Patterns of Mineral Transformations in Clay Gouge, with Examples from Low-Angle Normal Fault Rocks in the Western USA

Journal of Structural Geology 43 (2012) 2e32

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Journal of Structural Geology

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Review article Patterns of mineral transformations in clay gouge, with examples from low-angle normal fault rocks in the western USA

Samuel H. Haines*, Ben A. van der Pluijm

Department of Earth & Environmental Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109, USA article info abstract

Article history: Neoformed minerals in shallow fault rocks are increasingly recognized as key to the behavior of faults in Received 19 October 2011 the elasto-frictional regime, but neither the conditions nor the processes which wall-rock is transformed Received in revised form into clay minerals are well understood. Yet, understanding of these mineral transformations is required 27 April 2012 to predict the mechanical and seismogenic behavior of faults. We therefore present a systematic study of Accepted 12 May 2012 clay gouge mineralogy from 30 outcrops of 17 low-angle normal faults (LANF’s) in the American Available online 1 June 2012 Cordillera to demonstrate the range and type of clay transformations in natural fault gouges. The sampled faults juxtapose a wide and representative range of wall rock types, including sedimentary, Keywords: Fault rock metamorphic and igneous rocks under shallow-crustal conditions. Clay mineral transformations were Gouge observed in all but one of 28 faults; one fault contains only mechanically derived clay-rich gouge, which Clay formed entirely by cataclasis. Cordillera Clay mineral transformations observed in gouges show four general patterns: 1) growth of authigenic Normal faults 1Md illite, either by transformation of fragmental 2M1 illite or muscovite, or growth after the dissolution of K-feldspar. Illitization of fragmental illiteesmectite is observed in LANF gouges, but is less common than reported from faults with sedimentary wall rocks; 2) ‘retrograde diagenesis’ of an early mechanically derived chlorite-rich gouge to authigenic chloriteesmectite and saponite (Mg-rich tri-octahedral smectite); 3) reaction of mechanically derived chlorite-rich gouges with Mg-rich ﬂuids at low temperatures (50e150 C) to produce localized lenses of one of two assemblages: sepiolite þ saponite þ talc þ lizardite or palygorskite þ/ chlorite þ/ quartz; and 4) growth of authigenic di-octahedral smectite from alteration of acidic volcanic wall rocks. These transformation groups are consistent with patterns observed in fault rocks elsewhere. The main controls for the type of neoformed clay in gouge appear to be wall-rock chemistry and ﬂuid chemistry, and temperatures in the range of 60e180 C. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction other reactions, including polytype transformations (e.g., Solum et al., 2005), methods for radiometric dating (e.g., van der Pluijm The role of authigenic clay growth in clay gouges is increasingly et al., 2001, 2006; Haines and van der Pluijm, 2008) and fabric recognized as a key to understanding the mechanics of brittle quantiﬁcation (e.g., Haines et al., 2009) of clay gouge, which have faulting and fault zone processes, including creep and seismo- been applied in a range of geologic settings around the world, from genesis, and providing new insights into the ongoing debate about transform and normal faulting in the USA to reverse faulting in the frictional strength of brittle faults (e.g., Rice, 1992; Scholz, Spain and northern Tibet (e.g., Schleicher et al., 2010; Duvall et al., 2002). In the 20th Anniversary Special Issue of the Journal of 2011; Rahl et al., 2011; Verdel et al., 2011). A long history and robust Structural Geology, Vrolijk and van der Pluijm (1999) made a ﬁrst literature exist on clay mineralogy (e.g., Grim, 1953; Moore and attempt at exploring the implications of newly-recognized clay Reynolds, 1997; Meunier, 2005), but a systematic study of clay transformations in natural fault gouge, mostly involving illitization transformations and neomineralization in fault rocks has not been of smectitic gouge. New work since that time led to recognition of made. This study, therefore, is an attempt at examining and cata- loguing the range of clay occurrences and processes that take place in natural fault rocks, using low-angle normal faults of the south- * Corresponding author. Current address: Chevron Energy Technology Corpora- western USA as the target area. Patterns observed in the fault rocks tion, 1500 Louisiana St., Houston, TX 77002, USA. E-mail address: [email protected] (S.H. Haines). described in this study involve a range of rock types that are

0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2012.05.004 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 3 common in other fault settings, so the current study presents 1910; Longwell, 1945), but the low dips observed in the field are in representative clay mineralogies and patterns of mineral reactions apparent contradiction with widely accepted ideas about the that are a framework for the study of shallow fault rocks elsewhere. mechanics of rock fracture and rock friction. Experimental data Brittle fault rocks have been of interest to the structural geology and rock mechanics theory predict that normal faults in the brittle and geophysics communities for over a century, (e.g. Lyell, 1851; regime should not form at dips <45, and rock friction data Lapworth, 1885; Waters and Campbell, 1935; Reed, 1964; Wu et al., indicates that frictional slip of most geological materials is not 1975; Sibson, 1977), but the recognition that growth of authigenic possible at dips <30 (Byerlee, 1978; Sibson, 1985; Collettini and clay may be a fundamental process in the evolution of faults in the Sibson, 2001). Normal faults that apparently slip at low dips are brittle regime (<<300 C), and not just a product of local alteration, key to the argument about the strength of natural faults (e.g. Rice, is relatively recent (e.g. Vrolijk and van der Pluijm, 1999; van der 1992), as they challenge the results of conventional rock friction Pluijm, 2011). The frictional properties of clay-rich rocks within experiments. Continental low-angle normal faults (LANFs major fault zones have received increasing attention as a possible or detachment faults) were first recognized in the Basin and explanation for anomalously weak faults (e.g. Wu et al., 1975; Wu, Range of the western US in conjunction with Tertiary meta- 1978; Rutter et al., 1986; Chester and Logan, 1987; Chester et al, morphic tectonites (e.g. Anderson, 1971; Armstrong, 1972; Davis 1993; Evans and Chester, 1995; Chester and Chester, 1998; and Coney, 1979; Crittenden et al, 1980; Wernicke, 1981; Cladouhos,1999; Cowan,1999; Schulz and Evans, 2000; Cowan et al., Wernicke and Axen, 1988) and have since been recognized world- 2003; Solum et al., 2003, 2005; Hayman et al., 2004; Numelin et al, wide (e.g. Froitzenheim and Eberli, 1990; Lister et al., 1991; 2007; Hayman, 2006; Schleicher et al., 2006, 2010; Carpenter et al., Manatschal, 1999; Martinez et al., 2001; Collettini and 2011). However, most studies have focused on the description, Holdsworth, 2004). Typically, these faults juxtapose a mid- physical properties and bulk geochemistry of brittle fault rocks. By crustal footwall and an unmetamorphosed, usually sedimentary, comparison to the voluminous literature on fault rocks, far fewer hangingwall. The faults usually have displacements of 10e30 km studies have focused on the clay minerals and especially, the clay and expose a footwall of foliated and lineated mylonites that are mineral transformations within fault gouges (e.g. Wu, 1978; Rutter overprinted by a distinctive ductile to brittle structural history et al., 1986; Vrolijk and van der Pluijm, 1999; van der Pluijm et al, recorded by the successive overprinting of the mylonites (if 2001; Solum and van der Pluijm, 2009; Solum et al., 2003, 2005, present) by a bottom-to-top series of distinctive brittle fault rock 2010; Casciello et al., 2004; Hayman et al., 2004; Hayman, 2006; types: 1) A chlorite- and epidote-rich breccia that formed at Jacobs et al., 2006; Schleicher et al., 2006, 2009, 2010, 2012; Haines greenschist-facies conditions in a cataclastic flow regime that can and van der Pluijm, 2008, 2010; Solum and van der Pluijm, 2009; be tens of meters thick; 2) A fine-grained lithified cataclasite, Zwingmann et al., 2010; Buatier et al., 2011; Surace et al., 2011). frequently referred to as ‘microbreccia’; and 3) Clay-rich fault Typically, these studies have characterized a single site with a series gouge, recording deformation at progressively higher structural of techniques, among them XRD, XRF, SEM, TEM, X-ray texture levels as the footwall is exhumed (e.g. Crittenden et al., 1980; goniometry (XTG), AMS, and various isotopic measurements. This Axen, 2004; Hayman, 2006). Some low-angle normal faults clearly single-site approach, however, cannot identify systematic patterns evolved from mid-crustal shear zones (e.g., the Ruby Mountains, of mineral transformations in fault gouge that may be repeatable in NV, Death Valley ‘turtlebacks’, CA, Central Mojave metamorphic space and time. Three heritage studies (Wahlstrom et al., 1968; core complex, CA, Whipple Mts, CA, Buckskin-Rawhide Mts, AZ, Brekke and Howard, 1973; Wu, 1978) and three recent studies Harcuvar and Harquahalla Mountains, AZ, and Sierra Mazatán, (Zwingmann et al., 2010; Buatier et al., 2011; Surace et al., 2011)have Mexico; see Snoke, 1980; Davis et al., 1980; Dokka and Glazner, surveyed the mineralogy of a suite of clay gouge samples from 1982; Reynolds and Spencer, 1985; Vega Granillo and Calmus, multiple outcrops, but did not discriminate between neoformed and 2003) while others lack mylonitic footwalls and evolved almost fragmental minerals, or identify specific transformations or entirely in the brittle upper crust (e.g., the Chemehuevi Moun- systematic patterns of transformations. To this end, we study of the tains, CA, West Salton detachment, CA, Panamint Mountains, CA, mineralogy of clay-rich gouges in low-angle normal faults of the Dante’s View low-angle normal fault, CA, Amargosa detachment, western and southwestern US, collected from 32 exposures of a total CA; see John, 1987; Hodges et al., 1990; Topping, 1993; Axen and of 17 detachment faults, ranging from the Ruby Mountains meta- Fletcher, 1998; Cichanski, 2000). morphic core complex in NE Nevada to the Sierra Mazatán meta- While a voluminous and lively literature debates the morphic core complex in Sonora, Mexico. These faults juxtapose wall mechanics of low-angle normal faults, their potential for seis- rock of widely varying compositions, from sedimentary and volcanic micity and their very existence (e.g. Miller et al., 1983; Spencer, rocks to metamorphic and plutonic lithologies, and range in age 1984; Buck, 1988; Wernicke and Axen, 1988; Spencer and Chase, from Eocene to Recent, We aim to document the presence of both 1989; Yin, 1989; Liavaccari et al, 1993; Lister and Balwin, 1993; relict cataclastic and authigenic clay phases and to demonstrate Wernicke, 1995; Axen and Bartley, 1997; Collettini and Sibson, patterns of clay mineral transformations that are common to 2001; Axen, 2004; Walker et al, 2006; Christie-Blick et al., 2007; shallow faults with widely varying ages and wall rock compositions. Collettini, 2011), little attention has been paid to the mineralogy These transformations include those documented in previous site- of the brittle fault rocks found within these controversial struc- specific studies, and so suggest a predictable set of clay mineral tures. Many low-angle normal faults contain clay-rich gouges, transformations in fault gouge given the appropriate wall rock which can be an important component in the physical behavior of composition, fluid availability and temperature. These mineral brittle fault systems (e.g. Vrolijk, 1990; Saffer et al., 2001; Brown transformations also provide insights into the conditions in the fault et al., 2003; Saffer and Marone, 2003; Schleicher et al., 2008). zone during clay gouge formation, such as the potential for reaction- We use the term ‘clay gouge’ from a field perspective to refer to weakening mechanisms that facilitated low-angle normal fault slip visibly massive to crudely foliated clay-rich material found within in the elasto-frictional regime. brittle fault zones that, although variably cohesive and ranging in hardness from soft to quite hard at outcrop, disaggregates readily 2. Low-angle normal faults in water. Our use of the field term does not imply a cataclastic mechanism for the formation of gouge. The existence of normal faults with low dips (<30) has been The clay mineralogy of gouge in low-angle normal faults has recognized since the early years of the last century (Ransom et al., important implications for the frictional properties of the fault 4 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 zones. Chlorite and illite both have frictional strengths of 3.1. Ruby mountains m ¼ 0.25e0.55, significantly lower than the 0.6e0.8 observed by Byerlee’s law, and smectite has reported frictional strengths as low The Ruby Mountains are a well-exposed core complex in north- as m ¼ 0.05 (Scholz, 2002; Brown et al., 2003; Ikari et al., 2007, eastern Nevada (Howard, 1980; Snoke, 1980) extending w150 km 2009; Tembe et al., 2010; Lockner et al., 2011). Frictional experi- alongstrikefromSWtoNE(Fig.1). The core complex was exhumed by ments on unsaturated clay-rich low-angle normal fault gouges a large WNW-dipping, 1e2 km thick shear zone that strikes along the indicate frictional strengths of m ¼ 0.41e0.52 (Numelin et al., 2007) NW side of the range. The detachment was active from the late and recent work on SAFOD core shows values below 0.20 for Cretaceous to the Miocene (Mueller and Snoke, 1993; McGrew and smectitic gouge (Carpenter et al., 2011; Lockner et al., 2011; van der Snee, 1994; Colgan et al., 2010). The detachment fault that was active Pluijm, 2011). If low-friction clay minerals are growing in the fault in the elasto-frictional regime clearly evolved from a well-defined mid- zone during slip, they will affect the frictional properties of the fault crustal mylonite zone spectacularly exposed in the footwall of the zone and possibly allow slip at low dips. brittle detachment (Snoke and Lush, 1984). The main detachment (“Secret-4”), an overlying brittle low-angle detachment in the hangingwall (“Secret-2”) and a high-angle normal fault that soles into the 3. Geological setting and sample locations main detachment (“Secret-1”) are all exposed along Nevada Rt 229 west of Secret Pass. The upper detachment and high-angle normal fault Low-angle normal faults and metamorphic complexes are that soles into the main detachment both juxtapose siltstones against found in a discontinuous band extending from southeastern footwall shales, while the main detachment juxtaposes zeolitized tuffs British Columbia in the north through the Cordillera and into the against silici fied carbonate mylonites in the footwall. A separate eastern Basin and Range to northwestern Mexico in the south. exposure of the detachment is found at Clover Hill (“Clover-1”), 5 km Exhumation occurred during the Eocene in the northern meta- SW of Wells, NV, on NV Rt 231, where the detachment juxtaposes morphic core complexes (north of the Idaho border) and during Miocene conglomerates in the hangingwall against carbonate the Oligocene and Miocene in the central and southern portions mylonites in the footwall. A comprehensive description of the outcrops of the belt (Axen et al, 1993). The youngest documented exhu- was given in Haines and van der Pluijm (2010). mation is in the Death Valley area where Quaternary low-angle faults are preserved (Hayman et al., 2003). As reconnaissance work for this study and previous studies indicates that many of 3.2. Death Valley area detachments the northern core complexes north of the Ruby Mountains in Nevada have brittle fault rocks that are dominated by lithified The Black Mountains on the east side of Death Valley and the cataclasites (e.g. Vanderhaeghe et al., 1999; Manning and Bartley, western flank of Panamint Mountains on the west side of Death 1994), this work focused on the southern metamorphic core Valley have a suite of spectacularly exposed low-angle normal complexes where clay gouges are frequently preserved and faults that have been the source of considerable controversy exposed. A total of 17 detachment faults were sampled for this (Fig. 2a). The faults record significant extension in the Death Valley study at 30 outcrops, collecting and characterizing a total of 59 area from the Miocene to Recent times, although both the timing gouge samples, 27 hangingwall samples and 40 footwall samples and magnitude of extension remain the subject of controversy (see Table 1,(Figs. 1 and 2). Miller and Pavlis, 2005 for a treatment of the issues). Three low-

Table 1 Sample locations and sample names used in this study.

