Neogene tectonic rotation of the San Gabriel Region, California, suggested by paleomagnetic vectors

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.

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