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Home , Albuquerque Basin, Basin and Range Province, Black Range, East Potrillo Mountains, Latir volcanic field, Mogollon Mountains

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. B6, PAGES 6263-6276, MAY 10, 1986

Cenozoic Thermal, Mechanical and Tectonic Evolution of the

PAUL MORGAN1

Departmentof Geosciences,Purdue University,West Lafayette, Indiana

WILLIAM R. SEAGER

Departmentof Earth Sciences,New State University,Las Cruces

MATTHEW P. GOLOMBEK

Jet PropulsionLaboratory, CaliforniaInstitute of Technology,Pasadena

Careful documentationof the Cenozoicgeologic history of the in reveals a complexsequence of events.At least two phasesof extensionhave been identified.An early phase of extensionbegan in the mid-(about 30 Ma) and may have continuedto the early (about 18 Ma). This phaseof extensionwas characterizedby local high-strainextension events (locally, 50-100%,regionally, 30-50%), low-anglefaulting, and the developmentof broad, relativelyshallow basins, all indicatingan approximatelyNE-SW •-25ø extensiondirection, consistent with the regionalstress field at that time.Extension events were not synchronousduring early phase extension and were often temporally and spatiallyassociated with major .A late phaseof extensionoccurred primarily in the late Miocene(10-5 Ma) with minor extensioncontinuing to the present.It was characterizedby apparently synchronous,high-angle faulting givinglarge verticalstrains with relativelyminor lateral strain (5-20%) whichproduced the moderuRio Granderift morphology.Extension direction was approximatelyE-W, consistentwith the contemporaryregional field. Late phasegraben or half-grabenbasins cut and often obscureearly phasebroad basins.Early phase extensionalstyle and basin formation indicate a ductilelithosphere, and this extensionoccurred during the climax of Paleogenemagmatic activity in this zone.Late phaseextensional style indicates a more brittle ,and this extensionfollowed a middle Miocenelull in .Regional uplift of about1 km appearsto haveaccompanied late phase extension, andrelatively minor volcanism has continued to thepresent. We have estimated geotherms and calculated lithosphericstrength curves for the two phasesof extension,using geologic data to constrainearlier events andgeophysical data to constrainthe moderngeotherm and crustalstructure. A highgeotherm was deduced for early phaseextension, resulting in a shallowcrustal brittle-ductile transition and negligiblemantle strength.The lithospherecooled after early phaseextension, resulting in a deepercrustal. brittle-ductile transition,and, perhapsmore significantly,a considerablezone of strengthimmediately beneath the Moho. Theseresults indicate that early phaseextensional style was controlled by a crustaldecollement nearthe brittle-ductiletransition, which was prevented during late phaseextension by significantstrength in the uppermostmantle. Late Cenozoicuplift of the rift zonecannot be explainedby crustalthinning duringextension and geothermevolution predicted from simplecooling. However, this uplift doesnot appearto be restrictedto the , and Plioceneto Recentvolcanism and heat flow data suggest that uplift may be causedby magmaticthickening of the ,perhaps unrelated to rifting. The complex interrelationshipamong regional and local prerifting,synrifting, and postriftingevents in the Rio Grande rift suggeststhat rifting, at least in this ,should not be consideredin isolationof other geologic events.

INTRODUCTION tectonicstyles. We havecarefully documented these events with relevant geological and geophysicaldata to attempt to Much attention has recently been focused upon the understandthe factorswhich controlledtectonic style during mechanismsand processesof rifting. In this studywe present each extensionalevent through an analysisof the thermal and a relativelydetailed analysis of two areasof New Mexico which mechanicalevolution of the lithospherein theseareas. As these exhibit complex but similar Cenozoichistories of extensional data are crucialto the thermomechanicalmodeling presented tectonism.The locationsof theseareas are shownin Figure in this contribution,we presentdetailed documentation of the 1. The first study area is the Basin and Range provinceand geologichistory of the southernRio Grande rift study area southernRio Grande rift in southernNew Mexico; the second below, followed by a more concisehistory of rift evolutionin studyarea is the centralRio Granderift in centraland northern New Mexico, the southernSan Luis basin,the Espafiolabasin, northernNew Mexico [see also Morgan and Golo•bek, !984]. It is beyondthe scopeof thispaper to presentsimilarly detailed and the Albuquerque-Belenbasin. At least two documentationfor the Cenozoicevolution of all,.areasof the extensionalevents have occurred in eacharea with contrasting Rio Granderift: however,the general similarities in evolutionary historythat we have deduced for ourtwo study are• suggest that our resultsmay be generallyapplicable to the Rio Grande •Alsoat Lunarand Planetary Institute, Houston, . rift. • Our analysisof extensionin the Rio Granderift differsfrom Copyright1986 by the AmericanGeophysical Union. manyprevious rift modelingstudies in that we ignorethe causes Paper number 5B5473. of extension.Through this analysis,we show that, at least in 0148-0227/ 86/ 005B-5473505.00 our study areas,the extensionevents cannot be understoodin

6263 6264 MORGAN ET AL.: CENOZOIC EVOLUTION OF THE RIO GRANDE RIFT

I • SouthernSan andesiticflows and lahars from widespreadcentral volcanoes. By 35 Ma, calc-alkalinerhyolite and rhyodacitewere erupted

I I in hugevolumes, largely as ash flow sheetsfrom cauldron complexes.Individual sheets spread across thousands of square I Alamo. LuisBasi_•n kilometersof southwestern New Mexico, suggestingthat neither activeextension nor compressionwas producingtopography jNEW MEXICO •..•.:.:.:.11 Espan01a I that might control the distributionof volcanicrocks [e.g., Chapin,1974; Chapin and Seager,1975; Seager et al., 1984). Belen Basin entoFe Elston [1984a] and Engebretsonet al. [1984] believemiddle Albuquerque- Tertiaryarc magmatismwas accompaniedby stronginterarc extension.On theother hand, Price and Henry [1984] and Henry . • I• rque , I and Price,[this issue] present data from westTexas that indicates ENE compressionfrom Laramidetime throughthe main pulse of middle Tertiary volcanism 39-32 Ma. Becauseof these •IZ.• •RIO•RANDE RIFT contradictions,it is unclear what kind or orientation of stress I :"'•• ' ' •ocorro field accompaniedmiddle Tertiary volcanismin southwestern New Mexico. What does seemclear is that by about 30 q- 2 _ ";•'-'-j-• SouthernRio Ma changingpatterns of magmatism,sedimentation, and deformationsignify establishment of regionalextension as a • '.'•.•:'( C:.• • ' •'Gra nde Riftstable, dominating .

Early PhaseExtension

Magmatism. Several kinds of evidence document the existenceof an extensionalregime in southernNew Mexico in late Oligocenetime. One of theseis the strikingand locally thick sequence of "basaltic " that overlies middle Oligoceneash flow tuffsor associatedrocks almost everywhere. Foults odive in ' .'• Ranging in age from about 29 to 18 Ma, these marie flows loreQu•ernery :••••idio locally comprisecentral shield volcanoes or are associatedwith Fig. 1. Index map for study arc•. The southernRio Grande •t VOLCANIC dct•lcd study area is outlined by the box. The •buqucrquc-Bclcn, TIME STYLE VOLCANIC PRODUCTS TECTONIC STYLE Espafiolaand southernSan Luis b•ins comprisethe ccntr• Rio Grande m.y.b.p. •t studyarea (b•c map from Seagerand Morgan [1979]).

RiftIAikali-Oiivine BasaltCulmination of Rifting isolation of other geologicevents. We attempt to show that the interrelationshipbetween regional and local eventsmay be t RiftExtension: Rapid complex but that it is fundamental to the understandingof --10-- extensionaltectonics. The interrelationshipbetween local and Magmatic regionalevents may be the key to understandingsome of the Lull Slow Extension ? complexitiesand diversitiesof continentalrifting.

