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