Indus-Yarlung Suture Research Articles

The fate of the Indian lithosphere beneath western Tibet: Upper mantle elastic wave speed structure from a joint teleseismic and regional body wave tomographic study

We investigate the fate of the Indian lithosphere following its descent beneath western Tibet by means of tomographic imaging based on arrival times of body waves from regional and teleseismic sources recorded by a portable network deployed in the region from 2007 to 2011. We use a non-linear iterative algorithm that simultaneously models absolute, regional, and relative teleseismic arrival times to obtain a 3-D velocity structure in a spherical segment that extends from 26°N to 37°N, from 76°E to 89°E, and from the surface to 430km depth. We find that variations in P and S wave speeds in the upper mantle are similar, and identify a number of prominent fast anomalies beneath western Tibet and the adjacent Himalayas. We associate these fast anomalies with the mantle lithosphere of India that is likely colder and hence faster than the ambient mantle. Resolution tests confirm the ability of our dataset to resolve their shapes in the upper 300km, and the lack of significant downward smearing of these features. We interpret the presence of faster material below 300km as being consistent with former Indian lithosphere having reached these depths. There are two main fast anomalies in our model. One resembles a ∼100km wide sub-vertical column located directly beneath the India-Asia plate boundary. The other anomaly is thinner, and has the shape of a dipping slab that spans the north-south width of the Lhasa block. It dips towards the NE, starting near the Indus-Yarlung suture and ending north of the Bangong-Nujiang Suture at depths in excess of 300km. Another finding of our study is the absence of major fast anomalies west of ∼80°E, which our resolution tests show to be significant. Our results do not support the notion of a continuous body of formerly Indian lithosphere being presently underthrust northward, and extending all the way to the northern boundary of the plateau. Rather, shapes of fast anomalies in western Tibet suggest colder material beneath the northernmost Himalaya and the southernmost part of the plateau. The presence of multiple relatively small-scale anomalies, rather than a single large fast body aligned with the India-Asia boundary, rules out subduction of the intact Indian lithosphere. Our preferred scenario for the process that currently removes the mantle lithosphere of India from the collision zone envisages viscous behavior, with gravity-driven instabilities leading to the formation of multiple drips of different size. To reconcile our findings with evidence of past underthrusting of Indian lithosphere beneath Tibet we postulate a geologically recent change in the mode of lithosphere removal. Our scenario includes initial underthrusting, subsequent gravitational destabilization, and removal of the mantle lithosphere of Indian plate. At present the mantle lithosphere of India that continues to underthrust southern Tibet develops a drip-like instability that is seen as a fast columnar feature in our model.

Adakite-like geochemical signature produced by amphibole-dominated fractionation of arc magmas: An example from the Late Cretaceous magmatism in Gangdese belt, south Tibet

Late Cretaceous (~106–76Ma) adakite-like intrusive rocks in the middle-eastern Gangdese belt occur in an E–W trending belt paralleling the Indus–Yarlung suture, south Tibet. Their petrogenesis and geodynamic processes have been a subject of debate. We report here U–Pb zircon ages, geochemical and Sr–Nd–Hf isotopic data for adakite-like intrusive rocks as well as the normal arc rocks (gabbros and gabbroic diorites) in the middle Gangdese belt. LA-ICPMS U–Pb zircon analyses yielded an identical age of ~88Ma for two adakite-like rocks, which are slightly younger than the gabbro and gabbroic diorite (ca. 94–90Ma). Both the adakite-like rocks and the normal arc rocks have similar whole-rock Sr–Nd and zircon Hf isotope compositions, indicating that they have been derived from a common source. Similarly, the adakite-like and normal arc intrusive rocks in the eastern Gangdese belt also show similar Sr–Nd–Hf isotope compositions. In the middle-eastern Gangdese belt, the >85Ma Late Cretaceous intrusive rocks consist of a magma series from gabbro to granodiorite, including both normal arc rocks and adakite-like rocks. These rocks overlap in space and time that conform to a normal arc differentiation trend. In terms of major and trace elements, they also show a clear evolution from the normal arc magmatic into adakitic field. Thus, we suggest that these >85Ma Late Cretaceous intrusive rocks were ultimately derived from melting of the hydrated mantle wedge and the adakite-like rocks can be generated in normal arc magmas by amphibole-dominated fractionation. Taking into accounting for the spatial and temporal distribution of the Cretaceous magmatic rocks in the Lhasa terrane, we prefer a model of early Late Cretaceous rollback following Early Cretaceous low-angle oceanic slab subduction. At intermediate pressure and H2O-rich conditions, fractionation of amphibole changes the major and trace element compositions of arc magmas, and will efficiently drives basaltic composition to andesitic composition in arc magmas.

