Temporal and spatial variability of seafloor spreading processes in the northern south Atlantic

Abstract

The paper reports the results of an aeromagnetic study of the Mid‐Atlantic Ridge in the northern South Atlantic. The study consists of 38,000 km of low altitude magnetics tracks centered on the ridge covering nearly 700,000 km2 south of Ascension Island. Most tracks extend out to crust of anomaly 5B age. Large areal coverage coupled with high resolution allows comparison of crustal generation processes over space (along ridge strike) and time (across strike). Temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion. The total spreading rate increased sharply at 10 Ma and then decreased between 6 Ma and the present. The time variation of spreading rate is similar to that seen in other areas such as the North Atlantic and the southern and eastern Pacific. The plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones. In several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones. The periods of plate boundary modification correlate with the increase and decrease in spreading rate. Spatial heterogeneities are caused by the influence of the Ascension hotspot on the Mid‐Atlantic Ridge in the northern portion of the study area, and include two apparent seamount chains formed by the hotspot. Ascension Island and several of the seamounts appear to have been produced some distance away from the hotspot location (presently about 225 km southeast of Ascension Island) by channeled subaxial magma flow beneath the spreading center. The seamount chains, therefore, do not mark hotspot tracks. Large local variations in spreading rate and degree of asymmetry along strike are also found.