The Narooma Terrane offshore: a new model for the southeastern Lachlan Orogen using data from rocks dredged from the New South Wales continental slope

Abstract

Eight dredges from the southern New South Wales continental slope sampled the offshore extension of the Lachlan Orogen. Two rock suites were recovered: (1) lower greenshist facies limestones, felsic volcanics, sandstones, mudstones and Moruya Suite granodiorite correlate with the onshore Silurian to mid-Devonian orogenic phase; and (2) a strongly deformed greenschist to lower amphibolite facies mafic volcanics, cherts, marbles, pelites and serpentinites correlate in part with the CambroOrdovician Wagonga Group of the Narooma Terrane. The mafic volcanic rocks have ocean island, tholeiitic and boninitic basalt affinities. The offshore distribution of ocean island basalt that correlates with medial Cambrian basalt breccias at Batemans Bay suggests a large seamount or seamount complex. The boninites, tholeiites and ultramafics could be part of a forearc-generated ophiolite. The Narooma Terrane basement is interpreted as the part of the bonititic arc postulated to have collided with Vandieland in late early Cambrian time. Mid-Cambrian rifting of the oceanward part of this arc remnant, generated the AlburyÀBega Terrane oceanic basement exposed in the Howqua Valley in the west and Melville Point in the east. Overlying are upperÀmid-Cambrian to lowermost Ordovician black shale and chert, Lower Ordovician to Gisbornian Adaminaby Group quartz turbidites and Gisbornian to lower Bolindian Bendoc Group black shales. Batemans Bay exposures are reinterpreted as a dismembered basin margin succession onlapping the west-facing attenuated flank of the Narooma Terrane. The Narooma Cambro-Ordovician cherts and mudstones were initially deposited outboard on the more elevated seamount flank elevated above the clastic-filled basin to the west. Benambran deformation commenced in latest Ordovician time uplifting the outer Narooma Terrane, shedding debris from the seamount and its flanks, culminating in allochthonous displacement of chert masses to the basin's eastern margin to Narooma, and emplacing them as a succession of thrust sheets. Contemporaneously, silt and mud of the Bogolo Formation, deposited from the west, were mixed with olistostomal basalt and chert debris from the east. Early Silurian westward tectonic transport of the Narooma Terrane ruptured the Albury-Bega basin floor at Batemans Bay, thrusting it and its sedimentary cover over its eastern margin as a series of thrusts each floored by melange (mapped Bogolo Formation), derived from the slope debris and its overpressured sedimentary cover. Offshore, the metamorphosed Benambran phase rocks are unconformably overlain by Tabberabberan cycle sediments and volcanics intruded by granodiorite. Our interpretation of the boundary between the Albury-Bega and Narooma terranes as a thrusted passive margin accumulation is incompatible with models of a Narooma Accretionary Complex formed by the subduction of the Paleopacific Plate.

Mid to late Paleozoic shortening pulses in the Lachlan Orogen, southeastern Australia: a review

“…This pattern can be accounted for by linking the Bendigo and Tabberabbera zones in the proposed orocline of Cayley (2012). A similar pattern is evident from the South Coast of NSW, where boninites of inferred Cambrian age have been dredged from the continental slope (Packham & Hubble, 2016), whereas backarc basalts have been found in coastal exposures at Melville Point (Stokes, Fergusson, & Offler, 2015). Thus, the Cambrian boninitic volcanic rocks seem to form a marker horizon that can be traced around the inferred oroclines.…”

“…These oroclines were considered unlikely by Glen (2013) given their linking of the Bendigo and Tabberabbera zones. They were discussed by Packham and Hubble (2016) and considered incompatible with the tectonic setting for the Silurian-Devonian history of the Lachlan Orogen, amongst other issues.…”

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

“…Many workers argued for rollback in the mid- Silurian to mid-Devonian (e.g., Collins 2002aCollins , 2002bGlen 2005;Fergusson 2010), although Aitchison and Buckman (2012) argued for multiple flipping subduction zones instead. Several authors (e.g., Foster and Gray 2000;Collins and Hobbs 2001;Fergusson 2003Fergusson , 2014Gray and Foster 2004) regarded the Lachlan Orogen as the site of multiple coeval subduction zones from the Ordovician to the Silurian, an interpretation not favoured by Glen (2013) or Packham and Hubble (2016). Glen et al (1998) argued for Ordovician subduction in the Lachlan Orogen to account for the calcalkaline signature of Early to Late Ordovician volcanics (Macquarie Volcanic Province) that we now regard as inherited (Quinn et al 2014).…”

“…Melville Point (MP) from personal observation and I. Stewart, personal communication. Burrawarra Point (Burr) from Bischoff and Prendergast (1987) and Stokes et al (2015); offshore Packham and Hubble (2016). Peel Manning Fault System (PMFS) and Murrawong area, New England Orogen from Cawood (1983), Leitch and Cawood (1987), and Stewart (1995).…”

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

“…It is apparent from the recent literature that the tectonics of the Ordovician is still a much discussed and somewhat perplexing issue (Fergusson, 2014;Glen, 2013;Moresi et al, 2014;Packham & Hubble, 2016;Quinn et al, 2014). The Macquarie Arc has been interpreted as an island arc based on both palaeogeographic and geochemical grounds (Blevin, 2002;Carr et al, 1996;Fergusson, 2009;Glen et al, 1998Glen et al, , 2007b.…”