Detrital zircon U–Pb age and Hf isotopic composition from foreland sediments of the Assam Basin, NE India: Constraints on sediment provenance and tectonics of the Eastern Himalaya

Synorogenic Palaeogene–Neogene sediments of the Assam foreland basin, were derived by erosion of adjacent crustal and orogenic sources following the Greater India–Eurasia collision since ∼55Ma. To constrain source sediment influx, and its relation to Himalayan tectonics, from pre- to post-collision time, detrital zircon U–Pb geochronology and their Hf isotopic compositions were carried out. The varying detrital zircon spectral patterns analyzed from the Paleogene Jaintia, Barail and Neogene Surma and Tipam Groups, with sediment petrography, track source sediment derived from cratonic India, Gangdese and eastern Transhimalayan batholiths and the eastern Himalaya. These sources are tested against Cenozoic paleopositions proposed for the northeastward motion of the Indian plate.Precollisional cratonic detritus to Middle to Late Eocene Sylhet Formation shifted to Tethyan Himalaya and arc sources of the Gangdese and eastern Transhimalayan batholiths to Late Eocene Kopili and Barail Formations, consistent with the proposed paleoposition proximal to the Indus-Yarlung suture. This Sylhet–Kopili Formation transition, within the Jaintia Group, reflects one of the earliest Himalayan hinterland exhumation stages during the Late Eocene. Major shift in provenance to Higher Himalayan Crystalline and arc detritus is recorded from the Surma Group, constraining Mid Miocene Himalayan tectonic exhumation from the eastern Himalaya. Late Miocene Tipam Group preserves sediment of Higher Himalayan Crystalline detritus, ophiolite and likely Lesser Himalayan rocks.

Constraining the age of Liuqu Conglomerate, southern Tibet: Implications for evolution of the India–Asia collision zone

Controversy over the depositional age and provenance of the Liuqu Conglomerate along the major structural Indus–Yarlung suture zone in South Tibet clouds our understanding of the process of the India/Asia collision. Here, we report low-temperature thermochronometric data (apatite fission track, apatite and zircon (U–Th)/He for the Liuqu Conglomerate in the Xigaze area). Our new data constrain its depositional age to latest Oligocene–Early Miocene time, indicating that rather than having formed immediately following Paleogene India–Asia collision or collision between India and an intra-oceanic arc as previously proposed, the Conglomerate was probably deposited in an intermontane basin, at a slightly later time than the Gangrinboche Group to the north. The Liuqu Conglomerate should therefore not be used as a key horizon in models constraining the early stages of India/Asia collision. Our data together with previous studies suggest that the Liuqu Conglomerate was sourced from the Xigaze forearc basin, Indus–Yarlung suture zone, as well as the Tethyan Himalaya. Furthermore, our data indicate that exhumation of the Liuqu Conglomerate commenced at ∼<10–12 Ma, suggesting significant erosion in the Indus–Yarlung suture zone attributable to incision of the Yarlung Zangbo in Mid to Late Miocene time.

Cenozoic low temperature cooling history of the Northern Tethyan Himalaya in Zedang, SE Tibet and its implications

Major structures and climatic change during the Cenozoic (especially Miocene) interacted to shape the giant Tibetan Plateau, whose formation was triggered by convergence of Indian and Asian plates. In this work, we report a low-temperature thermochronology data set from the northern Tethyan Himalaya and Indus–Yarlung suture zone in the Zedang area, where structures are defined by two parallel thrusts (the Zhongba–Gyantse Thrust to the south and Great Counter Thrust to the north). Eighteen samples from the region constrain their low temperature cooling and exhumation histories and further elucidate the evolution of the southern Tibetan Plateau. Thermal history modelling reveals Eocene and Miocene cooling episodes. Based on the temporal and spatial patterns, Eocene cooling is probably linked to crustal thickening resulting in the Eocene anatexis in the northern Tethyan Himalaya; the rapid pronounced cooling episode between ca. 20Ma and 15Ma in the Luobusha area reflects the activity of the Great Counter Thrust (GCT); the ca. 14–10Ma cooling event of the Yardoi dome can probably be linked to rapid erosional denudation during that period; and pronounced cooling commencing in the middle Miocene was probably caused by incision of branches of the Yarlung–Zangbo River. A further cooling episode commencing in late Miocene time (ca. 5Ma) in Luobusha area and Yardoi dome, may reflect initial activity of the Cona Graben cutting through the region.

