The grabens of Canyonlands National Park, Utah: Geometry, mechanics, and kinematics

Abstract

The grabens of the Needles District, Canyonlands National Park, Utah occur in a 460 m‐thick plate of unfolded, brittle rocks overlying ductile evaporites. These rocks dip gently northwest towards Cataract Canyon of the Colorado River. The river has eroded entirely through the brittle plate, thus permitting downdip flow in the evaporites with consequent extension and faulting within the brittle plate producing systematically spaced grabens which in plan are curved, concave in the direction of dip. Stresses in the brittle plate are similar to those in the brittle surface ice of a valley glacier in extending flow, and stress analyses of crevasse geometry are applicable to the Canyonlands grabens; graben curvature is due to shear stresses acting on vertical planes parallel to the lateral boundaries of the zone of evaporite flow. Flow in the evaporites relaxes basal shear stress on the brittle plate, hence the spacing of grabens is a function only of the thickness of the brittle plate and the strength of the contained rocks. Because both are essentially uniform throughout the area, the grabens tend to be uniformly spaced. Evidence from the field and from experiments suggests that the graben faults initiated at or close to the contact between brittle and ductile rocks and propagated both upward and laterally within the brittle plate. At the surface, the resulting incipient grabens thus had initial widths that were functions only of thickness of the brittle plate and dips of the faults. Because plate thickness and mechanical properties of the rocks parallel to bedding do not vary significantly within the area, these initial widths were similar for all major grabens formed. Grabens terminate in ramps which retain this initial width. An equation based on this model of graben geometry and kinematics relates thickness of faulted plate to graben parameters measurable on photographs. Near the surface, graben geometry and trend indicate a compromise between a curved intermediate principal stress trajectory and two sets of pre‐existing, nearly rectilinear, vertical joints, resulting in grabens characterized by en‐echelon offsets and saw‐toothed walls and which have bounding faults that are vertical near the surface but dip inward at depth.