We used a torsional geometry to grow shear rupture in two fine-grained rock types. The sample has a slot that is loaded to produce a strong mode 3 shear stress concentration. The onset and growth of the damage zone is detected by monitoring the elastic stiffness of the samples. We find that the samples continue to harden after the onset of fracture. This is initially accompanied by oblique tensile cracks forming at the stress concentration. The peak in supported load apparently corresponds to the onset of strain localization, with the development of cross-cutting shear fractures. We attemped to estimate the partitioning of energy dissipation between elastic and frictional component with a fracture mechanics analysis. The elastic energy increase with normal stress, but scatter in the inelastic data does not allow identification of convincing trends. The sample is too small to develop a complete transition to a shear rupture. The geometry of the fractures is used to explain the energy relations as well as other experimental observations. ¿ American Geophysical Union 1988