Mesozoicnorth America(Intermontane Easternedge of Thealexander Terrane

Home , Intermontane Belt, Revillagigedo Island

TECTONICS, VOL. 11, NO. 3, PAGES 586-602, JUNE 1992

TECTONIC HISTORY OF THE EASTERN EDGE Intermontanecomposite terrane). Contractionaldeformation OF THE ALEXANDER TERRANE, SOUTHEAST alongthe lengthof the thrustbelt wasbroadly coeval with arc ALASKA magmatism, and thus records intra-arc tectonism. Late Paleoceneto early Eoceneigneous activity and extensional (?) deformationsubsequently affected the thrustbelt. CharlesM. Rubin! andJason B. Saleeby Division of Geologicaland PlanetarySciences, California Instituteof Technology,Pasadena INTRODUCTION

Stmcturallyimbricated metamorphic rocks are intermittently exposed along the western flank of the Coast Plutonic Abstract.Rocks exposed west of theCoast Plutonic Complex Complexand extendfor approximately2000 km alongstrike, in southern southeast Alaska form an imbricate thrust belt that from northern Washingtonto southeastAlaska (Figure 1). overprintsthe tectonicboundary between two of the largest Detailedstructural, stratigraphic, and geochronologic data from allochthonouscrustal fragments in the North American the metamorphicrocks are critical for understandingthe Cordillera,the Insularand Intermontane composite terranes. In accretionaryhistory of the northwesternCordillera. The the Alexander terrane (Insular compositeterrane), lower metamorphicrocks contain mantle-derived,juvenile crustal Paleozoicmetavolcanic and metasedimentaryrocks (Descon materialrepresented by the Alexanderterrane on the westand Formation)and dioritic plutons are unconformablyoverlain by fringing continental volcanic arc rocks and continentally Lower Devonian clastic strata (Karheen Formation). These derived slope-and-rise deposits of the North American rocks are overlainlocally by Upper Triassicbasalt, rhyolite continent on the east. One of the outstanding tectonic and marine clastic strata(Hyd Group). Upper Jurassicand problemsin the northwesternCordillera is the natureof the Lower Cretaceousmetavolcanic and metasedimentarystrata of boundarybetween allochthonous ensimatic crustal fragments, the Gravinasequence unconformably overlie the Alexander consistingof the Alexander and Wrangellia terranes(Insular terrane. The Gravinasequence forms a structuralpackage over compositeterrane of Wheeler and McFeely [1987]), and the 15 km thick and recordsintermittent arc volcanismalong the westernmargin of late MesozoicNorth America(Intermontane easternedge of theAlexander terrane. The Gravina sequence is compositeterrane of Wheeler and McFeely [1987]). The structurallyoverlain by upperPaleozoic and lower Mesozoic Ketchikan area providesa strategiclink acrossthis boundary metamorphosedbasaltic strata, marble, and argillite (Alava because it is relatively unaffected by Tertiary plutonism. sequence),and locallyby lowerPaleozoic supracrustal rocks Distinctive stratigraphicsequences record the late Mesozoic and orthogneiss(Kah Shakessequence). Together, these continent-marginhistory of westernNorth America. Recent constitute the Taku terrane which we correlate with the work [Crawford et al., 1987; Rubin et al., 1990a] shows that Intermontanecomposite terrane. Local unconformityof regionallymetamorphosed rocks form a lateMesozoic fold and Gravinasequence strata over the Alava sequencedemonstrates thrust belt and involve both crystalline basement and its that the Gravina sequenceoverlapped an earlier structural volcanic and basinal cover. Mid-Cretaceous thrust belt boundarybetween the Intermontaneand Insular composite deformationis well documentedand wasbroadly coeval with terranes. The rocks were deformedin the mid-Cretaceousby arc magmatism,involving the emplacementof west-directed west-vergentthrusting that was was broadlycoeval with arc thrust nappes over a structurally intact and relatively magmatism. Deformationinvolved emplacementof west- unmetamorphosedbasement. The presence of inverted directedthrust nappes over the structurallyintact and relatively metamorphicisograds beneath thrust faults indicates that hot unmetamorphosedAlexander terrane basement. Mid- metamorphic and plutonic rocks were translated over a Cretaceoustonalitc, granodiorite,and quartz diorite intrude relativelycold basement. rocks of the thrust belt and are locally affected by the In this paper,we describethe generalstratigraphy and broad deformation.Mid-Cretaceous deformation occurred during two structuralgeometry of metamorphicrocks from the Ketchikan episodesthat were contemporaneous with theemplacement of area that include the Alexander and Taku terranes and Gravina large sill-like plutons. Older structuresrecord ductile sequence. U-Pb zircon geochronologicdata and a relative southwest-vergent folding and faulting, regional structural chronology allow us to establish the timing of metamorphism,and developmentof axial-planarfoliation. deformation in relation to isotopically dated plutons. The second-generationstructures developed during the later Furthermore, by establishing the structural setting of stagesof southwest-directedthrust faulting, which juxtaposed syntectonicand posttectonicplutons and with the useof zircon rocksof contrastingmetamorphic pressures and temperatures. geochronology,we are able to establishthe absolutetiming Structural,stratigraphic, and geochronologic data indicate that and tectonic setting of thrust belt evolution. Stratigraphic the two phasesof regional thrusting in southeastAlaska correlations, regional structure, and U-Pb zircon occurred between 113 Ma and 89 Ma. Rocks in the western geochronology,discussed in detail by Rubin and Saleeby part of the thrustbelt were uplifted regionallyby 70 Ma. [1991a, b, c], are summarizedbelow. The datapresented in Deformationinvolved the collapseof a marginalbasin(s) and a this paper are based upon derailed geologic field mapping magmaticarc, and overprintedthe older tectonicboundary along the shorelines and ridges of Cleveland Peninsula, betweenthe Insularcomposite terrane and the late Mesozoic Revillagigedo,Annette, Gravina, and smalleradjacent islands western margin of North America (at that time the (Figure2).

GEOLOGIC SETYING 1Nowat Departmontof Goology, Contral Washington University,Ellensburg. The Alexanderterrane forms structuralbasement for mostof the rocks that lie west of the Coast Plutonic Complex. The Copyright1992 by the AmericanGeophysical Union. Alexander terrane consistsof a structurallyintact lower Paler number91TC02182. Paleozoicensimatic arc sequence overlain by middlePaleozoic 0278 -7407/92/91TC-02182 $10.00 clasticand carbonatestrata that are unconformablycapped by Rubinand Saleeby: Tectonic History of EasternEdge of AlexanderTerrane 587

•.•.•o.o[•qlntermontaneSuperterrone Stikine Insular Superterrane

QuesnelCache Creek • Alexanderterrane •l Wrangellia CoastPlutonic Complex

:• Fra•7mentsNW Cascadesinthe SystemSan Juan

Insular Superterrane

T'['laccretionaryCretaceous oceanic complexes Study Area • arcUpper & riftPz assemblages- lowerMzprimitive

I• islandLower arcPz, assemblageslower Mz primitive !:• CoastPlutonic Comt•lex Intermontane Superterrane

,• arcUpper assemblagesPz- lower Mz

,• accretionaryUpperPz- lower complexesPzoceanic

• basinalUpperPz assemblages- lowerMzmarginal ).'•ß LowerPzbasinal assemblages Pacific ':...•.•:,i.(x,/ Ocean

' Cc

Vancouver

I I 100 km

Fig. 1. Generalizedgeologic map of thenorthwestern Cordillera with study area indicated. Areas of mid- Cretaceousand younger deformation are stippled. Terrane map of theCanadian Cordillera is shownin inset. Pz is Paleozoic; Mz is Mesozoic. anUpper Triassic rift assemblage[Gehrels and Saleeby, 1987]. belt containingimbricated Gravina, Alava, and Kah Shakes In mostareas, rocks of the Alexanderterrane are only slightly sequencesoccurs along parts of the easternboundary of the deformedand are not highlymetamorphosed, except near the Alexanderterrane (Figure 3). The thrustbelt consistsof an easternboundary where they are overprinted by lateMesozoic imbricateseries of west-vergentthrust sheets; on Cleveland deformationalstructures. Upper Jurassic to Lower Cretaceous Peninsula the imbricated package has a total structural marinepyroclastic and basinal strata of the Gravinasequence thickness of over 15 km. On the basis of field relations and depositionallyoverlie the Alexanderterrane. The middle geochronologydiscussed here, the thrust belt was active over a Paleozoicand lower MesozoicAlava sequenceand lower to relativelyshort period of timeand was broadly coeval with arc mid-PaleozoicKah Shakessequence (part of the Taku terrane magmatism. of Berget al. [ 1988])structurally overlie the Alexander terrane and Gravinasequence. Locally, channel-fill deposits of the Gravinasequence overlie the Alava sequence and thus form an GEOLOGIC FRAMEWORK overlapbetween the Alexander terrane and the Alava sequence [Rubin and Saleeby, 1991a). The Kah Shakessequence Alexander Terrane locally occupieshigher structurallevels on northeast Cleveland Peninsula, western Revillagigedo Island and Lower Paleozoic metavolcanicrocks. The southernpart of northwesternPortland Peninsula (Figure 2) and consistsof Cleveland Peninsula and part of the western side of lower Paleozoic meta-silicic tuff, quartzite, marble, RevillagigedoIsland are underlain by metavolcanicrocks, with metabasalt,calc-silicate, and orthogneiss. subordinatephyllite and marble of Ordovician to Early A regionallyextensive mid-Cretaceous west-vergent thrust Silurian(?) age (Figure2). The lower contactof this mafic 588 Rubinand Saleeby: Tectonic History of EasternEdge of AlexanderTerrane

+ + + + + ß 132 ø + + + + +

+ + + + + + ß + + + + + + + + + + + + + + + + + + + UB' 52 .45

+ + + + + + + + + + + + +

+ + + + + + + + + +

+ + + + + + + + +

+ + + + + + + +

Ketchikan + + + + + + + +

+ + + + + +

lO km I I

+ + + + + +

55 ø' • t• + + + + + + + + + + +

PALEOCENEAND YOUNGERINTRUSIVE ROCKS U. PALEOZOIC& L. MESOZOICALA VA SEOUENCF (Coast Plutonic +l•'-•Tonalite, quartz diorite, &granodiorite Complex) <•l'• Metamorphosed maficpillow flows, tuff& breccia, • Pegmatitedike swarm argillite,marble, & quartzite MIDDLE CRETACEOUS INTRUSIVE ROCKS PALEOZOIC& L. MESOZOICALEXANDER TERRANF ?• Tonalite,granodiorite, diorite,& gabbro .• Triassicbasalt, &conglomerate, rhyolite siltstone,limestone, :?•-•'•Zoned ultramafic complexes / gabbro "• Devonian& marble conglomerate, sandstone, siltstone, U. JURASSIC& L. CRETACEOUSGRA VINA SEQ{IENCE .•"",'•Ordovician-Silurian basalticandesite tuff, !• conglomerate,Metamorphosedbasalt-andesitetuff, greywacke, tuff,argillite, breccia & breccia,pillowed flows, & hypabyssalrocks pillowflows, & hypabyssalintrusive rocks x}[•Silurian trondhjemite &local diorite '"'•Strike&dip of bedding'"• Geologic contact •] Ordovician-Siluriantonalite,diorite,& gabbro "x (dashedwhere inferred Strike&dip of foliation '"& dotted where covered) I•l Cambrian&older (?) meta-igneous rocks Strike&dip cross-cutting'•-•. ThrustFault PALEOZOICKAH SHAKES SEQUENGE cleavage .. Devonianorthogneiss, lower Paleozoic quartz- (dashed where inferred ::..:..:.•bearing psammitic rocks,silicic metavolcanic •,, Trend&plunge oflineation &dotted where covered) rocks,amphibolite, metapelite, quartzite & marble EASTBEHM CANAL GNEISS COMPLEX sampleU-Pbzirconlocation • • ... High(dashedAngle whereFault inferred j•{;• LowerPaleozoic, tonalite gneiss, diorite gneiss, & dottedwhere covered) amphibolite,& psammiticgneiss Rubinand Saleeby: Tectonic History of Eastern Edge of Alexander Terrane 589

Northern Cleveland Peninsula Union Bay

SW • O,o•%' O%e&• •o Ultramafic Complexo• Garnet - Out • o•.%,. EatonPlutonPoint NE '• +.• VixenInlet '½ A'

• • SouthornClovoland Poninsula I 2 km I SW • • ' • • • BellIsland • / • to'In or-/ øø• NRFZ B'

