Evolution of the Central Garlock Fault Zone, California: a Major Sinistral Fault Embedded in a Dextral Plate Margin

Evolution of the central Garlock fault zone, California: A major sinistral fault embedded in a dextral plate margin

Joseph E. Andrew1,†, J. Douglas Walker1, and Francis C. Monastero2 1Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA 28597 Timaru Trail, Reno, Nevada 89523, USA

ABSTRACT and the creation of new faults. The geologic A major hurdle to understanding the tec tonics slip rates for the Garlock fault in the LMSR of the Garlock fault has been establishing a The Garlock fault is an integral part of match with and explain along-strike varia- detailed slip history, because the age of all of the the plate-boundary deformation system tions in neotectonic rates. known slip markers predate the initiation of slip. inboard of the San Andreas fault (Califor- The total slip on the Garlock fault is ~64 km nia, USA); however, the Garlock is trans- INTRODUCTION using offset pre-Cenozoic features (see Fig. 1 versely oriented and has the opposite sense and Table 1 for descriptions and references); of shear. The slip history of the Garlock A quarter of the North American to Pacifi c however, slip initiated after 17 Ma (Monastero is critical for interpreting the deforma- plate displacement is actively accommodated by et al., 1997) and possibly at 11 Ma (Burbank tion of the through-going dextral shear of dextral shear in eastern California (Miller et al., and Whistler, 1987; Blythe and Longinotti, the Walker Lane belt–Eastern California 2001). The Garlock fault (Fig. 1 inset map) is an 2013). The Garlock has neotectonic slip rates of shear zone. The Lava Mountains–Summit active large-magnitude sinistral-slip fault (Hess, 4.5–14 mm/yr (Clark and Lajoie, 1974; McGill Range (LMSR), located along the central 1910; Hulin, 1925; Dibblee, 1952; Smith, 1962) and Sieh, 1993; McGill et al., 2009; Rittase Garlock fault, is a Miocene volcanic center embedded in and perpendicular to this zone of et al., 2014), but most of these rates are too rapid that holds the key to unraveling the fault dextral shear. The Garlock is a fundamental if they are applied to the 11 m.y. history of the slip and development of the Garlock. The structure in this deformation zone, as it separates Garlock fault (64 km over 11 m.y. = 5.8 mm/yr). LMSR is also located at the intersection of the dextral transpressive Eastern California shear This paper presents new stratigraphic, the NNW-striking dextral Blackwater fault zone to its south (Dokka and Travis, 1990) from structural, and geochronologic data to defi ne and contains several sinistral WSW-strik- the dextral transtensional Walker Lane belt to a detailed history for the Garlock fault. This ing structures that provide a framework its north (Stewart, 1988). Active NNW-striking study focuses on the central segment of fault in for establishing the relationship between dextral fault systems both north and south of the the Lava Mountains–Summit Range (LMSR) the sinistral Garlock fault system and the Garlock fault have abundant seismicity (Unruh area, a Miocene volcanic center located imme- dextral Eastern California shear zone. New and Hauksson, 2009) and offsets of 2–12 km diately adjacent to the Garlock fault (Smith, fi eld mapping and geochronology data (Monastero et al., 2002; Glazner et al., 2000; 1964; Dibblee , 1967; Smith et al., 2002). Pre- (40Ar/39Ar and U-Pb) show fi ve distinct Oskin and Iriondo, 2004; Casey et al., 2008; vious workers suggested that Miocene and suites of volcanic-sedimentary rock units in Frankel et al., 2008; Andrew and Walker, 2009). younger strata in the LMSR can be correlated the LMSR overlain by Pliocene exotic-clast Geodetic data show that dextral shear crosses the across the fault (Carter, 1994; Smith et al., 2002; conglomerates. This stratigraphy coupled Garlock fault unimpeded (Peltzer et al., 2001; Frankel et al., 2008), thus offering the potential with fi fteen fault slip markers defi ne a Gan et al., 2003) with an additional component to understand the slip history in detail. It is also three-stage history for the central Garlock of northwest-directed extension north of the Gar- an important study site because it contains the fault system of 11–7 Ma, 7–3.8 Ma, and lock fault (Savage et al., 2001). Despite the activ- northern terminus of the dextral Blackwater 3.8–0 Ma. Pliocene to recent slip occurs in a ity and signifi cant offsets on the NNW-striking fault, a major fault of the Eastern California ~12-km-wide zone and accounts for ~33 km faults, these faults nowhere cut the Garlock fault shear zone. This paper describes the stratig- or 51% of the total 64 km of left-lateral off- (Noble, 1926). The Garlock fault has a curved raphy of the LMSR and the geometry and set on the Garlock fault in the vicinity of trace, with its central and eastern segments kinematics of the structures, to identify numer- the LMSR since 3.8 Ma. This history yields rotated clockwise relative to its straight western ous offset markers to restore fault movements. slip rates of 6–9 mm/yr for the younger segment (Fig. 1). This change in strike has been The goal of this paper is to use these data and stage and slower rates for older stages. The interpreted as oroclinal bending that accommo- interpretations to construct the slip history of LMSR internally accommodates north- dates dextral shear in the Mojave Desert (Gar- the Garlock and associated faults. This history west-directed dextral slip associated with funkel, 1974; Schermer et al., 1996; Guest et al., is needed to resolve the initiation of slip on the the Eastern California shear zone–Walker 2003; Gan et al., 2003). Most models of this Garlock fault system and the current structural Lane belt via multiple processes of lateral oroclinal bending are based on dextral shear and confi guration, and to evaluate major changes in tectonic escape, folding, normal faulting, counterclockwise rotation of crustal blocks, but slip magnitude, fault geometry, and deformation the corners of these blocks with the Garlock fault style. We consider the implications of the cur- †E-mail: [email protected] have not been studied. rent structural confi guration to create a model

GSA Bulletin; Month/Month 2014; v. 1xx; no. X/X; p. 1–23; doi: 10.1130/B31027.1; 13 ﬁ gures; 4 tables; Data Repository item 2014297.

35°45′ : : QT Plio- Indian J 5 0K 10 kilometers 60 Pleistocene Wells J SPVF K Valley alley PR

Tm Tertiary : K D Southern : J e N (Miocene) Y at Cerro Coso fault fault Tm h

QT Va

Tp Tertiary : QT lle STS J y (Paleocene) : IDS Searles V Slate RangeE J fa

Tm Y u

K Cretaceous lt IDS ESTS lt u Garlock fault . a IDS : f J Figure 2 QT n ASF J AM J Jurassic o PKV

y Tm Granite Mountains . J Triassic n a Tp Garlock fault J

: Paleozoic C J . f QT J Tm f K

Proterozoic i Y l

Lava ECDS J C BF Goldstone . EC Lake fault – El Paso Mountains NevadaSierra Lane Walkerbelt MC QT Mountains San 1

2 Central

: K Tm

Tm 0 CC Andreas fault Basin & ° 36° DSF K Range K aso fault NV Sierra MDS El P RM CA Garlock fault Garlock shear Czone Eastern Nevada Harper Lake fault K fault a Summit lifornia Cantil fault K Pacific Range Ocean Mojave SESD 35°15′ Desert – PL K 1 Tm K QT –117° 0 200 km 16° K 34°K WGF –118° Tm BM

Figure 1. Simpliﬁ ed geologic map of the central and eastern Garlock fault (California, USA). Inset map shows the Garlock fault (thickest line), Figure 1 area (red line), geologic provinces, other features, and the border between California (CA) and Nevada (NV). Previously published total offset constraints for the Garlock fault are shown in bold text (see Table 1 for abbreviations and descriptions). Fault abbreviations: BF—Blackwater fault; WGF—western Garlock fault; BM—Black Mountain; DSF—Dove Spring Formation; ECDS—Eagle Crags dike swarm; MDS—megacrystic dike swarm; and SESD—Southeast Sierra dikes. Geology modiﬁ ed from Walker et al. (2002). Holocene and upper Pleistocene units are shown as white.

TABLE 1. PUBLISHED CONSTRAINTS OF LEFT-LATERAL OFFSET ON THE GARLOCK FAULT Amount Labels† Feature Authors (km)* Details North South Late Jurassic Independence dike swarm (IDS) in the Spangler Hills with dikes Independence dike swarm Smith (1962) 64 IDS IDS in the Granite Mountains Eastern boundary of domain of northwest-striking faults; Piute Line (PL) in Fault domain boundary Michael (1966) 74 the southeastern Sierra Nevada and Blackwater fault (BF) in the northern PL BF Mojave Desert Late Cretaceous Rand Schist in the Rand Mountains (RM) and in the southern Rand Schist Dibblee (1967) 72 #RM Sierra Nevada Smith et al. (1968); Davis Jurassic East Sierran thrust system (ESTS) in the southern Slate Range and East Sierran thrust system 52–64 ESTS ESTS and Burchfi el (1973) in the Granite Mountains Paleozoic eugeoclinal metasedimentary and metavolcanic rocks with thrust Smith and Ketner (1970); Eugeoclinal Paleozoic rocks 48–64 faults in Mesquite Canyon (MC) of the El Paso Mountains and south of Pilot MC PKV Carr et al. (1997) Knob Valley (PKV) Basal passive margin Neoproterozoic to early Paleozoic miogeoclinal sequences in the Panamint Jahns et al. (1971) 48–64 PR AM sequence Range (PR) and Avawatz Mountains (AM) Fault that juxtaposes NNW-striking faults that juxtapose Mesozoic rocks with Proterozoic rocks; Mesozoic and Proterozoic Davis and Burchfi el (1973) 64 Southern Panamint Valley fault (SPVF) with the Arrastre Spring fault (ASF) SPVF ASF rocks in the Avawatz Mountains Early Miocene Cudahy Camp Formation (CC) of Loomis and Burbank (1988) Early Miocene volcanic fi elds Monastero et al. (1997) 64 CC EC with the Eagle Crag Volcanics (EC) of Sabin (1994) *Amount—Left-lateral offset along the Garlock fault determined for each feature, either as best estimated or as minimum and maximum amounts. †Labels—Labels for these features on Figure 1. North and south refer to the offset features on the north and south sides of the Garlock fault. #This location is west of the area of Figure 1.

of dextral shear accommodation without dex- and simpliﬁ ed on Fig. 2) and geochronologic was collected in the Sierra Nevada (MDS on tral faults cutting the Garlock fault. Lastly, we analysis (locations on Fig. 2) of key rock units Fig. 1). New volcanic rock samples from the compare our interpreted long-term slip rates to were used to better deﬁ ne and delineate the stra- LMSR (Table DR1 in the GSA Data Reposi- published neotectonic rates along the Garlock. tigraphy of the area (Fig. 3) as well as reveal the tory1) were analyzed by the 40Ar/39Ar geochro- details of faulting. nology step-heating method, mostly at the STRATIGRAPHY New Mexico Geochronology Research Labo- Geochronology ratory (analytical methods in Heizler et al. Despite the volume of previous work, there [1999] and Brueseke et al. [2007]) and one were still many undated rock units and uncer- We establish age constraints in the LMSR tainties regarding the order and stratigraphic by dating samples using the 40Ar/39Ar and U-Pb 1GSA Data Repository item 2014297, additional relationships of the Miocene units and structures zircon geochronology methods and by reinter- geochronologic data, is available at http:// www 40 39 in the LMSR. New detailed geologic mapping preting previously published Ar/ Ar ages .geosociety.org /pubs /ft2014 .htm or by request to of this area (Andrew et al., 2014; summarized of Smith et al. (2002). An additional sample editing@ geosociety .org.

2 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault Ts Ka Ka Tp D_e Tl

Hills

Black Ts LV051210-6 Dg Ts (11.66 ±0.06 Ma)

35°30′N Tp 117°20′W Ka Ma) u fault; ted by Te Ma) Tpg Tb Ka u Ma) u

Te Canyon

LV030710-5 Christmas (18.84 ±0.24 Ma) LV030710-4 Te

LV051210-5 LV050710 RWFZ Blackwater fault Te (11.221 ±0.018 *b LV050810-4 P*h map area (19.63 ±0.32 Ma) Ma) Ka u Tb (6.507 ±0.070 (10.490 ±0.054 Oskin & Iriondo (2004) northern Tpg Tpg Te Tb Te JDW-20 Te Ts LV040910

NBF (7.479 ±0.023 (19.00 ±0.22 Ma)

Little Bird fault Tb NBF Tl Ka O_c Tb Tl Ts Ta Tl LV031510 NBF Metasedimentary rocks of Holland Camp (Permian-Pennsylvanian) (Pennsylvanian) Metasedimentary rocks of Benson Well Metasedimentary rocks of Gerbracht Camp (Devonian) Metasedimentary rocks of El Paso Peaks (Devonian to Cambrian) Metasedimentary rocks of Colorado Camp (Ordovician-Cambrian) (19.37 ±0.04 Ma) Ts

Dg *b Ma)

ult zone ult P*h u D_e O_c Tb a Tl Ma) u Tl (6.522 Ta of clast)

Tl ±0.026

LV051810-8 LV031410-B

LV030710-7 fault Garlock Tl (11.62 ±0.11 Ma f Ranch Browns (10.966 ±0.042 (N.A.) Tl Tb LM1132 Tlr Tlr Tl Jl

TWF

Jl Lava Te Mountains (N.A.) LM96-16 LM96-17 Ka (7.82 ±0.22 Ma)

Ta Ma) Savoy fault Savoy x Ar) 39 Ta Ar/ Tb Tl

40 Teagle Wash Teagle Te Tb Tl QTph SD040910-1 Rand Schist (Cretaceous) Atolia Quartz Monzonite (Cretaceous) Laurel Mountain granodiorite (Jurassic) Meta-Andesite of Goler Gulch (Permian) Ma)

x (12.74 ±0.05 Summit Range Summit Jl Tl Kr Ka Pg 5 km Te Ta

GFZ-080603-1

LM96-2 (11.24 ±0.43 Ma) Tlr Pre-Cenozoic Tc Ts (7.64 ±0.18 Jl Tb Tb Jl = U-Pb age (others are age (others are = U-Pb = older age due to excess argon excess = older age due to LM96-15 u x Tpr (10.66 ±0.12 Ma) Kr Te Tb Tb Kr Cerro Coso fault Ka Tpr Ma) x Tc Tpr QTph

ed geologic map of the Lava Mountains–Summit Range. Data from inside the dashed line are from Andrew et al. (2014); Andrew from inside the dashed line are ed geologic map of the Lava Mountains–Summit Range. Data from 117°40'W SD060810-1 Ma) x (11.93 ±0.14 Ma) SD120905-2 (7.32 ±0.02 ics (10.5–11.5 Ma) ics (10.5–11.5 Jl Tc SD120905-1 (7.24 ±0.83 Jl Tpr Figure 2. Simpliﬁ Figure deno sample locations are et al. (2002). Geochronologic Walker et al. (1997), and Dibblee (1967), Carr from additional data are NBF—northern Blackwater thick lines. Fault abbreviations: Faults are age in parentheses. stars with sample name and interpreted shown as white. Pleistocene and Holocene units are Younger fault. Wash TWF—Teagle fault zone; Wash RWFZ—Randsburg D_e GFZ-85 Dg Plio-Pleistocene Sedimentary rocks conglomerate of Hardcash Gulch sedimentary rocks undifferentiated conglomerate of Golden Valley conglomerate from Red Mountain gray lava flows red flow-banded lava flows associated intrusive rocks (7.5–8.0 Ma) Almond Mountain Volcanics Bedrock Spring Formation (7–10 Ma) Summitvolcan Range Lower Miocene volcanics (15–21 Ma) Eagle Crags Volcanics Cudahy Camp Formation Goler Formation (Paleocene to Eocene)

Lava Mountains Dacite flows (6.5 Ma) (11.32 ±0.94 Ma) Garlock fault Garlock *b

Tc El Paso El Mountains P*h Tl Ts Tc Tp Tg Tlr Ta Tb Te Tg

Tpr Tpg

QTph 35°25′N Tc Cenozoic

Tg N Tg Pg Tc Goler Gulch

Geological Society of America Bulletin, Month/Month 2014 3 Andrew et al.

