uncorrected proof

Journal of Structural Geology xxx (2018) xxx-xxx

Contents lists available at ScienceDirect

Journal of Structural Geology

PR OO F

journal homepage: www.elsevier.com

Relation between alternating open/closed-conduit conditions and deformation patterns: An example from the Somma-Vesuvius volcano (southern Italy) F.D.A. Tramparuloa⁠ ,⁠ ∗⁠ , S. Vitalea⁠ ,⁠ b⁠ , R. Isaiaa⁠ , A. Tadinic⁠ ,⁠ d⁠ ,⁠ 1⁠ , M. Bissond⁠ , E.P. Prinzib⁠ a

Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università degli Studi di Napoli Federico II, Via Cupa Nuova Cintia 21, 80126 Napoli, Italy Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira, 4, 50121 Firenze, Italy d Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Via della Faggiola, 32, 56126 Pisa, Italy b c

ABSTRACT

Keywords: Volcano-tectonics Structural analysis DTM lineament analysis Volcanic dykes

We present the results of a meso-scale systematic structural analysis of fractures, faults and dykes exposed at the Somma-Vesuvius volcano (southern Italy). Observed fractures include: (i) radial and tangential (with respect the caldera axis), sub-metric to metric joints associated with the edifice load and volcano-tectonic activity (i.e. inflation, deflation and caldera collapse stages) and (ii) decameter-scale fractures related to volcano flank instabilities. For the Somma-Vesuvius volcano, preexisting radial joints were commonly reactivated as transfer faults during the caldera formation, allowing different blocks to move toward the center of the collapsing area. Dykes occur with different geometries, including en-echelon structures bounding structural depressions. The orientation analysis of all structures indicates that they are preferentially oriented. Furthermore, we provide a morphological lineament analysis using high-resolution Digital Terrain Models of Somma-Vesuvius. Azimuth and spatial distribution of dykes and morphological lineaments were analyzed for comparison with the old Somma Crater and Gran Cono axes, respectively. Results highlight the overprinting of radial and clustered strain patterns recorded in different volcano-tectonic evolution stages. We suggest a possible deformation evolution model in which structures develop along either radial or preferential trends, highlighting different volcanic conditions: (i) where radial patterns occur, the structures developed during volcanic inflation cycles with a closed magmatic conduit condition whereas (ii) clustered patterns are probably associated with a regional strain field that overcomes the local deformation field, a situation typical in the case of open-conduit activity.

UN CO RR EC TE D

ARTICLE INFO

1. Introduction

Defining deformation patterns in a volcanic edifice is a useful tool for disclosing the stress field acting during the volcano activity, and the role of the local stress field with respect to the regional deformation pattern (e.g. Pollard et al., 1983; Gudmundsson, 1995; Marinoni and Gudmundsson, 2000). Volcanoes characterized by a well-defined, long-lived central conduit, such as Somma-Vesuvius (SV), are mainly characterized by radial deformation patterns of faults, fractures, dykes, and eruptive fissures (e.g. Acocella and Neri, 2009; Vezzoli et al., 2014; Tadini et al., 2017a; b). Such symmetric patterns usually form under uniform stress field conditions, where the maximum principal stress focuses above the magma chamber center. According to several authors

∗ 1

(e.g. McGuire and Pullen, 1989; Pinel and Jaupart, 2003), the main factor driving this phenomenon is the edifice load. Nevertheless, discrepancies from this simple model can exist. A strong tectonic regional stress field can considerably modify the radial pattern (e.g. Odé, 1957; Muller and Pollard, 1977; Nakamura, 1977); this could happen when, for example, there is synchronous activity of a volcanic radial pattern and a well-developed regional deformation field. The resulting strain field is a complex array of structures that can be revealed through specific orientation analyses that separate radial (volcanic) and unidirectional (regional) components (e.g. Quintà et al., 2012). In other cases, the occurrence of mega-landslides (Becerril et al., 2015) or pre-existing anisotropies in the host rock (Gudmundsson, 2006) can locally mask the radial pattern. In the case of SV, a local variation of the radial pattern could result from active gravitational spreading triggered by the

Corresponding author. Email address: [email protected] (F.D.A. Tramparulo) Current address: Laboratoire Magmas et Volcans, Campus Universitaire des Cézeaux 6 Avenue Blaise Pascal TSA 60026 – CS 60026 63178 Aubiere Cedex, France.

https://doi.org/10.1016/j.jsg.2018.05.008 Received 11 December 2017; Received in revised form 3 May 2018; Accepted 10 May 2018 Available online xxx 0191-8141/ © 2018.

F.D.A. Tramparulo et al.


(Rosi et al., 1993). During periods of open-conduit conditions (e.g. S. Maria cycle and the AD 1631–1944 period; see Cioni et al., 2008), the volcanic activity was characterized also by the development of eruptive fissures (Bianco et al., 1998; Ventura and Vilardo, 1999). These structures are located mostly to the south (AD 1760) and the west (AD 1697–1861), and are characterized by N-S and WSW-ENE directions, respectively (Fig. 2). Major fissures are also located on the NW and NE flanks with NW-SE and NE-SW directions, respectively, and ages between 22 and 19 ka BP (Santacroce, 1987; Bianco et al., 1998; Santacroce and Sbrana, 2003). Other minor fissures, hosted in the Gran Cono flanks, range in age between AD 1701 and AD 1899 (Santacroce, 1987; Santacroce and Sbrana, 2003; Acocella et al., 2006; Cioni et al., 2008), and their trends are N-S, E-W and WNW-ESE (Tadini et al., 2017b and references therein).

PR OO F

edifice load and the occurrence of a weak sedimentary layer underlying the volcano (e.g. Borgia et al., 2005; Marturano et al., 2009; Mathieu and van Wyk de Vries, 2009; D'Auria et al., 2014) or by flank failures (e.g. Ventura et al., 1999). This paper aims to describe the detailed deformation state of the SV volcano through a multi-scale approach that combines the orientation analysis of fractures, faults and dykes (collected during several field surveys) and the analysis of morphological lineaments derived from high-resolution Digital Terrain Models (DTMs). The results of this study may shed light on the connection between the current deformation pattern and the complex tectonic and eruptive history of the volcano, characterized by Plinian to effusive eruptions and alternating open- and closed-conduit stages. 2. Geological setting

