Brazilian and African passive margins of the Central Segment of the South Atlantic Ocean: Kinematic constraints

Abstract : The thinning of passive continental margins is usually explained by models using pure stretching or simple shear. These models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins (more than 250 km for the Brazilian and Angolan Margins). Refraction/reflection data together with the most recent and tightest pre-opening fit using continental and oceanic, geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part (upper continental crust) and an allochthonous part (exhumed material). The thinning process, which evolves in an elevated position of the system until at least the break-up, seems to be depth dependent and to mainly concern the lower/middle crust, which we postulate is exhumed. This exhumation does not explain the entire thinning of the system: horizontal motions cannot alone explain the formation of the huge Angolan–Brazilian Basin. Similar observations are made on the whole Central Segment of the South Atlantic Ocean. Part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process.