The University of Maine
DigitalCommons@UMaine
Earth Science Faculty Scholarship
Earth Sciences
6-1-1992
Thermal and Barometric Constraints on the
Intrusive and Unroofing History of the Black
Mountains: Implications for Timing, Initial Dip,
and Kinematics of Detachment Faulting in the
Death-Valley Region, California
Daniel K. Holm
J. Kent Snow
Daniel R. Lux
University of Maine - Main, [email protected]
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Repository Citation
Holm, Daniel K.; Snow, J. Kent; and Lux, Daniel R., "Thermal and Barometric Constraints on the Intrusive and Unroofing History of
the Black Mountains: Implications for Timing, Initial Dip, and Kinematics of Detachment Faulting in the Death-Valley Region,
California" (1992). Earth Science Faculty Scholarship. Paper 107.
http://digitalcommons.library.umaine.edu/ers_facpub/107
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TECTONICS,VOL. 11, NO. 3, PAGES507-522,JUNE 1992
THERMAL
AND BAROMETRIC
CONSTRAINTS
ON THE INTRUSIVE
AND
UNROOFING
HISTORY
OF THE BLACK
MOUNTAINS:
IMPLICATIONS
FOR
centralArizonaandsoutheastern
California. Beginningwith an
TIMING,
footwallflexuresoccurbelowa scoop-shaped
hangingwall
block.Oneconsequence
of thismodelis thatgentlydipping
ductilefabricsdeveloped
in themiddlecruststeepen
in the
uppercrustduringunloading.
Thisprocess
resolves
thelow
initialdipsobtainedherewithmappingwhichsuggests
transport
of theupperplateonmoderately
tosteeply
dipping
INITIAL
KINEMATICS
FAULTING
REGION,
DIP, AND
OF DETACHMENT
IN THE DEATH
VALLEY
CALIFORNIA
DanielK. Holml andJ. KentSnow
2
Department
of EarthandPlanetary
Sciences,
Harvard
initiallylistticgeometry,
a patternof footwallunroofing
accompanied
bydikeintrusion
progresses
northwestward.
This
patternmaybeexplained
by a modelwheremigration
of
surfacesin themiddleanduppercrust.
University,Cambridge,Massachusetts
INTRODUCTION
Daniel R. Lux
Department
of Geology,Universityof Maine,Orono
Determining
thetiming,pattern,andamountof unroofing
duringorogeny
isfundamental
tounderstanding
processes
of
lithospheric
deformation.
In theBasinandRangeprovince,
geologic
studies
usingfluidinclusions
[ParryandBruhn,1987]
Abstract.Unroofingof theBlackMountains,
DeathValley,
California,hasresultedin theexposureof 1.7 Ga crystalline
basement,
latePrecambrian
amphibolitefaciesmetasedimentary
andbalancedandreconstructed
crosssections
[Bartleyand
Wernicke,1984;WernickeandAxen, 1988]indicate10-15 km,
rocks,
andaTertiary
magmatic
complex.
The40Ar/39Ar
cooling
ages,obtained
fromsamples
collected
across
theentire
lengthof therange(>55km),combined
withgeobarometric
resultsfromsynextensional
intrusions,
providetime-depth
constraints
on the Mioceneintrusivehistoryandextensional
unroofingof theBlackMountains.Datafromthesoutheastern
BlackMountainsandadjacent
Greenwater
Rangesuggest
unroofing
fromshallowdepthsbetween9 and10Ma. To the
northwest
in thecrystalline
coreof therange,biotiteplateau
agesfrom~ 13 to 6.8 Ma fromrocksmakinguptheDeath
Valleyturtlebacks
indicatea midcrustal
residence
(with
temperatures
>300øC)priorto extensional
unroofing.Biotite
andperhaps
upto20 km,of footwallunroofing
forsome
mountain
ranges.As thesedepths
generally
correspond
to
temperatures
between
200øC
and500øC,
40Ar/39Ar
thermochronologic
analyses
of minerals
withclosure
temperatures
in thisrangeprovide
a testof theamount
and
timingof unroofing
duringCenozoic
extension
[Foster
etal.,
1990;Richardet al., 1990]. With a regionaldistribution
of
ages,
thermochronology
canalso
beused
todetenfiine
theareal
pattern
of cooling,
andthusprovide
a powerful
testof structural
models of extension.
The BlackMountainsof DeathValley,California(Figure1)
plutons
reveala diachronous
coolingpatternof decreasing
ages
towardthenorthwest,subparallel
to theregionalextension
direction.Diachronous
coolingwasaccompanied
by dike
arecentrally
located
in theDeathValleyextended
terrain
between
theSpringMountains
totheeastandtheSierraNevada
tothewest.Isotopic,
field,andpetrologic
studies
[Asmerom
et
al., 1990;HolmandWernicke,1990]showtheBlack
Mountains
toconsist
predominantly
of a Miocenemidcrustal
magmatic
system
(intermediate-mafic
tosilicic)intruded
into
intrusionwhichalsodecreases
in agetowardthenorthwest.
Proterozoicbasementandlesseramountsof miogeoclinal
strata
Thecoolingagepatternandgeobarometric
constraints
in
crystalline
rocksof theBlackMountains
suggest
denudation
of
10-15 km alonga northwest
directeddetachment
system,
consistent
with regionalreconstructions
of Tertiaryextension
andwithunroofingof a northwest
deepening
crustalsection.
Mica coolingagesthatdeviatefromthenorthwest
younging
metamorphosed
toamphibolite
facies[Otton,1977].Although
40Ar/39Ar
thermochronologic
studies
insome
surrounding
ranges
haveproventobeproblematical
[e.g.,DeWittetal.,
40Ar/39Ar
ages
fromboth
Precambrian
basement
andTertiary
trendareconsistent
with northwestward
transport
of rocks
initiallyat shallowercrustallevelsontodeeperlevelsalong
1988],preliminary
results
fromtheBlackMountains
metamorphic
core
complex
yielded
well-behaved
40Ar/39Ar
systematics
in bothProterozoic
basement
rocksandTertiary
plutons[Holmet al., 1989;Asmerom
et al., 1990;McKenna,
1990]. ThustheBlackMountainsprovidean excellent
splaysof thedetachment.
Thewell-known
Amargosa
chaos
andperhaps
theBadwater
turtleback
areexamples
of this
opportunity
tostudythethermal
history
of a rangeblock
"splaying"process.
cooling
agedataandbarometry
onsynextensional
intrusions
andcrystalline
basement
rocksfrom18localities
distributed
throughout
theBlackMountains.
Thesethermochronologic
Considering
thecurrentdistance
of thestmcturally
deepest
samples
awayfrommoderately
to steeplyeasttiltedTertiary
stratain the southeastern
Black Mountains,thesedataindicate
anaverage
initialdipof thedetachment
system
of theorderof
20ø, similarto thatdeterminedfor detachment
faultsin west
1NowatDepartment
of Geology,
KentState
University,
Kent, Ohio.
2 NowatDivisionof Geology
andPlanetary
Sciences,
CaliforniaInstituteof Technology,Pasadena,California.
Copyright1992 by the AmericanGeophysical
Union,
unroofed
during
Tertiary
extension.
Wepresent
40Ar/39Ar
andbarometric
resultsplaceimportant
constraints
onthetime,
amount,andstyleof intrusionandunroofingof footwallrocks
exposed
by crustalextension
in theDeathValleyregion.
GEOLOGIC
FRAMEWORK
Crustalextension
in theDeathValleyregionhasoccurred
principallybetween15Ma andthepresent.Reconstruction
of
Neogeneextension
in theregionbetweenthestableSpring
Mountainsblockandthe SierraNevadarestores
rangeblocks
Papernumber92TC00211
0278-7407/92/92TC-0021510.00
now scattered over an area 150 km wide into an area less than
10 km wide [Wernickeet al., 1988; SnowandWernicke,
508
Holmet al.: Unroofingof theBlackMountains,
DeathValley
ion•
direction
ß
ßFIGURE
,
2
........
ß
,,.,.,,.
ß ,
Quaternary
.,,
.,..
'.i[--•']Pre-Quaternary
Fig. 1. Index map of the DeathValley regiondepictingranges
andmajor faults.AbbreviationsareNFZ, NorthemDeath
Valley-FumaceCreekFault zone;SDF, SouthemDeathValley
fault;SI-IF,Sheephead
fault;GF, Garlockfault.
1989]. Accordingto thesestudies,
regionalwestwardmigration
of extensionalongdown-to-the-west
detachment
faultsresulted
in the sequentialdetachmentof crustal-scale
sliversfroma
westwardmigratinghangingwall block[e.g.,Asmeromet al.,
1990]. Otherstudiesin theregion,however,advocatemuch
lessextensionandfavortheevolutionof rangeblockssemiindependently
of eachother,eachwithseparate
deformational
and thermalhistories[e.g.,Wright, 1989].
The BlackMountainslie on theeastflankof DeathValley,
Califomia(Figure 1). Unroofingof footwallrockson the
westemportionof therangehasresultedin theexposure
of a
seriesof northwestplungingtopographic
andstructural
domes
knownas theDeathValley "turtlebacks"
(Figure2) [Curry,
1938;Wrightet al., 1974a]. Thesefeaturesarecomposed
predominantlyof 1.7 Ga gneissicandschistose
basement
anda
carapace
of deformedandmetamorphosed
Eocambrian
carbonate.
In thecoreof theBlackMountainsandlargelysurrounding
the
turtlebacks
to theeastis a batholithof gabbroicanddioritic
composition.A recentisotopicstudyindicatesthatthis
intrusion(theWillow SpringPluton)is mid-Miocenein age
(11.6 + 0.2 Ma [Asmeromet al., 1990])andthuswasemplaced
duringlarge-scaleextensional
tectonism
in theregion[Cemen
et al., 1985]. Intrudedinto theplutonandPrecambrian
basementrocksis a siliciccomplexof graniticto monzonitic
composition.