3. Structural analysis of the Somma-Vesuvius volcano

The SV volcano is located in the southern sector of the Campanian Plain (Fig. 1), a structural depression filled by Pleistocene to Holocene continental, volcanic and marine deposits bounded to the SE, E, and NE by several Meso-Cenozoic carbonate ridges (e.g. Santangelo et al., 2010; Vitale and Ciarcia, 2018 and references therein). The SV is a 1281-m-high stratovolcano whose products (lavas, scoriae and pyroclastic deposits) are ultrapotassic to potassic in composition, including leucititic tephrites, phonolites and trachytes (Barberi et al., 1981; Bernasconi et al., 1981; Santacroce, 1987; Santacroce et al., 2008). Geophysical and borehole data indicate that volcanic activity of SV postdates the emplacement of the Campanian Ignimbrite eruption products, (39.8 ka BP; Giaccio et al., 2017), which represents the major Plinian eruption of the nearby Campi Flegrei caldera recorded in the Campanian Plain. The present shape of SV is characterized by a younger cone, the Gran Cono, localized within an elliptical caldera oriented roughly E-W. This edifice is constrained to the north by an arc-shaped scarp, representing the remnants of the old Somma Crater (Cioni et al., 1999). The SV multistage summit caldera, located between ca. 560 and 870 m a.s.l., is the result of multiple caldera collapses that occurred in different areas of the volcano at the times of the four major Plinian eruptions (Cioni et al., 1999). The oldest Plinian eruption of the SV volcano is the 18 ka BP Pomici di Base (Bertagnini et al., 1998). This eruption, whose vent was located between the present Gran Cono edifice and the Mt. Somma scarp (Fig. 2; Cioni et al., 1999; Carniel et al., 2017), resulted in a caldera collapse that truncated the top of the old Somma Crater. Following the last Plinian eruption of SV (the AD 79 Pompeii eruption; Cioni et al., 1992), the Gran Cono started to grow discontinuously, with its present shape and structure defined mostly during the AD 1631–1944 eruptive activity. The oldest products of SV are exposed at the base of the Mt. Somma scarp, and date back to about 30 ka BP, while those belonging to the last cycle of activity (AD 1631–1944) are located on top of the scarp itself (Santacroce and Sbrana, 2003). Several dykes crosscut the volcanic succession exposed along the caldera wall; most of them are sub-vertical, while others are inclined with dip-directions both outward and inward with respect to the caldera center (Porreca et al., 2006). It is widely accepted that the SV volcano is currently characterized by a localized magmatic conduit at depth (Di Stefano and Chiarabba, 2002; Zollo et al., 2002; De Natale et al., 2004). The long-lived conduit has determined the whole volcanic activity, including both small-scale effusive events and large-scale explosive eruptions (Santacroce, 1987). Millennia-long periods of dormancy, with closed-conduit conditions, preceded the largest eruptions (e.g. Rosi et al., 1993). On the other hand, an open-conduit condition was associated with continuous moderate activity with small-scale eruptions (e.g. Cioni et al., 2008). The volcanic evolution, after the AD 79 Plinian eruption, was characterized by alternating periods of open-conduit conditions, with semi-persistent, mainly effusive, activity and periods of closed-conduit conditions preceding the sub-Plinian eruptions of AD 472 and AD 1631

3.1. Methods

UN CO RR EC TE D

Different processing methods have been used to analyze the structural data collected both in the field and by a DTM study. Analyses of fractures and faults, hosted both in lavas and pyroclastic rocks, include the study of the orientations by means of stereographic projections and rose diagrams. Fractures were analyzed as a complete dataset and as sub-groups. The area was subdivided in 200 × 200-m cells and, for each cell, a dataset (Dn⁠ ) of fractures was created. To mark eventual preferred orientations in every dataset, bipolar rose diagrams with azimuth bin intervals of 10° were produced, and the fracture/fault main orientations were identified as the bins with the highest frequency (Table1SM in Supplementary Material). With the aim to unravel a possible difference between the deformation pattern close to and far from the Gran Cono vent, the preferred orientations resulting for every cell were further analyzed, subdividing the area in three concentric circular sectors: (i) an inner circle corresponding to a simplified Gran Cono outline (0 ≤ radius < 1 km); (ii) a second circular sector comprising the Mt. Somma scarp (1 ≤ radius < 2 km) and (iii) an outer ring including the remaining fractures on the distal flanks (radius ≥ 2 km). Furthermore, in order to investigate a possible relation between the fracture development and age of the host rocks, we subdivided the total fracture data into two groups, older (group A) and younger (group B) than the last Plinian eruption (AD 79). Finally, the least principal stress axis (S3⁠ ) was estimated as the maximum concentration of fracture poles through Bingham statistics (Bingham, 1974). This analysis consists in reconstructing three mutually orthogonal symmetry axes. The maximum and minimum concentrations are indicated by the orientations of S3⁠ and S1⁠ axes, respectively. The remaining axis (S2⁠ ) is known as the orientation of intermediate concentration. Generally, detected fractures form one or two sets. In case of poles forming two separated or partially superposed clusters (mixed Bingham distribution), this analysis cannot be applied. In order to identify the different clusters and calculate the S1⁠ , S2⁠ and S3⁠ axes for each one, we used the clustering method proposed by Yamaji and Sato (2011) and Yamaji (2016). Each Dn⁠ dataset was analyzed by means of the software GArcmB (http://www.kueps.kyoto-u.ac. jp/∼web-bs/tsg/software/GArcmB/). Numerical results are listed in the Table1SM (Supplementary Material). Similarly to fractures, to disclose preferred orientations, fault data were studied as a whole and grouped in datasets. Furthermore, in order to establish the orientation of the stress field, they were analyzed through the P–B–T technique (Angelier and Mechler, 1977; Reiter and Acs, 1996–2003). This inverse method provides, for each single fault datum (plane attitude, slip orientation and kinematics), the three principal axes of strain: P (direction of maximum shortening), T (direction of maximum stretching) and B (intermediate axis, orthogonal to the P–T plane). Numerical results are listed in the Table1SM (Supplementary Material).

2


UN CO RR EC TE D

PR OO F


Fig. 1. Geological map of the southern sector of the Campanian Plain and Somma-Vesuvius volcano (after Vitale and Ciarcia, 2018 modified). WGS84-UTM Projection.

Dykes and lineaments were analyzed to determine how their azimuth varies in map view around the old Somma Crater and Gran Cono centers, respectively. This method (e.g. Quintà et al., 2012) consists in

evaluating three angular parameters: (i) the angle α defined as the angle between a line parallel to the north direction, and the line connecting the reference point (either the old Somma Crater or the Gran Cono 3


UN CO RR EC TE D

PR OO F


Fig. 2. Sketch map of Somma-Vesuvius showing dykes (this study), eruptive fissures (from Santacroce, 1987; Bianco et al., 1998), main vents (from Bianco et al., 1998; Paoletti et al., 2016); faults (from this study, Bianco et al., 1998, and ISPRA, 2018); and caldera and cone rims (from Cioni et al., 1999). WGS84-UTM Projection.

vents) and the dyke/lineament position (from 0° to 360°; Fig. 3a); (ii) the angle β, defined as the angle between the north direction and the strike of the dyke/lineament (from 0° to 180°; Fig. 3a) and (iii) the angle δ, which is the absolute value of the difference between α and β. Furthermore, using the δ angle, which ranges between 0 and 90°, the analyzed structures are classified into: i) radial (δ ≤ 30°); ii) oblique (30°≤δ ≤ 60°) and iii) tangential (δ ≥ 60°) groups. By means of the α-β diagram (Fig. 3b–d) the relation between principal directions of regional- and volcano-related stress fields can be evaluated. In the case of regional- or volcano-related patterns, the data will lie along horizontal (Fig. 3b) or oblique (Fig. 3c) lines, respectively. On the other hand, in the case of synchronous activity of regional and volcanic stress fields, the resulting mixed patterns will have sinusoidal shapes (Fig. 3d). In order to provide density contour maps of the α-β diagrams, we used the free OpenPlot software (http://www.openplot.altervista.org). Finally, we analyzed the morphological lineaments extracted by DTMs of the area. Elevation data for SV topography were derived from two different sources: i) the “Ufficio Sistema Informativo Territoriale (SIT)” of the “Provincia di Napoli” (Project “Centro Satellitare Cave della Provincia di Napoli – Ce.CO.SCA”); ii) the “Ministero dell'Ambiente e della Tutela del Territorio e del Mare (MATTM)” with license

