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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. 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