--20-- CENOZOIC GEOLOGIC HISTORY OF SOUTHERN RIO GRANDE AlkaliRhyolite RIFT & - Datii Prerift Tectonicsand Magmatism Backarc Extension: Backarc? Rapid

During the last 65 m.y., southwesternNew Mexico has been - --- 29- 30 ------30-- Mogollon•------29 -- - 30 ..... the site of almost constanttectonic and/or magmaticactivity Backarc? I Calc-A!kaline or Arc? I Basaltic Andesire (Figure 2). Most of southernNew Mexico was a cratonicback ------33 I arc region during the Laramide which failed under regional Calk-Alkaline No strong regional compressionby forming a seriesof northwesttrending, Wind Arc? stress (Arc?) River style basementblock uplifts and complementarybasins --40-- I J [Seaget, 1983; Seaget and Mack, 1986a]. Only in the I •Calk-Aikaline southwesterncorner of the state did a magmatic arc extend Andesire into the region,although isolated -plutonic centers occur Laramide Compression: Foreland-style outsidethe arc at Tyrone,Pinos Altos, SantaRita, and Hillsboro. block uplifts and basins; All of the magmatismis Late Cretaceousto Paleocenein age. overthrusting (?) Middle Tertiary magmatism affected virtually all of --50-- southwesternNew Mexico. Although locally sheetsof rhyolitic Fig. 2. Cenozoicvolcano-tectonic history of southwesternNew Mexico ash flow tuff are prominent, earliesteruptions were generally basedon geologicdata. MORGANET AL.: CENOZOICEVOLUTION OF THE RIO GRANDERIFT 6265

the remains of cinder cone fields or diatremes. At least one There is little evidenceto tie low-angle faulting genetically 27 Ma cinder cone was constructedon a throughgoingnormal to developmentof broad early rift basins.Few low-anglefaults [Seaget and Clemons, 1975], implying that extensional are known to cut early rift .Most occurrencesof low- stresses were active at this time. anglefaulting are in thevicinity of majorlate Oligocenevolcano- Although the basaltic andesitesare generally dark colored, plutonit complexes(e.g., Organ , East Potrillo they range in compositionfrom mafic andesitcto latite and, Mountains),whereas lacking such igneous centers also like their predecessortuffs and ,are calc-alkaline lack well-developedlow-angle faults (e.g., , [ Chapinand Seaget,1975; Bornhorst, 1980]. Strontium isotopic northern and central , Sacramento ratios are also similar to the older volcanics (0.706-0.709, Mountains). On the other hand, not all volcano-plutonic [$tinnett and $teuber, 1976]). High silica (rarely peralkaline) complexesare associatedwith low-anglefault systems(e.g., rhyoliticlavas and tuffs are interbeddedlocally with the , Emory cauldron,Tres HermanasMountains). Such inconsis- especiallyin the Mogollon-Datil Plateauwhere important ash tenciesmake it difficult to correlatedirectly low-angle faulting flow tuff cauldronsof early Miocene age are well known [e.g., with shallowplutonism and volcanism. Nevertheless, the overlap Ratte et al., 1984; Elston, 1984b]. As Elston [1984a,b] has of later phases of middle Tertiary volcanism with the emphasized,the passiveeraplacement of batholithic roots of developmentof closelyspaced low-angle faults seems significant. thesecauldrons implies an activeextensional stress field. We believethat the faultingwas probably a responseto extension The structural,stratigraphic, and chemical-isotopicdata from of an upper crust that had been thermallyweakened by 10- the "basaltic andesites" and related rocks discussed above 20 m.y. of shallowpluton emplacement and volcanism. indicatethat Oligocenecalc-alkaline magmatism continued into Amount of earlyphase extension. Estimatesof the amount early Miocenetime but modifiedto the extent that a regional, of earlyphase extension are difficult to make.Using assumptions stable extensionalstress field, firmly establishedabout 30 q- and argumentsof McKenzie[1978] and Morganand Golombek 2 Ma, promoted weakly bimodal but fundamentallybasaltic [1984], broad early rift basinsup to 1.9 km deep, suchas we andesitcvolcanism until about 18 Ma. Another important havedescribed, may indicate as much as 27% extension assuming manifestationof this extensionwas development of "early rift" a 100-km-thicklithosphere prior to extensionand that thermal basins. relaxationafter rifting did not deepenthe basins.Because of "Early rift" basins. Across broad tracts of southern New uncertainfault geometry,estimates of extensionresulting in low- Mexico, thick basinalsedimentary rocks intertongue with upper anglefaults also is imprecise.Using calculations described by Oligocene-lowerMiocene "basalticandesitc" flows. Locally as ThompsonI1960], Wernickeand Burchfiel[1982], and Angelier much as 1.9 km thick, these depositsaccumulated in broad and Colletta[1983] basedupon palinspasticreconstruction of "early rift" basinsas alluvial fan, alluvial fiat, and playa facies. mappedfault blocks, we believeextension of 50-100% was Clastscan be tracedto late Oligoceneto middle Miocene fault achievedlocally in areasof closelyspaced, low-angle faults such blocks,and they also recordthe erosional"unroofing" of these as BishopCap hills and . Much less blocks, some of which are ancestral to modern ranges. extensionis apparentin areasnot affectedby middleTertiary Fanglomerate wedges and interformational unconformities magmatismor low-anglefaulting. Thus 30% extensionmay be documentprogressive deformation along basin margins during a reasonableestimate for total earlyphase extension across the the Miocene. Upper parts of basinfill, however,lack evidence southernrift, and locallyextension may haveapproached 100%. of associatedvolcanism and, locally at least, representbroad, Orientation of early phase extension. We have used the stable basin floors contemporaneouswith the middle to late trendsof late Oligoceneto middle Miocenefaults, basins,and Miocene magmaticlull in southernNew Mexico [Chapin and dikesto judge the orientationof the regionalextensional stress Seaget, 1975; Seagetet al., 1984]. This period of possiblyless field during the early phase of rifting. Although there are vigoroustectonic activity and clear absenceof volcanismwas confusinganomalous trends, there also seemsto be a preferred followed in latest Miocene time by renewed faulting that orientation of structuresbetween N-S and N60øW, averaging modified,or in placescompletely disrupted, "early rift" basins. N30ø-40 ø W. This indicatesNE-SW extensionduring the early Low-angle normal faults. Whereas broad basins seem to phaseof rifting [Seager,1981; Newcomer et al., 1983;Seager havebeen a widespreadproduct of the early phaseof extension, and Mack, 1986b]. a more local but neverthelesssignificant additional feature is North-northeast directed extension in southern New Mexico low-anglefaulting. Faults dipping40 ø to near horizontalare duringlate Oligoceneto middleMiocene time is consistentwith exposedin severalranges of southernNew Mexico (Figure 3). the direction of extensiondetermined in the northern rift by The faultsare generallyclosely spaced in contrastto the widely Lipman [1983] and Golombeket al. [1983], in westTexas by spaced,high-angle faults which cut them and border modern Price and Henry [1984] and Henry and Price [this issue],and mountains.It is seldomclear whetherthe low dips in Arizonaby Rehrigand Heidrick[1976]. It is alsosupportive representfiat partsof listricfaults or domino-stylerotation of of Miocene stressfields determinedby Zoback et al. [1981] originallyhigh-angle faults. In general,we believethat the faults for the Basinand Rangeprovince in general. have beensubstantially rotated during the courseof early and/ or latephase extension, even if theywere originally listfie [Seager, Late Phase Extension 1981;Seager and Mack, 1986b]. Unfortunately,the low-angle faulting cannot be closely dated. The waning of early phaseextension and onsetof late phase About all that can be saidis that the faultsare latestOligocene extensionwas transitional across the spanof the middleMiocene or youngerand that they are olderthan the modernhigh-angle gap. Evidencefor the late phaseor episodeof rifting rangeboundary faults. They thus fall into the generalage of include:new, widespread,coarse-grained fanglomerate wedges betterdated (31-10 Ma) low-anglefaults reported by Chamber!in at the top of early rift basin fills; the appearanceof alkali- [1983]and Chamberlinand Osburn[1984] from the Rio Grande olivine flows (Selden Basalt, 9.6 Ma); major offset of rift near Socorro. upper Miocene rocks; and striking unconformitiesbetween 6266 MORGANETAL.: CENOZOIC EVOLUTION OFTHE Rio GRANDE RIFF

