Australia: The Land Where Time Began

A biography of the Australian continent 

Tasmanides of Eastern Australia - Refining Accretionary Orogen Models

In the southern Tasmanides of eastern Australia the stratigraphy is younger from southwest to Northeast over a present-day cross-strike distance of more than 1,500 km, and this has been used as evidence in support accretionary orogenesis models behind a Palaeopacific plate that is continually rolling to the east, though the authors1 suggest these accretionary models need modification. The oldest outcrops of supra-subduction rocks from the Cambrian, dated to about 530 Ma, are present in the outboard New England Orogen, that is presently about 900 km east of the next oldest supra-subduction rocks, 510-500 Ma, and this is not consistent with simple easterly rollback. The early history of the southern Tasmanides is characterised by a margin that is westwards-migrating between about 530 Ma and about 520 Ma that opened a vast backarc basin that was about 2,000 km wide that has remained open. Continental margin arcs that are stacked almost vertically, dating from the Ordovician to the close of the Carboniferous, within 100 kilometres of each other in the New England Orogen, indicate that in a Gondwanan reference frame there is a plate boundary that is constantly west-dipping. The actual boundary position has been inferred to have undergone advances that are contraction-related and retreats that are extension-related, these movements being estimated to have been about 500 km or less. Rollback that occurred in the Early Permian has not since been reversed, so arcs from the Late Permian-Triassic to Cretaceous lie further to the east in the most easterly part of eastern Australia, with rifted fragments being present in the Lord Howe Rise and in New Zealand. The northern Tasmanides are even more anomalous as they missed out on the retreat of the plate boundary that occurred in the south in the Middle Cambrian, with the result that their history from the Cambrian to the Devonian is concentrated in a strip that is 300 km wide immediately to the west of Precambrian cratonic Australia and overlying basement from the Precambrian. A plate boundary that is virtually stationary in a Gondwanan frame of reference is implied by the presence in this narrow belt, that dates from the Ordovician to Carboniferous, of continental margin arcs and backarc basins. The presence of a segmented Palaeopacific plate is suggested by the bipolar nature of the Tasmanides, with major transform faults that propagate into the Tasmanides as tear faults favourably oriented for local supra-subduction zone systems to form, and subsequently intraplate north-south shortening. According to this interpretation of the Tasmanides quartz-rich turbidites, from the Lower-Middle Ordovician, accumulated as fan sequences, and are not representative of multiple subduction complexes that developed above subduction zones present behind the plate boundary. The general absence of material accreted from the Palaeopacific plate, and by the dominance of recycled sedimentary rocks that were craton-derived, characterises the Tasmanides.

Sources & Further reading

  1. Glen, R. A. "Refining Accretionary Orogen Models for the Tasmanides of Eastern Australia." Australian Journal of Earth Sciences 60, no. 3 (2013/04/01 2013): 315-70.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated  18/06/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading