The Mesozoic South Atlantic Ocean and evolution of its continental margins

Abstract

Gravity and magnetic anomalies bordering the continental margins of the southern South Atlantic Ocean are compared, in detail, on conjugate sides of the ridge crest, and a model for the boundary between oceanic and continental basement is given. The area of study includes the predominantly sheared margins of the Agulhas‐Falkland fracture zone and the rifted margins of Argentina and southern Africa south of the Rio Grande Rise and Walvis Ridge, respectively. These margins have associated with them, for the most part, linear magnetic anomalies that can be modeled as edge effect anomalies separating oceanic from continental basement. Coincident with the magnetic anomalies are gradients in the isostatic gravity anomaly. We have taken the location of these geophysical lineaments on the African margin and rotated them clockwise to fit the anomalies on the Argentine margin. This fit, which gives us a new pole of total closing for the South Atlantic Ocean, obviates, for the most part, the gaps and overlaps observed in other reconstructions. The improved fit thereby suggests rigid plate behavior and minimum stretching of continental crust during the early opening of the southern South Atlantic Ocean. A zone of crustal stretching may be present in the southernmost Argentine and Cape basin margins. New poles of early opening for the South Atlantic Ocean have been determined (from 130 to 107 m.y. B.P. and from 107 to 80 m.y. B.P.) utilizing the above reconstruction as well as the strike of the Agulhas‐Falkland fracture zone where it is well determined. The earliest pole, which is located much farther south than previously determined early poles, satisfies not only the geophysical data in the southern regions but allows us to explain a number of outstanding problems north of the Rio Grande Rise‐Walvis Ridge area. These problems include the timing of the onset of sedimentation on the northern Brazil margin, the origin of the compressional features along the Venezuela margin, and the onset of open marine circulation between the North and South Atlantic oceans. Paleoreconstructions using the new early poles also align very well the seaward edge of the salt boundaries off Brazil and West Africa. The age of the salt, as inferred from the paleoreconstruction to its seaward boundaries, is younger than the age of magnetic anomaly M0. Furthermore, our paleoreconstructions show barriers for salt deposition not only across its southern termination (Walvis Ridge area) but also farther north in the equatorial regions. The salt, for the most part, has been deposited on oceanic crust. The new predrift reconstruction and early opening poles, taken together with the new identifications of the Mesozoic and Late Cenozoic sequences of magnetic anomalies, allow us to determine the magnitude and time interval of spreading center migration. In particular, we can demonstrate that ridge crest migrations of ∼1000 km have occurred along the strike of the Falkland escarpment. We demonstrate that the isostatic gravity gradient associated with the boundary between oceanic and continental basement is independent of the location of major sediment accumulations. We have modeled this anomaly as resulting from elevated oceanic basement adjacent to continental crust. This crustal model satisfies the limited upper crustal seismic data available near margins. The oceanic basement elevations are relics of a transient phenomenon associated with young rifted margins such as the East African and Red Sea rifts.