Australia: The Land Where Time Began

A biography of the Australian continent 

The Lachlan Supercycle¹

Molassic sedimentation of the old Delamerian Orogen began the began the Lachlan Supercycle, though the main characterising feature was rollback of the southern part of the Tasmanides and the formation of the Macquarie Arc, that was intraoceanic, opposite a part of the boundary that was 1000 km long and was a highly convergent part of the boundary dipping to the west. In the southern part of the New England Orogen, blueschists that were associated with this subduction zone are present as knockers in fault slices, suggested by the author¹ to probably be displaced by about 200 km to the north in the Benambran Orogeny dating to about the end-Ordovician. Quartz-rich turbidites and shales, of Ordovician age, formed in the southern Tasmanides in separate terranes, one suggested by the author¹ to be in situ, deposited in a back arc basin, the others being allochthonous formed off West Antarctica of the present. The Bega Terrane was transported to the north along a largely transform margin during the Late Ordovician, being accreted, along with the Macquarie Arc by combined thick and thin-skinned thrusting, with multiple deformation around the boundary of Ordovician-Silurian age in the Benambran Orogeny. In the Benambran Orogeny the Narooma terrane, that was largely oceanic from the Ordovician, was also accreted to the Bega Terrane. During the closure of ocean basins in the Benambran Orogeny ocean crust that had formed during the Middle Ordovician spreading of the seafloor were also imbricated in with other terranes and are represented by MORB-like mafic-volcanic-chert terranes. In the Benambran Orogeny the Bendigo Terrane, that was allochthonous, also underwent a thin-skinned deformation, south of its present position, though only in the western part. Deposition continued in the eastern part into the Early Devonian as part of  northwards strike-slip transport until accretion took place in the Tabberabberan Orogeny at the end of the Early Devonian. The North Queensland Orogen also preserved convergent margin relations, though with a continental margin arc and back arc basin that are recognised in the Early Ordovician and the Late Ordovician. An unconformity of Middle Ordovician age is situated between them.

In the Tabberabberan Cycle the enlarged eastern part of Gondwana, except for the Bendigo Terrane, underwent major tension that continued from the Early Silurian to the Middle Devonian. Crustal extension in the southern Tasmanides occurred in a wide developing back arc region that had formed as a result of rollback that took place in the southern section of the proto-Pacific Plate. As a result of this a new subduction zone, that dipped to the west, was relocated, and an intraoceanic arc and subduction complex rocks are now preserved by the southern section of the New England Orogen. The Macquarie Arc, of Ordovician age, was dismembered into several structural belts as a result of this extension in the Silurian-Devonian, as well as formation of sedimentary rifts and transtensional basins, and both I and S-type granitic batholiths were emplaced. The author¹ suggests the accretion of the Bendigo Terrane, marked by inversion of sedimentary and volcanic-rich rifts, deformation of granitoids and re-deformation and renewal of imbrication of older rocks, is probably reflected by the Tabberabberan Orogeny from the Middle Devonian.

Sources & Further reading

1. Glen, Dick, The Lachlan Orogeny: New Boundaries, new data, new ideas, new deposits.
2. Glen, R.A., 2005, The Tasmanides of Eastern Australia. In: Vaughn, A. P. M., Leat, P. T. & Pankurst, R. J. Terrane Processes at the Margins of Gondwana. Special publication of the Geological Society, London, 246, 23-96.