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JGR – revised 3 Di Bucci et al.
Tectonic evidence for the ongoing Africa-Eurasia convergence in Central
Mediterranean foreland areas: a journey among long-lived shear zones,
large earthquakes, and elusive fault motions
Daniela Di Bucci (1), Pierfrancesco Burrato (2), Paola Vannoli (2), Gianluca Valensise (2)
1) Dipartimento della Protezione Civile, Via Vitorchiano, 4 - 00189 Roma, Italy
2) Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605 - 00143 Roma, Italy
Corresponding Author:
Daniela Di Bucci
Dipartimento della Protezione Civile
Via Vitorchiano, 4
00189 - Roma, Italy
Phone.: ++39-06-68204761
Fax: ++39-06-68202877
e-mail: [email protected]
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Running title: Africa-Eurasia motion: tectonic evidence from Central Mediterranean
Index terms:
8123 Dynamics: seismotectonics
8107 Continental neotectonics (8002)
8111 Continental tectonics: strike-slip and transform
8158 Plate motions: present and recent (3040)
8175 Tectonics and landscape evolution
Keywords: Molise-Gondola shear zone, Vizzini-Scicli shear zone, Gargano Promontory, Hyblean
Plateau, slip reversal, Italy.
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Abstract
We investigate the role of the Africa-Eurasia convergence in the recent tectonic evolution of the
Central Mediterranean. To this end we focused on two sectors of the Adriatic-Hyblean foreland of the
Apennine-Maghrebian chain as they allow tectonic evidence for relative plate motions to be analyzed
aside from the masking effect of other more local tectonic phenomena (e.g. subduction, chain building,
etc.). We present a thorough review of data and interpretations on two major shear zones cutting these
foreland sectors: the E-W, Molise-Gondola (MGsz) in Central Adriatic, and the N-S, Vizzini-Scicli
(VSsz) in Southern Sicily. The selected foreland areas exhibit remarkable similarities, including an
unexpectedly high level of seismicity and the presence of the investigated shear zones since the
Mesozoic. We analyze the tectonic framework, active tectonics and seismicity of each of the foreland
areas, highlighting the evolution of the tectonic understanding. In both areas we find that current strains
at mid-crustal levels seem to respond to the same far-field force oriented NNW-SSE to NW-SE, similar
to the orientation of the Africa-Eurasia convergence. We conclude that this convergence plays a
primary role in the seismotectonics of the Central Mediterranean and is partly accommodated by the
reactivation of large Mesozoic shear zones.
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1. Introduction
The convergence between the Africa and Eurasia plates has dominated the evolution of the
Mediterranean basin since the Cretaceous, controlling the generation, spatial distribution and shape of
all mountain chains and of the intervening basins. During the past two decades space geodesy
observations have shown that the convergence is proceeding vigorously and have set strong constraints
on its modes and rates [Altamini et al., 2007]. In the Central Mediterranean, however, the extent of
orogenic deformation and the complexity of the resulting structures make it extremely hard to
document the ongoing convergence geologically. This is because the active tectonics signature that can
be retrieved from field data is dominated by processes that are strictly related to the evolution of the
mountain chains, such as the extension at the core of the orogenic stack and the compression at the
leading front of the accretionary wedge.
We aim at unearthing tectonic and seismotectonic evidence for the ongoing relative plate
motion between Africa and Eurasia over a large portion of the Central Mediterranean and showing that
this evidence is coherent with the motion inferred from space geodesy (Figure 1). We are well aware
that the effects of continental convergence are in competition with those of other tectonic processes
occurring at more local scale, such as the subduction of the Ionian lithosphere and the build-up of the
Apennines-Maghrebides fold-and-thrust belt. Although these processes ultimately result from the same
remote geodynamic cause, the Africa-Eurasia convergence, their strain rates are usually faster than
those at which deformation caused directly by the convergence takes place. Furthermore, the spacing of
major fault zones these processes generate or reactivate is expected to be smaller – and the field
evidence easier to appreciate – than that characteristic of shear zones directly driven by plate motion.
To achieve our goals we investigated in detail the current tectonics and the seismicity of two
portions of the Apennine-Maghrebian foreland in Italy. Investigations of foreland areas allow first
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order deformation related to plate convergence to be isolated from other tectonic processes occurring at
a more local scale. We analyzed in detail and compared two major foreland deformation zones, the
Molise-Gondola and Vizzini-Scicli shear zones (hereinafter MGsz and VSsz, respectively; Figure 1a).
Both shear zones extend from a genuine foreland area to an inflected foreland buried below the
foredeep deposits and the frontal part of an orogenic wedge.
The Central Mediterranean foreland areas we investigated (i.e. the Mesoadriatic region and the
Hyblean-Malta block) are the locus of significant historical and instrumental seismicity, including
some of the strongest earthquakes in Italian seismic history (up to magnitude ~7; unless otherwise
specified, all subsequent magnitudes are Maw from the CPTI catalogue [CPTI Working Group, 2004] or
Me from the CFTI4Med catalogue [Guidoboni et al., 2007]; both are assumed to coincide with modern
Mw). This severe seismicity is absent in other portions of the Adriatic foreland, such as southern
Apulia.
Despite their numerous similarities, the MGsz and VSsz have never been closely related to each
other, nor has their associated seismicity. In fact, their current state of activity was not correctly
appreciated until very recently, nor were they seen as the actual source of the large earthquakes that fall
near and around them. Why is it so? Both the MGsz and VSsz are long-lasting structures displaying a
strong morphological and structural overprint from a previous tectonic phase, but geological evidence
for the present activity is faint and can be detected only under especially favorable circumstances.
Our paper describes for the first time the tectonic effects of the current Africa-Eurasia
convergence onto the seismotectonics of a large portion of the Central Mediterranean. Previous papers
interpreted local tectonic features as a result of this convergence, but in our case the scale is that of the
Mediterranean plate tectonics.
2. Regional setting
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2.1. Preliminary remarks
The Italian portion of the foreland of the Apennine-Maghrebian fold-and-thrust belt can be subdivided
into three distinct crustal domains: Adriatic, Ionian and Hyblean, respectively from north to south
(Figure 1). The first and third domains are made of continental crust, whereas the second is made of
oceanic or thinned continental crust undergoing subduction beneath the Calabrian arc [Ben-Avraham et
al., 1990; Doglioni et al., 1999; Sartori, 2003; Faccenna et al., 2005]. The three foreland domains have
their counterpart in three corresponding sections of the fold-and-thrust belt. The Apennine-Maghrebian
chain is exposed next to the Adriatic and Hyblean foreland, but in the Calabrian arc, in correspondence
with the Ionian foreland, it is overlain by the Kabilo-Calabride nappes, which have a European-Alpine
origin (e.g. Elter et al., [2003]). In this work we deal with the Adriatic and Hyblean foreland, hosting
the MGsz and the VSsz respectively. These shear zones extend on- and off-shore (Figure 1) and are
spectacularly exposed, both at the outcrop scale and in satellite imagery.
The ca. 180 km-long MGsz is composed of four fault systems, respectively from east to west
(i.e. from the foreland to the chain; Figure 2): the Gondola Fault Zone (off-shore, surface-breaking), the
Mattinata Fault (on-shore, surface-breaking), the Chieuti High (on-shore, buried at shallow depth) and
the Eastern Molise fault system (on-shore, buried at deeper depth), responsible for the 31 October and
1 November 2002, Mw 5.8-5.7, Molise earthquakes.
The >110 km-long VSsz is composed of three fault systems, respectively from south to north
(i.e. from the foreland to the chain; Figure 3): the Scicli off-shore (surface-breaking), the Scicli on-
shore (surface-breaking) and the Vizzini Fault Zone (hidden).
