Dating fault rocks using optically stimulated luminescence: Insights from the northern Yangsan Fault, Korean Peninsula

Abstract

For decades, the luminescence dating of fault rocks has been challenging, mainly because it is difficult to prove whether luminescence signals of silicate minerals in fault rocks are effectively reset during fault slip. Although several previous studies have reported the potential of OSL (optically stimulated luminescence) dating of fault rocks with simulated or natural samples, its practical applicability for constraining the timing of fault slip remains in question. As a part of these ongoing attempts, we investigated the luminescence dating of fault rocks exposed at the Bogyeongsa site, located in the northern Yangsan Fault, southeastern Korean Peninsula, where previous researches suggested the most recent faulting occurred during the late Pleistocene - Holocene. Based on the macro- and microscopic observations, the fault rocks were classified into CZ (Cataclasite Zone), PSZ (Principal Slip Zone) and BZ (Brown gouge Zone). Samples from PSZ and BZ showed quartz De (equivalent dose) values below 2D0 (twice the characteristic dose, usually taken as the upper limit for reliable estimation of De values, thus luminescence ages), whereas the polymineral pIRIR290 signals (post-IR IRSL signals measured at 290 degrees C) remained saturated. In contrast, for the CZ, both quartz post-IR blue and polymineral pIRIR290 signals were below dose saturation level. While the quartz De values are significantly higher than the usual dose saturation level for quartz (- 150-200 Gy) and may represent experimental artifacts, the polymineral pIRIR290 De value of 500 f 30 Gy obtained from the CZ is well below saturation level (fraction of saturation = 0.67 f 0.02), yielding the age of 119 f 8 ka. Supported by both signal behavior and model simulation, this seems to indicate that the polymineral pIRIR290 signal of the CZ has been reset to some extent during at least one fault slip since -120 ka.