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Holocene earthquakes and late Pleistocene slip rate estimates on the Wassuk Range fault zone, Nevada, USA.
Jayne M. Bormann, Benjamin E. Surpless, Steven G. Wesnousky, and Marc W. Caffee
Corresponding Author: Jayne Bormann Center for Neotectonic Studies Nevada Geodetic Laboratory University of Nevada, Reno MS 178 Reno, NV 89557 [email protected]
Electronic supplement: Two tables and figures detailing the inputs and results for radiocarbon and cosmogenic analyses and photos of the rocks sampled for cosmogenic analysis. 1 2 Abstract
3 The Wassuk Range fault zone is an active, 80 km long, east-dipping, high-angle normal
4 fault that flanks the eastern margin of the Wassuk Range in central Nevada. Observations from
5 two alluvial fan systems truncated by the fault provide information on the uplift rate and
6 Holocene earthquake history along the rangefront. At the apex of the Rose Creek alluvial fan,
7 radiocarbon dating of offset stratigraphy exposed in two fault trenches shows that multiple
8 earthquakes resulted in 5.5-7.0 m of vertical offset along the fault since ~9400 cal yr B.P. The
9 southern trench records at least two faulting events resulting in a ~5.5 m scarp since ~9400 cal yr
10 B.P., with the most recent displacement postdating ~2800 cal yr B.P. The northern trench records
11 a ~1 m offset after ~600 cal yr B.P., allows an earlier event at ~1450 cal yr B.P., and records one
12 or more prior events. Although large variations in stratigraphy between trench exposures prevent
13 the development of a unique earthquake chronology, these observations result in a Holocene
14 uplift rate of 0.6 – 0.8 mm/yr. Approximately 30 km north, the range-front fault has truncated
15 and uplifted the Penrod Canyon fan remnant ~40 m since the surface was abandoned after ~113
16 ka, based on cosmogenic dating of two large boulders. These data permit a best estimate of the
17 late Pleistocene vertical uplift rate between 0.3-0.4 mm/yr along the Wassuk Range fault zone.
18
2
19 Introduction
20 The Wassuk Range fault zone is an active, east-dipping normal fault that strikes north-
21 northwest for a distance of over 80 km along the eastern margin of the Wassuk Range, forming
22 the western boundary of the Walker Lake basin (Figure 1). Thermochronologic analysis suggests
23 that rapid extensional deformation and uplift of the Wassuk Range occurred between ~15-12 Ma,
24 with renewed uplift along the present-day, high-angle, rangefront fault beginning ~4 Ma
25 (Surpless et al., 2002; Stockli et al., 2002). Reaching elevations of over 3,400 m, the Wassuk
26 Range is a major tectonic feature in the Central Walker Lane: a complex zone of transtensional
27 faulting that separates the extending Basin and Range from the rigid Sierra Nevada block and
28 accommodates up to 10 mm/yr of Pacific-North American relative right-lateral plate motion (e.g.
29 Thatcher et al., 1999; Bennett et al., 2003; Oldow et al., 2001; Hammond and Thatcher, 2007).
30 Strain in the Central Walker Lane is strongly partitioned into a zone of dextral-dominated
31 deformation to the east of the Walker Lake basin and extension-dominated deformation to the
32 west (e.g., Oldow, 2003; Wesnousky, 2005; Surpless, 2008). Previous geologic studies show no
33 evidence for significant dextral deformation along the Wassuk Range fault zone (e.g., Dilles,
34 1993; Stockli et al., 2002; Surpless, 2011), making the active, range-bounding fault ideal to
35 investigate vertical slip rates related to extensional deformation. We report observations from
36 two locations along the Wassuk Range that help constrain the earthquake history and uplift rate
37 along the fault in an effort to add information to regional seismic hazard analysis (Figures 1 and
38 2). At Rose Creek, two trenches excavated across the fault yield information about the size and
39 timing of Holocene earthquakes and an estimate of the Holocene uplift rate. The second site is at
40 Penrod Canyon, where cosmogenic dating of two large boulders on an uplifted, abandoned fan
41 remnant allows an estimate of the late Pleistocene uplift rate.
