Fire and Ice - Tectonic and glacial history of the Amundsen Sector, West Antarctica

Abstract

West Antarctica has gone through major tectonic changes beginning in the Cretaceous (145 - 66 Ma). With the West Antarctic Rift System, the area also holds one of the largest continental rifts known on Earth. However, the West Antarctic Rift System is almost completely buried beneath West Antarctic Ice Sheet, as a result details of its geodynamic history are sparse and its tectonic evolution is still poorly understood. Furthermore, with the 'Amundsen Sector', West Antarctica hosts one of the most rapidly changing parts of the West Antarctic Ice Sheet. With the fastest flowing ice streams in Antarctica, especially the eastern areas are characterized by rapid ice sheet thinning and grounding-line retreat. Like data on West Antarctica's geodynamics, constraints on the deglacial history are limited to either marine sedimentary data or a few isolated nunataks. Thus it is difficult to assess the long-term deglacial history of the area, too. In this study we reconstructed the shallow crustal (~15 - 1 km) evolution of the ~1000 km long Amundsen Sector of West Antarctica and alongside the rift system. Thereby we applied three lowtemperature thermochronology analysis: apatite (U-Th-Sm)/He, apatite fission track and zircon fission track, and combined their results in inverse thermal history models. We further utilized detrital information on the dominantly ice-covered hinterland through the analysis of ice rafted debris extracted from marine sediments. Finally, to address the glacial retreat after the Last Glacial Maximum (ca. 23 - 19 ka) we applied 10Be-surface exposure dating in two key areas onshore of the eastern and mid Amundsen Sector, namely Pine Island Bay and the Kohler Range. Together, all three thermochronology systems yield early Cretaceous to early Miocene ages (121 - 18 Ma), with a dominant cluster in the mid- to late Cretaceous. Zircon fission track data ranges between 108 - 80 Ma, apatite fission track between 121 - 28 Ma, with narrow track lengths distributions and mean track lengths of 14.9 - 13.1 um, apatite (U-Th-Sm)/He mean ages between 94 and 18 Ma. Dominant (subglacial) lithologies of the coastal Amundsen Sector are igneous rocks that presumable intruded into low-grade meta-sediments of early Paleozoic times, and from our data there is no indication for un-metamorphosed sedimentary bedrock beneath the ice. Thermal history modeling identifies two major cooling stages: beginning with a period of rapid cooling during the Cretaceous, which was replaced by a stage of relative tectonic stability and rather slow cooling in the Cenozoic. This phase of strong cooling is also present in the detrital data, indicating that the majority of the Amundsen Sector was strongly affected by the extension phase along the modern northern margin of the West Antarctic. The extension was part of the Gondwana break-up and ultimately lead to the separation of West Antarctica - New Zealand. Post-Cretaceous activity appears to be limited to block tilting along the Pine Island glacial trough and an isolated Miocene cooling signal within the mid Amundsen Sector, which is interpreted to reflect the exhumation of a fault-bound horst structure separated through fault zones that are resultant of or directly linked to activity in the West Antarctic Rift System. Apart from that our data implies a rather stable tectonic setting and shows no strong indication of significant glacial erosion for most of the Cenozoic. The 10Be surface-exposure ages indicate that the sampled nunataks in the Kohler Range are ice-free since about 13 and 9 ka, respectively. Exposure ages from Pine Island Bay range from 8 ka close to the Pine Island glacial trough to 9 ka from a more coastal island. This implies a general Holocene retreat and minimum long-term average thinning rates of ~3.3 cm/yr are one order of magnitude lower than recent rates based on satellite data. Our results thereby provide insights into the significance of local ice-sheet variations and suggest that the post-Last Glacial Maximum history in the Amundsen Sector was characterized by glacial thinning as well as lateral retreat in pre- to early Holocene times.