The BlackMountainscoredescribed
aboveis stmcturally
overlainalongnormalfaultsby theAmargosachaos[Noble,
1941;WrightandTroxel, 1984],a highlyextended
massof
unmetamorphosed
miogeoclinal
swamof Eocambrian
and
CambrianageandTertiaryvolcanicandsedimentary
rocks
(Figure2). The crystallineterrainwasunroofedalongthese
faultsin late Mioceneandyoungertime akinto other
metamorphic
corecomplexesin theCordilleranorogen.Holm
andWemicke[1990]proposed
thatrocksof theAmargosa
chaos have been translated 15-20 km northwestward from an
originalpositionin the southeastem
BlackMountains(Figure
2). Accordingly,thechaoswouldrepresent
a stranded
portion
of thehangingwall of an initiallywesterlydippingdetachment
faultalongwhichtheBlackMountainsweredenuded
duringlate
Miocenetime. The predominantly
shalloweastwarddip of the
faultcurrentlyexposedin theeasternBlackMountainswould
reflecteastwardtilting of the rangeasthe footwallwas
progressivelydenudedto the west. On thebasisof the current
configurationof thecrustalsectionandon geobarometry
of the
Willow SpringPluton,Holm andWemicke[1990]suggested
a
paleodepth
rangeof 10-30 km for thewestemportionof the
Black Mountainsprior to extension.This tectonicscenariofor
formationof the Black Mountainsis consistent
with previous
reconstructions
in the DeathValley regionwhichsuggest
transportof the PanamintMountainson the westemsideof
DeathValley (Figure1) off the topof theBlackMountains
duringTertiaryextension[Stewart,1983;SnowandWemicke,
1989]. In contrastto this scenario,Wright et al. [1987]
interpretthe Amargosachaosasoverlyinga northwesterly
movinglower platewith bothtop-to-the-east
denudation
of the
eastemBlack Mountainsandtop-to-the-west
denudation
of the
western Black Mountains.
ANALYTICAL
METHODS
In orderto revealthecoolingagepatternacross
theBlack
Mountains,
samples
werecollected
fromwidelyspaced
localitiesthroughout
therangein bothTertiaryplutonicrocks
and Precambrian basement. Because of the extensive
synextensionalintrusionslocatedin the centralBlack
Mountains,
moresamples
fromdifferentrocktypeswere
collected
therein ordertodifferentiate
between
cooling
ages
relatedto tectonic
unroofing,
topostmagmatic
cooling,
orto
thermalresetting.Eighteen
localities
weresampled
across
an
approximately
55 km northwest
trending
traverse
alongthe
lengthof themountain
range.Fromthesesamples,
30 age
determinations
weremadeonfivedifferent
mineralsystems
and
onewholerockseparate
(Table 1).
Mineralswereseparated
usingstandard
magnetic
andheavy
liquidseparation
techniques
andhandpicking.Sample
preparation,
irradiation,
andanalytical
procedures
for40Ar/39Ar
incremental
release
datingweredescribed
byLux [1986].
Samples
wereirradiated
in theUniversity
ofMichigan
reactor.
Variationsir•neutronflux duringirradiation
weremonitored
witha biotitemineralstandard
(University
of Mainelaboratory
standard,
247.6Ma). Plateauageswerecalculated
from
consecutive
gasincrements
thattogetherconstitute
>60% of
thetotalgasreleased.Uncertainties
arereported
at the20 level
andincludetheuncertainty
in thefluxmeasurement
(Jvalue).
Analytical
results
foreach
sample
aregiven
intheAppendixl.
Unalteredsamplesof the SmithMountainGranitewere
studied
petrographically
toselectthosewiththerequired
phase
assemblage
for Al-in-homblende
geobarometry.
ThetotalA1
1Appendix
isavailable
withentire
manuscript
on
microfiche.OrderfromAmerican
Geophysical
Union,N.W.,
Washington,
D.C. 20009. Document
T92-002;$2.50.Payment
must accompanyorder.
116ø47'
$60•8 '
Badwater
turtleback
A
Copper
Canyon
turtleback
Mormon
Point
turtleback
Figure 3
Ashford
Canyon
Region
L•,••..
....•.
of Amargosa
Chaos
Unmetamorphosed
Late
Precambrian
to
Tertiary sedimentary and volcanic
hangingwall rocks, Intact but steeply
east-tilted
in southern
Black
Mountains
';'•'-,:-••
Granitic
intrusive
rocks
/..'??..'..'..
.....
:?..'?.• •'7
'•:i:'.:,::....•,
i'"'"'"'"'"'"""'"'"':"'"'"'"'""'"'"::•
::::::::::::::::::::::::::::::
::::::::::::::::::::::::::::::::
:::::::::::::::::::::::::::
Willow
Spring
Diorite
•.:,::,:.?:::.:.:::.•
ß
:......::...:,.,:.,.:......:...:...,
•..:.:./
'::"'::'
:"'•"':i!'"//
[•'?
1,7
Ga
crystalline
rocks
including
amphibolite
_ • • •
Black Mountains
facies
Late
Precambrian
carbonate
rocks
surrounding the turtlebacks
detachment
SYMBOLS
Basal unconformity, Tertiary
volcanic sequence
ß
•
•
ß
•
fault
Basal unconformity, Pahrump
Group and younger units
Fig.2. Geologic
mapof theBlackMountains
andGreenwater
Rangesimplified
afterStreitzandStinson
[1974],Drewes[ 1963],WrightandTroxel[ 1984],Otton[1977]andHolmandWernicke[1990].
510
Holmet al.: Unroofing
of theBlackMountains,
DeathValley
Table1. Summary
ofBlack
Mountains
40Ar/39Ar
Data
Sample
Rock
Type
Locality
Mineral
Elevation,
m
Age*,Ma
Southeastern Black Mountains
IH1-BIO
pC basement
116ø25.6
' W, 35ø52.5'N
biotite
RH-BIO
pCbasement
116030.0
' W, 35ø55.0'
N
biotite
AM-UNC-M
AM2a-M
pC basement
pC basement
116ø39.6
' W, 35ø56.4
'N
116ø40.2
' W, 35ø57.5'
N
TPG-SM
granite
116o40.0
' W, 36ø02.7'
N
1,119
234.3+ 2.5 (tg)
671
232.3+ 5.9(ti)
195
317
1411.6+ 10.7(tg)
283.2+ 4.1 (ti)
988
8.7 + 0.1 (ti)
AshfordCanyon
muscovite
muscovite
Central Black Mountains Core
hornblende
biotite
CCR-AR
8.2 + 0.1 (ti)
gabbro-diorite
116o41.6
' W, 36ø06.1
'N
hornblende
1,585
TPL-BIO
latite dike
116o40.5' W, 36ø03.0'N
biotite
921
7.85 + 0.18 (tp)
89-CC
pC basement
116ø44.1
' W, 36ø06.6'
N
homblende
-60
> 14.31+ 0.81 (tg)
988-25
gabbro-diorite
116ø40.4
' W, 36ø03.1'
N
975
10.16+ 0.20 (tp)
89-MP
pC basement
116ø45.1
' W, 36ø02.0'
N
61
9.94+ 0.44 (tp)
988-20
gabbro-diorite
116ø40.4
' W, 36ø03.1'
N
homblende
975
10.42+ 0.31(tp)
biotite
plagioclase
1
plagioclase
2
orthoclase
biotite
biotite
homblende
6.63 + 0.13 (tp)
biotite
homblende
10.06+ 0.39(tp)
8.39 + 0.13 (tp)
biotite
8.03 + 0.14 (tp)
8.36 + 0.17 (tp)
6.70 + 0.61 (tp)
5.89 + 0.51 (tp)
felsicdike
116o42.3' W, 36ø07.1'N
TQM-HB
DH-3780
granite
gabbro-diorite
(deformed)
gabbro-diorite
(deformed)
116ø37.6
' W, 36o08.4
'N
116o40.4
' W, 36ø08.0'
N
90-38
dioritedike
116ø45.0
' W, 36ø16.3'N
wholerock
750
6.31 + 0.21 (tp)
BWT-B
DH-BWT-2
pC basement
pC basement
116o45.6
' W, 36ø17.1'
N
116o45.0
' W, 36ø16.3'
N
biotite
muscovite
265
732
13.23+ 0.23(tp)
24.00+ 0.40(ti)
DH-2650
116ø41.9'
W, 36ø07.5'
N
homblende
homblende
biotite
homblende
biotite
1,067
7.20 + 0.30 (ti)
6.90 + 0.20 (ti)
CCTB-B
1,670
1,152
808
8.69+ 0.31(tp)
8.05+ 0.28(tp)
7.00 + 0.10 (ti)
8.90+ 0.60(ti)
6.66 + 0.12 (tp)
Northern Black Mountains
biotite
12.70 + 0.20 (ti)
GreenwaterRange
TQM-GW-B grilni•;e
116ø21,•
' W, 35ø•59,9'
N
Di0•ite
902
9.76+ 0.25(tp_)
* All tp (plateau
age)andti (intercept
age)areconcordant
withinerrorwhenbothagesexistforindividualsamples;
tgrepresents
totalgasage;pC is Precambrian.
contentin rimsof homblende
grainsin thesesamples
was
obtained
usinga Camecathree-spectrometer
CAMEBAX
electronmicroprobe
at HarvardUniversity.
