Abstract

Only relatively rare fault rocks record paleoseismicity (e.g., pseudotachylyte), though most fault slip probably accumulates during earthquakes. We describe two fault-rock assemblages, made of common fault-rock types, and argue that fault-rock assemblages can provide criteria for inferring seismogenic slip on exhumed faults that are more common than individual fault-rock types. Such assemblages allow direct study of fault-slip products formed under various crustal conditions that are not accessible on active faults. One assemblage, of layered cataclasites, is preserved several meters below the detachment. It is rare in the rock record and is best interpreted as recording parts of multiple seismic cycles. A second assemblage is the two-part detachment fault core: inner fault-core ultracataclasite layers separated by principal slip surfaces and outer fault-core cataclasites. We hypothesize that this common assemblage forms mainly during seismogenic slip. We interpret a third assemblage of clay gouge and breccia as recording slip and alteration above the seismogenic zone.

Detailed structural analysis of the layered cataclasites shows that they formed and were deformed episodically, most consistent with multiple seismic cycles that probably were defined by West Salton detachment fault (WSDF) main shocks. Cataclasite layers formed sequentially at the expense of overlying quartz diorite, while older, deeper layers were abandoned, folded, and faulted. Upward growth of the stack of layers probably reflects both strain weakening of the overlying rocks and strain hardening of the layered cataclasites. Each layer formed in two stages: (1) Lithologically heterogeneous and/or cataclastically foliated, fault-bounded precursor slivers above the top layer record grain-size reduction by cataclasis and cataclastic foliation development and probably record inter-seismic shear. (2) One or more slip event(s), interpreted as seismogenic, transformed precursor slivers into layers of homogeneous, random-fabric cataclasite. Many layers are overprinted by weakly developed foliation, probably of postseismic origin.

The detachment fault core formed at ≥2.3–4 km depth, in the upper seismogenic zone. Pseudotachylyte along the detachment nearby also records multiple seismogenic WSDF slip events. Similarities between the cataclastic layers and detachment fault-core rocks support the interpretation of both as products of seismogenic slip. In contrast, clay-rich gouge and upper-plate breccias formed at depth ≤2–3 km. The footwall fault core is clay poor and thus apparently passed through the uppermost crust with little overprint.

The layered cataclasites and detachment fault core display only minor chemical alteration, and the layers record limited shear strain relative to fault-core rocks. Thus, the layered cataclasites may be good natural analogs for experimentally formed fault rocks. Both assemblages formed from intermediate plutonic protoliths, similar to “average” mid-crustal rocks, and from the same lithologies as cut by the active San Jacinto and Elsinore faults. Thus, these two assemblages provide accessible, paleoseismically formed analogs to fault rocks presumably present at seismogenic depth along active faults.

INTRODUCTION

Fault rocks formed in and above the brittle-plastic (or frictional-viscous) transition are of particular interest for understanding fault and earthquake processes (e.g., reviews by Cowan, 1999 and Rowe and Griffith, 2015): fault-zone evolution (e.g., Scholz, 1987; Chester et al., 1993; Scholz et al., 1993; Ben Zion and Sammis, 2003), fault stress states and strength, energy budget and constitutive relations (e.g., Axen and Selverstone 1994; Chester and Chester, 1998; Marone, 1998; Reches and Dewers, 2005; Collettini et al., 2009; Axen et al., 2015), shear sense (e.g., Petit, 1987), and fault-related fluid flow, fluid pressure, and alteration (e.g., Sibson et al., 1988; Sibson, 1989; Caine et al., 1996; Smith et al., 2008; Haines and van der Pluijm, 2012; Selverstone et al., 2012).

The factors controlling whether fault slip accumulates seismically versus aseismically are not fully understood: temperature, friction properties, fluid effects, and rock type(s) form a partial list. Exhumed fault zones allow direct, detailed, three-dimensional observations of natural fault-zone products developed at depth and thus yield key data for comparison to conceptual models of faulting, which are based largely upon results of laboratory experiments, theoretical considerations, indirect seismic observations, and/or modeling (e.g., Rice, 1992; Marone, 1998; Sibson, 1998; many others).

Mature faults in brittle upper-crustal rocks display a wide variety of rock types and textures, including (1) random-fabric cataclastic rocks; (2) phyllosilicate-rich material, commonly foliated, either smeared along the fault from a wall rock or grown in situ as alteration products; (3) minor to significant vein material; (4) frictional melt products and other less common materials interpreted to record paleoseismicity (e.g., amorphous silicates) (see Rowe and Griffith, 2015, and references therein).

We describe two fault-rock assemblages and interpret them as records of paleoseismicity. Such assemblages probably are more common than the few individual rock types known to form during earthquakes, but few are presently described (e.g., Smith et al., 2008; Rowe et al., 2011). Pseudotachylyte, formed by frictional melting along faults during fast slip, is the main, natural fault rock generally agreed to record paleoseismicity (Cowan, 1999, and references therein), but it is not particularly common. Others exist but are even more rare and are difficult to identify in natural faults (Rowe and Griffith, 2015, and references therein).

We focus on an assemblage of uncommonly well-layered cataclasites found a few meters below the West Salton detachment fault (WSDF), a late Neogene–Quaternary low-angle normal fault (Axen and Fletcher, 1998). Using textural analysis and structural arguments, we conclude that the layered cataclasites formed, and were deformed, sequentially during several seismic cycles. For several reasons, we infer that the two-part fault core at the top of the WSDF footwall also probably records seismic activity.

Low-angle normal faults exhume their footwalls directly, yielding abundant exposures of fault rocks that were processed in crustal levels ranging from the brittle-plastic transition to the near surface (e.g., John, 1987; Axen and Selverstone, 1994; Axen et al., 2001; Cowan et al., 2003; Numelin et al., 2007; Smith et al., 2008; many others). Many are young (Neogene or Quaternary) and thus provide a fault-rock record that is not overprinted by deformation related to later tectonic exhumation events or by chemical processes that may occur before or during slow erosional exhumation.

The top of the WSDF footwall displays a fault-rock assemblage commonly found along mature faults in quartzofeldspathic rocks: one or more principal slip surfaces within a relatively narrow inner fault core of ultracataclasite, surrounded by a thicker outer fault core of cataclasite (Chester and Chester, 1998; Luther et al., 2013). This two-part core, in turn, is underlain by a thicker fractured damage zone, in which fracture density decreases away from the core to a background value (Luther et al., 2013). We argue that WSDF slip exhumed its footwall from the upper seismogenic zone and that its fault core formed at those depths. Pseudotachylyte is found in several places along the WSDF (Fig. 1B; Axen et al., 1998; Prante et al., 2014), showing that it hosted earthquakes, consistent with our inference that the WSDF fault core records paleoseismicity. In contrast, fault rocks at the base of the upper plate in some exposures are breccia and clay gouge, formed from plutonic protoliths at depths ≤2–3 km. We interpret these rocks as formed above the seismogenic zone.

The WSDF footwall is nearly ideal for studies of brittle fault rocks. The upper several meters of the footwall commonly are well exposed and were processed while passing through upper seismogenic-zone depths. Intermediate plutonic rocks dominate the footwall and hanging wall (Fig. 1C); therefore, WSDF fault rocks formed from protoliths that represent typical continental crust. The San Jacinto and Elsinore fault zones cut the same rock types; thus the WSDF footwall may provide analogs for fault rocks formed at depth along these active faults. At the study site, these plutonic rocks lack preexisting mechanical anisotropy (e.g., Cretaceous mylonitic fabrics; Simpson, 1984), simplifying mechanical studies, and are little altered; therefore, chemical processes probably did not dominate fault-rock genesis. Post-WSDF deformation was relatively minor and localized near the younger dextral faults.

WEST SALTON DETACHMENT FAULT

The WSDF is a low-angle normal (detachment) fault that forms the boundary between the Peninsular Ranges (footwall) and the Salton Trough (upper plate) in southern California (Fig. 1; Axen and Fletcher, 1998), defining the northern segment of the Gulf of California rift system (Axen, 1995). It was active from ca. 5–8 Ma to ca. 1 Ma and accommodated ~10 km of slip (Axen and Fletcher, 1998; Lutz et al., 2006; Dorsey et al., 2007, 2012; Shirvell et al., 2009). The WSDF accommodated ~east-west extension within the southern San Andreas plate-boundary fault system (Axen and Fletcher, 1998; Axen et al., 1998). The stratigraphic record shows that the dextral San Jacinto and Elsinore fault zones dismembered the WSDF ~1.1–1.2 m.y. ago and indicates ~100–200 k.y. of temporal overlap of slip on the WSDF and these dextral faults (Lutz et al., 2006; Janecke et al., 2010; Dorsey et al., 2012).

WSDF slip was mainly top-ESE (~110° azimuth), parallel to ESE-plunging corrugations of the fault surface (Fig. 1B; Axen and Fletcher, 1998; Kairouz, 2005; Steely et al., 2009; Dorsey et al., 2012). The WSDF on the south flanks of the Yaqui Ridge and Whale Peak antiforms is cut by young faults of the San Andreas system, which partly controlled late folding (Steely et al., 2009). In contrast, the north flanks are little disrupted but probably were steepened during folding. The Whale Peak arch may have been a primary corrugation that was amplified by folding (Kairouz, 2005). This ~E-W extension and ~N-S shortening reflect the expected strain field around the NW-striking dextral San Andreas fault system (Fig. 1A; e.g., Savage et al., 1986). Folding began during WSDF activity and continued after younger faults crosscut the WSDF and activity on the detachment ended (Dorsey et al., 2007, 2012; Steely et al., 2009).

