Geophysical Journal International - jstock/Ge136a-2013/Ganev2012.pdf · Geophysical Journal International Geophys. J. Int. (2012) 190, 745–760 doi: 10.1111/j.1365-246X.2012.05494.x

Geophysical Journal InternationalGeophys. J. Int. (2012) 190, 745–760 doi: 10.1111/j.1365-246X.2012.05494.x

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Constancy of geologic slip rate along the central Garlock fault:implications for strain accumulation and release in southernCalifornia

Plamen N. Ganev,1∗ James F. Dolan,1 Sally F. McGill2 and Kurt L. Frankel3†1Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089, USA. E-mail: [email protected] of Geological Sciences, California State University San Bernardino, San Bernardino, CA 92407, USA3School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA

Accepted 2012 April 2. Received 2012 March 30; in original form 2011 April 25

S U M M A R YWe present new slip rate estimates for the central Garlock fault. The incised edge of an alluvialfan deposit with a 10Be cosmogenic radionuclide age of 13.3 +5.9/−1.1 ka is offset 70 ±7 m yielding a slip rate of 5.3 +1.0/−2.5 mm yr−1. A stream thalweg that is deeply incisedinto this surface is offset a lesser amount (58 ± 4 m) providing the lower bound for thisrate. Climate considerations, however, suggest that the deep incision of this thalweg may haveoccurred at a younger date, specifically at the end of the Younger Dryas period at ∼11.5 kaor at the onset of a summer monsoon rainfall pattern at ∼8–10 ka. If the deep incision wasrelated to one of these climate events, then the resulting slip rate would be 5.1 ± 0.3 and 6.6 ±1.2 mm yr−1, respectively. All of the slip rates are consistent with previous geologic sliprates, most of which suggest a latest Pleistocene–Holocene slip rate of ∼5–7 mm yr−1. Thesegeologic slip rates, however, contrast markedly with much slower (at most half as fast) short-term rates of elastic strain accumulation constrained by geodesy. The new slip rate thus addsto a growing body of evidence indicating that the Garlock fault is experiencing a period oftransient lack of strain accumulation, in which the lower crust (and mantle?) beneath the faultis deforming at a rate that is much slower than its long-term average rate. These observationssuggest that the Garlock fault experiences two modes of strain accumulation: the current ‘slow’mode, in which strain accumulates very slowly along the fault, and ‘fast’ mode, during whichthe fault must store and release elastic strain energy at much faster rates than the long-termaverage to account for the relatively rapid geologic slip rates measured along the central partof the fault.

Key words: Geomorphology; Continental neotectonics; Continental tectonics: strike-slip andtransform; Tectonics and climatic interactions; North America.

I N T RO D U C T I O N

One of the most fundamentally important, yet unresolved, issuesin active tectonics is the degree to which fault loading and strainrelease are constant in time and space. Although comparisons ofmillion-year plate-boundary rates (e.g. DeMets et al. 1990, 1994)with shorter term data suggest that rates are relatively constantalong most plate boundaries when averaged over the entire zone ofdeformation (e.g. Humphreys & Weldon 1994; Sella et al. 2002),comparison of geodetic and geologic rate data suggest that somefaults and fault systems exhibit transient strain accumulation andrelease over a range of timescales (e.g. Peltzer et al. 2001; Dawson

∗Now at: Aera Energy, LLC, 10 000 Ming Avenue, Bakersfield, CA 93311,USA.†Deceased.

et al. 2003; Friedrich et al. 2003; Oskin & Iriondo 2004; Meade& Hager 2005; Dolan et al. 2007; Oskin et al. 2007; McGill et al.2009; Saillard et al. 2009, 2011). Moreover, earthquake clusteringis increasingly observed at a wide variety of spatial and temporalscales, ranging from years to millennia, on both single faults (e.g.Barka 1992; Marco et al. 1996; Stein et al. 1997; Dolan et al. 1998;Hubert-Ferrari et al. 2002; Friedrich et al. 2003; Dolan & Bowman2004; Tsutsumi & Sato 2009; Kozaci et al. 2011), as well as regionalfault networks (e.g. Jackson & McKenzie 1984; Dolan et al. 1998;Mann et al. 1998; Rockwell et al. 2000; Nalbant et al. 2005; Dolanet al. 2007; Manaker et al. 2008; Ganev et al. 2010). In addition,in at least some regional fault systems, transiently elevated strainaccumulation appears to coincide with clusters of large-magnitudeearthquakes (e.g. Rockwell et al. 2000; Oskin et al. 2007; Dolanet al. 2007).

Such observations raise a number of basic questions. For ex-ample, what processes control strain accumulation and release at

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746 P.N. Ganev et al.

timescales of both individual earthquake cycles and over multiplecycles? Is strain release relatively constant, as might be expectedif faults are loaded steadily by plate motions? Or is strain accu-mulation and release markedly nonconstant, as might occur if faultloading is controlled by temporally transient mechanisms, such aslower crustal creep pulses (Dolan et al. 2007) or as feedback re-lated to earthquake clustering (Sammis & Dolan 2003; Oskin et al.2008)? Are transient effects limited to single faults, or small re-gions, or do they occur at much larger scales and over long timeintervals?

In this paper, we address these issues by determining a new sliprate along the central part of the Garlock fault in southern California.As we discuss, determination of this rate required consideration ofnot only the slip behaviour of the fault, but also of the climaticresponse of the landscape, including relationships between regionalclimate change and cycles of aggradation and incision of offsetgeomorphic features. These results have important implications forcomparisons of geologic and geodetic rates in southern California,and more generally for geologic slip rate determinations based onoffset geomorphic landforms.

A C T I V E T E C T O N I C S O F T H E G A R L O C KFAU LT : P R E V I O U S W O R K

The Garlock fault is a major left-lateral strike-slip fault that extendsin a broad arc for 250 km eastward from the San Andreas faultto the southern end of Death Valley in the Avawatz Mountains,and possibly even further to the east (Davis & Burchfiel 1973;Plescia & Henyey 1982). The fault marks a profound geologic and

physiographic boundary between the extended Basin and Rangeprovince to the north and the much less extended Mojave block to thesouth (Davis & Burchfiel 1973). Three potentially complementarytectonic models have been proposed to explain the Garlock fault’sexistence: (1) it forms a conjugate fault pair with the San Andreasfault that accommodates some of the convergence at the Big Bendsection of the plate boundary fault (Hill & Dibblee 1953; Stuart1991; McGill et al. 2009); (2) it is a transform fault between theextended Basin and Range Provence to the north and the Mojaveblock to the south (Davis & Burchfiel 1973); or (3) it is a structurethat accommodates clockwise block rotation in the northeasternMojave desert (Humphreys & Weldon 1994; Guest et al. 2003).

