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The deep structure of the continental collision between India and Asia and whether India's lower crust is underplated beneath Tibet or subducted into the mantle remain controversial. It is also unknown whether the active normal faults that facilitate orogen-parallel extension of Tibetan upper crust continue into the lower crust and upper mantle. Our receiver-function images collected parallel to the India-Tibet collision zone show the 20-km-thick Indian lower crust that underplates Tibet at 88.5-92°E beneath the Yarlung-Zangbo suture is essentially absent in the vicinity of the Cona-Sangri and Pumqu-Xainza grabens, demonstrating a clear link between upper-crustal and lower-crustal thinning. Satellite gravity data that covary with the thickness of Indian lower crust are consistent with the lower crust being only ∼30% eclogitized so gravitationally stable. Deep earthquakes coincide with Moho offsets and with lateral thinning of the Indian lower crust near the bottom of the partially eclogitized Indian lower crust, suggesting the Indian lower crust is locally foundering or stoping into the mantle. Loss of Indian lower crust by these means implies gravitational instability that can result from localized rapid eclogitization enabled by dehydration reactions in weakly hydrous mafic granulites or by volatile-rich asthenospheric upwelling directly beneath the two grabens. We propose that two competing processes, plateau formation by underplating and continental loss by foundering or stoping, are simultaneously operating beneath the collision zone.

We studied the crustal structure and tectonics in the north Tibetan Plateau from the Songpan-Ganzi terrane to the Qaidam Basin using teleseismic receiver-function imaging, across a major lithospheric boundary, the Kunlun-Qaidam boundary, where previous studies suggest a ~15–20-km change in crustal thickness from thicker crust in the Kunlun Mountains to thinner crust in the Qaidam Basin. We report P receiver functions for 70 stations, largely the International Deep Profiling of Tibet and the Himalaya (INDEPTH), phase IV, experiment. Our most dense station coverage is located along the roughly north-south INDEPTH-IV active-source seismic profile at approximately 95° E longitude. Azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and Vp/Vs from across the study area. Receiver functions show strong converters that we interpret as the Moho at ~70 km depth beneath the Qiangtang, Songpan-Ganzi terranes and Kunlun Mountains and at ~50 km depth beneath the central Qaidam Basin. This large change in crustal thickness occurs >50 km north of the North Kunlun strike-slip fault, on which the 2001 M8.1 Kunlun earthquake occurred. Receiver functions for some of the stations north of the thickness change at the Kunlun-Qaidam boundary also show a deeper ~70-km bright converter in addition to the 50-km converter. The two converters appear to overlap by up to ~30 km in some locations along the south Qaidam Basin. We combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north Tibetan Plateau including crustal flow and crustal injection. At depths imaged here, shallower than ~100 km, we see no evidence of southward subduction of Eurasian lithosphere.

To understand the along-strike variation of crustal deformation and tectonic processes in the India–Tibet continental collision zone, we deployed a linear array of broadband seismic stations along 92° E to image lithospheric structure. Our receiver-function cross-section reveals a prominent negative converter dipping ∼20° north from ∼10–55 km depth below sea-level, almost through the whole crust, beneath the southern Lhasa terrane. We interpret it to be a manifestation of the Yarlung–Zangbo suture zone (YZS) separating the continental crust of the Indian and Eurasian plates. This implies the hypothesized channel-flows of Indian middle crust extruding southwards from Tibet are limited at this longitude to the southernmost portion of the Lhasa terrane. A positive converter, consistent with previous suggestions of eclogite formation, is seen about 10–15 km above the Moho and continuing 50 km north of the 20°-dipping YZS converter. We image this positive converter continuously from ∼60 km south of the surface trace of the YZS to the vicinity of the Jiali fault, supporting the interpretation of sub-horizontal underplating of Tibetan crust by Indian crust to ∼31° N at 85° E on the Hi-CLIMB transect. However, we also show a negative mantle converter sub-parallel to the crustal YZS converter, from the northern limit of the underplating Indian lower crust to at least 140 km depth, that we interpret as the base of Tibetan lithosphere overlying an asthenospheric mantle wedge. Based on the lithospheric structure observed in this and other studies, we infer that Indian mantle lithosphere currently detaches from Indian lower crust at the “mantle suture” that is nearly 50 km south of the surface trace of the YZS at 92° E, south of the mantle suture suggested by INDEPTH transect beneath the surface trace of the YZS at ∼90° E, and far south of the mantle suture suggested to be at the 31° N northern limit of underthrusting Indian lower crust suggested by Hi-CLIMB transect at ∼85° E. A change from underplating in the west to steep subduction in the east can reconcile all these observations.

