Exploring the hidden underbelly – tectononorphic evolution of the Bellingshausen Sea sector of West Antarctica

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The Bellingshausen sector underlies a large part of the West Antarctic Ice Sheet, and it hosts several deeply incised troughs, which are interpreted as branches of the West Antarctic Rift System, one of the largest continental rift structures on Earth. Except for few nunataks close to the troughs, these rift branches are completely covered by km-thick ice, impeding direct geological investigations. The troughs connect the South Pacific with Antarctica´s continental interior, providing pathways for warm oceanic deep water, which causes basal melting of the overlying ice sheet. The ship icebreaker Polarstern provides us with the opportunity for field work and sample collection to investigate this remote part of Antarctica. In addition to the coastal nunataks, we will obtain material from the area hidden beneath the ice by collecting offshore clastic sediments from the glacial outlets draining into the Bellingshausen Sea. The samples will be studied by combined U/Pb, fission track and (U-Th)/He analysis. For efficiently including (U-Th)/He analysis on detrital minerals, we will apply the new technique of laser-ablation (U-Th)/He dating. Two main topics will be addressed: (i) Timing of activity, mode (continuous vs. episodic) and kinematic relationships of the different rift branches. The results will contribute to the understanding of this hitherto poorly constrained continental rift system, and will also provide information since when the current configuration connecting the oceanic realm with the continental interior via the deep rift troughs may have existed in the past. This in turn provides information on the past vulnerability of the West Antarctic Ice Sheet. (ii) Tectonomorphic evolution of the Bellingshausen sector. Plate tectonic reconstructions indicate that the tectonic history of the Bellingshausen sector was very different from that of the adjacent Amundsen sector, as the latter was situated along a rifted margin from ~100 Ma onwards, whereas the Bellingshausen sector remained in an active margin position until at least the Eocene. These different tectonic settings are only insufficiently incorporated into current paleotopographic models. However, understanding the topographic evolution of Antarctica is of major importance, because the existence of topography is the prerequisite to allow for West Antarctic glaciation under the warmer climate of the geological past. Our new data will be compiled with previously published thermochronology data from West Antarctica and the Transantarctic Mountains, partly remodelled for obtaining internally consistent exhumation patterns, and combined with offshore seismic and sedimentological data. From this we will generate maps which visualise patterns of onshore and offshore denudation and burial for various time slices, providing a comprehensive data base for understanding source-sink relationships, drainage patterns, and refined paleotopographic models.