EVIDENCE FOR LATE MIOCENE TRANSTENSION WITHIN THE BILL WILLIAMS MOUNTAINS AND CASTANEDA HILLS, WEST-CENTRAL ARIZONA

Post–12 Ma deformation within the lower Colorado River region is likely the result of diffuse dextral transtension. One area of documented dextral strain is the Buckskin-Rawhide system superimposed along the west-central Arizona metamorphic core complex belt. Deformation here has been postulated to connect with the ≤13 Ma Stateline fault system that comprises the northern boundary of the Eastern California shear zone, though how strain connects between both systems remains speculative. In an attempt to decipher this connection, geologic observations, geometric and kinematic fault data, and paleostrain analyses on minor and macro scale faults are presented from the Mohave Wash area in the Bill Williams Mountains and Castaneda Hills. Key units within the study area record the latest phases of detachment-related extension (ca. 12 Ma) to latest Miocene or Pliocene diffuse deformation (≤7 Ma). Published mapping and results show that the Bill Williams fault zone (BWFZ) is a NW-SE striking dextral feature, interpreted here to be a continuation of the Buckskin-Rawhide system. To the north, the NNW-SSE striking Little Black Mountain fault (LBMF) displays dextral shear criteria, and is interpreted to have splayed off the BWFZ. A tilted 10 Ma basalt surface is evident between both faults. At its northern terminus, the LBMF is observed to curve and transition into a N-striking normal fault. Faulting north of this terminus is characterized by E-W extensional paleostrain taken up on N-S striking normal faults, some of which cut ≤7 Ma basalt. Data at the northernmost part of the study area shows NW-SE striking dextral slip and a prevalence for NW-SE striking faults. Therefore, it is postulated that the northwestern end of the Buckskin-Rawhide system transferred strain northward via an extensional corridor coincident with Mohave Wash. Continuation of dextral strain 55 km further northwestward to the Needles deformation zone, a plausible dextral feature with Quaternary motion, is considered based on its along-strike geometry and geophysically determined faults. Results have implications for the northwestward continuation of dextral strain toward the Stateline fault, its deformational style, and the character of diffuse transtensional deformation inboard of the Pacific-North America plate boundary since the late Miocene.