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.
Highlights
Continuous deformation by folding causes progressive strain in rocks, and folds should allow study of evolving strain fields if structures from progressive, superimposed stages can be identified
We show that a total of four populations of structures record different kinematics and defor mation mechanisms, depending on orientation to sedimentary bedding within the first-order fold
We present the kinematic interpretations of deformation bands and frac tures linked to their deformation mechanisms, spanning across to shear system discussions
Summary
Continuous deformation by folding causes progressive strain in rocks, and folds should allow study of evolving strain fields if structures from progressive, superimposed stages can be identified. The uniqueness of our study lies in deeper investigation of distinct structural populations of progressive deformation stages and especially transitions in deformation style between deformation bands and frac tures This is revealed in the central part of the forelimb of the San Rafael Swell monocline, where the strain is at its highest. Failure of rock by fractures causes dilation Such structures can be divided into shear-fractures and joints, of which the latter records pure dilation, as for instance discussed in Aydin et al (2006) and Schultz and Fossen (2008), and commonly applied to analysis of deformation in folds (e.g., Hancock 1985). Some thin sections were analysed in a Scanning Electron Microscope (Hitachi SU5000 at the Dept. of Geo sciences, UiO), to extract high-resolution information on mineralogy and diagenetic sequences, including pore- and fracture-filling cements
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