Paper No. 202-11
Presentation Time: 10:45 AM
THE ROLE OF ORIENTATION AND MECHANICAL STIFFNESS CONTRAST IN JOINTING OF CATACLASTIC DEFORMATION BANDS
The Waterpocket Fold in southern Utah features sets of cataclastic deformation bands containing cross-cutting joints that terminate at the boundaries between the deformation bands and the surrounding sandstone. The mechanical contrast of weak sandstone host rock and stronger deformation bands is equivalent to interbedded sedimentary sequences where preferential jointing of the stronger layers with joints typically perpendicular to layer boundaries is known to occur. Deformation bands in the field area represent conjugate strike-slip shear zones, creating a three-dimensional network of mechanically strong zones in variable orientations. As a result joint orientation varies systematically as a function of deformation band strike, and the angle between deformation bands and joints is perpendicular to neither the deformation band nor the far-field extension direction. This study also uses 2D finite element analysis to confirm that local stress reorientation at boundaries between layers affects the mechanics of jointing. The stiffness contrast results in conditions for joint initiation confined in the deformation band, and also results in variable joint orientations in deformation bands with strike orientations oblique to far-field extension. A higher stiffness contrast results in a greater angle between deformation band strike and band-confined joint strike.