Indian Continental Lithosphere Research Articles

Late Oligocene to Pleistocene thermo-tectonic evolution of the Karakoram Fault Zone: New insights from basement and detrital apatite fission track thermochronology

The Karakoram Fault Zone displays distinctive topographic and tectonic characteristics, with intricate river drainage systems. Although there are extensive thermochronological datasets for the western Tibetan Plateau, limited investigations have been carried out to uncover the low-temperature thermal history of the Karakoram Fault Zone, which connects the Lhasa terrane with the Pamir. In this contribution, apatite fission track (AFT) thermochronology was conducted on 13 basement and 4 conglomerate samples collected from the western Gangdese batholith and the Ayi Mountain Range, on either side of the Karakoram fault. Inverse thermal history modeling results reveal that the western Gangdese batholith experienced two stages of rapid basement cooling at 27–23 Ma (late Oligocene-early Miocene) and 16–4 Ma (middle Miocene-Pliocene). The adjacent Ayi Mountain Range experienced cooling in the late Miocene (∼8 Ma) and during the Pliocene to Pleistocene (4–1 Ma). We suggest that tectonic forces associated with rollback and lateral break-off of the Indian continental lithosphere, and the dextral strike-slip activity of the Karakoram fault were responsible for the first cooling phase. The second accelerated cooling phase was due to the deep fragmentation of the Indian slab, the activity of the Karakoram normal fault, and associated ∼E-W extension. Additionally, since the late Oligocene, the Indus River incision has likely also contributed to the exhumation of the western Tibetan Plateau. Subsequently, the western Gangdese batholith reached high elevations, resulting in the reversal of the Yarlung River. The dextral-normal motion of the Karakoram fault, which is still active today, was the main cause of the Pliocene to Pleistocene accelerated cooling detected in the Ayi Mountain Range.

Geodynamic models of Indian continental flat slab subduction with implications for the topography of the Himalaya-Tibet region

The slab structure and high elevation of the Himalaya-Tibet region and their underlying mechanisms have been widely discussed. Many studies interpret a flat slab segment of Indian continental lithosphere located below the overriding plate, but interpretations of the northward extent of the flat slab differ substantially, with minimum estimates placing the boundary at the northern margin of the Himalaya (Indus-Yarlung Tsangpo suture), and maximum estimates placing it at the northern boundary of Tibet. In this study, we investigate for the first time if a flat slab segment of subducted buoyant Indian continental lithosphere below the Himalaya-Tibet region is geodynamically feasible and we quantify its northward extent, as well as its contribution to the high topography of the region. We conduct three large-scale fully-dynamic (buoyancy-driven) analogue experiments to simulate the subduction of the Indian continent. Our preferred, and geodynamically most feasible, model shows a continental flat slab extending northward up to ~ 320 km from the Himalayan thrust front, in agreement with recent estimates. Furthermore, it suggests that the positively buoyant flat slab segment of the Indian continent contributes some ~ 1.5–2 km to the high topography of the Himalaya-Southern Tibet region by providing an upward force to elevate the overriding Eurasian plate.

