Clay quantification and Ar–Ar dating of synthetic and natural gouge: Application to the Miocene Sierra Mazatán detachment fault, Sonora, Mexico

Direct dating of brittle fault rocks has been predicated on the assumption that illite in fault gouge is a mixture of two populations of clays: one detrital, derived from the wall rock, and the other authigenic, forming in the brittle fault zone during faulting. Owing to complex diagenetic histories of wallrock shales in previous fault-dating studies, this assumption has remained largely untested. We demonstrate the validity of our clay quantification technique using calculated WILDFIRE© patterns to accurately model both artificial mixtures of 2M1 and 1Md illite, and natural mixtures in young fault gouges. Using our quantification approach, we obtain well-defined detrital and authigenic ages from the Sierra Mazatán metamorphic core complex in Sonora, Mexico. Because the age population of the wall rock is well constrained by cooling ages, we confirm that the detrital component in the gouge is the age of the footwall granite, while the age of the authigenic component is the time at which faulting ceased, which is in agreement with field constraints. Based on the Ar–Ar age and the thermal conditions for the growth of the authigenic illite we also conclude that the Sierra Mazatán detachment continued slipping in the brittle regime at strain rates similar to those for the plastic regime (on the order of 10−14 s−1).