Indus-Yarlung Suture Research Articles

High-pressure, low-temperature metamorphism preserved in the Indus-Yarlung suture zone of the eastern Himalaya: Overprinting at amphibolite facies and comparison with occurrences in the western Himalaya

High-pressure, low-temperature metamorphism preserved in the Indus-Yarlung suture zone of the eastern Himalaya: Overprinting at amphibolite facies and comparison with occurrences in the western Himalaya

Pressure-temperature-time evolution of the Milin pelitic schists: Implications for the tectonic evolution of the eastern Indus-Yarlung suture zone, eastern Himalaya

We report an integrated comprehensive dataset composed of petrography, pressure-temperature (P-T) calculations, monazite U-Th-Pb ages, and trace-element data from pelitic schists in the eastern Indus-Yarlung suture zone in the Milin area of the eastern Himalaya. These rocks represent the exposure of subduction-related rocks within the eastern Indus-Yarlung suture zone accretionary complex. The dominant mineral assemblages of the pelitic schists are garnet + kyanite + staurolite + biotite + quartz and garnet + kyanite + staurolite + biotite + paragonite + sillimanite with quartz, plagioclase, and ilmenite assemblages. Phase equilibrium modeling of sillimanite-bearing pelitic schists yielded peak P-T conditions of ∼670−680 °C at ∼8.6 kbar, similar to that of kyanite-bearing schists (∼670 °C, ∼8.8 kbar). Monazite grains with complex internal structures retained variable ages ranging from 28 Ma to 15 Ma, which correlate systematically with changes in the concentrations of Y, Th, U, and heavy rare earth elements and ratios of Th/U. Combined with petrologic analysis, we conclude that the pelitic schists experienced a long-lived prograde metamorphism from ca. 28 Ma to ca. 22 Ma. Peak metamorphism occurred in the period 22−21 Ma, followed by quasi-isothermal decompression until 15 Ma. The discrepancies among metamorphic P-T-t paths in the eastern Indus-Yarlung suture zone indicate the presence of not only collision-related regional metamorphism at medium P-T conditions, but also subduction-related high-pressure−low-temperature terranes in the Milin region. These two domains experienced different P-T evolution and tectonic histories and were juxtaposed in the early Neogene during the India-Asia continental collision.

Late Cenozoic Cooling History of the Xigaze Fore-Arc Basin along the Yarlung–Zangbo Suture Zone (Southern Tibet): New Insights from Low-Temperature Thermochronology

Abstract The Tibetan Plateau is currently the widest and highest elevation orogenic plateau on Earth. It formed as a response to the Cenozoic and is still ongoing collision between the Indian and Eurasian plates. The Xigaze fore-arc basin distributed along the Indus–Yarlung suture zone in southern Tibet preserves important information related to the late Cenozoic tectonic and topographic evolution of the plateau. In this study, apatite fission track (AFT) thermochronology was carried out on twelve sandstone samples from the middle segment of the Xigaze basin and additionally on four sedimentary rocks from the neighboring Dazhuka (Kailas) and Liuqu Formations. Inverse thermal history modeling results reveal that the fore-arc basin rocks experienced episodic late Oligocene to Miocene enhanced cooling. Taking into account regional geological data, it is suggested that the late Oligocene-early Miocene (~27–18 Ma) cooling recognized in the northern part of the basin was promoted by fault activity along the Great Counter thrust, while mid-to-late Miocene-accelerated exhumation was facilitated by strong incision of the Yarlung and Buqu rivers, which probably resulted from enhanced East Asian summer monsoon precipitation. Sandstone and conglomerate samples from the Dazhuka and Liuqu Formations yielded comparable Miocene AFT apparent ages to those of the Xigaze basin sediments, indicative of (mid-to-late Miocene) exhumation soon after their early Miocene burial (> ~3–4 km). Additionally, our new and published low-temperature thermochronological data indicate that enhanced basement cooling during the Miocene prevailed in vast areas of central southern Tibet when regional exhumation was triggered by both tectonic and climatic contributing factors. This recent and widespread regional exhumation also led to the formation of the high-relief topography of the external drainage area in southern Tibet, including the Xigaze fore-arc basin.

