geology of the chatham sound region, southeast alaska and coastal british columbia

Geology of the Chatham Sound region, southeastAlaska and coastal British Columbia

George E. Gehrels

Abstract: The Coast Mountains orogen is thought to have formed as a result of accretion of the Alexander and Wrangelliaterranes against the western margin of the Stikine and Yukon–Tanana terranes, but the nature and age of accretion remaincontroversial. The Chatham Sound area, which is located along the west flank of the Coast Mountains near the Alaska – BritishColumbia border, displays a wide variety of relations that bear on the nature and age of the boundary between inboardand outboard terranes. Geologic and U–Pb geochronologic studies in this area reveal a coherent but deformed andmetamorphosed sequence of rocks belonging to the Yukon–Tanana terrane, including pre-mid-Paleozoic marble, schist,and quartzite, mid-Paleozoic orthogneiss and metavolcanic rocks, and upper Paleozoic metaconglomerate and metavolcanicrocks. These rocks are overlain by Middle Jurassic volcanic rocks (Moffat volcanics) and Upper Jurassic – Lower Cretaceousstrata of the Gravina basin, both of which also overlie Triassic and older rocks of the Alexander terrane. This overlaprelationship demonstrates that the Alexander and Wrangellia terranes were initially accreted to the margin of inboardterranes during or prior to mid-Jurassic time. Accretion was apparently followed by Late Jurassic – Early Cretaceousextension–transtension to form the Gravina basin, left-slip along the inboard margin of Alexander–Wrangellia, mid-Cretaceouscollapse of the Gravina basin and final structural accretion of the outboard terranes, and early Tertiary dip-slip motionon the Coast shear zone.

Résumé : On croit que l’orogène de la chaîne Côtière résulte de l’accrétion des terranes d’Alexander et de Wrangelliasur la limite ouest des terranes de Stikine et de Yukon–Tanana, mais la nature et l’âge de l’accrétion demeurent con-troversés. La région du passage Chatham, localisée le long du flanc ouest de la chaîne Côtière près de la frontière Alaska –Colombie Britannique, montre une grande variété de relations qui touchent à la nature et à l’âge de la limite entre lesterranes internes et externes. Des études géologiques et géochronologiques U–Pb dans cette région révèlent uneséquence de roches cohérente mais déformée et métamorphisée qui appartient au terrane Yukon–Tanana, incluant dumarbre, du schiste et du quartzite datant du pré-Paléozoïque moyen, des roches métavolcaniques et de l’orthogneissdatant du Paléozoïque moyen ainsi que des roches métavolcaniques et des métaconglomérats datant du Paléozoïquesupérieur. Ces roches reposent sous les roches volcaniques du Jurassique moyen (les volcaniques Moffat) et les stratesdu bassin de Gravina, Jurassique supérieur – Crétacé inférieur, ces deux dernières recouvrant également des rochesdatant du Trias et des roches plus anciennes du terrane Alexander. Cette relation de chevauchement démontre que lesterranes d’Alexander et de Wrangellia ont été tout d’abord accrétés à la marge des terranes internes au cours du ouavant l’époque du Jurassique moyen. L’accrétion aurait été suivie d’une extension–transtension au Jurassique tardif – Crétacéprécoce pour former le bassin Gravina, d’un décrochement senestre le long de la limite interne du terraned’Alexander–Wrangellia, de l’effondrement au Crétacé moyen du bassin Gravina et de l’accrétion structurale finale desterranes externes, puis d’un mouvement selon le pendage le long de la zone de cisaillement de la Côte, au Tertiaireprécoce.

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GehrelsIntroduction

Southeast Alaska and western British Columbia are underlainby a wide variety of terranes that record the progressiveaccretion of crustal fragments to the Cordilleran margin(Berg et al. 1972; Monger et al. 1982, 1991; Fig. 1). Themain terranes west of the Coast Mountains consist ofNeoproterozoic through mid-Mesozoic rocks of the Alexanderand Wrangellia terranes (Fig. 2). East of the central andnorthern Coast Mountains are pre-mid-Paleozoic continental

margin strata of the Yukon–Tanana (or Nisling) terrane,Paleozoic and lower Mesozoic strata of the Stikine terrane,and Jurassic–Cretaceous basinal strata of the Bowser basin(Fig. 2). Within and immediately adjacent to the central andnorthern Coast Mountains are metamorphosed equivalents ofthese assemblages, plus Permian–Triassic metavolcanic andmetasedimentary rocks of the Taku terrane, middle andupper Paleozoic metavolcanic and metasedimentary rocksthat belong to the Yukon–Tanana terrane, and Jurassic–Cretaceous basinal metasedimentary rocks of the Gravina

Can. J. Earth Sci. 38: 1579–1599 (2001) © 2001 NRC Canada

1579

DOI: 10.1139/cjes-38-11-1579

Received August 23, 2000. Accepted April 26, 2001. Published on the NRC Research Press Web site at http://cjes.nrc.ca onOctober 31, 2001.

Paper handled by Associate Editor L. Heaman.

G.E. Gehrels. Department of Geosciences, University of Arizona, Tucson, AZ 85721, U.S.A. (e-mail: [email protected]).

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Fig. 1. Tectonic map of the Coast Mountains and adjacent regions of the Insular and Intermontane belts (adapted from Monger and Berg 1987;Wheeler and McFeely 1991; Gehrels et al. 1992; and Currie and Parrish 1997).

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belt. All of these assemblages were regionally metamorphosed,penetratively deformed, and imbricated along various thrustand normal faults during emplacement of widespread Jurassicthrough Eocene plutons of the Coast Mountains batholith.

Most workers agree that there is a genetic relationshipbetween the regional metamorphism, deformation, andplutonism within the Coast Mountains orogen and the accretionof the Alexander and Wrangellia terranes (Berg et al. 1972;Monger et al. 1982; Crawford et al. 1987). Unraveling thedetailed history of this accretionary event, however, has been

one of the long-standing challenges in Cordilleran tectonics.For example, the location of the fundamental boundaryseparating Alexander and Wrangellia from inboard fragmentsis highly uncertain: it may be as far west as the outboardmargin of the Taku terrane, or it may be far to the east,within the core of the Coast Mountains (Fig. 1). The age ofaccretion is also uncertain, with previous estimates rangingfrom latest Triassic to early Tertiary time.

This study is an attempt to shed light on the accretionaryhistory of the Alexander–Wrangellia terrane and the evolution

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Fig. 2. Schematic columns showing the first-order tectonic units and structures near Chatham Sound. Dark borders represent the configurationof crustal fragments prior to the interpreted mid-Jurassic accretionary event. Thrust faults (long dashed lines) represent the final structuralaccretion of Alexander–Wrangellia during mid-Cretaceous time. Short dashed lines show the early Tertiary Coast shear zone and a normalfault along the east margin of the Coast Mountains. Information adapted from Crawford et al. (1987), Gehrels and Saleeby (1987)mGehrels et al. (1992), Gardner et al. (1988), Monger et al. (1991), Rubin and Saleeby (1992), and Currie and Parrish (1997).

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of the western Coast Mountains. The information presentedderives from geologic mapping and U–Pb geochronologicstudies along the west flank of the Coast Mountains in theChatham Sound region, which is near the border separatingsoutheast Alaska and coastal British Columbia (Fig. 1). Thisis a critical area because it contains units and relations thathave not been recognized elsewhere in Alaska and BritishColumbia. Many of these relations were first mapped at1 : 250 000 scale by Hutchison (1982) and Berg et al.(1988), and subsequently studied in more detail byWoodsworth et al. (1983), Woodsworth and Orchard (1985),Crawford et al. (1987, 2000), Hollister and Andronicos(1997), Klepeis et al. (1998), Chardon et al. (1999), andSaleeby (2000).

