structural and stratigraphic evolution of the calico ...€¦ · structural evolution of the calico...

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Structural and stratigraphic evolution of the Calico Mountains: Implications for early Miocene extension and Neogene transpression

in the central Mojave Desert, California

John S. Singleton*Phillip B. GansDepartment of Earth Science, University of California, Santa Barbara, California 93106, USA

459

Geosphere; June 2008; v. 4; no. 3; p. 459–479; doi: 10.1130/GES00143.1; 13 fi gures; 1 table; 1 plate; 1 supplemental fi gure.

*Present address: Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712, USA.

ABSTRACT

New geologic mapping, structural data, and 40Ar/39Ar geochronology document early Miocene sedimentation and volcanism and Neogene deformation in the Calico Moun-tains, located in a complexly deformed region of California’s central Mojave Desert. Across most of the Calico Mountains, volcaniclastic sediments and dacitic rocks of the Pickhan-dle Formation accumulated rapidly between ca. 19.4 and 19 Ma. Overlying fi ne-grained lacustrine beds (here referred to as the Cal-ico Member of the Barstow Formation) are bracketed between ca. 19 and 16.9 Ma, and are thus older than the type section of the Barstow Formation in the Mud Hills. Sev-eral 17.1–16.8 Ma calc-alkaline dacite domes intrude the Calico Member and represent a previously unrecognized volcanic episode in this region.

In the southern Calico Mountains, the Cal-ico fault (part of the Eastern California shear zone) forms a west-northwest– striking, trans-pressional restraining bend with ~3 km of right-lateral slip and perhaps 1 km of reverse (north side up) throw distributed on two main fault strands. Part of the Calico fault appears to have originated as an early Miocene normal fault that unroofed metavolcanic basement rocks in the footwall and created a hanging-wall basin in which Pickhandle Formation strata accumulated. This extensional slip must have largely ceased prior to deposition of the Calico Member, which unconformably overlies the Pickhandle Formation north of the Calico fault and directly overlies metavol-canic rocks south of the Calico fault. Deposi-tion of the Pickhandle Formation and at least part of the Calico Member was coeval with

rapid unroofi ng of the central Mojave meta-morphic core complex, yet extension in the Calico Mountains is minor and is overprinted by dextral faulting and transpression.

Calico Member beds north of the Calico fault are intensely folded into numerous east-west–trending, upright anticlines and synclines that represent 25%–33% (up to ~0.5 km) north-south shortening. Folds are detached along the base of the Calico Mem-ber and thrust over the Pickhandle Forma-tion, which dips homoclinally ~15–30°S to SE. The geometry and distribution of folds are most compatible with localized transpres-sion between the Calico Member and the Pickhandle Formation within a positive fl ower structure. Transpressional folding and faulting in the Calico Mountains postdate the ca. 17 Ma dacite intrusions and appear to be largely restricted to the area along the Calico fault restraining bend.

Keywords: Calico Mountains, Calico fault, Mo-jave Desert, Barstow Formation, transpression.

INTRODUCTION

The central Mojave Desert region in south-ern California records a complex deformation history that includes Cenozoic extension, con-traction, and strike-slip faulting. Early Mio-cene detachment faulting and extensional basin development generally preceded transform-dominated tectonics related to the Pacifi c–North American plate boundary, yet the timing, mag-nitude, and tectonic signifi cance of these dispa-rate modes of deformation remain controversial (see Glazner et al., 2002, for a review).

The central Mojave metamorphic core complex exposes a low-angle normal fault

(the Waterman Hills detachment fault) that juxtaposes tilted early Miocene volcanic and sedimentary rocks in the hanging wall against variably mylonitized basement rocks in the footwall. Based on apparent offsets of pre-Ter-tiary markers, several workers (Glazner et al., 1989; Walker et al., 1990; Martin et al., 1993) proposed that 40–60 km of northeast-directed normal slip occurred along the Waterman Hills detachment fault. The distribution of exten-sion is controversial. Dokka (1989) argued that regional extension occurred within an east-west–trending belt across most of the Mojave Desert region. In contrast, Glazner et al. (2002) suggested that extension was largely confi ned to an ~25-km-wide area centered around the central Mojave metamorphic core complex. Currently there is no strong consensus on the precise timing of extension in the central Mojave Desert. A few lines of evidence suggest that deformation associated with the central Mojave metamorphic core complex occurred between ca. 24 and 19 Ma. First, a dacite dike in the Mitchel Range and the Waterman Hills granodiorite are interpreted to have been emplaced synkinematically into the footwall of the central Mojave metamorphic core com-plex (Walker et al., 1990; Fletcher and Bartley, 1994); these intrusions have zircon U-Pb ages of 23.0 ± 0.9 Ma and 21.9 ± 0.8 Ma, respectively (Walker et al., 1990, 1995). Second, the Pick-handle Formation volcanic and sedimentary rocks in the hanging wall of the central Mojave metamorphic core complex are interpreted as synextensional deposits ranging in age from ca. 24 to 19 Ma (Fillmore and Walker, 1996). Younger (ca. 17–13 Ma) fi ne-grained lacustrine rocks of the Barstow Formation are considered postextensional deposits. Thermochronologic data of mylonitic rocks from the Mitchel Range

Singleton and Gans

460 Geosphere, June 2008

and Hinkley Hills indicate that the footwall of the central Mojave metamorphic core complex underwent rapid cooling (50–100 °C/m.y.) between ca. 21 and 17.5 Ma (Gans et al., 2005). This episode of cooling is interpreted to refl ect exhumation of the footwall during slip on the Waterman Hills detachment fault.

Strike-slip faulting associated with the Eastern California shear zone appears to have been the dominant mode of postextensional deformation in the Mojave Desert region. Northwest-trending right-lateral faults are ubiquitous and accom-modate a small percent of the relative motion between the Pacifi c and North American plates (Dokka and Travis, 1990b). East- to northeast-trending left-lateral faults are also common, par-ticularly in the northeastern Mojave Desert. The cumulative amount of northwest-directed dex-tral shear across the region is probably on the order of 50–75 km (Dokka and Travis, 1990a; Glazner et al., 2002). It is unclear when this faulting began, but some indirect evidence sug-gests that northwest- trending dextral faults may have locally been active as early as 19 Ma (Bart-

ley et al., 1990). Several strike-slip faults are considered to be active now (Jennings, 1994).

Neogene shortening in the Mojave Desert region has primarily been attributed to local transpression along northwest-striking dextral faults (e.g., Dibblee, 1980b, 1994), or regional north-south contraction (Bartley et al., 1990; Linn et al., 2002). The most common types of contractional structures are approximately east-west–trending folds, many of which occur in Miocene lacustrine rocks. Folds are wide-spread across the region, suggesting that con-traction is a regional phenomenon (Bartley et al., 1990). The magnitude of shortening repre-sented by these folds is not well documented, and the timing of folding is unclear. Gently folded Quaternary gravels indicate that some folding is related to active strike-slip faults, and north-south shortening may play an important role in present-day strain accumulation across the Eastern California shear zone (Oskin et al., 2007). The goal of this study is to understand the stratigraphic and structural evolution of the Calico Mountains, which arguably best exem-

plify synextensional deposition and transpres-sion in the central Mojave Desert.

GEOLOGIC OVERVIEW OF THE CALICO MOUNTAINS

Located ~15 km northeast of Barstow, the Cal-ico Mountains form a 15-km-long, northwest- trending range composed primarily of early Miocene sedimentary and volcanic rocks in the upper plate of the central Mojave metamor-phic core complex (Fig. 1). Dacite and coarse volcaniclastic sedimentary rocks of the Pick-handle Formation compose most of the north-ern and central Calico Mountains (Fig. 1). The type locality of the Pickhandle Formation is in the northwestern Calico Mountains, where the south- to southwest–dipping section is ~1500 m thick (McCulloh, 1952; Dibblee, 1994; Fig. 1). Overlying the Pickhandle Formation are fi ne-grained lacustrine rocks generally considered part of the Barstow Formation and referred to in this study as the Calico Member of the Barstow Formation. These lacustrine rocks are intruded

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Pickhandle Fm. (early Miocene, mostly coarse-grainedvolcaniclastic rocks)

Waterman Hills granodiorite(early Miocene)

Mylonitic rocks in the footwall of the central Mojave metamorphic core complex

Explanation

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Figure 1. Geologic map of the central Mojave Desert region near Barstow, California, compiled from McCulloh (1960, northern Calico Moun-tains), Dibblee (1968, Mud Hills; 1970, Lead Mountain–Elephant Mountain area, central Calico Mountains), Cox and Wiltshire (1993, Ele-phant Mountain area), Fletcher et al. (1995, Hinkley Hills, Mitchel Range, Waterman Hills), and this study (southern Calico Mountains).

Structural evolution of the Calico Mountains, Mojave Desert, California

Geosphere, June 2008 461

by several dacite domes that form most of the peaks in the southeastern Calico Mountains.

The dominant structure in the Calico Moun-tains is the Calico fault, a northwest-trending right-lateral fault with a maximum displace-ment of ~10 km in the Rodman Mountains (~30 km southeast of the Calico Mountains; Dibblee, 1964; Glazner et al., 2000). In the southern Calico Mountains, the Calico fault strikes west-northwest and forms a 10-km-long restraining bend (Fig. 1). Lacustrine rocks north of the Calico fault are folded into numer-ous anticlines and synclines, making the Calico Mountains one of the type localities for Neo-gene contractional deformation in the central Mojave Desert. The overall east-west trend of the folds and their proximity to the restraining bend in the Calico fault have led most geolo-gists to interpret the folding as transpressional (e.g., Tarman and McBean, 1994; Dibblee, 1994; Glazner et al., 1994).

PREVIOUS WORK

The geology of the Calico Mountains was mapped by McCulloh (1952, 1960, 1965) and later by Dibblee (1970). Prior to this investiga-tion, the fi ne-grained lacustrine section in the Calico Mountains was correlated with the type locality of the Barstow Formation in the Mud Hills (McCulloh, 1952; Dibblee, 1980a). Reyn-olds (2000) argued that a sequence of three marker beds suggest a stratigraphic correlation between the lacustrine sections in the Calico Mountains and the Mud Hills. However, the age of fi ne-grained lacustrine sedimentation in the Mud Hills Barstow Formation is bracketed between ca. 17 and 13 Ma (MacFadden et al., 1990), which is distinctly younger than what we determine to be the age of lacustrine beds in the southern Calico Mountains. In this paper, the lacustrine section in the Calico Mountains is referred to as the Calico Member of the Bar-stow Formation.

Fletcher (1986) mapped a 6 × 1 km area along the Calico fault west of Calico Ghost Town at a scale of 1:4800, focusing primarily on silver mineralization. Based on the apparent offset of two similar mineralized deposits, he estimated that 1.9–3.2 km of right-lateral offset existed along the Calico fault.

METHODS

Detailed geologic mapping provided the basis for understanding the stratigraphy and structural geology of the southern Calico Mountains. The folded lacustrine section in the center of the study area (Plate 1; Fig. 2) was mapped at a scale of 1:6000; surrounding areas were mapped

at 1:12,000. Axial traces of all folds with ampli-tudes ≥1 m were mapped, and the orientations of fold axes and axial surfaces were determined by measuring bedding orientations around the hinge of each fold and axial trace orientations on profi le views of folds.

The 40Ar/39Ar geochronology was used to determine the ages of Miocene sedimentation and volcanism in the Calico Mountains. Detailed step-heating experiments (typically 7–12 steps) were performed on pure separates of plagio-clase, biotite, and whole rock from 11 different volcanic samples (Table 1). Errors reported on plateau ages are ±2σ. The major and trace ele-ment geochemistry of selected volcanic rocks was determined by X-ray fl uorescence (XRF).

STRATIGRAPHY OF THE CALICO MOUNTAINS

Pickhandle Formation and Metavolcanic Basement Rocks

The Pickhandle Formation consists primar-ily of coarse-grained volcaniclastic deposits and silicic volcanic rocks that are generally inter-preted to have accumulated in extensional basins during rapid slip along the Waterman Hills detachment fault (e.g., Fillmore and Walker, 1996). The thickest and most widespread occur-rence of the Pickhandle Formation in the central Mojave Desert is in the Calico Mountains. In the northern part of the study area, the Pickhandle Formation forms a gently southeast- to south-dipping homocline, and is composed primarily of volcaniclastic sandstone, tuff breccia, and dacite domes (Plate 1). Most of the Pickhandle Formation beds are channelized and were likely deposited by alluvial or fl uvial processes.

