Crust In Southern Tibet Research Articles

Mg-Ca-Fe isotopes of post-collisional magmatic rocks record the crust-mantle interaction processes beneath southern Tibet

The non-traditional Mg, Ca and Fe isotopes have proven to be useful tools for investigating the petrogenesis of the mantle- and crust-derived rocks. Individual non-traditional isotopes in continental basalts are well-studied, particularly in subduction settings. In contrast, there is a significant lack of comprehensive research on various non-traditional stable isotopes related to magmatic processes within continental collision zones, especially during post-collisional stage. To address this knowledge gap, this study presents stable Mg-Ca-Fe isotopic data for the Miocene post-collisional magmatic rocks including ultrapotassic, potassic, and adakitic rocks from the Lhasa Terrane in southern Tibet. The ultrapotassic rocks exhibit slightly lower δ26Mg (−0.28 ± 0.07‰, 2SD, n = 18) and significantly lower δ44/40Ca (0.69 ± 0.03‰, 2SD, n = 18) values than the depleted mantle. The high CaO/Al2O3 and CaO/TiO2 ratios of the ultrapotassic rocks are typical characteristics of carbonate metasomatism, suggesting that the light Mg and Ca isotopic composition may be attributed to the involvement of recycled Ca-rich sedimentary carbonate in the mantle source. Additionally, these ultrapotassic rocks display δ56Fe values (0.026–0.163‰, 2SD, n = 18) ranging from the normal peridotite mantle to much higher values, suggesting a likely origin from a pyroxenite-bearing lithospheric mantle source. These findings imply that subducted oceanic crust and overlying Ca-rich sediments plausibly account for the formation of a mantle source characterized by distinctive Mg-Ca-Fe isotope compositions. The adakitic rocks exhibit the highest δ26Mg (−0.13 ± 0.10‰, 2SD, n = 15) and δ56Fe (up to 0.304‰) values, along with the lowest δ44/40Ca values (0.60 ± 0.07‰, 2SE, n = 15) among these post-collisional magmatic rocks. They also demonstrate strong correlations between δ56Fe and (Gd/Lu)N ratios, indicating a significant role of garnet in their source, consistent with their origin from the thickened lower crust associated with the collision between India and Asia continents. Given the consistent δ56Fe values (0.102 ± 0.067‰, 2SD, n = 5) observed in the adakitic rocks from the western Lhasa Terrane, we posit that it reflects the Fe isotopic composition of the thickened lower crust in southern Tibet. The potassic rocks display lighter Ca isotopes (δ44/40Ca = 0.67 ± 0.03‰, 2SE, n = 5) than the mantle, consistent with a carbonate metasomatized source. They exhibit varying δ26Mg (−0.31 ± 0.07‰ to −0.04 ± 0.07‰, 2SD) and relatively uniform δ56Fe (0.186 ± 0.051‰, 2SE, n = 6) values, falling within the range of nearly contemporaneous ultrapotassic and adakitic rocks. This suggests that potassic rocks may be formed through the mixing of mantle materials resembling ultrapotassic rocks and thickened crustal components resembling adakitic rocks in the lower crust. Building upon these findings, we propose that the distinctive geochemical characteristics, as well as the spatial and temporal distribution of post-collisional magmatic rocks in southern Tibet, are the outcome of partial melting within a carbonated pyroxenite-bearing lithospheric mantle, coupled with the involvement of thickened crust and their subsequent interactions.

Geochemical evidence for the Eocene surface uplift of the southern Lhasa subterrane, southern Tibet and the implications

