Lithospheric Structure of the Southeast Australian Lachlan Orogen along the Victorian Global Geoscience Transect

Abstract

Upper-crustal elements of the ∼35 km thick crust of the southern portion of the Lachlan Orogen consist of a chevron-folded and faulted turbidite package (15 to 17 km structural thickness) overlying imbricated Cambrian metabasites and cherts (∼5 km structural thickness). These are intruded by both Early and Late Devonian granites and are overlain by Upper Silurian (?) marine to continental clastics (Grampians Group) in the west, and Upper Devonian-Early Carboniferous silicic volcanics and continental redbed elastics in the east. The turbidites show a general younging to the east, 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 ∼100°C at 340-330 Ma in the west and 300 to 280 Ma in the east. Younger fission-track ages to the south, approaching the present coastline, reflect denudation during the opening of Bass Strait and the formation of the Cretaceous Otway and Gippsland basins. The major crustal discontinuities, the Woorndoo-Moyston and Mount Wellington fault zones, show significant Mesozoic reactivation and juxtapose regions of younger against older apatite fission-track ages. 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-Paleozoic tectonic accretion in a Southwest Pacific-style oceanic setting. Subduction-related oceanic thrusting produced the deformed and imbricated turbidite packages, and subduction-related magmatic underplating (perhaps during “rollback”) produced the large volumes of granite and volcanic rocks, and the localized high-T/low-P metamorphism.