Lithospheric structure of western Tibet – A brief review

Journal Pre-proofs Lithospheric structure of Western Tibet – A brief review Junmeng Zhao, Bhupati Neupane, Hongbing Liu, Deng Yan PII: DOI: Reference:

S1367-9120(19)30511-5 https://doi.org/10.1016/j.jseaes.2019.104159 JAES 104159

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Journal of Asian Earth Sciences

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Please cite this article as: Zhao, J., Neupane, B., Liu, H., Yan, D., Lithospheric structure of Western Tibet – A brief review, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes.2019.104159

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Lithospheric structure of Western Tibet – A brief review Junmeng Zhao 1, 2, 3, Bhupati Neupane 2, 3*, Hongbing Liu1, 2, Deng Yan2, 4

1 CAS 2 Key

Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China

Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau

Research, Chinese Academy of Sciences, Beijing 100101, China 3 University 4

of Chinese Academy of Sciences, Beijing 100101, China

State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 10029, China

*Corresponding author: Bhupati Neupane ([email protected])

Abstract Over the course of the last hundred years or so, there have been countless seismic experiments conducted under the aegis of different research programs, which have aimed to investigate the structure of the crust and uppermost mantle of the western Tibetan Plateau. However, there have been fewer detailed lithographic studies of the western sector of the Tibetan Plateau, and those that have been conducted have principally fallen under the research umbrella of the ANTILOPE (Array Network of Tibetan International Lithospheric Observation and Probe Experiments). In this paper, we summarize the important research findings that have been gathered using comprehensive geophysical profiles, and provide an overall view of the lithospheric structure of the western Tibetan Plateau that has resulted from the onset of tectonic collision and the

continuous convergence of the Indian and Eurasian plates since ~65 Ma ago. In western Tibet, the subducted Indian lithospheric mantle is moving towards the northern edge of the Plateau, colliding with the Tarim Basin at 80°E. However, the Tarim Basin shows different nature of subductions in eastern, central and western regions itself. The maximum Moho depth (~93 km) beneath the Qiangtang Terrane marks the northern margin of the decoupled underthrusting Indian Plate’s lower crust and the lithospheric mantle. The broader, stronger terrain of the western Tibetan Plateau may indicate an eastward-broadening shape and a lower Cenozoic shortening, and the relatively larger width of the Lhasa-Qiangtang terranes may be a key factor controlling the geodynamic evolution of the Tibetan Plateau. Keywords: Seismic Experiments; Lithospheric Structure; Western Tibetan Plateau; Moho Depth; Vp/Vs ratio 1. Introduction The Tibetan Plateau (TP) formed the large-scale crustal shortening and uplift resulting from the collision of the Indian-Eurasian continental plates which began between 65 and 50 Ma. The precise date of this collision continues to be debated. A widely accepted date is ~50 Ma (Yin and Harrison, 2000), but van Hinsbergen et al. (2012) proposed a later date (i.e., ~25-20 Ma). However, this younger estimate is at odds with a ~45 Ma estimate for the higher sections of the Lhasa Block (Hetzel et al., 2011; Rohrmann et al., 2012). The evolution of the TP has included the subduction of the Indian lithosphere, the thickening of the TP’s crust, and the eastward extrusion of the Tibetan lithosphere (Royden et al., 2008). From the Early to the Mid Miocene, the thickening of the TP’s crust to about twice its previous thickness occurred as a result of the northward migration of the continental plate, a N-S shortening, and the vertical ‘stretching’ of the Asian continent (Li et al., 2009). At the same time, the northward movement of the Indian

subcontinent was principally accommodated by a N-S shortening in the northern and southern areas of Tibet, with the TP itself rising ~2 km and undergoing an E-W extension. This process has lasted from the Early Pliocene to the present day (Zhang et al., 2011). Geophysical techniques are widely used to probe the structures of the Earth’s crust and upper mantle, in an attempt to understand the geodynamics of the present day and shed light on the crust’s tectonic evolution. Of the new breed of seismic studies, seismic tomography, converted wave studies and seismic anisotropy analysis have been proven to be able to provide significant findings. Furthermore, these techniques are capable of covering the entire lithosphere and asthenosphere, and can produce images of some important boundaries inside the Earth, such as the Moho discontinuity, the lithosphere-asthenosphere boundary (LAB), and the 410 km and 660 km discontinuities (Gao et al., 2013 and references therein). Due to the complex lithospheric structure which has resulted from the ongoing collision, capturing a fine lithospheric image of the crustal structure beneath the TP becomes fundamental to a full understanding of the dynamics of the IndiaEurasia collision. Deep seismic reflection imaging techniques were successfully applied in studies of the tectonic development of the southern TP in the early 1990s (Zhao et al., 1993; Brown et al., 1996; Nelson et al., 1996), but such studies of the central TP have neither been as forthcoming nor as successful (Ross et al., 2004). There are numerous geodynamic models which may be used to explain the deformation of the crust and upper mantle, as well as some uplift mechanisms and the evolution of the subduction, collision, and post-collision processes which have occurred on the TP. Argand (1932) is the first to put forward a model that posited the double thickening of the TP’s crust resulting from the convergence of, and collision between, the Indian and Eurasian plates. A model which, by its very nature, implied that the entire TP was uplifted by the underthrusting of the Indian Plate from the