Smith (1964) Smith et This study conglomerate of Hardcash Gulch* al. (2002) basalt clast in Pliocene(?) QThb = containing basalt clasts Qa Qa Qa conglomerate of QThf = containing metasedimentary clasts * Golden Valley Pre-Quaternary units in the LMSR Qvb Qg Qg Qcc 11.62 ±0.11 LV030710-7 Qva Qg

Quaternary Qcc Qcc QThb Tpr = fanglomerates from Red Mountain Quaternary Quaternary conglomerate of Golden Valley* Tl Tl QThf Lava Mountains Dacite Tf, Tt, u Tgb = containing basalt clasts Ttb 6.507 ±0.070 LV050810-4 & Tvb? Ta Ta Tpr Tgb 6.522 ±0.026u LV051810-8 Tgh = containing hornblende diorite clasts upper Tp 7.24 ±0.83x SD120905-1 Tgf = containing metasedimentary clasts Pliocene x Tbs Tsd Pliocene Tgf Tgh 7.32 ±0.02 SD120905-2 Lava Mountains Dacite

upper x Tv Miocene Ttw Tld Tli 7.64 ±0.18 LM96-2 Tli = sills middle Pliocene Ts Tad Almond Mountain Volc. Almond Mountain Volcanics Tbs 7.479 ±0.023 JDW-20u pTa Tba Tax Tat 7.82 ±0.22 LM96-16 Tad = dacite lava pre-Tertiary Tf | Tbf * Tv Summit Range volc. * = newly Tbc 10.490 ±0.054u LV050710 Bedrock Spring Formation

Quaternary middle Miocene Tsr upper 10.66 ±0.12 LM96-15 Tba = arkosic sandstone Qa = alluvium Tt Miocene Tsr Tsp # = newly Qva = andesite Ts u recognized Tsx Tsh Tsf 10.966±0.042 LV031410-B Tbc = conglomerate 11.22 ±0.02 LV030710-4 Summit Range volcanics* units within Cretaceous Ka 11.24 ±0.43 GFZ-080603-1 Qvb = basalt Tsd Tsa Tsx Tsr = reddish dacite lava the LMSR Paleozoic | 11.32 ±0.94 GFZ-85 Tsc Tsp = porphyritic felsite intrusion Qg = older gravels Mio. mid. 11.66 ±0.06 LV051210-6 Qcc = Christmas Canyon Fm. Tsb 11.93 ±0.14 SD060810-1 Tertiary Tcv Tcp 12.74 ±0.05x SD040910-1 Tf = felsite Tsd = bluish dacite lava domes Mio. lower Tct Eagle Crags Volc.- Tsa = arksoic sandstone # Cudahy Camp Fm. Tsc = conglomerate Ka 18.84 ±0.24 LV030710-5 Tvb = volcanic breccia

-zoic 19.00 ±0.22 LV040910 Meso Jlm # Tl = Lava Mountains Andesite 19.37 ±0.04 LV031510 Eagle Crags Volcanics-Cudahy Camp Formation Ta = Almond Mountain Volcanics P*h 19.63 ±0.32 LV051210-5 u 40 39 Tbs = Bedrock Spring Formation *b = U-Pb age (others are Ar/ Ar) Tv = ‘other’ volcanic rocks x = older age due to excess argon Ka = Atolia Quartz Monzonite#

Paleozoic DO_e O_c Ts = sediments older than Bedrock Jlm = Laurel Mountain quartz diorite# Spring Formation New units of Smith et al. (2002) P*h = metasedimentary rocks of Holland Camp# Pre-Cenozoic Tsd = Andesite of Summit Diggings *b = metasedimentary rocks of Benson Well# pTa = Atolia Quartz Monzonite Ttw = basalt of Teagle Wash DO_e = metasedimentary rocks of El Paso Peaks# Pz = Paleozoic metamorphic rocks Tsr = Dacite of Summit Range O_c = metasedimentary rocks of Colorado Camp#

Figure 3. Stratigraphic interpretations for correlation of units for the Lava Mountains–Summit Range (LMSR). New and newly interpreted geochronologic constraints are listed to the right with the age (in Ma) and sample name (for age interpretations, see Tables 2 and 3 and Figs. 4, 5, and 6). other at the Massachusetts Institute of Tech- 2011). Compiled single zircon crystal frac- Stratigraphic Units nology (MIT; analytical methods in Hodges tions give crystallization ages of these rocks et al. [1994] and Snyder and Hodges [2000]). (Table 3; Fig. 5). Paleozoic Metasedimentary Rocks A sample containing sanidine was analyzed by We reinterpret the step-heating 40Ar/39Ar Coherent basement of metasedimentary rocks single crystal fusion. Step-heating results were analyses of Smith et al. (2002); neither the occurs in the Christmas Canyon area (Figs. 2 interpreted using plateau and inverse isochron data nor interpretive plots were published, but and 7). These rocks have relatively low metamor- methods (Fig. 4; see Table 2 for interpreta- the data were available from author Monastero phic grades and consist mostly of meta-siltstone tion of 40Ar/39Ar age data) using the software (Table DR3), who was a collaborator on the with locally dominant meta-limestone beds. We Isoplot (Ludwig, 2012). Six samples of zircon Smith et al. (2002) work. These samples were correlate these rocks to the well-studied Paleo- from felsic rocks were analyzed by chemical analyzed at the MIT lab. Our rationale for rein- zoic rocks in the nearby El Paso Mountains abrasion thermal ionization mass spectrometry terpreting these ages is that hornblende and bio- (Dibblee, 1952; Carr et al., 1997). Distinctive (CA-TIMS) at the Isotope Geology Labora- tite from the same samples yielded very differ- units within this sequence allow a stratigraphic tory at the University of Kansas (Table DR2). ent ages (e.g., Fig. 6E), but Smith et al. (2002) correlation to rocks in the eastern El Paso Moun- Samples were annealed and leached following interpreted the ages based on the hornblende tains (Fig. 2); the details of this correlation are the protocols of Mattinson (2005), then spiked ages alone. The 40Ar/39Ar hornblende spectra described further in the fault slip section. with EarthTime ET535 tracer and dissolved show evidence of excess argon (i.e., saddle- using standard U-Pb double pressure-vessel shaped plateau and non-atmospheric 40Ar/36Ar Mesozoic Plutonic and Metamorphic Rocks digestion procedures. Column-puriﬁ ed U and intercept values on Fig. 6D) (e.g., Lanphere and A few small bodies of chlorite-altered quartz Pb were run together on a VG Sector TIMS Dalrymple, 1976; Harrison and McDougall, diorite intrude metasedimentary rocks in the run in single-collector ion-counting mode for 1981), so we reinterpret the ages of these rocks Christmas Canyon area. These rocks have simi- Pb and multicollector static mode for U. Data (Table 2B) by examining plots of age spectra lar mineralogy, textures, and alteration as the were reduced and interpreted using Tripoli and inverse isochrons using the biotite analyses Jurassic Laurel Mountain pluton (Carr et al., and U-Pb_Redux software (Bowring et al., (Fig. 6). 1997) that intrudes correlative metasedimentary

4 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault Ar A. LV051210-5 plagioclase B. LV030710-5 plagioclase Ar 28 I = 18.45 ±2.3 Ma 40 28 40 *P = 19.63 ± 0.32 Ma I = 19.59 ±0.90 Ma A (MSWD = 2.1, Ar/ B (MSWD = 2.1, A Ar/ (MSWD = 1.76, 60.3% 39Ar) 39 36 *P = 18.84 ± 0.24 Ma H 100% Ar) 36 A 60.3% 39Ar) B E (MSWD = 1.47, 97.8% 39Ar) 40Ar/36Ar intercept J 40Ar/36Ar intercept = 295 ±1 = 294 ±2 AB J D 0.002 C 0.002 G F F I F E H G C D G C E Apparent Age (Ma) Apparent Age (Ma) 0.001 C D E G 0.001 I F H D I H 12 B 12 J 0 0

*I = 19.00 ±0.22 Ma

C. LV031510 groundmass D. LV040910 groundmass Ar Ar 28 40 28 40 *I = 19.37 ±0.04 Ma (MSWD = 2.6, P = 19.40 ± 0.47 Ma A Ar/ Ar/ 39 39 (MSWD = 1.6, A 98.8% Ar) 36 36 P = 19.54 ± 0.12 Ma (MSWD = 2.37, 67.5% Ar) B 40 36 67.5% 39Ar) 39 Ar/ Ar intercept (MSWD = 3.15, 67.5% Ar) B I 40Ar/36Ar intercept J = 301 ±2 I F G J = 318 ±10 G G F 0.002 I C 0.002 C BC F H H D DEH DE J E J Apparent Age (Ma) Apparent Age (Ma) B I 0.001 0.001 H A G 12 A FE 12 C D 0 0

E. SD040910-1 groundmass F. SD060810-1 groundmass *I = 11.93 ±0.14 Ma Ar Ar 40 A 40 A (MSWD = 1.4, 18 *I = 12.74 ±0.05 Ma 18 P = 12.39 ± 0.25 Ma 39 Ar/ B 39 96.8% Ar) Ar/ 36 (MSWD = 6.7, (MSWD = 4.86, 96.8% Ar) C 36 B 40Ar/36Ar intercept P = 12.82 ± 0.13 Ma C 99.7% 39Ar) H 39 40 36 = 301 ±9 (MSWD = 7.54, 99.7% Ar) Ar/ Ar intercept J = 302 ±2 G 0.002 I H 0.002 G I C F E C D E J D B D

Apparent Age (Ma) D I Apparent Age (Ma) F H 0.001 0.001 12 B G J E H 12 E G F F 0 0

I = 11.70 ±0.07 Ma H. GFZ-080603-1 groundmass G. LV051210-6 groundmass Ar 14 Ar 40 (MSWD = 1.3, 40 18 B Ar/ A 93.5% 39Ar) Ar/ 36 40Ar/36Ar intercept 36 *P = 11.66 ± 0.06 Ma = 292 ±4 (MSWD = 1.18, 93.5% 39Ar) J A D E B I 0.002 C H 0.002 H A G G G B F F IJ *I = 11.24 ±0.43 Ma B C P = 11.01 ±0.29 Ma H

Apparent Age (Ma) (MSWD = 0.53,

Apparent Age (Ma) 39 C G 0.001 (MSWD = 0.62, 100% Ar) 39 F 100% Ar) E 12 40Ar/36Ar intercept D CD EF DE 6 = 285 ±18 H 0 0 0 Cumulative 39Ar Fraction 0.9 0 39Ar/40Ar 0.3 J. SD120905-2 groundmass 14 I = 7.24 ±0.05 Ma Ar I. LV030710-4 sanidine 40 A (MSWD = 1.8,

39 Ar/

100% Ar) 36 I 40 36 B Ar/ Ar intercept 11.221 ±0.018 Ma = 302 ±9 (MSWD = 0.64, n = 9) H *P = 7.32 ±0.02 Ma C 0.002 C (MSWD = 0.23, 61.6% 39Ar) D Data at 1-sigma,

Relative Probability results at 1-sigma Apparent Age (Ma) I G 0.001 E FGH 6 D F E B 0 8101214Age (Ma) K. SD120905-1 groundmass L. LV030710-7 groundmass J Ar Ar 40 14 A *I = 7.24 ±0.83 Ma 40 14 H I A Ar/ G (MSWD = 0.17, Ar/ 36 H FD 85.7% 39Ar) 36 B B 40 36 E C E Ar/ Ar intercept C I D JF P = 7.71 ± 0.11 Ma = 298 ±5 B C F I G E 39 G HD (MSWD = 0.91, 78.6% Ar) I 0.002 0.002 *P = 11.62 ± 0.11 Ma I = 11.99 ±0.52 Ma B C D F (MSWD = 0.90, 82.2% 39Ar) (MSWD = 0.95, 39 Apparent Age (Ma) Apparent Age (Ma) H 0.001 88.2% Ar) 0.001 E G 40Ar/36Ar intercept 6 6 A = 293 ±4 0 0 Cumulative 39Ar Fraction 0.9 0 39Ar/40Ar 0.3 0 Cumulative 39Ar Fraction 0.9 0 39Ar/40Ar 0.3 Figure 4. Plots of plateau diagrams and inverse isochrons for step-heating 40Ar/39Ar analyses of our new samples, except I, which is a frequency plot of analyses of sanidine (MSWD—mean square of weighted deviates). Interpreted ages are reported using plateau (P) and inverse isochron (I) methods with the preferred age denoted by (*). Double-headed arrows indicate the steps included in the plateau age. Fraction labels (i.e., A, B, C, etc.) in gray were not used in the age interpretation. The small gray box on the inverse isochron plots denotes the value of atmospheric argon with 40Ar/36Ar of 295.5.

Geological Society of America Bulletin, Month/Month 2014 5 Andrew et al. plateau with progressive steps decreasing ages (Fig. 4F) age (Fig. 4E) similar to sample SD060810-1. Inverse isochron a complicated plateau pattern (Fig. 4A), but the large errors allow 4L) that yields the preferred age of 11.62 ± 0.11 Ma. Inverse ± 0.11 4L) that yields the preferred age of 11.62 lyzed via total fusion of sanidine fractions (Fig. 4I). Frequency (Fig. 4B) but yields a plateau of 18.84 ± 0.24 Ma with an ample has an odd plateau behavior (Fig. 4H) with a large-volume, sample does not yield a valid plateau (Fig. 6G). A three-point A sample does not yield a valid plateau (Fig. 6G). m this sample has an odd-shaped plateau (Fig. 4K). The lowest age m this sample has an odd-shaped plateau (Fig. 4K). nit to the Lava Mountain Dacite. Groundmass from this sample has ass from this sample has a saddle-shaped plateau (Fig. 4D) that gives ass from this sample has a slight saddle-shaped plateau (Fig. 4C) .12 Ma (Fig. 6B) but a high MSWD because the three plateau steps ndmass from this sample yields a valid plateau age of 11.66 ± 0.06 Ma ndmass from this sample yields a valid plateau age of 11.66 Ar released. Steps F and G yield the preferred age for this rock of 7.64 elds a valid plateau age of 7.82 ± 0.22 Ma (Fig. 6C). The plateau diagram elds a valid plateau age of 7.82 ± 0.22 Ma (Fig. 6C). 94 Ma using steps E–G (Fig. 6A). This analysis has a large analytical 94 Ma using steps E–G (Fig. 6A). 39 isochron (Fig. 6H). This analysis does not yield a valid age. isochron (Fig. 6H). osamples.org/ using the International Geo Sample Number (IGSN). verse isochron has a similar age with many anomalous fractions and slightly the slightly higher MSWD. The preferred age is 19.63 ± 0.32 Ma from the plateau slightly higher MSWD. sample is similar in appearance to SD120905-1.This has odd r age steps may be the result of alteration or maybe contamination by slightly rrelation to the Eagle Crags Volcanics (Sabin, 1994; Monastero et al., 1997). rrelation to the Eagle Crags Volcanics s the other orthoclase megacrystic dacite (sample LV030710-4; Fig. 4I). s the other orthoclase megacrystic dacite (sample LV030710-4; mineralogically and texturally similar to the nearby sample SD120905-2, which he ages of other basalts in the base Summit Range Volcanics. s with the basalts at base of Summit Range volcanics. The inverse isochron yields a similar age to the plateaus but with slightly better s Dacite, based on its location and that it cuts the Almond Mountain Volcanics. s Dacite, based on its location and that it cuts the eau (Fig. 6F) and isochron ages of ca. 10 Ma, but has ~10% age error is ~2.7 . We interpret this age to be too old; see text for discussion. . We too old; see text for discussion. in the base of Summit Range volcanics. mmit Range volcanics. astero et al., 1997). heric argon. We interpret the plateau pattern as excess argon and therefore this heric argon. We ; Monastero et al., 1997). 1997). Ar. Step K has an age of 8.65 ± 0.67 Ma, just barely overlapping, within error, with the Step K has an age of 8.65 ± 0.67 Ma, just barely overlapping, within error, Ar. 39 Ar intercept value. The inverse isochron age of 19.00 ± 0.22 Ma is the preferred age. The The inverse isochron age of 19.00 ± 0.22 Ma is the preferred age. Ar intercept value. 36 Ar intercept value than the atmosphere. The inverse isochron age of 19.37 ± 0.04 Ma is the Ar intercept value than the atmosphere. 36 Ar/ 40 Ar/ 40 . i Ar 40 Ar/ 36 Ar GEOCHRONOLOGY INTERPRETATIONS Ar GEOCHRONOLOGY 39 Ar/ 40 error. This analysis does not yield a valid age. error. i Ar 36 Ar/ 40 TABLE 2. TABLE Ar samples from Smith et al. (2002) 39 Ar/ 40 value. Hornblende does not yield a valid plateau (Fig. 6D) because of step K, which released 79% the i (IGSN:JEAGFZ863). Sample of orthoclase megacrystic dacite lava that is part the Summit Range volcanics. Biotite from this s Ar samples Ar (IGSN:JEA0522MU). Sample of basalt lava below dacite lava. Groundmass from this sample has a disturbed plateau with decreasing (IGSN:JEA0522MT). Sample of basalt lava from the Summit Range, below dacite lava. Groundmass this sample has a disturbed (IGSN:JEA0522MS). Sample of vitrophyric dacitic intrusion with fi ne-grained (<1 mm) needles of hornblende. We correlate this u ne-grained (<1 mm) needles of hornblende. We (IGSN:JEA0522MS). Sample of vitrophyric dacitic intrusion with fi (IGSN:JEA0522MR). Sample of fi nely vitrophyric dacitic intrusion that we correlate to the Lava Mountain Dacite. Groundmass fro (IGSN:JEA0522MR). Sample of fi 39 (IGSN:JEA0522M7). Sample of basaltic andesite from below dacite lava. Plagioclase this sample has a saddle-shaped plateau (IGSN:JEA0522MM). Sample of basaltic andesite from below dacite lava in the Lava Mountains. Plagioclase this sample shows (IGSN:JEA0522M5). Basalt clast sample from the conglomerate of Golden Valley. Groundmass from this sample has a plateau (Fig. (IGSN:JEA0522M5). Basalt clast sample from the conglomerate of Golden Valley. (IGSN:JEA0522MK). Sample of the basal basalt lava fl ow in the Black Hills over tuff deposits and basaltic-andesite fl ows. Grou deposits and basaltic-andesite fl ow in the Black Hills over tuff (IGSN:JEA0522MK). Sample of the basal basalt lava fl This sample was ana (IGSN:JEA0522M6). Sample of orthoclase megacrystic dacite lava that is part the Summit Range volcanics. 40 (IGSN:JEA0522MB). Sample of dacite clast from silicifi ed breccia deposit from below dacite lava in the Lava Mountains. Groundm (IGSN:JEA0522MB). Sample of dacite clast from silicifi (IGSN:JEA0522MD). Sample of dacite clast from silicifi ed breccia deposit from below dacite lava in the Lava Mountains. Groundm (IGSN:JEA0522MD). Sample of dacite clast from silicifi Ar/ (IGSN:JEALM9616). Sample of dacitic volcanic debris-fl ow deposit in the Almond Mountain Volcanics. Biotite from this sample yi Almond Mountain Volcanics. ow deposit in the (IGSN:JEALM9616). Sample of dacitic volcanic debris-fl (IGSN:JEALM9617). Sample of tuff in Almond Mountain Volcanics interbedded with Bedrock Spring Formation. Hornblende from this Almond Mountain Volcanics in (IGSN:JEALM9617). Sample of tuff (IGSN:JEALM9615). Sample of dacite lava in the Summit Range volcanics. Biotite from this sample has a plateau age 10.66 ± 0 Ar/ (IGSN:JEALM1132). Sample from thick gray dacite lava fl ow. Biotite from this sample does not defi ne a valid plateau or inverse Biotite from this sample does not defi ow. Sample from thick gray dacite lava fl (IGSN:JEALM1132). 40 (IGSN:JEAGFZ085). Sample of dacite lava in the Summit Range volcanics. Biotite for this sample has a plateau age of 11.32 ± 0. (IGSN:JEAGFZ085). Sample of dacite lava in the Summit Range volcanics. Biotite for this sample has a plateau age 11.32 (IGSN:JEALM9602). Sample of dacitic sill. Biotite data do not yield a valid plateau (Fig. 6E), but steps F and G are 74.5% 36 Additional data for these samples are included inTables DR1 and DR3 (see text footnote 1) also accessible at http://www.ge Additional data for these samples are included inTables has a saddle shape, so this age may include an excess argon component and therefore this interpreted age may be too old. The in has a saddle shape, so this age may include an excess argon component and therefore interpreted be too old. m.y. older. We interpret the hornblende to have an inherited component. We correlate this intrusive unit with the Lava Mountain interpret the hornblende to have an inherited component. We We older. m.y. error of 7.66%. The inverse isochron has a low MSWD, but large age error of 8.6 m.y. The inverse isochron has a low MSWD, but large age error of 8.6 m.y. error of 7.66%. The inverse isochron has a higher MSWD. ne a slope. defi that gives a relatively high MSWD. Inverse isochron analysis yields better MSWD value with slightly higher a relatively high MSWD. Inverse isochron analysis yields better but still MSWD value and slightly higher a valid plateau with an acceptable mean standard weighted deviates (MSWD). Inverse isochron analysis gives similar age (Sabin, 1994; Mon ts with a correlation to the Eagle Crags Volcanics The age and stratigraphic position of this lava fi diagram. acceptable MSWD. The inverse isochron has higher errors and MSWD. The age and stratigraphic position of this lava fi t with a co The age and stratigraphic position of this lava fi The inverse isochron has higher errors and MSWD. acceptable MSWD. ± 0.18. The biotite inverse isochron gives a similar age but has a large MSWD. The hornblende for this sample yields valid plat The biotite inverse isochron gives a similar age but has large MSWD. ± 0.18. isochron yields an age of 9.98 ± 0.09 Ma with a large MSWD and preferred age. The age and stratigraphic position of this lava fi t with a correlation to the Eagle Crags Volcanics (Sabin, 1994 t with a correlation to the Eagle Crags Volcanics The age and stratigraphic position of this lava fi preferred age. plot gives an age of 12.74 ± 0.05 Ma with a slightly better fi t but has a high MSWD of 6.7 and slightly value for atmosp plot gives an age of 12.74 ± 0.05 Ma with a slightly better fi Ma) of other basalts and 11.93 ts with the slightly younger ages (11.66 This interpretation fi interpreted age is a maximum age. The anomalous olde t with a MSWD of 1.4. that shows two ages at ca. 15 Ma and 12 Ma. Inverse isochron plot gives a better fi ts with t This age fi (Fig. 4G) with a reasonable MSWD value, and inverse isochron gives similar age slightly higher MSWD. ± 0.29 Ma with a MSWD of 0.62. valid plateau for the entire step-heating yields 11.01 A high-temperature step with a lower age. ± 0.018 Ma. analysis of these data yields an age 11.221 high age and stratigraphic position of this lava fi t with a correlation to the Eagle Crags Volcanics (Sabin, 1994; Monastero et al., t with a correlation to the Eagle Crags Volcanics age and stratigraphic position of this lava fi plateau shape which may indicate a relatively young, but still signifi cant excess argon component. We interpret this age to be cant excess argon component. We plateau shape which may indicate a relatively young, but still signifi biotite plateau. The hornblende inverse isochron yields a ca. 7 Ma age, but with an anomalous biotite plateau. isochron analysis (without step A) gives a slightly higher MSWD but an older age with more error. The age of this basalt matche A) gives a slightly higher MSWD but an older age with more error. isochron analysis (without step a plateau age of 7.32 ± 0.02 Ma with low MSWD (Fig. 4J). The inverse isochron yields a similar age but with a higher MSWD. This The inverse isochron yields a similar age but with higher MSWD. a plateau age of 7.32 ± 0.02 Ma with low MSWD (Fig. 4J). older igneous rocks. The preferred age is 11.93 ± 0.14 Ma, which fi ts with the age range of other basalts in base Su ± 0.14 Ma, which fi The preferred age is 11.93 older igneous rocks. MSWD value. Therefore our preferred age is 11.24 ± 0.43 Ma age from the inverse isochron. This age is the same, within error, a This age is the same, within error, ± 0.43 Ma age from the inverse isochron. Therefore our preferred age is 11.24 MSWD value. This sample is t with a low MSWD and an age of 7.24 ± 0.83 Ma. The inverse isochron yields a good fi step (B) is 7.46 ± 0.33 Ma. exhibits a better plateau behavior and has an age similar to the of lowest step inverse isochron this sample Note: LM96-16 GFZ-85 LV051210-5 LV031510 A. New LM96-15 LM1132 LV040910 SD040910-1 SD060810-1 LM96-2 LV030710-5 LM96-17 LV051210-6 LV030710-4 SD120905-2 GFZ-080603-1 B. Reinterpreted LV030710-7 SD120905-1

