The control of pore geometry and orientation on compaction localization in triaxially deformed porous lava

The deformation behavior of volcanic rocks is governed by their microstructure (porosity, pore geometry, orientation) and external conditions such as pressure, temperature, and strain rate. At low confining pressures, porous rocks fail by shear fracturing, whereas higher pressures promote ductile processes, including cataclastic flow or compaction band formation. While these processes are well established in sedimentary rocks, compaction localization in volcanic rocks remains poorly understood. Here, we investigate how preferred pore orientation influences the mechanical behavior, compaction band development, and permeability of lava deformed in the ductile regime. We focus on a porous lava from Volvic, France, for which 3D imaging reveals a pronounced pore preferred orientation. Cylindrical samples were cored with the pore major axes parallel (VBY) and perpendicular (VBZ) to the maximum principal stress and permeability measurement direction. Triaxial deformation experiments show that VBY samples require significantly higher stress to initiate inelastic deformation. Post-deformation imaging reveals oblique compaction bands in VBY samples and diffuse, sub-perpendicular bands in VBZ samples, with microcracking being more abundant in VBZ samples, indicating that compaction localization is strongly influenced by pore preferred orientation. Despite similar porosities, initial permeability in VBY samples is an order of magnitude higher than in VBZ samples. Upon deformation, microcracking in VBZ samples reduces flow-path tortuosity and leads to an order-of-magnitude permeability increase, whereas permeability in VBY samples increases negligibly. Our findings demonstrate that preferred pore orientation affects strain localization, mechanical behavior, and fluid transport in lava, with implications for volcanic risk assessment and geothermal energy extraction.