Tsangpo Suture Research Articles

History of groundwater development in the Indian Himalayas: past, present and future scenarios

Since time immemorial, civilizations (Indus Valley, Mesopotamian, etc.) have flourished in and around rivers or water bodies, and the same at the time of flooding/flash flooding has been the cause of their destruction. The present paper highlights the history of groundwater development in the mountains of the Indian Himalayas. The attempt is based on studying the groundwater structures developed by our ancestors since time immemorial in the form of identifying and protecting natural springs. Later, wells dug initially in the inter mountain valley region gradually shifted to the peaks of mountains in the form of manually digging wells for the forts constructed in the 16th century. Sustainability of the water source in the forts had to be developed for 24 × 365 hours per year. There was gradual transformation from exploring wells in the inter mountain valley regions to the top of the mountain, and this transformation could be related to the flash flooding activity in the past which might have washed out the major civilization/societies/kingdoms located on the banks of rivers or in the inter mountain valley regions. The accuracy in the techniques for exploration and development of groundwater resources in the mountain peaks was at its peak until the early 18th century. This is evident from the wells found in all the forts located on the peaks of Banasar, Malaon, Bilaspur and Bhota visited by the author in the state of Himachal Pradesh. This traditional science of hydrogeological exploration, however, died an unnatural death in 1815 when the tanks of the British defeated the Gorkhas in the Battle of Malaon. The ancestral science of exploration was based on sound lithological knowledge coupled with intuition (dowsing) which was the basis of our traditional scientific knowledge. After the industrial revolution, the British brought pumps with them and lifted water from rivers/springs to the peaks of the hills of Subathu, Kasauli, Dagshai and Shimla, which were in one of the oldest cantonments in the Himalayan subcontinent. The hydrogeologists of the past were not able to communicate with the new rulers. Thus language and the advent of new technology was the cause of unnatural death of the traditional water wisdom. The scenario changed and there was major shift from developing groundwater resources to the tapping of surface water resources, and pumping machinery was developed to pump more and more water from different localities to the tops of the hills. The science of hydrogeology suffered and was almost a forgotten chapter in the mountains of the Himalayas. Until the early 1990s, the hills were considered to be devoid of groundwater resources. However, with the modernization of drilling equipment and the growing demand for water, an attempt was made to tap groundwater resources in the Himalayan region. In the first phase, Kangra Valley was identified and groundwater resources were developed there. However, later, exploration techniques were developed which were again using the traditional wisdom to locate the sites to tap groundwater resources in the mountains. Satellite imagery and geophysical methods are not at all applicable in the investigation of these groundwater resources in the mountain terrain. In 1997–1998 the first wells were explored and drilled by the author across the Indus Tsangpo suture zone. By then more than 25 000 wells had been drilled in different hydrostratigraphic zones of the Himalayas identified by author in 1996. The history of groundwater development can therefore be summarized as science which was at its peak before the arrival of the British in 1815 in the Indian Himalayan Kingdom. This was followed by death of this science for almost 200 years.

The youngest marine deposits preserved in southern Tibet and disappearance of the Tethyan Ocean

Fossil ages as young as Priabonian (38–34Ma) are reported for the last marine sedimentary rocks in southern Tibet. Correlation is based on examination of foraminifers and nannofossil biostratigraphy of youngest preserved sediments in sections at Gamba (Zongpu), Tingri (Qumiba) as well as a previously unreported section at Yadong. Our results demonstrate that a marine seaway remained in existence south of the Yarlung Tsangpo suture zone until at least Priabonian time. Notably this remains a maximum age estimate in this area as all sections are truncated by erosion or faulting. We compare our results with sections throughout the Himalaya region to demonstrate that shallow marine conditions existed widely during the Eocene period. In fact, it seems likely that the marine conditions in the Tethyan Himalaya did not entirely disappear by the end of Priabonian, especially in the eastern Himalaya. The data presented in this study place direct constraints on the elimination of the Tethyan Ocean and thus have important implications for timing of the India–Eurasia collision.

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.

