Displacement gradient and deformation in normal fault systems

Abstract

All fault systems contain faults with lateral (strike-parallel) displacement gradients. Lateral displacement gradients give rise to elongations parallel to fault cutoffs in either or both of the footwall and hanging wall fault blocks. We present a simple method for estimating cutoff parallel elongation based on geometric fault and fault-block elements. These elements are readily measured in the field or from maps and subsurface data, and include orientations of cutoff lines, faults, and displacement directions. Displacement gradients on overlapping normal faults produce relay ramps. Deformation within a relay ramp includes tilting, extension parallel to bounding cutoff lines, vertical axis rotation, and eventual breakthrough of the ramp. Relay ramp deformation is sensitive to fault displacement directions—oblique slip directions on the ramp bounding faults can cause contraction (restraining-step sense of displacement and overlap) or enhanced ramp extension (releasing-step sense of displacement and overlap). Relay ramp extension accommodated by brittle faulting and extension fracturing is important for locally altering porosity and permeability in fractured aquifers and reservoirs. Locally enhanced fault and extension fracture density can provide fast pathways for infiltration, percolation, and flow of groundwater, or barriers to fluid movement and can influence rock quality and stability of underground excavations. An example of strain localization in a displacement transfer zone from Yucca Mountain, Nevada, the proposed site for a high-level radioactive waste repository, is examined.