Long-wavelength tilting of the Australian continent since the Late Cretaceous

Abstract

Global sea level and the pattern of marine inundation on the Australian continent are inconsistent. We quantify this inconsistency and show that it is partly due to a long wavelength, anomalous, downward tilting of the continent to the northeast by 300 m since the Eocene. This downward tilting occurred as Australia approached the subduction systems in South East Asia and is recorded by the progressive inundation of the northern margin of Australia. From the Oligocene to the Pliocene, the long wavelength trend of anomalous topography shows that the southern margin of Australia is characterized by relative subsidence. We quantify the anomalous topography of the Australian continent by computing the displacement needed to reconcile the interpreted pattern of marine incursion with a predicted topography in the presence of global sea level variations. On the southern margin, long wavelength subsidence was augmented by at least 250 m of shorter wavelength anomalous subsidence, consistent with the passage of the southern continental margin over a north–south elongated, 500 km wide, topographic anomaly approximately fixed with respect to the mantle. The present day reconstructed position of this depth anomaly is aligned with the Australian Antarctic Discordance and is consistent with the predicted passage of the Australian continent over a previously subducted slab. Both the long-wavelength continental tilting and smaller-scale paleo-topographic anomaly on the southern Australian margin may have been caused by subduction-generated dynamic topography. These new constraints on continental vertical motion are consistent with the hypothesis that mantle convection induced topography is of the same order of magnitude as global sea level change.