Datapages, Inc.Print this page

Paleomagnetism and Tectonic Rotation of the Southern Coast Ranges, California

Donald R.Prothero
Occidental College Geology, 1600 Campus Road, Los Angeles, CA 90041

Although post-Oligocene clockwise tectonic rotations are widespread in the western Transverse Ranges of California, current tectonic models suggest that the blocks north of the Big Pine-Santa Ynez faults (southern Salinian block) should show no rotation, but only northwesterly translation parallel to the San Andreas fault. Over the years, however, a number of labs have produced paleomagnetic directions that suggest rotation in the Santa Maria Basin, the Caliente Range, and the Cuyama Badlands. I sampled most of the available Cenozoic sedimentary units in the Caliente Range block and the Cuyama Badlands block, and analyzed them paleomagnetically using both alternating field and thermal demagnetization. Most of the formations produced a stable remanence, and many passed a fold test for stability. Nearly every formation in these two blocks shows between 31° and 53° of clockwise rotation since the late Miocene, but there is no net change in rotation of these blocks from the Paleocene to the late Miocene. Since the late Miocene, these blocks underwent rapid rotation, and by1.7-2.0 Ma had rotated about 20°-23° clockwise. These rotations are not explained by any currently available tectonic model, but suggest that the tectonic blocks were relatively small and operate like “ball bearings” during dextral shear along the San Andreas fault. The initiation of this rotation at about 11 Ma also coincides with the movement along the San Gabriel fault, the opening of the Los Angeles basin, and the capture of the Guadalupe and Magdalena microplates.

 

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California