Description: Highlights • Crustal structure of Walvis Ridge reveals high seismic velocities in the lower crust intruding the African continent. • This modified crust is localized to approx. 100 × 100 km within the continent. • No indication for a large plume head observed The opening of the South Atlantic is a classical example for a plume related continental breakup. Flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at Tristan da Cunha. To determine the effect of the proposed plume head on the continental crust, we acquired wide-angle seismic data at the junction of the Walvis Ridge with the African continent and modelled the P-wave velocity structure in a forward approach. The profile extends 430. km along the ridge and continues onshore to a length of 720. km. Crustal velocities beneath the Walvis Ridge vary between 5.5. km/s and 7.0. km/s, a typical range for oceanic crust. The crustal thickness of 22. km, however, is approximately three times larger than of normal oceanic crust. The continent-ocean transition is characterized by 30. km thick crust with strong lateral velocity variations in the upper crust and a high-velocity lower crust (HVLC), where velocities reach up to 7.5. km/s. The HVLC is 100 to 130. km wider at the Walvis Ridge than it is farther south, and impinges onto the continental crust of the Kaoko fold belt. Such high seismic velocities indicate Mg-rich igneous material intruded into the continental crust during the initial rifting stage. However, the remaining continental crust seems unaffected by intrusions and the root of the 40. km-thick crust of the Kaoko belt is not thermally abraded. We conclude that the plume head did not modify the continental crust on a large scale, but caused rather local effects. Thus, it seems unlikely that a plume drove or initiated the breakup process. We further propose that the plume already existed underneath the continent prior to the breakup, and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts.

Description: A seismic refraction profile across Langeland (Denmark) obtained from land stations recording airgun shots allowed to resolve upper crustal velocities to a depth of 8 km. The profile traverses the proposed Caledonian Deformation Front and the Ringkoebing-Fyn High. The Ringkoebing-Fyn High is about 10 km wide and the top basement lies less than 2 km below the surface. Basement velocities as high as 6.4 km/s, at depths between 6 and 8 km, can be best explained by compositional changes between adjoining basement units to the north and south. South of the Ringkoebing-Fyn High another high velocity basement unit is encountered and most probably represents a basement affected by the Caledonian orogeny. Along this profile on Langeland the positions of the Caledonian Deformation Front and the northern limit of the Zechstein deposits coincide.

Description: Favourable sea-ice conditions gave way to an acoustic survey offshore NE Greenland during RV Polarstern ARK-XXIV/3 leg in 2009. The acquired data set clearly depicts sediment ridges in an area of app. 18 × 9 km. The ridges are found in water depths between 270 and 350 m. The sediment ridges are 2.5–9 km long, 50–250 m wide and 5–25 m high. In profile, most of these ridges are characterized by steep slopes towards Northwest and gentle slopes towards Southeast. Their internal structure, imaged by parametric echo-sounding data, shows that they are positive sedimentation features rather than erosive remnant structures. Arcuate shape, joint orientation and position on a basal till are indicative for end moraines. Because they are positioned within the Westwind Trough on a basal till that extends further east, we consider these ridges end moraines of the Westwind ice stream reported by Evans et al. (2009), Marine geophysical evidence for former expansion and flow of the Greenland Ice Sheet across the north-east Greenland continental shelf. Journal of Quaternary Science (2008), doi: 10.1002/jqs.1231.). Based on our hydro-acoustic data, we interpret these end moraines to be formed by short-lived re-advances during an overall recession of the ice margin. However, they could also be deposited during halts of the grounding line (comparable to De Geer moraines) though their morphological characteristics are slightly different from most published De Geer moraines. The ages for the moraine deposition can be inferred from a thin sedimentary drape indicating timing between Lateglacial and early Holocene. This set of end moraines is direct evidence for a dynamic behaviour of the marine-based ice stream during the last deglaciation on the NE Greenland shelf.