TECTONICS, VOL. 11, NO. 4, PAGES823-835, AUGUST 1992

UPPER JURASSIC-LOWER CRETACEOUS Wrangellia,and Peninsularterranes, collectively referred BASINAL STRATA ALONG THE to as the AWl:' (Figure 1), comprisethe latter group. CORDILLERAN MARGIN: IMPLICATIONS Mid-Cretaceouscrustal shortening and high-pressure metamorphisminvolving the AWP and inboardterranes FOR THE ACCRETIONARY HISTORY OF are cited by manyworkers as evidencefor AWP THE ALEXANDER-WRANGE LLIA- accretion[Davis et al., 1978;Coney et al., 1980;Csejtey PENINSULAR TERRANE et al., 1982;Monger et al., 1982;Saleeby, 1983; Jones et al., 1986; Crawford et al., 1987; Brandon et al., 1988]. In WilliamC. McClelland1 and George E. Gehrels contrast,arguments based on faunal and stratigraphic Departmentof Geosciences,University of Arizona, similaritiesand/or distributionof plutonicand volcanic Tucson arc rocks across terranes of the Canadian Cordillera have been usedin modelsfavoring pre-Early Cretaceous[e.g., JasonB. Saleeby Kleinspehn,1985; Armstrong, 1988] or pre-Late Jurassic Divisionof Geologicaland PlanetarySciences, California [e.g.,Anderson, 1976; Jeletzky, 1984; Tipper, 1984;van Institute of Technology,Pasadena der Heyden,1992] accretionof the AWl'. Additional modelspropose a Late Jurassicand/or Early Cretaceous migratingor zipperingsuture [e.g., Armstrong,1988; Wernicke and Klepacki,1988; Pavlis, 1989]. Abstract. Upper Jurassicand Lower Cretaceous bashal strata are preservedin a discontinuousbelt along Gehrelsand Saleeby[1985] suggested that a belt of discontinuoustranstensional basins containing Upper the inboardmargin of the Alexander-Wrangellia- Jurassicand Lower Cretaceousfiyschlike clastic and Peninsularterrane (AWP) in Alaskaand western Canada,on the outboardmargin of terranesin the volcanicstrata evolved along the Cordilleranmargin from southernAlaska to at least southernCalifornia during CanadianCordillera accretedto North America prior to Late Jurassictime, and alongthe Cordilleranmargin the northwardmigration of the AWl:' in Late Jurassic- from southernOregon to southernCalifornia. Nearly all Early Cretaceoustime. The followingpaper modifies of the basinalassemblages contain turbiditic strata and expandson theseearlier ideasby summarizingthe depositedbetween OxfordJan and Albian time. Arc-type age and stratigraphiccharacter of the basinal volcanicrocks and abundantvolcanic detritus in many of assemblagesobserved along the Cordilleranmargin and the Middle Jurassicto mid-Cretaceoustectonic setting in the assemblagessuggest deposition within or adjacentto a coevalarc complex. On the basisof the general whichthey evolved.The ensuingdiscussion presents a similaritiesbetween the basinalsequences, we propose generalmodel for the evolutionof the marginalbasins and accretionaryhistory of the AWP. that theyrecord involvement of the AWP in the Late Jurassic-EarlyCretaceous evolution of the Cordilleran margin. A geologicallyreasonable scenario for the PRE-LATE JURASSIC EVOLUTION OF THE accretionof the AWP includes(1) Middle Jurassic ALEXANDER-WRANGELLIA-PENINSULAR accretionto the Cordilleranmargin, in particularthe TERRANE Stikine and Yukon-Tanana terranes, in a dextral transpressionalregime, (2) Late Jurassic-Early The Alexanderand Wrangelliaterranes (Figure 1), Cretaceous overall northward translation of the AWP originallydefined by apparentdifferences in their and evolution of a series of transtensional basins within a Paleozoicto Late Triassichistories [Jones et al., 1•972, complexdextral strike-slip system along the Cordilleran 1977],were adjacentto one anotherby Pennsylvanian margin,and (3) mid-Cretaceousstructural imbrication of time (308 _ 6 Ma) [Gardneret al., 1988]. The earliest the AWP and inboard terranes that either terminated or demonstrableties betweenthe Peninsularand Wrangellia resultedin a changein the characterof depositionin the terranesare Late Triassicin age [Plafker et al., 1989]. marginalbasins. Mid-Cretaceous deformation along the The Paleozoicbasement history of the Alexanderterrane, inboardmargin of the AWP was broadlysynchronous whichis the most completeof the AWP triad, indicates with contractionaldeformation throughout the Cordillera that the crustalfragment evolved as an islandarc and mostlikely due to changesin subductionzone complex,perhaps in proximityto the Gondwanaorogenic parametersalong the Cordilleranmargin, outboard of the systemprior to Devoniantime [Gehrelsand Saleeby, AWP, rather than collisionof the AWP. 1987]. Nd isotopicstudies suggest an intraoceanic environmentfor the Alexander-Wrangelliaterrane INTRODUCTION throughLate Triassictime [Samsonet al., 1989, 1990]. Paleomagneticdata from Wrangellia[Hillhouse and Terranesof the CanadianCordillera [e.g., Coney et Gromm6, 1984]and the Alexanderterrane [Haeussleret al., 1980]are typicallydivided into (1) fragmentsaccreted al., 1989]suggest that the AWP was located to the continentalmargin prior to Late Jurassictime and approximately10 ø to 20ø north or southof the (2) fragmentsstructurally imbricated with inboard paleoequatorduring Late Triassictime. Assuminga terranesduring mid-Cretaceous (Albian-Cenomanian) northernLate Triassicpaleolatitude, paleomagnetic data time [e.g.,Monger et al., 1982]. The Alexander, for southernWrangellia (Vancouver Island) are not discordantwith respectto North America[May and Butler, 1986]. There are, however,no rigorous 1Nowat Departmentof Geological Sciences, constraints on the choice of northern versus southern Universityof California,Santa Barbara. hemisphereLate Triassicposition for the AWP. The southernoption is favoredon the basisof reported Copyright1992 differences between Late Triassic AWP and North by the American GeophysicalUnion. Americanfauna [Tozer, 1982;Newton, 1983; Silberling, Paper number92TC00241. 1985]. Placingthe AWP in an equatorialeastern Pacific 0278-7407/92/92TC-00241$10.00 positionwithin the faunalrealm of the SouthAmerican 824 McClellandet al.: BasinalStrata on the CordilleranMargin margin(southern hemisphere) in Late Triassictime is North Americafor Wrangelliaduring Early Jurassictime consistent with these differences. [Tayloret al., 1984;Smith and Tipper, 1986]. On the Comparisonof Pliensbachianammonites suggests a basisof faunaland stratigraphicsimilarities between northernhemisphere position and latitudinal Callovianbasinal strata observed from Wrangelliato the displacementof approximately2400 km with respectto eraton,Tipper [1984]and Jeletzky[1984] arguedthat