Discovery and genesis of helium-rich geothermal fluids along the India–Asia continental convergent margin

Large-scale helium accumulations are rarely reported in tectonically active continental subduction zones. This study shows geochemical variations in helium and other volatiles (N2, CO2, Ne and Ar) in hot springs from the India–Asia continental convergent margin (Indus–Yarlung suture, IYS) to the intracontinental extensional region in southern Tibet and identifies a 1200-km-long helium enrichment zone (He > 5000 ppm) within 50 km north of the IYS. In total, 144 gas samples were categorized into four groups: Group A, with dominant N2 species and He concentrations (0.07–2.31 vol%) 1.4–23 times higher than industrial-grade He resources (0.05–0.1 vol%); Group B, with dominant CO2 species and high He concentrations (0.05–1.57 vol%); and Groups C and D, depleted in He (<0.05 vol%) and dominated by CO2 and N2, respectively. The gas mixtures exhibit systematic variations from the extensional zone to the continental convergent margin, changing from deep-origin, He-depleted, CO2-dominated fluids (Group C) to relatively shallow crust-derived, He-rich, N2-dominated gases (Group A). The former exhibits small but detectable amounts of mantle-derived He (2.3 ± 2.6%) and CO2 (<3.2%), primarily atmosphere-derived N2 (δ15N = 0.0 ± 0.6‰) and Ar (40Ar/36Ar = 307 ± 25), and crustal CO2 (82 ± 9% from carbonate and 17 ± 9% from sediments/metasediments) from the subducted Indian slab-carbonate and Tibetan crust. The low He flux of Group C matches the steady release of He from the 70-km-thick crust into the geothermal waters during 0.02–43.5 ka. Shallow calcite precipitation and dissolution reactions cause CO2 loss and slight He enrichment in the extensional zone. Group A samples at the convergent margin show pure crustal He, more radiogenic 40Ar (40Ar/36Ar = 330 ± 67), and higher sedimentary contributions of 28 ± 23% for N2 and 37 ± 11% for CO2. A high He flux near the IYS requires an external source with high 4He/N2 (∼0.05) and 4He/20Ne (103–105) to provide >62% of the total He flux. This finding is best explained by He production in the south-dipping Gangdese granitic belt and deep crust, accumulation below the accretionary wedge, and episodic liberation along Great Counter thrust structures since 0.7 Ma by Tibetan Plateau uplifts and intense hydrothermal activity. The He-rich fluids mix with the sedimentary and atmosphere-derived N2 from the accretionary wedge, forming He-rich N2-type gases. This study systematically reports large-scale He enrichment along the continental convergent margin for the first time and demonstrates that complex crustal-scale structures in the convergent margin significantly controlling gas geochemistry. These results have profound implications for potential helium resource exploration and He-CO2-N2 recycling in continental subduction zones.

Cenozoic kinematic histories of the Tidding and Lohit thrusts in the northern Indo-Burma Ranges: Implications for crustal thickening and exhumation of Gangdese lower arc crust along the Indus-Yarlung suture zone

Crustal thickening has been a key process of collision-induced Cenozoic deformation along the Indus-Yarlung suture zone, yet the timing, geometric relationships, and along-strike continuities of major thrusts, such as the Great Counter thrust and Gangdese thrust, remain inadequately understood. In this study, we present findings of geologic mapping and thermo- and geochronologic, geochemical, microstructural, and geothermobarometric analyses from the easternmost Indus-Yarlung suture zone exposed in the northern Indo-Burma Ranges. Specifically, we investigate the Lohit and Tidding thrust shear zones and their respective hanging wall rocks of the Lohit Plutonic Complex and Tidding and Mayodia mélange complexes. Field observations are consistent with ductile deformation concentrated along the top-to-the-south Tidding thrust shear zone, which is in contrast to the top-to-the-north Great Counter thrust at the same structural position to the west. Upper amphibolite-facies metamorphism of mélange rocks at ∼9−10 kbar (∼34−39 km) occurred prior to ca. 36−30 Ma exhumation during slip along the Tidding thrust shear zone. To the north, the ∼5-km-wide Lohit thrust shear zone has a subvertical geometry and north-side-up kinematics. Cretaceous arc granitoids of the Lohit Plutonic Complex were emplaced at ∼32−40 km depth in crust estimated to be ∼38−52 km thick at that time. These rocks cooled from ca. 25 Ma to 10 Ma due to slip along the Lohit thrust shear zone. We demonstrate that the Lohit thrust shear zone, Gangdese thrust, and Yarlung-Tsangpo Canyon thrust have comparable hanging wall and footwall rocks, structural geometries, kinematics, and timing. Based on these similarities, we interpret that these thrusts formed segments of a laterally continuous thrust system, which served as the preeminent crustal thickening structure along the Neotethys-southern Lhasa terrane margin and exhumed Gangdese lower arc crust in Oligocene−Miocene time.

