Abstract

Computer simulation of the time-dependent deformations of layered viscous solids serves as the basis of a study of the mechanics of large-amplitude folds. Several models of a single viscous layer embedded in a less viscous matrix and a model of an infinite stack of layers of alternating viscosity have been studied. This study treats the plane finite deformations of the models as a series of small incremental deformations. The velocity fields of the incremental problems are found using the finite element method. With the single-layer models, the contrast of viscosity between the layer and matrix strongly influences the geometry of folds at the dominant wavelength. Fold geometries vary from concentric for large viscosity contrasts to nearly similar for low contrasts. In each model, the patterns given by directions perpendicular to the principal axes of maximum total compressive strain closely resemble axial-plane foliations in natural folds. Published fabric data for calcite-twin lamellae and quartz-deformation lamellae are consistent with stress directions in the theoretical models and support an interpretation that these fabric data record cumulative-intragranular flow during the course of folding.

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