2.2. MGsz regional setting
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The Adriatic foreland of Italy runs to the NE of the Apennines from the Po Plain to Apulia and
corresponds mainly to the Adriatic Sea (Figure 1a). It has an average crustal thickness of ca. 30 km
[Panza et al., 2007] and is characterized by a Meso-Cenozoic sedimentary cover resulting from
palaeogeographic domains of alternating carbonate platforms and pelagic basins [Pieri and Groppi,
1981; Bally et al., 1986; Mostardini and Merlini, 1986]. In particular, the study area hosting the MGsz
(Figure 2) roughly corresponds to the Apulia Platform, a ~6 km-thick succession of shallow-water,
Mesozoic carbonates [Ciaranfi et al., 1988; Ricchetti et al., 1988]. Slope deposits cropping out in the
Gargano Promontory are evidence of a progressive transition to a pelagic basin [Doulcet et al., 1990].
This few kilometers-thick sedimentary cover is underlain by fluvial-deltaic Permo-Triassic deposits
[Butler et al., 2004] and by an igneous/metamorphic Palaeozoic basement [Tiberti et al., 2005, and
references therein].
The oldest evidence for the activity of the MGsz, and particularly of its off-shore portion, dates
back to the Jurassic. During the subsequent Meso-Cenozoic tectonic phases, the MGsz slipped both as
a right- and as a left-lateral major discontinuity in the foreland of the Apennines and Dinarides chains
(see Patacca and Scandone [2004a] for a review). The algebraic sum of strike-slip displacements
estimated for the Gondola portion of the MGsz leads to ca. 15 km of right-lateral offset [De’ Dominicis
and Mazzoldi, 1987] (see subsection 3.3).
The Mattinata Fault is the sole part of the MGsz that is fully exposed. As such it has been the
object of a plethora of studies [e.g.: Funiciello et al., 1988; Winter and Tapponier, 1991; Bertotti et al.,
1999; Chilovi et al., 2000; Billi, 2003] (Figure 2). Mesostructural data, mainly from faults cutting
Mesozoic limestones, show that the strongest structural imprint at the mesoscale was given by the left-
lateral strike-slip kinematics, accompanied by secondary faults, a pull-apart basin and closely-spaced
dissolution-related cleavage [Billi, 2003]. Notice that this sense of shear, associated with contractional
tectonics across the Apennines [Billi et al., 2007], has long coexisted with the NW-SE Africa-Eurasia
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convergence [Mazzoli and Helman, 1994]. Faint yet unambiguous field evidence however shows that
slip has switched to right-lateral around the beginning of the Middle Pleistocene [Piccardi, 1998; 2005;
Tondi et al., 2005].
To the west of the Gargano Promontory, the foreland plunges with a dip of ~10° beneath the
Plio-Pleistocene deposits filling the most recent foredeep of the Southern Apennines [Mariotti and
Doglioni, 2000] (Figure 2), but an E-W ridge aligned with the Mattinata Fault is preserved at the top of
the buried Apulia Platform. This structure, known as Chieuti High, has been interpreted as a horst
[Casnedi and Moruzzi, 1978] and later as a push-up structure related to strike-slip motion [Patacca and
Scandone, 2004a].
Finally, a continuation of the MGsz further to the west as far as the Apennines front had been
hypothesized by Finetti [1982], but no additional constraints for this hidden part of the shear zone were
published before the occurrence of the damaging 31 October-1 November, 2002, Mw 5.8-5.7, Molise
earthquakes [Di Luccio et al., 2005]. These events were caused by right-lateral slip along buried E-W
faults. In the wake of the evidence supplied by these earthquakes Fracassi and Valensise [2007]
reinterpreted the seismotectonics of the 1456 earthquake sequence, possibly the largest in Italian
history, suggesting to extend the buried MGsz further to the west towards the core of the Apennines
(see subsection 3.3).
2.3. VSsz regional setting
The Hyblean foreland is part of the wider Pelagian block [Burollet et al., 1978]. This sector of the
northern African margin is bounded to the east by the Malta Escarpment (Figure 1), an inherited
Mesozoic tectonic boundary separating the continental crust of the Pelagian block from the oceanic
crust of the Ionian basin [Scandone et al., 1981; Ben-Avraham et al., 1990; Grasso, 1993]. The portion
of foreland corresponding to the Hyblean Plateau exhibits a crustal thickness of about 25-30 km
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[Cassinis, 1983] and includes a Meso-Cenozoic sedimentary cover consisting of a thick succession of
shallow and deep water carbonate deposits. Mafic effusive deposits of Triassic-to-Quaternary age,
mainly erupted underwater, crop out along the northern margin of the plateau; at lower stratigraphic
levels they are locally interfingered with the sedimentary sequence, which is then capped by younger
volcanic deposits [Schmincke et al., 1997; Behncke, 2004]. The carbonate succession attains a
maximum thickness of ca. 10 km, but it thins out toward the ancient platform margins [Bianchi et al.,
1987]. Similarly to what is observed in the Adriatic foreland (see subsection 2.2), facies changes within
the succession record vertical movements induced by the opening, evolution and closure of the
Neotethys Ocean and by the build-up of the Apennine-Maghrebian chain in Meso-Cenozoic times
[Yellin-Dror et al., 1997].
Both the on-shore and off-shore portions of the Hyblean foreland are cut by the VSsz, a roughly
N-S-oriented deformation belt (Figure 3) whose early development was reportedly caused by right-
lateral reactivation of a Cretaceous-Late Tertiary fault zone [Grasso and Reuther, 1988]. Grasso et al.
[1990] interpreted the Scicli fault systems, both on- and off-shore, as a reactivated structure coinciding
with a pre-existing crustal inhomogeneity, which comprised a zone of weakness since Mesozoic times.
The role played by this shear zone in Neogene-Quaternary times has been variously interpreted as: (i) a
right-lateral wrenching system [Ghisetti and Vezzani, 1980; Grasso et al., 1986; Grasso and Reuther,
1988]; (ii) a pseudo-transform fault zone induced by a change in crustal thickness at the collision front,
where differential underthrusting took place due to the buoyant Hyblean crust [Ben-Avraham and
Grasso, 1991]; or (iii) an abandoned western margin of the Adria block, acting since the Middle
Pleistocene as the boundary between Africa and Eurasia plates [Catalano et al., 2008b].
The off-shore portion of the Hyblean foreland extends southward through the Hyblean-Malta
continental shelf as far as the Malta Island [Ben-Avraham and Grasso, 1990] and is formed by 30-33
km-thick continental crust [Catalano et al., 2000a]. The Mesozoic to Quaternary overburden consists
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mainly of interlayered volcanic and marine sedimentary successions with a total thickness in excess of
5 km [Jongsma et al., 1985]. The origin of the pre-Triassic substratum within the Hyblean-Malta crust
is unknown [Ben-Avraham and Grasso, 1990].
In SE Sicily the Hyblean foreland comes ashore in the Hyblean Plateau (Figure 3). According
to the literature, the morphology of this elevated plateau is delineated by large normal fault systems
that separate it from the Gela-Catania foredeep to the west and northwest, and from the Ionian basin to
the east. The fault systems forming the on-shore portion of the VSsz are the Scicli on-shore and Vizzini
Fault Zone, which display ca. 70 km of cumulative length (Figure 3). These two fault systems are
composed of four main N-S– to N10°–oriented segments, both west and east facing, extending from
Palagonia to the north to Cava d’Aliga to the south, near the coastline [Catalano et al., 2008a]. The
overall geometry of the shear zone, with main strike-slip faults accompanied by secondary synthetic
and antithetic faults, folds and joint sets, supplies evidence for long-lived dextral shear [e.g. Ghisetti
and Vezzani, 1980].
The best-exposed portion of the VSsz is the 40 km-long Scicli on-shore fault system, which has
a strong morphological and structural imprint. Based on offset drainage patterns Catalano et al.
[2008b] estimated a right-lateral displacement of ca. 2,000 m along the central part of this fault system.
This offset was attained between 1.2-1.5 and 0.85 Ma, when its causative stress regime came to an end.
To the north, the surface evidence of the Vizzini Fault Zone becomes fainter as individual fault
planes are exposed discontinuously (Figure 3). Nevertheless, the presence of this fault zone is
suggested by various lines of evidence:
(i) the presence of foreland limestones, that crop out only to the west of the inferred fault trace;
they are uplifted along the western limb of the fault zone, downthrown and buried along the
eastern one;
(ii) the existence of large flows of basaltic pillow lavas blanketing only the eastern fault block;
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(iii) a N-S alignment of historical earthquakes, that fits well with the fault trace; and
(iv) the instrumental seismicity, that also concentrates along this trend.
