TECTONICS, VOL. 12, NO. 5, PAGES 1195-1208, OCTOBER 1993

DENALl FAULT SYSTEM OF SOUTHERN 1974; Lanphere,1978; Stoutand Chase,1980]. Despitethis : AN INTERIOR STRIKE.SLIP consensus,the tectonichistory of the DFS remainsrelatively STRUCTURE RESPONDING TO DEXTRAL unconstrained.Critical outcrops are rare, access is difficult,and AND SINISTRAL SHEAR COUPLING to this day muchof the regionis incompletelymapped at detailed scales. Grantz[1966] named or redefinedsix individualfault ThomasF. Redfieldand Paul G. Fitzgerald1 segmentscomprising the DFS. From west to eastthe Departmentof Geology,Arizona State University, segmentsare the Togiak/Tikchikfault, the Holitnafault, the Tempe Farewellfault, the fault (subdividedinto the McKinley andHines Creek strands), the Shakwakfault, andthe Dalton fault (Figure 1). At its westernend, the DFS is mappednot as a singleentity but ratherappears to splayinto a complex, Abstract.The Denalifault system (DFS) extendsfor-1200 poorlyexposed set of crosscuttingfault patterns[Beikman, km, from southeastto southcentral Alaska. The DFS has 1980]. Somewhatmore orderly on its easternend, the DFS beengenerally regarded as a fight-lateralstrike-slip fault, along appearsto join forceswith the ChathamStrait fault. This which postlate Mesozoicoffsets of up to 400 km havebeen structurein turn is truncatedby the Fairweatherfault [Beikman, suggested.The offsethistory of the DFS is relatively 1980],the present-dayNorth American plate Pacific plate unconstrained,particularly at its westernend. For thisstudy boundary[Plafker et al., 1978]. SoutheastAlaska (the we calculatedrelative motion vectors at discretepoints along "panhandle"),appears to be characterizedby manysubparallel, the lengthof the DFS, basedon the well-understoodkinematic coastparallel faults and shear zones [Beikman, 1980], andcould interactionbetween the North American, Pacific, and Kula perhapsbe considereda singlemegashear plate boundary. plates,and the following assumptions: (1) The arcuate Existingestimates of the senseof offsetacross each fault geometryof theDFS hasexisted essentially unchanged since segmentsince the LateCretaceous are dominantly right-lateral theLate Cretaceous; (2) The Yukon-Tananaterrane and other [Grantz, 1966; Stoutet al., 1973; Forbeset al. 1973a; 1973b; terranesnorth of the DFS werefixed, in situ,prior to the 1974;Reed and Lanphere, 1974; Turner et al. 1974;Warhaftig accretionof thesouthern Alaskan terranes; and (3) Tangential et al., 1975; Eisbacher1976; Hickman et al., 1977; 1990; andnormal relative motion vector components calculated for Plafkeret al., 1977; Lanphere,1978, Stout and Chase, 1980]. pointsalong the DFS usingthe platemodel of Kelley [1993] However,the timingof offsetand the total displacement across describethe platekinematics of theDFS sincethe Late eachindividual fault segmentsare unclear,and controversial. Cretaceous.The consequentkinematic model for the DFS For example,Csejtey et al. [1982] suggestedthat there are no predictsthat left-lateral stresses have acted upon the western end lateraldisplacements across the McKinleystrand of theDenali of theDFS for muchof its history,and conflicting senses of fault,while Jarrard [1986], Schultz and Aydin [1990], and shearexist between the easternand western ends of thesystem. Schollet al. [1992] notedthat in westernAlaska sinistral slip The offsethistory of thewestern end of theDenali fault system is predictedfrom platemotion models. shouldbe significantlydifferent than the history of thecentral The majorityof structuralstudies on the DFS havebeen sited andeastern sections; consequently, individual crustal blocks in eastof theDenali massif. Lanphere [1978 p. 817] statedthat southeastand southwest Alaska may have undergone, "thewestern section probably is theleast known part of the respectively,clockwise and counterclockwise rotations. The faultsystem." Grantz [1966] proposed a right-lateraloffset senseof rotationpredicted by ourmodel is in agreementwith historyfor the Togiak/Tikchik,Holitna, and Farewell faults, rotationsdetermined by palcomagneticstudies and provides an andnoted present-day scarps as highas 6 m. However,a clear alternativemodel to the "Alaskanorocline" hypotheses. recordof fight-lateraloffsets for thewestern section has not yet beenpresented in thereadily accessible literature. This paper

INTRODUCTION exploresthe possibilitythat the DFS was not, and is not, a fight-lateralstrike-slip fault along all its entirelength throughoutits Cenozoichistory. We presenta kinematic TheDenali fault system (Figure 1) haslong been recognized modelthat suggests many areas of thewestern end of the DFS asa majorcrustal structure transecting much of Alaska. haveexperienced sinistral shear stress, and possibly even left- Sainsburyand Twenhofel [1954] and St. Armand [1954] lateraloffset, stemming from plate tectonic convergence and simultaneouslyrecognized the regional importance offaults faultgeometry. These stress fields may have caused the local identifiedby earlier workers. St. Armand [1957] postulated 250 counterclockwise rotation of small crustal blocks described in km of right-lateraldisplacement and named the collective faults the palcomagneticliterature of westernAlaska. the Denali fault system(DFS). Sincethe recognitionof the importanceof the DFS to the tectonicsof southernAlaska, it hasbeen regarded as a fight-lateralstrike-slip fault of SHEAR STRESS AND BLOCK ROTATIONS continentalproportions [Grantz, 1966; Reed and Lanphere, Rotationof crustalblocks along the obliquely convergent Pacificmargin has long been recognized [Cox, 1957;Beck, 1 Nowat Department ofGeosciences, University ofArizona, 1976;Luyendyk et al., 1985;Wells and Coe, 1985]. Beck Tucson [1976, 1980, 1986] suggestedthat clockwise block rotations observedin thePacific Northwest are related to fight-lateral Copyright1993 by the AmericanGeophysical Union. transformand obliquely subducting plate boundaries. Beck's schematicmechanism (Beck's Ball Bearings),shown in Figure Paper number 93TC00674. 2, providesa readilyunderstandable conceptual framework 0278-7407/93/93TC-00674510.00 describingblock rotation and translation at obliquemargins. 1196 Redfieldand Fitzgerald: Denali Fault System, Alaska

5/• AlaskaRange Complex •m]]Yukon-Tanana Terrane • NixonFork Terrane • BrooksRange••• i"i•ß Stikine, Taku,and Tracy Arm Terranes ::"'"""•TogiakTerrane Bering(•DenaliFaultS•stem Tinti•aF•lt••;• YakutatBlock KaltagFault ._- I[ •1 PeninsularTerrane 1Mc_Grath •l[2 j[•j[l[ll[ll[][•l[11[l[[l[ i[1[[i[Illl""4,,12 It...... / . PrinceKuskokwim,,,u Alexander TerraneTerrane [d•tarod-N•xonForkFault •, -• ,, .-..• • 5,,•• • • Wrangellia _

-- Lower• , _ , •=., .,, ,. _,-'.-' ...... t'•','½',',•..:'1•?•½ ...... ,,.,,::'::• /•//• ' " .:,,:,..::.;::" '" A merlean •YukonRiver • 3 • ,•,•' • ' • ß •.... • •:'?:"•??:..

KuskokwimBay 2.' .:..:" ...... :...... 5':'• •$ ' Castle ,,, ....6 / '• ß 1 :'"i':*'"'? Mountain Border x.' Fault tContact RangeFaultFault • '/_*• • . • ::•::•:.?'" BristolBay Fairweather BruinBayFault'•••' Fault7 f 2 Pacific Plate 21 AI Kaltag/TintinaIditarod/NixonForkRMVsitesRMVsites 3 ß Denalifault system RMV sites 1 0 Kilometers 500 40 CastleMt./Fairweather RMV sites , , 5 Cl Presentday plate boundary RMV sites

Fig. 1. Simplified,schematic terrane map of Alaskaafter Coe et. al. [1985] andHowell et al. [1985], showingthe locationsof majortectonostratigraphic terranes, structures, relative motion vector (RMV) analysissites, and other localities mentioned in the text. Note thatthe Yakutatterrane extends offshore. RMV siteson the DFS are markedwith a soliddot. Site 1 of the DFS is on the Togiak/Tikchikfault. Sites 2 and 3 of the DFS are on the Holitna fault. Sites 4-7 of the DFS are on the Farewell fault. Sites 8-10 of the DFS are on the McKinley strandof the Denall fault. Sites11 and 12 of the DFS is on the Shakwak fault. Site 13 of the DFS is on the faiton Fault. Site 14 of the DFS is on the Chatham Strait fault.

