GNGTS 2018 - 37° Convegno Nazionale

6 GNGTS 2018 S essione 1.1 2. Contended Issues. The majority of the contended issues is related to the occurrence, number and extent of lithospheric tears along the western Ionian region. 2.1. How many lithospheric tears? At least three belts where lithospheric tearing is supposed to occur have been proposed on the western Ionian region, and their occurrence is considered not exclusive by some authors (Fig. 1). These fault zones can be named Malta Escarpment (Argnani and Bonazzi, 2005), Alfeo Fault System (Gutscher et al. , 2016) and Ionian Fault (Polonia et al. , 2012); they all occur within a distance of 100-150 km, that is the expected thickness of the old Ionian lithosphere (Argnani, 2005). The extent and magnitude of the supposed lithospheric tear on each of these fault zones vary according to the authors, going from a minimalist approach, with incipient tear mostly north of Siracusa (Argnani and Bonazzi 2005; Argnani, 2012), to a ca. 50 km-wide transtensional zone located between the Alfeo and Ionian faults (Polonia et al. , 2017). The occurrence of multiple, and narrowly-spaced, lithospheric tear faults in the old and thick Ionian lithosphere is unsound in many respects, and it has not been observed elsewhere in similar settings (e.g., Govers and Wortel, 2005). 2.2. Constraints on lithospheric tears. Tomographic evidence of slab tears are based on seismic tomography (e.g., Neri et al. , 2009), that suggests the occurrence of tears to the north and south of the Calabrian slab; these tears are intended as perpendicular to the strike of the subducted slab, and propagating into the foreland region. The data show that seismicity and high velocity anomaly fade out laterally along strike; as an alternative interpretation the lithospheric tear can have propagated along the strike of slab from the north and from the south (Argnani et al. , 2016). The efficient wave propagation along the slab waveguide from deep slab earthquakes and recorded on the stations in the foreland (Mele, 1998; Monna and Dahm, 2009) suggest that this interpretation is viable and that lithospheric tears may be mostly limited to the Tyrrhenian region. It is worth noting that the distribution of instrumental seismicity does not show major lineaments in the Ionian basin, whereas the sparse focal mechanisms show mostly strike-slip solutions with roughly NW-SE P axes; improved hypocentral locations, however, are desirable in the Ionian Sea. One issue that should be considered is that wrench faults are not uncommon in accretionary werges, remaining however confined within the wedge and above the basal decollement (e.g., Breen et al. , 1986); therefore, it may not always be obvious to image the surface expression of a lithospheric faults. Moreover, the lithospheric STEP is not a plain strike-slip fault, as some vertical displacement, increasing along the fault towards the hinterland, is expected. The pieces of geological evidence that support the occurrence of lithospheric tear are based on crustal-scale or shallower geophysical data, mostly seismic data. Fault activity along the Malta Escarpment have been documented by seismic data that show extensional basins filled by ca. 1000 m thick sediments (Argnani and Bonazzi, 2005). The activity of this fault system is likely responsible for the 1693 earthquake that affected eastern Sicily (Argnani et al. , 2012). The Alfeo fault system runs about 40 km from the Malta Escarpment offshore southeast Sicily and gets closer northward. This feature has some morphologic expression and in places it is documented by seismic data of variable quality (Gutscher et al. , 2016). A dominant right- lateral strike-slip motion is inferred, based on morphology and on the occurrence of small releasing-bend basins. Interestingly, the connection between the faults bounding the basins at shallow depth and the discontinuity observed at depth in the oceanic basement is not always obvious (e.g., Maesano et al. , 2016). The Alfeo fault system is supposed to connect to Mt. Etna, but there is little data supporting this interpretation in the northern segment of the fault system, where some structural complexity has been observed offshore Mt. Etna (Argnani et al. , 2013). The Ionian Fault system has been initially interpreted as a right-lateral lithospheric strike- slip fault separating the two lobes of the outer Calabrian Arc accretionary wedge (Polonia et