Abstract

Asymmetric fabrics in Franciscan Complex shale matrix mélange near San Simeon, central California, record synsubduction shear that has implications for the Late Cretaceous to early Tertiary tectonic evolution of the region. Outcrop-scale structural data including S-, C-, and C′-planes, slickenlines, block long axes, and offset blocks indicate that most asymmetric fabrics developed in a NE-dipping sinistral shear zone following Late Cretaceous chaotic mixing and accretion of the mélange. Approximately 70% of C-planes strike within ∼45° of the major NW-striking structures in the region—the Neogene San Gregorio–Hosgri fault and the Late Cretaceous–early Tertiary Nacimiento fault zone. Slip vectors on these NW-striking C-planes consistently record sinistral or oblique sinistral shear. This dominant shear regime accommodated E-W to NE-SW shortening and N-S to NW-SE extension. Angles between S- and C-planes are typically ∼25°–35° and range up to ∼45°, suggesting dominantly simple shear. Sinistral shear in the mélange is kinematically incompatible with Neogene to recent dextral faulting, indicating that these fabrics predate the early Miocene development of the Pacific–North America transform. We propose that sinistral shear in the mélange is related to latest Cretaceous to early Tertiary slip on the adjacent Nacimiento fault zone. These kinematic data are compatible with previous studies that have suggested that the juxtaposition of the Salinian block against the Nacimiento block was accommodated by hundreds of kilometers of sinistral slip on the Nacimiento fault zone.

INTRODUCTION

The synsubduction tectonic evolution of the Franciscan subduction complex in California remains disputed. It is clear that the Franciscan Complex developed primarily as an accretionary prism during late Mesozoic to early Cenozoic subduction of the Farallon plate beneath western North America (e.g., Hamilton, 1969; Ernst, 1970; Dickinson, 1970), but in many areas, late Cenozoic deformation related to the San Andreas transform system and discontinuous exposure obscure the structural history of the Franciscan Complex. One of the few major synsubduction structures that does not appear to have been significantly overprinted by late Cenozoic deformation is the Nacimiento fault in west-central California (herein referred to as the Nacimiento fault zone; Fig. 1). The Nacimiento fault zone, which juxtaposes the Salinian block to the northeast against the Nacimiento block to the southwest, defines a major break in the subduction-related lithotectonic architecture of California. The Salinian block is cored by Late Cretaceous granitoids (ca. 100–75 Ma) that have been displaced from the Sierra Nevada–Peninsular Ranges magmatic arc by the San Andreas fault system, whereas the Nacimiento block is composed dominantly of the Franciscan Complex and distal equivalents of the Great Valley Group forearc basin (e.g., Page, 1981; Vedder et al., 1983; Mattinson, 1990; Hall, 1991; Dickinson et al., 2005; Fig. 1). The apparent omission of most of the forearc basin and the westernmost arc between these two blocks suggests that significant displacement (>100 km) has occurred across the Nacimiento fault zone. The timing of major slip along the Nacimiento fault zone is constrained by displaced Salinian block granitoids as young as ca. 76 Ma (Mattinson, 1990; Kidder at el., 2003) and the probable overlap of the Lower Miocene Vaqueros Formation across the fault east of San Simeon (Dickinson et al., 2005; Fig. 1). The apparent overlap of Lower Eocene marine strata across the southern end of the Nacimiento fault zone (Vedder et al., 1983; Jacobson et al., 2011) and Paleocene unconformities in Salinian block strata suggest that slip on the Nacimiento fault zone ceased by ca. 62–56 Ma (Hall, 1991; Dickinson et al., 2005). Thus, the juxtaposition of the Salinian block against the Nacimiento block most likely occurred between ca. 75 Ma and 60 Ma, coeval with flat-slab subduction during the Laramide orogeny.

