A dislocation model applied to slump structures, Ainsa Basin, South Central Pyrenees

Features associated with gravity-induced slumping in deep-water Eocene sediments of the Ainsa basin are described and four aspects are selected as being especially significant. These are: extensional strain, contractional strain, strain overprinting and clastic dykes. Slump strain is interpreted as a consequence of the initiation, translation and termination phases of slump development and is explained in terms of a dislocation model. The initial phase of development involves the propagation of a failure through undisturbed sediment and this imparts a characteristic strain above the décollement surface. Translation of the failed body involves sequential velocity changes which also deform the moving sediment. During the termination phase a type of dislocation, here named an anti-dislocation, migrates along the basal failure when a slump regains cohesion with the substrate. Clastic dykes are interpreted as dewatering structures initiating at basal faults which have associated high pore fluid pressure. Dewatering of slumped décollement sheets may be a significant phase in the termination of movement of failed sediment bodies.