Search

Landforms produced beneath former ice sheets offer insights into inaccessible subglacial processes and present analogues for how current ice masses may evolve in a warming climate. Large subglacial channels cut by meltwater erosion (tunnel valleys [TVs]) have the potential to provide valuable empirical constraints for numerical ice-sheet models concerning realistic melt rates, water routing, and the interplay between basal hydrology and ice dynamics. However, the information gleaned from these features has thus far been limited by an inability to adequately resolve their internal structures. We use high-resolution three-dimensional (HR3-D) seismic data (6.25 m bin size, ~4 m vertical resolution) to analyze the infill of buried TVs in the North Sea. The HR3-D seismic data represent a step-change in our ability to investigate the mechanisms and rates at which TVs are formed and filled. Over 40% of the TVs examined contain buried glacial landforms including eskers, crevasse-squeeze ridges, glacitectonic structures, and kettle holes. As most of these landforms had not previously been detected using conventional 3-D seismic reflection methods, the mechanisms that formed them are currently absent from models of TV genesis. The ability to observe such intricate internal structures opens the possibility of using TVs to reconstruct the hydrological regimes of former mid-latitude ice sheets as analogues for contemporary ones.

We present the first multichannel seismic (MCS) reflection line that crosses the South Sandwich Trench, South Sandwich island arc, and East Scotia Sea backarc basin. The line is used in combination with earthquake catalogue data to interpret the strain distribution across the Sandwich plate and the relationship of forearc structures to processes operating at the trench. The MCS data reveal a 1.2-km-high fault scarp associated with a 20-km-wide arcward-tilted block in the mid-forearc; these features indicate large-scale gravitational collapse, and earthquake data are consistent with trench-normal extension at shallow depth in this area. There is, however, little evidence of distributed extension within the interior of the Sandwich plate. The MCS data show a small frontal wedge that achieves its maximum thickness only 18 km from the trench. Backarc magnetic data, mid-forearc extension, and the small size of the frontal wedge are all consistent with long-term and ongoing subduction erosion. Earthquake data suggest that this erosion is taking place in an environment of low interplate stress.

Multichannel seismic profiles across the Pacific margin of the Antarctic Peninsula show a series of oblique progradational sequences. These sequences exhibit a variety of unusual characteristics that suggest they were produced by the action of ice sheets grounded out to the shelf edge at times of glacial maximum. Reflection events from deeper stratigraphic levels, followed down the continental slope and onto the rise, overlie ocean crust of known age, showing that at least eight such glacial sequences have been deposited within the past 6 m.y. Similar groundings have probably occurred on most Antarctic margins, but the depositional record is particularly well preserved at this margin because of Pliocene-Pleistocene thermal subsidence. Neogene global sea-level fluctuations have been attributed to changes in volume of continental ice sheets. The depositional sequences on the Pacific margin of the Antarctic Peninsula are thought to record West Antarctic ice-sheet fluctuations directly. Further investigation of these sequences would assess the relation between fluctuations in ice volume and the low-latitude record of global sea-level change.