Modeling of Grabens Extending above Evaporites in Canyonlands National Park, Utah

Grabens in Canyonlands National Park, Utah, began extending above a layer of evaporites when the Colorado River cut through the overburden. Two-dimensional finite element models simulate the effects of geometry and rock properties on graben configuration and spacing. Only those models having a progressively increasing slope or no slope mimicked the natural upslope graben propagation. Typical rock properties produced the most realistic fault patterns: an initial friction angle of 31 degrees, a cohesion of 1 MPa, and strain weakening comprising cohesion loss and decrease of friction angle to 26 degrees. A tensile stress limit narrowed the grabens and reproduced the vertical upper portion of the natural faults. The viscous salt resisted overburden spreading and controlled its rate. Modeled grabens spread at typical rates of 1-2 min a(-1) for a salt viscosity of 1 x 10(18) Pa s, and the entire system strained at rates from 6.0 x 10(-14) s(-1) to 0.5 x 10(14) s(-1). The faults bounding a graben formed nearly simultaneously at the top surface and propagated downward. Salt rose beneath the grabens as reactive diapirs. Overburden adjacent to the canyon flexed as salt was expelled and formed an arching horst and graben. A corresponding horst has been found in the field. The model results scale to larger dimensions, except for the steep upper part of the faults. Reduced dimensions create vertical or no faults