AUSTRALIA/Tasman Orogenic Belt 247 Figure Ages and distribution of granite intrusives associated with the Tasman Orogen the highest-grade areas (at about 506 Ma) and was followed by post-tectonic plutonism (ca 490–480 Ma) Back-arc Basin Formation (520–500 Ma): Evolution of the Lachlan Orogen A magmatic arc grew on the eastern side of the Delamerian Orogen, starting at about 510–500 Ma, and included plutons and volcanic rocks of the Stavely Belt in Victoria (Figure 4) Shortly after this major crustal thickening the Delamerian belt began to extend rapidly Extension is suggested by, first, the dominantly alkaline and extension-related nature of the post-tectonic plutons (intrusion ages younger than about 490 Ma), second, the inferred rift setting of the Mount Stavely volcanic complex, and, third, thermochronological data indicating relatively rapid cooling of the higher-grade areas Post-orogenic extension may have been caused by subduction roll-back after the Delamerian event, and this probably formed most of the oceanic (back-arc) basement for the Lachlan turbidites that were deposited during the Late Cambrian and Ordovician Most of the mafic metavolcanic rocks and plutons of the central Victorian basement give ages of about 500 Ma Extension induced by slab roll-back may also have rifted small continental fragments away from the cratonized Delamerian Orogen and distributed them within the developing back-arc basin