Asian Lithosphere Research Articles

Eastward extrusion and northward expansion of the Tibetan Plateau—Discussions for the deep processes of the plateau uplift

The weak materials beneath the Tibetan Plateau have been escaping and extruding eastward from beneath the thickened and elevated central plateau driven by continuous convergence between India and Eurasia. This paper presents a detailed shear wave velocity cross-section image of crustal and uppermost mantle structure across the Tibet-Qinling transition zone that was generated from a dense linear seismic array. Based on our inverted seismic image together with previous geological and geophysical studies, we discussed about the material extrusion of the Tibetan Plateau, unveiling the depth extent in which the Qinling belt (which could serve as a flow channel) may accommodate the extrusion of the ductile material beneath the Tibetan Plateau, and suggested that how these dynamic processes might accommodate plateau uplift and expansion in the eastern margin of the Tibetan Plateau; furthermore, we compared the mechanisms of crustal deformation/thickening and the behavior patterns of lithospheric mantle between the eastern and northern margins of the Tibetan Plateau. Integrating our seismic image with previous observations, we can obtain the following findings: (1) A vigorous low velocity zone (LVZ) ( V s V s<4.2 km/s) resides beneath the Tibet-Qinling/Sichuan boundary area, contrasting to the high-velocity upper mantle to the east beneath the Qinling block and Sichuan basin. A synthetic analysis based on these findings and previous geological/geophysical observations leads to the corresponding conclusions: (1) Middle to lower crustal flow accompanied by regional fault-related strike-slip/thrusts may be occurring beneath the eastern margin of the Tibetan Plateau, and may be a significant deep seismogenic mechanism of the large earthquakes occurrence in the eastern margin of the plateau, but it has not vastly extruded farther beyond the Tibet-Qinling boundary into the lower crust of the Qinling belt. (2) The hot asthenospheric mantle material beneath the Tibetan Plateau may escape eastward driven by convergence/squeezing between India-Eurasia continents and flow to the Qinling belt, accelerating the delamination or thermal erosion of the base of the lithosphere beneath the Tibet-Qinling/Sichuan boundary area. Combination of the preceding two mechanisms, i.e., the extrusion of ductile middle to lower crustal materials accompanied by regional fault-related strike-slip/thrusts and isostatic buoyancy resulting from lithospheric detachment (triggered by asthenospheric flow), may have jointly engendered the plateau uplift and expansion in the Tibet-Qinling and Tibet-Sichuan transition zones. In contrast, uplift of the Qilian Shan in the northern margin of the Tibetan Plateau significantly depends upon upper crustal shortening processes above a weak midcrustal decollement, which is subject to the North China plate underthrusting beneath the plateau. Obviously, the deep process of material extrusion in the eastern margin of the Tibet Plateau is distinguishable from the deep mechanism responsible for the northward expansion within the Tibet-Alxa transition zone in the northern margin of the plateau. The fundamental geodynamic mechanism leading to the mode discrepancy of plateau uplift between the eastern and northern margins of the Tibetan Plateau may be attributed to the lateral variation of the nature of the lithospheres from central and eastern Tibet (interior part of the plateau) to the northern margin of the plateau. A relatively hot/weak Tibetan lithosphere underlies the Songpan-Ganzi and Qiangtang blocks in the central and eastern Tibet, while a relatively cold/rigid Asian lithosphere underlies the Qaidam and Qilian blocks (as well as the northern bordering Alxa block) in the northern margin of the Tibetan Plateau, with the boundary extending along the Kunlun-West Qinling range on a large-scale view.

Mantle kinematics driving collisional subduction: Insights from analogue modeling