These latter two points will be thoroughly discussed in subsections 3.2 and 3.4.
To the south, along the Hyblean-Malta continental shelf, the off-shore part of the Hyblean
foreland is affected by a number of SW-NE to SSW-NNE high-angle faults, some of which continue
on-shore [Ronco et al., 1990; Mattavelli et al., 1993] (Figure 3). To the west, the Hyblean-Malta
continental shelf margin corresponds roughly with one of these faults, the Scicli off-shore described
earlier [Grasso et al., 1990]. To the east, it is bounded by the Malta Escarpment, which marks the
progressive transition from continental to oceanic crust in the Ionian Sea [Catalano et al., 2000a;
Finetti, 2005] (Figure 1).
There is substantial agreement on considering the faults that dissect the Hyblean-Malta
continental shelf as predominantly strike-slip. In contrast, their sense of motion has been variously
interpreted (e.g., left-lateral in Ronco et al. [1990], right-lateral in Grasso et al. [1990]). Based on
evidence of syn-sedimentary activity, these faults are assumed to have operated since the Miocene.
Available seismic reflection lines allow the Scicli off-shore to be mapped for a minimum length of 40
km [Grasso et al., 2000a].
3. Active tectonics of the foreland shear zones
3.1. Major seismicity
The Adriatic and Hyblean foreland areas are the locus of two of the strongest earthquakes in Italian
seismic history: the 30 July 1627, Gargano (Mw 6.8, Imax X) and the 11 January 1693, Eastern Sicily
(Mw 7.4, Imax XI), respectively. They were both complex events generated by the rupture of adjacent or
nearby fault segments, and caused widespread destruction and significant effects on the environment
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[Guidoboni et al., 2007] (Figures 2 and 3). The 1627 Gargano earthquake was followed on the same
day by a Mw 5.8 aftershock (Imax VIII-IX); the sequence continued for about one year (Table 2a). The
1693 Sicily earthquake was felt all over eastern Sicily and was preceded by a strong foreshock on 9
January (Mw 6.2, Imax VIII-IX) [Boschi and Guidoboni, 2001]. The seismic sequence lasted until 1696.
Most likely the mainshock itself included two or more subevents, as suggested by the existence of two
separate concentrations of damage, but this hypothesis is not explicitly backed by the available
historical sources.
In spite of the severity of damage and the widespread environmental effects, for both these
earthquake sequences the identification of the causative source is still the object of a lively debate
owing to: (i) the absence of surface faulting; (ii) the proximity of the two mesoseismal areas to the
coast, causing an asymmetry of information; (iii) the occurrence of a tsunami following the
mainshocks, suggesting that the earthquake sources were located near- or off-shore; (iv) the presence of
multiple shocks; and (v) the long recurrence interval of major earthquakes in Southern Italy, typically
longer than a millennium [e.g. Basili et al., 2008; Galli et al., 2008], which prevents more than one
complete seismic cycle from being represented in the historical record, making the identification of
seismogenic sources especially challenging.
Due to the occurrence of a tsunami following the mainshock [Tinti et al., 2004] and to possible
coseismic coastal uplift [Mastronuzzi and Sansò, 2002], the source of the 1627 Gargano earthquake
was proposed to lie off-shore. Such a source, however, would explain the occurrence of the tsunami but
not the observed damage pattern, since the largest shaking took place inland, 10-20 km from the
coastline and south of the Lesina Lake (Figure 2). Most of the proposed sources for the 1627 Gargano
earthquake are instead on-shore [Salvi et al., 1999; Patacca and Scandone, 2004a; DISS Working
Group, 2010] and compare well with the macroseismic source and with most of the available structural
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evidence, but do not explain well the tsunami, for which further investigations and hypotheses are
needed (e.g., shaking-induced submarine landslides).
Based on the occurrence of a large tsunami, also the source of the 1693 Sicily earthquake was
thought to lie off-shore [e.g., Gutscher et al., 2006; Gerardi et al., 2008], and specifically along the
Malta Escarpment [Piatanesi and Tinti, 1998; Bianca et al., 1999; Azzaro and Barbano, 2000; Jacques
et al., 2001; Argnani and Bonazzi, 2005, among others]. Also in this case the main open question
regards the mismatch between the hypothesized off-shore source and the damage pattern, which
displays the highest intensities inland (Guidoboni [2001]). We remark that, similarly to the case of
1627, only an inland source is compatible with the observed intensity distribution. An off-shore source
(a) would not explain the observed intensity decay along the coast and (b) would require an
unrealistically high seismic moment to account for the very large intensities (X and XI) reported over
50 km west of the Ionian coast.
Also for the causative fault of the 1693 Sicily earthquake different analyses and input data
returned a spectrum of widely different hypotheses. Sirovich and Pettenati [2001] proposed a NNE-
oriented, 60 km long-fault nearly overlapping the VSsz and characterized by oblique motion. At the
opposite end of the spectrum, Gutscher et al. [2006] proposed that the 1693 Sicily earthquake was
caused by slip over a portion of the locked basal thrust of the the Calabrian arc. Lavecchia et al. [2007]
suggested that the 1693 Sicily earthquake was caused by a segment of the S-verging, N-dipping basal
thrust of the Apennine-Maghrebian chain, corresponding to the outermost thrust front (Figure 3).
Finally, based on the hypothesis that the 1693 was a complex earthquake, the Database of Individual
Seismogenic Sources (DISS version 3.1.1 [DISS Working Group, 2010]) proposes the activation of two
opposite-verging compressional faults: a SE-dipping reverse fault located beneath the northern margin
of the Hyblean Plateau, and a NW-dipping segment of the Apennine-Maghrebian thrust front near
Catania [Bousquet and Lanzafame, 1986] (Figure 3).
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Several other historical earthquakes hit the Gargano Promontory and the Hyblean Plateau
besides 1627 and 1693 (Table 2). In the Gargano, historical seismicity concentrates near the eastern
and northern coasts, testifying to the activity of the Mattinata Fault and of other still unknown
structures (Figure 2). Sizable earthquakes occurred on 31 May 1646 (Mw 6.2) and 6 December 1875
(Mw 6.1). The historical seismicity of the Hyblean foreland is much more significant (Figure 3) and
includes several destructive earthquakes, some followed by a tsunami [Tinti et al., 2004], peaking at
Mw 6.8 on 10 December 1542. Among them, the 3 October 1624, Mw 5.6, Mineo and the 1 March
1818, Mw 5.5, Hyblean earthquakes locate along the trace of the Vizzini Fault Zone, thus testifying to
the activity of the northernmost reach of the VSsz.
3.2. Active stresses and strains
Instrumental seismicity. The hypocentral distribution of selected instrumental earthquakes from the
Italian Seismic Catalogue (CSI [Castello et al., 2006]) shows that the seismogenic layer attains a
thickness of more than 20 km beneath the external areas of the Apennine-Maghrebian fold-and-thrust
belt in both the Adriatic and Hyblean forelands [Chiarabba et al., 2005].
In the region hosting the MGsz, earthquakes extend down to the depth of the Moho (ca. 30 km)
and concentrate between 15 and 25 km. Fault plane solutions indicate a prevailing strike-slip to oblique
tectonic regime with most of the P-axes striking NW-SE (Figure 2). Milano et al. [2005] showed that
the Gargano seismicity is generated by E-W, right-lateral strike-slip faults belonging to the MGsz, and
by NW-SE, normal to left-lateral second-order faults slipping in response to dominantly NW-SE
shortening. The 31 October-1 November 2002 Molise earthquake sequence shed light on the potential
of the westernmost reach of the MGsz (Figure 2). Two similarly large (Mw 5.8-5.7) and relatively deep
(16-18 km) mainshocks occurred 24 hours and few kilometers apart with the same pure right-lateral
focal mechanism on E-W nodal planes [Vallée and Di Luccio, 2005]. The aftershocks concentrated
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along a ca. 20 km-long, sub-vertical E-W fault zone, revealing that the mainshocks ruptured a right-
lateral fault zone composed of two segments; no surface faulting was reported. The focal mechanisms
of the mainshocks and of about 170 aftershocks showed coherent kinematics and consistently sub-
horizontal, NW-SE trending P-axes [Chiarabba et al., 2005; Di Luccio et al., 2005].
