VORTICITY ESTIMATION USING SHEAR BAND CLEAVAGES AS A MEANS TO CHARACTERIZE STRAIN PARTITIONING IN A MAJOR ARCHEAN GENERAL SHEAR ZONE

Sets of synthetic and antithetic shear band cleavages (SBC) developed in strongly anisotropic zones in the Marbanite shear zone from Val d'Or, Quebec, in the Abitibi greenstone belt, were used to estimate the vorticity of flow for distinct portions of the shear zone. Results suggest that although quantitative vorticity analysis using SBC may not be realistic, useful information about strain partitioning in a large shear zone without abundant kinematic indicators can be obtained using this method. Field observations suggest that synthetic and antithetic shear band cleavages initiate approximately parallel to the acute and obtuse bisectors of the irrotational lines of flow, respectively, during general shear. An estimate for the bulk vorticity of flow can be obtained by approximating the SBC initial orientations, assuming that the flow parameters remained constant throughout deformation. Analysis of shear band cleavage sets in the Marbanite shear zone suggests that deformation was strongly spatially partitioned early in the tectonic history of the area. Initial, dominantly coaxial deformation indicated by down-dip stretching lineations and upright folds was followed by major, dextral general shearing in which the central higher-strain portion of the shear zone experienced non-coaxial-dominant deformation while the lower-strain margins experienced coaxial-dominant deformation. These analyses, coupled with the sequence of development of structures within the shear zone, supports the idea that partitioned orogen-parallel deformation was long-lived in this area.