Since several decades, the processes allowing for the subduction of the continental lithosphere less dense than the mantle in a collision context have been widely explored, but models that are based upon the premise that slab pull is the prominent driver of plate tectonics fail. The India–Asia collision, where several episodes of continental subduction have been documented, constitute a case study for alternative views. One of these episodes occurred in the early collision time within the Asian plate where continental lithosphere not attached to any oceanic lithosphere subducted southward in front of the Indian lithosphere during its northward subduction that followed the oceanic subduction of the Tethys ocean. This process, known as collisional subduction, has a counter-intuitive behavior since the subduction is not driven by slab pull. It has been speculated that the mantle circulation can play an important role in triggering collisional subduction but a detailed, qualitative analysis of it is not available, yet. In this work we explore the southward subduction dynamics of the Asian lithosphere below Tibet by means of analogue experiments with the aim to highlight how the mantle circulation induces or responds to collisional subduction. We found that during the northward oceanic subduction (analogue of Tethys subduction) attached to the indenter (Indian analogue), the main component of slab motion is driven vertically by its negative buoyancy, while the trench rolls back. In the mantle the convective pattern consists in a pair of wide convective cells on both sides of the slab. But when the indenter starts to bend and plunge in the mantle, trench motion reverses. Its advance transmits the far field forces to two upper plates (Asian analogues). The more viscous frontal plate thickens, and the less viscous hinterland plate, which is attached to the back wall of the box, subducts. During this transition, a pair of sub-lithospheric convective cells is observed on both sides of the Asian analogue slab, driven by the shortening of the frontal plate. It favors the initiation of the backwall plate subduction. Such subduction is maintained during the entire collision by a wide cell with a mostly horizontal mantle flow below Tibet, passively advecting the Asian analogue slab. Experimental results suggest that once the tectonic far-field force related to the forward horizontal motion becomes dominant upon the buoyancy forces, trench advancing and the transmission of the tectonic force to the upper and backwall plates are promoted. This peculiar condition triggers the subduction of the backwall plate, despite it is light and buoyant.

Mantle Flow and Deforming Continents: From India-Asia Convergence to Pacific Subduction.

The formation of mountain belts or rift zones is commonly attributed to interactions between plates along their boundaries, but the widely distributed deformation of Asia from Himalaya to the Japan Sea and other back‐arc basins is difficult to reconcile with this notion. Through comparison of the tectonic and kinematic records of the last 50 Ma with seismic tomography and anisotropy models, we show that the closure of the former Tethys Ocean and the extensional deformation of East Asia can be best explained if the asthenospheric mantle transporting India northward, forming the Himalaya and the Tibetan Plateau, reaches East Asia where it overrides the westward flowing Pacific mantle and contributes to subduction dynamics, distributing extensional deformation over a 3,000‐km wide region. This deep asthenospheric flow partly controls the compressional stresses transmitted through the continent‐continent collision, driving crustal thickening below the Himalayas and Tibet and the propagation of strike‐slip faults across Asian lithosphere further north and east, as well as with the lithospheric and crustal flow powered by slab retreat east of the collision zone below East and SE Asia. The main shortening direction in the deforming continent between the collision zone and the Pacific subduction zones may in this case be a proxy for the direction of flow in the asthenosphere underneath, which may become a useful tool for studying mantle flow in the distant past. Our model of the India‐Asia collision emphasizes the role of asthenospheric flow underneath continents and may offer alternative ways of understanding tectonic processes.

Limited southward underthrusting of the Asian lithosphere and material extrusion beneath the northeastern margin of Tibet, inferred from teleseismic Rayleigh wave tomography

AbstractThe northeastern margin of Tibet is of key importance in understanding the uplift and expansion of Tibetan Plateau. In this research, we perform Rayleigh wave tomography from 20 s to 167 s using the data recorded by China Digital Seismic Network and several portable seismic arrays in Northeastern Tibet and adjacent areas. Our resultant 3‐D shear wave velocity model exhibits strong lateral variations in the lithosphere. High shear wave velocities are observed in the mantle of the cratonic Ordos and Yangtze Blocks, as well as the East Qinling Orogen. The Qilian Orogen is underlain by a ∼120 km thick lithosphere of intermediate velocities and a low‐velocity layer of asthenospheric mantle below the depth of 120 km. Low velocities are characteristic in the middle‐to‐lower crust of the Songpan‐Ganzi Terrane and the West Qinling Orogen, possibly representing the crustal flow penetrating the Kunlun Fault. An uppermost mantle low‐velocity zone is found in the same location, indicating either the asthenosphere upwelling or the transitional crust‐mantle boundary layer. Our observations have two tectonic implications. First, the large‐scale eastward Tibetan material extrusion into Eastern China through the East Qinling Orogen is unlikely to be an ongoing process above 200 km depth; second, the southward subduction of the North China Craton toward Northeastern Tibet may not have fully developed in this region. Furthermore, we propose that the difference in the lithospheric strength between the Qaidam Block and the West Qinling Orogen is a critical element to control the tectonics in the northeastern margin of Tibet.

