Late Quaternary Paleoseismology of the Southern ... - GeoScienceWorld

Bulletin of the Seismological Society of America, Vol. 97, No. 5, pp. 1662–1678, October 2007, doi: 10.1785/0120060202

Late Quaternary Paleoseismology of the Southern Steens Fault Zone, Northern Nevada by Stephen F. Personius, Anthony J. Crone, Michael N. Machette, Shannon A. Mahan, Jai Bok Kyung, Hector Cisneros, and David J. Lidke

Abstract The 192-km-long Steens fault zone is the most prominent normal fault system in the northern Basin and Range province of western North America. We use trench mapping and radiometric dating to estimate displacements and timing of the last three surface-rupturing earthquakes (E1–E3) on the southern part of the fault south of Denio, Nevada. Coseismic displacements range from 1.1 to 2.2 Ⳳ 0.5 m, and radiometric ages indicate earthquake times of 11.5 Ⳳ 2.0 ka (E3), 6.1 Ⳳ 0.5 ka (E2), and 4.6 Ⳳ 1.0 ka (E1). These data yield recurrence intervals of 5.4 Ⳳ 2.1 k.y. between E3 and E2, 1.5 Ⳳ 1.1 k.y. between E2 and E1, and an elapsed time of 4.6 Ⳳ 1.0 k.y. since E1. The recurrence data yield variable interval slip rates (between 0.2 Ⳳ 0.22 and 1.5 Ⳳ 2.3 mm/yr), but slip rates averaged over the past ⬃18 k.y. (0.24 Ⳳ 0.06 mm/year) are similar to long-term (8.5–12.5 Ma) slip rates (0.2 Ⳳ 0.1 mm/yr) measured a few kilometers to the north. We infer from the lack of significant topographic relief across the fault in Bog Hot Valley that the fault zone is propagating southward and may now be connected with a fault at the northwestern end of the Pine Forest Range. Displacements documented in the trench and a rupture length of 37 km indicate a history of three latest Quaternary earthquakes with magnitudes of M 6.6–7.1 on the southern part of the Steens fault zone. Introduction The Basin and Range province is a large region of extensional tectonics in western North America. The region is marked by hundreds of north-trending ranges and adjacent basins formed primarily by normal faulting in the late Tertiary and Quaternary (Fig. 1; Wallace, 1984). From geodetic surveys across the central Basin and Range, Bennett et al. (1998, 2003), Thatcher et al. (1999), Thatcher (2003), and Hammond and Thatcher (2004) determined that much of the 11–13 mm/yr of northwest-directed extension across the region is localized on the eastern and western province margins and along narrow belts of deformation such as the central Nevada seismic belt. Comparisons between short-term (geodetic) and long-term (geologic) extension rates conducted in the central Basin and Range show similar rates in some areas (Friedrich et al., 2003; Wesnousky et al., 2005) but not others (Bell et al., 2004; Friedrich et al., 2004). Such comparisons in the northern Basin and Range heretofore have not been possible because most fault slip rates are poorly constrained by reconnaissance studies (dePolo, 1998; dePolo and Anderson, 2000; Machette et al., 2003). Possible disparities between the geologic and geodetic deformation data are important because seismic-hazards maps of the region are based primarily on historical seismicity and paleo-

seismic data from mapped Quaternary faults (e.g., Frankel et al., 2005) but are starting to integrate geodetic data into hazards calculations. Our purposes in studying faults in the northern Basin and Range (e.g., Personius and Mahan, 2005) are twofold: to obtain geologic slip rates to compare with recently acquired geodetic data and to provide recurrence intervals and coseismic displacements to improve seismic-hazards maps of the region. We chose to investigate the Steens fault zone (Fig. 1) for several reasons: (1) it is the longest (192 km) and most topographically prominent normal fault system in the northern Basin and Range province; (2) it is crossed by the campaign geodetic survey of Hammond and Thatcher (2005, 2007); (3) it has been assigned high rates of vertical slip (0.3–3.7 mm/yr) by others (Hemphill-Haley, 1987; dePolo, 1998; dePolo and Anderson, 2000; Oldow and Singleton, 2005); and (4) Hammond and Thatcher (2005, 2007) used campaign geodetic data to model a regional domain (block) boundary that is coincident with the southern part of the fault zone. Although a few detailed investigations of Quaternary faulting have been conducted along the central Steens fault zone in the Alvord basin in southern Oregon (Hemphill-Haley et al., 1989, 2000), the study described

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Late Quaternary Paleoseismology of the Southern Steens Fault Zone, Northern Nevada

Figure 1. Map of Quaternary faults in northwestern Nevada and southeastern Oregon (modified from Personius et al., 2006). Fault data from this study and Quaternary fault database of U.S. Geological Survey (Machette et al., 2003; available online at http://earthquake.usgs.gov/regional/qfaults/). Trace of Alder Creek fault (age unknown) from Colgan et al. (2006a). Base map from 30 m digital elevation model (DEM) data of U.S. Geological Survey. Boxes denote locations of Figures 2 and 5. Light shaded areas mark maximum extent of late Pleistocene pluvial Lake Alvord, Lake Coyote, and Lake Lahontan (Reheis, 1999). White arrows mark flow path of overflow floods from pluvial Lake Alvord into Lake Coyote basin and Owyhee River (Reheis, 1999; Carter et al., 2006). White triangles denote mapped limits of Denio section of Steens fault zone (Personius et al., 2003). Seismicity data from USGS catalogs Eastern, Central, and Mountain states of the United States (SRA), 1534–1986, and USGS/NEIC (PDE), 1973 to present.

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S. F. Personius, A. J. Crone, M. N. Machette, S. A. Mahan, J. B. Kyung, H. Cisneros, and D. J. Lidke

herein is the first to determine earthquake recurrence intervals and slip rates from a trench investigation on the fault zone in Nevada. The Steens fault zone forms the eastern margin of Steens Mountain and the Pueblo Mountains; these major west-tilted blocks are capped with Miocene volcanic rocks, primarily Steens Mountain basalts of the Columbia River Basalt Group (Willden, 1964; Walker and Repenning, 1966; Burnam, 1970; Roback et al., 1987; Hart et al., 1989). Recent geologic and thermochronologic studies indicate that Basin and Range extensional faulting began 10–12 Ma in the region (Colgan et al., 2006a, b). The downfaulted Alvord Desert and Pueblo and Bog Hot valleys are filled with as much as a kilometer or more of Tertiary to Quaternary basin fill (Cleary et al., 1981a, b; Edquist, 1981; Oldow et al., 2005). In the middle and late Pleistocene, these valleys were occupied by pluvial Lake Alvord, which during high stands extended 120 km from the northern end of the Alvord Desert to the southern end of Bog Hot Valley (Fig. 1; HemphillHaley, 1987; Reheis, 1999). Rates of modern seismicity in the area are low, with only two earthquakes of M ⬍4.5 recorded in locations that might be attributable to slip on the Steens fault zone (Fig. 1).

Trench Setting, Structure, and Stratigraphy We sited the Bog Hot Valley trench across one of a series of right-stepping, 3- to 5-m-high fault scarps that extend south from the southern end of the Pueblo Mountains and traverse the floor of Bog Hot Valley (Fig. 2). The site was chosen because of the presence of well-preserved fault scarps in fine-grained lacustrine and fluvial deposits (Fig. 3) and the proximity of geodetic stations (Hammond and Thatcher, 2005, 2007) that straddle the fault zone. A second trench was located on a 5-m-high scarp in alluvial sediments 7.5 km south of the main trench (Fig. 2). Unfortunately this trench exposed loose sand and silt in the fault zone and collapsed during excavation. No additional information about the fault was obtained from this excavation. We also excavated two soil pits, one at the trench site to obtain data on the soil formed on the faulted lacustrine deposits (Fig. 4), and a second on a lacustrine barrier bar located in the southern Pueblo Valley about 23 km northeast of the trench site (Fig. 5). The latter pit was excavated to help define the age of pluvial Lake Alvord sediments in the trench. The history of pluvial Lake Alvord is complicated by a lack of chronologic control and the occurrence of an overflow flood or floods that partially drained the lake in the Pleistocene. Studies of lacustrine and flood features in the Alvord Desert and Coyote Lake basins indicate that Lake Alvord overtopped a sill at Big Sand Gap in the Alvord Desert (Fig. 1) some time in the late Pleistocene. This overtopping event lowered the elevation of the lake, probably in stages, from an elevation of 1292–1297 m to an elevation of about 1280 m during a large flood or series of floods that spilled the waters of Lake Alvord into adjacent Lake Coyote

