Core-Refracted Shear-Wave Anisotropy beneath the Korean Peninsula: Insights into its Tectonic Evolution

The tectonic history of the Korean Peninsula includes the Permo-Triassic collision between the North and South China blocks and the subsequently opening of the Yellow and East seas during the Late Oligocene and Miocene. Due to the lack of evidence and based on different geological and geophysical data, several models and mechanisms have been proposed to explain how the collision and openings happened. We studied seismic anisotropy from core-refracted shear-wave splitting to place constraints on lithospheric-scale and upper mantle structures and dynamics and provide insight into the tectonic evolution of the Korean Peninsula. We implemented the eigenvalue-based method to measure the splitting parameters and used the transverse energy minimization and cross-correlation techniques to validate our results. We found delay times ~1.4 s which is consistent with anisotropy residing in the asthenospheric and/or lithospheric mantle. Our results strongly suggest that the anisotropy signature of past tectonic events have been preserved and that the upper asthenosphere and lithosphere have undergone coherent deformation. Based on our model, we interpret that the Hongseong-Imjingang belt is part of the collision boundary, since we observed a lateral variation of the splitting parameters coinciding with it. We suggest two possible scenarios for the continuation of this collision suture: (1) one offshore with the boundary coinciding with the West Marginal Fault Zone, and (2) another one onshore along the southern limit of the Gyeonggi massif, going from the Hongseong to the Odesan belt. Our observations along the east and west coasts support a fan-shaped opening mechanism for the East Sea and an eastward post-collisional extension for the Yellow Sea, respectively. The fan-shaped opening mechanism, which implies a clockwise rotation of the Japanese Islands away from the Korean Peninsula, appears to have occurred in two stages: an approximately E-W rifting followed by a N-S spreading. Lastly, our splitting observations beneath the western Gyeonggi and Yeongnam Precambrian massifs appear to be in good agreement with a possible fossil anisotropy. The fast axes observed for the former might reveal the true direction of motion of the Nort China Block, while the ones observed for the latter appear to have been affected by post-collisional tectonic episodes since they are not parallel to the infer direction of motion of either the North or South China blocks.