The effect of sub-seismic fault slip processes on the isotopic signature of clay minerals – Implications for K-Ar dating of fault zones

Clay minerals are abundant in fault gouges and isotope dating of the radiogenic noble gases that they contain may be used to constrain timing of fault initiation, reactivation and to estimate recurrence intervals of earthquakes. The potential influence of fault slip and ambient temperature on the isotope signature of authigenic clay minerals, however, remains unknown. This study reports measurements of the K-Ar isotope signature of samples that have been sheared experimentally at sub-seismic slip rates within a range of ambient temperatures, as well as of samples undergoing standardised clay heating degas experiments in the investigation and comparison of the influence of sub-seismic frictional slip and external heating on clay gouge. The age data confirm that the sub-seismic frictional slip of clay gouge affects the clay microstructures and releases radiogenic 40Ar. The experimental data indicate that radiogenic 40Ar release due to external heating can be disregarded until ~350 °C, which is supported by Ar diffusion modelling. Frictional slip at sub-seismic slip rates and at room temperature results in a significant radiogenic 40Ar loss of ~56% and the remaining clay will inherit ~44% of radiogenic 40Ar. Sub-seismic frictional slip experiments at imposed elevated temperatures from 150 to 450 °C increase the amount of radiogenic 40Ar loss and decrease the amount of inherited radiogenic 40Ar. At a relatively high temperature of 450 °C, about 16% of radiogenic 40Ar will be present in the remaining sample portion, reinforcing the importance of rigorous clay grain size separation processes required prior to fault gouge dating.