Dyke propagation with distributed damage of the host rock

Observations of off-plane inelastic deformation around dykes motivate consi deration of models of fluid-driven crack propagation in a solid which can u ndergo material degradation, or damage. The application to dyke propagation of a recently proposed damage rheology [Lyakhovsky et al., J. Geophys. Res . 102 (1997) 27635-27649] based on thermodynamical principles and experimen tal measurements is discussed. The rate of accumulation of damage in this r heology is the product of a material-dependent parameter (Cd) and the squar e of the strain. For geological values, a dimensionless parameter c(d)eta/D elta P characterizing the ratio of a damage timescale to a flow timescale i s very small, where eta is the magmatic viscosity and Delta P the driving p ressure. As a result, significant rates of damage are confined to a small r egion near the dyke tip, where the strain is large. Consideration of possib le singularities in near-tip solutions, shows that the rate of propagation is governed by the viscous fluid mechanics. To a good approximation, the ra te has a value equal to that given by the zero-stress-intensity solutions o f previous models based on linear elastic fracture mechanics. Predictions f rom the damage rheology both of a narrow damage zone and of the rate of pro pagation are in good agreement with observations. (C) 1999 Elsevier Science B.V. All rights reserved.