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Glen R.A.,Geological Survey of New South Wales | Glen R.A.,Macquarie University
Australian Journal of Earth Sciences | Year: 2013

The well-known southwest-to-northeast younging of stratigraphy over a present-day cross strike distance of >1500 km in the southern Tasmanides of eastern Australia has been used to argue for models of accretionary orogenesis behind a continually eastwards-rolling paleo-Pacific plate. However, these accretionary models need modification, since the oldest (ca 530 Ma) outcrops of Cambrian supra-subduction zone rocks occur in the outboard New England Orogen, now ∼900 km east of the next oldest (520-510 Ma) supra-subduction zone rocks. This is not consistent with simple, continuous easterly rollback. Instead, the southern Tasmanides contain an early history characterised by a westwards-migrating margin between ca 530 and ca 520 Ma, followed by rapid eastwards rollback of the paleo-Pacific plate from 520 to 502 Ma that opened a vast backarc basin ∼2000 km across that has never been closed. From the Ordovician through to the end of the Carboniferous, the almost vertical stacking of continental margin arcs (within a hundred kilometres of each other) in the New England Orogen indicates a constant west-dipping plate boundary in a Gondwana reference frame. Although the actual position of the boundary is inferred to have undergone contraction-related advances and extension-related retreats, these movements are estimated to be ∼250 km or less. Rollback in the early Permian was never completely reversed, so that late Permian-Triassic to Cretaceous arcs lie farther east, in the very eastern part of eastern Australia, with rifted fragments occurring in the Lord Howe Rise and in New Zealand. The northern Tasmanides are even more anomalous, since they missed out on the middle Cambrian plate boundary retreat seen in the south. As a result, their Cambrian-to-Devonian history is concentrated in a ∼300 km wide strip immediately west of Precambrian cratonic Australia and above Precambrian basement. The presence in this narrow region of Ordovician to Carboniferous continental margin arcs and backarc basins also implies a virtually stationary plate boundary in a Gondwana frame of reference. This bipolar character of the Tasmanides suggests the presence of a segmented paleo-Pacific Plate, with major transform faults propagating into the Tasmanides as tear faults that were favourably oriented for the formation of local supra-subduction zone systems and for subsequent intraplate north-south shortening. In this interpretation of the Tasmanides, Lower-Middle Ordovician quartz-rich turbidites accumulated as submarine fan sequences, and do not represent multiple subduction complexes developed above subduction zones lying behind the plate boundary. Indeed, the Tasmanides are characterised by the general absence of material accreted from the paleo-Pacific plate and by the dominance of craton-derived, recycled sedimentary rocks. © 2013 Copyright Taylor and Francis Group, LLC.

Burton G.R.,Geological Survey of New South Wales
Australian Journal of Earth Sciences | Year: 2010

Paleozoic and older basement rocks in northwestern New South Wales and southern and central Queensland are largely obscured by sedimentary rocks of the Mesozoic Eromanga Basin and younger Cenozoic sediments. Interpretations of the basement geology for that area have been heavily reliant upon interpretations of the regional aeromagnetic and gravity data. Geophysically, northwestern New South Wales is characterised by an east-west-trending belt of magnetic features which trend northwest in the western part and northeast to east-northeast in the eastern part; and by an east-west-trending gravity ridge. These east-west-trending features have previously been interpreted to represent the southern part of the Thomson Orogen, a separate tectonic domain to the Lachlan Orogen to the south. However, apart from these east-west geophysical trends there is sufficient geophysical and lithological evidence to suggest continuity of the Lachlan Orogen into southern and central Queensland. A new model is herein proposed whereby the geophysical features are attributed to structures resulting from crustal thickening of a Cambro-Ordovician backarc package, west of a volcanic arc, which occurred during the Late Ordovician to Early Silurian Benambran Orogeny. While the principal stress direction was oriented east-northeast-west-southwest, the structures formed via a process of tectonic escape or extrusion due to differential strain caused by the shape of the Precambrian margin in western New South Wales. The model is consistent with field observations, geophysical imagery, borehole data and seismic data. Analogue models using sand and gelatine were constructed to illustrate the concept and demonstrate that it is feasible. The model permits the extension of the Delamerian and Lachlan Orogens into southern and central Queensland to join with similar rock packages in northern Queensland, consistent with some previous tectonicmodels for the Tasmanides. This brings into question the need to delineate a separate Thomson Orogen. © 2010 Geological Society of Australia.