Whether the crust of southern Tibet has been thickened during the Eocene and if so, the mechanisms of crustal thickening in this region need to be further investigated. This study presents geochronological and geochemical data for Eocene (45–41 Ma) granitic plutons, including the Aima diorites in the eastern part and Huo'er pluton (granites and mafic enclaves) in the western part of the southern Lhasa subterrane. The Aima diorites show adakitic affinities, such as moderate SiO 2 contents and relatively high Sr/Y values, with depleted 87 Sr/ 86 Sr (i) (0.704039–0.704054), εNd (t) (+2.9 to + 3.3) isotopic compositions, indicating that they are derived from a juvenile continental crust. The Huo'er host granites have high SiO 2 and low MgO contents. Meanwhile, they are characterized by the enriched initial 87 Sr/ 86 Sr (i) (0.707899–0.708618), εNd (t) (from − 6.4 to − 5.3) and variable zircon εHf (t) (from − 6.8 to + 5.0) values. These geochemical characteristics and the presence of mafic enclaves demonstrate that the Huo'er host granites were the result of melting of crust (both ancient and juvenile) of southern Lhasa subterrane with the involvement of mantle-derived materials. The Huo'er mafic enclaves display an igneous texture and contain acicular apatite, suggesting that they are quenching of mafic magmas that have co-mingled with the host granites. The mafic enclaves have arc-like trace-element patterns, together with their enriched 87 Sr/ 86 Sr (i) (0.707608–0.707826), εNd (t) (from − 4.8 to − 4.6) and zircon εHf (t) (from − 4.0 to − 1.7) isotopes, revealing that they were derived from an enriched lithospheric mantle source. Taking the regional tectonic evolution into consideration, we propose that the Eocene Aima diorites and Huo'er pluton are the products of the last stage of magmatism related to break-off of the subducted Neo-Tethyan oceanic slab. Our study of these two plutons, along with results from previous studies, suggests that the crust of the southern Lhasa subterrane was thickened during the Eocene. The thickened crust was mainly caused by underplating of mafic magmas and, to a lesser extent, by thrust-related accretion of continental crust of the Indian terrane to the Lhasa terrane. We further propose that the Eocene thickened crust of southern Lhasa subterrane has triggered the surface uplift of this region during the Eocene, which may have non-negligible impact on the Cenozoic evolution of the Indian Monsoon system. • The Eocene Aima diorites and Huo'er pluton were in response to the slab break-off. • The crust of the southern Lhasa subterrane became thickened during the Eocene. • Magma underplating and accretion of Indian crust contributed to crustal thickening. • The Eocene surface uplift is synchronous with intensification of the Indian Monsoon.

Interplay between oceanic subduction and continental collision in building continental crust

Generation of continental crust in collision zones reflect the interplay between oceanic subduction and continental collision. The Gangdese continental crust in southern Tibet developed during subduction of the Neo-Tethyan oceanic slab in the Mesozoic prior to reworking during the India-Asia collision in the Cenozoic. Here we show that continental arc magmatism started with fractional crystallization to form cumulates and associated medium-K calc-alkaline suites. This was followed by a period commencing at ~70 Ma dominated by remelting of pre-existing lower crust, producing more potassic compositions. The increased importance of remelting coincides with an acceleration in the convergence rate between India and Asia leading to higher basaltic flow into the Asian lithosphere, followed by convergence deceleration due to slab breakoff, enabling high heat flow and melting of the base of the arc. This two-stage process of accumulation and remelting leads to the chemical maturation of juvenile continental crust in collision zones, strengthening crustal stratification.

Permian arc magmatism in southern Tibet: Implications for the subduction and accretion of the Zhikong–Sumdo Paleo-Tethys Ocean

The tectonic evolution of the Zhikong–Sumdo Paleo-Tethys Ocean (Hereafter referred to as Zhikong–Sumdo Ocean) in southern Tibet is controversial due to the limited records of arc magmas. Here, the newly discovered Zhangda complex in the Zhikong area of Tibet is selected as the research object. Zircon U–Pb geochronology, zircon Hf isotopes, whole-rock geochemistry and Sr-Nd isotope analyses performed based on field geological surveys show that the Zhangda complex is composed of Early Permian hornblende gabbro (278.3 ± 1.1 Ma), cumulate gabbro (277.4 ± 1.7 Ma), tonalite dike (281.1 ± 1.9 Ma), Middle–Late Permian gabbro diorite (267.8 ± 2.9 Ma, 269.0 ± 3.1 Ma), and tonalite (258.0 ± 2.7 Ma). The Zhangda mafic rocks show weak fractionation of light and heavy rare earth elements, and they are enriched in large ion lithophile elements and depleted in high field strength elements. Meanwhile, the Zhangda tonalites show the characteristics of metaluminous calc-alkaline I-type granite. They are enriched in light rare earth and large ion lithophile elements and are depleted in heavy rare earth and high field strength elements. The ɛHf(t), ɛNd(t) and the initial 87Sr/86Sr values of the mafic rocks are + 9.3–+14.4, +4.2–+5.9 and 0.702803–0.704264, respectively, while the ɛHf(t), ɛNd(t) and initial 87Sr/86Sr values of the tonalites are + 9.8–+16.3, +5.5–+6.1 and 0.703105–0.704071, respectively. Combined with the regional geological survey results, the Permian Zhangda complex was found to be formed in a tectonic setting of subduction north of the Zhikong–Sumdo Ocean. The Zhangda mafic rocks were formed by partial melting of the depleted mantle modified by subducted metasomatic fluids, while the Zhangda tonalites were the products of partial melting of the juvenile crust. During the Zhikong–Sumdo Ocean subduction, mantle-derived magma underplating led to the growth of juvenile crust in southern Tibet.

