Asia Collision Research Articles

Overriding Lithospheric Strength Affects Continental Collisional Mode Selection and Subduction Transference: Implications for the Greater India–Asia Convergent System

The India–Asia collision, starting from 55 ± 5 Ma, leads to the formation of the Himalayas and Tibetan Plateau with great gravity potential energy and large forces acting on the surrounding blocks. However, the subduction transference/jump does not occur in the southern Indian continental margin or the northern Indian oceanic plate as supposed to happen repeatedly during the preceding Tethys evolution. Instead, the continental collision and orogeny continues until present day. The total amount of convergence during the India–Asia collision has been estimated to be ∼2,900–4,000 km and needs to be accommodated by shortening/extrusion of the Tibetan plate and/or subduction of the Greater Indian plate, which is a challenging issue. In order to study the collision mode selection, deformation partition, and continental mass conservation, we integrate the reconstruction-based convergence rate of the India–Asia collision into a large-scale thermomechanical numerical model and systematically investigate the effects of overriding Tibetan lithospheric strength and the amount of convergence. The model results indicate that the absence of subduction transference during the India–Asia collision may be attributed to strain localization and shortening of the rheologically weak Tibetan plate. In case of the India–Asia collision for ∼50 Myr with a total convergence of ∼2,900 km, the model with the intermediately weak Tibetan plate could reconcile the general deformation partition and continental mass balance of the Himalayan–Tibetan system. However, the longer period of India–Asia collision for ∼55 Myr leads to significant shortening of the overriding plate that is not consistent with the Tibetan observations, in which case an oceanic basin may be required for the Greater Indian continent.

Cenozoic two-phase topographic growth of the northeastern Tibetan Plateau derived from two thermochronologic transects across the southern Qilian Shan thrust belt

The formation of high topography on the northeastern margin of the Tibetan Plateau (the Qilian Shan) constrains the evolutionary history of the plateau. Although prominent exhumation occurred in the middle–late Miocene, the time at which deformation and relief development began in the Qilian Shan during the Cenozoic remains highly debated. Here, we use low-temperature thermochronology to reveal the histories of topographic growth in the Qilian Shan. Two transects were sampled in two primary thrust belts (Qaidam Shan and Danghenan Shan) in the southern Qilian Shan. New apatite fission track ages range from 16.6 to 113 Ma, and apatite (U-Th)/He ages range from 8.4 to 63.5 Ma. Age–elevation relationships and thermal modeling show enhanced rock exhumation in the late Paleocene–early Eocene and middle–late Miocene in both transects. The results indicate two pulses of enhanced thrusting on the major thrust belts in the southern Qilian Shan in the late Paleocene–early Eocene and middle–late Miocene. Our findings imply an early Cenozoic onset of topographic growth in the northeastern Tibetan Plateau, which was quasi-synchronous with the India–Asia collision.

New Paleomagnetic Constraints on the Early Cretaceous Paleolatitude of the Lhasa Terrane (Tibet)

New zircon U-Pb dating results from the Zonggei Formation volcanics indicate that the volcanic rocks formed at ∼114–110 Ma. Paleomagnetic data, petrography, and rock magnetism confirm the primary nature of isolated characteristic remanent magnetizations carried by titanomagnetite and hematite. A statistical analysis of the combined results from the Zonggei and Duoni formations reveals a group-mean direction of D±ΔD = 0.4° ± 6.0°, I±ΔI = 22.2° ± 5.6°, α95 = 5.6°, k = 35.2 after bedding correction based on 20 group-mean directions. The corresponding paleopole was calculated to be λp = 70.3°N, φp = 270.5°E with A95 = 5.2°. The interpretation of our data alongside the Cenozoic data from the Tethyan Himalaya indicates that the India–Asia collision initiated by 61.7 ± 3.0 Ma at 13.0° ± 1.8°N, assuming a single-collision model. Intracontinental crustal shortening totaling 1,770 ± 470 km took place on the Asian side since the onset of India–Asia collision. Furthermore, the data show that the Neo-Tethys Ocean reached its maximum N-S width of 7,100 ± 530 km at ∼132 Ma and shrank to 6,400 ± 550 km by ∼115 ± 5 Ma. This is consistent with previous estimates based on the geophysical images of the subducted Neo-Tethyan slab beneath Eurasia.