Provenance of the Upper Cretaceous to Lower Tertiary Sedimentary Relicts in the Renbu Mélange Zone, within the Indus-Yarlung Suture Zone

AbstractThe Upper Cretaceous to Lower Tertiary sediments in the Indus-Yarlung suture zone provide critical records on the history of accretion along the southern margin of Asia prior to, and during, the India-Asia collision. In this article, we report field and petrographic observations, in situ detrital zircon U-Pb ages, Lu-Hf isotopic analyses, and Cr-spinel electron microprobe data from the Upper Cretaceous to Lower Tertiary sedimentary rocks of the Renbu melange zone in east Xigaze, southern Tibet. The Renbu melange zone consists of two serpentinite melange subzones separated by a mud-matrix melange. Using similarities found in the compositions of detrital Cr-spinels and detrital zircon U-Pb ages, we propose to correlate the northern Renbu melange subzone with the upper part of the Xigaze forearc basin. Detrital zircons from sandstones in the southern Renbu melange subzone indicate an influx of Cretaceous–early Cenozoic zircon grains with juvenile Hf isotopic compositions, suggesting a provenance from...

The Moho beneath western Tibet: Shear zones and eclogitization in the lower crust

The Tibetan Plateau is formed by continuing convergence between Indian and Asian plates since ∼50 Ma, involving more than 1400 km of crustal shortening. New seismic data from western Tibet (the TW-80 experiment at 80°E) reveal segmentation of lower crustal structure by the major sutures, contradicting the idea of a mobile lower crust that flows laterally in response to stress variations. Significant changes in crustal structure and Moho depth occur at the mapped major tectonic boundaries, suggesting that zones of localized shear on sub-vertical planes extend through the crust and into the upper mantle. Converted waves originating at the Moho and at a shallower discontinuity are interpreted to define a partially eclogitized layer that extends 200 km north of the Indus–Yarlung Suture Zone, beneath the entire Lhasa block at depths of between 50 and 70 km. This layer is thinner and shallower to the north of the Shiquanhe Fault which separates the northern Lhasa block from the southern part, and the degree of eclogitization is interpreted to increase northward. The segmentation of the Tibetan crust is compatible with a shortening deformation rather than shear on horizontal planes. Unless the Indian-plate mantle lithosphere plunges steeply into the mantle beneath the Indus–Yarlung suture, leaving Indian-plate crust accreted to the southern margin of Tibet, then it too must have experienced a similar shortening deformation.

Himalayan detrital chromian spinels and timing of Indus-Yarlung ophiolite erosion

The geochemistry of detrital chromian spinels is commonly used to discriminate provenance from different tectonic settings of mafic and ultramafic igneous rocks. Detrital spinels in Cenozoic foreland-basin successions fed from the Himalaya Orogen were assertively interpreted as sourced from the ophiolitic rocks of the Indus-Yarlung suture zone. This study compares the geochemistry of detrital Cr-spinels from the Tethys Himalaya passive margin and Cretaceous Xigaze forearc successions with those from the Indus-Yarlung ophiolites. Cr-spinels in the Indus-Yarlung ophiolites have low TiO2 (mostly < 0.2%) and high Al2O3 (10–48%). Detrital Cr-spinels from the Tethyan Himalaya have instead high TiO2 (mostly > 0.2%) and low Al2O3 (mainly 6–23%), indicating a rift-related basaltic origin. Detrital Cr-spinels from the Xigaze forearc basin have either low TiO2 (mostly < 0.2%) and low Al2O3 (4–34%), suggesting provenance from a supra-subduction-zone peridotite, or high TiO2 (> 1.0%), indicating intra-plate basaltic origin. Compositional fingerprints of detrital Cr-spinels from Lower Eocene foreland-basin strata in the central-eastern Himalaya indicate provenance from the Lhasa Block without input from the Indus-Yarlung ophiolites. Only Cr-spinels from the Lower Eocene foreland-basin strata in the north-western Himalaya and the Upper Eocene–Lower Miocene remnant-ocean turbidites of the Bengal basin are mostly ophiolite-derived. The Indus-Yarlung ophiolites were thus emplaced and exposed to erosion since the Early Eocene (> 50 Ma) in the NW Himalaya, but only subsequently (50–38 Ma) in the eastern Himalaya.

