JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B8, PAGES 13,201-13,224, JULY 30, 1991

EvolvingGeographic Patterns of CenozoicMagmatism in the North American Cordillera: The Temporaland Spatial Association of Magmatism and MetamorphicCore Complexes

RICHARD LEE ARMSTRONG

Departmentof GeologicalSciences, University of British Columbia,Vancouver, Canada

U.S. Geological Survey,Menlo Park, California

Four maps are presentedhere that show the location and extent of magmaticfields between eastern Alaska and northernMexico during the successivetime intervalsof 55-40, 40-25, 25-10, and 10-0 Ma, and four others show the distributionof metamorphiccore complexesduring the same Cenozoic time intervals. The maps are based on U.S. Geological Survey and Canadian Cordilleran data bases contining about 6000 isotopic dates and extensive literature review. For nearly 60 Ma the developmentof metamorphiccore complexeshas coincided with the locus of areally extensive and voluminousintermediate-composition magmatic fields. The associationis suggestiveof a close link betweenmagmatism and core complexformation, namely that magma directly and indirectly lowers the strengthof the crust. Magmatism thus controlsthe location and timing of core complex formation. The stressesresponsible may be inherited from Mesozoic crustal thickening,locally createdby uplift and magmatic thickeningof the crust, and imposedby the global pattern of plate motions and driving forces. Since the Miocene, rates of magmatism,extension, and core complex formation have declined. The modern Basin and Range province is not a suitable model for the situation that existed during major magmatic culminations. The singular event of early Miocene time, the merging of two large magmatic fields, extinguishing the Laramide magmatic gap, explains several disconnected observations:the hyperextensionepisode of the ColoradoRiver corridor, rapid reorientationof stress patternsacross much of westernNorth America, and subsequentrapid tectonicmovements in California. Magma-triggeredbreakup of westernNo•h America lithospherecoincided with developmentof the San Andreas transform system. Thermal destructionof the Laramide magmatic gap created a California "microplate"about 22 Ma ago that moved rapidly away from North America. Thus two plate tectonic processes,thermal destructionof the lithosphere"bridge" and northwardgrowth of a transformsystem, interactedto produceMiocene and later tectonicpatterns and events.

INTRODU•ON the Cordilleran region and discussedthe significance of those evolving magmatic patterns for the Western Interior There have been many reviews of the Cenozoic magmatic Basin. The mandate of the first paper did not include the evolution of the western United States [Armstrong et al., Cenozoic, but the abundantdata on age and distribution of 1969; McKee, 1971; Lipman et al., 1971; Christiansen and magmatismdid not stop in the early Cenozoic. In fact, the Lipman, 1972;Lipman eta/., 1972;Armstrong and Higgins, time and spacedistribution of Cenozoicmagmatism is richly 1973; Snyder et al., 1976; Cross and Pilger, 1978; and accuratelydocumented. It was almost no extra effort to Armstrong, 1978; Coney and Reynolds,1977; Stewart and continuethe compilation,done up to 55 Ma ago for the Carlson, 1978; Christiansen and McKee, 1978; Luedke and Western Interior Basin paper, to the present. Moreover, Smith [1979a, 1979b, 1981, 1982, 1983, 1984, 1986], there were other interestingstories to tell. The maps that Luedke et al. [1983]; Lipman, 1980; Ewing, 1980; Smith are centralto both paperswere createdtogether from a large and Luedke, 1984; Mutschler et al., 1988], southeastern body of recently assembledinformation. Togetherthey Alaska [Brew, 1988] and some broad overviews of the provide a cinematicimage of evolvingmagmatic patterns, Cenozoic magmatic history of western Canada [Souther, presentedin uniform graphic style, for Late Triassic to 1967, 1970, 1977; Noel, 1979]. Nevertheless, there has presenttime. In discussingmagmatism we include data from never been a single presentation of the distribution of both intrusive and extrusive igneous rocks. For the magmatic activity over the entire Cordilleran region Cenozoic the data base is weighted toward volcanic rocks, extendingfrom southeasternAlaska to the Mexican border whereasin the Mesozoic, intrusiverocks are predominant. (Figure 1). None of the previousreviews has beenbased on Where studied in detail, intrusive bodies and thick lava as large a data baseas we have assembled.Our compilation accumulationsare coincident in time and are essentially for the Cenozoic is an outgrowth of work for anotherpaper coextensive. Distal volcanic ash layers must of course be [Armstrong and Ward, 1991] that synthesizedthe Late disregardedin mappingthe extent of magmaticallyactive Triassic to early Cenozoic(230-55 Ma) magmatichistory of areas. For many years the U.S. Geological Survey has been Copyright1991 by the AmericanGeophysical Union. building a RadiometricAge Data Bank (RADB) [Zartman et al., 1976]. Coveragehas graduallybeen compiled state by Paper number 91JB00412 state. Ward [1986, and unpublishedmanuscripts, 1986, 0148-0227/91/91JB-00412505.00 1988] recently assembled the state files for the entire

13,201 13,202 ARMSTRONGAND WARD: C'EN•IC MAGMATISM IN THE NORTH AMERICANCORD-.I .•RA

EXPLANATION

BOUNDARIES OF THICK-SKINNED

LARAMIDE DEFORMED BELT

EAST BOUNDARY OF LATE MESOZOIC

FOLD AND THRUST BELT

WEST BOUNDARY OF OLDER

PRECAMBRIAN BA8EMENT OF

NORTH AMERICA

EAST LIMIT OF TERRANES ACCRETED

IN PALEOZOIC TO EARLY TRIASSIC TIME

EAST LIMIT OF TERRANES ACCRETED

IN LATE TRIASSIC TO JURASSIC TIME

EAST LIMIT OF TERRANES ACCRETED

IN LATE JURASSIC TO CRETACEOUS TIME

- WRANGELL TERRANE IN NORTH

- FRANCISCAN COMPLEX IN SOUTH

EAST LIMIT OF TERRANES ACCRETED

IN CENOZOIC TIME

BARBS ARE ON UPPER PLATE OF MAJOR

TECTONIC CONTACTS AND THUS INDICATE

DIP DIRECTION OF THOSE CONTACTS.

BARBS ON BOTH SIDES INDICATE THAT

DIP OF TECTONIC CONTACT IS UNKNOWN,

Fig.1. Mapshowing tectonic belts and geographic base for Plates 1-4 [after Armstrong and Ward, 1991]. Cenozoic tectonicand magmatic features are labeled on subsequentFigures.

western United States and produced a series of computer- There are 11,500 age determinationsplotted on the maps plotted maps showingdistribution of dated rocks during a that we preparedspanning the last 230 Ma in the Cordillera. sequenceof time intervals. He discussedthe observed About 50% of those are within the 55-0 Ma time interval magmatichistory and possibleplate tectonicexplanations, that we focus on here. The data presented in this paper with particular emphasison the Cenozoicpatterns in the representa fourfold increasein the numberof isotopicdates western United States. compiled since the last similar syntheses[Snyder et al., The Canadian Isotopic Date Data File was convertedto 1976; Cross and Pilger, 1978], but there are certain computer-readableform by B entzen [1987], with support necessarydisclaimers. The data are not completefor many from the GeologicalSurvey of Canadaand British Columbia reasons. The intent of the compilers of RADB was to Ministry of Energy, Mines and PetroleumResources, during include only dates published in major journals with the winter of 1986-1987. We merged the two data banks to supportingdetails on analytical techniques,decay constants provide reasonably comprehensivecoverage for the entire used, and observedconsistency with other dates and geologic North American Cordillera between Mexico and Alaska relationships. The Canadianfile, in contrast,contains many (Figure 1). unpublished,or grey literature, items. Some large regions, ARMSTRONGAND WARD: CENOZOICMAGMATISM IN TIlE NORTH AMERICAN CORDII I F.RA 13,203 such as northern Washington, northern British Columbia, of magmatic belt boundaries from the previous interval to and the Yukon, are less well studiedand may not be properly the one being illustrated. Solid symbols show age representedin this type of summary. Young depositsmay determinationsbelonging to the early part of time intervals, obscure the older record in areas such as the Intermontane and open symbolsshow the age determinationsin the last 5- Belt of British Columbia, Columbia Plateau, Snake River 10 Ma of the intervals. This allows changes within any Plain, and Puget-WillametteTrough. Neither file is entirely interval to be directly visualized. These within-interval up to date or complete. For example, the Canadianfile on changes are highlighted by larger outlined arrows. Some magnetictape has been updatedonly to late 1984, did not larger arrows are based on literature discussions of the have unpublished data from universities other than the magmatic evolution of specific areas. Where open and University of British Columbia or from the Geological closed age determination symbols mingle on the maps, Survey of Canada, and was known to be incompletewith magmatism persisted throughout the time interval for the respectto datesfor ash layers in the WesternInterior Basin. map. The outlines of magmatic areas on Plates 1-4 have The RADB was last updated in 1986. There are unknown been extensively edited to conform to literature descriptions amountsof grey literature, unpublisheddata, and overlooked of magmatic activity in many areas where dates are not data that result in incomplete representation of the shown. information that exists at any time. Probably another 20-30 It has not been practical to compile volumetric and % could be added today to the data collection. Both large petrochemicaldata for this review. The papers cited in the files will contain errors and misleadingdates. The Canadian introductioncontain abundantinformation on petrochemistry file has been fairly heavily edited to flag any dubiousresult (especially Christiansen and Lipman [1972], Lipman et al. or results giving dates for metamorphicevents or partially [1972], Luedke and Smith [1979a, 1979b, 1981, 1982, reset dates. It is clear from study of the raw plots that the 1983, 1984, 1986], Luedke et al. [1983], Mutschler et al. U.S. file containsmany questionableitems; these have been [1988]). In this paper we make only passingmention of the assignedto the "ash layer or dubious"category, which are pe•xochemicalcharacter of magmaticfields. shown as small dots on the maps. An effort to bring both Throughoutthis discussionwe accept the Decade of North files completely up to date and to edit them exhaustively American Geology (DNAG) time scale calibration [Palmer, would involve years of scholarly work and is beyond the 1983] as most up-to-date, accurate, and conventional. A limits of this study. This is thus only a progressreport on revision of the Harland et al. [1982] time scale [Harland et an unendingprocess of data generation,systematization, and al., 1990] makes no substantialdepartures from the DNAG synthesis. Our goal here is to describethe time and space scale, in spite of an enlargeddata base. distribution of Cenozoic magmatism and focus on the implicationsfor Cenozoicmetamorphic core complexesand THE TIME INTERVAI• USED FOR COMPILATION: continental deformation. THE CHANGING TEMPO OF MAGMATISM

