Slip Rate of the Western Garlock Fault, at Clark Wash, Near Lone Tree Canyon, Mojave Desert, California

Slip rate of the western Garlock fault, at Clark Wash, near Lone Tree Canyon, Mojave Desert, California

Sally F. McGill1†, Stephen G. Wells2, Sarah K. Fortner3*, Heidi Anderson Kuzma1**, John D. McGill4 1Department of Geological Sciences, California State University, San Bernardino, 5500 University Parkway, San Bernardino, California 92407-2397, USA 2Desert Research Institute, PO Box 60220, Reno, Nevada 89506-0220, USA 3Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W Dayton St., Madison, Wisconsin 53706, USA 4Department of Physics, California State University, San Bernardino, 5500 University Parkway, San Bernardino, California 92407-2397, USA *Now at School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 S. Oval Mall, Columbus, Ohio 43210, USA **Now at Department of Civil and Environmental Engineering, 760 Davis Hall, University of California, Berkeley, California, 94720-1710, USA

ABSTRACT than rates inferred from geodetic data. The ously published slip-rate estimates from a simi- high rate of motion on the western Garlock lar time period along the central section of the The precise tectonic role of the left-lateral fault is most consistent with a model in which fault (Clark and Lajoie, 1974; McGill and Sieh, Garlock fault in southern California has the western Garlock fault acts as a conju- 1993). This allows us to assess how the slip rate been controversial. Three proposed tectonic gate shear to the San Andreas fault. Other changes as a function of distance along strike. models yield signifi cantly different predic- mechanisms, involving extension north of the Our results also fi ll an important temporal niche tions for the slip rate, history, orientation, Garlock fault and block rotation at the east- between slip rates estimated at geodetic time and total bedrock offset as a function of dis- ern end of the fault may be relevant to the scales (past decade or two) and fault motions tance along strike. In an effort to test these central and eastern sections of the fault, but inferred from longer term (>1 m.y.) offsets of models, we present the fi rst slip-rate estimate they cannot explain a high rate of slip on the geologic features. for the western Garlock fault that is con- western Garlock fault. strained by radiocarbon dating. A channel Tectonic Models (referred to here as Clark Wash) incised into INTRODUCTION a Latest Pleistocene alluvial fan has been left- Hill and Dibblee (1953) viewed the left-lateral laterally offset at least 66 ± 6 m and no more The tectonic role of the Garlock fault (Fig. 1) Garlock and Big Pine faults and the right-lateral than 100 m across the western Garlock fault, has been an intriguing question for several San Andreas fault as conjugate shears defi ning a indicating a left-lateral slip rate of 7.6 mm/ decades. Three primary models have been pro- regional strain pattern of north-south compres- yr (95% confi dence interval of 5.3–10.7 mm/ posed. It has been interpreted (1) as a conjugate sion and east-west extension (Fig. 2A). In this yr) using dendrochronologically calibrated shear to the San Andreas fault (Hill and Dibblee, view the Garlock and San Andreas faults both radiocarbon dates. The timing of aggrada- 1953) that helps to accommodate convergence accommodate eastward motion of the Mojave tional events on the Clark Wash fan corre- at the major restraining bend in the San Andreas block as it extrudes from the Transverse Ranges sponds closely to what has been documented fault in southern California (Stuart, 1991), (2) as restraining bend formed between the Pacifi c and elsewhere in the Mojave Desert, suggesting a transform fault accommodating extension in North American plates along the San Andreas that much of this activity has been climati- the Basin and Range (Davis and Burchfi el, fault in southern California. Consistent with this cally controlled. The range-front fault, located 1973), and (3) as a structure accommodat- model, left-lateral faulting of Quaternary age in a few hundred meters northwest of the Gar- ing block rotation in the northeastern Mojave California is largely confi ned to the vicinity of lock fault, has probably acted primarily as a (Humphreys and Weldon, 1994; Guest et al., this regional-scale restraining bend in the San normal fault, with a Holocene rate of dip-slip 2003). In contrast to all three geologic models, Andreas fault (Figs. 1 and 2A). Similar lateral of 0.4–0.7 mm/yr. The record of prehistoric geodetic data suggest that the region surround- extrusion models have been proposed for strike- earthquakes on the Garlock fault at this site, ing the Garlock fault is dominated by northwest- slip faults in other parts of the world (McKen- though quite possibly incomplete, suggests a oriented, right-lateral shear and that very little zie, 1972; Tapponnier et al., 1982), as well as for longer interseismic interval (1200–2700 yr) left-lateral strain is accumulating on the Garlock the Los Angeles basin and western Transverse for the western Garlock fault than for the fault (Savage et al., 1981, 1990, 2001; Gan et al., Ranges (Walls et al., 1998), although Argus et central Garlock fault. 2000; McClusky et al., 2001; Miller et al., 2001; al. (1999) argue that crustal thickening, rather The relatively high slip rate determined Peltzer et al., 2001). than westward extrusion, is the dominant mode here indicates that the western and central In an effort to test proposed models and to of convergence in the Los Angeles basin and segments of the Garlock fault show similar better understand the tectonic role of the Gar- western Transverse Ranges (Fig. 2A). rates of movement that are somewhat faster lock fault, we have measured the latest Pleisto- Other investigators proposed a second cene to Holocene slip rate on the western strand model in which the Garlock fault is a transform †E-mail: [email protected] of the Garlock fault, for comparison with previ- fault (Fig. 2B), with left slip on the Garlock

GSA Bulletin; March/April 2009; v. 121; no. 3/4; p. 536–554; doi: 10.1130/B26123.1; 14 ﬁ gures; 1 table; Data Repository item 2008251.

37°N Holocene faults California Owens Valley Quaternary faults GF slip-rate and paleoseismic sites BASIN AND RANGE Saline SAF Valley Area of fig. 1 Death Valley

Eastern CaliforniaPanamint Valley shear zone Sierra 36°N Nevada

San Andreas fault SV SR SSH El Paso Peaks Fault Searles Lake shoreline AM EPM Christmas Canyon Mesquite Canyon SM Bakersfield Koehn Lake SLB Garlock Oak Creek Clark Wash 35°N Twin Lakes Mojave PM Eastern CaliforniaBarstow shear zone San AndreasMOJAVE fault DESERT Big Pine fault

Los Angeles 34°N 10050 0 100 km

120°W 119°W 118°W 117°W 116°W Figure 1. Location of the Clark Wash site (large white circle) as well as other slip-rate and paleoseismic sites (small white circles) along the Garlock fault. AM—Avawatz Mountains; EPM—El Paso Mountains; GF—Garlock fault; PM—Providence Mountains; SAF—San Andreas fault; SLB—Soda Lake Basin; SM—Soda Mountains; SR—Slate Range; SSH—Salt Spring Hills; SV—Searles Valley.

fault accommodating differential extension in perpendicular to the northeast- to east-striking The Cenozoic extension direction in the the Basin and Range province (between the Garlock fault (Fig. 2B). Modern deformation Basin and Range province is west-northwest- Sierra Nevada and Death Valley) relative to in the portion of the Basin and Range province ward (Stewart, 1983; Burchfi el et al., 1987; the Mojave block (Hamilton and Myers, 1966; north of the Garlock fault is largely northwest- Jones, 1987; Wernicke et al., 1988; Snow and Troxel et al., 1972; Davis and Burchfi el, 1973). oriented dextral shear (Fig. 2D). Late Qua- Wernicke, 2000). This orientation more closely The location of the Garlock fault at the south- ternary extension north of the Garlock fault approaches the strike of the central and eastern margin of the western Basin and Range appears to be largely concentrated within pull- ern Garlock fault but is still at a 45° angle to province is consistent with the transform apart basins (Death Valley, Panamint Valley, the western Garlock fault. The transform fault model, as is the eastward termination of the and Saline Valley) between northwest-strik- model may thus be a partially viable model for Garlock fault at the eastern limit of signifi cant ing, right-lateral faults. It is these right-lateral the initiation of left slip on the central and east- Quaternary extension in the western Basin and faults, rather than the Garlock fault, that are ern Garlock fault (if that portion of the fault has Range province (Figs. 1 and 2B). The present- parallel to the present-day extension direction not been rotated—see third model below and day extension direction in the Basin and Range and appear to be serving as transform faults discussion section), but it is unable to explain province, however, is northwestward (Minster for the Late Quaternary extension north of the the orientation of the western Garlock fault, nor and Jordan, 1987; Dixon et al., 2000), nearly Garlock fault (Fig. 2B). does present-day extension seem capable of

