Completed Study



Walsh, P., and Schultz-Ela, D., in press, Mechanics of graben evolution in the Needles District of Canyonlands National Park, Utah: Geological Society of America Bulletin

Schultz-Ela, D., and Walsh, P., 2001, Modeling of grabens extending above evaporites in Canyonlands National Park, Utah: Journal of Structural Geology, v. 24, p. 247-275.

Modeling of Extensional Systems Detaching on Evaporites
Daniel Schultz-Ela and P. Walsh

(1) Mechanics of graben evolution in the Needles District
of Canyonlands National Park, Utah

Results of numerical models and field observations of regularly spaced grabens in Canyonlands National Park, Utah, demonstrate that salt flow beneath a brittle overburden accommodated recent and ongoing westward gravity spreading. Erosion of the Colorado River canyon differentially loaded the underlying viscous salt. In our models, the overlying brittle strata flexed downward toward the canyon, initiating faults near the surface that propagated downward toward the salt contact. Modeled grabens developed sequentially away from the canyon (eastward) as salt was expelled from beneath undeformed strata. After their eastern boundary faults broke through, horst blocks tilted in the opposite direction of initial flexure, resulting in increased symmetry of older grabens closer to the canyon. Continued extension formed a reactive diapir beneath each graben.

Field observations show that multiple faults bound grabens, indicating reactive diapirs beneath them. Topographic profiles and surveyed points along a stratigraphic layer show that horst blocks subsided as salt migrated toward the river canyon and into the diapirs. Field data from less evolved horsts imply that individual horst blocks responded to differential loading by progressive flexure and tilt, similar to the models. Horst-block flexures also vary along strike, and localized folds and faults formed where fault displacement changes abruptly.

(2) 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°, a cohesion of 1 MPa, and strain weakening comprising cohesion loss and decrease of friction angle to 26°. 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 to 2 mm a-1, for a salt viscosity of 1´1018 Pa s, and the entire system strained at rates from 6.0´10-14 s-1 to 0.5´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.

For more information, please contact Dan Schultz-Ela. Telephone 512-471-4335; e-mail
March 2003