The Influence of Clay Authigenesis on the Mechanical and Hydrological Properties of the Moab Fault, Utah

Understanding the development of clay-rich fault rocks is essential for understanding the mechanical and hydrological properties of faults. Algorithms such as the shale gouge ratio (SGR) or clay smear potential (CSP) have been used to characterize the incorporation of protolithic clays into fault rocks, however they do not account for fault-related clay authigenesis, which appears to be almost ubiquitous along the Moab fault.

A portable X-ray fluorescence device was used to look for qualitative enrichments/depletions in fault rocks relative to host rocks, a proxy for the presence of fluids. Fault rocks an ~ 30 km along strike section of the fault with throws from 60 to 900 m (R191, Corral Canyon, Courthouse Canyon, and Bartlett Wash) show enrichments or depletions in Rb and Sr. The presence of these chemical trends indicates that the fault rocks are not simply mechanically-disaggregated protolith.

Authigenesis at the R191 location is characterized by the formation of illite in the gouge and illite-smectite in the smear. Friction tests show that as fault-related clays increase the coefficient of friction decreases, suggesting that authigenesis influences fault strength. The variation in the fault-related mineralogy is significant as illite and montmorillonite exhibit different frictional properties. This suggests that authigenesis may also influence the failure mechanism of faults.

Both natural and experimental slip surfaces show that preferred orientation is extremely localized, extending at most a few tens of micrometers away from the slip surface. More significantly, natural slip surfaces (the product of mechanical and chemical processes) tend to be more continuous that their experimental analogs (the product of mechanical processes alone), suggesting that clay authigenesis may enhance the seal capacity of clay-rich fault rocks.