Deformation at Stromboli volcano (Italy) revealed by rock mechanics and structural geology

Deformation at Stromboli volcano (Italy) revealed by rock

mechanics and structural geology

A. Tibaldia,b,*, C. Corazzatoa, T. Apuanic, A. Cancellia

aDipartimento di Scienze Geologiche e Geotecnologie, Universita di Milano–Bicocca, Italyb Istituto per la Dinamica dei Processi Ambientali, CNR, Milan, Italy

cDipartimento di Scienze della Terra ‘‘A. Desio’’, Universita di Milano, Italy

Received 17 January 2002; accepted 11 October 2002

Abstract

We approach the reconstruction of the recent structural evolution of Stromboli volcano (Italy) and the analysis of the

interplay between tectonics, gravity and volcanic deformation. By tying together structural, lithostratigraphic and rock

mechanics data, we establish that since 100 ka BP, the edifice has faulted and jointed mainly along NE-striking planes. Faults

mostly dip to the NW with normal displacement. Taking also into account the presence of a NW-trending regional least

principal stress and of tectonic earthquake hypocenters inside the cone, we suggest that this fracturing can be related to the

transmission of tectonic forces from the basement to the cone. Dyking concentrated along a main NE-trending weakness zone

(NEZ) across the volcano summit, resembling a volcanic rift, whose geometry is governed by the tectonic field. In the past 13

ka, Stromboli experienced a reorganisation of the strain field, which was linked with the development of four sector collapses

affecting the NW flank, alternating with growth phases. The tectonic strain field interplayed with dyking and fracturing related

to unbuttressing along the collapse shoulders. We propose that tectonics control the geometry of dykes inside the cone and that

these, in turn, contribute to destabilise the cone flanks.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Stromboli; Rock mechanics; Strain field; Volcano collapse; Dykes; Joints

1. Introduction

Fracturing in volcanoes plays a fundamental role in

the creation of: (i) preferential pathways for magma

upwelling, (ii) weakness zones for lateral failure of the

cone, (iii) hydrothermal alteration zones and (iv)

preferential water migration fluxes. The reconstruc-

tion of fracture patterns in volcanoes is, thus, funda-

mental for the assessment of several geological

hazards such as eruptions and landslides, and for the

evaluation of economic and social potential such as

mineral and water exploration.

After the emplacement and cooling of volcanic

deposits, fractures in volcanoes can mainly be pro-

duced by gravity instability, by intrusion processes or

by transmission of regional tectonic forces to the

cone. Whereas gravity fractures linked to cone flank

erosion should mainly affect the surface zones of the

cone, intrusion-related fractures, tectonic fractures and

0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0040-1951(02)00589-9

* Corresponding author. Dipartimento di Scienze Geologiche e

Geotecnologie, Universita di Milano–Bicocca, P. della Scienze 4,

I-20126 Milan, Italy.

E-mail address: [email protected] (A. Tibaldi).

www.elsevier.com/locate/tecto

Tectonophysics 361 (2003) 187–204

deep-seated gravity processes can affect the whole

cone. The endogenous growth of magmatic masses

can play a substantial role in deforming a volcanic

edifice. Complexes of hundreds to thousands of dykes

were observed to form the core of volcanic edifices

exhumed by erosion or landslides (Walker, 1986,

1999; Takada, 1988; Walker and Eyre, 1995; Grasso

and Bachelery, 1995; Tibaldi, 1996; Maillot, 1999).

Geodetic and seismic monitoring of active volcanoes

such as Kilauea (Hawaii Islands), Piton de la Four-

naise (Reunion Island) and Mt. Etna (Italy) show that

successive intrusions deform and uplift these edifices

(e.g. Swanson et al., 1976; Decker, 1987; Lenat and

Bachelery, 1990; McGuire et al., 1991; Annen et al.,

2001). It is not clear how a volcanic edifice exactly

responds to magma intrusion, and it would be useful

to understand how the fractures related to tectonics,

gravity and intrusions are distributed in a cone.

Several works showed that tectonic-controlled

fractures can affect volcanic edifices. For example,

Nakamura (1977) and Nakamura et al. (1977) showed

that in many arc volcanoes, the island elongation and

rift zones are parallel with the trajectory of motion of

the plate on which they are situated being the ori-

entation of the maximum principal stress, even though

this is not systematically valid for all volcanic arcs.

