© 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 work in the (Medvecky Peaks) is situated in the Loewe Massif Lambert Graben area is restricted to apatite FT close to the Amery fault contact between the analyses in the northern PCM (Arne, 1994; Lisker et Precambrian basement and the late Palaeozoic Amery al., 2003). Furthermore, a new FT data set was Group sedimentary sequence (Fig. 2). The region is determined for the Beaver Lake/ northern PCM characterised by scattered nunataks ranging in altitude (Lisker & Brown, 2003), and initial data were between ~3000 m and sea level, with maximum

Fig. 2 – Location map of the northern Prince Charles Mountains as indicated in figure 1. Sample locations of the apatite fission track samples from the Corry Massif, Crohn Massif, Mount Kirkby, Medvecky Peak (Lisker et al., 2003) and the Beaver Lake area (Lisker & Brown, 2003) are indicated. (a) Age-elevation plot of the fission track samples. The sample symbols correspond with those of the main figure. All samples of the Porthos Range/ Medvecky Peak have the same age range, although they are exposed in different topographic elevations (Lisker et al., 2003). In spite of the close spatial vicinity, no correlation could be identified between the ages of the Beaver Lake samples and the topographic elevation or any regional tectonic structure (Lisker & Brown, 2003). A further set of similar data published by Arne (1994) covers the whole Beaver/Radok Lake area. (b) Schematic thermal histories of five representative vertical profiles of the northern Prince Charles Mountains analyzed by Arne (1994), Lisker et al. (2003) and Lisker & Brown (2003). Symbols of the locations as in figure 2a. © Terra Antartica Publication 48 F. Lisker et al. altitudes decreasing from the Porthos Range in the opposed to slow and continuous denudation since the east towards the Beaver Lake in the west. The AFT Cretaceous denudation (Arne, 1994). ages range from 315±12 to 183±9 Ma (Fig. 2a), thus being considerably younger than the Proterozoic metamorphic ages of the host rocks and the last FIELD WORK DURING PCMEGA metamorphic event which affected the PCM at ~500 AND FIRST RESULTS Ma. The AFT ages are generally correlated with the topographic altitude. In contrast to the systematic The 2002/03 German-Australian Antarctic pattern of the AFT ages, there is a significant expedition PCMEGA is the first large-scale expedition difference between the mean track lengths and to the southern PCM after a number of smaller corresponding standard deviations of the Porthos Russian (between 1983 and 1991) and Australian Range (12.4–13.6 µm/ 2.2–1.3 µm) samples and those (1960/61, 1970-74, 1998/99) field campaigns. The from the Loewe Massif (11.8–12.2 µm/ 2.2–1.7 µm) field work consisted of the recording and analysis of suggesting a similar thermal history of the two areas, kinematic indications, morphological observations, and although comprising a subtle difference in the style of a sampling campaign for thermochronological cooling. investigation. The modeled T-t paths indicate two denudation 1) Kinematic indications. These measurements phases during the Phanerozoic for both the Porthos focussed on the brittle structures (faults, thrusts, Range and the Loewe Massif (Lisker et al., 2003) joints) that potentially result from pan African(?) (Fig. 2b). Minimum Carboniferous–Triassic and post-pan African tectonic processes. Working denudation between 1.6 and 5.0 km corresponds areas were the Mawson Escarpment (Rofe Glacier, closely with the contemporaneous Gondwana Tingey Glacier), Mt. Stinear, Mt. Ruker, and sedimentation, and is interpreted to represent initial Cumpston Massif (Fig. 3). Synkinematic micas rifting in the Lambert Graben. Lisker et al. (2003) were sampled from faults in the Rofe Glacier and relate the initiation of rifting to the formation of the Tingey Glacier. 40Ar/39Ar age determinations will Ancestral Gamburtsev Mountains due to Variscan provide crucial timing on faulting in these areas. compression and substantial crustal thickening of the 2) Morphological observations. Although forming a East Antarctic Craton (cf. Veevers, 1994). A second sharp escarpment, the shoulders of the Lambert rifting stage associated with crustal denudation of 1.0 Graben are not aligned in a mountain chain-like to 4.5 km commenced in the Early Cretaceous. pattern like other large Antarctic rift structures Maximum denudation occurred ~20 km inland of the (e.g., West Antarctic Rift System, Rennick current master fault of the Lambert Graben. The pre- Graben). Instead, an extensive Triassic- rift geothermal gradient increased from 19˚Ckm-1 Jurassic?/Cretaceous-Early Cenozoic? erosion below the western part of the northern PCM (Porthos surface has been observed throughout the southern Range) to 29˚Ckm-1 along the immediate graben PCM. This palaeo-surface is particularly well shoulder (Loewe Massif). The Early Cretaceous developed on top of the Mawson Escarpment and denudation/rifting stage in the northern PCM was the Cumpston Massif and provides an important probably related to the activity of the Kerguelen geological constraint on interpretations of the plume (e.g., Duncan, 2002) and the beginning of the thermochronological data. The data set will Gondwana separation between India and Antarctica. provide crucial information on the relationship Moreover, the comparison of the denudation patterns between uplift and denudation during and of the Lambert Graben and the Indian Mahanadi subsequent to the graben formation. Additional Basin strongly support the existence of a common information and samples were collected from key intra-Gondwana rift during the Carboniferous–Triassic lithologies and landforms, including: (Lisker & Fachmann, 2001; Lisker, 2004). conglomerates from the Grove Mountains Additional AFT data from the Beaver/ Radok (Permian?), Cenozoic tillite and diamictite layers Lake area of the northern PCM were obtained by and moraines of varying thickness throughout the Arne (1994) and Lisker & Brown (2003) (Fig. 2a). whole PCM, and observations of valley-shape and Both AFT ages and track length distributions vary slope-form. Each of these will contribute to our considerably between 307±25 and 25±13 Ma, and 9.9 quantification and understanding of Phanerozoic and 15.0 µm, respectively. Thermal history modelling denudation processes in the southern PCM. shows a generally similar cooling history as outlined 3) Thermochronology. The outcome of the extensive above, with slightly higher amounts of Cretaceous sampling campaign during PCMEGA consists of cooling/ denudation. This probably refers to a less approximately 200 rock samples for 40Ar/39Ar uniform denudation pattern across the former rift dating (UK), fission track analysis on apatites, margin, and to a more varying heatflow in the zircons and sphenes (FL, UK), U/Th-He analysis vicinity of the Amery Fault. Moreover, the Cenozoic on apatites (DB, FL), and dating of cosmogenic ages and age components of the Beaver Lake favour isotopes in quartz (DB). The majority of the a final stage of discrete uplift and denudation as thermochronological samples were taken along 15 © Terra Antartica Publication Thermochronological Investigation around the Lambert Graben 49

