Low thermal Peclet number intraplate orogeny in central Australia

The late Phanerozoic Alice Springs Orogen in central Australia is an archetypal intraplate orogen characterised by a dense, granulitic core exhumed from beneath a carapace comprising a highly radiogenic granitic mid-upper crust and sediments deposited in a shallow intracratonic basin. Exhumation occurred in large part along a crustal penetrative fault system, the Redbank Shear Zone, producing one of the largest gravity anomalies (∼150 mgal) known from the continental interiors. The lithospheric strength implied by the preservation of this anomaly for more than 300 Myr raises the intriguing conundrum of what localised the intraplate deformation in the first place. Available biostratigraphic and thermochronologic data imply bulk convergence rates of less than 1 mm/yr for the orogen as a whole, several orders of magnitude lower than typical of plate margin orogens. The thermal and mechanical evolution of intraplate orogens deformed at such low thermal Peclet numbers differs in fundamental ways from plate margin orogens. In particular, at such low thermal Peclet numbers the conductive response to exhumation of heat sources cools the mid to deep crust during progressive orogenic activity. This is consistent with the hypothesis that the density structure and associated gravity anomalies may have been locked-in by virtue of the strength acquired during the orogenic process provided that the lithospheric strength changes associated with a reduction in average crustal temperature of 20–30°C are of the same order as the forces that drive intraplate deformation.