THERMOCHRONOLOGY
RESULTS
PreviousK-At isotopicstudiesof theBlackMountains
concentrated on volcanic rocks which flank the eastem and
northernsidesof thecrystallineterrane[Fleck,1970].
However,Armstrong
[ 1970]reported
K-Ar agesfromTertiary
intrusiverocksof 7.5 _+0.3 Ma (biotitein diorite)and6.3 +
0.4 Ma (graniteporphyrywholerock). Theresultsof this
studyarediscussed
belowwithrespectto theregional
distribution
of samplelocalitiessummarized
in Table1. Plate
1 is a summary
diagramof thethermochronologic
data
presented
in thispaper.Theagesareplottedona mapof the
BlackMountainswhichis thesameasFigure2 butwithout
thegeologicalpatterns.No diffusionexperiments
to determine
mineralclosure
temperatures
werecarriedoutin thisstudy,and
standard
closuretemperatures
of 500ø+ 25øC(hornblende),
350ø
_+25øC(muscovite),and300ø+ 25øC(biotite)areassumed
to
interpretthedata[McDougallandHarrison,1988].
Interpretation
of feldspar
releasespectra
andclosure
temperature
are discussed in the text.
Southeastern
BlackMountains.Two ageswereobtained
fromcoarsely
crystalline
micaschists
andquartz-feldspar
gneisses
from Precambrianbasementin the southeastern
Black
Mountains.Biotiteseparates
fromsamples
IH1-BIO andRHBIO yieldedsimilarreleasespectra
andtotalgasagesof 234.3+
2.5 Ma and230.8+ 6.1 Ma respectively.
Neithersample
yieldeda plateauage,butbothshowa monotonicincrease
in
age(from-210 to 250 Ma) in progressively
highertemperature
Holmet al.: Unroofingof theBlackMountains,
DeathValley
increments.SampleRH-BIO yieldedanisotopecorrelation
intercept
ageof 232.3+ 5.9Mawitha40Ar/36Ar
initialratio
of 304.5 + 38.9. The similar,althoughsomewhat
disturbed,
releasespectraandtotalgasagesof thesesamples,
aswell as
theconcordant
interceptagefor oneof thesamples,
leadusto
concludethatthisregionof the BlackMountainshasremained
below 300øC since 230-235
Ma.
AshfordCanyon. Two Precambrian
basement
sampleswere
collectedfrom thevicinityof AshfordCanyonwherelate
Precambrian
PahrumpGroupstratarestdepositionally
on
basement(Figure2). SampleAM-UNC-M is located~30 m
belowthenonconformity
(Figure3). A muscoviteseparate
from AM-UNC-M yieldeda totalgasageof 1411.6+ 10.7
Ma and a releasespectrain whichfive increments(with over
65% of thetotalgas)giveagesbetween1360and 1400Ma.
We interpretthisspectrumto indicatecoolingbelow350øCat
about1380+ 20 Ma. A muscoviteseparate
from sample
AM2a-M (Figure3) yieldeda saddie-shaped
spectrum
withan
ageminimumof 279.3+ 1.9 Ma. Saddle-shaped
spectraarea
typicalsignof excessargon[McDougallandHarrison,1988],
andthe40Ar/36Ar
initialratioof718.3+ 47.2forthissample
confirmsthis. However,an interceptageof 283.2+ 4.1 Ma
indicatesthatthe ageminimumfor thisspectrum
maybe a
meaningful
cooling
age.In addition,
when
a 40Ar/36Ar
initial
ratioof 718.3 is assumed,
thereleasespectrum
givesa totalgas
ageof 284 Ma withover90% of thegas(eightincrements)
fallingbetween280 and300 Ma.
0
1.0
I
km
511
CentralBlackMountainscore. Elevensampleswere
collectedfrom thecentralBlackMountainscrystallinecore
(Table 1), the regionoccupiedby the southerntwo turtlebacks
(MormonPointandCopperCanyon)andthe adjacentMiocene
intrusions(Figure2). Two samples(988-20 and988-25) of
undeformed11.6 Ma Willow SpringPlutoncollectedfromthe
sameoutcropnearWillow Springin Gold Valley (Figure2)
givehornblende
plateauagesof 10.42+ 0.31 Ma and 10.16+
0.20 Ma andconcordant
interceptagesof 10.3+ 0.2 Ma and
10.2+ 0.5 Ma. A hornblendeseparatefromanother
undeformedsample(CCR-AR, collected-6.4 km northof this
locality)givesa plateauageof 10.06+ 0.39 Ma anda
concordant
interceptageof 9.8 _+0.2 Ma. Biotiteseparates
from samples988-20 and988-25 giveconcordant
totalgas,
intercept,andplateauagesof 8.36 + 0.17 Ma and8.39 + 0.13
Ma respectively.We interprettheseagesasindicating
postcrystallization
coolingbelow500øCat 10.25_+0.15 Ma
and below 300øC at 8.40 _+0.15 Ma.
Releasespectrawerealsoobtainedfromplagioclase
and
orthoclase
separates
from sample988-20. The Willow Spring
Plutoncontainsonly minor amountsof orthoclaseand
separation
from plagioclasewasfraughtwith difficultyowing
to thepresence
of hydratedplagioclase
grains.A separate
consisting
of ~50% orthoclase
and~50% plagioclase
did not
givea plateaubut yieldedan interceptageof 7.2 + 0.3 Ma in
agreementwith its total gasage. While cautionmustbe used
in interpretingsuchdata,it is worth notingthat McKenna
[ 1990] obtainedan interceptageof 7.03 _+0.04 Ma for an
orthoclase
separate
froma nearbygranitewhichintrudesthe
Willow SpringPluton. Thus we tentativelyinterpretthesedata
to indicatecoolingbelow200ø-250øC(commonlydetermined
closuretemperature
rangefromdiffusionstudies
onK-feldspars
[e.g.,HubbardandHarrison,1989]) at 7.0-7.5 Ma.
Two splitsof plagioclasefrom sample988-20 yielded
saddie-shaped
spectra.The first splitgivesa plateauageof
5.89 + 0.51 Ma for nineincrements
consisting
of 76% of the
gasanda concordant
interceptageof 5.9 + 0.4 Ma. The second
split givesa plateauage of 6.70 + 0.61 Ma for eight
increments
consisting
of 70% of thegasandan interceptageof
6.4 + 0.3 Ma. The disparateagesfor two splitsof the same
separate
aswell asthesaddle-shaped
spectraleadto difficulties
in interpretingthesedata. In addition,thelowerdiscordant
age
from theorthoclase
separate(in spiteof a similaror possibly
evenhigherclosuretemperature
basedonempiricalevidence
[McDougallandHarrison,1988])indicateto usno appropriate
meaningfulinterpretation.
Intrudedinto the Willow SpringPlutonin the southern
GoldValley area(andunderlyingmuchof SmithMountain,
Figure2) is a coarsegrainedgranitewhichcommonlyexhibits
Rapakivitextureandlocallycontains
abundant
coarse
hornblende
grains.A hornblende
separate
(TPG-SM)givesan
interceptageof 8.7 + 0.1 Ma anda nearplateaufor thelast 10
increments
(78% of thegas)of 8.5 + 0.4 Ma with theoldest
increment
being8.72+ 0.15 Ma. Biotitefromthissample
givesan intercept
ageof 8.1 + 0.1 Ma anda plateauageof
+Ashford-'-"•
':
8.00+ 0.21Ma indicating
thatthegranitecooledquiterapidly.
Because
theadjacentWillow SpringPlutonintowhichthis
graniteintrudedcooledbelow 500øCat 10.1-10.4 Ma, we
Fig. 3. Geologicmapof AshfordCanyonregionshowing
samplelocalities(simplifiedafterWrightandTroxel[1984]).
PatternedafterFigure2.
interpretthe8.7 Ma hornblende
ageasrepresenting
the
approximate
timeof crystallization
of theintrusion.
A hornblende
separate
(TQM-HB) froma coarse-grained
silicicplutonontheeastsideof theBlackMountains
yieldeda
slightlysaddle-shaped
spectrum
witha plateauageof 10.11+
512
Holm et al.: Unroofingof the BlackMountains,DeathValley
0.73 Ma. However,the isotopecorrelationdiagram,when
regressed
throughtheplateaupoints,givesan interceptageof
8.7Ma andaninitial40Ar/36Arratioof 312.02+ 5.12
indicatingthepresenceof excessargon. Whenan initial
40Ar/36Ar
ratioof 312isassumed
fortheincremental
release
spectrum,
thesaddleshapedisappears
andtheplateauage
becomes8.69 + 0.31 Ma, concordant
with theinterceptage.
This pluton,like the SmithMountainGranite,intrudedinto
theWillow SpringPluton,andits hornblende
ageis similarto
the hornblendeageof the SmithMountainGranite. We
interpretthisageasrepresenting
thetimeof crystallization
of
severalsilicicplutonsin the Black Mountains.
Late-stagelatiteandrhyolitedikeswithbiotiteandfeldspar
phenocrysts
intrudeboththe Willow SpringPlutonandthe
SmithMountainGranitein the vicinityof Gold Valley. These
dikesexhibitan intensemyloniticfoliationwhichcommonly
doesnot extendinto undeformedcountryrock(Figure4a). A
coarsebiotiteseparatefroma dikesample(TPL-BIO) that
intrudestheWillow SpringPlutoncollectednearWillow
Spring(Table 1) yieldeda plateauageof 7.85 + 0.18 Ma anda
concordantinterceptageof 7.9 + 0.1 Ma. Theseagesare only
slightlyyoungerthanthe biotiteagesof the SmithMountain
Granite,theupperboundevenoverlapping
in errorwith the
plateauagealthoughnot with theinterceptage. We interpret
the dike age as the time of intrusionandcrystallizationinto
countryrock with ambienttemperatures
between200øCand
300øCasindicatedby theK-feldsparandbiotiteagesobtained
from the surrounding
Willow SpringPluton.
