Timing of orogenic events in the Lachlan Orogen

A substantial database of Ar-40/Ar-39 ages, collected recently from micas i n western and central Victoria, has been used in several recent papers as s upport for continuous, diachronous deformation across western and central V ictoria lasting through much of the Early Palaeozoic. This paper reviews th ese ages, together with field evidence collected over the last ten years. I t provides an alternative interpretation, that mica growth and overgrowth i n 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 the se times. These ages have been obtained from mica in regional cleavage, cre nulation cleavage and in quartz veins, and from across the entire width of the Stawell and Bendigo structural zones of western Victoria. A sharp chang e 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 diac hronous deformation are not related to deformation: an Ar-40/Ar-39 age of 4 17 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 def ormation, then the Ar-40/Ar-39 ages indicate episodic, not continuous, defo rmation 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 th e Melbourne Zone in the period (Ordovician to mid-Early Devonian) during wh ich the Stawell and Bendigo zones were undergoing deformation. Some correla tion also exists between the Ar-40/Ar-39 ages from western Victoria and wel l-constrained deformational events in the eastern Lachlan Orogen. The patte rn of deformation has important corollaries in any model that attempts to u nderstand 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 pro posed in recently published models. Rather, the observed pattern suggests t hat deformation happened in several very brief events, probably on semi-rig id plates.