I 2km I MothBay Flat.lyingaplito and quartz voins SW •.a•t..... Pluton• CarrollInlet NE intrusive rocks Alava sequence contactaureole SRFZ • C'

Alexanderterrane +F:'•'l intrusiveUltramafic rocks&gabbroic KahGravina Shakessequence sequence:• Mesozoic I 4kin Fig. 3. Geologiccross sections: A-A', northernCleveland Peninsula; B-B', southernCleveland Peninsula; C-C', CarrollInlet. Locationsof crosssections shown in Figure2. No verticalexaggeration.

sequenceis not exposed.The uppercontact is faultedagainst correlative strata on Prince of Wales Island. No fossils have theUpper Jurassic to Lower CretaceousGravina sequence and been recovered from these lower Paleozoic rocks on Cleveland is depositionallyoverlain by theDevonian Karheen Formation Peninsula. on western Cleveland Peninsula. The minimum age of Metaplutonic Complex. Metamorphosed gabbro, diorite, metavolcanicrocks on the southwesternpart of Cleveland and plagioclaseand quartz porphyritic granodioriteunderlie Peninsulais constrainedby a crosscutting trondhjemite dike, much of southwestern Cleveland Peninsula (Figure 2). with single-fractionU-Pb zirconage of 443 + 4 Ma (sample Plagioclase and quartz porphyritic granodiorite occurs as 1, Tables 1 and 2). Althoughthere was insufficientmaterial homogeneous, massive to foliated bodies containing to obtain multiple analyses,the internallyconcordant Late oligoclase, quartz, interstitial microperthite, and minor Ordovicianage is similarto agesof metaigneousrocks of the hornblende. Hererogenousbodies of foliated and layered DesconFormation on Prince of Wales Island [Saleeby et al., diorite,quartz diorite, and gabbro consist of texturallyvaried, 1984]. Similar metavolcanicand metasedimentaryrocks on fine- to medium-grainedplagioclase, hornblende, biotite, and centraland southernPrince of WalesIsland range in agefrom minorquartz. The widespreadfoliation is definedby micasand Early Ordovicianto Early Silurianand are assignedto the mafic phases. Minor hornblenditeand clinopyroxenitesills Descon Formation [Eberlein et al., 1983; Herreid et al., 1978; and dikes displaycomplex intrusive relations with the diorite Gehrels and Saleeby, 1987; Gehrelset al., 1987]. On the andgranodiorite. Interlayered foliated dikes and sillsof diorite basisof similarrock types,stratigraphic position, and age, we and gabbroare crosscut by porphyriticintrusive bodies. The assignthe mafic metavolcanicand metasedimentarystrata on quartzporphyritic and diorite plutonic rocks intrude screens and southwestern Cleveland Peninsula to the Descon Formation. septaof foliatedaugite-phyric metabasalt and marblethat are There is a possibilitythat the metavolcanicrocks on southern part of the DesconFormation. All intrusiveunits are, in turn, ClevelandPeninsula represent slightly deformed Wales Group, crosscutby leucogabbroand diorite pods and sills and by inasmuchas the Wales Group lies regionally beneath the quartzand feldsparveins. Zircon from a foliatedmetadiorite DesconFormation; however, suchan interpretationdoes not dike yieldsa U-Pb age of 445 + 8 Ma (sample2, Tables1 and fundamentallyaffect our analysis. 2). The metaplutonicrocks exposed on ClevelandPeninsula On Cleveland Peninsula, the Descon Formation consistsof are similar in composition,texture, lithology, and intrusive lower greenschistfacies metabasalt flows, breccia,and tuff. relationsto metaplutonicrocks exposedto the west across The mafic metavolcanicstrata contain euhedralaugitc and Clarence Strait on Kaasan Peninsula [Eberlein et al., 1983]. feldsparphenocrysts in a pale to dark greentuffaceous matrix On the basisof thesegeologic relations and the continuityof comprisedof albite, chlorite, epidote,and white mica. The exposurealong the southwesternside of ClevelandPeninsula mafic metavolcanicstrata are locally interlayeredwith marble and the eastside of KaasanPeninsula, the metaplutonicrocks and argillite;marble beds range in thicknessbetween 1 and 3 on ClevelandPeninsula are interpretedto be correlativewith m. The DesconFormation in this region containsa greater similar rocks on Kaasan Peninsula. proportionof mafic metavolcanicrocks than are presentin U-Pb zircon geochronology. All zircon fractionsfrom the

Fig.2. Geologicmap showing distribution of geologic units, major structures, and zircon sample locationson Cleveland Peninsula and Revillagigedo and adjacent islands. Abbreviations areAB, Alava Bay;B, BackIsland; Bn, Barton Island; BI, BellIshmd Pluton; BP, Bushy Point Pluton; C, CarolInlet; EP,Eaton Point Pluton; Gc, Gnat Cove, G, GeorgeInlet; MB, MothBay Pluton; POW, Prince of Wales Island;SB, Spacious Bay; SI, Spire Island; T, ThorneArm; UB, Union Bay Ultramafic Complex. NRFZ isnorthern Revillagigedo Island fault zone; SRFZ is southern Revillagigedo Island fault zone. Adapted fromBerg [1972, 1973] (parts of Annetteand Gravina islands), Gehrels and Saleeby [ 1987] (Prince of WalesIsland, parts of Annette,Duke, and Gravina islands), and this work (Cleveland Peninsula and Revillagigedoandadjacent islands). A-A', B-B', and C-C' indicate locations ofcross sections inFigure 3. 590 Rubinand Saleeby: Tectonic History of EasternEdge of AlexanderTerrane

TABLE 1. U-Pb GeochronologicSample Locations From the KetchikanArea

Sample Latitude Longitude FieldSeahag Zircon Properties

1 N55ø30'00" 131ø59 ' 6" Nonfoliated,medium-grained 2:1; Sub=An>Eu;irregular shapes; colorless, trondhjemitedike - intrudes grey-tint;inclusions common greenschistfacies metavolcanic rocks and marble

2 N55ø31'24" 13101 ' 61" Slightly alteredand foliated 2:1; Sub>Eu>An;irregular shapes; colorless, medium-grainedquartz diorite, in grey- to pink -int; inclusionscommon a heterogeneoussequence of diorite,amphibolite, homblendite, and pyroxenite

3 N55ø46'43" 131ø48' 3" Highly foliatedmedium-grained 1:1; An>Sub>Eu;rounded, irregular shapes; micaceousquartzite grey-to red-tint;inclusions common in all grains:Detrital zircon

4 N55ø54'15" 131ø11'6" Massive,very fine-grained 3:1;Eu>Sub>An;colorless, pink-tint inclusion gabbroicpods and dikesthat are free interstitial to homblendite associatedwith the Union Bay UltramaficComplex

5 N55ø55'00" 131033'20" Massive,slightly foliated, medium- 2:1; Eu>Sub>An;pale pink-tintinclusion free grainedbiotite tonalite,part of the Bell Island Pluton

6 N55ø59'57" 13203'5" Nonfoliated,medium-grained 3:1;Eu>Sub>An; colorless, pink-tint inclusion biotite granodiorite,part of the free Eaton Point Pluton

7 N55ø 7'58" 132ø51'13" Nonfoliated,medium-grained 3:1;Eu>Sub>An; colorless, pink-tint inclusion porphyriticgranodiorite, exposed free on the northernshore of Spacious Bay, par t of the EatonPoint Pluton

8 N55ø48'42" 131047'4" Massiveto slightlyfoliated, 3:1;Eu>Sub>An; colorless, very pale pink-tint, medium-to coarse-grained inclusionfree epidote-bearinggranodiorite, part of the BushyPoint Pluton

Abbreviationsare Eu, euhedral;Sub, subhedral; An, anhedral.2:1, length:widthratios of zircongrains. Color determined under reflectedlight.

Paleozoicsamples yield discordantU-Pb andPb-Pb data. Both and vein quartz. Basalticsills and dikesintrude the sequence. samples(samples 1 and 2) have Pb-Pb agesfrom 443 to 445 No fossilshave been recoveredfrom the clastic sequence.On Ma and U-Pb agesin the 330-430 Ma range(Table 2). Zircon the basis of similarities of stratigraphic position and in these samples lacks optically distinguishedcores or lithology,the clasticstrata are interpretedas part of the Lower compositionalzoning. We interpret the discordanceas the Devonian conglomeratic part of the Karheen Formation, result of episodic lead loss during Mesozoic and Cenozoic exposed on central Prince of Wales Island [Eberlein et al., time, due to a younger thermal disturbance. This 1983]. On Princeof Wales Island,similar strata consisting of interpretationis supportedby hornblendeand biotite K-Ar pebble to cobble conglomerate, limestone, shale, and ages,as youngas ~79 Ma, from PaleozoicAlexander terrane greywacke unconformably overlie metaplutonic diorite rockson Annetteand Duke islands[Smith and Diggles, 1981]. [Eberleinet al., 1983]. The conglomeraticstrata record uplift Both polyphasedeformation and mid-Cretaceousgreenschist and erosionof Ordovicianand Silurianrocks during the Klakas faciesmetamorphism have affected the Alexanderterrane rocks orogeny[Gehrels et al., 1987]. and perhapscontributed to the isotopic disturbance. This Hyd Group. Upper Triassic strata of the Hyd Group on interpretationis in accord with that of Gehrelsand Saleeby Annette and Gravina islands consist of limestone, fine- to [1987], who report similar disturbanceof U-Pb isotopic coarse-grainedclastic sedimentary rocks, and basaltto rhyolite systems in metaplutonic rocks of the Alexander terrane. volcanicrocks (Figure 2) [Berg, 1972, 1973]. Theserocks Alternatively, the U-Pb isotopic disturbancemay be due to unconformablyoverlie pre-Devonianstrata of the Alexander later mid-Teritary hyrdothermalactivity and associatedfluid terraneand are unconformablyoverlain by the UpperJurassic flow [Magaritzand Taylor, 1986]. to Lower CretaceousGravina sequence (see discussion below). Karheen Formation. Interbedded siliceous argillite and TheseUpper Triassic strata recently described by Gehrelset al. micaceouslimestone overlain by distinctivepebble to cobble [1987] are not present on Cleveland Peninsula and conglomeratelayers, 30-50 cm thick, unconformablyoverlie RevillagigedoIsland. metaplutonicrocks on the shorelinesouth of Niblack Hollow on western Cleveland Peninsula (Figure 2). Clasts in the Taku Terrane conglomerateare matrix supportedand are usuallyless than 5 cm in diameter in an argillaceouslimestone matrix. Clasts Alava sequence.The Alava sequence(part of the the Taku consistof roundedto subangularplutonic and volcanicclasts, terraneof Berg et al. [1988]) is a distinctiveupper Paleozoic Rubinand Saleeby: Tectonic History of Eastern Edge of AlexanderTerrane 591

and lower Mesozoic stratifiedsequence and, along with the with the Triassicportion of the Stikine terrane(e.g., Stuhini Gravinasequence, is exposedalong the easternboundary edge Group [Rubinand Saleeby,1991a]). of the Alexander terrane along its 100-km extent in the Kah Shakessequence. The Kah Shakessequence (part of the Ketchikanarea. The sequenceis fault boundedon the eastby Taku and Tracy Arm terranesof Berg et al. [ 1988] comprises lower to middle Paleozoicorthogneiss, and locally on eastern metamorphicrocks exposedalong the westernflank of the RevillagigedoIsland the sequenceis intrudedby Cretaceous CoastPlutonic Complex and forms screensand septawithin andyounger foliated tonalire and granodiorite. part of the Coast Plutonic Complex (Figures 1 and 2) The Alava sequence,in southernsoutheast Alaska, consists [Saleebyand Rubin, 1990a]. The sequenceis boundedon the of two members:a lower memberof Upper Pennsylvanianand westby a thrustfault in whichthe Alexanderterrane forms the Lower Permian metavolcanic and metasedimentary lower plate, and is intruded on the east by the voluminous assemblage,and a uppermember of Middleto UpperTriassic Cretaceousand Tertiary plutonicrocks of the Coastbatholithic mixedmetasedimentary and metavolcanic assemblage [Rubin belt. On southernmostRevillagigedo Island, the Kah Shakes andSaleeby, 1991a]. The lowermember consists of massive sequencestructurally overlies the Alexanderterrane along an crinoidalmarble interlayered with black argilliteand phyllite, east-dippingthrust fault. The sequenceconsists of a thick and mafic metavolcanicflows, breccia,tuff, pillowed flows, successionof quartziteand carbonate-richturbidires, siliceous and minor quartzite. The upper member comprises and quartzo-feldspathicschist, subordinate metabasalt, pelitic carbonaceousand siliceousnodular limestone, argillite, mafic schist, and orthogneiss (Figure 4). Highly deformed metavolcanictuff, breccia,and pillowedflows. The baseof micaceousquartzite from easternCleveland Peninsula yielded a the Alava sequenceis not exposed, and the sequence very small,heterogeneous zircon population (sample 3, Table structurally overlies either the Gravina or Kah Shakes 1). Becauseof the very smallsample size, one fraction(62-80 sequencesor the Alexanderterrane. Locally, strataof the gm) was analyzedfor its "average"isotopic properties. A Gravinasequence unconformably overlie the Alava sequence Precambrian207pb-206pb average age of 2.03Ga for the [Rubinand Saleeby,1991b, c]. quartziteis a minimum age for the oldestcomponent in the On thebasis of lithology,broad age constraints, and tectonic fractionand indicatesthat Precambrian rocks formed part of the setting,the Alava sequencemight be correlativewith the ultimate sourcefor the quartz-richdetritus. To the southon upperPaleozoic parts of the Yukon-Tananaterrane, whereas PortlandPeninsula, similar Precambrian 207pb-206pb ages thelower Mesozoic portion of the Alava sequenceis correlated on Kah Shakes quartzite have been reported [Saleeby and

