Abstract - Arrays of closely-spaced (approximately <70 mm) sub-parallel
cataclastic deformation bands are common structures in deformed, high-porosity
(10–35%) sandstones. The distribution of strain onto many small-displacement
deformation bands is thought by some to result from strain-hardening of the
cataclasite within individual bands. Examination of both normal and strike-slip
faults with displacements <7 m from southeastern Utah, USA, and the North
Island of New Zealand suggests, however, that clusters of deformation bands
systematically develop at fault geometric irregularities (e.g., fault bends,
steps, relays, intersections and zones of normal drag). The strain-hardening
model does not account for clustering of deformation bands at fault geometric
irregularities or the associated widespread coalescence of bands, and is not
unequivocally demonstrated by post-peak macroscopic mechanical responses in
laboratory rock deformation experiments. A geometric model is proposed in which
individual bands within clusters develop sequentially due to migration of
incremental shear strains at fault geometric irregularities as part of a slip
localisation, asperity removal and strain weakening process. The geometric model,
which does not require strain hardening of the fault rock, applies for the duration
of faulting and a range of rock types in the brittle upper crust.
Journal of Structural Geology, 55, 21-33, 2013.