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 perform ed in various places within the Centro-Iberian Zone of western Europe. The observed strain pattern shows that in the inner Ibero-Armorican a rc, as well as near the hinge zone, sinistral transpression produces c onstrictive ellipsoids. In the outer arc, away from the hinge zone, th e deformation regime produces plane strain to oblate strain ellipsoids . Some of these data clearly contradict existent kinematical models, w hich associate constriction with transtension, and consider transpress ion zones to generate oblate strain ellipsoids. However, newly develop ed models show that it is possible to obtain a wide range of finite st rain ellipsoids in transpression zones if one takes account of lateral escape, axial depression and volume change. The geodynamic implicatio ns of the strain analyses suggest generation of constriction in a tran spressive regime, caused by lateral expulsion in front of the Cantabri an 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, th e displacement by lateral escape was easily accommodated by subduction of the South Portuguese Terrane under the Ossa-Morena Zone of the Ibe rian Terrane.