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Implications for the Tectonics of West Antarctica

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Implications for the Tectonics of West Antarctica JOURNALOF GEOPHYSICALRESEARCH, VOL. 96, NO. Bll, PAGES17,935-17,954, OCTOBER 10, 1991 New PaleomagneticData FromThurston Island' Implications for the Tectonicsof West Antarcticaand Weddell Sea Opening A.M. GRUNOW1 ByrdPolar Research Center, Ohio State University, Columbus Lamont-DohertyGeological Observatory andthe Department ofGeological Sciences, Columbia University, New York I. W. D. D ALZlEL Institutefor Geophysics,University of Texasat Austin Palcomagneticdata from threeWest Antarcticcrustal blocks (Antarctic Peninsula (AP), ThurstonIsland-Eights Coast (TI), and the Ellsworth-Whitmore Mountains (EWM) indicate that there has been motion between the individualblocks and motion relative to EastAntarctica during the Mesozoic. A Triassicpalcomagnetic pole from theTI block(116øE, 61øS, A95 = 19.4ø,N = 3 VGPs)appears toindicate that the block has rotated -90 ørelative to EastAntarctica between 230 Ma and110 Ma. Ourpreviously reported Middle Jurassic palcomagnetic pole from the EWM block indicates that a 90 ø rotation relative to East Antarctica occurred sometime between the Cambrian and 175 Ma. We believe that the 90 ø counterclockwise EWM rotation occurred between-220 Ma and 175 Ma related to thedevelopment of post-GondwanideOrogeny shear zones. The motion of theAP, TI, andEWM blocksappears to be linked duringthe mid- to late Mesozoicto threemajor events in the evolutionof the southemocean basins. Openingin theMozambique-Somali-Weddell Sea basins may have produced major counterclockwise rotation of the TI blockwith respectto EastAntarctica between the Jurassic and Early Cretaceousbased on new LateJurassic (145øE,64.5øS, A95 = 7ø,N= 5 VGPs)poles. We believe that the TI rotation,as well as deformation inthe southem AP block, wascaused by collisionand shearingof the EWM blockagainst the othertwo asthe EWM block moved southwardwith East Antarctica. An Early Cretaceous palcomagnetic pole (232øE, 49øS, A95 = 7.9ø,N= 5 VGPs) from the TI blockrequires that between the Early andmid- Cretaceousthere was clockwise rotation, with respect to EastAntarctica, of the AP-TI-EWM blocks(an entity we callWeddellia). A changein theopening history of the WeddellSea basin caused by initiationof spreadingin theSouth Atlantic ocean basin at-130 Ma probablystarted Weddellia'sclockwise rotation. Two new - 110and -90 Mapoles from the TI block(210øE, 73 øS, A95 = 7.6ø,N= 7 VGPsand 161øE, 81øS,A95 = 3.9ø,N = 18VGPs, respectively) aresimilartoequivalent agepoles from the APblock andEast Antarctica and indicate that Weddellia was at ornearits present-day position with respect to East Antarctica by -110 Ma. This correspondsto a time of majorplate reorganizaion in the SouthAtlantic and southeast Indian Oceans.Based on both the new TI palcomagneticdata and previously reported data from Marie ByrdLand (MBL), dextralshearing would be expected to haveoccurred between MBL andWeddellia since the mid-Cretaceous. Pine IslandBay, the areabetween the TI andMBL blocks,marks a fundamentaland complex tectonic boundary in West Antarcticathat we proposehas largely been a zoneof transcurrentsheafing. ]_NTRODUCTION GEOLOGIC OVERVIEW OF WEST ANTARCTICA The positionof West Antarcticawith respectto cratonicEast The AP and TI blocks are composedprimarily of Mesozoic Antarctica prior to and after breakup of Gondwanaland is and Cenozoic arc intrusive and extrusive rocks associated with critical to the opening history of the Weddell Sea. West the Pacific convergent margin of Gondwanaland [Srnellie, Antarctica,however, is not a singletectonic entity but rather is 1981; Thornson and Pankhurst, 1983; Pankhurst, 1990]. Early composedof four major crustalblocks whose relation to each Paleozoic basementrepresenting the roots of a magmatic arc other and to East Antarctica is not well understood. These indicate that parts of the AP block have been along a crustal blocks or terranes are the Antarctic Peninsula (AP), the convergent margin since the Paleozoic [Pankhu.rst, 1982, Ellsworth-Whitmore Mountains (EWM), the Thurston Island- 1983; Harrison and Loske, 1988; Milne and Millar, 1991]. The EightsCoast (TI), andMarie ByrdLand (MBL) [Jankowskiand EWM block consistsprimarily of a Precambrian basement at Drewry, 1981;Dalziel and Elliot, 1982] (Figure 1). The focus Haag Nunataks [Millar and Pankhurst, 1987] and a Paleozoic of this paperis on new palcomagneticdata from the TI block sedimentarysuccession in the Ellsworth Mountains [Webers combined with existing palcomagnetic, geologic, and and Sporli, 1983] that has often been correlated with the geophysicaldata from Gondwanalandthat links the movement stratigraphic succession in the Pensacola Mountains, East of the West Antarctic crustal blocks to the