Download Crustal collapse, mantle upwelling, and Cenozoic extension in the

TECTONICS, VOL. 17, NO. 2, PAGES 311-321, APRIL 1998
Crustal collapse, mantle upwelling, and Cenozoic
extension
in the
North
American
Cordillera
Mian Liu and Yunqing Shen
Departmentof GeologicalSciences,Universityof Missouri,Columbia
Abstract. Gravitational collapse has been suggestedas the
major cause of Cenozoic extension in the North American
Cordillera and many other orogenic belts. Although both
crustal thickening and mantle upwelling may have contributed
to the Cordilleran extension,previous models of gravitational
collapse have focused on the former; the cause of mantle
upwelling and its relationship to crustal collapse remain
obscure.Here we attempt to addressthe questionof whether
gravitationalcollapseof an overthickenedcrust could induce
major mantle upwelling and whole-lithosphere extension.
Thermal-rheologicalcalculationsindicatethat crustalcollapse
may decouple from the mantle lithosphere, because the
extensional forces arising from an overthickenedcrust are
limited to the crust, while the rheology of continental
lithosphereis intrinsically stratified. Even when the mantle
lithosphere is mechanically coupled to the crust,
thermomechanicalmodeling indicates that strain is localized
in the weak lower crust during crustal collapse, and no
significant(<10 km) thinning of the mantle lithospheremay
be induced at the absenceof extensional forces from plate
boundaries. Crustal collapse of the Sevier-Laramide orogen
seems adequate to account for much of the mid-Tertiary
extensionin the Cordillera, including formation of many core
complexes,but it is unlikely to have been the major causeof
the more recent basin-and-rangeextension.We suggestthat a
strong pulse of mantle upwelling in the mid-Tertiary, as
indicated by the "ignimbrite flare-up," may have triggered
basin-and-rangeextensionby weakening the lithosphereand
providingexcessgravitationalpotentialenergy.The causeof
mantle upwelling remains uncertain, but the continued
extension
and volcanism
since mid-Miocene
in the northern
Basin and Range province favor an active mantle upwelling
with internal convective heating.
1. Introduction
Orogenic belts, the products of intensive compressional
tectonics, are often the sites of continental extension and
rifting, as first observedby Wilson [1966] in his studyof the
openingof the Atlantic Ocean. The theory of plate tectonics
offers no ready explanation for such extension, because
orogenicbelts usually form near convergentplate boundaries,
and extensionof orogenicbelts happenseven when plates are
Copyright1998by the AmericanGeophysicalUnion.
Papernumber98TC00313.
0278-7407/98/98TC-00313512.00
311
still convergingand the regionalstressfield is predominantly
compressional.This is illustrated by active extensionin the
centralAndes[Dalmayerand Molnar, 1981;Dewey, 1988] and
the Tibetan plateau [England and Houseman, 1989; Molnar
and Chen, 1983].
The popularexplanationis the hypothesisof gravitational
collapse[Dewey, 1988;Molnar and Lyon-Caen,1988]. Most
orogenicbelts in the world are supportedby an Airy-type
crustal root [Airy, 1855]. The isostatic balance of vertical
forces,however,doesnot meanmechanicalequilibriumin the
lateral directions. A thickened crust, with a higher
gravitational potential energy than the adjacent lowlands,
tendsto spreador collapseunderits own weight[Artyushkov,
1973; McKenzie, 1972]. Gravitational collapse is now
believedto haveplayeda majorrole in both synorogenic
and
postorogenicextensionin many orogenic belts [Dewey,
1988], including the Cenozoic extension in the North
American Cordillera (Figure 1) [Coney, 1987;Harry et al.,
1993; Livaccari, 1991; Sonder et al., 1987; Wernicke et al.,
1987].
Although gravitationalcollapseof an overthickenedcrust
is conceptually simple, its geodynamics are not well
understood. One major problem is the role of mantle
upwelling,which is often involvedin orogeniccollapseand
contributesto the driving forces[Dewey, 1988;England and
Houseman, 1989]. However, becausemantle processesare
difficult to constrain, most studies have avoided them or
included them in calculations without offering much
discussionof their causeand relationshipto crustalcollapse
[Sonderet al., 1987]. Various assumptions
have been made,
leading to divergentconclusions.Some workers,assuming
mechanical decoupling of the crustal processesfrom the
mantle lithosphere because of the weak lower crust, have
predictedquick flattening of crustal welts into "pancakes"
with little effect on the mantle lithosphere[Bird, 1991];
others, assuminga full crust-mantlecoupling, have shown
that extensionalcollapseof an overthickenedcrustcan lead to
major extensionof the whole lithosphere[Goversand Wortel,
1993; Harry et al., 1993].
In this study we attempt to constrainthe role of crustal
collapse and the relationshipbetween crustal collapse and
mantle upwelling. The well-established history of the
Cenozoic
extension
in
the
North
American
Cordillera
providesa goodexamplefor this study.The questions
we hope
to address include the following: (1) What is the role of
gravitationalcollapseof the Sevier-Laramideorogenin the
Cordilleran extension?(2) Could crustal collapse of the
Sevier-Laramide orogen have led to basin-and-range
extension?(3) What is the causeof mantleupwellingunderthe
Basin and Rangeprovince?
312
LIU AND SHEN: CRUSTAL COLLAPSE AND CORDILLERAN EXTENSION
50 ø
40 ø
30 ø
125 ø
115 ø
105 ø
Figure 1. Shadedrelief map of the North AmericanCordillera.The toothcurveshowsthe easternboundary
of the Sevier-Laramidefold-and-thrust
belts.The bold solidline indicatesthe boundaryof the Basinand Range
province.The stippledareasare the locationsof major metamorphiccore complexes,and the thin solid line
shows the main volcanic field of the ignimbrite flare-up during mid-Tertiary. Abbreviationsare as follows:
NBR, northern Basin and Range, also called the Great Basin (GB); SRB, southernBasin and Range; SRP,
Snake River Plain; and CP, Colorado Plateau.
2. Cenozoic
Cordillera
Extension
There are numerous
in the North
excellent
review
American
articles on extensional
tectonicsin the North American Cordillera [Burchfiel et al.,
1992; Burchfiel and Davis, 1975; Coney, 1987' Eaton, 1982;
Hamilton and Myers, 1966; Stewart, 1978' Thompson and
Burke, 1974; Wernicke, 1992; Zoback et al., 1981]. A
summary here highlights some of the problems pertinent to
our discussion.
During the Mesozoic and early Cenozoic (-165-55 Ma),
western No_rthAmerica experienced a protracted phase of
crustal compressionas the oceanic Farallon plate subducted
underneath
North
America.
The crustal contraction
involved
a
complex history of subduction-related deformation and
massive plutonism along the coastal margin [Burchfiel and
Davis, 1975]. Crustal shortening in the inland Cordillera
occurredin a zone stretchingfrom Canadato northernMexico;
it telescoped more than 200 km of crust and caused
progressivedevelopmentof the fold-and-thrustbelt [Elison,
1991]. In the hinterland of the orogenic belt the crustal
thicknesswas nearly doubledto more than 50 km [Coney and
Harms, 1984; Parrish et al., 1988].