Rapid Recycling of Subducted Sediments in the Subcontinental Lithospheric Mantle

ABSTRACT Subduction recycling of sediments plays a key role in the geochemical evolution of Earth. The presence of recycled terrigenous sediments in upwelling plumes has been cited to explain the EM2 signature in ocean island volcanics, characterized by particularly high 87Sr/86Sr (>0.706). However, the origin of such isotopic anomalies in continental regions and the role of subducted sediments in the subcontinental lithospheric mantle (SCLM) remain unclear. The Himalaya–Tibet orogen is one of the world’s best places for deciphering continental subduction and the fate of subducted crustal materials in the mantle. Here we present a systematic study of the geochronology, mineral chemistry (especially clinopyroxene), whole-rock chemistry and Sr–Nd–Pb–Hf–O isotopic compositions of Cenozoic potassic–ultrapotassic lavas from the western Kunlun area of northwestern Tibet. New secondary ion mass spectrometry (SIMS) zircon U–Pb dating, coupled with published age results, constrain the timing of volcanism from ~8.3 Ma to the present. These lavas show geochemical characteristics that closely resemble the EM2 mantle end-member represented by the Samoan hotspot. Both whole rocks and individual magmatic clinopyroxenes display arc-like trace-element patterns and remarkably enriched Sr–Nd–Pb–Hf isotope compositions (87Sr/86Sr ≥ 0.7080; εNd ≤ −4.8; 206Pb/204Pb ≥ 18.704; εHf ≤ −2.6). Together with high zircon δ18O values (6.3–10.4‰), the data point to a mantle source enriched by recycled sedimentary materials. Geochemical modeling and geophysical evidence further indicate that the sediments were directly derived from the subducted Indian continental lithosphere during India–Eurasia collision. Partial melting models assuming a hybridized mantle source that contains ~5% Indian continental crust suggest that the primary melts of the potassic–ultrapotassic lavas could be formed by melting of a phlogopite-bearing garnet lherzolite at low melting degrees (1–5%). The magma geochemistry is consistent with the model of mélange melting, implying that the subducted sediments may detach from the downgoing Indian slab and rise up diapirically into the overlying mantle lithosphere. Unlike traditional models of subducted sediments entering the deep mantle, the western Kunlun EM2-like lavas reveal that subducted sediments can be rapidly recycled into the SCLM during continental subduction (probably <50 Myr). We suggest that the SCLM could be an important reservoir for subducted sediments. The findings are important to our understanding of mantle circulation rates and chemical heterogeneities.

Basement and lithospheric structure of the central Ganga Basin between the Bundelkhand craton and the Sharda Deep by magnetotellurics

The geological and tectonic fabric of the Indian plate in the Ganga Basin is masked by alluvial sediments. Geophysical exploration of the basin for hydrocarbons yielded information on the presence of transverse basement ridges and Cenozoic as well as some pre-collision stratigraphy of the basin. However, the depth of the basement especially in the northern part of the basin and the nature of the underlying crust and mantle lithosphere is largely unknown. We have carried out a magnetotelluric (MT) study along a 300-km-long profile between the Bundelkhand craton (BC) and the Sharda Deep (at the Himalayan foothills) across the central Ganga Basin to delineate the basement and the lithospheric structure. The electrical resistivity image obtained by 2-D inversion of broadband MT data reveals significant lateral heterogeneity in the subsurface structure along the profile. The model yields a super-thick sedimentary basin in the Sharda Deep consisting of multiple stages of basin formation, including a Proterozoic paleo-rift, and a highly heterogeneous lithospheric structure of the Indian plate north of the BC. The model consists of two major upper mantle conductors connected to the conductors within the lower crust. It is surmised that enhanced conductivity could be due to the presence of fluids and the upper mantle conductors are feeding to the crustal conductors. However, an in-depth analysis of the genesis of these conductors would require other independent geophysical information which is not available at present. This is the first reporting of such a heterogeneous nature of the Indian continental lithosphere beneath the Ganga Basin at a regional scale. Our model further supports the presence of the electrical Moho in the BC and, together with earlier published MT results along another profile, suggests that the BC extends much farther north than its presently known northern limit.

Upper Mantle Velocity Structure Beneath the Yarlung–Tsangpo Suture Revealed by Teleseismic P-Wave Tomography

We applied teleseismic tomography to investigate the 3D P-wave velocity (Vp) structure of the crust and upper mantle at depths of 50–400 km beneath the Yarlung–Tsangpo suture (YTS), by using 6164 P-wave relative travel-time residuals collected from 495 teleseismic events recorded at 20 three-component broadband seismograms. A modified multi-channel cross-correlation method was adopted to automatically calculate the relative arrival-time residuals of all teleseismic events, which significantly improved the efficiency and precision of the arrival-time data collection. Our results show that alternating low- and high-Vp anomalies are visible beneath the Himalayan and Lhasa blocks across the YTS, indicating that strong lateral heterogeneities exist beneath the study region. A significant high-Vp zone is visible beneath the southern edge of the Lhasa block at 50–100 km depths close to the YTS, which might indicate the rigid Tibetan lithosphere basement. There exists a prominent low-Vp zone beneath the Himalayan block to the south of the YTS extending to ~150 km depth, which might be associated with the fragmentation of the underthrusting Indian continental lithosphere (ICL) and induce localized upwelling of asthenospheric materials from the upper mantle. In addition, significant low-Vp anomalies were observed beneath the Yadong–Gulu rift and the Cona–Sangri rift extending to ~300 km depth, indicating that the tearing of the subducted ICL might provide pathways for the localized asthenospheric materials upwelling, which contributes to the widespread distribution of north–south trending rifts and geothermal activities in southern Tibet.

