Single- and Multigrain Luminescence Dating of Sediments Related to

Bulletin of the Seismological Society of America, Vol. 100, No. 3, pp. 1051–1072, June 2010, doi: 10.1785/0120090310

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault, Eastern San Francisco Bay Area, California by Glenn W. Berger, Thomas L. Sawyer, and Jeffery R. Unruh

Abstract Near urban areas and extending ∼60 km along the eastern margin of the Livermore Valley, the Greenville fault is the easternmost right-lateral strike-slip fault of the San Andreas system in the greater San Francisco Bay area. Notwithstanding the 1980 Livermore earthquake sequence (mainshock ML 5.9) on the Greenville fault, there is no record of recency or of Holocene rates of activity on the Greenville fault, yet this fault exhibits clear geomorphic evidence of late Quaternary faulting. In trenches parallel and normal to the fault through alluvial fan deposits at the Laughlin Road site only pedogenic carbonate was available for 14 C dating. Therefore, we applied several photon-stimulated luminescence (PSL) sediment-dating procedures to the silt and sand fractions of six samples. The polymineral-fine-silt multi-aliquot age estimates are generally inaccurate, but the single-grain quartz (SGQ) and multigrain quartz single-aliquot regenerative-dose (SAR) ages from sand grains are in stratigraphic sequence. In trench 3A these SAR ages range from 125
11 yrs (before 2007) within the topmost unit L to 13:45
0:79 ka in the base of the lowermost channel-fill unit G. The SAR PSL results demonstrate the importance of the use of SGQ dating for such sediments and provide the first numerical ages used to constrain the slip rate on the Greenville fault. Trench exposures reveal that unit G is an alluvial sequence infilling a paleochannel offset in a right-lateral sense along the northern Greenville fault. Age estimates from upper and middle unit G bracket deposition of subunit Gb are between 11:12
0:55 ka and 10:64
0:85 ka and those from middle and lower unit G bracket deposition of subunit Go are between 11:12
0:55 ka and 13:45
0:79 ka. These SAR PSL age estimates and measurements of the lateral offset constrain a preliminary slip-rate estimate to about 2 mm=yr or higher for the northern Greenville fault zone. Introduction The Greenville fault (Fig. 1) is the easternmost rightlateral strike-slip fault in the San Andreas system in the greater San Francisco Bay area. The fault zone extends ∼60 km, largely along the eastern margin the Livermore Valley. The trace of this fault passes within 2 km of the Livermore National Laboratories, as well as near growing urban areas. The 1980 Livermore earthquake (mainshock ML 5.9; e.g., Scheimer et al., 1982) sequence on the Greenville fault demonstrated the potential of this fault to generate significant earthquakes, such as Mw 6.7–7.1 (e.g., Petersen et al., 1996) with a characteristic earthquake of Mw 6.9 (Working Group on California Earthquake Probabilities [WGCEP], 2003). ML refers to local magnitude as defined by Richter and Gutenberg and is applicable to magnitudes in the range 2–6. MW refers to moment magnitude and is applicable to magnitudes > 3:5. The Greenville fault is important for our understanding of the full distribution and transfer of slip among eastern strands of the San Andreas fault system. However, there is no

record of recency or of Holocene rates of activity on the Greenville fault (Schwartz, 2008), and this fault is among a few segments just east of San Francisco that “still have large critical data gaps” (Galehouse and Lienkaemper, 2003). The Greenville fault exhibits clear geomorphic evidence of late Quaternary faulting, including fault scarps, hillside benches and troughs, offset and deflected drainages, and several closed depressions (e.g., Cotton, 1972; Herd, 1977; Dibblee, 1980; Hart, 1981; Wright et al., 1982; Unruh and Sawyer, 1998), yet the timing of prehistoric earthquakes (and thus inferred slip rates) on the fault and the relation of the fault to other geologic structures are effectively unknown. At the Laughlin Road site (Fig. 2) trenches parallel and normal to the fault were excavated (Sawyer and Unruh, 2002a) through alluvial fan deposits. The fault-normal trenches documented the location of the Greenville fault and provided evidence of surface faulting from the 1980 earthquake sequence (Sawyer and Unruh, 2002b). The

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Figure 1. Satellite image map showing the location of the study site and the geographic relation of the Greenville fault (solid white line) to some other components of the fault systems (dashed white lines) near San Francisco. The color version of this figure is available only in the electronic edition. fault-parallel trenches exposed a correlative sequence of well defined and laterally restricted channel deposits that are offset by right-lateral displacement. Two channel-fill units and the margin of a large paleochannel are exposed in the faultparallel trenches and are right-laterally offset by 17–25 m. The geomorphic offset of the alluvial fan is ∼25 m, based on a 2 ft contour map (Sawyer and Unruh, 2002b). Determination of Holocene slip rates requires numerical dating of the fault-related deposits. Of the available accelerator mass spectrometric (AMS) radiocarbon (14 C) age estimates for deposits related to the aforementioned offsets, most are from pedogenic carbonate and one is from charcoal. These age estimates (Table 1) range from modern (near the surface) to ∼14 ka, with age reversals and other chronostratigraphic inconsistencies. Because of these problems with 14 C dating and because pedogenic carbonate can provide only minimum-age estimates, we applied several photon-stimulated luminescence (PSL) sediment-dating procedures to the sand and silt fractions of deposits from these trenches. PSL dating can provide the burial age (last daylight exposure) of unheated sediment directly in calendar years (e.g., Aitken, 1998). Here we describe our dating procedures, the results, our interpretation of the PSL data, and their implications for slip-rate calculations. These results represent the first application of singlegrain quartz (SGQ) PSL dating to deposits from this region.

Tectonic and Geomorphic Setting The Laughlin Road site (Fig. 2) was identified by Unruh and Sawyer (1998) as having the potential to yield paleoseis-

mic information. At this site the fault is marked by low scarps and seasonally prominent vegetation lineaments that in conjunction with observations from trench exposures indicate that the fault forms a groundwater barrier. The small alluvial fan cut by the fault at this site is one of several fans aligned along the fault in the area (Unruh and Sawyer, 1998). The axis of the fan west of the main fault is located ∼25 m north of the fan apex, such that active deposition occurs only on the south flank of the fan. Based on these observations and trench stratigraphy, the overall fan landform appears to have been dismembered and translated northwest along the main trace of the fault. The northern part of the fault was mapped in detail by Herd (1977) and Hart (1981). Traces of the fault in the Diablo Range (Fig. 1) were mapped by Cotton (1972), Dibblee (1980), Wagner et al. (1990), and Unruh and Sawyer (1998). Right-lateral displacement along the Greenville fault is indicated (Sawyer and Unruh, 2002a) by subhorizontal slickensides on shear planes associated with the fault zone; a small graben and a large sag pond formed by apparent right-releasing stepovers along fault strands; right-laterally offset and deflected drainages; and right-lateral surface offsets observed along traces of the fault following the 1980 earthquake sequence. Seismicity associated with the northern part of the fault is characterized by a subvertical alignment of epicenters extending to depths of ∼17–18 km (Bolt et al., 1981; Hill et al., 1990; Oppenheimer and MacgregorScott, 1992). Focal mechanisms indicate mainly dextral strike-slip motion on northwest-striking nodal planes (Oppenheimer and Macgregor-Scott, 1992). Some geologic and paleoseismic studies conducted after the 1980 earthquake (Sweeney, 1982; Wright et al., 1982) inferred that the slip rate on the fault was ∼ < 1 mm=yr and thus very low relative to that of other strands of the San Andreas fault system at this latitude. Late Quaternary slip rates in the northern San Andreas fault system to the west are ∼15–30 mm=yr (e.g., Grove and Niemi, 2005). Among other limitations noted by Unruh and Sawyer (1998), this low slip-rate estimate (< 1 mm=yr) is poorly constrained by climate-dependent models of rates of soil formation and thus lacks numerical dating. In contrast, hypothesizing a leftrestraining transfer of dextral slip from the Greenville fault to the nearby Concord fault (Fig. 1), Unruh and Sawyer (1997) suggested that the slip rate on the Greenville fault may be several millimeters per year or greater. Based on other indirect arguments, Peterson et al. (1996) estimated a slip rate of 2
1 mm=yr for the Greenville fault. A recently determined geologic slip rate for the Concord fault over the past 6.4 ka is 3:2
0:4 mm=yr (Borchardt, 2008), which is comparable to the aseismic creep rate of 3:0–3:5 mm=yr (Galehouse, 2002) on that fault. If the Concord fault derives the full rate of slip via a strain transfer from the Greenville fault, then the rate of crustal shortening across the 25 km wide Mt. Diablo stepover region (Fig. 1), as well as the rate of right slip on the Greenville fault, should be comparable. D’Alessio et al. (2005) recently estimated a considerably

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Figure 2. Map of the Laughlin Road study site (about 37.74° N, 121.72° W), showing the two trenches (T3A and T4A, white fill) of this article. Prior trenches excavated by Sawyer and Unruh (2002a) are shown with gray fill but here are unlabeled. Elevation contours are in meters. The color version of this figure is available only in the electronic edition.

Table 1 Radiocarbon (14 C) Age Estimates AMS Date 14 C

Sample

*

†

(14 C

LLNL Number (CAMS-)

Trench /Unit

Material

yr B.P.)