A distinct characteristic of the Pickhandle Formation in the southern Calico Mountains is that it primarily contains clasts of dacite. In addition, a signifi cant part of the Pickhandle Formation consists of biotite ± hornblende dacite domes and fl ows (unit Tpd; Plate 1). In the study area the thickest exposed section of Pickhandle sedimentary rocks is only ~300 m because dacite domes are widespread lower in the section. Some of these domes intrude Pick-handle sedimentary beds, and others are depo-sitionally overlain by volcaniclastic intervals. The association of domes, fl ows, volcaniclas-tic breccias, and sandstone beds suggests that the Pickhandle Formation in the southern Cal-ico Mountains represents a dacite lava-dome fi eld and associated pyroclastic and epiclastic apron deposits.

Immediately south of the Calico fault, rocks resembling the Pickhandle Formation do not exist, as the fi ne-grained lacustrine section

(Calico Member) directly overlies metavolcanic rocks and conglomerates and breccias composed largely of pre-Tertiary detritus (Fig. 3). The metavolcanic basement rocks range from basalt to rhyolite, but most are andesitic. The green-schist facies metamorphism that affected these rocks preserves volcanic textures, but resulted in widespread growth of metamorphic epidote, chlorite, and albite. It is unclear whether these metavolcanic rocks are part of the Jurassic Side-winder Formation (e.g., Schermer and Busby, 1994) or the upper Paleozoic Coyote Group described by McCulloh (1952). Similar rocks have been described in the Elephant Mountain area to the west-southwest (Fig. 1; Cox and Wiltshire, 1993).

Nature of the Pickhandle Formation–Calico Member Contact

The contact between the Pickhandle Forma-tion and directly overlying Calico Member is generally conformable, and maroon sandstone beds of the Pickhandle Formation appear to grade up into the white-gray volcaniclastic sandstone beds at the base of the fi ne-grained lacustrine section. However, on a larger scale, homoclinally southeast-dipping Pickhandle strata strike obliquely to the Calico beds, and the amount of section in the uppermost Pick-handle Formation subunit Tpsu varies from ~170 m to <30 m beneath the contact with the Calico Member (Plate 1). Volcaniclastic sand-stone beds at the base of the Calico Member are consistently present above the contact east of Calico Ghost Town (Fig. 1), so there does not appear to be any section omitted from the base of the Calico Member. These stratigraphic rela-tionships indicate that the bedding-subparallel Pickhandle Formation–Calico Member contact is a subtle angular unconformity. The geom-etry of this unconformity has most likely been modifi ed by transpressional movement along the contact.

40Ar/39Ar GeochronologyPrevious geochronologic studies of the Pick-

handle Formation in other parts of the central Mojave Desert suggest that volcanism and coarse volcaniclastic sedimentation occurred between ca. 24 and 19 Ma (Burke et al., 1982; Fillmore and Walker, 1996). Three new 40Ar/39Ar dates on dacite from the Pickhandle Formation in the Calico Mountains range from 19.35 ± 0.15 to 19.0 ± 0.1 Ma. An exogenous dacite dome at the base of the Calico Mem-ber near Old Borate yielded 40Ar/39Ar plateau ages of 19.13 ± 0.04 Ma and 19.0 ± 0.1 Ma on biotite and plagioclase, respectively (Plate 1; Table 1). Detritus of this dome is present in beds at the base of the lacustrine section, indicating

Singleton and Gans


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CM

-209

0

1 ki

lom

eter

00.

5

0.5

0.5

1 m

ile

C

on

tou

r in

terv

al =

20

feet

N

CALI

COCA

LICO

Cal

ico

faul

t

S. Cal

ico fa

ult

D U

Qs

con

tact

Geo

log

ic c

on

tact

(das

hed

wh

ere

app

roxi

m.

or i

nfe

rred

, do

tted

wh

ere

con

ceal

ed)

Fau

lt (h

ash

mar

k sh

ow

s d

ip o

f fau

lt;

arro

w

sho

ws

tren

d a

nd

plu

ng

e o

f slip

vec

tor;

D

=d

ow

n-t

hro

wn

sid

e, U

=u

p-t

hro

wn

sid

e)

Bed

din

g-p

aral

lel f

ault

an

d fo

ld d

etac

hm

ent

bet

wee

n t

he

Pick

han

dle

Fo

rmat

ion

(Tp

) an

d

the

Cal

ico

Mem

ber

(Tb

c)

Axi

al t

race

of a

n a

nti

clin

e (d

ott

ed w

her

e ap

pro

xim

ate

or c

on

ceal

ed, a

rro

w s

ho

ws

plu

ng

e d

irec

tio

n o

f fo

ld a

xis)

Axi

al t

race

of a

syn

clin

e

40A

r/39

Ar g

eoch

ron

olo

gy

& X

RF g

eoch

emis

try

sam

ple

loca

tio

n, n

ame,

& a

ge

(+ 2

sig

ma)

Geo

chro

no

log

y sa

mp

le lo

cati

on

, nam

e, &

ag

e

Geo

chem

istr

y sa

mp

le lo

cati

on

& n

ame

6048

CM

-190

17.1

0

.1 M

a+

CM

-42

19.0

0

.1 M

a+ C

M-2

09

35 78 2045

Stri

ke &

dip

of b

edd

ing

Stri

ke o

f ver

tica

l bed

din

g

Stri

ke &

dip

of o

vert

urn

ed b

edH

ori

zon

tal b

edd

ing

Ap

pro

xim

ate

stri

ke &

dip

St

rike

& d

ip o

f flo

w fo

liati

on

LEG

END

Tdi

Tdb

Tc4

Tdi c

Tdi b

Tc3

Tdc

= c

last

- an

d m

atri

x-su

pp

ort

ed c

on

glo

mer

ate

and

san

dst

on

e; b

asal

t cl

asts

do

min

ant,

o

ther

cla

sts

incl

ud

e m

etav

olc

anic

s, d

acit

e, g

ran

ite;

un

cert

ain

str

atig

rap

hic

po

siti

on

= c

last

-su

pp

ort

ed d

acit

e b

recc

ia; l

oca

lly m

on

olit

ho

log

ic; c

last

s m

ost

ly d

eriv

ed fr

om

Td

i an

dTd

i b; T

db

b =

dac

ite

bre

ccia

co

mp

ose

d e

nti

rely

of T

di b

cla

sts

= h

orn

ble

nd

e d

acit

e in

tru

sio

n; p

hen

ocr

ysts

: 13-

22%

pla

gio

clas

e, 4

-7%

ho

rnb

len

de

= h

orn

ble

nd

e d

acit

e in

tru

sio

n w

ith

pat

chy

cela

do

nit

e al

tera

tio

n; 2

0-25

%

ph

eno

crys

ts: ~

5% h

orn

ble

nd

e, <

1% b

ioti

te

= h

orn

ble

nd

e-b

ioti

te d

acit

e in

tru

sio

n; 1

8-25

% p

hen

ocr

ysts

: 4-7

% h

bl,

1-3%

bio

= p

oly

mic

tic

con

glo

mer

ate;

cla

sts:

bas

alt,

dac

ite,

met

avo

lcan

ics,

gra

nit

e; lo

wer

3/4

is

mat

rix-

sup

po

rted

w/

sst

and

slt

st in

terb

eds,

up

per

1/4

is m

ost

ly c

last

-su

pp

ort

ed

= d

acit

e w

ith

per

vasi

ve c

elad

on

ite

alte

rati

on

; sea

-gre

en c

olo

r is

do

min

ant;

maf

ic

min

eral

s h

eavi

ly o

xid

ized

(<3%

ho

rnb

len

de)

Qs

undifferentiated Calico Member

Tbc 1

Tbc 2

Tbc 3

Tbc 4

Tbc 5

Tbc 6

Tdb

bc

Calico Member of the Barstow Formation

Tbc O

B1

Tbc O

B2

Tbc O

B3

= u

nd

iffer

enti

ated

Qu

ater

nar

y se

dim

ent

dep

osi

ts; m

ost

ly g

rave

l-do

min

ated

al

luvi

um

; co

lluvi

um

an

d d

acit

e ta

lus

map

ped

ove

r th

e C

alic

o M

emb

er

Dacite of the Yermo volcanic center (~17 Ma)

Tbc s

si

Tbc s

l

Tbc s

shl

Tbc s

shl

Tbc l

sh

Tbc s

sh

Mule Canyon, N of the Calico faultOld Borate

= w

hit

e-g

ray

volc

anic

last

ic s

and

sto

ne

= s

and

sto

ne,

silt

sto

ne,

an

d li

mes

ton

e; c

her

t b

eds

com

mo

n n

ear b

ase

= li

gh

t re

d-b

row

n s

and

sto

ne

= li

gh

t o

live-

bro

wn

san

dst

on

e

= t

an s

and

sto

ne

= li

gh

t ta

n to

gre

en g

ray

silt

sto

ne

and

cla

ysto

ne

= d

acit

e b

recc

ia in

terb

edd

ed w

ith

Tb

c 6 an

d b

etw

een

Tb

c OB

2 an

d T

bc O

B3

= b

row

n p

laty

lim

esto

ne

and

tan

cal

care

ou

s sh

ale,

ch

ert

bed

s co

mm

on

nea

r bas

e

= t

an to

gre

en-g

ray

silt

sto

ne

and

cla

ysto

ne

= t

an li

mes

ton

e an

d s

hal

e; li

mit

ed e

xpo

sure

eas

t o

f Old

Bo

rate

; ove

rlie

s Td

bb

c

= s

and

sto

ne,

sh

ale,

an

d li

mes

ton

e

= b

row

n p

laty

lim

esto

ne

and

sh

ale

= s

and

sto

ne

and

lim

esto

ne

= li

thic

san

dst

on

e an

d s

iltst

on

e

= c

lays

ton

e an

d s

iltst

on

e

Tbc s

h*

= re

d/p

urp

le/y

ello

w-b

row

n c

alca

reo

us

shal

e, h

ydro

ther

mal

ly a

lter

ed

RO

CK

UN

ITS

Tbc1

Tbc

2

Tbc

3T

bc4

Tbc

5

Tbc

6

Tdb

bc

Tdb

bc

Tbc

6

Tbc

sshl

Tbc

sshl

Tbcs

shl

Tbcs

shl

Tbc

OB

1

Tbc

OB

2

Tbc

OB

2

Tbc m

= w

eakl

y m

iner

aliz

ed (s

ilica

+b

arit

e) s

and

sto

ne

and

sh

ale

Tbc

sl

Tbc

lsh

Tc4

Tbc

sshl

Tbc

sh

Tbc

sh*

Tbc

ssi

Tbc m

Tdc

sshl

Tdc

sshl

Tc2

Tdb

1Tc

1

Tmvb

Tgb

Ttb

g mv

= d

acit

e b

recc

ia in

ters

trat

ified

(?) w

ith

th

e C

alic

o M

emb

er; m

ost

ly c

last

-su

pp

ort

ed;

Tdb

1ms=

mat

rix-

sup

po

rted

.