Crustal structure of the Indus–Tsangpo suture zone and its ophiolites in southern Tibet

The >2000-km-long Indus–Tsangpo suture zone (ITSZ) in southern Tibet contains remnants of Neotethyan oceanic lithosphere, ophiolitic mélanges, flysch units and continental rocks, and has been interpreted as the boundary between the India and Eurasia plates. We have used field-based structural analyses and geological mapping in five key ophiolite massifs (Luobusa, Zedang, Xigaze, Jiding and Dongbo) and petrofabric studies of core samples recovered through the Luobusa Scientific Drilling Project to document the structural architecture and the deformation history of the ITSZ. The structural and kinematic analyses of the suture zone units indicate the occurrence of superposed deformation fabrics, which developed first during the ophiolite emplacement and then during the subsequent India–Asia continental collision. The suprasubduction zone ophiolites were emplaced southward onto the northern margin of the proto-Indian plate in the late Cretaceous. The continental collision between the ophiolite-laden proto-Indian margin and the Lhasa terrane of the Eurasian plate in the early Paleogene resulted in backthrusting and inversion of the structural order of the ITSZ. The ophiolite massifs, ophiolitic mélanges and flysch units were thrust northward as nappe sheets over the Eocene and younger forearc basin strata. The ITSZ shows, therefore, a complex structural anatomy characterized by both south and north-directed thrust faults, and ductile and brittle deformation fabrics.

Effects of Pleistocene climatic fluctuations on the phylogeography, demography and population structure of a high‐elevation snake species, Thermophis baileyi, on the Tibetan Plateau

AbstractAimOur aims were to investigate the geographical distribution, population history and demographics of the hot‐spring snake Thermophis baileyi, and to examine how the Pleistocene glacial stages and geomorphological configurations resulting from the uplift of the Tibetan–Himalayan orogen shaped the genetic diversity and evolutionary history of the species.LocationTibetan Plateau.MethodsWe surveyed potential habitat sites throughout Tibet. Sequence data were obtained for three mitochondrial markers (ND2, ND4 and cytb; 2185 bp) in 181 individuals from 18 populations across the species' distribution range. We conducted population‐genetic (samova; baps), phylogenetic (maximum likelihood; Bayesian), demographic (Tajima's D; Fu's FS; mismatch distribution) and ecological analyses (logistic regression for presence/absence data) to examine the recent distribution, evolutionary history and diversification of the species.ResultsThe geographical range of T. baileyi is a restricted area between the Transhimalaya and the Himalaya, along the central part of the Yarlung Tsangpo suture zone. The divergence times of sequences matched the Guxiang glaciation (300–130 ka) and the last glacial period (70–10 ka). We observed two genetically distinct lineages north of the Yarlung Tsangpo river. These lineages are known from previous work and are largely congruent with a division across the drainage divide of the Nyainqêntanglha rift. Haplotype grouping based on cluster analysis indicated the existence of riverine migration corridors between populations.Main conclusionsThe evolutionary history of T. baileyi is consistent with a model of range expansion from different refugia during interglacial and post‐glacial times. All lines of evidence suggest that Pleistocene glacial oscillations, triggered by tectonic activity during plateau uplift, have had the strongest influence on intraspecific differentiation. Physical barriers, such as high‐elevation mountains and the drainage system, however, also appear to have affected the population structure and distribution of this snake. Population clustering suggests that there has been long‐distance dispersal along riverine corridors.

The Cretaceous and Cenozoic tectonic evolution of Southeast Asia

Abstract. Tectonic reconstructions of Southeast Asia have given rise to numerous controversies that include the accretionary history of Sundaland and the enigmatic tectonic origin of the proto-South China Sea. We assimilate a diversity of geological and geophysical observations into a new regional plate model, coupled to a global model, to address these debates. Our approach takes into account terrane suturing and accretion histories, the location of subducted slabs imaged in mantle tomography in order to constrain the evolution of regional subduction zones, as well as plausible absolute and relative plate velocities and tectonic driving mechanisms. We propose a scenario of rifting from northern Gondwana in the latest Jurassic, driven by northward slab pull from north-dipping subduction of Tethyan crust beneath Eurasia, to detach East Java, Mangkalihat, southeast Borneo and West Sulawesi blocks that collided with a Tethyan intra-oceanic subduction zone in the mid-Cretaceous and subsequently accreted to the Sunda margin (i.e., southwest Borneo core) in the Late Cretaceous. In accounting for the evolution of plate boundaries, we propose that the Philippine Sea plate originated on the periphery of Tethyan crust forming this northward conveyor. We implement a revised model for the Tethyan intra-oceanic subduction zones to reconcile convergence rates, changes in volcanism and the obduction of ophiolites. In our model the northward margin of Greater India collides with the Kohistan–Ladakh intra-oceanic arc at ∼53 Ma, followed by continent–continent collision closing the Shyok and Indus–Tsangpo suture zones between ∼42 and 34 Ma. We also account for the back-arc opening of the proto-South China Sea from ∼65 Ma, consistent with extension along east Asia and the formation of supra-subduction zone ophiolites presently found on the island of Mindoro. The related rifting likely detached the Semitau continental fragment from South China, which accreted to northern Borneo in the mid-Eocene, to account for the Sarawak Orogeny. Rifting then re-initiated along southeast China by 37 Ma to open the South China Sea, resulting in the complete consumption of proto-South China Sea by ∼17 Ma when the collision of the Dangerous Grounds and northern Palawan blocks with northern Borneo choked the subduction zone to result in the Sabah Orogeny and the obduction of ophiolites in Palawan and Mindoro. We conclude that the counterclockwise rotation of Borneo was accommodated by oroclinal bending consistent with paleomagnetic constraints, the curved lithospheric lineaments observed in gravity anomalies of the Java Sea and the curvature of the Cretaceous Natuna paleo-subduction zone. We complete our model by constructing a time-dependent network of topological plate boundaries and gridded paleo-ages of oceanic basins, allowing us to compare our plate model evolution to seismic tomography. In particular, slabs observed at depths shallower than ∼1000 km beneath northern Borneo and the South China Sea are likely to be remnants of the proto-South China Sea basin.