The seismicity and stress regime of the Hyblean foreland (Figure 3) was investigated in detail
by Musumeci et al. [2005]. Similarly to the Gargano area, seismicity extends down to the depth of the
Moho, about 30 km, and concentrates in the 15-25 km depth interval. NNE-SSW and NE-SW left-
lateral strike-slip faulting mechanisms are common in the western and central sector of the Hyblean
Plateau, suggesting sinistral motion along the VSsz. Coexisting normal faulting on NNW-SSE planes is
observed near the Ionian coastline and along the Malta Escarpment. The 13 December 1990, ML 5.4
earthquake that occurred off-shore north of Siracusa (Figure 3), however, was caused by right-lateral
slip on a vertical E-W plane, or by left-lateral slip on a N-S plane [Amato et al., 1995]. In map view
this earthquake falls on or just to the west of the Malta Escarpment, but its depth (22 km) indicates that
it occurred well below the rather shallow NE-dipping normal faults that delineate the submarine scarp.
All these findings support the existence of a compressional regime in the Hyblean foreland, with the
maximum stress axis consistent with the direction of the Africa-Eurasia convergence (NNW-SSE to
NW-SE; Figure 1). Earthquakes located east of the Malta island display strike-slip focal solutions with
P-axes oriented roughly N-S [Vannucci and Gasperini, 2004; Pondrelli et al., 2006], in agreement with
the same tectonic regime.
Present-day stress field. The analysis of good quality breakout data acquired in the Hyblean foreland
shows a ENE-WSW to E-W oriented Shmin direction [Ragg et al., 1999; Montone et al., 2004] (Figure
3). No breakout data are available for the Adriatic foreland near the Gargano Promontory (Figure 2),
but to the south, within the same foreland, the stress map of Italy [Montone et al., 2004] lists a WSW-
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ENE oriented Shmin from break-out analysis. This is compatible with stress indicators obtained from
focal mechanisms of earthquakes with M >3, Quaternary structural data and in situ stress
measurements. Finally, Barba et al. [2008] computed the maximum horizontal stress azimuth in this
area through a finite-element model and compared it with available stress indicators, returning a NNW-
SSE striking ShMax.
Present-day strains. The evolution of eastern Sicily and of the Hyblean foreland during the Neogene
and Quaternary has traditionally been interpreted as the result of slow convergence of the Africa and
Eurasia plates in a roughly NW-SE direction of relative motion [e.g. Argus et al., 1989; DeMets et al.,
1994] (Figure 1b). At the scale of the country, studies using GPS velocities from campaigns
[Hollenstein et al., 2003] integrated with data from permanent sites [D’Agostino and Selvaggi, 2004;
Serpelloni et al., 2005; D’Agostino et al., 2008; Devoti et al., 2008] confirm that in a fixed Eurasia
reference frame the Hyblean foreland moves towards the N-NW, quite coherently with the Africa plate.
Moreover, recent analyses based on long GPS time-series demonstrate that the Africa-Eurasia
kinematics does control the long-wavelength tectonic deformation in the Central Mediterranean
[Marotta and Sabadini, 2008] (Figure 1b). GPS solutions for southern Sicily, between the Hyblean
foreland and the Apennine-Maghrebian chain, suggest NNW-SSE directed compression and ENE-
WSW extension, compatible with a transcurrent regime [Serpelloni et al., 2005].
At a more local scale, Ferranti et al. [2008] addressed the interseismic deformation of the
Hyblean Plateau by analyzing new campaign GPS velocities. The observed displacements suggest
active displacement along the buried thrust front of the Apennine-Maghrebian chain, in agreement with
the orientation of the Shmin from breakout analysis [Montone et al., 2004]. Ferranti et al. [2008],
however, could not infer any active motion along the VSsz, probably due to the large uncertainties
associated with the position of the temporary GPS sites straddling the fault zone.
16
JGR – revised 3 Di Bucci et al.
GPS studies at the scale of the country suggest that the central-southern part of peninsular Italy
is undergoing SW-NE extension nearly everywhere [Serpelloni et al., 2005, 2007; D’Agostino et al.,
2008; Devoti et al., 2008]. In the region hosting the MGsz, however, the GPS coverage by permanent
sites is poor and only studies based on campaign measurements supply observations of the present-day
velocity field. For instance, Giuliani et al. [2007] analyzed coseismic strains caused by the 2002
Molise seismic sequence by comparing GPS campaign data supplying positive evidence for
deformation induced by slip on a buried right-lateral fault, in agreement with seismological evidence.
Ferranti et al. [2008] analyzed the relationships between GPS site velocities and interseismic
deformation along the Mattinata Fault; the northernmost two sites among those measured straddle the
MGsz and yield a right-lateral slip rate of about 2 mm/a. Finally, to the south of Gargano, few GPS
data acquired in the Apulia area indicate NNE relative motion of this part of the Adriatic foreland
referred to a fixed Eurasia plate [Devoti et al., 2008].
3.3. Active tectonics of the MGsz
The MGsz appears as a ca. 15 km-wide and 180 km-long corridor, running at the latitude 41.6°N from
the Adriatic foreland off-shore to the core of the Apennines fold-and-thrust belt (Figure 2). The current
activity of its off-shore portion, the Gondola Fault Zone, is testified by recent studies based on high-
resolution seismic reflection data. These studies allowed the Middle-Late Pleistocene and Holocene
activity of this fault zone to be mapped with great accuracy over a length of ca. 50 km. The fault
exhibits dominant right-lateral motion and a horizontal slip-rate ranging between 0.08-0.72 mm/a
[Ridente and Trincardi, 2006; Ridente et al., 2008; Di Bucci et al., 2009].
Moving westward, the MGsz comes ashore in the Gargano Promontory as Mattinata Fault
(Figures 2). Most investigators agree on its present-day dominantly right-lateral sense of motion, also
supported by earthquake focal mechanisms (MEDNET [2006]; Pondrelli et al. [2006]; Figure 2), GPS
17
JGR – revised 3 Di Bucci et al.
data from permanent [Devoti et al., 2008] and non-permanent [Oldow and Ferranti, 2006; Ferranti et
al., 2008] stations, InSAR studies [Atzori et al., 2007], geological [Tondi et al., 2005] and
paleoseismological [Piccardi, 2005] investigations. The Mattinata Fault has also been interpreted as the
source of sizable historical earthquakes (e.g., 493 A.D., Baratta [1901], referenced in Piccardi [1998];
6 December 1875, Mw 6.1 [Valensise and Pantosti, 2001; DISS Working Group, 2010]), and
instrumental seismicity is frequently recorded within the uppermost 25 km of the crust throughout the
entire Gargano Promontory (see subsection 3.2).
Unlike the current fault kinematics, the determination of the cumulative horizontal displacement
associated with the ongoing deformation phase is an open issue. Chilovi et al. [2000] suggested that the
Mattinata Fault has acted as a right-lateral fault since the Late Pliocene and assigned to it a cumulative
post-Late Pliocene displacement of 15 km, implying an average slip-rate of about 6 mm/a. The 15 km
estimate, however, was originally proposed by De’ Dominicis and Mazzoldi [1987] as a cumulative
value for the entire slip history of the Gondola Fault Zone. In contrast, Piccardi [1998] used
observations of offset streams and subtle landscape features to propose a horizontal component of the
slip-rate in the order of 1 mm/a. Based on analogue modeling Di Bucci et al. [2006] supported the
hypothesis that the during its most recent phase of activity the MGsz has slipped at about 1.3 mm/a, in
agreement with the figure proposed by Piccardi [1998]. Tondi et al. [2005] used structural evidence to
estimate a late Pleistocene and Holocene slip rate of 0.7-0.8 mm/a for the Mattinata Fault.