Althoughlocalizedextensionmay have startedas early as
the Mesozoic [Hodgesand Walker, 1992], major extensionin
the Cordillera is postorogenic,occurring after the Laramide
orogeny[Zobacket al., 1981]. In many placesthe inception
of extensionis markedby developmentof metamorphiccore
complexes [Coney, 1987]. The occurrenceof many core
complexesalong the core zone of the Sevier-Laramideorogen
where crust was significantlythickenedstronglyindicatesa
causeof crustalcollapse[Coney and Harms, 1984]; however,
not all core complexes are formed in the overthickenedcore
zone. In the southernBasin and Range province, most core
complexesdevelopedin the midst of a deep-seatedthrustbelt
[Coney, 1980]; detachmentfaults and core complexesare also
found in the Mojave desertwhere there is no clear evidencefor
a significantly thickened crust before extension [Dokka and
Ross, 1995; Glazner and Bartley, 1984]. Furthermore, the
inception of major extension in the Cordillera was
diachronous,although the duration and the amount of crustal
contraction were remarkably uniform from southeastern
British Columbia to Nevada and Utah [Elison, 1991]. In the
southern Canadian Cordillera, northern Washington, Idaho,
and Montana, extensionbegan in early Eocene. Farther south,
the inception time was largely Oligocene in the Great Basin
LIU AND SHEN:CRUSTALCOLLAPSEAND CORDILLERAN
EXTENSION
a
(the northern Basin and Range province), and was slightly
later in the Mojave-Sonora desert region. Extension in areas
near the latitude of Las Vegas did not occuruntil mid-Miocene
[Wernicke et al., 1987]. Wernicke et al. [1987] have pointed
out that the timing of inceptionof extensionin the Cordillera
is apparently correlated with the abundanceof associated
plutonism,and numerousstudieshave stressedthe importance
of magmatism in core-complex formation [Armstrong and
Ward, 1991;Axen et al., 1993]. Much of the magmatismmay
have resulted from postorogenic thermal relaxation and
radioactive heating [Glazner and Bartley, 1985]; however, at
least
in the Canadian
Cordillera
where
extension
within a few million years after the orogeny, some thermal
Calc-alkaline volcanism was widespreadthrough much of
the early to middle Tertiary and culminatedwith eruption of
voluminous(>35,000 km3) silicictuff in the centralGreat
Basin between 34 and 17 Ma [Best and Christiansen, 1991].
Although eruption of mafic magma is rare, significant mantle
upwelling seems necessaryto provide heat for the extensive
crustal anatexis [Hildreth, 1981] and sourcematerials for some
of the silicic tuff [Grunder, 1995; Johnson, 1991]. The
relationship between extensional tectonics and volcanism is
controversial. Some workers find that extension was mainly
synvolcanical [Gans et al., 1989], others [Axen et at., 1993;
Best and Christiansen,1991; Taylor and Bartley, 1992] argue
for a poor spatial-temporalcorrelationbetweenvolcanismand
extension on a provincial scale. Liu and Furlong [1994]
suggested that the apparently poor correlation between
extension and volcanism may be partially attributed to the
competing effects of thermal weakening and rheological
hardening associated with intrusion and underplating of
mantle-derived magmas.
Since mid-Miocene (-17 Ma) another major phase of
extension, with characteristic deep-penetrating (10-15 km)
block faulting and association with bimodal (basalticrhyolitic) volcanism, has led to formation of the Basin and
Range province, where the total extensionis estimatedto be
between 50% and 300% [Hamilton and Myers, 1966;
Wernicke, 1992]. It is commonly recognized that this
younger phase of basin-and-rangeextension [Zoback et al.,
1981] is fundamentallydifferent from the earlier low-angle
detachment faults [Burchfiel et al., 1992; Coney, 1987],
although the change between these two types of extension
was gradualin many places[Zobacket al., 1981].
The cause of extension in the Cordillera is a subject of
intensivestudy and debate.One view attributesthe Cordilleran
tectonics to plate interactions along the western margin of
North America [Atwater, 1970; Severinghausand Atwater,
1990], where subduction of the Farallon plate under North
America has been replaced by the evolving San Andreas
transform fault since 25-30 Ma. Sonder et al. [1987] and
Wernicke et al. [1987], noting that much of the Cordilleran
extension happened when western North America was under a
compressive and transpressive tectonic regime, have
emphasizedthe role of gravitationalcollapse.Becauseof the
involvement of major mantle upwelling in the basin-andrange extension, there are also numerous suggestionsfor a
causativerole of mantle thermal perturbations[Parsonset at.,
1994; Saltus and Thompson,1995; Suppeet at., 1975].
b
B
Prn
occurred
perturbations from the mantle seem necessary [Liu and
Furlong, 1993].
313
A
P- pgz
:'
'
Figure 2. Conceptualmodel of crustal collapse. (a) Sketch
of a thickened crust with an Airy-type crustal root. (b)
Lithostatic pressures along vertical profiles across the
lowland crust (line A) and the mountain range (line B). The
differential pressureAp tendsto causecrustal collapse;the
lateral pressure gradient in the transition zone between the
mountain range and the lowland tends to drive lateral
extrusion of the ductile lower crust. Notice that Ap vanishes
below
the crustal welt.
The approachwe take here is to first constrainthe effectsof
crustal collapse of the Sevier-Laramide orogen, which
involves fewer uncertainties than either plate interactions or
mantle processes.By isolating the effects of crustal collapse,
we may reach a better understandingof the role of mantle
upwelling and plate interactions.
3. Crustal Collapse
3.1.
Driving
Forces
and Thermal-Rheological
Control
The dynamic instability of an overthickened crust at
isostaticequilibrium is illustratedin Figure 2. The crustal welt
is unstablebecauseat any depth above the compensationlevel
(taken to be at the Moho of the mountain range) the
lithostatic pressure under the mountain range is greater than
that under the surroundinglowland. This differential pressure
tends to drive gravitational collapse of the mountain range.
The vertical integral of the differential pressurerepresentsthe
total extensionalforce [Lynch and Morgan, 1987]:
L
l[Pt(Z)-Pr(Z)],:lZ
-h
(])
where Pt (z) and Pr (z) are the lithostaticpressureat depthz
under the mountain range and the lowland, respectively;h is
the elevation of the mountain range above the reference
lowland, and L is the compensationdepth. The extensional
forceF actson the sidesperpendicular
to the planeof Figure2
andtherefore
hasdimensions
of forceperunitlength(N m-•)
[Turcotteand Schubert,1982].The valueof F, representedby
the shadedarea in Figure 2b, is numerically equivalentto the
excess gravitational potential energy stored in a column of
the mountainrangerelative to that in the lowland [Molnar and
Lyon-Caen, 1988]. It is clear from Figure 2 that F is
dependent on the density of the crust and mantle and the
elevationof the mountainrange above the referencelowland,
314
LIU AND SHEN: CRUSTAL COLLAPSE AND CORDILLERAN EXTENSION
which may be related to crustal thicknessif a uniform crustal
densityand the Airy isostasycan be assumed:
h= (Pm
-Pc)(Ht-Hr)
collapse, because the continental lithosphere has an•
intrinsically stratified rheology (Figure 3). The strength
envelopesof the lithosphereare definedby
(2)
Pm
O'yield(Z)=
min(%,oa)
where H t and H r are the thicknessesof the thickenedand
referencecrust, respectively;Pm and Pc are the mantle and
crustaldensity,respectively.The extensionalforce defined in
(1) can be easily calculatedfrom the elevation and the density
contrastbetweenthe crust and the mantle [Molnar and LyonCaen, 1988]:
h+AH
/
F = pcgh H r +•
fold-and-thrust
belts was thickened
where o't, and o'a are the stressdifference((o'1- o3) / 2 )
needed for brittle and ductile extension at depth z,
respectively. Laboratory experiments [Brace and Kohlstedt,
1980] suggestthat ductiledeformationdependson lithology,
strain rate, and, especially,temperature:
(3)
2
where g is gravitationalaccelerationand Mar is the thickness
of the Airy-type crustalroot: Mar= H t -H r -h. For Airy-type
isostasy, Mar= hPc/(Pm-Pc)' The crustin the hinterlandof
the Cordilleran
(4)
to -60
km
near the end of the Sevier-Laramide orogeny [Coney and
Harms, 1984; Parrish et al., 1988]. If we take the reference
crust (the crust adjacentto the hinterland) to be 40 km thick,
where k is the strain rate; the typical value of • for
continental
deformation
is in therangebetween10-14s-1 and
10-16S-1.Theparameter
A is a constant,
H is the activation
enthalpy, R is the gas constant, and T is the absolute
temperature.For a given rangeof stressesthe parametern is a
constant associated with deformation mechanism (n --3 for
dislocation creep in the lithosphere). All results discussed
thecrustalandmantledensity
to be 2800kg m-3 and3300kg
m-3, respectively,
we obtainan elevation
of 3 km from(2);
thecorresponding
extensional
forceis about4.2x1012
N m-1
belowassume
a graniticcrust(A=10-8'8MPa-ns-1,H=123 KJ
from (3), comparablewith typical tectonicforces associated
with ridge pushand slabpull [Bott, 1993; Forsythand Uyeda,
Kronenberg, 1987; Korato et al., 1986]. Using other
publishedrheologicalparametersfor the lithospherewill not
1975].
affect the generalconclusionsdrawn here.