Geological Society of America Bulletin, March/April 2009 537

Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/121/3-4/536/3400247/i0016-7606-121-3-536.pdf by California Inst of Technology user on 12 February 2020 McGill et al. 40 km Clark site Wash 40 mm/yr late Quaternary faults early Quaternary faults CMM3 site 20 Kilometers late Quaternary faults early Quaternary faults 40 0 40 40 20 0 40 km 80 0 80 DV PV Mojave block Mojave SV Transform model prediction Transform Clark Wash site Sierra Nevada Present-day motion of Sierra Nevada block (~ 13 mm/yr at N47W) Nevada motion of Sierra Present-day 2000). stable North to and others, relative America (Dixon Present-day motion of Sierra Nevada block (11 ± 1 mm/yr at N37W) Nevada motion of Sierra Present-day 2000). and others, (Dixon Ely Nevada to relative Cenozoic maximum finite elongation direction (N73W), relative to to elongation direction relative maximum finite (N73W), Cenozoic 2000). Wernicke, and (Snow Plateau Colorado D B 80 km Quaternary faults late Quaternary faults early Quaternary faults 40 Kilometers SSH late Quaternary faults early Quaternary faults 80 0 80 80 40 0 Domains or of measured clockwise rotation inferred 40 km Clark Wash site LA SR + 80 0 80 No rotation WTR RS

KL of Garlock fault Garlock of

Clark Wash site Proposed initial orientation orientation initial Proposed WTR RS? A C

538 Geological Society of America Bulletin, March/April 2009

Figure 2. Proposed tectonic models for the Garlock fault. (A) Conjugate faulting (Hill and Dibblee, 1953). Convergence at the large restraining bend in the San Andreas fault is accommodated by a combination of eastward extrusion of the Mojave Desert and westward extrusion (Walls et al., 1998) and crustal thickening (indicated by large “+” symbol) (Argus et al., 1999) of the Transverse Ranges. LA—Los Angeles; WTR—western Transverse Ranges province. (B) Transform fault model (Davis and Burchﬁ el, 1973). DV—Death Valley; PV—Panamint Valley; SV—Saline Valley. (C) Model in which the eastern Garlock fault accommodates clockwise block rotation in the northeastern Mojave Desert (Guest et al., 2003). KL—Koehn Lake slip-rate site (Clark and Lajoie, 1974); RS—Rand schist; RS? —schist between stands of Garlock fault zone that may correlate with Rand schist; SSH—Salt Spring Hills; SR—Slate Range. (D) Geodetic velocity vectors from the Southern California Earthquake Center’s Crustal Motion Model, version 3 (CMM3) (http://epicenter.usc.edu/cmm3/) relative to a refer- ence frame deﬁ ned by 12 stations on the North American plate. Vectors shown have been arbitrarily selected from the complete data set to reduce clutter. Dashed polygons show the locations of two transects plotted in Figure 13.

driving a signifi cant amount of left slip on any has remained an important, throughgoing fault The rotating block model (Fig. 2C) primarily part of the Garlock fault (Figs. 2B and 2D). that has produced several large earthquakes in explains slip on the eastern Garlock fault and Since the recognition of the Eastern Califor- the past few thousand years (McGill and Sieh, predicts little or no slip on the western Garlock nia Shear Zone (Dokka and Travis, 1990), with 1991; McGill, 1992; Dawson et al., 2003) and is fault (Humphreys and Weldon, 1994). throughgoing, northwest-oriented, right-lateral still seismically active (Astiz and Allen, 1983). Our results from the Clark Wash site indicate shear both north and south of the Garlock fault Evidence for Holocene left-lateral slip is abun- that the Holocene slip rate of the western strand (e.g., Savage et al., 1990) (see Fig. 2D), it is dant and nearly continuous along the entire of the Garlock fault is at least as fast as previ- no longer easy to view the Garlock fault as an ~250-km length of the fault (Clark, 1970; Clark, ously published rates along the central section accommodation structure separating two prov- 1973; McGill and Sieh, 1991), whereas the right- of the fault. Neither the transform nor rotating inces with radically different tectonic regimes. lateral faults to the north and south die out (e.g., block models are capable of explaining the ori- Instead, the Garlock fault appears as an enig- Oskin and Iriondo, 2004) or exhibit possible left- entation of and rate of slip on the western Gar- matic anomaly within a single large province— lateral drag folding as in the case of the southern lock fault. Some combination of the above mod- the Eastern California Shear Zone. Both north Death Valley and Panamint Valley faults, as they els seems necessary to understand the tectonic and south of the Garlock fault, the Eastern Cali- approach the Garlock fault (Fig. 1). role of the fault as a whole. fornia Shear Zone (Fig. 1) has accommodated In light of recent understanding of dextral tens of kilometers of northwest-oriented, right- shear and vertical-axis block rotation in the SITE DESCRIPTION lateral shear in the late Tertiary (Stewart, 1983; Eastern California Shear Zone (Luyendyk et Burchfi el et al., 1987; Dokka and Travis, 1990), al., 1985; Carter et al., 1987; Ross et al., 1989; The site discussed here is informally named at rates of 5–10 mm/yr across the zone since the Dokka and Travis, 1990; Schermer et al., 1996), the Clark Wash site because the offset chan- late Pleistocene (Lee et al., 2001; Oswald and Humphreys and Weldon (1994) and Guest et al. nel used for the slip-rate measurement was Wesnousky, 2002; Oskin et al., 2006). Geodetic (2003) have suggested a third model for the Gar- fi rst noted by Clark (1973). The intersection of data (Fig. 2D) indicate that the Garlock region lock fault in which clockwise rotation of blocks Clark Wash with the Garlock fault is located is currently dominated by northwest-trending, in the northeastern Mojave Desert contributes near the center of section 26, T. 31 S., R. 36 right-lateral strain rather than by northeast- left slip to the eastern part of the Garlock fault E. (N35.205°, W118.087°), ~3.5 km southwest trending, left-lateral strain (Savage et al., 1981, (Fig. 2C). Guest et al. (2003) have shown that a of Lone Tree Canyon (Fig. 3), and ~18 km 1990, 2001; Gan et al., 2000; Miller et al., 2001; combination of large magnitude (~35° or more), northeast of the town of Mojave. The site is Peltzer et al., 2001). Present-day, northwest-ori- domino-style, clockwise rotation of blocks (and located ~3 km west of the western end of the ented, right-lateral shear has been documented of their left-lateral bounding faults) both south 3.5-km-wide left step in the Garlock fault that at rates of 9–11 mm/yr north of the Garlock fault and north of the Garlock fault and a moderate separates the western section of the fault from (Dixon et al., 2000; Meade and Hager, 2005) and amount of extension north of the Garlock fault the central section (Fig. 1). Koehn Lake occu- ~7–15 mm/yr south of the Garlock fault (Sauber between the Slate Range and the Salt Spring pies the center of a large closed depression that et al., 1986, 1994; Meade and Hager, 2005). Hills (Fig. 2C) can accommodate the full, has formed within this 3.5-km-wide dilational Despite the prominence of the Eastern Cali- 48–64 km of left slip on the Garlock fault with- stepover in the fault. Within the study area the fornia Shear Zone, the Garlock fault arcs north- out requiring a search for an elusive eastward dominant strand of the Garlock fault is the seg- easterly to easterly through this zone (Fig. 1), extension of the Garlock fault (e.g., Plescia and ment that enters the Koehn Lake stepover from with undeniable left-lateral bedrock offsets Henyey, 1982) that was previously thought nec- the west. However, the central segment of the of 48–64 km (Smith, 1962; Smith and Ketner, essary (Davis and Burchfi el, 1973). fault, which enters the Koehn Lake stepover 1970; Davis and Burchfi el, 1973; Carr et al., These three different models for the tectonic from the east, may continue westward into the 1993; Monastero et al., 1997; Jachens and Cal- role of the Garlock fault each result in different study area as a buried fault along the southern zia, 1998) and a Holocene slip rate between 4 predictions for the slip rate of the fault as a func- range front of the Sierra Nevada (Smith, 1964). and 9 mm/yr (Clark and Lajoie, 1974; McGill tion of position along strike. The conjugate fault This fault strand is referred to as the range- and Sieh, 1993). Dextral shear in the East- model (Fig. 2A) predicts a slip rate that would front fault in this paper. ern California Shear Zone may have rotated gradually decrease eastward, away from the San The Clark Wash site is located on an alluvial the central and eastern sections of the Garlock Andreas fault. The transform model predicts an fan complex that drains southeastward from the fault from an original northeastward strike to incrementally westward-increasing slip rate, as southern Sierra Nevada (Figs. 3 and 4). Clark their current east-northeast and eastward strikes each normal fault north of the Garlock fault con- Canyon, the drainage from which most of the (Fig. 2C) (Jones, 1987), but the Garlock fault tributes additional left slip to the fault (Fig. 2B). alluvial deposits discussed here emanated, drains

