Ordovician continental margin terranes in the Lachlan Orogen, Australia: Implications for tectonics in an accretionary orogen along the east Gondwana margin

Abstract

Four continental margin turbidite ± black shale terranes of the Lachlan Orogen in the southern Tasmanides of eastern Australia formed in two major systems along the east Gondwana margin and constrain the Ordovician assembly of this accretionary orogen. Key features are the dissimilar stratigraphies of the adjacent Bendigo and Melbourne terranes in the western system; the dissimilar stratigraphies of the adjacent Melbourne and Albury‐Bega terranes that reflect juxtaposition of the two systems during the Middle Devonian, and the presence of the Albury‐Bega Terrane both west and east of the Macquarie Arc in the eastern system that also includes the ocean floor Narooma Terrane and igneous ocean crust terrane(s). Repetition of the Albury‐Bega Terrane either side of the arc requires either rifting or orogen‐parallel, strike‐slip duplication of a once contiguous package. Terrane interactions began in the earliest Gisbornian with early docking, uplift, deformation, and exchange of detritus. Amalgamation occurred in the earliest Silurian Benambran Orogeny with accretion in the Middle Devonian. Over 40 Myr, discrete turbidite terranes aligned along the Gondwana margin in two systems were converted into a very wide orogen characterized by the along‐strike juxtaposition of superficially similar terranes.

Evolution of a Cambrian active continental margin: The Delamerian–Lachlan connection in southeastern Australia from a zircon perspective

“…Some recent work has raised questions about the vast Ordovician turbidite fan having formed adjacent to the Delamerian Fold Belt. It has been suggested that the deep-marine Ordovician siliciclastic rocks of the Lachlan Fold Belt formed one or more allochthonous terranes that have been transported along the East Gondwana margin (Baillie, 1985;Glen et al, 2009). Others suggested that long-distance sedimentary transport of sediment from the East -West Gondwana Pan-African collision zones thousands of kilometres away provided an explanation for the detrital U-Pb zircon ages in the Ordovician and other siliciclastic successions (Squire et al, 2006;Maidment et al, 2007;Williams and Pulford, 2008).…”

“…The relationship between the Macquarie arc and the surrounding Ordovician quartz turbidites are problematic with several explanations offered including juxtaposition by strike-slip faulting (Glen et al, 2009), island arc rotation and development of adjoining subduction complexes (Fergusson, 2009), and overthrusting of a "passive margin" by an island arc from the east (Aitchison and Buckman, 2012). For the Early Ordovician, a connection between the Gondwana margin and the Macquarie Arc has been argued by Glen et al (2011) who showed that the Mitchell Breccia at the base of the Macquarie Arc succession contains a detrital zircon age pattern similar to that documented for the Ordovician quartz turbidites.…”

“…The tectonic development of the Lachlan Fold Belt is a widely debated issue. Numerous suggestions have been given for the Ordovician tectonic development including formation of multiple subduction zones (Gray and Foster, 2004), strike-slip emplacement of continental margin terranes (Glen et al, 2009) and island arc -passive margin collision somewhat akin to the Delamerian Orogeny in Tasmania (Aitchison and Buckman, 2012). In the Cambrian, an oceanic realm developed resembling the modern western Pacific Ocean with island arcs, backarc basins and boninitic-tholeiitic successions (Crawford et al, 2003).…”

“…The East African Orogen has been considered a significant source for early Palaeozoic sedimentary units in Gondwana including North Africa and adjacent parts of the Middle East (Figure 1) (Squire et al, 2006;Meinhold et al, 2013) and may also have been a source for late Cambrian to earliest Ordovician sedimentary rocks along the Tethyan margin of north India (Cawood et al, 2007). It has also been argued that many of the early Palaeozoic units of the Lachlan Fold Belt are allochthonous and have been transported by hundreds of kilometres or more along strike-slip faults (Willman et al, 2002;Glen et al, 2009).…”

Thermochronology and stratigraphy of the Thomson Orogen, north-eastern Australia

“…: those of the Sunbury Group of Victoria (Glen et al, 2009). One prominent model for the source of these sediments is the so-called "Trans-Gondwanan Supermountain" (TGSM), which is proposed to have developed in Neoproterozoic to Cambrian time during the collision of East and West Gondwana (Squire et al 2006).…”