Sequential development of a mid-crustal fold-thrust complex: evidence from the Mitakoodi Culminationin the eastern Mt Isa Inlier, Australia

Abstract

The Eastern Fold Belt, Mt Isa Inlier preserves possible backarc inversion in the Mesoproterozoic, which occurred in a ca 1600 – 1500 Ma Isan Orogen that was distal to the plate margin. Like many of the Palaeoproterozoic to Mesoproterozoic orogens of the Australian continent, this orogen displays several unusual characteristics compared with modern convergent orogens. The thermal history is dominated by high-temperature, low-pressure metamorphism, which pre-dates maximum crustal thickening. Crustal thickening has been achieved by stacking younger and relatively hotter basin phases over older and colder basin phases during thin-skinned lateral crustal translations. The structural geometries produced during the Isan Orogeny suggest that wholesale inversion was accomplished via the development of a fold and thrust belt at mid-crustal levels. The early evolution involved thin-skinned (D1) deformation characterised by northwest-directed transport of nappes and thrusting of young supracrustal successions over older supercrustal successions along mylonitic overthrusts. As the fold and thrust belt evolved, crustal translations (D2) were facilitated along a subhorizontal structure (Argylla Detachment) located at the interface between crystalline basement and the overlying basin successions. Upright to inclined folds (F2) with north-northeast-trending axial traces formed above underlying ramps and as fault propagation folds in the hangingwall of the Argylla Detachment. D1 nappes and shallow dipping mylonites were refolded by F2 folds and translated towards the west. Continued deformation involved thick-skinned crustal shortening producing upright local F3 folds, broad flexures in the basement – cover interface and steeply east-dipping basement-rooted reverse faults. Whilst the architecture of the fold and thrust belt preserved in the Eastern Fold Belt resembles that preserved in the foreland of many orogenic belts, there are several notable differences. Younger and hotter sedimentary successions have been thrust over older sedimentary successions, rather than older over younger as we might expect. Stratigraphy has been excised, rather than repeated, across major mylonite overthrusts. These observations are consistent with an episode of lithospheric extension immediately before a switch to crustal thickening and orogenesis. In this interpretation, high-temperature metamorphism during the earliest stages of orogenesis may have been inherited from the earlier extensional regime, and extensional detachments may have been reactivated as thrusts during regional shortening. Such a model has important implications for understanding basin inversion processes at mid-crustal levels and provides a framework to interpret the timing and context of high-temperature, low-pressure metamorphism, which is a common characteristic in many Proterozoicz orogens of Australia.