ZHANG Xin-Yan,
GAO Rui,
BAI Zhi-Ming et al
.2017.Crustal structure beneath the Longmenshan area in eastern Tibet:new constrains from reprocessing wide-angle seismic data of the Aba-Longmenshan-Suining profile.Chinese Journal Of Geophysics,60(6): 2200-2212,doi: 10.6038/cjg20170614
Crustal structure beneath the Longmenshan area in eastern Tibet:new constrains from reprocessing wide-angle seismic data of the Aba-Longmenshan-Suining profile
ZHANG Xin-Yan1, GAO Rui1,2, BAI Zhi-Ming2,3, XU Tao3,4, LI Qiu-Sheng1
1. Key Laboratory of Earthprobe and Geodynamics, MLR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China; 2. School of Earth Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China; 3. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 4. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China
Abstract:The Longmenshan area in the eastern Tibetan Plateau experienced strong crustal deformation and faulting during Mesozoic and late Cenozoic epochs resulting in significant uplift of the Longmenshan Mountains, which serves as an important window to see the uplift and lateral expansion of the Tibetan Plateau. In this paper, the deep seismic sounding profile named the Aba-Longmenshan-Suning profile is reprocessed using a high-precision traveltime tomography for velocity structure with an irregular surface to reconstruct the crustal structure beneath this area. The obtained velocity structure along the profile could be divided into three major sections: the Songpan-Garzê block, Longmenshan fault zone and Sichuan basin. The sedimentary cover of the Songpan-Garzê block is about 10 km thick, with high-velocity bodies in 5~10 km depths, which correspond to the faults and likely the intrusion of Mesozoic granites. The undulating interface of the sediment may indicate the thrust-nappe tectonics. Velocity structures of the middle and lower crust change continuously with lower average velocity (about 6.26 km·s-1). In the Sichuan basin, the sediment in the west is thicker than that in the west. The sediment in the west even exhibits compression and torsion forms, which may be caused by compression and erosion. The middle and lower crust in the west is thinner than that in the east with higher average velocity (about 6.29 km·s-1). The Longmenshan fault zone is a transition zone of the crustal velocity and thickness, with the Moho uplifting about 13 km from the west to the east. Along the whole profile the Moho shows toughness winding, while the velocity of the middle and lower crust changes continuously in the lateral direction, which may indicate that the ancient Yangtze block has reached beneath the Songpan-Garzê block. In the Songpan-Garzê block, which is considered to be a passive margin, the middle and lower crust experienced ductile deformation under strong compression. The brittle upper crust with thrust-nappe belts is dragged by the middle and lower crust, while stress is accumulated and released on the faults, which may induce a large number of earthquakes.
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