In classic orogenic models, the mountain range is underlain by a deep crustal root. Here we present the crustal and upper mantle structures along two receiver function profiles across Qilian, an orogen experiencing recent growth at the northern margin of the Tibetan plateau. Opposite to an expected crustal root beneath the orogen, the Moho beneath Qilian is arch-like, shallower beneath the center and deepens by up to 10 km beneath its southern and northern boundaries. Additional velocity interfaces sub-parallel to the Moho are observed in the lower crust of the basins south of Qilian, which we interpret as the top of a mechanically strong lower crust thrusting several tens of kilometers underneath Qilian. In the north, the small lateral offset between the surface and mantle traces of the thrust system reveals a steep boundary, indicating that the North China cratonic crust acts as a strong resistance to the northward growth of the plateau, forcing the development of the left-lateral strike-slip Haiyuan fault south of the northern Qilian suture. The young Qilian orogen thus has been rising and growing progressively from the boundaries to the center, squeezed up by more rigid tectonic blocks in the north and south.

The northeastern boundary of the Tibetan high plateau is marked by a 2 km topographic drop and a coincident rapid change in crustal thickness. Surface tectonics are dominated by the Kunlun strike‐slip fault system and adjacent Kunlun concealed thrust. The main objective of the current study is to map lateral variations of seismic anisotropy parameters in this region along the linear INDEPTH IV array in order to investigate the link between surface and internal deformation in the context of crust and mantle structure. To achieve this aim, we performed Minimum‐Transverse‐Energy based SKS splitting measurements using 23 stations of the INDEPTH IV array deployed across the northeastern margin of Tibet. Average fast polarization directions and splitting time delays are obtained by averaging stacked misfit surfaces of all analyzed events at each station. The agreement of fast directions with the strikes of major active strike‐slip faults and strike‐slip focal mechanisms, but not with fossil structures such as the Jinsha suture, implies that the anisotropy records lithospheric petrofabric formed by recent deformation within the lithosphere rather than representing frozen‐in anisotropy or shear within the asthenosphere due to absolute plate motion. The distribution of large splitting delays throughout the northern plateau suggests that deformation is distributed rather than focused onto narrow shear zones associated with the Kunlun strike‐slip faults. The drop in splitting delays toward the Qaidam is then a natural consequence of the much lower degree of deformation there.

