Unique Layered Cataclasites Below the West Salton Detachment Fault, Southern California GEOSPHERE; V

Research Paper

GEOSPHERE A rock record of paleoseismic cycling: Unique layered cataclasites below the West Salton detachment fault, southern California GEOSPHERE; v. 14, no. 1 Gary J. Axen1, Jane Selverstone2, and Amy Luther1,* 1Department of Earth & Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA doi:10.1130/GES01499.1 2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

16 figures ABSTRACT as products of seismogenic slip. In contrast, clay-rich gouge and upper-plate CORRESPONDENCE: Gary.Axen@nmt.edu breccias formed at depth ≤2–3 km. The footwall fault core is clay poor and thus Only relatively rare fault rocks record paleoseismicity (e.g., pseudotachy- apparently passed through the uppermost crust with little overprint. CITATION: Axen, G.J., Selverstone, J., and Luther, lyte), though most fault slip probably accumulates during earthquakes. We The layered cataclasites and detachment fault core display only minor A., 2018, A rock record of paleoseismic cycling: Unique layered cataclasites below the West Salton describe two fault-rock assemblages, made of common fault-rock types, and chemical alteration, and the layers record limited shear strain relative to fault- detachment fault, southern California: Geosphere, argue that fault-rock assemblages can provide criteria for inferring seismogenic core rocks. Thus, the layered cataclasites may be good natural analogs for ex- v. 14, no. 1, p. 187–214, doi:10.1130/GES01499.1. slip on exhumed faults that are more common than individual fault-rock types. perimentally formed fault rocks. Both assemblages formed from intermediate Such assemblages allow direct study of fault-slip products formed under vari- plutonic protoliths, similar to “average” mid-crustal rocks, and from the same Science Editor: Shanaka de Silva ous crustal conditions that are not accessible on active faults. One assemblage, lithologies as cut by the active San Jacinto and Elsinore faults. Thus, these of layered cataclasites, is preserved several meters below the detachment. It two assemblages provide accessible, paleoseismically formed analogs to fault Received 20 January 2017 is rare in the rock record and is best interpreted as recording parts of multiple rocks presumably present at seismogenic depth along active faults. Revision received 24 August 2017 Accepted 16 October 2017 seismic cycles. A second assemblage is the two-part detachment fault core: Published online 22 November 2017 inner fault-core ultracataclasite layers separated by principal slip surfaces and INTRODUCTION outer fault-core cataclasites. We hypothesize that this common assemblage forms mainly during seismogenic slip. We interpret a third assemblage of clay Fault rocks formed in and above the brittle-plastic (or frictional-viscous) gouge and breccia as recording slip and alteration above the seismogenic zone. transition are of particular interest for understanding fault and earthquake pro- Detailed structural analysis of the layered cataclasites shows that they cesses (e.g., reviews by Cowan, 1999 and Rowe and Griffith, 2015): fault-zone formed and were deformed episodically, most consistent with multiple evolution (e.g., Scholz, 1987; Chester et al., 1993; Scholz et al., 1993; Ben Zion seismic cycles that probably were defined by West Salton detachment fault and Sammis, 2003), fault stress states and strength, energy budget and con- (WSDF) main shocks. Cataclasite layers formed sequentially at the expense stitutive relations (e.g., Axen and Selverstone 1994; Chester and Chester, 1998; of overlying quartz diorite, while older, deeper layers were abandoned, folded, Marone, 1998; Reches and Dewers, 2005; Collettini et al., 2009; Axen et al., and faulted. Upward growth of the stack of layers probably reflects both strain 2015), shear sense (e.g., Petit, 1987), and fault-related fluid flow, fluid pressure, weakening of the overlying rocks and strain hardening of the layered cata- and alteration (e.g., Sibson et al., 1988; Sibson, 1989; Caine et al., 1996; Smith clasites. Each layer formed in two stages: (1) Lithologically heterogeneous et al., 2008; Haines and van der Pluijm, 2012; Selverstone et al., 2012). and/or cataclastically foliated, fault-bounded precursor slivers above the top The factors controlling whether fault slip accumulates seismically versus OLD G layer record grain-size reduction by cataclasis and cataclastic foliation devel- aseismically are not fully understood: temperature, friction properties, fluid opment and probably record inter-seismic shear. (2) One or more slip event(s), effects, and rock type(s) form a partial list. Exhumed fault zones allow direct, interpreted as seismogenic, transformed precursor slivers into layers of homo detailed, three-dimensional observations of natural fault-zone products devel- geneous, random-fabric cataclasite. Many layers are overprinted by weakly oped at depth and thus yield key data for comparison to conceptual models of OPEN ACCESS developed foliation, probably of postseismic origin. faulting, which are based largely upon results of laboratory experiments, theo The detachment fault core formed at ≥2.3–4 km depth, in the upper seis- retical considerations, indirect seismic observations, and/or modeling (e.g., mogenic zone. Pseudotachylyte along the detachment nearby also records Rice, 1992; Marone, 1998; Sibson, 1998; many others). multiple seismogenic WSDF slip events. Similarities between the cataclas- Mature faults in brittle upper-crustal rocks display a wide variety of rock tic layers and detachment fault-core rocks support the interpretation of both types and textures, including (1) random-fabric cataclastic rocks; (2) phyllosili cate-rich material, commonly foliated, either smeared along the fault from a

This paper is published under the terms of the *Present address: Department of Geology and Geophysics, Louisiana State University, Baton wall rock or grown in situ as alteration products; (3) minor to significant vein CC‑BY-NC license. Rouge, Louisiana 70803, USA material; (4) frictional melt products and other less common materials inter-

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preted to record paleoseismicity (e.g., amorphous silicates) (see Rowe and these active faults. At the study site, these plutonic rocks lack preexisting me- Griffith, 2015, and references therein). chanical anisotropy (e.g., Cretaceous mylonitic fabrics; Simpson, 1984), simpli- We describe two fault-rock assemblages and interpret them as records of fying mechanical studies, and are little altered; therefore, chemical processes paleoseismicity. Such assemblages probably are more common than the few probably did not dominate fault-rock genesis. Post-WSDF deformation was individual rock types known to form during earthquakes, but few are pres- relatively minor and localized near the younger dextral faults. ently 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 WEST SALTON DETACHMENT FAULT fault rock generally agreed to record paleoseismicity (Cowan, 1999, and references therein), but it is not particularly common. Others exist but are even The WSDF is a low-angle normal (detachment) fault that forms the bound- more rare and are difficult to identify in natural faults (Rowe and Griffith, 2015, ary between the Peninsular Ranges (footwall) and the Salton Trough (upper and references therein). plate) in southern California (Fig. 1; Axen and Fletcher, 1998), defining the We focus on an assemblage of uncommonly well-layered cataclasites northern segment of the Gulf of California rift system (Axen, 1995). It was ac- found a few meters below the West Salton detachment fault (WSDF), a late tive from ca. 5–8 Ma to ca. 1 Ma and accommodated ~10 km of slip (Axen and Neogene–Quaternary low-angle normal fault (Axen and Fletcher, 1998). Us- Fletcher, 1998; Lutz et al., 2006; Dorsey et al., 2007, 2012; Shirvell et al., 2009). ing textural analysis and structural arguments, we conclude that the layered The WSDF accommodated ~east-west extension within the southern San An- cataclasites formed, and were deformed, sequentially during several seismic dreas plate-boundary fault system (Axen and Fletcher, 1998; Axen et al., 1998). cycles. For several reasons, we infer that the two-part fault core at the top of The stratigraphic record shows that the dextral San Jacinto and Elsinore fault the WSDF footwall also probably records seismic activity. zones dismembered the WSDF ~1.1–1.2 m.y. ago and indicates ~100–200 k.y. of Low-angle normal faults exhume their footwalls directly, yielding abundant temporal overlap of slip on the WSDF and these dextral faults (Lutz et al., 2006; exposures of fault rocks that were processed in crustal levels ranging from the Janecke et al., 2010; Dorsey et al., 2012). brittle-plastic transition to the near surface (e.g., John, 1987; Axen and Selver- WSDF slip was mainly top-ESE (~110° azimuth), parallel to ESE-plunging stone, 1994; Axen et al., 2001; Cowan et al., 2003; Numelin et al., 2007; Smith corrugations of the fault surface (Fig. 1B; Axen and Fletcher, 1998; Kairouz, et al., 2008; many others). Many are young (Neogene or Quaternary) and thus 2005; Steely et al., 2009; Dorsey et al., 2012). The WSDF on the south flanks provide a fault-rock record that is not overprinted by deformation related to of the Yaqui Ridge and Whale Peak antiforms is cut by young faults of the later tectonic exhumation events or by chemical processes that may occur be- San Andreas system, which partly controlled late folding (Steely et al., 2009). fore or during slow erosional exhumation. In contrast, the north flanks are little disrupted but probably were steepened The top of the WSDF footwall displays a fault-rock assemblage commonly during folding. The Whale Peak arch may have been a primary corrugation found along mature faults in quartzofeldspathic rocks: one or more principal that was amplified by folding (Kairouz, 2005). This ~E-W extension and ~N-S slip surfaces within a relatively narrow inner fault core of ultracataclasite, sur- shortening reflect the expected strain field around the NW-striking dextral San rounded by a thicker outer fault core of cataclasite (Chester and Chester, 1998; Andreas fault system (Fig. 1A; e.g., Savage et al., 1986). Folding began during Luther et al., 2013). This two-part core, in turn, is underlain by a thicker frac- WSDF activity and continued after younger faults crosscut the WSDF and ac- tured damage zone, in which fracture density decreases away from the core tivity on the detachment ended (Dorsey et al., 2007, 2012; Steely et al., 2009). to a background value (Luther et al., 2013). We argue that WSDF slip exhumed Trends of WSDF striae range widely from W to SE, with maxima in the north- its footwall from the upper seismogenic zone and that its fault core formed at ern quadrant and toward the ESE (Stinson and Gastil, 1996; Axen et al., 1998; those depths. Pseudotachylyte is found in several places along the WSDF (Fig. Kairouz, 2005; Steely et al., 2009; Fig. 2A). This scatter is attributed to episodi- 1B; Axen et al., 1998; Prante et al., 2014), showing that it hosted earthquakes, cally alternating WSDF slip directions: top-ESE normal slip and ~N-S reverse(?) consistent with our inference that the WSDF fault core records paleoseismicity. slip related to broadly coeval ~N-S shortening (Axen and Fletcher, 1998; Kairouz, In contrast, fault rocks at the base of the upper plate in some exposures are 2005; Steely et al., 2009). Paleostress analyses at Yaqui Ridge (Fig. 1B) are con- breccia and clay gouge, formed from plutonic protoliths at depths ≤2–3 km. We sistent, requiring two alternating local stress fields: a normal-faulting field with interpret these rocks as formed above the seismogenic zone. ~E-W extension and a ~N-S thrusting stress field (Luther and Axen, 2013). The WSDF footwall is nearly ideal for studies of brittle fault rocks. The Rocks described here are in the WSDF footwall, which comprises mid- upper several meters of the footwall commonly are well exposed and were crustal crystalline basement of the Peninsular Ranges (Silver and Chappell, processed while passing through upper seismogenic-zone depths. Intermedi- 1988; Todd et al., 1988), mostly intermediate plutonic rocks of the Middle Cre- ate plutonic rocks dominate the footwall and hanging wall (Fig. 1C); therefore, taceous La Posta tonalite-granodiorite suite (Todd et al., 1988). The amphibo- WSDF fault rocks formed from protoliths that represent typical continental lite-grade Cretaceous Eastern Peninsular Ranges mylonite zone (Sharp, 1979; crust. The San Jacinto and Elsinore fault zones cut the same rock types; thus Simpson, 1984) overprints the footwall farther north but dies out southward, the WSDF footwall may provide analogs for fault rocks formed at depth along and metamorphic fabrics are absent at our study site. Pre–La Posta lithologies

N B A California SJFZ SA F Ranges

NW Pa Ocean Part ona PD cific B Ariz Part C Baja

E WP California F B r 100 km aw

33 N le Z Quaternaryon Sy cover e ei sm AC Neogene upper-plate strata Upper-plateic basement West Salton detachment fault Lower-plate basement Peninsular W

