Abstract

A strike-slip fault is present outboard and subparallel to the Wassuk Range front within the central Walker Lane (Nevada, USA). Recessional shorelines of pluvial Lake Lahontan that reached its highstand ca. 15,475 ± 720 cal. yr B.P. are displaced ∼14 m and yield a right-lateral slip-rate estimate approaching 1 mm/yr. The strike-slip fault trace projects southeastward toward the eastern margin of Walker Lake, which is ∼15 km to the southeast. The trace is obscured in this region by recessional shorelines features that record the historical dessication of the lake caused by upstream water diversion and consumption. High-resolution seismic CHIRP (compressed high intensity radar pulse) profiles acquired in Walker Lake reveal ∼20 k.y. of stratigraphy that is tilted westward ∼20–30 m to the Wassuk Range front, consistent with ∼1.0–1.5 mm/yr (20–30 m/20 k.y.) of vertical displacement on the main range-bounding normal fault. Direct evidence of the northwest-trending right-lateral strike-slip fault is not observed, although a set of folds and faults trending N35°E, conjugate to the trend of the strike-slip fault observed to the north, is superimposed on the west-dipping strata. The pattern and trend of folding and faulting beneath the lake are not simply explained; they may record development of Riedel shears in a zone of northwest-directed strike slip. Regardless of their genesis, the faults and folds appear to have been inactive during the past ∼10.5 k.y. These observations begin to reconcile what was a mismatch between geodetically predicted deformation rates and geological fault slip rate studies along the Wassuk Range front, and provide another example of strain partitioning between predominantly normal and strike-slip faults that occurs in regions of oblique extension such as the Walker Lane.

INTRODUCTION

The Wassuk Range (Nevada, USA) is a north-trending ∼80-km-long mountain block within the central Walker Lane (Figs. 1 and 2), which is an ∼100-km-wide zone of discontinuous active faults and disrupted topography that trends along the east flank of the Sierra Nevada (e.g., Wesnousky, 2005). Based on geodesy, as much as one-fifth of the right-lateral relative plate motion between the Pacific and North America plates is accommodated east of the Sierra Nevada, with the majority localized within the Walker Lane (Bennett et al., 1999; Hammond and Thatcher, 2007; Thatcher et al., 1999). The eastern escarpment of the Wassuk Range is steep, abrupt, and bounded by a normal fault just west of Walker Lake (Fig. 2; e.g., Wesnousky, 2005). Here we present both terrestrial and lacustrine observations that place bounds on the late Pleistocene rate of uplift due to displacement on the range-bounding normal fault and document the presence of an active strike-slip fault system outboard and subparallel to the range front. The observations begin to reconcile an existing mismatch between geodetically predicted deformation rates compared to geologically determined slip rates reported from studies along the range-bounding fault (Bormann et al., 2012; Wesnousky et al., 2012). The study also provides another example of the partitioning of slip between primarily normal and strike-slip faults that occur in regions of oblique extension (e.g., Wesnousky and Jones, 1994).

OBSERVATIONS

Terrestrial

The characteristics of the strike-slip fault system originally mapped by House and Adams (2010) are elucidated here with Lidar (light detection and ranging) data reported and processed by Lopes and Smith (2007) and a section of a 1:40,000-scale low-sun-angle photograph. The Lidar digital elevation map defines a lineament that is east of and strikes subparallel to the range front (Fig. 3). The entire lineament is located below the prehistoric highstand of pluvial Lake Lahontan that filled the basin to an elevation of ∼1330 m ca. 13,070 ± 60 14C B.P. (e.g., Adams and Wesnousky, 1998), or ∼15,475 ± 720 cal. yr B.P. (Briggs and Wesnousky, 2004). Below the ∼1330 m pluvial highstand and above the ∼1262 m latest Holocene highstand of the lake mapped by House and Adams (2009, 2010), the lineament cuts wave-washed surfaces of pluvial Lake Lahanton and alluvial fan deposits developed since that time. The field expression of the lineament is characterized by the presence of alternate facing scarps along strike, which are typically associated with strike-slip displacement (e.g., Wallace, 1991). The lineament continues southeastward from the ∼1262 m shoreline for ∼1 km as an alignment of vegetation. Continuing farther southeast with a slight change in azimuth, and below the ∼1252 m A.D. 1868 historical highstand of the lake (Adams, 2007), a distinct lineament in the Lidar imagery is present (Fig. 3B). Though the Lidar reports horizontal and vertical positional accuracies of ∼1 m and 13–20 cm, respectively, field observation indicates that the lineament is an artifact of Lidar processing errors (e.g., swath stitching misalignment).

