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The Weddell Sea Rift was a major focus for Jurassic extension and magmatism during the early stages of Gondwana break-up and underlies the Weddell Sea Embayment, which separates East Antarctica from a collage of crustal blocks in West Antarctica. Newly-collected aerogeophysical data over the catchments of Institute and Möller ice streams reveal the inland extent of the Weddell Sea Rift against the Ellsworth-Whitmore block and a hitherto unknown major left-lateral strike slip boundary between East and West Antarctica. Aeromagnetic and gravity anomalies define the regional subglacial extent of Proterozoic basement, Middle Cambrian rift-related volcanic rocks, Jurassic intrusions and sedimentary rocks of inferred post-Jurassic age. 2D and 3D magnetic depth-to-source estimates were used to help constrain joint magnetic and gravity models for the region. The models reveal that Proterozoic crust similar to that exposed at Haag Nunataks, extends southeast of the Ellsworth Mountains to the margin of the Coastal Basins. Thick granitic Jurassic intrusions are modelled at the transition between the Ellsworth-Whitmore block and the thinner crust of the Weddell Sea Rift and within the Pagano Shear Zone. The crust beneath the inland extension of the Weddell Sea Rift is modelled as being either ~ 4 km thinner compared to the adjacent Ellsworth-Whitmore block or as underlain by an up to 8 km thick mafic underplate.

Gravity anomalies provide a tool to study crustal structure, effective elastic thickness, and isostatic and tectonic processes. Over the last 10 years major airborne gravity surveys were flown by the international community over several Antarctic frontiers. The longer-wavelength Antarctic gravity anomaly field is increasingly better resolved with satellite-gravity. These recent airborne and satellite gravity datasets provide novel perspectives on Antarctic crustal structure and geodynamic evolution. We review results from some of these surveys over the Gamburtsev Subglacial Mountains, Dronning Maud Land, the Wilkes Subglacial Basin, the Transantarctic Mountains and the West Antarctic Rift System and present gravity modelling outputs of crustal thickness for these regions. We contrast these gravity results with a seismically-derived estimation of Antarctic crustal thickness (Baranov and Morelli, 2013, Tectonophys). Anomalously thick East Antarctic crust lies beneath the Gamburtsev Mountains and parts of Dronning Maud Land (50-58 km). Crustal thickening may stem from the collision of a mosaic of East Antarctic crustal provinces in Meso to Neoproterozoic times (Ferraccioli et al., 2011, Nature), or during younger Edicaran to early Cambrian “Pan-African age” orogenic events. The preservation of such thick crust provides significant support for the high bedrock topography in East Antarctica. Additional flexural uplift along the flanks of the Permian to Cretaceous East Antarctic Rift System helps explain the enigmatic Gamburtsev Mountains. Lithospheric flexure along the flank of the West Antarctic Rift System (WARS) may explain the Transantarctic Mountains (TAM), the longest and highest non-compressional mountain range on Earth. Whether the Wilkes Subglacial Basin also developed in response to lithospheric flexure is debated. Our gravity models image thicker crust beneath the Transantarctic Mountains (TAM) (ca 40 km thick), compared to the relatively thinner crust (30-35

The bed of the West Antarctic Ice Sheet is, in places, more than 1.5 km below sea level1, 2. It has been suggested that a positive ice-loss feedback may occur when an ice sheet’s grounding line retreats across a deepening bed1, 2, 3. Applied to the West Antarctic Ice Sheet, this process could potentially raise global sea level4 by more than 3 m. Hitherto, attention has focussed on changes at the Siple Coast5, 6, 7 and Amundsen Sea embayment8, 9, 10 sectors of West Antarctica. Here, we present radio-echo sounding information from the ice sheet’s third sector, the Weddell Sea embayment, that reveals a large subglacial basin immediately upstream of the grounding line. The reverse bed slope is steep, with about 400 m of decline over 40 km. The basin floor is smooth and flat, with little small-scale topography that would delay retreat, indicating that it has been covered with marine sediment5, 11 and was previously deglaciated. Upstream of the basin, well-defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin. Evidence so far suggests that the Weddell Sea sector of the West Antarctic Ice Sheet has been stable, but in the light of our data we propose that the region could be near a physical threshold of substantial change.

