TECTONICS, VOL. 12, NO. 6, PAGES 1460-1478, DECEMBER 1993 KINEMATIC EVOLUTION OF THE contractionin outboardsupracrustal rocks, suggesting that the MILLER RANGE SHEAR ZONE, Neoproterozoicto earlyPaleozoic plate margin of Antarctica CENTRAL TRANSANTARCTIC wascharacterized by left-obliqueconvergence in whichstrain MOUNTAINS, ANTARCTICA, AND within the orogenwas partitioned into deep-levelstrike slip IMPLICATIONS FOR NEOPROTEROZOIC and shallow-level contraction. TO EARLY PALEOZOIC TECTONICS OF THE EAST ANTARCTIC MARGIN OF INTRODUCTION GONDWANA The TransantarcticMountains constitute a major orogenic belt in Antarctica,extending for -3000 km betweenthe JohnW. Goodge•, Vicki L. Hansen•, SimonM. Peacock2, present-dayWeddell and Ross seas. Although the modem BradK. Smith3, andNicholas W. Walkera physiographyof the TransantarcticMountains is dominantlya manifestationof Cenozoic rift tectonics[Stem and ten Brink, 1989], its fundamentalunderlying tectonic architecture is Abstract.High-grade ductile tectonites of thePrecambrian largelya resultof its Neoproterozoicto earlyPaleozoic Nimrod Group in the centralTransantarctic Mountains form history. The Neoproterozoic-earlyPaleozoic tectonic the Miller Rangeshear zone (MRSZ). With no exposed evolutionof thisorogen primarily involved interactions boundaries,this zone has a minimum structuralthickness of betweenthe stabilizedEast Antarctic craton and younger 12-15 kin. Shear-senseindicators record consistent top-to-the- tectonicelements along the Antarcticmargin of Gondwana, SE, or left-lateral,shear within the NW striking,moderately anda betterunderstanding of thistectonic history is relevantto SW dippingzone. Cylindricalfolds with axesnormal to evaluatingplate reconstructions that have been proposed as elongationlineation (L ½)are kinematicallyconsistent with modelsfor supercontinentalassembly, fragmentation, and othershear indicators. They may representearly stagesin the reassembly[Dalziel, 1992]. developmentof subordinatenoncylindrical sheath folds, which The TransantarcticMountains are notedfor multiple indicatelocally high bulk ductilestrain and a moderatestrain Neoproterozoicto earlyPaleozoic tectonic events related to gradient.Pervasive, open to tightcylindrical folds with axes riftingand convergence, as are counterparts in Africa and parallelto L½formed during shear and may reflect a component Australia[Stump, 1987]. The centralpart of the orogenin the of constrictionalstrain. Quartz c axis fabricsfrom micaceous NimrodGlacier area, however, is uniquein exposinga quartzitesshow asymmetric single girdles evident of Precambrianmetamorphic terrain of theEast Antarcticcraton dominantlyrhombohedral slip, with limitedbasal-plane slip, thatlies inboardof younger,deformed sedimentary sequences affirmingboth the consistency of shearsense and high-grade (Figure 1). Previously,three orogenic events were recognized syn-kinematicconditions. Deformation did not persistduring on the basisof geologicrelations, the PrecambrianNimrod, subamphibolitefacies cooling, as shownby (1) a lack of Neoproterozoic(?)Beardmore, and Cambro-Ordovician Ross basal-planeslip in ductilelydeformed quartz, (2) a lackof orogenies,although distinguishing these events temporally is quartzsubgrains and grain shape-preferred orientation, and (3) a formidableproblem because of poorage constraints, the presenceof orientedmuscovite "fish" included within discontinuousexposures, and ambiguous structural relations. polygonalquartz. grains, which show that quartz grain bound- In the traditionalview, theNimrod deformation affected high- ariesmigrated and annealed under static conditions following grademetamorphic rocks of theNimrod Group [Grindley and ductile shear. From the uniform Lc orientationand consistent McDougall, 1969;Grindley, 1972], whereassupracrustal rocks shearsense, we interpretthat ductile deformation resulted from depositedalong the outermargin facing the modemRoss Sea a single,kinematically simple, left-lateral (top-to-the-SE) (Beardmoreand Byrd groups) were only affectedby thelater shearevent. Together,the scale,high total strains(? > 5), Beardmoreand Ross contractional deformations [Grindley and fabricuniformity, and the widespreadpresence of asymmetric Laird, 1969; Laird et al., 1971; Stump, 1981; Stumpet al., microstructuresformed at high temperatures,all indicatethat 1986, 1991]. strainrates within the MRSZ were high and that it representsa Recentstudies, however, provide an emergingpicture of majorcrustal structure. Orogen-parallel displacements within punctuatedAntarctic-margin orogenesis during the this zone duringthe latestNeoproterozoic to Early Cambrian Neoproterozoicto Early Ordovicianthat is associatedwith wereat a highangle to penecontemporaneousorogen-normal