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.
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.
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.
Forster D.B.,Geological Survey of New South Wales |
Carr G.R.,CSIRO |
Downes P.M.,Geological Survey of New South Wales
Gondwana Research | Year: 2011
The Macquarie Arc of New South Wales hosts several major Au and Cu deposits. We present new Pb isotope results for Cadia, the Copper Hill, Little Copper Hill and Cowal deposits, along with data from the CSIRO database. The results generally plot close to established mantle growth curves and are similar to oceanic basalts. Data for individual deposits mostly have Pb model ages consistent with independent age constraints on mineralization. Intrusions associated with the Early Silurian mineralization at Cadia and Goonumbla have narrow and distinct Pb isotope signatures that we interpret to be the result of partial melting of already LILE-enriched mantle-like sources. The data suggest that deposits of the Macquarie Arc derived Pb from one or more long-lived mantle-like Pb isotope reservoirs without significant contributions of crustal Pb prior to the Benambran Orogeny. Data for the Copper Hill deposits includes unradiogenic, possibly old Pb and supports previous workers who suggested that old MORB-like basalts may occur at depth in the area. The Peak Hill deposit has the most unradiogenic signature and has the lowest 208Pb/ 204Pb and 207Pb/ 204Pb. These signatures closely match Cambrian MORB-like basalts in the Koonenberry Belt and are unlike Cambrian mafic rocks in Victoria. Similar rocks could form part of the substrate to other parts of the Macquarie Arc. © 2010.
Rawlinson N.,Australian National University |
Kennett B.L.N.,Australian National University |
Vanacore E.,Australian National University |
Glen R.A.,Geological Survey of New South Wales |
Fishwick S.,University of Leicester
Gondwana Research | Year: 2011
Distant earthquake data recorded by seven sub-arrays of the ongoing WOMBAT rolling seismic array deployment in southeast Australia are combined for the first time to constrain 3-D variations in upper mantle P-wavespeed via teleseismic tomography. The seven arrays comprise a total of 276 short period recorders spaced at intervals of approximately 50. km, thus allowing unprecedented resolution of the upper mantle over a large region. In the mantle lithosphere immediately below the crust (~. 50. km depth), dominant variations in velocity tend to strike east-west, and share little resemblance to Palaeozoic boundaries in the shallow crust inferred from surface geology and potential field data. A broad region of elevated wavespeed beneath northern Victoria may represent the signature of underplated igneous rocks associated with detachment faulting during the break-up of Australia and Antarctica. A distinct low velocity anomaly in southern Victoria appears to correlate well with the Quaternary Newer Volcanic Provinces. Towards the base of the mantle lithosphere, the dominant structural trend becomes north-south, and five distinct velocity zones become apparent. Of particular note is a transition from higher wavespeed in the west to lower wavespeed in the east beneath the Stawell Zone, implying that the Proterozoic lithosphere of the Delamerian Orogen protrudes eastward beneath the Western subprovince of the Lachlan Orogen. This transition zone extends northwards from southern Victoria into central New South Wales (the northward limit of the arrays), and is one of the dominant features of the model. Further east, there is a transition from lower to higher wavespeeds in the vicinity of the boundary between the Western and Central subprovinces of the Lachlan Orogen, which has several plausible explanations, including the existence of a Proterozoic continental fragment beneath the Wagga-Omeo Zone. The presence of elevated wavespeeds beneath the Melbourne Zone in Victoria, although not well constrained due to limited data coverage, provides some support to the Selwyn Block model, which proposes a northward extension beneath Bass Strait of the Proterozoic core of Tasmania. © 2010 International Association for Gondwana Research.
