Mechanism and sequence of formation of deformation bands into spatially localized or distributed sets: Ladders, Riedels, and echelon arrays of Utah

Recent fieldwork has shown that many of the geometric relationships previously observed in cataclastic deformation bands are based on a two dimensional view of a three dimensional structure. When the bands are viewed in the mode III direction lenses and inosculating bands are seen, as first noted by Aydin and Johnson [1978] and later by many others. When the bands are viewed mode II direction, “ladder structures” (linked mode II echelon stepovers) and conjugate structures are seen [Davis, 1999]. Our field observations reveal that the principal difference between the two is the relative distance separating the parallel echelon bands. Bands that are sufficiently close interact mechanically to promote formation of linking bands within the stepovers. We use boundary element models to simulate this in-plane growth of mode II deformation bands. Our results suggest that deformation bands (in mode-II sections) accumulate displacement as they form cataclastic zones from the surrounding, more porous host rock.