Brittle deformationof rocksis characterizedby slidingon
fracturesand faults and is generallyindependentof strainrate,
temperature, and lithology [Byerlee, 1978]:
Notice that the gravitationaldriving forcesarisingfrom an
overthickened crust alone are limited to the crust (Figure 2).
This simple fact has important ramifications for crustal
zSP(MPa)
Yield Strength(MPa)
0
100
200
300
tool-1,andn=3)andanolivine-dominated
mantlelithosphere
(A=103'28
MPa-ns-1,H=420KJ mo1-1,
andn=3)[Kirbyand
400
0
t•t, =ktt• n
(6)
100
wherektis the frictionalcoefficient(takento be 0.85) and t•n
0
is normal stresson the fault plane. Assumingfracturesoccur
• 20[ Ductile
ß
•
40
in all orientations[Brace, 1972], crn can be replacedby the
effective lithostatic stress(the lithostatic pressureminus the
pore fluid pressure). Recent studies indicate that the brittle
strengthbecomesless sensitiveto pressureas depthincreases
[Shimada, 1993]; the complexitiesof t•/• at higherpressures
are not critical
for our discussion here and therefore are not
considered.The yield strengthof the lithosphereis usually
60
defined as the vertical integral of the yield stressacrossthe
whole lithosphere [Ranalii, 1995]:
80
Figure 3. (left) Strength envelopesof a model continental
lithosphere.The dashedprofile is for the referencelithosphere
characterizedby a 30 km crust and an equilibriumgeotherm
witha surface
heatfluxof 60mWm-2.Theshaded
profiles
are
for 20 and 50 m.y. after an instant crustal thickening that
increased the crustal thickness to 50 km; reduction of the
lithospheric strength is due to postkinematic thermal
relaxationand radioactiveheatingand the mantlelithosphere
being pushedto a greater depth. Notice the thick channelof
ductile lower crust under orogens. (right) The differential
pressure(Ap) in Figure 2 is replotted.
0
S=I O'y
ield
(z)dz
l
(7)
where l is the baseof the referencelithosphere.We alsodefine
the crustal strength as the yield stressintegrated acrossthe
crust.
The "jelly sandwich"rheologicalstructureshownin Figure
3 is intrinsic to the continental lithospherebecauseof the
temperature dependence of rheology and different
compositionsof the crust and mantle. It is also clear from
Figure 3 that the ductile lower crustis particularlyweak and
LIU AND SHEN: CRUSTAL COLLAPSE AND CORDILI.ERAN
EXTENSION
315
thick under orogenic belts owing to heating associatedwith
crustal thickening (thermal relaxation, radioactive heating,
shearheating, etc. [see Liu and Furlong, 1993]) and the fact
that the mantle material is pushed down to a hotter regime
[Glazner and Bartley, 1985]. Suchrheologicalstructuresraise
the important questionof whether crustal collapse,driven by
extensionalforces arising within the crust, can lead to wholelithosphere extension.
of lateralductileextrusionwithin the lowercrust,drivenby an
initial lateral topographic gradient of 0.01. The inset in
Figure 4 showsthe typical velocity profile. For simplicity,
3.2
Crust-Mantle
Crustal
Collapse
becausethe extensionalforces and lithosphericstrengthare
mainly determined by the crustal thickness and thermal
1993]; this is a condition representativefor core-complex
formation in the Cordillera [Parrish et al., 1988; Wernicke et
al., 1987]. The decreaseof extensionalforce and lithospheric
strengthwith time in Figure 4 is due to thinningof the crustal
welt by crustal collapse,which in the model occursin the form
brittle deformation is not included; this is not critical here
Decoupling
and Evolution
of
The effects of crustalcollapseon mantle lithospheredepend
on mechanical coupling between the crust and mantle, which,
in turn, is influenced by thermal structuresof the lithosphere.
Predicting thermal structures of orogens inevitably involves
poorly constrainedfactors such as shear heating, erosion, and
heat input from the mantle [Liu and Furlong, 1993]. To derive
some general constraints,we considerhere two end-member
cases.The first caseis for a relatively cold lithospherewhere
the strengthof the uppermostmantle preventsit from flowing
together with the lower crust. This is the assumptionin most
models of ductile flow within the lower crust [Bird, 1991]. For
crustal collapseto induce extensionin the mantle lithosphere,
sufficient shear stressesneed to be transmitted through the
ductile lower crust.The channelPoiseuilleflow may be usedto
approximateductile flows within the lower crustdriven by the
lateral pressuregradient induced by topographicchanges;the
shear stress exerted on the top of the mantle lithosphereis
[Bird, 1991]:
z=bPcgC•x
x
structures and are not sensitive
to the details of extensional
processes.Since crustalcollapseis decoupledfrom the mantle
lithosphere, no asthenospheric upwelling is induced to
compensatefor the lost gravitationalpotentialenergy, so the
total extensional force decreasesrapidly; within 10 million
yearsor so it becomeslessthan the yield strengthof the crust,
and crustalcollapseis expectedto stop.We found that, within
reasonable ranges of crustal thickness (40- 60 km) and
thermalstructures,the predictedlifespanof crustalcollapseis
about 5-15 million years. Such a relatively short lifespanof
crustal collapse is comparable with that of core-complex
formation in the Cordillera [Parrish et al., 1988].
The other end-member case is for a relatively hot
lithospherewhere the uppermostmantle is sufficientlyweak
to flow togetherwith the lower crustduringcrustalcollapse;
in this sensethe crust and mantle are fully coupled.We have
modeledductile flow within the crust and mantle lithosphere
induced by crustal thickening at orogenic belts. Figure 5a
shows the model geometry and boundary conditions. The
ductile flows are driven by the lateral pressure gradient
(8)
where g is the gravitationalacceleration,c (equal to cos
(Moho slope)) is a small geometriccorrectionfactor, and
4.0
0
dh/dx is the topographicgradient.The parameterb is roughly
half the thickness
of the ductile
channel
for Newtonian
fluids
but is only aboutone fourth for power law fluids [Bird, 1991].
For an upper bound stress estimation, take b = 10 km, a
topographicgradientof 0.01, and a flat Moho, we find •=2.8
MPa.
Similar
results
can be obtained
from
a Couette
flow
3.0
55
0.0
Velocity
(mm/yr)
0.1
2.0
approximation,assumingthe lower crust flows by the shearof
the sliding upper crust [Hopper and Buck, 1996]. Given the
typical lithosphericstrength(>1012N m-1) [Lynchand
Morgan, 1987], such shear stressesare unlikely to cause
significant deformation in the mantle lithosphere. In this
case, crustal collapsemay be mechanicallydecoupledfrom the
mantle lithosphere, as assumedin previous models [Bird,
1991; Block and Royden, 1990].
