Strain Localisation in Porous Sandstone as a Function of Tectonic Setting, Burial and Material Properties

Abstract

The analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis are different and can be expressed as a function of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear enhanced (reverse) compaction bands, (2) a regularly spaced less dense network of reverse compactional shear bands, and (3) a localized network of normal shear bands. The field data show that the localization of strain is favored in an extensional setting and is characterized by shear bands with a large shear to compaction ratio and a low conjugate band angle. In contrast, distributed strain is favored in a contractional setting and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes. We propose a model of strain localisation in porous sandstone as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure.