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New Perspectives Workshop, September 2014
Presented by AIG Victoria and AusIMM Central Victoria Branch, Romsey Victoria

Evaluating tectonic evolution and resource exploration potential of the southern Thomson Fold belt

W.J. Collins*

NSW Institute for Frontiers Geoscience, University of Newcastle, Newcastle NSW, Australia

G. Rosenbaum

School of Earth Sciences, University of Queensland, St Lucia Queensland, Australia

S.E. Bryan

School of Earth & Environmmental Science, Queensland University of Technology, Brisbane Qld, Australia

C. Verdel

School of Earth Sciences, University of Queensland, St Lucia Qld, Australia

A.C. Hack

NSW Institute for Frontiers Geoscience, University of Newcastle, Newcastle NSW, Australia

R. Hegarty

Geological Survey of New South Wales, Maitland NSW, Australia

D Purdy

Geological Survey of Queensland, Brisbane Qld, Australia

Abstract

This project is a collaboration between three universities and two geological surveys, with Geoscience Australia also involved through major geophysical and drilling programs. It is one of the key initial projects of the national geoscience exploration initiative, UNCOVER.

A key unexposed region in eastern Australia is the central Tasmanides, an area straddling the NSW-Qld border (Fig. 1). In this region, the Southern Thomson Orogen (STO) links the southern with the northern Tasmanides, and hence plays a key role in understanding Tasmanides geodynamics. It is one of the most poorly exposed and least understood regions in eastern Australia.

The origin and evolution of the Tasmanides have traditionally been explained in the context of a palaeo- Circum-Pacific convergent margin, which was oriented ~N-S (in present coordinates) and mostly involved W-dipping subduction (eg., Veevers, 1984; Fergusson et al , 1986; Collins, 2002; Glen, 2005). Such models explain the general N-S structural grain of the Tasmanides, but have difficulty in explaining orthogonal (~E-W) tectonic features. The STO is the major, arcuate E-W oriented structure in the Tasmanides, extending ~750 km broadly parallel to the NSW-Qld border. The interplay with Paleozoic deformation within the Australian craton is just beginning to be investigated (eg. Cayley, 2012; Veevers, 2013), but to date, little consideration has been given to how orthogonal (N-S oriented) tectonic forces have impacted on Tasmanides development. As such, the origin of the STO is an unanswered question, resolution of which may change the way we understand the geodynamic evolution of the Australian continent during the Paleozoic.

Collins Fig 1

Fig 1. Map showing location of Southern Thomson Orogen (STO), which lies below the extensive cover of the Eromanga Basin. Only 1% of the STO crops out in the field area. Note the enigmatic E-W boundary with the Lachlan Orogen. (From Glen et al 2013).

Contrasting models of the central Tasmanides

Three hypotheses have recently been suggested for the origin and evolution of the STO:

  1. Development as an E-W oriented dextral strike-slip fault system at the southern margin of a westward retreating subduction margin (Glen et al., 2013). This geodynamic setting, according to Glen et al. (2013), was dictated by the geometry of a Neoproterozoic backarc system (Barcoo Basin) that was lying behind an eastern arc built on an extended ribbon of Precambrian continental basement.
  2. Reorientation of the Cambrian-Ordovician N-S trending convergent margin by mid-Paleozoic oroclinal deformation (Cayley, 2012). According to this scenario, the entire Lachlan Orogen was folded into a Z- shaped orocline that more than doubled the width of the orogenic system. The model predicts southward translation and clockwise rotation of an Ordovician oceanic arc in North Queensland (equivalent to Macquarie Arc in NSW), starting at ~435 Ma, and resulting in the major E-W trending arcuate structure of the STO.
  3. Formation of the E-W trending segment of the STO by “Benambran” ENE-WSW compression at 440-430 Ma. According to this model, Cambro-Ordovician turbidites were squeezed against the Precambrian salient of the Curnamona block, and were then extruded to the south (Burton, 2009).

