Heterogeneity-Driven Localization and Weakening in Scaly Clays From a Fossil Accretionary Prism

In accretionary prisms, scaly clays structure might be the result of strain localization and weakening or strain delocalization and hardening. Therefore, it is not clear how they influence the mechanical behavior of the accretionary prism. Here, we investigate the effect of rock fabric on the mechanical properties of scaly clays sampled from a fossil accretionary prism in a range of pressure-temperature conditions typical of burial within accretionary prisms. We performed triaxial experiments using a direct shear configuration on samples in which the natural scaly clay fabric and natural lithological boundaries were preserved, at confining pressures of 10–120 MPa and temperatures of 25–150°C. Samples with homogeneous composition and natural scaly fabrics display strain hardening behavior at all tested conditions. Shearing at high P-T displays lower strength with respect to shearing at low P-T. The samples containing a lithological contact display strain hardening at low P-T and weakening at high P-T conditions. After the experiments, the homogeneous samples show a distributed foliation and short, discontinuous shear surfaces at the contacts with the sample holder, while the samples including a contact developed a through-going shear zone composed of a series of en-echelon shear surfaces. A reduced smectite and water content is observed in the post-deformation samples, suggesting that weakening may correlate with water release during dehydration reactions of clay minerals at 150°C. This shows how both dehydration reactions at high P-T conditions and strain localization along lithological boundaries promote brittle behavior within scaly clays in subduction zones accretionary prisms.