India (Tethyan Himalaya Series) in Central Myanmar: Implications for the Evolution of the Eastern Himalayan Syntaxis and the Sagaing Transform‐Fault System

AbstractIn the Katha Range of central Myanmar, lithologic tracers and pressure‐temperature‐deformation‐time data identify Cambro‐Ordovician, Indian‐affinity Tethyan Himalaya Series, located ∼700 km from their easternmost outcrop in S‐Tibet, and ∼450 km from Himalayan rocks in the Eastern Himalayan Syntaxis. Metamorphism began at ∼65 Ma, peaked at ∼45 Ma (∼510°C, 0.93 GPa), and exhumation/cooling (∼25°C/Myr) occurred until ∼30 Ma in a subduction‐early collision tectonic setting. When the Burma microplate—part of the intra‐Tethyan Incertus arc—accreted to SE‐Asia, its eastern boundary, the southern continuation of the Indus‐Yarlung suture (IYS), was reactivated as the Sagaing fault (SF), which propagated northward into Indian rocks. In the Katha rocks, this strike‐slip stage is marked by ∼4°C/Myr exhumation/cooling. Restoring the SF system defines a continental collision‐oceanic subduction transition junction, where the IYS bifurcates into the SF at the eastern edge of the Burma microplate and the Jurassic ophiolite‐Jadeite belts that include the Incertus‐arc suture.

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.

Direct dating of the Sinongduo thrust system in southern Tibet: immediate response to India-Asia collision

ABSTRACT The Tibetan Plateau is a product of the continental collision between the Indian and Asian plates along the Indus-Yarlung suture zone that started in the early Cenozoic. This collision initially produced stress at the collisional boundary and later propagated with time on both the Indian plate and Asian plate. Many studies have reported tectonic deformation in the Himalayas representing the immediate response of continental collision on the Indian side near the collisional boundary. However, there have been only few studies on the early Cenozoic shortening structures in the Lhasa block that forms the southernmost part of the Asian plate. In this study, we performed Ar/Ar geochronology of syntectonic muscovite from the Sinongduo thrust system in the central Lhasa block. The results show that the Sinongduo thrust system was active in the Early Eocene. We interpret the Sinongduo thrust system as an immediate response to the India-Asia collision in the Lhasa block. This study reconciles the early Cenozoic India-Asia collision with the lack of tectonic response in the Lhasa block.

Deep electrical resistivity structure across the Gyaring Co Fault in Central Tibet revealed by magnetotelluric data and its implication

As one of the most prominent deformation features on Tibetan plateau, a series of N-S systematic rifts and V-shaped conjugate strike-slip faults are well developed in central-southern Tibet. However the mechanism for the formation of the rifts and conjugate strike-slip faults is still controversial. An east-west trending magnetotelluric (MT) array has been operated across the Gyaring Co Fault (GCF) at the northern end of the Xainza-Dingjye Rift (XDR) in Lhasa block. Three-dimensional (3-D) inversions are employed to image lithospheric resistivity structure. Combined with the previous north-south MT 3-D inversion result, electrical resistivity models reveal obvious conductive layer in the mid-to-lower crust beneath the XDR and apparent resistivity change in the vicinity of the GCF. The calculated depth-integrated conductivity shows that the weak mid-to-lower crust (~30 km- ~ 60 km) characterized by high conductance (≥ ~10,000 S) and melt fraction of ~5–13%, is mainly distributed between the Indus-Yarlung suture (IYS) and the GCF, whereas the relatively rigid crust characterized by low conductance (≤ ~2000 S) and melt fraction of ~2–4% appears to the northeast of the GCF. The weak mid-to-lower crust in southern Tibet and the abrupt difference in the resistivity characteristics of the mid-to-lower crust in the vicinity of the GCF, suggest the formation of the rifts in Lhasa block is closely associated with the weak mid-to-lower crust. Taken together with the upper mantle seismic features, the weak mid-to-lower crust is believed to be attributed to the underplated Indian plate. An alternative geodynamic model is presented to respond to the eastward extrusion during the ongoing N-S convergence between the Indian and Eurasian plates, that the rigid crust in central Tibet is sandwiched by the weak mid-to-lower crust in southern and northern Tibet, and the rifts form above the weak mid-to-lower crust, whereas the conjugate strike-slip faults develop on the rigid crust in central Tibet.