Plate 1 shows the distribution of magmatism in the first time interval illustrated in this paper, from 55 to 40 Ma. From the merged computerfiles [Ward, 1986; Armstrong, On histograms of Cordilleran magmatic activity for the 1988], two series of maps were plotted using Albers equal- region north of latitude 42øN [e.g., Armstrong, 1974a; area conic projections: one from 46ø to 67.5øN (western Armstrong et al., 1977; Armstrong, 1988] this time interval Canadaand adjacentparts of Alaska and the westernUnited brackets a prominent magmatic episode for which the States), and the other from 28ø to 50øN (western United compositename "Kamloops-Challis-Absaroka"is used. This Statesand adjacentparts of Canada and Mexico). All dates episodebegan in Canada and northern Washington about 60 used were calculatedor recalculatedusing InternationalUnion Ma ago, culminatedover a large region at 50+1 Ma and was of Geological Sciences(IUGS) conventionaldecay constants in declineby 45 Ma. By 40 Ma, magmatismwithin much of [Steigerand Jiiger, 1977]. One pair of maps was createdfor the previously active region had ceased, and a southward each 5 Ma time interval from 225 to 0 Ma. On the original transgressionof magmatisminto the future Basin and Range plots, distinctionwas made betweendates for intrusiverocks Province had begun [Lipman et al., 1972; Armstrong and and different types of extrusive rock (mafic, felsic, Higgins, 1973]. pyroclastic). There was also indicationof datesthat were no The time-subdivisions of the Cenozoic after 40 Ma are more than minimum or maximum values for the time of somewhat arbitrary. Although magmatism is always locally emplacementor thoseof dubioussignificance. episodic, the episodes blur into a continuum of magmatic These initial figures were then used to constructhistograms activity when the whole Cordilleran region is viewed during and compositetime interval maps to decide what simplified later Cenozoic time. No comprehensivelulls punctuate the time groupingswere meaningful, with distinction based on Cenozoic magmatic history. In contrast with the Mesozoic magmatic lulls or minima and changes in geographic [Armstrong and Ward, 1991], the discussion of Cenozoic pattern. For the final choice of compositetime intervals,a history relegatestalk of magmatic arcs and episodesto local seriesof maps for the entire Cordillera between Mexico and features and becomesdominated by descriptionof migrating Alaska was drafted (Figures 3 to 8 of Armstrong and Ward, patterns of activity, time-transgressive shifts of pattern, [1991] and Plates 1-4 of this paper) showingthe distribution waxing and waning magmatic loci, persistent loci, and of magmatismduring each of the compositetime intervals. magmatic gaps. A fine-scale episodicity has been discussed These maps are the basisfor the following discussion. for the Cascade to Snake River Plain region [McBirney et As a general rule, distal ash layers in sedimentarybasins al., 1974; Armstrong, 1975], but it is not clear from the have been deemphasized. The magmatic belts shown are larger body of isotopic data that we reviewed how applicable intendedto be the areas where magmatismoccurred, not the those details (Columbian, 16-13 Ma; Andean 10-9 Ma; entire areasinto which volcanic productswere transportedby Fijian 6-3 Ma; and Cascade 1-0 Ma episodesof McBirney Earth surfaceprocesses. [1978]) are to the Cordilleran region as a whole. Priest The plates are designedto show the extent of magmatism [1989] is even skeptical of their application in the Cascades within a time interval and significant changesbetween and in central Oregon. There does appear to be a reduction in during time intervals. Single-shaftedarrows facilitate the magmatic activity to a minimum at about 40 Ma before an comparisonof successiveplates by showing the movement increaseto a weak maximum in Oligocenetime (about 35-30 13,204 ARMSTRONGAND WARD: CENOY•IC MAGMATISM IN THENORTH AMERICAN CORDn J.ERA

