Autori: Dickinson, W.R., Ducea, M.N., Rosenberg, L., Greene H.G., Graham, S., Ernst, W.G., Kidder, S. and Weber, G.

Editorial: Geological Society of America Special Paper, 391, p.43p, 2005.

Rezumat:

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 154?2 km along the San Gregorio?Hosgri fault zone, a strand of the San Andreas transform system in coastal California. Slip rates over time are not well constrained, but were greater than at present during early phases of strike slip after fault initiation in late Miocene time. Strike slip propagated southward from the western flank of the San Francisco Peninsula to the offshore Hosgri fault in the offshore Santa Maria basin without significant diminution by transfer of strike slip into the California Coast Ranges. No diversion of San Gregorio?Hosgri slip to displacements along the Rinconada fault or other intra-Salinian faults can be demonstrated. Onshore segments of the San Gregorio?Hosgri fault are exposed as the Seal Cove and San Gregorio faults on the coast of the San Francisco Peninsula, and as the Sur and San Simeon fault zones along the coastal fringe of the Santa Lucia Range. Slivering of the Salinian block by San Gregorio?Hosgri displacements elongated its northern end and offset its western margin delineated by the older Nacimiento fault, which is probably a sinistral strike-slip fault of latest Cretaceous to Paleogene age.
Key cross-fault ties compatible with 154 km of San Gregorio?Hosgri fault offset include porphyritic granodiorite and overlying Eocene turbidites 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, Upper Cretaceous (Campanian) turbidites of the Pigeon Point Formation on the San Francisco Peninsula and the Atascadero Formation in the southern Santa Lucia Range, assemblages of Franciscan rocks exposed at Point Sur and at Point San Luis, and a lithic assemblage of Mesozoic rocks and their Tertiary cover exposed near Point San Simeon and at Point Sal, as restored for intrabasinal deformation within the Santa Maria basin.
North of its juncture with the San Andreas fault, dextral slip along the San Gregorio?Hosgri fault is additive to net San Andreas displacement, but alternate restorations of the Gualala block imply that nearly half the San Gregorio?Hosgri slip propagated along a postulated offshore Gualala fault strand lying west of the Gualala block, which is bounded on the east by the current master trace of the San Andreas fault. On the south, San Gregorio?Hosgri strike slip is transposed into crustal shortening associated with vertical-axis tectonic rotation of fault-bounded crustal panels that form the western Transverse Ranges and with kinematically linked deformation within the adjacent Santa Maria basin. The San Gregorio?Hosgri fault serves as the principal link between transrotational tectonism and simple strike slip within the San Andreas transform system of central California.

Cuvinte cheie: central California, tectonics, strike-slip faults