6 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault

TABLE 3. U/Pb ZIRCON GEOCHRONOLOGY RESULTS ing upstream of the dacite domes. Sediment Unit Age Age error transport in the Bedrock Spring Formation was Sample name IGSN* code† Geologic context (Ma) (± Ma) MSWD§ n to the northwest (Smith, 1964; this study), so the LV050810-4 JEA0522MI LMD Dacite lava fl ow 6.507 0.070 1.9 2 LV051810-8 JEA0522MO LMD Vitrophyric dacitic sill 6.522 0.026 2.8 3 domes would have formed topographic barriers. JDW-20 JEA0522MQ AMV Thick pink tuff 7.479 0.023 3.5 4 Dacitic volcanic deposits of the Almond LV050710 JEA0522MH SRV Upper dacite lava fl ow/dome 10.490 0.054 1.2 5 LV031410-B JEA0522MA SRV Lower dacite lava dome 10.966 0.042 2.4 5 Mountain Volcanics overlie and are intercalated CINCO JEACINCO1 SRV Megacrystic dacitic dike 11.383 0.028 1.2 6 with the upper parts of the Bedrock Spring For- Note: Additional data for these samples are included in Table DR2 (see text footnote 1). mation. These have pronounced facies changes *International Geo Sample Number data accessible at http://www.geosamples.org/. across the LMSR, ranging from 200-m-thick †Unit Code: Stratigraphic unit that the sample belongs to: ECV—Eagle Crag Volcanics; SRV—Summit Range volcanics; AMV—Almond Mountain Volcanics; LMD—Lava Mountain Dacite. complexes of multiple units (Fig. 8A) to sec- §MSWD—mean standard weighted deviates. tions with only a single pumice lapilli tuff bed (Fig. 8B). The thickest sequences occur in the southern Lava Mountains, with multiple vol- rocks in the eastern El Paso Mountains (Fig. 2). Upper Miocene Summit Range Volcanics. canic debris-fl ow deposits of clasts of light-purple A few small exposures of similar quartz diorite The basalts and older units in the Summit Range porphyritic dacite lava interbedded with several occur in the northeastern Summit Range adja- and Lava Mountains are overlain by a few- 2–10-m-thick pumice lapilli to lithic tuff beds cent to the Garlock fault (Fig. 2). All of the other meters-thick conglomerate containing rounded and local sandstone beds. In the northeastern basement in the LMSR is quartz monzonite to and polished cobble- to boulder-sized clasts Lava Mountains, volcanic interbeds are mostly granodiorite of the Cretaceous Atolia Quartz of felsic plutonic rocks and also of distinctive absent in the Bedrock Spring Formation except Monzonite (Hulin, 1925; Smith, 1964). fl ow-banded rhyolite. A sequence of volcanic for one ~15-cm-thick pumice lapilli tuff and a rocks occurs above beginning with a several- 2-m-thick bed of dacitic volcanic debris-fl ow Cenozoic Units meters-thick tuff containing plutonic lithic deposit. The Almond Mountain Volcanics are Lower Miocene volcanic-sedimentary rocks. blocks, pumice lapilli, and numerous lenses of not present in the Black Hills area and are rare The oldest Cenozoic units in the LMSR are wide- dacitic lava breccia. These units are overlain by elsewhere in the LMSR; a 10-m-thick bed of spread but discontinuous felsic ash and pumice and interbedded with several bodies of dacite tuff is exposed in the western Summit Range lapilli tuffs (Figs. 3 and 7). These tuffs are white, lava that have textures ranging from aphyric area and a 10–15-cm-thick tuff bed occurs in distinctively bedded with beds 5–15 cm thick, to porphyritic to megacrystic porphyritic. The the upper parts of arkosic beds in the Christmas and have local interbeds of arkosic sandstone. dacite lava fl ows are thick (50–150 m), have Canyon area. Dacitic lava above the Bedrock The tuffs are overlain by oxidized porphy- limited areal extent (less than 1000 m long), and Spring Formation has a 40Ar/39Ar of <7.82 ± ritic basaltic-andesite to andesite lava and are have the stratigraphic expression and textures 0.22 Ma on biotite (Fig. 6C; Table 2), and a tuff intruded by intermediate-composition porphyry of dacitic lava domes (Andrew et al., 2014). in the upper Bedrock Spring Formation has a (Dibblee, 1967). The lavas have 40Ar/39Ar ages Feldspar and biotite porphyritic dacite lavas U-Pb zircon age of 7.479 ± 0.023 Ma (Fig. 5C). of 19.63 ± 0.32 Ma on plagioclase (Fig. 4A) have 40Ar/39Ar ages of 11.32 ± 0.94 Ma (Fig. Upper Miocene Lava Mountains Dacite. A and 18.84 ± 0.24 Ma on plagioclase concentrate 6A) and 10.66 ± 0.12 Ma (Fig. 6B) on biotite thick porphyritic lava fl ow overlies the Bed- (Fig. 4B). Silicifi ed volcanic breccia deposits and U-Pb zircon ages of 10.966 ± 0.042 Ma rock Spring Formation and Almond Mountain overlie the tuffs in the Christmas Canyon area. (Fig. 5A; Table 3) and 10.490 ± 0.054 Ma (Fig. Volcanics. Smith (1964) defi ned this as the The clasts are dark colored and aphyric, and are 5B). Quartz-biotite-feldspar porphyritic dacite “Lava Mountain Andesite”, but geochemical dacites based on geochemical analyses (Monas- lavas containing distinctive 1–4 cm megacrysts data (Smith et al., 2002) show that it is a dacite; tero, unpub. data). These rocks have 40Ar/39Ar of orthoclase have 40Ar/39Ar ages of 11.24 ± therefore we propose the name “Lava Moun- ages of 19.37 ± 0.04 Ma on groundmass (Fig. 0.43 Ma on groundmass concentrates (Fig. 4H) tain Dacite”. This lava is a distinctive brown- 4C) and 19.00 ± 0.22 Ma on groundmass con- and 11.221 ± 0.018 Ma on sanidine (Fig. 4I). ish gray and has porphyritic plagioclase, horn- centrate (Fig. 4D), similar to ages of the altered We modify terminology of Smith et al. (2002) blende, and biotite with rare quartz and smaller lava fl ows (Table 2). We correlate these rocks and refer to these dacite dome complexes of and less-abundant pyroxene. It contains vari- to the Eagle Crags Volcanics, based on age, lava, tuff, and epiclastic deposits as the Summit able amounts of mafi c clots a few centimeters composition, and stratigraphic relations (Sabin, Range volcanics (Fig. 3). in diameter. The unit is voluminous, covering 1994). Monastero et al. (1997) correlated the Upper Miocene Bedrock Spring Formation– >50 km2 and 40–100 m thick. A sample from the Eagle Crag Volcanics to the Cudahy Camp For- Almond Mountain Volcanics. Arkosic sand- Lava Mountain Dacite has a U-Pb zircon age of mation (Cox and Diggles, 1986) in the El Paso stone of the Bedrock Spring Formation (Smith, 6.507 ± 0.070 Ma (Fig. 5D). Mountains. 1964) unconformably overlies the dome com- Another porphyritic dacite lava fl ow occurs Middle Miocene basalt. Vesicular basalt over- plexes and older rocks in the Lava Mountains, just beyond the northern exposures of Lava lies the lower Miocene units in the LMSR. Summit Range, and Christmas Canyon area. Mountain Dacite. This fl ow is distinguished These are fi nely porphyritic with olivine and The basal unconformity with the dacite domes from the Lava Mountain Dacite in that it has a red pyroxene and occur as 2–4-m-thick fl ows in the varies from disconformity to angular unconfor- color, pilotaxitic textures, and a 10–20-m-thick Summit Range and Lava Mountains but as a mity to buttress unconformity. Thin lenses (gen- basal vitrophyre. We associate this fl ow with 25-m-thick sequence in the Black Hills. Basalts erally <15 cm) of pebble to cobble conglomer- the Lava Mountain Dacite based on its similar in the Summit Range and Lava Mountains have ate occur throughout the section, as do locally phenocryst assemblage, thickness, location, and 40Ar/39Ar groundmass ages of <12.74 ± 0.05 Ma abundant layers of siltstone. Limestone beds stratigraphic relationships. Smith (1964) also (Fig. 4E; Table 2) and 11.93 ± 0.14 (Fig. 4F), and well-cemented arkose occur only along the included this with his Lava Mountains Andesite. and the oldest basalt in the Black Hills has an south sides of the dacite lava domes. These are Numerous vitrophyric sills intrude the Bed- age of 11.66 ± 0.06 Ma (Fig. 4G). likely lacustrine deposits resulting from pond- rock Spring Formation and Almond Mountain

Geological Society of America Bulletin, Month/Month 2014 7 Andrew et al.

A. LV031410-B zircon B. LV050710 zircon 7.32 ± 0.02 Ma (Fig. 4J) and 7.24 ± 0.83 Ma 0.00180 11.6 Ma (Fig. 4K) on groundmass concentrates (Table 2). z10 We postulate that the discrepancy between the U 10.7 Ma 40 39 238 z4 U-Pb and Ar/ Ar ages could reﬂ ect (1) an z11 0.00165

Pb/ z1 excess argon component, creating anomalously 40 39 206 U older ages, or (2) that the sills dated by Ar/ Ar

z13 238 z5 z12 better correlate to the Almond Mountain Vol-

0.00170 Pb/ z14 canics. The former is more likely because of the 206 z8 excess argon–interpreted hornblende 40Ar/39Ar weighted mean weighted mean 206 238 0.00161 Pb/ U (Th-corrected) 206Pb/238U (Th-corrected) ages, which are older than those of biotite from 10.966 ±0.042 Ma 10.6 Ma z9 10.3 Ma 10.490 ±0.054 Ma samples of Almond Mountain Volcanics and MSWD = 2.4, n = 5 MSWD = 1.2, n = 5 from one of these sills (Fig. 6C and 6E). 0.010 207Pb/235U 0.016 0.007 207Pb/235U 0.012 Plio-Pleistocene conglomerates. Pebble to boulder conglomerates overlie the Miocene units C. JDW-20 zircon D. LV050810-4 zircon in the LMSR area. These rocks contain no known weighted mean 0.00118 7.6 Ma 206Pb/238U (Th-corrected) datable units, but we infer them to be Pliocene U z1 6.507 ±0.070 Ma based on their deposition over the Lava Moun- 238 0.00105 MSWD = 1.9, n = 2 (z2 & z4) tain Dacite and their angular unconformable Pb/ contacts with overlying Quaternary units. We U 206 z21 z23

238 break out two different sets of these conglom- 6.6 Ma erates based on location and differences in clast z5 weighted mean Pb/ 0.00115 206Pb/238U (Th-corrected) 206 z4 types. Both units contain clasts of metasedimen- 7.4 Ma 7.479 ±0.023 Ma tary, meta-plutonic, plutonic, and volcanic rocks 0.00100 z4 MSWD = 3.5, n = 4 z2 exotic to the LMSR, and also contain placer gold 6.4 Ma deposits (Dibblee, 1967; Fife et al., 1988). Boul- 0.007207Pb/235U 0.009 0.005 207Pb/235U 0.008 ders and cobbles of felsic plutonic rocks, felsic hypabyssal intrusive rocks, and quartzite are E. LV051810-8 zircon 6.7 Ma F. CINCO zircon 12.0 Ma conspicuously well rounded and polished. weighted mean 0.00185 The informally defined conglomerate of 206Pb/238U (Th-corrected) z14b U Golden Valley is the eastern unit, which has 0.00103 weighted mean

6.522 ±0.026 Ma 238 206Pb/238U (Th-corrected) distinctive lineated and foliated hornblende

U MSWD = 2.8, n = 3 z2b 6.6 Ma Pb/ 11.383 ±0.028 Ma diorite clasts with a capping unit of boulders of 238 z10b 206 vesicular basalt. These clast types distinguish 11.6 Ma MSWD = 1.2, n = 6 Pb/ 0.00180 it from the informally deﬁ ned conglomerate of

206 z3 z2 z9b Hardcash Gulch in the Summit Range (Fig. 2), z3b 15 km to the west. Rittase (2012) reported a 6.5 Ma z8b z7b z1 z5b tephrochronology age of 3.14 Ma for a tuff over- z11b z12b lying exotic-clast conglomerates a few kilome- 0.00100 11.2 Ma ters east of Christmas Canyon. A sample from 0.0060 207Pb/235U 0.0080 z13b 207 235 0.010 Pb/ U 0.014 a basalt boulder of the conglomerate of Golden Valley has a 40Ar/39Ar age of 11.62 ± 0.11 Ma Figure 5. Concordia plots of U-Pb analyses of samples (MSWD—mean square of weighted (Fig. 4L), matching the age of the texturally and deviates). Unﬁ lled fraction error ellipses were not used to calculate the crystallization age. mineralogically similar basalt in the Black Hills.

