Tuesday 26th  August

12 – 1 pm

Rm 210.104

Karol Czarnota

Geoscience Australia, Canberra, Australia

Dynamic topography of Australia:
Surface expression of mantle convective circulation

Abstract

It is generally agreed that plate motion is maintained by convective circulation of the Earth’s mantle. However, the detailed spatial and temporal pattern of this circulation is poorly known.  Since dynamic topography is generated by the interplay between mantle convective circulation and plate motion, observational constraints should yield hitherto inaccessible insights into this convective process.  Australia’s isolation from active plate boundaries and its rapid northward motion within a hotspot reference frame make it a useful natural laboratory.  The present-day dynamic topography is best mapped offshore by measuring the residual depth of oceanic floor with respect to the well-known age-depth relationship.  Onshore present-day dynamic topography can be estimated using the relationship between gravity and topography at wave-lengths >350 km.  The temporal evolution of this topography can be constrained by interrogating passive margin architecture and inverting longitudinal river profiles for uplift histories.  The results show the amplitude of Australian Dynamic topography is ±1 km. Southwestern Australia appears to have been emerging from the dynamic to- pography low associated with the Australian-Antarctic Discordance over the last ~50 Myrs whereas northern Australia has been drawdown by up to 700 m from an unperturbed elevation over the last ~10 Myrs.  The Eastern Highlands were uplifted in two stages.  The Great Escarpment appears to be the expression of present-day dynamic support, which grew during and immediately prior to Cenozoic volcanism.  A discrete earlier phase of uplift is temporally associated with rifting leading to Tasman Sea floor spreading.  This history of vertical motions constrains the passage of thermal anomalies beneath the Australian plate and is consistent with palaeocoastline elevations, longterm river incision rates, basin sequence stratigraphy and thermochronological studies.