Fig. 3 – Operation area of PCMEGA (cf. Fig. 1) including sample localities (Arabian numbers: samples, Roman numbers: vertical profiles).

vertical profiles. The most important samples The combination of the methods outlined above locations are: Mawson Escarpment (Lines Ridge, will enable us to develop a much better understanding Berkell Platform, Rofe Glacier, Hay Hills, Harbour of the timing, dimension and spatial relationship of Bluff, Hellmore Glacier, McIntyre Bluff, Prior the processes shaping Mac.Robertson Land during the Bluff, and Casey Point), Mt. Stinear/Mt. Rymill, Phanerozoic. This particularly refers to (1) the Mt. Ruker/Mt. Rubin, Cumpston Massif, structure, initiation and development of the Lambert complemented by various single locations from the Graben, (2) the long-term evolution of the southernmost PCM (between Wilson Bluff and topography, and (3) the use of intra-Gondwanian rifts Komsomolski Peak) (Fig. 3). The thermal history (i.e., the Lambert and Mahanadi grabens) as a tool for models derived from the thermochronological data Gondwana reconstruction. will provide crucial information about timing, intensity, course and cause of rifting and graben formation, about the variation of the geothermal gradient along and across the Lambert Graben, Acknowledgements - At the first place, and most importantly, we wish to say Thank You to Dave Gardner, and about the graben geometry and segmentation. field training officer during PCMEGA. We were very sad If initial rifting was triggered by Variscan-age when we heard about the tragic accident, one year after a compression across the Ancestral Gamburtsev very special change of the year in the stormy bight below Mountains, and the Cretaceous rifting stage is Mt. Ruker. Dave, we will always remember you: your related to the Gondwana breakup between unfatiguing help in the field, your interest in our work, Antarctica and India, then the respective amounts enthusiastic spirit and comradeship, your adventurous of denudation should vary along the graben. We cooking, and your funny stories in the tent! For logistic would expect higher amounts of late Palaeozoic support we also wish to thank Dave’s colleagues Gary and Bill, and the crew of Helicopter Resources. The authors are denudation within in the southern Lambert graben very indebted to the Bundesanstalt für Geowissenschaften area than across the northern PCM, and in turn a und Rohstoffe and to the Australian Antarctic Division for more intense Cretaceous stage of rifting/ the invitation to participate in PCMEGA 2002/03. This denudation towards the continental margin, i.e. the ongoing research benefits from the financial support of the northern PCM. German Research foundation (DFG projects OL 25/ 15 and © Terra Antartica Publication 50 F. Lisker et al.

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