THRUST-RELATED STRAIN GRADIENTS AND THRUSTING MECHANISMS IN A CHEVRON-FOLDED SEQUENCE, SOUTHEASTERN AUSTRALIA

An E-directed, imbricate-fan thrust-system within a chevron-folded, monotonous quartz-rich turbidite succession shows marked gradients in strain from the upper to lower parts of individual thrust-sheets. Upper parts of thrust-sheets show low to moderate strains (XZ strains < 5.0:1), subvertical extensions defined by straight quartz-fibres in pressure-shadows on framboidal pyrite, and 50-65% shortening in the inferred thrust-transport direction. This shortening is reflected by upright, close chevron-folds (ILA almost-equal-to 35-50-degrees, where ILA is fold interlimb angle) with axial lines subparallel to major fault traces, spaced cleavage and both E- and W-dipping reverse faults. Lower parts of thrust-sheets are characterized by high strains (XZ > 30:1), tight inclined chevron folds (ILA < 30-degrees) with predominantly W-dipping axial surfaces, intense phyllitic cleavage with a strong down-dip mineral stretching lineation, and long curved tapering quartz pressure-shadows on pyrite. Such differences in fold geometry, the intensity of the cleavage and the geometry of quartz-fibres in pressure shadows, require thrust-sheet emplacement to be synchronous with the internal deformation of sheets. Thrust-sheets show a change from an approximately constant volume, plane strain coaxial deformation in their upper parts to non-coaxial high strain deformation with incremental (omega-i) between those expected for pure shear and simple shear (i.e. 0 < omega-i < 5.45-degrees) in their basal parts. Observed geometries and intensities reflect partitioning of strain and deformation mechanisms within individual thrust-sheet volumes both during the initiation and propagation of subsurface detachment-faults, and with their emplacement to shallower crustal levels. Strain softening, related to chevron-folding and more intense cleavage development within a 500 m basal zone, has controlled the emplacement of these turbidite-dominated thrust-sheets. Transport and internal deformation of the thrust-sheets did not involve gravitationally-induced spreading, but required either a push from the rear or some form of underthrusting.