HR: 0800h
AN: T41A-1278    [Abstracts]
TI: Crustal Structure of the Northeastern Margin of the Tibetan Plateau From the Songpan-Ganzi Terrane to the Ordos Block
AU: * Liu, M
EM: shane@usgs.gov
AF: Geophysical Exploration Center, China Earthquake Administration, 104 Wenhua Road, Zhengzhou, 450002 China
AU: Mooney, W
EM: shane@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Li, S
EM: shane@usgs.gov
AF: Geophysical Exploration Center, China Earthquake Administration, 104 Wenhua Road, Zhengzhou, 450002 China
AU: Okaya, N
EM: shane@usgs.gov
AF: Geophysical Exploration Center, China Earthquake Administration, 104 Wenhua Road, Zhengzhou, 450002 China
AU: Detweiler, S
EM: shane@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: The 1000-km-long Maqin-Lanzhou-Jingbian seismic refraction profile is located in the NE margin of the Tibetan Plateau, and crosses the northern Songpan-Ganzi terrane, the Qinling-Qilian fold zone, the Haiyuan arcuate tectonic region, and the stable Ordos block. The profile and calculated Poisson's ratio reveal many significant characteristics in the P-wave and S-wave velocity structure. The crustal thickness increases from northeast to southwest, with the average observed crustal thickness increasing from 42 km in the Ordos block to 63 km in the Songpan-Ganzi terrane. The crust becomes obviously thicker south of the Haiyuan fault and south of Zikog. The crustal velocities vary significantly along the profile. The average crustal P-wave velocities are 6.30 km/s in the Ordos block, 6.22 km/s in the Haiyuan arcuate tectonic region, 6.25 km/s in the Qilian block, 6.20 km/s in the Qinling block, and 6.10 km/s in the Songpan-Ganzi terrane. The average crustal velocity is 6.22 km/s along the profile, which is 0.23 km/s lower than the global average (6.45 km/s). Combined P-wave velocity and Poisson's ratio suggest that the crust is dominantly felsic in composition with an intermediate base. A mafic lower crust is absent in the NE margin of the Tibetan Plateau from the Songpan-Ganzi terrane to the Ordos block. Felsic lithologies are inferred to be greenschist-to-amphibolite facies. There are low velocity zones in the Qinling block and the Haiyuan arcuate tectonic region. The low velocity zones have low S-wave velocities and high Poisson's ratios, so it is possible that partial melting has caused the formation of the low velocity zones. The crust is divided into two layers, the upper and the lower crust, with crustal thickening mainly in the lower crust in the NE Tibetan Plateau. The profile shows that the thickness of the lower crust increasing from 22 km to 38 km as the crustal thickness increases from 42 km in the Ordos block to 63 km in the Songpan-Ganzi terrane south of the Kunlun fault. Both the Conrad discontinuity and Moho in the Qinling block and in the Haiyuan arcuate tectonic region are laminated interfaces, implying intense tectonic activity. The arcuate faults and large earthquakes in the Haiyuan arcuate tectonic region are the result of interaction between the Tibetan Plateau and the Ordos and Alxa block.
DE: 7205 Continental crust (1219)
DE: 8038 Regional crustal structure
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: Fall Meeting 2005