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, USA28597 Timaru Trail, Reno, Nevada 89523, USA

ABSTRACT

The Garlock fault is an integral part of the plate-boundary deformation system inboard of the San Andreas fault (Califor-nia, USA); however, the Garlock is trans-versely oriented and has the opposite sense of shear. The slip history of the Garlock is critical for interpreting the deforma-tion of the through-going dextral shear of the Walker Lane belt–Eastern California shear zone. The Lava Mountains–Summit Range (LMSR), located along the central Garlock fault, is a Miocene volcanic center that holds the key to unraveling the fault slip and development of the Garlock. The LMSR is also located at the intersection of the NNW-striking dextral Blackwater fault and contains several sinistral WSW-strik-ing structures that provide a framework for establishing the relationship between the sinistral Garlock fault system and the dextral Eastern California shear zone. New fi eld mapping and geochronology data (40Ar/39Ar and U-Pb) show fi ve distinct suites of volcanic-sedimentary rock units in the LMSR overlain by Pliocene exotic-clast conglomerates. This stratigraphy coupled with fi fteen fault slip markers defi ne a three-stage history for the central Garlock fault system of 11–7 Ma, 7–3.8 Ma, and 3.8–0 Ma. Pliocene to recent slip occurs in a ~12-km-wide zone and accounts for ~33 km or 51% of the total 64 km of left-lateral off-set on the Garlock fault in the vicinity of the LMSR since 3.8 Ma. This history yields slip rates of 6–9 mm/yr for the younger stage and slower rates for older stages. The LMSR internally accommodates north-west-directed dextral slip associated with the Eastern California shear zone–Walker Lane belt via multiple processes of lateral tectonic escape, folding, normal faulting,

and the creation of new faults. The geologic slip rates for the Garlock fault in the LMSR match with and explain along-strike varia-tions in neotectonic rates.

INTRODUCTION

A quarter of the North American to Pacifi c plate displacement is actively accommodated by dextral shear in eastern California (Miller et al., 2001). The Garlock fault (Fig. 1 inset map) is an active large-magnitude sinistral-slip fault (Hess, 1910; Hulin, 1925; Dibblee, 1952; Smith, 1962) embedded in and perpendicular to this zone of dextral shear. The Garlock is a fundamental structure in this deformation zone, as it separates the dextral transpressive Eastern California shear zone to its south (Dokka and Travis, 1990) from the dextral transtensional Walker Lane belt to its north (Stewart, 1988). Active NNW-striking dextral fault systems both north and south of the Garlock fault have abundant seismicity (Unruh and Hauksson, 2009) and offsets of 2–12 km (Monastero et al., 2002; Glazner et al., 2000; Oskin and Iriondo, 2004; Casey et al., 2008; Frankel et al., 2008; Andrew and Walker, 2009). Geodetic data show that dextral shear crosses the Garlock fault unimpeded (Peltzer et al., 2001; Gan et al., 2003) with an additional component of northwest-directed extension north of the Gar-lock fault (Savage et al., 2001). Despite the activ-ity and signifi cant offsets on the NNW-striking faults, these faults nowhere cut the Garlock fault (Noble, 1926). The Garlock fault has a curved trace, with its central and eastern segments rotated clockwise relative to its straight western segment (Fig. 1). This change in strike has been interpreted as oroclinal bending that accommo-dates dextral shear in the Mojave Desert (Gar-funkel, 1974; Schermer et al., 1996; Guest et al., 2003; Gan et al., 2003). Most models of this oroclinal bending are based on dextral shear and counterclockwise rotation of crustal blocks, but the corners of these blocks with the Garlock fault have not been studied.

A major hurdle to understanding the tec tonics of the Garlock fault has been establishing a detailed slip history, because the age of all of the known slip markers predate the initiation of slip. The total slip on the Garlock fault is ~64 km using offset pre-Cenozoic features (see Fig. 1 and Table 1 for descriptions and references); however, slip initiated after 17 Ma (Monastero et al., 1997) and possibly at 11 Ma (Burbank and Whistler, 1987; Blythe and Longinotti, 2013). The Garlock has neotectonic slip rates of 4.5–14 mm/yr (Clark and Lajoie, 1974; McGill and Sieh, 1993; McGill et al., 2009; Rittase et al., 2014), but most of these rates are too rapid if they are applied to the 11 m.y. history of the Garlock fault (64 km over 11 m.y. = 5.8 mm/yr).

This paper presents new stratigraphic, structural, and geochronologic data to defi ne a detailed history for the Garlock fault. This study focuses on the central segment of fault in the Lava Mountains–Summit Range (LMSR) area, a Miocene volcanic center located imme-diately adjacent to the Garlock fault (Smith, 1964; Dibblee , 1967; Smith et al., 2002). Pre-vious workers suggested that Miocene and younger strata in the LMSR can be correlated across the fault (Carter, 1994; Smith et al., 2002; Frankel et al., 2008), thus offering the potential to understand the slip history in detail. It is also an important study site because it contains the northern terminus of the dextral Blackwater fault, a major fault of the Eastern California shear zone. This paper describes the stratig-raphy of the LMSR and the geometry and kinematics of the structures, to identify numer-ous offset markers to restore fault movements. The goal of this paper is to use these data and interpretations to construct the slip history of the Garlock and associated faults. This history is needed to resolve the initiation of slip on the Garlock fault system and the current structural confi guration, and to evaluate major changes in slip magnitude, fault geometry, and deformation style. We consider the implications of the cur-rent structural confi guration to create a model

For permission to copy, contact editing@geosociety.org© 2014 Geological Society of America

1

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.

†E-mail: jeandrew@ku.edu

Andrew et al.

2 Geological Society of America Bulletin, Month/Month 2014

of dextral shear accommodation without dex-tral faults cutting the Garlock fault. Lastly, we compare our interpreted long-term slip rates to published neotectonic rates along the Garlock.

STRATIGRAPHY

Despite the volume of previous work, there were still many undated rock units and uncer-tainties regarding the order and stratigraphic relationships of the Miocene units and structures in the LMSR. New detailed geologic mapping of this area (Andrew et al., 2014; summarized

and simplifi ed on Fig. 2) and geochronologic analysis (locations on Fig. 2) of key rock units were used to better defi ne and delineate the stra-tigraphy of the area (Fig. 3) as well as reveal the details of faulting.

Geochronology

We establish age constraints in the LMSR by dating samples using the 40Ar/39Ar and U-Pb zircon geochronology methods and by reinter-preting previously published 40Ar/39Ar ages of Smith et al. (2002). An additional sample

was collected in the Sierra Nevada (MDS on Fig. 1). New volcanic rock samples from the LMSR (Table DR1 in the GSA Data Reposi-tory1) were analyzed by the 40Ar/39Ar geochro-nology step-heating method, mostly at the New Mexico Geochronology Research Labo-ratory (analytical methods in Heizler et al. [1999] and Brueseke et al. [2007]) and one

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San Andreas fault

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 pub-lished total offset constraints for the Garlock fault are shown in bold text (see Table 1 for abbreviations and descriptions). Fault abbrevia-tions: 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.

1GSA Data Repository item 2014297, additional geochronologic data, is available at http:// www .geosociety .org /pubs /ft2014 .htm or by request to editing@ geosociety .org.

TABLE 1. PUBLISHED CONSTRAINTS OF LEFT-LATERAL OFFSET ON THE GARLOCK FAULT

srohtuAerutaeFAmount

sliateD*)mk(Labels†

North South

46)2691(htimSmrawsekidecnednepednI Late Jurassic Independence dike swarm (IDS) in the Spangler Hills with dikes in the Granite Mountains IDS IDS

Fault domain boundary Michael (1966) 74Eastern boundary of domain of northwest-striking faults; Piute Line (PL) in

the southeastern Sierra Nevada and Blackwater fault (BF) in the northern Mojave Desert

PL BF

Rand Schist Dibblee (1967) 72 Late Cretaceous Rand Schist in the Rand Mountains (RM) and in the southern Sierra Nevada # RM

East Sierran thrust system Smith et al. (1968); Davis and Burchfi el (1973) 52–64 Jurassic East Sierran thrust system (ESTS) in the southern Slate Range and

in the Granite Mountains ESTS ESTS

Eugeoclinal Paleozoic rocks Smith and Ketner (1970); Carr et al. (1997) 48–64

Paleozoic eugeoclinal metasedimentary and metavolcanic rocks with thrust faults in Mesquite Canyon (MC) of the El Paso Mountains and south of Pilot Knob Valley (PKV)

MC PKV

Basal passive margin sequence Jahns et al. (1971) 48–64 Neoproterozoic to early Paleozoic miogeoclinal sequences in the Panamint

Range (PR) and Avawatz Mountains (AM) PR AM

Fault that juxtaposes Mesozoic and Proterozoic rocks

Davis and Burchfi el (1973) 64NNW-striking faults that juxtapose Mesozoic rocks with Proterozoic rocks;

Southern Panamint Valley fault (SPVF) with the Arrastre Spring fault (ASF) in the Avawatz Mountains

SPVF ASF

Early Miocene volcanic fi elds Monastero et al. (1997) 64 Early Miocene Cudahy Camp Formation (CC) of Loomis and Burbank (1988) with the Eagle Crag Volcanics (EC) of Sabin (1994) CC EC

*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.

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 3

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Andrew et al.

4 Geological Society of America Bulletin, Month/Month 2014

other at the Massachusetts Institute of Tech-nology (MIT; analytical methods in Hodges et al. [1994] and Snyder and Hodges [2000]). A sample containing sanidine was analyzed by single crystal fusion. Step-heating results were interpreted using plateau and inverse isochron methods (Fig. 4; see Table 2 for interpreta-tion of 40Ar/39Ar age data) using the software Isoplot (Ludwig, 2012). Six samples of zircon from felsic rocks were analyzed by chemical abrasion thermal ionization mass spectrometry (CA-TIMS) at the Isotope Geology Labora-tory at the University of Kansas (Table DR2). Samples were annealed and leached following the protocols of Mattinson (2005), then spiked with EarthTime ET535 tracer and dissolved using standard U-Pb double pressure-vessel digestion procedures. Column-purifi ed U and Pb were run together on a VG Sector TIMS run in single-collector ion-counting mode for Pb and multicollector static mode for U. Data were reduced and interpreted using Tripoli and U-Pb_Redux software (Bowring et al.,

2011). Compiled single zircon crystal frac-tions give crystallization ages of these rocks (Table 3; Fig. 5).

We reinterpret the step-heating 40Ar/39Ar analyses of Smith et al. (2002); neither the data nor interpretive plots were published, but the data were available from author Monastero (Table DR3), who was a collaborator on the Smith et al. (2002) work. These samples were analyzed at the MIT lab. Our rationale for rein-terpreting these ages is that hornblende and bio-tite from the same samples yielded very differ-ent ages (e.g., Fig. 6E), but Smith et al. (2002) interpreted the ages based on the hornblende ages alone. The 40Ar/39Ar hornblende spectra show evidence of excess argon (i.e., saddle-shaped plateau and non-atmospheric 40Ar/36Ar intercept values on Fig. 6D) (e.g., Lanphere and Dalrymple, 1976; Harrison and McDougall, 1981), so we reinterpret the ages of these rocks (Table 2B) by examining plots of age spectra and inverse isochrons using the biotite analyses (Fig. 6).

Stratigraphic Units

Paleozoic Metasedimentary RocksCoherent basement of metasedimentary rocks

occurs in the Christmas Canyon area (Figs. 2 and 7). These rocks have relatively low metamor-phic grades and consist mostly of meta-siltstone with locally dominant meta-limestone beds. We correlate these rocks to the well-studied Paleo-zoic rocks in the nearby El Paso Mountains (Dibblee, 1952; Carr et al., 1997). Distinctive units within this sequence allow a stratigraphic correlation to rocks in the eastern El Paso Moun-tains (Fig. 2); the details of this correlation are described further in the fault slip section.

Mesozoic Plutonic and Metamorphic RocksA few small bodies of chlorite-altered quartz

diorite intrude metasedimentary rocks in the Christmas Canyon area. These rocks have simi-lar mineralogy, textures, and alteration as the Jurassic Laurel Mountain pluton (Carr et al., 1997) that intrudes correlative metasedimentary

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This studySmith (1964)

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pre-Tertiary

Tf, Tt, Ttb

& Tvb?

|

Smith et al. (2002)

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|Paleozoic

basalt clast in Pliocene(?) conglomerate of Golden Valley*

11.62 ±0.11 LV030710-7

Lava Mountains Dacite6.507 ±0.070u LV050810-46.522 ±0.026u LV051810-87.24 ±0.83x SD120905-17.32 ±0.02x SD120905-27.64 ±0.18x LM96-2

Almond Mountain Volc.7.479 ±0.023 JDW-20u

7.82 ±0.22 LM96-16

Summit Range volc.*

10.490 ±0.054u LV05071010.66 ±0.12 LM96-15

10.966±0.042u LV031410-B11.22 ±0.02 LV030710-411.24 ±0.43 GFZ-080603-111.32 ±0.94 GFZ-85

11.66 ±0.06 LV051210-611.93 ±0.14 SD060810-112.74 ±0.05x SD040910-1

Eagle Crags Volc.- Cudahy Camp Fm.#

18.84 ±0.24 LV030710-519.00 ±0.22 LV04091019.37 ±0.04 LV03151019.63 ±0.32 LV051210-5

Tsd

Tba

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zoic

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Tgf

Tli

Tbf

Tax

Tsa

Tsh

Tcp

Tsp

QThb

Tgh

Tat

Tsf

Tsx

Tgb

Quaternary

Qa = alluvium

Qva = andesite

Qvb = basalt

Qg = older gravels

Qcc = Christmas Canyon Fm.

Tertiary

Tf = felsite

Tvb = volcanic breccia

Tl = Lava Mountains Andesite

Ta = Almond Mountain Volcanics

Tbs = Bedrock Spring Formation

Tv = ‘other’ volcanic rocks

Ts = sediments older than Bedrock

Spring Formation

Pre-Cenozoic

pTa = Atolia Quartz Monzonite

Pz = Paleozoic metamorphic rocks

conglomerate of Hardcash Gulch*QThb = containing basalt clastsQThf = containing metasedimentary clasts

Pre-Quaternary units in the LMSR

Tpr = fanglomerates from Red Mountainconglomerate of Golden Valley*

Tgb = containing basalt clastsTgh = containing hornblende diorite clastsTgf = containing metasedimentary clasts

Lava Mountains Dacite

Tli = sillsAlmond Mountain Volcanics

Tad = dacite lava

Bedrock Spring FormationTba = arkosic sandstone

Tbc = conglomerateSummit Range volcanics*

Tsr = reddish dacite lavaTsp = porphyritic felsite intrusion

Tsd = bluish dacite lava domesTsa = arksoic sandstoneTsc = conglomerate

Eagle Crags Volcanics-Cudahy Camp Formation#

Ka = Atolia Quartz Monzonite#

Jlm = Laurel Mountain quartz diorite#

P*h = metasedimentary rocks of Holland Camp#

*b = metasedimentary rocks of Benson Well#

DO_e = metasedimentary rocks of El Paso Peaks#

O_c = metasedimentary rocks of Colorado Camp#

* = newly

# = newly

recognized

units within

the LMSRCretaceous

New units of Smith et al. (2002)

Tsd = Andesite of Summit Diggings

Ttw = basalt of Teagle Wash

Tsr = Dacite of Summit Range

u = U-Pb age (others are 40Ar/39Ar)x = older age due to excess argon

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).