Crustal rheology controls on the Tibetan plateau formation during India-Asia convergence

The formation of the Tibetan plateau during the India-Asia collision remains an outstanding issue. Proposed models mostly focus on the different styles of Tibetan crustal deformation, yet these do not readily explain the observed variation of deformation and deep structures along the collisional zone. Here we use three-dimensional numerical models to evaluate the effects of crustal rheology on the formation of the Himalayan-Tibetan orogenic system. During convergence, a weaker Asian crust allows strain far north within the upper plate, where a wide continental plateau forms behind the orogeny. In contrast, a stronger Asian crust suppresses the plateau formation, while the orogeny accommodates most of the shortening. The stronger Asian lithosphere is also forced beneath the Indian lithosphere, forming a reversed-polarity underthrusting. Our results demonstrate that the observed variations in lithosphere deformation and structures along the India-Asia collision zone are primarily controlled by the strength heterogeneity of the Asian continental crust.

Asian collisional subduction: A key process driving formation of the Tibetan Plateau

Abstract Using silicone slabs as a model analogue for lithospheric plates subducting into a box of glucose syrup, as an analogue of the mantle, we explore the subduction of continental lithosphere in a context of intercontinental collision. The continental indenter pushed by a piston, reproducing the collision, attached to a dense oceanic plate, subducts to two-thirds of the depth of the mantle box. We show that, surprisingly, the continental plate attached to the back wall of the box subducts, even if not attached to a dense oceanic slab. The engine of this subduction is not the weight of the slab, because the slab is lighter than the mantle, but the motion of the piston, which generates horizontal tectonic forces. These are transmitted to the back wall plate through the indenter and the upper plate at the surface, and by the advancing indenter slab through the mantle at shallow depth. We define this process as collisional subduction occurring in a compressional context. The collisional subduction absorbs between 14% and 20% of the convergence, and represents an unexplored component of collisional mass balance. The transmission of tectonic forces far from the collision front favors the formation of a wide plateau. Our experiments reproduce adequately the amount and geometry of the Asian lithosphere subduction episodes inferred during the collision, leading us to conclude that it reproduces adequately the physics of such process.

Anisotropic regime across northeastern Tibet and its geodynamic implications

A dense linear array of 38 broadband seismograph stations was deployed to traverse the northeastern margin of Tibetan plateau (NE Tibet). Shear wave splitting measurements show significant lateral variations of seismic anisotropy across NE Tibet. Combined with previous tomography studies, the SKS travel-time analysis along the array supports the inference that a cold/rigid Asian lithosphere resides beneath the Qilian and Alxa blocks while a hot/soft Tibetan lithosphere resides beneath the Songpan-Ganzi (SPGZ) and Kunlun-West Qinling (KL-WQL) blocks. The observed variations of anisotropy along the array indicate the important role the major faults have been playing in the process of lithospheric deformation in NE Tibet. The West Qinling fault (WQLF) is reckoned as the boundary between the Tibetan lithosphere and the Asian lithosphere in the study area. The Kunlun fault to West Qinling fault (KLF-to-WQLF) zone may constitute a boundary accommodating the eastward extrusion of the Tibetan lithosphere, with the rigid Asian lithosphere in the north barrier to the northeastward tectonic flow of central-eastern Tibet. A significant character of two-layer anisotropy was identified in the Qilian orogen, which was inferred to be associated with the low velocity layer (LVL) acting as a thrust decollement in the mid-to-lower crust. A thorough analysis involving crustal anisotropy and the regional XKS splitting results in NE Tibet, in association with the crustal LVL feature, indicates that decoupling deformation may dominate the lithosphere beneath the Qilian orogen while coherent deformation may dominate the lithosphere beneath the WQL and SPGZ blocks. Anisotropy beneath the Qilian orogen seems consistent with recent deformation under the boundary forces related to possible lithosphere underthrusting from its northern margin. Our shear wave splitting analysis, combined with published results, reflects a regional anisotropic regime that emphasizes the dominance of eastward extrusion of lithospheric blocks on the present-day deformation in NE Tibet.

Discussion on the Special Lithosphere Type in Eastern China

In this paper, the authors proposed that there is a special type of thin lithosphere, with continental crust (about 35 km thick) and oceanic mantle (about 40 km thick) in eastern China. The characteristics of eastern China lithosphere is so special, it is different to most of continental lithosphere of globe. To detail research the characteristics and evolution of eastern Asian lithosphere is important, which will be helpful to develop the theory of global plate tectonics. This paper will expound the thickness change, tectonic characteristics and transformation period of lithosphere type, and to discuss its mechanism.