Further to the west, the foreland plunges below the Plio-Pleistocene deposits filling the foredeep
of the Southern Apennines. As stated earlier, the buried Chieuti High (Figure 2) is accompanied by
WNW-ESE-striking, SSW-dipping, dominantly extensional faults and by intervening downthrown
blocks. Patacca and Scandone [2004a] interpreted one of these extensional structures, the Apricena
Fault (to the south of “Ap” in Figure 2), as the source of the 30 July 1627 Gargano earthquake.
18
JGR – revised 3 Di Bucci et al.
Scattered clues of recent activity on E-W structures, both in this area and more to the west, are supplied
by the drainage pattern that shows consistent E-W trending anomalies [Valensise et al., 2004].
Finally, the Apulia Platform and the underlying basement deepen even further below the outer
front of the Apennines orogenic wedge (Figure 2), where the 2002 Molise earthquakes occurred.
3.4. Active tectonics of the VSsz
The N-S striking VSsz runs around the longitude 14.7°E and can be traced for a total length of ca. 110
km, both on-shore across the Hyblean foreland and off-shore (Figure 3). The off-shore portion of the
VSsz has been traced for a length of ca. 40 km by interpreting deep well logs and seismic reflection
lines [Grasso et al., 2000a]. Unlike other faults cutting the Hyblean-Malta continental shelf, the Scicli
off-shore appears to cut the main unconformity at the top of the Messinian evaporites and affect the
Plio-Pleistocene succession, occasionally up to the seafloor. Hence, the Scicli off-shore stands out
among several faults cutting the same tectonic domain as one that displays clear hints of recent-to-
active deformation. Unfortunately, the state of activity of the Scicli off-shore is not backed by
background seismicity data (CSI [Castello et al., 2006]; Figure 3). Nevertheless, the Italian CMT
dataset [Pondrelli et al., 2006] shows that two earthquakes that occurred near the southern end of the
Scicli off-shore (21 March 1972, Mw 4.8, and 29 October 1990, ML 4.3) were caused by left-lateral
motion along roughly N-S planes, therefore displaying the same tectonic style seen in the Hyblean
Plateau.
The Scicli on-shore fault system is composed by a N10°-striking principal fault zone and by
conjugate fault planes and associated secondary structures. This system can be subdivided into three
main segments: the Cava d’Aliga, Ragusa and Giarratana faults, respectively from south to north
[Catalano et al., 2008b] (Figure 3). The 40 km-long fault system running between Cava d’Aliga and
Giarratana is the best-exposed portion of the entire VSsz. Standard structural analyses show that the
19
JGR – revised 3 Di Bucci et al.
Scicli on-shore developed as a right-lateral shear zone under a NE-SW oriented ShMax [e.g. Ghisetti and
Vezzani, 1980], but the most recent kinematic indicators measured at several locations supply evidence
for present-day sinistral motion under a ShMax trending NW-SE [Catalano et al., 2008b]. Near the
coast, several strands of the Cava d’Aliga segment displace late Quaternary marine terraces [Grasso
and Reuther, 1988] and the drainage network with a total estimated sinistral offset of ca. 150 m and a
vertical offset of ca. 10 m, yielding a slip rate of ca. 1.2 mm/a [Catalano et al., 2008b]. To the north,
the drainage network crossing the Ragusa and Giarratana fault segments displays a total sinistral off-set
of ca. 1,250 m, yielding an average late Quaternary slip rate of ca. 1.4 mm/a [Catalano et al., 2008b].
The activity of the on-shore portion of the VSsz is testified also by relatively frequent
instrumental earthquakes showing consistent strike-slip kinematics [Musumeci et al., 2005; Pondrelli et
al., 2006] (see subsection 3.2) and by limited historical evidence (Figure 3). Based on macroseismic
patterns Azzaro and Barbano [2000] tentatively assigned to the Scicli on-shore fault system two
moderate-size earthquakes (8 October 1949, Noto, Mw 5.2, and 23 January 1980, Modica, Mw 4.6; see
also Figure 3 and Table 1b).
The structural and morphological evidence of the VSsz fades away to the north of Giarratana
(Figure 3); faint evidence for Quaternary activity is available only near the village of Vizzini [Grasso
and Reuther, 1988]. Nevertheless, a swarm of historical and instrumental earthquakes up to magnitude
5.6 aligns along a roughly N-S trend corresponding with the suggested fault trace.
Further to the north, the Hyblean foreland plunges beneath the Gela-Catania foredeep and the
external front of the Apennine-Maghrebian chain of Sicily (i.e., the Gela nappe) [Lentini, 1982;
Bianchi et al., 1987; Carbone et al., 1987; Bigi et al., 1992] (Figure 3). Whether and how the VSsz
continues across this area is unknown: no data are available to even speculate on this issue.
4. Discussion
20
JGR – revised 3 Di Bucci et al.
4.1. Comparing the MGsz and the VSsz
The data presented in the previous sections highlight the strong similarity between the MG and VS
shear zones (Figure 4). They are comparable from a geometric and kinematic point of view; both are
more than 100 km-long and formed by 30-50 km-long fault systems, which in some cases can be
further subdivided into 10-15 km-long fault segments, exhibit high angle-fault planes and are
dominantly strike-slip. Figure 5 schematizes the relation between Africa-Eurasia relative motion and
strike-slip motion along the MGsz and VSsz. The two shear zones strike perpendicularly to each other
and their sense of shear is opposite, as if they behaved mirror-like with respect to a NW-SE striking,
vertical symmetry plane.
The MGsz and VSsz are not only similar in many respects, but are also located in similar
tectonic environments. Both extend from an open foreland area to the outer front of a fold-and-thrust
belt. In detail, both shear zones display two fault systems in the foreland, the first of which is
submerged (Gondola Fault Zone and Scicli off-shore, respectively) whereas the second one is exposed
in the mainland (Mattinata Fault and Scicli on-shore, respectively). A third fault system is located near
the front of the chain, buried below the foredeep deposits or covered by pillow lavas (Chieuti High and
Vizzini Fault Zone, respectively). The MGsz also exhibits a fourth fault system buried below the
frontal part of the Apennines wedge.
Both shear zones display present-day activity due to reactivation of regional structures inherited
at least since Mesozoic times. These large regional fault zones, which dissect the foreland crust, have
experienced long-lasting activity under different tectonic regimes, that is to say, with different
kinematics at different times. For both, the inception of the present day activity and thus the most
recent slip-reversal occurred at about the same time, around the beginning of the Middle Pleistocene.
The most recent slip reversal of the MGsz has been interpreted as related to the contemporaneous
21
JGR – revised 3 Di Bucci et al.
deactivation of the Southern Apennines frontal thrust ramp (Di Bucci et al. [2006], with references). In
Sicily, the frontal thrust ramp of the Gela nappe in correspondence with the VSsz ceased its motion
roughly at the same time [Patacca and Scandone, 2004b]. Therefore, we hypothesize a similar reason
also for the slip reversal along the VSsz. Notice that the frontal thrust deactivation must not necessarily
correspond to the end of the contraction in the whole chain, as it depends on the critical taper of the
wedge front.
The current activity of the MGsz and VSsz is therefore extremely young, at least in comparison
with their long slip history. This circumstance has two main implications: (i) the morphological and
structural expression of both shear zones is mostly imprinted by the penultimate strike-slip kinematics
(left-lateral for the MGsz and right-lateral for the VSsz) and in fact this is the deformation style that
was described in early papers predating the development of seismotectonic studies and the current GPS
era; (ii) the evidence for current tectonic activity is quite elusive and can be detected with confidence
only through the integration of detailed and specific methods of analysis. In the absence of this
integration, the sense of shear and even the mere state of activity or quiescence of both the MGsz and
VSsz would still be debated; (iii) the horizontal slip-rates associated with the current style of activity
are also comparable and are in the order of 1 mm/a, in agreement with the typical rates of Italian faults
[Basili et al., 2008; Galli et al., 2008].