Tomographic Image of Shear Wave Structure of NE India Based on Analysis of Rayleigh Wave Data

The major scientific purpose of this work is to evaluate the geodynamic processes involved in the development of tectonic features of NE India and its surroundings. In this work, we have obtained tomographic images of the crust and uppermost mantle using inversion of Rayleigh waveform data to augment information about the subsurface gleaned by previous works. The images obtained reveal a very complicated tectonic regime. The Bengal Basin comprises a thick layer of sediments with the thickness increasing from west to east and a sudden steepening of the basement on the eastern side of the Eocene Hinge zone. The nature of the crust below the Bengal Basin varies from oceanic in the south to continental in the north. Indo-Gangetic and Brahmaputra River Valleys comprise ∼5–6-km-thick sediments. Crustal thickness in the higher Himalayas and southern Tibet is ∼70 km but varies between ∼30 and ∼40 km in the remaining part. Several patches of low-velocity medium present in the mid-to-lower crust of southern Tibet along and across the major rifts indicate the presence of either partially molten materials or aqueous fluid. Moho depth decreases drastically from west to east across the Yadong-Gulu rift indicating the complex effect of underthrusting of the Indian plate below the Eurasian plate. Crust and upper mantle below the Shillong Massif and Mikir Hills are at a shallow level. This observation indicates that tectonic forces contribute to the uprising of the Massif.

Microstructure and seismic properties of amphibole-rich rocks from the deep crust in southern Tibet

The crust of southern Tibet shows strong azimuthal and radial seismic anisotropies. However, their origin has remained ambiguous. Here we characterized the composition, microstructure, and seismic properties of amphibole-rich deep-crust xenoliths and exhumed rocks from a deep-crust terrane in southern Tibet. The xenoliths show typical magmatic textures and have amphiboles with homogenous chemical composition, low intracrystalline deformation, but strong crystallographic preferred orientations (CPOs). We interpret these CPOs as products of rotation of non-equant amphibole crystals in a deforming magma. The exhumed amphibolites display evidence for amphibole deformation by dislocation creep. Plagioclase in all samples shows nearly random CPOs. The calculated seismic properties of the studied amphibolites indicate that amphibole is likely a major contributor to the deep-crustal seismic anisotropies observed in southern Tibet. However, these anisotropies do not necessarily record solid-state flow. They may also result from magmatic flow in large synkinematic mafic plutons.

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.

Formation and composition of the Late Cretaceous Gangdese arc lower crust in southern Tibet

Arc lower crust plays a critical role in processing mantle-derived basaltic melts into the intermediate continental crust, yet can only be studied indirectly or in exposed arc sections. Compared with the relatively well-studied oceanic arc sections (e.g., Kohistan and Talkeetna), the composition and formation mechanisms of continental arc lower crust remain less clear. Here we present a geochronological and geochemical study on the Lilong Complex and the Wolong granitoids from the Gangdese arc deep crustal section in southern Tibet. The Lilong Complex is composed of the early (85–95 Ma) mafic-intermediate sequence and late (85–86 Ma) ultramafic sequence. The Lilong crustal section exposed crustal depth extending from ~ 42 to 17 km based on the geobarometry. The mafic-intermediate sequence is a damp (low H2O) igneous differentiation sequence characterized by the subsequent appearance of pyroxene → plagioclase → amphibole → biotite. The ultramafic sequence represents a wet igneous differentiation sequence composed of olivine → pyroxene → amphibole → plagioclase. The 74–84 Ma Wolong granitoids were formed by fractional crystallization of wet magma and intra-crustal assimilation. Calculated seismic properties of the Gangdese deep arc crust are comparable to the average continental crust at a similar depth. The average composition of the Gangdese arc lower crust is basaltic andesite with SiO2 of ~ 54 wt%. The highly incompatible elements in the Gangdese arc lower crust are systematically higher than those of the oceanic arc and are comparable with the estimates of lower continental crust, suggesting continental arc magmatism significantly contributes to the formation of continental crust.