Deep India meets deep Asia: Lithospheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia)

Subduction of buoyant continental lithosphere is one of the least understood plate-tectonic processes. Yet under the Pamir–Hindu Kush, at the northwestern margin of the India–Asia collision zone, unusual deep earthquakes and seismic velocity anomalies suggest subduction of Asian and Indian lithosphere. Here, we report new precise earthquake hypocenters, detailed tomographic images and earthquake source mechanisms, which allow distinguishing a narrow sliver of Indian lithosphere beneath the deepest Hindu Kush earthquakes and a broad, arcuate slab of Asian lithosphere beneath the Pamir. We suggest that this double subduction zone arises by contrasting modes of convergence under the Pamir and Hindu Kush, imposed by the different mechanical properties of the three types of lithosphere involved. While the buoyant northwestern salient of Cratonic India bulldozes into Cratonic Asia, forcing delamination and rollback of its lithosphere, India's thinned western continental margin separates from Cratonic India and subducts beneath Asia. This torn-off narrow plate sliver forms a prominent high-velocity anomaly down to the mantle transition zone. Our images show that its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab, providing a rare glimpse at the ephemeral process of slab break-off.

Is the Asian lithosphere underthrusting beneath northeastern Tibetan Plateau? Insights from seismic receiver functions

Whether or not the Asian lithosphere has underthrusted beneath the Tibetan Plateau is important for understanding the mechanisms of the plateau's growth. Using data from the permanent seismic stations in northeastern Tibetan Plateau, we studied seismic structures of the lithosphere and upper mantle across the plateau's northeastern margin using P and S receiver functions. The migrated P- and S-receiver function images reveal a thick crust and a diffuse lithosphere–asthenosphere boundary (LAB) beneath the Tibetan Plateau, contrasting sharply with the relatively thin crust and clear, sharp LAB under the bounding Asian blocks. The well-defined LAB under the Asian blocks tilts toward but does not extend significantly under the Tibetan Plateau; this is inconsistent with the model of Asian mantle lithosphere underthrusting beneath the Tibet Plateau. Instead, our results indicate limited, passive deformation of the bounding Asian lithosphere as it encounters the growing Tibetan Plateau.

Titanite petrochronology of the Pamir gneiss domes: Implications for middle to deep crust exhumation and titanite closure to Pb and Zr diffusion

©2015. American Geophysical Union. All Rights Reserved. The Pamir Plateau, a result of the India-Asia collision, contains extensive exposures of Cenozoic middle to lower crust in domes exhumed by north-south crustal extension. Titanite grains from 60 igneous and metamorphic rocks were investigated with U-Pb + trace element petrochronology (including Zr thermometry) to constrain the timing and temperatures of crustal thickening and exhumation. Titanite from the Pamir domes records thickening from ∼44 to 25 Ma. Retrograde titanite from the Yazgulem, Sarez, and Muskol-Shatput domes records a transition from thickening to exhumation at ∼20-16 Ma, whereas titanite from the Shakhadara dome records prolonged exhumation from ∼20 to 8 Ma. The synchronous onset of exhumation may have been initiated by breakoff of the Indian slab and possible convective removal of the Asian lower crust and/or mantle lithosphere. The prolonged exhumation of the Shakhdara and Muztaghata-Kongur Shan domes may have been driven by continued rollback of the Asian lithosphere concurrent with shortening and northwestward translation of the Pamir Plateau.

Petrogenesis of the Ramba leucogranite in the Tethyan Himalaya and constraints on the channel flow model