Currently, there is no consensus on the kinematic history of the Nacimiento fault zone. Previous studies have interpreted the Nacimiento fault to record dextral slip during mega-northwest transport (>2000 km) of the Salinian and Nacimiento blocks (e.g., Page, 1982; Vedder et al., 1983; Champion et al., 1984), ∼150–200 km of W-directed thrusting (e.g., Hall, 1991; Ducea et al., 2009), and ∼500–600 km of sinistral slip (Dickinson, 1983; Seiders and Blome, 1988; Dickinson et al., 2005; Jacobson et al., 2011). The mega-offset dextral translation interpretation arose primarily from paleomagnetic data that were not corrected for compaction (Butler et al., 1991; Kodama and Davi, 1995), whereas the thrust interpretation has centered on the Sur fault, a possible northwest continuation of the Nacimiento fault zone (Page, 1970). Hall (1991) and Ducea et al. (2009) interpreted the Sur fault as the fundamental boundary between the Salinian block and the Nacimiento block, whereas Dickinson et al. (2005) interpreted it as an intra-Salinian thrust unrelated to the block boundary. The majority of these interpretations are based on relatively ambiguous correlations and tectonic reconstructions. Previous studies lack kinematic data from the Nacimiento fault zone.

We present a structural analysis of Franciscan mélange shear fabrics in the Nacimiento block near San Simeon, California. These fabrics record a consistent kinematic regime that we interpret to be related to synsubduction slip on the Nacimiento fault zone. Although mélanges by definition are chaotic mixtures, several studies have identified systematic asymmetric fabrics that give insight into mélange formation processes and/or paleo–plate convergence directions (e.g., Kano et al., 1991; Kusky and Bradley, 1999; Onishi et al., 2001; Ujiie, 2002; Fukui and Kano, 2007; Tokiwa, 2009). Development of systematic shear fabrics at San Simeon postdates the chaotic mixing that has been attributed to either tectonic processes within a subduction channel shear zone (e.g., Cloos, 1984; Cloos and Shreve, 1988) or olistostrome deposition (e.g., Cowan, 1978). These postaccretion fabrics have potentially significant implications for the Late Cretaceous to early Tertiary tectonic evolution of California.

GEOLOGIC SETTING OF THE FRANCISCAN COMPLEX NEAR SAN SIMEON

Some of the best exposures of Franciscan Complex mélange are located along an ∼6-km-long stretch of coastline near San Simeon, central California (Fig. 2). Sea cliffs up to ∼15 m high expose fresh outcrops of mélange, which consists dominantly of graywacke and greenstone blocks within a foliated shale matrix. On average, the block to shale matrix ratio is ∼1. Chert, blueschist, and graphite schist blocks are also present, but they typically comprise <1% of the mélange. Locally, the mélange is overlain by Upper Cretaceous Cambria slab sandstone beds, which are interpreted as trench-slope basin deposits (Howell et al., 1977; Smith et al., 1979; Becker and Cloos, 1985; Dickinson et al., 2005). The youngest detrital zircon ages from both graywacke blocks in the mélange and the Cambria slab are 84–87 Ma (Morisani, 2006; Jacobson et al., 2011, see Data Repository item 2011005). Blueschist blocks within the mélange were metamorphosed at ∼300–350 °C and 5–7 kbar during the early history of subduction (ca. 155–135 Ma; Ukar, 2012), whereas the pumpellyite-bearing graywacke blocks and the shale matrix were metamorphosed at peak temperatures of ∼100–200 °C, most likely corresponding to ∼10–15 km depths (Cloos, 1982; Morisani, 2006).

Previous studies have identified the general characteristics of deformation within the mélange near San Simeon. Cowan (1978) recognized a dominant NE-dipping foliation and small-offset faults within the mélange. The presence of mélange diapirs within the Cambria slab (Becker and Cloos, 1985), shale matrix injections into blocks, and deformation of elongate graywacke blocks by particulate flow (Cloos, 1982; Harrington, 2001) indicate that the chaotic mixture was created while the shale matrix and graywacke clasts were underconsolidated with a high water content. In contrast, distortion of greenstone, blueschist, and chert was accommodated primarily by cataclastic flow along block margins (Cowan, 1978; Cloos, 1982; Harrington, 2001). Mélange blocks are variably deformed and commonly display pinch-and-swell structure, boudinage, and oblate shapes (Cowan, 1978; Cloos, 1982).

