Early Tasmanides evolution: Passive to convergent margin history in New South Wales, Australia

Greenfield, John1; Gilmore, Phil1; Musgrave, Robert1

1Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia.

Initial development of the Tasmanides in southeastern Australia involved Early Neoproterozoic rifting/break-up of the Rodinian supercontinent, expressed as tholeiitic dyke swarms and continental rifting in the Adelaide Rift Complex of eastern South Australia. This left highly-extended, transitional crust between the Gawler Craton and Curnamona Province, which became the depocentre for extensive platform carbonate and shallow marine clastic sedimentation. By ~700 Ma, a passive margin developed east of the Curnamona Province which saw the initiation of the palaeo-Pacific Ocean. A final NE–SW phase of rifting in the Late Neoproterozoic was associated with shallow marine platform sedimentation and alkaline magmatism (Mount Arrowsmith Volcanics), further attenuating the eastern Curnamona Province crust and presenting an angular continental salient towards the nascent ocean to the east.

This crustal configuration profoundly influenced the palaeogeography and tectonism that followed during the Delamerian Cycle, as passive margin clastic deposition in the early Cambrian gave way to west-facing subduction in the mid Cambrian. Elements of the resulting Cambrian volcanic arcs (Mount Wright and Loch Lily–Kars) are now immediately adjacent to the Curnamona Province margin. However, regional geological mapping in the Koonenberry Belt has shown that the Mount Wright Arc developed in a rift zone within the Curnamona Province that probably initiated during the last phase of Rodinia break-up in the Late Neoproterozoic. In contrast, the strike-equivalent Loch Lily–Kars Arc was developed in an intra-oceanic setting. Mapping, drillhole and geophysical data shows that this arc segment, along with forearc volcanic rocks of the Ponto Group, were oroclinally folded clockwise almost 90°, and thrust against the southeastern Curnamona Province margin during the Late Cambrian Delamerian Orogeny. If the original arc segments were part of a linear belt, it would have extended southeast from the oroclinal hinge at the Grasmere Knee Zone. Recently acquired and modelled AusLAMP magnetotelluric data show a strong lower crustal conductive anomaly aligned along this trend.

The Delamerian Orogeny caused strong ductile deformation of rocks deposited in the Delamerian Cycle. Areas that were highly attenuated during break-up suffered tight upright folding and oroclinal bending, which may also have been affected by clockwise rotation of the Curnamona Province. Proterozoic Olarian Cycle metamorphic rocks along the margins of the Curnamona Province and the eastern edge of the Gawler Craton also suffered upright open to tight folding during the Delamerian, with σ1 perpendicular to the outboard margin. In the Broken Hill Domain, early southwest-directed fold-thrusting switched to northwest-directed fold-thrust and strike-slip deformation at the end of the Delamerian Orogeny in the Early Ordovician.

Clearly the switch to a convergent setting, with arc accretion and terminal deformation in the Delamerian Orogen, caused extensive shortening of a highly attenuated margin that had been in extension for c. 300 Ma. The Curnamona Province, although acting as a salient in the early Delamerian Orogeny, was itself deformed and could not insulate the distal-foreland Flinders Ranges from deformation late in this event. This had cumulative effects on the ensuing cycles of subduction roll-back, extension and contraction that defined the Tasmanides throughout the Palaeozoic.


Biography

John leads the Geoscience Acquisition & Synthesis Unit in the Geological Survey of NSW, which collects and interprets geoscientific data from geological mapping, geophysical surveys, and specialist studies in mineral deposits, palaeontology, and petrography.

Unravelling the Tumut Trough: A Middle Ordovician age for the Brungle Creek Metabasalt, eastern Lachlan Orogen

Bruce, Michael,1 Percival, Ian1, Zhen, Yong Yi1

1Geological Survey of New South Wales, W.B. Clarke Geoscience Centre, Londonderry, Australia

Alpine-type ultramafic bodies are exposed in numerous localities throughout the Lachlan Orogen of New South Wales. Despite the tectonic significance of such oceanic lithosphere to the development of the orogen, few studies on the genesis of these bodies have been documented.