Petrogenesis of Eocene Wangdui adakitic pluton in the western Gangdese belt, southern Tibet: implications for crustal thickening

AbstractLower-crust-derived adakitic rocks in the Gangdese belt provide important constraints on the timing of Tibetan crustal thickening and on the relative contributions of magmatic and tectonic processes. Here we present geochronological and geochemical data for the Wangdui porphyritic monzogranites in the western Gangdese belt. Zircon U–Pb dating yields emplacement ages of 46–44 Ma. All samples have high Sr (321–599 ppm), low Yb (0.76–1.33 ppm) and Y (10.6–18.3 ppm) contents, with high La/Yb (51.1–72.3) and Sr/Y (21.0–51.4) ratios, indicating adakitic affinities. The low MgO (0.97–1.76 wt %), Cr (7.49–53.6 ppm) and Ni (4.75–29.1 ppm) contents, as well as high 87Sr/86Sr(i) (0.7143–0.7145), low ϵNd(t) (−10.4 to −9.8) and zircon ϵHf(t) (−17.7 to 0.4) values, suggest that the Wangdui pluton most likely originated from partial melting of the thickened ancient lower crust. In combination with previously published data, despite the east–west-trending heterogeneity of crustal composition in the Gangdese belt, the La/Yb ratios of magmatic rocks reveal that both western and eastern segments experienced remarkable crustal thickening in the Eocene. However, in contrast to the thickened juvenile lower crust in the eastern segment formed by the underplating of mantle-derived magmas, tectonic shortening plays a more crucial role in thickening of the ancient basement in western Gangdese. In fact, such Eocene-thickened ancient lower-crust-derived adakitic rocks are widely distributed in the central Himalayan–Tibetan orogen. This, together with the extensive development of fold–thrust belts, suggests that tectonic shortening might be the main mechanism accounting for the crustal thickening associated with the India–Asia collision.

Topographic Response of Hinterland Basins in Tibet to the India–Asia Convergence: 3D Thermo-Mechanical Modeling

A number of basins have developed in Tibet since the early stages of the India–Asia collision, and now, they have become integral parts of the Tibetan Plateau. Geophysical and geochemical data reveal that these basins are currently characterized either by strong or weak basements. However, it remains unclear how these hinterland basins evolved during the India–Asia collision and how they affected the post-collisional growth of the Tibetan Plateau. Here, we use 3D thermo-mechanical simulations to investigate the topographic response of a strength-varying hinterland basin imbedded in an orogenic plateau under the horizontal compression condition. Our results show that a strong hinterland basin experiences little deformation and develops into a lowland with respect to the surrounding plateau at the early stages of the collision. The lowland gradually shrinks and survives in the interior of the plateau for ∼30–40 Myr before merging into the plateau. In contrast, a weak hinterland basin uplifts soon after the initial collision and develops into a highland after ∼20 Myr of convergence. Topographic analysis reveals that the strong hinterland basin experiences an evident elevation drop after ∼20–30 Myr of convergence, followed by a rapid uplift. After compiling the paleoelevation data, we proposed that the Tibetan Plateau experienced a four-stage surface uplift, which was characterized by 1) the Gangdese and central watershed highlands isolating three lowlands during the Eocene, 2) the central lowland experiencing an elevation drop of ∼2000 m during the Oligocene, 3) the central lowland suffering a rapid uplift and merging into the Tibetan Plateau in the Early Miocene, and 4) the south and north lowlands rising and developing into a plateau similar to the modern Tibetan Plateau since the Middle Miocene.

Indian plate paleogeography, subduction and horizontal underthrusting below Tibet: paradoxes, controversies and opportunities.

