Strain patterns in models of spreading‐gliding Nappes

Three experiments have been carried out in order to study the progressive and finite strain in nappes where gravitational spreading and gliding occur together. These experimental models were made from analogue materials able to collapse under their own weight on an inclined plane. Throughout most of the models the stretch trajectories in a vertical plane parallel to the flow show a low‐amplitude sigmoidal pattern consistent with a previous theoretical model. Other stretch trajectories exist at the front and back ends of each model. The kinematic significance of all these trajectories is discussed in detail. Highest strain intensities are always found next to the base of the model. Strain paths calculated at different stages of flow indicate clearly that nowhere in the model is the motion simple. There is a complex combination in time and space of simple and pure shear, except at the base of the model where the motion approximates a simple shearing. Near the two parallel lateral sides of the model a lateral boundary effect can be observed from the strong curvature of the transverse markers (surface grid and vertical layers). Strain paths have also been estimated in this complex zone where wrench shearing is superimposed upon vertical shortening and thrust shearing. The geological implications of these experimental models are discussed in relation to recent theoretical studies and field work.