Intraplate deformation in central Australia, the link between subsidence and fault reactivation

摘要

Central Australia has experienced two intraplate orogenic events involving significant north-south shorterning: the late Neoproterozoic to Early Cambrian Petermann Orogeny and the Devonian to Carboniferous Alice Springs Orogeny. In each event pre-existing structures inherited from Mesoproterozoic terrain amalgamation were reactivated and basement rocks exhumed from beneath thick sedimentary successions accumulated in the Centralian Superbasin. The pattern of fault reactivation during these events shows a striking similarity to the pattern of subsidence in the overlying basin. Immediately prior to the Petermann Orogeny, the Centralian Superbasin was thickest in the vicinity of the Musgrave Block, the region in which deformation was subsequently localised. At the same time crustal-scale faults elsewhere in central Australia that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate the north-south shortening. Between the Petermann and Alice Springs Orogenies, subsidence patterns shifted, such that fault systems in the Arunta Block and also those in the southern Musgrave Block were buried by significant thicknesses of sediment, whereas the major structures that were exhumed during the Petermann Orogeny were not significantly buried. During the Alice Springs Orogeny reactivation once again occurred along the most deeply buried faults, even in the instances where those faults had remained inactive during the earlier Petermann Orogeny. Importantly the major Petermann-aged structures that were not buried during renewed subsidence remained inactive during the Alice Springs Orogeny. The record of reactivation implies that the presence of pre-existing crustal-scale faults alone was insufficient to localise deformation. Rather, fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale. The correspondence between the distribution of basement fault reactivation and subsidence patterns during both the Petermann and Alice Springs Orogenies implies a link between relatively thick sedimentation and long-term lithospheric weakening. We show that this link is compatible with the thermal effects of a thick sedimentary blanket. In the context of central Australia the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the Proterozoic basement.