LRS-1 LRS-14 LRS-7

n.a. 83316 83312

T3/unit YC T3/upper unit G T3/middle unit G

charcoal pedogenic CaCO3 pedogenic CaCO3

n.a. 3775
35 2865
35

LRS-6

83311

T3/basal unit G

pedogenic CaCO3

2600
40

LRS-9

83314

T3/basal unit G

pedogenic CaCO3

2760
40

LRS-8

83313

T3/upper unit PG

pedogenic CaCO3

11830
50

LRS-10

83315

T3/upper unit PG

pedogenic CaCO3

7675
40

(cal. B.P.)‡

modern§ 4148–3991 3060–2892 3155–2869 2765–2714 2782–2535 2917–2783 2946–2777 14046–13614 14075–13502 8536–8405 8540–8388

*LLNL, Lawrence Livermore National Laboratory. T3, eastern fault-parallel trench (Sawyer and Unruh, 2002a). ‡ Stuiver and Reimer (2000); the two (2σ) range estimates arise from the nature of the calibration program. § CAMS-LLNL; G. Seitz (personal comm., 2001). †

(mean, ka)

4.1 3.0 2.7 2.9 13.8 8.5

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G. W. Berger, T. L. Sawyer, and J. R. Unruh

higher, preferred-model slip rate of 5:4
1:0 mm=yr for the Greenville fault, based on 200 Global Positioning System (GPS)-derived surface velocities and data collected between 1993 and 2003. Sawyer and Unruh (2002b) estimated a Holocene slip rate of 4:1
1:8 mm=yr, or less, based on the offset channel deposits and the aforementioned problematic AMS 14 C age estimates on pedogenic carbonate (Table 1). Because these 14 C age estimates likely underestimate the ages of the offset deposits, due to postdepositional accumulation of pedogenic carbonate, this preliminary slip rate was considered an upper bound estimate or limiting-maximum rate for the main trace of the northern Greenville fault. Thus there is considerable uncertainty about Holocene slip rates on the Greenville fault and relations to nearby faults.

Relevant Principles of Luminescence Dating PSL dating (Aitken, 1998) (the term optically stimulated luminescence [OSL] was introduced in a dating context by Berger, 1986) determines the time elapsed since the last exposure to daylight of feldspar and/or quartz grains in unheated sediments. PSL is a more realistic acronym than OSL because the implicit phenomena are photonic processes within crystals. Infrared (IR) light stimulates PSL from feldspars (hence IR-PSL) but generally not from quartz. Blue (B) or green (G) light stimulate PSL from both feldspars and quartz (hence B-PSL and G-PSL). PSL provides a unique dating tool for Quaternary sediments, mainly for two reasons: (1) PSL is very sensitive to daylight, with as much as 90% of the quartz signal capable of removal in only 5–15 sec of full sunlight exposure (GodfreySmith et al., 1988); and (2) PSL dating reaches well beyond the 14 C maximum age (usually 35 ka) to ∼150 ka with quartz (Murray and Olley, 2002) and potentially higher with feldspar (e.g., Berger, 2001; Jacobs, 2008). One of the principal constraints on PSL dating accuracy is that single grains are not always exposed to full sunlight when transported in fluvial or sheet-wash systems (e.g., Wallinga, 2002) or if transport occurs in darkness. Thus typically in waterborne and colluvial sediment one can expect a mixed-age population of grains and a resultant age overestimate if many grains are analyzed together. Before the advent of single-grain dating procedures (e.g., Duller et al., 1999), one could accurately date fine-silt grains from several settings, but this required the assumption that all silt grains were equally well exposed to lengthy daylight illumination before burial. An ideal silt sample, such as loess, would have all grains exposed to many hours of daylight before burial. However, for many deposits (e.g., colluvial and fluvial) only some of the grains might be exposed adequately to daylight. Because it is impossible, at present, to isolate individual silt-size grains in PSL dating, to truly isolate the PSL signals from grains last exposed to daylight in alluvial and colluvial deposits, one must employ sand-size grains. Moreover, because feldspar grains can often contain an unstable component (leading to age underestimates) (e.g.,

Aitken, 1998), quartz grains are preferred. Thus when sand grains have been used in this project, attempts were made to isolate pure quartz. After burial of detrital grains, low-level, ambient ionizing radiation (mainly from the decay of K, U, and Th isotopes within the sediment) provides an effectively constant (over the Quaternary) dose rate (DR ) to buried mineral crystals. This ionizing radiation dislodges electrons from lattice-mineral sites into lattice charge traps (either light sensitive or not), some of which can be stable over more than a million years. With PSL dating (as distinct from the older thermoluminescence [TL] dating methods), only the light-sensitive charge traps are sampled. Under photonic stimulation in the laboratory a fraction of the trapped light-sensitive charges recombine with opposite charges at certain lattice sites, releasing photons. The intensity of this PSL is related to the time since last daylight exposure, which in suitable settings is equivalent to the last burial time. This PSL is scaled in the laboratory by use of calibrated radiation sources to yield a paleodose (DE or equivalent dose) value. DE (in units Gy or Gray) is a measure of the total absorbed energy from ionizing radiation that is stored in the crystal since the last daylight exposure, and DR (Gy=ka) is a measure of the rate of storage of ionizingradiation energy in the crystal. The burial age is t  DE =DR in calendar years. The burial dose rate will not be constant if there have been radioisotope decay-series disequilibria in the deposits (e.g., Olley et al., 1996), but this is uncommon in fine-grain sediments (e.g., Berger and Péwé, 2001; Krbetschek et al., 1994). With sandy sediments, the routine use of thick-source alpha-particle counting (TSAC) (e.g., Huntley et al., 1986) for determination of the U and Th isotope contributions to the dose rate is more accurate (when disequilibrium occurs) than are the more commonly used procedures that determine only the concentrations of the parent nuclides (e.g., neutron activation methods and inductively coupled plasma mass spectrometry). We employed TSAC to determine U and Th contributions and commercial atomic absorption spectrophotometry (AAS) to determine K contributions to DR . We estimated the small cosmic-ray contribution to DR per Prescott and Hutton (1988). In this project, all samples have been dated by two general PSL approaches: the multi-aliquot approach (for fine silts), and the single-aliquot approach (for sand grains). An aliquot consists of one to several grains on a single metal disc. We employed the multi-aliquot thermal-transfer (MATT) approach (e.g., Ollerhead et al., 1994; Aitken, 1998; Berger and Doran, 2001) of the IR-PSL procedures for polymineral fine silt, thus measuring only feldspar signals. This procedure is similar to the misnamed multi-aliquot additivedose (MAAD) procedure referred to in some publications. In MATT, a dose-response curve is extrapolated to a nonzero luminescence signal. Strictly, an additive-dose curve refers to laboratory-generated buildup from a zero signal (Aitken, 1998), such as encountered with heated materials (pottery, volcanic glass shards). For unheated sediments there is

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault always a residual luminescence signal. In TL this is mainly a postbleaching light-insensitive signal. In PSL, this is an artifact of laboratory heat treatments, usually termed thermal transfer. For these reasons, MATT is a more accurate acronym than MAAD for unheated sediments. To obtain accurate age estimates with the use of the MATT approach, we require that all silt grains be exposed equally to hours of daylight before burial. With MATT, 40–50 aliquots are needed to obtain a single DE value (hence, a single age estimate). The single-aliquot regenerative-dose (SAR) PSL procedure (Murray and Wintle, 2000, 2003), and its extension to single grains, is now an established method for dating quartz-bearing silts and sands (reviews by Bøtter-Jensen et al., 2003; Duller, 2004; Lian and Roberts, 2006; Fuchs and Owen, 2008; Jacobs, 2008; Preusser et al., 2008; Rittenour, 2008). Most quartz grains release 3–6 PSL signals having different release rates (e.g., Jain et al., 2003, 2005). The preferred component for SAR dating is the fast component (e.g., Murray and Wintle, 2000; Wintle and Murray, 2006), a signal usually released in the first 0.8 sec of typical bluediode stimulation. A dating precision of ca. 10% (sometimes better) can be attained routinely with multigrain SAR quartz methods (e.g., Murray and Olley, 2002) applied to eolian sand. The SAR age range for quartz generally is from as little as 8 yrs (e.g., Ballarini et al., 2003, 2007a) to ∼150 ka (Murray and Olley, 2002) for normal-radioactivity sediment. With SAR, each aliquot yields a distinct DE value and thus a distinct age estimate. In the limit of one grain per aliquot, single grains can be dated with the SAR approach. The concepts and advantages of single-grain dating are expounded clearly by Olley et al. (1999), Bøtter-Jensen et al. (2003), Duller (2004, 2008), and Berger et al. (2009). Porat et al. (2009) report the application of multigrain and SGQ PSL dating to fault-related colluvial wedge and alluvial fan deposits in the southern Negev. They described difficulties in interpreting multigrain SAR results from quartz and feldspar sand grains and found that only single-grain-quartz dating (of 8 samples) was able to produce stratigraphically

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reasonable age estimates. They obtained minimum-age estimates ranging from ∼0:5 ka to 5.7 ka.