The India–Asia collision zone is the archetype to calibrate geological responses to continent–continent collision, but hosts a paradox: there is no orogen-wide geological record of oceanic subduction after initial collision around 60–55 Ma, yet thousands of kilometers of post-collisional subduction occurred before the arrival of unsubductable continental lithosphere that currently horizontally underlies Tibet. Kinematically restoring incipient horizontal underthrusting accurately predicts geologically estimated diachronous slab break-off, unlocking the Miocene of Himalaya–Tibet as a natural laboratory for unsubductable lithosphere convergence. Additionally, three endmember paleogeographic scenarios exist with different predictions for the nature of post-collisional subducted lithosphere but each is defended and challenged based on similar data types. This paper attempts at breaking through this impasse by identifying how the three paleogeographic scenarios each challenge paradigms in geodynamics, orogenesis, magmatism or paleogeographic reconstruction and identify opportunities for methodological advances in paleomagnetism, sediment provenance analysis, and seismology to conclusively constrain Greater Indian paleogeography.

Calcite U–Pb dating of altered ancient oceanic crust in the North Pamir, Central Asia

Abstract. The North Pamir, part of the western syntax of the India–Asia collision zone, preserves remnants of a poorly understood Paleozoic intra-oceanic subduction zone. To constrain the age of this ancient ocean floor, we analyzed calcite phases in vesicular basalt and basaltic volcanic breccia with U–Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Dating of radial fibrous to equant spary calcite yielded three meaningful Visean–Serpukhovian ages. Rare-earth elements and yttrium (REE + Y) data reveal that the basaltic host rock of the calcite and oxidizing seawater are major sources of trace elements during calcite precipitation. U–Pb ages seem to be independent of REE + Y concentrations. Our results demonstrate the potential of calcite dating to constrain the age of ancient ocean floors. We challenge the hypothesis that a continuous early Paleozoic Kunlun Terrane extended from northern Tibet into the North Pamir.

Biomarker Records From Eocene Lacustrine Sequence in the Eastern Tibet Plateau and Its Implication for Organic Matter Sources

The Eocene is the initial stage of the Cenozoic global cooling. Compared with the abundant marine records, the continental records of Eocene are scarce. Throughout the Eocene, a series of continuous deposition of gypsum and volcanic tuff-bearing red clastic sediments have developed in the Nangqian Basin (NB). In this work, representative sediments were collected from the NB, and lipid biomarkers and compound-specific carbon isotopes of n-alkanes were analyzed. Based on the robust paleomagnetic age–depth model, from the early to the late Eocene, the compound-specific carbon isotopic compositions (δ13C23 and δ13C25) increased with the sedimentary facies changed. At the same time, the relative proportion of mid-chain length to the long-chain length homologs (Paq) decreased, and the peak carbon number (Cmax) shifted from nC21, nC22 or nC23 to nC16, nC25, nC27, or nC31. We ascribed these variations to the climate drying and water level turning high as indicated by the lithology change from fluvial to lacustrine facies and the terrestrial inputs from neighboring mountain belts in the middle–late Eocene. Moreover, we compared our n-alkane results with other records from the TP and the global sea level and marine benthic δ18O. We found that a nearly synchronous deformation and drying of the eastern TP caused by the India–Asia collision in the early Eocene was closely related to the arid conditions and topographically changed in the northern TP. The climate variations in the Eocene in the NB were mainly controlled by the global climate change and the uplift of the TP and affected by the Paratethys Sea on a long scale.