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 5

Rel

ativ

e P

roba

bilit

y

Age (Ma)

I. LV030710-4 sanidine

8 10 12 14

Data at 1-sigma, results at 1-sigma

11.221 ±0.018 Ma(MSWD = 0.64, n = 9)

BA

C

D

EF

GH

I

J

0.001

36A

r/40A

r

0

0.002

B. LV030710-5 plagioclase

C D EF

G

H*P = 18.84 ± 0.24 Ma(MSWD = 1.47, 97.8% 39Ar)

12

28

A B

I = 18.45 ±2.3 Ma (MSWD = 2.1,

100% 39Ar)40Ar/36Ar intercept

= 295 ±1

App

aren

t Age

(Ma)

A

BC

DE FGHI

J

F. SD060810-1 groundmass

BC

DE

F

G

H

P = 12.39 ± 0.25 Ma(MSWD = 4.86, 96.8% 39Ar)

12

18

*I = 11.93 ±0.14 Ma (MSWD = 1.4,

96.8% 39Ar)40Ar/36Ar intercept

= 301 ±9

App

aren

t Age

(Ma)

0.001

36A

r/40A

r

0

0.002

6

14

B DE F G H

I

J. SD120905-2 groundmass

*P = 7.32 ±0.02 Ma(MSWD = 0.23, 61.6% 39Ar)C

A

B

CD

EF

G

H

I

I = 7.24 ±0.05 Ma (MSWD = 1.8,

100% 39Ar)40Ar/36Ar intercept

= 302 ±9

App

aren

t Age

(Ma)

0.00136

Ar/40

Ar

0

0.002

App

aren

t Age

(Ma)

H. GFZ-080603-1 groundmass

6

14

P = 11.01 ±0.29 Ma(MSWD = 0.62, 100% 39Ar)

A BC D E

F GH

*I = 11.24 ±0.43 Ma (MSWD = 0.53,

100% 39Ar)40Ar/36Ar intercept

= 285 ±18

AB

CDE

FGH

0.002

36A

r/40A

r

0

G. LV051210-6 groundmass

12

18

B

C D E F

G

H

I J

*P = 11.66 ± 0.06 Ma(MSWD = 1.18, 93.5% 39Ar)

App

aren

t Age

(Ma) AB

C

DEF

GH

IJ

I = 11.70 ±0.07 Ma (MSWD = 1.3,

93.5% 39Ar)40Ar/36Ar intercept

= 292 ±4

0.001

36A

r/40A

r

0

0.002

A

B

C

D

E

F GH

IJ

E. SD040910-1 groundmass

B

C D EF G H

I

J

P = 12.82 ± 0.13 Ma(MSWD = 7.54, 99.7% 39Ar)

12

18 *I = 12.74 ±0.05 Ma (MSWD = 6.7,

99.7% 39Ar)40Ar/36Ar intercept

= 302 ±2

App

aren

t Age

(Ma)

0.001

36A

r/40A

r

0

0.002

A

B

CDE

FGHI

J

D. LV040910 groundmass28

12

A

B C D E F

G

H

I

J

P = 19.54 ± 0.12 Ma(MSWD = 3.15, 67.5% 39Ar)

*I = 19.00 ±0.22 Ma (MSWD = 2.6,

98.8% 39Ar)40Ar/36Ar intercept

= 301 ±2

App

aren

t Age

(Ma)

0.001

36A

r/40A

r

0

0.002

BA E

C

DFG

HI

J

A. LV051210-5 plagioclase

C D EF

GH

I

J

*P = 19.63 ± 0.32 Ma(MSWD = 1.76, 60.3% 39Ar)

B12

28 I = 19.59 ±0.90 Ma (MSWD = 2.1,

60.3% 39Ar)40Ar/36Ar intercept

= 294 ±2

App

aren

t Age

(Ma)

A

0.001

36A

r/40A

r

0

0.002

0.001

AB

C DEF

GHI

J

36A

r/40A

r

IJ

0

0.002

C. LV031510 groundmass

A

B

C D EFG

H

P = 19.40 ± 0.47 Ma(MSWD = 2.37, 67.5% 39Ar)

12

28 *I = 19.37 ±0.04 Ma (MSWD = 1.6,

67.5% 39Ar)40Ar/36Ar intercept

= 318 ±10

App

aren

t Age

(Ma)

A

BC

DEFG

HIJ

L. LV030710-7 groundmass

A

B C DE

F G

H

I

J

*P = 11.62 ± 0.11 Ma(MSWD = 0.90, 82.2% 39Ar)

14

6

App

aren

t Age

(Ma)

0.001

36A

r/40A

r

0

0.002I = 11.99 ±0.52 Ma

(MSWD = 0.95,88.2% 39Ar)

40Ar/36Ar intercept = 293 ±4

App

aren

t Age

(Ma)

6

14

B C D

E

F

GH

I P = 7.71 ± 0.11 Ma

(MSWD = 0.91, 78.6% 39Ar)

A

BCDE

FGHI

*I = 7.24 ±0.83 Ma (MSWD = 0.17,

85.7% 39Ar)40Ar/36Ar intercept

= 298 ±5

K. SD120905-1 groundmass

0.001

36A

r/40A

r

0.002

39Ar/40ArCumulative 39Ar Fraction 0.30 0.9 0 39Ar/40ArCumulative 39Ar Fraction 0.30 0.9 0

39Ar/40ArCumulative 39Ar Fraction 0.30 0.9 0

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 re-ported 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 interpre-tation. The small gray box on the inverse isochron plots denotes the value of atmospheric argon with 40Ar/36Ar of 295.5.

Andrew et al.

6 Geological Society of America Bulletin, Month/Month 2014

TAB

LE 2

. 40A

r/39

Ar

GE

OC

HR

ON

OLO

GY

INT

ER

PR

ETA

TIO

NS

A. N

ew 40

Ar/

39A

r sa

mpl

es

LV05

1210

-5 (

IGS

N:J

EA

0522

MM

). S

ampl

e of

bas

altic

and

esite

from

bel

ow d

acite

lava

in th

e La

va M

ount

ains

. Pla

gioc

lase

from

this

sam

ple

show

s a

com

plic

ated

pla

teau

pat

tern

(F

ig. 4

A),

but

the

larg

e er

rors

allo

w

a va

lid p

late

au w

ith a

n ac

cept

able

mea

n st

anda

rd w

eigh

ted

devi

ates

(M

SW

D).

Inve

rse

isoc

hron

ana

lysi

s gi

ves

a si

mila

r ag

e w

ith a

slig

htly

hig

her

MS

WD

. The

pre

ferr

ed a

ge is

19.

63 ±

0.3

2 M

a fr

om th

e pl

atea

u di

agra

m. T

he a

ge a

nd s

trat

igra

phic

pos

ition

of t

his

lava

fi ts

with

a c

orre

latio

n to

the

Eag

le C

rags

Vol

cani

cs (

Sab

in, 1

994;

Mon

aste

ro e

t al.,

199

7).

LV03

0710

-5 (

IGS

N:J

EA

0522

M7)

. Sam

ple

of b

asal

tic a

ndes

ite fr

om b

elow

dac

ite la

va. P

lagi

ocla

se fr

om th

is s

ampl

e ha

s a

sadd

le-s

hape

d pl

atea

u (F

ig. 4

B)

but y

ield

s a

plat

eau

of 1

8.84

± 0

.24

Ma

with

an

acce

ptab

le M

SW

D. T

he in

vers

e is

ochr

on h

as h

ighe

r er

rors

and

MS

WD

. The

age

and

str

atig

raph

ic p

ositi

on o

f thi

s la

va fi

t with

a c

orre

latio

n to

the

Eag

le C

rags

Vol

cani

cs (

Sab

in, 1

994;

Mon

aste

ro e

t al.,

199

7).

LV03

1510

(IG

SN

:JE

A05

22M

B).

Sam

ple

of d

acite

cla

st fr

om s

ilici

fi ed

brec

cia

depo

sit f

rom

bel

ow d

acite

lava

in th

e La

va M

ount

ains

. Gro

undm

ass

from

this

sam

ple

has

a sl

ight

sad

dle-

shap

ed p

late

au (

Fig

. 4C

) th

at g

ives

a r

elat

ivel

y hi

gh M

SW

D. I

nver

se is

ochr

on a

naly

sis

yiel

ds a

bet

ter

MS

WD

val

ue w

ith a

slig

htly

hig

her

40A

r/36

Ar

inte

rcep

t val

ue th

an th

e at

mos

pher

e. T

he in

vers

e is

ochr

on a

ge o

f 19.

37 ±

0.0

4 M

a is

the

pref

erre

d ag

e. T

he a

ge a

nd s

trat

igra

phic

pos

ition

of t

his

lava

fi t w

ith a

cor

rela

tion

to th

e E

agle

Cra

gs V

olca

nics

(S

abin

, 199

4; M

onas

tero

et a

l., 1

997)

.

LV04

0910

(IG

SN

:JE

A05

22M

D).

Sam

ple

of d

acite

cla

st fr

om s

ilici

fi ed

brec

cia

depo

sit f

rom

bel

ow d

acite

lava

in th

e La

va M

ount

ains

. Gro

undm

ass

from

this

sam

ple

has

a sa

ddle

-sha

ped

plat

eau

(Fig

. 4D

) th

at g

ives

a

rela

tivel

y hi

gh M

SW

D. I

nver

se is

ochr

on a

naly

sis

yiel

ds a

bet

ter

but s

till h

igh

MS

WD

val

ue a

nd a

slig

htly

hig

her

40A

r/36

Ar

inte

rcep

t val

ue. T

he in

vers

e is

ochr

on a

ge o

f 19.

00 ±

0.2

2 M

a is

the

pref

erre

d ag

e. T

he

age

and

stra

tigra

phic

pos

ition

of t

his

lava

fi t w

ith a

cor

rela

tion

to th

e E

agle

Cra

gs V

olca

nics

(S

abin

, 199

4; M

onas

tero

et a

l., 1

997)

.

SD

0409

10-1

(IG

SN

:JE

A05

22M

U).

Sam

ple

of b

asal

t lav

a be

low

dac

ite la

va. G

roun

dmas

s fr

om th

is s

ampl

e ha

s a

dist

urbe

d pl

atea

u w

ith d

ecre

asin

g ag

e (F

ig. 4

E)

sim

ilar

to s

ampl

e S

D06

0810

-1. I

nver

se is

ochr

on

plot

giv

es a

n ag

e of

12.

74 ±

0.0

5 M

a w

ith a

slig

htly

bet

ter

fi t b

ut h

as a

hig

h M

SW

D o

f 6.7

and

a s

light

ly h

igh

valu

e fo

r at

mos

pher

ic a

rgon

. We

inte

rpre

t the

pla

teau

pat

tern

as

exce

ss a

rgon

and

ther

efor

e th

is

inte

rpre

ted

age

is a

max

imum

age

. Thi

s in

terp

reta

tion

fi ts

with

the

slig

htly

you

nger

age

s (1

1.66

and

11.

93 M

a) o

f oth

er b

asal

ts in

the

base

of t

he S

umm

it R

ange

vol

cani

cs.

SD

0608

10-1

(IG

SN

:JE

A05

22M

T).

Sam

ple

of b

asal

t lav

a fr

om th

e S

umm

it R

ange

, bel

ow d

acite

lava

. Gro

undm

ass

from

this

sam

ple

has

a di

stur

bed

plat

eau

with

pro

gres

sive

ste

ps w

ith d

ecre

asin

g ag

es (

Fig

. 4F

) th

at s

how

s tw

o ag

es a

t ca.

15

Ma

and

ca. 1

2 M

a. In

vers

e is

ochr

on p

lot g

ives

a b

ette

r fi t

with

a M

SW

D o

f 1.4

. The

ano

mal

ous

olde

r ag

e st

eps

may

be

the

resu

lt of

alte

ratio

n or

may

be c

onta

min

atio

n by

slig

htly

ol

der

igne

ous

rock

s. T

he p

refe

rred

age

is 1

1.93

± 0

.14

Ma,

whi

ch fi

ts w

ith th

e ag

e ra

nge

of o

ther

bas

alts

in th

e ba

se o

f the

Sum

mit

Ran

ge v

olca

nics

.

LV05

1210

-6 (

IGS

N:J

EA

0522

MK

). S

ampl

e of

the

basa

l bas

alt l

ava

fl ow

in th

e B

lack

Hill

s ov

er tu

ff de

posi

ts a

nd b

asal

tic-a

ndes

ite fl

ows.

Gro

undm

ass

from

this

sam

ple

yiel

ds a

val

id p

late

au a

ge o

f 11.

66 ±

0.0

6 M

a (F

ig. 4

G)

with

a r

easo

nabl

e M

SW

D v

alue

, and

inve

rse

isoc

hron

giv

es a

sim

ilar

age

and

slig

htly

hig

her

MS

WD

. Thi

s ag

e fi t

s w

ith th

e ag

es o

f oth

er b

asal

ts in

the

base

of t

he S

umm

it R

ange

Vol

cani

cs.

GF

Z-0

8060

3-1

(IG

SN

:JE

AG

FZ

863)

. Sam

ple

of o

rtho

clas

e m

egac

ryst

ic d

acite

lava

that

is p

art o

f the

Sum

mit

Ran

ge v

olca

nics

. Bio

tite

from

this

sam

ple

has

an o

dd p

late

au b

ehav

ior

(Fig

. 4H

) w

ith a

larg

e-vo

lum

e,

high

-tem

pera

ture

ste

p w

ith a

low

er a

ge. A

val

id p

late

au fo

r th

e en

tire

step

-hea

ting

yiel

ds 1

1.01

± 0

.29

Ma

with

a M

SW

D o

f 0.6

2. T

he in

vers

e is

ochr

on y

ield

s a

sim

ilar

age

to th

e pl

atea

us b

ut w

ith s

light

ly b

ette

r M

SW

D v

alue

. The

refo

re o

ur p

refe

rred

age

is 1

1.24

± 0

.43

Ma

age

from

the

inve

rse

isoc

hron

. Thi

s ag

e is

the

sam

e, w

ithin

err

or, a

s th

e ot

her

orth

ocla

se m

egac

ryst

ic d

acite

(sa

mpl

e LV

0307

10-4

; Fig

. 4I)

.

