Geometry, classification and kinematics of S-C and S-C′ fabrics in the Mushandike area, Zimbabwe

Abstract

Planar fabrics in small dextral shear zones in the Mushandike area of the Zimbabwe Archaean craton consist of a sigmoidal primary foliation S displaced by discrete shears C or C′. These planar fabrics can be divided into porphyroclastic, megacrystic and banded types, depending mainly on the values of the angles between S- and C- or C′-surfaces, the structures that define S-surfaces, and the spacing of C- or C′-surfaces. The values of the angles between the foliations are proposed as the basis for a general classification of planar fabrics in shear zones. The values of the angles depend on microstructures and strain. Microstructural evidence shows that slip did not occur on S-surfaces, but only on C- or C′-surfaces. A model for the development of the fabrics proposes that S-surfaces lie parallel to the XY plane of the local finite strain ellipsoid in domains of simple shear between C- or C′-surfaces. C′-surfaces may form in the orientation of a Coulomb failure surface. The field evidence suggests that an increase in bulk strain is at least partly accommodated by an increase in local strain in domains between C- or C′-surfaces as the S-surfaces rotate with the local finite strain ellipsoid.