STRUCTURES Volcanics. Smith (1964) misinterpreted the ies, in the western Summit Range (Fig. 2), are intrusive front of one of these dacite sills (Fig. correlated to the Lava Mountain Dacite based Previous studies of the LMSR show that the 8C) as a lava fl ow front of a “Quaternary andes- on their vitrophyric texture and relationships Cenozoic units are deformed (Smith, 1964; ite”. This roll structure is intrusive: there is no to stratigraphic units. These have fi ne needles Dibblee, 1967; Smith et al., 2002) but did not basal fl ow breccia and the contacts on all sides of hornblende and intrude the Bedrock Spring determine kinematics of fault slip. New geologic are baked. We correlate these sills with the Lava Formation and Almond Mountain Volcanics. mapping (Andrew et al., 2014) better defi nes Mountain Dacite based on similar phenocryst Dibblee (1967) mapped these intrusions as these faults, identifi es several new faults (Fig. 2), mineralogy, location, and large volume, and basalt, but geochemical analyses show them to and identifi es many stratigraphic markers to use because they cut the Almond Mountain Vol- be dacitic (Monastero, unpub. data). in determining fault offset. Slip direction and canics. These intrusives form a WNW-trending A vitrophyric sill from the central Lava sense of shear given for each fault below was zone along the northern exposures of the Lava Mountains area has a U-Pb zircon age of 6.522 ± determined by combining measurements of fault Mountain Dacite (Fig. 2). The red pilotaxitic 0.026 Ma (Fig. 5E), but the biotite 40Ar/39Ar age striations with two or more shear-sense indica- lava occurs only along this zone of intrusions of a similar sill is 7.64 ± 0.18 Ma (Fig. 6E; Table tors such as Riedel shears, fault gouge foliation, and may therefore be the near-vent facies of the 2). The hornblende vitrophyric dacitic intrusions drag folding, fault-zone clast rotation, slicken- Lava Mountain Dacite. Two other intrusive bod- from the Summit Range have 40Ar/39Ar ages of fi bers, asperity tool marks, and small-scale offset

8 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault

15 A. GFZ-85 biotite D B. LM96-15 (Tvd) biotite A I = 10.98 ±0.53 Ma 14 C I = 11.3 ±8.6 Ma A (MSWD = 38, 39 E F G (MSWD = 0.20, 96% Ar) 40 36 H 39 J Ar/ Ar intercept D 88.3% Ar) B 40 36 = 277 ±17 Ar/ Ar intercept I D E = 287 ±18 F D 0.002 G H 0.002 Ar 40 E L Ar C *P = 11.32±0.94 Ma Ar/

H 40 (MSWD = 0.017, 88.3% 39Ar) *P = 10.66±0.12 Ma C G 36 G F Ar/ 39 Apparent Age (Ma) 0.001 (MSWD = 1.9, 91% Ar) 0.001 Apparent Age (Ma) 36 B K A,B,C A E F J 6 B HI 5 15 80 C. LM96-16 (Ta) biotite D. LM96-16 (Ta) hornblende I = 7.66 ±0.26 Ma I = 7.0 ±2.1 Ma (MSWD = 2.4, C H,I (MSWD = 0.48, A 39 91% 39Ar) 86% Ar) 0.003 40Ar/36Ar intercept 40Ar/36Ar intercept P = no plateau L A = 317 ±22 = 410 ±36 Ar step K = 8.65 ±0.67 Ma 0.002 40 with 79% of 39Ar 0.002 B D C Ar/ 36 E Ar Apparent Age (Ma) G B 40 A F B J Ar/ Apparent Age (Ma) 0.001 J E 20 J 36 K F,G,H K *P = 7.82 ±0.22 Ma I H G,I,L,F (MSWD = 0.27, 86% 39Ar) D 5 0 15 80 E. LM96-2 (Qa) biotite F. LM96-2 (Qa) hornblende I I = 7.66 ±0.32 Ma A I = 9.9 ±1.3 Ma Ar B A 40 C (MSWD = 3.5, (MSWD = 0.54, 39 B 39 Ar/ 95% Ar) 95% Ar) 36 40Ar/36Ar intercept B 40Ar/36Ar intercept

P = no plateau = 304 ±13 Ar E P = 10.3 ±1.2 Ma (MSWD = 0.68) = 316 ±16 40 *Steps F&G = 74.5% of 39Ar *step G = 10.23 ±0.62 Ma 0.002 Ar/ 39 0.002 with an age of 7.64 ±0.18 Ma D 36 with 74% of Ar I

Apparent Age (Ma) E

F G I F Apparent Age (Ma) C 0.001 H 20 H 0.001 D E J D E F G G C H A G F 5 0 80 10

G. LM96-17 (Tat) I = 9.98 ±0.09 Ma H. LM1132 biotite Ar

C 40 hornblende B (MSWD = 5.6, G 39 Ar/

85.9% Ar) 36 E 0.003 P = no plateau A 40Ar/36Ar intercept I = no intercept F E = 295 ±480 I F G D C H 0.002 E G H 0.002 H P = no plateau L F F A Ar Apparent Age (Ma) L D 40 D H I G Ar/

C J Apparent Age (Ma) 20 I D 0.001 K 36 I E B J K M C 0 0 01Cumulative 39Ar Fraction 00.339Ar/40Ar 01Cumulative 39Ar Fraction 00.339Ar/40Ar

Figure 6. Plots of plateau diagrams and inverse isochrons for step-heating 40Ar/39Ar analyses of the reinterpreted samples from Smith et al. (2002). See Figure 4 for symbol and abbreviation explanations. The gray blocks on C and E are the corresponding hornblende data for each sample.

(Tchalenko, 1970; Chester and Logan, 1987; planes (Fig. 9A). Fault scarps of the Garlock fault quartz diorite is similar to intrusive rocks of the Means, 1987; Petit, 1987). We also discuss folds in Quaternary alluvium (Rittase, 2012) show a Jurassic Laurel Mountain granodiorite and not of late Cenozoic units in the LMSR. similar orientation of fault planes (Fig. 9B). the more felsic and less altered Atolia Quartz Teagle Wash fault. The Teagle Wash fault Monzonite. Measured fault planes have low Geometry and Kinematics of Faults (named herein) occurs in the northeast Summit dips with low-rake fault striae (Fig. 9C). Top-to- Range (Fig. 2). It was previously interpreted as a the-ESE relative motion of this fault (Fig. 10A) Sinistral Faults thrust fault (Hulin, 1925; Smith, 1964) because equates to left-lateral oblique slip (Fig. 9C). Garlock fault. Data for the Garlock fault in it places granitic rock over Bedrock Spring Savoy fault. The Savoy fault (named herein) the western LMSR show left-lateral slip with low Formation, Eagle Crags Volcanics, and green- separates the Summit Range from the Lava rake and an associated set of normal-slip fault altered medium-grained quartz diorite. This Mountains (Fig. 2). It has moderate dips to the

Geological Society of America Bulletin, Month/Month 2014 9 Andrew et al.

u LV051810-8 (6.522 ±0.026 Ma) u = U-Pb age (others are 40Ar/39Ar) x LM96-2 (7.64 ±0.18 Ma) Tld x = older age due to excess argon LM96-15 (10.66 ±0.12 Ma) Central LM96-16 LV031410-B u s (10.966 ±0.042 Ma) r Summit Range (7.82 ±0.22 Ma) e GFZ-080603-1 (11.24 ±0.43 Ma) t Northeastern e

GFZ-85 (11.32 ±0.94 Ma) Northern Lava m Tlr Ta 0 Lava Mountains 0 SD060810-1 (11.93 ±0.14 Ma) LM96-17 (N.A.) 1 Mountains Christmas scale Vertical SD040910-1 (12.74 ±0.05x Ma) JDW-20 (7.479 ±0.023u Ma) Tpg Canyon SD120905-1 Tb LV030710-7 [clast] x LV050810-4 (7.24 ±0.83 Ma) u (11.62 ±0.11 Ma) Western (6.507 ±0.070 Ma) SD120905-2 Tpg x Summit Tld (7.32 ±0.02 Ma) Range Tlr Tb Ts Tld QTph Tb Tb Tb Ta Tb LV040910 Tb Ts (19.00 ±0.22 Ma) Ta Ts Tb Ts Te Te Ts Tpr J Te Tb Ts Te Eastern Te K Ts Summit K Te lock fault K Range Black Hills Gar Te Ts North-Central Ta LV051210-6 Tb Far Western Western Lava Savoy fault Mountains Lava Mountains (11.66 ±0.06 Ma) Lava Mountains K Tb Te Central Stratigraphic units K Lava K Plio-Pleistocene sedimentary rocks Mountains Eastern Lava Trpg Tp QTph (see Figure 2 for abbreviations) Mountains LV051210-5 Blackwa (19.63 ±0.32 Ma) Lava Mountains Dacite u Tld Tlr -red pilotaxitic LV050710 (10.490 ± 0.054 Ma) Ta Almond Mountain Volcanics LV030710-4 (11.22 ±0.02 Ma) Lithology ter fault Tb Bedrock Spring Formation fanglomerate dacite lava dome LV030710-5 (18.84 ±0.24 Ma) N Ts Summit Range volcanics hypabyssal dacitic conglomerate LV031510 (19.37 ±0.04 Ma) Te Eagle Crags Volcanics arkosic sandstone K Cretaceous Atolia Quartz Monzonite J Jurassic quartz diorite deposit Paleozoic metasedimentary rocks metasedimentary rocks plutonic rocks

Figure 7. Simplifi ed stratigraphic columns across the Lava Mountains–Summit Range plotted on an oblique view of the same area as Figure 2. The lithology of each unit is denoted by fi ll patterns, and the stratigraphic unit is denoted by fi ll color and an adjacent unit label. Geochronologic sample locations are denoted by stars with sample name and interpreted age in parentheses. Note that most of the contacts are unconformable and drawn only schematically. south, low-rake slip vectors (Figs. 9D, 10B, and faults indicate sinistral-oblique thrust kinematics strikes NNW-SSE, dips steeply, has low-rake 10C), and left-lateral sense of shear. The Savoy (Fig. 9F). We refer to this as the Little Bird fault fault striae (Fig. 9H), and has dextral motion. fault merges into the Garlock fault in the east- (Fig. 2). The fault is unconformably overlapped The Blackwater fault continues northward past ern LMSR. The Savoy fault continues westward by the conglomerate of Golden Valley. the Brown’s Ranch fault zone (Andrew et al., into Quaternary fault scarps of the Cantil fault 2014), in contrast to the interpretation of Oskin (Dibblee, 1952, 1967) and may be a continua- Normal Fault and Iriondo (2004) that it to loses slip before this tion of the main trace of the western Garlock Randsburg Wash fault zone. To the east of point. The northward continuation has a slightly fault (WGF on Fig. 1). the Blackwater fault, Smith (1964) and Oskin different geometry, with a more northwesterly Browns Ranch fault zone. The ~2-km-wide and Iriondo (2004) mapped a zone of faults ori- strike, lower dips, and right-oblique normal Brown’s Ranch fault zone has similar fault mea- ented similar to the Brown’s Ranch fault zone. slip (Fig. 9I). There are also sets of normal and surements and kinematics (Fig. 9E) to the Gar- This fault zone is poorly exposed, but limited thrust faults associated with the northern part of lock fault (Fig. 9A). Exposures of the Brown’s data show northeast-striking faults with dip-slip this fault (Fig. 9I). Ranch fault zone show a multi-stranded struc- motion (Fig. 9G) dissimilar to that of the oblique ture with spatially associated folding of all of strike-slip Brown’s Ranch fault zone. We con- Folds the Miocene units with axes roughly parallel to sider it to be a different structure and name it the the strike of this fault zone (Fig. 10D). Randsburg Wash fault zone (RWFZ on Fig. 2). Smith (1964) identifi ed folds in the Lava These faults are inferred to have slip as young Mountains that deform upper Miocene units Oblique Fault as late Pleistocene based on along-strike fault about WSW-trending fold axes (Fig. 9J). These Little Bird Fault. The boundary between the scarps 12 km to the east that Smith et al. (1968) folds form anticline-syncline pairs in the Bed- Paleozoic basement of the Christmas Canyon interpreted as late Pleistocene features. rock Spring Formation that have steeply over- area and the Atolia Quartz Monzonite basement turned SSE-facing central limbs. Miocene present in the rest of the LMSR is a WNW-strik- Dextral Fault bedding also show a second possible fold orien- ing fault that juxtaposes lower Miocene rocks Blackwater fault. The Blackwater fault juxta- tation that plunges shallowly to the NNW (Fig. against Bedrock Spring Formation. Limited poses Quaternary alluvium against Cretaceous 9J). Bedding for Miocene units in the Christmas fault measurements along this and subsidiary granitic and Miocene rocks (Fig. 2). This fault Canyon area are interpreted to have both of these

10 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault

Figure 8. Photographs of select late Miocene A dacite debris geologic units in the Lava Mountains–Sum- Lava Mountains flow deposits mit Range. (A) View of thickest section of Dacite flows Almond Mountain Volcanics. Scale varies; the steep cliff face is ~200 m tall. (B) View of the distal volcanic facies of the Almond Mountain Volcanics, where it occurs as a arkose tuff bed single thin tuff bed. Hammer (33 cm long) for scale. (C) View of roll structure of dacite arkose that Smith (1964) interpreted as a lava tuff bed ﬂ ow edge, but which is the lateral edge of a fault Bedrock Spring shallow-level intrusion. Formation Bedrock Spring Formation fold sets, although the WSW-trending set is less B tightly folded (Fig. 9K). The younger conglomerate of Golden Valley is folded only about the WSW-trending fold axes (Fig. 9L). The lack of the NNW-trending fold set in this Pliocene unit indicates that the NNW-trending folds occurred prior its deposition. Bedrock Spring Formation Magnitude and Rates of Fault Slip

Following the example of Andrew and Walker (2009), we identiﬁ ed as many markers pumice bluish as possible in order to obtain maximum reso- lapilli tuff lution on the amount and timing of motions of individual fault systems (see Table 4 for details Bedrock Spring and methods for determining uncertainty). Formation The above section on fault geometry and kine- C matics shows that the major faults in the LMSR, massive dacite except for the Randsburg Wash fault zone, are dominantly strike-slip faults; therefore we can approximate the fault slip as horizontal offsets in map view. The interpreted slip markers discussed below are shown mostly on Figure 11 with a few regional markers on Figure 1. These markers vary from blunt area restoration markers to relatively precise narrow linear feature restorations. Slip rates are calculated for all of the faults (Table 4) but are discussed only for the Garlock fault.

Bedrock Sinistral Faults cooling Spring Garlock fault—Christmas Canyon area. We 1 m columns Formation interpret that the Christmas Canyon basement rocks correlate to rocks in the eastern El Paso Mountains using distinctive Paleozoic metasedimentary units that are appropriate to use as slip the quartzite-slate marker bed along the Garlock Smith et al., 2002); this correlation can be used markers, because they are thin (60 ± 10 m), have fault is 32.9 ± 0.6 km (Table 4). Larger slip is to constrain slip on this segment of the Garlock steep dips, and strike roughly perpendicular to unlikely because metasedimentary rocks farther fault. The Pliocene conglomerate of Golden the Garlock fault. A quartzite–black slate bed west in the El Paso Mountains have higher meta- Valley contains clasts exotic to the LMSR and (A on Fig. 11) matches a correlative bed in the morphic grades, contain abundant metavolcanic placer gold (Fife et al., 1988). A subset of these eastern El Paso Mountains (A′ on Fig. 11). A rocks, and are intruded by ductilely deformed clasts has distinctive well-rounded and polished meta-conglomerate layer (B on Fig. 11) matches plutons (Dibblee, 1952; Carr et al., 1997). textures: felsic plutonic, felsic hypabyssal, and a narrow zone of rocks in the eastern El Paso We correlate a set of Pliocene rocks in the quartzite. The exotic clasts, placer gold, and well- Mountains (B′ on Fig. 11), but it is less well Christmas Canyon area to rocks in the eastern El rounded clasts match to a sediment source in exposed in the LMSR. Offset needed to restore Paso Mountains (Carter, 1994; Carr et al., 1997; the El Paso Mountains: the Paleocene Goler

Geological Society of America Bulletin, Month/Month 2014 11 Andrew et al.

A. Garlock fault in bedrock B. Garlock fault in Quaternary C. Teagle Wash fault sediments

n = 6 n = 6 n = 4

D. Savoy fault E. Browns Ranch fault zone F. Little Bird fault

Figure 9. Stereographs of fault and bedding data for the Lava Mountains–Summit Range. Stereographs A–I plot data for the main fault plane for each major fault plus a few associated faults. Fault planes are great circles with the corresponding fault striae plotted as a point on the great circle. n = 11 n = 8 n = 3 The motion of the hanging wall of each fault is indicated by the arrow away from the G. Randsburg Wash fault zone H. Blackwater fault I. Northern Blackwater fault zone center. Faults are color-coded by dominant slip type: sinistral = blue, dextral = red, normal = green, and thrust = black. Stereo- graphs J–L plot poles to bedding for different units with best-ﬁ t fold girdles in red and corresponding fold axes labeled β1 and β2.

n = 4 n = 4 n = 17

J. Miocene bedding in K. Miocene bedding in L. Pliocene bedding of central Lava Mountains Christmas Canyon area conglomerate of Golden Valley

β1 β1

β2 β2 β2

1% contours 1% contours 1% contours n = 74 n = 552 n = 82

Figure 10 (on following page). Photographs of faults in the Lava Mountains–Summit Range. Note the dot-in-circle and cross-in-circle symbols that represent motion out of and into the plane of view, respectively. (A) View of the Teagle Wash fault looking upward 30° from horizontal toward the southwest. This view is oblique to the fault striae. The fault juxtaposes Atolia Quartz Monzonite over Bedrock Spring Formation via left-lateral slip on a low- to moderate-angle fault (dipping away in this view). (B) Horizontal-looking view toward the south of the traces of two splays of the Savoy fault. Each splay has a few centimeters of gouge creating a horse of the Bedrock Springs Formation that is sheared and deformed in a left-lateral sense. (C) Detailed oblique view of the Savoy fault looking southwestward, at an angle to the strike. The kinematic indicators show normal oblique left-lateral slip. (D) Along-strike, northeast-looking, cross-sectional view of the Brown’s Ranch fault zone cutting volcanic debris-ﬂ ow deposits of dacite and arkosic sandstone of the Bedrock Spring Formation. Note the associated syncline. (E) Detailed view of the northern Blackwater fault, looking toward the southeast, with kinematic indicators showing a normal component of oblique slip.