LV03

0710

-4 (

IGS

N:J

EA

0522

M6)

. Sam

ple

of o

rtho

clas

e m

egac

ryst

ic d

acite

lava

that

is p

art o

f the

Sum

mit

Ran

ge v

olca

nics

. Thi

s sa

mpl

e w

as a

naly

zed

via

tota

l fus

ion

of s

anid

ine

frac

tions

(F

ig. 4

I). F

requ

ency

an

alys

is o

f the

se d

ata

yiel

ds a

n ag

e of

11.

221

± 0

.018

Ma.

SD

1209

05-2

(IG

SN

:JE

A05

22M

S).

Sam

ple

of v

itrop

hyric

dac

itic

intr

usio

n w

ith fi

ne-g

rain

ed (

<1

mm

) ne

edle

s of

hor

nble

nde.

We

corr

elat

e th

is u

nit t

o th

e La

va M

ount

ain

Dac

ite. G

roun

dmas

s fr

om th

is s

ampl

e ha

s a

plat

eau

age

of 7

.32

± 0

.02

Ma

with

low

MS

WD

(F

ig. 4

J). T

he in

vers

e is

ochr

on y

ield

s a

sim

ilar

age

but w

ith a

hig

her

MS

WD

. Thi

s sa

mpl

e is

sim

ilar

in a

ppea

ranc

e to

sam

ple

SD

1209

05-1

.Thi

s sa

mpl

e ha

s od

d pl

atea

u sh

ape

whi

ch m

ay in

dica

te a

rel

ativ

ely

youn

g, b

ut s

till s

igni

fi can

t exc

ess

argo

n co

mpo

nent

. We

inte

rpre

t thi

s ag

e to

be

too

old;

see

text

for

disc

ussi

on.

SD

1209

05-1

(IG

SN

:JE

A05

22M

R).

Sam

ple

of fi

nely

vitr

ophy

ric d

aciti

c in

trus

ion

that

we

corr

elat

e to

the

Lava

Mou

ntai

n D

acite

. Gro

undm

ass

from

this

sam

ple

has

an o

dd-s

hape

d pl

atea

u (F

ig. 4

K).

The

low

est a

ge

step

(B

) is

7.4

6 ±

0.3

3 M

a. T

he in

vers

e is

ochr

on y

ield

s a

good

fi t w

ith a

low

MS

WD

and

an

age

of 7

.24

± 0

.83

Ma.

Thi

s sa

mpl

e is

min

eral

ogic

ally

and

text

ural

ly s

imila

r to

the

near

by s

ampl

e S

D12

0905

-2, w

hich

ex

hibi

ts a

bet

ter

plat

eau

beha

vior

and

has

an

age

sim

ilar

to th

e ag

e of

the

low

est a

ge s

tep

and

inve

rse

isoc

hron

of t

his

sam

ple.

We

inte

rpre

t thi

s ag

e to

be

too

old;

see

text

for

disc

ussi

on.

LV03

0710

-7 (

IGS

N:J

EA

0522

M5)

. Bas

alt c

last

sam

ple

from

the

cong

lom

erat

e of

Gol

den

Val

ley.

Gro

undm

ass

from

this

sam

ple

has

a pl

atea

u (F

ig. 4

L) th

at y

ield

s th

e pr

efer

red

age

of 1

1.62

± 0

.11

Ma.

Inve

rse

isoc

hron

ana

lysi

s (w

ithou

t ste

p A

) gi

ves

a sl

ight

ly h

ighe

r M

SW

D b

ut a

n ol

der

age

with

mor

e er

ror.

The

age

of t

his

basa

lt m

atch

es w

ith th

e ba

salts

at t

he b

ase

of th

e S

umm

it R

ange

vol

cani

cs.

B. R

eint

erpr

eted

40A

r/39

Ar

sam

ples

from

Sm

ith e

t al.

(200

2)

GF

Z-8

5 (I

GS

N:J

EA

GF

Z08

5). S

ampl

e of

dac

ite la

va in

the

Sum

mit

Ran

ge v

olca

nics

. Bio

tite

for

this

sam

ple

has

a pl

atea

u ag

e of

11.

32 ±

0.9

4 M

a us

ing

step

s E

–G (

Fig

. 6A

). T

his

anal

ysis

has

a la

rge

anal

ytic

al

erro

r of

7.6

6%. T

he in

vers

e is

ochr

on h

as a

low

MS

WD

, but

larg

e ag

e er

ror

of 8

.6 m

.y.

LM

96-1

5 (I

GS

N:J

EA

LM96

15).

Sam

ple

of d

acite

lava

in th

e S

umm

it R

ange

vol

cani

cs. B

iotit

e fr

om th

is s

ampl

e ha

s a

plat

eau

age

of 1

0.66

± 0

.12

Ma

(Fig

. 6B

) bu

t a h

igh

MS

WD

bec

ause

the

thre

e pl

atea

u st

eps

defi n

e a

slop

e. T

he in

vers

e is

ochr

on h

as a

hig

her

MS

WD

.

LM

96-1

6 (I

GS

N:J

EA

LM96

16).

Sam

ple

of d

aciti

c vo

lcan

ic d

ebris

-fl o

w d

epos

it in

the

Alm

ond

Mou

ntai

n V

olca

nics

. Bio

tite

from

this

sam

ple

yiel

ds a

val

id p

late

au a

ge o

f 7.8

2 ±

0.2

2 M

a (F

ig. 6

C).

The

pla

teau

dia

gram

ha

s a

sadd

le s

hape

, so

this

age

may

incl

ude

an e

xces

s ar

gon

com

pone

nt a

nd th

eref

ore

this

inte

rpre

ted

age

may

be

too

old.

The

inve

rse

isoc

hron

has

a s

imila

r ag

e w

ith m

any

anom

alou

s fr

actio

ns a

nd a

slig

htly

hi

gh 36

Ar/

40A

r i va

lue.

Hor

nble

nde

does

not

yie

ld a

val

id p

late

au (

Fig

. 6D

) be

caus

e of

ste

p K

, whi

ch r

elea

sed

79%

of t

he 39

Ar.

Ste

p K

has

an

age

of 8

.65

± 0

.67

Ma,

just

bar

ely

over

lapp

ing,

with

in e

rror

, with

the

biot

ite p

late

au. T

he h

ornb

lend

e in

vers

e is

ochr

on y

ield

s a

ca. 7

Ma

age,

but

with

an

anom

alou

s 36

Ar/

40A

r i.

LM

96-2

(IG

SN

:JE

ALM

9602

). S

ampl

e of

dac

itic

sill.

Bio

tite

data

do

not y

ield

a v

alid

pla

teau

(F

ig. 6

E),

but

ste

ps F

and

G a

re 7

4.5%

of t

he 39

Ar

rele

ased

. Ste

ps F

and

G y

ield

the

pref

erre

d ag

e fo

r th

is r

ock

of 7

.64

± 0

.18.

The

bio

tite

inve

rse

isoc

hron

giv

es a

sim

ilar

age

but h

as a

larg

e M

SW

D. T

he h

ornb

lend

e fo

r th

is s

ampl

e yi

elds

val

id p

late

au (

Fig

. 6F

) an

d is

ochr

on a

ges

of c

a. 1

0 M

a, b

ut h

as ~

10%

age

err

or a

nd is

~2.

7 m

.y. o

lder

. We

inte

rpre

t the

hor

nble

nde

to h

ave

an in

herit

ed c

ompo

nent

. We

corr

elat

e th

is in

trus

ive

unit

with

the

Lava

Mou

ntai

ns D

acite

, bas

ed o

n its

loca

tion

and

that

it c

uts

the

Alm

ond

Mou

ntai

n V

olca

nics

.

LM

96-1

7 (I

GS

N:J

EA

LM96

17).

Sam

ple

of tu

ff in

Alm

ond

Mou

ntai

n V

olca

nics

inte

rbed

ded

with

Bed

rock

Spr

ing

For

mat

ion.

Hor

nble

nde

from

this

sam

ple

does

not

yie

ld a

val

id p

late

au (

Fig

. 6G

). A

thre

e-po

int

isoc

hron

yie

lds

an a

ge o

f 9.9

8 ±

0.0

9 M

a w

ith a

larg

e M

SW

D a

nd a

larg

e 40

Ar/

36A

r i er

ror.

Thi

s an

alys

is d

oes

not y

ield

a v

alid

age

.

LM

1132

(IG

SN

:JE

ALM

1132

). S

ampl

e fr

om th

ick

gray

dac

ite la

va fl

ow. B

iotit

e fr

om th

is s

ampl

e do

es n

ot d

efi n

e a

valid

pla

teau

or

inve

rse

isoc

hron

(F

ig. 6

H).

Thi

s an

alys

is d

oes

not y

ield

a v

alid

age

.

Not

e: A

dditi

onal

dat

a fo

r th

ese

sam

ples

are

incl

uded

inTa

bles

DR

1 an

d D

R3

(see

text

foot

note

1)

and

also

acc

essi

ble

at h

ttp://

ww

w.g

eosa

mpl

es.o

rg/ u

sing

the

Inte

rnat

iona

l Geo

Sam

ple

Num

ber

(IG

SN

).

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 7

rocks in the eastern El Paso Mountains (Fig. 2). A few small exposures of similar quartz diorite occur in the northeastern Summit Range adja-cent to the Garlock fault (Fig. 2). All of the other basement in the LMSR is quartz monzonite to granodiorite of the Cretaceous Atolia Quartz Monzonite (Hulin, 1925; Smith, 1964).

Cenozoic UnitsLower Miocene volcanic-sedimentary rocks.

The oldest Cenozoic units in the LMSR are wide-spread but discontinuous felsic ash and pumice lapilli tuffs (Figs. 3 and 7). These tuffs are white, distinctively bedded with beds 5–15 cm thick, and have local interbeds of arkosic sandstone. The tuffs are overlain by oxidized porphy-ritic basaltic-andesite to andesite lava and are intruded by intermediate-composition porphyry (Dibblee, 1967). The lavas have 40Ar/39Ar ages of 19.63 ± 0.32 Ma on plagioclase (Fig. 4A) and 18.84 ± 0.24 Ma on plagioclase concentrate (Fig. 4B). Silicifi ed volcanic breccia deposits overlie the tuffs in the Christmas Canyon area. The clasts are dark colored and aphyric, and are dacites based on geochemical analyses (Monas-tero, unpub. data). These rocks have 40Ar/39Ar ages of 19.37 ± 0.04 Ma on groundmass (Fig. 4C) and 19.00 ± 0.22 Ma on groundmass con-centrate (Fig. 4D), similar to ages of the altered lava fl ows (Table 2). We correlate these rocks to the Eagle Crags Volcanics, based on age, composition, and stratigraphic relations (Sabin, 1994). Monastero et al. (1997) correlated the Eagle Crag Volcanics to the Cudahy Camp For-mation (Cox and Diggles, 1986) in the El Paso Mountains.

Middle Miocene basalt. Vesicular basalt over-lies the lower Miocene units in the LMSR. These are fi nely porphyritic with olivine and pyroxene and occur as 2–4-m-thick fl ows in the Summit Range and Lava Mountains but as a 25-m-thick sequence in the Black Hills. Basalts in the Summit Range and Lava Mountains have 40Ar/39Ar groundmass ages of <12.74 ± 0.05 Ma (Fig. 4E; Table 2) and 11.93 ± 0.14 (Fig. 4F), and the oldest basalt in the Black Hills has an age of 11.66 ± 0.06 Ma (Fig. 4G).

Upper Miocene Summit Range Volcanics. The basalts and older units in the Summit Range and Lava Mountains are overlain by a few-meters-thick conglomerate containing rounded and polished cobble- to boulder-sized clasts of felsic plutonic rocks and also of distinctive fl ow-banded rhyolite. A sequence of volcanic rocks occurs above beginning with a several-meters-thick tuff containing plutonic lithic blocks, pumice lapilli, and numerous lenses of dacitic lava breccia. These units are overlain by and interbedded with several bodies of dacite lava that have textures ranging from aphyric to porphyritic to megacrystic porphyritic. The dacite lava fl ows are thick (50–150 m), have limited areal extent (less than 1000 m long), and have the stratigraphic expression and textures of dacitic lava domes (Andrew et al., 2014). Feldspar and biotite porphyritic dacite lavas have 40Ar/39Ar ages of 11.32 ± 0.94 Ma (Fig. 6A) and 10.66 ± 0.12 Ma (Fig. 6B) on biotite and U-Pb zircon ages of 10.966 ± 0.042 Ma (Fig. 5A; Table 3) and 10.490 ± 0.054 Ma (Fig. 5B). Quartz-biotite-feldspar porphyritic dacite lavas containing distinctive 1–4 cm megacrysts of orthoclase have 40Ar/39Ar ages of 11.24 ± 0.43 Ma on groundmass concentrates (Fig. 4H) and 11.221 ± 0.018 Ma on sanidine (Fig. 4I). We modify terminology of Smith et al. (2002) and refer to these dacite dome complexes of lava, tuff, and epiclastic deposits as the Summit Range volcanics (Fig. 3).

Upper Miocene Bedrock Spring Formation–Almond Mountain Volcanics. Arkosic sand-stone of the Bedrock Spring Formation (Smith, 1964) unconformably overlies the dome com-plexes and older rocks in the Lava Mountains, Summit Range, and Christmas Canyon area. The basal unconformity with the dacite domes varies from disconformity to angular unconfor-mity to buttress unconformity. Thin lenses (gen-erally <15 cm) of pebble to cobble conglomer-ate occur throughout the section, as do locally abundant layers of siltstone. Limestone beds and well-cemented arkose occur only along the south sides of the dacite lava domes. These are likely lacustrine deposits resulting from pond-

ing upstream of the dacite domes. Sediment transport in the Bedrock Spring Formation was to the northwest (Smith, 1964; this study), so the domes would have formed topographic barriers.

Dacitic volcanic deposits of the Almond Mountain Volcanics overlie and are intercalated with the upper parts of the Bedrock Spring For-mation. These have pronounced facies changes across the LMSR, ranging from 200-m-thick complexes of multiple units (Fig. 8A) to sec-tions with only a single pumice lapilli tuff bed (Fig. 8B). The thickest sequences occur in the southern Lava Mountains, with multiple vol-canic debris-fl ow deposits of clasts of light-purple porphyritic dacite lava interbedded with several 2–10-m-thick pumice lapilli to lithic tuff beds and local sandstone beds. In the northeastern Lava Mountains, volcanic interbeds are mostly absent in the Bedrock Spring Formation except for one ~15-cm-thick pumice lapilli tuff and a 2-m-thick bed of dacitic volcanic debris-fl ow deposit. The Almond Mountain Volcanics are not present in the Black Hills area and are rare elsewhere in the LMSR; a 10-m-thick bed of tuff is exposed in the western Summit Range area and a 10–15-cm-thick tuff bed occurs in the upper parts of arkosic beds in the Christmas Canyon area. Dacitic lava above the Bedrock Spring Formation has a 40Ar/39Ar of <7.82 ± 0.22 Ma on biotite (Fig. 6C; Table 2), and a tuff in the upper Bedrock Spring Formation has a U-Pb zircon age of 7.479 ± 0.023 Ma (Fig. 5C).

