Formation of Deformation Band Structures Normal to the Shear Plane

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 shear direction lenses and inosculating bands are seen, as first noted by Aydin [1978] and later by many others. When viewed normal to the shear direction "ladder structures" (linked mode II echelon stepovers or duplexes) and conjugate structures are seen [Davis, 1999]. Davis [1999] grouped these two geometries as different classes of deformation bands and many have interpreted the duplexes as strain localization in Riedel shear zones. Here we demonstrate an alternative explanation. When viewed normal to the shear direction, deformation bands display either mutually crosscutting ("conjugate") sets or contractional stepovers ("duplexes") between interacting mode-II bands. 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 linking bands within the stepovers. These linking bands are younger than the bounding echelon bands. Because the orientations of bands within the stepovers are the same as that of "conjugate" bands that are more widely spaced, we infer that the magnitude of stress within the stepovers was increased somewhat over background values, but that stress rotations within the stepovers were negligible. To accomplish this, the echelon deformation bands must be strong relative to the host rock and accommodate only small offsets, leading to only minor perturbations of the local stress state in their vicinity. Ladder structure, "radiator rock" [Davis, 1999], and linked echelon stepovers demonstrate a progression from distributed to more localized strain of the sandstone, with the scale dependent on the size and offset magnitude of individual bands. Occurrence of linked echelon stepovers along both conjugate directions in spaced arrays argues against Riedel shearing as a mechanism for localizing this class of deformation bands. Aydin, A., Small Faults Formed as Deformation Bands in Sandstone, Pure and Applied Geophysics, 116, 913-930, 1978 Davis, G., Structural Geology of the Colorado Plateau Region of Southern Utah: With Special Emphasis on Deformation Bands, GSA Special Paper 342, 157 pp., 1999