ThoughBeck's model is impracticalon a crustalscale subduction,detached slivers of continentalcrust can be [McKenzieand Jackson, 1989] and was never intended to translatedparallel to a convergentmargin [Jarrard, 1986; Beck, providean accuratedepiction of therotation process (M. L. 1983, 1989]. If actualtransport is lessthan the maximum Beck,personal communication, 1993), empirical evidence of margin-parallelcomponent of relativemotion, the sliverwill clockwisepaleomagnetic rotations in regionscharacterized in be subjectto shearand could tend to fractureinto rotating thepast by continental-scaleright-lateral shear implies that a blocks[Beck, 1989]. Jarrard[1986] andBeck [1986]suggested directrelationship between rotation and shear exists [Wells and thatresistance to slip is at leastpartly a functionof subduction Heller, 1988]. Experimentaldata concur: Cobbold et al. [1989] zoneage: Mature subduction zones offer lessresistance to slip, presenteda sandbox experiment yielding counterclockwise whileyounger convergence zones generate greater resistance and rotationsofup to 30 ø within aleft lateral simple shear couple. correspondingblock rotation. The resultsof this experimentclearly suggests that complex Lewis et al. [1988] showedthat in the Aleutianforearc deformationpatterns should be expectedwithin a shearcouple, accretionaryprism, foldinghas occurred along axes includingright-lateral domino offsets. perpendicularto plateconvergence, followed by thrustfaulting Jarrard[1986] discussedstrike-slip faulting at continental parallelto convergence,followed by strike-slipfaulting. marginsand concluded that obliquely subducting boundaries Right-lateralfaults dominate west of a majortransverse fault (at drivetoday's onshore strike-slip shear margins. Under oblique longitude161 ø) whileleft-lateral faults are dominant to the east. Redfieldand Fitzgerald: Denali Fault System, Alaska 1197

North 5.6 Ma: Dextral Lat, Long,Omega American regime switchesto Plate sinistralregime. PoleofRotation (Fixed) Crustal block beneathDenali (Mt. McKinley) beginsto respond to shear couple by anticlockwise rotation. Uplift of Denali begins (Fitzgeraldet al., 1993)causing glacial excavation. Pacific Plate (Moving) 3.0 Ma: Continued sinistral shear couple causes (a) additional anticlockwise rotation. Maintaining its drainagechannel, • SinistralStrike Slip I DextralStrike Slip the Muldrow glacier flowsto the right.

0 Ma: Denali reaches 6 km elevation. Anticlockwise rotation of the crustal block under a sinistral regime has createda right- lateral offset of the Muldrow Glacier. (b) (c) Fig.2. (a) Schematicdiagram showing a generalizedtectonic situation where both dextral and sinistral senseof shear might result from oceanic plate versus continental plate oblique plate tectonic convergence. Spaceproblems are alleviated by interveningstructures, such as the thrust fault shown. Note the different sensesof thetangential components of the two vector diagrams. (b) Block diagram (Beck's Ball Bearings, modifiedafter Beck [1980, 1989] shows conceptually how both clockwise and counterclockwise rotations of individualblocks could be expected along a singlestrike-slip fault as a functionof fault geometry and theangle of convergence.(c)Sequence showing how counterclockwise rotation of a blocktrapped within a left-lateralshear couple and the continued excavation ofpreexisting drainages might create right-lateral offsets,possibly explaining observed drainage patterns of theMuldrow and Peters Glaciers north of Denali (MountMcKinley).

Lewiset al. [1988] ascribedthe sense of deformationto REGIONAL GEOLOGY inheritedpre-Eocene structures. We speculate that similar processeshave operated in southernAlaska on crustalscales. Giventhe obliquely convergent plate tectonic regimes, the Muchof theknown kinematic history of theAlaskan terranes significantlength of theassociated subduction zones, and the isbased upon palcomagnetic studies, which have delineated systematicallyvariable plate boundary geometry, it is mobileand less mobile terranes throughout much of Alaska. reasonableto expect dextral and sinistral senses of shearalong The DFS, extendingfrom southeastern Alaska into south thesouthwest and southeast coasts of Alaskaduring the centralAlaska, and partly crosscutting theAlaska Range Cenozoic.In addition,given the mature age of theboth the complex(Figure 1), wouldappear to separatethe southern, southernAlaska subduction zone and the DFS, it is alsolikely trulyallochthonous terranes from para-autochthonous or even that much of the associatedclockwise and counterclockwise autochthonousterranes to the north. rotationstook place during the early development of the Southof theDFS lie Wrangellia,the Peninsular terrane, and system. mostof theAlaska Range complex. From palcomagnetic and 1198 Redfieldand Fitzgerald: Denali Fault System, Alaska paleontologicstudies, both Wrangellia and the Peninsular Koyukukarc was in placeby LatestCretaceous to Early terraneare suspected to haveundergone substantial northward Tertiarytimes. latitudinaltranslation [Packer, 1972; Hillhouse,1977; Panuska "The CenozoicDenali fault systemof Alaskaapparently has and Stone, 1981; Stone, 1982; Stone et al., 1982; Panuskaand developedon thealready accreted continental margin, partly Stone,1985; yon Hueneet al., 1985] and may havebeen alongan older,Cretaceous suture" [Csejtey et al., 1982,p. accretedto terranesof continentalaffinity northof the DFS as 3746]. This conclusionis supportedby recentgcophysical earlyas mid to Late Cretaceous[Plafker et al., 1989].The studies.Labson et al. [1988] andStanley et al. [1990] reported AlaskaRange complex encompasses parts of Wrangelliaand on magnetotelluricdata, suggesting that the metamorphicrocks thePeninsular terrane [Stone and Wallace, 1987] aswell as of theYukon-Tanana terrane represent a thin-skinnedthrust manystructural slivers and terrane fragments, many of which sheetoverlying conductive Upper Jurassic and Cretaceous mayalso have been translated significant distances [Jones et al., sedimentaryrocks. Beaudoinet al. [1992] integratedthe 1982]. Grantzet al. [1991,p. 6-7], citingwhat they consider electrical data cited above with seismic data from the Trans- to be "themost reliable and representative palcomagnetic data AlaskaCrustal Transect project, suggesting that the from Alaska," statedthat "the terranesthat have a well- underplatingof theYukon-Tanana terrane occurred before the documentedhistory of majornorthward displacement occur dockingof Wrangellia.Pavlis et al. [1993] suggestedthat south of the Denali fault." coolingages within the Yukon-Tanana terrane record tectonic Terranesnorth of theDFS aregenerally considered to have extension.However, this extensionwas completeby 110 Ma. formedmuch closer to their presentpositions than the terranes Hansen[1990] concluded that a largepart of theYukon-Tanana southof the DFS [Coe et al., 1985]. Dissentingviews are terraneis para-autochtonouswith a complexhistory of presentedby Churkinet al. [1982] andBox [1985]. For "dynamothermal"metamorphism during the middle to late example,Churkin et al. [1982] describethe Yukon-Tanana Mesozoicand hence should not be consideredsuspect. Taken terraneas a compositeof discretemicroterranes possibly together,these studies would suggest that the Yukon-Tanana accretedto the NorthAmerican plate at a southerlylatitude in terrane and other central Alaskan terranes north of the DFS were YukonTerritory, Canada, prior to theLate Cretaceous and in placeprior to the accretionof the mobilesouthern terranes. subsequentlytransported to theirpresent location by strike-slip On the aboveevidence, and for the sakeof exploringan faulting. Box [1985] suggestedthat much of Alaskanorth of intriguinghypothesis, we havemade the necessarilyspeculative theDFS mayhave resulted from an EarlyCretaceous collision assumptionthat the terranesnorth of the DFS havebeen of an active volcanic are with the North American continental effectivelyanchored, essentially in situ,since the Latest lithosphere.However, these interpretations do not seemto be Cretaceous,or at leastthe Early Tertiary. In our model,the supportedby geologicalor geophysicaldata. functionof theseterranes is to provideresistance (a "backstop") Hillhouseand Gromm6 [1982] concluded that all terranes againstwhich the stressesgenerated by platetectonic (e.g.,the Yukon-Tanana and Nixon Fork terranes), lying convergencewere resolved into normal and tangential beneathor northof thepresent-day exposures of Paleocene components.We speculatethat the tangentialcomponents CantwellFormation (on the northside of the DFS just northof wereprimarily responsible for impartingthe counterclockwise the Denali/Mt. McKinley massif),have undergone no detectable andclockwise rotations observed in the palcomagneticrecord of polewarddrift relative to theNorth American craton since the southernAlaska. The normal stresscomponents were probably Paleocene.Plumley and Coe [1982, 1983]and Blodgett absorbedby attenuationand shortening, perhaps initially behind [1983a,b] suggestedindependently, on palcomagnetic and (northof) thebackstop, and later within the accreting terranes paleontologicgrounds, that the Nixon Fork terranehas not south of the DFS. movedfar from its pointof origin. Palcomagneticdata presented by Coeet al. [1985,1989] supportthe conceptthat terranes north of the DFS havenot OFFSET HISTORY OF THE DENALI FAULT SYSTEM movedgreatly from theirpresent-day location. From a palcomagneticstudy of lowerCretaceous tuffs in theYukon Kuskokwimdelta region, Globerman et al. [1983]speculated At a few placesthe DFS displaysevidence of Holocene thatcentral and western Alaska were essentiallypart of the activity. Betweensome segments, pre-Holocene offset is NorthAmerican continent by Late Cretaceousto Paleocene documentedby correlatedgeologic formations and intrusions time. Data from the NowitnaLava flows (-64 Ma) [Moll et separatedby apparentstrike-slip motion. However,well- al., 1981] of the KuskokwimMountains (near McGrath, documentedexamples of offsetsacross the DFS, particularlyat Alaska)give a meandirection that is consistentwith little or its westernend, are not plentiful. no latitudinaldisplacement or rotationrelative to North Holocenescarps up to 6 m high characterizethe Holitna, America[Coe et al., 1985]. AlthoughStone et al. [1982] Togiak-Tikchik,and Farewell fault traces[Grantz, 1966]. The suggestedthat the present-day Kuskokwim terrane was displaced McKinley strandof the DFS displaysright-lateral horizontal from a locationof-15 ø north at --85 Ma, threeout of four offsetsof 50 to 60 m and 6 to 10 m of vertical throw, north- localitiessampled were nearLake Clark,south of theDFS. side-upsince 10,000 yearsago [Stoutet al., 1973]. The Only one sitewas northof the DFS, nearMcGrath. The westernend of the Shakwakfault segmentexhibits 87 m of resultsof thisstudy are therefore somewhat inconclusive with right-lateraloffset that occurred during the Holocene [Plafker et respectto the historyof the Kuskokwimterrane north of the al., 1977]. After a literaturestudy of the six majorstrands of DFS. Harriset al. [1987, p. 362] presentpalcomagnetic data the DFS, Lanphere[1978] concludedthat only the McKinley indicatingthat the Yukon-Koyukak province (between the strand and the western end of the Shakwak fault of the DFS BrooksRange and the Yukon-Kuskowim Delta) was in place exhibita reliablehistory of Holoceneoffset. Savageet al. by 56 Ma and "formedpart of the accretionarynucleus that [1981] andPlafker et al. [1989] suggestedcontemporary slip servedas a backstopfor theaccreted terranes of southern ratesalong the centralDenall fault (sites9 and 10, Figure1) to Alaska." Wallaceet al. [1989] suggestedthat the Yukon be of theorder of 1-2 cm/yr. Redfieldand Fitzgerald: Denali Fault System, Alaska 1199