TABLE 2. U-Pb ZirconIsotopic Age Data for theAlexander Terrane, Kah ShakesSequence and Mid-Cretaceous Plutons Concentrations,ppm Atomic Ratios IsotopicAges, Ma Amount 206pb 206pb, 207pb* 207pb* 206pb* 207pb* 207pb* SampleFractiøn,1gm Analyzed, mg 238U 206pb* 204pb 238U 235U 206Pb* 238U 235U 206pb* Alexander terrane TrondhjemiteDike 1 45-62 0.6 130 7.9 7110 0.07289(44) 0.05403 0.05578(14) 438.0 438.7 443+4

Foliated QuartzDiorite 2 45-62 2.7 1167 53.1 42995 0.052550(36) 0.40425 0.05581(72) 330.2 344.7 444+26 2 62-80 2.3 991 49.2 50280 0.053064(34) 0.44088 0.05582(19) 359.3 370.9 445+8 2 >80 1.0 1201 67.1 5742 0.064319(54) 0.49498 0.05583(04) 401.9 408.3 445+5

Kah Shakessequence MicaceousQuartzite 3 45-62 0.2 93 4.6 32 0.032484(07) 0.56088 0.12529(17) 206.1 452.1 2032

Mid-Cretaceousplutons UnionBay UltramaficComplex 4 45-62 2.8 462 5.5 7007 0.013690(07) 0.09061 0.048026(11) 87.7 88.1 101+6 4 62-80 3.1 508 5.8 14901 0.013219(07) 0.08053 0.048053(09) 84.7 85.2 101+4 4 80-100 3.7 587 6.9 6889 0.103470(08) 0.08926 0.048077(15) 86.3 86.8 102+7 replicate 80-100 2.7 434 5.1 12982 0.013476(07) 0.08917 0.048019(09) 86.3 86.7 99+10 4 100-120 3.2 493 5.8 10950 0.013676(07) 0.09074 0.048144(20) 87.6 88.2 105+11 4 2 >120 3.3 449 602 11894 0.015931(09) 0.10541 0.048011(18) 101.9 101.7 99+ 18

Bell Island Tonalite 5 45-80 4.3 1190 14.3 8037 0.014910(09) 0.09406 0.048132(07) 90.7 91.3 105+7 5 80-100 6.3 449 5.7 412 0.014082(11) 0.09331 0.048083(19) 90.2 90.6 102+11

Eaton Point Granodiorite 6 <25 3.6 692 8.9 6955 0.014784(10) 0.09850 0.04834(08) 94.6 95.4 115+15 6 25-45 5.2 898 11.2 7421 0.014368(09) 0.09524 0.04809(11) 92.8 92.6 103+13 6 45-62 9.0 725 9.5 5532 0.014976(10) 0.09897 0.04795(39) 95.8 95.8 95+26 592 Rubinand Saleeby: Tectonic History of EasternEdge of AlexanderTerrane

TABLE 2. (continued)

Concentrations,ppm AtomicRatios IsotopicAges, Ma Amount 206pb 206pb, 207pb, 207pb, 206pb, 207pb, 207pb, SampleFractiøn,1gm Analyzed, mg 238U 206pb* 204pb 238U 235U 206Pb* 238U 235U 206pb* Mid-Cretaceousplutons (continued) SpaciousBay PorphyriticGranodiorite

7 62-80 7.5 752 9.7 7598 0.014910(10) 0.09852 0.047941(22) 95.4 95.4 95+26 7 80-100 5.9 870 11.3 6468 0.01492(10) 0.09847 0.047819(18) 95.6 95.4 90+11 7 >100 2.9 536 6.9 9036 0.014935(10) 0.99129 0.048157(37) 95.6 95.6 106+35

BushyPoint Epidote-Bearing Granodiorite 8 62-80 2.4 2248 28.7 1713 0.01479(10) 0.09772 0.04793(09) 94.6 94.6 95+5 8 80-120 5.8 1368 17.5 1333 0.01476(10) 0.09774 0.04793(09) 94.4 94.4 93+5 i Fractionsseparated bygrain size and magnetic properties. Magnetic properties arenonmagnetic using20ø side and 2ø front slopes at 1.7 A on FranzIsodynamic Separator. Samples hand-picked to 99.9% purityprior to dissolution.Dissolution and chemicalextraction techniquemodified from Krogh[1973]. 2 Zirconfraction leached ina PFAteflon beaker on a hotplate with HF for 7 daysat ~100øC and fluxed twice for 2 daysin 6N HC1 prior to sampledigestion. 3 Radiogenic:nonradiogenic correction based upon 40 picogramblank Pb (1:18.78; 15.61; 38,50) and initial Pb approximations: 206/204 = 48.6; 207/204 = 15.6; 208/204 = 38.2. 4 Decayconstants usedin age calculation: 3,238U = 1.55125x 10©, 3,235U= 9.98465x 10© [Jaffeyet al., 1971];238U/235U atom= 137.88.Uncertainties (+)in radiogenic milos calculated byquadratic sum of total derivatives of238U and 206pb* concentration and207pb*/206pb* equations with error differentials defined as follows: (1) isotope ratio determinations fromstandard errors (B/N) of massspectrometer runs plus uncertainties in fractionationcorrections based on multipleruns of NBS 981,983, andU500 standards;(2) spikeconcentrations from rangeof deviationsin multiplecalibrations with normalsolutions; (3) Spike compositionsfrom external precisionsofmultiple isotope ratio determinations; (4)uncertainty innatural 238U/235U from Chen and Wasserburg [1981]; and (5) nonradiogenicPb isotopiccompositions from uncertaintiesin isotoperatio determinationsof blankPb and uncertaintiesin composition of initialPb from estimatesof regionalvariations based on referencesgiven above and consideration of rock type.

Rubin, 1989, 1990a]. On the basisof lithologicsimilarities, are angular to subangularin shape. The age of the lower the presenceof Precambriandetrital zircon, spatialproximity memberis poorly constrainedand, on the basisof lithologic and tectonicsetting, the Kah Shakessequence is interpretedas and stratigraphic similarities with Upper Jurassic strata correlative with the Yukon-Tanana terrane, which is now exposedon GravinaIsland [Berg, 1973], the lowermember on recognizedwithin roof pendantsand metamorphicscreens as ClevelandPeninsula is interpretedas Late Jurassicin age. far southas latitude 59 ø, east of Juneau[Wheeler and McFeely, The upper member consistsof argillite, tuff, slate, and 1987; Gehrels et al., 1990; Gehrels et al., 1991; Rubin et al., conglomerateand is well exposedon Cleveland Peninsula, 1990b;Rubin and Saleeby,1991a]. Revillagigedo, Gravina, and adjacent islands. The lower contactof the member is not exposed;however, locally the uppermember overlies both the lower memberof the Gravina GravinaSequence sequenceand unconformablyoverlies the upperPaleozoic and lower MesozoicAlava sequence.The uppercontact is a thrust Upper Jurassicand Lower Cretaceousstrata structurally fault with adjacent terranes. Lithologic units consist of overlie lower Paleozoic rocks on southeastern Cleveland argillaceousand tuffaceousturbidires and pebble to cobble Peninsulaand locally lie unconformablyover Triassicrocks conglomerate,with a total structural thicknessof 900 m. on Annetteand GravinaIslands (Figure 2) [Berg, 1972, 1973; Conglomeratebeds occur in a distinctivemappable horizon, Brew and Karl, 1987; Rubin and Saleeby, 1991c]. These exposed between southeastern Cleveland Peninsula and stratabelong to the Gravina-Nutzotinbelt [Berg et al., 1972] southeasternRevillagigedo Island. Clastsin the conglomerate which we informally call the Gravina sequencein southern yield U-Pb zircon ages of 154 Ma to 158 Ma [Rubin and southeasternAlaska. The sequenceis exposedalong the Saleeby, 1991c]. These Late Jurassicages for the granitic easternmargin of the Alexanderterrane in muchof southeast clasts provide a maximum age for deposition. Based on Alaska. Near Ketchikan,the Gravinasequence consists of two geologicrelations with similar strataexposed to the north and distinctive members; however, stratigraphic thickness is sparsefossil data in theKetchikan area [Berg and Cruz, 1982], uncertaindue to complexdeformation. an Early Cretaceousage is inferred for the upper member On Cleveland Peninsula, the lower contact of the Gravina clastic and tuffaceous strata. sequenceis a thrustfault with the lower Paleozoicrocks of the Alexanderterrane (Figure 5). There,the Gravinasequence has an approximate structural thickness of 15 km. The lower Mid-Cretaceous Intrusive Rocks memberconsists of argillite, calcareousargillite, waterlaid coarsepyroclastic deposits, tuff, lavas, and minor intrusive Mafic and ultramafic rocks. Mid-Cretaceousmafic and rocks. Pyroclastic volcanic deposits dominate the lower ultramafic bodies underlie part of Cleveland Peninsula, member on Cleveland Peninsula and contain massive to Revillagigedo,and Duke islands(Figure 2), The ultramafic pillowed flows and flow breccia,commonly with claststhat complexesform a linear belt that spansapproximately 560 Rubinand Saleeby: Tectonic History of EasternEdge of AlexanderTerrane 593

Fig. 4. Amphibolite-faciesrocks of quartziteand marble in theKah Shakessequence exposed north of RudyerdInlet on theeast- side of BehmCanal.

120 110 100 90 80 Ma I i i i i Sample # Cleveland ,/""/ breccia Peninsula/ / {-• , I Grav,.... q..... k ¾ / /;-.mArgillite 6•n Point 1•%_½ / •01•';;J•;• I• Maf,ctu,andvo, can,cf,ows • 7 • SpaciousBay • _ •y Pom, • gg2•nO / • 100...... , • • ,oliation=•,ng !sland Fig. 5. Geologic sketch map of the contact between the 4 [.•._..•__ __Tertiary Pb loss...•-r' Alexander terrane and the Gravina sequence,exposed in an unnamed cove northeast of Caamano Point, southern HFLeach Bay Cleveland Peninsula. Regional location is shown in inset I i i I ! 120 110 100 90 80 Ma (KetchikanC-6 quadrangle). 238U/2o6 Pb age Fig. 6. Concordiaplotted separatelyfor each sample,with km, from northwestern British Columbia to southeastAlaska scalefor 207pb/206pb ratio shown. Each segment contains [Taylor, 1967]. The complexesintrude rocks of the Alexander data points and error bars for indicatedsamples of igneous terrane,and Gravina and Alava sequences.The bodiesoccur as rocks from Cleveland Peninsula and Bell Island. Bars at ends zoned ultramaficcomplexes ranging in compositionfrom of concordiasegments show uncertainty in 207/206pb values dunite,in thecenter, to pyroxeniteand hornblende pyroxenite of concordiabased on uncertainties in 238U and 235U decay on the outer border [Taylor, 1967; Irvine, 1967, 1974]. constantsfrom Mattinson's[1987] treatmentof Jaffey et al. Interstitial gabbroicpods and lensesare presentin the [1971] data. Concordiadiagram after Tera and Wasserburg hornblendite.Plagioclase from the gabbrois extensively saussuritized.Zircon from gabbropods and veins within [1972]. Linear regressionand errors in lower and upper interceptsare adaptedfrom York [1969]. hornblenditein the Union Bay UltramaficComplex (Table 1) yielddiscordant 238U-206Pb ages ranging from 84.7 to 88.1 Ma (sample4, Table 2). A leachedcoarse zircon fraction yieldsa concordant238U_206Pb age of-101.5 Ma (sample 4, Cretaceouszircon populationsrather than inheritancefrom olderxenocrystic zircon. This interpretationis consistentwith Table2). Five fractionsare dispersed off concordia(Figure 6), the lack of compositionalzoning or opticallydistinguished which may reflect later disturbanceof the U-Pb isotopic coresin zircon. WidespreadMiocene volcanism, hydrothermal systemsin zircon. The 238U-206pb and 235U-207pb ages activity and associatedfluid flow have been recognizedin areslightly discordant at--- 86 Ma; however, the 207pb-206pb southeastAlaska [Magaritzand Taylor, 1986] and may have agesare olderand range between 99 and 105 Ma (Table2). beenresponsible for the isotopicdisturbance in the UnionBay Discordanceprobably reflects later isotopicdisturbance of zircon. On the basisof the foregoing,the gabbroicpods 594 Rubinand Saleeby: Tectonic History of Eastern Edge of Alexander Terrane

withinthe hornblenditeare assignedan approximateage of by Zen [1985, 1988], Zen andHammarstrom [1984], Hollister 102Ma, which represents theoldest 207pb-206pb agefor the et al. [1987], and Cook et al. [1991]. The rocks are sampleand the 238U-206Pb age for the leachedzircon characterizedby apple-green,euhedral epidote and contain fraction. K-Ar analyseson hornblendeand biotite of the quartz,plagioclase, microcline, hornblende, biotite, and locally ultramaficbodies yield 100-110Ma coolingages [Lanphere garnet. Magnetite,ilmenite, apatite, zircon, sphene,and and Eberlein, 1966]. K-Ar analyseson the Union Bay allaniteoccur as accessory minerals. The plutons are usually ultramaficbody are consistentwith the 108 Ma U-Pb zircon massivein theinterior and foliated along their margins and ageobtained from a pegmatiteassociated with the Duke Island intrudethe Gravinaand Alava sequences and, locally, the ultramaficcomplex [J.B. Saleeby, unpublished data, 1990]. Alexanderterrane. The bodiesdisplay intrusive contacts and Quartzdiorite and granodiorite.Northern Revillagigedo arelocally in faultcontact with adjacent metasedimentary and Islandand Cleveland Peninsula are underlainby a composite metaplutonicrocks. U-Pb zircon ages for theplutons range batholithcomposed of massiveto foliated,medium- to coarse- between94 Ma and 101 Ma, andlocally zircon systematics grainedhornblende quartz diorite, tonalite, and granodiorite revealProterozoic inheritance (e.g., the Moth Bay Pluton (Figure2). TheBell IslandPluton on northern Revillagigedo [Rubinand Saleeby, 1987]). A hornblendeAr40/Ar 39 plateau andBell Islandsand northern Cleveland Peninsula is partof ageof 97 Ma is reportedby Sutterand Crawford [1985] for the the compositebody (Figure 2). Subhorizontaldikes and veins Moth Bay Pluton. of quartz-feldsparpegmatite and subordinateagmatite are U-Pb zircongeochronology. Mid-Cretaceous ages were presentthroughout the batholith.The rockscontain varying determined for all four plutons on northern Cleveland proportionsof plagioclase,microcline, quartz, hornblende, and Peninsulaand Revillagigedo and Bell islands. The porphyritic brown biotite. Apatite, zircon, sphene, ilmenite, and granodioriteof SpaciousBay (sample7) shows95 Ma magnetiteoccur as accessory minerals. A leucocraticquartz concordanceof all the fractions,with slightdispersion of its diorite(Bell IslandPluton, sample 5, Table 1) yieldsU-Pb coarsefraction off concordiawhich probably reflects minor zircon age of 90.5 + 7 Ma (Table 2). These data are contaminationby olderzircon from its country rocks (Figure consistentwith a preliminaryU-Pb zircon age of 89 Ma 6). The EatonPoint granodiorite (sample 6) is discordant, reportedby Arthet al. [1988]. K-Ar analysesyield hornblende with its coarsefraction lying on concordiaat - 95.8 Ma cooling ages that range from 86 to 55.5 Ma, and biotite (Figure6). The BushyPoint epidote-bearinggranodiorite coolingages that range from 74 to 49 Ma [Smithand Diggles, (sample8) yieldsa U-Pbage of 94.5Ma, withPb-Pb ages of 1981]. The batholithintrudes the Gravinasequence on 95 + 5 and93 +5 Ma, respectively(Table 2). The two finer northernCleveland Peninsula, the Kah Shakessequence on fractionsare dispersed off concordia,which may reflect later east Behm Canal, and the Alava sequenceon western disturbanceof theU-Pb isotopicsystems in zircon,perhaps RevillagigedoIsland. due to the emplacementof the youngerBell Islandpluton Plagioclaseporphyritic granodiorite, quartz diorite, and (sample5) or Miocenehydrothermal activity discussed above. tonalite. Elongate stocks, plutons, sills, and dikes of A youngerisotopic disturbance is recordedby K-Ar ages plagioclaseporphyry and quartz diorite intrude much of [Smithand Diggles, 1981]. Quartzdioritic rocks yield K/Ar Revillagigedoand adjacentislands and ClevelandPeninsula agesof--- 74 Ma (biotite)and 81.8 to 79 Ma (hornblende)on (Figure 2). The Eaton Point Pluton, exposedon northern northernCleveland Peninsula and on adjacentislands ClevelandPeninsula, is assignedto this unit. Locally,the southwestof Bell Island (uncertaintiesin the K-Ar datesare rockscontain abundant plagioclase phenocrysts, which make reportedat the lc• level). The dispersionof the finer fractions up to 50% of the rock and range between 1 and 4 cm in may also be due in part to contaminationwith older zircon. diameter. The groundmasscontains fine-grained quartz, Disturbanceof the U-Pb isotopicsystems may havebeen microcline,biotite, minor hornblende,epidote, and garnet. morecomplex than a singleepisode of leadloss, perhaps due Euhedralgarnet occurs within cores of plagioclase,but it also to a combination of Pb loss and contaminationof older zircon. occurswithin the groundmass. Anhedral to euhedralepidote is Thequartz diorite of BellIsland (sample 5) yieldsa U-Pbage interstitial to plagioclasecrystals. The plutonsintrude of 90.5 Ma, with olderPb-Pb ages of 102 + 11 and 105 + 7 metasedimentaryand metavolcanicrocks assignedto the Ma, respectively(Table 2). A 90 Ma age is consistentwith Gravinaand Alava sequences, and locally the Alexander terrane nearlyconcordant 206pb/238U zircon ages of •- 89 and91 Ma (?), and have narrow contactaureoles that overprintthe [Arth et al., 1988];Pb-Pb ages were not reportedin their metamorphicfoliation. The sills and dikesare parallelwith study. foliationand locally crosscutthe metamorphicframework rocks, increasingin abundancetoward the east. The Eaton PointPluton, a medium-grainedgranodiorite, yields a U-Pb STRUCTURAL GEOLOGY zircon age of 95+3 Ma (sample6, Tables 1 and 2). A Only recentlyhave mid-Cretaceous basement-involved thrust plagioclaseporphyritic granodiorite phase of the EatonPoint faultsand associatednappe structures been recognized in Plutonyields a U-Pb ageof 95+1Ma (sample7, Tables1 and southernsoutheast Alaska [Berget al., 1972, 1988; Crawford 2). A granodioritephase of the BushyPoint Pluton (Bushy et al., 1987;Rubin et al., 1990a;Rubin and Saleeby, 1991b]. PointPluton, sample 8, Table 1) yieldsa U-Pb ageof 94.5 Near Ketchikan, deformation of lower Paleozoic to mid- Ma (Table2). Thesedata are consistentwith a preliminary Cretaceoussupracrustal and plutonic rocks is localizedalong discordant238U-206pb zircon age of 92.7Ma reported by the eastern boundary of the Alexander terrane and is Arth et al. [1988]. K-Ar analysesyield 82 + 2.5 Ma characterizedby an imbricateseries of west- to southwest- (hornblende)and 86 _+2.7 Ma (biotite)cooling ages [Smith vergent thrust sheets. Based on regional geologicand and Diggles, 1981]. geochronologicrelations, the west-vergentfold and thrust Epidote-hornblendetonalite, quartz diorite, and granodiorite. systemwas active between about 113 and 89 Ma. Younger Epidote-bearinggranodiorite, quartz diorite, and tonalite occur intrusiverocks do notdisplay the mid-Cretaceous thrust-related as elongateplutons on southwesternand part of northeastern fabrics. Both the younger intrusive rocks and the rocks RevillagigedoIsland (Figure 2). Thesedistinctive plutons affectedby mid-Cretaceousdeformation were subsequently continueas a beltfarther north into the Petersburg and Juneau affectedby late Paleoceneto earlyEocene deformation and area(Figure 1) [Brewand Morrell, 1983]. Thepetrology and associateduplift [Hollister, 1982; Crawford et al., 1987; mineralchemistry of theepidote-bearing plutons are discussed Gehrels and McClelland, 1988; Gehrels et al., 1992; Rubinand Saleeby: Tectonic History of Eastern Edge of Alexander Terrane 595

McClelland et al., 1992,a, b]. Deformation and plutonicbodies. Older fabricsrecord ductile southwest-vergent metamorphismwhich precededmid-Cretaceous tectonism are folding and faulting and regional metamorphism,whereas discussedby McClellandand Gehrels[1990] and Saleebyand youngerfabrics are characterizedby crenulationcleavage and Rubin [1990b]and appear to representpre-Late Jurassic dextral thrustfaulting and associatedfolding. Structuralstyle varies strike-slipdismemberment of the Alexanderterrane, possibly along the strike of the orogeniczone and betweenstructural during its accretion, and subsequentLate Jurassic-earliest levelsdue to changesin rock type,overall metamorphic grade, Cretaceousextension across the accretionary suture. and proximity to plutonic rocks. Cross sectionscannot be Mid-Cretaceous deformation occurredduring three main rigorouslybalanced due to (1) changesin bulk-rockvolume, episodesthat correspondwith the emplacementof sill-like (2) the presenceof ductilestrain and associated plastic flow, and (3) the absenceof exposedstep-up angles along thrust faults.

First-Generation Mid-Cretaceous Thrust Faults

The lowest thrust sheets consist of lower Paleozoic schist, marble, and metaigneousrocks of the Alexander terrane (Figures 3a and 3b). These rocks are characterizedby mesoscopicfolding, nonpenetrative foliation, and thrust faulting and display lower-greenschist facies mineral assemblages.Early deformational fabrics (D1) are characterized by mesoscopic northeast dipping foliation surfaces(S 1)and a bedding-foliationintersection lineation (L0xl). Thrustfaults trend northwest and dip moderately to the northeast. The Gravina sequencestructurally overlies the Alexanderterrane, and on ClevelandPeninsula the sequence consistsof six separatewest-vergent thrust sheets (Figure 3b). The basalthrust fault separatingthe Alexanderterrane from the Gravina sequence crosscutsthe metamorphic foliation, displaysa cataclasticfabric, and juxtaposes shallowly dipping Gravinasequence argillite over the moderatelydipping lower PaleozoicDescon Formation of the Alexanderterrane (Figure 5). Lower Paleozoicand upperProterozoic crystalline rocks of ß •=58 '• I,,I=$4 ß I,,1=$$ 0•=•"1 the Alexander terrane form structural basement to the thrust Fig. 7. Lower hemisphere,equal-area plot showing, (a) poles belt. In general, thrust faults trend northwest, and their to foliation surfaces on northwest Cleveland Peninsula, (b) orientation generally follows the regional metamorphic poles to foliation surfaceson southernCleveland Peninsula, foliation (Figure 2). (c) trendand plungeof small-scalefolds and polesto foliation There is a predominanceof moderate,northeast-dipping surfacesin higher level thrustnappes on northernCleveland foliation surfaces (Figure 7a)which are axial planar to Peninsula,and (d) poles to foliation surfaces,and trend and asymmetric, west- to southwest vergent folds. These plungeof elongationlineation in higherlevel thrustnappes. asymmetricfolds (Figure 8) are truncatedby thrustfaults and

:•;?.i:'•'""'"'•'•: ...... ":•?•!•!?;%...... ;:;:•%.•..• ...... •'•'•" ½:"'•"'?.::::::::::::::::::::::: •;":,•?,'•':•-•-"• ...... •.• .....'•?:::•-•.•. •. ',.::•"•:•':• •:.•"'• :.:::...':.:•-•:•::;:::.:•?.:...... '4:-•:...... '•;...... *:' ß...... '•'•,? ::•'.•..•: •." "•.%.: -* .... ½•....• ';•'"'":•"%?•;%-•....