Total documented sinistral displacement is 48–64 km (Smith1962; Smith & Ketner 1970; Davis & Burchfiel 1973; Carr et al.1993; Monastero et al. 1997), which initiated sometime between 17and 7 Ma (Burbank & Whistler 1987; Loomis & Burbank 1988;Monastero et al. 1997; Frankel et al. 2008). A prominent, ∼2-km-wide extensional left step-over in the vicinity of Koehn Lake anda ∼15◦ change in strike south of the Quail Mountains have beenused to separate the Garlock fault into western, central, and easternsegments (McGill & Sieh 1991; Fig. 1).

Despite abundant geomorphic and stratigraphic evidence forHolocene seismic activity, the Garlock fault has not generatedany large earthquakes during historic time (e.g. 1850). At theirpalaeoseismologic trench site in the El Paso Mountains alongthe central Garlock fault, ∼30 km east of the Koehn lake step-over, McGill & Rockwell (1998) and Dawson et al. (2003) docu-mented evidence for six surface ruptures during the past 7 ky Theirdata reveal markedly irregular recurrence. The most recent surface

Figure 1. Index map of the southern Eastern California shear zone and Garlock fault. The black rectangle in the central section of the Garlock fault indicatesour study area.

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Geologic slip rates from central Garlock fault 747

rupture at their site, which occurred between 1450 and 1640 A.D.,was preceded by a temporal cluster of three surface ruptures between25 and 950 A.D. This cluster was in turn preceded by a long seismiclull between 2–5 ka, with two older events occurring at ∼5 and 7ka. On the western segment of the Garlock fault, Madden & Dolan(2008) documented the occurrence of four palaeo-earthquakes attheir Twin Lakes palaeoseismologic site in the Tehachapi Moun-tains (Fig. 1). The most recent surface rupture at their site occurredafter 1450 A.D., and before the historic period (i.e. before ∼1800–1850 A.D.), and may be the same surface rupture observed at theEl Paso peaks site. Similarly, Madden & Dolan (2008) observed asurface rupture that occurred between 40 B.C. and 650 A.D., duringthe 1–2 ka cluster at El Paso Mountains. They also observed eventsat 770–360 B.C. and 3620–2040 B.C. These two events occurredduring the long seismic lull observed at the El Paso Mountains site,indicating that the Garlock fault may sometimes rupture in its en-tirety in large-magnitude earthquakes, or closely spaced sequencesof earthquakes, but that sometimes the western and central partsof the fault behave independently, perhaps in response to rupturetermination at the Koehn Lake step-over (Madden & Dolan 2008).

McGill & Sieh (1991) mapped small-scale offsets of geomorphicfeatures, such as shutter ridges, gullies, terrace risers, and alluvialfans, along the central and eastern Garlock fault where numerousoffsets are preserved. Relatively few offset measurements are cur-rently available from the western segment of the Garlock fault. Ahistogram of the small-offset data from the central section of thefault, south of El Paso Mountains, reveals marked groupings of dis-placement that suggest offsets of ∼7 m in the two most recent eventsand ∼4 m in the third event back (fig. 5 in McGill & Sieh 1991). Tothe east, however, along the easternmost 90 km of the Garlock fault,small-scale offsets suggest only 2–3 m of left-lateral slip during themost recent earthquake although an alternative model allows forvariable slip of 2–6 m in the most recent event (fig. 14 in McGill &Sieh 1991).

Several studies report late Pleistocene–Holocene slip rates fromthe Garlock fault. These range between 4 and 11 mm yr−1, withan emerging slip rate consensus of 5–7 mm yr−1 (e.g. Clark &Lajoie 1974; McGill & Sieh 1993; McGill et al. 2009). Specifically,Clark & Lajoie (1974) measured an offset shoreline berm alongthe margin of Koehn Lake and dated offset tufa deposits using 14Cgeochronology. Their slip rate of 4.5–6.1 mm yr−1 (after apply-ing dendrochronological calibration to their radiocarbon dates) isconsidered a minimum since the tufa was deposited at some unde-termined time before the offset. McGill & Sieh (1993) obtained awell-constrained slip rate of 4–9 mm yr−1, with a preferred rate of5–7 mm yr−1, using an offset late Pleistocene shoreline of SearlesLake and correlation of lake stands with radiocarbon-dated organicsediments from cores. At Clark Wash, a site along the westernsection of the Garlock fault about 20 km west of Koehn Lake,McGill et al. (2009) reported a slip rate of 7.6 +3.1/−2.3 mm yr−1

based on an offset incised channel and radiocarbon dating. Theseintermediate-scale geomorphic slip rates are not significantly dif-ferent than the million-year geologic rates of displacement. Forexample, Carter (1994) determined a slip rate of 5.5–8 mm yr−1

by dating early Hemphillian fossils (6–9 Ma) recovered from theoffset Bedrock Springs Formation. Burbank & Whistler (1987) andLoomis & Burbank (1988) inferred that progressive sinistral ro-tations of the Ricardo Group in the southwestern El Paso Moun-tains between 10 and 7 Ma indicate the initiation of sinistral slipon the Garlock fault. This initiation age, combined with the 48–64 km total displacement on the Garlock fault, would yield a sliprate of 5–9 mm yr−1. In addition, Monastero et al. (1997) and

Keenan (2000) reported on 17 Ma rocks that may be the youngestunits to exhibit the full fault displacement, which would yield aminimum slip rate of >3.8 mm yr−1.

Such intermediate- and long-term geologic slip rates, however,are inconsistent with most geodetically derived short-term slip rateson the Garlock fault, which indicate sinistral slip rates that are nomore than half the geologic slip rates (≤3 mm yr−1; McCluskyet al. 2001; Miller et al. 2001; Peltzer et al. 2001; Meade & Hager2005). These geodetic studies reveal a velocity field dominatedby northwest-oriented, right-lateral shear parallel to the easternCalifornia shear zone, and extending across the Garlock at a highangle. Such a zone of right-lateral shear could be related to the tem-poral and spatial clustering of earthquakes along the Garlock fault(Peltzer et al. 2001; Dolan et al. 2007); however, the precise natureof the relationship between the Garlock fault and the northwest-oriented, right-lateral faults of the ECSZ remains enigmatic.

