JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, B07401, doi: /2012jb009254, 2012

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2012jb009254, 2012 GPS velocity and strain fields in Sicily and southern Calabria, Italy: Updated geodetic constraints on tectonic block interaction in the central Mediterranean M. Palano, 1 L. Ferranti, 2 C. Monaco, 3 M. Mattia, 1 M. Aloisi, 1 V. Bruno, 1 F. Cannavò, 1 and G. Siligato 1,4 Received 23 February 2012; revised 18 May 2012; accepted 22 May 2012; published 3 July [1] We present an improved rendition of the geodetic velocity and strain fields in Sicily and southern Calabria obtained through the analysis of 18 years of GPS observations from continuous and survey station networks. The dense spatial coverage of geodetic data provides precise quantitative estimates of previously established first-order active kinematic features, including: i) a narrow east-west-elongated belt of contraction (1 1.5 mm/yr) extending offshore northern Sicily from Ustica to Stromboli across the Aeolian Islands; ii) a narrow east-west-trending contractional belt located along the northern rim of the Hyblean Plateau in southern Sicily, with shortening at up to 4.4 mm/yr; iii) right motion (3.6 mm/yr) on the Aeolian-Tindari-Letojanni fault (ATLF) system, a main shear zone extending from the Aeolian Islands to the Ionian coast of Sicily, with significant transpression and transtension partitioned between discrete sectors of the fault; iv) transtension (1 mm/yr) across the Sicily Channel between Sicily and North Africa. We use geodetic observations coupled to geological constraints to better elucidate the interplay of crustal blocks revealed in the investigated area. In particular, we focus on the ATLF, which forms the primary boundary between the Sicilian and Calabrian blocks. The ATLF juxtaposes north-south contraction between Sicily and the Tyrrhenian block with northwest-southeast extension in northeastern Sicily and Calabria. Contraction between Sicily and Tyrrhenian blocks probably arises from the main Europe-Nubia convergence, although Sicily has a component of lateral motion away from Nubia. We found that convergence is not restricted to the northern offshore, as commonly believed, but is widely accommodated between the frontal belt and the northern rim of the Hyblean foreland in southern Sicily. Geodetic data also indicate that active right shear on the ATLF occurs to the southeast of the mapped fault array in northern Sicily, suggesting the fault cuts through till the Ionian coast of the island. The small geodetic divergence between the Hyblean and Apulian blocks rimming on both sides the Calabria block and subjacent Ionian slab, coupled with marine geophysical evidences in the Ionian Sea lends credit to the proposed deep root of the ATLF and to a fragmentation of the Ionian domain. Citation: Palano, M., L. Ferranti, C. Monaco, M. Mattia, M. Aloisi, V. Bruno, F. Cannavò, and G. Siligato (2012), GPS velocity and strain fields in Sicily and southern Calabria, Italy: Updated geodetic constraints on tectonic block interaction in the central Mediterranean, J. Geophys. Res., 117,, doi: /2012jb Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Sezione di Catania, Catania, Italy. 2 Dipartimento di Scienze della Terra, Università di Napoli Federico II, Naples, Italy. 3 Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Sezione di Scienze della Terra, Università di Catania, Catania, Italy. 4 Now at Leica Geosystems S.P.A., Cornegliano Laudense, Italy. Corresponding author: M. Palano, Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Sezione di Catania, Piazza Roma 2, I Catania, Italy. (mimmo.palano@ct.ingv.it) American Geophysical Union. All Rights Reserved /12/2012JB Introduction [2] Southern Italy represents a key region to investigate the geodynamic processes related to the Late Cenozoic Africa- Eurasia convergence that characterizes the central Mediterranean area [e.g., Barberi et al., 1973; Patacca et al., 1990; Faccenna et al., 2001]. A large amount of geodetical data collected in the last decade, have allowed researchers to better understand and quantify these processes, including both large-scale plate motions [e.g., Hollenstein et al., 2003; Caporali et al., 2011; Serpelloni et al., 2005; Devoti et al., 2011] and regional deformation along the plate boundary [e.g., Hollenstein et al., 2003; D Agostino and Selvaggi, 2004; D Agostino et al., 2011; Goes et al., 2004; Mattia et al., 1of12

