Mechanics of Contrasting Structural Styles in Fold/Thrust Belts

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There are apparent differences in the style of deformation between various foreland fold and thrust belts. Despite these differences, the overall topology of foreland belts is remark- ably similar in that both thrust belts and fold belts show, (1) a cratonward decreasing systematic spacing of major structures, (2) a shared regional decollement and (3) multiple lesser internal decollements levels. The fundamental differences in style (folding vs. faulting) can be related to the strength, stacking order and rheology of the stratigraphy. By applying an analysis of the flow instability of a layered sequence for a non- linear viscous fluid, the initiation and localization of first order structures in both fold-dominated and fault-dominated fold/ thrust belts will be determined. This theory will predict the spacing of major structures, whether folds or thrusts, within a layered sequence for a range of geologically plausible represen- tations of the stratigraphic sequences of the Plateau Province of Pennsylvania and the Idaho-Wyoming-Utah thrust belt. Further analysis of the finite evolution of these structures for a non- linear rheology will focus on the transition from folding to faulting that may be responsible for the observed deformation in the Idaho-Wyoming-Utah thrust belt. Thus, this study will attempt to unify the mechanisms for the origin of foreland fold and thrust belts.