ESR dating of the Wangsan fault, South Korea

Abstract

The Wangsan fault is exposed at Kyeongju, Korea. The andesite is unconformably covered by Quaternary alternating conglomerate and sandstone deposits. The unconformity is cut by the thrust fault which displaces a hanging wall block of about 30 m. Exposed at the surface of this fault is a light gray and brown fault gouge, about 40–120 cm thick. In order to test the consistency of ESR ages from a single fault gouge zone, we collected six gouge samples systematically along and across the Wangsan fault. We found that six samples collected from the same gouge zone show consistent ESR date estimates (average 550 ka). Because fault rock is rare along faults in unconsolidated sediments due to low confining stress near the surface, we consider that this fault gouge has been moved up with the hanging wall block along the fault. The estimated average uplift rate of the hanging wall block is about 0.04 cm/year based on the age of the displaced Quaternary deposits (vertical separation; about 20 m) dated by OSL dating methods. The depth of the fault gouge at the time of reactivation, which was estimated from uplift rate and the ESR ages, is about 220 m. Therefore, we conclude that the results of ESR age estimates represent the time of reactivation of the fault gouge at a depth of 220 m in the past during fault movement, because later movements which occurred during uplift near the surface may not have zeroed ESR signals significantly for ESR dating of fault movements.