Finally, the foreland areas hosting the MGsz and VSsz are both marked by severe seismicity
that is nearly absent elsewhere in the Adriatic and Hyblean foreland. Some of these earthquakes (Table
2) have been positively associated with the fault systems comprising the investigated shear zones
(Figure 4). The complexity of the largest earthquakes and the moderate magnitude of the smaller ones
(see subsection 3.1) are compatible with the extreme state of fragmentation displayed by some of the
fault systems, suggesting that the reactivation of inherited shear zones in the Apennine-Maghrebian
foreland proceeds by limited fault segments.
22
JGR – revised 3 Di Bucci et al.
4.2. The shear zones engine
As outlined above, the analogies between the MGsz and the VSsz extend to several different topics.
Among these, (i) the crustal scale of the shear zones, (ii) their tectonic context, (iii) the nearly
contemporaneous inception of the current activity and (iv) the comparable slip rates, all drive the
discussion toward the hypothesis that the MGsz and the VSsz also share a similar geodynamic cause.
So far, all geodynamic interpretations for the current activity of these shear zones have considered each
of them separately: this paper is the first attempt to devise a unified interpretation.
The E-W deformation belt extending between the Gargano Promontory and the Central Adriatic
Sea has been traditionally considered related to lithospheric heterogeneities, and in particular to a
change in the thickness of the Adriatic lithosphere, that thins out towards the north [Calcagnile and
Panza, 1981; Doglioni et al., 1994]. More specifically, this deformation belt was interpreted as a right-
lateral transfer zone accommodating higher roll-back velocities in the northern Adriatic slab with
respect to the southern part [Doglioni et al., 1994]. A more recent view emphasizes the role of the NW-
SE Africa-Eurasia plate convergence in controlling the seismotectonics of the Italian peninsula. In this
perspective, motion on the MGsz has been interpreted as the reactivation of an inherited belt of
deformation induced by such convergence [Di Bucci and Mazzoli, 2003; Valensise et al., 2004; Ridente
et al., 2008; Scisciani and Calamita, 2009; Latorre et al., in press].
As far as the geodynamic role currently played by the VSsz is concerned, Catalano et al.
[2008b] proposed that this shear zone may have been acting since 850 ka as a local expression of the
Africa-Eurasia plate boundary. This interpretation is quantitatively based on the 2.1 mm/a NW-SE
shortening rate obtained by converting the slip rate estimated for the Scicli on-shore, a figure to be
compared with the 2.6 mm/a NW-SE shortening rate from GPS data. Identifying in detail the plate and
microplate boundaries in the Central Mediterranean is beyond the aims of this paper, which deals only
23
JGR – revised 3 Di Bucci et al.
with first order evidence of plate motion. GPS data are providing an invaluable contribution to this
debate, but the extraordinary geodynamic complexity of this part of the Mediterranean region still does
not allow GPS displacements to be unequivocally correlated with plate motions, as recently
demonstrated for the Adria block by Devoti et al. [2008]. Therefore, we prefer to interpret the VSsz
more conservatively, that is to say, as an intraplate deformation belt.
It is worth noticing that the most recent literature suggests a possible role played by the Africa
plate motion to explain at regional scale the activity of the MGsz or of the VSsz. In fact, the
improvement of GPS data from permanent stations nationwide is showing with increasing confidence
that the convergence between the Africa and Eurasia plates affects the entire Italian peninsula [Devoti
et al., 2008; Marotta and Sabadini, 2008], although the relatively low velocities of convergence
observed can be locally concealed by the superposition of other, more localized deformation
phenomena, such as the ongoing extension along the axis of the Apennines. In spite of this, also in
complex areas such as the Calabrian arc, where the ongoing subduction of oceanic crust is a dominant
feature, the convergence between the Africa and Eurasia plates has been positively recongnised as a
measurable background component of the subduction velocity [Devoti et al., 2008].
In this general framework, the northeastward motion detected along the Adriatic-facing side of
the Italian peninsula requires a separate discussion. We remark that in the southern portion of this side
of the peninsula GPS data [D’Agostino et al., 2008; Devoti et al., 2008] are partly measured on the
exposed foreland (e.g., see MATE in Figure 1) and partly on the Southern Apennines thrust belt. In the
northern part of the Adriatic side, GPS data are all measured on the Northern Apennines thrust belt. In
its turn, the core of the Apennines is undergoing SW-NE extension everywhere, whereas the outer front
is interpreted either as under active thrusting [Doglioni et al., 1994; Vannoli et al., 2004] or as sealed
by intervening sedimentation [Patacca and Scandone, 2001; Di Bucci et al., 2003]. Based on these
circumstances, the assumption that all GPS stations along the Adriatic side of the Italian peninsula may
24
JGR – revised 3 Di Bucci et al.
be used to define the Adria plate motion needs to be carefully considered. If this assumption is correct,
however, GPS data imply a general anticlockwise rotation of Adria as observed velocities decrease
toward the NW. We interpret this rotation as an effect of the Africa-Eurasia convergence combined
with the drag due to the westward drift of the lithosphere relative to the underlying mantle [Panza et
al., 2007].
The direction of the relative motion between Eurasia and Africa lies roughly at 45° with respect
to the direction of both the MGsz and VSsz (Figure 5), which act mirror-like with opposite sense of
shear, as expected. This peculiar configuration implies that, being the MGsz and VSsz similar, the
effect of the Africa-Eurasia convergence on one of them should be the same as on the other one,
exception made for the obvious mirroring effects. Therefore, the existing interpretation of the MGsz as
an inherited belt of deformation whose reactivation is induced by Africa-Eurasia plate convergence can
be adopted for the VSsz as well, thus providing a single remote cause for the current activity of both
shear zones. This is compatible with the strain partitioning of the Africa-Eurasia convergence, that
between the two shear zones is largely taken up by the subduction beneath the Calabrian arc.
Moreover, in the Adriatic foreland exposed in southernmost Apulia, quantitative analysis of
extensional joints showed that the the Africa-Eurasia convergence played a detectable role in the Upper
Pleistocene and possibly in the current tectonics [Di Bucci et al., 2010]. Finally, the 1990 Potenza
earthquake [Ekström, 1994] (Figure 1a) shows that the same deformation style also characterizes other
parts of the Adriatic foreland buried under the Southern Apennines south of the Gargano area.
4.3. Seismotectonic implications for the Italian foreland
In addition to providing a unified cause for the current activity of all foreland areas of the Apennines-
Maghrebian chain in the Central Mediterranean, the model proposed here implicitly suggests to
25
JGR – revised 3 Di Bucci et al.
consider the active tectonics of the foreland and of the related chain separately, both in the Italian
peninsula and in Sicily. The dynamics we are focusing on deals with long-wavelength intraplate
deformation, and therefore subtends the shallower and more local tectonic activity of the chain.
The distribution of major seismicity along the MGsz and VSsz shear zones indicates that the
Africa-Eurasia plate convergence is more likely to reactivate pre-existing zones of weakness within the
foreland crust than to generate brand-new faults. This means that, when dealing with the seismic hazard
of the Italian foreland areas, particular attention should be paid to those zones characterized by Meso-
Cenozoic fault systems at regional scale; our interpretation suggests that, if favorably oriented, they
could be reactivated and generate earthquakes. For instance, another E-W–striking regional fault
system cuts the Adriatic foreland to the south of the MGsz, running roughly from Taranto to Potenza
[Ciaranfi et al., 1983; Ambrosetti et al., 1987; Cinque et al., 2000; Servizio Geologico d’Italia, 2004;
Boncio et al., 2007; DISS Working Group, 2010] (Figure 1). As previously mentioned, between May
1990 and May 1991, a seismic swarm very similar to the 2002 Molise sequence struck the city of
Potenza. Hundreds of aftershock aligned along a previously unknown E-W, 30 km–long lineament
centered on the 5 May, Mw 5.8 mainshock, which displayed a pure strike-slip mechanism with right-
lateral slip on an E-W–trending plane [Azzara et al., 1993; Ekström, 1994; Boncio et al., 2007] (Figure
1 and Table 1a). The Potenza shear zone has already been interpreted as an equivalent of the MGsz [Di
Bucci et al., 2006], and can now be better understood in the wider geodynamic scheme proposed here
along with other major E-W lineaments cutting the southern Apennines [DISS Working Group, 2010].
