Crustal structure of the Ordovician Macquarie Arc, Eastern Lachlan Orogen, based on seismic-reflection profiling

Abstract

In the Eastern Lachlan Orogen, the mineralised Molong and Junee-Narromine Volcanic Belts are two structural belts that once formed part of the Ordovician Macquarie Arc, but are now separated by younger Silurian-Devonian strata as well as by Ordovician quartz-rich turbidites. Interpretation of deep seismic reflection and refraction data across and along these belts provides answers to some of the key questions in understanding the evolution of the Eastern Lachlan Orogen—the relationship between coeval Ordovician volcanics and quartz-rich turbidites, and the relationship between separate belts of Ordovician volcanics and the intervening strata. In particular, the data provide evidence for major thrust juxtaposition of the arc rocks and Ordovician quartz-rich turbidites, with Wagga Belt rocks thrust eastward over the arc rocks of the Junee-Narromine Volcanic Belt, and the Adaminaby Group thrust north over arc rocks in the southern part of the Molong Volcanic Belt. The seismic data also provide evidence for regional contraction, especially for crustal-scale deformation in the western part of the Junee-Narromine Volcanic Belt. The data further suggest that this belt and the Ordovician quartz-rich turbidites to the east (Kirribilli Formation) were together thrust over ?Cambrian-Ordovician rocks of the Jindalee Group and associated rocks along west-dipping inferred faults that belong to a set that characterises the middle crust of the Eastern Lachlan Orogen. The Macquarie Arc was subsequently rifted apart in the Silurian-Devonian, with Ordovician volcanics preserved under the younger troughs and shelves (e.g. Hill End Trough). The Molong Volcanic Belt, in particular, was reworked by major down-to-the-east normal faults that were thrust-reactivated with younger-on-older geometries in the late Early - Middle Devonian and again in the Carboniferous.