Abstract 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.

中文翻译：

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使用数字体积相关性在多孔安山岩中成像应变定位

摘要 应变局部结构，例如剪切裂缝和压实带，由于它们对渗透率的影响以及因此而被认为影响喷发方式的释气因素而具有重要意义。在这项研究中，我们的目标是使用在脆性和延性状态下变形前后获得的多孔安山岩（孔隙度 = 0.26）样品的 X 射线断层扫描图像，更好地了解多孔火山岩中的应变定位。这些 3D 图像首先被分析以提供未变形安山岩内孔隙结构的 3D 图像，该结构由一个大的、连接良好的孔隙主干和许多较小的孔隙组成，这些孔隙要么是孤立的，要么是通过薄的微结构元素连接到孔隙主干上的。例如，微裂纹）。变形后，样品的孔隙率分布分别显示在脆性和延性状态下变形的样品内的局部膨胀（孔隙率增加）和压实（孔隙率减少）。三轴变形前后图像的数字体积相关（DVC）用于量化张量应变场，位移场的增量发散（体积应变）和卷曲（用作剪切应变的指标）由DVC。这些场表明，在脆性状态下变形的样品中的应变局部化表现为约 1 毫米宽的膨胀剪切断裂，其方向与最大主应力成 40-45° 的角度。变形前和变形后渗透率测量表明，在脆性状态下变形的样品渗透率从 3.9 × 10-12 增加到 4.9 × 10-12 m2，推测与剪切断裂有关。对于在延性状态下变形的样品，应变局部化为约 1 毫米厚、起伏的压实带，其方向与最大主应力亚垂直，几乎没有剪切迹象。综上所述，我们的数据表明，这些带是在机械数据中看到的大应力下降期间形成的，在样品内的高孔隙率区域内，以及在大的、连接良好的孔隙率主干内。变形前和变形后渗透率测量表明，非弹性压实使样品的渗透率降低了约 3 倍。本研究的数据有助于了解多孔火山岩中的应变定位，其对渗透性（以及火山释气）的影响，