Precambrian
basement
samples
werecollectedfromthe
MormonPoint(sample89-MP) andCopperCanyon
turtlebacks(sample89-CC; Figure2). Theserockscontaina
coarselyrecrystallized
metamorphic
fabric. Basement
fromthe
MormonPointturtleback(sample89-MP) givesa hornblende
plateauageof 9.94 + 0.44 Ma with a concordant
interceptage
of 9.9 + 0.2 Ma similarto the hornblendeagesin theWillow
SpringPluton. A concordant
plateauandinterceptbiotiteage
from this sampleindicatessimplecoolingbelow300øCat 8.03
+ 0.14 Ma. Sample89-CC showsa complicatedhornblende
spectrumwith no interceptregression.The spectrum,
however,
appearsverysimilarto partiallyresetspectrafromotherstudies
wheretheminimumagecorresponds
to a knownlaterheating
episode[HarrisonandMcDougall,1980]. The minimumage
for thisspectrum(11.86 + 1.57Ma) corresponds
with theage
of intrusionof the Willow SpringPluton,andthereforewe
interpretthisspectrumto representcoolingbelow500øCby at
least14 Ma with partialresettingat 11.6Ma dueto reheating
by the gabbro-diorite
batholith. Biotitefromthissampleyields
a concordant
interceptandplateauageindicatingsimplecooling
below 300øCat 6.63 + 0.13 Ma. The CopperCanyon
turtlebackdatasuggestthatthebasementrocksin thisregion
Fig. 4. (a) Photomicrograph
of mylonitictexturecommonin
porphyriticdikesin thecentralBlackMountains(sampleTPLBIO). Lengthis 2.0 mm across.(b) Photomicrograph
of late
leucocratic
phaseof theWillow SpringPluton(sample88-23)
showingquartzin contactwith hornblende
(curvedarrows).
Straightarrowspointto examples
of K-feldspargrainsandKfeldsparinclusionsin plagioclase.Lengthis 0.5 mm across.
(c) Photomicrograph
of euhedralhornblende
crystalfromthe
SmithMountainGranite(sampleTPG-SM) in quartzplagioclase-potassium
feldspar-biotite
rich matrix. Lengthis
0.5 mm across.
werebelow 500øCprior to intrusionof theWillow Spring
Pluton. We interpretthe MormonPointhornblende
ageas
havingbeencompletelythermallyresetby thebatholithat 11.6
Ma, followedby slowcoolingto below500øCat 10 Ma (note
the similarhornblendeagesfrom the Willow SpringPluton).
Holmet al.: Unroofingof theBlackMountains,
DeathValley
A ductilelydeformedsampleof theWillow SpringPluton
(sampleDH-2650) obtainedbeneaththefoliatedEocambrian
carapace
onthenortheast
sideof theCopperCanyonturtleback
(Figure2) givesa hornblende
plateauageof 8.77+ 0.70 Ma
with a concordant
interceptageof 8.9 + 0.6 Ma. Biotitefrom
thissampleyieldeda plateauageof 6.66 + 0.12 Ma andan
interceptageof 6.8 + 0.1 Ma. Anotherdeformedgabbro-diorite
sample(DH-3780) froma ductileshearzone~3 km northwest
of sampleDH-2650 givesa hornblende
plateauageof 8.05+
0.28 Ma andan interceptageof 8.3 + 0.5 Ma. Biotitefrom
thissampledoesnotyielda plateauagebutdoesgivean
interceptageof 7.0 + 0.1 Ma. Thehomblende
agesfromthese
samplesaresubstantially
youngerthanthehornblende
ages
derivedfromundeformed
samples.Thebatholithintruded
at
11.6 Ma into countryrockwith ambienttemperatures
below
500øC and cooled below 500øC at 10.1-10.4 Ma. Therefore we
interprettheseyoungerhornblende
agesassociated
with
ductilelydeformedsamples
asindicating
resetting
during
mylonitization
with ambienttemperatures
between300øCand
500øC. Davidsonand Snee [ 1990] have demonstrated
that under
greenschist
faciesconditions
(>300øC)hornblende
canbereset
duringductilesheafingat ambienttemperatures
belowthe
homblende
closuretemperature.
Mylonitefabricsaretypically
formedat temperatures
abovetheclosuretemperature
for
diffusionin biotite. We thereforeinterpretthe6.7 and7.0 Ma
biotiteagesfromthesesamples
asrepresenting
simplecooling
followingdeformation.
The CopperCanyonturtleback
is intruded
by felsicdikes
whichareorientedperpendicular
to theaxisof theturtleback
513
basementrockscooledslowlythroughthe 350øCisothermat
~24 Ma with furthercoolingthrough300øCat ~13 Ma.
Fine-graineddioriticdikescut thebasementfoliationat a
highangleandare synkinematic
with late-stage
brittlefaulting
[Miller, 1991]. A wholerock analysisof oneof thesedikes
(sample90-38) givesa plateauageandconcordant
interceptage
that indicatescrystallizationat 6.3 + 0.2 Ma. Thesedata
suggest
thatthebasement
rocksof theBadwaterturtleback
were
within a few kilometersof theEarth'ssurfaceby at least6.3
Ma.
GreenwaterRange. The GreenwaterRangeliesjusteastof
theBlackMountains(Figures1 and2) andconsists
of Tertiary
silicicplutonsoverlainby 10-4 Ma vailablyfaultedmaficto
silicicvolcanicrocks. In the southemGreenwaterRange,8-9
Ma Shoshone
volcanicslie depositionally
on feldsparporphyry
granite[Figure2; L. Wright andB. Troxel,personal
communication].A biotiteseparatefrom thisgranite(sample
TQM-GW-B) gaveconcordant
totalgas,intercept,andplateau
agesof 9.76 + 0.25 Ma, consistent
with rapidcoolingfrom
above300øCat 9-10 Ma. This age is approximately1 m.y.
olderthanthe ageinterpretedfor majorsilicicplutonismin the
central Black Mountains.
GEOBAROMETRY
RESULTS
extensivelyalteredto chlorite,andcommonlyexhibit
A lowerboundon thedepthestimatefor crystallization
of
theWillow SpringPlutonat 9.5-12.5 km wasdetermined
by
Holm andWernicke[1990,Table 1] usingtheAl-in-homblende
geobarometer
[Hammarstrom
andZen, 1986;Johnson
and
Rutherford,1989]andassuming
anoverburden
densityof 2750
deformation features such as strain shadows and kink bands. A
kg/m
3. Asobserved
byHolmandWemicke
[1990]
and
biotiteseparate
fromoneof thesedikeswasconsidered
pure
enoughfor agedatingonlyafter2 daysof hand-picking.
SampleCCTB-B did notyielda plateauagebutdidgive
concordant
interceptandtotalgasagesof 6.9 + 0.2 Ma and6.8
+ 0.2 Ma, respectively.Theseagesoverlapwith thebiotite
coolingagesobtainedfrom thePrex:ambrian
basement
andthe
Willow SpringPlutonin thisregion. The CopperCanyon
turtlebackdikesarevolumetricallysmallandareconsidered
unlikelyto havecompletelythermallyresetthe surrounding
countryrock to above300øC. We interpretthe6.9 + 0.2 Ma
interceptageas the time of intrusionof thesedikesintorapidly
coolingcountryrock similarto dike intrusionin theGold
Valley region~1.0 m.y. earlier.
corroborated
by Wrightet al. [1991]themainportionof the
batholithdoesnotcontaintheappropriate
assemblage
required
for thisgeobarometer.
However,lateleucocratic
portionsof
whichare intrusiveinto the mainportiondo containthe
requiredassemblage.
We concurwithWrightet al. [1991]that
theirsamples
wereinappropriate
for usingthisgeobarometer
as
theywereunableto findanypotassium
feldsparandcouldnot
locatequartzin contactwith hornblende.However,samples
analyzedby Holm andWernicke[1990]do containpotassium
feldspar(asevidenced
fromoptical,microprobe,
andchemical
staininganalyses)aswell asquartzin contactwith hornblende
(Figure4b). In addition,successful
analysesandsimilarresults
usingthisgeobarometer
on theserockshavebeenreportedby
NorthernBlack Mountains. SamplesBWT-B andDHBWT-2 aremylonitizedPriambrian basement
fromthe
Badwaterturtlebackin the northemBlackMountains.Sample
BWT-B is locatedonlya few metersbelowthebrittle
detachment
surface,whereassampleDH-BWT-2 wascollex:ted
from a deeplyincisedcanyonin thelowerplate(approximately
200 m below the brittle detachmentsurface). Micas from both
localitiesare strainedintomica-fishprofilestypicalof
mylonitictextures.Muscovitefrom sampleDH-BWT-2
showsa typicaldiffusionlossprofilewith an ageminimumof
16.03+ 1.90 Ma, four increments(representing
72% of the
gas)with agesbetween23.4 and24.4 Ma, andan interceptage
of 24.0 + 0.4 Ma. Biotitefrom sampleDH-BWT-2 is
stronglydiscordantfrom themuscoviteagegivinga concordant
interceptandplateauageof 12.63+ 0.50 Ma. Biotitefrom
sampleBWT-B givesa slightlyolderplateauageof 13.23+
0.23 Ma anda concordant
interceptageof 13.4+ 0.1 Ma. We
interpretthesedataasindicatingthattheBadwaterturtleback
Meurer and Pavlis [ 1991].
fold [Drewes, 1963]. Biotite in thesedikes are often
Dated samplesof the SmithMountainGranite(TPG-SM)
alsocontainthepropermineralassemblage
n•essaryfor
applicationof theAl-in-homblendegeobarometer.
