RESEARCH Fault Geometry and Cumulative Offsets in the Central

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 disruption, 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 displacement 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;

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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 northward 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 density 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. Estimates of cumulative and other faults in the central California Coast changed only slightly (Jachens and Griscom, right-lateral displacement for the Reliz-Rinco- Ranges. Prominent aeromagnetic anomalies 1985; Oliver, 1973). Measurement distribution nada fault range from at least 18 km since the are caused by magnetic rock types that pri- onshore is on average 1 station per 2 km2 south Pliocene (Durham, 1965) to 60 km since the marily reside within the Mesozoic basement, of latitude 36°15′Ν (courtesy of the agency for- Early Tertiary (Dibblee, 1976, p. 40). In the such as the Great Valley ophiolite, certain plu- merly known as the Defense Mapping Agency). case of the Oceanic–West Huasna fault, esti- tons within the Salinian block, and metaba- North of latitude 36°15′Ν, measurement distri- mates of right-lateral displacement range from salts within the Franciscan Complex. Gravity bution onshore in the Gabilan and Santa Lucia 5 to 8 km (McLean, 1993) to 15 km (Hall et al., data are sensitive to the density distribution of Ranges may be as low as 1 measurement per 1995, p. 87), although others (Tennyson et al., the subsurface, primarily the contrast between 10 km2. Offshore, data are generally taken from 1991) cannot fi nd strike-slip offset on the fault, Cenozoic basin-fi ll and pre-Cenozoic basement gridded compilations of shiptrack gravity data

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37° 122° 122°

Quaternary deposits Santa Cruz Pliocene sedimentary rocks San Jose 0 25 km Miocene sedimentary rocks Mtns Miocene volcanic rocks Año Oligocene sedimentary rocks Eocene sedimentary rocks Nuevo

San Gregorio - - Gregorio San Cretaceous sedimentary rocks Franciscan Complex Santa A greenstone Cruz Salinian basement 121° 121°

MBFZ Coast Range ophiolite A’ Monterey 0 25 km Bay

Point Salinas Lobos Gabilan Range G’ Salinas Valley G Reliz - Rinconada Fault Point San Andreas Fault Sur 36° Santa Lucia

Range Greenfield

San Ardo Cape

San Simeon - San Martin Jolon Valley B’

San Simeon B OF Piedras

Blancas Santa Lucia Bank Fault Bank Lucia Santa 120° 120° Cambria Paso 36° Robles 36° C Estero C’ Bay

35° Fault Hosgri E E’ WHF F’ Morro S Bay LOF J-6 Point Buchon La Panza Pism Irish F Hills Range o Syncline San Luis Huasna Syncline San Luis Obispo Bay Santa CF ChF Point Barrett D Valley Ridge D’ Maria LHF Sal Stanley

Mountain EHF

SMRF

Point Lompoc Arguello Santa Ynez Mountains SCF Cuyama SYRF LPF San RafaelValley Santa 35°N Ynez Mountains Santa Barbara Channel

34° 34° Santa Barbara N SYF 119° 119°W A B

Figure 2. (A) Simpliﬁ ed geologic and (B) shaded-relief topographic maps of the study area. Geology is from Jennings et al. (1977). Faults shown in thin red lines are modiﬁ ed from Jennings and Bryant (2010). Thick black dotted line—Nacimiento fault; dashed magenta line—Rus- sell fault. Thick gray dotted line in A shows outline of onshore Santa Maria Basin. Abbreviations in A: CF—Casmalia fault; ChF—Chimineas fault; EHF—East Huasna fault; LHF—Lions Head fault; LOF—Los Osos fault; LPF—Little Pine fault; MBFZ—Monterey Bay fault zone; OF— Oceanic fault; SCF—South Cuyama fault; SMRF—Santa Maria River fault; SYF—Santa Ynez fault; SYRF—Santa Ynez River fault; WHF—West Huasna fault. Star in B is location of 2003 M 6.5 San Simeon main shock.

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37°30' (Roberts et al., 1990; Decade of North Ameri- 122° 37° can Geology Project, 1987). Outer Santa Aeromagnetic data consist of several sur- Cruz Basin Santa Cruz veys (U.S. Geological Survey, 1987, 1996a,

Mtns San Gregorio - - Gregorio San 1996b, 2001, 2005; Langenheim et al., 2009) that were ﬂ own at a nominal height of 150– 37°30' 305 m above ground along ﬂ ight lines spaced A 36°30' 121° 530–800 m apart. Accuracy of these surveys is 1 A’ nanotesla or better. The aeromagnetic data were adjusted to a common datum 305 m (1000 ft) above ground and then merged by smooth inter- PL Gabilan Range polation across survey boundaries to produce an G’ aeromagnetic map of the study area (Fig. 4).

G Salinas Valley 37° To defi ne the edges of displaced magnetic San Andreas Fault rock bodies that constrain cumulative offset 36° estimates on strike-slip faults, we used the Santa Lucia Isostatic Gravity (mGal) maximum horizontal gradient method (Cordell Range Greenfield and Grauch, 1983; Blakely and Simpson, 1986) 24 using the magnetic potential fi eld. Using the San 20 magnetic potential fi eld removes the dipole 122° Ardo RiF 36°30' 16 effect and mathematically transforms the mag- R1R1 12 netic fi eld into an equivalent gravity fi eld (also 35°30' Jolon 8 known as “pseudogravity”). For this transfor- Valley B’ 4 mation, we used an inclination of 59° and a R2R2 B SS 0 declination of 14° for Earth’s magnetic fi eld. OF -4 Gradient maxima occur directly over vertical -8 120° or near-vertical contacts that separate rocks of -12 contrasting magnetizations. For moderate to 36° -16 steep dips (45° to vertical), the horizontal dis- C C’ 35° E E’ -20 placement of a gradient maximum from the

F’ Hosgri Fault Hosgri PB WHF SJ-6 -24 top edge of an offset horizontal layer is always PismoLOF Syncline La Panza F -28 less than or equal to the depth to the top of the SLO Range -32 source (Grauch and Cordell, 1987). SLB P1 Huasna Syncline -36 -40 35°30' GEOPHYSICAL ANOMALIES PS -44 D D’’ -48 34°30' LHFL Isostatic residual gravity anomalies reﬂ ect SMRF -52 density variations in the upper 10–15 km (Simp-

PA P2P EHF son et al., 1986), with one of the most prominent SYRF density contrasts being that between low-density 121° Cuyama Neogene sedimentary rocks and dense Meso- Valley LPF zoic basement rock types. Pronounced gravity Santa BarbaraSanta C Ynez M San Rafael 35° lows within the central California Coast Ranges MountainsM coincide with deep sedimentary basins in the 34° ountains Cuyama, Santa Maria, Jolon, and Salinas Val- leys (Figs. 2B and 3). Gravity lows also coincide hannel with the Huasna and Pismo synclines (Fig. 3). The most prominent gravity low lies outside

Ventura 34°30'N the Coast Ranges study area within the Western Basin Transverse Ranges province and is associated 34° 120° 119°W with the Ventura Basin and its offshore exten- 0 50 KM sion beneath the Santa Barbara Channel (Fig. 3). Gravity highs coincide with exposures of Salin- Figure 3. Isostatic gravity map. Colored areas within map show gravity values for locations ian basement rocks in the Santa Lucia, Gabilan, within 5 km of a gravity measurement. Blue lines—modeled profi les. Gray line—seismic- and La Panza Ranges as well as with outcrops refl ection profi le SJ-6. Thick gray dotted line shows outline of onshore Santa Maria Basin. Thick of Franciscan Complex and Great Valley ophio- black dotted line—Nacimiento fault; dashed magenta line—Russell fault. R1 and R2 are gravity lite in the San Rafael, southern Santa Lucia, and lows correlated across the Reliz-Rinconada fault. P1 and P2 are gravity lows correlated across the West Huasna fault (WHF). Abbreviations: LHF—Lions Head fault; LOF—Los Osos fault; western Santa Ynez Mountains. Intermediate PA—Point Arguello; PB—Point Buchon; PL—Point Lobos; RiF—Rinconada fault; PS—Point Sal; gravity values over widespread exposures of SLB—San Luis Bay; SLO—San Luis Obispo; SS—San Simeon; SYRF—Santa Ynez River fault. Eocene and Cretaceous sedimentary rocks in the

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37°30' eastern Santa Ynez Mountains reﬂ ect intermedi- 37° ate densities between the low-density Neogene Año Santa Cruz rocks and the denser basement rocks. Nuevo Prominent aeromagnetic anomalies reﬂ ect Mtns the relative abundance of magnetic min-

San Gregorio - - Gregorio San erals, primarily magnetite, in rocks from the sur- 37°30' A face to mid- to lower crustal depths. In the Coast 36°30' 121° Ranges, these anomalies are generally associ- A’ Monterey ated with Mesozoic basement rocks. Ultramaﬁ c Bay rocks associated with the Coast Ranges ophiolite produce magnetic highs near Point Arguello, Point Sal, San Luis Bay, Piedras Blancas, and G’ Gabilan Range the western San Rafael Mountains (Figs. 2 and Point G Reliz - Rinconada Fault 37° 4). The magnetic high associated with the Point

Santa Lucia Sur San Andreas Fault Sal ophiolite can be traced nearly 50 km south- 36° Range Magnetic Field eastward beneath the sedimentary cover of the (nT) Santa Maria Basin. Metabasalt and interleaved slices of ophiolite within the Franciscan Com- 650 plex are responsible for the northwest-trending, 600 San narrow magnetic anomalies within the Santa 122° Ardo 36°30' 550 Lucia Range. CSM San Simeon - 500 Jolon Other magnetic sources lie within the base- Valley 450 35°30' ment of the Salinian block. The granodiorite of B’ 400 La Panza Range is associated with one of the 350 more prominent magnetic anomalies in the cen- B OF 300 tral Coast Ranges (Figs. 2 and 4). The basement 250 Piedras 120° of the Salinian block, however, is not uniformly 200 Blancas magnetic; exposures of batholithic rocks in the 36° 150 northern Gabilan and Santa Lucia Ranges coin- C C’ 100 35° E E’ cide with a generally ﬂ at magnetic ﬁ eld. Mag- 50 PB La PanzaF’ netic anomalies associated with the Salinian

