Lithospheric structure of the southeast Australian Lachlan Orogen along the Victorian Global Geoscience Transect

Upper-crustal elements of the similar to 35 km thick crust of the southern portion of the Lachlan Orogen consist of a chevron-folded and faulted turbi dite package (15 to 17 km structural thickness) overlying imbricated Cambri an metabasites and cherts (similar to 5 km structural thickness). These are intruded by both Early and Late Devonian granites and are overlain by Uppe r Silurian(?) marine to continental clastics (Grampians Group) in the west, and Upper Devonian-Early Carboniferous silicic volcanics and continental r edbed clastics in the east. The turbidites show a general younging to the e ast, as well as eastward vergence apart from a local reversal (Tabberabbera zone). The region has been relatively stable since the mid-Paleozoic with apatite fission-track data recording cooling below similar to 100 degrees C at 340-330 Ma in the west and 300 to 280 Ma in the east. Younger fission-t rack ages to the south, approaching the present coastline, reflect denudati on during the opening of Bass Strait and the formation of the Cretaceous Ot way and Gippsland basins. The major crustal discontinuities, the Woorndoo-M oyston and Mount Wellington fault zones, show significant Mesozoic reactiva tion and juxtapose regions of younger against older apatite fission-track a ges. The nature of the lower crust remains unclear, but there is increasing evidence that it is not underlain by thinned Proterozoic continental crust . The Lachlan Orogen is an example of mid-Paleazoic tectonic accretion in a Southwest Pacific-style oceanic setting. Subduction-related oceanic thrust ing produced the deformed and imbricated turbidite packages, and subduction -related magmatic underplating (perhaps during "rollback") produced the lar ge volumes of granite and volcanic rocks, and the localized high-T/low-P me tamorphism.