Trends of WSDF striae range widely from W to SE, with maxima in the northern quadrant and toward the ESE (Stinson and Gastil, 1996; Axen et al., 1998; Kairouz, 2005; Steely et al., 2009; Fig. 2A). This scatter is attributed to episodically alternating WSDF slip directions: top-ESE normal slip and ~N-S reverse(?) slip related to broadly coeval ~N-S shortening (Axen and Fletcher, 1998; Kairouz, 2005; Steely et al., 2009). Paleostress analyses at Yaqui Ridge (Fig. 1B) are consistent, requiring two alternating local stress fields: a normal-faulting field with ~E-W extension and a ~N-S thrusting stress field (Luther and Axen, 2013).

Rocks described here are in the WSDF footwall, which comprises mid-crustal crystalline basement of the Peninsular Ranges (Silver and Chappell, 1988; Todd et al., 1988), mostly intermediate plutonic rocks of the Middle Cretaceous La Posta tonalite-granodiorite suite (Todd et al., 1988). The amphibolite-grade Cretaceous Eastern Peninsular Ranges mylonite zone (Sharp, 1979; Simpson, 1984) overprints the footwall farther north but dies out southward, and metamorphic fabrics are absent at our study site. Pre–La Posta lithologies include quartz-biotite schist, orthogneiss, and lesser amphibolite and marble (Todd et al., 1988). Local Eocene Poway conglomerate deposits are nonconformable on the batholith (Abbott and Smith, 1989), showing that most exhumation from intrusive depths was pre-Eocene. (U-Th)/He thermochronometry of footwall rocks shows that the WSDF tectonically exhumed its footwall at least 2.3–4 km, but greater depths were possible at the onset of WSDF slip (Shirvell et al., 2009).

The WSDF upper plate comprises plutonic rocks and lesser metamorphic rocks, as in the footwall, nonconformably overlain by Neogene sedimentary strata (e.g., Winker and Kidwell, 1986) that mostly were deposited in WSDF-controlled basins (Axen and Fletcher, 1998; Dorsey et al., 2007, 2012). Eocene(?) Poway-type fluvial strata are found locally in the upper plate (Abbott and Smith, 1989; Kairouz, 2005). These Cenozoic strata show that the presently exposed upper plate was shallower than 2–3 km during WSDF activity.

Fault rocks derived from similar plutonic protoliths are different in the upper and lower plates. The basal upper plate is generally covered but mainly is either deformed Cretaceous plutonic rocks or late Neogene sedimentary strata. Excellent basal plutonic exposures in the east fork of Nolina Wash (Fig. 1C) contain several meters of fractured (mainly) and brecciated (near the WSDF), white quartz diorite that is punky, friable, and presumably heavily microfractured for ~10 m above the WSDF (makes a dull thud when hammered). Gray to greenish-gray clay gouge, consistent with generation at shallow depth and low temperature, is present as an irregular layer 0–25 cm thick immediately above the WSDF, along upper-plate fractures, and as breccia matrix. We infer that these fault rocks formed above the upper limit of the seismogenic zone, consistent with clay gouges being velocity strengthening (see review by Marone, 1998). Another good exposure of basal upper plate at Powder Dump wash (Fig. 1B) also contains highly fractured to punky, white plutonic rocks but lacks macroscopically visible clay gouge, and contains pseudotachylyte injection veins that end downward at the WSDF.

In contrast, a two-part fault core is commonly well exposed at the top of the WSDF footwall. In most places, the upper 2–20 m of the footwall comprise a distinctive, moderately erosionally resistant, light-gray to light greenish-gray rounded cliff or steep slope made of fractured damage zone below ~1–2 m of outer fault-core cataclasites capped by 10–75 cm of inner fault-core ultracataclasite (e.g., Luther et al., 2013). Footwall outer-fault-core cataclastic and fractured damage-zone rocks erode mainly by granular disintegration rather than by fracture-bound block falls, and both make a dull thud when hit by a hammer. Presumably these rocks are heavily microfractured, but alteration is minimal. The contact between damage zone and outer fault core typically is gradational over 1–2 m, and the contact between outer and inner fault-core rocks also typically is gradational over ~0.5–2 cm and, though probably sheared, generally is not a sharp slip plane. Clay minerals are sparse to absent in footwall fault rocks. This pseudo-stratigraphy permits detailed mapping of the WSDF (e.g., Fig. 1C) and provides a rich resource of WSDF footwall fault rocks, of interest here because they began to form at significantly greater depths than exposed upper-plate fault rocks.

The footwall ultracataclasite inner fault core typically contains or splits along several nonreflective slip surfaces that are parallel to the WSDF itself. These presumably served as principal slip surfaces at different times during WSDF evolution and commonly separate ultracataclasite layers of different colors. Many of these surfaces display weak to strong abrasive-wear striations. The ultracataclasites typically break into rhombs defined by closely spaced (0.5–2 cm) joints and the slip planes. Shervais and Kirkpatrick (2016) described a similar layered inner fault core along the La Quinta fault, which probably was active as a strand of the WSDF. We define the WSDF as the slip surface at the top of the ultracataclasites and below upper-plate rocks because (1) the character of the fault rocks changes there and (2) no evidence for incorporation of upper-plate lithologies into the inner core has been found where Neogene strata form the upper plate.

(U-Th)/He thermochronology (Shirvell et al., 2009) of apatite indicates that the footwall was exhumed by the WSDF at least 2.3–4 km (rapidly cooled from ~55–70 °C since ca. 12 Ma); zircon results show that the footwall resided at greater depth prior to WSDF activity (~5–7 km based on slow zircon cooling from ca. 51–31 Ma in its partial retention zone at ~150–190 °C). Thus, exposed footwall fault rocks should record processes in the upper kilometers of the seismogenic zone, consistent with pseudotachylyte found along the fault (discussed more below). The general lack of clay minerals or clay gouge in the footwall (as found in the basal upper plate) suggests that the footwall transited the shallow non-seismogenic zone with little overprint. Grain-size distributions of footwall fault-core samples suggest formation by constrained comminution overprinted by additional shearing (see Sammis et al., 1987; Sammis and King, 2007), consistent with confining pressure high enough (or pore pressure low enough) to prevent significant dilatancy during cataclasis (Luther et al., 2013).

Paleoseismic WSDF slip is recorded by pseudotachylyte around the NE tip of Yaqui Ridge (Frost and Shafiqullah, 1989; Axen et al., 1998, Kairouz, 2005; Prante et al., 2014). Prante et al. (2014) recently studied the Yaqui Ridge pseudotachylytes in detail, showing that bodies up to ~1 m thick occur, both as fault veins and injections into the upper and lower plates, that microscopic textures typical of pseudotachylytes are present (see also Kairouz, 2005), and that X-ray diffraction (XRD) patterns display a “glass hump.” Most of the tip of Yaqui Ridge is composed of foliated plutonic rock, but there are many small bodies of pre-plutonic metasedimentary rocks along the WSDF (mainly quartz-biotite schist and marble; Steely et al., 2009). Prante et al. (2014) suggested that the metasedimentary protoliths, rich in hydrous minerals, may have favored melting. A thick (~75 cm), fault-parallel pseudotachylyte body in Nude Wash (Fig. 1B) contains multiple thin (≤5 cm) pseudotachylyte layers, indicating repeated seismicity. This multi-layer pseudotachylyte body lies above metasediments, but the nearly ubiquitous inner-core ultracataclasites are absent there, suggesting that the typical WSDF core did not form everywhere (i.e., where frictional melting occurred and/or where plutonic protoliths are absent). In some places (e.g., Powder Dump Wash, Fig. 1B), off-fault pseudotachylyte veins in the basal upper plate end at the WSDF, where they were presumably generated.

Samples of pseudotachylyte from Yaqui Ridge yield Miocene K/Ar and 40Ar/39Ar total gas ages (Frost and Shafiqullah, 1989; Hazelton, 2003; Kairouz et al., 2003; Kairouz, 2005). The 40Ar/39Ar total gas ages are interpreted as maximum ages due to partial retention of mid-Cretaceous radiogenic Ar in mineral fragments and/or excess Ar contamination, but the ages of youngest individual heating steps are consistent with the late Neogene WSDF age known independently (Hazelton, 2003).

Seismic slip occurs on normal-sense, low-angle detachment faults; so the WSDF is not unique in this characteristic. The contemporaneous Cañada David detachment fault in northern Baja California (Siem and Gastil, 1994; Axen and Fletcher, 1998; Axen et al., 2000) defines the next rift segment to the south (Axen, 1995). It also was seismogenic, as shown by Quaternary fault scarps (Axen et al., 1998, 1999; Fletcher and Spelz, 2009). Wernicke (1995) and Axen (2004) summarized evidence for seismogenic slip on low-angle normal faults. Axen (1999) documented examples of triggered earthquakes on such faults, and Prante et al. (2014) listed nine low-angle normal faults worldwide that are ornamented with pseudotachylyte. Smith et al. (2008) interpreted fault rocks and veins along the Zuccale fault (Italy) as recording seismogenic slip. Thus, seismic slip on detachments is fairly common.

We describe in detail unique layered cataclastic rocks from the WSDF footwall in the west fork of Nolina Wash (Figs. 1B and 1C) and interpret them as recording multiple seismic cycles, due to the repeating, cyclic nature of processes that produced products such as cataclastic injections, cataclasite layers, formation of low-temperature foliations, and destruction (by cataclastic randomization) of those foliations. We then consider the WSDF fault core and suggest that this common fault-rock assemblage also probably records seismogenic slip.

