Comparison of Faulting Under Moderate and High Confinement: Lessons from Ice

Triaxial testing of laboratory grown granular and columnar polycrystalline ice at -10 °C has established two distinct modes of compressive brittle-like shear faulting. The first mode, termed Coulombic (C) or frictional faulting, develops under moderate levels of confinement, is oriented at ~30° from the direction of maximum shortening, and is comprised of a narrow band of microcracks. Mechanically C faulting demonstrates pressure hardening, grain size dependant strength, localized heat production, a low degree of cohesion, and the creation of a substantial amount of fault gouge. The second mode of faulting, termed plastic (P) or non-frictional faulting, develops under higher levels of confinement, is oriented ~ 45° to the direction of maximum shortening, and is comprised of a fine band of recrystallized grains. Mechanically P faulting demonstrates pressure and grain size independent strength, localized heat production, and a high degree of cohesion. The transition between C faulting and P faulting corresponds to the level of confinement required to suppress frictional sliding. Our observation of a transition to a new mode of faulting with increasing confinement is similar to and expands upon similar observations in rock, suggesting that under specific conditions localized plastic flow, rather than friction, determines the failure strength of the Earth’s crust.