Restoration of Early Miocene Paleotopography of Marie Byrd Land, Antarctica, using information from bedrock geology
Abstract
Analysis of sub-ice bed topography datasets of West Antarctica, undertaken as part of the international Antarctic paleotopography program, ANTscape, reveals subice landforms that may have implications for ice sheet history, ice dynamics, and isostatic responses of the crust of Marie Byrd Land (MBL) since onset of Antarctic glaciation. These include subglacial volcanoes, alpine glacier valleys upon volcano summits, and a flexural moat around the Executive Committee Range (ECR) volcanic field. Whereas the West Antarctic Ice Sheet initiated at ca. 34 Ma, most MBL volcanoes are 17 to 8 Ma; thus, the early WAIS did not nucleate upon an ice cap on the volcanic peaks. Once formed, the 2000+m volcanoes would have received higher precipitation and been sites of volcano-thermal perturbation conducive to warm-based glaciation, glacial erosion and formation of alpine glacier valleys during times of reduced ice sheet extent. These geological considerations guide our reconstruction of Early Miocene paleotopography of MBL, to be integrated into the series of 6 paleogeographic maps that ANTscape is compiling, for time intervals of paleoclimate and tectonic significance from 92 Ma to present. The geospatial representations of past landscapes will serve as datasets for use in ice sheet/ice shelf models and for visualizations of Antarctic paleoenvironments during key climate stages. Bed topography data for MBL (ASE05 and BEDMAP1_plus) show a pronounced asymmetrical, narrow downwarp around the ECR, suggesting the presence of a volcanic moat formed from the lithostatic load of the ECR on the lithosphere. Thus, our Early Miocene topographic reconstruction must both remove volcano elevation and allow isostatic rebound from volcano and ice sheet load. Also evident in the bed topography are narrow, steep-sided valleys that we postulate were formed by alpine glacier erosion at a time of reduced WAIS extent when the ECR hosted alpine glaciers. The major ECR volcanoes are <14 Ma, providing a maximum age for the time when warm-based ice retreated to the summit elevations. The finding of warm-based glacial erosion features formed at high elevations after the mid-Miocene climate transition (~14 Ma) -when there was a change to hyper arid climate and onset of cold-based mode of Antarctic glaciation-is a possible indication that basal thermal conditions were elevated in the vicinity of the volcanoes. After ~14 Ma, warm-based glacial erosion became focused within preexisting drainages at low elevation that are now occupied by outlet glaciers in narrow, incised troughs. Therefore, our paleotopography reconstruction restores the volumes of rock removed since mid-Miocene by low-elevation outlet glaciers and ice streams. The paleogeographic reconstruction for Early Miocene is a subdued landscape with <1000 m of vertical relief. Ongoing work assesses structural inheritance and the possibility that fault reactivation may be linked to glacial erosion-induced exhumation, tectonic influences, and fluvial or glacio-fluvial processes. Once all geological influences on bedrock elevation are accounted for, isostatic corrections for removal of volcanic rock and ice will commence.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMPP43A1563S
- Keywords:
-
- 0700 CRYOSPHERE;
- 8107 TECTONOPHYSICS / Continental neotectonics;
- 9310 GEOGRAPHIC LOCATION / Antarctica