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

Abstract

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.

The use of the Minnaert correction for land‐cover classification in mountainous terrain

“…Palaeomagnetic, as well as geological structure evidence, suggests that the Transverse Ranges have undergone an approximately 50u clockwise rotation since the middle Miocene. In the Santa Monica Mountains the rotation has been as much as 78u (Kamerling and Luyendyk 1979, Terres and Luyendyk 1985Hornafius et al 1986). Sedimentary formations dominate, typically sandstones, siltstones, and conglomerates.…”

Chapter 8: Whole-rock K-Ar ages and geochemical data from middle Cenozoic volcanic rocks, southern California: A test of correlations across the San Andreas fault

“…Volcanic flows in the Vasquez Formation show an apparent clockwise rotation of 37° (±12°; Terres and Luyendyk, 1985), although orientations differ significantly near faults. Data from the stacked sequence consisting of the Vasquez, Tick Canyon, and Mint Canyon Formations provide evidence that this apparent rotation may have resulted from an initial clockwise rotation of 53° and a subsequent counterclockwise rotation of 16° .…”

Paleotectonics of a complex Miocene half graben formed above a detachment fault: The Diligencia basin, Orocopia Mountains, southern California

“…1), which has been rotated ~40° clockwise about a vertical axis (Terres, 1984;Terres and Luyendyk, 1985;Luyendyk, 1991). The uppermost Oligocene/ Lower Miocene Vasquez Formation is rotated 40°, whereas the Middle Miocene Mint Canyon Forma tion has not been rotated, thus bracketing the time of clockwise rotation as 18-16 Ma; ~55° of clockwise rotation occurred at this time, with later counterclockwise rotation of ~15°, resulting in a net clockwise rotation of 40° (Terres, 1984;Terres and Luyendyk, 1985;Luyendyk, 1991). The western Transverse Ranges have been rotated clockwise more than 90°, beginning around 18 Ma (Luyendyk, 1991;Nicholson et al, 1994;Dickinson, 1996;Ingersoll and Rumelhart, 1999;Ingersoll, 2008a).…”