Subduction Beneath Southern Italy Close the Ending: Results from Seismic Tomography

Abstract

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 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 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.