2 Figure 1. Simplified tectonic map of the Sicilian-Calabrian area. Tectonic structures in the Ionian Sea redrawn from Polonia et al. [2011]. Instrumental seismicity since 1983 with magnitude 2.5 ( rm.ingv.it): white for events occurring at depth h <30 km; yellow for those occurring at depth ranging between 30 and 200 km; red for those at depth >200 km. Focal mechanisms (FM) of events with magnitude >3.0 are also reported according to the faulting styles based on definitions by Zoback [1992]: red for strike-slip, blue for thrust faulting and black for normal faulting. Abbreviations are as follows: ATLF, Aeolian-Tindari- Letojanni fault system; Ce, Cefalù, MtE, Mount Etna; CM, Capo Milazzo; Sa, Salina Island, Vu, Vulcano Island, Us, Ustica Island; HP, Hyblean Plateau. The Africa-Eurasia plate configuration is shown in the inset; CPA, Calabro-Peloritan Arc; Sar, Sardinia. 2008, 2009, 2012; Serpelloni et al., 2010]. The major findings related to active regional deformation ensuing from geodetical and seismological studies are (Figure 1): i) two narrow east-west-oriented belts of contraction, in the southern Tyrrhenian Sea from Ustica to the Aeolian Islands and, less well-known, along the northern border of the Hyblean Plateau in southern Sicily; ii) oblique convergence between eastern Calabria and Apulia, suggesting ongoing motion of the Calabro-Peloritan (inset in Figure 1) accretionary prism; iii) transtension across the Sicily Channel; iv) extension in western Calabria; v) right-oblique motion along the Aeolian- Tindari-Letojanni fault (ATLF) system. [3] Here, we present an up-to-date crustal velocity field of southern Italy and Sicily, based on an extensive combination of permanent and non-permanent GPS observations since 1994, which provides detailed velocity and strain rate fields. The good spatial coverage of geodetical data allows depicting the interplay of crustal blocks and better defining the role of main fault systems that bound them. In particular, we focus on the ATLF and give new insights into displacement mode 2of12

3 and rate of this major tectonic lineament. We conclude that the ATLF system represents the boundary between two different kinematic blocks (Sicilian and Calabrian), and extends further southward than what has been evidenced by geological data. Taking into account the regional tectonic constraints, a southward extension of the ATLF has relevant implications for different models proposed so far on the kinematic interplay between the two blocks. In addition, we investigate the kinematic interaction of Sicily and Calabria with the Tyrrhenian block to the north; in particular, our refined analysis allows better understanding how convergence is partitioned between two different shortening belts in northern and southern Sicily. 2. Tectonic Setting [4] The present-day tectonic framework of the Calabro- Peloritan Arc (CPA), which includes northeastern Sicily and Calabria (Figure 1) is the result of Neogene-Quaternary geodynamic processes related to the ca. N-S Africa-Eurasia convergence [e.g., Barberi et al., 1973; Malinverno and Ryan, 1986; Dewey et al., 1989; Boccaletti et al., 1990; Patacca et al., 1990]. Despite the Africa-Eurasia convergence occurring at a rate of 1 2 cm/yr during the last 5 6 Myr, the CPA experienced rapid E to SE motion at a rate of 5 6 cm/yr. Motion was related to roll-back of the subjacent Ionian transitional to oceanic slab and back-arc extension in the Tyrrhenian Sea basin behind [Gueguen et al., 1998; Faccenna et al., 2004; Rosenbaum and Lister, 2004]. However, during the middle-late Pleistocene, roll-back and subduction slowed to less than 1 cm/yr [Faccenna et al., 2001].Evidenceofa 70 NW-dipping Benioff-Wadati zone beneath the CPA has been presented since the early 1970s [Chiarabba et al., 2008, and references therein]. The slab is vertically defined by earthquakes, which occur down to 600 km depth [Frepoli et al., 1996]. Laterally, the seismically active portion of the slab is no longer than 250 km (from southern Calabria to the western Aeolian Islands), less than its vertical extent. The slab is not detected further west and east underneath western Sicily and the southern Apennines, respectively, supporting the idea of slab tears affecting the subducting Ionian lithosphere [e.g., Wortel and Spakman, 2000]. [5] In Sicily and Calabria, two main crustal domains characterized by different stress regimes can be recognized (Figure 1). Both seismological and geodetic data highlight a contractional domain in the northern Sicilian offshore and of an extensional domain in northeastern Sicily and southern Calabria [Neri et al., 2004; Pondrelli et al., 2006; Ferranti et al., 2008a; Mattia et al., 2009; Serpelloni et al., 2010; Cuffaro et al., 2011]. Whereas it is widely proposed that the contractional domain is connected with the convergence between Europe on one hand and Sicily or Nubia on the other hand, the causes for the extension in the CPA are still debated. Various alternative processes are suggested, ranging from rifting [Tortorici et al., 1995; Monaco and Tortorici, 2000; Jacques et al., 2001], to back-arc stretching in the Tyrrhenian Sea [Neri et al., 2003], to dynamic yet inadequate balance with regional, deep-induced uplift