A Sharp Melt‐Bearing Lithosphere–Asthenosphere Boundary Beneath the Korean Peninsula: Evidence From Teleseismic Receiver Functions

Abstract The lithosphere–asthenosphere boundary (LAB) provides key constraints on how continental lithosphere responds to tectonic processes. Located between the East Asian cratonic blocks and the western Pacific subduction system, the Korean Peninsula (KP) provides an important setting to examine continental lithospheric evolution, yet seismic constraints on its LAB structure remain limited. We use P‐ and S‐receiver functions to investigate crustal and lithospheric structure beneath the southern KP and adjacent volcanic islands. Beneath the southern KP, the LAB lies at ∼70 km depth and is marked by a sharp transition zone (<20 km) and a pronounced shear‐wave velocity ( V S ) reduction (7%–10%), most plausibly explained by partial melt near the LAB. Thermodynamic modeling predicts melt fractions near the LAB, consistent with independent estimates from V S reduction. These observations suggest that the southern KP represents the eastern continuation of a tectonically modified domain formed through long‐term subduction‐related thinning and removal of continental lithosphere. This modification varies laterally within the southern KP, where the southeastern block shows a deeper Moho, shallower LAB, and higher crustal P‐to‐S wave velocity ratio ( V P /V S ) than the northwestern block, indicating lithospheric overprinting by magmatic or thermal processes. Jeju Island retains inland‐like Moho and LAB depths, but shows complex V P /V S variations and a broad lithosphere–asthenosphere transition. In contrast, Ulleung Island exhibits a shallow Moho, low V P /V S , and an exceptionally shallow LAB, reflecting additional lithospheric modification associated with Miocene backarc extension. Our results link the present‐day lithospheric heterogeneity to the multi‐stage tectonic evolution of the eastern Eurasian margin.