The Crustal Structure beneath the South Margin of Ordos Block, China from P Wave Receiver Functions

Ordos block is in the west of north China craton, and effected by the India-Asia collision and west pacific subduction. Under the two dymanic systems, Ordos is very stable in the center, but highly deformates in the margin. Qinling Orogen is to the south of Ordos. The Shanxi graben is another import tectonic feature in the south margin of Ordos, which is the result of late Cenozoic extension in north China. From 2005 to 2007, we situated broadband seismic array in the south margin of Ordos and adjacent area, and obtained two longitudinal seismic profiles which are parallel to each other. By using P wave receiver functions analysis, the characters of the crustal structure in this region are generalized as follows: comparing the two profiles, the crust is thinner in the east than that in the west. The crust in Ordos is simple and stable. The crustal thickness is ~39km on the east profile and ~42km on the west profile. Average crustal poisson’s ratio is ~0.25. The Moho is almost flat in Qinling Orogen. And crustal thickness is ~36.5km in on the east profile and ~40km on the west profile. The poisson’s ratio is 0.21~0.24. The crustal structure in Shanxi graben is very complex. As the effect of Shanxi graben, P wave receiver functions are very particular. A strong positive phase arrives at 1~2s, following several weaker phases. We infer these features are the results of thick sedimentary layer. Though, the Moho conversional phase Pms is very weak, there is an evidential crustal thinning in the graben. Especially, there is an obviously crustal thickness variation near Weihe fault. It indicates that it dominates the evolution of the graben during late Cenozoic. The poisson’s ratio is 0.26~0.29. (Supported by National Natural Science Foundation of China, Grant No: 40634021, 40474017)