Detachment fault Range Outcrop locality Outcrop name Latitude Longitude Amargosa detachment Black Mts, CA Klippe north of Ashford Canyon Ashford Klippe 34 100 800 113 400 4600 Exclamation rock Exclamation rock 35 550 1100 116 320 3900 Virgin Springs wash Virgin Springs W 35 560 300 116 340 56400 Badwater turtleback Black Mts, CA Unnamed wash Badwater-1 36 150 25" 116 460 29" Natural Bridge wash Badwater-2 36 170 11" 116 450 47" Buckskin-Rawhide detachment Buckskin Mts, AZ “A-bomb canyon” A-Bomb 34 100 800 113 400 4600 Buckskin-Rawhide detachment Plomosa Mts, AZ Plomosa Drive roadcut Plomosa 33 480 2400 114 50 400 Buckskin-Rawhide detachment Rawhide Mts, AZ Near Swansea town site Swansea 34 100 800 113 500 3800 Bullard detachment Harcuvar Mts, AZ South side of Bullard Peak Bullard 34 30 3300 113 170 1900 Bullard detachment Harquahalla Mts, AZ Ojo de Aguila peak Aguila 33 530 1600 113 100 4100 Central Mojave core complex Waterman Hills, CA Below Waterman Hills radio towers WH-68 34 580 1600 117 20 3100 Chemehuevi detachment Chemehuevi Mts, CA N. end Lobeck Pass Lobeck pass 34 410 2800 114 370 400 Copper Canyon turtleback Black Mts, CA Unnamed wash south of Copper Canyon Copper 36 70 2800 116 440 3400 Dante’s View fault Black Mts, CA Dante’s View road, mile 11.3 Dante 36 130 2700 116 420 3900 Emigrant fault Panamint Mts, CA Unnamed wash on west side Tucki Mt. Tucki 36 310 1200 117 110 1200 off Wildrose Canyon road Wildrose 36 140 1200 117 130 3400 Gregory Peak fault Black Mts, CA “Size 36 canyon” Size 36 35 570 5000 116 400 3400 Mormon Point turtleback Black Mts, CA W side Mormon Point Mormon-1 36 20 4500 116 450 3900 E side Mormon Point Mormon-2 36 20 3700 116 440 1200 E side Mormon Point Mormon-3 36 20 3100 116 440 2000 Mosaic Canyon fault Panamint Mts, CA Mosaic Canyon Mosaic 36 340 1200 117 80 1800 Panamint range front LANF Panamint Mts, CA South Park Canyon South Park 36 00 500 117 120 100 Ruby Mts core complex Ruby Mts, NV Rt 231 roadcut Clover-1 41 30 4800 115 10 2100 Rt 229 roadcut Secret-1 40 520 1200 115 150 3600 Rt 229 roadcut Secret-2 40 520 600 115 150 2200 Rt 229 roadcut Secret-4 40 510 5400 115 150 1700 Salton Sea detachment Santa Rosa Mts, CA Wonderstone Wash 571 33 200 4400 116 80 1200 Sierra Mazatan core complex Sierra Mazatan, Sonora, Mexico Near Rancho La Feliciana Maz-1 29 90 5800 110 130 5800 Whipple detachment Whipple Mts, CA E end Whipple wash Whip-3 34 210 5500 114 160 4400 E end Whipple wash Whip-5 34 220 800 114 170 1100 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 5

Fig. 1. Schematic map of the western US showing major tectonic features and the location of ranges with detachment faults sampled in this study and other detachment faults mentioned in the text. SS is Salt Springs fault. Areas of Fig. 2a and b are shown. Major faults are shown in grey. Redrawn and modiﬁed from Davis and Coney (1979). angle normal faults known as ‘the turtlebacks’ are exposed in the Mesoproterozoic schistose gneisses in the footwall and quartzites Black Mountains, along the east side of Death Valley (Curry, 1938; of the Mesoproterozoic Crystal Springs formation. Wright et al, 1974, Miller and Pavlis, 2005). The Badwater, Copper The Panamint Mountains are bounded on their west ﬂank Canyon, and Mormon Point turtlebacks are exposed along the east partially by a series of west-dipping low-angle normal faults in side of Death Valley, while the Gregory Peak detachment and the Panamint Valley that formed during the latter stages of exhuma- Amargosa Detachment (also known as the Amargosa Chaos (Noble, tion (Hodges et al., 1989, 1990). The Mosaic Canyon fault, which 1941) are exposed further to the south (Fig. 2). Each ‘turtleback’ has dips moderately to the east and bisects the northern portion of the a distinctive turtleshell-like form with an anticlinal carapace of Panamint Mountains, has been argued to be an overturned west- mylonitic mid-crustal rocks separated from a hangingwall of dipping normal fault that may be contiguous with the Emigrant unmetamorphosed Miocene to Recent sediments by a ductile to fault on the west side of the range. The Mosaic Canyon fault was brittle normal fault. The Badwater detachment was sampled at an sampled at Mosaic Canyon (“Mosaic”), where the fault juxtaposes unnamed wash near the southern edge of the detachment and at Neoproterozoic metasediments (Johnnie Quartzite in the hang- Natural Bridge. At both outcrops (“Badwater-1” and “Badwater-2”, ingwall and Noonday Dolomite in the footwall). The Emigrant fault respectively) the fault juxtaposes mylonitic granite and schist in the was sampled in two localities, in an unnamed wash of the west footwall against Pleistocene gravels and fanglomerates. The Copper side of the Panamint Mountains south of Black Point (“Tucki”), and Canyon turtleback was sampled at the base of the turtleback in the Wildrose Canyon area (“Wildrose”), south of the point (“Copper”), where the turtleback fault descends into where the Emigrant fault and another low-angle detachment, the PlioceneeRecent alluvium. The fault juxtaposes Pliocene Towne Pass fault, merge. At both outcrops, the fault juxtaposes conglomerates and Quaternary alluvium in the hangingwall against Pliocene conglomerates of the middle Pliocene Nova Formation Proterozoic gneisses and marbles in the footwall. The Mormon against Neoproterozoic metasediments in the footwall. Both the Point detachment was sampled at three localities, (“Mormon-1”, Mosaic Canyon fault and Emigrant fault lack mylonitic footwalls “Mormon-2” and “Mormon-3”). At all localities the hangingwall and evolved entirely in the brittle upper crust at low dips from consists of Plio-Pleistocene gravels, while the footwall lithology is 11.4 Ma to < 3.3 Ma, coeval with the deposition of the supra- quite variable. At Mormon-1, at the western side of Mormon Point, detachment Nova Basin (Hodges et al., 1990; Snyder and Hodges, the footwall consists of felsic gneiss and interlayered dolomite, 2000). A separate low-angle fault outcrops further south along while at Mormon-2 the footwall consists of chlorite-altered diorite. the east side of Panamint Valley, the Panamint Valley low-angle At Mormon-3, the footwall consists of chlorite-altered dioritic fault (Cichanski, 2000). This fault was sampled in South Park gneiss and interlayered dolomite. The Gregory Peak and Amargosa Canyon (“South Park”), where it locally dips 18 west and juxta- detachments both lack mylonites in the footwalls and apparently poses Pliocene and older Pleistocene conglomerates against were active entirely in the brittle regime. The Gregory Peak a footwall of Proterozoic schists. The Panamint Valley low-angle detachment fault sampled at “Size 36 canyon” separates a brecci- normal fault lacks a mylonitic footwall and was apparently active ated, unmylonitized granite in the footwall and Pliocene gravels in entirely in the brittle regime. the hangingwall. The Amargosa detachment, the basal fault in the Amargosa Chaos suite of low-angle normal faults in the southern 3.3. Colorado river extensional corridor Black Mountains was sampled in Virgin Springs Wash (“Virgins Springs W”), at Exclamation Rock (“Exclamation Rock”), and at The Colorado River extensional corridor is a 100 km 300 km a klippe north of Ashford Canyon (“Ashford Klippe”, a.k.a. “Scotty region of extreme Oligocene-Miocene extension in southeastern Klippe” of Noble, 1941). At both outcrops the fault juxtaposes California and west-central Arizona straddling the Colorado River 6 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32

Fig. 2. A.) Sketch geologic map of the Death Valley and Panamint Mountains area, CA showing sample locations for low-angle normal faults in the Black and Panamint mountains and selected tectonic elements. Redrawn after USGS Misc. Field Studies map MF-2370. B) Sketch geologic map of the Colorado River extensional corridor, CA and AZ, showing sample locations for low-angle normal faults sampled in this study. Redrawn and modiﬁed after Carter et al. (2006). S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 7

(Howard and John, 1987)(Fig. 2b). Approximately 50 km of NE-SW Paleozoic greenschist- and amphibolite-facies metasediments in directed extension was accommodated by a single master detach- both the hangingwall and footwall. ment fault, or suite of faults, that dips 10e20 northeast as the footwall rocks were drawn out from underneath upper crustal 3.5. Central Mojave metamorphic core complex rocks located to the northeast (Spencer and Reynolds, 1991). The region exposes five major low-angle detachment faults, two in The Central Mojave metamorphic core complex is exposed in California (Chemehuevi detachment and the Whipple detachment) the Waterman Hills, Mitchell Range and Newberry Mountains of and three along strike in west-central Arizona (the Buckskin- southeastern California (Fig. 1). The detachment accommodated Rawhide detachment, the Harcuvar detachment and the Harqua- 40e50 km of NE-directed extension during the early Miocene hala detachment). Smaller sub-parallel detachments are frequently (Walker et al., 1990). The detachment fault trace trends roughly found above and below the main detachment at each core complex. northwest-southeast and dips gently to the east. The northern and All the detachment faults have a dip <30, a top-to-the-northeast central reaches of the fault expose greenschist-facies mylonites, sense of shear and common timing of exhumation from 23 to while the southern portion of the fault in the Newberry Mountains 12 Ma (e.g. Davis, 1988; John and Foster, 1993; Foster et al., 1993; lacks a mylonitic footwall and probably evolved entirely in the Carter et al, 2006), and it has been proposed that all the major brittle regime (Fletcher et al., 1995). The detachment fault was exhumed footwalls are range-scale “mega-mullions” of a single sampled in the northern Waterman Hills (outcrop “WH-68”) where detachment system (Spencer and Reynolds, 1991). The footwalls of it juxtaposes Tertiary rhyolites in the hangingwall and mylonitic the detachments are typically Proterozoic gneisses and schists, and granodiorite in the footwall. The temporal and kinematic rela- Cretaceous and Miocene plutonic rocks. Footwall rocks are mylo- tionship of the brittle fault to the mylonitic detachment is complex nitic in the Whipple, Buckskin-Rawhide, Harcuvar and Harquahala in that kinematic indicators associated with the brittle fault records Mountains, and unmylonitized in the Chemehuevi and Sacramento north-south extension as opposed to the northeast-southwest Mountains and smaller ranges to the north, indicating that the extension recorded by the mylonites (Anderson, 2007). central and southern portions of the Colorado River extensional corridor evolved from mid-crustal shear zones, while the northern reach was active entirely under brittle conditions and may have 3.6. Sierra Mazatán metamorphic core complex initiated at or near current dips <20 (John and Foster, 1993; Miller and John, 1999). The Chemehuevi detachment was sampled on the Sierra Mazatán is the southernmost Cordilleran metamorphic northwest side of Lobeck Pass (“Lobeck Pass”) where the fault core complex, (Anderson et al., 1980a, b; Nourse et al, 1994; Gans, juxtaposes a post 18.5 Ma monomict megabreccia (Miller and John, 1997; Wong and Gans, 2008; Fig. 1). The complex has a character- 1999) against a chloritized, but unmylonitized 19.1 Ma granite istic metamorphic core complex geometry, with a topographically (Foster et al, 1996). high shield-like lower plate of mid-crustal Paleocene (58 þ/ 3Ma Faults kinematically linked to the Whipple detachment were UePb age; Anderson et al., 1980a, b) granites and minor gneisses, sampled at the northeast corner of the core complex near where separated from Cretaceous metavolcanics and unmetamorphosed Whipple Wash reenters the hangingwall of the core complex. The Tertiary sedimentary strata by a shallowly west-dipping complex main detachment is not well exposed in the east, but high-angle fault zone with a plastic-to-brittle deformation history (Vega normal faults that sole into the main fault are well-exposed and Granillo and Calmus, 2003; Wong and Gans, 2003). The onset of contain clay gouge (“Whip-3” and “Whip-5”). The faults both exhumation at Sierra Mazatán is well constrained by K-feldspar juxtapose Miocene silicified monolithic breccias of the War Eagle 40Ar/39Ar ages, which indicate initial extension from 25 to 23 Ma, mega-landslide against chloritized, but unmylonitized granites of followed by rapid west-directed exhumation, beginning at 20 Ma in probable Cretaceous age that were tectonically incorporated into the east, and at 18 Ma in the west. The end of exhumation is con- the upper plate during slip (Davis et al., 1980). Mylonitized foot- strained by a 14.9 Ma age of clay gouge in the detachment fault wall rocks are abundant 100 m below the contact of footwall and (Haines and van der Pluijm, 2008) and a 12.4 Ma ignimbrite hangingwall lithologies. The BuckskineRawhide detachment was outcropping on the mylonitic carapace, indicating the mylonites sampled at three sites, at two poor outcrops near the ghost town were exposed at the surface by that time (Vega Granillo and of Swansea (“Swansea”) and in the Plomosa Range (“Plomosa”), Calmus, 2003; Wong and Gans, 2003). The detachment fault is and at “A-Bomb Canyon” (locality 7 of Spencer and Reynolds, 1986) exposed on the northwest side of the core complex at the base of where the fault juxtaposes a granite in the upper plate over Cerro Pelon (“Maz-1”). The gouge zone is 0.5e2 m thick, and a chlorite tectonic breccia in the footwall. The Harcuvar detach- separates Cretaceous metavolcanics from weakly mylonitized and ment (the Bullard detachment; cf. Reynolds and Spencer, 1985) extensively fractured Paleocene granite. The gouge is a yellowish- was sampled north of Bullard Peak (“Bullard”), where the brown hard clayey gouge with a crude visible fabric, and contains detachment places Miocene conglomerates over upper Cretaceous rounded and abraded clasts of granite and grains of quartz and to lower Tertiary variably mylonitized granitic rocks (Spenser and feldspar in a matrix of very fine-grained illitic clay. Reynolds, 1985). The Harquahala detachment was sampled on the In summary, the low-angle normal faults sampled in this study west slope of Eagle Eye Peak (“Aguila”) where the fault places are: 1) Faults that demonstrably evolved from mid-crustal mylo- a monomict silicified breccia over chloritized and variably mylonitic detachments (the Ruby Mountains Detachment, Whipple nitized schists. Mountains Detachment, BuckskineRawhide Mountains Detach- ment, Bullard Detachment in the Harquahala, and the Harcuvar 3.4. West Salton detachment fault Mountain Detachment); 2) Faults that evolved entirely in the brittle regime (the Mosaic Canyon fault, Emigrant fault, Panamint Valley The West Salton detachment fault is one of a suite of east- low-angle normal fault, the Gregory Peak and Amargosa Detach- dipping Pliocene and Pleistocene low-angle normal faults on the ments in the Black Mountains, the Chemehuevi Detachment, and western side of the Miocene-to-Recent Salton Trough, which forms the West Salton Detachment), and 3) Faults where the relationship the northern end of the Gulf of California (Axen and Fletcher, 1998; of the brittle detachment to underlying mylonites may be complex Shirvell et al., 2009; Fig. 1)). The detachment was sampled in (Black Mountains turtlebacks and the Waterman Hill exposure of Wonderstone Wash (outcrop “571”) where the fault juxtaposes the central Mojave metamorphic core complex detachment). 8 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32

