Nucleation, growth and structural development of mylonitic shear zones in granitic rock

Well-exposed shear zones in granitic rock of the Sierra Nevada are preserved in various degrees of development, facilitating interpretation of the mechanisms of shear-zone nucleation and growth. Field relations suggest that these shear zones formed by shear localization within pre-existing aplite dikes that served as tabular zones of inherent weakness. Crossing dikes provide a measure of the offset at several points along the shear zones. Apparently, the shear zones grew in width as they accumulated offset, and propagated longitudinally by end-to-end linkage of approximately coplanar aplite dike segments. Propagation caused discontinuous dikes or dike segments to be joined, giving the shear zones a forked, or eye-shaped, trace pattern. Some shear zones have developed opening fractures at their tips, analogous to the splay fractures that form near the tips of small faults. These shear zones provide an excellent opportunity to study the processes involved in natural shear-zone formation in granitic rocks where shear is localized along pre-existing zones of inherent weakness. They may serve as useful analogs for the growth and development of larger-scale faults that involve ductile shearing.