| Summary: | We show that time-dependent models of mantle upwellings above a cold downwelling in the New Zealand-Antarctica region since 80 Ma can explain anomalous geophysical observations: ~1.0 km of positive residual bathymetry at the Antarctica margin, a large Ross Sea geoid low, 0.5–0.9 km of excess tectonic subsidence of the Campbell Plateau since 80 Ma, and several seismic wave speed anomalies. Model results indicate that the largest mantle upwelling, centered in the Ross Sea, has an average temperature anomaly of 200°C and density anomaly of 0.6%, and it rose from midmantle depths at 80 Ma to a present depth of 400–1000 km. Anomalous Campbell Plateau subsidence requires a smaller hot anomaly evolving within the upper mantle under the region of the reconstructed Late Cretaceous Campbell Plateau. The excess subsidence of the plateau results from northward drift of New Zealand away from the dynamic topography high created by the smaller hot anomaly. To fit present-day geoid and residual topography observations, we require a large lower:upper mantle viscosity ratio of 100:1. We suggest that the distribution of temperature and viscosity is related to long-lived Gondwana subduction that accumulated high-density, high-viscosity lower mantle below a chemically altered upper mantle with anomalously low density and/or high temperature. Time-dependent observations enable constraints on absolute viscosities of 10^(23) Pa s and 10^(21) Pa s for the lower and upper mantle, respectively. © 2010 American Geophysical Union. Received 12 May 2009; accepted 18 December 2009; published 14 May 2010. All calculations carried out on the Caltech Geosciences Supercomputer Facility were partially supported by NSF EARâ€0521699. This work was supported through the New Zealand Foundation for Research Science and Technology, the Caltech Tectonics Observatory (by the Gordon and Betty Moore Foundation), the National Science Foundation (EARâ€0609707 and EARâ€0810303), StatoilHydro, and an ExxonMobil Science grant (to S.S.). The original ...
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