These rift zones on volcanoes are typically made of

parallel and subparallel faults, fractures, eruptive

fissures and aligned monogenic cones and craters.

Formation of faults and fractures due to the propaga-

tion of regional tectonic stress has also been tested

through laboratory experiments and modelling for

example by van Wyk de Vries and Merle (1996)

and Merle et al. (2001). They demonstrated that

normal faults linked with a basement tectonic rift

could propagate through volcanic cones. They also

showed that increased fault throw as the volcano is

approached is caused by an interaction of the regional

stress field with the one set up by the volcano mass. In

addition, strike-slip faults can propagate across a

volcanic cone with the formation of a typical sigmoid

pattern of fracturing (Lagmay et al., 2000).

The aims of our study are to quantitatively evaluate

the contributions of tectonics and gravity to fracturing

of Stromboli volcano, and to evaluate its interaction

with dyke intrusion. As a first step, we surveyed at

1:2000 scale all the island of Stromboli in order to

detect the main faults and joints. Then, we defined the

characteristics of all the joints affecting the rock

succession in 52 sites of measurement. With stand-

ardised rock mechanics procedure (ISRM—Interna-

tional Association of Rock Mechanics, 1978), we

determined the geometry of each joint (strike, dip

angle and dip direction), spacing, type of movement,

dilation amount, degree of alteration, presence and

nature of infill, number of joint sets, and representa-

tive orientation, in order to determine the geometric,

physical and mechanical characteristics of deforma-

tion. Next, we compared the characteristics of joints

and faults with those of dykes. Finally, we merged

together this information with lithostratigraphic data

in order to reconstruct the geological and deformation

evolution of the volcano.

2. Geological background

The island of Stromboli and two other volcanic

centres belong to a late Quaternary large volcanic

complex of mostly basaltic to basaltic–andesitic com-

position. This rises 2.6 km above the sea bottom at the

NE tip of the Aeolian archipelago. Strombolicchio is a

neck forming a small island located to the NW, and

Fig. 1. Location of Stromboli Island with respect to the submerged

part of the volcano, and scars of the eight collapses indicated by

Tibaldi (2001). The other two volcanic centres of Strombolicchio

(emerged) and Cavoni (submerged) are indicated. Bathymetry is

taken from Gabbianelli et al. (1993).

A. Tibaldi et al. / Tectonophysics 361 (2003) 187–204188

Cavoni is a submerged centre to the south localised by

marine geophysical data (Gabbianelli et al., 1993)

(Fig. 1).

Strong erosion mainly due to sea activity and

landsliding allows an exceptional view of the inner

structure of the upper part of the volcano. This

structure is complex because of the alternating build-

ing and destructive phases. The destructive phases

range from slow slope erosion to rapid slope failure

and have created several unconformities, which

enabled Rosi (1980), Francalanci (1987), Hornig-

Kjarsgaard et al. (1993) and Pasquare et al. (1993)

to distinguish a succession of lithostratigraphic units

representing the main volcanic cycles. These are, from

the oldest one, Paleostromboli (Pst I, Pst II and Pst

III), Vancori (Lower, Middle and Upper), Neostrom-

boli, and recent Stromboli. More recently, Tibaldi

(2001) added the Pst 0 unit, which represents an

undated, fractured lava deposit unconformably cov-

ered by the Pst I lavas, and a subdivision of recent

Stromboli products into RS I, RS II and RS III. The

latter subdivision marks major morphological and

structural changes in the cone evolution due to the

development of Holocene sector collapses.

Recent papers recognised also the occurrence of

summit vertical collapses of caldera type which pre-

dominantly characterised the earlier stages of Strom-

boli evolution (100–24 ka BP) (Pasquare et al., 1993;

Tibaldi et al., 1994). The occurrence of four lateral

collapses in the last 13 ka, the last one resulting in the

formation of the Sciara del Fuoco depression, have

been preliminary recognised on the base of geolog-

ical, structural and morphological studies (Tibaldi,

2001; Tibaldi and Pasquare, in press). The ongoing

research seems to confirm this hypothesis basing on

new findings in correspondence with the scarps of

volcaniclastic deposits probably related to the collapse

events whose study in detail will be soon performed.