Geochronologic analyses presented herein have beenconducted utilizing conventional isotope dilution, thermalionization mass spectrometry. Samples ranged from 5 to 10 kgin weight, and each sample was collected from a single outcrop.Mineral separation and sample analysis techniques generallyfollowed Gehrels (2000a). Data reduction and plotting utilizedprograms of Ludwig (1991a, 1991b). The analyses are plottedon Pb/U concordia diagrams in this paper.1 All ages arereported at the 95% confidence level. For concordant analyses,the age uncertainty is determined from the range of overlapof the error ellipses with concordia. For discordant analyses,the age and uncertainty are determined from a regressioncalculated using the programs of Ludwig (1991b).

Geologic framework

The general configuration of terranes, first-order structures,and magmatic belts is quite continuous along the length ofthe central and northern Coast Mountains (Fig. 1). Westernmostare pre-Jurassic rocks of the Alexander and Wrangellia terranes,which in many areas are deformed only slightly and meta-morphosed to low grade. The Alexander terrane consistslargely of Neoproterozoic – lower Paleozoic arc-type rocks,upper Paleozoic limestones and basinal clastic strata, andUpper Triassic rift-related volcanic and sedimentary rocks(Fig. 2, Gehrels and Saleeby 1987). Wrangellia, whichrecords a similar history and may be part of the same crustalfragment as the Alexander terrane, is located outboard of theAlexander terrane at the latitude of Chatham Sound. Importantunits recognized in this part of Wrangellia that are not foundin the adjacent Alexander terrane include latest Triassic toMiddle Jurassic arc-type volcanic rocks (Monger et al.1991). Rocks of Wrangellia were not studied during thisinvestigation.

The Alexander terrane is intruded by Late Jurassic andEarly Cretaceous plutons and is overlain to the east by UpperJurassic through mid-Cretaceous clastic strata (mainlyvolcanic-rich turbidites) and mafic volcanic rocks of theGravina belt (Fig. 1; Berg et al. 1972). These strata arebounded eastward by an east-dipping thrust system ofmid-Cretaceous age (Fig. 1; Crawford et al. 1987; Rubin etal. 1990; McClelland et al. 1991; Rubin and Saleeby 1992;Gehrels et al. 1992; Gehrels 2000b). The hanging wall ofthis thrust in most areas consists of Permo-Triassic arc-type

rocks of the Taku terrane or Alava sequence (Rubin andSaleeby 1992; Gehrels et al. 1992; Fig. 2).

Rocks of the Taku terrane are bounded to the east by asecond thrust system that carries higher grade rocks of theYukon–Tanana terrane southwestward (Fig. 1; McClelland etal. 1991; Gehrels et al. 1992; Rubin and Saleeby 1992;Crawford et al. 2000; Saleeby 2000; Gehrels 2000b). TheYukon–Tanana terrane in southeast Alaska is divided intopre-mid-Paleozoic quartzite, marble, and pelitic schist of theTracy Arm assemblage, mid-Paleozoic metavolcanic rocksof the Endicott Arm assemblage, and upper Paleozoicmetasedimentary rocks of the Port Houghton assemblage(Fig. 2).

Rocks of the Yukon–Tanana terrane are also found alongthe east flank of the northern Coast Mountains (Fig. 1; Mongerand Berg 1987). In this region, pre-mid-Paleozoic continentalmargin strata are referred to as the Florence Range suite,whereas middle and upper Paleozoic strata are referred to asthe Boundary Ranges suite (Fig. 2; Currie and Parrish 1997).Mortensen (1992) concludes that the contact between thesetwo assemblages is depositional, whereas Currie and Parrish(1997) argue that the Florence Range suite is separated fromthe Border Ranges suite by a mid-Jurassic accretionaryboundary.

Inboard of the central Coast Mountains are Devonianthrough Cretaceous volcanic and sedimentary rocks of theStikine terrane and Bowser basin (Fig. 1; Monger et al.1991). The Stikine terrane consists of mid-Paleozoic arc-typerocks, upper Paleozoic marine clastic strata, limestone, andsubordinate volcanic rocks, and widespread Triassic throughmid-Jurassic arc-type volcanic rocks. These rocks are overlainby mid-Jurassic through mid-Cretaceous strata of the Bowserbasin (Fig. 2). Rocks that are part of the contiguous Stikineterrane and Bowser basin were not mapped during the presentstudy.

Known and suspected linkages among the terranes withinand adjacent to the Coast Mountains are shown in Fig. 2.West of the Coast Mountains, in mainland Alaska and Yukon,the Alexander and Wrangellia terranes are linked by a �309Ma pluton that intrudes both terranes (Gardner et al. 1988).To the east, the Taku, Yukon–Tanana, and Stikine terranesare linked by (i) possible correlation of mid-Paleozoic volcanicassemblages in the Stikine and Yukon–Tanana terranes(Mortensen 1992; Currie and Parrish 1997); (ii) possiblecorrelation of upper Paleozoic (McClelland 1992) and Triassic(Brew et al. 1994) strata in the core of the northern CoastMountains with strata of the western Stikine terrane; (iii) thepresence of a clast in Triassic conglomerate of the Stikineterrane that is interpreted to have been derived from Yukon–Tanana rocks (Jackson et al. 1991); (iv) the presence of LateTriassic plutons in both the western Stikine and easternYukon–Tanana terranes (Wheeler and McFeely 1991); and(v) the presence of detrital zircons of 380–300 Ma in upperPaleozoic – lower Mesozoic strata of the Stikine (Greig andGehrels 1995), Yukon–Tanana (Gehrels and Kapp 1998),and Taku (G. Gehrels, unpublished data, 1999) terranes.Hence, it appears likely that by the end of Triassic time, Al-exander and Wrangellia were linked together as one crustal

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1 The table of isotopic data can be purchased from the Depository of Unpublished Data, CISTI, National Research Council Canada, Ottawa,ON K1A 0S2, Canada.

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fragment, and Stikine, Yukon–Tanana, and Taku terranes werecontiguous or at least in close proximity to each other. Ifthese relations are correct, the fundamental boundary be-tween inboard and outboard crustal fragments is located alongthe western margin of the Taku or Yukon–Tanana terranes(Fig. 2), which is presently delineated by a mid-Cretaceousthrust fault (Fig. 1).

The oldest linkage that extends across the Coast Mountainsand connects Alexander–Wrangellia with Taku, Stikine, andYukon–Tanana is the subject of considerable debate. Mostworkers view mid-Cretaceous and younger plutons of theCoast Mountains batholith as the oldest linkage, followingthe conventional interpretation that Alexander–Wrangelliawas accreted during mid-Cretaceous time (Berg et al. 1972;Monger et al. 1982; Crawford et al. 1987). In contrast,McClelland and Gehrels (1990), McClelland et al. (1992),van der Heyden (1992), Monger and Nokleberg (1996), andSaleeby (2000) argue that the outboard fragment was juxtaposedagainst inboard terranes between latest Triassic and mid-Jurassictime. As a third alternative, Cowan et al. (1997) andHollister and Andronicos (1997) propose that the outboardterranes were at the latitude of northern Mexico untilmid-Cretaceous time and did not arrive at their presentposition until the Late Cretaceous – early Tertiary. Approxi-mately 1500 km of northward motion is suspected to haveoccurred on or near the Coast shear zone, which is a steeplydipping shear zone in the western Coast Mountains (Fig. 1).

Geology of the Chatham Sound region

The Chatham Sound region was investigated because itexposes critical relations between rocks of the Alexanderterrane, Gravina belt, and Yukon–Tanana terrane (Fig. 3).Reconnaissance-style mapping and geochronology wereconducted throughout the region, and detailed studies wereconducted in the Dundas Island, Tongass Island, and DigbyIsland areas. Relations in these three areas are described inthis section, with an emphasis on protolith, regional structural,and geochronologic relations between the various units.Crawford et al. (2000) describe mainly the metamorphic andstructural aspects of the Chatham Sound region.