3.7. Metasomatic chloritic footwall alteration chlorite was established using similar random powder sample preparations as for illitic samples, but as the polytype-specific Chlorite metasomatic alteration and brecciation of the footwall peaks are much stronger than for illite, samples were scanned extending for meters to tens to hundreds of meters below the from 4 to 40 2Q at a scan rate of 1/minute, following the proce- detachment fault surface is a common feature of metamorphic core dures of Brindley and Brown (1980). The composition of chlorite complex detachment faults (e.g. Crittenden et al., 1980; Kerrich, was estimated from peak intensities from the same patterns using 1988). Chlorite alteration is found at all of the detachments the technique of Brindley and Brown (1980). sampled in this study that are thought to have evolved from mid- To determine the relative abundance of the various polytypes of crustal shear zones where foot lithologies are dioritic or granitic in illite (the 2M1 and 1Md polytypes) in each size fraction of illitic composition (Badwater, Copper Canyon, and Mormon Point turtle- gouge identified by XRD scans of the oriented mounts, diffraction backs, Whipple detachment, Buckskin-Rawhide detachment, patterns from random XRD patterns of the same material were Ó Harcuvar detachment, Harquahala detachment) as well as at the modeled using WILDFIRE . This program calculates three- Chemehuevi detachment, which mostly lacks mylonites in the dimensional X-ray diffraction patterns for randomly oriented footwall, although some are described locally (Campbell-Stone et al., grains and allows the user to vary many mineralogic properties to 2000). Mylonitic marbles that are locally present in the footwalls of capture the variability of structure ordering in illites, along with the Black Mountains, and that are intercalated with extensively varying the thickness of the diffracting crystallites, the randomness chloritized gneisses, are unaltered. The breccias contain a distinctive of the sample (also known as the Dollase factor), the percentage of assemblage of chlorite þ epidote þ K-feldspar þ calcite þ sphene interlayered smectite, its hydration state and any ordering of illite/ that overprints mylonitic fabrics and imparts a distinctive greenish smectite (Reichweite) (Reynolds, 1993). This multitude of options color to the rocks. Isotopic and fluid inclusion studies indicate the allows for better matching of powder patterns than previous alteration forms at greenschist-facies metamorphic conditions approaches that used empirical ratios of peak areas derived from (300e350 C, Kerrich, 1988; and 350e520 C, Morrison and a set of standards (see summary in Dalla Torre et al., 1994). Anderson, 1998)byaninflux of Fe-, Mg- and Mn-rich meteoric or igneous fluids (Smith et al., 1991; Morrison and Anderson, 1998). 5. Results

4. Methods Thirty separate fault outcrops were sampled, collecting a total of 59 gouge samples as many outcrops had two or three different 4.1. Sampling and clay separation gouge horizons visible at outcrop. Results from XRD identification of the clay minerals present in both the various horizons of the About 0.5 kg of clay gouge material was collected at each gouge and the wall rocks of the various detachment faults are detachment fault exposure, sampling visibly distinct regions presented in Table 2. All but one of the outcrops sampled contained separately. Samples were disaggregated by soaking in de-ionized significant clay minerals in at least one horizon. An outcrop of the water and suspended repeatedly until they were free of salts. The Buckskin-Rawhide detachment near Swansea, AZ (“Swansea”) had clay-sized material (<2 mm) was separated using Stoke’slaw granular material that resembled gouge in the field but XRD indi- techniques. The accumulated clay fraction was centrifuged into cated the material between the footwall and hangingwall to be three different size fractions, (2 mme0.4 mm, 0.4 mm0.05 mm e and a mixture of gypsum and Fe oxides. Another outcrop of the <0.05 mm) spanning roughly two orders of magnitude in grain size, Buckskin-Rawhide Mountains detachment in the Plomosa range to obtain both coarse-grained and very fine-grained size fractions. (“Plomosa”) contained clays, but it could not be definitively established in the field that the clays were derived from fault gouge 4.2. X-ray diffraction and not soils. 29 of the gouge samples from a total of 23 outcrops were then selected for separation into size fractions. All samples To characterize the main clay minerals present in each sub- separated into <0.05 mm fractions contained one of five clay region of a fault exposure, oriented clay slurry mounts of the minerals: the 1Md polytype of illite, illite-rich illiteesmectite, <2 mm fraction were prepared. Samples were scanned from 2 to 35 discrete smectite, chlorite, or chloriteesmectite. Nearly all of 2Q (Cu-ka) at a scan rate of 1/minute both air dried and after these <0.05 mm size fractions were dominated by only one of the ethelyne glycol salvation for 24 h. Samples were scanned using five clays. (Table 3). 15 of the 29 fine size fractions sampled were a Scintag X1 Q-Q powder diffractometer with an accelerating dominated by authigenic 1Md illite, with illite-rich I/S, smectite, voltage of 40.0 kV, and a filament current of 35.0 mA. The nature of and chlorite dominating the remainder in roughly equal numbers, illiteesmectite (I/S) and the effect of any ordering (R0, R1, etc) were five illite-rich I/S, four smectite and four chlorite. Chloriteesmectite determined using the program NEWMOD (Reynolds and Reynolds, was observed dominating the fine fraction in one sample. Only one 1996). NEWMOD calculates 1-dimensional XRD patterns for (00l) detachment outcrop with well-developed clay gouge (Mormon-2) reflections of illite and illiteesmectite (I/S) and allows the user to showed no evidence for a significant clay mineral transformation in vary the crystallite size, the composition of the clay phase, the any region of the gouge. The pervasive change in clay mineralogy percentage of illite in an illiteesmectite (I in I/S) and the hydration from the relatively coarse, relict wall-rock-dominated 2.0e0.4 mm state of that smectite, as well as any ordering in the I/S. Once the size fraction to the fine, authigenic-dominated <0.05 mm size principal clay minerals were identified, material was separated into fraction of at least one gouge horizon at nearly all the fault gouges size fractions as described above, and illitic and chloritic material sampled indicates that clay minerals in the gouges had undergone was selected for polytypism analysis. As the polytype-specific significant transformations. peaks used to establish the polytypism of illite and chlorite are non-00l peaks and are suppressed by the oriented mounts used to 5.1. Chloritic gouges identify the main clay minerals present, random sample preparations of the same material using a side-loaded sample packer Six outcrops sampled (Mormon-2, Mormon-3, Copper, Lobeck (Moore and Reynolds, 1997) were used to accentuate the non-00l Pass, A-Bomb Canyon, and Whip-5) have a chlorite-dominated peaks. Illitic samples were step-scanned from 16 to 44 2Q with gouge layer in at least part of the fault zone that has a distinc- a step size of 0.05 and a count time of 40 s per step. Polytypism of tive ‘scaly’ texture and that is frequently greenish in color. The Table 2 Table showing mineralogy as determined by XRD of all samples in this study. C ¼ chlorite, C/S ¼ chlorite/smectite, R0 ¼ disordered, R1 ¼ ordered, I ¼ illite, I/S ¼ illite/smectite, R ¼ Reichweit (R0, R1, etc), Sm ¼ smectite, Sep ¼ sepiolite, Pal ¼ palygorskite, T ¼ talc, L ¼ lizardite, K ¼ kaolinite, Q ¼ quartz, KF ¼ K-feldspar, P ¼ plagioclase, A ¼ analcime, Ca ¼ calcite, D ¼ dolomite, G ¼ gypsum, H/C ¼ heulandite/clinoptilite. Bold type indicates predominant phase. Polytype abundance and ordering of I/S is quanti fied by XRD modeling, non-clay abundance is estimated visually.

Detachment fault Outcrop Sample Size fraction Clay minerals Non-clay minerals C C/S Ordering I Polytype I/S (R) Sm Sep Pal T L K Q KF Pl A Ca D G H/C Amargosa detachment Ashford Klippe Hanging wall <2.0 mmx xX xx x Gouge 2.0e0.4 mmx2M1/1Md x X xx xx Gouge 0.4e0.05 mm X 2M1/1Md xxxx Gouge <0.05 mm X1Md Altered footwall <2.0 mm X x

Exclaimation rock Hanging wall Crushed whole rock X Gouge 2.0e0.4 mmx2M1/1Md X x Gouge 0.4 e0.05 mm X2M1/1Md x Gouge <0.05 mm X 2M /1M 1 d 2 (2012) 43 Geology Structural of Journal / Pluijm der van B.A. Haines, S.H. Clayey breccia <2.0 mm X xx Footwall Crushed whole rock x x 2M 1 X xx

Virgin Springs W Hanging wall Crushed whole rock X Greenish gouge <2.0 mm X x Reddish-brown gouge <2.0 mm X x Brownish gouge 2.0 e0.4 mmx2M1/1Md X x Brownish gouge 0.4 e0.05 mm X2M1/1Md x Brownish gouge <0.05 mm X2M1/1Md Brownish gouge <0.03 mm X 2M1/1Md Footwall Crushed whole rock x x 2M 1 X xx Badwater turtleback Badwater-1 Hanging wall <2.0 mm x X Gouge 2.0 e0.4 mmx x1Md X x Gouge 0.4 e0.05 mmx x1Md X x Gouge <0.05 mm X1Md Footwall <2.0 mm X x Footwall diabase Crushed whole rock x x X x

Badwater-2 Hanging wall <2.0 mmx x2M1 X xxx Gouge 2.0 e0.4 mmx2M1/1Md X xx Gouge 0.4 e0.05 mmx2M1/1Md X Gouge <0.05 mmx2M1/1Md x Footwall <2.0 mm X2M1 xx Footwall mylonite Crushed whole rock x X x e

Footwall diabase Crushed whole rock x x X x 32

Buckskin-Rawhide detachment A-Bomb H/W breccia 2.0e0.4 mmx x2M1 X xx H/W breccia 0.4 e0.05 mm X H/W breccia <0.05 mm X Upper red gouge <2.0 mmxR0 X Lower green gouge <2.0 mmxR0 X Footwall Crushed whole rock x X x

Buckskin-Rawhide detachment Plomosa Altered footwall <2.0 mmx2M1 X xx Buckskin-Rawhide detachment Swansea Veins <2.0 mm x X Altered footwall <2.0 mm X xx Altered footwall <2.0 mm X xx x Bullard detachment Bullard Hanging wall <2.0 mm x R0 x 2M 1 xx x Upper gouge <2.0 mmxR0X xxx Lower gouge 2.0e0.4 mmxR0X2M1/1Md xx (continued on next page ) 9 Table 2 (continued ) 10

Detachment fault Outcrop Sample Size fraction Clay minerals Non-clay minerals C C/S Ordering I Polytype I/S (R) Sm Sep Pal T L K Q KF Pl A Ca D G H/C

Lower gouge 0.4e0.05 mm X2M1/1Md x Lower gouge <0.05 mmxR0X 2M1/1Md xx Footwall <2.0 mm X R0 x 2M 1 xxxxxx Bullard detachment Aguila Hanging wall <2.0 mm X Upper gouge <2.0 mm X xx Center gouge <2.0 mmxx X x Lower gouge <2.0 mmxx X Footwall breccia Crushed whole rock x X xx Footwall Crushed whole rock x x 2M 1 X Central Mojave core complex WH-68 Hanging wall breccia <2.0 mmxxX x Hanging wall breccia <2.0 mmxxxX Grey gouge 2.0e0.4 mmx1Md X x ..Hie,BA a e lim/Junlo tutrlGooy4 21)2 (2012) 43 Geology Structural of Journal / Pluijm der van B.A. Haines, S.H. Grey gouge 0.4 e0.05 mm X1Md x Grey gouge <0.05 mm X1Md Brownish gouge <2.0 mmxX Brownish gouge 2.0e0.4 mmxX x X Brownish gouge <0.4 mmxX x

Chemehuevi detachment Lobeck Pass Hanging wall Crushed whole rock X xx Tan crust <2.0 mmx X xx x Reddish gouge <2.0 mm X R0 x x x x Reddish gouge <2.0 mm X R0 x x x Reddish gouge <2.0 mm X R0 x x x x Green gouge Crushed whole rock x X xx Green gouge 2.0e0.4 mm X xx Green gouge 0.4e0.05 mm X xx Green gouge <0.05 mm X Footwall Crushed whole rock x X xxx Footwall <2.0 mm X xR1x

Copper Canyon turtleback Copper Hanging wall 2.0e0.4 mmx X2M1 x Gouge 2.0e0.4 mmxX R1 x Gouge 0.4e0.05 mmxX R1 Gouge <0.05 mm X R0 Footwall Crushed whole rock x X x

Dante’s View fault Dante Hanging wall Crushed whole rock x x X Tan horizon Crushed whole rock x X x e

Upper gouge 2.0e0.4 mm X xx32 Upper gouge 0.4e0.05 mm X xx Upper gouge <0.05 mm X xx Lower gouge 2.0 e0.4 mm X xx Lower gouge 0.4e0.05 mm X xx Lower gouge <0.05 mm X xx Footwall Crushed whole rock X xx x

Emigrant Fault Tucki Hanging wall <2.0 mmx X2M1 xx Upper gouge <2.0 mm X x Lower gouge 2.0e0.4 mm X2M1/1Md x Lower gouge 0.4e0.05 mm X2M1/1Md x Lower gouge <0.05 mm X 2M1/1Md x Altered footwall <2.0 mmx xX Footwall <2.0 mm X2M1 x

Emigrant Fault Wildrose Hanging wall <2.0 mmx X2M1 x X xx Upper gouge 2.0e0.4 mmx xR3 xX xx Upper gouge 0.4e0.05 mm XR3 x Upper gouge <0.05 mm XR3 tr x Lower gouge <2.0 mm XR3 Footwall schist Crushed whole rock X xx

Gregory Peak fault Size 36 Hanging wall <2.0 mmxX xxx x Hanging wall <2.0 mmxX xxx x Upper gouge <2.0 mm X1Md xx Lower gouge 2.0e0.4 mmx x2M1/1Md X xx Lower gouge 0.4 e0.05 mmx2M1/1Md X xx Lower gouge <0.05 mm X1Md Footwall breccia 2.0e0.4 mmx x1Md xx X Footwall breccia 0.4 e0.05 mmx x1Md x X Footwall breccia <0.05 mm X1Md Footwall Crushed whole rock x X xx

Mormon Point turtleback Mormon-1 Hanging wall <2.0 mmxX x

Gouge 2.0e0.4 mmxxxxX x 2 (2012) 43 Geology Structural of Journal / Pluijm der van B.A. Haines, S.H. Gouge 0.4e0.05 mmxxxX xx Gouge <0.05 mm XR0 tr Footwall Crushed whole rock x X xx

