Development of monoclines: Part II. Theoretical analysis of monoclines

Monoclinal flexures, which are isolated asymmetric flexures, range in scale from a few millimetres in kink bands to hundreds of metres in monoclines on the Colorado Plateau. A general model of monoclinal flexuring of multilayers is proposed here; the multilayers include layers with various rheologies, densities, thicknesses, and strengths of contacts between layers. The multilayers are subjected to displacements at their base, stresses at their edges, and a free surface at their tops. We study in detail three modes of this general model, assuming linear, incompressible elastic or viscous multilayers: Drape folding, in which a monoclinal flexure develops over a vertical fault; buckling, in which an initial monoclinal flexure is amplified by layer-parallel compression; and kinking, in which monoclinal kink bands develop unstably by compression inclined to the layering. Selected solutions are presented for the first two modes, and previous research is summarized for the kinking mode. According to analyses of the three special cases of the general model, the profile of the monoclinal flexure, the displacement field, and the strain distribution within the flexure are useful criteria for distinguishing among the three modes of monoclinal flexuring. The Palisades monocline, described in detail in Part I (this volume), is interpreted to be a result of a combination of drape folding over a fault in Precambrian basement rocks and buckling, which together appear to account for most of the field observations. The Yampa monocline in Dinosaur National Park changes form along its length, but each form can be compared with characteristics of a combination of modes, including faulting at depth and layer-parallel compression. In some places it closely resembles a large kink band.