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 degrees 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 degrees 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. Plain Language Summary In subduction zones, among the rocks that constitute the accretionary prism are claystones with a peculiar texture called "scaly clays." We investigated these rocks preserving their original scaly texture at deformation and temperature conditions typical of those occurring in the frontal part of subduction zone. In the deformed samples, only when a contact between two different scaly clays is present and it is deformed at the highest pressure and temperature, a decrease of strength occurs. This behavior is related to the simultaneous presence of a pre-existing lithological contact across the sample and by the release of water by dehydration of the "scaly clays." The process, reproduced in the laboratory, might occur at a larger scale and control deformation within the natural subduction zones.