Son Diego Mr. 5000] RBF' T•• 4000 oJ

Southern Son Andres Mrs. 70OO

5000

3000 Organ Mts.

Bishop Cap hills p RBF , 5000 ..... " 4000 3000

NorthernFranklin Mts. r6ooo

•, ¾, O •4ooo ,\• \• 130oo

East Potrillo Mts. K RBF15000 , Q Q

'.

Easl Parrilia Mrs Q •,•'•,K• KK•.• QRBF J-5000 4000 Fig.3. Low-anglefaultsin south central New Mexico. Novertical exaggeration; elevationsin feet. Note different scales. SanDiego [after Seaget etal., 1971]; southern SanAndres Mountains, , andBishop Cap hills[after Seaget, 1981]; Franklin Mountains [afterHarbour 1972]; East Potrillo Mountains [afterSeaget andMack, 1985a].RBF, high-angle rangeboundary fault; PE•, rocks;Pz, rocksundifferentiated; PI,Lower Paleozoicrocks; Pu, Upper Paleozoic rocks; K, rocks;Tv, Oligocene volcanic rocks of Organ cauldron; Te, "earlyrift" basin sedimentary rocks; Q, Quaternary rocks. MORGAN ET AL.: CENOZOIC EVOLUTIONOF THE RIO GRANDE RIFT 6267 deformedupper Miocene rocks and undeformed upper and Bailey, 1976]. Subalpine floras of latest Oligocene-early strata or lavas. Miocene age from the (Hillsboro fauna [Meyer, 7tmingof late phase. Severalworkers along the length of 1983]) may be associatedwith topographicallyhigh volcanic the rift have bracketedthe latest phase of rifting, the major structures,probably relatedin part to the Emory cauldron,and episodeof faulting that outlinedthe modern basins,between thereforenot indicative of latest uplift. Basedon the middle 10 and 3 Ma [e.g., Chapin and Seager, 1975; Chapin, 1979; Miocene fossilpalm, ,4xelrod and Bailey [1976] estimate1100 Baltz, 1978; Manley, 1979; Manley and Mehnert, 1981; m of epeirogenicuplift of the northernRio Grande rift in the Golombek et al., 1983; Seageret al., 1984]. In southernNew last 13 m.y. or so. In the Socorro area, Meyer [1983] indicates Mexico, high-angleblock faulting was vigorously active between about 700 q- 300 m of uplift sincemiddle Miocene, basedon 9.6 and 4.5 Ma, continuingto the presentbut probablywith his study of fossil Junipers. Much of this regional uplift lessintensity [Chapin and Seager,1975]. Locally, Seager et al. documentedby ,4xelrod and Bailey [1976] and Meyer [1984] [1984] bracketthe main pulseof block faulting between9.6 probably accompaniedmovements on faults during the late and 7.1 Ma in the Las Cruces area. phaseof Rio Grande rift extension,although this has not yet Structuralstyle, amount and orientationof extension. The been demonstrated conclusively[Chapin and $eager, 1975; late phaseof blockfaulting produced relatively widely spaced, Chapin, 1979]. high-anglenormal faults that borderthe modernfault block mountainsand valleys.The upliftedblocks are simplehorsts CENOZOIC GEOLOGIC HISTORY OF CENTRAL RIO GRANDE or tiltedblocks, and the downdropped segments are either simple RIFT or compositegrabens or half-,generally narrow (5-20 Laramidecompressional deformation in mostof north central km wide) and 1 km or more deep. Although range boundary New Mexico and produceda provinceof foreland faultsare steep(650-75 ø ) and seeminglyplanar at presentlevels basementuplifts and basins known as the southern Rocky of exposure,some may flatten downward into a midcrustalzone Mountains [Baltz, 1978; Chapin and Cather, 1981]. These of extension.This is suggestedby broadlycurved fault traces; compressionaland strike-sliprelated basins are not coincident by moderate(20 ø ) rotationof fault blocks,by the development with later extensionalbasins [Stearns, 1953]. of zones of reversedrag and/or intenseantithetic faulting in basin fill adjacent to some boundary faults [Hamblin, 1965; Early PhaseExtension Seager, 1980, 1981], and by interpretationof seismicprofiles [Cape et al., 1983]. Five to twenty percentextension appears The early phase of extensionin the central Rio Grande rift to be characteristic of late phase faulting based upon occurredduring the latter part of a major phase of mostly reconstructionsof the preextensiongeometry of mappedfault Oligocenevolcanism. Evidence for this volcanicevent can be blocks and,.considering the small amount of observedtilt of found in the San Luis basin (rhyolitic through andesiticrocks) fault blocks,is consistentwith eitherplanar or listricfault models beneaththe Taos Plateau volcanicfield [Lipman and Mehnert, [ Wernickeand Burchfiel,1982]. 1979] and into Colorado [Tweto, 1979]; in the Latir volcanic Fault blocks and grabensof the late phasetrend northerly, field, adjacentto the San Luis basin(andesite through rhyolite) althoughmany fault segmentstrend northwest or eveneasterly. [Lipman, 1981]; in and around the Espafiolabasin (andesite, Overall, the structuralgrain is northerly, however, resultingin latite, limburgite, and basalt of the , Cerrillos structural truncation of older tectonic trends. It seems clear that Hills and other localitiesin the southeasternpart of the basin) the grabens,horsts, and tilted fault blocks are a product of [Stearns, 1953; Bachman and Mehnert, 1978; Baldridge et al., E-W extension in latest Miocene and Pliocene time, in contrast 1980; Kautz et al., 1981], and in and adjacentto the southern to the NE-SW extension that characterize early phase Albuquerque-Belenbasin [Osburn and Chapin, 1983]. deformation. Faulting in local areasduring the early phase of extension Late phase magmatism. Besides differences in stress waslow angleand eitherplanar (domino style) or curved(listric). orientationand structuralstyles between late and early phase Where this phase is well documented,in the Questa deformation,associated igneous activity is also very different. in the Latir [Lipman, 1981] and in the Lemitar The onsetof late phaseextension is marked by the appearance Mountains [Chamberlin, 1983], faulting was both locally of alkali-olivineto rarer tholeiitic"true" ,chemically and pervasiveand involvedvery large strains(greater than 100%). isotopically unlike the "basaltic andesites" of the early This combination of large strains on closely spaced faults extensionalphase [Chapin and $eager, 1975; $tinnet and producedhighly rotated strata. Constraintson the timing of $teuber,1976]. Although seemingly a productof the late phase thesefaulting events,Questa 23 Ma, and Lemitar 31-28 Ma, of extension,the basaltscuriously are more abundantafter 5.0 impliesthat they are not synchronousbut are temporally and Ma, after the intensityof block faulting waned. Apparently, spatiallyassociated with major magmaticevents. Other examples volcanismwas somehowdelayed until, or acceleratedafter, 5.0 attributableto thisphase of activityhave been suggested flanking Ma. At any rate, basalticvolcanism has continueduntil as the Albuquerquebasin [Baldridgeet al., 1984]. Betweenthese recentlyas 0.18 Ma in the southernrift. As Baldridgeet al. regionsthere is little evidenceof very large strains,and early [1984]point out, however,the volumeof volcanismgenerated phaseextension probably involved broad basin development in the latephase of riftingwas small in the southernrift. accompaniedby small to moderate(- 30%?)extension. Regionaluplift. Evaluationof regionaluplift associatedwith Broad shallow basinsformed along the trend of the central riftingis basedlargely on a few