Upper mantle anisotropy beneath North China from shear wave splitting measurements

Abstract

Polarization analysis of teleseismic data was used to determine the SKS fast polarization directions and the delay times between the fast and slow shear waves for each of the 52 seismic stations from both temporary and permanent broadband seismograph networks deployed in the North China Craton (NCC). The analysis employed both the Silver and Chan (1991) and stacking analysis methods. In this way, an image of upper mantle anisotropy in the NCC was acquired. The fast polarization directions are oriented along a WNW–ESE azimuth in the Eastern and Central Blocks and rotate to a NNW–SSE azimuth in the Western Block. The average fast polarization directions in the Eastern and Central Blocks and the Western Block are 107±9° and 153±12°, respectively. The average delay time in Eastern and Central Blocks is 0.98±0.19s, whereas it is 0.75±0.17s in the Western Block. In the Eastern and Central Blocks, the fast polarization direction is consistent with the absolute plate motion (APM) direction, implying that the WNW-trending asthenospheric mantle flow driven by the APM plays a major role in observed upper mantle anisotropy beneath the Eastern and Central Blocks, and makes the alignment of upper mantle peridotite lattice parallel to the deformation direction, and thus generates consistently the WNW-trending fast polarization direction. However, in the stable Western Block, the weak anisotropy observed is possibly attributed to the “fossil” anisotropy frozen in the thick lithosphere of ancient cratonic NCC.