that has affected the CPA since the Middle Pleistocene [Ghisetti, 1992; Westaway, 1993; Ferranti et al., 2010], to response to counterclockwise rotation of the Ionian block [D Agostino and Selvaggi, 2004;Goes et al., 2004]. The transition between the two domains occurs along the ATLF system which has been interpreted as a transfer crustal zone between the northern Sicily offshore thrust belt in the Tyrrhenian Sea and the accretionary wedge offshore eastern Calabria in the Ionian Sea [Goes et al., 2004; Neri et al., 2004; Billi et al., 2006], or as a lithospheric tear fault bounding the western edge of the subducting Ionian slab [Nicolich et al., 2000, Doglioni et al., 2001; Faccenna et al., 2004; Rosenbaum et al., 2008; Chiarabba et al., 2008]. Extension, however, although at an immature tectonic stage, also occurs in a narrow band west of the ATLF in northern Sicily as documented by seismology and structural studies [Neri et al., 2005;Billi et al., 2010]. Extension on the northern Sicily coast (Figure 1) is interpreted as an orogenic collapse in the rear of the active thrust belt [Lavecchia et al., 2007], or alternatively as stemming from reactivation of preexisting faults coupled to melt ascent beneath Mount Etna volcano in eastern Sicily [Billi et al., 2010]. [6] In the field, the ATLF is formed by NW-SE-oriented en-echelon segments characterized by prevailing righttranspressional movements in the Aeolian sector [Ventura, 1994; Mazzuoli et al., 1995] and by transtensional motion in the northern Sicily sector [Ghisetti, 1979]. Evidences that the ATLF is still active in its northern portion are provided by seismological, geological and geodetic data. Since 1983 (instrumental catalog), more than 1500 earthquakes (maximum Magnitude = 4.5) have been localized close to this tectonic structure (Figure 1). Fault plane solutions reveal reverse focal mechanisms between Salina and Vulcano Islands, where the ATLF system joins with the eastern continuation of the northern Sicily offshore thrust belt, dextral strike-slip mechanisms between Vulcano and Capo Milazzo [Mattia et al., 2009] and prevailing normal faulting coupled with dextral solutions along the in-land part of the ATLF [Neri et al., 2005; Giammanco et al., 2008]. Geological surveys along the in-land segment of the northern part of the ATLF system evidenced Holocene activity with predominantly extensional and subordinately strike-slip kinematic features [Billi et al., 2006]. [7] Although in the Aeolian Islands and in northern Sicily the ATLF system is well characterized by structural studies and seismicity, the evidence of the fault disappears southwards [Billi et al., 2006]. Recent GPS measurements [Mattia et al., 2009] across the southern part of the ATLF, however, have established a tensile and a dextral strike-slip component of and mm/yr respectively. [8] Notwithstanding the lack of field evidences, several authors extend the southern continuation of the fault system to the Ionian coast of Sicily near Mt. Etna [e.g., Govers and Wortel, 2005, and references therein], and link it with the seismically active Hyblean-Maltese escarpment fault (HMEF) system offshore eastern Sicily (Figure 1) [Lanzafame and Bousquet, 1997; Doglioni et al., 2001]. The HMEF represents a NNW-SSE-striking Mesozoic lithospheric boundary separating the Ionian oceanic basin from the thick Hyblean continental crust [Nicolich et al., 2000]. Multichannel seismic data have shown Cenozoic reactivation of the northern sector of the Mesozoic fault system, while along its southern sector, the HMEF appears as a steep escarpment that flattens out toward the Ionian basin [Scandone et al., 1981;Sartori et al., 1991; Reuther et al., 1993; Hirn et al., 1997; Bianca et al., 1999; Argnani and Bonazzi, 2005]. AccordingtoWestaway [1990], the HMEF belongs to a diffuse transtensional fault 3of12

4 Figure 2. GPS stations used in this work: red circles for survey GPS sites and white diamonds for continuous GPS stations. Abbreviations are as follows: ATLF, Aeolian-Tindari-Letojanni fault system; Ms, Messina Strait; MtE, Mount Etna; NM, Nebrodi Mts.; MM, Madonie Mts., St, Stromboli Island; Sa, Salina Island, Vu, Vulcano Island, Li, Lipari Island, HP, Hyblean Plateau; Ma, Malta Island; La, Lampedusa Island; Pa, Pantelleria Island; Us, Ustica Island; Sal, Salento. Inset shows location of IGS stations introduced into the GPS processing. system developed from southern Sardinia and eastern Sicily into the Sirte basin of the north African margin (inset in Figure 1), through the Sicily Channel [Corti et al.,2006].this fault system would accommodate the Neogene and the current separation of the Ionian domain from the Hyblean Plateau. 