Bertagnini and Landi (1996) already attributed a

volcaniclastic sequence to the last sector collapse of

Fig. 2. Main NE-trending weakness zone (NEZ) across Stromboli and secondary dyke injection zones with ages. Stereograms of the main faults,

Schmidt’s equal area projection, lower hemisphere. Boxes locate Figs. 4 and 10. Scars of the eight collapses after Tibaldi (2001) are numbered

from the oldest one. (A)– (B) locates the section in Fig. 9.

A. Tibaldi et al. / Tectonophysics 361 (2003) 187–204 189

about 5 ka BP, while Calvari and Tanner (2001, 2002)

distinguished a larger number of volcaniclastic se-

quences related to different lateral collapse events in

agreement with Tibaldi (2001). Calvari and Tanner

(2002) also described a volcaniclastic deposit related

to a tsunami wave caused by a recent lateral collapse.

This recent dominance of lateral collapses was attrib-

uted to the coexistence of a series of causes such as

asymmetric distribution of buttressed flanks (Romag-

noli and Tibaldi, 1994), postglacial sea-level varia-

tions and slope erosion (Tibaldi et al., 1994). Factors

influencing the collapse geometry at Stromboli in-

Fig. 3. Photos of NE-striking normal faults found at Stromboli. (A) At sea level along the southeastern flank, faults dip to the NW and offset the

late Pleistocene rocks of the Paleostromboli I succession. (B) In the summit area of the volcano, a main fault crops out along the scarp of the first

sector collapse (site d, Fig. 2). The fault, dipping at high angle to the NW, affects precollapse Vancori succession but it is cut by the 13-ka-old

Vancori collapse. The fault plane puts into contact lavas, lava–breccia and breccia deposits to the east with younger deposits to the west,

suggesting normal displacement. Downthrown is at least 15 m. A dyke is intruded close to the fault plane.


clude basement morphology, magma intrusions with a

preferential pathway and related lateral stress concen-

trations (Tibaldi, 1996, 2001).

3. Data

3.1. Faults

Rare faults with offset in the order of 1–15 m and

several microfaults with displacement in the order of a

few centimetres are present on Stromboli. Faults were

found in the eastern side of the island, 1 km south of

Stromboli village, and in the summit area of the

volcano (Fig. 2). At site ‘‘a’’, on the eastern coast,

four faults dipping 76–78j to the NW are present

(Fig. 3A). They offset lava and pyroclastic products

belonging to the Pst I succession. Displacement is

about 0.5 m on each fault plane, totalling 2 m.

Downthrown is normal to the northwest. No striae

are present on the motion planes.

In the same area at a distance of a few hundred

metres, other three normal faults dip 55–60j to the

west plus two faults dipping to the NW (site b, Fig. 2).

They displace the Pst I succession of about 0.7 m on

each plane, totalling 2 m. A hundred metres further

north, four normal faults dip to the SE and two faults

dip 70j to the NW (site c, Fig. 2). They displace the

Pst I succession of a few decimetres on each plane,

totalling 1 m.

In the summit area of the volcano, a main fault

crops out along the scarp of the first sector collapse

(site d, Fig. 2). This fault, striking NE, affects the pre-

first sector collapse Vancori succession (Fig. 3B). The

motion plane puts into contact lavas, lava–breccia and

breccia deposits to the east with younger lava–breccia

and breccia deposits to the west, resulting in normal

motions. Downthrown is at least 15 m. A dyke is

intruded close to the fault plane, and it is not possible

to establish if it postdates the faulting event.

Microfaults mostly strike NE with a dip ranging

50–80j. Displacement is always of normal type. A

few microfaults with different strike and kinematics

are present along the northern shoulder of the sector

collapse zone. They strike about E–W and offset the

Neostromboli succession with left lateral strike-slip

displacement of a few centimetres on each plane (Fig.

4). These E–W microfaults also offset: (1) joints

which can be related to lava cooling because they

have random strike distribution and affect single

depositional units, and (2) other joints which are

parallel to the collapse scarp (here, striking NNW)

and which can be referred to postdepositional defor-

mation because they cut more than one depositional

unit.

