Subglacial geology in Coats Land, East Antarctica, revealed by airborne magnetics and radar sounding

doi:10.1016/j.epsl.2006.01.068

Earth and Planetary Science Letters

Volume 244, Issues 1–2, 15 April 2006, Pages 323-335

https://doi.org/10.1016/j.epsl.2006.01.068 Get rights and content

Abstract

During the austral summer of 2001/02 five thousand line kilometres of airborne radio echo sounding and aeromagnetic data were collected in the region of three tributaries of Slessor Glacier, East Antarctica, which drains into the Filchner Ice Shelf. Basal topography and roughness estimates were obtained from the echo sounding data, which have been combined, here, with the subglacial geological units inferred from an analysis of the aeromagnetic data. This analysis included Euler and Werner deconvolution to determine the location and depth of sills, dykes and faults. In addition, we high-pass filtered the magnetic anomaly profiles to obtain information about the near surface subglacial geology. This revealed a prominent area of low magnetic susceptibility lying in a deep topographic basin underneath the northern tributary of Slessor Glacier. Forward and inverse magnetic models indicate that this area is likely underlain by a 3 km thick sedimentary basin. Ice flow modelling suggests that this sedimentary basin influences the ice dynamics in the tributary. These results have important implications for understanding the dynamics of this and other East Antarctic flow features that may be underlain by a weak sedimentary bed.

Introduction

The East Antarctic Ice Sheet (EAIS) contains about 80% of the land ice on the planet. Understanding the mechanisms that control the flow of the ice is, therefore, central to our ability to understand and model the past, present and future behaviour of the ice sheet. The motion of ice in Antarctica is the result of three mechanical processes: internal deformation of the ice (also known as creep), sliding of the ice over bedrock and shear of a layer of deformable sediment at the ice/bed interface. One or more of these processes may be present at any particular place across the ice sheet. The latter two processes, however, require the presence of water at the bed and, in the case of the third process, a substantial layer of water-saturated sediment. So while internal deformation takes place everywhere where there is horizontal ice motion, basal sliding and deformable sediments have additional requirements that are not met everywhere. When present, basal motion can, however, be responsible for 90% or more of the total ice velocity. As a consequence, understanding the factors that influence basal motion is crucial to our understanding of ice sheet dynamics and our ability to adequately model ice flow.

Most of the EAIS is grounded on bedrock that is above sea level. For the West Antarctic Ice Sheet (WAIS), the converse is the case and it is, consequently, often termed a marine ice sheet. As a consequence, much of the WAIS has the potential to be underlain by marine sedimentary deposits and these (where identified) have been linked with the existence and behaviour of active ice streams and glaciers [1], [2], [3]. Unlike the WAIS, very little is known about the subglacial geology underlying areas of enhanced flow beneath the EAIS. Partly to redress this gap in our knowledge, an airborne geophysical survey was undertaken in the austral summer of 2001/02 in the vicinity of two enhanced flow features in the Bailey/Slessor region of East Antarctica (Fig. 1). The key aerogeophysical instruments onboard were a radio echo sounding (RES) system, for measuring ice thickness and basal characteristics [2], and a magnetometer for determining total magnetic intensity (TMI). The RES data were processed to provide information on bed topography, basal roughness and returned power strength. The results of this have been presented elsewhere [2] and are not discussed in detail here. In this paper we present the results of the aeromagnetic survey and the implications, which they, in conjunction with the RES results and ice dynamics modelling, have for possible geological controls on enhanced flow in East Antarctica.

Section snippets

Airborne geophysical survey

Data were collected during five 1000 km survey flights covering an area of 200 × 280 km, about a base/re-fuelling camp located at 78° 58.60′ S, 007° 24.97′ W (Fig. 1). The survey was flown using a DeHavilland Twin Otter aircraft of the British Antarctic Survey. A near-constant terrain clearance of 85 m was maintained from the ice surface, monitored by the aircraft's radar altimeter system. Longitudes and latitudes on the WGS84 ellipsoid were recorded for each measurement by a GPS receiver and later

Methodology

The deeper tectonic setting was investigated by interpreting the magnetic anomaly map with the aid of Euler deconvolution [8]. To focus on the surface bedrock properties (which are what are relevant to the ice dynamics) we then analysed TMI profiles, which contain the higher-frequency information necessary to investigate shallower magnetic properties.

TMI anomalies are governed by the source's magnetic susceptibility, along with the geometry and distance between source and measurement instrument

Magnetic anomaly patterns and lineaments

The draped TMI map (Fig. 2a–b) reveals the marked contrast between the highly magnetic region labelled MH, and the flanking magnetic lows to the North and South, labelled MLN and MLS, respectively. MH is a predominantly positive, long wavelength feature. Superimposed onto this regional positive anomaly are relatively shorter wavelength anomalies of around 60–80 km wavelength, reaching peaks up to 300 nT. While MLN has mean values around − 50 nT, MLS is distinctly more negative with mean values

Regional geological setting

A simple structural elements map was compiled from aeromagnetic data analyses over the Slessor Glacier tributaries and inter-tributary regions and forms the basis for our discussion (Fig. 6). To discuss the regional geological setting in the Slessor Glacier tributaries area we also inspected magnetic anomaly signatures previously detected over the Shackleton Range and the Theron Mountains (Fig. 1) [20], [21], where geological investigations are fairly extensive [22].

The broad magnetic high,

Conclusions

Data from airborne radio echo sounding and aeromagnetics have been combined to provide an inferred picture of the subglacial geology and its relation to ice flow for a region covering the three tributaries of Slessor Glacier, a major East Antarctic outlet glacier [35]. The main subglacial geological units we infer include a Precambrian block, Jurassic dykes and sills, and a post-Jurassic(?) sedimentary basin. The first two geological units show no clear relationship with subglacial topography,

Acknowledgements

This work was funded by UK NERC grant GR3/AFI2/65. We thank Ian Joughin for providing the InSAR-derived velocity data used here. Fieldwork in Antarctica was made possible by the British Antarctic Survey (BAS). Hugh Corr is acknowledged for his assistance with the aerogeophysical survey. In the field, we are particularly grateful to David Leatherdale, Phil Jones and Paul Woodroffe.

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Subglacial geology in Coats Land, East Antarctica, revealed by airborne magnetics and radar sounding

Abstract

Introduction

Section snippets

Airborne geophysical survey

Methodology

Magnetic anomaly patterns and lineaments

Regional geological setting

Conclusions

Acknowledgements

Earth Planet. Sci. Lett.

Tectonophysics

Tectonophysics

Earth Planet. Sci. Lett.

Glob. Planet. Change

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Influence of subglacial geology on the onset of a West Antarctic ice stream from aerogeophysical observations

Nature

Basal topography and ice flow in the Bailey/Slessor region of East Antarctica

J. Geophys. Res.

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Geophys. Res. Lett.

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Geophys. Prospect.

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