An enlarged portion of a 1:40,000-scale Nevada Bureau of Mines and Geology low-sun-angle photo of the fault trace where it cuts recessional shorelines of Lake Lahontan is shown in Figure 4A. The fault trace cuts from northwest to southeast across the central portion of the image. Several of the tonal bands resulting from shadows cast by the recessional beach ridges are offset right laterally across the trace. To measure the amount of offset, the photo was georeferenced to existing U.S. Geological Survey digital raster graphics and orthophotoquads encompassing the site. The dashed line pairs in Figure 4B are along interpreted piercing point lines that follow the three most distinct tonal contrasts. The right-lateral offset of the tonal lineaments across the fault range from 11.7 to 15.2 m with an average of 13.5 m. In addition, we used a backpack global positioning system (GPS) to survey along our interpretation by eye of the crests and swales of a number of beach ridges (dotted line pairs in Fig. 4C). The range of 4 offsets measured in this way range from 12.3 m to 14.3 m, with an average of 13.9 m. Figure 4D shows the Lidar image at the same scale as Figures 4B and 4C. In this case, the offsets observed in the low-sun-angle image are not clearly evident in the Lidar image.

The offset shorelines postdate the highstand of pluvial Lake Lahontan (15,475 ± 720 yr B.P.). Dividing the ∼14 m offset of the shorelines by the age of the highstand yields a minimum fault slip rate equal to ∼0.9 mm/yr. The approximate fault slip rate assumes the total offset is due to multiple earthquakes.

Submarine CHIRP Survey

Methods

Approximately 200 line-km of seismic CHIRP (compressed high intensity radar pulse, acoustic variant) data were acquired in Walker Lake in 2012 and 2013 (Fig. 5). The survey employed Scripps Institution of Oceanography’s Edgetech SUBSCAN CHIRP profiler and was operated with a 50 ms swept pulse of 1–15 kHz, which provides decimeter vertical resolution with subbottom penetration >60 m. All data were digitally recorded in JSF (Java Server Faces) format (converted to Society of Exploration Geophysicists SEG-Y format) with real-time GPS navigation, providing location accuracy to within 5 m. Data were processed using SIOSEIS (Scripps Institution of Oceanography seismic processing software; Henkart, 2005) and imported to IHS Kingdom Suite and IVS Fledermaus software packages for interpretation. A nominal water and sediment velocity of 1500 m/s is assumed for all depth and sediment thickness conversions.

Line D01L02 was designed to traverse across the locations of two existing cores acquired in the lake (12 and 50 m long; Benson, 1988; Bradbury et. al., 1989; Benson et al., 1991) to correlate the lithostratigraphy and seismic stratigraphy (Fig. 6). Detailed analyses of both cores were performed, including radiocarbon age dating (Benson, 1988; Bradbury et al., 1989; Benson et al., 1991), which allowed us to determine the age of the reflectors imaged in the CHIRP data as well as lake history (Fig. 6). These data provide important age constraints for the deformational events imaged in the CHIRP profiles. Accordingly, an unconformity associated with the horizon shown in orange in Figure 6 is interpreted to be ca. 20 ka and the age of the green horizon is interpreted as ca. 10.5 ka. In the shallow portion of the sedimentary section, an interpolation of 14C-dated samples from the 12-m-long Benson et al. (1991) sediment core places the age of the blue horizon (erosional unconformity) as 3 ka and the age of the yellow horizon as ca. 3.9 ka. The distinct acoustic character of these reflectors allows for robust correlation of these horizons around the CHIRP seismic grid (e.g., Figs. 7 and 8).

Results

Characteristic track lines that best illustrate the structure and stratigraphy observed in the lake sediments are presented in Figure 6 (line D01L02), Figure 7 (line D01L01), and Figure 8 (line D02L13). All three lines cross the lake in the east-west direction (Fig. 5). There are several notable features observed in the CHIRP data. First and foremost, east-west profiles are marked by a strong divergence of sedimentary packages toward the Wassuk normal fault to the west. Superimposed on this westward tilt are broad folds, some faulted, with diminished amplitude upsection. The seismic imagery also reveals two erosional unconformities that appear to be related to periodic dessication of Walker Lake throughout the late Pleistocene and Holocene; these markers (orange, ca. 20 ka; blue, ca. 3.9 ka) also help place bounds on vertical slip rate across the Wassuk fault.

