Because of the proximity of the Euler poles of rotation of the Pacific and Antarctic plates, small variations in plate kinematics are fully recorded in the axial morphology and in the geometry of the Pacific-Antarctic Ridge south of the Udintsev fracture zone. Swath bathymetry and magnetic data show that clockwise rotations of the relative motion between the Pacific and Antarctic plates over the last 6 million years resulted in rift propagation or in the linkage of ridge segments, with transitions from transform faults to giant overlapping spreading centers. This bimodal axial rearrangement has propagated southward for the last 30 to 35 million years, leaving trails on the sea floor along a 1000-kilometer-long V-shaped structure south of the Udintsev fracture zone.
A pronounced regional bathymetric swell is a common feature of oceanic hotspot volcanism. Recently, the hypothesis of a buoyant sublithospheric swell-root has been favoured, the root being either a Ôrefracted plumeÕ of hot, buoyant and hence lowviscosity plume material embedded within surrounding higher viscosity asthenosphere, or a Ôrestite-rootÕ composed of hot and more viscous residues to hotspot melt-extraction from the hottest central portion of the upwelling plume. In this article, we present numerical experiments that show that these two scenarios predict different flow and melting patterns whenever the plume passes beneath an obliquely oriented fracture zone where the base of the overriding lithosphere changes in thickness. We find that the restite-root hypothesis predicts an asymmetric spatial pattern of persistent Ôarch volcanismÕ strikingly similar to that found in the Hawaiian chain surrounding the Molokai Fracture Zone. In contrast, the Ôrefracted plumeÕ hypothesis predicts more chaotic patterns quite different from the off-chain pattern of Hawaiian volcanism.
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