Abstract The Himalayan leucogranites are critical for understanding the geodynamic process of Himalayan orogenesis. In this article, we present petrological, geochronological and geochemical results of the Ramba leucogranites that crop out in the center of the Ramba dome within the Tethyan Himalaya. U–(Th)–Pb dating of zircon, monazite and xenotime revealed three episodes of leucogranitic magmatism in the Ramba dome at ca. 44 Ma, ca. 28 Ma and ca. 8 Ma, respectively. The ca. 44 Ma and ca. 28 Ma magmatisms are both represented by strongly deformed porphyritic two-mica granite gneiss dykes that intruded into the margin of the dome. These granite gneisses have high CaO and Sr contents, relatively high Na/K, Sr/Y and low Rb/Sr ratios, and much lower initial 87Sr/86Sr ratios but higher eNd(t) and eHf(t) values than the ancient Indian crustal material. These results indicate that the granite gneisses were not derived from metasedimentary rocks of the High Himalayan Sequence but most likely were derived from partial melting of mafic rocks at a thickened crustal condition. The ca. 8 Ma leucogranites consist of a two-mica granite pluton in the core of the dome and garnet-bearing granite dykes in the margin of the dome. Major and trace element compositions indicate that these Miocene leucogranites are typical S-type granites and were most likely derived from muscovite and/or biotite dehydration melting of metapelites. The relatively lower TiO2, TFeO, MgO and CaO contents, higher Rb concentration and Rb/Sr ratio, and more strongly negative Eu anomaly of the garnet-bearing granite indicate that it was more evolved than the coeval two-mica granite. The non-CHARAC (CHarge-And-Radius-Controlled) trace element behavior and the tetrad effect of the REE distribution of the garnet-bearing granite further suggest that it is a highly fractionated granite. Besides, our study detected some Asian-like materials in the Ramba leucogranites, such as Mesozoic-aged inherited zircons and the relatively depleted Sr–Nd–Hf isotopic compositions of some leucogranite samples. These features of leucogranites in the Tethyan Himalaya have been previously considered as direct geological evidence for the channel flow model. However, detailed geochronological and geochemical studies of the country rocks suggest that these Asian-like materials are most likely sourced from the country rock rather than the Asian lithosphere, thus they may not be used as solid evidence for the channel flow model.

Upper mantle seismic anisotropy beneath a convergent boundary: SKS waveform modeling in central Tibet

This study used SKS waveforms from the International Deep Profiling of Tibet and the Himalayas (INDEPTH) III dataset and a new 2D method for modeling seismic waves in anisotropic media to construct an image of anisotropic structures beneath central Tibet. A preferred model revealed three-segment anisotropic structures in the upper mantle beneath the study region. Waveform modeling demonstrated that the anisotropy was mainly generated by the lithosphere but not the asthenosphere, and that an anisotropic model with a flatter axis of symmetry provides a more consistent interpretation of the observations than models having steeply dipping symmetry axes. A relatively low velocity zone may underlie or intermingle with the anisotropic structures in the northern portion of the region. Synthetic tests also indicate that variations in the elastic constants and depth extent of the anisotropy assumed by the calculations do not affect the general conclusions, although trade-offs exist among certain model parameters. The modeling results suggest that the complex seismic structures in central Tibet were associated with underthrusting of the Indian lithosphere beneath the Asian lithosphere; the inferred flat symmetry axis of the anisotropy was likely generated during this collision process. If this were not the case, the inherited anisotropy would exhibit a steeply dipping axis of symmetry, parallel to the direction of underthrusting.

Deep burial of Asian continental crust beneath the Pamir imaged with local earthquake tomography

An inclined zone of intermediate-depth seismicity beneath the Pamir orogen in Central Asia has been interpreted as southward subduction of a slab of Asian lithosphere. However, it is not known whether Asian lithosphere subducts intact or only partially. We used arrival times of shallow and intermediate-depth earthquakes, recorded with a temporary (2008–2010) seismic network in this region, to invert for 3D models of seismic velocities in an attempt to answer this question. With local seismicity reaching depths of up to 240 km, the deep structure of the Pamir could be illuminated with high resolution.The resulting velocity models show a north–south contrast in crustal seismic velocities in the Pamir, with very low P velocities (5.7–5.9 km/s at 15–30 km depth), coupled with relatively low vp/vs (<1.70), at mid-crustal levels in the southern part of the orogen. At sub-Moho depths, we image an arcuate high-velocity (8.2–8.6 km/s) slab dipping south in the eastern Pamir and east in the Pamirʼs southwest, underlying the intermediate-depth earthquakes. On top of the high-velocity slab and just above the onset of deep seismicity, between a depth of 60 to 100 km, very low compressional wavespeeds (around 7.1 km/s) and high vp/vs ratios (⩾1.80) attest to subducted crustal rocks. Additionally, we carried out 2D numerical thermomechanical modeling of the continental collision in the Pamir, focusing on the fate of the crust and mantle lithosphere of the Asian and Indian plates. Seismic velocities were computed from the modeling results, and the resulting images were compared with the velocity distributions obtained from seismic traveltimes.Combining tomography and modeling results, we infer that a substantial amount of crustal material is pulled down beneath the Pamir by cold mantle lithosphere to depths of at least 80–100 km. From there on, only lower crust and mantle lithosphere continue their subduction, and earthquakes occur inside the lower crustal layer probably due to metamorphic reactions. The buoyant Asian upper and middle crust does not penetrate deeper into the mantle, but pools at this depth level, from where it might eventually exhume or relaminate.

