Abstract

The Moab Fault is one of the longest of several northwest-striking, northeast-dipping, Cenozoic normal faults in the Paradox Basin which are spatially associated with northwest-trending salt-cored anticlines. The fault can be subdivided into three sections based on geometric and kinematic considerations and geomorphic expression. The 10 mi (16 km) long northern section extends from Tenmile graben to Courthouse Rock, has the smallest total displacements, and reversed fault-topography created by significant post-faulting erosion. The 12 mi (19 km) long central section extends from Courthouse Rock to the Colorado River, has the greatest total displacement, and also shows evidence for significant post-faulting erosion. The 12 mi (19 km) long southern section, inferred to extend from the Colorado River to the southern end of Spanish Valley, is entirely buried by Quaternary basin deposits. Available evidence suggests that most, if not all, of the slip on the Moab Fault is pre-Quaternary. Although the age of most recent activity is not known because late Cenozoic erosion has removed significant sections of the stratigraphic record, the geomorphic expression of the fault indicates very low rates of activity. We used bedrock scarp-retreat rates of 0.6 to 1.6 ft (0.2 to 0.5 m) per 1000 years to estimate the time required for rocks in the footwall of the Moab Fault to erode back from the main fault trace to the present position of the bedrock escarpment. Resulting age estimates range from 1.2 to 7.5 Ma for the last movement on the fault.

Displacement patterns, crosscutting relations, and differences in structural style between salt-dissolution-subsidence deformation and deformation observed along the Moab Fault indicate that the primary displacement on the Moab Fault occurred before, and is unrelated to, Quaternary salt-dissolution subsidence in Moab-Spanish Valley. Additionally, stratigraphic data suggest that the primary fault movement post-dates major salt diapirism which formed multiple progressive unconformities in Permian through Jurassic rocks that flank the Moab salt-cored anticline. Based on all the geologic evidence, we believe that the primary movement on the Moab Fault is tectonic and occurred during a period of Tertiary extension. However, subsurface, map, and geophysical data also suggests that the Moab Fault probably soles into salt of the Paradox Formation at a shallow depth and is presently not capable of producing significant earthquakes.