Geological Society of America Bulletin, March/April 2009 539

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purpose of exposing the Qf1 soil where it was 118 o 05' W not buried by Qf2. ? The deposits exposed in trench 9 are inter- Lone Tree Canyon preted as belonging to unit Qf1 for several rea- N sons. The surface at trench 9 is elevated above the neighboring Ha surface to the northeast to a greater degree than is Qf2, and it is more dis- 1 km sected than Qf2 (Fig. 4). The two surfaces can also be distinguished because surveyed proﬁ les Clark Canyon indicate that Qf1 has a steeper slope (6.2°) than Qf2 (4.4°). Furthermore, the soil exposed in trench 9 displays stronger development of car- Range-front fault bonate, clay ﬁ lms, and structure than does the Clark Wash soil described on Qf2 in trench 7 (see GSA Data BARREN Repository Table DR11). Area of figure 4 In the latest Pleistocene or early Holocene, a

o Phillips o large channel incised deeply into Qf1 and Qf2

35 12' N Road 35 12' N (see Fig. 4 and trench 6 in Fig. 6). The channel Garlock fault 14 was fi lled with thickly bedded, gravelly sand of early Holocene age (Hoa). Within trench 6, these Range-front fault channel-fi ll deposits interfi nger with sandy colluvium derived from the channel wall and from 118 o 05' W the Garlock fault scarp. The base of the early Holocene channel was not exposed in any of the Figure 3. Location of offset channel of Clark Wash. Base map is the Mojave NE, trenches. The depth of incision was thus greater 15-minute topographic quadrangle published by the U.S. Geological Survey. The than 9 m beneath the top of Qf2, followed by site is located near the center of Section 26, T. 31 S., R. 36 E. Garlock fault location at least 5.4 m of aggradation of the channel-fi ll is from Clark (1973). Heavy lines mark the youngest fault traces; thinner lines deposits (Hoa) (see trench 6 in Fig. 6). Dates from mark older fault traces. Bar and ball on lower side of fault scarps with predom- this episode of channel fi lling range from 8.1 ka inantly horizontal movement. Hachures on relatively downthrown side of fault (sample 6N-44) to 6.8 ka (sample 1N-8/10). scarps with primarily vertical movement. Approximate location of range-front A younger episode of channel incision and fault is from Dibblee (1959), dotted where buried. Note that the canyon labeled fi lling occurred in the late Holocene. This is “Lone Tree Canyon” in this fi gure and on the Mojave 15-minute quadrangle is visible in trench 5 (Fig. 6), where a channel labeled as “Pine Tree Canyon” on the Mojave NE 7.5-minute quadrangle. incised into Hoa was fi lled with deposits (Hya) dated at ca. 2.5 ka. The late Holocene channel- fi ll deposits are also exposed in trenches 3 and an area of ~1 km2 that is underlain by granitic history of aggradation and incision at this site. 4. They are not exposed in trench 6; appar- bedrock of the Sierra Nevada batholith (Smith, Table 1 reports both conventional and calibrated ently the late Holocene channel lies beyond 1964). We excavated 11 trenches by backhoe to radiocarbon ages. All ages mentioned in the text the southwestern end of trench 6. The modern reveal the stratigraphic, tectonic, and pedogenic are calibrated. channel incised to a depth of 2–3 m beneath the history at the site. Trench locations are shown in top of Hya and has since been fi lled with 0–1 m Figures 4 and 5. STRATIGRAPHIC HISTORY OF THE of sediments. Soil profi le descriptions for Qf1, SITE Qf2, and Hya may be found in the data reposi- RADIOCARBON DATES tory (Table DR1 [footnote 1]). Much of the stratigraphic history of this site Twenty-nine charcoal and wood samples can be deciphered from the cross sections of the CULTURAL HISTORY OF THE SITE from the Clark Wash site have been radiocarbon fault-parallel trenches (Fig. 6). During the late dated (Table 1). In our judgment, seven of the Pleistocene, an alluvial fan (Qf1) was aggrad- Several areas of darkened sediment were 29 sample ages do not accurately represent the ing, with alternating layers of grain-supported exposed in trenches 1, 2, and 6. These darkened ages of the alluvial and colluvial deposits from stream deposits and matrix-supported debris zones ranged in size from ~5–30 cm thick and which they were collected. Most of these seven fl ows. Within trenches 3, 6, 7, and 8, a buried from ~30 to 100 cm long. The darkened areas samples were collected from bioturbated zones soil is exposed on the Qf1 surface and is, in in trenches 1 and 6 are labeled A through H or were suspected to be problematical for other turn, buried by an additional ~1 m of younger in Figure 6, and we interpret them as middens reasons before they were dated (see comments alluvial fan deposits (Qf2) (see trench 6 in in Table 1). Radiocarbon sample locations are Fig. 6). Aggradation of the Qf1 deposits began shown in Figures 6 and 7. For brevity, the H14 prior to 22.7 ka (sample 2W-2) and continued 1GSA Data Repository item 2008251, including a prefi x is omitted from the sample numbers in the to at least 13.3 ka (samples 9S-2/4). The lat- table of soil profi le descriptions as well as a fi gure and accompanying text documenting projections of a fi gures and in the following discussion. Nine- ter date comes from trench 9, which was a soil buried channel edge to the fault, is available at http:// teen of the 22 sample ages that we deem reliable pit located ~250 m upstream from most of the www.geosociety.org/pubs/ft2008.htm or by request are plotted in Figure 8, which summarizes the other trenches (Fig. 4) and excavated for the to [email protected].

540 Geological Society of America Bulletin, March/April 2009

b b Hyc Late Holocene colluvium 100 0 200 Hya Late Holocene alluvium b Meters Ha Holocene alluvium, undifferentiated Qf1 Qf2 Latest Pleistocene or earliest Holocene b alluvial fan deposits ? b Qf1 Late Pleistocene alluvial fan deposits Hya b Bedrock (felsic plutonic rocks) Ha Ha Contact, dashed where approximately located Fault, bar and ball on lowerside of scarp Hya Fault, buried b ? Fault, inferred A A' Location of topographic profiles for measuring vertical slip (Figure 8) Area of figure 11 ? Hya Qf1 Qf1 T10 Ha

T9 Hya Qf1 N A''' B'' Hya Figure 4. Simpliﬁ ed geologic map of Clark Wash. Base is a U.S. Geological Survey aerial photograph, courtesy of Mal- Qf2 colm Clark. A'' A'' Qf2 Ha A' 2nd Los Angeles aqueduct B' A' B' Hyc Hya Area of figure 4 Qf2 Qf2 Hya B A

868 Qf1 d Qf2 d Disturbed areas, including dirt roads T3 866 Hya Qf2 Modern channel 870 Hya Late Holocene alluvium 872 T2 d 66 + 6 m 864 Hyc Holocene colluvium, massive Qf2 72 m Qf2 Latest Pleistocene or early Holocene 103 m alluvial fan deposits 58 m T5 T7 Qf1 Late Pleistocene alluvial fan deposits d 862

868 T4 Garlock fault, Hya T1,6 Dashed where approximately located 866 860 T8 N Hyc Qf2 Dotted where concealed Contact Top of terracer riser, Qf2 864 Longer dash where projected 858 Piercing point Trench Hya Offset measurment 100 m 862

Figure 5. Geologic map showing left-lateral offset of the incised channel of Clark Wash across the Garlock fault. Base map is a photogrammetric map constructed by Advanced Digital Maps. Contour interval is 2 m. Trenches (T1–T6) and soil pits (T7 and T8) are shown. Trench 6 was a reexcavation of trench 1, making it longer and deeper.

Geological Society of America Bulletin, March/April 2009 541

e n i l a k l a

r o f Comments

l l a H m 6

h t o r o a t

e e l h

p m m o a r F S in Hoc burrow From

y l i m l a f a

Larrea o e c a r e a c h a Sample material r Charcoal None C e t s wood (creosote) wood A a y unit H channel Stratigraphic Stratigraphic 6 0 4 6 9 2 (%) Relative Relative probability 0 0 0 6 0 0 0 # 3 7 1 4 6 7 2 1 † – – – – 0 0 0 0 6 2 1 30–0 6 2 Range (yr B.P.) 1 4 2 2 6080–6000 5 6 8 7870–7580 95 2 7870–7620 94 b m o Calibrated age Calibrated b - 820 920–710600 100 670–540 Hoc 100 Charcoal Qf2 bioturbated Possibly Charcoal in Qf2 Sample from burrow 150 290–0 100 Hoc Uncharred t s Median (yr B.P.) o P TABLE 1. RADIOCARBON DATES FROM THE CLARK WASH SITE WASH THE CLARK FROM RADIOCARBON DATES 1. TABLE § u u u u Conventional C age (yr B.P.) 14 ‡ ) oo / o C ( −22.94 −251 ± 57 13 1 2 / / 8 0 6 8 3 2 8 8 1 1 R R18388/4 R18388/2 R R18388/3 R18388/5 R18388/6 AA-14558 −21.3 631 ± 54 AA-15822 −24.0 892 ± 52 AA-15823 −23.1 136 ± 73 NZA 3692 NZA 3693 −16.45 2419 ± 67 2490 2710–2340 100 Hya Charcoal None NZA 3691 −22.11 2412 ± 67NZA 3694 −24.14 2480 2449 ± 67NZA 3699 2710–2340 −16.05 2530 100 2448 ± 68 2710–2350 Hya 2530 100 2710–2350 Hya Charcoal 100 None Hya Charcoal None Charcoal None u 2 u u ‡‡ ## ††u §§‡‡ 4 ## 1 ? / 2 1 1 4 1 1 / / 6 0 1 4 H14-6S-20 AA-14548 −22.6 ± 71 5602 6390 6550–6280 99 Hyc Charcoal Hearth 6C-1 From 1 Samples from unit Hoc and Hoa H14-6N-60 H14-5S-1 H14-6N- Beta-74108 −24.9 ± 140 7170 8000 8300–8240 4 Hoc Charcoal Combination of three samples H14-6N-44 AA-16339 (−25.0) 7240 ± 59 8065 8170–7960 100 Hoa Charcoal None H14-4E-12 NZA 3514 −21.49 2534 ± 68 2590 2760–2430 94 Hya Charcoal None Samples from the modern channel deposits H14-3S-6 NZA 3196 −22.07 260 ± 61 310 490–260 74 Modern Sample from unit Qf2 H14-4W-2 H14-2E-11H14-2W-30H14-1N-8(&10?) AA-9170 AA-9172 AA-9173 (−25) (−25) −22.7 1920 ± 55 5950 ± 70 1025 ± 55 1860 6780 940 1990–1730 6970–6630 1060–790 100 100 100 Hyc Hyc Hyc Charcoal Charcoal Charcoal None or midden a burn layer From or midden Composite sample from a burn layer H14-6N-17 AA-14551 −23.6 ± 88 6888 7730 7930–7890 5 Hoc Charcoal 6G burn feature From H14-6N-133 AA-14552 −23.8 ± 71 6918 7760 7930–7890 6 Hoc Charcoal 6F burn feature From 1 H14-6S-1H14-6N-71 AA-14550 −24.3 ± 103 7056 7880 8050–7670 99.6 Hoc Charcoal 6E burn feature From H14-6N-145 H14-4E-17 Sample no. Laboratory no. Samples from unit Hya Samples from unit Hyc H14-6N- AA-14547 −22.0 ± 98 5516 6310 6500–6170 91 Hyc Charcoal 6A-1 burn feature From H14-4W-5 H14-6N-56 AA-14555 −23.6 ± 91 6812 7660 7840–7500 100 Hoa Charcoal None H14-6S-31 AA-14549 −25.3 ± 94 7040 7860 8020–7680 100 Hoc Charcoal 6D burn feature From H14-4W-4 H14-6N-50B AA-14553 −23.4 ± 109 6968 7800 7980–7600 100 Hoc Charcoal hearth 6H From

