Deformation band shear zone, East Kaibab monocline, Utah

Because the Navajo Sandstone is very porous (~20% porosity), its preferential mode of failure is through the formation of deformation band shear zones. Tectonic stress causes initial localized collapse of porosity along a very thin band that as a result is more rigid than the host rock. Miniscule slip (shear), on the order of millimeters of displacement, takes place along the deformation band, and then the zone of deformation will jump to an adjacent part of the host rock where the porosity is high. This repeats itself time and again, producing a zone of deformation bands. If tectonic stress continues to be applied, eventually there may be wholesale faulting along the deformation band shear zone, creating meter-scale offset and the formation of classical fault features (e.g., polish and slickenlines). ;In this rock specimen, the polygonal cavities represent places where porous and friable sandstone has been weathered and eroded. The ladder like bands are marked by strong ribs or struts, and these are the deformation band shear zones. Sense of offset can be determined based upon the ‘ladder’ geometry. The acute angle ‘points’ in the direction of differential shear.

During the period 1994-1999 I invested a lot of time in southern Utah, mapping and analyzing deformation band shear zones.  I collected this rock along the East Kaibab monocline along the Cockscombs near Candyland.  The nature of the shear is extensional, normal slip.  I like this rock because it drives home the illusion that deformation are veins, when in fact they are shear zones.  Moreover, the ladder structure is ubiquitous in rocks marked by pervasive deformation bands, and it was a challenge to try to figure out how to ‘read them.’

Age & Formation
Navajo Sandstone of Jurassic age, ~170 Ma, laced with deformation bands that formed in latest Cretaceous to Eocene, i.e., ~70-55Ma.