KINEMATICS AND KINEMATIC SIGNIFICANCE OF FOLIATIONS I. SORBY (1850's) - one of the first to interpret cleavage in rocks A. Phyllosilicate grains rotate to a preferred orientation parallel to the xy plane of strain and perpendicular to the z plane (max shortening) due to mechanical rotation (most foliations are the result of this) B. This interpretation precludes lateral displacement (i.e., shear) since principle strain axes are axes of no shear; i.e., lines are only shortened or lengthened C. Some cleavages, however, contain lineations and thus are shear surfaces; i.e., not formed purely by shortening D. Actually both interpretations are correct II. RAMSAY AND GRAHAM (1970, C.J.E.S., v. 7, p. 786-813) defined a ductile shear zone (dsz): A. No volume loss (DV = 0) B. No deformation outside ductile shear zone C. Zone has planar boundaries 1. Strain must be entirely simple shear (where shear plane, line of no finite elongation, remains unrotated) 2. If so, foliation should be inclined to the shear zone boundary; so one could use the attitude of the foliation (reference to undeformed rocks) to determine the amount of strain; if stretching lineations are also present, one could ideally get: a. Orientation of stress (tells us x, we know x and y and can find z stereographically b. Sense of shear c. How far moved by equation g = 2/tan2q'; and S =0òx gdx III. BERTHE et al. 1979 (J.S.G., v.1) - recognized that there are two foliations within a shear zone: A. Penetrative schistosity inclined to the shear zone boundary B. Spaced foliation parallel to the shear zone boundary; 1 typically bends into 2 resulting in phacoid shaped pattern C. The first is called S ("schistosite"); xy plane, perpendicular to z traces the incremental finite strain ellipse D. The second is called C ("cisaillement", shearing); microscopic shear zone (no volume loss, so not a case of pressure solution) shear band - very small-scale, subparallel, evenly spaced (<1-10cm) dsz’s that deflect some pre-existing anisotropy with a consistent sense of shear (i.e., displacement discontinuities) E.So; Sorby’s flattening foliation (S-planes) and shear foliation (C-planes) both occur together in some ductile shear zones F. A problem for Ramsay and Graham (1970); shear strain calculations can be made on dsz’s but values of displacement are minimum estimates G. Berthe et al’s work revolutionized structural geology because one could determine sense of shear on basis of a single hand specimen (no need to see displaced markers)