Alaska I/

HinesC•reek fault /

Denall fault

NUTZOTIN TK

Tintina fault - N R M T

YTN o 400

km RangesBorder Yukon fault

DEZADEASH

QSK BOWSER Duncan Canal BASIN shear zone

GRAVlNA British Columbia

YTN WHITESAIL LAKE &T REGION i i

BASlNAL ASSEMBLAGES Upper Jurassic & Lower Cretaceous QCl marine clastic strata and volcanic rocks (KH = Kahiltna basin) -TYAUGHT•N AWPW = Wrangellia;TERRANE P (A= Peninsular)=Alexander; t LATE JURASSIC-TERTIARY VI ETHOW

ACCRETIONARYINCLUDES: ChugachCOMPLEXES terrane in Alaska ,Fraser'- Straight & FranciscanComplex in California-Oregon SAN JUAN '-' ',Creek fault

JURASSIC-CRETACEOUS ARC ISLANDS

COMPLEXESINCLUDES: Goodnews,INWESTERN Koyokuk,ALASKANyac, CASCADES Tikchik, & Togiak terranes • _• BLUE •( MOUNTAINS SKAGITCOAST MOUNTAINSTERRANE BATHOLITHAND MVRTC RGtON TERRANES IN THE SOUTHERN COAST AND NORTHERN CASCADE MOUNTAINS INCLUDES: Bridge River, Cadwallader, Chilliwack, Harrison, & Shuksan terranes

PRE-LATE JURASSIC ACCRETIONARY COMPLEXES OF CACHE CREEK AFFINITY (CC = Cache Creek terrane) SIERRA GREAT OTHER INBOARD TERRANES: VAL D = Dillinger;M = McKinley& Windy; NX = Nixon Fork; QSK = Quesnellia,Slide Mountain,Kootenay, & related terranes; S = Stikine; T = Taku; YTN = Yukon-Tanana & Nisling

Fig. 1. Generalizcdtcrrane map of the westernCordillera showing the distributionof the Upper Jurassic-LowerCretaceous basinal assemblages and geographicregions discussed in the text. Figureis modifiedafter Joneset al. [1987],Wheeler et al. [1988],and Silberlinget al. [1987]. Abbreviationsare NMRT, northernRocky Mountain trench; PR, PrinceRupert; QCI, Queen Charlotte Islands;and VI, VancouverIsland. Other abbreviationsare definedin the legend. McClellandet al.: BasinalStrata on the CordilleranMargin 825

Wrangelliawas near its presentposition relative to Deformationof similarage may be recordedby the cratonalNorth Americaby late Middle Jurassictime. Middle JurassicKotsina conglomerate in the Wrangellia Paleomagneticdata from Upper Jurassicclastic strata and Peninsularterranes in southernAlaska [Plafker et (NaknekFormation) indicate 3500 (_ 1000) km of post- al., 1989]. Jurassic northward translation for the Peninsular terrane Middle Jurassic deformation and terrane accretion [Stoneand McWilliams,1989] using the referencepole of alongthe Cordilleranmargin marked the demiseof a Gordon et al. [1984]. Recalculatingthe estimateusing west facinglate Paleozoic-EarlyJurassic fringing arc the GlanceConglomerate reference pole [May and complexrepresented by disruptedarc and oceanic Butler, 1986]and accountingfor possibleinclination error assemblagesof CacheCreek affinity(Figure 1) and inducedduring deposition and compaction[Coe et al., generallyinboard island arc sequences(McCloud belt) 1985]yields nearly concordant results. Thus significant [Miller, 1987,and referencestherein]. The pre-Middle post-Jurassiclatitudinal translation of the Peninsular Jurassicposition of the Stikineterrane relative to rocks terranerelative to North America is permissiblebut not of CacheCreek affinityis uncertain(compare Wernicke required. From the constraintsoutlined above, we and Klepacki[1988] and Oldow et al. [1989]). We conclude'thatthe AWP was most likely positionedin the assumethat it occupiedits presentposition relative to easternPacific basin, south (•30') of the North inboardterranes by Late Jurassictime. On the basisof Americanmargin, during Late Triassictime and moved evidence for imbrication of the Alexander and Yukon- northwardto near its presentposition with respectto Tananaterranes and widespreaddeformation within the cratonalNorth Americaby Late Jurassictime. AWP, we concludethat the AWP was involvedin Middle Jurassicdeformation and accretionobserved along the MIDDLE JURASSIC TECTONISM ALONG THE Cordilleranmargin. CORDILLERAN MARGIN UPPER JURASSIC-LOWER CRETACEOUS BASINAL Middle Jurassicaccretionary events in the Canadian ASSEMBLAGES Cordillerainclude (1) eastdirected thrusting of the Quesnellia,Yukon-Tanana and Slide Mountain terranes Variably deformedUpper Jurassic-LowerCretaceous over cratonalNorth America [Price, 1981;Monger et al., basinalassemblages are preservedalong the AWP suture 1982;Brown et al., 1986;Armstrong, 1988], (2) obduction zone from southern Alaska to southern British Columbia of the Cache Creek terrane onto Stikine and Quesnellia and alongthe Cordilleranmargin at leastas far southas [e.g.,Mortimer, 1986;Cordey et al., 1987],and (3) the Klamath-SierraNevada region (Figure 1). Individual amalgamationof the Stikine,Bridge River, and assemblagesdiffer in pre-UpperJurassic basement rocks, Cadwalladerterranes [Rusmore et al., 1988] (Figure 1). provenance,and detailedstratigraphy, but a common Althoughstrongly modified by late Mesozoicand evolutionand tectonicsetting are inferred from the Cenozoictectonism, all terranes inboard of the AWP in generalsimilarity in age and lithologiccharacter. the CanadianCordillera appear to be tied to North Americawith no interveningsubduction zones by Late Alaska-Northwestern Canada Jurassic time. Terranes outboard of North American basementin Blue Mountainsregion of northeastern SouthernAlaska. The Kahiltnaterrane (Figure 1) Oregonand westernIdaho (Figure1) were reportedly includesKimmeridgian to Valanginianvolcaniclastic amalgamatedduring or prior to Late Jurassictime [e.g., turbiditesof the KoksetnaRiver sequence(Figure 2a) Av6 Lallemant,1992] and accretedto North America [Wallaceet al., 1989]and clasticstrata as youngas duringEarly Cretaceoustime [Lurid and Snee, 1988]. By Cenomanianthat are structurallyintermixed with small analogywith correlativeterranes in the Canadian blocksof Triassicand older strata of uncertainorigin Cordillera(see reviewby Oldowet al. [1989]),we [Joneset al., 1982]. The KoksetnaRiver sequence assume that these terranes were accreted to North apparentlywas derived from and depositedon the Americaby Late Jurassictime. Early to mid-Cretaceous Peninsularterrane [Wallace et al., 1989]. At its southern (118-88Ma) deformationalong the SalmonRiver suture margin,the Kahiltnaterrane is separatedfrom the zone [Lurid and Snee,1988] most likely reflects Wrangelliaand Peninsularterranes by southeastdipping modificationof the originalaccretionary boundary. A mid-Cretaceousthrust faults [e.g., Csejtey et al., 1982; Middle Jurassicarc in the Klamath-SierraNevada region Nokleberget al., 1985]. The Kahiltnaterrane is (Figure 1) was constructedon a diversebasement of currentlyjuxtaposed with the Yukon-Tananaand other previouslyaccreted Paleozoic and Upper Triassic-Lower terranesto the north alongthe Denali, Hines Creek, and Jurassicarc fragmentsand m61angecomplexes (see Chilchitnafaults (Figure 1). Depositionalties between reviewsby Burchfielet al. [1992]and Saleebyand Busby- the Kahiltna basin and the northern terranes have not Spera[1992]). The arc complex,active from been documented.Stanley et al. [1990]suggested that approximately177 to 159 Ma, was deformedduring a the Wrangelliaand Kahiltnaterranes were underthrust west-vergentcontractional event between 169 and 161 Ma beneaththe Yukon-Tananaterrane to the north during [Wright and Fahan, 1988]. mid-Cretaceoustime. Significantpre-Late Cretaceous Middle Jurassic deformation in the Alexander terrane (pre-95Ma) displacementalong the Hines Creek fault is recordedby a dextraltranspressional shear zone [Wahrhaftiget al., 1975]indicates that mid-Cretaceous (DuncanCanal shearzone of McClellandand Gehrels contractionaldeformation was accompaniedby dextral [1990])in centralsoutheastern Alaska (Figure 1) and strike-sliptranslation. imbricationof the Alexanderterrane and metamorphic Eastern Alaska. OxfordJan to Barremian volcaniclastic rocksequivalent to the Yukon-Tananaterrane in turbiditesof the Nutzotin Mountainssequence in eastern southernsoutheastern Alaska [Saleebyand Rubin, 1990]. Alaska(Figures 1 and2b) unconformablyoverlie and Southwestdirected Middle Jurassic(Aalenian-Bajocian) were in part shedfrom Wrangellia[Berg et al., 1972]. compressionaldeformation of Wrangelliais observedon Theserocks, apparently deposited in a northeastward the Queen CharlotteIslands [Lewis et al., 1991]. deepeningbasin, are overlainby Lower Cretaceousarc 826 McClelland et al.' BasinalStrata on the CordilleranMargin

