Late Cenozoic tectonic evolution of southwestern California

Abstract

We synthesize available geologic and geophysical data from southern California and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern Coast Ranges, western Transverse Ranges, and borderland regions of California since 30 Ma. Premises include specified relative plate motions for times prior to 10.5 Ma and after 3 Ma, a mid‐Tertiary bight in the continental margin of southern California, midcrustal detachment faults under much of California and adjoining regions, rotation of the western Transverse Ranges about an eastern pivot, and specified fault displacement histories. Prior to 18 Ma, about 90% of the tangential component of Pacific‐North America relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope. From 18 Ma to 5.5 Ma, dextral slip was accommodated predominantly on the offshore system but also on a second, inboard system that included the San Andreas fault. Fault‐bounded crustal blocks between the offshore and inboard fault systems underwent translation, rotation, and extension from 18 Ma to 11.5 Ma but not from 11.5 Ma to 5.5 Ma. Since 5.5 Ma, dextral slip has been accommodated predominantly on the inboard fault system; dextral slip ceased on the offshore fault system north of the Transverse Ranges but continues on faults west of or within Baja California. The San Gregorio‐San Simeon‐Hosgri fault system has accommodated less than 5 km of the late Cenozoic dextral slip. The distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction, and, to a lesser degree, changes in magnitude of Pacific‐North America relative motion. Based on our reconstuction, the tangential component of Pacific‐North America relative motion was ∼ 90mm/yr for the period 10.5 Ma to 5.5 Ma and ∼ 60mm/yr for the period 5.5 Ma to 3 Ma.

Neogene geohistory analysis of Santa Maria Basin, California, and its relationship to transfer of Central California to the Pacific Plate. Diatom biochronology of the Sisquoc Formation in the Santa Maria Basin, California, and its paleoceanographic and tectonic implications

“…The sequence of tectonic events documented in the Santa Maria area by others (see Namson and Davis, 1990;Sedlock and Hamilton, 1991;Clark and others, 1991) is reflected in vertical tectonic movement that is recorded at our study sites. In particular, all sites record an early Miocene subsidence event during inferred transtension and a middle Miocene interruption in sediment accumulation or a condensed interval.…”

“…As much as 115 km of slip on the San Gregorio-San Simeon segments may have occurred since the early Miocene (Graham, 1978;Graham and Dickinson, 1978). However, recent reevaluation of the southern Hosgri segment suggests that it has been primarily a reversefault system in the late Cenozoic (Neogene and younger) with no more than 5 km post-Oligocene dextral slip (Crouch and others, 1984;Clark and others, 1991;Sedlock and Hamilton, 1991). Regardless, the Hosgri segment juxtaposes basement rocks with different Neogene structural histories (McCulloch, 1987).…”

“…For most of Santa Maria province, detailed fault-slip data are not available; thus, although surface and shallow subsurface fault orientations are known in many cases, the exact direction and amount of slip, amount of extension or shortening, and the fault orientations at depth are not known. Also, some faults display evidence of reactivation with differing slip directions, and some faults display evidence of merging into a midcrustal detachment zone (Crouch and others, 1984;McCulloch, 1987;Sedlock and Hamilton, 1991;Mclntosh and others, 1991;Clark and others, 1991). Regional syntheses of available geological and geophysical data and more recent geochronology provide constraints summarized below.…”

“…In summary, estimates of cumulative post-Oligocene dextral slip in the southern Coast Ranges and offshore (excluding the San Andreas Fault) range from as little as 85 km to as much as 195 km (Sedlock and Hamilton, 1991;Vedder and others, 1991). Namson and Davis (1990) found no evidence in the southern Coast Ranges for significant strike-slip displacement during the late Pliocene and Quaternary and suggest that lateral slip in the Santa Maria province has been minimal (<l-2 km) since early Pliocene time.…”

“…This report uses the record of vertical tectonic and isostatic movement preserved in offshore and onshore Santa Maria basin strata, coupled with both recent published accounts of regional folding and faulting histories (Namson and Davis, 1990;Sedlock and Hamilton, 1991;Clark and others, 1991) and recent plate reconstructions (Atwater, 1989;Lonsdale, 1991;Fernandez and Hey, 1991), to develop insights into the interplay between plate kinematics and continental margin deformation styles and rates. Because this complex region has several major unresolved tectonic questions, our tectonic reconstruction must be considered preliminary.…”