On thebasis of separationof Cretaceousrocks and regional centraland southern Alaska, accompanied by adjustmentsin dragfeatures, Grantz [1966] concluded that the Holitna and northernAlaska [Grantz, 1966; Patton and Tailler, 1977]. Togiak/Tikchiksegments moved 20 - 30 km in a right-lateral Althougha simpleorocline theory is nowconsidered unlikely senseduring Late Cretaceousand Cenozoic time. Reedand [Coeet al., 1989],the concept of large-scalerotations of Elliot [1968] notedeast-northeast drag features south of the western Alaska is still considered viable. Farewellfault, implying right-lateral offset. Grantz[1966] Coeet al. [1985]and Coe et al. [1989]presented a case for suggested~100 km slip hasoccurred on the Farewellfault since oroclinalbending, by "mega-kinking,"based upon the the Late Cretaceous.However, Lanphere [1978, p. 821] noted predominantlycounterclockwise palcomagnetic rotations thatthe western end of theDFS displays"little evidence for the observedin southwestAlaska. Coeet al. [1989]cited studies morethan 300 kilometersof Tertiaryoffset recognized in theBristol Bay region,the Lake Clark region, the Lower elsewhereon the system".There appear to be no published YukonRiver region, and the Blackburn Hills (in southwestern accountsattempting a rigorouscorrelation of specific andwestern Alaska), that documented predominantly formationsor intrusionsalong the western end of the DFS. counterclockwise rotations. Additional studies in the Cantwell Warhaftig[1958] and Warhaftig et al. [1975]demonstrated Basin,the Talkeetna Mountains, and the McGrath region (in that significantright-lateral movement of the HinesCreek central,south central, and western Alaska), though less well strandof theDFS is pre-Paleocene(and probably pre-Late constrained,also suggested counterclockwise rotations have Cretaceous)in age. Their work led Lanphere[1978] to suggest occurred.Assuming that the westernend of the DFS was,and thatthe Hines Creek segment should be consideredan older is, a right-lateraltransform fault, Coe et al. [1985,1989] structure,not partof the DFS. Southof the HinesCreek interpretedthe counterclockwiserotations to reflectthe effects strand,major fight-lateral displacements may have occurred of continentalscale enechelon kink folding, a resultof alongthe McKinley strand since the Tertiary. The strongest convergencebetween and Eurasia. However, evidencesupporting Tertiary movement along the McKinley Scholland Stevenson [1991] suggested, as do we, thata strandis theoffset of the McGonagallpluton relative to the convincingexplanation for thecurvature of geotectonic Forakerpluton, requiring 38 km right-lateraloffset since 38 Ma structuresin Alaskaneed not invoke oroclinal bending. The two plutonswere correlated by Reedand Lanphere [1974, p. 1883]on the basisof "nearlyidentical mineralogy and chemistry"andK-At and 40Ar/39Ar biotite and hornblende age TECTONIC MODEL determinationssuggesting both simultaneous intrusion ages and similarcooling histories. Paleomagneticdata comprise the most compelling evidence Forbeset al. [1973a,b], Turneret al. [1974] andNockleberg supportingthe various orocline models. However, we suggest et al. [1985] suggestedthat 300 to 400 km of right-lateral analternative hypothesis integrating seafloor spreading models displacement(separating the Ruby Range and the McLaren andonshore paleomagnetic data. Our model, advancing the MetamorphicBelt) occurred since Early Cretaceous along the currentlyheretical notion that the westernend of the DFS has McKinley, Dalton,and Shakwaksegments of the DFS. Also, beenstrongly (but not totally) influenced by sinistralshear, Eisbacher[1976] correlated the Nutzina sequence with permitsadjacent small-block clockwise and counterclockwise sedimentaryrocks in theDezadeash basin. If so,these packages rotationsin southernAlaska to peacefullycoexist and predicts of Mesozoicflysch rocks along the DFS wouldrecord a an offsethistory for the DFS thatallows different amounts of minimumof 300 km right-lateraloffset since the Eocene apparentoffset to be observedalong the trace of thefault [Eisbacher,1976] or at leastsince the Paleocene [Lanphere, system. 1978]. Stoutand Chase [1980] noted that these last two offset Oceanicplate/continental plate relative motion vectors estimatestend to independentlycorrelate one another. (RMVs) calculatedfor southernAlaska suggest that the Late The abovesummary clearly shows that there are few solid Cretaceousand Cenozoic DFS wasdriven by bothright-lateral constraintsupon the offset history of theDFS. Alongdifferent andleft-lateral tangential components. West of thecentral arc segmentsof theDFS, theamount and timing of offsetvaries apex,left-lateral components dominated, while east of theapex by an orderof magnitude,from estimatesof tensof kilometres a primarilyright-lateral regime was in place(Table 12 in the for the western end to hundreds of kilometers for the eastern microficheand Figure 3). Offsetacross the DFS wasa end. In particular,the systemwest of the Denall/Mount functionof convergencedirection of theoffshore oceanic plate Mckinleymassif is poorlystudied, and its offset history is not (Pacificor Kula)and the local backstop azimuth. If so,small well constrained. crustal blocks in south-central Alaska would have resided in a dominantlyleft-lateral tectonic environment, conducive to counterclockwiserotation, while counterpart blocks in THE OROCLINE HYPOTHESIS southeasternAlaska would be affectedby a dextral,clockwise shearregime. The modelalso predicts Cenozoic "partitioning" Carey[1955] suggested that most of Alaskapivoted of thewestern end of theDFS intoregions of left-lateraland counterclockwise28 ø abouta pointnorth of theGulf of Alaska, right-lateralshear, providing a mechanismto explainlocalized, openingthe Canada basin north of Alaskaand bending the less common clockwise rotations. Alaskanmountain ranges (and presumably the trace of the The modelrequires three principal assumptions: 1. The DFS) intotheir present arcuate geometries. Several authors DFS hasexisted since the latest Cretaceous, and possibly built uponCarey's initial hypothesis.One setof modelscalls upona lateJurassic to mid-Cretaceoustectonic event opening theCanada basin and causing Brooks Range thrusting 2 Tables1 and2 areavailable with entire article on microfiche. [Rickwood,1970; Tailler, 1973; Mayfield et al., 1983]. Other Orderfrom the American Geophysical Union, 2000 Florida modelsaccomplish bending by requiring40 ø to 55ø Avenue,N.W., Washington,DC 20009. DocumentT93-003; counterclockwiserotation of individualfault-separated blocks in $2.50. Paymentmust accompany order. 1200 Redfieldand Fitzgerald: Denali Fault System, Alaska earlier,with muchof its presentday arcuate shape intact. This separatingplate A andplate B) providesthe tangential assumptionis supportedby Stoutand Chase [1980], who componentof motionat theplate margin (Figure 2). arguedthat the small circle outline of thecentral part of the Engebretsonet al. [1985]presented a plate tectonic model for DFS is an intrinsicfeature of thefault systemitself, and by thePacific Ocean basin that permitted a quantitativeassessment Csejteyet al. [1982],who suggested that the development of of theinteraction between the subdueting oceanic plates and the the Cenozoic DFS followed an older terrane suture line. continentalmargin. Their modelis dependenton the However,Coe et al. [1989]point out thaton a largescale, the fundamentalassumptions that hotspots have remained fixed fit of the D FS to smallcircles is notvery good, and that (with respectto themselves)relative to therate of motionof accretionof terranessouth of the DFS duringthe late theoverriding lithospheric plates and that sea floor spreading Cretaceousand early Tertiary may have indented and/or ratesbetween the existingPacific plate and its subdueted accentuatedthe arcuate shape of theDFS. 2. The Yukon- neighborswere symmetric and can be describedby synthetic Tanana terrane and other terranes north of the DFS formed an isoehrons.Errors in locationof variouscomponents of the existingbackstop against which the southernterranes were modelsummed up to a totaluncertainty of 900 km for 100 Ma acefeted. As describedabove, the northernterranes are not [Engebretsonet al., 1985]. New datahave helped reduce some suspectedof havingundergone significant latitudinal transport of theseuncertainties. Lonsdale [1988] reinterpreted the history relativeto NorthAmerica, while the palcomagnetic of the Kula plate,documenting a significantperiod of documentation of exotic travel histories for the southern asymmetricspreading, after which Kelley [1993] integrated terranesis, essentially,beyond dispute. Combining the first Lonsdale'sfindings into a newmodel, also incorporating a two assumptions,the traceof the DFS thusmarks, in a gross revisedtime scalefrom Cande and Kent [1992]. sense,the traceof the backstopmargin. 3. Relativemotion Althoughuncertainties created by slightvariations in vectors(RMVs) calculatedfrom the models of Engebretsonet backstopgeometry significantly exceed the differences in al. [1985]and Kelley [1993], for variousplaces along the above RMVs calculatedwith the two platetectonic models, we have backstopmargin are representative of the localstresses imposed usedthe up-to-datepoles of Kelley [1993]. With thesepoles, at the D FS backstopmargin. This assumptionincorporates RMVs were calculatedat 14 selectedpoints along the DFS additionaluncertainties. The RMV resultantswill alwaysbe (Figure1) for 11 timeperiods. In addition,RMVs were maximumvalues as coupling between converging plates is calculatedfor locationsalong the Kaltag/Tintinafault system, certainlyless than 100% efficient.The accretionof the the Iditarod/NixonFork fault, the Fairweather/CastleMountain southernterranes may have occurred as a seriesof discrete fault,and at today'sAlaska/Pacific plate margin (Figure 1). events,modifying the geometryof the the backstopmargin. In The resultsare presentedin Figures3, 4, and5, andTables 1 addition,known structures between the present-day plate margin and 2 in the microfiche. and the DFS, suchas the Lake Clark/Castle Mountain fault, The magnitudeand sign of thetangential components of the the Fairweatherfault, the BruinBay fault,and the Border RMV resultantsare sensitive to theazimuth of convergence, Rangesfault could absorb much of theeffective stress. (Indeed, andthus the orientation of thebackstop. For this study we it is possiblethat these faults are forming a new,concave- measured azimuths of the DFS from the State of Alaska northwardfault systemwhose behavior mimics that of theolder geologicmap [Beikman, 1980] and, by assumption, computed DFS). Splaysto the DFS, suchas the Totschundafault, and RMVs asif theD FS werethe plate margin. For the sake of thrustfaults such as the BroxonGulch thrust,add additional completeness,we havecalculated RMVs from 83 Ma to the tectoniccomplexity. Within theseassumptions, we presenta present.However, the uncertainties of ouranalysis increase speculativemodel for thetectonic development of Alaskasouth withthe age of themargin. When discussing the implications of the DFS. of ourmodel, we bearin mindthat speculation achieves new heightswith increasingage. In view of the aboveassumptions and the known geological METHODS complexities,we emphasizethat the quantitativeresults this analysispresents are maximumestimates for an assumedplate The relativemotion vector (RMV) V of a pointP on plateB margingeometry. The RMV datashould not be viewedas relativeto plateA maybe calculatedas follows: absolute.Rather, we presentthis analysis as a qualitative, conceptualframework that may perhaps lead to a fieldwork- Vglobal = coR(Ex P) basedreinterpretation of the offsethistory of the DFS. whereE is the Eulerpole of rotation,co is the angularrotation rate,and R is theradius of theEarth. Conversionfrom global DISCUSSION coordinatesto localcartesian coordinates is accomplishedby a 3 x 3 trigonometricmatrix T: The variablesense of shearalong the DFS predictedby the modelmay be reflectedin palcomagneticdata from southwest Vlocal= IT]Vglobal and south central Alaska. In this section we discuss the RMV datagenerated from specific locations along faults comprising Detailsof the methodmay be foundin Cox andHart [1986,p. theDFS in the contextof regionalgeology and associated 155-156]. The resultantrelative motion vector (RMV) palcomagneticstudies. describesthe interactionof plateB (e.g.,the Pacific or Kula The Togiak/Tikchikand Holitna segmentsof the DFS runs plate)relative to fixed-in-spaceplate A (e.g.,North from Bristol Bay to a point ~150 km inland,at azimuths America/Alaska).Appropriate trigonometry using the angle between210 and220 (sites1-3, Figure1). RMV calculations betweenthe RMV andthe perpendicular to thebackstop (e.g., for sites1 (azimuth210), 2, and3 (azimuths220" and225") the azimuthof the subductionzone or strike-slipfault showa consistentleft-lateral tangential component between 83 Redfieldand Fitzgerald: Denali Fault System, Alaska 1201

