Mesozoicseafloor Spread Ing Bordering Conjugate Continental Margins Of Angola And Brazil

ABSTRACT The seaward edges of the Brazil and Angola Gabon salt plateaus are defined by geophysical observations. Paleo-reconstruction of the South Atlantic Ocean to the salt edges suggests that the salt was formed in one continuous basin. After the deposition of the salt, an eastward migration of the paleo-mid-ocean ridge axis of ~200km caused a major asymmetry in part of what is now the salt basins. Magnetic lineations are observed on the Sao Paulo Plateau, which form a pattern symmetrical about the center of the constructed salt basins. The axis of symmetry of the magnetic anomalies delineates the position of the extinct spreading axis. We suggest that the magnetic anomalies were formed by seafloor spreading between the time of early opening of the South Atlantic in the Valanginian and the time of the salt deposition and ridge crest jump in the latest Aptian. INTRODUCTION Diapiric structures attributed to salt in origin are observed on seismic reflection records bordering the continental margin of Brazil between about 12°S to the southern extent of the Sao Paulo Plateau at about 28°S.1–4 Similar structures are present bordering the opposing continental margin off Africa.5–9 Based on paleo-reconstructions, 4–10 the similarities in the deep seismic reflectors bordering the two margins,11 and the similarities in age of the salt, it has been suggested that (i) the salt was originally deposited in one continuous basin, and (ii) after the deposition of the salt, an eastward migration of ~200km of the paleo-mid-ocean ridge axis caused an asymmetrical split in the salt basin. In this paper we will examine the geophysical measurements which traverse the Brazilian and African salt basins and show that these data, primarily the marine magnetics, are consistent with the concept that an extinct spreading axis & present beneath the Sao Paulo Plateau. This spreading axis appears to have been active from the time Africa and South America separated in the Valanginian until the time of an eastward ridge crest jump of 200km in the Aptian. REGIONAL SETTING The origin of the salt is closely tied to the early spreading history of the South Atlantic Ocean. Based on magnetic anomalies off Southern Africa and Argentina,14 the South Atlantic began opening in the Valanginian (~130mybp). The oldest identified seafloor spreading anomaly in the deep ocean basin north of the Rio Grande Rise and Walvis Ridge is anomaly 3415 (~80mybp). We show in figure 1 a paleo-reconstruction of the African continent with respect to South America at this time. Also, reconstructed to anomaly 34 time is the African salt basin and present day earthquake epicentres. Synthetic flow lines are given as well as the synthetic mid-ocean ridge axis for 80 mybp. This synthetic ridge axis assumes symmetrical seafloor spreading and is located midway between shelf edges on either side of the 80mybp South Atlantic Ocean.