The West Antarctic Rift System provides critical geological boundary conditions for the overlying West Antarctic Ice Sheet. Previous geophysical surveys have traced the West Antarctic Rift System and addressed the controls that it exerts on the West Antarctic Ice Sheet in the Ross Sea Embayment. However, much less is known about the rift system under the Amundsen Sea Embayment, a key sector of the West Antarctic Ice Sheet, which is thinning significantly today. New aerogravity data over the Pine Island Glacier region, one of the fastest flowing glaciers within the Amundsen Sea Embayment, sheds new light into the crustal structure under this dynamic part of the West Antarctic Ice Sheet. Three-dimensional (3-D) inversion of terrain-decorrelated free-air and Bouguer gravity anomaly data reveal significant crustal thinning beneath the catchment of Pine Island Glacier. Under the Byrd Subglacial Basin and the newly identified Pine Island Rift, Moho depth is estimated to be 19 ± 1 km. This is the thinnest crust observed beneath the West Antarctic Ice Sheet. Estimates of lithosphere rigidity (Te), based on isostatic models, yield a Te of 5 ± 5 km, which is comparable to values from modern rift systems such as the Basin and Range Province. Major crustal thinning, coupled with low lithosphere rigidity, attest to the considerable impact of continental rifting beneath this part of the West Antarctic Ice Sheet. In analogy with the better known Ross Sea segment of the West Antarctic Rift System we suggest that the Amundsen Sea Embayment was affected by distributed Cretaceous rifting, followed by Cenozoic narrow-mode rifting. Narrow-mode rifting within the Pine Island Rift is particularly important as it may serve as a geological template for enhanced glacial flow associated with Pine Island Glacier.

The Wilkes Subglacial Basin represents all approximately 1400 kin-long and Lip to 600 kill wide Subglacial depression, buried beneath the over 3 km-thick East Antarctic Ice Sheet. Contrasting models, including rift models and flexural models. have been previously Put forward to explain the tectonic origin of this enigmatic basin, which is located in the largely unexplored hinterland of the Transantarctic Mountains. A major aerogeophysical survey was flown during the 2005-06 austral summer to explore the Wilkes Subglacial Basin. Out, new airborne radar dataset reveals that the Wilkes Subglacial Basin contains several subglacial basins, which are considerably deeper than previously mapped. Major aeromagnetic lineaments are detected from total field, pseudo-gravity, tilt derivative and Euler Deconvolution maps. These aeromagnetic lineaments reveal that the Wilkes Subglacial Basin and its Sub-basins are structurally controlled. Comparison between aeromagnetic signatures over the Wilkes Subglacial Basin region and the Cordillera in North America, suggests that the basin contains a former broad backarc basin and fold-and-thrust belts, forming the transition between a Precambrian craton and the Ross Orogen. The eastern margin of the Wilkes Subglacial Basin is imposed upon the Prince Albert Fault System and the Priestley Fault. These faults May have been reactivated in the Cenozoic, as major strike-slip faults. The western margin of the Wilkes Subglacial Basin is located along the southern extension of the Precambrian-age Mertz Shear Zone and marks the edge of the Terre Adelie Craton. High-frequency aeromagnetic anomalies in the Wilkes Subglacial Basin image large Volumes of Jurassic tholeiites, which were intruded into and extruded over Beacon sediments in a possible rift setting. Depth-estimates of magnetic anomaly Sources and forward modelling indicate that major Cretaceous and Cenozoic rift basins with thick sedimentary infill, comparable to the deep Ross Sea Rift basin