Creative Commons 3.0 Italy (CC BY-SA-3.0IT). These models were provided in 5451 ASCII files (3615 collected in 2009 and 1836 collected in 2012), matrices of 500 rows x 500 columns having a cell size of 1 m and geocoded into WGS84 UTM ZONE 33 reference coordinate system. The elevation data of the MATTM instead were acquired during ALS flights occurred in 2008. Also, these data were provided in matrices with cell sizes of 1 m. We define “lineament” as any natural linear pattern visible by the shaded reliefs of the study area. Lineaments that clearly represent anthropogenic features (road cuts, terraces, buildings, etc.) or irregularities in the DTM (due to acquisition errors) were not included in this investigation. To reduce noise within the data, we selected an area of 71 km2⁠ characterized by more smooth topography and weak urbanization. The investigation scale was defined to 1:10,000. Natural lineament extraction involved two steps. First, all the clearly evident natural lineaments were traced on four shaded reliefs by illuminating the DTM with six different settings of azimuth from N (0°, 45°, 135°, 180°, 225° and 315°) and maintaining constant the illumination angle of 45°. Next, to magnify possible directional trends, three different gradient filters (N, NE and NW) were applied to all the shaded reliefs (to enhance the visibility of the features with directions opposite to that of the filter), combining a directional 3× 3 convolution kernel to each matrix of the shaded reliefs.

4


PR OO F


Fig. 3. (a) Scheme showing the construction of α and β angles. Relations between stress-field principal directions and angular coordinates (α and β) diagram in the case of (b) regional stress field; (c) volcanic stress field; (d) synchronous activity of regional and volcanic stress fields. The line colors in the sketch map are related to the line colors in the α−β diagrams. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

for group A (Fig. 6h) preferred NE-SW, E-W and NW-SE trends, while for group B (Fig. 6i), in addition to the same trends of group A, also the N-S direction occurs.

UN CO RR EC TE D

3.2. Fractures Fractures in lavas are generally high-angle structural elements (Fig. 4a, c), mostly orthogonal to the lava boundary surfaces, which form two orthogonal sets (Fig. 4a). Locally, plumose structures are present along the fracture walls (Fig. 4b). Fractures within dykes form roughly square or hexagonal systems made up of two and three sets, respectively (Fig. 4c), characterized by dip angles ranging from sub-horizontal to sub-vertical, usually orthogonal to the dyke margins (Fig. 4c). Other fractures, whose lengths are up to several tens of meters, crosscut the Mt. Somma scarp, and are either radial (Fig. 4d) or tangential (Fig. 4e) with respect to the caldera axis. Radial fractures also occur along the inner crater of the Gran Cono (Fig. 4f). Two decametric fractures, crosscutting the AD 1929 pahoehoe lavas along the eastern rim of the caldera (Fig. 4g–i), are N-S trending and show a total length of ∼72 m. The easternmost SW-dipping fracture is characterized by more than 1 m of horizontal displacement, a few centimeters of vertical slip, and a hanging-wall angular rotation of a few degrees (Fig. 4i). The westernmost fracture shows, instead, greater length but less than 1 m aperture. Furthermore, ash and pyroclastic deposits exposed in the quarries located along the SV flanks (Fig. 1) are crosscut by fractures that typically form two orthogonal sets. These fractures mainly developed within the fine-grained deposits (e.g. compact ash layers). The attitude of fractures in the lavas is consistent with an almost random pattern, with a slight prevalence of both E-W and NE-SW strike directions (Fig. 5a). In contrast, fractures within pyroclastic rocks show three main strike directions: NE-SW, NW-SE and E-W (Fig. 5b). The rose diagram (Fig. 5c) of the main directions of all fracture datasets (collected both in lavas and pyroclasts) marks the prevalence of E-W, ENE-WSW and NNE-SSW strike directions. The stereographic projection of the least principal stress axis (S3⁠ ), calculated by means of the clustering and Bingham analyses (see the methods paragraph) for all fracture datasets, indicates three main extensional directions: NNW-SSE; NW-SE and WNW-ESE (Fig. 5d). Fig. 6a and b shows both fracture and S3⁠ strike directions in the map as oriented segments. The Gran Cono area (Fig. 6a) is characterized by fractures with a main E-W strike direction and subordinate N-S, NE-SW and NW-SE strike directions (Fig. 6e). For the second circular area, the orientation analysis marks the dominant NNE-SSW and E-W strike directions (Fig. 6f); whereas for the external sector the main directions are ENE-WSW and WNW-ESE (Fig. 6g). The rose diagrams, obtained by subdividing the whole fracture data into two groups (A older than AD 79 and B younger than AD 79), indicate

3.3. Dykes

The dykes exposed along the Mt. Somma scarp and the crater walls of Gran Cono are characterized by different values of length, thickness, orientation, and geometry. A total of 100 dykes were surveyed in the field (Fig. 7a). They occur as single (Fig. 8a), parallel (Fig. 8b) or en-echelon (Fig. 8c–e). Dykes range in dip from sub-vertical to moderately and gently dipping both inside and outside the caldera. In some places, they appear as sills or with ramp-flat geometry (Fig. 8a). Two en-echelon conjugate dyke sets bound a NE-SW-striking graben (Fig. 8c). In the western sector of the Gran Cono (Fig. 8b), a main N-S sub-vertical dyke and a parallel minor structure crosscut the AD 1855–1872 volcanic deposits (Santacroce, 1987). Dyke attitude analysis indicates a dominant NE-SW strike direction (Fig. 7b); however, normalization of all data to the length of the dykes shows both NE-SW and NW-SE strike directions form the main directions (Fig. 7c). Taking into account that the formation of these dykes was synchronous with the eruptive fissures located on the NW and NE flanks (Porreca et al., 2006), whose ages are older than 18 ka BP, it follows that their formation predates the age of the first caldera collapse (Cioni et al., 1999). With this in mind, we investigated the relation between the directions of the dykes located in the Mt. Somma scarp and the axis of the old Somma Crater (Fig. 7a), through the radial analysis described in section 3.1. The α-β contour map shows that the regional NE-SW trend is the dominant pattern (Fig. 7e), whereas the δ-histogram (Fig. 7d) indicates that 59% of dykes are radial, 24% are oblique, and only 17% tangential. 3.4. Faults

Most of the faults were observed along the Mt. Somma scarp. Slickensides in the lava strata, oriented parallel to the caldera scarp, generally host steps and striations (Fig. 4a). These faults are characterized by high values of pitch, indicating a dominant dip-slip kinematics, whereas oblique kinematics, with pitch angles >45°, may occur along radially arranged planes (Fig. 4a). The larger structures are tens-of-m-long, high-angle normal faults, striking parallel to the caldera scarp and showing displacements up to several meters (Fig. 9b and c). Rare low-angle normal faults also occur, which displace preexisting dykes (Fig. 4d), but they are crosscut by sub-vertical radial fractures and

5


UN CO RR EC TE D

PR OO F


Fig. 4. Mt. Somma scarp: (a) ring fault and reactivated radial joint both showing slickenside striations; (b) plumose structure in lavas; (c) orthogonal sub-vertical cooling joint sets in dykes; (d) radial fractures meter-length and low-angle normal faults; (e) tangential fractures; (f) radial fractures hosted in the Gran Cono crater; (g) satellite image of the macro-fractures in the eastern side of the Mt. Somma scarp; (h–i) pictures of the macro-fractures.

