Processes of magmatic and tectonic accretion of oceanic lithosphere at mid ocean ridges : constraints from a seismic refraction study at he Mid Atlantic Ridge near 21.5° N Mid oceanic ridges are plate boundaries where new oceanic crust is created. Especially slow spreading ridges like the Mid Atlantic Ridge (MAR) reveal a complex structure denoted by magmatic and tectonic processes. In the working area of this seismic refractions study both types of crustal accretion are present. The northern segment (22.2ʿ N) compensates tectonically the tensional stresses caused by the plate tectonic movements of the African and the Northern American plates. So called detachment faults or oceanic core complexes (OCCs) develop during that tectonic phase. In the meantime the southern segment (21.5ʿ N) is a magmatically robust segment. The peculiarity of this segment is that it growths south and northwards along the ridge axis starting at about 5 m.y. ago. Ridge propagation was strong enough to break through a stable small offset transform fault. During propagation the transform migrated southwards leaving behind a V shaped structure the so called inner and outer pseudofaults. From five seismic refraction and wide angle profiles ridge parallel and ridge crossing the seismic velocity structure was observed. The results show a strong crustal variation. The ridge crossing profiles illustrate the temporal evolution of the crustal accretion within the magmatic robust segment. Past magmatic activities can be reconstructed. The different morphological and geological features of the area required different inversion and modelling procedures. A broad variety of methods for interpretation of the collected geophysical data were applied to gain a subsurface image and to allow a geological reconstruction ...