Crustal collapsethat decouplesfrom the mantle lithosphere
is expectedto be short-lived. This is illustrated in Figure 4,
which compares the total extensional force with the yield
strength of a model lithosphere determined in a onedimensional thermomechanical model, assuming complete
mechanical decoupling between the crust and mantle
lithosphere.In the model the crust with an initial thicknessof
35 km is thickened instantly to 55 km by overthrustingof a
20-km crustal sheet.Crustal collapseis allowed to occur when
temperatureat the Moho reaches650øC by postkinematic
thermal relaxation and radioactive heating [Liu and Furlong,
S1
1.0
Crustal Collapse ---•
0.0
0
5
10
15
Time (m.y.)
Figure 4. Evolution of the total extensionalforce (F) and
the yield strengthof the lithosphere
(Si) andCrust(Sc)during
crustal collapse that is mechanically decoupled from the
mantle lithosphere.The inset shows one typical velocity
profile of ductile extrusion in the crust (brittle extension is
not included in the model). Time is after the initiation of
crustal collapse.Crustal collapseis expectedto stop within
--10 million years when the extensionalforce is insufficient
to overcomethe crustalyield strength.
316
LIU AND SHEN: CRUSTAL COLLAPSE AND CORDILLERAN EXTENSION
Surface
•x.
T = 0øC
Brittle
Crust
u-v=0
Crust
Thickened
Reference
Crust
Mantle
•
Lithosphere
•
Base
ofLithosphere
u = ohv
/ c)y= 0 T = 1300øC
Distance
b
25-
crustalflows, we chosea rigid upperboundaryand a free lower
boundary so that the lateral extrusionof crustal material is all
balancedby upwelling of the mantle material under the crustal
welt. As shown in Figure 5b, the lateral flow is channelizedin
the lower crust and occurs mainly in the transition zone
between the thickened crust and the adjacent lowland, where
the lateral pressuregradientis the highest.The verticalflow is
induced by the lateral flow, as required by volume
conservation.Figure 6 shows the maximum flow rate and the
accumulative thinning of the mantle lithosphere under the
crustal welt for one experiment that started with a thermal
structure
characterized
by a surface
heatflux of 80 mW m-2.
The sharp drop of flow rate with time in Figure 6 is due to
decrease of the lateral pressuregradient as the ductile flow
reducesthe elevation gradient; the flow essentially stopsafter
10-20 million years. The total amount of thinning of the
mantle lithosphere is about 4 km. Within the reasonable
range of model parameters,including the amount of crustal
thickening (20-30 km) and the initial crustal temperature
(characterizedby an equilibriumsurfaceheat flux between60
and90 mW m-2),the totalamountof thinningof themantle
lithosphereis less than 10 km. In other words, even when the
mantle is fully coupledto the crust,crustalcollapsealone does
not induce significant thinning of the mantle lithosphereand
upwelling of the asthenosphere.
The short lifespan of crustal
collapseis consistentwith formationof somecore complexes
in the Cordillera [Parrish et al., 1988]; however, crustal
collapse of the Sevier-Laramide orogen seems unlikely to
have been the major causeof basin-and-rangeextension.
75-
0.0
0.5
1.0
1.5
4. Mantle Upwelling and Basin-and-Range
Distance (100 km)
Extension
Figure 5. (a) Model geometry and boundary conditionsfor
ductile deformation driven by the lateral pressuregradient
between the crustal welt and the lowland. Only the right half
of the model is solved becauseof the symmetry of the model.
The topographicgradient,not shownin Figure 5, is calculated
assuming local isostasy. The numerical mesh used for the
calculationsis 51x51. (b) Snapshotof the velocity field. The
thick shaded line indicates the initial Moho; the thin solid
line is the Moho at 12 m.y. The mantle lithosphereis fully
coupledto the crust,as shownby the continuousvelocityfield
There is little doubt that basin-and-range extension has
involved major mantle upwelling [Stewart, 1978]. Seismic
studies indicate that the lithospherein the Great Basin is as
thin as -65 km [Benz et al., 1990; Smith et al., 1989]. An
abnormallyhot mantle under the Basin and Range provinceis
also indicatedby the gravity [Eaton et al., 1978], high surface
feat flux (-90 mW m-2) [Lachenbruch
and Sass,1978],and
across the crust and mantle.
10.0
between the thickened crust and the lowland. Assuming
constant crustal and mantle density and local isostasy, this
lateral pressuregradient is related to the change of crustal
.=•
7.5
7.5
thickness:
5.0
cgP
Pc)dH
'•xx
=Pcg(1-p
mdx
(9)
5.0
Lithosphericthinnin
2.5
2.5•
.,•
Flow rate
where P is the pressure,x is the horizontaldistance,and H is
the crustal thickness.To isolate the effects of crustalcollapse,
we impose no lateral displacement (i.e., no shortening or
stretching)at the right-side boundary,which is also taken to
be a far-distanceboundary(calculatedto a distanceof 300 km)
so its influence on the velocity field near the crustal welt
(Figure 5b) is proven negligible.To maximize the amountof
thinning of the mantle lithosphere that may be induced by
0.0
o.o
0
lO
20
30
Time (m.y.)
40
50
Figure 6. The maximum flow rate and the accumulative
thinning of the mantle lithosphere during a model crustal
collapse that is mechanically coupled to the mantle
lithosphere.Time is after the initiation of crustal collapse.
LIU AND SHEN:CRUSTALCOLLAPSEAND CORDILLERANEXTENSION
seismic attenuationin the upper mantle under the Basin and
Range [Rornanowicz, 1979; Smith et al., 1989]. The high
elevation of the Basin and Range province (~1.5 km) is
largely compensatedby thermally inducedmassdeficiency in
the mantle, although some of the mass deficiency may be
related to compositional heterogeneities[Humphreys and
Dueker, 1994]. Although projecting the present lithospheric
structureback into the geologicalpast is difficult, significant
mantle upwelling since mid-Miocene is clearly indicated by
the widespreadbimodal volcanism associatedwith basin-andrange extension [Lipman, 1980]. There was probably strong
mantle upwelling just before basin-and-rangeextension, as
indicatedby the voluminousmid-Tertiary (34-17 Ma) volcanic
eruptionin the Great Basin.This so called ignimbriteflare-up,
317
a
16
12
recorded
by morethan35,000km3 of silicicvolcanic
ashflow
depositedover a region of >71,000 km2 [Best and
Christiansen, 1991], requires significant upwelling of the
asthenosphereto supply both the parental magmasfor some
40
of the volcanic tuff [Grunder, 1995; Johnson, 1991] and heat
for the extensive crustal anatexis [Hildreth, 1981; Liu, 1996].
Such a mantle upwelling could have played a major role in
driving basin-and-range extension. The mechanics of
continental rifting induced by mantle upwelling have been
Depth to the Upwelled Mantle (km)
intensively studied [Crough, 1978; Sengor and Burke, 1978]
and will not be discussed. Here we examine the general
gravitational instability of the lithosphereover an upwelled
asthenosphere.Figure 7 shows that an elevated lithosphere
isostatically supported by a buoyancy asthenosphere is
dynamically unstable and tends to collapse. The situation is
similar to that of an overthickenedcrust (see Figure 2) with
two major differences:(1) With asthenospheric
upwelling the
gravitational extensional forces are distributed across the
whole lithosphere, and (2) heat advected by the upwelling
80
120
b
0
Yield Strength(MPa)
50
100
0
AP(MPa)
50
• 50
a• 100.
a
B
b
A
P=pgz
I
Figure 8. (a) Total gravitational driving force (F) and
lithospheric strength (S) as functions of mantle upwelling,
calculatedassuming15 m.y. after an instantmantle upwelling
to various depths, with temperatureof the upwelled mantle
1
oho_
I
A
B
Upwelleld
• I
Asthenosphere•
I
•
Z
Figure 7. Conceptualmodel of gravitationalcollapseof the
lithospheredue to mantleupwelling.(a) Structureof the model
lithosphere. The uplifted topography is isostatically
supported by thermal buoyancy forces in the upwelled
asthenosphere.