Upper Miocene Lava Mountains Dacite. A thick porphyritic lava fl ow overlies the Bed-rock Spring Formation and Almond Mountain Volcanics. Smith (1964) defi ned this as the “Lava Mountain Andesite”, but geochemical data (Smith et al., 2002) show that it is a dacite; therefore we propose the name “Lava Moun-tain Dacite”. This lava is a distinctive brown-ish gray and has porphyritic plagioclase, horn-blende, and biotite with rare quartz and smaller and less-abundant pyroxene. It contains vari-able amounts of mafi c clots a few centimeters in diameter. The unit is voluminous, covering >50 km2 and 40–100 m thick. A sample from the Lava Mountain Dacite has a U-Pb zircon age of 6.507 ± 0.070 Ma (Fig. 5D).

Another porphyritic dacite lava fl ow occurs just beyond the northern exposures of Lava Mountain Dacite. This fl ow is distinguished from the Lava Mountain Dacite in that it has a red color, pilotaxitic textures, and a 10–20-m-thick basal vitrophyre. We associate this fl ow with the Lava Mountain Dacite based on its similar phenocryst assemblage, thickness, location, and stratigraphic relationships. Smith (1964) also included this with his Lava Mountains Andesite.

Numerous vitrophyric sills intrude the Bed-rock Spring Formation and Almond Mountain

TABLE 3. U/Pb ZIRCON GEOCHRONOLOGY RESULTS

Sample name IGSN*Unit

code† Geologic contextAge (Ma)

Age error(± Ma) MSWD§ n

LV050810-4 JEA0522MI LMD Dacite lava fl ow 6.507 0.070 1.9 2LV051810-8 JEA0522MO LMD Vitrophyric dacitic sill 6.522 0.026 2.8 3JDW-20 JEA0522MQ AMV Thick pink tuff 7.479 0.023 3.5 4LV050710 JEA0522MH SRV Upper dacite lava fl ow/dome 10.490 0.054 1.2 5LV031410-B JEA0522MA SRV Lower dacite lava dome 10.966 0.042 2.4 5CINCO JEACINCO1 SRV Megacrystic dacitic dike 11.383 0.028 1.2 6

Note: Additional data for these samples are included in Table DR2 (see text footnote 1).*International Geo Sample Number data accessible at http://www.geosamples.org/.†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.§MSWD—mean standard weighted deviates.

Andrew et al.

8 Geological Society of America Bulletin, Month/Month 2014

Volcanics. Smith (1964) misinterpreted the intrusive front of one of these dacite sills (Fig. 8C) as a lava fl ow front of a “Quaternary andes-ite”. This roll structure is intrusive: there is no basal fl ow breccia and the contacts on all sides are baked. We correlate these sills with the Lava Mountain Dacite based on similar phenocryst mineralogy, location, and large volume, and because they cut the Almond Mountain Vol-canics. These intrusives form a WNW-trending zone along the northern exposures of the Lava Mountain Dacite (Fig. 2). The red pilotaxitic lava occurs only along this zone of intrusions and may therefore be the near-vent facies of the Lava Mountain Dacite. Two other intrusive bod-

ies, in the western Summit Range (Fig. 2), are correlated to the Lava Mountain Dacite based on their vitrophyric texture and relationships to stratigraphic units. These have fi ne needles of hornblende and intrude the Bedrock Spring Formation and Almond Mountain Volcanics. Dibblee (1967) mapped these intrusions as basalt, but geochemical analyses show them to be dacitic (Monastero, unpub. data).

A vitrophyric sill from the central Lava Mountains area has a U-Pb zircon age of 6.522 ± 0.026 Ma (Fig. 5E), but the biotite 40Ar/39Ar age of a similar sill is 7.64 ± 0.18 Ma (Fig. 6E; Table 2). The hornblende vitrophyric dacitic intrusions from the Summit Range have 40Ar/39Ar ages of

7.32 ± 0.02 Ma (Fig. 4J) and 7.24 ± 0.83 Ma (Fig. 4K) on groundmass concentrates (Table 2). We postulate that the discrepancy between the U-Pb and 40Ar/39Ar ages could refl ect (1) an excess argon component, creating anomalously older ages, or (2) that the sills dated by 40Ar/39Ar better correlate to the Almond Mountain Vol-canics. The former is more likely because of the excess argon–interpreted hornblende 40Ar/39Ar ages, which are older than those of biotite from samples of Almond Mountain Volcanics and from one of these sills (Fig. 6C and 6E).

Plio-Pleistocene conglomerates. Pebble to boulder conglomerates overlie the Miocene units in the LMSR area. These rocks contain no known datable units, but we infer them to be Pliocene based on their deposition over the Lava Moun-tain Dacite and their angular unconformable contacts with overlying Quaternary units. We break out two different sets of these conglom-erates based on location and differences in clast types. Both units contain clasts of metasedimen-tary, meta-plutonic, plutonic, and volcanic rocks exotic to the LMSR, and also contain placer gold deposits (Dibblee, 1967; Fife et al., 1988). Boul-ders and cobbles of felsic plutonic rocks, felsic hypabyssal intrusive rocks, and quartzite are conspicuously well rounded and polished.

The informally defined conglomerate of Golden Valley is the eastern unit, which has distinctive lineated and foliated hornblende diorite clasts with a capping unit of boulders of vesicular basalt. These clast types distinguish it from the informally defi ned conglomerate of Hardcash Gulch in the Summit Range (Fig. 2), 15 km to the west. Rittase (2012) reported a tephrochronology age of 3.14 Ma for a tuff over-lying exotic-clast conglomerates a few kilome-ters east of Christmas Canyon. A sample from a basalt boulder of the conglomerate of Golden Valley has a 40Ar/39Ar age of 11.62 ± 0.11 Ma (Fig. 4L), matching the age of the texturally and mineralogically similar basalt in the Black Hills.

STRUCTURES

Previous studies of the LMSR show that the Cenozoic units are deformed (Smith, 1964; Dibblee, 1967; Smith et al., 2002) but did not determine kinematics of fault slip. New geologic mapping (Andrew et al., 2014) better defi nes these faults, identifi es several new faults (Fig. 2), and identifi es many stratigraphic markers to use in determining fault offset. Slip direction and sense of shear given for each fault below was determined by combining measurements of fault striations with two or more shear-sense indica-tors such as Riedel shears, fault gouge foliation, drag folding, fault-zone clast rotation, slicken-fi bers, asperity tool marks, and small-scale offset

0.007 0.009

0.00118

0.00115

z21

7.6 Ma

z5

z4

z23

7.4 Ma 7.479 ±0.023 MaMSWD = 3.5, n = 4

C. JDW-20 zircon

0.016

0.00180

0.010

weighted mean206Pb/238U (Th-corrected)

10.966 ±0.042 MaMSWD = 2.4, n = 5

z11

11.6 Ma

10.6 Ma

z1

z14z12

z13

A. LV031410-B zircon

0.007

0.00165

weighted mean

10.490 ±0.054 MaMSWD = 1.2, n = 5

z1010.7 Ma

10.3 Ma

z4

z8

z9

z5

B. LV050710 zircon

0.012

0.010

0.00180

0.00185

0.014

F. CINCO zircon

z14b

z7bz9b

z8b

z10bz2b

z11b z12bz5b

z3b

z13b

weighted mean

11.383 ±0.028 MaMSWD = 1.2, n = 6

11.2 Ma

11.6 Ma

12.0 Ma

206Pb/238U (Th-corrected)0.00161

weighted mean206Pb/238U (Th-corrected)

206Pb/238U (Th-corrected)

207Pb/235U

0.0080.005

z1

6.4 Maz2

z4

6.6 Ma

D. LV050810-4 zirconweighted mean

6.507 ±0.070 MaMSWD = 1.9, n = 2 (z2 & z4)

206Pb/238U (Th-corrected)

207Pb/235U

206 P

b/23

8 U206 P

b/23

8 U20

6 Pb/

238 U

206 P

b/23

8 U20

6 Pb/

238 U

207Pb/235U

0.0080

0.00103

0.001000.0060

z2z3

6.7 Ma

6.6 Ma

z1

E. LV051810-8 zircon

6.5 Ma

6.522 ±0.026 MaMSWD = 2.8, n = 3

206Pb/238U (Th-corrected)weighted mean

206 P

b/23

8 U

207Pb/235U

207Pb/235U 207Pb/235U

0.00170

0.00105

0.00100

Figure 5. Concordia plots of U-Pb analyses of samples (MSWD—mean square of weighted deviates). Unfi lled fraction error ellipses were not used to calculate the crystallization age.

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 9

(Tchalenko, 1970; Chester and Logan, 1987; Means, 1987; Petit, 1987). We also discuss folds of late Cenozoic units in the LMSR.

Geometry and Kinematics of Faults

Sinistral FaultsGarlock fault. Data for the Garlock fault in

the western LMSR show left-lateral slip with low rake and an associated set of normal-slip fault

planes (Fig. 9A). Fault scarps of the Garlock fault in Quaternary alluvium (Rittase, 2012) show a similar orientation of fault planes (Fig. 9B).

Teagle Wash fault. The Teagle Wash fault (named herein) occurs in the northeast Summit Range (Fig. 2). It was previously interpreted as a thrust fault (Hulin, 1925; Smith, 1964) because it places granitic rock over Bedrock Spring Formation, Eagle Crags Volcanics, and green-altered medium-grained quartz diorite. This

quartz diorite is similar to intrusive rocks of the Jurassic Laurel Mountain granodiorite and not the more felsic and less altered Atolia Quartz Monzonite. Measured fault planes have low dips with low-rake fault striae (Fig. 9C). Top-to-the-ESE relative motion of this fault (Fig. 10A) equates to left-lateral oblique slip (Fig. 9C).

Savoy fault. The Savoy fault (named herein) separates the Summit Range from the Lava Mountains (Fig. 2). It has moderate dips to the

I = 7.66 ±0.32 Ma (MSWD = 3.5,

95% 39Ar)40Ar/36Ar intercept

= 304 ±13

E. LM96-2 (Qa) biotiteB

C D EF G

A

P = no plateau*Steps F&G = 74.5% of 39Ar

with an age of 7.64 ±0.18 Ma

15

5

AC

D

E

I

FG

H

0.002

0.001App

aren

t Age

(Ma)

36A

r/40A

r

I = 7.66 ±0.26 Ma (MSWD = 0.48,

86% 39Ar)40Ar/36Ar intercept

= 317 ±22

C. LM96-16 (Ta) biotite

DE

F G

H,I

*P = 7.82 ±0.22 Ma(MSWD = 0.27, 86% 39Ar)

15

5

B

A

C

F,G,H I

J

D

E

B

C

A

0.002

0.001App

aren

t Age

(Ma)

36A

r/40A

r

I = 10.98 ±0.53 Ma (MSWD = 38,

96% 39Ar)40Ar/36Ar intercept

= 277 ±17

B. LM96-15 (Tvd) biotite

B

C

D

E

F G H

A

J

I

*P = 10.66±0.12 Ma(MSWD = 1.9, 91% 39Ar)

15

5

B

A

ED

CL

FG

HI

KJ

0.002

0.001App

aren

t Age

(Ma)

36A

r/40A

r

I = 9.98 ±0.09 Ma (MSWD = 5.6,

85.9% 39Ar)40Ar/36Ar intercept

= 295 ±480

B

C

D

E

F

G

HA

IJ K

L

G. LM96-17 (Tat) hornblende

P = no plateau

0

20

80

KJ

B

A

F

M

G

CD

EH

I

L 0.002

0.003

App

aren

t Age

(Ma)

36A

r/40A

r

F. LM96-2 (Qa) hornblende

B

E

F

GH

I

P = 10.3 ±1.2 Ma (MSWD = 0.68)*step G = 10.23 ±0.62 Ma

with 74% of 39Ar

0

20

80A

CD

E

I

FG

H

B

J

I = 9.9 ±1.3 Ma (MSWD = 0.54,

95% 39Ar)40Ar/36Ar intercept

= 316 ±16

0.002

0.001

App

aren

t Age

(Ma)

36A

r/40A

r

D. LM96-16 (Ta) hornblende

J

L

K

P = no plateaustep K = 8.65 ±0.67 Ma

with 79% of 39Ar

0

20

80

B

A

H

J

KG,I,L,F

0.003

0.002

I = 7.0 ±2.1 Ma (MSWD = 2.4,

91% 39Ar)40Ar/36Ar intercept

= 410 ±36

App

aren

t Age

(Ma)

36A

r/40A

r

App

aren

t Age

(Ma) 36

Ar/40

Ar

0.002

0.001

H. LM1132 biotite

I

C

D EF

G H

P = no plateau

10

0

I

C

D

E

F

GH

I = 11.3 ±8.6 Ma (MSWD = 0.20,

88.3% 39Ar)40Ar/36Ar intercept

= 287 ±18

A. GFZ-85 biotite

E F GH

*P = 11.32±0.94 Ma (MSWD = 0.017, 88.3% 39Ar)

6

14

H

C

D

G

E F

D

A,B,C

0.002

0.001App

aren

t Age

(Ma)

36A

r/40A

r

AB

I = no intercept

0 10 1 Cumulative 39Ar FractionCumulative 39Ar Fraction 0 0.339Ar/40Ar0 0.339Ar/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.

Andrew et al.

10 Geological Society of America Bulletin, Month/Month 2014

south, low-rake slip vectors (Figs. 9D, 10B, and 10C), and left-lateral sense of shear. The Savoy fault merges into the Garlock fault in the east-ern LMSR. The Savoy fault continues westward into Quaternary fault scarps of the Cantil fault (Dibblee, 1952, 1967) and may be a continua-tion of the main trace of the western Garlock fault (WGF on Fig. 1).

Browns Ranch fault zone. The ~2-km-wide Brown’s Ranch fault zone has similar fault mea-surements and kinematics (Fig. 9E) to the Gar-lock fault (Fig. 9A). Exposures of the Brown’s Ranch fault zone show a multi-stranded struc-ture with spatially associated folding of all of the Miocene units with axes roughly parallel to the strike of this fault zone (Fig. 10D).

Oblique FaultLittle Bird Fault. The boundary between the

Paleozoic basement of the Christmas Canyon area and the Atolia Quartz Monzonite basement present in the rest of the LMSR is a WNW-strik-ing fault that juxtaposes lower Miocene rocks against Bedrock Spring Formation. Limited fault measurements along this and subsidiary

faults indicate sinistral-oblique thrust kine matics (Fig. 9F). We refer to this as the Little Bird fault (Fig. 2). The fault is unconformably overlapped by the conglomerate of Golden Valley.