S U M M I T R A N G E S T U DY A R E A – S I T E4 4 9 1 0 0

In comparison to the western and eastern parts of the Garlock fault,the central section, east of Koehn Lake and west of the Quail Moun-tains, is structurally simpler. For the most part, it exhibits a single,east-northeast-striking trace within Holocene deposits, with localsubordinate parallel strands, most of which accommodate dip-slipmotion (Fig. 2). This relative structural simplicity, coupled with themountain-front location of the fault trace along this reach, resultsin well-defined sinistral offsets of numerous geomorphic features(e.g. rills, incised stream channels, alluvial fans), with displace-ments ranging from a few meters to >500 m (Clark 1973; McGill& Sieh 1991).

We used the recently acquired GeoEarthScope lidar data set toexamine an especially promising, 7-km-long stretch of the Garlockfault located along the northern edge of the Summit Range eastof Trona Road, where offsets of several tens of meters are particu-larly prominent and well-preserved (Fig. 2). Several of these siteswere previously identified by Clark (1973). In this region, the Gar-lock fault strikes ∼80◦ and offsets mostly upper Quaternary alluvialdeposits rather than bedrock. In the 7-km-long stretch that we stud-ied, we identified at least four offset channels that are displaced 65–70 m, whereas several channels and one alluvial fan are displaced by40–50 m (some of which are visible in Fig. 2). The alluvial deposits,consisting mostly of sand, gravel, pebble, and silt, are transportednorthwards from the northern slopes of the Summit Range towardsthe adjacent valley (Searles Valley), which is bounded by the Sum-mit Range to the south, the Spangler Hills to the north and the SlateRange to the east. Small outcrops of folded Quaternary conglom-erate are present at several locations on the north side of the fault(unit QTc in Fig. 3). These contain clasts from the Garlock series(Carr et al. 1993), which is not exposed locally. These conglomer-ates have been interpreted by Carter (1994) as being left-laterallyoffset 17 km from an area 1–2 km west of the mouth of ChristmasCanyon.

One of the most prominent offset landforms that we examinedwith the lidar data is a sharply offset, deeply incised stream channellocated at 35◦28′44.9′′N and 117◦33′36.73′′W (Fig. 3). We infor-mally call this the “channel 449100” site, after the UTM easting ofthe site. The active stream that flows northwards through the offsetchannel is one of the main channels that drain the central part ofthe Summit Range. The channel is incised into several abandonedalluvial fan deposits that also emanated from this source drainage.The abandoned alluvial fan surfaces are moderately well-preserved,

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Figure 2. (a) Annotated lidar image of our study site and the vicinity. The location of Fig. 3 is outlined by the white dashed line. (b) Aerial photograph of thesame area as in (a). (c) Topographic map (5 m contour interval) derived from the lidar dataset of the same area as in (a).

with limited erosion near the edges formed by the incised channels;however, anthropogenic disturbances (all-terrain-vehicle tracks) areprominent at certain locations. The offset channel is incised into de-posits Qf1 and Qf2 (Fig. 3), which formed during late Pleistocene

periods of alluvial fan aggradation (see discussion below). The Qf1deposit, which is composed primarily of granitic clasts ≤5 cm indiameter with oxidized grains, is the most predominant depositat the site. The Qf1 surface exhibits subdued topographic relief

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Figure 3. (a) Map of the Summit Range site with mapped Quaternary deposits, including the 449100 offset channel. (b) Retrodeformed map of our study site.Restoration of the displacement (70 ± 7 m) is based on the preserved edge of the Qf2 surface (indicated by the paler orange color). (c) Topographic map; (d)70-m retrodeformed topographic map and (e) 58-m retrodeformed topographic map of the study site rendered from the lidar DEM (1-m contour interval).

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(typically <15 cm) on well-preserved parts of the alluvial fan.Natural stream-bank exposures along the channel that is incisedinto the Qf1 and Qf2 deposits provide the opportunity to observe thestratigraphic relationship between these two fan deposits. The Qf2deposit is a small alluvial fan emanating from the 449100 drainagethat crops out on both sides of the offset channel. This small fan,which is no more than 3 m thick, was deposited locally atop theolder, more laterally extensive Qf1 fan surface. The Qf2 depositconsists of 10–30-cm granitic clasts, which are well-cemented in acarbonate matrix in the basal ∼10–20 cm of the unit at ∼3 m belowthe surface.

Fault displacement

The lidar data (available online at http://www.opentopography.org)collected along the Garlock fault north of the Summit Range showthe prominent sinistral offset of the main channel (449100), aswell as a set of an additional six deflected channels to the eastthat are incised into the older Qf1 deposit (Fig. 3). The sinistraloffsets are largely localized along a single strand, though two south-dipping reverse fault strands are also present north of the mainfault (Fig. 3). These reverse strands do not appear to contribute anysignificant amount of lateral slip to the total fault displacement. Thesix deflected channels merge downstream into the main drainageabout 200–300 m north of the fault, as the main drainage changesflow direction towards the northeast due to the presence of theQTc outcrop (Fig. 3). These channels are incised into the olderQf1 deposit. All channel displacements were measured in ArcGIS(version 9.2) directly from the one-meter digital elevation model(DEM) derived from the lidar data set.

The offset of channel 449100 was previously estimated at∼45 m (reported as 150 ft), based on the offset of the channel thal-weg (Clark 1973). Analysis of the lidar data indicates that the trueoffset is larger, and suggests the possibility of a more complicatedfault offset history of this site. Specifically, restoration of the deeplyincised channel thalweg by 58 ± 4 m yields a sedimentologicallyplausible configuration for the incised channel with a minimum ofchannel curvature at the fault (Fig. 3). Our error estimates on thisoffset are based on the width of the narrow, deeply incised channelthalweg.

The geometry of the preserved edge of the Qf2 surface, however,as well as the geometrical relationship between the stream and thefault, suggests that this offset is a minimum. In a situation suchas this, with a north-flowing stream cutting across a west-striking,left-lateral strike-slip fault with a small component of north-side-up slip at the fault, the corners of the stream bank that will bepreferentially eroded are the eastern bank downstream from thefault, and the western bank of the channel upstream from the fault(McGill & Sieh 1991). Thus, the current configuration of the deeplyincised channel may partially reflect incremental straightening ofthe channel segments during fault offset, resulting in the channel-thalweg offset being an underestimate of the true fault offset sinceinitial incision.