Pervasive Miocene melting of thickened crust from the Lhasa terrane to Himalaya, southern Tibet and its constraint on generation of Himalayan leucogranite

The Oligocene to Miocene period is one of the most important stages during the formation and evolution of the Tibetan Plateau which generated the tectonic framework similar to the present. The rise of southern Tibet at this time was accompanied by widespread and intense tectonomagmatic activities. The Himalayan region was dominated by leucogranites considered as pure metasediments derived S-type granites, while the Lhasa terrane developed coeval thickened crust derived high Sr/Y calcalkaline rocks and metasomatized mantle derived ultrapotassic rocks. It is puzzling why the adjacent amalgamated regions show contrasting magmatic and dynamic process.This contribution presents a systematic study of newly found various dikes, including minette, aplite, granodiorite porphyry and granite porphyry, which are widespread from the southern Lhasa terrane, across the Yarlung Tsangpo suture zone (YTSZ) and into the Tethyan Himalaya. These dikes yield 16–11 Ma U-Th-Pb ages from zircon, titanite and monazite. The aplite and granodiorite porphyry dikes (high Sr/Y calcalkaline) and minette dikes (ultrapotassic) previal from the Lhasa terrane to the YTSZ. They are identical to the coeval high Sr/Y and/or ultrapotassic rocks from the Lhasa terrane, indicating a similar melting process of thickened lower crust and metasomatized lithospheric mantle. To the south of YTSZ, the middle Miocene granite porphyry in the Tethyan Himalaya exhibit high Sr/Y ratios (50–138), relatively low initial 87Sr/86Sr ratio (0.7064–0.7098), high εNd(t) value (−8.31 to −1.91) and positive εHf(t) value (+1.55 to +4.33), in contrast to the high 87Sr/86Sr and low εNd(t) Himalayan leucogranites. They were likely generated by partial melting of thickened lower crust dominated by amphibolite with significant contributions from juvenile magma. The two-mica granite of Bendui pluton from the Tethyan Himalaya is similar to the granite porphyry in element composition, while the highly evolved muscovite granite there resembles the Himalayan leucogranite. Together, the granite porphyry, Bendui two-mica granite and leucogranite from the Tethyan Himalaya show a transition trend in elemental and Sr-Nd isotopic compositions, implying that the high Sr/Y granite porphyry dikes may represent the more primitive magma of Himalayan leucogranites. In this case, the Himalayan leucogranites, at least in part, represent highly evolved I-type granites with extensive assimilation of ancient crustal materials instead of pure metasediment-derived partial melts. Therefore, we suggest that the thickened lower crust in southern Tibet underwent pervasive partial melting during Oligocene to Miocene as a result of the removal of subducting Indian plate.

Constructing the Early Mesozoic Gangdese Crust in Southern Tibet by Hornblende-dominated Magmatic Differentiation

Constructing the Early Mesozoic Gangdese Crust in Southern Tibet by Hornblende-dominated Magmatic Differentiation

Present-day distribution of deformation around the southern Tibetan Plateau revealed by geodetic and seismic observations

