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 number 92TC00241. 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 BorderRanges 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