Direct dating of Eocene reverse faulting in northeastern Tibet using Ar-dating of fault clays and low-temperature thermochronometry

Paired together, fault gouge dating and low-temperature thermochronometry overcome the limitations and assumptions inherent in each independent technique. Here we establish timing of brittle faulting along the West Qinling fault of northeastern Tibet by dating several size fractions of fault gouge clay that represent variable populations of illite polytypes. Results show that the authigenic or fault-generated component of illite formed at 50 ± 8 Ma and that the detrital component formed at 236 ± 7 Ma indicating a Middle Eocene age of faulting and a Middle Triassic age of the wall rocks. Comparing this dataset with published thermochronology from hanging wall rocks supports the interpretation that the West Qinling fault initiated at ~ 50 Ma and continued until at least Middle Miocene time and that authigenic clay growth occurred at ambient temperatures of ~ 110 °C. Lack of overprinting of younger clay ages at this site may indicate that rocks were out of the thermal window for authigenic clay formation during later faulting episodes. The potential for temperature to control illite growth has implications for interpretation of authigenic illite ages and their relationship to deformation episodes within fault zones.