The South Shetland Islands Region

This subregion includes the Bransfield Strait and the continental margin around the South Shetland Islands (Fig. AP-1). Bransfield Strait is a 2,000 m deep rift basin that is actively extending at 7mm/y. (Dietrich et al., 2004). The time of initial extension and the oldest age of basin sediments are uncertain, but may be 4 Ma (Barker and Dalziel, 1983) or ~6Ma (Larter and Barker, 1991a). Gamboa and Maldonado (1990) speculate that

Bransfield Strait may have opened earlier, during the early Miocene, and have been continuous with mid-shelf basins to the southwest.

From MCS data, Gamboa and Maldonado (1990) identify 'rift' and 'drift' sequences in Bransfield Strait. 'Drift' sequences prograde the shelf and are about 1km thick. In SCS data, Jeffers and Anderson (1990) define four glacio-eustatic sequences within the 'drift' sequences, and interpret all four sequences as being younger than 3 Ma. Prieto et al. (1999) use different SCS data to define eight seismic units that comprise interfingering slope and basinal deposits within the 'drift' sequences. They interpret the slope units as having been deposited directly from grounded ice during glacial periods, and interpret the basinal units as having been deposited by mass flow processes during deglaciations and interglacial periods.

From SCS data, Banfield and Anderson (1995) identify sediment mound features that they infer to be glacial grounding moraines in up to 1,000 m water depth on the southeastern flank of Bransfield Strait. They speculate that mounds at ~700m depth mark the maximum advance during the LGM. Deep troughs with mega-scale lineations are incised into the shelf and are interpreted as the paths of palaeo-ice streams (Banfield and Anderson, 1995; Canals et al., 2002).

MCS profiles across the continental slope NW of the South Shetland Islands reveal a forearc basin, with more than 1.5 km sediments, that is bounded to the NW by a small accretionary prism (Maldonado et al., 1994a, b). The prism overthrusts trench-fill sediments that may have been deposited rapidly and are up to 1 km thick (Maldonado et al., 1994a, b; Kim et al., 1995).

Other seismic-reflection surveys have been done in the region by British, Polish, German, Spanish, US, Italian, Chinese and Korean research groups, but published results focus on the tectonic evolution of the region and tectonic processes (Barker, 1976; Guterch et al., 1985; GRAPE Team, 1990; Acosta et al., 1992; Henriet et al., 1992; Grad et al., 1993; Barker and Austin, 1994, 1998; Bochu et al., 1995; Gracia et al., 1996; Jin et al., 1996; Jin and Kim, 1998; Prieto et al., 1998; Jin et al., 2002), on gas hydrates (Lodolo et al., 1993, 2002; Tinivella et al., 1998, 2002; Jin et al., 2003), and on a large submarine slide (Imbo et al., 2003).

Swath bathymetry data exist over most deep-water parts (> 1,000 m) of Bransfield Strait (Lawver et al., 1996; Gracia et al., 1997). Along Boyd Strait, 'bundle structures' and mega-scale glacial lineations occur and confirm palaeo-ice stream flow during glacial periods (Canals et al., 2000; COHIMAR/SEDANO Scientific Party, 2003). MCS data along outer Boyd Strait reveal glacial progradation of the margin (Maldonado et al., 1994a, b), and deep-tow boomer profiles reveal a glacier grounding zone wedge near the shelf edge (Vanneste and Larter, 1995).

The only scientific drilling in the South Shetland Islands region was done at SHALDRIL-I Site 1 in Maxwell Bay (Fig. AP-1), where an expanded sequence of Holocene diatomaceous muds, ~105m thick, overlying a clay-rich diamicton was sampled (Shipboard Scientific Party, 2005).

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