Ordovician‐Silurian accretion tectonics of the Lachlan Fold Belt, southeastern Australia

Abstract

The Lachlan Fold Belt of southeastern Australia contains a remnant ocean basin with Cambrian igneous basement of intra‐arc and backarc boninitic to tholeiitic volcanics and island‐arc calc‐alkaline volcanics overlain by ?Upper Cambrian to dominantly Ordovician siliciclastic turbidite successions derived from Gondwana and deposited in a huge submarine turbidite fan(s). Much of the remnant ocean basin is preserved in accretionary subduction complexes that are characterised by abundant coherent successions with relatively sparse mélanges. The lack of chaotic rocks reflects the accretion of thick (2–5 km) siliciclastic turbidite successions and low rates of underthrusting. Three previously recognised accretionary subduction complexes are located in western Victoria (Stawell and Bendigo Zones), eastern Victoria (Tabberabbera Zone) and along the eastern coastline of southeast Australia (Narooma subduction complex). In addition, a short‐lived west‐dipping subduction zone is proposed to account for the areally restricted Howqua River Zone along the eastern margin of the Melbourne Zone in eastern Victoria. The Howqua River Zone contains gently dipping mélanges and subduction‐related blueschist fragments. The subduction complex in the Tabberabbera Zone is considered to extend throughout the remnant ocean basin succession of the Wagga‐Omeo Zone of the central Lachlan Fold Belt. Apart from the Howqua River Zone, these subduction complexes are an end‐member in the spectrum of accretionary complexes that contrast with more chaotic assemblages as preserved in southwest Japan.