Kaltag/TintinaFault System

, , Iditarod/Nixon Fork Fault ' ' '8••enaliFaultSystem.[!iii'"'i2i[iiiiiii'iiiiiiiii!

,

Castle Mountain/Fairweather Fault System

Relative Motion Vectors between North America (fixed) and the Pacific plate, 5.6 Ma to 0 Ma • LeftLateral (anticlockwise) "'":":•Right Lateral (clockwise) PresentDay Margin Backstop

Direction of relative ß convergence Tangential component E Normal component S Site

Fig. 3. Schematicdiagram showing RMV diagramsalong the major curved faults of Alaskabetween 5.6 Ma andthe present. Note that tangential slip velocities are greatest at eitherend of a givencurved fault system,becoming more compressional toward the "armpit" of Alaska.Generation of tangentialand normalvectors utilizes plate tectonic models of Kelley[1993] and Engebretson et al. [1985],as discussed in the text.

Ma andthe present (Table 1 in the microficheand Figure 3). 5.6 Ma (Figure3). A compressionalregime is implied The regionbetween the sitesis structurallycomplex, with betweensites 1 and4 duringthis time period,providing a manysemiorthogonal crosscutting fault patterns[Beikman, possiblemechanism for generatingsome of the structural 1980]. Palcomagneticdata in thisarea are limited. Globerman complexitynear the Bristol Bay region by thrustingand reverse andCoe [1983] reportedcounterclockwise rotations from an faulting. Site5 (placedon the mostnortherly trending segment upperCretaceous unnamed volcanic series in theTogiak terrane of the bendin the Farewellfault; backstop azimuth 230 ø) nearnorth Bristol Bay, resultsconsistent with left-lateralshear. displaysconsistent left-lateral tangential components from 83 The Farewellfault (sites4-7, Figure1) connectsthe western Ma to the present.Site 6 (azimuth250 ø) favorsright-lateral andcentral portions of the DFS, crosscuttingboth Cretaceous shearbetween 33.3 Ma and5.6 Ma. Tangentialcomponents andPaleozoic rocks along its trace[Beikman, 1980]. Sites4, are quitesmall; in thisarea, strain accumulation may not 5, and6 wereselected to investigatethe effect of a gentlebend alwayshave been relieved by strike-slipmotion. Csejteyet al. in fault geometry(Figure 6). At site4 (azimuth242ø), a right- [1982]suggested that Tertiary right-lateral strike-slip lateralRMV tangentialcomponent is favoredbetween 33.3 and displacementdies out alongthe Farewell fault, replaced by 1202 Redfieldand Fitzgerald: Denali Fault System, Alaska

West Denali fault systemsites East

Time Intervals 12345678 9 0.00 to 5.60 ma 51 55 53 59 55 59 53 57 "•'•"'"'•"'"5:•ii'•?!!, 5.60 to 10.21 Ma 17 8 11":"":"'""'"":;•'i'i;:;:2iiii:•i 1 ...... •:---'-'"•Jiii 2 i'"'"'""":'"'"'•':'.:4!ii i"";:":'"":'""'•':;-2:-iii!i:i':':'"':';:'"'""•;'-:•iiii!i ...... 10.21 to 19.62 Ma

19.62 to 26.17 Ma 26.17 to 33.30 Ma

33.30 to 43.20 Ma

43.20 to 52.74 Ma

52.74 to 58.00 Ma 97 109 107 125 118 •27 H9 128 92 84 87 59 76 49 78 55 42 27 58.00 to 67.61 Ma

67.61 to 73.78 Ma 57 77 71 113 91 122 89 117 128 137 101 92 95 66 84 56 85 62 49 33 73.78 to 83.00 Ma I SinistralI Variable I Dextral I

15 km/myright-lateral tangential component and '""'"'"'""'i•...... 63model.km/my Dextral normal shear componentcouple predicts predictedclockwise from rotations. 22 km/myleft lateraltangential component and 71 2L•km/mySinistral normalshear couple componentpredicts predictedanticlockwise from model.block rotations.

Fig. 4. Blockdiagram synthesizing RMV datafor the Denalifault systemin spaceand time. Note that thewestern end of the DFS hasexperienced entirely sinistral tangential components, while the easternend hasalways been characterized by dextralcomponent:;. The partitioningof themiddle regions into sinistral anddextral regimes is a functionof fault/backstopgeometry (see Figure 6). Sourcedata for the computationof tangentialand normal vectors are from plate tectonic models of Kelley[1993] and Engebretsonet al. [1985], as discussedin the text.

reversefaulting. The largenormal components of the RMV direction,or minorgeometric irregularities in thebackstop, datawould tend to supporttheir view. could be sufficient to reverse the sense of shear and create Sites4, 5, and6 (Figure6) demonstratethe azimuth opposinglocal stress regimes. This appearsto be reflectedin sensitivityof our analysis.The azimuthsof relative thepalcomagnetic record of southwesternAlaska: of seven convergencealong the westernend of the DFS areclose to the studiesreported by Coe et al. [1989], six demonstrated "crossoverazimuth," where tangential components change from counterclockwiserotations while onerecords a weakapparent sinistralto dextral. Smallvariations in plateconvergence clockwiserotation. Thrupp and Coe [1986] alsoshowed that Redfieldand Fitzgerald: Denali Fault System, Alaska 1203

West East Kaltag- sites West East

Iditarod-Nixon Fork fault sites

- • Time Intervals Time Intervals 1 2 3 4 0.00 to 5.60 Ma 0.00 to 5.60 Ma 25 14 55 57 5.60 to 10.21 Ma 5.60 to 10.21 Ma

10.21 to 19.62 Ma 10.21 to 19.62 Ma

19.62 to 26.17 Ma 19.62 to 26.17 Ma ::::::::::::::::::::::::::.;-:::.:-::::-::: :--.. 40 .•?•.i:;$1i'•:•.ii:•ii 26.17 to 33.30 Ma 26.17 to 33.30 Ma 12 3 49 49 33.30 to 43.20 Ma 33.30 to 43.20 Ma 26 18 42 44 43.20 to 52.74 Ma 43.20 to 52.74 Ma 41 29 63 72 52.74 to 58.00 Ma 52.74 to 58.00 Ma 47 25 115 125

58.00 to 67.61 Ma 49 50 27 ...... 58.00 to 67.61 Ma 78 64 91 91 99 68 81 67.61 to 73.78 Ma 74 75 44 67.61 to 73.78 Ma 113 95 88 107 122 122 137 ...... 73.78 to 83.00 Ma 73.78 to 83.00 Ma 87 72 96 94 70 83

West East West East (presentday Pacific/North Bear Mt./Castle Mt./Fairweather fault sites American plate boundary )sites

o

Time Intervals 12 345 67 Time Intervals 123 45 67 0.00 to 5.60 Ma 0.00 to 5.60 Ma 5.60 to 10.21 Ma 5.60 to 10.21 Ma

10.21 to 19.62 Ma 10.21 to 19.62 Ma

15 km/myright-lateral tangential component and

'":'•"':•model.63km/my Dextralnormal shear componentcouple predicts predictedclockwise from rotations. 22 km/myleft lateraltangential component and 71 km/mySinistralnormalshear couple componentpredicts predictedanticlockwise from model.block rotations.