Some support for this idea is provided by the geometry of theincised edge of the Qf2 surface west of channel both north and southof the fault. This incised edge downstream from the fault and westof the channel will have been preferentially preserved during faultoffset. As shown in Fig. 3, this edge of the Qf2 surface is notablylinear. The preserved upstream edge of the Qf2 surface west of thechannel is also relatively linear, away from the eroded corner nearthe fault. We suggest that projection of these incised Qf2 surfaceedges west of the channel on either side of the fault provide a robust

measure of the maximum-possible displacement since initial inci-sion of the Qf2 surface, and may provide the best estimate of the trueamount of offset since Qf2 was abandoned. Restoration of 70 m ofsinistral offset yields a plausible configuration of the locus of initialchannel incision into the Qf2 surface following fan abandonment(Fig. 4). We estimate conservative errors on this measurement of±7 m based on the fact that restoration of 77 m closes off anynorthward flow through the channel incised into the Qf2 surface,and restoration of 63 m results in a sedimentologically unlikelysharp leftward deflection at the fault of the Qf2 surface edge on thewest side of the channel. At a restoration distance of 70 m, the thal-weg of the channel exhibits a pronounced and sedimentologicallyunlikely ∼10 m deflection to the right at the fault crossing, consis-tent with our suggestion that the deeply incised channel may haveexperienced incremental straightening during fault offset. Never-theless, to be conservative, we use the 58 ± 4 m restoration of thedeeply incised channel thalweg as a minimum offset in our slip-ratecalculations below, including the rate calculated from it within theuncertainty bounds on our reported slip rate for the past 13.3 ka.We use 70 ± 7 m as our preferred amount of offset since 13.3 ka.If, alternatively, the 58 ± 4 m offset of the deeply incised channelrecords the full offset since abandonment of the Qf2 fan surface at13.3 + 5.9/−1.1 ka, then the resulting minimum slip rate would be4.4 + 0.7/−1.6. As noted above, we think the offset is larger thanthis minimum estimate, and we include this rate estimate here onlyfor the sake of completeness.

TCN geochronology

We quantified the ages of the Qf1 and Qf2 alluvial fan surfaces bymeasuring the concentration of in-situ-produced terrestrial cosmo-genic 10Be concentrations in samples collected from depth profilesthrough the top ∼2 m of the deposits (e.g. Lal 1991; Andersonet al. 1996; Gosse & Phillips 2001). Estimation of the inheritedcomponent of 10Be is of critical importance for accurate determina-tion of the age of abandonment of the fan surface. The productionof 10Be diminishes quasi-exponentially with depth beneath the sur-face (Anderson et al. 1996; Gosse & Phillips 2001), and thereforea model surface age can be determined by extrapolating a regres-sion of measured 10Be concentrations versus depth to the surface(e.g. Anderson et al. 1996; Hancock et al. 1999; Hidy et al. 2010).Because the production of 10Be below ∼2 m in alluvium is muchlower, any 10Be concentration measured below this depth allows foran estimation of inheritance (e.g. Anderson et al. 1996).

We collected bulk samples of unconsolidated sand from twopits excavated into the Qf1 deposit and one pit excavated intothe Qf2 deposit. The ∼2-m-deep pits were hand excavated at lo-cations where the original fan surface was best preserved, andaway from any obvious erosional features. Samples were col-lected from depths of 1.9, 1.5, 1.2, 0.9, 0.6 and 0.3 m be-low the fan surface. The samples, which were processed at theGeorgia Institute of Technology Cosmogenic Nuclide Geochronol-ogy Laboratory, were first crushed, then ground and sieved to agrain size fraction of 250–750 μm. Quartz was isolated follow-ing the procedures described in Kohl & Nishiizumi (1992) andberyllium was extracted from the samples by ion exchange chro-matography, precipitated as Be(OH)2, and oxidized to BeO (e.g.Bierman et al. 2002). After ignition of the BeO, the samples werethen mixed with niobium and packed in stainless-steel cathodes.The 10Be/9Be ratios were measured at the Lawrence LivermoreNational Laboratory Center for Accelerator Mass Spectrometry.Surface exposure ages were modelled from the depth profiles

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Figure 4. 5-m incremental slip restorations (b–h) of offset channel 449100 incised into Qf2 fan.

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using the Monte Carlo approach outlined in Hidy et al.(2010) (available online at http://geochronology.earthsciences.dal.ca/downloads-models.html). Currently, there is no consensus withregard to which time-variable production-rate model is most ac-curate (e.g. Lal 1991; Stone 2000; Dunai 2001; Desilets & Zreda2003; Pigati & Lifton 2004; Lifton et al. 2005; Desilets et al. 2006;Staiger et al. 2007). Therefore, we chose to use a time-invariantproduction-rate model to determine the ages of the deposits (Lal1991; Stone 2000). However, for the timescales discussed herein,ages from the various production rate models would be within ∼10per cent of each other (i.e. overlapping within the age uncertainty;e.g. Balco et al. 2008).

TCN results

Qf2 surface

Depth profile PG1209 was used to constrain the age of abandon-ment of the Qf2 fan surface (Fig. 5 and Table 1; see Fig. 3 forlocation). Hidy et al.’s (2010) model, which we used to determinethe surface age, incorporates site-specific geologic parameters (e.g.altitude, latitude, shielding, erosion/aggradation) to calculate thebest-fit values for exposure age, erosion rate and inherited nuclideconcentration. In addition, the model explicitly propagates the errorin all variables. The surface exposure age for the Qf2 deposit wasdetermined by the best-fit of 100 000 Monte Carlo simulations usingthe following model constraints: (1) we assumed uniform densitywith depth, but allowed the density to vary from 1.8 to 2.4 g cm−3;(2) we used as inputs to the range of models tested by the MonteCarlo simulation a minimum possible age of 5 ka and maximumpossible age of 20 ka based on fan morphology and soil develop-ment. Specifically, the absence of any argillic horizon in the Qf2deposit suggests that the Qf2 surface is unlikely to be older thanthe latest Pletistocene, as argillic horizons are typically developedin deposits of late Pleistocene age; (3) the surface erosion rate wasallowed to vary from −1 (net aggradation on the fan surface) to2 cm/ka (net erosion of fan surface); (4) the total erosion thresh-old was allowed to vary between 30 (30 cm of net aggradation onfan surface) and −30 cm (30cm of net erosion of fan surface) and(5) inheritance was constrained in the model to values ranging from1000 to 175 000 atoms/g SiO2 based on the 10Be concentration(22.19 ± 0.56 × 104 atoms/g SiO2) in the deepest sample in thedepth profile.