and eventually into the Owyhee River (Fig. 1; HemphillHaley, 1987; Reheis, 1999; Carter et al., 2006). The most prominent young shoreline features in Bog Hot and Pueblo valleys are barrier bars and wave-cut shorelines located at elevations of 1295–1297 m (Figs. 2 and 5), indicating that Lake Alvord rose above the elevation of the trench site (1292 m) to an elevation of 1295–1297 m before the flood. The age of this flood is poorly known, but it likely preceded the age of the regional Last Glacial Maximum (Oxygen Isotope Stage II) pluvial highstand (Carter et al., 2006), which is undated in the Lake Alvord basin but in the Chewaucan basin 150 km to the northwest is thought to have occurred about 12,000 radiocarbon years ago (Licciardi, 2001), and in the Lahontan basin 15–20 km to the south, about 13,000 radiocarbon years ago (Thompson et al., 1986; Benson, 1993; Adams and Wesnousky, 1998). We used these ages, estimated uncertainties of Ⳳ200 years, and the computer program OxCal (v.3.10; Bronk Ramsey, 1995, 2001), to determine calibrated age ranges of about 13,400–14,600 and 14,800–16,100 cal yr B.P., respectively, for the high stands of Lake Chewaucan and Lake Lahontan. We conclude from these regional correlations that the lacustrine sediments at the trench site were deposited prior to 13.4–16.1 ka. Three normal faults were exposed in the trench (Fig. 4). The primary fault (F1) dips east and offsets all units except colluvial units 7 and 8. Two west-dipping antithetic faults (F2 and F3) form nested grabens, are buried by colluvial units 5, 6, 7, and 8, and have no surface expression. Only fault F1 was active during all of the surface-faulting earthquakes documented at the trench site. A scarp profile across the fault scarp yielded a surface offset of 4.3 m and correlation of the base of unit 4 across the fault zone yielded a similar stratigraphic vertical separation of 4.4 Ⳳ 0.2 m (Fig. 4). The Bog Hot Valley trench exposed a faulted, conformable sequence of flat-lying, well-stratified lacustrine and fluvial(?) sediments (units 1–4) deposited during fluctuations of pluvial Lake Alvord (Fig. 4) (See Personius et al. [2006] for more detailed trench logs, soils, and unit descriptions). Unit 1 consists of poorly sorted fluvial(?) sandy pebble gravel with lenses of better sorted silt and sand. Unit 2 consists of a 30- to 50-cm-thick lower massive lacustrine silt deposit overlain by 30–50 cm of parallel-bedded sand and fine pebble gravel. Unit 3 consists of moderately sorted, parallel-bedded fluvial(?) sand, and channel fills of sandy pebble gravel. Unit 4 consists of massive lacustrine silt. We found numerous liquefaction features in these sediments throughout the trench and soil pit. These features consist of tongues of sand and gravel (units 1 and 3), probably injected into overlying silt beds (units 2 and 4) during earthquakeinduced ground shaking. Hanging-wall deposits in the trench (Fig. 4) consist of three fault-scarp colluvial deposits (units 5–7). The oldest colluvial deposit (unit 5) consists of poorly sorted pebbly silty sand and contains several blocks of massive silt (unit 4); these sediments were eroded from units 3 and 4 in the


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Figure 2.

Map showing Quaternary faults and selected shoreline features in the vicinity of Bog Hot Valley, Humboldt County, Nevada (modified from Personius et al., 2006). Base map from parts of Alder Creek Ranch, Bog Hot Springs, Denio, and Vicksburg Canyon, Nevada, 1:24,000-scale quadrangles; contour intervals 10, 20, and 40 feet. See Figure 1 for location. Heavy lines are faults with latest Quaternary (⬍15 ka) movement. Geodetic (GPS) stations from Hammond and Thatcher (2005).

footwall of fault F1 and unit 4 in the footwall of fault F2. In places unit 5 sediments are crudely bedded. The unit is capped by a discontinuous silty Av horizon with veinlets of gypsum and calcium carbonate. Unit 6 consists of poorly sorted sandy pebble and minor cobble gravel eroded from units 2, 3, and 4 in the footwall of fault F1. The unit also contains several blocks of gypsum- and calcium-carbonatebearing silt eroded from the soil formed on unit 5 in the footwall. In places unit 5 sediments are crudely bedded. The unit is capped by a nearly continuous calcium-carbonatebearing silty Av horizon. Unit 7 consists of several facies: (1) a 20- to 50-cm-thick basal debris wedge of poorly sorted

pebbly sand and sandy pebble and minor cobble gravel, deposited against the eroded free face of fault F1; these sediments were eroded from unit 1 in the footwall of fault F1; (2) 40–140 cm of poor to moderately sorted, wash-facies pebbly silty sand, eroded from units 1, 2, 3, 4, and 6 in the footwall of fault F1; and (3) 20–70 cm of well-sorted grabenfill facies sandy silt, eroded from units 2 and 4 in the footwall of fault F1 and unit 4 in the footwall of fault F3. The grabenfill sediments are deposited against the eroded free face of fault F3 and interfinger with and are overlain by the washfacies sediments east of trench meter 10.5. Unit 7 is capped by 40–50 cm of modern slope wash pebbly sandy silt (unit

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Figure 3. Map of trench site (see Fig. 2 for location). Base map is rectified digital orthophoto of part of Alder Creek Ranch, Nevada, 1:24,000-scale quadrangle. Map units: LPHa, latest Pleistocene and Holocene (⬍13.4 ka) alluvium; LPpl, latest Pleistocene (13.4–20(?) ka) pluvial lacustrine deposits. 8) and a silty A horizon. We found no evidence, such as extensive erosional unconformities, to indicate any incompleteness in the fault-scarp colluvial record.

Dating We used radiocarbon, luminescence, and regional correlation methods to determine the ages of colluvial and lacustrine deposits in the trench. We radiocarbon dated four samples of finely disseminated charcoal from two thin (⬍1cm-thick) burn layers in colluvial unit 7 (Table 1, Fig. 4). These samples yielded similar ages, indicating that deposition of the burn layers was closely spaced in time. We give priority to our radiocarbon ages in our dating analysis for two reasons: (1) hundreds of prior studies have established the reliability of this technique, and (2) these burn layers formed either from combustion of in situ vegetation or airfall from nearby range or forest fires and thus are unlikely to contain reworked charcoal. We used luminescence-dating techniques to determine the ages of lacustrine and colluvial units that did not contain datable organic material. Thermoluminescence (TL), infrared stimulated luminescence (IRSL)—a type of optically stimulated luminescence (OSL) unique to feldspars, and blue-light OSL (hereafter referred to as OSL) techniques date

the last time sediment is exposed to sunlight, presumably during deposition (Berger, 1988; Forman et al., 2000). A polymineralic, fine-silt-size (4–11 lm) fraction was isolated for each TL and IRSL sample; the fine quartz sand (90– 250 lm) fraction was isolated for OSL sample PT01. Samples were subjected to combinations of sunlight sensitivity tests, anomalous fading tests (Wintle, 1973), total bleach and partial bleach experiments (Wintle and Huntley, 1980; Singhvi et al., 1982) for TL, and additive dose experiments for IRSL (Aitken, 1998). Complications that reduce the precision and accuracy of luminescence dating include anomalous fading, incomplete zeroing prior to deposition, and fluctuations in water content following deposition (Forman et al., 2000). Our TL, IRSL, and OSL analyses (Table 2) yielded mixed results. A significant problem with anomalous fading was observed in some TL and IRSL samples, so fading tests were conducted in the laboratory on most samples. Ages were then corrected for fading following the methods of Huntley and Lamothe (2001). OSL dating is preferred because much shorter light exposures are necessary for zeroing the luminescence signal prior to deposition and because quartz grains are not susceptible to anomalous fading. Unfortunately all of the trench samples were deficient in clean quartz grains, probably as a result of the predominant volcanic source rocks in the area, so we based most of our preferred ages (Table 2) on the fading-corrected TL and IRSL dating results. We obtained a single OSL age (sample PT01) from correlative deposits in a granitic source area in nearby Pueblo Valley where quartz-bearing sediments are more abundant. We chose to weight average most of our luminescence ages to reduce uncertainties for several reasons: (1) unlike our radiocarbon analyses, both techniques were applied to the same sample; (2) both determinations usually yielded similar glow curves; (3) we found no clear age trends, such as one technique yielding consistently younger or older ages; and (4) all averaged pairs passed chi-square tests for contemporaneity. Our fading-corrected TL and IRSL ages are in stratigraphic order, with four exceptions: colluvial samples BT06 and the IRSL age of sample BT05 (not used in the analysis) yielded ages several hundred years younger than overlying radiocarbon ages, and lacustrine silt samples BT01 and BT04 yielded ages significantly younger than correlative or overlying deposits. We constrained the age of lacustrine deposits in the trench with two luminescence ages (BT02 and PT01) and with correlations to better dated pluvial lake sequences in the region.

Paleoseismology Sequence of Events The history of faulting at the trench site began some time after deposition of silty sediment during the last occupation of pluvial Lake Alvord in Bog Hot Valley (unit 4, Figs. 4 and 6a). This occupation was followed by final withdrawal of pluvial lake water from the valley following over-


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Figure 4. Logs of excavations and scarp profile from the Bog Hot Valley trench site (see Personius et al. [2006] for larger-scale logs and more detailed unit and soil descriptions).

flow flooding elsewhere in the basin. The Lake Alvord flood likely preceded the Last Glacial Maximum highstand of the lake, which in other dated pluvial lakes in the region occurred 13.4–16.1 ka. In Bog Hot and Pueblo valleys, we obtained two luminescence ages (Table 2) that support a latest Pleistocene age for prefaulting lacustrine deposits at the trench site and also provide upper-limiting ages on the Lake Alvord flood. One luminescence age from the trench (BT02) indicates the lacustrine silts near the top of the prefaulted section (unit 4) were deposited 17,870 Ⳳ 1,140 yr. In addition, we obtained an OSL age of 17,360 Ⳳ 4,120 yr

from sediment in a barrier bar in Pueblo Valley (PT01) that we correlate with lacustrine sediment at the trench site (Fig. 5). If these sediments are indeed correlative, then a weighted mean of 17,830 Ⳳ 1,100 yr yields a reasonable composite age for the youngest lacustrine sediment at the Bog Hot Valley and Pueblo Valley sites. However, given the large anomalous fading correction needed to obtain the age of sample BT02 and the large laboratory uncertainties of sample PT01, we round this age to the nearest 500 years and use a 4r uncertainty to conclude that the youngest lacustrine sediment in the trench was deposited about 18 Ⳳ 2.2 ka.

