HISTORY OF RIDGE-CREST OFFSET AT THE FALKLAND-AGULHAS FRACTURE-ZONE FROM A SMALL-CIRCLE GEOPHYSICAL PROFILE

Summary. The magnetic and bathymetric profiles are described along a single ship track following a small circle of South Atlantic opening about 100 km south of the Falkland‐Agulhas Fracture Zone (FAFZ). The magnetic anomalies reveal a 700 km westward jump of the crest of the Mid‐Atlantic Ridge at about 61–64 Ma ago and suggest a second 100 km westward jump 58–60 Ma ago. The destinations of both jumps are now occupied by narrow rough‐topped ridges standing about 2 km above the adjacent ocean floor. It is suggested that these were formed at the times of the jumps, by volcanic and intrusive igneous activity associated with the rupture of older oceanic crust. The original offset of the ridge crest at the FAFZ was 1400 km, the length of the continental Falkland Plateau; of this only about 200 km remains. 800km of offset was eliminated by the two ridge jumps already detected, leaving 400 km unaccounted for. Under reasonable assumptions, a further westward ridge jump of 380 km at about 98 Ma is proposed, the destination of which is marked by the only other similarly anomalous elevations on the bathymetric profile, one of which is the Agulhas Plateau. Existing sediment samples from the Agulhas Plateau are consistent with this interpretation of its origin. Magnetic anomalies directly east of the Falkland Plateau are identical to those in the eastern Cape Basin, which suggests strongly that the South Atlantic spreading system extended some distance south of the FAFZ from its inception. The anomalous elevations extend about 600 km south of the FAFZ indicating the southward extent of the ridge jumps and perhaps therefore of the spreading system at 98 and 60 Ma. In many ways the elevations resemble others commonly identified as hotspot phenomena, some of which may also have formed initially at ridge jump destinations. The correspondence between the observed magnetic profile and a synthetic profile generated from the LaBrecque time scale is particularly good for the past 40 Ma. This allows a detailed comparison; using the enhanced sensitivity of a reduced distance plot, which reveals several departures from linearity. Some revision of the LaBrecque time scale may be required. The reduced distance plot also shows details of the asymmetric spreading which, being of the same degree and in the same sense on both sides of the FAFZ, did not change the offset significantly. The asymmetry resembles that found south of Australia in being without discernible ridge jumps but displaying abrupt changes in degree, and in that the more slowly accreting (African) plate is the more elevated. Copyright © 1979, Wiley Blackwell. All rights reserved