Topographic and structural conditions in areas of gravitational spreading of ridges in the Western United States

Abstract

Gravitational spreading of steep-sided ridges produces characteristic geomorphic forms including grabens and depressions along ridge crests, trenches, and uphill-facing, as well as downhill-facing scarps, on the mountain flanks, and outward bulging of the lower slopes. These sackung-type features occur in a variety of geologic settings in the Western United States. Those discussed here occur principally in high, linear ridges separated by glaciated valleys. The ridges are underlain by hard, but closely jointed, Precambrian igneous rocks. Topography is the primary determinant of the location and direction of the trenches and scarps, but the topographic grain of the terrane is, itseH, determined in part by rock structures, such as joints and faults. In the Sawatch Range in Colorado, some valleys in the study area follow the direction of primary joint systems and, in turn, determine the direc- tion of trenches and scarps parallel to slope contours. The principal joint sets are, themselves, parallel to microcracks in the rocks. The relation of sackung features to structural elements is close in the Sawatch and Williams Fork Mountains in Colorado, not obvious at the one site examined in the Sangre de Cristo Mountains of New Mexico, close in the Stillwater Complex in Montana, and apparently close in a zone around the Straight Creek fault in the northern Cascade Mountains in Washington. Elastic-plastic stress analysis indicates that uphill-facing scarps may develop in the upper extending parts of a slope preferentially over downhill-facing scarps.