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Figure 5. Location map (a), photograph (b), and generalized description of sediments (c) in soil pit on 1295 m lacustrine barrier bar in southern Pueblo Valley (see Fig. 1 for location). We use similar elevation, soil development, and luminescence ages to infer correlation of these deposits with lacustrine sediment (unit 4) in trench. Base map is rectified digital orthophoto of part of Wilder Creek Ranch, NevadaOregon, 1:24,000-scale quadrangle. See Personius et al. (2006) for detailed soil description. Stadia rod numerated in decimeters. Our luminescence results and regional correlations indicate that the Lake Alvord flood occurred before 13.4–16.1 ka and after 18 Ⳳ 2.2 ka. Withdrawal of Lake Alvord from Bog Hot Valley was followed by soil development on the exposed lacustrine sediments (unit 4). The first earthquake surface rupture (E3) recorded in the trench resulted in formation of a narrow graben between faults F1 and fault F2 (Fig. 6b) and deposition of scarpcolluvial unit 5 onto lacustrine unit 4 in the floor of the

graben (Fig. 6c). The lack of a soil on the top of unit 4 in the graben probably indicates some minor scarp-parallel erosion of the graben floor after the earthquake, but the exposed thickness of unit 4 in the graben (⬃60 cm) only allows for a maximum of about 30 cm of missing section. Ground motions from this earthquake probably caused the liquefaction features found throughout the trench, because groundwater levels must have been higher during earthquake E3 than later surface-rupturing earthquakes in the Holocene. A maximum-limiting age of earthquake E3 is provided by our composite age of 18 Ⳳ 2.2 ka derived from luminescence ages from lacustrine sediment in the trench (unit 4) and a correlative deposit in Pueblo Valley (Fig. 7). This age probably is not a close maximum estimate of the timing of earthquake E3 because it dates to the occupation of the site by Lake Alvord, whereas the sedimentary fabric of colluvial unit 5 is similar to other colluvial units in the trench (units 6 and 7) and thus indicates subaerial deposition some time after withdrawal of Lake Alvord from Bog Hot Valley. A minimum-limiting age of earthquake E3 is provided by a luminescence age on sample BT09 of 11,130 Ⳳ 360 yr from scarp-colluvial unit 5 (Fig. 7). Earthquake timing is better constrained by this age for several reasons: (1) this sample exhibited a minimal amount of anomalous fading (Table 2); (2) the sample was taken from the lower 10 cm of unit 5 and thus was likely deposited no more than a few hundred years after the earthquake; and (3) our experience in dating scarp-colluvial sequences (i.e., Personius and Mahan, 2005) indicates that carefully selected luminescence samples from closely spaced locations above and below earthquake horizons commonly yield overlapping ages (also see discussion of earthquake E2 in the following). We round up the age of sample BT09 to the nearest 500 years, estimate an uncertainty of Ⳳ2000 yr based on the possibility of erosion of unit 4 in the graben floor and the lack of a close maximum-limiting age, and conclude that earthquake E3 occurred about 11.5 Ⳳ 2.0 ka. Earthquake E3 was followed by accumulation of unit 5 in the graben, soil formation on the upper part of unit 5, and continued soil formation on unit 4 east of fault F2. A second earthquake surface rupture (E2) then occurred on fault F1 (Fig. 6d). Rupture E2 was followed by deposition of scarpcolluvial unit 6 (Fig. 6e). The E2 earthquake horizon (contact between units 5 and 6) is marked by the buried soil on the upper part of unit 5 and by westward rotation of unit 5 as indicated by a sharp change in dip (40–50⬚) of bedding across the earthquake horizon (Personius et al., 2006). The unusual shapes of units 5 and 6 may indicate that these deposits extended somewhat further east than they are presently mapped (Fig. 4). However, several lines of evidence indicate minimal erosion of these or underlying unit 4 deposits: (1) soil horizons are preserved on the upper parts of units 5 and 6 west of fault F2 (Personius et al., 2006); (2) we observed no evidence of channeling or alluvial fills at the top of unit 4 east of fault F2; and (3) unit 4 is 5–10 cm

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Table 1 Radiocarbon Data from the Bog Hot Valley Trench Sample No.

R04 R03 R02 R01

Lab No.*

CAMS 107169 OS-44875 OS-45467 OS-44943

Reported Age† (14C yr B.P.)

3430 3920 3680 3960

Ⳳ 35 Ⳳ 30 Ⳳ 90 Ⳳ 50

Calibrated Age‡ (cal yr B.P.)

Comments

3580–3830 4240–4430 3720–4300 4240–4530

Fine disseminated charcoal from upper burn layer (UBL) Fine disseminated charcoal from UBL Fine disseminated charcoal from lower burn layer (LBL) Fine disseminated charcoal from LBL

*Laboratory identifiers: OS, National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution; CAMS, Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory. † 1r analytical uncertainties reported by accelerator mass spectrometry laboratory. ‡ 2r calibrated age range; calibration from computer program OxCal (version 3.10, Bronk Ramsey, 1995, 2001) and the IntCal 2004 atmospheric data set (Reimer et al., 2004).

thicker between faults F2 and F3 than to the east of fault F3. A more likely explanation is that thin distal remnants of the upper part of unit 5 and/or the lower part of unit 6 were incorporated into the soil formed on unit 4 between faults F2 and F3. This would explain the slight thickening of unit 4 and would also explain the apparently anomalous luminescence age of sample BT04 (⬃6 ka) taken from the top of unit 4 in this location. The timing of earthquake E2 is tightly constrained by overlapping luminescence ages of 6,050 Ⳳ 260 yr and 6,140 Ⳳ 310 yr sampled directly above and below the earthquake horizon, respectively (Fig. 7). We use these nearly identical ages and an estimate of uncertainty of Ⳳ500 yr based on assumptions we made about anomalous fading of these samples (Table 2) to conclude that earthquake E3 occurred about 6.1 Ⳳ 0.5 ka. After accumulation of unit 6 and soil formation on its upper part, a third earthquake surface rupture (E1) occurred on faults F1 and F3 (Fig. 6f). Earthquake E1 was followed by deposition of scarp-colluvial unit 7 (Fig. 6g). The E1 earthquake horizon (contact between units 6 and 7) is marked by the buried soil on the upper part of unit 6. The timing of earthquake E1 is constrained by radiocarbon ages above and luminescence ages above and below the earthquake horizon (Fig. 7). Four charcoal samples from two burn layers in unit 7 located about 20 cm and 50 cm above the earthquake horizon yielded calibrated ages of 3580–4530 cal yr B.P. Luminescence ages of 3690 Ⳳ 360 yr and 4790 Ⳳ 830 yr from unit 7 are in stratigraphic order and consistent with nearby radiocarbon ages. A luminescence age of 3930 Ⳳ 270 from the soil on the upper part of unit 6, 10 cm below the earthquake horizon, is similar to but slightly younger than those above it. We cannot precisely reconcile all of these ages in their present stratigraphic positions (Figs. 4 and 7), but the clustered ages indicate that the earthquake likely occurred in the time range of 4–5 ka. To account for all laboratory uncertainty, we use the midpoint of the 2r range of all ages that constrain the earthquake horizon (3580–5620 yr) to conclude that earthquake E1 occurred about 4.6 Ⳳ 1.0 ka.

Earthquake Recurrence Our preferred earthquake times (E3, 11.5 Ⳳ 2.0 ka; E2, 6.1 Ⳳ 0.5 ka; and E1, 4.6 Ⳳ 1.0 ka) yield recurrence intervals of 5.4 Ⳳ 2.1 k.y. between earthquakes E3 and E2 and 1.5 Ⳳ 1.1 k.y. between earthquakes E2 and E1. The E2–E1 interval appears especially anomalous given the interval between the deposition of unit 4 and earthquake E3 (6.5 Ⳳ 3.0 k.y.) and an elapsed time of 4.6 Ⳳ 1.0 k.y. since the youngest earthquake (E1). Even with the large uncertainties in our earthquake age estimates, these intervals appear to indicate irregular recurrence of ground-rupturing earthquakes on this part of the Steens fault zone. Coseismic Fault Displacements We use displacements of distinctive stratigraphic units, the geometries of colluvial wedges, and reconstructions of faulting to estimate vertical displacements during the three earthquake surface ruptures at the Bog Hot Valley site. We know from a scarp profile and the deformation of the contact between units 3 and 4 in the trench (Fig. 4) that the last three ruptures resulted in a total vertical separation of 4.4 Ⳳ 0.2 m. We found no evidence of significant erosion in the hangingwall deposits, so if we assume the paleoseismic record is complete and all three surface ruptures were about the same size, then each rupture caused about 1.5 m of vertical separation. However, differences in colluvial wedge thickness also may indicate variations in displacement. The colluvial wedges from the first two ruptures (units 5 and 6) have similar cross-sectional areas and thicknesses of about 1 m, indicating that vertical displacements during each earthquake may have been similar in size. The colluvial wedge from the third rupture (unit 7) has a much larger cross-sectional area and is thicker (1.5 m) than the two older wedges. These differences could be the result of many factors, including variations in accumulation time (recurrence interval), variations in climate-driven sedimentation rate, or variations in displacement. If the latter is the case, then stratigraphic reconstructions based on the thicknesses of preserved colluvial wedges (Fig. 6) suggest vertical separations of about 1.1 m

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7

7

6

6

5

4, 5?, 6?