Mormon Point turtleback Mormon-2 Hanging wall Crushed whole rock x 2M 1 X xx Green gouge Crushed whole rock x X xx Reddish gouge <2.0 mm X x Green gouge 2.0e0.4 mm X Green gouge 0.4 e0.05 mm X Green gouge <0.05 mm X Footwall Crushed whole rock x X xx Footwall Crushed whole rock x X xx Footwall Crushed whole rock x X xx Footwall Crushed whole rock X xx Footwall Crushed whole rock x X xx Footwall Crushed whole rock x X x Footwall Crushed whole rock x X xx

Mormon Point turtleback Mormon-3 Hanging wall Crushed whole rock x 2M 1 X xx Green gouge 2.0 e0.4 mm X Green gouge 0.4 e0.05 mm X Green gouge <0.05 mm X Whitish gouge 2.0 e0.4 mmx X xxx Whitish gouge 0.4e0.05 mm X xxx Whitish gouge <0.05 mm X xxx e

Footwall Crushed whole rock x x 2M 1 X xx 32 Mosaic Canyon fault Mosaic Hanging wall Crushed whole rock X Gouge <2.0 mmx X tr x Gouge 2.0e0.4 mm X2M1/1Md tr tr Gouge 0.4e0.05 mm X2M1/1Md tr Gouge <0.05 mm X 2M1/1Md tr Gouge <2.0 mmx X tr x Footwall Crushed whole rock X

Panamint range front LANF South Park Hanging wall Crushed whole rock x 2M 1 X xx Upper gouge <2.0 mm X1Md x Central gouge 2.0 e0.4 mmx1Md x X xx Central gouge 0.4 e0.05 mm X 1Md xx x Central gouge <0.05 mm X1Md (continued on next page ) 11 Table 2 (continued ) 12

Detachment fault Outcrop Sample Size fraction Clay minerals Non-clay minerals C C/S Ordering I Polytype I/S (R) Sm Sep Pal T L K Q KF Pl A Ca D G H/C

Altered footowall <2.0 mm X1Md xx Footwall Crushed whole rock X2M1 x

Ruby Mts core complex Clover-1 Hanging wall Crushed whole rock x 2M1/1Md x X Brownish gouge 2.0 e0.4 mmx2M1/1Md Brownish gouge 0.4e0.05 mmx2M1/1Md Brownish gouge <0.05 mm X1Md Brown/yellow gouge contact <2.0 mmtr X xx Yellowish gouge <2.0 mm X xx Footwall Crushed whole rock X

Ruby Mts core complex Secret-1 Hanging wall 2.0e0.4 mmxR3xxX Hanging wall 0.4e0.05 mm XR3 xtrx Hanging wall <0.05 mm XR0 tr ..Hie,BA a e lim/Junlo tutrlGooy4 21)2 (2012) 43 Geology Structural of Journal / Pluijm der van B.A. Haines, S.H. Greenish gouge 2.0e0.4 mm X R1/R3 xx Greenish gouge 0.4 e0.05 mm X R1/R3 tr Greenish gouge <0.05 mm XR0 Brownish gouge <2.0 mmxXR1&R3 xx x Footwall 2.0e0.4 mm X 2M1/1Md xx x Footwall 0.4e0.05 mm X 2M1/1Md x Footwall <0.05 mm X1Md

Secret-2 Hanging wall <2.0 mmxR3xxX Upper grn gouge <2.0 mm XR1 xxxx Middle gouge <2.0 mm XR1 xxx x Lower gouge 2.0e0.4 mm x R1/R3 x X x Lower gouge 0.4e0.05 mm XR1 xx x Lower gouge <0.05 mm XR0 Footwall <2.0 mm X 2M1/1Md xx x Ruby Mts core complex Secret-4 Hanging wall Crushed whole rock X Upper brn gouge <2.0 mm X xx x Greenish gouge <2.0 mm X Black gouge 2.0e0.4 mm x R3 x X x Black gouge 0.4e0.05 mm XR1 x Black gouge <0.05 mm XR0 Footwall Crushed whole rock X xx

Salton Sea detachment 571 Gouge <2.0 mmx x X tr x x e

Sierra Mazatan core complex Maz-1 Gouge 2.0e1.0 mmx2M1/1Md X x 32 Gouge 1.0 e0.5 mmx2M1/1Md X x Gouge 0.5 e0.1 mm X2M1/1Md tr Gouge 0.1 e0.05 mm X 2M1/1Md Gouge <0.05 mm X 2M1/1Md Whipple detachment Whip-3 Hanging wall Crushed whole rock X x Reddish gouge <2.0 mm X xxxx Cataclasite Crushed whole rock xx X Greenish gouge <2.0 mm X xxx Footwall Crushed whole rock x x X x

Whip-5 Hanging wall Crushed whole rock x x X x Reddish gouge <2.0 mm X x x Greenish gouge 2.0e0.4 mm X x1Md tr tr Greenish gouge 0.4e0.05 mm X x1Md Greenish gouge <0.05 mmx X1Md Footwall Crushed whole rock x X xx S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 13

Table 3 XRD analysis of the fine fraction (<0.05 mm) of the 29 gouge samples separated into size fractions. Note that all samples contain one of four clay minerals in the finest size fraction which concentrates authigenic minerals. C ¼ chlorite, C/S ¼ chlorite/smectite, R0 ¼ disordered, R1 ¼ ordered, I ¼ illite, I/S ¼ illite/smectite, R ¼ Reichweit (R0, R1, etc), Sm ¼ smectite, Sep ¼ sepiolite, Pal ¼ palygorskite, T ¼ talc, L ¼ lizardite, K ¼ kaolinite, Q ¼ quartz, KF ¼ K-feldspar, P ¼ plagioclase, A ¼ analcime, Ca ¼ calcite, D ¼ dolomite, G ¼ gypsum, H/C ¼ heulandite/clinoptilite. Bold type indicates predominant phase. Polytype abundance and ordering of I/S is quantified by XRD modeling, non-clay abundance is estimated visually.

Detachment fault Outcrop Sample Clay minerals Non-clay minerals

Chl C/S Ordering ill p-type I/S (R) Sm Sep Pal T L K Q KF Pl A Ca D G H/C

Amargosa detachment Ashford Klippe Gouge X1Md Amargosa detachment Exclamation Rock Gouge X 2M1/ 1Md Amargosa detachment Virgin Springs W Brownish gouge X2M1/ 1Md Badwater turtleback Badwater-1 Gouge X1Md Badwater turtleback Badwater-2 Gouge X 2M1/ x 1Md Buckskin-Rawhide A-Bomb Breccia X detachment

Bullard detachment Bullard Upper gouge x R0 X 2M1/ xx 1Md Central Mojave core WH68 Grey gouge X1Md complex Central Mojave core WH68 Altered footwall X x complex Chemehuevi Lobeck Pass Gouge X detachment Copper Canyon Copper Gouge X R0 turtleback Dante’s View fault Dante Upper gouge X xx Dante’s View fault Dante Lower gouge X xx

Emigrant Fault Tucki Lower gouge X 2M1/ x 1Md Emigrant Fault Wildrose Upper gouge XR3 tr x

Gregory Peak fault Size 36 Lower gouge X1Md Gregory Peak fault Size 36 Footwall breccia X1Md Mormon Point Mormon-1 Gouge XR0 tr turtleback Mormon Point Mormon-2 Green gouge X turtleback Mormon Point Mormon-3 Green gouge X turtleback Mormon Point Mormon-3 Lower gouge X xxx turtleback

Mosaic Canyon Mosaic Gouge X 2M1/ tr fault 1Md Panamint range South Park Gouge X1Md front LANF

Ruby Mts core Clover-1 Brownish gouge X1Md complex Ruby Mts core Secret-1 Greenish gouge XR0 complex Ruby Mts core Secret-2 Gouge XR0 complex Ruby Mts core Secret-4 Gouge XR0 complex

Sierra Mazatan Maz-1 Gouge X 2M1/ core complex 1Md Whipple detachment Whip-5 Greenish gouge x X1Md

‘scaly’ gouge is usually in contact with chlorite-metasomatized Pass, the lower half of the fault zone in contact with chlorite- and brecciated footwall (see Fig. 3a, b for photos). The metasomatized footwall consists of green scaly gouge. The clay Mormon-2 outcrop preserves a pristine scaly gouge in its fraction of the green scaly gouge consists entirely of chlorite entirety, while the other ﬁve fault outcrops that have a green (Fig. 4). Separation of the green scaly gouge at both localities into scaly gouge layer all evidence clay mineral transformations either size fractions indicates that the scaly gouge at both Mormon-2 within another layer of the gouge zone (at A-Bomb Canyon, and Lobeck Pass consists almost exclusively of chlorite in all Mormon-3, and Lobeck Pass) or within the green scaly gouge size fractions (see Fig. 4). XRD powder patterns of crushed scaly layer itself (at Copper and Whip-5). At Mormon-2, a 1.0 m thick gouge at Mormon-2 and at Lobeck Pass indicate that both scaly gouge zone has visibly distinct reddish and greenish layers at gouges are dominated by an assemblage of IIb-polytype chlorite, outcrop (see Fig. 3a). XRD analysis indicates that the reddish and quartz, feldspar, and minor amphibole (see Fig. 4). When the greenish layers have almost identical chlorite compositions; the diffraction patterns of crushed scaly gouge and chlorite-altered differences in color can be attributed to differing oxidation states footwall at both Lobeck Pass and Mormon-2 are compared, of Fe oxy-hydroxides on grain surfaces (see Table 2). At Lobeck they are remarkably similar (see Fig. 4), indicating that the scaly 14 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32

Fig. 3. Outcrop photographs of several detachment faults sampled in this study. A.) Mormon Point turtleback (Mormon-2). B.) Chemehuevi detachment, (Lobeck Pass). C.) Copper Canyon turtleback (Copper). D.) Buckskin-Rawhide detachment (A-Bomb Canyon). E.) Mormon Point turtleback, (Mormon-3). Box shows area of f.) F.) Close-up photo of detachment outcrop at Mormon-3. G.) Ashford Klippe. H.) Whip-5. I.) Mosaic Canyon. Arrow indicates full-sized backpack for scale. J.) Badwater Turtleback (Badwater-1). K.) Dante Peak detachment (“Dante”). L.) Waterman Hills detachment (WH-68). gouge is derived from cataclasis of the chlorite-altered footwall between chlorite in the footwall and chloritic gouge indicates that without significant mineralogical transformation. scaly chloritic gouge horizons are almost entirely fragmented in To further demonstrate the common origin of the chlorite in the origin and are not the result of chlorite growth in the fault zone green scaly gouges and footwall, the compositions of chlorite in during brittle deformation. the footwall and in chloritic gouge at A-Bomb Canyon, Copper Canyon, Lobeck Pass, Mormon Point and the Whipple detachment 5.2. Chlorite alteration gouges are estimated from (00l) intensities using the technique of Brown and Brindley (1980). Chlorites in the footwall are somewhat Fe- Outcrops at 3 of the 6 detachment faults with a scaly chloritic rich ((Mg1e1.5,Fe4.5e5)6[Si, Al)8O20](OH)16 and chlorite from chlo- gouge layer (A-Bomb Canyon, Lobeck Pass and Mormon-3) contain ritic gouge at A-Bomb Canyon, Mormon-2, Mormon-3 and the at least one other gouge layer that is compositionally distinct and Whipple outcrop is essentially identical in composition to that in contains neoformed clay minerals derived from the fragmented the footwall in all size fractions (Fig. 5). Chlorite in the finest size chlorite preserved in the scaly layer (See Figs. 3 and 6). At Lobeck fractions at Lobeck Pass expands slightly on glycolation and is Pass, a 20 cm thick reddish gouge layer is found directly above the slightly more Mg-rich, consistent with some alteration of frag- scaly chloritic layer (see Fig. 3b). The reddish layer contains chlorite mented chlorite to Mg-rich chlorite-rich chloriteesmectite in the and tri-octahedral smectite (saponite). The (001) reflection of the finer size fractions. At Copper Canyon, the chlorite in the gouge is chlorite in the clay fraction is stronger than the (002) reflection, significantly more Mg-rich than that in the footwall as a conse- indicating some alteration of the chlorite towards a vermiculite-like quence of later transformation to corrensite and composition (Moore and Reynolds,1997), and the chlorite peaks are chloriteesmectite, and will be discussed further below. The simi- narrow (FWHM ¼ 0.17e0.26) indicating a large diffracting domain larity of bulk mineralogy, chlorite composition and polytypism size. The reddish gouge is a mixture of variably altered fragmented S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 15

Fig. 3. (continued). chlorite and authigenic saponite. The reddish layer is itself bisected The yellowish layer consists of an Mg-rich clay mineral assemblage by 5e10 cm thick hard tan horizon. XRD indicates the tan layer to be of saponite (tri-octahedral smectite) and sepiolite with minor predominantly palygorskite with minor chlorite, quartz and calcite amounts of lizardite, talc and chlorite (Fig. 8). A 9.3e9.4 Å basal (Fig. 6). As the palygorskite-rich layer is cut by striated Reidel spacing and a narrow (001) peak width (full-width half- fracture surfaces consistent with ENE-directed normal slip, the maximum ¼ 0.16) for the talc phase indicates the phase is talc, palygorskite grew before the end of fault slip. Palygorskite is an Mg- sensu stricto and not kerolite (Mg3Si4O10(OH)4$nH2O), which is and Al-rich phyllosilicate with a fibrous morphology that has been often associated with Mg-smectites and sepiolites in evaporative reported in brittle faults only twice previously (García-Romero lacustrine environments (Brindley et al., 1977). Chlorite is only et al., 2006; Jacobs et al., 2006). At the A-Bomb Canyon exposure present in the coarse size fraction of the Mg-rich layer whereas the of the Buckskin-Rawhide detachment (Fig. 3c) a 50 cm thick scaly other Mg-rich minerals are all present in the medium and fine size chloritic breccia layer is preserved at the contact with the hang- fractions (Fig. 8), indicating that the chlorite is fragmented and ingwall granite, while a 20 cm-thick lower layer of the gouge derived from the green scaly gouge, while the sepiolite, saponite, outcrop consists predominantly of smectite; chlorite is present in lizardite and talc are all authigenic. As the nearest ultramafic body small amounts in the coarse size fraction of the lower layers, but is that could be a source for detrital fragmented lizardite is in the absent in the <0.4 mm size fraction. The smectitic gouge has shear foothills of the western Sierra Nevada, >100 km from the Mormon indicators consistent with local dextral shear, indicating the smec- Point turtleback, a fragmented origin for the lizardite is unlikely. tite formed prior to the end of slip on the detachment. A (060) The nearest known outcrop of talc-rich rocks to Mormon Point are reflection at 1.53 Å indicates that the smectite is tri-octahedral and found in the contact aureole of the Kingston Peak pluton, 73 km to is saponite (Fig. 7). At Mormon-3, a 10 cm to 1.0 m thick scaly the southeast (Calzia et al.,1987), and a relict wall-rock origin for the chloritic gouge layer with clear folding structures is preserved along talc is thus similarly improbable. the contact with the hangingwall, and a 20e50 cm thick hard waxy The other two outcrops have greenish gouges with scaly textures yellowish layer is found along the contact with the footwall (Fig. 3a). identical in the field tothose described above, but more complex clay 16 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32