METHODS

Our analysis of layered cataclasites is based mainly upon field observations. Local context for the study site comes from detailed geologic maps at 1:12,000 scale on enlarged U.S. Geological Survey (USGS) topographic maps (Fig. 1C) and co-registered orthophotoquads or topographic contour maps (2 m interval) at 1:1000 scale made from high-resolution (~5 m spacing) NextMap™ digital elevation data (Fig. 3). Very large-scale “outcrop maps” drawn on photographs of key exposures show details of the cataclastic layering, top and basal contacts, overlying deformed plutonic rocks, and structures and crosscutting relationships.

Additional preliminary results are presented, from (mostly) oriented samples of the cataclasites and their plutonic protoliths. Thin sections were examined optically and with backscattered electron (BSE) images. BSE imaging and sparse mineral spot analyses were done on the electron microprobe in the New Mexico Bureau of Geology and Mineral Resources. Two-dimensional grain-size distributions were derived from photomicrographs and BSE images following methods of Keulen et al. (2007), as described in Luther (2012) and Luther et al. (2013). Backscattered electron images of freshly broken surfaces were obtained on the scanning electron microscope (SEM) in the Institute of Meteoritics at University of New Mexico; energy-dispersive X-ray spectroscopy (EDS) allowed for mineral identification.

STRUCTURAL SETTING OF LAYERED CATACLASITES

The study site lies in the uppermost WSDF footwall, between two western branches of Nolina Wash (Fig. 1C). In that area, the WSDF and its adjacent footwall rocks are well exposed in a dip-slope outcrop pattern on the north flank of the Whale Peak arch (Fig. 1B). At the study site, the WSDF dips ~30° north and ESE-WNW–trending striae record dextral-normal oblique slip (Figs. 1C and 2A).

The layered cataclasites crop out in two zones, separated by a ridge where they are covered by structurally overlying rocks (Fig. 3). The best exposures are in the northwestern outcrop band. There, the layered cataclasites, their upper and lower bounding faults (the “top” and “bottom contacts”, respectively), and surrounding rocks are folded coaxially into an open, gently NW-plunging “outcrop antiform” with a NE limb formed and attenuated by WSDF drag (Figs. 2B–2D, 3, and 4). A gully follows the hinge and provides good exposures of both limbs (Fig. 5). Outcrop-scale observations below were made there.

Next, we describe in detail the structural units and fault contacts that separate them, beginning at the WSDF and descending structurally. Significant differences exist between the structures and structural styles on the NE (attenuated) and SW limbs of the outcrop antiform. The SW limb (Fig. 6) is of particular interest because lack of overprinting on it preserved structural relationships that record formation processes of the layered cataclasites.

WSDF at the Study Site

The WSDF is a sharp slip surface at the top of ~10–30 cm of brown ultracataclasite that splits into sub-layers ~1–3 cm thick along sharp, planar slip surfaces parallel to the WSDF. Sparse striae on these surfaces and the WSDF trend ESE (Figs. 1C and red lines in 2A). The ultracataclasites are underlain, across an abrupt transition ~0.5–2 cm thick, by ~1 m of cataclasite. This sequence is like the typical WSDF core elsewhere; therefore, we infer that the mechanical evolution of the WSDF at the study site was similar to that of most other parts of the fault. The WSDF is not folded by the outcrop antiform.

Quartz Diorite below the WSDF and above the Top Contact

This unit thickens south across the outcrop antiform because the WSDF is not folded, but the top contact is. Map-scale structures within this unit (Figs. 3 and 4) include a low-angle footwall splay (thin black line with tick marks left of gully in Fig. 5) from the WSDF that cuts, in its footwall, a NW-down normal-separation fault that, in turn, truncates the NW end of the outcrop antiform and the layered cataclasites. The footwall splay resembles the WSDF, being underlain by 2–8 cm of brownish to olive-green ultracataclasite that grades down across ~1 cm to cataclastic quartz diorite. This similarity suggests that the splay evolved similarly to the WSDF. The splay cannot be traced far into the footwall and ultracataclasite along the splay is thinner than that along the adjacent (or typical) WSDF. We conclude that the splay probably has less slip than the WSDF (see Evans, 1990, for cautionary use of thickness-displacement relationships). The splay probably is not folded by the outcrop anticline, although it dips slightly more gently south of the antiform than to the north (Fig. 3). The simplest timing interpretation of these relationships is that the layered cataclasites and the outcrop anticline formed before the fault that terminates them, which is older than the WSDF-like splay.

Rocks on the SW flank of the outcrop antiform, and immediately below the WSDF footwall splay, are cataclasites that grade down into weakly deformed (microfractured?) quartz diorite that looks intact macroscopically but makes a dull thud when hammered and is locally so weak that it can be excavated with fingernails. In thin section, this “punky” quartz diorite shows kinked (but whole) biotite grains, microcracks, and quartz with weak undulatory extinction (Fig. 7A). Plagioclase in plane-polarized light locally shows very fine-grained hazy sericitic(?) alteration. Some plagioclase grains show micro-porosity in alternate albite twin lamellae, as seen in SEM images. In thin section, these form plucked, planar zones contaminated with opaque polishing compound (Fig. 7A). These textures, except undulatory extinction, are absent in fresh plutonic rocks farther from the WSDF.

The microfractured quartz diorite on the SW flank grades down into increasingly brecciated and cataclastic rocks (up to ~4 m above the top contact) that have random fabrics and reduced matrix grain size. The cataclasites are poorly sorted, with clasts ranging up to a few centimeters in diameter. Many individual clasts are fractured internally and composed of subclasts, either still in place relative to one another or slightly offset or rotated relative to one another (crackle and jigsaw textures, respectively). Biotite typically is dismembered into thinner, smaller cleavage flakes than in the protolith and is commonly kinked.

A penetrative foliation affects the bottom ~5–50 cm of the quartz diorite above the top contact on the SW flank (Figs. 8 and 9B), suggesting significant shear strain related to the contact. The foliation is weak at the top of the zone, where it overprints random-fabric cataclasites. Foliation intensity increases downward to the top contact, the maximum grain size decreases to ~1 mm, and foliation planes rotate into near-parallelism with the top contact (Figs. 7C, 8, and B9B). Thus, shear strain increases down toward the top contact.

Microscopically (Figs. 7C and 7D), foliated quartz diorite on the SW flank is composed of angular grains down to micron sizes, with foliation defined mainly by aligned biotite flakes and, to a lesser extent, other grains (most of which are subequant). Biotite grains appear largely unaltered but commonly are kinked (especially those not parallel to the foliation) or deformed between and/or wrapped around harder grains (Fig. 7D). Biotite cleavage planes commonly are open and filled with either fine-grained cataclasite or with microscopically unstrained calcite (probably the same age as late calcite veins described below). Fabrics representative of crystal plasticity are absent, with the exception of weak undulatory extinction in quartz, which likely predates WSDF slip. Thus, these foliations also are low-temperature cataclastic textures, and we interpret them as S-foliations that record mainly flattening at the grain scale.

Primary Reidel shears (R1) are common in the SW-flank foliated zone, and some extend above it into the random-fabric cataclasites (Fig. 8). These dip moderately SW (Fig. 2I) and cut cataclastic quartz diorite, truncate outsized clasts, and cut the S-foliation, which locally is developed more strongly adjacent to R1 shears and shows normal drag adjacent to them (e.g., above the R1 shear above the pen in Fig. 8A). Downward, some R1 shears merge with or terminate against the top contact, but others bend into parallelism with the S-foliation centimeters above the top contact (Fig. 8), so the S-foliation locally was reutilized in shear. These relationships indicate that the R1 shears formed in concert with slip on the top contact. These R1 shears are not interpreted as C′ shears (which are mechanically similar but develop with S–C–C′ composite fabrics) because (1) C shears generally are absent on the SW flank and (2) many R1 shears extend upward into unfoliated breccias. Secondary Reidel shears are rare. The quartz diorite above the top contact on the SW flank (whether punky, cataclastic, or foliated) also is cut by many small faults with fairly random orientations (Fig. 2I).

Two pegmatite dikes cut the quartz diorite above the SW-flank top contact (Figs. 5B and 6). More than ~1 m above the top contact, these dikes are roughly planar, dip steeply, and strike ~E-W (Fig. 2J). As the top contact is approached, they are dramatically thinned, cataclasized and locally foliated, cut by R1 shears, and drag-folded into parallelism with the top contact. Decimeter-scale cataclastic injection veins (discussed more below) fill mode-1 cracks subparallel to the axial surface of the folds (Figs. 6A, right side) but are not deformed and thus postdate most or all of the folding.

Together, the Reidel shears, foliation, and drag-folded pegmatites record generally dextral, top-NNW, downward-increasing shear above the SW-flank top contact. In the section on the top contact (below), we use the Reidel orientations to infer shear sense on that contact.

Structures on the attenuated NE flank, in the quartz diorite below the WSDF and above the top contact, are different from on the SW flank. “Intact” (but punky and microcracked) quartz diorite is mostly absent. Poorly sorted breccias and cataclasites grade down into foliated cataclasite present in a zone ~0.3–2 m thick above the top contact.

Foliation intensity generally increases downward toward the top contact, again suggesting that development was related to shear on the top contact. NE-flank foliated rocks are coarser grained and more poorly sorted than on the SW flank, unfoliated clasts remain, and a composite S–C–C′ fabric exists: a penetrative S-foliation (flattening plane) occurs in lithons bounded by C and/or C′ shear planes (Figs. 9A and 10). C-planes are spaced 5–25 cm apart, and striae on the C-planes generally trend ESE; their relation to S-foliations indicates top-ESE shear sense during foliation development (Fig. 2H), consistent with slip sense and direction on the WSDF but different from shear sense on the SW flank. A few S-planes display striations that record shear on S (which is not expected), but these are oriented differently than most S-planes, which cluster well (Fig. 2H).