3.2. Eruptive fissures and dykes

The summit feeding system is revealed by eruptive

fissures and dykes. These are not arranged in a typical

radial pattern as observed in the summit zone of other

composite volcanoes (e.g. Chevallier and Verwoerd,

1988; Manetti et al., 1989; Ferrari et al., 1991), but are

preferentially developed along three zones which

show different ages of magma injection already estab-

lished by Tibaldi (1996). The majority of dykes and

fissures have developed from 100 ka to the present

along a rectilinear NE-trending weakness axial zone

Fig. 4. Lines of horizontal projection of dilation direction measured

along joints, fissures, microfaults and dykes around the sector

collapse zone. Numbers of collapses are the same of the other

figures. Location in Fig. 2.


Fig. 5. Stereograms of poles of about 3000 joints measured at 52 sites in the island. Schmidt’s projection, lower hemisphere. In (a), rock mechanics data are represented, while (b)

reports the stereograms related to the structural data. In text, they are discussed as a whole.

A.Tibaldiet

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361(2003)187–204

192

Fig.5(continued).


(NEZ) dividing the volcano into two parts (Fig. 2).

Another dyke zone developed with a N–S trend in the

southern part of the island between 85 and 55 ka BP.

The third dyke zone developed after 13 ka BP along

the shoulders of the sector collapse amphitheatre.

Inside the NEZ, dykes strike NE, ESE and SSE in

decreasing order of frequency. These dykes crop out

in the southern and central parts of the NEZ. Prolon-

gation of the NEZ towards northeast is based on the

following data: (a) the composite recent pyroclastic

cone located on the summit of Stromboli is elongated

in a NE direction, (b) single active craters are NE-

elongated and aligned, (c) the eruptive fissure opened

in 1985 strikes NE (De Fino et al., 1987), (d) other

NE-striking eruptive fissures on the northeastern flank

strike NE and are aligned along the NEZ. Dyke dip

along the NEZ is mostly >80j. Dyke thickness rangesfrom a few decimetres to 10 m and cumulative dyke

thickness has a NW–SE horizontal maximum. Along

the southern part of the NEZ, a few dykes dip to the

SW, subparallel to the local cone slope.

Around the Sciara del Fuoco depression, dyke

azimuths concentrate in three groups in decreasing

order of frequency: ESE, NNE, and SSE. These dykes

are mostly parallel or subparallel to the scarp trace of

the sector collapses. Dykes mostly dip at high angle

towards the Sciara del Fuoco. Dyke thickness ranges

from 0.5 to 3 m.

3.3. Rock mechanics data

Rock mechanics data comprise dip, inclination,

fracture mechanism, dilation, spacing and infill of

about 3000 joints measured at 52 sites. Data cover

the majority of the island with exclusion of a few

zones with heavy logistical difficulties. Dip and

inclination for rock mechanics and structural data

are plotted in Fig. 5a and b as projection on equal

area Schmidt’s stereograms of poles to planes. These

stereograms reveal a complicate joint pattern with the

presence of two to four sets in each considered rock

mass. In each rock mass, the different joint sets are

characterised by strong differences in the amount of

joint population. The most frequent joint set strikes

NE and dips >75j towards N280–300j. This is

representative of the 80% of the investigated sites.

In addition, in the few sites where it is not the

dominant set, it is still represented as the second set

in order of frequency. The remaining 20% of the sites

show the dominance of other joint sets: one dipping

75–85j towards N50–80j characterises the southern

shoulder of Sciara del Fuoco, another dominates along

the northern shoulder of the Sciara del Fuoco and

strikes NNW to N. Other joint sets dominate in the

southwestern coast dipping 75j towards N270j and

N210j.In order to better display the joint population, in

Fig. 6, the data were condensed into six homogeneous

structural areas of the volcano. These areas embrace

the northern and southern shoulders of the collapse

zone (areas I and II), the summit area (III), the

northern (IV), eastern (V) and southern (VI) coastal

flank areas. It is noteworthy that the main NE-striking

joint set is widespread all over the island and shows a

very narrow strike dispersion. The other joint sets

instead show a higher strike dispersion from one area

to another. In particular, in areas I and II, the joint sets

striking NNW and WNW, respectively, are parallel to

the orientation of the two shoulders of the Sciara del

Fuoco collapse zone. In area III, a secondary set

strikes NNW, while in areas IV and V, the NNW- to

NW-striking joint sets are parallel to the local volcano

flank. This is particularly evident also in area VI

where a NNE- and a NW-striking sets are exactly

parallel to the main trend of the coast.

