MIDDLE TERTIARY HYDROTHERMAL ACTIVITY AND UPLIFT OF THE NORTHERN FLINDERS RANGES, SOUTH-AUSTRALIA - INSIGHTS FROM APATITE FISSION-TRACK THERMOCHRONOLOGY

Apatite fission-track thermochronology reveals that hydrothermal activity was pervasive during mid-Tertiary time within the Paralana Fault zone and adjacent country rocks of the northern Flinders Ranges, South Australia. Apatite fission-track ages from the Mt Painter block, west of the Paralana Fault zone, range between approximately 175 and 260 Ma. The mean lengths of confined fission-tracks for these samples are approximately 12 mum with s.d. of the track length distributions of approximately 2 mum. These data suggest that rocks exposed in the Mt Painter block cooled slowly though the apatite partial annealing zone (approximately 110-50-degrees-C) starting before 200 Ma and continuing into the Tertiary. In contrast, apatite samples from the Paralana Fault zone yield apparent ages as young as 25-30 Ma and mean track lengths mostly < 12 mum, with s.d. approximately 3 mum. These young apparent ages and relatively short track lengths require mid-Tertiary palaeotemperatures of 90-105-degrees-C for the fault zone, assuming a heating time in the order of 1 Ma. This heating is interpreted to be the result of hydrothermal activity within the fault zone when it was buried beneath Mesozoic and Tertiary sedimentary strata. At least one stage of uplift of the northern Finders Ranges during Miocene time elevated these rocks above the basin sediments, resulting in cooling due to a significant reduction in hot springs activity.