HR: 0800h
AN: T41A-1275    [Abstracts]
TI: Crustal Structure of the NE Tibetan Plateau, China, From Deep Crustal Seismic Profiling Data
AU: * Mooney, W
EM: mooney@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Wang, Y
EM: mooney@usgs.gov
AF: China University of Geoscience, 29 Xueyuan Lu, Beijing, 100081 China
AU: Yuan, X
EM: mooney@usgs.gov
AF: China Geological Survey, 31 Xueyuan Lu, Beijing, 100081 China
AU: Okaya, N
EM: mooney@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: We discuss crustal structure across the northeastern Tibetan plateau based on active-source seismic data recorded along a 1600-km-long profile crossing, from NW to SE, the southern Tarim basin, the NE Tibetan plateau, and the Sichuan basin. Within the Tibetan plateau, from NW to SE, are the western flank of the South-Qilian Shan, the NE margin of the Qaidam basin, the East-Kunlun Shan (mountain range containing the Kunlun fault), and the Songpan-Ganzi terrane. The crustal P- and S-wave velocity structure and Poisson's ratio outline the characteristics of the crustal structure and provide constraints on the crustal composition. The derived crustal cross section shows several significant features. (1) The crustal thickness varies considerably. North of the Kunlun fault variations in crustal thickness and topography correlate well. The crust thickens from 48 km below the Tarim basin to 70 km beneath the northeastern margin of the Qaidam basin, and then thins to about 56 km depth beneath the eastern flank of the Qaidam basin. The crust subsequently thickens again to 70 km beneath the East-Kunlun Shan. Across the Songpan-Ganzi terrane, the crust steadily thins from 70 km just south of the Kunlun fault to 48 km beneath the Sichuan basin, despite relatively constant topography across the Songpan-Ganzi terrane followed abruptly by a 10% drop in elevation in the Sichuan basin. (2) North of the Kunlun fault differences in crustal thickness are mainly caused by variations in lower crustal thickness, whereas south of the Kunlun fault they are caused by variations in upper-, middle- and lower-crustal thickness. (3) North of the Kunlun fault we detect a mid-crustal low-velocity zone, which is not apparent south of the fault. Across the plateau the Poisson's ratio is nearly constant with a value of 0.24-0.25 in the upper and middle crust, indicating a felsic bulk composition. In the lower crust the Kunlun fault seems to act as a boundary, with a Poisson's ratio of 0.29 north of the fault (Kunlun-Qaidam) and 0.26 south of the fault (Songpan-Ganzi). The Poisson's ratio and P-wave velocity values suggest that the lower crust throughout the Tibetan plateau (South-Qilian Shan, margins of the Qaidam Basin, East-Kunlun Shan, Songpan-Ganzi terrane) is of intermediate composition. Thus, along our profile, the NE Tibetan plateau is missing a mafic crustal layer at the base of the crust. In contrast, the Tarim basin, which borders the Tibetan plateau to the north, shows a typical platform-like crustal structure with a felsic upper and middle, and a mafic lower crust. The Sichuan basin, which borders the Tibetan plateau to the east, also has a felsic upper and middle crust, and an intermediate or mafic lower crust.
DE: 7205 Continental crust (1219)
DE: 8038 Regional crustal structure
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: Fall Meeting 2005