5

4

2

BT03

BT05

BT06

BT08

BT07

BT04

BT09

BT02

BT01

Sample No*

Trench Unit†

Dose Rate㛳 (grays/k.y.)

4.61 Ⳳ 0.06

4.38 Ⳳ 0.06 4.53 Ⳳ 0.08 4.30 Ⳳ 0.08

4.97 Ⳳ 0.08 4.72 Ⳳ 0.08 4.89 Ⳳ 0.08

4.65 Ⳳ 0.07 5.01 Ⳳ 0.07

4.76 Ⳳ 0.07 4.30 Ⳳ 0.06

4.09 Ⳳ 0.06 4.40 Ⳳ 0.06 4.18 Ⳳ 0.06 4.64 Ⳳ 0.08 4.41 Ⳳ 0.08 4.89 Ⳳ 0.11

4.64 Ⳳ 0.11

Equivalent Dose§ (grays)

16.75 Ⳳ 1.36

15.47 Ⳳ 0.92 16.51 Ⳳ 1.39 10.68 Ⳳ 0.29

9.50 Ⳳ 0.79 15.29 Ⳳ 0.46 24.03 Ⳳ 1.01

22.99 Ⳳ 0.43 23.09 Ⳳ 1.84

24.09 Ⳳ 0.56 25.28 Ⳳ 2.07

24.39 Ⳳ 0.73 44.56 Ⳳ 1.08 42.64 Ⳳ 0.47 18.54 Ⳳ 1.36 18.76 Ⳳ 0.49 56.41 Ⳳ 1.27

52.64 Ⳳ 0.74

TL-tb

IRSL TL-tb

IRSL

TL-tb

IRSL

TL-tb

IRSL TL-tb

IRSL TL-tb

IRSL TL-tb

IRSL TL-tb

IRSL TL-tb

IRSL

Dating Method‡

Table 2

11,340 Ⳳ 610

4,250 Ⳳ 270 11,540 Ⳳ 740

10,200 Ⳳ 400 3,990 Ⳳ 600

5,960 Ⳳ 400 10,140 Ⳳ 570

5,060 Ⳳ 270 5,880 Ⳳ 970

4,940 Ⳳ 240 4,610 Ⳳ 740

4,910 Ⳳ 440

3,240 Ⳳ 220

⬎1,910 Ⳳ 320

2,480 Ⳳ 160

3,530 Ⳳ 430 3,650 Ⳳ 630

3,640 Ⳳ 600

TL, IRSL, or OSL Age# (yr)

3

17 3

2 17

1 2

5*** 1

5*** 5***

5***

5

5

5

1 5

1

Fading** (%/decade)

13,020 Ⳳ 700

18,190 Ⳳ 1280 13,250 Ⳳ 850

11,150 Ⳳ 440 16,590 Ⳳ 2540

6,180 Ⳳ 420 11,080 Ⳳ 620

6,210 Ⳳ 330 6,090 Ⳳ 1005

6,060 Ⳳ 300 5,640 Ⳳ 910

6,020 Ⳳ 540

3,930 Ⳳ 270

⬎2,290 Ⳳ 380

3,220 Ⳳ 210

3,650 Ⳳ 450 4,790 Ⳳ 830

3,770 Ⳳ 620

Fading-Corrected Age†† (yr)

Luminescence Data from the Bog Hot Valley and Pueblo Valley Sites

—

17,870 ⴣ 1,140

11,130 ⴣ 360

—

6,140 ⴣ 310

6,050 ⴣ 260

3,930 ⴣ 270

4,790 ⴣ 830

3,690 ⴣ 360

Preferred Age Estimate‡‡ (yr)

No preferred age; regional pluvial lake history and overlying sample BT02 suggest all ages too young

Weighted mean of fadingcorrected TL and IRSL ages

Weighted mean of fadingcorrected TL and IRSL ages

No preferred age; pluvial lake history suggests all ages too young; sample may incorporate remnants of units 5 or 6

Weighted mean of fadingcorrected TL and IRSL ages; fading estimated from sample BT06

Overlying calibrated radiocarbon ages yield similar age Overlying calibrated radiocarbon ages indicate IRSL age is too young; not averaged—fails chi-square test No stable plateau; minimum age only; not averaged Overlying radiocarbon ages indicate age may be too young by several hundred years Weighted mean of fadingcorrected TL and IRSL ages; fading estimated from sample BT06

Weighted mean of fadingcorrected TL and IRSL ages; calibrated radiocarbon ages from same horizon yield similar age

Comments

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OSL-E —

bar sand —

—

Dose Rate㛳 (grays/k.y.)

3.96 Ⳳ 0.07 3.96 Ⳳ 0.07 —

Equivalent Dose§ (grays)

76.4 Ⳳ 14.1 76.4 Ⳳ 14.1 —

16,390 Ⳳ 5038 17,830 Ⳳ 1100

19,310 Ⳳ 7150

TL, IRSL, or OSL Age# (yr)

na —

na

Fading** (%/decade)

na —

na

Fading-Corrected Age†† (yr)

18,000 ⴣ 2,200

17,360 ⴣ 4,120

Preferred Age Estimate‡‡ (yr)

Weighted mean of BT02 (fading-corrected TL and IRSL ages) and PT01 (linear and exponential fit ages); rounded to 18 Ⳳ 2.2 ka (4r uncertainty); see text for discussion.

Weighted mean of linear and exponential fit OSL ages

Comments

*All BT samples are from south wall of Bog Hot Valley trench; PT sample is from 95-cm depth in soil pit on lacustrine barrier bar in Pueblo Valley, 23 km northeast of trench site (Fig. 5). † All unit designators are from trench (Fig. 4) except sample PT01, which is description of sediment in Pueblo Valley soil pit (Fig. 5). ‡ TL-tb, thermoluminescence, total bleach; IRSL, infrared-stimulated luminescence; OSL-L, blue-light optically stimulated luminescence, single aliquot regeneration, linear fit; OSL-E, blue-light optically stimulated luminescence, single aliquot regeneration, exponential fit. § Uncertainties are Ⳳ1r. 㛳 Trench dose rates from in situ measurements with Exploranium GR-256 gamma ray spectrometer at field moisture (1–20%), then recalculated to 30% moisture (except BT02 and BT08, which use 20% moisture); sample PT01 dose rate measured by laboratory analysis (high-resolution gamma spectrometry) of bulk sediment sample at field moisture, recalculated to 20% moisture; uncertainties are Ⳳ1r. # Uncertainties are Ⳳ2r. **Anomalous-fading factor (Huntley and Lamothe, 2001) calculated from laboratory fading tests; values marked with *** are estimated from nearby samples; na, not applicable to OSL sample. †† Corrected ages; anomalous-fading factor in previous column applied to each age (Huntley and Lamothe, 2001); na, not applicable to OSL sample. ‡‡ Preferred ages; reasons for preference explained in comments and text; mean ages are error weighted, with Ⳳ2r uncertainties (formula of Geyh and Schleicher, 1990); –, not calculated, ages likely in error.

OSL-L

barrier

Dating Method‡

PT01

Sample No*

Trench Unit†

Table 2 Continued

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Figure 7.

Timing diagram for three most recent surface-rupturing earthquakes at the Bog Hot Valley trench site. Gray boxes represent earthquakes marked by boundaries between individual fault-scarp colluvial wedges (triangular polygons). Luminescence ages (filled circles) are shown with 2r uncertainties; sample PT01 in unit 4 is from correlative 1295 m lacustrine barrier bar deposit in southern Pueblo Valley (Fig. 5). Radiocarbon ages from unit 7 (filled triangles) are 2r calibrated age ranges of two analyses each from upper and lower burn layers. Symbols and unit numbers are same as shown in Figure 4; short squiggly lines denote soil development. See text for basis of preferred event times.

Figure 6.

Simplified sequence of three most recent earthquake ruptures at the Bog Hot Valley trench site; fault and unit numbers are same as shown in Figure 4. Short vertical hachures denote soil development. Note injection of sand and gravel (units 1 and 3) into overlying units 2 and 4 during liquefaction caused by earthquake E3 (b).

for the first two surface ruptures and 2.2 m for the third rupture. Uncertainties in these displacement estimates are difficult to determine, but are conservatively estimated at Ⳳ0.5 m. With the exception of minor changes in the thickness of units 3 and 4 across the fault zone (Fig. 4), we found no slip indicators in the trench that would allow measurement of oblique displacement. The right-stepping pattern of faulting across the floor of Bog Hot Valley (Fig. 2) suggests a left-lateral component of slip, but this pattern contrasts with recent geodetic studies that indicate a component of right-lateral shear in the region (Hammond and Thatcher, 2005, 2007).

