Paleozoic Rocks of Northern Chukotka Peninsula, Russian Far East: Implications for the Tectonicsof the Arctic Region

TECTONICS, VOL. 18, NO. 6, PAGES 977-1003 DECEMBER 1999

Paleozoic rocks of northern Chukotka Peninsula, Russian Far East: Implications for the tectonicsof the Arctic region

BorisA. Natal'in,1 Jeffrey M. Amato,2 Jaime Toro, 3,4 and James E. Wright5

Abstract. Paleozoicrocks exposedacross the northernflank of Alaskablock the essentialelement involved in the openingof the the mid-Cretaceousto Late CretaceousKoolen metamorphic Canada basin. domemake up two structurallysuperimposed tectonic units: (1) weaklydeformed Ordovician to Lower Devonianshallow marine 1. Introduction carbonatesof the Chegitununit which formed on a stableshelf and (2) strongly deformed and metamorphosedDevonian to Interestin stratigraphicand tectoniccorrelations between the Lower Carboniferousphyllites, limestones, and an&site tuffs of RussianFar East and Alaska recentlyhas beenrevived as the re- the Tanatapunit. Trace elementgeochemistry, Nd isotopicdata, sult of collaborationbetween North Americanand Russiangeol- and texturalevidence suggest that the Tanataptuffs are differen- ogists.This paperpresents the resultsof one suchstudy from the tiatedcalc-alkaline volcanic rocks possibly derived from a mag- ChegitunRiver valley, Russia,where field work was carriedout matic arc. We interpretthe associatedsedimentary facies as in- to establishthe stratigraphic,structural, and metamorphicrela- dicativeof depositionin a basinal setting,probably a back arc tionshipsin the northernpart of the ChukotkaPeninsula (Figure basin. Orthogneissesin the core of the Koolen dome yielded a 1). The ChegitunRiver regionexposes an Ordovicianto Lower Devonian(between ~369 and ~375 Ma) U-Pb zirconage which is Carboniferouscarbonate and metacarbonate,shale, phyllite, and similarto the agesof the Tanataptuffs as well as graniticplutons thin-beddedturbidite sequencewith rare interbeddedtuffs that formedwithin a Devonianactive continentalmargin of northern were metamorphosedand deformedduring the Cretaceous.This Alaska. The stratigraphyof the Chegitununit is similarto that of sequenceis in fault contact with a mid-Cretaceousto Late the Novosibirskcarbonate platform which overlies the Late Cretaceoushigh-grade metamorphic complex, the Koolendome. Precambrian Bennett-Barrovia block. The basement of the block The geologyof the high-gradecomplex was discussedby the is exposedin Chukotkawhere ortogneissin the ChegitunRiver Bering Strait GeologicalField Party (BSGFP) [1997]. valley yielded Late Proterozoic(•-650 to 550 Ma) U-Pb ages. Stratigraphiccorrelations between Paleozoic sequences in the These two tectonic units form the shelf of the Chukchi and East ChegitunRiver region and similar successionsin Chukotka,in Siberian Seas and may continue into northern Alaska as the the various islands of the Chukchi and East Siberian Seas,and in Hammond subterrane. The deep-waterTanatap unit can be northernAlaska bear on an importantunsolved problem for this tracedalong the southernboundary of the Bennett-Barroviablock region:the presenceor absenceof a large Precambrianblock on from the Novosibirsk Islands to northern Alaska. This basin was which theselower Paleozoiccarbonate platform deposits accu- pairedwith a Devonianmagmatic arc that existedfarther to the mulated. south. The northernmargin of the Bennett-Barroviablock col- The Paleozoicframework of the regionsheds light on the ge- lidedwith NorthAmerica in the Late Silurianto Early Devonian. ometryand sizeof the Chukotka-ArcticAlaska microplate whose In Chukotka,during Middle to Late Carboniferoustime the re- Early Cretaceousdisplacement is thoughtto be responsiblefor constructedDevonian arc-trench system at the southernedge of the openingof the CanadaBasin (Figure 1) (see Lawyer et al. the Bennett-Barrovia block collided with an unknown continental [1990] for a review of the differentmodels proposed for the object,fragments of which now occurto the southof the South openingof the CanadaBasin). In this paperwe investigatethe Anyui suture.Triassic to Cretaceousdeformation strongly modi- correlation of Late Proterozioc and Paleozoic rocks of the fied the Paleozoicunits. Our resultsprovide new constraintson Chegitun River valley with rocks of similar ages in the the geometryand Paleozoichistory of the Chukotka-Arctic Siberian/northernAlaska sector of the Arctic region,the depositional setting of these strata, and the geochemistryand geochronologyof Devonian magmatism on the Chukotka 1 Departmentof Geology,Istanbul Technical University, Istanbul, Peninsula. These data are then used to discuss the Paleozoic Turkey. tectonic evolution of northeastern Russia and northern Alaska. 2 Departmentof Geological Sciences, New Mexico State University, We will address the Cretaceous thermal evolution and structural Las Cruces. 3 Departmentof Geologicaland Environmental Sciences, Stanford historyof the ChegitunRiver valley metamorphicrocks in a later University,Stanford, California. paper. 4 Nowat Departmentof Geologyand Geography, West Virginia University,Morgantown. 5 Departmentof Geology and Geophysics, Rice University, Houston, 2. Tectonic Setting of the Study Area Texas. Prior to the now widely acceptedhypothesis of the counter- clockwise rotation of the Chukotka-Arctic Alaska block for 60 ø Copyright1999 by theAmerican Geophysical Union. abouta pole at the McKenzie delta (Figure 1) [e.g., Tailleur and Papernumber 1999TC900044 Brosge,1970], the idea of a Hyperboreanplatform underlain by 0278-7407/99/1999TC900044512.00 Precambrianbasement in the centralpart of theArctic Oceanwas

977 978 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 979 proposedby Shatsky[ 1935] to explainthe sinuosityand disposi- igneousand sedimentaryprotoliths metamorphosed to upperam- tion of fold belts(e.g., Chukotkafold belt,the BrooksRange) and phiboliteand locally granulitegrade. The age of metamorphism high-grademetamorphic rocks in the Arctic region. Eardley was previouslyestimated to rangefrom Archean[Shuldiner and [1948] referred to this Precambrianplatform as "Ancient Nedornolkin,1976; Nedornolkin, 1977] to Paleozoic [Gnibidenko, Arctica•" Americangeologists proposed the existenceof an area 1969; Bunker et al., 1979] or with an additionalepisode in the called "BarroviaLand" to providea northemsource region for Mesozoic [Gelman, 1973; Natal'in, 1979], but it is now known to Paleozoicand Mesozoicclastic rocks in northemAlaska [e.g., haveoccurred during mid-Cretaceous time on both the Chukotka Tailleur, 1973]. However,if a rotationalorigin for the Chukotka- Peninsula [BSGFP, 1997] and on the Seward Peninsula [Arnatoet Arctic Alaska block abouta pole in the McKenzie delta is as- al., 1994; Arnatoand Wright, 1998]. sumed, the North American craton is a suitable source for these Paleozoicrocks and high-grademetamorphic complexes on sediments. Zonenshain et al. [1990] defined a microcontinent, the Chukotka Peninsulacomprise part of a broader tectonic "Arctida," as one of several Precambriancratons, such as the East provincein the RussianFar Eastand northernAlaska: the former Europeanand Siberiancratons, which was later disruptedowing Chukotka-ArcticAlaska microplate. The southernboundary of to the openingof the oceanicbasins in the Arctic Ocean. •engiir thisancient microplate is definedby the NeocomianSouth Anyui and Natal'in [1996] supportedthe necessityof an old continental suture in northeast Russia and the Late Jurassic to Neocomian block but definedit with a shapeand size differentfrom that of Kobuk suturein Alaska (Figure 1). For a review of the tectonics Arctida, They namedit after the Bennettmassif which is located of northeastAsia, seethe work of Parfenovand Natal'in [1985], in the northwesternpart of the East SiberianSea [Vinogradovet Zonenshainet al. [1990], Fujita and Cook [1990], and •enggir al., 1974] but notedthat it may be contiguouswith Precambrian and Natal'in [19%]. For a review of the tectonics of northern crustin the northeasternpart of the Chukchi Sea [Grantz et al., Alaska, see the work of Moore et al. [ 1994, 1997b]. 1990]. In this paperwe usethe term Bennett-Barroviablock for this Precambrian block. It should be stressed that the Bennett- Barrovia block is not the same as the Arctic Alaska terrane 3. Lithotectonic units of the Chegitun River Valley [Newmanet al., 1977;Moore et al., 1994].The latteris a younger tectonicelement not assembleduntil Devoniantime (seebelow). From the southeastto the northwest,three principal fault- The Chegitun River valley exposescarbonate and metacar- boundedlithotectonic units are exposedin the ChegitunRiver bonate rocks which we correlate with the early to middle valley (Figure3): (1) the High-Gradeunit, which consistsof Ul> PaleozoicNovosibirsk carbonate platform. The Novosibirskcar- per amphibolitefacies metamorphicrocks along the northern bonateplatform covers the Bennett-Barroviablock as definedby flank of the Koolen metamorphicdome; (2) the Tanatap unit, •engb'r and Natal'in [1996]. This platform includesrocks ex- which consistsof Devonianto Lower Carboniferousgreenschist posedon the Novosibirsk(New Siberian)Islands of the East faciesmetasedimentary rocks; and (3) the Chegitununit, which SiberianSea, on Wrangel Island, and in severallocalities of consists of virtually unmetamorphosedOrdovician through northernAlaska including the SewardPeninsula and the western Lower Devonianshallow marine carbonate rocks. It was previ- and centralBrooks Range (YM and HM, Figure 1). The wide ously believed that the high-grade rocks representedthe geographicdistribution of rocksof similarage and composition, Precambriandepositional basement for the Paleozoicsedimentary in additionto geophysicaldata, suggests that this early Paleozoic rocks [Belyi, 1964; Tiltnan, 1973; Shuldiner and Nedornolkin, carbonateplatform underlies the entire East Siberianshelf. On 1976; Nedornolkin,1977]. In this framework the Tanatap and the basisof stratigraphicand fossil evidence, both Durnoulin and Chegitununits were regardedas a continuousPaleozoic succes- Harris [ 1994], for the Alaskanterranes, and •;engOrand Natal'in, sion [Nedornolkin, 1977; Oradovskaya and Obut, 1977]. for the whole region, agree that these widely scatteredpre- However, although the High-Grade unit does include Late Middle-Devonian carbonatesuccessions were part of a single Proterozoicprotoliths, our researchhas shown that there is no ev- carbonateplatform, rather than disparate,far-traveled terranes as idenceof a straightforwardbasement/cover relationship between had been previously proposed [Fufita and Cook, 1990]. the high-grademetamorphic rocks and the weakly metamor- However, the Paleozoictectonic history of this extensiveplat- phosedsedimentary rocks that bear Paleozoicfossils. In addition, form hasnot beeninvestigated in greatdetail. both the lithology and paleodepositionalenvironment of the The Paleozoic rocks of northern Chukotka Peninsula are in Tanatap unit are significantly different from those of the fault contact with the Koolen metamorphicdome (Figure 2) Chegitun unit and the inferred sedimentaryprotoliths for the [Natal'in, 1979], one of severalhigh-grade metamorphic com- high-grademetamorphic rocks. These differencesin lithology plexes on the Chukotkaand SewardPeninsulas [Belyi, 1964; and depositionalenvironment allow us to define three fault- Drabkin, 1970b; Tilttan, 1973; Shuldiner and Nedornolkin, 1976; bounded lithotectonic units which have internal contacts that are Nedornolkin, 1977; Natal'in, 1979; Arnato et al., 1994]. These truncatedby the boundingfaults. These units are describedin metamorphiccomplexes expose Late Proterozoicto Paleozoic sections3.1-3.3 in orderof decreasingmetamorphic grade.

