Abstract

We use vertical axis rotations from new paleomagnetic data to constrain off-fault deformation within the San Andreas fault system in central California. Samples were collected from 177 sites in the Miocene Monterey Formation adjacent to the Rinconada fault. Reliable means from 57 sites have 3 prominent patterns: (1) the largest clockwise rotations are close to the Rinconada and Nacimiento faults; (2) no significant rotation is observed near the San Andreas fault; and (3) counterclockwise rotations are observed at several sites northwest of Paso Robles. These paleomagnetic results are compared to two other measures of off-fault deformation where rotation can be calculated. Results from a fold-based kinematic model show increasing clockwise rotations toward the Rinconada fault, consistent with pattern 1. Few folds are observed in rocks on Salinian basement near the San Andreas fault, suggesting that little deformation has occurred, and providing an explanation for the negligible paleomagnetic rotations in pattern 2. Rotations calculated from the global positioning system velocity field predict counterclockwise rotations that coincide with those observed from paleomagnetic data in pattern 3. Broad patterns in the velocity field appear to be controlled by the transition from creeping to locked behavior along the San Andreas fault, and the region of counterclockwise rotation is linked to this transition. Thus, we suggest that the creeping segment has been aseismic over geologic time scales in order to produce the observed paleomagnetic rotations. The integration of all three data sets demonstrates that the San Andreas fault borderlands record an important portion of fault-parallel plate motion over geologic and geodetic time scales.

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