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Measurements of shear wave splitting at 43 three–component seismic stations show very big difference in anisotropy on both sides of the Indus–Yarlung Zangbo suture (ITS), but little difference on both sides of the older Bangong–Nujiang suture (BNS) and the Jinsha River suture (JS) to its north. Obvious discrepancy exists between the anisotropy direction and the superficial tectonic trends, which is not explicable directly by the coherent upper mantle deformation usually supposed to occur in consistency with the trend of a collisional belt. On the other hand, strong spatial relationships are observed from the anisotropy results, such as the orthogonal directions of anisotropy on both sides of ITS and the good correlation between the region of larger magnitude of anisotropy and the zone of inefficient Sn propagation of Qiangtang as well as the systematic rotation of the directions of anisotropy, which should testify some much more complicated aspects of the continental convergence mechanism To the best of our data, we tend to suppose that the Qinghai—Tibet plateau might result from a mechanism complicated by the coexistence of Argand's underthrusting and Dewey's diffuse deformation.

This paper introduces 8 major discoveries and new understandings with regard to the deep structure and tectonics of the Himalayas and Tibetan Plateau obtained in Project INDEPTH, They are mainly as follows. (1) The upper crust, lower crust and mantle lithosphere beneath the blocks of the plateau form a "sandwich" structure with a relatively rigid-brittle upper crust, a visco-plastic lower crust and a relatively rigid-ductile mantle lithosphere. This structure is completely different from that of monotonous, cold and more rigid oceanic plates. (2) In the process of north-directed collision-compression of the Indian subcontinent, the upper crust was attached to the foreland in the form of a gigantic foreland accretionary wedge. The interior of the accretionary wedge thickened in such tectonic manners as large-scale thrusting, backthrusting and folding, and magmatic masses and partially molten masses participated in the crustal thickening. Between the upper crust and lower crust lies a large detachment (e.g. the Main Himalayan Thrust in southern Tibet, 5-8 km thick) or a very thick shear-schistose zone (e.g. the Main Qiangtang Thrust-MQT in northern Tibet, up to 20 km thick), which causes the decoupling of the upper crust and lower crust and separation of tectonic activities. (3) During the collision-compression, the Indian mantle lithosphere was delaminated into two layers from where the crust thickened most rapidly (beneath the High Himalayas). The upper layer extends to 34.5°N and the lower layer to 33.5°. They have been underthrust to depths of 250-300 km into the asthenosphere. Meanwhile the Asian lithosphere (possibly the Qaidam terrane) has also been subducted southwards. Very thick mantle lithosphere does not exist beneath the plateau. (4) The oceanic lithosphere, in light of its lithology and dynamic behaviour, might be close to those of the continental lithosphere and its front might enter the asthenosphere before the continental lithosphere. (5) A 150-200 km deep low-velocity body below 35°N and a wide low-velocity zone below the area between 33.5° and 35°N dip north at very steep angles. Volcanism took place frequently in northern Tibet and anisotropy variations are prominent at depths, which might indicate a zone of large-scale eastward transfer of deep-seated materials.

Numerical simulation of fluid migration during the ore-forming process of the Carboniferous exhalation-sedimentary (Sedex) massive sulfide deposits in the Tongling district shows that fluid and thermal activities in lying-wall rocks were limited to a small area around the main draining passage, which led to weak mineralization and alteration in the lying-wall rock. Temperature and fluid fields indicate that mineralization and alteration in the lying-wall rock of the Sedex-type deposits are usually weaker than those of volcano-hosted massive sulfide deposits. Fluid migration involves the following processes: seawater penetrating and leaching the lying-wall rocks, then mixing with ascending hydrothermal fluids in the main draining passage, and finally jetting into seafloor. Although fluid activity-influenced area is rather small, the content of metals leached out from the lying-wall rocks is high enough to form large-scale ore deposits. Tensional contemporaneous faults accompanied with strong heat flows controlled the formation and distribution of Sedex deposits. The tensional tectonic regime on the northern margin of the Yangtze block during the Hercynian provided Sedex deposits with a prerequisite geodynamic condition.

Sixteen 3-component, 5-sec stations along a profile crossing the Altyn Tagh fault (ATF) at ∼90°E were used to measure shear wave splitting beneath Tibet's northern edge. Splitting delay times (δt) of 0.3 to 1.3 sec show a marked increase in a narrow zone centered on the fault, implying anisotropy in the lithospheric mantle. Fast Polarization Directions (FPD) (ϕ) swing counterclockwise, from a mean azimuth of ∼110°E in the Qaidam basin south of the fault, to a mean azimuth of ∼60°E in the Altyn mountains. This swing occurs in less than 40 km. Both the δt increase and FPD swing are consistent with ductile sinistral shear along the 40 km-wide, low P-wave velocity anomaly (−6–8%) imaged to a depth of ∼140 km beneath the fault by teleseismic tomography. This confirms that the ATF guides the northeastwards extrusion of the Tibet-Qaidam's lithosphere relative to the Tarim's.

