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 b etween the Central Iberian and Ossa-Morena zones of the Iberian Massif (Variscan belt). The three units have been named the Northern Unit, C entral Unit, and Southern Unit. The Northern Unit corresponds to the b order of the Central Iberian Zone; it evolved at low-temperature and i ntermediate- or low-pressure metamorphic conditions and was affected b y top to the SE ductile shearing. The Central Unit, placed under the N orthern Unit, preserves high-pressure Silurian eclogitic assemblages r etrograded by high- to medium-temperature and, finally, low-temperatur e ductile shearing with top to the NW sense of movement (oblique left- lateral). The Central Unit is superposed on the Southern Unit. The lat ter corresponds to the border of the Ossa-Morena Zone and underwent lo w-pressure, intermediate-temperature metamorphism synchronous with rig ht-lateral (top to the SE) ductile shearing. The envisaged tectonic ev olution is as follows: after a stage of lower Paleozoic rifting, subdu ction of the Central Unit under the Northern Unit took place in Siluri an times. As a result of the crustal thickening, a gravitational insta bility developed, giving way to left-lateral extensional shearing that affected the entire Central Unit. The combined action of oblique thru sting at tile front of the wedge and oblique extensional shearing at t he rear caused the exhumation of the high-pressure rocks of the Centra l Unit. This tectonic evolution reveals that the boundary between the Central Iberian and Ossa-Morena zones is a suture of the Variscan belt .