The modern kinematics in and around the Tibetan Plateau have been widely investigated; however, the deformation distribution pattern describing the crustal kinematics in this region is controversial. Here, we quantify the deformation distribution in the southern Tibetan Plateau by adopting the partitioning ratio of internal to boundary potency rates instead of the individual slip rate data on major faults used in previous studies. The study area was divided into several blocks by major faults based on geological surveys, seismic data and a significance statistical analysis. For each block, the partitioning ratio of the potency rates was calculated by a microplate model using 294 GPS velocities from 13 studies. The deformation distribution derived from the GPS velocities was validated by surface-wave data from 407 earthquake events. After excluding the potency rates from the boundary between the Tibetan Plateau and Indian Plate, the ratio of intrablock potency rates within the southern Tibetan Plateau was 51%. This suggests that the Tibetan deformation is diffusely distributed over a large range. A seismic data analysis indicated that 53% of the historical seismic moments were released in the modeled blocks since 1901, which supported the conclusion based on the GPS velocities. The F-test indicated that the internal strain in most Tibetan blocks cannot be neglected to fit the surface velocities. Thus, the present-day crust in southern Tibet is probably far from a pure rigid block. This is supported by the relatively slow strike-slip rates of two major faults: the Karakorum fault (5.9 ± 0.8 mm yr−1) and Karakorum-Jiali fault (4.4 ± 0.8 mm yr−1 in the east segment, 9.3 ± 0.8 mm yr−1 in the west segment). The large-scale diffuse deformation in the southern Tibetan Plateau reproduced by the microplate model with significant internal strain, with seismic imaging or structural analyses, could further clarify the geodynamic processes in the India-Asia collision zone.

Lithium content and isotopic composition of the juvenile lower crust in southern Tibet

The concentrations and isotopic geochemistry of Li are potentially useful geochemical tracers of geological processes. To fully utilize Li isotopes as geochemical tracers, it is necessary to characterize the Li isotopic compositions of the various geological reservoirs. However, the Li isotopic composition of the juvenile lower crust is currently poorly constrained. Given that lithospheric architecture of the Tibetan Plateau includes Indian upper/lower crust, Tibetan upper/lower crust and juvenile lower crust, it is necessary to determine the Li isotopic composition for each geological endmember underneath southern Tibet. Among them, the juvenile lower crust was formed directly by underplating of mantle-derived basaltic magma, which is likely to be the critical factor to control the Cu-Au mineralization in southern Tibet and is responsible for crustal thickening beneath southern Tibet. Here, we report the Li concentration and isotopic composition of the juvenile lower crust in southern Tibet. Based on whole-rock major element, trace element, and Sr–Nd–Pb isotopic data, we infer that the Yeba basalts and Gangdese gabbros were derived from partial melting of metasomatized lithospheric mantle, and have compositions similar to the juvenile lower crust. In contrast, the Dianzhong andesites and Gangdese diorites originated from partial melting of the juvenile lower crust. Therefore, these units may be considered representative of the juvenile lower crust. The juvenile lower crust has Li concentrations of 7.1–37.2ppm (mean=15.4ppm), consistent with the Li concentration for the lower crust (13ppm). Li isotopic compositions (δ7Li) vary from +0.8‰ to +6.6‰ (mean=3.0‰), similar to values for the EMI/EMII mantle. The Li isotopic compositions of the analyzed samples were not significantly affected by alteration, metamorphism, crustal assimilation, or magmatic differentiation, and therefore represent the isotopic compositions of the juvenile lower crust. The Li systematics of the juvenile lower crust may be attributed to partial melting of subcontinental lithospheric mantle that has undergone metasomatism by Li-rich fluids derived from subducted oceanic crust and marine sediments. Our study also demonstrates near-identical Li isotopic compositions for juvenile lower crust and metasomatized lithospheric mantle, resulting from the lack of Li isotope fractionation during basalt generation and differentiation.

Hot Paleocene-Eocene Gangdese arc: Growth of continental crust in southern Tibet

The 1600 km-long Gangdese magmatic belt features extensive Paleocene–Eocene I-type intrusive rocks and coeval volcanic successions, which can be divided into Group I (~69–53 Ma), Group II (~53–49 Ma), and Group III (~49–43 Ma), corresponding to Neo-Tethyan slab rollback, Neo-Tethyan slab breakoff, and ongoing Indian-Asian collision, respectively. The magmas from these three groups show significant variations in geochemical and isotopic compositions, which provide the information of the growth of continental crust in southern Tibet. The most voluminous magmatism in the Gangdese belt occurred during ~53–49 Ma. High zircon saturation temperature (up to 800 °C) and Ti in zircon temperature (up to 980 °C) estimations suggest there is a period of thermal anomaly during ~53–49 Ma. Starting from ~53 Ma, magmas have increased K2O contents, and their zircons have decreased Th/U ratios, and Y and Yb contents. Zircons from Group II have the most heterogeneous Hf isotopic compositions (εHf(t) = −5.3 to 15.1). These are evident of ingress of asthenosphere mantle in the arc, extensive crustal melting, and magma mixing. Magma underplating during this time is the main mechanism for the growth of continental crust. With the Indian-Asian collision going on, the magmas in Group III show high Th/Y and La/Yb ratios and K2O contents, but significantly low Tzr and T(ti-zr) values (mostly below 750 °C). These features suggest the water-fluxed melting of early arc residues occurred in the late stage of growth of continental crust. The crust has been thickened and nearly mature at this stage. This study has great implication on understanding of growth of continental crust in orogenic belts.

