2-D thermal modeling of HT–LP metamorphism in NW and Central Iberia: Implications for Variscan magmatism, rheology of the lithosphere and orogenic evolution

Abstract

A model for the P–T–t evolution of NW and Central Iberia is presented to better understand its metamorphic evolution from initial Barrovian stage through HT–LP conditions. The model is constrained by the well-known succession of structural events and geochronology of the Variscan deformation, standard physical parameters, and conservative values for the surface heat flow and radiogenic heat production. The goals are to test an orogenic model derived from observations against a physical reality constrained by the thermal properties of the lithosphere, and to explore the implications for Variscan orogenic evolution. A600km section, parallel to the axial surface of the Central Iberian arc is modeled to the scale of the continental lithosphere. The model outcomes are presented as sections showing the evolution of isotherms over 65Ma, as the variation of the lithospheric geotherm and as P–T–t paths for significant parts of the model section and for points originating at different depths. Profiles of lithosphere strength have been constructed combining the maximum shear stress for brittle fracture with published flow–stress–depth profiles of characteristic rocks and temperatures obtained for the different depths in the model.Quantitative constraints from 2-D thermal models suggest that the metamorphic and magmatic evolution can be explained by the succession of three orogenic stages, namely crustal thickening, a period of thermal relaxation, and subsequent extensional collapse, providing that the first stage doubles the thickness of the continental crust. Thrusting of a thick nappe stack, well preserved in NW Iberia, seems to be the likely cause of the high temperatures reached later during the HT–LP event that occurred during late orogenic extension. The late addition of heat from asthenospheric upwelling remains a possibility, but the model results suggest that it is not necessary to explain the Variscan P–T–t paths.