Trench Samples Stratigraphy All four trenches at the Laughlin Road site exposed similar stratigraphy. From top to bottom these are divided into Holocene/Pleistocene units L, C, W, G (with subunits Gb and Go), and CP (Figs. 3 and 4). An additional alluvial unit, PG, has been logged at the northwest end of trench 3 of Sawyer and Unruh (2002a), occurring between units CP and overlain by unit G. Underlying these beds is sandstone of the Miocene Cierbo formation. The Holocene alluvial units are correlated across the fault based on their relative stratigraphic position, unique stratigraphic and/or pedogenic characteristics, and a matching of the transverse morphology of stream channels. The uppermost alluvial deposit exposed at the site, unit L (PSL sample 5, Table 2), is a southward-thickening layer of fine sandy loam to silty fine sand, up to 55 cm thick, found in trench 3A and, previously, in the southern part of trench 3 and the overlapping portion of trench 4 (not named in Fig. 2). Unit L represents the youngest depositional lobe on the alluvial fan. The distribution and thickness of unit L indicate that recent deposition has occurred only on the south flank of the fan, as expected if this landform has been translated northwestward by dextral movement on the fault. The unit is dark grayish brown (10YR4=2) to dark brown (7:5YR4=2), with the lower 5–7 cm being noticeably lighter in color, suggesting leaching by elluvial pedogenic processes. The lower contact of unit L is planar, sharp to distinct, and commonly marked by basal parting. Only one colluvial unit was encountered at the Laughlin Road site, which was shed off the colluvial slope directly north of the site, and partly buries the axis of the alluvial fan. Unit C is a southwestward thinning layer of recent

Figure 3. Simplified stratigraphic log for trench 3A, identifying the stratigraphic units (see text) and showing the locations of the luminescence (PSL) samples, and the most probable PSL age estimates (lower left) for each sample.

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Figure 4. Stratigraphic log of the portion of trench 4A from which PSL sample 6 was extracted. surficial colluvium overlying unit W in trenches 4A and 5 and, previously, in the northernmost parts of trenches 3 and 4. This wedge-shaped unit is composed of reddish brown, clayey matrix supporting gravel-sized clasts. The deposition of unit C appears to postdate the most-recent (pre1980) surface-rupturing earthquake on the Greenville fault. Underlying unit L in trench 3A, unit YC is a generally fining upward sequence of pebbly sand to sandy silt that fills a narrow ∼1 m deep trough incised through unit W and into unit G. The lower ∼2=3 of the channel fill exhibits crossbedded lamina that locally have been disrupted by bioturbation. Sawyer and Unruh (2002a) identified a half meter long fine sand layer containing scattered small fragments of detrital charcoal in the uppermost deposit associated with unit YC, that yielded a modern AMS radiocarbon age (Table 1). Alluvial unit W (PSL samples 4 and 6) is ∼1 m thick, dark reddish brown to black (5YR2:5=2–5YR2:5=1), clay to sandy clay that is hard to extremely hard when dry. This unit is associated with a vertisolic (self-churning) soil characterized by strong, coarse to extremely coarse, prismatic to columnar structure. Individual soil peds are up to 50 cm wide by 50 cm long. Pedogenic slickensides were observed on wedge-shaped (sphenoid) structural aggregates, a diagnostic of vertisols. The sedimentary structure of unit W has been extensively disrupted by pedoturbation, and only discontin-

uous remnants of gravel layers remain locally in this alluvial deposit. The lower contact with unit G is gradual to diffuse. Unit G (PSL samples 3, 2, and 1, Table 2) occurs at depths 0.8–1.3 m in all four trenches but is absent in the relatively downthrown (west) side of the fault in trench 2, where apparently it lies deeper than 2 m. The unit fills a sizable channel > 6 m wide and > 1 m deep. Unit G is incised into the Cierbo formation sandstone and in trench 3 is incised into subadjacent units CP and PG. Unit G is a relatively thick channel-fill sequence, > 2 m thick that represents a significant aggradational period of alluvial fan deposition. This unit is hard to very hard, dense, pebbly sand to silty clay. Clasts range from 0.3 to 3 cm and many siliceous clasts are well rounded and probably were reworked from clastic beds within the Cierbo formation. The upper 50–60 cm of unit G (PSL sample 3) is reddish brown (5YR4=4) and the lower part is yellowish brown (10YR5=6). The locally present Bt horizon exhibits moderately distinct blocky soil structure with infrequent, discontinuous, reddish brown (5YR4=3) clay films on ped faces. Underlying this soil horizon is a juvenile Bk horizon with carbonate-cemented nodules and rhizoliths and rare carbonate coats (calcens) on ped faces. Two distinctive gravel layers within unit G, subunits Gb and Go (Figs. 3 and 5) were classified because of their importance in constraining stratigraphic offsets along the Greenville fault. These subunits are present only in the southern part of trenches 3 and 3A and in the northern part of trenches 4 and 4A. The effects of the gravels on the gamma (environmental) dose rate for the PSL samples near the gravels (Fig. 5) were incorporated into the dose-rate calculations by use of dosimetry data from small samples of the gravels coupled with estimates of the relative geometric volume of gravel within a 30 cm radius of the corresponding PSL samples. Opposite the fault from trenches 3 and 3A these subunits do not occur to the south in trenches 4 and 4A. Both subunits have abrupt and distinct southern pinchouts, especially subunit Go. The abrupt pinchout of subunit Gb on either side of the fault provides a measure of the amount and sense of slip averaged over several earthquake cycles. Sawyer and Unruh (2002a) interpret these subunit traits to indicate a minor amount of vertical offset (0:4
0:1 m). However, a right-lateral offset of subunit Go of 22:8
2:0 m implies a preliminary Holocene slip rate of ∼4
2 mm=yr or less for the main trace of the fault at this site, using the available 14 C age estimates (Table 1).

Table 2 Luminescence Samples Sample

Depth (m,
0:02)

LVM03-5 LVM03-4 LVM03-6 LVM03-3 LVM03-2 LVM03-1

0.20 0.43 0.62 0.93 1.27 1.76

Description

Trench Trench Trench Trench Trench Trench

3A, 3A, 4A, 3A, 3A, 3A,

surface L unit, 0.22 m above sample 4, contains fine rootlets upper W unit, ∼8 cm below contact of L unit middle W unit, 38 cm above W base, and 42 cm below C upper G unit, above coarse part of subunit Gb, and 34 cm above sample 2 between main subunits Gb and Go, above coarse part of Go, and 49 cm above sample 1 10–12 cm above sandy unit SS, below base of Go subunit

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault

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of these carbonates was conducted (Sawyer and Unruh, 2002a) (Table 1). Unit CP, exposed only in the central, bottom part of trenches 3 and 3A, is composed of gravelly to locally cobbly alluvium with a silty clay loam matrix. This unit is associated with a calcic paleosol that ubiquitously engulfs the deposit in pedogenic carbonate. Carbonate occurs also as scattered nodules and rhizoliths. The abundance of carbonate and secondary clay suggests a protracted period of soil formation and indicates a late Pleistocene age. Luminescence Samples Six samples were collected by G. W. Berger in July 2003 (Fig. 5). These are listed in Table 2 in the order of stratigraphic position, top to bottom. Small samples were collected adjacent to and ∼20 cm above and below the luminescence samples. These small samples were employed for measurement of in situ and saturation water concentrations, as well as elemental concentrations. These data are required for dose-rate calculations. The small samples above and below the PSL samples provided data used for estimating the environmental (or gamma) dose rate from around the PSL samples. Luminescence samples were extracted from cleaned (8–20 cm of surface was first removed) trench walls by use of light-tight tins. Only the ∼5 mm thick ends of the blocks within each tin were exposed to light during sampling. Consequently, ∼5 mm of this end material was removed under filtered laboratory lighting (Berger and Kratt, 2008) before processing of the interior sample for luminescence measurements.

Analytical Procedures Silt Grains

Figure 5. Merged photographs of that portion of trench 3A yielding the PSL samples. Small samples were collected above, at, and below the PSL samples for dosimetry. The color version of this figure is available only in the electronic edition.

Alluvial unit PG was found only in the central part of trench 3 (not shown), where it infills two relatively small stream channels scoured into unit CP. Unit PG is distinguished from unit CP based on color and substantially lower pedogenic carbonate content and from overlying unit G by the increased presence of translocated pedogenic clay and locally lower gravel content. The dimensions of these channels are similar to that of the active channel on the alluvial fan. Unit PG is ∼1 m thick and ∼6 m wide. This gravelly channel-fill deposit is a brown, silty clay loam with pedogenic carbonate filaments and nodules. 14 C dating of some

For the MATT approach, the polymineral, noncarbonate, detrital 4–11 μm diameter size fraction was prepared using laboratory procedures outlined elsewhere (e.g., Berger, 1990). For U and Th measurements, the aforementioned TSAC technique was applied to dried powders. K was determined by commercial AAS. The past average water concentration ratio was estimated to be ∼50% of the measured saturation values for all samples. Saturation of samples in the Reno laboratory is attained routinely by use of capillary uptake of deionized water into the preperforated field-sample tins collected for dosimetry around and adjacent to the PSL sample. Water concentrations in the collected dosimetry samples varied from ∼8% to 15% and saturation values varied from ∼20% to 40%, depending upon texture (mainly) and depth excavated into the trench face. As the trenches had been open for several months before PSL sampling, the ascollected water values were probably lower than the presentday (fresh trench) in situ values would be, even though we attempted to compensate for section-face drying by excavating into the trench wall. Furthermore, given the uncertainty in the possible annual variation in in situ water concentration