The offshore trace of the NW-striking San Gregorio–San Simeon–Hosgri fault zone—a major strand of the San Andreas transform system—is located ∼1–2 km southwest of the coastline near San Simeon (Fig. 1). This fault zone is interpreted to have ∼150–160 km of Neogene dextral offset (Clark, 1998; Dickinson et al., 2005). Near San Simeon Point, the San Gregorio–San Simeon–Hosgri fault zone is ∼120 m wide and cuts Pleistocene terrace deposits (Hall, 1976; Hanson and Lettis, 1994; Hall et al., 1994). Approximately 13–16 km northeast of the San Simeon coastline, the Nacimiento fault zone defines the boundary between the Nacimiento block and the Salinian block, locally juxtaposing Upper Cretaceous sedimentary rocks against Franciscan Complex mélange (Fig. 1).

DESCRIPTION OF MÉLANGE FABRICS

Fabrics in the mélange near San Simeon consist of a scaly cleavage in the shale matrix, elongate clasts aligned parallel to the cleavage, and sets of subparallel, small-offset faults (Fig. 3). The scaly cleavage is defined primarily by the alignment of clay-sized micas and detrital silt/sand grains (Harrington, 2001). Fine (narrow) striations are locally present along cleavage planes. The subparallel, small-offset faults are widespread in the mélange and are typically spaced ∼2–15 cm apart. Apparent displacements observed along these faults are generally ∼2–25 cm, and most have little to no gouge or breccia. The mélange foliation is consistently oriented ≤∼45° from the fault sets, and the foliation typically curves toward parallelism with the faults (Fig. 3). Faults are common along the tops and bottoms of clasts, resulting in asymmetric or sigmoidal clast shapes (Fig. 3). Less common synthetic faults that cut across the foliation at angles of ∼45°–60° merge with or are cut by the more pervasive sets. This mélange foliation and the sets of faults resemble an S-C-C′ fabric very similar to others described in shale-matrix mélanges (e.g., Kano et al., 1991; Kusky and Bradley, 1999; Fukui and Kano, 2007) and non-coaxial ductile shear zones in general (e.g., Berthé et al., 1979; Lister and Snoke, 1984). In these asymmetric fabrics, the sets of small-scale faults are C-planes parallel to the overall shear direction, and the faults that cut across the foliation at a higher angle are C′-planes (Fig. 4). S-C-C′–type fabrics are also common in brittle fault gouge zones, where S-planes (foliation) and C- and C′-type shears are typically referred to as a P foliation and Y and R1 (Riedel) shears, respectively (e.g., Rutter et al., 1986; Chester and Logan, 1987; Cowan and Brandon, 1994; Fig. 4). The fine-grained mélange matrix at San Simeon is not a gouge zone produced by mechanical milling (Cloos, 1982, 1983), so we prefer the S-C-C′ terminology to describe this fabric. The small-scale faults that we interpret as C- and C′-planes appear to correlate with the D2 shear fractures previously described by Cowan (1978).

The S-C-C′ shear fabric is also evident at the microscopic scale (Fig. 5). In an oriented thin section cut parallel to the calculated S-C slip vector (see following section), sets of microscopic C- and C′-planes deflect and cut across the matrix cleavage with a shear sense parallel to that observed at the outcrop scale (Fig. 5). Locally, minor amounts of calcite and fine-grained chlorite are present along both C- and S-planes, suggesting these planes developed synchronously.

KINEMATICS OF MÉLANGE FABRICS

Overview of Data

Detailed structural data were collected across almost all segments of mélange exposures between San Simeon State Park and San Simeon Pier (Fig. 2). These data include orientations of S-, C-, and C′-planes, slickenlines on these planes, clast long-axis trends, and kinematic observations. Foliation in the mélange is somewhat irregular at the outcrop scale, particularly where the matrix wraps around more rigid clasts. The dominant foliation dips moderately (∼30°–70°) NE to ENE, with domains of E- and SE-dipping foliation present locally (Fig. 6A). C-planes have similar overall NE-dipping orientations to the foliation, but the mean C-plane strike is ∼20°–30° counterclockwise of the mean S-plane strike (Fig. 6B). Most C′-planes strike counterclockwise of the C-planes, dipping moderately to steeply N to NNE (Fig. 6B).