Dundas Island areaThe Alexander terrane is represented in the Dundas Island

area by a variety of lower Paleozoic through lower Mesozoicrocks. The oldest units are mafic metavolcanic rocks andmetagraywacke intruded by quartz diorite, diorite, gabbro,and granodiorite plutons. All of these rocks are metamorphosedto greenschist facies and display a variably developed foliationthat generally strikes northeast and dips southeast (Fig. 4). Asample of quartz diorite from northeastern Dundas Island(sample 1) yields a U–Pb (zircon) age of 410 ± 3 Ma(Fig. 5). Collectively, these metavolcanic and metaplutonicrocks are very similar to the Ordovician–Silurian DesconFormation and related intrusions of the Alexander terrane insouthern southeast Alaska (Gehrels and Saleeby 1987).

These lower Paleozoic rocks are unconformably overlainby a sequence of mid-Paleozoic through Upper Triassicstrata that are described in detail by Woodsworth andOrchard (1985). Mapping during the present study refinedthe outcrop pattern of these rocks on Dunira Island and

smaller islands to the north and east (Fig. 4) and recognizeda thin horizon of conglomeratic sandstone at the base of thesequence on low-tide outcrops north of Randall Island.Based on rock type and stratigraphic position, this unit ismost likely the Lower Devonian Karheen Formation, whichoccurs in adjacent areas of southeast Alaska (Gehrels andSaleeby 1987).

The main new findings in the Dundas Island area bear onthe nature and age of the Mesozoic metavolcanic andmetasedimentary rocks (Fig. 4). Woodsworth and Orchard(1985) presented a U–Pb analysis of zircon grains from ametarhyolite on eastern Randall Island, referred to as theMoffat rhyolite, which they interpreted as �188 Ma. Thisage has been considered problematic because Jurassic rhyolitehas not been recognized elsewhere in the Alexander terrane,and because the age is interpreted from a single U–Pbdetermination. The presence of Jurassic volcanic rocks inthe region is confirmed, however, by a U–Pb age of 177 ± 4 Mafrom a metarhyolite at the southeast end of the MoffatIslands (sample 2, Fig. 6). Mapping on Randall, Moffat, andMelville islands (Fig. 4) demonstrates that the Jurassic rocksconsist of a several kilometres thick sequence of interlayeredbasalt (mainly pillow flows and fragmental tuff), rhyolite(mainly quartz- and locally plagioclase-porphyritic tuff), andsubordinate tuffaceous mudstone and shale. This sequence isherein referred to as the Moffat volcanics. These rocks overlieUpper Triassic(?) tuffaceous rhyolite (green phyllite unit ofWoodsworth and Orchard 1985) along a sheared contact onRandall Island and are faulted against upper Paleozoic strataon eastern Dunira Island.

The Moffat volcanics are overlain along an unexposedcontact by a monotonous sequence of conglomeraticmetaturbidites that are exposed on Whitesand Island andadjacent smaller islands (Fig. 4). Based on map patterns,these strata apparently truncate a large thickness of theunderlying volcanic sequence due to either erosion along abasal unconformity or faulting. An unconformable relationship,perhaps modified by faulting, is supported by analyses ofdetrital zircons from a metagraywacke (sample 3, Fig. 7)exposed on Whitesand Island (Fig. 4). The ages define twogroups of approximately 220–200 Ma and 172–158 Ma,which are similar to the ages of underlying Jurassic and possiblyTriassic(?) volcanic rocks. These ages are also similar to thedominant ages of detrital zircons in strata belonging to theGravina belt in southeast Alaska (Kapp and Gehrels 1998).Based on similar stratigraphy and detrital zircon ages, theWhitesand metaturbidites are correlated with the Gravinabelt in southeast Alaska. This correlation was previouslyproposed by Woodsworth and Orchard (1985) and Crawfordet al. (2000).

The youngest major units in the Dundas Island area areCretaceous granite–granodiorite-dioritic plutons (Figs. 3, 4).These bodies yield U–Pb ages of 103 ± 2 Ma on southwesternDundas Island (sample 4) and 101 ± 1 Ma on northernStephens Island (sample 5; Figs. 3, 8).

Tongass Island areaThis region displays a wide variety of stratigraphic and

structural relations that bear on the accretionary history ofthe Alexander–Wrangellia terrane. These relations are

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Fig. 3. Sketch map of the geology of the Chatham Sound region. Adapted from Hutchison (1982), Woodsworth and Orchard (1985),Crawford et al. (1987, 2000), Berg et al. (1988), Wheeler and McFeely (1991), and Saleeby (2000).

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Fig. 4. Geologic map of the Dundas Island area (adapted from Hutchison 1982; and Woodsworth and Orchard 1985).

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shown in map view in Fig. 9 and in cross-section view inFig. 10.

Cape Fox – Harry Bay regionThe westernmost rocks exposed in this area consist of

lower Paleozoic metaigneous rocks of the Alexander terranethat are well exposed near Cape Fox and on the Lord Islands(Fig. 9). The oldest rocks are metavolcanic units belongingto the Descon Formation which are intruded by a �430 Matrondhjemite body at Cape Fox (Saleeby 2000) and byOrdovician–Silurian(?) dioritic rocks. Along the west shoreof Harry Bay (Fig. 9), these lower Paleozoic rocks are overlain

unconformably by a sequence of less-deformed sandstonewith subordinate limestone and pebble conglomerate. Clastsin the conglomerate are predominantly trondhjemite derivedfrom the nearby Cape Fox pluton. The sequence becomesmore volcanic-rich upsection (toward the east) and is cappedby mafic pillow flows and breccia. Based on lithic types andstratigraphic position, these rocks are correlated with theKarheen Formation and overlying Lower–Middle Devonian

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Fig. 5. Concordia diagram of quartz diorite from northeasternDundas Island (sample 1). Age is interpreted from the twoconcordant single grains. Discordance is interpreted to resultfrom Pb loss (mean square of weighted deviates (mswd) = 0.57).Data reduction and plotting on all concordia diagrams from Ludwig(1991a, 1991b).

Fig. 7. Concordia diagram of abraded single zircon grains from ametagraywacke of the Gravina belt on Whitesand Island (sample 3).The grains apparently define two age groups of 220–200 and172–158 Ma.

Fig. 8. Concordia diagrams of two Cretaceous plutons from DundasIsland (sample 4) and Melville Island (sample 5). The ages areassigned on the basis of the clusters of concordant grains. Sample 4also contains a multigrain fraction and a single grain that haveinherited components with an average age of 508 ± 129 Ma(mswd = 1.9).

Fig. 6. Concordia diagram of abraded single zircon grains frommetarhyolite on the southern Moffat Islands (sample 2). The assignedage is derived from the cluster of apparently concordant grains.The additional younger grain is interpreted to have experiencedPb loss.

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Fig. 9. Simplified geologic map of the Tongass Island area, which is based mainly on shoreline traverses conducted during this study.Figure 10 shows the relations in cross section along A–A′.

Fig. 10. Simplified cross section of the Tongass Island area. Line of section is along A–A′ in Fig. 9. As shown in this cross section,structural relations in the area suggest that the large syncline in the region formed prior to the Harry Bay, Tingberg, and Lincoln Chan-nel thrust faults.

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mafic volcanic rocks, which are common in adjacent areasof southern southeast Alaska (Gehrels and Saleeby 1987).Rocks of the Alexander terrane are referred to by Crawfordet al. (2000) as the Digby sequence.

