A KINEMATIC MODEL FOR THE RINCONADA FAULT SYSTEM IN CENTRAL CALIFORNIA BASED ON STRUCTURAL ANALYSIS OF EN ECHELON FOLDS

Numerous en echelon folds are exposed adjacent to the Rinconada fault in the Santa Lucia Range of central California. We studied two specific locations within these folded areas near the Espinosa segment of the Rinconada fault at Williams Hill and near the San Marcos segment at Lake San Antonio. The average obliquity between fold hinges and the local orientation of the Rinconada fault varies from 14-27° at these two sites. The orientation and displacement recorded by fault populations at each site suggests that the maximum horizontal extension is achieved parallel to the local fold hinges. The magnitude of extension ranges from 4-9% based on systematic fault displacement measurements along several roadcuts that strike ±25° from the local fold hinge orientations.

The orientation and stretch of fold hinges was used to construct a kinematic model for distributed deformation adjacent to the Rinconada fault, assuming deformation occurred in monoclinic transpression. This modeling suggests a 20-50° angle of oblique convergence, 4-5 km of total fault-parallel wrench deformation, and 3-4 km of fault-perpendicular shortening. Bulk clockwise rotation of 3-15° is predicted in this model due to the wrench component of deformation and supported by paleomagnetic analyses in the region. Integrating these fault-parallel and fault-perpendicular offsets with the regional tectonics predicts a distributed wrench deformation rate of 0.8-1.0 mm/yr and a shortening rate of 0.5-0.8 mm/yr since the Pliocene. Comparing the fault-parallel deformation due to folding with the discrete offset on the Rinconada fault suggests that 15-40% of the total fault-parallel motion is accommodated by distributed deformation.