30 km 6 Other faults, mostly dextral 11 Antiforms & synforms 30 WSDF; arrow: dip & dip direction, Qa, Qc Quaternary alluvium & colluvium C Figure 1. (A) Location of part B in southern bar & ball: trend of striations California. (B) Generalized tectonic map of Qt1–3 Quaternary fluvial terrace deposits Other low-angle faults southern exposures of the West Salton detachment fault (WSDF). Pseudotachylyte is Brittle shear zone PQc Plio-Quaternary Canebrake Cgl. found at Nude Wash (NW), Powder Dump 37 Strike & dip: Hi-T solid-state or fc Fault-rocks, top of WSDF lower plate Wash (PD), and elsewhere around the tip magmatic foliation of Yaqui Ridge (YR). Hot springs issue from Kqd Cretaceous quartz diorite the WSDF at Agua Caliente County Park Strike & dip: Mylonitic foliation; Pre-plutonic rocks (AC). Other abbreviations are EF—Elsinore 33 arrow: trend of stretching lineation pKm fault; SJFZ—San Jacinto fault zone; WP— N W

olin Main exposures ash N Whale Peak. (C) Geologic map of the study Kqd a area and surroundings made at 1:12,000 363 595 000 of layered fork Qt1 scale, showing area of Figure 3. Kqd NW cataclasites Qt1 E fo 22 Qa 30 fork rk 30 Qt1 Qa SW Qc 28 29 Qc 26 fc Kqd PQc Qt2 28 ? PQc 40 fc Kqd Fig. 3 pKm Exposed 58 ? basal fc Qc Kqd ? upper 10 14 plate 57 32 pKm Qa Kqd fc pKm 32 pKm Qt3 13 5

pKm 63 6

3 fc

fc 00 0 35 44 N 37 33 26 366585 000000 0 77 0 0.25 0.5 km 27

Kt? 5 Topographic base from USGS 64

Kqd 00 0 Whale Peak 7.5′ Quadrangle 32 S Contour interval: 40 feet

A WSDF B Top Fold axis: 18, 322 C Bottom Fold axis: 22, 340 Contact Contact

Best-fit WSDF in study area: Figure 2. Lower-hemisphere, equal-area 296, 32NE stereonet plots of orientation data. See lower-right panel for symbols. (A) The West Salton detachment fault (WSDF) (great circles and poles): black (n = 24) Best-fit pole are from area of Figure 1C, and red (n = 5) in study area: are from area of Figure 3. Best-fit pole to 58, 206 those from Figure 3 is 58→206. (B) Top n = 29 n = 24 n = 8 contact (great circles and poles) defines Folds in Faults in axis of outcrop antiform as 18→322. Note D Cataclasite E F that most striae cluster around fold axis. Layers Fold axis: Layers Layers (C) Bottom contact (great circles and 11, 319 poles) defines axis of outcrop antiform as 22→340. (D) Cataclasite layers define axis of outcrop antiform as 11→319. (E) Folds in layered cataclasite. Great circles and filled squares are axial planes (n = 9) and their poles, respectively. Small filled circles are fold axes (n = 15). (F) Sharp faults (black; n = 19) and granular shears (red; n = 34) cutting cataclastic layers. (G) Late veins and cracks that cut layered cataclasites and rocks above top contact. (H) Struc- n = 12 tures above the top contact (TC) on the NE flank of the outcrop antiform. Top G Late Veins H NE Flank, Top Contact & I SW Flank Above Top contact (blue dashed line) has possible & Cracks S-C Contact slip line (blue filled circle) shown 90° from R1 intersection with best-fit C-plane. Cata- clastic S–C composite fabric: S-foliation TC TC (black, n = 9; striae, n = 2) and C-planes (red, n = 9; striae, n = 7). Note that striae S trend on C-planes is subparallel to slip C on WSDF. (I) Structures above the top contact on the SW flank of the outcrop antiform (mostly from in or near area of

Fig. 6). Primary Reidel (R1) shears (blue,

n = 8; measured R1 striae, n = 4). Best-fit 1R (dashed blue line) is 208, 48W. Top contact (red, n = 2, with measured striae trending n = 15 NW). Top-contact slip lines (large red-filled circles), shown 90° from best R plane, J Folded Pegmatites, SW K NE-Flank 1 SYMBOLS trend NNW-ESE. Other faults are black Flank Faults (n = 20). (J) Two pegmatite dikes above Planes top contact, showing drag folding as top contact is approached. Thicker pegmatite (in Fig. 6) shown black (n = 8; fold axis: Poles to Planes 46→291). Thinner pegmatite shown blue Best Fit (n = 13; fold axis: 42→287). (K) Faults that cut NE-flank top contact. Fold Axes or Striae Best Fit Axis or Calculated Slip n = 11 Line

include quartz-biotite schist, orthogneiss, and lesser amphibolite and marble The footwall ultracataclasite inner fault core typically contains or splits (Todd et al., 1988). Local Eocene Poway conglomerate deposits are noncon- along several nonreflective slip surfaces that are parallel to the WSDF itself. formable on the batholith (Abbott and Smith, 1989), showing that most exhu- These presumably served as principal slip surfaces at different times during mation from intrusive depths was pre-Eocene. (U-Th)/He thermochronometry WSDF evolution and commonly separate ultracataclasite layers of different of footwall rocks shows that the WSDF tectonically exhumed its footwall at colors. Many of these surfaces display weak to strong abrasive-wear striations. least 2.3–4 km, but greater depths were possible at the onset of WSDF slip The ultracataclasites typically break into rhombs defined by closely spaced (Shirvell et al., 2009). (0.5–2 cm) joints and the slip planes. Shervais and Kirkpatrick (2016) described The WSDF upper plate comprises plutonic rocks and lesser metamorphic a similar layered inner fault core along the La Quinta fault, which probably rocks, as in the footwall, nonconformably overlain by Neogene sedimen- was active as a strand of the WSDF. We define the WSDF as the slip surface tary strata (e.g., Winker and Kidwell, 1986) that mostly were deposited in at the top of the ultracataclasites and below upper-plate rocks because (1) the WSDF-controlled basins (Axen and Fletcher, 1998; Dorsey et al., 2007, 2012). character of the fault rocks changes there and (2) no evidence for incorporation Eocene(?) Poway-type fluvial strata are found locally in the upper plate (Abbott of upper-plate lithologies into the inner core has been found where Neogene and Smith, 1989; Kairouz, 2005). These Cenozoic strata show that the presently strata form the upper plate. exposed upper plate was shallower than 2–3 km during WSDF activity. (U-Th)/He thermochronology (Shirvell et al., 2009) of apatite indicates that Fault rocks derived from similar plutonic protoliths are different in the up- the footwall was exhumed by the WSDF at least 2.3–4 km (rapidly cooled from per and lower plates. The basal upper plate is generally covered but mainly is ~55–70 °C since ca. 12 Ma); zircon results show that the footwall resided at either deformed Cretaceous plutonic rocks or late Neogene sedimentary strata. greater depth prior to WSDF activity (~5–7 km based on slow zircon cooling Excellent basal plutonic exposures in the east fork of Nolina Wash (Fig. 1C) from ca. 51–31 Ma in its partial retention zone at ~150–190 °C). Thus, exposed contain several meters of fractured (mainly) and brecciated (near the WSDF), footwall fault rocks should record processes in the upper kilometers of the white quartz diorite that is punky, friable, and presumably heavily micro seismogenic zone, consistent with pseudotachylyte found along the fault fractured for ~10 m above the WSDF (makes a dull thud when hammered). (discussed more below). The general lack of clay minerals or clay gouge in Gray to greenish-gray clay gouge, consistent with generation at shallow depth the footwall (as found in the basal upper plate) suggests that the footwall and low temperature, is present as an irregular layer 0–25 cm thick immedi- transited the shallow non-seismogenic zone with little overprint. Grain-size ately above the WSDF, along upper-plate fractures, and as breccia matrix. We distributions of footwall fault-core samples suggest formation by constrained infer that these fault rocks formed above the upper limit of the seismogenic comminution overprinted by additional shearing (see Sammis et al., 1987; zone, consistent with clay gouges being velocity strengthening (see review by Sammis and King, 2007), consistent with confining pressure high enough (or Marone, 1998). Another good exposure of basal upper plate at Powder Dump pore pressure low enough) to prevent significant dilatancy during cataclasis wash (Fig. 1B) also contains highly fractured to punky, white plutonic rocks (Luther et al., 2013). but lacks macroscopically visible clay gouge, and contains pseudotachylyte Paleoseismic WSDF slip is recorded by pseudotachylyte around the NE tip injection veins that end downward at the WSDF. of Yaqui Ridge (Frost and Shafiqullah, 1989; Axen et al., 1998, Kairouz, 2005; In contrast, a two-part fault core is commonly well exposed at the top of Prante et al., 2014). Prante et al. (2014) recently studied the Yaqui Ridge pseudo- the WSDF footwall. In most places, the upper 2–20 m of the footwall comprise tachylytes in detail, showing that bodies up to ~1 m thick occur, both as fault a distinctive, moderately erosionally resistant, light-gray to light greenish-gray veins and injections into the upper and lower plates, that microscopic textures rounded cliff or steep slope made of fractured damage zone below ~1–2 m of typical of pseudotachylytes are present (see also Kairouz, 2005), and that X-ray outer fault-core cataclasites capped by 10–75 cm of inner fault-core ultracata- diffraction (XRD) patterns display a “glass hump.” Most of the tip of Yaqui clasite (e.g., Luther et al., 2013). Footwall outer-fault-core cataclastic and frac- Ridge is composed of foliated plutonic rock, but there are many small bodies tured damage-zone rocks erode mainly by granular disintegration rather than of pre-plutonic metasedimentary rocks along the WSDF (mainly quartz-biotite by fracture-bound block falls, and both make a dull thud when hit by a hammer. schist and marble; Steely et al., 2009). Prante et al. (2014) suggested that the Presumably these rocks are heavily microfractured, but alteration is minimal. metasedimentary protoliths, rich in hydrous minerals, may have favored melt- The contact between damage zone and outer fault core typically is gradational ing. A thick (~75 cm), fault-parallel pseudotachylyte body in Nude Wash (Fig. over 1–2 m, and the contact between outer and inner fault-core rocks also typi 1B) contains multiple thin (≤5 cm) pseudotachylyte layers, indicating repeated cally is gradational over ~0.5–2 cm and, though probably sheared, generally seismicity. This multi-layer pseudotachylyte body lies above metasediments, is not a sharp slip plane. Clay minerals are sparse to absent in footwall fault but the nearly ubiquitous inner-core ultracataclasites are absent there, suggest- rocks. This pseudo-stratigraphy permits detailed mapping of the WSDF (e.g., ing that the typical WSDF core did not form everywhere (i.e., where frictional Fig. 1C) and provides a rich resource of WSDF footwall fault rocks, of interest melting occurred and/or where plutonic protoliths are absent). In some places here because they began to form at significantly greater depths than exposed (e.g., Powder Dump Wash, Fig. 1B), off-fault pseudotachylyte veins in the basal upper-plate fault rocks. upper plate end at the WSDF, where they were presumably generated.

