Seafloor spreading in the Weddell Sea and southwest Atlantic since the Late Cretaceous

Geosat radar altimetry defines a pattern of closely spaced curvilinear gravity anomalies in the northern Weddell Sea and adjacent parts of the southwest Atlantic. These anomalies are caused by a fracture zone pattern resulting from seafloor spreading, mainly on the southern flank of a ridge system which now exists only to the east of the South Sandwich Trench. Flowlines derived from these anomalies are combined with isochrons resulting from an interpretation of all available magnetic anomaly profiles in the region, to produce a new tectonic summary chart. This chart reveals major changes in spreading direction during (i) the Cretaceous magnetic superchron, (ii) the latest Cretaceous/Paleocene (∼ 65 Ma) and (iii) the mid-Eocene (∼ 50 Ma). The latter two changes enclose a period of very slow spreading, and define a kink in the flowlines which is discernable throughout most of the Weddell Sea. Preliminary modelling indicates that trends from the western Weddell Sea to near the South Atlantic triple junction can be explained by the separation of just two plates. There is some disagreement between the observed flowlines and those predicted by rotations published for the South Atlantic and Southwest Indian Ridge, but abrupt changes in spreading direction and a slowing of rates are observed on all three branches of the South America-Africa-Antarctica plate system between the times of anomalies C31 and C21. This suggests that the lack of closure results from errors in the calculated rotations, and that the seafloor of the Weddell Sea and southwest Atlantic was created as a direct consequence of South America-Antarctica plate motion. The data do not support the existence of a Late Cretaceous Malvinas Plate within the Weddell Sea, although microplates in the northernmost Weddell Sea do appear to have formed briefly as a result of interactions between the former spreading ridge and a subduction zone located at the South Scotia Ridge.