Investigation of quartz luminescence properties in bedrock faults: Fault slip processes reduce trap depths, lifetimes, and sensitivity

Abstract

Quantitative constraints on the timing and temperatures associated with Quaternary fault slip inform earthquake mechanics and seismic hazard analyses. Optically stimulated luminesce (OSL) and thermoluminescence (TL) are tools that can provide these constraints from fault gouge and localized slip surfaces. This study investigates the quartz luminescence properties of five 2-mm-thick slices of rock as a function of distance perpendicularly from a discrete, m-scale mirrored fault surface that cuts quartz-rich conglomerate along the Hurricane fault, UT, USA. We use pulsed annealing linearly modulated OSL experiments to determine the response of OSL signals to annealing temperatures. Results were used to estimate trap depths and trap lifetimes. We also calculated changes in OSL and TL sensitivity across the fault-perpendicular transect. All five subsamples show a strong initial fast component peak following annealing steps of 200–300 °C, which is absent following higher pre-heat steps of 320–420 °C. The fast component trap lifetimes and depths indicate they are stable over the Quaternary and suitable for OSL dating. Data exhibit increasing trap depth, trap lifetime, and sensitivity with distance from the fault surface. We suggest mechanical processes, fluids, and/or elevated temperatures during seismicity work constructively to transform fault materials and affect the quartz luminescence properties at a mm-scale from this fault surface. Results highlight the importance of assessing the scale of fault-related impacts on host rock and luminescence properties when applying trapped-charge techniques to recover fault-slip chronologies and/or paleotemperature information.