GEOMETRICAL ANALYSIS OF FOLD PROPAGATION IN A NATURAL ALTERNATING COMPETENT AND INCOMPETENT MULTILAYERED MEDIUM DURING SIMPLE SHEAR

Asymmetrical folded structures observed in a regular alternating compe tent and incompetent multilayered medium, constituting the lower part of the volcano-sedimentary sequence from Bordardoue (Belle-lle-en-Mer, Southern Brittany) exhibit a spectacular development of different sta ges of fold propagation within a shear zone. Geometrical analysis allo ws us to propose a non-linear correlation between the angle of rotatio n (alpha) of a given layer at a given point and the distance (d) of th is point from the position (R) of the angle of maximum rotation (alpha (max)) in each structure. Mathematical analysis of alpha = f (d) curve s shows that, when alpha greater than or equal to 30 degrees, the abov e correlation fits with an exponential variation. It is proposed that fold propagation towards the principal extension during simple shear f ollows an exponential empirical law. The physical meaning of this law is discussed in the light of current theoretical and experimental work s.