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

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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]. They alsomodify the notionof a discrete Glacierarea include (Figure 1): (1) theNimrod Group d6collement,including both the Endurance thrust of Grindley [Grindleyand Warren, 1964], a lithologicallydiverse complex [1972]and the ---1-kin-wide Endurance shear zone of Goodgeet of stronglydeformed high-grade gneisses and schists of al. [1991a]. Our structuraldata, in conjunctionwith Precambrianprotolith age [Gunnerand Mattinson, 1975; geochronologicdata [Walker and Goodge, 1991; Goodge and Gunner,1983; Goodge et al., 1990, 1991a;Walker and Daftmeyer,1992; Goodge et al., 1993] and structuralrelations Goodge,1991]; (2)the Neoproterozoic(?)Beardmore Group, a withinoutboard supracrustal rocks, provide the framework for thinsequence of metacarbonateand quartzite (Cobham a tectonicmodel involving Neoproterozoic to earlyPaleozoic Formationof Laird et al. [1971]) conformablyoverlain by obliqueplate interactions and strainpartitioning at different low-grade,unfossiliferous turbiditic greywacke and slate crustallevels manifested by synchronousorogen-parallel and (GoldieFormation of Gunnand Walcou [1962]); (3) theByrd orogen-normaldisplacements. Group [Laird, 1963],Lower Cambrianshallow-water 1462 Goodgeet al.: KinematicEvolution, Transantarctic Mountains carbonates(Shackleton Limestone) [Laird and Waterhouse, post-762Ma andpre-Early Ordovician. Geologic and 1962; Rowell and Rees, 1989; Rees et al., 1989] geochronologicdata have thus cast doubt on theoccurrence of unconformablyoverlain by a sequenceof Middle to Upper a definitiveNeoproterozoic Beardmore deformation. Cambrianclastic units (Starshot, Dick, and Douglas Early studiesof theNimrod Group indicated that these rocks formations)[Skinner, 1964; Rowell and Rees, 1989, 1990]; werecomplexly deformed during a high-grademetamorphic and (4) the 550480 Ma Hopegranite suite, part of the event(Nimrod orogeny of Grindleyand Laird [1969]) thatis regionallyextensive Granite Harbour Intrusive Complex notexpressed in youngersedimentary units [Grindley and [Gunn and Warren, 1962; Gunner, 1976; Faure et al., 1979; Warren, 1964; Grindleyand McDougall, 1969; Grindley, Borg, 1983; Borg et al., 1990; Goodgeet al., 1993]. In terms 1972]. K-Ar metamorphicmineral ages indicated this of lithology,age, metamorphism and structural relations, the deformationmay be as old asNeoproterozoic in age [Grindley Nimrod Groupis uniquewithin the TransantarcticMountains, andMcDougall, 1969]. Goodgeand Dallmeyer [1992] showed althougha metamorphicterrain with similarlithologies exists thatincorporation of extraneousargon was the likely causeof in northernVictoria Land [e.g.,Grew et al., 1984; theNeoproterozoic K-Ar ages,and they provided evidence of Kleinschmidt and Tessensohn,1987; Kleinschmidt et al., Nimrod coolingbelow ,--500øCbetween 525-480 Ma. A ,--1.7 1987]. Rocksof theNimrod and Beardmore groups are not in Ga orthogneissthat crosscuts Nimrod metasedimentary rocks exposedcontact, yet Grindleyand Laird [1969]inferred that indicatesa periodof crypticPaleoproterozoic tectonism Beardmorestrata unconformably overlie the Nimrod Group, [Goodgeet al., 199la], but the mostsevere Nimrod whereasStump et al. [1991] suggesteda correlationbetween deformation,the subjectof thispaper, was in progressby the CobhamFormation and part of the Nimrod Group. An about540 Ma [Goodgeet al., 1993]. unconformable relation between Goldie clastic rocks and The relationshipsbetween these orogenic events remain overlyingShackleton Limestone [Laird, 1963;Laird et al., enigmatic. Doubt aboutthe Goldie-Shackletonunconformity 1971;Laird andBradshaw, 1982; Stump et al., 1991] indicates raisesthe possibilitythat the Ross and Beardmore events are that the Goldie is olderthan Early Cambrian. A gabbroicunit the same,or perhapsclosely related in time. Temporaloverlap associatedwith pillow basaltin the Goldiestrata yielded a Sm- of Byrd andNimrod groupdeformations, although of different Nd isochronage of 762 Ma [Borget al., 1990], indicatingthat structuralstyles and crustallevels, suggests cogenetic the Goldie rocksare probably Neoproterozoic in age. This age tectonism.Thus evidence is mountingfor protractedbut assignmentfor the Goldieassumes, however, that the dated punctuatedNeoproterozoic to earlyPaleozoic tectonism. mafic rocksare volcanicin origin. In additionto timing, the plate tectonicsettings for these Amongthe proposedNeoproterozoic to earlyPaleozoic orogenicepisodes are uncertain.The structuralstyles deformation events in the central TransantarcticMountains, the associatedwith the latestProterozoic to earlyPaleozoic mostconspicuous is theearly Middle Cambrianto Early deformationevents are compatiblewith an overallconvergent OrdovicianRoss orogeny. This regionalcontractional plate-marginregime, perhaps involving subduction and/or arc deformationis recognizedby open,upright folds and a steep accretion.Many workershave suggested that the early eastdipping (toward Ross Ice Shelf)cleavage in Beardmoreand PaleozoicAntarctic plate margin was the site of convergence Byrd supracrustalrocks [Laird et al., 1971;Rowell et al., involvingsubduction of oceaniclithosphere beneath the East 1986; Reeset al., 1989; Stumpet al., 1991; Goodgeet al., Antarctic craton [Elliot, 1975; Bradshawet al., 1985; Gibson 199lb], andit culminatedin regionalCambro-Ordovician andWright, 1985; Borg et al., 1987, 1990; Kleinschmidtand magmatism[McDougall and Grindley, 1965; Gunner and Tessensohn,1987]. Structuralrelations are an importantway Martinson,1975; Gunner,1976; Borg et al., 1990]. Recent to establishthe relativetiming of tectonicevents, and they studiessuggest that the so-calledRoss event, as definedby alsoprovide a measureof geometricand kinematic compati- deformationof supracrustalsedimentary rocks, was punctuated, bility amongevents. Here, we documentthe structuraland involvingas manyas threediscrete pulses of activityduring kinematicevolution of Nimrod Grouprocks that mustbe andafter Early to Middle Cambriandeposition of ByrdGroup establishedin orderto developplausible larger-scale tectonic sediments[Rowell and Rees, 1989; Rowell et al., 1991]. The models. ageof thebroadly defined Ross deformation is constrainedby deformedLower to MiddleCambrian sedimentary rocks MILLER RANGE SHEAR ZONE [Rowell et al., 1992] and the emplacementof-500 Ma posttectonicplutons [Gunner, 1976; also J. Martinson, The NimrodGroup, exposed in theMiller andGeologists personalcommunication]. The Beardmoreorogeny was ranges(Figure 1 andPlate 1), is a metamorphicbasement originallydefined on thebasis of an angularunconformity complexcomposed of schist,metacarbonate, orthogneiss, and betweensedimentary rocks of the Beardmoreand Byrd groups compositionallylayered gneiss [Goodge et al., 1990, 1991a, [Grindleyand McDougall, 1969;Laird et al., 1971; Stumpet b]. Grindleyet al. [1964] gavesimilar map units formational al., 1991], althoughsome of theseboundaries are now status,but we defineda greatervariety of unitson thebasis of recognizedto be faults [Rowell et al., 1986]. Beneaththis rockcomposition and cross-cutting relations. We emphasize, allegedunconformity where it wasidentified south of Nimrod however,that severalof the metamorphicmap unitsshown in Glacier [Laird et al., 1971; Stumpet al., 1991], closeto tight Plate 1 are recognizedon thebasis of theiroverall lithologic foldsin Beardmorestrata are nearlycoaxial with Ross character,and they may be quiteheterogeneous on a smaller contractionalstructures. Stump et al. [1986] suggestedthat scale. Major lithologiesamong these map units include Neoproterozoicintrusive rocks cut deformedGoldie-equivalent peliticschist, micaceous quartzite, amphibolite, banded strata,but reanalysisindicates that these intrusives are quarmofeldspathicto mafic gneiss, homogeneous (garnet-) probablyLate Cambrianto Ordovicianin age (502 + 5 Ma, biotite-hornblendegneiss, granitic to gabbroicorthogneiss, Rb-Sr) [Pankhurstet al., 1988]. Thereforeas presently migmatite,calc-silicate gneiss, and marble. Structurally known,deformation of Goldie rocksis only constrainedto be enclosedwithin Nimrod rocks, particularly along the boundary Goodgeet al.: KinematicEvolution, Transantarctic Mountains 1463 separatingquarmofeldspathic gneiss from schists,are discrete andoutcrop evidence such as mafic mineralselvedges and tectonicblocks (0.5-50 m) of mafic and ultramafic serrate contacks. These features indicate that dike and sill composition(Plate 1). Many mafic blockscontain relict intrusionoccurred at severalstages during ductile deformation. eclogitefacies mineral assemblages [Goodge et al., 1992], as The greatabundance of thesesynkinematic dikes and sills may notedearlier by Grindley [ 1972], but metamorphicgrade in all have thermallyenhanced ductility in someareas. otherrock typesranges from upperamphibolite to lower granulitefacies [Goodge ctal., 1992]. Grindley[ 1972] firstdocumented structural relations of Tectonite Fabrics Nimrod Group rocksin the Miller Range. He describedfive phasesof deformation,the first threeof which he ascribedto Nimrod Grouptectonites contain monoclinic to triclinic the Nimrod orogeny,and the latertwo phasesof faultingand fabricscomposed of tectonitefoliation (S), elongation flexurehe interpretedas minorevents during regional early lineation(Lc), fabricasymmetries, and mesoscopic folds. S is Paleozoictectonism. According to Grindley[1972], structures composedof phyllosilicateand amphibolepreferred orientation formedduring high-grade metamorphism include both NW and in schistsand gneisses, and lenticularfeldspar porphyroclasts NE trendingfight to isoclinalfolds. Two fold generations(F1 in orthogneisses.L• is formedby roddedquartz, crenulated andF2) with NW trendingaxes were interpreted as theresult of mica,and elongate or alignedfeldspar, amphibole, sillimanite NE directedshortening along a structurehe termedthe or kyanite,depending on hostlithology. L• is contained Endurancethrust, placing banded gneisses upon the other units within S (XY plane), commonlycoincident with of theNimrod Group. Grindley'sthird phase of Nimrod compositionallayering. The planenormal to S andparallel to folding(F3), markedby NE trendingfold axesand no apparent L• (XZ plane) commonlycontains microstructural asymmetries axial planeschistosity, was a productof late-stageSE directed thatreflect ductile shear vorticity, and it is thereforecalled the movementfollowing the main-phaseEndurance displacements. motionplane [Arthaud,1969]. The planenormal to both S In contrastto Grindley'sfindings, field studyby Goodgeet and the motionplane (YZ plane) may containa lineation,a al. [ 1991a] showedthat (1) the Endurancethrust is a distributed symmetricplanar fabric, or an axial-profileview of folds. ductile shear zone with a structuralwidth of-1 km, rather than Fabricasymmetry within the motionplane records the a discretethrust surface (referred to by Goodgeet al. [ 1991a]as directionof tectonictransport during ductile deformation [e.g., theEndurance shear zone); and (2) kinematicfeatures within Eisbacher,1970; Berth• et al., 1979; Simpsonand Schmid, thiszone record principal top-to-the-SE, or left-lateral, 1983;Hanmer and Passchier, 1991]. Motion-plane displacement. asymmetriesin the Nimrod tectonitesare observedat the Buildingon thisearlier work, detailed field and petrofabric macroscopic,mesoscopic, and microscopicscales. Shear-sense studyshows that rocks of the NimrodGroup throughout the indicatorsuseful for kinematicinterpretation of Nimrod Miller andGeologists ranges display well-formed ductile L-S tectonitesinclude mesoscopic to microscopicasymmetric folds tectonitefabrics characterized by compositionlayer-parallel with axesnormal to L•; rotatedclasts and winged inclusions shearfoliation (S) and mineralelongation lineation (Lo) (Plate [Hanmer and Passchier,1991]; stretched,imbricated, and 1). S generallydips moderatelyto the SW, andLc plunges locally foldedpull-aparts [Hanmer, 1986]; foliation "fish" gentlyto the NW and SE (seefabric diagrams, Plate 1). [Hanmer,1986]; typesI andII S-C fabrics[Lister and Snoke, DuctileL-S tectonitefabrics are pervasivein the Nimrod 1984]; shearbands [Platt and Vissers,1980], developedmostly Group,and we will hererefer to theserocks generally as in paragneisstectonitcs; o and b feldsparmegacrysts [Passchier Nimrod tectonites;tectonite exposure and dip constraina andSimpson, 1986], developedin orthogneisstectonites; minimum structural thickness of 12-15 km before the fabrics mica,amphibole, and kyanite "fish"; mineral grain shape- disappearbeneath snow and ice. The Nimrod tectonitesformed preferredorientations; and quartz c axisfabrics. In all cases, duringnoncoaxial ductile shear, as developed below, and, theseasymmetries record top-to-the-SE, or left-lateral,shear collectively,they comprisethe Miller Rangeshear zone parallelto Lc (meanN46øW). (MRSZ), a term we introducein orderto conveythe large Nimrodtectonites may be describedgenerally as ductile L-S regionover which these rocks are exposed. We document tectonites,but ultramylonitesoccur within the Enduranceshear relationshipsbelow which showthat tectonitesof the zone. Asymmetricmotion-plane structures are nearly Enduranceshear zone record extremely high strain, and we thus ubiquitousat the mesoscopicscale, and we interpretedthem retainuse of thisseparate term to delineatea zoneof relatively with confidencein the field (Plate 1). The typesof higherstrain within the MRSZ. mesoscopicstructures displayed by the Nimrod tectonites, Nimrodductile tectonism occurred under moderate-P, high-T describedbelow, are typicalof thoseseen in otherlarge-scale conditions(P _>8 kbar;T = 700øC)in the upperamphibolite shearzones [e.g., Davidson,1984]. Althoughtectonite fabrics to lower granulitefacies, as shownby synkinematickyanite + displayinginterpretable kinematics occur throughout the garnet+ muscovite+ biotite+ quartzin pelites,hornblende + Nimrodcomplex, fabric intensity varies greatly at the outcrop plagioclase+ garnet+ clinopyroxene+ clinozoisitein mafic andlarger scales. This fabricheterogeneity may be theresult rocks,and by thermobarometry[Goodge et al., 1992]. Late of variationsin eitherstrain intensity or lithologicrheology, synkinematicgrowth of sillimaniteafter kyanite reflects and we are unable to differentiate these mechanisms. However, waningdeformation along a decompressionpath. A varietyof the ubiquitouspresence of tectonitefabrics indicates that the graniticto pegmatiticdikes and sills crosscut the Nimrod entireNimrod complex forms a singlelarge-scale shear zone, tectonites;these intrusive bodies display variable L-S fabric the locusof which may be representedby theEndurance shear formation,and they are planarto boudinagedor folded. zone,but the boundariesof which are not exposed. Formationof somepegmatitic melts may havebeen related to Asymmetricand imbricatedpull-aparts, including both local in situ partialmelting, consistent with the high-grade boudinageand pinch-and-swell structures [Hanmer, 1986], metamorphicconditions that accompanied shear deformation occurthroughout the MRSZ. Thesestructures are locally 1464 Goodgeet al.: KinematicEvolution, Transantarctic Mountains associatedwith SE vergentfolds, and themselves record SE andcompositional layering that are boudinaged, broken, folded, directeddisplacement. Pull-apans are preserved in fourstages stacked,or warped,often pinching off alonglayer strike. One of formation:(1) pull-apartboudinage of a rigidlayer; (2) layermay display folds or pinchingand swelling along its rotationof pull-apansinto an en echelon series, indicating the length,whereas an adjacentlayer is boudinaged,broken, and vorticityof noncoaxialshear (Figure 2a); (3) imbricationand stacked.In eachcase, brittle failure is withincertain layers and stackingof tabulardetached boudins; and (4) deformationof the comprisespart of a largerregime of ductilefailure. These adjacentductile layer, and locally the boudins themselves, into fabricheterogeneities provide evidence that local brittle failure asymmetricSE vetgentfolds. Pull-apartsobserved within the accompanieddominant ductile tectonism. MRSZ rangein sizefrom 1-10cm rotatedamphibolite inclusions(Figure 2b) to relicteclogitic blocks ranging from Folds 1-50 m in size (Figure2c). Foliation"fish" [Hanmer, 1986], alsocommon within the MRSZ (Figure2d), allow confident Threetypes of foldsoccur within the MRSZ (Figures3 and kinematicinterpretation in thefield. 4): (1) pervasiveopen to tight,cylindrical folds of Types I and II S-C fabrics [Lister and Snoke, 1984] in compositionallayering with axesparallel to L•, presentat the granodioriteorthogneiss, and paragneiss and schist, mapto hand-samplescale; (2) SE vergentcylindrical folds respectively,consistently record top-to-the-SE displacement. withaxes normal to L•, observedat themacroscopic to Kinematicallycompatible shear bands [Platt and Vissers, microscopicscale; and (3) subordinatenoncylindrical sheath 1980]are most common in paragneiss,yet they are developed foldselongated parallel to L•, foundat themesoscopic scale. locallyin orthogneissas well. Type I S-C fabrics, We referto thesefolds as types1, 2, and3, respectively. documentedin theCamp Ridge orthogneiss of theEndurance Map-scaletype I foldsdeform compositional layering as shearzone [Goodge et al., 1991a,Figure 3], arealso observed open,gently NW plungingto subhorizontalfolds in the in otherorthogneiss units. Largeorthogneiss bodies of this Geologistsand northern Miller ranges,and open to close, typehave semitabular shapes that appear to parallel the gentlyNW plungingfolds in thesouthern Miller Range(Plate regionalmetamorphic foliation (Plate 1). Someorthogneiss 1). Thesefolds show no preferred vergence or asymmetry; units,however, display less strongly formed tectonite fabrics theyverge NE in theGeologists and northern Miller ranges, characterizedchiefly by linearelements. The presence of S-C andSW in thesouthern Miller Range(Figures 3a and3b). fabricsin orthogneissunits indicates that their parent igneous Locally,outcrop and hand-sample scale type 1 foldsare bodiesintruded prior to, or during,Nimrod tectonism. extremelycomplex with a largevariation in axialplane Althoughsome orthogneiss bodies in the Miller and orientation(Figure 3c), yetdue to thecylindrical nature of Geologistsranges appear petrographically and texturally thesefolds the associated L•-parallel view in eachcase is similar,they are not all of thesame age. TheCamp Ridge structurallysimple (Figure 3d). Mesoscopictype l folds granodioriteorthogneiss that contains strong type I S-C fabrics displayopen to tight interlimbangles. In somecases, well- withinthe Endurance shear zone yielded a zircon2ø7pb?ø6Pb developedL•-parallel rods or mullionsare exposed on fold-limb dateof-l.7 Ga [Goodgeet al., 1991a],whereas a granodiorite surfaces.Rootless type 1 folds,refolded by otherfolds of type orthogneisswith similarigneous protolith characteristics and I geometry,occur locally. Type 1 foldsalways deform tectonitefabrics in thesoutheastern Miller Rangenear Orr compositionallayering, which in manycases is parallelto Peakyielded a zirconU-Pb date of-540 Ma [Goodgeet al., tectonitefoliation (S). 1993]. Type 1 foldsdisplay three different relationships with S as Orthogneisstectonites of theMRSZ alsodisplay feldspar o observedin axial profile. porphyroclasts[Passchief and Simpson, 1986] that record SE 1. Openfolds deform coplanar compositional layering and directed shear. Within the Endurance shear zone both •Jand o S. A seriesof suchfolds resembles a corrugated surface. porphyroclastsrecord sinistral, or SE-vergent,vorticity. In 2. Foldsdeform compositional layering, locally with orderfor 5 porphyroclaststo form, the ratio of recrystallization coplanarS, but an axial planartectonite foliation is also rate to deformation rate must be low and shear strain values developed.These relations occur typically in fightto isoclinal mustbe high( ¾> 5-10) [Passchierand Simpson, 1986]. examples. Becausemineral re. crystallization rates were probably relatively 3. Tightto isoclinalfolds of compositionallayering, highin thiscase, given the high-temperaturesynkinematic containedwithin phacoidal domains, are truncated by discrete conditions[Goodge et al., 1992],the presence of 5 shearzones that are asymptoticto theirlimbs. porphyroclastsindicates relatively high strain rates during Type2 foldsare common within the MRSZ, but they are deformation within at least the Endurance shear zone. presentchiefly within a 5-km-thickzone centered approxi- Themotion-plane views of thesetectonites are relatively matelyon the Endurance shear zone near Camp Ridge and simple,comprised of foliationswith or withoutfolds and rarelyin otherareas (Plate 1). Type2 foldsare cylindrical, microstructuresindicating consistent shear sense. In contrast, tightto isoclinalfolds with axesnormal, or at a highangle, to theplane perpendicular to foliation and !ineation may display L•. S is coplanarwith type2 axialplanes. These structures, fabricdisruptions on the scale of a handsample to a thatof a whichfold compositional layering and, locally, tectonite cliff face.These heterogeneities take many forms: (1) foliation,display consistent SE vergencewith axial planes at a foliationphacoids, in whichfoliation and compositional shallowangle to S (Figure3e). Type2 foldsare observed at layeringare truncated along discrete shear surfaces resulting in outcrop,hand-sample, and microscopic scales. In outcroptype lensoidaldomains (Figure 2e); (2) discreteblocks, which 2 foldsare most commonly associated with strongly themselvesare foliated and locally boudinaged, enclosed in a contrastingcompositional layers [e.g., Goodge et al., 1991a; secondtectonite foliafion that wraps completely or partially Figure3], andthey are locally associated with pull-apart or aroundthe foliated clast (Figure 20; (3) foliafionthat is folded boudinagestructures. The consistent SE vetgentasymmetry aroundlineaftøn-parallel folds, described below, or foliafion viewednormal to L• indicatesthat these folds formed during

