Models of mantle convection incorporating plate tectonics: The Australian region since the cretaceous

Abstract

We propose that the anomalous Cretaceous vertical motion of Australia and distinctive geochemistry and geophysics of the Australian-Antarctic Discordance (AAD) were caused by a subducted slab which migrated beneath the continent during the Cretaceous, stalled within the mantle transition zone, and is presently being drawn up by the Southeast Indian Ridge. During the Early Cretaceous the eastern interior of the Australian continent rapidly subsided, but must have later uplifted on a regional scale. Beneath the AAD the mantle is cooler than normal, as indicated by a variety of observations. Seismic tomography shows an oblong, slab-like structure orientated N-S in the transition zone and lower mantle, consistent with an old subducted slab. Using a three-dimensional model of mantle convection with imposed plate tectonics, we show that both of these well documented features are related. The models start with slabs dipping toward the restored eastern Australian margin. As Australia moves east in a hot spot reference frame from 130-90 Ma, a broad dynamic topography depression of decreasing amplitude migrates west across the continent causing the continent to subside and then uplift. Most of the slab descends into the deeper mantle, but the models show part of the cooler mantle becomes trapped within the transition zone. Prom 40 Ma to the present, wisps of this cool mantle are drawn up by the northwardly migrating ridge between Australia and Antarctica. This causes a circular dynamic topography depression and thinner crust to develop at the present position of the AAD. The AAD is unique within the ocean basins because it is the only place where a modern ridge has migrated over the position of long term Mesozoic subduction. Our study demonstrates the predictive power of mantle convection models when they incorporate plate tectonics.