Mantle flow pattern and geodynamic cause of the North China Craton reactivation: Evidence from seismic anisotropy

We investigate the mantle flow pattern and geodynamic cause of North China Craton (NCC) reactivation using shear wave splitting measurements from 140 broadband stations. Using a newly developed method for simulating wave propagation in a two-dimensional anisotropic medium, we first examined the influence of sedimentary structures on SKS splitting measurements. The simulations show that a sedimentary layer, whether isotropic or anisotropic, significantly influences the waveform; however, the shear wave splitting parameters can be retrieved with negligible errors. As a result, this study included new splitting measurements at stations which were deployed within basins and not used previously. Among 121 broadband stations that contribute valid splitting results, 55 stations are newly added which were deployed within basins. This significantly improved the sampling coverage on the NCC. The complicated spatial patterns of the splitting parameters indicate that complex upper mantle deformation has occurred in the NCC. To obtain both deep kinematic and geometric information, we interpret our splitting measurements in the context of new high-resolution tomographic results for the NCC. By comparing our observations with three end-member conceptual models (upwelling, wedge flow, and lithospheric delamination), we found that the observed anisotropy pattern beneath the NCC is not completely consistent with any of them. Thus, we prefer a hybrid mantle flow model, where the subduction of the Pacific Plate causes a mantle wedge flow beneath the eastern Archean block and a regional upwelling beneath the central block which has been imaged as a low velocity anomaly in seismic tomography. Thus we speculate that the subduction of the Pacific Plate, compared to the NCC-Yangtze Craton amalgamation and the India-Eurasian collision, is most likely the geodynamic cause of the reactivation of the NCC eastern block during the Late Mesozoic to Cenozoic.