Mantle fluids in the Karakoram fault: Helium isotope evidence

The Karakoram fault (KKF) is the 1000km-long strike-slip fault separating the western Himalaya from the Tibetan Plateau. From geologic and geodetic data, the KKF is argued either to be a lithospheric-scale fault with hundreds of km of offset at several cm/a, or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper-crustal localization of distributed deformation at depth. Here we show 3He/4He ratios in geothermal springs along a 500-km segment of the KKF are 3–100 times the normal ratio in continental crust, providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle. Mantle enrichment is absent along the Indus–Yarlung suture zone (ISZ) just 35km southwest of the KKF, suggesting that the mantle fluids flow only within the KKF. Within the last few Ma, the KKF must have accessed tectonically active Tibetan mantle northeast of the “mantle suture” which we therefore locate vertically beneath the KKF, very close to the surface trace of the ISZ. Hence, in southwestern Tibet, Indian crust may not now be underthrusting substantially north of the ISZ, even though Miocene underthrusting may have placed Indian crust north of the ISZ in the lower half of the Tibetan Plateau crust. This is in significant contrast to central and eastern Tibet where underthrust Indian material not only forms the lower half of the Tibetan crust but is also currently underthrusting for ∼200km north of the ISZ. Our new constraint on KKF penetration to the mantle allows an improved description of the continuously evolving Karakoram fault, as a tectonically significant yet perhaps geologically ephemeral lithospheric structure.

Eocene–Oligocene granitoids in southern Tibet: Constraints on crustal anatexis and tectonic evolution of the Himalayan orogen

Tectonic models for the evolution of the Himalayan orogen interpret the Greater Himalayan crystalline complex (GHC) to be the result of either thick-skinned thrusting involved Indian basement, thin-skinned thrusting involving exotic terranes, middle-crustal ductile flow, or wedge extrusion of the Indian crust during India–Asia collision. Two key pieces of information needed to test the validity of these models is the temporal–spatial distribution of, and the identification of the dynamic mechanisms involved in, regional melting under southern Tibet. Here, we document an Eocene–Oligocene melting event in southern Tibet, which forms a 150-km-long, NW–SE-trending granitoid belt along the Zedong–Lhunze traverse between the Indus–Yarlung suture (IYS) and the south Tibetan detachment (STD). U–Pb dating of magmatic zircons indicates that this granitoid belt youngs northward from ∼46Ma (in Lhunze) to ∼30Ma (in Zedong). 40Ar/39Ar dating of deformed biotite within 42–46Ma granitoids constrains the timing of shearing to ∼39–41Ma.The granitoid belt of southern Tibet is dominated by Eocene two-mica granites in the Tethyan Himalaya, with minor ∼30Ma granodiorites along the IYS and ∼35Ma granites in the Yelaxiangbo dome, where Indian mid-crustal rocks are exposed. The ∼35Ma granites are characterized by variable Na2O/K2O ratios (1.03–4.44), relatively high Sr concentrations, and high Sr/Y (14.0–126.3) and La/Yb (11.1–42.8) ratios, which distinguish these granitoids from Miocene leucogranites in the Himalaya. Comparison of the Sr–Nd isotopic compositions of these granites with mid-crustal amphibolites exposed in the Yelaxiangbo dome suggests that the granites were derived from melting of the amphibolites at ∼880°C and ∼10kbar. The ∼30Ma granodiorites and ∼42–46Ma two-mica granites are Na-rich and peraluminous, and are adakitic. They contain inherited Proterozoic zircons, and have a much wider range in εNd(t) of –14.9 to –2.5 and (87Sr/86Sr)i of 0.7062–0.7188, and have a Nd isotopic model age of 1486–1978Ma, indicating that these magmas were derived from a thickened Indian lower crust and were subsequently mixed with amphibolite-derived granite melts or were contaminated by the middle crust under southern Tibet. An apparent northward-younging age trend and shearing of the Eocene–Oligocene granitoids requires the southward migration of slices of middle crustal material, in which the Eocene granitoid magmas were emplaced and stored. Our data, along with structural, metamorphic, and intrusive histories of the Himalaya, lead us to propose a model for crustal anatexis and tectonic evolution of the Himalayan orogen, controlled by a number of large-scale events, such as slab break-off, buoyancy-driven uplift, lateral movement, and subsequent exhumation of slices of the subducted Indian crust during Indo-Asia collision at 55–40Ma.

