out of sequence thrusts. Recent investigations, using a multitechnique approach, allowed to recognize a tectonic and metamorphic discontinuity, localized in the mid GHS, with a top-to-the SW sense of shear (Higher Himalayan Discontinuity: HHD) (Carosi et al., 2010; Montomoli et al., 2013). U-(Th)-Pb in situ monazite ages provide temporal constraint of the acitivity of the HHD from ~ 27-25 Ma to 18-17 Ma. Data on the P and T evolution testify that this shear zone affected the tectono-metamorphic evolution of the belt and different P and T conditions have been recorded in the hanging-wall and footwall of the HHD. The HHD is a regional tectonic feature running for more than 700 km, dividing the GHS in two different portions (Iaccarino et al., 2015; Montomoli et al., 2015). The occurrence of even more structurally higher contractional shear zone in the GHS (above the HHD): the Kalopani shear zone (Kali Gandaki valley, Central Nepal), active from ~ 41 to 30 Ma (U-Th-Pb on monazite) points out to a more complex deformation pattern in the GHS characterized by in sequence shearing. The actual proposed models of exhumation of the GHS, based exclusively on the MCT and STD activities, are not able to explain the occurrence of the HHD and other in-sequence shear zones. Any model of the tectonic and metamorphic evolution of the GHS should account for the occurrence of the tectonic and metamorphic discontinuities within the GHS and its consequences on the metamorphic paths and on the assembly of Himalayan belt. References Godin L., Grujic D., Law, R. D. & Searle, M. P. 2006

Modeling the Evolution of Localized Strain in Orogenic Wedges: From Short-term Deformation to Long-term Tectonic States

NASA Astrophysics Data System (ADS)

Weiss, J. R.; Ito, G.; Brooks, B. A.; Olive, J. A. L.; Foster, J. H.; Howell, S. M.

2015-12-01

Some of the most destructive earthquakes on Earth are associated with active orogenic wedges. Despite a sound understanding of the basic mechanics that govern whole wedge structure over geologic time scales and a growing body of studies that have characterized the deformation associated with historic to recent earthquakes, first order questions remain about the linkage of the two sets of processes at the intermediate seismotectonic timescales. Numerical models have the power to test the effects of specific mechanical conditions on the evolution of observables at active orogenic wedges. Here we use a two-dimensional, continuum mechanics-based, finite difference method with a visco-elasto-plastic rheology coupled with surface processes to investigate the spatiotemporal distribution of deformation during wedge growth. The model simulates the contraction of a crustal layer overlying a weak base (décollement) against a rigid backstop and the spontaneous nucleation and evolution of fault zones due to cohesive, Mohr-Coulomb failure with strain weakening. Consistent with critical wedge theory, the average slope across the wedge is controlled by the relative frictional strengths of the wedge and décollement. Initial calculations predict changes in wedge deformation on short geologic timescales (103-105yrs) that involve episodes of widening as new, foreland-verging thrusts nucleate near the surface beyond the wedge toe and propagate down-dip to intersect the décollement. All the while, the wedge thickens via slip on older, internal fault zones. The aim of this study is to identify the parameters controlling the timescales of 1) episodic widening versus thickening and 2) nucleation and life-span of individual fault zones. These are initial steps needed to link earthquake observations to the long-term tectonic states inferred at various orogenic belts around the world.

Suspended sediment chemistry from large Himalayan Rivers

NASA Astrophysics Data System (ADS)

Tipper, E.; Bickle, M.; Bohlin, M.; Andermann, C.

2016-12-01

Recent work has demonstrated that weathering in areas with the highest physical erosion rates are the most sensitive to climatic feedback parameters (both rainfall and temperature) because they are not limited by a supply of material. The Himalayan region is central to this work because of 1) the high erosion rates, 2) high monsoonal rainfall, and 3) high temperatures in the Ganges plain in front of the main range, where much of the weathering takes place. The material that is weathered in the Ganges plain is delivered as sediment from the mountain front. Therefore, detailed understanding of the chemistry of the sediment leaving the high mountains is essential. Interest has been renewed not least because of the magnitude 7.8 (25/4/15) and 7.3 (12/5/2015) earthquakes in Nepal in 2015 which triggered thousands of landslides, likely causing major perturbations to sediment and chemical loads carried by the local Himalayan rivers. We collected both sediment and water samples in 2015 and 2016 in a transect across Nepal, including depth profiles of suspended sediment in the Narayani, Kosi and Karnali Rivers. The Narayani and Kosi rivers which drain the earthquake-hit area carry > 40% of the total bicarbonate flux input to the Ganges from the Himalayan mountains. Here we present our initial findings on the chemistry of the sediment from the 2015 and 2016 field seasons and compare it to published data sets.

Poverty, development, and Himalayan ecosystems.

PubMed

Sandhu, Harpinder; Sandhu, Sukhbir

2015-05-01

The Himalayas are rich in biodiversity but vulnerable to anthropogenic pressures. They are also host to growing number of rural poor who are dependent on forest and ecosystem services for their livelihood. Local and global efforts to integrate poverty alleviation and biodiversity conservation in the Himalayas remain elusive so far. In this work, we highlight two key impediments in achieving sustainable development in the Himalayas. On the positive side, we also highlight the work of Ashoka Trust for Research in Ecology and the Environment (ATREE), a research organization based in India that seeks to integrate biodiversity concerns with livelihood security. For impediments, we draw on two examples from the Darjeeling district, India, in Eastern Himalayan region to illustrate how development organizations are failing to simultaneously address poverty and environmental issues. Based on the success of ATREE, we then propose a conceptual framework to integrate livelihood generating activities with sustainable and equitable development agenda. We recommend developing a Hindu-Kush Himalayan Ecosystem Services Network in the region to formulate a strategy for further action. We conclude by offering measures to address the challenge of integrating livelihood and environment issues through this network.

Map and Database of Probable and Possible Quaternary Faults in Afghanistan

USGS Publications Warehouse

Ruleman, C.A.; Crone, A.J.; Machette, M.N.; Haller, K.M.; Rukstales, K.S.

2007-01-01

The U.S. Geological Survey (USGS) with support from the U.S. Agency for International Development (USAID) mission in Afghanistan, has prepared a digital map showing the distribution of probable and suspected Quaternary faults in Afghanistan. This map is a key component of a broader effort to assess and map the country's seismic hazards. Our analyses of remote-sensing imagery reveal a complex array of tectonic features that we interpret to be probable and possible active faults within the country and in the surrounding border region. In our compilation, we have mapped previously recognized active faults in greater detail, and have categorized individual features based on their geomorphic expression. We assigned mapped features to eight newly defined domains, each of which contains features that appear to have similar styles of deformation. The styles of deformation associated with each domain provide insight into the kinematics of the modern tectonism, and define a tectonic framework that helps constrain deformational models of the Alpine-Himalayan orogenic belt. The modern fault movements, deformation, and earthquakes in Afghanistan are driven by the collision between the northward-moving Indian subcontinent and Eurasia. The patterns of probable and possible Quaternary faults generally show that much of the modern tectonic activity is related to transfer of plate-boundary deformation across the country. The left-lateral, strike-slip Chaman fault in southeastern Afghanistan probably has the highest slip rate of any fault in the country; to the north, this slip is distributed onto several fault systems. At the southern margin of the Kabul block, the style of faulting changes from mainly strike-slip motion associated with the boundary between the Indian and Eurasian plates, to transpressional and transtensional faulting. North and northeast of the Kabul block, we recognized a complex pattern of potentially active strike-slip, thrust, and normal faults that form a

Fluid-driven normal faulting earthquake sequences in the Taiwan orogen

NASA Astrophysics Data System (ADS)

