The finite-amplitude instability in the folding of layered rocks

Abstract

Examination and comparison of several computational studies of finite-amplitude single-layer folding bring to light an important unifying concept, that of the finite-amplitude instability. If the fold-causing stresses are constant throughout the development of a fold, the overall shortening strain-rate due to the folding will first show a marked increase with increasing fold amplitude and then decrease as the fold becomes tightly appressed; on the other hand, if the shortening strain-rate is held constant, the stresses necessary to maintain the constant shortening rate will decrease and then increase again. Considering the layer-in-medium system as a whole, this instability results in a bulk-effective viscosity with respect to layer-parallel compression, which is first high, then decreases to a minimum, and then rises again, These changes in bulk-effective viscosity are large, nearly two orders of magnitude.Effects of the finite-amplitude instability on the internal stresses within the layer and surrounding medium are as follows. First, there will probably be an early, low-amplitude stage with high layer-parallel stresses within the competent layer. The major development of deformation fabrics may occur during (his early stage. Second, development of plastic yield in the crestal region of the fold is strongly controlled by the details of external load pattern and therefore by the mechanism causing the folding. Third, the increase in effective bulk viscosity of the layer-in-medium system corresponds to a major rearrangement of the deformation patterns in both the medium and the layer.Consideration of décollement folding and multi-layer folding suggests that the concept of finite-amplitude instability may be applicable, at least qualitatively, to these more common fold types. If this is true, then the concept is important in considering more general geologic questions such as the dating of an orogenic episode by angular unconformity.