542 Geological Society of America Bulletin, March/April 2009

remaining from Native American occupation of the site. In some of the middens (especially H, F, and C), we observed closely spaced charred cobbles within the wall of the trench. We excavated middens H, C-1, and C-2 in three dimen- sions and showed them to be unambiguous cultural hearths composed of circular collec- tions (fi lled circles, not rings) of tightly packed charred cobbles. Gardner et al. (2002) provide more detailed descriptions, sketches, and pho- tographs of the hearths. The 7.7–8.3 ka age of Comments hearth 6H (sample 6N-140/141/142) makes it one of the oldest radiocarbon-dated archeo- logical features in the western Mojave Desert (Gardner et al., 2002). Perhaps sample is from backfi ll of T1? ll of sample is from backfi Perhaps in by backhoe. in by below surface. below Sample fell from wall. May have been brought have May from wall. Sample fell C normal. Carbon yield low for charcoals. for Carbon yield low C normal.

13 HOLOCENE SLIP RATE OF THE e δ a

e WESTERN GARLOCK FAULT c a ) b a ples were dated using accelerator mass spectrometry dated using accelerator ples were F on 5.0.1 (Stuiver and Reimer, 1993) using Northern and Reimer, on 5.0.1 (Stuiver

Offset Measurement d ombustion. ombustion. e family r reason. Sample r material a continued h Our Holocene slip-rate estimate for the west- c n

U ern Garlock fault depends critically on the measurement of the offset of the channel incised into Qf1 and Qf2. This channel is left-laterally sepa- 1 f unit Q rated by 66 ± 6 m along the western segment of the Garlock fault (Fig. 5). We measured the Stratigraphic Stratigraphic separation both by using surveyed points along facility (“AA” prefi x) or at Rafter Radiocarbon Laboratory at the Department of Scientifi c x) or at Rafter Radiocarbon Laboratory at the Department of Scientifi prefi (“AA” facility the top of the channel wall and by using the top-

(%) ographic contours on the photogrammetric map Relative Relative

probability (with a 0.5-m contour interval) to deﬁ ne the top of the channel wall incised into Qf1 and Qf2 on # the eastern side of Clark Wash. The southwest- † ern wall of the channel has been more greatly 20–0 6 Range (yr B.P.) 170–150 2 220–140 21 modiﬁ ed by erosion, but projection of the top of 19,610–19,080 88 the southwestern channel wall to the fault sug- b

m gests a similar separation of 65 ± 7 m (Fig. 5). o Calibrated age Calibrated b uncertainties and are rounded to the nearest 10 yr. Ranges separated by less than 10 yr were combined. combined. less than 10 yr were by Ranges separated uncertainties and are rounded to the nearest 10 yr. - 380310 490–280 480–260 98 73 Qf1 Qf1 Charcoal Charcoal T1. T6 that reoccupied Sample from area of age unknown. anomalously young Reason for t

σ Both of these separations are minimum s Median (yr B.P.) o

P estimates of the tectonic offset because the

§ upstream, northeastern channel wall and the u u u

C was also low. This could be due to the species of plant charcoal came from. also low. C was downstream, southwestern channel wall may TABLE 1. RADIOCARBON DATES FROM THE CLARK WASH SITE ( WASH THE CLARK FROM RADIOCARBON DATES 1. TABLE 13 δ have been subject to lateral erosion by the late Holocene channel, which would lessen both Conventional C age (yr B.P.) of the apparent offsets. No deposits of Hoa or 14 uncertainties. ‡

) Hoc are preserved along the northeastern chan- σ oo / o nel wall, on the northwest side of the fault (see

C ( trench 3 in Fig. 6). Presumably they were pres- 13 ent at one time but were removed during the incision of the late Holocene channel. This suggests that some amount of Qf1 may also have

R18260/1 been eroded by this channel, thus making the AA-15311AA-15824 −23.6 −22.81 Post-bomb AA-14559 −22.5 300 ± 51 257 ± 55 present separation less than the true, tectonic offset of the early Holocene channel wall. u u

u The tectonic offset can be no more than 103 m, however, because any offset larger than Soil stuck to sample. Acid wash only. Sample too small for further pretreatment (no alkaline wash). Red-brown residue after c Red-brown further Sample too small for pretreatment (no alkaline wash). only. Acid wash to sample. Soil stuck further Sample too small for pretreatment. only. Acid wash charcoals and to normal low for Carbon yield was pretreatment. any Sample too small for only. Combustion Calibrated age ranges are reported in years before 1950, with 2 are reported age ranges Calibrated before in years See comments for collected. unit from which it was indicator of the age stratigraphic Sample judged to be an unreliable All samples calibrated with the University of Washington Quaternary Isotope Laboratory’s Radiocarbon Calibration Program versi Quaternary Program Radiocarbon Calibration Washington Isotope Laboratory’s of with the University All samples calibrated measured. were All other values are in parenthesis. Assumed values ages are reported with 1 Conventional this would require the northeastern wall of # †† ‡‡ §§ ## u † ‡ § Laboratory sample number with “Beta” prefi x was dated by Beta Analytic using conventional radiocarbon techniques. All other sam techniques. radiocarbon Beta Analytic using conventional dated by x was prefi “Beta” Laboratory with sample number H14-6N-68 H14-6N-67 H14-10W-1 AA-15827 −21.15 ± 149 16,264 19,420 19,810–19,650 12 Qoc Charcoal None methods at the University of Arizona’s National Science Foundation–Accelerator Mass Spectrometry National Science Foundation–Accelerator of Arizona’s methods at the University x). prefi Zealand (“NZA” and Industrial Research in New Hemisphere terrestrial dataset (intcal04.14c) of Reimer et al. (2004). Hemisphere terrestrial dataset (intcal04.14c) of Reimer et al. Sample no. Laboratory no. Samples from unit Qf1 H14-1N-1 Sample from unit Qoc H14-3S-4 NZA 3197 −22.16 18,295 ± 143 21,760 22,190–21,220 100 Qf1 Charcoal None H14-9S-2/4 AA-15825/6 11,470 ± 105 13,330 13,570–13,130 100 Qf1 Charcoal samples 1.25 and 1.5 m Combination of two H14-2W-2 AA-9171 (−25) 19,075 ± 220 22,660 23,400–22,210 100 Qf1the Charcoalchannel None on the upstream side of the fault to have initially incised in a location that is southwest of the projection to the fault of the downstream, southwestern wall of the channel

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Qf2 SW Trench 3 NE 868 m

Hya 866 m 3S-6 (0.3 ka) 1,2,5 3,4 3S-4 (21.7 ka) Hya Modern channel 864 m Modern channel deposits Qf1 862 m Hya Late Holocene alluvium Hyc Mid- to late Holocene colluvium Meters Hoa Early to mid-Holocene alluvium Hoc Early to mid-Holocene colluvium SW Trench 5 NE Qf2 Latest Pleistocene or early Holocene fan deposits Hya 864 m Modern channel Qf1 Late Pleistocene fan deposits Hyc 862 m 6S-1 Dated radiocarbon samples Hya A1 Hearth or midden Hoa? 860 m 5S-1 (2.5 ka) a Qf1? Meters

