Natale, Jacopo (2022) Interaction between tectonics and volcanic activity in the Campi Flegrei caldera and northeastern sector of Roccamonfina. [Tesi di dottorato]

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This thesis presents a study on two research areas: Campi Flegrei and Roccamonfina (southern Italy). The evolution of these volcanic areas results from the complex interplay between tectonics and volcanic processes. In particular, in the area of Campi Flegrei, the research was split into two main branches: (1) the reconstruction of an integrated stratigraphy of the last 15 kyr of the marine and volcaniclastic infill of the Pozzuoli Gulf, which represents over one-third of the overall volcanic field, and (2) the reconstruction of the structural pattern in the offshore caldera, to disentangle the formation of the caldera during significant volume eruptions. This study has been performed analyzing a set of over 150 seismic profiles acquired in the Pozzuoli Gulf between 2008 and 2013 that investigate the submerged portion of the caldera down to the first 350 m. The interpretation of seismic data was integrated with the stratigraphic data of the thoroughly studied uplifted marine sequence of the La Starza unit and the most up-to-date tephrostratigraphy of the last 15 kyr. The following results were achieved in agreement with the stratal architecture, stacking patterns, and unit-bounding unconformities. The main results of this study consist of an integrated on-land-offshore stratigraphy, able to explain and detail the several volcano-tectonic processes that followed the collapse of the Neapolitan Yellow Tuff (NYT) eruption. In particular, the almost complete sedimentary record describes that ground deformation is coupled with volcanism and that large erupted magma volumes intra-caldera Plinian eruptions triggered the main ground movements, with a scale-invariant pattern resemblance. Both Agnano-Pomici Principali (12 ka) and Agnano-Monte Spina (4.55 ka) eruptions were followed by significant ground subsidence that favored marine transgression and then followed by roughly rapid uplift, which was locally recorded by erosional unconformities. It is important to stress that the general pattern of ground deformation in the last 10 kyr shows a radial symmetry, as observed today (i.e. bradyseism), with maximum uplift values and subsidence recorded near Pozzuoli town. The stratigraphy allowed to detail the stratigraphic position of debated coastal eruptions, such as Banco di Nisida and Nisida, to the East, and Bacoli, Porto Miseno, and Capo Miseno, to the West, respectively. Furthermore, two offshore intrusion features, namely the Bagnoli cryptodome and the Punta Pennata structure, were characterized both time and process-wise. This work is a robust attempt to overwhelm the lack of direct stratigraphic data (i.e., well-logs) within the offshore sector and substantially contribute to the comprehension of the recent evolution of Campi Flegrei caldera. The same seismic reflection dataset was used for the second research branch concerning the offshore sector of the Campi Flegrei caldera. The aim was to thoroughly reconstruct the structural framework of the Campi Flegrei offshore and unravel a potential contribution from the tectonic field. As an outcome, two main fault systems were recognized: the ring fault system, furtherly divided into inner, medial, and outer fault zones, and the dome fault system. The ring fault system is a 2.5 km wide arcuate ring fault zone, massively involved by fluid uprise, which mainly down threw the central part of the caldera, joining with the continental structural highs of Posillipo and Monte di Procida, to the East and the West, respectively. Stratigraphic analysis corroborates that the collapse has occurred in two distinct episodes, first with the Campanian Ignimbrite (CI) eruption (40 ka), and then the same structures have been reactivated during the NYT collapse (15 ka), although with a lower amount of subsidence, ranging from one-third to one-quarter. For the NYT collapse, the structural analysis suggests that the process not only reactivated the main ring fault of the CI collapse but also occurred in a differential manner, with the sinking proceeding from outside-to-inside, showing hints of fault growth on the external rims, whereas depicting a late collapse on the innermost rim, in agreement with on-land studies. The dome fault system develops in the central part of the caldera, and it is the offshore continuation of a complex array of faults accommodating the volcano-tectonic collapses of the central sector. In particular, this area is a NE-SW to NNE-SSW trending, piece-meal-like high-angle normal fault array that accommodates the sudden subsidence that closely followed the Plinian eruption of Agnano-Monte Spina (4.55 ka). The structural analysis points out that this collapsed area is associated with discrete episodes of volcano-tectonic collapses instead of being ascribed to analog models-like resurgence-related graben formation. Furthermore, over 220 fault plane data have been analyzed with the radial analysis, pointing out the influence of N-S tectonic field on the formation of the CI caldera and for volcano-tectonic collapses, as furtherly supported by the systematic reactivation of adequately oriented faults (i.e., E-W), and the formation of grabens within the fault zones. This work substantially contributes to the comprehension of the overall caldera structure and greatly addresses the volcano-tectonic processes following the major caldera collapse. Finally, this work highlights how the offshore fault zones may connect the surface to the deep-seated hydrothermal system, conveying considerable amounts of fluids on a much larger area than that currently monitored, and suggest that this area should not be ruled out as a potential eruption site. Finally, as concerned the study conducted in the NE sector of Roccamonfina volcano, the aim was to investigate a beautifully exposed magmatic dike within an active cement quarry on the Mt Cesima ridge. During three years of excavations, the quarry cultivation progressively exposed more and more outcrops along the several excavation steps and fronts. Systematic structural and UAV-based (Unmanned Aerial Vehicles) surveys shed light on the complex interaction between preexisting and syneruptive fault zones, with dike intrusion and fissure eruption. In particular, the main outcome of this work is that a reasonably large magmatic dike of lc-tephrite composition has intruded within an E-W fault zone during a rifting event characterized by N-S extensional direction. The dike, minor segments, and fingers intruded within a Mesozoic limestone host rock, overlain by early Pleistocene slope breccia (“Breccia Mortadella”). Its lateral path was deflected during its propagation by a pre-existing NE-SW cataclastic fault zone. A large amount of extension and the vicinity to the surface caused the dike to thicken, reaching a width exceeding 25 m. Nonetheless, the dike had managed to reach the surface and feed an Icelandic/Strombolian eruption that dispersed in the vicinity a 30 m thick pyroclastic succession constituted by scoriaceous spatters and lapilli. Large ballistics of carbonate lithics, lava blocks, and metric spatters characterize the proximal facies of this deposit. The eruption also built a series of welded spatters, ramparts, and cones, which are barely preserved from erosion. One of the most remarkable outcomes of this work is that the dike products overlay a series of paleosols and trachyte Plinian fallout deposits ascribable to the White Trachytic Tuff (WTT) eruptions of Roccamonfina, whose younger age is 230 ka. Furthermore, a thick reddish paleosol interposes between that pumice-lapilli layer with the proximal Strombolian succession, suggesting to have occurred way after it. The whole succession, including the host rock and Strombolian tephras, are intensely faulted, with a prevailing contribution from N-S and E-W faults. This tectonic phase is likely related to the uplift phase that exhumed the carbonate ridge of Mt Cesima, giving its present-day configuration. In previous petrological studies, these products were ascribed by inference to Epoch 1 of Roccamonfina, i.e., older than 350 ka due to similarity with the HKS products erupted in the early stages of the volcano activity. However, this study demonstrates that this remarkable extensional tectonic pulse occurred, hinted by the dike intrusion, to be younger than the last Plinian eruptions of Roccamonfina, the WTT, whose composition belong to the less alkaline KS series. Therefore, this study suggests that the rift-related extensional pulse managed to melt and involve HKS magma in this rejuvenation of the activity. Finally, these results open the way to the comprehensive understanding of volcano-tectonic processes in the Tyrrhenian margin of the Campania coastal half-graben system.

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