RESEARCH Fault geometry and cumulative offsets in the central Coast Ranges, California: Evidence for northward increasing slip along the San Gregorio–San Simeon– Hosgri fault V.E. Langenheim, R.C. Jachens, R.W. Graymer, J.P. Colgan, C.M. Wentworth, and R.G. Stanley U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025, USA ABSTRACT Estimates of the dip, depth extent, and amount of cumulative displacement along the major faults in the central California Coast Ranges are controversial. We use detailed aeromagnetic data to estimate these parameters for the San Gregorio–San Simeon–Hosgri and other faults. The recently acquired aeromagnetic data provide an areally consistent data set that crosses the onshore-offshore transition without disrup- tion, which is particularly important for the mostly offshore San Gregorio–San Simeon–Hosgri fault. Our modeling, constrained by exposed geology and in some cases, drill-hole and seismic-refl ection data, indicates that the San Gregorio–San Simeon–Hosgri and Reliz-Rinconada faults dip steeply throughout the seismogenic crust. Deviations from steep dips may result from local fault interactions, transfer of slip between faults, or overprinting by transpression since the late Miocene. Given that such faults are consistent with predominantly strike- slip displacement, we correlate geophysical anomalies offset by these faults to estimate cumulative displacements. We fi nd a northward increase in right-lateral displacement along the San Gregorio–San Simeon–Hosgri fault that is mimicked by Quaternary slip rates. Although overall slip rates have decreased over the lifetime of the fault, the pattern of slip has not changed. Northward increase in right-lateral dis- placement is balanced in part by slip added by faults, such as the Reliz-Rinconada, Oceanic–West Huasna, and (speculatively) Santa Ynez River faults to the east. LITHOSPHERE; v. 5; no. 1; p. 29–48 | Published online 2 October 2012 doi: 10.1130/L233.1 INTRODUCTION began ca. 5.5 Ma (McCrory et al., 1995). How (defi ned by the aftershock distribution) dipping this deformation is accommodated depends on 45° to 60° to the north-northeast, likely the Oce- The central California Coast Ranges are the depth, extent, geometry, and cumulative dis- anic fault (McLaren et al., 2008). At the other an incompletely understood part of the North placement of the faults within this region. end of the spectrum, the Hosgri fault and other American–Pacifi c plate margin compared to the Although the San Gregorio–San Simeon– faults are postulated to be vertical to steeply relatively well-studied San Francisco and Los Hosgri fault strikes nearly parallel to the direc- dipping, deeply penetrating faults that primar- Angeles urban areas. The region is roughly tri- tion of modern Pacifi c–North American plate- ily accommodate strike-slip deformation, as angular, bounded by the San Andreas fault on margin motion (Fig. 1), estimates of the depth, originally proposed by Hill and Dibblee (1953). the east, the San Gregorio–San Simeon–Hos- extent, and geometry of this fault and others in Hanson et al. (2004) presented evidence based gri fault on the west, and the Western Trans- the central California Coast Ranges are contro- on Quaternary deposits that the Hosgri fault is verse Ranges on the south (Fig. 1). Within this versial. One end-member model posits that the steeply dipping. Relocated seismicity (Harde- region, multiple north-northwest–striking to San Gregorio–San Simeon–Hosgri and other beck, 2010) also supports a steep to vertical dip west-northwest–striking faults, including the faults, such as the Reliz-Rinconada (Fig. 2A), between 3 and 12 km for the Hosgri fault, at least Reliz-Rinconada and the Oceanic–West Huasna are primarily northeast-dipping, compressional where the fault is associated with microearth- faults (Fig. 2A), cut Cenozoic rocks that over- structures, becoming listric at more than 5–10 quakes between Piedras Blancas and Point Sal lie three main Mesozoic basement types (Fran- km depth and rooting into a regional thrust or (Fig. 2B). The fault dip and depth extent are criti- ciscan Complex, Coast Range ophiolite with detachment fault (Crouch et al., 1984; Nam- cal parameters for seismic hazard assessment, as overlying Great Valley Sequence, and Salinian son and Davis, 1988a, 1990). Cross sections by these infl uence patterns of ground shaking. basement with its overlying cover). This region Namson and Davis (1990) indicate as much as The amount of offset on the faults of the must somehow have accommodated deforma- 20%–30% contraction across the region, with central California Coast Ranges since the tion produced by the ~90° clockwise rotation of one section (Namson and Davis, 1988b) show- inception of the transform margin is also con- the Western Transverse Ranges that began in the ing the San Andreas fault displaced at depth by troversial. Estimates for strike-slip offset on the early