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Ma], the ignimbrite flareup, followed by a steady decline inland to the Rocky Mountain region and greatly diminished toward the end of the Cenozoic. The Oligocene maximum in intensity [Coney and Reynolds, 1977]. The magmatic was previously emphasizedby Gilluly [1973] becausehis gap was to remain a prominent feature of Cordilleran data base was confined largely to Basin and Range latitudes. magmaticpatterns until about 22 Ma ago. White and McBirney [1978] and Verplank and Duncan [1987] Laramide-styleuplifts of basementrock in the Montana and discuss the general decline since 35 Ma in the Cascade WyomingRocky Mountains appear to have ceasedto rise by region. The voluminousbasalt, eruptedbetween 17 and 14 55 Ma. In Wyoming the uplifts are unconformablyoverlain Ma in the Columbia Plateauregion [Christiansenand McKee, by volcanic rocks and volcaniclastic sedimentary rocks of 1978; Barrash and Venkatakrishnan, 1982; Barrash et al., 55-50 Ma age [Dickinson et al., 1988]. Farther south, the 1983], is a local anomaly,which is also evident in increased upliftsin the Southernor Colorado-NewMexico Rocky' magmatism in adjacent areas but not over the entire Mountains may have continued to rise actively until Cordilleranregion. In the Basin and Rangeand Rio Grande somewhatlater, until about40 Ma accordingto Dickinson et Rift areas a lull at about 17 Ma has been reportedby several al. [1988]. By 55 Ma the Laramidemagmatic episode had workers [McKee et al., 1970; Chapin and Seager, 1975; virtually endedin Arizona [Damon and Mauger, 1966], had Aldrich et al., 1986]. In many areas there is evidence of a contracted to a persistent magmatic locus in central Pliocenelull followed by acceleratedmagmatism during the Colorado,and was waning in the Black Hills region of South last 2-3 Ma [McBirney et al., 1974; Armstrong, 1975; Dakota and Wyoming [Shafiquillahet al., 1980; Snyder et Smith and Luedke, 1984], but this is a detail not resolved in al., 1976; Mutschler et al., 1988]. our compilation. Moreover, becauserocks of very young ageare difficultto dateaccurately by K-At, they areprobably 40-55 Ma (Plate 1) underrepresentedin the data base. For Plates2 to 4 we have somewhatarbitrarily chosen time The Kamloops-Challis-Absaroka volcanic episode intervals of 40-25, 25-10, and 10-0 Ma. The 40-25 Ma [Armstrong, 1974a] dominates this time interval. At its interval capturesthe main episodeof ash flow volcanismin culminationthis magmaticbelt extendedfrom southernIdaho the Great Basin, southern Arizona, and New Mexico and in to Alaska. Its westernmagmatic front is sharpand precisely the Eastern Rocky Mountains, as well as the establishment located in the Coast Plutonic Complex of British Columbia. of the Cascade volcanic arc. Between 25 and 10 Ma the It generally varies in compositionfrom calc-alkaline on the most dramaticmagmatic event was the eruptionof basaltsin west to distinctly alkaline on the east, but the chemical the Columbia Plateau region. Over much of the Cordillera patterns are locally complicated [Larsen, 1940; Lipman et south of the Colorado Plateau this was also the time of al., 1971, 1972; Ewing, 1981; Holder and Holder, 1988; changefrom intermediate-compositionash flow to bimodal, O'Brien et al., 1991]. The curvature of the axis of the fundamentally basalt eruptions [Lipman et al., 1971; magmatic belt into central Montana is a notable feature, Christiansenand Lipman, 1972]. Basin and Range and Rio partly inheritedfrom the Laramidemagmatic pattern, a locus Grande rifting were prominent developments that of exceptionally intense magmatism that swept accompaniedthe changein magmaticstyle. The magmatic northeastward from the Sierra Nevada into central Montana patternsfrom 10 Ma to the presentare essentiallythose of over the time interval 80-65 Ma [Armstrong and Ward, the presentday, with only evolutionarymodifications. 1991]. After 45 Ma, magmatismin the Kamloops-Challis- Absaroka belt waned dramatically, and at the same time a REVIEW OF MAGMATIC EVOLUTION southward transgression of intermediate-composition magmatism,including voluminous ash flows from calderas, Prior to 55 Ma began [Lipman et al., 1972; Armstrong and Higgins, 1973]. This is shownby the larger outlined arrows on Plate 1. In We begin this synthesiswith a brief review of magmatic Alaska the name "Skagway-Ketchikan-PrinceRupert" [Brew, patternsjust before 55 Ma that are discussedby Armstrong 1988] is used for a part of the much larger Pal½ogcn½ and Ward [1991]. Their figures have the same format as magmatic belt. thosepresented here. A major turningpoint in Cordilleran Dying remnantsof Laramide magmaticfields show on Plate tectonicsoccurred about 55 Ma ago. During the Mesozoic, 1 in the Black Hills (wherea westwardmovement of activity magmatismin the Cordillerahad beenepisodic on a scaleof is hinted at by a short arrow), in centralColorado (last gasp tens of million years and had been distributedin long linear of the Laramideepisode in the Coloradomineral belt), and in belts parallel to the entire length of the continentalmargin. southernArizona (end of the Laramideepisode of Damon and Between 85 and 55 Ma the behavior of U.S. and Canadian Mauger [1966]). Along the southernedge of the map the sectorsdeparted from this simple pattern. In Canada the Laramideactivity was followedby a shift of magmatisminto magmaticbelt remainedessentially continuous and parallel southwesternTexas in middle Eocene time, about 4740 Ma to the continentalmargin. A weak culminationof magmatic ago [Wilson et al., 1968; Hoffer, 1970; Henry and activity at about 80 Ma was followed by a decline to a McDowell, 1986]. This completed the late Mesozoic to pronouncedminimum about 65 Ma, closeto the Cretaceous- early Cenozoic eastward sweep of magmatism noted by Tertiary boundary[Armstrong, 1988]. By 60 Ma the tempo Coney and Reynolds[1977]. of magmatismin the north acceleratedrapidly, and at some The remainingmagmatic features of Plate 1 are three belts time between 60 and 55 Ma the tectonic style north of 42ø along the western margin of the continent. The reversed dramatically, from crustal shortening that southernmostis the Coast Range belt of basaltic island constructed the fold and thrust belt of the Rocky and volcanosof Oregon and Washington[Duncan, 1982] and Mackenzie Mountains to crustal extension and strike-slip Metchosin belt of southern Vancouver Island [Muller, 1980; faulting that continuedto be the dominanttectonic themes Massey, 1986]. This region was not firmly attachedto throughthe rest of the Cenozoic[Ewing, 1980; Okulitch, North ,america at the time of construction of the older 1984; Journeay, 1986; Carr, 1989]. volcanoes (basalt accumulation spanned 62 to 41 Ma) In the United States south of 42 ø, the continuity of the [Duncan, 1982; McElwee and Duncan, 1984; Heller et al., Mesozoic magmaticbelt was disruptedafter 80 Ma by the 1987]. These volcanic accumulations are notable for the Laramidemagmatic gap [Armstrong, 1974b], which spread large and variable tectonic rotations that have accumulated over California, Nevada, and Utah as magmatism shifted during Cenozoic time [Wells and Heller, 1988]. The amount ARMSTRONGAND WARD: CENOZOIC MAGMATISM IN TIlE NORTH AMERICAN CORDII.!.ERA 13,209 of northward motion of the component parts of this belt cases [e.g., Rouse and Mathews, 1988; Ross, 1983; with respect to North America is uncertain. A northward Mathews, 1989] but are volumetrically small. A few basalt movement of a few hundred kilometers is suspected but dikes of this time interval are also found in the Vancouver difficult to quantify [Beck, 1984]. Accretion occurred in area of southwestern British Columbia and on northern middle Eocene time, about 50-42 Ma ago [Muller, 1980; Vancouver Island. Granitic and lamprophyric dikes and Heller and Ryberg, 1983; Armentrout, 1987]. small plutons of Oligocene to Miocene age have been The Flores Volcanic-Catface Intrusive belt on Vancouver reportedin several studiesof the Coast Mountains near and Island [Carson, 1973; Isachsen, 1987; Massey and in the southeastern tip of Alaska (Portland Canal area) Armstrong, 1989] and the Fairweather-Baranofbelt in Alaska [Brew, 1988; Lull and Plafker, 1988]. [Brew, 1988] are continent-margin calc-alkaline magmatic The two larger Oligocene magmatic fields north of latitude belts of enigmatictectonic setting. The 55-40 Ma magmatic 49øN are the Masset volcanic field (culminating35-25 Ma), rocks on Vancouver Island and in the foothills of the on and near the Queen Charlotte Islands [Young, 1981; Cascade Range in Washington appear to be almost exactly Hickson, 1988], and the Glacier Bay-Tkope magmatic belt coincident in age with the inland Kamloops-Challis- (42-24 Ma, shifting northeastwardduring this time interval), Absarokamagmatic belt. The Alaskan magmatic belt is the in southeastern Yukon, northwesternmost British Columbia same, to slightly younger in age (49-39 Ma according to and adjacentAlaska [Brew, 1988]. Brew [1988]). Cowan [1982] suggestedthat the Alaskan Eocenemagmatic belt is offset by strike-slip faulting during 10-25 Ma (Plate 3) later Cenozoic time from original continuity with the geologicallysimilar VancouverIsland Eocenemagmatic belt. There are three highlights of this time interval: the merging of the Arizona and Nevada magmatic fields along 25-40 Ma (Plate 2) the Colorado River corridor about 25-22 Ma ago, the eruption of basalt in the Columbia Plateau 17-14 Ma ago, By 35 Ma [Smith et al., 1980; Priest and Vogt, 1983; and the northwards transgressive change in tectonic and Richards and McTaggart, 1976] the Cascade volcanic arc magmatic behavior that affected much of the western United became establishedclose to its present location in Oregon, States at about 20-17 Ma ago [Christiansen and Lipman, Washington,and southernBritish Columbia. It remained a 1972; Armstrong and Higgins, i973]. persistentmagmatic feature for the rest of Cenozoic time, The merging of the magmatic fields, a singular event of undergoingonly modest shifts in position and variations in profound significance for metamorphic core complex intensity [McBirney et al., 1974; Armstrong, 1975; development, was coincident with the inception' of Armstronget al., 1985]. magmatism in the Mojave Desert [Armstrong and Higgins, The southwardtransgression of an east-west trending belt 1973; Glazner and Bartley, 1984; Dokka and Woodburne, of magmatismin Nevada and Utah is a dominantfeature of 1986; Dokka, 1988]. Plate 2. This transgression was first documented by An aspect of the merged volcanic belt that has received Armstrong et al. [1969] and Armstrong [1970] and later considerable discussion is the change in volcanic discussedby Lipman et al. [1972], Noble [1972], Stewart et composition and eruptive style from predominantly al. [1977], and Burke and McKee [1979], among others. intermediate chemistry with typical ash flow eruption style Magmatic activity in the San Juan Mountains and nearby to one of bimodal chemistry, predominantly basaltic areas in Colorado and in the Basin and Range and Rio eruptions. This was first discussed by McKee [1971], Grande rift areas of Arizona and New Mexico expanded Lipman et al. [1971, 1972], Noble [1972], and Christiansen dramatically at the same time as the Great Basin ignimbrite and Lipman [1972], and the time-transgressivepattern of flareup, but the two major Oligocene magmatic fields this change is shown by Armstrong and Higgins [1973]. In remained separated until after 25-22 Ma by the Laramide the southernmost areas the transition occurred earliest, magmatic gap. Over the time interval illustrated, the perhapsas early as 25 Ma ago [Crowe et al., 1979]. The Coloradoand Arizona-New Mexico magmaticfields expanded change spread rapidly northward between 20 and 17 Ma, independently and eventually merged into one as the Rio connectingwith the Columbia Plateau basalt eruptions at 17 Grande rift began its development. As part of this Ma. By 15-12 Ma the change to bimodal, mostly basalt expandingactivity, magmatismin southernArizona migrated magmatic style had occurredover most volcanic fields of the westward at 3-4 cm/a [Shafiqullah et al., 1980]. Several western United States, the principal exception being the Colorado Plateau laccolithic centers and diatremes were CascadeArc [Priest, 1989]. A brief interruptionof magmatic emplaced, mostly later in this time interval, in an alkalic, activity at this time of changing magmatic style is small-volume, scattered-centermagmatic field that almost recognized in several papers [McKee et al., 1970; Chapin linked the large southern (Arizona-Colorado) and northern and Seager, 1975; Aldrich et al., 1986]. (Nevada-Utah) magmatic regions [Naeser, 1971; Laughlin et Also approximately coincident with the merging of the al., 1986; Sullivan, 1987]. larger magmatic fields was a magmatic episode (24-22 Ma The other notable features of magmatism in the western according to Stanley [1987]) of considerable extent but United States on Plate 2 (25-40 Ma time interval) are the modest volume on the Pacific plate, west of the San Andreas southwesternMontana locus of persistent magmatism and fault in southwestern California [Johnson and O'Neil, 1984; nearby singular magmatic spots and the area of late stage Fox et al., 1985; Stanley, 1987]. The Mojave field was basalt and alkalic magmatism of the Oregon Coast Range presumably coextensive as well as coincident in time with [McElwee and Duncan, 1984]. this volcanic area. The two fields are interpretedto be one, In southwestern California, west of the San Andreas fault, split by movement along the San Andreas fault [Matthews, magmatismstarted only at the very end of the time interval 1973]. Pilger and Henley [1979] and Tennyson [1989] for Plate 2 and is thus more reasonablyincluded in the next emphasize that this magmatic event preceded the arrival of time interval to be discussed. transform faulting in southwestern California. In middle Oligocene magmatismin Canada and southwesternAlaska Miocene time (15 Ma ago) a north migrating and expanding was restricted in volume and extent. A few parts of the magmatic field appeared just northeast of the San Andreas Eocenebelt remained sites of basalt eruption. These features fault in central California. of the map (Plate 2) are confirmedby multiple dates in most As the Basin and Range magmatic field expanded both 13,210 ARMSTRONGAND WARD: CENOZOICMAGMATISM IN THE NORTH AMERICAN CoRDn •I.ERA