Predictions of and tests for each model

  1. Timing of arc volcanism: The major E-W, geophysical anomaly is interpreted as an arc in all models, but the age is unknown. Model 1 suggests that it is Neoproterozoic (580 Ma) or possibly an older passive margin (Anakie equivalents). Model 2 suggests that it is Ordovician (Macquarie Arc equivalent). Model 3 predicts that the region is dominated by Cambro-Ordovician turbidites. Tests will involve U-Pb geochronology of appropriate volcanic sequences and coeval granitoids (e.g., Warraweena volcanics).
  2. Nature of basement: Model 1 suggests that the basement is a Neoproterozoic passive margin. Model 2 suggests a dominant Cambrian basement, with possible remnants of Neoproterozoic crust. Model 3 predicts Cambro-Ordovician oceanic crust. Tests will seek inheritance age spectra in magmatic rocks to target for Hf isotopes, and maximum depositional ages of adjacent tectono-stratigraphic packages. We will also conduct geophysical and thermal modelling to place broad constraints on the nature of the basement (e.g., high heat flow associated with old radiogenic continental crust).
  3. Relation to Koonenberry Zone (KZ) (Delamerian Orogen: Fig 1) and its eastern termination: Model 1 could incorporate the eastern KZ as the along-strike Cambrian equivalent to the Neoproterozoic arc; Models 2 and 3 predict no Koonenberry equivalents in the STO. Tests will involve age/provenance of tectono-stratigraphic packages.
  4. Nature of the domain northward of the inferred arc: Model 1 predicts Neoproterozoic-Cambrian Barcoo Basin sediments; Model 2 predicts Ordovician Girilambone Group or the Bendigo-Ballarat zone sediments of Victoria, as does model 3. Tests will involve detrital zircons on tectono-stratigraphic packages and/or the chemistry of inferred ultramafic/mafic schists along the Culgoa lineament
  5. Nature of the domain directly southward of the inferred arc: Model 1 implies a Neoproterozoic- Cambrian forearc; Model 2 predicts Mallacoota-Adaminaby zone turbidite equivalents of the Lachlan Orogen (LO); Model 3 predicts Girilambone Group of the LO. Alternatively, is this KZ equivalent? Tests will involve detrital zircons on tectono-stratigraphic packages in Brewarrina region
  6. Nature of the original suture zone between the Thomson & Lachlan orogens: Is the suture marked by the Culgoa Lineament (NE-trending bright lineament in central-east of Fig. 1)? Where does it continue westward? Tests will involve kinematic analysis and Ar-Ar age constraints on the timing of deformation.
  7. Nature of the inferred bounding fault between the STO and Lachlan Orogen (Olepoloko Fault) (dashed curvi-lineament running E from Binerah Downs): Model 1 predicts the boundary is a Neoproterozoic transform fault that was reactivated during later (Devonian) N-S thrusting and had a final Carboniferous overprint. Model 2 predicts an early Silurian sinistral fault, evolving into a major Devonian thrust fault (without the requirement for Carboniferous movement). Model 3 predicts an early Silurian fault. Tests will involve kinematic analysis and Ar-Ar ages for the timing of deformation.
  8. Structural evolution and timing of orogenesis. Model 1 implies that the major E-W structures are dominantly Devonian-Carboniferous, perhaps beginning in the middle Ordovician (460-360 Ma range). Model 2 implies Silurian-Devonian (440-390 Ma) ages for these structures, and Model 3 implies only Early Silurian (~440-430 Ma) ages. Devonian basins in the STO (e.g., Paka Tank Tough) provide useful time markers that can help reconstructing the structural evolution. In particular, thermochronology can provide key information on when the Thomson Orogen became cratonised. Tests will involve constraints on the timing of deformation (schistosity/cleavage) and syn- to post-kinematic granites. In addition, we will constrain maximum depositional ages on zircons from Devonian basins and thermochronolgy U-Pb and Ar/Ar geo- and thermochronology of Devonian basins.
  9. Nature of E-W trending plutons. Model 1 predicts that these plutons should be pre-thrusting (pre- Devonian); Model 2 predicts plutonism to ~390 Ma, when the orocline is inferred to stop. Model 3 predicts plutonism at ~440-430 Ma, associated with Benambran “tectonic escape”. Tests will involve U- Pb geochronology of late granites.
  10. Existence of Early Devonian rift sequences. The Devonian sedimentary basins provide additional clues to STO evolution. The potential input from surrounding orogens (Central Australia, Lachlan ± New England) can also be assessed. Also, by understanding the thermal history of the STO, we will be better able to understand how and when tectonic activity terminated in the Thomson Orogen and shifted eastwards.