Samples of pseudotachylyte from Yaqui Ridge yield Miocene K/Ar and that produced products such as cataclastic injections, cataclasite layers, forma- 40Ar/39Ar total gas ages (Frost and Shafiqullah, 1989; Hazelton, 2003; Kairouz tion of low-temperature foliations, and destruction (by cataclastic randomiza- et al., 2003; Kairouz, 2005). The 40Ar/39Ar total gas ages are interpreted as max- tion) of those foliations. We then consider the WSDF fault core and suggest that imum ages due to partial retention of mid-Cretaceous radiogenic Ar in mineral this common fault-rock assemblage also probably records seismogenic slip. fragments and/or excess Ar contamination, but the ages of youngest individual heating steps are consistent with the late Neogene WSDF age known inde- pendently (Hazelton, 2003). METHODS Seismic slip occurs on normal-sense, low-angle detachment faults; so the WSDF is not unique in this characteristic. The contemporaneous Cañada David Our analysis of layered cataclasites is based mainly upon field observations. detachment fault in northern Baja California (Siem and Gastil, 1994; Axen and Local context for the study site comes from detailed geologic maps at 1:12,000 Fletcher, 1998; Axen et al., 2000) defines the next rift segment to the south scale on enlarged U.S. Geological Survey (USGS) topographic maps (Fig. 1C) (Axen, 1995). It also was seismogenic, as shown by Quaternary fault scarps and co-registered orthophotoquads or topographic contour maps (2 m interval) (Axen et al., 1998, 1999; Fletcher and Spelz, 2009). Wernicke (1995) and Axen at 1:1000 scale made from high-resolution (~5 m spacing) NextMap™ digital (2004) summarized evidence for seismogenic slip on low-angle normal faults. elevation data (Fig. 3). Very large-scale “outcrop maps” drawn on photographs Axen (1999) documented examples of triggered earthquakes on such faults, of key exposures show details of the cataclastic layering, top and basal con- and Prante et al. (2014) listed nine low-angle normal faults worldwide that are tacts, overlying deformed plutonic rocks, and structures and crosscutting re- ornamented with pseudotachylyte. Smith et al. (2008) interpreted fault rocks lationships. and veins along the Zuccale fault (Italy) as recording seismogenic slip. Thus, Additional preliminary results are presented, from (mostly) oriented sam- seismic slip on detachments is fairly common. ples of the cataclasites and their plutonic protoliths. Thin sections were ex- We describe in detail unique layered cataclastic rocks from the WSDF foot- amined optically and with backscattered electron (BSE) images. BSE imaging wall in the west fork of Nolina Wash (Figs. 1B and 1C) and interpret them as re- and sparse mineral spot analyses were done on the electron microprobe in cording multiple seismic cycles, due to the repeating, cyclic nature of processes the New Mexico Bureau of Geology and Mineral Resources. Two-dimensional

31 quartz diorite Upper Plat B 32 B A e 840 23 ) Transpor footwall footwall spla WSDF splay t 830 y ? 30 Layered 820 49 34 elevation (m Top Layers Figure 3. Detailed geologic map (made Cataclasite 0 at 1:1000 scale) of the layered cataclasite 51 810 81 in Antiform no vertical exaggeration Disrupted Layers locality (area shown in Fig. 1C), showing 21 15 48 location of cross section (inset). Upper plate of West Salton detachment fault upper-plate (WSDF; heavy line with double tick marks) is pink, and lower plate is blue. Footwall quartz diorite 820 splay faults and top and bottom contacts A of the layered cataclasite body are shown ? 830 WSDF 38 by medium-weight black lines (with single Covered ticks where gently to moderately dipping; 19 N brittlely foliated arrows show dip lines). Layered cataclasite quartz diorite 18 19 body is magenta, with outcrop antiform 36 shown by yellow line. Curved line pattern ? shows cataclastically foliated rocks above ? top contact. 80

quartz diorite quartz diorite 025 50 m

850 contour interval: 2 m 840

grain-size distributions were derived from photomicrographs and BSE images differences exist between the structures and structural styles on the NE (atten- following methods of Keulen et al. (2007), as described in Luther (2012) and uated) and SW limbs of the outcrop antiform. The SW limb (Fig. 6) is of particu- Luther et al. (2013). Backscattered electron images of freshly broken surfaces lar interest because lack of overprinting on it preserved structural relationships were obtained on the scanning electron microscope (SEM) in the Institute of that record formation processes of the layered cataclasites. Meteoritics at University of New Mexico; energy-dispersive X-ray spectroscopy (EDS) allowed for mineral identification. WSDF at the Study Site

STRUCTURAL SETTING OF LAYERED CATACLASITES The WSDF is a sharp slip surface at the top of ~10–30 cm of brown ultra cataclasite that splits into sub-layers ~1–3 cm thick along sharp, planar slip sur- The study site lies in the uppermost WSDF footwall, between two western faces parallel to the WSDF. Sparse striae on these surfaces and the WSDF trend branches of Nolina Wash (Fig. 1C). In that area, the WSDF and its adjacent foot- ESE (Figs. 1C and red lines in 2A). The ultracataclasites are underlain, across wall rocks are well exposed in a dip-slope outcrop pattern on the north flank of an abrupt transition ~0.5–2 cm thick, by ~1 m of cataclasite. This sequence is the Whale Peak arch (Fig. 1B). At the study site, the WSDF dips ~30° north and like the typical WSDF core elsewhere; therefore, we infer that the mechanical ESE-WNW–trending striae record dextral-normal oblique slip (Figs. 1C and 2A). evolution of the WSDF at the study site was similar to that of most other parts The layered cataclasites crop out in two zones, separated by a ridge where of the fault. The WSDF is not folded by the outcrop antiform. they are covered by structurally overlying rocks (Fig. 3). The best exposures are in the northwestern outcrop band. There, the layered cataclasites, their up- Quartz Diorite below the WSDF and above the Top Contact per and lower bounding faults (the “top” and “bottom contacts”, respectively), and surrounding rocks are folded coaxially into an open, gently NW-plunging This unit thickens south across the outcrop antiform because the WSDF is “outcrop antiform” with a NE limb formed and attenuated by WSDF drag (Figs. not folded, but the top contact is. Map-scale structures within this unit (Figs. 2B–2D, 3, and 4). A gully follows the hinge and provides good exposures of 3 and 4) include a low-angle footwall splay (thin black line with tick marks left both limbs (Fig. 5). Outcrop-scale observations below were made there. of gully in Fig. 5) from the WSDF that cuts, in its footwall, a NW-down normal- Next, we describe in detail the structural units and fault contacts that sepa separation fault that, in turn, truncates the NW end of the outcrop antiform rate them, beginning at the WSDF and descending structurally. Significant and the layered cataclasites. The footwall splay resembles the WSDF, being

R1 shears Figure 4. Schematic block diagram, viewed pegmatit approximately to W, showing the layered top footwall contact splay cataclasite body and its relationship to t features such as the outcrop antiform, faults below the West Salton detachment s top contac fault (WSDF), primary Reidel (R1) shears, top layersted layer and the folded pegmatite dikes. Top layers m disrupted layers are shown red, and distorted layers are top contac disrupbotto shown pink. Small faults that cut the bottom contact and, probably, the top contact in the subsurface on the NE flank are not contac t WSDF shown. t contacts bottom top and faulted as at surface probably ~5 m

A B

Layere

d Layered to Figure 5. (A) Oblique photo looking up p Area of B gully that branches south from the northwest fork of Nolina Wash, where best ex- Cataclasitescontact Area of Fig. 6 posures of layered cataclasites exist. Top contact is visible on right side of the gully. Cataclasit (B) Closer view of the gully, the layered Fractured cataclasites and deformed rocks above ? them, with area of Figure 6 shown. ? Upper es Plate

WSDF

Northwest fork, Nolina Wash

underlain by 2–8 cm of brownish to olive-green ultracataclasite that grades whole) biotite grains, microcracks, and quartz with weak undulatory extinction down across ~1 cm to cataclastic quartz diorite. This similarity suggests that (Fig. 7A). Plagioclase in plane-polarized light locally shows very fine-grained the splay evolved similarly to the WSDF. The splay cannot be traced far into the hazy sericitic(?) alteration. Some plagioclase grains show micro-porosity in al- footwall and ultracataclasite along the splay is thinner than that along the adja ternate albite twin lamellae, as seen in SEM images. In thin section, these form cent (or typical) WSDF. We conclude that the splay probably has less slip than plucked, planar zones contaminated with opaque polishing compound (Fig. the WSDF (see Evans, 1990, for cautionary use of thickness-displacement rela- 7A). These textures, except undulatory extinction, are absent in fresh plutonic tionships). The splay probably is not folded by the outcrop anticline, although rocks farther from the WSDF. it dips slightly more gently south of the antiform than to the north (Fig. 3). The The microfractured quartz diorite on the SW flank grades down into in- simplest timing interpretation of these relationships is that the layered cata creasingly brecciated and cataclastic rocks (up to ~4 m above the top contact) clasites and the outcrop anticline formed before the fault that terminates them, that have random fabrics and reduced matrix grain size. The cataclasites are which is older than the WSDF-like splay. poorly sorted, with clasts ranging up to a few centimeters in diameter. Many Rocks on the SW flank of the outcrop antiform, and immediately below the individual clasts are fractured internally and composed of subclasts, either still WSDF footwall splay, are cataclasites that grade down into weakly deformed in place relative to one another or slightly offset or rotated relative to one an- (microfractured?) quartz diorite that looks intact macroscopically but makes other (crackle and jigsaw textures, respectively). Biotite typically is dismem- a dull thud when hammered and is locally so weak that it can be excavated bered into thinner, smaller cleavage flakes than in the protolith and is com- with fingernails. In thin section, this “punky” quartz diorite shows kinked (but monly kinked.

A top fractured fbx cont peg act top ? GSZs layers

? catclastic ? ? disrupted la ? r injection ? granular covered shea

complexly ye boulders in shear bottom rs foreground granular deformed meters contact 0 1 2 covered fractured B

Figure 6. (A) Outcrop map (view to SW) of representative part of the SW flank of the outcrop antiform. The top layers are subparallel to the top contact and overlie the disrupted layers along a narrow zone of shear. Disrupted layers are isoclinally and recumbently folded and are cut by both sharp faults (blue lines, with ticks where gently dipping) and granular shear zones (GSZs; outlined by blue dashed lines). A pegmatite dike (peg, upper right) is folded and attenuated by shear above the top contact. Red lines show traces of fold axial surfaces. Thin white lines on left show some of the late fractures and calcite-gypsum veins. Scale bar is only approximate due to photo distortion. (B) Same photo as (A), without lines.

A B WNW ESE C SE NW

1 mm 1 mm

D SE NW E

1 mm 1 mm 1 cm

F G SE NW H SE NW I plag

1 mm 1 mm 1 mm 50 µm

Figure 7. Photomicrographs (plane-polarized light) and scanning electron microscope (SEM) image of textures. (A) Sample 35-78-1 (4× magnification): Microcracked and punky quartz diorite proto lith showing slightly kinked biotite grains (right), fractured quartz (top center), and plagioclase (with partially plucked albite twins contaminated with opaque polishing compound). (B) Sample 35-93-1 (4× magnification): Cataclastic foliation in quartz diorite above top contact on NE flank of outcrop anticline, showing kinked and dismembered poorly aligned biotite grains and one grain of cataclasite (lower right). (C) Scanned thin section containing top contact (arrows) on SW flank of outcrop anticline. Note aligned biotite grains both above and below. (D) Sample 35-91-1 (4× magnification): Cataclastic foliation ~3 cm above top contact on SW flank of outcrop anticline. Note kinked biotite with calcite growing in open cleavage planes. (E) Sample 35-68-1B (4× magnification): Millimeter-scale cataclastic layers from ~4 cm below top contact on SW flank of outcrop anticline. Note that largest grains appear well sorted in each layer. Fine-grained layer in center has well-aligned biotite grains and probably experienced more shear strain and comminution than layers above and below. (F) Sample 35-87-2 (4× magnification): Clast of ultracataclasite in layered cataclasite is indented by stronger mineral grains. (G) Sample 35-86-2 (4× magnification): Layered cataclasite sample from uppermost disrupted layers, immediately below contact, on SW flank of outcrop antiform. Note two very fine-grained layers that define a central layer with weak grain-shape preferred orientation. (H) Sample 35-106-2 (4× magnification): Weak S-foliation in layered cataclasite is defined by aligned biotite grains at ~45° to layer boundaries (which are parallel to top and bottom of frame). From SW flank of outcrop antiform. (I) SEM “topographic” backscattered electron (BSE) image of broken surface of layered cataclasite, showing micropores (arrows) in plagioclase (plag) and euhedral zeolite crystals (z) in large central pore. These zeolites probably are laumontite based on similar crystal habit to grains with energy-dispersive spectroscopy spot analyses (from other samples).