Normal FaultRandsburg Wash fault zone. To the east of

the Blackwater fault, Smith (1964) and Oskin and Iriondo (2004) mapped a zone of faults ori-ented similar to the Brown’s Ranch fault zone. This fault zone is poorly exposed, but limited data show northeast-striking faults with dip-slip motion (Fig. 9G) dissimilar to that of the oblique strike-slip Brown’s Ranch fault zone. We con-sider it to be a different structure and name it the Randsburg Wash fault zone (RWFZ on Fig. 2). These faults are inferred to have slip as young as late Pleistocene based on along-strike fault scarps 12 km to the east that Smith et al. (1968) interpreted as late Pleistocene features.

Dextral FaultBlackwater fault. The Blackwater fault juxta-

poses Quaternary alluvium against Cretaceous granitic and Miocene rocks (Fig. 2). This fault

strikes NNW-SSE, dips steeply, has low-rake fault striae (Fig. 9H), and has dextral motion. The Blackwater fault continues northward past the Brown’s Ranch fault zone (Andrew et al., 2014), in contrast to the interpretation of Oskin and Iriondo (2004) that it to loses slip before this point. The northward continuation has a slightly different geometry, with a more northwesterly strike, lower dips, and right-oblique normal slip (Fig. 9I). There are also sets of normal and thrust faults associated with the northern part of this fault (Fig. 9I).

Folds

Smith (1964) identifi ed folds in the Lava Mountains that deform upper Miocene units about WSW-trending fold axes (Fig. 9J). These folds form anticline-syncline pairs in the Bed-rock Spring Formation that have steeply over-turned SSE-facing central limbs. Miocene bedding also show a second possible fold orien-tation that plunges shallowly to the NNW (Fig. 9J). Bedding for Miocene units in the Christmas Canyon area are interpreted to have both of these

N

GFZ-85 (11.32 ±0.94 Ma)

LM96-15 (10.66 ±0.12 Ma)

LM96-17 (N.A.)

LM96-2 (7.64 ±0.18x Ma)

LM96-16 (7.82 ±0.22 Ma)

SD120905-1(7.24 ±0.83x Ma)

SD120905-2(7.32 ±0.02x Ma)

GFZ-080603-1 (11.24 ±0.43 Ma)

LV030710-4 (11.22 ±0.02 Ma)

LV030710-7 [clast](11.62 ±0.11 Ma)

SD060810-1 (11.93 ±0.14 Ma)

SD040910-1 (12.74 ±0.05x Ma)

LV030710-5 (18.84 ±0.24 Ma)

LV040910(19.00 ±0.22 Ma)

LV031510 (19.37 ±0.04 Ma)

LV051210-5(19.63 ±0.32 Ma)

JDW-20(7.479 ±0.023u Ma)

LV050710 (10.490 ± 0.054u Ma)

LV031410-B(10.966 ±0.042u Ma)

LV051810-8 (6.522 ±0.026u Ma)

LV050810-4(6.507 ±0.070u Ma)

LV051210-6(11.66 ±0.06 Ma)

WesternSummitRange

CentralSummit Range

EasternSummitRange Black Hills

ChristmasCanyon

Northeastern Lava

MountainsNorthern

Lava Mountains

Eastern LavaMountains

CentralLava

Mountains

Western LavaMountains

North-CentralLava MountainsFar Western

Lava Mountains

Garlock fault

Savoy fault

Blackw

ater fault

sret

em

00

1

u = U-Pb age (others are 40Ar/39Ar)x = older age due to excess argon

Ve

rtic

al s

cale

Plio-Pleistocene sedimentary rocks

Tld TlrTaTbTsTeKJ

Tld

Tlr

Ta

Tb

TsTe

K

Tpr

QTph

Tb

Tb

Tb

Tb

Tb

Tb

Tb

Tb

Tpg

Tb

Te

Te

Te

TeTe

Te

Te

Ts

Ts

TsTs Ts

Ts

Ts

K

K

K

K

K

Ta

Ta

Tld

Tld

Tb

Ta

Tlr

Cretaceous Atolia Quartz MonzoniteEagle Crags VolcanicsSummit Range volcanicsBedrock Spring FormationAlmond Mountain Volcanics

Lava Mountains Dacite

Stratigraphic units

-red pilotaxitic

Jurassic quartz diorite

dacite lava domeconglomerate

plutonic rocks

fanglomeratehypabyssal dacitic

deposit

arkosic sandstone

metasedimentary rocks

Lithology

Tpg

QTphgpT rpT (see Figure 2 for abbreviations)

J

Paleozoic metasedimentary 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.

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 11

fold sets, although the WSW-trending set is less tightly folded (Fig. 9K). The younger conglom-erate 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.

Magnitude and Rates of Fault Slip

Following the example of Andrew and Walker (2009), we identifi ed as many markers as possible in order to obtain maximum reso-lution on the amount and timing of motions of individual fault systems (see Table 4 for details and methods for determining uncertainty). The above section on fault geometry and kine-matics shows that the major faults in the LMSR, 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 dis-cussed below are shown mostly on Figure 11 with a few regional markers on Figure 1. These markers vary from blunt area restoration mark-ers 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.

Sinistral FaultsGarlock fault—Christmas Canyon area. We

interpret that the Christmas Canyon basement rocks correlate to rocks in the eastern El Paso Mountains using distinctive Paleozoic metasedi-mentary units that are appropriate to use as slip markers, because they are thin (60 ± 10 m), have steep dips, and strike roughly perpendicular to the Garlock fault. A quartzite–black slate bed (A on Fig. 11) matches a correlative bed in the eastern El Paso Mountains (A′ on Fig. 11). A meta-conglomerate layer (B on Fig. 11) matches a narrow zone of rocks in the eastern El Paso Mountains (B′ on Fig. 11), but it is less well exposed in the LMSR. Offset needed to restore

the quartzite-slate marker bed along the Garlock fault is 32.9 ± 0.6 km (Table 4). Larger slip is unlikely because metasedimentary rocks farther west in the El Paso Mountains have higher meta-morphic grades, contain abundant metavolcanic rocks, and are intruded by ductilely deformed plutons (Dibblee, 1952; Carr et al., 1997).

We correlate a set of Pliocene rocks in the Christmas Canyon area to rocks in the eastern El Paso Mountains (Carter, 1994; Carr et al., 1997;

Smith et al., 2002); this correlation can be used to constrain slip on this segment of the Garlock fault. The Pliocene conglomerate of Golden Valley contains clasts exotic to the LMSR and placer gold (Fife et al., 1988). A subset of these clasts has distinctive well-rounded and polished textures: felsic plutonic, felsic hypabyssal, and quartzite. The exotic clasts, placer gold, and well-rounded clasts match to a sediment source in the El Paso Mountains: the Paleocene Goler

tuff bed

Bedrock Spring

Formation

Lava MountainsDacite flows

tuff bed

arkose

arkose

Bedrock Spring

Formation

bluish pumice

lapilli tuff

massivedacite

Bedrock SpringFormation

coolingcolumns

Bedrock SpringFormation

fault

A

C

B

1 m

dacite debrisflow deposits

BedrockSpring

Formation

Figure 8. Photographs of select late Miocene geologic units in the Lava Mountains–Sum-mit Range. (A) View of thickest section of 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 single thin tuff bed. Hammer (33 cm long) for scale. (C) View of roll structure of dacite that Smith (1964) interpreted as a lava fl ow edge, but which is the lateral edge of a shallow-level intrusion.

Andrew et al.

12 Geological Society of America Bulletin, Month/Month 2014

A. Garlock fault in bedrock

n = 6 n = 6

B. Garlock fault in Quaternarysediments

n = 4

C. Teagle Wash fault

D. Savoy fault

n = 11

E. Browns Ranch fault zone

n = 8

n = 552

β2

β1

J. Miocene bedding in central Lava Mountains

1% contours

G. Randsburg Wash fault zone

n = 4

I. Northern Blackwater fault zone

n = 17

H. Blackwater fault

n = 4

n = 3

F. Little Bird fault

β2 β2

n = 82

L. Pliocene bedding of conglomerate of Golden Valley

1% contours

β1

n = 74

K. Miocene bedding in Christmas Canyon area

1% contours

Figure 9. Stereographs of fault and bed-ding 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. The motion of the hanging wall of each fault is indicated by the arrow away from the 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 differ-ent units with best-fi t fold girdles in red and corresponding fold axes labeled β1 and β2.

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 cut-ting 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.

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 13

volc

anic

deb

ris fl

ow

of d

acite

cla

sts

(Alm

ond

Mou

ntai

nVo

lcan

ics)

ash

tuff

ash

tuff

sync

line

5mD E

volc

anic

deb

ris fl

ow

of d

acite

cla

sts

(Alm

ond

Mou

ntai

nVo

lcan

ics)

volc

anic

deb

ris fl

ow

of d

acite

cla

sts

(Alm

ond

Mou

ntai

nVo

lcan

ics)

fels

ic p

orph

yry

(Sum

mit

Ran

gevo

lcan

ics)

fels

ic p

orph

yry

(Sum

mit

Ran

gevo

lcan

ics)

Ato

liaQ

uart

zM

onzo

nite

quar

tzm

onzo

nite

quar

tz

mon

zoni

te

25 c

m

A

25 c

m

daci

tela

va

goug

e of

Ato

lia Q

uartz

Mon

zoni

te

dacit

e go

uge

Lava

Mou

ntai

nsD

acite

Bed

rock

Spr

ing

Form

atio

n

alte

red

Ato

liaQ

uartz

Mon

zoni

te

5 m

25 c

m

over

turn

edre

cum

bent

antic

line

blea

ched

quar

tzm

onzo

nite

goug

e

B C

over

turn

edbe

ddin

gbe

ddin

g

over

turn

ed

bedd

ing

R-shear

R′-shear

R-shea

r

R-shear

Bed

rock

Sprin

g Fm

.

Bed

rock

Sprin

g Fm

.

Faul

t str

iae

Faul

tsu

rfac

e

Folia

ted

goug

e

Fig

ure

10.

Andrew et al.

14 Geological Society of America Bulletin, Month/Month 2014

Formation (Fig. 2). The basal Goler Formation contains coarse conglomerates derived from the El Paso Mountains and the Sierra Nevada Mesozoic batholith, quartzite clasts of unknown provenance (Cox, 1982), and placer gold (Dib-blee, 1952). The farther-traveled clasts of the Goler Formation (felsic plutonic and quartzite) have well-rounded and polished textures (Cox, 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 of clasts from the Sierra Nevada batholith and quartzite clasts.

Clasts of altered volcanic rock are also pres-ent in the conglomerate of Golden Valley. A possible source for these clasts is the Cudahy Camp Formation in the El Paso Mountains, which overlies the Goler Formation (Dibblee, 1952; Loomis and Burbank, 1988; Monastero et al., 1997). A potential specifi c source for all of these clasts is Goler Gulch (Fig. 2), which cuts through the Goler and Cudahy Camp For-mations. Goler Gulch is the largest drainage system of the eastern El Paso Mountains and the only one to cut across the El Paso Mountains southern escarpment. We thus envision it to be a long-lived feature capable of supplying sedi-ment to the southern El Paso Mountains where the Garlock fault would subsequently trans-port it away.

The clasts in the conglomerate of Golden Valley in the Christmas Canyon area (C on Fig. 11) are offset 30.2–39.2 km (Table 4) from Goler Gulch in the eastern El Paso Mountains (C′ on Fig. 11). This offset of a wide-area fea-ture agrees with the 32.9 ± 0.6 km offset of the Paleozoic marker beds. This implies that slip on this segment of the Garlock fault initiated after deposition of the conglomerate of Golden Valley (6.5–3.14 Ma). Simple slip-rate calcula-tions using these data yield long-term slip rates of 5.0–10.7 mm/yr (Table 4). These are similar 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 (McGill and Sieh, 1993).

Garlock fault—Summit Range. The Atolia Quartz Monzonite in the Summit Range matches across the Garlock fault to similar rocks in the southeastern Sierra Nevada (Fig. 1; Saleeby et al., 2008). Garlock fault offset for restoring these rocks is poorly constrained to >42 km using the easternmost exposures of Cretaceous plutonic rocks in the Sierra Nevada (Fig. 1). A megacrystic dacite lava dome in the Summit Range is cut by the Garlock fault (D on Fig. 11), so it probably continued north-ward but there are no known dacite lavas on

TAB

LE 4

. SLI

P C

ON

ST

RA

INT

S F

OR

FA

ULT

S IN

LA

VA

MO

UN

TAIN

S–S

UM

MIT

RA

NG

E A

RE

A

tluaffoedis

etisoppon o

eru ta efd etal err o

Cerut ae f

c igo lo egt ni artsnocpi l

SS

lip

(km

)§

[err

or]

Age

(M

a)#

[err

or]

Slip

ra

te

setoN

)ry/m

m(La

bel*

Des

crip

tion

D†

(km

)La

bel*

Des

crip

tion

D†

(km

)G

arlo

ck fa

ult a

long

Chr

istm

as C

anyo

n ar

ea

AQ

uart

zite

and

bla

ck s

late

bed

s in

met

a-si

ltsto

ne

and

met

a-lim

esto

ne s

eque

nce*

*0.

91A

′Q

uart

zite

and

bla

ck s

late

of m

iddl

e m

embe

r of

the

met

ased

imen

tary

roc

ks o

f El P

aso

Pea

ks (

Car

r et

al.,

199

7)

0.37

32.9

[± 0

.6]

ca. 4

60T

his

mea

sure

men

t inc

lude

s a

faul

t with

~80

0 m

of

left-

late

ral o

ffset

in th

e ea

ster

n E

l Pas

o M

ount

ains

(C

arr

et a

l., 1

997)

BM

etac

ongl

omer

ate

in m

eta-

silts

tone

seq

uenc

e in

Chr

istm

as C

anyo

n ar

ea1.

85B

′M

etac

ongl

omer

ate

of th

e m

etas

edim

enta

ry

rock

s of

Hol

land

Cam

p (C

arr

et a

l., 1

997)

1.11

32.4

[± 0

.8]

ca. 3

40

CD

istin

ctiv

e bo

ulde

r cl

ast a

ssem

blag

e in

co

nglo

mer

ate

of G

olde

n V

alle

y. A

lso

roun

ded

boul

der

text

ures

and

pla

cer

gold

.

0.80

C′

Dep

ositi

on a

cros

s G

arlo

ck fa

ult f

rom

Gol

er G

ulch

w

hich

cut

s cl

ast s

ourc

es o

f Gol

er F

orm

atio

n an

d C

udah

y C

amp

For

mat

ion.

See

text

di

scus

sion

.

