Tectonic thinning of a crust slice at high pressure and high temperature by ductile-slab breakoff (Cabo Ortegal Complex, northwest Spain)

Abstract

The upper tectonic unit of the Cabo Ortegal Complex (northwest Spain) comprises an ordered rock sequence of ultramafic and mafic rocks and quartzofeldspathic gneisses. The entire assembly forms part of a transitional mafic crust. In an early Paleozoic subduction episode, most of this crust was metamorphosed at eclogite facies and high-pressure granulite facies conditions. Field and microstructural observations suggest that this metamorphism occurred simultaneously with deformation, producing a pervasive tectonic fabric that developed under a strain regime of bulk coaxial deformation. Downdip extension of the crust in the subduction zone is inferred to have thinned the rock sequence. The relatively high pressures and high temperatures (770–900 8C and .1.2–1.7 GPa) during deformation at peak conditions, in addition to the significant thinning of the units, suggest a tectonic setting of ductile slab breakoff as the cause of the intense and widespread deformation.