Geologic evidence for the plate‐driving mechanism: The continental undertow hypothesis and the Australian‐Antarctic discordance

Abstract

Investigations of the pattern of flow in the mantle have been carried out almost exclusively by theoretical geophysicists, mainly fluid dynamicists, because mantle flow has been considered too deep and too disconnected to leave direct traces in surface tectonic geology. This geophysical approach has not yet provided a detailed understanding of mantle flow. The assumption of invisibility of mantle flow from the surface should be reassessed. The tectonic history of gaps in the circum‐Pacific ring of continents (Caribbean, Scotia Sea, Australian‐Antarctic Discordance) suggests that upper mantle return flow is escaping from the Pacific hemisphere as the Pacific Ocean contracts and is avoiding the regions beneath continents. Major continental masses may thus have the deep roots suggested by some seismologists. If these roots are coupled to the lower mantle, plates that include continents may be driven by “continental undertow.” In this case, continental motions may tell us the pattern of lower mantle flow, and the resulting four‐celled lower mantle picture is remarkably simple. A key area for testing the continental undertow hypothesis is the Australian‐Antarctic Discordance, a low saddle on the spreading ridge between these two continents. In the continental undertow view, upper mantle flow escaping the Pacific is met in the discordance by upper mantle flow originating at the Java‐Sumatra trench, with the bathymetric low representing the sink toward which this converging flow is moving. Two new lines of evidence support this interpretation: (1) Isotope geochemistry of ridge basalts from the discordance shows that mantle sources with distinctive Pacific Ocean and Indian Ocean characters meet precisely at the discordance. (2) Seismic tomography reveals cool upper mantle under the discordance, which argues for feeding of this ridge segment laterally, rather than from beneath. Hot spot tracks in and just east of Australia appear to argue against the continental undertow hypothesis but do not allow an unambiguous test of the continental undertow hypothesis. Mantle flow patterns may in fact be partly visible from the surface, and careful evaluation of key regions by tectonic geologists may provide a fruitful approach to understanding mantle dynamics.