SW Trench 6 NE 864 m Qf2 862 m 6N-133 6N-116/112 (6.3 ka) Qf1 Hya Modern channel Hya 1N-1 B1 B2 1N-8 A2 A1 6N-145 Hyc A3 858 m Hoa 6N-56 (7.7 ka) 6S-20 C1,2 G F 6N-71 6N-68 856 m Hoc E D 6N-44 6N-50b 6N-60 6N-67 6N-17 6N-140/141/142 (8.0 ka) 6S-1 6S-31 Meters Figure 6. Cross sections along the northwestern walls of the fault-parallel trenches (T3, T5, and T6). The three trenches are shown with the northeastern edge of the modern channel (which ﬂ ows relatively straight across and roughly perpendicular to the Garlock fault) aligned. The fault is located between trenches 3 and 5. Note the left-lateral shift (between trenches 3 and 6) of the buried wall of the channel that incised into Qf1. All dated radiocarbon samples from these trenches are shown. Samples that were collected from the southeastern wall of a trench have an “S” within the sample number. They have been projected to an equivalent position on the northwestern wall. The median calibrated radiocarbon dates (rounded to the nearest 100 yr) for a few representative samples are shown. See Table 1 for the dates of other samples. To provide additional context, the surface topography and additional exposed stratigraphic relationships are sketched in a schematic way beyond the limits of the portions of trenches 3 and 6 that were quantitatively documented. Buried channel features used to reconstruct the offset of the Hya deposits are marked by arrows labeled “a” in trench 5 and “1,2,5” and “3,4” in trench 3 (see footnote one).

(Fig. 5). Allowing for an initial width of the Age Constraints 8065 yr B.P. (two-sigma range of 8170–7960 yr channel of at least 3 m (which is narrower than B.P.; Table 1). Seven other samples from slightly the modern channel at present), we use 100 m We now examine the constraints on the age of higher stratigraphic levels within Hoa and Hoc as a maximum offset. Because both walls of this offset channel. We were unable to obtain any are slightly younger and are thus consistent with the channel become well aligned when ~66 m reliable dates from Qf2. The youngest reliable this date (Table 1 and Fig. 6). Two samples from of slip is restored, we suspect that modifi cation date from deposits that predate the incision of the Hoc are much younger (6N-60 and 6N-71) and of the channel wall is minor and that the tec- channel is 13.3 ka (two-sigma range of 13,570– are interpreted as being emplaced by bioturba- tonic offset that has occurred since the initial 13,130 yr before present, from samples 9S-2/4, tion and thus not representative of the deposi- incision of this channel is substantially closer from trench 9; see Table 1). This is a combined tional age of Hoc. We use the age of sample to 66 m than to 100 m. date measured on two samples neither of which 6N-44 to constrain the minimum age of initial The vertical displacement of the top of the was large enough to date independently. The two incision. We acknowledge that the channel wall channel wall is negligible. Topographic pro- samples came from depths of 1.5 m (9S-2) and segment located upstream from the fault may fi les constructed from the photogrammetric 1.25 m (9S-4) beneath the surface of Qf1. have been modifi ed by lateral erosion after this map indicate that the vertical displacement A minimum age for the initial incision of the 8065-yr B.P. minimum age of initial incision, of the top of the channel wall across the main offset channel may be obtained from the oldest but this fact is accounted for in our use of a fault strand (after restoring the left-lateral slip) deposits that fi ll the channel. The stratigraphi- maximum offset of 100 m. is 0 ± 1 m on the northeast side of Clark Wash cally lowest dated sample is 6N-44 (bold label In our calculation of the slip rate, we do not and 1.3 ± 0.7 m northwest-up on the southwest in Fig. 6). The median of the probability density attempt to “correct” the age of sample 6N-44 side of the wash (Fig. 9). function for the calibrated age of this sample is with an estimation of any potential inherited

544 Geological Society of America Bulletin, March/April 2009

NW Trench 4 SE 866 m

Road-cut 865 m Event A 4W-2 Hya Dirt road 864 m 4E-17 (2.5 ka) Event B?

4W-4 4W-5 863 m Event C

4E-12 (2.6 ka) Hya 862 m

Meters

NW Trench 2 SE

864 m Fault scarp Hyc? Dirt road Road berm Qf1 2E-1 Burn layer or midden 862 m Hyc Hyc

Qf1 860 m Qf1 2W-2 (22.6 ka) Meters

SE Trench 10 NW

Granitic bedrock Qf Meters Qoc 10W-1 (19.4 ka) Range-front fault zone

Meters Figure 7. Cross sections along fault-perpendicular trenches (T2, T4, and T10). White-ﬁ lled circles show the locations of radiocarbon-dated charcoal samples. Dated samples that were collected from the opposite wall of the trench have been projected to an equivalent position on the wall that is shown. The median calibrated radiocarbon dates (rounded to the nearest 100 yr) for a few representative samples are shown. See Table 1 for the dates of other samples.

Geological Society of America Bulletin, March/April 2009 545

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3 2 A Top of Qf2 1 Top of Qf1 0 –1 10W-1 9S-2/4 Top of Hya –2 3S-4 Top of Hoa –3 –4 2W-2 Figure 8. (A) History of aggradation and –5 incision at Clark Wash. Filled squares show dated samples from trenches 1–8, near the –6 Garlock fault. Open circles show dated –7 Lowest exposure samples from trenches 9 and 10, located 250 –8 of Hya Lowest exposure of Qf1 Lowest exposure of Hoa/Hoc and 350 m farther upstream, respectively. Elevation relative to top of Qf1 (m) relative Elevation –9 (B) Comparison of timing of aggradational –10 pulses at Clark Wash with other sites in the B Mojave Desert (Soda Lake basin, Harvey Clark Wash Hoa Qf1 Qf1 Qf2 Hya and Wells, 2003; Providence Mountains, Qf1 Soda Lake basin LM1 LM2 Qf2 YS Qf3 Qf4 Qf5 McDonald et al., 2003; Mojave Desert, Bull, Qf4Providence Mountains Qf5 Qf6 Qf7 1991) and with lakes stands of Lake Mojave Mojave Desert (LM1—Lake Mojave I; LM2—Lake Mojave II; YS—youngest shoreline; Harvey 3000030,000 2500025,000 20,000 20000 1500015,000 1000010,000 5000 0 and Wells, 2003). Calibrated age, yr before A.D. 1950

Tops and bases of stratigraphic units in T1-8, where exposed Lowest exposures of stratigraphic units in T1-8 (units extend deeper than this) Portions of the aggradational and incisional history that are well-constrained Portions of the aggradational and incisional history that are poorly constrained Aggradational pulses Stands of Lake Mojave

Northwest Southeast 890 B'' Qf2 885 Northeast of channel A''' Southwest of channel 880 Road Qf1 875 Qf1 Qf2 Figure 9. Topographic proﬁ les B' A'' northeast and southwest of Clark Road Qf2 1.3 m, NW-up 870 Wash, showing vertical displace- Fault Qf2 ment of the Qf2 surface across the A' Qf2 0 ± 1 m Garlock fault after restoration of Elevation (m) 865 Road left-lateral slip. See Figure 4 for Road Fault Qf2 proﬁ le locations. 860 Qf2 B 855 Qf2 A 850 0 50 100 150 200 250 300 350 400 Distance southeastward along profile (m)

546 Geological Society of America Bulletin, March/April 2009

age it may have had at the time of deposition. 95% conﬁ dence bounds on the offset. We then We acknowledge that sample 6N-44 may have add to this distribution tails on each end that 0.04 been deposited after its calibrated radiocarbon extend another 6 m on each side, and which age of 8065 yr B.P., perhaps up to a few hun- each contain 2.5% of the total area under the 0.03 dred years later. However, we are trying to date curve, to allow for a 5% chance that the true 0.02 the initial incision of the offset channel, which offset lies outside of the range 60–100 m. 0.01 occurred prior to deposition of this sample, and Our probability density function for the age of 0