ALASKA - NORTHWESTERN CANADA SOUTHERN COA ST MTNS. (a) (b) (c) (d) (e) (f) (g)

CENOMANIAN

ALBIAN

APTIAN ? R'•'•'"•BA

HAUTERIVIAN

VALANGINIAN

BERRIASIAN

TITHONIAN KIMME•

OXFORDIAN

CALLOVlAN w CT

SAN JUAN ISLANDS KLAMATH-SIERRA REGION (h) (i) (j) (k) (I) (m)

91 I I 7.5 CENOMANIAN

113 ALBIAN 7:: APTIAN/ EXOTIC BARREMIAN SLIDE BLOCKS ,:'.:.::/i 9 131HAUTERIVlAN VALANGINIAN BERRIASIAN T THONIAN

KIMMERIDGIAN 156-152 I F'••'"•R•ox • N 163 CALLOVIAN

MID-JURASSIC & OLDER ARCS & ACCRETIONARY COMPLEXES

DOM I NANTLY ASSEMBLAGE :??::.•CLASTICTURBI DITES ROCKS, I INTRA-ARC OPHIOLITIC• UNCONFORMITY

VOLCANICROCKS, DOMINANTLY&VOLCANICLASTIC ARC-TYPE J•J TUFFACEOUS PELAGICROCKSANDCHERT Fig.2. Generalizedcolumns compiling known or inferredage (time scale of Palmer[1983]) and basementof UpperJurassic-Lower Cretaceous clastic and arc-type volcanic rocks in the (a) Kahiltnaterrane, (b) NutzotinMountains sequence and Chisana Formation, (c) Dezadeash Formation,(d) Gravinabelt, (e) Tyaughtonbasin, (f) Methowbasin, (g) HarrisonLake region (GamblerGroup), (h) Decaturterrane, (i) ConstitutionFormation, (j) SpiedenGroup, (k) Galice Formation,(1) Mariposa Formation, and (m) GreatValley sequence. See Figure 1 for locations and basementterrane abbreviations. Columns adapted from Arthur [1986]-g;Berg et al. [1972]- b, c, d; Brandonet al. [1988]-h,i, j; Eisbacher[1976]-c; Garver [1988a, 1989]-e, h; Harperand Wright[1984]-k; Hopson et al. [1981,unpublished data, 1992]-m; Johnson [1981]-j; McClelland et al. [1992a]-d;McGroder et al. [1990]-f;Saleeby [1986]-1, m; Sharp[1988]-1; Wallace et al. [1989]- a; and other referencescited in text.

volcanic and volcaniclastic rocks of the Chisana similar to that observed in southern and southeastern Formationthat may recordeast directed subduction Alaska. Stratigraphicrelationships between the beneaththe outboardmargin of the AWP [Berg et al., DezadeashFormation and inboardterranes (Yukon- 1972;Barker, 1988]. Similarbut unfossiliferousvolcanic Tanana)have not beendemonstrated. and volcaniclasticstrata along the Denali fault mark the SoutheasternAlaska. The Gravina belt in northwesterncontinuation of this belt [Nokleberget al., southeasternAlaska (Figure 1) includesOxfordJan to 1985]. Albian(and possibly Cenomanian) argillite, volcanic- The DezadeashFormation, exposed northeast of the lithicgraywacke turbidites, and minorconglomerate Denalifault in Yukon(Figure 1), comprisesOxfordian to (Figure2d) [Berget al., 1972;Brew and Karl, 1988; Valanginianargillite and graywacke turbidites (Figure 2c) Gehrelset al., 1992]. Mafic volcanicrocks are that were derived from a source area to the southwest dominantlyEarly Cretaceous in age [e.g.,Brew and Karl, and are likelycorrelative with the NutzotinMountains 1988;Gehrels et al., 1992]but are in part of probable sequence[Eisbacher, 1976]. The Dezadeashstrata are Late Jurassicage [e.g.,Rubin and Saleeby,1991b; structurallyoverlain to the northeastby the Kluaneschist McClellandet al., 1992a]. Berg et al. [1972]and most whichmay havebeen in part derivedfrom the subsequentworkers concluded that the Gravinabelt Dezadeashturbidites. We infer that metamorphismand depositionallyoverlies the Alexanderterrane. To the imbrication of the Kluane schist and Dezadeash east,the Gravinabelt is separatedfrom the Yukon- Formation are related to mid-Cretaceous deformation Tanana,Stikine, and Taku terranesby east dippingmid- McClellandet al.:Basinal Strata on the CordilleranMargin 827