Kanaganut, Tingberg, and Tongass islandsToward the east, units exposed on the east shore of Harry

Bay and on Kanaganut and Tongass islands are interpretedto define a large syncline that trends northwest–southeastand has isoclinal limbs that dip steeply northeastward(Fig. 10). The oldest units exposed on both limbs aredacitic(?) metavolcanic rocks associated with discontinuouslayers of gray marble and pelitic schist. A sample ofquartz-porphyritic metadacite from westernmost TingbergIsland yields a U–Pb age of 354 ± 10 Ma (sample 6,Fig. 11), whereas quartz-porphyritic metarhyolite on the eastlimb of the fold, on eastern Kanaganut Island, yields an ageof 367 ± 5 Ma (sample 7, Fig. 11). The latter sample alsocontains a highly discordant xenocrystic(?) grain with a207Pb/206Pb age of �1515 Ma and an upper intercept age of�2896 Ma when projected from 367 Ma. The metavolcanicunits along the east shore of Kanaganut Island are referredto as the Delusion Bay sequence by Crawford et al. (2000).

Overlying the metavolcanic rocks on the eastern limb ofthe syncline is a thick sequence of metaconglomerate andmetasandstone that grades upward into mafic to intermediatemetavolcanic rocks with minor metapelite and marble. Thisunit traces northward to the east shore of Harry Bay (Fig. 9),and also occurs to the south on the mainland near PortSimpson (Fig. 3). Two samples of metasandstone have beenanalyzed for detrital zircons. One sample, from the south tipof Kanaganut Island, comes from �5 m above thedepositional contact with the 367 ± 5 Ma (sample 7)metavolcanic unit. Conglomerate clasts at this localityconsist mainly of felsic to intermediate-compositiongranitoids and felsic metavolcanic rocks. Seven abraded singlegrains from the metasandstone matrix yield concordant analysesof 375 ± 7 Ma (sample 8, Fig. 12). A second sample comesfrom 2 km farther north and �40 m above the base of thesequence (sample 9, Fig.12). Four single grains have beenanalyzed from this sample: three are 386 ± 7 Ma and one isapparently concordant at �364 Ma (Fig. 12). The clasts inthis horizon are mainly schistose felsic metavolcanic rocksand interlayered quartzite and marble, and some show clearsigns of deformation and metamorphism prior to inclusion inthe host metaconglomerate (Fig. 13). The presence of thesefoliated clasts suggests that the underlying metavolcanicsequence was metamorphosed and deformed prior to depositionof the conglomeratic strata.

The sequence of units on Kanaganut Island is very similarto that present between Petersburg and Juneau in centralsoutheast Alaska. In this region, the Endicott Arm assemblageconsists of felsic metavolcanic rocks of 375–367 Ma(McClelland et al. 1991; Gehrels et al. 1992) interlayeredwith siliciclastic sediments that yield detrital zircons withages of 370–350 Ma and 2.67–1.78 Ga (Gehrels and Kapp1998). These rocks are unconformably overlain by a lowergrade and less deformed sequence of Carboniferous(?)metaconglomerate, metapelite, mafic metavolcanic rocks,and marble, which are included in the Port Houghton assem-blage. Detrital zircons in the basal metaconglomerate are

mainly 365–330 Ma and 2.68–1.71 Ga, reflecting derivationfrom the underlying metavolcanic rocks and from nearbymid-Paleozoic metaigneous rocks. The similarity in age,protoliths, and relative degrees of metamorphism and defor-mation lead to the interpretation that the units on KanaganutIsland are correlative with the mid-Paleozoic Endicott Armassemblage and the Carboniferous(?) Port Houghton assem-blage.

Overlying the conglomeratic sequence on westernKanaganut Island is a sequence of quartz-porphyriticmetarhyolite interlayered with subordinate pelitic schist andmetabasalt. Crawford et al. (2000) interpret this contact as awest-vergent thrust, whereas relations observed during thepresent study suggest that this contact is depositional.Abraded single grains from a sample of this metarhyolite onnorthern Tongass Island yield a U–Pb age of 170 ± 5 Ma

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Fig. 11. Concordia diagram of abraded single zircon grains fromfelsic metavolcanic rocks on western Tingberg Island (sample 6)and southeastern Kanaganut Island (sample 7). The ages are basedon the clusters of concordant grains. One additional highly discordantgrain from each sample (not shown) defines a Pb-loss trajectoryfor sample 6 and an inheritance trajectory for sample 7.

Fig. 12. Concordia diagram of abraded single zircon grains fromtwo metaconglomerate samples from southern Kanaganut Island(samples 8 and 9). The ages are interpreted from the clusters ofconcordant grains for each sample, except for the single �364Ma grain in sample 9.

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(sample 10, Fig. 14). There are also grains with apparentages of �220–212 Ma in this sample that are interpreted tobe detrital grains incorporated into the tuffaceous layers duringdeposition. Both sets of grains were very similar in morphology,shape, size, etc., and could not be distinguished optically.These rocks are interpreted to be correlative with the Moffatvolcanics based on similar stratigraphic relations and U–Pbages. Their southward continuation occurs along the shorelinenortheast of Port Simpson (Fig. 3). Crawford et al. refer tothese rocks as the Venn sequence.

This mid-Jurassic metarhyolite is overlain conformably bya thick sequence of metaturbidites. Graded beds in the sequenceindicate that the strata on much of northwestern KanaganutIsland and western Tongass Island face westward and areaccordingly overturned. The degree of deformation in theserocks increases westward into a zone along the westernmostshore of northern Kanaganut Island, which is characterizedby penetrative tight folds with gently northwest- andsoutheast-plunging axes. West of this �20 m wide zone, thedegree of deformation decreases, and graded beds demonstratethat the strata are east facing and right side up. The highlyfolded zone coincides with the axial trace of the largesyncline defined by the pattern of units on these islands(Figs. 9, 10).

A sample of conglomerate within the metaturbidites wasanalyzed for detrital zircons (sample 11). Fourteen grainswere analyzed, of which 12 are 160–150 Ma and two are �205Ma (Fig. 15). The 160–150 Ma grains were apparently shedfrom magmatic sources that are slightly younger than the �175Ma Moffat volcanics underlying the clastic sequence. Simi-larities in protolith and provenance with strata on WhitesandIsland and in southeast Alaska suggest that thesemetasedimentary rocks belong to the Gravina belt (or Vennsequence; Crawford et al. 2000).

Conformably below the Gravina strata to the west, onTingberg Island, is a �20 m thick band of quartz-porphyriticmetarhyolite. A U–Pb sample of the metarhyolite from

Tingberg Island (sample 12, Fig. 16) yields an age of 173 ± 8Ma. On westernmost Tingberg Island this metarhyolite isbounded to the west by an east-dipping thrust fault (describedby Klepeis et al. 1998), referred to herein as the Tingbergthrust, that juxtaposes these rocks over mid-Paleozoicmetarhyolite (�354 Ma, sample 6), marble, and metapelite(Fig. 9). The Port Houghton(?) conglomeratic sequence thatstratigraphically separates the two units on the east limb ofthe syncline is structurally omitted along this thrust(Fig. 10).

The relations described above indicate that Tongass andKanaganut islands preserve a highly deformed but apparentlyintact stratigraphic linkage between the Yukon–Tananaterrane and the Gravina belt. Such a linkage has been proposedby Kapp and Gehrels (1998) on the basis of detrital zirconsin Gravina strata, which were clearly derived from theYukon–Tanana terrane. The Tongass–Kanaganut area, however,is apparently the only place where stratigraphic relationsbetween these two units may be preserved (althoughCrawford et al. 2000 suggest that the sequence is morehighly faulted than described herein). In addition, the presenceof �175 Ma Moffat volcanics above both the Yukon–Tananaterrane (on Kanaganut Island) and the Alexander terrane (onRandall Island) provides a tie between these two terranesthat extends back to Middle Jurassic time. This is consistentwith the observation of Saleeby (2000) that both Alexanderterrane and Yukon–Tanana terrane rocks are intruded by162–139 Ma felsic dikes near Boca de Quadra (Fig. 1), justnorth of Harry Bay.

