Forelimb deformation in some natural examples of fault-propagation folds

Abstract

An analysis of two natural examples of fault-propagation folds from the Pyrenees and the Cantabrian zone (N Spain) is presented. These folds possess common features despite their different geological settings, i.e. related to thrust tectonics in the Pyrenean example and to strike-slip faulting in the Cantabrian zone. Transition from fault-bend to fault-propagation folding along individual thrusts is related to changes in lithology. The fault-propagation folds studied are tight and markedly asymmetric with respect to limb thickness and shape. The forelimbs are curvilinear and display progressive thinning of beds towards the fault surface, which can be interpreted in terms of a characteristic heterogeneous strain superimposed onto flexural shear, and restricted to them. This strain has been factorized by means of grid construction, and is found to be a combination of thrust-parallel heterogeneous simple shear and pure shear, attributed here to a shear zone with diverging walls and extrusion. In contrast with previous models, it is shown that thrust-parallel simple shear is not necessarily distributed above the entire fault surface but may instead be restricted to regions where motion was inhibited such as tip zones or ramps.