THE MECHANISMS OF FINITE BRITTLE STRAIN

The mechanical processes that lead from first fracture in an undeformed rock mass to the fault gouge observed in a highly sheared fault zone are outlined. Tensile fracture, dilation, rotation, the collapse of beams and filling of voids are the basic mechanical elements. Repeat many times, over a wide range of scales, they accommodate finite strain and create the complex fabrics observed in highly deformed rocks. Defects that nucleate tensile cracks in the earth are both spatially clustered and occur on a wide range of scales. This inhomogeneity is responsible for features that distinguish deformation of rocks from deformation of laboratory samples. As deformation proceeds, failure at one scale leads to failure at another scale in a process of evolving damage. Abrupt catastrophic failure never extends indefinitely throughout the earth as it does in rock samples. The mechanics of the interactions between scales are investigated. Approximate expressions are modified from engineering damage mechanics for this purpose and their validity is demonstrated by detailed numerical modeling of critical examples. The damage that results as deformation proceeds extends over a range of scales and is consistent with the observed fractal nature of fault systems, joints and fault gouge. The theory for the mechanical evolution of fractal fault gouge which is based on' the mechanical interaction of grains of different sizes is discussed. It is shown that the damage mechanics description and the granular deformation mechanism are alternative descriptions of the same process. They differ mainly in their usefulness in describing different stages of damage evolution. Field examples of features described in the geological literature as faults, joints, fault gouges, megabreccias and melanges are shown to be plausibly explained by the mechanical processes described.