Kink Bands and Major Folds, Broken Hill, Australia

Abstract

Microscopic kink bands form an axial-surface foliation to major folds in layering and schistosity in the Mt. Robe area, Broken Hill. The kink bands, spaced at intervals of 1 to 30 mm, are concentrated in narrow zones (200 to 400 m thick) which pass through the hinges of the folds. The axes of the kink bands are statistically parallel to axes of the major folds. The major folds apparently formed by rotation of one limb while the other limb remained static. The sense of rotation of a deformed limb is the same as the constant sense of rotation of kink bands in the same fold. The orientation of kink bands varies systematically and can be resolved into domains of constant orientation. Within each domain, all kink bands have a constant width and show approximately the same amount of rotation of foliation segments. Thus, a domain is made up of kink bands representing equal increments of strain while the total strain differs in each domain. The orientation of the principal axes of stress during formation of kink bands can be inferred from deformation lamellae in synchronously deformed quartz. The maximum principal stress made an angle of 65° with the initial orientation of schistosity and an angle of 55° with kink boundaries. Measurement of the angles between two sets of lamellae in some grains suggests that lamellae may parallel the basal pinacoid, unit rhombohedra, rhombohedra, and trigonal bipyramids. Analysis of minor folds and lineations indicates that the major folds, and associated kink bands, formed during the last of three episodes of deformation. The first episode included intense isoclinal folding and the formation of schistosity (axial-surface) and mineral lineation. The folding effected almost complete transposition of layering at all scales to parallel layering. The lineation is generally oblique to fold axes; the mode of deformation of calc-silicate boudins with the same orientation suggests extension parallel to, and normal to, lineation in the schistosity. A second episode of folding produced numerous minor isoclinal folds but left the fabric at a larger scale unaffected.