Beschreibung: The structure of the Mid-Atlantic Ridge at 5°S was investigated during a recent cruise with the FS Meteor. A major dextral transform fault (hereafter the 5°S FZ) offsets the ridge left-laterally by 80 km. Just south of the transform and to the west of the median valley, the inside corner (IC – the region bounded by the ridge and the active transform) is marked by a major massif, characterized by a corrugated upper surface. Fossil IC massifs can also be identified further to the west. Unusually, a massif almost as high as the IC massif also characterizes the outside corner (OC) south of the inactive fracture zone and to the east of the median valley. This OC massif has axis-parallel dimensions identical to the IC massif and both are bounded on their sides closest to the spreading axis by abrupt, steep slopes. An axial volcanic ridge is well developed in the median valley both south of the IC/OC massifs and in an abandoned rift valley to the east of the OC massif, but is absent along the new ridge-axis segment between the IC and OC massifs. Wide-angle seismic data show that between the massifs, the crust of the median valley thins markedly towards the FZ. These observations are consistent with the formation of the OC massif by the rifting of an IC core complex and the development of a new spreading centre between the IC and OC massifs. The split IC massif presents an opportunity to study the internal structure of the footwall of a detachment fault, from the corrugated fault surface to deeper beneath the fault, without recourse to drilling. Preliminary dredging recovered gabbros from the scarp slope of the rifted IC massif, and serpentinites and gabbros from the intersection of this scarp with the corrugated surface. This is compatible with a concentration of serpentinites along the detachment surface, even where the massif internally is largely plutonic in nature.

Beschreibung: New multibeam bathymetric data from the southeastern Weddell Sea show significant differences in surface morphology of the outer continental shelf and slope between two adjacent cross-shelf troughs. These are the Filchner Trough and a smaller trough to the east which we refer to as the ‘Halley Trough’. Multibeam bathymetric data, acoustic sub-bottom profiler and seismic data show major differences in the incidence and morphologies of submarine gullies, channel systems, submarine slides and iceberg scours, and in sediment deposition. These large-scale differences suggest significant variation in slope and sedimentary processes and in the environmental setting between the two troughs, leading to much greater deposition at the mouth of the Filchner Trough. Bedforms, including a terminal moraine and scalloped embayments on the outer shelf of the Halley Trough, provide insight into the relative timing and extent of past ice-sheet grounding and point to grounded ice near to the shelf edge during the Late Quaternary. The new data reveal two large-scale submarine slides on the upper slope of the eastern Crary Fan, a trough mouth fan offshore from the Filchner Trough. Both slides head at the shelf edge (~ 500 m water depth), with the largest slide measuring 20 km wide and with an incision depth of 60 m. Multibeam and seismic data show elongate slabs on the seafloor surface of the mid-slope. The lack of a discernible sedimentary cover suggests that they were generated after the Last Glacial Maximum (LGM). This is unusual because post-LGM submarine slides are very rare on the Antarctic continental margin, and to our knowledge, no other post-LGM slides have been documented on an Antarctic trough mouth fan. Because the slides occur on a part of the continental slope where the deposition of glacial debris was greatest, we speculate that weaker, unconsolidated sedimentary layers within the subsurface are important for slide initiation here.