TECTONIC ROTATIONS IN SOUTHERN CALIFORNIA AND THE MOJAVE DESERT DURING NEOGENE TIME

Paleomagnetic data from southern California south of 35.5° N latitude suggest that eight crustal domains have rotated clockwise at various times during the Neogene. Earthquake data suggest that some domains may be continuing to rotate today. Rotated domains include the western and eastern Transverse Ranges blocks, a crustal block near Morro Bay, Catalina Island block, San Gabriel Mountains, Tehachapi Mountains, the central Mojave Desert and the northeast Mojave Desert. These rotated domains comprise about 25% of the surface area of this region. Paleomagnetic data constrain the amounts of net clockwise rotation to be as much as 100 degrees in the California Borderland and western Transverse Ranges and from 60 to 40 degrees for the other domains. The average rotation rate can be estimated for the western Transverse Ranges at 5-6 degrees/m.y. since 17 Ma.

These rotations have occurred during three distinct tectonic regimes. Early Miocene rotations and extension in the central Mojave may have been related to a slab window that formed beneath the region after the Pacific and North American plates came into contact; the Tehachapi and Morro blocks began rotating in this regime also. Capture of the Monterey Plate by the Pacific Plate caused the initiation of rotation under transtension of the western Transverse Ranges, San Gabriel and Catalina blocks at 17 Ma. Shift of the Pacific-North American plate boundary eastward to open the Gulf of California at about 5 Ma created a restraining geometry to the plate boundary. This caused clockwise rotation of the eastern Transverse Ranges and the northeast Mojave Desert and smaller counterclockwise rotation of portions of the western Mojave Desert and the San Gabriel block.