Fig. 5. Blockdiagram synthesizing RMV datafor theKaltag/Tintina, Iditarod-Nixon Fork, Castle Mountain/Fairweatherfaults, and the present-day North America-Pacific plate boundary, in timeand space. Sourcedata for thecomputation of tangentialand normal vectors are from plate tectonic models of Kelley [1993] andEngebretson et al. [1985], asdiscussed in the text. 1204 Redfieldand Fitzgerald: Denali Fault System, Alaska

Bering

Farewell Fault

.55 W ......

Granitic intrusive rocks, undated

Cretaceousvolcanic greywacke sandstone and mudstone

Cambrianto Devoniansedimentary and meta-sedimentaryrocks

Fig. 6. Schematicgeologic map of a portionof theFarewell fault [afterBeikman, 1980], showing the faultazimuth dependency of thisstudy. Gentle variations in azimuthcan result in conflictingtangential components(see Table 1 in the microfiche).

counterclockwiserotations south of LakeClark occurred during favoredright-lateral components. Site 9 (Denali/Deborah, the Paleocene,a timewhen large sinistral tangential azimuth259 ø) hashosted a dextralcomponent since 33.3 Ma. componentswould be expected(Figure 3 andTable 1 in the The dextralRMV regimeat theDenali site (site 8) maysupport microfiche) the well-known offset correlation of the Foraker and As discussedabove, the Hines Creek strandof the Denali McGonagallplutons [Reed and Lanphere, 1974]. At fault hasprobably been inactive for muchof the Cenozoic approximately5.6 Ma, a changein Pacific-NorthAmerica plate [Lanphere,1978]. Consequently,for thisstudy, RMVs were relativemotion [Cox and Engebretson, 1985] caused the generatedfor theMcKinley strand alone (sites 8-10, Figure1). tangentialcomponent near Denali to changefrom dextralto Strictlyspeaking, the backstop for the McKinleystrand would sinistral,joining the KichatnaSpires in left-lateralpersuasion. appearto be composedof manytectonic slivers trapped between Combinedwith a geometriclocation near the centerof the DFS the McKinleyand the HinesCreek strands [Jones et al., 1982; arc,this increase in (or focusingof) compressionmay have had Hickmanet al., 1990]. However,north of the HinesCreek a spectaculartectonic result. Apatite fission track data strandis theYukon-Tanana terrane. This terrane probably acted [Fitzgeraldet al., 1993] suggestthat at ~6 Ma the Denali asthe control against which the Late Cretaceous-early Cenozoic regionexperienced rapid uplift and denudation, producing the DFS developed. Denali(Mount McKinley)massif. Another fission track study Site7 (KichatnaSpires, azimuth 232") has experienced in the samearea [Plafker et al., 1992]also suggests denudation entirelyleft-lateral tangential components since 83 Ma. at about this time. This observation is in accord with a model Between33.3 and5.6 Ma, site8 (Denali,azimuth 249 ø) by Schultzand Aydin [1990], showing mean stress due to slip Redfieldand Fitzgerald: Denali Fault System, Alaska 1205 alongthe DFS at a maximuminside the apexof the arc. 1992]. Stoutand Chase [1980] alsocite thrustswest of the Paleomagneticstudies in theDenali area are equivocal. BroxonGulch thrust postulated by Csejtey[1976] asadditional Hillhouseand Gromm6 [1982] suggestedan counterclockwise structureswhich could have accommodated shortening. rotationoccurred in theCantwell basin (near site 8), while unnamedlava flows in theTalkeetna Mountains (near site 9) TECTONIC IMPLICATIONS yieldedan ambiguousresult [Hillhouse and Gromm6, 1983; Hillhouseet al., 1983, 1985]. However,in thesestudies error Becauseof the lack of constraintsin a largenumber of the estimatesexceeded the amount of postulatedcounterclockwise assumptionsmade, the RMV datapresented in thispaper rotation. Anotherstudy in the samearea by Panuskaet al. shouldbe interpreted,geologically speaking, in a conservative, [1990] suggesteda postEocene counterclockwise rotation, a nonquantitativemanner. However, as a conceptuallynovel way conclusionconsistent with a periodof sinistralshear between of viewingthe tectonicframework of southernAlaska, our 53 and33 Ma. Offsetglaciers draining the northernflanks of modelinspires us to drawthe followingobservations: Denali(e.g., the Muldrow and Peters glaciers) suggest an 1. The DFS hasexperienced a consistent left-lateral apparentdextral offset across the Denali strand, although this tangentialcomponent along its westernend. This senseof couldbe explainedby counterclockwiserotation of a block shearis conduciveto counterclockwiserotation, though the southof the Denali strandresulting from the impositionat 5.6 largeassociated normal component probably resulted in an Ma of a left-lateralshear stress (Figures 2 and3). overallcompressire regime. Sites9 through14 (Denali/Deborahthrough Petersburg) have 2. The DFS hasshown a consistentright-lateral tangential experiencedconsistent right-lateral RMV components(Figure componentalong its easternend since 83 Ma. This senseof 3). Site9 marksa turningpoint of thedominant shear regime shearis conduciveto clockwiserotation. RMV tangential alongthe DFS. Betweensite 1 (Togiak/Tikchik)and site 8 componentson the easternend were probably of sufficient (Denali)we calculatefor all timeintervals primarily left-lateral magnitudeto transportterranes along dextral strike-slip faults. componentswith largenormal components. The 3. The centralportion of theDFS hasexperienced a variable paleomagneticrecord, suggesting counterclockwise rotations of history. Between83.0 and33.3 Ma, the apexof the arc and the TalkeetnaMountains, Denali National Park, Lake Clark, areawest of the apexwere subjected to entirelysinistral BristolBay, andthe Mc13rathregion relative to NorthAmerica tangentialcomponents. Changes in platemotions after 33.3 sincethe Paleocene[Hillhouse et al., 1985; Hillhouse,1987] Ma resultedin periodiclocal reversals of the tangential wouldseem to agree.Between sites 10 and 14, RMV data components,permitting variable senses of rotation. suggestthat the DFS hasexperienced large, right-lateral 4. As a directresult of its arcuateshape, offsets across the tangentialcomponents. The long-timedextral regime on the DFS wouldbe expectedto differ in magnitude,as well asin easternend of the DFS is in agreementwith the multiplicityof sense,between the westernand eastern ends of the fault system. studiesthat have proposed large right-lateral offsets across the Thesespeculations would imply thatthe apexof the DFS DFS sincethe Late Cretaceous[e.g. Lanphere, 1978], andwith tracearc would form a pointof focus,implying a net paleomagneticstudies documenting clockwise rotations in the compressionalregime. This tectonicsituation would clearly WrangellMountains [Hillhouse, 1977, Hillhouseand Gromm6, presenta spaceproblem. The arcapex is a logicalarea to 1984],and the Hound Island volcanics near Petersburg, Alaska expectthe complex thrusting and tectonic slivering reported by [Haeussleret al., 1992]. Joneset al. [1982]. Additionally,Stout [1972] andStout and In summary,RMV datapredicting that the western end of the Chase[1980] notedthat the Broxon 13ulch thrust may be a DFS shouldbe characterizedlargely by sinistralshear appear to majortectonic break extending to mantledepths and may have be supportedby thepaleomagnetic record. While largenormal movedsimultaneously with the Denalifault. Plafkeret al. RMV componentssuggest that compression has been a [1992] suggestedthat uplift of thecentral in the dominant tectonic feature of the western end of the DFS LateCenozoic occurred along a hypotheticalnortheast striking, (perhapsreflected in observedvertical fault scarp northwestdipping thrust splay fault thatintersects the Denali morphologies),it alsoappears that the tangential sense of shear strandat an angleof 20ø . Thesethrusts, and perhaps other as is left-lateral,perhaps reflected in theobserved counterclockwise yetunrecognized structures, may have relieved the space blockrotations. The sameRMV datapredict that the eastern problemgenerated by compression. endof the DFS hasbeen an activeright-lateral strike-slip fault The postMesozoic backstop geometry of southernAlaska sincethe Late Cretaceous.The magnitudesof the tangential hasundoubtly been influenced by theaccretionary history of componentsalong the Shakwak and Dalton faults often exceed terranes south of the DFS. As terranes and crustal blocks thoseof the normalcomponents; major dextral offsets are dockedand moved along the backstop of the DFS, local favored(and indeed, may be observedin thegeologic record). deviationsfrom theconvex northward arc we postulatein this Stoutand Chase [1980] proposeda modelin whichright-lateral papercould create regions where tangential components were motionalong the DFS maybe describedby threerigid plates, thereverse of whatour model predicts. However, we suggest whosemotions are in smallcircles defined by the traceof the thatas the southernmargin of Alaskacontinued to build,the DFS. Althoughit maybe difficultto considerthe lithosphere DFS arcbecame more and more the controlling factor to southof the DFS to be composedof rigid plates,we notethat rotationand translation. A corollaryof thismodel compared to their modelis not completelyincompatible with ours. Both observedgeologic constraints is thatdisplacements both within theirDenali and McKinley plates must be, ultimately,driven terranes[Thrupp and Coe, 1986] and between terranes played an by couplingwith thePacific plate; our resultsimply that the importantrole in the postaccretionarytectonics of southern McKinleyplate would meet additional resistance to thewest. Alaska,perhaps through rotation of blocksand attenuation of This resistancewould have to be relievedby additional crustalong thrust or high-anglefaults. thrusting.The BroxonGulch thrust, located near the apexof As the DFS becameprogressively insulated by thesouthern the DFS arc, can be invokedto accommodateconsiderable accretedterranes, outboard faults may have formed along crustalshortening [C. !3. Chase,personal communication, youngersutures. We speculatethat the Fairweather fault 1206 Redfieldand Fitzgerald: Denali Fault System, Alaska

