CONSTRICTION IN A TRANSPRESSIVE REGIME - AN EXAMPLE IN THE IBERIAN BRANCH OF THE IBERO-ARMORICAN ARC

Detailed strain analyses in the Armorican quartzites have been performed in various places within the Centro-Iberian Zone of western Europe. The observed strain pattern shows that in the inner Ibero-Armorican arc, as well as near the hinge zone, sinistral transpression produces constrictive ellipsoids. In the outer arc, away from the hinge zone, the deformation regime produces plane strain to oblate strain ellipsoids. Some of these data clearly contradict existent kinematical models, which associate constriction with transtension, and consider transpression zones to generate oblate strain ellipsoids. However, newly developed models show that it is possible to obtain a wide range of finite strain ellipsoids in transpression zones if one takes account of lateral escape, axial depression and volume change. The geodynamic implications of the strain analyses suggest generation of constriction in a transpressive regime, caused by lateral expulsion in front of the Cantabrian indenter that produced the Ibero-Armorican Arc. This mechanism was responsible for the Variscan wrench component in western Europe, which was mainly sinistral in Iberia and dextral in Britanny. In Iberia, the displacement by lateral escape was easily accommodated by subduction of the South Portuguese Terrane under the Ossa-Morena Zone of the Iberian Terrane.