Strong intracontinental lithospheric deformation in South China: Implications from seismic observations and geodynamic modeling

摘要

Classical plate tectonics theory predicts concentrated deformation at plate boundaries and weak deformation within plates. Yet, the existence of intracontinental orogens shows that highly deformed regions can occur within continental plates, which is geodynamically incompletely understood. Shear wave splitting measurements in South China show belt-parallel (i.e. NE-SW) fast directions beneath the Wuling-shan-Xuefengshan Belts, while no dominant fast direction is found in the cratonic Sichuan Basin. Tomographic studies in the mantle in the same area show that the thickness of lithosphere beneath the intracontinental orogen is larger than that beneath the cratonic Sichuan Basin. In order to better understand these seismic observations, we performed numerical experiments of intracontinental lithospheric deformation with the presence of cratonic basin. By comparing systematic model results with topography data and constraints on the accumulated amount of shortening in the study region, we find that the best-fitting result is obtained for models with a strong upper and a wet lower crust and with a weak mantle lithosphere beneath the orogen belt, which has an effective viscosity that is 2 orders of magnitude smaller than that of the craton. Our numerical model successfully explains the belt-parallel anisotropy and high velocity anomaly beneath intracontinental orogen and suggests that significant intracontinental lithospheric deformation occurred in South China.