southeastward and westward (in step with the outward slowly expand. Its early "cores", in Nevada and southern propagationof extensionalstrain) [Armstronget al., 1969; Arizona, have become virtually extinct. This pattern is Armstrong, 1970; Scott et al., 1971; Armstrong et al., illustrated in Plate 4. The waning stages are typically 1972], the Rio Grande Rift magmatic belt propagated represented by small vents erupting basanitic, nodule- northwardand graduallybecame more narrowly focused. The bearing basalt [Wilshire and Shervais, 1975; Smith and last of the Colorado Plateau laccolith centerswere emplaced Luedke, 1984]. Magmatism, mostly of small volume and early in the time interval of Plate 3. bimodal-basaltic character, has concentrated near the In the northwestern states and southern British Columbia, expanding margins of the magmatic field. The east eruptionof basalt from fissuresgreatly expandedthe area of migrating easternmargin of the Basin and Range in Idaho, magmatic activity both eastward and northward. The Wyoming, and northern Utah is tied to the migrating Columbia River Basalt of the Columbia Plateau region is the Yellowstone hot spot [Pierce et al., 1988; Anders et al., better known and most voluminousresult of these eruptions 1989; Westaway, 1989a, 1989b]. Elsewheremagmatism and [Christiansen and McKee, 1978; Hooper and Swanson, uplift appear to advance sooner into stable areas than 1987], but lesser volumes of contemporaneousand younger extensional strain, as for example along the eastern, basalt occur throughout much of the Intermontane Belt of western, and southwestern edge of the Colorado Plateau British Columbia as older parts of the "Plateau Basalt" or [McKee and Anderson, 1971; Best and Hamblin, 1978; "Chilcotin Basalt" [Mathews, 1988, 1989]. The Columbia Rowley et al., 1978; Best et al., 1980]. Rates of River Basalt spreadacross the Cascadesinto coastalOregon extensional strain declined overall during this interval and Washington, and contemporaneousbasalts were erupted [Zoback eta/., 1981; Eaton, 1984; Wernicke et al., 1988]. as well in northern California and Nevada. The time- A notable feature of this time is the developmentof the transgresslyebimodal magmatism of the Snake River Plain Snake River Plain, documentedby Armstrong et al. [1975]. began about 15 Ma ago in westernIdaho at the time of the Silicic magmatism, followed by bimodal and basaltic Columbia River Basalt event, and bimodal volcanism became eruptions, has migrated northeastward up the axis of the widespreadover much of easternOregon. easternSnake River Plain to a present locus in Yellowstone The Cascadearc continuedthroughout this time interval as Park in northwesternWyoming. A much weaker and parallel a stable feature, stretching from Oregon to southwestern migration of magmatism has occurred into the Clayton- British Columbia, in virtually the same position as its Raton area of northeastern New Mexico and southeastern Oligocenepredecessor. There is disagreementas to whether Colorado [Suppeet al., 1975]. Lipman [1980] has argued Cascade magmatism was interrupted [Priest, 1989] or that this is not the end of a long-lived hot spot trace. accelerated [McBirney et al., 1974] at the time of the Almost perpendicular is the trend of migration and Columbia Plateau basalt episode. The persistentmagmatic expansionof the magmaticfield that lies just east of the San locus in Montana finally became extinct during the Miocene. Andreasfault in central westernCalifornia. Activity there is At 54ø-55øN during the time interval of Plate 3 the Masset currently centered in the The Geysers-Clear Lake area magmatic activity of the Queen Charlotte Islands diminished [McLaughlin and Donnelly-Nolan, 1981; Fox et al., 1985; to near extinction. Magmatic activity shifted from the Stanley, 1987]. Masset centerseastward to begin the Anahim Magmatic Belt The other magmatic features south of 42øN are scattered hot spot trace [Bevier et al., 1979; Souther, 1986] as the bimodal volcanic occurrences in southwestern California and Masset area itself was displaced northward on strike slip around the Gulf of California, the remnants of the central faults that parallel the continental margin [Young, 1981; Colorado-RioGrande rift persistentmagmatic locus, and one Yorath and Hyndman, 1983]. Early Miocene basaltsoccur in field of peculiar extremely potassic alkaline magmas in the interior of British Columbia at these latitudes [Church, southern Wyoming [McDowell, 1971]. In the northwestern 1973], but they are volumetrically insignificant. states,the Cascadearc has persistedthrough the last 10 Ma, Early in the late Oligocene to late Miocene time interval of with its magmaticfront migrating eastwardin central Oregon Plate 3 scatteredcalc-alkaline to alkaline magmatic activity [McBirney, 1978; Priest, 1989] and jumping westward in continued in southeastern Alaska [Brew, 1988] and southernBritish Columbia [Armstronget al., 1985; Green et northwesternmostBritish Columbia. In addition, early to al., 1988]. From 10 to 4 Ma the average rate of Cascade middle Miocene basalt has been discovered on the east shore magmatismdeclined, reaching a minimum in Pliocene time. of Atlin Lake, east of the Coast Mountains in northwestern In Quaternary time, magmatism acceleratedto current rates. British Columbia [Bultman, 1979; University of British The Cascade arc now extends down into northern California Columbia (UBC), unpublisheddata, 1980]. to virtually merge with the expanded Basin and Range After about 15 Ma, magmatism shifted northeastwardto magmatic field. In eastern Oregon, the area where silicic form the calc-alkaline Wrangell belt, which initially magmatism occurs has systematically contracted over the extended from Alaska southeastward across the southwestern last 10 Ma into the Newberry area, just east of the Cascades corner of the Yukon into northwestern British Columbia [McKee et al., 1976]. The younger basaltic centers in [Skulski, 1988]. About 15 Ma ago a large peralkaline easternOregon are largely locatedin a narrowbelt extending volcanic shield (Level Mountain) began growing in east-southeastfrom Newberry volcano [Guffanti and Weaver, northwestern British Columbia (unpublished UBC K-At 1988]. Two areas of late basalt eruption on the Columbia dating for the GeologicalSurvey of Canada)and by 10-8 Ma Plateau are shown on Plate 4. That field is now extinct. that magmatic field had expandedsouthward into the Mount In central and southernBritish Columbia the eruption of Edziza area [Souther et al., 1984]. Volcanic rocks at several "Plateau" and "Valley" basalts has continued into the localities further north in the Yukon have been inferred to be Holoceneover an extensiveregion, but the volumes erupted Miocene [e.g., Tempelman-Kluit, 1974, 1976; Noel, 1979], are small [Bevier, 1983; Mathews and Rouse, 1986; but are actually Cretaceous [Grond et al., 1984]. Mathews, 1988, 1989]. The Anahim Belt has migrated to the Nazko area [Souther, 1984] and another vent cluster has 0-10 Ma (Plate 4) appearedin the Clearwater area [Hickson and Souther, 1984]. These features are shown on Plate 4 by the larger outlined During the last 10 Ma there have been few major changes alTOWS. in the overall magmatic pattern. The areas of Quaternary A short-lived magmatic belt on northern VancouverIsland, magmatism are much the same as those of late Miocene the Alert Bay Belt, was active about3+1 Ma ago, during the time. The Basin and Range magmaticfield has continuedto westwardjump of the Cascade Belt from the Pembertonto ARMSTRONGAND WARD: CENO7•IC MAGMATISM IN TItE NORTHAMERICAN CORDII J.ERA 13,211 the Garibaldi axes [Armstronget al., 1985]. Scatteredyoung Eocenetime, coincidentwith the Eocenemagmatic episode. basalt fields occur on the Queen Charlotte Islands and in At the internationalboundary (49øN latitude) the area affected southeasternAlaska. The Quaternary Edgecomb field, of by ductiledeformation of Eoceneage is nearly as wide as the bimodal chemistry, lies farthest west, close to the currently magmatic arc, extendingfrom Cascadeand Coast Mountains active strike-slip North American plate boundary [Kosko, (Ross Lake and Bridge River areas) on the west to the 1981; Meyers and Marsh, 1981]. eastern flank of the Shuswap Complex (Valhalla and In northern British Columbia, Level Mountain has become Spokane domes) on the east. extinct, and there has been an expansion of the Edziza field [Souther et al., 1984] from peralkaline central volcanos to 25-40 Ma (Figure 3) widespreadsmall-volume basalt vents (unpublishedUBC K- Ar dating for the Geological Survey of Canada). The Ductile extensionstructures of this age occur in two areas, Wrangell arc has contracted so that its activity is now each within major magmatic fields. The large core entirely within Alaska [Skulski, 1988]. Scattered late complexes of southern Idaho, eastern Nevada, and Miocene to Pleistocene basalt vents occur in the southern northwesternUtah formed synchronouslywith the ignimbrite Yukon, east of the Coast Mountains. transgressionthrough Nevada. In southern Arizona, some ductile strain is reported to be as old as the time interval MAGMATIC CORE COMP•S IN TIME AND SPACE illustratedhere, but most is immediatelylater.