south, thus doubling the crustal thickness and increasing the surface altitude. This thesis preceded the revolutionary plate tectonics theory by 40 years. However, with no in-depth geophysical data to hand, it was quite difficult for Argand to support his crust-thickening theorem. Some 36 years later, in 1958, the deep seismic reflection program, jointly organized by the Chinese Academy of Sciences (CAS) and the ex-Ministry of Petroleum of China, successfully constructed the first deep seismic reflection profiles for the Qaidam Basin (Zhang et al., 2011 and references therein). Since the 1980s, and over the last 20 years, various geophysical prospecting programs (i.e., national or international) have been undertaken in order to determine the deep structure of the TP. A recent study employed a high-resolution P travel time tomographic image to plot the high-velocity Indian lithosphere beneath the western TP, and was able successfully to map the entire plateau down to a depth of 300–400 km (Huang and Zhao, 2006; Li et al., 2008; Zhou and Lei, 2016) . This location was used because beneath the eastern TP the front of the Indian Plate is located south of the Yarlong–Zangbo Suture (YZS) (Li et al., 2008). The seismic modeling of the TP’s lithosphere has designated a number of continental or island-arc type blocks that appear to have originated from progressive accretion before the Indian continental plate came into direct contact with that of Eurasia, or before any stepwise subduction of the Eurasian Plate had occurred (Allegre et al., 1984; Tapponnier et al., 2001). Previous research related to the north-central and eastern TP has led to the conclusion that this zone may have acted/be acting as a deformable crush zone between the colliding plates (Zhao et al., 2010). GPS measurements using satellite observations have indicated that, in the central to western parts of the TP, velocities decrease sharply from south to north. This would imply that the shortening of the TP has accommodated a sizeable proportion of the northeastward advancement of the Indian Plate (Gan et al., 2007). The TP’s poor shear-wave transmission capabilities would

suggest that this region is underlain by an anomalously hot upper mantle (Willett and Beaumont, 1994). The crustal materials along the TP’s northeastern margins are moving NE-NNE at a velocity of 15-19 mm/yr relative to the South China Block, whereas the western sector of the northeastern TP is experiencing a counterclockwise rotation at a velocity as high as 30-40 mm/yr (Gan et al., 2007). Over the entire TP, any structural interior deformation has, as a general rule, occurred along the axis of the Plateau’s ESE-WNW extension and has been accompanied, in addition, by a slightly slower NNE-SSW shortening, whereas the actual material within the TP’s interior has moved roughly eastward with speeds that increase toward the east, and then flowed southward around the eastern end of the Himalaya (Zhang et al., 2004). A complex geological structure, allied to the rugged topography of the TP, makes geophysical investigation a complicated and challenging task. In this paper, we report on research which has focused on seismic investigations of the western TP over the last two decades in an attempt to understand better the region’s lithospheric structure and the active tectonics which have characterized, and continue to characterize, the Indian-Eurasian continental collision. This research is not only important for establishing a comprehensive collation of previous research results, but is also crucial for understanding the mechanisms controlling this continent-continent collision, and its consequences for the TP and its neighboring areas. 2. Regional tectonic setting The TP, with a mean crustal thickness of 60-75 km, and a continuously elevated area of nearly 3500 km by 1500 km in extent, is one of the largest and highest plateaus in the world (Yin and Harrison, 2000; Royden et al., 2008; Zhao et al., 2010; Xu et al., 2011). The TP is bounded by the deserts of the Tarim and Qaidam basins to the north, and the Himalaya, Karakoram, and

Pamir mountain chains to its south and west (Fig. 1). It consists of five nearly E-W stretching tectonic blocks successively accreted to Eurasia, namely the Himalaya, Lhasa, Qiangtang, Songpan-Ganzi and Kunlun-Qaidam blocks. These structures are separated by five sutures: the Indus-Yarlung Suture (YZS); the Bangong-Nujiang Suture (BNS); the Jinsha River Suture (JS); the Anyimaqen-Kunlun-Mutztagh Suture (AKMS); and the South Qilian Suture (SQS) (Yin and Harrison, 2000; Lin et al., 2009). The western TP includes different geological structures, such as the Late Precambrian to Early Paleozoic sedimentary and meta-sedimentary rocks of the Himalayan orogeny (Yin and Harrison, 2000; Murphy and Copeland, 2005), magmatic rocks and ophiolitic sequences (~130 to 40 Ma) (Kapp et al., 2007), and metamorphic rocks sourced from marine sediments in the Qiangtang Block, which include younger (Triassic) flysch deposits and older (Late Paleozoic) shallow marine deposits (Yin and Harrison, 2000). The southward subduction/underthrusting of the Asian lithosphere beneath the TP has been proposed by a number of authors (e.g., Wittlinger et al., 2004). Three types of geodynamic models have been proposed to explain the uplift and outgrowth of the TP. The first is the rigid block extrusion, or rigid plate, model (Tapponnier et al., 1982; Tapponnier et al., 2001), which suggests that the deformation of the TP’s crust has been accommodated by a combination of crustal thickening, underthrusting and rigid block motion along large-scale strike-slip faults. The second is a viscous flow, or lower crust channel flow, model (England and Houseman, 1989; Clark and Royden, 2000; Royden et al., 2008), which implies that the deformation has been accommodated by a folding and thrusting in the upper crust contributed by a lower crust plastic flow. The third model proposes a uniform thickening and shortening of the entire lithosphere (Dewey et al., 1988; England and Houseman, 1989) which has led to a vertically coherent deformation across the entire lithosphere, i.e., the N-S convergence has