A penetrative foliation affects the bottom ~5–50 cm of the quartz diorite tures, and we interpret them as S-foliations that record mainly flattening at above the top contact on the SW flank (Figs. 8 and 9B), suggesting significant the grain scale.

shear strain related to the contact. The foliation is weak at the top of the zone, Primary Reidel shears (R1) are common in the SW-flank foliated zone, and where it overprints random-fabric cataclasites. Foliation intensity increases some extend above it into the random-fabric cataclasites (Fig. 8). These dip downward to the top contact, the maximum grain size decreases to ~1 mm, moderately SW (Fig. 2I) and cut cataclastic quartz diorite, truncate outsized and foliation planes rotate into near-parallelism with the top contact (Figs. 7C, clasts, and cut the S-foliation, which locally is developed more strongly adja

8, and 9B). Thus, shear strain increases down toward the top contact. cent to R1 shears and shows normal drag adjacent to them (e.g., above the

Microscopically (Figs. 7C and 7D), foliated quartz diorite on the SW flank R1 shear above the pen in Fig. 8A). Downward, some R1 shears merge with is composed of angular grains down to micron sizes, with foliation defined or terminate against the top contact, but others bend into parallelism with mainly by aligned biotite flakes and, to a lesser extent, other grains (most of the S-foliation centimeters above the top contact (Fig. 8), so the S-foliation

which are subequant). Biotite grains appear largely unaltered but commonly locally was reutilized in shear. These relationships indicate that the R1 shears

are kinked (especially those not parallel to the foliation) or deformed between formed in concert with slip on the top contact. These R1 shears are not inter- and/or wrapped around harder grains (Fig. 7D). Biotite cleavage planes com- preted as C′ shears (which are mechanically similar but develop with S–C–C′ monly are open and filled with either fine-grained cataclasite or with micro- composite fabrics) because (1) C shears generally are absent on the SW flank

scopically unstrained calcite (probably the same age as late calcite veins and (2) many R1 shears extend upward into unfoliated breccias. Secondary described below). Fabrics representative of crystal plasticity are absent, with Reidel shears are rare. The quartz diorite above the top contact on the SW flank the exception of weak undulatory extinction in quartz, which likely predates (whether punky, cataclastic, or foliated) also is cut by many small faults with WSDF slip. Thus, these foliations also are low-temperature cataclastic tex- fairly random orientations (Fig. 2I).

f A f f

f f f Figure 8. (A) Annotated close-up view (to SW) of rocks above the top contact on the SW flank of the outcrop antiform, showing primary Reidel shears (blue lines dipping right) that flatten and/or disappear into cataclastic foliation (wavy black lines) above the top contact (blue line top with tick marks). Ultracataclasite lenses layeredgranular contact cataclasite shown by pink overlay; macroscopically poorly sorted quartz diorite breccia shown B by small polygons; quartz diorite clasts shown with cross-dash pattern; fractured quartz diorite bodies labeled “f”; bodies of granular cataclasite shown by closed circles or black arrows (where along other- wise sharp faults). Pen for scale. (B) Same as (A), but without annotation. Box shows location of Figure 9B.

Fig. 9B

A B

R1 bx

R1 Figure 9. Cataclastic foliation types. Coins are 2 cm in diameter. Red-filled arrows point to ultracataclasite lenses; white C′ arrows show width of granular to foliated fbx shear zones; black arrows show sharp bx shear planes; thin black lines show traces of foliation. (A) Composite S–C–C′ fabric above NE-flank top contact. Many outsized C′ C clasts remain, and maximum grain size commonly is similar to protolith quartz C′ fbx diorite. (B) Foliated (fbx) and brecciated (bx) quartz diorite above SW-flank top contact. Some outsized clasts remain, C bx C′ but overall grain size is more strongly re- bx fbx duced and more even than on NE flank (A). bx Arrows show basal part of a primary Reidel bx uc uc shear that is ornamented by ultracata fbx clasite (red arrows), approaches parallel- contact bx ism with top contact, and changes laterally top from a foliated shear zone of finite width (white arrows) to a sharp plane (black C D arrows). Base of upper plate is a thin, fault- bounded, heterolithologic unit of breccia, foliated breccia and ultracataclasite (uc) above the top contact. Location shown in Figure 8B. (C) Weakly developed S-foliation in a medium-grained cataclastic layer, at ~45° to top and bottom boundaries of the layer. The boundaries are thinner, fine- grained recessive cataclasite layers, which probably equate to C-planes. At top of photo, thin layers (one is also foliated) are truncated downward against the upper, bounding, recessive C-plane. (D) Top is a coarse-grained cataclastic layer that contains thinner layers of finer-grained cata clasite within it, as well as a weak foliation similar to that in (C). A thin layer of ultracataclasite on the left is truncated at both ends, and the lower coarse-grained layer contains flattened clasts of ultracataclasite (lower right).

coin size

Two pegmatite dikes cut the quartz diorite above the SW-flank top contact but is cut by several small faults on the NE flank, and the shear sense is in- (Figs. 5B and 6). More than ~1 m above the top contact, these dikes are roughly ferred to differ on the two flanks. planar, dip steeply, and strike ~E-W (Fig. 2J). As the top contact is approached, The top contact on the SW flank of the outcrop antiform (Fig. 6) is cut by

they are dramatically thinned, cataclasized and locally foliated, cut by R1 only a few fractures with centimeters of offset. Striations on the SW-flank top shears, and drag-folded into parallelism with the top contact. Decimeter-scale contact formed by abrasive wear and trend NNW, subparallel to the fold hinge cataclastic injection veins (discussed more below) fill mode-1 cracks subpar- defined by the top contact (Fig. 2B). Dextral, top-NNW shear sense on the SW-

allel to the axial surface of the folds (Figs. 6A, right side) but are not deformed flank top contact is inferred from R1 orientations above it, by assuming that

and thus postdate most or all of the folding. the slip line is 90° from the intersection with the best-fit 1R plane (Fig. 2I). This Together, the Reidel shears, foliation, and drag-folded pegmatites record direction is similar to striae orientations (Figs. 2B and 2I). This shear sense is generally dextral, top-NNW, downward-increasing shear above the SW-flank consistent with drag folding and attenuation of two pegmatite dikes above top contact. In the section on the top contact (below), we use the Reidel orien- the SW-flank top contact (Figs. 2J and 6). The top contact on the SW flank tations to infer shear sense on that contact. is locally ornamented with ultracataclasite, and injections of cataclasite and Structures on the attenuated NE flank, in the quartz diorite below the WSDF ultracataclasite into overlying rocks (discussed more below) terminate down and above the top contact, are different from on the SW flank. “Intact” (but at the top contact. punky and microcracked) quartz diorite is mostly absent. Poorly sorted brec- The top contact on the NE flank is cut by several moderately NE-dipping, cias and cataclasites grade down into foliated cataclasite present in a zone normal-separation faults (Figs. 2K and 10) with decimeters of offset. Striations ~0.3–2 m thick above the top contact. were not found on the NE-side top contact. Slip sense and direction are inferred Foliation intensity generally increases downward toward the top contact, to be top-ESE, parallel to the shear recorded by immediately overlying S–C again suggesting that development was related to shear on the top contact. fabrics (Fig. 2H and discussion above) and on the WSDF itself. However, S–C NE-flank foliated rocks are coarser grained and more poorly sorted than on development may have been synchronous with or later than the small faults the SW flank, unfoliated clasts remain, and a composite S–C–C′ fabric exists: a that crosscut the NE-flank top contact but do not cut the overlying, well-devel- penetrative S-foliation (flattening plane) occurs in lithons bounded by C and/or oped S–C foliation (Fig. 10). Thus, these small faults may be entirely older than C′ shear planes (Figs. 9A and 10). C-planes are spaced 5–25 cm apart, and striae the foliation, in which case assignment of this shear sense to the NE-flank top on the C-planes generally trend ESE; their relation to S-foliations indicates top- contact is questionable. Alternatively, they may have functioned as C′ planes ESE shear sense during foliation development (Fig. 2H), consistent with slip during S–C development, as they appear in Figure 8, but their orientations are sense and direction on the WSDF but different from shear sense on the SW not what one would expect of C′ planes (compare Figs. 2H and 2K). flank. A few S-planes display striations that record shear on S (which is not The change of top-contact shear sense and direction across the outcrop expected), but these are oriented differently than most S-planes, which cluster antiform suggests that the two limbs of the outcrop anticline sheared inde- well (Fig. 2H). pendently of one another. The SW-flank striae and slip direction (inferred from

Microscopically, foliated quartz diorite from the NE flank of the antiform R1 shears) are subparallel to the antiformal axis; so unfolding (rotating the SW is composed mostly of angular clasts and contains clasts of cataclasite (Fig. flank into parallelism with the NE flank) does not align them with the shear 7B) and thus is interpreted as formed by low-T cataclastic processes. Biotites direction inferred on the NE-flank top contact. Even if unfolding would align generally are aligned, with some showing fish-like shapes; misaligned biotites the shear directions, the shear senses differ on the two flanks (Fig. 3, cross are commonly kinked (Fig. 7B). NE-flank foliated cataclasites commonly have section). 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 Layered Cataclasites 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 The layered cataclasite body is thicker on the south limb of the outcrop formed along C-planes and been injected into foliated lithons. A thin (deci- antiform than on the north. Layered cataclasites on the south limb are divided meters) random-fabric breccia zone separates these foliated rocks from brec- into structurally higher, weakly deformed “top layers” and structurally lower, ciated diorite above. 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 The Top Contact limb, where disrupted layers directly underlie the top contact. The bottom contact (see below) also is a sharp fault (Figs. 3 and 6). The top contact of the layered cataclasites is a sharp fault (Figs. 8, 9B, and Cataclastic layers are defined mainly by grain-size changes from layer to 10) that is folded by the outcrop antiform, dipping north on the NE flank and layer: thin (1–5 mm), fine-grained, recessive layers separate thicker (2–25 cm), WSW on the SW flank (Figs. 2B and 3). It is nearly unfaulted on the SW flank more resistant, coarser-grained layers (Fig. 6). Locally, ultracataclasite is

A scaly

foliation ? ?

ufc Area of part C ufc

uc scaly foliation

fbx fbx Figure 10. (A) Outcrop map of representative part of the NE flank of the outcrop a antiform (view to NE). S–C composite fabric (inset) affects rocks above the top a a S contact (blue lines with tick marks). Traces of S-foliation, which is variably developed and wraps around outsized, unfoliated p C to act quartz diorite clasts (ufc), are shown by ? cont short black lines; traces of C-planes are Other faults shown by blue lines. See text for discus- disrupted layered cataclasite or C′ - R1? sion of small faults cutting the top contact. Ultracataclasite (uc, pink overlay color) occurs in irregular bodies that com- B C monly terminate against C-planes, sug- uc gesting they may have been generation surfaces. A dismembered and attenuated aplite dike (a) is shown by white overlay; fbx—foliated breccia. (B) Same photo as (A), without lines. (C) Detail of S–C foliation (area shown in A), showing ultracata clasite lenses parallel to, and presumably injected into, S-foliation.

2 cm uc

05centimeters 0

present along the recessive, fine-grained layers. Grain size of cataclastic layers >500 cm (Fig. 11). Because the 3D shapes of layers are not known, and because is described macroscopically with sandstone nomenclature, using the average some layers are truncated by younger faults, these lengths and thicknesses largest grains visible to the naked eye or in hand lens. At this observation scale, probably are minimal. Layer thickness and length are only weakly correlated, individual cataclasite layers appear moderately well to well sorted and clast even if fine-grained recessive layers and obvious outliers are excluded. Layer supported: largest grains in a given layer have a restricted grain-size range and length decreases downward, but lower layers are more disrupted (see below), cover a large percentage of any surface. Maximum grain size in coarse layers so this probably is not a primary difference. (excluding scattered larger clasts) is typically ~2 mm. The “average largest grain Microscopically, the layered cataclasites are very poorly sorted, with clasts size” is not well correlated with layer thickness or length (Fig. 11). However, that range in size continuously down to microns, although parts of some ap- layer thickness and grain size are positively correlated to some degree, because pear clast supported even in thin section (Fig. 7E). This appearance contrasts thin, fine-grained, recessive layers separate thicker, coarser, resistant layers. with the results of grain-size distribution analysis discussed below. Clasts are Cataclastic layer dimensions (Fig. 11) were measured mostly in the little- angular to subangular and mostly composed of individual mineral grains in the disrupted top layers. Macroscopically, maximum layer thickness ranges from general proportions found in the protolith quartz diorite. Larger-than-average ~1–2 mm to ~25 cm; some layers taper rapidly near their ends, and others quartz diorite clasts occur. Sparse clasts of older, previously lithified, cataclasite taper gradually along their lengths. Measured layer lengths range from ~30 to 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 600 (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 500 subparallel to the layer, suggesting high shear strain. These microscopic observations are consistent with the conclusion, drawn below from macroscopic Grain Size observations, that grain size generally is related inversely to net shear strain. Resistant Layers Preliminary scanning electron microscope (SEM) imaging of freshly bro- 400 Coarse Coarse - Medium ken surfaces shows open inter-grain pores and some plagioclase grains with Medium micropores (Fig. 7I). Euhedral, prismatic to tabular and micro-fibrous crystals Medium - Fine were found in pores (Fig. 7I) and in open cleavage planes in biotite (like un-