Glen R.A.,Geological Survey of New South Wales |
Glen R.A.,Macquarie University |
Saeed A.,Macquarie University |
Quinn C.D.,Geological Survey of New South Wales |
Griffin W.L.,Macquarie University
Gondwana Research | Year: 2011
The Ordovician Macquarie Arc in the eastern subprovince of the Lachlan Orogen, southeastern Australia, is an unusual arc that evolved in four vertically stacked volcanic phases over ~37millionyears, and which is flanked by coeval, craton-derived, passive margin sedimentary terranes dominated by detrital quartz grains. Although these two terranes are marked by a general absence of provenance mixing, LA-ICPMS analysis of U-Pb and Lu-Hf contents in zircon grains in volcaniclastic rocks from 3 phases of the arc demonstrates the same age populations of detrital grains inherited from the Gondwana margin as those that characterise the flanking quartz-rich Ordovician turbidites. Magmatic Phase 1 is older, ~480Ma, and is characterised by detrital zircons grains with ages of ~490-540 with negative ε Hf from 0 to mainly -7.78, 550-625Ma ages with negative ε Hf from 0 to -26.6 and 970-1250Ma (Grenvillian) with ε Hf from +6.47 to -6.44. We have not as yet identified any magmatic zircons related to Phase 1 volcanism. Small amounts of detrital zircons also occur in Phase 2 (~468-455Ma), hiatus 1 and Phase 4 (~449-443Ma), all of which are dominated by Ordovician magmatic zircons with positive ε Hf values, indicating derivation from unevolved mantle-derived magmas, consistent with formation in an intraoceanic island arc. Because of the previously obtained positive whole rock ε Nd values from Phase 1 lavas, we rule out contamination from substrate or subducted sediments. Instead, we suggest that during Phase 1, the Macquarie Arc lay close enough to the Gondwana margin so that volcaniclastic rocks were heavily contaminated by detrital zircon grains shed from granites and Grenvillian mafic rocks mainly from Antarctica (Ross Orogen and East Antarctica) and/or the Delamerian margin of Australia. The reduced nature of a Gondwana population in Phase 2, hiatus 1 and Phase 4 is attributed to opening of a marginal basin between the Gondwana margin and the Macquarie Arc that put it out of reach of all but rare turbiditic currents. © 2010.
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 |
Kars M.,Center for Advanced Marine Core Research
Geochemistry, Geophysics, Geosystems | Year: 2016
Core disturbance, drilling overprints, postdepositional acquisition of remanence, authigenic growth of magnetic iron sulfides, and alteration all contribute challenges to recognizing the primary magnetostratigraphy in marine sediments. We address these issues in a sequence of tuffaceous muds and volcaniclastics at International Ocean Discovery Program Site U1437 and produce the longest continuous magnetic polarity stratigraphy in the history of scientific ocean drilling. Remanence measurements were filtered to remove intervals affected by fluidization, plastic sediment disturbance, and core biscuiting. Drilling overprints are concentrated in the disturbed annulus surrounding intact core material. Bioturbation was limited to a vertical extent of at most 15 cm. Changes in sediment color, stiffness, and magnetic hysteresis all suggest that remanence was locked in within a few meters of the sediment-water interface. We did not observe any systematic offset between magnetostratigraphic and biostratigraphic datums. Authigenic growth of greigite, in response to both initial sulfate reduction in the upper 50 m of the sediment column and to deeper resupply of sulfate, has led to magnetic overprinting. Anomalous polarity artefacts, extending <5 m and occurring within about 20 m below a real polarity transition, appear to be due to a chemical remanence acquired by greigite produced during early diagenesis. Diagenetic magnetic mineral alteration resulted in the progressive loss of fine-grained magnetite, which enhanced susceptibility to drilling and postdrilling overprints and increased the resistance of these overprints to removal by conventional demagnetization. We recovered the magnetostratigraphic record from many samples with resistant overprints through low-temperature demagnetization through the Verwey transition. © 2016. American Geophysical Union.
Pickett J.W.,Geological Survey of New South Wales
Geologica Belgica | Year: 2016
The early ontogeny of the solitary rugose coral Symplectophyllum from the late Tournaisian-early Viséan of New South Wales is described. The genus is frequently associated with the phaceloid tabulate coral Syringopora, individuals of the former genus occurring within the coralla of the latter, and growing in tandem. A similar association also occurs between Symplectophyllum and the rugosan genera Cionodendron and Pickettodendron, though less commonly. The phaceloid genera suffer periodic mortality events, causing breaks in vertical growth; the settlement of Symplectophyllum larvae appears to be associated with these events. Larvae settle on algal incrustations of the epitheca of the phaceloid corals, not on the epitheca itself. The manner in which Symplectophyllum corallites acquired space for growth suggests the presence of sweeper tentacles in Rugosa. © 2016, Taylor's University. All rights reserved.