been largely accommodated by both horizontal shortening and vertical thickening. Integrating most recent seismic results, a big mantle wedge structure has been proposed under eastern Tibet to better understand the deep dynamics of eastern Tibet (Lei et al., 2009; Lei and Zhao, 2016; Lei et al., 2019) because several tomographic models demonstrate high velocity anomalies in the upper mantle under Burma arc and extending down to the mantle transition zone (e.g., Li et al., 2008; Lei et al., 2009, 2019). Such results have been supported by the thickened mantle transition zone (Shen et al., 201; Hu et al., 2013). In order to conduct a detailed investigation of the lithospheric structure of the western TP, a total of 80 broadband seismic stations were installed 10 km apart along the array representing the first stage of the Array Network of Tibetan International Lithospheric Observation and Probe Experiments (ANTILOPE-1) profile. This profile lies perpendicular to the main tectonic boundaries such as the Lhasa, Qiangtang and Songpan-Ganzi terranes on the western TP and the Tarim Basin in the north (Fig. 1), recording seismic data from October 2006 to November 2007. The profile was used to obtain a cross-sectional image of the Moho topography and crustal composition that lie across the principal terrines of the western TP. 3. Seismic investigations Since the last century, numerous seismic experiments have investigated the structure of the crust and uppermost mantle of the TP. These have included a Sino-American project (Owens et al., 1993), the Hi-CLIMB (Nábělek et al., 2009), INDEPTH (Zhao et al., 1993; Nelson et al., 1996; Zhao et al., 2001; Tilmann et al., 2003) and ANTILOPE (Zhao et al., 2010; Zhao et al., 2014; Xu et al., 2015) programs. These experiments focused mainly on both active source and passive source seismology to obtain a detailed picture of the crust and upper mantle structure,

including the seismic velocity structure of the TP, as well as to reveal the geodynamic processes behind the inter-continental collision. Different projects used different techniques such as teleseismic P and S receiver functions (Kind et al., 2002; Nábělek et al., 2009; Shi et al., 2009; Xu et al., 2010; Zhao et al., 2010; Xu et al., 2011), seismic refraction/wide-angle reflection (Yuan et al., 1997; Zhao et al., 2001; Zhao et al., 2013), seismic tomography (Tilmann et al., 2003; Zhao et al., 2014), and magnetotelluric sounding (Wei et al., 2001; Unsworth et al., 2004; Unsworth, 2010; Xiao et al., 2011; Xiao et al., 2013). 3.1 The crustal and upper mantle structure beneath the western Tibet Over the last 20 years, much research has been conducted in the eastern and central parts of the TP in order to construct a detailed lithospheric structure (Huang and Zhao, 2006; Li et al., 2008; Lei et al., 2014; Zhou and Lei, 2016), but little research has been conducted in the TP’s western sector (Table 1). There have, however, been a few attempts to fill this gap, with scientists working on different aspects of the abovementioned profile. Wittlinger et al. (2004) observed teleseismic images, based on the crust and upper mantle, using the common conversion point (CCP) time to depth migrations of radial receiver functions (RRF) from within the deep structure below the TP along the section of the profile between the Tarim Basin and the Karakoram range in western Tibet. The data obtained were for the period July to November 2001. The results suggested that the Tarim lithospheric mantle plunges down to a depth of ~300 km beneath northwestern Tibet. The western Kunlun lithospheric mantle appears to have been obliquely subducted beneath an upward-extruding thrust wedge of the Tarim crust. Zhao et al. (2010) constructed two densely-spaced seismic profiles for the September 2005 to November 2007 period for the western TP (Fig. 1) using approximately 150 seismology stations

to record 478 teleseismic earthquakes with high signal-to-noise ratios. This research applied the P and S receiver function common conversion point (CCP) stacking method, which can be used to investigate the Earth’s internal structure (Yuan et al., 2006). On the basis of the resultant seismic tomography, deep seismic discontinuities such as that which occurs at the crust-mantle boundary (the Moho discontinuity), and at the lithosphere-asthenosphere boundary (LAB), it was concluded that the boundary between the Indian and Asian tectonic plates lies below the TP, and that a crustal shortening in the south of this region has allowed the underthrusting of the Indian crust below the Asian crust, possibly further north than the YZS (Zhao et al., 2010). Zhao et al. (2014) presented high-resolution tomographic images of the crust and upper mantle structure of the western TP using P and S wave travel time tomography. They used a large quantity of high-quality arrival-time data recording local, regional and teleseismic events that were measured by 68 seismology stations which form part of the ANTILOPE project. These measurements were conducted between October 2006 and November 2007. Zhao et al. concluded that, after the breaking-away of the Indian Plate, it continued to advance northward and has now reached sub-horizontally as far as the JRS at depths of ~100 to ~250 km, suggesting that the subduction process has evolved over time. Additionally, they suggested that the high-velocity anomalies under southern Tibet could reflect the presence of a cold crust that acts as a barrier to crustal flow, whereas the presence of an underthrusting Indian Plate, and the mantle upwelling under the Songpan-Ganze terrane, may result in the weak and warm crust observed in northwestern Tibet. Nonetheless, Zhao et al. determined that the Eurasian Plate appeared to have been subducted beneath the Tarim Basin down to a depth of ~100 km. Zhang et al. (2014) obtained new, high-resolution seismic data from western Tibet by applying a TW-80 experiment at 80°E (Western Tibet). An array of broadband seismographs was