The results
of microprobe
analyses
of coarseeuhedralhomblendes
(Figure
4c) fromthreedoublypolishedsex:tions
of thegranitearegiven
in Table2. Total A1 contentfrom rim analysesof homblende
indicatesa pressureof 2.76 + 0.5 kbar (usingthemodified
equationof JohnsonandRutherford[ 1989,p. 838]) or a depth
estimateof 10.0+ 1.8km usingan averageoverburden
density
of2750kg/m
3. ThetotalA1content
obtained
hereis
substantially
greaterthanthatrecentlyreportedfor theSmith
MountainGraniteby Wright et al. [1991]. A possible
explanation
for thisdiscrepancy
mightbe thatdifferentphases
of thegraniteweresampled.Considering
thatintrusion
occurredsimultaneously
with unroofing,it seemsquite
possiblethatvariousphasesmightrex:ord
differentdepthsof
emplacement.
514
Holmetal.: Unroofing
oftheBlackMountains,
DeathValley
TABLE 2. CationProportions
FromRim Analysesof Hornblendein SmithMountainGranite
Cation
Sample
90-SMG-4
90-SMG-1
91-SMG-3
90-SMG-3
Si
A1
Fe
Mg
Ca
Na
K
Ti
6.85
1.46
2.04
2.65
1.94
0.27
0.15
0.10
0.06
Mn
6.82
1.45
1.82
2.82
1.92
0.31
0.17
0.16
0.06
6.76
1.45
2.05
2.69
1.90
0.34
0.17
0.15
0.07
6.75
1.49
2.01
2.70
1.89
0.36
0.18
0.16
0.07
6.71
1.51
2.04
2.71
1.94
0.33
0.19
0.15
0.07
6.78
1.47
2.04
2.63
1.87
0.39
0.18
0.16
0.07
6.81
1.41
1.99
2.74
1.93
0.28
0.17
0.14
0.07
6.77
6.78
1.52
1.49
1.93
2.01
2.69
2.72
1.93
1.89
0.32
0.29
0.18
0.18
0.14
0.14
0.07
0.06
6.78
1.50
2.06
2.65
1.88
0.35
0.17
0.13
0.07
6.78
1.44
1.93
2.77
1.92
0.34
0.17
0.16
0.07
6.74
1.53
2.06
2.63
1.92
0.27
0.18
0.14
0.07
6.77
1.53
2.05
2.61
1.92
0.30
0.16
0.13
0.07
6.99
1.23
1.92
2.83
1.98
0.24
0.13
0.09
0.07
6.96
1.29
1.84
2.91
1.87
0.26
0.14
0.12
0.06
6.74
1.51
2.03
2.66
1.93
0.34
0.17
0.15
0.07
6.75
1.52
2.00
2.68
1.90
0.34
0.18
0.14
0.07
6.78
1.50
1.97
2.68
1.87
0.35
0.18
0.16
0.07
6.83
6.80
1.37
1.49
1.90
1.81
2.85
2.83
1.93
1.88
0.28
0.34
0.16
0.18
0.15
0.15
0.06
0.06
Proportions
areper23 oxygens.
DISCUSSION
duringregional
extensional
tectonism
andunroofing
inthe
DeathValley region[Cemenet al., 1985;HolmandDokka,
Timing,Depth,andDeformationof MioceneIntrusions
Intrusions
in theBlackMountains
areentirelyTertiaryin
age,theoldestbeingtheWillow SpringPlutondatedat 11.6+
0.2 Ma [Asmerom
et al., 1990]. Previous
mapping
distinguished
at leastthreemajorsilicicintrusive
bodies
younger
thantheplutonbasedoncompositional
differences
and
dikerelations[Drewes,1963;Otton,1977].
WillowSpringPluton. The minimumhornblende
ageof
~14.3Ma fora themallyresetPrecambrian
basement
sample
(89-CC) in the centralBlack Mountainsindicatesambient
temperatures
below500øCpriorto intrusion
of Tertiary
plutons.Thisestablishes
anupperboundfor thedepthof
intrusionof theWillow SpringPlutonat-16.7-20 km
assuming
typicalBasinandRangegeothermal
gradients
after
Lachenbruch
andSass[1978]of 25-30øC/km
priorto
intrusion.Considering
elevated
geothermal
gradients
during
Miocenetimewouldmaketheupperboundforintrusion
depth
evenless. The 8.4-6.7 Ma biotiteagesfromthepluton
(samples
988-20, 988-25, DH-2650, andDH-3780) aretoo
youngto be attributedto simpleposternplacement
conductive
coolingof an 11.6Ma intrusion.We therefore
interpret
the
biotiteagesasrepresenting
coolingassociated
withextensional
unroofing.Thisinterpretation
impliesthattheplutonintruded
intocountryrockwithambienttemperatures
above300øC.
Again,assuming
25-30øC/kmgeothermal
gradients,
weobtain
a lowerboundfor intrusion
eraplacement
depthof 10-12km.
Thislowerboundis consistent
withgeobarometric
results
ona
latecrystallization
phaseof thepluton.Allowingfor some
upliftbetweencrystallization
of themainandlateleucocratic
phases
of thepluton,considering
it crystallized
andcooled
1991],weconsider
anintrusion
depthforthemainphase
of the
plutonof 10-15 km mostlikely.
The DeathValley turtlebacks
containa thick,welldeveloped
myloniticfoliationin basement
rocksthatis
subparallel
to theoverlyingcarbonate
carapace
andbrittle
detachmentsurface. Involvementof foliated and lineated
WillowSpringPlutonin thesefootwallmylonites
(sample
DH-2650) indicatesthat at leastsomeof the foliationformed
duringMioceneextension[HolmandWernicke,1989;Holm
andLux, 1991].Myloniticdeformation
appears
tohavebegun
priorto intrusion
of theSmithMountain
Granitecomplex
as
suggested
by the-8.9 Ma hornblende
agefromdeformed
gabbro-diorite
(sampleDH-2650), consistent
withthe
observation
thattheWillowSpringPlutonis moreductilely
deformed
thantheyounger
granitic
complex
[Holmand
Wernicke, 1990].
SmithMountainGranitecomplex.The 8.7 Ma hornblende
agesobtainedfromgraniticplutonsin thecentralBlack
Mountains
(samples
TQM-HB andTPG-SM)signifya major
silicic intrusive event coeval with extensivevolcanismin the
Greenwater
range.The SmithMountainGraniteandcoeval
plutons
in thecentralBlackMountains
intruded
intocountry
rockwithanambient
temperature
above300øCasindicated
by
the8.4 Ma biotiteagesobtained
fromtheWillowSpring
Plutoncountry
rockintowhichit intruded.We interpret
the
8.2 Ma biotiteageobtainedfromthe SmithMountainGranite
asindicating
continued
posternplacement
cooling
afterthe
country
rockhadcooled
through
300øC.The40Ar/39Ar
coolingagedataandgeobarometry
ontheSmithMountain
Granite,
takentogether,
areconsistent
witha depth
range
for
crystallizationof 8.2-11.8 km.
3O
MUSCOVITE
DH-BWT-2
988-20
18
Tg =23.14+0.44 Ma
HORNBLENDE
BWT-B
Ti = 24.0 + 0.4 Ma
Tg = 11.03ñ 0.88 Ma
Tp = 10.42ñ 0.31 Ma
No Plateau
Ua
•
I
I
I
I
Ti = 10.3 ñ 0.2 Ma
,
2O
BIOTITE I
Ua
Tg = 12.58 + 0.22 Ma
Tp = 12.63 + 0.50 Ma
BIOTITE I
BIOTITE
Ti = 12.7 + 0.2 Ma
Ti = 13.4ñ0.1
lO
I
I
I
o
I
ol
Tg = 13.22 ñ 0.29 Ma
Tp = 13.23 ñ 0.23 Ma
I
Ma
I
I
lOO
% 39 Ar
% 39 Ar
100
,I
Tg = 8.35 ñ 0.17 Ma
Tp= 8.36ñ0.17 Ma
Ti =8.4:1:0.1
I
I
I
I
Ma
I
I
I
lOO
% 39 Ar
lO
988-20
I 90-38
PLAGIOCLASE
12
HORNBLENDE
DH-3780
Ti = 6.4 ñ 0.3 Ma
Ti = 8.3 ñ 0.5 Ma
Ma
Ua
Tg = 7.77 ñ 1.05 Ma
Tp = 6.70.ñ 61 Ma
Tg = 7.30 ñ 0.44 Ma
Tp = 8.05 ñ 0.28 Ma
WHOLE ROCK
Tg = 6.51 + 0.65 Ma
Tp = 6.31 + 0.21 Ma
Ti = 6.2 + 0.1 Ma
BlOTITEl
i Tg
=6.72
ñ0.95
Ma
Tg = 6.18ñ0.15 Ma
Tp = 5.89 ñ 0.51 Ma
Ti = 7.0 ñ 0.1 Ma
No Plateau
0
% 39 Ar
100
0
16
DH-2650
Ti = 5.9 ñ 0.4 Ma
% 39 Ar
HORNBLENDE
100
Tg = 8.34 + 1.63 Ma
Tp = 8.77 + 0.70 Ua
Ma
988-20
Ti = 8.9 + 0.6 Ma
1 oo
% 39 Ar
ORTHOCLASE
•
16
Ti = 7.2ñ0.3
TQM-HB
BIOTI•
III I
Tg = 6.72d:0.21 Ma
III I
Tp= 6.66+ 0.12Ua
0
% 39 Ar
\
/
Ua
/
I
I
Tg
=6.06
Ma J
Tp = 8.69 ñ 0.31 Ma
101
0
• 89-CC
HORNBLENDE
•
0
Tg = 14.31 + 0.81 Ma
,
] , ] , ] ' '
% 39 Ar
No Plateau
Ua
40Ar/36Ar = 301 ñ 7.7
Ti = 8.7 ñ 0.3 Ma
2O
36/40
100
lO
( .