The Coolac Serpentinite is an Alpine-type ultramafic intrusion that marks the eastern edge of the Tumut Trough in the eastern Lachlan Orogen. Recent petrological, geochemical and geochronological studies into the massive harzburgite (Bruce 2018) that makes up most of this body reject any ophiolitic association with rocks of the North Mooney Complex. These rocks are traditionally ‘lumped in’ as part of the proposed Coolac Ophiolite Suite, largely because of their physical location and resemblance to a layered, crustal ophiolitic sequence. Instead, a 2-stage melting model is proposed for the origin of the Coolac harzburgites with a late Cambrian latest melting event inferred from an allochthonous block (501 ± 2.6 Ma; U/Pb zircon) with petrological links to the harzburgites.

The ‘block’ has been incorporated into the Silurian Jackalass Slate within the trough, which was previously thought to be simple sedimentary trough fill, but is now partly interpreted as a sedimentary-matrix melange incorporating much older blocks. This interpretation is supported by blocks of chert found within the Jackalass Slate that contain conodont elements (Periodon aculeatus) of late Darriwilian to early Gisbornian age. Slightly older conodont assemblages with diagnostic elements of Periodon hankensis, indicating a late Dapingian to early Darriwilian age, are also found within chert lenses of the structurally underlying Brungle Creek Metabasalt.

Chert blocks within the Jackass slate and chert lenses within the Brungle Creek Metabasalt show near-identical, REE chondrite normalised abundances and patterns as well as evidence of significant terrigenous mafic volcanic and hydrothermal input. This implies that the Brungle Creek Metabasalt is coeval with chert deposition and is thus early Middle Ordovician in age (465−468 Ma). The presence of Cambrian basement, widespread cataclastite in and around the Brungle Creek Metabasalt and structurally underlying Bullawyarra Schist and identical chert units in the Brungle Creek Metabasalt and younger Jackalass Slate, as well as lenses of chert−volcanic clast conglomerate within the Brungle Creek Metabasalt, all support the structural interpretation of Stuart-Smith (1990), who suggested uplift, collapse and extension along a low-angle detachment fault. In addition, it is suggested that older, collapsed blocks have later been re-sedimented into younger Silurian basin strata.

Bruce M.C. 2018. Petrology, geochemistry and a probable Cambrian age for harzburgites of the Coolac Serpentinite, New South Wales, Australia. Australian Journal of Earth Sciences 65, 335−355.

Stuart-Smith P.G. 1990. Evidence for extensional tectonics in the Tumut Trough, Lachlan Fold Belt, NSW, Australia. Australian Journal of Earth Sciences 37, 147−167.


Biography

Michael Bruce currently works for the Geological Survey of NSW, Department of Regional NSW. Michael joined this organisation in 2005, having completed a PhD at the University of Queensland. His interest includes petrology and geochemistry of igneous rocks but is particularly focused on the genesis of mafic and ultramafic rocks.

Life Cycle of the Ordovician Macquarie Arc, Lachlan Orogen, Eastern Australia

Qing Zhang1, 2, Allen Nutman1, Solomon Buckman1

1School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

The Ordovician intra-oceanic Macquarie Arc is preserved in a tectonostratigraphic terrane, faulted to the west and east against coeval, quartz-rich turbidites of the Adaminaby Group, within the Lachlan Orogen of eastern Australia. Debates exist concerning the allochthoneity of the Macquarie Arc, the polarity of its related subduction and the nature and exact timing of collision with Gondwana. These key problems are addressed by the integrated application of field observations, petrography, zircon U-Pb-Hf isotopes and whole rock geochemistry to key units within the Macquarie Arc stratigraphy. By these approaches, it has been possible to answer (i) the timing and juvenility of the arc initiation, (ii) the timing of arc-continent collision, and (iii) the allochthoneity and emplacement mechanism of the Macquarie Arc onto the eastern edge of Gondwana. The new results confirm that the Macquarie Arc was initiated far from the continent with no continental contamination, most likely via outboard (eastward) subduction at high dip angle. The arc started colliding with the eastern Gondwana during the Late Ordovician (~456 Ma), indicated by the trench-fill sedimentary protoliths of the Triangle Formation. Preservation of juvenile island arc on continental margins is aided by outboard subduction that results in emplacement of the arc complex as a klippe in an upper plate position on top of the passive margin sequence, instead of an autochthon extending deep to the mantle, amalgamated with the continent through a back-arc closure.