SUMMARY From the S-wave data collected along a 270-km-long profile spanning the Kunlun mountains in NE Tibet, 14595 Sg phase arrivals and 21 SmS phase arrivals were utilized to derive a whole-crustal S velocity model and, together with a previously derived P velocity model, a Poisson's ratio (σ) model beneath the profile. The final tomogram for the upper 10–15 km of the crust reveals the lower velocities associated with the predominantly Neogene-Quaternary sediments of the Qaidam basin to the north and the higher velocities associated with the predominantly Palaeozoic and Mesozoic upper crustal sequences of the Songpan-Ganzi terrane and Kunlun mountains to the south. This study finds no evidence that the Kunlun mountains are involved in large-scale northward overriding of the Qaidam basin along a shallow south-dipping thrust. The σ in the upper 10–15 km of the crust are often lower than 0.25, indicating a preponderance of quartz-rich rocks in the upper crust beneath the profile. Below 10–15 km depth, the remainder of the crust down to the Moho has an average σ of 0.24 beneath the Songpan-Ganzi terrane and Kunlun mountains and 0.25 below the Qaidam basin. These low σ are similar to other low σ found along other profiles in the northeastern part of the plateau. Assuming an isotropic situation and no significant variation in σ between 10–15 km depth and the Moho, then the lower crust between 25–30 km depth below sea level and the Moho with P velocities varying from 6.6 km s−1 at the top to around 6.9 km s−1 at the base and σ of 0.24–0.25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part. As the pre-Cenozoic Qaidam basin crust has probably not lost any of its lower crust during the present Himalayan orogenic cycle in the Cenozoic and only has a σ of 0.245–0.25, then it appears that the pre-Cenozoic Qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average σ of 0.265–0.27 and the Tarim and Sichuan basin crusts. This situation then probably facilitates the collision and promotes the formation of new high plateau crust at the NE margin of Tibet. South of the Qaidam basin, the crust of the Songpan-Ganzi terrane and Kunlun mountains has an even lower average crustal σ of 0.23–0.24 and is thus presumably even weaker and more easily deformable than the crust beneath the Qaidam basin. This then supports the hypothesis of Karplus et al. that ‘the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust'.

In the past ten years, three major earthquakes with devastating impacts have taken place along the margins of the Bayan Har Terrane in NE Tibetan Plateau, China. To understand the geodynamic background controlling the NE Tibetan Plateau, we processed a large amount of broad-band seismic data in and around the Tibetan Plateau, and constructed a three dimensional lithospheric S-wave velocity model using a regional surface-wave tomographic tool. The velocity model produces images of important structures. First, a weak discontinuity at ∼110 km depth seems to exist, which separates the abnormally thick Tibetan upper-mantle lithosphere into two layers. Second, the upper-mantle lithospheric velocity structures have certain correlations with seismicity and fault activity. Regions with a low-velocity upper-mantle lithosphere have a stronger seismicity than regions with a high-velocity upper-mantle lithosphere. The earthquake focal mechanisms or faults in the interior of the low-velocity regions typically display normal faulting with a strike-slip component, while reverse faults, sometimes with a small strike-slip component, typically occur in the low-to-high velocity transition zone. The correlation of seismicity and fault activity with upper-mantle lithospheric velocity anomalies suggests that the strength of the upper-mantle lithosphere can control the deformation mode of the overlying crust where most earthquakes occur.

During 2006 to 2007, a small array of broadband seismic stations was deployed across the eastern Qaidam–Kunlun border. Receiver functions derived from this data set show a coherent P-to-S converted phase from the Moho at ~ 65 km to ~ 58 km below sea level (b.s.l.) from south to north beneath the Kunlun mountains. The Moho is observed at ~ 45 km b.s.l. beneath the Qaidam basin. A sharp offset of ~ 15 km in the Moho depth is observed just beneath the Qaidam–Kunlun border. The close correlation between the surface topography and the sharp Moho offset substantiates the view that the Kunlun crust is weaker than that of the Qaidam basin and thickens vertically in response to compression between India (through the Tibetan plateau) and the Qaidam basin. Furthermore, a strong steeply north-dipping converted phase can be seen from ~ 5 to 15 km b.s.l. north of the North Kunlun Fault, which is likely a major south-directed thrust fault partially responsible for the thickening of the Kunlun crust.

