Late Cambrian – Middle Ordovician extension of the northern Tasmanides: thinned crust that facilitated intense Silurian (Benambran) shortening deformation

Henderson, Robert1, Fergusson, Chris2 and Withnall, Ian3

1Department of Earth and Environmental Sciences, James Cook University, Townsville, Australia, 2School of Earth, Atmospheric and Life Sciences, University of Wollongong, Australia, 3Geological Survey of Queensland, Brisbane, Australia

The Charters Towers and Greenvale Provinces of the Thomson Orogen provide the most extensive exposure of early Paleozoic rocks in the northern Tasmanides. Upper crust represented by the Charters Towers Province developed a thick, Proterozoic passive margin, sedimentary assemblage with a minor contribution from mafic igneous rocks. This assemblage was subsequently overprinted by active margin tectonism commencing in the mid Cambrian (~510 Ma), reflecting a broad scale Tasmanide regime change of that age. The overprinting regime generated thick (>15 km) basinal infill, inclusive of a substantial intermediate to silicic volcanic complement, of the upper Cambrian – Middle Ordovician Seventy Mile Range Group and other upper Cambrian basinal relicts identified for the Charters Towers and Argentine Metamorphics. Magmatic arc plutonism is widely developed as the Ravenswood Batholith and Fat Hen Creek Complex. Regional metamorphism and penetrative fabric development of Early Ordovician age overprinted all of the Proterozoic passive margin assemblage and part of the Cambrian basin fill, with structural analysis favouring its association with extensional strain. Coeval basin formation, plutonism and metamorphism across the province reflects an enduring episode of crustal extension.  

The Greenvale Province consists largely of metavolcanic and metasedimentary tracts with protoliths of Early Ordovician age. Lower Ordovician granites are also represented and in broad aspect, rock units of the province both resemble, and are correlative with, the active margin assemblages of the Charters Towers Province.  Structural analysis shows a deformation history matching that of the Charters Towers Province with the dominant foliation similarly attributed to extensional strain.  Development of the province represents thick basinal infill developed on thinned crust of a continental margin. Contrary to earlier publications, no rocks older than Ordovician are known for the province. Amphibolite and ultramafic units interleaved with metasediments on its eastern (outboard) side suggest it may have developed on oceanic crust.

For both provinces thin crust had developed by the Middle Ordovician, weakening its resistance to subsequent Benambran contraction. Cambro-Ordovician rock assemblages of both provinces were affected by structural/metamorphic overprint with local generation of mylonite in wide shear zones. Contraction is dated as Silurian by Ar–Ar mineral ages, by fabric development in deformed granites of known age and by the age of overlap strata. Tight upright folds and accompanying foliation, generated by shortening, deformed tracts affected by Ordovician extensional strain, steepening previously formed fabrics. Basin fill represented by the Seventy Mile Range Group was inverted. For the Greenvale Province, widespread amphibolite grade metamorphism indicates that much of it has been exhumed from considerable crustal depth (>15km). In contrast exhumation of the Charters Towers Province was heterogeneous, ranging from <5 km (unmetamorphosed Seventy Mile Range Group) to >15 km (amphibolite facies metamorphism and local migmatite). Silurian structural trends, considered by some authors as registering an orocline, are attributed to strain partitioning consequential on oblique convergence.


Based in Townsville, the presenter has actively pursued research interests in the Tasmanides for over 40 years, with a focus in particular on the Mossman, Thomson and New England Orogens which are extensively developed in Queensland. Mush of this activity has involved collaboration with Chris Fergusson and Ian Withnall.

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