Progressive growth of San Clemente Island, California, by blind thrust faulting: implications for fault slip partitioning in the California Continental Borderland

Abstract

We find that the genesis of San Clemente Island and its surrounding submarine platform is consistent with progressive slip on two, southeast-striking, southwest-dipping, blind thrust fault segments. Since their inception 2 to 5 Ma, 3 km of compression normal to the N150°E fault strike has been accommodated with 1700 m of domal uplift of the San Clemente Anticlinorium. The existence of an extensive suite of Pleistocene marine terraces provides evidence that slip and uplift are continuing today. Based on direct terrace fossil age determinations and correlations of terrace heights with global sea-level curves, we estimate that San Clemente Island is currently uplifting at between 0.2 and 0.5 mm yr−1. This translates into 0.6–1.5 mm yr−1 of thrusting on the causative blind thrusts beneath the island. Unlike the situation at nearby Palos Verdes, where a simple twist in a regional strike-slip fault accommodated both fault-parallel and fault-normal motions, the shallow dips of the thrusts suggest that, if regional strike-slip motion on the San Clemente Fault exists, it must be partitioned onto through-going surfaces distinct from the thrusts. Current GPS data are sparse and equivocal, but they indicate that 1–4 mm yr−1 of compression and 4–7 mm yr−1 of strike slip are absorbed in the California Continental Borderland. With the Palos Verdes Fault taking some 3 mm yr−1 from the strike-slip budget, 1–4 mm yr−1 of motion could be present on a through-going San Clemente Fault. When translated into an annual moment release rate using Kostrov's formula, GPS strains predict that between 2.5 and 4.9 times 1017 N m yr−1 of earthquake potential is available offshore from San Diego to the Santa Barbara Channel. Distribution of this moment budget among various earthquake magnitudes is arguable, but we predict that M > 6 quakes in the Borderland could recur between 30 and 80 years, and M > 7 quakes might be found every 310 to 580 years.