Thermal characteristics of the Main Himalaya Thrust and the Indian lower crust with implications for crustal rheology and partial melting in the Himalaya orogen

The Main Himalaya Thrust (MHT) is the current tectonic boundary between the subducting Indian lithosphere and the overlying Himalayan orogenic prism and the Tibetan crust. We present thermo-kinematic calculations and metamorphic P–T–t paths of the Indian lower crust (ILC) that constrain the thermal structure of the MHT and the southern Tibetan crust (Lhasa Block) and explain the origin of a thin, seismic low velocity zone that was revealed by the recent Hi-CLIMB experiment from receiver functions of teleseismic waves. Northward of the Himalayas, the low velocity zone occurs within the ductile regime of the crust and is thought to extend along the MHT into the Lhasa Block. In the Lhasa Block, the low velocity zone occurs directly above the ILC. Predicted evolution of mineralogy of the ILC along its subduction P–T–t path shows that its dehydration can potentially induce wet melting within the orogenic prism above the inclined portion of the MHT. However, north of the Yarlung Tsangpo Suture (YTS) below the southern Lhasa Block, where subduction of the ILC is flat, the ILC is predicted to be anhydrous eclogite and therefore, it cannot supply H2O to the overlying crust. The seismic low velocity zone above this portion of the ILC is best explained by dehydration melting due to strain heating. The MHT there appears to be localized by the rheological contrast between the ductile lower Lhasa Block and the strong eclogitic ILC.Southward thrusting of the Himalaya orogenic prism, which contains accreted Indian upper crust, causes advection of hot middle-crustal rocks to shallower levels, thereby producing a shallow ductile regime between the Himalayas and the YTS. The shallow ductile regime is evident in the limit of upper crustal earthquake foci to shallow depths in this region.

Tomographic imaging of the underthrusting Indian slab and mantle upwelling beneath central Tibet

To better understand the pattern of convergence between the Indian and Eurasian plates, we determine a high-resolution P-wave tomography of the crust and upper mantle under southern to central Tibet using a large number of high-quality data of local earthquakes and teleseismic events collected by the ANTILOPE-II and Hi-CLIMB projects. A significant low-velocity zone is detected above the northward underthrusting Indian slab beneath the Indus–Tsangpo suture, which may reflect fault zones or (incomplete) fragmentation as well as melts and/or fluids associated with the dehydration of the underthrusting Indian slab. The variations in the diving depth and extending distance of the Indian slab, under the ANTILOPE-II and Hi-CLIMB seismic profiles, may be caused by the differences in the viscosity contrast between the Indian slab and the surrounding mantle as well as strong structural heterogeneities in the upper mantle under the study region. Both high and low velocity anomalies are revealed in the mid- to lower crust under southern Tibet, which may reflect the complex pattern of crustal structure. The rifts in southern Tibet exhibit different velocity images, suggesting that different structures exist beneath those rifts.