Wang, Ling-hua; Rau, Ruey-Juin; Lee, En-Jui

2017-04-01

Seismicity in the Central Range of Taiwan shows normal faulting mechanisms with T-axes directing NE, subparallel to the strike of the mountain belt. We analyze earthquake sequences occurred within 2012-2015 in the Nanshan area of northern Taiwan which indicating swarm behavior and migration characteristics. We select events larger than 2.0 from Central Weather Bureau catalog and use the double-difference relocation program hypoDD with waveform cross-correlation in the Nanshan area. We obtained a final count of 1406 (95%) relocated earthquakes. Moreover, we compute focal mechanisms using USGS program HASH by P-wave first motion and S/P ratio picking and 114 fault plane solutions with M 3.0-5.87 were determined. To test for fluid diffusion, we model seismicity using the equation of Shapiro et al. (1997) by fitting earthquake diffusing rate D during the migration period. According to the relocation result, seismicity in the Taiwan orogenic belt present mostly N25E orientation parallel to the mountain belt with the same direction of the tension axis. In addition, another seismic fracture depicted by seismicity rotated 35 degree counterclockwise to the NW direction. Nearly all focal mechanisms are normal fault type. In the Nanshan area, events show N10W distribution with a focal depth range from 5-12 km and illustrate fault plane dipping about 45-60 degree to SW. Three months before the M 5.87 mainshock which occurred in March, 2013, there were some foreshock events occurred in the shallow part of the fault plane of the mainshock. Half a year following the mainshock, earthquakes migrated to the north and south, respectively with processes matched the diffusion model at a rate of 0.2-0.6 m2/s. This migration pattern and diffusion rate offer an evidence of 'fluid-driven' process in the fault zone. We also find the upward migration of earthquakes in the mainshock source region. These phenomena are likely caused by the opening of the permeable conduit due to the M 5

Himalayan orogeny and palaeovegetational changes: a relook into the factors controlling global expansion of C4 grasslands.

NASA Astrophysics Data System (ADS)

Singh, S.; Awasthi, A.; Parkash, B.; Kumar, S.

2012-04-01

The Himalayan orogeny constitutes a significant tectonic event in the Earth's Cenozoic history which encompasses a series of events resulting in long-term climatic cooling and drying. Establishing synchroneity of palaeoecological events through floral and faunal changes in proxy-records could help in documenting factors responsible for this change in global climate. Based on geological evidences, various workers in different parts of the world have established C4 grassland appearance during late Cenozoic, though the expansion is confined largely to Late Miocene. However, causes of this worldwide C4 grassland expansion have remained controversial since its discovery. Resolution of such controversies ultimately lies in undertaking more detailed local palaeo-vegetational studies of Cenozoic sediments and subsequent correlation at regional and global scale. The aim of the present work is to study the Himalayan Cenozoic sediments of India and the results are then compared with other similar studies done in different parts of the world. Carbon isotope analysis of soil carbonate, largely nodules, had been carried out from Samba-Mansar (S-M) section in the Jammu & Kashmir state of India which is placed laterally ~ 40-50Km along strike from another comparable Jammu-Nandni (J-N) section. Analyses of a total of 141 samples in the Ramnagar sub-basin, spanning a period from ~ 12Ma to ~ 0.4Ma, have been coherent so as to have a better view of palaeovegetational change across the sub-basin, both at comparable temporal and spatial regional scale. Herein the isotopic results show the dominance of C3 vegetation pre-7Ma and C4 vegetation post-5Ma with first appearance of C4 plants at ~6.8Ma. Percentage abundance of C4 vegetation was less than 20% pre-7Ma but was increased to more than 40% post-5Ma reaching up to 100% in the youngest analyzed sediments. The results are in conformity with patterns of change in vegetation documented in other parts of the Himalayan belt. These indicate

On the potential asthenospheric linkage between Apenninic slab rollback and Alpine topographic uplift: insights from P wave tomography and seismic anisotropy analysis

NASA Astrophysics Data System (ADS)

Malusa', Marco Giovanni; Salimbeni, Simone; Zhao, Liang; Guillot, Stéphane; Pondrelli, Silvia; Margheriti, Lucia; Paul, Anne; Solarino, Stefano; Aubert, Coralie; Dumont, Thierry; Schwartz, Stéphane; Wang, Qingchen; Xu, Xiaobing; Zheng, Tianyu; Zhu, Rixiang

2017-04-01

The role of surface and deep-seated processes in controlling the topography of complex plate-boundary areas is a highly debated issue. In the Western Alps, which include the highest summits in Europe, factors controlling topographic uplift still remain poorly understood. In the absence of active convergence, recent works have suggested a potential linkage between slab breakoff and fast uplift, but this hypothesis is ruled out by the down-dip continuity of the Alpine slab documented by recent tomographic images of the upper mantle beneath the Alpine region (Zhao et al. 2016). In order to shed light on this issue, we use a densely spaced array of temporary broadband seismic stations and previously published observations to analyze the seismic anisotropy pattern along the transition zone between the Alps and the Apennines, within the framework of the upper mantle structure unveiled by P wave tomography. Our results show a continuous trend of anisotropy fast axes near-parallel to the western alpine arc, possibly due to an asthenospheric counterflow triggered by the eastward retreat of the Apenninic slab. This trend is located in correspondence of a low velocity anomaly in the European upper mantle, and beneath the Western Alps region characterized by the highest uplift rates, which may suggest a potential impact of mantle dynamics on Alpine topography. We propose that the progressive rollback of the Apenninic slab induced a suction effect and an asthenospheric counterflow at the rear of the unbroken Alpine slab and around its southern tip, as well as an asthenospheric upwelling, mirrored by low P wave velocities, which may have favored the topographic uplift of the Alpine belt from the Mt Blanc to the Ligurian coast. Zhao L. et al., 2016. Continuity of the Alpine slab unraveled by high-resolution P wave tomography. J. Geophys. Res., doi:10.1002/2016JB013310.

Impacts after four years of experimental trampling on alpine/sub-alpine environments in western Tasmania.

PubMed

Whinam, Jennie; Chilcott, Nicole M

2003-04-01

Experimental trials were undertaken over four years to assess the impact of recreational trampling in undisturbed alpine and sub-alpine vegetation communities in the Western Arthur Range, western Tasmania. Data on 'pad' formation due to human trampling were collected using vegetation cover assessments, biomass estimates and detailed cross-sectional surface profiles. In sub-alpine buttongrass and alpine herbfield, prolonged and sustained damage may occur after 100 passes by walkers. The environmental threshold of the flat alpine herbfield site was breached after 200 passes. Plant morphology was one determinant of resistance and resilience, with upright woody shrubs and tall tussock graminoids most vulnerable to sustained trampling damage. Cushions are susceptible to trampling impacts at 500 passes. Loss of vegetation cover peaks 6-12 months after trampling. Our results show that pads formed with as few as 30-100 passes per annum and tracks form at between 100 and 500 passes per annum. Two years after the cessation of trampling, there is some small recovery in vegetation cover after 30 and 100 passes per annum applied for three years, but no evidence of recovery at the 500 pass treatments. The low trampling threshold and slow recovery rates in western Tasmania suggest that concentrating walkers on a minimal number of sites may be the best management option for these untracked alpine and sub-alpine environments.

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

A Mesozoic orogenic cycle from post-collision to subduction in the southwestern Korean Peninsula: New structural, geochemical, and chronological evidence

NASA Astrophysics Data System (ADS)

Park, Seung-Ik; Kwon, Sanghoon; Kim, Sung Won; Hong, Paul S.; Santosh, M.

