Crustal Flow Between the Ordos and Sichuan Blocks in Western China

No earthquakes of magnitude > 4.5 have been recorded in the region of Qinling orogen between the Ordos and Sichuan blocks in western China. On the other hand, both modern and historical earthquake activity (ca. 1831 BCE onward) is present along the margins of the blocks, with only a few earthquakes in the interior of the blocks (Gu, 1983). In order to study causal relationship between earthquakes and lithospheric processes in the region, we determine intracrustal density variations by taking into account gravity effects of the Moho variation and sedimentary basin response and relate them to seismic properties and GPS velocities. The crust has lower densities along the Qinling orogen in comparison to the Ordos and Sichuan blocks to the north and south, respectively. Receiver function analysis along a 1000 km long N-S profile with 32 seismic stations from the northeastern Sichuan basin to the Ordos block has revealed a region of high crustal Vp/Vs ratio ( 2.0, Jia et al., 2015) that corresponds to the above low intracrustal density region. The lithospheric structure with S-wave receiver function using waveforms from the seismic arrays shows that the amplitudes of mid-crustal reflectors significantly increase at the depth of about 25 km in the southernmost Ordos block in comparison to the neighboring non-cratonic areas (Shen et al., 2017). The location of the strong reflectors also corresponds to the higher density of the southernmost part of the rigid Ordos block. In the neighboring non-cratonic regions on the profile, mid-crustal and Moho reflectors are also discontinuous and have lower intracrustal densities. High Vp/Vs (>1.85) can be indicative of a dominant mafic composition or of partial fluid content; Vp/Vs ratios of about > 2 can generally be associated with the orientations of cracks (Wang et al., 2012 GRL) or the presence of less viscous material (Clark and Royden, 2000, Geology). The low density region in the Qinling gap between the Ordos and Sichuan blocks excludes the possibility of mafic composition. Because the GPS velocities in the region of the gap are higher than those within the blocks and are directed eastward, we interpret that the low density rocks in between the two blocks may be related to the flow of ductile rocks from the Tibetan region eastward in a channel between the Ordos and Sichuan blocks.