NUCLEATION, GROWTH AND STRUCTURAL DEVELOPMENT OF MYLONITIC SHEAR ZONES IN GRANITIC ROCK

Well-exposed shear zones in granitic rock of the Sierra Nevada are pre served 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 pr e-existing aplite dikes that served as tabular zones of inherent weakn ess. Crossing dikes provide a measure of the offset at several points along the shear zones. Apparently, the shear zones grew in width as th ey accumulated offset, and propagated longitudinally by end-to-end lin kage of approximately coplanar aplite dike segments. Propagation cause d discontinuous dikes or dike segments to be joined, giving the shear zones a forked, or eye-shaped, trace pattern. Some shear zones have de veloped opening fractures at their tips, analogous to the splay fractu res that form near the tips of small faults. These shear zones provide an excellent opportunity to study the processes involved in natural s hear-zone formation in granitic rocks where shear is localized along p re-existing zones of inherent weakness. They may serve as useful analo gs for the growth and development of larger-scale faults that involve ductile shearing. (C) 1997 Elsevier Science Ltd.