The impact of unloading stresses on post-caldera magma intrusions

open6siAcknowledgments. This research was supported by the GFZ German Research Centre for Geosciences and the ERC Volcapse project, and by the program Forschungsstipendium für Doktorat from Deutscher Akademischer Austausch Dienst DAAD awarded to AG. AG and SHH wish particular thanks to members of the Liverpool MAGMA Laboratory Elliot Wood and Simon Martin for their assistance in the development of this work. SHH acknowledges support from the Malaysian Government and the National University of Malaysia. AG especially thanks to Mehdi Nikkhoo for long discussions and advice related to this work and other topics and for help with conducting earlier models. We thank Steffi Burchardt and Craig Magee for thoughtful reviews which improved the manuscript, and we thank Tamsin Mather for editorial support.Calderas represent morphological depressions several kilometers in diameter, and the unloaded crustal stresses they produce can form rapidly (e.g. Pinatubo, 1990) or slowly (e.g. Hawaii, 2018). Active calderas are known as sites of persistent magma intrusions, and yet the dynamics of their shallow plumbing system is not well constrained. We use scaled laboratory experiments to study how experimental intrusions are created beneath a caldera by injecting dyed water (magma analogue) into the base of an elastic gelatin solid (crust analogue) with a cylindrical cavity in its surface to mimic a caldera-like topography. The evolving dike geometry and stress field were qualitatively determined using polarized light, and digital image correlation allowed the incremental and total strain to be quantified by tracking passive-tracer particles in the gelatin that fluoresced in a thin 2D vertical laser sheet. Our results show that the unloaded stress field from a caldera can cause a divergence of vertical dikes, and leads to circumferential dikes and cone sheets. When the caldera was large the initially vertical dike became arrested, then grew laterally via circumferentially-propagating en echelon segments; these eventually joined to complete a cone sheet that was parallel to, but extended outside and beneath, the large caldera. When the caldera was small, a circumferential dike erupted, producing a short fissure which was outside, but parallel to, the caldera. We suggest that the distinct curved geometry, velocity, strain and stress characteristics of circumferential dikes and cone sheets can be used to interpret the origin and growth of post-caldera magmatism and the likelihood of eruption in caldera systems.openGaete A.; Kavanagh J.L.; Rivalta E.; Hilmi Hazim S.; Walter T.R.; Dennis D.J.C.Gaete A.; Kavanagh J.L.; Rivalta E.; Hilmi Hazim S.; Walter T.R.; Dennis D.J.C