Recognition and definition of orogenic events in the Lachlan fold belt

Unconformities have been used to establish an orogenic framework for the Lachlan Fold Belt consisting of four main orogenic pulses starting at 440 Ma and ending with stabilisation by 340 Ma. These unconformities provide information about tectonic instability in different tectonic/structural positions but do not delimit fold-belt-wide orogenic events. Ar-40/Ar-39 geochronologic data from cleaved slates and white mica in syntectonic quartz veins in the chevron-folded turbidites of the western subprovince of the Lachlan Fold Belt, have necessitated a complete review and revision of the existing orogenic framework. The orogenic framework is now redefined as continuous over the extent of the belt through latest Ordovician to Late Devonian times (Lachlan Orogeny). Deformation occurs as two subduction-related west-to-east migrating waves within the western and eastern/central subprovinces of the fold belt. The Middle Devonian 'Tabberabberan' event (ca 380-370 Ma) has reduced significance representing limited deformation during amalgamation of the western and central/eastern subprovinces of the fold belt at this time. It is not a paroxysmal cataclysmic orogenic event responsible for the regional 'Lambian' unconformity, which probably results from post-crustal-thickening isostatic adjustment. Orogeny over much of the Lachlan Fold Belt can be considered as progressive, ongoing and subduction-controlled in a complex oceanic, southwest-Pacific-style setting. Such orogeny is analogous to migrating deformation and sedimentation in accretionary wedges above subduction zones.