Proterozoic strain localisation during the Kimban Orogeny: a structural analysis of Williams Island, Eyre Peninsula, South Australia

© 1997 Matthew HarrowfieldStrain localisation and shear zone development is spatially controlled by rheological inhomogeneity of the crust. Shear zone development is found to be localised by the distribution of mafic dykes within the deformed Palaeoproterozoic granitoids and orthogneisses of the Lincoln Batholith, outcropping on Williams Island, southern Eyre Peninsula, South Australia. These granitoids have experienced regional upper amphibolite metamorphism, episodic ductile deformation and exhumation of the Lincoln Batholith during the Kimban Orogeny (c. 1850-1700 Ma). In the area mapped, crustal shortening has been accommodated by dextral strike-slip transport along the dyke-hosted Williams Shear Zone, and by the transferring of strain into oblique south-up thrusts. Strain analysis of granite orthogneiss was undertaken using both finite strain methods and by examination of strain-induced orthogneiss fabric development. It was found that large strains have been localised within mafic dykes, whilst comparatively little strain has been accommodated within the host rock. Although finite strain estimation met with limited success, results demonstrate a partitioning of flattening and constrictional strain between granite gneisses and the mafic dykes and an asymmetry of strain intensity across dyke-hosted shear zones. Temporal evolution of the granite/dyke rheological contrast results in a progression of boudinage structures which may be used to chart the variation or temperature and strain rate during metamorphism and deformation. Such evolution is mimiced within orthogneiss fabric by changes in the quartz/feldspar rheology contrast and grain-scale microstructure. Placing the deformation observed on Williams Island into a regional perspective has proven difficult due to the limited scale of mapping and an incoherence of kinematic data from previous work within the batholith. It seems plausible that the strike-slip movement recorded on the Williams Shear Zone occurred in response to regional exhumation of the Lincoln Batholith during the Kimban Orogeny