Sitklan Passage – Pearse Canal regionOn the east shore of Kanaganut and Tongass islands, the

mid-Paleozoic metavolcanic rocks grade eastward into asequence of biotite–garnet schist (derived from black shale)with thin layers of gray marble and dacitic(?) metatuff. The

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Fig. 13. Photograph of 5 cm long cobble of interlayered quartz-ite and marble in a moderately deformed metasandstone fromsample locality 9 on southwestern Kanaganut Island. The folia-tion and layering in the cobble were clearly acquired prior to in-corporation in the surrounding metasandstone.

Fig. 14. Concordia diagram of abraded single grains from ametarhyolite on Tongass Island (sample 10). The crystallizationage is interpreted from the cluster of concordant grains, whereasthe older grains are interpreted to have been incorporated whilethe tuffaceous material was deposited.

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proportion of marble and metavolcanics decreases eastward(apparently downsection), with monotonous biotite schistdominating both western and eastern shorelines of LincolnChannel and several small islands within the channel(Fig. 9). Klepeis et al. (1998) and Crawford et al.(2000) concluded that Lincoln Channel is controlled by athrust fault based on the occurrence of kyanite in peliticrocks on the east shore but not in pelitic strata on the westshore. Toward the east, the pelitic strata grade back intomafic and intermediate-composition metavolcanic rocks thatdisplay relict fragmental textures and pillows. The similarityof protoliths across the Lincoln Channel thrust suggests thatit does not juxtapose formerly disparate units. The timing of

ductile deformation along the Lincoln Channel thrust isconstrained by relations with a felsic dike at sample locality13 (Fig. 9). As originally recognized by M.L. Crawford (oralcommunication, 1994), this dike clearly intrudes across thedominant foliation in the pelitic country rocks and contains aweakly developed foliation only where it intrudes parallel tolayering. Four zircon fractions from this dike yield concordantanalyses suggesting an age of 89 ± 2 Ma (Fig. 17).

Toward the east, the metavolcanic rocks are intruded by avariety of metaplutonic rocks (Fig. 9). Westernmost bodies,locally exposed along the east shore of Lincoln Channel, aretrondhjemitic. Two U–Pb samples from these plutons yieldages of 374 ± 10 Ma (sample 14, Fig. 18) and 380 ± 8 Ma(sample 15, Fig. 19). Both samples have inherited components,with upper intercepts of �2.0 Ga and �869 Ma, respectively.Toward the east, on the mainland north of Sitklan Passageand on Wales Island (Fig. 9), orthogneisses dominate andonly small screens of country rock are preserved. Thegneisses are quite variable in composition (mostlygranodiorite and tonalite, with subordinate trondhjemite,diorite, and gabbro) and texture. Four U–Pb samples werecollected from tonalitic phases of these orthogneisses alongthe north shore of Sitklan Passage. As shown in Fig. 19,eight abraded single zircon grains have been analyzed fromeach sample. One of the samples yields concordant analyses(sample 16) with an age of 424 ± 8 Ma. The other three(samples 17–19) are discordant, presumably due to MesozoicPb loss and (or) growth of new zircon. The upper-interceptages for these samples are 420 ± 32 Ma (sample 17), 440 ± 119Ma (sample 18), and 429 ± 47 Ma (sample 19). The age of424 ± 8 Ma is similar to the age of �422 Ma reported bySaleeby (2000) for a tonalite at the same locality.

The two 380–374 Ma trondhjemite bodies are generallycoeval with the ages of volcanic rocks on Kanaganut Islandand with volcanic and plutonic rocks throughout the Yukon–Tanana terrane. The 440–420 Ma ages are problematic interms of terrane assignments, as igneous rocks of this ageare not known from the Yukon–Tanana terrane in Yukon andAlaska (Mortensen 1992) but are common in the Alexanderterrane to the west (Gehrels and Saleeby 1987). In terms ofcountry rocks, however, the units in Sitklan Passage bear astrong resemblance to rocks of the Yukon–Tanana terrane.Saleeby (2000) reported a similar relationship in the areabetween Boca de Quadra and Ketchikan (Fig. 1), wheremetaplutonic and metavolcanic rocks of Yukon–Tananaaffinity (Kah Shakes sequence) yield U–Pb ages of 424–412 Ma.The orthogneisses and metamorphic rocks in Sitklan Passageare accordingly interpreted to belong to the Yukon–Tananaterrane. In contrast, Saleeby interprets the units in easternSitklan Passage to be part of the Alexander terrane, thrustwestward over units of Yukon–Tanana affinity. Documentingthe protolith and structural relations in this complex areawill clearly require more detailed structural, geochronologic,and perhaps Nd or Sr isotopic analyses.

Tonalitic orthogneisses were also analyzed from westernPearse Canal, at sample localities 20 and 21. These bodieshave a strong foliation but are more homogeneous incomposition than the orthogneisses in Sitklan Passage.Eight single-grain analyses from sample 20 yield an age of186 ± 5 Ma based on two concordant grains and on theupper intercept of a well-constrained discordia (Fig. 20).

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Fig. 15. Concordia diagram of abraded single grains from aconglomeratic metagraywacke of the Gravina belt on Tongass Island(sample 11). The grains are interpreted to be mainly 155 ± 7 Ma,with a subordinate population at �205 Ma.

Fig. 16. Concordia diagram of abraded single grains from ametarhyolite on western Tingberg Island (sample 12). The age isinterpreted from the cluster of concordant grains.

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This intrusion is approximately the same age as other plutonswithin the Coast Mountains in the Prince Rupert – Ketchikanarea (Gareau 1989; M.L. Crawford, oral communication,1999). Sample 21 yields low-precision but apparentlyconcordant analyses, with an age of 72.3 ± 2.0 Ma(Fig. 21). This tonalite is the westernmost part of theQuottoon tonalite, which underlies much of western PearseCanal and extends along strike for tens of kilometres to thenorthwest and southeast (Hutchison 1982; Klepeis et al. 1998).

Country rocks intruded by these trondhjemitic and tonaliticorthogneisses change dramatically in western Sitklan Passage(Fig. 9). Detailed mapping along this shoreline indicates that

the 380–374 Ma trondhjemites at localities 14 and 15 intrudemafic to intermediate-composition metavolcanic rocks andgraphitic schist, which are a continuation of the mid-Paleozoicmetavolcanic rocks on Kanaganut Island (Delusion Baysequence of Crawford et al. 2000). In contrast, the tonaliticbodies (samples 16–21) to the east intrude a sequence ofthin-layered marble, pelitic schist, and quartzite. Thewesternmost tonalitic gneiss (sample 16) intrudes bothmetavolcanic and metasedimentary units.

A sample from a 20 m thick quartzite horizon within themarble–schist–quartzite unit was analyzed for detrital zirconages (sample 22). Twenty-two abraded single grains wereanalyzed, with the results shown in Fig. 22. The analysesfrom this quartzite are very similar to the results fromquartzites elsewhere in the Coast Mountains in that they arehighly discordant and clearly include components of bothArchean and Proterozoic age. Five grains with the oldest207Pb*/206Pb* ages appear to lie along a discordia line, withan upper intercept of �2.72 Ga. The other 17 grains liewithin a discordia band, with upper intercepts between 2.0Ga and 800 Ma, but clusters of ages are not apparent. Inter-estingly, these grains, which are clearly of detrital origin, arecolorless, perfectly faceted euhedral grains, which show nosign of abrasion or rounding. This suggests that discordanceis due at least in part to growth of new zircon duringmetamorphism. Several grains in this sample yield <300 Ma206Pb*/238U ages, which suggests that they consist largely ofmetamorphic zircon.

