Multiphase cooling and exhumation of the southern Adelaide Fold Belt: constraints from apatite fission track data

Data from apatite fission track analysis are presented for 20 outcrop sampl es collected in the southern Adelaide Fold Belt, South Australia. Interpret ation of these data, with the aid of numerical models which allow inference of multiphase cooling histories, indicate three discrete cooling events th at are likely to correlate with sedimentation events in surrounding deposit ional settings. An event beginning some time after 85 Ma (Late Cretaceous) was characterized by cooling throughout the study area from temperatures of roughly 50 to 70 degrees C. An event beginning at 300-270 Ma (Late Palaeoz oic) was characterized by cooling from temperatures > 120 degrees C in all areas except for the Mount Lofty Ranges and Murray Bridge region, where pea k temperatures were only 95-115 degrees C prior to Palaeozoic cooling. Some samples from these subregions of relatively cool Late Palaeozoic temperatu res also retain evidence for even earlier cooling from temperatures > 120 d egrees C, beginning prior to 350 Ma. We interpret the post 85-Ma event as t he consequence of regional exhumation from a depth of 1.0-1.6 km. The Late Palaeozoic event (300-270 Ma) is interpreted as cooling associated with the termination of the Alice Springs Orogeny, while cooling prior to 350 Ma pr obably represents the final stages of Early Middle Palaeozoic unroofing of the southern Adelaide Fold Belt. The results highlight the importance of regional, episodic postorogenic exh umation of Palaeozoic fold belts, where - in some cases - conventional meth ods have erroneously suggested relatively long-term stability.