12 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault 25 cm Formation

Bedrock Spring dacite gouge dacite gouge of Monzonite Atolia Quartz

Dacite

altered Atolia altered Quartz Monzonite Quartz

Lava Mountains

bedding

overturned

R-shear bedding 5 m lava dacite quartz gouge anticline bleached monzonite C recumbent B overturned Figure 10. Figure quartz monzonite 25 cm 5m 25 cm Volcanics) Bedrock ash tuff Spring Fm. of dacite clasts (Almond Mountain volcanic debris flow volcanics)

felsic porphyry (Summit Range

Foliated gouge Foliated overturnedbedding

quartz monzonite syncline Volcanics) of dacite clasts

(Almond Mountain R-shear R-shear volcanic debris flow Bedrock Spring Fm. Fault surface R′-shear ash tuff volcanics)

felsic porphyry

(Summit Range Fault striae Fault Atolia Quartz Volcanics) Monzonite of dacite clasts (Almond Mountain A E D volcanic debris flow

Geological Society of America Bulletin, Month/Month 2014 13 Andrew et al. ) Formation (Fig. 2). The basal Goler Formation contains coarse conglomerates derived from

the El Paso Mountains and the Sierra Nevada continued ( Mesozoic batholith, quartzite clasts of unknown provenance (Cox, 1982), and placer gold (Dib- blee, 1952). The farther-traveled clasts of the s e

Goler Formation (felsic plutonic and quartzite) t o have well-rounded and polished textures (Cox, N slip with the timing of 1982). We rule out a bedrock source for the ′ exotic clasts in the LMSR based on the inter- mixture of clast types, which are derived from bedrock sources in both the western and eastern ends of the El Paso Mountains, and the presence Paso Mountains. Modern Goler Gulch has an alluvial fan that is 1.8 km wide. conglomerate of Golden Valley of left-lateral offset in the eastern El Paso of left-lateral offset lavas and younger volcanic-sedimentary successions are no specifi c features to restore are no specifi on the Garlock fault in this area (McGill et al., 2009) Mountains (Carr et al., 1997) Mountains, sample JDW-20 and 7.5 Ma for slip between 11.4 dome offset of Golden Valley Mountains, sample LV051810-8 This conglomerate is sourced from the eastern El A-A Slip rate uses Age from U-Pb zircon age of vitrophyre in Lava This correlation has low precision because there These domes are covered by younger dacite These are much slower than neotectonic rates Similar source interpretation as the conglomerate of clasts from the Sierra Nevada batholith and in lava Age from U-Pb zircon age of thick tuff from megacrystic dacite bed offset Subtracted tuff ) r quartzite clasts. y / m 9.1 0.9 3.4 3.9 1.3 Slip rate 10.7 1.65 5.0 to 0.9 to 3.8 to 2.2 to Clasts of altered volcanic rock are also pres- 0.7 to m 1.33 to ent in the conglomerate of Golden Valley. A ( possible source for these clasts is the Cudahy # 6.5 6.5 6.5 Age 11.8 (Ma) 7.5 to 6.522 3.742 to 2.2 1.8 to 7.479 [error] 11.221 11.383 3.14 to 3.14 to ca. 340 Camp Formation in the El Paso Mountains, ca. 460 This measurement includes a fault with ~800 m [± 0.026] [± 0.028] [± 0.018] [± 0.023] which overlies the Goler Formation (Dibblee, 1952; Loomis and Burbank, 1988; Monastero § 40 Slip 32.9 10.7 (km) [± 10] [error] [± 4.5] [± 0.6] [± 0.8] [± 0.7] [± 0.9] [± 0.6] [± 1.8] [± 2.5] [± 0.8] et al., 1997). A potential specifi c source for all [± 1.3] of these clasts is Goler Gulch (Fig. 2), which †

cuts through the Goler and Cudahy Camp For- D 1.11 32.4 0.99 34.7 2.31 16.7 0.65 6.0 0.99 15.5 0.37 32.9 0.69 43.7 0.37 7.3 mations. Goler Gulch is the largest drainage (km) system of the eastern El Paso Mountains and the t l u

only one to cut across the El Paso Mountains a f f southern escarpment. We thus envision it to be o e d a long-lived feature capable of supplying sedi- i s e ment to the southern El Paso Mountains where t i s the Garlock fault would subsequently trans- o p p Ar sanidine) orthoclase o port it away. 39 n o The clasts in the conglomerate of Golden Ar/ 40 e r u

Valley in the Christmas Canyon area (C on t a e

Fig. 11) are offset 30.2–39.2 km (Table 4) from f d e

Goler Gulch in the eastern El Paso Mountains t a l

′ e

(C on Fig. 11). This offset of a wide-area fea- r r o which cuts clast sources of Goler Formation discussion. and Cudahy Camp Formation. See text rocks of Holland Camp (Carr et al., 1997) Formation in Summit Range with ESE strike See text discussion. (Murdoch and Webb, 1940; Samsel, 1962) (Murdoch and Webb, of the metasedimentary rocks El Paso Peaks (Carr et al., 1997) pilotaxitic fabric and basal vitrophyre over Bedrock Spring Formation Gulch which cuts clast sources of Goler Formation and Cudahy Camp Formation. megacrystic dacite dike swarm in granitic rocks western El Paso Mountains megacrystic dacite dome in Lava Mountains ows overlying ca. 12 Ma basalt lava ﬂ

ture agrees with the 32.9 ± 0.6 km offset of the C Deposition across Garlock fault from Goler Gulch Metaconglomerate of the metasedimentary Quartzite and black slate of middle member ~70-m-thick tuff bed within the Bedrock Spring ~70-m-thick tuff Deposition across Garlock fault from Goler 11.383 ± 0.028 Ma (U-Pb) orthoclase 11.383 Correlate to Dove Spring Formation in the Red plagioclase porphyritic lava ﬂ ow with Red plagioclase porphyritic lava ﬂ Paleozoic marker beds. This implies that slip ± 0.18 Ma ( 11.221 †† †† ′ ′ ′ ′ ′ ′ on this segment of the Garlock fault initiated ′ F E B A C D after deposition of the conglomerate of Golden DSF Valley (6.5–3.14 Ma). Simple slip-rate calcula- tions using these data yield long-term slip rates † D 0.80 C 1.85 0.00 MDS 0.00 0.00 2.30 0.91 0.60 TABLE 4. SLIP CONSTRAINTS FOR FAULTS IN LAVA MOUNTAINS–SUMMIT RANGE AREA RANGE MOUNTAINS–SUMMIT IN LAVA CONSTRAINTS FOR FAULTS 4. SLIP TABLE of 5.0–10.7 mm/yr (Table 4). These are similar (km) Label* Description to neotectonic slip rates for the central Gar- lock fault of 7–14 mm/yr (Rittase et al., 2014) and the slightly longer-term rate of 5–9 mm/yr e r u (McGill and Sieh, 1993). t a e Garlock fault—Summit Range. The Atolia f c i

Quartz Monzonite in the Summit Range g o l o e matches across the Garlock fault to similar Ar biotite) orthoclase Ar biotite) orthoclase g 39 39 t

rocks in the southeastern Sierra Nevada (Fig. 1; n i Ar/ Ar/ a 40 40 r Saleeby et al., 2008). Garlock fault offset for t s n restoring these rocks is poorly constrained to o c p i

>42 km using the easternmost exposures of l Cretaceous plutonic rocks in the Sierra Nevada S conglomerate of Golden Valley. Also rounded conglomerate of Golden Valley. boulder textures and placer gold. Formation Range, overlying ca. 12 Ma basalt lava fl ows Range, overlying ca. 12 Ma basalt lava fl megacrystic dacite lava dome in Summit rounded textures and placer gold. in Christmas Canyon area pilotaxitic fabric and basal vitrophyre Ma dacite lavas deposited on top of 10–11 Range, overlying ca. 12 Ma basalt lava fl ows Range, overlying ca. 12 Ma basalt lava fl in conglomerate of Hardcash Gulch. Also in conglomerate of Hardcash Gulch. and meta-limestone sequence** megacrystic dacite lava dome in Summit (Fig. 1). A megacrystic dacite lava dome in volcanics Easternmost exposures of clast assemblage the Summit Range is cut by the Garlock fault Bedrock Spring Formation and Summit Range

(D on Fig. 11), so it probably continued north- ″ Fow with Red plagioclase porphyritic lava ﬂ A Quartzite and black slate beds in meta-siltstone B Metaconglomerate in meta-siltstone sequence E bed in upper Bedrock Spring ~80-m-thick tuff C Distinctive boulder clast assemblage in D ± 0.43 Ma ( 11.24 D ± 0.43 Ma ( 11.24 C

ward but there are no known dacite lavas on Label* Description Garlock fault along the Summit Range Garlock fault along Christmas Canyon area Savoy fault

14 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault

the north side of the fault. There is, however, a ″ , I

′ 1.3-km-wide swarm of north-trending dacitic dikes in the southeastern Sierra Nevada (Sam- sel, 1962; MDS on Fig. 1) that has a similar phenocryst assemblage and orthoclase megacrysts (Murdoch and Webb, 1940). These s e

t dikes could be feeder dikes for the mega- o

N crystic dacite dome in the Summit Range. A sample from these dikes has a U-Pb zircon age of 11.383 ± 0.028 Ma (Fig. 5F; Table 3), compared to the 11.24 ± 0.43 Ma 40Ar/39Ar

′′′ age of the mega crystic dacite lava in the Sum- mit Range (Fig. 4H). These domes and dikes lava ﬂ ow across wide fault zone lava ﬂ age. The fault zone is 480 m wide. age. and I includes El Paso fault, faults in the LMSR, and Canyon fault Cliff rocks to El Paso Mountains from the offset of rocks to El Paso Mountains from the offset the El Paso Mountains to Pilot Knob includes El Paso fault and all of the faults in Lava Mountains–Summit Range (LMSR) Estimated slip restoring edge of 80–100-m-thick Tuff correlation to the thick tuff with a U-Pb zircon correlation to the thick tuff Tuff From Oskin and Iriondo (2004) Subtracted offset of Christmas Canyon Paleozoic Subtracted offset Note, there are three strands denoted by I From Oskin and Iriondo (2004) are the same age within error. The slip of the ) r

y Garlock fault required to juxtapose the ortho- / ) m 0.7 0.3 6.2 Slip rate 0.5 to 0.2 to 5.9 to clase megacrystic dikes of the Sierra Nevada m ( to orthoclase megacrystic dacite domes in the # Summit Range is 43.7 ± 0.8 km (Table 4). continued 7.8 Age 11.4 3.77 (Ma) 6.522 7.5 to 7.479 6.5 to [error] ca. 18 slip of the central Garlock fault zone: Total [± 1.1] ca. 260 slip across the central Garlock fault zone: Total [± 0.11] The Bedrock Spring Formation of the LMSR [± 0.026] [± 0.023] has been correlated with the upper Dove Spring § Formation in the El Paso Mountains (DSF on Slip 29.8 (km) [error] [± 0.7] [± 0.1] [± 0.3] [± 0.6] [± 1.1] [± 1.7]

tor Zone 11 North. The fault slip constraints were projected to the trace, North. tor Zone 11 Fig. 1) (Smith et al., 2002; Frankel et al., 2008). [+5.1/–3.8] We ﬁ nd that the Bedrock Spring Formation ent. The error measurement is the interpreted maximum and minimum extreme ent. †

D matches well in time, composition, and sedi- 0.0 0.3 to 1.8 7.2 0.1 2.1 1.13 1.9 1.83 3.9 0.68 62.7 0.37 67.6 (km) n sedimentary textures but have been partially recrystallized. ment transport direction (Smith, 1964; Loomis

ciﬁ c points of line and area features, errors in horizontal projection using feature azimuths, ciﬁ and Burbank, 1988; Whistler et al., 2009) with t l u

a member 5 of the Dove Spring Formation. The f f

o lower four members of the Dove Spring Forma- e d

i tion correlate in time with the Summit Range s e

t volcanics, but these do not have dacite lava i s

o domes or the associated near-vent volcanic p p

o rocks. An 11.8 ± 0.9 Ma (Loomis and Burbank, n

o 1988) thick felsic tuff occurs in member 2 of e r u

t the Dove Spring Formation, which could have a e

f erupted from the lava domes of the Summit d e

t Range volcanics (Frankel et al., 2008). These a l e

r correlations loosely constrain offset of the Gar- r o quartz monzonite overlying Bedrock Spring Formation distal facies of the Almond Mountain Volcanics, Almond Mountain Volcanics, distal facies of the dips 52° into fault zone Spring Formation against the side of dacite domes Formation on the east side of a fault contact with greenstone (Carr and Poole, 1992) 980 m wide. Nevada (Samsel, 1962). Azimuth of 289, and Nevada (Samsel, 1962).

C lock fault to be 30–50 km (Table 4). ENE-striking buttress unconformity of Bedrock ow over North edge of thick glassy dacite lava fl Eastern edge of Lava Mountain Dacite lava fl ow Eastern edge of Lava Mountain Dacite lava fl Early Miocene(?) felsic dike swarm in the Sierra 4-m-thick bluish pumice lapilli tuff bed of the 4-m-thick bluish pumice lapilli tuff Northern edge of basalt lava fl ow Northern edge of basalt lava fl Metaconglomerate of the Robbers Mountain 0.1 1.8 The Pliocene conglomerate of the Hardcash †† †† †† ′ ′ ′ Gulch contains clasts consistent with a source ′′′ H I G from Goler Gulch in the eastern El Paso Moun- tains (Fig. 11) based on the presence of exotic

† clasts, well-rounded textures of boulders, and D 0.0 J 0.00 0.1 0.62 1.57 PKV (km) Label* Description 18.5 SESD placer gold (see earlier discussion of Pliocene conglomerates). Garlock fault offset needed to TABLE 4. SLIP CONSTRAINTS FOR FAULTS IN LAVA MOUNTAINS–SUMMIT RANGE AREA ( AREA RANGE MOUNTAINS–SUMMIT IN LAVA CONSTRAINTS FOR FAULTS 4. SLIP TABLE deposit the easternmost exposure of the conglomerate of Hardcash Gulch is 15.5 ± 0.9 km e r u t (C″ to C′ on Fig. 11; Table 4). The age of this a e f conglomerate is unknown, but it is younger than c i

g the Lava Mountain Dacite–correlated intrusive o l o

e rocks it overlies, and the western parts may be g t

n as young as early Pleistocene. Slip-rate calcula- i a r t tions for minimum and maximum values yields s n

o 2.2–9.1 mm/yr (Table 4). The younger ages c p i

l therefore are more consistent with the neotec- S tonic slip rates for the central Garlock (McGill quartz monzonite Spring Formation against side of dacite facies of the Almond Mountain Volcanics. Almond Mountain Volcanics. facies of the This bed dips moderately to the southeast. domes Crags (Sabin, 1994; Monastero et al., 1997). Azimuth of 281, and 990 m wide. Formation on east side of a fault with Devonian(?) greenstone (Carr et al., 1997)

overlying Bedrock Spring Formation and Sieh, 1993; Rittase et al., 2014). Metaconglomerate of the Robbers Mountain 18 Ma “high-silica rhyolite dikes” in the Eagle Northern edge of basalt lava ﬂ ow Northern edge of basalt lava ﬂ 0.0 BM Teagle Wash fault. There are no fault slip †† †† †† I ENE-striking buttress unconformity of Bedrock

Jow over North edge of thick glassy dacite lava fl markers for the Teagle Wash fault. The presence H of the distal 4-m-thick bluish pumice lapilli tuff Gow Eastern edge of Lava Mountain Dacite lava fl These constraints are located outside the area of Figure 11; see Figure 1 for their locations. These constraints are located outside the area of Figure 11; BM D is the projection distance of feature to fault. Merca Transverse ArcGIS database projected in Universal The amount of fault slip was measured using map data in a georeferenced This is the age of fault-slip constraint geologic feature. MC *Label identifi es slip constraint features shown on Figure 11. *Label identifi **We use the “meta- + (sedimentary rock)” terminology of Carr et al. (1997), because this best describes these rocks that retai **We † § # †† Label* Description

ECDS of Jurassic Laurel Mountain granodiorite below Inferred fault south of Christmas Canyon Garlock fault between El Paso Mountains and Pilot Knob area geometric extrapolations of slip markers to the fault trace in question, and it encapsulates errors exact correlation spe and uncertainties associated with elevation differences. Browns Ranch fault zone Blackwater fault Garlock fault between Sierra Nevada and Pilot Knob area and the distance between them was measured along the trace of the fault. The slip amount shown is our preferred offset measurem The slip amount shown is our preferred offset and the distance between them was measured along trace of fault.

Geological Society of America Bulletin, Month/Month 2014 15 Andrew et al.

