Structures and deformation characteristics of the active fault, Hwalseongri area, Gyeongju, Korea

Abstract

The active fault that displaces the unconsolidated Quaternary sediments can develop across different materials, such as inside bedrock, between bedrock and unconsolidated sediments, and inside unconsolidated sediments. We conducted structural observation and material analysis on the active fault, Hwalseongri area, Gyeongju, to understand how the structure and deformation characteristics can vary depending on the location along the fault. At the place where the active fault is observed inside the bedrock (Location 1), fault gouge rich in clay minerals occurs in the bedrock's cataclasis zone, and a 10-20 mu m-wide principal slip zone develops inside it. Next, where the fault develops between the bedrock and the Quaternary sediment (Location 2), a protocataclasis zone consisting of breccia showing weak cataclastic foliation is observed in the bedrock. Both deformed weathered gravels and the preferred orientation of some minerals are identified in the Quaternary sediment. At Location 3, where the fault is observed inside the Quaternary sediment, the weathered gravels and the preferred orientation of minerals are identified, as in the sediment at Location 2. However, such structures appear weak and do not extend to the top and end up within the sediment. The above observations indicate that the fault materials are deformed by frictional sliding (e.g., along the clay minerals) or by cataclasis and cataclastic flow in the bedrock's slip zone at depths and that unconsolidated sediments are deformed by granular flow at the very shallow depth. Fault slip is localized along the principal slip zone of the clay-rich gouge in the bedrock. In contrast, shear deformation may occur in a more distributed manner, while passing through the unconsolidated sands and gravels, and eventually terminated. This aspect should be considered when estimating the timing of active fault movement based on sediments' absolute age and whether the fault crosscuts the sediments.