3 42
43 Rose Creek alluvial fan
44 Rose Creek drains the highest portion of the Wassuk Range and has produced a large fan
45 on the eastern flank of the rangefront (Figure 2). The Wassuk Range fault zone cuts the apex of
46 the Rose Creek fan at an elevation of ~1525 m, well above the ~1330 m 13 ka late Pleistocene
47 Lake Lahontan highstand (Figure 2; Adams and Wesnousky, 1999). The fault is expressed by
48 scarps with vertical separations of 1-2 m and 5.5-7 m in Holocene alluvial fan deposits Qy2 and
49 Qy1, respectively (Figure 3). We excavated and mapped trench exposures across the small and
50 large scarps to quantify the timing, displacement, and recurrence of slip on the fault.
51
52 Rose Creek North trench
53 The Rose Creek North (RCN) trench was excavated across a ~1 m scarp cutting the Qy2
54 alluvial fan surface to the northwest of Rose Creek (Figure 3). The ~20 m long trench exposed
55 alluvial fan gravels offset across a series of normal fault strands (Figure 4a). At the base of the
56 footwall, unit 1 is composed of a fine-grained, alluvial gravel layer overlain by a coarse debris
57 flow deposit and a younger fine-grained, alluvial gravel layer. Sitting above and in fault contact
58 with unit 1 across fault strand B, unit 2 is scarp-derived colluvium and fissure fill composed of
59 loosely consolidated pebbles, cobbles, and small boulders in a sandy matrix. A coherent block of
60 the coarse debris flow member of unit 1 is entrained in the unit 2 fissure fill. Unit 3 is a coarse
61 debris flow deposit that overlies unit 2. Fault strand B offsets footwall units 1, 2, and 3. Units 2
62 and 3 are further offset and truncated by fault strand A. In the hanging wall, the beds of unit 4 are
63 similar in composition to footwall units 1, 2, and 3, though correlation of individual beds across
64 the fault zone is ambiguous. A wedge-shaped package (unit 5) of east dipping and upward-fining
4
65 scarp-derived colluvium extends eastward from fault strand A to overlie unit 4. A fissure
66 extending downward from the base of the wedge is filled with unit 5 sands and gravels (subunit
67 5’). Unit 6 is a charcoal-bearing, alluvial deposit that overlies units 4 and 5. It is composed of
68 brownish–tan, sandy matrix-supported fan gravels that become increasingly silt-rich adjacent to
69 the fault. Subunit 6a is a waterlain, reworked-tephra bearing lens that overlies a charcoal-rich
70 burn layer ~0.15 m above the base of unit 6. Tephra samples RCN-T1 and -T2, taken from unit
71 6a, are regionally correlated with late Holocene Mono Craters volcanism ~600-2000 14C yr B.P.
72 (Wesnousky, 2005; J. Bell, Nevada Bureau of Mines and Geology, Reno, Nevada, personal
73 communication, 2010). The youngest unit in the trench, unit 7, rests on the scarp face of fault
74 strand A and is a wedge-shaped package of scarp-derived colluvium composed of tan, silty/sandy
75 matrix-supported pebble gravels. Subunit 7’ is a small fissure filled with unit 7 gravels along the
76 westernmost fault strand (strand B). The basal contact of unit 7 overlies a weakly developed Av
77 horizon that caps unit 6. Unit 7 contains two distinct burn layers, depicted as dark grey lenses
78 (Figure 4a). Upslope from the fault, the matrix of unit 7 becomes more sand rich, reflecting an
79 influx of fan material from a small alluvial cone at the base of the rangefront adjacent to the
80 trench (Figure 3). The cone alluvium covers the hanging wall fan surface (unit 3) near the trench
81 and obscures the 1 m fault scarp.
82 The hanging wall stratigraphy in the north trench exposure is interpreted to record at least
83 two surface-rupturing earthquakes. The wedge-shaped unit 7 and fault-bounded subunit 7’ are
84 interpreted to be scarp-derived colluvium and fissure fill resulting from the most recent
85 movement on fault strands A and B (Figure 4a). Displacement for the event estimated by
86 thickness of the colluvial wedge is ~1.0 m, approximately the same height as the surficial scarp
87 near the trench. The radiocarbon analysis of the youngest charcoal sample taken from the upper
5 88 portion of unit 6, ~0.3 m below the basal contact of unit 7, is 61457 cal yr B.P. (sample RCN-
89 RC14 in Figure 4a; Table S1). Charcoal samples from the burn layer in unit 7 yield modern ages
90 and thus are of limited utility in further constraining the age of faulting (samples RCN-RC10 and
91 RCN-RC13 in Figure 4a and Table S1). This limits the occurrence of the most recent
92 displacement to be post ~614 cal yr B.P.
