Influence of mechanical stratigraphy on folding vs faulting

The competing processes of folding and faulting during shortening of a layered sequence are investigated using elastic-plastic models, in order to determine the influence of mechanical stratigraphy on the development of fold belts vs thrust belts. The models consist of a strong middle layer and weaker décollement and cover layers. The mechanical behaviour of each layer is determined by its plastic hardening modulus h and its elastic shear modulus G. With bonded contacts, fold amplification at low shortening strain (relatively large h) is greatest for a thin cover layer and a thick décollement layer. At higher shortening strain (h ⪡ G), the middle layer may reach its peak strength before the other layers, causing it to become relatively weaker in shortening and causing fold amplification to be greatest for a thick middle layer. Free-slip contacts between the layers enhance folding and fold amplification is greatest if the slip surfaces are roughly equally distributed throughout the sequence. A relatively weak décollement layer enhances folding, consistent with the fact that fold belts usually include a salt décollement layer. Dominant wavelengths increase with increasing strength contrasts between the layers and with increasing thicknesses of the décollement and strong layers.