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Terra Antartica Publication Terra Antartica 2005, 12(2), 45-50 © Terra Antartica Publication Terra Antartica 2005, 12(2), 45-50 Thermochronological Investigation around the Lambert Graben: Review of Pre-Existing Data and Field Work during PCMEGA F. LISKER1*, D.X. BELTON2 & U. KRONER3 1FB Geowissenschaften, Universität Bremen, PF 330440, 28334 Bremen - Germany 2School of Earth Sciences, University of Melbourne, 3052 Parkville, Victoria – Australia 3Institut für Geowissenschaften, Technische Universität Bergakademie Freiberg, Bernhard-von-Cottastr. 2, 09596 Freiberg - Germany Abstract - The Lambert Graben is the largest known rift structure within the East Antarctic Craton. The north-western shoulder segment, the northern Prince Charles Mountains (PCM), experienced two major denudational episodes during the Permo-Carboniferous and the Cretaceous that are apparently related to the initial rifting and graben formation, and its reactivation due to the Gondwana breakup between India and Antarctica, respectively. Structural field work and morphological observation carried out during the joint German-Australian PCMEGA expedition 2002/03 as well as subsequent thermochronological analyses (40Ar/39Ar, fission track, and U/Th-He analyses) shall unravel the rifting history of the much less understood southern graben segment. The main topics of the future investigation are: (1) Structure, initiation and development of the Lambert Graben, (2) Long-term evolution of the topography, and (3) Using intra- Gondwanian rifts as a tool for Gondwana reconstruction. INTRODUCTION The Lambert Graben in Mac.Robertson Land is one of the most remarkable crustal-scale structures of Antarctica. Its importance results partly from its sheer dimension as the world’s largest glacial drainage system, but it is as well one of the planet’s largest continental rift structures. Its current geometry with respect to the Antarctic continent, together with the juxtaposed Indian Mahanadi basin, strongly suggests that it might also represent an example of an immense intra-Gondwana rift system that has been dissected as a result of the breakup of the former supercontinent. Although the peripheral regions of the Lambert Graben have been the subject of international Antarctic research over the last five decades, little has been done on the interior reaches of the structure. Limited accessibility of the region has meant that previous research activities have been concentrated oh the coastal areas of Mac.Robertson Land between Prydz Bay and northern Prince Charles Mountains (PCM) (Fig. 1). The rocks of the southern PCM remain one of the most poorly understood outcrop areas in East Antarctica. The joint German-Australian PCMEGA (Prince Charles Mountains Expedition of Germany-Australia) expedition 2002/03 offered an excellent opportunity to investigate the structure, the thermo-tectonic history and the long-term landscape Fig. 1 – Location map of the Lambert Graben area/ East evolution of the central and southern graben Antarctica, approximate margins of the Graben after Fedorov et al. segments. (1982). Bedrock exposures in black, elevation contours in m. The dotted frame represents the area of figure 2 (northern Prince A major problem in establishing the origin and the Charles Mountains), the bold one the area of the PCMEGA field subsequent history of the Lambert Graben is that the campaign (Fig. 3). *Corresponding author ([email protected]) © Terra Antartica Publication 46 F. Lisker et al. geology is obscured by the thick ice cover of the the northern PCM. These rocks have undergone a Lambert Glacier infilling the graben, and thus, “the multistage tectono-metamorphic history with granulite- relationship between Permian sedimentation and the facies events at 3000, 2500 and 1100 Ma (Sheraton et crustal structures which controlled initiation of the rift al., 1984). Both Prydz Bay and the PCM have been is not clear” (Fedorov et al., 1982). Consequently, the overprinted by a main metamorphic episode of Pan- origin and evolution of the Lambert Graben is African age (540–490 Ma/ e.g., Boger et al., 2002). unlikely to be resolved solely on the basis of In contrast to the general agreement regarding the structural analyses and geophysical data, and early history of Mac.Robertson Land, the Phanerozoic sedimentological and palaeontological evidence is history is a matter of considerable controversy. meagre. A powerful approach for reconstructing the Various authors postulate at least five stages of long-term morphotectonic history of basement terranes thermo-tectonic activity that are important for the is the application of thermochronological techniques, rifting history of the Lambert Graben and the long- such as 40Ar/39Ar, fission track (FT) and U/Th-He term landscape evolution of the surrounding