The depth of isostaticcompensationis at the
base of the referencelithosphere.(b) Pressureprofiles across
the referencelithosphere(line A) and the thinned lithosphere
(line B).
150,
kept constant at 1300øC. The initial thickness of the
lithosphere is 150 km. (b) The strengthenvelope (left) and
the differential pressure(right) for mantle upwelling to 60 km
depth.The numbersshow the values of the total driving force
andyieldstrength
( in 1012N m-l).
asthenosphere
may significantly weaken the lithosphere.The
total extensional force and the lithospheric strength depend
mainly on the amountof asthenospheric
upwelling(Figure 8).
The resultsin Figure 8 are derived with a model lithosphere
that is initially 150 km thick; the driving force and the
lithosphericstrengthare calculatedfor 15 million years after
an instant asthenosphericupwelling, comparableto the time
interval between the peak mid-Tertiary volcanism and basinand-rangeextension[Bestand Christiansen,1991]. Figure8a
indicates that asthenosphericupwelling to depths shallower
318
LIU AND SHEN: CRUSTAL COLLAPSE AND CORDII.LERAN EXTENSION
than 70 km depth would provide sufficient extensionalforces
to cause whole-lithospheric extension. Figure 8b shows the
vertical distributionof the extensionalforces (shown by the
differential pressure between the thinned and the reference
lithosphere) and lithospheric strength for asthenospheric
upwelling to 60 km depth. Although uncertaintiesof mantle
compositions under the Basin and Range province and the
exact amount of basaltic magma involved in the ignimbrite
flare-up make it difficult to place tight bounds on the
magnitudeof mantle upwelling during the peak mid-Tertiary
volcanism, asthenosphericupwelling to around 60 km depth
is not unreasonable. Decompressional partial melting of
typical mantle materials would require mantle upwelling to
less than 50 km depth [Liu and Furlong, 1992; McKenzie and
Bickle, 1988], and a significant amount of basaltic magmas
was likely involved in the eruption of voluminous midTertiary volcanic rocks in the Great Basin [Feeley and
dimensionaladvectionmodel; the changeof elevationin the
extensionalregion was calculatedassuminglocal isostasy.
The extensionalforce and the lithosphericstrengthwere then
calculatedby integratingthe differentialpressureand the yield
strength across the lithosphere according to (1) and (7).
Figure 9 suggeststhat lithosphericextensionmay not last for
more than 10 million years for case B, because thermal
buoyancy forces in the upwelled mantle quickly diminish
through conductive cooling during extension. The total
amount of extension(vertically averagedhorizontal strain) is
less than 30% (equalto 10-•5 s-• x 10 m.y.) in caseB.
upwelling would be sufficient to trigger basin-and-range
Conversely,more than 20 million years of extensioncan be
expectedfor caseA, with -80% extension.Thesevaluesvary
mainly with the amount of mantle upwelling and the initial
thicknessof the crustin the extensionalzone and the adjacent
reference lithosphere. Initial thermal structures of the
lithosphereare not critical here, becausethey are quickly
overprintedby heat advectedby the upwellingasthenosphere.
The initial extensionalforce would be higher for a greater
extension.
contrast of crustal thickness between the extensional zone and
The causeof mantle upwelling under the Basin and Range
remainsuncertain.Our resultsargueagainstcrustalcollapseof
the Sevier-Laramideorogenbeing the major cause.Harry et al.
[1993] have shown that gravitational collapseof a thickened
crust could lead to significant mantle upwelling and
lithospheric extension, but that may be attributed to the
constant lateral stretching imposed in their model. Other
causesinclude delaminationof the mantle lithosphere[Bird,
1979], convective thinning [Housemanet al., 1981], mantle
upwelling in a slablesswindow [Dickinsonand Snyder, 1979]
or slabgap [Severinghausand Atwater, 1990] associatedwith
the migration of the Mendocino triple junction, subductioninduced mantle upwelling in a back arc setting [Stewart,
1978], and a mantle plume [Parsons et al., 1994; Saltus and
Thompson, 1995; Suppe et al., 1975]. Unfortunately, many
of these processesare difficult to test. We may group the
proposed mantle processes into two major categories
according to thermal evolution within the upwelling
asthenosphere:(1) passive mantle upwelling, where mantle
upwells adiabatically and then cools by conduction;and (2)
active mantle upwelling, where mantle upwelling is adiabatic
or superadiabatic, such as in a mantle plume, and the
temperaturein the upwelled mantle is maintained by some
the adjacentlowland. In any case the predictedextensionfor
active and passivemantleupwellingis significantlydifferent.
Grunder, 1991; Grunder, 1995; Johnson, 1991]. Such a mantle
kind of convective
The
continued
extension
and volcanism
in the Basin
and
Range province since mid-Miocene and the large total strain
(50% to 300%) across the Cordillera are more consistentwith
an active mantle upwelling.
5. Discussion
Gravitational collapse of an overthickened crust is
conceptuallysimple and geologically observable,a major
reason for gravitational collapse to have gained much
3.0
2.t)
flows. The thermal histories between these
two types of mantle upwelling are significantlydifferent so
that some general constraintsmay be derived by comparing
the predicted stability of the lithospherewith the history of
basin-and-rangeextensionin the Cordillera.
Figure 9 shows the predicted total extensional force and
yield strengthof a model lithospherewith an active (case A)
and a passive(case B) mantle upwelling. Both casesstarted
with an instantmantleupwellingto 60 km depth.The crustis
initially 40 km thick in the extensional zone and is 35 km
thick in the referencelowland. In caseA, temperaturewas
maintainedto be 1300øC at 60 km depthwith an adiabatic
geothermwithin the upwelled asthenosphere,
while in case B
conductivecooling of the upwelled mantle was allowed after
its initial ascension.Assuming pure shear extensionof the
lithospherewith a uniform strain rate of 10-15s-l, the
transient thermal structure was calculated using a one-
0o0
0
10
20
Time (m.y.)
Figure 9. Evolution of the total extensionalforce (F) and
lithosphericstrength(S) during lithosphericextensionwith
an active(caseA) anda passive(caseB) mantleupwelling.In
both cases, extension starts 15 m.y. after an instantaneous
upwelling of the asthenosphere
to 60 km depth. During
lithosphericextensionthe upwelledasthenosphere
is kept at
1300øC at 60 km depthin caseA but coolsconductively
in
case B.
LIU AND SHEN: CRUSTAL COLLAPSE AND CORDILLERAN EXTENSION
popularity in recent years. Although mantle upwelling is
often involved [Dewey, 1988; England and Houseman,1989],
its relationship with crustal collapse is ambiguous. It has
been shownin somemodels[Goversand Wortel, 1993; Harry
et al., 1993] and implied in other studies[Livaccari, 1991;
Molnar and Chen, 1983] that extensional collapse of the
overthickenedcrust of the Sevier-Laramide orogen could have
led to basin-and-rangeextension.Our results suggestthat the
effects of crustal collapse are more limited than previously
thought.Crustal collapseof the Sevier-Laramideorogencould
have accounted for much of the localized, short-lived mid-
Tertiary (>mid-Miocene) extension in the Cordillera,
including formation of many metamorphiccore complexes;
however, it is unlikely to have been the major causeof basinand-range extension. A different cause for basin-and-range
extension
is consistent
with its fundamental
differences
with
the detachment faults associated with core-complex
formations [Coney, 1987; Zoback et al., 1981].
