JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B8, PAGES 13,509-13,528,
JULY 30, 1991
Limited ExtensionDuring PeakTertiary Volcanism,
Great Basin of Nevada and Utah
MYRON O. BEST AND ERIC H. CHRISTIANSEN
Department
of Geology,BrighamYoungUniversity,
Provo,Utah
Therelativetimingandmagnitude
of middleTertiaryextension
andvolcanism
in theGreatBasin(northern
BasinandRangeprovince)
of thewestern
UnitedStates
remaincontroversial.
To constrain
thetiming,wepresent
31 stratigraphic
sections
fromthecentralpartof theprovince,
together
withdatafromotherstudies
in theGreat
Basin.Especially
significant
inthisrecord
ofregional
paleogeographic
andassociated
tectonic
conditions
arethick
sections
ofmanywell-dated
ashflowsheets
emplaced
duringtheperiodof themostvoluminous,
orpeak,volcanic
activityabout31-20 Ma. Fromthesedatawemakethefollowing
conclusions:
(1) Extension
priortotheperiodof
peakvolcanism
wasapparently
localized.
(2) Extension
duringpeakvolcanism
(theignimbrite
flareup)wasminor
andinplaces
possibly
relatedtomagmatic
processes
intheshallow
crust,ratherthantoregional
tectonic
processes.
Angular
unconformities
andinterbedded
epiclastic
deposits
withinsequences
of volcanic
rocksfrom31toabout
22-20 Ma thatwouldmanifestsynvolcanic
faulting,tilting,anderosionarelimited.(3) In theGreatBasinasa
whole,majorextension
andpeakvolcanism
correlate
poorlyin space
aswellastime.(4) Essentially
dip-slipfaults
cutting
theentireconformable
volcanic
sequence
arecommon
intheGmatBasinandindicate
awidespread
episode
of extension
afterpeakvolcanism.
Southward
sweeping
Tertiaryvolcanism
in theGreatBasinreflectsmigration
of
the mantlemagmasupplythat poweredcrustalmagmasystems.
We suspect
this migrationwasrelatedto
progressive
southward
foundering
andsteepening
of dipof a subducting
oceanic
plate(afteranearliest
Tertiary
near-horizontal
configuration
beneaththe continental
lithosphere)
andconsequent
backflowof asthenospheric
mantleintothewideningwedgebetweentheplates.In thenorthern
GreatBasin,wherethesweepwasrapid,we
postulate
thatrelativelysmallvolumes
of mantle-derived
magmawereinserted
asdikesintothelower,locally
extending
crustwhichwasunusuallywarmbecause
of Mesozoiccompressional
thickening;
crustalmagma
systems
sopowered
wererepeatedly
tapped
tofeedmodest
volumeeruptions
of chieflyintermediate
composition
lavaandminorsilicicashflow tuff. As thesweepstagnated
in thecentralsouthern
GreatBasin,copiousvolumesof
maficmagmawereinsertedinto the crust,apparently
mostlyas extensivehorizontalsheets,or sills, in a
nonextending,
upliftingcrustin a stateof nearlyisotropic
horizontal
stress.
Thesesillsandthehighmantlepower
inputoptimized
crustal
magma
generation,
creating
hugevolumes
ofsilicicmagma
thatvented
aslargevolumeash
flows,chieflyabout31-20 Ma. Afterabout22-20 Ma, thevolcanic-capped
plateaucollapsed
in a widespread
networkof northstrikingextensional
faultsasplateboundary
compressive
forceswereovercome
by spreading
forceswithintheuplift.Eruptionof lavaagainbecamethedominant
modeof volcanism.
p. 1] and"... theonsetof extensional
faultingat anygivenlatitude
in the BasinandRangegenerallycoincidedwith, or immediately
Because
of itslonghistoryof platesubduction,
magmatism,
and postdated,
voluminous
eruptions
of intermediate
to silicicvolcanic
tectonism
duringtheMesozoicandintotheTertiary,westernNorth rocks"[Gansetal. 1989,p. 45] (seealsoChristiansen[ 1989], Gans
INTRODUCTION
America continues to be a focus of much attention. The northern
andMahood, [ 1989], andZobacket al. [ 1981]). The generalization
BasinandRangeprovinceof NevadaandwesternUtah, hereafter of Ganset al. [1989] is basedupon(1) detailedstudyof a 15,000
referredto astheGreatBasinbecauseof the nearlycoextensivearea klTl
2 areain eastcentralNevada(Figure1), whereof theorderof
of internaldrainage(Figure1), harborswidespread
calc-alkaline 250% extensionoccurredconcurrentlywith andfollowingeruption
rocks of intermediateto silicic compositioncappedlocally by of about1000km3of daciticlavaandminorashflow tuff, mostly
basalticandrhyoliticrocks.For nearlytwo decades
[e.g., Chris- 36-35 Ma, in theearlyOligoceneand(2) reviewof work by other
tiansenandLipman,1972;Noble, 1972]thiscompositional
transi- geologists
in sevenotherlocal areaswithin the BasinandRange
tion duringthe Miocenein the westernUnited Stateshas been (Figure1) whichareinterpreted
to support
theconceptof synexten-
attributedto a shiftfrom convergentto a youngertransformplate
margin,with associated
crustalextension
inland.
Thetectonomagmatic
evolutionof theGreatBasindifferssignificantlyfromthesouthern
BasinandRangeprovince[Glaznerand
Bartley, 1990], an areanotconsidered
here.
Recently,somegeologists
haveconcluded
thatpartsof theGreat
Basinexperienced
majorextension
priorto the Mioceneandthat
"... major[volcanic]eruptions
appearto be bothtemporallyand
spatiallyassociated
withlarge-magnitude
extension"
[Gans,1987,
sionalmagmatism.
Gans [1987] and Gans et al. [1989] recognizedthat in the San
JuanandMarysvalevolcanicfields(Figure1) andevenwithinsome
areasof the BasinandRangeprovince,voluminousvolcanismdid
not accompanysubstantialcrustalextension.Nonetheless,Gans
[1987] concludedthatall Tertiaryvolcanismin the westernUnited
Statesis fundamentally
synextensional,
becauseof a highlymobile
and activelyextendinglower crustand mantlelithosphere;weak
couplingwiththelowercrustallowsareasof extremeuppercrustal
extensionto be interspersed
amongareasthat lack normalfaults
at the surface.This tectonomagmaticviewpoint constitutessome
Copyright1991by theAmericanGeophysical
Union.
Papernumber91JB00244.
0148-0227/91/91 JB-00244505.00
departurefrom the conventionalconceptof a subductionsetting
of little or no extensionin which intermediatecompositioncalc-
alkalinemagmasaregenerallyconsidered
to form [e.g., Hildreth,
1981, Figure15] andthereforedemandsclosescrutiny.
13,5O9
13,$ l 0
BEST AND CHRISTIANSEN:EXTENSIONAND VOLCANISMIN GREAT BASIN
In contrastto the modelof synextensional
magmatismandactive
rifting, Taylor et al. [1989, p. 7771] concludedthat "There is a
generallack of correlationbetweenextensionalperiodsandvolcanism" in the southeastern
Great Basin. They (seealsoBartley et al.
Nevadaandpartsof CaliforniaandUtahwereincluded.Digitized
time-areasummaries
of thesemapsare shownin Figure3b. In
addition
toillustrating
thewell-known
ignimbrite
flareupof thelate
Oligocene-early
Miocene, these maps clearly demonstratethe
[1988])examined
anareaofabout8000km2insoutheastern
Nevada sparse
areaof exposure
of sedimentary
deposits
of thistimeperiod,
(Figure 1) and found crustalextensionto be episodicduring four
lessthanin the brieferolderandyoungerones.Similarlyscarce
periodsof time, as follows: prevolcanicbefore 32 Ma, early syn- sedimentary
deposits
aredisclosed
in theTertiarypartof 18 strativolcanic 30-27 Ma, immediately postvolcanicabout 16-14 Ma,
and Plioceneto Quaternary.Although the timing and amountof
extensionare not accuratelyquantifiablewith respectto the entire
transect,extensionoccurredduringeachperiodin somepart of it.
Becausevolcanic activity was time transgressivethrough the
Great Basin [Cook, 1965; Armstrong et al., 1969; Stewart and
Carlson, 1976;Bestet al., 1989b], inadequatesamplingof areasin
spaceand time could yield misleadingconclusions
regardingthe
relativetiming of volcanismandextension.
The purposesof this paper are (1) to presentstratigraphicdata
for the Tertiary of a large part of the central Great Basin, in part
encompassing
areasof previousstudies,(2) to usestratigraphicdata
to demonstrate
that crustalextensionwas episodic,not continuous,
during the Tertiary, as Taylor et al. [1989] found in part of the
province, (3) to determinethe relative timing of volcanismand
extensionand thereforethe casefor passiveversusactive rifting,
and (4) to proposean alternativetectonomagmatic
model for the
Tertiary of the Great Basin.
OTHER PERTINENT PREVIOUS WORK
Three decadesago, Mackin [1960] first conceptualizedthe significanceandutility of Tertiary volcanicrocks,particularlydatable
ash flow sheetsthat are widespreadin both time and space, in
elucidating the structural evolution of the Basin and Range
province.Mackin [ 1960, p. 114] notedthat"... theyprovidea key
(a) to suchproblemsaswhethertheblockfaultinghasbeenepisodic
in a regionalsenseor randomin distributionin spaceandtime, (b) to
the relationshipof the block faulting to eruptiveactivity.... "The
possibilityof two or more episodesof normal faulting in some
placesand lack of suchfaulting in otherswas considered[Mackin
[1960, p. 113].
Usingonlybasicphysicalcharacteristics
andwithoutthebenefit
of radiometricages,[Cook, 1965], a studentof Mackin, correlated,
named, and determinedthe distributionand volume of many ash
flow sheetsfoundin the areaof Figure5. In thispioneeringeffort,
he constructednumerousstratigraphiccolumns, 13 of which are
includedherein updatedform. Cook [ 1965, p. 55] recognizedlocal
prevolcanicfaulting, suggested
someaccompanied
volcanism,but
graphicsectionsin theeasternGreatBasinof Utah [Hintze, 1988].
