evolution of the central garlock fault zone, california: a...
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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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:
::
:
::
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K
K
K
K
K
K
K
K
K
K
K
J
JJ
J
J
J
KK
J
J
Y
Y
Tm
Tm
Tm
Tm
Tm
TmTp
J
J
RM
–117
°
–118
°
35°45′
35°15′
IDS ESTS
SPVF
ASF
MC
PKVESTS
IDS
BF
HarperLake fault
Goldstone
Lake fault
Sear
les
Valle
y fa
ult
Sier
raNe
vada
Slat
e Ra
nge
El Paso Mountains
Granite Mountains
Lava
Mountains
Figure 2
CC
EC
PL
AM
SummitRange
IndianWellsValley
DSF
Garlock fault
Cerro Coso fault
Clif
fCan
yo
n fault
Southern
DeathValley fault
MD
S
SESD
ECDS
CretaceousJurassic
Paleozoic
0 10 kilometers 605
Tertiary(Paleocene)
Triassic
Tertiary(Miocene)
Plio-Pleistocene
QT
QT
QT
QTQT
Proterozoic
BMQT
WGF
PR
Cantil faultGarlock fault
Garlock fault
JTm
TmJ
El Paso fault
IDS
PacificOcean
NVCA
0 200 km
K
Tm
Tp
QT
J
Y
N
Garlockfault
Sierra
Nevada
MojaveDesert
CentralBasin &Range
–116°
°021
–
36°
34°
Eastern
California
shear zone
Walker
Lane belt
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
Savo
y fa
ult
Blackw
ater fa
ult
Browns
Ran
ch fa
ult zo
ne
NBF
Lava
Mou
ntai
ns
El P
aso
Mou
ntai
nsSu
mm
it Ra
nge
Gar
lock
fau
lt
RWFZ
Cerro C
oso f
ault
Bla
ckH
ills
TWF
Gar
lock
fau
lt
Osk
in &
Irio
ndo
(200
4) n
orth
ern
map
are
a
Teag
le W
ash
NBF
NBF
Chr
istm
asC
anyo
n
Goler Gulch
Littl
e Bi
rd fa
ult
117°40'W
35°3
0′N
35°2
5′N
117°20′W
LM11
32(N
.A.)
GFZ
-85
(11
.32
±0
.94
Ma
)
LM96
-15
(10
.66
±0
.12
Ma
)LM
96-17
(N.A
.)
LM96
-2(7
.64
±0
.18
x Ma
)
LM96
-16
(7.8
2 ±
0.2
2 M
a)
SD12
0905
-2(7
.32
±0
.02
x Ma
)
SD12
0905
-1(7
.24
±0
.83
x Ma
)
GFZ
-080
603-1
(11
.24
±0
.43
Ma
)
LV03
0710
-4(1
1.2
21
±0
.01
8u M
a)
LV03
0710
-7(1
1.6
2 ±
0.1
1 M
ao
f cl
ast
)
SD06
0810
-1(1
1.9
3 ±
0.1
4 M
a)
SD04
0910
-1(1
2.7
4 ±
0.0
5x M
a)
LV03
0710
-5(1
8.8
4 ±
0.2
4 M
a)
LV04
0910
(19
.00
±0
.22
Ma
)
LV03
1510
(19
.37
±0
.04
Ma
)
LV05
1210
-5(1
9.6
3 ±
0.3
2 M
a)
JDW-20
(7.4
79
±0
.02
3u
Ma
)LV05
0710
(10
.49
0 ±
0.0
54
u M
a)
LV03
1410
-B(1
0.9
66
±0
.04
2u M
a)
LV05
1810
-8(6
.52
2
±0
.02
6u M
a)
LV05
0810
-4(6
.50
7 ±
0.0
70
u M
a)
LV05
1210
-6(1
1.6
6 ±
0.0
6 M
a)
Cenozoic