Cretaceousthrust faults [Gehrelset al., 1990, 1992; Southern Coast Mountains, Cascades,and San Juan Rubin et al., 1990;Rubin and Saleeby,1992; McClelland Islands et al., 1992b]. Gravinastrata reportedly depositionally overliethe Taku terrane[Rubin and Saleeby, 1991a] and Upper Jurassic-LowerCretaceous strata in the detritalzircon studies suggest that the Gravinabelt southernCoast Mountains,northwest Cascades, and the receiveddetritus from and thereforemay have San Juan Islandsare generallycoeval but vary in depositionallyoverlain the Yukon-Tananaand Stikine stratigraphyand basementcharacter. They are divided terranes[Gehrels and Greig, 1991]. into (1) the Tyaughton-Methowbasin which is likely tied The Gravina-Nutzotin belt is inferred to have been to the Stikineand inboardterranes, (2) strataassociated depositedin an intra-arcbasinal setting [Berg et al., with varied Middle Jurassic and older arc terranes in the 1972;Rubin and Saleeby,1991b; McClelland et al., southernCoast Mountainsand northern Cascades,and 1992a]. Late Jurassicand Early Cretaceousplutons (3) a westernassemblage associated with disrupted representingintrusive components of the arc system complexesand Middle-LateJurassic ophiolitic rocks in intrudedthe Wrangelliaand Alexanderterranes in the SanJuan Islands and westernCascades (Figure 1). southeasternAlaska [Berg et al., 1972;Gehrels and Berg, Tyaughton-Methowbasin. The Tyaughtonand 1988]. In southernAlaska and Yukon, the arc is Methow basins,offset approximately 100 km by dextral expressedas the Tonsina-Chichagofbelt [Hudson,1983] displacementon the Fraser-StraightCreek fault system and SaintElias suite[Dodds and Campbell,1988]. (Figure 1) [Priceet al., 1985;Kleinspehn, 1985], include Tithonian-Valanginiancomponents of the Chugach OxfordJanto Albian volcaniclasticsandstone, siltstone, terraneincluding the McHugh Complexin southern argillite, and conglomeratewith minor andesiticvolcanic Alaska are preservedoutboard of the AWP as scattered rocks(Figures 2e and 2f) [Garver, 1989;Umhoefer, 1989; fragmentsalong the BorderRanges fault (Figure1). McGroderet al., 1990,and referencestherein]. Thesefragments are interpretedas remnantsof the CallovianTyaughton strata apparently depositionally accretionarycomplex associated with the westfacing overliethe Cadwalladerterrane [Rusmore et al., 1988] Gravina-Nuzotinarc [Berget al., 1972;Pavlis, 1982; whichis correlativewith the Stikineterrane [Umhoefer, Gehrelsand Berg,1988; Plafker et al., 1989]. Likely candidates for the northern or western continuation of 1990;Rusmore and Woodsworth,1991]. Oxfordianand the basinalarc complexinclude Late Jurassic-Early youngerstrata in the Methow basinare separatedfrom Cretaceousarc fragmentsin the Togiak,Nyac, and adjacentterranes by the Ross Lake, Hozameen,and Koyukukterranes in westernAlaska (Figure 1) [Box and Pasaytenfaults. Lithologicsimilarities between Lower to Patton,1989; Plafker et al., 1989;Wallace et al., 1989]. MiddleJurassic strata in the Methowbasin (Ladner Central British Columbia. Clastic strata that are Group) and coevalstrata to the east (Ashcroft probablycorrelative with the Gravinabelt are observed Formation)[Monger, 1986; McGroder, 1991] suggest west of the Coast Mountains batholith as far south as depositionof Upper Jurassicand youngerMethow strata the PrinceRupert region(Figure 1) [Woodsworthand on or at least near the accretedQuesnellia, Stikine, and Orchard,1985; Crawford et al., 1987]. Extensionof CacheCreek terranes. Stratawithin the Tyaughtonand these rocks into the batholith at this latitude has been Methow basinswere apparentlyderived from an eastern proposedthrough correlation of the Khutzeymateen sourceuntil Albian time [e.g.,Kleinspehn, 1985] although Groupwith the Gravinabelt [Douglas,1986]. This Jeletzkyand Tipper [1968]suggested that Hauterivian correlationis tenuoussince quartzite observed in this stratain the Tyaughtonbasin were in part westerly sequenceis characteristicof rocksequivalent to the derived. Clasticrocks in the westernTyaughton basin Yukon-Tananaterrane in the PrinceRupert region [M. interfingerwith Lower Cretaceousvolcanic rocks L. Crawford,personal communication, 1988; Gareau, indicatingthat the Tyaughtonbasin may have been 1991] rather than the Gravina belt. Late Jurassicand boundedby a volcanicarc to the west [Garver et al., Early Cretaceousplutons representing the southern 1988]. A major changein Tyaughton-Methow continuation of the Gravina arc are observed south of sedimentationin Albian time reflectedby a double-sided PrinceRupert [vander Heyden,1992] and on the Queen (west and east) infilling of coarseclastic material CharlotteIslands (Figure 1) [Andersonand Greig, 1989]. [Garver, 1989]was coincidentwith the developmentof Tithonianto Upper Cretaceousclastic strata on the the mid-Cretaceous eastern Cascades fold belt Queen Charlotte Islands are inferred to have been [McGroder, 1989]. depositedin a forearcsetting, west of the Early Southern Coast Mountains and north central Cascades. Cretaceousarc [Haggart,1991; Lewis et al., 1991]. An Mafic and minor felsic volcanicrocks, volcaniclastic influxof Upper Cretaceousconglomerates (Honna graywacke,and argilliteof the Gainbierassemblage Formation)likely reflectsuplift of the CoastMountains (includingthe NooksackGroup) rangefrom Oxfordianto due to mid-Cretaceouscrustal thickening [Higgs, 1990, Albian in age [e.g.,Misch, 1966;Wheeler and McFeely, and referencestherein]. 1987;Woodsworth and Monger, 1992]. Gambler strata Middle Jurassic(Bathonian) to Cenomanianmarine in the southern Coast Mountains and northern Cascades and nonmarine clastic and minor volcanic rocks are occuras fault slicesin a west-vergentmid-Cretaceous preservedeast of the Coast Mountainsin the Bowset thrustsystem [e.g., Misch, 1966;Brandon et al., 1988; Basin(Figure 1) [Evenchick,1991, and references Journeay,1990]. Oxfordianto Valanginianvolcaniclastic therein]. This sequencewas likely depositedin a rocks and Lower Cretaceous volcanic rocks west of foredeepformed in responseto Middle Jurassicwest HarrisonLake (Figure2g) [Arthur, 1986;Monger, 1989] directedemplacement of the Cache Creek terrane on the stratigraphicallyoverlie the Middle JurassicHarrison Stikineterrane [e.g., Eisbacher, 1985]. Bathonianto Lake Formationwhich is likely correlativewith strata in Kimmeridgianand Hauterivianto Albian volcaniclastic Wrangellia[Friedman et al., 1990]. Hauterivian- and volcanicrocks along the westernmargin of the Barremianarc-type volcanic and volcaniclasticrocks, BowserBasin [Anderson, 1989; Bassett, 1991] may record largelypreserved as pendants,are widespreadacross the proximityof the basinto the Late Jurassic-Early CoastMountains [Woodsworth and Monger, 1992]. Cretaceousarc developedon the AWP to the west. Gainbier