Sediment provenance of pre- and post-collisional Cretaceous–Paleogene strata from the frontal Himalaya of northwest India

Whereas the timing for India–Asia collision remains debated, contrasting collisional models provide testable predictions in terms of sediment source contributions during the accumulation of Paleocene to middle Eocene deposits now exposed in the frontal Himalayan system. Within the Lesser Himalaya and frontal thrust system of northwest India, discontinuous exposures of the Cretaceous Singtali Formation and upper Paleocene–middle Eocene Subathu Formation yield a record of these early collisional stages. To test competing collisional models, we analyze the provenance of these deposits with new detrital zircon U-Pb and Hf isotopic data which can distinguish among Indian plate, Asian plate, Kohistan-Ladahk arc, and various Himalayan sources. Detrital zircon age distributions for the Singtali Formation are dominated by Paleoproterozoic zircons with a distinct Cretaceous age component, whereas age data from the Subathu Formation record (1) a marked increase in the relative abundance of Cambrian–Neoproterozoic grains, (2) a decrease in the proportion of Paleoproterozoic grains, and (3) distinct Permian and Late Cretaceous–Paleocene age components. All Cretaceous grains from the Singtali Formation yielded crustal Hf isotopic signatures, indicating a distinctive pre-collisional source of Cretaceous grains of Indian affinity. Zircon Hf isotopic signatures from <320 Ma grains in the Subathu Formation show a significant lower to middle Eocene increase in source diversity, including juvenile grains most likely originating from the Asian plate and Kohistan-Ladakh arc. This requires inception of India-Asia collision by ∼44–50 Ma—the depositional age of the uppermost Subathu Formation. This major provenance shift is similar to that observed for pre- and post-collisional Tethyan Himalayan strata in the north, which is suggestive of a single contiguous basin linking the Lesser Himalaya by ∼44–50 Ma and contests the notion that the Lesser and Tethyan Himalaya were separated by a “Greater India Basin” during the Eocene. When coupled with the well documented Paleocene–early Eocene provenance record of the Tethyan Himalaya, these new data provide support for a collisional model in which Asian detritus reached the northernmost edge of India by ∼59 Ma with terminal closure of both the Shyok and Indus-Yarlung suture zones by ∼54 Ma.

Attenuation of high frequency body waves in the crust of western Tibet

The attenuation characteristics of western Tibet have been analyzed by estimating the seismic attenuation parameters (Qp−1 and Qs−1) at five different central frequencies (1.5, 3, 6, 12 and 18 Hz). The extended coda normalization method has been used on 469 three component local earthquakes which are recorded at Y2 network of broadband stations deployed in western Tibet during years 2007–2011. We have estimated the frequency dependent Qp−1 and Qs−1 values at each station. The results indicate strong body wave attenuation in the medium below western Tibet. The highest attenuation is observed at WT19 and WT13 stations located near to the Karakoram fault (KKF) and Indus-Yarlung suture (IYS). Across the KKF, we have subdivided the entire study area into two regions to explore the lateral variations of crustal attenuation properties. The first part covers the northeastern of KKF referred as Region 1 while the second part covers the southwestern of KKF referred as Region 2. The frequency dependent relations are found as follows: Qp−1 = (19.4 ± 4.4).10−3f−(0.9±0.2), Qs−1 = (13.3 ± 2.1).10−3f−(0.8±0.1) for Region 1 and Qp−1 = (18.4 ± 1.8).10−3f−(0.8±0.1), Qs−1 = (13.2 ± 2.2).10−3f−(0.8±0.1) for Region 2. Both the regions show strong frequency dependent nature of Q−1 values with no significant lateral variations. It may suggest the presence of similar tectonic complexities and heterogeneities beneath both the regions. We also find that all the estimated values of Qp−1/ Qs−1 ratios are greater than unity at all the frequency range which are likely to be associated with both the intrinsic and scattering phenomenon in the crust of western Tibet.