The orientation patterns of S-, C-, and C′-planes are compatible with a dominant NW-directed sinistral shear regime, which is consistent with dozens of observations of centimeter-scale apparent sinistral displacement on C- and C′-planes (Fig. 3). At several localities, slickenlines on calcite-coated C-planes directly record the slip vector associated with shear fabric development (Fig. 7A). These C-plane slickenlines and slickenlines on S-planes trend mostly NW-SE, S, or E, recording strike-slip–dominated motion. The C-plane slip vector was also determined from the S- and C-plane orientations. At each outcrop, the mean S- and C-planes from several measurements were calculated. The slip vector was calculated as the direction on the mean C-plane oriented 90° from the intersection between the mean S- and C-plane (e.g., Moore, 1978). These slip vectors determined from S-C fabric orientations also trend dominantly NW-SE to S and E, consistent with the slickenlines measured on C-planes (Fig. 7B). The trends of the long axes of clasts within the mélange (measured on exposures subparallel to foliation planes) parallel these dominant slip vector trends (Fig. 7C). To evaluate the significance of these kinematic data with respect to the major regional structures, we subdivided the data into two groups: one group where C-planes strike within ∼45° of the average trace of the Nacimiento fault zone ∼15 km northeast of San Simeon (also subparallel to the San Gregorio–Hosgri fault zone), and another group where C-planes strike >45° from the Nacimiento fault zone trace. This subdivision was also chosen because it created the most coherent group of kinematic data: C-planes striking <45° to Nacimiento fault zone.

Sinistral Shear Subparallel to the Nacimiento Fault Zone

Approximately 70% of all shear planes in the mélange (C and C′) strike within ∼45° of the NW-striking Nacimiento fault zone. The majority of these shear planes dip moderately to steeply NE, and slickenlines and calculated S-C slip vectors typically plunge <40° from the NW and SE (Fig. 8A). Altogether 81 slip vectors (23 from slickenlines and 58 from S-C fabrics) were determined from C-planes subparallel to the Nacimiento fault zone. The kinematic data from these shear fabrics are remarkably consistent: 79/81 slip vectors record a sinistral component of slip (Fig. 8B). In addition, 18/18 shear planes where only an apparent offset was determined record apparent sinistral shear. The most common slip vector rake on these sinistral shear planes is <10°, and most of these slip vectors (67%) have <30° rakes (Fig. 9A). Oblique sinistral-reverse and sinistral-normal slip vectors are equally common. These data indicate that the dominant kinematic regime in the mélange is sinistral shear with no systematic component of transpression or transtension.

Mean angles between foliation and C-planes subparallel to the Nacimiento fault zone range from ∼13° to 47°, with the most common angle between 25° and 35° (Fig. 9B). The continuous range of these S-C angles up to a maximum of ∼45° suggests that these fabrics record dominantly simple shear. Assuming that the mélange foliation records the finite strain x-y plane, the foliation should initiate at 45° to the C-planes during simple shear and progressively rotate in the shear direction. The largest S-C angles (45°–47°) may be due to anomalous foliation attitudes or antithetic rotation of the foliation by C′-planes. Slip along C′-planes also records a component of stretching parallel to the shear zone and shortening normal to the shear zone, so the bulk strain across the shear zone is not 100% simple shear. It is also possible that strain was partitioned into simple shear along C- and C′-planes and sub–simple shear between shear planes. Assuming simple shear kinematics along the C-planes, shortening and extension axes (P and T axes) calculated from slip vectors indicate dominantly E-W to NE-SW shortening and N-S to NW-SE extension associated with slip subparallel to the Nacimiento fault zone (Fig. 8C).

