Initiation of cataclastic flow in a mylonite zone

Abstract

At the mylonite-cataclasite transition in the homogeneous quartzo-feldspathic mylonites of the Great Slave Lake shear zone, northwest Canadian Shield, arrays of distributed macroscopic fractures evolved into zones of cataclastic flow. Fracture arrays occupy elliptical areas (metres by 10s of centimetres) aligned in the mylonite foliation plane. Fracture segments rotated either forwards or backwards in response to deformation. Where fractures rotated backwards, they remained planar and cataclasite did not develop. In the case of forward-rotation, only the central part of each fracture rotated. The rotated fracture segments are notably shorter and more closely spaced (1–2 mm) than those which have not rotated and the fracture array has a sigmoidal geometry. Cataclasite developed by the further deformation of such sigmoidal, forward-rotated fracture arrays. Back-rotated fractures were initially planar, regularly spaced (2–10+ cm) and oriented at 70° to the mylonite foliation; they correspond to extensional faults. Forward-rotated fractures were initially planar, regularly spaced (2 cm) and oriented perpendicular to the mylonite foliation; they do not correspond to classical arrays of extensional or contractional faults. Both the initial orientation of the fractures and their subsequent rotational behaviour were constrained by the ability/inability of the wallrock to extend and by the ability/inability of the fractures to dilate. Within the forward-rotated fracture arrays, cataclasite was produced after the rotated fracture segments had locked. It is proposed that a second perpendicular fracture set cut the earlier sigmoidal fractures resulting in stubby fragments of mylonite. The angular fragments acted as stress raisers within the otherwise homogeneous mylonite. This allowed the transition from distributed macroscopic fracture to impingement-induced localized microfracture and reduction of the fragment size with further strain. When a sufficient volume of interconnected fine material had formed, significant displacements could be accommodated by cataclastic flow, leading eventually to the development of thick (1–100s m) zones of cataclasite. Models of cataclasis developed for porous and carbonate media predict initial grain-scale micro-fracture, slip and fragment rotation leading to frictional wear. The observations made in the Great Slave Lake shear zone suggest that such models may not be directly applicable to the initiation of cataclastic flow developed within non-porous, homogeneous, quartzo-feldspathic mylonite belts in crustal-scale fault zones, at the elevated confining pressures typical of the transition from the plastic to the brittle deformation regimes.