Late Cretaceous Adakitic Granites of the Southeastern Tibetan Plateau: Garnet Fractional Crystallization of Arc‐Like Magmas at the Thickened Neo‐Tethyan Continental Margin

The tectonic setting of Cretaceous granitoids in the southeastern Tibet Plateau, east of the Eastern Himalaya Syntax, is debated. Exploration and mining of the Laba Mo–Cu porphyry‐type deposit in the area has revealed Late Cretaceous granites. New and previously published zircon U–Pb dating indicate that the Laba granite crystallized at 89–85 Ma. Bulk‐rock geochemistry, Sr–Nd isotopic data and in situ zircon Hf isotopic data indicate that the granite is adakitic and was formed by partial melting of thickened lower crust. The Ca, Fe, and Al contents decrease with increasing SiO2 content.These and other geochemical characteristics indicate that fractional crystallization of garnet under high‐pressure conditions resulted in the adakitic nature of the Laba granite. Cretaceous granitoids are widespread throughout the Tibetan Plateau including its southeastern area, forming an intact curved belt along the southern margin of Eurasia. This belt is curved due to indenting by the Indian continent during Cenozoic, but strikes parallel to both the Indus–Yarlung suture zone and the Main Frontal Thrust belt. It is therefore likely that Cretaceous granitoids in both the Gangdese and southeastern Tibetan Plateau areas resulted from subduction of Neo‐Tethyan lithosphere.

Geochemical evidence for thin syn-collision crust and major crustal thickening between 45 and 32 Ma at the southern margin of Tibet

Geochemical data on widely distributed igneous rocks of southern Tibet are used to reconstruct paleo-crustal thickness during the 50+ million years that have elapsed since the onset of the India-Asia collision. We use two approaches, one based on Nd isotopes and an assimilation-recharge model for granitic magma genesis and another empirical method based on trace element geochemistry (La/Yb). The focus is on granitic rocks of two age ranges in a segment of the southern Lhasa Block between approximately 89.5° and 92.5°E longitude. One age range, 45 to 62 Ma, spans the time of the onset of collision and for which we infer the geochemistry of granitic rocks reflects mainly pre-collision structure. The other age range is 21 to 9 Ma for the Nd isotopic approach, and 32 to 9 Ma for La/Yb, where the geochemistry must reflect post-collision structure. Our results suggest that the pre- and syn-collision southern margin of the Lhasa block, that portion now located within 50–60 km of the Indus-Yarlung suture (IYS) and south of 29.8°N latitude was relatively thin, about 25–35 km thick until 45 Ma. At approximately 29.8°-29.9°N latitude there was a pronounced crustal discontinuity, and north of that latitude (for a distance that we cannot constrain), the inferred crustal thickness was greater, at least 50–55 km, as indicated by latest Cretaceous and Early Tertiary granitoids and ignimbrites that have large fractions of assimilated continental crust and high La/Yb ratios. Post-collision Nd isotopic and La/Yb data from granitoids younger than 32 Ma suggest that the southern margin south of 29.8°N was thickened substantially to at least 55–60 km (based on Nd isotopes) and possibly as much as 70–75 km (based on La/Yb) by Early to mid-Miocene time. These observations require that thickening of the southern Lhasa Block margin in the period 45–32 Ma was non-uniform; the crust now within 60 km of the suture was thickened by approximately 40 km whereas the crust north of 29.9°N latitude was thickened much less, or not at all. The region currently between 29.8°N and the YTS may have been the highest elevation mountain terrane in the period from roughly 30 to 20 Ma. The amount of Miocene denudation reflects this difference, as there is evidence of substantially more denudation near the IYS than in the region north of 29.9°N. Some of the difference in thickening could be due to magmatic additions from the mantle in the region south of 29.8°N, but there is need for at least 30 km of tectonic thickening between 45 and 32 Ma. The non-uniform thickening suggests that the high elevations at the southern margin of the Himalaya-Tibet orogen propagated southward by about 200 km, from north of Lhasa to their present position, during the period from 50 to 20 Ma. Present crustal thickness requires an additional 10–15 km of more uniform post-Miocene thickening.

