Cenozoic Evolution of the American-Antarctic Ridge (Paleoceanographic Mapping Project Report No. 09-0286)

New bathymetric and magnetic anomaly data for the American-Antarctic Ridge region of the South Atlantic are compiled and presented. The magnetic anomaly data are used to produce poles of rotation and angles of opening for six stages for the Cenozoic between anomaly 5 time (9. 7 Ma) and anomaly 21 time (50.3 Ma). Synthetic spreading centers and fracture zones are generated, some of which give very close agreement with noted SEASAT derived gravity anomalies. Two large symmetrical regions of the American-Antarctic Ridge area could not have been formed at the present-day active American-Antarctic Ridge spreading centers. We conclude that the 560 kilometer long Bullard Fracture Zone was created just prior to anomaly 6 (20.4 Ma) time eliminating two or more spreading segments of the American-Antarctic Ridge. The change in spreading and direction which precipitated the realignment of transform faults was caused by the subduction of about 25% of the American-Antarctic Ridge at anomaly 6A or 6B time (21.0-23.0 Ma) at Jane Bank. An additional section of the ridge was subducted near Discovery Bank at about 7-10 Ma and about half of the remaining American-Antarctic Ridge will be subducted between 17 and 20 million years from now if present-day rates persist.