Timing and extent of Neogene tectonic rotation in the western Transverse Ranges, California

Abstract

Paleomagnetic data collected from lower Miocene rocks throughout the western Transverse Ranges Province have consistently clockwise-deflected magnetic declinations, which suggests that the entire province has experienced clockwise tectonic rotation during Neogene time. Paleomagnetic declinations in lower Miocene rocks increase westward from 33.5 C ± 11° in the San Gabriel block to 91.7° ± 7° in the western Santa Ynez Range, suggesting a westward increase in the net amount of clockwise tectonic rotation. Paleomagnetic data that constrain the tuning of rotation suggest that the entire western Transverse Ranges rotated uniformly 50°–60° clockwise during the middle Miocene. This implies a minimum of 200 km of distributed dextral shear along the California coast and offshore during middle Miocene time. Little tectonic rotation appears to have occurred in the western Transverse Ranges during late Miocene time. Since Miocene time, continued clockwise rotation in the western Santa Ynez Range (31.3°), combined with counterclockwise rotation in the San Gabriel block (−16°), has: oroclinally bent the Transverse Ranges to the west of the San Andreas fault. The Pliocene-Pleistocene clockwise rotation implies an additional 30–60 km of distributed dextral shear across the southern California Coast Ranges during post-Miocene time.