Structure and development of an anastomosing network of ductile shear zones

A detailed structural analysis of an anastomosing shear zone network in metagabbros from the Archean Rainy Lake zone (Canada) revealed the existence of prevalent dextral and minor sinistral conjugate shear zones with the obtuse angle (>130°) facing the main shortening direction. A typology of shear zone intersections, confluences, and other features shows that all shears formed during a single deformation event, with dextral and sinistral shears being active together or in an alternating fashion. In spite of the difficulty of establishing a complete kinematic sequence, early and late shears can be distinguished. The final angular pattern between dextral and sinistral shears is not an original feature. Dextral and sinistral shears formed at nearly right angles, and the angles progressively opened towards the extension direction as a result of increasing strain. The obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge-shaped domains of lesser-deformed rock bounded by the shears. Through time, there was an increasing prevalence of dextral shears over sinistral ones. The studied pattern and sequential analysis indicate that the bulk deformation was noncoaxial with a deformation regime evolving from a pure shear-dominated dextral transpression to a higher vorticity dextral transpression.