(whichtruncates the southeasternmostend of theDFS) has DFS providedan existing backstop; (3) relativemotion vectors joinedforces with the Lake Clark-CastleMountain fault system (RMVs) calculatedalong the DFS are representative of the to createa new variable-shearstrike-slip fault. Field drivingstresses generated by interactionof theKula or Pacific observationsalong the Contactfault [Bol andRoeske, 1993] plateswith continentalNorth America. The resulting suggesta recordof dextralslip, as do RMVs calculatedat kinematicmodel suggests that, while the easternend of the representativesites along these faults for the last20 million DFS hasprobably been characterized by right-lateralshear, the years(Table 2, Figure5). It is likely thatcrustal blocks westernend of thesystem has experienced dominantly left- againstfaults south of the DFS havebehaved in a similar lateralor compressiveregimes. Absolute offsets across the fashionto thosealong the DFS throughoutthe Cenozoic.Net DFS may be variablebetween the southwestern and the compressionin the arcapex and a largeregion of increased southeasternends of the system,possibly explaining some of meanstress [Schultz and Aydin, 1990] maybe reflectedin the conflictingestimates of strike-slipdisplacements across the southcentral Alaskan topography. DFS. In westernAlaska, along the Togiak/Tikchik,Holitna, For the sakeof completeness,we haveextended the modelto andFarewell faults, local supracrustal tectonics may havebeen the Kaltag/Tintinaand the Iditarod/NixonFork fault systems. controlledby locallyconflicting slip vectors. In southcentral RMV resultantssuggest that duringthe early Cenozoicboth Alaska,near the apexof theDFS arc,opposing senses of shear faultsmay have experienced sinistral stress regimes (Figure 4). would havecaused space problems. As a resultof the However,both fault systemswould appear to havehosted approximately5 Ma changein motionof the PacificPlate dextralslip componentsbetween 26.2 Ma and 5.6 Ma. If the [Cox andEngebretson, 1985; Kelley, 1993], increasedmean RMV datathis far northof the margincan be extendedinto the stressat the arc apex[Schultz and Aydin, 1990] may have earlyand middle Cenozoic, net westwardtransport might be resultedin the uplift of the present-dayDenali massif[Plafker predictedbetween the largelyright-lateral Kaltag/Tintina fault et al., 1992; Fitzgeraldet al., 1993]. andthe left-lateral western DFS. This is in accordwith a study The senseof shearcalculated using RMVs at any onepoint by Meislinget al. [1987] inferring130 km right-lateralslip alongthe DFS is dependentupon the azimuthof the fault. alongthe Kaltagfault and 200-kin right-lateralslip on the Pointsof convergenceand divergence (where right-lateral shear Tintinafault since75 Ma. Present-daycomponents suggest switchesto left-lateralshear, or viceversa) are a functionof that the Kaltag/'rintinafault systemis partitionedin a similar changingbackstop geometry as well asvarying direction of manner as the DFS and that the Iditarod/Nixon Fork fault is plateconvergence that existed along the DFS in southwestand influencedby an entirelyleft-lateral stress field (Figure3). south central Alaska. As the southern terranes were accreted to Shultzand Aydin [1990] showedcomposite mean stress inside theAlaskan margin, their palcomagnetic rotations may have the arc of the Kaltag/Tintinafault systemto be lowerthan beenat leastpartially determined by theprevailing regime: insidethe DFS for bothindependent slip andinteractive slip right-lateralor left-lateral. Palcomagneticclockwise or models.Their model,combined with therelatively weak counterclockwiserotations would therefore be, in part,a tangentialcomponents and the complexityof outboard functionof wherea givenblock was accreted. In our model, structures,might explain the lack of significanttopography theobserved palcomagnetic data are interpreted by local insidethe arc apexof the Kaltag/'rintinafault system. accretionor passiverotation (an apparentrotation acquired One implicationof thisanalysis is thatterranes accreted duringlatitudinal translation of a giventerrane) rather than againsta backstopsubparallel to the westerntrace of the DFS invokinga 40øoroclinalbend of muchof southwesternAlaska. priorto 33.3 Ma wouldhave experienced a left-lateralregime In conclusion,we noteagain that our hypothesisis model- favorable to counterclockwise rotation. Between 33.3 and 5.6 dependent,built upona numberof assumptions.However, the Ma, structuralcomplexities caused by dextral/sinistral simplicityof the idea intriguesus. While unequivocalproof partitioningcould have created areas where clockwise rotations linkingthe counterclockwiseblock rotations of westernAlaska were favored.The modelprovides an alternativemechanism to with paleosinistralstrike-slip shear coupling may be difficultto explainexisting palcomagnetic data and obviates the needto obtain,we speculatethat future field studiesalong the western invokeoroclinal bending of southwesternand south central endsof the curvedfault systemsmay helpclarify the Alaska. geotectonicdevelopment of westernAlaska throughout the Cenozoic. CONCLUSIONS Acknowledgments.This paperhas greatly benefitted from The platetectonic model of Engebretsonet al. [1985] and its discussionwith Bob Grimm, Clem Chase,Peter Coney, directsuccessor [Kelley, 1993] canbe usedto placesignificant SimonPeacock, and Myrl Beck. DavidStone and Dave Scholl constraintsupon the evolutionof the Denali fault system arethanked for thoroughand very supportivereviews. Kevin (DFS) if certainassumptions are made. Theseassumptions Kelley andDave Engebretsonkindly provided new stagepoles include(1) the geometryof the DFS hasremained essentially beforetheir work sawpublication. Support is from NSF grant unchangedsince the LateCretaceous; (2) terranesnorth of the DPP 8821937 and DPP-9017763.

REFERENCES

Beaudoin,B., G. Fuis, W. Mooney,W. positionsas evidencefor regional shearin the transportin zones of oblique subduction, Nockleberg,and N. Christensen,Thin, low- western Cordillera of North America, Am. J. Tectonophysics,93, 1-11, 1983. velocity crust beneaththe southernYukon- $ci., 276, 694-712, 1976. Beck, M. E., Model for Late MesozoicEarly Tanaria terrane, east central Alaska: Results Beck, M. E., Paleomagneticrecord of plate margin Tertiary Tectonicsof coastalCalifornia and from the Trans-Alaska Crustal Transcot tectonicprocesses along the westernedge of westernMexico and speculationson the origin refraction/wide-anglereflection data, J. North America,J. Geophys.Res., 85, 7115- of the , Tectonics, 5, 49-64, Geophys. Res., 97, 1921-1942, 1992. 7131, 1980. 1986. Beck, M. E., Discordantpaleomagnetic pole Beck, M. E., On the mechanismof tectonic Beck, M. E., Block rotations in continental crust: Redfieldand Fitzgerald: Denali Fault System, Alaska 1207