During the two years of Sino-French cooperative research program (1992-1993), total of 108 digital seismic stations were deployed in two phases along the profile from Tingri, the south of Tibet, to Xietiesam, the north of Qinghai province. Four months of field recording resulted in hundreds of teleseismic events. Obvious SKS and PKS splitting has been observed on the seismograms of the 43 three-component stations with more than one event. Measurements of the fast polarization and the delay time show a very big difference on the two sides of Indus-Tsangpo suture (ITS) but little difference on the elder Bangong-Nujiang suture (BNS) and Jinshariver suture (JS). Obvious inconsistency exists between the anisotropy directions and the superficial tectonic/geological trends, which might not be explicable directly by the coherent upper mantle deformation usually supposed in a collisional belt. On the other hand, however, strong spatial relationships were revealed by our anisotropy results, such as the perpendicular directions of anisotropy on both sides of ITS and the good correlation between the region of larger magnitude of anisotropy and the zone of inefficient Sn propagation of Qiangtang as well as the systematic rotation of its directions of anisotropy, which should testify much more complicated aspects of the continental convergence mechanism. To the best of our data, we tend to suppose that Qinghai-Tibet plateau might result from a mechanism complicated by the coexistence of Argand's underthrust and Dewey's diffuse deformation, and Qiangtang region might be most probably undergoing the delamination.

Although the crust and mantle of the Tibet Plateau reveal vital information for understanding the interplay of dynamic processes that has governed its recent uplift and growth, their deep, physical and thermal structure remains poorly understood. In order to throw light on the structure and, hence, to constrain the processes and models tied to them, we performed a teleseismic experiment on a 600 km long profile across the northern part of the plateau, the Kunlun range and the Qaidam basin. The 400 km deep tomographic image we obtain has a resolution < 50 km, over one order of magnitude better than achieved in previous, broader-scale studies. At relatively shallow depth (< 100 km), the tomographic cross-section clearly reveals that the mid-lower crust and upper lithosphere of northern Tibet is an assemblage of blocks with different velocities and thicknesses, hence different natures, histories and ages. The crust of the Qiantang block appears to be thickest (≈ 70 km), with the lowest velocity. Along the north edge of that block, the Jinsha suture, although not remarkable in the local geology and topography, stands as a particularly sharp and prominent crustal boundary. The crust and lithosphere of the region between that suture and the Qaidam (Bayan Har-Songpan, South Kunlun) appear to be thinnest (≈ 50 km) and with the highest velocity. At greater depth (> 150 km), our experiment confirms the existence of a bulky low-velocity zone in the mantle beneath the northernmost reaches of the plateau. The tomogram helps assess with unprecedented accuracy the location and shape of this low-velocity anomaly, which forms a 250–300 km wide dome rising to about 150–200 km, south of the Kunlun range, roughly coincident with the region where the strongest shear-wave splitting, hence horizontal anisotropy, has recently been found. That dome lies between two relatively high velocity zones with opposite dips, one towards the north, under the Qaidam basin, the other towards the south, beneath the Quangtang platform. The implications that this new, detailed image of the deep structure of northern Tibet has upon the uplift and thermo-mechanical evolution of the plateau are discussed.

Seismic tomography across the Altyn Tagh fault, at the north edge of the Tibetan Plateau, reveals a low P -wave velocity anomaly below the fault down to 140 kilometers. This anomaly probably reflects strike-slip shear in the lithosphere. Slip-partitioning may also induce a wedge of crust from the Tarim Basin to plunge into the mantle.

The lower crust and Moho are the most active boundary layers in the process of continental evolution, in which marks left by tectonic and magmatic activities during the process are preserved. The evolutionary process of the continental lithosphere may be reconstructed by exploring the structures of the lower crust and Moho. According to a study of the deep seismic reflection data obtained from the middle-lower Yangtze Valley, the authors find bright layered reflections ubiquitous in the lower crust and think that the bright reflections are caused by un-derplating of basic or ultrabasic magmas, which might be related to delamination of the lithosphere. On the basis of an integrated analysis of the geophysical and geological data of the region, the authors propose a model for geodynamic evolution of the middle-lower Yangtze Valley. This model suggests that the middle-lower Yangtze Valley had undergone such geodynamic processes as collision-compression, delamination-extension and underplating-melting since the end of the Permian, finally forming the gigantic middle-lower Yangtze Valley metallogenic belt.

The deep seismic profiling across the Tongling ore district reveals a complex crustal structure. Strong contrasting dipping layered reflections (4—11 s, TWT)，which dominate the lower crust of the northern part of the profile, are interpreted as the underplating of the basalt related to extensional tectonism. In the south of the profile, the Yangtze craton is characterized by strong reflections in the middle crust, showing a distinct two-layer crustal structure. Over the region of the Tongling uplift, there appear the complex arc shape reflections, suggesting folded, faulted and intruded structures, and the transparent zone below them reveals the existence of batholith. The south dipping strong reflections between the upper crust and lower crust (4—7 s, TWT) suggest a detachment between them. The detachment provided space for the magma intrusion, and caused the formation of the batholith. The Yangtze craton has a clear Moho reflection, while the Tongling uplift has a weak Moho, whereas below the reflective lower crust in the northern part of the profile, there are sub-Moho reflections. The abrupt variation of the Moho characteristics within a short distance indicates the complexity of magmatic activity.