%•','.:':..... "'":'}::?;'•:•[::;•g}-:•5:::;;.:-•-'.-: :•:.::-'---.-:-::..:';:....-...:...::.....-.-.,.,.• ..... :•:•:.;..:•:½:.,' ...... ½i• -•:'"'"';•'•'--'*•' %":"}•:;•.;.'½-.:-% '• :' • ''.'.' ½'"'.;:4'• '"•;:;*'".*• • ...... •':'%..i'- . ..:L;:•:%.,.-:.:•.:. :-;•u..E•:.%•'::•...... ?..:::.:•:::-:•?•.::.:;:..•;;%•:::::•:;.:...•:•,•;...::•:,•½.•.•:.%..,::• ...... :.:-'::;•.r,':•:•½...: ½,'-.•,•-...--:-:..----.----• ... •:•.... '•,. '-!.•}....;:•.•:.•;:.;•:....:..;½:. : ..--•'. . %•4- •.• ½ß •-.-...... :.;.. -•". ß.'•..... ,...' ...... ":'½•::..•..:.. . ½';•:':"•5. .:-..,:•..:....•...... -.';¾'""?.?;;•;;;•:;;'":v. •. :-.•• .• .•:•:?½•;½½?':'":.:;:'::•;"½ •...... •:...... '-?..'½'•:•'½*'"?•. . ..•...... '-... •"•"- ,:. •.'•'...: • •, .•r ..• •. .•. •_•';" ...... ,. -,:.:.... .: :.,•:-.2:...... •:•;4;...... :.:.;:.:.:-:•.:. ;.:-•;.•..:;;...½ .•, • ...... :- .%... . •. -•:. ...•: _ .-..•...... :•...:•...... •:•...... :.:...... :.-.;•.,. •.... ,.• •,. . :.:

::. ..., • . ,--:.•;• ....• s:•:..• •, .•-:½&' .•.... ½'-.• .. '•:'•.:;{:8 '". "- •b ß

...... ;:- ...... 4:•. ß.:,•..• ...... , •-..;..;;•:;;.•...:;.;..:.: .:•..?';.•:•."....•,;;- .; : ...... : .:. ""' •%• .... '...... ; ;...'...... :½:,:':•}•:•,.."':'";::•i':'i.• ...... •.::;::•;:•:,." ,;' ß•...... •:•,. '.,'...Q•-.•:?.-.:•...... ::..,½L::'.;::,. . ,.•..-.... }.&. %.... .•..

... ..';;:•':;?•;•:::;•-; .....: ....•;:•:• ...:• ß •.--,%}::.•.-•.,;•: ...... -,-•.'

:•::•-•'•½•½:•.::;, ...... ,• ...... •:

Fig. 8. Asymmetricfolds truncatedby moderatelydipping thrust faults in the Permianpart of the Alava sequence,exposed on RevillagigedoIsland. This fault zoneplaces Permian marble and metabasaltover Triassiccarbonaceous pelitic schist. 596 Rubin•md Saleeby' Tectonic History of EasternEdge of AlexanderTerrane appearkinematically linked to thethrust faults. In well-bedded pressureestimates obtained for the Moth Bay Pluton, and strata, compositionallayering and foliation are commonly based on aluminium content in hornblende [Zen and subparallel,whereas massive volcanic flows andbreccias do Hammarstrom, 1984], and the experimental calibration of not display a foliation and were deformedas rigid bodies. Johnsonand Rutherford[1989] imply pressureof 5 to 6 kbar Foliation parallelsflattening fabrics that are well developed and temperaturesof--- 550øC. With the samegeobarometer, within conglomeratichorizons. The S1 foliationis definedby pressureestimates for the BushyPoint Pluton, locatedto the mica grainsand elongate,slightly flattened plagioclase grains northeast(Figure 2) [Zen and Hammarstrom,1984], yield that are parallel or subparallelto compositionallayering. pressuresin excessof 6 kbar. In higherstructural levels, on Rocksof the Gravina sequenceare structurallyoverlain to the northernCleveland Peninsula and Revillagigedo Island (Figure northeast,along the Point Francisfault system(Figure 2), by 2), kyanite + sillimaniteschist occur as eitherbladed crystals upper greenschist facies rocks belonging to the upper up to 1 cm long or bundles of rods which crosscutthe Paleozoicand lower MesozoicAlava sequence.Moderately dominant northeast trending foliation. The presenceof dipping,west- to southwest-vergentductile fault zonesare kyanite-staurolitemineral assemblages,midcrustal level characteristicof the structurallyhigher thrust sheets. There is plutons(e.g., Bushy Point Pluton), and the occurrenceof a predominanceof moderate northeast-dippingfoliation tonaliresills and dikesthat lack contactaureoles suggest upper surfaces(S1) which are axial planar to asymmetric,west- to amphibolitetemperatures of at least550 ø C andpressures of- southwest-vergent folds (Figures 7b and 7c). These 6-7 kbar [e.g., Turner, 1981] in the higher structuralthrust asymmetricfolds are truncatedby thrustfaults (Figure 8) and sheets. appearto be kinematicallylinked, as above,to thrustfaults. The Alava sequenceis locally structurally overlain by amphibolitefacies rocks assigned to the lower PaleozoicKah Second-Generation Mid-Cretaceous Thrust Faults Shakes sequence. Kah Shakesrocks are characterizedby attenuatedfold limbs and a moderatelydeveloped down-dip The SouthernRevillagigedo Island fault zone (Southern elongationlineation that is parallelto fold axes. On northern Revillagigedo Island shear zone of Cook et al. [1991]), Cleveland Peninsula and northeastRevillagigedo Island exposedon southwesternRevillagigedo Island between Thorne (Figure2), Kah Shakesfoliation trends northwest. Arm and Carroll Inlet (Figures2 and 9), is a major structural Metamorphicgrades increase from lowerto higherstructural boundary in the Ketchikan area. The fault zone dips levels. In the lowest structurallevel, phyllite in the Gravina moderately northeastwardand juxtaposes mid-Cretaceous sequencecontains white mica, chlorite,epidote, plagioclase, tonaliticsills, dikes,and a largepluton, along with their host rocks, over nondeformed mid-Cretaceous sills and dikes and and calcite which are typical of lower greenschistfacies metamorphism. Fossils are preserved in carbonaceous Gravina sequencemetasedimentary and metavolcanicrocks limestoneand argillite. Contact metamorphicaureoles are (Figure 3b). To the northwest,the fault zone cuts tonalitic presentnear plutonsand are best observedon Spire Island sills and segmentsof the Moth Bay Pluton. Regional located northeastof Annette Island (Figure 2). There relict metamorphicand structuralfabrics are truncatedby the fault andalusite,replaced by white mica, occursin phyllite in a zone. Fabricsproduced by simpleshear are well preservedin narrow zone adjacent to the Spire Island diorite, which the fault zone. Igneousrocks in the fault zone commonly indicates low pressure and low to moderate temperature displayblastomylonitic textures, and, locally, relict igneous conditions. Garnet porphyroblastsoccur in phyllite of the textures are completely overprinted by ultramylonite Gravina sequenceadjacent to the Union Bay ultramafic consistingof very fine grainedquartz-bioite-epidiote mineral assemblages.Rotated plagioclase porphyroclasts, asymmetric complex. There, the regional fabric, definedby the planar recrystallizedtails, and S-C fabrics in the igneousrocks orientationof biotite and white mica, overprintsthe contact suggesttop to the northwestor an oblique, senseof motion. metamorphicaureole, showingthat the emplacementof the Br•ciated, oblate clastsof weakly deformedplutonic rocks ultramaficcomplex pr•eded regionalmetamorphism. (Figure 10) are surroundedby a foliated micaceous,locally Mafic volcanicrocks are widespreadin the lower structural mylonitic matrix and commonly display S-C fabrics which levels and contain relict euhedral phenocrysts of indicatea similarsense of shear. The timingof motionon the clinopyroxene,hornblende, and plagioclasein a fine-grained Black Mountain fault is constrainedby two lines of evidence. microgranular matrix of relict pyroxene, amphibole, The fault zoneaff•ts plutonicrocks with a U-Pb zirconage of metamorphicactinolite, chlorite, epidote, albite, quartz,white 101 Ma [Rubin and Saleeby, 1987], so much of the mica,and calcite, typical of low-temperaturegreenschist-facies deformation must have postdatedthe emplacementof the metamorphism. Relict clinopyroxene and hornblende pluton. Although muchof the deformationpostdates the 101 phenocrystsin the volcanic strata are unstrainedand are partially replacedby chloriteand white mica. Preliminary oxygen isotopic analyses on mafic phenocrystsindicate SWAndalusite SouthernIsland FaultRevillagigedo Zone P> 6 kbar NE primary igneous •5180 values which imply that the contact aureole Moth Bay Pluton phenocrystsdid not interactwith hydrothermalfluids (C.M. Rubin and H. P. Taylor, Jr., unpublished data, 1987). Andalusite in contact aureoles,lower greenschistfacies metamorphic mineral assemblages,and abundant relict phenocrystsin volcanicstrata imply pressuresno greaterthan 3.5 kbar [e.g., Holdaway, 1971] and generally low temperatures(<400øC, [e.g., Turner, 1981]) in the lower , lkm • structural levels. :::/•OGravinaphyllite &sequence schist W Alavasequence schist Toward the east, in successivelyhigher structurallevels, biotite and garnetappear, indicating upper greenschist facies • Alexandermetaplutonicterranerocks •"•Q quartzMid-Cretaceous diorite, & tonalite,granodiorite metamorphism.Locally, stauroliteis presentin pelitic strata, and hornblende-plagioclase-garnet+ epidote mineral Fig. 9. Geologic cross section across the Southern assemblagesare presentin mafic metavolcanicrocks. These Revillagigedo Island fault zone, exposedon southwestern lower amphibolite mineral assemblages,combined with RevillagigedoIsland. Rubinand Saleeby: Tectonic History of EasternEdge of AlexanderTerrane 597

:i:...... -•.:.::•. -•,'.•.:•::•:•,..::..:•.•u•.• 7}•' :, .? ..•.. "' : Fig. 10. Weakly deformedtonalitic clasts in a myloniticmatrix. Thesefabrics developed within the SouthernRevillagigedo Island fault zone on southemRevillagigedo Island.

