Geodynamic Model of the New Zealand-Antarctica Conjugate Margin Since the Late Cretaceous

Present-day plate boundaries in the Southwest Pacific were created through continental rifting since Late Cretaceous. Spreading on the Pacific-Antarctic Ridge and the southeast Indian Ridge started around 95-90 Ma and continues to the present. The New Zealand and Antarctica conjugate margins are characterized by a number of anomalous observations. The Antarctica margin and adjacent sea floor is approximately 1 km shallower than the conjugate Campbell plateau. Backstripping of the sediments from the boreholes in Campbell plateau indicates anomalously high tectonic subsidence, with a rapid subsidence phase in the period 70-40 Ma, coincident with northward drift of the Campbell plateau. Finally, the Antarctic margin is associated with a large negative geoid anomaly and low S-wave seismic velocities. We developed a three-dimensional geodynamic model of the New Zealand-Antarctica conjugate margins for last 100 million years. We used global finite-element models of mantle flow (with CitcomS) while imposing surface velocities based on the plate reconstructions from GPlates. Models are iteratively updated based on the discrepancy between predicted dynamic topography and observed borehole subsidence on the Campbell plateau and the Antarctic margin topography. Additional constraints on the model are imposed based on the discrepancies between predicted and observed present-day geoid. Long-wavelength features of anomalous observations on the New Zealand-Antarctica conjugate margin can be attributed to time-dependent evolution of a large-scale mantle upwelling. The mantle upwelling at the present day is located on the Antarctic margin, causing a relatively long-lived topography high. Campbell plateau experiences tectonic subsidence as it drifts away from this mantle upwelling. Geoid low observed on the Antarctic margin is associated with both mantle upwelling and lower mantle "slab graveyards".