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Subduction Beneath Southern Italy Close the Ending: Results from Seismic Tomography G. Neri, B. Orecchio, C. Totaro, G. Falcone, and D. Presti

G. Neri,1 B. Orecchio,1 C. Totaro,1 G. Falcone,1,2 and D. Presti1 INTRODUCTION The western Mediterranean area located in the contact belt between the slowly convergent African and Eurasian plates (Calais et al. 2003; Nocquet and Calais 2004; Serpelloni et al. 2007) has been the site of a continental-scale lithospheric subduction process, the evolution of which in the last 30 million years is marked by the eastward migration of the retreating subduction hinge shown in Figure 1 (Wortel and Spakman 2000). In this framework, the Tyrrhenian Sea formed as an extensional back arc basin that opened during the last 10 million years due to the roll-back of the Ionian portion of the subducting lithosphere (Malinverno and Ryan 1986; Faccenna et al. 2005). The widely shared model shown in the partial, 3D sketch view of Figure 1 indicates that most of the subduction system has already undergone detachment of the subducting lithosphere with the exception of the central, most arcuate portion of the system, the Calabrian arc in southern Italy, where the state of subduction is doubtful and needs further exploration (larger question mark in the same sketch view). The distribution of intermediate and deep seismicity in the Calabrian arc region (Anderson and Jackson 1987; Giardini and Velonà 1991; Selvaggi and Chiarabba 1995; Chiarabba et al. 2005) evidenced a narrow (~200 km) and steep (~70°) WadatiBenioff plane striking NE-SW and dipping northwest down to 500 km depth. Hypocenter locations and high-velocity anomalies (HVAs) revealed by tomographic investigations (Piromallo and Morelli 2003; Spakman and Wortel 2004; Cimini and Marchetti 2006; Montuori et al. 2007) have furnished overall pictures of the subducting structure, but an accurate knowledge of its geometry and eventual in-depth continuity is still lacking. Beneath the southern Apennines (Figure 1) the previous tomographies (Cimini 1999; Wortel and Spakman 2000; Cimini and Marchetti 2006) revealed (i) the lack of subcrustal seismicity and HVAs up to 200 km depth and (ii) a southwestward dipping high-velocity body at deeper depths. These indications, and specifically the contrast with the apparently continuous pattern of seismicity and HVAs detected beneath the Calabrian Arc (Cimini 1999; Wortel and Spakman 2000; Cimini and Marchetti 2006), were related to different states 1. Dipartimento di Scienze della Terra, Università di Messina, Messina, Italy 2. Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy doi: 10.1785/gssrl.80.1.63

of the subduction process in the respective areas due to lateral heterogeneities of the foreland lithosphere that is oceanic in the Ionian and continental in the southern Apennines. Several investigators have suggested that a lateral slab tear may have propagated southeastward from the northern Apennines (Spakman and Wortel 2004; Faccenna et al. 2005; Cimini and Marchetti 2006) (Figure 1). On the southwestern side of the subduction system, the information available underneath northwestern Sicily (Piromallo and Morelli 2003; Spakman and Wortel 2004; Montuori et al. 2007) indicates that subcrustal seismicity and HVAs are absent until 150–200 km depth. Previous researchers have proposed that a mechanism similar to that suggested for the Apennines is at work here, assuming a lateral slab tear propagating eastward from the northern coast of Africa (Wortel and Spakman 2000; Faccenna et al. 2005; Lucente et al. 2006). To summarize, the seismic and seismotomographic data available on the Apennines-Maghrebides subduction system indicate that the Calabrian arc is the only sector of the system where the subducting lithosphere may still be undetached. We note that several authors (Spakman and Wortel 2004 and references therein) argue that the very low outward migration velocity of the Calabrian arc and its widely documented strong uplift may suggest shallow detachment of the subduction slab in this region. According to these authors, detachment may not have been detected because it may have occurred (and is located) in a depth-range not well-resolved by the available tomographies, which are primarily based on inversion of teleseismic data. After investigation of geological and morphostructural differences between southern and northern Calabria, Guarnieri (2006) proposed a different evolution of the subduction process in these respective sectors (Figure 2). In his attempt to model the processes over the whole arc structure, the author considered that 1) the subduction hinge retreat may have been locked in northern Calabria and central-western Sicily about 2 million years ago due to continental collision, and 2) strong lithosphere deformation may then have occurred in correspondence to two tear faults separating the portion still capable of retreat (southern Calabria) from the confining locked segments (Figure 2). Guarnieri’s (2006) reconstruction also includes a counterclockwise rotation of the retreating subduction hinge in recent times (Figure 2). Very recently, Chiarabba et al. (2008) published a seismic tomography covering the whole region of south Italy down to a depth of 350 km. The tomography we present in the next section focuses with greater detail on the smaller but crucial area

