Interaction of deformation bands and fractures during progressive strain in monocline - San Rafael Swell, Central Utah, USA

Abstract

Folds in porous sandstone in cases allow identification of progressive deformation in an evolving strain field. In the Navajo Sandstone of the km-scale Laramide-style monocline of the San Rafael Swell (Utah, USA), four populations of small-scale structures record different kinematics and deformation mechanisms, depending on orientation to bedding within the first-order fold. Small-scale structures span from cataclastic (shear-) compaction and shear-isochoric deformation bands to dominant disaggregation (shear-) dilation bands. Extension and shear fractures record transformations from band to fracture formation, adding to the structural diversity.

Early structures record semi-penetrative shear deformation guided by bedding and lamination in eolian deposits, consistent with layer-parallel shortening. Subsequent deformation is localized and at a higher angle relative to bedding, recording forward-directed and subsequently backward-directed shear structures within the east-verging monocline. Final deformation is highly localized and appears as a conjugate set of sub-vertical shear zones with shortening-extension axes oblique to the monocline.

For the given conditions in a progressive shear system in highly porous sandstones, interactions of deformation bands and fractures suggest a revival of deformation bands by mutual shear band-fracture systems as developing band swarms rotate into an extensional strain sector during folding. In cases of deformation by shear-dilation strain, deformation bands may evolve directly into fractures, as grain contacts are lost.