Collins Fig 2

Fig. 2. TMI map of Southern Thomson Orogen showing major geophysical E-W anomalies plus location of drillholes available for sampling (yellow dots). Traverses for proposed MT and AEM surveys, and for AEM array also shown. Drilling program will largely follow the geophysical surveys, plus areas of shallow basement.

Implications For Resource Potential

Cu deposits in BIF-like metamorphosed metasediments might be expected if Anakie equivalents exist in the basement (Model 1).

VHMS base metal deposits, and Ni in ultramafic schists (eg., Culgoa Magnetic Lineament) if the Warraweena volcanics are of Cambrian age. Similarly, it is likely that the Cobar Basin extends northward at least to the Olepoloko Fault, and possibly to the Culgoa Lineament(linear magnetic feature in central- east of Fig 2).

Porphyry copper-gold deposits might be expected if the arc is a correlative of the Macquarie arc.

Orogenic gold deposits are likely to occur in the deformed extensive turbidite piles to the N of the arc system, given the potential correlations with the Bendigo-Ballarat zone (models 2 and 3).

Volcanic or sediment-hosted Cu deposits equivalent to Girilambone group copper deposits are predicted by model 3 for sediments located south of the arc system.

Intrusion-related gold deposits associated with ~430 Ma granites are already recognised in the Tibooburra region, and along-strike to the east.

Tin deposits have been discovered in the 420 Ma old Brewarrina granite, and similar syn-kinematic granites are predicted from the geophysical maps of the region.

References

Burton, G. R.(2010) ‘New structural model to explain geophysical features in northwestern New South Wales: implications for the tectonic framework of the Tasmanides, AJES 57: 23-49.

Cayley, R. 2012. Oroclinal folding in the Lachlan Fold Belt: Consequence of SE-directed Siluro-Devonian subduction rollback superimposed on an accreted arc assemblage in eastern Australia. In: Selywn Symposium 2012. Geol. Soc. Aust. Abst, 103, 34-43.

Collins W.J. 2002. Nature of extensional accretionary orogens. Tectonics 21 (4);1258-1272 (10.1029/2000TC001272).

Fergusson et al.., 1986 Fergusson, C.L., Gray, D.R. and Cas, R.A.F., 1986, Overthrust terranes in the Lachlan Fold Belt, southeastern Australia: Geology, 14, 519-522.

Glen, R.A., 2005. The Tasmanides of eastern Australia. Geological Society Special Publication, 246, p. 23–96.

Glen R.A., et al. (2013) Geodynamic significance of the boundary between the Thomson Orogen and the Lachlan Orogen, northwestern New South Wales and implications for Tasmanide tectonics, AJES 60, 371-412,

Hegarty, R. 2010. Preliminary geophysical–geological interpretation map of the Thomson Orogen. In: Thomson Orogen—Release of Provisional and Preliminary Information June 2010. DVD, Geological Survey of New South Wales.

Veevers, J.J. (Ed.), 1984. Phanerozoic Earth History of Australia. Clarendon Press, Oxford, pp. 418.

Veevers, J.J., 2013 Pangea: Geochronological correlation of successive environmental and strati-tectonic phases in Europe and Australia. Earth-Science Reviews 127, 48–95

About the Speaker

Professor Bill Collins is the Director of the NSW Institute for Frontiers Geoscience at the University of Newcastle. He has focussed his research on the interplay between magmatism and tectonics, using the integrated information to highlight the role of protracted extensional tectonics in the origin of the Tasmanides. More recently, he has been using Hf isotopes in zircon to provide new avenues to understand Tasmanides tectonics, and is applying this isotopic approach to understand global geodynamics. He was a member of the implementation committee for progressing the UNCOVER initiative, which has a focus on exploration geoscience in Australia. He will be discussing how he has used the UNCOVER initiative to gain Australian Research Council (ARC) linkage project success.

 

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