Percival I.G.,Geological Survey of New South Wales
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2012

Early to Middle Ordovician cherts and cherty siltstones associated with distal turbidite deposition in back-arc basins, are widespread in the Hermidale and Albury-Bega Terranes of the Lachlan Orogen in New South Wales. Study of more than 2500 bedding plane-parallel thin sections prepared to a thickness of 50 μm from these cherts enables recognition of four conodont zones that range in age from the late Tremadocian to latest Darriwilian. Comparable cherts are present in two small remnants of oceanic derivation now exposed on the coast of New South Wales, in the Narooma Terrane (Furongian to Darriwilian), and in the New England Orogen at Port Macquarie in allochthonous blocks (Late Ordovician). Associated fauna include radiolaria, sponge spicules, lingulide and acrotretide brachiopods, fragmentary graptolites, and rare filaments attributed to cyanobacteria. Some of these organisms were pelagic, or may have been attached to floating material, and hence became entrapped in siliceous ooze on the sea floor when they settled under gravity. Others (e.g. the brachiopods) may have been attached to sponges growing on the sea floor. The presence of burrows and bioturbation demonstrates that the deep-sea environment in the Middle Ordovician was well-oxygenated, though this contrasts with Lower Ordovician environments where evidence for infauna is lacking. Predominant colouration of the cherts examined in thin section ranges from honey and yellow-brown (typical of semitransparent cherts) through cream-coloured translucent lithologies to opaque varieties. Dark brown cherty rocks that show evidence of burrowing or bioturbation tend to have a higher silt component. The Ordovician is also a time of extensive chert deposition elsewhere, including terranes in Kazakhstan (commencing in the Late Cambrian); these siliceous sediments display many of the features described from eastern Australia. © 2011.

Glen R.A.,Geological Survey of New South Wales | Glen R.A.,Macquarie University | Roberts J.,University of New South Wales
Journal of the Virtual Explorer | Year: 2012

Most of the New England Orogen comprises a convergent margin, in which Middle Devonian to latest Carboniferous continental margin arcs, forearc basin and subduction complexes were developed above a west-dipping subduction zone. The subsequent history is marked by an early and middle Permian hiatus in arc magmatism, followed by a resumption of west-dipping subduction from the late Permian to Triassic. The geometry of the southern New England Orogen is dominated by a northern, well-established oroclinal fold pair, developed in a subduction complex and overlying Permian rocks, and by a southern oroclinal fold pair, more controversial in acceptance. Our data sustain the presence of the two southern oroclines or megafolds, and suggest that they formed by anticlockwise fold rotation over a possible time span of ~40 million years, beginning in the latest Carboniferous and continuing into the middle Permian. By generating an oroclinal model that takes into account the fold-thrust deformation style of the forearc basin, along with multiple deformation, variations in directions and amounts of shortening, as well as vergence variations, we suggest that the Manning and Hastings oroclinal folds in forearc basin and subduction complex rocks developed as amplified buckle folds of large amplitude, the hinges of which can be tracked south-southwest along their axial traces into smaller amplitude folds along the old arc/forearc boundary. Rather than forming in response to either sinistral or dextral simple shear slip of hundred of kilometres on an inferred N-trending onshore or offshore master fault, these oroclines are reflections of changes in directions and amounts of shortening that occurred along the western margin of the New England Orogen during a lull in convergent margin tectonism.

