Intra-arc and back-arc volcano-tectonics: Magma pathways at Holocene Alaska-Aleutian volcanoes

The reconstruction of magma pathways at active volcanoes is of paramount importance for the comprehension of their structure and for geohazard assessment. Magma plumbing systems at volcanic arcs may be particularly complicated since the magma rises along fractures that can be consistent with the coeval regional state of stress, the local state of stress, or can form dykes that instead follow pre-existing structures. Magma path orientation can be stable over time or can vary as the consequence of external events like large earthquakes or important modifications in volcano morphology. In order to advance understanding of these issues, we reviewed all available information on the Holocene volcano-tectonics of the Alaska-Aleutian arc and back-arc zones, based on published seismological, interferometric and geological-structural data, geological maps, and official reports. We completed our review with some new measurements of Holocene eruptive fissures, faults, dykes, and morphometric characteristics of pyroclastic cones and volcanic domes aimed at better defining the possible shallow magma paths of the recent-active volcanoes. Finally, we reviewed the possible parameters and models that explain the path configurations. At 32 volcanoes, magma paths strike NW-SE, perpendicular or oblique to the arc but parallel to the regional greatest principal stress. At 20 volcanoes magma paths are parallel to the arc, and 19 volcanoes form rows of coalescent cones that also suggest ascent of magma parallel to the arc. Eight volcanoes display both directions (normal and parallel to the arc), and seismological data indicate that at some volcanoes there has been a rotation of the magma pathway over time. Integration of all data shows that the regional ambient tectonic stress field promotes dyke intrusions normal to the trench. Dykes can also intrude parallel to the trench following stress unclamping from large earthquakes. Trench-parallel dykes and rows of volcanoes can be generated by magma batches that are aligned parallel to the trend of the subduction zone. Once a dyke or a sill is intruded, it locally perturbs the stress field facilitating successive intrusion along a perpendicular direction.