Figure 1. Principaltectonic units of northeasternRussia and northern Alaska [afterNatal'in, 1981, 1984;Moore et al., 1994, Plafkerand Berg, 1994]). The insetshows the Chukotka-ArcticAlaska block. AN, Angayuchamterrane; BD, Baird Group;HM, Hammondsubterrane; KG, Kigluaikdome; KO, Koolendome; KM, Kolyuchin-Mechigmen zone; MA, Arctic mid-oceanridge; MD, McKenzie delta; NP, North Pole; PR, PrimorskBasin; SE, Senyavin Uplift; SN, SvaytoyNos; YM, York terrane;YU, PaleozoicYarakvaam and Aluchinblocks. 980 NATAL'IN ET AL.: PALEOZOICROCKS OF NORTHERNCHUKOTKA

174 ø NESHKAN 172ø C. Seshan 170ø DOME ChegitunRiver KOOLEN 'X, region,Fig.3 DOME

170 ø

Bay

,avrentiya Bay

Mechigmen Bay 65 ø %%%%%%%%%%%"•"1--•% %%%

%%%%%%%%%% •- •ß , ' %%%%%%%%%%% %%%-%%%% %%%%%%%%%%%%%%%%%%%% %%%%%%%%%%' %%%%-• %%%%%%

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%%%%%% •%%%%%% 172 ø % % % % % % %%%%4 ,%%%%% %%%%%%% %%%%7 %%%%%%%% %•%-" %." %."% •% •

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Continuation • %%%%%% of the South •# Anyui suture • SENYAVIN 65 ø 176 ø UPtlFT 174ø 64ø ['"--'"'""-'-"-'"'"-"1Cenozoiccover • Chegitununit Devoniancarbonate andclastic rocks I'•'•'•'•t Okhotsk-Chukotka••"'•• Tanatap unit •'-'-'-'-_• volcanic belt ,,,,,,,,• HIGH-GRADE METAMORPHIC ROCKS

1•.•.'.•.'.•.'.•."1•-•,-3•.•.."•.."1plutonsCretaceous IIIIIIIIII Mechigmen•o.•o,,n- zone •'•...... •'•l:r'•Etelkhvyleut Series and equivalents

TriassicChukotkarocks fold ofbelt Triassic-Jurassicoceanic rocks Lavrentiya Seriesand equivalents j Fault J Strike-slipfault j Normalfault • Structuraltrend

Figure2. Geologicmap of the Chukotka Peninsula modified after Gorodinsky [1980] showing thelocation ofde- tailedgeologic mapping inthe Chegitun River valley area of Figure 3. TheEtelkhvyleut Series isexposed inthe coreof the Koolen dome and the Lavrentiya Series represents flanks of the dome. Circles indicate locations ofsam- plesfor U-Pb age determination: 1,M18-94K; 2, F45-94K;and 3, 95JT4a. NATAL'IN ET AL.' PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 981

HIGH-GRADE UNIT CHEGITUN UNIT 171 LavrentiaSedes: •[•• LowerDevonian dolomilJc limestones •• calc-silicateand marblesgneisses Upper SiludanOdan Formation; 66ø30 ' limestones dolomites .r'••schistsbiolJte-gamet :•• and •4t•2;•quartzites III!:•'.111 blackLower shalesSilurian Putukuney Formation;

•-'• Upperreef limestonesOrdovician ChegitunFormalion'

,.[•"•biolJte-gametparagneisses m•'• limestonesMiddle Ordovician and dolomitesIsseten Form.; EtelkhvyleutSedes: ?.;•;• brecciated -.•.?.•.m-• carbonates '• augenuartzo-fel gneissesdspatic TANATAP UNIT .-/.-/.-• Lower Carboniferous limestones and ,-,...-..iC.•'.•'i ,-,-• calcareous shales; Utaveem Formation Middle-UpperDevonian m• IkychurenFormation; Umestone and marbles MiddleDevonian Tanatap Formalion: Ratkhat F. II,'-,-'-,--, I i i recrystallizedlimestones • and marbles

•...... turbiditesphyllites and •m•'•undivided . Structural symbols

traceof contact /'-'-

66o20 ' inferredcontact .•' ' Nynykin F. ' / l'l' l' coveredcontact /--- "' ' Tanatap R.' senseofandslip dip-slipon strike-slip faults •

normalfault,lowerbarbs plate on / 71 ø30' thrustfaults,upperbarbs plateon • unclassifiedfaults/ •..' inferredfault / Cretaceousgranites coveredfault . .." stdkeanddip of bedding •-• % % % % 0 2 4 6 8 km

the High-Gradestdkeanddip and of TanatapS1 folialJon unitsin

Figure3. Geologicmap of the ChegitunRiver valley.

3.1. High-Grade Unit. EtelkhvyleutSeries and the LavrentiyaSeries; (2) thinly banded schistsand quartzofeldspathicparagneisses that includelenses of The high-grademetamorphic rocks in the ChegitunRiver val- ultramafic rocks, marbles, amphibolites,and rare quartzite ley are very similarto rocksin the coreand southemflank of the (metachert?)[BSGFP, 1997], which togethermay representac- Koolendome that are exposedin the KoolenLake region(Figure crefionaryprism material [•engi•r and Natal'in, 1996]; and (3) 2) [Shuldiner and Nedomolkin, 1976; Nedomolkin, 1969, 1977; marble and calc-silicate rocks that resemble the less metamor- Gelman, 1973; Natal'in, 1979; BSGFP, 1997]. The Koolen phosedlower to middle Paleozoicshallow marine sequenceof metamorphiccomplex was divided into two major units by the Novosibirskcarbonate platform as exposedon the Chukotka Shuldinerand Nedomolkin[1976]. A structurallylower succes- Peninsula[•'engOr and Natal'in, 1996]. Mid-Cretaceousgranites sionof mostlyorthogneiss is calledthe EtelkhvyleutSeries, and a intrude the Koolen metamorphic complex [Shuldiner and ,structurallyhigher, dominantlyparagneiss, schist, and marble- Nedomolkin, 1976; Nedomolkin, 1977; BSGFP, 1997] and the bearingsuccession is calledthe LavrentiyaSeries. In the Koolen Tanatapunit in the ChegitunRiver area. Lake region,three types of rock assemblageshave been observed Along the Chegitunvalley, in the southwestempart of the map in ascendingorder: (1) granitic orthogneissesof Cretaceous area (Figure 3), biotite-bearinggranitic augen gneisses intefiay- [BSGFP, 1997] and Devonianages (see below) within both the ered with 3- to 30-m-thickhorizons of coarse-grainedwhite mar- 982 NATAL'INET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA

'-oCLO

x /

'- 6o-• o•-

I i i I • •

o o 0 0 • • o 0• NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 983 ble constitutethe lowestexposed structural unit (map unit "aog" LavrentiyaSeries. In general,we agreewith this correlation,but in Figures3 and4). We carriedout ion microprobe(super high the lithologyand successionof the rocksin the Chegitunvalley resolutionion microprobe-reversegeometry (SHRIMP-RG)) U- are differentin detail from thoseof the LavrentiyaSeries in the Pb dating of zircon grains from this augen gneiss (sample Koolen Lake region where thick quartzo-feldspaticparagneisses 95JT4a)and obtained 2ø6pb/238U ages between -650 and~540 Ma (referto Table 1 for analyticalmethods and to Figures5a and 0.14

ß rims 800 Ma , 5b for dataplots). This broadrange of agesfrom a singlesample 0.13 - ß cores is probablydue to a combinationof two effects:(1) the incorpo- = concordia rationof somewhatolder zirconsby a latestProterozoic granitic 0.12 magma (inheritance)and (2) loss of radiogenic lead during youngermetamorphic events. These two phenomenawill con- 0.11 spireto spreadthe agesparallel to concordia(Figure 5a), making it impossibleto assigna precisecrystallization age to the pro- 010 6OO tolith of the augen gneiss. In any case,these new data demon- stratethe occurrenceof Late Proterozoicgranitic magmatism in 0 09 I Chukotka Peninsula. A few lensesof plagioclase-bearingamphibolites, probably 0.08 500• metagabbro,occur within the marbles. We correlatethese rocks with the Etelkhvyleut Series of the Koolen Lake region 0.07 [Shuldiner and Nedomolkin, 1976; Nedomolkin, 1969, 1977; 400 ChegitunRiver Orthogneiss a BSGFP, 1997], but the augen gneisses and interlayered 0 o6 i i i i i 95JT4ai I i i i i I i i i i I i i i i I i i i i (I i )i i i 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 metasedimentaryrocks of the Chegitunvalley containless peg- 207pb/235U matiteor otherevidence for partial meltingthan equivalentunits of the Koolen Lake region. The augengneiss unit is overlainby 0.20 ' ' ' " I ' " ' " I ' " " " I ' " " ' I " ' ' ' I " " " ' 95JT4a a poorly exposedsection of biotite gneissescontaining subordi- ChegitunRiver Ortho.c•neiss N=17 nate horizonsof augen gneisses(map unit "bgn"). Overlying rocks, from the structuralbase upward, include intercalatedbi- otite gneisses,calc-silicate rocks, and marblesthat gradeupward >,0.15 to pure coarse-grainedmarbles (map unit "mb"). Muscovite- bearinggamet schists and quartzitesform a lenticularbody in the marblesin the upper part of the structuralsection (map unit "qv"). Biotiteand garnet-bearingschists (map unit "bsc")overlie 0.10 the marbleunit. Peak metamorphismreached upper amphibolite faciesin the deepestpart of the structuralsection exposed in the Chegitunvalley and in the core of the Koolen dome.This meta- morphicevent has been datedas occurringbetween 104 and 94 0.05 Ma in the KoolenLake region[B SGFP, 1997]. In the Chegitunarea, Nedomolkin[1969, 1977] assignedall rocks that lie structurallyabove the augen gneissunit to the 0.00 3OO 400 500 600 700 800 900 2ø6pb/238UAge (Ma) Figure5. U-Pb concordiadiagrams from theKoolen orthogneiss. 0.066 (a) Concordiadiagram of ion microprobe(super high resolution ' I ' I ' I ' I ' I ' I ' ion microprobe-reversegeometry (SHRIMP-RG)) data from {•) F45•4K369.6 +_1.2 Ma (MSWD =3.3) M sample 95JT4a of Late Proterozoic augen gneiss from the 0.062 Chegitunvalley. Eachdata point represents a single ~20 Hm spot O M18/94K 374.8 +_0.5Ma (MSWD = 1.2) on a zirconcrystal. The discordanceand wide spreadof agesis probablydue to the combinedeffects of inheritanceof older zir-

con and lead loss during Cretaceoushigh-grade metamorphism. 0.058 (b) Cumulativeprobability diagram of 2ø{ipb/238Uages of zir- 36O cons from sample 95JT4a from the Chegitun valley. This his-

togramis calculatedby takingthe individualdata pointsand as- 340 suminga Gaussiandistribution of unit area with width propor- 0.054 tional to the error. (c) U-Pb concordiadiagrams from the Koolen orthogneiss,F45/94K and M18/94K. MSWD standsfor

meanstandard weighted deviation, a measureof the goodnessof 0 o5o fit of the chord. Both setsof analysesare discordant,but upper interceptsof 376 and 375 Ma agree within analytical error. KoolenLake Orthogneisses (C) Thesedata show that the orthogneissof the EtelkhvyleutSeries is neitherPrecambrian nor Cretaceousas hasbeen previously spec- 0.046 I , I , I , I , I , I , I 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 ulated. These Devonian ages are within the stratigraphicage limits of the Tanatapmetatuff. See text andTable 3 for details. 207pb/ 235U 984 NATAL'IN ET AL.' PALEOZOIC ROCKS OF NORTHERN CHUKOTKA NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 985