Joint spacing is represented in Fig. 7 as average

spacing of the dominant joint set (NE–SW) measured

at various sites along two NW–SE traverses across

the island. In the same graphs, the topographic profile

along the traverse is represented. Both traverses show

that joint spacing is maximum on the NW flank of the

island, whereas it is minimum in the central summit

region corresponding to the NEZ. In the northern

traverse (Fig. 7A), the NE-striking joints are rare on

the NW flank, but suddenly increase at the NEZ

reaching a spacing of 50 cm. Farther SE, spacing

strongly increases to 150 cm in correspondence with

the topographic top of the island and on SE coast, it

decreases to 60 cm. In the southern traverse (Fig. 7B),

the only difference is given by a lower joint spacing in

correspondence with the northwestern coast.

Joint dilation is represented in Fig. 8 as average

dilation of the joints measured at various sites along

two NW–SE traverses across the island and a more

limited one to the south. Topographic profiles are also

shown. All traverses show that the average joint


dilation is in the order of 0.5–3 cm in the NW flank of

the island. In the central part of the island correspond-

ing to the NEZ, the average dilation is very variable,

spanning from 1 to 8 cm. In the SE flank of the island

where older rocks crop out (Fig. 9), the average

dilation is the highest, mostly between 2.5 and 11.5

cm. Generally, dilation joints strike NE or are locally

parallel to the slopes. Along the shoulders of the

Sciara del Fuoco collapse depression, dilation fissures

mostly strike parallel to the depression scarp. Joints

are usually empty, mostly without traces of volcanic

fluids. Exception is represented by some joints dis-

tributed along the summit part of the NEZ where

sulphur deposits mantle the joint surfaces. Hydro-

thermal alteration and deposits are particularly wide-

spread along the numerous joints that affect the active

pyroclastic cone and some conterminous outcrops of a

previous tuff cone.

Fractures with dilation in the order of some deci-

metres are present in the upper part of the Sciara del

Fuoco depression. Here, in 2000, a 150-m-long frac-

ture opened along the trace of the last sector collapse

(Fig. 10). Southwest of the active craters, this fracture

strikes N30j and opened with pure dilation totalling

20 cm in November 2000. Farther southwest, this

fracture bends to an average N110j strike. In detail,

Fig. 6. Stereograms of poles resuming the joint population, condensed into six homogeneous structural areas of Stromboli. Schmidt’s projection,

lower hemisphere. Arrows give the dilation direction. These areas embrace the northern and southern shoulders of the collapse zone (areas I and

II, respectively), the summit area (III), the northern (IV), eastern (V) and southern (VI) coastal areas.


this N110j portion of the fracture has a zigzag

pattern: the segments striking more E–W show a

component of dilation whereas those striking more to

the SE show no dilation or are associated with small

push ridges (Fig. 10c). A boulder standing above the

fracture displays a push ridge on one side and a small

graben on the other, giving evidence of anticlockwise

rotation on a vertical axis. Some offset blocks show a

displacement trend of N115–150j. These data indi-

cate left lateral strike-slip motion along the N110jportion of the fracture with a lower amount of

dilation. The amount of strike-slip measured at an

offset block was 20 cm in November 2000. By April

2001, the N30j fracture had propagated northeast-

wards through the formation of four main parallel

fractures exhibiting a total of 45 cm of pure dilation

with a minor component of downthrown to the NW.

These fractures are detectable up to the base of the

active pyroclastic cone where they could be con-

cealed by the continuous scoria fall, whereas farther

north and to the west, they meet unconsolidated

breccia deposits. The N110j fracture also moved

further with dominant transcurrent motion totalling

35 cm.

3.4. Holocene strain field around Sciara del Fuoco

The Holocene strain field was reconstructed with a

particularly high detail along the shoulders of the

Sciara del Fuoco. This detail was possible due to the

presence of several microfaults, joints, fractures and

dykes which are three-dimensionally exposed along

the rock walls of the sector collapse depressions.

