A Palaeozoic Tectonic Model for the Ruby Gap Duplex and the Entia Dome in Central Australia

In the eastern central Australian intracontinental orogen, the Entia Dome, the Ruby Gap Duplex and the intervening north dipping Illogwa shear zone are interpreted as a compressional thrust stack formed during the Alice Spring Orogeny (ASO, 450-300 Ma). However, extensional deformation in the dome has been overlooked.

The Entia Dome is a high-grade gneiss dome structured into a double-dome with a median high-strain zone separating the sub-domes. The dome comprises a Paleoproterozoic igneous core underneath a Neoproterozoic cover sequence - the Harts Range Group, which has been deformed and metamorphosed in the amphibolite to granulite facies during the ASO. To the south, the Ruby Gap Duplex consists of greenschist-grade Neoproterozoic sedimentary rocks deformed into a south-verging fold-thrust belt during the late ASO. The intervening Illogwa shear zone consists of sheared and retrogressed Paleoproterozoic gneiss. On the southern section of the shear zone, near Ruby Gap, Ar-Ar on muscovite reveals a cooling age of 327 Ma.

Structural analysis of the Entia dome suggests a syn-partial-melting double-dome structure and an exhumation history involving an extensional setting. Recumbent to sub-recumbent folding and horizontal boudinage in and around the sub-domes suggest a strong vertical shortening, post-dating upright folds and foliation. The architecture of the double dome with a median high strain zone can be reproduced by 2D and 3D coupled thermo-mechanical numerical modelling of continental extension in the context of a hot geotherm. Age-dating of syn-exhumation leucosome veins in metatexite from the Entia Dome points to ASO ages, including late ASO ages coeval with Ar-Ar ages on the Ruby Gap Duplex, and the Illogwa shear zone.

Structural data shows a structural coherence from the dome to the duplex, with the Harding Springs slide acting as a south dipping normal detachment connecting to the north dipping Illogwa shear zone across a broad synform of Bruna gneiss. Existing late ASO geochronology data from the dome to the duplex supports a tectonic model documenting a genetic link between the Entia Dome and the Ruby Gap Duplex. The juxtaposition of a mid-Carboniferous extensional domain to the north and a mid-Carboniferous contractional domain to the south suggests a possible geodynamic model in which the exhumation of the deep crust in the dome contributes to contractional deformation to the south. In this model, the south-dipping Harding Springs detachment on the southern margin of the Entia Dome is structurally linked to the north-dipping Illogwa shear zone on the north of the Ruby Gap duplex, which functions as the roof-thrust of the Ruby Gap Duplex.