The mechanisms of finite brittle strain

Abstract

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. Repeated 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.