Abstract

The current controversy regarding kinematic interpretation of lineation in schists hinges on the significance of symmetry in tectonite fabrics. Interpretation of such fabrics by Sander and Schmidt is based on an assumption that symmetry of fabric reflects symmetry of internal movements accompanying deformation. In identifying b lineation, which is the symmetry axis of certain monoclinic fabrics, as the principal direction of movements concerned in the evolution of those fabrics, some recent writers have neglected or rejected Sander's postulate regarding symmetry. This course seems unjustified for two reasons: (1) Those who identify regional lineation with movement direction have done so for Precambrian or Paleozoic rocks whose metamorphic and deformational history is ambiguous and often highly complex; the “direction of movement” in such rocks has not been established independently of fabric evidence. (2) Since it was proposed 30 years ago, the symmetry concept of Sander and Schmidt has become strengthened by evidence accruing from studies on fabrics of experimentally deformed metallic aggregates and ceramic bodies. Additional supporting evidence now comes from fabrics of experimentally deformed marble. In deformed marble cylinders symmetry of fabric is identical with symmetry of movement as inferred from measurable strain. The pattern of strain is controlled by the orientation of the cylinder (a mechanically anisotropic aggregate of grains) in relation to applied force. Even in highly deformed material (e.g., where elongation exceeds 500 per cent) the influence of the original anisotropy on symmetry of experimentally induced movement and strain is obvious in the final fabric.

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