0.99

34.7

[± 4

.5]

3.14

to

6.5

Thi

s co

nglo

mer

ate

is s

ourc

ed fr

om th

e ea

ster

n E

l P

aso

Mou

ntai

ns. M

oder

n G

oler

Gul

ch h

as a

n al

luvi

al fa

n th

at is

1.8

km

wid

e.

32.9

[± 0

.6]

3.14

to

6.5

5.0

to

10.7

Slip

rat

e us

es A

-A′ s

lip w

ith th

e tim

ing

of th

e co

nglo

mer

ate

of G

olde

n V

alle

yG

arlo

ck fa

ult a

long

the

Sum

mit

Ran

ge

D11

.24

± 0

.43

Ma

(40A

r/39

Ar

biot

ite)

orth

ocla

se

meg

acry

stic

dac

ite la

va d

ome

in S

umm

it R

ange

, ove

rlyin

g ca

. 12

Ma

basa

lt la

va fl

ows

0.00

MD

S††

11.3

83 ±

0.0

28 M

a (U

-Pb)

ort

hocl

ase

meg

acry

stic

dac

ite d

ike

swar

m in

gra

nitic

roc

ks

(Mur

doch

and

Web

b, 1

940;

Sam

sel,

1962

)

0.69

43.7

[± 0

.8]

11.3

83[±

0.0

28]

3.8

to

3.9

The

se a

re m

uch

slow

er th

an n

eote

cton

ic r

ates

on

the

Gar

lock

faul

t in

this

are

a (M

cGill

et a

l.,

2009

)B

edro

ck S

prin

g F

orm

atio

n an

d S

umm

it R

ange

vo

lcan

ics

DS

F††

Cor

rela

te to

Dov

e S

prin

g F

orm

atio

n in

the

wes

tern

El P

aso

Mou

ntai

ns40

[± 1

0]7.

5 to

11

.8T

his

corr

elat

ion

has

low

pre

cisi

on b

ecau

se th

ere

are

no s

peci

fi c fe

atur

es to

res

tore

C″

Eas

tern

mos

t exp

osur

es o

f cla

st a

ssem

blag

e in

con

glom

erat

e of

Har

dcas

h G

ulch

. Als

o ro

unde

d te

xtur

es a

nd p

lace

r go

ld.

0.00

C′

Dep

ositi

on a

cros

s G

arlo

ck fa

ult f

rom

Gol

er

Gul

ch w

hich

cut

s cl

ast s

ourc

es o

f Gol

er

For

mat

ion

and

Cud

ahy

Cam

p F

orm

atio

n.

See

text

dis

cuss

ion.

0.99

15.5

[± 0

.9]

1.8

to

6.5

2.2

to

9.1

Sim

ilar

sour

ce in

terp

reta

tion

as th

e co

nglo

mer

ate

of G

olde

n V

alle

y

Sav

oy fa

ult

D11

.24

± 0

.43

Ma

(40A

r/39

Ar

biot

ite)

orth

ocla

se

meg

acry

stic

dac

ite la

va d

ome

in S

umm

it R

ange

, ove

rlyin

g ca

. 12

Ma

basa

lt la

va fl

ows

2.30

D′

11.2

21 ±

0.1

8 M

a (40

Ar/

39A

r sa

nidi

ne)

orth

ocla

se

meg

acry

stic

dac

ite d

ome

in L

ava

Mou

ntai

ns

over

lyin

g ca

. 12

Ma

basa

lt la

va fl

ows

2.31

16.7

[± 1

.8]

11.2

21[±

0.0

18]

1.33

to1.

65T

hese

dom

es a

re c

over

ed b

y yo

unge

r da

cite

la

vas

and

youn

ger

volc

anic

-sed

imen

tary

su

cces

sion

sE

~80

-m-t

hick

tuff

bed

in u

pper

Bed

rock

Spr

ing

For

mat

ion

0.00

E′

~70

-m-t

hick

tuff

bed

with

in th

e B

edro

ck S

prin

g F

orm

atio

n in

Sum

mit

Ran

ge w

ith E

SE

str

ike

0.65

6.0

[± 0

.7]

7.47

9[±

0.0

23]

0.7

to0.

9A

ge fr

om U

-Pb

zirc

on a

ge o

f thi

ck tu

ff in

lava

M

ount

ains

, sam

ple

JDW

-20

FR

ed p

lagi

ocla

se p

orph

yriti

c la

va fl

ow w

ith

pilo

taxi

tic fa

bric

and

bas

al v

itrop

hyre

de

posi

ted

on to

p of

10–

11 M

a da

cite

lava

s

0.60

F′

Red

pla

gioc

lase

por

phyr

itic

lava

fl ow

with

pi

lota

xitic

fabr

ic a

nd b

asal

vitr

ophy

re o

ver

Bed

rock

Spr

ing

For

mat

ion

0.37

7.3

[± 1

.3]

6.52

2[±

0.0

26]

0.9

to1.

3A

ge fr

om U

-Pb

zirc

on a

ge o

f vitr

ophy

re in

Lav

a M

ount

ains

, sam

ple

LV05

1810

-8

10.7

[± 2

.5]

3.74

22.

2 to

3.4

Sub

trac

ted

tuff

bed

offs

et fr

om m

egac

ryst

ic d

acite

do

me

offs

et fo

r sl

ip b

etw

een

11.4

and

7.5

Ma

(con

tinue

d)

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 15

the north side of the fault. There is, however, a 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 mega-crysts (Murdoch and Webb, 1940). These dikes could be feeder dikes for the mega-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 are the same age within error. The slip of the Garlock fault required to juxtapose the ortho-clase megacrystic dikes of the Sierra Nevada to orthoclase megacrystic dacite domes in the Summit Range is 43.7 ± 0.8 km (Table 4).

The Bedrock Spring Formation of the LMSR has been correlated with the upper Dove Spring Formation in the El Paso Mountains (DSF on Fig. 1) (Smith et al., 2002; Frankel et al., 2008). We fi nd that the Bedrock Spring Formation matches well in time, composition, and sedi-ment transport direction (Smith, 1964; Loomis and Burbank, 1988; Whistler et al., 2009) with member 5 of the Dove Spring Formation. The lower four members of the Dove Spring Forma-tion correlate in time with the Summit Range volcanics, but these do not have dacite lava domes or the associated near-vent volcanic rocks. An 11.8 ± 0.9 Ma (Loomis and Burbank, 1988) thick felsic tuff occurs in member 2 of the Dove Spring Formation, which could have erupted from the lava domes of the Summit Range volcanics (Frankel et al., 2008). These correlations loosely constrain offset of the Gar-lock fault to be 30–50 km (Table 4).

The Pliocene conglomerate of the Hardcash Gulch contains clasts consistent with a source 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 placer gold (see earlier discussion of Pliocene conglomerates). Garlock fault offset needed to deposit the easternmost exposure of the con-glomerate of Hardcash Gulch is 15.5 ± 0.9 km (C″ to C′ on Fig. 11; Table 4). The age of this conglomerate is unknown, but it is younger than the Lava Mountain Dacite–correlated intrusive rocks it overlies, and the western parts may be as young as early Pleistocene. Slip-rate calcula-tions for minimum and maximum values yields 2.2–9.1 mm/yr (Table 4). The younger ages therefore are more consistent with the neotec-tonic slip rates for the central Garlock (McGill and Sieh, 1993; Rittase et al., 2014).

Teagle Wash fault. There are no fault slip markers for the Teagle Wash fault. The presence of Jurassic Laurel Mountain granodiorite below

TAB

LE 4

. SLI

P C

ON

ST

RA

INT

S F

OR

FA

ULT

S IN

LA

VA

MO

UN

TAIN

S–S

UM

MIT

RA

NG

E A

RE

A (

cont

inue

d)

tluaffoedis

etisoppono

er utae fdet aler ro

Ceruta ef

ci go loe gtnia rtsn ocpil

SS

lip

(km

)§

[err

or]

Age

(M

a)#

[err

or]

Slip

ra

te

setoN

)ry/m

m(La

bel*

Des

crip

tion

D†

(km

)La

bel*

Des

crip

tion

D†

(km

)B

row

ns R

anch

faul

t zon

e

GE

aste

rn e

dge

of L

ava

Mou

ntai

n D

acite

lava

fl ow

ov

erly

ing

Bed

rock

Spr

ing

For

mat

ion

0.62

G′

Eas

tern

edg

e of

Lav

a M

ount

ain

Dac

ite la

va fl

ow

over

lyin

g B

edro

ck S

prin

g F

orm

atio

n1.

833.

9[±

0.7

]6.

522

[± 0

.026

]0.

5 to

0.7

Est

imat

ed s

lip r

esto

ring

edge

of 8

0–10

0-m

-thi

ck

lava

fl ow

acr

oss

wid

e fa

ult z

one

Bla

ckw

ater

faul

t

H4-

m-t

hick

blu

ish

pum

ice

lapi

lli tu

ff of

the

dist

al

faci

es o

f the

Alm

ond

Mou

ntai

n V

olca

nics

. T

his

bed

dips

mod

erat

ely

to th

e so

uthe

ast.

0.00

H′

4-m

-thi

ck b

luis

h pu

mic

e la

pilli

tuff

bed

of th

e di

stal

faci

es o

f the

Alm

ond

Mou

ntai

n V

olca

nics

, di

ps 5

2° in

to fa

ult z

one

1.13

1.9

[± 0

.3]

7.47

9[±

0.0

23]

0.2

to0.

3Tu

ff co

rrel

atio

n to

the

thic

k tu

ff w

ith a

U-P

b zi

rcon

ag

e. T

he fa

ult z

one

is 4

80 m

wid

e.

IE

NE

-str

ikin

g bu

ttres

s un

conf

orm

ity o

f Bed

rock

S

prin

g F

orm

atio

n ag

ains

t sid

e of

dac

ite

dom

es

0.1

I′′′

EN

E-s

trik

ing

buttr

ess

unco

nfor

mity

of B

edro

ck

Spr

ing

For

mat

ion

agai

nst t

he s

ide

of d

acite

do

mes

0.1

2.1

[± 0

.6]

7.5

to

7.8

Not

e, th

ere

are

thre

e st

rand

s de

note

d by

I′, I

″an

d I′′

′

JN

orth

edg

e of

thic

k gl

assy

dac

ite la

va fl

ow o

ver

quar

tz m

onzo

nite

0.0

J′N

orth

edg

e of

thic

k gl

assy

dac

ite la

va fl

ow o

ver

quar

tz m

onzo

nite

0.0

0.3

to 1

.87.

2[±

1.1

] F

rom

Osk

in a

nd Ir

iond

o (2

004)

BM

††N

orth

ern

edge

of b

asal

t lav

a fl o

w

0.0

BM

††N

orth

ern

edge

of b

asal

t lav

a fl o

w

0.1

1.8

[± 0

.1]

3.77

[± 0

.11]

Fro

m O

skin

and

Irio

ndo

(200

4)

Gar

lock

faul

t bet

wee

n E

l Pas

o M

ount

ains

and

Pilo

t Kno

b ar

ea

MC

††M

etac

ongl

omer

ate

of th

e R

obbe

rs M

ount

ain

For

mat

ion

on e

ast s

ide

of a

faul

t with

D

evon

ian(

?) g

reen

ston

e (C

arr

et a

l., 1

997)

1.57

PK

V††

Met

acon

glom

erat

e of

the

Rob

bers

Mou

ntai

n F

orm

atio

n on

the

east

sid

e of

a fa

ult c

onta

ct

with

gre

enst

one

(Car

r an

d P

oole

, 199

2)

0.68

62.7

[± 1

.7]

ca. 2

60To

tal s

lip a

cros

s th

e ce

ntra

l Gar

lock

faul

t zon

e:

incl

udes

El P

aso

faul

t and

all

of th

e fa

ults

in th

e La

va M

ount

ains

–Sum

mit

Ran

ge (

LMS

R)

Gar

lock

faul

t bet

wee

n S

ierr

a N

evad

a an

d P

ilot K

nob

area

EC

DS

††18

Ma

“hig

h-si

lica

rhyo

lite

dike

s” in

the

Eag

le

Cra

gs (

Sab

in, 1

994;

Mon

aste

ro e

t al.,

199

7).

Azi

mut

h of

281

, and

990

m w

ide.

18.5

SE

SD

††E

arly

Mio

cene

(?)

fels

ic d

ike

swar

m in

the

Sie

rra

Nev

ada

(Sam

sel,

1962

). A

zim

uth

of 2

89, a

nd

980

m w

ide.

0.37

67.6

[+5.

1/–3

.8]

ca. 1

8To

tal s

lip o

f the

cen

tral

Gar

lock

faul

t zon

e:

incl

udes

El P

aso

faul

t, fa

ults

in th

e LM

SR

, and

C

liff C

anyo

n fa

ult

Infe

rred

faul

t sou

th o

f Chr

istm

as C

anyo

n

29.8

[± 1

.1]

6.5

to

11.4

5.9

to6.

2S

ubtr

acte

d of

fset

of C

hris

tmas

Can

yon

Pal

eozo

ic

rock

s to

El P

aso

Mou

ntai

ns fr

om th

e of

fset

of

the

El P

aso

Mou

ntai

ns to

Pilo

t Kno

b*L

abel

iden

tifi e

s sl

ip c

onst

rain

t fea

ture

s sh

own

on F

igur

e 11

.† D

is th

e pr

ojec

tion

dist

ance

of t

he fe

atur

e to

the

faul

t.§ T

he a

mou

nt o

f fau

lt sl

ip w

as m

easu

red

usin

g m

ap d

ata

in a

geo

refe

renc

ed A

rcG

IS d

atab

ase

proj

ecte

d in

Uni

vers

al T

rans

vers

e M

erca

tor

Zon

e 11

Nor

th. T

he fa

ult s

lip c

onst

rain

ts w

ere

proj

ecte

d to

the

faul

t tra

ce,

and

the

dist

ance

bet

wee

n th

em w

as m

easu

red

alon

g th

e tr

ace

of th

e fa

ult.

The

slip

am

ount

sho

wn

is o

ur p

refe

rred

offs

et m

easu

rem

ent.

The

err

or m

easu

rem

ent i

s th

e in

terp

rete

d m

axim

um a

nd m

inim

um e

xtre

me

geom

etric

ext

rapo

latio

ns o

f slip

mar

kers

to th

e fa

ult t

race

in q

uest

ion,

and

it e

ncap

sula

tes

erro

rs in

exa

ct c

orre

latio

n of

spe

cifi c

poi

nts

of li

ne a

nd a

rea

feat

ures

, err

ors

in h

oriz

onta

l pro

ject

ion

usin

g fe

atur

e az

imut

hs,

and

unce

rtai

ntie

s as

soci

ated

with

ele

vatio

n di

ffere

nces

.# T

his

is th

e ag

e of

the

faul

t-sl

ip c

onst

rain

t geo

logi

c fe

atur

e.**

We

use

the

“met

a- +

(se

dim

enta

ry r

ock)

” te

rmin

olog

y of

Car

r et

al.