(cm) Fine strained calcite seen in thin section). Energy dispersive X-ray spectroscopy 300 Very Fine (EDS) spot analyses indicate these crystals are calcic zeolites (probably huelan- Recessive

Length Layers dite, laumontite, and chabazite). These zeolites formed late, after deformation. We suspect that ongoing work also will show broken, syncataclasis zeolites. Mixed Slice, Base 200 Grain-size distributions (GSDs) were determined for three samples of of Upper Plate 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 100 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 (de- 0 scribed below). 0510 15 20 25 Most cataclasite layers are deformed at scales smaller than the outcrop Maximum Thickness (cm) antiform. The intensity of this deformation increases structurally downward, Figure 11. Plot of cataclasite layer length versus thickness, mostly showing that relative age of layers increases downward. This is particularly from top layers. Symbol colors indicate macroscopic grain sizes apparent on the SW flank. of cataclastic layers, except gray swath on left that encompasses Top layers on the SW flank (Fig. 6) form a stack ~15–40 cm thick. These most recessive, very fine-grained layers that bound coarser-grained layers. Red diamond shows one heterogeneous, fault-bounded layers generally are little deformed and mostly are subparallel to the top con- sliver immediately above top contact. tact. The top layers are more strongly deformed in the northwestern several

A trated 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). Figure 12. (A) Grain-size distribution Distributed granular shear probably is more common in coarser-grained (GSD) plot (in mm) of one layered cata layers because fracture propagation is more easily stalled by large grains clasite (with 2D fractal dimension of 2.14) and representative 2D images used to that “block” a propagating fracture tip and/or because larger grains provide obtained GSDs (B and C). (B) Photomi- greater leverage on weak inter-grain bonds, favoring grain rotation and dis- crograph (plane light) showing isolated tributed granular shear over localization. These faults and granular shear fractured quartz grains (Q) and one ultra zones commonly form sets of rotated blocks with bookshelf-style geometries. cataclasite grain (uc). (C) Electron micro probe backscattered electron (BSE) image, Many merge and “flatten” into recessive fine-grained layers above or below showing angular, isolated grain of micro- (Fig. 13D), which separate the bookshelf arrays from surrounding longer, un- porous plagioclase (P), quartz (Q), and faulted layers. Original layer thickness is commonly reduced in zones with opaque grains (white; probably ilmenite). Two elongate quartz grains are cracked many granular shears (Figs. 13D and 13E), indicating a complex strain field of but barely offset (arrows), and the large B C both discrete and distributed shear and suggesting that cohesion in primary quartz grain at the bottom (labeled Q) layers was low. may have a sheared fragment above and P to the right. Q Much evidence indicates that layer-parallel shear was concentrated in the uc finer, recessive layers. As noted above, numerous faults and granular shears that cut across some layers merge above or below into fine-grained layers. Q Ultracataclasite found locally along these recessive fine-grained layers also 1.5 mm 50 µm Q 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 meters of the outcrop, approaching the fault that truncates the layered body, cut. Thus maximum grain size of layers is, at least crudely, inversely propor- but this may be purely coincidental. This NW increase in disruption is mainly tional to net layer-parallel shear strain. These relationships also suggest that by small faults and granular shears that crosscut some layers and then merge finer-grained layers are weaker than coarser-grained ones, favoring re-slip and with layer-parallel shears, with the top contact or with the contact at the base localization of slip, in accord with the cores of many large faults being made of the top layers. Some centimeter-scale folds also appear in this zone. Farther of ultracataclasite. southeast, the top layers are little disrupted internally. Many cataclasite layers display a weak but macroscopically visible, folia- The contact between top and disrupted layers is marked by an abrupt tion at an acute angle (commonly ~45°) to the layer boundary. This foliation downward increase of deformation intensity across a thin zone (0.5 to ~10 cm). is defined mainly by aligned biotite fragments and by other (less common) This zone is generally finer grained than cataclasites above or below (partic- elongate grains (Figs. 7H and 9C). We interpret this as a low-strain flattening ularly where it is thinner), and many layers, especially in the disrupted layers foliation (S) that developed in concert with shear strain along fine-grained, re- below, are truncated at this zone; thus we infer that the contact below the top cessive (C) layers, forming a composite S–C fabric. In the top layers, the shear layers is a shear zone. sense inferred qualitatively (top-NW) is consistent with that obtained for the The disrupted layers, which form most of the outcrop, are intensely de- top contact and rocks above it, but orientation of this weak S-foliation is very formed by several meter-scale tight to isoclinal folds (Figs. 6, 13A, and 13B), difficult to measure accurately. In the disrupted layers, the foliation orientation by fewer, generally smaller, open folds, by sharp faults, and by granular shear (where present) is scattered, consistent with disruption by folds and faults be- zones of width from 2 to 3 mm (Fig. 13) to ~20 cm (Fig. 6). Deformation inten- ing younger than these S–C fabrics. sity in the disrupted layers generally increases downward; near the base of the A second foliation, developed locally in cataclastic layers, is defined by layered cataclasites, it can be difficult to define the original layering. thin (generally <3 mm) sub-layers of differing grain sizes that are oblique to Tight to isoclinal folds mostly have gently inclined axial surfaces and axes the layer that contains them (Figs. 7G and 9D). These sub-layers commonly that trend E-W to NW-SE (Fig. 2E). These record significant N-S to NE-SW contain subparallel aligned biotite flakes. This foliation appears to have shortening within the disrupted layers and some vertical thickening. formed by dismemberment and rotation of older layers by small normal- Most faults and granular shear zones that cut the disrupted layers (Figs. 6 separation faults, which commonly flatten into shear zones above and be- and 13C–13E) have normal separation and, in contrast to the tight folds de- low, allowing these oblique sub-layers to become incorporated into younger, scribed above, thin the disrupted layers. Strikes are scattered but concen- thicker layers.

Figure 13. Examples of layer disruption A Area of B B cs by folding and faulting; views to ~SW late on SW flank of outcrop antiform. Traces of fold axial surfaces shown by red lines; sharp faults shown by blue lines; layer contacts shown by thin black lines. Knife fracture for scale in (A) to (C) is ~8 cm long. (A) Re- cumbently folded layers above layers that are highly disrupted by small faults and granular shear zones. Contact between cs the two domains, shown by blue arrows, is abrupt but irregular suggesting it also cs is disrupted. (B) Close-up view of fold cs hinge (area shown in A). Remnants of a coarse-grained, dismembered, layer (cs) are outlined. Dismemberment presumably preceded folding. (C) Steeply dipping lay- C a ers cut by sharp faults and granular shears (area shown in A). A distinctive but dis- a continuous, medium-grained layer, apparently formed mainly from aplite (a) proto- a a lith, is outlined. (D) Rotated domino-style blocks bounded by normal-separation faults and granular shears. These faults merge, both upward and downward, with fine-grained recessive shear zones marked by pen (white), mechanical pencil (red), and by blue arrows. This style of disrup- Area of C tion is commonly accompanied by abrupt lateral changes in layer thickness (shown by different lengths of red lines drawn D E perpendicular to a correlative, coarse- grained layer) and by complete attenuation of some layers (red arrows). Black arrows point to zones of distributed granular shear and layer attenuation. (E) Faults usually are sharp in fine-grained layers or where fine-grained layers are juxtaposed against coarse-grained layers but com- Area of E monly turn into granular shear zones of finite width in coarse-grained layers, as shown by sharp plane above and right of coin (~1.4 cm diameter). Traced down into the coarse layer, that sharp plane turns into a granular shear of finite width (between blue arrows) and attenuates the coarse layer. Red arrows as in (D).

Bottom Contact Cataclasite and Ultracataclasite Injections

The bottom contact is a sharp fault that puts disrupted layers above de- Injections of cataclasite, generally millimeters in width and up to a few centi formed quartz diorite. It is exposed only over several meters of outcrop length. meters in length, locally cut the layered cataclasites. Injections also are found Some of the small faults and granular shear zones in the disrupted layers cut along and above the top contact. Figure 14 shows decimeter-scale injections of the bottom contact, which also is cut by larger-displacement (decimeters), cataclasite that acutely crosscut foliated and fractured rocks above the top con- NE-dipping, normal-separation faults and by faults that strike subparallel to tact, thin upward, and are truncated at their bases by the top contact, suggest- the fold hinge. These relations suggest that the bottom contact is older than ing they originated along the top contact. They fill mode-I fractures that could the top contact on the SW side of the antiform. not have been generation surfaces. The largest is graded: finer grained at the

A fractured, brecciated and foliated 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. Granular injections ~parallel to fractures Other injections above the top contact are irregular in shape. One injection fbx and foliation 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 top that they originated below it, thus, this injection was one of the last events top la Area of along the top contact. The older layering is preserved in this injection (unlike ye rs part B contact the others that were fluidized granular injections), suggesting that the layers had developed some cohesiveness but were still ductile during injection. The B 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 be- headed by the upper fault. Injections of ultracataclasite also occur above the top contact (Fig. 15A).

Figure 14. (A) Veins of granular cataclasite In one, ultracataclasite fills a crack that is perpendicular to and cuts across a in deformed quartz diorite end downward thin (3–4 cm) layer of breccia immediately above the top contact (between the at the top contact, which cuts them. Pre- heavy opposing arrows in Fig. 15A). This injection is continuous with ultracata sumably they were formed along it or in the uppermost layers. View to SW. Box clasite along the top contact and along the top of the breccia layer (Fig 15A) shows area of (B). Foliated breccia—fbx. and therefore probably was generated by slip on one or both of those surfaces. (B) Lack of shear offset of light-colored A larger body of ultracataclasite that contains breccia clasts resides above an aplite vein shows injection into tensile Area of crack. Box shows area of (C); mechanical R1 shear (Fig. 15A) and may also have been injected. pencil for scale. (C) Inward-coarsening part C The relations described above, along with a general lack of injections more grain-size sorting suggests fluidized gran- than ~50 cm above the top contact, suggest that the cataclastic and ultracata- ular flow during injection with higher clastic injections originated along and/or below the top contact or on R shears shear strain (rate?) near vein margins. 1 near the top contact.

Late Mineral Veins

The top and disrupted layers commonly are cut by planar veins of late- C 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.

A bx bx uc bx bx

uc Figure 15. (A) Detail of structural relation- bx ships along top contact on the SW flank uc R1 of the outcrop antiform (view to SW). layered, folded Cataclasite (c) and foliated cataclasite R1 ductile injection bx (fc) layers of different grain sizes (sepa- bx rated by thin black lines) are truncated by bx uc R1 uc the top contact at an acute angle, unlike bx most of the length of the top contact. Pri- mary Reidel shears (R1) cut poorly sorted breccia (bx) above the top contact. One c uc top c of these opened a pull-apart gap. Ductile c (folded) cataclasite layers were injected contact area of B across the top contact, into the pull-apart, layered and truncate the top contact. On the left, fc cataclasite ultracataclasite (uc) is present along the top contact and a parallel fault above it c (one or both presumably being generation c surfaces) and was injected across the ex- centimeters tended breccia layer (between heavy blue 0 5 10 c uc arrows). A large body of ultracataclasite (left of box) probably also is an injection. B C bx (B) Detail of Reidel shear that opened the pull-apart (area shown in A). Weak cataclastic foliation formed near a Reidel shear weak foliatio shows normal drag. Tip of pen for scale. bx (C) Detail of folded cataclasite layers in uc n bx the pull-apart. Layers can be traced across the top contact, showing that injection top occurred during the final slip event. The contact ultracataclasite body at the top of the pull- bx uc apart also may have been injected or may top contact have formed along the Reidel shear before the pull-apart opened.

area of C

Rocks immediately above the top contact reveal processes of cataclastic homogenized during overprinting by cataclastic foliation (described above).

layer formation. One process apparently involves slip on R1 shears that bend The foliated rocks immediately above the top contact generally are macro- into parallelism with the top contact several centimeters above it but do not scopically well sorted, a characteristic of most cataclastic layers also. merge with it (Fig. 8), defining fault-bounded layers of foliated rock above and Elsewhere, slices of heterogeneous rocks occur immediately above and

parallel to the top contact. These R1 shears flatten and disappear into the cata parallel to the top contact, bounded above by shear planes subparallel to the clastic foliation, which generally also is subparallel to the top contact within top contact and within ~10 cm of it (Figs. 9B, 15, and 16). These slices contain

several centimeters of it (Figs. 8 and 9B). Slip on foliation-parallel parts of R1 granular cataclasite, breccia, foliated cataclasite, ultracataclasite, and injected shears and, probably, lateral propagation and linking of these parts isolated material, and have sharp to granular shears within them (Figs. 9B and 16).

rock slices composed mainly of cataclastically foliated quartz diorite. Grain size Some R1 shears flatten into the tops of these heterogeneous slices, but others in such slivers was first reduced during brecciation and further reduced and cut across them and merge with the top contact (Fig. 15A).