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Table 3 Slip Rate Data from Bog Hot Valley Area, Southern Steens Fault Zone Slip Rate Description*

Average rate, vertical separation of unit 4 Average rate, postearthquake E3 Interval rate between earthquakes E3 and E1 Interval rate between earthquakes E3 and E2 Interval rate between earthquakes E2 and E1 Long term (Miocene)

Vertical Separation (m)

4.4 3.3 3.3 1.1 2.2 1750

Ⳳ 0.2 Ⳳ 0.5# Ⳳ 0.7# Ⳳ 0.5 Ⳳ 0.5 Ⳳ 250

Vertical Slip Rate† Horizontal Extension Rate†‡ Horizontal Extension Rate†§ (mm/yr) (60⬚ dip, mm/yr) (45⬚ dip, mm/yr)

Time Span (k.y.)

18 Ⳳ 2.2㛳 11.5 Ⳳ 2.0 6.9 Ⳳ 2.2 5.4 Ⳳ 2.1 1.5 Ⳳ 1.1 10,500 Ⳳ 2,000

0.24 Ⳳ 0.06 0.29 Ⳳ 0.13 0.48 Ⳳ 0.37 0.2 Ⳳ 0.24 1.5 Ⳳ 2.3 0.2 Ⳳ 0.1

0.14 Ⳳ 0.04 0.17 Ⳳ 0.08 0.28 Ⳳ 0.21 0.12 Ⳳ 0.15 0.79 Ⳳ 1.5 0.1 Ⳳ 0.05

0.24 Ⳳ 0.06 0.29 Ⳳ 0.13 0.48 Ⳳ 0.37 0.2 Ⳳ 0.24 1.5 Ⳳ 2.3 0.2 Ⳳ 0.1

*See text for discussion. All calculations use formulas of Geyh and Schleicher (1990). † Rates shown with Ⳳ2r uncertainties. ‡ Horizontal extension calculated from measured or estimated vertical separation assuming pure dip-slip on fault dipping 60⬚; dip estimate from local geologic and geophysical data (Burnam, 1970; Edquist, 1981; Hulen, 1983). § Horizontal extension calculated from measured or estimated vertical separation assuming pure dip-slip on fault dipping 45⬚; dip estimate from Thatcher and Hill (1991). 㛳 Weighted mean of luminescence ages BT02 and PT01, rounded to nearest 500 years with 4r uncertainty. # Uncertainties are different because post-E3 calculation is result of difference between measured separation of unit 4 and earthquake E3 separation estimate, whereas E3–E1 interval calculation is sum of E2 and E1 separation estimates.

Latest Quaternary Slip Rates Fault slip rates commonly are calculated as either average rates based on the offset of a datum of known or estimated age, or as interval rates determined between two or more dated earthquakes. Average rates are based on slip over several seismic cycles, but the interval used in the calculation (usually the age of the faulted landform) contains open intervals of unknown length at both ends. Interval rates are more narrowly defined by age determinations at both ends of the seismic cycle, but sample a shorter span of fault history. Average rates better represent long-term fault behavior. Interval rates usually are more tightly constrained and provide a measure of variability of slip through time. Both types of rates are used in seismic hazards evaluations (e.g., Wong and Olig, 1998). Slip-rate determinations are affected by poorly understood properties of fault mechanics, such as stress interactions between nearby faults and along-strike variations in fault slip. However, because fault scarp profiles on the Bog Hot Valley scarps yield consistent surface offsets of 3–5 m (Personius et al., 2006), we believe the slip data from the trench is representative of slip on the southern 10–15 km of the Steens fault zone in Bog Hot Valley. We first calculate an average vertical slip rate using the stratigraphic separation of the contact between trench units 3 and 4 (4.4 Ⳳ 0.2 m), and the weighted mean of two luminescence ages from our excavations. Separation of 4.4 Ⳳ 0.2 m and an age of 18 Ⳳ 2.2 ka yield an average vertical slip rate of 0.24 Ⳳ 0.06 mm/ yr for the post-Lake Alvord history of the southern Steens fault zone (Table 3). We use post-E3 slip data (3.3 Ⳳ 0.5 m of vertical separation since 11.5 Ⳳ 2.0 ka) to calculate a more narrowly defined average rate of 0.29 Ⳳ 0.13 mm/yr. These average rates are similar to a vertical slip rate of 0.3 mm/yr estimated from the heights of faceted spurs along the east flank of the Pueblo Mountains (dePolo, 1998; dePolo and Anderson, 2000).

We also determined three interval slip rates, two individual rates between earthquakes E3 and E2 and E2 and E1, and one multiple-interval rate between earthquakes E3 and E1 (Table 3). The interval between earthquakes E3 and E2 (5.4 Ⳳ 2.1 k.y.) culminated in a vertical separation of 1.1 Ⳳ 0.5 m, and yields an interval slip rate of 0.2 Ⳳ 0.24 mm/ yr. The short interval between earthquakes E2 and E1 (1.5 Ⳳ 1.1 k.y.) culminated in a vertical separation of 2.2 Ⳳ 0.5 m and yields an interval slip rate of 1.5 Ⳳ 2.3 mm/yr. The interval between earthquakes E3 and E1 (6.9 Ⳳ 2.2 k.y.) culminated in a cumulative vertical separation of 3.3 Ⳳ 0.7 m and yields a multiple-interval slip rate of 0.48 Ⳳ 0.37 mm/yr. The large variations in the individual interval slip rates are a result of the irregular recurrence times and amounts of displacement of the last three earthquakes. In contrast to the short interval between earthquakes E2 and E1, the long elapsed time since the most-recent earthquake (E1, 4.6 Ⳳ 1.0 k.y.) and the interval between the deposition of unit 4 and earthquake E3 (6.5 Ⳳ 3.0 k.y.) indicates that through most of its latest Quaternary history, the southern Steens fault zone is probably best characterized by our average and multiple-interval vertical slip rates (0.24–0.48 mm/yr). Our rates are lower than rates on high-slip faults on the eastern and western margins of the Basin and Range province (e.g., Machette et al., 1992; Ramelli et al., 1999), but are higher than rates on most faults in the interior of the province (e.g., Machette et al., 2003; Personius and Mahan, 2005; Wesnousky et al., 2005). Long-Term (Miocene) Slip Rates Long-term slip rates are difficult to determine in the vicinity of the trench because of burial of both the hanging wall and footwall by basin-fill deposits. Therefore we determined a long-term vertical slip rate in the vicinity of Baltazor Hot Spring (12 km north of the trench site, Fig. 2) because

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the footwall geology in the Pueblo Mountains has been mapped in detail and subsurface data obtained during exploration of geothermal resources are available in this area. The topographic relief of the southern Pueblo Mountains increases northward from zero where Nevada Highway 140 crosses the fault, 4 km north of the trench site, to about 500 m at Baltazor Hot Spring. Gravity data suggest depths to basement of less than a kilometer in the hanging wall beneath the floor of Bog Hot Valley (Edquist, 1981; Saltus and Jachens, 1995). Our interpretations of published cross sections (Burnam, 1970; Edquist, 1981; Hulen, 1983) yield a total throw estimate of the Miocene volcanic section across a 60⬚-dipping range-front fault of 1.75 Ⳳ 0.25 km in the vicinity of Baltazor Hot Spring. The time of initiation of movement on the Steens fault zone in Bog Hot Valley is not precisely known. The oldest rocks exposed in the footwall of the Pueblo Mountains are Jurassic metamorphic rocks, unconformably overlain by a more than 1200-m-thick sequence of ⬃15–17 Ma basalts and less voluminous rhyolitic ash-flow tuffs (Burnam, 1970; Rytuba and McKee, 1984; Roback et al., 1987; Hart et al., 1989). The Miocene volcanic sequence dips uniformly westward about 20⬚ and is unconformably overlain by gently west-dipping basin-fill sediments (Burnam, 1970). These sediments were initially thought to be as young as early Pliocene in age, but later work in the Virgin Valley/Thousand Creek area 20–40 km to the west showed that correlative sediments contain early Hemphillian fossils and are overlain by 10 Ma volcanic rocks (Greene, 1984), and thus are late Miocene in age. Burnam (1970) suggested that deposition of these sediments signaled the initiation of range-front faulting in the southern Pueblo Mountains; if correct, then the age relations of these sediments to the west suggest an age of fault initiation of ⬎10 Ma. Rytuba and McKee (1984), Hemphill-Haley (1987), and Hemphill-Haley et al. (1989) used the widespread occurrence of the 9.3 Ma Devine Canyon Tuff (Walker, 1979) on both sides of the Steens/ Pueblo valleys in Oregon to infer an age of fault initiation of ⬍9.3 Ma. However, in more recent work in the nearby Pine Forest Range (Fig. 2), Colgan et al. (2006a, b) used fission track and U-Th/He (He) thermochronology along with structural data to infer an age of fault initiation of 10– 12 Ma on the normal fault that bounds the eastern margin of the range. From these data, we use an estimated age of fault initiation of 10.5 Ⳳ 2.0 Ma and a throw of 1.75 Ⳳ 0.25 km to determine a long-term vertical slip rate of 0.2 Ⳳ 0.1 mm/yr on the Steens fault zone in the vicinity of Baltazor Hot Spring. This rate, averaged over 8.5–12.5 Ma, is nearly the same as our average late Quaternary slip rate from the trench (Table 3). The prominent topographic relief (1750 m) of Steens Mountain, 80–90 km north of the trench site in southern Oregon (Fig. 1), indicates that long-term slip rates probably are higher on the central part of the Steens fault zone. Hemphill-Haley (1987) and Hemphill-Haley et al. (1989) projected the top of the Miocene volcanic sequence in the