2018-05-01

The Early to Middle Mesozoic basins, distributed sporadically over the Korean Peninsula, preserve important records of the tectonic history of some of the major orogenic belts in East Asia. Here we present a comprehensive study of the structural, geochemical, geochronological, and paleontological features of a volcano-sedimentary package, belonging to the Oseosan Volcanic Complex of the Early to Middle Mesozoic Chungnam Basin, within the Mesozoic subduction-collision orogen in the southwestern Korean Peninsula. The zircon U-Pb data from rhyolitic volcanic rocks of the complex suggest Early to Middle Jurassic emplacement age of ca. 178-172 Ma, harmonious with plant fossil taxa found from the overlying tuffaceous sedimentary rock. The geochemical data for the rhyolitic volcanic rocks are indicative of volcanic arc setting, implying that the Chungnam Basin has experienced an intra-arc subsidence during the basin-expanding stage by subduction of the Paleo-Pacific (Izanagi) Plate. The Jurassic arc-related Oseosan Volcanic Complex was structurally stacked by the older Late Triassic to Early Jurassic post-collisional basin-fill of the Nampo Group by the Jangsan fault during basin inversion. The Late Jurassic to Early Cretaceous K-feldspar and illite K-Ar ages marked the timing of inversion tectonics, contemporaneous with the magmatic quiescence in the southern Korean Peninsula, likely due to flat-lying or low-angle subduction. The basin evolution history preserved in the Mesozoic Chungnam Basin reflects a Mesozoic orogenic cycle from post-collision to subduction in the southwestern Korean Peninsula. This, in turn, provides a better understanding of the spatial and temporal changes in Mesozoic tectonic environments along the East Asian continental margin.

Strain distribution across a partially molten middle crust: Insights from the AMS mapping of the Carlos Chagas Anatexite, Araçuaí belt (East Brazil)

NASA Astrophysics Data System (ADS)

Cavalcante, Geane C. G.; Egydio-Silva, Marcos; Vauchez, Alain; Camps, Pierre; Oliveira, Eurídice

2013-10-01

The easternmost part of the Neoproterozoic Araçuaí belt comprises an anatectic domain that involves anatexites (the Carlos Chagas unit), leucogranites and migmatitic granulites that display a well-developed fabric. Microstructural observations support that the deformation occurred in the magmatic to submagmatic state. Structural mapping integrating field and anisotropy of magnetic susceptibility (AMS) revealed a complex, 3D structure. The northern domain displays gently dipping foliations bearing a NW-trending lineation, southward, the lineation trend progressively rotates to EW then SW and the foliation is gently folded. The eastern domain displays E-W and NE-SW trending foliations with moderate to steeply dips bearing a dominantly NS trending lineation. Magnetic mineralogy investigation suggests biotite as the main carrier of the magnetic susceptibility in the anatexites and ferromagnetic minerals in the granulites. Crystallographic preferred orientation (CPO) measurements using the electron backscatter diffraction (EBSD) technique suggest that the magnetic fabric comes from the crystalline anisotropy of biotite and feldspar grains, especially. The delineation of several structural domains with contrasted flow fabric suggests a 3D flow field involving westward thrusting orthogonal to the belt, northwestward orogen-oblique escape tectonics and NS orogen-parallel flow. This complex deformation pattern may be due to interplay of collision-driven and gravity-driven deformations.

Localized collision vs regional heating: the paradoxical aspects of 2720-2670 Ma geological evolution in the Kaapvaal craton, southern Africa.

NASA Astrophysics Data System (ADS)

Vezinet, Adrien; Nicoli, Gautier; Moyen, Jean-François; Laurent, Oscar; Stevens, Gary

2014-05-01

The Kaapvaal craton (KC) in South Africa, one of the oldest cratonic nuclei on Earth, is bounded to the North by the Limpopo belt, whose Southern Marginal Zone (SMZ) is regarded as a ca. 2.7Ga collisional orogeny. This is substantiated by structural data, such as the south verging thrust system that bounds the SMZ to the South (HRSZ); metamorphic data, demonstrating that metapelites from the SMZ underwent a very fast (˜40Ma) clockwise PTt loop at ca. 2.7Ga, with a peak of metamorphism dated at 2713±8Ma. The SMZ is therefore interpreted as representing a partially molten orogenic channel behaving like modern Himalaya. However, a review of geochronological data of Limpopo Belt, KC and Zimbabwean Craton shows that geological activity at around 2.7Ga was not restricted to the Limpopo belt, but was instead scattered throughout the KC. That is not in agreement with a Himalayan model but could be a response to a general thermal event, which is recorded by: 1) The massive flood basalts of the Vendersdorp Supergroup in central South Africa. This supergroup crops out from Johannesburg in the North to Prieska in the South and records the most widespread sequence of volcanic rocks of the KC. This massive basaltic extrusion occurs between 2714±8Ma (Klipriviersberg Group) and ˜2650Ma (Transvaal Supergroup lower limit). 2) A series of granitic plutons immediately south of the HRSZ, emplaced at 2670-2680Ma; all of them include a mantle-derived component whose composition is similar to the Ventersdorp basalts. A similar and synchronous pluton, the Mashishimale, emplaces South-East of the HRSZ. 3) Further afield, in the Eastern KC, slightly older plutons (I-type (high-Ca) granitoids) emplace in Swaziland between 2720 and 2700Ma. 4) Granulite-facies metamorphism in Swaziland at ca. 2.7Ga. 5) Finally, in the Zimbabwe Craton intermediate to felsic lavas erupted at the same time as well as gneisses and granitoids from northern Botswawa However, interactions of Zimbabwe craton with

Carboniferous - Early Permian magmatic evolution of the Bogda Range (Xinjiang, NW China): Implications for the Late Paleozoic accretionary tectonics of the SW Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Wali, Guzalnur; Wang, Bo; Cluzel, Dominique; Zhong, Linglin

2018-03-01

The Late Paleozoic magmatic evolution of the Bogda Range (Chinese North Tianshan) is important for understanding the accretionary history of the Central Asian Orogenic Belt. We investigated the Carboniferous and Lower Permian volcanic and sedimentary sequences of the Daheyan section, southern Bogda Range, and present new zircon U-Pb ages and whole-rock geochemical data for the volcanic rocks. One Carboniferous rhyolite is dated at 298 ± 8 Ma; a Permian basalt yielded many Proterozoic zircon xenocrysts, and its maximum age (∼297 Ma) is constrained by the detrital zircon ages of the sandstone that stratigraphically underlies it. These volcanic rocks belong to calc-alkaline series. We further synthesize previous geochronological, geochemical and isotopic data of magmatic and sedimentary rocks in the Bogda Range. The available data indicate that the magmatism occurred continuously from 350 Ma to 280 Ma. A comprehensive analysis allows us to propose that: (1) the Carboniferous to Early Permian magmatic rocks of the Bogda Range generally show consistent arc-type features; (2) increasing mantle input through time suggests intra-arc extension in a supra-subduction zone; (3) the localized occurrence of Early Permian alkaline pillow basalts and deep water sediments close to the major shear zone advocate a transtensional crustal thinning during the transition from Carboniferous convergence to Early Permian transcurrent tectonics; (4) occurrence of a large number of Proterozoic zircon xenocrysts in the Late Paleozoic magmatic rocks, and Proterozoic detrital zircons in the coeval clastic sediments suggest a continental or transitional basement of the Bogda Arc; (5) subduction in the Bogda area terminated prior to the deposition of Middle Permian terrestrial sediments.

Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin)

NASA Astrophysics Data System (ADS)

Lupker, Maarten; France-Lanord, Christian; Galy, Valier; Lavé, Jérôme; Gaillardet, Jérôme; Gajurel, Ananta Prasad; Guilmette, Caroline; Rahman, Mustafizur; Singh, Sunil Kumar; Sinha, Rajiv

2012-05-01

We present an extensive river sediment dataset covering the Ganga basin from the Himalayan front downstream to the Ganga mainstream in Bangladesh. These sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column. Mineral sorting is the first order control on the chemical composition of river sediments. Taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain. By comparing sediments sampled at the Himalayan front with sediments from the Ganga mainstream in Bangladesh it is possible to budget weathering in the floodplain. Assuming a steady state weathering regime in the floodplain, the weathering of Himalayan sediments in the Gangetic floodplain releases ca. (189 ± 92) × 109 and (69 ± 22) × 109 mol/yr of carbonate bound Ca and Mg to the dissolved load, respectively. Silicate weathering releases (53 ± 18) × 109 and (42 ± 13) × 109 mol/yr of Na and K while the release of silicate Mg and Ca is substantially lower, between ca. 0 and 20 × 109 mol/yr. Additionally, we show that sediment hydration, [H2O+], is a sensitive tracer of silicate weathering that can be used in continental detrital environments, such as the Ganga basin. Both [H2O+] content and the D/H isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions. By comparing the chemical composition of river sediments across the floodplain with the composition of the eroded Himalayan source rocks, we suggest that the floodplain is the dominant location of silicate weathering for Na, K and [H2O+]. Overall this work emphasizes the role of the Gangetic floodplain in weathering Himalayan sediments. It also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into

Olivine fabrics and tectonic evolution of fore-arc mantles: A natural perspective from the Songshugou dunite and harzburgite in the Qinling orogenic belt, central China

NASA Astrophysics Data System (ADS)

Cao, Yi; Jung, Haemyeong; Song, Shuguang

2017-03-01

To advance our understanding of the deformation characteristics, rheological behaviors, and tectonic evolution of the fore-arc lithospheric mantle, we analyzed mineral fabrics for a large spinel-bearing ultramafic massif in the Songshugou area in the Qinling orogenic belt, central China. In the spinel-poor coarse-grained dunite, stronger A/D-type and weaker C-type-like fabrics were found, whereas the spinel-rich coarse-grained dunite displayed a comparatively stronger B-type-like fabric. These olivine fabrics are high-T fabrics influenced by the presence of melt, in which B and C-type-like fabrics are inferred to be produced by melt-assisted grain boundary sliding during synkinematic high-T melt-rock reactions. In contrast, the spinel-poor porphyroclastic and fine-grained dunites present weak AG and B-type-like fabrics, respectively. Their olivine fabrics (low-T fabrics) are inferred to transform from A/D-type fabric in their coarse-grained counterparts possibly through mylonitization process assisted by low-T fluid-rock reactions, during which strain was accommodated by the fluid-enhanced dislocation slip and/or fluid-assisted grain boundary sliding processes. Combined with the tectonic results of our previous work, the high-T olivine fabrics are probably related to a young and warm fore-arc mantle where intense partial melting and high-T boninitic melt-rock reactions prevalently occurred, whereas the low-T olivine fabrics likely reflect the evolving tectonic settings through the cooling fore-arc mantle to a continental lower crust in a collisional orogeny where low-T fluid-rock reactions were pervasively activated. These low-T olivine fabrics imply that though cold, the fore-arc lithospheric mantle may be locally weak (˜20-30 MPa), allowing ductile deformation to occur at a geologically significant strain rate.

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

NASA Astrophysics Data System (ADS)

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

2017-07-01

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

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.

A Review of Tectonic Models and Analytical Data from Almora-Dadeldhura Klippe, Northwest India and Far Western Nepal.

NASA Astrophysics Data System (ADS)

Bosu, S.; Robinson, D.; Saha, A.

2017-12-01

Tectonic models developed from the Himalayan thrust belt constitute three models- critical taper, channel flow and wedge extrusion. Their differences are manifested in predicted minimum shortening, deformation propagation style and tectonic architecture across the thrust belt. Recent studies from isolated synformal klippen composed of Greater and Tethyan Himalayan rock within the Himalayan thrust belt disagree over the tectonic history, especially in the Almora-Dadeldhura klippe, which is the largest klippe in the thrust belt. These recent studies are limited to one transect each, and two or fewer types of analytical data to justify their models. Due to the limited spatial coverage, these studies often reflect a narrow perspective in their tectonic models; thus, combining the data from these studies provides a holistic view of the regional tectonic history. This study compiled the available data across the 350 km wide Almora-Dadeldhura klippe, using petrology, stratigraphy, metamorphic history, microstructure, U-Pb ages of intrusive granite, monazite and muscovite ages of the shear zones, and exhumation ages from apatite fission track, along with original field observations, microstructure and microtexture data from 5 different transects in northwest India and far western Nepal. The review of the compiled data suggests that the Himalayan thrust belt in northwest India and far western Nepal is a forward propagating thrust system, and that the analytical data support the critical taper model.

Inferred Early Permian Arc Rifting in Bogda Mountain, Southernmost of the Central Asia Orogenic Belt: Evidence from a Peperite Bearing Volcano-Sedimentary Succession

NASA Astrophysics Data System (ADS)

Memtimin, M.; Guo, Z.

2017-12-01

Late Paleozoic tectonic history, especially Carboniferous-Permian periods, of the Central Asia Orogenic Belt (CAOB) is considered to be the turning point for the termination of terrane amalgamation and closure of the Paleoasian Ocean. However, the debate about the paleoenvironment and tectonic setting of the region during the period is still not resolved. In this study, we report a set of volcano-sedimentary sequence in the Bogda Mountain of the southernmost of CAOB, which is associated with contemporaneous subaqueous emplacement of and interaction between mafic lava and carbonate sediments. The succession contains four distinct facies including closely packed pillow basalts, pillow basalts with interstitial materials, hyaloclastites and peperites. We discuss their formation and emplacement mechanism, interaction between hot magma-water/unconsolidated sediments and thermal metamorphism during the interaction. Textural features of the sequence, especially hyaloclastites and peperites, provide clear evidence for in situ autofragmentation of lava flows, synvolcanic sedimentation of carbonates, fuel coolant interaction when hot magma bulldozed into wet unconsolidated sediments, and represent autochthonous origin of the succession. Lateral transition of the lithofacies indicate a progressively deepening subaqueous environment, resembling a stepwise evolution from early stage of volcanic intrusion with lower lava flux in shallower water level to increasingly subsiding basin with more lava flux in greater depth. Previous studies determined that the mafic magma was intruded around the Carboniferous-Permian boundary ( 300Ma), and geochemical studies showed the magma was originated from dry depleted mantle with little crustal contamination. Nevertheless, the succession was thought to be fault related allochthones formation which was transferred in as part of a Carboniferous intraplate arc. Combining our findings with the previous study results, we propose a new model to

Ancient Himalayan wolf (Canis lupus chanco) lineage in Upper Mustang of the Annapurna Conservation Area, Nepal

PubMed Central

Chetri, Madhu; Jhala, Yadvendradev V.; Jnawali, Shant R.; Subedi, Naresh; Dhakal, Maheshwar; Yumnam, Bibek

2016-01-01

Abstract The taxonomic status of the wolf (Canis lupus) in Nepal’s Trans-Himalaya is poorly understood. Recent genetic studies have revealed the existence of three lineages of wolves in the Indian sub-continent. Of these, the Himalayan wolf, Canis lupus chanco, has been reported to be the most ancient lineage historically distributed within the Nepal Himalaya. These wolves residing in the Trans-Himalayan region have been suggested to be smaller and very different from the European wolf. During October 2011, six fecal samples suspected to have originated from wolves were collected from Upper Mustang in the Annapurna Conservation Area of Nepal. DNA extraction and amplification of the mitochondrial (mt) control region (CR) locus yielded sequences from five out of six samples. One sample matched domestic dog sequences in GenBank, while the remaining four samples were aligned within the monophyletic and ancient Himalayan wolf clade. These four sequences which matched each other, were new and represented a novel Himalayan wolf haplotype. This result confirms that the endangered ancient Himalayan wolf is extant in Nepal. Detailed genomic study covering Nepal’s entire Himalayan landscape is recommended in order to understand their distribution, taxonomy and, genetic relatedness with other wolves potentially sharing the same landscape. PMID:27199590

Responses Of Alpine Vegetation To Global Warming: Insights From Comparing Alpine-Restricted And Broad-Ranging Herbs Along Snowmelt Gradients

NASA Astrophysics Data System (ADS)

Butz, R. J.; Reinhardt, K. S.; Germino, M. J.; Kueppers, L. M.

