Syn-collisional extensional collapse parallel to the orogenic trend in a domain of steep tectonics: the Salamanca Detachment Zone (Central Iberian Zone, Spain)

Abstract

The area south of the town of Salamanca is a zone of the Spanish Variscan Belt characterized by a first compressional event which gave rise to steep structures, followed by the development of an extensional ductile shear zone, more than 4 km thick, that is described as the Salamanca Detachment Zone. The strain associated to the detachment is analyzed using quartz pebbles in conglomerates. Comparing the measured strain values with theoretical strain paths, leads to the conclusion that the deformation was approximately of plane strain type. Quartz c-axis fabrics and kinematic criteria indicate that simple shear was a very important component of the deformation, though coaxial components were probably involved. The extensional character of the detachment is indicated by the geometry of the metamorphic zones and the metamorphic evolution. Taken together, the data indicate that the extensional event was related to gravitational collapse induced by the thickening of the continental crust, and that it was syn-collisional. The movement of the hangingwall unit was to the southeast, parallel to the trend of the fold 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 the preservation of low-grade metamorphic conditions in the hangingwall, preventing it from being affected by extensive brittle tectonic processes.