Invited research articlePost-collapse evolution of a coastal caldera system: Insights from a 3D multichannel seismic survey from the Campi Flegrei caldera (Italy)
Introduction
Caldera-forming explosive eruptions are considered as one of the most catastrophic natural event to affect the Earth's surface and human society (Rampino, 2002, Self, 2015). Also in their post-collapse phase, a large number of calderas remain active as evident by caldera resurgence (Smith and Bailey, 1968, Acocella et al., 2001, Cole et al., 2005), short-term episodes of unrest involving seismicity, ground deformation, and hydrothermal activity (Newhall and Dzurisin, 1988, Acocella et al., 2015), as well as volcanic intrusions and eruptions (Cole et al., 2005). Such active calderas pose high concern for imminent volcanic activity. Major structural features such as caldera ring-faults or regional tectonic fractures represent preferential magma ascent pathways (Moore and Kokelaar, 1997, Saunders, 2004), thereby depicting potential post-collapse eruption sites. Hence, the acquisition of veridical information on the structural framework of calderas is crucial to be able to assess the location and timing of future eruptions. Moreover, understanding the past volcanic eruption history, including the areal distribution and volumes of products from former eruptions, is essential for a reliable hazard and risk assessment. However, to fully comprehend such a staggeringly complex system, a detailed 3D investigation of the subsurface is indispensable.
The partly submerged Campi Flegrei caldera (CFc), located in southern Italy, represents one of the world's most active calderas (De Natale et al., 2006, Del Gaudio et al., 2010) and, thus, provides an ideal natural laboratory to study post-collapse evolutionary processes at an active caldera. Generally, the CFc is regarded as nested-caldera system formed by two major eruptions namely the Campanian Ignimbrite (CI) and the Neapolitan Yellow Tuff (NYT) eruptions at 39 and 15 ka, respectively (e.g. Rosi et al., 1983, Barberi et al., 1991, Orsi et al., 1996, Deino et al., 2004). Since the last caldera-forming event related to the NYT eruption, the CFc has been modified by various post-collapse processes including long-term caldera resurgence (Orsi et al., 1996, Acocella, 2010), ~ 60 volcanic eruptions (Di Vito et al., 1999), sea-level variations (D'Argenio et al., 2004b, Steinmann et al., 2016), as well as recent unrest since the 1950s (Del Gaudio et al., 2010, Chiodini et al., 2012). In particular, the linkage between the magmatic and hydrothermal systems as well as their contribution to the recent unrest episode remain a matter of discussion, thereby impairing a reliable evaluation of the eruption potential and assessment of volcanic risks (e.g. Lima et al., 2009, D'Auria et al., 2015, Chiodini et al., 2016). A detailed 3D understanding of the CFc architecture in particular – but also of calderas in general – is scarce in the current literature; however, essential for unravelling the interconnection between the deep magmatic-hydrothermal systems and the surface as well as potential eruption pathways. In fact, the caldera's subsurface structure including the location of the main caldera ring-faults – in particular their offshore continuation – remains debated and inhomogeneous (Orsi et al., 1996, Bellucci et al., 2006, Perrotta et al., 2006, Acocella, 2008, Vitale and Isaia, 2014, De Natale et al., 2016). Even though, the approximate offshore extent of the caldera margin could be circumscribed (Fig. 1) based on geophysical studies including seismic refraction tomography (e.g. Zollo et al., 2003, Judenherc and Zollo, 2004, Dello Iacono et al., 2009), gravity (e.g. Florio et al., 1999, Capuano and Achauer, 2003, Capuano et al., 2013) and magnetic (e.g. Florio et al., 1999, Secomandi et al., 2003, Aiello et al., 2005) measurements, interpretational uncertainties remained due to the dataset's low resolution and, thus, lack of structural detail. More detailed insights into the shallow structures of the submerged CFc were provided by high-frequency 2D marine reflection seismic investigations (D'Argenio et al., 2004b, Milia, 2010, Aiello et al., 2012, Sacchi et al., 2014, Steinmann et al., 2016). These reflection seismic studies stand out due to their high-resolution, however, they lacked spatial coverage, which is crucial in order to fully comprehend a complex caldera system. Moreover, the volume and dispersal of post-collapse volcanic eruptions of the CFc is still controversial and mainly based on onshore field observations and/or borehole data (e.g. Rosi et al., 1983, Orsi et al., 1996, Di Vito et al., 1999), thereby providing only selective insights and neglecting the marine portion, which represents approximately half of the entire caldera structure.