93 The interpretation of a second earthquake recorded in the hanging wall sediments is
94 based on unit 5’s wedged shape, association with a fissure (subunit 5’), and fault bound contact
95 with unit 2. These features indicate unit 5 was formed by colluvium shed off a scarp produced
96 by slip on fault strand A and was subsequently displaced during the most recent event. The
97 radiocarbon age of charcoal sampled from directly above the basal contact of unit 6 is 146168
98 cal yr B.P. (sample RCN-RC1 in Figure 4a; supplemental Table S1). This sample constrains the
99 deposition of unit 6 to after ~1461 cal yr B.P. and limits the minimum age of the earthquake that
100 produced colluvial unit 5, hereafter referred to as the pre 1461 cal yr B.P. event. The surficial
101 geology near the trench suggests that the fan alluvium of unit 6 sourced from a small alluvial
102 cone to the west of the trench (Figure 3) and was deposited atop the scarp-derived colluvium of
103 unit 5 as growth stratigraphy against the pre 1461 cal yr B.P. event scarp. This interpretation
104 suggests the combined thickness of units 5 and 6 adjacent to fault strand A reflects a minimum
105 event displacement of ~1.6 m for pre 1461 cal yr B.P. event (Figure 4a). However, the observed
106 increase in the silt content of unit 6 near fault strand A may represent a pulse of sedimentation
107 resulting from an additional scarp forming earthquake that occurred closely in time to ~1461 cal
108 yr B.P. (as suggested by the location of sample RCN-RC1 at the base of unit 6). If units 5 and 6
109 resulted from separate earthquakes, the minimum event displacement related to the pre 1461 cal
110 yr B.P. and ~1461 cal yr B.P. ruptures is respectively ~1.3 m and ~0.3 m.
6
111 In the footwall, the truncation of unit 1 along fault strand B and the wedge-shape of unit 2
112 record the occurrence of at least one additional earthquake prior to ~1461 cal yr B.P. The
113 absence of dateable materials within units 1 and 2 and correlative hanging wall deposits
114 precludes further characterization of the earthquakes recorded in footwall stratigraphy.
115
116 Rose Creek South Trench
117 The Rose Creek South (RCS) trench was excavated across a ~5.5 m scarp cutting the Qy1
118 alluvial surface to the southeast of Rose Creek (Figure 3). The ~30 m long trench exposes a thick
119 package of coarse debris flow and colluvium deposits (Figure 4b). The oldest unit exposed in the
120 trench (unit 1) is a debris flow deposit capped by a 20-30 cm thick, black, organic-rich, charcoal
121 bearing, peat-like layer at the base of the trench. This unit is overlain by a thick package of
122 matrix-supported, debris flows containing angular, large boulders (unit 2). Slight reddening at
123 the contacts of individual flow deposits (~1.0 m average thickness) reveals a generally tabular
124 fabric in the otherwise massive package. Footwall units 1 and 2 are truncated by an eastward-
125 dipping normal fault. The hanging wall is similar in composition and texture to unit 2 and is
126 labeled unit 3. Slight reddening along contacts between individual sediment packages within unit
127 3 is evident toward the easternmost portion of the exposure, but evidence of bedding or
128 horizonation is absent in the western portion of the unit, closer to the fault. Where observed, the
129 average bedding thickness in unit 3 (~0.5 m average) is distinctly less than that recorded in unit
130 2. Fissures filled with loose, reddish-brown sands and vertically aligned clasts (unit 4) cut units 2
131 and 3. Unit 5 is a slope-wash deposit that overlies the eastern end of unit 3 and is buried by a
132 fluvial, reworked tephra-bearing deposit (unit 6). Modern slope-wash and aeolian deposits (unit
133 7) cap the entire exposure.