6


UN CO RR EC TE D

PR OO F


Fig. 5. Lower hemisphere, equal-area projections, contour plots and rose diagrams of fractures and S3⁠ axes. Contour intervals (%/1% area)" are as shown in legends.

faults (Fig. 4d). The low-angle normal faults show rare centimeter-thick damage zones, characterized by a higher fracture density (Fig. 9a) with respect to neighboring rocks. The pyroclastic rocks, located along the SV flanks, are crosscut by sparse syn-sedimentary normal faults, as shown by growth strata and soft-sediment deformation localized at the hanging walls (Fig. 9e). Slickenside striations and steps, although very rare, indicate extensional kinematics (Fig. 8d). The faults, hosted both in lavas and pyroclastic rocks, are characterized by a dominant E-W strike direction (Fig. 10a); however, calculation of the main fault directions for every 200 × 200 m cell identifies dominant E-W and N-S directions and secondary NE-SW and NW-SE directions (Fig. 10b). By means of the P-B-T method, we calculated, for every 200 × 200 m cell, the T-axes (extension). Numerical results are listed in the Table2SM (Supplementary Material). The stereographic projection of all T-axes indicates NNE-SSW and NNW-SSE dominant di

rections (Fig. 10c). Finally, Fig. 6c and d shows the maps of fault main directions and T-axis directions, respectively. 3.5. Morphological lineaments To highlight possible connections between the volcano-tectonic activity and the morphology of the SV, we conducted lineament analysis using DTMs of the volcano. Previous analyses on SV were performed to study either volcano-tectonic (Ventura et al., 1999) or geomorphological features (Ventura et al., 2005; Alessio et al., 2013). Following the two steps described in the methods section, a database of 4095 natural lineaments was assembled (Fig. 11). Each lineament has been analyzed using (i) azimuth (expressed in degrees with respect to north); and (ii) spatial location. The orientation analysis, with respect to the volcano center, was calculated as well. The method used for this analysis is the

7


UN CO RR EC TE D

PR OO F


Fig. 6. Maps of (a) fracture main directions; (b) S3⁠ axis directions; (c) fault main directions and (d) T-axis directions. The 200 × 200 m grid is also shown. Rose diagrams of main directions for fractures included in the (e) area 1; (f) area 2 and (g) area 3. Rose diagrams of main directions for fractures included in the (h) deposits older than AD 79 and (i) deposits younger than AD 79. WGS84-UTM Projection.

same as that previously applied to the dykes, assuming as reference point the Gran Cono vent. In order to analyze the relation between lineament orientations and distance from the Gran Cone axis, six different circular areas were identified with radii ranging between 0 and 1, 0.5–1.5, 1–2, 1.5–2.5, 2–3 and 3–4 km. We used more circular areas than the fracture orientation analysis because of the large amount of data uniformly distributed in the study area. The α-β contour maps (Fig. 12) show a superposition between radial and regional configurations. The radial patterns are marked by the oblique alignment, defined by equal values for the α and β angles (point dot lines; Fig. 12); whereas the horizontal pattern, characterized by a constant β-angle (dotted lines; Fig. 12), indicates lineament directions possibly related to the regional stress field. The latter features are characterized, especially for radii 2 km). Grouping the fracture data by age of the host rock (group A with ages older than AD 79 and group B with ages younger than AD

8


UN CO RR EC TE D

PR OO F


Fig. 7. a) Map of analyzed dykes, WGS84-UTM Projection; b-c) rose diagram of dyke directions; d) histogram of the δ-frequency; e) contour map of α (dyke azimuths) vs β diagram (see the text for the explanation); scale color bar indicates the percentage with respect to the maximum density value. R: radial; O: oblique; T: tangential. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 8. Mt. Somma scarp: a) single dyke with ramp-flat geometry; b) two parallel dykes hosted in the AD 1855–1872 deposits of Vesuvius; c) panoramic view of en-echelon dykes; d-e) pictures of the dextral and sinistral en-echelon dyke arrays (the location of these dykes is shown in Fig. 7a).

9


UN CO RR EC TE D

PR OO F


Fig. 9. Mt. Somma scarp: (a) normal fault plane developing a few centimeter-size damage zone; (b–c) high-angle normal faults (∼10 m long) parallel to the caldera scarp. (d) Normal fault plane showing slickenside striations in scoriae (Pollena quarry). (e) Syn-sedimentary normal fault with growing strata hosted in 8 ka Mercato Pumice eruption pyroclastic deposits (Ranieri quarry).

79) shows that group A fractures have NE-SW, E-W and NW-SE trends whereas, in younger deposits of group B, an additional N-S trend is present. Some of the analyzed fractures, orthogonal to the lava/dyke boundaries and showing columnar patterns (Fig. 4c), can be interpreted as cooling joints. For those joints not related to contraction, the occurrence of some preferred orientations suggests that they are related to well-defined volcano-tectonic stress fields. In particular, they commonly show radial and tangential attitudes with respect to the volcano axis. These fractures can form simply, related to the edifice load (e.g. McGuire and Pullen, 1989; Pinel and Jaupart, 2003), or associated with volcanic activity, such as during inflation, deflation or caldera-collapse events (Komuro, 1987; Martì et al., 1994; Stewart, 2006). Furthermore, some pre-existing high-angle fractures can be reactivated as normal and reverse faults during caldera formation in which the central part of the edifice collapses (e.g. Acocella, 2007 and references therein). Finally, the open decametric fractures, observed along the NE flank of the Mt. Somma edifice and characterized by a shear component, can be related to ground instability along the slope. This is vali

dated by the occurrence of tilted hanging walls, which suggests that the shallow sub-vertical fracture planes become listric surfaces at depth, typical of rotational landslides (Varnes, 1978). The results of dyke-orientation analysis indicate a radial pattern, similar to that found by Porreca et al. (2006), in which both ESE-WNW and NE-SW strike directions can be distinguished (anisotropic radial pattern; Acocella and Neri, 2009). These strike orientations are comparable to those that characterize the two eruptive fissures located along the NW and NE flanks, respectively. These fissures erupted between 22 and 19 ka BP (Santacroce, 1987; Bianco et al., 1998), when most of the dykes exposed along the Mt. Somma scarp were probably emplaced (Porreca et al., 2006). On the other hand, the dykes within the Gran Cono, characterized by a N-S strike direction, are hosted in the AD 1855–1872 rocks (Santacroce, 1987). The observed anisotropic radial pattern of dyke directions in SV is a feature observed in several other volcanoes around the world, such as Etna (Acocella and Neri, 2003) and Stromboli (Tibaldi, 2003) in Italy, Fuji in Japan (Takada et al., 2007) and Erta Ale in Ethiopia (Acocella, 2006).

