Intracrystalline folding and cataclasis in biotite of the Santa Rosa mylonite zone: HVEM and TEM observations

Abstract

High voltage electron microscope (HVEM) and transmission electron microscope (TEM) analyses indicate that grain size reduction of biotite in deformed granodiorite of the Santa Rosa mylonite zone, southern California, was in part accomplished by Intracrystalline folding and cataclasis. Through this process, deformation takes place within discrete parallel-sided or lens-shaped zones up to 6 μm wide. Within the zones, deformation begins with the opening of cavities along cleavage planes. Further deformation causes layers parallel to (001) to become detached from the surrounding crystal along cleavage planes or ruptured along micro-shear zones at an angle to (001). The layers may be strongly folded, branching, or completely fragmented, the latter suggesting cataclastic behavior. The smallest measured lens of cataclasite is 0.05 μm in width. High resolution TEM images show that (001) lattice fringes fold into these regions. Zones of Intracrystalline folding and cataclasis are visible petrographically. Because of the many orientations of biotite within these regions, the optical properties are effectively averaged. The zones are characterized by little or no change in color with rotation of the optical microscope stage and, in most orientations, by higher refractive indices than the surrounding crystal.