Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan)

Abstract

The permeability of magma in volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2 m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ∼ 1 m wide, highly sheared region with relatively low porosity and permeability, as well as an outer 20 cm wide cataclastic fault zone. The low-porosity, highly sheared rock further exhibits an anisotropic permeability network, with slightly higher permeability along the shear plane (parallel to the conduit margin), and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3 m long by ∼ 9 cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with permeability enhanced by ∼ 3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e. ductile) shear during ascent up to the point of rupture, which was estimated by Umakoshi et al. (2008) at ∼ 500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into and along the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the core of the spine, leaving a slight permanent displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.