Microscopically, foliated quartz diorite from the NE flank of the antiform is composed mostly of angular clasts and contains clasts of cataclasite (Fig. 7B) and thus is interpreted as formed by low-T cataclastic processes. Biotites generally are aligned, with some showing fish-like shapes; misaligned biotites are commonly kinked (Fig. 7B). NE-flank foliated cataclasites commonly have maximum grain sizes similar to that of the protolith quartz diorite and are moderately to poorly sorted at the maximum grain size. Irregularly shaped bodies of ultracataclasite are present in the foliated zone (Fig. 10). These are mostly truncated by C-planes but locally are elongate along C-planes so may have formed along C-planes and been injected into foliated lithons. A thin (decimeters) random-fabric breccia zone separates these foliated rocks from brecciated diorite above.

The Top Contact

The top contact of the layered cataclasites is a sharp fault (Figs. 8, 9B, and 10) that is folded by the outcrop antiform, dipping north on the NE flank and WSW on the SW flank (Figs. 2B and 3). It is nearly unfaulted on the SW flank but is cut by several small faults on the NE flank, and the shear sense is inferred to differ on the two flanks.

The top contact on the SW flank of the outcrop antiform (Fig. 6) is cut by only a few fractures with centimeters of offset. Striations on the SW-flank top contact formed by abrasive wear and trend NNW, subparallel to the fold hinge defined by the top contact (Fig. 2B). Dextral, top-NNW shear sense on the SW-flank top contact is inferred from R1 orientations above it, by assuming that the slip line is 90° from the intersection with the best-fit R1 plane (Fig. 2I). This direction is similar to striae orientations (Figs. 2B and 2I). This shear sense is consistent with drag folding and attenuation of two pegmatite dikes above the SW-flank top contact (Figs. 2J and 6). The top contact on the SW flank is locally ornamented with ultracataclasite, and injections of cataclasite and ultracataclasite into overlying rocks (discussed more below) terminate down at the top contact.

The top contact on the NE flank is cut by several moderately NE-dipping, normal-separation faults (Figs. 2K and 10) with decimeters of offset. Striations were not found on the NE-side top contact. Slip sense and direction are inferred to be top-ESE, parallel to the shear recorded by immediately overlying S–C fabrics (Fig. 2H and discussion above) and on the WSDF itself. However, S–C development may have been synchronous with or later than the small faults that crosscut the NE-flank top contact but do not cut the overlying, well-developed S–C foliation (Fig. 10). Thus, these small faults may be entirely older than the foliation, in which case assignment of this shear sense to the NE-flank top contact is questionable. Alternatively, they may have functioned as C′ planes during S–C development, as they appear in Figure 8, but their orientations are not what one would expect of C′ planes (compare Figs. 2H and 2K).

The change of top-contact shear sense and direction across the outcrop antiform suggests that the two limbs of the outcrop anticline sheared independently of one another. The SW-flank striae and slip direction (inferred from R1 shears) are subparallel to the antiformal axis; so unfolding (rotating the SW flank into parallelism with the NE flank) does not align them with the shear direction inferred on the NE-flank top contact. Even if unfolding would align the shear directions, the shear senses differ on the two flanks (Fig. 3, cross section).

Layered Cataclasites

The layered cataclasite body is thicker on the south limb of the outcrop antiform than on the north. Layered cataclasites on the south limb are divided into structurally higher, weakly deformed “top layers” and structurally lower, strongly deformed “disrupted layers” (Figs. 4 and 6). The top layers on the SW flank are generally subparallel to the top contact but are absent on the north limb, where disrupted layers directly underlie the top contact. The bottom contact (see below) also is a sharp fault (Figs. 3 and 6).

Cataclastic layers are defined mainly by grain-size changes from layer to layer: thin (1–5 mm), fine-grained, recessive layers separate thicker (2–25 cm), more resistant, coarser-grained layers (Fig. 6). Locally, ultracataclasite is present along the recessive, fine-grained layers. Grain size of cataclastic layers is described macroscopically with sandstone nomenclature, using the average largest grains visible to the naked eye or in hand lens. At this observation scale, individual cataclasite layers appear moderately well to well sorted and clast supported: largest grains in a given layer have a restricted grain-size range and cover a large percentage of any surface. Maximum grain size in coarse layers (excluding scattered larger clasts) is typically ~2 mm. The “average largest grain size” is not well correlated with layer thickness or length (Fig. 11). However, layer thickness and grain size are positively correlated to some degree, because thin, fine-grained, recessive layers separate thicker, coarser, resistant layers.

Cataclastic layer dimensions (Fig. 11) were measured mostly in the little-disrupted top layers. Macroscopically, maximum layer thickness ranges from ~1–2 mm to ~25 cm; some layers taper rapidly near their ends, and others taper gradually along their lengths. Measured layer lengths range from ~30 to >500 cm (Fig. 11). Because the 3D shapes of layers are not known, and because some layers are truncated by younger faults, these lengths and thicknesses probably are minimal. Layer thickness and length are only weakly correlated, even if fine-grained recessive layers and obvious outliers are excluded. Layer length decreases downward, but lower layers are more disrupted (see below), so this probably is not a primary difference.

Microscopically, the layered cataclasites are very poorly sorted, with clasts that range in size continuously down to microns, although parts of some appear clast supported even in thin section (Fig. 7E). This appearance contrasts with the results of grain-size distribution analysis discussed below. Clasts are angular to subangular and mostly composed of individual mineral grains in the general proportions found in the protolith quartz diorite. Larger-than-average quartz diorite clasts occur. Sparse clasts of older, previously lithified, cataclasite or ultracataclasite also occur (in some cases larger than the average maximum grain size, but mostly not) (Fig. 7F), indicating that cataclasis occurred repeatedly. Some ultracataclasite grains are indented by quartz or plagioclase grains (Fig. 7F), so were relatively weak. Microscopic layers exist that are ≤1 mm thick, many of which also appear to have well-sorted largest average grain sizes (Fig. 7E). The thinnest microscopic layers form fine-grained sheared boundaries between coarser layers (Fig. 7G). Biotite flakes in such layers commonly are subparallel to the layer, suggesting high shear strain. These microscopic observations are consistent with the conclusion, drawn below from macroscopic observations, that grain size generally is related inversely to net shear strain.

Preliminary scanning electron microscope (SEM) imaging of freshly broken surfaces shows open inter-grain pores and some plagioclase grains with micropores (Fig. 7I). Euhedral, prismatic to tabular and micro-fibrous crystals were found in pores (Fig. 7I) and in open cleavage planes in biotite (like unstrained calcite seen in thin section). Energy dispersive X-ray spectroscopy (EDS) spot analyses indicate these crystals are calcic zeolites (probably huelandite, laumontite, and chabazite). These zeolites formed late, after deformation. We suspect that ongoing work also will show broken, syncataclasis zeolites.

Grain-size distributions (GSDs) were determined for three samples of layered cataclasite. Photomicrographs and electron microprobe BSE images (Figs. 12B and 12C) show relatively few similarly sized grains in direct contact and many cracked grains with little evidence of shear across the cracks. These samples yielded exponential GSDs with 3D fractal dimensions of ~2.75, ~3.15, and ~3.2, suggesting that cataclasis occurred by constrained comminution overprinted by shearing (Sammis et al., 1987; Sammis and King, 2007). This is consistent with weak foliations observed in some layered cataclasites (described below).

Most cataclasite layers are deformed at scales smaller than the outcrop antiform. The intensity of this deformation increases structurally downward, showing that relative age of layers increases downward. This is particularly apparent on the SW flank.

Top layers on the SW flank (Fig. 6) form a stack ~15–40 cm thick. These layers generally are little deformed and mostly are subparallel to the top contact. The top layers are more strongly deformed in the northwestern several meters of the outcrop, approaching the fault that truncates the layered body, but this may be purely coincidental. This NW increase in disruption is mainly by small faults and granular shears that crosscut some layers and then merge with layer-parallel shears, with the top contact or with the contact at the base of the top layers. Some centimeter-scale folds also appear in this zone. Farther southeast, the top layers are little disrupted internally.

The contact between top and disrupted layers is marked by an abrupt downward increase of deformation intensity across a thin zone (0.5 to ~10 cm). This zone is generally finer grained than cataclasites above or below (particularly where it is thinner), and many layers, especially in the disrupted layers below, are truncated at this zone; thus we infer that the contact below the top layers is a shear zone.

The disrupted layers, which form most of the outcrop, are intensely deformed by several meter-scale tight to isoclinal folds (Figs. 6, 13A, and B13B), by fewer, generally smaller, open folds, by sharp faults, and by granular shear zones of width from 2 to 3 mm (Fig. 13) to ~20 cm (Fig. 6). Deformation intensity in the disrupted layers generally increases downward; near the base of the layered cataclasites, it can be difficult to define the original layering.

Tight to isoclinal folds mostly have gently inclined axial surfaces and axes that trend E-W to NW-SE (Fig. 2E). These record significant N-S to NE-SW shortening within the disrupted layers and some vertical thickening.

Most faults and granular shear zones that cut the disrupted layers (Figs. 6 and 13C–13E) have normal separation and, in contrast to the tight folds described above, thin the disrupted layers. Strikes are scattered but concentrated in NNE-SSW direction and most dip moderately WNW (Fig. 2F), indicating that the layered cataclasites have been extended ESE. Individual shears commonly change updip or downdip from sharp slip planes in fine-grained layers to finite-width granular shear zones in coarse-grained layers (Fig. 13). Distributed granular shear probably is more common in coarser-grained layers because fracture propagation is more easily stalled by large grains that “block” a propagating fracture tip and/or because larger grains provide greater leverage on weak inter-grain bonds, favoring grain rotation and distributed granular shear over localization. These faults and granular shear zones commonly form sets of rotated blocks with bookshelf-style geometries. Many merge and “flatten” into recessive fine-grained layers above or below (Fig. 13D), which separate the bookshelf arrays from surrounding longer, unfaulted layers. Original layer thickness is commonly reduced in zones with many granular shears (Figs. 13D and 13E), indicating a complex strain field of both discrete and distributed shear and suggesting that cohesion in primary layers was low.

