Inferring the melt accumulation zone feeding the Auckland Volcanic Field through magma pathway retrogression

<jats:p>Distributed volcanic fields are hazardous and enigmatic geological features due to their puzzling vent location patterns and magma source conditions. Here we propose a new method to constrain the location of the magma source feeding eruptive vents by simulating magma pathways in reverse, from the vents back through the crust to the depth of magma generation inferred from geochemical data. The approach involves first constraining the elastic stress field by numerically modelling the primary stress mechanisms; then, a simplified magma trajectory model backtracks magma pathways, aligning them perpendicular to the local axis of least compression, also accounting for magma buoyancy. We show the potential of this backtracking method at the Auckland Volcanic Field, New Zealand, which consists of at least 53 vents and an undefined source location. We find the dominant stress factors are (weak) regional extension and unloading of the adjacent Hauraki Rift. The computed reversed magma pathways initially descend subvertically beneath the AVF, then tilt and become subhorizontal, ultimately reaching a common location at ∼30-70 km depth beneath the offshore Hauraki Rift. Our results show that the variability in vent distribution within the AVF may derive from perturbations in the ratios between different stress sources. Our physical modeling suggests that magma ascent may be slow at depth, where trajectories are subhorizontal, and relatively fast during the late-stage, near-vertical ascent period, which is consistent with previous geochemical analyses of erupted scoria. Our magma pathway retrogression offers a new approach to illuminate the deep origins of volcanic fields.</jats:p>