The Quartz Lamella Fabrics in a Concentric Fold

Lamellar structures in quartz grains occurring in a concentric fold (multiple-layered system made of the alternation of mica-rich layers and quartz-rich layers) found in the Sangun metamorphic formation at Atetsu, Okayama Pref., Southwest Japan, have been described. Two types of lamellar structure have been found in quartz grains of the quartz-rich layers, that is, the Böhm lamellae and the deformation bands. The crystallographic location of the deformation bands has been tentatively determined on the basis of assumption that the rotational axis of the change in the lattice orientation from the band to the host crystal coincides with either the a-axis or the a*-axis. At this time the deformation bands show a wide range of the crystallographic location, but many of them are inclined at high angles to the c-axis. Quartz grains containing the deformation bands are found in very sharply restricted parts in the fold, and these parts all correspond approximately to the "knee of fold". The distribution of quartz grains containing the Böhm lamellae is also sharply restricted in two narrow zones through the fold, which are approximately placed on the knee of fold, though the distribution area is more enlarged to the limbs than that for the deformation bands. The Böhm lamellae occurring in the central part of the knee are different in geometric properties from those occurring in the marginal parts of the knee and in the limbs. Namely, most of the former is inclined at high angles to the c-axis, and, in the diagram for the pole of lamellae and the c-axis for the former, great circles containing these axial data for individual grains have a constant direction and sense, while the latter shows a wide range of the crystallographic location and the diagram in question does not show any regularity. In the specimens described so far in the literature, the Böhm lamellae referred to the type of the former and these to the type of the latter appear to be commonly coexisting. The rule for establishing the directions of the principal normal stresses developed in the system concerned during the deformation related to the formation of the lamellae, previously introduced by the author (1961a and 1963), has been successfully used also for the present specimen. The cause of the fact that quartz grains containing the deformation bands or the Böhm lamellae are found in some very sharply restricted parts in the fold (especially the "knee of fold") has been also discussed with reference to the mechanics of rock folding in the final phase of meta-morphic deformation.