Internal Structure and Kinematic Evolution of the Hambaeksan Fault, Taebaeksan Basin, South Korea

The Hambaeksan Fault is a major structural feature in the Taebaeksan Basin, South Korea, characterized by a significant right-lateral strike separation of approximately 3–5 km. Although it truncates the Baekunsan Syncline, the precise timing and kinematic history of the fault remain poorly understood. This study investigates the internal structures, fault rock characteristics, and kinematic indicators of the Hambaeksan Fault through field observations and microstructural analysis at the Sorotgol road outcrop in Taebaek.

At the study site, the fault juxtaposes the limestone of the Duwibong Formation against the shale of the Geumcheon-Jangseong Formation. The fault zone strikes N-S with a 65°E dip, comprising a ~30 cm thick fault core and a damage zone exceeding 10 m. The fault core consists of limestone breccia, shale breccia, and mixed breccia. Linear structures within the shale breccia (N5°E, 10°) confirm a dominant strike-slip movement.

Microstructural analysis reveals a complex deformation history characterized by several distinct features. A well-defined, linear Principal Slip Zone (PSZ), approximately 300 μm thick, is developed within the shale breccia and sharply truncates earlier clasts. Within the mixed breccia, the presence of clay-rich matrices exhibiting fluidized textures and injection structures suggests the occurrence of seismic slip, possibly involving rapid fluidization. Furthermore, kinematic overprinting is evident in the shear bands adjacent to the PSZ, where sinistral shear senses are superimposed on earlier dextral shear senses. These observations indicate that the Hambaeksan Fault has experienced a multi-stage evolution, beginning with a primary dextral strike-slip movement followed by localized subsequent deformation within the mechanically weaker shale units.

These findings suggest that the Hambaeksan Fault underwent at least one episode of seismic slip along the lithological boundaries (mixed breccia) prior to the formation of the current PSZ in the shale unit. The observed kinematic overprinting indicates that after the initial juxtaposition of the formations via dextral strike-slip movement, subsequent deformations were localized within the mechanically weaker Geumcheon-Jangseong shale. This study provides critical insights into the seismic behavior and structural evolution of major crustal faults in the Korean Peninsula.