Closely-spaced receiver-function profiles in the east-central India–Tibet collision zone reveal drastic west–east changes of the crustal and upper mantle structure. West of ∼91.5°E, we show the Indian crust-mantle boundary (Moho) extending subhorizontally from ∼50 km depth below sea level under the High Himalaya to ∼90 km under the central Lhasa terrane. Further north, this boundary transitions to become the top of the Indian lithospheric mantle and, becoming faint but still observable, it can be tracked continuously to ∼135 km depth near ∼31.5°N. The top of the Indian lithospheric mantle is clearly beneath the Tibetan Moho that is also a conspicuous boundary, undulatory at 60–75 km depth from the central Lhasa terrane to the north end of our profile at ∼34°N. This geometry is consistent with underthrusting of Indian lower crust and underplating of the Indian plate directly beneath southern Tibet. In contrast, east of ∼91.5°E, the Indian Moho is only seen under the southernmost margin of the Tibetan plateau, and eludes imaging from ∼50 km south of the Yarlung-Zangbo suture to the north. The Indian lower crust thins greatly and in places lacks a clear Moho. This is in contrast to our observation west of ∼91.5°E, that the Indian lower crust thickens northwards. A clear depression of the top of the Indian lower crust is also observed along west–east oriented profiles, centered above the region where the Indian Moho is not imaged. Our observations suggest that roll-back of the Indian lithospheric mantle has occurred east of ∼91.5°E, likely due to delamination associated with density instabilities in eclogitized Indian lower crust, with the center of foundering beneath the southern Lhasa terrane slightly east of 91.5°E.

Summary Surface waves recorded by the nine broad-band stations of the INDEPTH II experiment are analysed to study the crustal structure of southern Tibet. Their frequency range is between approximately 0.015 and 0.050 Hz (i.e. between 20 and 60 s period). Phase velocity dispersion curves are calculated for the regions north and south of the Tsangpo suture, using Wiener filtering of fundamental mode Rayleigh waves incident upon the array with different backazimuths. The two dispersion curves are inverted to obtain the S-wave crustal models north and south of the suture. They show that a low-velocity layer is present in the lower crust north but not south of the Tsangpo suture. To confirm these findings, variations in the amplitudes of the Rayleigh waves across the suture are interpreted by numerical simulation. The strong amplitude variations at a 30 s period are reproduced using a model with a low-velocity layer in the lower crust north of the suture, a normal high-velocity layer to the south, and a sharp transition between these layers.

Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) has collected over 300 km of multichannel, deep seismic reflection data using explosive sources as part of a multidisciplinary effort to image the structure of the crust and uppermost mantle of the Tibetan plateau. The reflection profiles lie within the Yadong-Gulu rift and were acquired in the summers of 1992 and 1994. Data processing utilized typical industry tools, and a new method was used to migrate the data. Both unmigrated and migrated sections are presented here in large format to facilitate further interpretations.

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep seismic reflection profiles show that the Indian lithosphere is underthrusting the central Himalaya along a gently north dipping decollement that is traceable northward beneath the Tethyan belt to ∼28.6°N and to a depth of about 45 km. The decollement carries in its hanging wall a near-crustal-thickness slice of internally deformed Indian continental basement and cover represented in outcrop by the Greater Himalayan belt and structurally overlying Tethyan belt. Geometric relationships suggest that the decollement probably ramps downward beneath the northern Tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from Indian mantle lithosphere that presently underlies southern Tibet to the north. The North Himalayan anticlinorium (Kangmar dome) appears to be a large duplex ramp anticline in the hanging wall of the decollement. The South Tibetan Detachment appears to have been imaged in two areas. In the Wagye La area it appears to follow the Tethyan belt/Greater Himalayan belt contact northward in the subsurface and to have been folded over the Kangmar dome. In the Zherger La area it appears to have been active relatively recently as a ∼30° north dipping normal fault that cuts deeply into the Greater Himalayan belt allochthon. Palinspastic reconstruction indicates a minimum of about 326 km of shortening of the Indian basement across the Himalaya since the initiation of the Main Central Thrust. Hence Indian continental lithosphere, largely stripped of its overlying crust, can extend northward beneath the Tibetan plateau to at least 32°N, where earthquake seismological observations show that the properties of the upper mantle change markedly [e.g., McNamara et al., 1995]. Taken together, the seismic reflection and earthquake seismological observations lend support for the view that the Indian mantle lid mechanically underplates roughly the southern half of the Tibetan plateau. The INDEPTH reflection data do not show that Indian crust attached to this lithosphere extends north of southernmost Tibet (Kangmar dome), nor do they negate this possibility. Similarly, the reflection data do not yield direct evidence for or against lower-crustal subduction beneath the Himalaya.

INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

[1] The International Deep Profiling of Tibet and the Himalaya Phase IV (INDEPTH IV) active source seismic profile in northeast Tibet extends 270 km roughly north-south across the Songpan-Ganzi terrane, the predominantly strike-slip North Kunlun Fault (along the Kunlun suture), the East Kunlun Mountains, and the south Qaidam Basin. Refraction, reflection, and gravity modeling provide constraints on the velocity and density structure down to the Moho. The central Qaidam Basin resembles average continental crust, whereas the Songpan-Ganzi terrane and East Kunlun Mountains exhibit thickened, lower-velocity crust also characteristic of southern Tibet. The crustal thickness changes from 70 km beneath the Songpan-Ganzi terrane and East Kunlun Mountains to 50 km beneath the Qaidam Basin. This jump in crustal thickness is located ∼100 km north of the North Kunlun Fault and ∼45 km north of the southern Kunlun-Qaidam boundary, farther north than previously suggested, ruling out a Moho step caused by a crustal-penetrating North Kunlun Fault. The Qaidam Moho is underlain by crustal velocity material (6.8–7.1 km/s) for ∼45 km near the crustal thickness transition. The southernmost 10 km of the Qaidam Moho are underlain by a 70 km reflector that continues to the south as the Tibetan Moho. The apparently overlapping crustal material may represent Songpan-Ganzi lower crust underthrusting or flowing northward beneath the Qaidam Basin Moho. Thus the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust.

INDEPTH seismic reflection profiling shows that the decollement beneath which Indian lithosphere underthrusts the Himalaya extends at least 225 kilometers north of the Himalayan deformation front to a depth of approximately 50 kilometers. Prominent reflections appear at depths of 15 to 18 kilometers near where the decollement reflector apparently terminates. These reflections extend north of the Zangbo suture to the Damxung graben of the Tibet Plateau. Some of these reflections have locally anomalous amplitudes (bright spots) and coincident negative polarities implying that they are produced by fluids in the crust. The presence of geothermal activity and high heat flow in the regions of these reflections and the tectonic setting suggest that the bright spots mark granitic magmas derived by partial melting of the tectonically thickened crust.

Teleseismic primary (P) to secondary (S) converted waves recorded on the INDEPTH III seismic array have been used to detect lithospheric-scale deformation structures of the central Tibetan Plateau from the central Lhasa terrane to the central Qiangtang terrane. A south-dipping crustal converter is seen from the upper crust near the 500-km-long metamorphic core complex exposures in the Qiangtang terrane to the lower crust near the Bangong-Nujiang suture. At deeper depths, a southeast-dipping mantle converter is seen extending from ∼50 km north of the Bangong- Nujiang suture at the depth of the Moho, to a depth of ∼180 km, ∼100 km south of the Bangong-Nujiang suture. We found the observations to be most consistent with a model of lithospheric deformation involving (1) southward subduction of the Tibetan lithospheric mantle along the Bangong-Nujiang suture and (2) thickening of the central Tibetan crust through a thick-skinned, crustal accretionary thrust-wedge structure in response to the India9s collision with Asia.

Seismic data from central Tibet have been combined to image the subsurface structure and understand the evolution of the collision of India and Eurasia. The 410- and 660-kilometer mantle discontinuities are sharply defined, implying a lack of a subducting slab beneath the plateau. The discontinuities appear slightly deeper beneath northern Tibet, implying that the average temperature of the mantle above the transition zone is about 300°C hotter in the north than in the south. There is a prominent south-dipping converter in the uppermost mantle beneath northern Tibet that might represent the top of the Eurasian mantle lithosphere underthrusting the northern margin of the plateau.

