North-south–directed extension, timing of extension, and vertical-axis rotation of the southwest Cady Mountains, Mojave Desert, California

Models to account for regional tectonic rotation in the Mojave Desert of California hypothesize rotation synchronous with early Miocene regional extension, whereas one model proposes that rotation occurred after major extension. Three sections of rocks from the southwest Cady Mountains, in the upper plate of the regional extensional system, give paleomagnetic data suggesting 127 ± 30, 130 ± 13, and 67 ± 15 of clockwise rotation (oldest to youngest). These data indicate that two episodes of clockwise vertical axis rotation occurred. The older of these rotations (64 ± 19 clockwise) occurred in early Miocene time after tilting due to extension and is associated with a regional tectonic rotation event. The younger rotation (67 ± 15 clockwise) occurred post–14.0 Ma, is of local extent, and is probably due to active northwest-directed right-lateral shear through this region. These data, combined with geologic evidence, indicate that rotation took place after tilting because of extension in the southwest Cady Mountains and that early Miocene extension was originally north-south directed. Correlation of rocks and structures in the southwest Cady Mountains to those in surrounding ranges implies that the early Miocene regional extension was originally approximately north-south directed as well.