Zobacket al. [ 1981], Eaton [ 1982], Engebretson
et al. [ 1984],
and Wernickeet al. [ 1987] advocatedconcurrentcrustal extension
andcalc-alkaline
magmatism
duringtheTertiary.Thesestudies
will
be examined in a later section.
BRIEF OUTLINE OF TERTIARY VOLCANISM
IN THE GREAT BASIN
Volcanismbegannorthof the GreatBasinin the Paleoceneand
sweptsouthward,
enteringthe northernpartof theprovincein the
Eoceneabout43 Ma [CrossandPilger, 1978;Bestet al., 1989b],
andcontinued
a southward
transgression
alonganarcuate,
roughly
east-west
frontinto southern
Nevadaby Miocenetime (Figures1
and 2). Age determinations
and chemicalanalyses[Bestet al.,
1989b;seealsoMcKee et al., 1970;McKee, 1971;Noble, 1972],
small-scalemaps [Stewartand Carlson, 1976], and estimatesof
volumesof largeandvery largeashflow tuff deposits
(Figure3)
showthe following:(1) Until about31 Ma mostof the erupted
magmawashigh-K andesite,dacite,andrhyolitelavasandminor
ß
ß ß
ß
ß
ß
wrestled with the cause and effect relation.
In a studyof an area (Figure 1) near thoseinvestigatedby Gans
et al. [1989] and Taylor et al. [1989] but two decadesearlier,
Moores et al. [1968] concluded that limited normal faulting
occurredduringtheperiodof maximumpyroclasticactivity(37-25
Ma) but was followed by greaterdeformationduringthe Miocene
Fig. 1. Indexmapshowingsomeareas(diagonalruling;datasourcesfrom
Bestet al. [1989b, Figure 2] andBest et al. [1989a, Figure 1]) of southwesternUnitedStatessubjected
to voluminousvolcanicactivityduringthe
middleTertiaryand areasof studyreferredto in text. Great Basinareaof
internal drainageis indicatedby heavy dotted line (Fenneman [1931];
and Pliocene(?).
California part is not consideredin this report). Late Oligocene-early
McKee et al. [ 1970] first pointedout that the (1) overall scarcity Miocene (34-17 Ma) Reno-Marysvalezone in the Great Basin extends
of sedimentarydepositsin the Great Basin older than the middle eastwardfromReno,Nevada.SmallsolidareasbetweenMarysvaleandSan
Miocene, (2) widespreaddistributionof thin ashflow sheets,and Juanvolcanicfieldsrepresentdioriticlaccolithsemplaced32-21 Ma [Sullivan et al., 1991]. Arrows off southern California coast are vectors of
(3) sameamountof deformationof old andyoungTertiaryrocksin convergenceof oceaniclithosphere(Vancouverplate) beneathcontinent
most places indicate the existenceof little relief and therefore from59 to 36 Ma (relativespeeds,indicatedby arrowlength,anddirections
30-35 fromStockand Molnar [1988, Table 15]). For comparison,vectors
limitedfaultinguntil late in the Tertiary.
In a seriesof time-periodmaps,necessarilygeneralizedbecause of convergence(50-25 Ma) from Engebretsonet al. [1985, Figure. 8] are
alsoshown.Solidline openboxesare areasof crustalextensionalreviewed
of the small 1:1,000,000 scale, Stewart and Carlson [ 1976] showed
by Ganset al. [ 1989], fine dottedline box by Taylor et al. [ 1989], andsolid
thelithologiccharacterof Tertiaryrocksin Nevada,therebyinclud- line ellipsebyMooreset al. [ 1968]. Dashedline in westernNevadaencloses
ing muchof theGreatBasin.Smaller-scale
time-lithologicmapsof areaof earlyMioceneextensiondocumentedby Johnet al. [ 1989].
BEST AND CHRISTIANSEN:EXTENSION AND VOLCANISM IN GRF•T BASIN
SOUTH
LATITUDE
37
4,5
38
39
NORTH
[ o N)
40
42
41
45
•
13,$11
thereforegenerallypredatesthe extrusionof basalt. A transition
from orogenic(calc-alkaline) to anorogenic(rare metal) rhyolite
occurredabout21 Ma [Christiansenet al., 1986]. Thus tectonic and
35
35
'-' 25
25
0
miles
0
100
magmatictransitionsin the Great Basin are only looselycorrelated
duringMiocene time.
STRATIGRAPHIC DATA
The 31 stratigraphicsectionslocatedand shownin Figures5 and
6 (see also Tables 1 and 2) constitutea sample of the Cenozoic
1(•0
kilometers
10-
GREAT
SOUTH
TUFFS
NORTH
o
o
o
Fig. 2. Deceleratingsouthwardtransgression
of calc-alkalinevolcanismin
the GreatBasin. (Top) Space-timedatafrom Bestet al. [ 1989b, Figures3
and 7] in a time-latitudeplane along the Nevada-Utah stateline.Sites of
datedlava flows (dots) and sourcesof regional ash flow tuffs (lines and,
wherepyroclasticactivitywasextendedin time, boxes)locatedon Precambriancrusthavebeenprojectedontothe plane. A bestfit line throughthe
(a)
BASIN
> 50,000 km3
_
o
10
age (Ma)
datashowsthatthesouthward
sweepnearlystagnated
(speedonlyabout3
km/m.y.)
inthesouth
central
Great
Basin
intheReno-Marysvale
zone
20-
wherethe ignimbriteflareupoccurred.(Bottom)A speculativeandhighly
schematic
cross-sectional
model,in aboutthesameplaneastopdiagram,
accountingfor the deceleratingtransgression
[cf. Lipman, 1980]. During
GREAT
the
Tertiary,
the
oceanic
plate
isbelieved
tohave
fallen
away,
orfoundered,
progressively
southward
fromtheoverlying
continental
lithosphere.
Mafic
nentalmagmasystemsare presumedto havebeengeneratedin the south-
LAVAS
290
magmas
thatsupplied
massandthermalenergyto thetransgressive
conti-
BASIN
ages
10-
ward
migrating
wedge
ofasthenospheric
backflow
overlying
thesubducting
plate.Becauseof considerable
uncertaintyin theamountof east-westcrustal
extension
since
peak
volcanism
intheGreat
Basin,
wehave
notattempted
to
adjustdatapointsto a preextension
configuration.Nonetheless,compensation for any amountof east-westextensionmakes the sweepmore south-
westerly
through
theGreat
Basin
sotheReno-Marysvale
zoneismore
nearly
perpendicular
to thedirectionof convergence
(seeFigure1) of theoceanic
40
3'0
2'0
plate.The founderinglip of the subducting
oceanicslabin the GreatBasin
areais considered
to havehad a somewhatarcuate,subhorizontal
hinge;a
Fig. 3. Peakvolcanismoccurredin theGreatBasinfromabout31 to 20 Ma.
complexwarpin the descending
slabmaynothaveexisted.
(a) Volumesof dated,regionallyextensiveashflow tuff sheetsestimated
from numerousmeasuredsectionsand geologicmapsare shownin top of
figureandpublished
andunpublished
agesof lavaflowsin bottompart(both
fromBestet al. [ 1989b]). Becauseof samplingbiases,the agehistogram
pyroclasticmatedhal,chiefly rhyolitic. (2) From about31 to about doesnot accuratelyportrayvolumesof lavas but suggeststhat lesswere
20 Ma, similar lavaswere extruded,but their volume was clearly extrudedduringpeakashflow activitythanbeforeandafter. (b) Areasof
tuff, lava, and sedimentarydepositsin the Great Basinfrom the mapsof
subordinate
to a muchgreater
volume,morethan35,000km3, of Stewart
and Carlson[1976] plottedin termsof total areafor the map time
rhyoliticanddaciticashflow deposits.In theGreatBasin,aswell as intervaldividedby the durationof the time interval (43 Ma - 34 Ma = 9
othermajorvolcanicfieldsin southwestern
North America(Figure m.y., etc.). Areasof lavaflowsandtuffs arefrom theirsmaller-scale
maps
depositsfrom the larger-scalemap
4), this late Oligocene-earlyMiocene periodrepresentspeak vol- of Utah andNevadaandof sedimentary
of Nevada(notechangein scalefor sedimentary
deposits).Theseareasdo
canicactivity,the"ignimbdhte
flareup".(3) Followingabout20 Ma,
notaccurately
represent
trueeruptedvolumesof magmain the GreatBasin
ashflow volcanism,chieflyrhyolitic,wanedin theGreatBasin,and
extrusions
of lavaflourished.Increasinglyalkalic,especiallysodic,
lavasof a broaderspectrumof silica concentrations
were erupted
[Bestet al., 1989b]. However,a well-developedbimodalcompositional spectrumand truebasalt(InternationalUnion of Geological
Sciencesclassification)did not appearuntil afterthelatestMiocene,
about8-6 Ma. Most dark coloredlavascontainingpyroxeneand
locally olivine phenocrystsof pre-Plioceneage are not basalts
becauseof their relatively high contentof alkalies and silica. The
inceptionin theGreatBasinof widespreadextensionalfaultingand
creationof local basinsin which erosionaldebriscollectedbeginningabout17 Ma [McKeeet al., 1970;Stewartand Carlson, 1976]
becauseof (1) variablethicknesses
(somelarge calderasharboras much as
2 km of tuff), (2) preferentially
greatererosionandburialof olderdeposits,
(3) voluminous extrusion of lavas in the Cascade and Snake River
Plainprovinces
adjacentto thewesternandnorthernmargins,respectively,
(4) a considerable
volumeof ashcarrieddownwind
outof theprovince.,
and
(5) regionallyvariable amountsof extensionsincevolcanism.For these
reasons,
theareaof 34-17Ma ashflowtuff(71,000km2)probably
underrepresents
the volumeof magmaeruptedrelativeto volumeof extruded
17-6 Ma lavaflows(areaof 64,000km2).Aboutone-half
of theareaof
34-17 Ma lavaflowsarein theGreatBasinpartof theMarysvalefield. Half
of the 34-17 Ma sedimentary
materialis associated
with knowncalderas,
makingthe amountpossiblyrelatedto regionaltectonismduringthis time
periodevenlessthanshownandtrivialcompared
to theamountfrom 17to 6
Ma, unlessit is preferentially
buriedin geologicallyrecentbasins.
