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Poster
DDC:
550 - Earth sciences
Abstract:
P-wave receiver functions (PRFs) from project INDEPTH IV (International Deep Profiling of Tibet and the Himalaya, phase IV) broadband seismic data constrain lithospheric structure from the northern Qiangtang terrane to the southern Qaidam Basin in Tibet. Using this data, we explore structural variations, the extent and types of lithospheric deformation in north Tibet, and mechanisms for growth of the north plateau. The Tarim and Sichuan Basins are widely considered to be rheologically strong lithospheric blocks that impede the extrusion of material from beneath Tibet or allow steep topographic gradients to form at the plateau margin. The role of the intraplateau Qaidam Basin in the escape of material from beneath central Tibet and the accommodation of convergence in the Kunlun and Qilian Mountains is less well studied. Two INDEPTH IV broadband datasets are used: 1) two dense linear broadband profiles with 6-km spacing (30 stations across the Kunlun-Qaidam boundary and Kunlun suture and 18 across the Jinsha suture) deployed for 14 months and 2) a regional array of broadband stations scattered across the Songpan-Ganzi, Kunlun Mountains, and Qaidam Basin deployed for up to 24 months. In this work, we clarify structural and rheological variations near the south margin of the Qaidam Basin and beneath the Kunlun Mountains. We calculate p-wave receiver functions using an iterative time-domain deconvolution and depth-convert them by back-projection using an average INDEPTH IV active-source p-wave velocity model. The Kunlun PRF profile clearly shows a crustal thickness change of about 15-20 km at the Kunlun-Qaidam boundary (from about 70 km beneath the Kunlun to 50 km beneath Qaidam), with complex Moho structure and possible overlapping or dipping strong Moho converters. We examine the azimuthal variation of prominent Moho and other converters near the location of the crustal thickness change in south Qaidam. There is a strong, negative-polarity converter at about 20-km depth beneath the Qiangtang and Songpan-Ganzi, extending at least as far north as the Kunlun suture, perhaps indicating fluids in the mid-crust as seen in southern Tibet. We compare synthetic receiver functions with the data-based receiver functions. We use these data and methods to test whether the Kunlun Mountains lower crust is injected northwards beneath an indenting, stronger Qaidam crustal wedge, a hypothesis derived from the INDEPTH IV active-source results.