Transition from oceanic to continental lithosphere subduction in southern Tibet: Evidence from the Late Cretaceous–Early Oligocene (~ 91–30 Ma) intrusive rocks in the Chanang–Zedong area, southern Gangdese

Little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the Gangdese Belt of southern Tibet (GBST). Here, we report zircon U–Pb age, major and trace element and Sr–Nd–Hf isotopic data for Late Cretaceous–Early Oligocene (~91–30Ma) intermediate-acid intrusive rocks in the Chanang–Zedong area immediately north of the Yarlung–Tsangpo suture zone. These rocks represent five magmatic episodes at ~91, ~77, ~62, ~48, and ~30Ma, respectively. The 91–48Ma rocks have slightly lower initial 87Sr/86Sr (0.7037 to 0.7047), and higher εNd(t) (+1.8 to +4.3) and εHf(t) (+3.5 to +14.7) values in comparison with those (0.7057 to 0.7062, −3.3 to −2.5 and +2.2 to +6.6) of the ~30Ma intrusive rocks. The ~91, ~62 and ~30Ma rocks are geochemically similar to slab-derived adakites. The ~91Ma Somka adakitic granodiorites were likely derived by partial melting of the subducting Neo-Tethyan oceanic crust with minor oceanic sediments, and the ~91Ma Somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite. The ~77Ma Luomu diorites were probably generated by partial melting of juvenile basaltic lower crust. The ~62Ma Naika and Zedong adakitic diorites and granodiorites were likely generated mainly by partial melting of thickened juvenile mafic lower crust but the source region of the Zedong adakitic rocks also contained enriched components corresponding to Indian continental crust. The ~48Ma Lamda granites were possibly generated by melting of a juvenile basaltic crust. The younger (~30Ma) Chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of Early Oligocene northward-subducted Indian lower crust beneath the southern Lhasa Block. Taking into account the regional tectonic and magmatic data, we suggest that the Gangdese Belt of southern Tibet (GBST) underwent a tectonodynamic transition from oceanic subduction to continental subduction between 100 and 30Ma. It evolved through four stages: 100–65Ma roll-back of subducted Neo-Tethyan oceanic lithosphere; 65–60Ma initial collision between Indian and Asian continents; 60–40Ma breakoff of subducted Neo-Tethyan oceanic lithosphere; and ~30Ma northward subduction of the Indian continent.

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.

Zircon SHRIMP geochronology and Nd-Pb isotopic characteristics of the meta-basalt in the central part of Tibetan Plateau’s Qiangtang region

A small-sized meta-basic rock system is discovered in Qilongwuru Gully of Central Qiangtang’s Shuanghu region and contains a meta-basalt and garnet-bearing plagioclase amphibolite. The zircon U/Pb age of this meta-basalt by SHRIMP analysis is 463.3±4.7 Ma, suggesting that this lava formed in the Middle Ordovician, and is consistent with that of the meta-basic rocks in the Taoxing Lake and Guoganjianian Mountain ophiolite found in the Qiangtang plate. As this lava system bears similar geochemistry to N-MORB, it might be a component of ophiolite that represents the trail of the extinction of the Proto-Tethys, suggesting that the formation of Proto-Tethys oceanic basin in the Longmu Co-Shuanghu suture zone could date as far back as to the Middle Ordovician. Isotopic geochemical analysis indicates that the magma source area consists of both depleted mantle (DM) and enriched mantle (EMII) end members and bears Dupal anomaly, similar to that of the Paleo-Tethys in the Neo-Tethys represented by the Yarlung-Tsangpo suture zone, the Paleo-Tethys represented by the Changning-Menglian suture zone, and the Paleo-Tethys in Sanjiang region. This suggests that they have inherited the attribute of the Proto-Tethys mantle domain, and the Longmu Co-Shuanghu suture zone may be a representative of the northern boundary of Gondwana.

The formation mechanism of garnet porphyroblast in quartz schist, Namche Barwa, east Himalaya

The quartz schist in the Indus-Yarlung Tsangpo suture zone, from Namche Barwa, east Himalaya undergone extensive mylonization with well developed, foliation and lineation; its S-C fabric, "σ" and "δ" type porphyroblasts and asymmetrical folds indicate the northwestward normal slip ductile shear deformation. The electron backscattered diffraction (EBSD) data, lattice preferred orientation (LPO) patterns for quartzes in the matrix foliation (or external foliation, S) and inclusion-trails in the rim of the garnet porphyroblasts (S) both show a top-to-NW (normal) shear sense. While LPO patterns for quartzes of the inclusion-trails in the garnet porphyroblast cores (S) show an opposite shear sense (top-to-SE). The garnet compositional zonation indicates a growth zoning characteristic from energy dispersive spectroscopy (EDS) analysis. By connecting the inclusion-trails in garnet cores, an initial foliation trace with asymmetrical fold shape can be obtained. It suggests that these garnet porphyroblasts with snov, ball structure formed on the asymmetrical folds by the rotation of external shear sense rather than the rotation of garnet porphyroblasts. Gamet porphyroblast with snowball structure proposed that the rocks from the lndus Yarlung Tsangpo suture zone west side of the Namche Barwa syntaxis have undergone southeastward thrusting and later followed by northwestward normal slipping.

