Moho depth and Poisson’s ratio distribution in the western edge of Ordos block

This paper collects P-waveforms of teleseismic events occurred from January 2015 to January 2016 recorded by 21 Ningxia regional seismic network stations and 183 China-Array seismic stations deployed in western edge of Ordos block, and extracts receiver functions by deconvolution, and then calculates the Moho depth and Poisson’s ratio in the studied area by using H-Kappa stacking method. The results show that the Moho depth ranges 36−58 km, which is presented as thick in the south and west and thin in the north and the east, exhibiting obvious subarea characteristics. Bounded by the Xiangshan-Tianjingshan fault, the Moho depth has a distinct change in the north-south direction, and to the south of the fault the crust-thicken way is probably related to upper crust overlap due to the horizontal extrusion from Tibetan Plateau. On the both sides of Helanshan eastern piedmont fault, the Moho depth varies largely, which may be caused by the joint impact from the extrusion in NE direction from northeastern Tibetan Plateau and the tension in NW-SE direction between Alxa block and Ordos block. In the studied area, the Poisson’s ratios are relatively high in general, and the average of the Poisson’s ratio of Ordos block is higher than that in northeastern Tibetan Plateau. On the other hand, high Poisson’s ratio in Yinchuan graben is possibly related to Huanghe-Lingwu fault as an ultracrustal one. The Moho depth is positively correlated to the surface elevation, suggesting that the crust is in the better state of isostasy, but there is no obvious linear correlation between Poisson’s ratio and Moho depth in the whole studied area. However, further study finds that there are two distinct areas, in which Poisson’s ratio is inversely correlated with the Moho depth, illustrating that the tectonic extrusion or tension is easier to concentrate on the felsic incompetent beds, which leads to the crust thickening or thinning. Consequently, from the inverse relationship between the Poisson’s ratio and Moho depth, it is believed that the crustal composition consists chiefly of felsic rocks beneath the zone from Haiyuan fault to Xiangshan-Tianjingshan fault and on the both sides of Helanshan eastern piedmont fault.