13,$12
BESTAND CHRISTIANSEN:
EXTENSION
ANDVOLCANISM
IN (•RF..AT
BASIN
erosional
debristhatcouldbeconfused
withproducts
of regional
tectonicprocesses.
Althoughwe have avoidedknownlarge
(b)
4
GREAT
calderas,the possibilityremainsthat the rockrecordin somesec-
BASIN
tionshasbeeninfluenced
by localmagmatic
processes;
twosuch
TUFFS
sectionsmightbe numbers18 and 26 (seediscussion
below and
Figures6 and 7).
2
Stratigraphic
Recordof Synvolcanic
Extension
•
Significant
synvolcanic
brittleextension
ofthecrustwouldproduceconspicuous
angular
unconformities,
fanning
dips,andsedi-
-
• 01 4'0 '
;m
3'0 '
2'0 '
1•) '
mentarydeposits
withinthevolcanicsequence
[Ganset al., 1989,
Figure 18] (seealsoEaton [1982, Figure6C] andNoble et al.
[1990b]).Slideblocksmayalsooccurbetween
sedimentary
de-
GREAT
BASIN
GREAT
BASIN
LAVASrhyolite
•
FF
,,..• >50
._c
LAV• 50?
o
i
MARYSVALE <.
'•.
',
0
,
•
s
ß
40
I
•
30
20
SAN
10
NEV^D^
I
SEDIMENTARY
DEPOSITS
40-
J
(partly tuffaceous)
o I 4•
'
3•
'
2•
'
1•
'
4O
30
MA
20
10
Age (Ma)
Fig. 3.
(continued)
>6
MOGOLLONDATIL
geologicrecordin the central Great Basin where peak volcanism
with respect to the entire province occurred during the late
Oligocene-earlyMiocene [Bestet al., 1989b]. Thesedatafurnisha
regionalstratigraphicframeworkfor evaluationof the relationbetween extension
and volcanism.
Thirteen
of these sections were
examinedby Cook [ 1965] andsomein westernUtah areincludedin
SIERRA
the work by Hintze [1988]. The 31 have good chronologicconMADRE
straintsand were chosento representthe longestintervalsof time
OCCIDENTAL
and the mostcompletesequenceof ashflow sheetsin a particular
area. With the exceptionof sections9- 13, which representtime
periodsof only a few million years, most cover about 10 m.y.
during the peak late Oligocene-earlyMiocene volcanism;some
sectionscovera greaterinterval of time.
Fig. 4. Highly generalizedtime-volumerelationsof lava flow and tuff
It iscriticallyimportant
inevaluating
thetimingofvolcanism
and depositsin volcanicfields of the southwesternUnited Statesand Mexico
regionaltectonism
to excludefromthedatasetstratigraphic
sec- (SierraMadreOccidental[fromMcDowelletal., 1990]).Volumeestimates
in thousandsof cubic kilometers(sourcesof data are in Best et al.
tionslocatedwithinrecognized
calderas
andothermagmatic
cen- are
[ 1989b, Table 1] andRatt• et al. [ 1989]). Note nearcoincidencein time of
ters.In suchplaces,subsidence
andpostcollapse
resurgence
and voluminous
ashflowactivity(ignimbrite
flareup)in thefiveareas[Noble,
associated
faultinghaveproduced
localangularunconformities
and
1972].
BEST
AND
CHRISTIANSEN:
EXTENSION
AND
VOLCANISM
INGREAT
BASIN
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BF•TANDCHRISTIANSEN:
EXTENSION
ANDVOLCANISM
INGREAT
BASIN
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BEST AND CHRISTIANSEN:EXTENSIONAND VOLCANISM IN GRF.•T BASIN
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13,$16
BESTAND CHRISTIANSEN:
EXTENSIONAND VOLCANISMIN GREATBASIN
TABLE 1. Datafor Stratigraphic
Sections(Figures5 and6)
Section
Topographic
Quadrangle/Reference
1
McKee[1976, Figures26,31,and32]
2
Horse Heaven Mountain, Nevada
3
4
Dixon et al. [ 1972]
Moody Peak, Nevada
Pancake Summit, Nevada
5
6
Brown Knoll, Nevada;Kellogg [ 1964]
7
Moores et al. [ 1968] and M. G. Best
8
(unpublished
data, 1991)
Kellogg[1964] andBestetal. [1989b]
9
10
11
Dutch John Mountain, Nevada
Loucksetal. [1989]
Burbank Hills, Utah; L. F. Hintze
12
13
14
Hintze [ 1974]
Hintze and Best [ 1987]
Best and Hintze [ 1980]
15
16
17
18
19
20
21
22
WaterGap NE, Nevada(seeFigure8)
Silver King Well, Nevada
Latitude
North
Latitude
West
39o12 '
39o03 '
38o53 '
39ø01 '30"
39 ø19' 20"
38045'20"
38037'30"
116o49 '
116ø18'
116o10 '
115o51 '
115044 '
114ø54'20"
115022 '
38o33 '
38o27 ' 30"
38o39 '
38054'20"
114o56 '
114o43 '
114o28 '
113049'23"
38o50 '
38o32 '
38o27 '
38o13'30"
38019 '
38020 '
38012 '
38005 '
37049 '
37039 '
37019 '
37019 '
37ø51'
113o28 '
113o55 '
113o51 '
115o18 '
114058 '
113033 '
113032 '
113027'30"
113013 '
113032 '
114003 '
114003 '
114ø21'
37o45 '
114o36 '
37o55 '
37o52 '
37o40 ' 30"
37026 '
37015 ' 10"
37005 '
37045 '
114o39 '
115ø01 '15
115o00 '
115022 '
115013'20"
114051 '20"
115036 '
(unpublisheddata, 1991)
23
Abbott et al. [ 1983]
Best et al . [ 1987c ]
Hintze et al. (in press)
Mackin and Rowley [ 1976]
Sidersetal. [1990]
DodgeSpring,Utah-Nevada;R. E. Anderson
andL. F. Hintze (unpublisheddata, 1990)
CondorCanyon, Nevada;G. J. Axen
(unpublisheddata, 1991)
24
Bennett Pass, Nevada; K. J. Burke
25
26
27
(unpublisheddata, 1991)
Ely Springs,Nevada;Tayloret al. [ 1989]
White River Narrows, Nevada; Taylor [ 1989]
PahrocSpring,Nevada;Williams[ 1967,frontispiece]
28
Hancock Summit, Nevada
29
30
31
Alamo, Nevada;A. S. Jayko(unpublisheddata, 1990)
Delamar 3 SE, Nevada;Swadleyet al. [ 1990]
TempiuteandMeeker Peak, Nevada
Wheresectionsarecompositeandcoverseveralsquarekilometerscoordinates
areonlygivento nearestminute.
Data for sectionsindicatedby topographicquadrangleare from unpublishedwork of M. G. Best and, where
indicated,by othergeologists.
posits,reflectingtectonicerosionof topographic
highsandaccumu- canic units, older than about 31 Ma. The aggregatethicknessof
lation in lowlands.Ash flow sheetswould have significantlateral theseolder sedimentarydepositsis roughlyone half of the total
material.These18 sectionsare scattered
bevariationsin thicknesswithin and between adjacentfault blocks Tertiarysedimentary
andtheirlateralextentwouldbe restrictedby suchblocks.Someof tweeninterveningsectionsin which volcanicrockslie on a clean
thesefeatureshavebeenfoundin eastcentralNevadaby Ganset al. erosion surfacecut into Paleozoic rocks. Four Tertiary sections
rock, all in southwestern
[1989]. However, major angularunconformitiesare not present (19-22) restingonMesozoicsedimentary
withintheirvolcanic
sections,
perhaps
reflecting
therelativelyshort Utah,have100m or moreof Eocene-Oligocene
continental
seditimeinterval(1-2 m.y.) duringwhichmostof thevolcanicrocksin mentaryrock(theClaronFormation)
beneathor partlyintervening
thisareawereerupted
andtheirbetterexposure
in lessextended
and between
thevolcanic
deposits.
Theserelations
suggest
thatasmuch
upliftedareas.
asa fewhundreds
of metersof reliefexistedontopof pre-Tertiary
Thepossibility
existsthatthestratigraphic
sections
portrayed
in rockwithinthecentralGreatBasinareaof Figure5. Conglomerate
Figure6 justhappen
to bepositioned
withinrelativelyundeformedcontaining
rounded
clastsonlyof Paleozoic
rocklie betweenvolblocksin whichtiltingof tuff sheetswasminimal.As we havenot canicunitsmorethanabout31 Ma in sections
1, 6, 8, 11, 12, 13,
investigated
logdatafromdrillholesin present
basins
thispossibil- 14, and17. Mostof theseeightlie in thenortheast
partof Figure5
ity cannotbe fully evaluated.Unextended
blockswouldhavere- and probablyreflectsheddingof erosionaldebrisoff a nearby
mainedhigh,thusprecluding
sediment
accumulation
onthembut highland
exposing
Paleozoic
rocksduringemplacement
of volcanic
alsorestricting
or evenpreventing
accumulation
of ashflow mate- rocksabout34-31 Ma. Fromthestratigraphic
dataaloneit cannot
rial, depending,
of course,
onfactorssuchasreliefandthickness
of be decided
whetherthedetrituswasshedoff (asrecentlyasearly
residualtopographic
highsassociated
with Mesozoic
the ashflow. Oldestashflow deposits,generallyolder than about Oligocene?)
tectonism
or highsproduced
by extensional
faulting
31 Ma, showevidenceof topographic
relief butthedistributionand compressional
thickness
of youngersheetsimply limitedreliefexistedduringtheir immediatelyprecedingpeakvolcanism.
In thepre-31 Ma partsof the Tertiarystratigraphic
sectionsin
deposition(seealsobelow).
Figure6, nomarkedangularunconformities
exist.Significantvariations in thicknessof older sedimentaryand volcanic deposits
reflectdepositionin topographiclows in the erodedterraneof
Eighteenof the 31 sectionsincludelocally tuffaceoussedimen- Paleozoicstrata,especiallyobviousin sections1, 12, 13, 17, 18,
tary depositslying beneathor intercalatedbetweenthe oldestvol- 25, and 30.