= v

olc

anic

co

ng

lom

erat

e; p

red

om

inan

tly

clas

t-su

pp

ort

ed; ~

2/3

clas

ts a

re b

asal

t, ~

1/3

clas

ts a

re m

etav

olc

anic

s; s

ecti

on

fin

es u

pw

ard

s in

to c

oar

se s

and

sto

ne

= m

etav

olc

anic

bre

ccia

; pre

do

min

antl

y cl

ast-

sup

po

rted

; bro

wn

lim

y m

atri

x an

d t

hin

p

laty

lim

esto

ne

inte

rbed

s co

mm

on

= p

oly

mic

tic

con

glo

mer

ate,

mo

stly

cla

st-s

up

po

rted

; cla

sts

incl

ud

e m

etav

olc

anic

ro

cks,

gra

nit

e, re

wo

rked

ash

fall

tuff

= g

ran

ite

bre

ccia

; mo

no

lith

olo

gic

, cla

st-s

up

po

rted

= b

recc

ia c

on

sist

ing

of r

ewo

rked

ash

fall

tuff

wit

h ~

3% b

ioti

te

= le

uco

gra

nit

e; m

iner

als:

k-fe

ldsp

ar, q

uar

tz, p

lag

iocl

ase,

hyd

roth

erm

al m

usc

ovit

e

= u

nd

iffer

enti

ated

met

avo

lcan

ic ro

cks,

bas

alt

to rh

yolit

e; a

nd

esit

ic la

vas

and

bre

ccia

s m

ost

co

mm

on

; pro

pyl

itic

ally

alt

ered

, gre

ensc

his

t-fa

cies

wit

h c

hlo

rite

an

d e

pid

ote

Tps

Tpd

Pickhandle Formation

Tps u

= P

ickh

and

le F

m. s

edim

enta

ry ro

cks

(un

diff

eren

tiat

ed);

mar

oo

n/t

an v

olc

anic

last

ic

san

dst

on

e &

san

dy

dac

itic

bre

ccia

; Tp

s l =

Pic

khan

dle

Fm

. sst

& b

recc

ia b

elo

w T

ps u

= u

pp

erm

ost

Pic

khan

dle

Fo

rmat

ion

sed

imen

tary

rock

s (s

ame

lith

olo

gy

as T

ps)

; se

par

ated

fro

m T

ps l

by

a m

arke

r bed

vbT

bv

Pre-Tertiary

South of the Calico fault

= d

acit

e d

om

es, l

ava

flow

s, an

d c

oar

se, m

on

olit

ho

log

ic d

acit

e b

recc

ia; p

hen

ocr

ysts

o

f pla

gio

clas

e +

bio

tite

+ h

orn

ble

nd

e +

qu

artz

Mule C

Mule C

Mule C

Mule CC

Mule C

Mule C

Mule C

Mule C

Mule C

le C

Mule CCC

Mule Can

yon

anyon

anyon

anyonyo

nyan

yon

anyon

anyo

anyony

Rd.Rd.Rd Rd Rd R

Tdb

bc

Tbc

OB

3

Tbc

OB

2

Tbcsshl

Tbcsshl

Tbc

sshl

Tbc

sshl

Pla

te 1

. A G

eolo

gic

map

of t

he s

outh

ern

Cal

ico

Mou

ntai

ns, Y

erm

o Q

uadr

angl

e, c

entr

al M

ojav

e D

eser

t, C

alif

orni

a. I

f you

are

vie

win

g th

e P

DF

of th

is p

aper

or

read

ing

it o

ffl in

e,

plea

se v

isit

htt

p://d

x.do

i.org

/10.

1130

/GE

S001

43.S

P1

(Pla

te 1

) or

the

ful

l-te

xt a

rtic

le o

n w

ww

.gsa

jour

nals

.org

to

view

Pla

te 1

.



Figure 2. Geologic cross sections of the southern Calico Mountains. Section lines and map units are shown in Plate 1.

?

?

A (S)

A' (N)

Tps TpdTdic Tbcslsh

Tbc6

2500

2000

1500

3000

Elevation (ft.)2500

2000

1500

Calico fault

S. Calico

f.

2500

2000

1500

3000

1000

Tbc1

Tbc 2

Tbc 3Tbc 6

Tdbbc

Tps

Tpd

Tdib

Calico fault

Tdc

TdiTdi

S. Calico

fault

Tbc 5 Tb

c 4

B (S) B' (N)

Tpsu

Tbcslsh

Tbcslsh

Tbc2

?

Tbc1

Tbc6

Tbc5

Tbc4

?

C (S)

2500

2000

1500

1000

Elevation (ft.)

C' (N) 3000

2500

2000

1500

1000

?

S. Calico

fault

Tpd

Tps

Tc3

Tlcssh

Tdic

Tdi

Tpsu

mv

Tc2

Tdb1Tdbbc

?

Tbc3

Tb

cslsh

Tc2

mv

Bend in cross section

D' (N) 3000

2500

2000

1500

1000

?

D (SSW)

2500

2000

1500

1000

Elevation (ft.)3000

TpsTpd

TdiTdi

Tdb

Tdi

Tdib

Tps

Qs

Tbcssh

Tbcsl

Tbc5

Calico fault

S. Calico fault

TbcOB1

Tbc5/TbcOB2

Tbc4

Tbc(undifferentiated)

?

2500

2000

1500

Elevation (ft.)

3000

3500

mv

Tdi

Tdi

Tps

Tpd

Tdi

Tdb

Tc2 2500

2000

1500

3000

3500

TbcOB1

TbcOB2

E' (NNE)E (SSW)

Tbcssh

Calico fault

Tbc

Tps

?

Tbc

(undiff.)

Elevation (ft.)

2500

2000

1500

3000

2500

1500

3000

3500

F' (N)Bend in section

?

?

Tdib Tdi

Tp

Tp

TpTc1

Tdi

mv

F (S)

Calico fault

2000

Tmvb

mv

Tbcs

Tbcsh*

Tbclsh

Tbc ls

h

Tdb

?

0 500 m 1000 m

no vertical exaggeration

Singleton and Gans


that the oldest part of the Calico Member is ca. 19 Ma or younger. A compositionally dif-ferent dacite dome in the north-central part of the study area yielded a plagioclase plateau age of 19.0 ± 0.1 Ma (Plate 1; Table 1). Locally this dome appears to have intruded Pickhandle sedimentary beds ~150 m beneath the contact with the Calico Member. In the northwestern Calico Mountains, a dacite fl ow near the base of the Pickhandle Formation along Fort Irwin Road yielded a plateau age of 19.35 ± 0.15 Ma on plagioclase. There is ~1 km of volcanic and volcaniclastic sedimentary rocks between the top of this dacite fl ow and the base of the over-lying Barstow Formation (McCulloh, 1952, 1960). Assuming that the uppermost Pickhan-dle Formation here is ca. 19 Ma, as it is in the Mud Hills (MacFadden et al., 1990) and east-ern Calico Mountains, then the Calico Moun-tains were the site of very rapid volcaniclas-tic sedimentation (~2.5 mm/yr) from ca. 19.4 to 19.0 Ma. This rapid sedimentation is most likely a product of both abundant dacitic volca-nism and extensional basin development.

Calico Member of the Barstow Formation

The Calico Member of the Barstow Forma-tion consists primarily of siltstone, sandstone, and limestone (Plate 1; Fig. 4). Lateral and verti-cal facies changes are common, although some distinct groups of beds can be mapped for a dis-tance of ~4 km (Plate 1). We have divided the internal stratigraphy of the Calico Member north of the Calico fault into different subunits based on distinct lithostratigraphic and color char-acteristics (Plate 1; Fig. 4). In the center of the study area the Calico Member is ~375 m thick (near cross-section C–C'; Plate 1; Fig. 4). East of Old Borate, exposure of the section is not as complete, but the thickness appears to be at least 450 m. South of the Calico fault, the minimum thickness of the fi ne-grained lacustrine section is ~300 m. The Calico Member north of the fault decreases in thickness westward toward Calico Ghost Town. For example, a distinct set of brown sandstone beds that is ~50 m thick and ~130 m stratigraphically above the top of the Pickhandle Formation near Mule Canyon is only ~20 m

thick and 35–40 m above the Pickhandle Forma-tion near Calico Ghost Town (Fig. 4).

The Calico Member is generally considered part of the middle Miocene Barstow Forma-tion, which is ~1000 m thick at its type locality in the Mud Hills (Dibblee, 1968; Woodburne et al., 1990). The age of the Barstow Forma-tion and potential stratigraphic correlations are important because the Barstow Formation in the Mud Hills provides the basis for the Barstovian land- mammal age. Although stratigraphic cor-relations between the Mud Hills and Calico Mountains have been suggested (Reynolds, 2000), there are a few clear lithostratigraphic differences between the Calico Mountains lacustrine rocks and the type section of the Barstow Formation. Lacustrine rocks in the Calico Mountains contain less granitic detritus than the Mud Hills Barstow Formation, and the prominent water-laid ash-fall tuff beds in the Mud Hills are not present in the Calico Moun-tains. Sandstone beds in the Calico Mountains are typically dominated by dacitic detritus, and rare ash-rich beds are all strongly reworked. In

TABLE 1. 40

AR/39

AR GEOCHRONOLOGY OF VOLCANIC ROCKS IN THE CALICO MOUNTAINS

Sample Description UTM

coordinates Separate Age (Ma) K/Ca ratio

Radiogenic yield(%)

CM-80 hbl dacite dome that intrudes the upper Calico Member north of the Calico fault

0515958 E 3866603 N

plag WR

16.9 ± 0.15 16.8 ± 0.2

.04–.07 1.1–3.4

25–74 18–52

CM-96 hbl dacite clast from breccia overlying Calico Member

0516844 E 3867615 N

plag WR

16.85 ± 0.15 17.0 ± 0.5

.033–.046.46–1.1

31–80 47–65

CM-98 hbl dacite dome that intrudes Calico Member south of the Calico fault

0515214 E 3865298 N

plag 16.9 ± 0.1 .037–.047 45–74

CM-124 celadonite-altered hbl dacite dome south of the Calico fault

0514621 E 3865596 N

plag 16.85 ± 0.15 .03–.05 40–84

CM-185 celadonite-altered hbl dacite dome that intrudes Calico Member west of Sunrise Canyon

0518768 E 3864670 N

plag 17.00 ± 0.08 .051–.063 66–84

CM-190 hbl dacite dome that intrudes Calico Member north of Sunrise Canyon

0518420 E 3866144 N

plag 17.10 ± 0.12 .027–.031 57–71

CM-200 hbl-bio-qtz dacite that intrudes Calico Member south of the Calico fault

0513528 E 3866290 N

biotite 17.11 ± 0.06 15–95 80–92.5

CM-42 hbl-bio dacite dome in Pickhandle Formation 0515063 E 3868152 N

plag 19.0 ± 0.1 .059–.068 56–92

CM-90 bio-qtz dacite flow and/or dome underlying Calico Member near Tin Can Alley

0517190 E 3868238 N

biotite plag

19.13 ± 0.04 19.0 ± 0.1

6–130 .055–.06

78–93 72–87

PGMJ-57 basal vitrophyre of dacite lava in Pickhandle Formation near Jackhammer Gap, northwestern Calico Mountains

0509733 E 3876297 N

plag 19.35 ± 0.15 .068–.077 42–79

CM-298

breccia consisting of reworked ash-fall tuff; underlies Calico Member south of the Calico fault

0517906 E 3864832 N

biotite 24.9 ± 0.1 13–450 74–95

Note: Universal Transverse Mercator (UTM) coordinates are based on the North American Datum, 1927 (NAD 27), zone 11. Mineral abbreviations: plag—plagioclase; bio—biotite; WR—whole rock; hbl—hornblende; qtz—quartz. Mineral separates were prepared and analyzed at University of California–Santa Barbara. Ages listed are calculated weighted mean plateau ages from incremental heating experiments and are relative to flux monitor Taylor Creek Rhyolite with an assigned age of 27.92 Ma. Uncertainties are ± 2σ. K/Ca ratios are based on

39Ar/

37Ar ratios. For age spectra and isochron plots, see Supplemental Figure S1

1.

1If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00143.S1 (Fig. S1) or the full-text article onwww.gsajournals.org to view Supplemental Figure S1.



addition, borates are present in the upper part of the lacustrine section in the Calico Mountains (Fig. 4), whereas the Mud Hills Barstow Forma-tion apparently lacks borate mineralization.

The new 40Ar/39Ar data in this study (Table 1) bracket the age of the Calico Member north of the Calico fault primarily between ca. 19 and 17 Ma, demonstrating that this section is older than the fi ne-grained Barstow Formation in the Mud Hills. However, the Calico Member overlaps in time with the Owl Conglomerate Member at the base of the Barstow Formation (Dibblee, 1968; MacFadden et al., 1990). We propose that the fi ne-grained lacustrine section in the Calico Mountains be given a new designa-tion, Calico Member of the Barstow Formation, to indicate that the Calico Mountains lacustrine rocks are not age correlative to fi ne-grained lacustrine rocks at the type locality of the Bar-stow Formation.