Other active regional, NW-SE–striking, right-lateral fault zones of the northern Adriatic foreland, such
as the Dinaric System in Western Slovenia [Burrato et al., 2008; Kastelic et al., 2008; Scisciani and
Calamita, 2009] and the Schio-Vicenza Line in the northern Po Plain of Italy [Viganò et al., 2008] may
similarly fall within the new interpretative model.
26
JGR – revised 3 Di Bucci et al.
In contrast, no other major regional shear zones parallel and comparable to the VSsz are known
at present, possibly because this foreland domain is less extended than the Adriatic domain. Based on
the interpretation of seismic lines Argnani et al. [1986] recognized a N-S Plio-Quaternary transcurrent
belt to the west of the Gela off-shore. Finetti and Del Ben [2005] suggested that left-lateral strike-slip
faults similar to those dissecting the Hyblean Plateau occur in the Pelagian block, off-shore
southwestern Sicily. Moreover, Catalano et al. [2009] postulated the occurrence of NNE–trending
normal faults in the Sicily Channel based on the distribution of volcanic submarine centres and
morphotectonic analyses of bathymetric maps. These structures deserve further investigations to
constrain better their state of activity and kinematics. The detection of any feeble evidence of activity
comparable with that of the VSsz, however, is made difficult by the opening of the Sicily Channel rift
zone [Grasso et al., 1985] (Figure 1a), that has left a strong imprint on the current tectonics of a large
part of this region.
A more general issue that deserves to be addressed in the light of our interpretative model is
whether or not the Africa-Eurasia plate convergence may reactivate (part of) the regional fault zone
comprising the Malta Escarpment (Figure 1a and 3). This Mesozoic passive margin separates the
continental crust of the Hyblean foreland from the oceanic crust of the Ionian foreland (Figure 5) by
means of NNW-SSE-striking, ENE-dipping normal faults [Scandone et al., 1981; Casero et al., 1984;
Catalano et al., 2000b]. These faults have been progressively tilted with a bookshelf mechanism up to
a dip angle of 30° [Argnani and Bonazzi, 2005], an unfavorable geometry for a strike-slip reactivation
within the Africa-Eurasia convergence. Therefore, we might expect either no current activity or a mild
left-lateral transpression on this inherited regional fault system. In fact, the most recent and accurate
marine geology investigations of the Malta Escarpment [Argnani and Bonazzi, 2005] showed no
evidence of current activity south of Siracusa. To the north of Siracusa, seismic reflection profiles show
normal faults parallel to the Malta Escarpment and deeply rooted beneath the Calabrian arc that have
27
JGR – revised 3 Di Bucci et al.
experienced tectonic inversion characterized by mild left-lateral transpression during the Quaternary.
Mesostructural analyses of youthful on-shore deposits support an active strike-slip regime with 1
striking NW-SE [Adam et al., 2000]. Our interpretative model fits well both with these data and with
the few focal mechanisms available for the same area, showing dominant strike-slip with P-axis
oriented NW-SE (Figure 3). Further to the north, the tectonic setting becomes more complex, and the
superposition of different tectonic features (the Malta Escarpment: 1- is buried under the Sicilian
portion of the Calabrian arc, which is active; 2- is overlain by Mount Etna; and 3- is not far from the
retreating hinge of the Tyrrhenian slab; Figure 5) precludes the detection of any component of
deformation due to the Africa-Eurasia convergence. Our interpretation implies that Ionian and Hyblean
foreland domains move jointly to the NNW-NW, broadly in accordance with the Africa plate motion.
This circumstance has been recently invoked to interpret GPS data [Oldow et al., 2002; Oldow and
Ferranti, 2006; D’Agostino et al., 2008].
On the northeastern side of the Ionian foreland, the other passive margin which separates the
oceanic crust from the continental crust of the Adriatic foreland is buried below the Southern
Apennines [Van Dijk et al., 2000] (Figure 5). Only a small portion of it located SE of the submerged
front of the Calabrian arc could therefore be envisaged as an equivalent of the Malta Escarpment
[Finetti et al., 1996]. An additional regional fault zone affecting the Adriatic foreland that deserves to
be looked at is the Apulia Escarpment, a long fault zone running roughly NW-SE bounding the lateral
ramp of the Calabrian arc and formed by SW-dipping normal faults (Figure 1a). In our view the Apulia
Escarpment should not be influenced by the relative Africa-Eurasia convergence, being parallel to it. In
accordance with Doglioni et al. [1999] and Argnani et al. [2001], we suggest that the activity of some
of the normal faults of this system, which affect Plio-Quaternary deposits and disrupt the seafloor,
should be seen simply as the result of the flexure of this part of the lithosphere about a NW-SE axis.
28
JGR – revised 3 Di Bucci et al.
5. Final remarks
Plate tectonics of the Central Mediterranean has always been puzzling. This is largely due to the
juxtaposition of various first order structures, including: (i) the Adriatic-Ionian-Hyblean foreland,
formed by both continental and oceanic lithosphere; (ii) a slab plunging into the Southern Tyrrhenian
asthenosphere; (iii) the Tyrrhenian Sea, a young oceanic-type basin that is still undergoing significant
stretching; (iv) a continuous fold-and-thrust belt with variable strike, degree of shortening and uplift
rates; (v) Mount Etna and other active volcanoes. We suggest that the Africa-Eurasia convergence acts
in the background of all these structures, playing a primary and unifying role in the seismotectonics of
the whole region. This circumstance stems from a thorough investigation of foreland areas, where the
effects of plate convergence are not masked by other regional-scale deformation phenomena.
Central Mediterranean foreland areas have been the object of intense investigations for over two
decades. We reviewed the results and hypotheses put forward by a large number of investigators. This
knowledge basis allowed us to develop a new interpretative model that sees the Africa-Eurasia
convergence as the fundamental engine for the active tectonics of Central Mediterranean foreland
areas. We remark that our model is based on the very conclusions of many different papers, without
which capturing the unified nature of current tectonic strains over such a large region would have been
impossible. For the first time the tectonic effects of the current Africa-Eurasia convergence are
interpreted as a unifying key for understanding the long wavelength seismotectonics of the Central
Mediterranean. The new model rests on an improved perception of the current deformation styles and
of the hierarchy of coexisting tectonic phenomena, and may have significant implications for the
assessment of seismic hazard over the entire region.
29
JGR – revised 3 Di Bucci et al.
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Table captions
Table 1. a. Selected instrumental earthquakes of the Adriatic foreland cited in text. b. Selected
instrumental earthquakes of the Hyblean foreland cited in text.
Table 2. a. Selected historical earthquakes of the Adriatic foreland cited in text. 3b. Selected historical
earthquakes of the Hyblean foreland cited in text.
Figure captions
Figure 1. Schematic structural map of Italy and location of the Molise-Gondola and Vizzini-Scicli
shear zones (MGsz and VSsz, respectively). KL, Kefallonia Line; NOTO, MATE: International GPS
Service reference sites (from Devoti et al. [2008]). The focal mechanism shown refers to the largest
shock of a seismic swarm that struck the city of Potenza between May 1990 and May 1991 (see text
and Table 1 for details). b. GPS baseline rate of change based on the ITRF2005, redrawn after Marotta
and Sabadini [2008]. Black and grey indicate shortening and elongation, respectively. The length of
each bar is proportional to the magnitude of the baseline change rate, given in mm a−1. The rates,
mostly calculated along baselines trending SE–NW and E–W, highlight the compression due to Africa–
Eurasia convergence, testified by the diffuse SE–NW shortening and by the extension between Sardinia
(CAGL) and the Adriatic Sea (MATE and VENE). The ITRF2005 solution thus supports active
compression along the NW-SE axis of the Italian peninsula and extension roughly perpendicular to it.
