SYN-COLLISIONAL EXTENSIONAL COLLAPSE PARALLEL TO THE OROGENIC TREND IN A DOMAIN OF STEEP TECTONICS - THE SALAMANCA DETACHMENT ZONE (CENTRALIBERIAN ZONE, SPAIN)

The area south of the town of Salamanca is a zone of the Spanish Varis can Belt characterized by a first compressional event which gave rise to steep structures, followed by the development of an extensional duc tile shear zone, more than 4 km thick, that is described as the Salama nca Detachment Zone. The strain associated to the detachment is analyz ed using quartz pebbles in conglomerates. Comparing the measured strai n values with theoretical strain paths, leads to the conclusion that t he deformation was approximately of plane strain type. Quartz c-axis f abrics and kinematic criteria indicate that simple shear was a very im portant component of the deformation, though coaxial components were p robably involved. The extensional character of the detachment is indic ated by the geometry of the metamorphic zones and the metamorphic evol ution. Taken together, the data indicate that the extensional event wa s related to gravitational collapse induced by the thickening of the c ontinental crust, and that it was syn-collisional. The movement of the hangingwall unit was to the southeast, parallel to the trend of the f old belt, and the translation was of the order of one to a few dozens of km. Though important, this is not exceedingly large, and allowed th e preservation of low-grade metamorphic conditions in the hangingwall, preventing it from being affected by extensive brittle tectonic proce sses.