These results enriched the knowledge of continental growth of eastern Gondwana that it involved the episodic addition of juvenile oceanic terranes via east-dipping subduction, and emphasized that the detrital zircon ages could record the process. By establishing the arc chronology via these zircons is a major contribution to understanding the geodynamic setting of this Paleozoic arc-related copper mineralization along the Pacific margin.


Biography

Qing Zhang is a postdoc at the Institute of Geology and Geophysics, Chinese Academy of Sciences (2020.7-), working with the SIMS research group. She is interested in understanding the evolution of orogens, with particular interests in using zircon U-Pb ages and Hf-O isotopes methods.

Revisiting the break-up evolution of the SE Australian rifted margin: New perspectives from larger data sets

Gleadow, Prof. Andrew1, McMillan, Malcolm1, Boone, Dr Samuel1, Kohn, Prof Barry1

1School of Earth Sciences, University of Melbourne, Melbourne, Australia

Studies of the apatite fission track (AFT) thermochronology of the SE rifted margin of NSW in the early 1980s were amongst the first to show a relationship between low temperature thermochronology (LTT) and the processes of continental rifting.  Similar studies have followed in other parts of the world. Several studies on the SE Australian margin have centered on a transect across the Bega Batholith from Nimmitabel via Bega to Tathra-Bermagui on the coast.  Like many rifted margins, this area shows an uplifted plateau of low relief separated by a major erosional escarpment near Brown Mountain from a broad coastal plain of moderate relief to the present Tasman Sea coast. 

Apatite fission track ages along this transect fall into three zones, the first on the uplifted plateau with AFT ages of ~270-300 Ma, mean track lengths (MTL) of ~13 µm and unimodal length distributions.  The second, from the escarpment towards the coast, with AFT ages of 150-250 Ma and broader, often bimodal, length distributions with means of 10-12µm.  The third zone, within ~10 km of the cost, has AFT ages of 90±10 Ma with narrow length distributions and long MTLs of ~14µm.  These zones define the original ‘boomerang’ trend of MTL with AFT age where the MTL reaches a minimum for the intermediate ages and represent the progressive replacement of an older age on the plateau (~300 Ma) by a younger cooling event (~90) near the coast representing samples that must have been at temperatures >100°C prior to rapid cooing in the mid Cretaceous.  Similar patterns, with some significant variations, have been found on rifted margins in other parts of the world, but are by no means universal.

The tripartite zoning in the AFT data approximately coincides with the principal geomorphic elements of this area which has long suggested that the two records are related.  Denudation from around the time of Tasman Sea rifting has been invoked to explain exposure of relatively deeper levels along the coast with much older ages preserved on the plateau. Important corollaries of this interpretation are that the boomerang trend and the youngest ages must always lie below the erosional escarpment, and that the youngest ages should approximately date at least the onset of denudation. 

Much larger regional scale AFT data sets now show, however, that the particular relationship between the AFT zones and the escarpment observed around Bega is atypical and probably fortuitous.  Further north the boomerang trend diverges inland and crosses the escarpment implying that the underlying thermal history, tied to the ~90 Ma cooling event, must pre-date formation of the escarpment and be independent of the subsequent post-breakup landscape evolution. The minimum of the boomerang trend closely parallels the western margin of the Sydney Basin and we propose that the AFT thermal history relates largely to Cretaceous erosion of a formerly more extensive Permo-Triassic sedimentary cover.  Evolution of the present-day landscape is poorly constrained by the thermochronology and may be of substantially younger age, probably Cenozoic.


Biography

Andy Gleadow has pioneered fission track thermochronology in Australia for understanding the thermal and tectonic evolution of the continental crust. His work has provided tools that are now used routinely by earth science researchers around the world to extend our knowledge of the age and evolution of earth materials. 

