In the western Grand Canyon, the Colorado River cuts through a series of moderate offset normal faults that mark the transition between the Colorado Plateau and the Basin and Range tectonic provinces. Both hanging wall and footwall rocks are exposed over > 1,000 m of vertical section, and contain relatively simple pre-existing structures. The area thus presents an opportunity to investigate processes of continental extension without cross-cutting fault patterns and large rotations typical of much of the Basin and Range.

We have mapped 8 km of the Froggy Fault, including ~ 600 m of vertical section, and deformed strata of the Esplanade fm. over a 32 km² area by integrating field observations, GPS surveying, and a high-resolution DEM. The 3D structure of the site is documented over well-exposed areas with no more than 10 m between data points and meter-scale precisions. In cross-section the hanging wall rolls into the fault from a regional dip of 2° to a maximum dip of 25°. Along strike, steeper dips are associated with synthetic faulting and lower dips with antithetic faulting. The footwall also is folded with a maximum dip of 12°, away from the fault, near the eroded scarp. Throw on the Froggy Fault ranges from 50 to 230 m across the field area, and fault dips are consistently steep, typically > 70°.

We fit the hanging wall fold shape with both kinematic and mechanical models, however only the mechanical models predict the footwall deformation. Kinematic models (variably inclined shear) require a smoothly curving fault to generate the observed hanging wall structure, with the depth to detachment dependent upon the shear angle. Steep fault dips and the geometric assumptions of this method limit permissible shear plane dips to between 75° in the opposite direction and 70° in the same direction as the fault. Alternatively, the observed folding may be fit by a planar fault in an elastic half space with slip over an interval from 0 to 1.2 km depth. This model predicts both footwall and hanging wall deformation, while honoring the observed near-planar fault geometry. Predicted strain gradients are high, suggesting that, although elastic stresses appear to play an important role in the development of the fold shape, a time-dependent (e.g. viscoelastic) rheology may provide better models of this structure.