Abstract

The San Benito Islands, a group of three islands off the Pacific coast of Baja California, are of significant geologic interest because of their proximity to the continental crust—oceanic crust interface. The islands consist of a complexly folded and sheared eugeosynclinal sequence of graywacke, chert, basalt, altered basalt and carbonate, serpentine, and glaucophane rock.

The western two thirds of West Island consist of a folded sequence of graywacke beds which are terminated on the east by a major shear zone, hereafter called the San Benito shear zone. The eastern third of the island consists of basalt, altered basalt, chert, sheared graywacke, and minor occurrences of glaucophane and serpentine. The areal relationship of these rocks is predominantly controlled by shearing.

The distribution of rocks on Central Island and the northern third of East Island is also controlled by tectonic movements, with the same juxtaposition of rock types as occurs on the eastern third of West Island. The southern two thirds of East Island consist of large blocks of glaucophane-bearing rock embedded in serpentinite in a chaotic manner.

West Island, Central Island, and the northern third of East Island are characterized by structural elements which generally trend N. 30°W. Both the anticlinal axes and the shear zone on West Island exhibit this trend, with the shear zone showing right-lateral movement. Structural elements east of the shear zone on West Island, on Central Island, and on northern East Island tend to swing to the north. The foliation of the remainder of East Island trends from north-south to N. 30°W. and faulting is apparently random in direction.

Tectonic movements have affected the islands during the Pleistocene, as evidenced by at least three levels of elevated terraces which are tilted in different directions on the three islands. Bathymetric surveys around the islands suggest the presence of other terraces below the present sea level as well as the extension of the San Benito shear zone to the north. The bathymetric survey also indicates the possible termination to the south of the folded graywacke sequence on West Island.

From field investigations it is concluded that at least some of the bedded chert on the San Benito Islands is a product of the chertification of graywacke beds and that most of the glaucophane-bearing rock is a metamorphosed graywacke. The association of rocks on the San Benito Islands is strikingly similar to the Franciscan rocks of California and is therefore tentatively identified as such.

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