footwall across the fault zone, used gravity data of Cleary et al. (1981a, 1981b) to infer depth to bedrock in the hanging wall, and determined a total vertical displacement of 2.75– 3.13 km in the vicinity of Steens Mountain. They then used the age of the 9.3 Ma Devine Canyon Tuff as the age of fault initiation to calculate a long-term vertical slip rate of 0.33 mm/yr. More recently, Whipple and Oldow (2004) and Oldow et al. (2005) used gravity and geologic data to infer an age of fault initiation of 4–7 Ma and calculated a longterm vertical slip rate of about 1 mm/yr across the Steens fault zone in the same area. Comparison of Geologic Slip Rates with Geodetic Velocities We calculated horizontal extension rates across the Steens fault zone from our vertical slip rates (Table 3) to use in comparisons with horizontal velocities documented in recent campaign Global Positioning System (GPS) surveys (Hammond and Thatcher, 2005, 2007). Our calculations assume pure dip-slip on 45⬚- and 60⬚-dipping faults; the lower dip is from a worldwide compilation of Thatcher and Hill (1991), and the higher fault dip is from local exposures of bedrock faults and subsurface data in the vicinity of Baltazor Hot Spring (Burnam, 1970; Edquist, 1981; Hulen, 1983). Our extension rates are variable, but average rates of 0.14 Ⳳ 0.04 to 0.29 Ⳳ 0.13 mm/yr and multiple-interval rates of 0.28 Ⳳ 0.21 to 0.48 Ⳳ 0.37 mm/yr (Table 3) are probably representative of latest Quaternary deformation. These rates are consistent with low extension rates (0.1 Ⳳ 0.3 mm/yr on a fault dipping 45⬚) modeled by Hammond and Thatcher (2007) in the vicinity of Bog Hot Valley. Our geologic sliprate uncertainties are unlikely to improve much in the near future, but because the geodetic rates of Hammond and Thatcher (2005, 2007) are based on a single campaign reoccupation spanning 4 years, future GPS surveys will result in more precise measurements and make future rate comparisons more meaningful. Earthquake Magnitude Estimates of earthquake magnitudes commonly are based on fault rupture lengths or coseismic surface displacements (e.g., Wells and Coppersmith, 1994). Estimating past fault rupture lengths is complicated by poor preservation of small displacements at the ends of faults, but we can estimate the length of the surface ruptures documented in the trench by examining the age of most recent faulting mapped along the Steens fault zone. Latest Quaternary fault scarps extend from the southern end of Bog Hot Valley northward about 37 km to the vicinity of Pueblo Mountain (Fig. 1); this part of the fault zone is termed the Denio section by Personius et al. (2003). If this length represents the total length of ruptures during the past few earthquakes, then magnitudes of M 6.9 (normal-fault regression, Wells and Coppersmith, 1994) are indicated.

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Coseismic surface displacements may yield better constraints on earthquake magnitude estimates (e.g., HemphillHaley and Weldon, 1999), but this method depends on whether displacements are considered average or maximum values (Wells and Coppersmith, 1994). Given the consistent 3- to 5-m surface offsets across scarps in Bog Hot Valley (Personius et al., 2006), we assume that our displacements probably are closer to average rather than maximum values. Given this caveat, estimates of displacements from the trench (1.1–2.2 Ⳳ 0.5 m) yield earthquake magnitudes of M 6.6–7.1 (normal-fault regression, Wells and Coppersmith, 1994) for the last three surface-rupturing earthquakes on this part of the Steens fault zone.

Discussion Evidence of Complex Faulting A 5.5-km-long zone of primarily west-facing, 0.5- to 2m-high fault scarps is present on latest Quaternary alluvial and lacustrine deposits a few kilometers southeast of the trench site along the eastern margin of Bog Hot Valley (Fig. 2). These scarps could be the result of earthquakes on a fault bordering the western margin of the Pine Forest Range, but their short length, location on the basin floor, and the apparent antiquity of a possible nearby range-front fault indicate that they probably were formed by slip on a buried west-dipping fault zone during a large earthquake elsewhere in the region. We have not studied these scarps in detail, but we use morphometric data from a single scarp profile (Personius et al., 2006) to estimate the age of the scarp using the linear-plus-cubic diffusion model of Andrews and Bucknam (1987). Assuming formation during a single earthquake rupture, profile parameters (surface offset ⳱ 1.9 Ⳳ 0.5 m, maximum slope angle ⳱ 17 Ⳳ 1⬚, far-field slope angle ⳱ 2.4 Ⳳ 1⬚, and a diffusion constant ⳱ 0.49 Ⳳ 0.03 m2/1000 yr (McCalpin, 1996) yield a scarp age of 3.8 Ⳳ 0.8 ka. While admittedly simple, this analysis results in an age that is consistent with faulting during the most recent earthquake (E1 4.6 Ⳳ 1.0 ka) on the southern Steens fault zone. Such complex rupturing of multiple fault zones during a single earthquake sequence is common in the historic earthquake record of the Basin and Range (e.g., 1915 Pleasant Valley, Nevada, 1954 Fairview Peak, Nevada, and 1955 Hebgen Lake, Montana earthquakes; de Polo et al., 1991), and has been inferred elsewhere in the paleoseismic record (e.g., Personius and Mahan, 2005). This phenomenon probably is the origin of many short, disconnected normal fault scarps mapped throughout the Basin and Range province. Evidence for Southward-Directed Fault Propagation Our trench data and the tectonic geomorphology of the region strongly suggest that the Steens fault zone is propagating southward. Fault slip rates in the Alvord Desert region (Fig. 1) appear to be higher than elsewhere on the fault zone (Oldow and Singleton, 2005), but no evidence of late Qua-

ternary ruptures has been documented on the northern half of the fault north of Steens Mountain (Personius et al., 2003). South of Steens Mountain, a 17-km gap in late Quaternary scarps is coincident with lower topography in the footwall in the northern Pueblo Mountains. Latest Quaternary faulting resumes from Pueblo Mountain southward along the eastern margin of the southern Pueblo Mountains and extends across 10 km of the nearly flat floor of Bog Hot Valley to an east-facing bedrock escarpment at the northwestern end of the Pine Forest Range (Figs. 1 and 2). This southermost part of the fault zone has experienced three surface-rupturing earthquakes in the last 11.5 Ⳳ 2.0 ka across the floor of Bog Hot Valley, despite the lack of a bedrock footwall escarpment and no evidence of significant throw across the fault in gravity maps of the area (Edquist, 1981; Ponce and Plouff, 2001). The lack of a bedrock range front suggests that the fault has recently propagated southward from the southern end of the Pueblo Mountains. At the southern end of Bog Hot Valley, late Quaternary fault scarps extend to the northern end of the east-dipping, northnortheast-striking Alder Creek normal fault (Fig. 1) of Colgan et al. (2006a). We did not evaluate this fault for evidence of Quaternary movement, but if these two faults are now kinematically connected, then the length of the Steens fault zone would extend and additional 32 km. Surface ruptures on normal faults that extend onto valley-floor positions are common in both the historic record (e.g. 1887 Sonora, Mexico, 1954 Rainbow Mountain/Stillwater, Nevada, and 1982 Borah Peak, Idaho earthquakes; de Polo et al., 1991), and on mapped late Quaternary faults (e.g., faults in Desert, Kings River, and Quinn River valleys; Fig. 1). Such propagation may be an underappreciated source of surfacerupture hazard in areas without mapped Quaternary fault scarps. Pluvial Lake Alvord History Paleoseismic investigations in the Basin and Range commonly provide serendipitous information on the history of pluvial lakes in the region (e.g., Briggs and Wesnousky, 2004; Caskey et al., 2004), and this study is no exception. We use our reconnaissance mapping of shoreline features and limited luminescence dating of shoreline and quiet-water deposits in Bog Hot Valley and southern Pueblo Valley to estimate the lake level (1295–1297 m) and timing (⬎13.4– 16.1 ka, ⬍18 Ⳳ 2.2 ka) of the long-postulated overflow and subsequent flood event that drastically lowered the level of pluvial Lake Alvord in the late Pleistocene.