Tp Jl C Tp Te P*h D_e O_c Tpg A Tg Jl B *b Dg Christmas Tpg Tc Tc Teagle Wash Canyon Jl Garlock fault Little Bird fault Te Tb Tc NBF Tb D′ Tb Jl QTph NBF Buried inferred fault(?) Tc TWF Savoy fault Ts Tb NBF Ts Ts C″ Ka I”’ Tpg D_e Jl Tlr I′ I″ WFZ F R Tg D Te Summit Range Ka El Paso E Ka Tb I Goler Ts Te Tb Gulch A′ Tb F′ Tlr Mountains Tb Te H H′ Ts *b Dg E′ Tl Tl G QTph Tl Ta Ts Tb G′ B′ Tlr ns Te C′ Lava Tb Te Garlock fault Tpr MountaiTl Tl Blackwater fault Black Ka Tpr Savoy Tprfault Hills Pg Tb Tl Ta Ka P*h Tpr Kr Ts Tb Ta Tl Browns Ranch faultTa zone Ta Ta(?) Tb Te Tl Ka Ta Tl Tl Kr Ta J′ J Tb Ka Ka Tb Te 5 km Te

Figure 11. Fault-slip restoration points plotted on the simpliﬁ ed geologic map of the Lava Mountains–Summit Range (Fig. 2). The labeled thick circles are reconstruction points for slip constraints derived from this new study, where, for example, X restores adjacent to X′, X″, and X′′′′′′ (see text and Table 4 for descriptions). Slip marker C is the outcrop area of the conglomerate of Golden Valley along the Garlock fault.

this fault requires a much larger amount of strike the megacrystic dacite in the Lava Mountains Brown’s Ranch fault zone. Determining slip slip, because the map distance, as measured on and for the age constraint of the offset on the for the Brown’s Ranch fault zone is diffi cult the north side of the Garlock fault, between Savoy fault. because it is oriented parallel to the strike of the the Jurassic granodiorite in the eastern El Paso Younger units can be correlated across the upper Miocene units. There is an apparent off- Mountains and the Atolia Quartz Monzonite– Savoy fault. An 80-m-thick tuff bed occurs set of the Lava Mountain Dacite in map view correlated plutons in the Sierra Nevada is 34 km in the upper part of the Bedrock Spring Forma- (Figs. 2 and 11; Table 4). We estimate that 3.9 ± (Fig. 1). tion in the Lava Mountains where it is cut by the 0.7 km of left-lateral offset is needed to restore Savoy fault. The Savoy fault cuts all of the Savoy fault (E on Fig. 11). A similarly thick tuff the eastern edge of the exposures of the Lava Miocene and Pliocene units of the LMSR. The bed within Bedrock Spring Formation is exposed Mountain Dacite (G and G′ on Fig. 11; Table 4) only locations of Summit Range volcanics in the western Summit Range (E′ on Fig. 11). across the Brown’s Ranch fault zone. with ca. 12 Ma basalt fl ows overlain by ca. The offset needed to juxtapose these tuff beds Little Bird fault. The Little Bird fault does 11 Ma dacite dome complexes are in the cen- is ~6.0 ± 0.7 km (Table 4). The age of this tuff not have any specifi c geologic features that can tral Summit Range and in the northeastern Lava bed is unknown, but we interpret it to correlate be used to measure offset across it. It juxtaposes Mountains. Dacite lava with distinctive mega- to the only other thick tuff within the Bedrock Paleozoic metasedimentary rocks with Creta- crystic orthoclase occurs in the central Summit Spring Formations, which has an age of 7.479 ± ceous granitic rocks; these rocks are >26 km Range (D on Fig. 11) with an age of 11.24 ± 0.023 Ma (Fig. 5C). The distinctive red fl ow- apart to the north of the Garlock fault. 0.43 Ma (Fig. 4H) and also in the northeastern banded lava fl ow of the 6.5 Ma Lava Mountain Lava Mountains (D′ on Fig. 11) with an age of Dacite can also be correlated across the Savoy Dextral Fault 11.221 ± 0.018 Ma (Fig. 4I). We correlate these fault (F and F′ on Fig. 11, respectively), requir- Blackwater fault. The facies of the Almond dacites along with their associated lava domes ing 7.3 ± 1.3 km of slip (Table 4), similar within Mountain Volcanics are mismatched across the and basalt fl ows of Summit Range volcanics error to that of the tuff offset. Because we have northern portion of the Blackwater fault. East across the Savoy fault for left-lateral offset of offset markers of different ages, we can calculate of the Blackwater fault there is only a single 16.7 ± 1.8 km (Table 4). The megacrystic dacite two intervals of slip (Table 4). Combining these bluish-colored pumice lapilli tuff bed within in the Summit Range correlates to the mega- yields slip in the interval between 11.2 and ca. the Bedrock Spring Formation (H on Fig. 11). crystic dacitic dikes in the Sierra Nevada that 7 Ma of 10.7 ± 2.5 km. Using the total slip esti- This tuff dips southwest at a moderate angle, is have a U-Pb zircon age of 11.383 ± 0.028 Ma; mate above for the fault yields motion of ~6 km ~4 m thick, and is part of the distal facies of the therefore we take ca. 11.4 Ma as the best age for between ca. 7 Ma and present. Almond Mountain Volcanics. The west side of

16 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault the Blackwater fault exposes both the distal and IMPLICATIONS FOR THE CENTRAL 16.7 ± 1.8 km of slip of ca. 11.4 Ma features near-vent facies of the Almond Mountain Vol- GARLOCK FAULT ZONE (see the age discussion in the “Magnitude and canics. At the northern extent of volcanic debris- Rates of Fault Slip” section), and the Brown’s fl ow units in the Bedrock Spring Formation Inferred Fault South of Christmas Canyon Ranch fault zone has 3.9 ± 0.7 km of slip of ca. there is a 4–5-m-thick tuff that ends eastward at 6.5 Ma features (Table 4). Adding these offsets the Blackwater fault (H′ on Fig. 11). This same The slip data for Paleozoic rocks at Christmas to the 43.7 km offset for the Summit Range seg- tuff bed is dextrally offset in repeated slivers Canyon require the presence of an unexposed ment of the Garlock fault gives cumulative left- across a ~600-m-wide fault zone. Total offset of fault south of the Christmas Canyon area. These lateral slip of 64.3 km, similar to the ~64 km the tuff is 1.9 ± 0.3 km (Table 4). Paleozoic rocks do not continue southward, as total slip value derived from Paleozoic, Meso- Farther north along the projection of the the Black Hills have Cretaceous Atolia Quartz zoic, and early Miocene offset markers outside Blackwater fault there are right-lateral offsets of Monzonite as basement (Fig. 11). A large expo- the distributed zone of faulting of the LMSR the northeast-striking buttress unconformity of sure of Paleozoic rocks occurs east of the LMSR (Table 1). This amount of slip supports the the Bedrock Spring Formation deposited against on the south side of the Garlock fault in the Pilot interpretation that motion on the Garlock fault the steep southern sides of the lava domes of the Knob Valley area (PKV on Fig. 1), which are began at or after ca. 11 Ma (e.g., Burbank and Summit Range volcanics (I, I′, I″ and I′′′ on Fig. offset 62.7 ± 1.7 km (Table 4) from the El Paso Whistler , 1987) and that sinistral slip is distrib- 11). The total offset of these points across the Mountains. There are no direct ties of the Paleo- uted across a 12-km-wide fault zone. Thus, for zone of faulting is 2.1 ± 0.6 km (Table 4). The zoic rocks of Christmas Canyon with the Pilot this area we refer to the sinistral faults together lava domes partially blocked the northward fl ow Knob area, but the Christmas Canyon rocks are as the Garlock fault zone (GFZ). in the basin of the Bedrock Spring Formation displaced 32.9 ± 0.6 km from the El Paso Moun- as shown by the presence of lacustrine lime- tains by the Garlock fault. Thus, a fault with Initiation of Regional Dextral Shear stone only along this northeast-trending buttress 29.8 km of left-lateral slip (Table 4) presumably unconformity. These ~2 km right-lateral slip exists south of Christmas Canyon to restore it to The GFZ is embedded in an active zone values are similar to those obtained by Oskin Pilot Knob Valley. of dextral shear (see references in Gan et al., and Iriondo (2004) from farther south along the Based on outcrop patterns, this inferred fault 2003). Initiation of dextral shear occurred in Blackwater fault (Table 4) using slip markers of must have a ENE strike in an area that is cov- Death Valley, to the east of the LMSR (Fig. 1), 7.2 ± 1.1 Ma (J to J′, Fig. 11) and 3.77 ± 0.11 Ma ered by the Pliocene conglomerate of Golden at ca. 7 Ma (Holm et al., 1993; Topping, 1993), (BM on Fig. 1). Valley and Quaternary alluvial deposits. This although Mancktelow and Pavlis (1994) argued conglomerate overlaps the Little Bird fault and that this initiated at ca. 11 Ma. The initiation age Regional Garlock Fault probably also the inferred fault. Juxtaposition of is younger in areas west of Death Valley: ca. Additional slip markers are needed for the the Christmas Canyon area to the north of the 4 Ma in Panamint and Searles Valleys (Burch- central Garlock faults to evaluate the fault Black Hills by this inferred fault was completed fi el et al., 1987; Hodges et al., 1990; Zhang slip in the LMSR. The Paleozoic rocks in the prior to deposition of the capping basalt boul- et al., 1990; Snyder and Hodges, 2000; Walker El Paso Mountains have been correlated to der layer of the conglomerate of Golden Valley et al., 2014), and 2–3 Ma farther to the west in similar Paleozoic rocks in the Pilot Knob area because it has basalt clasts without dacite lava Indian Wells Valley (Monastero et al., 2002). (Fig. 1) for a left-lateral offset of 48–64 km on clasts; the Black Hills are the only exposure This westward migration of dextral shear possi- the central Garlock fault (Smith and Ketner , of ca. 12 Ma basalt without overlapping dacite bly also occurred in the Eastern California shear 1970; Carr et al., 1997; Table 1). Geologic domes (Fig. 2). zone, but the initiation ages are poorly known: map data show a steeply dipping fault in the beginning after 6 Ma (Dokka and Travis , 1990; El Paso Mountains that places Mississippian Initiation of the Garlock Fault Zone Schermer et al., 1996; Glazner et al., 2002), meta-conglomerate of the Robbers Mountain 3.8 Ma (Oskin and Iriondo, 2004), or even Formation against Devonian(?) greenstone Slip markers in the LMSR yield offsets for younger (Miller and Yount, 2002). The age of (Carr et al., 1997). A steeply dipping fault in the Garlock fault that differ from the total off- the initiation of dextral shear in the LMSR is the Pilot Knob area juxtaposes the same units set of ~64 km (Table 1). Sinistral slip on the interpreted to be <3.8 Ma, the time for initia- (Carr and Poole, 1992). Matching these faults Garlock fault required to juxtapose the mega- tion of the dextral slip on the Blackwater fault (MC and PKV, respectively, on Fig. 1) yields crystic dacite domes in the Summit Range with (Oskin and Iriondo, 2004; this paper), similar to 62.7 ± 1.7 km of offset (Table 4). megacrystic dikes in the Sierra Nevada is 43.7 ± ca. 4 Ma initiation in Searles Valley (Fig. 1) just Another offset marker for the central Gar- 0.8 km (Table 4), whereas ca. 18 Ma high-silica north of the LMSR. lock fault is an 18 Ma swarm of WNW-striking, dikes southeast of the LMSR appear to record +5.1 “high-silica rhyolite dikes” (ECDS on Fig. 1) in the full offset of the Garlock (67.6 /–3.8 km) Change in Deposition Systems the Eagle Crags (Sabin, 1994; Monastero et al., to the Sierra Nevada. This difference may be 1997). A set of dikes with similar width, azi- explained in two ways: (1) the Garlock fault slip There is a profound change in the deposition muth, and composition occurs in the southeast- began between 18 and 11.4 Ma, so the mega- systems along the GFZ after ca. 7 Ma. The Bed- ern Sierra Nevada (SESD on Fig. 1) intruded crystic dacite is too young to record the full Gar- rock Spring and Dove Spring Formations along into Cretaceous granite (Samsel, 1962). These lock fault offset; or (2) slip in the vicinity of the the central GFZ are thick successions of arkosic dikes are correlated to early Miocene volcanism LMSR is distributed among several faults, with sandstone deposited from 11.5 to 7.0 Ma (Fig. (Dibblee, 1967), as this swarm projects into a the megacrystic dacites of the Summit Range 12), with sediment transported northwestward lower Miocene volcanic center (Coles et al., within and the 18 Ma dikes outside of this fault from granitic bedrock sources in the central 1997). A slip marker using these dikes yields zone. We favor the second hypothesis because Mojave Desert area (Smith, 1964; Loomis and +5.1 67.6 /–3.8 km left-lateral slip on the central of the presence of several faults with left-lateral Burbank, 1988; Whistler et al., 2009; this paper). Garlock fault (Table 4). offset within the LMSR: the Savoy fault has The conglomerate of Golden Valley occurs as a

Geological Society of America Bulletin, Month/Month 2014 17 Andrew et al.

11 Ma 109876543210

Christmas Canyon-Garlock fault 32.9 km

Little Bird fault >5 km

open folds with NNW axes open folds with WSW axes

10.7* km 19.1* km inferred fault

East of Blackwater fault conglomerate of Golden Valley

Blackwater fault –2 km

El Paso Mountains: Dove Spring Fm. Lava Mountains conglomerate of Bedrock Spring Fm. Dacite Hardcash Gulch Summit Range Almond Bedrock volcanics Mountain monoclines with WSW axes Volcanics Spring Fm. open folds with NNW axes

19.1 km 9.1 km

pre-conglomerate of Hardcash Gulch Summit Range-Garlock fault post-conglomerate of Hardcash Gulch West of Blackwater fault West 15.5 km Summit Range-Garlock fault Teagle Wash fault >2.2 km

10.7 km pre-Lava Mountains Dacite Savoy fault

post-Lava Mountains Dacite Savoy fault 6.0 km

Browns Ranch fault zone 3.9 km Lithology Interpreted Deformation Conglomerate Period 1: Basin and Range transform Initiate Volcanic rocks Period 2: Transition to dextral shear LMSR area Reorganize Garlock fault Garlock fault Initiate dextral

Arkosic sandstone shear in central Period 3: Regional dextral shear

Figure 12. Summary of timing constraints for the Lava Mountains–Summit Range (LMSR) area. Faults, rock units, and folds are arranged from east to west relative to the Blackwater fault. Box outlines denote the maximum and minimum timing constraints for rock units and deformation. Pattern ﬁ lls indicate the interpreted timing of each feature based on the three-stage deformation history model of the Garlock fault zone. The slip amount interpreted for each fault in each stage is given. Slip amounts are negative for dextral slip, and amounts with asterisks are calculated from other slip constraints. The vertical gray bars denote key times in the history. The 9.1 km for the Garlock fault between 4 and 2 Ma is derived by subtracting the total slip of the Summit Range segment of the Garlock fault and subtracting the 7–3.8 Ma and 2–0 Ma slip values (19.1 km and 15.5 km, respectively) for the same fault.

local basin sourced from a now-displaced local Current Structural Confi guration of Teagle Wash fault is somewhat different and uplift along the GFZ, as discussed above. The the Garlock Fault Zone has more similarity with the Little Bird fault in modern deposition systems along the GFZ are that it juxtaposes different-age basement rocks similar, having local closed basins and sediment The confi guration of faults is different on (i.e., Cretaceous versus Jurassic), implying a sources in nearby uplifts (Fig. 1). The topogra- either side of the Blackwater fault (Fig. 11): signifi cant left-lateral slip. Slip on the Teagle phy of the central GFZ therefore changed from to the west, the Summit Range segment of the Wash fault resolves onto the Savoy and Gar- one of low relief that did not signifi cantly dis- Garlock fault is active, with additional slip on lock faults, and so its history is not critical to turb transport and deposition to a system with several other sinistral strike-slip faults (Teagle this analysis. WSW-trending folds occur in all locally high relief creating uplifts and closed Wash fault, Savoy fault, and Brown’s Ranch Miocene (Figs. 9J and 9K) and Pliocene (Fig. basins. We postulate that the change to dextral fault zone); to the east, there are only the Gar- 9L) rock units and have been interpreted to shear could have led to creation of higher-relief lock and dip-slip Randsburg Wash faults. The have been active as recently as the Pleistocene topography along the GFZ, and therefore this Garlock, Savoy, and Randsburg Wash faults (Smith, 1964, 1991). conglomerate may have formed during the initi- are all correlated with Quaternary fault scarps, This young deformation can be more fully ation of local dextral shear at ca. 3.8 Ma (Oskin and the Brown’s Ranch fault zone was inter- explored in the context of a regional deforma- and Iriondo, 2004). preted by Smith (1964) to have young slip. The tion model. The model is based on two obser-