prior to the deposition of at least 0.5 m of Hoa initial incision of the channel is triangular with density Probability deposits (and possibly much more, because the a peak at 9300 yr B.P. This refl ects our judg- 50 60 70 80 90 100 110 base of the offset channel was not exposed in ment that the age of incision is much closer to A Left-lateral offset (m) any of the trenches). We thus assume that any the younger bounding age of 8.1 ka than to the overestimation of the age of initial incision of older bounding age of 13.3 ka. It seems likely the offset channel as a result of the potential that the events that occurred between 13.3 ka inherited age of sample 6N-44 is compensated and incision of the channel (deposition of 0.0004 for by the underestimation of the age of inci- ~1.25 m of Qf1, development of a soil profi le on 0.0003 sion as a result of the time required to deposit Qf1, and followed by deposition ~1 m of Qf2) 0.0002 the sediment that had fi lled the channel prior to required more time than the deposition of the 0.0001 deposition of sample 6N-44. >0.5 m of Hoa that had fi lled the channel prior 0 to deposition of sample 6N-44. Within the range 7000 12,000 Slip Rate Calculation of reasonable choices for the age at which the density Probability Calibrated age (yr B.P.) peak of the triangular distribution is placed, the B To derive a preferred slip rate for the main specifi c choice does not greatly affect the slip strand of the Garlock fault, we construct a pre- rate or its confi dence limits. We chose 9300 yr ferred probability density function (PDF) for the B.P. based on a tentative correlation of climati- 0.04 offset of the northeastern channel wall and for the cally driven pulses of alluvial fan deposition 0.03 age of initial incision of that channel wall. Each within the Mojave Desert (Harvey and Wells, is constrained by quantitative measurements yet 2003). As shown in Figure 8B and described in 0.02 also shaped by our subjective judgment regarding the discussion section on paleoclimatic impli- 0.01 the geologic history of the site. We then construct, cations, Qf2 at Clark Wash may correlate with 0 using a spreadsheet, a joint probability density alluvial unit Qf2 in the Soda Lake Basin, which Probability density function for offset and age. Each cell in the two- has an estimated age of 11.5–9.3 ka (Harvey and 051015 dimensional joint PDF contains the probability Wells, 2003). This choice also puts the incision C Slip rate (mm/yr) that the offset and age both fall within the range of Clark Wash into Qf2 during the youngest of offsets and ages spanned by that cell: lake stand of Lake Mojave (Wells et al., 2003; Figure 10. Probability density functions for located in the Soda Lake basin and east of the left-lateral offset (A) and age (B) of Clark p(O,A) = p(O)dO p(A)dA. Soda Mountains; see Fig. 1). The relatively high Wash that were assigned on the basis of rainfall inferred for this period is consistent with quantitative constraints and subjective judg- We then sum the probabilities contained in all the high ratio of runoff to sediment availability ment (see text), and the resulting probability of the cells in the joint PDF that have offsets that would have been necessary for the incision density function for the slip rate of the Gar- and ages that contribute to a particular range of of this channel. lock fault (C). slip-rate values, in order to obtain the probabil- From this peak, the triangular density function ity that the slip rate falls within that range. This for age of incision slopes downward and reaches allows us to calculate a PDF and cumulative zero probability at the outer tails of the probabil- probability distribution for the slip rate, from ity density functions for the bounding dates. At PDF for slip rate, these extreme bounds corre- which we can obtain the mean and 95% confi - the older end, our PDF reaches zero probabil- spond to a 99.95% confi dence interval. dence intervals for the slip rate. The probability ity at 13,700 yr B.P. because 99.9% of the area density functions we constructed for offset and beneath the PDF for the date on sample 9S-2/4 is PREHISTORIC EARTHQUAKES age and the resulting PDF for slip rate are shown younger than this date. At the younger end, our in Figure 10. The following paragraphs describe PDF reaches zero probability at 7880 yr B.P., Although the primary focus of this study our reasoning in choosing the particular shapes because 99.9% of the area beneath the PDF for was to measure the slip rate of the fault, trench and boundaries that we used for the probability the date on sample 6N-44 is older than that date. 4 also revealed evidence for at least two prehis- density functions for offset and age. The mean of the resulting slip-rate distribu- toric faulting events (Fig. 7). The most recent Our probability density function for the tion is 7.6 mm/yr, and the 95% confi dence inter- event (event A) disrupted the younger Holocene offset of the northeastern wall of the channel val for the slip rate is 5.3–10.7 mm/yr. Simple channel-fi ll deposits (Hya) within a 1-m-wide has a fl at-topped peak from 66 to 76 m, thus division of the maximum offset by minimum age shear zone that extends nearly up to the mod- including the possibility of up to 10 m of lat- and division of the minimum offset by the maxi- ern ground surface on the Hya terrace. The evi- eral erosion of the channel wall within the most mum age yield a broader range of possible slip dence for an older event (event C) includes a probable offset estimate. From here the density rates (4.4–12.6 mm/yr), but these extreme val- fi lled fi ssure and upward termination of minor function slopes down to 60 m and 100 m, which ues represent much more conservative bounds faults. All of these features were visible on are considered to be subjectively equivalent to than a 95% confi dence interval. In our preferred both walls of the trench. In the northeastern

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wall of trench 4, a single fault strand terminates Event C in trench 4 (Fig. 7) occurred at about range-front fault projects to the range-front fault upward at a stratigraphic level intermediate this same time. Thus the 2–16 m offset of the somewhere between ~896.2 and 897 m elevation. between the event-A horizon and the event-C Hya deposits was accomplished by at least one If no vertical slip has occurred on the range-front horizon. However, that same fault strand termi- earthquake (event A) and more likely by two or fault, then the top of the channel wall northwest nates at a lower stratigraphic level on the oppo- more earthquakes (events A and C and possibly of the fault would also have originally intersected site wall of the trench. While this fault strand event B or additional events at the stratigraphic the fault at the point where the 896.2- to 893-m may indicate the occurrence of an additional level of event A). contour intersected the fault. Considering the faulting event (event B), it is also possible that erosion that may have occurred since the chan- it represents slip during event A that did not HOLOCENE SLIP RATE OF THE nel incised, several possible projections of these propagate all the way to the surface. RANGE-FRONT FAULT contours to the fault are possible (Fig. 11). These Four detrital charcoal samples constrain constrain the left-lateral separation of the chan- the ages of these faulting events (samples The left-lateral slip rates reported in the previ- nel wall across the range-front fault to between 4E-17, 4W-4, 4W-5, and 4E-12; Table 1 and ous sections apply only to the main strand of the 17 and 34 m, if no vertical slip occurred. Fig. 7). The layers from which samples 4W-5 Garlock fault at this site. We must consider the However, if northwest-side-up displacement and 4E-12 were collected bracket the event-C possibility that additional left-lateral slip may has occurred on the range-front fault, then less horizon. If we assume that the radiocarbon age have occurred on the range-front fault. Discon- left-lateral slip is required to restore the chan- of these samples was close to zero at the time tinuous, crudely aligned notches and breaks in nel wall to its original position. For example, if they were deposited, then event C occurred at slope along the range front suggest the presence 5–6 m of northwest-side-up displacement has ~2555 ± 205 yr B.P. The date of event A is not of a broad, range-front fault zone (Fig. 4), which occurred, then no left-lateral slip is required to well constrained, but historical data indicate may be continuous with the central strand of the restore the channel wall to a position in which that event A must have occurred before the Garlock fault, which forms the northern side of it could reasonably have been cut. Topographic beginning of the twentieth century (Hanks et the stepover in the Koehn Lake basin (Fig. 1). relief across the range-front fault suggests that al., 1975). The interseismic interval between Left-lateral defl ections of stream channels are several hundred meters of northwest-side-up events A and C is between 1200 and 2700 yr. present at some locations along the range-front displacement has occurred along that fault dur- However, the recurrence interval could easily fault. We excavated trench 10 across one strand ing its history. On the northeast side of Clark be less than 1200 yr, if event B is real, if more of the range-front fault just northeast of Clark Canyon, the maximum elevation of Qf1 depos- than one event occurred at the stratigraphic Wash (Figs. 4, 7, and 11). At the range front, its on the northwest side of the fault is 3 m level of event A without any intervening depo- a prominent fault zone was visible within the higher than the maximum Qf1 elevation on the sition, if other events went undetected in the trench. Near the northwestern end of trench 10, southeast side (Fig. 11). In addition, the surface trench, or if the charcoal samples had an inher- a 1-m-wide fi ssure within extremely weathered of Qf1 is not preserved on the northwest side ited age at the time of deposition. and fractured granitic bedrock (saprolite) was of the fault, so the maximum elevation of Qf1 fi lled with unconsolidated colluvium (Fig. 7). there may well have been a few meters higher LATERAL SLIP IN THE MOST RECENT The southeastern wall of the fi ssure is nearly than the top of the existing deposits prior to FAULTING EVENTS vertical, but the northwestern wall dips ~45° to erosion. On the southwest side of Clark Can- the southeast (away from the mountain front). If yon, the range-front fault steps northwestward, None of the individual strata or channel this dip is representative of the dip at depth, it and a probable fault scarp in Qf1 has a verti- edges in trench 5 could be correlated with any indicates that any vertical displacement on this cal surface offset of 5.5 m (northwest-side-up). certainty to specifi c strata or channel edges strand of the range-front fault is normal in sense Thus, it appears that the amount of dip-slip on within the channel fi ll in trench 3. However, rather than reverse. the range-front fault since abandonment of Qf1 the deposits exposed in the southwestern two- About 10 m farther southeast, two nearly and the incision of the early Holocene chan- thirds of trench 5 (Hya) generally resemble vertical fault strands offset both the weathered nel is suffi cient to restore the channel wall to the character of the channel fi ll in trench 3 bedrock and an overlying layer (~0.3 m thick) of a position in which it could reasonably have (Fig. 6). Various projections of channel trends well-consolidated colluvium (Qoc). The collu- been cut, without any left-lateral slip on the to the fault yield left-lateral offsets of 2–16 m vial layer shows apparent vertical displacements range-front fault. We thus infer that the range- for the late Holocene channel (see Figure DR1 of ~10–15 cm southeast-side down on each of front fault is primarily a dip-slip fault. and accompanying text [footnote 1]). The large the two fault strands. A detrital charcoal sample We can make a crude estimate of the rate uncertainty in the offset estimate for the late from this well-consolidated, faulted colluvial of dip-slip on the range-front fault as follows. Holocene channel results from the fact that deposit has a calibrated age of 19.4 ka (range Assuming a fault dip somewhere between 90° all reasonable channel trends and bends in the 19.1–19.8 ka; sample 10W-1 in Table 1). This and 45°SE (as exposed in trench 10), and using channel are considered and from the fact that indicates that the range-front fault system has the measured 8° slope of the Qf1 surface near the the trends are projected over a 25 m distance been active in the past 20 ka. range-front fault scarp southwest of Clark Can- between trenches 3 and 5. A three-dimensional The early Holocene channel incised into Qf1 yon, a 5.5-m vertical surface offset translates into excavation of the late Holocene channel wall shows a slight left-lateral separation at the range a vertical displacement between 5.5 m (for a 90° will be necessary to measure its offset exactly. front (Figs. 4 and 11). Because very little of the dip) and 6.4 m (for a 45° dip), and into a dip-slip Detrital charcoal sample H14-5S-1 (Table 1), Qf1 deposits are preserved on the northwest side displacement between 5.5 m and 9.1 m (Fig. 12). from a portion of the late Holocene channel-fi ll of the range-front fault, it is diffi cult to measure This displacement has accumulated since the deposits (Hya) in trench 5 that stratigraphically precisely the amount of separation. Nonethe- time that Qf1 was abandoned, which was some- overlies the channel features that appear to be less, the following arguments can be made. A time after 13.3 ka (sample 9S-2/4). This suggests offset 2–16 m, has a median age of 2530 yr topographic profi le along the top of the channel a vertical slip rate of at least 0.4–0.5 mm/yr and a B.P. (two-sigma range: 2710–2350 yr B.P.). wall incised into Qf1 on the southeast side of the dip-slip rate of at least 0.4–0.7 mm/yr.