Rapid Eocene erosion, sedimentation and burial in the eastern Himalayan syntaxis and its geodynamic significance

The lower Bomi Group of the eastern Himalayan syntaxis comprises a lithological package of sedimentary and igneous rocks that have been metamorphosed to upper amphibolite-facies conditions. The lower Bomi Group is bounded to the south by the Indus–Yarlung Suture and to the north by unmetamorphosed Paleozoic sediments of the Lhasa terrane. We report U–Pb zircon dating, geochemistry and petrography of gneiss, migmatite, mica schist and marble from the lower Bomi Group and explore their geological implications for the tectonic evolution of the eastern Himalaya. Zircons from the lower Bomi Group are composite. The inherited magmatic zircon cores display 206Pb/238U ages from ~74Ma to ~41.5Ma, indicating a probable source from the Gangdese magmatic arc. The metamorphic overgrowth zircons yielded 206Pb/238U ages ranging from ~38Ma to ~23Ma, that overlap the anatexis time (~37Ma) recorded in the leucosome of the migmatites. Our data indicate that the lower Bomi Group do not represent Precambrian basement of the Lhasa terrane. Instead, the lower Bomi Group may represent sedimentary and igneous rocks of the residual forearc basin, similar to the Tsojiangding Group in the Xigaze area, derived from denudation of the hanging wall rocks during the India–Asia continental collision. We propose that following the Indian–Asian collision, the forearc basin was subducted, together with Himalayan lithologies from the Indian continental slab. The minimum age of detrital magmatic zircons from the supracrustal rocks is ~41.5Ma and their metamorphism had happened at ~37Ma. The short time interval (<5Ma) suggests that the tectonic processes associated with the eastern Himalayan syntaxis, encompassing uplift and erosion of the Gangdese terrane, followed by deposition, imbrication and subduction of the forearc basin, were extremely rapid during the Late Eocene.

Upper- and mid-crustal radial anisotropy beneath the central Himalaya and southern Tibet from seismic ambient noise tomography

Through analysis of the Rayleigh wave and Love wave empirical Green's functions recovered from cross-correlation of seismic ambient noise, we image the radial anisotropy and shear wave velocity structure beneath southern Tibet and the central Himalaya. Dense ray path coverage from 22 broadband seismic stations deployed by the Himalayan Nepal Tibet Seismic Experiment project provides the unprecedented opportunity to resolve the spatial distribution of the radial anisotropy within the crust of the central Himalaya and southern Tibet. In the shallow subsurface, the obtained results indicate significant radial anisotropy with negative magnitude (VSV > VSH) mainly associated with the Indus Yarlung Suture and central Himalaya, possibly related to the fossil microcracks or metamorphic foliations formed during the uplifting of the Tibetan Plateau. With increasing depth, the magnitude of radial anisotropy varies from predominantly negative to predominantly positive, and a mid-crustal layer with prominent positive radial anisotropy (VSV < VSH) has been detected. The top of the mid-crustal anisotropic layer correlates nicely with the starting depth of the mid-crustal lower velocity layers detected in our previous study. The spatial correlation of the positive radial anisotropy layers and mid-crustal lower velocity layers might suggest lateral crustal channel flow induced alignment of mineral grains, most likely micas or amphiboles, within the mid-crust of the central Himalaya and southern Tibet. This observation provides independent seismic evidence to support the thermo-mechanical model, which involves the southward extrusion of a low viscosity mid-crustal channel driven by the denudation effect focused at the southern flank of the Tibetan Plateau to explain the tectonic evolution of the TibetanHimalayan orogen.

Geochemistry and provenance of bed sediments of the large rivers in the Tibetan Plateau and Himalayan region

We investigated the geochemical characteristics of major, trace and rare earth elements and Sr–Nd isotope patterns of bed sediments from the headwaters and upper reaches of the six large rivers draining the Tibetan Plateau (the Jinsha River—Yangtze, Lancang River—Mekong, Nujiang River—Salween, Huang He—Yellow, Indus, and Yarlung Tsangpo—Brahmaputra). By using Ca/Al versus Mg/Al, La/Sc versus Co/Th, and 87Sr/86Sr versus eNd (0) binary differentiation diagrams of provenance, some typical contributors to the different catchment sediments can be identified. In the Three-River (the Jinsha, Lancang, and Nujiang Rivers) tectonomagmatic belt, acidic–intermediate-acidic volcanic rocks are very important provenance of sediments. Carbonate rocks and Permian Emeishan basalts are dominant in the Jinsha River. The Yellow River sediments have similar geochemical characteristics with loess in catchments. The Indus and Yarlung Tsangpo Rivers sediments are mainly from ultra-K volcanic rocks and Cenozoic granitoids widely distributed in the Indus–Yarlung suture. The intensity of chemical weathering in these river catchments is evaluated by calculating the chemical indices of alteration (CIA) of sediments and comparing them with bedrocks. The CIA values of the six river sediments are from 46.5 to 69.6, closing to those of bedrocks in the corresponding catchment, which indicates relatively weak chemical weathering intensity. Lithology, climate, and topography affect the chemical weathering intensity in these river catchments.