K‐Rich Adakite‐Like Rocks in Central Tibet: Fractional Crystallization of a Hydrous, Alkaline Primitive Melt

AbstractK‐rich adakite‐like rocks (KARs) in post‐collisional settings, such as in Tibet, have been widely linked with the melting of pre‐existing thickened crust. Here, we investigate geochemical data of the Late Eocene (38–34 Ma) volcanic rocks including KARs from the southern Qiangtang terrane (SQT) of central Tibet. The data reveal that: (a) the volcanic rocks define a fractionation trend from high‐K alkaline basalt to high‐K calc‐alkaline rhyolite, with a continuous compositional range and (b) they are characterized by a narrow range of depleted Sr–Nd isotopic compositions relative to the pre‐Eocene SQT crust. We contend that the KARs in the SQT resulted from fractional crystallization of hydrous, alkaline melts derived from the lithospheric mantle where fractionation was dominated by amphibole and plagioclase. Partial melting of the lithospheric mantle beneath the SQT was possibly triggered by thermal perturbations owing to the north‐directed subduction of the Indian continental lithosphere beneath southern Tibet.

Geochemical characteristics and water quality assessment of trace elements in geothermal springs in the Gulu-Yadong rift, Tibetan Plateau

Thermal springs are enriched in many trace elements that are closely related to environmental and human health. The occurrence of trace elements in groundwater systems is one of the most significant environmental and public health concerns. The northward subduction of the Indian continental lithosphere beneath the Asian continent has produced an ongoing continental subduction zone. The Tibetan Plateau serves as the ‘Water Tower of Asia’, supplying drinking water to more than one billion residents in Asia. In this study, eighteen geothermal springs in the Gulu-Yadong Rrift (GYR) were selected for investigation of the geochemical characteristics of their trace elements. The geological process–related geochemical characteristics of trace elements in geothermal springs in the GYR were quantitatively assessed. The results indicated that there were high concentrations of Cr, As, B, and F in the geothermal waters in the GYR. The concentrations of Cr, As and B greatly exceeded the World Health Organization (WHO) guidelines for drinking water. The water quality throughout the whole rift was spatially heterogeneous. The concentrations of trace elements in only six geothermal springs were within the guidelines of the WHO, while the concentrations of trace elements in eleven geothermal springs considerably exceeded the WHO guidelines. Continental subduction–related magmatic processes, leaching and water-rock interactions may be the main factors controlling trace elements in the geothermal water. The deep circulation of the thermal water facilitated the addition of upwelling hot materials and magmatic fluids into the shallow aquifers, as well as enhancing the water-rock interactions. The results of this study indicate that geological processes have a large influence on the geochemical characteristics of trace elements in geothermal springs in the Tibetan Plateau. It is a non-negligible environmental and health issue in the plateau.

Sunda subduction drives ongoing India-Asia convergence

The Himalaya and the Tibetan plateau, the highest mountain range on Earth, have been growing continuously for the last 55 Myr since India collided with Eurasia. The forces driving this protracted mountain building process are still not fully understood. Although subduction zones are considered the main driving force for plate tectonics, mantle flow and plate boundary migration, their role in driving the Indian indentation and the northward movement of the collisional plate boundary is yet to be tested with geodynamic models. Here, we use four-dimensional geodynamic physical models to show that active subduction of the Indo-Australian plate along the Sunda subduction zone is probably the main driver of the India-Asia convergence, Indian indentation, and the consequent growth of the Himalaya-Tibet mountains, and also the present-day eastward crustal displacement of southeast Asia. Our experiments show that at least 880 km of northward indentation of India would not have ensued in the absence of the lateral subduction zones. Our experiments with lateral subduction zones show that subduction of the Indian continental lithosphere is maximum close to the eastern and western syntaxes, which ranges between 450 and 500 km. Based on our model results we propose that the protracted growth of collisional mountains on Earth, like the Himalaya, is highly dependent on nearby active subduction zones.

