U.S. Geological Survey and The National Academies; USGS OF-2007-1047, Short Research Paper 105; doi:10.3133/of2007-1047.srp105 Denudation and uplift of the Mawson Escarpment (eastern Lambert Graben, Antarctica) as indicated by apatite fission track data and geomorphological observation F. Lisker, 1 H. Gibson, 2 C. J. Wilson, 3 and A. Läufer 4 1 Universität Bremen, FB 5, Postfach 330440, 28334 Bremen, Germany (flisker@uni-bremen.de) 2 GeoIntrepid, Unit 2, 1 Male Street, Brighton, Victoria 3186, Australia 3 School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia 4 Bundesanstalt für Geowissenschaften und Rohstoffe, Postfach 510153, 30631 Hannover, Germany Abstract Analysis of three vertical profiles from the southern Mawson Escarpment (Lambert Graben) reveals apatite fission track (AFT) ages ranging from 102±20 to 287±23 Ma and mean lengths of 12.2 to 13.0 µm. Quantitative thermal histories derived from these data consistently indicate onset of slow cooling below 110°C began sometime prior to 300 Ma, and a second stage of rapid cooling from paleotemperatures up to ≤100°C to surface temperatures occurred in the Late Cretaceous – Paleocene. The first cooling phase refers to Carboniferous – Jurassic basement denudation up to 5 km associated with the initial rifting of the Lambert Graben. The presence of the ancient East Antarctic Erosion Surface and rapid Late Cretaceous – Paleocene cooling indicate a second denuda- tional episode during which up to 4.5 km of sedimentary cover rocks were removed, and that is likely linked to the Cretaceous Gondwana breakup between Antarctica and India and subsequent passive continental margin formation. Citation: Lisker, F., , H. Gibson, C.J. Wilson, and A. Läufer (2007), Denudation and uplift of the Mawson Escarpment (eastern Lambert Graben, Antarctica) as indicated by apatite fission track data and geomorphological observation, in Antarctica: A Keystone in a Changing World – Online Proceedings of the 10th ISAES, edited by A.K. Cooper and C.R. Raymond et al., USGS Open-File Report 2007-1047, Short Research Paper 105, 6 p.; doi:10.3133/of2007-1047.srp105 Introduction The Lambert Graben transects the metamorphic basement of the East Antarctic Craton perpendicular to the continental margin in the vicinity of Prydz Bay (Fig. 1). Hosting the largest glacial drainage system of the world, it also represents one of the largest rift systems of Antarctica. The western shoulder of the Lambert Graben is exposed for more than 700 km. It consists of the northern Prince Charles Mountains (PCM), a group of nunataks around the Fisher Massif, and the southern PCM, and shows significant geomorphological differences. The northern PCM consist of mountainous massifs with partially spiky summits, while the Fisher Massif area comprises only small nunataks and ridges. The southern PCM expose a landscape of high peaks with altitudes up to >3000 m a.s.l. in the west and lower, flat summits towards the Lambert Glacier in the east. In contrast, the eastern rift shoulder of the Lambert Graben is represented only by a single, homogeneous landscape feature, the Mawson Escarmpent. The Mawson Escarpment is a ~150 km long escarpment with a topographic offset, that partially exceeds a height of 1 km, towards the Lambert Graben. It is covered by a flat erosion surface of regional extent (Wellmann & Tingey, 1981) that gently declines towards the east (Fig. 2). Field evidence (effusion of 55 Ma vesiculous eucritic rocks) indicates an early Cenozoic age of this “East Antarctic Erosion Surface”. Our knowledge of the long-term landscape evolution of the Lambert Graben depends very much on accessibility and geomorphological appearance of both shoulders of this rift structure. While there exist detailed thermochronological studies from the logistically less challenging northern PCM (Arne, 1994; Lisker et al., 2003), the landscape evolution of the southern graben segments and the eastern graben shoulder is largely unknown. This paper presents the first apatite fission track (AFT) data from the Mawson Escarpment with the aim to determine the timing and magnitude of maximum paleotemperatures, to derive burial depths, and to interpret the denudation history of the southern Mawson Escarpment. The low-temperature thermal, spatial and structural pattern of the southern Mawson Escarpment, together with data from the northern PCM and the Vestfold Hills and geomorphological indications, bear essential information about the timing, style and magnitude of rifting of the Lambert Graben, as well as the reactivation of rifting processes, the uniformity of the tectonic activity, and the mechanisms triggering the regional long-term landscape development. Geological overview The basement of the Lambert Graben is composed by a melange of Precambrian terranes of similar rheological behaviour. It comprises a broad spectrum of metamorphic rocks ranging from low-grade Archaean and early Proterozoic granitic basement and meta- sedimentary rocks in the southern PCM and the Mawson Escarpment to high-grade late Proterozoic granulites and charnockites in the north. These rocks underwent a multistage tectono-metamorphic history with granulite- facies events at 3000, 2500 and 1100 Ma (e.g., Sheraton, 1984). Both Prydz Bay and the PCM were overprinted by a main metamorphic episode of Pan-African age (540–490 Ma/ e.g., Boger et al., 2002).