strata in the east central Coast Mountains are 828 McClellandet al.:Basinal Strata on the Cordilleran Margin juxtaposedagainst the Stikine terrane along east-vergent a complexdextral strike-slip fault systemactive in the Late Cretaceousthrust faults [Crawford et al., 1987; forearcregion of the Early Cretaceousarc alongthe Rusmore and Woodsworth,1989; van der Heyden, 1992] Cordilleranmargin [e.g., Rusmoreand Cowan,1985; suchthat primarystratigraphic relationships between the Brandon,1989]. Stikine terrane and Gambler assemblageare uncertain. Oxfordian to Hauterivian volcaniclasticstrata, breccia Lower Cretaceous calc-alkaline volcanic rocks in the and conglomerateof the SpiedenGroup were deposited Gambier assemblageand Late Jurassic(165-145 Ma) and within or adjacentto a Late Jurassicarc complex(Figure Early Cretaceous(120-114 Ma) plutons[Armstrong, 2j) [Johnson,1981]. The SpiedenGroup presentlylies 1988; Friedman, 1989;van der Heyden, 1992] in within the footwallof the San Juan thrust system,but Wrangelliamark the southerncontinuation of the correlationof theserocks with Wrangelliais uncertain Gravinaarc. In the WhitesailLake region(Figure 1), [Brandonet al., 1988]. Callovianto Albian clasticrocks Late Jurassicplutons intrude metamorphic rocks likely of Wrangelliaon VancouverIsland (Figure 1) lack derivedfrom the Stikine terrane and thus providean evidencefor coevalarc volcanismand may be in part additional Late Jurassic link between the AWP and correlative with forearc strata on the Queen Charlotte Stikineterranes [van der Heyden,1992]. We infer that Islands[Muller et al., 1981]. Similaritieswith coeval the Gambler arc evolvedalong the easternedge of rocksin the HarrisonLake region[Johnson, 1981] Wrangelliaand westernmargin of the Tyaughton- suggestthat the SpiedenGroup was deposited along the Methow basin on the Stikine and Quesnellia terranes. western flank of the Gambier arc. San Juan Islands. Upper Jurassic-LowerCretaceous strata comprisingthe ConstitutionFormation, Lummi Blue Mountains Region Group, and SpiedenGroup are preservedin thrust slices of the mid-Cretaceous northwest Cascades-San Juan Callovian to Oxfordian turbidites in the Blue thrustsystem (Figure 1). Althoughthese units and their Mountainsregion (Figure 1) overlieMiddle Jurassic basementassemblages are fault-bounded,most are forearcdeposits and older arc assemblagesin the similarto other componentsin the Cordillera. Wallawa(also SevenDevils) and Izee terranes[Vaillet, The Decatur terrane includesthe Fidalgo Complex 1977;Dickinson and Thayer, 1978]. Late Jurassic-Early and Lummi Group (Figure 2h) [Garver, 1988a,and Cretaceousbasinal rocks are absentin this region, referencestherein]. Back-arcophiolitic basement of the presumablydue to Late Jurassicdeformation [e.g., Av6 FidalgoComplex is intrudedby arc-relatedplutons and Lallemant,1992] that was followedby emplacementof overlainby volcanicrocks of Callovianto Oxfordianage Early Cretaceous(144 Ma and younger)arc-related [Brandonet al., 1988; Garver, 1988a]. The volcanic plutons(N. W. Walker, personalcommunication, 1992). sequenceis interlayeredwith and overlainby Oxfordian Widespreaddeposition of Albian to Cenomanian to Tithonian argillite,chert, and volcaniclasticrocks that synorogenicconglomerate reflects mid-Cretaceous are in turn unconformablyoverlain by Tithonianto deformation[Lund and Snee,1988, and references Valanginianvolcaniclastic strata of the Lummi Group therein]. [Garver, 1988a]. The FidalgoComplex and similar ophioliticcomplexes in the westernand central Cascades Sierra Nevada-KlamathMountains Region [Whettenet al., 1980;Miller, 1985]may have evolvedas small pull-apartsegments along transform faults [e.g., Upper Jurassic-LowerCretaceous basinal strata in Brandonet al., 1988]. Direct correlationof the Lummi the Klamath Mountains and western Sierra Nevada Group-FidalgoComplex with the Great Valley sequence- regioninclude the Galiceand Mariposaformations, CoastRange ophiolite(described below) proposedby Great Valley sequence,and Myrtle Group (Figure1). Garver [1988b]implies significant northward translation Theseclastic strata are typicallyunderlain by Middle- of the FidalgoComplex during Hauterivian to Aptian early Late Jurassicophiolites and coevalvolcanic arc time. Similar northwardtranslation has been proposed complexes.In the KlamathMountains, the Galice for blueschist-grademetamorphic rocks of the Shuksan Formationand relatedunits consist of argillite, metamorphicsuite in the Cascades[Brown and Blake, volcaniclasticgraywacke, and minor conglomerateand 1987] and clasticrocks of the westernm61ange belt in chertof late Oxfordian-earlyKimmeridgian to latest the westernCascades [Jett and Heller, 1988]. Jurassicage (Figure2k) [e.g.,Harper and Wright,1984; The ConstitutionFormation consists of Upper Wyld and Wright, 1988]. This sequenceand its probable Jurassic-LowerCretaceous volcaniclastic sandstone, equivalentsdepositionally overlie the 161-164Ma mudstone,chert, and minor pillow basalt(Figure 2i) Josephineand DevilsElbow ophiolites[Saleeby, 1982, [Brandonet al., 1988]. Althoughcurrently fault- 1992;Harper and Wright, 1984;Wright and Wyld, 1986; bounded,the presenceof detritusand slideblocks likely Wyld and Wright, 1988]and possiblythe PrestonPeak shedfrom adjacentterranes in the San Juan Islandsand mafic complex[Snoke, 1977]. In addition,the clastic northernCascades suggests deposition of the sequence rocksinterfinger with and overlie the Rogue arc near the Cordilleranmargin [Brandonet al., 1988]. The sequence[Garcia, 1982]. Oxfordianto lower Constitution Formation is similar to the Pacific Rim Kimmeridgianslate, graywacke, and argillitein the Complexand PandoraPeak unit on VancouverIsland westernSierra Nevadafoothills region (Mariposa [Rusmoreand Cowan, 1985; Brandon,1989], western Formation)typically contain abundant chert and m61angebelt in the westernCascades (see Tabor et al. metamorphicclasts but at leastlocally contain a [1989]for review),Rimrock Lake inlier in the southern significantvolcanic component (Figure 21) [Saleeby,1986; Cascades[Miller, 1989],and Yolla Bolly terrane (part of Sharp,1988]. Theserocks overlie and interf'mgerwith the FranciscanComplex) in southwesternOregon and Callovianto lower OxfordJanvolcanic rocks deposited California[e.g., Blake et al., 1985]. Althoughdistinct in within an extensional arc constructed on Middle Jurassic stratigraphicdetail, most of theseassemblages are and older ophiolitem61ange basement of the Sierran associated with or contain detritus from Late Jurassic arc foothillsand Calaveraschert and argillitem61ange complexes.The m61angesare inferredto have formedin [Saleebyand Busby-Spera,1992, and referencestherein]. McClellandet al.: Basinal Strata on the Cordilleran Margin 829