2009-12-01

Many alpine plant species face habitat fragmentation and loss, and even extinction because their narrow elevation, precipitation, and temperature tolerances limit their geographic distribution. In order to assess the impacts of climate change on sensitive native alpine communities we used a variety of methods to look at the seasonal timing of life stages (phenology) and the stress responses (physiology) of alpine species along a natural environmental gradient at Niwot Ridge in the Colorado Rocky Mountains to address the following question: Will alpine plants be impaired in their existing range as a result of climate change? We collected data on date of snowmelt and vegetative and flowering phenology of all alpine species present from snowmelt to senescence in 80 1m2 plots above treeline. In addition, we measured soil temperature and moisture, plant water potential and leaf-level gas exchange early, mid, and late-season on three alpine-restricted and three broader-ranging alpine species: Geum rossii, Artemisia scopulorum, Carex rupestris, Lewisia pygmaea, Tetraneuris grandiflora, and Sibbaldia procumbens. In 2009, the natural variation in snowmelt timing was 40 days (approximately 5.5 weeks) over the 80 plots. Our results suggest that with earlier snowmelt, the number of vascular species per plot increases. However, this increase is almost exclusively attributable to wider ranging species not restricted to the alpine. Plots with intermediate natural snowmelt dates had a higher diversity of alpine-restricted species, photosynthesis, and water-use efficiency, thereby potentially increasing long-term survival rates amongst alpine species. Water stress increased in all species as the season progressed, especially in plots where snow melted earliest. Photosynthetic productivity and diversity of alpine-restricted species was greatest in plots having intermediate melt dates. These findings suggest that shifts in snowmelt date under a warming climate will likely impact the

Taking a step back: Himalayan erosion as seen from Bangladesh

NASA Astrophysics Data System (ADS)

Lupker, M.; France-Lanord, C.; Lavé, J.; Blard, P.; Galy, V.

2012-12-01

The Himalayan range represents the archetype of mountain building and is considered in many studies as the locus of intense interactions between climate, denudation and tectonics. A better understanding of these interactions requires that the flux of material removed from the system through erosion is known. The products of Himalayan erosion are exported to the Bengal fan and the Indian Ocean by two major rivers: the Ganga and Brahmaputra. These rivers provide the opportunity to quantify the Himalayan denudation rates as they integrate surface and tectonic processes across the entire basin. Basin wide erosion or denudation rates have classically been derived from the gauging of sediments fluxes. By coping with the inherent spatial and temporal variability of sediment concentration in rivers, sediment budgets yield average denudation rate over the observational period ranging from years to decades. Cosmogenic nuclides such as 10-Be allow the estimation of basin-wide denudation rates averaged over typical time scales of hundreds to thousand of years, from a single measurement in river sediments. We compare these methods for the case of the Ganga basin that drains the central part of the Himalayan range. By using a distal point of view, i.e. by sampling and evaluating the sediment flux at the outlet of the Ganga in Bangladesh we are able to propose an average denudation rate of the entire, central part of the Himalayan range. This sampling location offers the benefit of integrating the entire basin and its distance from the sediment source makes it also less prone to perturbations in the headwaters. However, the effects of 500 to 1000 km floodplain transfer on the sedimentary signal needs to be correctly evaluated. The gauged sediment flux can mainly be impacted by the sequestration of sediments in the floodplain. For the Ganga basin, sequestration is limited to ca. 10 % of the eroded sediment flux as deduced from geochemical mass balance approaches [1]. On their side

Partial melting of amphibolites in the Eastern Segment of the Sveconorwegian orogen, southern Sweden.

NASA Astrophysics Data System (ADS)

Brophy, E.; Hansen, E. C.; Möller, C.; Huffman, M.

2017-12-01

Mafic migmatites with amphibolitic melanosome and tonalitic leucosome are a common feature in continental collision orogenic zones. However, the anatexis of mafic rocks has received much less attention than anatexis in felsic, intermediate or pelitic compositions. We examined mafic migmatites along a traverse within the Eastern Segment of the 1.14-0.9 Ga Sveconorwegian orogen, between Forsheda and Fegen southern Sweden. This traverse occurs in the center of a >150 km metamorphic transition from sub-greenschist facies in the east to high-pressure granulite and eclogite facies in the west (Möller and Andersson, unpublished metamorphic map). The Eastern Segment is a parautochthonous belt made up of rocks of the Fennoscandian shield that were deformed and metamorphosed during the Sveconorwegian orogeny. Within the traverse amphibolite bodies occur within migmatitic felsic to intermediate orthogneisses. The first appearance of tonalitic leucosome in amphibolite was observed towards the eastern edge of the traverse and continued to occur sporadically westward ranging in abundance (by outcrop area) from 0 to 25 %. The mineral assemblage in amphibolite is hbl + plag ( An30) + qtz + bt ± grt ± ilm ± ttn ± py ± SO2-rich scp. No examples of peritectic pyroxene associated with leucosome were found. The lack of peritectic pyroxene suggests that a water-rich phase was present at the onset of anatexis. The highly variable amount of leucosome further suggests that the amount of melt generated was determined by the amount of water available. Together these suggest that partial was driven by the local influx of a water-rich fluid. In the higher grade portions further west migmatitic amphibolite with tonalitic leucosome occurs in two varieties: one with peritectic pyroxene and relatively small amounts of leucosome, interpreted as forming by water-undersaturated dehydration melting, and another without peritectic pyroxene and with larger amounts of leucosome which is interpreted

Paleogene palaeogeography and basin evolution of the Western Carpathians, Northern Pannonian domain and adjoining areas

NASA Astrophysics Data System (ADS)

Kováč, Michal; Plašienka, Dušan; Soták, Ján; Vojtko, Rastislav; Oszczypko, Nestor; Less, György; Ćosović, Vlasta; Fügenschuh, Bernhard; Králiková, Silvia

2016-05-01

The data about the Paleogene basin evolution, palaeogeography, and geodynamics of the Western Carpathian and Northern Pannonian domains are summarized, re-evaluated, supplemented, and newly interpreted. The presented concept is illustrated by a series of palinspastic and palaeotopographic maps. The Paleogene development of external Carpathian zones reflects gradual subduction of several oceanic realms (Vahic, Iňačovce-Kričevo, Szolnok, Magura, and Silesian-Krosno) and growth of the orogenic accretionary wedge (Pieniny Klippen Belt, Iňačovce-Kričevo Unit, Szolnok Belt, and Outer Carpathian Flysch Belt). Evolution of the Central Western Carpathians is characterized by the Paleocene-Early Eocene opening of several wedge-top basins at the accretionary wedge tip, controlled by changing compressional, strike-slip, and extensional tectonic regimes. During the Lutetian, the diverging translations of the northward moving Eastern Alpine and north-east to eastward shifted Western Carpathian segment generated crustal stretching at the Alpine-Carpathian junction with foundation of relatively deep basins. These basins enabled a marine connection between the Magura oceanic realm and the Northern Pannonian domain, and later also with the Dinaridic foredeep. Afterwards, the Late Eocene compression brought about uplift and exhumation of the basement complexes at the Alpine-Carpathian junction. Simultaneously, the eastern margin of the stretched Central Western Carpathians underwent disintegration, followed by opening of a fore-arc basin - the Central Carpathian Paleogene Basin. In the Northern Hungarian Paleogene retro-arc basin, turbidites covered a carbonate platform in the same time. During the Early Oligocene, the rock uplift of the Alpine-Carpathian junction area continued and the Mesozoic sequences of the Danube Basin basement were removed, along with a large part of the Eocene Hungarian Paleogene Basin fill, while the retro-arc basin depocentres migrated toward the east

