Effective elastic thickness and mechanical anisotropy of South China and surrounding regions

Abstract

South China and surrounding regions extend from the eastern Tibetan plateau, through the tectonically stable Sichuan basin and the broad Mesozoic magmatic and fold belt, to the trench-arc-basin system in the western Pacific which provide an ideal place to study deformation of the continental lithosphere under long-term magmatism and oceanic subduction. We obtained the effective elastic thickness (Te) and its anisotropy of South China and surrounding regions from the analysis of coherence between topography and satellite gravity using wavelet methods. The Te values of the study area vary from 2 to 75km, with relatively low Te values (≤30km) along the tectonic boundaries, the North–South Gravity Lineament (NSGL) and seismic zones, and in the regions with high surface heat flow. The evenly low Te values in the Lower Yangtze region and the Cathaysia block can be attributed to the long-lived subduction of the Paleo-Tethys and the Paleo-Pacific ocean basins beneath the South China block (SCB). The NSGL in the SCB may separate the unmodified (high Te) and thermally weakened (low Te) continental lithosphere due to oceanic subduction. Despite different distances to the tectonic boundaries, earthquakes occur more frequently in regions with Te values of 10–30km, implying strain concentration in the low-Te regions. A positive correlation between seismic activity and the magnitude of Te anisotropy suggests that a highly anisotropic mechanical structure will promote strain localization and brittle failure in the lithosphere. The poor correlation between the weak axis of Te anisotropy and the dynamic indicators of the present tectonic regime (the shear-wave splitting direction, the maximum horizontal compressive stress direction) confirms that Te anisotropy mainly reflects tectonic inheritance of the continental lithosphere.