OROGEN-SCALE STRAIN PARTITIONING AND AN ANALOGY TO SHEAR-BANDS IN THETORNGAT OROGEN, NORTHEASTERN CANADIAN SHIELD

Shear-bands are discrete small-scale shear zones oriented at an acute angle to the main plane of anisotropy of a foliated rock. While this r ock undergoes non-coaxial deformation, if the bulk direction of shear is not parallel to the main plane of anisotropy, which, in most cases, is a preferential locus for gliding, the strain will be partitioned b etween this plane and subsidiary slip planes, namely shear-bands. As o rogenic belts become intrinsically anisotropic after a severe deformat ion, similar strain partitioning on a megascopic scale is expected whe n they undergo further bulk regional non-coaxial deformation. Such a p rocess occurred in the lower Proterozoic Torngat Orogen of the northea stern Canadian Shield, where gneisses with a regular planar foliation developed through regional sinistral non-coaxial shear under granuliti c conditions. Shearing during the waning stage of high-grade metamorph ism, once the main planar fabric developed, leads to partitioning of t he strain increments into the main fabric and discrete map-scale shear zones developed at an acute angle with the regional fabric. Coeval sl ip between these shears and reactivated main fabrics is supported by t he oriented growth of hornblende upon stretched hypersthene crystals w ithin both fabrics, indicating that shearing occurred under the same m etamorphic conditions along both the main fabric and the shear-bands.