separatethe augenorthogneisses from the overlyingmarbles [see shalewith rare late Middle Devonian(Givetian) coralsand bra- BSGFP, 1997]. chiopods;and (3) the UtaveemFormation, consisting of Lower A thick structural section of calc-silicate rocks and marble is Carboniferousgray to blackfine-grained fossiliferous limestone, exposedin the upper reachesof the Tanatap River (Figure 3). calcareoussandstone, and phyllite (Figures3 and 4). Drabkin Individual horizons of foliated marble are internally homoge- [1970a] and Markov et al. [1980] infer that Upper Devonian neousand oftenmore then 10 m thick, suggestingthat their pro- rocksare presentin the IkychurenFormation, although no Late toliths were shallow water carbonaterocks. Lower-grademeta- Devonian fossils have been found so far in northern Chukotka. morphismmay indicatethat the calc-silicaterocks and marbles In the ChegitunRiver region (Figure3) the Tanatapunit is belongto the upperpart of the structuralsection (Figure 4). This boundedby the Nynykinfault to the southeastand by what we conclusionis supportedby the correlationwith the KoolenLake call the Ratkhatfault in thenorthwest, which are bothinterpreted region where marbles occur at the highest structural level asright-lateral strike-slip faults (Figure 3). Rocksof theTanatap [BSGFP, 1997]. unit possessan intricatestructure produced by polyphase,inho- The4øAr/39Ar analyses indicate that the high-grade rocks near mogenousdeformation and greenschist facies metamorphism that the ChegitunRiver cooled from 500øC to 350øC between 108 resultedin formation of muscovite,chlorite, albite, and actinolite. and 104 Ma [Natal'in et al., 1997]. These cooling ages are Numerousisoclinal folds, shear zones, and faults make it impos- slightlyolder than those(90-92 Ma) of the southemflank of the sibleto reconstructdetails of the initial stratigraphicsuccession. Koolen dome. An unconformitywas proposedto exist at the base of the Senseof sheardetermined from stretchinglineations is domi- Utaveem Formation [Drabkin, 1970a;Nedomolkin, 1969, 1977]. nanfly top-to-south,similar to the kinematicspreviously de- In our field area the Lower Carboniferousrocks indeed occupy scribed for the southern flank of the dome [BSGFP, 1997]. the higheststructural level (Figure3); however,the lower contact Taking into considerationthe structuralrelationships and litho- of the UtaveemFormation is not exposed.Like the underlying logic similarity, the calc-silicateand carbonateunits of the Devonian rocks, the limestones of the Utaveem Formation dis- LavrentiyaSeries are tentativelycorrelated with the Ordovician play spacedanastomosing cleavage, and the calcareousphyllites to Lower Devonian carbonaterocks of the Chegitun unit (see have a penetrativefoliation. This indicatesthat the Utaveem below), whichare exposedto the northwest.Both of theseunits Formationwas deformedand metamorphosedtogether with un- are interpretedto representpart of the Novosibirskcarbonate derlyingDevonian rocks. If the unconformitydoes exist, it does platformof •'engOrand Natal'in[1996] (Figure1). not likely reflecta penetrativedeformational event. Despitepoor outcrops,the map pattern,together with struc- In general,graphitic phyllites and thinly beddedmetacarbon- tural data (B.A.Natal'in et al., manuscriptin preparation,1999) atesare the predominantlithologies of the TanatapFormation. In and the fact that internalcontacts of the High-Gradeunit are at an zonesof lower strain,sedimentary structures are locally well pre- angleto the contactwith the Tanatapunit, indicatesthat a fault served.Turbidites consisting of BoumaC, D, andE intervalsare (here namedthe Nynykin fault) separatesthe two tectonicunits commonly observed. In a few localities, flame structuresand (Figure3). The NynykJnfault cutsthe mid-Cretaceousfoliation matrix-supporteddebris flow depositsseveral meters thick and in the high-graderocks and is conjugatewith north-weststriking rich in limestonepebbles were observed. extensional faults and dikes. The dikes have been dated as Late Metalimestoneof the Tanatap and Ikychuren Formationsis Cretaceous [Nedomolkin, 1969]. On the other hand, the exten- mainly thinly beddedand ofteninterbedded with clearexamples sionalstructures have the sameorientation as Tertiary extensional of carbonate turbidites. Medium- and thick-bedded metalime- structuresin the Hope Basin (Figure 1) [Grantz et al., 1990]. stoneis rare. The metalimestonecontains little terrigenousmate- Thus we infer the Late Cretaceousto Tertiary age of the Nynykin rial and very few macrofossils. Both formationscontain uni- fault. Kinematiccriteria indicate that the fault hasa right-lateral formly disseminatedsulfides that, togetherwith a high contentof senseof displacement. carbonin metapeliticrocks, are interpretedas evidenceof an anoxic depositionalenvironment. These sedimentologicalfea- turessuggest that they were depositedin a deep-waterrestricted 3.2. Tanatap Unit marine basin. The Tanatapunit (Figure 3) consistsof polydeformedgreen- In the TanatapFormation we found 2- to 35-m-thick horizons schistfacies rocks exposedalong the ChegitunRiver from the of reddishand greenandesitc tuff containingabundant volcanic southwesterncomer of the map area to the Chukchi Sea coast. clasts(see below). Gray and dark gray metalimestoneof the Similar rocks are also exposedalong the northernflank of the Lower CarboniferousUtaveem Formation is rich in macrofossils, Koolen dome (Figure 2) [Nedomolkin, 1%9, 1977; Natal'in, mainly shallowmarine corals [Drabkin, 1970a]. We recovered 1979]. Accordingto previousaccounts and unpublishedgeologic Cavusgnathusunicornis, Cavusgnathus, and Bispathodusstabilis maps, three stratigraphicunits metamorphosedto lower green- or Bispathodusutahensis conodonts from this unit, thuscorrobo- schist fades have been recognized within the Tanatap unit rating a mid-Famennianto Meramecianage (latest Devonianto (Figure 4) [Drabkin, 1970a; Nedomolkin, 1969, 1977; early late Mississippian)(A. Harris, written communication, Oradovskayaand Obut, 1977; Krasny and Putintsev,1984]: (1) 1997). the Tanatap Formation,consisting of graphiticphyllite, calcare- Middle to Upper (?) Devonianand Lower Carboniferousrocks ous slate,limestone turbidites, and recrystallizedlimestone dated are also exposedalong the Chukchi seacoast to the southeastof as early Middle Devonian(Eifelian) on the basisof extremely the Chegitun River region (Figure 2). In that area, Devonian rare brachiopods and coral remnants; (2) the Ikychuren rocksthat are similar to thoseof the ChegitunRiver valley are Formation,consisting of thinly beddedlimestone and calcareous overlain by Lower Carboniferous(Visean) limestonesthat are 986 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA

rich in shallowmarine fossils. The limestonesgrade up into thin tion and reflectsthe deepeningof the basinover time. To the intercalatedblack phyllites and slates, sericitic and chlorific northwestof the ChegitunRiver, limestonebreccia with a phyllite, and quartzo-feldspathicsandstones [Drabkin, 1970a]. dolomitematrix is reported as the most abundant rock type within Cleavageand schistositydeveloped at greenschistfacies meta- the lower part of the formation. The rocks contain Middle morphic conditionsare reported from these rocks [Drabkin, Ordoviciancorals (Late Llanvimian to Llandeilo),graptolites, 1970a; Natal'in, 1979]. Thus we assign these rocks to the brachiopods,gastropods, and trilobites which are very similar to Tanatapunit. thefaunal assemblages of the Siberian platform and Kolyma re- We infer that greenschistfades metasedimentaryand gion[ Oradovskayaand Obut,1977]. A Middle Ordoviciancon- metavolcanicrocks that are exposedto the northwestand north of odontassemblage was recoveredfrom this unit, including the Chegitununit, in the SeshanCape region (Figure 2), are Protopanderodussp. indet., Panderodus sp. indet., equivalentto thoseof the Tanatapunit. Oradovskayaand Obut Acanthocodina(?) sp. indet.,Acanthocordylodus sp. indet., [1977] distinguishedthere the SeshanFormation, which contains Drepanoistodussp., and possibly Erismodus sp. (A. Harris,writ- phyllites,rhyolites, and probablybasaltic tuffs, and overlying, ten communication,1997). This Middle Ordovicianconodont as- dark gray to black shale, phyllites, and limestonesof the semblageis characteristicof warm shallow water environments Ikoluvrun Formation. No fossils have been found in either of of the Siberianplatform. theseformations. Their Cambrian-EarlyOrdovician age was in- The Upper OrdovicianChegitun Formation (225-350 m) ferredfrom their structuralposition beneath fossil-bearing Upper (Figure4) conformablyoverlies the Isseten Formation and is rely Ordovicianrocks at SeshanCape [ Oradovskayaand Obut, 1977]. resentedby medium-to coarse-graineddark gray bioclastic and However, the lithologicalsimilarity of the Ikoluvrun Formation reefallimestone and rare dolomite. Dark, organic-rich silicified to the Tanatap Formationas well as strongdeformation at the concretionsare commonly present in reefallimestones. Bedding contactwith the overlying Ordovicianrocks [see Oradovskaya surfaceswithin this unit are wavy. Reefallimestones form lenses and Obut, 1977, Figure 7] allows the inferencethat there is a severalmeters thick. The rocksare generallyrich in recrystal- fault contact between the Ikoluvrun Formation and the lizedpentamerids, corals, and gastropods of Ashgilian (Upper Ordovicianrocks. Thus an age determinationbased on structural Ordovician)age which are similar to thoseof theKolyma region positionmay not be valid. The lithologicand structuralfeatures, [Oradovskayaand Obut, 1977]. Oradovskayaand Obut [1977] greenschistfacies metamorphism and fault relationshipswith the infera disconformityat the base of theChegitun Formation. Ordovicianrocks of the Chegitununit, as well as the presenceof The ChegitunFormation is, in turn,conformably overlain by volcanic horizons in the Seshan Formation, indicate that the the SilurianPutukuney Formation (60-70 m) (Figure4). The Seshanand Ikoluvrun Formationsmay also correlatewith the contactbetween formation is very sharp. The Putukuney TanatapFormation. Formationconsists of blackcalcareous shale and dark, thinly Someresearchers believe that the southernpart of the Tanatap bedded,flaggy argillaceouslimestone which containsnumerous unit containsRiphean rocks. This belief is basedon findingsof graptolitesindicating Late Llandoverian,Wenlockian, to Early oncolites[lvanov and Kryukov,1973] or acritarchs[Gorodinsky, Ludlovianage. Light grayto yellowishthick-bedded limestone 1980; Krasny and Putintsev, 1984], the location of which have anddolomite (300-315 m) of theLate Silurian (Upper Ludlow) not beenspecified. Within the areaof supposedlyRiphean rocks OrlanFormation overlies the PutukuneyFormation. Lacking we observedrocks that are lithologicallyidentical to rocksof the fossilevidence, Oradovskaya and Obut [1977] determinedthe TanampFormation and in placescontaining remnants of crinoids. ageof theOffan Formation on the basis of its structuralposition Thus we dispute the existenceof the Riphean rocks in the betweenthe PutukuneyFormation and fossil-bearingLower Tanatapunit. Devonianrocks. Silurian conodonts recovered from our samples The 4øAr/39Ardating of metamorphicwhite mica from the of the Orlan Formationinclude Ozarkodinus sp. indet and Tanatapunit has shownthat the age of deformationand meta- Walliserodussp. indet (A. Harris,written communication, 1997). morphismin the ChegitunRiver valley is Early Cretaceousat 124 At thetop of theChegitun unit are Lower Devonian (200-250 m) Ma [Natal'in et al., 1997]. No traces of earlier deformational dolomitic limestones containing stromatoporoidcorals eventshave been detected by structuralanalysis. [Oradovskayaand Obut, 1977; Krasny and Putintsev, 1984]. In contrastto the Tanatapunit, the internalstructure of the 3.3. Chegitun Unit Chegitununit is very simple. There are northwestand north- southstriking open folds, wavelength of whichis of hundredsof The Chegitununit is representedby unstrained,fossil-rich, metersto kilometerscale. These folds are oblique to thebound- shallow marine Middle Ordovician to Lower Devonian lime- ariesbetween the lithotectonic units and to theprincipal north- stone,dolomite, and minor shale (Figures 3 and4). The baseof easterntrends in the Tanampunits. Northweststriking normal theChegitun unit is not exposed inthe Chegitun River valley, but faultsand northeast striking steeply dipping strike-slip faults are Oradovskayaand Obut [1977] assume that the unit rests upon the superimposedonthe above mentioned folds. Despite the simple inferred Upper Cambrianto Lower OrdovicianSeshan and map-and outcrop-scale structure and the lack of anypenetrative IkoluvrunFormations. These relationships have been disputed in metamorphicfabric, the conodontsrecovered from rocksof the section3.2, in which the Seshanand Ikoluvrun Formationsare Chegitununit have conodont alteration indexes (CAIs) ranging correlatedwith the Middle Devonian Tanatap Formation. from5 to 6 (A. Harris,written communication, 1997). These At the baseof the Chegitununit lies the430 - to 540-m-thick valuescorrespond topaleotemperatures of300ø-435øC [Rejebian IssetenFormation (Figure 4), whichconsists of dark gray to gray et al., 1987].We attribute this high CAIs to local heating of the pelitomorphicand bioclasticlimestone (locally argillaceous), Chegitun unit during emplacement ofthe mid-Cretaceous granitic dolomite,marl, and rare sedimentary breccia with clasts of lime- intrusionsthat rim the Koolen and Neshkan metamorphic domes stone,and dolomite. Marl is presentin theupper part of thesec- (Figure2). NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 987

4. Devonian Magmatism on the Northern Table 2. Major and Trace ElementGeochemistry for Chukotka Peninsula Devonian IgneousRocks