Local ExtensionBefore31 Ma Prior to Peak Volcanism
BESTANDCHRISTIANSEN:
EXTENSION
ANDVOLCANISM
IN GRF.
AT BASIN
TABLE 2. StratigraphicUnits andAges
Symbol
Nameof Unit
Age, Ma
HI
Hiko Tuff
18.6
HH
PL
HarmonyHills Tuff
tuff of Pahranagat
Lakes
CondorCanyonFormation
21
22.65
CB
CS
CG
LC
Bauers Tuff Member
Swett Tuff Member
tuff of ClipperGap
LeachCanyonFormation
22.74
22.8
24 ___
BMB
Bates Mountain Tuff
D unit
C unit
B unit
SPU
ShinglePassTuff
upper
26.00
SPL
IS
lower
Isom Formation
26.68
26.96
MO
MonotonyTuff
27.31
LU
Lund Formation
27.9
WW
Wah Wah SpringsFormation
29.5
CW
WB
PS
SC
Cottonwood Wash Tuff
Windous Butte Formation
Pancake Summit Tuff
Stone Cabin Formation
30.6
31.37
34.73
35.40
BMD
BMC
25.11
AgesmostlyfromBestet al. [ 1989b]andalsounpublished
determina-
tions
ofA.L. Deinobylaserfusion
of single
crystals
for4øAr/39Ar.
Uncertaintiesin conventionalK-Ar values,citedto nearesttenthof a million years,
areabout3%; uncertainties
in 4øAr/39Ar
determinations,
citedto nearest
hundredth
of a millionyears,areabout0.1%.
Othergeologistshavefoundevidencefor Tertiary faultingpredatingpeakvolcanismin the GreatBasin. While it is beyondthe
scopeof this paperto documentall instances,a few examplesare
sufficientto characterizethisperiodof extension.In the vicinity of
the Grantand southernEganranges,represented
by sections6-8,
themostlyepiclasticSheepPassFormationof Eoceneto Paleocene
age(topossiblylateCretaceous
[Stewart,1980])recordscontemporaneousblockfaulting [Mooreset al., 1968;Newman, 1979]. Near
section8, Kellogg [1964] mapped a 3-km-long slide block of
Paleozoicrock restingon the SheepPass;this block, which he
presumed
to havecalvedoff thenearbyancestralShinglePassfault
scarp,is in turncoveredby tuffaceous
sediments
andthenby lava
flows, conglomerate,and finally severaltuff cooling units depositedabout31-23 Ma. Southof Eureka,Nevada, betweensections2 and 5 in the southernFish Creek Range, J. Bartley (written
communication,March 1990) hasfound what he believesto be slide
massesof Paleozoicrock at the baseof the SheepPassFormation.
He hasalsomappedin the vicinity of section13 numerousNNW
striking,high-anglenormalfaultswith generallysmalloffsetsthat
are overlainby a 32.6 Ma tuff. Justnorthof the area of Figure 5,
depositionof the Eocene-Oligocene
Elko and Indian Well forma-
13,517
tions [Solomonet al., 1979; Palmer et al., 1991] apparentlywas
governedin partby localuplift via blockfaulting. In theIndian Well
sequence,an angulardiscordanceof about 15 existsbetweentuff
unitsdepositedabout36 Ma andthoseabout31 Ma. In the areanear
Pioche,Nevada, Taylor [ 1989, pp. 107-141 ], Taylor et al. [ 1989],
and Axen et al. [1988] have found evidence for extensional faults
which on the basisof compatibletiming and similarityin geometry
and kinematics,they believe constitutea southerncontinuationof
the Oligoceneextensionalfault systemof Gans and Miller [ 1983]
andGanset al. [ 1989] in eastcentralNevada that was active mostly
36-35 Ma. Taylor [1989, p. 139] proposedthat regionalextension
was roughly synchronousfor 200 km or more southwardalong
strikeof this fault systemand that becauseof the southwardtransgressionof volcanismin the Great Basin (Figure 2), extensionwas
synvolcanicin eastcentralNevadabut prevolcanicnearPioche.
Limited Extension About 31-22
Ma Concurrent
with Peak Volcanism
Examinationof the sectionsin Figure 6 discloseslittle stratigraphicevidencefor significantregionaltectonicextensionfrom
about 31 Ma to about 22-20
Ma in the central Great Basin when the
greatestvolumeof magmawas erupted.This lack of evidencefor
extensionwasacknowledgedby Ganset al. [ 1989, p. 48]. There is
a strikingcontrastbetweenthe expectedstratigraphicsequencein a
synextensional
volcanicterrain[Ganset al., 1989, Figure 18] and
the observedsectionsin Figures 6 and 8.
Sedimentarymaterial intercalatedwithin volcanic rocks after
30.6 Ma (the time of depositionof the CottonwoodWash Tuff)
occursonly in sections6, 8, 22, 26, and possibly1; the maximum
aggregatethicknessof thesesedimentarydeposits,someof which
might have been shedoff older topographichighs, is 1% of the
maximumcumulativethicknessfor all Tertiary sedimentarymaterial in the 31 sections.The possibilityexists, of course,that erosional debris from topographichighs producedby synvolcanic
faults was continuouslyand virtually wholly flushed outsidethe
central Great Basin area during emplacementof the ash flows,
particularlyif the region was concurrentlyuplifting (see below).
We haveno way of independently
evaluatingthis, althoughfaultrelatedangulardiscordances
wouldremain.
Marked angulardiscordances
within the post-31 Ma part of the
volcanicsequence
occuronly in section18 whichrepresentsa local
areaof earlyMiocene(about22-18 Ma) extensionvia blockfaulting [Bestet al., 1987b] that is intimatelyassociatedin spaceand
time with quasi-bimodalmagmatism.
West of Pioche, Nevada, in the Northern PahrocRange, Taylor
[1989, pp. 90-91], Taylor et al. [1989], and Taylor and Bartley
[ 1988] find evidencefor minor extensionalfaulting 30-27 Ma that
producedan angulardiscordanceof less than 10 in the volcanic
sequence
andan intercalatedcoarsesedimentarydepositasmuchas
C
26
eters
•0
Fig. 7. Stratigraphic
sections
in theNorthPahrocandSeaman
Ranges[afterBartleyet al., 1988,Figure8]. Locationof sections
shownin Figure5. SeeFigure6 andTable 2 for otherdesignations.
13,518
BESTAND CHRISTIANSEN:EXTENSIONAND VOLCANISMIN GREAT BASIN
Fig. 8. View lookingeastat northernGoldenGateRange,section15. A conformable
sequence
of nineashflow deposits
rangingin
agefrom30.6 to 22.74 Ma in highesthill is cappedby 20.7 Ma lavas(seenon hill to south)anddisconformably
overliesPaleozoic
carbonatestratato northexposedin low hill. Entire sectionis gentlytilted.
60 m thick(Figure7). Mappedfaultsaccountfor considerably1 km
of extensionovera distanceof severalkilometers.AlthoughTaylor
[1989, p. 91] consideredthe possibilitythat the faulting might be
related to the nearby, mostly slightly older Indian Peak caldera
complex[Bestet al., 1989a], we believethat it is more likely to be
related to emplacementof magmas in the upper crust, or their
eruption,27-24 Ma, to form severallocal lava flows and ash-flow
crustalsags,grabens,or block faultsthat formedconcurrentlywith
pyroclasticactivity. No directevidencefor suchcontrollingfaults
have been found, althoughsuchstructuresdo locally exist in the
easternGreatBasin[e.g., Bestet al., 1987b]; whetherthey formed
becauseof associatedmagmaemplacementin the uppercrustcan-
tuff sheetsin this immediate area (see Best et al., [ 1989b ], Table
flow sheets in the Great Basin [Best et al.,
R3, PetroglyphCliff, Hancock Summit, and trachydacitetuffs).
Alternatively, this extensionalfaulting might reflect a regional
tectonicpulseof limited magnitude.In the southernGrant Range
fartherto the northwest,the NW strikingWadsworthRanch fault
is an obliqueslip fault that diesnorthwardfrom a probablecaldera
[Taylor et al., 1989; Bartley and Gleason, 1990]. It cuts 31.8 Ma
tuff and is cut by a large dike whoseage may be about 26 Ma.
Whetherthis fault was causedby the nearbymagmaticactivity is
evidencethat during the latestOligoceneand early Miocene little
topographicrelief existedand thereforelittle extensionalfaulting.
For example,the correlativeNine Hill Tuff andD unit of the Bates
not be ruled out.
The wide distributionof thin, generallylessthan 10-20 m, ash
1989b] is indirect
Mountain Tuff (25.110.02 Ma [Deino, 1989]) extend from the
westernfoothills of the Sierra Nevada to within 40 km of Ely,
Nevada,andthetuff of ClipperGap (22.8 Ma) cropsoutfrom 35 km
southeastof Austin, Nevada, to within 8 km of the Utah stateline
[Gromm• et al., 1972; C.S. Gromm6, unpublisheddata, 1990].
certain.
In southwesternNevada, Robinsonand Stewart [ 1984] found no
indicationof late Oligocene-earlyMiocene faulting exceptin the
CandelariaHills where eruptionof 25-23 Ma ash flows seemsto
havebeenassociated
with subsidence
of a local trough.
The distributionof two regional ash flow sheets (the Lund
emplacedabout28 Ma and the Leach Canyon about 24 Ma) in the
southeastern
Great Basin are much more elongateeast-westthan
older and younger sheets derived from nearby sources [Best,
1988b;Bestet al., 1989a,b ]. Althoughthe distributionof outflow
sheetsis influencedby ventposition,eruptiondynamics,andother
factors,it is nonethelessa possibilitythat the dispersalof the ash
flows or preservationof thesesheetswas influencedby east-west
ExtensionAfterAbout22-20 Ma PostdatingPeak Volcanism
Dip-slipfaultsthatcuttheentiresequence
of volcanicandolder
rockarewidespread
in theGreatBasinandareresponsible
for the
characteristic
topographyof this part of the Basin and Range
province[e.g., Stewart,1978].Thesefaultsindicatethatpostvolcanic deformationis both commonand one of the most significant
elementsin the aggregatecrustalextension.Major detachment
faultsoccurin manyplaces[e.g., Tayloret al., 1989]. Someareas
[e.g., Noble et al., 1990b] haveclearlybeenextendedbetween
timesof deposition
of ash-flowsheetsafterpeakvolcanismin the
middle to late Miocene.
