SYNCING FAULT ROCK CLOCKS; DIRECT COMPARISON OF U-PB CARBONATE AND K-AR ILLITE FAULT DATING METHODS

The timing of slip on brittle faults in the Earth’s upper crust is difficult to constrain and is commonly found by either indirect methods or by dating fault-generated materials using radiometric dating. Both methods often yield results that can be difficult to interpret. Here we make the first direct comparison between K-Ar dating of fault gouge clay (authigenic illite) and U-Pb dating of carbonate slickenfibres and veins from the same fault, the Big Creek fault, a NW-striking, dextral strike-slip fault system in Yukon Territory, Canadian Cordillera. Both methods yielded dates at ~73 Ma and ~60–57 Ma, representing at least two periods of fault slip which form part of a complex fault and fluid-flow history. The Cretaceous result lies within indirect estimates for major slip on the fault. The Paleocene-Eocene result coincides with the estimated timing of slip of the nearby Denali and Tintina faults, large-scale, NW-striking dextral faults, indicating Big Creek fault reactivation during regional faulting. The coincidence of periods of carbonate-crystallizing fracturing and fluid flow with intervals of seismic, gouge-generating slip supports the fault valve model where fault strength is mediated by fluid pressures, and fluid emplacement requires seismic pumping in otherwise impermeable aseismic fault zones. The reproducibility of slip periods for different fault-generated materials using distinct decay systems indicates that these methods provide complimentary results and can be reliably applied to date brittle fault slip, opening new opportunities for investigating fault conditions with associated mineralizing fluid events.