NET DEXTRAL SLIP, NEOGENE SAN GREGORIO-HOSGRI FAULT ZONE, COASTAL CALIFORNIA: GEOLOGIC EVIDENCE AND TECTONIC IMPLICATIONS

Reinterpretation of onshore and offshore geologic mapping, examination of a key offshore well core, and revision of cross-fault ties jointly indicate net Neogene dextral strike slip of 156±4 km along the San Gregorio-Hosgri fault (SGHF) zone, a strand of the San Andreas transform system in coastal California.

Key cross-fault ties compatible with ~155 km of SGHF offset include porphyritic granodiorite and overlying lower Eocene strata at Point Reyes and at Point Lobos, the Nacimiento fault contact between Salinian basement rocks and the Franciscan Complex within the Outer Santa Cruz basin and in the vicinity of Esalen on the California coast, Upper Cretaceous (Campanian) turbidites of the Pigeon Point Formation in the central Santa Cruz Mountains and the Atascadero Formation in the southern Santa Lucia Range, and assemblages of Franciscan rocks exposed at Point Sur and at Point San Luis.

Merging of the San Andreas and San Gregorio faults at Bolinas Lagoon north of the Golden Gate indicates that slip on the two structures is additive north of San Francisco Bay, largely accounting for apparently greater net San Andreas slip in northern California than in central California. SGHF displacements in effect lengthened the Salinian block of plutonic and metamorphic basement rocks by slivering its western edge.

The difference between long-term and current slip rates implies that SGHF motion has slowed over time from a peak as high as 25 mm/yr in late Miocene (10 Ma) time. Thus, the SGHF was probably a principal strand of the San Andreas system in late Miocene time and initially a more important structural element of central California than today.