Slide 7 of 22

A simple active continental margin In simple case the subducting plate is moving perpendicularly to the margin. Sediments are scraped off the oceanic plate to form an accretionary prism due to the action of folds, reverse faults and thrusts. At first, folds and faults have an opposite vergence (direction of movement) to that of the subducting plate. As the prism growth wider and higher it resists deformation and new folds and faults develop verging in the same direction than that of the subducted slab. The prism develop a fan symmetry. A volcanic arc develop on the continental margin above the subducted slab. The basaltic magma is generated by the partial melting of the mantle wedge above the subducted oceanic slab. The partial melting is caused by the release of metamorphic-related fluids from the oceanic slab into the mantle. Deformation always localises in the weakest portions of the continental margin. Interestingly these weakest portions move through times because thickening and heating change locally the strength of the lithosphere. Indeed the formation of an accretionary wedge creates an internal force (gravitational force) that resists deformation. Stresses are transferred in other portion of the system: the back arc basin or alternatively the volcanic arc.