18 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault vations: (1) the trace of the Garlock is curved; mation older than 3.8 Ma in the LMSR does Garlock Fault Zone Slip Rates and (2) dextral faults in the areas to the north not accommodate dextral slip. Unfortunately, and south do not cut the Garlock (i.e., slip on long-lived strike-slip fault systems like the Slip rates calculated in Table 4 are based on the Blackwater fault ends northward before the GFZ do not preserve a complete detailed his- the minimum and maximum timing constraints Garlock fault). The curved trace of the Garlock tory of slip. To aid interpretation we assume for fault motion, but these do not specify when fault is an outcome of progressive bending of that the older GFZ was a simple strike-slip the faults were active. When these slip esti- an originally northeast-trending fault by trans- fault with only one active fault strand because mates are considered in light of the three-stage versely oriented dextral shear distributed along it did not need to accommodate dextral slip deformation scenario proposed above (Fig. the central and eastern segments (Gar funkel, and shear (see Fig. 12 for interpreted timing of 12), we can use the entire data set to estimate 1974; Dokka and Travis, 1990; Gan et al., fault slip and Fig. 13 for interpreted time-slice slip rates within the fault system. The earliest 2003). Young deformation in the LMSR must maps). The single-strand assumption is sup- stage involved the Savoy fault with a slip rate accommodate the dextral faulting of the Black- ported by observing that the western Garlock of 3.1 mm/yr (10.7 km over the interval from water fault without cutting the Garlock fault as fault has this character and is outside the zone 10.5 to ca. 7 Ma). If the single-strand assump- well as larger-scale clockwise bending of the of dextral shear. tion for the early Garlock fault is not true, then trace of the Garlock fault. The slip constraints from the LMSR using additional slip would have occurred on the Sum- Accommodation of dextral slip in the LMSR the volcanic-sedimentary assemblages as the mit Range segment of the Garlock fault at this necessitates NNW-SSE–oriented shortening of key time markers allow a view of two incre- time, which would increase the slip rate for this western side of the Blackwater fault relative ments in the pre–3.8 Ma history of the GFZ. interval. The slip rate for the Garlock fault in the to the eastern side. The NNW-SSE shortening The oldest offset markers are ca. 11.4 Ma intermediate stage is a maximum of 6.0 mm/yr could be taken up by the WSW-trending fold- megacrystic dacite domes and feeder dikes, (19.1 km over the interval from ca. 7 to 3.8 Ma); ing and also by lateral escape of fault slivers but the relative locations of slightly younger this rate would decrease if some slip on these between the Summit Range segment of the (10.5 Ma) dacite domes across the Savoy fault faults occurred in the earlier stage. Youngest- Garlock fault, the Savoy fault, and the Brown’s could give a better maximum age. The end of stage slip in the eastern LMSR is interpreted to Ranch fault zone (Fig. 11). NNW-SSE elonga- deposition of the Bedrock Spring Formation have been on the Christmas Canyon segment of tion east of the Blackwater could be partially demarks the end of the early slip of the Savoy the Garlock fault with a slip rate of 8.7 mm/yr taken up by normal slip on the Randsburg Wash fault and the beginning of slip on the Little (32.9 km since 3.8 Ma). This slip-rate accelera- fault. These deformation mechanisms would Bird fault. The 7.5 and 6.5 Ma slip markers for tion is linked to the initiation of dextral faulting absorb the slip of the Blackwater so that the the Savoy fault have indistinguishable offsets in the LMSR area. West of the Blackwater fault Garlock fault is not offset and would allow the within error; for simplicity in this discussion, the sinistral slip of the GFZ is divided across trace of the Garlock to be bent by increasing we average these ages to ca. 7 Ma. The next- three main faults and fault zone: the Summit clockwise amounts eastward. younger marker unit is the conglomerate of Range segment of the Garlock fault (43.7 km – The slip history of faults in the Christmas Golden Valley whose age is not well known. 19.1 km = 24.6 km of slip) has a rate of 6.5 Canyon appears to also ﬁ t with this dextral slip We interpret its deposition to be linked with the mm/yr, the Savoy fault (6.0 km of slip) has a accommodation model. The Paleozoic rocks of beginning of the dextral deformation, because rate of 1.6 mm/yr, and the Brown’s Ranch fault the Christmas Canyon area were north of the the same deposition system that created this zone (3.9 km of slip) has a rate of 1.0 mm/yr. main trace of the GFZ system until after deposi- conglomerate is still active from Goler Gulch. The distribution of sinistral slip across mul- tion of the conglomerate of Golden Valley when We assume that deposition began at ca. 4 Ma tiple faults in the western LMSR creates dis- the Christmas Canyon segment of the Garlock in this analysis. crepancies in the slip rates along the modern fault formed. Block transfer across the GFZ by a These assumptions allow a glimpse of the GFZ. The single-stranded Garlock fault east of left step in the main trace of the GFZ effectively earlier slip history of the GFZ. The earliest the LMSR has neotectonic slip rates, depending adds material to the east side of the Blackwater slip was 10.7 km on the Savoy from 10.5 to ca. on the age of the marker, of 4–9 mm/yr (McGill fault. This breaking of a new fault also allows a 7 Ma. This amount of slip was taken up in the and Sieh, 1993) and 7–14 mm/yr (Rittase et al., more continuous trace to the active strand of the eastern LMSR on the inferred fault discussed 2014), which match with our longer-term, Garlock fault. We envision that these processes above. Afterward from ca. 7 Ma until 3.8 Ma, model-interpreted slip rate of 8.7 mm/yr. West could apply to other intersections of dextral slip occurred on the Summit Range segment of of the LMSR the neotectonic slip rates on the faults with the GFZ and possibly to the larger- the Garlock, Teagle Wash, and Little Bird faults central Garlock fault are 4.5–6.1 mm/yr (Clark scale clockwise bending of the trace of the Gar- and the inferred fault. The intermediate-stage and Lajoie, 1974), which agrees with the model- lock fault. This model implies that the wide, GFZ had a maximum sinistral slip of 19.1 km calculated slip rate of 6.0 mm/yr for the Summit multiple-fault structure of the GFZ formed in (subtraction of the 10.7 km of Savoy fault slip Range segment of the Garlock fault. Our model- response to dextral slip of the Blackwater fault, from the 29.8 km calculated slip for the inferred calculated rates for the Summit Range segment which began after 3.8 Ma (Oskin and Iriondo, fault) if the Summit Range segment of the Gar- of the Garlock fault plus those for the Savoy fault 2004; this paper). lock fault was active only after the ﬁ rst stage add to 7.6 mm/yr, which agrees with the 5.3–10.7 of slip on the Savoy fault. NNW-trending folds mm/yr neotectonic rate for the western segment Slip History of the Garlock Fault Zone formed during the intermediate-stage slip in the of the Garlock fault (McGill et al., 2009). areas along the Little Bird fault. We speculate The current structural configuration of that this fold set may be due to the ESE strike of CONCLUSIONS the LMSR, as outlined above, implies a link the Little Bird fault. This orientation would have between the wide, multi-stranded GFZ and caused the Little Bird fault to have a restraining The LMSR area is a Miocene to Pliocene the accommodation of dextral slip and clock- orientation in the sinistral fault system, forming volcanic-sedimentary complex adjacent to the wise bending of the Garlock. Therefore defor- NNW-trending folds. Garlock fault that contains rocks created before,

Geological Society of America Bulletin, Month/Month 2014 19 Andrew et al.

Restore: Slate A 11 Ma Range Thrust faults: Mesquite Restored fault slip in central Garlock fault region; Canyon (MC) & Pilot prior to slip on GFZ and ECSZ and extension in the Knob Valley (PKV) ESTS western Basin and Range IDS Independence dike swarm 0102030 (IDS) km detachment Eastern Sierra thrust Future Slate Range system (ESTS) N ESTS 18 Ma silicic dike swarm in SE Sierra Nevada (SESD) along trend of IDS Eagle Crag dike swarm (ECDS) Summit Range volcanics Granite (SRV) with megacrystic Mountains dacitic dike swarm (MDS) El Paso MC Mountains Christmas Slate Canyon Range Sierra Summit Nevada Range ESTS

MDS PKV

Slate Range Future Savoy/inferred fault system detachment SESD ESTS Eagle SRV Crags IDS ECDS Lava Mountains Black Hills

QT Plio-Pleistocene conglomerate Ts Upper Miocene arkose Tu 6.5-8 Ma volcanic rocks inferred fault Tm 10-12 Ma volcanic rocks Tl Lower Miocene volcanic & Restore: sedimentary rocks 6.5 to 8 Ma units of Te Eocene Goler Formation Bedrock Spring UDSF Paleocene Goler Formation Fm. (BSF) & K Cretaceous metamorphic & upper Dove future Garlock fault plutonic rocks

future Cliff future

Spring Fm. fault Canyon Ji Jurassic plutonic rocks (UDSF). BSF Jv Jurassic metavolcanic rocks Note that the ^ Triassic metaplutonic rocks Summit Range & Savoy fault P Permian metamorphic rocks Christmas * Pennsylvanian metamorphic rocks Canyon blocks M Mississippian metamorphic rocks are located north D Devonian metamorphic rocks of the main trace O_ Cambrian & Ordovician of the GFZ. B 7 Ma metamorphic rocks Region after initial slip on Savoy and inferred faults and extension on the Slate Range detachment.

Figure 13 (on this and following page). Interpreted time-slice maps using new fault displacement data (Table 4) and interpreted fault slip chronology (Fig. 12). Thick black lines shown in each time slice are faults that were active before the time-slice ﬁ gure, and dashed faults are active after. Red-ﬁ lled dots are the locations of the megacrystic dacite lava domes in the Summit Range and Lava Mountains for each time-slice map. Note that rock unit labels are only on the time slice in D. The interval 4–0 Ma is interpreted to have local clockwise vertical-axis rotations; blocks with rotations are shown with a curved arrow in C. Panel D shows the area interpreted to be the wide active Garlock fault zone as diagonal hatching. Fault displacement data from the Slate Range are from Andrew and Walker (2009) and Andrew et al. (2011). Data for the Cerro Coso fault are from Casey et al. (2008) and Andrew et al. (2011). Data for the Cliff Canyon fault are reconnaissance data (collected by the authors of this study). The right-lateral slip of the Goldstone Lake fault is an interpreted amount in order to juxtapose and align the Eastern Sierra thrust system (ESTS) across the Garlock fault. GFZ—Garlock fault zone; ECSZ—Eastern California shear zone.

20 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault

C 4 Ma N t Region after strike-slip displacement on the Garlock fault and extension on the Cliff Canyon fault. Valley faul Initiation of uplift along the: Sediment from Goler Gulch (GG) future Searles is shed across the GFZ for deposition of the conglomerate of Golden Valley (CGV) on recently juxtaposed Lava Mountains and Christmas Canyon blocks. future Cerro Coso fault Future: Summit Range segment of the Garlock future Marine Gate fault fault (SR-GF) Christmas Canyon segment GG of the Garlock fault inferred fault (CC-GF) future Goldstone future CC-GF CGV Lake fault

CGV

futureCliff reactivated Canyon faultfuture El Paso fault

Blackwater fault future SR-GF future Jv

future reactivated Savoy fault Searles

Valley fault Valley future Browns Ts Ranch fault zone Ji Jv

Cerro Coso fault Ji D 0 Ma QT Region after initiation of ECSZ Spangler Hills and the creation of the Marine Gate fault Tl Te Garlock fault Tl wide and complex PKV GG Ji Garlock fault Garlock fault D Tl Tm QT Tl Eagle zone. Tm O_ Summit RangeTs Ts Crags Tu ECDS ^ MC P * CHG Tm Tl Savoy faultLava Blackwater fault K El Paso D QT Mountains Black QT Mountains Tu Hills K Ts ^ Tu Tu Sierra Ts Nevada fault Canyon Cliff Garlock fault Browns Ranch fault K Ts El Paso fault Harper Lake fault Rand MDS Mountains Cantil fault Continued uplift along GFZ: K sediment from Goler Gulch (GG) K deposited as the conglomerate of SESD Lockhart fault Hardcash Gulch (CHG) and then this conglomerate is displaced by Garlock fault subsequent left-lateral slip.

Figure 13 (continued).

during, and after slip on faults in the GFZ. Geo- nected to the inferred fault via the Teagle Wash slip history occurred after 3.8 Ma when the GFZ logic features yield constraints on fault slip that and Little Bird faults. The ESE strike of the changed into a locally wide, multi-stranded indicate a three-stage history. The GFZ began Little Bird fault was a constrictional bend in the fault zone to accommodate dextral offset of the as a single-stranded fault after 10.5 Ma with GFZ as recorded by NNW-trending folds. This Blackwater fault and regional clockwise orocli- slip on the Savoy fault continuing eastward on constrictional bend was eventually abandoned nal bending of the GFZ. This stage had ~33 km a now-buried inferred fault south of Christmas as the sinistral fault system stepped leftward, of slip on the GFZ, which is roughly half of the Canyon. We interpret an intermediate stage of creating the Christmas Canyon segment of the total offset. Topography along the GFZ changed deformation between ca. 7 Ma and 3.8 Ma when Garlock fault to the north of Christmas Canyon. at the beginning of this stage of deformation to the active strand of the GFZ became the Summit The ﬁ rst and intermediate stages have ~30 km one of local high relief, creating a new pattern Range segment of the Garlock fault which con- of cumulative sinistral slip. The last stage in the of depositional systems. The slip rates for the

Geological Society of America Bulletin, Month/Month 2014 21 Andrew et al.

recent motion on the Garlock fault calculated physics, Geosystems, v. 12, Q0AA19, doi:10 .1029 Frankel, K.L., Glazner, A.F., Kirby, E., Monastero, F.C., /2010GC003479 . Strane, M.D., Oskin, M.E., Unruh, J.R., Walker, J.D., using the three-stage deformation scenario are Brueseke, M.E., Heizler, M.T., Hart, W.K., and Mertzman, Anandakrishnan, S., Bartley, J.M., Coleman, D.S., 6.0 mm/yr for the Summit Range segment and S.A., 2007, Distribution and geochronology of Oregon Dolan, J.F., Finkel, R.C., Greene, D., Kylander-Clark, 8.7 mm/yr for the Christmas Canyon segment. Plateau (USA) fl ood basalt volcanism: The Steens Ba- A., Marrero, S., Owen, L.A., and Phillips, F., 2008, Ac- salt revisited: Journal of Volcanology and Geothermal tive tectonics of the Eastern California shear zone, in The multi-stranded confi guration of the GFZ Research, v. 161, p. 187–214, doi: 10 .1016 /j .jvolgeores Duebendorfer, E.M., and Smith, E.I., eds., Field Guide west of the Blackwater fault explains the slower .2006 .12 .004 . to Plutons, Volcanoes, Reefs, Dinosaurs, and Possible slip rate of the central Garlock fault along the Burbank, D.W., and Whistler, D.P., 1987, Temporally con- Glaciation in Selected Areas of Arizona, California, strained rotations derived from magnetostratigraphic and Nevada: Geological Society of America Field El Paso Mountains (Clark and Lajoie, 1974) data: Implications for the initiation of the Garlock Guide 11, p. 43–81, doi: 10 .1130 /2008 .fl d011 (03) . compared to sites east of the Blackwater fault fault: Geology, v. 15, p. 1172–1175, doi:10 .1130 /0091 Gan, W., Zhang, P., Shen, Z., Prescott, W.H., and Svarc, J.L., -7613 (1987)15 <1172: TCTRDF>2.0 .CO;2 . 2003, Initiation of deformation of the Eastern California (McGill and Sieh, 1993; Rittase et al., 2014) Burchfi el, B.C., Hodges, K.V., and Royden, L.H., 1987, Shear Zone: Constraints from the Garlock fault geom- and farther west where the multi-stranded GFZ Geology of Panamint Valley–Saline Valley pull-apart etry and GPS observations: Geophysical Research Let- narrows to the single-stranded western Gar- system, California: Palinspastic evidence for low-angle ters, v. 30, 1496, doi: 10 .1029 /2003GL017090 . geometry of a Neogene range-bounding fault: Journal Garfunkel, Z., 1974, Model for the late Cenozoic tectonic lock fault. of Geophysical Research, v. 92, p. 10,422–10,426, doi: history of the Mojave Desert, California, and its rela- The younger deformation system of the 10 .1029 /JB092iB10p10422 . tion to adjacent regions: Geological Society of Amer- LMSR can be modeled as a zone of strain Carr, M.D., and Poole, F.G., 1992, Geologic map of the pre- ica Bulletin, v. 85, p. 141–188, doi: 10 .1130 /0016 -7606 Tertiary rocks in Pilot Valley, California: U.S. Geologi- (1974)85 <1931: MFTLCT>2 .0 .CO;2 . accommodation, taking up the 2 km of dextral cal Survey Bulletin 2015, scale 1:24,000. Glazner, A.F., Bartley, J.M., and Sanner, W.K., 2000, Nature slip on the Blackwater fault without cutting the Carr, M.D., Christiansen, R.L., Poole, F.G., and Goodge, of the southwestern boundary of the central Mojave J.W., 1997, Bedrock geologic map of the El Paso Tertiary province, Rodman Mountains, California: Geo- Garlock fault. Although much of the regional Mountains in the Garlock and El Paso Peaks 7.5′ quad- logical Society of America Bulletin, v. 112, p. 34–44, dextral strain is accommodated by bending of rangles, Kern County, California: U.S. Geological Sur- doi: 10 .1130 /0016 -7606 (2000)112 <34: NOTSBO>2.0 the trace of the Garlock fault, the area of the vey Miscellaneous Investigations Map I-2389, scale .CO;2. 1:24,000 with pamphlet. Glazner, A.F., Walker, J.D., Bartley, J.M., and Fletcher, LMSR must internally deform to allow bending Carter, B., 1994, Neogene offsets and displacement rates, J.M., 2002, Cenozoic evolution of the Mojave block of of the Garlock fault trace and dextral offset of central Garlock fault, California, in McGill, S.F., and southern California, in Glazner, A.F., Walker, J.D., and the Blackwater fault. The transfer of the Christ- Ross, T.M., eds., Geological Investigations of an Ac- Bartley, J.M., eds., Geologic Evolution of the Mojave tive Margin: Redlands, California, San Bernardino Desert and Southwestern Basin and Range: Geologi- mas Canyon block across the Garlock fault zone County Museum Association, p. 348–356. cal Society of America Memoir 195, p. 19–41, doi:10 during the initiation of the last stage of slip may Casey, Z., Walker, J.D., and Taylor, M., 2008, Kinematics .1130 /0 -8137 -1195 -9 .19 . of the Cerro Coso fault and its intersection with the Guest, B., Pavlis, T.L., Golding, H., and Serpa, L., 2003, have been created by a defl ection of the trace Garlock fault, southern Indian Wells Valley, CA: Geo- Chasing the Garlock: A study of tectonic response to of the Garlock fault during the initial stages of logical Society of America Abstracts with Programs, vertical axis rotation: Geology, v. 31, p. 553–556, doi: regional dextral shear. v. 40, no. 1, p. 99. 10 .1130 /0091 -7613 (2003)031 <0553: CTGASO>2 .0 Chester, F.M., and Logan, J.M., 1987, Composite planar .CO;2 . 40 ACKNOWLEDGMENTS fabric of gouge from the Punchbowl Fault, California: Harrison, T.M., and McDougall, I., 1981, Excess Ar in Journal of Structural Geology, v. 9, p. 621–634, doi: 10 metamorphic rocks from Broken Hill, New South 40 39 This work was partly supported by a grant from the .1016 /0191 -8141 (87)90147 -7 . Wales: Implications for Ar/ Ar age spectra and the Clark, M.M., and Lajoie, K.R., 1974, Holocene behavior of thermal history of the region: Earth and Planetary Geothermal Program Offi ce of the U.S. Department the Garlock fault: Geological Society of America Ab- Science Letters, v. 55, p. 123–149, doi: 10 .1016 /0012 of the Navy, China Lake, and the National Science stracts with Programs, v. 6, p. 156–157. -821X (81)90092 -3 . Foundation EarthScope Program. Andrew Sabin and Coles, S., Prothero, D.R., Quinn, J.P., and Swisher, C.C., III, Heizler, M.T., Perry, F.V., Crowe, B.M., Peters, L., and Steve Alm facilitated logistics and access permissions 1997, Magnetic stratigraphy of the middle Miocene Appelt, R., 1999, The age of Lathrop Wells volcanic to areas of the China Lake Naval Weapons Station. Bopesta Formation, southern Sierra Nevada, Califor- center: An 40Ar/39Ar dating investigation: Journal of Matt Heizler and the New Mexico Geochronology nia, in Girty, G.H., Hanson, R.E., and Cooper, J.D., Geophysical Research, v. 104, p. 767–804, doi: 10 .1029 Research Laboratory provided analyses and inter- eds., Geology of the Western Cordillera: Perspectives /1998JB900002. pretations for most of the argon data in this paper. From Undergraduate Research: Pacifi c Section SEPM Hess, F.L., 1910, Gold mining in the Randsburg quadrangle, (Society for Sedimentary Geology), v. 82, p. 21–34. California, in Hayes, C.W., and Lindgren, W., ed., Tandis Bidgoli helped with early reviews of the paper. Cox, B.F., 1982, Stratigraphy, sedimentology, and structure Contributions to Economic Geology: U.S. Geological Finally, we acknowledge Jeff Lee, Terry Pavlis, and of the Goler Formation (Paleocene), El Paso Moun- Survey Bulletin 430-I, p. 23–47. Nadine McQuarrie for comments leading to great im- tains, California: Implications for Paleogene tectonism Hodges, K.V., McKenna, L.W., and Harding, M.B., 1990, provement in the manuscript. on the Garlock fault zone [Ph.D. thesis]: Riverside, Structural unroofi ng of the central Panamint Moun- University of California, 248 p. tains, Death Valley region, southeastern California, REFERENCES CITED Cox, B.F., and Diggles, M.F., 1986, Geologic map of the El in Wernicke, B.P., ed., Basin and Range Extensional Paso Mountains wilderness study area, Kern County, Tectonics near the Latitude of Las Vegas, Nevada: Geo- Andrew, J.E., and Walker, J.D., 2009, Reconstructing late California: U.S. Geological Survey Miscellaneous logical Society of America Memoir 176, p. 377–390, Cenozoic deformation in central Panamint Valley, Cali- Field Studies Map MF-1827, scale 1:24,000. doi: 10 .1130 /MEM176 -p377 . fornia: Evolution of slip partitioning in the Walker Lane: Davis, G.A., and Burchfi el, B.C., 1973, Garlock fault: An Hodges, K.V., Hames, W.E., Olszewski, W.J., Burchfi el, Geosphere, v. 5, p. 172–198, doi: 10 .1130 /GES00178 .1 . intracontinental transform structure, southern Califor- B.C., Royden, L.H., and Chen, Z., 1994, Thermo- Andrew, J.E., Walker, J.D., and Monastero, F.C., 2011, Slip nia: Geological Society of America Bulletin, v. 84, barometric and 40Ar/39Ar geochronologic constraints history model of the Garlock fault zone: Basin and p. 1407–1422, doi: 10 .1130 /0016 -7606 (1973)84 <1407: on Eohimalayan metamorphism in the Dinggyê area, Range extension overprinted by Eastern California GFAITS>2 .0 .CO;2 . southern Tibet: Contributions to Mineralogy and Petrol- shear zone transtension: Geological Society of Amer- Dibblee, T.W., Jr., 1952, Geology of the Saltdale quadrangle, ogy, v. 117, p. 151–163, doi: 10 .1007 /BF00286839 . ica Abstracts with Programs, v. 43, no. 2, p. 209. California: California Division of Mines and Geology Holm, D.K., Geissman, J.W., and Wernicke, B.P., 1993, Tilt Andrew, J.E., Rittase, W.M., Monastero, F.M., Bidgoli, T., Bulletin 160, 66 p. and rotation of the footwall of a major normal fault sys- and Walker, J.D., 2014, Geologic map of the Northern Dibblee, T.W., Jr., 1967, Areal geology of the western Mo- tem: Paleomagnetism of the Black Mountains, Death Lava Mountains and Summit Range, San Bernardino jave Desert, California: U.S. Geological Survey Pro- Valley extended terrane, California: Geological So- County, California: Geological Society of America fessional Paper 522, 153 p. ciety of America Bulletin, v. 105, p. 1373–1387, doi: Digital Map and Chart 19, scale 1:20,000, doi: 10 .1130 Dokka, R.K., and Travis, C.J., 1990, Late Cenozoic strike- 10 .1130 /0016 -7606 (1993)105 <1373: TAROTF>2 .3 /2014 .DMCH019 . slip faulting in the Mojave Desert, California: Tecton- .CO;2 . Blythe, A.E., and Longinotti, N., 2013, Exhumation of the ics, v. 9, p. 311–340, doi: 10 .1029 /TC009i002p00311 . Hulin, C.D., 1925, Geology and ore deposits of the Rands- southern Sierra Nevada–eastern Tehachapi Mountains Fife, D.L., Ruff, R.W., and Unruh, M.E., 1988, Late Ter- burg quadrangle, California: California State Mining constrained by low-temperature thermochronology: tiary epithermal(?) gold mineralization associated with Bureau Bulletin 95, 152 p. Implications for the initiation of the Garlock fault: the Garlock fault zone, Christmas Canyon quadrangle, Jahns, R.H., Troxel, B.W., and Wright, L.A., 1971, Some Lithosphere, v. 5, p. 321–327, doi: 10 .1130 /L252 .1 . San Bernardino County, California, in Gregory, J., and structural implications of a late Precambrian–Cam- Bowring, J.F., McLean, N.M., and Bowring, S.A., 2011, Baldwin, E.J., eds., Geology of the Death Valley Re- brian section in the Avawatz Mountains, California: Engineering cyber infrastructure for U-Pb geochronol- gion: South Coast Geological Society Guidebook 16, Geological Society of America Abstracts with Pro- ogy: Tripoli and U-Pb_Redux: Geochemistry, Geo- p. 397–407. grams, v. 3, p. 140.