The role of crustal collapse is limited because the
extensional forces arising from an overthickened crust are
limited to the crust, while the rheology of continental
lithosphere is intrinsically stratified. Although any
calculations of the lithospheric rheology inevitably involve
uncertainties with extrapolating the laboratory-determined
rheologic parameters, there are abundant geological and
geophysical observations indicating the existence of a
locally weak, ductile crustand mechanicaldecouplingbetween
the upper crust and the mantle lithosphere(see Kirby and
Kronenberg [1987] for a review). Crust-mantledecouplingis
thought to be important in thin-skinned thrusting [Ranalli
and Murphy, 1987] and crustal deformation at convergent
orogens[Royden, 1996]. The casefor crust-mantledecoupling
is especially strong for crustal collapse of orogenic belts,
where the ductile lower crust is particularly thick and weak.
Mechanical decoupling may also occur between finer-scale
rheological layers resulting from magmatism and
compositional heterogeneities in the crust [Lister and
Baldwin, 1993], allowing crustal collapse to occur at much
lower deviatoric stressesthan those required to overcome the
yield strength of the whole lithosphere.This may help to
explain the diffusive crustalextensionin the Great Basin that
spansa greater spaceand time than the few well-developed
metamorphiccore complexes[Axenet al., 1993].
Crustal collapse that decouples mechanically from the
mantle lithospherewould causethrustingnear the marginsof
orogenic belts, as required by the volume conservation.
Thrusting concurrentwith extensionis observedin the Andes
and the Tibet Plateau [Burchfiel et al., 1992; Molnar and
Lyon-Caen, 1988] and in some Neogene extensionalbasinsin
the Mediterranean [Platt and Vissers, 1989]. Evidence for
thrusting at the margins of the Cordilleran hinterland coeval
with core-complexextension is scarce;this may be partly
attributed to the difficulties in interpreting faults from
complicatedtectonic overprinting in the Cordillera [Coney,
1980]. Concurrent stretching of the mantle lithosphere by
processesrelated to plate interactions [Dokka and Ross,
1995] would also mitigate the volume problem created by
crustal collapse.
Eliminating crustal collapse of the Sevier-Laramideorogen
as the direct cause of mantle upwelling under the Basin and
319
Range province brings us a step closer to the causeof basinand-range extension. Localized mantle upwelling under the
Cordillera may have startedin the Eoceneor even earlier. Liu
and Furlong [ 1993] find that an increasedmantleheatflux was
needed to account for the high crustal temperature and
plutonism associated with the Eocene crustal extension and
core-complex formation in the southwestern Canadian
Cordillera. The mantle upwelling could have been causedby
delamination of the mantle lithosphere [Bird, 1979] or
convective thinning [Houseman et al., 1981]; in either case,
basin-and-rangeextensioncan be regardedas having a similar
origin to the mid-Tertiary extension, both resulting from the
dynamic instability of a thickened lithosphere[Sonderet al.,
1987]. However, the models of delamination or convective
thinning are difficult to test; conversely, the southward
migration of Tertiary volcanism in the Great Basin may be
more easily explained as a result of the retreating Farallon
plate [Best and Christiansen, 1991; Lipman, 1980]. In any
case, the intensive mid-Tertiary ignimbrite flare-up indicatesa
strong pulse of mantle upwelling, which may have triggered
basin-and-range extension by providing the excess
gravitational potential energy and by thermally weakening
the lithosphere [Liu, 1996].
While our discussionhas been focusedon crustal collapse
and mantle upwelling, there is no doubt that the evolving
tectonic setting in western North America has played an
important role in the Cordilleran extension. The crustal
collapse of the Sevier-Laramide orogen was probably
triggered by the drop of compressionalstressesat the end of
the Laramide orogenyas a result of the reducedconvergentrate
between the Farallon and the North American plates [Coney,
1987]; the basin-and-rangeextensionis closely related to the
change of plate geometry at the western margin of North
America, where convergencebetween the Farallon and North
American plates has been gradually replaced by the San
Andreastransformfault [Dickinsonand Snyder,1979; Zoback
et al., 1981]. This change of tectonic setting may have
facilitated basin-and-range extension by reducing the
compressional stresses; however, it generates no major
extensional forces [Sonder et al., 1986]. If the major forces
driving basin-and-rangeextension are the thermal buoyancy
forces in the upwelling mantle, our resultsindicate that some
kinds of convective flows within the upwelling mantle were
necessaryto sustainthe gravitational potential energy.
6.
Conclusions
1. Gravitationalcollapseof an overthickenedcrustmay be
largely decoupled from the mantle lithosphere; crustal
collapse alone cannot induce major mantle upwelling and
whole-lithosphereextension. Most crustal collapse is shortlived (10-15 million years)and may have concurrentthrusting
at the margins of orogens. Crustal collapse of the SevierLaramide orogenis consistentwith the mid-Tertiary extension
and formation of metamorphic core complexes in the
Cordillera; however, it is unlikely to have been the major
causeof basin-and-rangeextension.
2. The strongpulse of mantle upwelling indicatedby the
ignimbrite flare-up may have triggered basin-and-range
extension by weakening the lithosphere and providing the
excessgravitationalpotential energy. The causeof the mantle
320
L1U AND SHEN: CRUSTAL COLLAPSEAND CORDILLERAN EXTENSION
upwelling is uncertain, but the continuous volcanism and
extension since mid-Miocene in the Basin and Range
province favor an active mantle upwelling with internal
Acknowledgments.
This work was supported
by the NSF grant
EAR-9506460andtheACS/PRFgrant27925-G2administrated
by the
AmericanSocietyof Chemistry.
We thankClemChasefor helpful
discussionand P. Coney, L. Royden, J. Platt, D. Scholl, and an
convective heating.
anonymous
reviewerfor theirhelpfulreview.
References
Airy, G.B., On the computation
of the effectof the
attractionof mountainmasses,as disturbingthe
geodetic surveys, Philos. Trans. R. Soc.
Coney, P.J., and T.A. Harms, Cordilleran
metamorphic core complexes: Cenozoic
extensional relics of Mesozoic compression,
Geology, 12, 550-554, 1984.
London, Ser. B, 145, 101-104, 1855.
Crough,S.T., Thermalorigin of mid-platehotspot
apparent astronomical latitude of stations in
Armstrong, R.L., and P. Ward, Evolving
geographicpatternsof Cenozoicmagmatismin
theNorthAmericanCordillera:The temporaland
spatial association of magmatism and
metamorphiccore complexes,J. Geophys.Res.,
96, 13,201-13,224,
1991.
tectonic evolution
of western North
America, Geol. Soc. Am. Bull., 81, 3513-3536,
1970.
1978.
Dalmayer, B., and P. Molnar, Parallel thrust and
normal faulting in Peru and constrainson the
state of stress,Earth Planet. Sci. Lett., 55, 473481,
Artyushkov, E.V., Stressesin the lithosphere
causedby crustalthicknessinhomogeneities,
J.
Geophys.Res., 78, 7675-7690, 1973.
Atwater, T., Implicationsof plate tectonicsfor the
Cenozoic
swells, Geophys.J. R. Astron. Soc., 55, 451469,
1981.
Dewey, J.F., Extensional collapse of orogens,
Tectonics, 7, 1123-1139,
1988.
slabs related
to the San Andreas
transform, J. Geol., 87, 609-627, 1979.
Dokka, R.K., and T.M.
PASSCAL
21,823-21,842,
1990.
Best, M.G., and E.H. Christiansen,
Limited
extensionduringpeak Tertiary volcanism,Great
Basin of Nevada and Utah, J. Geophys.Res.,
96, 13,509-13,528, 1991.