LOF WHF Hosgri Fault Hosgri 0 Range block tend to be broader and blockier than the F -50 SLO elongate anomalies associated with the Francis- SLB -100 can Complex. -150 Other possible magnetic rock types, such 35°3 -200 PS 0' as Tertiary basaltic rocks, are generally thin D D’D -250 34°30' LHF (<150 m) within the study area (Cole and Stan- SMRF -300 ley, 1998). Several drill holes have penetrated

EHF thicker sections of Tertiary volcanic rocks in PA the Pismo syncline, along the Santa Maria River SYRF LPFLPL 121° San Rafael and Santa Ynez River faults, and west of the F F Mountains 35° Hosgri fault (Cole and Stanley, 1998), although the volcanic section there appears to consist mainly of weakly magnetic tuffs (Ron Cole, 34° 2011, written commun.). Sedimentary rocks are weakly magnetic, except for locally magnetic conglomerate and sandstone that coincide with a narrow, northwest-trending magnetic anomaly located on the southwest fl ank of the La Panza 34°30'N Range. Thus, the magnetic data allow us to infer 120° 34° 119°W basement type beneath the Cenozoic basin fi ll 0 50 KM in most places. Potential-fi eld gradients mark stretches of the major faults in the region. The Hosgri fault Figure 4. Aeromagnetic map. White line—coastline. Gray line—seismic-refl ection profi le not only coincides locally with magnetic gradi- SJ-6. Blue lines—modeled profi les. Thick gray dotted line shows outline of onshore Santa Maria Basin. Thick black dotted line—Nacimiento fault; dashed magenta line—Russell ents along its length, but it also separates more fault. Abbreviations: LHF—Lions Head fault; LOF—Los Osos fault; LPF—Little Pine fault; westerly trending anomalies to the east from OF—Oceanic fault; SYRF—Santa Ynez River fault; WHF—West Huasna fault; CSM—Cape more northerly trending anomalies to the west. San Martin; PA—Point Arguello; PB—Point Buchon; PS—Point Sal; SLB—San Luis Bay; The more westerly trending anomalies appear SLO—San Luis Obispo. to be truncated by the fault, especially between

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Point Arguello and Cape San Martin (Fig. 4). Pine fault (Fig. 4). The Little Pine fault is also to be within reasonable ranges for various rock The Hosgri fault thus marks a signifi cant mag- marked by a gravity gradient indicating north- types (e.g., Dobrin and Savit, 1988). We assume netic domain boundary. The San Gregorio fault, east side up. that remanent magnetization is a minor compo- which we interpret to be a northern continuation Other faults do not coincide with prominent nent of the total crustal magnetization, which is of the Hosgri fault via the San Simeon fault, magnetic or gravity gradients, notably por- supported by measurements of natural remanent coincides with a fairly broad magnetic gradient tions of the Quaternary traces of the Los Osos magnetization on the granodiorite of the La that trends northwest across the mouth of Mon- and Lions Head faults. In the case of the Lions Panza Range (10−2 to 10−3 A/m; Edward Manki- terey Bay. The combined Hosgri–San Simeon– Head fault, this may indicate that the fault trace nen, 2008, oral commun.; Koenigsberger ratio San Gregorio fault is also well expressed in the is poorly exposed or mapped (see Sylvester and of less than 0.01) and Koenigsberger ratios of gravity fi eld (Fig. 3). North of San Simeon, it Darrow, 1979), except where it is intercepted less than 1 for Coast Ranges ophiolite (Beebe, separates the prominent gravity high of the by drill-hole data (Hall, 1982). Where it is well 1986; Langenheim et al., 2012). Assignment of Santa Lucia Range to the east from gravity lows exposed at its northwest end, it places ophiolite magnetic susceptibilities was guided by more offshore to the west. South of San Simeon, the against Miocene sedimentary rocks and coin- than 1000 magnetic susceptibility measure- fault lies near the base of a signifi cant gravity cides with a strong magnetic gradient. To the ments from hand samples and outcrops in the gradient for a stretch of 70 km, consistent with southeast, the map trace is poorly constrained study area. For areas with complexly interleaved northeast-side-up slip and a steep northeast dip. by surfi cial data and deviates from the south- ophiolite and Franciscan Complex, we used an South of Point Sal, the gravity gradient con- west edge of a magnetic body, which we sug- average susceptibility that matched both the tinues in a more diffuse fashion ~30–40 km to gest is a better indicator of the location of the amplitude and gradient of the anomaly. Maxi- Point Arguello, where it begins to change to a fault. In contrast, the Quaternary map trace of mum depth of the magnetic rocks is limited by more easterly strike and wraps around the coast- the Los Osos fault is well known where it forms the Curie isotherm for magnetite. Modeling of line uninterrupted, merging with the striking the northeast margin of the Irish Hills (Fig. 2). heat fl ow across the Coast Ranges suggests that east-west gravity gradient associated with the We interpret the absence of prominent gravity temperatures are lower than 580 °C to depths of northern margin of the offshore Ventura Basin and magnetic gradients along this fault, except 20–25 km (Erkan and Blackwell, 2009)—essen- (Fig. 3). along its northeasternmost extent, to refl ect at tially to the base of the crust. We did not try to fi t The Reliz-Rinconada fault is marked by most a small amount of cumulative displace- every short-wavelength magnetic anomaly, but magnetic gradients for a stretch of nearly 100 ment, consistent with a low slip rate, recent ini- instead concentrated on matching the shape and km from San Ardo to the vicinity of San Luis tiation, or both. amplitude of the magnetic anomalies that per- Obispo (Fig. 4). It bounds the southwest side tain to fault geometry. of the prominent magnetic high associated with GEOPHYSICAL MODELS Assignment of densities for the Mesozoic the granodiorite of the La Panza Range (Fig. 4). basement and overlying sedimentary cover is About 35–40 km to the north near Jolon Valley, Approach guided by more than 700 hand sample mea- it bounds the east side of an areally equivalent, surements from published sources (Ross, 1972, but lower-amplitude, magnetic high, the source We present several joint gravity and mag- 1982) and this study as well as from well logs. of which is concealed beneath Cenozoic and netic models across the central California Coast For the Neogene sedimentary rocks, densities Mesozoic sedimentary rocks. Gravity gradients Ranges, focusing in particular on the geom- from hand samples are often biased toward also mark this stretch of the fault zone. Continu- etry and depth extent of the San Gregorio–San higher values due to diffi culty in obtaining a ity of the gravity gradient separating high values Simeon–Hosgri and Reliz-Rinconada faults. hand sample in loosely consolidated materials. in the San Rafael Mountains from gravity lows Constraints on the geometry and depth of these The most direct measure of the density of the in the Huasna syncline suggests that the Rinco- faults are derived primarily from magnetic data, Neogene sedimentary sequence comes from nada fault joins with the East Huasna fault to the although gravity data provide information on fi ve borehole gravity surveys in the Santa Maria southeast as a continuous fault system. North of fault dip for the upper 2–3 km of the crust, espe- Basin (Beyer et al., 1985) and a handful of den- Greenfi eld, the northern, Reliz segment of the cially on profi les C-C′ and G-G′. sity logs in northern Salinas Valley (Tiballi and fault lies at the base of a 40-km-long east-facing We used a 2.5-dimensional simultaneous Brocher, 1998). gravity gradient (Fig. 3). gravity and magnetic modeling program based The depth extent modeled for the crustal Magnetic and gravity data support linking on generalized inverse theory. The program sources depends on the assigned density and the Oceanic, West Huasna, Santa Maria River, requires an initial estimate of model parameters magnetization contrasts. A higher density and and Little Pine faults as one major structure. (depth, shape, magnetization, and density of magnetization contrast is needed if the modeled Like the Hosgri fault, this fault system forms suspected sources), and then selected parame- source is thinner, which in turn can lead to a a boundary between magnetic domains. This ters are varied manually in an attempt to reduce shallower dip for the physical property contrast. composite fault marks the sharp southwestern the weighted root-mean-square misfi t between The geometry of the base of modeled sources is margin of a prominent magnetic high associated the observed and calculated potential fi elds. poorly resolved by the potential-fi eld data. with ophiolite north of San Luis Obispo. South The initial model estimate is based on mapped of San Luis Obispo, it separates shorter-wave- geologic relationships, physical property infor- Results length anomalies to the west from a smoother mation, and well intercepts. The amplitude of magnetic fi eld to the east for a distance of ~60 an anomaly is not the only attribute to match; San Gregorio–San Simeon–Hosgri Fault km. Southeast of Santa Maria Valley, it again gradients and infl ections are critical parameters Potential-fi eld models, described in more forms the southwestern margin of a prominent constraining the depth to the top of the source detail in the following, indicate that the San magnetic high that coincides with exposures and its shape. Simeon and Hosgri faults are vertical to steeply of serpentinite and Franciscan Complex in Magnetic properties are assigned to match northeast dipping in the upper 3–15 km. The the hanging wall (northeast side) of the Little the amplitudes of the observed anomalies and San Gregorio fault appears to dip moderately to