Neogene South Asian monsoon rainfall and wind histories diverged due to topographic effects

The drivers of the evolution of the South Asian Monsoon remain widely debated. An intensification of monsoonal rainfall recorded in terrestrial and marine sediment archives from the earliest Miocene (23–20 million years ago (Ma)) is generally attributed to Himalayan uplift. However, Indian Ocean palaeorecords place the onset of a strong monsoon around 13 Ma, linked to strengthening of the southwesterly winds of the Somali Jet that also force Arabian Sea upwelling. Here we reconcile these divergent records using Earth system model simulations to evaluate the interactions between palaeogeography and ocean–atmosphere dynamics. We show that factors forcing the South Asian Monsoon circulation versus rainfall are decoupled and diachronous. Himalayan and Tibetan Plateau topography predominantly controlled early Miocene rainfall patterns, with limited impact on ocean–atmosphere circulation. The uplift of the East African and Middle Eastern topography played a pivotal role in the establishment of the modern Somali Jet structure above the western Indian Ocean, while strong upwelling initiated as a direct consequence of the emergence of the Arabian Peninsula and the onset of modern-like atmospheric circulation. Our results emphasize that although elevated rainfall seasonality was probably a persistent feature since the India–Asia collision in the Paleogene, modern-like monsoonal atmospheric circulation only emerged in the late Neogene. A modern-like South Asian Monsoon only appeared when East African and Middle Eastern uplift led to the establishment of the Somali Jet around 13 million years ago, according to Earth system modelling using a range of regional palaeogeographies.

Multi-stage crustal magma reservoirs of ultrapotassic rocks recorded by zoned clinopyroxene

(a) Main crustal processes within magmatic plumbing system underneath the ultrapotassic Zhuopan pluton . (b) Petrogenetic model for the 37–30 Ma mafic potassic–ultrapotassic volcanic and plutonic rocks in western Yunnan, highlighting their distinct magmatic ascent paths and processes in the crust. • Clinopyroxene zoning and textures record a complex magma history. • Clinopyroxenes crystallized at two crustal levels with distinct depths. • Crustal contamination and magma recharge took place in the magma system. • The Zhuopan pluton originated from a metasomatized lithospheric mantle. • Potassic–ultrapotassic volcanic and plutonic rocks differ in element systematics. Ultrapotassic rocks that are rare and volumetrically minor have attracted much attention due to their genetic link with the terrestrial mantle evolution and specific tectonic settings. However, crustal processes that may be involved in the generation of these rocks are poorly known despite extensive studies on their mantle sources. The ultrapotassic Zhuopan pluton in western Yunnan, southeastern Tibetan Plateau, consists chiefly of clinopyroxene-rich mafic–intermediate rocks and well preserves clinopyroxene textures and zoning, implying its potential record of valuable information on crustal processes. This pluton was emplaced at ∼ 36 Ma, suggesting its formation in the post-collisional setting associated with the India–Asia collision. Clinopyroxenes have three distinct types and form complex zoning patterns, with colorless clinopyroxene at the core, pale green at the mantle, and dark green at the rim. The clinopyroxene–liquid thermobarometer indicated that the clinopyroxenes crystallized in two magma reservoirs at depths of ∼ 18 and ∼ 10 km. The in situ clinopyroxene Sr isotopes revealed that wall-rock contamination had taken place and then was punctuated by recharge with a relatively evolved magma, leading to the resorption of pre-existing clinopyroxenes. The Zhuopan plutonic rocks were formed through evolution within multi-stage crustal reservoirs from the primary mafic magma derived from a subduction-metasomatized lithospheric mantle. Together with the coeval (37–30 Ma) potassic–ultrapotassic volcanic counterparts, we determined that the two differ in elemental systematics. This may have resulted from distinct magmatic ascent paths and processes in the crust. We also concluded that the generation of these coeval potassic–ultrapotassic rocks was triggered by massive lithospheric delamination.

Introduction to the special issue “Tibetan tectonics and its effect on the long‐term evolution of climate, vegetation and environment”

AbstractThe long‐term evolution of the Tibetan Plateau significantly influenced Asian climate, nearby ocean physics and chemistry, and terrestrial biodiversity. This range of impacts has attracted research attention from a correspondingly broad range of disciplines, providing important new insights into prolonged and emerging debates concerning the Himalayan–Tibetan morphotectonic evolution and its impacts on the long‐term evolution of climate, biodiversity, and the environment on regional to global scales. To communicate the latest advances on this coupled tectonic, climatic and biological system, we have launched a special virtual issue in Terra Nova and solicited submissions of 20 papers in total. The papers cover a wide range of topics that fall in the following, partially overlapping, categories: pre‐India–Asia collision tectonic configuration; post‐collision deformation; sedimentary system: source to sink studies, climatic forcing, and river incision; and climatic and biospheric influence of the Tibetan Plateau.

