Nucleation of ductile shear zones on dilatant fractures

Small shear zones in granitic rocks of the Sierra Nevada, California, and near Roses (Rosas), northeast Spain, display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation. Many zones have sharp, nearly planar boundaries between their highly deformed interiors and the undeformed wall rock. The Roses shear zones narrow continuously to hairline fractures at their ends. Mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode. Dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation. Gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleation of the shear zones. We suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes, thereby promoting chemical alteration and/or hydrolytic weakening.