Pre-Cenozoic
Ran
d S
chis
t (C
reta
ceou
s)A
tolia
Qua
rtz M
onzo
nite
(Cre
tace
ous)
Laur
el M
ount
ain
gran
odio
rite
(Jur
assi
c)M
eta-
And
esite
of G
oler
Gul
ch (P
erm
ian)
Met
ased
imen
tary
rock
s of
Hol
land
Cam
p (P
erm
ian-
Pen
nsyl
vani
an)
Met
ased
imen
tary
rock
s of
Ben
son
Wel
l (P
enns
ylva
nian
)M
etas
edim
enta
ry ro
cks
of G
erbr
acht
Cam
p (D
evon
ian)
Met
ased
imen
tary
rock
s of
El P
aso
Pea
ks (D
evon
ian
to C
ambr
ian)
Met
ased
imen
tary
rock
s of
Col
orad
o C
amp
(Ord
ovic
ian-
Cam
bria
n)
5 km
Plio
-Ple
isto
cene
Sed
imen
tary
rock
s
cong
lom
erat
e of
Har
dcas
h G
ulch
un
diffe
rent
iate
d se
dim
enta
ry ro
cks
co
nglo
mer
ate
of G
olde
n Va
lley
co
nglo
mer
ate
from
Red
Mou
ntai
nLa
va M
ount
ains
Dac
ite fl
ows
(6.5
Ma)
gr
ay la
va fl
ows
re
d flo
w-b
ande
d la
va fl
ows
as
soci
ated
intru
sive
rock
sA
lmon
d M
ount
ain
Volc
anic
s (7
.5–8
.0 M
a)B
edro
ck S
prin
g Fo
rmat
ion
(7–1
0 M
a)S
umm
it R
ange
vol
cani
cs (1
0.5–
11.5
Ma)
Low
er M
ioce
ne v
olca
nics
(15–
21 M
a)
Eag
le C
rags
Vol
cani
cs
Cud
ahy
Cam
p Fo
rmat
ion
Gol
er F
orm
atio
n (P
aleo
cene
to E
ocen
e)
Tg
Tg
Tc
Tc
Tc
Te
Te
Te
Te
Te
Te
Te
Te
Tb
Tb
Tb
Tb
Tb
Tb
Tb
Tb
Ta
Ta
Ta
Tl
Tl
Tl
Tl
Tl
Tl
Tp
r
Tp
r
Tp
r
QT
ph
Tpg
Tpg
Ka
Ka
Ka
Ka
Ka
Ka
Ka
Kr
Jl
Jl
Jl
Jl
Pg
P*
h*b
Dg
D_
e
D_
eP
*h
*b
Tb
Ta
Ta
Tb
Tl
O_
c
Tp
Tpg
Tlr
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Tlr
Tlr
Tc
Pg
P*
h
*b
Dg
D_
e
Kr
Ka
O_
c
JlT
g
Te
Ts
Tb
TaTl
QT
ph
Tp
r
Tlr
Tc
Tp
g
Tp
Tg
Tp
r
Kr
KaT
e
Tb
Ta
Tl
Tl
Tl
Tl
Tl
Tl
Tb
Tb
Tg
Tc
Tc
Te
Te
Jl
Jl
N
u =
U-P
b a
ge
(o
the
rs a
re 4
0A
r/3
9A
r)x =
old
er
ag
e d
ue
to
exc
ess
arg
on
QT
ph
Ka
Tp
Tl
Dg
Fig
ure
2. S
impl
ifi ed
geo
logi
c m
ap o
f th
e L
ava
Mou
ntai
ns–S
umm
it R
ange
. Dat
a fr
om in
side
the
das
hed
line
are
from
And
rew
et
al. (
2014
);
addi
tion
al d
ata
are
from
Dib
blee
(19
67),
Car
r et
al.
(199
7),
and
Wal
ker
et a
l. (2
002)
. G
eoch
rono
logi
c sa
mpl
e lo
cati
ons
are
deno
ted
by
star
s w
ith
sam
ple
nam
e an
d in
terp
rete
d ag
e in
par
enth
eses
. Fau
lts
are
thic
k lin
es. F
ault
abb
revi
atio
ns:
NB
F—
nort
hern
Bla
ckw
ater
fau
lt;
RW
FZ
—R
ands
burg
Was
h fa
ult
zone
; TW
F—
Teag
le W
ash
faul
t. Y
oung
er P
leis
toce
ne a
nd H
oloc
ene
unit
s ar
e sh
own
as w
hite
.