10


UN CO RR EC TE D

PR OO F


Fig. 10. Lower hemisphere, equal-area projections, contour plots and rose diagrams of faults and T-axes. Contour intervals (%/1% area)" are as shown in legends.

Dykes generally appear as single planar features at places with a ramp-flat geometry, but several dykes exposed in the wall of the Mt. Somma scarp were emplaced as conjugate en-echelon segments. This geometry implies that magma flow within the en-echelon apophyses was dominantly sub-horizontal, such as shown by Anisotropy of Magnetic Susceptibility (Porreca et al., 2006). Due to the lack of evidence for shear displacement along the dyke planes, we interpret these structures as formed by mode I opening (e.g. Hudson and Harrison, 1997) in conjugate brittle-plastic extensional shear zones bounding graben structures. The NE-SW direction of the dominant dyke swarm, including en-echelon dykes (Fig. 7a), suggests a main NW-SE extension direction during their emplacement (22–19 ka BP). According to several authors (e.g. Dvorak and Nakamura, 1987; Acocella et al., 2001; Porreca et al., 2006), an open volcanic conduit stage favors the formation of propagating dykes in shallow rocks (Fig. 13a); conversely, a closed-conduit condition increases the magma pressure, triggering volcano inflation and the prevalence of a radial field with the formation of mainly radial fractures (Fig. 13b). Most of the analyzed faults show tangential and radial orientations with respect to the caldera axis. Tangential planes host slickenside striations (Fig. 4a), indicating normal dip-slip kinematics, whereas the radial planes show oblique kinematics (Fig. 4a). We interpret the former structures as ring faults whose development, as observed in several volcanic settings, is related to caldera collapse processes (e.g. Lipman, 1984; Martì et al., 1994; Walter and Troll, 2001). In particular, it is reasonable to assume that these ring faults, defined by strike directions parallel to the Mt. Somma scarp, were formed during the caldera col

lapse related to the 8 ka BP Mercato Pumice Plinian eruption that shaped the present arcuate Mt. Somma (Cioni et al., 1999). On the other hand, radial faults behaved as transfer structures (Fig. 13c and d) that allowed different blocks to independently move toward the center of the collapsing caldera. We suggest that these transfer faults can be the reactivation of preexisting radial fractures formed as result of the edifice load or volcanic inflation-deflation processes (Fig. 13a and b). Hence they predate the 8 ka BP Mercato Pumice Plinian eruption. However, it is not possible to exclude the possibility that some structures were formed also in the inflation-deflation and caldera-collapse stages following this event. Faults were also observed crosscutting pyroclastic deposits on the volcano slopes. Since they are commonly associated with plastic deformation features, we interpret them as synchronous to the eruption, as documented in other volcanic settings (e.g. Campi Flegrei, Vitale and Isaia, 2014; Solfatara volcano, Isaia et al., 2015). Another type of structure is represented by low-angle normal faults. Despite their length, they show small displacements, crosscutting dykes and in turn being cut by radial fractures and faults and ring faults. Low-angle normal faults are probably related to the inflation stage that triggered gravitational instabilities along the volcano flanks (Fig. 13b). Finally, the orientation analysis of the T-axis (Fig. 10c) shows that two main extensional directions (NNW-SSE and NNE-SSW) occur. However, these directions probably are associated with the radial uniform extensional field that occurred during the caldera collapse that formed the present Mt. Somma scarp. Most of measured faults are tangential structures hosted along this arcuate morphological structure (Fig. 6c–d).

11


UN CO RR EC TE D

PR OO F


Fig. 11. Study area and traced lineaments. WGS84-UTM Projection.

that some of the radial lineaments can be the expression of valleys and drainages generally parallel to the slope direction. However, some radial lineaments should be the intersection of the radial fractures, related to the volcano activity, with the edifice topography, as seen at similar volcanoes (e.g. Beneduce and Giano, 1996). The clear separation between radial and clustered configurations based on orientation analysis (Fig. 12) suggests that they were recorded in different volcano-tectonic conditions, although the local and regional stress fields acted simultaneously. This feature is also marked for the Vulture Volcano located on the front of the southern Apennine chain, where middle Pleistocene volcanic activity was synchronous with extensional tectonics affecting the eastern sector of the chain (e.g. Beneduce and Schiattarella, 1997; Schiattarella et al., 2005; Sinisi et al., 2016). We suggest that, even if the local and regional stress fields acted synchronously, the radial pattern prevailed during the volcano inflation (closed conduit), eventually leading to a caldera collapse (Fig. 13b and c); on the other hand, a regional (clustered) pattern, characterized by a prevailing unidirectional extension, was favored by open-conduit conditions (Fig. 13a) during which the radial stress field was weak.

4.2. Alternating open/closed-conduit conditions and deformation mechanisms

At this point, the relation between faults, caldera and main vent centers and eruptive fissures should be considered. The SV eruptive history was characterized by an alternation of open- and closed-conduit stages with four caldera collapses (Cioni et al., 1999). However, the caldera and main vent centers were localized in different sectors of the present caldera, depicting a triangular area bounded by ca. E-W, N-S and NE-SW sides (Fig. 2). Several geophysical studies (e.g. Ciaranfi et al., 1981; Cassano and La Torre, 1987; Bruno et al., 1998; Bruno and Rapolla, 1999) reconstructed the buried faults underlying the volcano edifice and hosted in the Meso-Cenozoic basement. These faults are mainly characterized by the well-known Apenninic (NW-SE) and anti-Apenninic (NE-SW) trends (e.g. Tadini et al., 2017a; b). Our data suggest that, in addition to these two main regional directions, well-marked by the strike directions of the two dyke swarms exposed in the Mt. Somma scarp, N-S and E-W trends should also be considered (Fig. 2). The (i) caldera centers of Mercato Pumice (8 ka BP; Cioni et al., 2008) and Pompeii (AD 79) eruptions, the (ii) vent centers of old Somma Crater and the recent Gran Cono, the (iii) eruptive fissures located in the southern flank of SV and the (iv) dykes within the Gran Cono are all aligned along a N-S direction. The N-S and E-W trends are also well defined in the preferred directions of fractures located close to the Gran Cono and by the rose diagram of eruptive fissures (Tadini et al., 2017a; b). Finally, the lineament analysis carried out on high-resolution DTMs marks a superposition of (i) a clustered pattern, characterized by NNE-SSW and ENE-WSW directions, and (ii) a radial pattern. It is possible

5. Conclusions

This study provides a general framework of meso-scale deformation at Somma Vesuvius volcano. Different kinds of fractures and faults were observed and measured. Fractures are hosted in lava, dykes and pyroclastic rocks; metric-scale fractures generally occur as radial and tangential with respect to the caldera/cone axis. Decametric open fractures, exposed at the eastern SV flank, are related to rotational landslides. Metric to decametric high-angle normal ring faults, with a few meters of displacement, bound the caldera rim. Sporadic decametric

12


UN CO RR EC TE D

PR OO F


Fig. 12. Contour map of α (lineament azimuths) vs β diagram (see the text for the explanation). R: radial; O: oblique; T: tangential. Histogram of the δ-frequency and rose diagram of lineament directions calculated for six different circles (centered on the Gran Cono) enclosing areas (a) 0–1 km, (b) 0.5–1.5 km, (c) 1–2 km, (d) 1.5–2.5 km, (e) 2–3 km and (f) 3–4 km from the Gran Cono center.