The modelled Qf2 surface exposure age is based on the best-fitcurve through the six samples collected between 1.9 and 0.3 mdepth below the surface. The resulting best-fit 10Be model age is13.3 +5.9/−1.0 ka (errors are based on the minimum and maximumbest-fit curves) and the inheritance is 1.35 +0.06/−0.09 × 105

atoms/g SiO2 (Fig. 5). Thus, the Qf2 surface was abandoned at ∼13ka, providing a maximum age for the initial incision of the offsetchannel. As discussed below, we use this age to calculate the sliprate of the Garlock fault at the Summit Range study site.

Qf1 surface

Depth profiles from two locations (PG0110 and PG0410) help tosemi-quantitatively determine the timing of abandonment of theQf1 surface (Fig. 5 and Table 1; see Fig. 3 for locations). For eachof the two depth profiles, modelling parameters were identical tothose used for Qf2 deposit (PG1209) except that the minimum andmaximum ages were constrained between 5000 and 100 000 years,respectively. For PG0110, although we collected six samples in a

depth profile extending down to 200 cm depth, the bottom threesamples were located below a ∼20-cm-thick carbonate-cementedlayer located 100 cm below the surface. The base of the carbonatesoil appears to correspond to a buried older surface that may haveacted as an aquaclude to help localize soil development. Abandon-ment of this older surface was superseded by renewed depositionof the uppermost meter of alluvial fan sediment and subsequentreabandonment of the fan. Thus, the exposure histories of the olderand younger fan material will record different 10Be concentrations.In an attempt to determine the age of the Qf1 surface, we usedonly the top three samples because of the extreme discordance withthe theoretical change in nuclide concentration with depth observedin the depth profile (the 10Be concentrations of the lower three sam-ples are shown in Fig. 5, but they are not used in the model toestimate the age of Qf1). The 10Be model age for depth profilePG0110 is ∼36 ka with inheritance equal to ∼0.8 × 105 atoms/gSiO2 (Fig. 5). The model age and inheritance for this profile, andparticularly the uncertainties, are poorly constrained due to the smallnumber of samples.

The model age of depth profile PG0410 is ∼43 ka (inheritanceis equal to ∼1.0 × 105 atoms/g SiO2) and is based on the best-fit curve through five samples (Fig. 5). The top sample, located0.3 m below the surface, yielded a much lower-than-expected 10Beconcentration, which we suspect is due to mixing of sediment. Theremoval of the top sample from the data set allows for a widerange of surface exposure ages, which is reflected in the poorlyconstrained error limits for the age. Because of the complicateddepositional and exposure history of the Qf1 deposit at PG0110, andthe wide spread in sample ages at PG0410, the resulting 10Be agesprovide only approximate estimates of the actual surface exposureage. Nevertheless, these results suggest that the Qf1 surface wasabandoned several tens of thousands of years ago, most likely at30–40 ka (range 15–80 ka).

D I S C U S S I O N

Slip rate since abandonment of Qf2 (13.3 ka)

We determine a slip rate of 5.3 +1.0/−2.5 mm yr−1 for the off-set 449100 channel incised into Qf2 surface by combining the13.3 +5.9/−1.1 ka abandonment age of the Qf2 surface with ourpreferred 70 ± 7 m offset of the preserved edge of the Qf2 surfacewest of the channel. The error ranges include our estimate of 58 ±4 m offset of the deeply incised channel thalweg as a minimum-possible offset of the channel. We use 70 ± 7 m as our preferredoffset since 13.3 ka because restoration of this offset realigns theincised edge of the Qf2 surface, which has been dated at ∼13.3 ka.The channel thalweg, which is offset 58 ± 4 m, could be a youngerfeature, as discussed previously and below. The 5.3 +1.0/−2.5 mmyr−1 rate is based on the assumption that initial incision that formedthe edge of the Qf2 surface west of the channel into the Qf2 surfacebegan immediately upon fan abandonment at ∼13 ka. If, however,fan abandonment was due to climatic causes (as discussed below)and initial incision that formed the edge of the Qf2 surface west ofthe channel actually occurred significantly later than fan abandon-ment at ∼13 ka, the resulting rate would be faster.

Palaeoclimate considerations and implications forGarlock fault slip rate

Our results are consistent with the record of major climate changesin the Mojave region. Specifically, major drying occurred at the

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Figure 5. Plots of sample depth versus 10Be concentration from depth profiles in displaced alluvial fans along the central Garlock fault. (a) Profile PG-1209from the Qf2 deposit, location: 35◦28′42.2′′N/ 117◦33′34.1′′W; (b) Profile PG-0110 from the Qf1 deposit, location: 35◦28′48.5′′N/ 117 ◦33′38.7′′W; (c) ProfilePG-0410 from the Qf1 deposit, location: 35◦28′50.98′′N/ 117◦33′19.85′′W. Grey regions represent the solution space for 100 000 Monte Carlo profile-regressionsimulations through samples (black circles) in the depth profile following methods outlined in Hidy et al. (2010). Solid red line represents the best-fit profile.Vertical grey lines represent the inheritance (solid) and associated uncertainties (dashed). Note that the poorly constrained model ages in profiles PG-0110 andPG-0410 provide only a rough estimate of the surface exposure age for the Qf1 deposit. Please see text for discussion.

Table 1. Analytical results of terrestrial cosmogenic nuclide 10Be depth profile geochronology forthe Summit Range site.

Sample Depth Shielding Quartzc,d Be 10Be/9Bef ,g 10Be Concentrationh

namea (cm) factorb (g) Carriere (×10−13) (×104 atoms/g(mg) SiO2)

Qf2∗PG1209-1 200 1 60.71 0.3504 3.85 ± 0.09 14.69 ± 0.37PG1209-2 150 1 60.19 0.3529 3.93 ± 0.9 15.25 ± 0.39PG1209-3 120 1 60.22 0.3501 4.11 ± 0.10 15.80 ± 0.41PG1209-4 90 1 60.68 0.3540 4.62 ± 0.12 17.87 ± 0.51PG1209-5 60 1 60.03 0.3529 5.11 ± 0.12 19.93 ± 0.50PG1209-6 30 1 60.15 0.3525 5.71 ± 0.13 22.19 ± 0.56Qf1∗∗PG0110-1 190 1 60.06 0.3571 5.58 ± 0.13 22.02 ± 0.55PG0110-2 150 1 60.02 0.3568 7.10 ±0.16 28.05 ± 0.70PG0110-3 120 1 59.97 0.3545 7.37 ± 0.16 28.96 ± 0.69PG0110-4 90 1 60 0.3556 5.15 ± 0.12 20.23 ± 0.51PG0110-5 60 1 59.99 0.3540 7.36 ± 0.17 28.88 ± 0.73PG0110-6 30 1 60.14 0.3538 9.03 ± 0.15 35.33 ± 0.68Qf1∗∗∗PG0410-1 190 1 59.98 0.3600 3.44 ± 0.08 13.64 ± 0.36PG0410-2 150 1 60.16 0.3600 4.73 ± 0.11 18.76 ± 0.47PG0410-3 120 1 60.23 0.3601 5.63 ± 0.09 22.33 ± 0.43PG0410-4 90 1 60.08 0.3600 6.01 ± 0.14 23.93 ± 0.60PG0410-5 60 1 60.01 0.3602 6.87 ± 0.16 27.40 ± 0.69PG0410-6 30 1 60.07 0.3592 6.62 ± 0.15 26.28 ± 0.66