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the northeast where it is modeled in Monterey using ocean-bottom seismometers (Begnaud et northeast dip of ~65°–70° for the Hosgri fault in Bay. Depending on the physical property con- al., 2000; Simila et al., 2006) and with northeast the upper 3 km of the crust, while a vertical fault trast chosen, the fault can be modeled to extend dips on seismic-refl ection profi les (C. Sorlien leads to a substantial mismatch (~2 km) in the as deep as the base of the seismogenic zone at and A. Smith, 2012, electronic commun.). We location of the gravity gradient. The magnetic depths of 12–15 km. note here that the magnetic gradient becomes anomaly associated with the Hosgri fault can be In Monterey Bay, modeling across profi le steeper to the south and north of profi le A-A′, fi t in two ways: (1) moderately magnetic mate- A-A′ (Fig. 5) indicates a moderate northeast suggesting that the fault dip steepens away from rial (0.0075 SI) extending vertically from depths dip for the contact between magnetic rocks on the modeled section. of 3 km to 15 km, representing an average sus- the southwest (undivided Franciscan Complex Farther south, along profi le B-B′ (Fig. 6), the ceptibility for interleaved Franciscan Complex and ophiolite, serpentinite) against nonmagnetic San Simeon fault, in one of the few places where and ophiolite, or (2) very magnetic material rocks to the northeast (basement of the Salinian this fault system is exposed, truncates mag- (0.025 SI; ophiolite and serpentinite; polygon block). The top of the magnetic source is equal netic anomalies associated with ophiolite that labeled C in Fig. 7) extending from depths of to or deeper than the base of the Miocene rocks, is exposed in outcrop (Mattinson and Hopson, 3 km to 8 km. In both models, magnetic mate- here defi ned by seismic-refl ection data (Aiello, 2008) on its west side. Modeling of the mag- rial does not extend along the Hosgri fault to the 2005) and constrained loosely by the gravity netic high indicates that the ophiolite is bounded surface, but instead surfaces to the east of the data. The dip of the fault depends on the depth by a vertical fault from 4 to 10 km depth (Fig. fault, along an offshore projection of the Los extent of the magnetic property contrast. If the 6). A small gravity gradient is associated with Osos fault, consistent with results of Watt et al. magnetic property contrast (0.01 SI) extends to the fault, but it does not help to constrain the dip (2011a) based on seismic-refl ection and more 15 km, the dip is ~55°; if a slightly higher con- of the fault. detailed, marine magnetic data. trast (0.0126 SI) extends to 10 km, the modeled In Estero Bay (profi le C-C′ in Fig. 2), the The southernmost model D-D′ extends dip is less, ~45°. A vertical fault leads to a much Hosgri fault marks the base of a steep gravity across the Hosgri fault at the latitude of Point steeper gradient that is west of the observed gra- gradient and the western side of a broad mag- Sal. The top of the magnetic source is equal to dient. Our results are consistent with estimated netic high. It places Cenozoic sedimentary or deeper than the base of the Miocene section, dips of 50° to 70° for the San Gregorio fault in rocks to the west against Mesozoic basement to here defi ned by seismic-refl ection data (C.R. Monterey Bay based on relocated seismicity the east. Model C-C′ (Fig. 7) indicates a steep Willingham, 2011, written commun.). Mod- eling of the magnetic data (Fig. 8) indicates a near-vertical fault that dips 80°–85° to the northeast and extends to a depth of 10 km. A A’ W E 100 Observed Reliz-Rinconada Fault Potential-fi eld models across the Reliz- Rinconada fault indicate that it is generally 0 steeply dipping, but its dip direction changes Calculated Calculated for along strike. Three models cross the fault in the dashed body vicinity of the La Panza Range, where model- Magnetics (nT) -100 ing indicates a near-vertical dip (Figs. 7, 9, 0 Gravity high based on and 10). The southernmost model F-F′ (Fig. -10 one measurement 10) crosses through the apex of the prominent Observed north of profile -20 Calculated magnetic anomaly of the La Panza Range, where the Rinconada fault juxtaposes Francis- -30 Calculated for can Complex and its sedimentary cover to the -40 dashed body southwest against granodiorite of the La Panza Gravity (mGals) Range to the northeast. In order to match both San Gregorio Fault Zone the position of the magnetic gradient along the QTs MONTEREY D=2670 BAY 0 Rinconada fault and the amplitude of the mag- water netic high, granodiorite with a magnetic suscep- QTs Jsp Kgr D=2270 D=2570 tibility matching the average measured value 5 KJf/Jo D=2670 KJf S=0.0126 for these rocks must surround a signifi cantly D=2670 D=2660 D=2720 D=2600 Depth (km) S=0.0057 more magnetic core that extends to a depth of V.E.=1 S=0.0057 S=0.0057 10 20 km. The base of the magnetic body could be Jsp to 15 km depth 0 15 40 as shallow as 15 km, as shown in models C-C′ Distance (km) and E-E′, but this would require an increase in Figure 5. Gravity and magnetic model across profi le A-A′. D and S are density and magnetic suscepti- the modeled susceptibility. The base of the mag- bility in kg/m3 and SI units. If no value for S is given, S equals zero. The eastern edge of the magnetic netic body may coincide with a band of gently rocks (unit Jsp) projects to the base of the Quaternary and Tertiary sedimentary strata (unit QTs) along east-dipping refl ections imaged along seismic the San Gregorio fault zone. Dashed black line on cross section is an alternate geometry of the mag- profi le SJ-6 at a depth of ~15–20 km (Trehu and netic rocks if they extend to 15 km depth. This geometry produces the dashed gray curves in the upper Wheeler, 1987). Regardless of the depth extent, panels. Units in cross section: QTs—Quaternary and Tertiary sedimentary rocks; Kgr—crystalline basement of the Salinian block; KJf—Franciscan Complex; Jsp—serpentinite; KJf/Jo—ophiolite, undivided modeled magnetic susceptibility, or presence of Franciscan Complex and Coast Ranges ophiolite. Note gravity high 10 km west of fault may be an arti- a concealed more magnetic core, a near-vertical fact because it is not refl ected in the magnetic or seismic-refl ection data. V.E.—Vertical exaggeration. dip is needed to match the location of the

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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. K B’ NE J QTp D=2070 I Kgr D=2670 S=0.013 H (Y=2-10km) Calculated Calculated G equals zero. Dotted Dotted equals zero. F S E D , 05300109 Exxon-Mobil McCool 1 J Kgr D=2670 , 05320466 TEPI Rosenberg (NCT-1) 34 TEPI Rosenberg (NCT-1) , 05320466 , 05300110 Exxon-Mobil San Ardo 23-35 Exxon-Mobil San , 05300110 I H is given, is given, , 05301220 Exxon-Mobil A. Orradre Et Ux 1 , 05301220 Exxon-Mobil S , 05320266 Energy Prod & Sales Burreson 4 K G QTs D=2270 Ts Wells shown Wells D=2270/2420 Tv D=2420 Tertiary sedimentary rocks; Ks—Cretaceous Ks—Cretaceous sedimentary rocks; Tertiary te; Jsp—serpentinite. Bodies marked by dotted dotted by Bodies marked Jsp—serpentinite. te; Ts C Kgr Rinconada Zone Fault D=2670 S=0.013 D=2070/2420 B Kgr magnetic data but is based on relocated aftershocks of aftershocks magnetic data but is based on relocated Kgr D=2670 S=0.013 D=2670 S=0.013 QTs , 05300102 Exxon Aurignac 1 , 05300102 Exxon D=2070 E Kgr 40 D=2670 , 05301222 Exxon Mobil U.S.L. 25-9 s in cross section: QTs—Cenozoic sedimentary rocks and depos- sedimentary rocks QTs—Cenozoic section: s in cross D , 05301407 TEPI Aurignac (NCT-1) 29 Aurignac (NCT-1) TEPI , 05301407 , 05301431 Chevron U.S.A. Inc. Hall 1 F C , 05301147 C.H. Chamberlin Shepherd 1 , 05301147 QTs , 05320579 Chevron Stimson Ranch Et Al. , 05320579 Chevron Stimson Ranch Et A D=2170 B Fault Jolon and SI units, respectively. If no value for for If no value respectively. and SI units, 3 Kgr D=2670 S=0.013 Tm D=2270 KTs Observed A D=2470/2570 survey boundary Kgr Ks D=2670 KJf D=2570

Nacimiento FaultD=2670 Kgr D=2670 Ks D=2670 Distance (km) Jsp/Jo D=2720 S=0.013 KJfv D=2670 S=0.013 20 are density and magnetic susceptibility in kg/m are KJf S D=2690 S=0.004 Observed and D . ′ le B-B le Oceanic Fault KJf D=2700 San Simeon Zone Fault Jo D=2750 S=0.013 coastline KJf D=2700 Ts V.E.=1 V.E.=1 D=2270 water 0 B SW 0 5 10 0 0

-20 -40

250 125 -125

Depth (km) (km) Depth Gravity (mGals) Gravity Magnetics (nT) Magnetics Figure 6. Gravity and magnetic model across proﬁ and magnetic model across Gravity 6. Figure Unit the south. that lies out of the plane section to within the Salinian block end is a magnetic pluton body at eastern and KTs—Cretaceous Formation; Tv—Vaqueros sedimentary rocks; Ts—Tertiary Formation; Tm—Monterey Robles Formation; QTp—Paso its; and gravity by is not constrained of the Oceanic fault that the geometry Note basement. Mesozoic overlying rocks denote pattern exaggeration. V.E.—Vertical the 2003 San Simeon earthquake. sedimentary rocks; Kgr—Salinian basement; KJf—Franciscan Complex; KJfv—metavolcanic rocks of the Franciscan Complex; Jo—ophioli Complex; of the Franciscan rocks KJfv—metavolcanic Complex; KJf—Franciscan Kgr—Salinian basement; sedimentary rocks;

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CC’tly east-dipping reﬂ ections on SJ-6, just east of SW NE the surface trace of the Rinconada fault and with higher terrain east of the Rinconada fault. About 50 km to the northwest, geologic 300 Observed analysis and modeling of magnetic data along ′ 200 B-B (Fig. 6) may indicate a more complicated geometry and history for the Rinconada fault. Magnetic data indicate a near-vertical dip for 100 the magnetic contrast under the surface trace of the Rinconada fault below ~3 km. Steeply dip- 0 Calculated (B) ping fault strands in the upper 3 km are based