Much evidence indicates that layer-parallel shear was concentrated in the finer, recessive layers. As noted above, numerous faults and granular shears that cut across some layers merge above or below into fine-grained layers. Ultracataclasite found locally along these recessive fine-grained layers also suggests concentrated shearing in them. In addition, truncated layers typically terminate against finer-grained layers, and the grain size of faults and granular shears that cut disrupted layers is typically finer than the layers they cut. Thus maximum grain size of layers is, at least crudely, inversely proportional to net layer-parallel shear strain. These relationships also suggest that finer-grained layers are weaker than coarser-grained ones, favoring re-slip and localization of slip, in accord with the cores of many large faults being made of ultracataclasite.

Many cataclasite layers display a weak but macroscopically visible, foliation at an acute angle (commonly ~45°) to the layer boundary. This foliation is defined mainly by aligned biotite fragments and by other (less common) elongate grains (Figs. 7H and 9C). We interpret this as a low-strain flattening foliation (S) that developed in concert with shear strain along fine-grained, recessive (C) layers, forming a composite S–C fabric. In the top layers, the shear sense inferred qualitatively (top-NW) is consistent with that obtained for the top contact and rocks above it, but orientation of this weak S-foliation is very difficult to measure accurately. In the disrupted layers, the foliation orientation (where present) is scattered, consistent with disruption by folds and faults being younger than these S–C fabrics.

A second foliation, developed locally in cataclastic layers, is defined by thin (generally <3 mm) sub-layers of differing grain sizes that are oblique to the layer that contains them (Figs. 7G and 9D). These sub-layers commonly contain subparallel aligned biotite flakes. This foliation appears to have formed by dismemberment and rotation of older layers by small normal-separation faults, which commonly flatten into shear zones above and below, allowing these oblique sub-layers to become incorporated into younger, thicker layers.

Bottom Contact

The bottom contact is a sharp fault that puts disrupted layers above deformed quartz diorite. It is exposed only over several meters of outcrop length. Some of the small faults and granular shear zones in the disrupted layers cut the bottom contact, which also is cut by larger-displacement (decimeters), NE-dipping, normal-separation faults and by faults that strike subparallel to the fold hinge. These relations suggest that the bottom contact is older than the top contact on the SW side of the antiform.

Cataclasite and Ultracataclasite Injections

Injections of cataclasite, generally millimeters in width and up to a few centimeters in length, locally cut the layered cataclasites. Injections also are found along and above the top contact. Figure 14 shows decimeter-scale injections of cataclasite that acutely crosscut foliated and fractured rocks above the top contact, thin upward, and are truncated at their bases by the top contact, suggesting they originated along the top contact. They fill mode-I fractures that could not have been generation surfaces. The largest is graded: finer grained at the margins and coarser in the center (Fig. 14C), consistent with inward-decreasing shear-strain rates and suggesting that the material was fluidized granular slurry when injected.

Other injections above the top contact are irregular in shape. One injection immediately above the top contact fills a “pull-apart” formed by slip on an R1 shear where it intersects the top contact (Fig. 15), causing extension of a sliver of mixed lithologies above the top contact. It is filled with folded millimeter-scale layers of cataclasite that cut across the top contact (Fig. 15C), showing that they originated below it, thus, this injection was one of the last events along the top contact. The older layering is preserved in this injection (unlike the others that were fluidized granular injections), suggesting that the layers had developed some cohesiveness but were still ductile during injection. The uppermost layer in this pull-apart is composed of ultracataclasite that also may have been injected into it, presumably earlier because it is terminated by the top contact. Another sorted injection (Fig. 16A) lies between the top contact and a subparallel fault a few centimeters above it, suggesting that it was beheaded by the upper fault.

Injections of ultracataclasite also occur above the top contact (Fig. 15A). In one, ultracataclasite fills a crack that is perpendicular to and cuts across a thin (3–4 cm) layer of breccia immediately above the top contact (between the heavy opposing arrows in Fig. 15A). This injection is continuous with ultracataclasite along the top contact and along the top of the breccia layer (Fig 15A) and therefore probably was generated by slip on one or both of those surfaces. A larger body of ultracataclasite that contains breccia clasts resides above an R1 shear (Fig. 15A) and may also have been injected.

The relations described above, along with a general lack of injections more than ~50 cm above the top contact, suggest that the cataclastic and ultracataclastic injections originated along and/or below the top contact or on R1 shears near the top contact.

Late Mineral Veins

The top and disrupted layers commonly are cut by planar veins of late-stage calcite ± gypsum. Most fill tensile cracks, but some show millimeters of shear offset. Most dip steeply ESE and thus opened parallel to the WSDF transport direction (Fig. 2G). They typically are <2 mm wide and may be up to ~2 m long. They clearly postdate the cataclastic deformation of main interest here.

STRUCTURAL FORMATION OF CATACLASTIC LAYERS

The SW-flank exposures provide an excellent record of processes related to the genesis and subsequent disruption of layered cataclasites. We conclude in this section that cataclastic layers formed from rocks just above the top contact, consistent with the interpretation (from downward-increasing layer disruption) that cataclastic layers are youngest at the top and generally older downward.

Rocks immediately above the top contact reveal processes of cataclastic layer formation. One process apparently involves slip on R1 shears that bend into parallelism with the top contact several centimeters above it but do not merge with it (Fig. 8), defining fault-bounded layers of foliated rock above and parallel to the top contact. These R1 shears flatten and disappear into the cataclastic foliation, which generally also is subparallel to the top contact within several centimeters of it (Figs. 8 and 9B). Slip on foliation-parallel parts of R1 shears and, probably, lateral propagation and linking of these parts isolated rock slices composed mainly of cataclastically foliated quartz diorite. Grain size in such slivers was first reduced during brecciation and further reduced and homogenized during overprinting by cataclastic foliation (described above). The foliated rocks immediately above the top contact generally are macroscopically well sorted, a characteristic of most cataclastic layers also.

Elsewhere, slices of heterogeneous rocks occur immediately above and parallel to the top contact, bounded above by shear planes subparallel to the top contact and within ~10 cm of it (Figs. 9B, 15, and 16). These slices contain granular cataclasite, breccia, foliated cataclasite, ultracataclasite, and injected material, and have sharp to granular shears within them (Figs. 9B and 16). Some R1 shears flatten into the tops of these heterogeneous slices, but others cut across them and merge with the top contact (Fig. 15A).

We interpret both foliated and heterogeneous layers above the top contact as “frozen” precursors to cataclastic layers. Heterogeneous layers have along-strike changes in grain size and lithology and thus provide a mechanism to incorporate outsized clasts and clasts of cataclasite or ultracataclasite into layers. One heterogeneous layer above the top contact has dimensions comparable to cataclastic layers (red diamond in Fig. 11). Some of these slices have a foliated texture at their SE ends, with foliation disappearing and textures becoming more random to the NW: at their SE end, they resemble rocks above the top contact, and at their NW end, they resemble the cataclasite layers. In contrast, foliated layers above the top contact are lithologically homogeneous and well sorted (at the largest grain sizes) and provide suitable precursors to well-sorted cataclastic layers.

All evidence is consistent with new layers having formed from fault-bounded slices above the top contact and being added to the top of the stack. Thus, the top contact migrated episodically upward into the overlying rocks. As new layers formed, older, structurally lower layers progressively were abandoned and disrupted by folds and faults. We infer that the first cataclastic layer formed along a fault oriented about the same as the SW-flank top contact, the first layer strengthened relative to overlying rocks (discussed below), and new layers formed at the expense of rocks above the top contact.

We conclude that formation of each cataclastic layer required the following two events: (1) preparation processes above the top contact, including: (a) cataclastic grain-size reduction of the quartz diorite; (b) overprinting by foliation with downward-decreasing maximum grain size and downward-increasing intensity and macroscopic sorting; (c) slip on R1 shears and other faults; and (d) formation of a slice-bounding fault above and parallel to the top contact; (2) one or more relatively large rapid-slip events, on the top contact and/or the slice-bounding fault, that homogenize each slice, destroy preexisting foliations, transform the slice into a random-fabric cataclastic layer, and cause the top contact to episodically “jump” upward. In some layers, a weak, shear-related foliation formed later.

Structural relationships on the NE flank of the outcrop antiform are most easily interpreted as showing that the antiform developed due to WSDF-related drag of the NE limb, rotating it into parallelism with the detachment, attenuating the NE limb and removing the top layers, and overprinting primary, layer-related fabrics with WSDF shear-related ones.

Grain-size distributions (GSDs) in cataclasite yield information about conditions of comminution (Sammis et al., 1987; Sammis and King, 2007). Log-normal GSDs suggest that cataclasis was accompanied by dilation; thus, grains can move around and past one another. Dilation during cataclasis, and reduction of effective normal stress between grains, may occur along subsurface faults if pore-fluid pressure is high or if the fault becomes dynamically dilatant. In contrast, exponential GSDs suggest that dilation was not important and that grains crush each other, failing mainly on tensile fractures. This “constrained comminution” leaves few same-size grains in contact: larger grains become surrounded and “insulated” from crushing by smaller grains (Sammis et al., 1987).