Fig. 13. Cartoons showing deformation structures formed during (a) open-conduit condition; (b) close conduit condition with volcano inflation; (c) close conduit condition with caldera collapse.

low-angle normal faults crosscut early dykes on the Mt. Somma scarp, and are, in turn, deformed by radial fractures and faults and ring faults. Finally, centimetric, syn-eruptive normal faults occur in pyroclastic deposits on the SV flanks. Dykes occur as single structures or sets with parallel or en-echelon geometries. Generally, the latter structures are located at the boundaries of structural depressions, suggesting a synchronous normal shear component during their development. Orientation analyses highlighted a clustered arrangement for all the studied structures when analyzed

with respect to their spatial location (this latter performed by subdividing the area in cells and in circular areas) or grouped in two temporal classes (older or younger than AD 79, age of the well-known Plinian Pompeii eruption). Results indicate that there are four dominant directions (NW-SE, NE-SW, N-S and E-W). The well-known Apenninic (NW-SE) and anti-Apenninic (NE-SW) trends are prevalent for structures located in older rocks (>AD 79) and in sectors far from the Gran Cono (>2 km), while N-S and E-W directions are better recorded in younger rocks, mainly in those close to the Gran Cono. The stereographic distri

13



bution of the S3⁠ axis, calculated for every fracture dataset by means of clustering and Bingham analyses, indicates dominant extensions defined by the NNW-SSE; NW-SE and WNW-ESE directions. The orientation of the dykes and morphological lineaments were studied with respect to the old Somma Crater and Gran Cono axes, respectively. These analyses mark a superposition between radial and clustered strain patterns. Part of the lineament radial pattern can be related to the morphological evolution of the volcano or to the edifice load, but we cannot exclude the possibility that some lineaments are expressions of the volcanic activity itself. With this in mind, we envisage that these two dominant patterns (radial and clustered) can be related to different volcanic evolution stages. The radial pattern forms during a closed conduit stage, when the volcanic inflation triggers a uniform strain field related to stress localization above the magma chamber, largely exceeding the synchronous unidirectional regional strain field. On the other hand, the clustered pattern develops during open-conduit activity, when the intensity of radial pattern is very low, and the regional deformation pattern prevails. In the first case, the volcano inflation mainly produces radial fractures that, in a possible subsequent caldera-collapse stage, can be reactivated as oblique faults acting as transfer structures. In the second case, the occurrence of an open-conduit allows the development of dykes with horizontal magma flows and eruptive fissures often forming en-echelon sets. Finally, we suggest that: (i) dykes exposed along the Mt. Somma scarp that indicate a main NW-SE extension were formed before the 8 ka BP Mercato Pumice eruption and caldera collapse that shaped this morphological structure; the age is probably the same as that of the eruption fissures located on the north flank of Mt. Somma dated at 22–19 ka BP; (ii) some of radial fractures that crosscut these dykes were formed before this event, probably related to an early inflation stage, as modelled before; (iii) ring faults, parallel (in the map) to the Mt. Somma scarp and radial fractures and reactivated as transfer faults developed synchronously to the caldera collapse and finally (iv) radial fractures, eruptive fissures and dykes exposed on the Gran Cone walls and decametric open fractures located along the West flank of the Mt. Somma, all characterized by N-S and/or E-W trends, are related to the youngest activity of the volcano.