examplesfrom westernNorth America, in and Alaska, Can. J. Earth ScL, 13, 1495-1513, Conference,edited by D. G. Howell,D. L. PaleomagneticRotations and Continental 1976. Jones,A. Cox, and A. Nur, pp. 111-114, Deformation,edited by C. Kiesseland C. Laj, Engebretson,D.C., A. Cox, and R. G. Gordon, StanfordUniversity Publications in Geological pp. 327-342, Kluwer Academic, Boston,Mass., Relative motions between oceanic and Sciences, vol. 18, 1983. 1989. continentalplates in the Pacific basin,Geol. Hillhouse, J., C. S. Gromm6, and B. Csejtey, Beikman,H., Geologicmap of Alaska, scale Soc. Am. Spec. Pap., 206, 59 pp., 1985. Tectonicimplications of palcomagneticpoles 1:2,500,000, Division of Geologicaland Fitzgerald,P. G., E. Stump,and T. F. Redfield, from lower Tertiaryvolcanic rocks, south GeophysicalSurveys, State of Alaska Dep. of Late Cenozoicuplift of Denali and its relation central Alaska,J. Geophys.Res., 90, 12,523- NaturalResources, Anchorage, 1980. to relativeplate motionand fault morphology, 12,535, 1985. Blodgett, R. B., Paleobiogeographicaffinities of Science, 259, 497-499, 1993. Howell, D. G., D. L. Jones, and E. R. Schermer, Devonian fossils from the Nixon Fork terrane, Forbes,R. B., D. L. Turner, J. H. Stout,and T. E. Tectonostratigraphicterranes of the Circum- southwestAlaska, in Pre-Jurassic Rock in Smith,Cenozoic offset along the Denalifault, Pacific Region,in Tectonostratigraphic WesternNorth America Suspect Terranes, edited Alaska (abstract),Trans. AGU, 54, 495, 1973a. Terranesof the Circum-PacificRegion, edited by C. H. Stevens,pp. 125-130, Pacific Forbes,R. B., D. L. Turner, T. E. Smith,J. H. by D. G. Howell, pp. 3-30, Circum-Pacific Section,Society of EconomicPaleontologists Stout, and F. R. Weber, The Denall fault offset Councilfor Energyand Mineral Resources, and Mineralogists,Tulsa, Oklahoma, 1983a. problem,in U.S. GeoLSurv. Alaska Program, Houston, 1985. Blodgett, R. B., Paleobiogeographicimplications U.S. GeoLSurv. Circ. 683, p. 46, 1973b. Jarrard,R. D., Relationsamong subduction of Devonian fossils from the Nixon Fork Forbes,R. B., T. E. Smith, and D. L. Turner, parameters,Rev. Geophys.,24, 217-284, terrane,southwestern Alaska (abstract),Geol. Comparativepetrology and structureof the 1986. Soc. Am. Abstr. Programs,15, 428, 1983b. McLaren,Ruby Range, and Coast Range belts: Jones,D. L., N.J. Silberling,W. Gilbert, and P. Bol, A., and S. Roeske,Strike slip faultingand implicationsfor offsetalong the Denali fault Coney, Character,distribution, and tectonic block rotation along the Contactfault system, system(abstract), GeoL Soc.Am. Abstr. significanceof accretionaryterranes in the eastern Prince William Sound, Alaska, Programs, 6, 177, 1974. centralAlaska Range,J. Geophys.Res., 87, Tectonics, 12, 49-62, 1993. GIGherman,B., and R. S. Coe, Palcomagnetic 3709-3717, 1982. Box, S. E., Early CretaceousGrogchic belt in resultsfrom upperCretaceous rocks in northern Kelley, K. P., Relative motionsbetween North NorthwesternAlaska: Internal organization, BristolBay, SW Alaska, and tectonic Americaand oceanplates of the PacificBasin lateral extent, and tectonic interpretation,in implications,in Proceedingsof the Circum during the past 130 million years, M.S. thesis, TectonostratigraphicTerranes of the Circum- PacificTerrane Conference, edited by D. G. 101 pp., Western WashingtonUniversity, Pacific Region, editedby D. G. Howell, pp. Howell,D. L. Jones,A. Cox,and A. Nur, pp. Bellingham, 1993. 137-145,Circum-Pacific Council for Energy 98-102, StanfordUniversity Publications in Labson,V., M. Fisher,and W. Nockleberg,An and Mineral Resources,Houston, 1985. GeologicalSciences, vol. 18, 1983. integratedstudy of the Denali fault from Cande,S. and D. Kent, A new geomagnetic Globerman,B., R. Coe, J. Hoare, and J. Decker, magnetotelluricsoundings, seismic refraction, polaritytime scalefor the Late Cretaceousand Palcomagnetismof lower Cretaceoustuffs from and geologicmapping (abstract), Trans. AGU., Cenozoic,J. Geophys.Res., 97, 13,917- Yukon Kuskokwimdelta region, western 69, 1457, 1988. 13,951, 1992. Alaska, Nature, 305, 516-520, 1983. Lanphere,M. A., Displacementhistory of the Carey,S. W., The oroclinchypothesis in Grantz,A., Strikeslip faultsin Alaska,U.S. Denall fault system,Alaska and Canada,Can. J. geotectonics,Proc. R. Soc. Tasmania,89, 255- Geol. Surv. OpenFile Rep.,267, 82 pp., 1966. Earth Sci., 15, 817-822, 1978. 288, 1955. Grantz, A., T. Moore, and S. Roeske, Continent- Lewis,S. D., J. W. Ladd,and T. R. Bruns, Churkin,M., H. L. Foster,and R. M. Chapman, ocean Transect A-3: Gulf of Alaska to Arctic Structuraldevelopment of an accretionaryprism Terranes and suture zones in east central Alaska, Ocean, Geol. Soc. Am. Centennial by thrustand strike-slip faulting: Shumagin J. Geophys.Res., 87, 3718-3730, 1982. Continent/OceanTransect 15 and Explanatory region,Aleutian Trench, GeoL Soc. Am. Bull., Cobbold,P., J. Brun,P. Davy, G. Fiquet,C. Parepict,72 pp., GeologicalSociety of 100, 767-782, 1988. Basile,and D. Gapais,Some experiments on America, Boulder, Colo., 1991 Lonsdale,P., Paleogenehistory of the Kulaplate: blockrotation in the brittle uppercrust, in Haeussler,P., R. Coe, and T. Onstott, Offshoreevidence and onshore implications, PaleomagneticRotations and Continental Palcomagnetismof the Late TriassicHound GeoL Soc.Am. BulL, 100, 733-754, 1988. Deformation,edited by C. Kiesseland C. Laj, Island volcanics:Revisited, J. Geophys.Res., Luyendyk,B., M. Kammerling,R. Terres,and J. pp.145-155, Kluwer Academic, Boston, Mass., 97, 19,617-19,639, 1992. Hornafius,Simple shearof southernCalifornia 1989. Hansen,V. L., Yukon-Tananaterrane: A partial duringNeogene time suggested by Coe, R. S., B. R. Globerman,P. W. Plumley,and acquittal,Geology, 18, 365-369, 1990. paleomagneticdeclinations, J. Geophys.Res., G. A. Thrupp, Palcomagneticresults from Harris, R. A., D. B. Stone, and D. L. Turner, 90, 12,454-12,466, 1985. Alaska and their tectonicimplications, in Tectonicimplications of palcomagneticand Mayfield,C. F., I. L. Tailleur,and I. Ellersieck, TectonostratigraphicTerranes of the Circum- geochronologicdata from the Yukon-Koyukuk Stratigraphy,structure, and palinspastic Pacific Region,edited by D. G. Howell, pp. 85- province,Alaska, Geol. Soc.Am. Bull., 99, synthesisof the westernBrooks Range, 108, Circum-PacificCouncil for Energyand 362-375, 1987. Alaska,U.S. GeoLSurv. Open File Rep.,83- Mineral Resources, Houston, 1985. Hickman, R. G., C. Craddock, and K. W. 779, 58 pp., 1983. Coe, R. S., B. R. Globerman,and G. A. Thrupp, Sherwood,Structural Geology of the Nenana McKenzie, D., and J. Jackson,The kinematicsof Rotation of central and southern Alaska in the River segmentof the Denali fault system, distributeddeformation, in Paleomagnetic early Tertiary: Oroclinalbending by central Alaska Range, Geol. Soc.Am. Bull., Rotationsand ContinentalDeformation, edited megakinking?in PaleomagneticRotations and 88, 1217-1230, 1977. by C. Kiesseland C. Laj, pp. 17-31,Kluwer ContinentalDeformation, edited by C. Kiessel Hickman, R. G., K. W. Sherwood,and C. Academic, Boston, Mass., 1989. and C. Laj, pp. 327-342, Kluwer Academic, Craddock,Structural evolution of the early Meisling,K., M. Gardner,G. Cushing,and S. Boston, Mass., 1989. Tertiary Cantwellbasin, south central Alaska, Bergman,A reconstructionof southernAlaska at 75 Ma (abstract),Geol. Soc.Am. Abstr. Cox, A., Remnantmagnetism of Lower to Middle Tectonics, 9, 1433-1449, 1990. Programs, 16, 769, 1987. Eocenebasalt flows from Oregon,Nature, 179, Hillhouse,J. W., Palcomagnetismof the Triassic 685-696, 1957. Nikolai Greenstone,McCarthy Quadrangle, Moll, E. J., M. L. Silberman,and W. W. Patton, Cox, A., and D.C. Engebretson,Change in Alaska, Can. J. Earth Sci., 14, 2578-2592, Chemistry,mineralogy, and K-Ar agesof 1977. igneousand metamorphic rocks of theMedfra motionof PacificPlate at 5 m.y.B.P., Nature, 313, 472-474, 1985. Hillhouse, J., Accretion of southernAlaska, quadrangle,Alaska, U.S. Geol.Surv. Open File Cox, A., and B. Hart, Plate tectonics:How it Tectonophysics,139, 107-122, 1987. Rep., 80-811C, 19 pp., 1981. works, 392 pp., Blackwell Scientific, Boston, Hillhouse, J. W., and C. S. Gromm6, Limits to Nockleberg,W. J., D. L. Jones,and N.J. northward drift of the Paleocene Cantwell Mass., 1986. Silberling,Origin and tectonicevolution of the Formation,central Alaska, Geology,10, 552- McLaren and Wrangelliaterranes, eastern Csejtey,B., Tectonicimplications of a late 556, 1982. Paleozoic volcanic arc in the Talkeetna Alaska Range,Alaska, GeoL Soc.Am. BulL, Hillhouse, J. W., and C. S. Groinre6, 1983, 96, 1251-1270, 1985. Mountains, south-centralAlaska, Geology,4, Wrangelliain southernAlaska 50 my ago Packer,D. R., Palcomagnetismof the Mesozoicin 49-52, 1976. (abstract),Trans. AGU., 64, 687, 1983. Alaska, Ph.D. dissertation,160 pp., University Csejtey,B., D. P. Cox, R. C. Evarts,G. D. Hillhouse,J., and C. Oromm6,Northward of Alaska, Fairbanks, 1972. Stricker, and H. L. Foster, The Cenozoic Denali displacementand accretion of Wrangellia:new Panuska,B.C., and D. B. Stone, Late Paleozoic fault systemand the Cretaceousdevelopment of palcomagneticevidence from Alaska, J. palcomagneticdata for Wrangellia:Resolution southernAlaska, jr. Geophys.Res., 87, 3741- Geophys.Res., 89, 4461-4477, 1984. of the polarityambiguity, Nature, 293, 561- 3752, 1982. Hillhouse,J. W., C. S. Oromm6,and B. Csejtey, 563, 1981. Eisbacher,G. H., Sedimentologyof the Deazadesh Palcomagnetismof early Tertiary volcanic Panuska,B.C., and D. B. Stone, Latitudinal flyschand its implicationsfor strike-slip rocks in the Northern Talkeetna Mountains,in Motion of the Wrangelliaand Alexander faultingalong the Denalifault, Yukon territory Proceedingsof the CircumPacific Terrane Terranesand the southernAlaska Superterrane, 1208 Redfieldand Fitzgerald: DenalL Fault System, Alaska