Bird, P., Continental
delamination
and
the
ColoradoPlateau,J. Geophys.Res., 84, 75617571, 1979.
Bird, P., Lateral extension of lower crust from under
high topography in the isostatic limit, J.
Geophys.Res., 96, 10,275-10,286, 1991.
Block, L., and L.H. Royden, Core complex
1995.
Eaton,G.P., The Basinand RangeProvince:Origin
and tectonic significance, Annu. Rev. Earth
Planet. Sci., 10, 409-440, 1982.
Eaton,G.P., R.R. Wahl, H.J. Prostka,D.R. Mahey,
and M.D. Kleinkopf, Regional gravity and
tectonicpatterns:Their relationto late Cenozoic
epeirogenyand lateral spreadingin the western
Cordillera,in CenozoicTectonicsand Regional
Geophysicsof the Western Cordillera, edited
by R.B. Smith and G.P. Eaton,Mem. Geol. Soc.
Am., 152, 51-91, 1978.
Elison, M.W., Intracontinental
contraction
in
western North America: Continuity and
episodicity, Geol. Soc. Am. Bull., 103, 12261238,
1991.
England, P.C., and G.A. Houseman, Extension
geometriesand regional scale flow in the lower
during continental convergencewith special
referenceto the Tibetan plateau,J. Geophys.
crust, Tectonics, 9, 557-567, 1990.
Res., 94, 17,561-17,597,
Bott,
M.H.P.,
Modeling
the
plate-driven
mechanism, J. Geol. Soc. London, 150, 941951, 1993.
Brace, W.F., Laboratorystudiesof stick-slip,and
their
application
to
earthquakes,
Tectonophysics,14, 189-200, 1972.
Brace, W.F.,
and D.L.
Kohlstedt,
Limits
on
lithospheric stress imposed by laboratory
experiments,J. Geophys.Res., 85, 6248-6252,
1980.
Burchfiel,
B.C.,
and G.A.
Davis, Nature and
controls of cordilleran orogenesis, western
United
States:
Extensions
of
an
earlier
synthesis,Am. J. Sci., 275, 363-396, 1975.
Burchfiel, B.C., D.S. Cowan, and G.A. Davis,
Tectonicoverviewof the Cordilleranorogenin
westernUnited States,in The Geologyof North
America, vol. G-3, The Cordilleran Orogen:
Conterminous U.S., edited by B.C. Burchfiel,
M.L. Zoback, and P.W. Lipman, pp. 407-480,
Geol. Soc. of Am., Boulder, Colo., 1992.
Byerlee, J.D., Friction of rocks, Pure Appl.
Geophys., 116, 615-626, 1978.
Coney, P.J., Cordilleran metamorphic core
complexes:An overview, Mere. Geol. Soc. Am.,
153, 7-31, 1980.
Coney, P.J., The regional tectonic setting and
possible causes of Cenozoic extension in the
North American Cordillera, in Continental
ExtensionalTectonics,editedby M.P. Coward,
J.F. Dewey, and P.L. Hancock,Geol. Soc.Spec.
Publ., 28, 177-186, 1987.
1992.
crustflow on continentalextensionandpassive
margin formation, J. Geophys. Res., 101,
1996.
Houseman,G.A., D.P. McKenzie, and P. Molnar,
Convective
instabilityof a thickened
boundary
layer and its relevancefor the thermalevolution
1075-1078,
seismic experiment, J. Geophys. Res., 95,
Hodges, K.V., and J.D. Walker, Extensionin the
Cretaceous Sevier orogen, North American
Cordillera, Geol. Soc.Am. Bull., 104, 560-590,
of continentalconvergent
belts,J. Geophys.
the San Andreasfault system,Geology, 23,
Range province from the 1986
Hildreth,W., Gradientsin silicicmagmachambers:
Implicationsfor lithosphericmagmatism,J.
Geophys.Res., 86, 10,153-10,192, 1981.
core complexes,the Sierra Nevada,orocline, and
the western United States, Geol. Soc. Am. Bull.,
105, 56-72, 1993.
Crustal structure of the northwestern Basin and
and structural evolution of the Great Basin,
Earth Planet. Sci. Lett., 117, 59-71, 1993.
of earlyMiocenedetachment
faults,metamorphic
extensionand magmatismin the Great Basin of
Benz, H.M., R.B. Smith, and W.D. Mooney,
North America:Implicationsfor the magmatic
20,175-20,194,
Ross, Collapse of
southwestern North America and the evolution
Axen, G.J., W. Taylor, and J.M. Bartley, Spacetime patternsand tectoniccontrolsof Tertiary
mechanics of continental extension in western
Hopper, J.R., and W.R. Buck, The effectsof lower
Dickinson,W.R., and W.S. Snyder,Geometryof
subducted
Harry, D.L., D.S. Sawyer,and W.P. Leeman,The
1989.
Feeley, T.C., and A.L. Grunder, Mantle
contributionto the evolutionof Middle Tertiary
silicic magmatism during early stages of
extension:The Egan Range volcanic complex,
east-central Nevada, Contrib. Mineral. Petrol.,
106, 154-169, 1991.
Forsyth, D., and S. Uyeda, On the relative
importance of the driving forces of plate
motion, Geophys.J. R. Astron. Soc., 43, 163200, 1975.
Gans, P.B., G.A.
Mahood, and E. Schermer,
southwesternUnited States, Tectonics, 3, 385396, 1984.
Glazner, A.F., and J.M. Bartley, Evolution of
lithosphericstrengthafter thrusting,Geology,
13, 42-45, 1985.
Govers, R., and M.J.R. Wortel, Initiation of
asymmetricextensionin continentallithosphere,
Tectonophysics,223, 75-96, 1993.
Grunder, A.L., Material and thermal roles of basalt
in crustalmagmatism:Case studyfrom eastern
Nevada, Geology, 23, 952-956, 1995.
Hamilton,W., and W.B. Myers, Cenozoictectonics
of the westernUnitedStates,Rev. Geophys.,
4.,
1966.
Res., 99, 9635-9650,
1994.
Johnson,C.M., Large-scalecrustal formation and
lithospheremodificationbeneathMiddle to Late
Cenozoiccalderasand volcanic fields, western
North America, J. Geophys.Res., 96, 13,48513,508,
1991.
Kirby, S.H., and A.K. Kronenberg,
Rheologyof
the lithosphere:
Selectedtopics,Rev. Geophys.,
25, 1219-1244, 1987.
Korato,S.I., M.S. Paterson,and J.D. Fitzgerald,
Rheology of synthetic olivine aggregates:
Influenceof grain size and water,J. Geophys.
Res., 91, 8151-8176, 1986.
Lachenbruch, A.H., and J.H. Sass, Models of an
extending lithosphere and heat flow in the
Basin and Range Province, in Cenozoic
Tectonicsand Regional Geophysicsof the
Western Cordillera, editedby R.B. Smith and
G.P. Eaton, Mem. Geol. Soc. Am., 152, 209250,
1978.
Lipman, P.W., Cenozoic volcanismin the western
United States: Implication for continental
tectonics, in Studies in Geophysics:
ContinentalTectonics,pp. 161-174,Nat. Acad.
Sci., Washington,D. C., 1980.
Lister, G.S., and S.L. Baldwin, Plutonism and the
originof metamorphic
corecomplexes,
Geology,
21, 607-614, 1993.
Synextensionalmagmatismin the Basin and
RangeProvince:A casestudyfrom the eastern
Great Basin, Spec. Pap. Geol. Soc. Am., 233,
58 pp., 1989.
Glazner, A.F., and J.M. Bartley, Timing and
tectonic setting of Tertiary low-angle normal
faulting and associated magmatism in the
509-549,
Res., 86, 6115-6132, 1981.
