OROCLINAL BENDING AND STRAND ABANDONMENT ALONG THE PINTO MOUNTAIN FAULT ZONE—IMPLICATIONS FOR FAULT INTERACTION WITHIN THE EASTERN CALIFORNIA SHEAR ZONE, SOUTHERN CALIFORNIA

The 105-km-long sinistral Pinto Mountain fault zone (PMfz) has long been recognized as the structural boundary between dextral faulting in the Mojave Desert and San Bernardino Mountains and sinistral faulting in the rotated eastern Transverse Ranges, and are important elements of the Eastern California Shear Zone (ECSZ). Two curious features distinguish the PMfz from many typical fault zones: a 1) prominent curved trace in the central part and 2) ~35-km-long abandoned strand that appears be to an eastern remnant of an older PMfz. Also notable is the pattern of dextral fault activity north of the PMfz. From west to east, faults are generally 1) inactive, 2) active interspersed with inactive faults, and 3) inactive. I interpret these dextral faults as a means to transfer strain to the PMfz. I propose the curved trace is an oroclinal bend due to kinematic interaction with the ECSZ and the abandoned zone resulted from westward progression of ECSZ fault abandonment. Here I propose a model that explains the oroclinal bend and the westward retreat of the active east part of the PMfz.

Although the data are sparse, I suggest this pattern of faulting is broadly consistent with Dixon & Xie (2018) who suggested this pattern can be explained by E-SE passage of the Mojave block over a ductile shear zone in the lower crust in response to N-S compression of the Mojave Block. I therefore conclude 1) the curved trace in the PMfz is an oroclinal bend resulting from kinematic interaction with the dextral faults, 2) the abandoned zone resulted from westward progression of ECSZ fault abandonment as these faults passed over the ductile shear zone, and 3) dextral faulting has progressed into the San Bernardino Mountains, but presently inactive.