1058 at this site (we lacked such data), our choice of 50% saturation value (consistent with G. W. Berger’s decades of experience) and of conservative error estimates (30–80% [2σ] of the chosen water concentration values) is reasonable. To convert the sample’s fossil light to an equivalent absorbed energy, calibrated laboratory beta (90 Sr=90 Y) and alpha (241 Am) sources were used. Signals in the MATT IR-PSL experiments were recorded with an automated, highcapacity Daybreak Nuclear Model 1150 reader using an EMI 9235Q photomultiplier tube (PMT). The MATT IR-PSL signals from feldspars were detected at deep blue wavelengths near the known 410 nm emission from most K-rich feldspars (e.g., Aitken, 1998; Krbetschek et al., 1997). Sand Grains Quartz-rich fractions were prepared by first destroying any carbonates and organic material by use of, respectively, 1N HCl acid and 30% H2 O2 (with dionized-water rinses between and after). For most samples, a hand magnet was employed to remove mafic minerals and thus concentrate quartz–feldspar mixtures before HF acid treatment. Later in the project the laboratory acquired a Frantz isodynamic magnetic separator, and subsequently this more efficient approach was employed to obtain nonmagnetic subfractions prior to treatment with 48% HF acid for dissolution of feldspars (e.g., Aitken, 1998), and subsequent treatment by 20% HCl acid to remove any fluorides. We found that the efficiency of destruction of feldspars in this way was sample dependent, so that additional quartz-purification steps were required for one sample. With the SAR procedure, all signals are recorded in the ultraviolet (UV) (360 nm center wavelength used here) where quartz PSL signals are strongest. As feldspar signals are generally highly unstable in the UV every effort was made to eliminate them, in ways outlined subsequently. The 105–210 μm grain-diameter fraction of quartz extracts was used for the SAR experiments. After HF acid treatment, representative multigrain portions of each sample were tested for the possible presence of residual (contamination) feldspar by first administering a 10 Gy radiation dose, then, after a 2 day delay, stimulating the test portions with IR at 80 or 125°C. If the sample was contaminated significantly and if enough sample remained, the HF treatment was repeated. Only sample 4 was treated more than once with HF acid. Other relevant details are given in footnotes to the subsequent tables. In addition to these post-HF IR multigrain tests for feldspar contamination, during multigrain SAR analysis, a hightemperature (125°C) IR wash step (e.g., Olley et al., 2004; Wang et al., 2006), but with stimulation times much longer than those of Olley et al. (2004), was included to reduce further any remaining small feldspar signal. Additionally, to monitor the effectiveness of this IR wash, some SAR steps were added at the end of the PSL readout sequence to permit comparison of postirradiation B-PSL signals (ostensibly from

G. W. Berger, T. L. Sawyer, and J. R. Unruh

quartz) with and without prior IR washing. Rather than the comparison of PSL intensities before and after an IR wash as introduced by Duller (2003) (his IR depletion ratio test), we compared L=T (test-dose-normalized luminescence signals) ratios after and before an IR wash step. Aliquots evidencing significant feldspar signals (L=T after/before ratios > 1:0 by more than 2σ) were rejected from age calculations. Similarly, in the single-grain experiments comparable additional SAR steps recorded beta-induced B-PSL with and without prior IR washing (Table 3). This expanded readout sequence facilitated rejection of any single-grain signals likely arising from feldspars. The main SAR parameters included the following choices (Table 3): (1) use of the 40 sec blue-diode wash step of Murray and Wintle (2003) at 20°C above the preheat temperature; (2) use of a suitable cut heat (e.g., 160°C); and (3) use of preheating temperatures (with 10 sec holds) in the range 180–240°C. The choice of preheating temperature for each sample was based on preliminary tests and the experiences of others with samples of similar age. Almost without exception, preheats of 180–200°C with cut heats of 160°C are suitable for very young samples and preheats up to 260°C (with cut heats up to 20°C less than the preheat [Wintle and Murray, 2006]) yield accurate DE values in doserecovery tests for older samples. We conducted one singlegrain dose-recovery test (see the Results section) using a preheating of 220°C. Several quality-control criteria were employed to reject PSL signals and resultant SAR DE values. Data rejection criteria were similar to those in common practice (e.g., Wintle and Murray, 2006). We accepted data having recycle ratios within 2σ of 1.0 (equivalent to the criterion of Ballarini et al., 2007a), recuperation ratios (e.g., Aitken, 1998) within 2σ of zero when recuperation was > 20% of the normalized natural signal (L0 =T 0 ratio), and test-dose-signal errors < 35%. We forced dose-response curves through the origin. In single-grain data processing we included a conservative Table 3 Modified-SAR Procedure for Single-Grain Signal Readout, Cycles i  0–n Step

1 2 3 4 5 6 7 8 9 10

Comment

(90 Sr=90 Y

Regenerative dose β source, skip in cycle i  0) Preheat (210°C or 220°C for 10 sec at 125°C) IR wash with diodes (875 nm, 90% power, ≥ 150 sec at 125°C) Measure Li using green laser (532 nm) stimulation (90% power for 1 sec at 125°C) Test dose (β source, e.g., 40 Gy at room temperature) Cut heat (160°C or 180°C, recording TL) IR wash with diodes (90% power, typically for 150 sec at 125°C) Measure T i using green-laser stimulation (90% power for 1 sec at 125°C) Blue wash with diodes (470 nm, 90% power for 40 sec at preheat 20C°) Repeat from step 1. In last cycle (second recycle), skip step 3, stop after step 8.

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault 12% single-grain measurement error (Thomsen et al., 2007) in the DE calculation. To minimize contributions from nonfast quartz-signal components within an SAR readout curve, an early light subtraction (ELS here, or EBS in Ballarini et al., 2007b; Berger, 2009) procedure was employed in data processing. The ELS procedure concentrates the most stable and most light sensitive of the quartz PSL component signals—the fast component. In the single-grain ELS approach, we subtracted the signal in the 0.2–0.3 sec interval from the first 0.06 sec of signal to calculate L=T ratios. This approach attempts to reduce the signal contribution from any of the less lightsensitive medium and slow components that might be present. Although all quartz PSL components may be zeroed during eolian transport, only the fast and medium components appear to be zeroed during fluvial transport (Singarayer et al., 2005). The distinction between recuperation and thermal transfer in SAR experiments is outlined succinctly by Wintle and Murray (2006). We believe that our choices of preheat temperatures and our use of the ELS data-processing procedure minimize or eliminate any possible thermal-transfer effects in our samples. Whether or not a slight thermal-transfer effect remains in the DE values from our youngest sample (LVM03-5) is not critical because the calculated age estimate (see the Results section) is consistent stratigraphically with what is known of this near-surface sediment horizon. Signals in the multigrain SAR experiments were recorded with an automated, high-capacity Daybreak Nuclear Model 2200 reader using an EMI 9235Q PMT or with automated Risø Model DA-20 PSL reader systems, also using an EMI 9235Q PMT, and blue light-emitting diode (LED) stimulation (B-PSL). For each of samples 1, 2, 3, 4, and 6, a multigrain SAR experiment was conducted also using the DA-20 blue-LED and IR-LED pulsed-diode facility (Thomsen et al., 2008), employing 50 μ sec on–off pulse intervals. The pulsed-diode approach can eliminate feldspar contributions to the B-PSL when HF acid treatments fail to remove all feldspar grains from the sample. Single-grain SAR experiments were conducted with the DA-20 reader systems, employing green-laser stimulation (e.g., Bøtter-Jensen et al., 2003; Duller, 2004) (G-PSL) (Table 3). Framework for Interpretation of DE Data from Sand Grains An important way to visualize the DE data obtained from SAR dating experiments can be a histogram plot (e.g.,

Olley et al., 1998), but a more statistically realistic plot for very young samples (e.g., Berger, 2009; Pietsch, 2009) is a probability-density (PD) plot (e.g., Galbraith, 1998; BøtterJensen et al., 2003; Jacobs et al., 2003). For samples older than modern or, for example, >∼100 yrs, the PD plot distorts the distribution of DE values by overemphasizing the relative significance (probability) of the lowest DE values compared to the higher DE values. This distortion arises because DE values

1059

for nonmodern samples conform more to a lognormal distribution than a Gaussian (statistically normal) distribution and because the errors in DE values are often proportional to the DE values. Consequently, Berger (2010) introduced a transformed-probability-density (TPD) plot, for which logarithms of DE values and their relative (not absolute) errors are employed. The TPD plot thus represents the relative structure of the DE distribution more accurately than does the PD plot, and TPD plots provide a graphical representation invoking a more intuitive visualization of the relative statistical significance of data than does the radial plot (e.g., Galbraith et al., 1999), which accurately represents the statistical significance of the individual DE values. Therefore, we employ here the TPD plot (coupled with a plot of ranked DE values and their absolute errors) as a visually intuitive graphical tool for exploratory, diagnostic, or descriptive purposes. That is, like the radial plot (which also employs relative errors and logarithms of DE values) the TPD plot can assist in the choosing of formal numeric hypothesis-testing procedures. For reasons outlined next, we employ the TPD plot as a visual aid in recognizing any possible grouping of youngest-age DE values within a given distribution. As explained elsewhere (e.g., Olley et al., 1998; Wallinga, 2002), with alluvial deposits many grains will retain a relict-age signal (they are not exposed to much or any daylight during final transport to the burial horizon), leading to a positively skewed distribution of SAR DE values. For this reason we dwell on the sand-grain results in the following discussions because only for sand grains are we able to resolve the signals of well-exposed grains from those of the poorly exposed grains. For most noneolian deposits, only the DE values defining the lowest DE cluster (component) may yield the true age (age of last daylight exposure). This concept of interpretation of skewed DE distributions provides the main guide for our age interpretations of SAR data. We use the TPD plot to visually identify any cluster of youngest-age DE values from a given experiment. When we have identified a smallest-DE cluster of data points within a TPD plot, then we calculate a weighted (by inverse variance) mean DE value for that cluster for use in age determination. Weighted means and standard errors of these means are calculated according to the equations summarized by Topping (1962). Weighted means are appropriate because the error estimates for DE values are independent of each other. One may quibble about whether or not the standard error of the weighted mean or the larger standard deviation is appropriate, but that is a topic for future, more detailed comparative-dating tests. In this project, the lack of any accurate independent numerical ages (except for the surface unit L) makes any stratigraphically acceptable ages from PSL dating valuable.