3. GPS Network, Data Processing, Velocity and Strain Rate Fields [9] In 2004 the INGV (Istituto Nazionale di Geofisica e Vulcanologia) began the deployment of a permanent GPS network, called RING ( aimed at improving the understanding of current deformation and seismogenesis in Italy. At present, RING consists of about 150 stations with a major concentration along the Apennines where station spacing is in the order of km. To increase station density in the studied area, we included available measurements carried out on non-permanent GPS networks since 1994 [Mattia et al., 2008, 2009, 2012; Serpelloni et al., 2010] (Figure 2). Resulting network configuration shows an excellent albeit variable station density in eastern Sicily, while in western Sicily station density is lesser. [10] All GPS data, spanning the , were processed by using the GAMIT/GLOBK software [Herring et al., 2010] according to the strategy described by Palano et al. [2011]. To improve the overall configuration of the network and to tie the regional measurements to an external global reference frame, data coming from 10 continuously operating IGS stations (AJAC, CAGL, GRAS, GRAZ, 4of12

5 Figure 3. (a) GPS velocities and 95% confidence ellipses in a fixed Central Europe frame [Nocquet and Calais, 2003]. (b) Geodetic strain rate parameters: background color reports the magnitude of the rate of areal change for each grid cell, while arrows represent the greatest extensional (ɛhmax) and contractional (ɛhmin) horizontal strain rates. LAMP, MATE, MEDI, NOTO, NOT1 and ZIMM; inset on Figure 2) were introduced in the processing. To adequately show the crustal deformation pattern over the investigated area, estimated GPS velocities were aligned to a fixed Eurasian reference frame [Nocquet and Calais, 2003]. The velocity map is reported in Figure 3a. In addition, the 2D strain rate field was computed by interpolating the GPS velocities onto a 0.2 spaced rectangular grid and taking the derivatives at the center of each grid cell, using the method of Beavan and Haines [2001]. The velocity field was interpolated by removing from the computation all sites with fewer than 2.5 years of data and/or because of their suspicious movements with respect to nearby sites. The estimated horizontal strain rates are shown in Figure 3b: the arrows show the principal axes of the horizontal component of strain rates, while the contours in the background show the rate of areal change. [11] A striking aspect of the velocity field (Figure 3a), as already noted by previous authors [e.g., Hollenstein et al., 2003; Cuffaro et al., 2011, and reference therein] is the fanshaped pattern depicted by the divergent motion between Sicily and Calabria: in the former region, the velocity vectors are directed to NNW with rates diminishing northward; in Calabria, vectors are directed to NNE with rates that are small along the Tyrrhenian coast and increase toward the Ionian coast. [12] Beside this primary pattern, other important features were recognized by previous studies and are here more precisely defined. Motion between sites MALT and LAMP in the Sicily Channel show about 1.4 mm/yr of extension along the NE-SW direction (Figure 4a). Both sites are characterized by NW-SE convergence (2.9 mm/yr) with respect to site PZIN. The relative motion of the three sites results in a weak 2D areal contraction (Figure 3b). [13] A more regionally significant, east-west-oriented belt of contraction, extending from Ustica to Stromboli across the Aeolian Islands and northwestern Sicily, is recognized (Figure 3b). More in detail, a shortening of about 1 mm/yr is absorbed between northwestern Sicily (MILO and PING) and Ustica (USIX) (Figure 4b), with ɛhmin strain axes aligned to the NW-SE direction (Figure 3b). To the east, the magnitude of contraction increases to about 4.6 mm/yr between Vulcano (VSER and VMOL) and Lipari (LSAN and LMAZ) Islands, with ɛhmin strain axes almost N-S oriented (Figure 3b). North of this location, contraction decreases to a more background value of about 1.4 mm/yr between Lipari (LSAN and LMAZ) and Stromboli (SVIN and STDF) Islands. [14] In southern Sicily, along the northern border of the Hyblean Plateau, we detect a narrow but significant belt of contraction of about 4.4 mm/yr (Figure 4c) with ɛhmin strain axes aligned between NNW-SSE and NNE-SSW (Figure 3b). [15] In northern Sicily, a belt parallel to the coastal area and comprising the Madonie and Nebrodi Mountains is characterized by extension to right-transtensional displacements. More in detail, the strain rate pattern shows a prevailing N-S extension, coupled with a small orthogonal shortening component in the Madonie Mountains to the west, and a NNE-SSW extension in the Nebrodi Mountains, close to the western side of the ATLF. East of the ATLF, the strain rate pattern is characterized by a N-S trending belt of oblique contraction, before passing to a regional NW-SE extension that stretches along the Tyrrhenian side of Calabria and the Apennines, with ɛhmax axes orientation always perpendicular to the mountain belt (Figure 3b). [16] To better evidence the regional deformation field and the major crustal boundaries, we aligned the horizontal GPS velocity field with respect to a fixed Hyblean-Malta block, 5 of 12

