Growth of deformation bands into echelon and ladder geometries

Abstract

We investigate the growth of cataclastic deformation bands in porous sandstone into Riedel, ladder, and “radiator rock” arrays. New field observations and mechanical modeling of DB growth using the distortional strain energy density criterion demonstrate that the physical control of these geometries is a contractional (mode‐II) stepover between two echelon bands. Propagation of bands under pre‐peak conditions, before Coulomb frictional sliding (faulting) begins, is not impeded by the echelon stepover geometry (as it is for brittle fractures under peak stress); continued in‐plane growth of echelon bands creates ladders from stepovers. Successive addition of stepovers/ladders along their shearing direction produces Riedel geometries, whereas addition normal to strike produces radiator rock (the damage zone) as a consequence of strain hardening of bands and linked stepovers. The new framework has implications for fluid flow in groundwater aquifers, hydrothermal and precious metals systems, and petroleum fields.