Fig. 4. XRD patterns from random preparations of scaly gouge at Mormon-2 and Lobeck Pass separated into size fractions A.) Mormon-2, crushed whole gouge and crushed footwall. B.) Lobeck Pass, crushed whole green scaly gouge and footwall. C.) <2 mm fraction of Mormon-2 separated into size fractions. D.) <2 mm fraction of green scaly gouge at Lobeck Pass separated into size fractions C ¼ chlorite, K ¼ K-feldspar, Ca ¼ calcite, filled circles are plagioclase peaks. mineral assemblages. At Copper Canyon, a 2e5 m thick gouge layer form the disordered C/S found in the finest size fraction. At the consists of chlorite, corrensite (ordered chloriteesmectite) in the Whipple detachment outcrop (Whip-5) the gouge horizon ranges coarse and medium size fractions and an Mg-rich chlorite with from 0.1 to 1.2 m in thickness below a sharp striated upper surface. smectite interlayers in the fine size fraction (see Fig. 6). XRD patterns The upper region of the gouge is reddish in color, and the lower, from the chlorite-metasomatized footwall gneisses indicate the generally thicker, unit is greenish and scaly (Fig. 3G). Both the presence of chlorite, but lack any chloriteesmectite phases. The reddish and greenish gouges contain chlorite and illite in the clay hangingwall gravels contain a minor amounts of a 2M1 polytype 10 Å fraction. Upon separation into size fractions, both the red and green mica (presumably relict wall-rock muscovite), but otherwise lack gouges consist primarily of fragmented chlorite in the coarse size clay minerals. The XRD patterns of the gouges thus indicate exten- fraction and authigenic 1Md illite in the fine fraction (Table 3; Fig. 9), sive transformation of fragmental chlorite derived from the footwall indicating authigenic illite growth from the breakdown of feldspar, to relatively large packets of corrensite (found in the 2.0e0.4 and which will be discussed further below. 0.4e0.05 mm size fractions), and transformation of both fragmental A Mg-rich clay mineral gouge assemblage is also found at the chlorite and/or corrensite to a disordered Mg-rich chlorite-rich Dante’s View detachment (“Dante”) exposure where a hangingwall chloriteesmectite that is the only clay phase in the <0.05 mm size rhyolite is juxtaposed against a smectitic tuff by a 50 cme1 m thick fraction. The age relationship of the corrensite and disordered gouge zone with a distinct 5e10 cm thick tan hard horizon with chloriteesmectite is unclear, although the absence of corrensite in striated surfaces at the sharp contact with the hangingwall the finest size fraction indicates that it may be being consumed to (Fig. 3K). Chloritic alteration of the footwall is absent. The hard tan S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 17

Fig. 5. Graphs of chlorite composition in low-angle normal fault gouge and wallrock composition as estimated from XRD intensities of (00l) peaks of random preparations (Brown and Brindley, 1980). Data are shown from Chemehuevi detachment (Lobeck Pass), Whipple detachment (Whip-5), Mormon Point turtleback (Mormon-2, Mormon-3), Buckskin- Rawhide detachment (A-Bomb Canyon) and Copper Canyon turtleback (Copper). Plotted are D (anisotropy of Fe distribution between octahedral sheet and hydroxide sheet) on the X axis against Y, total Fe per (Mg, Fe)6 on the Y axis. layer along the hangingwall contact consists of palygorskite and reflection at 1.50 Å (61.90) indicates that the smectite is di-octa- quartz. The palygorskite vein is authigenic in origin and formed hedral and is thus distinct from the tri-octahedral smectites found before the end of faulting, as it forms a thin vein in the gouge zone, in gouges derived from chlorite microbreccias (See Fig. 7). The di- is striated, and is not found in either wall rock. The gouge clay octahedral smectite probably formed from low-temperature alter- mineral assemblage in both the upper and lower region of the ation of either tuffs derived from the footwall or rhyolite derived gouge (Fig. 3K) consists predominantly of a di-octahedral smectite from the hangingwall. The possibility that some of the smectite is (montmorillonite) and palygorskite with minor illite and zeolites fragmented with respect to the brittle deformation, having formed (heulandite or clinoptillite). Smectite and palygorskite are present during possible earlier, pre-faulting alteration low-temperature in the finest size fraction in roughly equal proportions to those alteration of the tuffs cannot be excluded. found in the coarse fraction, indicating the palygorskite in the Two detachment fault outcrops with chloritic footwalls do not gouge matrix itself is also certainly authigenic. Much of the smec- preserve a distinct scaly chloritic gouge layer per se, but do have tite in the gouge is undoubtedly authigenic, as the clay gouge is gouges that contain significant fragmented chlorite in the coarse much more clay-rich than either wallrock at outcrop. A (060) and medium fractions and chloriteesmectite and/or tri-octahedral 18 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32

Fig. 6. Representative XRD patterns from gouges from faults with chlorite-altered footwalls. Top left: <2 mm fractions of gouges from visible layers of gouge at A-Bomb Canyon (see Fig. 3D). Top right <2 mm fractions of gouges from visible layers of gouge at Lobeck Pass (see Fig. 3b). Bottom left: <2 mm separated into size fractions of gouge from Copper Canyon. C ¼ chlorite, Cor ¼ corrensite, P ¼ palygorskite, Q ¼ quartz, Ca ¼ calcite, ﬁlled circles are K-feldspar peaks. Field photos of outcrops are provided in Fig. 3.

smectite minerals in the fine fractions. At the Harquahala 5.3. Illitic gouges detachment (Aguila), the footwall is a chlorite-metasomatized, variably mylonitic schist and the hangingwall is a silicified The remaining 19 detachment exposures have gouges that are breccia. The gouge layer is a 1 m thick and consists of three layers dominated by authigenic illite or illite-rich illiteesmectite. Of these, visible at outcrop, one dominated by partially vermicularized 6 detachment exposures have chlorite-metasomatized footwalls chlorite (001) > (002) with subsidiary tri-octahedral smectite, and and gouges that are predominantly illitic (Badwater-1, Badwater-2, two very similar layers with predominantly tri-octahedral smectite Bullard, Wildrose, Mosaic, and Whip-5 e also discussed above). The and subsidiary altered chlorite (see Table 2). At the Wonderstone other 13 exposures have widely varying footwall compositions: Wash outcrop of the Salton Sea detachment (571), the gouge is granite (Ashford Klippe, WH-68, Maz-1, Size 36), quartzo- 20e50 cm thick and consists predominantly of a tri-octahedral feldspathic schist or gneiss (Exclamation Rock, Mormon-1, Virgin smectite and minor kaolinite with partially vermicularitized Springs, Wildrose Canyon), greenschist-facies metapelites (Tucki, chlorite and a fragmented 10 Å mica, either illite or muscovite South Park) shales (Secret-1, Secret-2) and variably silicified (Table 2). The smectite is most likely derived from extensive marbles (Clover-1, Secret-4). Two main classes of illitic gouges may alteration of chloritic footwall schists. At both of these outcrops, be distinguished, those in which a transformation from the high- when chlorite is found in gouge, it is variably altered, with the temperature 2M1 polytype of illite (or muscovite) to the low- (001) reflection stronger than the (002) reflection, similar to that temperature 1Md polytype of illite, and gouges where a trans- described at Lobeck Pass. formation from K-feldspar to the 1Md polytype of illite occurs. Both S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 19

10000 Tri- Di- C Smec C Smec C Lobeck (reddish gouge)

Salton (Salton 571)

Aguila (Aguila-1)

A-Bomb (lower gouge) Counts

Mormon (Mormon 3-1)

Ruby Mts (Sec-3)

Waterman Hills (WH-68-1)

Dante Peak (Dante-1) 0 58 59 60 61 62 63 2-theta (degrees)

Fig. 7. (060) peaks of smectites from clay gouges sampled in this study. Upper 5 samples are from faults with chlorite microbreccia footwalls, lower three are from faults with acid volcanic rocks in at least one wall rock. Ranges of position of (060) reflections for tri-octahedral smectites and di-octahedral smectites are shown in grey. C ¼ chlorite. transformations occur in faults with and without chlorite- metasomatized footwalls. Seven detachment fault exposures have gouges where a transition from 2M1 illite to 1Md illite occurs. At Ashford Klippe a hangingwall of Pliocene conglomerates is separated from a footwall of altered granite by a 1 m thick zone of variegated reddish, yellowish and greenish gouge. The gouge consists predominantly of illite and minor kaolinite. When separated into size fractions the gouge shows a clear transition from predominantly the 2M1 polytype in the coarse and medium size fractions to an almost pure 1Md polytype in the finest size fraction (Figs. 9 and 10). Similar transitions from a coarse-grained fragmental illite or muscovite are observed at Mosaic, Maz-1, Tucki, Clover-1 Exclamation Rock and Virgin Springs W (Table 3). Eight detachment fault exposures have gouges where a transformation can be observed from fragmented feldspar-dominated material to the authigenic 1Md polytype of illite. These gouges conspicuously lack the 2M1 polytype of illite in any size fraction. At Badwater-1, a chlorite-metasomatized quartzo-feldspathic gneiss is separated from a hangingwall of weakly consolidated gravels and fluvially-reworked tuffs by a 20 cm thick clayey gouge (Fig. 9). The gouge consists predominantly of feldspar and illite, with chlorite present in amounts only slightly above the limits of XRD detection (see Table 2). Separation of the gouge into size fractions indicates that the coarse and medium size fractions of the gouge are dominated by microcline with subsidiary illite, and the fine size fraction consists entirely of a 1Md illite (Fig. 9). Although the strong feldspar peaks do interfere with the area where the polytype-specific peaks < m fi Fig. 8. A.) XRD patterns of oriented preparations of the 2.0 m fractions of the green for the 2M1 polytype of illite are found, no 2M1-speci c peaks are scaly gouge and whitish waxy gouge at Mormon-3. Grey line ¼ air-dried, black identifiable. The same reaction is observed in gouge at the line ¼ after glycol solvation. C ¼ chlorite, S ¼ smectite, Sep ¼ sepiolite, Waterman Hills outcrop of the Central Mojave core complex Corr ¼ corrensite, I/S, L ¼ lizardite, T ¼ talc, Ca ¼ calcite. B.) XRD patterns of oriented preparations of size fractions of the whitish waxy gouge at Mormon-3. Grey line ¼ air- detachment fault (WH-68), where no 2M1 peaks are observed, but dried, black line ¼ after glycol solvation. the polytype-specific peaks at 24.3 and 29.1 can be observed in all size fractions, although the peaks are much less intense in the coarse and medium size fractions owing to the presence of 20 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32

A B

Fig. 9. Representative XRD patterns from faults with illitic gouges. Shown are random preparations of size fractions at: A.) Amargosa detachment (Ashford), B.) Mosaic Canyon fault

(Mosaic), C.) Waterman Hills detachment (Waterman Hills), D.) Badwater Turtleback (Badwater-1) E.) Whipple detachment (Whip-5). Arrows in A.) and B.) show position of 2M1- speciﬁc illite peaks. ill ¼ illite, Q ¼ quartz, K ¼ K-feldspar, C ¼ chlorite, Ca ¼ calcite, G ¼ gypsum. S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 21

Canyon). At Secret Pass three faults associated with the detachment are exposed (see Haines and van der Pluijm, 2010 for details on outcrops and clay mineralogy), the authigenic illiteesmectite is a 1-water layer disordered illite smectite (85% I in I/S, R0) and forms by the illitization of hangingwall illiteesmectite at Secret -1 and Secret-2, while at Secret-4, where the detachment juxtaposes zeolitized tuffs in the hangingwall against silicified carbonate mylonites in the footwall. The tuffs are altered to an effectively pure montmorillonite in the upper layer of gouge at the outcrop and the lower layer consists almost exclusively of an illite-rich illiteesmectite. The source of the illite is not problematic as <50 m along strike, the hangingwall lithology changes from the Miocene zeolitized tuff to Mississippian shales, which were smeared along the fault plane and transformed to the authigenic I/ S phase while the fault was active, evidenced by 11e13 Ma AreAr ages, consistent with regional constraints on the time of fault movement (Haines and van der Pluijm, 2010). At Wildrose Canyon, a hangingwall of Pliocene conglomerates is separated from a footwall of chloritized amphibolitic schist intercalated with mylonitic marble. The gouge zone is 30 cm thick and contains predominantly quartz, feldspar (both plagioclase-and microcline) with lesser amounts of R3 illiteesmectite and chlorite. Upon separation into size fractions, the abundance of the R3 I/S increases from coarse to fine size fractions, and the abundance of chlorite decreases (Table 2). Trace amounts of talc are also present in the medium and fine size fractions. The gouge contains minerals clearly derived from both walls of the fault. The microcline is derived from the hangingwall as the footwall lacks þ potassic minerals, and is the most reasonable source of K for the illiteesmectite, while the chlorite in the gouge is derived from the þþ footwall. Mg required for talc growth is most likely derived from the footwall dolomitic marbles. No age relationship between the authigenic illite and authigenic talc can be determined from outcrop or the XRD patterns.

