Ring fault-related deformation mechanisms in active calderas: new insights from a deep section of the Mt. Epomeo Green Tuff resurgent block (Ischia Island)

In active calderas, deformation structures associated with collapse are seldom exposed at the surface. One of the deadliest landslides on the island of Ischia (southern Italy), which occurred in 2022 at Mt. Epomeo, exceptionally exposed these structures. This work presents a field study of a ring fault zone associated with caldera collapse during the Mt. Epomeo Green Tuff eruption (MEGT; 62–56.5 ka). The subsequent resurgence of the central part of the caldera furnished the unique opportunity to investigate these rocks and structures. The studied outcrops expose the deepest rocks filling the caldera floor, corresponding to marine deposits (Cava Celario unit, ECV) buried below the MEGT sequence and exhumed during block resurgence. We carried out stratigraphic, structural, and palaeoecological investigations on these deposits, which crop out in the Cava Celario engraving, located along the northern slope of Mt. Epomeo, recently exposed following the dramatic landslide on 26 November 2022. The stratigraphic survey indicates that the ECV unit is composed of two members, separated by a reworked magmatic and pyroclastic debris flow deposit. Lithified varved sediments characterize the lower member (ECV1), whereas the upper member (ECV2) is a massive deposit, both of which are formed by hydrothermally altered, very fine, reworked volcanic rocks. The structural analysis reveals that the ring fault zone is characterized by deformation structures, including folds and faults, the latter defined by both reverse and normal kinematics, suggesting a strain field associated with gravitational collapse, as observed in caldera formation. Finally, studying the fossil content and the inferred paleoenvironment of the ECV unit enabled us to reconstruct the vertical displacement curve of Mt. Epomeo before and during the block resurgence (from ~ 70 ka to the present). The upper member ECV2 terminates with a turbiditic layer containing fossils that indicate an offshore environment and an inferred paleo-bathymetry of 100–200 m b.s.l. The reconstructed vertical displacement indicates first subsidence during the ECV deposition, followed by an uplift (doming) predating the initial rapid subsidence associated with the caldera formation. After that, Mt. Epomeo experienced a general uplift (block resurgence) interspersed with subsidence periods and volcanism along its bounds, accumulating a net uplift of ~ 930 m during the last ~ 56 kyr.