Flow stress, strain rate and effective viscosity evaluation in a high-pressure metamorphic nappe (Cabo Ortegal, Spain)

High-pressure deformation and metamorphism in the Cabo Ortegal allochthonous nappe pile (NW Spain) occurred during eo-Hercynian (395-365 Ma ago) subduction/collision. On the basis of thermobarometry, geochronology and a combined petrostructural study of deformed quartz veins from eclogite-bearing deep crustal units, deformation began under confining pressures of 1.5-1.7 GPa, temperatures well above 600 degrees C, low estimated differential stress (10 MPa, based on palaeopiezometry) and slow estimated strain rates (10(-15)-10(-13)s(-1)) Localized deformation continued to lower temperatures, c. 520 degrees C at higher differential stresses (greater than or equal to 25 MPa) and faster strain rates (between 10(-14) and 10(-12)s(-1)). The effective viscosity evolved from 1.8 x 10(19.7+/-1.1) to 1.0 x 10(18.7+/-0.8) Pa . s during cooling and deformation localization. Shear-zone-parallel displacement rates in the allochtinonous nappe pile (an initially 2.5-km-thick movement zone dipping 30 degrees) varied from 0.79 to c. 7.9 mm/year. Exhumation rates of 0.41-4.11 mm/year accompanied cooling from 800 +/- 45 degrees C to 325 +/- 50 degrees C at a rate of c. 17 degrees C/Ma. Exhumation was initially driven by compression and later by extension. These thermomechanical results support rheological models of the lithosphere depicting a spatially heterogeneous medium composed of high-viscosity and relatively undeformed regions separated by low-viscosity and strongly deformed zones.