Results Luminescence dose-rate data are listed in Table 4 with details in footnotes. Dose rates are in the range

1060

G. W. Berger, T. L. Sawyer, and J. R. Unruh

Table 4 Dosimetry Data for the Sediment Samples Sample

Water*

K2 O (%)

Ct † (ks1 cm2 )

LVM03-5

0:14
0:03 0:14
0:03 (sand quartz) 0:14
0:03 0:14
0:03 (sand quartz) 0:16
0:05 0:16
0:03 (sand quartz) 0:13
0:05 0:20
0:03 (sand quartz) 0:14
0:03 0:14
0:03 (sand quartz) 0:14
0:02 0:14
0:02 (sand quartz)

3:05
0:05 2:52
0:05

LVM03-4

LVM03-6

LVM03-3

LVM03-2

LVM03-1

(ks1 cm2 )

b Value‡ (pGy m2 )

Dose Rate§ (Gy/ka)

0:3734
0:0088 0:500
0:012

0:171
0:028 0:225
0:037

0:787
0:081

3:44
0:12

2:52
0:05 2:61
0:05

0:500
0:012 0:465
0:045

0:225
0:037 0:214
0:021

0:745
0:072

3:47
0:11 3:37
0:15

1:84
0:05 1:81
0:05

0:596
0:017 0:605
0:019

0:180
0:048 0:271
0:010

0:695
0:080

3:25
0:11 3:19
0:18

2:11
0:05 2:16
0:05

0:621
0:011 0:5150
0:0085

0:242
0:035 0:170
0:020

0:811
0:084

2:81
0:15 3:33
0:12

2:56
0:05 1:96
0:05

0:4307
0:0094 0:524
0:010

0:141
0:026 0:184
0:020

0:662
0:070

2:96
0:11 3:16
0:11

3:01
0:05 2:81
0:32

0:510
0:011 0:399
0:055

0:156
0:032 0:152
0:030

0:768
0:068

3:12
0:10 3:63
0:13

Cth

†

3:54
0:13

*Estimated historic average ratio of weight of water/weight of dry sample, based on measured as-collected and saturated values. Uncertainties here and elsewhere are
1σ. The first two rows for each sample provide the data for calculation of the dose-rate to fine-silt (4–11 μm diameters) grains. The second of these rows provides data used to calculate the γ dose-rate component (the so-called environmental or external component). Gamma rays penetrate ∼30 cm within typical sediment, thus material within 30 cm of the luminescence sample is important. These second-row data were obtained from small dosimetry samples collected above and below the PSL samples (e.g., Berger and Péwé, 2001). † Total and thorium count rates from finely powdered samples for TSAC method (Huntley and Wintle, 1981). Cu  Ct  Cth . These count rates are used directly in the dose-rate equations (Berger, 1988). ‡ Alpha effectiveness factor (Berger, 1988; Huntley et al., 1988). For sand quartz, this is set to zero and an internal dose-rate component is adopted§. § The dose rate was calculated with the conversion factors given by Adamiec and Aitken (1998) using the equations of Berger (1988) and includes a small cosmic-ray component estimated from the data of Prescott and Hutton (1988). In down-table order, these cosmic-ray dose-rate components are 0:20
0:01, 0:190
0:008, 0:19
0:01, 0:18
0:01, 0:17
0:01, and 0:165
0:010. All sand-quartz dose rates include a component  0:06
0:03 Gy=ka (A. Murray, personal comm., 2004) representing the small contributions from internal-quartz radio-elements. Also for the sand grains, appropriate β-attenuation factors were used (Aitken, 1985) because the range of β rays is typically 2 mm in sediment. For sample 6, there is an additional (∼10%) correction to the dose-rate calculation arising from the presence of 7:0%
0:1% organic matter. The effect of organic matter on the dose rate is analogous to that from intergranular water, and the correction calculation is carried out as detailed in Berger and Anderson (2000, p. 15,448).

2:8–3:5 Gy=ka, comparable to those from most terrestrial sediments (2–4 Gy=ka; Aitken, 1985, 1998). PSL data and calculated ages are listed in Table 5 with details in footnotes. In Table 5 a PSL age is calculated for each of several SAR runs for sample 4 and for the two SAR runs for samples 1 and 3. This listing aids the discussion of the results from these two samples, whereas for samples 2 and 6 a comparison of the SAR DE values suffices (dose rates being constant within a sample group of SAR DE values). As a check on our choice of preheats, we conducted a single-grain doserecovery test with sample LVM03-1(105–150 μm). Using a preheat of 220°C, a cut heat of 160°C, an applied β dose of ∼43 Gy (following blue-diode bleaching for 100 sec at 125°C), and 300 grain holes, we obtained 31 acceptable DE values normally distributed about the applied-dose value, yielding a dose-recovery ratio of 0:995
0:086. This result supports our choices for preheat temperatures and cut heats (see discussion in Wintle and Murray, 2006). The PSL age estimates range from ∼125 yrs (before 2007) to ∼20 ka. The sand-grain results seem to be more

accurate than the silt-grain age estimates, primarily because of a realistic age for the topmost sample (LVM03-5) and the lack of an apparent age reversal in unit G (samples 3, 2, and 1) when sand-grain results are chosen. Certainly these sandgrain results are more accurate than the available AMS 14 C age estimates. Both the sand and silt results indicate a significant age hiatus above unit G (above sample 3), but the sand data place that hiatus after 10.6 ka, rather than sometime after 15–20 ka (MATT silt data). The sand data indicate that unit G began to accumulate at 13.5 ka (SGQ age for sample LVM03-1) and aggregated until ∼10:6 ka (multigrain SAR age for sample LVM03-3) or later.

Discussion of PSL Systematics Luminescence from Silt Grains The MATT age estimates for fine-silt feldspars would be accurate only if all silt grains were exposed to at least several

LVM03-1

LVM03-2

LVM03-3

LVM03-6

LVM03-4

LVM03-5

Sample

150=2d 260=220 220=160

150=2d 240=220 240=180 150=2d 240=200 240=200 240=180 220=160

2 mm=105–185 2 mm=105–210 1 mm=105–210

2 mm=105–185

Size‡

SGQ 105–210 MATT SAR 2 mm=105–185 SAR-P 1 mm=105–185 Weighted-mean SAR  MATT SAR 2 mm=105–185 SAR-P 1 mm=105–185 MATT SAR 2 mm=105–150 SAR 0:4 mm=105–150 SAR-P 0:4 mm=105–150 SGQ 105–150 Weighted-mean SAR 0:4 mm and SGQ  MATT SAR 2 mm=105–150 SAR-P 1 mm=105–150 Weighted mean above multigrain SAR 

MATT SAR MATT SAR SAR SAR-P

145=2d 200=160 145=2d 220=170 230=200 210=180 210=180 145=2d 235=200 200=160

Mode†

Heating* (°C)

Table 5

6:44
0:39 0:432
0:034 (20) 20:08
0:94 12:92
0:37 (6) 10:83
0:28 (9) 14:25
0:65 (18)-a 15:25
0:62 (24)-b 14:00
0:57 (99) 12:95
0:75 7:46
0:39 (48) 5:91
0:38 (23) 6:66
0:27 68:9
3:1 42:9
1:1 (16) 31:5
1:9 (19) 48:6
2:7 44:21
0:70 (32) 37:9
2:0 (11) 37:4
2:9 (9) 33:6
1:0 (224) 34:7
1:3 75:6
1:7 66:9
2:7 (16) 68:8
4:6(21) 67:4
2:3

DE § (Gy)

∥

– 6711 3

– 25 13 – –

– 9:9 32:06:3

– –

–

– – 14:940:81 0:76

0:390:17 0:07

MAM

19:04
0:95

11:12
0:55 20:8
1:2

2:37
0:16 20:7
1:2 14:49
0:65 10:64
0:85 15:4
1:0 14:17
0:51

4:31
0:23 4:06
0:33

1:87
0:13 0:125
0:011 5:96
0:39 3:98
0:18 3:33
0:14 4:38
0:25

PSL Age # (ka, before 2007)

Multi-Aliquot, Single-Aliquot, and Single-Grain Data with Apparent Ages 14 C

(continued)

2.6–2.9

3:0
0:2

4:1
0:1

modern

Age (ka, cal. B.P.)

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault 1061

Mode†

SAR-P SGQ

Heating* (°C)

220=160 220=160 **

36:2
6:7 (4) 47:6
2:2 (128)

DE § (Gy)

Table 5 (Continued) 1 mm=105–150 105–150

Size‡

–

MAM

∥

13:45
0:79

PSL Age # (ka, before 2007)

14

C Age (ka, cal. B.P.)