Magnetics (nT) Calculated on folded Cenozoic sedimentary strata and on –100 Calculated drill-hole data and a seismic-refl ection profi le (C) 5–10 km to the northwest of B-B′ (Graham et –200 al., 1991). Gravity data are consistent with over- 0 all southwest-side-up displacement on the fault Calculated if vertical and with a steep southwest dip for the eastern- Hosgri most strand of the Rinconada fault. Although –10 fault the magnetic contrast beneath 3 km may refl ect Observed a vertical plutonic contact rather than a fault, the magnetic gradient produced by this contrast is –20 roughly collinear with the surface trace of the Calculated if Rinconada fault for at least 25 km along strike Hosgri fault dips NE (Fig. 4), suggesting a fault origin is more likely. Gravity (mGal) in upper 3 km –30 The steeply dipping strands inferred from fold attitudes and other data likely refl ect transpres- sional slip across the Rinconada fault since HF Jsp coast WHF RF the late Miocene (Titus et al., 2007), while the QTs D=2570 Ks QTs D=2570 D=2320 S=0.013 A B C D=2370 deeper magnetic contact may refl ect older, more 0 purely strike-slip displacement along the fault. D=2670 Kgr S=0.005 The idea of an older (Miocene), near-vertical D=2670 Kgr C D=2770 Jo fault was also proposed by Graham et al. (1991), KJf NF S=0.005 D=2690 D=2670 likely because the dipping Rinconada fault was S=0.03 S=0.019 KJf/Jo not deemed compatible with tens of kilometers Depth (km) 10 D=2670 KJf D=2670 D=2670 B of strike slip attributed to the fault. S=0.0075 ? The northernmost model G-G′ (Fig. 11) V.E.=1 crosses the Reliz segment of the Reliz-Rincon- ada fault where it lies at the base of a gravity gra- 0 10 D=2670 20 30 40 50 60 S=0.0075 dient arising from the density contrast between Distance (km) Wells shown Salinian block basement to the southwest and A, 07900192, Colony Oil J.R. Davis 1 Cenozoic deposits to the northeast. The fault is B, 07900384, Gene Reid Drilling Heilman 1 C, 07900104, Amerada Hess Co. Mcwilliams 1 not readily imaged by the magnetic data along the profi le. We show two models for the geom- Figure 7. Gravity and magnetic model across profi le C-C′. D and S are density and magnetic suscep- etry of the Reliz fault, assuming two different tibility in kg/m3 and SI units, respectively. If no value for S is given, S equals zero. Dashed black line B on cross section represents the alternative interpretation of a northeast-dipping, rather than ver- density contrasts. Assuming a typical density tical, Rinconada fault and produces the dashed black curve in the uppermost panel. White dashed contrast of −400 kg/m3 between the basement body C on cross section is a magnetic body with a susceptibility of 0.025 SI units and produces the complex and the overlying sediments indicates calculated curve C in the uppermost panel. Faults: HF—Hosgri fault; NF—Nacimiento fault; RF— a fault that dips moderately (45°) to the south- Rinconada fault; WHF—West Huasna fault. Units in cross section: QTs—Quaternary and Tertiary west. Assuming a somewhat lower density con- sedimentary rocks; Ks—Cretaceous sedimentary rocks; Kgr—Salinian block granitic rocks; KJf— trast of −300 kg/m3, the dip steepens to ~70°. Franciscan Complex; Jsp—serpentinite; Jo—ophiolite. Note that the Nacimiento fault is not constrained by density or magnetic model and is projected into section. V.E.—Vertical exaggeration. A lower density contrast is supported by several density logs in the broader Salinas Valley region (Tiballi and Brocher, 1998). Regardless of the density contrast chosen, the Reliz fault dips to magnetic gradient along the Reliz-Rinconada anomaly with moderate southwest or northeast the southwest in the upper 1–2 km of the crust. fault. A more magnetic core is not needed for dips of the Rinconada fault; a steep dip of 77° models E-E′ and C-C′, which cross the magnetic to the northeast provides a calculated curve Other Faults anomaly where it has a lesser amplitude. that best fi ts the observed magnetic data (inset Our models locally provide constraints on Along seismic profi le SJ-6, model E-E′ in Fig. 9). A slight northeast dip is consistent the attitudes of other faults in the central Cali- shows the degree of mismatch of the magnetic with the apparent termination of the deep, gen- fornia Coast Ranges. Models C-C′ (Fig. 7) and

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D D’ SW NE 200 100 0 Calculated –100 Observed –200 Magnetics (nT) 0 –10 –20 –30

Gravity Observed (mGal) –40 Calculated –50 HF LHF coast CF

A B C D 0 QTS QTs D=2320 D=2270 Jo D=2750 Jo/KJf KJf S=0.0138 D=2700 D=2670 KJe? KJf 5 S=0.0126 ? D=2670 D=2670 Depth (km) KJf? D=2670

V.E.=1 10 0 10 20 30 Wells shown A, 08321609, Union Oil Copen 2-29 Distance (km) B, 08320604, Argo Petroleum Union Sugar 3 C, 08320409, Argo Petroleum Donovan 1 D, 08304126, Union Donovan 1

Figure 8. Gravity and magnetic model across proﬁ le D-D′. D and S are density and magnetic susceptibility in kg/m3 and SI units, respectively. Units in cross section: QTs—Quaternary and Tertiary sedimentary rocks; KJe—Espada Formation; Jo—ophiolite; KJf—Franciscan Complex; Jo/KJf—ophiolite and Franciscan Complex, undivided. Faults: CF—Casmalia fault; HF—Hosgri fault; LHF—Lions Head fault. V.E.—Vertical exaggeration.

F-F′ (Fig. 10) cross the West Huasna fault where Huasna fault may become more listric at depths to the southeast of profi le B-B′. However, the it forms the southwestern margin of a prominent greater than 6 km, below the sensitivity of our signifi cant seismic-velocity contrast across the magnetic anomaly originating in serpentinite model. fault is not accompanied by an equally signifi - and ophiolite. In both models, the fault dips The Nacimiento fault, although primarily cant gravity gradient. The Nacimiento fault even steeply northeast (70°–80°) to a depth of ~6 km. active during the Late Cretaceous (Dickinson, strikes across a major, continuous gravity high Generally higher topography northeast of the 1983), is a major structure that separates the in the Santa Lucia Range (Fig. 4). The absence fault along much of its length (Fig. 2B) suggests Franciscan Complex from the Salinian block. of a signifi cant density contrast suggests that the a reverse component of slip. Geologic mapping The geometry of the Nacimiento fault is locally fault places relatively coherent, higher-velocity shows that the West Huasna fault is vertical to constrained by modeling of magnetic data along Salinian block rocks (granite and gneiss) against northeast-dipping, but with down-to-the-north- profi le B-B′ (Fig. 6). Downward projection of fractured, lower-velocity, but equally dense east displacement in the Tar Spring Ridge and serpentinite and ophiolite exposed just southwest Franciscan Complex rocks (as exemplifi ed by Lopez Mountains quadrangles (McLean, 1994, of the fault precludes a vertical or southwest dip units of mélange and broken formation). 1995), located a short distance southeast of F-F′ for the Nacimiento fault in the upper 5–6 km. Model D-D′ (Fig. 8) sheds light on the geom- (Fig. 10). Our modeled dip is steeper than the The concealed southwest margin of magnetic etry of the Lions Head and Casmalia faults in the focal mechanism determined for the 2003 San Salinian block basement (Kgr in Fig. 6) requires Santa Maria Basin. Although the profi le crosses Simeon main shock (46° northeast; McLaren the fault to dip steeply. Geologic mapping in the both faults obliquely, it crosses the Lions Head et al., 2008), ~40 km northwest of our modeled southern Santa Lucia and San Rafael Mountains fault where it coincides with a pronounced mag- profi les and where the Oceanic fault—inter- (Vedder et al., 1991) also indicates a northeast netic gradient caused by the southwest edge of preted here as a northwestward continuation dip for the Nacimiento fault. The steep north- the Point Sal ophiolite. The difference in loca- of the West Huasna fault—has a more westerly east dip of the fault derived from the magnetic tion between the gravity and magnetic gradi- trend and presumably becomes more transpres- data is consistent with results from a seismic- ents along the northeast margin of the ophiolite sional or compressional. Alternatively, the West refraction study (Howie et al., 1993) 50 km suggests that the Casmalia fault truncates the

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E E’ Hills. Along F-F′, the surface trace of the fault SW NE coincides with a modeled, near-vertical mag- 300 best-fit (77˚NE) netic boundary (Fig. 10), but it deviates away fit with 52˚SW dip 200 from the more linear magnetic boundary along 100 strike out of the cross section. This relationship 0 fit with 52˚NE dip suggests that the Los Osos fault is not a major -100

Magnetics (nT) structure with signiﬁ cant cumulative displacement. Rather, we suggest that the pronounced Observed 0 linear magnetic boundary, slightly northwest of -10 the mapped Los Osos fault, is a major structure, possibly as old as Mesozoic. This boundary -20 Calculated coincides with an alignment of Oligocene vol-

Gravity (mGals) -30 Rinconada canic necks (the Morro Rock–Islay Hill volca- Qs Ks Ts Fault nic complex of Ernst and Hall, 1974). D=2570 D=2270 D=2070 A B C D The southwest margin of the Irish Hills corresponds with a linear magnetic boundary 0 Jsp D=2670 QTs that lies just offshore. The southern edge of S=0.013 Ks QTs D=2270 the magnetic body, as modeled along profi le NF D=2570 D=2170 F-F′, is nearly vertical and also coincides with the Shoreline fault, a linear and near-vertical Kgr granodiorite of La lineament defi ned by seismicity (Hardebeck, D=2670 Jo Panza Range 2010) and scarps on the seafl oor (Nishenko et D=2670 D=2670 5 S=0.019 S=0.020 al., 2010). The model indicates a depth extent of 6 km, although the body could be thin- Franciscan Complex ner, deeper, and more magnetic, and still fi t D=2670 the data. For example, Watt et al. (2011a) fi t

Depth (km) more detailed marine and helicopter marine

120 data by extending the magnetic source to 8 km Inset 52˚NE d depth, with thin, near-vertical slices of ophio- 100 10 lite extending nearly to the surface. Note that 80 ip the magnetic gradient caused by the southwest 60 margin of the ophiolite extends farther to the 52˚SW dip 40

RMS error (nT)RMS error southeast than does the relocated seismicity.

