The Gravina Sequence Remnants of a Midmesozoic Oceanic Arc In
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B9, PAGES 14,551-14,568, AUGUST 10, 1991 The GravinaSequence' Remnants of a Mid-MesozoicOceanic Arc in Southern Southeast Alaska CHARLESM. RUBIN1 AND JASONB. SALEEBY Divisionof Geologicaland PlanetarySciences, California Institute of Technology,Pasadena Fragmentsof UpperJurassic to Lower Cretaceousvolcanic and basinal strata constitute the Gravinabelt in southeastAlaska. In the Ketchikanarea the Gravina belt is made up of two lithotectonicunits. The lower unit consistsof coarsemarine pyroclasticand volcaniclasticstrata, mafic flows, breccia,and fine- grainedtuff which are locally intrudedby hypabyssalbodies of diorite and quartz diorite. The volcanic rocksare characterizedby tholeiitic arc basalts,lack felsic volcanicstrata, and overlie Upper Triassicand older strataof the Alexanderterrane. Augire and/orhomblende-bearing porphydtic rocks are commonand locally intrude the Alexander terrane basement,where they are thought to representthe intrusive equivalentsof lavas within the section. Age constraintsfor the volcanicunit, basedon structuraland stratigraphicrelations with adjacentunits, are late Middle to Late Jurassic.The Gravinabelt upperunit consistsof fine- to coarse-grainedturbidRes and relatedconglomeratic channel-fill deposits. The basinal rocks unconformablyoverlie Permianand Triassic rocks of the Taku terrane and remnantsof the lower volcanicpart of the Gravinasequence which overlie the Alexanderterrane. The conglomerateunits contain mostlyvolcanic and plutoniclithic clasts,some of whichyield Pb-U zirconages of 154-158Ma. The predominanceof pyroclasticdeposits interbedded with massiveflows, tuff, breccia,and argillaceous turbidires,and the lithologic and chemical compositionof the volcanic rocks indicate a submarine volcanicarc settingfor the Gravinasequence. The basinalpyroclastic rocks are inferredto have beenshed from submarinestratovolcanos during the Late Jurassic.Epiclastic rocks were depositedas submarinefans, derivedin partfrom erosionof a magmaticarc. The presenceof fine-grainedtuffaceous turbidires implies ongoing,but distant,volcanism. The pyroclasticand volcaniclasticrocks represent remnants of a Late Jurassicoceanic arc constructedon a compositebasement consisting of the Alexanderand Taku terranes. The strataaccumulated in an intra-arcbasin on the eastemedge of the Alexanderterrane. The volcanic and basinal rocks were deformedduring a major mid-Cretaceousintra-arc contractionalevent, in conjunction with the emplacementof a distinctlyyounger, arc-related plutonic suite. INTRODUCTION adjacent to North America [see summary by Rubin et al., 1990a]. This compositeterrane was accretedto North America The paleotectonic setting and stratigraphic affinity of in Early to Middle Jurassictime [Monger et al., 1982] and thus variably deformedand metamorphosedUpper Jurassicto Lower formed the western margin of the North American continent Cretaceousmetavolcanic and metasedimentarystrata exposed during late Mesozoic time. for nearly 750 km along the easternedge of the Alexanderand Recent tectonic syntheses of Mesozoic paleogeography Wrangellia terranes(Figure 1) [Berg et al., 1972)] have been have been limited by uncertaintiesregarding the stratigraphic the subjectof much debate. Theserocks are called the Gravina- and structuralevolution of the Gravina belt [e.g., Monger et Nutzotin belt [Berg et al., 1972] and consist of interbedded al., 1982; Pavlis, 1982; Plafker et al., 1989a]. The initial marine basaltic breccia, flows, tuff, and turbidires. The stratigraphic and tectonic relations between the Insular volcanic belt unconformablyoverlies both the Alexander and composite terrane and North America are obscured by mid- Wrangellia terranes [Berg et al., 1972; Monger and Berg, Cretaceouspolyphase deformation and metamorphism;thus 1987], which collectively form the Insular compositeterrane the initial tectonicboundary is highly modified. For example, [Wheeler and McFeely, 1987; terrane II of Monger et al., the timing and style of accretion of the Insular composite 1982]. The Insular compositeterrane, consistingof juvenile, terrane to the western North American continental margin mantle-derived volcanic arc and rift assemblages[Samson et remains enigmatic. Gravina belt rocks play a critical role in al., 1989], had no palcogeographicrelation to North America addressing these tectonic questions, due to their unique until Mesozoic time [Monger et al., 1982; Saleeby, 1983; stratigraphicand paleogeographicposition along the eastern Gehrels and Saleeby, 1987a]. Rocks that lie to the east of the margin of the Insular compositeterrane. If Gravina belt strata Insular composite terrane belong to the Intermontane overlap both the Insular and Intermontanecomposite terranes, compositeterrane [Wheeler and McFeely, 1987; terrane I of then a pre-LateJurassic tie betweenthe Insularsuperterrane and Monger et al. 1982], consistingof lower Paleozoic continent- the western margin of North America is implied. One derived slope-and-rise deposits, upper Paleozoic to lower objective of this study of the Gravina belt is to provide a Mesozoic ensimatic arc assemblagesthat probably formed critical tie betweenthe Insular superterraneand westernNorth America. 