UN CO RR EC TE D

PR OO F

Alessio, G., De Falco, M., Di Crescenzo, G., Nappi, R., Santo, A., 2013. Flood hazard of the Somma-Vesuvius region based on historical (19-20th century) and geomorphological data. Ann. Geophys. 56, S0434. Angelier, J., Mechler, P., 1977. Sur une méthode graphique de recherche des contraintes principales égalment utilisable en tectonique et en seismologie: La méthode des dièdres droits. Bull. Soc. Géol. Fr. 19, 1309–1318. Barberi, F., Bizouard, H., Clocchiatti, R., Metrich, N., Santacroce, R., Sbrana, A., 1981. The Somma–Vesuvius magma chamber: a petrological and volcanological approach. Bull. Volcanol. 44, 295–315. Becerril, L., Galindo, I., Martí, J., Gudmundsson, A., 2015. Three-armed rifts or masked radial pattern of eruptive fissures? The intriguing case of El Hierro volcano (Canary Islands). Tectonophysics 647–648, 33–47. Beneduce, P., Giano, S.I., 1996. Osservazioni preliminari sull'assetto morfostrutturale dell'edificio vulcanico del Monte vulture (Basilicata). Il Quat. 9, 325–330. Beneduce, P., Schiattarella, M., 1997. Relazioni tra tettonica regionale quaternaria e deformazione vulcanogenica nelle aree Dei Campi Flegrei, Isola di Ustica e Monte Vulture (Italia meridionale). Il Quat. 10, 585–590. Bernasconi, A., Bruni, P., Gorla, L., Principe, C., Sbrana, A., 1981. Risultati preliminari dell'esplorazione geotermica profonda nell'area vulcanica del Somma-Vesuvio. Rendic. della Soc. Geol. ltaliana 4, 237–240. Bertagnini, A., Landi, P., Rosi, M., Vigliargio, A., 1998. The Pomici di Base plinian eruption of Somma-Vesuvius. J. Volcanol. Geotherm. Res. 83, 219–239. Bianco, F., Castellano, M., Milano, G., Ventura, G., Vilardo, G., 1998. The Somma–Vesuvius stress field induced by regional tectonics: evidences from seismological and mesostructural data. J. Volcanol. Geotherm. Res. 82, 199–218. Bingham, C., 1974. An antipodally symmetric distribution on the sphere. Ann. Stat. 2, 1201–1225. Borgia, A., Tizzani, P., Solaro, G., Manzo, M., Casu, F., Luongo, G., Pepe, A., Berardino, P., Fornaro, G., Sansosti, E., Ricciardi, G.P., Fusi, N., Di Donna, G., Lanari, R., 2005. Volcanic spreading of Vesuvius, a new paradigm for interpreting its volcanic activity. Geophys. Res. Lett. 32, l03303. ,. ,. Bruno, P.P., Rapolla, A., 1999. Study of the sub-surface structure of Somma-Vesuvius (Italy) by seismic reflection data. J. Volcanol. Geotherm. Res. 92, 373–387. Bruno, P.P., Cippitelli, G., Rapolla, A., 1998. Seismic study of Mesozoic carbonate basement around Mt. Somma-Vesuvius, Italy. J. Volcanol. Geotherm. Res. 84, 311–322. Carniel, R., Guzmán, S., Neri, M., 2017. FIERCE: FInding volcanic ERuptive CEnters by a grid-searching algorithm in R. Bull. Volcanol. 79, https://doi.org/10.1007/ s00445-017-1102-3. Cassano, E., La Torre, P., 1987. Phlegrean fields geophysics. Quad. la Ric. Sci., CNR 114, 103–131. Ciaranfi, N., Cinque, A., Lambiase, S., Pieri, P., Rapisaldi, L., Ricchetti, G., Sgrosso, I., Tortorici, L., 1981. Proposta di zonazione sismotettonica dell'Italia Meridionale. Rendic. della Soc. Geol. Ital. 4, 493–496. ,. Cioni, R., Marianelli, P., Sbrana, A., 1992. Dynamics of the AD 79 eruption: stratigraphic, sedimentological and geochemical data on the successions from the Somma-Vesuvius southern and eastern sectors. Acta Vulcanol. 2, 109–123. Cioni, R., Santacroce, R., Sbrana, A., 1999. Pyroclastic deposits as a guide for reconstructing the multi-stage evolution of the Somma-Vesuvius Caldera. Bull. Volcanol. 60, 207–222. Cioni, R., Bertagnini, A., Santacroce, R., Andronico, D., 2008. Explosive activity and eruption scenarios at Somma-Vesuvius (Italy): towards a new classification scheme. J. Volcanol. Geotherm. Res. 178, 331–346. ,., De Natale, G., Kuznetzov, I., Kronrod, T., Peresan, A., Sarao, A., Troise, C., Panza, G.F., 2004. Three decades of seismic activity at Mt. Vesuvius: 1972-2000. Pure Appl. Geophys. 161, 123–144. Di Stefano, R., Chiarabba, C., 2002. Active source tomography at Mt. Vesuvius: constraints for the magmatic system. J. Geophys. Res. 107, https://doi.org/10.1029/ 2001JB000792. Dvorak, J.J., Nakamura, A., 1987. A hydraulic model to explain variations in summit tilt rate at Kilauea and Mauna Loa volcanoes. Prof. Pap Geol. Surv. (U.S.) 1350, 1281–1296. D'Auria, L., Massa, B., De Matteo, A., 2014. The stress field beneath a quiescent stratovolcano: the case of Mount Vesuvius. J. Geophys. Res. 119, 1181–1199. . Giaccio, B., Hajdas, I., Isaia, R., Deino, A., Nomade, S., 2017. High-Precision 14C and 40Ar/39Ar Dating of the Campanian Ignimbrite (Y-5) Reconciles the Time-Scales of Climatic-Cultural Processes at 40 Ka. Sci. Rep. 7, 45940. Gudmundsson, A., 1995. Infrastructure and mechanics of volcanic systems in Iceland. J. Volcanol. Geotherm. Res. 64, 1–22. Gudmundsson, A., 2006. How local stresses control magma-chamber ruptures, dyke injections, and eruptions in composite volcanoes. Earth Sci. Rev. 79, 1–31. ,. Hudson, J.A., Harrison, J.P., 1997. Engineering Rock Mechanics: an Introduction to the Principles. Pergamon, Oxford, UK. Isaia, R., Vitale, S., Di Giuseppe, M.G., Iannuzzi, E., Tramparulo, F.D.A., Troiano, A., 2015. Stratigraphy, structure and volcano-tectonic evolution of Solfatara maar-diatreme (Campi Flegrei, Italy). GSA Bull. 127, 1485–1504. ISPRA, 2018. Carta Geologica D'Italia Alla Scala 1:50.000, Foglio 448 “Ercolano. http:// www.isprambiente.gov.it/Media/carg/448_ERCOLANO/Foglio.html. Komuro, H., 1987. Experiments on cauldron formation: a polygonal cauldron and ring fractures. J. Volcanol. Geotherm. Res. 31, 139–149.

Acknowledgements

We thank the Editor Toru Takeshita for his careful editorial handling. We would like to express our sincere thanks to the referees Fabrizio Agosta and Rich Walker for their constructive and thorough reviews and useful suggestions, from which we have benefited greatly in revising our manuscript. Furthermore, we are grateful to Michael Ort for the English editing. Finally, we thank Maria Monda for her contribution during the structural survey. Appendix A. Supplementary data

Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jsg.2018.05.008. References

Acocella, V., 2006. Regional and local tectonics at Erta Ale caldera, Afar (Ethiopia). J. Struct. Geol. 28, 1808–1820. Acocella, V., 2007. Understanding caldera structure and development: an overview of analogue models compared to natural calderas. Earth Sci. Rev. 85, 125–160. Acocella, V., Neri, M., 2003. What makes flank eruptions? The 2001 Etna eruption and its possible triggering mechanisms. Bull. Volcanol. 65, 517–529. Acocella, V., Neri, M., 2009. Dike propagation in volcanic edifices: overview and possible developments. Tectonophysics 471, 67–77. Acocella, V., Cifelli, F., Funiciello, R., 2001. The control of overburden thickness on resurgent domes: insights from analogue models. J. Volcanol. Geotherm. Res. 111, 137–153. Acocella, V., Porreca, M., Neri, M., Mattei, M., Funiciello, R., 2006. Fissure eruptions at Mount Vesuvius (Italy): insights on the shallow propagation of dikes at volcanoes. Geology 34, 673–676.

