Upper mantle anisotropy and tectonic deformation beneath the Ordos Block in the western North China Craton: Constraints from wide-angle seismic data

The Ordos Block is located in the northern part of the North China Craton and its interior is highly stable. A series of fault basins and faults has developed around the periphery, and seismic activity is frequent. To research the dynamic mechanisms that caused the special geomorphology and geological features of the Ordos Block further, this study processed data from two nearly orthogonal profiles laid out in this area and obtained the P-wave velocity structure characteristics of the area. The results showed that the crust-mantle velocity structure, interface depth, and the relationship with the peripheral regions obtained from the two profiles are rather complex: the internal crust structure of the Ordos Block is relatively simple, the low-velocity layer is not developed, the interface does not undulate much, and the crust thickness of the Ordos Block is about 42.0 km. These results show that the Ordos Block is part of a sinking residual feature of the ancient stable continental lithosphere. The crust-mantle structure of the block and its peripheral coupled region shows complex crustal structure features. The velocity structure is abnormally disordered and the interface shows declining or depressed anomalies, revealing the background characteristics of the active structure of the block and the periphery. Another major feature is that the velocity of the Pn wave at the uppermost mantle near the intersection of the two profiles has directional and anisotropic features, i.e., the value of the velocity in the east-west direction is significantly greater than that in the north-south direction and the difference is 0.25 km/s. The reason for this phenomenon may be that since the Mesozoic Era the Ordos Block has been mainly subject to east-west compressive stress, forming a series of north-south structural deformations.