D-values for the layered cataclasites are ~3, indicating either that shear localization followed and overprinted constrained comminution, resulting in a higher proportion of smaller grains (Sammis and King, 2007) or that high shear strain occurred during cataclasis (Storti et al., 2003). These D-values are similar to those determined for footwall fault-core rocks from both the WSDF and the Whipple detachment (Luther et al., 2013), suggesting that the layered cataclasites formed in mechanical conditions similar to those experienced by fault-core rocks from mature, large-slip detachments.

FORMATION OF LAYERED CATACLASITES IN REPEATED SEISMIC CYCLES

The observations above show that several different deformation events were required to create each layer of cataclasite. These events probably occurred at different strain or slip rates. We infer below that one (possibly more) rapid (seismogenic) and chaotic slip event(s) on the top contact and/or the faults bounding the top of precursor slices was required to homogenize lithologies and randomize fabrics when each cataclastic layer was created.

The seismic cycle at any point on a fault is defined by main shocks that rupture that place, with inter-seismic strain accumulating between them. The largest earthquakes probably would have been on the WSDF, just meters above the layered cataclasites. Foreshocks sometimes precede main shocks, and aftershock sequences follow them, with some aftershocks on/near the main-shock plane and others off the plane (in the fractured damage zone) (e.g., Savage et al., 2017). Aftershocks commonly concentrate near the main-shock rupture margins. Aftershocks and/or aseismic slip in the fractured damage zone presumably contribute to its development (e.g., Savage and Brodsky, 2011). Small aftershocks are much more common than large ones, and aftershock magnitudes generally decline with time. Relatively rapid aseismic “afterslip” is reasonably common in the months or years following main shocks; later, relatively slow “inter-seismic creep” may occur (also aseismic). Afterslip and inter-seismic creep usually are detected geodetically and are modeled as occurring on the main fault, but some of both occurring off-fault cannot be excluded.

We are unaware of reasons to think that the layered cataclasites entirely pre-dated the WSDF, and we conclude that the layers formed in the WSDF damage zone, suggesting that WSDF main shocks would have defined the local seismic cycle. Layered pseudotachylyte nearby along the WSDF requires that it was repeatedly seismogenic (Prante et al., 2014). Episodically formed and mutually overprinting structures and fabrics described here are consistent with cyclic and episodic processes, and with the seismic cycle. Rowe et al. (2011) also interpreted mutually overprinting fault-rock types along an exhumed subduction thrust as evidence for repeated seismic cycles.

In this context, we assign some of the textures described above, and their formative processes, to fast versus slow deformation rates. Injections of cataclasite and ultracataclasite are the structures most likely to record paleoearthquakes. Off-fault injections of pseudotachylyte are common, require rapid injection during an earthquake (before the veins can solidify in seconds or minutes), and are associated with cataclastic injections (e.g., Rowe et al., 2005). Rowe et al. (2012) described pseudotachylyte and cataclasite injections in subduction-zone mélange that were generated during seismic slip on a plane where they end. They have aspect ratios of ~0.2, much higher than aspect ratios of 0.001–0.0001 for dikes. Griffith et al. (2012) documented similar pseudotachylyte injections into tonalite with aspect ratios of ~0.015–0.5. Both argued that such injections require dynamic deformation of the wall rock in order to form.

The injections in Figure 14 end at the top contact, where they probably were generated, and have aspect ratios of ~0.07 and 0.09, consistent with a coseismic interpretation. Their angles to the fault are, however, much lower than expected from coseismic tensile cracks that host injections (e.g., Di Toro et al., 2005; Griffith et al., 2009; Rowe et al., 2012). The low angle in our study site probably was controlled by anisotropy in the host rocks: those injections are in the hinge zone of a folded pegmatite, where many fractures are subparallel to the injections (Fig. 14B).

If seismogenic slip events on the top contact were needed to form the injections, then the same or similar rapid slip events probably formed homogeneous, random-fabric cataclastic layers from heterogeneous and/or foliated rocks. Such events probably were aftershocks to WSDF main shocks.

In contrast, formation of foliations requires penetrative strain that probably accumulated slowly. Foliation above the SW-flank top contact pre-dates formation of cataclastic layers, but foliation also overprinted some cataclastic layers, requiring multiple episodes of slow strain. Other structures may have formed either quickly or slowly: Reidel shears, other sharp faults, granular shear zones, and various folds.

Some textures and structures preserved in the layered cataclasites and in the rocks above the SW-flank top contact probably record generally decreasing energy release (whether seismic, aseismic, or both), consistent with temporally decreasing aftershock magnitudes and with decreasing afterslip rates. For example, it probably required more energy to form and homogenize any given layer than to subsequently overprint it with a weakly developed foliation, fold it, or cut it by small faults. Many of the delicate structures in the layered cataclasites probably would not be preserved if formed along a long-lived fault because they would be destroyed by subsequent rapid, high-energy slip events. Similarly, delicately folded millimeter-scale layering injected across the top contact (Fig. 15C) probably records the final small slip event, as the injection opened, on that part of the top contact before it was completely abandoned.

Given the large number of layers, a comparable or larger number of seismic slip events were needed to form the whole layered stack. The body of layered cataclasites is ~2–3 m thick on average, and the average thickness of individual layers is ~4 cm, suggesting that ~50–75 layers exist in the stack. In the disrupted layers, isoclinal folding repeated some layers and probably thickened the stack, but normal faulting also thinned it; so this estimate is very rough. However, if at least one earthquake is needed to form each layer (more may be required), then a minimum of ~50–75 seismic events were needed to form the body, suggesting multiple seismic cycles.

DISCUSSION

Rare Preservation of Common Products or Preservation of Rare Products?

Do these outcrops display rarely preserved products of common processes, and therefore potentially are of general significance, or do they preserve products of rare processes and are not of general importance? We support the former view for the following reasons.

First, the layered cataclasites are similar to typical cataclasites formed from quartzofeldspathic protoliths (cf., Sammis et al., 1987; Luther et al., 2013) in several ways (grain-size distribution, grain shapes, overprinting weak foliation, etc.), implying that they formed by common fault-zone processes. However, most of the cataclastic layers and many of the textures and crosscutting relations documented in them probably would have been destroyed if the SW-flank top contact had not progressively migrated up into its hanging wall, with successively older layers being abandoned. We argue below that this was due to strength contrasts across the top contact. Thus, the strength evolution may have been special, but we doubt that the processes that formed the layered cataclasites were.

Second, many fault-core rocks are layered. Along the WSDF, layering is common within the ultracataclastic inner core, which is separated into layers (commonly of different colors) by slip surfaces parallel to the WSDF (see also Shervais and Kirkpatrick, 2016). Ultracataclasites along the WSDF also yield evidence for cyclic processes similar to those that we infer operated here. For example, Kairouz (2005) and Hazelton (2003) described clasts of ultracataclasite in ultracataclasite layers and rare, weak foliations overprinting ultracataclasite, along both the WSDF and the Whipple detachment. Prante et al. (2014) describe multiple layers of fault-vein pseudotachylyte from the WSDF.

Given these commonalities, we find ourselves in disagreement with the often-cited view of Cowan (1999) that pseudotachylyte is the only fault rock that compellingly records paleoseismicity. We agree with Rowe and Griffith (2015) that this viewpoint is too strong, especially given the knowledge that many (most?) continental faults accumulate slip primarily seismically (e.g., nearly all of the San Andreas system, except creeping sections). We therefore suggest that a paleoseismic interpretation should be preferred where strong evidence exists for both cyclic formation of fault rocks and for cyclic changes in strain rate (e.g., Rowe et al., 2011). It takes considerable effort to document the full suite of fault-rock textures and overprinting relationships needed to infer seismic cycling at a given locality. Regardless, we view it as likely that seismogenic fault-rock assemblages are more common than single-lithology paleoseismic indicators (pseudotachylyte, amorphous silicates, etc.).

Seismic Cycling and the Rock Record

Only very rough estimates can be made of earthquake magnitudes needed to form the layered cataclasites because the fault dimensions and slip magnitudes related to their formation are poorly known. Decreasing energy release during aftershock sequences suggests that a wide range of magnitudes can be anticipated. Scalar seismic moment of an earthquake, M0, is given by the product of shear modulus (μ), average slip (S), and fault area (A), with shear modulus given by the product of shear-wave velocity (VS) squared and density (ρ) (e.g., Shearer, 1999, p. 21 and 169). We use VS = 3.5 km/s and ρ = 2700 kg/m3, appropriate for intermediate plutonic rocks. Minimum rupture area can be constrained crudely from layer lengths (themselves minima; Fig. 11), but it is very likely that layer-forming rupture areas are larger. We use A ranging from 102 to 106 m2 (for fault lengths of ~10–1000 m). Slip magnitude is poorly constrained; we use the range 0.1–1 m. These numbers yield a moment range for layer-forming earthquakes of ~3.3 × 1011 to 3.3 × 1016 N-m, or moment magnitude (Mw) of 1.6–5.0 (Kanamori, 1977; Shearer, 1999, p. 187). This range is too high for many of the smaller preserved structures above the top contact (e.g., Fig. 15A) or in the disrupted layers (Fig. 13), with slip <0.1 m and outcrop lengths suggestive of areas <<10 m2. However, these ranges may be reasonable for events that formed and homogenized cataclastic layers.

