Geometric and mechanical-stiffness controls on jointing in cataclastic deformation bands

Abstract

Cataclastic deformation bands on the Waterpocket Fold in southern Utah contain cross-cutting joints that terminate at the contacts between the deformation bands and surrounding sandstone. The mechanical contrast between sandstone host rock and stronger deformation bands is analogous to inter-bedded weak and strong layers in a sedimentary sequence, a situation known to result in joints preferentially forming in the stronger layers with joints perpendicular to layer boundaries. Deformation bands in the field area represent conjugate strike-slip shear zones, many with internal Riedel shear geometry, creating a three-dimensional network of mechanically strong zones in variable orientations. Joint attitudes were found to vary systematically as a function of deformation band strike, and the angle between each deformation band and its associated joints is perpendicular to neither the deformation band nor the far-field extension direction.Field observations are confirmed by 2D finite element models showing that local stress reorientation at boundaries between layers affects the mechanics of jointing. The stiffness contrast results in conditions for initiating joints contained in the deformation band, and also results in variable joint orientations in deformation bands with strike orientations oblique to the far-field extension direction. A higher stiffness contrast results in a greater angle between deformation band strike and band-contained joint strike.