TECTONIC OVERPRESSURE IN COMPETENT MAFIC LAYERS AND THE DEVELOPMENT OF ISOLATED ECLOGITES

Variation in the state of stress during heterogeneous deformation shou ld be reflected in variation in the effective pressure of metamorphic reactions, whether this is mean stress or the normal stress acting acr oss the reacting interface. The magnitude of this pressure variation w ill determine whether it is discernible in the preserved metamorphic m ineral assemblages of heterogeneously deformed rocks. The magnitude of the mean stress difference across a non-slipping interface between tw o materials with viscosity ratio >c. 20:1 is effectively equal to the maximum shear stress for flow in the more viscous material. Progressiv e shortening of the interface results in a higher mean stress in the m ore competent material, whereas extension results in a lower mean stre ss. For high-P/low-T eclogite facies conditions, current experimental data indicate that clinopyroxene- and garnet-rich layers of eclogite s hould 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 (b y a factor approaching the viscosity ratio). Extrusion of material bet ween more rigid plates, which has been proposed as a regional mechanis m of lateral 'continental escape' for both the Alps and the Himalayas, should also be accompanied by a lateral gradient in effective pressur e; otherwise extrusion could not occur. Maximum mean stresses with mag nitudes that are many times the maximum shear stress required for plas tic flow should develop for deformation zones that are long relative t o 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, migh t help explain the occurrence of isolated layers and pods of low-T ecl ogite (<550 degrees C) with estimated peak pressures markedly in exces s of those in the surrounding matrix. It cannot explain the occurrence of isolated high-T eclogites, because at temperatures >c. 550 degrees C, the dramatic weakening of clinopyroxene in the power-law creep fie ld precludes the development of significant overpressures in eclogite layers.