I
6.7
/
/•'
(,..•---/
TPL-BIO
JT
•",
lO
I---I
Ma
-/
BIOTITE I
("•-,
8.4
• z?/
v'
Ti = 6.7 + 0.1 Ma
i
i
f
(/'.,,/
I
'
r
, IF
5
BIOTITE
,b5
Tg = 6.59 + 0.13 Ma
Tp = 6.63 + 0.13 Ma
i
L
I
Tg= 7.69+ 0.25Ma
Tg = 7.85+ 0.18 Ma
(..
Ti =7.9+0.1
I
Ma
lOO
% 39 Ar
lO
I
I
I
I
I
I
I
I
% 39 Ar
CCTB-[••
Ua
I
14
\
BIOTITE
lOO
TQM-GW-B
'3
Tg = 6.80 ñ 0.23 Ma
Ti = 6.9 ñ 0.2 Ma
No Plateau
Ma
-f\., 234
i
i
i
i
i
0
i
i
i
\\ j
i
% 39 Ar
100
'•'
8
,.._.,,
BIOTITE
Tg = 9.54:1:0.29Ma
Tp = 9.76 ñ 0.25 Ma
Ti = 9.8 ñ 0.1 Ma
0t[ [ [
2O
f[89-MPHORNBLENDE
Ma'
41111
I
I
Tg=10.14ñ
0.50Ma
Tp= 9.94.ñ 0.44Ma
//
/
10-
/
•,
/
/
k,..
I
I
0
I
I
I
I
I
lOO
% 39 Ar
/
\
/
300
=%39 Ar
__100
Ma
400
AM2a-M
\
•
/
:
200 -
BIOTITE
BIOTITE []
Tg = 7.88 ñ 0.23 Ma
Tp = 8.00 + 0.21 Ma
Tg = 234.3 ñ 2.5 Ma
No Plateau
TI =8.1 +-0.1Ma
MUSCOVITE
Ma'
IH1-BIO
14
-TTPG-SM
•ORNBLENDE
Tg = 299.6 ñ 2.3 Ma
Ti = 283.2 ñ 4.1 Ma
100
No Plateau
300'
•
I
I
0
'
N
200
I
I
% 39Ar
100
lOO
%39 Ar
20
•CCR-AR
lO
16oo
I
-I
1:3
i
I
I
I
I
I
100
300
RH-BIO
Ua
, I ] ] j I •
I
km
MUSCOVITE
Tg= 1411.6ñ 10.7Ma
0
0
Tg = 10.30ñ 1.16Ma
Tp = 10.06ñ 0.39Ma
Tg = 230.8 ñ 6.1 Ma
Ti=9•
No Plateau
% 39 Ar
Ti = 232.3 ñ 5.9 Ma
100
i
i
% 39 Ar
i
lOO
i
i
i
i
i
% 39 Ar
No Plateau
500
I
BIOTITE
•_
Ua
lOOO
I
% 39 Ar
2O0
AM-UNC-M
ß
I
0
I
lOO
Plate
1.The40Ar/39Ax
age
spectra
and
intercept
diagram
ofdated
minerals
and
rocks
with
sample
localities
plotted
onthegeologic
map
from
Figure
2(unpattemed
here).
Numbers
adjacent
tosome
localities
areages
(inmillions
ofyears)
plotted
forease
ofviewing
theoverall
cooling
age
pattern
for
biotite.
i
1 oo
Holmet al.' Unroofingof theBlackMountains,
DeathValley
Greenwaterplutons.Graniticplutonsin theadjacent
Greenwater
range(Figure1) aredepositionally
overlainby 8-9
Ma Shoshone
volcanicsandyielda biotiteplateauageof 9.76
+ 0.25 Ma, substantiallyolder thanboththe hornblendeand
biotiteagesfrom the SmithMountainGranitecomplex.
Because field relations indicate that all silicic intrusions in the
BlackMountainsare youngerthantheWillow SpringPluton
[Drewes,1963],we inferthattheGreenwater
plutonsrepresent
crystallization
of a graniticbodyintermediate
in agebetween
theWillow SpringPlutonand the SmithMountainGranite
complex.Althoughno basement
rocksareexposedin the
vicinityof thispluton,we notethatit appearsto be locatedin
thesamestructuralpositionor depthbeneaththelate
Precambrian
nonconformity
andoverlyingMiocenevolcanic
rocks as the basement rocks in the southeastern Black
Mountains. It seemsIlkely thereforethatthisplutonintruded
at shallowlevelsinto countryrock colderthan300øCandthat
the9.76 Ma biotiteagerepresents
a nearcrystallization
age.
The nearlyconcordant
ageof theoverlyingShoshone
volcanics
indicatesunroofingandexposure
priorto 9 Ma, soonafter
crystallization.
Myloniticdikes. Numerousporphyriticlatiteandrhyolite
dikesin the centralBlack Mountainscommonlyexhibita
myloniticfoliationwhich is usually(althoughnot always)
absentin the countryrock adjacentto thesedikes. A 7.85 Ma
biotiteageobtainedfroma dikenearWillow Springin Gold
Valley indicatescrystallization
of thesedikeswithin 1.0 m.y.
aftercrystallization
of theSmithMountainGranitecomplex.
The dikenatureandtheporphyritictextureof theselate-stage
intrusions
indicatecolderambienttemperatures
andtherefore
likely a shallowerdepthof intrusionthantheolderplutonic
complex.At Gold Valley, thesedikesintrudedintocountry
rockundergoing
rapidcoolingratesof 35ø-100øC/m.y.
based
on the ageandclosuretemperature
differences
betweenbiotite
andK-feldspar.We interpretdikeintrusionasfractureinfilling
of crustbrittlelydeformingduringrapidunroofing.Becausethe
temperature
of thedikeswouldbe substantially
higherthanthe
surrounding
countryrockimmediatelyafteremplacement,
these
dikesdeformedductilelyin an overallbrittleregime,thus
havinga myloniticfoliationimpartedto themwhichis largely
nonexistentin the adjacentcountryrock.
Thermochronologic
datafromTertiaryplutons,combined
with geobarometric
data,providetime-depth
constraints
on the
Mioceneintrusivehistoryof theBlackMountainscrystalline
terrain. At 11.6 Ma, the Willow SpringPlutonintrudedat a
depthrangeof 10-15 km into countryrock with an ambient
temperature
below 500øC. At ~8.7 Ma the SmithMountain
Granitecomplexintrudedat a depthof 8.2-11.8 km into
countryrock with temperatures
above300øC. Thesetimeand
depthconstraints
indicateonly minorto moderateamountsof
unroofing(0-6.8 km) at averageratesof 0-2.2 mm/yrbetween
11.6 Ma and 8.7 Ma.
517
exclusivelyon unmetamorphosed
strataof Paleozoic,
Eocambrian,or latePrecambrianage,suggesting
significant
preextensional
depths.Oneexceptionis theareaoccupied
by
theAmargosachaosat thevicinityof AshfordCanyonsouthof
the Mormon Point turtleback(Figure2). Here late
Precambrian
PahrumpGroupstrataandoverlyingTertiarystrata
aredeposited
onbasement
whichcooledbelow350øCat
approximately
1380Ma (sampleAM-UNC-M). Theserocks
lie only5--6km southof exposures
of theWillow Spring
Plutonandtheregionallymetamorphosed
NoondayDolomite
on the MormonPointturtleback(Figure3). This areaoccupies
thesouthernboundaryof theNopahUpland(Figure5),
described
asa regionof crystallinerockupliftedin thelate
Precambrian
(pre-Noonday
time)andflankedto thesouthby
PahrumpGroupstrata[Wrightet al., 1974b]. At Ashford
Canyonandelsewhere,
unmetamorphosed
PahrumpGroup
stratalie only 2-3 km belowtheTertiaryunconformity,
implyingshallowdepthsat theonsetof extension.Wrightand
Troxel [ 1984]havearguedthattherelativepositions
of
Clasts of both the Smith Mountain
GranitecomplexandtheWillow SpringPlutonin theCopper
CanyonFormationto thenorth[HolmandLux, 1991]indicate
exposureof theserockspriorto 5 Ma. Thusdatafromthe
GoldValley areaindicatean increase
of unroofing
ratesto
greaterthan2.3-3.2 mm/yr after 8.7 Ma.
Translationof theAmargosaChaos
Holm andWernicke[ 1990]observed
thatthecrystalline
rocksin the centralBlack Mountainslie well away from
Tertiaryunconformities
of pre-10 Ma stratawhichlie
Fig. 5. (a) Paleogeologicmap of theDeathValley region,
representing
timethatimmediatelypreceeded
deposition
of the
NoondayDolomite [afterWright et al., 1974b]. Filled circles
representbasalnonconformity
of PahrumpGroupstrata.(b)
CrosssectionX-X' just prior to Tertiaryvolcanismshowing
theshallowdepthrequiredfor theNopahUplandassuming
stratigraphic
depthequalsstructuraldepth. Dashedlines
represent
latePrecambrian
strata.Shadedregiondepictsthe
"missing"
depthdifferential(7-13 km) considering
thatthe
11.6 Ma Willow SpringPlutonintrudedat a depthof 10-15
km.
See text for discussion.