Upper Tithonian to Valanginianturbiditic strata of the AWP and is similar in some aspectsto modelspresented basalGreat Valley sequenceoverlie OxfordJan to by Saleebyand Gehrels [1988], Brandonet al. [1988], Tithonianpelagic, volcanic, and volcaniclasticrocks and Pavlis[1989], Plafker et al., [1989],Wallace et al. [1989], the 170-160Ma Coast Range ophiolite(Figure 2m) [e.g., van der Heyden [1992],Burchild et al. [1992],Saleeby Hopsonet al., 1981, 1991]. The turbiditesare dominated and Busby-Spera[1992], and others. by chert and metamorphicdetritus shed from the As outlinedabove, paleobiogeographic and adjacentCordilleran margin [Dickinsonand Rich, 1972; paleomagneticdata suggestthat the AWP was Ingersoll,1983; Seiders, 1988]. The Wild Rogue considerablysouth of its presentlocation with respectto ophioliteand overlyingarc-type volcanic rocks and Upper North America in Late Triassictime and migrated Jurassic-LowerCretaceous clastic strata (Myrtle Group) northwardduring Late Triassic-MiddleJurassic time. in southwesternOregon (Figure 1) are similarto the Accretion of the AWP to the Cordilleranmargin prior to CoastRange ophioliteand Great Valley sequence[Blake Late Jurassictime is suggestedby (1) imbricationof the et al., 1985;Saleeby, 1992]. Alexander and Yukon-Tanana terranes and deformation Middle to Late Jurassicophiolites and extensionalarc within the AWP prior to depositionof Oxfordianstrata complexesin the Klamath-SierraNevada region are in the Gravinabasin [McClellandand Gehrels,1990; interpretedto haveformed in a forearcto intra-arcand Saleebyand Rubin, 1990],(2) emplacementof Late interarctranstensional rift systembeginning at Jurassicarc-related plutons in both the Wrangelliaand approximately170 Ma (Bathonian)and continuingto Stikine terranes[Armstrong, 1988; van der Heyden, early Oxfordiantime [Saleeby,1992, and references 1992],(3) the presenceof detritusderived from the therein]. Most of the ophiolitesare clearlyassociated Yukon-Tanana and Stikine terranes in the Gravina belt with arc complexesbut someocean floor remnants,such [Gehrelsand Greig, 1991]and clastsderived from the as the Point Sal (southernCoast Range) ophiolite,are Quesnellia-equivalent(?)Chilliwack terrane in Middle separatedfrom overlyingarc-related rocks by a JurassicWrangellia-equivalent strata beneaththe substantial hiatus and remained distal to arc Gambier assemblagein the Harrison Lake region sedimentation[Hopson et al., 1991]. Arc volcanicswere [Monger, 1989;Friedman et al., 1990]. Sincethere is typicallyonlapped by turbiditicstrata between Oxfordian currentlyno evidencefor a subductioncomplex and Tithonian time. Late Jurassic deformation in the separatingthe AWP and Stikineterrane, we conclude Klamath-SierraNevada regionterminated clastic that the initial Middle JurassicAWP-Cordilleran margin depositionof the Galice and MariposaFormations and is juxtapositionresulted from either obliquesubduction reflectedby a changein basingeometry and deposition alongthe margin southof the Stikine terrane followedby of the lower Great Valley sequence. northwarddisplacement along a dextralstrike-slip fault systemor migrationof the AWP along the Cordilleran OVERVIEW OF THE UPPER JURASSIC-LOWER marginas a forearcfragment above a coevaleast dipping CRETACEOUS BASINAL ASSEMBLAGES subductionzone (see Saleebyand Busby-Spera[1992] for discussion).Oblique or transpressionalaccretion of the Upper Jurassic-LowerCretaceous basinal assemblages AWP, most likely alongthe paleo-Oregon-Washington along the Cordilleranmargin from southernAlaska to margin,was apparentlysynchronous with Middle Jurassic southernCalifornia are dividedinto three main groups accretion of the Stikine and inboard terranes. Middle basedon their basementcharacter: (1) thosedeposited Jurassic contractional deformation in the Klamath-Sierra on the easternedge of the AWP, (2) thosedeposited on Nevadaregion may record accretionof the southern inboard terranes that were accreted to North America AWP terrane [Saleebyand Busby-Spera,1992]. prior to Late Jurassictime, and (3) thosedeposited on FollowingButler et al. [1989],we favor Middle Jurassic Middle-Late Jurassicophiolitic basement or pre-Late accretion of the AWP and Stikine terrane within 1000 Jurassicaccretionary complexes. Despite variancein km of their presentposition with respectto North local provenance,stratigraphy, and basementtype, the America rather than >_2400km to the southas implied basinalassemblages share several common elements: (1) by the Baja BritishColumbia model of Umhoefer [1987]. sedimentationin all of the basinsinitiated or changedin Closelyfollowing accretion of the AWP, arc characterduring Oxfordian-Kimmeridgian time, (2) magmatismin all but the westernmostportion of the depositionof arc-typevolcanic rocks or detritusshed Stikine terrane ceased[Armstrong, 1988] and Late from coevalarc complexesin many of the basinsduring Jurassicarc magmatismapparently shifted westward to Oxfordian-Albiantime, and (3) clasticsedimentation in the AWP (Figure3a). This westwardshift likely reflects all of the basinschanged in characterduring mid- initiationor resumptionof east directedsubduction Cretaceoustime. Productsof Late Jurassic-Early outboardof the AWP that is recordedby the oldest Cretaceousarc magmatismare preservedin the AWP, accretionarycomplex fragments preserved in the Chugach southwesternStikine terrane, and the Klamath-Sierra terrane. The Late Jurassic arc constructed on the AWP Nevada region. The clasticbasins are inferred to have and Stikine terranes mark the northern continuation of evolvedin closeassociation with this arc systemwhich the Late JurassicKlamath-Sierra Nevada arc. Along the recordsnortheast directed subduction along the southernsegment of the arc, Callovianto early Cordilleranmargin from Late Jurassicto mid-Cretaceous Oxfordianinterarc to forearcophiolite complexes evolved time [e.g.,Engebretson et al., 1985]. within a dextral transtensionalregime during or immediatelyfollowing AWP accretion(Figure 3a) ACCRETIoNARY HISTORY OF THE AWP [Saleeby,1992]. The ophiolitesand their overlying pelagicand volcaniccover moved rapidly northward in The followingsection discusses a geologically early Late Jurassictime [Hopsonet al., 1991;Pessagno reasonable scenario for the Middle Jurassic accretion and and Blome, 1990]within the margin-paralleltransform northwardmigration of the AWP along the Cordilleran system. The dextral fault systemis inferred to have marginduring Late Jurassic-EarlyCretaceous time. It is extendednorthward along the AWP-Cordilleranmargin by no meansa unique solutionto the accretionof the suturezone utilizingthe pre-LateJurassic accretion- (a) EARLY LATE JURASSIC (b) LATE EARLY CRETACEOUS