Ma Moth Bay pluton, contactmineral assemblageslocally pressures on rocks exposed on either side of the fault, overprintblastomylonitic fabrics in the fault zone,implying however,a qualitativedisplacement estimate can be obtained. that intrusion and contact metamorphismwere partly The presenceof kyanite-stauroliteschist gives a minimum contemporaneouswith faulting. pressureof 4.5 kbar for the hangingwall metasedimentary The fault zone probablyextends to the north along the rocks. In addition,thermobarometric estimates for the upper southwestside of RevillagigedoIsland (Figure 2), where a plate were determinedby Cook et al. [1991] and rangefrom thrust fault strikes approximatelyN20ø-30øW. Associated -7.9 + 0.6 kbar to -8.5_+1 kbar. On the basis of aluminum southwest-vergentasymmetric folds trend S60ø-70øEand contents in hornblende from the Moth Bay Pluton plungemoderately to the southeast(Figure 1 la). Numerous [Hammarstromand Zen, 1986; Cook et al., 1991] and using quartz veins are present in the fault zone. In the footwall, the experimentalcalibration of Johnsonand Rutherford[1989], Gravinasequence rocks are well bedded and are characterized by a pressurerange from 6.0 to 7.9 kbar is recordedfor the pluton lowermost greenschistfacies mineral assemblagesand exposed in the upper plate of the thrust. The ranges in unstrainedrelict phenocrysts.On the basisof theserelations, pressuremay be due to variabilityof AIT in hornblende. the footwallblock appearsto have undergoneminor finite Thesepressures are in agreementwith the inferredestimated strain and metamorphism. In contrast, the upper plate crystallizationpressure for the Moth Bay plutonbased on the containsrocks that locally have biotite-grade greenschist facies presenceof magmaticepidote [Zen and Hammarstrom,1984]. mineralassemblages and well-developedfoliation surfaces and Pressuresof 3.0-3.5 kbar in the lowerplate can be inferredby that are cut by numerousdeformed sills and dikes of diorite. the andalusitecontact aureole and associatedregional lower As the faultzone is approached,abundant quartz veins appear greenschistfacies mineral assemblages. Assuming that in the metamorphosedhanging wall lithologies. pressureestimates are correctand were locked in prior to The amountof displacementon the fault zone is difficult to faulting,the hangingwall was uplifted-14 km with respectto estimate.By usingdifferences in metamorphicand igneous the footwall. A minimum dip-slip displacement of

N N N

INtel ON•22 *N.7 IN.•4 N•51 CJ.. 40'

Fig. 11. Lowerhemisphere equal-area plot showing(a) polesto foliationsurfaces, small-scale folds, and lineationsalong the southernRevillagigedo fault zone, (b) polesto foliationsurfaces and small-scale folds of D1 and D2 fabrics,and (c) Contourof polesto dike orientation.C.I. is confidenceinterval [after Kamb, 1959]. Solidcircles are S1 foliation;Open squares are S2 foliation;Asterisks are trendand plungeof smallscale folds; Open circles are trend and plunge of linearion. 598 Rubinand Saleeby: Tectonic History of Eastern Edge of Alexander Terrane

approximately 24 km is inferred across the Southern fault zone is difficult to estimate, due to the absenceof RevillagigedoIsland fault zone,assuming the fault wasnot stratigraphiccutoffs along the fault zone. A minimumof subsequentlyrotated during post-Cretaceous time. Although displacementof 7 km betweenthe hanging wall andfootwall elongationlineations trend oblique to the dip of the fault, is inferred,on thebasis of metamorphicpressure estimates on concurrentstrike-slip displacement across the fault cannotbe pelitic supracrustalrocks exposed on eitherside of the fault quantified. zone. Strike-slip displacementacross the fault cannotbe quantified;however, on thebasis of thesoutheast plunging stretchinglineation associatedwith asymmetricfolds, Third-Generation Ductile Fault Zones substantialdisplacement may be likely. Steeplydipping, north- to northwest-strikingductile fault zones crosscutthe earlier (S 1) fabrics in the easternand Zonationof the ThrustBelt northernpart of thethrust belt (Figure2). The faultzones are characterizedby a stronglydeveloped foliation which reorients High-anglereverse faults, represented by thenorthern and earlierS 1 fabrics. Structuralrelations suggest that reverse southernRevillagigedo fault zones,divide the thrustbelt into faulting occurredafter F1 folding and associatedthrust three of the four mid-Cretaceousstructural domains shown in faulting, and possiblyduring the late stagesof the Figure13. The subsurfacegeometry of the thrustfaults and emplacementof the mid-Cretaceoustonalire plutons. The ductilereverse faults is notaccurately known. By combining relationshipsbetween thrust faulting, regional metamorphism, availablegeologic and petrologic constraints, a reasonable and andlater reverse faulting described below suggest that these are internallyconsistent subsurface geometry can be constructed. likelythe result of a singleprogressive deformational event. Domain1, locatedon thewestern portion of thethrust belt, Thenorthern Revillagigedo Island fault zone, exposed on the consistsof rocksthat have undergone little finite strainand northwesternportion of RevillagigedoIsland (Figure 2), dips thatcontain low-temperature and low-pressure metamorphic steeplynortheastward. In the fault zone, earlier northwest- mineralassemblages. Typically, rocks in thisdomain are well trendingS 1 fabrics(regional foliation) are preserved within a beddedand consistentirely of the Alexanderterrane and compositionallybanded biotite schist and are completely Gravinasequence. The centralregion, domain 2, consistsof transposedand tightly refoldedwithin a seriesof ductileshear imbricatethrust nappes of rocksthat are assignedto the zones(Figure 12). The metamorphicrocks are cut by Alexanderterrane and Gravina and Alava sequences. The rocks numerousquartz veins in whichnorthwest-trending S1 quartz displaywell-developed metamorphic fabrics and contain veinsare folded and attenuated; later quartz veins formed along mediumpressure and temperature-sensitivemetamorphic theaxial surfaces of S2 folds. Mostaxial planes of S2 folds minerals. The appearanceof mid-Cretaceoustonalite and trendbetween N40 ø to 60øWand dip from 70 ø to thenortheast granodioriteis characteristicof this domain. Higher- to 90ø. Folds in bandedbiotite schistare disharmonic, temperatureand higher-pressure (?) mineralassemblages are generallytight to isoclinaland commonly asymmetric. The recorded by rocks in domain 3, which consists of the foldaxes trend S40 ø to 80øEand plunge moderately to the Alexanderterrane, and Kah Shakes,Alava, and Gravina southeast.The limbs of thefolds are extremely attenuated and sequences. There, amphibolitefacies rocks are dominant. locallydisplay isolated asymmetric floating hinges. A well- Well-developedelongation lineations, highly attenuated fold developedelongation lineation is associatedwith S2 foliation limbs, and polyphase fabrics characterizethis domain. Metamorphicrocks are intrudedby tonaliticsills and dikes, andtrends S40 ø to 60øE,plunging moderately to steeplyto the southeast. and,locally, lit-par-lit structures are well developedin the high-gradegneisses of domain3. Typically,rocks in Domain The northernRevillagigedo Island fault zoneseparates 3 are intruded by late Paleoceneintrusive rocks and are supracrustalmetamorphic sequences of widely differing metamorphicgrade. In the footwall block, southof the fault overprintedby earlyTertiary extensional deformation. zone, supracrustalrocks display garnet greenschist-facies Mid-Cretaceousshortening was accommodated through the mineralassemblages, whereas to the northin the hanging developmentof crenulationcleavage and is bestexposed near wall, kyanite-bearingamphibolite grade schist and the mid- NahaBay along the northwestern shore of RevillagigedoIsland CretaceousBell Islandpluton are present.On the basisof (Figure2). This fabricis definedby mesoscopic,northeast thesegeologic relations, top to thenorth or a reverse-senseof trendingspaced cleavage (S2), S 1- S2 intersectionlineation motionis recorded.The magnitudeof displacementon the (Llx2), andassociated west- to northwest-vergentasymmetric kinkfolds (Figure 1 lb). Thecleavage forms small-scale folds in phyllite,whereas in massivemetavolcanic rocks a planar Youngershear zones (S2) widelyspaced cleavage is present(Figure 1 lb). Thecleavage Domainalshear zones with andquartz veins is definedby theconcentration of phyllosilicates in fold limbs. isolated fold hinges New mineralgrowth parallel to the kink fold axial planes consistof fine-grainedbiotite and garnet.Quartz grains are recrystallizedand slightly flattened to theaxial planes.

LatePaleocene Deformation view towards the southeast Paleoceneand younger(?) deformationhas affected rocks on

0 30 cm thewestern margin of theCoast Plutonic Complex (Figure 2). Folded S1 foliation and quartz veins I I Deeperstructural levels are exposed along the eastern edge of theAlexander terrane and the Alava sequence and are dominated Fig. 12. Geologicsketch of transposedfoliation (S1) and by low to moderateeast-dipping sheets of schist,gneiss, and associatedisoclinal folds in bandedkyanite schist along the pht/llonitetermed the east Behm Canal gneisscomplex NorthernRevillagigedo fault zone. Elongation lineation dips [Saleeby, 1987; Saleeby and Rubin, 1990a]. Shallow moderatelyto the southeast. northeast-plunging elongation lineations are common; Rubinand Salecby: Tectonic History of EasternEdge of AlexanderTerrane 599

Domain I Domain II Domain III Domain IV

I SouthernRevillagigedo NorthernRevillagigedo bendinsection FaultZone FaultZone •,• Pt.Francis Fault \ Med.pressure Low-med. pressure Lowpressuretemperature \ Med. pressuretemperature LateTeritary •,• • Hightemperature Low strain (?) (?) Low strain •1 Moderatestrain mafic dikes \ • High strain

6km SouthernRevillagigedo Imbricatethrust sheet• \ Kyanite ' }]' - '•' ' "CoastPlutonic nT mgals Fault Zone mid-Cretaceous Complex 50,000 0 54,000 -60 Bøugtte•vityProfi/e 0 0 0 0 0 0 0 -'"-'"'•O'""•'•O'•-Oøroo,, 56,000 -100 AeromagneticProfile(after U.S.G.S., 1977)

DOMAIN I: AT & GS DOMAIN III: AT, KSS, AS, & GS (?) DOMAIN II: AT, GS, & AS DOMAIN IV: KSS & AT ,[• Mid-Cretaceous• Tertiaryintrusive rocks intrusive rocks CMR '89