Kinematics and dynamics of the Namche Barwa Syntaxis, eastern Himalaya: Constraints from deformation, fabrics and geochronology

Field observations, deformation and fabric analyses, and precise age data acquired by zircon SHRIMP, LA–ICP-MS U–Pb and 40Ar– 39Ar dating methods have yielded new constraints on the kinematics and dynamics of the Namche Barwa Syntaxis (NBS) which is the eastern corner of the Himalaya. A two-stage model has been established to explain the formation and evolution of the NBS. The northward indentation of the Indian plate beneath the Lhasa terrane began at 55–40 Ma, and crustal materials at this corner were subducted to depths > 70 km where they experienced HP (UHP?) metamorphism. Since 40 Ma, large-scale, right-lateral strike–slip along the Sagaing fault has accommodated the rapid northward movement of the eastern Indian plate corner with respect to the Indochina block. This caused significant and progressive bending of the Indus-Yarlung suture zone (IYSZ) such that it became the Dongjiu-Milin left-lateral, strike–slip, shear zone (DMSZ) in the west and the Aniqiao-Motuo right-lateral, strike–slip, shear zone (AMSZ) in the east. Both zones underwent strong mylonitization. Meanwhile, the HP (UHP?) metamorphic rocks were rapidly exhumed, first into the deep crust at 22–18 Ma and then to the shallow crust to form an antiformal dome at 6–2 Ma. Our model provides new insight into the processes of post-collisional crustal thickening related to the formation of the Himalayan orogenic belt.

Oligocene-Miocene Kailas basin, southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung suture zone

Research Article| July 01, 2011 Oligocene–Miocene Kailas basin, southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung suture zone P.G. DeCelles; P.G. DeCelles † Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA †E-mail: decelles@email.arizona.edu Search for other works by this author on: GSW Google Scholar P. Kapp; P. Kapp Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar J. Quade; J. Quade Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar G.E. Gehrels G.E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (7-8): 1337–1362. https://doi.org/10.1130/B30258.1 Article history received: 25 Jan 2010 rev-recd: 26 May 2010 accepted: 24 Jun 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation P.G. DeCelles, P. Kapp, J. Quade, G.E. Gehrels; Oligocene–Miocene Kailas basin, southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung suture zone. GSA Bulletin 2011;; 123 (7-8): 1337–1362. doi: https://doi.org/10.1130/B30258.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Kailas basin developed during late Oligocene–early Miocene time along the Indus-Yarlung suture zone in southwestern Tibet. The >2.5-km-thick basin-filling Kailas Formation consists of a lower coarse-grained proximal conglomerate and more distal fluvial sandstone member, a lacustrine shale and sandstone member, and an upper red-bed clastic member. Felsic tuffs and trachyandesite layers are locally present. Detrital and igneous zircon U-Pb ages indicate deposition of most of the Kailas Formation between ca. 26 and 24 Ma. The Kailas Formation was deposited by alluvial-fan, low-sinuosity fluvial, and deep lacustrine depositional systems in buttress unconformity upon andesitic volcanic (ca. 67 Ma) and granitoid (ca. 55 Ma) rocks of the Gangdese magmatic arc. Abundant organic material, fish and amphibian fossils, and sparse palynomorphs suggest that Kailas lakes developed in a warm tropical climate, quite different from coeval basins in central Tibet, which formed at high elevation in a dry climate. Provenance and paleocurrent data indicate that the bulk of the Kailas Formation was derived from the northerly Gangdese magmatic arc (Kailas magmatic complex). Only during the latest stages of basin filling was abundant sediment derived from the southerly Tethyan Himalayan thrust belt in the hanging wall of the Great Counter thrust. Kailas basin stratigraphy resembles a classic lacustrine sandwich and is most consistent with deposition in an extensional or transtensional rift that developed along the suture zone some 30 m.y. after the onset of Indo-Eurasian intercontinental collision. Correlative coarse-grained syntectonic strata similar to the Kailas Formation crop out along a >1300 km length of the Indus-Yarlung suture zone, suggesting that the basin-forming mechanism recorded by the Kailas Formation was of regional significance and not exclusively related to local kinematics near the southeastern end of the Karakoram fault. We propose that extension of the southern edge of the Eurasian plate was caused by southward rollback of underthrusting Indian continental lithosphere, followed by slab break-off. Alternating episodes of hard and soft collision, associated with regional contraction and extension, respectively, in the Tibetan-Himalayan orogenic system may have been related to changing dynamics of the subducting/underthrusting Indian plate. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Isotopic and structural constraints on the late Miocene to Pliocene evolution of the Namche Barwa area, eastern Himalayan syntaxis, SE Tibet

