Tectonic overpressure in competent mafic layers and the development of isolated eclogites

Abstract

Abstract Variation in the state of stress during heterogeneous deformation should be reflected in variation in the effective pressure of metamorphic reactions, whether this is mean stress or the normal stress acting across the reacting interface. The magnitude of this pressure variation will determine whether it is discernible in the preserved metamorphic mineral assemblages of heterogeneously deformed rocks. The magnitude of the mean stress difference across a non‐slipping interface between two materials with viscosity ratio >c. 20:1 is effectively equal to the maximum shear stress for flow in the more viscous material. Progressive shortening of the interface results in a higher mean stress in the more competent material, whereas extension results in a lower mean stress. For high‐P/low‐T eclogite facies conditions, current experimental data indicate that clinopyroxene‐ and garnet‐rich layers of eclogite should be very strong and that pressure differences of up to 800 MPa (8 kbar) between competent layer and weaker matrix may be possible. Such high values can be obtained in widely separated competent layers for values of bulk stress in the overall multilayer that are much lower (by a factor approaching the viscosity ratio). Extrusion of material between more rigid plates, which has been proposed as a regional mechanism of lateral ‘continental escape’for both the Alps and the Himalayas, should also be accompanied by a lateral gradient in effective pressure; otherwise extrusion could not occur. Maximum mean stresses with magnitudes that are many times the maximum shear stress required for plastic flow should develop for deformation zones that are long relative to their width (e.g. around 20 times for a width‐to‐thickness ratio of 10). Tectonic overpressure in progressively shortened competent layers, particularly in regions of extrusion between more rigid plates, might help explain the occurrence of isolated layers and pods of low‐T eclogite (<550°C) with estimated peak pressures markedly in excess of those in the surrounding matrix. It cannot explain the occurrence of isolated high‐T eclogites, because at temperatures >c. 550°C, the dramatic weakening of clinopyroxene in the power‐law creep field precludes the development of significant overpressures in eclogite layers.