Humphreys,
E.D., and K.G. Dueker,Physicalstate
of the westernU.S. uppermantle,J. Geophys.
Liu, M., Dynamic interactions between crustal
shortening,extension,and magmatismin the
North American Cordillera, Pure Appl.
Geophys., 146, 447-467, 1996.
Liu, M., and K.P. Furlong,Cenozoicvolcanismin
the California Coast Ranges: Numerical
solutions, J. Geophys. Res., 97, 4941-4957,
1992.
Liu, M., and K.P. Furlong,Crustalshortening
and
Eocene extension in the southeastern Canadian
Cordillera: Some thermal and mechanical
considerations, Tectonics, 12, 776-786, 1993.
Liu, M., and K.P. Furlong, Intrusion and
underplating of mafic magmas: thermalrheological
effectsandimplications
for Tertiary
tectonomagmatism in the North American
Cordillera, Tectonophysics, 273, 175-187,
1994.
Livaccari, R.F., Role of crustalthickeningand
extensionalcollapsein the tectonicevolutionof
the Sevier-Laramideorogeny,westernUnited
States,Geology, 19, 1104-1107, 1991.
LIU AND SHEN:CRUSTALCOLLAPSEAND CORDILLERANEXTENSION
Lynch, H.D., and P. Morgan, The tensile strength
of the lithosphere and the localization of
extension,
in
Continental
Extensional
Tectonics, edited by M.P. Coward, J.F. Dewey
and P.L. Hancock, Geol. Soc. Spec. Publ., 28,
53-66, 1987.
McKenzie,
D.P.,
Active
tectonics
of
the
Mediterranean region, Geophys. J. R. Astron.
Soc., 30, 109-185, 1972.
McKenzie, D., and M.J. Bickle, The volume and
compositionof melt generatedby extensionof
the lithosphere,J. Petrol., 29, 625-679, 1988.
Molnar, P., and W.-P. Chen, Focal depthsand fault
plane solutions of earthquakes under the
Tibetan plateau, J. Geophys. Res., 88, 11801196,
1983.
Soc. Am., 218, 179-207, 1988.
Parrish, R.R., S.D. Cart, and D.L.
extensional
Parkinson,
tectonics
and
1988.
Parsons,T., G.A. Thompson,and N. Sleep, Mantle
plume influence on the Neogene uplift and
extension of the U.S.
western Cordillera?,
Geology, 22, 83-86, 1994.
Platt,
J.P.,
and
R.L.M.
Vissers,
132, 281-295,
1987.
Romanowicz, B.A., Seismic structureof the upper
mantle beneath the United States by threedimensional inversion of body wave arrival
times, Geophys. J. Royal Astron. Soc., 57,
1979.
Wernicke, Mere. Geol. Soc. Am., 176, 1-22,
17,679-17,705,
Shimada, M., Lithosphere strength inferred from
fracture strength of rocks at high confining
pressures and temperatures, Tectonophysics,
Thompson, G.A., and D.B. Burke, Regional
Geophysicsof the Basin and Range province,
Annu. Rev. Earth Planet. Sci., 2, 213-238,
C.A.
Baker,
and
D.G.
Gants,
Geophysical and tectonic framework of the
eastern Basin and Range-Colorado PlateauRocky Mountain transition, in Geophysical
Framework of the Continental United States,
edited by L.C. Pakiser and W.D. Mooney,
Mere. Geol. Soc. Am., 172, 205-233,
Continuum
calculations
1989.
of
continental
deformation in transcurrent environments, J.
Geophys. Res., 91, 4797-4818, 1986.
Sonder, L.J., P.C. England, B. Wernicke, and R.L.
Christiansen, A physical model for Cenozoic
of
western
Turcotte, D.L., and G. Schubert, Geodynamics:
Applications of Continuum Physics to
Geological Problems, 450 pp., John Wiley,
New York, 1982.
Wernicke, B.P., Cenozoic extensional tectonics of
the U.S. Cordillera, in The Geology of North
America, vol. G-3, The Cordilleran Orogen:
Conterminous United States, edited by B.C.
Burchfiel,M.L. Zoback,and P.W. Lipman,pp.
553-581, Geol. Soc. of Am., Boulder. Colo.,
1992.
Smith, R.B., W.C. Nagy, K.A. Julander, J.J.
extension
western Utah, Geol. Soc. Am. Bull., 104, 255266, 1992.
North
America,
in
Continental Extensional Tectonics, edited by
M.P. Coward, J.F. Dewey, and P.L. Hancock,
Geol. Soc. Spec.Publ., 28, 187-201, 1987.
Stewart,J.H., Basin and Range structurein western
North America: a review, in Cenozoic Tectonics
and Regional Geophysics of the Western
Cordillera, edited by R.B. Smith and G.L.
Wernicke, B., R.L. Christiansen,P.C. England,
and L.J. Sonder,Tectonomagmaticevolutionof
Cenozoic
Cordillera,
extension
in
in
1996.
Suppe, J., C. Powell, and R. Berry, Regional
topography, seismicity, quaternary volcanism,
and the present-day tectonics of the western
United States, Am. J. Sci., 275, 397-436, 1975.
the North
Continental
American
Extensional
Tectonics,edited by M.P. Coward,J.F. Dewey,
and P.L. Hancock, Geol. Soc. Spec. Publ., 28,
203-221, 1987.
Wilson, J.T., Did the Atlantic close and then re-
open?, Nature, 211,
Zoback, M.D.,
R.E.
676-681, 1966.
Anderson, and G.A.
Thompson, Cenozoic evolution of the state of
stressand style of tectonismof the Basin and
Ranges Province of the western United States,
Philos. Trans. R. Soc. London, Ser. A, 300,
407-434,
1981.
M. Liu and Y. Shen, Dept. of Geological
Sciences, University of Missouri, 101 Geology
Building, Columbia, MO 65211. (e-mail:
[email protected],
[email protected])
Eaton, Mere. Geol. Soc. Am., 152, 1-31,1978.
Royden, L., Coupling and decouplingof crust and
mantle in convergentorogens:Implicationsfor
strainpartitioningin the crust,J. Geophys.Res.,
I01,
slabs beneath western North America, in Basin
and Range Extensional Tectonics Near the
Latitude of Las Vegas,Nevada,edited by B.P.
Viveiro,
Taylor, W.J., and J.M. Bartley, Prevolcanic
extensional Seaman breakaway fault and its
geological implications for easternNevada and
1974.
1978.
Severinghaus, J., and T. Atwater, Cenozoic
geometryand thermal state of the subducting
Sonder, L.J., P.C. England, and G.A. Houseman,
Extensional
collapseof thickenedcontinentallithosphere:A
working hypothesisfor the Alboran Sea and
Gibraltar arc, Geology, 17, 540-543, 1989.
Ranalli, G., Rheology of the Earth, 413 pp.,
Chapmanand Hall, New York, 1995.
Ranalii, G., and D.C. Murphy, Rheological
stratificationof the lithosphere,Tectonophysics,
479-506,
Sengor,A.M.C., and K. Burke, Relative timing of
rifting and volcanismon earth and its tectonic
implications,Geophys.Res. Lett., 5, 419-421,
217, 55-64, 1993.
geochronologyof the southern Omineca belt,
British Columbia and Washington, Tectonics,
7, 181-212,
1995.
1990.
Molnar, P., and H. Lyon-Caen, Some simple
physical aspectsof the support, structure,and
evolution of mountain belts, Spec. Pap. Geol.
Eocene
Saltus, R.W., and G.A. Thompson, Why is it
downhill from Tonopahto Las Vegas?:A case
for mantle plume supportof the high northern
Basin and Range, Tectonics, 14, 1235-1244,
321
(received October 16, 1997;
revised January6, 1998;
acceptedJanuary 28, 1998.)