Folding in regions of extension

Abstract

SUMMARY Many extension zones have been subjected to folding and shortening in a direction perpendicular to the stretching. Such deformation can be accounted for by the extension of a thin superficial elastic layer overlying a substrate that has small elastic moduli or that deforms in a viscous regime. Laboratory experiments are used to document the wavelength and amplitude of the folds for a range of geometrical configurations. Folding is observed even for very small amounts of extension (less than 1 per cent) with characteristics that are consistent with finite-amplitude scaling laws. Because of the intrinsically 3-D nature of the deformation field, the size of the region affected by folding and the direction of the fold axes depend on the orientation of the extension with respect to the rigid blocks that bound the deforming region. For regions of extension where the elastic thickness is about 10 km, as in the Basin and Range province for example, it is predicted that folding occurs with wavelengths in a 20–40 km range, such that it induces little deformation in the lower crust and maintains a flat Moho discontinuity. These predictions are consistent with the observations. The characteristics of faulting that is associated with such deformation are discussed.