BEST AND CHRISTIANSEN:EXTENSIONAND VOLCANISM IN GREAT BASIN
EPISODIC VERSUS CONTINUOUS EXTENSION
DURING THE TERTIARY
In southeastern
Nevada, Taylor et al. [ 1989] foundevidencefor
episodic
extension,
themostsignificant
occurringbefore34-32 Ma
and, with loose constraints,sometimeafter about 22-15 Ma, de-
pendingonthe area.Limitedextensionthatwaspossiblyrelatedto
localmagmaticactivityoccurredin the interim. Southof the area
studiedby Taylor et al. [1989], dayko[1990] documentedsimilar
episodicextensionandfoundno evidencefor late Oligocene-early
Miocene
extension.
In the Yeringtondistrictjust southof Reno, Nevada(Figures1
and9 [Proffett, 1977;Proffettand Dilles, 1984]), the stratigraphic
record is unusuallycompleteand entirely compatiblewith the
interpreted
episodicdeformation,anditstiming,in thesoutheastern
partof Nevada.Basalclasticdepositsoverlainby voluminouslate
OligoceneSilicicash flow tuff sheetsare reminiscentof many
sections
in eastern
Nevada.Thesmallamountof epiclastic
deposits
andlackof angulardiscordances
withinthe2 + km sequence
of tuffs
indicaterelative tectonicquiescenceduring the period of peak
82 _.+ 11-8Ma
i111111
122_+
400+19-17Ma
0-183
0_160
2•j•
60-300
244-412
I
I
30-90
13,:519
regional volcanism in the Great Basin. Also recorded in the
Yerington sectionis major extension, locally more than 100%,
betweenabout17 and 11 Ma andthereforewell after peakregional
volcanism.
The Yeringtondistrictis actuallyonly oneof 13 areasin western
Nevada(Figure 1) whereextensionalfaultingproducedwidespread
angular unconformitiesafter the regional peak volcanism [John
et al., 1989]. From about20 to 19 Ma the styleof volcanicactivity
shifted from eruptionof silicic pyroclasticflows to intermediate
compositionlavas. John et al. [1989, Table 2] also indicatedthat
only two of the 13 areasexperiencedextensionalfaulting during
ash-flowactivity, once againdemonstrating
that only local deformationoccurredduringpeak volcanismin the Great Basin.
Gans et al. [1989, Figure 19] show major extensionin eastcentralNevada continuingfor about 15 m.y. after the time about
35 Ma when widespreadvolcanicdepositsaccumulatedto serveas
chronotectonicmarkers. However, this interpretation is based
[Ganset al., 1989, pp. 28-29] uponsedimentarybasinsequences
containingonly threedatedashbedswhich indicatelower partsare
middleto latestOligocene;upperpartshaveunconstrained
ages.
Zoback et al. [ 1981, Table 1] presenteddata intendedto "prove
or supportan interpretationof pre-basin-rangeextension"through
the OligoceneandMiocene in the Great Basin. However, reliability
of the citedevidencefor regionaltectonicextensionis variable. For
example, extensioninferred from passivelyemplacedbatholiths
andfaultingnearcalderasmay not be relatedto regionaltectonism.
Extensionin the Needle Range area of westernUtah (sections13,
14, and 17-19 in Figures5 and 6) and in Lincoln County, Nevada
(sections23-27), was almost entirely after the early Miocene
accordingto our data (Figure 6). Anderson [1983] documented
interstratifiedearly Oligoceneconglomerateand tuff in several
placesin the easternGreat Basin aswell as "Oligoceneor younger"
and Miocene extension. All of these data can be interpretedto
conformto an episodicpatternof extensionin which only a limited
amountoccurredduringthe periodof mostvoluminousvolcanism.
Wernicke et al. [1987, pp. 212-213] discussedevidence for
pre-middleMioceneextensionin theGreatBasin.Onceagain,all of
their data are compatiblewith two major episodesof extension:
beforeandafter peaklate Oligocene-earlyMiocene volcanism.For
example,they notethat
I
I
I
i
I
I
I
i
ß . . in the northernDeath Valley area, no normal faults of earlyMioceneor Oligoceneageareknown, butthedepositionof up to 1000
m of Oligoceneand lower Miocene(?)Titus CanyonFormation...
may recordthe onsetof extensionaltectonismin this area. The basal
Titus Canyon containslensesof non-volcanicmegabreccia,above
whichoccurfossilsof early-Oligoceneage. The top of the formation
is overlainby volcanicrocksbetween22 and20 Ma old.
I
The casefor continuous,significant,regionalextensionduring
the Tertiary in the Great Basin is thereforenot compellingand we
concludethatit wasepisodic(Figure 10). Locally significantexten-
0-46
30-113
60-143
.,•
0-21 ':'"' '.'
0-27
0-50
0-520•a
0-120
0-90•-•-
0-183•
Fig. 9. Generalizedstratigraphicsectionof rocksin the Yeringtondistrict
[after Proffett, 1977] in format of Figure 6. Light wavy line is erosional
unconformity,andheavywavy line is angularunconformity.
sion occurred sometime before 34-32 Ma and, with loose constraints,sometimeafter about 22-15 Ma, dependingon the area.
The latter period of extensionhas been widespreadand of major
magnitude,producingthe presenttopographyof rangesandbasins
in the Great Basin. Limited, local extensionthat was possibly
relatedto shallowcrustalmagmaticactivity in someareasoccurred
in the interim, but hardly justifies, in our view, the qualifier
"synextensional"
in describingthe accompanyingvolcanism.
RELATION BETWEEN EXTENSION AND VOLCANISM
IN SPACE AND TIME IN THE GREAT BASIN
Becauseof the southwardtransgressivesweepof volcanismin
the Great Basin (Figure 2), datable markers are not everywhere
13,$20
BEST AND CHRISTIANSEN:EXTENSIONAND VOLCANISMIN GREAT BASIN
ß
'•--
ß
ß
EXTENSION
CA NV // I
ß•
,
I
i
t
j
•
•t
!
19
ß
,I
[
._
e
tuf•
n' 4'0
3•
o
ß
--
ß
(a)
-
I
I
t
ß
I
"'"""""'"':
ß
,///,/,..... ß
'"
lava
10
/
•.. •; .'...•".'
•
2'0
ß
'- 'o' '.'' I.'."
VOLCANISM
• ? -- -- 9
,
ß
/
_=
.0
0
AGE (Ma)
Azl
Fig.
Schematic
timing
ofdiachronous
extension
and
volcanism
the
entire10.
Great
Basin. Generalized
relative volumes
of volcanic
rocks arefor
based
*
•.:X',x
.'"*:
:t'
ß
.... '"'
onFigure
3and
essentially
represents
anintegrated
picture
ofvolcanism
ß
. .....-.x.
x &r.•J 0 KILOMETERS
alonga north(older)to south(younger)sectionthroughthe GreatBasin.
ß ..
3OO
'i-•
0
ß '''
--
. -MILES
I
200
ß ' ' ' '1; ' ' '
present
to delineate
all detailsof thetimingandintensity
of extension.Despitethisinadequacy,
ourperspective
ofhowextension
and
volcanismarerelatedin spaceandtime in the GreatBasindiffers
significantly
fromconclusions
drawnby someothergeologists.
Ourinterpretation
of whatwe consider
to berepresentative
data
in theGreatBasinis thata periodof limitedextension,andpossibly
evenregionaltectonicquiescence,prevailedfrom at leastas early
as about31 Ma and lasteduntil roughly22-20 Ma, essentially
coincidingwiththeperiodof maximum,voluminousvolcanismthat
was manifestin maximumpyroclasticactivity (Figure 10). Local
extensionoccurredbeforethis tectoniclull and ignimbriteflareup
but widespreadextension,includingmajor detachmentfaultingin
places,afterwards.On a province-widescale,maximumextension
andthemostvoluminousvolcanismarepoorlycorrelated,not only
in time butalsoin space(Figure 11a). In theGreatBasin,volcanism
wasdominatedby extrusionof lavasat 43-34 and 17-6 Ma [Stewart and Carlson, 1976] duringextension(Figure 10). However,
there is little spatial correlation between areas of lava-dominant
volcanismandhighly extendedterranes(Figure 11b). Someareas
of olderandyoungervolcanismhave,asyet, no documented
major
detachmentfaults. Extendedareas do not everywheremanifest
•-
;7-'.
ßß
'
ß
[1989].
PREVIOUS TECTONOMAGMATIC
u-r,,
øO
øøøøIo
ß
i'' '
volcanism.
On thebasisof availableinformation,it thusappearsthatextensionandvolcanismduringthe Tertiary in the GreatBasinwere not
closelycoupledprocesses;
this lack of close couplingwas also
suggested
by Bartley et al. [ 1988], Taylor [ 1989], andTaylor et al.
- ß43-34
Ma
.
0
.
KILOMETERS
300
I
Fig. 11. (a) Detachmentfault terranes(stippled;from Davis and Lister
[1988]; see also Wust [1986]) and areas of voluminous volcanic rocks
deposited
34-17 Ma (diagonalruling;fromStewartandCarlson[ 1976];and
D.M. Miller (written communication, 1989)). The 34-17 Ma volcanism
was chieflypyroclastic,exceptin centralUtah wherelavasdominate.(b)
Detachment
faultterranescorrespond
poorlyto lava-dominantareasformed
at 43-34 Ma (horizontalruling)and 17-6 Ma (verticalruling).
MODELS
If extensionand volcanismare looselycoupledon a provincewidebasis,thensimple,end-membermodelsof passiveandactive
continentalrifting, suchasproposedby SengorandBurke [1978],
aredifficultto applyin the GreatBasin.