ß Alaska Alaska TK-- ß Ka YU KON-TANANA

ß 9 YUKON-TANANA ß ß , --.

9 Yuko Yukon ....'.. .. 80..:2...

..:.•

B.C. B.C.

70 ø N Ch

BLUE MTNS.

MTNS.

KLAMA TH-SIERRA REGION KLAMA TH- SIERRA5--•o N Fr REGION

0 500 1000 ß I I , ,I km (approximate)

•, THRUSTBELT ...:.•:..•ß ANDALEXANDER PENINSULAR (A),WRANGELLIA (P) TERRANES (W), E• STIKINETERRANE :i;•ß BASlNALUPPER JURASSIC-LOWERASSEMBLAGES CRETACEOUS ß APPROXIMATE ARC ß LOCATION CACHECREEK AFFINITY ASSEMBLAGES

•, SUBDUCTIONZONE YUKON-TANANA,MOUNTAIN, AND RELATEDQUESNELLIA, TERRANESSLIDE

Fig. 3. Schematicreconstructions depicting inferred relations along the Cordilleranmargin during (a) early Late Jurassicand (b) late Early Cretaceoustime. Basinalassemblages are Bo, Bowser Basin;De, Dezadeash;Ga, Galice; Gm, Gambier; Gr, Gravina;GV, Great Valley; Ka, Kahiltna; Ma, Mariposa;My, Myrtle; Nu, Nutzotin;SJ, Lummi and Constitution;and T-M, Tyaughton- Methow. Accretionarycomplexes are Ch, Chugachterrane and Fr, FranciscanComplex. The sketchesare not intendedto be palinspasticreconstructions; however, the mid-Cretaceous configurationof major terranesis approximatedassuming the followingLate Cretaceous-Tertiary displacements:Tintina fault-northernRocky Mountain trench (T'r-NRMT) = 500-1000km, Denali fault (DF) = 450 km, and Fraser-StraightCreek fault system(F-SCF) = 100 km [Priceet al., 1985;Gabrielse, 1985; NoMeberg et al., 1985]. See Figure I for terraneabbreviations. Figure 3a depictsMiddle Jurassic accretion of the AWP followedby evolutionof dextraltranstensional basinsalong the Cordilleranmargin. The locationof the Late Jurassicarc reflectseast-directed subductionalong the margin,outboard of the AWP. Figure3b depictsEarly Cretaceous northward translation of the A•hrP and San Juan terranes and mid-Cretaceous contractional deformation. McClellandet al.: BasinalStrata on the CordilleranMargin 831 related structures. Clasticbasins along the inboard 1989]. Fundamentalproblems with this interpretation marginof the AWP likely evolvedin this early Late includeevidence reviewed above supporting pre-Late Jurassicdextral regime. Regionseast of the AWP were Jurassic accretion of the AWP and the distinct lack of apparentlyaffected as well: the Bowserbasin is inferred any structuralevidence for an Early Cretaceous to haveevolved during a periodof dextraltranstension subductionzone within or adjacentto the Tyaughton- within the Stikineterrane [Greig et al., 1991] and Late Methow basin. Early Cretaceous(130-120 Ma) Rb-Sr Jurassicplutons east of the Omenicabelt were emplaced and K-Ar metamorphicages for the Shuksansuite are in a dextralregime [Vogl and Simony,1991]. alternativelyinterpreted to recordvariable mid- Marine clasticsedimentation beginning in Oxfordian Cretaceousresetting of late Middle Jurassic(170-160 time reflectsthe evolutionof transtensionalbasins along Ma) blueschistmetamorphic assemblages. We the Cordilleranmargin (Figure 3a). Depositionin accordinglyinterpret the SpencesBridge arc to reflect ophiolite-flooredbasins south of the AWP is recordedby eastwardmigration Gambler arc magmatismassociated Oxfordianto Kimmeridgianturbiditic strata that typically with a singlesubduction zone outboardof the AWP. interfingerwith or overlie arc-relatedvolcanic rocks or East directedunderthrusting of the AWP, Gravina- containa significantcomponent of arc-deriveddetritus. Nutzotin belt, and Kahiltna basin beneath the Yukon- The Constitutionand similar assemblageswere likely Tanana and Stikine terranes and northwest directed depositedin forearcbasins that evolvedwithin the imbrication in the northwest Cascades-San Juan thrust ophiolitebasin frameworksouth of the AWP. systemin mid-Cretaceoustime resultedin a regionally Concurrent with establishment of a Late Jurassic arc on extensivethrust belt extendingfrom southernAlaska to the AWP, Upper Jurassicstrata were depositedalong the northwesternWashington [Rubin et al., 1990;Gehrels et inboardAWP marginin the Kahiltnaand Gravina- al., 1992;McClelland et al., 1992b;Rubin and Saleeby, Nutzotinbasins. Upper Jurassicvolcaniclastic rocks in 1992](Figure 3b). Kinematicrelations suggest that the HarrisonLake region(Gambler assemblage) and San thrustingin the northwestCascades-San Juan systemmay Juan region(Spieden Group) were depositedin proximity haveoccurred in a right-lateraltranspressional setting to the southernextension of the AWP arc complex. The [Brown,1987; Brown and Talbot, 1989]. A similar Tyaughton-Methowand Bowsetbasins continued to settingis envisionedfor the mid-Cretaceous evolveeast of the Late Jurassicarc. Depositional underthrustingof the AWP in southeasternAlaska continuitybetween the northernbasins (e.g., Gravina- [Coney,1989; McClelland et al., 1992b],although a Nutzotin-Bowser-Tyaughton-Methowbasin of Eisbacher sinistralcomponent of underthrustingis locallyrecorded [1985]) was likely but is not demonstrable.Establishing [Saleebyand Busby-Spera,1992]. The thrustbelt is originalbasin widths is not possibledue disruption coincidentwith a belt of high pressuremid-Cretaceous resultingfrom mid-Cretaceousand youngerdeformation arc-typeplutons that extendssouthward into the Blue inboard of the AWP. Mountainsregion [Zen, 1988]which also experienced Argumentsfor left-lateralshear within and along the mid-Cretaceouscrustal thickening possibly in a dextral margin of the Cordilleraduring Late Jurassicto Early regime [Lund and Snee, 1988]. Cretaceoustime are presentedby Engebretsonet al. Mid-Cretaceousdeformation along the entire [1985],Av6 Lallemantand Oldow [1988], and May et al. Cordilleranmargin either terminateddeposition in the [1989]. Saleebyand Busby-Spera[1992] and Plafker et marginalbasins or is generallymarked by a si,qmlficant al. [1989] discussedgeologic evidence for latest Jurassic unconformitybeneath Albian or youngercoarse clastic to earliestCretaceous sinistral displacement along the strata. Deformationalong the inboardmargin of the southernCordilleran margin and within Wrangellia, AWP was broadlysynchronous with contractionin the respectively.A return to northwardtranslation of Stikineterrane (Skeena fold belt of Evenchick[1991]) outboardterranes along the Cordilleranmargin is and Omenicabelt [Archibaldet al., 1983] (Figure3b). recordedby Hauterivianto Aptian northwarddispersal of In the southernCordillera, west directed thrustsinvolving the Decatur terranefrom the CoastRange ophiolite- the Great Valley-CoastRange ophioliteand Franciscan Great Valley sequence[Garver, 1988b] and Shuksan Complexwere broadlysynchronous with contractional metamorphicsuite from the FranciscanComplex [Brown deformationin the Sevierorogenic belt [Lawton, 1985; and Blake, 1987]. Early Cretaceous(post-150 Ma and Heller et al., 1986]and its hinterland[e.g., Miller and pre-110 Ma) dextraldisplacement along the Mojave- Gans, 1989](Figure 3b). Thus deformationalong the SnowLake fault in easternCalifornia [Lahren et al., inboardmargin of the AWP reflectsinvolvement in a 1990] may have accommodatedthe northwardtranslation Cordilleran-widecontractional event related to a change of thesefragments [Burchfiel et al., 1992]. in subductionzone parametersoutboard of the AWP Arc magmatismshifted eastward in Early Cretaceous (e.g.,increased coupling with the downgoingoceanic time suchthat, with the exceptionof the Kahiltna slab) rather than the collisionof the AWP. terrane, Lower Cretaceousvolcanic rocks are common to all of the assemblagesalong the easternmargin of the CONCLUSIONS AWP and interfingerwith clasticstrata in the western exposuresof the Tyaughtonand Methowbasins (Figure Discrepanciesbetween models for the accretionary 3b). Aptian-Cenomanian(•125-90 Ma) plutonicand historyof the AWP have for the mostpart centeredon volcanicrocks east of the Tyaughton-Methowbasin argumentsfavoring pre-Late Jurassicor mid-Cretaceous [Greig, 1989;Thorkelson and Smith, 1989;Hurlow and accretionor collision. Uncertaintyconcerning the AWP Nelson,1991] and Early Cretaceousblueschists in the accretionaryhistory largely results from extensivemid- Shuksanmetamorphic suite in the northernCascades Cretaceousand youngerdisruption, overprinting, and [Armstrongand Misch, 1987] have been cited by modificationof possiblepre-mid-Cretaceous accretion- numerousworkers as evidencefor Early Cretaceousarc related structures. On the basis of evidence outlined magmatism(Spences Bridge arc) associatedwith east above, we concludethat the AWP was accretedto the directedsubduction and closureof a marginalbasin east Cordilleranmargin during Middle Jurassictime. of the AWP [e.g.,Monger, 1986;Thorkelson and Smith, Regionalsimilarities in the characterand age of Late 832 McClellandet al.' BasinalStrata on the CordilleranMargin