Goodgeet al.: KinematicEvolution, Transantarctic Moufitains 1467

..o

Fig. 2. Photographsof asymmetricmotion-plane structures or fabricsthat are usedfor determiningshear sensewithin the MRSZ tectonites.All photographsviewed to SW, unlessnoted. (a) Tabularmafic- layerboudins showing tilt to form en echelonseries resulting from progressiveductile shear after initial separation.Hammer is 33 cm long. (b) Detachedtectonic inclusion of amphibolitein calc-silicate gneiss,showing sinistral rotation. Inclusionis 4 cm in diameter. (c) Mafic boudinswithin layered tectonites,showing detached and tailed form. Theseboudins contain relict eclogiticmineral assemblages.Hammer is 33 cm long. (d) Asymmetricfoliation "fish" of amphibolitein light-colored calc-silicatehost. Note planartail truncafionsparallel to S. Pen is 14 cm long. (e) Outcropof foliationphacoid (center) in whichfoliafion and compositional layering are truncated along discrete shear surfacesat top and bottom. Personstanding in upperleft corner. View is to NW downplunge of L•. (f) Outcropshowing elliptical block (center) that is foliatedand boudinaged (light-colored layer), and whichis enclosedby a secondtectonite foliation that wraps around it. Hammeris 33 cm long. View is to NW down plungeof L•. Fig. 3. Axial-profileviews of foldsin Nimrodtectonites. Type 1 foldsshown in Figures3a-3c, lookingdown the plunge of Leto theNW. (a) Type1 foldsexposed along Endurance Cliffs in the GeologistsRange, showing steep to moderatelydipping axial planes and generally NE foldvergcnce. Cliff faceis about300 m high. (b) Type1 foldsexposed along Gerard Bluffs in thesouthern Miller Rfinge.Folds in thiscliff face are subordinate tolarger NW plungingfolds shown on Plate I. Note steepto moderatelydipping axial planes and SW fold vergence. Outcrop is about 200 m high.(c) Type 1 mesofoldsin layeredgneiss, showing variably oriented axial planes and no preferred fold vergence. Notestructural complexity and layer discontinuity due to boudinage.Outcrop height is about1 m. (d) Motion-planeview (looking SW) of outcropshown in Figure3c at samescale. Note extreme planar aspectto layering,general structural simplicity, and left-lateral (top-to-the-SE) fabric asymmetry in centraldark layer. Outcrop height is about1 m. (e) Motion-planeaxial-profile view of type2 mesofoldsin layeredschistose tectonites (looking SW). Axialplanes are nearly parallel to S in tectonites,and folds show SE vergence. Sunglasses are 14 cm long. (f) Type3 noncylindricalsheath foldsviewed down tectonite L• (to NW). Notenear complete closure of foldlimbs, tight to isoclinal intefiimbangle, and foliation-parallel axial plane. Other type 3 foldsshow complete limb closure and SE vergentasymmetry. Goodgeet al.: KinematicEvolution, Transantarctic Mountains 1469