Gans et al. [1989, pp. 47-48] recognizedthat significant
supracrustal
extensiondid notoccurin somelarge,middleTertiary
eruptivecentersin the southwesternUnited States,suchas the San
Juanvolcanicfield in Colorado,partsof the Mogollon-Datilvolcanicfield in New Mexico, andmuchof the Oligoceneto Miocene
ashflow tuff provincein the centralGreatBasin.They askedwhy
theseareasdid not extendandif therewasanythingfundamentally
differentaboutthemagmatism.It is beyondthescopeof thispaper
to considerdetailsof the largeSanJuanandMogollon-Datilfields.
But we considernext whether there is anythingfundamentally
differentin the GreatBasinbetweenTertiaryvolcanismassociated
in spaceandtime with majorextension,asin theeastcentralNevada
areastudiedby Ganset al. [ 1989] andFeeleyand Grunder[ 1991],
and volcanismnot accompaniedby extension.As a test, we comparetime-compositional
propertiesof the eastcentralNevadafield
with the Indian Peak volcanic field [Best et al., 1989a]; the latter
field surrounds
the Indian Peak calderacomplex(Figure6), was
createdby episodiceruptionsabout32-27 Ma, and haslittle evidence for concurrent extension.
The Indian Peak field, like others in the Great Basin formed
duringpeakvolcanicactivity [Bestet al., 1989b], wascreatedby
eruptionof thousandsof cubic kilometersof silicic ash flows and
only minorandesiticthroughrhyoliticlavasfrom a localizedmagma system,now markedby a large calderacomplex[Bestet al.,
1989a]. Figure12 demonstrates
thatfew contrastsexistbetweenthe
BESTAND CHRISTIANSEN:EXTENSIONAND VOLCANISMIN GRF_.AT
BASIN
400
12
10
300
13,521
ß
•
Trachyte
• Trachv-
•
•
u
.......•.:.:•
[]
a'ndesite
• •'•"'•'-'-"•iiiiiii:':"":'"?-.'•?•iii.:•iiii
i
ß-:.
:.:.:.:-:-:-:-:.:.:-:.'.'-':
-''
":'"":::.•:•:i•::::'"
=================================
[]
100
[]
Dacite
Basaltic
•
./•na;•ii•e
. Andesite
10
600
8
500
300
4
200
2
100
600
8
7
500
6
400
•
5
[oo
200
3
2
100
1
1.4
1.2
3O
•
1.0
20 -
(:•0.60.4-
100.2-
0
50
55
60
65
Si02(wt%
)
0
75
80
50
55
60
65
70
75
Si02(wt%)
Fig. 12a. Comparisonof compositions
of volcanicrocksin the eastcentralNevada and Indian Peak volcanicfields. Shadedarea
represents
samplesfromtheeastcentralNevadafield [Ganset al., 1989;FeeleyandGrunder, 1991] andsquares
representtheIndian
Peakfield [Bestet al., 1989, alsounpublisheddata, 1991]. IUGS classificationin upperleft.
80
13,522
BEST AND CHRISTIANSEN:EXTENSIONAND VOLCANISMIN GREAT BASIN
model include(1) southwarddeceleratingsweepof calc-alkaline,
lOOO
Indian Peak V F
i'•i?•ii:•iiiiiiiiiii?•iii
subduction-related
volcanism(Figure 2), (2) coincidencein space
East Central NV
•
and time of most voluminous
volcanism
with the area where the
sweepslowedandstagnated
in the southernGreatBasin34-17 Ma,
forming the Reno-Marysvale volcanic zone (Figure 1), (3) decreasedspeedof convergencebetweenoceanicFarallon-Vancouver
-o lOO
o
and continentalNorth American platesthroughthe Tertiary [Stock
and Molnar, 1988; see also Engebretsonet al., 1985]; the north
velocitycomponent(parallelto planeof Figure 2) at 45N and 38N
o
probablydecreasedaboutthreefoldfrom the earliestPaleoceneto
latest Eocene, while no significantchange occurredin the east
lO
component;during this time, the direction of movementof the
oceanicplate shiftedto more easterly(Figure 1), (4) mostvolumiBa
' RbTh..............
K NbTa La Ce Sr Nd P Sm Zr Hf Ti Tb • Yb
, nousvolcanismdominantlypyroclastic,i.e., the "ignimbriteflareup" (Figure3), (5) mostvoluminousvolcanismin the GreatBasin
in spaceand time with significantcrustalextension
Fig. 12b. Chondritenormalized[Thompson,1982] traceelementpatterns not associated
for lava flows with less than 65% SiO2. Indian Peak field ruled; lines are
(the samewastrue of the SanJuanandpartsof the Mogollon-Datil
representative
analysesfrom the eastcentralNevadafield.
volcanic fields [Gans et al., 1989]. In the Sierra Madre Occidental
in Mexico, there is little sedimentarymaterial and angulardiscordancein the centralplateau, which is as much as 3 km abovesea
compositions
of the broadlycalc-alkalinemagmaseruptedin this level, althoughthe easternand westernmarginsare cut by many
field and the east central Nevada field. One notable difference is the
normal faults (F. McDowell, personal communication,March
late eruptionin the Indian Peak field of pl+cpx+opx trachyteto 1990)), and (6) extensionand volcanismduring the Tertiary not
trachydacite
ashflowsformingthe IsomFormation.Althoughthe closelycoupledin spaceandtime.
lasteruptions
in theeastcentralNevadafield werealsoof relatively
Crossand Pilger [1978] andLipman [1980, Figure 14.6] sugalkalinemagmas,they do not havehigh concentrations
of Zr, Ti,
gestedthat the southwardsweepof volcanismin the Great Basin
and incompatibleelementslike the Isom [Christiansenet al.,
was relatedto progressivesteepeningand founderingof the sub1988]. Moreover, the late dacitesin the east central Nevada field
ducting oceanic lithospherebeneath the continental lithosphere
containmorebiotitethanpyroxene.Figure 12bcomparesthe trace (Figure 2) as the rate of plate convergencediminishedin the Terelementpatternsfor intermediatecomposition(65% SiO2) lava tiary. Mafic, mantle-derivedmagmasthat thermallypoweredand
flowsfrom the two regions.Again, no majordifferencesare appar- providedmassto the migrating continentalmagma systemswere
ent. Andesitesfrom both fields displaypronounceddepletionsof associated
in someway with the southwardgrowingasthenospheric
Nb, Ta, and Ti comparedto adjacentelements;suchpatternsare wedgeoverlyingthe steepeningplate. However, Glazner [1983]
typicalof subduction-related
calc-alkalinemagmaticsuites.
cautionedagainstassumingan inverserelationbetweenslabdip and
Althoughnomajorcontrasts
existin thecompositional
natureof convergencerate, exceptin a qualitativesense,becauseof a study
volcanismin the two testareas,thereare significantdifferencesin of 17 modem subductionzones by Tovish and Schubert [1978].
the relativevolumesand modesof eruption(Figure 13). Volumi- They showedthat for 12 zones,dips are 450-80ø for convergent
nouspyroclasticeruptionsdominatedin the Indian Peak field, velocitieslessthanabout10 cm/yr but 50-30 øfor five slabswhere
whereasa lesservolumeof eruptedmagma,mostlylava, formedthe velocitiesare 10-11 cm/yr. For comparison,duringthe Tertiary of
smallereastcentralNevadafield. Suchcontrasts,however,merely westernNorth America, Engebretsonet al. [ 1985] found that from
reflecttemporalvariationsin the GreatBasinasa whole.
26øNto 42øNlatitudethe total convergentvelocitydecreased
from
about 15 to 5 cm/yr from 50 to 25 Ma. Stockand Molnar [1988]
ALTERNATIVE TECTONOMAGMATIC MODEL
showedthat the north componentof velocity for aboutthe same
from about10 to 3 cm/yr from 68 to 42 Ma.
Fundamentalpropertiesof Great Basinvolcanismandtectonism latituderangedecreased
duringtheTertiary which mustbe incorporatedinto an interpretive Thus we seeno significantinconsistencybetweenthe geophysical
EAST
CENTRAL
NEVADA
VOLCANIC
FIELD
INDIAN
PEAK
VOLCANIC
tuff
R
FIELD
25
-L_
lava
R
_
lava
30
R
.T
•A
O
R
40-1-R
-
A
I
1OOO
,
CUBIC
,
,
,
I
KILOMETERS
Fig. 13. Comparisonof rock volumesin synextensional
eastcentralNevadavolcanicfield andnonextensional
Indian Peakvolcanic
field. Volumesfor unitsin eastcentralNevadafield takenfrom Ganset al. [ 1989,Figure4] by scalingthicknesses
of minorunits
againstthickness
of twomajortuff andlavaunitsof citedvolume.Datafor IndianPeakfieldfromBestet al. [ 1989a]; timingand
volumesof lavaextrusions
areschematic
andtimeof inceptionof volcanismisuncertainbutcloseto 33 Ma. A, andesite;
D, dacite;T,
trachyte;R, rhyolite.
BESTAND CHRISTIANSEN:EXTENSIONAND VOLCANISM IN GREAT BASIN
dataandthe proposedmodelof a founderingand steepeningslab
(Figure2).
Furtherdynamicconsiderations
of middle Tertiary volcanism
andtectonismin the Great Basincanbe viewed with respectto the
late Cenozoic evolution of the central Andes, also a site of shallow
subductionof oceaniclithosphere.
The Altiplano-Punaof westernSouthAmericais a highplateau
rangingfrom 3.5 to 4.7 km in elevationandextendingfrom about
200 to 600 km inland from the trench [Froidevaux and Isacks,
1984;Isacks, 1988]. Episodictectonicshorteningduringthe late
13,523
faulting [Molnar and Lyon-Caen, 1988; England and Houseman,
1989]. North and southof the Altiplano-Puna, mean elevation is
less, crust is thinner, and andesitic rocks dominate over ash flows.
Isacks[1988, p. 3228] concluded
There is no evidencefor major crustalrifts that might result if large
amountsof materialwere injectedinto the crustasdikes. The physiographiccharacteristics
of the [Altiplano-Puna]plateauthus indicate
that substantialmagmaticcontributionswould have to be in the form
of sill-like intrusionsor horizontallydistributedunderplatingof material ratherthanlargeverticaldikelike intrusions.