Shear Transverse to the Nacimiento Fault Zone

Approximately 30% of all shear planes in the mélange strike >45° from the Nacimiento fault zone. These shear planes, which are largely restricted to the northwestern half of the study area (Fig. 2), dip primarily E to SE and have roughly N-S–trending slip vectors (Fig. 10). Angles between average S- and C-planes in this group range from ∼17° to 40°. The kinematic data from these S-C fabrics are more variable than from those subparallel to the Nacimiento fault zone. SE- to E-dipping C-planes with oblique sinistral-reverse or sinistral slip comprise ∼61% (20/33) of this group (Fig. 10A). Assuming simple shear along the C-planes, this S-C fabric population records approximately SW-plunging extension axes and N- to NW-plunging shortening axes (Fig. 10A). The next most common kinematic regime in this group (∼21%) is characterized by dextral and oblique dextral slip on E-dipping C-planes (Fig. 10B). This S-C fabric population records NW-plunging extension axes and S- to SW-plunging shortening axes (Fig. 10B). The remaining ∼18% of S-C fabrics in this group record shear that appears to be kinematically incompatible with all previously described populations, including oblique dextral-normal slip on SE-dipping C-planes (n = 3; Fig. 10C). Some of this kinematic heterogeneity is probably related to large blocks that locally perturb the displacement field. Relative timing relationships between different S-C fabric geometries are not clear from field relationships, and aside from geometry, the outcrop-scale characteristics of these fabrics are very similar to those subparallel to the Nacimiento fault zone.

DISCUSSION

The structural data presented here indicate that the majority (∼70%) of asymmetric fabrics in the Franciscan mélange near San Simeon formed during sinistral shear within a NE-dipping, non-coaxial shear zone. This sinistral shear is subparallel to the adjacent Neogene San Gregorio–Hosgri fault and Late Cretaceous–early Tertiary Nacimiento fault zone. Surprisingly, mélange S-C fabrics record little to no dextral shear related to movement on the San Gregorio–Hosgri fault and related faults. It is possible that some of the E-dipping dextral shear fabrics (Fig. 10B) are Neogene, but these shear planes strike ∼30°–35° from the trace of the San Gregorio–Hosgri fault. NW-striking, high-angle faults inferred to be Neogene in age cut the mélange (e.g., Cowan, 1978; Coppersmith, 2008), but these structures are associated with well-defined cataclasite zones in the shale matrix rather than penetrative fabrics. The dominant sinistral shear recorded in the mélange fabrics is kinematically incompatible with Neogene to recent faulting in the region, indicating that this deformation must have predated the development of the Pacific–North America dextral transform.

The timing of sinistral shear is loosely bracketed by: (1) geochronologic evidence that most of the mélange formed post–ca. 85 Ma (Morisani, 2006) and (2) reconstructions indicating that subduction transitioned to transform tectonics at ca. 20 Ma near San Simeon (e.g., Atwater and Stock, 1998). Chaotic mixing of the mélange occurred in a distributed, high-strain subduction shear zone when the matrix was poorly consolidated (Cloos, 1982, 1984). The localized slip along C- and C′-planes and the modest strain recorded by the sinistral shear fabrics indicate that they postdate the chaotic mixing that characterizes the mélange. We interpret the fabrics to have developed during the final stages of mélange compaction following accretion. The relatively high angles between S- and C-planes (up to ∼45°; Fig. 9B) suggest that relatively little compaction could have occurred following the fabric development.

Given these timing constraints and the location of the study area, we favor the interpretation that the asymmetric mélange fabrics record shear associated with very latest Cretaceous to early Tertiary slip on the Nacimiento fault zone. The systematic sinistral shear of the mélange is consistent with the model of major sinistral slip on the Nacimiento fault zone (Fig. 11). Hundreds of kilometers of slip on the Nacimiento fault zone would likely be associated with distributed shear within some parts of the accretionary prism. Restoration of 500–600 km of sinistral slip on the Nacimiento fault zone brings the Mesozoic lithotectonic belts into alignment and places the Nacimiento block against similar Mesozoic rocks in the Diablo Range (Dickinson, 1983; Fig. 11). Additional evidence for the sinistral slip model includes the apparent restoration of linear trends of Mesozoic granitoid initial strontium isotopic ratios (Dickinson, 1983), possible correlation of distinct Upper Cretaceous conglomerates in the Golden Gate–Gilroy block and Nacimiento block (Seiders and Blome, 1988), and apparent restoration of southeastward-younging age patterns of the underplated Catalina-Rand-Pelona-Orocopia schists (Jacobson et al., 2011). The shear fabrics in the mélange near San Simeon further support the interpretation of Late Cretaceous to early Tertiary sinistral slip on the Nacimiento fault zone.