The divergence, rotation, and systematic increase in dip with depth of reflectors observed along the western portion of the lake (Figs. 6 and 7) are consistent with the presence of a down-to-the-east normal fault along the Wassuk Range front (e.g., Bormann et al., 2012). In this regard, the most recent deformation event preserved in the lake stratigraphy may be recorded by an ∼2.5-m-thick sediment wedge above the blue horizon in Figure 6, which is dated as ca. 3 ka. Bormann et al. (2012) recognized at least one surface displacement during this period of time along the range-front fault at Rose Creek, and thus it is possible these events correlate. Alternately, the package of sediment above the sediment lense may reflect the infilling of a river channel that flowed across the dessicated lake surface that is represented by the blue horizon (erosional unconformity). The absence of reflectors and thus possible structures along the westernmost portion of the lake is attributed to the presence of gas in the sediments (Fig. 6).

The horizon depicted in orange (Fig. 6) exhibits a regional dip that increases toward the west near the core sites. The observed regional tilt to the west is ∼20–30 m. Given that the horizon marks an erosional unconformity that was originally horizontal, the tilting would suggest a vertical component of displacement along the Wassuk Range front of 1.25 ± 0.25 mm/yr or more (20–30 m/20 k.y.). This value is more than twice that interpreted from displaced terrestrial deposits along the range-front fault (∼0.2–0.8 mm/yr; Bormann et al., 2012; Fig. 2). The difference in the estimated slip rate may reflect that the rates reported in each study are averaged over different periods of time or the limited data available to document the age of offset features in each study.

The deepest reflectors (horizons in red and below; Fig. 6) exhibit synclinal folds that diminish upsection: one in the case of line D01L02 (Fig. 6) and three in line D01L01 (Fig. 7). The layers of this deeper fold are concordant and maintain constant thickness. The easternmost limb of the easternmost fold in each section is truncated by an erosional unconformity (late Pleistocene, ca. 20 ka) marked for clarity in Figure 6 by an orange horizon that is most pronounced toward the east. This erosional unconformity is most likely due to wave-base erosion during an episode of lower lake level, possibly near Tioga glacial maximum of ca. 20 ka (Phillips et al., 1996).

The synclinal fold appears to continue upsection above the erosional unconformity, but with diminished amplitude. Clear evidence of tectonic folding is absent above the horizon in green; reflectors onlap within depressions on that horizon and appear to reflect the depositional infilling of the underlying fold (Fig. 6). The deepest basal reflectors are offset by a fault (f1) along the easternmost fold on all three lines (Figs. 6–8). The different pattern of throw along this fault (f1) (a different downthrown side is observed in D01L01 [Fig. 7] with respect to D02L13 [Fig. 8]) is consistent with strike-slip deformation. Deformation observed along this fault does not offset the orange horizon. In line D01L01 (Fig. 7), both of the two labeled synclines are offset at their western limbs with the cessation of deformation delineated by the green horizon (early Holocene, ca. 10.5 ka). Deduced age constraints for the orange and green horizons postdate the last deformational event associated with the deep folds and fold-related faults observed on all three lines ca. 20 ka (f1) (Fig. 6) and 10.5 ka (f2 and f3) (Fig. 7).

The locations and orientations of fault and fold axes determined by mapping of these features through the entire CHIRP data set are summarized in Figure 5. The fault orientations and fold axes are oriented ∼N35°E. The faults are discontinuous, are no more than 2–5 km long, and exhibit an en echelon character in map view (Fig. 5).

The CHIRP profiles also record lake-level fluctuations of Walker Lake in the late Quaternary that are consistent with several core studies (Benson, 1988; Benson et al., 1991). Benson (1988) reported high lake levels between 12 and 15 ka; this correlates to the acoustically transparent layer (homogeneous fine-grained sediments) below the horizon depicted in green (Figs. 6–8; ca. 10.5 ka). The green horizon is an onlap surface that separates the acoustically transparent highstand deposits below from the acoustically laminated onlapping sediments above. The high-amplitude reflectors within the acoustically laminated sequence are coarse layers emplaced during low-lake-level conditions (e.g., Brothers et al., 2011; Fig. 6). Some high-amplitude sequences are also observed above the orange layer, suggesting another lake-level lowstand that correlates with a desiccation period between 15 and 20 ka (Benson, 1988). Furthermore, the transparent layer below the orange horizon correlates to the highstand reported by Benson (1988) as older than 20 ka (Fig. 6). The acoustic character and erosional surfaces (i.e., orange and blue horizons, Figs. 6–8) observed in the CHIRP data appears in accord with the lake environment history as presented by Benson (1988) in terms of lake elevation and facies predictions from the late Pleistocene through Holocene.