Similarities in protoliths and detrital zircon ages withunits in other parts of the Coast Mountains indicate that themarble–schist–quartzite sequence in Sitklan Passage andPearse Canal belongs to the Tracy Arm assemblage of theYukon–Tanana terrane. This is important because the Silurian–Devonian orthogneiss bodies clearly intrude both these rocksand the Endicott Arm metavolcanic rocks to the west,thereby demonstrating that the two assemblages have beentogether since mid-Paleozoic time. The occurrence of TracyArm rocks and Silurian orthogneisses both east and west ofthe main strand of the Coast shear zone is also important, asthis relationship makes large-scale (�1500 km; Hollister andAndronicos 1997) displacement on this part of the Coastshear zone unlikely.

Digby Island areaDigby Island and nearby regions are underlain by many of

the same units described near Dundas and Tongass islands.In the region south of Tongass Island, the main units divergeas the large syncline opens southward (Fig. 3). This producesa broad expanse of metamorphosed metasedimentary rocksin the Port Simpson area that are interpreted to belong to theGravina belt. Alternatively, Crawford et al. (2000) refer tothese rocks as the Delusion Bay sequence, which is interpretedto be correlative with the Taku and (or) Yukon–Tanana terranes.

The Gravina strata on Whitesand Island also continuesouthward and underlie a broad region on northern DigbyIsland and Tsimpsean Peninsula (Figs. 3, 23). Near DigbyIsland, these units converge southward such that the width ofGravina strata becomes quite narrow at the mouth of theSkeena River.

The western part of Digby Island is underlain by felsic tomafic metavolcanic rocks that resemble Ordovician–Silurian

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Fig. 17. Concordia diagram of zircon grains from a leucogranitedike that intrudes across deformational fabrics related to the LincolnChannel thrust fault (sample 13, Fig. 9). The age is interpretedfrom the cluster of concordant analyses.

Fig. 18. Concordia diagram of zircon grains from a leucotrondhjemitebody along the east shore of Lincoln Channel (sample 14). Theage is interpreted from the lower intercept of a regression throughthe three nearly concordant grains and five larger and older singlegrains. Excluded from the regression are three fractions of smallergrains that clearly do not lie along the discordia. A similar agewould be obtained from the average of the three nearly concordantgrains.

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rocks of the Descon Formation of the Alexander terrane(Digby sequence of Crawford et al. 2000). A fragmentalmetarhyolite from locality 23 yields a U–Pb age of 472 ± 13 Mabased on analysis of eight abraded single zircon grains(Fig. 24). At the south end of Digby Island, these rocks areunconformably overlain by a less deformed sedimentarysequence that is dominated by conglomeratic sandstone andmudstone with thin-bedded fossiliferous limestone layers.One of these layers, collected from the locality shown inFig. 23 (Geological Survey of Canada locality C-302159),yields conodonts of Early Devonian (probably late Pragian)age (M.J. Orchard, oral communication, 1995). These strataare apparently part of the Karheen Formation. Younger rocksof the Alexander terrane are also exposed on DevastationIsland (Fig. 23). Here, as described by Woodsworth et al.(1983) and Woodsworth and Orchard (1985), upper Paleozoic(?)brachiopod- and coral-bearing marble on the west is overlaineastward by conglomeratic strata in which rhyolite cobblesare enclosed in a matrix of marble, sandstone, and graphiticphyllite. The Triassic(?) conglomerate is overlain conformably

by metarhyolite (Moffat volcanics?) that makes up the easternpart of Devastation Island.

Eastern and northern Digby Island and western TsimpseanPeninsula are underlain by a thick sequence ofmetaturbidites of the Gravina belt (Venn sequence ofCrawford et al. 2000). These rocks are commonly conglom-eratic, with most clasts derived from rhyolite and graniticprotoliths, and there are scattered layers of quartz-porphyriticmetarhyolite. One of the metarhyolite layers from Jap Point(Fig. 3) yields a U–Pb age of 168 ± 5 Ma (sample 24,Fig. 25). An additional zircon grain yields a much older207Pb*/206Pb* age (Fig. 25; Table 1), which suggests that aPrecambrian inherited component, perhaps derived from theYukon–Tanana terrane, is present.

Detrital zircons have been analyzed from two conglomeratichorizons. Eleven abraded single grains from a pebblymetasandstone (sample 25) on Carr Island (Fig. 23) yieldages of 450–405 Ma (n = 5) and 390–365 Ma (n = 6;Fig. 26). Twenty-two grains from a second sample (sample26), collected from a peninsula west of Anian Island

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Fig. 19. Concordia diagrams of abraded single zircon grains from a leucotrondhjemite (sample 15) and four tonalitic orthogneisses(samples 16–19) along the north shore of Sitklan Passage (Fig. 9). The age of sample 15 is interpreted from the lower intercept of aregression through the eight analyses (mswd = 0.38). The age of sample 16 is interpreted from the cluster of concordant analyses. Theages of samples 17, 18, and 19 are interpreted from regression lines through each of the eight analyses (mswd = 0.40, 2.62, and 1.61,respectively).

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(Fig. 23), yield non-overlapping and variably discordantages (Fig. 27). The preferred interpretation, given the discor-dance patterns of this sample and the ages in other Gravinasamples, is that discordance results from Pb loss in grains thatare �430 Ma (n = 1), �380 Ma (n = 9), �225 Ma (n = 4), and�175 Ma (n = 8).

Separating the two belts of Gravina strata on TsimpseanPeninsula is a belt of metavolcanic and metasedimentaryrocks of uncertain tectonic affinity (Fig. 3). As shown in

Fig. 23, these rocks can be divided into four separate mapunits. From west to east these include (i) a heterogeneousunit of crinoidal marble (locally conglomeratic with rhyoliteclasts), pelitic schist, and mafic and felsic metavolcanicrocks; (ii) a thick section of quartz-porphyritic and locallyfragmental metarhyolite; (iii) mafic metavolcanic brecciaand pillow flows; and (iv) pelitic schist with minormetarhyolite, metabasalt, and marble layers. These rocks areinterpreted to be upper Paleozoic and Triassic strata that arecorrelative with Alexander terrane units on DevastationIsland (Fig. 23) and in the Dundas Island area (Fig. 4), overlainby Jurassic metavolcanic and metasedimentary rocks of theMoffat volcanics. This interpretation differs from the corre-lations of Crawford et al. (2000), who refer to these units asthe Delusion Bay sequence and suggest that they correlatewith the Taku and (or) Yukon–Tanana terranes. These rocksare bounded by a west-vergent thrust on the west and by thePrince Rupert shear zone to the east.

The Prince Rupert shear zone is a major structure thatjuxtaposes kyanite-bearing schist in the hanging wall overlower grade rocks to the west (Crawford et al. 1987, 2000).This thrust is known to be mid-Cretaceous in age because ittruncates Gravina belt strata, is syntectonic with respect tothe �91 Ma Ecstall pluton (Fig. 3), and produced metamorphicassemblages that yield �90 Ma Ar–Ar cooling ages(Crawford et al. 1987, 2000). The other thrusts in the region,which merge southward with the Prince Rupert shear zone(Fig. 3), are also interpreted to have moved during mid-Cretaceous time. Although the Prince Rupert shear zone hasa broad zone of deformation in the Prince Rupert area(Fig. 23), its northern continuation is uncertain. The interpretedtrace, as shown in Fig. 3, continues southwest of PortSimpson, where an east-dipping thrust fault separatesGravina strata to the east from Jurassic(?) rocks to the west.