deployed with a mean station interval of 15-20 km between November 2011 and November 2013 along a 400 km profile, from the southwestern Himalaya (Zarda) to northwestern Tibet (Quanshuigou). To summarize, they suggested that the major tectonic sutures cut through the crust to the Moho. They also posited that a converted Moho phase beneath the Lhasa Block might represent the upper surface of a lower crustal layer that has been progressively eclogitized. Zhang et al. observed significant changes in the crustal structure and Moho depth along the profile which revealed zones of localized shearing on sub-vertical planes; these extended through the crust and into the upper mantle. The mantle lithosphere of the Indian Plate appeared to plunge steeply into the Asian mantle beneath the IYS, whereas the crust of the Indian Plate seemed to have accreted to the southern margins of the TP. Xu et al. (2015) studied the detailed crustal structure and images of the boundary between the converging Indian and Eurasian plates using RRF technique along with a temporary seismic array (ANTILOPE-2). The ANTILOPE-2 profile is located across the central TP, and was operational from September 2005 to October 2006. It consisted of ~80 portable broadband seismology stations with a mean station spacing of ~10 km. Xu et al. noticed an intra-crustal interface at ~60 km beneath the Lhasa Terrane which followed southward through the MHT and connected with the MBT at the surface. This led them to conclude that the edge of the Indian crust was underthrusting the Asian crust south of the BNS. They also discovered that the Moho appeared to rise at an increasing gradient north of the Himalaya Terrane. It was measured at depths of between ~50 km and ~80 km, reaching depths of ~85 km beneath the IYS to the south, ~80 km across the Lhasa Terrane, and ~70 km under the Qiangtang Terrane to the north. A mid-crustal low-velocity zone was observed at depths of 14-30 km beneath the Lhasa and Himalaya terranes which was posited to be a product of partial melt and/or aqueous fluids.

Zhang et al. (2016) presented P and S wave velocity tomography in 3-D view format, and P wave anisotropy, along a N-S trending profile across the western TP (i.e., the ANTILOPE-I profile). They used a sizeable quantity of P and S arrival time data for local earthquakes and teleseismic events. The entire profile (i.e., ANTILOPE-I) was divided into three segments according to each segment’s convergent and divergent nature. Whereas central-southern Tibet showed a divergent nature, the northern TP appeared to be partly divergent, and the Tarim Basin evinced a convergent nature. Zhang et al. observed multiple low-velocity zones within the middle crust, but these were also visible in the lower crust beneath the northwestern TP, suggesting the existence of significant heterogeneities and complex patterns of flow. According to the P and S wave tomographies, and the P wave azimuthal and radial anisotropy results, they concluded that, due to shear heating, a low-velocity gap existed in the upper mantle of the Indian and Eurasian plates. Zhou et al. (2016) presented a high-resolution Pn anisotropic tomographic model of the uppermost mantle beneath China which using data obtained from seismic stations in Tibet and the Tienshan orogenic belt. After using, an anisotropic tomographic method Zhou et al observed prominent high Pn velocities under the stable cratonic blocks, and remarkable low Pn velocities in the tectonically active areas. The high Pn velocities and an arc-shaped fast Pn direction in the west Tibet were reflecting to the Indo-Asian collision, whereas in the eastern Tibet, low Pn velocities and a range-parallel Pn fast direction shows the northward underthrusting of the Tarim lithosphere and the south ward underthrusting of the Kazakh lithosphere. Razi et al. (2016) investigated the fate of the Indian lithosphere beneath the western TP, using tomographic imaging based on the arrival times of body waves derived from regional and teleseismic sources measured by a portable network deployed in the region from 2007 to 2011.

They used a non-linear iterative algorithm to obtain a 3-D velocity structure in a spherical segment beneath the study area from the surface to a depth of 430 km. They found similar variations in P and S wave speeds in the upper mantle as well as a number of notably fast anomalies beneath the western TP and the adjacent Himalaya, potentially suggesting that the mantle lithosphere of the Indian crust was likely colder and hence able to move faster than the ambient mantle. Razi et al. discovered two significantly fast anomalies: the first one resembled a ~100 km wide sub-vertical column and was located beneath the Indian-Eurasian plate boundary; and the second anomaly was thinner and contained a dipping slab shape which spanned the N-S width of the Lhasa Block at depths in excess of 300 km. The absence of significantly fast anomalies beneath the western TP (at ~80°E) might suggest the presence of a colder material beneath the northernmost Himalaya and the southernmost part of the TP. Razi et al. posited that the absence of single large-scale fast anomalies, in contrast with the presence of multiple relatively small-scale anomalies, would most likely indicate the subduction of the intact Indian lithosphere, and that the mantle lithosphere of the Indian Plate continues to underthrust the southern TP today, developing a ‘drip-like’ instability. Xu et al. (2017) investigated the lithospheric structure beneath the western TP using P and S receiver function techniques. In order to recover the teleseismic waveform data, they used 42 temporary seismology stations along the Y2 and ANTILOPE-1 profiles. They observed the Moho as a concave shape, with depths of between 55 and 82 km, reaching its deepest location 82 km below the surface at a point north of the IYS. An intracrustal discontinuity was also observed at a depth of ~55 km below the southern Lhasa Terrane, potentially connected to the MHT south of the IYS and therefore possibly representing the upper boundary of the underthrusting Indian crust. A mid-crustal low-velocity zone was observed beneath the Tethyan Himalaya and southern Lhasa terranes, most likely signifying a layer of partial melts which could be decoupling the thrust/fold

deformation of the upper crust from the shortening and underthrusting occurring in the lower crust. The LAB appeared to occur at depths of 130-200 km, suggesting that the Indian lithospheric mantle is underthrusting beneath the southern TP with a ‘flat ramp’ shape, and that it may be detached from the lower crust just under the surface of the IYS. Murodov et al. (2018) obtained images of the Moho depth and Vp/Vs ratio along the ANTILOPE-1 profile beneath the western TP. Data were derived from receiver function analysis, using measurements from 54 broadband seismology stations for the period October 2006 to November 2007. They identified variations in the Moho depth of between ~50 km and 93 km; offsets of ~15 km and ~20 km observed in the central Lhasa and Qiangtang terranes, respectively, which were taken to signify the northern extent of the underthrusting Indian lower crust and lithospheric mantle. Murodov et al. explained the changes in the mean crustal Vp/Vs ratio beneath the Himalaya and Lhasa terranes and the Tarim Basin by suggesting that these were indicative of a felsic-to-intermediate composition. The N-S variations in the Moho depth and Vp/Vs ratio were taken to be reflective of the joint effects of Indian lower crust underthrusting, crustal shortening and thickening, and the low-velocity zone of the mid-upper crust, respectively. Ju et al. (2018) presented seismic anisotropy measurements of the crust and upper mantle beneath the western TP, using teleseismic shear wave splitting measurements. The data were obtained from 31 seismology stations belonging to the temporary Y2 array profile for the period from 2007 to 2011. They divided the study region into three segments according to the E-W rapid polarization directions evident, and posited that these implied a decoupling between the crust and mantle lithosphere below the western TP and explained the changing behavior of the Indian lithospheric mantle shown at ~82°E, where it appears to be underthrusting beneath the Karakoram Fault. Due to the absence of any variation in these rapid polarization directions and delay times in

