Structure of an accreted intra-oceanic arc: potential-field model of a crustal cross-section through the Macquarie Arc, Lachlan Orogen, southeastern Australia

Abstract

Construction of a 3D geological model for the eastern Lachlan Orogen provided a new reference model for a combined magnetic and gravity model of the Macquarie Arc, an intra-oceanic arc accreted to Gondwana during the Late Ordovician to early Silurian Benambran Orogeny. Geologically constrained, iterative 2.5 D forward modelling represented the entire thickness of the crust over a west-to-east profile extending for 230 km, approximately coinciding with seismic reflection profiles 99AGSL1 and 99AGSL2. The output model, referred to here as the Forbes model, confirms west-dipping thrust stacks of the Junee-Narromine and Molong volcanic belts of the Macquarie Arc, in packages that also comprise underlying arc basement, overlying Ordovician turbidites, and overlying Silurian–Devonian volcanic and sedimentary units. Ordovician rocks in the Parkes Fault Zone are distributed in wedges within a flower-fault structure. These structural elements are broadly similar to an earlier interpretation of the seismic profiles and corresponding potential-field models but differ in the nature of the substrate to the Macquarie Arc. The arc substrate is best fitted with physical properties broadly that of an intermediate calc-alkaline rock, rather than mid-ocean ridge basalt (MORB) as previously inferred. Highly magnetic metamafic rocks of the Jindalee Group appear to occupy relatively narrow fault-bounded slivers and may correspond to mafic volcanic rocks in the forearc rather than fragments of MORB basement. The thick intermediate-composition crustal profile required by the Forbes model cannot distinguish between an Izu-Bonin-like thickened arc or a compound of an Ordovician arc developed on a pre-existing Cambrian arc, but the simple MORB substrate previously assumed for the Macquarie Arc can be excluded, as can obduction of the Macquarie Arc as a nappe over the Wagga Belt. KEY POINTS A 3D model for the Lachlan Orogen provides geological constraints on joint magnetic and gravity model across the accreted intra-oceanic Macquarie Arc. The substrate of the arc is best fitted with magnetic susceptibility and density typical of an intermediate composition, in contrast with previous interpretations that the substrate preserves an oceanic crust basement. Western and central belts of the Macquarie Arc are stacked by west-dipping thrusts, inconsistent with emplacement as an allochthonous nappe.