$ $

W

T•pe2 b Type 1 Type 3

Fig.4. Blockdiagrams summarizing different mesoscopic fold types relative to geometry of L-S tectonitefabrics. Open arrows show sense of ductileshear. (a) Opentype 1 cylindricalfolds with axes parallelto elongationlineation (L ½that) signify minor L ½-normalstrain during dominant L ½-parallel flow. (b) Asymmetrictype 2 cylindricalfolds with axes normal to L½which, may evolve to noncylindricaltype 3 sheathfolds with nosesparallel to L½at highstrains. noncoaxialductile shear, and their asymmetry records shear- ductiletectonism. Types 2 and3 foldsare themselvesin inducedvorticity. Some folds of type2 styleare refolded by kinematicagreement, and they are consistent with top-to-the- relativelyyounger type 2 foldswithin the Endurance shear SE (left-lateral)displacement parallel to L½.In addition,these zone.We interpretthat the refolded type 2 foldsreflect foldsdisplay SE vergenttectonite fabrics on eachlimb as particularlyhigh shear strains resulting from progressive SE viewedwithin the motion plane. Therefore we interpretthat directed flow. thesetwo fold typesformed during ductile shear deformation. Type3 foldsoccur locally within the Endurance shear zone, Foldssuch as these are common in shearzones [e.g., Cobbold andthey are most common at theboundary between andQuinquis, 1980; Hanmer and Passchier, 1991]. The compositionallydistinct units such as felsiteand biotite proximityof types2 and3 foldsto theEndurance shear zone gneiss,or marbleand mica schist (Figure 313. They are providesevidence of relativelyhigher strain within this noncylindricalsheath folds [Quinquis et al., 1978;Cobbold subzone of the MRSZ. andQuinquis, 1980], with axial planes parallel to S, isoclinal Type1 folds,which fold compositional layering and, intefiimbangles, and asymmetric profiles in sectionsparallel locally,S, showno preferred overall asymmetry and their fold to L½.Rarely observed profiles normal to L½show limb axesconsistently parallel L ½.L ½-parallelfolds are common in closure;where exposure permits a three-dimensionalview, Lc- large-scaleshear zones [Christie, 1963; Eisbacher, 1970; Bell, parallelprofiles record top-to-the-SE (left-lateral) ductile flow. 1978; Mattaueret al., 1981; Bell and Hammond,1984; Thesefolds transpose compositional layering and S, andthey Sylvesterand Janecky, 1988; Hansen, 1989], particularly those probablyrecord extreme ductile shear strain within the characterizedby largebulk shear strains. If all type1 foldshad Enduranceshear zone (? > 10) [Cobboldand Quinquis, 1980]. formedby rotation of preexistingfolds during shear [e.g., Bryantand Reed, 1969; Bell, 1978],fold interlimbangles Foldtiming. Deformation of Nimrodmetamorphic rocks wouldbe expectedto tightenduring rotation, resulting in tight produceda complicated set of structures,including not only the to isoclinalcolinear folds [e.g., Escher and Watterson, 1974; shearfabrics and folds described above, but also geometrically Bell, 1978;Skjernaa, 1980; Jamieson, 1987]. Mosttype 1 complexre-folded and/or rootless folds observed locally. These folds,however, exhibit open to closefold profiles; therefore latterstructures may have formed either prior to orduring thesetype 1 foldslikely do notpredate ductile shear progressiveductile shear deformation, but we are unableto deformation.Because S is locallydeformed by type1 folds, documentan unambiguousrecord of presheardeformation. somemust have formed after at leastthe earliest stages of Likewise,some type 1 foldswith tight interlimb angles could noncoaxialshear. If thetype 1 foldshad formed after ductile predateductile shearing with their present geometry due to shearceased, opposing shear sense should be preserved on structuralreorientation in the MRSZ. However,because of oppositefold limbs. However,opposite limbs in all cases thenature of progressiveductile deformation, we cannot prove thatwe examinedrecord the same top-to-the-SE shear. eitherthat they are the product of anearlier tectonic event or Thereforetype 1 foldsmust have formed during Nimrod thattheir present orientation within the MRSZ hasany tectonismwith theiraxes initially parallel to L½.Axes of type significancewith respect to priordisplacements. 1 folds,like L½,thus approximately represent the maximum Despitethese ambiguities, the three types of foldsdescribed elongationaxis of finite strain. aboveare each interpreted to haveformed during Nimrod Type1 foldscorrespond geometrically and descriptively to 1470 Goodgeetal.: KinematicEvolution, Transantarctic Mountains