the western sector of their study area, Ju et al. 2018 posited that the underthrusting of the Indian lithospheric mantle(ILM)had crossed to the BNS. Huangfu et al. (2018) analyzed the lithospheric evolution beneath the western and centraleastern TP, using two profiles from the western and central-eastern TP. They performed 2-D thermomechanical numerical experiments to investigate the potential correlation between the postcollisional crustal-lithospheric structure and the across-strike multi-terrane configuration observed in collision zones. Their delineation of post-collisional magmatism and deep mantle tomography indicated that the width of the relatively stronger Lhasa-Qiangtang terranes is a key control of the geodynamic evolution of the TP. They constructed a model, which predicted the complete detachment of the terrane at points where the underthrusting, cold Indian lithosphere beneath the collision zone would cause relatively low orogenic thermal conditions. Huangfu et al. (2018) noted that large-scale magmatism was not widespread in the western TP. 3.2 Crustal structure of the Tarim Basin The Tarim Basin, positioned between Tien Shan in the north and Tibet in the south in the western China, is a complex basin with a number of uplifts and depressions developed due to the long and complex tectonic history (Jia et al., 1998). Although, the active convergence occurred in Tien Shan and Tibetan regions, Tarim basin appears to have remained a rigid block that experiences little or no shortening (Kao et al., 2001). The convergence mechanics are still debated and several competing models have been proposed (Ni and Barazangi, 1984; Molnar, 1988; Jin et al., 1996). There are few work done for knowing crustal structures under Tarim, which applies gravitational and magnetic data (Caporali, 1995; Curtis and Woodhouse, 1997), a broadband

seismic deployment (Kao et al., 2001) and 3D seismic data (Zeng et al., 2011). Zhao et al. (1993) used INDEPTH data to map the north-dipping reflectors whereas Jin et al. (1996) interpreted of gravity data, Zhu and Helmberger (1996) used rare intermediate earthquakes. Wittlinger et al. (1998) widely discussed about relationship between Tibet and Tarim and hypothesized to be one wherein subduction plays an important role. Because of the high and low anomalies in the shearwave speed observed beneath southern and north-central Tibet (Curtis and Woodhouse, 1997; McNamara et al., 1997) and significant differences in velocity structures between western and eastern Tibet (Ritzwoller and Levshin, 1998), subduction processes are more complex than the simple configuration to understand in Tibet (Kao et al., 2001). Kao et al. (2001) applied a broadband seismic deployment in 1998–1999 in southwestern Tarim, which provided data for imaging the crust and upper mantle across the boundary between the Tarim block and the TP. It is observed that the Tarim crust is estimated to be ~ 42 km in thickness, Moho discontinuity beneath the southwestern Tarim basin that reaching ~50 km beneath the northern part of the Kunlun foreland. The observed north- and south-dipping upper mantle structures under the Kunlun foreland and Kunlun Shan region can be explained as a result of duplexing of the crustal materials, which signifies with a model the lithospheric collision in which the crust and the upper mantle on both sides interpenetrate and deform (Kao et al., 2001). Subduction of Tarim Basin beneath the TP shows a unique in nature from the eastern to western regions of the TP. The results obtained from the Zhao et al. (2006) described the Southeastern Tarim Basin subducted beneath the Qaidam Basin near the Altyn Tagh fault (Fig. 2a) at the long distance. Gao et al. (2000) analyzed a deep seismic reflection profile crossing the boundary between Tarim Basin and the West Kunlun Mountains. The deep seismic reflection profile indicated that the tilted reflection structure, V-shaped basin-and-range coupling relation between

the Tarim Basin and the West Kunlun Mountains signify the continent-continent collision (Gao et al., 2000). The unexpected results of reflections indicate that West Kunlun inclined to the north and Tarim Basin inclined to the south. This constitutes the structure pattern of the face-to-face subduction which is absent in the TP lithosphere. Based on the deep seismic reflection profile, Gao et al. (2000) argued that the continental lithosphere might be arised from India that is subducting to the north along the Main West Kunlun Thrust (MWKT) (Fig. 2b). The upward projection along the dip of MWKT shows that the north-inclining reflection corresponds to the Qiangtang-Hoh Xil terrane (Fig. 2b). From the result obtained using S wave receiver functions of ANTILOPE-I, Zhao et al. (2010) proposed that there is no subduction occurred between Tarim Basin and TP to the western regions (Fig. 2c). 4. Discussion The E-W inclination of the Indian-Eurasian lithospheres may indeed change, but the Indian lithosphere steepens from west to east, whereas the Eurasian LAB dips more shallowly from west to east. The difference in surface topography between the western and eastern parts of the TP may also be controlled by the geometry of the Indian-Eurasian collision (Zhao et al., 2010). The rigid mantle lithosphere of the western TP presents a more rugged and a higher-altitude topography than the weaker lithosphere of the eastern TP, which is gentler and broader. The terranes of the western TP appear steeper, stronger and narrower than those of the central-eastern TP. Due to the eastwardbroadening shape of the TP, these terranes probably inherited a geometric shape similar to that of the Lhasa-Qiangtang terranes during the incipient collision. The Cenozoic shortening of the central-eastern Lhasa-Qiangtang terranes is generally more significant than that of the western terranes (Huangfu et al., 2018). 4.1 Laterally varying crustal thickness

