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.

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