Jurassic-EarlyCretaceous marginal basins common to the AWP was related to a changein subductionzone AWP and Cordilleranmargin reflect the Middle Jurassic parametersoutboard of the AWP rather than its collision accretion of the AWP. The basins are inferred to have with the Cordilleranmargin. In our view, accretionof evolvedwithin or proximalto transtensionalbasinal arc the AWP was a protractedprocess of juxtaposition, complexesthat recordeast directedsubduction along the translation, and imbrication that occurred from Middle Klamath-SierraNevada margin and outboardmargin of Jurassicto mid-Cretaceoustime. Althoughthere are still the AWP. Variationsin stratigraphyand provenance many uncertaintiesregarding the proposedaccretionary observedbetween basins along the marginresulted from historyof the AWP, the availableevidence demands that differencesin basementcomposition and proximityto the AWP was intimately involvedin the Late Jurassic- coeval arc volcanism as well as local variation in Early Cretaceousevolution of the Cordilleranmargin. transtensionaland transpressionaldisplacement along the This involvementmust be incorporatedinto models margin. A first-orderdifference between the northern concerningMesozoic evolution of the Cordilleranmargin. and southern Cordillera resulted from the kinematic Acknowledgments.The ideaspresented in this paper historyof the AWP outlinedherein. Post-Middle were stronglyinfluenced by discussionswith P. J. Coney, Jurassic evolution of the northern Cordillera was J. W. H. Monger, and C. R. Rubin and are the direct dominatedby the presenceof the AWP in a forearc outgrowthof researchin southeasternAlaska fundedby positionwhereas to the south,a thinnerbelt of Middle GeologicalSociety of America,Sigma Xi, and ARCO to Late Jurassicinterarc ophiolite-floored basins evolved researchgrants awarded to McClelland and NSF Grants and servedas basementfor the youngerGreat Valley EAR-8616473 and EAR-8903794 awarded to G. E. forearc basin. Gehrels and EAR-860538 and EAR-8803834 awarded to Evidence for Middle JurassicAWP accretion, J. B. Saleeby. C. H. Hopson,J. L. Jackson,and M. M. involvementof the AWP in Late Jurassic-Early Miller providedvaluable comments on early versionsof Cretaceousbasinal evolution along the Cordilleran the manuscript.We thank G. A. Davis,J. O. Wheeler, margin,and similarityin timingof mid-Cretaceous and P. van der Heyden for providingunpublished deformationthroughout the Cordillerasuggest that mid- manuscripts,and J. I. Garver and K. L. Kleinspehnfor Cretaceousdeformation along the inboardmargin of the reviews.

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