Conclusions We use detailed trench mapping and radiocarbon and luminescence dating to estimate the vertical displacements and timing of the past three surface-rupturing earthquakes on the southern part of the Steens fault zone in northernmost Nevada. Three surface ruptures were associated with indi-

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vidual vertical displacements of 1.1–2.2 Ⳳ 0.5 m of latest Quaternary lacustrine, alluvial, and colluvial deposits. Radiocarbon and luminescence ages bracket the last three earthquake horizons and yield estimated ages of 11.5 Ⳳ 2.0 ka, 6.1 Ⳳ 0.5 ka, and 4.6 Ⳳ 1.0 ka for earthquakes E3, E2, and E1, respectively. These data yield recurrence intervals of 5.4 Ⳳ 2.1 k.y. between earthquakes E3 and E2, 1.5 Ⳳ 1.1 k.y. between earthquakes E2 and E1, and an elapsed time of 4.6 Ⳳ 1.0 k.y. since earthquake E1. Although the record is short (three earthquakes) and age uncertainties are large, our data appear to indicate variable recurrence on this part of the fault zone. Estimates of surface-rupture length and vertical displacements indicate that the last three large earthquakes had probable magnitudes of M 6.6–7.1. We calculated a wide range of interval and average vertical slip rates (0.2–1.5 mm/yr), but through most of its latest Quaternary history, the southern Steens fault zone is probably best characterized by our average and multiple-interval vertical slip rates (0.24–0.48 mm/yr). These rates are similar to both long term (post-Miocene) slip rates and recently modeled GPS velocities in the region (Hammond and Thatcher, 2007). Evidence of complex faulting patterns and the southward propagation of earthquake surface ruptures across the floor of Bog Hot Valley attest to the dynamic nature of Quaternary faulting along the southern Steens fault zone. Surface ruptures on faults in valley-floor positions and induced rupture on nearby apparently inactive faults are common phenomenon in both historic and paleoseismic earthquake records in the Basin and Range province. Such features have implications for siting critical facilities in areas where evidence of surface rupture is subtle to nonexistent.

Acknowledgments This research was supported by the Earthquake Hazards Reduction Program of the U.S. Geological Survey under a Memorandum of Understanding between the Central Geologic Hazards Team of the USGS and the Winnemucca District Office of the Bureau of Land Management. We wish to thank personnel of the Winnemucca District Office for assistance in conducting this research on BLM lands, Richard Dart for GIS support, and Bill Hammond for help with GPS data. The report was improved by comments from USGS reviewers Kathy Haller and Alan Nelson, BSSA reviewers Richard Briggs and Keith Kelson, and BSSA associate editor Mark Hemphill-Haley.

References Adams, K. D., and S. G. Wesnousky (1998). Shoreline processes and the age of the Lake Lahontan highstand in the Jessup embayment, Nevada, Geol. Soc. Am. Bull. 110, 1318–1332. Aitken, M. J. (1998). An Introduction to Optical Dating—The Dating of Quaternary Sediments by the Use of Photon-Stimulated Luminescence, Oxford University Press, Oxford, 267 pp. Andrews, D. J., and R. C. Bucknam (1987). Fitting degradation of shoreline scarps by a nonlinear diffusion model, J. Geophys. Res. 92, 12,857– 12,867. Bell, J. W., S. J. Caskey, A. R. Ramelli, and L. Guerrieri (2004). Patterns and rates of faulting in the central Nevada seismic belt, and paleo-

seismic evidence for prior beltlike behavior, Bull. Seism. Soc. Am. 94, 1229–1254. Bennett, R. A., B. P. Wernicke, and J. L. Davis (1998). Continuous GPS measurements of contemporary deformation across the northern Basin and Range province, Geophys. Res. Lett. 25, 563–566. Bennett, R. A., B. P. Wernicke, N. A. Niemi, A. M. Friedrich, and J. L. Davis (2003). Contemporary strain rates in the northern Basin and Range province from GPS data, Tectonics 22, 1008, doi 10.1029/ 2001TC001355. Benson, L. (1993). Factors affecting 14C ages of lacustrine carbonates— timing and duration of the last highstand lake in the Lahontan basin, Quat. Res. 39, 163–174. Berger, G. W. (1988). Dating Quaternary sediments by luminescence, in Dating Quaternary Sediments, D. J. Easterbrook (Editor), Geol. Soc. Am. Spec. Pap. 227, 13–50. Briggs, R. W., and S. G. Wesnousky (2004). Late Pleistocene fault slip rate, earthquake recurrence, and recency of slip along the Pyramid Lake fault zone, northern Walker Lane, United States, J. Geophys. Res. 109, B08402, doi 10.1029/2003JB002717. Bronk Ramsey, C. (1995). Radiocarbon calibration and analysis of stratigraphy—the OxCal program, Radiocarbon 37, 425–430. Bronk Ramsey, C. (2001). Development of the radiocarbon program OxCal, Radiocarbon 43, 355–363. Burnam, R. (1970). The geology of the southern part of the Pueblo Mountains, Humboldt County, Nevada, Master’s Thesis, Oregon State University, Corvallis, 114 pp. Carter, D. T., L. L. Ely, J. E. O’Connor, and C. R. Fenton (2006). Late Pleistocene outburst flooding from pluvial Lake Alvord into the Owyhee River, Oregon, Geomorphology 75, 346–367. Caskey, S. J., J. W. Bell, A. R. Ramelli, and S. G. Wesnousky (2004). Historic surface faulting and paleoseismicity in the area of the 1954 Rainbow Mountain-Stillwater earthquake sequence, central Nevada, Bull. Seism. Soc. Am. 94, 1255–1275. Cleary, J., I. M. Lange, A. I. Qamar, and H. R. Krouse (1981a). Gravity, isotope, and geochemical study of the Alvord Valley geothermal area, Oregon—summary, Geol. Soc. Am. Bull. Part I 92, 319–322. Cleary, J., I. M. Lange, A. I. Qamar, and H. R. Krouse (1981b). Gravity, isotope, and geochemical study of the Alvord Valley geothermal area, Oregon, Geol. Soc. Am. Bull. Part II 92, 934–962. Colgan, J. P., T. A. Dumitru, M. McWilliams, and E. L. Miller (2006a). Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada—new constraints from the northern Pine Forest Range, Geol. Soc. Am. Bull. 118, 126–139, doi 10.1130/B25681.1. Colgan, J. P., T. A. Dumitru, P. W. Reiners, J. L. Wooden, and E. L. Miller (2006b). Cenozoic tectonic evolution of the Basin and Range province in northwestern Nevada, Am. J. Sc. 306, 616–654. dePolo, C. M. (1998). A reconnaissance technique for estimating the slip rate of normal-slip faults in the Great Basin, and application to faults in Nevada, U.S.A., Ph.D. Dissertation, University of Nevada–Reno, 199 pp. dePolo, C. M., and J. G. Anderson (2000). Estimating the slip rates of normal faults in the Great Basin, USA, Basin Res. 12, 227–240. dePolo, C. M., D. G. Clark, D. B. Slemmons, and A. R. Ramelli (1991). Historical surface faulting in the Basin and Range province, western North America—implications for fault segmentation, J. Struct. Geol. 13, 123–136. Edquist, R. K. (1981). Geophysical investigation of the Baltazor Hot Springs known geothermal resource area and the Painted Hills thermal area, Humboldt County, Nevada, Master’s Thesis, University of Utah, Salt Lake City, 90 pp. Forman, S. L., J. Pierson, and K. Lepper (2000). Luminescence geochronology, in Quaternary Geochronology—Methods and Applications, J. S. Noller, J. M. Sowers, and W. R. Lettis (Editors), Vol. 4, American Geophysical Union Reference Shelf, 157–176. Frankel, A. D., M. D. Petersen, C. S. Mueller, K. M. Haller, R. L. Wheeler, E. V. Leyendecker, R. L. Wesson, S. C. Harmsen, C. H. Cramer, D. M. Perkins, and K. S. Rukstales (2005). Seismic-hazards maps for