5.4. Dioctahedral smectite gouges

Gouges at three outcrops (WH-68, Secret-4, and “Dante”) consist of at least one horizon that is dominated by di-octahedral smectite (montmorillonite). All of these outcrops lack chloritic metasomatic alteration of footwall lithologies. At the Waterman Hills detachment (WH-68) the detachment juxtaposes a Miocene granodiorite in the footwall and a Miocene rhyolite in the hangingwall. The gouge zone Fig. 10. XRD patterns of gouge from Ashford Klippe (previous figure) modeled using is 0.3e1.0 m thick and internally complex, containing dark grey clay- WILDFIRE (grey). Percentages in upper right are relative abundances of polytypes in rich horizons that appear to pinch and swell in a matrix of lighter modeled pattern. Note large shift in relative abundance of 2M1 and 1Md polytypes between the coarse and fine size fractions. K ¼ kaolinite, KF ¼ K-feldspar, Q ¼ quartz, brown clay and lithic fragments (Fig. 3L). No clear age relationship ¼ fi I illite, arrows are 2M1 illite-speci c polytype peaks. between the lighter and darker material is evident at outcrop. XRD patterns indicate that the dark grey lenses are illite, while the lighter brown matrix is di-octahedral smectite (Table 2). Upon separation fragmented feldspar in those size fractions (Fig. 9). The same K- into size fractions, the illitic material shows a clear transition from K- feldspar / 1Md illite reaction is observed in gouges at Badwater-2, feldspar in the coarse and medium fractions to 1Md illite in the finest Bullard, Mormon-1, Size 36, Canyon, South Park, and Whip-5 e see size fraction (see Fig. 9). The lighter brown material is exclusively di- Fig. 9). The absence of the 2M1 polytype in any size fraction and the octahedral smectite in the <2.0 mm size fraction (Table 2; Fig. 7). At presence of the 1Md polytype in all size fractions indicates that the the main detachment in the Ruby Mountains detachment system authigenic 1Md illite is growing directly from the breakdown of (outcrop Secret-4) the detachment juxtaposes zeolitized tuffs in the fragmental feldspar and not from the replacement of a fragmented hangingwall from silicified carbonate mylonites of the footwall. 2M1 polytype. Oriented mounts of illitic material from both types of Three distinct zones can be distinguished in the 1.0 m thick gouge transformations indicate some expansion upon glycolation, indi- zone, both visually and on the basis of clay composition determined cating the authigenic illite frequently contains a few percent by XRD. The upper region of the gouge zone (sample Sec 4-1) interlayered smectite layers, typically <10% as determined by the consists of a hard light brown mixture of smectite and kaolinite, with D2Q method of Srodon (1980). very minor illite and quartz. The central region of the gouge zone Four detachment fault exposures contain gouge where the (sample Sec 4-3) is a light green, very stiff, essentially pure smectite. dominant clay mineral is an authigenic illite-rich illiteesmectite The smectite is di-octahedral on the basis of a (060) reflection at (the three exposures of the Ruby Mountain detachment fault at 1.499 Å (61.88 Cu ka e Fig. 7), and SEM/EDS analysis indicates that Secret Pass (Secret-1, Secret-2 and Secret-4), and at Wildrose the primary cations are Mg and Fe with subordinate K and Ca, 22 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 warranting the montmorillonite designation. The lower region or the less-common illitization of illiteesmectite. This trans- (sample Sec 4-2) is a dark grey, hard clayconsisting of illite-rich illite/ formation of fragmental 10 Å mica derived from either sedimentary smectite and quartz. The contact between the upper smectite- protoliths, such as shale, or mica-rich metamorphic or igneous dominated layers and the basal illitic layer is sharp, but lacks any lithologies is widespread, both in the gouges sampled in this study cross-cutting relationships, and strongly suggests that both clay and in the majority of previous reports of authigenic clay in fault assemblages formed contemporaneously The Dante’s View gouges. This transformation may be the most common clay mineral detachment outcrop is described above. transformation in clay gouges, owing to the abundance of the wall rock lithologies, particularly in sedimentary environments. 6. Discussion The systematic change in many of the gouges in this study in which the coarse and medium fractions are dominated by K-feld- The clay mineral assemblages found in low-angle normal fault spar and the fine fraction is dominated by 1Md illite, and the gouges indicate that mineral transformations are widespread and absence of the 2M1 polytype of illite in any size fraction, indicates almost ubiquitous. Only one exposure, Mormon-2, lacks evidence for that K-feldspar is broken down in the presence of water to form asignificant clay mineral transformation in the fault gouge. Various illite (see also Wintsch et al., 1995). The presence of this trans- transformations occur in fault gouges that can be broadly grouped formation at seven separate detachment faults indicates that this into four series of mineralogical transformations: 1) 1Md illite growth process is common and perhaps under-recognized process in fault reactions, either from the dissolution of 2M1 illite/mica or K-feldspar; gouges. The growth of authigenic 1Md illite from the breakdown of 2) Chlorite / corrensite, disordered chlorite/smectite or tri- K-feldspar is, however, common in hydrothermal systems, espe- octhedral smectite; 3) Di-octahedral smectite growth from the cially associated with epithermal ore deposits (e.g. Kuehn and Rose, breakdown of acid volcanic wall rocks and 4.) Mg-rich 1992; Bove et al., 2002). Large-scale K-feldspar metasomatism fluid þ fragmented phyllosilicates / Mg-rich phyllosilicate assem- locally associated with economic Au, Ag, Cu deposits and extensive blage (palygorskite or sepiolite, talc, lizardite, tri-octahedral smec- precipitation of hematite and Fe oxihydroxides, has been docu- tite). The illitic and di-octahedral smectite transformations appear to mented in the hanging walls of three detachment faults sampled in be near-isochemical and do not require large external fluid fluxes. The this study (the Whipple detachment, Bullard detachment and the chloriteecorrensiteesmectite transformations can be thought of as Buckskin-Rawhide detachment, Beretan, 1999) and at several other near-isochemical ‘retrograde diagenesis’ of fragmented chlorite, metamorphic core complexes (e.g. the Picacho and Santa Catalina whereas the low-temperature Mg-rich assemblages are a response to metamorphic core complexes AZ, Brooks, 1986; Roddy et al., 1988). external fluid fluxes. All of these neoformed clay minerals are fric- The K-feldspar to 1Md illite transformation in fault gouges is not, tionally weaker than is typical for silicate and carbonate lithologies however, spatially associated with regions of large-scale upper- (e.g. Byerlee, 1978), so offer the possibility for significant reaction- plate metasomatism, indicating the two processes are distinct softening of detachment surfaces that facilitates slip at low dips. phenomena. Geochemical data from the Black Mountains gouges demonstrate an enrichment of illite in fault gouge relative to the 6.1. Fragmental gouges footwall, but only a minor change in the K2O content of the gouge relative to wallrock lithologies, indicating that mineral trans- Gouges that are mineralogically indistinguishable from wallrock formations are occurring, but the fluids responsible for growth of lithologies and lack evidence for growth of neoformed clays are hydrous phases are not ‘flooding’ the gouges with externally- þ considered ‘fragmental’, i.e. cataclastically-reworked wall rock. sourced K as might be expected from K-feldspar metasomatism Chlorite-rich gouge in which the bulk gouge mineralogy is identical somewhere in the detachment system (Hayman, 2006). to the footwall, and the chlorite is mineralogically and compositionally indistinguishable from that found in the footwall is found 6.3. Chlorite ‘retrograde diagenesis’ reactions at 6 detachment fault exposures in this study. The presence of fragmental gouges at multiple outcrops indicates that the ‘classical’ Chlorite-dominated fragmental gouge is typically transformed to idea of gouge genesis as purely cataclastic remains relevant for one or more of several mixed-layer chloriteesmectite and discrete some stages of LANF gouge evolution. Notably, though, only at one tri-octahedral smectite (saponite) phases. The initial chlorite in the exposure (Mormon-2) is fragmental gouge the only gouge type metasomatic breccias and fragmental gouge is somewhat Fe-rich as present in the fault core. At the other 7 exposures of faults with evidenced by the intensities of the various (00l)reflections, and the fragmental gouges, zones of authigenic-dominated material are reaction sequence of chlorite to mixed-layer chlorite smectite pha- also present, all later than the fragmental gouge based on cross- ses to smectite results in a gradual removal of Fe and enrichment of cutting relationships. It is thus probable that the fragmental Mg from the original fragmental chlorite. This process of Mg gouges formed earlier in the history of the detachment, and are enrichment of fault gouges of the Black Mountains is also apparent then extensively altered to authigenic-dominated assemblages from geochemical data (Hayman, 2006). The reaction series can be during further deformation and the influx of fluids. The Mormon-2 thought of as ‘retrograde diagenesis’ of tri-octahedral phyllosili- exposure is unusual in relation to the others described herein, but cates. The prograde diagenetic transformation of saponite to chlorite several classic examples of fault gouges lacking significant miner- in mafic rocks has been extensively documented (e.g. Shau and alogical transformations are known (e.g. Engelder, 1974; Brock and Peacor, 1992) and the saponitic and corrensitic gouges seen at Engelder, 1977; Anderson et al., 1980a, b). many detachment outcrops is the diagenetic reaction series in reverse, as the fragmental chloritic gouge is transformed into a lower 6.2. 1Md illite gouges temperature assemblage when gouge is brought to progressively shallower crustal levels during detachment slip. A similar trans- The growth of the authigenic 1Md polytype of illite in fault formation of chlorite to tri-octahedral smectite has been docu- gouge is now widely documented (e.g. Solum et al., 2005; Haines mented in gouge from the Alpine Fault (Warr and Cox, 2001) and in and van der Pluijm, 2008, 2010; Duvall et al., 2011; Rahl et al., several sedimentary basins (e.g. Nieto et al., 2005). The presence of 2011). All previous demonstrations of the growth of illite in fault fragmental chlorite in several gouges with (001) > (002) (indicating gouge have documented a transformation from the high- the chlorite hydroxyl layer is partially replaced by hydrated cations) temperature 2M1 polytype to the low-temperature 1Md polytype indicates that fragmental chlorite preserved in smectite-dominated S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 23 gouges is sometimes transformed to chlorite-rich chlorite/vermic- either as a solid-state transformation of smectite or as dissolu- ulite. Modeling the relative intensities of the (001), (002), (003) and tion of smectite and precipitation of palygorskite. As the sheet- (004) peaks with NEWMOD indicates that chlorite/vermiculites are like palygorskite occurrences include minor amounts of chlorite 70e90% chlorite. The age relationship of this transformation to the and quartz, both of which are fragmental in origin, it is probable various chlorite / chlorite/smectite and chlorite / chlorite/ that the palygorskite sheets grew through localized trans- vermiculite transformations is unclear, and it is possible that several formation of smectite and the sheet-like morphology is not occurred concurrently (e.g. Aspandiar and Eggleton, 2002). As necessarily indicative of high fluid pressures in low-angle normal vermiculite is also a common mineral in soils, a post-tectonic near- fault cores. X-ray texture goniometry measurements on the surface vermicularitization of fragmental chlorite cannot be palygorskite ‘sheets’ indicate that the palygorskite in sheets has excluded. a weak preferred orientation parallel to the fault core margins, identical to that in the surrounding smectitic gouge (Haines et al., 6.4. Mg-rich gouges 2009). The similarity of preferred orientation between the palygorskite in sheets and other clay minerals in the surrounding Two distinct assemblages of Mg-rich phyllosilicates are recognized maxtrix further suggests the palygorskite formed as the result of in fault gouges, an authigenic saponite þ sepiolite þ talc þ lizardite transformation of smectite and not as vein fill. Palygorskite is assemblage and authigenic palygorskite, either as fault-parallel stable below 220 C(Imai and Otsuka, 1984), and forms at ocean- syntectonic veins or as a gouge matrix in association with smectite. bottom conditions in deep marine sediments (e.g. Bonatti and Both formed from circulation of Mg-rich fluids along the fault plane at Joensuu, 1968) and in cave-fill sediments and on cave walls. temperatures <<200 C and very late in the active slip history of (Polyak and Guven, 2000). As it is found either cross-cutting the fault. Sepiolite and palygorskite are both fibrous Mg-rich phyllo- smectitic gouge, or found together in a gouge matrix with silicates that can form at very low temperatures (<25e150 C) and are demonstrable authigenic smectite, the palygorskite-bearing rarely reported in fault rocks. They are principally found in association assemblages must form <130 C, indicated by the stability of with evaporative carbonate and phosphatic environments in smectite. The source of the Mg-rich fluid for either assemblage is continental settings and as authigenic minerals in shallow and deep uncertain, although dolomitic marbles found in the footwall at marine sediments, although vein occurrences of both minerals are Mormon Point are a likely source of Mg in the Black Mountains known (Jones and Galan, 1988).Sepioliteismorecommoninconti- turtlebacks (Hayman, 2006). A meteoric or basinal origin for the nental settings (e.g. Hay et al., 1986; Pozo and Casas, 1999; Cuevas Mg-rich fluids seems far more likely than an igneous or meta- et al., 2003), especially evaporative lacustrine environments, morphic source owing to the low temperatures and alkali-rich whereas palygorskite is often reported as an authigenic mineral in oxidizing nature of the conditions required for the growth of marine sediments (e.g. Couture, 1977). In both settings, sepiolite and either assemblage. As palygorskite contains significantly more Al palygorskite form as a result of the interaction of Si- and Mg-rich alkali than sepiolite, and the palygorskite assemblage is never found in (pH 7e9) oxidized fluids with fragmental minerals such as chlorite conjunction with the sepiolite þ smectite þ talc þ lizardite and illite at temperatures <<200 C(Jones and Galan, 1988). Authi- assemblage, the two assemblages my be the result of a com- genic talc is similarly reported from evaporative lacustrine settings mon Mg-rich fluid interacting with relatively Al-poor (Bailev, 1949; Braitsch, 1971) as well as lizardite (Fuchtbauer and chloritic gouge and wall rocks at Mormon Point to form Goldschmidt, 1956). The lowest possible temperature limit of the sepiolite þ smectite þ talc þ lizardite, and interacting with gouge sepiolite þ smectite þ talc þ lizarditeassemblageisthusindicatedby partially derived from more Al-rich wall rocks (granites and reported occurrences of neoformed talc, lizardie and/or sepiolite in rhyolites) at Lobeck Pass and “Dante” to form palygorskite. Both evaporative lacustrine settings synthesis of sepiolite and talc at 25 C Mg-rich assemblages probably formed quite late in the history of from fluids of pH > 8.5 (Siffert and Wey, 1962). A precise temperature the fault, although prior to the end of fault slip, as meteoric or limit for the formation of the sepiolite þ smectite þ talc þ lizardite basinal fluids descended the detachment. assemblage is uncertain, but is likely in the range of 25e150 C. Fault-hosted palygorskite has been described at several other 6.5. Wall rock composition and clay gouge transformations localities. At the Seratta de Nijar fault, Spain (García-Romero et al., 2006) palygorskite is found as veins in secondary faults A marked difference in gouge mineralogy can be distinguished sub-parallel to the main fault surface. A hydraulic pumping between gouges from this study with a chlorite/epidote ‘micro- mechanism of Mg-rich fluids scavenged from Mg-rich volcanic breccia’ footwall and those without. Where chlorite-rich wallrock is wall rocks was suggested for palygorskite growth. Palygorskite is cataclastically incorporated into the fault zone, the resulting gouges also reported in gouge from faults in the western San Bernardino contain tri-octahedral clays (saponite, chlorite/smectite and cor- Mountains in California, together with tri-and di-octahedral rensite), with some exceptions. Where chlorite-rich wall rocks are smectites and vermiculite (Jacobs et al., 2006). Similar occur- absent, the gouges are dominated byauthigenic illite or illite-rich I/S. rences are reported from the San Jacinto fault zone, also in Cal- The consistency of the relationship between wall rock lithology and ifornia, where palygorskite is reported as a prominent damage- observed clay gouge mineralogy indicates that wallrock composi- zone vein-infilling phase in granitic wall rocks (Morton et al., tion and clay gouge mineralogy are strongly linked. A recent study of 2012). Palygorskite occurrences are also reported as vein fill clay gouge mineralogy in the central Alps supports our observations. from greenschist and amphibolite-facies mineral deposits, Surace et al. (2011) report that in fault with quartzo-feldspathic wall although the age of the palygorskite relative to mineralization is rock, gouges contained primarily illite and smectite, while in gouges unclear in most cases (Post and Crawford, 2007). The paly- with amphibolitic wall rocks contain primarily tri-octahedral chlo- gorskite occurrences in low-angle normal faults are different in rite and corrensite, and gouges in faults in ultramafic wall rocks that they occur either as a single sheet-like layer sub-parallel to contain primarily talc and serpentine as gouge minerals. While the the main fault surface, or are disseminated in the gouge matrix. neoformed nature of all the clay phases is not explicitly demon- In outcrop, field relations are ambiguous as to whether the strated in the Surace et al. (2011) study, their data support our palygorskite grew as fill of an open vein (sensu stricto), or grew as observations of a clear link between wallrock chemistry and gouge the result of smectite-rich gouge reacting with Mg-rich fluid mineralogy. Geochemical studies of clay gouge mineralogy have also passing along a through-going shear surface within the gouge, indicated that gouge/wallrock transformations involve significant 24 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 mineralogical changes, but that many of the transformations are Several apparently similar field localities were reported by Wu mostly isochemical, albeit with varying volume loss (Evans and (1978) elsewhere in the San Andreas system where wallrocks are Chester, 1995; Manatschal et al., 2000; Hayman, 2006; Schleicher Franciscian units and predominantly tri-octahedral clay mineral et al., 2009). Changes in composition of chlorite between wallrock assemblages were found in gouge (chorite and chlorite/vermicu- and gouge elucidated from XRD data similar to ours, suggestive of lite) although details are sparse. Tri-octahedral smectite is also the growth of authigenic Fe-poor chlorite have been reported from reported as the dominant clay mineral in gouge at exposures of the gouge the Chelungpu thrust fault in Taiwan, whereas wall-rock Gokash-Arashima Tectonic Line in Japan, where ultramafic rocks chlorites are Fe-rich (Hashimoto et al., 2008), suggesting that chlo- are juxtaposed against arc-derived plagioclase-and chlorite-rich rite can be authigenic in some cases, but details are scarce. siliciclastic rocks, as at SAFOD (Sone et al., 2011). Shigematsu Faults with sedimentary siliciclastic wall rocks usually contain et al. (2012) also report similar saponite-dominated gouges with authigenic illite or I/S as the authigenic mineral, as at Secret Pass, minor chlorite from from the Median Tectonic Line in SW Japan, Previously documented examples include the Moab Fault and Little where metabasic schists and serpentinites are juxtaposed against Grand Wash fault in Utah, thrust faults in the Canadian and tonalites. American Rockies, the West Qinling Fault in Tibet, and thrust faults Several faults with intermediate metamorphic or plutonic wall in the Pyrenees, (Vrolijk and van der Pluijm, 1999; Solum et al., rocks juxtaposed against sedimentary lithologies have gouges that 2005; Solum and van der Pluijm, 2007; Dockrill and Shipton., are predominantly tri-octahedral clay-dominated (saponite, chlo- 2010; Duvall et al., 2011; Rahl et al., 2011). Where intermediate- rite/smectite or corrensite þ/ palygorskite). No examples are re- to felsic igneous and/or micaceous metamorphic lithologies rocks ported in this study, as all localities with chlorite retrograde are found as footwall rocks, 1Md illite or illite-rich illite/smectite is diagenesis gouges have significant chlorite in the footwall, either at also observed as the authigenic phase in the resulting gouges, as at outcrop or locally up-dip. Several examples, however, exist in the 10 localities in this study (see above). Similar transformations are literature, such as the Punchbowl Fault and San Bernardino faults in described from the Carboneras Fault in SE Spain, where wall rocks California (Solum et al., 2003; Jacob et al, 2006), and the Nojima are predominantly mica schists (Rutter et al., 1986; Solum and van Fault in Japan (e.g. Othani et al., 2000; Matsuda et al., 2004; der Pluijm, 2009). Apparently similar clay assemblages are reported Mizoguchi et al., 2009). These wall rock units lack the Mgþ at other exposures of the San Andreas system (Wu, 1978) and from necessary to explain the authigenic clay growth as isochemical, and heritage gouge mineralogy reports in the engineering geology instead require an outside Mgþ fluid flux or preferential break- literature (Wahlstrom et al., 1968; Brekke and Howard, 1973), down of relatively Mg-rich minerals such as biotite or hornblende although the nature of any transformations are not described. More to explain the Mg-rich nature of the tri-octahedral clays found in recent studies in the central Alps, Mongolia and southern California gouge. Previous geochemical studies on gouge mineralogy have also document gouges in quartzo-feldspathic wallrocks dominated indicated that Mg enrichment of gouge is common (Manatschal by neoformed illite (Zwingmann et al., 2010; Buatier et al., 2011; et al., 2000; Hayman, 2006; Schleicher et al., 2009), but not ubiq- Surace et al., 2011; Morton et al., 2012). In these environments, uitous (Matsuda et al., 2004). Smectite of uncertain composition fragmental micas, fragmental feldspars, and/or smectite-rich I/S are (di- or tri-octahedral), along with apparently detrital chlorite is þ all potentially abundant and provide the K necessary for illite reported from gouge of the southern San Andreas Fault at Lit- growth. Verdel et al. (2011) reported seemingly similar illitic gouge tlerock, CA, where granodiorites are juxtaposed against Tertiary assemblages and granitic footwall rock compositions from the Salt fluvial sediments (Wechsler et al. (2011). Springs fault in NW Arizona. They argue that illitic gouge is derived The effect of wall rock composition is also evident in the from concentration of hanging wall clays by a scraping mechanism predominantly di-octahedral nature of the authigenic smectite from a proximal conglomeratic succession, which contrast with our found in faults with acid volcanics or plutonic rocks in either the observations of gouge in structurally similar Death Valley Turtle- hanging wall or footwall, contrasted with the predominantly tri- back detachments, in which authigenic clays are derived from the octahedral smectites found in gouges with chlorite microbreccias footwall on the basis of geochemistry (Hayman, 2006; this paper). in the footwall (see Fig. 7). Where wallrocks are more magnesium-rich such as chlorite-rich Kaolinite (or its polymorphs dickite and hayallosite) has not units (e.g. ‘chlorite microbreccias’), or where serpentine-rich been found as an authigenic mineral in gouges in this study, except lithologies are juxtaposed against siliciclastic units, or where in exposures where the kaolinite can clearly be related to formation dolomitic limestones and marls are juxtaposed, tri-octahedral clays in near-surface weathering conditions on the basis of field rela- are instead found as the predominant authigenic phases. For many tions. Previous reports of kaolinite in fault rocks relative to other of the low-angle normal faults sampled in this study, footwall clay minerals are relatively rare, and are found in 3 environments: lithology is more closely linked with gouge mineralogy than 1) Faults with extensive high-temperature hydrothermal alteration hanging wall lithology is with gouge mineralogy, perhaps sugges- and mineralization (e.g. Jacobs and Kerr, 1965; Matsuda et al., 2004; tive that gouge mineralogy formed before current footwall and Buatier et al., 2011) 2) Surface exposures of active strike-slip faults hangingwall lithologies were juxtaposed. At the SAFOD site, very where the effects of weathering cannot be ruled out (e.g. Wu, 1978; magnesian rocks (serpentinites) are juxtaposed against silici- Buatier et al., 2011) or 3) Geotechnical reports from tunnel expo- clastics, and all of the principle clays documented in this study tri- sures where the definition of a ‘fault’ and ‘fault gouge’ is more and di-octahedral clays are documented (saponite, chlorite/smec- operational, that is, it causes significant engineering problems, tie, corrensite, illite/smectie and illite, as well as apparently frag- typically excavation stability or significant water inflow (inflow mental talc and lizardite e Schleicher et al., 2009, 2012; Carpenter >>liters/minute) (Wahlstrom et al., 1968; Brekke and Howard, et al., 2011; Holdsworth et al., 2011; Lockner al., 2011). It is thus 1973). The high flow rates of water in the operationally defined likely that the locally highly variable clay assemblage at SAFOD is ‘faults’ in many tunnel exposures are likely responsible for local a result of locally variable amounts of highly magnesian and sili- post-faulting kaolinization of older authigenic clays in the upper ciclastic lithologies admixed within the fault core and damage 1 km of the crust. Kaolinite is thus not likely a significant authigenic zone. Moore and Rymer (2012) document the same trans- mineral in clay gouges, but rather the result of post-faulting near- formations at a surface exposure of clay gouge near the SAFOD site, surface alteration of gouge clay minerals. which also juxtaposes serpentinite with siliciclastic sedimentary The relative proximity of the three Mormon Point outcrops wall rocks, and has a gouge matrix of tri-octahedral smectite. ( <5 km along strike) and yet the distinct clay mineral populations S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 25 found at each, further illustrates the roles of both wallrock temperature (w110 C) (Sinclair et al. 2005; Duvall et al., 2011; composition and fluid circulation in gouge mineralogy and mineral Rahl et al., 2011). At the SAFOD drilling site, authigenic illite from transformations. The sepiolite þ smectite þ talc þ lizardite the vicinity of the fault core and thermochronology indicate that assemblage at Mormon-3 is only 300 m along strike from Mormon- authigenic illite formed at 8.0 þ/ 1.3 Ma and 4.0 þ/ 4.9 Ma, and 2, the only outcrop sampled where an ‘fragmental’ mechanically- that fault zone has had w1kmexhumationinthelast8Ma derived chlorite gouge is preserved without later reactions occur- (Blythe et al., 2004; Blythe and Burgmann, 2008; Schleicher et al., ring, Similarly, Mormon-1 is within 2 km of both of the other 2010). As the sampled gouge was found at w3000 m depth, and outcrops and yet has a gouge dominated by authigenic 1Md illite sampled at down-hole temperatures of 110e114 C, a geothermal and lacks a fragmented chloritic component of the gouge entirely. gradient of 33e35 C/km is reasonable. Assuming a constant This rapid change of gouge composition along strike indicates that geothermal gradient over the last 8 Ma, at the time of gouge Mg-rich fluid fluxes may be localized phenomena and perhaps formation (8e4 Ma), the material would have been at tempera- controlled by local wall-rock chemistry and not a detachment-scale tures of 116e140 C. Zwingmann et al. (2010) report multiple fluid circulation system. As footwall chemistry differs strongly gouge KeAr ages from authigenic 1Md illite in gouges exposed in between the three outcrops, and yet the hangingwall rocks are all a trans-Alpine rail tunnel with quartzo-feldspathic wall rocks Pliocene fanglomerates with a presumed common source area, it is with w6 Ma apatite-fission track ages, suggesting formation of likely that footwall chemistry drives clay gouge composition at gouge illite at or slightly above the 60e110 C partial retention Mormon Point much more than hangingwall composition. Footwall zone of apatite. Thus, for 1Md illite in fault zones, a temperature rocks have been observed to contribute much more material to the range of 90e180 C for authigenic illite growth appears likely, gouge of low-angle normal faults than hangingwall rocks in the with some occurrences as low as w60 C. Apparently authigenic Black Mountains and at Sierra Mazatán (Hayman, 2006; Haines and illite in gouge has been reported elsewhere in faults that were van der Pluijm, 2008) and the observation can be applied to almost seemingly active at shallower depths (Casciello et al., 2004, 2011; all the detachments sampled in this study, excepting Badwater-1 Caine and Minor, 2009), but firm temperature constraints are and Size 36 Canyon, where physical incorporation of hanging wall difficult to evaluate. Casciello et al. (2004) reported authigenic material in gouge is documented (Hayman, 2006). illite in clay gouge from the Scoriabuoi fault, a left-lateral strike- slip fault in the southern Apennines, Italy that was exhumed from 6.6. Temperatures of LANF clay gouge formation depths <2km(Onofrio et al., 2009; Casciello et al., 2011). Caine and Minor (2009) reported illite enrichment in clay gouge in For the temperature of formation of the authigenic clay to be the San Ysidro Fault, a normal fault in the Rio Grande Rift with adequately constrained, both the age of the authigenic clay (ob- <1 km exhumation. Although Caine and Minor attribute the clay- tained by direct dating of authigenic, usually K-bearing, clays) and rich fault core to clay smear (sensu strictu, e.g. Lindsay et al., the time-temperature history of the wall rock (typically from 1993), they note that the clays in the fault core are composi- apatite fission-track or (U/Th)/He thermochronology) must both be tionally distinct from wallrock clay-rich horizons, and thus an known. For non-K-bearing clay phases (smectite, corrensite, authigenic origin for some of the San Ysidro fault core clays disordered chloriteesmectite, and the various Mg-rich phases), cannot be excluded. which cannot be dated directly, the upper and lower temperature Illite-rich illite/smectite is known in gouge from 4 faults with stability limits observed for these phases in other geological envi- good temperature constraints at the time of gouge formation, the ronments place somewhat less precise constraints on the temper- Moab and Little Grand Wash faults in Utah, likely forming at ature of formation of clay-rich gouges in low-angle normal faults temperatures of 90e110 C, at SAFOD, forming at temperatures of (Fig. 11). 110e155 C, and the Lewis thrust in Canada, forming at tempera- Authigenic 1Md illite offers the best temperature constraints on tures of 114e153 C(Vrolijk and van der Pluijm, 1999; Schleicher its growth in clay gouges. Thermal constraints on the growth of et al., 2009 and see above). The temperature of formation of the 1Md illite are derived primarily from low-temperature thermo- Moab Fault and Lewis thrust examples is determined from an chronology systems (apatite fission-track and apatite (UeTh)/He) unpublished industry apatite-fission-track study described in measurements on wall rocks, and less commonly vitrinite reflec- Vrolijk and van der Pluijm (1999), while the Little Grand Wash fault tance measurements and regional basin models. Of the faults example temperature is constrained by a 38 Ma age of the authi- sampled in this study, only the Sierra Mazatán detachment has genic illite-rich I/S (Vrolijk et al., 2005) and a regional basin model both reliable ages for clays and a thermal history for the wall rocks (Nuccio and Condon, 1996) which places temperatures of the derived from thermochronology sufficient for an estimate of the juxtaposed units at between 100 and 125 C at 38 Ma. Neoformed temperature of clay formation. At the Sierra Mazatan metamorphic illite-rich I/S is also described from other faults with less well- core complex, a combination of authigenic gouge dating and K- defined thermal histories: the Punchbowl Fault, CA, exhumed feldspar cooling data indicate that the authigenic illite most likely from 2 to 4 km depth (Solum et al., 2003), and the San Gabriel fault, formed at 110e160 C(Wong and Gans, 2003; Haines and van der CA, exhumed from 2 to 5 km depth (Chester et al., 1993). Estimates Pluijm, 2008). Gouge ages from the Ruby Mts are reliable, but the of the temperature of formation at the Punchbowl and San Gabriel nearest reliable thermochronometer ages are from rocks 50 km to sites is problematic, owing to significant zeolite mineralization in the SW along strike, and are thus of limited value in assessing the the gouge, suggestive of local hydrothermal circulation and wallrock conditions at the time of gouge formation (Colgan et al., possible deviation from a regional geothermal gradient. Neoformed 2010). illite-rich I/S is also reported from faults exhumed from shallower For faults in other tectonic environments, Vrolijk and van der depths. At the Scoriabuoi fault in Italy, exhumed from depths Pluijm (1999) report temperature estimates for authigenic 1Md <2kmand<60 C (Casciello et al., 2004, 2011), discrete illite is growth in gouges from the Canadian Rockies ranging from 115 to reported in gouge whereas I/S minerals are found in wall-rocks on 180 C, based on vitrinite reflectance measurements and apatite either side. On the basis of observations of illite growing prefer- fission-track paleothermometry, and 100 þ/ 10 C for gouge entially along P surfaces and infilling open Riedel surfaces, shear from the Moab Fault in UT. Authigenic 1Md in fault gouges in the stress is posited to facilitate the smectite to illite transformation Pyrenees and northern Tibet formed contemporaneously when (Casciello et al., 2011). R1 illiteesmectiteevermiculte is also re- wall rocks cooled through the apatite fission-track annealing ported from as an authigenic phase in gouge in a small- 26 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32