Seismological Research Letters Volume 80, Number 1 January/February 2009 63

▲▲ Figure 1. Map of the Mediterranean region evidencing the western Mediterranean plate boundary evolution in the last 30 million years (redrawn from Wortel and Spakman 2000). The black arrows indicate the present motion of Africa relative to Europe (Calais et al. 2003; Nocquet and Calais 2004). The solid curves with the sawtooth pattern indicate the location of the western Mediterranean convergent boundary at different times, a consequence of rollback of the subducting lithosphere. The sawteeth point in the direction of subduction or underthrusting. Black sawteeth indicate continuous subducting slab. White sawteeth indicate plate boundary segments where slab detachment has already occurred. Gray sawteeth indicate the doubtful situation of continuous or recently detached slab (Calabria). The white arrow shows the inferred direction of lateral migration of slab detachment. The dashed curves with the sawtooth pattern indicate the present location of the convergent boundary in the Hellenic and Carpathian arcs, information outside the scope of the present study but displayed for completeness. The top-left inset shows, on a different scale, the southern Italy portion of the Apennines-Maghrebides subduction system. The lower-right inset shows a 3D cartoon of the imaged positive mantle anomalies below the Alps, the Apennines, and the southern Tyrrhenian Sea, summarizing also the interpretation of their geodynamic significance (taken from Spakman and Wortel 2004). The above-mentioned doubtful situation of subduction beneath Calabria is indicated with the larger question mark.

of the Calabrian arc, taking advantage of the greater amount of data available to us in this specific sector. Our dataset may allow us to specifically answer the question of whether the subduction slab is still undetached beneath Calabria.

SEISMIC DATASET AND TOMOGRAPHIC INVERSION Data and recordings relative to the intermediate and deep earthquakes that occurred in southern Italy between January 1975 and December 2006 have been collected from the International

Seismological Centre (ISC) bulletin (http://www.isc.ac.uk), the Italian national catalogs and databases (http://www.ingv. it), and the databases of the local seismic networks operating in Calabria and Sicily (Bottari et al. 1981; Guerra et al. 1991; Chiodo et al. 1994; Barberi et al. 2004). We selected from the collected dataset the events with a minimum of 12 readings, the quality of which was in the majority of cases checked directly on the recordings. The final inversion dataset consisted of 15,859 P-wave arrival times from 1,360 earthquakes recorded at a total of 199 stations (Figure 3A). The P-wave ray-paths displayed in

64 Seismological Research Letters Volume 80, Number 1 January/February 2009

(A)

(B) ▲▲ Figure 2. Map view of the space-time evolution of the central portion of the western Mediterranean subduction system (corresponding to the Calabrian arc, southern Italy) according to Guarnieri (2006). In the author’s reconstruction the subduction hinge retreat was locked by continental collision in northern Calabria and central-western Sicily about 2 million years (2My) ago, and this has caused strong lithosphere deformation in correspondence to two tear faults separating the portion still capable of retreat (southern Calabria) from the confining locked segments. Guarnieri (2006) also proposed counterclockwise rotation of the retreating subduction hinge in recent times.