Mesozoic evolution Antarctica (Figure 1) [Schopf, 1969; Dalziel and Elliot, 1982]. of the southern ocean basins. MBL consistsof Paleozoic sedimentary and igneous rocks that were intrudedbY mid-Cretaceousgranitoids; it is mostclosely XNowat Departmentof Earth Sciences, University of Oxford,Oxford, associatedwith the Paleozoicrocks of North Victoria Land England. (EastAntarctica) and with New Zealand[Bradshaw et al., 1983]. Copyright1991bythe American Geophysical Union. Variouspositions havebeen suggested forthe West Antarctic Papernumber 91JB01507 crustalblocks ina prebreakup Gondwanaland reconstruction 0148-0227/91/91JB-01507505.00 [Schopf,'1969; Smith and Hallam, 1970; De Wit, 1977;Barron 17,935 17,936 GRUNOWET AL.: NEW PALEOMAGNETICDATA FROM THURSTONISLAND et al., 1978; Norton and $clater, 1979; Dalziel and Elliot, EWM-TI blocks may have acted a tectonic entity which we 1982; Powell et al., 1982; LaBrecque, 1985; Elliot, 1991]. refer to as Weddellia, between the Middle Jurassic and Norton and Sclater [1979] showed that the du Toit [1937] and Cretaceous, insofar as can be resolved paleomagnetically Smith and Hallam [ 1970] fit of the major continentalfragments [Grunow et al., 1987b]. If the AP, EWM, and TI blocks are of Gondwanalandwas essentiallycorrect but that an joined in their present-dayrelative positions(rigid Weddellia), unacceptableoverlap of the AP block with the FalklandPlateau there is an overlap of the AP block with southern South occurs'if West Antarcticais left in its present-dayposition America. Geosat, marine, and airborne gravity data [Sandwell with respectto East Antarctica.Together with the geographic and McAdoo, 1988; Bell et al., 1990] indicate that the isolation and anomalous structural trend of the Ellsworth continentalmargin of the Antarctic Peninsulais much narrower Mountains fold belt, this overlap constituted the primary than necessarily inferred by the imprecise location of the r•asonto/:onsider relative motion among the crustal blocks of shelf-slope break in ice-covered waters, and this lessens the West Antarctica [Schopf, 1969; Dalziel and Elliot, 1982]. pre-breakup overlap shown for the AP block with southern Palcomagneticdata supporta 90ø counterclockwiserotation of South America [Grunow et al., 1987b]. A reconstruction with the EWM block from the Transantarctic margin of East the blocks separated(mosaic Weddellia) fits the geologic and Antarctica(probably near CoatsLand; Figure 1) sincethe Late paleomagneticdata better but requiresa more complex opening cambrian[Watts and Brarnall, 1981; Grunow et al., 1987a]. history for the Weddell Sea basin [Grunow et al., 1987b; Palcomagnetic data obtained from Middle Jurassicrocks in Lawyer and Scotese, 1987; Lawyer et al., 1991]. the EWM block yielded a palcomagneticpole very similar to a Middle Jurassic pole from the AP block [Longshaw and GEOLOGIC SETrING OF TI BLOCK Griffiths, 1983; Grunow et al., 1987a]. An Early CretaceousTI pole [Grunow et al., 1987b] was indistinguishable from the The TI block consists of the area known as Thurston Island Middle JurassicAP and EWM poles and was interpretedby us as and the Eights Coast between ~255øE (Pine Island Bay) and possibly indicating little apparent polar wander for these ~270øE and from 71øS to ~ 75ø S. ThurstonIsland is separated blocks between the Middle Jurassic and Early Cretaceous from the Eights Coast by the Abbot Ice Shelf, which is found [Grunow et al., 1987b]. This led us to suggestthat the AP- within a graben or half graben structure(Figure 1). The rocks Paleomagneticlocalities Otherlocalities mentioned Antarctic Peninsula MaudLand Weddell Land THURSTON ISLAND Mtns. Thurston_.,., Island• EASTANTARCTIC •,• 1s.,• • CRATON ' Bay MarieByrd • O'o4.,.4•oLand Sea C ' Sherman•.......................... '" ':•'•••'••'"'"'! .... 72o30'S-- B¸T ICE SHELF N LN 0 50kmEIGHTS COAST 100øWI 96oWI '•• 92oWI 73ø30'S Fig. 1. Samplelocations on ThurstonIsland: BN, BelknapNunatak; DI, DustinIsland (ek, EhlersKnob); HG, Hale Glacier,HN, HarrisonNunatak; JM, JonesMountains; LN, LepleyNunatak; LP, LandfallPeak; MB, MountaBtamhall; MB, MountDowling; MI, McNamaraIsland (pb, PelletsBluff); MS, Mount Simpson;PP, ParkerPeak; SH, SheltonHead. Otherlocations: HK, HendersonKnob; MN, MountNoxom. Insetshows the WestAntarctic cmstal blocks (boldly outlined) in theirpresent-day positionswith respectto EastAntarctica. EWM, Ellsworth-WhitmoreMountains; EM, EllsworthMountains; NVL, North Victoria Land;asterisk, Haag Nunataks. Stippled lines show mountain trends; slanted lines show location of ice shelves. GRUNOWET AL.' NEWPALEOMAGNETIC DATA FROM THURSTON ISLAND 17,937 on Thurston Island crop out along two general east-west The Mesozoic rocks in the JonesMountains are unconformably strikingzones, parallel to theAbbot Ice Shelf.The majority of overlain by glacial till and flat-lying Tertiary alkaline
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