scatteredpaleobotanical studies. rift by late Oligocene time [e.g., Chapin, 1979]. The best Palms and other tropical plants,suggestive of near sea level documentedof theseis the Espafiolabasin, which began forming conditions,are present in Eoceneand middle Oligocene volcanic about 26 Ma [e.g., Baltz, 1978; Baldridgeet al., 1980; Manley rocks in southernNew Mexico and have also been reported and Mehnert, 1981]. The early Espafiola basin was a broad, from middle Miocene strata in northernNew Mexico [,4xelrod shallow,north trendingdownwarp that was roughlycoincident 6268 MORGANET AL.: CENOZOIC EVOLUTION OF THE RIO GRANDERIFT with the presentbasin (but wider).Evidence for thisgeometry interpretedfrom rocksand structuresformed within the last comesfrom early rift sedimentsextending beyond the present 30 q- 2 m.y.. The olderregime is characterizedby emplacement basinmargins and thin deposits of rift sedimentsalong the basin of basaltic andesitc and related flows with relatively high edges[e.g., Manley, 1979; Kelley, 1978]. strontiumisotope ratios, and lesser,more siliceousrocks; by The leastprincipal stress direction during the early phase of formationof broad, shallownorthwest trending basins; and by extensionis poorlydocumented. Lipman [1981] has shown that the local developmentof closelyspaced, low-angle faults, priorto 5-10Ma, NW trendingnormal faults were predominant generally in theareas of volcano-plutonic complexes. This regime in and adjacentto the San Luisbasin and thus imply a NE- evolved under an extensional stress field oriented NE-SW and SW extension direction. extensionof the order of 30% was characteristicand may locally haveapproached 100% or morein regionsof highheat flow Late Phase Extension and closelyspaced faulting. This earlyextension phase closely matchesin bothstyle, timing, and associated volcanism a similar The late phaseof extensionin the centralRio Granderift phaseof extensionin theBasin and Range (e.g., pre-Basin and followeda mid-Miocenelull (or at leasta significantreduction) Rangeevent of Zobacket al. [1981]that lastedfrom 30 to in magmatismthat lastedfrom 20 to 13 Ma [e.g.,Chapin, 1979; 13 Ma). Althoughit cannotbe demonstratedthat low-angle Baldridgeet al., 1980].In the centralRio Granderift thisphase faults are related to the formation of the early broad basins, of extensionproduced narrow elongatefault boundedbasins the similarityin timingbetween early basin formation and low- alongthe locusof the earliershallow broad basins. The present- anglefaulting, coupled with the sameregional association in daybasins in thenorthern rift arenorth trending basins arranged partsof the Basinand Range [e.g., Zoback et al, 1981;Eaton, en echelonto the right resultingin an overallNNE strike to 1982]implies a geneticrelationship. the rift [Kelley, 1979, 1982]. In the Espafiolabasin, faulting The youngerepisode or phase of extensionseemingly producedthe present narrow basin about 10 Ma [Manley,1979], representsa renewal or accelerationof blockfaulting and later as indicatedby a 9.8 Ma [Bachmanand Mehnert, 1978] volcanismbeginning about 9 to 10 Ma, aftera longtransition intrudedalong a NE trendingwestern border fault. Local and periodduring the middleMiocene when volcanism was absent regionalapproaches indicate that this phaseof extensionwas (or minor)and tectonism perhaps less vigorous. This latest phase quiteaccelerated at about 10 Ma [e.g., Golornbeket al., 1983; resultedin segmentationof earlierrift basinsinto narrower horsts Golornbek,1984; Dethier and Martin, 1984;Gardner and Goff andgrabens, formation of northerlytrending modern rift uplifts 1984].This period of acceleratedactivity coincides with a regional and basinsby movementon widelyspaced, high-angle faults, changein the leastprincipal stress direction from WSW-ENE and the later renewal of volcanism, this time dominated by to WNW-ESE [e.g., Zoback et al., 1981; Lipman, 1981; relativelyprimative alkali-olivine basalt. Late phase deformation Golornbeket al., 1983]. followeda changeto E-W extensionand wasaccompanied by In contrastto the earlierphase of extension,faulting during epeirogenicuplift of 700-1100m. Also at about10 Ma, this the late phaseresulted in much lesstilted and rotated strata changein stressdirection resulted in openingof the northern (implyinghigh anglefaults). Althoughthere is somedebate Basinand Rangeprovince [Zoback et al., 1981]that can be concerningthe exactform of the faults at depth(i.e., listric relatedto a changein the predominantlytransform motion or planar[e.g., Woodward,1977; Brown et al., 1980;Brocher, between the Pacific and North American plates. Reduced 1981; Capeet al., 1983;De Voogdet al., this issue],there is geothermalgradients and thicker, brittle upper crust are little questionthat verticalmovements were more important indicatedby the surficialstyle of faultingand by lesservolcanic than block rotation during the late phase.The narrow basins activitycompared to the earlyphase of deformation.Only 5- producedare typicallyasymmetric with majorborder faults on 20% extensionresulted from late phaseextension, but vertical oneside only (half- )[e.g., Kelley, 1979,1982]. Attempts displacementswere in generalat leastas greator greaterthan to estimatethe extensionacross the central rift suggestroughly in early phaseextension. 10% of extension[Woodward, 1977; Brocher,1981; Cordell, 1982; Golornbek, 1981; Golornbek et al., 1983]. Given the GEOPHYSICAL AND OTHER EVIDENCE FOR MODERN structural relief acrossthe modern basins(several to 10 km) LITHOSPHERIC STRUCTURE [e.g.,Birch, 1982; Cordell, 1979; Kelley, 1977; Woodward, 1977], the late phaseof extensioncan be characterizedas having small Geophysicaldata relatingto crustalstructure in the Rio amountsof extensionand large verticaldisplacements. Unlike Grande rift were summarizedby Cordell [1978] and in more the early extensionevent, volcanismis, in general, poorly detail for the southernRio Grande rift by Seagerand Morgan associatedwith the late phase.If anything,most volcanicsin [1979].These data showanomalous crustal and uppermantle structure associated with the southern Rio Grande rift. The the centralrift are low-lyingbasalt fields that formedafter the seismic data indicate that the crust of the southern Basin and acceleratedphase of activityended (in the last 5 m.y.) [e.g., Rangeand southernRio Granderift (23-28 km) is significantly Kudo, 1982;Lipman and Mehnert,1979; Aubele, 1979]. Regionaluplift hasaccompanied the latephase of extension thinnerthan the adjacentColorado Plateau (40 km) and Great in the centralRio Granderift. Fossilflora [Axelrodand Bailey, Plainsprovinces (50 km) [Toppozadaand Sanford, 1976].New seismic refraction data indicate that the crust thickens from 1976],uplifted erosion surfaces [Scott, 1975; Taylor, 1975], and fissiontrack dates[Kelley and Duncan, 1984;this issue]all approximately23 km in the Basinand Range of southwestern indicaterelatively rapid uplift of more than 1 km in the past [Sinno et al., 1981] to approximately31 km at the Arizona-New Mexico border[Gish et al., 1981]but thinsagain 7-10 m.y. to between 27 and 28 km beneath the southern Rio Grande