Figures 3B and 3C provide initial evidence of the suitability of the ray-coverage to investigate the 40–260 km depth range beneath the Calabrian arc region. We used the standard Simulps code (Evans et al. 1994) to invert the arrival time data for the P-wave velocity structure at the depth levels 65, 105, 150, 200, and 260 km. Figure 4A displays the map view of the inversion grid, oriented N60°W to approximately follow the direction of subduction. The grid horizontal spacing was fixed to 50 km. As a starting model for inversion we used (i) the 3D local velocity structure of Barberi et al. (2004) in the upper 40 km and (ii) the ak135 1D velocity model of Kennett et al. (1995) at deeper depths. So that poorly constrained nodes did not bias the velocity estimates at the potentially well-constrained ones, we decided to perform the inversion only in the sectors with large ray density. The final inversion nodes were characterized by derivative weight sum

(C) ▲▲ Figure 3. (A) Epicenters of the earthquakes of southern Italy used for inversion (gray dots) and recording seismic stations (triangles). (B) and (C) display, respectively, the horizontal and vertical views of the P-wave ray tracing in the ak135 model. The white dashed line in (B) indicates the profile of the N-S vertical section given in (C) where vertical and horizontal distances are given in kilometers.

values (Toomey and Foulger 1989) greater than 100. The velocity patterns resulting from tomographic inversion are displayed in Figures 4 and 5. The tomographic inversion full-resolution matrix has been used to evaluate the reliability of the model, making a combined


(A)

(B)

(C)

(D)

(E)

(F)

▲▲ Figure 4. (A) Map view of the inversion grid nodes, the depths of which are fixed to 65, 105, 150, 200, and 260 km. (B–F) Plates corresponding to different depths (lower-right corner) of the tomographic results reported in terms of percentage variation of P-wave velocity with respect to model ak135. The black curve contours the zone where the spread function (SF) values are not larger than 3.25. Dots represent the relocated earthquakes separated according to the following depth ranges in kilometers: 45–75 (A), 75–135 (B), 135–165 (C), 165–230 (D), >230 (E).


▲▲ Figure 5. Vertical sections of the tomographic model along the profiles indicated in the map. In the upper 40 km we report the shallow velocity structure locally estimated by Barberi et al. (2004). At depths deeper than 40 km we report the results of the present study in terms of percentage deviation of P velocity from ak135. The black curve contours the zone where the SF values are not larger than 3.25. The dots indicate the earthquakes located within ± 25 km of the vertical planes.


DISCUSSION OF RESULTS AND CONCLUSION

▲▲ Figure 6. This figure focuses on the central portion (Calabrian arc, southern Italy) of the western Mediterranean subduction system represented in Figure 1. The results of tomographic inversion displayed in Figures 4 and 5 have allowed us to clarify the state of subduction beneath the Calabrian arc, so that we can include the new information in this figure. Please note, in fact, that the gray sawteeth used in Figure 1 to indicate the doubtful subduction setting in the Calabrian arc have now (Figure 6) been changed to black (= in-depth continuous slab) in southern Calabria and to white (= detached slab) in northern Calabria and northeastern Sicily.

use of the spread function (SF) (Toomey and Foulger 1989; Michelini and McEvilly 1991) and the resolution smearing contours (Reyners et al. 1999). The reverse of the SF indicates how strong and peaked the resolution is in correspondence to each individual node; therefore low SF at a given node indicates locally high resolution. Previous investigators (see, e.g., Toomey and Foulger 1989) showed that the SF upper bound contouring the well-resolved domain of the tomographic model needs to be determined in any specific investigation because it depends on the model parameterization. To properly address the choice of the SF upper bound in the present study, we estimated the 70% smearing contours at the individual nodes with the procedure of Reyners et al. (1999) and compared these contours with the SF values, as was done by previous investigators (Eberhart-Phillips and Reyners 2001; Husen et al. 2003; Reyners et al. 2006; Sherburn et al. 2006). We found that in our case, SF values no larger than 3.25 always correspond to nodes without smearing from beyond the adjacent nodes, e.g., to well-resolved nodes. Therefore we assumed the SF value of 3.25 as a conservative upper bound to contour the well-resolved zones of the tomographic model (Figures 4 and 5). Finally, we checked the model reliability in the SF ≤ 3.25 zones by standard synthetic tests like restore resolution and checkerboard tests (see, e.g., Zhao et al. 1992), which gave additional evidence of the high stability of the model and suitability of the inversion grid.

