Lithospheric Deformation and Destruction of North China Craton Revealed by Ambient Noise and Earthquake Surface Wave Array Tomography
Abstract:The lithosphere of North China Craton (NCC) has undergone significant thinning since the Mesozoic. However, it still remains unclear about the geodynamic causes of the destruction of NCC as well as the patterns of lithospheric deformation. Here we use both ambient noise and earthquake surface wave data from a dense array (with about 200 stations) to investigate the lithospheric structure and azimuthal anisotropy in NCC. Since surface wave tomography can better reveal depth-dependent anisotropy (Yao et al., 2010), it provides essential constraints on deformation patterns at depths and evolution history of the crust and upper mantle in NCC.
At short and intermediate periods (e.g., T < 50s), we perform ray-based eikonal tomography (Lin et al., 2009) using phase traveltimes from both ambient noise (T = 5 – 40 s) and earthquake surface wave data (T = 20 – 50 s) since the effect of finite frequency is not very important. However, at long period (T > 50s), the finite frequency effect of surface wave propagation is no longer negligible, therefore we perform Helmholtz tomography (Lin & Ritzwoller, 2011) for earthquake surface wave data with both phase and amplitude information used in the tomography.
Apparent depth-varying azimuthal anisotropy is observed in NCC with very different fast directions in the upper crust and uppermost mantle. Both upper crust and uppermost mantle in NCC exhibit strong azimuthal anisotropy, while much weaker azimuthal anisotropy but strong radial anisotropy (Cheng et al., 2013) exists in the middle-to-lower crust. The fast polarization axes in the upper crust seem to be controlled by the orientations of major fault systems and strikes of mountains and rifts. Deformation of the middle and lower crust is probably affected by the extension and destruction of NCC in late Mesozoic. The fast axes in the upper mantle, as inferred from surface wave tomography and shear wave splitting, seem to be consistent with the absolute plated motion. These results will help us better understand the patterns of deformation in the crust and upper mantle in NCC as well as the causes of the destruction of NCC.