Stress-controlled reaction pattern in the layered lower crust: Field evidence

Abstract

Stress can strongly modify the mechanical and transport properties of rocks. This effect is particularly important for metamorphic equilibria in subduction zones where metamorphic reactions and deformation are often concomitant. However, the impact of stress on the propagation of high pressure metamorphic reactions remains largely under-explored. The island of Holsnøy (Norway) shows incipient eclogitization affecting layered continental granulites along shear zones and puzzling finger-shaped structures. While eclogite shear zones suggest that reaction progress is controlled by strain, a mechanism is still required to explain the propagation of finger-shaped reaction fronts in adjacent low-strain domains. Here, we present a detailed structural analysis of the partially eclogitized Holsnøy massif that highlights the relationship between fingers and shear zones in the anisotropic granulite. We show that these structures are not randomly distributed. Finger-shaped eclogite fronts preferentially propagate along the granulite foliation when layering is at high angle to the local maximal principal stress σ₁. This feature can be described using the Damköhler number from the theory of reactive transport. Conjunction of anisotropic eclogitization kinetics and anisotropic stress state actually controls the possible development of finger-shaped reaction fronts.