Inferring mantle properties with an evolving dynamic model of the Antarctica-New Zealand region from the Late Cretaceous

Abstract

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.