WEST AFRICAN AND BRAZILIAN MARGINS: PRE-DRIFT TO EARLY DRIFT PLATE CONFIGURATIONS AND CONJUGATE SEISMIC PAIRINGS

Seismic reflection data and GETECH gravity imagery have been used to rigidly close the Equatorial South Atlantic Ocean to pre-drift and early drift configurations. The closures are the first in this area to employ extensive seismic data bases from both margins to constrain models of closure.

Prior to 120 MY, rift deformation was dominated by left-lateral dislocation along a proto-transform fault zone parallel to the Brazilian margin between 80-130 S. This zone, the "Jacuipe-Sergipe- Alagoas Transform " (JSAT), has a length of about 400 km, with a reconstructed width of 100 km. Conjugate fits of paired seismic lines from the two margins suggest 100+ km of dislocation prior to oceanic crustal genesis. The duration of the deformation probably exceeds several MY. The JSAT was a complex zone of transcurrent fault branches, relays, pull-apart basins, and compressional ridges. Elements of all are suggested in the seismic data.

Both the Jatoba-Tucano-Reconcavo (JTR) rift zone north of the JSAT and the Jequitinhonha-Cumuruxatiba/Gabon (JC-G) margins to the south were obliquely extended during this transform phase. De-stretching the offshore segment of the Gabon margin by 100% parallel to the JSAT requires about 100 km of shortening, consistent with the inferred dislocation along the JSAT. Conjugate seismic pairings across the adjoined JT-G margins suggest a simple shear extensional style . The low-angle main detachment dips toward the Gabon side and deepens beneath it, dividing a narrow band of abruptly extended Brazilian Sao Francisco cratonic crust (lower plate) from a broad zone of extended Congo Fold Belt rocks (upper plate).

At 120 MY the direction of plate motion changed by 420 to almost E-W relative to the South American plate. This marked the onset of seafloor spreading in this sector of the Atlantic, abandonment of the JTR rift zone, cessation of extension of the JT-G margin sectors, and the end of evaporite deposition. Soon thereafter, the margins along the prior JSAT gravitationally collapsed on both plate sides, locally sliding sheets of pre-spreading, rift -phase rocks and early post-rift salt sequences seaward onto the developing oceanic plate sectors. The Brazilian plate segment south of the JSAT also experienced abrupt collapse during the early opening phase.