Fracture kinematics was reconstructed through meas-

uring the dip of the displacement vector expressed by

homologous irregularities on both sides of the frac-

ture. Dyke opening direction was measured with

standardised methods (e.g. Marinoni, 2001).

Fig. 7. Joint spacing is represented as average spacing of the dominant joint set (NE–SW) measured at various sites along two NW–SE

traverses across the island. The topographic profiles along the traverses are represented. Both traverses show that fracture spacing is maximum

at the NW flank of the island, whereas it is minimum in the central summit region in correspondence of the NEZ.


Fig. 4 shows the Holocene strain field as horizontal

direction of dilation. It is very clear that dilation

direction is perpendicular to the strike of the Holocene

sector collapse scarps. In the uppermost part of the

collapse zone, the NW–SE dilation direction is nor-

mal to the NEZ strike. The resulting general pattern is

expressed by converging lines of dilation with a centre

located about in the middle of the collapse amphi-

theatre. Deformation is concentrated along the shoul-

der of the collapse depression, following local

Fig. 8. Joint dilation is represented as average dilation of the joints measured at various sites along two NW–SE traverses across the island and a

more limited one to the south. Topographic profiles are also shown. All traverses show that average joint dilation is in the order of 0.5–3 cm in

the NW flank of the island. In the central part of the island in correspondence with the NEZ, average dilation is very variable spanning from 1 to

8 cm. In the SE flank of the island where older rocks crop out (see also Fig. 9), average dilation is the highest, mostly between 2.5 and 11.5 cm.


Fig. 9. NW–SSE geological section across the island of Stromboli, data from Tibaldi and Pasquare (2002). Note the presence of older rocks cropping out in the southeastern flank of

the cone that can explain some rock mechanics characteristics of the studied joints such as an increase in cumulative dilation. The section is located in a profile of the volcano

including the submerged portion, and the hypothesized sliding surfaces are indicated after Tibaldi (2001). Further research dealing with numerical modelling is in progress in order to

assess the most probable shape and depth of these surfaces.

A.Tibaldiet

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361(2003)187–204

198

unbuttressing. It is mostly expressed by dykes and

joints striking parallel to the collapse scarp, and

dipping 80–70j towards the depression. Dominant

kinematics is, thus, given by pure dilation, but also

transcurrent kinematics is present along some E–W-

striking microfaults. They show left lateral strike-slip

motion of 1–3 cm. Their spacing is in the order of

60–100 cm. The deformation field resulting from the

interplay between the E–W-striking microfaults and

the NNW- to N–S-striking joints is given by west-

wards differential dilation. In this case, the E–W-

striking structures acted as transfer microfaults bound-

ing the rock blocks that moved at different velocities

towards the collapse depression.

4. Discussion and conclusions

By means of combining data from structural geol-

ogy, rock mechanics and stratigraphy, we were able to

quantitatively appraise the deformation history of

Stromboli volcano both in terms of spatial and tem-

poral evolution. Only the simultaneous comparison of

these data related to different disciplines allowed us to

correctly interpret the significance of the studied

features.

Apart from deposition surfaces (set I, Fig. 11) and

cooling joints, the rock succession of Stromboli is

pervasively affected by other discontinuities, namely:

joints, microfaults and faults in decreasing frequency,

which cut more than one depositional unit. The origin

of these discontinuities can, thus, be found in the

propagation of postdepositional stresses in the cone.

These stresses created brittle failure, which further

decreased the rock strength. Gravity can explain

slope–parallel joints as due to erosion unbuttressing

(set II, Fig. 11). But the most frequent joint set strikes

NE dipping at high angle mostly to the NW (set III,

Fig. 11). It is also noteworthy that the great majority

of the faults found in the island dip to the NW. Set III

structures could be produced by gravity and tectonic

forces. Gravity could explain the dominance of NE-

striking structures because Stromboli is elongated in

the same direction. The influence of the shape of

volcanoes on stress distribution within the cone has

been already put forward since Fiske and Jackson

(1972). These authors suggested the possibility that

the stress field generated by gravity acting on the

edifices as they grow exerts an important or even

dominant influence upon the orientation and growth

of volcanic rifts and, thus, of large volcanoes. This

influence is particularly relevant on dyke orientation.