14


Journal of Structural Geology xxx (2018) xxx-xxx Tectonophysics 690, 2016, 206-218. Stewart, S.A., 2006. Implications of passive salt diapir kinematics for reservoir segmentation by radial and concentric faults. Mar. Petrol. Geol. 23, 943–853. Tadini, A., Bisson, M., Neri, A., Cioni, R., Bevilacqua, A., Aspinall, W.P., 2017a. Assessing future vent opening locations at the Somma-Vesuvio volcanic complex: 1. A new information geodatabase with uncertainty characterizations. J. Geophys. Res. Solid Earth 122, 4336–4356. Tadini, A., Bevilacqua, A., Neri, A., Cioni, R., Aspinall, W.P., Bisson, M., Isaia, R., Mazzarini, F., Valentine, G.A., Vitale, S., Baxter, P.J., Bertagnini, A., Cerminara, M., de Michieli Vitturi, M., Di Roberto, A., Engwell, S., Esposti Ongaro, T., Flandoli, F., Pistolesi, M., 2017b. Assessing future vent opening locations at the Somma-Vesuvio volcanic complex: 2. Probability maps of the caldera for a future Plinian/sub-Plinian event with uncertainty quantification: Vent Opening Probability map for Vesuvio. J. Volcanol. Geotherm. Res. 122, 4357–4376. Takada, A., Ishizuka, Y., Nakano, S., Yamamoto, T., Kobayashi, M., Suzuki, Y., 2007. Characteristic and evolution inferred from eruptive fissures of Fuji volcano, Japan. In: Aramaki, S. (Ed.), Fuji Volcano. Yamanashi Institute of Environmental Sciences, pp. 183–202. Tibaldi, A., 2003. Influence of cone morphology on dikes, Stromboli, Italy. J. Volcanol. Geotherm. Res. 126, 79–95. Varnes, D.J., 1978. Slope movement types and processes. In: Schuster, R.L., Krizek, R.J. (Eds.), Special Report 176: Landslides: Analysis and Control. Transportation and Road Research Board, National Academy of Science, Washington D. C., pp. 11–33. Ventura, G., Vilardo, G., 1999. Slip tendency analysis of the Vesuvius faults: Implication for the seismotectonic and volcanic hazard assessment. Geophys. Res. Lett. 26, 3229–3232. Ventura, G., Vilardo, G., Bruno, P.P., 1999. The role of flank failure in modifying the shallow plumbing system of volcanoes: an example from Somma-Vesuvius, Italy. Geophys. Res. Lett. 26, 3681–3684. Ventura, G., Vilardo, G., Bronzino, G., Gabriele, G., Nappi, R., Terranova, C., 2005. Geomorphological map of the Somma-Vesuvius volcanic complex (Italy). J. Maps 30–37. Vezzoli, L., Renzulli, A., Menna, M., 2014. Growth after collapse: the volcanic and magmatic history of the Neostromboli lava cone (island of Stromboli, Italy). Bull. Volcanol. 76, 1–24. Vitale, S., Ciarcia, S., 2018. Tectono-stratigraphic setting of the Campania region (southern Italy). J. Maps 14, 9–21. Vitale, S., Isaia, R., 2014. Fractures and faults in volcanic rocks (Campi Flegrei, southern Italy): Insight into volcano-tectonic processes. Int. J. Earth Sci. 103, 801–819. Walter, T.R., Troll, R., 2001. Formation of caldera periphery faults: an experimental study. Bull. Volcanol. 63, 191–203. Yamaji, A., 2016. Genetic algorithm for fitting a mixed Bingham distribution to 3D orientations: a tool for the statistical and paleostress analyses of fracture orientations. Isl. Arc 25, 72–83. Yamaji, A., Sato, K., 2011. Clustering of fracture orientations using a mixed Bingham distribution and its application to paleostress analysis from dike or vein orientations. J. Struct. Geol. 33, 1148–1157. Zollo, A., Marzocchi, W., Capuano, P., Lomax, A., Iannaccone, G., 2002. Space and time behaviour of seismic activity and Mt. Vesuvius volcano, Southern Italy. Bull. Seismol. Soc. Am. 92, 625–640.

UN CO RR EC TE D

PR OO F

Lipman, P.W., 1984. The roots of ash flow calderas in western North America: windows into the tops of granitic batholiths. J. Geophys. Res. 89, 8801–8841. Marinoni, L., Gudmundsson, A., 2000. Dykes, faults and palaeostresses in the Teno and Anaga massifs of Tenerife (Canary Islands). J. Volcanol. Geotherm. Res. 103, 83–103. . Martì, J., Ablay, G.J., Redshaw, L.T., Sparks, R.S.J., 1994. Experimental studies of collapse calderas. J. Geol. Soc. Lond. 151, 919–930. Marturano, A., Aiello, G., Barra, D., Fedele, L., Grifa, C., Morra, V., Berg, R., Varone, A., 2009. Evidence for Holocenic uplift at Somma-Vesuvius. J. Volcanol. Geotherm. Res. 184, 451–461. Mathieu, L., van Wyk de Vries, B., 2009. Edifice and substrata deformation induced by intrusive complexes and gravitational loading in the Mull volcano (Scotland). Bull. Volcanol. 71, 1133–1148. McGuire, W.J., Pullen, A.D., 1989. Location and orientation of eruptive fissures and feeder-dykes at Mount Etna: influence of gravitational and regional stress regimes. J. Volcanol. Geotherm. Res. 38, 325–344. Muller, O.H., Pollard, D.D., 1977. The stress state near Spanish Peaks, Colorado, determined from a dike pattern. Pure Appl. Geophys. 115, 69–86. Nakamura, K., 1977. Volcanoes as possible indicators of tectonic stress orientation-principle and proposal. J. Volcanol. Geotherm. Res. 2, 1–16. Odé, H., 1957. Mechanical analysis of the dike pattern of the Spanish Peaks area, Colorado. GSA Bull. 68, 567–576. Paoletti, V., Passaro, S., Fedi, M., Marino, C., Tamburrino, S., Ventura, G., 2016. Sub-circular conduits and dikes offshore the Somma-Vesuvius volcano revealed by magnetic and seismic data. Geophys. Res. Lett. 43, 9544–9551. Pinel, V., Jaupart, C., 2003. Magma chamber behavior beneath a volcanic edifice. J. Geophys. Res. 108, https://doi.org/10.1029/2002JB001751. Pollard, D.D., Delaney, P.T., Duffield, W.A., Endo, E.T., Okamura, A.T., 1983. Surface deformation in volcanic rift zones. Tectonophysics 94, 541–584. Porreca, M., Acocella, V., Massimi, E., Mattei, M., Funiciello, R., De Benedetti, A.A., 2006. Geometric and kinematic features of the dike complex at Mt. Somma, Vesuvio (Italy). Earth Planet. Sci. Lett. 245, 389–407. Quintà, A., Tavani, S., Roca, E., 2012. Fracture pattern analysis as a tool for constraining the interaction between regional and diapir-related stress fields: Poza de la Sal Diapir (Basque Pyrenees, Spain). In: Alsop, G.I., Archer, S.G., Hartley, A.J., Grant, N.T., Hodgkinson, R. (Eds.), Salt Tectonics, Sediments and Prospectivity. vol. 363, Geological Society London Special Publications, pp. 521–532. Reiter, F., Acs, P., 1996–2003. TectonicsFP. Software for Structural Geology. Innsbruck University, Austria. Rosi, M., Principe, C., Vecci, R., 1993. The 1631 Vesuvius eruption. A reconstruction based on historical and stratigraphical data. J. Volcanol. Geotherm. Res. 58, 151–182. Santacroce, R., 1987. Somma Vesuvius. C.N.R., Rome, 251. Santacroce, R., Sbrana, A., 2003. Carta Geologica del Vesuvio, Scala 1:15000, Progetto CARG, Servizio Geologico Nazionale-Consiglio Nazionale delle Ricerche. Santacroce, R., Cioni, R., Marianelli, P., Sbrana, A., Sulpizio, R., Zanchetta, G., Donahue, D.J., Joron, J.L., 2008. Age and whole rock-glass compositions of proximal pyroclastics from the major explosive eruptions of Somma-Vesuvius: a review as a tool for distal tephrostratigraphy. J. Volcanol. Geotherm. Res. 177, 1–18. Santangelo, N., Ciampo, G., Di Donato, V., Esposito, P., Petrosino, P., Romano, P., Russo Ermolli, E., Santo, A., Toscano, F., Villa, I., 2010. Late Quaternary buried lagoons in the northern Campania plain (southern Italy): evolution of a coastal system under the influence of volcano-tectonics and eustatism. Italian J. Geosci. 129, 156–175. Schiattarella, M., Beneduce, P., Giano, S.I., Giannandrea, P., Principe, C., 2005. Assetto strutturale ed evoluzione morfotettonica quaternaria del vulcano del Monte Vulture (Appennino lucano). Boll. della Soc. Geol. Ital. 124, 543–562. ,. Sinisi R., PetrulloA.V., Agosta F., Paternoster M., Belviso C., Grassa F. Contrasting fault fluids along high-angle faults: a case study from Southern Apennines (Italy),

15