Evolution of the Southwest Indian Ridge from the Late Cretaceous (Anomaly 34) to the Middle Eocene (Anomaly 20) (Paleoceanographic Mapping Project Progress Report No. 25-0987)

The determination of the motion of Antarctica relative to Africa is particularly important when considering the breakup of Gondwana. Two models have been proposed that describe the pattern of seafloor spreading between Africa and Antarctica during the Late Cetaceous (starting at chron 34, 84 Ma) through to the Middle Eocene (chron 20, 46 Ma). In the flrst model, the motion of Antarctica relative to Africa can be simply described by a rotation about a single pole of rotation. In the second model, which we favor, the relative motion of Antarctica and Africa is more complex, and a major change in spreading direction between chron 32 (74 Ma) and chron 24 (56 Ma) times is required. In this paper we present 10 plate tectonic reconstructions of the Southwest Indian Ridge that were produced using a new compilation of magnetic, bathymetric, and satellite altimetry data, in combination with interactive computer graphics. These reconstructions illustrate that spreading directions started to change at chron 32 time (74 Ma). Between chrons 31 and 28 (69 to 64 Ma), spreading was very slow ( <1 cm/yr) and the direction of spreading changed from NE-SW to a more N-S direction. Between chrons 26 and 24 (61 to 56 Ma) the direction of spreading shifted back to a NE-SW orientation. These changes in spreading direction suggest that the present-day fracture zones in the area of the Prince Edward fracture zone are younger features (Eocene) than their lengths might imply. Our results also provide important constraints concerning the Mesozoic reconstructions of the Indian Ocean and the motion of South America relative to Antarctica prior to the Eocene.