7 134 The coarse and massive deposits exposed in the trench prevent from the extraction of a
135 detailed earthquake chronology from the exposure. Deposition of the Qy1 surficial fan deposits,
136 in which the trench was excavated, postdates the deposition of the peat-like layer that caps unit 1,
137 radiocarbon dated at 9400±97 cal yr B.P. (RCS-RC1 in Figure 4b; Table S1). Accordingly, the
138 5.5-7 m scarp that truncates the Qy1 fan surface must result from displacements occurring after
139 9400±95 cal yr B.P. The massive nature of unit 3 near the contact with the footwall is consistent
140 with an interpretation that unit 3 is fault-derived colluvium resulting from at least one
141 earthquake. The eastward gradation of unit 3 into a number of distinct layers capped by incipient
142 soil development allows a speculative interpretation that each layer represents aggradation at the
143 distal end of a colluvial wedge subsequent to individual earthquake offsets; however, this
144 interpretation is complicated by the observation that the deposits appear to be emplaced by
145 energetic debris flow processes associated with flash floods rather than colluvial deposition.
146 Withstanding this uncertainty, fissures (filled by unit 4) that cut units 2 and 3 are capped by
147 modern deposits and record at least one earthquake subsequent to 2806±50 cal yr B.P, the
148 radiocarbon age of a charcoal sample found within a displaced soil block in the fissure fill
149 (Sample RCS-RC10 in Figure 4b and Table S1). There is a possible small colluvial deposit (unit
150 4?) associated with the fissure-producing earthquake along the upward continuation of the main
151 fault strand, but this relationship is obscured by disturbed stratigraphy due to the presence of a
152 large boulder in the trench wall. A reworked tephra deposit in the hanging wall (unit 6, samples
153 RCS-T1 and RCS-T2) is correlated with Mono Craters volcanism between ~600-2000 14C yr
154 B.P. Deposition of the tephra in the trench wall postdates faulting associated with the
155 development of unit 3. However, the age relationship between the tephra bearing gravels
8
156 (subunit 6a within unit 6) and the event or events that produced the fissures (unit 4) is
157 ambiguous.
158
159 Holocene Uplift Rate
160 The fault scarp on the Qy1 surface adjacent to the south trench shows a vertical
161 separation of 5.5-7 m (Figure 3). The radiocarbon date obtained from the base of the hanging
162 wall in the southern trench (sample RCS-RC1 in Figure 5b and Table S1) constrains the age of
163 the fan surface to be less than ~940095 cal yr B.P. The stratigraphy in both the north and south
164 trenches indicate that the vertical separation of the Qy1 surface results from multiple
165 earthquakes. Dividing 5.5-7 m by 9400±95 yr yields an estimate of Holocene vertical uplift rate
166 at 0.6-0.8 mm/yr.
167
168 Penrod Canyon alluvial fan
169 At the mouth of Penrod Canyon, Quaternary uplift is recorded by a fault scarp that
170 truncates an abandoned alluvial fan surface (Figure 5). The escarpment displays eroded wave-cut
171 benches created by late Pleistocene pluvial lake high-stands (Figure 5) and strikes approximately
172 N30E at a right step in the NNW-striking range front fault system (Figure 2). The large
173 escarpment cuts middle to late Pleistocene fan and lacustrine platform veneer deposits (House
174 and Adams, 2009) and is paralleled by 3-7 m fault scarps that offset the adjacent Holocene fan
175 deposits (Figure 5). The proximal location and parallel orientation of the young fault scarps
176 relative to the wave-modified scarp indicate the predominantly tectonic origin of the large
177 escarpment and abandoned fan surface (Wesnousky, 2005). Detailed topographic profiles across
178 the larger scarp reveal approximate offsets of 30 m, 40 m, 41 m, and 39 m (Figure 5). Cross
9 179 section A-A’ is based on a profile from the highest elevation of the abandoned alluvial fan
180 surface southeast across the escarpment to the active alluvial fan. At this location, ~40 m of
181 vertical uplift is evident between the abandoned and active alluvial surfaces (Figure 5).