Late Cretaceous Metamorphism and Anatexis of the Gangdese Magmatic Arc, South Tibet: Implications for Thickening and Differentiation of Juvenile Crust

AbstractMagmatic arcs are the primary sites of growth of post-Archean continental crust; however, the mechanisms and processes for transforming primary arc crust into mature continental crust are subject to disagreement. We conducted a detailed petrologic and geochronological study on mafic and felsic migmatites from the eastern Gangdese magmatic arc, which is typical of continental arcs worldwide. The studied mafic migmatites contain amphibole, garnet, plagioclase, epidote, white mica, quartz, rutile and ilmenite in melanosomes, and plagioclase, garnet, epidote, amphibole, white mica, and quartz in leucosomes. The leucosomes occur as diffuse patches, concordant bands, or concordant and discordant networks and veins in the melanosomes. The migmatites have protolith ages between ~157 and ~86–87 Ma, and metamorphic ages of ~83–87 Ma and underwent high-pressure granulite-facies metamorphism at peak P–T conditions of ~850–880°C and 15–17 kbar. Heating, burial, and associated partial melting preceded near-isobaric cooling with residual melt crystallization. Significant melt (>16 wt.%) generated during heating and loading had a granitic composition. Compositional comparison to low-grade meta-gabbros implies that any extracted melt had adakitic affinities (high Sr/Y and highly fractionated REE patterns). The eastern Gangdese magmatic arc experienced crustal thickening during Late Cretaceous late-stage evolution of the arc due to magma loading and tectonic shortening and thrusting of the arc crust. Crustal thickening and chemical differentiation of the Gangdese arc occurred during late subduction of the Neo-Tethys, prior to the India–Asia collision. Metamorphism nearly completely erased all prior igneous mineralogy and mineral chemistry, and consequent partial melting represents a potential source for Late Cretaceous granitoids of the upper arc crust. Although prior studies demonstrate the significance of fractional crystallization, deep-seated metamorphic processes largely drove chemical differentiation to produce mature continental crust in the Gangdese arc during the late Cretaceous.

Palaeogeographic reconstructions of the Eocene-Oligocene Tarim Basin (NW China): Sedimentary response to late Eocene sea retreat

The Paleogene sedimentary successions of the Tarim Basin of western China document a major regression of the proto-Paratethys Sea, significantly changing the land-sea distribution and thereby the palaeoclimatic conditions in Central Asia. However, a comprehensive reconstruction of the palaeogeographic evolution of the basin is lacking because only few exposures are known from the central part of the basin that is mostly covered by the Taklamakan Desert. Here, we reconstruct the main palaeoenvironmental changes following Eocene sea retreat by determining the sedimentological facies of 69 wells that are internally correlated by using seismic profiles and externally to published sections of the exposed southwestern and northern margins of the Tarim Basin. Two major depocentres existed in the Southwest Foredeep and the Kuqa Depression, separated from each other by the Bachu Uplift that was most likely not a particularly high terrain during the Paleogene. We confirm that a major transition from marine to continental-lacustrine sedimentation took place during the Eocene-Oligocene transition in the central parts of the Tarim Basin. We provide new palaeogeographic maps with Eocene and Oligocene facies distributions in the Tarim Basin, which will facilitate a better understanding of the sedimentary response to Paleogene sea-level change and tectonic events of the neighboring mountain ranges. Our reconstruction confirms that both tectonic and eustatic forcing played an active role in controlling the late Paleogene transgression-regression in the Tarim Basin. Provenance and palaeocurrent analyses, combined with sediment distribution patterns, indicate gradual growth and continuous uplift of the Tibetan Plateau following India–Asia collision in the early Paleogene. The demise of the Tarim Sea at ~41 Ma correlates with palaeoclimatic and environmental changes observed in several basin of central Asia. After sea retreat, the Tarim region progressively transformed into a large and shallow lake system that potentially persisted up to the (early) Oligocene. • Sedimentary facies of 69 drill holes from the Tarim Basin of SW China are analysed. • Palaeogeographic reconstructions are based on Eocene-Oligocene facies distribution. • Facies patterns indicate late Eocene tectonic activity of the Pamir and West Kunlun. • Early Bartonian sea retreating from Tarim relates in time to inner Asian aridification. • During the early Oligocene a large lake persisted in the Tarim Basin.