*For the MATT fine-silt experiments, this preheating temperature is held (in a dark oven) for 2 days with readout conducted later. For the SAR multigrain experiments, this is the preheating (10 sec hold) followed by the cut heating (10 sec hold). For the SAR single-grain experiments, the cut heating hold was 0 sec (Murray and Wintle, 2000). A signal readout temperature of 125°C was employed in all SAR experiments and 30°C in the MATT experiments. † MATT denotes use of the multi-aliquot thermal-transfer additive-dose IR-PSL procedures. SAR-P denotes use of the pulsed-diode technology for multigrain SAR runs. SGQ denotes use of the microfocused laser SGQ technology. ‡ For multigrain SAR runs, the aliquot diameter (circular zone of monolayer of grains) is followed by the grain-size-diameter fraction (micrometers) used. The estimated number of grains per aliquot is stated in the text (Discussion of PSL Systematics section). § For the MATT IR-PSL experiments, this is the weighted-mean equivalent dose plus or minus the average error (1 standard deviation), typically for the first 30–40 sec of signal release. The DE values were determined as outlined in Berger and Doran (2001), using a bleaching time of 5 hr. For SAR, this is the weighted mean plus standard error of the mean (Topping, 1962) corresponding to the selected number of aliquots (multigrain data) or selected number of grains (single-grain data) shown in parentheses. Only the largest of the two standard-error estimates (internal or external; Topping, 1962) is reported here. The selected DE values are represented by the filled circles shown in the TPD plots or those data points under the horizontal bar in these plots. For LVM03-4 (SAR-P), a and b refer to bars in Figure 8b. ∥ MAM (four parameter), of Galbraith et al. (1999) using an Excel version courtesy of S. Huot. MAM was applied only to some of the SAR data sets, as explained here. The errors are at 1σ, and “–” indicates that the MAM returned no error estimate. The absence of an MAM estimate (“–”) indicates that the MAM calculation failed (Namely, for the SGQ runs, MAM parameter p  0 implying bulk of data points are consistent with a normal distribution. The TPD plots for SGQ runs show this.) or was not applied because the TPD plot is sufficiently informative. # DE divided by the corresponding dose rate from Table 4. **The weighted mean for the four lowest DE data points in Figure 13.

Sample

1062 G. W. Berger, T. L. Sawyer, and J. R. Unruh

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault tens of minutes of daylight before final burial, and only if any possible feldspar instability (due to anomalous fading, Aitken, 1985) has been removed, minimized, or is absent. In settings such as these samples represent, the most likely source of inaccuracy in MATT age estimates for silt would be incomplete daylight exposure of all silt grains. The 1:87
0:13 ka MATT estimate for near-surface sample 5 clearly indicates a mixed-age population of silt grains, and one that carries a very large relict-age signal. That is, most of these silt grains likely were not exposed to daylight before final burial. This light shielding of a significant fraction of silt grains is expected in most alluvial/colluvial deposits, depending on transport distance and the possibility of transport of a portion of the grains at night. Moreover, there is an apparent MATT age reversal between samples 3 (upper unit G) and 2 (middle unit G). The relatively young 4:06
0:33 ka MATT result for sample 6 (unit W, trench 4A) may reflect the nature of this sedimentary unit, which we discuss subsequently in the context of the sand-grain results. We think that the MATT age estimates are unreliable, mainly because of a sometimes failure of the clock-zeroing assumption in these fan and channel-fill deposits.

1063

mean of the filled-circle data points yields the most probable minimum age of this sample. The deconvolution statistical four-parameter minimum-age model (MAM column, Table 5) yields a result of ∼0:4 Gy, not different (but of lower precision) from the preferred weighted-mean result of 0:432
0:034 Gy. The different representations of the DE data by the PD and TPD plots are compared in Figure 6. Hereafter we employ only the TPD plot. The use of ELS with this sample produced large errors and poor data due to a lower signal/ background ratio. The 125
11 yr (before 2007) age estimate for this sample is consistent not only with its stratigraphic position (within the modern root zone) but also with the modern age estimated from associated 14 C charcoal dating (Table 1). This result gives us confidence that the multigrain and single-grain SAR results for the other samples at this site can reveal the true age of deposition of the sandy matrix of the deposits. Note that neither of the middle and highest DE peaks in the TPD plot (Fig. 6) have any geological significance but merely represent aliquots containing an arbitrary mixture of ages (partially reset PSL).

Luminescence from Sand Grains Complications in interpretation of DE distributions (e.g., in TPD or radial plots) can arise from choices of laboratory parameters or from geological factors. Deposits such as buried soil horizons can represent geological factors that complicate interpretation of TPD plots. In this case, because of prolonged sediment overturn (with the evolving surface) by various agents (e.g., earthworms), the most realistic SAR PSL age may be represented by the central DE component (cluster), rather than the youngest component of the DE distribution. The samples (4 and 6) from vertisol unit W present such geologically generated complications and are discussed subsequently. Apart from such geological complications, the principal factor affecting the interpretation of multigrain SAR DE data distributions is the size (number of grains per disc) of the chosen aliquot (e.g., Duller, 2008; Berger et al., 2009). Based on preliminary sensitivity tests, we selected multigrain SAR aliquot sizes (constant within each SAR experiment) large enough to yield usefully precise PSL signals, but small enough to create a reasonably large interaliquot spread of DE values. For the 105–150 μm size fractions (Table 5), each of the 2 mm, 1 mm, and 0.4 mm aliquots was calculated to hold, respectively, ∼250, 60, and ∼20 grains. For the 105–210 and 105–185 μm size fractions, each of the 2 mm and 1 mm aliquots was calculated to hold, respectively, ∼200 and 50 grains. Microscopic observation of 0.4 mm aliquots confirms these estimates. Sample 5 (Unit L, Trench 3A). This sample’s quartz sand grains yielded a classical positively skewed distribution of DE values in a TPD plot (Fig. 6), for which the weighted

Figure 6.

Ranked DE values (with 1σ errors) from a multigrain single-aliquot experiment on aliquot set B of near-surface sample LVM03-5, comparing the conventional relative-PD or PD plot (dashed curve) with the statistically more accurate representation of relative probability distribution by the TPD plot (Berger, 2010). Each aliquot of this size range of quartz grains contains ∼250 grains. All 25 of the 25 aliquots yielded DE values that met the data-acceptance criteria. The DE values in this experiment were derived using the late light subtraction (LLS) procedure for reasons stated in the text. The age estimate in Table 5 is calculated from the weighted mean (0:432
0:034 Gy) of the DE values (filled circles) under the bar, corresponding to the main minimum-DE -value compound probability peak. Note that the PD plot (dashed) overemphasizes the statistical importance of the lowest DE values.

1064

G. W. Berger, T. L. Sawyer, and J. R. Unruh

The radial plot representation of the DE values in Figure 6 is shown in Figure 7. Comparative features are outlined in the figure captions. Sample 4 (Upper W Unit, Trench 3A). The SAR results for sample 4 in Table 5 are listed in the chronological order in which the experiments were conducted. The first SAR age estimate (3.98 ka, 2 mm aliquots) derived from a complex distribution that yielded only six youngest-age data points. Pre-SAR IR feldspar-contamination tests indicated that this quartz-extract fraction contained a few feldspar grains. Thus the intra-SAR IR wash steps may not have removed all feldspar signals from the B-PSL signals (although Olley et al., 2004, show that IR wash can), and this age estimate could be somewhat younger than the true age. Therefore, because there was sufficient material remaining, a second 48% HF treatment and a hydrofluosilicic acid (H2 SiF6 [HSF]; Berger et al., 1980) treatment were applied. The repeated SAR experiment (2 mm aliquots, 105–210 μm fraction, Table 5) yielded a larger number (nine) of youngest-age data points (Fig. 8a) and also a statistically lower mean DE value (10.8 Gy, compare with 12.9). This lower DE was surprising because the previous result was expected to contain a contribution from feldspar grains, which would yield age underestimates. Consequently, a third multigrain SAR experiment was conducted, with two main changes: use of smaller aliquots (1 mm diameter) and use of pulsed diodes. The use of smaller aliquots should help resolve more clearly the young-age subpopulation of DE values. The use of pulsed diodes should

Figure 7.

Radial plot (Galbraith et al., 1999) of the DE values in Figure 6. In radial plots, data points with relatively high precision plot to the right. The center of the
2σ hashed bar is drawn to 0.432 Gy (Table 5). Line a indicates the upper range of DE values chosen in Figure 6 for calculation of the weighted mean. The radial plot shows that four of the six data points lying under the tallest peak in Figure 6 are outside the
2σ range of values. The uppermost line indicates those four DE values highlighted by the middle peak in Figure 6.

Figure 8. TPD plots for sample LVM03-4. These fractions were given two HF acid treatments plus an HSF acid treatment. Multigrain single-aliquot results are shown in (a) and (b), with singlegrain results in (c). A 1 mm aliquot contains ∼50–60 grains. The TPD plot in (b) suggests two possible choices (a and b) of DE values for calculation of youngest-age DE values. DE values were derived using the ELS procedure. In (c), only 108 DE values met data-acceptance criteria, out of 1000 grain holes.

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault

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skewness might have been expected based on the multigrain results in Figure 8a and b. Possibly the skewness in an SGQ experiment on this sample might increase if a larger number of grain holes were analyzed. If so, such a result would merely indicate the inhomogeneity of the sand-grain population (with regard to daylight exposure) between the multigrain and single-grain portions arbitrarily selected for SAR experiments. Possibly the use of a larger regenerative-dose range (e.g., up to 500 Gy) in the SGQ experiment would have revealed the presence of larger DE values.

Figure 9.

