Is an Extinct Rift Embedded on the Western Flank of the Southernmost Mid-Atlantic Ridge?

Two spreading corridors north of the Bouvet triple junction display elevated topography and magnetic anomaly patterns that suggest a possible eastward jump of the Mid-Atlantic spreading ridge (MAR) after Chron 6b. In the grid of estimated depths (Smith and Sandwell, JGR, 1994) the elevated topography appears remarkably like an extinct rift with an axial valley- the valley trends N10° W along 10° W longitude (between about 52° -55° S), with seafloor deepening away from the ridge axis. Ship tracks JR09B, A1076, JR177, and C1314 that traverse this feature confirm the local valley flanked by deepening seafloor. Track CN154 on the eastern flank of the MAR traverses another locally shallow region, but it is not prominent in the estimated depth grid. Our examination of available ship magnetic anomaly data in these southernmost spreading corridors reveals symmetry about the proposed extinct rift on the western flank of the MAR - we tentatively identify anomaly 6b at the extinct rift axis, with conjugate anomalies out to anomaly 8. Further, anomalies 5a and 5b are missing. On the eastern flank of the MAR, anomalies 5a and 5b are repeated, and 6b through 8 are missing. Although these anomalies are difficult to identify conclusively, we have nevertheless developed a spreading model that can explain these observations. We suggest that the MAR jumped eastward after Chron 6b, followed by a short jump westward at Chron 5. Additional magnetic data from future ship cruises can help confirm if these proposed ridge jumps did occur.