Timing of orogenic events in the Lachlan Orogen

A substantial database of 40Ar/39Ar ages, collected recently from micas in western and central Victoria, has been used in several recent papers as support for continuous, diachronous deformation across western and central Victoria lasting through much of the Early Palaeozoic. This paper reviews these ages, together with field evidence collected over the last ten years. It provides an alternative interpretation, that mica growth and overgrowth in western Victoria was not continuous but episodic, occurring at ca 455 Ma, 440 Ma and 425 Ma, with little or no mica growth recorded from between these times. These ages have been obtained from mica in regional cleavage, crenulation cleavage and in quartz veins, and from across the entire width of the Stawell and Bendigo structural zones of western Victoria. A sharp change in mica ages occurs at the Mt William Fault, east of which no mica growth older than about 380 Ma is recorded. Several ages used in support for diachronous deformation are not related to deformation: an 40Ar/39Ar age of 417 Ma from Chewton is from the aureole of a Devonian granite, and an age of 410 Ma from the Melbourne Zone is shown to contain a substantial amount of inherited mica. If it is accepted that mica growth can be used to date deformation, then the 40Ar/39Ar ages indicate episodic, not continuous, deformation in western Victoria (Stawell and Bendigo Zones). The sharp decrease in the deformation age in the Melbourne Zone, east of the Mt William Fault, agrees weil with field evidence that shows continuous sedimentation in the Melbourne Zone in the period (Ordovician to mid-Early Devonian) during which the Stawell and Bendigo zones were undergoing deformation. Some correlation also exists between the 40Ar/39Ar ages from western Victoria and well-constrained deformational events in the eastern Lachlan Orogen. The pattern of deformation has important corollaries in any model that attempts to understand what drives the deformation. While plate convergence must be the ultimate driving force, the pattern is guile inconsistent with deformation of a crust that was being drawn progressively into subduction zones, as proposed in recently published models. Rather, the observed pattern suggests that deformation happened in several very brief events, probably on semi-rigid plates.