First Steps toward a Reassessment of the Seismic Risk of the City of Dushanbe (Tajikistan)

Online Material: Table detailing station installation sites and recording times. The country of Tajikistan is located in the Asia–India continental collision zone where the northward‐moving Indian plate indents the Eurasian plate (Molnar and Tapponnier, 1975). The Asian lithosphere being impinged upon by the shallow northward underthrusting Indian lithosphere has resulted in high‐mountain topography, which is subject to active deformation and contemporary faulting, and, consequentially, frequent earthquakes (e.g., Gubin, 1960; Burtman and Molnar, 1993). According to the Global Seismic Hazard Map (Giardini, 1999), almost the entire country exhibits a high‐hazard level with intensities of VIII–IX for a 5% exceedance in 50 years. Most of the earthquakes that occur in these fault zones are crustal events with dominating thrust faulting along the Pamir front, whereas on the eastern edge of the Pamir the style of deformation is generally characterized by oblique thrusting with a component of strike‐slip motion. Thrust‐type events beneath the Tadjik Depression, a compressional intermountain basin in the western part of the country, indicate that both the sedimentary rocks and the basement are involved in shortening (Fan et al. , 1994; Delvaux et al. , 2013). Although archaeological remnants dating to the fifth century B.C. have been discovered in this area, there is little to suggest that Dushanbe, the Tajik capital, was more than a small village until the early twentieth century. Therefore, there exists only a relatively short record of strong earthquakes in the area (e.g., the 1907 M 7.3 Karatag earthquake and the 1949 M 7.4 Khait earthquake) in comparison with other parts of Central Asia. The General Seismic Zoning map for the former Soviet Union (GSZ‐78) associated an intensity of IX to Dushanbe, with a recurrence interval of 1000 years (Bune and Gorshkov, 1980). Recently, Nurmangambetov et al. (1999) calculated refined return periods for intensities IX and …

Amount of Asian lithospheric mantle subducted during the India/Asia collision

Body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle. Focusing on the India/Asia collision zone, we postulate the existence of several Asian continental slabs, based on seismic global tomography. We observe a lower mantle positive anomaly between 1100 and 900km depths, that we interpret as the signature of a past subduction process of Asian lithosphere, based on the anomaly position relative to positive anomalies related to Indian continental slab. We propose that this anomaly provides evidence for south dipping subduction of North Tibet lithospheric mantle, occurring along 3000km parallel to the Southern Asian margin, and beginning soon after the 45Ma break-off that detached the Tethys oceanic slab from the Indian continent. We estimate the maximum length of the slab related to the anomaly to be 400km. Adding 200km of presently Asian subducting slab beneath Central Tibet, the amount of Asian lithospheric mantle absorbed by continental subduction during the collision is at most 600km. Using global seismic tomography to resolve the geometry of Asian continent at the onset of collision, we estimate that the convergence absorbed by Asia during the indentation process is ~1300km. We conclude that Asian continental subduction could accommodate at most 45% of the Asian convergence. The rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction/underthrusting processes. Continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like Eurasia.

Source-side shear-wave splitting and upper-mantle flow beneath the Arakan slab, India-Asia-Sundalind triple junction

Shear-wave splitting of S waves from earthquakes in the Arakan slab is consistent with strong asthenospheric anisotropy that developed as a consequence of the India-Asia collision. Global positioning system (GPS) site velocities in the India-Asia-Sundaland triple junction region show that deformation along the Arakan subduction zone is partitioned into dextral strike-slip motion, as India moves northwards with respect to Asia, and contraction across the Arakan trench and Chittagong-Tripura fold belt. Indian Ocean lithosphere comprising the Arakan slab is dismembered into three segments as a result of its collision with Asian lithosphere at the East Himalayan syntaxis. Offsets of intermediate-depth earthquake hypocenters at two locations delineate slab segments that form a left-stepping en echelon structure. Arakan slab focal mechanisms are consistent with slab sinking and along-strike compression and bending, and, south of 25°N, dextral strike-slip along the slab. Two regions of N-S contraction within the slab appear to be localized at the slab segment offsets. Teleseismically recorded S waves from earthquakes within the three slab segments, and surroundings, are split systematically: once corrected for receiver-side splitting, fast shear trends are predominantly trench-parallel beneath the east-dipping slab segments; are more nearly trench-normal on the Sundaland (east) side of the Arakan lithosphere; parallel the southern ∼E-W gap between Arakan slab segments; and turn sharply around the extreme northern and southern edges of subducted Arakan lithosphere. Source-side shear-wave splitting beneath India is consistent with observed ∼E-W–trending fast shear polarizations of SK(K)S splitting in northeastern India. The general pattern of both surface site velocities from GPS and shear-wave splitting studies is consistent with material flow around the eastern Himalayan syntaxis and into the mantle wedge above the Arakan slab, and around the northern terminus of the Arakan slab. The upper mantle may also flow through the gap between the central and southern Arakan slab segments.