Grindley's[1972] early phase(F• andF2) folds,which he TypesI andII S-C fabricsare interpretedwith confidencein attributedto NE displacementalong the Endurance thrust. thin section(Figures 5d to 5f). Kyanite"fish" in pelitic From the distributionof thesefolds throughout the MRSZ, tectonitescommonly show tapered ends that terminatealong C well awayfrom theEndurance thrust as mapped by Grindley planes,and locally they are bent, showundulose extinction, [1972], and the generallack of systematicvergence, we andare segmentedby cross-grainfractures (Figure 5d). These interpretthat the type 1 foldsdid not form in responseto texturesindicate prekinematic to synkinematicgrowth of majorNE directeddisplacement. Rather, they formed as a kyanite. Sillimaniteneedles or bundlesare bentinto resultof regionalNE-SW constrictionperpendicular to the parallelismwith C planes(Figure 5e). Sillimanitegenerally directionof penetrativetranslational shear, perhaps lacksundulatory extinction and broken fibers, indicating that accommodatedby movementalong minor shearsnormal to Le sillimanitegrowth was synkinematic to postkinematic. (Figure4a). Furthermore,it is importantto emphasizethat Sillimanitewas never observed overprinting asymmetric fabric type 1 fold axial profilesare viewedwithin the planenormal to elements;however, sillimanite commonly replaces kyanite. tectoniteLe and not within the motionplane; hence these folds Thussillimanite growth was probably late synkinematic. do not record shear sense. Smalland large garnets alike exhibitellipsoidal shapes with Furtherevidence for establishingthe nature and timing of longaxes inclined, together with pressureshadows, in the foldformation comes from examination of widespread directionof shearas in Figures5c and5e. Kyaniteand pegmatiticdikes and sills. Theseintrusive rocks, typically muscovite"fish," and type II S-C fabricsin garnet-kyanite- containingquartz + K-feldspar+ biotite+ garnet+ tourmaline, muscovitepelitic schist, provide evidence that high-grade arecommon within orthogneisses, schists, and gneisses. They metamorphicconditions accompanied the strongnoncoaxial areparticularly abundant near zones of migmatiteand relatively ductiledeformation (Figures 5d to 50. The synkinematic isotropicgranite, but theyare alsocommon as swarmswithin assemblagekyanite + muscovitemay existat T = 700øConly themetamorphic tectonites, both concordant and discordantto at moderateto high P; hencemuscovite remained stable even S. Dikesand sills range in sizebut most commonly are <1 m at the hightemperatures of deformationthat at lowerP might wide. Pegmatiticdikes and sills show a texturalrange from inducereaction to anhydrousphases. Despite well-preserved unmodifiedigneous textures to strongL-S tectonitefabrics shear-senseindicators, all samplesobserved in thin section concordantto wall-rockfabrics. In somelocations, dikes show lack strongquartz grain shape-preferred orientation. Quartz, successivestages of foldingand boudinage, in whichearly chieflypreserved as equidimensional polygonal grains with formed dikes that have been folded and dismemberedare straightto coarselyserrated grain boundaries, generally crosscutby younger,generally planar dikes that are subparallel displaysflat-field extinction (e.g., Figures 5a and5b). to the youngestshear foliations. The foldeddikes are geometricallyand stylistically similar to thetype 1 folds Quartz c Axis Fabrics observedin layeredtectonites, as shown by fold-axis parallelismwith L•, a lackof asymmetry,and open interlimb We analyzedquartz c axisfabrics in 12 specimensof angles.We considerall of theseintrusive types to be quartziteselected from differentstructural levels throughout the generallysynkinematic, although some of the moststrongly MRSZ (Plate 1 andFigure 6). Sampleswere cut in two deformedexamples could predate ductile deformation by a perpendicularsections, parallel to themotion plane and normal considerableperiod. These relationships are evidence of to S andL• (theXZ andYZ planes,respectively). Quartz c multipleinjections of pegmatiticigneous melts during folding axisorientations were measured optically using the universal and ductile shear deformation within the MRSZ. stage[Turner and Weiss, 1963]. Data measuredin the YZ planewere rotated into the XZ planeand are shown together in Microstructural Fabrics lower-hemisphereprojections with theXZ planeas the perimetergreat circle (Figure6). Microscopicexamination of Nimrod tectonitefabrics MRSZ c axisfabrics indicate that L• is indeedan elongation confirmsthe left-lateral displacement sense interpreted in the linearion(i.e., thatit is parallelto X) becausethe fabric girdle field throughoutthe MRSZ. The form andorientation of for eachsample intersects S normalto Lc [Behrmannand Platt, mica,kyanite, and amphibole "fish" in typeII S-C mylonitic 1982]. Furthermore,quartzite c axisfabrics show well-defined quartzite,pelite, and amphibolite provide evidence of asymmetricsingle girdles which confirm that noncoaxial proannealing,noncoaxial, left-lateral deformation. ductileflow wasin a top-to-the-SE,or left-lateral,direction Intragranularand intergranular muscovite crystals in [e.g.,Lister and Williams, 1979;Schmid and Casey, 1986]. (garnet-)micaquartzite are needle-shaped with their long axes In additionto kinematicinformation, quartz c axisfabrics parallelto L•, or parallelto theinterpreted direction of alsoprovide a meansto qualitativelyevaluate relative thermal maximumfinite strain(X; Figures5a and5b). Despitethe conditionsand finite strainwithin the MRSZ. Quartzc axis extremeaspect ratio (e.g., x:y:z = 30:3:1)of theirgrain patternsare a functionof intracrystallineslip, temperature, shapes,locally preserved populations of asymmetricmica watercontent, strain rate, recrystallization rate, and "fish" record consistent left-lateral shear sense. Preferred deformationpath [Tullis et al., 1973; Nicolasand Poirer, mineralgrain shape is alsoshown by garnetobserved in 1976; Lister and Hobbs, 1980; Hobbs, 1985; Law, 1990]. garnet-biotitequartzite (Figure 5c). In theserocks, rounded Asymmetricsingle-girdle fabrics can form as theresult of an garnetsshow aspect ratios of 1.5:1:1 to 3:1:1. Within the increasingnoncoaxial component of strain,or as a resultof motionplane, the long axes of thegarnets consistently plunge increasingstrain during simple shear [Schmid and Casey, NW relativeto S, andthey record a top-to-the-SEdirection of 1986]. Within the MRSZ, the only indicationof prominently shear.Asymmetric biotite pressure shadows on garnetreflect higherrelative strain is within the Enduranceshear zone as similarshear sense in thesequartzites, as do quartzc axis markedby thepresence of sheathfolds, ultramylonite, and orientations(see below). extremelyattenuated asymmetric fold profiles. Elsewhere,S ' -,•"'...... •" ':-•.•t ".':' '"'*•' .:"'-:' •'-'.s '*,,'• -"*.• ½' - . ..

o,.-..;.,•,..•;..•..•....-•:•?,,. :•!'.'•,,• ,•'.:-...,-'!;:•-;:4...... ':'""% ....:...... :•.,.•...-..,.-.•. ..,., - ,½...... •.. ½:--. ..:.-"• , .,...• -...;..

• • -.

• ...::- • - -• • • %•.:.•-. • ..%.,'•. , ....•.;.,•;••

....•?..•"',., ..... :::•<'-".J?"•"*%• •:,•...'--•: . 4'

'%• ½ .:...... • ...... 4..... , , •, - ,. j•;•--.. ß...... -,-,-•-...... ,-"•:•'.','-'-•.•.... .•;.• ß ...... •..,...... •.= .. , . •. . .

:•;:•. ...• .... . -..::.• .....

.i;. • ...... •.•..... '":• ,• '-.•.½....,,,,,.•*•.•-•...• "•;•-......

'":'.• ::;•,•:& ½• .... - ' '½"- ;"•-..L •"• ....•...... '"' ;': •.• •'• •... • ..... - .• ß ...... •.•.• "......

Fig. 5. Photographsof Nimrod tectonite microstructures. All photographsare parallel to themotion plane(XZ) unlessnoted. (a) (Garnet-)muscovite quartzite (89GGR14D; crossed nicols; 8 mmwide), withmuscovite grain shape showing extreme aspect ratio (up to 30:1)with long axis parallel to S, and intragranularnature of muscovitein quartz.Quartz exhibits flat-field extinction and relatively straight, locallypolygonal, grain boundaries. Rare very fine-grained muscovite "fish" preserved in lowerright comershow dextral (top-SE) shear as viewed. Quartz c axisfabric diagram for thissample shown in Figure6a. (b) View of quartziteshown in Figure5a lookingdown Lc in sectionperpendicular to motion-plane(XY). Crossed-nicols;5 mm wide. Notesmaller aspect ratio of muscovite(2:1 to 5:1) thanthat shown in motion-planesection. (c) Garnet-biotitequartzite with asymmetricbiotite pressure shadowson ellipsoidal garnets. Plane-polarized light; 10 mmwide. Garnetshave long axes inclined relativeto S in thedirection of shear(dextral as viewed), and they are circular in thesection normal to themotion plane and Lc. Quartzgrains are equidimensional polygons with flat-field extinction in crossed-nicolsview. Biotiteis preservedin bothintragranular and intergranular positions. (d) Gamet- kyanite-muscoviteschist showing left-lateral type II S-C fabricsand kyanite crystals (dark gray) that are tapered,bent, and segmented. Plane-polarized light; 16 mm wide. (e) Garnet-sillimanite-muscovite schist.Sinistral shear sense indicators include grain shape-preferred orientation of ellipsoidalgarnets (black),garnet pressure shadows of quartz,and type II S-Cfabrics, deforming sillimanite. Crossed nicols view; 16 mmwide. (f) Gamet-kyanite-muscoviteschist showing kyanite and muscovite "fish" and type II S-C fabrics. Shearsense is dcxtralas viewed. Plane-polarizedlight; 16 mm wide. 1472 Goodgeet al.: KinematicEvolution, Transantarctic Mountains

89GGR14D 0.5, 1.5, 3, 89GGR22E 0.5, 2, 4, 89VGR32A 0.5, 1,2, 3, (500).•••-'""•-•6,8 , ,

a) c)

89GMR37 0.2, 1,2, 3, 4, 89GMR48E 0.5, 1, 2, 3, 89GMR51D 0.2, 1,2, 3,

d) e)

89GMR68.7 0.2, 3, 6, 9, 89VMR100B 0.2, 1,2, 3, 89VMR100B 0.5, 2, 4, 6, , , ,5,6,7

g)