20 Implications of Potential-Field Modeling 0 30SW50SW 70SW 90 70NE 50NE 30NE Potential-fi eld modeling indicates that the V.E.=1 Dip 15 San Gregorio–San Simeon–Hosgri and Reliz- 0 5 10 15 20 Rinconada faults are generally characterized Wells shown A, 07900262, S.E. Hogue 1 by steep dips through seismogenic depths. The Distance (km) B, 07900162, Canon Drilling Co. Heilman 1 C, 07900103, Amerada Hess Corp Creston Community 5-1 San Gregorio–San Simeon–Hosgri fault, where D, 07900201, A.J. Crose 1 it is not vertical, always dips to the northeast. Figure 9. Gravity and magnetic model across profi le E-E′ and the Rinconada fault, coincident with The Reliz-Rinconada fault, where not vertical, seismic profi le SJ-6. D and S are density and magnetic susceptibility in kg/m3 and SI units, respec- tends to dip southwest along its northern stretch tively. If no value for S is given, S equals zero. Units in cross section: Qs—Quaternary sediments; and northeast along its southern stretch. Mod- QTs—Quaternary and Tertiary sedimentary rocks; Ts—Tertiary sedimentary rocks; Ks—Cretaceous eled dips are consistent with those imaged by sedimentary rocks; Jo—Jurassic ophiolite; Jsp—serpentinite. Inset: Plot of root mean square error microearthquakes during the past three decades, (RMS) from magnetic model versus dip of the Rinconada fault. Best-fi t dip is 77° to the northeast. V.E.—Vertical exaggeration. primarily along the Morro Bay stretch of the San Gregorio–San Simeon–Hosgri fault (Hard- ebeck, 2010). Because the contacts modeled ophiolite at ~3 km depth. Modeling of the mag- Lastly, model F-F′ (Fig. 10) highlights struc- by the potential-fi eld data represent long-term netic data indicates steep dips for both faults in tures that bound the Irish Hills. The model displacements, the similarity in fault geometry the upper 5 km, with the Lions Head fault dip- crosses the Pismo syncline, characterized by a between the potential-fi eld data and the micro- ping to the northeast and the Casmalia fault dip- gravity low originating in Miocene and younger seismicity suggests that the fault geometry has ping to the southwest. Our results are consistent sedimentary rocks. Modeling of the magnetic been roughly constant over the lifetime of the with a seismic-refl ection profi le interpreted by data indicates that fairly magnetic rocks under- faults in these two places. This is contrary to Seeber and Sorlien (2000) ~25 km to the south- lie the syncline, likely the same ophiolite and interpretations of shallow dips based on seismic- east of D-D′ that also shows steep, inward-dip- serpentinite that are exposed in uplifted blocks refl ection data along the Hosgri fault (Crouch et ping attitudes for the Lions Head and Casmalia on both sides of the syncline. The Los Osos al., 1984) and balanced cross sections across (called the Orcutt fault in Seeber and Sorlien, fault, a Quaternary reverse or thrust fault, forms both the Hosgri and Rinconada faults (Namson 2000) faults in the upper 5 km. the northern topographic margin of the Irish and Davis, 1988a, 1988b, 1990). The shallow

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F F’ or even subduction-related fault. The geometry SW NE of the northern stretch of the Reliz-Rinconada 600 fault may reﬂ ect transpression that began dur- 500 ing the late Miocene or possibly a complicated transfer of slip from the Reliz-Rinconada fault 400 Observed 300 to the San Gregorio–San Simeon–Hosgri fault through the Santa Lucia Mountains. In either 200 case, we speculate that more moderate dips 100 may be the result of local fault interactions or 0 transfer of slip between faults. If so, deviations

Magnetics (nT) –100 Calculated from near-vertical faults should be considered –200 along other stretches of these faults where –300 fault interaction or slip transfer is known, even 10 though physical property contrasts are lacking or microseismicity is absent. Calculated 0 Steeply dipping faults that cut the whole seismogenic crust indicate that much of the –10 displacement on these faults should be characterized by strike-slip displacement, rather than –20 by mostly reverse or normal offsets. Excep- tions may be the result of fault interactions, as Gravity (mGal) Observed noted in the previous paragraph, or of subse- –30 quent deformation of previously vertical fault planes by regional transpression that began Ts Ks during the late Miocene. Next, we estimate Pismo D=2640 D=2370 D=2570 S=0.009 cumulative strike-slip offset along major faults SF syncline LOF D=2660 WHF RF Ts Ts Tv D=2600 D=2320 D=2320 by correlating magnetic and gravity anomalies Ts S=0.006 A 0 D=2270 and then compare these estimates with those D=2710 D=2670 derived from geologic observations. 5 S=0.013 D=2640 S=0.013 Jo D=2670 D=2720 D=2650 ESTIMATES OF CUMULATIVE FAULT D=2650 Granodiorite of La Panza Range S=0.013 S=0.026 S=0.007 S=0.044 OFFSET 10 D=2720 D=2670 Depth (km) S=0.013 S=0.019 D=2680 15 S=0.038 Identifying offset magnetic and gravity Franciscan Complex and ophiolite anomalies across faults is analogous to identify- D=2700 ing offset geologic features at the surface. We 20 search along the trace of a candidate fault for 0 5 10 15 20 25 30 35 40 45 50 55 60 65 elongate magnetic anomalies that are subper- V.E.=1 Distance (km) Well shown pendicular to the fault and that are truncated A, 07920564, BP Exploration Inc. Upton Canyon 1 at the fault trace. We then search the opposite fault block for similar anomalies that are trun- Figure 10. Gravity and magnetic model across profi le F-F′. D and S are density and magnetic sus- cated against the opposite side of the fault and 3 ceptibility in kg/m and SI units, respectively. If no value for S is given, S equals zero. Bodies west estimate the magnitude of offset by measuring of the Rinconada fault consist of undivided Franciscan Complex and ophiolite (unless otherwise labeled). These bodies are set to a depth extent of 6 km. Unit labels: Jo—Jurassic ophiolite; Ks— the along-fault separation between correspond- Cretaceous sedimentary rocks; Ts—Tertiary sedimentary rocks; Tv—volcanic rocks of the Morro ing distinctive features of the anomaly. In most Rock–Islay Hill volcanic complex. LOF—Los Osos fault; RF—Rinconada fault; SF—Shoreline fault; cases, that distinctive feature is the inferred lat- WHF—West Huasna fault. V.E.—Vertical exaggeration. eral boundary of the causative magnetic body. Such boundaries (black dots on Fig. 12A) are determined automatically from the digital aero- dips interpreted by Crouch et al. (1984) could be the San Gregorio fault in Monterey Bay and magnetic grid by the technique of Blakely and out-of-plane refl ectors or artifacts of processing; possibly the northern part of the Reliz-Rincon- Simpson (1986), modifi ed slightly to focus the however, only line drawings of the data across ada fault. The moderate dip of the San Gregorio technique on the top edge of the magnetic body. the fault are published. A key assumption in pro- fault likely steepens to the south and north in This technique has been used to measure off- ducing the area-balanced fault-bend fold-style Monterey Bay as indicated by tightening of the sets along faults in the Mojave Desert, yielding cross sections by Namson and Davis (1988a, magnetic gradient to the south and north. The results for right-lateral offset along these faults 1988b, 1990) was that no material has moved in profi le crosses the San Gregorio fault close to its consistent with geologic data (Jachens et al., or out of the line of the section, an assumption intersection with the Monterey Bay fault zone, 2002). Uncertainty in the offset estimate arises that is violated by signifi cant strike slip. and the deviation from a steep dip may refl ect from (1) possible misidentifi cation of offset fea- Although our models indicate generally interaction between the two fault systems, rather tures, (2) imprecision in defi ning the offset fea- steep dips for major faults, two exceptions are than from reactivation of an earlier extensional tures from the magnetic data, (3) dip-slip offsets

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G G’ on the Hosgri fault. Furthermore, interpretation SW NE of seismic-reﬂ ection data suggests 3.5 km of strike-slip offset since ca. 4 Ma between Point 40 observed from grid survey boundary Sal and Point Conception (Sorlien et al., 1999a), 20 based on restoring folds that deform the top of (nT) calculated the Sisquoc Formation. An equally viable, and 0

Magnetic Field our preferred, interpretation is that the Hosgri fault offsets the magnetic basement (presumed –10 calculated to be Mesozoic) horizontally by no more than –20 5–10 km, which is consistent with the interpre- observed –30 tation of the offshore seismic-reﬂ ection data. (mGal)

Gravity from grid –40 The next prominent magnetic anomaly east of the Hosgri fault north of Point Arguello is the SANTA LUCIA RANGE Rinconada-Reliz SALINAS VALLEY band of concealed magnetic rocks that bisects fault fault the Santa Maria Basin and surfaces at the Juras-

