Semi-brittle deformation of anhydrite-halite shear zones simulating mylonite formation

Abstract

A series of new shear experiments on similar volume mixes of anhydrite and halite have been run at 100° and 400°C, at the same constant loading rate and constant confining pressure of 200 MPa. Increasing temperature is found to decrease the viscosity contrast between anhydrite and halite, the “hard” and “soft” phases, respectively. The mechanical response is similar at all temperatures, regardless of volume proportions of mix; an initial high stress steady state portion of the stress/strain curve is followed by a stress drop and then by a second steady state stress interval, at a much lower value. That is nearly equal to the strength of the softer phase, halite. Individual mineral behaviours and deformation mechanisms appear to correlate with specific portions of the stress/strain curve as the fabric(s) evolve with increasing strain. Mechanism(s) of strain accomodation at all temperatures is semi-brittle; deformation mechanisms change gradually with conditions, with shear dominated by stable sliding along R 1 and R 2 shear fracures (especially in mixes of high anhydrite content) at 100°C, while homogeneous ductile flow along S- and C-surfaces is observed at 400°C in all variations of composition. These features are very similar to those observed in natural shear zones.