THE Himalaya and adjacent Tibetan plateau, constituting Earth's largest region of elevated topography and anomalously thick crust, formed as a consequence of Cenozoic collision between India and Asia—itself considered the archetypal continent–continent collision1–3. Here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling. Our ∼100-km-long profile, acquired in the Tethyan Himalaya, shows a mid-crustal reflection that prob-ably marks the active thrust fault along which the Indian plate is underthrusting southern Tibet; upper-crustal reflections with geo-metries suggestive of large-scale structural imbrication of the upper crust; and Moho reflections from the base of the double-normal-thickness crust underlying the region. These results lend substantial support to the view that crustal thickening beneath southernmost Tibet was accomplished by wholesale underthrusting of Indian continental crust beneath the structurally imbricated upper crust comprising the Tethyan Himalaya.

Summary In the summer of 1998, project INDEPTH recorded a 400 km long NNW–SSE wide-angle seismic profile in central Tibet, from the Lhasa terrane across the Banggong-Nujiang suture (BNS) at about 89.5°E and into the Qiangtang terrane. Analysis of the P-wave data reveals that (1) the crustal thickness is 65 ± 5 km beneath the line; (2) there is no 20 km step in the Moho in the vicinity of the BNS, as has been suggested to exist along-strike to the east based on prior fan profiling; (3) a thick high-velocity lower crustal layer is evident along the length of the profile (20–35 km thick, 6.5–7.3 km s−1); and (4) in contrast to the southern Lhasa terrane, there is no obvious evidence of a mid-crustal low-velocity layer in the P-wave data, although the data do not negate the possibility of such a layer of modest proportions. Combining the results from the INDEPTH III wide-angle profile with other seismic results allows a cross-section of Moho depths to be constructed across Tibet. This cross-section shows that crustal thickness tends to decrease from south to north, with values of 70–80 km south of the middle of the Lhasa terrane, 60–70 km in the northern part of the Lhasa terrane and the Qiangtang terrane, and less than 60 km in the Qaidam basin. The overall northward thinning of the crust evident in the combined seismic observations, coupled with the essentially uniform surface elevation of the plateau south of the Qaidam basin, is supportive of the inference that northern Tibet until the Qaidam basin is underlain by somewhat thinner crust, which is isostatically supported by relatively low-density, hot upper mantle with respect to southern Tibet.

Records of densely spaced shots along the Sino-US reflection line INDEPTH II at offsets between 70 and 130 km parallel to the main profile provide an image of the crust straddling the Indus-Yarlung suture. The major features are prominent reflections at about 20 km depth beneath and extending out to about 20–30 km north and south of the surface exposure of the suture, and north-dipping reflectors north of the suture. Various interpretations for the reflections are possible. (i) They represent a decollement, possibly of the Gangdise thrust system. In this scenario, the surface expression of the Gangdise thrust as mapped in eastern south Tibet is a splay with the decollement continuing southwards and either ending as a blind thrust or ramping up as one of the thrusts within the northernmost Tethyan shelf sequence. (ii) The reflections represent fabrics within gneisses, partly obliterated by intrusions reaching various levels of the crust. The reflection bands may be interpreted in terms of deformation or sedimentary structures belonging to the Indian crust, the accretionary complex, and the basement of the Gangdise belt. The intrusions could be related to the Tethyan leucogranites south of the suture (Rinbung leucogranite), and to the Gangdise magmatic arc to the north of the suture. (iii) The reflections represent a fortuitous coincidence of different features north and south of the suture. South of the suture, the reflections may record the basement–cover interface of the Indian crust or a thrust system in the Tethyan shelf. North of the suture, they may comprise different levels within the Gangdise belt and its basement. Although it is not possible to discriminate between the suggested scenarios without additional information, the seismic mapping points to the importance of post-collisional (Oligocene–Miocene) tectonics, which reshaped the suture.

