FAULT ROCK GENERATION, FRICTIONAL PROPERTIES, AND PERMEABILITY IN FAULT ROCKS OF THE MOAB FAULT, UTAH

The mechanical and hydrologic evolution of fault rocks is strongly dependent on rock type and deformation mechanism. In the Moab fault, Utah, fault rocks were derived from sandstone and shale. Fault rocks in sandstone are associated with cataclasis of quartz and feldspar in either deformation bands or due to abrasion of joint surfaces reactivated in shear. Fault rocks in shale are associated with: 1) Shale smear which consists of pervasive ductile deformation that attenuated and entrained shale into the fault zone but largely preserved sedimentary layering and 2) Clay gouge generated by abrasion of shale, typically against slip surfaces in sandstone, which also contains with authigenic clays. The frictional strength (μ) and permeability (k) of samples of these fault and host rocks were measured at effective confining pressures of 10 - 40 MPa consistent with the burial depth during faulting at ~60 Ma. In the Jurassic aeolian sandstone, μ ranges from 0.71 to 0.91. k of experimentally deformed sandstone varies between units from ~9E-18 to ~120E-18 m². In clay-rich fault rocks, μ is ~0.39 in the shale smear and ~0.37 in the clay gouge. k in the clay gouge is ~3E-21 m² at 40 MPa confining pressure. The fault rocks derived from sandstone have remained distinct from those derived from shale. These clay-rich fault rocks are present in all exposures of the fault juxtaposed or offset past shale. This partitioning and apparent continuity suggest that once developed, strain is concentrated in clay-rich fault rocks as expected from their relatively low μ. Authigenesis of clays in the gouge represents continued development beyond mechanical disruption and grain size reduction that promotes clay gouge continuity and changing μ and k. Precipitation of minerals such as calcite that contribute to the regeneration of fault zone strength (by increasing cohesion and μ) and reduce permeability (by filling pores) are largely restricted to relays and intersections between segments of the Moab fault subjected to increased dilatancy evidenced by joints and breccia. Thus, in the damage zone μ and k vary strongly due to the influence of distinct deformation mechanisms in both rock types. In the fault core, it appears likely that μ and k evolved to a steady state dominated by clays that maintained seal integrity and fault weakness over geologic time spans.