Crustal architecture of basin inversion during the Proterozoic Isan Orogeny, Eastern Mount Isa Inlier, Australia

Abstract

The folded and metamorphosed supracrustal rocks of the eastern margin of the Mount Isa Inlier, NW Queensland, Australia, provide a record of basin inversion processes at high temperatures, from the foreland toward the core of a developing orogen. Deep water, turbiditic metasedimentary rocks of the lower Maronan Supergroup (ca. 1.71–1.66 Ga) occupy the hanging wall of a crustal-scale thrust, the Cloncurry Overthrust, which separates them from calc-silicate and meta-evaporitic rocks of the Doherty Formation (ca. 1.74–1.72 Ga) in the footwall. During the early stages of the Isan Orogeny (ca. 1.60–1.58 Ga) the rocks of the Maronan Supergroup were expelled from their basin and thrust to the north-northwest. Shortening was restricted to the hanging wall and produced a 10 km scale north to northwest verging nappe with a crumpled frontal zone of upright east–west trending folds. This morphology is consistent with a push-from-the-rear mechanism of nappe emplacement with the simple shear component of deformation increasingly accommodated within the hanging wall, down dip toward the core of the orogen. Metamorphism associated with this folding reached upper amphibolite facies temperatures at moderate to low pressures (4–6 kbar). Metamorphic zones in the hanging wall of the Cloncurry Overthrust were antiformally folded with continuing deformation. The relationship between faulting, folding and metamorphic zoning across the Cloncurry Overthrust are consistent with a thrust system, despite that fact that younger rocks are juxtaposed over older. Basin inversion provides the best explanation for this phenomenon. Within this context metamorphism during the early stages of orogenesis can be explained by the inheritance of elevated geothermal gradients produced by lithospheric thinning and sediment loading during formation of the basin. The overall orogenic architecture was controlled by a combination of thermal softening, which focussed early deformation within the basin, and weak zones at the base of the basin and the base of the crust. This resulted in crustal-scale thrusts of opposite vergence that partitioned upper crustal and lower crustal deformation.