518
Holmet al.: Unroofingof theBlackMountains,
DeathValley
basementrocksandvariousPahrumpGroupsections
in this
area have remained intact since Precambrian time. If it is
assumed
thatstratigraphic
depthequalsstructural
depth,the
paleogeography
accordingto Wrightet al. [1974b]would
requirethePrecambrian
basement
rocksmakingup the
MormonPointandCopperCanyonturtlebackto lie onlya few
kilometersbelow thesurfaceat theonsetof extension(Figure
5). However,
the40Ar/39Ar
dataandgeobarometry
on
Mioceneintrusions
presented
hereindicatea midcrustal
depth
(10-15 km) for theserocksduringMiocenetime.
It seemsunlikelythatMioceneintrusiverocksbeneaththe
Precambrian
nonconformity
at AshfordCanyoncouldhavea
differentialdepthof 7-13 km overa distanceof only 5-6 km
betweentheirexposurealongtheMormonPointturtleback
and
AshfordCanyon(Figure3). HolmandWernicke[1990]
attributedtheproximityof theunmetamorphosed
andregionally
metamorphosed
PahrumpGroupstratato translation
of the
unmetamorphosed
rocksnorthwestward
awayfromanoriginal
positionin the southeastern
BlackMountains.Translation
occurred
alonga detachment
zoneunderlying
thehighlyfaulted
Amargosachaos,thedetachment
zonehavinglocallycutinto
basementrocksbeneaththe nonconformity,
thusjuxtaposing
allochthonous
basement(andtheoverlyingnonconformity)
ontointactbasementin the vicinityof AshfordCanyon(Figure
3). We emphasizethatthisdetachment
faultwouldbe difficult
to mapwherebasementwasfaultedontobasement.However,
the~283 Ma muscoviteageminimumof sampleAM2a-M
approaches
agesobtainedfromthebasement
in thesoutheastem
BlackMountains(samplesRH-BIO andIH1-BIO) suggesting
equivalence.In addition,thissample,collectednearerto the
inferredfault thansampleAM-UNC-M, showsa disturbed
theCopperCanyonturtleback.Thiscoolingagepatternis
visiblein samplesfrom boththeplutonandthePrecambrian
basementrockssuggesting
thattheplutonthermally
equilibrated
with thecountryrockat temperatures
abovethe
closuretemperature
for biotite. We interpretthenorthwest
youngingof biotitecoolingagesto reflectprogressive
tectonic
unroofingof the CopperCanyonand MormonPoint
turtlebackstowardthenorthwestfrom midcrustaldepths
(temperatures
above300øC),consistent
with theoverall
regional
extension
direction
andwithregional
westdirected
tectonicdenudation
suggested
by Wrightet al. [1984].
Progressive
unroofing
wasaccompanied
by footwalldike
intrusionwhichalsodecrease
in agetowardthenorthwest.
Mica coolingagesfromthenorthern
BlackMountains
(the
Badwaterturtleback)are substantially
olderthanmicasfrom
boththeCopperCanyonandMormanPointturtlebacks
in the
centralBlack Mountains. The dissimilarcoolingagessuggest
thatrocksmakinguptheBadwaterturtleback
lay at a shallower
level in the crustprior to intrusionof theWillow Spring
Plutonandtheonsetof unroofing
thanweretherocksof the
CopperCanyonandMormonPointturtlebacks.
Thereforethe
CopperCanyonandMormonPointturtlebacks
wouldrepresent
thedeepest
exposed
levelof theBlackMountains
crustunroofed
duringTertiaryextension;
thisis in contradiction
to the
westward
deepening
crustalsectionhypothesis
presented
by
Holm andWemicke [ 1990]. A possibleexplanation
for the
oldermicaageswhichwouldresolvethiscontradiction
considers
theBadwaterturtlebackas allochthonous
withrespect
to thesoutherntwo turtlebacks,
havingbeentranslated
northwestward
in thedirectionof overallregionalextension
similarto theAmargosa
chaosdescribed
above.Restoration
of
spectra
withexcess
40Ar,acharacteristic
common
infaulted
the Badwater turtleback southeastwardto a location either above
andshearedrocks. We concludethatthethermochronologic
datapresented
here,whentakenin conjuction
withdepth
determinations
for Mioceneintrusions,
arebestexplainedby
restorationof the Amargosachaos(andthelatePrecambrian
nonconformity
exposedat AshfordCanyon)15-20 km
southeastward
to equivalentrocksin thesoutheastern
Black
or southeast
of theCopperCanyonturtlebackwouldaccount
for thecorrectpositioning
of oldermicaagesandwouldalso
explaintheformationandconcentration
of high-temperature
Mountains.
CoolingAge Pattern
Samplesfrom thesoutheastern
andcentralBlackMountains
showa patternof coolingwith biotiteagesyoungingtoward
the northwest(Plate 1). Biotitetotalgasagesof 230-235 Ma
ductiledeformationfabricsaroundtheCopperCanyon
turtleback
4-5 m.y.aftertheBadwater
turtleback
cooledbelow
300øC.Alternatively,thenorthwest
dippingdetachment
fault
mayhaverampedupto a shallower
levelin thecrusttowardthe
northeast
in thevicinityof theBadwaterturtleback.As dike
intrusionseemsto haveaccompanied
unroofing
andcoolingin
theGoldValley andCopperCanyonturtleback
regionsat 7.9
Ma and6.8 Ma, respectively,
theyoungerdikeslocated
onthe
Badwater
turtleback
(6.3 Ma) mayreflectthelaterunroofing
of
thatareaalbeitfrom shallowerdepths(<300øC).
from Precambrian basement in the southern Black Mountains
(samplesRH-BIO andIH1-BIO) suggest
temperatures
below
300øCwell beforethe onsetof extensionin thisregion.
Shallowdepthsfor theserockspriorto Tertiaryunroofing
wouldbe expectedgiventheirproximityto unmetamorphosed
latePrecambrian-Cambrian
andTertiarysedimentary
rocks
whichdepositionallyoverliebasementin thesoutheastern
Black Mountains.Taking300øCas themaximumtemperature
of thesesamplespriorto Tertiaryunroofing
andconsidering
a
low geothermalgradient,we obtaina depthmaximumof 12
km priorto extension.Thermochronology
andfieldrelations
in theadjacentGreenwater
Rangeindicateunroofingof~10 Ma
shallowlevel granitesbetween9 and 10 Ma while thewestern
BlackMountainrocksremaineddeeperthan10 km.
In the centralBlack Mountains,biotiteageswithin the
Willow SpringPlutonshowa youngingtowardsthenorthwest
with samplescollectedfrom Gold Valley coolingthrough
300øC~1.5 m.y. beforesamplescollectedat andnortheast
of
CoolingAge versusElevation
Hornblende
agesfromundeformed
WillowSpringPluton
samples
suggest
uniformcoolingof theintrusion
below500øC
at 10.1-10.4 Ma, approximately1.0 to 1.5 m.y. after
crystallization.
Similarhornblende
coolingagesfrom
undeformed
samples
of theplutonwereobtained
by McKenna
[ 1990]. Thereis nodiscernable
agedifference
between
samples
collectedoveran elevationdifferenceof ~ 1.5km (Figure6).
Thisis perhapsnot surprising
considering
thatthepluton
intrudedintocountryrockwithtemperatures
below500øCat its
base.Intrusionof theplutonwouldhavecreateda temporary
unstable
geotherm
withaninvertedisotherm
at itsbase.While
it is certainlypossiblethatsomecoolingmightbeattributed
to
denudation,
we feel thatat thesehightemperatures
coolingwas
probablydominatedby conductive
coolingof theplutonasit
thermallyequilibratedwith thecountryrock. Rapid
Holm et al.: Unroofingof the Black Mountains,DeathValley
2.0-
biotite
519
footwallrelativeto changesin structuraldepthacrossstrike
allow an estimateof theaverageinitial dip whenvariablessuch
o hornblende
E
as amount of internal extension and amount of overburden are
considered. For instance,a thick sectionof overburdenwill
givea lowerestimatefor theinitial dip of thedetachment
beneaththe nonconformity.Also, intemalextensionof the
rangeblockwouldcausethecurrentwidthof exposed
footwall
to be greaterthanit initially wasandthusmakethedip
estimatelower. Assumingonly 1-2 km of overburden
above
the nonconformityanda liberalestimateof 50% intemal
0.5
6
I
I
I
I
I
7
8
9
10
11
Age, Ma
Fig. 6. Coolingageversuselevationdatafrom thecentral
Black Mountains. Data pointsare from this studyandfrom
McKenna [ 1990]. Hornblendeagesare from theWillow Spring
Plutononly. Biotiteagesare from theplutonandfrom
Precambrian basement.
equilibration
of a steepor invertedgeotherm
coolingthrough
500øCwouldlikely maskanycompeting
effectof cooling
associated with vertical denudation and therefore no difference in
hornblende
coolingageswithelevationwouldbe expected
[cf.
Wright et al., 1991;McKennaandSnee,1991].
