Upper mantle anisotropy beneath Abag area in Inner Mongolia from shear wave splitting

Abag area suffered a strong crustal and mantle deformation, however, the mechanism of this deformation is poorly understood. A total of 120 pairs of shear wave splitting measurements and 113 null measurements are obtained at 32 portable seismic stations. Delay times vary from 0. 4 s to 1. 4 s with an average value of 0. 77 +/- 0. 21 s; while fast directions trend from N101 degrees W to N45 degrees E. One group of fast directions in line with the strike of regional faults, trending N82. 0 degrees E 112. 3 degrees, is caused by lattice-preferred orientation of mantle crystals; the other group of fast directions observed in the North China Craton with an average value of N146. 8 degrees E +/- 9. 5 degrees are parallel to the direction of lithospheric deformation in Early Cretaceous, which may origin from fossil anisotropy remaining in the lithosphere. In addition, solely null measurements in the northern part of study area may suggest small-scale hot mantle upwellings which partially eroded the fossil anisotropy in lithosphere.