Conference Report: Relationship between brittle and ductile processes in rocks

Report of meeting held by the Tectonic Studies Group at the Department of Geological Sciences, University of Birmingham on 3 November 1982. The transition from fracture-dominated deformation at high levels in the crust to flow-dominated deformation as temperature and confining pressure increase with depth is a concept familiar to all geologists. Nevertheless, the nature of this transition, and the ways in which ‘brittle’ and ‘ductile’ processes interact and compete at different levels in the lithosphere, are still poorly understood. The ten papers presented at the meeting addressed these difficult issues and included experimental and theoretical approaches as well as a number of carefully documented field and petrographic studies. The meeting attracted about 80 participants. Murrell opened the meeting with a wide-ranging review of experimental rock deformation but made the point that, in spite of the wealth of experimental data, a detailed understanding of the conditions under which brittle processes are superseded by crystalplastic processes is lacking. Rutter argued that the entrenched ‘brittle’-‘ductile’ terminology should be replaced by a 4-fold classification of failure mode based on deformation mechanism (cataclastic or crystal-plastic) and on whether or not the failure is localized. He illustrated this scheme with examples from experimentally and naturally deformed rocks, and drew attention to apparent scale differences. Both Murrell and Rutter emphasized that fracture-dominated deformation involves dilation and is therefore very sensitive to effective confining pressure but rather insensitive to temperature and strain-rate; crystal-plastic flow, on the other hand, is very sensitive to temperature and strain-rate but insensitive to