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FIGURE 1 Global bathymetry (from Smith and Sandwell, 1997). Convergent plate boundaries shown in orange. Arrow indicates the pathway of kinematic connection between the African and Pacific hemispheres that did not contain destructive boundaries during the Cenozoic era. Oblique Mercator projection.

FIGURE 2 Gravity anomalies (from Sandwell and Smith, 1997).

very large predicted plate displacements at greater distances (e.g., at equatorial latitudes). The validity of Antarctic reconstructions must be consistent with other motions in the South Pacific, because Antarctica is part of a closed plate-motion circuit that includes Australia and New Zealand.

This paper presents a model for the block motion of Marie Byrd Land relative to the East Antarctic craton since 74 Ma. The model is simplistic by design, because the primary purpose of this paper is to propose a new and quantitative hypothesis for motion on an intra-Antarctic plate boundary during the interval 74-50 Ma. It is accepted that refinement to this model will be necessary, to fit observations of crustal strain and the timing of deformation in detail. The hypothesis is presented and then tested against crustal geology of the Antarctic continent, the geometry of seafloor in the South Pacific plate-motion circuit, and the global motions of plates relative to hotspots. A plausible tectonic explanation for why such a model makes physical sense is briefly discussed. Finally, global implications of the hypothesis are explored with regard to subduction budgets since the late

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