Substrate of the Macquarie Arc: geophysical evidence and implications for tectonic setting

Musgrave, Dr Robert1

1Geological Survey Of New South Wales, Maitland, Australia

Ordovician to early Silurian calc-alkaline volcanic-, associated intrusive-, and shallow-marine sedimentary rocks comprise the Macquarie Arc, the principal arc system in the Lachlan Orogen. Although Pb-isotope ratios and positive εNd suggest intraoceanic origins, there are competing hypotheses regarding the arc’s tectonic setting. The range of proposed tectonic models include:

  1. simple intraoceanic models, in which subduction initiated in the Ordovician over a west-dipping subduction zone, an east-dipping zone that later reversed polarity, or a south-dipping zone within a back-arc that later rotated anticlockwise;
  2. two-stage intraoceanic models, in which the Ordovician arc developed on an earlier intraoceanic Cambrian arc;
  3. the Lachlan Orocline model, which suggests the Macquarie Arc arose close to the Cambrian convergent margin of Gondwana;
  4. or in a very different viewpoint, the Macquarie ‘Arc’ may have been an extensional system in a back-arc, far behind the frontal arc located outboard of the New England Orogen.

Seismic reflection and refraction surveys across the Macquarie Arc, with corresponding potential-field models, were previously interpreted to indicate a MORB-like substrate, consistent with proposed tectonic models of type 1 and (arguably) 4. However, petrophysical parameters yielded by these potential-field models are inconsistent with this interpretation. Long-wavelength, deeply sourced signals from a variety of geophysical techniques can clarify the composition of the basement, and these differing datasets show remarkable agreement. Long-wavelength aeromagnetic features qualitatively indicate a basement of low magnetic susceptibility. Quantitative modelling limits this to values typical of intermediate intrusive rocks characteristic of either mature arc or continental basement. Seismic velocity from the AuSREM model maps out a belt of low p-wave velocity, consistent with an intermediate chemistry, in the mid- to lower crust, which corresponds closely to the areas of magnetic susceptibility lows. Long-wavelength, long period MT signals recorded by the AusLAMP array map out conductivity highs that continue from mid (10 km) to lower (40 km) crustal depth and broadly correspond to the Macquarie Arc.

Petrophysical interpretation of an intermediate composition in the middle and lower crust below the Macquarie Arc is incompatible with a MORB-like composition and requires the basement of the Macquarie Arc to be either a pre-existing arc, a continental fragment, or a very thick arc in which little of any original MORB remains in situ. Tectonic models of types 1 and 4 are therefore ruled out. Recently reported magmatic Cambrian zircons from Macquarie Arc rocks are consistent with models of either type 2 or 3, but Pb isotope links between the Macquarie Arc and the forearc of the Cambrian Mount Wright Arc on the Gondwana margin suggest support for a version of model type 3, the Lachlan Orocline model. The Eocene to Recent forearc to rear-arc system of the Izu-Bonin Arc, south of Japan, shares geochemical, geophysical and scale similarities with the Macquarie Arc. This analogy suggests a genetic linkage between the Macquarie Arc, the Cambrian boninite–tholeiite–calc-alkaline belts in Victoria and offshore NSW, and the Cambrian Ponto Group on the margin of the Delamerian orogen in northwest New South Wales.


Biography

Robert (Bob) Musgrave: Research Geophysicist with the Geological Survey of New South Wales. Multidisciplinary geophysicist and tectonic interpreter, with more than 30 years’ experience in palaeomagnetism, tectonics, magnetic and gravity interpretation and modelling, and marine geophysics. Bob built and runs the PALM palaeomagnetic/petrophysics laboratory at the University of Newcastle.