(199

7), b

ecau

se th

is b

est d

escr

ibes

thes

e ro

cks

that

ret

ain

sedi

men

tary

text

ures

but

hav

e be

en p

artia

lly r

ecry

stal

lized

.††

The

se c

onst

rain

ts a

re lo

cate

d ou

tsid

e th

e ar

ea o

f Fig

ure

11; s

ee F

igur

e 1

for

thei

r lo

catio

ns.

Andrew et al.

16 Geological Society of America Bulletin, Month/Month 2014

this fault requires a much larger amount of strike slip, because the map distance, as measured on the north side of the Garlock fault, between the Jurassic granodiorite in the eastern El Paso Mountains and the Atolia Quartz Monzonite–correlated plutons in the Sierra Nevada is 34 km (Fig. 1).

Savoy fault. The Savoy fault cuts all of the Miocene and Pliocene units of the LMSR. The only locations of Summit Range volcanics with ca. 12 Ma basalt fl ows overlain by ca. 11 Ma dacite dome complexes are in the cen-tral Summit Range and in the northeastern Lava Mountains. Dacite lava with distinctive mega-crystic orthoclase occurs in the central Summit Range (D on Fig. 11) with an age of 11.24 ± 0.43 Ma (Fig. 4H) and also in the northeastern Lava Mountains (D′ on Fig. 11) with an age of 11.221 ± 0.018 Ma (Fig. 4I). We correlate these dacites along with their associated lava domes and basalt fl ows of Summit Range volcanics across the Savoy fault for left-lateral offset of 16.7 ± 1.8 km (Table 4). The megacrystic dacite in the Summit Range correlates to the mega-crystic dacitic dikes in the Sierra Nevada that have a U-Pb zircon age of 11.383 ± 0.028 Ma; therefore we take ca. 11.4 Ma as the best age for

the megacrystic dacite in the Lava Mountains and for the age constraint of the offset on the Savoy fault.

Younger units can be correlated across the Savoy fault. An 80-m-thick tuff bed occurs in the upper part of the Bedrock Spring Forma-tion in the Lava Mountains where it is cut by the Savoy fault (E on Fig. 11). A similarly thick tuff bed within Bedrock Spring Formation is exposed in the western Summit Range (E′ on Fig. 11). The offset needed to juxtapose these tuff beds is ~6.0 ± 0.7 km (Table 4). The age of this tuff bed is unknown, but we interpret it to correlate to the only other thick tuff within the Bedrock Spring Formations, which has an age of 7.479 ± 0.023 Ma (Fig. 5C). The distinctive red fl ow-banded lava fl ow of the 6.5 Ma Lava Mountain Dacite can also be correlated across the Savoy fault (F and F′ on Fig. 11, respectively), requir-ing 7.3 ± 1.3 km of slip (Table 4), similar within error to that of the tuff offset. Because we have offset markers of different ages, we can calculate two intervals of slip (Table 4). Combining these yields slip in the interval between 11.2 and ca. 7 Ma of 10.7 ± 2.5 km. Using the total slip esti-mate above for the fault yields motion of ~6 km between ca. 7 Ma and present.

Brown’s Ranch fault zone. Determining slip for the Brown’s Ranch fault zone is diffi cult because it is oriented parallel to the strike of the upper Miocene units. There is an apparent off-set of the Lava Mountain Dacite in map view (Figs. 2 and 11; Table 4). We estimate that 3.9 ± 0.7 km of left-lateral offset is needed to restore the eastern edge of the exposures of the Lava Mountain Dacite (G and G′ on Fig. 11; Table 4) across the Brown’s Ranch fault zone.

Little Bird fault. The Little Bird fault does not have any specifi c geologic features that can be used to measure offset across it. It juxtaposes Paleozoic metasedimentary rocks with Creta-ceous granitic rocks; these rocks are >26 km apart to the north of the Garlock fault.

Dextral FaultBlackwater fault. The facies of the Almond

Mountain Volcanics are mismatched across the northern portion of the Blackwater fault. East of the Blackwater fault there is only a single bluish-colored pumice lapilli tuff bed within the Bedrock Spring Formation (H on Fig. 11). This tuff dips southwest at a moderate angle, is ~4 m thick, and is part of the distal facies of the Almond Mountain Volcanics. The west side of

Savoy faultBlackwater fault

Browns

Ran

ch fa

ult zo

ne

NBF

Lava

Mountains

El Paso

Mountains Summit Range

Garlock fault ChristmasCanyon

RWFZ

BlackHills

TWF

Garlock fault

Teagle Wash

Savoy faultNBF

NBF

Little Bird faultBuried inferred fault(?)

5 km

A

A′

D′

H′

F

J

H

D

F′

J′

GG′

B

B′

I

I″I”’

E

E′

I′

Go

ler

Gu

lch

C′

C″

C

Tg

Tg

TcTc

Tc

TeTe

Te

Te

Te

Te

Te

Te

Tb

Tb

Tb

Tb

Tb

TbTb

Tb

Ta

Ta

Ta

Tl

Ta(?)

Tl

Tl

Tl

TlTpr

Tpr

Tpr

QTph

Tpg

Tpg

Ka

Ka

Ka

Ka

Ka

Ka

KaKr

Kr

Jl

Jl

Jl

Jl

PgP*h

*b Dg

D_e

D_eP*h

*b

Tb

Ta

TaTb

Tl

O_c

Tp

Tpg

Tlr

Ts

Ts

Ts

Ts

Ts

TsTs

Tlr

Tlr

Tc

Tpr

Ka Ta

Tl

TlTl

Tl

Tb

Tb Tb

Tb TeTe

Ta

TbJl

QTph

Tp

Dg

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.

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 17

the Blackwater fault exposes both the distal and near-vent facies of the Almond Mountain Vol-canics. At the northern extent of volcanic debris-fl ow units in the Bedrock Spring Formation there is a 4–5-m-thick tuff that ends eastward at the Blackwater fault (H′ on Fig. 11). This same tuff bed is dextrally offset in repeated slivers across a ~600-m-wide fault zone. Total offset of the tuff is 1.9 ± 0.3 km (Table 4).

Farther north along the projection of the Blackwater fault there are right-lateral offsets of the northeast-striking buttress unconformity of the Bedrock Spring Formation deposited against the steep southern sides of the lava domes of the Summit Range volcanics (I, I′, I″ and I′′′ on Fig. 11). The total offset of these points across the zone of faulting is 2.1 ± 0.6 km (Table 4). The lava domes partially blocked the northward fl ow in the basin of the Bedrock Spring Formation as shown by the presence of lacustrine lime-stone only along this northeast-trending buttress unconformity. These ~2 km right-lateral slip values are similar to those obtained by Oskin and Iriondo (2004) from farther south along the Blackwater fault (Table 4) using slip markers of 7.2 ± 1.1 Ma (J to J′, Fig. 11) and 3.77 ± 0.11 Ma (BM on Fig. 1).

Regional Garlock FaultAdditional slip markers are needed for the

central Garlock faults to evaluate the fault slip in the LMSR. The Paleozoic rocks in the El Paso Mountains have been correlated to similar Paleozoic rocks in the Pilot Knob area (Fig. 1) for a left-lateral offset of 48–64 km on the central Garlock fault (Smith and Ketner , 1970; Carr et al., 1997; Table 1). Geologic map data show a steeply dipping fault in the El Paso Mountains that places Mississippian meta-conglomerate of the Robbers Mountain Formation against Devonian(?) greenstone (Carr et al., 1997). A steeply dipping fault in the Pilot Knob area juxtaposes the same units (Carr and Poole, 1992). Matching these faults (MC and PKV, respectively, on Fig. 1) yields 62.7 ± 1.7 km of offset (Table 4).

Another offset marker for the central Gar-lock fault is an 18 Ma swarm of WNW-striking, “high-silica rhyolite dikes” (ECDS on Fig. 1) in the Eagle Crags (Sabin, 1994; Monastero et al., 1997). A set of dikes with similar width, azi-muth, and composition occurs in the southeast-ern Sierra Nevada (SESD on Fig. 1) intruded into Cretaceous granite (Samsel, 1962). These dikes are correlated to early Miocene volcanism (Dibblee, 1967), as this swarm projects into a lower Miocene volcanic center (Coles et al., 1997). A slip marker using these dikes yields 67.6 +5.1/–3.8 km left-lateral slip on the central Garlock fault (Table 4).

IMPLICATIONS FOR THE CENTRAL GARLOCK FAULT ZONE

Inferred Fault South of Christmas Canyon

The slip data for Paleozoic rocks at Christmas Canyon require the presence of an unexposed fault south of the Christmas Canyon area. These Paleozoic rocks do not continue southward, as the Black Hills have Cretaceous Atolia Quartz Monzonite as basement (Fig. 11). A large expo-sure of Paleozoic rocks occurs east of the LMSR on the south side of the Garlock fault in the Pilot Knob Valley area (PKV on Fig. 1), which are offset 62.7 ± 1.7 km (Table 4) from the El Paso Mountains. There are no direct ties of the Paleo-zoic rocks of Christmas Canyon with the Pilot Knob area, but the Christmas Canyon rocks are displaced 32.9 ± 0.6 km from the El Paso Moun-tains by the Garlock fault. Thus, a fault with 29.8 km of left-lateral slip (Table 4) presumably exists south of Christmas Canyon to restore it to Pilot Knob Valley.

Based on outcrop patterns, this inferred fault must have a ENE strike in an area that is cov-ered by the Pliocene conglomerate of Golden Valley and Quaternary alluvial deposits. This conglomerate overlaps the Little Bird fault and probably also the inferred fault. Juxtaposition of the Christmas Canyon area to the north of the Black Hills by this inferred fault was completed prior to deposition of the capping basalt boul-der layer of the conglomerate of Golden Valley because it has basalt clasts without dacite lava clasts; the Black Hills are the only exposure of ca. 12 Ma basalt without overlapping dacite domes (Fig. 2).

Initiation of the Garlock Fault Zone

Slip markers in the LMSR yield offsets for the Garlock fault that differ from the total off-set of ~64 km (Table 1). Sinistral slip on the Garlock fault required to juxtapose the mega-crystic dacite domes in the Summit Range with megacrystic dikes in the Sierra Nevada is 43.7 ± 0.8 km (Table 4), whereas ca. 18 Ma high-silica dikes southeast of the LMSR appear to record the full offset of the Garlock (67.6 +5.1/–3.8 km) to the Sierra Nevada. This difference may be explained in two ways: (1) the Garlock fault slip began between 18 and 11.4 Ma, so the mega-crystic dacite is too young to record the full Gar-lock fault offset; or (2) slip in the vicinity of the LMSR is distributed among several faults, with the megacrystic dacites of the Summit Range within and the 18 Ma dikes outside of this fault zone. We favor the second hypothesis because of the presence of several faults with left-lateral offset within the LMSR: the Savoy fault has

16.7 ± 1.8 km of slip of ca. 11.4 Ma features (see the age discussion in the “Magnitude and Rates of Fault Slip” section), and the Brown’s Ranch fault zone has 3.9 ± 0.7 km of slip of ca. 6.5 Ma features (Table 4). Adding these offsets to the 43.7 km offset for the Summit Range seg-ment of the Garlock fault gives cumulative left-lateral slip of 64.3 km, similar to the ~64 km total slip value derived from Paleozoic, Meso-zoic, and early Miocene offset markers outside the distributed zone of faulting of the LMSR (Table 1). This amount of slip supports the interpretation that motion on the Garlock fault began at or after ca. 11 Ma (e.g., Burbank and Whistler , 1987) and that sinistral slip is distrib-uted across a 12-km-wide fault zone. Thus, for this area we refer to the sinistral faults together as the Garlock fault zone (GFZ).

Initiation of Regional Dextral Shear

The GFZ is embedded in an active zone of dextral shear (see references in Gan et al., 2003). Initiation of dextral shear occurred in Death Valley, to the east of the LMSR (Fig. 1), at ca. 7 Ma (Holm et al., 1993; Topping, 1993), although Mancktelow and Pavlis (1994) argued that this initiated at ca. 11 Ma. The initiation age is younger in areas west of Death Valley: ca. 4 Ma in Panamint and Searles Valleys (Burch-fi el et al., 1987; Hodges et al., 1990; Zhang et al., 1990; Snyder and Hodges, 2000; Walker et al., 2014), and 2–3 Ma farther to the west in Indian Wells Valley (Monastero et al., 2002). This westward migration of dextral shear possi-bly also occurred in the Eastern California shear zone, but the initiation ages are poorly known: beginning after 6 Ma (Dokka and Travis , 1990; Schermer et al., 1996; Glazner et al., 2002), 3.8 Ma (Oskin and Iriondo, 2004), or even younger (Miller and Yount, 2002). The age of the initiation of dextral shear in the LMSR is interpreted to be <3.8 Ma, the time for initia-tion of the dextral slip on the Blackwater fault (Oskin and Iriondo, 2004; this paper), similar to ca. 4 Ma initiation in Searles Valley (Fig. 1) just north of the LMSR.

Change in Deposition Systems

There is a profound change in the deposition systems along the GFZ after ca. 7 Ma. The Bed-rock Spring and Dove Spring Formations along the central GFZ are thick successions of arkosic sandstone deposited from 11.5 to 7.0 Ma (Fig. 12), with sediment transported northwestward from granitic bedrock sources in the central Mojave Desert area (Smith, 1964; Loomis and Burbank, 1988; Whistler et al., 2009; this paper). The conglomerate of Golden Valley occurs as a

Andrew et al.

18 Geological Society of America Bulletin, Month/Month 2014

local basin sourced from a now-displaced local uplift along the GFZ, as discussed above. The modern deposition systems along the GFZ are similar, having local closed basins and sediment sources in nearby uplifts (Fig. 1). The topogra-phy of the central GFZ therefore changed from one of low relief that did not signifi cantly dis-turb transport and deposition to a system with locally high relief creating uplifts and closed basins. We postulate that the change to dextral shear could have led to creation of higher-relief topography along the GFZ, and therefore this conglomerate may have formed during the initi-ation of local dextral shear at ca. 3.8 Ma (Oskin and Iriondo, 2004).