Dacite of the Yermo Volcanic Center

Dacite domes and dacite breccias that are younger than the Calico Member form an

approximately east-west–trending belt that covers ~20 km2 in the southeastern Calico Mountains (Plate 1). This dome fi eld is desig-nated as the Yermo volcanic center (after the nearby town of Yermo; Fig. 1). The domes intrude Calico beds that are generally steep-ened and baked (hardened and oxidized) against the margins of the domes. At least 15 different dacite intrusions are present (Fig. 1). Coarse, clast-supported, monolithologic brec-cias that overlie the Calico Member are com-positionally identical to these dacite intrusions and were most likely shed from the domes as rock avalanche deposits and block and ash fl ows. In some areas it is diffi cult to distinguish these breccia deposits from the brecciated por-tions of the domes. Some of the breccia sheets appear to be as thick as 150 m, but these sheets are concentrated around domes and do not appear to be laterally extensive.

The dacites of the Yermo volcanic fi eld are mineralogically and geochemically homoge-neous. All contain phenocrysts of plagioclase (10%–22%) and hornblende (4%–7%), and some contain sparse (1%–3%) biotite pheno-

crysts. The XRF analyses of eight samples from the Yermo volcanic center indicate an IUGS classifi cation range from dacite to trachydacite (Fig. 5), with a narrow range of SiO

2 content

(65.2–67.7 wt%) and generally calc-alkaline geochemical signatures (Singleton, 2004).

The Yermo volcanic center was active dur-ing the fi nal stages of lacustrine sedimentation preserved in the Calico Member of the Bar-stow Formation. Clast-supported dacite breccia sheets (unit Tdbbc) are locally interbedded with shale in the upper part of the lacustrine sec-tion (Plate 1; Fig. 4). The clasts in this breccia are compositionally indistinguishable from the Yermo domes that intrude the Calico Member and do not resemble dacite from the Pickhandle Formation. Assuming that this breccia sheet was shed from a Yermo dacite dome, at least 60–80 m of fi ne-grained lacustrine beds were deposited west and east of the Yermo volcanic center after the initiation of volcanic activity. It appears that dacitic volcanism locally shut off lacustrine sedimentation in the southeastern Calico Mountains, but sedimentation distal to the Yermo volcanic center continued and may

Figure 3. Composite stratigraphic columns north and south of the Calico fault in the southern Calico Mountains.

19.13 0.04 Ma+

Dacite (Td)

Dacite (Td)

Hypabyssal granite (g)

Predominantly clast-supported; clasts up to ~0.5 m; ~66% clasts = basalt; ~33% clasts = metavolcanic rocks

K-spar + quartz + plag + sparse muscovite; granophyric texture

dacite breccia (Tdbbc)

siltstone and claystone

sandstone

limestone,siltstone, andsandstone

unconformity

?

Conglomerate (Tc3)

maroon to tan volcani-clastic sandstone, matrix-supported dacite breccia, and dacite domes/flows,clasts commonly <3 cm,channelized beds common

>1 km thick

Granite breccia (Tgb)

Metavolcanic breccia (Tmvb)

Tuff breccia (Ttb)

Monolithologic; clast-supported;most clasts < 0.5 m; crudely stratified in parts; source = hornblende dacite intrusions Polymictic; upper 40 m = mostly clast-supported; up to 40 cm clasts, clast types=basalt (~50%), hbl-bio dacite, metavolcanic rocks, granite

Limey sandstone and siltstone interbeddedw/ matrix-supported conglomerate

DESCRIPTION

Primarily platy limestone interbedded w/limey siltstone; near Mule Canyon Road = limey sandstone and siltstone

Clast-supported; monolithologic

Basalt to rhyolite lavas and breccias; andesitic rocks most common; propylitically altered, greenschist-facies with abundant chlorite &epidote; Jurassic Sidewinder Formation?

Polymictic conglomerate (Tc1)mostly clast-supported; clasts include metavolcanic rocks, granite, reworked ash-fall tuff

breccia consisting of reworked ash-fall tuff w/ ~3% biotite; bio age = 24.9 + 0.1 Ma

North of the Calico fault South of the Calico fault

Dacite breccia (Tdb)

Dacite intrusion (Tdi)phenocrysts: 13-22% plag,4-7% hbl, 0-2% biotite

16.8 - 17.1 Ma

16.9 0.2 Ma+

Calico Member(lacustrine rocks)

phenocrysts: 15-20% plag, 4% biotite, 1% qtz

Dacite (Tpd)

19.13 0.04 Ma

Pickhandle Fm. (Tps)

biotite hornblende qtz phenocrysts

Dacite (Tpd)

Calico Member(lacustrine rocks)

Conglomerate (Tc4)

Metavolcanic basement (mv)

Dacite sill (Tdi)

Dacite breccia (Tdb)

Dacite intrusion (Tdi)phenocrysts: 13-22% plag,4-7% hbl, 0-2% biotite

16.8 - 17.1 Ma

+

++

0 m

800 m

400 m

200 m

600 m

0 m

800 m

200 m

600 m

Singleton and Gans


have been synchronous with deposition the Bar-stow Formation beds in the Mud Hills.

40Ar/39Ar GeochronologyNine mineral separates from seven different

Yermo dacite units yielded 40Ar/39Ar plateau ages that range from 16.8 ± 0.1 to 17.1 ± 0.1 Ma (Plate 1; Table 1), indicating that the Yermo vol-canic center was short-lived. This ca. 17 Ma volcanic activity has not previously been recog-nized in the central Mojave Desert. Prior to this study, it was thought that most volcanism in the Barstow area occurred predominantly between 24 and 20 Ma and had ceased by ca. 18 Ma (Glazner et al., 2002).

These new 40Ar/39Ar data provide a clear upper age bracket on lacustrine sedimentation in the southern Calico Mountains. North of the Calico fault, a dacite dome that intrudes the upper part of the Calico Member yielded pla-teau ages of 16.8 ± 0.1 and 16.9 ± 0.1 Ma on a whole-rock separate and plagioclase, respec-tively. A clast from dacite breccia that caps the Calico Member near Old Borate yielded a pla-teau age of 16.9 ± 0.2 Ma (plagioclase). This particular breccia layer contains clasts of baked

0

100 m

200 m

300 m

400 m

chert

limestone &limey siltstone

dacite(Pickhandle Fm.)

siltstone & claystone

dacite breccia

borate horizon (colemanite)

gypsiferous

EAST

E-E' (Old Borate Canyon)

TbcOB2

TbcOB1

Tpd

dacite breccia

dacite breccia

reworked ash-fall tuff

limestone

sst &sandy sltst

siltstone& claystone

Tbc3

Tbc4

Tbc5

Tbc6

C-C' (Mule Canyon Rd.)

Tdb

Tdbbc

Pickhandle Formationmostly volcaniclastic sst & matrix-supported dacite breccia

lithic sst

sltst, sst, & chert

Tbc1

Tbc2

Tpsu

borate horizon (howlite)

Calico fault

limey siltstone

sandstone

dacite breccia

claystone & siltstone

limestone

B-B'

Pickhandle Fm.

Calico fault

Pickhandle Fm.

lithic sandstone

sandstone

limestone

A-A' (Calico Ghost Town)

WEST

1300 m 770 m 3000 m

limey siltstone

Figure 4. Stratigraphic columns indicating west to east variations in thickness and lithology of the Calico Member of the Barstow Forma-tion, north of the Calico fault. Most thicknesses were measured from cross sections (Plate 1; Fig. 2) and illustrate the original vertical strati-graphic succession after folding is removed. Plate 1 map units are listed on the left side of columns C–C' and E–E'.

Picro-basalt

BasaltBasalticandesite

AndesiteDacite

Rhyolite

Trachyte

Trachy-andesite

Basaltictrachy-andesiteTrachy-

basalt

TephriteBasanite

Phono-Tephrite

Tephri-phonolite

Phonolite

Foidite

35 40 45 50 55 60 65 70 750

2

4

6

8

10

12

14

16

Na

2O

+K

2O

SiO2

Trachydacite

Figure 5. Total alkalies-silica diagram showing International Union of Geological Sciences volcanic rock classifi cation of Yermo dacite samples (ca. 17.1–16.8 Ma). The most alkalic sample of the group contains 6.56% K2O and has probably undergone minor potassic alteration. Geochemical analyses were determined by X-ray fl uorescence at the Washington State University GeoAnalytical Laboratory.



shale, indicating that the breccia clast age is younger than fi ne-grained sedimentation. Two dacite domes that intrude the Calico Mem-ber near Sunrise Canyon yielded ages of 17.0 ± 0.1 and 17.1 ± 0.1 Ma. Two dacite intrusions south of the Calico fault yielded ages of 17.1 ± 0.1 and 16.9 ± 0.1 Ma. These ages indicate that the most of the exposed lacustrine rocks in the southern half of the Calico Mountains are older than ca. 17 Ma, although post–17 Ma lacustrine sedimentation continued both east and west of the Yermo volcanic center.

STRUCTURAL GEOLOGY

The Miocene rocks in the southern Calico Mountains record a complex structural history that involves extension, strike-slip faulting, and shortening. Northwest-striking, high-angle (≥45°) normal faults are common in the Pick-handle Formation (Fig. 6). However, strike-slip and transpressional faulting strongly overprint the extensional fault system and represent the dominant style of deformation in the study area. Northwest-striking dextral and oblique dextral faults are particularly common. The most sig-nifi cant structure in the area is the Calico fault, which strikes west-northwest and forms a left bend in the southern Calico Mountains. Based on the orientation of this apparent restraining bend, the Calico fault is thought to accommodate trans-pressional deformation, although the amounts of heave and throw along this segment of the fault have not been previously estimated. Dibblee (1994) suggested that this restraining bend was

formed by slip on the Manix fault, a major east-northeast–striking sinistral fault that is inferred to intersect the Calico fault at the southeastern end of the Calico Mountains (Fig. 1).

The Calico Mountains are probably best known for the folded lacustrine rocks that are spectacularly exposed along Mule Canyon Road and in the parking lot of the Calico Ghost Town (Plate 1; Fig. 7). These folds are located north of the restraining bend in the Calico fault and have been cited as an example of contraction associ-ated with a strike-slip fault (Tarman and McBean, 1994; Dibblee, 1994; Glazner et al., 1994).

Faults

The majority of faults in the Calico Moun-tains strike northwest and dip steeply (Fig. 6). These northwest-striking faults include normal faults in the Pickhandle Formation and oblique dextral faults throughout the study area. East- to northeast-striking faults are also present and typ-ically have oblique sinistral slip. The intensity of faulting is strongly dependent on rock type; the Pickhandle Formation and the Yermo dacite rocks are highly faulted, whereas faults are rare within the Calico Member (Plate 1). Northwest-striking dextral faults that cut the Pickhandle Formation and Yermo dacite appear to die out abruptly in the Calico Member. Individual Cal-ico beds that show little or no evidence of brittle offset persist along strike for >2 km (Plate 1).

Dextral and oblique dextral-reverse shear-ing are clearly the dominant modes of brittle deformation. The Calico fault system accounts

for at least 3 km of dextral slip (see following), and the cumulative amount of dextral shear in the study area (across a northeast transect from C–E'; Plate 1) is estimated to be ~4.1 km. The largest northeast-trending fault in the study area is an oblique sinistral-reverse fault with ~650 m apparent heave. Other east- to northeast- trending faults appear to have small offsets, and the cumu-lative amount of sinistral slip across the study area is probably ≤750 m. Locally the Yermo vol-canic center is shortened by the combination of dextral slip along the Calico fault and sinistral slip along the largest east- northeast–trending fault. However, the dominance of dextral and dextral-reverse shearing over sinistral shear-ing indicates that conjugate strike-slip fault-ing does not play a major role in north-south shortening. In addition, although the east- to northeast- trending sinistral faults are oriented as a conjugate set to the dextral faults, dextral faulting appears to have postdated most of the sinistral faulting. The Calico fault and Southern Calico fault are not cut by any east- or northeast- trending faults, whereas the Calico fault cuts three northeast-trending faults (including the largest oblique sinistral fault; Plate 1). Outside of the Calico fault system, east- to northeast-trending faults are commonly cut by northwest-trending dextral faults (Plate 1). These timing relationships suggest that sinistral and dextral faulting were generally not mutually active.