90VMR120 0.2, 1.5, 3, 4.5, 90VMR135 0.5, 1,2, 90VMR136C 0.2, 2, 4, 6, 8,

j) • k) I)

Fig. 6. Lower-hemisphere,equal-area stereonet projections of quartzc axisfabrics from selected quartzite tectonitessampled at differentstructural levels in theGeologists and Miller ranges(locations shown on Plate1). Eachdiagram shows combined c axesmeasured from both the motion plane (XZ), andthe planenormal to themotion plane and Lc (YZ). Numberof totalc axesmeasured in parentheses. Densitycontours indicated by thenumbers to theupper right of eachplot, in percent.Fabric diagrams shownin a structuralreference frame with respectto S (horizontalline) andL ½(dot; downplunge direction).Paired arrows indicate shear sense determined from the inclination of thec axisgirdle to S. Diagramsplotted using a sphericalGaussian function. andL• appearequally well developed,and on thisbasis, we resultof high-strainsimple shear (7 > 5) undermedium- to interpretno obviousvariation in strainintensity within the high-temperatureconditions. Medium- to high-temperature bulkof theMRSZ. However,several quartzite samples simpleshear is consistentwith the simultaneousdevelopment displayequally well-developed single-girdle c axis fabrics of muscoviteand kyanite "fish,"and with estimatesof despitetheir relative proximity to, or distancefrom, the temperatureduring Nimrod tectoniteformation [Goodge et al., Enduranceshear zone (Figures 6a, 6b, 6d, 6f, 6g, 6h, 6i and 1992]. 6k). By comparisonwith simulatedfabric diagrams [Jessel and The activityof differentslip systemsin quartzduring Lister, 1990],we interpretthese single-girdle fabrics as the ductiletectonism is principallyrelated to temperature,strain Ooodgeet al.: KinematicEvolution, Transantarctic Mountains 1473 rate, and water content[Hobbs, 1985]. The dominantsingle- temperatureductile deformation and syntectonic to posttectonic girdle c axis patternsin MRSZ quartzitesare evidenceof magmatism.Thus ductile deformation was diminishing during principaldeformation by rhombohedraland/or prismatic slip the intervalbetween about 540 to 520 Ma, althoughwhen this [Schmidand Casey,1986]. Many MRSZ quartzc axis fabrics deformationbegan is unknown.Deformation in thisinterval alsoshow a lack of polesalong or nearthe greatcircle at a duringthe Early Cambrian(based on new criteriafor the small angleto Z (Figures6a, 6b, 6d, 6f, 6g, 6j and 61). An Precambrian-Cambrianboundary) [Compston et al., 1992; absenceof polesof thisorientation indicates a lack of basal- Cooperet al., 1992]is broadlysynchronous with contractional planeslip [Schmidand Casey, 1986], which occurs principally deformationof Byrd Groupsupracrustal rocks to the east. at moderateto low temperaturesof deformation[Tullis, 1977; Tectonicimplications of thistemporal overlap are discussed Jesseland Lister, 1990]. The lack of evidentbasal-plane slip below. is consistentwith the interpretationof high-T deformation,but it also impliesthat Nimrod tectonitesdid not experience Strain Rate intracrystallinestrain at lower subpeaktemperatures (i.e., duringthermal retrogression). This providesindirect, yet A uniformLc orientationand widespread,consistent shear important,evidence that crystal-plasticflow endedwhile the sense indicate that ductile deformation in the MRSZ resulted Nimrod tectoniteswere still at high temperatures.These from a single,kinematically simple, top-to-the-SE shear relativeconstraints on the kinematicand cooling histories of event. Generalshear strains within the MRSZ were probably the Nimrod tectonitesare valuablefor decipheringpossible moderateto high (? > 5), and locally very high (? > 10), as tectonicpaths of deformation,as discussedbelow. indicatedfrom structuressuch as type 3 sheathfolds, extreme Quartzwithin eachof the measuredsamples occurs as fold attenuation,ultramylonite,/5 porphyroclasts, and quartz c polygonalgrains that lack grainshape-preferred ohentation and axis fabrics. High relativestrain rates may alsobe showchiefly flat-field extinction(e.g., Figures5a and b), and qualitativelyinferred. MRSZ fabricscomprise a thickzone intragranularmuscovite "fish" are common.However, with greatgeometric uniformity and consistency of muscovitegrains probably pinned quartz grain boundaries asymmetricmicrostructures formed during high-temperature duringdeformation, and they were mostlikely intergranular ductiledeformation, in placesassociated with synkinematic duringductile flow. The preservedquartz textures may be the magmatism.S ynkinematic mineral assemblages indicate that resultof movementof grain dislocations,and grain and upperamphibolite to granulitefacies conditions (T -• 700øC) subgrainboundaries, resulting in annealingof petrographic accompaniedductile deformation [Goodge et al., 1992]. The textures.Annealing probably postdated ductile deformation preservationof well-developedmicrostructural ductile shear becausequar• grainslack grain shape-preferred orientation. fabricsrequires that the ratio of strainrate to mineral Thereforewe suggestthat quartz c axisfabrics record the recrystallizationrate be high,where recrystallization rate is ductileshear, whereas quartz, petrographic textures resulted phmarilya functionof temperature.Nimrod tectonites formed from postkinematicthermal annealing. This interpretation at temperaturessufficient to promoterapid mineral wouldexplain the polygonal quartz grain boundaries, flat-field recrystallization.In orderto preservea microstructuralrecord extinction,a lack of grainshape-preferred ohentation, and only of noncoaxialshear, high-T shear zones must undergo strain at minor subgrainformation, as well as intragranularmuscovite, a fasterrate thantheir lower-T counterparts;if not, suchzones andit is consistentwith the lack of evidencefor intracrys- wouldlikely notpreserve asymmetric microstructures. Thus talline slip along quartzbasal planes. Thus coolingappears to thewell-preserved, high-T microstructures described here are havefollowed, rather than accompanied, deformation. compatiblewith high strainrates.

TemporalRelations DISCUSSION

The absolutetiming of ductiledeformation is constrained To summarize structural relations within the MRSZ, by mineralgeochronometer data (available age data from NimrodGroup tectonites were ductilely deformed by Nimrod unitsare shownin Plate 1). Goodgeet al. [1993] dominandynoncoaxial shear within a broadzone (12-15 km describedseveral Nimrod orthogneiss units that exhibit wide) undermoderate-P, high-T conditions in the upper variablyformed ductile deformation fabrics, ranging from amphiboliteto lower granulitefacies. Althoughthe orthogneissbodies displaying well-formed L-S fabrics boundaresof thiszone are not observed,its exposedstructural kinematicallyconsistent with theirhost tectonites, to weakly widthis comparablewith thatof theGrenville Front Tectonic deformedorthogneiss bodies expressing only linearor planar Zone [Davidson, 1984] and the Great Slave Lake ShearZone fabrics,to pegmatitesills containingno solid-stateductile [Hanmer,1988]. L-S tectonitefabrics are pervasive,and only deformationfabrics. U-Pb agedata obtained from igneous minorstrain gradients are apparent.Relative displacement zirconsin theserocks indicate a periodof progressivelywaning acrossthe zonewas left-lateral,or top-to-the-SE,accompanied ductiledeformation between about 540 and 520 Ma. Kyanite- by localperpendicular constrictional strain (Figure 7). On the zonepelitic schistscontaining well-formed L-S fabricscontain basisof well-formedmicrostructures and quartz c axisfabrics, metamorphicmon•ites that yieldedU-Pb agesof-524 Ma. we estimatethat total strainswere high, generallyof the order Theseage data are consistentwith petrologicand of ? > 5, with ? > 10 locally within the Enduranceshear zone, geothermometdcconstraints indicating that Nimrod ductile as shownby ultramylonite,sheath folds, and/5 porphyroclasts. deformationoccurred at high temperatures(,--700øC). The High totalstrains, fabric uniformity, consistent shear sense, 4ømr/39Arcooling ages of 525-487Ma thatwere obtained from andhigh synkinematic temperatures collectively argue for high hornblendeand muscovite[Goodge and Dallmeyer, 1992], strainrates within the MRSZ. Microstructuraland quartz earlierinterpreted as evidenceof magmaticheating by crystallographicfabrics, with little evidenceof quartzbasal- posttectonicplutons, probably record cooling following high- planeslip, indicatethat crystal-plastic flow withinthe MRSZ 1474 Goodgeet al.: KinematicEvolution, Transantarctic Mountains

