continental convergence
Convergence accommodated by subduction
Subduction zones accommodate convergence between oceanic and continental lithospheres. The angle of the Benioff-Wadatti plane is a critical parameter in the dynamic of subduction-orogens as it controls the shear traction at base of the overriding plate.
Shalow subduction zones involve a large shear traction and therefore a strong coupling between the overriding plate and the subducting plate. Shortening affects not only the continental margin but also adjacent continental area. The belt is as a symmetric geometry with thrusts of opposed vergence at both end of the orogen.
Steep subduction zones involve a weaker coupling between the two plates. Despite short bursts of contractional deformation, the overriding plate is under extension. The weakening of the mantle wedge above the subduction zone via release of metamorphic fluid transform the lithospheric mantle into weaker "asthenospheric" mantle further reducing the coupling. The percolation of water throug the mantle wedge enhances partical melting boosting volcanism.
Through time and space, the subduction angle may change due slab break-off, or the arrival of oceanic plateaux, seamount, or simply younger, thinner, more buoyant oceanic lithopshere. It is also under the influence of the asthenospheric flow around the subducting slab. Therefore, the tectonic regime in the overriding plate varies from contractional to extensional. The South American coordillera is build over short burst of contractional deformation seperated by long stages where extensional tectonic dominates.
The block diagrams on the right show two sections at different latitude across the South American Cordillera (Mattauer, 1989; Monts et Merveilles, Herman edt.).