Microfacies analysis and paleoenvironmental significance of palustrine carbonates in the Thakkhola-Mustang Graben (Nepal Himalaya)

The Thakkhola-Mustang Graben represents the extensional tectonic phase of the Tibetan Plateau uplift and whole Himalayan orogeny. It is situated at the northern side of the Dhaulagiri and Annapurna Ranges and south of the Yarlang Tsangpo Suture Zone. Stratigraphically, the oldest sedimentary units are the Tetang and Thakkhola Formations (Miocene), while the Sammargaon, Marpha and Kaligandaki Formations lying disconformably above these formations represent Plio-Pleistocene units. In this study, different lacustrine carbonates and calcretes were investigated within different lithological units and depositional environments to interpret the palaeoenvironmental and palaeoclimatological evolution of the area.Geological mapping, construction of columnar sections and carbonate sampling were carried out in the field, and stable oxygen and carbon isotope analyses and thin section analyses were done in the laboratory. Lacustrine facies contained abundant pelletal, charophytic algae, oncolitic algal micritic palustrine limestones with ostracods, and micritic mudstones with root traces. Stable carbon and oxygen isotope analysis from the carbonates show a range of δ13C values from −0.6‰ to 11.1‰ (V-PDB) and δ18O values from −13.5‰ to −25‰ (V-PDB).Discontinuous growth of oncolites and spherical pellets (25–40μm in diameter) in micritic limestone, algal mats and charophyte algae indicate the presence of both shallow and deep water carbonates. Ostracods in dark micritic carbonates indicate quiet and calm water conditions. Microfabrics of the carbonates suggest that they were deposited in a flat and shallow lacustrine environment. The δ18O values of the investigated limestones of the Thakkhola-Mustang Graben suggest that it attained the current elevation level prior to the east-west extension of the Himalaya.

U–Pb zircon ages for Yarlung Tsangpo suture zone ophiolites, southwestern Tibet and their tectonic implications

Ophiolite complexes preserved along the Yarlung Tsangpo suture zone (YTSZ) and obducted onto the northern continental margin of India in southern Tibet represent the remnants of the once extensive Permian–Mesozoic Neo-Tethyan Ocean that separated India from Asia. Complete ophiolite successions are preserved near Xigaze, whereas the rest of the belt is essentially represented by mantle rocks with subordinate disrupted lower crustal rocks. U–Pb zircon LA-MC-ICP-MS geochronology on two gabbro samples from the Luobusa ophiolite yielded concordant data with mean 206Pb/238U ages of 149.9±1.4 (2σ) Ma and 150.0±5.0Ma. These ages are in contrast to a younger age of 131.8±1.0Ma obtained from a pegmatitic gabbro in Xigaze. Five U–Pb zircon TIMS ages from gabbroic samples in the western portion of the ophiolite belt reveal that the Dangxiong ophiolite formed between 126.7±0.4Ma and 123.4±0.8Ma. Zircons from the Jungbwa ophiolite have similar ages of 123.4±0.8Ma and 123.9±0.9Ma. A single zircon analysed from a gabbro in Kiogar has an age of 159.7±0.5Ma. Geochronological data reported here show YTSZ ophiolites formed in association with intra-oceanic subduction zone systems and are related a significant tectonic episode within the Tethyan Ocean during Late Jurassic to Early to mid Cretaceous times.

Crustal thickness and Poisson’s ratio variations across the northwest Himalaya and eastern Ladakh

Crustal thickness and Poisson’s ratios are estimated across the northwest (NW) Himalaya and eastern Ladakh applying H-k stacking method on receiver functions of teleseismic earthquakes recorded at 16 broadband seismological stations. The results show significant lateral variation of crustal thickness from the Lesser and Higher Himalaya (∼50 km thick) to Ladakh (∼80 km thick) through the Indus Tsangpo Suture Zone (ITSZ). The Indian Moho is continuously traceable across the ITSZ which is consistent with the underthrusting of the Indian plate beyond the surface collision boundary. The estimated Poisson’s ratios in the Lesser and Higher Himalaya are low (0.249–0.253), suggesting felsic composition of the crust. The Poisson’s ratio is intermediate in the Tethyan Himalaya (0.269–0.273) and high beneath Ladakh (0.280–0.303), indicating the effect of aqueous fluid/partial melt present in the crust.