Within the Namche Barwa area, SE Tibet, the Indus–Yarlung suture zone separates the Lhasa terrain in the north from the Himalayan unit including the Tethyan (sedimentary and volcanic rocks), Dongjiu (greenschist to lower amphibolite facies), Namche Barwa (granulite facies), Pei (amphibolite facies) and Laiguo (greenschist facies) sequences in the south. Two fault systems were distinguished in the Namche Barwa area. The former includes a top-down-to-the-north normal fault in the north and two top-to-the-south thrust zones in the south named as Upper and Lower Thrusts, respectively. The Namche Barwa and Pei sequences were exhumed southwards from beneath the Dongjiu sequence by these faults. Thus, the fault system is regarded as a southward extrusion structure. Subsequently, the exposed Dongjiu, Namche Barwa, Pei and Laiguo sequences were displaced northwards onto the Lhasa terrain by the top-to-the-north fault system, thus, marking it as northward indentation structure. Monazite TIMS U–Pb dating demonstrates that the normal fault and the Lower Thrust from the southward extrusion system were probably active at ~ 6 Ma and ~ 10 Ma, respectively. Zircon U–Pb SHRIMP and phlogopite K–Ar ages further suggest that the Upper Thrust was active between 6.2 ± 0.2 Ma and 5.5 ± 0.2 Ma. The northward indentation structures within the core portion of the eastern Himalayan syntaxis were perhaps active between 3.0 Ma and 1.5 Ma, as inferred by published zircon U–Pb SHRIMP and hornblende Ar–Ar ages. The monazite from upper portions of the Pei sequence dated by U–Pb TIMS indicates that the precursor sediments of this sequence were derived from Proterozoic source regions. Nd isotopic data further suggest that all the metamorphic rocks within eastern Himalaya (εNd = − 13 to − 19) correlate closely with those from the Greater Himalayan Sequences, whereas the western Himalayan syntaxis is mainly comprised of Lesser Himalayan Sequences. The two indented corners of the Himalaya are, thus, different.

Shear-wave birefringence and current configuration of converging lithosphere under Tibet

New data from west-central Tibet show that birefringence of S-waves has two pronounced increases in magnitude toward the hinterland. Null birefringence persists to about 75km north of the Indus–Yarlung suture (IYS) between the Indian shield and the Lhasa terrane of southern Tibet. A second, rapid increase occurs about 100km farther north of the Bangong–Nujiang sutures between the Lhasa terrane and the Qiangtang terrane in central Tibet. The latter feature is consistently observed along three long transects that collectively span a lateral (orogen-parallel) distance of about 600km and is likely to mark the northern, leading edge of sub-horizontally advancing mantle lithosphere of the Indian shield (the “Greater India”) — an interpretation consistent with the latest results of finite-frequency tomography using both P- and S-wave travel-times, previous results of modeling gravity anomalies, and a host of other seismic observations. Similarly, complementary constraints indicate that the sudden onset of significant birefringence north of the IYS is likely to be the southern termination of Eurasian mantle lithosphere. Curiously, the shortest of three transects showed null birefringence through much of the Lhasa terrane, a pattern inconsistent with those of He isotopes and gravity.

Midcrustal low‐velocity layer beneath the central Himalaya and southern Tibet revealed by ambient noise array tomography

Ambient noise tomography has been becoming an important tool to image the shallow lithospheric structure of the Earth. Using 2 months of ambient noise data from 20 stations of the Himalayan Nepal Tibet Seismic Experiment, we investigate the upper and middle crustal structure in the central Himalaya and southern Tibet. About 120 interstation Rayleigh wave empirical Green's functions with sufficient signal‐to‐noise ratio are obtained and used for group velocity dispersion analysis in the period range 6–25 s using frequency‐time analysis technique. The obtained dispersion data are then used to construct 2‐D group velocity maps. At the short periods from 9 to 15 s, the distribution of Rayleigh wave velocities delineates several distinct low‐ and high‐velocity zones separated mainly by geological boundaries. The high group velocity zone is located mainly around regions with plutonic rocks, and the low group velocity zone is located around regions with sedimentary or metasedimentary rocks. Finally, we invert for the shear velocity structure in the upper and middle crust along a N‐S trending cross section at the longitude 86.5°E. We observe a clear low‐velocity layer in the middle crust (about 10–25 km depth) distributed on both sides of the Indus Yarlung Suture zone. The existence of this midcrustal low‐velocity zone suggests a mechanically weaker middle crust beneath the central Himalaya and southern Tibet, which might decouple the upper crustal deformation from that of the lower crust in the Tibetan‐Himalayan orogenic processes.

