Characteristics of Brittle Fault Zone and Fault-related Rocks

Abstract

Generally, fault zone is composed of wall rock, damage zone and fault core. Fault core consist of fault rocks such as fault gouge and breccia with random fabric and grain size reduction. Fault rocks are considered to arise from concentration of strain within a shear zone. Fault rocks from different depths in the crust will record different mechanisms. So, fault rock studies are very helpful for evaluation of the fault zone characteristics. In this study three sites including Igidae, Wolseong and Daeheuksan Island are examined in the aspects of fault zones and fault-related rocks in brittle condition. The first site, Igidae, was undertaken to understand the Cenozoic deformation history. The area is composed of andesitic rocks from the Yucheon group, which are in turn part of the Gyeongsang supergroup. Developing relationships between fractures were examined from well exposed wave-cut outcrops. Based on cross-cutting relationships, interpretation of tree structures and veins, the fractures developed in this area have the following deformation history: (Ⅰ) E-W extension → (Ⅱ) NE-SW compression → (Ⅲ) NW-SE compression → (Ⅳ) WNW-ESE compression → (Ⅴ) ENE-WSW compression. Comparison with the previous studies indicated that the Igidae area experienced a very similar paleostress and deformation history to the other areas in the eastern part of the Yangsan fault. The second site, Wolseong, examines mechanisms that resulted in asymmetric fault breccia patterns in Cretaceous sedimentary rocks of the Gyeongsang supergroup in southeastern part of Korea. Several reactivated faults are present, including a 1 km-long, 2 m-thick, N-S striking fault zone that shows various fault gouge bands and S-C fabrics that indicate discrete stages of normal displacement during SE extension and was later reactivated to reverse displacement during NNW compression. Major fault sets do not cross-cut each other; instead, the fault gouges within the fault zone split and merge into other fault zones. The braided fault patterns are generally symptomatic of mature fault systems. Breccias and gouges within the brittle fault zones have irregular patterns, which include lens-shapes blocks, and asymmetric distributions of grain size and fracture density across the fault zone, suggests a mechanical model that involved several episodes reactivation.. The third site, Daeheuksan Island, was undertaken to understand the characteristics of fractures that filling the pseudotachylytes. Daeheuksan Island is composed of quartzite from the Okcheon supergroup, Precambrian. Generally, pseudotachylyte is the rock which is related with the earthquake characteristic fault. These rock are not reported in Korea before. But, the inferred materials were detected in Daeheuksan Island. Pseudotachylytes in this area were injected in fault plane, secondary fractures and vicinity fractures. Based on fracture relationship and interpretation, the fractures developed have the following deformation history: (Ⅰ) NW-SE compression → (Ⅱ) NNW-SSE compression → (Ⅲ) E-W compression. The microstructures of pseudotachylyte are represented by a sperulite that generally morphological features of pseudotachylyte. The overall architecture of fractures that kinematic features, development orientation of fault and fault rock fabrics are play an important role in understands about dynamical and tectonical character that ground stability and fault activity related to major industries facilities. Also, useful method in controlling fluid flow and exploration of resources within faulted and fractured rock volumes.