Deep reflections from within oceanic crust are observed on two widely separated multichannel seismic reflection profiles in the southern Angola Basin. On University of Texas Institute for Geophysics line AM-26-C, the deep reflection can be traced for 9 km as it slopes gently to the west in an interval 0.25–0.40 s below the interpreted basement-sediment interface. Various explanations for the intracrustal reflection, such as an artifact of data processing, a multiple, the presence of high-velocity sediments immediately overlying basement, and sideswipe or diffraction from the basement surface, are each examined and rejected. A 5.0 km/s interval velocity calculated for the basement material overlying the deep reflection, in conjunction with refraction studies of ocean crust in the Atlantic and recent Deep Sea Drilling Project Leg 83 results from the Costa Rica Rift (site 504B), all suggest that the deep reflection on AM-26-C originates from the layer 2B/layer 2C boundary.
The deep reflection on Bundesanstalt für Geowissenschaften und Rohstoffe line 78-41 occurs 1.60 to 1.90 s below the oceanic basement surface, is identifiable as a subhorizontal surface for more than 50 km, and appears generally less continuous and of lower amplitude than the deep reflection on AM-26-C. This deep reflection exhibits “velocity pull-up” characteristic of primary reflections beneath varying thicknesses of sediment cover of lower velocity, and it cannot be a multiple or a diffraction. We conclude that this deep reflection originates from within layer 3.