Faults within the Pickhandle FormationMost faults in the Pickhandle Formation

strike northwest, dip ≥45° (dominantly to the

Figure 6. Fault data from the southern Calico Mountains. (A) Poles to all measured fault planes. Boxes—fault planes in the Pickhandle Formation. Triangles—fault planes along the Calico fault system. Solid circles—all other fault planes (mostly in dacite rocks of the Yermo volcanic center). (B) Measured fault planes and striae in the Pickhandle Formation. The majority of faults strike northwest, dip >45° and have downdip striae. Approximately 40% of measured surfaces with downdip striae also have subhorizontal striae. (C) Measured fault planes and striae along the Calico fault system. The red fault represents an average of fault data collected from the main Calico fault north-west of the restraining bend (northwest of the study area). Most fault planes dip north-northeast and have striae that rake obliquely from the west-northwest, indicating dextral-reverse slip.

Equal Area

A B C

Equal Area Equal Area

Singleton and Gans


NS

PICKHANDLEPICKHANDLEFORMAFORMATIONTION

0 ~25 mParking lot

Calico fault

NN

1 m

CB

A

D

Figure 7. Selected views of folds in the Calico Member. (A) Folded lacustrine rocks in the Calico Ghost Town parking lot. The photo is a composite of three shots with distortion. The line drawing is a composite profi le view sketch that is approximately true to scale. View is looking west. (B) Anticline and syncline in chert beds near the base of Tbc2. (C) Anticline in lacustrine sandstone, siltstone, and limestone beds (subunit Tbc2). 1.5 m Jacob’s staff for scale. (D) Anticline and syncline in subunits Tbc5 and Tbc6 along Mule Canyon Road. Folds have an amplitude of ~30 m and a north-vergent asymmetry. View is looking northwest.



southwest), and have downdip striae (Fig. 6B). Offset markers and fault plane kinematic indi-cators (e.g., Reidel shears) suggest that most of these dip-slip faults have normal offset, although the amount of normal throw along individual faults is typically <50 m and often only a few meters. The total magnitude of northeast-south-west extension calculated from fault dips and apparent offsets is ~5% (~100 m over a distance of ~2 km). Northwest-striking normal faults are rare in the Calico Member and Yermo dacite, suggesting that much of this faulting occurred prior to ca. 19 Ma and was related to extension in the central Mojave metamorphic core complex. However, the gentle south and southeast tilting of Pickhandle Formation strata is not compatible with northeast-directed extension, suggesting that the tilting may be the composite effect of both extensional and transpressive deformation.

Of measured fault planes with downdip striae, ~40% also have subhorizontal striae (Fig. 6B). On fault planes where the relative timing of slip could be determined, dextral movement overprints dip-slip (mostly normal) movement. These relationships are consistent with the idea that dextral faulting related to the Eastern Cali-fornia shear zone was superimposed on early Miocene northeast-southwest extension.

Calico FaultThe Calico fault is one of the largest strike-

slip faults in the Mojave Desert. In the Rodman Mountains (~30 km southeast of the Calico Mountains) the Calico fault has a maximum dis-placement of 9.8 km (Dibblee, 1964; Oskin et al., 2007). Displacement decreases to the north-west, and the Calico fault appears to break into several strands in the Mud Hills (Fig. 1). The segment of the fault northwest of the restrain-ing bend in the Calico Mountains was estimated to have 1.9–3.2 km of dextral slip based on the apparent offset of the Waterloo and Langtry barite-silver deposits (Fletcher, 1986). In the eastern Mud Hills the main strand of the Calico fault appears to have 1.3 km of slip, based on the apparent offset of a dacite dome at the top of the Pickhandle Formation.

In the study area the Calico fault cuts lacus-trine rocks of the Calico Member, domes of the Yermo volcanic center, and, in the southeastern end of the range, metavolcanic rocks (Plate 1). Two major strands of the Calico fault system exist along the restraining bend, including a poorly exposed fault that fl anks the southern edge of the Calico Mountains east of Calico Ghost Town (Plate 1). This fault is referred to as the Southern Calico fault. The fault previously identifi ed by McCulloh (1965) and Dibblee (1970) as the main Calico fault is referred to as the Calico fault in this study.

Geometry and KinematicsSoutheast and northwest of the study area, the

Calico fault has an average strike of ~N35–40W (Fig. 1). Along the restraining bend in the south-ern Calico Mountains, the Calico fault has an average strike of ~N70W and dips 45º–70°NNE (Plate 1; Fig. 6C). Fault plane striae on the Calico fault and subsidiary fault strands consis-tently rake obliquely from the west-northwest, indicating oblique dextral-reverse slip (Fig. 6C). The reverse (north side up) component of slip can also be inferred from the north-dipping, overturned beds on the south side of the Calico fault, west of Calico Ghost Town (Plate 1). This fault geometry and fault slip data are compatible with a transpressional strain regime in which the principal shortening direction is approximately north-south. Northwest of the restraining bend, the Calico fault dips ≥70°NE and has subhori-zontal striae, suggesting that the transpressional slip regime is primarily restricted to the restrain-ing bend in the southern Calico Mountains (Fig. 6C). The offset Pickhandle Formation–Barstow Formation contact in the easternmost Mud Hills does not appear to be uplifted across the Calico fault, providing further evidence that transpressional shortening across the Calico fault is localized along the restraining bend. However, reverse slip has also been reported along the Calico fault in the Rodman Moun-tains (Glazner et al., 2000; Oskin et al., 2007), indicating that locally the Calico fault accom-modates shortening without the presence of a restraining bend or stepover.

Fault striae on the Southern Calico fault also rake obliquely from the west-northwest, but the dip of this fault varies from near vertical to 48°N over short distances, and near Mule Canyon Road the fault makes an abrupt bend (Plate 1). This irregular geometry suggests that the South-ern Calico fault may have been an older strand of the Calico fault system that became inactive and was deformed.

OffsetThe clearest offset markers along the Cal-

ico fault are found between Mule Canyon and Ghost Town Road. A distinctive 17.1 ± 0.1 Ma dacite dome south of the main Calico fault can be matched to a dacite dome north of the fault at Camp Rock, ~1.2 km to the east-southeast (Plate 1). These domes contain 20%–25% phenocrysts, including 3%–5% hornblende, 2%–4% biotite, and ~1% anhedral quartz. No other dacite rocks near the Calico fault have more than 1% biotite phenocrysts or more than a trace amount of quartz. Both offset domes along the Calico fault intrude a calcareous interval within the Calico Member, and on their eastern margin are in intrusive (?) contact with a distinc-

tive hydrothermally altered dacite (Plate 1). This older dacite is highly fractured and weathered and has a distinct sea-green color from pervasive celadonite alteration (unit Tdc). The northwest-ern margins of both intrusive domes are faulted against crudely stratifi ed, purple dacite breccia that appears to be interbedded with lacustrine rocks on the south side of the Calico fault (Plate 1). Two sets of northeast-striking faults that are cut by the Calico fault northwest of the offset domes are likely the same (Plate 1). These dis-tinct sequences of rocks on opposite sides of the Calico fault match well if 1.1–1.2 km of right-lateral slip along the Calico fault is restored. East of Mule Canyon, the amount of slip most likely increases due to additional northwest-striking faults that either merge with or are cut by the Calico fault. A northeast-striking, northwest-dipping fault along Mule Canyon Road south of the Calico fault may be the offset portion of a similarly oriented fault north of the Calico fault, which would indicate ~1.5–1.6 km of cumula-tive right-lateral slip (Plate 1).

Exposure south of the Southern Calico fault is poor, but two distinctive dacite domes crop out along opposite sides of the fault east of Ghost Town Road and west of Mule Canyon Road (Plate 1). Mineralogically, these domes are similar to most dacite of the Yermo volca-nic center (~5% hornblende phenocrysts, <1% biotite, no quartz), but both domes are distinct in outcrop because of their patchy celadonite alteration and liesegang banding. Assuming that these domes are the same, the southern Calico fault has 1.8–1.9 km of right-lateral slip. Thus, the Calico fault system in the southern Calico Mountains has 3 ± 0.1 km of cumulative right-lateral slip distributed between two faults, and restoring this right-lateral slip concentrates the Yermo dacite dome fi eld into an approximately east-west–trending ellipse. This offset estimate is similar to the 1.9–3.2 km of offset estimated by Fletcher (1986) along the Calico fault north-west of the study area. However, an offset of 3 km should be considered a minimum estimate because additional strands of the Calico fault may exist under alluvium southwest of the Cal-ico Mountains.

The amount of reverse (north side up) slip along the Calico fault system is ambiguous. Based on average slickenline rakes, the main strand of the Calico fault should have 0.2–1 km of reverse slip, corresponding to ~100–500 m of approximately north-south shortening. The Southern Calico fault should have 0.7–1.3 km of north-side-up throw, but the amount of horizon-tal shortening across this fault is unknown due to the variable dip of the fault plane. Thus, if slip vectors measured along the Calico fault system are representative of the entire slip history of

Singleton and Gans


the fault, the cumulative amount of reverse slip along the Calico fault system is ~1–2 km. How-ever, the Calico fault juxtaposes fi ne-grained lacustrine rocks <500 m thick that are presum-ably part of the same section, suggesting that reverse slip along this strand is <500 m. Oblique dextral-reverse slip vectors may only refl ect the most recent movement along the Calico fault system. It is possible that the Manix fault bent the Calico fault into a restraining bend orienta-tion after the Calico fault had been active as a more northwest-trending, purely strike-slip fault (Dibblee, 1994). Accordingly, 1–2 km would be an overestimate of the cumulative amount of reverse slip along the Calico fault system. Given that the amount of reverse slip along the Calico fault is most likely <500 m, the total amount of north-side-up slip along the Calico fault system is probably closer to 1 km, corresponding to ~500 ± 150 m of north-south shortening.

Pretranspressional Slip on the Calico FaultThe signifi cant stratigraphic mismatch of

pre-lacustrine rocks across the Calico fault in the southeastern Calico Mountains strongly suggests there was uplift on the south side of the fault prior to dextral and/or transpressional movement. Metavolcanic basement rocks underlie the Calico Member south of the Calico fault, whereas a thick section of Pickhandle Formation underlies the Calico Member north of the fault. Calico beds across the fault are most likely correlative, indicating that either metavolcanic basement rocks were unroofed of Pickhandle deposits prior to the deposition of the Calico Member (beginning ca. 19 Ma), or that the area south of the Calico fault was a

structural high during Pickhandle deposition. We favor the interpretation that an early Mio-cene proto-Calico fault was a basin-bounding, northeast- to north-northeast–dipping normal fault that enabled >1 km of Pickhandle For-mation rocks to accumulate to the north. The present ~45°–70°NNE dip of the Calico fault is consistent with the inferred dip of this hypo-thetical proto-Calico normal fault, and in one location in the southeastern Calico Mountains, the main Calico fault plane has downdip striae that are overprinted by striae from oblique dextral-reverse movement. The sense of slip on the downdip striae is unclear, but this older dip-slip movement may record early Miocene nor-mal faulting. Northwest of the restraining bend in the Calico Mountains, Pickhandle Formation rocks are present on both sides of the Calico fault (Fig. 1), suggesting that only the restrain-ing bend segment of the fault was a Pickhandle basin-bounding normal fault.

Folds

Folds are common structures across the Mojave Desert, particularly in Miocene lacus-trine rocks (e.g., folds in the Calico Mountains, Alvord Mountains, Lead Mountain area, Mud Hills, Kramer Hills, and Black Canyon area; for locations see Bartley et al., 1990). In most of these areas the age and structural signifi cance of folding are poorly understood. The Calico Member rocks in the southern Calico Mountains expose some of the tightest folds in the central Mojave Desert (Fig. 7). This folding is generally thought to be related to transpression along the Calico fault (Tarman and McBean, 1994; Dib-

blee, 1994; Glazner et al., 1994), yet based on fi eld work to the west of Calico Ghost Town, Weber (1976) proposed that folding was due to south-directed gravitational sliding of the Calico Member off of the underlying Pickhandle For-mation. This interpretation of folding was sup-ported in part by Tarman and McBean (1994), who noted that west of Calico Ghost Town, a south-dipping polished plane separates the Pick-handle Formation from Calico Member, and that some beds appear to be missing from the base of the Calico Member, suggesting that gravity slumping may have formed some folds. How-ever, Tarman and McBean (1994) argued that the upright axial surfaces of many of the folds are not compatible with gravity folding. Another possible explanation for folding is that forceful emplacement of dacite domes into the Calico Member ca. 17 Ma folded the lacustrine rocks.