The correlationof Paleozoic magmatic events between the Koolen Lake Orthogneiss Tanatap Metatuff Russian Far East and North America and the search for a tectonic frameworkthat can help us to understandthe dispositionand F45-94K M 18-94K 95JT-3 9 95JT-48 origin of Paleozoicmagmatic belts remain an outstandingprob- lem. This is especiallyimportant in Chukotkawhere pre-Triassic SiO2t 60.37 64.02 57.27 58.91 rocksare generallyburied by the Mesozoicsedimentary rocks of AI20 3 17.39 16.27 20.03 20.02 the Chukotka fold belt and by the mid-CretaceousOkhotsk- TiO 2 1.04 0.89 1.05 1.06 Chukotkavolcanic belt. Devonianvolcanic and plutonic rocks of FeO* 5.16 4.81 7.12 4.99 the Russian Far East have been correlated with similar rocks MnO 0.11 0.11 0.06 0.05 known from California to northernAlaska [e.g., Tilman, 1973; CaO 4.28 3.36 2.34 2.60 Rubin et al., 1990]. We providehere new agesfor Devonianor- thogneissesfrom the coreof the Koolendome, and we reportan- MgO 1.96 1.57 3.53 3.84 desitictuffs from the Tanatapunit of the ChegitunRiver valley. K20 3.57 4.45 4.42 3.48 TheseDevonian magmatic rocks are the first recognizedfrom the Na20 4.43 3.88 0.44 1.08 ChukotkaPeninsula and providean importantlink betweenthe P205 0.39 0.33 0.15 0.15 mid-Paleozoictectonic history of Chukotkaand northernAlaska. Sum 98.70 99.69 96.41 96.18 Ba* 1271 1540 460 500 4.1. The Etelkhvyleut Series Orthogneiss Rb 199.30 140.40 177.90 137.20 The Etelkhvyleutorthogneiss of the Koolen dome (Figure 2) Th 58.20 18.38 11.19 12.18 consistsof metamorphosedand deformed biotite granodiorite and Nb 8.52 16.97 17.37 18.28 granite, in places exhibiting migmatization [Shuldiner and Ta 2.56 2.69 1.17 1.25 Nedomolkin,1976]. Protolithages for this unit were suspectedto Sr 360 536 172 479 be Precambrian,but BSGFP [1997] consideredthe possibility Zr 460 339 162 188 that some of these orthogneisseswere highly deformed and Hf 9.79 7.40 4.58 5.20 metamorphosedPaleozoic or Mesozoic plutonic rocks. U-Pb Tb 0.88 1.10 1.08 1.06 geochronologyrevealed that mostof the orthogneissesare highly Y 18.62 32.23 35.25 33.83 deformedCretaceous granites related to the high-grademetamor- La 82.89 36.04 49.59 phic event [BSGFP, 1997]. However, samplesF45-94K and M18-94K yieldedDevonian ages. These two samplesare from a Ce 155.35 71.08 93.36 metaluminousbiotite granitic orthogneiss(unit Egog) [see Pr 15.77 8.04 10.44 BSGFP, 1997, Figure 4]. SiO2 is 60 wt % and 64 wt % in the Nd 57.81 31.60 40.63 two rocks,respectively, so the originalcomposition was likely a Sm 10.44 7.00 8.54 granodiorite(Table 2). Accessoryphases include sphene, zircon, Eu 2.41 1.56 1.34 andapatite. Gd 7.70 6.01 6.28

We datedsix fractionsof zirconfrom eachof two Etelkhvyleut Tb 1.10 1.08 1.06 Seriesorthogneisses along the northand southernsides of Koolen Dy 6.10 6.54 6.48 Lake (Table3 and Figure2). Becauseof the fairy limited spread Ho 1.17 1.31 1.29 in U-Pb datesand the completeagreement within analyticalerror of the207pb*/zø6pb* dates from analyzed fractions of each Er 3.03 3.71 3.71 Tm 0.45 0.51 0.53 samplewe int•er•ret the age of these samples tobe the weighted meanof the20/pb*/206pb* dates.The sixzircon fractions ana- Yb 2.67 3.20 3.40 lyzed from sample F45-94K give an age of 369.6 +_ 1.2 Ma Lu 0.43 0.49 0.53 (MSWD = 3.3) whereasthe 5 zirconfractions from sampleM18- Ni 68 63 94K give an age of 374.8 + 0.5 Ma (MSWD = 1.2). Thesedata Cr 118 126 establish a Devonian age for the Etelkhvyleut Series or- Sc 15 13 thogneissesin this area. The initial Sr for thesetwo samplesis V 184 184

0.706, andthe end is +0.2 (F45) and -0.3 (M18, Table 4). Ga 27 31

Cu 1 487 4.2. The Tanatap SeriesMetatuff Zn 33 37

At leastfour distincthorizons of alternatingred and greenan- Pb 1.6 1.3

desitetuffs 2- 35 m thickare presentin theTanatap Formation in U 3.5 3.5

the ChegitunRiver valley; whereasa thicker (340 m) body of Cs 6.9 6.3 volcanicrocks has been reported off SeshanCape [ Oradovskhya and Obut, 1977]. In places,these horizons contain altered clasts ?Major element data were collected by XRF. * All Fe was calculated as FeO. with relict porphyritictexture. The tuffs have been metamor- Traceelement data were collected by inductivelycoupled phosedto greenschistfacies, and the identifiablemineralogy is plasma mass spectrometryexcept for metatuff rare earth element mainly white mica, calcite/dolomite,and chlorite. The layered (REE) data,which were collectedby isotopedilution at the University fabric is similar to unmetamorphosedvolcanoclastic rocks, al- of Wisconsinusing a mixed REE spike. 988 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA

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thoughsubsequent tectonic deformation may havecontributed to the formation of the foliation. Major elementgeochemistry on two of the volcanicsamples indicatesthat the tuffs are calc-alkalineandesites (SiO2 = 59-61 wt % anhydrous)significantly enriched in potassium(Table 1). Becauseof greenschistfades metamorphism,it is not knownto whatdegree this potassium enrichment is the resultof posterup- tive metasomaticprocesses. Some notable aspects of the geo- chemistryinclude Cr concentrationsof 118 and 126 ppmand Ni concentrationsgreater than 60 ppm. It is unclearwhat phases are responsiblefor theseconcentrations, which are elevatedwith re- spectto normalandesites [e.g., Ewart, 1982]. It is possible,however, that the whole rock SiO2 concentrationis elevatedbecause of the inadvertent inclusion of felsic clasts in the whole rock analysis. If this is true, the tuffs may be of a morebasaltic-an- desitecomposition, for whichsuch high Cr andNi concentrations maybe morecharacteristic. These rocks are enriched in thelight

,• o rare earthelements (LREE) (Figure6), with La/Lu ratiosof 74- o o 94 anda negativeEu anomaly. Althoughwe reportSr isotopicdata for thetuffs (Table 2), the mobilityof Rb duringgreenschist facies metamorphism may have affected the measured Sr ratios. Nd and the REE are less mobileduring metamorphism, so we believethe Nd isotopicdata is muchmore robustß Although the tuffs are foliated,there are no structural discontinuities or shear zones between the tuffs and the surroundingrocks of the Tanatap Formationfrom which rare findingsof MiddleDevonian fossils have been reported [Nedomolkin,1969, 1977; Oradovskaya and Obut, 1977]. Soif weassume anage for the tuffs similar to thatfor the orthogneisses(~375 Ma), theinitial r:Nd for bothmetatuff samples isabout-7. This is significantlydifferent from the values mea- es 0 suredfrom the Koolen Lake orthogneiss and would seem to pre- cludea commonsource regionß However, the tuffs could have interactedwithsignificant amounts ofolder crustal material dur- ingtheir ascent and eruption. Alternatively, theisotopic analysis o of thetuff could have been contaminated by the inclusion of crustalmaterial of a differentage or origin. These two possibili- tiescould have the effect ofdecreasing theend. The question of

E [] 95JT-39 Iß 100 ß 95JT-48

o o IO 10

I SmEu Gd TbDy Ho Er TmYb Lu i Figure6. R=eeach element pattern from T•atap me•mff. Lightrare •h elementenriched pa•ern with a negativeEu anom•yis commonlyobserv• in continen•arc vol•nic rocks •d mayreflect de•vation from • alreadyfractionat• sour• or a mixtureof a pfim• ma•a rutholder, evolved cms• p•tid melts. No•dization factors are from An•rs and Ebih•a [ •98•]. 990 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA whetheror not the tuff and the orthogneissare comagmaticis NeocomianSouth Anyui suture[Parfenov and Natal'in, 1977, thusnot resolvableusing the isotopicdata. We believethat the 1985;Natal'in, 1981, 1984]. spatialrelationships and the similar compositionspoint to a In northernAlaska all Proterozoicto EarlyCretaceous tectonic commonorigin for the orthogneissand tuff. unitslocated to thenorth of theAngayucham terrane (Figure 7) Trace elementgeochemistry can be usefulfor identifyingthe are assignedto the ArcticAlaska terrane [Moore et al., 1994; tectonicsetting of volcanicrocks, though large-ion lithophile el- Plafkerand Berg, 1994]. The Seward Peninsula is generally cor- ementssuch as Ba andRb are to be avoidedowing to bothpost- relatedwith the southern part of theArctic Alaska terrane [ Moore depositionalalteration and the likelihood that the primary magma et al., 1994;Plafker and Berg, 1994]. The ArcticAlaska terrane interactedwith continentalcrust during its ascent. This crustal is unifiedby itsLate Devonian through Jurassic passive margin interactionmay significantlyalter the magmaticvalues and thus stratigraphy(the Ellesmerian sequence). This sequence is pre- providemisleading information. High field strengthelements datedby Middle(in placesupper Lower Devonian) to Upper andREE are lessaffected by alterationand metamorphism, and a Devonianclastic rocks, shale, and mafic igneous rocks which comparisonof the La/Th and La/Nb ratioswith otherorogenic formedin extensionalenvironment and accumulated atop of andesitecompositions indicates that the Tanataptuffs overlap stronglydeformed basement [Anderson et al., 1994;Moore et al., with high-K andesitesfrom otherareas [Gill, 1981]. 1994;1997b]. Grantzet al. [1991]and Moore et al. [1994] Texturalevidence, major and trace element chemistry, and Nd suggestthat pre-Middle-Devoniantectonic history of northern isotopicdata all suggestthat the Tanatap Series tuffs are differen- Alaskawas a resultof accretionof continentalfragments of tiatedcalc-alkaline andesites possibly derived from a continental Siberianaffinity to the continentalmargin of NorthAmerica. marginmagmatic arc. The degreeof metamorphismand alter- Followingthis interpretation, the mostsignificant pre-Middle- ationprecludes a more rigorous assessment of the tectonic setting Devoniantectonic units are the following (Figure 7): (1) a frag- basedon the tuffs alone. If we takeinto consideration the deep- mentof theupper Proterozoic to lower Paleozoic passive conti- water,anoxic, fine-grained nature and the lack of a terrigenous nentalmargin rocks of theNorth American craton exposed in the clasticcomponent in the hostrocks, it seemspossible that these Sadlerochitand Shublik mountains, northeastern Brooks Range; arcvolcanic rocks were deposited in a backarc basin. (2) lowerPaleozoic rocks of oceanicand island arc affinity that aretectonically mixed with upperProterozoic to lowerPaleozoic passivecontinental margin deposits in the Franklinianfold belt 5. Paleozoic and Mesozoic Tectonic Units and (Figure7) andwhich are exposed tothe south of this fragment of Correlation of the Structures of Chukotka and theNorth Amehcan margin in theRomanzoff Mountains and in Northern Alaska theDoonerak window of theBrooks Range and underlie part of the ColvilleBasin; (3) a complexlyimbricated and metamor- Therocks exposed in theChegitun River valley are an impor- phosedassemblage of upper Proterozoic to Mississippian rocks tant elementin Paleozoictectonic reconstructions, because in mostlyof continentalaffinity that make up the Hammond subter- many other areasof Chukotkaand northemAlaska, Mesozoic raneof theBrooks Range; and (4) Proterozoicto Mississippian magmatism,metamorphism, and deformation obscure earlier tec- (protolithage) schist, phyllite, metasandstone, and metacarbonate tonicfeatures. In addition,important tectonic elements may lie rocks with subordinatemetaquartzite, metabasite, and felsic hiddenwithin the broad continental shelves of theregion. In this metavolcanicrocks of the Coldfoot subterrane.Units 3 and 4 and sectionwe describe the important tectonic units of thisregion, at- at leastpart of unit2 havebeen considered asbeing of Siberian temptto correlatethese features between Russia and Alaska, and origin[Grantz et al., 1991]. To the southof the Coldfootsubter- proposea Paleozoic tectonic evolution for the region. ranethere are quartzosephyllite, schist, slate, quartzite, minor The major Paleozoictectonic units of the Russiansector of the metabasite,and chert of theSlate Creek subterrane which yield Chukotka-ArcticAlaska microplate are the following (Figure 7): sparseEarly and Middle Devonianand Mississippianfossils. (1) Precambriancrust which is inferred to underlie the Paleozoic The Hammond, Coldfoot, and Slate Creek subterraneswere af- andMesozoic sedimentary cover (the Bennett-Barrovia block), fectedby intense deformation and blueschist to greenschist facies (2) Ordovicianto Devonianshelf carbonates and shales(the metarnorphismduring the Brookian orogeny in LateJurassic to Novosibirskcarbonate platform) [•engSr and Natal'in,1996], Early Cretaceoustime. and(3) a middlePaleozoic arc-trench system stretching along the The Late Jurassicto Early CretaceousKobuk suture, reacti- southemboundary of the Novosibirskcarbonate platform vatedby mid-Cretaceousnormal faulting [Grantz et al., 1991], [Natal'inet al., 1997]. Triassic-EarlyJurassic passive continental separatesArctic Alaskafrom the Angayuchamterrane and, to- margindeposits which now make up the Chukotkafold belt getherwith the SouthAnyui suture,defines the southernbound- overliethese units and are boundedin the southby the aryof theChukotka-Arctic Alaska block [Parfenov and Natal'in,