In the areathat becamethe Great Basin, compressionalepisodes
occurredduringthePaleozoicandMesozoic[Stewart, 1980] andin
volcanic
deposits,
including
atleast10,000
klTl
3oflatest
Miocene
to the easternpart shorteningof 104-135 km occurred150-50 Ma
Pleistoceneash flow tuff [de Silva, 1989a]. Some large-volume [Levyand Christie-Blick, 1989]. The amountof associatedcrustal
tuff sheetsare of crystal-richdacite, whereascontemporaneous thickeningwasprobablyvariable;quantificationis problematicand
andesiticlavasarevolumetricallysubordinate,asin the late Oligo- dependsupon uncertainassumptions[Sonderet al., 1987; Guns,
cene-earlyMioceneof the Great Basin [de Silva, 1989b; Chris- 1987; Coney and Harms, 1984]. Mesozoic granitic intrusionsare
tiansenandBest, 1989]. Sincemajorcompressional
faultingin the widely scattered[Bartonet al., 1988], particularlyin southeastern
lateMiocene,theplateaublockhasbeentectonicallyrelativelyinert Nevada and westernUtah, but local bodiesof two-mica graniteare
andcurrentshallow(65 km) seismicityis very limited [Froidevaux thoughtto be relatedto thickenedcrust[e.g., Armstrong,1983;Lee
andIsacks, 1984];onlylocalminorcompressional
andextensional and Christiansen, 1983] and seem to be associatedin some way
faultinghas occurred(Jordanet al. [1983]; but seeMoore and with highlyextendedterranes[Anderson,1988].
The causeof local extensionthat occurredin the early Oligocene
Wernicke[ 1988]). Isacks[ 1988] concludedthatsignificantpost-late
Mioceneuplift wasa consequence
of crustalthickeningassociated is uncertainbut possiblywas related to gravitationallyunstable
with crustalshorteningof the orderof 100 km as well as thermal thickenedcrustor to momentaryrelaxationof compressionalforce
thinningof themantlelithosphere
relatedto convectivetransportof as the speedof plate convergencestartedto diminish after about
50 Ma. Volcanismaccompanyingthis episodewas dominatedby
heat from the underlyingwedge of asthenospheric
mantle. Althoughthe horizontalforceperpendicular
to a plateausuchas the extrusionsof lava (Figure 10). Mantle-derivedmagmasmay have
Altiplano-Punacan be constantduringplate convergence,lateral invadedthecrustasdikes;only the moreevolvedmagmasleakedto
variationsin crustalthicknessand densityacrossit can modify the the surface [Glazner and Ussler, 1988] and before very large
orientationsof principal stressesso that stressstatesare locally volumesof magmawere produced(Figure 14).
During the late Oligocene-earlyMiocene, significantchanges
horizontallyisotropicor conduciveto extensionalor strike-slip
Cretaceousand Cenozoic [Noble et al., 1990a] produceda crust
about70 km thick that is cappedby a relatively thin veneerof
mantle
input
oxo •o•
2X
mantle
input
vertical
exaggeration
Fig. 14. Speculativecoinparisonof ii•agii•aticsystemsin the eastcentralNevada volcanicfield [after Gunset al., 1989] and the
Reno-Marysvale
zonewherepeakvolcanismin theGreatBasinoccurred.Largeinputof mantle-derived
basalticmagmastagnating
as
sillsat a level of neutralbuoyancyspawnedlargesilicicmagmabodiesin the Reno-Marysvalezone. Smallermantlemagmainputin
theextendingeastcentralNevadacrustformeddikesandsmallersilicicmagmabodies.Dottedlinesfor crustin Reno-Marysvalezone
is a 50-km-thickreferencethicknessto showthat intrudedmantle-derivedmagmathickenedandelevatedcrust.
13,524
BESTAND CHRISTIANSEN:
EXTENSIONAND VOLCANISMIN GREATBASIN
vale, Utah, areaare J- andT-shaped[Anderson,1986], suggesting
local horizontallyisotropicstressratherthan a minimum principal
stressnorth-south[Best, 1988a]. Local, limited synvolcanicextension west of Pioche, Nevada [Taylor et al., 1989], indicates an
east-westto northeast-southwest
minimum principal stress.Indirectly, suchinformationsuggestsa more or lessisotropicstateof
stressin the GreatBasinthatvariedslightlyfrom placeto placeand
possiblyin time as well. Deviatoric statesof variable orientation
resulted,but nowherewere magnitudessufficientto producemajor
deformation.This interpretation
is reminiscentof thecontemporary
state in the Altiplano-Punaof South America discussedabove.
Mantle-derivedbasalticmagmasmight haveinitially ascendedinto
1988].
the lower crustin the Reno-Marysvalecorridoras vertical dikes.
If sucha large volumeof mantle-derivedmagmawas emplaced But continuedcopiousinjectionof mafic magma,ascentto a level
asdikesinto the lower crustin the Reno-Marysvalezone, it would of neutralbuoyancy,andconsequentswellingagainstthe wall rock
be expectedto be accomodated
by extension.But accordingto our could have increasedthe horizontal principal stressesuntil the
data, extensionwas limited duringpeak volcanism.We do not yet vertical stress became least, in which situation sills formed and
have requisitetraceelementand isotopicdatato evaluateapproxi- grew repeatedlyas long as the mafic magma supply lasted (for
matelyhowmuchmantle-derivedmagmamusthavebeenemplaced furthermechanicalargumentsseeMcCarthy and Thompson[ 1988,
into the crustto producethe extrusivevolume of volcanicrock in p. 1370] andShaw [ 1980, pp. 229-230]).
Reorientationof principal stressesduring diking is even more
the Reno-Marysvalezone [cf. Riciputi and Johnson,1990]. Nonetheless,a crudeestimatecanbe madeusingthe numericalmodelof likely in a crustundermore or lessisotropicstressif the effect of
Feeley and Grunder [1991] and assumingsimilar proportionsof thermalexpansionof the wall rock is considered;the horizontal
crustand mantle componentswere involved becausevolcanicrock thermal stress normal to the vertical dike can exceed the ambient
compositions
are similar(seesectionon Previoustectonomagmatic deviatoric stress[Tucotte and Schubert, 1982, p. 87; see also
models).
In anareaof about150km2intheEganRange
northwestEtheridge, 1983].
Rinehartet al. [ 1979] found seismicevidencefor a sill at a depth
of Ely, Nevada, Feeley and Grunder [1991] concludedthat to
occurredin platetectonicandcrustalprocesses
thatwe believehada
profoundmagmaticand tectoniceffect in the Great Basin. As the
southwardsweepingmantle magma supply to the crust nearly
stagnatedin the southcentralGreat Basin, as much as an orderof
magnitudemore mantle-derivedmafic magma might have been
insertedinto thecrust,comparedto the regionof fastersweepto the
north. This greatermantlemagmainput into the Reno-Marysvale
zone is reflectedin the contrastingvolumesof the erupted,more
evolvedmagmabetweenthenorthern(43-34 Ma) andsouthcentral
Great Basin (34-17 Ma) (Figure 10). The ascendingbasaltic
magmamay havebeentrappedat midcrustaldensitydiscontinuities
where the magma had neutral buoyancy [Glazner and Ussler,
produce
about
40km3ofvolcanic
rock(mostly
lavaflows,average of
thickness 0.27 km) at least 4.5-9 km of basalt was added to the
about 20 km beneath the Rio Grande rift. The sill has a thickness
of 0.6-1.2 km if themagmais whollyliquidanda lateralextentof at
thatalthough
therifthasformed
in
crust,ignoringthat involvedin crustalmelting.In the wholeReno- least1700km2.Theyconclude
Marysvale
zone,at least35,000km3 of rock(whichdoesnot anextensionalstressregimesincelateOligocene,thecontemporary
includean untoldvolumeof air fall tuff outsidethe zone)is spread stateappearsto be compressive(alhorizontalnorth-south)or possioveranareaofroughly
80,000km2(compensated
foranestimatedbly hydrostatic.Howard [1991] reportedintrusionsof mafic rock
50% east-westcrustalextension)for an averagethicknessof 0.44 exposedover a large area of Arizona and southeastern
California
km. This would imply that 7.3-14.7 km of basaltwas addedto the that were emplacedas horizontal sheets.Goodwin et al. [1989]
crustin the zone. If thisbasaltwasinjectedasverticaldikesthrough interpretedprominantseismicreflectorsin the subsurfaceof west
20 km of the lower crust, which is probably an excessivedepth central Arizona as horizontal mafic sheets.
interval,eachonekm2 cross-sectional
verticalcolumnof lower
Thus,althoughwe havenodirectevidencefor widespread
sillsof
crust would contain dikes 0.37-0.73 km in width, for a horizontal
Tertiarymafic rock in the lower crustin the Reno-Marysvalezone
extensionof 37-73%. If theassumptions
andapproximations
in this (Figure 14), their existenceseemslikely and could be testedby
model are reasonable,then diking shouldhave been manifest in appropriateseismicinvestigations[Goodwin et al., 1989]. In a
brittleextensionof the uppercrust.However, our stratigraphicdata lower crustunder more or less isotropicstressand still relatively
allow for only limited extension,in all probability less than this warm, andthereforeweak, dueto Mesozoicthrusting[Glaznerand
Bartley, 1985; Sonderet al., 1987; Gaudemeret al., 1988], condicalculatedrange.
Entrapmentof a large volume of magma in the crust without tionsfor silling of mafic magmaintrudedat a high rate at a level
accompanyinghorizontalextensionis possibleif the intrusionsare of neutralbuoyancyseemespeciallyfavorable.
subhorizontalsheets,or sills; the crust simply inflates vertically.
