Tessensohn, F.

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Polarforschung 60 (2): 63-66,1990 (erschienen 1992)The Antarctic Expedition GANOVEX VI:Introduction to Field ResultsBy Franz Tesscnsohn'1. INTRODUCTIONThe Ross Sea area (in the Pacific sector of Antarctica) is particularly suitable for investigations on the structureand evolution of the Antarctic lithosphere. The present structure of the lithosphere is largely characterized bythe existence of a large continental rift, a feature that allows the study of the initial processes of lithosphericthinning associated with continental split and break-up, This stage is highly relevant for the later formation of anew ocean. The Ross Sea rift system wh ich is active wirhin an apparently stable and aseismic plate is comparablein size to the Basin and Range province of North America and the East African rift. For an intracontinental systemit is unique for the asymmetry of the shoulder uplift.The evolution of the West Antarctic lithosphere to its present stage is characterized by:a) the generation of new crust at the active Gondwana margin bordering the Pacific 01' its predecessor andb) the fragmentation of the lithosphere which finally led to the present mosaic of crustal blocks. Both processescan be studied well in the working area.1.1 Generation of new crust in West AntarcticaThe Antarctic Ross Orogen forms one of the longest preserved segments of the mobile zones along the Pacificouter rim of Gondwana. This margin was active from the late Precambrian on. In the Ross Sea area, Precambrianand lower Paleozoic rocks form the basement of the Transantarctic Mountains. Metasediments and migmatitesof the Wilson Group crop out mainly in the westem inland areas of North Victoria Land and probably continuesouth to the region of the Dry Valleys in South Victoria Land. Adjacent to the east in North Victoria Land, thebasement is formed by the very low grade rocks of the Bowers Supergroup and the Robertson Bay Group (Fig.I).These three units occur in zones separated from each other by faults and are often regarded as tectonostratigraphicterranes, However, all three terranes were deformed and metamorphosed during the Ross Orogeny in the EarlyOrdovician. At the same time, granites were formed in the Wilson Terrane. At this time the various terranes hadalready been welded to the outer rim of Gondwana. To provide further evidence for this assumption it was plannedfor GANOVEX VI to compare the age of deformation in the three terranes by Ar/Ar dating of newly formedmicas in the folded rocks. Only if there are large differences in the age of deformation, the terranes should beregarded as truly exotic. Another approach related to the same question is the geochemical and geochronologicalcomparison of Devonian stitching granites wh ich intrude all three terranes along the north coast of N0I1h VictoriaLand.The mode of attachement of the three terranes (strike-slip accretion or subduction accretion) is still much in thedebate. During GANOVEX Vaseries of thrustplanes had been discovered in Oates Land and the Terra NovaBay region. These thrusts can better be explained by subduction than by strike-slip movements. Because of theimportance of these structures more detailed investigations were planned for GANOVEX VI.Since Italian scientists have discovered lower crustal granulite rocks in the Terra Nova Bay region there was::< Dr. Franz Tessensohn, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-W-3000 Hannover 51, FRG.63renewed interest in the problem how to distinguish relict shield rocks from high grade metamorphie rocks of theRoss Orogen. Because of the later Ross overprint dating of the reliet rocks is only possible by the U/Pb methodon zireons. Therefore a sampling program was set up for this expedition.Petrographical and geochemical investigations for later p-T detenninations were devoted to the problem of granite/migmatite relations and the question whether there is an older generation of Granite Harbour Intrusives (arouncl530 Ma), apart from the bulk of the granites which have intrusive ages around 500 Ma.1.2 Fragmentation of the Wesl Antarctic LithosphereThe following evolutionary steps can be clistinguished in the Ross Sea area:a) Ferrar volcanism as part of a Gondwana-wicle break-up event.b) Basin fonnation possibly related to Andean back-are processes.e) Separation of Tasmania/New Zealand from Antarctiea.d) Extensional processes within the Antarctie plate resulting in the formation of the Ross Sea rift and theTransantarctic Mountains.Significant problems related to the fragmentation of the lithosphere and to be addressed by GANOVEX VIincluded:Nature and thickness of the crust beneath the Transantarctic Mountains.Geometry of the faultplane bounding the basins of the Ross Sea rift from the rift shoulder of the TransantarcticMountains.Time of major uplift of the Transantarctie Mountains.Age and episodicity of rift volcanism.Local structure of the lithosphere derived from mantle xenoliths.Nature of the lithosphere and the under1ying mantle as derived from isotope investigations of plutonic andvolcanie rocks.Recent heatflow in the rift shoulder as compared to the Ross Sea.Style of young teetonism in the Transantarctie Mountains.Crustal gradient between Ross Sea and Polar Plateau as derived from regional gravity data,Integration of subglacial and submarine areas by aeromagnetic surveys.Under-ice-morphology inland of the Transantarctic Mountains as derived from radio-echo-sounding.Glaeial geology in relation to uplift.2. EXPEDITION CONCEPTBased on these open questions the following 20 programs were fonnulated for the expedition (partieipatinginstitutions in brackets):1) Aeromagneties over the Ross Ice Shelf to the east and south of McMurdo (BGR, AWI, USGS).2) Aeromagneties and airbome radio-echo-sounding over the Oates Coast, LowerRennick G1acier (BGR, AWI, USGS).3) Regional gravity survey on ice, Lower Rennick Glacier (BGR, USGS).4) Radar-echo-sounding in support of gravity program, Lower Rennick Glacier (BGR).5) Glacial geology on the flanks of the lower Rennick Glacier, Daniels, Morozumi, Everett ranges (RGD).6) Geochronology of deformation events to eompare three suspect Paleozoicterranes (BGR).7) Teetonic transport at the deformed terrane boundaries, Bowers Mtns., Wilson Hills (Uni F).8) Granite-migmatite relations in the Wilson Terrane, Daniels Range, Wilson Hills (Uni HB, WÜ).9) Sampling of enigmatic granites in the Robertson Bay Terrane, Surgeon Island,Cooper Bluffs, Sputnik Island, Yennak Point (BGR).64Sou hRossSeaV L B~~dOStationFig. 