Fig. 11. Schematic figure showing patterns of transformation identified in this study and known temperature constraints on the stability of identified neoformed clay minerals found in clay gouges in this study. Constraints from studies cited in text. Solid lines indicate known constraints, dashed where observations are sparse or speculated. Ft. Z. ¼ fault zone, Sed. ¼ sedimentary basin, Hydrot. ¼ hydrothermal system. displacement (9 m) normal fault in the Black Diamond Mine in CA greenschist-facies footwall rocks against near-surface sediments (Eichhubl et al., 2005). The wallrocks are deltaic sandstones and (often not completely consolidated), constraining the depth or mudstones, and the maximum depth of burial is <1 km, implying temperature of formation is difficult, and analogy from other very low temperatures, <50 C. tectonic environments must be made. At the SAFOD site on the San Constraints on the temperature for the growth of tri-octahedral Andreas, currently at temperatures of 110e114 C, authigenic di-and clays are less precise. As most of the faults in this study with tri- tri-octahedral smectites together with corrensite and ordered illite- octahedral clay growth juxtapose rocks exhumed from rich illite/smectite are observed in the actively deforming fault core, S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 27 in addition to lizardite and talc (Schleicher et al., 2009, 2012; vein-filling mineralization characteristic of hydrothermal systems Carpenter et al., 2011; Lockner et al., 2011; Holdsworth et al., 2011). is observed, neoformed clay minerals such as smectite are possibly Corrensite, together with tri-octahedral smectite was also reported stable to higher temperatures than for most faults in this study. It in gouge from the Peramola thrust in the Spanish Pyrenees, where should be noted that the extensive carbonate and zeolite veining the thrust juxtaposes dolomitic limestone over marls (Haines et al., typical of continental-scale strike-slip faults is conspicuously 2009). Constraints on temperature at the time of thrusting are absent from the LANF’s sampled for this study. imprecise, but estimates of <4 km of overburden and an absence of evidence for hydrothermal alteration at outcrop indicates 6.7. Frictional strength of authigenic clays a temperature <120 C(Burbank et al.,1992; Haines et al., 2009). Co- existing disordered chloriteesmectite and illiteesmectite are The growth of authigenic illite and smectite minerals and other observed as authigenic phases in gouge from the Punchbowl Fault, phyllosilicates in low-angle normal fault gouge has important CA (Solum et al., 2003). Authigenic di- and tri-octahedral smectites, implications for the frictional strength of low-angle normal faults. together with palygorskite are reported from faults in the San Ber- The main clay minerals observed in this study (illite, illite-rich illite- nadino Mountains, CA, exhumed from depths of 1e2km(Jacobs smectite, smectite, chloriteesmectite minerals, and chlorite) have et al., 2006). The temperature stability limits indicated above coefficients of friction that are far lower than the typical m ¼ 0.65 to suggest that clay gouges form below 200 C and most probably at or 0.80 for most rock-forming minerals (Byerlee, 1978). Reported below 150e180 C(Fig. 11). Previous temperature estimates for the measurements of the frictional strength of illite range from m ¼ 0.50 formation of low-angle normal fault gouges have ranged as high as to 0.60 for dry powdered material (Saffer and Marone, 2003; Ikari 230 C, but did not assess the fragmental versus authigenic nature of et al., 2007)tom ¼ 0.27e0.32 for saturated materials at the components that were interpreted to be in equilibrium (e.g., 10e60 MPa effective normal stress (Kopf and Brown, 2003; Ikari Damon and Shafiqullah, 2006). et al. 2009; Tembe et al., 2010). Reported measurements of the It should be noted that in sedimentary and hydrothermal frictional strength of smectite vary, probably as a function of environments, the temperature ranges for the observed stability of experimental configuration, but are always <0.30 for both unsatu- these phases are far larger. 1Md illite has been reported to have rated and saturated conditions and have been reported as low as grown at temperatures as low as 15e30 C in sediments in the 0.03 (Logan and Rauenzahn, 1987; Morrow et al, 1992; Kopf and Barbados accretionary prism and the southern North Sea Brown, 2003; Ikari et al., 2007, 2009; Carpenter et al., 2011; (Kantorowicz, 1984; Buatier et al., 1992), although most 1Md illite Lockner et al., 2011; Sone et al., 2011). Reported measurements for growth in sedimentary basins is reported at temperatures of the strength of chlorite are similarly variable, ranging from 0.6 at dry 100e140 C (e.g. Hower et al., 1976; Freed and Peacor, 1989; Franks conditions (Morrow et al., 1992) to 0.38 and 0.27e0.32 for saturated and Zwingmann, 2010) and the upper limits on the temperature of conditions (Moore and Lockner, 2004; Ikari et al., 2009). The fric- growth in sedimentary basins are uncertain, but certainly below tional strengths of interlayered minerals (illite-rich illiteesmectite 200 C(Meunier, 2005). Apparently cogenetic 1Md, 1M and 2M1 and the various chloriteesmectite phases) are not directly known, are, however, known from hydrothermal systems active at but are likely similar to those of their constitutive components. The 270e330 C(Lonker and Fitz Gerald, 1990). As neither the 1M nor frictional strengths of either sepiolite or palygorskite are not known, the 2M1 polytype is observed as an authigenic phase in any of the but, as they share structural similarities with phyllosilicates, it is gouges sampled for this study, the thermal conditions for 1Md illite likely they too may be weaker than typical Byerlee friction values. As growth in gouges must be lower than those observed by Lonker and the distribution of sepiolite and palygorskite is highly localized, it is Fitz Gerald (1990). Ordered illiteesmectite can form in sedimentary unlikely that the strength of these minerals has a significant control basins at temperatures as low as 45 C and can persist to temper- on LANF frictional behavior. LANF clay-rich gouges do contain atures of 140 C(Freed and Peacor, 1989; Sandler and Sarr, 2007), ‘survivor clasts’ and ‘survivor grains’ (Cladouhos, 1999) of higher- although examples are known from hydrothermal systems to friction silicate minerals, predominantly quartz and feldspars or forming at 100e285 C(Junfeng et al., 1997). Discrete smectite can carbonates, and that the clast-clay ratio is variable within a single form at surface conditions and can persist to 135 C in sedimentary outcrop. However at least some areas of gouge at many outcrops is basins and to 200 C in hydrothermal systems (Hein et al., 1979; clay-rich, and, if shear localizes in the clay-rich areas (e.g. Schleicher Simmons and Browne, 2000; Aplin et al., 2006). Smectite is et al., 2010), then it is likely that low-friction gouges provide an observed in the cores of continental-scale strike-slip faults such as explanation for normal fault slip at low-angles. Most of the common the Punchbowl and San Bernadino faults in California, and the authigenic minerals in LANF gouges have sufficiently low coeffi- Median Tectonic Line and Nojima faults in Japan in conjunction cients of friction to explain slip at dips <30, and possibly as low as with extensive zeolite and carbonate mineralization, although age 10, assuming displacement at over-pressured conditions. relationships are complex, and co-precipitation cannot be assumed In addition to being frictionally weak, the presence of significant (Ohtani et al., 2000; Solum et al., 2003; Shigematsu et al., 2012). authigenic clay in fault gouges has important implications for the Fluid inclusion homogenization temperatures from carbonate veins frictional stability of clay rich gouges, that is, the degree to which in the fault core at the Nojima drill hole where smectite is observed a frictional instability could nucleate within the gouge and propagate range from 88 Cto197C(Ohtani et al., 2000). Corrensite can form to seismic velocities. Measurements of frictional strength over at temperatures between 60e70 C and 160 C in sedimentary a range of sliding velocities for a given material indicate if the basins (Helmond and van der Kamp, 1984; Chang et al., 1986), material becomes frictionally stronger with increasing sliding although examples from open hydrothermal systems are observed velocity (so-called ‘velocity strengthening’, suppressing the at higher temperatures of 150e230 C(Tomasson and tendency of an instability to reach seismic velocity), or becomes Kristmanndóttir, 1972; Keith et al., 1984). Disordered weaker with increasing sliding velocity (‘velocity-weakening’,thus chloriteesmectite can form in soils and is observed in hydro- permitting an instability to ‘run away’ and reach seismic velocities). thermal systems at temperatures of 100e230 C(Tomasson and For the principal authigenic clay minerals reported in clay gouges, Kristmanndóttir, 1972; Bettison and Schiffman, 1988). Observed most all are velocity-strengthening, with the rate-dependent fric- temperatures of clay mineral occurrence in hydrothermal systems tional variable (aeb) being positive over velocities of 0.003 mm/s to are consistently higher than those estimated for the same clay 300 mm/s (Morrow et al., 1992; Saffer and Marone, 2003; Ikari et al., species when found in sedimentary basins and clay gouges. Where 2007, 2009). Measurements of natural gouge materials further 28 S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 support this conclusion (Numelin et al., 2007; Verbene et al., 2010; 3. Illitization of fragmental illiteesmectite to form authigenic Carpenter et al., 2011; Lockner et al., 2011; Sone et al., 2011). It is thus illite-rich illiteesmectite. possible that clay-rich gouges suppress the formation of seismic instabilities and that clay-rich gouges deform primarily by aseismic Perhaps specific to the detachment faults studied, where the creep and not by seismic slip (Schleicher et al., 2010; Ikari et al., 2011). brittle detachment evolved from a mid-crustal shear zone with a distinctive chlorite-metasomatized footwall rocks, we identify 6.8. Implications for fault slip mechanisms a pattern of early chlorite-rich gouge that is fragmental in origin, which is transformed to a ‘retrograde diagenetic’ assemblage of The neoformed clay phases we identify in clay-rich gouges form chloriteesmectite or discrete tri-octahedral smectite (saponite). The at temperatures ranging from w60 to 180 C. This observed early fragmental gouge is preserved in gouge zones at several core temperature range places some limits on the region of faults where complexes and is mineralogically indistinguishable from footwall the mechanical paradox of low-angle normal faulting might be chlorite breccias. This fragmental chlorite is then frequently trans- resolved by weak fault zone material. At conventional geothermal formed into either chloriteesmectite (both random gradients of 20e35 C/km this temperature interval corresponds to chloriteesmectite and corrensite) or discrete smectite. The frag- depths of approximately 3e9kmat20C/km and to only depths of mental chloritic gouge can also be transformed to one of two Mg-rich 2e5kmat35C/km. If low-angle normal faults advect significant assemblages, a sepiolite þ smectite þ talc þ lizardite assemblage, or heat with the uplifting footwall (e.g., Grasemann and Mancktelow, palygorskite þ/ chlorite þ/ quartz. The Mg-rich assemblages form 1992), then the temperature range estimated for authigenic clay at very low temperatures (50e150 C) by interaction of Mg-rich fluids growth would represent shallower depths for clay-assisted slip on with fragmental chloritic gouge. We note that talc and lizardite are these low-angle normal faults. The presence of 1Md illite, free of present only as minor constituents in the Mg-rich assemblages and interlayered smectite in several gouges in this study and are probably not a major factor in fault frictional properties. chloriteesmectite, both as a neo-formed mineral in this study and as Transformations in clay gouge can produce clay-rich fault rocks coatings in other fault rocks, such as the San Andreas Fault (e.g., with lower coefficients of friction than is common for silicate rocks, Schleicher et al., 2012), suggest that clays may indeed be mechan- and thus, where they are present, have the potential to significantly ically active at depths up to 10 km, because of the higher tempera- weaken the surfaces of pre-existing faults, permitting slip at low ture stability of both 1Md illite and corrensite (Velde et al.,1986). The angles in detachment systems and at low shear stresses in general. repeating observation in this study that quartz and calcite behave as The temperature range over which neoformed clays grow in fault fragmental, cataclastic phases in nearly all of the gouges sampled gouges indicates that the depths in the crust where the weak fault suggests that these gouges either a) formed at temperatures below paradox may be resolved is from w2 km to as much as 9 km depth, where quartz and calcite dissolve and re-precipitate rapidly, or b) above the depths where many earthquakes nucleate. formed at temperatures where quartz and calcite can form diage- netically, but are then rapidly tectonically exhumed by continued Acknowledgments slip to lower temperatures where quartz and calcite only behave in a brittle manner. The depth range where these neoformed clays are Support for this work was provided by National Science Foun- thought to form is partially above the depth at which earthquakes dation grant EAR-0738435 and EAR-1118704, a GDL Foundation typically nucleate (5e15 km) (Jackson and White,1989). Perhaps this grant to Haines, and the Scott Turner Fund at the University of separation of clay-rich gouges from regions of active seismicity is not Michigan. GSA and AAPG Grants-In-Aid grants further supported surprising, given the observed tendency of clay minerals to fieldwork for Haines. We are particularly grateful to numerous strengthen with increased sliding velocity, thus suppressing rupture colleagues for directing us to key localities and for stimulating propagation in the uppermost brittle crust (e.g. Ikari et al., 2009). discussions; their generous help was central to the study. We acknowledge use of the EMAL facility and the support of Carl 7. Conclusion Henderson and Anja Schleicher at the University of Michigan.

