OBSERVATIONS OF RIDGE-HOTSPOT INTERACTIONS IN THE SOUTHERN-OCEAN

The evolution of ridge-hotspot systems is not well understood. In this investigation, satellite-derived marine gravity data are used in conj unction with underway bathymetric and magnetic anomaly profiles to inv estigate the nature of ridge-hotspot interaction at four sparsely expl ored systems in the Southern Ocean. These systems illustrate three dif ferent stages of ridge-hotspot interaction in which a migrating spread ing center approaches a hotspot (Pacific-Antarctic/Louisville), passes over or is captured by the hotspot (Mid-Atlantic/Shona-Discovery), an d ultimately migrates away from the hotspot (Southeast Indian/Kerguele n). All of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relo cate toward the hotspot by asymmetric spreading. Interestingly, these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge-hotspot systems. A simple model predicts that typi cal plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge ju mp. The presence of previously uncharted, obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot suppor ts the notion of asthenospheric flux from the plume to the spreading c enter both before and after the time when the hotspot is ridge centere d. The azimuths of the aseismic ridges cannot be explained by plate ki nematics alone; they consistently extend from the ends toward the cent ers of the adjacent spreading segments suggesting some interaction bet ween plume derived asthenospheric flux and local lithospheric structur e. The features discussed here also indicate that the transfer of asth enospheric material from the plume to the spreading center is influenc ed by the local plate boundary configuration and interaction with tran sform offsets.