Upper Mantle beneath the Myanmar and Surrounding Tomography: New Insight into Plate Subduction and Volcanism

Myanmar and its surrounding areas have complex topography and strong tectonic movement, which has always been a challenge to most geoscientists. We used teleseismic tomography to study the subsurface velocity structure in this area. We present a new P-wave tomographic model beneath Myanmar and the surrounding areas by inverting 129,788 arrival-time data recorded by 372 stations. We found an inclined high-velocity subducting plate beneath central Myanmar, where the dip angle becomes smaller near 25°~26°N, and the seismic depth is limited below 200 km. The Indian oceanic lithosphere is being detached from the Indian continental lithosphere, which limits the depth of the earthquake. The active Tengchong volcano is underlain by a prominent low-velocity (low-V) anomaly in the shallow mantle, which may be caused by the subduction and dehydration of the Burma microplate (or Indian plate). The formation of the Singu volcano is related to the mantle flow of the Qinghai–Tibet plateau and the tearing of the Indian plate. The Yangtze craton (beneath the Sichuan Basin) shows a high-velocity anomaly, and both the shallow and deep parts have been destroyed, which may be related to the upwelling of deep heat flow.

Eocene magmatism in the Himalaya: Response to lithospheric flexure during early Indian collision?

Abstract Eocene mafic magmatism in the Himalaya provides a crucial window for probing the evolution of crustal anatexis processes within the lower plate in a collisional orogen. We report geochemical data from the earliest postcollision ocean-island basalt–like mafic dikes intruding the Tethyan Himalaya near the northern edge of the colliding Indian plate. These dikes occurred coeval, and spatially overlap, with Eocene granitoids in the cores of gneiss domes and were likely derived from interaction between melts from the lithosphere-asthenosphere boundary and the Indian continental lithosphere. We propose that these mafic magmas were emplaced along lithospheric fractures in response to lithospheric flexure during initial subduction of the Indian continent and that the underplating of such mafic magmas resulted in orogen-parallel crustal anatexis within the Indian continent. This mechanism can explain the formation of coeval magmatism and the geologic evolution of a collisional orogen on both sides of the suture zone.

Age, petrogenesis, and metamorphic modelling of high‐pressure garnet‐amphibolite from the Tethyan Himalayan Sequence of Bhagirathi Valley, Western Garhwal Himalaya

The Cenozoic Himalayan Orogeny is a manifestation of subduction and subsequent collision of the Indian lithosphere with the Asian Plate in the Eocene, resulting in the formation of intra‐terrane shear zones and regional metamorphic belt along the strike of the entire orogen. This metamorphic belt contains high‐pressure (HP) metamorphic rocks (garnet‐amphibolite, late‐stage granulite, and host gneisses) at various structural levels in the Himalayas. In the present study, we carried out geochemical, chronological, and metamorphic modelling of an exotic tschermakite‐bearing garnet‐amphibolite unit exposed within the Tethyan Himalayan Sequence of Western Garhwal Himalaya of India. Bulk‐rock geochemistry suggests the protolith's tholeiitic fractionation trend and trachy‐andesitic nature. U–Pb geochronology of zircon reveals an age spectrum with pronounced clusters at ~1,550 Ma, ~1,350 Ma, 1,100–1,150 Ma, 900–930 Ma, and ~550 Ma. There is a possibility that the protolith of the HP garnet amphibolite is magmatic. However, near‐concordant ages at certain intervals might suggest crustal components from multiple sources. Integrated geothermobarometry and P–T pseudosection modelling indicate high‐P/medium‐T conditions (~1.65 GPa and 650°C) of peak metamorphism. Based on these results, we infer that this garnet‐amphibolite represents pre‐Himalayan Indian lower crust and during India–Asia collision. The leading edge of the Indian continental lithosphere subducted where mafic and some of the felsic lithologies underwent HP garnet‐amphibolite facies metamorphism, and after a possible slab break‐off, these rocks exhumed during the Himalayan orogeny.