Radial plot of the DE values in Figure 8b. The hashed bar is centered on the weighted-mean estimate (15.25 Gy, Table 5) from bar b in Figure 8b. The single line denotes the upper limit to the range of DE values under bar b in Figure 8b.

eliminate any feldspar signal contribution to the B-PSL (Thomsen et al., 2008). This capability is especially important if any grains contain feldspar microinclusions, which chemical treatments can never remove. The result (Fig. 8b) is that a larger number of data points now delimit the lowestage TPD peak, but the presence of a compound peak suggests that either of groups a or b could be selected for weightedmean calculation. The weighted means for groups a and b (respectively, 14:25
0:65 and 15:25
0:62 Gy, Table 5) do not differ at 1σ. The MAM-4 result (∼14:9 Gy, Table 5) agrees with these means. The radial plot for the data in Figure 8b is shown in Figure 9. It does not change the interpretation possible from Figure 8b, especially because of the overriding importance of the geological factors discussed subsequently (sample 6 and unit W) and because of the lack of any reasonably accurate independent age estimates from AMS 14 C dating. Because each of the a and b weighted-mean DE values is larger than the previous 2 mm results, it appears that the use of pulsed diodes has eliminated any remaining feldspar signals from the UV detection window. To validate the pulsed-diode result in Figure 8b, a singlegrain SAR experiment was conducted on the same fraction. The result (Fig. 8c) indicates that only 10% of the grain holes in this sample gave acceptable DE values. The mean singlegrain DE (14:00
0:57 Gy, Table 5) is concordant with that of group a from the pulsed-diode experiment, and thus an age estimate of 4:31
0:23 ka (SGQ data) for sample 4 appears to be sound. The weighted mean is not too sensitive to the range of data points selected because of the relatively large errors in the data points above the main TPD peak. Interestingly, the SGQ distribution in Figure 8c is only somewhat (not dramatically) positively skewed. Greater

Sample 6 (Middle W Unit, Trench 4A). Although from the same stratigraphic unit as sample 4 (but in a different trench), sample 6 was collected in a bed that was richer in clayey silt and organic matter than that from which sample 4 was collected. Overall, unit W represents a vertisol and thus subsamples of the unit will manifest heterogeneous, prolonged soil-development processes. In particular, vertisols are selfchurning (stratigraphically inverting) soils due to significant shrink-swell of the clayey matrix. The expected complexity in sand-grain daylight-exposure history is reflected in the DE distributions for sample 6 (and for sample 4 in Fig. 8a). The multigrain SAR TPD plot (Fig. 10a) shows a dominant cluster of data points toward the center of the plot without any clearly resolved youngest-age subpopulation. This distribution is reminiscent of distributions reported elsewhere (Lomax et al., 2007) from mixed-age deposits for which a median-age population interpretation was the most realistic, based on comparisons with 14 C data. However, we could not rely on independent 14 C ages to aid in the interpretation of the TPD plot in Figure 10a. Therefore, we repeated the SAR experiment using smaller aliquots (1 mm) and pulsed diodes. The use of pulsed diodes with 1 mm aliquots appears to shift the dominant TPD cluster (under the bar in Fig. 10b) only slightly to younger-age values than in Figure 10a, but the analytical errors in each data point are larger because of lower signal/background ratios. The resultant mean DE (5.9 Gy, Table 5) is significantly lower (at 1σ) than that (7.5 Gy) from the 2 mm experiment, and it appears, therefore, that feldspar contamination did not play a large role in the data set in Figure 10a (or the reverse shift would have been observed). Rather, as would be expected from a vertisol, the whole sample is displaying a different mix of populations than the idealized positively skewed distribution one expects from fluvial or colluvial deposits. The resultant SAR PSL age for sample 6 of 2:37
0:16 ka (Table 5) is stratigraphically inverted compared to sample 4, probably manifesting the heterogeneity within this unit and the more open system behavior of sample 6 sand grains, long after the initial fan deposition. Ironically, the silt-grain MATT result (∼4 ka) for sample 6 is closer to the sand-grain SAR age estimate for unit W in trench 3A (sample 4) than are the sand-grain results from sample 6, but this is likely only a coincidence. Nonetheless, the single-grain result from sample 4 seems to provide the most reasonable estimate (4.5 ka) for the

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G. W. Berger, T. L. Sawyer, and J. R. Unruh

Figure 11. A TPD plot for the pulsed-diode 1 mm single-aliquot experiment on sample LVM03-3. The weighted mean for the data under the bar is 31:5
1:9 Gy (Table 5). Clearly, unlike sample 4, the change in aliquot size, not the use of pulsed diodes, had the strongest effect on the DE distribution. The use of smaller aliquots permitted clearer resolution of the youngest-age subpopulation, which apparently was masked by the use of 2 mm aliquots. The pulseddiode weighted-mean DE value (31.5 Gy) concords with the lower precision MAM-4 result (32 Gy, Table 5). Absent a single-grain SAR experiment (which our limited financial resources precluded), we consider this 1 mm aliquot age estimate (10:64
0:85 ka) to be the most accurate age for upper unit G. Figure 10.

TPD plots for sample LVM03-6, using (a) 2 mm

aliquots and (b) 1 mm aliquots plus pulsed diodes. The bars denote the range of values used to calculate the corresponding weighted means listed in Table 5 (7.46 Gy and 5.91 Gy, respectively).

last-daylight exposure age of unit W, though not for the age of the prechurned fan. Sample 3 (Upper G Unit, Trench 3A). The first SAR experiment (2 mm aliquots) yielded a central-age TPD-plot distribution (not shown), rather than a prominent youngest-age subpopulation distribution. The resultant age estimate of 14:49
0:65 ka (Table 5) is much greater than the corresponding 14 C age estimate of ca. 4.1 ka. However, this 14 C age estimate is only a minimum because it is from pedogenic carbonate. To check this SAR PSL result, we repeated the experiment but reduced the aliquot size to 1 mm and employed pulsed diodes. The TPD plot of DE values (Fig. 11) reveals a positively skewed distribution and a prominent youngest-age subpopulation. The corresponding youngest-age estimate of 10:64
0:85 ka (Table 5) is stratigraphically reasonable.

Sample 2 (Middle G Unit, Trench 3A). We were able to fully test the effect of aliquot size on resolution of the youngest-age subpopulation for sample 2. We conducted multigrain SAR experiments using 2 mm and 0.4 mm aliquots and an SGQ experiment, with corresponding TPD plots of resulting DE data shown in Figure 12. A central-age population is evident from the use of 2 mm aliquots (Fig. 12a), with a mean DE value of 44.2 Gy. The corresponding age estimate of 14:17
0:51 ka is not significantly different (at 1σ) from the age estimate of 15:4
1:0 ka from the fine-silt multigrain MATT experiment (Table 5). The use of 0.4 mm aliquots revealed a distribution (Fig. 12b) with a youngest-age mean DE of 37.9 Gy, significantly lower than the 2 mm SAR result (44.2 Gy, Table 5). The MAM-4 result (∼25 Gy, Table 5) for the data in Figure 12b is merely consistent with the weightedmean result but otherwise uninformative. The MAM calculations (using the available Excel implementation, see footnote ∥ for Table 5) failed for the other distributions in Figure 12. A pulsed-diode experiment using 0.4 mm aliquots (Fig. 12c) affirms the result in Figure 12b but because of the low signal/background ratios there are relatively few acceptable

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault

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Figure 12. TPD plots for sample LVM03-2, showing the results of the use of (a) 2 mm aliquots, (b) 0.4 mm aliquots, (c) 0.4 mm aliquots and pulsed diodes, and (d) single grains . In (d), the poor-precision lowest DE value was omitted in calculation of the weighted mean of 33.6 Gy (Table 5), though its inclusion would make no difference. DE values and relatively large analytical errors in the data points. For these reasons, an SGQ experiment was carried out on the same fraction. The result (Fig. 12d) is a positively skewed distribution having a youngest-age mean DE of 33:6
1:0 Gy (Table 5). Within 2σ, this mean is not different from the 0.4 mm multigrain SAR results (Fig. 12b and c). Consequently the weighted-mean DE value of the single-grain and of the two 0.4 mm multigrain SAR results is considered to provide the best youngest-age estimate (11:12
0:55 ka, Table 5) for this sample (i.e., subunit Go). Affirming the utility of these SAR techniques, this age estimate is stratigraphically consistent (and overlapping) with the age estimate of 10:64
0:85 ka for the overlying sample 3. Sample 1 (Base of G Unit, Trench 3A). This is the oldest sample of the set from trench 3A. Unlike for samples 2 and 3, reduction in multigrain SAR aliquot size apparently did not lead to a change in youngest-component mean DE values (66.9–68.8 Gy, Table 5) for sample 1. However, the 2 mm aliquot experiment revealed the presence of a single statistical outlier DE value (from one aliquot) at ∼30 Gy in the

TPD plot (not shown). If this reflected the presence of con-

taminant feldspar grains, then use of pulsed diodes should affirm the main-subpopulation result (66.9 Gy, Table 5) from the 2 mm experiment. The use of 1 mm aliquots in combination with pulsed diodes produced a TPD plot (Fig. 13) that affirms the ∼67 Gy result from the use of 2 mm aliquots if we take the mean over the range represented by the horizontal bar in Figure 13. However, the mean multigrain SAR age of 19:04
0:95 ka (Table 5), surprisingly close to the fine-silt MATT age estimate of 20:8
1:2 ka (Table 5), presents a challenge to stratigraphic interpretation. It seems unreasonable to presume that unit G required 8–9 kyr to build. Accepting the 19 ka age estimate for sample LVM03-1 (basal G unit) implies the existence of an apparent hiatus between 19 ka and 11 ka (LVM03-2, middle G). An hiatus in lower G unit was first identified by Sawyer and Unruh (2002a) based on the scoured unconformity separating the basal section of the unit G from subunit Go. However, the approximately 8 kyr duration of this hiatus suggested by the previous PSL results is unexpected. It is unexpected because no evidence is recognized in any of the seven trenches supporting a lengthy hiatus during deposition of

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Figure 13. A TPD plot for pulsed-diode experiment on sample LVM03-1 from the use of 1 mm aliquots.