A r covered bx bx shea shear

bx granular granular bx peg bx bx bx

c peg uc bx uc fc peg fbx sorted A top cont B bx injection act c bx centimeters bx layered cataclasite C c 0 10 20 B

Figure 16. (A) Annotated detail of a heterogeneous slice ~5–8 cm thick along the top contact on the SW flank of the outcrop antiform (view to SW). Fractured quartz diorite (cross dashes), breccia (bx), well-sorted cataclasite (c), pegmatite breccia (peg bx), foliated cataclasite (fc; wavy lines show trace of foliation), injected cataclasite (above A, internally sorted, coarser in center), and injected(?) ultracataclasite (uc), right of B, are all present in the slice. Injections record extension of the slice, but the injection at (A) probably continued farther up originally and has been displaced. A basal layer of cataclasite in the slice (above C) has a wavy, folded(?) upper contact suggesting ductile granular behavior. Granular shear zones cut rocks above the slice. (B) Same view but without annotation.

We interpret both foliated and heterogeneous layers above the top con- contrast, foliated layers above the top contact are lithologically homogeneous tact as “frozen” precursors to cataclastic layers. Heterogeneous layers have and well sorted (at the largest grain sizes) and provide suitable precursors to along-strike changes in grain size and lithology and thus provide a mechanism well-sorted cataclastic layers. to incorporate outsized clasts and clasts of cataclasite or ultracataclasite into All evidence is consistent with new layers having formed from fault- layers. One heterogeneous layer above the top contact has dimensions com- bounded slices above the top contact and being added to the top of the stack. parable to cataclastic layers (red diamond in Fig. 11). Some of these slices have Thus, the top contact migrated episodically upward into the overlying rocks. a foliated texture at their SE ends, with foliation disappearing and textures As new layers formed, older, structurally lower layers progressively were becoming more random to the NW: at their SE end, they resemble rocks above abandoned and disrupted by folds and faults. We infer that the first cataclastic the top contact, and at their NW end, they resemble the cataclasite layers. In layer formed along a fault oriented about the same as the SW-flank top con-

tact, the first layer strengthened relative to overlying rocks (discussed below), The seismic cycle at any point on a fault is defined by main shocks that rup- and new layers formed at the expense of rocks above the top contact. ture that place, with inter-seismic strain accumulating between them. The larg- We conclude that formation of each cataclastic layer required the follow- est earthquakes probably would have been on the WSDF, just meters above ing two events: (1) preparation processes above the top contact, including: the layered cataclasites. Foreshocks sometimes precede main shocks, and (a) cataclastic grain-size reduction of the quartz diorite; (b) overprinting by aftershock sequences follow them, with some aftershocks on/near the main- foliation with downward-decreasing maximum grain size and downward- shock plane and others off the plane (in the fractured damage zone) (e.g., Sav-

increasing intensity and macroscopic sorting; (c) slip on R1 shears and other age et al., 2017). Aftershocks commonly concentrate near the main-shock rup- faults; and (d) formation of a slice-bounding fault above and parallel to the top ture margins. Aftershocks and/or aseismic slip in the fractured damage zone contact; (2) one or more relatively large rapid-slip events, on the top contact presumably contribute to its development (e.g., Savage and Brodsky, 2011). and/or the slice-bounding fault, that homogenize each slice, destroy preexist- Small aftershocks are much more common than large ones, and aftershock ing foliations, transform the slice into a random-fabric cataclastic layer, and magnitudes generally decline with time. Relatively rapid aseismic “afterslip” is cause the top contact to episodically “jump” upward. In some layers, a weak, reasonably common in the months or years following main shocks; later, rela shear-related foliation formed later. tively slow “inter-seismic creep” may occur (also aseismic). Afterslip and inter- Structural relationships on the NE flank of the outcrop antiform are most seismic creep usually are detected geodetically and are modeled as occurring easily interpreted as showing that the antiform developed due to WSDF- on the main fault, but some of both occurring off-fault cannot be excluded. related drag of the NE limb, rotating it into parallelism with the detachment, at- We are unaware of reasons to think that the layered cataclasites entirely tenuating the NE limb and removing the top layers, and overprinting primary, pre-dated the WSDF, and we conclude that the layers formed in the WSDF layer-related fabrics with WSDF shear-related ones. damage zone, suggesting that WSDF main shocks would have defined the lo- Grain-size distributions (GSDs) in cataclasite yield information about condi- cal seismic cycle. Layered pseudotachylyte nearby along the WSDF requires tions of comminution (Sammis et al., 1987; Sammis and King, 2007). Log-normal that it was repeatedly seismogenic (Prante et al., 2014). Episodically formed GSDs suggest that cataclasis was accompanied by dilation; thus, grains can and mutually overprinting structures and fabrics described here are consis- move around and past one another. Dilation during cataclasis, and reduction tent with cyclic and episodic processes, and with the seismic cycle. Rowe of effective normal stress between grains, may occur along subsurface faults et al. (2011) also interpreted mutually overprinting fault-rock types along an if pore-fluid pressure is high or if the fault becomes dynamically dilatant. In exhumed subduction thrust as evidence for repeated seismic cycles. contrast, exponential GSDs suggest that dilation was not important and that In this context, we assign some of the textures described above, and their grains crush each other, failing mainly on tensile fractures. This “constrained formative processes, to fast versus slow deformation rates. Injections of comminution” leaves few same-size grains in contact: larger grains become sur- cataclasite and ultracataclasite are the structures most likely to record paleo rounded and “insulated” from crushing by smaller grains (Sammis et al., 1987). earthquakes. Off-fault injections of pseudotachylyte are common, require D-values for the layered cataclasites are ~3, indicating either that shear rapid injection during an earthquake (before the veins can solidify in seconds localization followed and overprinted constrained comminution, resulting in or minutes), and are associated with cataclastic injections (e.g., Rowe et al., a higher proportion of smaller grains (Sammis and King, 2007) or that high 2005). Rowe et al. (2012) described pseudotachylyte and cataclasite injections shear strain occurred during cataclasis (Storti et al., 2003). These D-values are in subduction-zone mélange that were generated during seismic slip on a similar to those determined for footwall fault-core rocks from both the WSDF plane where they end. They have aspect ratios of ~0.2, much higher than as- and the Whipple detachment (Luther et al., 2013), suggesting that the layered pect ratios of 0.001–0.0001 for dikes. Griffith et al. (2012) documented similar cataclasites formed in mechanical conditions similar to those experienced by pseudotachylyte injections into tonalite with aspect ratios of ~0.015–0.5. Both fault-core rocks from mature, large-slip detachments. 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 FORMATION OF LAYERED CATACLASITES IN were generated, and have aspect ratios of ~0.07 and 0.09, consistent with a co- REPEATED SEISMIC CYCLES seismic 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., The observations above show that several different deformation events 2005; Griffith et al., 2009; Rowe et al., 2012). The low angle in our study site were required to create each layer of cataclasite. These events probably oc- probably was controlled by anisotropy in the host rocks: those injections are curred at different strain or slip rates. We infer below that one (possibly more) in the hinge zone of a folded pegmatite, where many fractures are subparallel rapid (seismogenic) and chaotic slip event(s) on the top contact and/or the to the injections (Fig. 14B). faults bounding the top of precursor slices was required to homogenize lithol- If seismogenic slip events on the top contact were needed to form the in- ogies and randomize fabrics when each cataclastic layer was created. jections, then the same or similar rapid slip events probably formed homo-

geneous, random-fabric cataclastic layers from heterogeneous and/or foliated to strength contrasts across the top contact. Thus, the strength evolution may rocks. Such events probably were aftershocks to WSDF main shocks. have been special, but we doubt that the processes that formed the layered In contrast, formation of foliations requires penetrative strain that proba- cataclasites were. bly accumulated slowly. Foliation above the SW-flank top contact pre-dates Second, many fault-core rocks are layered. Along the WSDF, layering is formation of cataclastic layers, but foliation also overprinted some cataclastic common within the ultracataclastic inner core, which is separated into layers layers, requiring multiple episodes of slow strain. Other structures may have (commonly of different colors) by slip surfaces parallel to the WSDF (see also formed either quickly or slowly: Reidel shears, other sharp faults, granular Shervais and Kirkpatrick, 2016). Ultracataclasites along the WSDF also yield ev- shear zones, and various folds. idence for cyclic processes similar to those that we infer operated here. For ex- Some textures and structures preserved in the layered cataclasites and in ample, Kairouz (2005) and Hazelton (2003) described clasts of ultracataclasite the rocks above the SW-flank top contact probably record generallydecreasing in ultracataclasite layers and rare, weak foliations overprinting ultracataclasite, energy release (whether seismic, aseismic, or both), consistent with tempo- along both the WSDF and the Whipple detachment. Prante et al. (2014) de- rally decreasing aftershock magnitudes and with decreasing afterslip rates. For scribe multiple layers of fault-vein pseudotachylyte from the WSDF. example, it probably required more energy to form and homogenize any given Given these commonalities, we find ourselves in disagreement with the layer than to subsequently overprint it with a weakly developed foliation, fold often-cited view of Cowan (1999) that pseudotachylyte is the only fault rock that it, or cut it by small faults. Many of the delicate structures in the layered cata- compellingly records paleoseismicity. We agree with Rowe and Griffith (2015) clasites probably would not be preserved if formed along a long-lived fault be- that this viewpoint is too strong, especially given the knowledge that many cause they would be destroyed by subsequent rapid, high-energy slip events. (most?) continental faults accumulate slip primarily seismically (e.g., nearly all Similarly, delicately folded millimeter-scale layering injected across the top of the San Andreas system, except creeping sections). We therefore suggest contact (Fig. 15C) probably records the final small slip event, as the injection that a paleoseismic interpretation should be preferred where strong evidence opened, on that part of the top contact before it was completely abandoned. exists for both cyclic formation of fault rocks and for cyclic changes in strain Given the large number of layers, a comparable or larger number of seis- rate (e.g., Rowe et al., 2011). It takes considerable effort to document the full mic slip events were needed to form the whole layered stack. The body of suite of fault-rock textures and overprinting relationships needed to infer seis- layered cataclasites is ~2–3 m thick on average, and the average thickness of mic cycling at a given locality. Regardless, we view it as likely that seismogenic individual layers is ~4 cm, suggesting that ~50–75 layers exist in the stack. fault-rock assemblages are more common than single-lithology paleoseismic In the disrupted layers, isoclinal folding repeated some layers and probably indicators (pseudotachylyte, amorphous silicates, etc.). 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 Seismic Cycling and the Rock Record may be required), then a minimum of ~50–75 seismic events were needed to form the body, suggesting multiple seismic cycles. Only very rough estimates can be made of earthquake magnitudes needed to form the layered cataclasites because the fault dimensions and slip magni- DISCUSSION tudes related to their formation are poorly known. Decreasing energy release during aftershock sequences suggests that a wide range of magnitudes can

Rare Preservation of Common Products or be anticipated. Scalar seismic moment of an earthquake, M0, is given by the Preservation of Rare Products? 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 den-

Do these outcrops display rarely preserved products of common processes, sity (ρ) (e.g., Shearer, 1999, p. 21 and 169). We use VS = 3.5 km/s and ρ = 2700 and therefore potentially are of general significance, or do they preserve prod- kg/m3, appropriate for intermediate plutonic rocks. Minimum rupture area can ucts of rare processes and are not of general importance? We support the for- be constrained crudely from layer lengths (themselves minima; Fig. 11), but mer view for the following reasons. it is very likely that layer-forming rupture areas are larger. We use A ranging First, the layered cataclasites are similar to typical cataclasites formed from from 102 to 106 m2 (for fault lengths of ~10–1000 m). Slip magnitude is poorly quartzofeldspathic protoliths (cf., Sammis et al., 1987; Luther et al., 2013) in constrained; we use the range 0.1–1 m. These numbers yield a moment range several ways (grain-size distribution, grain shapes, overprinting weak foliation, for layer-forming earthquakes of ~3.3 × 1011 to 3.3 × 1016 N-m, or moment mag-

etc.), implying that they formed by common fault-zone processes. However, nitude (Mw) of 1.6–5.0 (Kanamori, 1977; Shearer, 1999, p. 187). This range is most of the cataclastic layers and many of the textures and crosscutting re- too high for many of the smaller preserved structures above the top contact lations documented in them probably would have been destroyed if the SW- (e.g., Fig. 15A) or in the disrupted layers (Fig. 13), with slip <0.1 m and outcrop flank top contact had not progressively migrated up into its hanging wall, with lengths suggestive of areas <<10 m2. However, these ranges may be reason- successively older layers being abandoned. We argue below that this was due able for events that formed and homogenized cataclastic layers.