In plate tectonics, a lateral variation in the Moho discontinuity and upper mantle is supposed to preserve and signify first-order information, and can therefore be used to reconstruct complicated geodynamics. Much research has been conducted throughout the region which lies between the Himalaya and the TP; this research has obtained Moho discontinuity measurements using P and S receiver functions (Fig. 3). Zhang et al. (2011) demonstrated that there was a very clear crustal thickness variation across the E-W TP profile. The crust of the western TP appears to thicken to ~75 km, but is only ~55 km for the southeastern TP, and 60-65 km for the Qiangtang, Songpan-Ganzi and Qilian blocks of the north and northeastern TP. This would signify that the Indian crust is moving towards the northern margins of the TP and has collided with the Tarim Basin at 80°E, which has slowed down its push to reach the BNS belt at 88°E (Zhang et al., 2011). This may reflect the current penetration of the Indian Plate beneath the Eurasian Plate and the location of the leading edge of the underthrusting Indian crust (Teng et al., 1985). The LAB geometry of the western TP profile (i.e., ANTILOPE) would suggest that the Indian lithosphere is underthrusting beneath the southern TP with a ‘flat ramp’ geometry and may have reached the JRS (Zhao et al., 2010). In contrast, it has also been shown that the Indian lithospheric mantle beneath the Himalaya is ~70 km thick (Kumar et al., 2013), and that it detaches from the lower crust just under the IYS (Xu et al., 2017). The particular combination of variations in the depth, shape and northern boundaries of the Indian LAB along typical N-S sections clearly suggests that the geometry of the underthrusting Indian lithosphere exhibits a significant E-W variation (Xu et al., 2011). Huangfu et al. (2018) stated that the horizontal distance of the Indian lithosphere underthrusting the TP appears to decrease from west to east, whereas the Indian lithosphere underlies the entire western TP; it is evident under the BNS on the central TP and subducts steeply around the Indus-Yarlung Suture (IYS) in eastern Tibet.

The dip angle of the leading edge of the subducting Indian slab decreases from east to west. In eastern-central Tibet, the Indian lithosphere plunges steeply into the mantle beneath the TP, but in western Tibet, the Indian lithosphere is horizontally underthrusted beneath the TP without any significant sinking (Zhao et al, 2010; Zhao et al., 2014a,b). 4.2 Moho depth Many authors, using different profiles, have agreed that the Moho depth beneath the southern Himalaya lies at ~50 km (Rai et al., 2006; Zhang et al., 2014). Nábelek et al. (2009) observed that the Moho discontinuity rapidly deepens from ~50 km beneath the Himalaya to ~70 km under the IYS, and remains relatively flat beneath the Lhasa Terrane. In the western and central TP, the Moho discontinuity dips from ~50 km beneath the Himalaya to ~75-80 km under the IYS zone (Zhao et al., 2010; Zhao et al., 2014a,b; Xu et al., 2015, 2017). Identical results have been obtained from picking out the Moho discontinuity from the shear velocity model (Gilligan et al., 2015), and from the joint inversion of the receiver function and surface wave dispersion (Rai et al., 2006). However, Huangfu et al. (2018) described a Moho discontinuity at depths of ~60-70 km that would suggest that the crustal thickening is driven by the combined effects of a previous crustal shortening of the accreted terranes and the subsequent underthrusting and duplexing of the Indian lower crust. Murodov et al. (2018) identified variations in the Moho depth between ~50 km and 93 km, where offsets of ~15 km and ~20 km observed in the central Lhasa and Qiangtang terranes would signify the northern extent of the underthrusting Indian lower crust and lithospheric mantle (Fig. 4). The Moho topography from the IYS zone remains relatively horizontal at ~75 km, including a small undulation beneath the Lhasa Terrane, before deepening slightly to ~80-85 km beneath the Qiangtang Terrane (Kind et al., 2002; Nábělek et al., 2009; Zhao et al., 2010).

Wittlinger et al. (2004) found that the deepest Moho lay beneath the Qiangtang Terrane on the western TP, along the southwestern margins of the Tarim Basin. The Moho remains at a depth of ~70 km along the northern margins of the TP, with a minor undulation beneath the northern Qiangtang and Songpan-Ganzi terranes, and abruptly shallows to ~45 km near the Altyn Tagh Fault. The crustal thickening of the northern TP is most likely due to pure shear shortening, without much underthrusting. The ~25 km Moho offset observed in the Altyn Tagh mountain range might indicate the separation of the TP from the Tarim Basin (Wittlinger et al., 1998; Tapponnier et al., 2001; Jiang et al., 2004). Shear shortening and underthrusting are quite different mechanisms from the dominant mechanism of crustal thickening which has been observed on the southern TP, where the Moho depth increases from ~50 km to 80 km on the central TP, shallowing again to the north (Zhao et al., 2010). The recent study along the ANTILOPE-1,which crosses the major tectonic boundaries, found that the Moho depth on the western TP suddenly decreases from ~80 km to ~66 km under the central Lhasa Terrane before gradually deepening northward to ~93 km (its maximum depth) beneath the Qiangtang Terrane (Murodov et al., 2018). However, a similar study also posited that the deepest Moho would most likely be located beneath the westernmost part of the TP near the southwestern margins of the Tarim Basin (Wittlinger et al., 2004; Zhang et al., 2014; Wei et al., 2016). Along a similar profile, a high velocity body was shown extending from the Himalaya to the central Qiangtang Terrane, where ~15 km and ~20 km offsets of the Moho depth might indicate the northern frontiers of the decoupled underthrusting Indian lower crust and lithospheric mantle (Zhang et al., 2016; Murodov et al., 2018). 4.3 Crustal velocity structure, Vp/Vs ratio

