Physical models of regional fold superposition: the role of competence contrast

Abstract

Physical models of superposed folds demonstrate that rheologic contrast strongly controls the style of fold interference. Rheologic contrast affects strain partitioning between layer-parallel shortening and buckling and affects the development of structural anisotropies parallel to first-generation folds. Laminates with insignificant competence contrast are characterized by circular to elliptical domes and basins (Type 1 interference). In addition to folding, the layers accommodate shortening by layer-parallel thickening, preferentially in the fold culminations. Laminates with significant competence contrast are characterized by buckled fold hingelines and axial surfaces (Type 2 interference). The less competent layers thicken in the hinges; the more competent layers maintain their initial thickness. In plan view, the F 1) hingelines are refolded in a lobate—cuspate to box style, whose axial traces form conjugate pairs. The models are dynamically scaled to represent folds with km-scale wavelengths. The evolution of surface structures, map patterns at different depths, and equal-area projections synthesized from structure contour maps were used for analysis, comparable to regional analysis common to field studies. Gravitational body forces effectively damp the vertical amplitudes of both the first and second generation folds, thereby enhancing the formation of Type 2 interference.