Exhumed caldera structures reveal shallow syn-eruptive dyke emplacement in caldera-forming eruptions of Ischia Island

Nested structures are a common feature of large calderas, but scientists often infer their existence only through indirect observations, as later eruptive deposits cover the ring-fault zones that bound the different collapsed blocks. The inherent structural complexity of caldera systems makes them challenging to investigate directly. The Island of Ischia (southern Italy) provides a unique opportunity to examine caldera-related structures, such as ring faults and dykes, because block resurgence has uplifted the caldera floor, exhuming its deepest rocks and structures. Due to the dramatic landslide that occurred in 2022 at Casamicciola, a series of previously unobserved dykes hosted in the Mt. Epomeo Green Tuff deposits (MEGT, 62-56.5 ka) were exposed, offering an extraordinary opportunity to study their geometries, intrusion mechanisms, and petrological significance in relation to the dynamics of caldera-forming eruptions. These dykes, located along the northern slope of Mt. Epomeo, situated in the central part of the island, exhibit various shapes, including ball-chained structures, ramp-flat geometries, and orthogonal strands. They are cm-thin and meters long, intruding in the pyroclastic deposits of the intermediate part of MEGT. A notable feature is the presence of a cataclastic shell that encases the cores, composed of fragmented tuff and crystals within a glassy matrix. Petrological and geochemical analyses indicate that the dyke compositions are consistent with those of the Monte Epomeo Green Tuff (MEGT) eruption, thereby linking them to the last phase of the caldera-forming eruption. Our findings unravel the connections between these dykes and the MEGT eruption, identifying this fault zone as a ring-fault zone of the MEGT caldera and, in turn, providing new insights into the nested caldera structure of Ischia Island and the role of the ring-fault zone during subsequent block resurgence. We propose a model for the origin of the dykes, involving circumscribed volatile-poor injections approaching the surface from a larger feeder dyke that fed the MEGT eruption. These structures, together with the deformation structures identified in the marine deposits beneath the tuff sequence, provide compelling evidence that this area represents a segment of the MEGT caldera ring-fault zone formed during the caldera-forming eruption, and that caldera block-resurgence has been accommodated by the inversion of this ring-fault zone. • We studied obsidian dykes hosted in the Mt. Epomeo Green Tuff in the Ischia Island caldera • Geometric features indicate they formed near the surface and that magma movement was horizontal • A particularity of these dykes is the presence of a cataclastic shell that encases the cores • Petrological analysis shows that they have the same major and trace elements chemistry as the MEGT ignimbrite • Dykes are associated with the last phase of the caldera-forming MEGT caldera-forming eruption