182 We sampled two large granitic boulders on the abandoned fan remnant (Figure 5) to
183 determine the length of time these rocks have been exposed to cosmogenic radiation on the fan
184 surface (Gosse and Phillips, 2001). The samples were prepared and the amounts of 10Be and 26Al
185 in the rocks were measured at the PRIME Lab of Purdue University (Sharma et al., 2000). BeO
186 and Al2O3 were purified from the quartz portion of the samples, following procedures developed
187 by Kohl and Nishiizumi (2000). The 10Be and 26Al exposure ages were determined using the
188 CRONUS-Earth online exposure age calculator, version 2.2 (hess.ess.washington.edu/math/)
189 (Balco et al., 2008). We calculated 10Be and 26Al exposure ages assuming no erosion for each of
190 the three samples taken from the two boulders, resulting in a total of six ages. The individual
191 10Be and 26Al concentrations, model ages, and sample information are detailed in Table S2,
192 available with field photos of the boulders in the electronic supplement to this paper. Four of the
193 age estimates result from one boulder (samples C2 and CWL1). This boulder yields 10Be ages of
194 84.5±1.7 ka and 111.9±2.7 ka and 26Al ages of 108.8±4.8 ka and 118.1±6.4 ka for samples C2
195 and CWL1, respectively. Three of these four exposure ages agree to within the 1σanalytical
196 uncertainties, but the C2 10Be age falls outside the 1σ range. It is unusual for sample replicates
197 from the same boulder to vary this much, especially given the concordance of the 26Al results;
198 however there were no obvious problems with the chemistry or the measurements. We sum the
199 individual probability density functions to determine the mean age for the boulder and calculate
200 the reduced χ2 statistic to determine the significance of the age groupings (Balco et al., 2009 &
201 2011; Schaefer et al., 2009; Rood et al., 2011a). Using all four C2 and CWL1 samples results in
10
202 a mean age for the boulder with a 1σ uncertainty of 105.8±13.5 ka and reduced χ2 value of 26.5.
203 If the C2 10Be age is excluded from the calculations, the resulting mean age of the three-sample
204 set is 112.9±6.25 ka with a reduced χ2 value of 0.785 (summary probability density function
205 diagrams and statistics available in the electronic supplement). The low reduced χ2 value of the
206 three-sample estimate gives us confidence in three-sample grouping, and we use the age of
207 112.9±6.25 ka for the boulder. The 10Be and 26Al age for boulder C1 are 75.7±1.7 ka and
208 89.6±4.6 ka. These estimates do not agree to within 1σ uncertainties. Because the 26Al dataset is
209 concordant for the C2/CWL1 samples and the C1 and C2 sample 26Al/10Be ratios are higher than
210 the accepted production ratio of 6.75 (Nishiizumi et al., 2007), we use the 26Al age of 89.6±4.6
211 ka as the exposure age of the C2 boulder. Although the ages of the two boulders are not in tight
212 agreement it is not unusual to see spread in boulder age estimates for depositional features of this
213 age (e.g. Heyman et al., 2011; Rood et al., 2011a). Surface exposure ages may be affected by a
214 number of geological processes. Weathering, exhumation, and shielding of surfaces by sediment
215 or snow will lead to exposure ages that are less than the true age of the landform (e.g. Heyman et
216 al., 2011; Owen et al., 2011). In contrast, surface exposure of rocks prior to deposition on a fan
217 surface will result in an overestimation of the landform. Recognizing these uncertainties and the
218 small sample size (n=2 boulders), we simply assume the age of the fan surface is best
219 represented by our age estimate for the older boulder (sample C2/CWL1) of 112.9±12.5 ka (2σ).
220
221 Late Pleistocene Uplift Rate
222 The vertical offset and exposure age data permit an estimate of a time-averaged late
223 Pleistocene uplift rate along the Wassuk Range fault zone. Dividing the scarp height (~40 m) by
224 the fan surface age estimate of 112.9±12.5 ka results in an initial estimate of Late Pleistocene
11 225 range-front uplift equal to about 0.3-0.4 mm/yr. Because our cosmogenic exposure ages assume
226 zero erosion on the sampled boulder, the ages are minimum estimates. In light of this
227 uncertainty, the actual uplift rate may be lower than 0.3-0.4 mm/yr if the fan surface is older than
228 112.9±12.5 ka.