The TP is supposed to be formed of an assembly of terranes that are different in age, temperature, composition and rheology, and have experienced strong tectonic deformation since the beginning of the Indian-Eurasian continental collision (Dewey et al., 1988; Royden et al., 2008; Yin and Harrison, 2000; Zhang et al., 2011), with the Vp/Vs ratio varying between different regions. Previous research has implied that the mean crustal Vp/Vs ratio beneath the Himalaya, the Lhasa Terrane and the Tarim Basin is nearly identical to the global continental mean (i.e., 1.75), potentially indicating the presence of felsic and intermediate compositions (Wittlinger et al., 2004), as well as a lack of widespread partial melt (Nábělek et al., 2009). The higher Vp/Vs ratio (1.83) observed beneath the Qiangtang and Songpan-Ganzi terranes would suggest the potential joint effects of a more mafic composition and partial melt within the crust (Owens and Zandt, 1997; Zhao et al., 2011; Yue et al., 2012; Zheng et al., 2015; Murodov et al., 2018). Vergne et al. (2002) applied crustal velocity structure models to describe the Qiangtang Terrane as underlain by a mafic lower crust that exerts a substantial influence on the increases in the Vp/Vs ratio in this area (Fig. 5). The high Vp/Vs ratio beneath the Qiangtang Terrane is supposed to be caused by the widely-distributed magmatic rocks resulting from the northward-subducting Lhasa Terrane (Ding et al., 2003; Yue et al., 2012). Nowack et al. (2010) applied Gaussian-Beam migration images of teleseismic P-RFs along the Hi-CLIMB profile, which suggested that the upper mantle’s mafic materials would most likely have been incorporated into the middle to lower crust in the vicinity of the BNS where the Moho was disrupted during Cenozoic collisions. Due to the presence of high Vp/Vs ratios, it could be posited that the presence of fluids or partial crustal melting would significantly decrease the shear wave velocity in relation to the compressional wave velocity (Watanabe, 1993). Yang et al. (2012) applied a 3-D shear wave velocity model, and, using

Rayleigh wave phase velocity dispersions and magnetotelluric data, observed a low-velocity zone at depths of 20-40 km, in addition to high conductivity anomalies in the middle and lower crust beneath the Qiangtang and Songpan-Ganzi terranes (Wei et al., 2001; Unsworth et al., 2004). Equally, Wang et al. (2015) used the petrological and geochemical data from Pliocene-Quaternary felsic rocks to suggest that this low velocity, high conductivity zone originated from the partial melting of crustal rocks. It could be argued that any large-scale magmatism has been limited by the underthrusting of the cold Indian lithosphere beneath the collision zone, thereby leading to relatively low orogenic thermal conditions. This might be especially applicable to the western TP, where the distribution of post-collisional volcanic rocks is limited. The main magmatic event that occurred during the Early Miocene (Malz et al., 2012) might have been induced by the detachment of accreted terranes prior to the underthrusting of the Indian lithosphere beneath the collision zone (Rutte et al., 2017; Huangfu et al., 2018). A notable feature of the shear-wave velocity model extend from west to east Tibet is the presence of a lateral variation low velocity zone (LVZ) which occurred at the depths of 10-40 km in the crust (Zhang, 2011; Mir et al., 2017; Gilligan & Priestley, 2018) that might be representing the decollement and related the uplift of Tibet Plateau. 5. Conclusions We collated the interpretative results of seismic investigations into the TP that have been conducted over the past two decades in an attempt to understand better the crustal structure of the western TP. Due to the absence of many studies of the western TP, those that have been conducted are of especial importance when evaluating the uplift of the TP and its unique tectonic environment. From the evidence available, we concluded that the lithospheric structure of the western TP can reasonably be summarized as follows:

1. A relatively horizontal Moho topography exists in the IYS zone at a range of ~75 km, and this decreases by up to ~14 km under the central Lhasa Terrane, with a maximum Moho depth of ~93 km beneath the Qiangtang Terrane, signifying the northern margins of the decoupled underthrusting Indian lower crust and lithospheric mantle. 2. The mean crustal Vp/Vs ratio varies in different regions of the TP, i.e., the northern Himalaya, Lhasa Terrane and Tarim Basin exhibit values similar to the global continental mean, whereas the Qiangtang and Songpan-Ganzi terranes yield values higher than that of global mean. The higher Vp/Vs ratios of the latter terranes probably indicate the joint effect of a more mafic composition and partial melt within the crust supposedly derived from the presence of magmatic rocks which have arisen from the actions of the northward subducting Lhasa Terrane. However, the terranes with ratios similar to the global continental mean are most probably characterized by felsic and intermediate compositions, with no widespread partial melt. 3. The location of the LAB at depths of 130-200 km might be explained by the underthrusting of the Indian lithospheric mantle beneath the southern TP with a ‘flat ramp’ shape that has become detached from the lower crust below the surface of the IYS. 4. The broader, stronger terranes of the western TP may be explained by the eastward-broadening shape of the TP and lower Cenozoic shortening at the onset of the Indian-Eurasian collision. 5. Along the boundary between the Tarim Basin and the TP shows a different contact relation. The Eastern margin of the Tarim Basin has subducted beneath the Qaidam Basin near the Altyn Tagh fault at the long distance, whereas further west, i.e. near Qiemo, the Tarim Basin has subducted to the TP at a short distance and with a face to face pattern; and still further west ,i.e. near Minfeng, no subduction has occurred between Tarim Basin and TP.