Lithologically controlled strength variation and the Himalayan megathrust geometry: an analogue modeling approach

NASA Astrophysics Data System (ADS)

Ghosh, Subhajit; Das, Animesh; Bose, Santanu; Mandal, Nibir

2017-04-01

A moment magnitude (Mw) 7.8 earthquake associated with a Mw 7.3 aftershock hit the Gorkha region near Kathmandu, Nepal on April 25, 2015. The rupture propagated eastward for about 140 km and caused thousands of deaths. The focal mechanism of the Gorkha earthquake shows thrust sense over the mid-crustal steeply dipping ramp on the basal décollement known as the Main Himalayan Thrust (MHT). The MHT is the largest and fastest slipping continental megathrust over which the southward tapering Himalayan thrust wedge similar to the accretionary wedges is moving. The MHT ramps up to the surface beneath the Siwalik group of rocks as the Main Frontal Thrust (MFT). Below the MFT the basal décollement is flat until it reaches the mid-crustal ramp ( 20°) below the Himalayan klippen and then again it becomes flat. This geometry of the décollement is consistent with the balanced cross sections, microseismic data, magnetotelluric images, INDEPTH seismic reflection profile, present day stress distribution and fits well with the prominent topographic break (physiographic transition) in the Lesser Himalaya. Lithologically stratified sedimentary sequences in the upper crust are mechanically heterogeneous. It has been long known that the mechanical properties of the stratigraphic succession influence the resultant structural architecture of the fold and thrust belts. The rheologically weak stratigraphic horizon generally contains the basal décollement due to its relatively low frictional strength. Hence, any vertical or lateral change in frictional property may control the effective strength and the positions of the décollement in space. In the present study, we used non-cohesive sand and mica dust layers as analogue materials for simulating the strong and weak layers respectively in the sandbox apparatus. Experimental results with relatively high basal friction (μ=0.46) show that such a weak horizon at a shallow depth perturbs the sequential thrust progression, and forces a

Constraining the strength of megathrusts from fault geometries and application to the Alpine collision zone

NASA Astrophysics Data System (ADS)

Dielforder, Armin

2017-09-01

Using Coulomb wedge solutions, we show that the effective strength of megathrusts (μb‧) can be determined from the geometry of out-of-sequence thrusts cutting through an accretionary or orogenic wedge. The method is first tested on central Chilean margin for which it yields a frictional strength of μb‧ = 0.053 (+ 0.043 / - 0.024). The inferred value agrees well with previous strength estimates and with the tectonic response of the central Chilean wedge to 2010 Mw 8.8 Maule earthquake. We then use the approach to constrain the strength of the collision megathrust of the central European Alps ∼30-20 million years ago. We find that the collision megathrust had a strength of μb‧ = 0.065 (+ 0.035 / - 0.026), which is similarly low than the strength of subduction megathrusts. The result is integrated into a static force balance model to examine potential implications of a weak megathrust for the Alpine orogeny. The model results suggest that the Alpine megathrust supported a mean maximum elevation of ∼2,000 m and that growth of the wedge up to this elevation supported a switch from contractional to extensional tectonics in the interior of the Alps around 20 Ma. Finally, using the example of the Himalayas, we show how the strength of megathrusts may be also derived from the geometry of crustal ramps, which provides a valuable alternative if details on out-of-sequence thrusts are missing.

Subduction initiation, recycling of Alboran lower crust, and intracrustal emplacement of subcontinental lithospheric mantle in the Westernmost Mediterranean

NASA Astrophysics Data System (ADS)

Varas-Reus, María Isabel; Garrido, Carlos J.; Bosch, Delphine; Marchesi, Claudio; Hidas, Károly; Booth-Rea, Guillermo; Acosta-Vigil, Antonio

2015-04-01

Unraveling the tectonic settings and processes involved in the annihilation of subcontinental mantle lithosphere is of paramount importance for our understanding of the endurance of continents through Earth history. Unlike ophiolites -- their oceanic mantle lithosphere counterparts -- the mechanisms of emplacement of the subcontinental mantle lithosphere in orogens is still poorly known. The emplacement of subcontinental lithospheric mantle peridotites is often attributed to extension in rifted passive margins or continental backarc basins, accretionary processes in subduction zones, or some combination of these processes. One of the most prominent features of the westernmost Mediterranean Alpine orogenic arcs is the presence of the largest outcrops worldwide of diamond facies, subcontinental mantle peridotite massifs; unveiling the mechanisms of emplacement of these massifs may provide important clues on processes involved in the destruction of continents. The western Mediterranean underwent a complex Alpine evolution of subduction initiation, slab fragmentation, and rollback within a context of slow convergence of Africa and Europe In the westernmost Mediterranean, the alpine orogeny ends in the Gibraltar tight arc, which is bounded by the Betic, Rif and Tell belts that surround the Alboran and Algero-Balearic basins. The internal units of these belts are mostly constituted of an allochthonous lithospheric domain that collided and overthrusted Mesozoic and Tertiary sedimentary rocks of the Mesozoic-Paleogene, South Iberian and Maghrebian rifted continental paleomargins. Subcontinental lithospheric peridotite massifs are intercalated between polymetamorphic internal units of the Betic (Ronda, Ojen and Carratraca massifs), Rif (Beni Bousera), and Tell belts. In the Betic chain, the internal zones of the allochthonous Alboran domain include, from bottom to top, polymetamorphic rock of the Alpujarride and Malaguide complexes. The Ronda peridotite massif -- the

The deep structure of the Sichuan basin and adjacent orogenic zones revealed by the aggregated deep seismic profiling datum

NASA Astrophysics Data System (ADS)

Xiong, X.; Gao, R.; Li, Q.; Wang, H.

2012-12-01

The sedimentary basin and the orogenic belt are the basic two tectonic units of the continental lithosphere, and form the basin-mountain coupling system, The research of which is the key element to the oil and gas exploration, the global tectonic theory and models and the development of the geological theory. The Sichuan basin and adjacent orogenic belts is one of the most ideal sites to research the issues above, in particular by the recent deep seismic profiling datum. From the 1980s to now, there are 11 deep seismic sounding profiles and 6 deep seismic reflection profiles and massive seismic broadband observation stations deployed around and crossed the Sichuan basin, which provide us a big opportunity to research the deep structure and other forward issues in this region. Supported by the National Natural Science Foundation of China (Grant No. 41104056) and the Fundamental Research Funds of the Institute of Geological Sciences, CAGS (No. J1119), we sampled the Moho depth and low-velocity zone depth and the Pn velocity of these datum, then formed the contour map of the Moho depth and Pn velocity by the interpolation of the sampled datum. The result shows the Moho depth beneath Sichuan basin ranges from 40 to 44 km, the sharp Moho offset appears in the western margin of the Sichuan basin, and there is a subtle Moho depression in the central southern part of the Sichuan basin; the P wave velocity can be 6.0 km/s at ca. 10 km deep, and increases gradually deeper, the average P wave velocity in this region is ca. 6.3 km/s; the Pn velocity is ca. 8.0-8.02 km/s in Sichuan basin, and 7.70-7.76 km/s in Chuan-Dian region; the low velocity zone appears in the western margin of the Sichuan basin, which maybe cause the cause of the earthquake.

Aulacogens and aulacogeosynclines: Regularities in setting and evolution

NASA Astrophysics Data System (ADS)

Milanovsky, E. E.