229
230 Discussion and Conclusions
231 Observations from the Rose Creek trenches provide information on the frequency, size,
232 and timing of surface rupturing earthquakes on the Wassuk Range fault zone. Both trench
233 exposures display records of multiple Holocene earthquakes, although the correlation of
234 individual events between the two trenches is problematic. The most recent event recorded in the
235 northern trench occurred after 614±57 cal yr B.P. with ~1 m of displacement. Prior fault
236 displacement may be explained by either the occurrence of a single >1.6 m displacement prior to
237 1461±68 cal yr B.P. or two events of >0.3 m and >1.3 m at and before about 1461±68 cal yr
238 B.P, respectively. The footwall stratigraphy shows evidence for at least one additional earlier
239 earthquake; however, the lack of datable materials within the footwall prevents further
240 development of an earthquake history for the fault. The ~1 m surface scarp at the northern trench
241 location represents slip from only the most recent event. The southern trench shows that multiple
242 surface rupturing earthquakes have resulted in a ~5.5-7 m scarp on the Qy1 fan surface since
243 9400±95 cal yr B.P., with the most recent displacement post-dating 2806±50 cal yr B.P. The
244 height of the scarp and the thickness of the hanging wall debris flow deposits (unit 3, Figure 4b)
245 suggest the likelihood of additional faulting events between 9400±95 cal yr B.P. and the fissure-
246 producing event, but the lack of distinct colluvial stratigraphy and age constraints in the exposure
247 prevents further characterization of individual displacements during this time. The limited
12
248 interpretation of the southern exposure is consistent with the earthquake history in the northern
249 trench. Our results support the interpretation of the geomorphology and soils by Demsey (1987)
250 suggesting that the Rose Creek fan head scarps record multiple earthquakes on Holocene age
251 alluvial surfaces.
252 Two lines of evidence point to a late Pleistocene-Holocene vertical uplift rate along the
253 Wassuk Range fault zone between 0.3 and 0.8 mm/yr. The multiple event scarp in the Qy1
254 surface at Rose Creek records 5.5-7 m of vertical separation since 9400±95 cal yr B.P., providing
255 the basis to estimate a Holocene uplift rate of 0.6-0.8 mm/yr. At Penrod Canyon, cosmogenic
256 exposure dating suggests that the age of the abandoned fan’s upper surface is 112.9±12.5 ka, and
257 scarp profiles indicate that was fan was tectonically uplifted 40 m above the correlative lower
258 surface now buried by alluvium. Dividing the offset by the cosmogenic exposure age results in
259 an estimated uplift rate of 0.3-0.4 mm/yr. It is likely because of the short time period over which
260 the Rose Creek rate is determined and the small number of boulders sampled (n=2) at Penrod
261 Canyon that the formal uncertainties attached to the rate estimates are less than the actual
262 uncertainties. Thus, we hesitate to conclude that the rates are significantly different. In this
263 regard, the observations presented in this paper suggest a late Pleistocene-Holocene uplift rate of
264 ~0.3-0.8 mm/yr. This rate is very similar to the estimated post-Pliocene time-averaged uplift rate
265 of 0.5 – 0.75 mm/yr for the Wassuk Range fault system (Stockli et al., 2002) and is consistent
266 with a previous Holocene uplift rate estimate of 0.4-0.5 mm/yr based on fault zone
267 geomorphology (Demsey, 1987).
268 dePolo and Anderson (2000) find that normal faults with the fastest vertical uplift rates in
269 Nevada are generally located within the Walker Lane. In the Walker Lane, late Pleistocene
270 uplift rates for the normal fault-bounded basins range from 0.2-3 mm/yr, with the fastest rates on
13 271 the Sierra Nevada frontal fault system (Ramelli et al., 1999; Brothers et al., 2009; Dingler et al.,
272 2009; Rood et al., 2011b; Wesnousky and Caffee, 2011; Sarmeinto et al., 2011). In contrast,
273 uplift rates in the interior portion of the Great Basin are an order of magnitude lower than in the
274 Walker Lane, generally between 0.01-0.4 mm/yr (dePolo and Anderson, 2000). The Wassuk
275 Range Holocene-late Pleistocene vertical uplift rate of 0.3-0.8 mm/yr is similar to previously
276 reported uplift rates for normal faults in the Walker Lane and is significantly higher than uplift
277 rates on faults in the interior Great Basin. Thus, with evidence for at least two Holocene surface
278 rupturing earthquakes and relatively high long-term vertical slip rates, the Wassuk Range fault
279 zone is a significant source of seismic hazard in the Central Walker Lane.
280
281 Data and Resources
282 All data used in this paper was collected during the duration of this study or came from
283 published sources listed in the references.
14
284 Acknowledgements
285 This work was supported by the National Science Foundation grant EAR-0635757 as part
286 of the EarthScope project. We are grateful to the Hawthorne Army Depot and John Peterson for
287 permitting us access to the Rose Creek site. Thanks to Alex Sarmiento for field assistance and
288 many insightful comments. Thank you to John Bell for helpful conversation regarding the age of
289 faulting along the Wassuk Range and to Kyle House for his wealth of knowledge regarding the
290 use of LiDAR data. Thanks also to Steve Personius, Rich Briggs, Ryan Gold, Tony Crone, R.