Devonian-Carboniferous regional deformation in the northeastern Lachlan Orogen, southeastern Australia

Fergusson, Chris1 & Colquhoun, Gary2

1School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2Geological Survey of New South Wales, Mining, Exploration & Geoscience, Department of Regional NSW, Maitland, Australia

The timing of the regional deformation of the turbiditic Silurian-Devonian Hill End Trough in the northeastern Lachlan Orogen has been a contentious issue with one view ascribing the regional north-south folds and axial planar foliation to Middle Devonian basin inversion. Alternatively, it has been argued that given the low-angle discordance between Lower and Upper Devonian units on the Capertee and Molong highs, and the development of a dominant episode of folding mapped from the Hill End Trough into the adjoining highs, the major regional deformation is latest Devonian to early Carboniferous and predates intrusion of Bathurst-type granites at 358–314 Ma. We have approached the problem by analysing the gentle, upright, southeast-trending folds and deformation patterns in Devonian units of the northern Capertee High (Cudgegong area), drawing a cross-section across the Hill End Trough and reviewing the structure of two areas in the Molong High (west of Orange, and south of Wellington). Upper Devonian units on the highs are affected by fold patterns that extend into the underlying successions and have been interpreted as continuous into the central Hill End Trough, confirming the extent of the latest Devonian to early Carboniferous regional deformation in the northeastern Lachlan Orogen. Constraints from timing of deposition and radiometric ages are ambiguous and imply deformation was overlapping with sedimentation, as has also been inferred from sedimentary features of the Upper Devonian succession. Steeply dipping faults along the highs predated the Late Devonian and were probably formed in extension during formation of the Hill End Trough and were reactivated during basin inversion and potentially during Upper Devonian sedimentation. Our conclusion is that the simplest hypothesis is that the regional north-south deformation was of latest Devonian to early Carboniferous age, although poorly understood basin inversion occurred in the Middle to Late Devonian.


Biography

Chris Fergusson graduated in geology in the 1970s/early 1980s and has been lecturing in geology at the University of Wollongong up until retirement earlier this year. He has maintained long-standing interests in the tectonics of orogenic belts (including the Tasmanides) but also in deep-sea drilling, and Sydney Basin geology.

New constraints on the tectonic and metallogenic history of Lachlan Orogen

Habib, Umer1, Leslie, Chris1, Meffre, A/Prof. Sebastien1, Schaap, Thomas1, Wells, Tristan1

1CODES, University of Tasmania, Hobart, Australia

The Lachlan Orogen has a long and complicated geological history involving:

  • Cambrian collision and orogenesis
  • Ordovician continental-derived sediment deposition
  • Ordovician island arc development formation of porphyry and epithermal deposits
  • Silurian orogenesis and extension and formation of volcanic-hosted massive sulphide deposits
  • Devonian orogenesis and formation of granite-related deposits

This geological framework is well-supported by data from many previous studies. However, the exact plate configurations responsible for these geological events remain poorly constrained. These constraints are required to make predictions about the location of ore deposits and to gain a better understanding of the structure and composition of the continental crust.

New data acquired throughout the Lachlan orogen over recent years combined with data from previous studies have helped to improve scientific knowledge showing that: 

  • During the earliest geological history of the Lachlan Orogen there was at least 2 Cambrian arcs: one continental and the other oceanic. The continental arc extended along the western edge of the orogen. The intra-oceanic island was dismembered after it collided with the Selwyn Block and Tasmania.
  • Geochemical and geochronology data from the Melbourne zone and the Selwyn block in Victoria show that most of this zone is likely underlain by crust containing juvenile components rather than thick Mesoproterozoic continental crust.
  • The intra-oceanic Macquarie Arc began approximately 10 Ma after the end of the Cambrian magmatism and was active for a further 40 million years.
  • Magmatism in Macquarie Arc began to be contaminated by continental material starting at 450 Ma in the Molong area. Continental contamination and porphyry development occurred at different times within the magmatic history of the arc.
  • The Lachlan Orocline model explains much of the tectonic evolution of the area from the latest Ordovician through to Devonian periods.

Although these constraints are useful in refining the tectonic models, many details remain unresolved and uncertain.


Biography

Sebastien Meffre is Associate Professor at the University of Tasmania. He is the head of Earth Sciences and also works within the Centre for Ore Deposit and Earth Sciences (CODES). His current research interests include: the U-Pb isotopic system, understanding the plate tectonic processes and interactions and geochemistry.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.

Conference Managers

Please contact the team at Conference Design with any questions regarding the conference.
© 2020 Conference Design Pty Ltd