Late Ordovician to mid-Silurian Benambran subduction zones in the Lachlan Orogen, southeastern Australia

Abstract

The Late Ordovician to early Silurian Benambran Orogeny had widespread effects in the Lachlan Orogen of southeastern Australia, including the Bendigo, Tabberabbera and Narooma zones. Much of the deformation in these zones has been attributed to formation of subduction complexes from accretion of Ordovician turbidites and has been a subject of ongoing controversy. The Narooma Zone subduction complex is more widely accepted because of the abundance of disrupted units, evidence for imbricated units and its metamorphic character. Imbricated and disrupted units consistent with a subduction accretion model also characterise the Tabberabbera Zone. A subduction complex model for the Bendigo Zone has been contested. Deep seismic profiles show a largely transparent upper crust down to ∼18 km of inferred Ordovician turbidites with a layered middle to lower crust of mafic volcanic rocks as shown by the Heathcote Greenstone Belt being traced from the surface to the middle crust. During the Benambran Orogeny, the Bendigo Zone is interpreted as a rapidly formed mega-subduction complex with ∼50 km crustal thickness. This is an exceptional thickness and much thicker than subduction complexes in Japan and the Barbados Accretionary Complex but more comparable with southeastern Alaska where the Chugach terrane (subduction complex) and underplated mafic igneous rocks have a present-day crustal thickness up to 30 km. Anomalous thick crust of the Bendigo Zone subduction complex is related to the huge volume of subduction inputs, with Cambrian fore-arc–backarc crust and the overlying thick Ordovician turbidite fan, resulting in a double décollement associated with the megathrust now represented by the Mount William Fault. The Benambran Orogeny is an example of widespread shortening reflecting rapidly initiated subduction zones with a short-lived history rather than long-lived subduction typical of the modern eastern Pacific Ocean.