291 Jayangondaperumal, Ricardo Civico, and the participants of the 2010 Pacific Cell Friends of the
292 Pleistocene field trip for valuable discussion, observations and questions that further refined this
293 research. Rich Koehler read an early draft of this manuscript and provided many thoughtful
294 insights and suggestions.
295
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18
364 Stockli, D. F., B. E. Surpless, and T. A. Dumitru (2002). Thermochronological constraints on the
365 timing and magnitude of Miocene and Pliocene extension in the central Wassuk Range,
366 western Nevada, Tectonics 21, doi: 10.1029/2001TC001295.
367 Surpless, B. E., D. F. Stockli, T. A. Dumitru, and E. L. Miller (2002). Two-phase westward
368 encroachment of Basin and Range extension into the northern Sierra Nevada, Tectonics 21,
369 doi: 10.1029/2000TC001257.
370 Surpless, B.E. (2011). Cenozoic tectonic evolution of the central Wassuk Range, western
371 Nevada, USA, Int. Geol. Rev., doi: 10.1080/00206814.2010.548117.
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373 Present-day deformation across the Basin and Range province, western United States,
374 Science 283 1714-1718, doi: 10.1126/science.283.5408.1714.
375 U.S. Geological Survey, Nevada Bureau of Mines and Geology, and the California Geological
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377 http://earthquake.usgs.gov/regional/qfaults/.
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385
19 386 Author’s Affiliations and Addresses
387 Jayne M. Bormann1,2, Benjamin E. Surpless3, Steven G. Wesnousky1, and Marc W. Caffee4
388
389 1Center for Neotectonic Studies
390 University of Nevada, Reno
391 Mail Stop 169
392 Reno, Nevada 89557
393
394 2Nevada Geodetic Laboratory
395 Nevada Bureau of Mines and Geology
396 University of Nevada, Reno
397 Mail Stop 178
398 Reno, NV 89557
399
400 3Deparment of Geosciences
401 Trinity University
402 One Trinity Place
403 San Antonio, TX 78212-7200
404
405 4Dept. of Physics, PRIME Lab
406 Purdue University
407 Lafayette, IN 47906
408
20
409 Figure Captions
410
411 Figure 1. The Wassuk Range fault zone in relation to faults within the Walker Lane. Box in inset
412 map shows area of detail and major faults in Nevada and California. Dashed lines mark the
413 boundaries of the Walker Lane. Strike slip faults of the Central Walker Lane are black, normal
414 faults are white. Lake Tahoe (LT), Walker Lake (WL), and Mono Lake (ML) are shown for
415 geographic reference. Faults are modified from the USGS (2006).
416
417 Figure 2. Map of the Wassuk Range fault zone. Faults are shown in relation to Quaternary
418 surficial deposits. Location of the Rose Creek and Penrod Canyon study sites are outlined with
419 black boxes and labeled.
420
421 Figure 3. Rose Creek alluvial fan paleoseismic and slip rate site. See Figure 2 for site location.
422 (a) Aerial photograph showing the location of the north (RCN) and south (RCS) trenches along
423 the fault scarp in relationship to Rose Creek. (b) Map showing the relationship of the fault to
424 Quaternary surficial deposits at the mouth of Rose Canyon. Mapping is based on field
425 observations and low sun angle air photos, modifying the work of Demsey (1987). The fault is
426 marked as a black line with ticks on the down-thrown side. Trench locations are indicated with
427 thick lines and are labeled RCN (north trench) and RCS (south trench). Numerical annotations
428 indicate vertical separation across surveyed fault scarp profiles (small circles). 1m topographic
429 basemap constructed from a lidar derived DEM of Walker Lake (USGS, 2008).
430
21 431 Figure 4. Sketch logs of the (a) northern and (b) southern trench exposures across the Wassuk
432 Range fault zone at the Rose Creek fan. Trench locations are shown in Figure 3. Unit label
433 numbers correspond to descriptions in text. Tephra (gray stars) and radiocarbon (black circles)
434 sample locations are shown. In the northern trench (a), the major fault strands are labeled (A-B).
435 See text for discussion.
436
437 Figure 5. Penrod Canyon alluvial fan slip rate site. See Figure 2 for location. (upper) Abandoned
438 fan surface truncated by the Wassuk Range fault zone at the mouth of Penrod Canyon. Location
439 of the boulders sampled for cosmogenic nuclide analysis is marked with a white star. Numerical
440 annotations indicate vertical separation across surveyed scarp profiles (dashed black lines).