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.

Crustal and upper mantle structure of southeast Tibet from Rayleigh wave tomography

We have applied the two‐plane wave inversion technique to fundamental mode Rayleigh waves in southeast Tibet. Phase velocity variations are obtained at 15 periods from 20 to 143 s and are used to construct three‐dimensional shear wave velocity structure. A low‐velocity zone is imaged in the middle crust of southeast Tibet, in which velocity perturbation varies from −4% to 3%. The slow area might be associated with high temperature and partial melt and can flow ductilely, while the high‐velocity region could be relatively cold and mechanically strong enough to pass crustal strain, implying that coherent lithospheric deformation is possible with the presence of complicated crustal flow. Strong negative anomalies in the shallow crust are imaged at 92°E, correlating with the location of a north‐south rift zone. A fast mantle lid to a depth of ∼120 km is present beneath most of the study area. The lithosphere is significantly slower in the central part of the study area than in the west and east. To the east of 96°E, a subvertical high‐velocity zone is imaged at 120–240 km depths under the Bangong‐Nujiang Suture (BNS), which is more like the subduction of the Asian lithosphere. We interpret the high‐velocity anomalies to the south of the BNS as the subducted Indian lithosphere and those to the north as the Asian lithosphere. A vertical low‐velocity column is observed in the central part of the region from the lower crust to the midupper mantle, which is probably in part responsible for the rift at 92°E. The slow anomaly is likely associated with asthenosphere upwelling following the opening of a slab window that could have developed when the slab was torn apart vertically because of the significant change of subducting direction at the eastern syntaxis.

Palaeomagnetism and 40Ar/39Ar geochronology of upper Palaeogene volcanic rocks from Central Tibet: implications for the Central Asia inclination anomaly, the palaeolatitude of Tibet and post-50 Ma shortening within Asia

SUMMARY Palaeomagnetic results from upper Palaeogene volcanics from three localities in the Qiangtang terrane of Central Tibet record a maximum likelihood palaeolatitude of 28.7 ± 3.7°N. These are the first volcanic-based palaeomagnetic results from the plateau interior that appear to average secular variation well enough to yield a reliable palaeolatitude estimate. This palaeolatitude estimate is significantly lower than the ∼36 ± 3°N predicted for Central Tibet by late Palaeogene Eurasia reference poles, consistent with the so-called Central Asia inclination anomaly, but it is consistent with values predicted from upper Palaeogene reference poles for Mongolia. The mean inclination of coeval sedimentary rocks from Central Tibet is ∼15 ± 5° shallower than our volcanic results, similar to the magnitude of sedimentary inclination shallowing estimated from Elongation/Inclination analysis of remanent directions in clastic rocks from the same region. We conclude that most of the inclination anomaly observed in volcanic rocks and that which remains after removal of sedimentary flattening in clastic rocks probably is due to the combined effect of reference poles that are not representative of stable Asia and by Cenozoic intracontinental shortening within Central Asia on the order of several hundred (not more than 1000) kilometers. Our compilation of palaeomagnetic data from the Qiangtang terrane further suggest that the southern margin of the Lhasa terrane (the southern margin of pre-India Asia) was located as far south as 20°N throughout Eocene time. These palaeolatitudes, in conjunction with the upper crustal shortening record for Asia, indicate that only approximately one third of the post-50 Ma India–Asia convergence is partitioned in Asian lithosphere. Low latitudes also imply that much of the proto-Tibetan plateau occupied southerly, tropical to subtropical latitudes, which has important implications for the weathering and erosion of the Tibetan plateau, palaeoaltimetry proxies and early Cenozoic climate dynamics.