THE BRITTLE-DUCTILE TRANSITION IN HIGH-LEVEL GRANITIC MAGMAS: MATERIAL CONSTRAINTS

Abstract

Granitic magmas residing at high levels in the continental crust may be subjected to stress transients on a wide range of time-scales. The origins of the stress transients may be internal, deriving from the volume evolution of the magmatic system, or external such as the rapid release of stresses during tectonic and volcanically derived earthquakes. The strain response of the granitic magma is critically dependent upon its viscosity and the magnitude and time-scale of the application or release of the stress. The response of the magma may be either ductile or brittle. Prediction of the expected response of the magma requires an accurate and reliable method for determining the location in temperature and time-scale of the transition of the response of the melt from ductile to brittle by analogy to the glass transition determined by frequency domain relaxation studies. The application of the Maxwell criterion, which treats the magma as a viscoelastic medium in the linear stress–strain regime, provides such a method. Application of this method requires accurate viscosity and elastic modulus data for the magma. The consequence of intersecting the glass transition in a system where the total strains are significant is brittle failure of the melt phase. Here the material constraints on the brittle ductile transition of granitic magma are reviewed.