CRUSTAL STRUCTURE OF THE NORTHEASTERN MARGIN OF THE TIBETAN PLATEAU FROM THE SONGPAN-GANZI TERRANE TO THE ORDOS BASIN

We report results from a 1000-km-long seismic refraction profile located on the NE margin of the Tibetan Plateau. This profile crosses the Songpan-Ganzi terrane, the Qinling-Qilian fold zone, and the stable Ordos block. The P-wave and S-wave velocity structure and Poisson's ratios reveal many significant characteristics of the crust. The crustal thickness increases significantly from northeast to southwest. The average crustal thickness is 42 km in the Ordos block and reaches 63 km in the Songpan-Ganzi terrane. The crust thickens in a step-like fashion southwest of the Haiyuan fault. The crustal velocities have significant variations along the profile. The average crustal P-wave velocities are 6.3 km/s in the Ordos block, 6.22 km/s in the Haiyuan arcuate tectonic region, 6.25 km/s in the Qilian block, 6.2 km/s in the Qinling block, and 6.1 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-to-mafic composition in the lower crust. However, 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 in greenschist-to-amphibolite metamorphic facies. There are mid-crustal 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 they are due to partial melting. The crust may be divided into two layers, the upper and the lower crust, with crustal thickening mainly in the lower crust beneath the NE Tibetan Plateau. The thickness of the lower crust increases 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 region are laminated interfaces. The faults and large earthquakes in the Haiyuan region are the result of compression between the Tibetan Plateau and the Ordos block.