horizontal Althoughmajor displacements within the MRSZ werenormal to the downplungeprojection, a constrictionalcomponent of strainis apparentfrom the large-scale type 1 folds. 2. Mafic andultramafic tectonic blocks (marked by stars) areconcentrated along the boundary between schist and layered gneissunits, which in thewestern Miller Rangeis generally coincident with the Endurance shear zone. The blocks are out of petrologicequilibrium with theirhost tectonites, within whichthey are boudinaged as relatively rigid bodies. The presenceof these"exotic" blocks suggests that the Nimrod Grouptectonites may mark a fundamentaltectonic boundary, althoughthe origin of thisboundary may predate formation of the MRSZ. 3. Pretectonicto syntectonicplutonic bodies, generally Fig. 7. Compositeblock diagram of Nimrodstructural smalland conformable with tectonitelayering, and elements.View is to SW, lookingin dip directionof S (broad posttectonicplutons, exhibiting subhorizontal lenticular blockfaces on topand bottom). L ½is consistendyoriented shapes,are readily distinguished. Older plutonic bodies are SE-NW, andface oriented N45øW is the motionplane. commonlydistended and folded (many occur as layer-parallel Simplifieddownplunge projection shown on Plate1 is viewed bodieswithin gneissic units, too smallto showat the scaleof lookingat the faceoriented S45øW. Shearsense is Plate 1), whereasposttectonic bodies lacking deformation consistentlytop-to-the-SE (black arrows on motionplane), featuresclearly crosscut all metamorphicunits as theirascent givinga generaldirection of ductileflow shownby largegray throughthe crust was arrested. The downplungeprojection arrows. Also shownare types1 and2 folds. thusnot only offersa revealingview of MRSZ structures,but it providesa windowinto the middleto lowercrust of the Antarcticplate-boundary regime during the Neoproterozoic- did not takeplace during progressive cooling; rather, the Cambrian transition which contains elements of a collisional tectonitescooled to temperaturesrepresentative of the or subduction-relatedmagmatic-arc system. greenschistfacies only afterbulk ductiledisplacement within Isotopiccompositions of posttectonicplutons reinforce the the zonehad ceased. This coolingpattern is verifiedby conceptthat the MRSZ may havereactivated a crustal-scale thermochronologicdata, which indicate that high-temperature boundary,in thatthese plutons are isotopicallydistinct from Nimrod ductile tectonism waned between about 540 to 520 Ma theNimrod tectonitesthey intrude [Borg et al., 1990]. These [Goodgeet al., 1993] but that postkinematiccooling to -500 Ma plutonshave average ENd values of - 11 andyield Sm- >350øCoccurred by --485 Ma [Goodgeand Dallmeyer, 1992]. Nd depleted-mantlemodel ages (TI>m) of about2.0 Ga, The largescale, high strain, and inferred deep-crustal considerablydifferent from any Nimrod lithologies(ENd = -21 conditionsof the MRSZ indicatethat it representsa major to -24 and TI>n= 2.7 Ga). This contrastin isotopic crustalstructure. This zone may haveoverprinted a compositionappears to precludethe presenceof significant preexistingcrustal boundary, as expressed by thepresence of ' Nimrod Groupmelt componentsin the •-500 Ma granites. mafic and ultramafic tectonic blocks distributed within it. Conversely,a •-1.7 Ga orthogneissnear Camp Ridge (Plate 1) Mafic blockswith relict eclogitefacies parageneses [Goodge et hasa Sm-Nd modelage similarto otherNimrod units(TI•m = al., 1992] and ultramaficbodies containing high-temperature 2.73 Ga) [Borget al., 1990]. The regionaldip of MRSZ anthophyllite-talcassemblages [Goodge et al., 1990]represent tectonitesto the SW, the absenceof ductiletectonites in any entrainedlower-crustal or upper-mantlefragments within the of the outboardsedimentary assemblages, and the inferred Nimrod Group. High-pressuremetamorphism of theseblocks presenceof isotopicallydistinct source materials beneath the (12-25 kbar) [Goodgeet al., 1992]probably predates ductile Nimrod tectonitesall suggestthat the MRSZ representsa flow within the MRSZ and may signifyan earliercollisional majorcrustal structure dipping toward the cratonthat eventat substantiallydeeper crustal levels. Theseblocks may juxtaposesreworked cratonic basement materials and outboard haveinitially beentectonically incorporated in theirhosts supracrustalassemblages. It mustbe emphasized,however, withineither a Franciscan-typesubduction zone setting or thatno constituentsof knownsupracrustal assemblages alongan Alpine-typecollisional suture belt. Whatevertheir (Beardmoreand Byrd groups) are present within the MRSZ, as origin,the high-pressuretectonic blocks may markan ancient recognizedeither on lithologic,metamorphic, structural, or crustal-penetratingsuture that was reactivated as an intraplate isotopicgrounds. The geometryand strike-slipkinematics of structureby the MRSZ. The relativetiming between such the MRSZ may thereforereflect accommodation of principally eventsis completelyunconstrained at present.It is possible margin-parallelmotion along an olderlithospheric boundary. that an event which led to entrainment of mafic and ultramafic In orderto understandthe tectonicregime in whichthe rocksis substantiallyolder than ductile deformation within the MRSZ formed,it is necessaryto recall deformationpatterns in MRSZ, or theseevents may reflectdifferent stages between the outlyingsupracrustal rocks. Regionally,rocks of the Byrd earlycollision and subsequentshear. Despiteremaining andBeardmore groups are affectedby opento tightfolds that ambiguities,the scaleand geometryof the MRSZ in its larger parallelthe present Transantarctic Mountains [Grindley and structuralcontext are well illustratedin the downplunge Laird, 1969; Oliver, 1972; Stump, 1981' Stumpet al., 1986; projection(mean L ½)shown on Plate 1. This projection Reeset al., 1987],although the specificages of these showsseveral key featuresof theMRSZ: structuresare uncertainin mostcases. In the vicinity of 1. Lithologiclayering and foliationof MRSZ tectonites Nimrodglacier, Byrd Group rocks are deformedby upright havean apparentdip to the SW (towardthe polar plateau). NNW-NW trendingfolds with subhorizontalaxes [GrindIcy Goodgeet al.: KinematicEvolution, Transantarctic Mountains 1475 and Laird, 1969; Laird et al., 1971; Rees et al., 1987], the Mountainswas oneof left-obliquesubduction beneath the East hallmarksignature of contractionalRoss deformation in this Antarcticcraton (Figure 8). In this model,strain partitioning area. In moredetail, however, Byrd Grouprocks appear to be accommodatedcontemporaneous deformation of basementand affectedby multipledeformations [Rees et al., 1987, 1989; supracrustalsequences [e.g., Fitch, 1972;Jarrard, 1986], in Rowell et al., 1992], includingseveral generations of tilting, which strike-parallelductile deformation within the MRSZ at folding,and thrust-faultdisplacement. Beardmore Group rocks deepcrustal levels occurred at the sametime as high-level are deformedby broadfolds that parallel regional Ross shorteningof the supracrustalBeardmore and Byrd sequences. structuraltrends [Grindley and Laird, 1969;Laird et al., 1971; Ductile tectoniteswithin the MRSZ representthe diffuse Stumpet al., 1991]; however,they reportedly underlie an equivalentsof discrete,higher level strike-slipfaults within the unconformityat the baseof Lower CambrianByrd Group upperplate, perhaps transecting the magmaticarc represented strata,indicating an earlier(Beardmore) deformation of pre- by syntectonicto posttectonicGranite Harbour plutons. The Early Cambrianage [Laird et al., 1971; Stumpet al., 1991]. orientationof MRSZ tectonitessuggests that the strike-slip Structuraldata from Beardmorerocks in the CobhamRange faultsare listric towardthe craton. Orogen-parallel andat Kon-Tiki Nunatak(Figure 1) [Goodgeet al., 1991b] displacementswithin the MRSZ are consistentwith a lack of indicatethat deformation there was west vergent, resulting in pronouncedmetamorphic gradients across the zone,as would large-scalefolds, flexural-slip mesofolds, en echelontension be expectedin thecase of a large-scale,purely dip-slip gashes,and a steepeast dipping cleavage. These relations, and collisionalsuture. Shorteningwithin the supracrustal similar observationsreported by others[Laird et al., 1971; sequencesprobably occurred within a forearcsetting, where Edgerton,1987; Stumpet al., 1991], suggestthat the latest bothwest and eastvergent thrust displacements are possible. majorsupracrustal deformation involved west directed An activeforearc environment with syntectonicconglomeratic shortening.Inadequate exposure, poor biostratigraphic control, andsiliciclastic sediment deposition is a plausiblesetting for and difficult accessmake it impossibleat this time to resolve at leastpart of the Byrd Group. The outeraccretionary prism the vergenceof otherlatest Proterozoic to earlyPaleozoic of sucha convergent-marginsystem has not beenidentified, deformationswithin the supracrustalsequences; until further eitherbecause it hasyet to be recognizedor becauseof studyis completed,it may only be statedwith assurancethat subsequenttruncation and translation.An obliquely theserocks were involvedin orogen-normalcontraction. The subductingplate margin in whichupper-plate transcurrent Early Cambrianalong-strike displacements we have faultspass downward into distributed zones of ductileflow documentedfor the MRSZ are thereforeat a highangle to the couldhelp to explainisotopic patterns observed in the ~500 principaldirections of latestProterozoic to Early Ordovician Ma posttectonicplutons [Borg et al., 1990]. If the MRSZ orogen-normalshortening recorded in outboardsupracrustal representsa large-magnitude,strike-slip structural boundary rocks. within the upperplate, it may be possiblethat granitic Based on structural relations of the MRSZ and the magmasgenerated by subductionwould carryan isotopic supracrustalrocks described above, we proposea tectonic signatureof more outboard,partly subductedmaterials. modelthat may serveto explainthe relation between generally Graniticrocks in thiscase were apparently derived from synchronousbut stylisticallydistinct deformations. Because youngermaterials that may presentlyunderlie the Nimrod of manyuncertainties, several assumptions must first be Group. stated.First, because crystalline metamorphic rocks of the An obliquelysubducting margin requires a low angleof NimrodGroup are nowhere in exposedcontact with the subduction(_<25 ø) in orderto generatethe platecoupling forces outboardsupracrustal sequences, the plausibility of our thatlead to developmentof strainpartitioning within the upper tectonicmodel rests on the temporaloverlap between ductile plate[Jarrard, 1986]. In addition,generation of granitoid MRSZ deformationand supracrustal shortening [Goodge et al., magmasalong this margin during and after orogenic 1993], rather thanon direct structuralobservation. Second, the deformationsuggests an arc-trenchdistance >200 km, basedon TransantarcticMountains have undergone only minor simplegeometric construction; distances of thismagnitude or nonverticaltectonic modification since the Devonian; the greaterare ob•rved in modemmargins characterized by oblique modern Transantarctic Mountains front is the result of subduction(e.g., Sumatranmargin of Eurasia). Thoseparts of differentialuplift along orogen-parallcl post-Triassic normal the orogenexposed in the present-dayTransantarctic faults,as shownby subhorizontalBeacon Group strata Mountains,although complicated by youngerextension, are overlyingthe Kukri peneplain[GrindIcy and Laird, 1969;Laird greaterthan 200 km in width;thus the breadthof thisorogen et al., 1971]. From this, we infer that the orientations of is compatiblewith generallyshallow subduction and magma tectonitefabrics within the MRSZ, whichare as youngas generation.Ductile tectonitefabrics within the MRSZ reflect earlyMiddle Cambrian in age,are not greatly different from a clearcomponent of shearsubparallel to the orogenthat is whenthey formed. Third, thegeneral tectonic setting for these nearlyorthogonal to the directionof maximumshortening in deformationsis oneof a convergentplate margin in which supracrustalrocks. Furthermore,this ductile deformation oceaniclithosphere of theproto-Pacific ocean was subducted occurredat significantcrustal depths (>26 kin). If the depthof beneaththe EastAntarctic continent [Elliot, 1975;Borg et al., Nimrod deformationwas actuallybelow that at which strainis 1987, 1990]. It is possible,however, that sucha margindid effectivelypartitioned, depending on relativeplate velocities, not havea simpleconfiguration as in the caseof the modem temperature,and geometry,it is possiblethat the SE directed Andeanmargin of SouthAmerica but perhaps was more (mean134 ø in present-daycoordinates) displacements recorded complicated,involving fringing volcanic arcs and multiple by ductilestrain parameters in Nimrod tectonitesin fact subductionzones as along the modernPacific-Eurasian plate preservethe true plate convergence direction, rather than a boundary. partitionedcomponent. Whichever case is morelikely, there Given theserelations, we arguethat the lateNeoproterozoic remainsplausible evidence for partitioneddeformation at to earlyPaleozoic tectonic setting of the centralTransantarctic differentcrustal levels within the upperplate of an oblique 1476 Goodgeet al.: KinematicEvolution, Transantarctic Mountains