SL Basement of the sic Point Sal ophiolite. As shown by the mag- Salinian Complex QTs netic data, the edges of this body extend off- D=2270 shore northwest of Point Sal and are truncated 1 S=0 D=2720 D=2670 at the Hosgri fault (body 1 in Fig. 12B). The S=0 S=0.00025 next prominent magnetic anomaly with similar Depth (km) 2 character west of the fault is located near San D=2670 Simeon (body 1′ in Fig. 12B). The anomalies S=0.0019 3 V.E.=1 ? ? are the same width where truncated by the 0 5 10 Hosgri and San Simeon faults. One of the fi rst Distance (km) studies that estimated strike-slip offset along the Hosgri fault zone correlated the ophiolite and Figure 11. Gravity and magnetic model across profi le G-G′. D and S are density and magnetic sus- its overlying sedimentary section at Point Sal ceptibility in kg/m3 and SI units, respectively. Dashed black line on bottom panel is basement contact with density of basin fi ll (unit QTs) increased to 2370 kg/m3 and susceptibility of basement with a similar ophiolite at San Simeon, giving a increased from 0.0019 to 0.0025 SI units; this confi guration produces the dashed calculated curves minimum estimate of 80 km of strike-slip offset in the upper two panels. V.E.—Vertical exaggeration. (Hall, 1975). Correlation of the magnetic anomalies supports that geologic estimate and argu- ably refi nes the estimate to 86–89 km because that give rise to apparent strike-slip offsets, and nounced gravity low (Fig. 3) and long-wave- the magnetic data can be used to extrapolate the (4) nonrigid behavior of the blocks on either length magnetic highs of the Ventura Basin and ophiolite exposed at Point Sal more than 10 km side of the fault, and (5) location errors in the Santa Barbara Channel (Fig. 4) wrap west and to the northwest offshore where it is truncated original magnetic surveys, which are less than northwest around the coastline without inter- by the Hosgri fault. 10 m. Given these factors, 5 km is a reasonable ruption, suggesting that the Hosgri fault does Recent, high-precision U-Pb zircon ages upper bound of the estimate of offset uncer- not continue south to the Channel Islands with for the ophiolites at Point Sal and San Simeon tainty. In the central California Coast Ranges, any signifi cant offset. This interpretation of the are indistinguishable at 165.580 ± 0.038 Ma strike-slip offsets defi ned on the basis of mag- potential-fi eld data supports previous interpre- (Mattinson and Hopson, 2008), supporting the netic and gravity data are generally in good tations of seismic-refl ection profi les (Steritz correlation of at least the Jurassic ophiolites. agreement with and in several cases refi ne those and Luyendyk, 1994; Sorlien et al., 1999b) As discussed by Sedlock and Hamilton (1991), from geologic data for the major through-going indicating that the fault zone does not extend however, much of this displacement may be faults (Table 1). across the Santa Barbara Channel. older than Neogene and predate initiation of We start our correlation of magnetic fea- slip on the San Andreas system. The key cor- San Gregorio–San Simeon–Hosgri Fault tures across this fault system at its southern end. relation is therefore that between the Tertiary Zone The southern extent of the Hosgri fault forms nonmarine conglomerates (Lospe Formation) the western edge of a magnetic body near Point that overlie the ophiolite. The type Lospe For- This fault zone is more than 400 km long, Arguello (PA on Fig. 12B) that consists of Fran- mation is located south of Point Sal, where iso- extending north from Point Arguello to just ciscan Complex and serpentinite exposed south topic and biostratigraphic data indicate depo- offshore of San Francisco, where it merges of the Santa Ynez River fault to the south of sition between 18 and 17 Ma (Stanley et al., with the San Andreas fault (Fig. 1). Seismic- Lompoc. On the west side of the Hosgri fault 1996). The conglomerates of the type Lospe refl ection data south of Point Arguello imply at this latitude, there is a broad magnetic high Formation near Point Sal include clasts from that the fault widens as it curves around the underlying the offshore Santa Maria Basin, the ophiolite and from various rock types of coast and, although poorly imaged, may be a the edge of which most clearly coincides with the Franciscan Complex, whereas the so-called northeast-dipping thrust (Steritz and Luyen- the Hosgri fault ~20–30 km north of the Point Lospe section near San Simeon contains clasts dyk, 1994). At its southern end, the Hosgri thus Arguello magnetic high. However, the poorly mostly of ophiolitic debris. The differences becomes either a westward continuation of the defi ned edges of the western magnetic body in clast compositions between the two areas North Channel fault system or is truncated by may refl ect in part relief on the surface of the probably refl ect local differences in sediment that fault system (Sorlien et al., 1999b). A pro- magnetic basement rather than horizontal offset source areas during the deposition of the Lospe

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122°30' 122° 121°30' 121° 120°30' 120° 119°30'W Magnetic Field 0 10 20 30 40 50 km (nT) 320 300 37°N 280 260 240 220 200 36°45' 180 160 140 120 100 36°30' 80 60 40 20 0 36°15' -20 -40 -60 -80 36° -100 -120

35°45'

35°30'

35°15'

35°

34°45'

A 34°30'

Figure 12 (on this and following page). (A) Aeromagnetic map with maximum horizontal gradients (black dots) that mark edges of magnetic bodies. Brown lines—faults (solid—Quaternary; dotted—older); blue line—coastline. (B) Annotated aeromagnetic map with correlated magnetic anomalies. Dark-gray lines outline correlated anomalies (labeled a, a′, etc.) across the Reliz- Rinconada, Chimineas, East Huasna (EHF), and South Cuyama (SCF) faults; purple lines mark edges of magnetic bodies (labeled 1, 1′, etc.) correlated across the San Gregorio–San Simeon–Hosgri and West Huasna faults. EHF—East Huasna fault; RF—Russell fault; SCF—South Cuyama fault; SCM—Santa Cruz Mountains; SM—Stanley Mountain; SYRF—Santa Ynez River fault. Anoma- lies PA and SRM are discussed in text. Red lines—faults (solid—Quaternary; dotted—older); blue line—coastline. (C) Reconstruc- tion of horizontal offset on faults based on matching magnetic anomalies. No offset was restored on the Santa Ynez River fault. Two positions of the Western Transverse Ranges are shown, one at its present-day location and the other after restoring 90° of clockwise rotation. Note that north is rotated ~45° counterclockwise.

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122°30' 122° 121°30' 121° 120°30' 120° 119°30'W Magnetic Field 01020304050 km (nT) 3'3' SCM 320 300 37°N Año Nuevo 280 San Gregorio 260 240 220 36°45' 200 San Andreas Fault 180 160 140 120 36°30' 100 Reliz - Rinconada Fault 80 60 Point 40 Sur Lockwood 20 36°15' 2'2' Valley 0 high -20 -40 -60 -80 36° -100 -120 Cape 3 San Martin La Panza 35°45' 1'1' a body Nacimiento Fault

b 35°30' San Simeon Barrett West Huasnaa'a' Fault Ridge 2 b' a'a' 35°15' c Point Chimin Buchon a"a" eas Fault Hosgri Fault 2 c' SM 35° EHF 1 SC Point F Sal 2"2" Naci RF m ien 34°45' to Fault SRM Point Arguello PA SYRF B 34°30'

N Barrett southern end Ridge of the San Joaquin ult Santa Barbara Cha Valley San Andreas Fa ult Fa Chimineas - Russell ( rotation restored) Lockwood S lt a Valley La Panza u nt yama F a high body th Cu a Sou SCM SM Y nnel a Fault n ad E Huas e on ast na z Fau ca.18 Ma inc lt - R SRM eliz R i R Cape v na Fault Point e San Huas r est Sal ic - W F Martin ean a Oc Point u S Buchon lt Año an Gre gorio Simeon- Nuevo - San Ho l today sgri d) Fault e hanne 50 km Point estor Sur ra C PA San otation r C Simeon Santa Barba (no r