The Mechanism and Dynamics of N‐S Rifting in Southern Tibet: Insight From 3‐D Thermomechanical Modeling

AbstractN‐S trending rifts are widely distributed in southern Tibet, suggesting that this region is under E‐W extension, behind the N‐S collision between the Eurasia and India plates. Geophysical anomalies and Miocene magma extrusions indicate the presence of dispersed weak zones in the middle to lower crust in southern Tibet. These weak zones are partially located underneath the N‐S rifting systems. In order to study the formation of rifts in collision zones, we have developed a high‐resolution 3‐D thermomechanical model of continental lithosphere with bidirectional compressional‐extensional deformation, and spatially localized weak and low‐density zones in the middle to lower crust. Our numerical experiments systematically reproduce the development of N‐S trending rifts. Model results reveal that the weak middle to lower crust triggers the development of normal faults in the upper crust and surface uplift, whereas regions without such weak layer or with small‐scale weak zones are characterized by strike‐slip faulting. Geodynamic properties (density, depth, and geometry) of the weak middle to lower crust and Moho temperature notably influence the rifting pattern. In addition, rifting formation is critically controlled by large E‐W extension, with the ratio of extensional to compressional strain rate larger than 1.5 in the model with continuous weak middle crust. Our simulated rifting patterns correlate well with the observations in southern Tibet; we conclude that a combination of the bidirectional compression‐extension and the presence of locally weak middle to lower crust triggered the development of the rifting systems in southern Tibet.

Tectonic evolution of the middle crust in southern Tibet from structural and kinematic studies in the Lhagoi Kangri gneiss dome

Field, structural, kinematic, and deformation temperature analyses were conducted on rocks from the Lhagoi Kangri gneiss dome (southern Tibet) in order to establish the geologic history of the dome, identify major phases of deformation within the dome, and to relate these phases of deformation to the tectonic evolution of the Himalayan middle crust. The Lhagoi Kangri dome, one of a series of gneiss-cored domes in southern Tibet, records stratigraphy and structural features similar to previously studied north Himalayan gneiss domes. Field mapping reveals a sequence of rocks that comprise a cover of unmetamorphosed to amphibolite-grade siliciclastic and minor carbonate rocks overlying a core predominantly composed of foliated and lineated orthogneiss intruded by relatively undeformed granite, which also intrudes the cover rocks both concordantly and discordantly. Field observations and microstructural analyses suggest that the contact between the core and cover rocks was originally a nonconformity, but we do not rule out the possibility of subsequent slip along the surface, as has been reported for correlative structures in other domes. Lhagoi Kangri rocks were pervasively deformed during at least two major tectonic phases. The earliest deformation event (D1) resulted in shortening and thickening of crust, the record of which is largely eliminated, particularly in lower structural levels, by transposition and recrystallization during the second phase of deformation (D2). Ductile deformation during D2 is characterized at higher structural levels by crenulation cleavage that tightens with depth, while at lower structural levels D2 manifests as a distributed shear zone that records some evidence of both plane strain and coaxial flattening, possibly indicating overall heterogeneous general shear. The shear zone is ∼3 km thick and contains rocks with mostly symmetrical top-to-north and top-to-south shear sense indicators with a dominant top-to-north component at lower structural levels. Microstructural analyses and quartz c-axis fabrics indicate a range of D2 shear zone temperatures from 200 to 300 °C at the upper boundary to ≥630 °C at the lowest structural levels sampled with minimal evidence of lower temperature overprinting. The interpreted temperatures define a wide range in thermal field gradients (18–90 °C/km) that suggest that temperature indicators were locked in at relatively late stages of D2. The structural framework and kinematic history of the Lhagoi Kangri dome are similar to previously studied north Himalayan gneiss domes as well as to transects through the South Tibetan detachment system, which supports previous interpretations of structural continuity between the north Himalayan gneiss domes and other middle crustal exposures in the Himalaya. The Lhagoi Kangri distributed shear zone, in particular, may represent a deeper ductile manifestation of the South Tibetan detachment system.

