PALEOMAGNETISM AND NEOGENE CLOCKWISE ROTATION OF THE SOUTHERN COAST RANGES, CALIFORNIA

It is well documented that there has been about 90 degrees of clockwise rotation in the Western Transverse Range block since the early Miocene, but most tectonic models suggest that the southern Coast Ranges, just north of the Santa Ynez fault, should be unrotated, and simply slide northwestward to accommodate dextral shear on the San Andreas fault. Many recent paleomagnetic studies in the region, however, show that there is widespread clockwise rotation in these supposedly unrotated terranes. The Caliente Formation (in the western Tujunga Terrane) shows progressive rotation from as great as 39 ± 7 degrees at 15 Ma to 23 ± 7 degrees at 4 Ma. In the Caliente Range to the north (southeast Salinian block), both the 60 Ma Pattiway Formation and the 23 Ma Soda Lake Shale Member of the Vaqueros Formation show about 60 degrees of rotation. Clockwise rotations are found in the middle Miocene Monterey shales, and the early Miocene Morro volcanics in the Santa Maria basin (southern Sur-Obispo terrane). Yet the Cretaceous rocks of the central Salinian block supposedly show no rotation. These results, plus new paleomagnetic data, show that clockwise rotations are widespread in the southern Coast Ranges, and call into question models where these ranges simply slide along the San Andreas. Instead, the complex model of smaller blocks rotating in ball-bearing fashion (originally applied to the Western Transverse Ranges) may also apply north of the Santa Ynez fault.