Abstract

Multilayered rock complexes with regular alternations of competent and incompetent layers of thickness t1 and t2, respectively, and with high ductility contrast form folds of the chevron style when subjected to compression along the layering. The geometric forms of progressively developing chevron folds are analyzed using a model whose properties are based on the geometric forms of naturally deformed rock layers. It is found that the chevron fold style is only stable where no strongly marked variations in competent layer thickness t1 exists. The thickness of the incompetent layer exerts no influence on fold model stability. Slight variations of competent layer thickness can be accommodated by local modifications of the fold style, such as limb faults, bulbous hinge zones, or layer boudinage, but if any strongly marked variation exists, the fold limbs become curved. The chevron model involves dilation at the hinge zones, and saddle reef formation, incompetent layer flow into the hinge, or slow hinge collapse generally results from actual or potential dilation. The speed of development of chevron folds is calculated under conditions of constant stress and of constant load. Folding starts slowly but rapidly accelerates; the later stages are characterized by a progressive slowing in shortening rate and fold growth, leading either to a stage of locking up of the fold or to modification of its geometry by limb thinning and hinge thickening; toward a more similar geometric style. The strains in the hinge zone region are related to the rates of shear taking place in the fold limbs; the geometric model is not completely stable throughout the fold development, and complex progressive strain increments can occur in the hinge zones and lead to the development of superposed small-scale structures indicating reversals of principal axes of incremental strain.

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