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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'. [ABSTRACT FROM AUTHOR]

Analyzes the crustal structure of Tibet, China as derived from project INDEPTH wide-angle seismic data. Purpose of the wide-angle seismic measurements; Indications of the crustal thickness beneath the INDEPTH III profile; Factors affecting seismic refraction.

In the deep crust, temperature, which is among the key parameters controlling lithospheric dynamics, is inferred by extrapolation 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 °C or higher are exceeded in the upper and middle continental crust composed of quartz-rich felsic rocks, the α-β quartz transition (ABQT) will occur, generating a measurable 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 below the INDEPTH III profile in central Tibet. This finding implies that a temperature of 700 °C 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 °C. It thus appears that this seismic boundary representing the ABQT is the result of recent geologic processes rather than being a lithologic boundary. [ABSTRACT FROM AUTHOR]

A tomographic image of the upper mantle beneath central Tibet from INDEPTH data has revealed a subvertical high-velocity zone from ~100- to ~400-kilometers depth, located approximately south of the Bangong-Nujiang Suture. We interpret this zone to be downwelling Indian mantle lithosphere. This additional lithosphere would account for the total amount of shortening in the Himalayas and Tibet. A consequence of this downwelling would be a deficit of asthenosphere, which should be balanced by an upwelling counterflow, and thus could explain the presence of warm mantle beneath north-central Tibet. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]