Tectonic evolution of the boundary between the Central Iberian and Ossa-Morena zones (Variscan belt, southwest Spain)

Three tectonometamorphic units can be differentiated in the boundary between the Central Iberian and Ossa-Morena zones of the Iberian Massif (Variscan belt). The three units have been named the Northern Unit, Central Unit, and Southern Unit. The Northern Unit corresponds to the border of the Central Iberian Zone; it evolved at low-temperature and intermediate- or low-pressure metamorphic conditions and was affected by top to the SE ductile shearing. The Central Unit, placed under the Northern Unit, preserves high-pressure Silurian eclogitic assemblages retrograded by high- to medium-temperature and, finally, low-temperature ductile shearing with top to the NW sense of movement (oblique left-lateral). The Central Unit is superposed on the Southern Unit. The latter corresponds to the border of the Ossa-Morena Zone and underwent low-pressure, intermediate-temperature metamorphism synchronous with right-lateral (top to the SE) ductile shearing. The envisaged tectonic evolution is as follows: after a stage of lower Paleozoic rifting, subduction of the Central Unit under the Northern Unit took place in Silurian times. As a result of the crustal thickening, a gravitational instability developed, giving way to left-lateral extensional shearing that affected the entire Central Unit. The combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high-pressure rocks of the Central Unit. This tectonic evolution reveals that the boundary between the Central Iberian and Ossa-Morena zones is a suture of the Variscan belt.