The Meander anticline, Canyonlands, Utah: An unloading structure resulting from horizontal gliding on salt

Abstract

The axial trace of the Meander anticline, which has a cumulative length of 25.5 mi, coincides with the Colorado River in Canyonlands National Park. On the southeast side of the river, eight tributary canyons also contain anticlines. In all cases, the anticlines occupy canyons that are eroded between 400 and 1,800 ft into the Pennsylvanian-Permian section. A system of closely spaced grabens has developed in the Pennsylvanian-Permian rocks which comprise the plateaus adjacent to the deeper canyons. The grabens are most extensively developed southeast of the Colorado River, where they follow the strike of the Meander anticline for 17 mi and extend 9 mi into the plateau. The Pennsylvanian Paradox evaporites, consisting of 85% salt, underlie the entire anticline-graben complex. Principal stress axes can be deduced from the intersections between conjugate thrust faults in the cores of the anticlines and from the downward intersections of normal faults bounding the grabens. The maximum principal stresses associated with the anticlines are horizontal and perpendicular to the axial planes regardless of the strikes of the anticlines. In contrast, the maximum principal stresses associated with the grabens are near vertical and normal to the strikes of the faults. The conclusions of this study are that (1) the anticlines are unloading structures, and (2) the mechanism which best accounts for the observed stresses is one involving the breakup of the brittle plate above the Paradox salts and its simultaneous gliding toward nearby canyons. The breakup and motion within the brittle plate involve a three-dimensional deformation which is analogous to the deformation that takes place within large unbounded landslides. Through compaction of the section, largely accounted for by the downdropping of grabens, the deforming mass expands laterally, yet maintains horizontal continuity. The result is the establishment of horizontal stress gradients at depth which produce compression within the rocks that bridge the floors of the deep canyons regardless of the orientations of the canyons. Through a combination of horizontal shortening and arching, the compressed rocks fold into the observed anticlines. Salt flowage into the cores of the anticlines is not the causative mechanism in their development.