MICROSTRUCTURAL EVOLUTION AND RHEOLOGICAL BEHAVIOUR OF MARBLES DEFORMED AT DIFFERENT CRUSTAL LEVELS

Abstract

Microstructures from naturally deformed marbles were investigated from a nappe pile with inverted Barrovian metamorphic zones. Detailed microstructural and textural work combined with existing PT data allows us to correlate microstructural types with tectonic events. Type 1 microstructure related to D1 continental underthrusting shows highly asymmetrical grain boundaries, increasing grain size and increasing intensity of lattice-preferred orientation with increasing metamorphic grade. These characteristics suggest that dislocation creep with concommitant grain boundary migration operated during the simple shear dominated underthrusting regime. Paleopiezometric and strain rate estimates show a decrease of flow stress with increasing metamorphic temperature and a strain rate of around 10-14s-1. Thrust related microstructure Type 2 is developed in Upper and Lower Nappes and is characterised by grain size reduction, increase of grain boundary symmetry and weaker textures. Microstructural and textural studies indicate dislocation creep with possible contribution of grain boundary sliding. Sub-grain rotation paleopiezometry and strain rate estimates show a decrease in flow stress with respect to the Type 1 microstructure and strain rate increase to around 10-12 to 10-13s-1. Type 3 microstructure is developed in thrust related shear zones in the Parautochthon and syn-convergent extensional zones in the top of the nappe pile. This microstructure shows a high contribution of diffusion creep indicating low values of flow stress and significant weakening of marble layers. The general feature of studied area is the localisation of deformation into marbles during thrusting in low metamorphic grades while in higher grades marbles are not exploited as lubricating layers.