548 Geological Society of America Bulletin, March/April 2009

50 m N Qc Plutonic bedrock Ha

Qf1 17 m

Hya

34 m T10 Dirt road Hya Late Holocene alluvium Qf1 Ha Holocene alluvium, undivided Qc Quaternary colluvium Hya Qf1 Late Pleistocene alluvial fan deposits Plutonic basement rock Fault, dashed where approximately located Contact, dashed where approximately located d Top of channel wall Projection of top of channel wall to fault Projection of 896.2-m and 897-m topographic contours to fault Piercing point for maximum left-lateral offset Piercing point for minimum left-lateral offset, if no vertical slip occurred

Figure 11. Surveyed topographic map of northeast edge of incised channel of Clark Wash at the range-front fault. Contour interval is 1 m.

Ground surface Definitions y α α = surface slope (8o) θ θ = fault dip (90o-45o SE) h = vertical surface offset (5.5 m) α h +y = vertical slip h d = dip slip b d Fault surface Derivations b = h cos α dip slip = d = b / sin (θ– α) = h cos α / sin (θ– α) = 5.5–9.1 m θ Ground surface vertical slip = h+y = d sin = h cos α sin θ / sin (θ- α)=5.5–6.4 m

Figure 12. Illustration of the conversion of vertical surface offset (h) to dip-slip (d) and vertical slip (h+y).

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DISCUSSION have measured at Koehn Lake on the central left slip on the fault. Present-day deformation Garlock fault (see KL in Figs. 2C and 13). This reveals little or no Garlock-parallel extension Tectonic Role of the Garlock Fault contention is further supported by the fact that north of the Garlock fault (Figs. 2D and 14). 48- to 64-km bedrock offsets are noted along Summation of Late Quaternary fault slip rates Our result at Clark Wash indicates a slip the central section of the Garlock fault (Smith, both north and south of the Garlock fault (Hum- rate of at least 5.3 mm/yr for the western Gar- 1962; Smith and Ketner, 1970; Carr et al., 1993; phreys and Weldon, 1994) also indicates that lock fault. This is at least as fast as previously Monastero et al., 1997), west of the region in the amount of Garlock-parallel extension north published Holocene and Late Quaternary rates which block rotation is occurring. In addition, of the fault during the Late Quaternary is not along the central strand of the fault (Fig. 13). Dibblee (1967) suggests a 72-km bedrock offset suffi cient to explain the measured slip rate of This indicates that block rotation in the north- along the western Garlock fault to align Rand 5.3–10.7 mm/yr on the western Garlock fault at eastern Mojave Desert, while important, can- schist with schist exposed within the western Clark Wash. More recently published slip rates not be the primary explanation for left slip Garlock fault zone (Fig. 2C), although Pow- from the Eastern California Shear Zone (Lee et on the Garlock fault. The east-west–striking, ell (1993) regards this as less convincing than al., 2001; Oswald and Wesnousky, 2002; Oskin left-lateral faults that accommodate clockwise other bedrock offsets across the Garlock fault et al., 2006; Frankel et al., 2007) do not change rotation in the northeastern Mojave Desert are and suggests no more than 12 km of left slip at this conclusion. only capable of explaining left slip on the east- the western end of the Garlock fault. We suggest In our view, it seems likely that the western ernmost one-third or less of the Garlock fault that clockwise rotation of blocks in the north- Garlock fault initiated as a northeast-striking (Fig. 2C). The lack of rotation in the central eastern Mojave accommodates the termination, conjugate fault (Hill and Dibblee, 1953). This Mojave (Wells and Hillhouse, 1989; Ron and at the eastern end of the fault, of left slip that model appears to be the only model capable Nur, 1996) implies that some other mechanism occurs along much of the length of the fault and of explaining a relatively high slip rate on the is necessary to explain the relatively high slip originates for other reasons. northeast-striking, western Garlock fault. Stu- rate that we have measured on the western Gar- The transform model for the Garlock fault is art (1991) has shown that the stress conditions lock fault, and that Clark and Lajoie (1974) also incapable of explaining northeast-oriented at the northern Big Bend of the San Andreas fault are appropriate for the initiation and growth of a left-lateral fault with the approximate length and orientation of the Garlock 36°N PV DV fault. Cox (1998) has identifi ed a set of pre- Sierra existing, northeast-trending structures that cre- Nevada ated a zone of weakness, which may have been another factor that infl uenced the location at which the Garlock fault formed. Gravity data 1.1 + 1.9 OLF: ~2.5 indicate the presence of a basin several kilo- SL:4-9 meters deep just east of the intersection of the 1.8 + 1.5 CC: > 0.8 MC: 5.5-8 Garlock and San Andreas faults (Jachens and Bakersfield KL: 4.5-6.1 Calzia, 1998), which would be consistent with eastward extrusion of the Mojave block. CW: 5.3-10.7 35°N 3.2 + 1.5 The central and eastern sections of the fault OC: 1.6-3.3 may also have initiated as a northeast-striking San Andreas fault conjugate fault and then have been rotated to Barstow their present orientations by dextral shear in the Eastern California Shear Zone (Jones, 1987; Big Pine fault Guest et al., 2003). Clockwise block rotations

10050 0 100 km both north and south of the eastern Garlock fault (Guest et al., 2003) and east-vergent thrusting in the Avawatz Mountains south of the fault (Troxel 119°W 118°W 117°W and Butler, 1998) appear to accommodate the Figure 13. Comparison of slip-rate estimates for the Garlock fault. The three values in ital- eastward termination of slip on the fault. ics, associated with boxes that outline sections of the fault, are the slip rates and formal uncertainties from Meade and Hager’s (2005) best-fi tting elastic block model of available Comparison with Geodetic Measurements geodetic data. They report, however, that experience with a range of models suggests that of Present-Day Deformation true uncertainties are ~3 mm/yr. White-fi lled circles mark the locations of Holocene and Late Quaternary geologic slip-rate estimates. The Holocene rates that are constrained by Our results confi rm previous indications that radiocarbon dates and are thus considered most reliable are shown in bold (CW—Clark the Holocene slip rate of the Garlock fault is Wash, this study; KL—Koehn Lake, Clark and Lajoie, 1974; SL—Searles Lake, McGill faster than the rate of left-lateral strain accu- and Sieh, 1993). Other Holocene and Late Quaternary rates are shown in plain text (CC— mulation on the fault. Elastic block modeling Christmas Canyon, Smith [1975], minimum rate based on an offset channel incised into a of geodetic data (Meade and Hager, 2005) sug- dated shoreline that may be much older than the channel; MC—Mesquite Canyon, Carter gests that only 3.2 ± 1.5 mm/yr of left-lateral et al. [1994], age control from fossil identifi cation; OC—Oak Creek, La Violette et al. [1980], strain is accumulating on the western Garlock age control from degree of soil development; OLF—Owl Lake fault, McGill, [1998], age fault, and that left-lateral strain accumulation control from radiocarbon dating of rock varnish). rates are even less on the central and eastern