Contemporaneous east–west extension and north–south compression at 43 Ma in the Himalayan orogen

It was previously proposed that the onset of east–west extension in the Himalayan orogen occurred after ca. 19 Ma. Here we present a Re–Os isochron age of ca. 43 Ma for sulfides from the Zhaxikang Zn–Pb–Sb–Ag polymetallic deposits, which occur within N–S–striking normal faults that reflect E–W extension in the Tethyan Himalayan orogen. This age is consistent with previously reported ages for regional metamorphism (ca. 48–42 Ma), granitoids (ca. 47–40 Ma) and gabbros (ca. 45 Ma) occurring along the Indus–Yarlung suture. In addition, this type of structure also broadly occurs in the Lhasa terrane, and plays a critical role in the Eocene porphyry–(skarn) copper polymetallic mineralization there. By ca. 45–43 Ma, the north–south–directed horizontal deviatoric stresses had peaked and subducted Neo–Tethyan slab breakoff had occurred at great depth, leading to abrupt slowdown of Indo–Asia convergence (ca. 45 Ma). E–W extension initially developed at shallow depths, resulting in formation of the north–south trending normal faults in the plateau. This type of extension would lead to collapse of the Tibetan plateau and decrease in altitude, subsequently resulted in Central Asian and the global climate changes. Our study places new constraints on the timing and nature of early Cenozoic tectonics and metallogenesis in the Himalayan orogen and climate changes in Central Asia.

Post-collisional exhumation of the Indus-Yarlung suture zone and Northern Tethyan Himalaya, Saga, SW Tibet

The Cretaceous-Eocene sedimentary sequences of the Sangdanlin section at Saga, SW Tibet, within the India-Asia collisional zone, have been studied in detail for constraining the timing of initial collision. However, the post-depositional history has not been investigated. Here, we report new low temperature thermochronology data along the classic Sangdanlin section, to reveal the cooling/exhumation history of the Indus-Yarlung suture zone (IYSZ) and northern Tethyan Himalaya. Samples from the IYSZ, all yield latest Oligocene-Miocene ages, with most zircon (U-Th)/He ages ranging between ~18 and 27 Ma and apatite fission track (AFT) ages of ~12–9 Ma. Thermal history modelling indicates a consistent Middle-Late Miocene cooling episode related to post-Great Counter Thrust (GCT) incision of the Yarlung River. To the south, samples from the northern Tethyan Himalaya Mesozoic sequences yield older ZHe ages of ~32–36 Ma, signifying activity of the Tethyan Himalayan fold-thrust zone, while two granites linked to formation of the North Himalayan gneiss domes yield Late Miocene AFT ages. The regional low temperature thermochronology data reveal different cooling patterns for tributaries on the northern and southern sides of the Yarlung River. This points to the strong influence of Early Miocene tectonism along the GCT, as a major controlling factor on river drainage reorganization at this time.

Oligocene-Miocene burial and exhumation of the southernmost Gangdese mountains from sedimentary and thermochronological evidence

The Kailas conglomerates crop out ubiquitously along the southernmost boundary of the Gangdese batholith. They unconformably overlie the Gangdese batholith and are displaced by the Great Counter thrust (GCT) fault, forming a fault contact with the Xigaze forearc basin, the associated subduction complex and the Tethyan Himalayan sequence. These strata furnish a record of uplift and paleoenvironmental change in the Indus-Yarlung suture zone during the Oligocene-Miocene. Our new and previously published low-temperature thermochronometric data from the Gangdese batholith and the Kailas conglomerates indicate a period of rapid exhumation beginning approximately 17–15Ma centered on the southern margin of the Gangdese batholith, whereas regional uplift commenced significantly earlier during the deposition of the Kailas conglomerates, based on the presence of an abrupt facies transition from deep-water lacustrine deposits to red alluvial fan or fluvial deposits. The period of rapid exhumation probably lagged behind the initiation of faster uplift, while the related changes in the depositional environment were most likely recorded immediately in the basin stratigraphy. Subsequently, the Kailas conglomerates were buried in association with the development of the north-directed Great Counter thrust, while rapid exhumation was facilitated by efficient incision by the paleo-Yarlung river at approximately 17–15Ma.

