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. 536 For permission to copy, contact [email protected] © 2009 Geological Society of America 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 Slip rate of the western Garlock fault 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 east- ern 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