Abstract

The anomalous position of continent-derived Paleogene sedimentary rocks on islands in the southern California borderland (SCB) has been explained by two hypotheses, both involving large-scale horizontal displacement during the Miocene. One hypothesis translates a tectonic “island block” westward 120 km from an original position west of Los Angeles and derives the Paleogene sediments from the same source as those of the Santa Ana and Santa Monica Mountains. The other translates the block by 180 km of right slip from an original position west of San Diego and derives the Paleogene sediments from the same source as those of the San Diego embayment.

Reconstructions of Late Cretaceous and Paleocene paleogeography fit either hypothesis except they do not permit a ridge of Catalina Schist between the islands and San Diego prior to Miocene time which is implied by right slip. Eocene reconstructions depend upon the recognition of criteria which would distinguish the Santa Ana Mountains source from the San Diego source of “Poway” conglomerate clast assemblages in the islands. Data on island conglomerates are insufficient to attribute them to one source in preference to the other, although clast assemblages from Eocene sandstones of the islands resemble those in the Santa Ana Mountains and not those at San Diego. The Oligocene Sespe Formation and the early Miocene “Vaqueros-Rincon” sequence of the northern Channel Islands are clearly related to the rocks of the Santa Ana Mountains, Santa Monica Mountains, and the southern Ventura basin; these have no counterpart in the San Diego area or northern Baja California. Reconstructions of Oligocene and early Miocene paleogeography are simple and straightforward under the extension hypothesis; complex and convoluted under right slip. The San Onofre Breccia of Saucesian-Relizian age has been used as evidence of offset by right slip; under extension the San Onofre, together with volcanic and volcaniclastic rocks, accompanied rifting and therefore are not used for piercing-point evidence of offset.

The fault proposed as the site of right slip cannot be traced beyond the American portion of the SCB, either to the south in Baja California or north in the Transverse Ranges. A major fault is possible east of the Vizcaino Peninsula of Baja California, but the Upper Cretaceous shoreline is not offset by right slip across this fault. A right-slip fault may be invoked to explain the presence of Eocene and Cretaceous rocks on the Santa Lucia Bank west of Santa Maria basin, but these may also be explained by east-west extension.

There is no evidence of major northwest right-slip displacement in southern California during the time proposed for large-scale horizontal displacement in the SCB. On the contrary, east-west extension accompanied by volcanism was the major tectonic style in the southern Great Basin and the Gulf of California. An appropriate model for middle Miocene tectonics in the SCB is not the San Andreas fault which is not accompanied by volcanism and high heat flow, but the rifts of the Salton depression, Great Basin, and East Africa which are characterized by extension, volcanism and high heat flow.