Davis and Coney [1979], Coney [1979, 1980], Davis 10-25 Ma (Figure 4) [1980], and Armstrong [1982] have reviewed the general characteristicsof Cordilleran metamorphic core complexes. Some ductile strain in the eastern Great Basin continued These are exposures of ductilely deformed middle crust into this time interval, but the focus of ductile (and brittle) exposedin structural culminationsin an extensional tectonic extensionwas in the ColoradoRiver Corridor and adjacent environment. It has been long known that the core parts of southern California and western and southern complexesbecome younger southwardsin the United States Arizona [Glazner and Bartley, 1984;Howard and John, 1987; and occur in close association with extensive regional Sherrod et al., 1987; Miller and John, 1988; Davis, 1988; magmatism. The association of magmatism and large- and Davis and Lister, 1988]. Davis and Lister [1988] infer magnitude extension was first recognized by Anderson rates of extension greater than 1 crn/a in the Whipple [1971] (with discussion and reply by Thompson and Mountains between 20 and 18 Ma ago. Brittle extension Anderson [1971]), and this associationwith core complexes occurredover a much wider area [Proffett, 1977; Rowley et is commentedon in all the reviews cited and is emphasized al., 1978; Morgan et al., 1986], and this is usuallyviewed in recent papers by Coney [1987], Hamilton [1987], Gans as the main periodof developmentof the Basin and Range [1987], Brown and Journeay [1987], Wernicke et al. [1987], structure [e.g., Eaton, 1984]. A major reorientation of Gans et al. [1989], McCarthy and Thompson [1988], regional stress in the western United States occurred 26-17 Thompsonand McCarthy [1990], Lachenbruch and Morgan Ma ago (Eaton et al. [1978] give approximately17 Ma for [1990], and Lucchitta [1990]. Table 1 lists a number of core the time of changein the Great Basin;Wust [1986a] gives complexesor closely related areasof ductile deformationand 24 Ma for the core complexes;Aldrich et al. [1986] give low-angle faulting where structural and geochronometric about 23 Ma for the Rio Grande rift region; Best [1988] studieshave defined the age of ductile deformationassociated gives26-18 Ma for westernUtah; andRen et al. [1989] give with uplift and rapid cooling of the metamorphic rocks. 30 to 14 Ma for the easternGreat Basin). This changeis This information has been obtained by U-Pb, K-At, fission stress orientation is notable in the contrast between Eocene track, and Rb-Sr dating of prekinematic, synkinematic and and Oligocene metamorphic core complexes, with WNW postkinematic igneous phases and minerals to determine lineations, and Miocene and younger metamorphic core crystallization ages and cooling histories. The common and complexes with WSW lineations, as shown on the long-recognized association of igneous rocks and compilation of Wust [1986a]. Further reorientation occurred deformation in metamorphiccore complexeshas often been about 10 Ma ago in the northernBasin and Range [Zoback the key to providing rocks whose ages closely bracket the et al., 1981], probably in responseto migration of the times of deformation. Most of the literature cited was Yellowstone plume and to increasedcoupling to the San publishedwithin the last decade. Not listed in Table 1 are Andreas transformsystem. many areas where K-At dates have been reset but where evidence for synchronousductile strain is lacking and areas 0-10 Ma (Figure 5) where brittle extension has occurred but which lack exposed rocks with ductile strain. Areas where ductile strain of this age is documentedare few The locations and times of ductile deformation are plotted and nearly all are in the Death Valley to Salton Sea region on a seriesof maps (Figures 2-5), similar in format to those of southernCalifornia [Hodges et al., 1987; Asmeromet al., showing the magmatic evolution of the Cordillera between 1990]. Ratesof extensionover the entire Basin and Range Alaska and Mexico. The point to be made is simple and are inferred to be much less than earlier in the Miocene emphatic. The core complexesform only within the largest [Zoback et al., 1981; Wernicke et al., 1988]. magmaticbelts and fields. They do not form where there is DISCUSSION little or no magmatic activity, and there is often magmatic activity without the formation of core complexes. The The associationof magmatismand core complexesleads to figures are a graphical demonstration of the association. the deductionthat magmatismis a necessaryprecondition to Extension directions for most of these metamorphic core the formation of metamorphiccore complexes,because it complexes have been compiled and illustrated by Wust leadsto elevatedgeotherms and a consequentweakening of [1986a]. the entire lithosphereso that extensionmay occur. The effectsof heat on lithospherestrength were emphasizedby 40-55 Ma (Figure 2) Burchfiel and Davis [1975], Cross and Pilger [1978], and Davis [1980] and are discussedmore recentlyby Lipman et From central British Columbia to central Idaho all the al. [1986], Lynch and Morgan [1987], and Gaudemeret al. metamorphic core complexes formed in early to middle [1988]. 13,212 ARMSTRONGAND WARD: CENOZOIC MAGMATISM IN TIlE NORTHAMERICAN CORDll I.ERA

TABLE1. Timesof DuctileDeformation in Metamorphic Core TABLE1. (continued) Complexes .... Tme of Tme of Ductile Ductile No. Locality Deformation Reference .No. Locality Deformation Referen• Canada 1 Nulki <58Ma R.Friedman and R.L. SouthernCalifornia and Southwestern Arizona 22 Colorado River 22-17 Ma Howard and John Armstrong, Corridor [1987] (unpublishedU-Pb 22-19Ma Howardetal. [1990] dating,1990) 23 Whipple 19-16Ma Andersonetal. [1988] 2 Coast Plutonic 51 Ma vander Heyden [1989] Mountains 20-18Ma Davisand Lister Complex [19881 3 Tatla Lake 56-47 Ma Friedman and 26-16 Ma Davis [1988] Armstrong [1988] 24 Rawhide 4 Valhalla 59-52 Ma 16-10 Ma Shackelford [1980] Parrish et al. [1988] Mountains 5 Okanagan 60-47 Ma Parrish et al. [1988] 25 Big Maria 20-16 Ma Hamilton [1982] 52-47 Ma Templeman-Kluitand Mountains Parkinson [1986] 26 Central Mojave 20 Ma Glaznerand Bartley Monger et al. [1984] 6 BridgeRiver 45-40 Ma [19841 and Potter [1986] 22-17 Ma Dokka [1988, 1989]

Washington Salton Sea Area North Cascades -45 Ma Babcock et al. [1984] 27 Yuba Desert <6 Ma Isaac et al. [1986] OkanoganDome -48, <65 Ma Hansenand Goodge 28 Peninsular <24,>5 Ma Engel and Schultejann [19881 Range [ 1984] and Keule Dome Late Cretaceous Cheney [1980] and Schultejann [1984] to Eocene Rhodes and Cheny [1981] Southern Arizona 10 Newportfault- 48-45 Ma Miller [1971], Harms 29 South Mountain 25-20 Ma S.J. Reynoldset al. SpokaneDome and Price [1983], [ 1986] >50 Ma Armstrong et al. 30 Pinaleno <_25Ma Glaznerand Bartley [ 1987], Bickford et al. [19841 [1985], and Rhodes 31 Rincon 28-24 Ma Glaznerand Bartley and Hyndrnan [1988] [19841 Eocene Rehrig et al. [19871 32 Catalina 21-17 Ma Anderson et al. [1988] 25-20 Ma Reynoldset al. [1986] 24-16 Ma Dickinson and Northern and Central Idaho Shafiqullah [1989] 11 Bitterroot 46-44 Ma Garmezyand Sutter 35-22 Ma Davy et al. [1989] Range [1983] 33 Bullardfault 25-17 Ma Reynoldsand Spencer 57?-49-46 Ma Hyndmanand Myers [19851 [19881 52-42 Ma Chase et al. [1983] A much longer list of areas of crustal extensionor low-angle 12 Pioneer -40 Ma Wust [1986b] faulting could be compiled,as, for exampleby Glazner and Bartley Mountains <68,>40 Ma O'Neill and Pavlis [1984], but such information would only diffuse the focus of this [19881 paper on the metamorphiccore complexes. 45-48 Ma and Silverberg [1990] The cross reference numbers are plotted on Figures 2-5, as 36-33 Ma appropriate. There is not enough room on the figures to include either names or times of ductile deformation.

