Abstract:

Seismic tomography is an important tool for studying the structure and dynamics of the Earths interior. We gave a brief introduction to the main methods used for seismic tomography of the lithosphere. Relative to seismic exploration with controlled sources, seismic tomography with passive natural sources is an effective and economic method for imaging the deep structure of the basement and the surrounding of a sedimentary basin. Weve recently obtained a highresolution Swave velocity model of the lithosphere in China from surfacewave tomography, which combines data from earthquakes and ambient noise correlations with much improved ray coverage. Our results show major common features of the three basins (Tarim, Qaidam, and Sichuan) surrounding the Tibetan Plateau in the western China: (1) the upper crust of the basins shows slow velocities corresponding to the thick sedimentary layers; (2) the midcrust, lower crust, and in particular, the uppermost mantle show faster speeds than the surrounding mountains; (3) the basins have thinner crust than the surrounding mountains and the Moho depth change is rapid. Significant lateral structure also exists within the basins, in particular, the Tarim and Sichuan Basins. The eastwest paleosuture in the central part of the Tarim is clearly visible in the crustal and mantle lithosphere velocities and in Moho depth variation. These results suggest that regional compression may involve the entire lithosphere. We propose a mechanical model (called compressional upliftsubsidence model) for the formation of the basins: under the regional compressional stress, the weaker lithosphere blocks surrounding the basins are thickened and uplifted, and the stronger basin blocks subside relative to the surrounding mountains under
the lithosphere isostacy, forming an intraorogenic basin. The compressional stress may have also deformed the giant Tarim and Sichuan Basins and created folds in a lithosphere scale. In the long geologic history of the formation of the major basins in the Western China, the lithospherescale compression from the Cenozoic IndiaEurasian collision and several orogenic episodes since the NeoProterozoic may have played a determining role in the formation and evolution of the basins.

Key words: basins in western China, Swave velocity, seismic tomography