Notes. aSamples consisted of 0.25–0.75 mm grains. bNo geometric shielding correction fortopography necessary (horizon <20◦ in all directions). cModelled with uniform density with depth,but allowed density to vary from 1.8 to 2.4 g cm−3. dErosion allowed to vary from −1 (netaccumulation on fan surface) to 2 cm/ka (net erosion of fan surface). eBe carrier concentrationequals 1155 ppm. f Isotope ratios normalized to 10Be standards prepared by Nishizumi et al. (2007)with a value of 2.85 × 1012 and using a 10Be half-life of 1.36 × 106 yr. gUncertainties reported atthe 1σ confidence level. hA mean blank value of 89 077 ± 19 866 10Be atoms used to correct forbackground concentrations.∗Location of pit: 35.478387◦N/117.55947◦E, elevaton: 1005 m above sea level. ∗∗Location of pit:35.480139◦N/117.56075◦E, elevaton: 978 m above sea level. ∗∗∗Location of pit:35.480500◦N/117.55500◦E, elevaton: 976 m above sea level.

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onset of the Younger Dryas (13–11.5 ka) in the Mojave region andelsewhere throughout western North America and the rest of theNorthern Hemisphere (e.g. Mott et al. 1986; Johnsen et al. 1992;Edwards et al. 1993; Kenett & Ingram 1995; Lowell et al. 1995;Osborn 1995; Peteet 1995; Benson et al. 1997; Mikolajewicz et al.1997). The onset of this relatively dry period at ∼13 ka terminatedextensive alluvial fan aggradation throughout the region (Van De-vender & Spaulding 1979; Benson et al. 1997, Benson et al. 1998).We observe evidence for this event in the ∼13 ka abandonmentage of the Qf2 fan surface. A similar age for fan abandonment isobserved 60 km to the west–southwest at McGill et al.’s (2009)Clark Wash site, where fan aggradation processes continued to∼13.3 ka. These data suggest that one major impact of the onset ofthe Younger Dryas period was cessation of fan deposition through-out the Mojave at ∼13 ka. The 13.3 ka abandonment age providedby the Qf2 depth profile at the channel 449100 site is thus consis-tent with the idea of cessation of alluvial deposition at the site andabandonment of the Qf2 surface at the onset of the Younger Dryasperiod.

As with the Qf2 10Be results, the results of the Qf1 depth pro-files are broadly consistent with palaeoclimate observations fromthe Mojave region. Specifically, during latest Pleistocene time (30–13 ka), a major aggradational pulse is observed from Clark Wash(McGill et al. 2009) 60 km west–southwest of our site, to Soda Lakebasin (Harvey & Wells 2003), Soda Mountains (Wells et al. 1987,1990) and Providence Mountains (McDonald et al. 2003), 150–200 km to the east of our site. During this period, fluvial and sheet-flood processes formed relatively large alluvial fans throughout theregion (e.g. Ore & Warren 1971; Brown et al. 1990; Harvey & Wells1994; McDonald et al. 2003). At the Summit Range site, althoughnot well-constrained, we observe a similar period of extensive allu-vial fan aggradation recorded by the wide-spread deposition of theQf1 deposit, which we interpret as being deposited and abandonedat ∼30–40 ka. The uncertainties in our 10Be dates indicate that thisalluvial fan unit could potentially be equivalent to unit Qf1 at SodaLake basin, whose estimated age based on soil-profile developmentis 19–30 ka (Harvey & Wells 2003).

Slip rate since deep incision of the thalweg (past 8–11.5 ka)

As noted above, the deeply incised thalweg of channel 449100 isoffset 58 ± 4 m, significantly less than the preserved edge of theQf2 fan surface along the western edge of the channel, which isoffset 70 ± 7 m. This geometry suggest the possibility that themain incision event that resulted in the current, deeply incisedchannel occurred after approximately 10–15 m of Garlock faultsinistral offset had accumulated following abandonment and ini-tial incision of the Qf2 surface. In as much as the deep incisionwas probably controlled by climate change, we have examined theclimate literature for latest Pleistocene–early Holocene changes inthe Mojave region for possible causative events. We suggest twopossible climate-based scenarios for the age of the deep incisionevent. The end of the Younger Dryas period at ∼11.5 ka resultedin a generally more humid climate in the desert southwest, sug-gesting the possibility that deep incision began during the returnto post-Younger Dryas wetter conditions (e.g. Mott et al. 1986;Johnsen et al. 1992; Edwards et al. 1993; Kenett & Ingram 1995;Lowell et al. 1995; Osborn 1995; Peteet 1995; Benson et al. 1997;Mikolajewicz et al. 1997). If the main incision event for channel449100 occurred at ∼11.5 ka, this would result in a slip rate of 5.1 ±0.3 mm yr−1. Alternatively, the most pronounced climate change in

the region following the end of the dry conditions prevailing duringthe Younger Dryas occurred between 10 and 8 ka, when summermonsoonal rainfall patterns were initiated (e.g. King 1976; Spauld-ing & Graumlich 1986; VanDevender et al. 1987; Wells et al. 1987,1990; Bull 1991; Harvey & Wells 2003; McDonald et al. 2003).Based on this, we suggest the possibility that the deep incision eventat channel 449100 may have occurred as recently as 10–8 ka. If cor-rect, the 58 ± 4 m offset of the deeply incised thalweg, combinedwith the 10–8 ka age of onset of the wet summer monsoon weatherpattern, would yield a slip rate of 6.6 ± 1.2 mm yr−1, slightly fasterthan (but overlapping with) the rate we infer from 70 ± 7 m ofslip that occurred since abandonment and initial incision of the Qf2surface at 13.3 ka. However, although we think these regional cli-mate events likely controlled the deep incision of channel 449100,the absence of definitive age constraints on the timing of deepincision renders these climate-based slip-rate estimates somewhatspeculative.

