Strain Localization and Complex Asthenospheric Flow Around Small-Scale Lithospheric Thickness Variations beneath the Korean Peninsula

The continental lithosphere has undergone long-term structural evolution through interaction with the underlying viscous asthenosphere, and its stability has commonly been attributed to the presence of thick cratonic roots. However, cratonic stability is not absolute, as cratonic keels can weaken or fail under certain conditions, implying episodic reorganization of the continental lithosphere by mantle dynamics. Most previous discussions have focused on interactions between large, laterally extensive cratonic roots extending deep into the upper mantle and the surrounding asthenospheric mantle. In contrast, small-scale thickness contrasts (< 100 km lateral scale) can induce edge-driven convection (EDC), enhancing basal drag and localizing strain, and thus may plays an important role in the long-term evolution of the continental lithosphere.

Seismic anisotropy records interactions between lithospheric deformation and asthenospheric flow. In this study, we measured seismic anisotropy beneath the southern Korean Peninsula (SKP) using shear-wave splitting analysis and compared the observations with numerical mantle flow simulations. The Korean Peninsula, located on the eastern margin of the southeastward-moving Eurasian plate and adjacent to the western Pacific and Philippine Sea plates, exhibits a small-scale (~50 km lateral scale) lithospheric thickness contrast, with a thick lithosphere (~130 km) in the southwest relative to thinner lithosphere (~80 km) in the east. An average delay time of ~1 s is observed across the SKP, with predominantly N–S fast directions in the eastern SKP and NW–SE fast directions in the southwestern SKP. Numerical mantle flow simulations that explicitly incorporate lateral lithospheric thickness variations generate density-driven asthenospheric flow with corresponding N-S and NW-SE directions, consistent with the observed splitting patterns. In addition, both observations and simulations reveal complex anisotropy patterns localized around the thick lithosphere, characterized by rapid lateral changes in fast direction and flow direction around the lithosphere. Such complexity reflects localized mantle flow perturbations and enhances basal shear generated by lateral lithospheric thickness variations. We suggest that asthenospheric-flow-induced basal drag promotes strain localization within the surrounding lithosphere, potentially enhancing basal lithospheric erosion and weakening long-term cratonic stability.