Tectonic transformation from orogen-perpendicular to orogen-parallel extension in the North Himalayan Gneiss Domes: Evidence from a structural, kinematic, and geochronological investigation of the Ramba gneiss dome

The North Himalayan Gneiss Domes consist of a series of isolated domes cored by leucogranite and/or gneiss body. The Ramba dome core comprises gneiss and two-mica granite, mantled mainly by schist sequences, and the degree of deformation decreases outward from pervasive mylonitization to weak mylonitization. Pre-doming deformation is characterized by the formation of regional-scale top-to-the-north shearing (D1) in the Tethyan Himalayan Sequence, locally preserved strong folding (F1), and intrusion of 46.3 Ma-40.6 Ma leucogranite. The D2 deformation consists of map-scale ductile shearing fabrics and top-to-the-north detachment along the South Tibetan Detachment System (STDS) in the middle and lower units of the dome. Intrusion of 26.1–21.0 Ma syn-kinematic leucogranite veins in the schist units occurred during the D2 north-directed extensional episode. The D3 deformation was characterized by top-to-the-east shear during 12.0–10.2 Ma, concurrent with the initiation of activity in the N–S-trending Yadong–Gulu rift. The latest deformation (D4) involved the ascent of two-mica leucogranite through diapirism at about 8.17 Ma. Microstructural analyses, as well as quartz c-axis fabrics measured by EBSD (Electron backscatter diffraction), indicate deformation temperature ranges of D2 extensional shearing from 500 to 600 °C in the middle unit of the Ramba dome to >600 °C in the lower unit. These new data imply that this type of doming in the Ramba region represents a classic type of transformation from orogen-perpendicular to orogen-parallel extensional deformation during Miocene India–Asia collision.

Late Cretaceous K-rich rhyolitic crystal tuffs from the Chuduoqu area in Eastern Qiangtang subterrane: evidence for crustal thickening of the central Tibetan Plateau prior to India–Asia collision

Late Cretaceous K-rich rhyolitic crystal tuffs from the Chuduoqu area in Eastern Qiangtang subterrane: evidence for crustal thickening of the central Tibetan Plateau prior to India–Asia collision

Palaeomagnetic indication for India–Asia collision at 12°N and maximum 810 km Greater India extent in the western suture zone