6 Figure 4. GPS velocities and 95% confidence ellipses in a fixed Central Europe frame: (a) detail of the Sicily Channel area; (b) detail of the northern Sicily - southern Tyrrhenian area; red line represents the ATLF; (c) detail of the Hyblean Plateau area; (d) decimated GPS velocities and 95% confidence ellipses in a fixed Hyblean-Malta block (see auxiliary material for details) of the eastern Sicily and southern Calabria area. GPS velocities in the fixed Hyblean-Malta block for the entire investigated region are also available as map and as supplementary material. 6of12

7 whose Eulerian pole was estimated by using seven GPS sites located in the Hyblean area and Malta Island (see Figure 4d and auxiliary material for additional details). 1 The velocity field reveals a general SE-directed motion of southern Calabria and northeastern Sicily sites located eastward of the ATLF (Figure 4d). On the other hand, sites located west of the ATLF (TIND, CAPO, CALA, NOVA) together with those located in southern Vulcano exhibit a NNE-directed motion, evidencing a dextral motion across the ATLF. Finally, the velocity field reveals about 1 2 mm/yr of residual velocity on the Apulian area with respect the fixed Hyblean-Malta block (see Auxiliary materials for details). 4. Velocity and Strain Rates Fields: New Constraints for Interpreting the Tectonic Block Interaction in the Central Mediterranean [17] The new GPS velocity field allows refining the current views on the kinematic setting of Sicily and southern Calabria. In the following we discuss the main findings and their implications for various regional and local deformation processes so far proposed Independent Hyblean Block [18] Southern Sicily shows a small but detectable divergence from the general motion of the Nubian plate. This is probably related to the occurrence of active extension orthogonal to the Sicily Channel. Rifting of this area started around 5 6 Myr [Argnani, 1990; Tricart et al., 1994] and geodetic data indicate that it still occurs today at about 1.4 mm/yr in the NE-SW direction. Extension, however, is coupled to about 2.9 mm/yr of contraction parallel to the Nubia-Eurasia convergence. It appears that NW-SE shortening of the Sicily Channel is occurring at a rate which is twice that of NE-SW directed extension. [19] Geodetic motion of Sicily with respect to Nubia has been interpreted as evidence of an independent Sicilian microplate [Goes et al., 2004; Serpelloni et al., 2005; Devoti et al., 2008], or as the expression of a larger Apulian-Ionian- Hyblean plate [D Agostino et al., 2008]. Taking into account that i) LAMP site (Figure 3a) has a velocity consistent with the Nubian plate (its motion shows a residue of about 0.2 mm/yr with respect to a Nubian-fixed reference frame (see D Agostino and Selvaggi [2004] for details) and ii) the occurrence of a residual geodetic velocity of about 2 mm/yr on the Apulian area with respect to a fixed Hyblean-Malta block (see Auxiliary materials for details), we support the contention of an independent southern Sicilian (Hyblean) block with respect to both Nubia and Apulia Convergence Between the Sicilian-Hyblean and Tyrrhenian Blocks [20] Active contraction occurs on a narrow east-westoriented belt north of Sicily, from Ustica to Stromboli across the Aeolian Islands. Our background rate estimation of mm/yr is consistent with previous studies (about 2.1 mm/yr and about 1.6 mm/yr as estimated by Serpelloni et al. [2005] and Devoti et al. [2011], respectively). The west-central side of this belt is characterized by frequent, 1 Auxiliary materials are available in the HTML. doi: / 2012JB moderate-sized crustal (15 20 km of depth) thrust earthquakes [Pondrelli et al., 2006], with P axes constantly trending NW-SE (Figure 1). On the other hand, seismicity which nucleates close to the Lipari-Vulcano complex shows prevailing strike-slip and subordinately reverses faulting solutions [Neri et al., 2005; Mattia et al., 2008]. [21] Several investigators have pointed out that the eastwest-trending contractional belt offshore northern Sicily formed as a consequence of locking of the Sicilian frontal accretion during the last Myr [Tortorici et al., 2001; Catalano et al., 1996; Pepe et al., 2004] and transfer of convergence to the north [i.e., Goes et al., 2004]. The formation of the contractional belt along the northern Sicily offshore was probably favored by the presence of crust thinned by back-arc extension in the Tyrrhenian Sea. The crustal thickness beneath the mountain chain in Sicily is higher than 30 km, but beneath the active offshore contractional belt it is less than 20 km [Finetti et al., 2005; Di Stefano et al., 2009]. In addition, the presence of preexisting faults cutting and weakening the crust may have helped localization of current shortening [i.e., Rosenbaum and Lister, 2004]. The absence of deep seismicity below northern Sicily southern Tyrrhenian (Figure 1), as well as subducted lithosphere material, as suggested by tomography studies [Wortel and Spakman, 2000; Piromallo and Morelli, 2003], is an indication that the ongoing convergence reflects a continental collision process. [22] In the East, we detect a contraction of about 4.6 mm/yr across Vulcano and Lipari Islands, which is larger than the regional value of the offshore contractional belt from Ustica to Stromboli as discussed above. Structural analyses carried out on Vulcano and Lipari [Ventura, 1994] have shown that the islands formed within an en echelon configuration bounded by NNW-SSE to N-S trending faults which represent the northern branch of the ATLF system. Because the en echelon configuration is left-stepping and occurs in a right-lateral kinematic pattern, growth of local restraining bends is predicted. Indeed, evidence of recent transpressive structures has been detected offshore between Capo Milazzo and Vulcano Islands, where seismic profiles show two overstepping broad anticlines that have recently inverted pre-existing Pliocene extensional structures [Argnani et al., 2007]. These geologic evidences support previous [Mattia et al., 2008] and our inference that shortening is here amplified by transpressional motion across the en echelon fault system. [23] In southern Sicily, GPS data across the frontal thrust belt indicate contraction along the northern border of the Hyblean Plateau at up to 4.4 mm/yr (Figure 4c). Based on geological data, the frontal thrust belt in Sicily was considered locked since the Middle Pleistocene [Butler et al., 1992; Lickorish et al., 1999; Tortorici et al., 2001], and it was thought to be as such in geodetic and seismotectonic studies [Hollenstein et al., 2003; Goes et al., 2004; Billi et al., 2006]. However, recent geological studies have shown that E-W striking normal faults as well as NNE-SSW-striking right-lateral faults cutting the northern rim of the plateau have been reactivated in reverse motion during the last 0.85 Myr [Catalano et al., 2008]. Based on survey-mode GPS data, Ferranti et al. [2008a] have estimated a NW-SE contraction at 3 mm/yr between the frontal thrust belt and the Hyblean Plateau, an estimate later refined to 1.1 mm/yr by Devoti et al. [2011] using permanent GPS stations. 7of12

8 Active contraction at the front has also been proposed based on reappraisal of historical and instrumental seismicity data [Lavecchia et al., 2007; Visini et al., 2009] although the estimated rate is largely lower (0.2 mm/yr). Although it is not possible from geodetic data to infer a motion of the thrust front proper, our analysis supports the contention of Ferranti et al. [2008a] of substantial shortening still occurring across the Sicilian frontal belt, at least along the eastern part of the northern Hyblean Plateau. The scarce geodetic data available on the frontal belt of western southern Sicily preclude precise quantification of the process in this somewhat weakly deforming sector, where nonetheless active thrusting and transpression has been proposed based on seismotectonic [Monaco et al., 1996] and geodetic [Ferranti et al., 2008a; Mattia et al., 2012] data. [24] In summary, geodetic data indicate that regional contraction between the Tyrrhenian and Sicilian (Hyblean) blocks occurs along two distinct belts located in the northern Sicily offshore (1 1.5 mm/yr background value) and across the Sicily front (4.5 mm/yr). Although internal deformation in western Sicily is small, the Sicilian orogen is deforming in response to the block convergence. The combined 5 6 mm/yr shortening rate closely matches the estimated convergence rate between Nubia and Eurasia [i.e., Sella et al., 2002; Reilinger and McClusky, 2011], suggesting that even if partly independent because of stretching in the Sicily Channel, Sicily is nonetheless trapped between the larger plate collision Extension in Central-Northern Sicily [25] On the Tyrrhenian side of central Sicily, our analysis highlights a narrow extensional area (about 4 mm/yr) affecting the Madonie and Nebrodi Mountains, in agreement with previous seismologic [Neri et al., 2005], geodetic [Devoti et al., 2011] and geological [Billi et al., 2010] data. Although Lavecchia et al. [2007] view this incipient extension as ensuing from upper crustal stretching above an active thrust belt, Billi et al. [2010] favor reactivation of pre-existing faults and upwelling of melt mantle material beneath Mount Etna. [26] We note that the area of geodetic (Figure 3b) and seismologic (Figure 1) extension is confined to central northern