Structure and thermochronology of an E-W profile through the Mount Painter Province, Northern Flinders Ranges, South Australia: is this a southern example of deformation and exhumation driven by the Alice Springs Orogeny?

Abstract

The Mount Painter Province in the Northern Flinders Ranges, South Australia is composed of Palaeoproterozoic to Mesoproterozoic basement overlain by 7-12 kilometres of Neoproterozoic to Cambrian sedimentary rocks and is associated with high lateral geothermal gradients. During the Early Paleozoic, deformation and metamorphism reached greenschist to amphibolite facies during the ~500 Ma Delamerian Orogeny. This study focuses on the subsequent thermal history of the area by studying an E-W profile through the Mount Painter Province using the widely used techniques of structural mapping, micro-structural analysis and 40Ar/39Ar thermochronology to characterise and date deformation and cooling (as a proxy for exhumation). The E-W trending profile, known as the Hamilton Fault, is south dipping oblique slip with a normal and dextral component overprinted by younger brittle structures and brecciation which is seen in the structural and micro-structural analysis.. It is proposed to have a very active past and there is evidence of movement in the Adelaidean due to an apparent formation offset of ~600 m. The regional context of the Hamilton Fault having a dextral and normal component suggests an ε3 uplift, an ε2 extension SW to NE and ε1 NW-SE shortening. This is similar in character to the N-S shortening which is seen in the Alice Springs Orogeny (ASO). Results from the 40Ar/39Ar thermochronology show the basement metasedimentary rocks have cooling ages of around ~350 Ma between 300 to 400 °C and 312 Ma at 150 °C. Interestingly, the younger Adelaidean metasedimentary rocks have an older cooling age of 390 Ma between 300 to 400 °C. The thermochronology data suggests differential cooling has occurred. The observations suggest that exhumation is driven following the Delamerian folding event and forced the earlier cooling of shallower samples at a slower rate and later cooling of the deeper samples at a faster rate, a process caused by differential tilting. The cooling paths are well represented in this example as shown by converging cooling paths. Overall I attribute this subsequent thermal history and structural similarity to the ASO, a major widespread dramatic orogenic event which has not been widely recognized as a significant tectonic event in the Adelaide Fold Belt.