This study describes and catalogues systematic patterns of clay mineral transformations in clay gouge from low-angle normal Appendix A. Supplementary data faults that involve conditions and lithologies that are common to shallow faults setting around the world, varied both in space and Supplementary data associated with this article can be found, in time. At least five clay minerals have been found to grow in the the online version, at doi:10.1016/j.jsg.2012.05.004. faults sampled in this study: 1Md illite, illite-rich illiteesmectite, discrete smectite (montmorillonite or saponite), disordered References chloriteesmectite, corrensite (ordered chloriteesmectite). Other Mg-rich phases (sepiolite, lizardite, talc, palygorskite) have been Aplin, A., Matenaar, I., McCarty, D., van der Pluijm, B., 2006. Influence of mechanical found, but are rare relative to these five. Chlorite has not been compaction and clay mineral diagenesis on the microfabric and pore-scale properties of deep-water Gulf of Mexico mudstones. Clays and Clay Minerals found as an authigenic mineral, but is common as a fragmental 54 (4), 500e514. phase in clay gouge. Kaolinite was not found in any gouge sampled Anderson, J., Osborne, R., Palmer, D., 1980a. Petrogenesis of cataclastic rocks within for this study, except where it could clearly be related to surficial the San Andreas Fault zone of southern California, U.S.A. Tectonophysics 67, e weathering. Authigenic 1M illite and illite-rich illite-smectite are 221 249. d Anderson, T., Silver, L., Salas, G., 1980b. Distribution and UePb isotopic ages of some found in many fault gouges, both in detachments that evolved from lineated plutons, northwest Mexico. Geological Society of America Memoir 153, mid-crustal shear zones and in faults that formed in the brittle 269e283. regime, as well as many faults in sedimentary environments. Anderson, R., 1971. Thin skin distension in Tertiary rocks of southeastern Nevada. Geological Society of America Bulletin 82, 43e58. Authigenic illite and illite-smectite form by three processes: Anderson, R., 2007. A blister hypothesis for the Central Mojave metamorphic core complex near Barstow, California. Geological Society of America Abstracts with Programs 39, 227. 1. Transformation of fragmental 2M1 illite or muscovite to Armstrong, R., 1972. Low-angle (denudation) fault, hinterland of the Sevier orogenic authigenic 1Md illite; belt, eastern Nevada and western Utah. Geological Society of America Bulletin 2. Breakdown of K-feldspar to form authigenic 1Md illite; 83, 1729e1754. S.H. Haines, B.A. van der Pluijm / Journal of Structural Geology 43 (2012) 2e32 29

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