Understanding Antarctic volcanoes is important as they provide a window on magmatic and tectonic processes of the Antarctic plate and contain datable records of ice-sheet changes. We present the results from the first detailed airborne radar and gravity surveys across James Ross Island, northern Antarctic Peninsula, which is dominated by Mt Haddington, an ice-covered Miocene-Recent alkaline stratovolcano. The surveys provide new insights into the subsurface structure of the volcano and hence its development, which are unavailable from the surface geology alone. We show that Mt Haddington is associated with a significant negative Bouguer gravity anomaly (>= 26 mGal), which suggests that there has not been significant pooling and solidification of a dense shallow-level mafic magma chamber during the growth of the volcano over at least the past 6 m.y., which is consistent with independent geochemical evidence. Simple flexural isostatic models cannot explain the localised negative Bouguer anomaly. 3D modelling techniques show that the negative anomaly is best explained by a shallow, low-density intra-crustal body with its top close to, or at, the surface. Although comparable gravity anomalies are commonly associated with large (similar to 20 km) ash-filled calderas, as seen at Yellowstone or Toba, there is no geological evidence on James Ross Island for a similar structure. We therefore propose that the James Ross Island volcanic edifice subsided into the thick underlying pile of relatively soft Jurassic and Cretaceous sediments, which were displaced by low-density hyaloclastite breccia. The type of deformation envisaged is similar to that associated with Concepciou, or lwaki volcanoes in South America, although Mt Haddington is much larger. (C) 2009 Elsevier B.V. All rights reserved.

Recent satellite observations of Thwaites Glacier in the Amundsen Sea Embayment, West Antarctica, have shown that the glacier is changing rapidly. The causes of its dynamic behavior are uncertain but are of concern because this glacier has the most negative mass balance of all Antarctic glaciers. To better understand Thwaites Glacier's subglacial setting, we conducted a multi- instrumented aerogeophysical survey of its catchment and present here the first gravity results. We employed a new gravimeter, and it performed well despite extreme conditions and an unusual survey design. The unleveled free- air gravity anomalies have a 2.3 mGal RMS error and a 9 km spatial resolution. Despite slightly higher than standard noise levels, the free- air anomalies correlate well with radar- derived subglacial topography. The new airborne gravity data assist in interpreting radar- identified bedrock features and are an ideal basis for future studies of subglacial geology and its control on the dynamics of Thwaites Glacier.

As we enter the IPY geological boundary conditions for the stability of the East Antarctic Ice Sheet (EAIS) remain largely unknown. During the 2005/06 field season a major new aerogeophysical survey was performed over the Wilkes Subglacial Basin, the Transantarctic Mountains and Dome C. Over 60,000 km of new airborne radar, aeromagnetic and aerogravity data were collected. We will describe the survey layout and methodologies and present new geophysical images as a tool to address the highly contentious stability of the EAIS during warmer than present palaeoclimates, the underlying crustal structure, and to study subglacial lakes

The polar regions play an important role in Earth's geodynamic and climatic systems. Modern airborne geophysical surveys combine radio-echo sounding, aeromagnetic and aerogravity methods to explore the geology of these regions. This paper reviews some recent aerogeophysical investigations undertaken by the British Antarctic Survey to: 1) Image subglacial rifts of Jurassic age in western Dronning Maud Land, which were associated with early Gondwana break-up; 2) Investigate crustal growth over the Antarctic Peninsula by Cretaceous arc magmatism and terrane accretion along the paleo-Pacific margin of Gondwana; 3) Analyse geological boundary conditions for presentday ice dynamics over Coats Land.

The West Antarctic Ice Sheet is currently undergoing rapid change in particular over the Amundsen Sea Embayment (ASE). Previous aerogeophysical investigations over the Ross Sea Embayment reveal that the underlying geology may modulate ice sheet dynamics and hence stability. But what are the interplays between sub-ice geology and the apparently thinning and retreating glaciers of the ASE region? We will present new aerogeophysical data to provide a window on the “lithospheric cradle” for this part of the West Antarctic Ice Sheet, thereby contributing towards studying the largest glaciated continental rift system on Earth, the West Antarctic Rift System.

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