Jim Austin

Cloncurry Fault Zone: strain partitioning and reactivation in a crustal-scale deformation zone, Mt Isa Inlier

The Cloncurry Fault Zone is a north-northwest-trending zone of complex deformation over 100 km long and up to 7 km wide near the eastern edge of the Mt Isa Inlier. The zone includes mylonites in an anastomosing shear-zone system with variably plunging mineral lineations within a north-northwest subvertical girdle that formed synchronously with north-northwest- and south-southeast-plunging folds. No clear overprinting of lineations in different orientations is observed, and the complexity of the penetrative fabrics can be attributed to strain partitioning during east-northeast contraction, rather than requiring a more complex history of overprinting relations. The mylonites formed at temperatures of 350–500°C, below the ∼650°C metamorphic peak that occurred during the regional D2 event, and they are superimposed on Maramungee-aged granites (1555–1545 Ma), implying that the majority of fabrics formed during D3. D3 was followed by the development of a D4 sinistral Riedel strike-slip fault system involving east-southeast contraction, which was coincident with massive Na–Ca alteration and brecciation within the zone. Reactivation with a normal component of movement occurred some time after the Jurassic. The Cloncurry Fault Zone is a crustal-scale feature of the Mt Isa inlier that records strain partitioning and a deformation history lasting over 1.5 Ga.