441 Location of illustrated scarp profile marked A-A’. Photograph modified from Wesnousky
442 (2005). (lower) Elevation profile across the ~40 m large wave modified fault scarp. Approximate
443 location of the Holocene (H) and late Quaternary (Q) fault traces marked with red dashed lines.
444
22 Figure Figure 1
120°W 119°W 118°W
West Tahoe fault Reno Walker Lane Basin Carson and LT Singatse Range fault Genoa fault Penrod Canyon City Fairview Peak fault study site Range Smith Valley fault 39°N Wassuk Range fault zone Gumdrop Hills fault
Antelope Valley fault Petrified Springs fault
Benton Springs fault N WL Rose Creek study site Hawthorne 0 102030 km Nevada Rattlesnake California Flat fault California Nevada
Excelsior fault
Candelaria fault ML Coaldate fault 38°N Figure 2
5 miles
White Mountain Qy 39°00' 118°45' Qo
QL Schurz N
QL Qo
Qh
QL R ee se R ive n r Canyo Penrod Canyon slip rate site QL Canyo Fig. 6 rod n en P Qo Qh ?
QL
n nyo Ca W per p Co a
l
k
e QL r
L
a
k
e W
a
QL
s
Rose Creek alluvial fan
s Site of RCN and RCS trenches.
QL u Figs. 3, 4, and 5 k ee r C Qh k ose ? R
Mt. Grant
3425m QL R ?
Hawthorne a
n 38°30'
g 118°45'
e
Qi n o y n a C h ort N Qh Historical lake deposits
Youngest alluvial deposits Qy and surfaces
Pleistocene lake deposits QL and surfaces Qi
Intermediate age alluvial deposits
Qi t and pediment surfaces a l
F
Oldest alluvial fan deposits and
Qo Qy y pediment surfaces k
s i Rx Undifferentiated, generally bedrock. h W
Faults and wave-modified fault scarps
Qi modified from Wesnousky, 2005 Figure 3
(a)
North South trench trench
Rose Creek
1540 1540 Rx Rx (b)1555 1535 1545 Qa 1530 1550 Qy2 RCN
Rx Qa RCS
1540 RCS Qy1 1535 Qa Qy1 1530 (1) (5) (5)(2) (5.5) (3.5) 1525 (7) Qy1 Qy2 Qa 1520 Qa 1515 Qo 1510 Qy1 Qy1
1505
Rose Creek 1510 1510 Active wash and Oldest fan deposits and Qa Qo fan deposits pediment surfaces Undifferentiated, Qy2 Young fan deposits Rx generally bedrock Qy1 Holocene fan deposits, <~9,400 cal yrs BP 90 m N (a) RCN-T1 & T2 SamplesFigure from 4 a reworked tephra RCN-RC10 RCN-RC13 143 +/- 111 deposit regionally correlated 144 +/- 112 cal yr BP with Mono Craters volcanism cal yr BP 14 RCN-RC14 600-2000 C yr BP. 6 RCN-RC15 614 +/- 57 7’ 799 +/- 104 cal yr BP RCN-RC1 3 5 cal yr BP 2 1461 +/- 68 7 cal yr BP RCN-T2 3 RCN-T1 1 4 6 5 2 3 6a W Depth (m) 1 2 4 Rose Creek 4 AB North Trench Log 1 Region obscured by Spoils large boulder. Inset E 5’ 650796 427344911N UTM NAD 1983 12 area is benched back 7 1 2 3 456 7891011 12 13 14 15 16 17 18 19 about 1m. 11 Distance (m) 2’ 10 (b) Possible MRE colluvium 7 9 4 RCS-T1 & T2 RCS-RC10 4? 2 Samples from a reworked tephra 2806 +/- 50 cal yr BP 8 deposit that are regionally ? correlated with Mono Craters W 7 14 4 volcanism 600-2000 C yr BP. 7 6 Depth (m) 5 5 4 3 1 7 ? ? RCS-RC1 4 6 ? 9400 +/- 95 cal yr BP ? ? ? ? 3 3 ? ? 2 E Rose Creek South Trench Log 1 650638 4273340 11N UTM NAD 1983
1 2 3 456 7891011 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Distance (m) Supplemental Material Common.Links.ClickHereToDownload