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

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.