Does the Karakoram fault interrupt mid-crustal channel flow in the western Himalaya?

Variations in the volume and age of Miocene granites and in mid-crustal conductance from the northwest Himalaya to southeastern Tibet imply lateral differences in late orogenic processes. The change from west to east occurs near the Gurla Mandhata dome, where the Karakoram fault terminates and merges with the Indus–Yarlung suture zone. The ‘channel flow’ model, developed in southeastern Tibet, predicts that anatectic partial melts beneath the Tibetan plateau are gravitationally-driven south to a topographic erosional front and are exposed as leucogranites in the Greater Himalaya Sequence; upwellings of these channel granites occur as gneiss domes in the Tethyan Himalaya Sequence. Published magnetotelluric profiles show high conductivity 30–40 km deep beneath Tibet from c. 400 km north of the Main Frontal thrust south across the suture zone, beneath the Himalayan gneiss domes, and to the topographic front; this conductive middle crust corresponds to 2–4% partial melt in the northwest Himalaya and 5–12% melt in southeastern Tibet, sufficient in the latter case to weaken rock for flow. East of the Karakoram termination leucogranites are abundant and are as young as 7 Ma; west of the termination, channel granites are less abundant and no younger than 18 Ma. Middle Miocene (16–14 Ma) leucogranites are found in the Karakoram fault zone located north of the suture zone and south of the proposed anatectic melt source. The initiation of motion on the crustal-penetrating Karakoram fault at 25–21 Ma may have created a barrier to the southward flow of mid-crustal melts and acted as a vertical conduit for these same melts.

Tectonic Evolution of Metasediments from the Gangdise Terrane, Asian Plate, Eastern Himalayan Syntaxis, Tibet

The Eastern Himalayan Syntaxis of Namche Barwa carries critical information for understanding the geodynamics of the Indian-Asian collision. In this syntaxis, the Nyingchi Group is a sequence of medium- to high-grade metasediments, located on the north side of the Indus-Yarlung suture zone. Results from zircon U-Pb ages coupled with their Hf isotope systematics for the Nyingchi Group and a two-mica granite intruding the paragneisses reveal strong contrasts in provenance and depositional age within the sequence. Detrital zircons from a garnet paragneiss of the lower Nyingchi Group display 206Pb/238U ages from 493 Ma to 2466 Ma, with peaks at 506 Ma, 821 Ma, and 1032 Ma. The minimum detrital zircon age of 493 Ma constrains the maximum depositional age of the lower Nyingchi Group. Detrital zircons from a paragneiss of the middle Nyingchi Group yield 206Pb/238U ages ranging from 55 Ma to 1362 Ma, with peaks at 60 Ma, 95 Ma, 138 Ma, and 171 Ma. This indicates that the maximum depositional age is less than 55 Ma. Zircons from a two-mica granite, intruding the upper Nyingchi Group, yield a magma crystallization age of 22 ± 1 Ma and inherited zircon ages of 52 Ma to 911 Ma. Because the Hf isotopic ratios together with the ages of detrital zircons suggest an origin from anatexis of the middle Nyingchi Group sediments, these data reduce the maximum depositional age of the middle Nyingchi Group to less than 52 Ma. Results preclude the Nyingchi Group from representing Precambrian basement of the Gangdise terrane. The detrital zircon and inherited zircon age distributions, combined with their Hf isotopic compositions, reveal that whereas the lower Nyingchi Group may be correlated with crystalline units from the Himalayan terrane, the middle-upper Nyingchi Group is a much younger deposit derived from the Gangdise magmatic arc. In order to explain the medium- to high-grade metamorphism of the middle-upper Nyingchi Group, we suggest that following the Indian-Asian collision, a thrust slice from the leading edge of the Asian continental slab (the middle-upper Nyingchi Group) became detached and subducted, together with Himalayan lithologies from the Indian continental slab. These results provide new insight into the tectonic evolution of the Eastern Himalayan Syntaxis.