Conservation and redistribution of crust during the Indo-Asian collision

We evaluate the mass balance of the Indo-Asian orogen by reconstructing the Indian and Asian margins prior to collision using recently published paleomagnetic and surface shortening constraints, and subtracting modern crustal volumes derived from gravity inversions and deep seismic soundings. Results show a ~30% deficit between original and modern orogen volumes if the average global crustal thickness of 41 km is assumed prior to collision, even once eastward extrusion and crustal flow are considered. Such a large discrepancy requires crustal recycling of a magnitude that is greater than one half of the modern orogenic mass, as others have previously suggested. Proposals for extensive high elevations prior to or soon after the collision further exacerbate this mismatch and dramatically increase the volume of material necessary to be placed into the mantle. However, we show that this discrepancy can be eliminated with a 23–29 km thick crust within the orogen prior to collision along with a thick southern Tibet margin (the Lhasa and Qiangtang terranes). Because of the relatively low magnitude of surface shortening in Asia, an initially thin crust would require underplating of Indian crust in southern Tibet and displacement of a highly mobile lower crust to the north and east in order to explain modern crustal thicknesses. The contrast between a proposed thinner Asian interior and older and thicker lithosphere of the North China block may have defined the distal extent of deformation at the time of collision and since.

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~50km to 80km beneath the Himalaya terrane, and locally reaches ~85km beneath the Indus–Yalung suture. It remains at ~80km depth across the Lhasa terrane, and shallows to ~70km depth under the Qiangtang terrane. An intra-crustal interface at ~60km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.

Petrogenesis of Cretaceous adakite-like intrusions of the Gangdese Plutonic Belt, southern Tibet: Implications for mid-ocean ridge subduction and crustal growth

We have conducted a whole-rock geochemical, U–Pb zircon geochronological, and in situ zircon Hf–O isotopic compositional study of rocks in southern Tibet from the Langxian igneous suite (including a lamprophyre dyke, mafic enclaves, a granodiorite, and a two-mica granite) and the Nuri igneous suite (a quartz–diorite). U–Pb zircon dating indicates that the timing of crystallization of the mafic enclaves and host granodiorite of the Langxian suite are ca. 105Ma and 102Ma, respectively, that the Langxian lamprophyre dyke and the two-mica granite were emplaced at ca. 96Ma and 80–76Ma, respectively, and that the Nuri quartz–diorite was emplaced at ca. 95Ma. With the exception of the lamprophyre dyke and mafic enclaves in the Langxian area, felsic rocks from the Langxian and Nuri igneous suites all show signs of a geochemical affinity with adakite-like rocks. The high Mg-numbers, high abundance of compatible elements, high εNd(t) (2.7 and 2.8) and δ18O (8.9 and 9.2‰) values, elevated zircon εHf(t) (11.0–17.0) values, and low 87Sr/86Sr(i) ratios (0.7040), collectively indicate that the Nuri adakite-like quartz–diorite was derived from partial melting of the low temperature altered Neo-Tethyan oceanic crust, and that these dioritic magmas subsequently interacted with peridotite as they rose upwards through the overlying mantle wedge. The observation of identical differentiation trends, similar whole-rock Sr–Nd and zircon Hf isotopic compositions, and consistently low (Dy/Yb)N ratios among the Langxian igneous suite rocks, indicates that the adakite-like granodiorite was produced by low-pressure fractional crystallization of precursor magmas now represented by the (relict) mafic enclaves. However, relatively high Al2O3 contents, low MgO, Cr and Ni contents, and low (La/Yb)N and (Dy/Yb)N values indicate that the two-mica granite was derived from partial melting of the southern Tibetan mafic lower crust in the absence of garnet, while isotopic data suggest that at least 70% of the magma source region was juvenile materials. Combined with the presence of HT (high temperature) charnockitic magmatism, HT granulite facies metamorphism, and large volumes of Late Cretaceous batholiths, the oceanic-slab-derived Nuri adakitic rocks indicate a substantial high heat flux in the Gangdese batholith belt during the Late Cretaceous, which may have been related to subduction of a Neo-Tethyan mid-ocean ridge system. According to this model, hot asthenosphere would rise up through the corresponding slab window, and come into direct contact with both the oceanic slab and the base of the overlying plate. This would cause melting of both the oceanic slab and the overlying plate by the addition of heat that was ultimately linked with peak magmatism and the significant growth and chemical differentiation of juvenile crust in southern Tibet during the Late Cretaceous (105–76Ma). In addition, the petrogenesis of the Langxian adakite-like two-mica granite indicates that the southern Tibetan crust was still of normal thickness prior to the emplacement of these intrusions at ca. 76Ma. This probably means that large parts of southern Tibet were not very highly elevated prior to the Indian–Asian collision.

