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We have studied the paleomagnetism of various Neogene age rocks in southern California as a means of determining the amounts of Oligocene and younger tectonic rotation and translation which has occurred in this region. Our results suggest that fully 25% of this area, in particular the Transverse Ranges, has undergone extreme clockwise rotation. Work in southeastern California implies that 40° of clockwise rotation has occurred here, although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200°. The crustal block bounded by the San Gabriel and San Andreas fault has undergone a net clockwise rotation of 35° although the data here can be interpreted to show that an original early Miocene rotation of about 50° was followed by a late Miocene or Pliocene counterclockwise rotation of 15°. Paleomagnetic results from the offshore islands suggests that San Clemente, Santa Barbara, and San Nicolas islands have not rotated but Santa Catalina has undergone about 90° of clockwise rotation. All of the northern Channel Islands, including Anacapa, Santa Cruz, Santa Rosa, and San Miguel, are implied to have rotated 70° or 80° clockwise. This result is also found for the Santa Monica Mountains east of Anacapa Island. In the Santa Ynez Range north of the Channel Islands, paleomagnetic study of the Monterey Formation also indicates large clockwise rotations of near 90°. These data also suggest that the Santa Maria Basin is not rotated and that the north boundary of the rotated region is the Santa Ynez River fault. Stratigraphic control on the paleomagnetic data from the Monterey Formation implies that the rotation began about 16 m.y. ago and may be continuing today in the western region. Paleomagnetic inclination data from our study show that the northern Channel Islands, in particular, may have translated 15° northward since middle Miocene time. However, equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections, non dipole magnetic field behavior and right offset on the San Andreas fault system. Palinspastic reconstruction of southern California regions for the early Miocene implies that parts of the Transverse Ranges structures were once aligned with north trending extensional structures in the southwestern United States. We propose that Pacific‐American plate interactions both rifted the continental crust to create this pattern and rotated the western‐most structures within a dextral simple shear zone which had a half width of about 400 km.

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Previous paleomagnetic work in the western Transverse Ranges and the islands offshore of California has suggested that these regions have experienced large clockwise rotations since middle Miocene time. This study is aimed at testing how far east these rotations extend into the continent. Neogene volcanic rocks from the region bounded by the San Gabriel and San Andreas faults were selected for paleomagnetic study. The sampled units are lavas from the late Oligocene to early Miocene continental Vasquez Formation, and water‐laid tuff beds from the middle to late Miocene Mint Canyon Formation. These rocks possess a stable remanent magnetization which was acquired soon after deposition and is believed to be primary. Seventy‐four lava flows from eleven locations in the Vasquez Formation were studied. The average direction for seven locations near the center of the study area suggests that this area has rotated a net clockwise amount of 37.1°±12.2° since the early Miocene. There is no suggestion of north or south translation. A location west of the central region is rotated almost 90° clockwise, although this is believed to be a local effect. Detailed magnetic stratigraphy at the western and central location can be correlated over 6 km. Locations in the east near the San Andreas fault yielded highly dispersed directions which cannot now be interpreted in terms of rotation. The mean paleomagnetic direction from six sites (four different beds) in the Mint Canyon formation west of the Vasquez study area is deflected in a counterclockwise sense by 16°±30°. These data are consistent with a proposed model in which the San Gabriel region first was rotated clockwise 53° (37° + 16°) in the early Miocene and later rotated 16° counterclockwise as it slid northward into the leftward bend region of the San Andreas fault. The clockwise rotation is believed to be due to dextral shear within the Pacific‐North American plate shear zone; counterclockwise rotation in the Mojave region is believed to be responsible for the bend in the San Andreas fault.