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Fig. 20. Concordia diagram of abraded single grains from a foliatedtonalite along northern Sitklan Passage (sample 20). The age isinterpreted from a regression through the six discordant analyses(mswd = 0.40) and the two apparently concordant grains.

Fig. 22. Concordia diagram of abraded single grains from amedium-grained ultramature quartzite from the Safa Islands, westernPearse Canal (sample 22). The analyses are shown as boxes becausethe individual error ellipses are too small to be visible in the diagram.Also shown for reference is the field of single-grain analyses fromother samples of quartzite in the Coast Mountains (from Gehrelset al. 1991; Gehrels and Kapp 1998; and Gehrels 2000b).

Fig. 21. Concordia diagram of abraded single grains from a foliatedtonalite at the eastern end of Sitklan Passage (sample 21). Theanalyses are of fairly low precision because of the low U andhigh common Pb content of the zircons. The age is interpretedfrom the cluster of concordant analyses.

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Fig. 23. Simplified geologic map of the Digby Island area (adapted from Hutchison 1982; Woodsworth et al. 1983; and Crawford et al.1987, 2000). The conodont sample yielded an age of Early Devonian, as determined by M.J. Orchard of the Geological Survey of Canada(written communication, 1997).

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This younger-on-older segment of the fault may overprint anoriginal depositional contact along the base of the Gravinabelt, as is suspected in the Dundas Island area (Fig. 4).Alternatively, Crawford et al. (2000) show the lithic breakalong the Prince Rupert shear zone as tracing northeast ofPort Simpson, separating rocks of the Kaien sequence (tectonicaffinity uncertain) to the northeast from rocks of the DelusionBay sequence (Taku and (or) Yukon–Tanana terranes) to thesouthwest.

Tectonic implications

Relations in the Chatham Sound area have importantimplications for the accretionary history of the Alexander–Wrangellia terrane and the evolution of the Coast Mountainsorogen. Most recent scenarios invoke accretion of Alexander–Wrangellia during mid-Cretaceous time, as recorded by thewidespread metamorphism, deformation, thrusting, and

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Fig. 24. Concordia diagram of abraded single zircon grains froma metarhyolite along the west shore of Digby Island (sample 23).The age is interpreted from a regression through the discordantanalyses (mswd = 0.70).

Fig. 25. Concordia diagram of abraded single zircon grains froma metarhyolite at Jap Point (Fig. 3; sample 24). The age is interpretedfrom the cluster of concordant analyses. A regression through thiscluster and a highly discordant xenocrystic(?) grain yields an upperintercept of 1945 ± 363 Ma.

Fig. 27. Concordia diagram of abraded single grains from aconglomeratic metagraywacke of the Gravina belt in Venn Passage,north of Digby Island (sample 26, Fig. 23). The analyses are concordantto moderately discordant and non-overlapping and cannot beassigned crystallization ages. Based on the discordia patterns andthe ages of grains in other samples from the Gravina belt, it islikely that these grains are �430, �380, �225, and �175 Ma.The regressions shown are projected from a 0 Ma lower intercept.

Fig. 26. Concordia diagram of abraded single zircon grains froma metagraywacke of the Gravina belt on Carr Island, north of DigbyIsland (sample 25, Fig. 23). The ages are interpreted to be 450–405and 390–365 Ma.

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plutonism of this age along the west flank of the CoastMountains (Berg et al. 1972; Monger et al. 1982; Crawfordet al. 1987; Rubin and Saleeby 1992). This model has beenchallenged by two opposing views.

One alternative view, based largely on interpretations ofpaleomagnetic data, is that Alexander–Wrangellia was locatedat the latitude of northern Mexico during mid-Cretaceoustime and did not arrive in its present position until the LateCretaceous – early Tertiary (Irving et al. 1996; Cowan et al.1997; Hollister and Andronicos 1997). Northward motion of�1500 km is suspected to have occurred along the Coastshear zone or related structures within the Coast Mountains(Cowan et al. 1997; Hollister and Andronicos 1997). Theexistence of large-scale strike-slip displacement on the mainstrand of the Coast shear zone is difficult to reconcile in thestudy area with the apparent continuity of protoliths acrossthe western Coast Mountains near Chatham Sound. Specificunits that are apparently continuous include the mid-Paleozoictonalitic orthogneiss in Sitklan Passage and the marble–schist–quartzite assemblage that makes up pendants throughout theaxial region of the Coast Mountains and also occurs west ofthe Coast shear zone along Sitklan Passage (Figs. 9, 10).This correlation is supported by detrital zircon analyses aswell, in that grains from a quartzite in western Boca deQuadra (west of the Coast shear zone, �20 km north ofHarry Bay; Fig. 1) yield ages (Saleeby 2000) that are similarto those of grains in the quartzite from Pearse Canal (sample22). These apparent linkages suggest that little orogen-parallelmotion has occurred along the western portion of the centralCoast Mountains since at least mid-Cretaceous time. Therelations do not, however, rule out the existence of structureswith large-scale motion inboard of the main trace of theCoast shear zone. A second alternative view is that initial ac-cretion of Alexander–Wrangellia occurred during latest Tri-assic to mid-Jurassic time (McClelland and Gehrels 1990;McClelland et al. 1992; van der Heyden 1992; Mihalynuk etal. 1994; Monger and Nokleberg 1996; Kapp and Gehrels1998; Saleeby 2000). Relations used to support this modelinclude (i) the presence of generally coeval latest Triassic toMiddle Jurassic volcanic-arc assemblages on all of these terranes(van der Heyden 1992; Monger and Nokleberg 1996), (ii) acoherent eastward-younging trend of Middle Jurassic – EarlyCretaceous plutonic belts across the western Coast Mountainssouthwest of Prince Rupert (van der Heyden 1992),(iii) evidence for a broad dextral shear zone of mid-Jurassicage along the inboard margin of the Alexander terrane nearPetersburg (McClelland and Gehrels 1990; Fig. 1), (iv) thepresence of detrital zircons derived from both the Alexanderand Yukon–Tanana terranes in Upper Jurassic – LowerCretaceous strata of the Gravina belt (Kapp and Gehrels1998), and (v) the presence of 162–139 Ma felsic dikes thatintrude rocks belonging to both the Alexander and Yukon–Tanana terranes in the Boca de Quadra area (Fig. 1; Saleeby2000).

Relations in Chatham Sound are strongly supportive ofthis view. The critical relationship is that the �175 Ma Moffatvolcanics overlie Triassic rocks of the Alexander terrane (onRandall and adjacent islands (Fig. 4) and probably onDevastation Island and southern Tsimpsean Peninsula(Fig. 23)) and upper Paleozoic(?) rocks of the Yukon–Tananaterrane (on Kanaganut and Tongass islands and the mainland

to the north (Fig. 9) and south (northeast of Port Simpson,Fig. 3)). As shown in Fig. 2, the metavolcanic rocks areapparently part of a regional assemblage of Aalenian–Bajocian (lower Middle Jurassic) volcanics that also occurson the Queen Charlotte Islands (Yakoun Formation) and in thewestern part of the Stikine terrane (Monger et al. 1991).Plutons of this age within the Coast Mountains (Fig. 2) maybe subvolcanic equivalents of the volcanic rocks. This overlaprelationship requires Alexander–Wrangellia to have been nearthe Yukon–Tanana and Stikine terranes by mid-Jurassic time.