Central and Northern Nevada 13 Wood Hills 56-47 Ma Thorman and Snee [19881 14 Ruby 26-24 Ma Dokka et al. [1986] Magmatism does not of itself necessarily cause Mountains 45-20 Ma Dallmeyer et al. [1986] 45-23 Ma Snoke and Miller convergenceor extension. That dependsmostly on the far- [1988] field plate tectonic forces acting on a region [Molnar and 15 SnakeRange 36-<24 Ma Lee et al. [1987] and Atwater, 1978; Chase, 1978; Cross and Pilger, 1982] and Miller et al. [ 1988] inheritedtopography and crustalthickness (as reviewedby Dewey [1988]). Magmatismmay contributeto uplift and

Southern Idaho and Northern Utah crustal thickening and, consequently,favor extensionbut is not alone decisive. Magmatic heating allows deformationto 16 Albion Range 34-17 Ma Armstrong [1976] 30-15 Ma Snoke and Miller occur. During the Mesozoicthere was abundantmagmatism [19881 without large-scaleextension. The Cordilleran region was 17 Raft River 50?-20 Ma Comptonet al. [1977] evidently under compression (maximum principal stress Mountains- --25 Ma Todd [1980] generallyeast-west) and once weakenedby heating,failed in Grouse Creek convergentstyle [Burchfiel and Davis, 1975;Allmendinger Range and Jordan, 1981]. The maximum principal stressaxis in the upper crust has apparentlybeen vertical during most of SouthernNevada to Death Valley Cenozoictime, startingin northernareas about 55 Ma ago, 18 Mineral Ridge .-17 Ma McKee [1983] and migrating southwardthrough the western United States 19 Trappman Hills .-14 Ma McKee [1983] 20 Bullfrog Hills -11 Ma McKee [1983] by about 40 Ma ago. Extension has occurred wherever and 21 Grapevine- 11-7 Ma Glaznerand Bartley wheneverweakness prevailed. After about 30 Ma ago the Panamint [ 1984] growth of the San Andreas transform [Atwater, 1970; 12-<5 Ma M.W. Reynoldset al. Dickinson, 1981] system added further complication to [19861 Cordilleran tectonic and magmatic patterns but did not ARMSTRONGAND WARD: CENOZOICMAGMATISM 1NTHE NORTH AMERICAN CORD-.!.ERA 13,213

1988; Davis, 1988; Davis and Lister, 1988]. Why is this •'OO phenomenaso extremely developedin this particular area and at one certain time, about 20-18 Ma ago? Contemplationof the maps showing changing patterns of magmatic activity provides an explanation that further elaborates on the theme that cold lithosphere is strong and areas of a magmatically sustainedsteep geothermalgradient are thoroughly weak. The unique circumstancethat began the time of hyperextensionis the merging of Arizona and Nevada magmaticfields at about 25-22 Ma, first documented by Armstrong and Higgins [1973]. This created a new

/ / I I I I • I /

i / / i / / i

I

/ / i / I /

/ I

loo

Fig. 2. Magmaticbelts and locationof ductile deformationin Cenozoicmetamorphic core complexesfor the time interval40-55 Ma ago (earlyand middleEocene time). The core complexesoccur in the areasshown by a darkerpattern. The numbersin thoseareas are keyedto the complexeslisted in Table 1. fundamentallychange the extensionaltectonic regime. The explanations for magmatism and stress within the Cordilleranlithosphere may be interrelatedbut must also be somewhatindependent of one another. lo o BURNING THE I/rHOS• B P,mG• Fig. 3. Magmatic belts and location of ductile deformation in The rapidproduction of corecomplexes of amazingextent Cenozoic metamorphiccore complexesfor the time interval 25-40 in the Colorado River corridor has been commented on by Ma ago (late Eoceneto latest Oligocenetime). The core complexes many workers,often in puzzlement[Davis, 1980;Glazner occur in the areas shown by a darker pattern. The numbers in those and Bartley,1984; Howard and John,1987; Miller andJohn, areas are keyed to the complexeslisted in Table 1. 13,214 ARMSTRONGAND WARD: CENOZOIC MAGMATISM IN THENORTH AMERICAN COIlDO ] I•.RA

state with forces resisting plate motion was achieved. In early to middle Miocene time California becamea separate microplate, able to rotate and move away from North II o America at rates that may have approached several centimetersper year. It is noteworthythat large tectonic rotationsin the TransverseRanges occurred during middle Miocene time [Hornafius et al., 1986] and that the last episodeof accretionof the FranciscanComplex, in the King Range area, occurred just after middle Miocene time [McLaughlin et al., 1982]. These events coincidedwith the brief time interval during which California was a rapidly

/ / / / / / /' Ii C / / / • / /

I I I i I / / / I / • I I

/ I / / I i

/ I I i'

/ ---4.- / / I .._. / I I / I i / / / / 700

Fig. 4. Magmatic belts and location of ductile deformationin Cenozoicmetamorphic core complexesfor the time interval 10-25 Ma ago (latest Oligoceneto early late Miocene time). The core complexesoccur in the areas shown by a darker pattern. The / iI1('ql':(•./ • numbersin thoseareas are keyed to the complexeslisted in Table 1.