SUMMARY Knowing the pre-collisional extent of the northern Indian Plate margin (‘Greater India’) is vital to understanding the tectonic evolution of the India–Asia collision and the formation of the Himalayan–Tibetan orogen. However, suitable geological units for palaeomagnetic investigations along the Himalayan belt are limited, which makes it difficult to reconstruct Greater India during the pre-collisional period in Late Cretaceous to Palaeogene. Often the palaeomagnetic results from the Zongpu Formation at Gamba in southern Tibet (∼88.5°E) were used for estimates of Greater India, but their validity was recently questioned. As a contribution to closing the data gap, we performed a palaeomagnetic study of the Palaeocene/Lower Eocene Dibling limestone (DL) in the western Tethyan Himalaya of Zanskar (34.0°N/76.6°E). The results from 27 sites revealed a well grouping (k = 71.7) syntectonic magnetization with best grouping at 52 per cent unfolding. The remagnetization of the DL was acquired shortly after ∼54 Ma, at the latest at ∼49 Ma, and is probably carried by fine-grained magnetite formed during the early orogenic phase. Assuming proportional tilting of the fold limbs, the corresponding palaeolatitude of 11.8 ± 2.4°N suggests a maximum Greater India extent of 810 ± 420 km and a first continental contact with the southern Eurasian margin at ∼12°N in the western part of the suture zone. The tectonostratigraphic equivalence of the DL with the Zongpu Formation at Gamba and a great similarity in their magnetic properties supports a secondary origin of the Gamba results. Through understanding the mechanism of remagnetization in the DL, an early orogenic remanence acquisition is also indicated for the Zongpu Formation, and thus the Gamba results deserve further credit for Greater India reconstructions. However, we note a large inconsistency of the available Late Cretaceous and Palaeogene palaeolatitude data from the Tethyan Himalaya by up to ∼20°, corresponding to differences of up to ∼2000 km in the size of Greater India. These discrepancies require further palaeomagnetic work in the Tethyan Himalaya, and in particular we recommend comparative studies at same locations and of same units.

Temporal and Spatial Variations of Enriched Source Components in Linzizong Volcanic Succession, Tibet, and Implications for the India–Asia Collision

Abstract The temporal and spatial distribution of enriched source components at sites of continent–continent collision provides critical insights into mantle dynamic processes associated with subduction- and collision-related events. However, determining the origin of such enriched components remains a significant challenge. We report a comprehensive dataset of the Linzizong volcanic succession (LVS) from four locations with varying distance across-strike to the Indus–Yarlung suture in southern Tibet, which marks the exposed surface expression of the India–Asia collision zone. The LVS rocks in this study can be divided into two groups: a calc-alkaline Group 1 (69–55 Ma), mainly including basaltic–andesitic varieties, and a shoshonitic Group 2 (52–50 Ma), consisting predominantly of silicic rocks with minor mafic compositions. Group 1 samples are likely derived from the fractional crystallization of primitive basaltic melts as a result of the partial melting of a metasomatized mantle wedge. These samples are decoupled in Nd–Hf isotopic compositions, suggesting an incorporation of subducting sediment-derived melts into the mantle wedge. The influence of sediment-derived melt on the mantle source increases away from the suture zone toward Asia (i.e., from the south to the north) as indicated by the more enriched Sr, Nd, Pb, and Hf isotopic compositions, as well as elevated Th/La and La/Sm ratios. The heavy δ26Mg values, and high Ba/Th and Sr/Th ratios of samples close to the suture coincide with the dehydration of the subducting Neo-Tethyan slab. Group 2 mafic samples have depleted and coupled εNd–εHf isotopic compositions and are characterized by elevated (La/Yb)N and Dy/Yb ratios as well as low Zr/Nb ratios, indicating an origin of enriched garnet-bearing lithospheric mantle with contributions from asthenosphere-derived materials. Group 2 silicic samples are isotopically enriched both near and far away from the suture, which can be attributed to the involvement of ancient lower crust-derived melt from Tethyan Himalaya and central Lhasa subterrane basement, respectively. Our results show that the LVS are the magmatic response to late subduction (Group 1), and to initial India–Asia collision and slab breakoff (Group 2). Negative trends in the whole-rock Nd and zircon Hf isotopic compositions at ~51 Ma should be treated with caution for geodynamic interpretations, depending on the distance between the samples and the India–Asia suture.