1992-12-01

Aulacogens and aulacogeosynclinal zones represent intra-continental palaeorifts and inverted palaeorifts. They were formed in the process of the break-up of continents during the Late Proterozoic. Inversion of such basins occurred variably during the Baikalian, Caledonian, Hercynian and Alpine orogenic cycles in response to the build-up of intraplate compressional stresses emanating from orogenic belts. Aulacogens were first recognized on the stable Russian Platform where they correspond to linear, graben type depressions filled with Proterozoic sediments, often buried beneath a thick cover of Palaeozoic and younger sediments. Some aulacogens have experienced a short period of reactivation during the Palaeozoic. Aulacogeosynclines differ from the genetically related aulacogens by their greater dimensions and the amount of lithospheric extension that can go to the extent of opening of small ocean basins; consequently they are characterized by a greater degree of destruction of the continental crust, the depth of subsidence and the complexity of their post-rift development which commonly terminates with their compressional deformation, giving rise to the development of intracontinental linear folded and thrusted structures, frequently associated with low-grade metamorphism and small intrusions. Many ancient platforms are characterized by aulacogens; metaplatforms are dissected by aulacogens and aulacogeosynclines, whereas geosynclinal troughs are the precursors of geosynclinal mobil belts. Aulacogens, aulacogeosynclines and geosynclinal troughs represent a group of negative, linear structures which are characterized by an increasing degree of crustal destruction during rifting processes and increasing mobility during lithospheric contraction.

Dating Paleogene Subduction in the Alborán Domain (Alpujárride Complex, S. Spain)

NASA Astrophysics Data System (ADS)

Williams, J. R.; Ashley, K.; Loewy, S. L.; Platt, J. P.

2016-12-01

The multimineral 87Rb/86Sr method has been used in recent studies to date subduction in high-pressure (HP) metamorphic belts of the Mediterranean region. In the Alpujárride Complex, the largest tectonic unit of the Alborán Domain, southern Spain, the timing of burial and HP metamorphism is controversial, with published 40Ar/39Ar white mica ages that range from 48Ma to 25Ma. Dating the HP event is complicated by a pervasive high-temperature (HT) metamorphic overprint (23-19Ma) associated with late-orogenic extension. We have identified 5 rock samples for 87Rb/86Sr dating which preserve a HP equilibrium assemblage: a garnet-staurolite-chloritoid schist, two calcareous Mg-chloritoid schists and two calcareous phyllites with previous 40Ar/39Ar ages of 48Ma and 41Ma. Improved constraints on the time gap between HP and HT metamorphism are important to test geodynamic models of the Alborán Domain, which range from prolonged thickening of continental lithosphere followed by extensional collapse, to punctuated subduction followed by back-arc extension. Furthermore, determining the onset and duration of HP metamorphism has broad implications for whether the Alborán Domain formed in the context of a single Alpine belt, or a separate and local accretionary setting. Lastly, this study will test the advantages and limitations of the 87Rb/86Sr method in a HP domain with a late HT overprint, a very common issue in orogenic systems.

Oblique contractional reactivation of inherited heterogeneities: Cause for arcuate orogens

PubMed Central

Sokoutis, D.; Willingshofer, E.; Brun, J.‐P.; Gueydan, F.; Cloetingh, S.

2017-01-01

Abstract We use lithospheric‐scale analog models to study the reactivation of pre‐existing heterogeneities under oblique shortening and its relation to the origin of arcuate orogens. Reactivation of inherited rheological heterogeneities is an important mechanism for localization of deformation in compressional settings and consequent initiation of contractional structures during orogenesis. However, the presence of an inherited heterogeneity in the lithosphere is in itself not sufficient for its reactivation once the continental lithosphere is shortened. The heterogeneity orientation is important in determining if reactivation occurs and to which extent. This study aims at giving insights on this process by means of analog experiments in which a linear lithospheric heterogeneity trends with various angles to the shortening direction. In particular, the key parameter investigated is the orientation (angle α) of a strong domain (SD) with respect to the shortening direction. Experimental results show that angles α ≥ 75° (high obliquity) allow for reactivation along the entire SD and the development of a linear orogen. For α ≤ 60° (low obliquity) the models are characterized by the development of an arcuate orogen, with the SD remaining partially non‐reactivated. These results provide a new mechanism for the origin of some arcuate orogens, in which orocline formation was not driven by indentation or subduction processes, but by oblique shortening of inherited heterogeneities, as exemplified by the Ouachita orogen of the southern U.S. PMID:28670046

Association of deformation and fluid events in the central Brooks Range fold-and-thrust belt, Northern Alaska

USGS Publications Warehouse

Moore, Thomas E.; Potter, Christopher J.; O'Sullivan, Paul B.; Shelton, Kevin L.; Underwood, Michael B.

2003-01-01

Ocentral Brooks Range consists of two superposed north-directed contractional orogens, one formed between 140-120 Ma and the other at ~60-45 Ma. The older orogen was an arc-continent collisional zone characterized by far-traveled allochthons and relatively low structural relief. The younger orogen is a retroarc thrust belt with relatively low amounts of shortening and high structural relief. Folding and thrusting of the younger episode is superimposed on the thin-skinned deformational wedge of the earlier orogen and also produced a frontal triangle zone in a thick sequence of mid-Cretaceous foreland basin sediments to the north. Stable isotope compositions of calcite and quartz veins indicate two fluid events including: (1) an earlier, higher-temperature (~250-300° C) event that produced veins in deformed Devonian clastic rocks, and (2) a younger, lower-temperature (~150° C) event that deposited veins in deformed Mississippian through Albian strata. The fluids in the first event had variable d18O values, but nearly constant d13C values buffered by limestone lithologies. The vein-forming fluids in the second event had similarly variable d18O values, but with distinctly lower d13C values as a result of oxidation of organic matter and/or methane. Zircon fission track ages demonstrate cooling to temperatures below 200° C between 140-120 Ma for the Devonian rocks, whereas zircon and apatite fission track ages show that Mississippian to Albian rocks were never heated above 200° C and cooled below 110-90° C at ~60-45 Ma. These data are interpreted as indicating that the older, high-temperature fluid event was active during thrusting at 120-140 Ma, and the younger fluid event during deformation at ~60-45 Ma. The data and results presented in this poster will be published in early 2004 in Moore and others (in press).

Crustal structure and evolution of the Trans-Hudson orogen: Results from seismic reflection profiling

NASA Astrophysics Data System (ADS)

Baird, D. J.; Nelson, K. D.; Knapp, J. H.; Walters, J. J.; Brown, L. D.

1996-04-01

A 400-km-long deep seismic reflection transect across northeastern Montana and northern North Dakota reveals the crustal-scale structural fabric of the Early Proterozoic Trans-Hudson orogen beneath the Williston basin. Comparison with deep seismic reflection data across the Canadian portion of the same orogen ˜700 km to the north reveals first-order similarities in crustal architecture but documents significant along-strike variation in orogenic evolution. Both transects display a broad crustal-scale antiform axial to the orogen. In the north, geologic data suggest that this antiform is cored by an Archean microcontinent. In the south, west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the Archean Wyoming craton. Within the Wyoming craton, the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the Early Proterozoic Trans-Hudson orogen as interpreted from potential field and drill core data. On the basis of subsurface geochronologic data from the crystalline basement and by analogy with the Glennie domain within the exposed Trans-Hudson orogen in Canada, we suggest that the southern antiform is cored by an Archean crustal fragment that was caught up in the terminal collision of the Wyoming and Superior cratons during Hudsonian orogeny. The eastern side of the Trans-Hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections. We interpret the easterly dip of these reflections as evidence that the Superior province was thrust westward over the interludes of the orogen during terminal collision. Although juvenile Early Proterozoic terranes characterize the exposed segment of the Trans-Hudson orogen in Canada, limited drill core information within the Dakota segment of the orogen shows a predominance of granulitic

Tectonomorphic evolution of the Eastern Cordillera fold-thrust belt, Colombia: New insights based on apatite and zircon (U-Th)/He thermochronometers

NASA Astrophysics Data System (ADS)

Ghorbal, B.; Stockli, D. F.; Mora, A.; Horton, B. K.; Blanco, V.; Sanchez, N.

