The brittle-ductile transition in rocks: Recent experimental and theoretical progress

The transition in failure mode from brittle fracture to plastic flow is important in understanding seismic source mechanics, the strength of the crust, and the style of deformation at the field scale. Recent studies in three areas have yielded new insight into the physics and mechanics of the brittle-plastic transition: acquisition of high quality mechanical data, systematic observations of deformation microstructures, and advances in the the oretical modeling of the failure of rocks in compression. Mechanical behavior over the brittle-ductile transition has been characterized for some rocks by measuring the pressure and temperature sensitivity of strength and by observing failure mode; particular advances have resulted from the measurement of volumetric strain. However, there have been few comprehensive studies of semibrittle flow in silicate rocks at high pressures and temperatures, or of the effect of variations of pore fluids, strain rate, and grain size on strength and rheology. The strengths at the transition from brittle fracture to semibrittle flow and from semibrittle flow to fully plastic flow are apparantly linear functions of pressure, but the physical bases for the relations are not well established. Qualitative microstructural observations have provided information on the conditions under which various deformation mechanisms operate, estimates of strain partitioning, and identification of crack nucleation mechanisms. Recent quantitative microstructural measurements place important constraints on the micromechanics of deformation in the semibrittle field, but important gaps in our understanding remain. Theoretical treatments of rock failure based on bifurcation analyses and fracture mechanics model successfuly only some of the experimental observations.