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Denali Fault System of Southern Alaska an Interior Strikeslip

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Denali Fault System of Southern Alaska an Interior Strikeslip TECTONICS, VOL. 12, NO. 5, PAGES 1195-1208, OCTOBER 1993 DENALl FAULT SYSTEM OF SOUTHERN 1974; Lanphere,1978; Stoutand Chase,1980]. Despitethis ALASKA: 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 Denali 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.
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