SUMMARY OF GEOLOGIC HISTORIES rift [Cook et al., 1979;Daggett, 1982; Sinno et al., this issue]. Long-wavelengthBouguer gravity data are alsoconsistent with In summary,two extensionalregimes of differentorigin (but crustalthinning beneath the southernRio Granderift [Ramberg transitional with each other through the Miocene) can be et al., 1978;Daggett, 1982; Daggett et al., thisissue]. Thus there MORGAN ET AL.: CENOZOIC EVOLUTION OF THE RIO GRANDE RIFT 6269

WEST EAST

150

Surface Heat Flow Southern New Mexico

• 100 Western NM mean 90 + 19mWm -2

< 50 --O Eastern NM meanßß ß ß ß ß 45+ 5mWm-: ß

B&R• RGR • GP

0 150 300 450 600 DISTANCE, km Fig. 4. Surfaceheat flow data from southernNew Mexicoprojected onto a westto eastprof'fie showing little apparent differencebetween the southernRio Granderift andadjacent Basin and Range(redrawn from Daggett,[1982]).

is good evidencefor anomalouscrustal and rift andadjacent Basin and Rangeprovince are thermally distinct. structureassociated with the late phaseextension event which As shown in Figure 5, there is no systematiccorrelation for formed the modem southern Rio Grande rift, and similar, but heat flow-heat generationdata from the southernrift, but data less definitive data suggestsimilar anomalous lithospheric from the adjacentBasin and Range show the samecorrelation structureto the north [e.g., Baldridgeet al., 1984]. as data from other areasof the Basinand Rangeprovince where Heat flow data and deep electricalsounding data indicate young volcanism has not occurred [e.g., Roy et al., 1972; high lower crustal and upper mantle temperaturesassociated Blackwell,1978]. These data indicategenerally high, but laterally with the Rio Granderift and adjacentBasin and Rangeprovince. variable, reduced heat flow in the southern Rio Grande rift, In the south, deep electricalsounding data indicate that the with elevated,but fairly uniform,heat flow in the adjacentBasin thermal anomaly is more intense beneath the rift than the and Range province. adjacentBasin and Range [Seagerand Morgan, 1979], but the surface heat flow data do not make a clear distinction between the Basin and Range and Rio Grande rift provinces:contour 160 maps of thesedata are somewhatsubjective (compare Reiter CPO et al. [1975], Seagerand Morgan [1979], Swanberg[1979], and Sasset al. [1981]). A large scatterin the surfaceheat flow data ORO 1 from southwesternNew Mexico makes an objectiveanalysis of thesedata very difficult, as shownin Figure 4. The Great 120 L Plains and southern Rio Grande rift provinces are clearly thermally distinct, but there are no major thermal boundary 0 betweenthe southernRio Grande rift and adjacentBasin and HCH TY Range province.Decker and Smithson[1975] suggestedthat the southernRio Grande rift is characterizedby a heat flow 80 OR02 of approximately100 mW m-2, and a crustalgeotherm derived LOR SR 0 from thisvalue is consistentwith lowercrustal pressure o AP and temperaturedata [Padovaniand Carter, 1977;Seager and SH Morgan, 1979].This geothermexceeds temperatures of 1000øC just belowthe Moho, however,which together with uppermantle 40 xenolith pressureand temperaturedata indicatesa convective 0 I 2 3 heat transferregime in the upper mantle beneaththe southern Rio Granderift [Seagerand Morgan, 1979].Scatter in the heat HEAT GENERATION, uW m flow in the southern Rio Grande rift around the mean value Fig. 5. Compilationof heat flow-heat productiondata for southern New Mexico and West Texas. Points from sites outside areas of Rio possiblyresults in part from magmaticactivity associated with Grande rift volcanism(less than 15 Ma; solid circles),i.e., LOR, SR this convectiveregime. To the north a clearlyidentified surface and TY, fall closeto the "Basinand Range"heat flow-heat production heatflow high is associatedwith the rift, and calculationsbased line, indicatinga predictablelithospheric thermal structure at thesesites. uponthese data suggestgeotherms that exceedthe crustalsolidus Points from Rio Granderift sites(open circlesand bars) showlarge scatterindicating disturbed geotherms. Key to data: CP, Cooks Peak; above the Moho [e.g., Clarkson and Reiter, 1984; Decker et LOR, Lordsburg;OR, OrganMountains; ORO 1 andORO2, Orogrande; al., 1984;Morgan and Golombek,1984]. SH, Shaefer,Texas; SR, Santa Rita; HCH, Hachita;LV, ; Reducedheat flow data suggestthat the southernRio Grande TY, Tyrone;AP, AnimasPeak (data from Cook et al. [1978]). 6270 MORGAN ET AL.: CENOZOIC EVOLUTION OF THE RIO GRANDE RIFT

locallyin isostaticequilibrium. Downwarp is causedby thinning Yield Strength MPa of thebuoyant crust during extension[McKenzie, 1978]. Narrow, deep basinscan most easily be explained by fault-controlled 00 400 800 1200 subsidencein local isostaticdisequilibrium. By this mechanism, br local downwarp is purely a function of fault geometry,with high-anglenormal faults resulting in largevertical displacements for relativelysmall lateral movements[Vening Meinesz, 1950;

O N -•-.•-•• _ $111ClC 1 Bott and Mithen, 1983]. This mechanismmay be expectedto prevail in stronger,thicker lithosphereas flexural strengthin the lithospheresupports local isostaticdisequilibrium, and isostaticequilibrium will only be obtainedon a regionalscale, du with someof the basin downwarpcompensated by upwarp of the flanks of the basin. On a regionalscale the net for thefault downwarpmodel will bethe sameas for thestretched 40 crust model, but the fault subsidencemodel producesgreater local subsidencethan the stretching model for the same extensional strain [Morgan and Golombek, 1984]. More complete analysesof the stretching-subsidencemechanism is 60 - givenby McKenzie[ 1978]and of thefault subsidencemechanism by Bott [1976] and Bott and Mithen [1983]. Quantitativemodeling of subsidenceassociated with extension is limited by a number of unknown or poorly constrained parameters.For example,uncertainties in fault geometriesand 80 du - displacementsmay make an accurateestimate of the amount of extensiondifficult. Initial model parametersmay also be difficult to define, as rifting perhaps rarely takes place in "normal" lithosphere,as we emphasizein our examplesfrom

i i I I TEMPERATURE, øC Fig. 6. Calculatedlithospheric strength curves for the 0 400 800 1200 (solid curve) and early Tertiary (dashed curve) assuminga shieldgeotherm [from Lachenbruchand Sass,1978]. Heavy curvescalculated for strainrate of 10-]5 s-]. Lightcurves above and belowheavy curve calculated for strainrates of 10-•s and 10-]4 S-], respectively.Depths to Great Plains and ColoradoPlateau Mohos are indicatedby MGP and MCP, respectively(depths from Cordell[ 1978]).

In summary,geophysical and other subsurfacedata indicate thatthe present crustal and .lithospheric structure is most strongly controlledby the late phase,Rio Granderift extensionevent. ß . Perhapsrather surprisingly,the much larger magnitudeearly phaseextension event has left little permanent imprint on gross m • at10 km \ \ crustalstructure. We nowdiscuss the mechanical implications e•

of the two extensionevents and use the geologichistory of 4O • 30km crust \ \\ the regionand constraintsfrom modemcrustal and lithospheric x 45km crust structureto attemptto reconstructthe thermaland mechanical ? historyof the lithospherein this region.