Dykes usually develop entirely inside the volcanic

edifice, and because of such shallow emplacement,

the direction of dyke propagation should be strongly

influenced by the gravitational stresses within these

edifices.

We note that the analysis of Fiske and Jackson

(1972) relates to volcanoes, such as the several shield

cones of Hawaii that pierced a thick, sloping apron of

Fig. 10. (a) Location map of the fissures developed in 2000–2001.

Numbers in centimeter show cumulative amount of pure dilation

(divergent arrows) or strike-slip motion (parallel arrows). These

fissures opened along the trace of the last sector collapse (no. 8 in

Fig. 2). Location in Fig. 2. (b) Photo of the fissure (black arrows)

opened along the southern Sciara del Fuoco scarp. To the left (i.e. to

the south), the highly inclined lateral scarp of the last sector

collapse. (c) Sketch in plane view of the left lateral strike-slip main

component of motion along the N110j portion of the fissure.


preexisting neighbouring volcanics. Dykes cluster

into well-defined rifts oriented roughly perpendicular

to the downslope direction of these aprons. Moreover,

the gravity force exerts its maximum effect on

strongly elongated volcanoes. Stromboli has a low

value of ellipticity and there is no evidence of it

growing on the apron of a preexisting volcano.

Strombolicchio actually represents the 200-ka-old

remnant of a volcanic conduit, but probably for a

long time, this centre grew contemporaneously to

Stromboli. This interpretation is based on the fact that

the oldest rocks of the island of Stromboli date back to

about 100 ka BP (Gillot and Keller, 1993), but this

must be considered a minimum since it must be taken

into account the necessary time to construct the

submerged part of the cone. In addition, the bathy-

metric data do not show any interruption in the base of

the volcano (Fig. 1), suggesting a single edifice

constituted by two main centres. Strombolicchio

centre ceased its activity 200 ka ago or less (more

recent deposits may be eroded), whereas Stromboli

continues its activity. Even in the case that Strombo-

licchio volcano preceded Stromboli growth, the NE

rift would be parallel to the downslope direction of the

supposed apron, and not perpendicular as could fol-

low from Fiske and Jackson (1972). Another main

point of discussion is the fact that faults at Stromboli

dip to the NW. If faulting was related to gravity forces

alone, we should expect to find symmetric inward

dipping faults, as in the case of axial rifts related to

volcano spreading (Borgia, 1994; Merle and Borgia,

1996; Borgia et al., 2000). Moreover, Stromboli nor-

mal faults are present also at sea level, i.e. 900 m

below the cone summit where spreading rifts usually

develop.

On the other hand, if we take into account the

presence of a NW-trending tectonic regional least

principal stress (r3) (Caccamo et al., 1996) and of

tectonic earthquakes hypocenters inside the cone

(Falsaperla et al., 1999), the systematic geometry of

the normal faults and joints of set III suggests that

they can be related to the transmission of tectonic

Fig. 11. Sketch resuming the postdeposition sets of joints and microfaults found at Stromboli (cooling joints not represented). For a comparison,

typical bedding attitude of each zone is portrayed (set I). Set II is made of joints and microfaults with strike and dip parallel to the local slope,

which thus can be interpreted as produced by local slope unbuttressing. Set III is given by high angle NE-striking structures widespread in the

entire island and its geometry is interpreted as deeply influenced by tectonics. Set IV gathers joints developed in the last 13 ka around the sector

collapse zone.


forces from the basement to the cone more than to

pure gravity. Actually, there is geophysical evidence

(Gabbianelli et al., 1993) of NE-striking normal faults

in the basement testified by offset deposits, by a

deepening of the Tyrrhenian basin toward NW, and

by NE-striking lineaments such as canyons.

We retain that gravity forces exert a very high

influence on brittle failures and dyking along the

shoulders of the sector collapse depression. This is

demonstrated by the strong parallelism between post-

13 ka joints and dykes and the collapse surfaces and

by the fact that also sector collapses formed after 13

ka BP (Fig. 12). Based on field relationship, we

suggest that these dykes and joints developed along

the shoulders of the collapse zone especially when it

was re-infilled. Presently, it can be only speculated

that this was due to the difference in density and

compaction between the younger infill and the older

deposits external to the collapse zone, and that dyking

and jointing secondarily occurred when the collapse

depression was empty due to unbuttressing. Dyke

emplacement was also favoured by the presence of

first-order discontinuities such as the collapse surfa-

ces.