margin-paralleldisplacements. First, stratigraphic, structural, andgeochronologic data show evidence of multipletectonic eventsduring the latestProterozoic and early Paleozoic that do •Departmentof GeologicalSciences, Southern Methodist not fit into a simpleNimrod-Beardmore-Ross orogenic University,Dallas, Texas. successionlRees et al., 1987; Rowell and Rees, 1989; Rowell 2Departmentof Geology,Arizona State University, Tempe, et al., 1991, 1992; Goodgeet al., 1993]. At leastfour discrete Arizona. tectonicevents that occurred over a time spanof-80 m.y., 3Instituteof EarthSciences, State University of Utrecht, straddlingthe Precambrian-Cambrian boundary, are now Utrecht, Netherlands. recognizedin basementand supracrustal rocks of thecentral 4Departmentof Geological Sciences, Brown University, TransantarcticMountains [Rowell et al., 1988, 1991, 1992; Providence, Rhode Island. Goodgeet al., 1993]. Second,stratigraphic, structural and Copyright1993 by the AmericanGeophysical Union. isotopicevidence indicates that orogenic development involved a componentof strike-slipdeformation in additionto a clear recordof regionalcontraction [Rowell and Rees, 1990; Goodge Papernumber 93TC02192. et al., 199la]. Thusthe concept of a narrowlydefined event 0278-7407/93/93TC02192.$10.00 suchas theRoss orogeny, regarded as the mostpronounced Goodgeet el.: KinematicEvolution, Transantarctic Mountains 1461 rl rl i i i i ! i i i ii i i i i ' _:*-':._]•=::'i• , i i,: .::,::'--= ::..:..;•:: ' -;--= •. ! :,.i )..':i?,•i':.'/ '":'•:'-•:• GeologistsR •& .'i :i•4-T I-:-'"'•• • 190E)----':-:/'••G:•,,q 90 W • •o.Ti•i• ) '• - I .. l•eo ), Nu;,t•k/ _ _0•7.... I. :•• -•m..' ....rodGlacier 'I •-'tSL[•/WEs•DAELL'0ø I . -"-.. '------"-:-/--'-"- _• DevoniantoTriassic >•••,•,\x•\\•x•- • w•..:....:..•........•..•i•i•11!:;:.iNii::tl/i•i!•j.i_i....• •.• t .--.:.-.• ..1•-_-._=_1CambrianBeaconSupergroupOrdovician MillerR. • '•'••C' '••':'•'"''"' "':'' ' :" '' .'" ]-'i:11'!""//'i-:ii-• tliii["•-'"•/''-"'"•""'•'• • CambrianByrd •'• ,' / ,•7---"•'"• '• •""-"•••••••••:•••••••'""::.':'•" ::..-:• Neoproterozoic / '/•/ /-_•;•L.'-_.'•:;!!?'/•:?!37-"/Y' .... !" ./•¾•.-'•i'?•:!'• ICE I.-'•:•';.::-:•Beardmore Group • / •,•, • %•] • • "••••••••••••••"'• .•'I-IPlP • Precambrian '• '" ............... !•,%•",1 ' POL4•• •• '•!':"21[•?i?:' Ross-agefolds • •.. Cenozoicfaults •'_-_-•.--•-••-.,•-•------=-:=-•c.-----__.._••(- 0 20 ..4..0 20 80 100 km •, Nimrodlineations Fig. 1. Generalizedgeologic map of thecentral Transantarctic Mountains in thevicinity of Nimrod Glacier[from Grindley and Laird, 1969]. Arrowsshow trends of representativeNimrod Group mylonitic lineations.Inset shows location in Antarctica;EA denotesEast Antarctica; NG denotesNimrod Group; TM denotesTransantarctic Mountains; and WA denotesWest Antarctica. alongthe entire margin, should perhaps be abandonedin favor TECTONIC SETTING of a diachronousseries of tectonicpulses that may have includednonorthogonal displacements. Rocksof the PrecambrianNimrod Groupcomprise an In thispaper we discussthe structuraland kinematic history isolatedhigh-grade metamorphic complex in the central of L-S (L = elongationlineation; S = foliation)tectonites from TransantarcticMountains, and theyhave no provencounterpart the PrecambrianNimrod Group,exposed in theMiller and in the RossSea sector of the TransantarcticMountains (Figure Geologistsranges of the centralTransantarctic Mountains 1). In theNimrod Glacierregion, exposures of metamorphic, (Figure1). We documentfield relations,ductile shear fabrics, plutonicand sedimentaryunits are limited,due to extensive the relationof thesefabrics to fold development,and kinematic coverby ice, andby generallyflat-lying, Devonian-Jurassic criteriato showthat Nimrod tectonismis bestexplained by a Gondwanaoverlap strata. Precambrian metamorphic rocks and singletectonic event involving synmetamorphic, penetrative, Neoproterozoicto lowerPaleozoic sedimentary units have long andprogressive ductile flow within the 12 to 15-kin-wide beenknown to recordregional tectonism, reflected by Miller Rangeshear zone. This ductileshear zone represents a contrastinglithologic assemblages, abrupt transitions in largecrustal structure with primarilytop-to-the-SE, or left- metamorphicgrade, magmatism, and large-scale fold-belt lateral,displacement that was accompanied by minorNE-SW structures[Gunn and Walcott, 1962;Grindley et al., 1964; constriction.Deformation occurred under high-T conditions Grindleyand McDougall, 1969; Gunner, 1969; Laird et al., within the middle to lower crust,probably at relativelyhigh 1971;Grindley, 1972; Rowell et al., 1988;Goodge et al., strainrates. Theseinterpretations are in contrastto those 1991a, b]. whichexplain Nimrod tectonismby NE directedshortening The majorpre-Devonian lithotectonic units of theNimrod [Grindley,1972].