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TABLE 1. COMPARISON OF FAULT OFFSETS FROM GEOLOGIC (AND OTHER) DATA AND GEOPHYSICAL fault with the somewhat less prominent mag- INTERPRETATIONS (FIG. 12B) netic high that encompasses the Lockwood Val- Fault Offset from geologic data Offset from geophysical data (km) ley high west of the Reliz-Rinconada fault (Fig. (km)*† Magnetic anomalies Gravity anomalies 12B). This correlation indicates 38–42 km of right-lateral displacement on the Reliz-Rinco- San Gregorio–San Simeon–Hosgri fault Point Arguello <5a <10 <10 nada fault. The higher estimate comes from Point Sal <10a–155b (90i) 86–89 matching the southern edges of the bodies and Point Buchon <10c–115j–155b 122–128 may be lower given the oblique angle between Cape San Martin <10c–160k (156b) 148–154 the southern edge of the western body and the Reliz-Rinconada 18d–74e 39–43 20–25 fault. The amplitude of the anomaly west of f e San Juan–Chimineas–Russell 13 –37 28 the Reliz-Rinconada fault is lower because the West Huasna 5g–15h 25–30 25–30 East Huasna NA 26 source is everywhere concealed beneath Ceno- zoic and Cretaceous cover; however, drill holes *Most recent estimate in parentheses. †Superscript numbers indicate the following: a—since ca. 4 Ma (Sorlien et al., 1999a); b—since ca. 12 Ma confi rm the presence of granitic rocks along the (Dickinson et al., 2005); c—since 38 Ma (Underwood and Laughland, 2001); d—Pliocene and younger (Durham, Lockwood Valley high. Dibblee (1976) also cor- 1965); e—since Late Cretaceous (Schwade et al., 1958); f—since Oligocene (Bartow, 1974; Dibblee, 1976); related the granitic basement of the Lockwood g—since Miocene (McLean,1993); h—Miocene and younger (Hall et al., 1995); i—since Jurassic (Mattinson and Hopson, 2008); j—since early Miocene (Graham and Dickinson, 1978); k—Clark (1998). Valley high with a similar basement high north of Paso Robles (35 km of right-lateral offset), but our data allow more precise mapping of the Lockwood high than the scattered drill holes. Formation. The youngest detrital zircon ages (body 3′ in Fig. 12B). We attribute the difference Our cumulative offset estimate is also within the of the conglomerates in the type Lospe Forma- in character between the two anomaly patterns error of Graham’s (1978) estimate of 43 ± 4 km tion at Point Sal and the conglomerates that to the difference in level of exposure, with base- based on offset early Miocene paleo-isobaths, overlie the San Simeon ophiolite are the same ment rocks exposed at Cape San Martin versus suggesting that our estimate, which can only (J.P. Colgan, 2011, written commun.) and thus an entirely concealed source for the magnetic be constrained as post-Cretaceous, is refi ned to are consistent with the two sections at Point anomaly beneath the outer Santa Cruz Basin be post–early Miocene. Similar estimates argue Sal and San Simeon being part of the same late offshore of Año Nuevo (expressed nicely as a against pre-Miocene displacement along the early Miocene depositional system. gravity low in Fig. 3). The offset estimate based Reliz-Rinconada fault, such as might be inter- The next prominent magnetic anomaly east on the correlation of these magnetic anomalies preted from earlier estimates of 64–72 km of of the fault north of Point Sal is near Point is within uncertainty of the 156 ± 4 km off- post-Cretaceous offset (Schwade et al., 1958; Buchon, with a pair of Mesozoic ophiolites that set estimated by Dickinson et al. (2005) done Dibblee, 1976). bracket the Pismo syncline. The anomaly pat- by correlating Cretaceous sandstones (Pigeon Another geophysical correlation of gravity tern consists of two magnetic highs bracket- Point and Atascadero Formations southwest and lows (R1 and R2 in Fig. 3), originating mostly ing a low (body 2 in Fig. 12B). The cross-fault northeast of the fault, respectively) and Lower in Miocene sedimentary rocks across the Reliz- counterpart is located 122–128 km to the north- Miocene basaltic volcanic rocks. Note that Rinconada fault (Jachens et al., 1998), provides west along the Hosgri–San Simeon fault near another of their correlations for this locality, the an estimate of ~20–25 km of presumably post- Point Sur, which also shows a double-peaked Pescadero felsite with the Oligocene Cambria Miocene displacement. This estimate is similar magnetic high (body 2′ in Fig. 12B). Geo- felsite, is no longer tenable because zircon ages to the estimate of 18 km of offset of Pliocene logic support for correlation of these magnetic show the Pescadero felsite to be Cretaceous in facies rocks along the Rinconada fault by Dur- anomalies is indicated by Miocene sedimentary age (Ernst et al., 2011). ham (1965). As pointed out by Graham (1978), strata overlying the Franciscan Complex near Others (Underwood et al., 1995; Underwood these offset estimates suggest episodic slip on Point Sur that are correlated with the Edna and and Laughland, 2001) have suggested that differ- the Reliz-Rinconada fault. Miguelito Members of the Pismo Formation ences in thermal history based on vitrinite refl ec- Continuation of slip south of the Rinconada exposed between Cambria and Pismo Beach, tance data preclude lateral displacements of more fault onto the East Huasna fault is supported by some 90–160 km to the southeast (Hall, 1991). than 5–10 km on the San Gregorio–San Simeon– the offset Nacimiento fault. The Nacimiento Based on their similar depositional environ- Hosgri fault zone, correlating the southwest mar- fault, forming the western margin of Salin- ments, Dickinson et al. (2005) argued that the gin of a thermal high at Cape San Martin (which ian rocks, appears to be offset ~35 km along a Cretaceous Point Sur rocks correlate with the they interpret as an overprint of a Late Creta- connected Rinconada–East Huasna fault and Point San Luis slab, indicating 155 km of dis- ceous thermal event since 38 Ma) with equivalent aligns two, relatively low-amplitude, curvilin- placement. We concur with this correlation, but paleothermal estimates near San Simeon. Cor- ear magnetic anomalies northeast of the fault. we refi ne the offset estimate to 122–128 km by relations based on the vitrinite refl ectance data, The magnetic high at Stanley Mountain, how- mapping the Mesozoic magnetic basement of however, suffer from very limited outcrops avail- ever, appears to be offset ~26 km from a broad the Port San Luis slab beneath water and sedi- able for sampling on the west side of the fault and high west of the East Huasna fault and may ment cover offshore to the Hosgri fault. no sampling in the Año Nuevo area. suggest that some slip is also partitioned onto The northernmost magnetic anomaly on the other faults. A magnetic high in the lower plate east side of the Hosgri—San Simeon—San Gre- Other Faults of the South Cuyama fault appears to have been gorio fault system for which we fi nd an offset underthrust in a southeast direction (direction equivalent is at Cape San Martin (body 3 in Fig. Reliz-Rinconada Fault of strike slip) by ~10 km. In this same area, the 12B), and its cross-fault counterpart is located We correlate the prominent magnetic high of fault is clearly a thrust or reverse fault, as shown 148–154 km to the northwest near Año Nuevo the La Panza Range east of the Reliz-Rinconada by well data (Vedder and Repenning, 1975). If

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this offset correlation is valid, this might argue Juan fault offset occurred during the same time data set that covers the entire fault system, both for underthrusting accommodating strike slip. frame. We also suggest that the full 26–29 km onshore and offshore. The few rock-unit corre- These correlations are diffi cult to test with exist- of offset from the concealed Russell fault was lations, although critical for determining the age ing geologic information because the sources of transferred to the Chimineas fault, rather than of displacement, are limited by the paucity of these correlated magnetic anomalies (except for half partitioned onto a postulated fault east of outcrops west of the fault and by uncertainty in Stanley Mountain) are not exposed. Barrett Ridge (Yeats et al., 1989). The magnetic offshore projections of onshore geology to the anomaly pattern is most easily explained by off- eastern side of the fault. Nevertheless, our geo- San Juan–Chimineas–Russell Fault set solely on the Chimineas fault. physical correlations across the San Gregorio– We posit that the prominent magnetic anom- San Simeon–Hosgri fault are supported by other aly at Barrett Ridge is offset from the equally West Huasna Fault data sets, and most are supported by rock-unit prominent magnetic high of the La Panza Range We correlate a magnetic high-low-high pat- correlations. by the San Juan–Chimineas–Russell fault, giv- tern across the West Huasna fault (bodies 2 and Northward-increasing right-lateral displace- ing a maximum estimate of 28 km of right-lat- 2″ in Fig. 12B) for an estimated right-lateral ment on the San Gregorio–San Simeon–Hosgri eral slip since the Late Cretaceous. Our estimate offset of ~25–30 km since the Mesozoic. This fault helps to resolve the discrepancy that arises is similar to that based on drill holes by Yeats et anomaly pattern is the southwest continuation if 156 ± 4 km of Neogene right-lateral offset al. (1989), 26–29 km, for the concealed Russell of the pair of ophiolites that bracket the Pismo were to be accommodated by crustal shortening fault to the south. Schwade et al. (1958, p. 85) syncline. We also note that the gravity low of across the Santa Maria Basin (Fig. 12 of Dick- estimated 37 km based on offset of the contact the Neogene Pismo syncline also appears to inson et al., 2005) versus the amount of shorten- between the granite and overlying Cretaceous be offset 25–30 km from a gravity low east of ing that has been documented. More than 50% strata, whereas Dibblee (1976, p. 21) reduced the fault (P1, P2 in Fig. 3). Other published shortening (or ~70 km) distributed from Morro the estimate to 13 km based on his mapping of estimates for offset are considerably lower, but Bay to Point Arguello is implied if 156 ± 4 km the contact. Bartow (1974, p. 139) refi ned the not well documented. Based on distribution of of offset is reduced to zero at Point Arguello. estimate to 13–15 km using a Vaqueros (Oli- various Oligocene and Miocene strata, Hall et Balanced cross sections assuming that folds are gocene) subcrop. The estimates based on the al. (1995, p. 87) inferred ~15 km of right-lateral kinematically linked to ramp-fl at faults at depth unconformity may suffer from inherent uncer- offset. In an abstract, McLean (1993) stated that indicate only 9.2 km of shortening across the tainty because of the oblique angle between the the outcrop distribution of an erosion-resistant Santa Maria Basin (Namson and Davis, 1990). northwest trends of the contacts between the andesite within the Obispo Formation appeared More recently, Graymer et al. (2010) estimated basement rocks and the overlying strata and to limit offset on the West Huasna fault to 5–8 only 4 km of Neogene shortening between Point the Chimineas fault (Powell, 1993, p. 51) and km. Clearly, further work is necessary to test Sal and Point Arguello based on restoration of because of incomplete knowledge of the Oligo- these offset estimates. compression of Miocene strata. To reconcile cene erosion surface. correlation of the San Simeon with the Point Others have postulated that the San Juan– DISCUSSION Sal ophiolites and maintain a constant offset of Chimineas–Russell fault may have reactivated 156 km on the Hosgri fault, at least 35 km of an older, pre-Cenozoic structure because the The correlation of magnetic anomalies sug- shortening is needed between Morro Bay and basement at Barrett Ridge (mostly gneiss) is gests that right-lateral offset on the San Grego- Point Sal. Drill holes and seismic-refl ection data petrologically dissimilar to much of what is rio–San Simeon–Hosgri fault zone increases in this area indicate a gentle southward tilting exposed (granodiorite) in the La Panza Range northward, from nearly zero south of Point of the Neogene section beneath the Santa Maria (Ross, 1972). The two are nearly identical, Arguello to ~155 km between Cape San Mar- Basin north of Point Sal, consistent with ~2 km however, in terms of their magnetic signa- tin and Año Nuevo since the Mesozoic, and we of shortening since the early Pliocene (Seeber ture. Furthermore, the easternmost outcrops posit, based on supporting correlations of Mio- and Sorlien, 2000). Furthermore, large amounts of the granodiorite of the La Panza Range are cene and younger strata discussed herein, since of convergence within the Santa Maria Basin interleaved with mica schist, and are darker the initiation of the fault during the Miocene before the Pliocene are not supported by the and more inclusion rich than typical La Panza (Clark, 1998; Dickinson et al., 2005). This set of stratigraphic record, which argues for transten- granodiorite, leading Ross (1972, p. 27) to state geophysical correlations suggests that previous sion and subsidence since ca. 18 Ma (Stanley et “the east end of the La Panza outcrop indicates estimates ranging from <10 km to 155 km may al., 1996; McCrory et al., 1995). nearness to signifi cant amounts of metamor- all be correct, depending on the location along The southward decrease in slip on the San phic rocks that are not now exposed.” These the fault. Even greater displacement north of Gregorio–San Simeon–Hosgri fault is mim- may be the rocks that were displaced to the our study area has been proposed by correlating icked by Quaternary slip rates (albeit poorly southeast by the San Juan–Chimineas–Russell the magnetic anomaly beneath the Santa Cruz constrained) that range from 4 to 11 mm/yr for fault. The nearly identical ages of the alaskite Mountains (SCM in Fig. 12B; likely originating the San Gregorio fault near Año Nuevo (Weber, that intrudes the gneiss at Barrett Ridge (U/Pb in Mesozoic ophiolite) with a prominent mag- 1990) to 1–3 mm/yr at San Simeon to 0.5–2 mm/ age of 80 Ma; Mattinson and James, 1985) and netic anomaly in the Gualala block (see Fig. 1 yr between Point Sal and Point Arguello (Han- the La Panza granodiorite (79 Ma; Colgan et for location), 175 km to the north (Jachens et son et al., 2004). Slip was initiated on the fault al., 2012) indicate that these rocks were of the al., 1998). at ca. 11 Ma, based on the age of the youngest same intrusive event, possibly now displaced by Our correlations disagree with previous unit (ash bed within the Monterey Formation), post–80 Ma displacement. Given that the offset studies that argue for a constant amount of which is offset as much as older units between estimate from the magnetic anomalies is similar offset along the entire length of the fault, such Point Lobos and Point Reyes (Clark, 1998). This to the estimate of 26–29 km between 23 and 4 as that of 156 ± 4 km since 12 Ma postulated suggests that, although slip rates have decreased Ma on the Russell fault to the south (Yeats et most recently by Dickinson et al. (2005). Our over the lifetime of the fault, the distribution of al., 1989), we suggest that the Chimineas–San correlations benefi t from an areally consistent slip has not changed.