Grey arrows in the lower right corner show the convergence vectors between Africa and Eurasia
48
JGR – revised 3 Di Bucci et al.
according to the Nuvel1A model [DeMets et al., 1994] and to the GPS pole of rotation proposed by
D’Agostino and Selvaggi [2004].
Figure 2. Seismotectonic setting of the Molise-Gondola shear zone. Key: CD, recent-to-present
continental and marine deposits; FL, sedimentary succession of the Adriatic foreland; FD, siliciclastic
deposits of the Bradanic foredeep; AM, sedimentary successions of the Apennine-Maghrebian fold-
and-thrust belt. Thick red lines indicate faults belonging to the MGsz. Fault systems: EM, Eastern
Molise (2002 earthquakes sources [DISS Working Group, 2010]); CH, Chieuti High [Patacca and
Scandone, 2004a]; MF, Mattinata Fault [Tondi et al., 2005]; GFZ, Gondola Fault Zone [Ridente et al.,
2008]. Squares are historical earthquakes scaled with magnitude: in black earthquakes from CFTI4Med
Catalogue [Guidoboni et al., 2007], in white earthquakes from CPTI Catalogue [CPTI Working Group,
2004]. Small brown dots are instrumental earthquakes from INGV Bulletin (1983-2007). Focal
mechanisms of selected events with Ml ≥ 4.1 and corresponding epicenters (yellow stars) are also
shown (data from Gasparini et al. [1982], Frepoli and Amato [2000], Di Luccio et al. [2005], Pondrelli
et al. [2006] and Scognamiglio et al. [2009]). Shmin directions are from Montone et al. [2004].
Instrumental and historical earthquakes are discussed in text and listed in Tables 1 and 2, respectively.
Localities: Ap, Apricena; Sa, Sannicandro.
Figure 3. Seismotectonic setting of the Vizzini-Scicli shear zone. Key: VD, volcanic deposits; CD,
recent-to-present continental and marine deposits; FL, sedimentary successions of the Hyblean
foreland; FD, siliciclastic deposits of the Gela-Catania foredeep and of the foreland basins; AM,
sedimentary successions of the Apennine-Maghrebian fold-and-thrust belt; KC, units of the Kabilo-
Calabride nappes. Thick red lines indicate faults belonging to the VSsz. Fault systems: VFZ, Vizzini
Fault Zone [Grasso and Reuther, 1988]; PIR, Pozzallo-Ispica-Rosolini fault system; AV, Avola fault;
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JGR – revised 3 Di Bucci et al.
MR, Marina di Ragusa graben; SL, Scordia-Lentini graben (all from Catalano et al., [2008a]); TFA,
Terre Forti Anticlines. Scicli on-shore after Catalano et al. [2008b]; Scicli off-shore after Grasso et al.
[2000a]. Squares and small dots are historical and instrumental earthquakes, respectively (same as in
Figure 2). Focal mechanisms of selected events with Ml ≥ 3.3 and corresponding epicenters (yellow
stars) are also shown (data from Amato et al. [1995], Pondrelli et al. [2004] and Musumeci et al.
[2005]). Shmin directions are from Montone et al. [2004]. Instrumental and historical earthquakes are
discussed in text and listed in Tables 1 and 2, respectively. Localities: Au, Augusta; CdA, Cava
d’Aliga; Ge, Gela; Gi, Giarratana; Pa, Palagonia; Ra, Ragusa; Sc, Scicli; Vi, Vizzini.
Figure 4. Schematic comparison between the MG and VS shear zones. Each shear zone is formed by
fault systems, each of which is further subdivided into fault segments, i.e. segments that may rupture
during individual damaging earthquakes or during a complex sequence. The double white arrows
bound individual fault systems. Where defined, the segments forming each fault system have been also
reported. The shear zones are not rigorously drawn to scale, but their total length is similar.
Figure 5. Geodynamic model proposed in this work, including a schematic representation of the
location and current kinematics of the MGsz and VSsz. Dashed lines are depth contours of the
subducting slab (from D’Agostino and Selvaggi [2004]). A gray line runs along the axis of the
Apennine-Maghrebian chain, which is currently undergoing extension. The relative Africa plate motion
(thick black arrow) is from Devoti et al. [2008]. Fault systems: EM, Eastern Molise (2002 earthquakes
sources); CH, Chieuti High; MF, Mattinata Fault; GFZ, Gondola Fault Zone; VFZ, Vizzini Fault Zone.
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JGR – revised 3 Di Bucci et al.
Table 1a. Selected instrumental earthquakes of the Adriatic foreland cited in text.
Date Name Magnitude Main reference Remarks 23 July 1930 Irpinia 6.7 (MW) Pino et al., 2008 --- 19 June 1975 Gargano 4.9 (ML) Gasparini et al., 1985 Mattinata Fault 5 May 1990 Potenza 5.7 (MW) Ekström, 1994 ---
30 September 1995 Gargano 5.4 (ML) CSI - Castello et al., 2006 Mattinata Fault 31 October 2002 Molise 5.8 (MW) Vallee and Di Luccio, 2005 Molise 2002
1 November 2002 Molise 5.7 (MW) Vallee and Di Luccio, 2005 Molise 2002 29 May 2006 Gargano 4.6 (MW) Ridente et al., 2008 Mattinata Fault
4 October 2006 Gargano 4.3 (MW) Pondrelli et al., 2006 Mattinata Fault
Table 1b. Selected instrumental earthquakes of the Hyblean foreland cited in text.
Date Name Magnitude Main reference Remarks 8 October 1949 Noto 5.2 (ML) Azzaro and Barbano, 2000 Scicli on-shore 21 March 1972 Sicily Channel 4.8 (ML) Pondrelli et al., 2006 Scicli off-shore
23 January 1980 Modica 4.6 (ML) Azzaro and Barbano, 2000 Scicli on-shore 29 October 1990 Sicily Channel 4.3 (ML) Azzaro and Barbano, 2000 Scicli off-shore
13 December 1990 Eastern Sicily 5.4 (ML) Amato et al., 1995 ---
Table 2a. Selected historical earthquakes of the Adriatic foreland cited in text.
Date Name Mw Main reference Remarks 30 December 1456 Molise 7.0 Fracassi and Valensise, 2007 W prolongation of MGsz
30 July 1627, h 10:50 Gargano 6.8 Guidoboni et al., 2007 Chieuti High
Shock 1 in Fig. 2
30 July 1627, h 11:05 Gargano 5.8 Guidoboni et al., 2007 Chieuti High
Shock 2 in Fig. 2
7 August 1627 Gargano 6.0 Guidoboni et al., 2007 Chieuti High
Shock 3 in Fig. 2
6 September 1627 Gargano 5.8 Guidoboni et al., 2007 Chieuti High
Shock 4 in Fig. 2 31 May 1646 Gargano 6.2 CPTI Working Group, 2004 ---
6 December 1875 S. Marco in Lamis 6.1 CPTI Working Group, 2004 Mattinata Fault 10 August 1893 Gargano 5.4 Guidoboni et al., 2007 Gondola Fault Zone
Table 2b. Selected historical earthquakes of the Hyblean foreland cited in text.
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JGR – revised 3 Di Bucci et al.
52
Date Name Mw Main reference Remarks
10 December 1542 Siracusano 6.8 Boschi and Guidoboni, 2001
Guidoboni et al., 2007 Vizzini Fault Zone?
3 October 1624 Mineo 5.6 Guidoboni et al., 2007 Vizzini Fault Zone
9 January 1693 Val di Noto 6.2 Boschi and Guidoboni, 2001
Guidoboni et al., 2007 Scicli on-shore? Shock 1 in Fig. 3
11 January 1693 Sicilia Orientale 7.4 Boschi and Guidoboni, 2001
Guidoboni et al., 2007
Mount Lauro Fault? Terre Forti Anticlines?
Shock 2 in in Fig. 3
20 February 1818 Catanese 6.0 Boschi and Guidoboni, 2001
Guidoboni et al., 2007 Shock 1 in Fig. 3
1 March 1818 Monti Iblei 5.5 Boschi and Guidoboni, 2001
Guidoboni et al., 2007 Vizzini Fault Zone Shock 2 in Fig. 3