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Yaolin Shi, Graduate University of Chinese Academy of Sciences (China)
Jianling Cao, Graduate University of Chinese Academy of Sciences (China)
Meijian An, Graduate University of Chinese Academy of Sciences (China)
Huai Zhang, Graduate University of Chinese Academy of Sciences (China)
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Thickness and rheological structure are very important problems in the study of continental geodynamics. Low velocity zone are usually used to identify the thickness of the lithosphere, however, ambiguity exists in choosing the exact location of the bottom of the lithosphere: Is it at the depth of initial reduction of velocity, or minimum velocity, or the depth of maximum negative velocity gradient, or just a fixed standard of certain velocity? Temperature can be used to define the thermal lithosphere, such as taking the 1300°C adiabat as the bottom of the lithosphere. However, error in estimation of temperature increases with depth if using surface heat flow data to extrapolate temperatures at depth greater than 100 km. At tectonic active regions, thermal state may not reach steady state and temperature evaluation becomes even more difficult. In this paper, we use seismic tomographic data and laboratory high temperature/pressure rock creep experiment results to estimate temperatures in the lithospheric mantle beneath continental China. Combined with surface measurements of heat flow, we construct a new geotherm model of China. Defining the 1300°C adiabat as coinciding with the lithospheric base, we estimate the seismic-thermal lithosphere thickness. The estimated thickness shows obvious dependence on the tectonic settings. Beneath eastern China, it has a thickness of ∼100 km; and beneath the Qinghai-Tibet plateau and south to the Tarim craton, it has a thickness of ∼160∼220 km. The lithospheric thicknesses of the three large para-platforms/cratons range from ∼170 km for the western Yangtze, ∼140 km for Tarim, and ∼100 km for Sino-Korean. Based on lithology model of Gao et al. (1999), and strain rate from GPS measurements (Zhu and Shi, 2006), we further construct a 3-D rheological model of China. The effective viscosity of the middle and lower crust usually range 1021-1024 Pa.s and 1021-1022 Pa.s respectively. The lower crust of Tibetan plateau has a low viscosity of 1019-1020 Pa.s, which is in agreement with the previous conclusion that there is ductile lower crustal flow in Tibetan plateau. Three dimensional finite element model based on the rheology model, characterized by an extremely ductile lower crust beneath Tibet, did explain the Tibetan clockwise rotation of GPS velocity around the eastern syntax of the Himalaya.
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