The final 3D velocity model from tomographic inversion is reported in Figures 4 and 5 in terms of percentage velocity variation with respect to the 1D reference model ak135. The vertical sections in Figure 5 also include the shallow structure of the study area computed by Barberi et al. (2004). The earthquake locations obtained by inversion are given both in the maps and the vertical sections. The existence of high-velocity anomalies in the upper mantle is generally interpreted as due to the cold (dense) oceanic lithosphere, which penetrates into a warm (soft) astenosphere characterized by lower velocity (see, among others, Hirahara and Hasemi 1993; Lay 1994; Cimini and Marchetti 2006). Analogously, we interpret the HVAs evidenced by the tomographic images of Figures 4 and 5 as signatures of the Ionian lithospheric slab penetrating into the mantle beneath the Calabrian arc area. The vertical section CC’ in Figure 5 indicates that the subducting slab is undetached beneath southern Calabria. Conversely, the slab is detached beneath northeastern Sicily (AA’) and northern Calabria (EE’). A transitional situation can be detected beneath the Messina straits (BB’) and central Calabria (DD’). As expected, the seismicity appears to be mostly located within or close to the high-velocity bodies. In particular, we note a high concentration of earthquakes in the thinnest portion of the slab at 120–180 km depth beneath southern Calabria (CC’), and this leads us to speculate about the possibility of an incipient slab detachment in this part of the southern Italy subducting system, where detachment does not appear to have occurred. The map views of the velocity patterns in Figure 4 reproduce the main features evidenced by the vertical sections and furnish additional details. In particular, HVA location at a depth of 150 km highlights the continuity of the subduction system beneath the southern Calabria NE-striking 100-kmlong segment. The high-velocity body reproducing the Ionian lithospheric slab shows its smallest along-strike extent at the 105 and 150 km depth levels (Figures 4C and 4D). The slab portions detected at these depths show an excellent agreement in terms of orientation and horizontal extent with the subduction hinge location about 2 million years ago reconstructed by Guarnieri (2006) on the basis of geological and morphostructural evidence (Figure 2). At 65 km depth (Figure 4B) the NNW-trending high-velocity anomaly marks the Ionian lithosphere approaching the in-depth penetration zone located beneath the Tyrrhenian coast of Calabria. In this case too, the HVA matches well with Guarnieri’s (2006) reconstruction and specifically with the present location and orientation of the subduction hinge (Figure 2). In other words, the HVA distribution inferred from our tomography and the consequent detection of the subduction system at different depths appear to furnish useful constraints for reconstruction of the subduction process in the last few million years. Finally, an additional observation from Figures 4 and 5 is that locations of the low-velocity anomalies are compatible with toroidal circulation of astenospheric


materials around the slab suggested by previous investigators (Civello and Margheriti 2004; Faccenna et al. 2005; Lucente et al. 2006; Montuori et al. 2007). In conclusion, using a denser dataset than that used by previous investigators in the Calabrian arc area we improved the knowledge of the state of subduction in this still-active portion of the western Mediterranean subduction system. As schematically shown in Figure 6, local earthquake tomography revealed that the subducting lithosphere remains undetached only with respect to a 100-km-long segment of southern Calabria (central portion of the Calabrian arc). Detachment has already occurred at the northern and southwestern edges of the arc (northern Calabria and northeastern Sicily, respectively). Final detachment beneath southern Calabria appears to be geologically close, implying the forthcoming end of the subduction process in the western Mediterranean region.

ACKNOWLEDGMENTS We thank SRL editor Dr. Luciana Astiz for comments and suggestions that significantly improved the manuscript.

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Dipartimento di Scienze della Terra Università di Messina Salita Sperone 31 98166 Messina Italy [email protected]

(G. N.)