A model of strain localization in porous sandstone as a function of tectonic setting, burial and material properties; new insight from Provence (southern France)

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 within the bands differ and can be expressed as functions 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 strain localization is favored in an extensional regime and is characterized by shear bands with a large shear to compaction ratio and a small conjugate band angle. In contrast, distributed strain is favored in a contractional regime 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 in a frictional porous granular material. We propose a model of strain localization in porous sands as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure. Our model suggests that stress reduction, inherent to extensional regime, favors strain localization as shear bands, whereas stress increase during contraction favors development of compactional bands.

► Three types of deformation bands sets formed under extension and contraction, in two different sands and at shallow depth.
► These bands sets differ in their spatial distribution, their degree of cataclasis and the obliquity of force chains.
► Ratio of shear to compaction for shear-enhanced compaction bands is 1, compactional shear bands is 10, and shear bands is 25.
► Stresses were quantified as a function of Earth pressure coefficient at rest, burial depth and tectonics environment.
► Strain localization in sands reflects burial conditions, tectonic stresses and material properties.