BASIN GEOMETRY AND EXTENSIONAL STYLE Fig. 7. Simplifiedestimated geotherms for southwesternNew Mexico at 30-25 Ma for different crustal thicknesses and with a batholith in Two distinctphases and stylesof Cenozoicextension have the crust at 10 km. The 30-km crust representsthe crust at the time of earlyphase extension, assuming no crustalthinning during extension been identified in the Rio Grande rift. During the first phase (thicknessmaintained by addition of ).The 45-km crust of extension, broad relatively shallow basins were formed representsprobable maximum crustalthickness assuming maximum associatedwith local high extensional strains. In contrast, crustalthinning during extension for 50%extension. Fine line on right relativelysmall extensional strains formed deep narrow basins representsthe basaltdry solidus[from Lachenbruchand Sass,1978], and geothermsare constrainedto intersectthis solidusat the Moho, during late phase extension. As discussedby Morgan and basedon the geochemicalevidence for crustalmelting and magma Golornbek[1984], thesecontrasting basin styles,both formed contamination. The batholith was assumed to be intruded at the solidus by extension, can be described in terms of end-member temperaturefor tonaliteof 760øCat 10 km [from Wyllie,1977]. All mechanismsfor extensionalbasin formation. Broad, relatively curvesare schematic,especially at highertemperatures, and do not shallow basinscan most easily be explainedby a mechanism accuratelyrepresent the transient effectsof intrusions.However, the strengthcurves generated from thesegeotherms are not sensitiveto in which extensional strain is distributed over a wide area and the geothermat high temperaturesand are consideredto be valid for in which the lithosphereis thin and weak, so that it is always purposesof qualitativecomparisons. MORGAN ET AL.: CENOZOIC EVOLUTION OF THE RIO GRANDE RIFT 6271

New Mexico(compare McKenzie [1978]). In addition,the TEMPERATURE, øC lithospheremay not be a closedsystem during extension, and 0 400 800 1200 material may be added by magmatism prior, during or postextension[e.g., Lachenbruchand Sass, 1978; Royden and Keen,1980]. Thus, at thispoint we useonly the observation that two distinct extensional styles are overprinted in the southern Rio Grande rift, an early ductile extension event indicatinga thin weak lithosphereand a later high-anglefault- controlledevent indicatinga strongerthicker lithosphere.We attemptto reconcilethe geologichistory of the regionwith this observation.

GEOTHERM EVOLUTION AND IMPLICATIONS

The overprintingof the two extensionevents suggest that the controlon extensionalstyle is not fundamentalto the crust 40 but is a temporallyvarying parameter. The two main temporally variableparameters which control the mechanicalstrength of the lithosphere are strain rate and temperature. Strength increasesas strainrate increases,but decreasesas temperature increases(see the appendix).In New Mexico, the high strain Fig.9. Calculatedgeotherms for southwesternNew Mexico,assuming conductivecooling after earlyphase extension and the modernsouthern (early extension) event was characteristicof a thin weak Rio Granderift geotherm.Curve A, assumedgeotherm at 25 Ma (see lithosphere.The low strain (late phaseextension) event was Figure7); curveB, modernRio Granderift geotherm[from Seager characteristicof a thicker, strongerlithosphere. Thus, if total and Morgan, 1979];curve C, calculatedgeotherm for 10 Ma; curve strain can be taken as a measure of strain rate, the behavior D, calculatedgeotherm for 0 Ma. Curves C and D were calculated of the lithosphereduring these two extensionevents is opposite from curveA assumingconductive cooling of the lithospherestarting at 25 Ma, usingthe coolingsolution given by McKenzie[1978] and to that expectedfrom the strain. As a first approximation, assuminga crustal thermal conductivity of 2.5 W m-1 K -1, a diffusivity therefore,we ignorestrain rate in our analysisand concentrate of 25km 2 my-1, and an equilibrium lithospheric thickness of 125 km. on temperaturechanges, or the evolutionof the geotherm.The CurveD closelymatches the predictedBasin and Rangegeotherm for techniquesused in this study for calculationof lithospheric a reducedheat flow of 59 mW m-2 [Royet al., 1972].See note in captionto Figure7 regardingvalidity of geotherms. strengthcurves from lithosphericstructure and the geotherm are givenin the appendix. Our resultsare useful in a qualitativerather than a quantitative sense,and we usea referencelithosphere with whichto compare changinggeotherms and changinglithospheric strength curves. For thislithosphere, we usea shieldgeotherm appropriate to the GreatPlains province [Lachenbruch and Sass,1978], from whichthe lithosphericstrength profile shown in Figure6 was YIELD STRENGTH, MPa calculated.For this strengthcurve and all other curvesin our analysis,we have assumed a strain rate of 10-5 s -1 (approximately o 200 400 3%/m.y.).This strain rate is probablyreasonable for latephase o extension and is assumed constant to allow examination of the effectof the changinggeotherm. To determinethe geothermat the start of the early phase extension(approximately 30 Ma), we mustrely upon indirect evidencefor lithospherictemperatures. Approximately 15 m.y. of intensemagmatic activity preceded this extensionevent, and we interpretstrontium isotdpe ratios of the magmasjust prior to extensionto indicatethat crustalmelting was caused by this lO magmaticactivity just prior to extension.As a lower bound to temperature,therefore, we use the solidesttemperature at the M oho and assumethat thermalequilibrium was developed in thelithosphere during this long period of magmaticactivity. Severalmajor upper crustalbatholiths were implacedat this time, indicatinghigher temperatures, and a highergeotherm associatedwith this magmaticactivity is alsoconsidered. One final unknownat thistime is crustalthickness. If no magma 2o was addedto the crust during extension,from the regional extensionalstrain of approximately30-50% and the modern crustalthickness of 30 km (for the southernrift), we estimate Fig. 8. Calculatedlithospheric strength curves for southwesternNew a preextensioncrustal thickness of 40-45 km. This may be Mexico at 30-25 Ma basedupon the geothermsshown in Figure 7 15 1 andassuming an extensionalstrain rate of 10- s-. Solidcurve represents regardedas an upper bound to crustalthickness, and a lower 45-kmcrust; dashed curve represents 30-km crust; dotted curve represents boundis the moderncrustal thickness, or 30 km, assuming crust with batholith at 10 km for both crustal thicknesses. all extensionto be accommodatedby the additionof magma 6272 MORGAN ET AL.: CENOZOIC EVOLUTIONOF THE Rio GRANDE RIFT to the crust.Schematic geotherms based upon thesegeological lithosphereassociated with thesouthern Rio Granderift, possibly inferencesare shown in Figure 7, and strengthcurves based postrifting,related to recentmagrnatism. on thesegeotherms are shownin Figure8. Similargeotherms and strengthcurves with a thickercrust applicable to the rift CONTROLS ON STRUCTURAL STYLE OF EXTENSIONAL in northernNew Mexico weregiven by Morgan and Golombek [1984]. Thesestrength curves indicate that prior to early phase Early PhaseExtension extension,the lithospherein the area of the rift zone of New Mexico was very weak relativeto the Great Plains, with no The lithosphericstrength curves shown in Figure 8 for the significantmantle strength and a very shallowcrustal brittle- time of early phaseextension in New Mexico suggesta simple ductiletransition (10-12 km). This brittle-ductiletransition may mechanismfor the generationof broad relativelyshallow basins havebeen locally much shallower, associated with higherupper during this extensionevent. As illustratedin Figure 1l a, only crustaltemperatures around recently intruded batholiths. the uppermostcrust will be expectedto behavebrittlely with To calculatethe geotherm and strengthcurves at the start shallow detachment and ductile extension in the remainder of of the latephase extension, we haveassumed that thelithosphere thecrust and upper mantle. To accommodatethe transformation conductivelycooled following early phaseextension. Starting from brittle to ductile extension, a decollement would be with the 30-km-thick crust geothermfrom Figure 7, we have expectedto form at the brittle-ductiletransition, with normal used the coolingequations of McKenzie [1978] to calculate faults in the uppermostcrust becominglistric and flattening geothermat 10Ma andthe present, assuming that cooling started at this depth. Faultingin the uppermostcrust causes primarily at 25 Ma, theapproximate age of thestart of waningof magmatic block rotation with little net vertical relief. Subsidence is caused activity.These geotherms and the modernsouthern Rio Grande primarily by the isostaticeffect of crustal necking.The close rift geothermare shownin Figure 9, and the strengthcurves associationof high strain with intrusionsduring this phaseof calculatedfrom these geothermsare shownin Figure 10. A extensionsuggests that some subsidencemay even have been comparisonof the pre-earlyphase and pre-late phase extension reducedby the additionof magmato the crustat theselocations. strengthcurves in Figures8 and 10shows that the crustal brittle- ductile transitiondepth increasedfrom around 10-12 km to Late Phase Extension about 14 km between the extensionevents. Perhaps the most The tectonic style of crustal extension at the time of late importantdifference between the strengthcurves, however, is phaseextension is lesseasy to deducefrom the strengthcurves the appearanceof significantuppermost mantle strength in the predictedfor this period. The growth of lower crustal and pre-lateextension curves and a zone of lower cruststrength. significantuppermost mantle strengthduring cooling of the The coolingmodel predicts that the geothermshould continue lithospherefrom earlyto latephase extension and the contrasting to cool and that the lithosphereshould continue to strengthen stylesof the two extensionevents suggest that uppermostmantle to thepresent. The modernpredicted geotherm shown in Figure strengthplayed an important role in controllingtectonic style 9 is consistentwith the reducedheat flow data from regions duringlate phaseextension. We suggestthat the growthof this of the Basinand Rangeprovince of southwesternNew Mexico lower crustal and uppermostmantle strengthprevented crustal whereyoung (<12 Ma) volcanismhas not occurred.Petrologic decollement,as predictedfor the early phaseextension, and and surface heat flow data from the southern Rio Grande rift this in turn preventedflattening of the normal faults associated which has recentvolcanism indicate a highermodern geotherm with extension.As actual fault failure is expectedto be a very for the rift as shown. Similar geothermsand strengthcurves high strain rate event,brittle failure may have penetratedthe are applicableto Rio Granderift evolutionin northernNew crustinto the uppermantle (Figure 11b) [ VeningMeinesz, 1950]. Mexico, as presentedby Morgan and Golombek[1984]. Alternatively,high-angle normal faults in the upper crustmay However, thicker crust in the north resultsin lessstrength in be terminatedby ductile extensionand perhapsflattening in the uppermostmantle for all stagesof geothermevolution. As thelower crust (Figure 11c) [Bott and Mithen, 1983]as suggested discussedbelow, we considerthe late Cenozoicreheating of by minor rotation of fault blocks (see above). The exact thelithosphere in theRio Granderift to beprimarily a postrifting mechanismof crustal failure is unimportant for the present phenomenonbased upon recent volcanism following the middle arguments,as ductile relaxation of thelower crust and uppermost Miocene volcanic lull in this zone and the different reduced mantle will be expected to produce similar final crustal heat flow characteristicsof the Rio Grande rift and adjacent geometries from either throughgoing crustal faulting or Basinand Range(Figure 5). uppermost crustal faulting only. The important concept The mainresults of ourgeotherm and strength curve evolution suggestedby the predictedstrength curves and the observed deductionscan be summarizedas follows: (1) Prior to early structuralstyles is the control of normal fault geometryby the phaseextension, the geotherm was very high and the lithosphere growth of strengthin the lower crust and uppermostmantle extremelythin and weak, its weakestpoints associated with asthe lithospherecools. the intrusionof uppercrustal batholiths, (2) if the lithosphere is assumedto have conductivelycooled after early phase DISCUSSION extension,lithospheric strength is predictedto have increased prior to late phaseextension, with significantstrength in the From our analysisof extensionaltectonics in the framework uppermostmantle as well asthe uppercrust and somestrength of the geologicalevolution of New Mexico, we suggestthat in the lower crust,(3) the model of conductivecooling from extensionalstrain may be localizedby earliergeological events. theearly extension event to thepresent is consistent with reduced Extensionin the Basinand Rangeand Rio Granderift provinces heat flow data from the of was precededby major magmatic activity which raised the southwesternNew Mexico where there are no recent volcanics, geotherm and reduced lithosphericstrength [Morgan and and (4) there appearsto have been a reheatingevent of the Golombek,1984; Baldridge et al., 1984].This magmaticactivity MORGAN ET AL.: CENOZOIC EVOLUTION OF THE RIO GRANDE RIFT 6273