Change in Subduction Dip Angle of the Indian Continental Lithosphere Inferred From the Western Himalayan Eclogites

The occurrence of ultrahigh-pressure (UHP) and high-pressure (HP) rocks in the Himalayan orogen has been conventionally attributed to the different subduction dip angles along the strike. The western Himalayan UHP eclogites point to a steep continental subduction in the Eocene. The present-day geophysical data show low subduction dip angles of the Indian lithosphere beneath southern Tibet and Karakoram, implying that a shift from steep to low-angle subduction probably happened in the western Himalaya. However, the timing and mechanism of such a subduction-angle change are still unknown. Here we present a combined analysis of zircon geochronology and geochemistry of eclogites and gneiss in the Stak massif, western Himalaya. Metamorphic zircons equilibrated with garnet and omphacite show flat heavy rare earth element patterns without Eu anomalies and, thus, yield similar eclogite-facie ages of ca. 31 Ma. The Stak HP eclogite-facie metamorphism is at least 15 Ma younger than those measured in the western Himalayan UHP eclogites, but broadly contemporaneous with other Himalayan HP rocks. Therefore, all the Himalayan HP rocks record higher peak geothermal gradients and younger ages than those of the UHP rocks. Our new data, combined with the magmatic lull observed in the Kohistan–Ladakh–Gangdese arc and with the convergent rate of the Indian plate, suggest a change in subduction dip angle over time. Consequently, we suggest that the entire Indian continental lithosphere experienced an approximately coherent shift from steep to low-angle subduction after the breakoff of the Neo-Tethyan slab since the middle Eocene. This critical change in subduction geometry is interpreted to be responsible for the transition from continental subduction to collision dynamics.

Indian plate structural inheritance in the Himalayan foreland basin, Nepal

AbstractThe Himalaya, the Earth's largest active orogen, produces a deep but relatively unexplored foreland basin by loading the Indian Plate. Newly available two‐dimensional seismic data (ca. 5,180 line km) spanning 900 km of the Nepali lowlands allow mapping and interpretation of several regional subsurface markers in two‐way‐travel time and estimated depth. Isopach maps for the major intervals allow us to interpret the interplay between basement structure, flexure, and faulting within the Ganga Basin. The Indian continental lithosphere beneath the foreland basin contains basement ridges oriented at high angles to the thrust belt. These basement structural highs and intervening depressions, tens to hundreds of kilometres wide, influenced deposition of the Precambrian Vindhyan strata and overlying Paleozoic to Mesozoic successions. The overlying Miocene to Quaternary foreland basin shows along‐strike thickness variations across the basement features. Because the foreland basin sediments were mainly deposited in an alluvial plain close to sea‐level, accommodation, and therefore thickness, was predominantly controlled by subsidence of the Indian Plate, providing evidence that the basement features controlled foreland basin development. Subsidence varied in time and space during Neogene basin development. When combined with flexural modelling, these observations imply that the subsidence history of the basin was controlled by inherited lateral variations in the flexural rigidity of the Indian Plate, as it was translated northward beneath the Himalayan Orogen. Basement features continue to play a role in higher levels of the thrust belt, showing that basement features in a down‐going plate may produce non‐cylindrical structures throughout orogen development.

Indian continental lithosphere and related volcanism beneath Myanmar: Constraints from local earthquake tomography

The Indian plate descends obliquely eastward beneath the Eurasian plate along the Burmese arc. Previous tomographic results revealed a high-velocity structure that plunges eastward into the deep mantle beneath Myanmar. However, the shallow structure beneath Myanmar remains unclear due to the lack of local seismic observations. Based on the local seismic data recorded by a newly deployed dense array in Myanmar, we obtain a three-dimensional velocity structure of the crust and lithospheric mantle above 100 km by employing the double-difference tomography method. Our imaging results support the existence of the Indian continental lithosphere to a depth of at least 100 km with a dip angle of ∼25° beneath the Indo-Burma Ranges and the Central Myanmar Basin. At the deep end of the continental slab, the high VP, high Vs and rather low average VP/VS ratio indicate that the lowermost portion of the crust from 80 to 120 km may have experienced partially metamorphic eclogitization. The imaging results also provide direct seismic evidence for the origin of the last Monywa volcanic activity in the Holocene. This subduction-related volcanism is characterized by a prominent low Vs anomaly in the lithospheric mantle, which indicates continental lithosphere dehydration and partial melting in mantle wedge. These findings provide new seismic constraints to understand the continental collision system between India and Eurasia under Myanmar and the related magmatic activities of the Monywa volcano.

