Asymmetric extension of the Middle Proterozoic lithosphere, Mount Isa terrane, Queensland, Australia

The Middle Proterozoic Mount Isa Basin, of the Mount Isa terrane, hosts several large Pb-Zn-Ag deposits and is arguably one of the richest mineral provinces in the world. The deformed remnants of this basin extend from the eastern margin of the Leichhardt River Fault Trough through to the Murphy Tectonic Ridge in the far north of the terrane. The Mount Isa Basin initially evolved in response to NW-SE-directed extension during the Mount Isa Rift Event. This event began before similar to 1708 Ma and had ceased by similar to 1653 Ma. A sag basin continued to evolve thereafter until similar to 1595 Ma. Regional analysis of the highest level cover rocks of the Mount Isa Basin reveals a notable difference in the locus of syn-rift sedimentation, syn-rift magmatism, and post-rift subsidence. Although crustal extension was widespread across the Mount Isa Basin, tectonic subsidence was focussed along the similar to N-S-oriented Mount Isa Rift. Approximately 3-5 km of fluvial to shallow marine elastic sediments were deposited into isolated rift basins. Bimodal volcanism and emplacement of shallow level plutons occurred along the western and northwestern margins of the Mount Isa Rift. Magmatic provinces mark the locus of significant subcrustal lithospheric thinning, asthenospheric upwelling, and mafic underplating. Within these magmatic provinces the syn-rift sequences are fewer and thinner (750-2000 m) and were dominantly deposited in subaerial environments, suggesting a relatively stable uplift and subsidence history. The position of maximum subcrustal lithospheric extension is determined by the position of greatest post-rift subsidence. This occurred beneath the northern Mount Isa terrane where the thickest post-rift sequences are preserved and the depositional history is more protracted. We propose that the evolution of the Mount Isa Basin is a consequence of asymmetric extension of the Middle Proterozoic lithosphere. (C) 1998 Elsevier Science B.V. All rights reserved.