Geologic slip rate comparisons from the Garlock fault

Our Garlock fault slip rate of 5.3 +1.0/−2.5 mm yr−1 for the past∼13.3 ka, and our climate-controlled slip rates of 5.1 ± 0.3 mmyr−1 (assuming a deep incision event at 11.5 ka) or 6.6 ± 1.2 mmyr−1 (deep incision event at 8–10 ka), are in general agreement withmost previously determined geologic slip rates, including other latePleistocene–Holocene rates and longer term Miocene rates (Fig. 6).For example, as noted above, Carter (1994) determined a slip rateof 5.5–8.0 mm yr−1 based on the offset of 6–9 Ma Bedrock SpringsFormation, whereas Burbank & Whistler’s (1987) and Loomis &Burbank’s (1988) suggested 7–10 Ma age of initiation of Garlockfault slip yields a slip rate of 6–9 mm yr−1, based on a cumula-tive fault displacement of 48–64 km (Smith 1962; Smith & Ketner1970; Davis & Burchfiel 1973; Monastero et al. 1997). Similarly,published late Pleistocene–Holocene slip rates range from 4–9 mmyr−1 (Clark & Lajoie 1974; McGill 1992; McGill & Sieh 1993;McGill et al. 2009). These late Pleistocene–Holocene slip rateswere measured at multiple sites spanning a 100 km length of theGarlock fault, from the eastern portion of the western segment ofthe Garlock fault at Clark Wash (McGill et al. 2009), through theKoehn Lake step-over (Clark & Lajoie 1974) and our study site inthe Summit Range, and eastward to Searles Lake (McGill 1992;McGill & Sieh 1993).

The similarity of these rates suggests remarkably con-sistent displacement along the Garlock fault during latestPleistocene–Holocene time. We note, however, that all of these ratesare averaged over many seismic cycles, and shorter term palaeoseis-mologic data suggest the possibility that the Garlock fault slip ratemay not always be consistent over timescales spanning fewer earth-quakes. For example, Dawson et al. (2003) report four earthquakes(events W, U, R and Q) which occurred between A.D. 1450–1640and A.D. 25–275. A histogram of small-scale offset measurementsfrom the El Paso Mountains (McGill & Sieh 1991) reveals peaks at7, 14 and 18 m, suggesting cumulative offsets in the past one, twoand three earthquakes that sum to these values. Dawson et al. (2003)used these data to infer displacements at their El Paso Peaks trenchsite of 7, 7 and 4 m for the three most recent surface ruptures at theirsite. If we use this same inference, we can use the palaeoearthquakeages and inferred displacements to calculate a geologic slip rateaveraged over the short period represented by the past three earth-quakes. If the cumulative slip in the past three palaeoseismologicallydocumented earthquakes is 18 ± 2 m (McGill & Sieh 1991), this

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Figure 6. Locations of slip-rate sites along the Garlock fault. Values in black are Holocene or latest Pleistocene rates, summarized in the table below the map.The three values in white are the slip rates and uncertainties from Meade & Hager’s (2005) elastic block model that best fits the available geodetic data.

slip would have accumulated over the time interval between eventsQ (25–275 C.E.) and W (1450–1640 C.E.) at the El Paso Peaks site(Dawson et al. 2003). Dividing the cumulative slip by this 1175-to 1615-year time interval yields a slip rate of 13 ± 2 mm yr−1.This could indicate that the Garlock fault has been in a period ofenhanced activity for the past ∼2 ka.

Alternatively, the 7 and 14 m offsets measured by McGill &Sieh (1991) at El Paso Peaks may record fault slip that has ac-crued in more events than just the two most recent earthquakes.This inference is at least partially supported by fault offset datafrom east of the El Paso Mountains. Specifically, in Searles Valley,∼20 km east of the El Paso Mountains, McGill & Sieh (1991) re-port displacements in the most recent event that average only 2 ±0.5 m. Similarly, in Pilot Knob Valley, ∼50 km east of the Daw-son et al. (2003) trench site, McGill & Sieh (1991) report peaksin cumulative displacements in past events at 3.4, 5.3 and 8.6 m,indicating offsets in the past three surface ruptures of ∼3.5, 2 and∼3.5 m. Based on these much smaller displacements measured tothe east of the El Paso Mountains, an alternative interpretation ofthe small-scale offset data is that slip per event along the centralpart of the Garlock fault is typically on the order of 2–4 m, and

that the 7 m displacements observed in the El Paso Mountains areareflect rates of incision of new drainages that are slower than therepeat time of surface ruptures, as suggested recently for the cen-tral San Andreas fault by Grant Ludwig et al. (2010) and Zielkeet al. (2010). In other words, the 7 m incremental offsets observedby McGill & Sieh (1991) near the Dawson et al. (2003) trench sitemay each record slip in more than one earthquake. This would resultin a slower geologic slip rate measurement. For example, if slip ineach of the past three earthquakes observed at the El Paso Moun-tains site was only 3.5 m (assuming 7 m of displacement recordstwo events), then the short-term geologic slip rate of the fault wouldbe ∼6–9 mm yr−1 (3 events × 3.5 m per event = 10.5 m, dividedby 1175–1615 yr). Additional short-term slip-rate data are required,however, before drawing any firm conclusions about whether or notthe Garlock fault has had an elevated slip rate over the past ∼2 ka.

Comparisons of geologic and geodetic rate data from theGarlock fault

Geodetic measurements of the rate of elastic strain accumulationon the Garlock fault have consistently indicated rates that are sig-

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Figure 7. Compilation of slip rates from the Garlock fault. Upwards- and downwards-pointing arrows indicate minimum and maximum slip rates, respectively.White circles indicate preferred slip rates as indicated by the authors of each publication, whereas white ellipses indicate the ranges of preferred slip rates asindicated by the authors of those publications. The error bars are error margins as reported by the authors of the publications. For simplicity, the geodeticallydetermined slip rates are plotted on a decadal time scale. The slip rates come from the following sources: (a) Meade & Hager (2005); (b) McClusky et al.(2001); (c) Miller et al. (2001); (d) based on age data from Dawson et al. (2003); (e) Clark & Lajoie (1974); (f) McGill et al. (2009); (g) this study, site 449100;(h) McGill & Sieh (1993); (i) Carter (1994); (j) Burbank & Whistler (1987), Loomis & Burbank (1988); (k) Keenan (2000) and (l) Monastero et al. (1997).