Huppert and Sparks [1988] concludedthat mafic magma inIndependentevidencefor suchsheetsin the Great Basin is, how- truded as sills, comparedto dikes, providesa more promising
ever, only circumstantial.
situationfor extensiveand geologicallyrapid melting of crustal
It is commonlyassumedthat horizontalintrusivesheetsrequire rocks.As large "ponds"of silicic crustalmagmasformed, subsethatthe leastprincipalstressbe vertical. Unfortunately,the stateof quentbatchesof mafic magmaascendinginto the lower crustwere
stressin the Great Basin during the time of most voluminous buoyantlyblocked and stagnated,which led to further heating
volcanismis difficult to evaluatedirectly. The speedof plate con- and melting and large-scalemixing of mafic and silicic magmas
vergencehad diminishedprior to, and perhapscontinuedto de- [Best et al., 1989a; Feeley and Grunder, 1991]. Becauseof the
creaseduring,early peakvolcanism,reducingthemagnitudeof the prewarmedcrust, the depth interval over which partial melting
compressional
force on the continentandthusfavoringextensional occurredwaspossiblygreaterrelativeto normalcrustaswell asthe
conditionsdue to gravitationalinstability of previouslythickened proportionof melt producedand the potentialamountof mixing.
crust[Englandand Houseman, 1989, Figures lb and lc]. Bartley Thus generationof dacite-rhyolitemagma was maximized in this
[1990] suggestedthat a north-southcomponentof "active spread- regime of large mantle input where the lower crust was already
ing" associatedwith the peak volcanismmight have been super- relativelywarm.
posedon the crustalstressfield otherwisecontrolledby buoyancy
Accordingto the calculationsof Huppert and Sparks [1988,
forces in a thickened crust. East-west vertical dikes emplaced p. 621], "... melting occursin the thermalboundarylayer and
duringpeak volcanismare sparseregionally;a few in the Marys- simultaneous
crystallizationmust occur in the interior of the melt
BF_3TAND CHRISTIANSEN:EXTENSIONAND VOLCANISMIN GREAT BASIN
13,525
layerasitforms... largevolumesof crystal-richmagmacouldbe the SanJuancrustdid largecaldera-formingsilicicmagmasystems
generated
in a sourceregionin very shorttime periods."As shown developthere[Lipman, 1975].
by Bestet al. [1989a,b ] muchof the pyroclasticmaterialerupted
CONCLUSIONS
duringtheperiodof mostvoluminousvolcanismin theGreatBasin
was crystal-richdaciteof the Monotony type. Such vast volumes
Tertiary extensionin the Great Basin was interruptedduringthe
(individual
eruptions
asmuchas3000km3)ofdacite
magma
imply
late Oligocene-earlyMioceneby a largelyignoredor unrecognized
largemantleinput. Gravitationalinstabilityof hugemagmabodies
periodof virtual tectonicquiescenceduring which time the most
of presumedsill form (Figure 14) ultimately led to eruptionand voluminous volcanism in the Great Basin occurred. Maximum
associated
calderacollapse.
extensionandmaximumvolcanismin the Great Basin appearto be
Eruptionsof the very largevolumeMonotony-typeashflows in
largely mutually exclusive in space and time. Extension during
the Reno-Marysvalezone, chiefly 31-27 Ma, were succeededby
the peak volcanism,or ignimbriteflareup, was so limited that the
widespreadventingof trachyticashflows of the Isom type, mostly
conceptof "synextensional
volcanism"is hardly appropriate.The
27-23 Ma [Best et al., 1989b; Christiansen and Best, 1989].
expectedstratigraphicrecordfor synextensional
volcanism[Gans
Preliminaryinterpretationsof compositionaldata suggest[Chriset al., 1989, Figure 18] is strikinglydifferent from the observed
tiansenet al., 1988; Best et al. [1989a] that theseunusual,highrecordin the middleTertiary (Figure 6).
temperaturecalc-alkalinemagmaswere createdby fractionationof
An older episodeof extensionbeforepeak volcanism,probably
pyroxenefrom mafic parentswith limitedinvolvementwith silicic
in the early Oligoceneabout 36 Ma, was apparentlymostly localcrust.This interpretationis compatiblewith our envisagedmassive
ized in easternNevada.More widespreadpostpeakextensionin the
input of mantle-derivedmagma into the crust along the Reno- Great Basin was associated in time with lava dominant volcanism
Marysvalecorridorwherethe Isom magmaseventuallyevolved.
(Figure 10), but spatial correspondenceis poor (Figure 11). As
Buoyancyof the Great Basinlithospherestemmingfrom MesoTaylor et al. previouslyconcluded[e.g., Taylor et al., 1989], for a
zoic compression
andresultantcrustalthickeningand heatingwas
local area, volcanismand extensionalfaulting were not correlative
greatly augmentedduring Tertiary magmatismand founderingof
in spaceandtime throughmostof the Tertiary of the GreatBasin.
the subductingoceanicplate beneaththe continentallithosphere.
First-ordertectonomagmatic
propertiesof the Great Basinin the
Thermalerosionof thelithosphere
ashotterasthenosphere
movedin
middleTertiarycanbe relatedto plate interactions.As the speedof
under it added another componentof buoyancy that no doubt
oceanic-continent
convergencediminished,the subductingplate is
culminatedin optimumconditionsfor extensionalcollapseof the
presumedto have droppedaway from the continentalplate and
elevatedcrust [Englandand Houseman, 1989; Molnar and Lyonassumed
a steepening
dip, causinga deceleratingsouthwardsweep
Caen, 1988]. Yet anotherfactorallowingfor crustalextensionafter
of volcanicactivity throughthe Great Basin. As the sweepnearly
peakvolcanismwas progressiveshutoff of subductionafter about
stagnatedin the southcentralGreat Basin in the Reno-Marysvale
28 Ma [e.g., StockandMolnar, 1988] alongthecontinentalmargin,
corridorin the late Oligocene-earlyMiocene, we postulatethat a
greatlyreducing,if not effectivelyeliminating,compressiveforce
copiousvolume of mantle-derivedmagma lodged as horizontal
againstthecontinent.This ongoingextensionaleventprobablyhad
sheetsin the lower crustin a nearly isotropicstateof stressthat was
a diachronous,but difficult to date, inceptionand has occurredin
previouslywarmedby Mesozoiccrustalthickening.Theseoptimal
separatedomainsat differenttimes; it has been obviouslymore
conditionsproducedhuge silicic magmabodiesthat eruptedthouwidely distributedandmorepervasivethroughout,andbeyond,the
sandsof cubic kilometersof pyroclasticmaterial, chiefly crystalGreat Basin area than the prepeakvolcanic extensionalepisode
rich dacite(Monotonytype).
[Wernickeet al., 1987; Levy and Christie-Blick, 1989].
Lava dominant volcanism before and after this ignimbrite
The shift to a widespreadstate of lithosphericextensionhas
flareup was associatedwith a crust in a stateof extensionwith a
allowed extrusionof basalticlavas only since approximatelythe
lower mantleinput. Suchconditionsengenderedsmaller, "leaky"
middle Miocene. Complete thermal erosion of the lithospheric
crustalmagma systemswhich did not mature into huge, sill-fed,
mantleandreplacementby asthenosphere
hasapparentlyoccurred
Monotony-typesystems.
locally in the Great Basin [Fitton et al., 1988]. Only sincepeak
Simplisticmodelsof active versuspassiverifting are inapprovolcanismcantheGreatBasinpossiblyqualify asan exampleof an
priateto thecomplexlyevolvingGreatBasinduringthe Tertiary.
active rift, but its evolutionhas been so complex throughthe
Tertiary that simplisticrift modelsare difficult to apply [Bartley,
1990].
In the Oligocene-earlyMiocene Marysvale field just eastof the
Great Basin (Figure 1), volcanismwas dominatedby extrusionof
Acknowledgments.Gordon Eaton and Earl Pampeyanhelped obtain
copiesof Earl Cook's unpublishedstratigraphicsectionswhich guidedour
initial field work. Bart Ekrenprovided encouragement
in the beginning.
On-going
highprecision
4øAr/39Ar
agedeterminations
by AlanDeinoto-
getherwith paleomagnetic
analysesby ShermanGromm6andJonHagstrum
in early stagesof the project,have providedquantitativeconfirmationsof
many otherwise suspectpetrographicand stratigraphiccorrelationsof
ashflow tuff unitsin the GreatBasin.The friendlycompanionship
of Alan,
Jon, andSherman,togetherwith the assistance
of Mark Loucksand especially PeterNielsen,madefield work morepleasant.Many geologistshave
generouslyprovidedhelpful guidancein the field, unpublisheddata, and
stimulatingdiscussions
regardingtheir favorite areas;they include John
volcanism as was the crust beneath the Great Basin because of little
Bartley, Ed du Bray, Allen Glazner, Dick Hardyman,Angela Jayko, Ted
crustalthickeningassociatedwith thrusting.Presumably,the un- McKee, Don Noble, Pete Rowley, Bob Scott, Wanda Taylor, and Steve
derlying mantle lithospherewas also cooler and apparentlyless Weiss.Bart Kowallis helpeddigitizemap data. Dave Tingey providedhigh
mantle-derivedmagmainvadedthe crustat the easternend of the quality X ray fluorescencechemicalanalysesat BYU. Helpful comments
on a preliminarydraft of the manuscriptwere provided
Renø-Marysvaiezone. Consequently,less thermal energy was andencouragement
by Lee Allison, Gary Axen, JohnBartley, ShermanGromm6, Lehi Hintze,
availableto power crustalmagma systems,and silicic pyroclastic Keith Howard, AngelaJayko,Bart Kowallis, Fred McDowell, Bob Scott,
eruptionswere subordinateto eruptionof andesiticlava. Likewise, andWandaTaylor. Scrutinyof a laterdraftwasprovidedby CharlesChapin,
only after an enormousamountof mafic magmawas emplacedin Allen Glazner, and Don Noble. The financial supportof BYU, the U.S.
lava flows, rather than ash flows. Can this contrastbe reconciled in
our dynamicmodel?It is probablyno coincidencethat the eastern
limit of Mesozoic-earliestTertiary thrustingand presumedcrustal
thickeningroughly separatesthe Marysvale field from the Great
Basin [e.g., Levy and Christie-Blick, 1989]. The lower crust beneaththeMarysvalefield wasprobablynot warmedprior to major
13,526
BESTAND CHRISTIANSEN:
EXTENSIONAND VOLCANISMIN GIIF_.AT
BASIN
GeologicalSurvey,and the NationalScienceFoundationthroughgrants
EAR-8604195,-8618323, and-8904245is gratefullyacknowledged.
Christiansen,
E.H., M.G. Best,andJ.D. Hoover,Petrologyof a trachytic
tufffroma calc-alkaline
volcaniccenter:TheOligoceneIsomtuff, Indian
Peakvolcanicfield, Utah andNevada, Eas Trans. AGU, 69, 1494, 1988.
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