Figure7. Tectonicmap of the Siberian-Alaskan sectorof Arctic. Abbreviations areas follows: AM, Alyarmaut Uplift;AN, Angayuchamterrane; BD, BairdMountains; BF, Barrowfault zone; BI, BelkovIsland; BN, Belkovskyi-NerpalakhTrough; DN, Doonerakwindow; HM, Hammondsubterrane; HRT, Herald thrust; HT, HenriettaIsland; KC, Kiber Cape; KG, Kigluaik dome; KI, Kotel'nyiIsland; KM, Kolyuchin-Mechigmenzone; KO, Koolendome; KU, KuyulUplift; NT, Nutesyn;PR, Primorsk Basin; RC, RauchuaBasin; RZ, Romanzof Mountains;SA, SouthAnyui suture zone; SE, SenyavinUplift; SS, Shublikand Sadlerochit mountains; TP, Topolevka;YR, YorkMountains; YU, YarakvaamUplift. NATAL'IN ET AL.' PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 991

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3NOZ .L•111:!A3.Ld•V 992 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA

1977, 1985; Natal'in, 1984, Rowley and Lottes, 1988]. In the the basementfor lessdeformed and lessmetamorphosed Silurian sections5.1-5.4 we discussmainly the Russiansector of the re- to Lower Devonianand younger rocks [Kos'ko et al., 1993]. The gion. Details of the geologicalstructure of the Alaskan sector fold belt consistsof felsic to intermediate metavolcanic rocks, (the Arctic Alaska terrane)are given by Moore et al. [1994, slate,schist, quartzite, and conglomerate,intruded by mafic sills 1997b] and Grantz et al. [1990]. and dikesas well as by small bodiesof graniteyielding a U-Pb age of 699 Ma [Cecile et al., 1991b]. The discoveryof Late

5.1. Bennett-Barrovia Block Proterozoicprotolith ages in the Etelkhvyleutorthogneiss in the ChegitunRiver valley indicatesthat a Late Proterozoicgranitic Two distincttectonic hypotheses have been proposedfor the magmaticbelt extendsto Chukotka. evolution of this portion of the Arctic. In one, an intact In the eastem Chukchi Sea (Barrovia in Figure 7), another Precambrian basement block forms the basement of the fragmentof Precambriancrust coveredby undeformedUpper Novosibirsk Islands, Wrangel Island, the Chukchi and East Proterozoicto Cambrian [Grantz et al., 1990] or lower Paleozoic Siberian Seas, and the Hammond subterrane in northern Alaska [Sherwood, 1994] carbonaterocks can be inferred from seismic [e.g., Zonenshainet al., 1990;•engOr and Natal'in, 1996]. In the reflectionprofiling. Dips of foresetbeds in overlyingPaleozoic other, this sameregion is an amalgamationof terranesthat ac- clastic rocks indicate that the source area for them was to the cretedto Asia or North America duringPaleozoic-Mesozoic time northwest(in the internalpart of the Bennett-Barroviablock). and then rifted away during a subsequentevent [Fujita and In the Chukotkasector the northernboundary of the Bennett- Newberry,1982; Fujita and Cook, 1990]. We believethe "intact Barroviablock is maskedby deepCretaceous to Cenozoicbasins Precambrianbasement block" hypothesisis valid for the follow- which stretchalong the shelfedge. On the basisof geophysical ing reasons: (1) The lower Paleozoicstratigraphy and lithology data, Vinogradovet al. [1974] definedthe Henriettafold belt to indicatesa tectonicallystable shallow marine environmentthat the northeastof BennettIsland (Figures 7 and9). The only expo- persistedthroughout the area,from the NovosibirskIslands to the sure of the belt in HenriettaIsland is representedby sandstone, Hammond subterranein Alaska (see below), thus the different tuffaceoussandstone, shale, conglomerate with numerouslavas, exposuresare not likely to representdisparate terranes, but rather sill, and dikes of basalt,andesite, and diabase. Conglomerate reflect depositionon a unified basementblock; (2) faunal evi- containspebbles of granite,greenschist, and gneiss.A Paleozoic dence placesthese exposuresnear each other and adjacentto age is evidencedfrom recrystallized,presumably Carboniferous, Siberia during Paleozoictime [Oradovskayaand Obut, 1977; foraminiferaand K-Ar ages of volcanicrocks of 450-310 Ma Krasny and Putintsev,1984; Moore et al, 1994]; and (3) low- [ Vinogradovet al., 1975]. The Ordovicianage of volcanicrocks amplitude,broad E-W or NW strikingmagnetic anomalies over hasbeen confirmed by recent4øAr/39Ar dating (A. Kaplan,oral most of the central and southern Chukchi and East Siberian Seas communication, 1998). The rocks of the Henrietta Island are [Vinogradovet al., 1974; see also Fujita and Cook, 1990] in&- markedly different from the Paleozoic cover of the Bennett- catean absenceof sutures,which would be requiredin the terrane Barroviablock and have previouslybeen interpretedas an inde- hypothesis. pendentterrane [Fujita and Cook, 1990]. We infer here that The Bennett-Barrovia block underlies the shelf of the East Ordovician basalt to andesite volcanic rocks of the Henrietta belt Siberian and Chukchi Seas (Figure 7). The boundariesof the may indicate a magmatic arc along the northern side of the block and its main componentscan be inferred from (1) expo- Bennett-Barrovia block sures of Paleozoic rocks, which reveal no evidence of extensive 5.1.2. Canadian Arctic Islands. The lower Paleozoic rocks pre-Late Jurassicto Early Cretaceousdeformation and thus of HenriettaIsland may representa link to Arctic Canada,where formed in rather stable tectonic environment [Kos'ko et al., the exotic Pearyaterrane (PE in Figure 9) providesa recordof a 1993]; (2) dispositionand geometryof the wide Mesozoicexten- Middle Ordovician collision of a Precambrian block with an sionalbasins (Figure 7); and (3) similarityof geophysicalcharac- Early to Middle Ordovicianisland arc [Trettin, 1991]. After their teristicsof the underlyingbasement [Vinogradov et al., 1974, collisiona new Middle Ordovicianto Silurianarc of southernpo- Grambergand Pogrebitzkiy,1984]. The boundariesand descrip- larity [Klaper, 1992; BjOrnerudand Bradley, 1994] was con- tion of thisblock will be separatelydescribed for Russia,Alaska, structedabove the suture within the Pearya terrane [Trettin, and the Canadian Arctic islands. 1991]. Anotherarc (CM in Figure 9) was activein the area be- 5.1.1. Russia. The westemboundary of the Bennett-Barrovia tweenthe Pearyaterrane and the passivecontinental margin of blockcoincides with theLaptev rift zone,which is a continuation North Americaduring the Ordovicianand Silurian. This arc also of the Arctic mid-oceanridge (Figure9). Zonenshainet al. had southernpolarity, as is indicatedby the vergenceof the [1990]included the NorthTaimyr Peninsula and the Sevemaya ClementsMarkham fold belt [Trettin, 1991;Klaper, 1992]. Zemlya Archipelago within their Arctida microcontinent,but Trettin [1991] correlatedpre-Middle-Ordovician rocks of the bothof theseregions are characterizedby a thick,strongly de- Pearyaterrane with the Caledonidesof the northernAtlantic. He formedsuccession of Ripheanto LowerOrdovician flysch and inferredthat the collisionof Pearyawith the ClementsMarkham rare red bedsdeposited on a passivecontinental margin. Such arc happenedbefore the Late Silurianin a sinistraltranspres- rocksare completelyabsent from the regionsthat we includein sional environment. In tum, the Clements Markham arc collided the Bennett-Barroviablock. In the BennettIsland region, in the with the passivecontinental margin of Arctic Canadain the Late East SiberianSea (Figure7), consolidatedPrecambrian crust un- Silurian to Early Devonian. Structuralstudies revealed no evi- derlyingPaleozoic and Mesozoicsedimentary cover can be in- denceof sinistralregime of these collisions[Klaper, 1992]. ferredfrom gravityand aeromagnetic anomalies [Tkachenko and Consequently,we infer that the Pearya terraneand Clements Egiazarov, 1970; Vinogradovet al., 1974; Gramberg and Markham arc were at least in the positionbetween Bennett- Pogrebitzkiy,1984]. We infer thatthis basement is exposedin Barrovia and Arctic Canada and thus relevant to the Bennett- Wrangel Island where a Late Precambrianfold belt constitutes Barrovia block evolution. NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 993

5.1.3. Alaska. Proterozoic rocks have been found in two quartzosesandstones in the Ordovician rocks on Bennett Island structurallyimbricated units in the westernpart of the Hammond may be interpretedas evidenceof facieschanges from an areaof subterranein northernAlaska [Moore et al., 1994]. The lower carbonatedeposition in Kotel'nyiIsland toward an upliftedarea unit consists of metacarbonate rocks with subordinate siliciclastic in the north[Kos'ko, 1994]. Startingin the Early Silurian,two and metavolcanicrocks. The upperunit is made up of metasedi- depositionalzones existed in Kotel'nyi Island [Kos'ko, 1977; mentaryrocks and metabasites.The rocksof the upperunit are Kos'ko et al., 1990]. In the northwesternzone, shallow marine intrudedby graniticrocks yielding a U-Pb age of 750 Ma. The and lagoonal carbonateaccumulated while in the southwestern amphiboliticfacies metamorphism that affectedthese rocks oc- zonefine-grained limestone was deposited with deep-water shale, curred at about 655-594 Ma [Moore et al., 1994]. These rocks siltstone,and siliceousshale. Middle Devonianlimestone, debris are very different from a more or lesscontinuous succession of flow deposits,and conglomerate conformably or unconformably Proterozoic to lower Paleozoic carbonate rocks of the North overlie the older rocks of both zones and mark another tectonic Americancontinental margin that are exposedin the Sadlerochit cycle [Kos'koet al., 1990]. Mountainsin the northeasternBrooks Range. Silurian to Lower Devonian shallow marine carbonates and Granitoidsyielding a 705 Ma U-Pb age also have been re- clasticrocks on Wrangel Island which formed in stableshelf, ported as intruding metasedimentaryand metavolcanicrocks in shallowmarine environments[Kos'ko et al., 1993] can also be the Coldfoot terrane [Moore et al., 1994]. In addition,Nd and Sr assignedto the Novosibirskplatform. The unconformablylying isotopic data from younger magmatic rocks, as well as on the Proterozoicrocks the Lowerto UpperDevonian unit (the Proterozoic inherited zircons found in the Devonian or- oldestfauna is Givetian)consists of conglomeratesand sand- thogneisses,suggest that thesemagmatic rocks were in part de- stones.Silurian to LowerDevonian sandstones consist predomi- rived from Proterozoic crust [Dillon et al., 1987; Nelson et al., nantlyof quartzand feldsparand are compositionallysimilar to 1993; Toro, 1998]. In Alaska, theserocks form the mostproba- the Ordoviciansandstones in the BennettIsland. Conglomerate ble basementfor the Paleozoic Baird Group carbonaterocks and sandstoneof the Lower to Upper Devonianunit, in contrast, [Nelson et al., 1993; Moore et al., 1994]. The timing of Late are lithic and includefragments of volcanic,volcanoclastic, and Proterozoicgranitoid magmatism in the BrooksRange is similar graniticrocks derived from the underlyingbasement [Kos'ko et to a magmaticevent recordedin rocksfound on Wrangel Island al., 1993]. [Kos'ko et al., 1993] and Chukotka. Late Proterozoic or- 5.2.2. Alaska. A characteristic succession of lower to middle thogneissesalso have been reportedfrom the SewardPeninsula Paleozoic shallow marine carbonate rocks is found in the York [Till and Durnoulin,1994; Arnatoand Wright, 1998]. Mountainsof the SewardPeninsula [Till and Durnoulin,1994], in In Alaska, Cambrian to Silurian rocks of island arc and the BairdMountains of thewestern Brooks Range, and in several oceanicorigin are exposedbetween the Hammondsubterrane and localitiesin the centraland eastemBrooks Range within the the fragmentof the North America continentalmargin in the Hammond subterrane[Dumoulin and Harris, 1994; Moore et al., northeasternBrooks Range (the Franklinianbelt in Figure 7) 1994] (Figures7 and 8). In the York Mountainsthe succession [Grantz et al., 1991; Moore et at., 1994]. These rocks mark an startswith Arenigiandeepening-upward subtidal to supratidal ocean which was closed at the end of the Silurian to Early limestoneswhich are overlainby shallowing-upwardLower Devonian,as it is evidencedby a sharpunconformity at the base Llanvimian shalesand thin-beddedlimestones [Durnoulin and of the Middle Devonian rocks [Anderson et al., 1994]. The su- Harris, 1994]. Rocks of this age are missingon Chukotka ture between the Bennett-Barrovia block and North America is Peninsulabut exist on the Kotel'nyiIsland [Kos'ko,1977]. The inferredto be locatedat the northernboundary of the islandarc Upper Llanvimian to Lower Llandeilian black limestoneof the and oceanicrocks [Grantz et al., 1991]. This conclusionis sup- York Mountains correlates well with the Isseten Formation of portedby Cambrianfossils of Siberiantype found in the island Chukotka.The overlyingUpper Ordovician bioclastic fossilifer- arc rocks [Dutro et al., 1984] and by their position near the ous limestonesare similar to the ChegitunFormation of the boundarywith the Hammondsubterrane. ChukotkaPeninsula Lower Upper Ordovicianrocks are not documentedor missingin both regions. Silurian(Ludlovian)

