THE TRANSPORT BLOCKAGE ON THE UPPER FLAT OF A FAULT-PROPAGATION FOLD - A POSSIBLE CAUSE OF OUT-OF-SEQUENCE THRUSTS

The classical Absaroka Thrust Sheet cross section (Rocky mountains USA ) exhibits an out-of-sequence thrust which occurred when a fault-propa gation fold, passively transported upon an upper flat (= residual fold ), collided with a previously formed frontal ramp. Such a relationship is also observed in other mountain fronts like the Variscan front (No rthern France, Belgium). We suggest that all these events are closely linked: the movement takes place as if the residual fold could not com pletely step over the frontal ramp. In the Boulonnais area, the transp ort of a residual fold over the ''ecaille des Moines'' appears to be r elated to the blocking of the ''Haut-Banc'' thrust. At the contact, th e forelimb of the fold is overturned and underwent a thinning of about 45-50%. This value is compatible with a geometrical modelling showing that the passing of overturned beds over a new ramp induces a simple shear and thinning. This abrupt increase of internal deformation favou rs the development of an out-of-sequence thrust located on a hinterlan d deformed zone. Typically, the out-of-sequence thrust acts as a break through generated from a previous ramp.