Regarding joint spacing, we can interpret its de-

crease at the summit of the cone as the effect of

maximum concentration of ruptures along the NEZ.

We suggest that this is related to the superimposition

of tectonic and magma-related forces. Magma upwell-

ing in correspondence of the main conduit located

near the top of Stromboli, and propagation of dykes

along the NEZ, induce further fracturing of the rocks.

Here, rock strength should also be lower due to pore

pressure increase during magma expansion phases

and hydrothermal alteration. Looking also at joint

dilation data, we can recognise that in the northwest-

ern flank of the volcano, joint spacing is high and

dilation is low, along the NEZ, spacing is low and

dilation is variable, while at the southeastern flank,

spacing is low and dilation is high. This can be

interpreted in the following terms: in the northwestern

flank, the rock succession is younger than in the

opposite flank (Fig. 9), thus, it had less time to record

the incremental deformation. In the southeastern

flank, the oldest rocks of the island crop out, thus,

cumulative deformation can explain the presence of

several joints with high dilation and faults. Along the

NEZ, deformation concentrated both in terms of

higher fracture frequency, locally high dilation

amount and presence of the faults with the largest

offset.

As regards to the evolution processes, we can

argue that no faults were found in the post-Vancori

deposits (younger than 13 ka), whereas systematic

joints are present in the whole succession. This

demonstrates that here, systematic jointing can be

independent from faulting, thus, alternation of shear

and pure tensional stresses can be invoked (Fig. 13).

This can be interpreted as: (1) a decrease in tectonic

force magnitude through time, thus, passing from

faulting-capable forces to jointing-capable forces, (2)Fig. 12. 3-D sketch of fractures and dykes around the sector collapse

zone whose emplacement was favoured by gravity forces.


a different attitude to deformation of the volcano

through time or (3) a faulting recurrence time higher

than 13 ka. Faults do exist in the basement surround-

ing Stromboli, but no historical and instrumental data

account for large seismic events in the upper crust of

the area (Barberi et al., 1993) testifying a low crustal

tectonic seismicity explained as the effect of condi-

tions which allow only a limited storage of stress

(Falsaperla and Spampinato, 1999). Although at

present, we can only reject the third explanation, we

retain, therefore, the first hypothesis as the most

probable. In this case, a higher magnitude of tectonic

forces in the Pleistocene following a NW–SE r3 at

upper crustal level could have facilitated the formation

of the NEZ. Once the NEZ had established, a feed-

back system acted between it, dykes and sector

collapses: the NEZ attracted dykes, which in turn

contributed to induce the volcano flank collapses,

and fluids, which should have further weakened the

NEZ by hydrothermal processes. A NE elongation of

the summit hydrothermal system was recently sug-

gested by Finizola et al. (2002). The unbuttressing on

the NW flank due to sector collapses in turn favoured

the formation of NE-striking joints along the summit

part of the NEZ and joints with other orientation along

the shoulders of the collapse zone. As concerns the

new fractures developed along the Sciara del Fuoco

rim in 2000–2001 A.D., at the present state of knowl-

edge, it is not possible to give a sound explanation

regarding their genesis.

Acknowledgements

We acknowledge the Gruppo Nazionale per la

Vulcanologia of I.N.G.V. for the economic contri-

bution to this research. This study was performed

in the framework of UNESCO-IUGS-IGCP Project

no. 455 ‘‘Effects of basement structural and strati-

graphic heritages on volcano behaviour and impli-

cations for human activities’’. The authors wish to

thank S. Falsaperla and B. van Wyk de Vries for

their constructive reviews.

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Fig. 13. 3-D sketch of faults, joints and main weakness zone whose

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fact that no faults were found in the post-Vancori deposits (younger

than 13 ka) whereas systematic joints are present in the whole

succession, an alternation of shear stresses and pure tensional

stresses can be invoked.


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Deformation at Stromboli volcano (Italy) revealed by rock mechanics and structural geology

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