Damage zones around en echelon dike segments in porous sandstone

We investigate arrays of en echelon dike segments and their associated defo rmation in porous sandstone to infer the segmentation mechanism and the sta te of stress during dike emplacement. The en echelon arrays are interpreted as breakdown segments of planar parent dikes that propagated from greater depth under mixed-mode conditions. Typically, an array consists of either c ontinuous nonoverlapping stepped segments (offset smaller than segment thic kness) or overlapping connected segments (offset larger than segment thickn ess). The deformation associated with the nonoverlapping stepped segment ar rays consists of newly documented fan-like patterns of deformation bands (l amellae of crushed detrital quartz grains), whereas the overlapping connect ed segment arrays consists of net-like patterns of deformation bands. Thus the patterns of deformation are related to offset geometry and are likely t o be diagnostic of the states of stress. We simulated the stress and deform ation fields around interacting breakdown segments by applying a continuum damage mechanics model. The simulation results mainly illustrate the stress dependence of the damage distribution and the sensitivity of the damage di stribution to the geometry of the segment offset and the mutuality of segme nt propagation. By changing the applied stress and by controlling the segme nt tip growth, symmetric and asymmetric distributions of damage were produc ed. We describe which aspects of the generated damage zones satisfactorily correlate with field observations. Damage mechanics simulations are useful tools for studying the state of stress during dike emplacement.