INFLUENCE OF DEFORMATION BAND SHEAR ZONES ON JOINT DEVELOPMENT

Deformation band shear zones (DBSZs) in porous sandstones localize penetrative, systematic joints. Orientations of DBSZ-confined joints are influenced primarily by local or regional strain-release directions, and to a lesser degree by orientations of the DBSZs in which the joints form. We compare data from two field sites in southern Utah, Cottonwood Canyon on the East Kaibab monocline and Hillsdale Canyon near the Ruby’s Inn thrust system, to distinguish relationships among timing, orientation, and tectonic cause of DBSZs and DBSZ-confined joints.

At Cottonwood Canyon NE-striking, steeply NW- and SE- dipping sets of DBSZs in Jurassic Navajo Sandstone developed along the crest of the Laramide-age East Kaibab monocline. The DBSZs are conjugate normal faults that allowed stretching of the outer arc of the fold. Confined within these DBSZs are NW-striking, vertical joints that are perpendicular to both the DBSZs and to the N65E Laramide shortening direction. The joints clearly formed later than the DBSZs, and are interpreted as post-Laramide release joints.

At Hillsdale Canyon sets of N- and S- dipping, ESE-striking DBSZs in Cretaceous Straight Cliffs Formation show conjugate thrust displacement. The DBSZs are elements of the Miocene Ruby’s Inn thrust system, a shallow system of thrust faults bordering the southern and eastern margins of the Marysvale volcanic center. The Miocene shortening direction at Hillsdale was SSW and is recognized by slickenlines on striated surfaces in Cretaceous sandstones, by the line of intersection of conjugate thrust faults, and by attitudes of coal cleats in Cretaceous and Tertiary strata. DBSZ-confined joints at Hillsdale Canyon are vertical and strike N8E, parallel to Miocene shortening. We infer that DBSZs and DBSZ-confined joints at this site formed together in response to Miocene thrust faulting.

Joints at both field sites are generally perpendicular to DBSZs, although the orientations, tectonic causes, and relative timing of the features differ. We conclude that (1) DBSZs behave as stiff mechanical layers, developing penetrative, systematic joints to accommodate layer-parallel extension, and that (2) the joints provide an additional structural tool for tracking stress directions and deformation history, particularly in regions of low strain.