SYSTEMATIC GROWTH OF FAULTS IN POROUS SANDSTONES AS REVEALED BY FIELD OBSERVATIONS AND FRACTURE MECHANICS

The high-quality exposures of deformation bands and subsequent structures on the Colorado Plateau are motivating an improved and systematic physical framework for understanding the growth of faults in porous sandstones. The geometry and sequence from individual cataclastic bands, through strain-hardening damage zone, though faulted bands as commonly observed in the field are well explained when it is recognized that bands function mechanically as discontinuities, or fractures, within the host rock, with the effect of the discontinuity on its surroundings depending on the magnitude and sense of normal or shear displacement across the band.

Deformation bands form in porous granular rocks by localized inelastic yielding that is illustrated by using a Cam cap model modified from soil mechanics. Band nucleation can then be understood and predicted by calculating the volumetric strain energy density in a deforming host rock. Similarly, band propagation as surfaces of plastic yielding can be predicted by calculating the distortional strain energy density around them, especially at their tips. Bands of all five types (dilation bands, dilation bands with shear, shear bands, compaction bands with shear, and compaction bands) propagate in their own planes unless perturbed by the presence of nearby bands; such perturbations lead in mode-II shear bands to backward-breaking echelon geometries (“ladder structures”) with linking bands forming within compressive stepovers. Continued growth of bounding bands beyond the stepovers leads to thicker zones of subparallel bands and widening of the growing damage zone in all three dimensions. Localized instability and resulting failure within the damage zone, forming individual slip patches at the band-host rock interface, and the growth and linkage of these slip patches into through-going faults, consistent with field relations, can be understood phenomenologically by incorporating an evolving frictional failure criterion into the Cam cap approach.