Interpretation of deformation history based on the structures and characteristics of fracture-controlled dykes

Dykes are injected into host rocks either vertically or laterally. Several case studies suggest that dyke injection and dyke induced faulting play major roles in shaping the volcanic/tectonic process and surface morphology in a particular area. The processes and sequences associated with magmatic emplacement can be understood based on the geometry and kinematics of dike-induced fractures. In this preset study, an attempt has been made to understand the relationship between dyke intrusion and associated faulting for a 6 km long strike-slip fault around the southern part of the Gyeongju city in SE Korea. During the ongoing paleoseismic investigation along the fault, named Chongundong Fault, we observed that the fault is associated with injection of dykes on several sections along the fault. To understand the relationship between dyke injection and faulting in more detail, two trench sections, showing dyke injection and displaced Quaternary layers, were analyzed.

From the 10m long and 4m deep trench section, six sets of dyke were observed, which intruded into the hornblende granite (bedrock). A few cm thick of fault gouges are developed along the boundaries of the dykes, and the Quaternary deposits are displaced along these faults. Some fracture sets show shear component, which cut the dykes with a trend of N118°/35°S. Also, some dykes are injected along a fracture set with sill geometry. Based on the structural analysis at this site, the dykes were mainly controlled by fracture sets. Similarly, the other fault perpendicular trench site shows four dykes and well developed fault gouges along the boundaries between dykes and the bed rock. Deformed dykes are only recognized in the upper part of hanging wall, indicating post-intrusion fault reactivation. Based on the structural analysis, it can be interpreted that the dyke was intruded into the early developed fault zone and the fault was reactivated at a later stage, probably during the Quaternary. On the basis of preliminary analysis of fracture system along this fault, cross-cutting relationship of dykes and related faults, we propose that the deformation history in this area is as follows: early fault and fracture sets development → dyke intrusion along the pre-existing fractures and fault → deformation of the dyke by fault reactivation cutting the Quaternary deposits.