Brittle behaviour of granitic magma: The example of Puente del Congosto, Iberian Massif, Spain

Abstract

The magmatic structures appearing in the Puente del Congosto granitic outcrop (central Iberian Massif) are described in this work. They are interpreted as the result of a complex interplay between viscous (Newtonian) and brittle behaviour of granitic magma, which allowed the newer magma pulses to intrude and deform older magma batches. A model of Newtonian magma intruding a linear viscoelastic host rock may be extended with some confidence to the case of magma into magma emplacement. The resulting structures combine the characteristics of dykes and diapirs. The formation of large batholiths might be initiated or entirely accomplished by this process. In order to investigate the influence of the general stress conditions characteristic of a given tectonic regime on the strength of granitic magma, an over-simplified macroscopic model considering mixed Newtonian and brittle behaviour has been developed in this work. The brittle response is simulated by the Modified Griffith criterion, so that only rough estimates of the critical differential stresses for brittle magma behaviour can be gained. The results of this model suggest that the brittle response of viscous granitic magmas is possible for any type of tectonic regime (specially under contractional tectonics). A comprehensive, physically sound model for this viscous-brittle behaviour of granitic magma is not yet available. Integrated theoretical, experimental and field-based studies are the best way to arrive at such a complete model.