Folding in the Calico Mountains is primar-ily restricted to the Calico Member north of the Calico fault restraining bend. Folds are detached from the underlying Pickhandle Formation, which homoclinally dips ~15–30°S to SE beneath the Calico Member (Plate 1; Fig. 8A). Calico beds south of the Calico fault are steeply tilted but not folded into the same scale of anticlines and synclines that characterize deformation in the Calico beds north of the Calico fault (Fig. 8).

GeometryThe geometry of folds in the Calico Mem-

ber varies considerably, but several important generalizations can be made. Most folds have steeply dipping axial surfaces (>75°) and shal-lowly plunging axes that trend east-west ±30° (Fig. 9). One exception to this fold orientation is

Equal Area Equal AreaEqual Area

A B C

Figure 8. (A) Poles to measured bedding attitudes in the Pickhandle Formation. Beds generally dip 15–30°S to SE and are not folded. (B) Poles to bedding attitudes in the folded Calico Member north of the Calico fault. Figure only shows attitudes plotted in Plate 1 and does not include bedding from fold hinges. The greater number of south-dipping beds refl ects the overall north-vergent asymmetry of folds in the Calico Member. (C) Poles to bedding attitudes in the Calico Member south of the Calico fault. The triangles represent poles to overturned Calico beds west of Calico Ghost Town (see Plate 1). Folds south of the Calico fault are rare, and west-dipping beds are common.



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orm

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n

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ks

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trus

ions

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ccia

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(Tp)

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al s

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n =

132

n =

111

01

km

Singleton and Gans


Tdb

Figure 10. Contact between the Calico Member and overlying dacite brec-cia (Tdb). Note that Calico beds become more shallowly dipping closer to the contact. The solid white line marks the Calico Member–dacite breccia contact. Dashed white lines are form lines in the Calico Member. View is looking southwest.

a small set of north-south–trending folds on the east side of Mule Canyon Road (Plate 1; Fig. 9). These north-south folds plunge into the core of an east-west–trending syncline, suggesting that they have been refolded by the syncline. Folds are cylindrical and systematically oriented on short length scales, although fold axis orienta-tion, axial surface attitude, and interlimb angle commonly change along the axial trace of a fold (Plate 1; Fig. 9).

One of the most distinct characteristics of the Calico folds is their relatively small size. The largest anticlines and synclines have amplitudes up to ~30 m, but the majority of folds have ampli-tudes of 2–12 m. Most fold sets have wavelengths <100 m, and axial traces are mostly ≤0.75 km long. These small-scale folds are very different than isolated, kilometer-scale folds such as the Barstow syncline in the Mud Hills (Dibblee, 1968), the Box Canyon syncline in the Rodman Mountains (Dibblee, 1964), and the Lenwood anticline near Barstow (Dibblee, 1967).

Many of the folds have a clear north-vergent asymmetry. Although axial surfaces dip nearly equally to the north and south (Fig. 9), north-dipping limbs are usually shorter than south-dipping limbs (Plate 1, Fig. 2). In certain areas folds are symmetric, but south vergence is rare. The majority of folded sandstone, limestone, and chert beds are classifi ed under Ramsay’s class 1B (parallel folds), whereas low compe-tence shale layers are commonly thickened in the hinge area, producing class 2 and class 3 folds (Ramsay, 1967; Fig. 7). Angular, chevron-like fold hinges are common in competent beds (Fig. 7). The presence of some parasitic folds and the strong planar mechanical anisotropy in the Calico Member indicate that folding was accommodated largely by fl exural slip. Most folds have an interlimb angle between ~45° and 90° (average = ~60°–70°). Based on eight restorable cross sections of the Calico beds, the average amount of north-south horizontal shortening represented by the Calico folds is ~25%–30% (see Singleton, 2004, for details). The total shortening due to folding between the Calico fault and the Pickhandle Formation is ~150–500 m.

Map View PatternsIn the eastern half of the study area, folds

parallel the contact with the dacite domes and breccias of the Yermo volcanic center (Plate 1; Fig. 9). This map view pattern is particularly evident north of the volcanic center, where axial traces form a sigmoidal shape that is parallel to the overall contact with the dacite domes and breccias. East of cross section C–C', axial traces wrap around the western edge of the Calico Member–dacite breccia contact, trending

and Odessa Canyon Roads the amount of north-south shortening ranges from 27% to 33%, and the amount of north-northeast–south-southwest shortening near Old Borate is 24%–29%. Along transects where folding accounts for ~500 m north-south shortening (near C–C'; Plate 1; Fig. 2), folding appears to die out near the Pickhandle Formation–Calico Member contact, suggesting that 500 m is an approximate upper limit on shortening due to folding.

Stratigraphic ControlsStratigraphic controls on folding are signifi -

cant in the Calico Mountains. The most obvious contrast in deformation can be seen between the Pickhandle Formation and the Calico Member. The Pickhandle Formation is clearly not folded, whereas Calico beds stratigraphically 5–15 m above the Pickhandle Formation are folded into a series of anticlines and synclines (Plate 1; Figs. 2 and 8). Thus, there is a detachment hori-zon between the top of the Pickhandle Forma-tion and lower part of the Calico Member. The homogeneous, massively bedded Pickhandle Formation lacks the marked planar mechanical anisotropy of the thinly bedded lacustrine sec-tion and was rheologically less able to deform by folding.

The amount of shortening by folding decreases in the upper ~30–100 m of the Cal-ico Member. The dips of beds become con-sistently shallower up toward the contact with the overlying dacite breccia (Fig. 10). Folds rarely exist within 15 vertical m of this contact,

approximately northeast-southwest to the north-west of the contact and northwest-southeast to the southwest of the contact (Plate 1; Fig. 9). West of the Yermo volcanic center, axial traces are consistently more east-west oriented; thus the map view geometry of the folds appears to have been infl uenced by the presence of the Yermo volcanic center. The map view pattern of axial traces is also refl ected by the contact with the underlying Pickhandle Formation (Plate 1; Fig. 9). North of the Yermo volcanic center, the Pickhandle Formation–Calico Member contact also forms a sigmoidal shape that is parallel to the contact with the Yermo dacite rocks.

Axial surface attitudes vary across the study area, but folds with similar axial surface ori-entations generally occur in groups (Fig. 9). There does not appear to be a systematic spa-tial pattern of north-dipping or south-dipping axial surfaces (Fig. 9). Neither fold interlimb angles nor the amount of shortening due to folding vary systematically across the study area (Fig. 9). The folds with the largest aver-age interlimb angles (75°–90°) that represent the smallest percent of north-south shortening (20%–23%) are near Calico Ghost Town at the western end of the folded Calico Member (Fig. 9). The tightest folds (average interlimb angle = 45°–60°) occur in an ~1.5-km-long belt where folds trend northeast-southwest to east-west between the contacts with the under-lying Pickhandle Formation and overlying dacite breccias (Fig. 9). These folds represent 30%–40% shortening. Between Mule Canyon



and shale beds typically dip <30° beneath the breccia (Plate 1). The decrease in dip in the uppermost part of the Calico Member appears to be gradual, but in one location at the south end of Old Borate Canyon, the decrease in shortening is accommodated by a discrete sub-horizontal detachment ~15 m below the Calico Member–dacite breccia contact. Below this detachment shale beds are folded into a small-scale anticline and syncline pair, whereas above the detachment beds dip shallowly toward the dacite breccia. The shallowing of dips and decrease of shortening in the uppermost part of the Calico Member appear to be restricted to beds near the Yermo volcanic center that are capped by dacite breccias (Plate 1).

Timing of FoldingThe shallowing of dips in the uppermost

Calico beds could be explained if sedimentation were synchronous with folding; however, fold growth strata do not appear to be present. Dacite breccias related to the Yermo volcanic center are overall conformable with the underlying Calico Member and do not cut across any fold struc-tures. The dacite breccia sheet (Tdb

bc) interbed-

ded with shale in the upper Calico Member is clearly folded. These observations indicate that folding took place after deposition of the old-est Yermo breccias (ca. 17 Ma). Upper age con-straints on folding are poor due to the lack of post–Calico Member rocks.

Interpretation of FoldingMap view patterns and stratigraphic varia-

tion. The map view pattern of fold axial traces suggests there is a geometric relationship between the folds and the dacite rocks of the Yermo volcanic center. One possible explana-tion for the parallelism between the axial traces and the contacts with the dacite domes and breccias is that folding was caused by the force-ful intrusion of dacite into the Calico Member ca. 17 Ma. Calico beds adjacent to intrusions are steepened and generally strike parallel to the margins of the intrusions, but this deforma-tion appears to be largely restricted to an area within ~150 m of the intrusion contact (Plate 1). Some small-scale folds occur in beds deformed by dome emplacement, but unlike most folds in the Calico Member, the geometry of these intrusion-parallel folds is highly irregular, and the axial traces are <150 m long. Dome emplacement as a mechanism for folding also does not explain the presence of east-west–trending folds far west of the dome fi eld, nor does this mechanism account for the overall lack of folding south of the Calico fault. In addition, evidence that folding took place fol-lowing deposition of the oldest Yermo dacite

fault are not tighter than northeast-southwest–trending folds, and fold interlimb angles do not decrease along strike to the west (closer to the Calico fault; Fig. 12). Other observations that argue against a wrench folding model include: (1) east-west– and east-southeast–west-north-west–trending folds north of the Yermo volca-nic center are not located near a zone of higher shear strain, and (2) east of cross-section C–C', northwest-southeast–trending folds are located due south of northeast-southwest–trending folds (Plate 1; Fig. 9), resulting in a map view pattern that is incompatible with progressive clockwise rotation of folds.

The most likely explanation for the map view pattern of fold axial traces is that the Yermo dac-ite domes and breccias resisted shortening, forc-ing the Calico beds to wrap around these rocks. This buttressing infl uence of the rigid Yermo volcanic center during north-south shortening caused folds to become oriented parallel to the overall contact of the dacite domes and brec-cias. The decrease in shortening in the upper-most part of the Calico Member can be similarly explained by assuming that the thick breccia sheets adjacent to domes resisted folding. Cal-ico beds close to the dacite breccia contact were not able to fold due to the mechanical rigidity of the overlying breccias. West of cross-section C–C' (west of the Yermo volcanic center; Plate 1), folds are consistently oriented approximately

Figure 11. Relationship between fold trend and fold interlimb angle using data from folds in the Calico Member north of the Calico fault. There is no systematic relationship between fold trend and fold tightness. If folding was related to wrench faulting, folds that parallel the trace of the Calico fault theoretically should have smaller interlimb angles than the northeast-southwest–trending folds.

breccias suggests that folding is younger than dome emplacement.

Another explanation for the overall sigmoidal pattern of fold axial traces involves the forma-tion and subsequent rotation of originally north-east-southwest–trending folds by right-lateral shear. If folds were originally oriented more northeast-southwest, the east-west– trending folds west of cross-section C–C' (Plate 1) could have rotated clockwise due to higher dextral shear strain adjacent to the Calico fault. This model interprets the folds as wrench folds along the Calico fault. Most experimental models pre-dict that folds associated with wrench faults will initially form at ~45° to the trace of the mas-ter fault (i.e., perpendicular to the incremental shortening direction; Odonne and Vialon, 1983; Jamison, 1991; Tikoff and Peterson, 1998). With progressive shear, folds rotate toward parallel-ism with the master fault, becoming tighter as they rotate (Fig. 11). If folding in the southern Calico Mountains was produced by this mecha-nism, folds that parallel the trace of the Calico fault (~N70W) should have smaller interlimb angles than the northeast-southwest–trending folds, and folds should become progressively tighter westward (closer to the Calico fault). However, there are no systematic relationships between the fold orientation, interlimb angle, and distance from the Calico fault (Figs. 9, 11, and 12). Folds that are subparallel to the Calico

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160 180

Fold trend vs. interlimb angle

N-S N-S

E-W

Fold trend

Inte

rlim

b a

ng

le (

deg

rees

)

average strike of the

Calico fault (range:100-130O)

hypothetical pattern due to tightening

of wrench folds with progressive rotation

(based on calculations by Jamison, 1991)

30

30

50

55

63

60

wrench fault (strike = 110o)

Singleton and Gans


ber to be thrust over the Pickhandle Formation along the south-dipping contact–folding detach-ment. The dominant north-vergent asymmetry of folds is compatible with this sense of movement along the detachment. The total amount of north-directed reverse slip along the detachment must be at least equal to north-south shortening due to folding in the hanging wall (up to ~500 m). In Mule Canyon, there is evidence that folding dies out just south of the contact (e.g., cross-section C–C'; Fig. 2), suggesting that the detachment is a blind fault with slip progressively decreas-ing northward to where the Tp-Tbc detachment becomes a normal stratigraphic contact. A right-lateral component of movement along the Tp-Tbc contact can be inferred from the change in the orientation of Pickhandle beds at the con-tact. West of Mule Canyon, southeast-dipping Pickhandle beds abruptly bend into parallelism with south-dipping Calico beds adjacent to the contact (Plate 1), consistent with drag due to right-lateral slip along the contact.