5.2. Novosibirsk Carbonate Platform dolostoneof the York Mountainsmay be correlatedwith the dolomites of the Odan Formation of Chukotka. Till and The earlyto middlePaleozoic Novosibirsk carbonate platform Durnoulin[ 1994] reportedthat in additionto carbonaterocks the wasdefined by •'engdirand Natat'in [1996] as coverdeposits of Silurian section of the York Mountains includes mudstone and the Bennett-Barroviablock. The platformincludes mainly shelf that Middle Silurian fossils have been documented from this sec- andlagoonal carbonate rocks that are exposedin bothRussia and tion. Theserocks may be equivalentto thePutukuney Formation Alaska (Figures7 and 8). shalesof the ChegitunRiver area 5.2.1. Russia. Ordovician to Lower Devonian shallow marine In the Baird Mountains there are two fault-bounded carbonate and lagoonal carbonaterocks and shales have been described sequences. The west central sequenceconsists of Lower from Kotel'nyi Island within the NovosibirskArchipelago Ordovicianto Devonianshallow to deeperwater platformal car- [Tkachenkoand Egiazarov, 1970; Volnov, 1975; Kos'ko, 1977; bonaterocks, and its stratigraphyis very similarto that of the Kos'ko et al., 1990]. On Kotel'nyi Island the oldestrocks are York Mountainson the SewardPeninsula [Durnoulin and Harris, Lower to Middle Ordovician,but fartherto the north,on Bennett 1994] and the Chegitununit on the ChukotkaPeninsula (Figure Island, shale and siltstone are interbedded with rare limestone 8). The Lower to Middle Ordovicianand Upper Silurianrocks containingMiddle Cambriantrilobites. These rocks are uncon- are representedby limestoneand dolomite,and the Upper formably(?) overlainby Lower to Middle Ordovicianshale, silt- Ordovicianconsists of limestone.The northeasternsequence of stone,and quartzose sandstone [Tkachenko and Egiazarov, 1970; the Baird Mountains consists of Lower to Middle Ordovician Kos'koet al., 1990]. Thin bedsof limestoneand the presenceof shaleand chert which gradeup into inner shelf metalimestone 994 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA

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Bennett-Barrovia passive margin block •-•:• craton • ofbeltthe Franklinian Novosibirsk :2.:2.•blocksbasement with Precambrian platform •• suture Lower Paleozoic oceanic crust []]]]• block'smargin shelf carbonate in Arctic

Figure9. The Bennett-Barroviablock and tectonic units used in the paleotectonicreconstructions. Abbreviations are as follows:AR, Alpha Ridge;CB, Canadabasin; CC, ChukchiCap; CM, ClementsMarkham belt; FL, Franz JosefLand; HT, Henriettafold belt; LM, Lomonosovridge; LR, Laptevrift; MA, Arctic mid-oceanridge; MR, MendeleevRidge; NF, Nixon Fork terrane;NP, North Pole, NW, NorthwindRidge; OH, Okhotskmassif; OK, Omulevkaterrane; OL, Oloy zone;OM, Omolonmassif; PE, Pearyaterrane; RB, Rubyterrane; SA, SouthAnyui suture;SZ, SevemayaZemlya; TA, Tuva-Mongolarc; TM, Taimyr;VR, Verkhoyanskpassive continental margin.

[Dumoulinand Harris, 1994]. This changeof faciesis different northern Asia and North America for the Late Ordovician bra- thanthat of the previouslydescribed regions. The upperUpper chiopods.Dumoulin and Harris [1994, p.56] notedthat "both Ordovicianto Lower Devoniancarbonate rocks are represented 'Siberian' and 'North American' faunal affinities occur at different by shallowmarine facies. The Middle to UpperCambrian metal- times and in different fossil groups within all of the imestonehave been described from this region. Dumoulinand (meta)carbonatesuccessions" in northem Alaska making it likely Harris [1994] report that the Cambrian and the Ordovician to that the Novosibirskplatform formed in a positionbetween Lower Devonian rocks of the eastern Baird Mountains are similar Siberia and North America to the rocksof the SnowdenMountain in the easternpart of the In the northeasterncorner of the Chukchi Sea (Barrovia in Hammond terrane. Figure7), seismicreflection profiling revealed two [Grantzet al., LowerPaleozoic fauna from rocks of theNovosibirsk platform 1990]or three[Sherwood, 1994] subhorizontalpre-Mississippian reveal a similarity with the Siberian fauna [Tkachenkoand seismostratigraphicunits. Thereis a controversyin the agede- Egiazarov, 1970; Oradovskayaand Obut, 1977; Krasny and terminationof theseunits. Grantz et al. [1990] infer, on the ba- Putintsev,1984; Moore et al., 1994; Till and Dumoulin, 1994]. sis of seismicvelocities and the characterof the reflectors,that Oradovskayaand Obut [1997] emphasizeda similarity of the lower unit is madeup of Precambrianor Cambriancarbonate Chukotkanand Alaskan(Kuskokwim and Yukon-Porcupinere- rocks and that the upper unit consistsof Ordovician to Silurian gions(NF in Figure9) and SewardPeninsula) Middle Ordovician clastic rocks. In contrast,Sherwood [1994] infers a Proterozoic brachiopods,Late Ordoviciancorrals, and Siluriangraptolites. (?) to Middle Devonianage for the carbonateunit and a Middle Rozman [1977] determined the Kolyma-Alaska xegion to LateDevonian age for theclastic rocks in whichhe recognized (Novosibirsk Islands, Chukotka and Seward Peninsula, two unitsseparated by a low-anglethrust. Both authorsagree Kuskokwim,and Yukon-Porcupineregions) as a uniqueone in that the NW strikingBarrow fault (BF in Figure7) separates 996 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA theseunits from stronglydeformed and slightlymetamorphosed In Chukotka the knowledgeof the distributionof Devonian Ordovician to Silurian argillite, phyllite, slate, chert, and plutonic and volcanic rocks is limited by the scarcity of graywackein the basementof the Colville basin. Regardlessof exposuresof Paleozoicrocks and the lack of high-qualityU-Pb theage determination, both interpretations corroborate the idea of geochronology.We have describedthe Devonianplutons in the the Novosibirskcarbonate platform as the sedimentarycover of core of the Koolen dome and the Devoniantuffs in the Tanatap the Bennett-Barrovia block. unit. In the Cape Kiber area (KC in Figure7), biotite granites We consider the regions listed above in Russia, Seward intrudeclastic rocks that are similar to the rocksbearing Middle Peninsula,and northernAlaska as part of the continuousBennett- Devonian fossils in the central and easternparts of the uplift. Barrovia block coveredby the Novosibirskcarbonate platform Clastsof the same granitesappear in the Lower Carboniferous (Figure9). The originalsize of thisblock could have b•n much conglomerateexposed nearby [Cecile et al., 1991a]. A Rb-Sr larger,as otherexposures of similarrocks may alsobe a part of whole rock age of the graniteshas beendetermined as 439 Ma thesetectonic units. For example,in Alaska the Nixon Fork ter- (Early Silurian)with an initial Sr ratio of 0.7042 [Tibilov et al., rane (Figure9) consistsof the Ordovicianto Lower Devonian 1986]. Subsequentstudies of the Cape Kiber granite[Cecile et shallow marine carbonate, calcareous turbidite, and shale. al., 1991a]questioned the Silurianage and recently a U-Pb zircon Fossils from these rocks indicate a link to Siberia [Dumoulin et agehas b•n determinedat 360 Ma [L. Lane, personalcommuni- al., 1998], and thus this terrane could also be a part of the cation,1998]. Compositionof the graniteincluding REE datain- Novosibirskplatform. dicatesthat the granite is subduction-related.Small fault- boundedblocks of tuffsand lavas ranging in compositionfrom 5.3. Middle PaleozoicArc-Trench System basaltto dacite containinglenses of limestonewith Lower Carboniferous(Visean) corals are exposed atthe northern margin 5.3.1. Magmatic arc. Devonianand lower Mississippian of theSouth Anyui suture (TP in Figure7) [Natal'in,1984]. volcanicand plutonicrocks are found throughoutthe North We agree with the many authors[e.g., Dusel-Baconand AmericanCordillera, from Californiato northernAlaska [Rubin Aleinikoff,1985; Rubin et al., 1990;Plafker and Berg, 1994] who et al., 1990]. These magmaticrocks and related basinal strata proposedthat a Devonian,or in placesDevonian to Early havebeen interpreted as remnantsof an arc systemdeveloped Carboniferous,active continentalmargin existed along the near the edgeof the Proterozoiccraton on continentalcrust, tran- Proterozoiccraton of NorthAmerica and Alaska, and we suggest sitionalcrust, or oceaniccrust [Dusel-Bacon and Aleinikoff that this arc systemcan be extendedinto northeasternRussia. 1985;Dillon et al., 1987;Rubin et al., 1990;Plafker and Berg, Somegeologists believe that in Alaskathe magmatic arc was ac- 1994]. Devonianplutonic rocks from both northernAlaska and tive only duringthe Early to Middle Devonianand that it was Russiaare briefly describedhere. succeededin the Middleto LateDevonian by extensionthat led In northernand central Alaska, Devonian granites are widely to theformation of thepassive continental margin [Moore et al., scattered through the Hammond and Coldfoot subterranes 1994, 1997b]. However,the agesof the subduction-and exten- (recently,parts of the Hammondsubterrane where granites are sion-relatedigneous rocks in the Hammondand Coldfoot subter- exposedwere assignedto the Coldfootsubterrane [Moore et al., ranes overlap. If euhedral zircons of 370-360 Ma did have 1997a]) and on the SewardPeninsula [Till and Dumoulin, 1994; graniticsources [Moore et al., 1997a],gramtic intrusions and Miller, 1994] (Figure7). A few graniticbodies of uncertainori- eruptionof enrichedMORB-type basaltsin the sametectonic gin are knownfrom the North Slopesubterrane [Moore et al., zone(rift/passive continental margin tectonic setting according to 1994].'Felsic to intermediatevolcanic rocks in theeastern part of Moore et al. [1997a]) seemsunlikely. Thuswe infer that exten- the Hammondterrane (Nutirwik Cr•k unit) yield the Early to sion in northern Alaska could be a result of the back arc basin Middle Devonian U-Pb zircon ages [Moore et al., 1997b]. formationrelated to the Devonianarc. In Chukotkathe mag- MiddleFrasnian conodonts have been collected from the upper maticarc stretchesalong the southernmargin of the Bennett- partof their section.Petrologic features of theserocks indicate a Barroviablock. In Alaska the Coldfootsubterrane, in whichthe subduction-relatedorigin of therocks. Their age partly overlaps majorityof theDevonian granites are exposed, is to thesouth of the age of the extension-relatedigneous rocks of the Hammond the Hammond subterrane,and one possiblerestoration of subterrane. Early to Middle Devonian granitoids(now or- Mesozoicthrusting places it in the sameposition [Moore et al., thogneiss)vary in compositionfrom dioriteto granite[Moore et 1997b]. al., 1994]. They yield U-Pb agesof 398-383 Ma and reveal a 5.3.2. Back arc basin. In the Novosibirsk Islands the rocks subduction-relatedsignature although mixing of some other of the Novosibirskcarbonate platform are boundedin the south magmatype with partialmelts from preexistingsubduction-re- by the Belkovskyi-NerpalakhTrough (BN in Figure7). The latedrocks is possible[Moore et al., 1997b]. Youngergranitic troughis filled with thick (8900 m) UpperDevonian to Lower rocksin the Coldfootterrane are Late Devonian(discordant U-Pb Carboniferouslimestone, shale, sandstone, and conglomerate that zirconage is 366 Ma) [Mooreet al., 1994]. Theirmajor element in the Devonianpart of the sequencecontain dikes and sillsof and isotopiccomposition indicates that thesegranites incorpo- low-potassiumgabbro [Tkachenkoand Egiazarov, 1970; ratedlarge components of continentalcrust [Nelson et al., 1993]. Vinogradovet al., 1974; Volnov, 1975]. The mafic intrusionsare Euhedraldetrital zircons from quartzitesin the Coldfootsubter- here interpretedas evidencefor an extensionalorigin of the fane varyingin age from 370 to 360 Ma may havevarious ig- Belkovskyi-NerpalakhTrough. The subsidenceof the southern neoussources, and granite plutons are considered among the pos- marginof the Novosibirskplatform began in the Silurianas evi- sible ones [Moore et al., 1997a]. Orthogneisson the Seward dencedfrom facieschanges of shallowmarine Lower Silurian Peninsulayields a 381 Ma U-Pb zirconage [Till and Dumoul#•, carbonaterocks in the centraland northernpart of Kotel'nyi 19941. Island to the deep-watershales and limestonesin the southwest- NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 997