1Alsoat Departmentof Geology,Stanford University, Stanford, This paper presents new stratigraphic, structural, California. geochronologic and geochemicaldata on the Gravina belt in southern southeast Alaska. Although rocks of the Gravina Copyright1991 by the AmericanGeophysical Union. sequencehave been metamorphosedto greenschistand lower Papernumber 91JB00591. amphibolite facies, locally original bedding features are 0148-0227/91/91JB-00591 $05.00 preserved.This study is based on detailed geologic mapping 14,551 14,552 RUBINAND $.AI•Y: GRAVINASEQUENCE Area of Figure I tI i i Insular Superterrane • accretionaryCretaceousoceanic complexes •] arcUpper & riftPz assemblages- lowerMz primitive • islandLower arcPz, assemblageslower Mz primitive i• CoastPlutonic Complex Intermontane Superterrane ,[• arcUpper assemblagesPz- lower Mz • accretionaryUpperPz- lower complexesPz oceanic ß ß ]• basinalUpperPz assemblages- lowerMz marginal ß e •..:•Lower Pzbasinal assemblages ß ß ß i Pacific Ocean ß ß ß ß ß ß I 150km I t 150ktm• Fig. 1. Location map of the Gravina sequencein the northwesternCordillera, showingregions and features referred to in text. (Adapted from Beikman [1980] and Monger and Berg [1987]). Abbreviations are BI, Banks Island; CP, Chilkat Peninsula; EL, Etolin Island; J, Juneau; K, Ketchikan; P, Petersburg;and QI, Queen Charloue Islands. Regional extent of the Gravina-Nutzotin belt shown in the upper right inset. CV is Chitna Valley. Location of major lithotectonicelements in the northernCordillera shownin lower right inset. QC-F is the QueenCharloue-Fairweather Fault System; TT-NRMT is the Tintina -Northern Rocky Mountain Trench System;Ytt is the Yukon-Tanana terrane; At is the Alexanderterrane; and Wr is Wrangellia. RUBIN AND S^I •J•.•Y: (]RAVINA SEQUENCE 14,553 .r-•lYounger Intrusive Rocks • UDDerPalaezoic &Lower Mesozoic A!ava Seouence MiddleCretaceous Intrusive Rocks • •. argillite,Metamorphosedmarble, &maficquartzite pillow flows, tuff &breccia, -• Tonalite,granodiorite, diorite,&gabbro I• Lower Palaezoic KahShakes Seauence "•ii• Devonianorthogneiss, lowerPaleozoic quartz-bearing ?• Zonedultramafic complexes / Gabbro I• metapelite,psammitic rocks,quartzitesilicic & marble metavolcanic rocks,amphibolite, UDDer Jurassic & Lower Cretaceous Gravina Seauence . _ ,• Metamorphosedgreywacke,argillite, Palaezoic&Lower Mesozoic Alexander Terrane conglomerate,& pillow flows, basalt-andesite& hypabyssal intrusivetuff, breccia rocks r• Triassic conglomerate, limestone, basalt, & rhyolite ßß 132ø • breccia,Ordovician-Silurian pillowed flows,basaltic & hypabyssalandesite tuff, rocks ß •Ordovician-Siluriantonalite,diorite, &gabbro U B ß Siluriantrondhjemite & local diorite 52 45 Strike& dip of foliation Geologic contact (dashed where inferred 45 ' & dottedwhere covered) • Thrust(dashedFault where inferred •1•& dottedwhere covered) ß ß 57 High(dashedAngle whereFault inferred • & dotted where covered) 34 Union Bay ultramaficcomplex Gc = Gnat Cove ß • • • • Bl=BackIsland ' • • • • • AI = AnnetteIsland GI = Gravina Island GI = George Inlet Cl = Carol Inlet 10 k m ß TA = ThorneArm 22 Fig. 2. Geologicmap of ClevelandPeninsula, Revillagigedo Islands and adjacentislands. Adapted from Berg [1972, 1973] (partsof Annetteand GravinaIslands], Gehrels and Saleeby[1987b] (partsof Annette, Duke, and Gravina islands], C. M. Rubin (unpublishedmapping, 1985, 1986, 1987; ClevelandPeninsula and adjacentislands); C. M. Rubin and J.B. Saleeby(unpublished mapping, 1986, 1987, 1988; Revillagigedo and adjacentIslands). along the shorelinesof Annette, Gravina, and Revillagigedo comparisonto age-correlativevolcanic sequenceselsewhere in islands, adjacent smaller islands, and Cleveland Peninsula the Cordillera, and providesnew constraintson mid-Mesozoic (Figure 2). This paper characterizesthe alepositionalsetting of paleogeographyand paleotectonicsetting of the northwestern the Gravina sequence, provides a reference section for Cordillera. 14,554 RUBIN AND SALEEBY:GRAVINA SEQUENCE PREVIOUS WORK slightly deformed and are not highly metamorphosed,except near the easternedge of the terranewhere they are overprinted Rocks of the Gravina belt in southeast Alaska were by late Mesozoic deformation [Rubin andSaleeby, 1987, originally described by Buddington and Chapin [1929]. Saleeby, 1987].