ANALYSIS OF COMPOSITE MEGALANDSLIDES IN THE COLORADO RIVER CORRIDOR, ARIZONA

The Grand Canyon (GC) Region of the Colorado Plateau contains numerous examples of composite megalandslides with basal sliding zones developed in the overconsolidated (OC) Bright Angel (Cba)and Chinle Formations. Larger slides displace over 600 m of strata. Although these slides are mapped and noted in literature, past analysis has lacked sufficient detail to fully explain their likely genesis.

Finite element models (FEM) have been employed by others, and suggest that a southerly regional dip between Deer and Havasupai Creeks might explain asymmetric cliff retreat along the Colorado River. This work didn't attempt to explain landsliding throughout the GC. A landslide at RM 205 on river left is the largest yet identified in GC. It appears to be structurally controlled by the Granite Park Fault and had a volume of ~2 billion m3 at failure. Variables, such as dip of strata with respect to canyon alignment, faults, perched water, springs/seeps, variability within formational units, and the regional joints appear to play a role in triggering the slides. Most landslides appear to be Pleistocene age or earlier. Other variables, such as a wetter paleoclimate or seismicity, may have contributed to their instability. We are collecting data over a wide area and noting the above-cited variables at locations with and without deep-seated landslides.

Conventional limit equilibrium analyses are unreliable in terrains of large scale and extreme topographic relief. Megalandslides generally undergo progressive failure due to strain softening which initiates at the toe of the slope and progresses upslope until a critical threshold is reached whereby the entire mass translates downslope. Landslides in OC shales experience about 1.5% strain before macro rupture occurs. We plan to use FEM to examine triggering mechanisms of these slides and finite difference and discontinuous deformation analyses to analyze the slides after the basal slip surfaces propagate and plastic deformation takes over.

A prominent incipient landslide with basal rupture flooring in the Cba has been mapped in Peach Springs Canyon. This block has dropped 15 to 20 m and a circular headscarp separation has developed (mapped as a fault), indicating motion shy of macro rupture. Understanding this feature may aid researchers in studying the triggering mechanisms responsible for megalandslides in the region.