These examples of microstructures and textures as­sociated with reaction weakening come from the Gessaman fault in southern Arizona, U.S.A., and were first described in Janecke and Evans (1988). Phyllonitic textures are concen­trated in a 2-3 m wide zone formed in Proterozoic Oracle Granite. The protolith granite consists of 10-25 mm diameter quartz grains, 5-12 mm diameter orthoclase grains, 5-10 mm plagioclase grains, and minor amounts of biotite and acces­sory minerals. Deformation occurred during the early Terti­ary, probably at temperatures of 250-400°C.