Influence of the structural framework on the origin of multiple fault patterns

We demonstrate that the general equation for three-dimensional strain by slip on orthorhombic faults can be rearranged to take a form that applies to two-dimensional strain due to slip on pre-existing planes of weakness. Therefore, either two-dimensional or three-dimensional strain may result from the same stress state. We deduce that the kinematic interaction between planes of weakness in a body is a fundamental factor to determine the type of strain produced by a stress state. Whether deformation occurs by forming new fractures or by slip on existing planes depends upon which requires a lower stress difference. The stress difference necessary to initiate slip along a plane is highly sensitive to variations in orientation, cohesion and depth. We propose a model for crustal deformation composed of an anisotropic body with existing planes of weakness that interact kinematically. The critical stress difference necessary to initiate sliding is that required by the interacting plane that needs the highest stress difference to slip. Because the stress difference will rise until it reaches a value that can cause slip on all interacting planes, once slip initiates it will occur simultaneously on all planes that require stress differences lower than the critical value. The anisotropic body model proposed here provides a mechanism for forming multiple fault sets and may pertain to the formation of low-angle normal faults.