Geometric and kinematic evolution of asymmetric ductile shear zones in thrust sheets, southern Adelaide Fold–Thrust Belt, South Australia

The Southern Adelaide Fold–Thrust Belt, South Australia contains unusually asymmetric shallow to moderately inclined low-grade phyllonitic shear zones, uniformly floored by basal imbricate thrusts. The fabrics and minor structures that reveal the geometric and kinematic evolution of these shear zones are described, analysed and presented in detail. Incipient fabrics and structures are developed in upper transitional zones and then develop progressively more intensely downwards into the shear zones. They ultimately intensify along the lower boundary thrusts that make up the base of all of the shear zones. Microstructural analysis of multiple cleavages and grain-scale geometries from within the shear zones demonstrate that deformation is spatially and temporally concentrated along the lower boundary thrusts. The shear zones initiated by strain-softening at the base of a number of stacked thrust sheets during Mid- to Late-Cambrian orogenic shortening of the area when the fold–thrust belt was generated. Kinematic indicators confirm consistent NW-directed transport. The shear zones progressively developed upwards by further superposition of shear and flattening strain during which time deformation was localised within the base of the thrust sheets to develop distinctively asymmetric shear zones.