UPLIFT AND EROSION HISTORY IN MARIE BYRD LAND AS A KEY TO POSSIBLE MID-CENOZOIC PLATE MOTION BETWEEN EAST AND WEST ANTARCTICA

Throughout the past 25 m.y., volcanism along the coast of Marie Byrd Land (MBL) has been accompanied by uplift of a broad dome, which now stands ~3 km a.s.l. and is ~800 km across. The dome crest is the central point of two patterns of volcanic activity that provide a means of estimating rate and timing of uplift. Uplift and volcanism are believed to be products of mantle plume activity beneath a stationary plate.

The Dorrel Rock gabbro (DR) is exposed on the eastern flank of the MBL dome, ~470km east of the dome crest, which is the center of Neogene uplift. It is the only known intrusive body related to Cenozoic magmatism in MBL, and probably represents the inception of magmatism and uplift in this region. 40Ar/39Ar dating indicates an emplacement time of 34-37 Ma for the gabbro. We estimate that it crystallized at a depth of ~4 km and required ~5 km of uplift and erosion to bring it to its present elevation. These events seem to have taken place between ~35-25 Ma, based on comparing the rate of erosion that exhumed DR, with observable rates of the past 25 m.y. At DR, erosion kept pace with uplift until 4+ km of overburden was removed and the gabbro exposure reached 800 m elevation. At the dome crest, by contrast, erosion has not kept pace with uplift since 25 Ma, even though the ~100m/m.y. rate of uplift is very slow. Topography still reflects the magnitude and sense of structural offset, e.g. a very low relief Late Cretaceous erosion surface is preserved at the dome crest, 2700m a.s.l., where it is overlain by weakly indurated 25-28 Ma tuff breccias. The implication of this history is that the locus of uplift shifted westward, from DR to the crest of the MBL dome sometime before 25 Ma, perhaps representing plate motion of West Antarctica over a plume head in MBL. The timing correlates with the 43-26 Ma time of ~180 km extension across the Adare trough, coupled with an eastward shift of West Antarctica, reported by Cande et al., (2000), but the inferred magnitudes of plate motion disagree by almost a factor of three.