Himalayan Miocene adakitic rocks, a case study of the Mayum pluton: Insights into geodynamic processes within the subducted Indian continental lithosphere and Himalayan mid-Miocene tectonic regime transition

AbstractThe Himalayan Orogen is a typical continental collision orogenic belt that formed during India-Asia collision. The orogeny involved a transition in tectonic regime, which led to E-W–trending extension during the middle Miocene. At the same time, widespread post-collisional adakitic magmatism occurred in southern Tibet, particularly in the Lhasa Terrane, and these rocks provide a valuable record of the collisional geodynamic processes within the lithosphere. Few studies have focused on the middle Miocene adakitic rocks of the Himalayan Orogen, so further research is required to constrain their origin and geodynamics. This study presents new geochronological and geochemical data, including the whole rock Sr-Nd and zircon Hf isotopic compositions from the Mayum pluton in the Mayum-Gong Tso area, Northern Himalaya. Zircon U-Pb ages show that the Mayum granodiorite porphyries crystallized at 16.67 ± 0.14 Ma (mean square weighted deviation [MSWD] = 0.82, n = 29) and 16.68 ± 0.16 Ma (MSWD = 1.13, n = 28), indicating that they formed during the middle Miocene. The major and trace element characteristics are as follows: SiO2 = 65.79–67.31 wt%, Al2O3 = 15.28–16.00 wt%, MgO = 1.77–1.89 wt%, Y = 12.0–13.5 ppm, Yb = 0.11–0.99 ppm, Sr = 719–822 ppm, (La/Yb)N = 21.89–27.02, Sr/Y = 56.1–65.5, and the rocks have weak negative Eu anomalies (Eu/Eu* = 0.76–0.85), indicating that they are adakitic rocks and show high Sr-Ba granite affinity. The Mayum granodiorite porphyries have high K2O contents (3.42–3.65 wt%), Na2O (3.76–4.04 wt%), and K2O/Na2O ratios of 0.91–0.95, indicating a high-K calc-alkaline affinity. All samples are enriched in large-ion lithophile elements and depleted in high field strength elements. The initial 87Sr/86Sr ratios are 0.709262–0.709327 and εNd(t) values are −6.36 to −7.07, which correspond to two-stage Nd model ages [TDM2(Nd)] of 1405–1348 Ma. In situ zircon Lu-Hf isotopic compositions are variable, with 176Hf/177Hf ratios of 0.2823845–0.282824, εHf(t) values of −13.37 to +2.17, and two-stage Hf model ages [TDM2(Hf)] of 1704–841 Ma. These geochemical and Sr-Nd-Hf isotopic characteristics indicate that the Mayum granodiorite porphyries may be derived from partial melting of the subducted Indian thickened ancient mafic lower crust (∼92%) mixed with depleted mid-ocean ridge basalt mantle material input (∼8%) slightly. Crust-mantle interaction was induced by asthenospheric upwelling followed by the underplating of the Himalayan lower crust beneath Southern Tibet during the middle Miocene in response to significant changes in the geodynamics of the India-Asia collisional orogen. These deep geodynamic processes reflect the break-off or rollback of the subducted Indian continental slab, which caused asthenospheric upwelling, the input of juvenile heat, and the addition of depleted mantle material. This study indicates that the middle Miocene Himalayan adakitic rocks, which include the Miocene Yardoi two-mica granite, Gyangzê granite porphyry dike, Bendui two-mica granite, Langkazi enclave, Kuday dacitic dike, Lasa pluton, and Mayum pluton, form a belt of adakitic rocks in the Northern Himalaya to the south of the Indus-Tsangpo Suture Zone. These adakitic rocks have similar temporal and spatial distributions, geochemical features, and Sr-Nd isotopic compositions, indicating that their petrogenesis and geodynamic settings were similar. At that time there was widespread initiation of N-S–trending rifting, exhumation of central Himalayan eclogites, and large changes in the compositions of Himalayan leucogranites in the Himalayan Orogen. These can be attributed to significant changes in the characteristics of the subducted Indian continental lithosphere during the middle Miocene. The middle Miocene Himalayan adakitic rocks provide valuable insights into this transition in the tectonic regime and deep geodynamic processes.

