DISTRIBUTED NEOGENE DEXTRAL FAULTING ACROSS ARIZONA’S METAMORPHIC CORE COMPLEXES: SUPERPOSITION OF THE PACIFIC-NORTH AMERICA PLATE BOUNDARY ON THE SOUTHERN BASIN AND RANGE

NW-striking dextral faults are pervasive across the belt of Miocene metamorphic core complexes between the South Mountains and Buckskin-Rawhide Mountains in Arizona. These faults postdate middle Miocene slip along a regional top-NE directed detachment fault system and locally reactivate steeply NE-dipping normal faults and joints formed during core complex extension. Kinematic data from across the region indicate that most dextral faults have shallow (<20°) slickenline rakes, although domains of oblique dextral-normal and dextral-reverse (NE-side-up) faulting are present. Distributed dextral slip along these faults reoriented the topographic trend of core complex corrugations, which trend clockwise of the detachment fault slip direction. In the South Mountains (Phoenix), NW-striking dextral faults account for ~30% of the brittle fault population. The misalignment between the well-defined topographic trend of the range (~062) and the core complex extension direction (~060) suggest that 650±350 m of dextral slip accumulated across the 18 km-long range, largely distributed across faults with cm-scale offset. Dextral shear across the belt of core complexes progressively increases along strike to the NW. Based on the misalignment between the topographic trend of corrugations and the core complex extension direction, the Harquahala Mountains record ~2–4 km of dextral slip across ~31 km, and the Buckskin-Rawhide Mountains record ~7–8 km of dextral slip across ~36 km. This zone of distributed dextral faulting is along strike with the Stateline fault system to the NW, which is considered the northeastern margin of the Eastern California Shear Zone. The superposition of dextral shear across the core complexes is likely related to the diffuse Pacific-North America plate boundary, which began to influence deformation in the southern Basin and Range after ~12 Ma.