ABSTRACT

Southern California’s Transverse and Peninsular mountain ranges separate the deserts from the coastal valleys and mesas. The origin of the Transverse Ranges is directly linked to the development of the San Andreas Fault and the formation of the Gulf of California that began to appear as a narrow basin around 13 million years ago. By 5.5 million years ago, active widening of the gulf was underway.

All of Southern California west of the San Andreas Fault, and all of Baja California to the south, including the mountains of the Peninsular Range, comprise a unified block that rides northwest on the Pacific Plate. As the Gulf of California opened, the entire block rafted north where it encountered the massively deep, granite batholiths (basement) of the Sierra Nevada, and the San Bernardino Mountains. The only option was for the block to shift directions to the west where it compressed the terrain higher and higher into the Transverse Ranges of today. Among other consequences were the formation of the Tehachapi Mountains and the right-lateral slip of the Mojave block.