EVIDENCE FOR DEXTRAL TRANSTENSION IN THE SOUTHERN PLOMOSA MOUNTAINS, WEST-CENTRAL ARIZONA

Previous studies in the southern Plomosa Mountains have noted km-scale, NW-striking faults with apparent dextral offset, but little data about the timing and kinematics of these faults exist. To characterize the faulting regime within this region, we conducted detailed geometric and kinematic analyses on measurements of small-scale faults from moderately-consolidated post-Miocene alluvial deposits and Mesozoic bedrock units. Alluvial deposits are poorly sorted and comprised of angular to sub-angular, gravel to boulder-sized clasts corresponding to the local bedrock lithology. Paleocurrent analysis of locally imbricated clasts at two alluvial outcrops indicates an E- to S-directed paleocurrent, consistent with S- to SE-dip of bedding. Our structural data show that faults in alluvium are dominantly NW-striking and steeply dipping dextral and normal faults. Conjugate N-S to NE-SW-striking sinistral faults and oblique-slip faults are also common. Locally, steeply-dipping faults, characterized by planes defined by the alignment of rotated clasts, are present in the alluvium and suggest some faulting occurred prior to lithification. The mean shortening axis (220/sub-vertical) and intermediate kinematic axis (350/sub-horizontal) for faults in the alluvium have similar eigenvector eigenvalues. This is compatible with a transtensional faulting regime that accommodated E-W extension and dextral shear. The kinematic data derived from NW-striking fault zones in the Mesozoic bedrock are complex, but the dominant faulting regime is consistent with dextral slip along N- to NE-dipping faults.

The kinematic data for small-scale faults highlights the occurrence of dextral slip in a region with prevalent NW-striking faults. Syndepositional faulting in the alluvium deposits provides evidence for post-Miocene (Pliocene?) dextral transtension, which is not well understood in this region. The relatively young fault system exhibits a kinematic resemblance to the San Andreas transtensional fault system to the southwest, and may record diffuse intracontinental accommodation of dextral- to dextral-oblique shear associated with Pacific-North American plate motion.