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

The Middle Proterozoic Mount Isa Basin, of the Mount Isa terrane, hosts sev eral large Pb-Zn-Ag deposits and is arguably one of the richest mineral pro vinces in the world. The deformed remnants of this basin extend from the ea stern margin of the Leichhardt River Fault Trough through to the Murphy Tec tonic 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 E vent. 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 159 5 Ma. Regional analysis of the highest level cover rocks of the Mount Isa B asin reveals a notable difference in the locus of syn-rift sedimentation, s yn-rift magmatism, and post-rift subsidence. Although crustal extension was widespread across the Mount Isa Basin, tectonic subsidence was focussed al ong the similar to N-S-oriented Mount Isa Rift. Approximately 3-5 km of flu vial 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 pro vinces the syn-rift sequences are fewer and thinner (750-2000 m) and were d ominantly deposited in subaerial environments, suggesting a relatively stab le uplift and subsidence history. The position of maximum subcrustal lithos pheric extension is determined by the position of greatest post-rift subsid ence. This occurred beneath the northern Mount Isa terrane where the thicke st post-rift sequences are preserved and the depositional history is more p rotracted. We propose that the evolution of the Mount Isa Basin is a conseq uence of asymmetric extension of the Middle Proterozoic lithosphere. (C) 19 98 Elsevier Science B.V. All rights reserved.