I I tectonic regime, a continuousmagmatic belt from Mexico to I Canada for the first time since about 80 Ma ago. Until I about 22 Ma the Laramide magmatic gap remained as a I "bridge" of cold lithosphere constraining extension in 1o 0 adjacentareas. When that bridge ceasedto exist (referredto above as "burning the bridge"), the constraintwas removed. Fig. 5. Magmatic belts and location of ductile deformation in Extensionof unusualrate and magnitudefollowed linkage of Cenozoicmetamorphic core complexesfor the time interval 0-10 Ma the magmatic fields within a few million years, and ago (late Miocene to Holocenetime). The core complexesoccur in continued for several million years, until the previously the areas shown by a darker pattern. The numbersin thoseareas are accumulatedstresses were accommodatedand a new steady keyed to the complexeslisted in Table 1. ARMSTRONGANDWARD: CENOZOIC MAGMATISM INTHE NORTH AMERICAN CORDH I.ERA 13,215 westwardmoving microplate. Palcomagnetic measurements Margin (hinted at by Scholzet al. [1971] and explicitly cannotdetect purely longitudinal motion, but reconstructioninvoked by Bestand Hamblin [1978],Dickinson and Snyder of the intersection of the Mendocino Fracture Zone with the [1979a], Lipman [1980], Seageret at. [1984], and Fox et edge of the continent must take these westward at. [1985]). This latter idea wouldnot seemto be applicable displacementsof Californiainto account[e.g., Stock and to Eocenemagmatism and core complexformation and so Molnar, 1988]. cannot be a general explanation for crustal extension. Thorkelsen and Taylor [1989] present unconstrained COMMEN'IZION EXPLANATIONSFOR MAGMATISM speculationthat a north-migratingEocene slab window did exist under southern British Columbia, but this does not fit The abundant literature on Cenozoic magmatism and observedmagmatic patterns. faultingin the Cordilleranregion is full of explanationsfor Other authors argue in a more general way for the the phenomenaobserved, and the truth probably importance of oblique convergence,which may be incorporatesmany of the proposedprocesses. In this partitionedinto subductionand strike slip components section we comment on the various proposals and the [Jarrard, 1986a, 1986b], with consequencesthat are similar observationsthat they explain. to the San Andreas-Basinand Range interaction. This has Largemagmatic belts parallel to thecontinental margin are been used to explain the Eocene events north of 42øN conventionallyviewed as magmaticarcs of Andeanstyle. [Ewing, 1980;Price and Carmichael,1986] andmany details Hamilton [1969] andDickinson [1970] emphasizedthis and of Cordilleran history [Page and Engebretson, 1984; theywere hardly the first to recognizethe similarity.It can Engebretsonet al., 1985;Wells and Heller, 1988]. be found in much earlier papers, for example, by D aly Migratingtriple junctions, especially the Mendocinotriple [1933] and Stifle [1940]. This is a traditionalfirst-order junctionof the evolvingSan Andreastransform fault system, view, supportedby studyof modemarcs, and generally are given special emphasisin numerouspapers [Coney, acceptedtoday for theCascades. Lipman et al. [1971,1972] 1976, 1987; Snyder et al., 1976; Dickinson and Snyder, so liked this idea that they proposeddouble magmatic arcs 1979b;Ingersoll, 1982;Glazner and Supplee,1982; Zandt (onenormal and one cryptic,without surface expression of and Furlong, 1982;Glazner and BartIcy,1984; Johnson and subduction),based on volcanic chemistry, for the early O'Neil, 1984, Fox et al., 1985; Ward, 1986]. A few papers Cenozoic.Burchfiel and Davis [1975],Snyder et al. [1976], assignimportance to overridingof the EastPacific Rise by Crossand Pilger [1978],Leemah [1982b], and many other North America [Menard, 1960; Cook, 1966; Damon, 1971] authorsfurther support the magmatic arc viewpoint but (and more recently,Gough [1984] andWilson [1988]), but usuallyin the simplerform of a singleslab being subducted most marine geologists view the Rise as an entirelypassive underthe Cordilleranregion. Lipman [1980], likewise,now feature that has no existence between offset active segments advocatesa single-slabmodel. [Atwater, 1970]. These mechanisms for uplift and Extension within active arcs is a common observation magmatismwould be applicableonly for the last 30 Ma of [Healey, 1962;Tobisch et al., 1986;Smith et al., 1987], the nearly 60 Ma durationof magmatismassociated with and is not incompatiblewith subduction[Molnar and Lyon- extension and core complexesand only from California Caen, 1988; Sdbrier eta/., 1988]. In fact, subductionof southward. oldercrust may evencreate trench suction and cause within- Explanationsusing the northwardmigration of subducted arc and back arc extension[Dewey, 1980; Cross and Pilger, rise and triple junctionsare mostattractive and suitablefor 1982;Burchfiel and Royden,1982]. Back arc extensionhas featuresof the magmaticand tectonichistory that migrate beenemphasized in discussionsof the GreatBasin by $cholz northward (e.g., ages of low-angle faulting in the et al. [1971], Thompsonand Burke [1974],Stewart [1978], southwesternUnited Statesdiscussed by Glazner and BartIcy Crossand Pilger [1978],Davis [1980],and Eaton [1980] and [1984] and the switchoff of arc magmatismin California in many later papers. discussedby Snyder et al. [1976]), in step with the The Laramidemagmatic gap is usually explainedby a migrationof the triple junction,and movingeastward with changefrom normal(> 30ø dip of downgoingplate) to a the velocityexpected for the subductedridge. shallowlydescending plate, as in modernnonvolcanic Another factor responsiblefor magmaticfields is deep sectorsof the Andes [Snyder et al., 1976; Coney and mantleplumes [Armstrong et al., 1975;Suppe et al., 1975; Reynolds,1977; Dickinson and Snyder,1978; Keith, 1978]. Leemah, 1982a; lyer, 1984; Brandon and Goles, 1988; The closelyrelated theme of controlof magmaticpatterns Westaway,1989a, 1989b].These are invokednot only for by detailsof absoluteand relativevelocity of convergent their magmaticcontribution but also as significantplate plates and the age and shapeof the subductedplate drivingforces by Matthewsand Anderson[1973] and Smith (changingrate of subduction,migration of flexures, and Sbar [1974]. Someauthors envision a moregeneralized segmentationof the downgoingplate, subductionof large-scalemantle upwellingas important[Thompson, 1972; aseismicridges or fracture zones, growth of accretion Smith, 1978; Eaton et al., 1978; Best and Hamblin, 1978; wedges,etc.) is furtherdeveloped in numerouspapers [e.g., Gough, 1986], and this overlapsconceptually with the slab- Vogt, 1973; Dickinson, 1973, 1979; Cross and Pilger, window and subductedocean ridge explanations,being only 1978, 1982; Coney, 1978;Pilger and Henley, 1979;Dewey, a little more vague as to the specific physical cause of the 1980; Wells et al., 1984; Page and Engebretson, 1984; upwelling. Mantle upwelling is generallyprescribed as the Elston 1984; Glazner and Schubert, 1985; Verplanck and explanationof the bimodal, predominantlybasalt magmatic Duncan, 1987;Guffanti and Weaver,1988]. association closely linked with crustal extension SinceHamilton and Myers [1966] and Atwater [1970], [Christiansen and Lipman, 1972; Leemah, 1982b]. Magma many discussionsinvoke the effectsof the enlargingSan volumes in purely extensional continental regimes above Andreastransform fault system,distributed shear [Lipman et upwelling mantle, apart from areas blamed on deep and al., 1971, 1972; Noble, 1972; Suppe et al., 1975; Stewart, energetic plumes, are generally small. The rhyolite 1978; Zoback and Thompson,1978; Livaccari, 1979;Pilger componentis attributedto extreme fractionationof primary and Henley, 1979; Coney, 1987], relaxed compression basalt or melting of lower crust by heat from the mantle- [Scholz et al., 1971; Thompson, 1972, Stewart, 1978; derived basalt. As the systemwanes nodule-bearing basanite Coney, 1987], and the enlargingslab windowcreated by becomes common [Smith and Luedke, 1984; Wilshire and changefrom subductionto strike slip along the Pacific Shervais, 1975]. 13,216 ARMSTRONGAND WARD: CENOZOIC MAGMA'rISM IN THENORTH AMERICAN CORDfl.! -•RA