Along the Tp-Tbc contact to the east of Calico Ghost Town, there does not appear to be a dis-crete slip surface along which reverse-dextral movement occurred, suggesting that movement may have been largely distributed throughout the sandstone beds at the base of the Calico Member. West of Calico Ghost Town, the absence of the basal sandstone beds and the presence of a south-dipping, bedding-parallel fault surface between the Pickhandle Formation and the Calico Mem-ber indicate that slip along the contact–folding detachment in this area probably occurred along a discrete fault. Two sets of striae are present on this surface; one set rakes 67° from the west and a more subtle set rakes 36° from the east. The east-raking striae are compatible with dextral-reverse slip along the folding detachment. As the detach-ment approaches the Calico fault (e.g., near Cal-ico Ghost Town), it steepens and is inferred to root into the main Calico fault (Fig. 2), forming a fault zone geometry that resembles a positive fl ower structure.

The north-vergent asymmetry of folds and the lack of numerous folds south of the Calico fault indicate that the Tp-Tbc contact played a fundamental role in folding. If slip along the main Calico fault was primarily responsible for folding, folds might have a south-vergent asym-metry, and lacustrine rocks south of the fault would most likely also be folded into small-scale anticlines and synclines. The presence of north-vergent folds only north of the Calico fault can be explained with a model in which north-south compression between the Calico Member and the more rigid Pickhandle Formation above a basal detachment drove folding of the lacustrine rocks. Calico beds south of the Calico fault were not thrust over a south-dipping basal detachment

Figure 12. Along strike variation in fold interlimb angle. Only interlimb angles from the relatively continuous belt of folds shown in the map inset were plotted. If the approxi-mately east-west–trending folds west of the Yermo dacite domes and breccias (Ty) rotated from more northeast-southwest orientations, there should be a progressive tightening of folds to the west (closer to the Calico fault). However, the tightest folds are located in a northeast-southwest– to east-west–trending belt between the dacite rocks and the Pick-handle Formation (Tp).

Ca

lico

Gh

ost T

ow

n

Old

Bo

rate

Ca

nyo

n

E-W position (km)

11

6°

50'

W E

Along strike variation in fold interlimb angle(Interlimb angle vs. E-W position)

Inte

rlim

b a

ng

le (

deg

rees

)

Distance from Calico fault (m): < 250 300-800 800-1200 1200-2000 > 2000

E-W trending folds NE-SW folds E-W trending folds ESE-WNW trending folds

TpOnly folds shown in map inset plotted

Ty

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8

east-west, and several close folds are present in the inferred uppermost part of the section. These observations are consistent with the idea that the geometry of Calico beds in this area was not infl uenced by dacite intrusions and breccias from the Yermo volcanic center.

The presence of a folding detachment hori-zon between the Calico Member and the Pick-handle Formation might be used to argue that south-directed gravity gliding was responsible for folding the Calico beds. If folding were due to the Calico Member sliding off of the south- dipping Pickhandle Formation, folds would most likely have south-vergent asymmetry and axial surfaces that dip north, away from the direction of transport. Alternatively, if gravity folding occurred when the Calico beds were still unconsolidated, folds might have highly irregu-lar geometries that are characteristic of soft sed-iment deformation and slumping. None of these hypothetical geometries is consistent with folds in the Calico Mountains. Folds in the Calico Member are systematically oriented and have upright axial surfaces that dip nearly equally to the north and south (Fig. 9). Moreover, the dom-inant sense of fold asymmetry is north vergent,

suggesting north-directed (reverse) transport above the folding detachment.

Transpression. The location of folds adjacent to a transpressional restraining bend constitutes the most basic evidence that folding is tectonic and related to the Calico fault system. Calico Member beds northwest of the Calico fault restraining bend are generally not intensely folded or overturned (Dibblee, 1970), sug-gesting that shortening in the southern Calico Mountains is fundamentally related to trans-pression within the restraining bend.

The Pickhandle Formation (Tp)–Calico Member (Tbc) contact represents an important break in the style and magnitude of deforma-tion. Calico beds north of the Calico fault have undergone 25%–33% north-south shortening due to folding, whereas the Pickhandle Forma-tion generally lacks folds and reverse faults. Slip along the Tp-Tbc contact is necessary in order to accommodate detachment folding. A few observations suggest that this contact has both a reverse and right-lateral component of move-ment that is compatible with transpressional slip along the Calico fault. North-south shortening would presumably have forced the Calico Mem-



and therefore deformed differently than their counterparts north of the fault (Fig. 8).

DISCUSSION

Interpretive Stratigraphic and Structural History of the Southern Calico Mountains

Based on geologic mapping, fi eld observa-tions and 40Ar/39Ar geochronology, a schematic Neogene geologic history of the southern Calico Mountains is presented in Figure 13. Between ca. 19.4 and 19 Ma, a thick (>1 km) section of coarse volcaniclastic rocks and dacite domes and fl ows of the Pickhandle Formation accumu-lated. The absence of the Pickhandle Formation south of the Calico fault suggests a model in which a northeast- or north- northeast-dipping proto-Calico normal fault uplifted metavol-canic basement rocks in the footwall and cre-ated a Pickhandle basin in the hanging wall (Fig. 13A). Alternatively, the proto-Calico fault may have unroofed metavolcanic basement rocks of Pickhandle deposits. In either scenario, slip along this inferred proto-Calico fault must have ceased prior to the deposition of fi ne-grained lacustrine rocks of the Calico Member, which unconformably overlie the Pickhandle Formation north of the Calico fault and directly overlie footwall metavolcanic rocks south of the Calico fault. Pickhandle dacite domes emplaced ca. 19 Ma most likely formed topographic highs that marked the northern margin of the shallow lake in which Calico Member sediments accu-mulated (Fig. 13A). Both the Pickhandle For-mation and at least the older part of the Calico Member were deposited during rapid slip along the Waterman Hills detachment fault (Gans et al., 2005), although most normal faulting in the southern Calico Mountains appears to predate deposition of the Calico Member.

The Yermo volcanic center became active dur-ing the waning stages of lacustrine sedimentation. Between 17.1 Ma and 16.8 Ma several dacite domes intruded Calico Member sediments north and south of the Calico fault (Figs. 13B, 13C). Coarse dacite breccias shed from the domes locally fi lled the shallow lake and precluded lacustrine sedimentation. East and west of the Yermo volcanic center, dacite breccia sheets are overlain by as much as 60 m of Calico Member beds, indicating that lacustrine deposition con-tinued locally during volcanic activity.

Strike-slip and transpressional deformation in the southern Calico Mountains postdates the formation of the Yermo volcanic center (post-16.8 Ma) and refl ects the dominant style of post–early Miocene deformation in the central Mojave Desert. A portion of the proto-Calico normal fault was reactivated as a dextral-reverse

fault (Fig. 13D). Transpressional faulting and folding are fundamentally related to the west-northwest–striking restraining bend in the Calico fault system. It is possible that the proto-Calico fault was west-northwest striking, which would indicate that the restraining bend is an original feature of the Calico fault system. If the restrain-ing bend is original, transpressional deforma-tion may have initiated early in the history of the right-lateral Calico fault system, perhaps in the middle or late Miocene. Alternatively, the restraining bend may be the result of coun-terclockwise rotation due to movement on the east-northeast–striking, left-lateral Manix fault (Dibblee, 1994), implying that transpressional deformation may postdate some right-lateral movement on the Calico fault. Approximately 3 km of right-lateral slip and perhaps 1 km of reverse slip have occurred along two major strands of the Calico fault system.

Transpression along the Calico fault restrain-ing bend forced the Calico Member north of the fault to detach along its base and move over the south-dipping Pickhandle Formation–Calico Member contact in a reverse-dextral sense (Fig. 13D). The thinly bedded Calico Mem-ber responded to this transpression by folding into numerous anticlines and synclines that account for 25%–33% north-south shortening. The Pickhandle Formation and dacite rocks of the Yermo volcanic center resisted folding and acted as rigid buttresses against which folds in the Calico Member developed. Post–16.8 Ma deformation within the Pickhandle Formation and Yermo dacite rocks consisted primarily of strike-slip and oblique strike-slip-reverse fault-ing that was broadly synchronous with folding in the Calico Member.

Miocene Stratigraphic Framework for the Central Mojave Desert

The prevailing tectonostratigraphic frame-work of early to mid-Miocene rocks in the central Mojave Desert is based on northeast-southwest extension associated with the central Mojave metamorphic core complex. Coarse volcaniclastic rocks of the Pickhandle Forma-tion have been interpreted as synextensional deposits ranging in age from ca. 24 to 19 Ma (Fillmore and Walker, 1996), whereas overly-ing, fi ne-grained Barstow Formation rocks are considered postextensional deposits that infi lled remnant basins (Fillmore and Walker, 1996; Ingersoll et al., 1996). This tectonostratigraphic model is fl awed in detail. Thermochronologic data indicate that rapid slip on the Waterman Hills detachment fault and extensional unroofi ng of the central Mojave metamorphic core com-plex footwall occurred largely between ca. 21

and 17.5 Ma (Gans et al., 2005). Based on new geochronologic data from this study, the thick section of Pickhandle Formation in the Calico Mountains accumulated rapidly from ca. 19.4 to 19 Ma, and ~400 m of fi ne-grained lacustrine rocks were deposited between ca. 19 and 17 Ma. Thus, the Pickhandle Formation and at least the older part of the Calico Member of the Barstow Formation accumulated during large-magnitude extension in the central Mojave metamorphic core complex. The association of coarse-grained volcaniclastic deposits with extensional basin development and fi ne-grained lacustrine depos-its with postextensional sedimentation can be misleading. Assuming that extension in the cen-tral Mojave metamorphic core complex did not begin until ca. 21 Ma (Gans et al., 2005), Pick-handle Formation rocks that range in age from 24 to 21 Ma may be preextensional deposits that refl ect proximity to local volcanic centers rather than extensional basin development. Similarly, the coarse breccia sheets that overlie the Calico Member in the southeastern Calico Mountains apparently postdate extension and are instead a consequence of steep volcanic topography cre-ated by the Yermo volcanic center.

Although most of the Barstow Formation was deposited after extension in the central Mojave metamorphic core complex had ended, synex-tensional, 19–17 Ma Barstow Formation lacus-trine deposits appear to be more widespread than previously recognized. In the easternmost Mud Hills and northwestern Calico Mountains the Pickhandle Formation grades into comform-ably overlying fi ne-grained lacustrine beds (Sin-gleton, 2006, personal observ.), suggesting that Calico Member sedimentation extended from the southeastern Calico Mountains to the east-ernmost Mud Hills (Fig. 1). Van Pelt and Gans (2005) bracketed the timing of fi ne-grained lacustrine sedimentation in the Lead Mountain area between ca. 19.3 and 17.2 Ma (Fig. 1), and north of Daggett Ridge several hundred meters of fi ne-grained lacustrine beds comformably overlie the ca. 18.5 Ma Peach Springs Tuff (Wells and Hillhouse, 1989; Dibblee, 1970). Lacustrine deposits in the central Mojave Des-ert are commonly assumed to correlate to the postextensional type section of the Barstow For-mation in the Mud Hills, yet many of these cor-relations appear to be inaccurate and overlook earlier synextensional lacustrine sedimentation.