to middle Miocene (Hornafi us et al., 1986), an inferred low-angle or horizontal detachment San Gregorio–San Simeon–Hosgri fault since in addition to regional transtension that accom- fault. An example might be the causative fault(s) ca. 4 Ma range from 5 km or less (Sedlock and panied the demise of the Farallon-Pacifi c spread- of the 2003 M 6.5 San Simeon earthquake; the Hamilton, 1991; Sorlien et al., 1999a; Under- ing ridge (Nicholson et al., 1994; Wilson et al., main shock was located near the base of seis- wood and Laughland, 2001) to 80–185 km 2005) and regional transpression and uplift that micity at a depth of nearly 10 km along a fault (Hall, 1975; Graham and Dickinson, 1978; LITHOSPHEREFor permission to| Volumecopy, contact 5 | Number [email protected] 1 | www.gsapubs.org | © 2012 Geological Society of America 29 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/5/1/29/3723410/29.pdf by guest on 02 October 2021 LANGENHEIM ET AL. 124° 123° 122° 121° 120° 119° 118° 117°W in this area. Together, these data refl ect density Cenozoic sedimentary rocks underlain by: and magnetization contrasts across faults that batholith and pre-batholithic metamorphic rocks have signifi cant (>1 km) vertical and/or hori- Franciscan (accreted) or Great Valley (forearc) Gualala ? complexes zontal displacements and thus allow us to look block batholith and pre-batholithic rocks structurally over at the geometry and cumulative slip of these ? metamorphosed Franciscan/Great Valley equivalent Basement rocks 39°N faults over their lifetime (since the early Mio- Franciscan or Great Valley Complex cene and even earlier with the magnetic data). Pt. Arena Mesozoic batholith and pre-batholithic rocks This study benefi ts from the recent acquisition Mesozoic ultramafic rocks of Franciscan/ of detailed aeromagnetic data that cover the Great Valley complex ? Major fault or fault zone entire central California Coast Ranges, provid- Nacimiento fault 38° ing an areally consistent data set that crosses the 10 Ma Pt. GREAT VALLEY Reyes SIERRA NEVADA onshore-offshore transition without disruption. 0 50 km ? SF This is particularly important for the San Grego- rio–San Simeon–Hosgri fault, which lies mostly offshore and offers few opportunities to estimate San Gregorio 37° offset based on geologic mapping. ? Based on our analysis of the aeromagnetic Salinian and gravity data, the San Gregorio–San Simeon– 16 Ma block Hosgri and Rinconada faults dip steeply to mid- crustal depths. Displacement on these faults is San Andreas Fault 36° San Simeon primarily strike slip. Based on correlation of ? magnetic anomalies on either side of the fault, we propose that displacement on the San Grego- 20 Ma paleosubduction zone Hosgri Fault rio–San Simeon–Hosgri fault increases north- ward because of slip on subsidiary faults to the 35° east, accommodating the clockwise rotation of Garlock Fault ? MOJAVE BLOCK the Transverse Ranges. SanSan Andreas Fa ult GEOPHYSICAL DATA WESTERN TRANSVERSE Gabriel RANGES Fault 34° LA PEN More than 500 new gravity measurements 28 Ma CALIFORNIA CONTINENTAL RANGESINSULAR in the region were added to earlier coverage BORDERLAND (Roberts et al., 1990; Langenheim et al., 2002; Pan-American Center for Earth and Environ- PACIFIC OCEAN 33° mental Studies, 2010; McPhee et al., 2011; Watt et al., 2011b) for this study. Together, nearly 25,000 gravity measurements were used to create an isostatic residual gravity map of the Figure 1. Index map of the central California Coast Ranges. LA—Los Angeles; SF—San Francisco. region (Fig. 3). The isostatic correction (using Arrow shows motion of the Pacifi c plate relative to the North American plate. Numbers in italics a sea-level crustal thickness of 25 km, a crustal show the position of the edge of the subducting slab from Atwater and Stock (1998) that migrates 3 northward and serves as a proxy for the development of the San Andreas transform margin. density of 2670 kg/m , and a mantle-crust den- sity contrast of 400 kg/m3) removes the long- wavelength effect of deep crustal and/or upper- mantle masses that isostatically support regional Clark et al., 1984; Jachens et al., 1998; Dickin- instead showing that the fault was a locus of topography, assuming an Airy-Heiskanen son et al., 2005; Burnham, 2009). Other faults, subsidence during the early Miocene. model of isostatic compensation (Jachens and such as the Reliz-Rinconada and the Oceanic– In this study, we use aeromagnetic and grav- Griscom, 1985). Modifying the parameters or West Huasna faults, also have ranges of esti- ity data to examine the dip, depth extent, and using a Pratt-Hayford model of compensation mated strike-slip offset, although not as widely cumulative offsets (since the late early Mio- produces changes of such long wavelength divergent as those for the San Gregorio–San cene) of the San Gregorio–San Simeon–Hosgri that the shapes of the residual anomalies are Simeon–Hosgri fault.