Timing of Mesoproterozoic tectonic activity in the northwestern Gawler Craton

Abstract

An array of northeast-trending shear zones in the northwest Gawler Craton of South Australia has been implicated in widely disparate Proterozoic continental reconstructions. Quantitative constraints on timing of movement along these shear zones are sparse, but potentially provide tests for contrasting reconstructions. New 40Ar/39Ar data provide improved age constraints for movement along these shear zones and are interpreted to indicate that the Karari, Tallacootra and Coorabie shear zones were active at ~1440 Ma. This timing is ~100 Ma younger than suggested in several published reconstructions and is also significantly older than Grenvillian-age tectonism in adjacent provinces to the west and north. The shear zones of the northwest Gawler Craton are interpreted to have formed in response to sinistral transpression, and dissect the region into several geological domains of contrasting metamorphic grade. Despite distinct differences in metamorphic grade the various domains exhibit evidence of a shared event-history prior to deformation at ~1440 Ma, albeit at different crustal levels. Movement along the shear zones of the northwest Gawler Craton at ~1440 Ma therefore appears to have dissected and reorganised a previously coherent crustal block rather than representing a suture zone between distinct terranes.