Transantarctic margin (~500 Ma)

Strike-slip Magmatic faults Forearc Trench EastAntarctic arc.rc ß:':'.:

100

km ...... •:i•i!½ ' .::•::::'!:iii•!•:?•...... ½•:•:•:•

Fig. 8. Schematicdiagram of TransantarcticMountains plate-boundary regime at-500 Ma. In this model,relative plate motions involving left-oblique subduction between the East Antarctic craton and proto-Pacificoceanic lithosphere is partitionedinto both contractional and strike-slip components of deformation.Inboard strike-slip faults that become shallow with depthaway from the subducting plate diffuseinto broad zones of ductileshear within the root zone of a magmaticarc represented at depth by theGranite Harbour intrusives. The Miller Rangeshear zone formed as a resultof strike-parallelductile displacementsat depths >25 km. Deformationin theforearc region occurred by punctuatedcontraction of supracrustalsedimentary rocks, including possible Neoproterozoic deformation of rift-faciesBeardmore Groupclastics and Middle Cambrian to EarlyOrdovician fold and thrust deformation of pretectonicto syntectonicByrd Group sediments. Rocks of an accretionaryor subducfioncomplex are not preserved. convergentplate boundary. postulatedearly Paleozoic strike-slip displacements for Rowell and Rees [1990] and Rowell et al. [1992] used northernVictoria Land, the kinematicsof ductile deformation sedimentologicalevidence to arguethat parautochthonous Byrd within the MRSZ providethe firstevidence for a relativesense Groupsediments accumulated along an activeEarly Cambrian of motionalong this plate boundary,that of left slip. Finally, plate marginof East Antarctica. Carbonateunits contain left-obliquemotion along this marginis compatiblewith, but featuresindicative of platformdeposition, but siliciclasticand notconfirmed by, a Cambrianplate reconstruction [Dalziel, conglomeraticfacies may have been deposited in structurally 1992] in whichNeoproterozoic rift separationof East controlledbasins distributed along a transform(or Antarcticafrom anothercontinent (perhaps Laurentia) initiated transpressive)margin. Rowell et al. [1992] definedtwo proto-Pacificocean basin spreading along an axisat an angle Cambriansedimentary bells ("marginal cratonic belt" and to theEast Antarctic margin that may have led to oblique "QueenMaud terrane")that are subparallelto thepresent trend subruction. of the Transantarctic Mountains and that coincide with crustal ageprovinces defined by Borget al. [1990]. Independent Acknowledgments.This studywas supported by a grant evidencethat Early Cambriandeposition of themarginal from the U.S. National Science Foundation Division of Polar cratonicbelt occurred in a tectonicallyactive strike-slip or Programs(DPP88-16807). Mike Robertsprovided transpressiveenvironment is thussupportive evidence that outstandinglogistical and geological assistance in the field. deformation in the Transantarctic Mountains occurred in an We thankRick Law for discussionof quartzc axispatterns. oblique-subductionsetting. Although Weaver et al. [1984] SimonHanmar and Peg Rees provided constructive reviews.

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