2010-12-01

The Eastern Cordillera (EC) of Colombia marks the eastern boundary of Cenozoic fold-thrust deformation in the northern Andes. It is a classic example of an inversion belt formed in the retro-arc region, in this case superimposed on a Triassic/Jurassic to Cretaceous intracontinental rift system of northern South America. Ongoing thrust reactivation (inversion) in this contractional orogen provides an excellent opportunity to study the patterns of deformation and influence of preexisting anisotropies (Mora et al., 2006). The objective of this detailed (U-Th)/He study is to unravel the tectonic and thermal evolution of the EC from the Magdalena Valley basin in the west to the Llanos foreland basin in the east and reconstruct the temporal and spatial progression of deformation in the EC fold-thrust belt. Furthermore, the Subandean or foothills zone of Colombia is key for understanding the petroleum systems in the complex frontal zone of the inverted fold-thrust belt. We present detailed apatite and zircon (U-Th)/He thermochronometric data from surface samples along a ~220 km WNW-ESE transect across the EC from the frontal fold-thrust belt at the edge of the Llanos basin to the western edge of the EC, the Magdalena basin. Surface and borehole zircon and apatite (U-Th)/He data, integrated with structural data, show that the EC fold-thrust belt propagated foreland-ward from the axial zone to the modern edges of the fold-thrust belt from at least the early Oligocene to the early Miocene. Detailed apatite and zircon (U-Th)/He data from surface samples and borehole samples in the foothills-Llanos transition zone and the Middle Magdalena Valley basin, between the large-displacement Guaicaramo and Pajarito-Chámeza thrusts in the east and the La Salina fault system in the west show a temporally complex evolution. The frontal fold-thrust belt was characterized by continued progressive foreland-ward migration of deformation and an apparent phase of major out-of-sequence motion

Mountain Building in Central and Western Tien Shan Orogen: Insight from Joint Inversion of Surface Wave Phase Velocities and Body Wave Travel Times

NASA Astrophysics Data System (ADS)

Wu, S.; Yang, Y.; Wang, K.

2017-12-01

The Tien Shan orogeny, situated in central Asia about 2000 km away from the collision boundary between Indian plate and Eurasian plate, is one of the highest, youngest, and most active intracontinental mountain belts on the earth. It first formed during the Paleozoic times and became reactivated at about 20Ma. Although many studies on the dynamic processes of the Tien Shan orogeny have been carried out before, its tectonic rejuvenation and uplift mechanism are still being debated. A high-resolution model of crust and mantle beneath Tien Shan is critical to discern among competing models for the mountain building. In this study, we collect and process seismic data recorded by several seismic arrays in the central and western Tien Shan region to generate surface wave dispersion curves at 6-140 s period using ambient noise tomography (ANT) and two-plane surface wave tomography (TPWT) methods. Using these dispersion curves, we construct a high-resolution 3-D image of shear wave velocity (Vs) in the crust and upper mantle up to 300 km depth. Our current model constrained only by surface waves shows that, under the Tien Shan orogenic belt, a strong low S-wave velocity anomaly exists in the uppermost mantle down to the depth of 200km, supporting the model that the hot upper mantle is upwelling under the Tien Shan orogenic belt, which may be responsible for the mountain building. To the west of central Tien Shan across the Talas-Fergana fault, low S-wave velocity anomalies in the upper mantle become much weaker and finally disappear beneath the Fergana basin. Because surface waves are insensitive to the structures below 300 km, body wave arrival times will be included for a joint inversion with surface waves to generate S-wave velocity structure from the surface down to the mantle transition zone. The joint inversion of both body and surface waves provide complementary constraints on structures at different depths and helps to achieve a more realistic model compared with

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

NASA Astrophysics Data System (ADS)

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

2017-12-01

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

Structure, metamorphism, and geochronology of the Cosmos Hills and Ruby Ridge, Brooks Range schist belt, Alaska

USGS Publications Warehouse

Christiansen, Peter B.; Snee, Lawrence W.

1994-01-01

The boundary of the internal zones of the Brooks Range orogenic belt (the schist belt) is a fault contact that dips toward the hinterland (the Yukon-Koyukuk province). This fault, here referred to as the Cosmos Hills fault zone, juxtaposes oceanic rocks and unmetamorphosed sedimentary rocks structurally above blueschist-to-greenschist facies metamorphic rocks of the schist belt. Near the fault contact, schist belt rocks are increasingly affected by a prominent, subhorizontal transposition foliation that is locally mylonitic in the fault zone. Structural and petrologic observations combined with 40Ar/39Ar incremental-release geochronology give evidence for a polyphase metamorphic and deformational history beginning in the Middle Jurassic and continuing until the Late Cretaceous. Our 40Ar/39Ar cooling age for Jurassic metamorphism is consistent with stratigraphic and other evidence for the onset of Brooks Range orogenesis. Jurassic metamorphism is nearly everywhere overprinted by a regional greenschist-facies event dated at 130–125 Ma. Near the contact with the Cosmos Hills fault zone, the schist belt is increasingly affected by a younger greenschist metamorphism that is texturally related to a prominent foliation that folds and transposes an older fabric. The 40Ar/39Ar results on phengite and fuchsite that define this younger fabric give recrystallization ages ranging from 103 to less than 90 Ma. We conclude that metamorphism that formed the transposition fabric peaked around 100 Ma and may have continued until well after 90 Ma. This age for greenschist metamorphism is broadly synchronous with the depositional age of locally derived, shallow-marine clastic sedimentary strata in the hanging wall of the fault zone and thus substantiates the interpretation that the fault zone accommodated extension in the Late Cretaceous. This extension unroofed and exhumed the schist belt during relative subsidence of the Yukon-Koyukuk province.

The potential for retreating alpine glaciers to alter alpine ecosystems in the Colorado Front Range

NASA Astrophysics Data System (ADS)

Hall, E.; Baron, J.

2013-12-01

Glaciers are retreating at an unprecedented rate. In mid-latitude alpine ecosystems the presence of glaciers and rock glaciers govern rates and ecology of alpine and sub-alpine ecosystems. Changes in the thermal environment due to the loss of isothermal habitat and inputs from glacier melt chemistry are altering alpine ecosystems in unpredictable ways. In particular, glacier may be a source of nitrogen that is altering alpine ecosystem dynamics. Loch Vale Watershed (LVWS) located within Rocky Mountain National Park. LVWS contains a surface glacier (Andrew's glacier) and a rock glacier (Taylor's glacier) at the headwater of each of the two drainages within the watershed. We collected precipitation from a National Atmospheric Deposition Site and surface water from multiple alpine lakes and streams during a particularly high and low snow year in the Colorado Front Range. We also sampled stream and lake sediments at each site to analyze the associated microbial community. Concentrations of nitrate and ammonium, relative abundance of amoA (the gene responsible for a key step in the microbial nitrification pathway), and the dual isotope signal to nitrate all point to snow melt as a key deliverer of nitrogen to ecosystems along the Colorado Front Range. However, late summer surface water chemistry is isotopically similar to the chemistry of glacial ice. This suggests that retreating glacier may be an additional source of N to alpine ecosystems and have the potential to alter microbial community composition, biogeochemical rate processes, and ecosystem function. These dynamics are most likely not unique to the Colorado Front Range and should be globally distributed as glaciers continue to retreat in high altitude ecosystems around the world.

Lithologic controls on landscape dynamics and aquatic species evolution in post-orogenic mountains

NASA Astrophysics Data System (ADS)

Gallen, Sean F.

2018-07-01