Orogenesis in Paleo-Mesoproterozoic Eastern Australia: A response to Arc-Continent and Continent-Continent Collision During Assembly of the Nuna Supercontinent

Abstract

Northern and southern Australia preserve a common record of 1790–1655 Ma intracontinental rifting and backarc extension culminating in formation of a marginal sea that subsequently collapsed following a 1650–1640 Ma reversal in plate motion and onset of arc‐continent collision. Arc‐continent collision was accompanied by intermediate‐pressure Barrovian‐type metamorphism (6–8 kb) and widespread crustal thickening as reflected in a clockwise pressure‐temperature‐time path. Later metamorphism during the 1620–1580 Ma Isa and Olary orogenies occurred under intrusion‐enhanced lower pressure conditions (4–6 kb) and followed a counterclockwise pressure‐temperature‐time path incompatible with either significant amounts of crustal thickening or tectonic models which have Australia and Laurentia commencing collision at this time. Continent‐continent collision more likely followed on closely behind arc‐continent collision (Riversleigh Tectonic Event), precipitating a switch in crustal shortening from northeast‐southwest to west‐east as the older rift template and underlying basement structures were reactivated in a transpressive or multicollisional tectonic regime not unlike that which produced the Himalayan‐Tibet orogenic system. As in the latter, ongoing collision was accommodated by thrusting, extensional collapse and lateral extrusion of still thermally weak crust on orogen‐parallel strike‐slip faults, resulting in formation of the west‐east‐striking Isa Superbasin, isothermal decompression and granite intrusion from 1620 Ma. A comparable record of deformation, metamorphism, and magmatic intrusion from 1650 until 1600 Ma has been documented for northern and southern North America (Forward/Racklan and Mazatzal orogenies) indicating that Laurentia and Australia were likely proximal to each other throughout this period.