The structure and evolution of small-displacement strike-slip faults in porous sandstone

Abstract

The early-evolution of fault structure is inferred from analysis of detailed maps of portions of strike-slip faults with uniform displacements ranging from mm to decimeter in porous quartzose sandstone. Emphasis is on assessing the spatial relationship between the progressive addition of subsidiary fault segments (deformation bands) and earlier-formed linkage structures. The along-strike variability and distribution of fault structure are documented and used to assess the role of early fault geometry on the evolution of fault structure. The study faults evolve in the initial stage by linkage of an early-formed array of isolated, en echelon fault segments with small relative spacing that step opposite to the sense of shear. The initial configuration of early fault segments favors the development of a geometrically distinct set of linkage structures denoted as Type 2 linkage structures. A simple Type 2 linkage structure consists of two, curved, overlapping and mutually abutting, synthetic extensions of the adjacent primary fault segments. Increasing displacement promotes a progressive increase in the internal extent of cataclastic deformation, structural complexity, and size of Type 2 linkage structures. Cumulative frequency curves of linkage structure dimensions indicate a progressive increase in the mean length, mean width, and mean ratio of length to width with increasing displacement. The along-strike distribution of deformation alternates from a section consisting of a single fault segment to a section consisting of a cluster of either multiple fault segments or a pod of cataclasis. A lacunarity analysis quantitatively demonstrates a progressive increase in the amount of deformation within clusters with increasing fault displacement. With increasing displacement subsidiary fault segments are preferentially added in close proximity to or within earlier-formed linkage structures and are not added adjacent to single fault segments. Accreted segments typically are arcuate and abut earlier segments at a high angle. Early linkage structures represent geometric irregularities (roughness) along the evolving fault that are interpreted to result in geometric stress concentrations that preferentially localize formation of new fault segments. This conceptual model explains evolution of a systematic variation of along-fault structure with increasing fault displacement without requiring the strain-hardening hypothesis commonly invoked by other workers.