em part of the island [Kos'ko, 1977]. Underlying Ordovician certain. Ultramafic rocks also exist in the lower part of the rocks do not reveal suchchanges. The Middle Devonian rocks LavrentiyaSeries in the KoolenLake region. Fromthese data we include debris flow depositsand limestoneconglomerate and infer thatmiddle Paleozoic accretionary prism material is present breccia with local unconformities at the base of these rocks in the metamorphiccomplexes of the Chukotka Peninsula [Kos'koet al., 1990] that may also indicate tectonicactivity re- [•eng6r and Natal'in, 1996]. Lenticular bodiesof Devonian or lated to the formationof the Belkovskyi-NerpalakhTrough. In oldermafic andultramafic rocks of oceanicorigin (metavolcan:,c spite of the large distancefrom the NovosibirskIslands, the rocks of NMORB and EMORB type) and a melangein the Chegitunand,the Tanatap units on the ChukotkaPeninsula reveal Coldfoot subterranein northern Alaska [Moore et al., 1997b] similarages, composition of rocks,and timing of events. suggestthat a part or all of the Coldfoot subterranemay have In the Alyarmautuplift (AM in Figure7), metaclasticrocks of oncecomposed a middlePaleozoic accretionary prism which was amphiboliteto greenschistfacies are presumablyof Devonian in front of the Devonian arc. age, because they are conformably overlain by Lower Carboniferousmetalimestones and black phyllites [Tilman, 5.4. Chukotka Fold Belt 1973]. These rocks contain actinolite schists [Markov et al., 1980] formedfrom volcanic(?)rocks. In the Kuyul Uplift (KU in The Chukotkafold belt (Figures1 and7) is a zoneof Triassic Figure 7), Lower to Middle Devonianrocks up to 1300 m thick to Lower Jurassicpassive continental margin deposits (Middle are representedby siltstone and shale with minor limestone. Triassicrocks are not confirmedpaleontologically) deformed be- Upper Devonian rocks (t200 m) consistof arkoseand quartz forethe mid-Cretaceous during closure of the SouthAnyui suture [Natal'in, 1981, 1984; Parfenov and Natal'in, 1977, 1985; sandstone[Rogozov and Vasilyeva,1968]. Unconformablyover- Zonenshain et al., 1990]. Thick (5.5 km) Triassic to Lower lying theserocks are Lower Carboniferous(2500 m) arkosesand- Jurassicturbidites unconformably overlie the Devonianto Middle stone, black shale with abundant sulfides, siliceous and calcare- Carboniferousrocks exposed in the Kuyuland Alyarmaut uplifts ousshale, dolomite, limestone, and conglomerate[Markov et al., 1980]. These rocks are twice as thick as coeval rocks on [Sadovsky,1965; Tilman, 1973; Drabkin, 1970a]. Upper WrangelIsland [Kos'ko et al., 1993]. Exposuresof Devonianto CarboniferousandPermian rocks are almost completely absent. LowerCarboniferous rocks in theAlyarmaut and Kuyul upliftsin Numeroussills and dikes of gabbroand diabase and sparse lava central and western Chukotka [Drabkin, 1970a; Krasny and flowsoccur among Lower to Middle(?)Triassic rocks. These Putintsev, 1984] are in an intermediateposition between the magmaticrocks indicate a rifting event that can also be recog- NovosibirskIsland and ChukotkaPeninsula and arguefor former nizedalong the northern boundary of theSiberian craton, within continuitybetween the DevonianBelkovskyi-Nerpalakh Trough the Siberiancraton itself, in FranzJosef Land, and in the andTanatap basins. SeremayaZemlya Archipelago in the Arctic Ocean [Gramberg In Alaskathe tectonicequivalent of thesebasins may be an ex- andPogrebitzkiy, 1984; Milanovsky, 1987; Zonenshain et al., 1990]. In the east the Chukotkafold belt terminatesat the tensionalbasin filled with the Givetian(?)(late Middle Devonian) to Early Frasnian(early Upper Devonian)Beaucoup Formation Kolyuchin-Mechigmenzoneof theChukotka Peninsula (Figures 2 and7). In the westit narrowstoward the NovosibirskIslands and associatedmetasedimentary and metavolcanicrocks within the Hammondsubterrane (Deitrich River phyllite in the eastem (Figure7) whereTriassic argillite, sandstone, limestone, and part of the subterrane)[Moore et al., 1994, 1997b]. Theserocks basalt(in LowerTriassic part of thesection) and Lower Jurassic are intrudedby sills, dikes,and stocksof metadiabaseand gab- mudstone,siltstone, and sandstonehave been reported bro. Compositionand elementalabundance pattems of igneous [Tkachenkoand Egiazarov; 1970, Kos'ko et al., 1990]. The rocksare similarto mafic magmatismin an extensionalenviron- Nutesynzone (Figure 7), consistingof the MiddleJurassic to ment throughlithosphere with a previoushistory of subduction Neocomianbasalt to daciteof islandarc affinity, tuff, volcan- magmatism[Moore et al., 1997b]. Detailedstudies in the eastem oclasticsandstone, conglomerate, andshale, rests unconformably part of the Hammondsubterrane have shownthat the extension- atopof theTriassic rocks at thesouthern margin of thecentral relatedrocks occur as a seriesof thrust-boundedsheets emplaced part of the Chukotkabelt but does not continueto the west from the south during the Brookian orogeny [Moore et al., [Natal'in,1981, 1984]. Similarvolcanic rocks exist in the Late 1997a]. Jurassicto EarlyCretaceous Rauchua Basin (Figure 7) andin smallbasins along the southern margin of theeastern part of the 5.3.3. Fore are region. It is noteasy to discerna forearcre- Chukotkabelt. All thesedata allow the reconstructionof a Late gionfor the reconstructedmiddle Paleozoic magmatic arc in Jurassic-EarlyCretaceous magmatic arc along the southern mar- Chukotkaand Alaska. All structuresto the southof the arc were gin of the easternpart of the Chukotkabelt [Natal'in, 1981, stronglyreworked by Triassicrifting in Chukotkaand by 19841. Jurassic-EarlyCretaceous thrusting and mid-Cretaceousexten- To ourknowledge, an equivalentof thisearly Mesozoic tur- sionaldeformation both in Chukotkaand in Alaska. Amphibolite bidite(plus gabbro) succession of theChukotka belt does not ex- andultramafic rocks have been reported in metamorphicrocks of ist in northernAlaska. There the Carboniferousto Lower theSenyavin Uplift of theChukotka Peninsula (SE in Figure7) CretaceousEllesmerian sequence is continuous,and the thickness [Akinin,1995; Calvert and Gans, 1996]. The Rb-Sr isochron age ofTriassic rocks does not exceed 200 m [Mooreet al., 1994]. of theserocks is 365 Ma [Akinin,1995], but it is unclearwhether In Chukotka,Triassic rifting led to the formation of a passive thisage reflects the protolith age or a subsequentmetamorphic continental margin along the southemside of the Bennett- event. Devonianfossils have been found in theweakly meta- Barroviablock and the opening of theSouth Anyui ocean. From morphosedlimestone and shale of theSenyavin uplift [Egiazarov thelocation ofthe middle Paleozoic arc-related rocks at the very and Dundo,1985], but their relationshipwith the ultramafic boundaryof theSouth-Anyui suture zone (TP in Figure7) and rocksoccurring within the Senyavin metamorphic complex isun- thelocation where there is anapparent absence of arc/forearctec- 998 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA tonic zones in the Novosibirsk Islands sector (a back arc exten- 6. Summary of the Paleozoic Tectonic History of sionalbasin exists there), we suggestthat this rifting wasoblique Northeastern Russia and Northern Alaska to the Paleozoic structures of the Bennett-Barrovia block and the middle Paleozoicarc-trench system and thus fragmentsof the The Paleozoic tectonic evolution of the Late Proterozoic arc-trenchsystem could have beenrifted away. In the present- Bennett-Barroviablock in somerespects resembles the tectonic day structurethey may be presentedon the oppositeside of the evolutionof the Arctida microcontinentoutlined by Zonenshain SouthAnyui suture.Indeed, to the southof the suturewithin the et al. [1990]. They proposedthat in the early Paleozoic,Arctida Oloy zone (Figure 7), numerousfault-bounded fragments of was separatedfrom the Siberiancraton. This is supportedby the Devonian to Middle Carboniferousarc-related magmatic rocks, existenceof a pre-Ordovicianunconformity and Ordovicianfel- Paleozoicophiolite, and flyschoccur among Mesozoic volcanic sic volcanic rocks in the Severnaya Zemlya Archipelago rocks [Natal'in, 1984; Zonenshainet al., 1990]. Their Paleozoic [Milanovsky,1987]. Alternatively,•engOr and Natal'in [1996] fauna assemblagesare similar to Siberianand Chukotkanfauna infer a Vendian(latest Precambrian) age of separationby linking assemblages[Krasny and Putintsev,1984] which fits a hypothe- it to the main rifting event betweenthe Russianand Angara cra- sis of a local origin of these blocks. To the north of the tons. Omulevka terrane, in the basement of the Primorsk Cenozoic Figure 10 showsthe Late Ordovicianposition of the Bennett- basin(PR in Figure7), continentalblocks, island arc complexes, Barrovia block with respect to the Angara craton, the North andophiolites are inferred from geophysical data [Litinsky and American craton, and the tectonic units of northeasternRussia. Raevsky,1977; Spector et al., 1981]. The Chukotkapassive con- The shapeof the Bennett-Barroviablock is correctedby restora- tinentalmarlgin collided with Siberiaat the end of Neocomian tion of Mesozoicleft-lateral strike-slipfaults in the Chukotkabelt time along the SouthAnyui suture[Parfenov and Natal'in, 1977; [Sutyginand Tibilov, 1969; Parfenovand Natal'in, 1985] and by Natal'in, 1981, 1984; Seslavinsky,1980]. unbendingthe Bering Straitorocline [Patton and Tailleur, 1977].

NF2 -- NF1 Beringor=Strait \ Ordov,c,n. . \ r• • 460Ma

'"•.;•'--•• • --{ .i:.•.:.i...

. •:?craton'•:•'•'"':"'["'•• • • / margin•,•pE /

Figure10. LateOrdovician paleotectonic reconstruction ofthe Bennett-Barrovia block. The positions ofmajor continentsare after Lawyer et al. [1990].The reconstruction of the Tuva-Mongol arc, Okhotsk, Omolon, and Omulevkaterrane isafter •engdr and Natal'in [1996]. Abbreviations arethe same as those in Figure 9. Positions of theChukchi Cap and Northwind Ridge are after Grantz et al. [1998].BB - Bennett-Barrovia;CP- Chukotka Peninsula;WI -WrangelIsland. NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 999

Thereare no paleomagneticdata to constrainthe position of the Late Ordovician magmatic arc Bennett-Barrovia block Our inference about its location is based (Pearya, shallow marine KlementsMarkhan, passive on the followingreasons. rbonate Henrietta) margin 1 ,. Facies indicate that lower Paleozoic rocks of the block ac- cumulatedin a warm dimate, and fossilassemblages are similar with the Omulevka terrane, Siberia, and North America 2. The Omulevkaterrane is characterizedby Ordovicianto Lower Carboniferous reefal limestone, shale, sandstone,and mi- b Silurian EarlyDevonian nor gypsum[Zonenshain et al., 1990]. Thus it is reasonableto subsidence (Kotel'nyiI ) Ellesmerian placethe Bennett-Barroviablock eitherin front of the Omulevka magmatice•en .sion_ al I shallowmarine orog/eny terraneor asa continuationalong strike. We havechosen the lat- ter becauseit makesa simplersubsequent tectonic evolution (see are?••.as• •[ •..,•.ar•onat•,• below). '3.. The Omulevka terrane was rifted away from the Verkhoyanskpassive margin in the Late Devonian[Parfenov and Natal'in, 1985; Zonenshain et al., 1990]. If we infer that the C magmaticarc LateDevonian (Koolen extensionalbasin Bennett-Barrovia and Omulevka blocks were close to each other Cape•Kiber) (Tanatap, Franldinian during the early Paleozoic and the beginning of the middle forearc I Belkovsky-Naralpakh) orogen region Paleozoic,the line of the rifting of the Omulevkaterrane would be on the continuation of the Devonian extensional basin of the Bennett-Barrovia block. •[•1• •'- BB•NAC 1 4'. The lower Paleozoic carbonate rocks of the Bennett- Barrovia block are similar to platformal carbonatesuccessions within the Central Compositeterrane, including the Ruby and d collisionuplift locallyLate Paleozoic Nixon Fork terranes [Plafker and Berg, 1994; Moore et al., ...... • •C,.,ukotka• shallow marine carbonate molasse'.• • magm. / clast•crocks and 1997a], which containsboth Siberianand North American fossils continentai• ["J arc / andlimestones clasticrocks (Kotel'nyi Wrangel) (Arctic Canada) [Dumoulin et al., 1998]. Therefore we keep the westernpart (Ordovician paleocoordi nates) of the Bennett-Barroviablock closer to North America. I The Ruby and Nixon Fork terranescould have two possible positions:(1) to thenorth of the Hammondand Coldfoot subterranes(NF1 andRB 1 in Figure10), from whichthese subterranes rifting EarlyTriassic M.Paleozoic (diabasein sandstone, could be rifted away during the opening of the Angayucham forearc Chukotka) shale, sandstone, (Oloy)M. PaleozoicJ basalt. shale ocean [Patton et al., 1994, and referencestherein] or (2) on the I magm.arc continuation of the Hammond and Coldfoot terranes (NF2 and • (O.Ioy)