Current Structural Confi guration of the Garlock Fault Zone

The confi guration of faults is different on either side of the Blackwater fault (Fig. 11): to the west, the Summit Range segment of the Garlock fault is active, with additional slip on several other sinistral strike-slip faults (Teagle Wash fault, Savoy fault, and Brown’s Ranch fault zone); to the east, there are only the Gar-lock and dip-slip Randsburg Wash faults. The Garlock, Savoy, and Randsburg Wash faults are all correlated with Quaternary fault scarps, and the Brown’s Ranch fault zone was inter-preted by Smith (1964) to have young slip. The

Teagle Wash fault is somewhat different and has more similarity with the Little Bird fault in that it juxta poses different-age basement rocks (i.e., Cretaceous versus Jurassic), implying a signifi cant left-lateral slip. Slip on the Teagle Wash fault resolves onto the Savoy and Gar-lock faults, and so its history is not critical to this analysis. WSW-trending folds occur in all Miocene (Figs. 9J and 9K) and Pliocene (Fig. 9L) rock units and have been interpreted to have been active as recently as the Pleistocene (Smith, 1964, 1991).

This young deformation can be more fully explored in the context of a regional deforma-tion model. The model is based on two obser-

Ea

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wa

ter

fau

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est

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ate

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10 9 8 7 6 5 4 3 2 1 011 Ma

Lithology

Conglomerate

Volcanic rocks

Arkosic sandstone

Interpreted Deformation

Period 1: Basin and Range transform

Period 2: Transition to dextral shear

Period 3: Regional dextral shear

conglomerate of Golden Valley

conglomerate of Hardcash Gulch

El Paso Mountains: Dove Spring Fm.

Bedrock Spring Fm.

Summit Rangevolcanics

AlmondMountainVolcanics

Lava MountainsDacite

inferred fault

Christmas Canyon-Garlock fault

pre-Lava Mountains Dacite Savoy fault

post-conglomerate of Hardcash Gulch Summit Range-Garlock fault

Browns Ranch fault zone

Blackwater fault

19.1* km

19.1 km

monoclines with WSW axes

Little Bird fault

10.7* km

9.1 km

open folds with NNW axes

15.5 km

post-Lava Mountains Dacite Savoy fault

3.9 km

6.0 km

32.9 km

–2 km

10.7 km

Teagle Wash fault >2.2 km

pre-conglomerate of Hardcash Gulch Summit Range-Garlock fault

>5 km

Init

iate

Ga

rlo

ck f

au

lt

Init

iate

de

xtra

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ea

r in

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al

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rea

Re

org

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Ga

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open folds with NNW axes

BedrockSpring Fm.

open folds with WSW axes

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.

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 19

vations: (1) the trace of the Garlock is curved; and (2) dextral faults in the areas to the north and south do not cut the Garlock (i.e., slip on the Blackwater fault ends northward before the Garlock fault). The curved trace of the Garlock fault is an outcome of progressive bending of an originally northeast-trending fault by trans-versely oriented dextral shear distributed along the central and eastern segments (Gar funkel, 1974; Dokka and Travis, 1990; Gan et al., 2003). Young deformation in the LMSR must accommodate the dextral faulting of the Black-water fault without cutting the Garlock fault as well as larger-scale clockwise bending of the trace of the Garlock fault.

Accommodation of dextral slip in the LMSR necessitates NNW-SSE–oriented shortening of western side of the Blackwater fault relative to the eastern side. The NNW-SSE shortening could be taken up by the WSW-trending fold-ing and also by lateral escape of fault slivers between the Summit Range segment of the Garlock fault, the Savoy fault, and the Brown’s Ranch fault zone (Fig. 11). NNW-SSE elonga-tion east of the Blackwater could be partially taken up by normal slip on the Randsburg Wash fault. These deformation mechanisms would absorb the slip of the Blackwater so that the Garlock fault is not offset and would allow the trace of the Garlock to be bent by increasing clockwise amounts eastward.

The slip history of faults in the Christmas Canyon appears to also fi t with this dextral slip accommodation model. The Paleozoic rocks of the Christmas Canyon area were north of the main trace of the GFZ system until after deposi-tion of the conglomerate of Golden Valley when the Christmas Canyon segment of the Garlock fault formed. Block transfer across the GFZ by a left step in the main trace of the GFZ effectively adds material to the east side of the Blackwater fault. This breaking of a new fault also allows a more continuous trace to the active strand of the Garlock fault. We envision that these processes could apply to other intersections of dextral faults with the GFZ and possibly to the larger-scale clockwise bending of the trace of the Gar-lock fault. This model implies that the wide, multiple-fault structure of the GFZ formed in response to dextral slip of the Blackwater fault, which began after 3.8 Ma (Oskin and Iriondo, 2004; this paper).

Slip History of the Garlock Fault Zone

The current structural configuration of the LMSR, as outlined above, implies a link between the wide, multi-stranded GFZ and the accommodation of dextral slip and clock-wise bending of the Garlock. Therefore defor-

mation older than 3.8 Ma in the LMSR does not accommodate dextral slip. Unfortunately, long-lived strike-slip fault systems like the GFZ do not preserve a complete detailed his-tory of slip. To aid interpretation we assume that the older GFZ was a simple strike-slip fault with only one active fault strand because it did not need to accommodate dextral slip and shear (see Fig. 12 for interpreted timing of fault slip and Fig. 13 for interpreted time-slice maps). The single-strand assumption is sup-ported by observing that the western Garlock fault has this character and is outside the zone of dextral shear.

The slip constraints from the LMSR using the volcanic-sedimentary assemblages as the key time markers allow a view of two incre-ments in the pre–3.8 Ma history of the GFZ. The oldest offset markers are ca. 11.4 Ma megacrystic dacite domes and feeder dikes, but the relative locations of slightly younger (10.5 Ma) dacite domes across the Savoy fault could give a better maximum age. The end of deposition of the Bedrock Spring Formation demarks the end of the early slip of the Savoy fault and the beginning of slip on the Little Bird fault. The 7.5 and 6.5 Ma slip markers for the Savoy fault have indistinguishable offsets within error; for simplicity in this discussion, we average these ages to ca. 7 Ma. The next-younger marker unit is the conglomerate of Golden Valley whose age is not well known. We interpret its deposition to be linked with the beginning of the dextral deformation, because the same deposition system that created this conglomerate is still active from Goler Gulch. We assume that deposition began at ca. 4 Ma in this analysis.

These assumptions allow a glimpse of the earlier slip history of the GFZ. The earliest slip was 10.7 km on the Savoy from 10.5 to ca. 7 Ma. This amount of slip was taken up in the eastern LMSR on the inferred fault discussed above. Afterward from ca. 7 Ma until 3.8 Ma, slip occurred on the Summit Range segment of the Garlock, Teagle Wash, and Little Bird faults and the inferred fault. The intermediate-stage GFZ had a maximum sinistral slip of 19.1 km (subtraction of the 10.7 km of Savoy fault slip from the 29.8 km calculated slip for the inferred fault) if the Summit Range segment of the Gar-lock fault was active only after the fi rst stage of slip on the Savoy fault. NNW-trending folds formed during the intermediate-stage slip in the areas along the Little Bird fault. We speculate that this fold set may be due to the ESE strike of the Little Bird fault. This orientation would have caused the Little Bird fault to have a restraining orientation in the sinistral fault system, forming NNW-trending folds.

Garlock Fault Zone Slip Rates

Slip rates calculated in Table 4 are based on the minimum and maximum timing constraints for fault motion, but these do not specify when the faults were active. When these slip esti-mates are considered in light of the three-stage deformation scenario proposed above (Fig. 12), we can use the entire data set to estimate slip rates within the fault system. The earliest stage involved the Savoy fault with a slip rate of 3.1 mm/yr (10.7 km over the interval from 10.5 to ca. 7 Ma). If the single-strand assump-tion for the early Garlock fault is not true, then additional slip would have occurred on the Sum-mit Range segment of the Garlock fault at this time, which would increase the slip rate for this interval. The slip rate for the Garlock fault in the intermediate stage is a maximum of 6.0 mm/yr (19.1 km over the interval from ca. 7 to 3.8 Ma); this rate would decrease if some slip on these faults occurred in the earlier stage. Youngest-stage slip in the eastern LMSR is interpreted to have been on the Christmas Canyon segment of the Garlock fault with a slip rate of 8.7 mm/yr (32.9 km since 3.8 Ma). This slip-rate accelera-tion is linked to the initiation of dextral faulting in the LMSR area. West of the Blackwater fault the sinistral slip of the GFZ is divided across three main faults and fault zone: the Summit Range segment of the Garlock fault (43.7 km – 19.1 km = 24.6 km of slip) has a rate of 6.5 mm/yr, the Savoy fault (6.0 km of slip) has a rate of 1.6 mm/yr, and the Brown’s Ranch fault zone (3.9 km of slip) has a rate of 1.0 mm/yr.

The distribution of sinistral slip across mul-tiple faults in the western LMSR creates dis-crepancies in the slip rates along the modern GFZ. The single-stranded Garlock fault east of the LMSR has neotectonic slip rates, depending on the age of the marker, of 4–9 mm/yr (McGill and Sieh, 1993) and 7–14 mm/yr (Rittase et al., 2014), which match with our longer-term, model-interpreted slip rate of 8.7 mm/yr. West of the LMSR the neotectonic slip rates on the central Garlock fault are 4.5–6.1 mm/yr (Clark and Lajoie, 1974), which agrees with the model-calculated slip rate of 6.0 mm/yr for the Summit Range segment of the Garlock fault. Our model-calculated rates for the Summit Range segment of the Garlock fault plus those for the Savoy fault add to 7.6 mm/yr, which agrees with the 5.3–10.7 mm/yr neotectonic rate for the western segment of the Garlock fault (McGill et al., 2009).

CONCLUSIONS

The LMSR area is a Miocene to Pliocene volcanic-sedimentary complex adjacent to the Garlock fault that contains rocks created before,

Andrew et al.

20 Geological Society of America Bulletin, Month/Month 2014

LavaMountains

MC

PKV

MD

S

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SESD

IDS

IDS

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Mountains

GraniteMountains

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N

A 11 Ma

Restored fault slip in central Garlock fault region;

prior to slip on GFZ and ECSZ and extension in the

western Basin and Range

B 7 MaRegion after initial slip on Savoy and inferred faults

and extension on the Slate Range detachment.

ESTS

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Independence dike swarm (IDS)

Eastern Sierra thrust system (ESTS)

18 Ma silicic dike swarm in SE Sierra Nevada (SESD) along trend of Eagle Crag dike swarm (ECDS)

Summit Range volcanics (SRV) with megacrystic dacitic dike swarm (MDS)

Restore:6.5 to 8 Ma units of

Bedrock Spring Fm. (BSF) & upper Dove Spring Fm. (UDSF).

Note that the Summit Range & Christmas Canyon blocks are located north of the main trace of the GFZ.

SRV

Plio-Pleistocene conglomerateUpper Miocene arkose6.5-8 Ma volcanic rocks 10-12 Ma volcanic rocksLower Miocene volcanic &

sedimentary rocksEocene Goler FormationPaleocene Goler FormationCretaceous metamorphic &

plutonic rocksJurassic plutonic rocksJurassic metavolcanic rocksTriassic metaplutonic rocksPermian metamorphic rocksPennsylvanian metamorphic rocksMississippian metamorphic rocksDevonian metamorphic rocksCambrian & Ordovician

metamorphic rocks

0 10 20 km30

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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 recon-naissance 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.

Central Garlock fault

Geological Society of America Bulletin, Month/Month 2014 21

during, and after slip on faults in the GFZ. Geo-logic features yield constraints on fault slip that indicate a three-stage history. The GFZ began as a single-stranded fault after 10.5 Ma with slip on the Savoy fault continuing eastward on a now-buried inferred fault south of Christmas Canyon. We interpret an intermediate stage of deformation between ca. 7 Ma and 3.8 Ma when the active strand of the GFZ became the Summit Range segment of the Garlock fault which con-

nected to the inferred fault via the Teagle Wash and Little Bird faults. The ESE strike of the Little Bird fault was a constrictional bend in the GFZ as recorded by NNW-trending folds. This constrictional bend was eventually abandoned as the sinistral fault system stepped leftward, creating the Christmas Canyon segment of the Garlock fault to the north of Christmas Canyon. The fi rst and intermediate stages have ~30 km of cumulative sinistral slip. The last stage in the

slip history occurred after 3.8 Ma when the GFZ changed into a locally wide, multi-stranded fault zone to accommodate dextral offset of the Blackwater fault and regional clockwise orocli-nal bending of the GFZ. This stage had ~33 km of slip on the GFZ, which is roughly half of the total offset. Topography along the GFZ changed at the beginning of this stage of deformation to one of local high relief, creating a new pattern of depositional systems. The slip rates for the

C 4 MaRegion after strike-slip displacement on the Garlock fault and

extension on the Cliff Canyon fault.

D 0 MaRegion after initiation of ECSZ

and the creation of the

wide and complex

Garlock fault

zone.

Initiation of uplift along the: Sediment from Goler Gulch (GG) is shed across the GFZ for deposition of the conglomerate of Golden Valley (CGV) on recently juxtaposed Lava Mountains and Christmas Canyon blocks.

Future: Summit Range segment of the Garlock

fault (SR-GF) Christmas Canyon segment

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El PasoMountains

SpanglerHills

SierraNevada

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Lockhart fault

Ts

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ECDS

GG

K

K

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Figure 13 (continued ).

Andrew et al.

22 Geological Society of America Bulletin, Month/Month 2014

recent motion on the Garlock fault calculated using the three-stage deformation scenario are 6.0 mm/yr for the Summit Range segment and 8.7 mm/yr for the Christmas Canyon segment. The multi-stranded confi guration of the GFZ west of the Blackwater fault explains the slower slip rate of the central Garlock fault along the El Paso Mountains (Clark and Lajoie, 1974) compared to sites east of the Blackwater fault (McGill and Sieh, 1993; Rittase et al., 2014) and farther west where the multi-stranded GFZ narrows to the single-stranded western Gar-lock fault.

The younger deformation system of the LMSR can be modeled as a zone of strain accommodation, taking up the 2 km of dextral slip on the Blackwater fault without cutting the Garlock fault. Although much of the regional dextral strain is accommodated by bending of the trace of the Garlock fault, the area of the LMSR must internally deform to allow bending of the Garlock fault trace and dextral offset of the Blackwater fault. The transfer of the Christ-mas Canyon block across the Garlock fault zone during the initiation of the last stage of slip may have been created by a defl ection of the trace of the Garlock fault during the initial stages of regional dextral shear.

ACKNOWLEDGMENTS

This work was partly supported by a grant from the Geothermal Program Offi ce of the U.S. Department of the Navy, China Lake, and the National Science Foundation EarthScope Program. Andrew Sabin and Steve Alm facilitated logistics and access permissions to areas of the China Lake Naval Weapons Station. Matt Heizler and the New Mexico Geochronology Research Laboratory provided analyses and inter-pretations for most of the argon data in this paper. Tandis Bidgoli helped with early reviews of the paper. Finally, we acknowledge Jeff Lee, Terry Pavlis, and Nadine McQuarrie for comments leading to great im-provement in the manuscript.

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SCIENCE EDITOR: CHRISTIAN KOEBERL

ASSOCIATE EDITOR: STEFANO MAZZOLI

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