in TectonostratigraphicTerranes of the Circum. NorthSlope of Alaska,edited by W. L. Adkison the DenalLfault system,Can. J. Earth Scl,,17, PacificRegion, edited by D. O. Howell, pp. 85- and M. M. Brosgt,pp. L1-L11, American 1527-1537, 1980. 108, Circum-PacificCouncil for Energyand AssociationPetroleum Geologists, Pacific Stout,J. H., J. B. Brady,F. Weber,and R. B. Mineral Resources,Houston, 1985. Section, Tulsa, Okla., 1970. Page,Evidence for Quaternarymovement on the Panuska,B., D. B. Stone, and D. L. Turner, Sainsbury,C. L., and W. S. Twenhofel,Fault McKinley strandof the DenalLfault in the Delta Palcomagnetismof Eocenevolcanic rocks, patternsin southeastAlaska (abstract), Geol. River area, Alaska, Geol. Soc.Am. Bull., 84, TalkeetnaMountains, Alaska, J. Geophys. Soc. Am. Bull,, 6.$, 1300, 1954. 939-948, 1973. Res., 95, 6737-6750, 1990. St. Armand, P., The tectonicsof Alaska as deduced Tailler, I. L., Probablerift origin of the Canada Patton, W. W., and I. L. Tailler, Evidence in the from seismicdata (abstract), GeoL Soc. Am. Basin,Arctic Ocean,in Arctic Geology,edited Bearing Strait region for differentialmovement Bull., 65, 1350, 1954. by M. G. Pitcher,pp. 526-535,Mere. Am. betweenNorth America and Eurasia, Geol. Soc. St. Armand,P., Geologicaland geophysical Assoc. Pet. Geol., 19, 1973. Am. Bull,, 88, 1289-1304, 1977. synthesisof the tectonicsof portionsof Thrupp,G. A., andR. S. Coe,Early Tertiary PayILs,T., V. Sisson,and H. Foster, Mid- BritishColumbia, the YukonTerritory, and palcomagneticevidence and the displacement Cretaceous extensional tectonics of the Yukon- Alaska, Geol. Soc. Am. Bull., 68, 1343-1370, of southernAlaska, Geology,14, 213- 217, Tanana terrane, Trans-Alaska Crustal Transect 1957. 1986. (TACT), eastcentral Alaska, Tectonics, 12, Savage,J., M. Losowski,and W. Prescott,Strain Turner, D. L., T. E. Smith, and R. B. Forbes, 103-122, 1993. accumulationacross the DenalLfault systemin Geochronologyof offsetalong the DenalLfault Plafker,O., T. Hudson,and D. H. Richter, the Delta River Canyon,Alaska, J. Geophys. systemin Alaska (abstract),Geol. Soc.Am. Preliminaryobservations on late Cenozoic Res., 86, 1005-1014, 1981. Abstr. Programs, 6, 268-269, 1974. displacementsalong the Totschundaand DenalL Scholl, D., and A. Stevenson,Tectonic evolution von Huene, R., G. Keller, T. R. Bruns, and K. fault systems,U.S. Geol. Surv. Cite. 751.B, of the Pacific'sAlaska-Bering Sea rim in terms McDougall, Cenozoicmigration of Alaskan B67-B69, 1977. of large-scaleplate-boundary driven terranesindicated by paleontologicstudy, in Plafker,G., T. Hudson,T. Bruna,and M. Rubin, transpressivedeformation, Geol. Soc. Can. TectonostratigraphicTerranes of the Circum- Late Quaternaryoffsets along the Fairweather ProgramsAbstr., 16, All2, 1991. PacificRegion, edited by D. G. Howell,pp. fault and crustalplate interactionsin southern Scholl, D., A. Stevenson,S. Mueller, E. Geist, D. 121-136,Circum-Pacific Council for Energy Alaska, Can. J. Earth Sci., 1.$, 805-816, 1978. Engebretson,and T. Vallier, Exploringthe and Mineral Resources,Houston, 1985. Plafker,G., W. J. Nockleberg,and J. S. Lull, notion that southeastAsian type escape Wallace,W. K., C. L. Hanks,and J. F. Rogers, Bedrockgeology and tectonicevolution of the tectonicsand trenchclogging are involvedin The southernKahiltna terrane: Implications for Wrangellia,Peninsular, and Chugachterranes regionalscale deformation of Alaska and the the tectonic evolution of southwesternAlaska, along the trans-Alaskacrustal transect in the formationof the Aleutian BearingSea region, Geol. Soc. Am. Bull., 101, 1389-1407, 1989. ChugachMountains and SouthernCopper River in SoutheastAsia Structures,Tectonics, and Warhaftig,C., Quaternarygeology of theNenana basin,Alaska, J. Geophys.Res., 94, 4255- Magmatism(abstract), pp. 1-5, Oeodynamics River and adjacentparts of the AlaskaRange, 4295, 1989. ResearchInstitute Symposium, A&M U.S. Geol. Surv. Prof Pap., 293-A, 68, 1958. Plafker, G., C. Naeser,R. Zimmermann,J. Lull, University, College Station, Texas, 1992. Warhaftig,C., D. L. Turner,F. R. Weber,and T. E. andT. Hudson,Cenozoic uplift historyof the Schultz,R., and A. Aydin, Formationof interior Smith, Nature and timing of movementon the Mr. McKinley Area in the CentralAlaska Range basins associatedwith curved faults in Alaska, HinesCreek strand of the DenalLfault system, basedon fissiontrack dating, U.S. Geol. Surv. Tectonics, 9, 1387-1407, 1990. Alaska, Geology,3, 463-466, 1975. Bull., 2041, 202-212, 1992. Stanley,W., V. Labson,B. Nockleberg,B. Wells, R., and R. Coe, Palcomagnetismand PlumIcy,P. W., and R. S. Coe, Palcomagnetic Csejtey,and M. Fisher,The DenalLfault system geologyof Eocenevolcanic rocks of southwest evidencefor reorginizationof the north and Alaska Rangeof Alaska:Evidence for Washington, implications for mechanismsof Cordilleran borderland in Late Paleozoic time underplatedMesozoic fiysch from tectonic rotation,J. Geophys.Res., 90, 1925- (abstract),Geol. Soc.Am. Abstr.Programs, 14, magnetotelluricsurveys, Geol. Soc.Am. Bull., 1947, 1985. 224, 1982. 102, 160-173, 1990. Wells, R., and P. Heller, The relative contribution PlumIcy,P. W., and R. S. Coe, Palcomagneticdata Stone,D. B.,Triassicpalcomagnetic data and of accretion, shear, and extension to Cenozoic from Paleozoicrocks of the Nixon Fork terrane, palcolatitudesfor Wrangellia,in ShortNotes tectonic rotation in the Pacific Northwest, Alaska, and their tectonicimplications on Alaska Geology,1981, Geol. Rep., 73, Geol. Soc. Am. Bull., 100, 325-338, 1988. (abstract),Geol. Soc.Am. Abstr.Programs, 15, Alaska Division of Geologyand Geophysics 428, 1983. Surveys,55-62, 1982. P. G. Fitzgerald,Department of Geosciences, Reed, B. L., and R. L. Elliot, Geochemical Stone,D. B., and W. K. Wallace,A geological Gould-SimpsonBuilding, University of anomaliesand metalliferousdeposits between frameworkof Alaska,Episodes, 10, 283-289, Arizona, Tucson, AZ 85721. Windy Fork and PostRiver southernAlaska 1987. T. F. Redfield,Department of Geology,Arizona Range,Alaska, U.S. Geol. Surv. Circ., 569, 22 Stone, D. B., B.C. Panuska, and D. R. Packer, State University,Tempe, AZ 85287-1404. pp., 1968. Palcolatitudes versus time for southernAlaska, Reed,B. L., andM. A. Lanphere,Offset plutons J. Geophys.Res., 87, 3697-3707, 1982. and historyof movementalong the McKinley Stout, J. H., Regional metamorphismand structure segmentof the DenalLfault system,Alaska, along the DenalLfault, eastcentral Alaska Geol. Soc. Am. Bull,, 85, 1883-1892, 1974. Range(abstract), Geol. Soc.Am. Abstr. (ReceivedOctober 28, 1992; Rickwood,F. K., The PrudhoeBay field, in Programs,4, 678, 1972. revisedMarch 9, 1993; Proceedingsof the GeologicalSeminar on the Stout, J. H., and C. G. Chase, Plate kinematicsof acceptedMarch 17, 1993.)