Abstract

The Yangsan Fault is known to have experienced kinematic changes due to tectonic stress variations since its formation in the Late Cretaceous. However, direct kinematic evidence based on the microstructure of fault rocks is rare. In this study, we analyzed the fault rocks of the Yangsan Fault using core samples collected from Yeonghae-eup, Yeongdeok-gun, in Southeast Korea, and aimed to obtain kinematic information through observations of rock slab-scale and micro-scale structures. In the inclined core samples with a length of 73.2 meters, fault rocks appear over a width of approximately 25 meters, consisting of foliated cataclasite derived from gneiss and foliated gouge derived from sedimentary rock. Rock slab-scale observations revealed dextral shear deformation in the cataclasite and sinistral shear deformation in the fault gouge. Micro-scale observations identified a principal slip zone approximately 1 mm wide, along which three units with different foliation orientations were found, indicating a dextral-sinistral-dextral sequence of slip sense changes. These findings provide geological evidence of slip-sense reversals in the northern segment of the Yangsan Fault, alongside similar reversals observed in the Bogyeongsa area, Pohang, approximately 30 km from the study area, and the identified sequence of slip senses is crucial for establishing the kinematic history of the Yangsan Fault.

Keywords Yangsan Fault, slip-sense inversion, Yeonghae, fault rock, principal slip zone