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.

Origin of Late Oligocene adakitic intrusives in the southeastern Lhasa terrane: Evidence from in situ zircon U–Pb dating, Hf–O isotopes, and whole-rock geochemistry

We describe the whole-rock geochemistry, biotite Ar–Ar and zircon U–Pb geochronology, and in situ Hf–O isotopic compositions of the Linzhi two-mica granitoid in the southeastern Lhasa terrane. The granitoids are calc-alkaline to high-K calc-alkaline in composition, and they have geochemical affinities with adakites. The whole-rock geochemistry, with high SiO2 and Al2O3 contents, low MgO, Cr, and Ni contents, low Mg# values, negative εNd(t) (−6.2 to −2.9), and low 87Sr/86Sr(i) (0.7059–0.7074), together with the in situ zircon Hf–O isotopic data (εHf(t)=−2.7 to 2.7; δ18O=7.2‰–8.8‰), indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the Lhasa terrane. This lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs (<55%). SIMS zircon U–Pb analyses of two samples indicate that the crystallization of the Linzhi granitoid took place around 26Ma, implying that the crust of the Lhasa terrane was already thickened up to 50km prior to emplacement of the adakitic rocks. 40Ar/39Ar ages for biotites, and the presence of magmatic epidote, suggest the granitoid was intruded at mid-crustal depths, indicating that the eastern Lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22Ma. The geochemical features of the Linzhi adakitic rocks are significantly different from those of Eocene adakitic rocks from the north Himalayan belt, and this suggests that we can rule out the possibility of crustal thickening in the eastern part of Lhasa terrane, caused by impingement of the Indian lower crust. Post-collisional (26–10Ma) adakitic rocks occur within an E–W-trending magmatic belt that runs parallel to the Yarlung–Tsangpo suture in the Lhasa terrane. These rocks have distinctive variations in geochemistry along the strike of the Lhasa terrane, indicating that the lower crust is isotopically heterogeneous from west to east along the terrane. These geochemical variations probably result from variable degrees of input, along strike, of juvenile mantle-derived melts into an old lower crust. The middle part of the Lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle-derived melt, underplated during Neo-Tethyan oceanic crust subduction. In contrast, the lower crust below the western and eastern parts of the terrane is relatively ancient. Therefore, crustal thickening caused by underplating of mantle-derived magmas could play an important role in the middle segment of the terrane, whereas crustal compression and shortening as a result of the collision of the Indian and Asian plates could be the major cause for crustal thickening in the western and eastern segments.

Age and thermal history of Eo- and Neohimalayan granitoids, eastern Himalaya

Four distinct granitoid suites outcrop across a N–S transect in the eastern Himalaya between the Main Central Thrust (MCT) and the Indus Tsangpo suture. The Arunachal and Tsona leucogranites outcrop, respectively, in the MCT hanging-wall and adjacent the South Tibetan Detachment. Both crystallized at about 19Ma, and appear to be lateral equivalents of the High Himalayan Leucogranites. In the central Tethyan Himalaya, the Dala granitoids form a suite of undeformed plutons emplaced into deformed Tethyan Himalayan Sequence metasediments. In the northern Tethyan Himalaya, the Yala-Xiangbo granitoids are part of an igneous complex in the core of the Yala-Xiangbo dome. Both the Dala and Yala-Xiangbo granitoids have Eocene magmatic ages (44.1±1.2Ma and 42±5Ma, respectively) that are notably older than the vast majority of post-collisional granitoids along the main Himalayan Arc. Pelitic units from the core of the Yala-Xiangbo dome record two episodes of amphibolite-grade metamorphism and monazite growth coinciding with well-documented episodes of Eohimalayan and Neohimalayan metamorphism. The thermochronological histories of the Dala and Yala-Xiangbo units suggest that the eastern Tethyan Himalaya was tectonically quiescent from ca. 40Ma until ca. 15Ma. Miocene exhumation of the Dala and Yala-Xiangbo units, and all North Himalayan Domes studied to date, appears best explained by a structural control, such as concurrent north-directed motion on the Great Counter Thrust. We hypothesize that an extension of the Ninety East Ridge may have been responsible for Eocene high heat-flow beneath the eastern Himalaya and explain the localization of magmatism there.