Ordovician Macquarie Arc and turbidite fan relationships, Lachlan Orogen, southeastern Australia: stratigraphic and tectonic problems

Modelling the Palaeozoic tectonic evolution of the Lachlan Orogen

“…Synchronous to this was the early development of the Macquarie Arc overlying the former Cambrian Arc basement in the Palaeo-Pacific. While this arc was likely initially striking parallel to the Gondwanan margin, a change in subduction polarity is invoked by Fergusson and Colquhoun (2018) with ensuing rollback and clockwise rotation of the arc, allowing for Gondwana-derived turbidite sequences to surround the Macquarie Arc. A second subduction reversal brought the Plaeo-Pacific crust back towards Gondwana, culminating in the Benambran Orogeny (Figure 2c-d; .…”

“…Cayley, et al 2018), while others invoke multiple concurrent subduction zones that experience polarity reversals and retreats (e.g. Fergusson and Colquhoun 2018).…”

Lachlan Orogen, Eastern Australia: Triangle Formation Records the Late Ordovician Arrival of the Macquarie Arc Terrane at the Margin of Eastern Gondwana

“…The Adaminaby Group is ≥1,000 m thick, but the nature of the underlying basement rocks is unresolved (Fergusson & Fanning, 2002;Pogson & Watkins, 1998;Thomas & Pogson, 2012). The turbidite unit age has been assigned to be older than late Darriwilian (462 Ma; Figure 3), as constrained by the fossil assemblages in the overlying Bendoc Group (Fergusson & Colquhoun, 2018, and references therein).…”

“…The Ordovician Macquarie Arc is a distinct island arc terrane within the Lachlan Orogen of eastern Australia (Crawford, Cooke, et al, 2007;Fergusson & Colquhoun, 2018;Glen, Crawford, et al, 2007;Meffre et al, 2007). However, it is juxtaposed against coeval, mature, quartz-rich turbidites of the Adaminaby Group to both its west and east.…”

“…Various models have been proposed to explain the juxtaposition of the Macquarie Arc as interleaved segments between the Adaminaby Group. These include (A) an episode of compression during long-lived, west dipping subduction (Cawood et al, 2009;Collins, 2002;Glen, 2013), (B) models with multiple subduction zones (Fergusson, 2003;Glen, Meffre, et al, 2007;Gray & Foster, 2004;Meffre et al, 2007), (C) juxtaposition by strike-slip faulting (Glen et al, 2009;Meffre et al, 2007), (D) arc rotation followed by an orocline (Fergusson, 2009;Fergusson & Colquhoun, 2018;Moresi et al, 2014), and (E) Aitchison and Buckman (2012) who proposed an arccontinent collision event as a result of east dipping subduction, before the allochthonous arc was emplaced onto the passive margin. Validation of any tectonic model requires accurate age and provenance data of (Glen, 2005(Glen, , 2013.…”