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.