Propagating fractures and lineament controls have been Arizona into Mexico) and most of the thick-skinned, advocatedas an alternative to plume tracesby Smith [1978], basement-upliftstructures in the Eastern Rocky Mountains Christiansen and McKee [1978], and Lipman [1980], but (betweenMontana and New Mexico). Magmatismdeclined many deeply rooted fractures are not associated with to a minimumat aboutthe Cretaceous-Tertiaryboundary. magmatismand fracture alone would seem inadequateas the During the early and middle Eocenea distinctmagmatic cause of a thermal perturbation capable of generating large culmination,peaking at 50 Ma, formed the Kamloops- amounts of magma. Challis-Absaroka belt north of 42 ø. Its sout//ern end curved Thick crust, inherited from Mesozoic orogenesis, was into cratonicparts of Montana,mimicking a Late Cretaceous suggested as one of the controls on extension and core to Paleocenemagmatic trend. Along the continentmargin complex formation by Armstrong [1972] and recently by an oceanic basaltic island chain formed and was Coney and Harms [1984], Coney [1987], Brown and synmagmaticallyaccreted in Oregon, Washington, and Journeay [1987], Malavielle [1987], and Gaudemer et al. southernBritish Columbiaand magmatismoccurred near the [1988]. This can be only a partial explanation for core edge of the continent from the foothills of the Cascadesin complex location and nucleation. It provides a simple Washington to Alaska. In Colorado and the Black Hills explanationfor locally higher crustal temperaturesand local region and in southernArizona, Laramidemagmatic belts forces favorable to extension. But a magmatic trigger and waned and shifted somewhateastward during this time suitable regional stressesare also necessary to explain the interval. association we observe between magmatism and core During late Eoceneto late Oligocenetime, magmatic complexesand their large strains. activity in the north shifted into Cascadearc and Great Basin The interplay of magmatism and crustal extension is fields. Severalcontinent margin magmaticfields persisted emphasizedin recent discussionsof the geophysicalimages from Vancouver Island northward, and inland there was of extensionalorogens [Klemperer eta/., 1986; Potter et al., scatteredbasaltic magmatism. Magmatic fields in Arizona- 1986; Allmendinger et al., 1987; McCarthy et al., 1987; New Mexico and the San JuanMountains regions expanded McCarthy and Thompson, 1988; de Voogd et al., 1988; and mergedalong the future trendof the Rio Granderift. The Valasek et al., 1989; Thompson and McCarthy, 1990]. large southern and northern magmatic fields were almost Magma in these interpretationsplays multiple roles: space connectedby scatteredmagmatic centersacross the Colorado filling and magma inflation of the lower crust solvesvolume Plateau. and isostatic problems, and recently eraplaced horizontal In latest Oligocenethrough most of Miocene time the most intrusive sheets may be the lower crust layers imaged as notablemagmatic development was the mergingof the large horizontal reflectors of seismic energy. A flat Moho, northern and southernmagmatic fields into one continuous apparentlyundisturbed beneath intensely faulted upper crust, and vigorous magmatic belt extending from Mexico to is a puzzling observationin all the extensionalorogens that Canada. Farther north, magmatism occurred in scattered have been studied. The lower crust must be extremely fields of hot spot or within-plate character and in the ductile, and what more suitable material to redistribute by Wrangell arc. The flood basalt episode of the Columbia flow, than magma, or rocks softenedby injection of magma? Plateau occurred, the Snake River Plain magmatic This carries the view of conditions in the roots of core transgressionbegan, and the Cascadearc persistedin place. complexes a step beyond the recognition that lower crust Magmatismrapidly spreadover much of California southwest strengthis reducedby heating. When magmaaccumulates in of the San Andreasfault, ColoradoPlateau activity declined abundance in the lower crust its strength must virtually and Rio Granderift activity becamemore focusedon modem disappear. de Voogd et al. [1986] present evidence that patterns. After the mergerof the two major magmaticfields, magma sheetsare presenttoday in the Death Valley region, about22 Ma ago, there has been both a generaldecline in where the youngest ductile extension structures are found. the amountof magmaticactivity and a changein most areas Weak lower crust is a conclusion dictated by core complex to the bimodal, predominantly basaltic, petrochemical' geometry [Block and Royden, 1990] and seismic reflectors association. in the crust and mantle IReston, 1990]. Time relationshipssuggest a link betweenmerging of the two large magmatic fields, the consequenttectonic and SUMMARY AND CONCLUSIONS magmaticeffects in the Great Basin, and the subsequent eruption of the basalts of the Columbia Plateau. The The complex and changingpattern of magmaticevolution northwardpropagation of acceleratedrifting and changein of the North American Cordillera requires a multifaceted magma petrochemistry reached the northwestern United explanation. In this paper we have presentedevidence, in Statesjust at the momentthe voluminousbasalt eruptions the form of maps showing the localities where isotopic began(17 Ma). The plateaubasalt event cannotbe the cause dating has been done, and from that data, and extensive of the other phenomenabecause it is the last to occur. If it literature review, we have outlined the magmatic fields that has an independentorigin (e.g., plume breakout[Morgan, existed during four successivetime intervals (55-40, 40-25, 1981; Richards et al., 1989; Loper, 1989] or meteorite 25-10, and 10-0 Ma). We have not attemptedto explain impact [Alt et al., 1988]) the timing must be pure thesepatterns in terms of a specific plate motion model, nor coincidence. We cannot explain how the northward have we dealt with any of the detailed questions of propagationof new tectonic and magmaticpatterns could petrogenesis. Our emphasishas been on the associationof cause, or trigger, a localized episode of plateau basalt magmatism and metamorphic core complexes and the volcanism. We only observe the sequenceof events and implications of that association. offer that observationas a topic for futurespeculation. Early Cenozoic magmatic patternswere inherited from the Since late Miocene time, magmatismhas been much like- late Mesozoic. After approximately 80 Ma the continuous modempatterns. In the north, the Wrangellarc has shrunk magmatic belt that lay parallel to the continent margin into Alaska. Several continentmargin and within-plate broke up into two belts, separatedby a Laramidemagmatic fields have waxed and waned. Small-volumehot spot and gap that linked California to the cratonic interior of North back arc predominantlybasaltic magmatismhas occurred America. Orogenic compressioncontinued between Mexico over much of southern British Columbia. The Cascade arc and Alaska until 55 Ma, creatingthe last stagesof the fold persistedfrom Canada to northern California, while back arc and thrust belt (from Montana northward and from southern activity in the United States has diminished to little more ARMSTRONGAND WARD: CENOZOIC MAGMATISM IN THENORTH AMERICAN CORDn I.ERA 13,217 than the easternplateaus of Oregon. The Snake River Plain growth and magmatic breakup of western North America, has continued to expand eastward. Magmatic activity has interacted. Miocene tectonics were not simply a tended to be localized on the margins of the Basin and consequenceof the growing transform system and triple Range and Colorado Plateau provinces. In southwestern junction migration. Tectonic models that focus on the California a continent margin magmatic belt has moved developmentof the San Andreas system as the singular northwestwardto The Geysers-ClearLake area. explanationof continentalmargin geology are missinghalf Since nearly 60 Ma ago, the developmentof metamorphic the story! core complexes has followed the locus of the largest voluminous intermediate compositionmagmatic fields. The Acknowledgments. The majority of data on which this synthesis associationis suggestiveof a close, necessarylink between restshas been producedby a few highly productivegeochronometry magmatismand crustal extensionof sufficient magnitude to labs over two decades, namely, the Menlo Park, Denver, and producethe structuresassociated with thesecomplexes. The Washington,D.C., laboratoriesof the U.S. GeologicalSurvey; the logical inference is that the emplacementof magma into the Ottawa laboratory of the Geological Survey of Canada; and crust raises crustal temperatures, and, moreover, its very university laboratoriesat Berkeley, Columbia, Arizona, Yale, Texas, presence lowers the strength of the crust. Thus existing Oregon State, and Ohio State, in the United States, and British stressesare allowed to deform the crust. Magmatism thus Columbia,Alberta, and Queens,in Canada. We owe specialthanks to Dick Marvin for his role in compiling U.S. data into the RADB. explains the location and timing of core complex formation. W. H. Mathews, Jamie Allen, H. M. Iyer, Mary Lou Zoback, John The stressesmay be inherited (magmaticallyor orogenically Bartley, Felix Mutchler, and Peter Lipman reviewedthis manuscript thickenedcrust) or immediately created (magmatic inflation, and provided many helpful comments. We thank Jan Ashdown for uplift above anomalousmantle) or imposed by the global typing the long referencelist. Researchin geochronometryat the pattern of plate motions and driving forces (the logical University of British Columbia is funded by a Canadian Natural recourse when local explanations are inadequate). The Sciencesand EngineeringResearch Council operatinggrant to R. L. interplay of magmatismand crustal extensionis emphasized Armstrongand by grants and contractswith the GeologicalSurvey in numerouspapers cited above. Since the Miocene, the of Canada and British Columbia Ministry of Energy, Mines and PetroleumResources. G. Caldwell startedthe processleading to this rates of magmatism, extension, and core complex formation paper by asking R. L. A. for a talk on Cordilleran magmatismand have declined. The thickened and magma-impregnatedcrust its implications for the Western Interior Basin. We did not realize of earlier Cenozoic time has now collapsed, one of the at that time what the subsequenteffort would lead us to. Ideas in extensionaldriving forces is neutralized by the collapse, and this paper were stimulated by participation in the Geological the crusthas becomestronger. The modem Basin and Range Society of America Penrose Conference on Metamorphic Core Province is not a suitable model for the situation that Complexes(1987, Elko, Nevada), but many of the issueshave been existed during local magmatic culminations. There must festering during years of work on core complexesand regional have been a much shallower brittle-ductile transition in the geochronometry. crust of the developing core complexes than exists today. Our sympathies for a theological model for core complex times would thus lie more with the model of a brittle zone Aldrich, M.J., Jr.,C.E. Chapin, and A.W. Laughlin, Stresshistory overlying a ductile zone in the crust (pure shear models of and tectonic developmentof the Rio Grande rift, New Mexico, J. Thompsonand Burke [1974] and Eaton [1980]) (graphically Geophys. Res.,91, 6199-6211, 1986. displayedby Davis and Lister [1988] as a melting chocolate Allmendinger, R.W., and T.E. Jordan, Mesozoic evolution, bar (MarsTM) in a frying pan) than with the idea that discrete hinterlandof the Sevier orogenic belt, Geology, 9, 308-313, 1981. low angle faults penetratethe entire crust to root in mantle Allmendinger,R.W., K.D. Nelson, C.J. Potter, M. Barazangi,L.D. asthenosphere(simple shear model of Wernicke [1981, Brown, and J.E. Oliver, Deep seismicreflection characteristics of 1985]. Wernicke [1990] now also favors a large degree of the continentalcrust, Geology, 15, 304-310, 1987. compensationwithin weak crust. Alt, D., J.M. Sears, and D.W. Hyndman, Terrestrial maria: The The singular event of early Miocene time, the merging of origins of large basalt plateaus,hotspot tracks and spreading magmatic fields, explains several disconnectedobservations. ridges, J. Geol. 96, 647-662, 1988. The time and place of the hyperextension episode of the Anders, M.H., J.W. Geissman, L.A. Piety, and J.T. Sullivan, Colorado River corridor are explained by the coincidenceof Parabolic distribution of circumeastern Snake River Plain magmatism and the release of a tectonic constraint, the seismicityand latest Quaternaryfaulting: Migratory patternand associationwith the Yellowstonehotspot, J. Geophys.Res., 94, thermal destruction of the Laramide magmatic gap. This 1589-1621, 1989. eliminated the cooler and consequentlystronger lithosphere Anderson,J.L., A.P. Barth, and E.D. Young, Mid-crustalCretaceous "bridge" that had connected California to central North roots of Cordilleranmetamorphic core complexes,Geology, 16, America for the previous50-60 Ma. The dramaticchange in 366-369, 1988. plate boundary conditions resulted in early Miocene Anderson, R.E., Thin skin distension in Tertiary rocks of reorientationof stresspatterns across much of western North southeasternNevada, Geol. Soc. Am. Bull., 82, 43-58, 1971. America. California was freed to move rapidly westward as Armentrout,J.M., Cenozoic stratigraphy,unconformity bounded an independent microplate. Within western California this sequences,and tectonic history of southwesternWashington, in, precipitated dramatic tectonic movements - rotation of SelectedPapers on the Geologyof Washington,edited by J.E. Schuster,Bull. Wash. Div. Geol. Earth Resour., 77, 291-320, TransverseRanges crustal blocks, rapid basin development, 1987. and a late episode of subduction in the northern Coast Armstrong,R.L., Geochronologyof Tertiary igneousrocks, eastern Ranges. The merger of magmatic fields and the magmatic Basin and Range province, western Utah, easternNevada, and flareup in southern California came before the onset of vicinity, U.S.A., Geochim. Cosmochim. Acta, 34, 203-232, transform faulting in southwesternCalifornia [Pilger and 1970. Henley, 1979; Tennyson, 1989]. The rapid westwardmotion Armstrong,R.L., Low-angle (denudation)faults, hinterland of the of California coincidedwith developmentof the San Andreas Sevier orogenicbelt, easternNevada and western Utah, Geol. Soc. transform system. Northward migration of the southern Am. Bull., 83, 1729-1754, 1972. magmatic field may have been linked to the northward Armstrong,R.L., Geochronometryof the Eocenevolcanic-plutonic episodein Idaho, Northwest Geol., 3, 1-15, 1974a. migration of the Mendocino triple junction, but the Armstrong, R.L., Magmatism, orogenic timing, and orogenic southward migration of the northern magmatic field was diachronismin the Cordillera from Mexico to Canada, Nature, certainly not. The two plate tectonic processes,transform 247, 348-351, 1974b. 13,218 ARMSTRONGAND WARD:CENOZOIC MAGMA'HSM IN 'rite NORTHAMERICAN CORDII I ERA

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