Schistosity and Penetrative Mineral Lineation as Indicators of Paleostrain Directions
Field evidence indicates that the progressive deformation of most schistose rocks consists of two principal phases: (1) the anisotropy-creating phase, during which schistosity and mineral lineation develop and (2) the anisotropic phase, during which numerous translational-slip surfaces are generated parallel to schistosity. This discontinuous deformation may be regarded as simple shear on a scale that is much larger than the average spacing of neighboring slip surfaces.Schistosity and mineral lineation can be produced normal to the direction of greatest finite shortening and parallel to the direction of greatest finite extension, respectively. Subsequent simple shearing tends to change this relationship such that both structural features become oblique to the principal directions of finite strain, although the mineral lineation remains parallel to the major axis of the finite-strain ellipse on the schistosity plane. Thus the lineation rather than the schistosity may be a reliable indicator of a (non-principal) direction of total strain, for a given period of progressive deformation.