Net dextral slip, Neogene San Gregorio–Hosgri fault zone, coastal California: Geologic evidence and tectonic implications

Abstract

Reinterpretation of onshore and offshore geologic mapping, examination of a key offshore well core, and revision of cross-fault ties indicate Neogene dextral strike slip of 156 ± 4 km along the San Gregorio-Hosgri fault zone, a major strand of the San Andreas transform system in coastal California. Delineating the full course of the fault, defi ning net slip across it, and showing its relationship to other major tectonic features of central California helps clarify the evolution of the San Andreas system. San Gregorio-Hosgri slip rates over time are not well constrained, but were greater than at present during early phases of strike slip following fault initiation in late Miocene time. Strike slip took place southward along the California coast from the western fl ank of the San Francisco Peninsula to the Hosgri fault in the offshore Santa Maria basin without signifi cant reduction by transfer of strike slip into the central California Coast Ranges. Onshore coastal segments of the San Gregorio-Hosgri fault include the Seal Cove and San Gregorio faults on the San Francisco Peninsula, and the Sur and San Simeon fault zones along the fl ank of the Santa Lucia Range. Key cross-fault ties include porphyritic granodiorite and overlying Eocene strata exposed at Point Reyes and at Point Lobos, the Nacimiento fault contact between Salinian basement rocks and the Franciscan Complex offshore within the outer Santa Cruz basin and near Esalen on the fl ank of the Santa Lucia Range, 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 onshore Santa Maria basin. 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, a sinistral strike-slip fault of latest Cretaceous to Paleocene age. North of its juncture with the San Andreas fault, dextral slip along the San Gregorio-Hosgri fault augments net San Andreas displacement. Alternate restorations of the Gualala block imply that nearly half the net San Gregorio-Hosgri slip was accommodated along the 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. With San Andreas and San Gregorio-Hosgri slip restored, there remains an unresolved proto-San Andreas mismatch of ~100 km between the offset northern end of the Salinian block and the southern end of the Sierran-Tehachapi block. 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 adja...

Tectonic underplating of trench sediments beneath magmatic arcs: the central California example

“…In places the two faults are mapped as a single fault, the 'Sur-Nacimiento fault' (Page 1970(Page , 1981Page et al 1979aPage et al , b, 1998. Locally, the Sur fault also overlaps with another fault that is exclusively Cenozoic in origin, the San Gregorio-Hosgri fault (Dickinson et al 2005). Poor outcrop patterns prevent detailed structural analysis of the fault, and there are no good piercing points to estimate the Late Cenozoic lateral offset.…”

“…This translation episode includes, in addition to the San Andreas fault, numerous right-lateral and high-angle reverse faults within the Monterey terrane, some with displacements as large as 150 km (i.e. San Gregorio-Hosgri, Dickinson et al 2005). In addition, several episodes of sedimentation, deformation and minor magmatism occurred; they affected the area throughout the Cenozoic.…”

“…Armstrong and Suppe 1973). However, slivering of the block along internal faults (Dibblee 1976;Dickinson et al 2005) can easily account for 300 km right-lateral slip. In either case, the genetic linkage between the Monterey terrane and southern California is not violated.…”

“…The San Andreas is unambiguously the eastern boundary of this block ( Figure 2); however no obvious fault uniquely separates the block's large-scale fabric from outboard geologic domains to the west. The San Gregorio-Hosgri (Dickinson et al 2005) is a well-documented significant right-lateral fault of Late Cenozoic age (,150 km displacement) and it parallels other, less significant faults in the area (Hall 1991). The offshore relationship of the Pacific plate to the terrane are largely unresolved (Greene et al 2002).…”

Late Quaternary landscape development along the Rancho Marino coastal range front (south‐central Pacific Coast Ranges, California, USA)

“…Offshore from Rancho Marino is the San Gregorio-Hosgri fault zone (Fig. 1), a major NNW-SSE to NW-SE orientated strike-slip fault that forms the western margin of the seismotectonic domain (Clark et al, 1994;Jennings, 1994;Hanson et al, 1995;Dickinson et al, 2005). This structure comes onshore in the San Simeon region (Figs 1 and 2) where displacement of late Quaternary marine terraces have been used to quantify fault-related surface uplift and fault slip movements (Hanson et al, 1994).…”