A notable acoustic feature observed in the CHIRP data appears to be linked to distribution of tufas and/or bioherms in Walker Lake (Figs. 5 and 8). The pattern is well documented in line D02L13 (Fig. 8), where carbonate growth has resulted in a rough hummocky surface that obscures the sediments below. The attenuation of acoustic energy from the hummocky horizon is significantly different than acoustic attenuation by gas (Figs. 6–8). Acoustically, gas in the subsurface has a blotchy return and obscures reflectors beneath it. The hummocky surface has a marked change in roughness and a sharp acoustic impedance contrast. The distribution of the hummocky acoustic return is shown in Figure 5.

DISCUSSION AND CONCLUSION

Geodesy shows that at the latitude of the Wassuk Range, right-lateral strain accumulation across the width of the Walker Lane is equivalent to 6–7 mm/yr, with a lesser component distributed across the Wassuk Range front (Wesnousky et al., 2012). The Wassuk Range is oriented oblique to the shear field. As a result, it may be expected that both normal and strike-slip components of displacement are required to accommodate the ongoing shear. Prior geological estimates of fault slip rate are few, and focused on the component of normal displacement along the range-bounding fault (Bormann et al., 2012), suggesting that the vertical late Pleistocene slip rate along the range front is as much as ∼0.7 mm/yr. Our observations of the divergence, rotation, and systematic increase in dip with depth of reflectors observed along the western portion of the lake allow that the vertical rate may be twice this value. An additional ∼1.1 ± 0.4 mm/yr of right-lateral strike slip parallel to the range front is required by geodesy to account for the entirety of the geodetic strain field (Wesnousky et al., 2012). The terrestrial and lacustrine observations we have put forth here allow the suggestion that this additional ∼1.1 ± 0.4 mm/yr may all, or in large part, be accounted for by slip on this strike-slip fault system outboard of the range front. The observed offset of the remnant Lake Lahontan shorelines is ∼14 m. Dividing this value by the age of the Lake Lahontan highstand yields a slip rate of ∼1 mm/yr; the rate assumes that the offset is the result of 2 or more earthquakes. Lesser offsets of these presumed events are not documented along the trace.

The partitioning of oblique deformation between subparallel normal and strike-slip faults was first noted by Fitch (1972) in his study of oblique convergence at plate boundaries; it has since been frequently observed in both transtensional and transpressional environments (Jones and Wesnousky, 1992; Wesnousky and Jones, 1994). In this particular case, it is analogous to the pairing of the Owens Valley and Sierran range-front faults farther south in the Walker Lane.

Projection of the strike-slip fault ∼15 km southeastward toward Walker Lake is problematic. The trace is obscured over this distance by recessional shorelines features that record the historical dessication of the lake caused by upstream water diversion and consumption. Direct evidence for the existence of a northwest-directed right-lateral strike-slip fault is not present in the CHIRP profiles, although a set of folds and faults aligned along a N35°E trend, conjugate to the trend of the strike-slip fault observed to the north, is superimposed on the west-dipping strata. The presence of the faults and folds illustrates the presence of deformation within the lake. However, the explanation of the pattern and trend of folding and faulting beneath the lake is not simple, and may be due to development of Riedel shears in a zone of northwest-directed strike slip. While illustrative of late Pleistocene displacement, the faults and folds within the lake appear to have been inactive during the past ∼10.5 k.y. If it is assumed that the onshore and submarine traces share the same earthquake history (i.e., that large earthquakes along the dextral strike-slip fault are driving deformation in the Riedel shear zone), it would imply that the ∼14 m of offset observed onshore occurred during the same short period of time, perhaps due to a punctuated period of earthquakes lasting no more than ∼5 k.y. The lack of a preserved strike-slip scarp below the 1260–1262 m middle to late Holocene shoreline (Fig. 3) supports the assumption underlying the idea.

This is University of Nevada Center for Neotectonics Contribution 65. Funding for the seismic CHIRP (compressed high intensity radar pulse) survey on Walker Lake was provided by Southern California Earthquake Center award SCEC-10166. Support for Shaopeng Dong was provided by the China Earthquake Administration Grant No. IGCEA 1220 and the National Natural Science Foundation of China Grant No. 41372220. We thank Ken Adams for a constructive review and criticisms regarding the ages of the late Holocene and historical highstands of Walker Lake that were valuable in improving the paper. Constructive reviews by Angela Jayko and Cooper Brossy also improved the paper. We thank Danny Brothers for an internal U.S. Geological Survey review and Ernest Aaron for his equipment expertise.