the unit G channel-fill sequence (e.g., period of soil formation, large-scale cut-fill relationship, changes in the nature or composition of alluvial materials). Sample LVM03-1 was collected from alluvial deposits directly overlying permeable sandstone of the Cierbo formation, where stratigraphic and pedogenic relationships appeared to have been altered within a 20–30 cm wide zone of leaching. Within this zone of apparently intense leaching the deposits had been whitened, soil structure largely obliterated, and the clay content appears to have been reduced. Given these unavoidable sedimentary complications, there appear to be three possible interpretations of this 19 ka age estimate: (1) that this part of the bed was mislogged originally as unit G instead of the stratigraphically underlying unit CP (Fig. 3); (2) that the basal parts of unit G manifest highly heterogeneous depositional conditions, with this single sample representing a mass-flow deposit reworked at night from some upslope, older bed (e.g., 19–20 ka old), and deposited at ∼12 ka (slightly before the 11 ka age of middle G sample LVM03-2); or (3) the true (last daylight exposure) age of basal G sample LVM03-1 is represented by the weighted mean of the four lowest DE data points in the TPD plot of Figure 13 (where these four data associate with a shoulder on the low side of the TPD peak). These four data points yield a mean DE of 36:2
6:7 Gy and a corresponding age estimate of 10:2
1:9 ka. Possibility (1) seems unlikely. Option (2) is possible but would require additional field sampling (too late: the trench is closed) at horizontally separated (by decimeters) locations within basal G unit. Considering option (3), it is interesting that the weighted mean of the four lowest DE data points in

G. W. Berger, T. L. Sawyer, and J. R. Unruh

Figure 13 yields an age estimate of 10:2
1:9 ka that is stratigraphically more reasonable than the 19 ka age estimate, but choosing this 10 ka estimate would be highly subjective, given the nature of the TPD plot in Figure 13. To help resolve this interpretive challenge, we conducted our last experiment, a single-grain SAR analysis of a remaining portion of the 105–150 μm fraction used in the pulseddiode experiment of Figure 13. We expected that not only would an SGQ analysis clarify the interpretation of Figure 13, but it could also test previous interpretive option 2. The SGQ experiment on sample LVM03-1 revealed a positively skewed DE distribution (Fig. 14) but with one caveat. The lowest DE value (obscured by the scale of Fig. 14) seems anomalously disparate (to be a statistical outlier) from the group defining the main TPD peak. The statistical distinction of this lowest DE data point is clearly shown in the companion radial plot (Fig. 15). We have therefore excluded this lowest DE value from the calculation of the weighted mean. As in some other previous SGQ examples, for the purposes of this first application of SGQ dating to this site, the exact choice of the upper bound to the range of data points under the SGQ bar (lowest value excluded) does not greatly alter the weighted mean. The rejected lowest datum could represent the inclusion in the laboratory fraction of a grain exposed to light during sample collection (at the ends of the containers) but accidently not eliminated during laboratory sample preparation. In any case we calculated a weighted-mean DE of 47:6
2:2 Gy (n  128, Table 5), corresponding to an age estimate for the base of unit G of 13:45
0:79 ka. This is stratigraphically more reasonable than the older 19 ka age estimate derived from the DE distribution in Figure 13.

Figure 14.

A TPD plot for the single-grain DE values from sample LVM03-1. The bar indicates the range of DE values used to calculate a weighted mean of 47:6
2:2 Gy (Table 5), with the lowest DE value omitted (see Fig. 15).

Single- and Multigrain Luminescence Dating of Sediments Related to the Greenville Fault

Figure 15.

Radial plot of the single-grain DE values for sample LVM03-1 in Figure 14. The
2σ hashed bar is centered on the weighted-mean DE value of 47.6 Gy (Table 5). This plot shows clearly that the lowest DE value (open circle, below the hashed bar) is a significant outlier. For this reason this datum is omitted from the calculation of the weighted mean of the data under the bar in Figure 14.

Implications of the PSL Age Estimates Stratigraphic Age Interpretation As mentioned, the SAR PSL result for sample 6 in the vertisol unit W in trench 4A is less reliable (as an estimate of unit W deposition time) than that for sample 4 in unit W in trench 3A, which itself may be yielding only a minimum estimate of the age. Considering only the PSL results for sand grains from trench 3A, it appears that sand grains continued to be exposed to daylight within the topmost unit L until the last ∼100 yrs. Because this uppermost alluvial fan unit is within the modern root zone, the SAR PSL age does not necessarily indicate the deposition age of this unit, but probably the last daylight exposure of sand grains that have been translocated downward within root channels or by surface organisms. Only multiple spatially distinct sampling within unit L may reveal the true fan-deposition age of this unit. The SAR PSL age for the upper part of alluvial fan unit W appears to be ∼4:3 ka (sample 4). Whether the unit W began to be deposited in the middle Holocene is indeterminate on the basis of only one sample. Certainly its deposition began after ∼10:6 ka (sample 3 within the upper part of the underlying unit G). The middle part of unit G was deposited at ∼11 ka (sample 2), but this estimate is not older statistically (at 1σ) than the age estimate for overlying sample 3. Thus there appears to have been a maximum hiatus of ∼6 kyr between deposition of unit G and unit W. This apparent depositional hiatus between units G and W is consistent with the large-scale buttress unconformity partially exposed between these units in trench 3A. There unit W is inset at least

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a meter into unit G. In the previous trench 4 (not identified in Fig. 2), the basal contact of unit W appears to truncate the sedimentary structure in the upper part of unit G, a truncation that is generally consistent with the occurrence of a considerable hiatus. The ∼2:4 ka SAR age for sample 6 from unit W in trench 4A is consistent with the self-churning character of this vertisol, where portions of the deposit can continue to be overturned long after initial deposition. Based on the stratigraphically self-consistent sand-grain SAR results (preferring SGQ results when available), we think that the entire unit G could have been deposited over the interval 13.5–10.6 ka. This is consistent with stratigraphic and other evidence mentioned previously. These SAR PSL age estimates for the quartz sand grains from trench 3A are highly discordant with available 14 C ages from pedogenic carbonate. This is reasonable because the 14 C age estimates (Table 1), being from pedogenic carbonate, are expected to be underestimates of the burial ages. Implications for Slip Rates In addition to constraining the depositional age of stratigraphic units exposed in trenches at the Laughlin Road trench site, the SAR PSL age estimates are the first numerical ages available to constrain the slip rate on the Greenville fault. Particularly significant to constraining the slip rate are the age estimates from unit G, a channel-fill sequence with two subunits and a paleochannel margin, all showing discrete right-lateral offset along the fault. Age estimates from upper and middle unit G bracket the deposition of subunit Gb between 11:12
0:55 ka and 10:64
0:85 ka and those from the middle and lower unit G bracket the deposition of subunit Go between 11:12
0:55 ka and 13:45
0:79 ka. The SAR PSL age estimates for the two subunits and unit G paleochannel margin, along with measurements of the lateral offset, constrain a preliminary slip-rate estimate of about 2 mm=yr or higher for a continuous trace of the northern Greenville fault zone (T. L. Sawyer, J. R. Unruh, and G. W. Berger, unpublished manuscript, 2010).

Conclusions The fine-silt MATT age estimates are generally inaccurate, exceeding the sand-grain age estimates. The preferred sand-grain age estimates range from 125
11 yrs within the topmost unit in trench 3A to 13:45
0:79 ka in the base of unit G. Within trench 3A there appears to be a temporal gap (depositional hiatuses?) between upper unit G and upper unit W (10.6–4.3 ka). Furthermore, there appears to have been little preservation (and deposition?) between upper unit W (4.3 ka) and the surface unit L (∼100 yrs). Finally, the sand-grain PSL age estimate (2.4 ka, sample 6) from middle unit W within trench 4A appears to be stratigraphically too young compared to the upper unit W result (4.3 ka) from trench 3A. This apparent anomaly may be understood in terms of the vertisolic nature of unit W.

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G. W. Berger, T. L. Sawyer, and J. R. Unruh

The preferred sand-grain age estimates from unit G bracket the ages of subunits Gb and Go and of the unit G paleochannel margin. These three stratigraphic features are discretely offset in a right-lateral sense along the Greenville fault. Thus these SAR PSL age estimates provide the first numerical ages for constraining the Holocene slip rate on the Greenville fault, a minimum of ∼2 mm=yr. This is consistent with the long-term rate of slip on the Greenville fault (Peterson et al., 1996; WGCEP, 2003) but is less than half the modern slip rate of 5:4
1:0 mm=yr estimated by D’Alessio et al. (2005) from GPS data and about 10% of the late Quaternary/Holocene rates estimated for the northern part of the San Andreas fault system to the west.

Data and Resources All data used in this article came from published sources listed in the references except the satellite image map underlying Figure 1, U.S. Geological Survey NEHRP Web page (http://earthquake.usgs.gov/hazards/qfaults/, last accessed May 2009).

Acknowledgments The fine-silt dating analyses were funded by a contract from Piedmont Geosciences. The dating analyses represent a part of our recent and ongoing paleoseismic studies of the northern Greenville fault zone funded by the USGS National Earthquake Hazard Reduction Program (NEHRP) through grants (00HQGR0055 and 03HQGR0108) awarded to Piedmont Geosciences. The sand-grain dating analyses were funded by the Desert Research Institute via IPA funds and via sabbatical-leave funds granted to G. W. Berger. G. W. Berger thanks research assistants Tessa Black, Daniel Scott, Nicki Holihan, and Ryan Loux for help in sample preparation. We thank two anonymous reviewers for their helpful comments toward improving the first version of this manuscript.

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