Earthquake data collected by the INDEPTH-II Passive-Source Experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern Tibet. North of the Zangbo suture, beneath the southern Lhasa block, a midcrustal low-velocity zone is revealed by inversion of receiver functions, Rayleigh-wave phase velocities, and modeling of the radial component of teleseismic P-waveforms. Conversely, to the south beneath the Tethyan Himalaya, no low-velocity zone was observed. The presence of the midcrustal low-velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern Yadong-Gulu rift and possibly much of southern Tibet.

Three-component wide-angle seismic data acquired in southern Tibet during Project INDEPTH show strong P-to-S converted reflections from reflectors that are aligned at a depth of approximately 15 kilometers beneath the northern Yadong-Gulu rift. These converted reflections are locally higher in amplitude than the corresponding P-wave reflections. Modeling of reflection mode conversion as a function of incidence angle indicates that this condition obtains for a reflector that is a solid over fluid interface; it is not typical of a solid-solid interface. The likely candidates for a fluid trapped within the crystalline crust of southern Tibet are granitic magma and water (brine).

In the deep crust, temperature, which is among the key pa- rameters controlling lithospheric dynamics, is inferred by extrap- olation from the surface using several assumptions that may fail in regions of active tectonics and fluid migration. In the rare case that temperatures of 700 8C or higher are exceeded in the upper and middle continental crust composed of quartz-rich felsic rocks, the a-b quartz transition (ABQT) will occur, generating a mea- surable seismic signature and offering the possibility for precisely estimating temperature from the known ABQT phase diagram. Here it is shown that all expected seismic features of the ABQT are met by the boundary between the upper and middle crust be- low the INDEPTH III profile in central Tibet. This finding implies that a temperature of 700 8C is achieved at a depth of 18 km under the southern Qiangtang block, which agrees with the depth to the top of a high electrical conductivity anomaly, likely representing partially melted crust. To the south in the northern Lhasa block, the ABQT is at 32 km depth, corresponding to a temperature of 800 8C. It thus appears that this seismic boundary representing the ABQT is the result of recent geologic processes rather than being a lithologic boundary.

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep reflection data in the Yangbajain-Damxung graben of southern Tibet yield evidence of magmatism and deformation beneath the southern Lhasa terrane and Yarlung-Zangbo suture. Shallow reflections and low-velocity first arrivals indicate a thin, Quaternary graben fill of generally less than a few hundred meters thickness. Underlying stratified reflections, extending to a maximum of about 11.5 km depth, likely originate from deformed Paleozoic-Cenozoic supracrustal strata of the Lhasa terrane. A prominent, undulatory band of reflections within the crystalline basement extends beneath the length of the Yangbajain-Damxung graben (depth ranges from ∼12 km to ∼18 km). This horizon has been interpreted to mark the top of a midcrustal partial-melt zone underlying southern Tibet. The undulatory character of this horizon additionally suggests that it may be tectonically deformed. A somewhat deeper, subhorizontal, wide-angle reflection extends southward beneath the outcrop of the Yarlung-Zangbo suture, indicating that the suture is cut off or superposed by a younger structure at depth. Three gently north dipping reflections at 19 km, 24 km, and 27 km depth beneath the Gangdese batholith are suggestive of a midcrustal duplex and may mark the northward (downdip) extension of the late Oligocene-early Miocene Gangdese thrust system. A prominent ∼40° north dipping reflection imaged in the deep crust beneath the Gangdese batholith, between 40 and 60 km depth, might mark the downdip expression of the Yarlung-Zangbo suture or, alternatively, a younger reverse fault in the lower crust (or both). Viewed in aggregate, the reflection data are suggestive of moderate, postcollisional shortening of the upper crust of the Lhasa terrane, accompanied by melting of the middle crust. Although the data are permissive of wholesale underthrusting or fluid injection of Indian continental crust beneath the Lhasa terrane, they show no direct evidence for this having occurred.