Fig. 13. Aeromagneticand gravity profilesand geologiccross section across the westernwall rocksof the Coast PlutonicComplex showingthe major mid-Cretaceousstructural domains. Abbreviationsare AS, Alava sequence;AT, Alexanderterrane; GS, Gravinasequence; and KSS, Kah Shakessequence. Med. is medium. Domain IV is not discussedin thispaper. however, penecontemporaneousrecrystallization overprints margin of North America prior to depositionof the Upper thisfabric. There is a predominanceof moderatewest-dipping Jurassicto Lower CretaceousGravina sequence. foliation surfaceswhich are axial planar to asymmetriceast- The record of sedimentation,magmatism, and deformation verging folds. These east-vergingfabrics have reoriented within the Gravina belt spansan intervalof almost50 m.y., earliermid-Cretaceous fabrics. Late Paleocenepegmatite dikes from the Late Jurassicto the end of the Early Cretaceous. are highly deformedand are affectedby theseeast- and west- Beginningin the Late Jurassic,primitive arc-type(?) basaltic dipping structures(Figure 2) [Saleeby and Rubin, 1989]. to basaltic andesite volcanic rocks of the Gravina sequence Exposureof the gneisscomplex may have resultedfrom east- were depositedon basinalepiclastic strata that unconformably vergentPaleocene deformation, when deep levels of the mid- overlie Triassic and older portionsof the Alexanderterrane Cretaceous thrust system were transportedupward along (Insularcomposite terrane) and the upperPaleozoic and lower younger east-vergent structures. A swarm of Tertiary Mesozoic Alava sequence(e.g., Yukon-Tanana and Stikine hornblende-bearingdiabase dikes crosscutsall structuresand terranes and Intermontane composite terrane [Rubin and fabrics. These dikes trendnortheast (Figure 1lc) and mark a Saleeby, 1991a]). Thus the Insular and Intermontane regionalchange in the overalltectonic setting during Miocene compositeterranes were adjacentduring constructionof the time. overlying Late Jurassic to Early CretaceousGravina arc. Coarse-grainedepiclastic and volcanic basinal strataof the Gravinasequence overlie the oldervolcanic rocks. Provenance DISCUSSION AND TECTONIC HISTORY of the clastic rocks indicates uplift erosion of the older plutonic arc edifice, perhaps within an extensionaltectonic Three major lithetectonicassemblages, the Alexander and setting[Rubin and Saleeby,1991c]. Following depositionof Taku terranesand the Gravina sequence,are presentalong a the Gravinasequence, zoned mafic and ultramaficcomplexes northwest-trendingbelt on the western side of the Coast were emplacedinto the Alexanderterrane on Duke and Annette PlutonicComplex. The upperPaleozoic portion of the Alava Islands (e.g., the Duke Island ultramafic complex) and the sequencemay representthe westernmostextent of the Yukon- Gravina sequence(e.g., the Union Bay ultramaficcomplex) Tananaterrane, an assemblageof lower Paleozoiccontinental and are present as numerous smaller bodies that intrude slope-and-risedeposits and dismembered fragments of a mid- to adjacentterranes. These enigmaticbodies form a linear belt late Paleozoic arc system. The lower Mesozoic part of the parallelto the Gravinasequence along the easternedge of the Alava sequencehas lithelogic similarities to parts of the Alexander terrane and may have been emplacedduring the Stikine terrane. Structuralbasement is compositeand consists onset of mid-Cretaceous deformation, possibly in an of the Alexanderand Taku (Alava and Kah Shakessequences) extensionalsetting. After depositionof the Gravinasequence terranos. The Alexanderterrane forms depositionalbasement and after the emplacementof the zonedultramafic complexes, to the Gravina sequence;locally the epiclasticstrata of the significantmid-Cretaceous deformation and metamorphism Gravinasequence lie unconformablyover the upperPaleozoic affectedthese rocks and their basement components. and lower Mesozoic Alava sequence[Rubin and Saleeby, Mid-Cretaceous deformation is recorded on Cleveland 1991c], thus forming an overlapbetween the two basement Peninsula,Revillagigedo Island, and the easternportions of constituents(Alexander terraneand Alava sequence). These Annette, Gravina, and adjacentislands. Asymmetricwest to data suggestthe Alexanderterrane was adjacentto the western southwest-vergentfolds formedduring thrust faulting. Thrust 600 Rubinand Saleeby: Tectonic History of EasternEdge of AlexanderTerrane sheetsof the lower PaleozoicKah Shakessequence overlie the SW NE Alexander terrane and Gravina and Alava sequencenappes; 120 120 however, locally on Portland Peninsula, the Kah Shakes [] •a,,--ExceeeArgon ? sequencelies structurallybelow deformedAlexander terrane [Saleebyand Rubin, 1990b]. Complex thrustgeometries and 100 lOO out of sequencethrust faults may have resulted from the + 4-4. reactivation of different primary basement components. [] [] Deformation is recorded on Cleveland Peninsula and 80 ß 80 Revillagigedoand adjacentislands by (1) west- to southwest- Isotopic Age vergentthrust faulting, (2) pervasivemetamorphism, ranging Determinations from lower greenschist to amphibolite facies mineral 60 [] K-Ar biotite 60 assemblages,(3) emplacementof calc-alkalinesills, dikes, and I K-Ar hornblende plutons,(4) high-anglereverse faulting, and (5) uplift that 4' U-Pb zircon 4O • I • I i I , • , 40 recordsat least 14 km of verticaltransport by Late Cretaceous o 20 40 60 80 100 time. Available age constraintsfrom Revillagigedo and adjacentislands indicate that this deformafional event began in Distance across thrust belt (km) the Albian (~113 Ma), the age of the youngeststrata involved Projected onto a SW- NE section in thrustfaulting, and musthave ceasedby ~74 Ma, basedon Fig. 14. U-Pb zircon agesand K-Ar coolingages for 100-90 biotite K-Ar cooling ages [Smith and Diggles, 1981] from Ma plutonsin the Ketchikanarea. U-Pb agesare after Arth et mid-Cretaceousplutons. Asymmetric west- to southwest- al. [1988] and Rubin and Saleeby(this paper and unpublished vergentfolds formed during this faulting. The presenceof data).K-Ar dataare from Smithand Diggles [1981]. thrustsheets composed of crystallinebasement may imply a zone of recouplingwithin middle levels of the continental crustduring the mid-Cretaceous.The extent of southwest- beneaththe arc coeval during the eastwardmigration of directed thrusting of the Yukon-Tanana terrane over the magmatism.In thisview, the eastwardchange of K-At biotite Gravina sequenceand its compositebasement is difficult to ages may reflect not regional uplift but flattening of the assess; however, it is possible that a subhorizontal subductingslab and the resultantunderflow of a relativelycold d6collementseparating the Yukon-Tanana and Alexander subductingplate (for example,see Dumitru [1990]). Present- terrane extends beneath northern British Columbia and day thermalgradients in muchof the regionare low [Yorathet southernYukon Territory [Rubinand Saleeby,1991b]. al., 1985], consistentwith a forearc settingfor the western The emplacementof mid-Cretaceoussills, dikes, and part of the CoastPlutonic Complex. elongateplutons was broadlycoeval with deformation.The Limitedgravity and aeromagnetic data along the trendof the details of the relation between deformation and pluton thrustbelt help definethe present-daydeeper crustal structure emplacement,however, are complicated [Crawford and [Barnes, 1972a, b; 1977; United States Geological Survey Crawford, 1991]. These calc-alkalineintrusive rocks formed (USGS), 1977]. The dramaticdecrease in Bouguergravity in responseto plateconvergence along the western margin of anomaly values, from about +10 mGal in the nondeformed the Alexander terrane and at middle crustal levels of a mid- portionsof the Alexanderterrane to about-115 mGal in the Cretaceouscontinent-margin arc [Barker and Arth, 1984]. interiorof the thrustbelt, alongthe westernedge of the Coast Mid-Cretaceousages reported here, together with olderU-Pb PlutonicComplex (Figure 13), may reflecta thickenedcrust datesfrom the Ketchikanarea [C. M. Rubin, unpublisheddata, and associatedlarge volumesof graniticmaterial. A strong 1988] indicatean overallprogression of olderto youngermid- Bouguergravity gradient coincides with the thickerparts of the Cretaceousigneous ages from the southwestto northeast thrustwedge and the presenceof mid-Cretaceousand younger (Figure 14). This trendmay reflect a northeastmigration of intrusiverocks. A steepmagnetic field gradientfrom low the mid-Cretaceousarc between 100 and 90 Ma. Steeply magneticstrata of nondeformedAlexander terrane on the west dippingreverse faults became active during the late stagesof to highly magneticeastern portions of the thrustbelt and the thrustfaulting. Vertical motionon thesefaults resulted in the westernedge of the CoastPlutonic Complex [USGS, 1977] juxtapositionof differing crustallevels acrossthe thrustbelt mirrors the Bouguergravity gradient. Bouguergravity data and wasperhaps responsible for the initial uplift of the deeper [Barnes, 1977] suggesta crustal thicknessof ~ 25-30 km levels of the arc by the latest Cretaceous,reflected by an beneaththe southeastAlaska archipelago(i.e., Alexander average uplift rate of ~ 0.9 mm/yr [Rubin and Saleeby, terraneand Gravinasequence). Crustal thickness beneath the 199lb]. K-Ar coolingages indicate that the westernside of CoastPlutonic Complex may be as thick as 40 km; however, the thrustbelt cooledbelow the biotite closuretemperature by gravity data alonecannot resolve Moho depths. The presence ~ 74 Ma (Figure 14) [Smith and Diggles, 1981]. Highly of thinnedlithosphere in which midcrustallevels are exposed deformed late Paleocene to early Eocene tonalitic and on the Earth's surfaceposes fundamental questions on the granodioriticintrusives are presenton easternRevillagigedo characterand natureof the lithosphericstructure beneath the Island and western Portland Peninsula, and belong to a Coast Plutonic Complex. Tertiary and younger (?) crustal discontinuousbelt of sills and elongate plutons that were thinningmay havebeen accomplished by extension;however, intrudedinto overthickenedcrust along the westernflank of the the role of extensionis presently difficult to assess. A Coast Plutonic Complex. Rapid uplift accompaniedthe northeast-trendingswarm of late Tertiary mafic dikescuts all emplacementof theseplutons [Hollister, 1982; Crawfordet Cretaceousand early Tertiary structuralelements and records al., 1987] andmay recordthe gravitationalcollapse of the mid- northwest-southeastextension. This dike swarm may reflect Cretaceousthrust belt, although a contractionalsetting has regionalextension associated with the openingof HecateStrait alsobeen proposed. An abruptstep in the K-Ar coolingages during the late Tertiary. The modern structuralsetting is indicatesthat the easternpart of the thrustbelt passedthrough dominated by dextral strike-slip motion on the Queen the biotite closure temperatureby ~58 Ma. Uplift in the Charlottefault system. region was probablyaccommodated by east-side-upfaults. The mid-Cretaceous southeast Alaska orogen was Alternatively,it is possiblethat the older K-At coolingages characterized by thrust faulting, metamorphism, and the may reflect cooling due to underthrustingof cold material emplacementof arc-relatedtonalitic and granodiorific plutons. Rubinand Salecby: Tcctonic History of EasternEdge of AlexanderTerrane 601

Igneous activity and deformation were broadly coeval, Acknowledgments.Parts of this researchwere supportedby suggestingan intra-arcsetting. Mid-Cretaceous rocks exposed National ScienceFoundation grants EAR 86-05386 and EAR at the surfacetoday were formed in middle crustallevels of a 88-034834 (to Saleeby). Additional support(to Rubin) was continent-marginarc andwere involvedin tectonicreworking providedby a GeologicalSociety of AmericaPenrose grant, a of an oldercrustal boundary between the Alexander terrane and Sigma-Xi grant-in-aid, by the U.S. Geological Survey, the westernmargin of North America. Most penetrative Alaska Branch, and by the U.S. Forest Service, Ketchikan deformationoccurred where structural reactivation of differing Ranger District. Rick Allmendingergraciously provided the basementcomponents, such as the Alexander terraneand the stereonetprograms. We thankFred Barker,Henry Berg, Dave Kah Shakessequence, resulted in complexfault geometries. Brew, Darrel Cowan,Weecha Crawford, John Garver, George Emplacementof the early Tertiarytonalitic to graniticplutons Gehrels,Linc Hollister, Bill McClelland, Meghan Miller, Jim followedmid-Cretaceous crustal shortening and uplift. These Monger, George Plafker, and Margi Rusmore for helpful data clearly indicatethat deformationand magmatismwere discussions on northwest Cordilleran stratigraphy and integral parts of the ongoing tectonic evolution of a tectonics.Jim Wright providedencouragement and adviceon convergentcontinental margin and did notresult from a short- U-Pb zircon leaching techniques. Jeff Marshall assistedin lived collisionalevent. Becausecrustal shorteningwas mappingpart of the area during the summerof 1987; Mark broadlycontemporaneous with arc magmatism,the subduction Fahnestockand Jon Nourse provided able field assistance of oceaniclithosphere played a majorrole in crustalshortening during the summer of 1986. Field discussionwith Bill in a noncollisionalsetting. During the evolution of the McClelland and MeghanMiller are appreciated.A review by orogen, mid-Cretaceousmagmatic processesthermally Weecha Crawford significantlyimproved the clarity of an weakenedand allowed compressional failure of theoverriding earlier versionof this paper. Hank Berg and Margi Rusmore continentalplate. thoroughlyreviewed the manuscript.

