250 AUSTRALIA/Tasman Orogenic Belt in the oceanic crust and imbrication in the overlying turbidite wedge Shallow-angle subduction had probably initiated in the west, under the backstop of the Delamerian Orogen, by about 460 Ma, when major faults began forming in the western Lachlan Orogen Sedimentation was continuing outboard in the basin as the western Lachlan sediment wedge thickened and the basal de´ collement propagated eastwards A magmatic arc associated with this westwards-dipping subduction zone did not develop until Late Silurian times, probably because the subduction was of very shallow dip and the ocean basin that closed was relatively small, so the amount of subduction was limited Shallow-dipping subduction is consistent with the moderately high metamorphic pressure– temperature ratios The eastwards-dipping subduction system of the central Lachlan Orogen resulted in the high-temperature–low-pressure metamorphism and elongated north-westwards-trending Late Silurian to Early Devonian granitoids of the Wagga-Omeo Metamorphic Belt (Figures and 8) as well as the emplacement of ophiolite slices (e.g Howqua, Dookie, and Tatong greenstones; Figure 4) and the Howqua blueschists (Figure 7) Andean-type Margin (400–380 Ma): Evolution of the Lachlan Orogen During the Early Devonian, at around 400 Ma, the last undeformed part of the marginal basin in the west collided with the arc–metamorphic complexes of the central Lachlan Orogen (Figure 10A) This caused some crustal thickening and deformation of the younger sediments (former Melbourne ‘trough’) Combined structural thickening and the removal of the oceanic lithosphere of the marginal basin formed crust of continental thickness and character, and led to a switch to continental sedimentation and the development of a major regional fold-belt-wide angular unconformity These effects are related to the Tabberabberan orogenic event Collision followed by attainment of freeboard over most of the Lachlan Orogen led to the development of an Andean-type margin (Figure 10B) Continued westwards-dipping or craton-directed subduction of proto-Pacific lithosphere beneath this part of the Gondwanan margin along the major outboard longlived subduction zone caused the magmatism that was responsible for the extensive northwards-trending elongated granitoids in the eastern Lachlan Orogen (Figure 8) An easterly younging in these granitoids suggests associated slab rollback during the mid-Late Silurian to Early Devonian, with rollback rates of the order of mm year Thrusting was also taking place in the eastern Lachlan Orogen at this time Roll-back and Gondwanan Margin Post-Orogenic Extension (365–340 Ma) Marked crustal-scale extension at around 380 Ma caused rifts, basin-and-range faulting, voluminous explosive volcanism and caldera development, and highlevel plutonism in the western and central Lachlan Orogen (e.g the Central Victorian magmatic province; Figure 8) By 340 Ma subduction in the eastern Lachlan Orogen had probably stopped, and reefs were growing around the volcanic edifices The major subduction zone at this time probably stepped east more than 1000 km to form the New England Orogen (Figure 10B) Andean Margin (350–280 Ma) and Arc–Continent Collision (260–230 Ma): Evolution of the New England Orogen During the Carboniferous, westwards-dipping subduction beneath the Gondwanan margin led to silicic and intermediate volcanism from 350 Ma to 310 Ma, coincident with an inboard belt of Late Carboniferous rocks (330–325 Ma) At around 300 Ma magmatic activity migrated outboard into the developing associated Devonian–Carboniferous accretionary prism Migration of the volcanic arc was associated with widespread extension in the Late Carboniferous to Early Permian due to slab retreat along the long-lived subduction zone, with the development of an outboard intraoceanic arc (Gympie Belt) Collision of this arc with the Gondwanan margin in the Late Permian led to the Hunter–Bowen Orogeny (260–230 Ma), which juxtaposed and accreted the Carboniferous flank–forearc sequence (Yarrol–Tamworth Belt), the Devonian arc complex, and the Devonian–Carboniferous subduction complex (Wandilla–Gwydir Belt) by cratondirected overthrusting Margin overthrusting led to molasse sedimentation in a developing foreland basin (Sydney–Bowen Basin) Cordilleran-style granites (e.g New England batholith) reflect renewed subduction magmatism at this time, with the development of a Late Permian–Early Triassic magmatic arc This was followed by Late Triassic extension, which is recorded by silicic caldera-related volcanism and granitic plutonism as well as the development of small extensional basins with bimodal volcanism See Also Andes Antarctic Australia: Proterozoic; Phanerozoic Gondwanaland and Gondwana Tectonics: Convergent Plate Boundaries and Accretionary Wedges; Mountain Building and Orogeny