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
Qa
Qg Qcc
Tld
Tpr
Qu
ate
rna
ryP
lioce
ne
Tp
QThf
This studySmith (1964)
Qa
QgQcc
Tl
Ta
Tv
Tbs
Ts
pTa
QvaQvb
Qu
ate
rna
ryu
pp
er
Plio
cen
em
idd
leP
lioce
ne
pre-Tertiary
Tf, Tt, Ttb
& Tvb?
|
Smith et al. (2002)
Qa
Qg
Qcc
Tl
Ta
Tsd
Tbs
Tv
Tsr
Ts
Ka
Tt
Ttw
Qu
ate
rna
ryu
pp
er
Mio
cen
em
idd
leM
ioce
ne Tf
|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
Tsr
Tsx
Tsc
Tbc
Tsb
Tcv
Tct
Ka
Jlm
up
pe
rM
ioce
ne
mid
.M
io.
Me
so-z
oic
low
er
Mio
.
P*h
*b
DO_e O_cPa
leo
zoic
Tad
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
st o
f B
lack
wa
ter
fau
ltW
est
of
Bla
ckw
ate
r fa
ult
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
lsh
ea
r in
ce
ntr
al
LMS
R a
rea
Re
org
an
ize
Ga
rlo
ck f
au
lt
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
ECDS
SESD
IDS
IDS
ESTS
Future Savoy/inferre
d fault s
ystem
Futu
re S
late
Ran
ge
de
tach
me
nt
SummitRange
ChristmasCanyon
El Paso
Mountains
GraniteMountains
SlateRange
Sie
rra
Ne
vad
a
EagleCrags
ESTS
BlackHills
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
SlateRange
IDS
ESTS
UD
SF
Savoy fault
inferred fa
ult
future Garlock fa
ult
Sla
te R
ang
ed
eta
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Restore: Thrust faults: Mesquite
Canyon (MC) & Pilot Knob Valley (PKV)
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
K
Ji
Jv
QTTs
Tu
Tm
Tl
Te
^
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D
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BSF
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
of the Garlock fault (CC-GF)
future El Paso fault
future CC-GF
futu
re Marin
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futu
re S
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inferred fault
future Goldstone
Lake fault
future
Blackwater fault
futu
re B
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ns
Ran
ch fa
ult z
one
futu
re reactiv
ated Savoy fault
future Cerro
Coso fault
futu
re SR-GFfu
ture
rea
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ated
Cliff
Can
yon
fau
lt
CGV
CGV
Savoy fault
Garlock fa
ult
Garlock fault
Harper Lake fault
Blackwater fault
Brow
ns Ranch
fault
Cantil fault
Garlock faultMarine Gate fault
Garlock fa
ult
El Paso fault
Cerro Coso fault
Cliff
Ca
nyo
n f
au
lt
Se
arl
es
Va
lley
fau
lt
N
MD
S
SESD
MC
PKV
LavaMountains
Summit Range
El PasoMountains
SpanglerHills
SierraNevada
EagleCrags
BlackHills
CHG
RandMountains
Lockhart fault
Ts
GG
ECDS
GG
K
K
K
K
K
Ji
Ji
JiJv
Jv
QT
Ts
Tu
Tm
Tl
Te
^
P *
D
O_^
TlTl
Tl
TlD
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QT
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Ts
Continued uplift along GFZ: sediment from Goler Gulch (GG) deposited as the conglomerate of Hardcash Gulch (CHG) and then this conglomerate is displaced by subsequent left-lateral slip.
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
MANUSCRIPT RECEIVED 4 NOVEMBER 2013REVISED MANUSCRIPT RECEIVED 8 JUNE 2014MANUSCRIPT ACCEPTED 16 JULY 2014
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