In this study we present a 3D grid of high-resolution multichannel seismic data (Fig. 1B) from the submerged sector of the CFc, providing an ideal combination of high vertical resolution (~ 2 m) and good spatial coverage and, thus, the best means to obtain 3D information on the caldera system. This marine dataset enables a detailed spatial investigation of the structural and stratigraphic framework down to a subsurface depth of 200 m without the challenges posed on land by intense subaerial erosion or urbanization (i.e. inaccessibility). The main aims of this 3D–investigation are to examine the caldera's subsurface structure and post-collapse (< 15 ka) deformational processes, the impact of hydrothermalism and magmatism (e.g. intrusions) on the offshore sector of the caldera, as well as to reassess the volume, dispersal and explosivity of coastal post-caldera eruption. The here presented work complements a previous study from Steinmann et al. (2016), which focused on a conceptual reconstruction of the caldera formation since CI eruption at 39 ka based on five selected seismic lines from the presented 3D grid. While the preceding study addressed fundamental aspects regarding the structural evolution (collapse, resurgence) of the CFc in interplay with sea-level changes in the 2D domain, the here presented work aims at providing detailed, 3D–insights on the shallow caldera structure and post-collapse processes (hydrothermalism, magmatism, volcanic activity, deformation). Moreover, the previously established stratigraphic and structural framework (e.g. seismic units, inner caldera ring-fault, and resurgent dome) could be spatially extended over the entire marine portion of the CFc and a higher stratigraphic resolution in the uppermost interval was achieved.
Section snippets
Regional setting
The CFc is a Quaternary caldera located within the graben-like structure of the Campanian Plain (southern Italy) between the Tyrrhenian Sea and Apenninic Mountain range (Fig. 1A). It is situated in a densely populated region with the city of Naples at its eastern border and the town of Pozzuoli in its centre (Fig. 1C). As proven by recent episodes of unrest, the Campi Flegrei district represents one of the world's most active calderas (Troise et al., 2008). With 360,000 people living within the
Material and methods
Here we present a grid of high-resolution multichannel seismic profiles collected during an oceanographic cruise (CAFE-7/3) on the R/V URANIA in the Gulf of Naples and the Gulf of Pozzuoli in 2008 (Fig. 1B). This study is based on in total 88 seismic profiles of which 48 are part of a densely spaced (75–150 m spacing) N-S oriented grid crossing over the offshore sector of the CFc. Data acquisition was carried out using the high-resolution multichannel seismic system of the Faculty of Geosciences
Results
For the sake of clarity, we use the same nomenclature for the seismic units as established in the preceding study by Steinmann et al. (2016). In total, we refer to four previously identified marine sedimentary units (M1–M4) and two volcaniclastic units (NYT, V2) with four associated unconformities (U1–U4) as well as major structural features such as the inner caldera ring-fault and a resurgent dome with an apical fault swarm. The sedimentary deposits likely consist to some extent of reworked
Discussion
The focus of this study lies on post-NYT-collapse (< 15 ka) caldera processes, hence, the seismostratigraphic interpretation addresses mainly the corresponding units (M2–M4.2, V2–V4). A summary of volcanic activity, sea-level variations and deformational processes in context of the established seismostratigraphy is provided in Fig. 7. For a more detailed analysis of the depositional environment prior the NYT eruption as well as caldera-forming processes associated with the CI and NYT eruptions,
Conclusions
In this study, we presented the first 3D high-resolution multichannel seismic dataset from a partly submerged caldera setting. Our investigation of the marine portion of the Campi Flegrei caldera (CFc) provided novel insights into the caldera's shallow (< 200 m) subsurface structure and post-collapse (< 15 ka) deformational processes, the manifestation of magmatic and hydrothermal processes in the shallow subsurface, as well as the volume, dispersal and explosivity of coastal post-collapse
Acknowledgements
The seismic dataset was acquired during the CAFE-7/3 expedition funded by the Italian Research Council through the CNR Shiptime Programme (Oceanographic Cruise_CAFE_07), seismic acquisition was supported by German Research Foundation (DFG) within the ICDP priority programme (Grant No. SP296/30-1). Data processing and analysis was funded through the DFG's ICDP priority programme (Grant No. SP296/34-1; SP296/34-2). Additional support was provided by the Bremen International Graduate School for
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