Tectonite

a rock in which the minerals have acquired a certain orientation under the action of deep-seated forces of the earth. This action is usually accompanied by recrystallization, the formation of foliation, or comminuation (as in mylonites and tectonic breccias). The study of tectonites through structural analysis (seePETROTECTONICS) permits determination of (1) the orientation of the compressive and shearing stresses acting during dynamic metamorphism and (2) the later stages of deformation.

Three groups of tectonites are distinguished according to the nature of mineral orientation. The groups reflect types of differential movement of matter. In the first case, platy minerals, such as mica, are arranged parallel to glide planes so as to produce cleavage. In the second case, the gliding in the minerals occurs along two intersecting planes. The third case is characterized by the rotation of grains around axes that are usually oriented along the trends of folds.