tectonic evidence for the ongoing africa-eurasia ... · mediterranean plate tectonics. 2. regional...

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.

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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


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


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


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


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


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


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


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


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


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


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


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


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


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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

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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;

49


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.

50


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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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

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