Hales Discontinuity in the Southern Indian Continental Lithosphere: Seismological and Petrological Models

AbstractWe model the depth and Vs structure of the Hales discontinuity (H‐D) beneath Eastern Dharwar Craton (EDC) and Southern Granulite Terrain (SGT) using P wave receiver function (P‐RF) analysis and joint inversion with Rayleigh wave phase velocity dispersion. We calculate P‐RFs at higher frequency (fmax = 0.46 Hz), compared to previous studies, to show that the H‐D P‐to‐S converted phase (Phs) is distinct from crustal reverberations. Phs at stations Hyderabad (HYB) and Gauribidanur (GBA), in the EDC, arrive at ∼11.5 s and ∼12 s, respectively. From joint inversion, the H‐D is modeled at 107 ± 5 km and 113 ± 4 km depth, with ∼3% and ∼4% Vs increase, beneath HYB and GBA, respectively. For station Kodaikanal (KOD), in SGT, the Phs destructively interferes with the negative midcrustal reverberation at most ray‐parameters, which explains its apparent absence in previous studies. We isolated P‐RFs where Phs is distinct at ∼11 s and model it at 102 ± 3 km depth. Common conversion point stack profiles constructed by depth migrating P‐RFs through the Vs model show an undulatory nature of the H‐D. From data of mantle xenoliths (Wajrakarur kimberlite field), we calculate Vs of mantle peridotite and eclogite, using published bulk rock compositions. At the H‐D depth and temperature derived from Indian shield geotherm, we observed a good match to the Vs structure of the H‐D. Our results support the geodynamic model of the H‐D being an interface of paleosubducted eclogitic oceanic crust embedded within the upper mantle peridotite. Global observations of mantle reflectors within the continental lithosphere, at depths similar to H‐D, have been related to relict subduction and independently support our model.

Crustal structure of the Indian plate underneath the alluvial plains of the central Ganga basin by broadband magnetotellurics

The Ganga basin consists of thick Cenozoic sediments that mask the tectonic fabric of the underlying Indian plate. Despite the availability of fairly good information on the intra-basin structure, especially at the foothills of the Himalaya due to hydrocarbon exploration activities, very little is known about the basement and the crustal/lithospheric structure although composition and thermomechanical properties of the underthrusting Indian plate are expected to have a significant role in controlling the deformation and seismicity along the Himalayan arc. Here, we present a two-dimensional geoelectric model of the crustal structure along a 285-km-long profile across the central Ganga basin. The results were obtained by inversion of decomposed TE- and TM-mode impedance tensors derived from broadband magnetotelluric data recorded at 45 sites along the profile. The results reveal three main features: (i) the highly resistive Bundelkhand massif (BKC) extends northward beneath the sediments up to Lucknow; (ii) the massif is constituted of two blocks separated by a 10-km-wide moderately resistive nearly vertical zone; and (iii) a super-deep sedimentary basin constituted of highly conductive sedimentary succession exists in the northern segment of the profile. We report the presence of an electrical Moho at 40–45 km depth for the buried BKC and a crustal shear zone within the craton masked by the alluvial sediments. These results shall facilitate in furthering our understanding of the evolution of the BKC as well as the lithospheric/flexural deformation of the Indian continental lithosphere.

Syn-collisional magmatic record of Indian steep subduction by 50 Ma

Abstract Subduction of Indian continental lithosphere during the Asia-India collision played an important role in the formation and evolution of the Himalaya-Tibetan orogen. However, the geometry of early Indian continental subduction remains debated. Given that the Indian continent is characterized by enriched isotope ratios (87Sr/86Sr > 0.730, εNd(t) < −10), relative to those in subducted oceanic materials (87Sr/86Sr < 0.704, εNd(t) ≈ +8), changes in the composition of magmatic rocks with time, in particular their radiogenic isotope ratios, is used to constrain the timing and nature of continental subduction. This study reports the field relations, zircon U-Pb ages and geochemical composition of a syn-collisional batholith that crosscuts the central Indus-Yarlung Zangbu suture in the Saga area of southern Tibet. Zircon U/Pb ages for the batholith mainly range from 50 to 46 Ma. Samples from the Lopu Range batholith have enriched zircon Hf (εHf(t) = −0.4 to −8.6) and whole rock 87Sr/86Sri = 0.7094–0.7121 and εNd(t) = −7.3 to −9.8, suggesting that they were derived from a mixture of juvenile Gangdese and isotopically enriched Indian crustal materials. This result indicates that subduction of Indian crustal rocks occurred before 50 Ma in the central Himalaya. The geochemical composition and distribution of high volume ca. 51 Ma magmatism in the Gangdese belt, combined with thermal models of the subduction zone, suggests a steepening of the subducted Indian continental lithosphere occurred between the onset of India-Asia collision (59 Ma) and 46 Ma in the central-eastern Himalaya.

