Reflection seismic investigations in the Weddell Sea and along the Antarctic peninsula

Within the framework of the first phase of the Belgian Antarctic Research Programme (1986-1988), the Renard Centre of Marine Geology (RCMG) at the State University of Ghent has participated in two cruises organized by the Alfred-Wegener-lnstitut fur Polar- und Meeresforschung (Bremerhaven). Both cruises addressed different marine geological domains: the passive margin of the eastern Weddell Sea (cruise Antarktis V/ 4) and the active western margin of the Antarctic Peninsula (cruise Antarktis VI/2). In the Weddell Sea some 2650 km of high-resolution reflection seismic profiles have been shot, partly in connection with the drilling operations of ODP Leg 113 off Cape Norvegia. This site has been used as a stratotype for an integrated seismic-stratigraphic reassessment of the Weddell Sea Basin, a joint venture of German, Norwegian and Belgian research teams. The study off Cape Norvegia also allowed to propose a new tectonic model for the origin of the Explora-Andenes Escarpment and the associated outer high. Another study region in the eastern Weddell sea was the distal part of Crary Fan, off Halley Bay. The very high resolution achieved there over the fan units allowed a detailed analysis of sediment deformations and mass transport. Large unconformities, calibrated by tentative long- range correlations to ODP Site 693, might yield some clues about the paleoceanography of the Mesozoic and Cenozoic South Atlantic domain. They seem to support the hypothesis of a Late Mesozoic Transantarctic initiation of an early Antarctic Circumpolar Current. Cenozoic unconformities are well controlled by paleoclimatic factors. The Antarctic Peninsula survey involved the recording of some 1800 km of reflection data with larger penetration, both in Bransfield Strait and along the Bellingshausen margin. The reassessment of the seafloor magnetic anomalies helped to constrain the seismic-stratigraphic analysis and also yielded some new insight in the local dynamics of spreading and subduction, e.g. suggesting phenomena like a slab pull induced spreading accelleration preceding ridge-trench collision. The seismic-stratigraphic interpretation of the oceanic sediments is in good agreement with the results found at DSDP well 325 and with those proposed by Japanese studies further to the west. Seismic profiles across Anvers and Hero Fracture Zones illustrate different aspects of thermal plate contraction, including thermal bending and magmatic diapirism. The seismic evidence that the submarine ridge of Hero F.Z. might be a diapiric serpentinite body, similar to the situation at Vema F.Z. in the central Atlantic, has several implications for the segmentation of the western margin of the Antarctic Peninsula. One new aspect about this segmentation confirmed by the present study is the presence of a fore-arc basin on the shelf south of Hero F .Z.