6. Although the mechanisms and patterns of deformation beneath the western TP have been obtained, many questions are still hanging there, therefore, further comprehensive geophysical explorations are still needed to ensure a better understanding of the lithospheric structure and geodynamic processes under the whole TP.

Acknowledgements

We are grateful to a number of scientists whose work we have referenced here. The National Natural Science Foundation of China (Grant No. 41661144026) and the major program of the National Natural Science Foundation of China (Grant No. 41490611) funded this study. This study also supported by the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No.XDA20070302). Additionally, author B. Neupane is supported by the Chinese Academy of Sciences President's International Fellowship Initiative (Grant No. 2019PT0018).

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List of Figures and Table Fig. 1 Tectonic framework of the Tibetan Plateau. The thick red dashed line delineates the western margin of the Tibetan Plateau; the blue dashed lines mark the major suture zones, thus: IYS Indus-Yarlung Suture; BNS - Bangong-Nujiang Suture; JRS - Jinsha River Suture; AKMS – Anyimaqen-Kunlun-Mutztagh Suture; SQS - South Qilian Suture; NQS - North Qilian Suture. The blue lines show major tectonic faults, thus: KKF - Karakoram Fault; ATF - Altyn Tagh Fault; KF - Kunlun Fault. Fig. 2. Showing a relation between Tarim Basin and adjacent regions. a) From the seismic refraction/wide-angle reflection profile, the northern margin of the Tarim basin subducted beneath the eastern margin of the Qaidam basin (Zhao et al., 2006). b) From the deep seismic reflection profile, Tarim Basin subducted beneath the northwest margin of the Qinghai-Tibetan Plateau (Gao et al., 2000). c) From the S wave receiver functions, in the western margin the Tarim Basin have nosubduction (Zhao et al., 2010). Fig. 3. Spatial distribution of the undulating Moho beneath the TP as measured (a) using inversion of gravity measurements, and (b) deep seismic soundings (adapted from Zhang et al., 2011). Fig. 4. A schematic cross-section of the western TP showing its lithospheric structures. This cross-section was prepared using seismic data. The crustal tectonic structure was derived from Huangfu et al. (2018 and references therein). The Indian mantle lithosphere is marked blue; the Eurasian mantle lithosphere is marked brown. The Indian and Eurasian crusts are marked in red and chartreuse; the Tibetan crust is in light green, and the green color signifies the Indian lower crust.

Fig. 5. Cross-sections of the Moho depth (Zhao et al., 2010) and Vp/Vs ratio (Murodov et al., 2018) beneath the western TP. The green dashed line shows the location of the Moho depth, and the green and orange polygons indicate the Vp/Vs ratios. Table Table 1: Seismic investigation information for the western TP for the last 20 years.

SN 1 2 3

Profile Yarkand (Tarim) - Upper Indus valley N-S densely-spaced seismic profiles N-S densely-spaced seismic profiles

Method(s) used Common conversion point (CCP) time to depth migrations of radial receiver functions (RRF) and teleseismic travel time tomography P and S receiver function common conversion point (CCP) stacking method

Project

Year

INSU

2001 (JulyNovember)

CAS

2005-2007

P and S wave travel time tomography

ANTILOPE, CAS

2006-2007

CAS

2011-2013

4

Zarda - Quanshuigou

P and S wave travel time tomography

5

ANTILOPE-2

Radial receiver function (RRF) technique

5

ANTILOPE-I

P and S wave velocity tomography

7

Northern Himalaya, Lhasa and Qiangtang blocks

P and S wave arrival times from regional and teleseismic events

8

IYS - Qiangtang (Y2 array)

P and S receiver function techniques

9

ANTILOPE-1

Vp/Vs receiver function analysis

10

Temporary Y2 array profile

Teleseismic shear wave splitting measurements

NSFC, CAS

2007-2011

11

Western Tibet and centraleastern Tibet

Two-dimensional (2D) thermomechanical numerical experiments, post-collisional magmatism, and deep mantle tomography

CAS

2018

ANTILOPE, CAS ANTILOPE, CAS NSF, New Brunswick ANTILOPE, CAS ANTILOPE, CAS

2005-2006 2006-2007 2007-2011 2007-2011 2006-2007

Table 1: Seismic investigation information for the western TP for the last 20 years

References Wittlinger et al., 2004 Zhao et al., 2010 Zhao et al., 2014 Zhang et al., 2014 Xu et al., 2015 Zhang et al., 2016 Razi et al., 2016 Xu et al., 2017 Murodov et al., 2018 Ju et al., 2018 Huangfu et al., 2018

Graphical abstract

Highlights 

A maximum Moho depth of ~93 km beneath the Qiangtang terrane signifies the northern frontiers of the decoupled underthrusting Indian lower crust and lithospheric mantle



The higher Vp/Vs ration of the Songpan-Ganzi terrane indicates a presence of joint effects of the more mafic composition and partial melt within the crust



The shear-wave velocity model extend from west to east Tibet, where a lateral variation low velocity occurred at depths of 10–40 km in the crust.

Author Contributions:

Conceptualization: Junmeng Zhao and Bhupati Neupane; Methodology: Junmeng Zhao and Bhupati Neupane Software: Bhupati Neupane, Hongbing Liu, Deng Yan Validation: Bhupati Neupane and Junmeng Zhao Formal analysis: Junmeng Zhao, Bhupati Neupane, Hongbing Liu Writing—original draft preparation: Junmeng Zhao, Bhupati Neupane, Deng Yan