We suggest that most WSDF slip occurred during episodic large earth- abandoned when a higher layer formed. In contrast, the WSDF apparently lo- quakes, given the common occurrence of pseudotachylyte elsewhere along calized to such an extent that the fault core preserved at the top of the footwall the WSDF (e.g., Prante et al., 2014) and the evidence presented here for for- typically is ≤1 m thick. mation of layered cataclasites in the WSDF damage zone during multiple One approach to estimating shear strain needed to form layered cataclasites seismic cycles. What are the reasonable ranges of main-shock magnitude and is to assume that their aspect ratios (Fig. 11) are related to shear strain. For a frequency that could be expected from the WSDF? The WSDF is ~100 km long thin (5 mm), 5-m-long, fine-grained, high-strain layer, the aspect ratio is 103; N-S (Fig. 1B does not show northern parts), with downdip length of ~20 km for a thick (250 mm), 2-m-long, coarse-grained layer, the ratio is 8 (~101). This (for 30° dip and 12 km seismogenic crust), yielding a maximum rupture area range may be higher than the actual shear strain that formed individual layers, of ~2000 km2. A small main shock might rupture only a 100 km2 patch. Most if the slip needed to form them was less than their lengths. historical normal-fault earthquakes occurred on steeper faults (Jackson and In contrast, if 10–15 km of WSDF slip was distributed through ~2 m of fault- White, 1989) and have ≤3 m maximum offset on surface scarps (Yeats et al., core thickness (assuming that a mirror-image, ~1-m-thick, hanging-wall fault 1997, chapter 9) but Wernicke (1995) argued that maximum slip on low-angle core was left behind at depth), then shear strain would be 5–7.5 × 103. If WSDF normal faults can be higher. For S of 0.5–5 m, A of 100–2000 km2, and other shear was distributed through only ~0.5 m of ultracataclastic inner fault core, parameters as above, the range of WSDF main-shock moment release is 1.7 × the shear strain would be 2–3 × 104. This range probably is higher than actu- 18 20 10 to 3.3 × 10 N-m, or Mw range of 6.1–7.6. If WSDF net slip is 10–15 km, all ally formed the inner-core ultracataclasites, because much slip arguably was main shocks ruptured the whole WSDF area in 5 m slip events, and the WSDF concentrated on very thin (<1 mm) slip surfaces within the ultracataclasites. was active for 7 m.y. total (from ~8 to ~1 m.y.), the minimum average recur- Thus, net shear strain for formation of the WSDF fault core probably was

rence interval for Mw = 7.6 main shocks would be ~2.3–3.5 k.y. significantly higher than that needed to form cataclastic layers. Interestingly, Assuming that WSDF slip accumulated mainly seismogenically, it follows the two rock assemblages have very similar D-values for their grain-size dis- that the very typical two-part WSDF fault core also formed mainly during earth- tributions. quake slip. Thus, we hypothesize that mature faults in quartzofeldspathic rocks with a layered ultracataclastic inner core and a cataclastic outer core should Evolution of Strength during Faulting be considered as paleoseismogenic. Such an assemblage was described by Chester and Chester (1998) along the Punchbowl fault (an abandoned strand In this section, we discuss the temporal and spatial evolution of strength in of the San Andreas fault), which they assumed was seismically active. Many and around the layered cataclasites and the WSDF in the context of the seismic low-angle normal faults with quarztofeldspathic footwalls have similar fault cycle and varying deformation rates. cores at their tops (e.g., Davis and Lister, 1988; many others). These are de- Cataclastic layers formed at the expense of rocks above the top contact and scribed as comprising a “microbreccia ledge” (inner core) above a “chlorite older, lower layers were preserved and abandoned. Thus, new layers presum- breccia zone” (including the outer fault core and, usually, part or all of the ably were stronger than overlying deformed quartz diorite. We infer that strain fractured damage zone). hardening of the layered cataclasites and/or strain softening in the rocks above If both the layered cataclasites and (more speculatively) the WSDF fault the top contact were essential for formation of the layers. core formed seismogenically, then what controlled the differences between Penetrative cataclastic foliation development and grain-size reduction

them? The most likely controls are elastic energy released in seismic events probably were strain-softening mechanisms above the top contact. R1 shears and the percentage of this expended as work during cataclasis, net shear that flatten into the well-developed foliation above and parallel to the top con- strain, and degree of localization (which affects the net shear strain). tact show that the foliation was weak. Foliation-parallel weakness probably The amount of finely comminuted ultracataclasite is much greater along was caused by grain-size reduction and alignment of biotite grains near the top the WSDF than in the layered cataclasites. Therefore, work expended forming contact. Collettini et al. (2009) showed that foliation in clay gouge dramatically WSDF fault-core rocks probably was much greater than that needed to create reduces friction, and we suspect that the same effect weakened the foliated cataclastic layers, although this is impossible to quantify at present. This is rocks above the top contact, where biotite is aligned. However, foliation above consistent with the estimates made above for earthquake magnitudes needed the top contact probably took many seismic cycles to develop, and certainly to form the layers seismogenically versus possible on the WSDF. was not present initially. Penetrative microfractures in the quartz diorite, which In contrast, the shear strains needed for layer formation and for WSDF net may have begun to form very early in the deformation history, would also have slip can be constrained roughly. The layered cataclasites probably record low reduced the strength of the quartz diorite enough to allow preservation of the values of integrated cataclastic shear strain relative to that which formed the first-formed layer. WSDF core. In addition, the layered cataclasites preserve a record of ~3 m of We argue above for repeated shearing on recessive, fine-grained layers, migration, measured approximately perpendicular to the fault plane along which separate resistant, coarser-grained, and presumably stronger layers; which they formed, and thus each was active only temporarily before being thus, strength contrasts existed among the layered cataclasites, with strength

apparently correlated to grain size. Thus, grain-size reduction probably was a velop foliation. Luther and Axen (2013) concluded from different arguments strain-weakening mechanism both above and below the top contact. There- that WSDF earthquakes had large stress drops. Second, inter-seismic strain fore, strong foliation development (alignment of biotites) above the top con- may have accumulated outside the WSDF fault core by small earthquakes or tact in addition to grain-size reduction probably account for late weakness of fracture creep events in the fractured damage zone, where foliation is gen- those rocks relative to the stack of layers. erally absent. This can be explained if WSDF fault-core healing (increase of It seems likely that compaction of cataclastic layers following their forma- cohesion, resistance to distributed shearing in the ultracataclasites, and/or tion strengthened them. This is in accord with increases of friction seen after inter-seismic increase of static friction) was fast and efficient relative to healing hold periods in slide-hold-slide experiments on thin gouge zones (e.g., Naka in the fractured damage zone. Fault surfaces within ultracataclasite may re- tani, 1998; Richardson and Marone, 1999; Sleep et al., 2000; many others). gain cohesion faster than coarser-grained fault rocks for several reasons. Tiny Mair and Marone (1999) present data that suggest greater strengthening grains can indent across slip surfaces a shorter distance, which may lead more during holds with low shear stress than with high shear stress; so perhaps quickly to geometric interlocking. This effect may be enhanced because tiny foliation-free cataclastic layers record top-contact earthquakes with larger grains are more likely to have individual strengths closer to crystallographic stress drops, leaving shear-stress levels during the compaction phase too low maxima, due to less internal damage in each grain (e.g., fewer microfractures for foliation development. The foliation in many cataclastic layers probably and other crystalline defects preserved). Last, healing by chemical processes records inter-seismic shear during compaction. It is weakly developed, and such as pressure solution and precipitation probably occurs faster over the at ~45° to layering (Fig. 9C), records small shear strain, suggesting that the shorter distances and higher surface areas of very fine-grained fault rocks cataclasite layers strengthened when it developed. In contrast, the well-devel- (Rutter, 1983; Bos et al., 2000). Also, the top contact had a different orienta- oped and strongly rotated foliation above the top contact records much higher tion than the WSDF while they were active and may simply have been more shear strain, suggesting that those rocks were weak relative to the layered favorably oriented for slip during the inter-seismic periods. Alternatively, re- cataclasites and that distributed inter-seismic shear strain was more strongly peated rupturing of the inner core may have been controlled by rupture-tip localized there. propagation processes rather than by typical measures of rock strength, but it The top contact also must have been stronger than the rocks above it, is unclear how to assess this. at least at times or locally, in order for cataclasite slices to have formed and We are not aware of mature faults that migrated systematically perpendic- been transferred into the layered stack. If grain size was a significant control ular to themselves. This probably explains why interleaved slices of rock from on strength, then strength was probably variable along the top contact, as in- both sides of faults are rare in the rock record. Multiple slip surfaces within the dicated by grain size along it ranging from aphanitic (ultracataclasite) up to inner core require small (centimeter-scale) “jumps,” but these may have been fairly coarse. controlled by processes within the fault core (e.g., Shervais and Kirkpatrick, The arguments above, for strength contrasts between the layered cata 2016). Occasional transfers of material from the outer fault core to the inner clasites and top contact (stronger) and the overlying deformed quartz diorite fault core are required in order for the inner core to grow in thickness without (weak) causing upward “jumps” of the top contact, are analogous to recent incorporating upper-plate lithologies; but how this happens is not known. results showing that pseudotachylyte solidification strengthens faults enough Random-fabric or sorted cataclastic injections require loss of cohesion and that they are abandoned and slip moves to new surfaces (Mitchell et al., 2016; fluidized cataclastic flow and transport. If flow is driven by a pressure gra- Proctor and Lockner, 2016). dient, then cataclasite-generation surfaces may have been, at least momen- In contrast, slip remained localized in the WSDF core, suggesting that it tarily, zones of high normal stress, as is inferred for formation and flow of remained weak relative to its surroundings. The most obvious differences pseudotachylyte (e.g., Sibson and Toy, 2006; Rowe et al., 2012). This may drive between layered cataclasites and the WSDF fault-core rocks are that (1) the transient pressure gradients required for coseismic off-fault injections, similar WSDF fault core has a much greater thickness of very fine-grained ultracata to off-fault pseudotachylyte veins. However, some injections fill dilatant pull- clasite, and (2) WSDF fault-core rocks are only rarely foliated. Neither contains apart structures (Fig. 15), and dynamic rupture propagation can cause tensile significant amounts of clay minerals (as are present in the base of the upper cracks to form in the absence of gouge (e.g., Griffith et al., 2009). Therefore, plate); therefore, clays are probably not the cause of WSDF weakness. Thick transiently reduced pressure in off-fault injection sites may also contribute to WSDF ultracataclasite (1) supports arguments already made that fine-grained coseismic pressure gradients. cataclastic rocks are weaker than coarser ones and is probably explained by WSDF fault-rock formation during larger-slip, higher-energy seismic events Mechanical versus Chemical Processing than created the layers. General lack of foliation in WSDF fault-core rocks (2) may be explained The degree of alteration in the layered cataclasites and surrounding plu- two ways. First, stress drop in most WSDF main shocks may have been nearly tonic protoliths is surprisingly small, suggesting that mechanical processes complete, and residual shear stress was too low to shear core rocks and de- dominated formation of the layered cataclasites. This may allow meaningful