Detailed Configuration of the Underthrusting Indian Lithosphere Beneath Western Tibet Revealed by Receiver Function Images

AbstractWe analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE‐1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55–82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus‐Yarlung suture (IYS). An intracrustal discontinuity is observed at ~55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At ~20 km depth, a midcrustal low‐velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere‐asthenosphere boundary (LAB) can be recognized at depths of 130–200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp‐flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

Chapter 3 Regional context of the geology of the Andaman–Nicobar accretionary ridge

Abstract The Andaman–Nicobar accretionary ridge forms the eastern boundary of the Bay of Bengal and is presently being constructed by accretion and underplating of sediments off-scraped from the obliquely colliding Bengal Fan. Net accretion is relatively low ( c. 28%) with the rest subducted mostly into the upper mantle. Although subduction initiated along the margin at c. 95 Ma, large-scale subduction accretion likely accelerated during the Early Miocene by which time wedge top basins had formed. Prior to that time, sediment off-scraped during the Eocene against the ophiolitic backstop was probably derived from the adjacent magmatic arc of Burma. There was also some erosion of continental sources at that time, probably from the Sibumasu Block which forms the western edge of Sundaland. The scale of this accretion was small and potentially interrupted by times of tectonic erosion during the Palaeogene. The influence of continental erosion increased into the Oligocene, potentially accompanied by modest flux from the Indus–Yarlung Suture via the Irrawaddy River. Drainage capture in eastern Tibet in the Early Miocene and opening of the Andaman Sea, probably in the Late Miocene, has removed these source areas to the Andaman Trench.

Tectonic and erosional history of southern Tibet recorded by detrital chronological signatures along the Yarlung River drainage

The Indus-Yarlung suture zone, within the larger Indo-Asia collision zone in southern Tibet, is characterized by a central depression with two oppositely directed axial rivers: the eastward-flowing Yarlung River and the westward-flowing Indus River. The axial valley is flanked by high-elevation ranges of the southern Lhasa terrane to the north and the Tethyan Himalaya to the south. This study analyzes the detrital geochronological and thermochronological signatures of rivers draining into the Indus-Yarlung suture zone as a proxy for the timing of tectonic, magmatic, and erosional processes in southern Tibet. Zircon U-Pb ages reflect source area crystallization ages, and their distribution is proportional to the relative area of source rocks exposed in the catchment areas. Rivers draining the northern side of the collision zone are dominated by ages between ca. 40 Ma and ca. 60 Ma, similar to ages of rocks in the Gangdese arc, whereas one river sample draining the southern side records a Tethyan Himalayan signature characterized by age clusters at ca. 500 Ma and ca. 1050 Ma. U-Pb zircon ages from the modern drainages are similar to signals preserved in the Oligocene−Miocene Liuqu and Kailas basins, suggesting that the geology of the Cenozoic drainages was similar to modern, albeit with significant erosion in the Miocene. New apatite fission-track (AFT) ages from some of the same rivers show an early Miocene regional exhumation signature, which is interpreted to record regional uplift-induced erosion coupled with efficient river incision by the Indus-Yarlung fluvial system as a result of renewed underthrusting (following rollback) of India under Asia. Late Miocene (ca. 8 Ma) apatite (U-Th)/He (AHe) ages are consistent with cooling and exhumation associated with E-W extension followed by a decrease in erosion after ca. 6 Ma.

Eocene Neo-Tethyan slab breakoff constrained by 45 Ma oceanic island basalt–type magmatism in southern Tibet

Slab breakoff is one of the primary processes in the evolution of many collisional orogens. In the Tibet-Himalaya orogen, the timing of breakoff of the Neo-Tethyan slab remains controversial because of a scarcity of solid evidence. This study reports the discovery of Eocene gabbros, dated at 45.0 ± 1.4 Ma (in-situ U-Pb age of titanite) using secondary ion mass spectrometry, from the eastern segment of Tethyan Himalaya in southern Tibet. These rocks show geochemical characteristics similar to those of HIMU (high μ)–type oceanic island basalt and have depleted Sr-Nd isotopes [87Sr/86Sr(t) = 0.70312-0.70317; eNd(t) = +4.9 to +5.0]. It is suggested that the gabbros stand as the first direct evidence for partial melting of the asthenosphere followed by rapid magma ascent with negligible crustal contamination. This event, combined with results from relevant studies along the Indus-Yarlung suture zone, is best explained by a sudden and full-scale detachment of subducted Neo-Tethyan slab at great depth. The breakoff model may account for coeval tectonomagmatic activities (development of small-scale, short-lived magmatism and subsequent termination of the Gangdese arc magmatism) in southern Tibet and for the abrupt slowdown (ca. 45 Ma) of Indo-Asia convergence.