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The pattern of northward-increasing right- One possible solution is to route much of the Hosgri and Santa Ynez River faults intersect lateral displacement on the San Gregorio–San 90 km of slip onto a proto-Hosgri fault. Steritz (Fig. 12C). Clearly, detailed work in the Santa Simeon–Hosgri fault can be explained in part by and Luyendyk (1994) suggested that signifi - Maria Basin will be crucial to resolving how right-lateral displacement on subsidiary faults cant slip (~60 km) transferred from the Hosgri slip is accommodated south of Point Sal. east of the fault. For example, the difference in onto the Santa Lucia Bank fault to the west; offset between Cape San Martin (148–154 km) this transfer, however, would be located to the CONCLUSIONS and Point Buchon (122–128 km) could be bal- north of San Simeon, rather than between Point anced by 20–32 km of right-lateral slip on the Arguello and Point Sal. Another possibility Modeling of magnetic anomalies across the Oceanic–West Huasna fault. This amount of off- is the Santa Ynez River fault, which separates San Gregorio–San Simeon–Hosgri and Reliz- set is similar to our estimate of 25–30 km based very different stratigraphic sections, those of the Rinconada faults indicates steep dips that extend on correlation of gravity and magnetic anoma- Santa Maria Basin, where Neogene sedimentary throughout the seismogenic part of the crust. lies across the fault. rocks lie directly on Franciscan or Great Val- These results suggest that steep or near-vertical Farther north, the difference in offset ley basement, versus those of the Santa Ynez dips of these faults determined from the decades- between Cape San Martin (148–154 km) and Mountains, where a thick Paleogene section lies long record of seismicity are characteristic of the the Santa Cruz Mountains (175 km) is 21–27 between the Mesozoic basement and the Neo- dip of the fault throughout its lifetime (Neogene) km. The Reliz-Rinconada fault, however, gene section. The amount and even the sense of and that much of the movement on these faults appears to have 38–42 km of right-lateral dis- offset on this fault are poorly known. Dickinson has been characterized by strike slip. Exceptions placement, at least south of San Ardo. Sev- (1979) suggested 70 km of net right-lateral off- are (1) a moderate dip for that stretch of the San eral lines of evidence suggest that the fault set on the Santa Ynez River fault based on pre- Gregorio fault in Monterey Bay and (2) a pos- changes character to the north and may reduce sumed displacements of Paleogene depositional sible moderate dip for the northern stretch of the the amount of strike slip that reaches the San systems. Dickinson (1983) later dismissed this Reliz-Rinconada fault. We speculate that these Gregorio fault (see also Rosenberg and Clark, correlation as being spurious because of clast deviations from steep to near-vertical dips are 2009). North of Greenfi eld, late Pleistocene compositions in Oligocene rocks. We suggest likely related to local changes in fault geometry fans are not displaced laterally (Tinsley and that a value of 70 km of offset is supported by or interactions with nearby faults, a hypothesis Dohrenwend, 1979), and the gravity model the possible correlation of an unusual Paleo- that should be tested with rigorous modeling of across G-G′ allows for a moderate fault dip cene and Eocene algal limestone (Sierra Blanca fault interactions. (Fig. 11), suggestive of some component of Limestone) that lies unconformably on Meso- Magnetic ophiolites of Mesozoic age are reverse slip. Sparse subsurface data suggest zoic rocks and underlies Eocene shales. One truncated along the east side of the San Gre- no more than 23 km of right slip of schist of such exposure is near Point Arguello, south of gorio–San Simeon–Hosgri fault at Cape San Mesozoic age across the fault zone (Ross, the Santa Ynez River fault (Keenan, 1932); the Martin, near Point Buchon, and near Point Sal. 1984). Thus, some of the right slip from the other lies 70 km to the east, north of the fault Cross-fault counterparts of their associated Reliz-Rinconada fault may have been accom- in the San Rafael Mountains (Keenan, 1932; magnetic anomalies (supported in most cases modated by crustal shortening or transfer of Walker, 1950). We also note that the Paleo- by rock-unit correlations) west of the fault slip onto other faults in the Santa Lucia Range. gene stratigraphic correlation is supported by suggest southward-decreasing apparent right- Dickinson et al. (2005, p. 36) calculated as apparent right-lateral displacement of magnetic lateral offsets of these ophiolites of 148, 125, much as 9 km of transpressive strike slip ori- anomalies (PA and SRM in Fig. 12B). Con- and 89 km, respectively, with uncertainties of ented parallel to the Reliz-Rinconada fault. glomerate clast compositions in the Eocene to <5 km. The differences in offsets can be bal- Farther south, structures that could balance Lower Miocene Sespe Formation are consis- anced, in part, with slip added from faults east the differences in offset between Point Sal (86–89 tent with strike-slip displacement (Howard, of the fault system. The mechanism by which km) and Point Buchon (122–128 km) and Point 1995), but they are not suffi cient to constrain the the observed reduction in slip along the Hosgri Arguello (<10 km) and Point Sal (86–89 km) amount or even sense of displacement. fault is accomplished south of Point Buchon have not been robustly determined. As mentioned It is highly unlikely that 70 km of Neo- is not completely understood, but some of the earlier, Neogene shortening within the Santa gene right-lateral offset passed through the slip may have been transferred onto the Santa Maria Basin can account for no more than 10 km present confi guration of the Hosgri and Santa Ynez River fault. of slip. Clockwise rotation of small crustal blocks Ynez River faults. Such an abrupt bend would within the Santa Maria Basin could account for produce very large amounts of uplift and con- ACKNOWLEDGMENTS some of the slip, as suggested by Sorlien et al. vergence, which are not observed. Clockwise (1999a), although paleomagnetic studies in the rotation of the Western Transverse Ranges We thank the National Cooperative Geologic Santa Maria Basin are few and indicate highly complicates a model that places right-lateral Mapping Program of the U.S. Geological Sur- variable clockwise rotations, some with large slip on the Santa Ynez River fault given (1) vey for its continuing support of this and related error bars (e.g., 9° ± 27° for Miocene rocks near predictions of left-lateral slip from transrota- projects. We also gratefully acknowledge fi nan- the Lion’s Head fault; Hornafi us et al., 1986). tional models (e.g., Hornafi us et al., 1986) and cial and logistical support from the Pacifi c Gas The Casmalia fault and the faults that form the (2) scattered kinematic indicators of left slip and Electric Company, in particular for provid- southern margin of the Irish Hills could account from striations on related faults (Sorlien et al., ing a substantial amount of funding for aero- for some of the slip discrepancy (33–42 km) 1999a). Reconstruction of slip on the various magnetic surveys. We thank Don Sweetkind, between Point Buchon and Point Sal, although faults from matching geophysical anomalies Dan Scheirer, and two anonymous reviewers for no defi nitive piercing points or blobs have yet highlights the space problems in the Santa helpful suggestions and comments that greatly been identifi ed to quantify that slip. Even more Maria Basin, which are accentuated by the tightened up the presentation and merit of the diffi cult to explain is the nearly 90 km decrease in large clockwise rotation of the Western Trans- manuscript. Chris Sorlien provided additional offset from Point Sal to Point Arguello. verse Ranges and the acute angle at which the insight into the dip of the San Gregorio fault.

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