YIELD STRENGTH, MPa phaseis consistentwith the contemporaryregional stressfield [Zoback and Zoback, 1980]. Thus strain was localizedduring 400 800 early phaseextension by earlier and concurrentheating of the lithosphere.Some localization of strainin late phaseextension may have resultedfrom resistanceof the Colorado Plateau to east-west extension. The Rio Grande rift sharesits two-phaseextension history with much of the Basinand Rangeprovince [e.g., Eaton, 1982; Zoback et al., 1981;Golornbek et al., 1983].Possible association of a changinggeotherm with changingextensional style has also been suggestedfor the Basin and Range [e.g., Lucchitta 2O and $uneson,1982], and we suggestthat the conceptsdeveloped ! / for the Rio Grande rift in the presentstudy may be generally applicableto the Basinand Rangeprovince. The lithospheric geotherm may be a strong factor in controllingthe style of extensionaltectonics. Ductile extension producingbroad relativelyshallow basin is favored by a high geotherm, and fault block tectonicsproducing narrow deep 4O basinsis favoredby a coolergeotherm. This observation suggests that it may be difficult to predictthe initial thermal condition for ductileextension and basinformation, and it is interesting to notethat in manyapplications of McKenzie• [1978]stretching model, it has beennecessary to invoke mechanismsthat in effect Fig. 10. Calculated lithosphericstrength curves for geotherms introduce more heat into the system,such as dike intrusion calculatedfor 10 Ma (solid curve) and 0 Ma (dashedcurve) shown or two-layerstretching [e.g., Roydenand Keen, 1980;Hellinger in Figure9, assuminganextensional strain rate of 10-25 s-•. wasnot widespreadin the adjacentGreat Plainsprovince, and we suggestthat the Great Plainslithosphere was resistantto brittlefailure extensionalstresses by virtue of high strengthresulting from its cool geotherm.The closespatial association of high-strain ductile extension earlyphase extension and magmatic intrusions suggests a strong M ./ localizationof highstrain by elevatedupper crustal temperatures ductile extension during this event.The directionof extensionalstrain in this earlyphase event is consistentwith the regionalstress field at thattime [Zoback and Zoback, 1980], but rapidlateral variations in extensionalstrain suggest isolation of the areasof strainfrom the relativelyrigid blocksof the Great Plains and perhapsthe ColoradoPlateau during this event. brittlefailur•e• Wherevera boundarybetween a rigid and deformedblock quasi-brittle',failure has a componentof strikeparallel to the axis of extension(or be compression),a strike-slipdecoupling between the two blocks M must exist to accommodate the difference in strain across the quasi-brittlev,failure boundary.Thus, to accommodateearly phase extensionin ductile extension' southwesternNew Mexico, there must have been strike-slip decouplingbetween the ColoradoPlateau and the area to the southeast,in the region now coveredby the Datil-Mogollon volcanics,although strike-slip faulting of the sameage as early phase extensionhas not yet been recognizedin the Datil- brittlefailure'• Mogollonfield. ? During late phase extension,extensional strain was still C. ductile extension confinedto areasof early and mid-Cenozoicmagmatic activity, but extensionwas not stronglycontrolled locally by magmatic M ductileextension activity. We tentatively suggestthat extensionalstrain has continuedlater in the Rio Granderift (and the Basinand Range of the ) than in the adjacentsouthern Basin and Fig. 11. Relationshipsbetween strength curves (left) and stylesof Rangeduring late phaseextension due to the stabilizingeffect extensionaltectonics (right). (a) Early phaseextension. (b) and (c) Late of the Colorado Plateau: East-westextension requires strike- phaseextension. Not to scale.See text for explanation.Upwarp of slipdecoupling of the ColoradoPlateau from the southernBasin the Moho (M) is shownschematically in Figures1 l(a) and 11(c)Only the direct effectsof faulting on the Moho are shownin Figure 11(b) and Range(the regionsouth of the plateau),whereas areas east Solid strengthcurves represent southern New Mexico (Figures8 and and westof the plateauare not influencedby the plateauduring 10); dashedcurves represent northern New Mexico [from Morgan and east-westextension. The directionof extensionduring the late Golornbek,1984]. 6274 MORGAN ET AL.: CENOZOIC EVOLUTION OF THE RIO GRANDE RIFT and Sclater,1983]. Difficulties inherent in predictingthe beforefaulting, which allows the following strength relationships preextensiongeotherm may thuslimit the usefulnessof these to beused [from Lynch, 1983 and Lynch and Morgan, 1986]. extension-subsidence models. Brittlestrength under extension Sb is givenby Further problemswith simpleextensional-subsidence models areevident when the regionalelevation history of southernNew Sb = 16 Z MPa Mexico is examined.Up to 1.25 km of regionalsubsidence is predictedby 30%regional crustal extension during early phase wherez is depthin kilometers. extensionand more if greater extensionis assumed.No such Ductile strengthSo is givenby regionalsubsidence has yet beendocumented, suggesting that • •/3 Q*+293z crustalextension may have been accommodated by the addition So= (•) exp ( )MPa of newmaterial to thecrust. In fact,Elston [1984a] has suggested eo 3RT uplift of at least1-2 km by magmaticcrustal thickening during this early phase of extensionin southwesternNew Mexico. wherei is strainrate (s-X), R is thegas constant (J mol-x K-X), T is absolutetemperature (K), and to and Q* are constants Additionalsubsidence is predictedduring late phaseextension, dependenton rock typegiven by: and the 2-3 km of crustalthinning indicatedby seismicand grayity data associatedwith the southernRio Grande rift are