Detrital Zircon Provenance of the Cenozoic Sequence, Kotli, Northwestern Himalaya, Pakistan; Implications for India–Asia Collision

This study reported the detrital zircon U-Pb geochronology of the Cenozoic sequence exposed in Kotli, northwestern Himalaya, Pakistan, which forms part of the Kashmir foreland basin. The U-Pb detrital age patterns of the Paleocene Patala Formation show a major age cluster between ~130–290 Ma, ~500–1000 Ma and ~1000–1500 Ma, which mainly resembles the lesser and higher Himalayan sequence. However, the younger age pattern (~130–290 Ma) can be matched to the ages of the ophiolites exposed along the Indus–Tsangpo suture zone. In addition, two younger grains with 57 Ma and 55 Ma ages may indicate a contribution from the Kohistan-Ladakh arc. The detrital zircons in the upper Tertiary sequence show the increased input of younger detrital ages <100 Ma, with more pronounced peaks at ~36–58 Ma, ~72–94 Ma and ~102–166 Ma, indicating the strong resemblance to the Asian sources including the Kohistan–Ladakh arc, Karakoram block and Gangdese batholith. This provenance shift, recorded in the upper portion of Patala Formation and becoming more visible in the upper Tertiary clastic sequence (Kuldana and Murree formations), is related to the collision of the Indian and Asian plates in the northwestern Himalayas. Considering the age of the Patala Formation, we suggest that the Indian and Asian plates collided during 57–55 Ma in the northwestern Himalayas, Pakistan.

Northward Growth of the West Kunlun Mountains: Insight From the Age–Elevation Relationship of New Apatite Fission Track Data

The Cenozoic collision between India and Asia promoted the widespread uplift of the Tibetan Plateau, with significant deformation documented in the Pamir Plateau and West Kunlun Mountains. Low-temperature thermochronology and basin provenance analysis have revealed three episodes of rapid deformation and uplift in the Pamir–West Kunlun Mountains during the Cenozoic. However, there is very little low-temperature thermochronology age–elevation relationship (AER) data on fast exhumation events in this area—especially in the West Kunlun Mountains— leading to uncertainty surrounding how these events propagated within and around the mountain range. In this study, we produced an elevation profile across granite located south of Kudi, Xijiang Province, China, to reveal its exhumation history. Apatite fission track AER data show that a rapid exhumation event occurred at ∼26 Ma in the southern West Kunlun Mountains. When combined with published data, we interpret that the initial uplift events related to the India–Asia collision began in the central Pamir, southern West Kunlun, and northern West Kunlun regions during the Late Eocene, Oligocene, and Middle Miocene periods, respectively. Therefore, the Cenozoic northward growth process occurred from south to north around West Kunlun.

Early mesozoic arc–back-arc system in the leading edge of the Tibetan Plateau

The southern Lhasa subterrane is the leading edge of the Tibetan Plateau and preserves important records of the Neo-Tethyan oceanic subduction and India–Asia collision. However, the early tectonic evolutionary history of the Neo-Tethyan oceanic subduction remains unclear. Here we present results from a systematic study on the early Mesozoic (Middle Triassic–Late Jurassic) magmatic and sedimentary rocks in the southern Lhasa subterrane. These rocks are distributed zonally along the southern and northern parts of the subterrane, and have been classified into the north and south belts. The magmatic rocks in the north belt show a back-arc basin affinity, with accreted bimodal volcanic sequence and sediments from central and southern Lhasa subterranes, whereas the magmatic rocks in the south belt reveal typical magmatic arc characteristics. The sediments were derived from southern Lhasa subterrane, without the input of clastic components from the central Lhasa subterrane. Based on geochronological, geochemical, and sedimentary records from both belts, we conclude that the early Mesozoic magmatism was triggered by the northward subduction of the Neo-Tethyan oceanic slab, and its initial subduction likely occurred before the Middle Triassic (ca. 240 Ma). During the Early–Middle Jurassic (ca. 193–165 Ma), a short-lived episode of back-arc extension, we attribute to roll-back of the subducting Neo-Tethyan slab, resulted in the single magmatic arc that existed during the early stages (ca. 240–194 Ma) and evolved gradually into an arc–back-arc system. The north belt represents a back-arc basin setting associated with a continental arc, whereas an intra-oceanic and continental (island) arc simultaneously developed in the south belt, and both these belts were possibly generated under the same subduction system.