Sicily, and does not extend in western Sicily. In addition, the extensional area is nearly coincident with the westward tapering of the regional uplift pattern in the CPA [Ferranti et al., 2010]. Although stretching above a deep thrust belt [e.g., Lavecchia et al., 2007] and magmatic contribution cannot be dismissed at least in part, we suggest a possible relation of this extensional process to the encroaching uplift domain [Ferranti et al., 2010]. [27] On the other hand, the weak extension observed along the central part of the Hyblean Plateau, limited by the contractional belt at its northern border (Figure 3b), might be a product of flexure of the crust trapped in collision Extension in Northeastern Sicily and Western Calabria [28] We document moderate but regionally consistent active extension in northeastern Sicily and southern Calabria, where geodetic ɛ Hmax axes show orientation always perpendicular to the Apenninic chain (Figure 3b). Our results are in agreement with previous seismological [Pondrelli et al., 2006; Giampiccolo et al., 2008] and with geological studies, which document the presence of several NNE-SSW-striking active faults [Westaway, 1993; Tortorici et al., 1995; Monaco and Tortorici, 2000; Jacques et al., 2001; Ferranti et al., 2007, 2008b]. As above discussed, the causes for this extensional domain are still debated, and geodetic data cannot provide quantitative kinematic constraints. [29] Extension in western Calabria is associated to a 2 mm/yr southeastward motion of the Ionian margin of Calabria toward the trench, measured relative to Apulia [D Agostino et al., 2011], which might reflect movement of the Ionian accretionary complex [Polonia et al., 2011] Tectonic Significance of the ATLF [30] Our analysis highlights the key role played by the ATLF in the recent kinematics of the CPA. By applying a vector decomposition of the velocities of sites located across the ATLF (CALA, TIND and MONT on the western side and NOVA and RODI on the eastern one, Figure 4d), and by taking into account a mean N35 W fault trend, we estimate about 3.6 mm/yr of motion along the N126 E direction, resulting in dextral transtension on the NNW-striking ATLF. This result agrees with and refines the earlier estimate of Mattia et al. [2009]. These sites are located in northern Sicily, where, beside geodetic observations, active transtension has already been documented by seismology [Neri et al., 2005] and structural studies [Billi et al., 2006]. The southern continuation of the ATLF toward the Ionian coast of Sicily, on the other hand, was so far poorly constrained by geodesy [Mattia et al., 2009] and only conjectured by geological investigation [Ghisetti, 1992]. [31] Our geodetic analysis indicates the southward extension of the ATLF is feasible, at least on-land, as shown by about 3.7 mm/yr of right-lateral motion observed along the Ionian coast between sites TAOR and 9532, located westward and eastward of the ATLF, respectively (Figure 4d). To explain the different geodetic velocity field between the Sicilian and the Calabrian blocks, Goes et al. [2004] invoked up to 5 mm/yr of regional right-lateral differential motion between them. Our analysis points out that as much as 70% of this motion is accommodated across the ATLF. [32] The offshore continuation of the ATLF is of course unconstrained by geodesy. However, the occurrence in the Ionian basin of NNW-SSE to NW-SE trending structures (Figure 4d) [Nicolich et al., 2000; Argnani and Bonazzi, 2005; Polonia et al., 2011], to which several earthquakes with prevailing strike-slip focal mechanisms can be associated (Figure 1) [Scarfì et al., 2009], suggests the possible offshore extension of the ATLF system. These segments could play the role of lithospheric boundary between the Sicilian-Hyblean and Ionian-Apulian blocks [Nicolich et al., 2000; Goes et al., 2004; Rosebaum and Lister, 2004; Chiarabba et al., 2008]; or, alternatively, accommodate differential movements between the Calabria extensional belt from the northern Sicily contractional belt and connecting this latter to the frontal arc located on the Ionian basin [Lanzafame and Bousquet, 1997; Bousquet and Lanzafame, 2004; Wortel and Spakman, 2000; Neri et al., 2004, 2009, 2012; Goes et al., 2004; Billi et al., 2006]. 5. Conclusions [33] The detailed geodetic analysis highlights the presence of distinct deformation belts separating the Tyrrhenian, 8of12

