David A. Ferrill¹, Alan P. Morris², Darrell W. Sims¹, John A. Stamatakos¹

(1) CNWRA, Southwest Research Institute, San Antonio, TX
(2) Division of Earth and Physical Sciences, University of Texas at San Antonio, San Antonio, TX

ABSTRACT: The Geometric Strength of Fault Systems

Faults optimally oriented for slip within an ambient stress field are geometrically weak compared with other fault orientations. Evolving fault systems undergo changes that lead to both geometric strengthening and weakening of faults. Given a single stress regime, geometrically weak faults grow and accumulate displacement, and geometrically strong faults tend to be abandoned. This natural selection process results in a well-organized fault pattern that reflects the ambient stress field. Initial fault surfaces are often en echelon and not connected. These faults grow by segment nucleation, growth, and connection by curved propagation or connecting fault formation. Fault propagation prior to linkage produces local perturbations that modify continuing fault propagation. As fault segments link, local stress field perturbations are relieved and certain fault patches become geometrically stronger in the ambient stress field. These poorly oriented fault patches are bypassed by newly formed cutoff faults that straighten the overall fault surface. This process results in an active fault surface that is smoother and geometrically weaker, and that has a smaller surface area. In addition, slip on an array of parallel normal faults progressively reorients fault planes to shallower and less favorable dips, causing the initiation of new, more favorably oriented faults. Analyses of natural fault systems from the Basin and Range Province illustrate this fault system evolution.

Work supported by the U.S. NRC (Contract NRC-02-97-009). This is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado