Propagation energies inferred from deformation bands in sandstone

Abstract

Propagation energies for five examples of deformationbandsinporoussandstonesfromfourfield sites in Nevada, Utah, and France were calculated by using the J-integral approach. Pure compaction bands that accommodate primarily closing displace- ments from the Valley of Fire (Nevada) site have a closing-mode band propagation energy of JI = 5.5 ± 1.6kJ/m 2 . Shear-enhanced compaction bands having subequal amounts of normal (closing) and shear strains across them from the same site have propagation energies of JI = 6.11 ± 1.8kJ/m 2 and JII = 1.52 ± 0.5kJ/m 2 for closing and shearing modes respectively. Closing- and shearing-mode band propagation energies for shear-enhanced compaction bands from the Buckskin Gulch (Utah) site, 5.63 ±1.7 and 1.91 ± 0.57kJ/m 2 and the Uchaux (France) site, 11.0 ± 3.3and2.35 ± 0.7kJ/m 2 respectivelyarecom- parabletothevaluesforsimilarstructuresfromtheVal- ley of Fire site. Cataclastic deformation bands at the Orange (France) site accommodate significantly larger values of shear offset than do shear-enhanced compac- tion bands, with a ratio of shear to closing offset that exceeds 7. The calculated closing- and shearing-mode propagation energies for these shear bands, 17.8 ±5.4 and 3.3 ± 1.0kJ/m 2 respectively are comparable to those for pure or shear-enhanced compaction bands deformed at approximately the same depth of bur- ial. Strain softening of cataclastic compactional shear bands implies unstable shearing, whereas stable and perhaps strain-hardening shearing may be inferred for shear-enhanced compaction bands. Displacement- lengthscalingrelationsforshear-enhancedcompaction bands of the form D ∝ √ L are consistent with the