9 Figure 5. Conceptual sketch of the different tectonic scenarios suggested in this work. (a) The Ionian domain is rigidly connected with the Hyblean-Malta block: this scenario suggests that the current convergence along the Calabrian trench should adsorb a large part of the horizontal motion measured across the ATLF, which connects the north Sicilian contractional belt to the arc front located on the Ionian basin. (b) The Ionian domain diverges from the Hyblean-Malta block, moving to the northeast with respect to Europe together with the Calabrian block. This scenario, though still compatible with convergence along the Calabrian trench, would imply the prolongation of the ATLF to the south. Sicilian-Hyblean and Calabrian blocks, which join in northeastern Sicily. [34] Deformation between the Sicilian-Hyblean and Tyrrhenian blocks is segregated between two distinct belts in the northern Sicily offshore and at the northern rim of the Hyblean plateau. Deformation is instead widespread in the extensional belt from northeastern Sicily to southern Apennines, partly in contrast with seismicity (Figure 1). Conversely, deformation is more concentrated along the ATLF oblique strike-slip zone extending between the Aeolian Islands and the Ionian coast of Sicily. In this context, the ATLF system juxtaposes NNW-SSE contraction between Sicily and the Tyrrhenian blocks with NW-SE extension along the CPA. To the south, motion may occur on the continuation of the ATLF along the Ionian offshore of Sicily. [35] The ATLF has probably formed as a consequence of the Middle Pleistocene tectonic reorganization in the southcentral Mediterranean, characterized by the slowing or cessation of Calabrian roll-back and subduction, and back-arc Tyrrhenian extension [Westaway, 1993; Wortel and Spakman, 2000; Goes et al., 2004]. This process has also caused i) the partial jumping of the Sicilian thrusting from the front to the rear of the chain (southern Tyrrhenian contractional belt); ii) high-rate extension and uplift in western Calabria and northeastern Sicily; iii) the change in the chemical composition of magmatic products on the eastern Aeolian Islands [De Astis et al., 2000]; iv) the triggering of Mt. Etna volcanism [Gvirtzman and Nur, 1999; Doglioni et al., 2001]. [36] In this context, a problem is the role played by the Ionian domain south of the accretionary prism [see also D Agostino and Selvaggi, 2004] that i) could still be a rigid part of the Hyblean-Malta block or ii) alternatively diverges from the Hyblean-Malta block, moving to the northeast with respect to Europe together with the Apulian block. The first scenario (Figure 5a) implies that the ATLF only connects the north Sicilian contractional belt to the arc front located on the Ionian basin, accommodating differential movements within the contractional belt [Goes et al., 2004; Jenny et al., 2006; Billi et al., 2006]. Recent GPS analysis [D Agostino et al., 2011], based on 2 mm/yr southeastward motion of eastern Calabria relative to Apulia, proposes that shortening between Calabria and Apulia is still active, and is associated to 1.4 mm/yr extension of the forearc. This scenario suggests that the current convergence along the Calabrian trench should adsorb a large part of the horizontal motion measured across the ATLF. [37] The second scenario (Figure 5b), although still compatible with convergence along the Calabrian trench, implies the prolongation of the ATLF a long way to the south, as far as North Africa [Westaway, 1990], with the involvement of HMEF and possibly other, sub-parallel faults located further east in the Ionian Sea, which cut the Ionian crust beneath the Calabria wedge [Argnani and Bonazzi, 2005; Polonia et al., 2011; Neri et al., 2012]. [38] Taking into account the apparent lithospheric nature of transtensional and normal faults occurring along the eastern Sicily offshore [Nicolich et al., 2000], the small geodetic divergence between the Hyblean and Apulian blocks further east lends credit to the southward extension of the ATLF system and a deep fragmentation of the Ionian domain. However, the lack of islands in the rigid Ionian domain precludes the use of GPS observations to verify the independent motion of this domain. Further data (i.e., tomographic and seismic profiling, GPS measurements in 9of12

10 North Africa) are necessary to better constrain the geometrical and tectonic features of the crustal block occurring and interacting in this active region of the Mediterranean. Testing these two hypotheses through the use of simple elastic dislocation models and/or more complex approaches such as the Finite Element Method could be a principal objective of future studies. The use of these approaches could also be useful in determining whether the small velocity gradients which characterized some regions of the study area reflect temporary effects of transient elastic strain accumulation and/or long-term tectonics. [39] Acknowledgments. We thank two anonymous reviewers and the Associated Editor, Isabelle Ryder, for their comments that helped to clarify some aspects of the work. We are indebted to all individuals and institutions contributing in the GPS data, particularly to technical staff of INGV who continue to maintain the RING network. 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