BRITTLE FAULTING IN THE SOUTH MOUNTAINS METAMORPHIC CORE COMPLEX, PHOENIX, ARIZONA: EVIDENCE FOR DISTRIBUTED DEXTRAL SHEAR ASSOCIATED WITH THE PACIFIC – NORTH AMERICAN PLATE BOUNDARY

The South Mountain metamorphic core complex in Phoenix, Arizona is dominated by variably mylonitized crystalline rocks exposed below a Miocene extensional detachment fault. We examined brittle faults in three different lithologies near Telegraph Pass in central part of the range to understand the evolution of Neogene brittle deformation in the core complex. Altogether we measured 244 fault planes along with 237 slickenline sets. We were able to determine the sense of slip on 104 sets of slickenlines using kinematic indicators such as offset markers, Riedel shears, T-fractures, and slickenfiber steps, and we assumed a sense of slip on 46 faults based on close geometries and proximity to faults with a known sense of slip. Approximately 70% of measured faults strike in the NW-SE quadrants, and >90% of faults dip >45˚. Many of these faults appear to have reactivated extension fractures, which are abundant and have a similar geometry. Of the faults with a determined slip sense, ~48% are NW-striking dextral faults, and ~5% are N-striking sinistral faults that we interpret to be conjugate to the dextral faults. NW-striking normal faults account for ~28% of the measured fault population. These normal faults accommodate NE-SW extension parallel to the core complex extension direction. NW-striking reverse or high-angle dip-slip faults with a NE-side-up sense of slip account for ~11% of the fault population. Approximately 72% of the reverse/NE-side-up faults also have a dextral component of slip. We interpret the dextral faulting and reverse/NE-side-up faulting to postdate middle Miocene detachment faulting. Dextral faulting may account for the ~3˚ misalignment between the orientation of the range (N63E) and the orientation of the core complex extension direction (N60E), which would indicate 0.5-1.0 km of total dextral shear across the ~18-km-long range. Distributed dextral shear in the South Mountains is most likely associated with the Pacific-North American plate boundary. At a distance of ~300 km from the San Andreas fault, the South Mountains may represent the easternmost documented extent of dextral shear associated with this plate boundary.