We havealsoplottedbiotitecoolingages(fromboth
Precambrianandgabbro-diorite
samplesfrom thecentralBlack
Mountains)versussampleelevationusingdatafromthisstudy
andfrom thatof McKenna[ 1990]. At first sightthemappears
to be considerable
scatterin thebiotitedatawithrespectto
elevation(Figure6). However,coolingagesobtainedfrom
specificregions(8-9 Ma agesfromtheGoldValley/Mormon
Pointarea,and6.7-7.0 Ma agesfromtheCopperCanyon
region)appearto showa steeppositivecorrelation
with
elevation.A possibleinterpretation
is thatthe steepslopes
represent
locallya verticalcomponent
of motion(present
day
coordinates)of the 300øCisotherm.If the motionwere
approximately
verticalandrelatedto unroofing,
theageversus
elevationdatafrom thisstudysuggests
unroofingratesof ~2.54.0 km/m.y. We emphasize,however,thatcautionmustbe
usedwheninterpretingsuchdatain theBlackMountains
especially
considering
therecentevidencefor Mioceneand
youngerfoldingof the pluton[Holm andLux, 1990;Pavlis,
extension
of therangeblock,weobtaina maximum
average
initialdipangleof ~30ø. Considering
3-5 km of overburden
andlessintemaldisruption
(10-20%) givesinitialaverage
anglesof ~15ø-20ø. By usingthisandothertechniques,
similar
averageinitialdipshavealsobeendetermined
for detachment
faults in west central Arizona and southeastemCalifornia
[DokkaandBaksi, 1988;Miller andJohn,1988;Fosteret al.,
1990; Richardet al., 1990].
Recentmappingof theBadwaterturtleback
suggests
that
transport
of theupperplateoccurred
onmoderately
to steeply
dippingsurfaces
in themiddleanduppercrust,favoring
only
moderate
amounts
of extension
in theDeathValleyregion
[Miller, 1991]. In addition,the Black Mountainshavebeen
citedasanexample
of largefootwallupliftbeneath
a planar
high-angle
normalfault,againrequiring
onlymoderate
to
minorhorizontal
extension
[KingandEllis, 1990]. These
conclusions
conflictwithregionalinterpretations
thatsuggest
largeamounts
of extension
in theregion[SnowandWemicke,
1989;Stewart,1983;Wemickeet al., 1988]andwith thelow
averageinitialdip of thedetachment
obtainedhere. The
conflicting
results
of theabovestudies
mayberesolved
in part
by considering
theinitialgeometry
of thedetachment
andits
evolutionduringextension.It seemsapparent
thatnearthe
preextensional
surfacein the southeastem
Black Mountainsthe
detachment
initiallydippedmoderately
to steeply,
asindicated
by themoderately
to steeplyeasttiltedTertiarystrataandsteep
fault-bedangles.Considering
initialdipsin theupper5-10 km
of 400-90ø, theaverage
initialdipbelowthatdepthwouldbeof
theorderof 6ø-13ø(Figure7a). Theseconsiderations
implyan
initiallylistricgeometry
similarto thatcommonly
observed
on
reflection
seismograms
of normalfaultsin theBasinandRange
andelsewhere[SmithandBruhn,1984].
As extensionandunloadingof thefootwalloccurs,isostatic
forcescausethefootwallto flex anduplift;thisresultsin the
migration
of a monoclinal
flexurethrough
thefootwallin the
19911.
directionof transport[Hamilton,1988;Buck, 1988;Wemicke
Initial Dip and Kinematicsof DetachmentFaulting
rapidcoolingpattern,a resultof unroofing
occurring
progressively
across
therange.Thedifferential
cooling
A principalfocusof interestin continental
extension
concemsthe initial geometryandangleof dip of major
detachmentzones[Buck, 1988; Wemicke and Axen, 1988;
King and Ellis, 1990; Miller, 1991;Holm, 1991]. The
thermochronologic
andgeobarometric
constraints
presented
here
suggestlimits on the averageinitial dip of theBlack
Mountainsdetachment.The datapresented
aboveindicatethat
thedeepestportionsof theBlackMountains(theregion
adjacentto andoccupyingthenorthernportionof theCopper
Canyonturtleback)wereat a maximumdepthof 15 km priorto
the onsetof unroofing.Projectedontoa crosssection
subparallel
to theregionalextension
directiontheserocks
currentlylie -45 km awayfrom thetiltednonconformity
in the
southeastern
BlackMountains.The currentwidthof exposed
represented
inthe40Ar/39Ar
data,
aswellastheaccompanying
and Axen, 1988]. This would be reflectedin a diachronous
footwalldikeintrusion
in theBlackMountains,
mayreflect
thisprocess.Anotherconsequence
wouldbethatgently
dippingto subhorizontal
ductilefabricsdeveloped
in themiddle
crust(wheretheaverage
initialanglewasverylow)during
extension
wouldsteepen
in thebrittleuppercrustduring
unloading
(Figure7b). As unroofingcontinued
andthe
steepened
fabricswerebroughtnearerthesurface,
theywouldbe
tiltedonceagainto loweranglesastheflexurecontinued
to
migratethroughtheuppercrust.Thereforethehigh-angle
orientation
wouldnotbetheoriginalorientation
asargued
by
Miller [1991];ratherit istheresultof anearliersteepening
as
therockswereunroofedfroman originallylow-angle
orientationin the midcrust(Figure7).
520
Holmetal.: Unroofing
oftheBlackMountains,
Death
Valley
A
!
Mylonitization
priorto 12.5 Ma
•' '"' '"' '"' (currentlyexposedon Badwaterturtleback)
Future sliver of
Tertiary strata
AmargosaChaos
Precambrian
strata
Mylonitizationat 8-9 Ma
(currentlyexposedon CopperCanyonturtleback)
BWT
CCT
lO
GV
(a)
km
15
I
I
40
6.7-7.0
Ma
I
3,0 km
7.5-8.5
20
I
10
Ma
Unroofing
between
Unroofingand dike intrusion
10-9 Ma
(b)
N
N
Fig.7. (a) Schematic
crosssection
(alongA-A' of Figure2) showing
likelyaverage
initialdipand
geometry
of detachment
faultresponsible
forunroofing
of theBlackMountains
andreconstructed
position
of Badwater
turtleback
rocks(BWT)priortounroofing.
GV isGoldValleyregion;CCTisCopper
Canyonturtleback
rocks.Depthestimates
areobtained
fromgeobarometry
andfromthermochronology.
(b) Diagrammatic
representation
of preferred
modelforunroofing
of theBlackMountains
[afterHamilton,
1988; Buck, 1988; and Wemicke andAxen, 1988]. Seetext for discussion.
CONCLUSIONS
Wehaveused
40Ar/39Ar
agespectrum
data,
combined
with
geobarometry,
to constrain
theintrusionandunroofing
history
of theBlackMountains.DatafromtheGoldValleyregionof
the central Black Mountains indicate intrusion of the 11.6 Ma
Willow SpringPlutonintocountryrockwithtemperatures
below500øCandat depthsof 10-15 km. A graniticcomplex
datedat 8.7 Ma intrudedintocountryrockabove300øCat 8.211.8km depths.Intrusionof fine-grained
porphyritic
latite
dikesindicateshallowcrustallevelsby 7.9 Ma. Thesecooling
depthdatasuggest
slowto moderate
unroofing
rates(0-2.2
mm/yr)between11.6and8.7 Ma andmorerapidrates(>2.33.2 mm/yr)after8.7 Ma for theGoldValleyregion.
The southeastern Black Mountains were at shallow crustal
levels(<300øC)well beforetheonsetof Tertiaryextension
with basement
biotitecoolingagesof 230-235 Ma.
Thermochronology
andfield relationsindicatethat~10 Ma
shallowlevel graniticplutonsin thesouthern
Greenwater
Rangewereunroofedshortlyaftercrystallization
(by ~9 Ma).
In thecentralBlackMountainsbiotitecoolingagesin rocksof
boththePrecambrian
basement
andsurrounding
Miocene
plutonsyoungto thenorthwest
indicating
diachronous
unroofing
between8.5 and6.7 Ma. Diachronous
unroofing
wasaccompanied
by dikeintrusions
whichalsodecrease
in age
towardthenorthwest.Biotiteagesfrom13Ma to asyoungas
6.7Ma fromtheturtlebacks
indicate
a midcrustal
originprior
to extensional
unroofing.Preextensional
depthsof theorderof
10-15 km andthenorthwest
younging
coolingagepatternare
consistent
withpalinspastic
reconstructions
of Tertiary
extensionwhichrequire~80 km of westdirectedhorizontal
transport
of thePanamintMountains
off thetopof theBlack
Mountains[Stewart,1983; SnowandWemicke, 1989].
The coolingagepatternanddepthestimates
obtainedhere
from acrossthe Black Mountains indicate northwestward
detachment
with an averageinitialdipon theorderof 20ø.
Startingwithan initiallylistricgeometry,
progressive
migration
of a footwallflexurein thedirection
of transport
(expressed
by thediachronous
coolingagepattern)accomodates
theobservations
of moderately
to steeply
dippingTertiarystrata
Holmetal.: Unroofing
oftheBlackMountains,
DeathValley
in the southeasternBlack Mountains [Holm and Wernicke,
1990],andfield relationssuggesting
an earlierhigh-angle
detachment
orientation[Miller, 1991],with thelow average
initial dipsobtainedhere. Mica agesdiscordant
from the
overallnorthwest
youngingtrend(agesfromtheAmargosa
chaosandBadwaterturtleback)
areexplained
by northwestward
transportof initially shallowercrustallevelsontodeeperlevels
alongsplaysof the majordetachmentzone.
521
Aknowledgments.
This work wassupported
by a National
ScienceFoundation
grant(EAR89-71227)awardedtoBrian
Wernicke.We thanktheNationalParkServicefor allowing
samplingandfield mappingin theBlackMountains.We thank
DavidWest for assistance
in thelaboratoryanddiscussion,
BrianWernickefor discussion
andcomments
on an early
versionof themanuscript,
andJohnBartleyandCalvinMiller
for thoughtfulreviews.
REFERENCES
Armstrong,R.L., Geochronology
of Tertiary
igneousrocks,easternBasinandRange
Province, westernUtah, easternNevada, and
vicinity, U.S.A., Geochim. Cosmochim.
Acta, 34, 203-232,
1970.
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