Imaging strain localisation in porous andesite using digital volume correlation
Résumé
Strain localisation structures, such as shear fractures and compaction bands, are of importance due to their influence on permeability and therefore outgassing, a factor thought to influence eruptive style. In this study, we aim to develop a better understanding of strain localisation in porous volcanic rocks using X-ray tomographic images of samples of porous andesite (porosity = 0.26) acquired before and after deformation in the brittle and ductile regimes. These 3D images have been first analysed to provide 3D images of the porosity structure within the undeformed andesite, which consists of a large, well-connected porosity backbone alongside many smaller pores that are either isolated or connected to the porosity backbone by thin microstructural elements (e.g., microcracks). Following deformation, porosity profiles of the samples show localised dilation (porosity increase) and compaction (porosity reduction) within the samples deformed in the brittle and ductile regimes, respectively. Digital volume correlation (DVC) of the images before and after triaxial deformation was used to quantify the tensor strain fields, and the incremental divergence (volumetric strain) and curl (used as an indicator of shear strain) of the displacement fields were calculated from the DVC. These fields show that strain localisation in the sample deformed in the brittle regime manifested as a ~ 1 mm-wide, dilatational shear fracture oriented at an angle of 40–45° to the maximum principal stress. Pre- and post-deformation permeability measurements show that permeability of the sample deformed in the brittle regime increased from 3.9 × 10−12 to 4.9 × 10−12 m2, which is presumed to be related to the shear fracture. For the sample deformed in the ductile regime, strain localised into ~1 mm-thick, undulating compaction bands orientated sub-perpendicular to the maximum principal stress with little evidence of shear. Taken together, our data suggest that these bands formed during large stress drops seen in the mechanical data, within high-porosity zones within the sample, and within the large, well-connected porosity backbone. Pre- and post-deformation permeability measurements indicate that inelastic compaction decreased the permeability of the sample by a factor of ~3. The data of this study assist in the understanding of strain localisation in porous volcanic rocks, its influence on permeability (and therefore volcanic outgassing), and highlight an important role for DVC in studying strain localisation in volcanic materials.
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