Abstract

Seismic reflection profile data indicate that the insular terrace surrounding San Clemente Island, California, postulated as a wave-cut terrace during the late Pleistocene–Holocene lowering of sea level and marine transgression, has been affected by Holocene tectonism. Offsets of the terrace floor have developed by faulting along this shelf, along with a recent southwestward tilt of about ¼ degree, possibly accompanied by upbowing. The entire tilt of the crustal block occurred from the latter part of the Pleistocene to Holocene. The average rate of change of slope at the northeast margin is about ⅓ m per thousand years.

A submarine canyon off the west side of the island (herein named “Eel Ridge Canyon”), forms the only complete break in the terrace along this side of the island. The canyon, considered to be primarily structurally controlled, is related to a major unconformity in offshore Miocene sedimentary rocks. Faulting, pivotal in part, is apparent in alignments both parallel and oblique to the axis of the canyon. The seismic reflection profiles suggest that the canyon predates the offshore terrace, but it is conjectural whether the present configuration is due more to recent tectonism or submarine processes by preferential erosion along the fault zone.

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