Major Thrust Faults and the Vertical Zonation of the Middle to Upper Proterozoic Crust in Central Australia

Abstract

The vertical crustal structure of, and correlations between, the Musgrave and Arunta blocks, central Australia, are considered in the light of tectonic, isotopic, thermobarometric and geophysical data. A crustal model is tentatively favoured including [1] granulite facies basal meta-supracrustals and orthogneiss intruded by mafic/ultramafic layered intrusions, namely the Giles Complex in the Musgrave Block and smaller-scale analogues in the Arunta Block; [2] amphibolite facies and retrogressed granulite facies supracrustals extensively invaded by granites. The major Bouguer gravity anomalies of central Australia, featuring amplitudes of up to 140 mgal, have been studied by seismic reflection surveys, indicating ‘thick skinned’ intra-sialic thrust tectonics which result in displacement of the MOHO at depths of 30–50 km and in the juxtaposition of infracrustal zone [1] rocks with mesocrustal zone [2] rocks. It is suggested that the granitic magmas formed by anatexis along the granulite- amphibolite metamorphic front. Rb-Sr ages show a wide scatter between 1.8–0.9 b. y. and may be interpreted in terms of different timing of thermal events in the Musgrave and Arunta blocks. However, in the lack of definitive U-Pb zircon data it is tentatively considered likely that the two blocks are broadly contemporaneous. Low to moderate initial Sr87/Sr86 ratios indicate short [<200 m. y.] crustal prehistories. The major thrusting events are considered to have culminated during the late Devonian-early Carboniferous. The deep level layered intrusions of the Giles Complex in the Musgrave Block and smaller scale analogues in the Arunta Block are considered to have triggered major anatectic events. These layered intrusions, emplaced at intermediate to deep crustal levels [above 6 kb], may signify the proximity of the MOHO in the upper Proterozoic.