Petrogenesis and geodynamic implications of the intrusions related to the Pusangguo skarn Cu‐dominated polymetallic deposit in Tibet: Constraints from geochronology, geochemistry, and Sr–Nd–Pb–Hf isotopes

Pusangguo is a high‐grade skarn‐type Cu‐dominated polymetallic deposit and the only large‐sized Cu‐Pb‐Zn‐(Co‐Ni) deposit in the Gangdese metallogenic belt (GMB); the magmatic rocks associated with this deposit are poorly documented, and their petrogenesis and geodynamic setting are unclear. To explore these issues, we present zircon U–Pb ages along with Hf isotopic, whole‐rock geochemical, and Sr–Nd–Pb isotopic data for Pusangguo biotite granodiorite (PBG) and Pusangguo diorite porphyrite (PDP) in this deposit. Zircon U–Pb dating for these rocks indicates that they were emplaced during the Miocene (13.6–14.6 Ma). Geochemically, both PBG and PDP show characteristics of adakites, with intermediate SiO2 (65.55–67.30 wt% and 60.56–58.27 wt%, respectively), relatively high Al2O3 (15.28–15.85 wt% and 17.71–16.71 wt%, respectively) and Sr (599–723 ppm and 687–1,616 ppm, respectively) and low Y and Yb. In addition, they show relatively low ratios of (87Sr/86Sr)i and (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i and relatively low values of εNd(t) and εHf(t). The Sr‐Nd‐Pb‐Hf isotopes of PBG and PDP are similar to those of the Miocene adakitic rocks associated with the porphyry–skarn deposits in the GMB, indicating that they were predominantly generated by partial melting of the thickened juvenile Lhasa Terrane lower crust and the PDP were likely to be generated through mixing between the magma parental to the PBG and a mafic end‐member. By combining our results with the previously reported results, we suggest that the model of delamination of the overthickened mantle lithosphere of Lhasa Terrane and convective upwelling of mantle asthenosphere which may involve Indian continental lithosphere.

Gravity Anomalies, Isostasy and Density Structure of the Indian Continental Lithosphere

Gravity anomalies across the Indian region depict most of the geological and tectonic domains of the Indian continental lithosphere, which evolved through Archean cratonic nucleation, Proterozoic accretion, Phanerozoic India-Eurasia plate convergence, and modification through many thermal perturbations and rifting. Integrated analysis of gravity and geoid anomalies together with topographic and heat flow data led to deciphering the mechanism of isostatic compensation of topographic and geological loads, lithospheric structure, and composition. This study discusses the nature of gravity (free-air, Bouguer and Isostatic) and geoid anomalies in relation to the topography, geology, and tectonics, and presents a lithospheric density model across the peninsular India and Himalaya. Southern peninsular Indian region shows relatively low Bouguer gravity anomalies compared to the northern region. The mobile belts are generally observed to have relatively higher Bouguer gravity anomalies, e.g., Eastern Ghats Mobile Belt compared to the shield regions. The gravity lows are observed over topographic features like the Western Ghats and Himalaya, while some of the topographic highs like Aravalli show positive gravity anomaly. The Indian Ocean Geoid Low varies from -82 m over Dharwar Craton to -98 m over the Southern Granulite Terrain and finally reaches a significant low of -106 m in the Indian Ocean. Flexural isostatic compensation with variable Effective Elastic Thickness (EET) ~10 km to 50 km prevails over the stable continental region. The lithospheric thickness varies from 80 km along the coastal region to 120-130 km beneath the Saurashtra Plateau, the Southern Granulite Terrain, and the Eastern Indian Shield, and reaches to more than 200 km under the Himalayan orogenic belt in the north. From Dharwar Craton to Bundelkhand Craton in central India, the lithospheric thickness varies between 160 and 180 km.