Sarah Wigginton, Elizabeth Petrie, James P. Evans

We examine the mechanics of thrust fault initiation and development in sedimentary rocks which accounts for vertical variation in mechanical strength of the rocks. We use numerical mechanical models of mechanically layered rocks to examine thrust ramp nucleation in competent units, and fault propagation upward and downward into weaker units forming folds at both fault tips. We investigate the effects of mechanical stratigraphy on stress heterogeneity, rupture direction, fold formation, and fault geometry motivated by the geometry of the Ketobe Knob thrust fault in central Utah. The study incorporates finite element models to examine how mechanical stratigraphy, loading conditions, and fault configurations determine temporal and spatial variation in stress and strain. We model the predicted deformation and stress distributions in four model domains: (1) an intact, mechanically stratified rock sequence, (2) a mechanically stratified section with a range of interlayer frictional strengths, and two faulted models, (3) one with a stress boundary condition, and (4) one with a displacement boundary condition. The models show that a dramatic increase in stress develops in the competent rock layers whereas the stresses are lower in the weaker rocks. The frictional models reveal that the heterogeneous stress variations increase contact frictional strength. Faulted models contain a 20° dipping fault in the most competent unit. The models show an increase in stress in areas above and below fault tips, with extremely high stresses predicted in a ‘back thrust’ location at the lower fault tip. These findings support the hypothesis that thrust faults and associated folds at the Ketobe Knob developed in accordance with the ramp-first kinematic model and development of structures was significantly influenced by the nature of the mechanical stratigraphy.