Deformation band formation and strength evolution in unlithified sand: The role of grain breakage

We report on laboratory experiments designed to investigate the strength evolution and formation mechanisms of cataclastic deformation bands hosted in unlithified sand, with particular focus on the role of grain breakage. Cataclastic deformation bands are characterized by particle size reduction and increased resistance to weathering compared to parent material. We recovered bands intact from late Quaternary, nearshore marine sand in the footwall of the active McKinleyville thrust fault, Humboldt County, California. Tabular samples 3-5 mm thick and 5 cm × 5 cm in area were sheared at normal stresses representative of in situ conditions, 0.5-1.8 MPa, sliding velocities from 10 m/s to 10 mm/s, and to shear strain up to 20. Cataclastic deformation bands are stronger than parent material (coefficient of internal friction μ_i = 0.623 and μ_i = 0.525, respectively) and exhibit a peak strength followed by weakening. Parent material exhibits significant strain hardening; the frictional yield strength increases up to 9% for a shear strain of 10. Detailed particle size analyses show that strain hardening in parent material is coincident with increased fine particle abundance, resulting from pervasive grain breakage. Our results support the hypothesis that cataclastic deformation bands are stronger than the surrounding parent material due to shear-driven grain breakage during their formation. We suggest that the combination of strain localization during band formation and strain hardening on individual bands results in dense networks of deformation bands.