Fragments of hot and metasomatized mantle lithosphere in Middle Miocene ultrapotassic lavas, southern Tibet

Uplift of the Tibetan Plateau and its infl uence on our global climate have been the focus of numerous studies. Miocene potassic to ultrapotassic volcanism is widespread in southern Tibet and has been generally attributed to convective removal of collision-thickened Asian lithosphere, which is also responsible for the uplift of the plateau. An implicit assumption of this model is the existence of a hydrous, metasomatized (i.e., phlogopite bearing) lithospheric mantle that remained after the convective thinning and was subsequently heated to form small-volume melts. If such a lithospheric mantle was present in the Miocene, it implies further change since that time, as seismic velocities indicate that cold and strong upper mantle occurs beneath the thick crust in southern Tibet. Here we describe peridotite xenoliths entrained in Middle Miocene ultrapotassic lavas from Sailipu, southern Tibet. The results suggest the existence of hot, highly metasomatized lithospheric mantle beneath southern Tibet during the Middle Miocene, and thus support the idea that convective thinning of the lithosphere was responsible for the uplift of the plateau. The relict mantle was later removed or squeezed northward by the underthrusting Indian continental lithosphere, which terminated magmatism in southern Tibet and played a role in creating the entire plateau.

Midcrustal reflector on INDEPTH wide‐angle profiles: An ophiolitic slab beneath the India‐Asia suture in southern Tibet?

The wide‐angle seismic experiment of Project INDEPTH (International Deep Profiling of Tibet and the Himalayas) focused on the structure of the India‐Asia continental collision suture (Yarlung Zangbo suture) in southern Tibet. Three‐component portable seismographs recorded the explosive sources of the INDEPTH seismic reflection profile across the suture zone. We image a prominent subhorizontal reflector, the “Yarlung Zangbo reflector” (YZR), dipping ∼4°N at ∼20‐km depth beneath the outcrop position of the Yarlung Zangbo suture, with a total area of at least 90 × 80 km. Possible geological interpretations for the YZR include fault/shear zones, fluids (magmatic, metamorphic, or hydrothermal), or lithotectonic contacts. High‐amplitude reflections off the YZR out to postcritical offsets and refracted phases with a velocity of ∼7 km s−1 provide evidence that it is a solid‐solid interface and the top of a mafic to ultramafic lithological unit. We suggest that the YZR is a Tethyan ophiolitic slab involving several kilometers of mafic‐ultramafic rocks. We interpret the origin of the slab and its position in the midcrust in the context of the emplacement history of oceanic lithosphere and accreted material onto the northwestern and northern Indian passive margin and the tectonic history of the Yarlung Zangbo suture zone. The origin of the YZR body and its position in the midcrust are explained by the emplacement of an ophiolite nappe‐accretionary wedge complex onto India's passive margin prior to continental collision, the subduction of this nappe complex beneath the forearc basin and the Asian (Gangdese) magmatic arc, and its further burial by the Gangdese arc from the north and the Indian passive margin sedimentary sequence from the south. Our model of the YZR constituting an ophiolitic sheet can explain several features of southern Tibetan tectonics, namely, the pre‐collisional deformation within the northern Indian passive margin, the lack of strong deformation during collision, and the bivergent thrust belt along the suture zone and doming within central southern Tibet (e.g., Kangmar dome). Our interpretation implies that the Yarlung Zangbo suture is subhorizontal in the middle crust in southern Tibet.