nificantly slower than geologically determined slip rates (e.g. Figs 6and 7; McClusky et al. 2001; Miller et al. 2001; Peltzer et al. 2001;Meade & Hager 2005). For example, Meade & Hager (2005) usedan elastic block model constrained by Global Positioning Systemmeasurements of interseismic deformation to determine a rate ofelastic strain accumulation along the central Garlock fault of 1.8 ±1.5 mm yr−1. This rate is much slower than the 4–9 mm yr−1 geo-logic slip rates measured along the central part of the fault (Clark &Lajoie 1974; McGill 1992; McGill & Sieh 1993; McGill et al. 2009;this study). Moreover, the velocity field in the region is dominatedby north–south right-lateral shear oriented almost perpendicular tothe trace of the Garlock fault (Dokka & Travis 1990; Humphreys& Weldon 1994; Hearn & Humphreys 1998; Thatcher et al. 1999;Dixon et al. 2000, 2003; McClusky et al. 2001; Miller et al. 2001;Bennett et al. 2003; Wesnousky 2005a,b). These observations sug-gest that the Garlock fault is currently experiencing a period oftransient slow strain accumulation, in which the lower crust (andmantle lithosphere?) beneath the fault is deforming at a rate thatis much slower than its long-term average rate. Thus, the Garlockfault apparently experiences two modes of strain accumulation; thecurrent ‘slow’ mode, in which strain accumulates at a lower than av-

erage rate along the fault, and alternating ‘fast’ modes, during whichthe fault must store elastic strain energy at rates much higher thanthe long-term average to account for the relatively rapid geologicslip rates measured along the central part of the fault.

Multiple hypotheses have been proposed in an attempt to ex-plain this apparent slip rate discrepancy but no consensus has beenreached thus far. One of those ideas calls for switching of seis-mic activity between the fault networks of the eastern Californiashear zone and the Garlock and San Andreas fault systems (Peltzeret al. 2001; Dolan et al. 2007) due to fluctuations in the loadingrate at depth associated with cycles of strain hardening and anneal-ing (Dolan et al. 2007). Another idea suggests northeast-trending,sinistral strain release driven by northwest-trending, dextral strainaccumulation (e.g. Savage et al. 2001; McGill et al. 2009), whichresults in conjugate faulting between the Garlock and San Andreasfaults (Hill & Dibblee 1953; McGill et al. 2009). The findingsfrom this study cannot distinguish between these potentially com-plementary hypotheses. Rather, they provide additional evidence ofconstant strain release along strike in the central part of the Garlockfault over a Holocene–late Pleistocene timescale. This along-strikeconsistency further highlights the discrepancy between current rates

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of elastic strain accumulation and longer term rates of strain releaseon the Garlock fault.

C O N C LU S I O N S

A 10Be depth profile age for abandonment of a small alluvialfan in the Summit Range that crosses the central Garlock fault,combined with lidar- and field-based mapping of a stream incisedinto the abandoned fan, yields a latest Pleistocene–Holocene sliprate of the central Garlock fault of 5.3 +1.0/−2.5 mm yr−1 that ispotentially a minimum rate, and a more speculative maximum rateof 6.6 ± 1.2 mm yr−1 based on a climatically inferred age for theoffset of the deeply incised channel thalweg and climate consider-ations. These rates are similar to other late Pleistocene–Holocenerates measured along a 100-km-long section of the central Garlockfault, highlighting the spatial consistency of slip along this part ofthe fault system when averaged over 105 year timescales. These rel-atively consistent latest Pleistocene–Holocene rates, however, aremuch faster than the current rate of elastic strain accumulation de-termined from geodetically constrained block models, indicatingthat the Garlock fault is currently experiencing a strain transient inwhich the lower crust (and mantle lithosphere?) beneath the faultare deforming much more slowly than the long-term average rate. Ifcorrect, this observation implies that the Garlock fault experiencestwo modes of strain accumulation, with the fault currently in a‘slow’ mode. Such slow modes, however, must be balanced by peri-ods of commensurately faster strain release. The available geologicslip rates, including those documented in this paper, are all aver-aged over multiple earthquake cycles and 105 year timescales. Thisobservation, coupled with the fact that the ∼10 ka fault slip ratesgenerally match much longer term (6–9 Ma) Garlock fault slip ratesbased on bedrock offsets, suggests that any ‘fast’ periods of elasticstrain accumulation probably occur at shorter timescales, on theorder of only a few earthquake cycles. Combining the small-scaledisplacement data (McGill & Sieh 1991) with palaeoseismologicdata (Dawson et al. 2003) from the El Paso Peaks area 20 km westof our study site suggests that the slip rate of the central Garlockfault may have been much faster (13 ± 2 mm yr−1) than the long-term average during a cluster of four earthquakes that occurredbetween ∼25 and 1650 AD, perhaps reflecting an elevated rate ofelastic strain accumulation. Alternatively, these small-scale offsetdata used to calculate the slip rate may have each accumulated inmore than one earthquake, which would result in a slower slip rateestimate of ∼6–9 mm yr−1. Additional shorter term slip rate datawill be required to test for possible correlations between earthquakeclustering and periods of elevated slip rate.

A C K N OW L E D G M E N T S

This research was supported by the Southern California Earth-quake Center. SCEC is funded by NSF Cooperative AgreementEAR-0106924 and USGS Cooperative Agreement 02HQAG0008.In addition, funding was also provided by the University of South-ern California Department of Earth Sciences (Ganev) and NationalScience Foundation EAR-0929960 (Frankel). Lidar data were col-lected by NCALM and are based on services provided by the PlateBoundary Observatory operated by UNAVCO for EarthScope andsupported by the NSF (EAR-0350028 and EAR-0732947). We aregrateful to Katelyn Carman and Lee McAuliffe for their help in thefield, to Penny Everett for her hospitality, to Tina Colbert and DylanRood for assistance with sample preparation and analysis and toDuncan Agnew, Mike Oskin and Vincent Regard for their thought-

ful and constructive reviews. The SCEC contribution number forthis paper is 1483.

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S U P P O RT I N G I N F O R M AT I O N

Additional Supporting Information may be found in the onlineversion of this article:

Figure S1. Field photograph of the Garlock fault, fault trace in-dicated by white arrows, and the Summit Range in the distance.Photograph taken from Trona Rd. looking south.Figure S2. Field photograph of the displaced channel across theGarlock fault (fault trace indicated by white arrows).

Please note: Wiley-Blackwell are not responsible for the content orfunctionality of any supporting materials supplied by the authors.Any queries (other than missing material) should be directed to thecorresponding author for the article.

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