Musgrave R.J.,Geological Survey of New South Wales
Journal of Structural Geology | Year: 2015

Oroclines are features of orogenic belts that exhibit curvature in plan on a map scale, with the additional requirement that this curvature resulted from relative rotation of the limbs around a vertical or near-vertical axis. Much debate has centred on contrasts between thin-skinned and thick-skinned systems, and on the question of whether the structural fabric was imposed prior to or synchronous with the development of curvature, leading to a variety of inconsistent terminology. A simple hierarchy, in which all map-scale curved orogenic features which pass a test for rotation are termed oroclines (as distinct from primary arcs), and in which these are then subdivided by timing into progressive (synchronous fabric and curvature) and secondary (pre-existing fabric is then rotated), and by mechanism into thin- or thick-skinned, is applied to a suite of curved orogenic features in the Tasmanides of eastern Australia. This analysis includes the first formal application of the "orocline test" (palaeomagnetic or palaeocurrent directions plotted against structural strike) to the Tasmanides.The Neoproterozoic to early Mesozoic Tasmanides comprise four orogens, each of which contains curved orogenic features which are highlighted by extensive aeromagnetic datasets. The Delamerian Orogen, developed on the Gondwana margin, exhibits two curved structures in the Neoproterozoic to Cambrian Adelaide Fold Belt, termed the Fleurieu and Nackara arcs: previous structural interpretations have indicated that the Nackara Arc is non-rotational, and hence not oroclinal. Re-examination of existing palaeomagnetic data, including a positive palaeomagnetic orocline test, shows the Nackara Arc to in fact be a hinge of the Adelaide orocline. Curvature in the Adelaide orocline was dominantly progressive and thin-skinned, but may have been influenced by secondary rotation of the pre-existing, partly underlying Curnamona Craton. Mega-kinks in a buried Delamerian arc-volcanic chain revealed in aeromagnetic images follow the curvature of the Nackara Arc, suggesting an accompanying thick-skinned, secondary oroclinal deformation further from the continental margin.Orogenic curvature in the Palaeozoic Lachlan Orogen has only been widely recognised since the advent of tilt-filtered aeromagnetic images, and a model of oroclinal folding with two hinges (Riverina and Tambo) and strike-slip offset during the Silurian Bindian orogeny has recently been proposed to explain perplexing repetitions of sequence and inversions of structural vergence and directions of sediment provenance. Palaeomagnetic data indicates rotation of elements within the proposed orocline, and a preliminary study indicates a positive palaeomagnetic orocline test for the Riverina hinge. Palaeocurrent data around the Tambo hinge provide a positive, if scattered, orocline test. An independently conceived numeric model of the roll-back response to congestion of a subduction zone has generated an oroclinal structure which shares many features with the proposed Lachlan orocline, and deep crustal ambient-noise seismic data confirms that the major curved and strike-slip features persist throughout the crust. The Lachlan orocline appears to be secondary and thick-skinned.Orogenic curvature in the largely covered Thomson Orogen occurs along its southern border with the Lachlan Orogen (the putative Olepoloko orocline) and around its geometrically complex boundary with the Mossman Orogen in northern Queensland (the Charters Towers orocline). Rotation of the Olepoloko structure cannot be confirmed, and competing models for its formation leave open the question of whether this is a primary arc, a thin-skinned progressive orocline, or a thick-skinned secondary orocline. Palaeomagnetic data from the Charters Towers orocline are limited to two poles, but a change in declination between the Silurian pole and the Devonian pole matches the apparent rotation of the orocline, which appears to have been thick-skinned and secondary.In contrast to the other orogens, consensus that at least part of the curvature in the Palaeozoic to Triassic New England Orogen is oroclinal has been broad, although not universal. There remains substantial debate over the number of hinges (from two to four) and mechanism. Palaeomagnetic poles have previously been cited as evidence of rotation of blocks within the orocline, but this paper presents the first formal palaeomagnetic orocline test, which is positive for the Manning and Texas hinges. A palaeocurrent orocline test of the Manning hinge, in younger rocks than the palaeomagnetic sample, is negative, constraining rotation in the southern, Manning hinge to the Carboniferous before 322 Ma, while rotation in the northern, Texas hinge appears to be latest Carboniferous or Permian. The existence of a Hastings hinge is questionable, but if real, its rotation also appears to be younger than that of the Manning hinge. Thick-skinned, secondary rotation of the hinges of the New England orocline appears to have been diachronous. © 2015 .Published by Elsevier Ltd.

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