comparison of these natural fault rocks to those created experimentally but described zeolite veins in mafic or intermediate rocks in and near fault zones. with too little time for significant chemical processes to occur. Calcic and sodic zeolites (mainly in veins) have been found in intermediate Mafic minerals (biotite, hornblende, and clinopyroxene) are essentially un- plutonic rocks around the Gole Larghe fault zone, Italy (Dempsey et al., 2014), a altered (though comminuted, broken, and bent). Chlorite is optically absent, famous site for study of pseudotachylyte (e.g., Di Toro and Pennacchioni, 2005; but its presence as a minor alteration product in biotite is suggested by low-K Smith et al., 2013). Zeolites there, and fluid flow related to their precipitation in values of some biotites. Preliminary SEM imaging of plagioclase shows lo- fractures and veins, are interpreted to be one manifestation of paleoseismicity cal development of microporosity, suggestive of dissolution, and some weak, (Dempsey et al., 2014). hazy alteration is observed in thin section (probably sericite). Euhedral calcic zeolite minerals in pores in layered cataclasites (Fig. 7I) CONCLUSIONS 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 hydro- Uniquely preserved layered cataclastic fault rocks are found below the thermal fluid circulation up the fault zone and possibly occurred at the expense WSDF, in an outcrop-scale antiform that folds them, the surrounding, de- of microporous plagioclase. In this context, it is noteworthy that hot springs formed quartz diorite protolith, and the faults that separate the two. Layers emanate from the WSDF at Agua Caliente County Park, near the south end of were formed sequentially and episodically along the top contact, by cataclasis the WSDF. Wood (2014) identified zeolites in the basal upper plate there, which and homogenization of rock slices previously above that contact. Layers were is altered for several meters above the fault. Footwall fault rocks there display transferred down into the layer stack, as their generation surface (top contact) a moderately developed cataclastic S–C foliation and stretching lineation, and migrated stepwise up into deformed quartz diorite. Older, structurally lower we recently found zeolites in these rocks (XRD analyses), but their ages rela- layers are increasingly deformed downward, by folds that record ~NNW-SSE tive to fault-related deformation are not yet known. Shervais and Kirkpatrick shortening and vertical thickening and by small faults that record ~ESE-WNW (2016) found laumontite in inner fault-core rocks of the La Quinta fault and extension and vertical thinning of the layer stack. normal-sense shears that splay from it. They suggested that this phase of La The layered cataclasites, as a fault-rock assemblage, record dozens of seis- Quinta slip occurred below 250 °C and was related to WSDF slip. mic cycles. Evidence for this includes many overprinting textures and cross- Zeolites commonly form in hydrothermal systems (among other occur- cutting relationships that show cyclic alternations between slow, progressive rences; Chipera and Apps, 2001). Hydrothermal calcic zeolites from wells near strain accumulation that presumably records inter-seismic strain (e.g., foliation Yellowstone are presumed to be in equilibrium with ambient temperatures in protolith quartz diorite that strengthens and rotates as the top contact is and subsurface geothermal fluids (Bargar and Kieth, 1995). Laumontite, which approached) and rapid strain or slip events that formed cataclastic injections is present in WSDF fault rocks, is present at Yellowstone at temperatures of and created homogeneous and random-fabric cataclasite layers from foliated ~160–200 °C, consistent with other studies (see Chipera and Apps, 2001). For and/or heterogeneous protolith. reasonable geothermal gradients of 40° to 25 °C/km, this suggests zeolite The layered cataclasites are a useful but atypical rock record of common deposition along the WSDF at depths of 4–8 km, consistent with independent processes (and thus are of general significance) rather than a rare product of estimates of paleodepth of these rocks summarized above (of course, the geo- uncommon processes. In particular, the sequential formation, preservation, therm in a fault-controlled fluid pathway may have been higher). Huelandite and deformation of cataclastic layers yield timing relationships and history (also in WSDF fault rocks) probably is stable at lower temperatures and depths that typically are difficult or impossible to determine in well-developed fault- (Chipera and Apps, 2001). core rocks. As such, the stack of layers is a unique record of the seismic cycle. Zeolites in fault rocks have been studied only sparsely. Calcic zeolites are The layered cataclasites apparently formed mainly by mechanical processes common as fracture and vein fill in core from the Cajon Pass borehole and with limited alteration and may provide good analogues to mechanically in surrounding outcrops (James and Silver, 1988), where they replace plagio (de)formed fault rocks studied experimentally. clase. There, the downward transition from stilbite to laumontite occurs at The WSDF was a repeatedly seismogenic low-angle normal fault, as shown ~100 °C (present well temperature) and was interpreted to be in equilib- by presence of layered pseudotachylyte along it (Axen et al., 1998; Prante rium at that temperature (James and Silver, 1988). Laumontite alteration of et al., 2014) and by the presence of these paleoseismic layered cataclasites host rock reduced the strength of intact Cajon Pass core samples (Vernik and just meters below the detachment. Simple calculations suggest that WSDF

Zoback, 1989). Morrow and Byerlee (1988) showed that zeolites at Cajon Pass main shocks could reasonably have had moment magnitudes Mw of 6.1–7.6 (M0 reduced permeability significantly and probably allowed for fluid compart- range of 1.7 × 1018 to 3.3 × 1020). In contrast, the large, fast-slip events inferred

mentalization (Kharaka et al., 1988). Morrow and Byerlee (1991) showed that to have formed individual cataclasite layers were probably much smaller (Mw 11 17 crushed laumontite artificial gouge from Cajon Pass has friction in accord with range of 1.6–5.0 and M0 range of 3.3 × 10 to 3.3 × 10 ). Net shear strains Byerlee’s Law, with no discernable effects of pore-fluid pressure increase. We to form layered cataclasites (order 101 to 103) also probably were significantly are not aware of rate-state experiments on zeolites. Walker et al. (2013a, 2013b) 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 detach- Axen, G.J., 1999, Low-angle normal fault earthquakes and triggering: Geophysical Research Let- ment fault cores (Luther and Axen, 2013) have similar fractal dimensions to ters, v. 26, p. 3693–3696, doi:10.1029/1999GL005405. Axen, G.J., 2004, Mechanics of low-angle normal faults, in Karner, G., Taylor, B., Driscoll, N., and these layered cataclasites, suggesting formation by constrained comminution Kohlstedt, D.L., eds., Rheology and Deformation of the Lithosphere at Continental Margins: overprinted by shear strain (e.g., Sammis and King, 2007). However, this mea- New York, Columbia University Press, p. 46–91, doi:10.7312/karn12738-004. sure does not reflect the overall much more significant grain-size reduction of Axen, G.J., and Fletcher, J.M., 1998, Late Miocene–Pleistocene extensional faulting, northern Gulf or California, Mexico, and southern California: International Geology Review, v. 40, WSDF and Whipple detachment core rocks versus cataclastic layers. We con- p. 217–244, doi:10.1080/00206819809465207. clude that the main difference between the WSDF fault-core rocks and the lay- Axen, G.J., and Selverstone, J., 1994, Stress state and fluid-pressure level along the Whipple de- ered cataclasites resulted from differences in net shear strain and energy con- tachment fault, California: Geology, v. 22, p. 835–838, doi:10.1130/0091-7613(1994)022<0835: SSAFPL>2.3.CO;2. sumed in comminution. Earthquake slip on the WSDF localized in a narrow, Axen, G.J., Fletcher, J.M., and Martín-Barajas, A., 1998, Late Miocene–Pleistocene detachment weak zone and resulting high rates of energy release and high shear strains faulting in the northern Gulf of California and its role in evolution of the Pacific–North Amer- formed decimeters of ultracataclasite. In contrast, strain softening above the ican plate boundary: Long Beach, California, California State University, Cordilleran Section top contact and strain hardening of the cataclastic layers prevented continued of the Geological Society of America, Guidebook to Fieldtrip #6, 29 p. Axen, G.J., Fletcher, J.M., Cowgill, E., Murphy, M., Kapp, P., MacMillan, I., Ramos-Velázquez, E., overprinting and comminution of cataclastic layers and allowed their preser- and Aranda-Gómez, J., 1999, Range-front fault scarps of the Sierra El Mayor, Baja California: vation at relatively early stages. Thus, average grain size, average largest grain Formed above an active low-angle normal fault?: Geology, v. 27, p. 247–250, doi:10 .1130/0091 size, and/or ultracataclastic rock volume may provide useful measures for de- -7613(1999)027<0247:RFFSOT>2.3.CO;2. Axen, G.J., Grove, M., Stockli, D., Lovera, O.M., Rothstein, D.A., Fletcher, J.M., Farley, K., and Ab- velopment of quantitative, predictive relationships among fault rocks, energy bott, P.L., 2000, Thermal evolution of Monte Blanco dome: Low-angle normal faulting during expended in cataclasis, and earthquake processing. Gulf of California rifting and Late Eocene denudation of the eastern Peninsular Ranges: Tec- The WSDF has a second fault-rock assemblage that we hypothesize formed tonics, v. 19, p. 197–212, doi:10.1029/1999TC001123. Axen, G.J., Selverstone, J., and Wawrzyniec, T., 2001, High-temperature embrittlement of exten- mainly seismogenically: a two-part footwall fault core with an inner core of sional Alpine mylonite zones in the midcrustal brittle-ductile transition: Journal of Geophysi ultracataclasite separated into layers by multiple slip surfaces and an outer cal Research, v. 106, p. 4337–4348, doi:10.1029/2000JB900372. core of random-fabric cataclasite. It resembles the cores of other mature faults Axen, G.J., Luther, A., and Selverstone, J., 2015, Paleostress directions near two low-angle nor- developed in quartzofeldspathic hosts (e.g., the Punchbowl fault; Chester mal faults: Testing mechanical models of weak faults and off-fault damage: Geosphere, v. 11, p. 1996–2014, doi:10.1130/GES01211.1. and Chester, 1998) and many other low-angle normal fault cores composed Bargar, K.E., and Kieth, T.E.C., 1995, Calcium zeolites in rhyolitic drill cores from Yellowstone Na- of “microbreccia” inner cores and “chlorite breccia” outer cores (e.g., Davis tional Park, Wyoming, in Ming, D.W., and Mumpton, F.A., eds., Natural Zeolites ’93: Occurrence, and Lister, 1988). We suggest that this common fault-rock assemblage may Properties, Use: Brockport, New York, International Committee on Natural Zeolites, p. 69–86. Ben-Zion, Y., and Sammis, C., 2003, Characterization of fault zones: Pure and Applied Geophysics, record paleoseismicity where found. Upper-plate fault rocks near the layered v. 160, p. 677–715, doi:10.1007/PL00012554. cataclasites are clay rich and probably formed above the seismogenic layer. Bos, B., Peach, C.J., and Spiers, C.J., 2000, Slip behavior of simulated gouge-bearing faults The lack of comparable volumes of clays in the footwall fault core suggests under conditions favoring pressure solution: Journal of Geophysical Research, v. 105, p. 16,699–16,717, doi:10.1029/2000JB900089. that footwall fault rocks passed through the crust above the seismogenic zone Caine, J.S., Evans, J.P., and Forster, C.B., 1996, Fault zone architecture and permeability struc- relatively unchanged. ture: Geology, v. 24, p. 1025–1028, doi:10.1130/0091-7613(1996)024<1025:FZAAPS>2.3.CO;2. Chester, F.M., and Chester, J.S., 1998, Ultracataclasite structure and friction processes of the Punchbowl fault, San Andreas system, California: Tectonophysics, v. 295, p. 199–221, doi:10 ACKNOWLEDGMENTS .1016/S0040-1951(98)00121-8. Chester, F.M., Evans, J.P., and Biegel, R.L., 1993, Internal structure and weakening mechanisms This study was funded by National Science Foundation awards EAR-0809638 (G.A.) and EAR- of the San Andreas fault: Journal of Geophysical Research, v. 98, p. 771–786, doi:10 .1029 0809220 (J.S.). Ashley Griffith and an anonymous reviewer each carefully and thoughtfully re- /92JB01866. viewed two versions of the manuscript, greatly improving it. Discussions with R. Holdsworth Chipera, S.J., and Apps, J.A., 2001, Geochemical stability of natural zeolites, in Bish, D.L., and about fault-rock genesis are appreciated. Support from the Anza-Borrego Desert State Park and Ming, D.W., eds., Natural Zeolites: Occurrence, Properties, Applications: Reviews in Mineral- the California State Parks Colorado Desert District was provided in the forms of sampling permits, ogy & Geochemistry, v. 45, p. 117–161. access to their research laboratory in Borrego Springs, California, and by the generally helpful Collettini, C., Niemeijer, A., Viti, C., and Marone, C., 2009, Fault zone fabric and fault weakness: staff there, especially G. Jefferson, L. Murray, and J. Manning. Jimmy Smith of Borrego Springs, Nature, v. 462, p. 907–910, doi:10.1038/nature08585. California assisted in the field. M. Spilde of the University of New Mexico Institute of Meteoritics Cowan, D.S., 1999, Do faults preserve a record of seismic slip?: A field geologist’s opinion: Jour- aided sample preparation for SEM work, and N. Dunbar and L. 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