mountain analyses (e.g., Arnaud et al., 1993; Gallagher et al., ranges (northern and southern PCM, Mawson 1998; Wolf et al., 1998). From the data pattern it is Escarpment): possible to constrain the chronology and style of 1) The intrusion of Late Ordovician-Silurian alkaline cooling of a rock for temperatures below ~300°C. to mafic dykes associated with the N-S to NE-SW Moreover, the application of different thermo- trending meridional fault system has been chronological methods covering different annealing interpreted as initial rifting of the Lambert Graben temperature ranges, and maximum palaeotemperature by Hofmann (1991). estimates for a series of apatite FT samples from 2) A Late Carboniferous–early Permian tectono- vertical profiles allow us to constrain the palaeo- magmatic episode included crustal uplift and geothermal gradient that existed at the time of the basement exhumation (Arne, 1994; Lisker et al., maximum temperature. Generally, cooling to low 2003), emplacement of mafic dykes (Andronikov temperatures results from denudation of the upper & Belyatsky, 1995), and crustal fracturing crust. Amounts and rates of denudation can be (Hofmann, 1991). Overall this may represent a derived from a vertical sequence of samples (using an stage of epeirogenetic uplift (e.g., Tewari & assumed or calculated palaeogeothermal gradient), or Veevers, 1993) or rifting (Lisker et al., 2003). from ages yielded by different thermochronological 3) In the Late Permian and the Triassic the coal methods. The regional pattern of denudation bearing sedimentary sequence of the Amery Group constructed from regional sample arrays provides was deposited in the Beaver Lake area (northern valuable information about the mechanism of PCM). Approximately 2500 m of conglomerate, denudation (i.e., surface erosion and/or normal sandstone and minor shale are preserved in this faulting) and long-term denudation rates, and on region (e.g., Mond, 1972; McKelvey & spatial variations in these rates within drainage basins. Stephenson, 1990). The subsidence in the Early Triassic was accompanied by the re-activation of the meridional fault system, cataclasis and THE EVOLUTION OF mylonitisation of the basement and the Permian THE LAMBERT GRABEN sediments, as well as the intrusion of Early Triassic porphyries (Hofmann, 1991). Most The Lambert Graben extends at least over a 700 x stratigraphic workers suggest this episode as main 100 km, largely ice-covered area from Prydz Bay rifting stage of the Lambert Graben. towards the East Antarctic Craton (Fig. 1). Seismic 4) A late stage of magmatic activity, possibly and gravimetric surveys indicate that the continental connected to the formation of the East Antarctic crust beneath this failed rift (Stagg, 1985) is thinned continental margin in the Early Cretaceous, led to from 35-40 km at the undisturbed basement to 25 km volcanism, emplacement of monchiquites, renewed along the rift axis (Bentley, 1983). Structurally, the extension and exhumation of the Lambert Graben Lambert Rift represents a typical half-graben with an (Arne, 1994) and the formation of a latitudinal internal horst-and-graben structure, bordered by strike-slip fault system in the northern PCM meridional fault systems (Craddock, 1972). The (Hofmann, 1991). Based on kinematic exposed western margin displays a vertical investigation, Wilson et al. (1999) propose that displacement of 10-12 km. The morphological this Late Cretaceous event is the main rifting expression of the rift shoulders, the PCM, the responsible for formation of the Lambert Graben. Mawson Escarpment and the exposed continental 5) Based on kinematic data, Hofmann (1990) margin of Prydz Bay, mainly consists of Precambrian postulated Cretaceous to Quaternary multiphase metamorphic rocks. They range from low-grade block movements and general uplift due to a final Archaean and early Proterozoic granitic basement and re-activation of the meridional fault system. On metasedimentary rocks in the southern PCM to high the other hand, using palaeostress determination grade late Proterozoic granulites and charnockites in techniques, Boger & Wilson (2003) interpreted the © Terra Antartica Publication Thermochronological Investigation around the Lambert Graben 47 Lambert Graben to represent a failed Cretaceous obtained from the Mawson Escarpment (Belton et al., transtensional rift arm that emanated from a triple 2003). junction between India and Antarctica. The denudational history of the northern PCM was quantified on the basis of thermal history modelling of 31 basement rock samples from four vertical PREVIOUS profiles (Lisker et al., 2003). Three of the four THERMOCHRONOLOGICAL STUDIES profiles (Corry Massif, Crohn Massif, Mount Kirkby) were taken from the Porthos Range, the fourth Published thermochronological
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