3-D crustal shear wave velocity and azimuthally anisotropic structure of the Pearl River Delta onshore-offshore area and its tectonic implications

Abstract

The Pearl River Delta (PRD) onshore-offshore area lies in the transition zone between South China and northern continental margin of the South China Sea. To better understand the tectonic and deformation patterns at depth, we developed a 3-D high-resolution crustal shear wave velocity (Vs) and azimuthal anisotropy model by ambient noise tomography in this region. The data sets comprise ∼30 days of seismic ambient noise recorded by 124 land seismic stations and 28 Ocean Bottom Seismometers (OBS). Our results reveal a significant lateral contrast in azimuthal anisotropy in the crust. The fast axes of the azimuthal anisotropy in land show an approximately NW-SE direction, which may be mainly controlled by the regional stress field. In the offshore PRD, however, the fast axes of anisotropy strike nearly in the NE-SW direction, likely dominated by the shearing of the littoral fault zone. Combined with other previous results, the anisotropic model reveals three major crustal deformation processes in this region since the Yanshanian. Furthermore, an upper-middle crustal frozen mafic magma chamber with high Vs (∼3.85–4.0 km/s) is imaged beneath Shenzhen, Zhuhai, and Hong Kong. It is interpreted as a source of intense magmatic activities in and around Hong Kong during the late Mesozoic. Our new model provides important constraints for understanding the crustal architecture and the multistage deformation processes in and around the PRD onshore-offshore area.