We suggest that most WSDF slip occurred during episodic large earthquakes, given the common occurrence of pseudotachylyte elsewhere along the WSDF (e.g., Prante et al., 2014) and the evidence presented here for formation of layered cataclasites in the WSDF damage zone during multiple seismic cycles. What are the reasonable ranges of main-shock magnitude and frequency that could be expected from the WSDF? The WSDF is ~100 km long N-S (Fig. 1B does not show northern parts), with downdip length of ~20 km (for 30° dip and 12 km seismogenic crust), yielding a maximum rupture area of ~2000 km2. A small main shock might rupture only a 100 km2 patch. Most historical normal-fault earthquakes occurred on steeper faults (Jackson and White, 1989) and have ≤3 m maximum offset on surface scarps (Yeats et al., 1997, chapter 9) but Wernicke (1995) argued that maximum slip on low-angle normal faults can be higher. For S of 0.5–5 m, A of 100–2000 km2, and other parameters as above, the range of WSDF main-shock moment release is 1.7 × 1018 to 3.3 × 1020 N-m, or Mw range of 6.1–7.6. If WSDF net slip is 10–15 km, all main shocks ruptured the whole WSDF area in 5 m slip events, and the WSDF was active for 7 m.y. total (from ~8 to ~1 m.y.), the minimum average recurrence interval for Mw = 7.6 main shocks would be ~2.3–3.5 k.y.

Assuming that WSDF slip accumulated mainly seismogenically, it follows that the very typical two-part WSDF fault core also formed mainly during earthquake slip. Thus, we hypothesize that mature faults in quartzofeldspathic rocks with a layered ultracataclastic inner core and a cataclastic outer core should be considered as paleoseismogenic. Such an assemblage was described by Chester and Chester (1998) along the Punchbowl fault (an abandoned strand of the San Andreas fault), which they assumed was seismically active. Many low-angle normal faults with quarztofeldspathic footwalls have similar fault cores at their tops (e.g., Davis and Lister, 1988; many others). These are described as comprising a “microbreccia ledge” (inner core) above a “chlorite breccia zone” (including the outer fault core and, usually, part or all of the fractured damage zone).

If both the layered cataclasites and (more speculatively) the WSDF fault core formed seismogenically, then what controlled the differences between them? The most likely controls are elastic energy released in seismic events and the percentage of this expended as work during cataclasis, net shear strain, and degree of localization (which affects the net shear strain).

The amount of finely comminuted ultracataclasite is much greater along the WSDF than in the layered cataclasites. Therefore, work expended forming WSDF fault-core rocks probably was much greater than that needed to create cataclastic layers, although this is impossible to quantify at present. This is consistent with the estimates made above for earthquake magnitudes needed to form the layers seismogenically versus possible on the WSDF.

In contrast, the shear strains needed for layer formation and for WSDF net slip can be constrained roughly. The layered cataclasites probably record low values of integrated cataclastic shear strain relative to that which formed the WSDF core. In addition, the layered cataclasites preserve a record of ~3 m of migration, measured approximately perpendicular to the fault plane along which they formed, and thus each was active only temporarily before being abandoned when a higher layer formed. In contrast, the WSDF apparently localized to such an extent that the fault core preserved at the top of the footwall typically is ≤1 m thick.

One approach to estimating shear strain needed to form layered cataclasites is to assume that their aspect ratios (Fig. 11) are related to shear strain. For a thin (5 mm), 5-m-long, fine-grained, high-strain layer, the aspect ratio is 103; for a thick (250 mm), 2-m-long, coarse-grained layer, the ratio is 8 (~101). This range may be higher than the actual shear strain that formed individual layers, if the slip needed to form them was less than their lengths.

In contrast, if 10–15 km of WSDF slip was distributed through ~2 m of fault-core thickness (assuming that a mirror-image, ~1-m-thick, hanging-wall fault core was left behind at depth), then shear strain would be 5–7.5 × 103. If WSDF shear was distributed through only ~0.5 m of ultracataclastic inner fault core, the shear strain would be 2–3 × 104. This range probably is higher than actually formed the inner-core ultracataclasites, because much slip arguably was concentrated on very thin (<1 mm) slip surfaces within the ultracataclasites.

Thus, net shear strain for formation of the WSDF fault core probably was significantly higher than that needed to form cataclastic layers. Interestingly, the two rock assemblages have very similar D-values for their grain-size distributions.

Evolution of Strength during Faulting

In this section, we discuss the temporal and spatial evolution of strength in and around the layered cataclasites and the WSDF in the context of the seismic cycle and varying deformation rates.

Cataclastic layers formed at the expense of rocks above the top contact and older, lower layers were preserved and abandoned. Thus, new layers presumably were stronger than overlying deformed quartz diorite. We infer that strain hardening of the layered cataclasites and/or strain softening in the rocks above the top contact were essential for formation of the layers.

Penetrative cataclastic foliation development and grain-size reduction probably were strain-softening mechanisms above the top contact. R1 shears that flatten into the well-developed foliation above and parallel to the top contact show that the foliation was weak. Foliation-parallel weakness probably was caused by grain-size reduction and alignment of biotite grains near the top contact. Collettini et al. (2009) showed that foliation in clay gouge dramatically reduces friction, and we suspect that the same effect weakened the foliated rocks above the top contact, where biotite is aligned. However, foliation above the top contact probably took many seismic cycles to develop, and certainly was not present initially. Penetrative microfractures in the quartz diorite, which may have begun to form very early in the deformation history, would also have reduced the strength of the quartz diorite enough to allow preservation of the first-formed layer.

We argue above for repeated shearing on recessive, fine-grained layers, which separate resistant, coarser-grained, and presumably stronger layers; thus, strength contrasts existed among the layered cataclasites, with strength apparently correlated to grain size. Thus, grain-size reduction probably was a strain-weakening mechanism both above and below the top contact. Therefore, strong foliation development (alignment of biotites) above the top contact in addition to grain-size reduction probably account for late weakness of those rocks relative to the stack of layers.

It seems likely that compaction of cataclastic layers following their formation strengthened them. This is in accord with increases of friction seen after hold periods in slide-hold-slide experiments on thin gouge zones (e.g., Nakatani, 1998; Richardson and Marone, 1999; Sleep et al., 2000; many others). Mair and Marone (1999) present data that suggest greater strengthening during holds with low shear stress than with high shear stress; so perhaps foliation-free cataclastic layers record top-contact earthquakes with larger stress drops, leaving shear-stress levels during the compaction phase too low for foliation development. The foliation in many cataclastic layers probably records inter-seismic shear during compaction. It is weakly developed, and at ~45° to layering (Fig. 9C), records small shear strain, suggesting that the cataclasite layers strengthened when it developed. In contrast, the well-developed and strongly rotated foliation above the top contact records much higher shear strain, suggesting that those rocks were weak relative to the layered cataclasites and that distributed inter-seismic shear strain was more strongly localized there.

The top contact also must have been stronger than the rocks above it, at least at times or locally, in order for cataclasite slices to have formed and been transferred into the layered stack. If grain size was a significant control on strength, then strength was probably variable along the top contact, as indicated by grain size along it ranging from aphanitic (ultracataclasite) up to fairly coarse.

The arguments above, for strength contrasts between the layered cataclasites and top contact (stronger) and the overlying deformed quartz diorite (weak) causing upward “jumps” of the top contact, are analogous to recent results showing that pseudotachylyte solidification strengthens faults enough that they are abandoned and slip moves to new surfaces (Mitchell et al., 2016; Proctor and Lockner, 2016).

In contrast, slip remained localized in the WSDF core, suggesting that it remained weak relative to its surroundings. The most obvious differences between layered cataclasites and the WSDF fault-core rocks are that (1) the WSDF fault core has a much greater thickness of very fine-grained ultracataclasite, and (2) WSDF fault-core rocks are only rarely foliated. Neither contains significant amounts of clay minerals (as are present in the base of the upper plate); therefore, clays are probably not the cause of WSDF weakness. Thick WSDF ultracataclasite (1) supports arguments already made that fine-grained cataclastic rocks are weaker than coarser ones and is probably explained by WSDF fault-rock formation during larger-slip, higher-energy seismic events than created the layers.

General lack of foliation in WSDF fault-core rocks (2) may be explained two ways. First, stress drop in most WSDF main shocks may have been nearly complete, and residual shear stress was too low to shear core rocks and develop foliation. Luther and Axen (2013) concluded from different arguments that WSDF earthquakes had large stress drops. Second, inter-seismic strain may have accumulated outside the WSDF fault core by small earthquakes or fracture creep events in the fractured damage zone, where foliation is generally absent. This can be explained if WSDF fault-core healing (increase of cohesion, resistance to distributed shearing in the ultracataclasites, and/or inter-seismic increase of static friction) was fast and efficient relative to healing in the fractured damage zone. Fault surfaces within ultracataclasite may regain cohesion faster than coarser-grained fault rocks for several reasons. Tiny grains can indent across slip surfaces a shorter distance, which may lead more quickly to geometric interlocking. This effect may be enhanced because tiny grains are more likely to have individual strengths closer to crystallographic maxima, due to less internal damage in each grain (e.g., fewer microfractures and other crystalline defects preserved). Last, healing by chemical processes such as pressure solution and precipitation probably occurs faster over the shorter distances and higher surface areas of very fine-grained fault rocks (Rutter, 1983; Bos et al., 2000). Also, the top contact had a different orientation than the WSDF while they were active and may simply have been more favorably oriented for slip during the inter-seismic periods. Alternatively, repeated rupturing of the inner core may have been controlled by rupture-tip propagation processes rather than by typical measures of rock strength, but it is unclear how to assess this.

We are not aware of mature faults that migrated systematically perpendicular to themselves. This probably explains why interleaved slices of rock from both sides of faults are rare in the rock record. Multiple slip surfaces within the inner core require small (centimeter-scale) “jumps,” but these may have been controlled by processes within the fault core (e.g., Shervais and Kirkpatrick, 2016). Occasional transfers of material from the outer fault core to the inner fault core are required in order for the inner core to grow in thickness without incorporating upper-plate lithologies; but how this happens is not known.