22 Geological Society of America Bulletin, Month/Month 2014 Central Garlock fault

Lanphere, M.A., and Dalrymple, G.B., 1976, Identifi cation nia: Carnegie Institute of Washington Year Book, v. 25, gested by offset sections of similar metasedimentary of excess 40Ar by the 40Ar/39Ar age spectrum technique: p. 415–428. rocks, in Geological Survey Research 1970, Chapter Earth and Planetary Science Letters, v. 32, p. 141–148, Oskin, M., and Iriondo, A., 2004, Large-magnitude transient D: U.S. Geological Survey Professional Paper 700-D, doi: 10 .1016 /0012 -821X (76)90052 -2 . strain accumulation on the Blackwater fault, Eastern p. D1–D9. Loomis, D.P., and Burbank, D.W., 1988, The stratigraphic California shear zone: Geology, v. 32, p. 313–316, doi: Smith, G.I., Troxel, B.W., Gray, C.H., Jr., and von Huene, evolution of the El Paso Basin, southern California: 10 .1130 /G20223 .1 . R., 1968, Geologic reconnaissance of the Slate Range, Implications for Miocene development of the Garlock Peltzer, G., Crampe, F., Hensley, S., and Rosen, P., 2001, San Bernardino and Inyo Counties, California: Cali- fault and uplift of the Sierra Nevada: Geological Soci- Transient strain accumulation and fault interaction fornia Division of Mines and Geology Special Report ety of America Bulletin, v. 100, p. 12–28, doi: 10 .1130 in the Eastern California shear zone: Geology, v. 29, 96, 33 p. /0016 -7606 (1988)100<0012: TSEOTE>2 .3 .CO;2 . p. 975–978, doi: 10 .1130 /0091 -7613 (2001)029 <0975: Snyder, N.P., and Hodges, K.V., 2000, Depositional and tec- Ludwig, K.R., 2012, Isoplot 3.75—A geochronological TSAAFI>2.0 .CO;2 . tonic evolution of a supradetachment basin: 40Ar/39Ar toolkit for Microsoft Excel: Berkeley Geochronology Petit, J.P., 1987, Criteria for the sense of movement on fault geochronology of the Nova Formation, Panamint Center Special Publication 5, 74 p. surfaces in brittle rocks: Journal of Structural Geology, Range, California: Basin Research, v. 12, p. 19–30, Mancktelow, N.S., and Pavlis, T.L., 1994, Fold-fault rela- v. 9, p. 597–608, doi: 10 .1016 /0191 -8141 (87)90145 -3 . doi: 10 .1046 /j .1365 -2117 .2000 .00108 .x . tionships in low-angle detachment systems: Tectonics, Rittase, W.M., 2012, Neogene to Quaternary tectonics of the Stewart, J.H., 1988, Tectonics of the Walker Lane belt, v. 13, p. 668–685, doi: 10 .1029 /93TC03489 . Garlock Fault and the Eastern California shear zone in western Great Basin: Mesozoic and Cenozoic defor- Mattinson, J.M., 2005, Zircon U-Pb chemical abrasion (CA- the northern Mojave Desert, California [Ph.D. thesis]: mation in a shear zone, in Ernst, W. G., ed., Metamor- TIMS) method: Combined annealing and multi-step Lawrence, University of Kansas, 224 p. phism and Crustal Evolution of the Western United partial dissolution analysis for improved precision and Rittase, W.M., Kirby, E., McDonald, E., Walker, J.D., Gosse, States: Englewood Cliffs, New Jersey, Prentice-Hall, accuracy of zircon ages: Chemical Geology, v. 220, J., Spencer, J.Q.G., and Herrs, A.J., 2014, Temporal p. 681–713. p. 47–66, doi: 10 .1016 /j .chemgeo .2005 .03 .011 . variations in Holocene slip rate along the central Gar- Tchalenko, J.S., 1970, Similarities between shear zones of McGill, S., and Sieh, K., 1993, Holocene slip rate of the lock fault, Pilot Knob Valley, California: Lithosphere, different magnitudes: Geological Society of America central Garlock fault in southeastern Searles Valley, v. 6, p. 48–58, doi: 10 .1130 /L286 .1 . Bulletin, v. 81, p. 1625–1640, doi:10 .1130 /0016 -7606 California: Journal of Geophysical Research, v. 98, Sabin, A.E., 1994, Geology of the Eagle Crags volcanic (1970)81 [1625: SBSZOD]2.0 .CO;2 . p. 14,217–14,231, doi: 10 .1029 /93JB00442 . fi eld, northern Mojave Desert, China Lake Naval Air Topping, D.J., 1993, Paleogeographic reconstruction of the McGill, S.F., Wells, S.G., Fortner, S.K., Kuzma, H.A., and Weapons Station, California [Ph.D. thesis]: Golden, Death Valley extended region: Evidence from Miocene McGill, J.D., 2009, Slip rate of the western Garlock Colorado School of Mines, 209 p. large rock-avalanche deposits in the Amargosa Chaos fault, at Clark Wash, near Lone Tree Canyon, Mojave Saleeby, J., Ducea, M.N., Busby, C., Nadin, E., and Wet- Basin, California: Geological Society of America Bul- Desert, California: Geological Society of America Bul- more, P.H., 2008, Chronology of pluton emplace- letin, v. 105, p. 1190–1213, doi: 10 .1130 /0016 -7606 letin, v. 121, p. 536–554, doi: 10 .1130 /B26123 .1 . ment and regional deformation in the southern Sierra (1993)105 <1190: PROTDV>2 .3 .CO;2 . Means, W.D., 1987, A newly recognized type of slickenside Nevada batholith, California, in Wright, J.E., and Unruh, J., and Hauksson, E., 2009, Seismotectonics of an striation: Journal of Structural Geology, v. 9, p. 585– Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: evolving intracontinental plate boundary, southeastern 590, doi:10 .1016 /0191 -8141 (87)90143-X . A Tribute to Cliff Hopson: Geological Society of California, in Oldow, J.S., and Cashman, P.H., eds., Michael, E.D., 1966, Large lateral displacement on Garlock America Special Paper 438, p. 397–427, doi: 10 .1130 Late Cenozoic Structure and Evolution of the Great fault, California, as measured from offset fault sys- /2008.2438 (14) . Basin–Sierra Nevada Transition: Geological Society of tem: Geological Society of America Bulletin, v. 77, Samsel, H.S., 1962, Geology of the southeast quarter of the America Special Paper 447, p. 351–372, doi:10 .1130 p. 111–114, doi: 10 .1130 /0016 -7606 (1966)77 [111: Cross Mountain quadrangle, Kern County, California: /2009 .2447 (16) . LLDOGF]2.0.CO;2 . California Division of Mines and Geology Map Sheet Walker, J.D., Black, R.A., Berry, A.K., Davis, P.J., Andrew, Miller, D.M., and Yount, J.C., 2002, Late Cenozoic evolu- 2, scale 1:39,350. J.E., and Mitsdarfer, J.M., 2002, Geologic maps of the tion of the north-central Mojave Desert inferred from Savage, J.C., Gan, W., and Svarc, J.L., 2001, Strain accumu- northern Mojave Desert and southwestern Basin and fault history and physiographic evolution of the Fort lation and rotation in the Eastern California Shear Zone: Range Province: Geological Society of America Mem- Irwin area, California, in Glazner, A.F., Walker, J.D., Journal of Geophysical Research, v. 106, p. 21,995– oir 195, p. 297–299, scale 1:250,000, doi:10 .1130 /0 and Bartley, J.M., eds., Geologic Evolution of the Mo- 22,007, doi:10 .1029 /2000JB000127 . -8137 -1195 -9 .297 . jave Desert and Southwestern Basin and Range: Geo- Schermer, E., Luyendyk, B.P., and Cisowski, S., 1996, Late Walker, J.D., Bidgoli, T.S., Didericksen, B.D., Stockli, D.F., logical Society of America Memoir 195, p. 173–197, Cenozoic structure and tectonics of the northern Mo- and Andrew, J.E., 2014, Middle Miocene to recent ex- doi: 10 .1130 /0 -8137 -1195 -9 .173 . jave Desert: Tectonics, v. 15, p. 905–932, doi:10 .1029 humation of the Slate Range, eastern California, and Miller, M.M., Johnson, D.J., Dixon, T.H., and Dokka, R.K., /96TC00131 . implications for the timing of extension and the transi- 2001, Refi ned kinematics of the Eastern California Smith, E.I., Sanchez, A., Keenan, D.L., and Monastero, F.C., tion to transtension: Geosphere, v. 10, p. 276–291, doi: shear zone from GPS observations, 1993–1998: Jour- 2002, Stratigraphy and geochemistry of volcanic rocks 10 .1130 /GES00947 .1 . nal of Geophysical Research, v. 106, p. 2245–2263, in the Lava Mountains, California: Implications for the Whistler, D.P., Tedford, R.H., Takeuchi, G.T., Wang, W., doi: 10 .1029 /2000JB900328 . Miocene development of the Garlock fault, in Glazner, Tseng, Z.J., and Perkins, M.E., 2009, Revised Miocene Monastero, F.C., Sabin, A.E., and Walker, J.D., 1997, Evi- A.F., Walker, J.D., and Bartley, J.M., eds., Geologic biostratigraphy and biochronology of the Dove Spring dence for post–early Miocene initiation of movement Evolution of the Mojave Desert and Southwestern Ba- Formation, Mojave Desert, California: Museum of on the Garlock fault from offset of the Cudahy Camp sin and Range: Geological Society of America Memoir Northern Arizona Bulletin, v. 65, p. 331–362. Formation, east-central California: Geology, v. 25, 195, p. 151–160, doi: 10 .1130 /0 -8137 -1195 -9 .151 . Zhang, P., Ellis, M., Slemmons, D.B., and Mao, F., 1990, p. 247–250, doi: 10 .1130 /0091 -7613 (1997)025 <0247: Smith, G.I., 1962, Large lateral displacement on the Gar- Right-lateral displacements and the Holocene slip EFPEMI>2 .3 .CO;2 . lock fault, California, as measured from an offset dike rate associated with prehistoric earthquakes along the Monastero, F.C., Walker, J.D., Katzenstein, A.M., and Sabin, swarm: The American Association of Petroleum Geol- southern Panamint Valley fault zone: Implications for A.E., 2002, Neogene evolution of the Indian Wells Val- ogists Bulletin, v. 46, p. 85–104. southern Basin and Range tectonics and coastal Cali- ley, east-central California, in Glazner, A.F., Walker, Smith, G.I., 1964, Geology and volcanic petrology of the fornia deformation: Journal of Geophysical Research, J.D., and Bartley, J.M., eds., Geologic Evolution of Lava Mountains, San Bernardino County, California: v. 95, p. 4857–4872, doi: 10 .1029 /JB095iB04p04857 . the Mojave Desert and Southwestern Basin and Range: U.S. Geologic Survey Professional Paper 457, 97 p. Geological Society of America Memoir 195, p. 199– Smith, G.I., 1991, Anomalous folds associated with the SCIENCE EDITOR: CHRISTIAN KOEBERL 228, doi:10 .1130 /0 -8137 -1195 -9 .199 . east-central part of the Garlock fault, southeast Cali- ASSOCIATE EDITOR: STEFANO MAZZOLI Murdoch, J., and Webb, R.W., 1940, Notes on some minerals fornia: Geological Society of America Bulletin, v. 103, MANUSCRIPT RECEIVED 4 NOVEMBER 2013 from southern California, II: The American Mineralo- p. 615–624, doi: 10 .1130 /0016 -7606 (1991)103 <0615: REVISED MANUSCRIPT RECEIVED 8 JUNE 2014 gist, v. 25, p. 549–555. AFAWTE>2 .3 .CO;2 . MANUSCRIPT ACCEPTED 16 JULY 2014 Noble, L.F., 1926, The San Andreas rift and some other ac- Smith, G.I., and Ketner, K.B., 1970, Lateral displacement tive faults in the desert region of southeastern Califor- on the Garlock fault, southeastern California, sug- Printed in the USA

Geological Society of America Bulletin, Month/Month 2014 23