Extensional Deformation

The paucity of large normal faults and the gentle tilts of strata in the Pickhandle Forma-tion suggest that much of the Calico Mountains behaved as a fairly coherent block during exten-sion in the central Mojave metamorphic core

Singleton and Gans


metavolcanic rocks (mv)

mv?

mv

mv

Tps

Pickhandle

Fm. (Tps)

~19 Ma dacite intrusion (Tpd)

Tpd

proto-Calico fault

Tc1

Calico Member (Tbc)

Tdb

Tc1

1 (SSW) (NNE) 1'

2 (SSW) (NNE) 2'

0 1 2 km

N

26 A)

B0 1 2 km

0

1

2km

TbcTbc

ca. 17.2 Ma

ca. 16.8 Ma

21-17.5 Ma NE-SW regional extension

granite (gr)

gr

post-16 Ma

0

1

2km

Tdi Tpd

A

C

Tpd

2

Calico Member sediments(beneath shallow lake)

Pickhandle Formation

dacite breccia

15

proto-Calico fault

ca. 16.8 Ma

Yermo volcanic center

2'

Yermo volcanic center

2'

D 0 1 2 km

0

1

2km

mv

Tps

TdiTpd

Calico fault

S. Calico faultTc3

Tc1

Tc2

3 (SSW) 3' (NNE)

folded Calico Member

N-S shortening

0 1 2 km

Tpd

Figure 13. Schematic reconstruction illustrating sequential stratigraphic and structural evolution of the southern Calico Mountains. (A) Cross-section 1–1': geometry of early Miocene sedimentation prior to the formation of the Yermo vol-canic center (ca. 17.2 Ma). More than 1 km of dacite rocks and volcaniclastic sediments of the Pickhandle Formation accumulated rapidly in a basin formed by normal slip along a north-northeast–dipping proto-Calico fault. Extensional faulting within the Pickhandle Formation is minor. Beginning ca. 19 Ma, slip along the proto-Calico fault decreases or stops, and fi ne-grained lacustrine sedimentation begins. Approximately 200–400 m of lacustrine sandstone, shale, and limestone accumulate in a lake that extends into the footwall of the proto-Calico fault, where the Calico Member is deposited directly on metavolcanic basement rocks. (B, C) Schematic cross-section (2–2') and geologic map after the formation of the Yermo volcanic center (ca. 16.8 Ma). Note that the map scale is one-half the scale of cross-section 2–2'. Between 17.1 and 16.8 Ma, the approximately east-west–trending dacite dome fi eld erupted into a shallow lake. Breccia sheets shed from the domes were deposited on lacustrine sediments, locally fi lling up the lake. West of the Yermo volcanic center, fi ne-grained lacustrine sedimentation continued during dome emplacement. (D) Cross-section 3–3': schematic geometry of post 16.8 Ma transpressional deformation. The proto-Calico fault was reactivated as an oblique dextral-reverse fault within a restraining bend of the Calico fault system. Transpression resulted in reverse-dextral movement between the Calico Member and the south- to southeast-dipping Pickhandle Formation, folding the fi ne-grained lacus-trine rocks into small-scale anticlines and synclines. Northwest-trending normal faults within the Pickhandle Formation were reactivated as dextral faults. See Plate 1 for key to rock unit symbols.



complex. High-angle, northwest-striking normal faults of inferred early Miocene age are present within the Pickhandle Formation, yet northeast-southwest extension associated with these faults is minor (~5%), and the dominant sense of normal slip (top to the southwest) is antithetic to shear across the central Mojave metamor-phic core complex. In the study area the Pick-handle Formation generally dips <30°SE and S (Fig. 8A). Across the central Calico Mountains, Pickhandle beds dip <20° in various directions (Dibblee, 1970). With the exception of the sec-tion along Fort Irwin Road, the Pickhandle Formation in the Calico Mountains never dips homoclinally to the southwest, which would be expected for northeast-directed extension in the upper plate of the central Mojave metamorphic core complex.

The apparent lack of signifi cant extension may be somewhat surprising given the proxim-ity of the Calico Mountains to lower plate rocks of the central Mojave metamorphic core com-plex, but the geometry of Pickhandle Formation strata in several other areas is also incompatible with northeast-southwest extension. For exam-ple, Pickhandle strata in the Lead Mountain area are folded and generally strike northeast-south-west (Fig. 1; Dibblee, 1970). The Pickhandle Formation in the Gravel Hills (northwest of the Mud Hills) dips gently to the south (Dib-blee, 1968). It is possible that strike-slip and/or transpressional deformation overprinted exten-sion, or that some of these south- to southeast-dipping strata rotated counterclockwise about a vertical axis from previous southwest-dipping orientations. However, most paleomagnetic data from the central Mojave Desert suggest clockwise rotation (see Glazner et al., 2002, for a review). More detailed mapping is needed to shed light on the style and magnitude of upper plate extension in the central Mojave Desert.

Faulting

Right-lateral slip along northwest-striking faults represents the dominant style of post–early Miocene deformation across the central Mojave Desert. New fi eld data from the south-ern Calico Mountains suggest that many of these faults may have complex histories that date back to at least the early Miocene. The stratigraphic mismatch of pre–Calico Member rocks across the Calico fault restraining bend cannot be explained with dextral and/or reverse slip. Prior to transpressional deformation, there appears to have been a signifi cant normal (southwest side up) component of slip along the Calico fault. Similarly, northwest-striking, early Miocene (?) normal faults in the Pickhandle Formation are overprinted by right-lateral slip. Early Miocene

northeast-southwest extension associated with the central Mojave metamorphic core complex most likely created numerous northwest- striking normal faults, many of which may have been reactivated as right-lateral faults.

Folding and Transpression

Several geologists have argued that north-south crustal contraction associated with strike-slip faulting dominates post–early Miocene deformation across most of the Mojave Desert region (e.g., Bartley et al., 1990; Glazner et al., 2002). East-west–trending folds are generally thought to be a consequence of this regional, crustal contraction, yet there are very few areas where the detailed geometry, style, and timing of folding have been documented. New geo-logic mapping, structural data, and interpreta-tions presented here illustrate several important characteristics of folds in the southern Calico Mountains, including the following.

1. The numerous approximately east-west–trending, upright folds are largely restricted to the fi ne-grained lacustrine beds north of the Calico fault and are detached from the under-lying Pickhandle Formation. Basement rocks are clearly not involved in the folding and are instead shortened by transpressional faulting.

2. Folding in the Calico Member represents ~25%–33% north-south shortening (up to 0.5 km), and thus does not account for several kilometers of shortening, as proposed by Glazner et al. (1994). Shortening due to reverse slip along the Calico fault system is not as well constrained, but is estimated to be ~0.5 km. Thus, transpres-sional folding and faulting across the southern Calico Mountains probably accounts for ~1 km of shortening. This shortening most likely absorbed some of the dextral shear along the Calico fault system and may be partly respon-sible for the northward decrease of dextral offset along the Calico fault (Oskin et al., 2007).

3. The map-scale geometry of folds in the southern Calico Mountains is strongly infl u-enced by boundary conditions, including the presence of a volcanic dome fi eld (the Yermo volcanic center) that acted as a rigid buttress and resisted folding. The interpretation of folds in the Mojave Desert should take into consid-eration specifi c boundary conditions that may affect deformation.

4. Folding in the southern Calico Mountains postdates the formation of the Yermo volcanic center (post ca. 17 Ma) and is temporally and spatially associated with transpressional slip along the Calico fault system. However, this folding is not classic wrench folding, where folds initiate obliquely to a strike-slip fault and then rotate toward parallelism with the fault during

progressive shear. Transpressional folding and faulting in the southern Calico Mountains is also inconsistent with a kinematic model in which regional-scale shortening is accommodated by conjugate dextral and sinistral faulting. Instead, folds are localized north of the restraining bend in the Calico fault system and accommodate a component of north-south shortening associated with transpressional, dextral-reverse faulting. In the Calico Mountains transpression appears to be restricted to this restraining bend region, and is not indicative of regional contraction, as proposed by Bartley et al. (1990). Although north-south contraction may be a regional phe-nomenon in the Mojave Desert region (Bartley et al., 1990), some of the best examples of short-ening in the central Mojave Desert may refl ect localized transpression more than regional con-traction. For example, the Su Casa basement arch and east-west–striking overturned beds are localized adjacent to the restraining bend in the Camp Rock fault (Dibblee, 1970; Dokka, 1986). The Barstow syncline occurs in a stepover zone between the Calico fault and the Blackwater fault zone (Dibblee, 1968), and the Lenwood anticline is located along a left-stepping bend in the Len-wood fault (Dibblee, 1967; Glazner and Bartley, 1994). Shortening does not appear to be homo-geneously distributed across the Mojave Desert, and localized transpression may play an impor-tant role generating contractional structures.

CONCLUSIONS

The Neogene geologic history of the Calico Mountains includes synextensional early Mio-cene sedimentation and volcanism, followed by transpressional faulting and folding. New 40Ar/39Ar geochronology ages indicate that most of the type section of the Pickhandle Forma-tion accumulated rapidly between ca. 19.4 and 19.0 Ma. The lack of a thick section of Pickhan-dle Formation beneath the Calico Member of the Barstow Formation south of the Calico fault suggests that a northeast- or north- northeast–dipping proto-Calico normal fault unroofed metavolcanic basement rocks in the footwall and created a Pickhandle Formation basin in the hanging wall. This inferred extensional basin development must have ceased prior to the deposition of fi ne-grained lacustrine rocks of the Calico Member, which unconformably overlie the Pickhandle Formation north of the Calico fault and directly overlie metavolcanic rocks south of the Calico fault. The 40Ar/39Ar geochronology brackets the age of fi ne-grained lacustrine sedimentation north of the Calico fault between ca. 19 and 16.9 Ma, indicating that the Calico Member is older than the type section of the Barstow Formation in the Mud Hills. The

Singleton and Gans


Pickhandle Formation and at least the older part of the Calico Member were deposited dur-ing rapid slip along the Waterman Hills detach-ment fault, yet northeast-southwest extension within the Pickhandle Formation is only ~5%. The Yermo volcanic center in the southeastern Calico Mountains was the site of calc-alkaline dacite dome emplacement between 17.1 and 16.8 Ma. This dacite volcanism was active dur-ing the late stages of lacustrine sedimentation. Previous geochronologic investigations have not recognized ca. 17 Ma volcanism in the cen-tral Mojave Desert.

Strike-slip faulting and transpression are the dominant styles of post–early Miocene defor-mation in the southern Calico Mountains. The north-northeast–dipping Calico fault system forms a restraining bend that accommodates dextral-reverse slip. Based on the apparent off-set of dacite domes of the Yermo volcanic cen-ter, the Calico fault restraining bend system has ~3 km of right-lateral slip and perhaps 1 km of reverse slip distributed on two main faults. The cumulative amount of dextral shear across the southern Calico Mountains is ~4.1 km.

Deformation within the Calico Member north of the Calico fault is taken up primarily by folding. Numerous approximately east-west–trending, upright folds represent 25%–33% north-south shortening (up to ~0.5 km). Dacite domes and breccias of the Yermo volcanic cen-ter resisted folding and deformed instead by strike-slip and transpressional faulting. Folds in the Calico Member are detached from the homoclinally south- to southeast-dipping Pick-handle Formation. We interpret the Pickhandle Formation–Calico Member contact as a reverse-dextral fault zone that is part of a positive fl ower structure along the Calico fault restraining bend. Transpression within this fl ower structure was responsible for folding the Calico Member. The geometry and map view pattern of folds are not compatible with gravity folding, folding due to dacite dome emplacement, or wrench folding. Transpressional folding and faulting in the Cal-ico Mountains is localized along the Calico fault restraining bend and is not indicative of regional north-south contraction.

ACKNOWLEDGEMENTS

This work was funded by Rio Tinto Industrial Min-erals Exploration. We thank several Rio Tinto borate exploration geologists, particularly John Reynolds, for valuable discussions on Mojave geology. Thought-ful reviews by Mike Oskin and John Fletcher have improved this manuscript and are greatly appreciated.

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