A potential regional-scale problem with Jurassic linkagesacross the Coast Mountains arises from the presence of Middleand Upper Jurassic marine strata in the Bridge River terranealong the inboard margin of the southern Coast Mountains(Monger et al. 1991). However, a possible explanation ofthis relationship that does not require an open ocean basinbetween the two crustal fragments has been offered by Mongeret al. (1994). These workers suggest that Bridge River stratamay have accumulated along the continental margin south ofthe Alexander and Wrangellia terranes and were then trappedinboard of these terranes by Early Cretaceous left-lateral motionon orogen-parallel strike-slip faults.

A reasonable scenario based on the mid-Jurassic linkageof Alexander–Wrangellia with Stikine and Yukon–Tananainvolves (i) Early–Middle Jurassic juxtaposition of theoutboard terranes with the inboard Stikine and Yukon–Tanana terranes; (ii) Middle Jurassic eruption of Moffatvolcanics and correlative volcanic sequences across much ofthe Alexander, Wrangellia, Yukon–Tanana, and Stikine terranes;(iii) Late Jurassic extension and (or) transtension creatingthe Gravina basin; (iv) left-lateral motion on strike-slip faultsalong the boundary between inboard and outboard terranes(Monger et al. 1994; Chardon et al. 1999); and (v) collapseof the Gravina basin and final structural accretion of Alexander–Wrangellia during mid-Cretaceous time (McClelland andGehrels 1990; van der Heyden 1992; McClelland et al. 1992;Saleeby 2000). The obvious record of this accretionary eventwest of the Coast Mountains includes a Jurassic dextralshear zone in the easternmost Alexander terrane near Peters-burg (McClelland and Gehrels 1990) and the lack of Lowerand Middle Jurassic strata beneath rocks of the Gravina beltin most of southeast Alaska (Berg et al. 1972). Within theCoast Mountains, Saleeby (2000) has recognized ductiledeformational fabrics within the Alexander and Yukon–Tanana terranes that are crosscut by less deformed 162–139Ma dikes, and Currie and Parrish (1997) describe a ductileshear zone along the east flank of the northern Coast Moun-tains that moved in Middle Jurassic time. To the east, in theStikine terrane, there is a major unconformity separatingUpper Triassic strata from overlying Lower–Middle Jurassicvolcanic rocks (Monger et al. 1991). In addition, clasticwedges of the appropriate age have been recognized inboardof both the northern Coast Mountains (Lower to Middle Ju-rassic Takwahoni Formation) and southern Coast Mountains(Lower Jurassic Harrison Lake Formation; Monger et al.1991).

Collectively, the available evidence suggests that the Earlyor Middle Jurassic arrival of Alexander–Wrangellia alongthe margin of Yukon–Tanana and Stikine terranes did notinvolve regional thrusting, development of high-grademetamorphic rocks, or widespread uplift and erosion of

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rocks within the ancestral Coast Mountains. Strike-slipjuxtaposition seems a likely explanation of the availableevidence. However, it is possible that the widespreadCretaceous – early Tertiary deformation, metamorphism,and plutonism have obscured evidence of a more significantJurassic orogenic event within the Coast Mountains.

Although these relations suggest linkages across the CoastMountains beginning in Early–Middle Jurassic time, they donot necessarily record the addition of Alexander–Wrangelliato the North American continent. As summarized in recenttectonic syntheses by Mihalynuk et al. (1994) and Mongerand Nokleberg (1996), the inboard Stikine and Yukon–Tananaterranes were probably not firmly attached to the NorthAmerican continent until Middle Jurassic time. Hence,interactions between Alexander–Wrangellia and the Stikineand Yukon–Tanana terranes may have begun while thesecrustal fragments were still a considerable distance from thecontinental margin. Alternatively, accretion of Alexander–Wrangellia to Stikine and Yukon–Tanana may have beenpart of a Middle Jurassic accretionary event that involvedmost terranes outboard of the miogeocline in the CanadianCordillera.

Conclusions

The Chatham Sound area displays relations that are criticalto understanding the tectonic evolution of the northernCordillera. In the present study, these relations have beenanalyzed through regional and detailed geologic mappingtogether and U–Pb geochronologic analyses of igneousrocks and detrital zircon grains. Conclusions based on thiswork support the general findings of Saleeby (2000) andCrawford et al. (2000) but differ in some of the specific ageassignments, structural relations, and tectonic correlations.Findings of regional significance are outlined as follows.(1) Rocks interpreted to belong to the Yukon–Tanana

terrane are well preserved in the Tongass Island area.Although regionally deformed and metamorphosed, theunits form a westward-younging sequence that appearsrelatively intact and coherent. Main components includea marble – pelitic schist – quartzite sequence that containsdetrital zircons of mainly �2.72 Ga and 2.0–0.8 Ga.These rocks are intruded by mid-Paleozoic tonalitic–trondhjemitic plutons and overlain by a sequence ofmid-Paleozoic basaltic to dacitic metavolcanic rockswith minor pelitic schist and marble. The metavolcanicrocks are overlain unconformably by a less deformedCarboniferous(?) sequence that grades upsection frommainly metaconglomerate and sandstone to mainlymetabasalt, pelitic schist, and marble. The unconformitybelow the conglomeratic strata apparently records a majorDevono-Mississippian orogenic event. These threeassemblages correlate well with rocks of the Tracy Arm,Endicott Arm, and Port Houghton assemblages in centralsoutheast Alaska.

(2) A sequence of �175 Ma metarhyolite and metabasalt,referred to as the Moffat volcanics, rests depositionallyon both Triassic strata of the Alexander terrane and upperPaleozoic(?) rocks of the Yukon–Tanana terrane. Thesemid-Jurassic volcanic rocks appear to be part of awidespread overlap sequence that is also found in the

Wrangellia terrane and the western Stikine terrane. Thisrelationship strongly supports previous findings (van derHeyden 1992; McClelland et al. 1992; Mihalynuk et al.1994; Monger and Nokleberg 1996; Kapp and Gehrels1998; Saleeby 2000) that Alexander and Wrangelliawere in close proximity to Yukon–Tanana and Stikineby Middle Jurassic time. Juxtaposition during latest Triassicto Middle Jurassic time was apparently followed byLate Jurassic rifting–transtension to form the Gravinabelt basin, Early Cretaceous left-lateral motion on orogen-parallel strike-slip faults, and a series of mid-Cretaceousevents including collapse of the Gravina basin, finalaccretion of Alexander–Wrangellia, and formation of thethrust belt, regional deformation, high-grade metamorphism,syntectonic plutons, and rapid uplift that characterizethe Coast Mountains orogen.

(3) The Coast shear zone is apparently developed largelywithin rocks of the Tracy Arm assemblage (Yukon–Tananaterrane) in the Sitklan Passage – Pearse Canal area. Thecontinuity of the marble–schist–quartzite sequence andthe Silurian tonalitic orthogneiss across the main strandof the shear zone in this area is difficult to reconcilewith the interpretation that this part of the Coast shearzone has large-scale (i.e., �1500 km) dextral offset(Hollister and Andronicos 1997).

Acknowledgments

I would like to thank M.L. Crawford for initiating ourcollaborative work in the Chatham Sound area and for sharingthe results of her many years of work in the Coast Mountains.I have also benefitted greatly from conversations about thegeology of the region with R.F. Butler, W.A. Crawford, L.S.Hollister, K.A. Klepeis, M.E. Rusmore, J.B. Saleeby, andG.J. Woodsworth. M.J. Orchard (Geological Survey ofCanada) graciously analyzed the limestone sample fromDigby Island for conodonts. B. Darby, P. Kaminski, P. Kapp,B. Lareau, and M. Spurlin were excellent undergraduate assis-tants in the field and U–Pb laboratory. This research wasconducted as part of the ACCRETE project with support fromthe National Science Foundation (EAR-9303824 and EAR-9526263). The paper was reviewed by J.W.H. Monger andR.M. Friedman.

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geology of the chatham sound region, southeast alaska and coastal british columbia

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