RB2slippedinalong Figure the 10); dextralfrom Proto-Tintina which these fault terranes [Grantz couldet al.,be 1991].strike- In the Late Silurian to Early Devonianthe Bennett-Barrovia block collidedwith North America (Figures11a and lib). This Figure 11. Tectonic evolution of Chukotkansector of the Bennett-Barroviablock: (a) The Bennett-Barroviablock is sepa- collisionled to the Ellesmerianorogeny of northernAlaska and ratedby an oceanfrom North America. The earlyPaleozoic sub- the CanadianArctic Islands. In Arctic Canada,episodes of corn- duction beneath the Bennett-Barroviablock created magmatic pressionaldeformation related to thecollision have been recorded arcs (CanadianArctic Islands,Henrietta belt, northernAlaska). until the Early Carboniferous[Trettin, 1991]. In northernAlaska, (b) The collision of the Bennett-Barroviablock and North compressionaldeformation terminated by the Middle Devonian, America led to the Elesmerianorogeny in Alaska and the and then it was followed by extensionaldeformation and the ac- CanadianArctic Islands.The arc-trenchsystem originated along cumulationof clastic rocks [Andersonet al., 1994; Moore at al., the southern(present-day coordinates) margin of the Bennett- 1994, 1997b]. However, in places(western Baird Mountains), Barroviablock. (c) The Devonianto Early Carboniferousevolu- shelf carbonatedeposits persist from Silurian until Middle tion of the arc-trenchsystem. (d) The collisionof an uknown continentalobject and the Bennett-Barroviablock. (e) The Early Devonian [Dumoulin and Harris, 1994]. Thus a part of the Triassicrifting and the originationof the South-Anyuiocean. Bennett-Barroviablock in northernAlaska was not affectedby Sliversof the middle Paleozoicarc-trench system and the conti- the extensionaldeformation. Tracesof the Ellesmerianorogeny nentalobject were rifted away. See text for discussion.B B, may be detectedon Wrangel Island by an unconformityat the Bennett-Barrovia; NAC, North American craton. baseof the Middle to Upper Devonianunit and the drasticchange in the compositionof clasts in sandstonesand conglomerates [Kos'koet al., 1993]. On Kotel'nyiIsland, stable shelf sedimentation was succeeded,in placesunconformably, by coarse-grained terpretedas evidence that the effect of theEllesmerian orogeny in clastic rocks in the Middle Devonian [Kos'ko et al., 1990]. No the Chukotkanpart of the Bennett-Barroviablock diminished unconformitieshave been reported for Devonian sectionsof away from the zone of Ellesmeriancollision. Chukotka [Rogozov and Vasilyeva, 1968; Drabkin, 1970a; After collision of the Bennett-Barrovia block with North Markov et al., 1980;Nedomolkin, 1977]. All of this may be in- America,the middlePaleozoic arc-trench system and the exten- 1000 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA

Figure12. EarlyTriassic paleotectonic reconstruction of the Bennett-Barroviablock. The positionsof majorconti- nentsafter Lawver et al. [1990]. The reconstructionof the Tuva-Mongolarc, Okhotsk,Omolon, and Omulevkater- rane is after •'engi•r and Natal'in [1996]. Triassic rift zones in the Western Siberian basin (WB) are after Zonenshainet al. [1990]. HM, Hammondand Coldfoot subterranes;KI, Kotel'nyi Island; RF, rifted fragmentsof the fore-arcregion of the middlePaleozoic arc-trench system of the Bennett-Barroviablock and unknown continental blockcollided with it; SP, SewardPeninsula. See Figures 9 and 10 for otherabbreviations.

sionalbasin were established along the oppositeside the Bennett- However, the suddenappearance of the enigmatic Nuka Land Barroviablock (Figure1 l c). The overlapof agesof the mag- [Moore et al., 1994] during the Early/Middle Carboniferousin matic arc and the extensional basin allow the inference that the the very southernpart of the margindoes not fit a model of the extensional basin formed as a back arc basin. stabledevelopment of the passivemargin facing the Angayucham In Alaska,arc magmatismstarted in the Early Devonian. In ocean. In contrast,in the Chukotkasegment of the southernmar- Chukotka,dated granitic plutons are Late Devonianand Early gin of the Bennett-Barroviablock, an unknowncontinental object Carboniferous,but the extensional(back arc) basin is Eifelian collided with the magmatic arc in the Middle to Late (MiddleDevonian) or older. In theKuyul Uplift the stratigraphic Carboniferous(Figure 1ld). succession of the extensional basin includes the Lower Devonian The following lines of evidencecan be usedto show that the clasticrocks. The Belkovskyi-NerpalakhTrough (Figure 1lc) evolutionof the activecontinental margin at the southernbound- formedin the early Late Devonian(Frasnian). Viseanandesilies ary of the Bennett-Barrovia block in Chukotka ended in the nearthe contact of theChukotka fold beltand SouthAnyui suture Middle to Late Carboniferoustime: (1) There are no Middle (TP in Figure 7) and Lower Carboniferousclastic rocks and tur- Carboniferousto Permianrocks in the Chukotkafold belt except biditesin the Belkovskyi-NerpalakhTrough and Tanatapbasin presumablyPermian shale, coaly shale,and sandstone[Drabkin, suggestthat the arc-trenchsystem was still active during the 1970a] which indicatethe existencethere of an upliftedarea for Early Carboniferous. •-70 Ma. (2) There is an unconformityat the baseof the Middle In Arctic Alaska, Devonianmagmatism and extensionalbasin Carboniferousclastic rocks, includingconglomerate and lime- formationended with the establishmentof a passivemargin stone breccia and sparse felsic lavas, on Kotel'nyi Island whichlasted from the Late Devonianto the Early Cretaceous.In [Vinogradov et al., 1974; Kos'ko et al., 1990]. (3) There is an turn, this passivemargin sedimentation ended with the closureof unconformityat the base of the 200 m thick Permian marine the Angayuchamocean and the onsetof the BrookJanorogeny. shalepredominant section on Kotel'nyiIsland and ,--150km to the NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 1001 southon the BolshoyLyakhovsky Island (Figure 7); Permian formed in an extensionalbasin most likely behinda Devonian rocksare 1200 m thick and representedby sandstoneswith coal magmaticarc. Afterbeing separated from Siberia in the Vendian seamsand phyllite [Kos'ko et al., 1990;Fujita and Cook,1990]. or in the early Paleozoic,the Bennett-Barroviablock collided Thusthe late Paleozoicuplift and formationof continentalbasins with NorthAmerica during Late Silurianto Early Devoniantime followedthe middlePaleozoic active margin development along andcaused the Ellesmerianorogeny. Devonian granites in north- the southernedge of the Bennett-Barroviablock. The process eastemBrooks Range and nearthe BarrowArch may be related thatwas responsible for theseevents is not well understood,but to this collision. In the late Paleozoic an unknown continental the collisionof a continentalmass is a viablehypothesis. objectcollided with the southernside of the ChukotkanBennett- The objectwhich deformedthe Devonianarc is inferredto Barroviablock. Triassicrifting truncated the southemportion of have beenremoved, together with sliversof the forearcregion, the Bennett-Barrovia block In the Mesozoic this block evolved duringEarly-Middle Triassic rifting (Figures11e and 12). This asa partof theChukotka-Arctic Alaska microplate. rifting couldhave transferred both the continentalmass in ques- Similarityof Paleozoicstratigraphy and timing of eventsin the tion and the forearcregion of the Devonianactive continental vast area between the Novosibirsk Islands and the Hammond marginto theAsian side of the SouthAnyui suture.Immediately subterranein Alaska definesthe unity of this tectonicprovince to the south of the suture within the Yarakvaam Uplift (YU, and imposesconstraints on the geometryand shape of the Figure 7), Devonianand Lower Carboniferousandesites and Chukotka-ArcticAlaska block. Severalmodels for the opening rhyolitesare intrudedby graniteand dioriteand unconformably of the Canada Basin have been proposed[see Lawyer and overlainby UpperCarboniferous shallow marine and terrestrial Scotese,1990]. Fujita and Newberry [1982] were the first to tuffaceoussandstone and conglomerate[Markov et al., 1980; notice that the Chukotka-ArcticAlaska block is too long to fit Natal'in, 1984]. The first unit may be consideredas a rifted againstthe marginof the CanadianArctic Islandsas is assumed fragmentof the middlePaleozoic active margin of the Bennett- in the rotational model. To solve this space problem, they Barrovia. We interpretthe late Paleozoicgranites and molasse diminishedthe size of the Chukotkansector of the block, placing depositsof the secondunit as evidencefor thecollision. Farther the westemboundary of the Chukotka-ArcticAlaska block across to the south,there are fragmentsof Paleozoicoceanic rocks, is- the Chukotka fold belt. The same solution of the problem is land arc volcanicrocks, and ophioliteswhich may belongto the typicalfor all rotationalmodels of the Chukotka-ArcticAlaska aforementionedcontinental mass and its margin. block [Tailleur and Brosge,1970; Newmanet al., 1977; Rowley Triassicrifting led to the formationof the SouthAnyui ocean and Lottes, 1988; Grantz et al., 1990]. The basis of such a and the Triassicto Early Jurassicpassive continental margin of solutionlies either in the lack of adequateknowledge of the the Bennett-Barroviablock (Figure 12). Note that this zone of geologyof northeastemRussia or in the inferencethat the South rifting lies on the continuationof rifts in the basementof the Anyui sutureabruptly changes in striketo N-S beyondits last WesternSiberia Basin [e.g., Zonenshainet al., 1990]. Triassic exposurein westernChukotka [e.g., Rowley and Lottes,1988]. diabaseand gabbro in SevemayaZemlya, Franz Josef Land, and Magneticanomalies which mark the sutureand the continuation NovayaZemlya show a link betweenthese two regions.During of the Jurassicto EarlyCretaceous magmatic arc whichis paired the Late Jurassic,fragments rifted from the margin of the with the sutureand stretches to the SvaytoyNos Capearea (SN in Bennett-Barroviablock (RF in Figure 12) collided with the Figure 1), do not allow this last solution[NatalVin, 1981, 1984; Omulevka terrane, leading to the oroclinal bending of the Parfenovand Natal'in, 1985;Zonenshain et al., 1990]. Our cor- Omulevkaterrane [•'engSr and Natal'in, 1996]. relation of the Paleozoic structures corroborates the real dimen- sions of the Chukotka-Arctic Alaska block and shows that the rotationalmodel still hasa significantspace problem. 7. Conclusions Acknowledgments. The work was funded by the Continental The PaleozoicChegitun and Tanatap units from the studyarea DynamicsDivision of the NationalScience Foundation, International formedin differenttectonic settings. Both units were strongly Division of the National ScienceFoundation, as part of award EAR- deformedin the Mesozoic,and their relationswith the largertec- 93170087-005to E. Miller and S. Klemperer,and EAR-9316573to J. E. tonic units and history were reconstructedon the basisof sedi- Wright,as well asT•3BiTAK-iT•3 joint research unit GloTec. Amato ac- knowledgesthe support of theAlbert and Alice Weeks Foundation at the mentologicaland stratigraphicdata and regionalcorrelation. The U. of Wisconsin and C. Johnsonfor the use of his laboratory. We are Ordovician to Lower Devonian shallow marine carbonates of the gratefulto E. Miller, S. Akinin,A. Calvert,L. Gahagan,K. Fujita,M. Chegitununit belong to the Novosibirskcarbonate platform that Gelman,A. Grantz,A. Kaplan,L. Lane, L. Lawver,and L. Popekofor covers the Precambrian basement of the Bennett-Barrovia block discussionsof differentaspects of the geologyof Alaskaand Chukotkao [•:engOrand Natal'in, 1996]. This unit underliesthe Chukchi E. Miller andC. Oengtrread the first versionof the paperand their con- structiveremarks improved the final version.The paperwas considerably shelf and the East Siberian Sea and continues into northern improvedowing to commentsand suggestions of A. B. Till, T. E. Moore, Alaska as the Hammond subterrane. The Devonian to Lower andW. K. Wallace. We are alsograteful to manypeople from the towns Carboniferousdeep-water sedimentary rocks of theTanatap unit of Lavrentiyaand Provideniya who helped us during the fieldwork.

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