The Plate Tectonics Theory is undoubtedly the most important discovery in Earth Sciences. It is based on the concept of tectonic plates (lithosphere), which float and move over the asthenosphere. Although the lithosphere is a basic building block of the plate tectonics theory, its nature, its thickness, and the defining character of its boundary with the asthenosphere ("lithosphere-asthenosphere boundary: LAB") remain a matter of debate. In order to address this fundamental problem, we have proposed a combined approach of acquiring ultra-deep seismic reflection, refraction (OBS), teleseismic (BBOBS) and magnetotelluric (MT) data across the Atlantic Ocean to study the evolution of the oceanic lithosphere from zero age at the ridge axis to 75-80 Ma near the continental margin. We have chosen the equatorial Atlantic region for this study as it contains large fracture zones that maintain their azimuths over more than 2000 km, and between which 2D profiles could be shot as straight lines. These combined datasets will allow us to image the LAB at different scales continuously from 0-80 Ma. These data will also allow to image melt lenses in the mantle the beneath the ridge axis, and provide quantitative information on processes of ageing of the oceanic crust and lithosphere, especially hydration processes. A profile across large-offset fracture zones will provide insight into both the shallow and structure of these fracture zones, and will detect anticipated variations in lithospheric thickness across them.

A total 2775 km of ultra-deep reflection data were acquired in March-April aboard Schlumberger-WesternGeco's vessel M/V Western Trident, using a 12 km long 4-component streamer, covering 0-75 Ma of oceanic lithosphere in E-W direction and crossing three major fracture zones (St Paul, Romanche and Chain) of the Mid-Atlantic ridge in N-S direction. In  2016, 39 BBOBS and 40 MT instruments were deployed for one year in the form of a grid in the same area. Then in late 2017, a long 2D seismic refraction profile coincident with a part of the ultra-deep seismic data was shot: this 1200-km long profile equipped with 70 OBS will image 0-50 Ma lithosphere on the African plate. To complete the data acquisition part of this project, here we propose to acquire two complementary refraction profiles (550 km and 800 km, respectively) along the rest of the ultra-deep seismic reflection lines, using 60 OBS: a first profile extending over 0-25 Ma of oceanic lithosphere of the South American plate and a second profile across the large-offset equatorial fracture zones. We propose to use a large airgun source with a shot interval 300-400 m, allowing to record arrivals up to 400 km offset with rays penetrating down 30-50 km depth.