ALBERT

Description: Walvis Ridge is a prominent aseismic ridge in the South Atlantic, stretching across the whole oceanic crust from the African continent to the Mid Atlantic Ridge, representing the trail of the Tristan da Cunha hotspot. This proposed deep mantle plume emplaced the Parana flood basalts in South America and the Etendeka flood basalts on the African continent, prior and during the breakup of Gondwana. This temporal proximity indicates a causal relationship between the arriving plume head and the continental breakup. To estimate the influence of the plume, a large-scale geophysical experiment was conducted in 2011. The P-wave velocity models derived from seismic refraction data are presented here. A 480 km long profile consisting of 27 ocean bottom stations crosses the ridge approximately 600 km west of the coast, while a second profile is located ridge-parallel along its crest with an extension on the continent. 28 ocean bottom stations, 50 land receiver and 8 dynamite shots are distributed along the total length of 730 km. Crustal velocities beneath Walvis Ridge range between 5.5 km/s and 7.0 km/s, which are typical velocities for oceanic crust. The crustal thickness, however, is approximately three times larger than of normal oceanic crust: 17 km in the western part increasing to 22 km towards the continent. The continent ocean transition is characterized by 30 km thick crust with a high velocity body (HVB) in the lower crust and seismic velocities up to 7.5 km/s. The western boundary of the HVB is at a similar longitude as HVBs observed south of Walvis Ridge. But different from those, the eastern boundary lies well within the continental domain, at the 40 km thick crust of the Kaoko fold belt. Here, the variation of seismic velocities indicates that hot material intruded the continental crust during the initial rifting stage. However, beyond this relatively sharp boundary (40 km wide), the remaining continental crust seems unaffected by intrusions. The cross-profile indicates that Walvis Ridge might be broader than its topographic expression and that the northward lying seamounts are part of the ridge. A HVB can only be found at the northern flank of the ridge, but not at its base. We conclude, that the postulated arriving plume head did not modify the continental crust on a large scale, but was a rather regional anomaly.