A potential problem with the sinistral slip model is that most reconstructions suggest that the relative Farallon–North America plate convergence in the Late Cretaceous to Paleocene was head-on (Page and Engebretson, 1984; Engebretson et al., 1985), or had a component of dextral motion (Stock and Molnar, 1988; Norton, 1995). Jacobson et al. (2011) proposed that sinistral slip on the Nacimiento fault zone could be attributed to tectonic escape driven by subduction of an aseismic ridge. In this scenario, sinistral slip could have been coeval with head-on subduction. We believe that sinistral slip could have also been driven by oblique convergence. The Farallon convergence vector during the Late Cretaceous to early Tertiary is not well constrained, but it is clear that the convergence rate was rapid (∼12 cm/yr; e.g., Engebretson et al., 1985; Stock and Molnar, 1988). If the convergence vector was ∼16° clockwise of head-on and resolved into trench-parallel and trench-perpendicular components, 500 km of sinistral slip on the Nacimiento fault zone could have occurred over ∼15 m.y. (e.g., 75–60 Ma). Another important uncertainty is the trend of the Nacimiento fault zone during the Late Cretaceous to early Tertiary. Palinspastic reconstructions of southern California by Grove et al. (2003) and Jacobson et al. (2011) suggest that near San Simeon, the Nacimiento fault zone was WNW-striking prior to Neogene faulting. At this orientation, 500 km of sinistral slip on the Nacimiento fault zone could have occurred in just ∼7 m.y., given the Late Cretaceous convergence vectors determined by Engebretson et al. (1985) (∼12 cm/yr at N58E). Existing constraints on the Late Cretaceous to early Tertiary plate geometry and kinematics do not preclude plate-driven sinistral slip within the accretionary prism and along the Nacimiento fault zone.

CONCLUSIONS

New structural data from Franciscan shale-matrix mélange near San Simeon document S-C-C′ shear fabrics that are kinematically incompatible with Neogene to recent dextral faulting in California. Dominant fabric orientations, slickenlines, calculated S-C slip vectors, offset blocks, and block long axes indicate that most mélange fabrics formed within a NE-dipping sinistral shear zone. Approximately 70% of slip vectors in the mélange record sinistral shear along NE-dipping C-planes, accommodating E-W/NE-SW shortening and N-S/NW-SE extension. The remaining 30% of slip vectors record variable shear regimes including oblique sinistral-reverse or sinistral slip along SE-dipping C-planes. Angles between S- and C-planes range from ∼13° to ∼45° with a maximum frequency of 25°–35°, suggesting dominantly simple shear. These shear fabrics must have developed after chaotic mixing and incorporation of graywacke blocks as young as ca. 85 Ma (Morisani, 2006) but prior to the initiation of the Pacific–North America dextral transform at ca. 20 Ma (Atwater and Stock, 1998). The dominant NE-dipping sinistral shear zones are subparallel to the Nacimiento fault zone, a major Late Cretaceous to early Tertiary structure that juxtaposes arc-related granitoids of the Salinian block against accretionary Franciscan rocks in the Nacimiento block. Previous studies have suggested that the juxtaposition of these blocks was accomplished by 500–600 km of sinistral slip on the Nacimiento fault zone (e.g., Dickinson, 1983). We propose that sinistral shear fabrics in the mélange record distributed shear within the Nacimiento block during major sinistral slip on the Nacimiento fault zone. This sinistral slip may have been driven by subduction of an aseismic ridge (Jacobson et al., 2011) and/or oblique convergence between the Farallon and North America plates during Late Cretaceous to early Tertiary flat-slab subduction.

Thoughtful reviews by Carl Jacobson and an anonymous reviewer have improved this manuscript and are very much appreciated. The first author acknowledges financial support from the Jackson School of Geosciences at the University of Texas.