1: Working area and important localities ofGANOVEX VI in northem andccntral VictoriaLand.Insel shows thc rectonic setfing of the area.BT = Bowcrs Terrane. RBT ::::: Robensou BayTerranc. \VT :::: Wilson Tcrranc. CB = CentralBasin. EB :::: Eastcm Busin. VLB = VictoriaLand Basin.Abb. 1: Das Arbeitsgebiet von GANOVEX VIim nördlichen und zentralen Victoria-Land mitden wichtigsten Lokationenlindeiner Skizze destektonischen Rahmens. BT = Bowcrs Terranc.RBT = Robertsob Bay Terrane, WT = WilsonTerrane, CB = Zentral Becken. EB :::: Ost-Bck-ken. VLB = Victoria-Land-Bccken.10) Heat flow measurements on stagnant glacial plateaus south of the Reeves Glacier (BGR).11) Gravity survey on ice and accompanying ice thickness determinations by radar,Terra Nova Bay area (BGR).12) Inventory of Cenozoic structures in the Ross Sea coastal areas (ZIPE).13) Dating of Cenozoic volcanics related to rift tectonism (BGR).14) Distribution of Kirkpatrick lavas south of the Reeves Glacier (BGR).15) Geochemistry and petrology of Jurassie and Cenozoic volcanic rocks (Uni MZ, Uni Napoli).16) Mapping, petrography and dating of pre-Ross relicts (Uni MS).17) Granite-migmatite relations in the Wilson Terrane, Campbell/Priestley Divide (Uni HB, Uni WÜ).18) Major tectonic boundaries in the deformed Ross Orogen, Mt. Murchison, Mariner Plateau, S of Reeves Glacier(UniF).19) Glacial geology, Mt. Joyce, Archambault Ridge (RGD).20) Fission track sampling of Cenozoic granites at the Ross Sea coast (Uni HB).AWI =Alfred-Wegener-Institut für Polar- und Meeresforschung; BGR =Bundesanstalt für Geowissenschaftenund Rohstoffe; RGD = Rijks Geologische Dienst (Netherlands); USGS = United States Geological Survey; UniF = University of Frankfurt; Uni HB = University of Bremen; Uni MS = University of Münster; Uni MZ =University of Mainz; Uni WÜ = University of Würzburg; ZIPE = Zentralinstitut für Physik der Erde.653. WORKING SCHEME AND FORCE MAJEURE EVENTSThe actual field work was structured into four different sections (Fig. I):a) Aeromagnetics in the McMurdo area (Nov.zl'iec. 1990).b) Fieldwork around the Gondwana Station (Dec./Jan.),c) Aeromagnetics and fieldwork at the Pacific coast around Cape Williams, lower Lillie Glacier (Jan. 1991).d) Fieldwork in the Prince Albert Mountains south of the Drygalski Ice Tongue (Dec./Febr. 1991).The fieldwork during the expedition was strongly affected by a number of adverse events:About a month delay in the arrival of the expedition aircraft because of bad weather en route;Damage of one helicopter beyond repair;Grounding of another helicopter because of technical problems;A difficult ice situation which kept the expedition vessel about 50 km away from the working area at thenorth coast;A fuel depot which had to be uncovered from 4 rn of snow;Extremely bad weather during most of January with snow and fog in the coastal areas and strang katabaticwinds inland;Complete damage of one of the fixed wing aircraft during the operation on a snow runway.The combined effects of all these events brought the activities of some groups to a complete halt for some periodof time, others had to reduce or change their programs, and almost all groups had to cope with some kind ofemergency alterations and activities on an opportunity basis, sometimes in completely unexpected areas. Despitecontinuous bad weather the last period in February produced a rather continuous working program and someuseful new results.We are glad that, inspite of all these difficulties, we are able to present our field results here in a summarizedform. More detailed interpretation will follow after the laboratory work on our sampies will be completed.4. AcknowledgmentsThanks are due to the Alfred Wegener Institute (AWI) for the cooperative support of the aerogeophysical surveyin terrns of logistics, scientific equipment and personnei; to the U.S. Antarctic Program (USAP) for the SUPPOltduring the transfer flights of the Polat-Z and Polar-e aircrafts and the provision of the McMurdo facilities forthe first part of the survey.In a difficult situation in Antarctica, one is particularly thankful for the help of other groups active in the samearea. We would particularly like to thank our Italian colleagues at Terra Nova Bay and Helicopters (N.-Z) forsome unbureaucratic field support in a true Antarctic spirit; we would also like to thank the US personnel inMcMurdo and Christchurch for the transport of urgently needed spare parts and for some medical cross-examinations of an injured person. The crews ofHSF helicopters and the Polar Queen did a greatjob in recoveringcompletely the damaged fixed wing aircraft, albeit in 4 single sections. As coordinator of the field activities Iwould like to express my personal thanks to all expedition members for keeping up their spirits in spite of thedisappointment about the reduction of the planned programs. This made the remaining work possible.66

The Antarctic Expedition GANOVEX VI: Introduction to Field Results

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