Seismic anisotropy evidence for modified lithosphere below the Bohai Sea region, eastern North China Craton

The Bohai Sea is proposed as the center of deformation associated with destruction of the North China Craton, and its tectonic history is essential for understanding the response of craton evolution to oceanic subduction. However, interior deformation of the craton remains unclear due to a lack of high-resolution imaging in this region. Here, we present high-resolution 3-D azimuthally-anisotropic shear-wave velocity structure of the crust and uppermost mantle using direct inversion of Rayleigh-wave phase-velocity dispersion from ambient noise. The anisotropic velocity structures reveal significant consistency between the regional strain field and seismic activity in the shallow crust. The N-S fast direction and relatively high velocity anomalies observed in the uppermost mantle beneath the Tanlu fault zone, as well as prominent Moho uplift, support the conclusion that the Tanlu fault zone has facilitated lithospheric modification of the eastern North China Craton since late Mesozoic. Anisotropy images also indicate that the northern and southern Bohai Bay Basin have undergone different deformation histories. The strong and consistent azimuthal anisotropy within the uppermost mantle beneath the southern Bohai Bay Basin may originate primarily from re-orientation arrangement of the olivine induced by new lithosphere formed from rapid cooling of upwelling asthenosphere after lithosphere delamination due to the subduction of the (Paleo-) Pacific Plate.