Crustal shear (S) velocity and Poisson's ratio structure along the INDEPTH IV profile in northeast Tibet as derived from wide-angle seismic data

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