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. 40Ar/39Ar 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.
Ar‐Ar data from fabric‐forming white mica in slates, syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism, deformation, and exhumation in the Lachlan orogen, Australia. These data also help define the tectonic evolution of the Tasmanides during Paleozoic time. The Lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to Gondwana during the closing of a small marginal ocean or back arc basin. This tectonic setting was similar to the present western and southwestern Pacific region. Accretion of the Lachlan orogen to Gondwana occurred by closing of the basin system by subduction‐accretion processes and some translation. The process is typified in the western Lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement/melange zone and is recorded by Ar‐Ar mica ages ranging from ∼455 Ma in the western part to ∼390 Ma in the eastern part. In the central Lachlan orogen, deformation migrated southwestward from ∼440–430 to 405 Ma away from the high‐temperature Wagga‐Omeo metamorphic complex, where deformation/metamorphism occurred between >440 and 400 Ma. In the north, ∼400 Ma mica ages record deformation and inversion of structures in the Cobar basin. In the eastern Lachlan orogen, Ar‐Ar mica dates range from 450 to 340 Ma. Ages of 455–445 Ma are yielded by the Narooma accretionary complex, 405–390 Ma ages are found along the major thrust faults bounding high‐grade metamorphic complexes, and 360–340 Ma cooling ages are found in the inverted extensional basins (e.g., Hill End) and related structural zones. The Ar‐Ar results also document periods of reactivation on early‐formed structures during later deformation elsewhere in the orogen.
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