POLYPHASE DEFORMATION AND SHEAR ZONE REACTIVATION, CURNAMONA PROVINCE, AUSTRALIA

The evolution of superimposed orogenic events can lead to episodic reactivation and accompanying development of new high strain and/or shear zones. This results in the sequential development of increasingly complex inter-linked shear zones, often with different orientations. Such shear zones preserve highly variable kinematic histories and considerable geometric complexity resultant from the same deformation event. Complex structural patterns and strain partitioning is developed and can lead to over-interpretation of the deformational evolution of an orogen in such circumstances.

Shear zone reactivation has played an important part in the evolution of the Curnamona Province, Australia, which has undergone multiple episodes of deformation and metamorphism; including the Paleoproterozoic Olarian Orogeny (~1600Ma), and the Cambrian Delamerian Orogeny (~520Ma). The earliest recognisable shear zones have exerted a major influence on deformation styles and strain partitioning as these orogens evolved. The first generation of shear zones are bedding parallel and formed under high temperature, low pressure metamorphic conditions, probably during extension of the lithosphere. Shear zone reactivation during nappe formation resulted in a combination of strain partitioning at different structural levels and folding of high temperature shear zones within the nappes. Favourably oriented shear zones folded within the nappe structures were reactivated during transpression into conjugate strike slip faults and associated folds and thrusts. These shear zones were then further reactivated during the Delamerian Orogeny as thrust faults juxtaposing Neoproterozoic Adelaidean sequences against Paleoproterozoic Willyama Supergroup sequences. This study demonstrates that deformation partitioning and associated geometries developed during polyphase deformation is strongly controlled by the orientation of original and reactivated segments of early shear and high strain zones.