Geophysical models for the tectonic framework of the Lake Vostok region, East Antarctica

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Abstract

Aerogeophysical and seismological data from a geophysical survey in the interior of East Antarctica were used to develop a conceptual tectonic model for the Lake Vostok region. The model is constrained using three independent data sets: magnetic, seismic, and gravimetric. A distinct change in the aeromagnetic anomaly character across Lake Vostok defines a crustal boundary. Depth to magnetic basement estimates image a 400-km-wide and more than 10-km-deep sedimentary basin west of the lake. Analysis of teleseismic earthquakes suggests a relatively thin crust beneath Lake Vostok consistent with predictions from kinematic and flexural gravity modelling. Magnetic, gravity, and subglacial topography data reveal a tectonic boundary within East Antarctica. Based on our kinematic and flexural gravity modelling, this tectonic boundary appears to be the result of thrust sheet emplacement onto an earlier passive continental margin. No data presently exist to date directly either the timing of passive margin formation or the subsequent shortening phase. The preserved thrust sheet thickness is related to the thickness of the passive margin crust. Because a significant amount of time is required to erode the thrust sheet topography, we suggest that these tectonic events are Proterozoic in age. Minor normal reactivation of the thrust sheet offers a simple mechanism to explain the formation of the Lake Vostok Basin. A low level of seismicity exists in the vicinity of this tectonic boundary. The existence of a crustal boundary in the Antarctic interior provides new constraints on the Proterozoic architecture of the East Antarctic craton.

Introduction

East Antarctica, the size of the conterminous United States, remains the largest unexplored part of the Earth’s crust. Our present understanding of the tectonic development of East Antarctica is mostly derived from remote sensing information and from the geological record as preserved in sparse rock outcrops along the perimeter of the Antarctic continent (Fig. 1). Knowledge of geological structures from the surrounding continents, such as Australia, southern Africa, and South America, complements the limited geologic information of East Antarctica. To date, geophysical mapping of the ice-covered continent’s interior was limited to reconnaissance expeditions during the International Geophysical Year in 1957, isolated over-ice traverses, and aerogeophysical profiling projects funded by the United States, the United Kingdom, Denmark, and Russia during the 1970s (e.g. [1]). Progress towards a detailed structural framework of East Antarctica is slow because of its remoteness and harsh climatic conditions.

The East Antarctic craton formed part of the Paleozoic/Mesozoic supercontinent Gondwana until the middle Jurassic to Cretaceous breakup of Africa, East Antarctica, India and Australia (e.g. [2], [3]). Sparse outcrops along the perimeter consist of mainly Precambrian metamorphic and intrusive rocks (e.g. [4] and references herein). Evidence for post-breakup tectonic activity in the interior of the Precambrian East Antarctic craton is not known (e.g. [5]). The East Antarctic craton is generally considered to be aseismic, although the large East Antarctic ice sheet may suppress seismic activity. Seismometer distribution within the Antarctic continent is also a problem. Within the interior of East Antarctica the most prominent subglacial landforms are the Gamburtsev Subglacial Mountains with peaks ranging up to 3500 m, and the Aurora Subglacial Basin at −1000 m (Fig. 1). Midway between these two physiographic features is Lake Vostok, the largest subglacial lake on Earth. The tectonic framework of the Lake Vostok region was the focus of a geophysical survey carried out during the austral summer of 2000/01. The data set consists of broadband seismic recordings and aerogeophysical measurements. The aerogeophysical data include gravity, magnetics, ice-penetrating radar and laser altimeter measurements. The nearest rock outcrops to our study area are over 800 km away in the Transantarctic Mountains and 1200 km along the coast (Fig. 1). Our goal is to derive a conceptual tectonic model for the Lake Vostok region based on geophysical data.

Section snippets

Data sets

The aerogeophysical data were acquired by the US National Science Foundation’s Support Office for Aerogeophysical Research (SOAR) using a ski-equipped De Havilland Twin Otter aircraft as survey platform. The survey consisted of a main grid for detailed mapping of the subglacial lake (discussed in [6]) and 12 regional lines, extending well outside of the main grid between 180 and 440 km (Fig. 2). The extent of the regional lines was limited by the range of the survey aircraft. Flight elevations

Geophysical constraints on the crustal structure of the Lake Vostok region

Previous models for the crustal and lithospheric structure of East Antarctica have been derived from teleseismic body waves and the dispersion of surface waves (e.g. [9], [10], [11], [12], [13]). The radial and lateral resolutions of these studies are poor (≥2°×2°) because of a lack of both permanent seismic observatories and intraplate seismicity within East Antarctica. The closest teleseismic sources are located along the Antarctic plate margin. Inversion of satellite gravity measurements

Conceptual tectonic models from gravity modelling

The regional gravity anomaly plays an important role in understanding the nature of the boundary beneath Lake Vostok. The gravity of the entire region is characterized by a positive–negative dipole, with the maximum just to the east of the lake and the minimum 150–250 km west of the lake (Fig. 6, Fig. 7). Superimposed on this long wavelength anomaly is an intermediate wavelength positive anomaly immediately west of Lake Vostok. A third characteristic of the observed gravity is a narrow minimum

Seismic activity

The broadband seismic data include the first-ever proximal recording of an earthquake and aftershock in the interior of East Antarctica. Prior to these events, only three earthquakes had been reported in the vicinity of Lake Vostok [29] (Fig. 1, Fig. 2). Near-absent seismicity, however, might be an artifact of the biased distribution of recording stations on the continent. In addition, continental ice sheets also have a tendency to suppress seismic activity [30]. On January 5, 2001, we recorded

Conclusions

Analysis of a recent geophysical survey in the interior of East Antarctica suggests a possible tectonic boundary along the eastern margin of Lake Vostok. We have developed a conceptual tectonic model for the Lake Vostok region that is consistent with the geophysical constraints from three independent data sets: seismological, magnetic, and gravimetric. A distinct change in the aeromagnetic anomaly across Lake Vostok defines a westward dipping boundary between two major crustal blocks. Depth to

Acknowledgements

We thank the Support Office for Aerogeophysical Research for acquisition and reduction of the aerogeophysical data. Ron Sweeney, Vicky Rystrom and Carol Finn (USGS, Denver) helped reducing the magnetic data. Christian Müller (Alfred Wegener Institute) and Wen-Xuan ‘Wayne’ Du (Lamont-Doherty Earth Observatory) helped locating the earthquake. Won-Young Kim, the Lamont-Doherty Cooperative Seismographic Network, and the PASSCAL Instrument Center provided equipment for the seismic survey. Art

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    Present address: Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japan.

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