Richard Glen, Geological Survey of New South Wales (Australia)
Tony Crawford, University of Tasmania (Australia)
Laurie Hutton, Geological Survey of Queensland (Australia)
The Tasmanides occupy the eastern third of the Australian continent, covering an area 3600 km long x 1200 km wide and form the world's best known collage of accretionary orogens. The Tasmanides consist of five orogenic belts, plus an internal Permian to Triassic rift-foreland basin system that record the evolution of the boundary between part of East Gondwana and the proto-Pacific Plate. From ∼750 Ma (Neoproterozoic) until 520 Ma (Cambrian), the Tasmanides evolved from a rift into a passive margin as Rodinia broke up. Upon the final suturing of West Gondwana, compressive stresses were transferred to the East Gondwana margin, the Australian part of which became largely convergent quite late from about 515 Ma. From then on it recorded convergent-to transform margin tectonism until the Late Triassic, after which the plate boundary migrated into the southwest Pacific. Over these 240 million years, the over-arching process in the southern Tasmanides is the long term easterly roll back of the east Gondwana-proto-Pacific plate boundary, so that the proto-Pacific Ocean never closed. This is reflected by the development of accretionary orogens, marked by general eastward younging of strata and ages of deformation across orogenic belts, and especially by the easterly migration of generally west-dipping suprasubduction zone systems, from Cambrian in the west (Delamerian Orogen) through Ordovician in the Lachlan Orogen to Silurian to Carboniferous in the east (New England Orogen). By the Late Devonian supra-subduction zone systems had evolved from intraoceanic to continental margin style.
There are three important key features:
i) There is no sign of rollback in the North Queensland Orogen where Neoproterozoic to Permian rocks are stacked on top of each other, and remain close to the cratonic margin. This indicates that rollback of the proto-Pacific plate only occurred in the south, and reflect segmentation or oblique spreading of that plate.
ii) Contractional deformations interrupted long periods of rift basin formation and supra-subduction zone activity and were generally short multiphase events of the order of 10 million years.
iii) Extensional roll back of the southern part of the proto-Pacific plate occurred after contractional-transpressional deformation and was accompanied by rifting of older fragments and their incorporation as basement to the developing new supra-subduction zone outboard of the old margin.
iv) Orogen-parallel accretion of continental margin turbidites was an important process in widening of orogenic belts.
v) Major world-class orebodies occur in accreted arcs, in deformed Ordovician turbidites and in Silurian-Devonian rift basins.