RockType to,MPa -3 s -• Q*, J mol, -• consistentwith approximately10% extension during this event. Silicic 2.5 X 10-8 1.4 X 105 However, the elevationchanges predicted by the late phase Mafic 3.2 X 10-3 2.5 X 105 extensionevent cannot explain the approximately 1 km of uplift Ultramafic 1.0 X 103 5.5 X 105 detei'minedfor thelast 10-13 m.y. in theRio Granderift. Post- middle Miocene uplift may be associatedwith postrifting, Lithosphericyield strengthS is then givenby: magmaticthickening of the crust, and the modernhigh heat flow and anomalousuppermost mantle beneath the southern S = Sb Sb > Sd Rio Granderift may be associatedwith this magmatic activity S = Sd Sb < Sd rather than directlywith late phaseextension (P. Morgan et al.•manuscript in preparation,1986) [see also Cook et al., 1978; Strength curvesused in this paper have been based upon Morgan and Golornbek, 1984; Morgan and Swanberg, 1985]. the arbitraryassumption that the upper half of thecrust is silicic The recent rift uplift is perhapsnot restrictedto the rift zone in compositionand the lower half of the crust is mafic in but includesthe ColoradoPlateau [e.g., Morgan and Swanberg, composition.Different (reasonable)assumptions about crustal 1985],the SouthernRocky Mountains,and muchof the western compositionchange the detailsof the curvesbut do not change : If the uplift in all this region is related to the qualitativeresults that we extractfrom the curves.The effect magrnatism,the youngmagmatism in the rift may not be directly of allowingstress relaxation in the lithosphereis to make the related to rifting. The complex interrelationshipbetween the brittle-ductiletransition(s) more shallowand changethe shape regional stress field and preextension and postextension of the sectionof the strengthcurve relating to ductilestrength magmatic eventssuggests that the Rio Grande rift can only [e.g., Lynch, 1983; Kusnir and Park, 1984], but again, the be explained in terms of the interrelationshipbetween local qualitativeconclusions that we derivefrom the strengthcurves -lithosphereconditions and the regional stress are not changedby thesemodifications to the models. field. Acknowledgements.Many of the conceptsdeveloped in this study CONCLUDING REMARKS havebeen suggested by previousstudents of Basinand Range and Rio Granderift geology,and in particular,we have benefited from discussions Detailed geologicalinformation and reasonablegeophysical with Ivo Lueehitta.Our analysesof lithosphericstrength have relied control of lithosphericstructure have allowed us to examine heavilyupon studiesof this subjectby H. D. Lynch. We are grateful the sequenceand styleof extensionaltectonics in New Mexico to E Cook and an anonymousreviewer for their careful reviewsof thismanuscript. P.M. startedthis study while at theLunar and Planetary duringthe Cenozoic.We are ableto producea consistentmodel Institute, which is operated by the Universities Space Research of geothermevolution and tectonic style, but thismodel suggests Association under contract NASW-3389 from the National Aeronautics a complexinterrelationship between regional and local geologic and SpaceAdministration. Work by M.P.G. on this studywas earfled events.Some of the paradoxesapparent from analyses of outat the Jet Propulsion Laboratory, Institute ofTechnology, under contractfrom the National Aeronauticsand SpaceAdministra- different may be resolvedby these models,but in turn tion. This is LPI contribution 582. they suggestthat modelsof extensionaltectonics may only be applicableon a local rather than a global basis.Rifting events REFERENCES cannot be analyzedin the isolation of the geologicevents that precedeand follow crustalextension. Angelier,J., and B. Coileta, Tensionfractures and extensionaltectonics, Nature, 301, 49-51, 1983. Aubele,J.D., The Cerrosdel Rio volcanicfield, FieM Conf Guideb. N.M. Geol. Soc., 30, 243-252, 1979. APPENDIX: LITHOSPHERIC STRENGTH CURVES Axelrod, D.I., and H.P. Bailey,Tertiary vegetation, climate and altitude of the Rio Grandedepression, New Mexico-Colorado,Palcobiology, Lithosphericyield strength curves used in thisstudy are based 2, 235-254, 1976. upon a compilationof laboratory and field experimentaldata Bachman,G.P., and H.H. Mehnert, New K-Ar datesand the late Pliocene by Lynch[ 1983]and Lynch and Morgan [1986]for brittlefailure to Holocenegeomorphic history of the centralRio Granderegion and ductile creep.For essentiallyqualitative comparison uses New Mexico, Geol.Soc. Am. Bull., 89, 283-292, 1978. Baldridge,W.S., P.E. Damon, M. Shafiqullahand R.J. Bridwell, of lithosphericstrength curves, we have assumeda constant Evolution of the central Rio Grande rift, New Mexico: New strainrate and that no stressrelaxation occurs in the lithosphere potassium-argonages, Earth Planet.Sci. Lett., 51, 309-321,1980. MORGAN ET AL.: CENOZOICEVOLUTION OF THE RIO GRANDE RIFT 6275

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