Late Quaternary Paleoseismology of the Southern Steens Fault Zone, Northern Nevada the conterminous United States, U.S. Geol. Surv. Scientific Investigations Map 2883. Friedrich, A. M., J. Lee, B. P. Wernicke, and K. Sieh (2004). Geologic context of geodetic data across a Basin and Range normal fault, Crescent Valley, Nevada, Tectonics 23, TC2015, doi 10.1029/2003 TC001528. Friedrich, A. M., B. P. Wernicke, N. A. Niemi, R. A. Bennett, and J. L. Davis (2003). Comparison of geodetic and geologic data from the Wasatch region, Utah, and implications for the spectral character of Earth deformation at periods of 10 to 10 million years, J. Geophys. Res. 108, 2199, doi 10.1029/2001JB000682. Geyh, M. A., and H. Schleicher (1990). Absolute Age Determination, Springer-Verlag, New York, 503 pp. Greene, R. C. (1984). Geologic appraisal of the Charles Sheldon wilderness study area, Nevada and Oregon, U.S. Geol. Surv. Bull. 1538-A, 13–34. Hammond, W. C., and W. Thatcher (2004). Contemporary tectonic deformation of the Basin and Range province, western United States—10 years of observation with the Global Positioning System, J. Geophys. Res. 109, B08403, doi 10.1029/2003JB002746. Hammond, W. C., and W. Thatcher (2005). Northwest Basin and Range tectonic deformation observed with the Global Positioning System, 1999–2003, J. Geophys. Res. 110, B10405, doi 10.1029/2005 JB003678. Hammond, W. C., and W. Thatcher (2007). Crustal deformation across the Sierra Nevada, northern Walker Lane, Basin and Range transition, western United States measured with GPS, 2000-2004, J. Geophys. Res. 112, B05411, doi 10.1029/2006JB004625. Hart, W. K., R. W. Carlson, and S. A. Mosher (1989). Petrogenesis of the Pueblo Mountains basalt, southeastern Oregon and northern Nevada, in Volcanism and Tectonism in the Columbia River Flood Basalt Province, S. P. Reidel and P. R. Hooper (Editors), Geol. Soc. Am. Spec. Pap. 239, 367–378. Hemphill-Haley, M. A. (1987). Quaternary stratigraphy and late Holocene faulting along the base of the eastern escarpment of Steens Mountain, southeastern Oregon, Master’s Thesis, Humboldt State University, Arcata, California, 84 pp. Hemphill-Haley, M. A., and R. J. Weldon, II (1999). Estimating prehistoric earthquake magnitude from point measurements of surface rupture, Bull. Seism. Soc. Am. 89, 1264–1279. Hemphill-Haley, M. A., W. D. Page, R. Burke, and G. A. Carver (1989). Holocene activity of the Alvord fault, Steens Mountain, southeastern Oregon, Final Technical Report to U.S. Geol. Surv., Contract 14-080001-G1333, 45 pp. Hemphill-Haley, M. A., W. D. Page, G. A. Carver, and R. M. Burke (2000). Paleoseismicity of the Alvord fault, Steens Mountain, southeastern Oregon, in Quaternary Geochronology—Methods and Applications, J. S. Noller, J. M. Sowers, and W. R. Lettis (Editors), Vol. 4, American Geophysical Union Reference Shelf, 537–540. Hulen, J. B. (1983). Structural control of the Baltazor hot springs geothermal system, Humboldt County, Nevada, Geothermal Resources Council Trans. 7, 157–162. Huntley, D. J., and M. Lamothe (2001). Ubiquity of anomalous fading in K-feldspars and the measurement and correction for it in optical dating, Can. J. Earth Sci. 38, 1093–1106. Licciardi, J. M. (2001). Chronology of latest Pleistocene lake-level fluctuations in the pluvial Lake Chewaucan basin, Oregon, USA, J. Quat. Sci. 16, 545–553. Machette, M. N., K. M. Haller, R. L. Dart, and S. B. Rhea (2003). Quaternary fault and fold database of the United States, U.S. Geol. Surv. Open-File Rept. 03-417 (on line database available at http://earth quake.usgs.gov/regional/qfaults/) (last accessed June 2007). Machette, M. N., S. F. Personius, and A. R. Nelson (1992). Paleoseismology of the Wasatch fault zone—a summary of recent investigations, interpretations, and conclusions, in Assessment of Regional Earthquake Hazards and Risk along the Wasatch Front, Utah, P. L. Gori and W. W. Hays (Editors), U.S. Geol. Surv. Profess. Pap. 1500, A1– A71.

1677

McCalpin, J. P. (1996). Paleoseismology, Academic Press, New York, 588 pp. Oldow, J. S., and E. S. Singleton (2005). Ground-based LIDAR determined fault-offsets of paleo-lake terraces—late middle Pleistocene to Holocene deformation rates in the Alvord extensional basin, southeastern Oregon (abstract), EOS Trans. AGU 86, G33D-06. Oldow, J. S., E. S. Singleton, and K. L. Whipple (2005). Integration of digital data resources to estimate the history and rates of deformation in the Alvord extensional basin, southeastern Oregon (abstracts with programs), Geol. Soc. Am. 37, 416. Personius, S. F., and S. A. Mahan (2005). Unusually low rates of slip on the Santa Rosa Range fault zone, northern Nevada, Bull. Seism. Soc. Am. 95, 319–333. Personius, S. F., A. J. Crone, M. N. Machette, J. B. Kyung, H. Cisneros, D. J. Lidke, and S. A. Mahan (2006). Trench logs and scarp data from an investigation of the Steens fault zone, Bog Hot Valley and Pueblo Valley, Humboldt County, Nevada, U.S. Geol. Surv. Scientific Investigations Map 2952 (available online at http://pubs.usgs.gov/sim/ 2006/2952/) (last accessed June 2007). Personius, S. F., R. L. Dart, L.-A. Bradley, and K. M. Haller (2003). Map and data for Quaternary faults and folds in Oregon, U.S. Geol. Surv. Open-File Report 03-095, scale 1:750,000, 579 pp. (available online at http://pubs.usgs.gov/of/2003/ofr-03-095/) (last accessed June 2007). Ponce, D. A., and D. Plouff (2001). Bouguer gravity map of Nevada—Vya sheet, Nevada Bureau of Mines and Geol. Map 128, scale 1:250,000. Ramelli, A. R., J. W. Bell, C. M. dePolo, and J. C. Yount (1999). Largemagnitude, late Holocene earthquakes on the Genoa fault, westcentral Nevada and eastern California, Bull. Seism. Soc. Am. 89, 1458–1472. Reheis, M. (1999). Extent of Pleistocene lakes in the western Great Basin, U.S. Geol. Surv. Misc. Field Studies Map MF-2323, scale 1:800,000. Reimer, P. J., M. G. L. Baillie, E. Bard, A. Bayliss, J. W. Beck, C.J.H. Bertrand, P. G. Blackwell, C. E. Buck, G. S. Burr, K. B. Cutler, P. E. Damon, R. L. Edwards, R. G. Fairbanks, M. Friedrich, T. P. Guilderson, A. G. Hogg, K. A. Hughen, B. Kromer, F. G. McCormac, S. W. Manning, C. Bronk Ramsey, R. W. Reimer, S. Remmele, J. R. Southon, M. Stuiver, S. Talamo, F. W. Taylor, J. van der Plicht, and C. E. Weyhenmeyer (2004). IntCal04 Terrestrial radiocarbon age calibration, 26-0 ka BP, Radiocarbon 46, 1029–1058. Roback, R. C., D. B. Vander Meulen, H. D. King, D. Plouff, S. R. Munts, and S. L. Willett (1987). Mineral resources of the Pueblo Mountains Wilderness Study Area, Harney County, Oregon and Humboldt County, Nevada, U.S. Geol. Surv. Bull. 1740-B, 30 pp. Rytuba, J. J., and E. H. McKee (1984). Peralkaline ash flow tuffs and calderas of the McDermitt volcanic field, southeast Oregon and north central Nevada, J. Geophys. Res. 89, 8616–8628. Saltus, R. W., and R. C. Jachens (1995). Gravity and basin-depth maps of the Basin and Range Province, Western United States, U.S. Geol. Surv. Geophys. Investigations Map GP-1012, scale 1:2,500,000. Singhvi, A. K., Y. P. Sharma, and D. P. Agrawal (1982). Thermoluminescence dating of sand dunes in Rajasthan, India, Nature 295, 313–315. Thatcher, W. (2003). GPS constraints on the kinematics of continental deformation, Int. Geol. Rev. 45, 191–212. Thatcher, W., and D. P. Hill (1991). Fault orientations in extensional and conjugate strike-slip environments and their implications, Geology 19, 1116–1120. Thatcher, W., G. R. Foulger, B. R. Julian, J. Svarc, E. Quity, and G. W. Bawden (1999). Present day deformation across the Basin and Range Province, Western United States, Science 283, 1714–1718. Thompson, R. S., L. Benson, and E. M. Hattori (1986). A revised chronology for the last Pleistocene lake cycle in the central Lahontan Basin, Quat. Res. 25, 1–9. Walker, G. W. (1979). Revisions to the Cenozoic stratigraphy of Harney Basin, southeastern Oregon, U.S. Geol. Surv. Bull. 1475, 35 pp. Walker, G. W., and C. A. Repenning (1966). Reconnaissance geologic map of the west half of the Jordan Valley quadrangle Malheur County,

1678


Oregon, U.S. Geol. Surv. Misc. Geologic Investigations Map I-457, scale 1:250,000. Wallace, R. E. (1984). Patterns and timing of late Quaternary faulting in the Great Basin province and relation to some regional tectonic features, J. Geophys. Res. 89, 5763–5769. Wells, D. L., and K. J. Coppersmith (1994). New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bull. Seism. Soc. Am. 84, 974–1002. Wesnousky, S. G., A. D. Barron, R. W. Briggs, S. J. Caskey, S. Kumar, and L. Owen (2005). Paleoseismic transect across the northern Great Basin, J. Geophys. Res. 110, B05408, doi 10.1029/2004JB003283. Willden, R. (1964). Geology and mineral deposits of Humboldt County, Nevada, Nevada Bureau of Mines and Geol. Bull. 59, 154 pp. Wintle, A. G. (1973). Anomalous fading of thermoluminescence in mineral samples, Nature 245, 143–144. Wintle, A. G., and D. J. Huntley (1980). TL dating of ocean sediments, Can. J. Earth Sci. 17, 348–360. Whipple, K. L., and J. S. Oldow (2004). Late Cenozoic extension and structural evolution of the Alvord basin, southeastern Oregon (abstracts with programs), Geol. Soc. Am. 36, 28. Wong, I. G., and S. S. Olig (1998). Seismic hazards in the Basin and Range province—perspectives from probabilistic analyses, in Proceedings Volume Basin and Range Province Seismic-Hazards Summit, W. R. Lund (Editor), Utah Geol. Surv. Misc. Publication 98-2, 110–127.

U.S. Geological Survey Box 25046, MS 966 Denver Federal Center Denver, Colorado 80225 [email protected] (S.F.P., A.J.C., M.N.M., D.J.L.) U.S. Geological Survey Box 25046, MS 974 Denver Federal Center Denver, Colorado 80225 (S.A.M.) Korea National University of Education 363-791 Chongwon-Gun, South Korea (J.B.K.) Universidad Nacional de San Luis D5700HHW San Luis, Argentina (H.C.)

Manuscript received 26 September 2006.