550 Geological Society of America Bulletin, March/April 2009

sections of the fault (1.8 and 1.1 mm/yr, respec- 16.0 tively; see Fig. 13). North Transect 14.0 Several hypotheses that could explain a strain South Transect accumulation rate that is lower than the Holo- 12.0 cene slip rate have been proposed, including temporally oscillating strain patterns (Peltzer et 10.0 al., 2001; Dolan and others, 2007), earthquake 8.0 cycle effects on a viscoelastic medium (Miller et al., 2001), and northeast-trending, left-lateral 6.0 strain release driven by northwest-trending, 4.0 right-lateral strain accumulation (Savage et al., 2001). The latter explanation (Savage et al., 2.0

2001) is consistent with our interpretation that (mm/yr) N75E to parallel Velocity 0.0 the Garlock fault primarily acts as a conjugate –120.0 –119.0 –118.0 –117.0 –116.0 –115.0 fault within a system dominated by dextral shear Longitude (degrees) (Hill and Dibblee, 1953). Figure 14. Component of geodetic velocity vectors that are parallel Comparison with Older Geologic Slip-Rate to the central Garlock fault (N75E) for transects north and south Estimates of the Garlock fault. See Figure 2D for transect locations. At any given longitude (an approximate proxy for distance along strike of The Holocene slip rate reported in this study the Garlock fault), the Garlock-parallel component of the velocity is consistent with some estimates of the longer is roughly equal on both sides of the fault, indicating no signifi cant term, geologic slip rate, but is faster than other Garlock-parallel extension is presently occurring north of the fault. estimates. The youngest rocks that are known to exhibit the full 64-km displacement are early Miocene volcanic and sedimentary rocks, which indicate that left slip on the Garlock fault began Paso Peaks (Dawson et al., 2003) paleoseismic next, much smaller, pulse of aggradation (Qf2) after 17 Ma (Monastero et al., 1997). This sug- sites (Fig. 1) are located ~29 km and 48 km north- at Clark Wash most likely correlates with the gests a long-term slip rate >3.8 mm/yr. Progres- east of the Clark Wash site, respectively, on the next younger depositional pulse at those three sive sinistral rotation of rocks of the Ricardo central segment of the Garlock fault. A 3.5-km- sites (also labeled Qf2 at Soda Mountains and Group (southwestern El Paso Mountains, Fig. 1) wide dilational stepover in the fault separates Soda Lake, and dated at 11.5–9.3 ka; Harvey starting at 10 Ma and continuing to 7 Ma, have these sites from the Clark Wash site, leaving less and Wells, 2003). Bull (1991) also recognizes a been interpreted to suggest that the Garlock reason to expect these sites to share an identi- depositional pulse at about this time (12–11 ka) fault became active during this interval (Bur- cal earthquake history with the Clark Wash site. throughout the Mojave Desert, which he associ- bank and Whistler, 1987; Loomis and Burbank, Both sites on the central Garlock fault suggest ates (based on fossil plants in packrat middens) 1988), which would yield a long-term slip rate more frequent earthquakes than are recorded at with the transition from the full-glacial climate of 6–9 mm/yr, consistent with the Holocene rate. the Clark Wash site on the western Garlock fault, with most moisture precipitating in the winter, However, Keenan (2000) reports late Miocene but once again, this could be explained by a pos- to an early Holocene monsoonal climate that volcanic rocks (10.3 Ma) that are offset only sibly incomplete record at Clark Wash. More was warmer but was still fairly moist as a result 32–39 km, suggesting that the left slip on the signifi cantly, event C at Clark Wash occurred of monsoonal thunderstorms in the summer Garlock fault began prior to 10.3 Ma, and that during a 3300-yr seismically quiescent period at (King, 1976; Spaulding and Graumlich, 1986; the long-term slip rate has been 3.1–3.8 mm/yr. El Paso Peaks (Dawson et al., 2003), indicating VanDevender et al., 1987). that the western and central sections of the fault At both Clark Wash and in the Soda Lake Comparison with Previous Paleoseismic sometimes rupture separately. basin (Harvey and Wells, 2003), there was a Results major transition from fan aggradation in the Paleoclimatic Implications late Pleistocene to fan-head incision and down- Paleoseismic events appear to have occurred stream progradation of the locus of deposition somewhat less frequently at Clark Wash than The history of alluvial aggradation and inci- during the Holocene. This transition, however, at Twin Lakes (Fig. 1), which is the only other sion at the Clark Wash site (Fig. 8) is largely occurred a bit later at Clark Wash than in the published paleoseismic site on the western seg- consistent with observations elsewhere in the Soda Lake basin. The >9-m incision of the offset ment of the Garlock fault (LaViolette et al., Mojave Desert, including the Soda Lake basin channel at Clark Wash occurred just after depo- 1980; Madden et al., 2005). However, given that (Harvey and Wells, 2003), the Soda Mountains sition of Qf2. In the Soda Lake basin, however, the Clark Wash record may be incomplete, it is (Wells et al., 1987, 1990), and in the Providence this transition occurred on most (but not all) fans quite possible that the earthquake history is the Mountains (McDonald et al., 2003). The major just prior to deposition of the unit (also labeled same at these two sites, which are located only aggradational episode (Qf1) on the Clark Wash Qf2) that we think correlates with Qf2 at Clark 47 km apart on the same fault section. fan (from >22.7 ka to <13.3 ka) is similar in Wash (Harvey and Wells, 2003). In contrast, it is harder to reconcile the paleo- age to a major pulse of alluvial fan aggradation The two channel-fi lling events (Hoa and Hya) seismic record at Clark Wash with records on at the three sites mentioned above, although at Clark Wash may also correlate with Holo- the central section of the fault. The Koehn Lake aggradation appears to have continued later at cene pulses of alluvial deposition in the eastern (Burke and Clark, 1978; Burke, 1979) and El Clark Wash than at the other three sites. The Mojave. The age of the Hoa deposits at Clark

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Wash (from 8.1 to 6.8 ka) match well with age component of left slip to the central and eastern U.S. Geological Survey Miscellaneous Geological of alluvial unit Qf3 in the Soda Lake basin (9.3– segments of the fault. Block rotation is also not Investigations Map I-741, scale 1:24,000, 3 sheets. Clark, M.M., and Lajoie, K.R., 1974, Holocene behavior 5.2 ka; Harvey and Wells, 2003). Bull (1991) also responsible for left slip on the western Garlock of the Garlock fault: Geological Society of America recognizes a depositional pulse around 8–9 ka fault, although it probably plays a major role in Abstracts with Programs, v. 6, p. 156–157. throughout the Mojave Desert, which he associ- slip on the eastern Garlock fault and in absorb- Cox, B.F., 1998, Late Cretaceous and early Paleogene tectonics and sedimentation near the Garlock fault, Califor- ates with a change from monsoonal climate to ing the termination of left slip near the eastern nia—Did Neogene strike-slip faulting exploit an older the present arid climate, based on fossil plants end of the fault. structural zone?, in Calzia, J.P., and Reynolds, R.E., eds., Finding faults in the Mojave: San Bernardino County in packrat middens. The Hya deposits at Clark Museum Association Quarterly, v. 45, p. 45–51. Wash (ca. 2.5 ka) are slightly younger than the ACKNOWLEDGMENTS Davis, G.A., and Burchfi el, B.C., 1973, Garlock fault: An next depositional pulse (Qf4) in the Soda Lake intracontinental transform structure, southern Califor- Tonya Daneke, Joseph Grant, Roger Johnson, Glenn nia: Geological Society of America Bulletin, v. 84, basin (4.3–3.5 ka; Harvey and Wells, 2003). Roquemore, Michael Slates, and Joseph Stroud pro- p. 1407–1422, doi: 10.1130/0016-7606(1973)84<1407: Depositional pulses of similar ages are also rec- vided extensive assistance with the fi eld work. Joseph GFAITS>2.0.CO;2. ognized at Soda Mountains (Reheis et al., 1989; Liebhauser and Michael Hogan facilitated the process Dawson, T.E., McGill, S.F., and Rockwell, T.K., 2003, Irreg- ular recurrence of paleoearthquakes along the central Wells et al., 1997, 1998), and in the Providence of obtaining excavation permits from the U.S. Bureau Garlock fault near El Paso Peaks, California: Journal Mountains (McDonald et al., 2003). of Land Management. This work was supported by of Geophysical Research, v. 108, p. 2356–2385, doi: U.S. Geological Survey National Earthquake Hazards 10.1029/2001JB001744. The similarities in timing of aggradational Research Program (NEHRP) award 1434-92-G-2210 Dibblee, T.W., Jr., 1959, Preliminary geologic map of the pulses between Clark Wash and the eastern and by National Science Foundation grant EAR- Mojave Quadrangle, California: U.S. Geological Sur- Mojave Desert suggest that the aggradational 9205669. The paper was substantially improved by vey Miscellaneous Field Studies Map MF-0219. Dibblee, T.W., Jr., 1967, Areal geology of the western and incisional events at Clark Wash are probably detailed and constructive reviews from Glenn Biasi, Carol Prentice, and the Associate Editor. Mojave Desert, California: U.S. Geological Survey largely related to climatic changes. The bedrock Professional Paper 522, 153 p. exposed in the drainage basin is biotitic granite Dixon, T.H., Miller, M., Farina, F., Wang, H., and Johnson, REFERENCES CITED D., 2000, Present-day motion of the Sierra Nevada (Smith, 1964). 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