PARTITIONING OF TRICLINIC DEFORMATION IN A HIGH-STRAIN ZONE INTO MONOCLINIC AND ORTHORHOMBIC DEFORMATION DOMAINS: AN EXAMPLE FROM THE SUPERIOR BOUNDARY ZONE, MANITOBA, CANADA

In progressive deformation, planar and linear fabrics, such as foliations, lineations and fold hinge lines, follow complex rotation patterns, especially in triclinic shear zones. Because triclinic shear zones are common, rotation patterns are expected to be complex in most shear zones. However, deformation across a shear zone may be partitioned into various domains. We describe shear zones along the Superior Boundary Zone (SBZ) in which the overall triclinic flow is partitioned into approximately monoclinic and orthorhombic domains.

The SBZ is a transitional zone of deformation between the Archean Superior Province and the Paleoproterozoic Trans-Hudson Orogen. Mylonitic shear zones within it show evidence for thinning within across the zones. The Aiken River shear zone (ARSZ) is a 1–1.5 km wide ~E-W trending, dextral, north-side-up, mylonite zone. SE-plunging folds in the Split Lake Block, north of the ARSZ, consistently become tighter, and turn to horizontal, towards the ARSZ, indicating a shear zone-parallel horizontal stretching component. Lineations and fold hinge lines within the mylonite zone generally plunge moderately to the west, indicating the simple shear direction. The shear zone thus has an overall triclinic symmetry.

If simple shear had played a significant role adjacent to the mylonite zone, the fold hinge lines of the Split Lake Block would not have rotated directly towards horizontal orientations, without rotating towards a moderately westerly plunge first. Flow was therefore approximately orthorhombic (pure shear). If pure shear had played a significant role within the mylonite zone, lineations and fold hinge lines would have rotated towards horizontal. Flow within the mylonite zone was therefore approximately monoclinic (simple shear). We conclude that shear zones with triclinic symmetry may be partitioned into approximately monoclinic and orthorhombic domains. A transitional domain with triclinic symmetry is likely to exist, but may be narrow. Furthermore, as exemplified by the transpressive shear zones of the SBZ, simple shear may be concentrated in relatively narrow zones with relatively sharp boundaries, while pure shear is distributed over broad zones with diffuse boundaries.