Timing of orogenic events in the Lachlan Orogen

Abstract

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 well 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 quite inconsistent with deformation of a crust that was being drawn pro‐ gressively 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.

Crustal architecture of central Victoria: results from the 2006 deep crustal reflection seismic survey

“…This indicates that, beginning in the Late Ordovician, a new landmass emerged to the west of the Melbourne Zone-that is, from the vicinity of the present-day Bendigo Zone. We interpret this as the most precise, though indirect, age control on the age of deformation, uplift and consequent emergence above sea-level of the Bendigo Zone (VandenBerg 1999;Cayley et al 2002). With these constraints, the timing and character of deformation in the Bendigo Zone best matches the Benambran orogenic cycle, a sequence of east-west shortening events with onset timing well constrained to around 440 Ma in the eastern Lachlan Orogen (Glen et al 2007c).…”

“…This model explains the apparent conflict between the concept of progressive deformation across the belt proposed by , and the observation that isotopic ages dating deformation and cleavage development appear to show tight time clusters that are widely distributed across the whole region, beginning at 455 to 440 Ma (VandenBerg 1999). The important 440 Ma deformational event, which is marked by a pulse of mica-growth dated by 40 Ar/ 39 Ar data (VandenBerg 1999), may represent the point where the deformational style of the Stawell and Bendigo Zones changed from progressive low-angle thrust imbrication of oceanic rocks, to distributed fault reactivation and cleavage formation fully confined between converging 'vice' jaws in an emerging continental setting. This change in deformation style would have coincided with rapid crustal thickening and Barrovian-style regional metamorphism, driving devolatilisation reactions deeper in the crust.…”

Magnetotelluric survey for undercover structural mapping, Central Victoria

“…How much of it was episodic and how much was continuous has been the subject of debate, with Foster (1998) andVandenBerg (1999) summarising the arguments for and against continuous deformation, respectively. It now seems possible that both arguments have some validity, with deformation being more pronounced in the period 455 to 440 Ma, but continuing outside this time bracket.…”

Pre-1.91 Ga deformation and metamorphism in the Palaeoproterozoic Vammala Migmatite Belt, southern Finland, and implications for Svecofennian tectonics

“…However,t he most significant deformation of the sequence (traditionally known as the Benambran Orogeny, cf. Gray et al, 1997;VandenBurg, 1999;Collins and Hobbs, 2001;Willman et al, 2002), which closed the marginal basin, took place in the late Ordovician to early Silurian. It is comparable to the~1.92-1.90 Ga (D E )deformation and metamorphism that closed the Svionian Basin in the Svecofennian province.…”