Random-fabric or sorted cataclastic injections require loss of cohesion and fluidized cataclastic flow and transport. If flow is driven by a pressure gradient, then cataclasite-generation surfaces may have been, at least momentarily, zones of high normal stress, as is inferred for formation and flow of pseudotachylyte (e.g., Sibson and Toy, 2006; Rowe et al., 2012). This may drive transient pressure gradients required for coseismic off-fault injections, similar to off-fault pseudotachylyte veins. However, some injections fill dilatant pull-apart structures (Fig. 15), and dynamic rupture propagation can cause tensile cracks to form in the absence of gouge (e.g., Griffith et al., 2009). Therefore, transiently reduced pressure in off-fault injection sites may also contribute to coseismic pressure gradients.

Mechanical versus Chemical Processing

The degree of alteration in the layered cataclasites and surrounding plutonic protoliths is surprisingly small, suggesting that mechanical processes dominated formation of the layered cataclasites. This may allow meaningful comparison of these natural fault rocks to those created experimentally but with too little time for significant chemical processes to occur.

Mafic minerals (biotite, hornblende, and clinopyroxene) are essentially unaltered (though comminuted, broken, and bent). Chlorite is optically absent, but its presence as a minor alteration product in biotite is suggested by low-K values of some biotites. Preliminary SEM imaging of plagioclase shows local development of microporosity, suggestive of dissolution, and some weak, hazy alteration is observed in thin section (probably sericite).

Euhedral calcic zeolite minerals in pores in layered cataclasites (Fig. 7I) have been identified, but it is not yet known if zeolite growth entirely postdated cataclasis or also accompanied it. Zeolite growth probably was aided by hydrothermal fluid circulation up the fault zone and possibly occurred at the expense of microporous plagioclase. In this context, it is noteworthy that hot springs emanate from the WSDF at Agua Caliente County Park, near the south end of the WSDF. Wood (2014) identified zeolites in the basal upper plate there, which is altered for several meters above the fault. Footwall fault rocks there display a moderately developed cataclastic S–C foliation and stretching lineation, and we recently found zeolites in these rocks (XRD analyses), but their ages relative to fault-related deformation are not yet known. Shervais and Kirkpatrick (2016) found laumontite in inner fault-core rocks of the La Quinta fault and normal-sense shears that splay from it. They suggested that this phase of La Quinta slip occurred below 250 °C and was related to WSDF slip.

Zeolites commonly form in hydrothermal systems (among other occurrences; Chipera and Apps, 2001). Hydrothermal calcic zeolites from wells near Yellowstone are presumed to be in equilibrium with ambient temperatures and subsurface geothermal fluids (Bargar and Kieth, 1995). Laumontite, which is present in WSDF fault rocks, is present at Yellowstone at temperatures of ~160–200 °C, consistent with other studies (see Chipera and Apps, 2001). For reasonable geothermal gradients of 40° to 25 °C/km, this suggests zeolite deposition along the WSDF at depths of 4–8 km, consistent with independent estimates of paleodepth of these rocks summarized above (of course, the geotherm in a fault-controlled fluid pathway may have been higher). Huelandite (also in WSDF fault rocks) probably is stable at lower temperatures and depths (Chipera and Apps, 2001).

Zeolites in fault rocks have been studied only sparsely. Calcic zeolites are common as fracture and vein fill in core from the Cajon Pass borehole and in surrounding outcrops (James and Silver, 1988), where they replace plagioclase. There, the downward transition from stilbite to laumontite occurs at ~100 °C (present well temperature) and was interpreted to be in equilibrium at that temperature (James and Silver, 1988). Laumontite alteration of host rock reduced the strength of intact Cajon Pass core samples (Vernik and Zoback, 1989). Morrow and Byerlee (1988) showed that zeolites at Cajon Pass reduced permeability significantly and probably allowed for fluid compartmentalization (Kharaka et al., 1988). Morrow and Byerlee (1991) showed that crushed laumontite artificial gouge from Cajon Pass has friction in accord with Byerlee’s Law, with no discernable effects of pore-fluid pressure increase. We are not aware of rate-state experiments on zeolites. Walker et al. (2013a, 2013b) described zeolite veins in mafic or intermediate rocks in and near fault zones. Calcic and sodic zeolites (mainly in veins) have been found in intermediate plutonic rocks around the Gole Larghe fault zone, Italy (Dempsey et al., 2014), a famous site for study of pseudotachylyte (e.g., Di Toro and Pennacchioni, 2005; Smith et al., 2013). Zeolites there, and fluid flow related to their precipitation in fractures and veins, are interpreted to be one manifestation of paleoseismicity (Dempsey et al., 2014).

CONCLUSIONS

Uniquely preserved layered cataclastic fault rocks are found below the WSDF, in an outcrop-scale antiform that folds them, the surrounding, deformed quartz diorite protolith, and the faults that separate the two. Layers were formed sequentially and episodically along the top contact, by cataclasis and homogenization of rock slices previously above that contact. Layers were transferred down into the layer stack, as their generation surface (top contact) migrated stepwise up into deformed quartz diorite. Older, structurally lower layers are increasingly deformed downward, by folds that record ~NNW-SSE shortening and vertical thickening and by small faults that record ~ESE-WNW extension and vertical thinning of the layer stack.

The layered cataclasites, as a fault-rock assemblage, record dozens of seismic cycles. Evidence for this includes many overprinting textures and crosscutting relationships that show cyclic alternations between slow, progressive strain accumulation that presumably records inter-seismic strain (e.g., foliation in protolith quartz diorite that strengthens and rotates as the top contact is approached) and rapid strain or slip events that formed cataclastic injections and created homogeneous and random-fabric cataclasite layers from foliated and/or heterogeneous protolith.

The layered cataclasites are a useful but atypical rock record of common processes (and thus are of general significance) rather than a rare product of uncommon processes. In particular, the sequential formation, preservation, and deformation of cataclastic layers yield timing relationships and history that typically are difficult or impossible to determine in well-developed fault-core rocks. As such, the stack of layers is a unique record of the seismic cycle. The layered cataclasites apparently formed mainly by mechanical processes with limited alteration and may provide good analogues to mechanically (de)formed fault rocks studied experimentally.

The WSDF was a repeatedly seismogenic low-angle normal fault, as shown by presence of layered pseudotachylyte along it (Axen et al., 1998; Prante et al., 2014) and by the presence of these paleoseismic layered cataclasites just meters below the detachment. Simple calculations suggest that WSDF main shocks could reasonably have had moment magnitudes Mw of 6.1–7.6 (M0 range of 1.7 × 1018 to 3.3 × 1020). In contrast, the large, fast-slip events inferred to have formed individual cataclasite layers were probably much smaller (Mw range of 1.6–5.0 and M0 range of 3.3 × 1011 to 3.3 × 1017). Net shear strains to form layered cataclasites (order 101 to 103) also probably were significantly lower than those that formed the WSDF fault-core rocks (5 × 103 to 3 × 104).

Grain-size distributions of fault rocks from the WSDF and Whipple detachment fault cores (Luther and Axen, 2013) have similar fractal dimensions to these layered cataclasites, suggesting formation by constrained comminution overprinted by shear strain (e.g., Sammis and King, 2007). However, this measure does not reflect the overall much more significant grain-size reduction of WSDF and Whipple detachment core rocks versus cataclastic layers. We conclude that the main difference between the WSDF fault-core rocks and the layered cataclasites resulted from differences in net shear strain and energy consumed in comminution. Earthquake slip on the WSDF localized in a narrow, weak zone and resulting high rates of energy release and high shear strains formed decimeters of ultracataclasite. In contrast, strain softening above the top contact and strain hardening of the cataclastic layers prevented continued overprinting and comminution of cataclastic layers and allowed their preservation at relatively early stages. Thus, average grain size, average largest grain size, and/or ultracataclastic rock volume may provide useful measures for development of quantitative, predictive relationships among fault rocks, energy expended in cataclasis, and earthquake processing.

The WSDF has a second fault-rock assemblage that we hypothesize formed mainly seismogenically: a two-part footwall fault core with an inner core of ultracataclasite separated into layers by multiple slip surfaces and an outer core of random-fabric cataclasite. It resembles the cores of other mature faults developed in quartzofeldspathic hosts (e.g., the Punchbowl fault; Chester and Chester, 1998) and many other low-angle normal fault cores composed of “microbreccia” inner cores and “chlorite breccia” outer cores (e.g., Davis and Lister, 1988). We suggest that this common fault-rock assemblage may record paleoseismicity where found. Upper-plate fault rocks near the layered cataclasites are clay rich and probably formed above the seismogenic layer. The lack of comparable volumes of clays in the footwall fault core suggests that footwall fault rocks passed through the crust above the seismogenic zone relatively unchanged.

ACKNOWLEDGMENTS

This study was funded by National Science Foundation awards EAR-0809638 (G.A.) and EAR-0809220 (J.S.). Ashley Griffith and an anonymous reviewer each carefully and thoughtfully reviewed two versions of the manuscript, greatly improving it. Discussions with R. Holdsworth about fault-rock genesis are appreciated. Support from the Anza-Borrego Desert State Park and the California State Parks Colorado Desert District was provided in the forms of sampling permits, access to their research laboratory in Borrego Springs, California, and by the generally helpful staff there, especially G. Jefferson, L. Murray, and J. Manning. Jimmy Smith of Borrego Springs, California assisted in the field. M. Spilde of the University of New Mexico Institute of Meteoritics aided sample preparation for SEM work, and N. Dunbar and L. Heizler of the New Mexico Bureau of Geology and Mineral Resources assisted with electron microprobe BSE imaging.

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Gold Open Access: This paper is published under the terms of the CC-BY-NC license.