REFERENCES

Arth, J. G., F. Barker, and T. W. Stem, Coast during 1987, edited by J.P. Galloway, and T. geochronology of detrital zircons from the Batholith and Taku plutons near Ketchikan, D. Hamilton, U.S. Geol. Surv. Circ., 1016, Yukon crystalline terrane in the northem Coast Alaska: Petrography, geochronology, 143-146, 1987. Mountains batholith, southeastern Alaska, geochemistry,and isotopic character, Am. J. Brew, D. A., and R. P. Morrell, Intrusive rocks Can. J. Earth Sci., 28, 899-911, 1991. Sci., 288-A, 461-489, 1988. and plutonic belts in southeasternAlaska, Gehrels, G. E., W. C. McClelland, S. D. Samson, Barker, F., and J. G. Arth, Preliminary results, U.S.A., Circum-Pacific Plutonic terranes, P. J. Patcherr,and M. J. Orchard, Geology of CentralGneiss Complex of the CoastPlutonic editedby J.A. Roddick, Geol. Soc. Am. Mere., the western flank of the Coast Mountains batholith, southeastern Alaska: the roots of a 159, 171-194, 1983. between Cape Fanshaw and Taku Inlet, high-K, calc-alkalinearc?, Phys. Earth Planet. Chen, J. H., and G. J. Wasserburg, Isotopic southeasternAlaska, Tectonics,in press1992. Inter., 35, 191-189, 1984. determinationof uranium in picomole and Hammarstrom, J. M., and E-A. Zen, Aluminum in Barnes,D. F., Simple Bouguer gravity anomaly subpicomole quantities, Anal. Chem., 53, hornblende: An empirical igneous map of Ketchikan 1:250,000 quadrangle, 2060-2067, 1981. geobarometer,Am. Mineral., 71, 1297-1313, southeastern Alaska, showing station Cook, R. D., M. L. Crawford, G. I. Omar, and W. 1986. locations,anomaly values, and generalized10- A. Crawford, Magmatism and deformation, Herreid, G., T. K. Bundtzen, and D. L. Tumer, milligal contours,U.S. Geol. Surv. Open File southern Revillagigedo Island, SE Alaska, Geology and geochemistryof the Craig A-2 Map, 1972a. Geol. Soc. Am. Bull., 103, 829-841, 1991. quadrangleand vicinity, Prince of Wales Barnes,D. F., Simple Bouguergravity anomaly Crawford, M. L, and W. A. Crawford, Magma Island, southeasternAlaska, Geol. Rep. Alaska map of Craig 1:250,000 quadrangle, emplacementin a convergenttectonic orogen, Div. Geol. Geophys.Surv., 48, 49p., 1978. southeastern Alaska, showing station southern Revillagigedo Island, southeastern Holdaway, M. J., Stability of andalusiteand the locations,anomaly values, and generalized10- Alaska,Can. J. Earth Sci., in press,1991. aluminumsilicate phase diagram, Am. J. Sci., milligal contours,U.S. Geol. Surv. Open File Crawford, M. L., L. S. Hollister, and G.J. 271, 97-131, 1971. Map, 1972b. Woodsworth,Cmstal deformation and regional Hollister,L. S., Metamorphicevidence for rapid Barnes, D. F., Interpretation of the available metamorphism across a terrane boundary, (2 mm/yr) uplift of a portion of the Central gravity, in Mineral Resourcesof the Tracy Coast Plutonic Complex, British Columbia, Gneiss Complex, Coast Mountains, British Arm-Fords Terror wildemess study area and Tectonics, 6, 343-361, 1987. Columbia, Can. Mineral., 20, 319-332, 1982. vicinity, edited by D. A. Brew, A. L. Kimball, Dumitru, T. A., SubnormalCenozoic geothermal Hollister, L. S., G. C. Grisson,E. K. Peters,H H. D. Grybeck, and J. C. Still,. Geol. Surv. Open gradients in the extinct Sierra Nevada arc: Stowell, and V. B. Sisson, Confirmation of the File Rep., 77-649, 300 pp, 1977. Consequencesof laramide and post-laramide emprical correlation of A1 in homblende with Berg, H. C., Geologic map of Annette Island, shallow-angle subduction,J. Geophys.Res., pressure solidification of calc-alkaline Alaska, scale 1:63,000, U.S. Geol. Surv. Misc. 95, 4925-4941, 1990. plutons, Am. Min., 72, 231-239, 1987. Geol. Invest. Map, 1-684, 8p., 1972. Eberlein, G. D., M. Churkin, Jr., C. Carter, H. C. Irvine, T. N., The Duke Island ultramafic complex, southeasternAlaska, in Ultramafic Berg, H. C., Geology of Gravina Island, Alaska, Berg, and A.T. Ovenshine, Geology of the Craig quadrangle, Alaska, scale 1:250,000, U.S. Geol.Surv. Bull. 1373, 41 pp., 1973. and RelatedRocks, edited by P. J. Wyllie, pp. U.S. Geol. Surv. OpenFile Rep., 83-91, 53p, 84-97, John Wiley, New York, 1967. Berg, H. C., and E. L., Cruz, Map showing 1983. Irvine, T. N., Petrology of the Duke Island locations of fossil collections and related Gehrels,G. E., and W. C. McClelland, Outline of ultramafic complex, southeastern Alaska, samples in the Ketchikan area and Prince the Takuterrane and Gravina belt in the Cape Mere. Geol. Soc. Am. 138, 1974. Rupert quadrangles,southeastern Alaska, scale Fanshaw-WindhamBay region of central Jaffey, A. H., K. F. Flynn, L. E. Glendenin,W. C. 1:250,000, U.S. Geol. Surv. Open File Rep., southeastern Alaska, Geol. Soc. Am. Abstr. Bentley, and A. M. Essling, Precision 82-1088, 27p., 1982. Programs,20, 163, 1988. measurement of half-lives and specific Berg, H.C., D. L. Jones, and D. H. Richter, Gehrels,G. E., and J. B. Saleeby,Geology of activitiesof 235Uand 238U, Phys. Rev. C, 4, Gravina-Nutzotinbelt: Tectonic significance Prince of Wales Island, southeasternAlaska, 1889-1906, 1971. of an upperMesozoic sedimentary and volcanic Geol. Soc. Am. Bull., 98, 123-137, 1987. Johnson, M. C., and M. J. Rutherford, sequencein southernand southeasternAlaska, Gehrels,G. E., J.B. Saleeby,and H. C. Berg, Experimental calibration of aluminium-in- U.S. Geol. Surv. Prof. Pap. 800-D, D1- D24, Geology of Annette, Gravina, and Duke 1972. homblendegeobarometer with applicationto Islands, southeasternAlaska, Can. J. Earth Long Valley caldera (California) volcanic Berg,H. C., R. L. Elliott,and R. D. Koch, Sci.,24,866-881, 1987. rocks, Geology, 17, 837-841, 1989. Geologicmap of theKetchikan and Prince Gehrels,G. E.,W. C.McClelland, S. D. Samson,Kamb, W. B., Theory of preferredorientation Rupertquadrangles, Alaska, scale 1:250,000, P.J. Patchett,and J. L. Jackson,Ancient developed by crystallization under stress, J. U.S. Geol. Surv.Misc. Invest. Set., Map, 1- continentalmargin assemblage in thenorthem Geol., 67, 153-170, 1959. 1807, scale 1:250,000, 1988. Coast Mountains, southeast Alaska and Krogh, T. E., A low-contamination method for Brew,D. A., andS. M. Karl, Reexaminationof northwestCanada, Geology, 18, 208-211, hydrothermal decompositionof zircon and the contacts and other features of the Gravina 1990. extraction of U and Pb for isotopic age belt, southeasternAlaska, in Geologicstudies Gehrels,G. E., W. C. McClelland,S. D. Samson, determinations,Geochim. Cosomochim. Acta, in Alaska by the U.S. Geological. Survey P.J. Patchett, and J. L. Jackson, U-Pb 37, 485-494, 1973. 602 Rubinand Saleeby: Tectonic History of Eastem Edge of Alexander Terrane

Lanphere,M. A., and G. D. Eberlein, Potassium- Implicationsfor continent-margintectonism, complexes of southeasternAlaska, in argon ages of magnetite-bearing ultramafic Geology,18, 276-280, 1990a. Ultrarnaficand RelatedRocks, edited by P. J. complexesin southeastemAlaska, Spec.Pap. Rubin,C. M., M. M. Miller, andG. E. Smith, Wyllie,pp. 209-230,John Wiley, New York, Geol. Soc. Am. 87, 94, 1966. Tectonic development of Cordilleran mid- 1967. Magaritz, M., and H. P. Taylor, Jr., Oxygen Paleozoic volcanoplutonic complexes: Tera, F., and G. S. Wasserburg,U-Th-Pb 18/Oxygen16 and D/I-I studiesof plutonic Evidencefor convergentmargin tectonism,in systematicsin three Apollo 14 basaltsand the granitic and metamorphic rocks across the Paleozoicand early Mesozoic palcogeographic problemsof the initial Pb in lunar rocks,Earth Cordilleran batholiths of southern British relations;Sierra Nevada, Klamath Mountains, Planet. Sci. Lett., 14, 281-304, 1972. Columbia, J. Geophys Res., 91, 2193-2217, andrelated rocks, edited by D. S. Harwoodand Turner, F. J., MetamorphicPetrology, 2nd ed., 1986. M. M. Miller, Spec.Pap. Geol. Soc.Am., 225, 524 pp., publisher,New York, 1981. Martinson, J. M., U-Pb ages of zircon: a basic 1-16, 1990b. United States Geological Survey (USGS), examination of error propagation, Chem. Saleeby,J. B., The inner boundaryzone of the Aeromagneticmap of the Ketchikan, Prince Geol., 66, 151-162, 1987. Alexander terrane in southern SE Alaska: Part 11 Rupert, and northeasternCraig quadrangles, McClelland,W. C., andG. E. Gehrels,Geology of southernRevillagigedo Island (RI) to CapeFox Alaska, scale 1:250,000, U.S. Geol. Surv. the Duncan Canal shear zone: Evidence for (CF), Geol. Soc. Am. Abstr. Programs,19, Open File Rep. Map 77-359, 1977. Early to Middle Jurassic deformation of the 828, 1987. Wheeler, J. O., and P. McFeely, Tectonic Alexander terrane, southeasternAlaska, Geol. Saleeby,J. B., and C. M., Rubin, The western assemblagemap of the Canadian Cordillera, Soc. Am. Bull., 102, 1378-1392, 1990. marginof the CoastPlutonic Complex (CPC) Geol. Surv. Can. OpenFile Rep. 1565, 1987. McClelland,W. C., G. E. Gehrels,S. D. Samson, in southernmostSE Alaska, Geol. Soc. Am. Yorath, C.J., G. J. Woodsworth, R. P. and P. J. Patcherr, Structural and Abstr. Programs,21, 139, 1989. Riddihough,R.G. Curde, R. D. Hyndman,G. C geochronologicrelations along the westem Saleeby,J. B., and C. M., Rubin, The East Behm Rogers, D. A. Seemann, and A.D. Collins, flank of the Coast Mountains batholith: Canal Gneiss Complex (southernsoutheast Centennial Continent/Ocean Transect # 8, Stikine River to Cape Fanshaw, central Alaska) and some insights into basement Intermontane Belt (Skeena Mountains) to southeasternAlaska, J. Struct.Geol.,in press1992a. tectonicsalong the Insular suturebelt, Geol. Insular Belt (Queen Charlotte Islands), Geol. McClelland,W. C., G. E. Gehrels,S. D. Samson, Assoc. Can. Abst. Programs, 15, All6, Soc. America. Centennial Continent/Ocean and P. J. Patchett, Protolith relations of the 1990a. TransectSer., 8, 7 pp. GeologicalSociety of Gravina belt and Yukon-Tanana terrane in Saleeby, J. B., and C. M. Rubin, Tectonic America, Boulder,Colo., 1985. central southeasternAlaska, J. Geol., 1992b. intercalationof crystallinenappes and basinal York, D., Least-squaresfitting of a straightline Rubin, C. M., and J. B. Saleeby, The inner overlap relations in southern SE Alaska: with correlatederrors, Earth Planet. Sci. Lett., boundary zone of the Alexander terrane in Implicationsfor the initial stagesof Insular 5, 320-324, 1969. southernSE Alaska, A newly discoveredthrust superterraneaccretion, Eos Trans. AGU, 71, Zen, E-A., Implicationsof magmaticepidote- belt, Geol. Soc. Am. Abstr. Programs, 19, 1591, 1990b. bearing plutons on crustal evolution in the 455, 1987. Saleeby,J. B., G. E. Gehrels,G. D. Ebefiein,and accreted terranes of northwestern North Rubin, C. M., and J. B. Saleeby, Tectonic H. C. Berg, Progressin lead/uraniumzircon America, Geology,12, 266-269, 1985. frameworkof the Upper Paleozoicand Lower studies of lower Paleozoic rocks of the Zen, E-A., Tectonicsignificance of high-pressure Mesozoic Alava sequence:A revisedview of southernAlexander terrane, U.S. Geol. Surv. plutonit rocks in the westerns(sic) cordilleran the polygenitic Taku terrane in southern Circ., 868, 110-113, 1984. of North America, in Metamorphism and southeastAlaska, Can. J. Earth Sci., 28, 881- Silbefiing,N.J., Wardlaw,B. R., andH. C. Berg, CrustalEvolution of the WesternUnited States, 893, 1991a. New paleontologicage determinationsform Rubey Volume7, editedby W. G. Ernst,pp. Rubin, C. M. and J. B. Saleeby,Thrust tectonics the Taku terrane,Ketchikan area, southeastem 41-68, Prentice-HaH,Englewood Cliffs, N.J., and intracontinentalshortening in southeast Alaska, U.S. Geol. Surv.Circ., 844, 117-119, 1988. Alaska and westernBritish Columbia,in Thrust 1982. Zen E-A., and J. M. Hammarstrom,Magmatic Tectonics, editedby by K. McCLay, Unwin Smith,J. G., andM. F. Diggles,Potassium-argon epidote and its petrologle significance, Hyman, London,in press.1991b. determinations in the Ketchikan and Prince Geology, 12, 515-528, 1984. Rubin, C. M., and J. B. Saleeby, The Gravina Rupertquadrangles, southeastem Alaska, U.S. Sequence - Remnants of a mid-Mesozoic C. M. Rubin, l•partrnent of Geology,Central Geol. Surv. Open File Rep., 78-73N, 1-6, WashingtonUniversity, Ellensburg, WA 98926. oceanic arc in southem southeast Alaska, J. 1981. J. B. $aleeby,170-25 Geology, California Geophys. Res., 96, 14551-14568, 1991c. Sutter,J.F., and M. L. Crawford,Timing of InstituteofTechnology, Pasadena, CA91125. Rubin, C. M., J. B. Saleeby,D. S. Cowan,M. T. metamorphismand uplift in the vicinity of Brandon,and M. F. McGroder,Regionally PrinceRupert, British Columbia, Geol. Soc. (ReceivedFebruary 21, 1991; extensivemid-Cretaceous west-vergent thrust Am. Abstr.Programs, 17, 411, 1985. revisedAugust 5, 1991; system in the northwestern Cordillera: Taylor, H. P., Jr., The zoned ultramarie acceptedAugust 8, 1991.)