Geodynamic setting
To illustrate the proposed geodynamic model of the Southern Apennines, the geological cross-section has been framed in the large scale lithospheric setting suggested by the available geophysical and geochemical data.
Although geodynamic interpretations that do not consider the subduction below the Apennines have been proposed, in this paper only a subduction model will be considered. Indeed, the simple and indisputable observation that about 190-210 km of crystalline basement (the former substratum of allochthonous units) are missing strongly support the westward subduction of the Apulo-Adriatic continental lithosphere under the Southern Apennines, as also suggested by the several independent geophysical datasets. The proposed large scale section (Fig. 9), is constrained with the following geophysical information.
The geometry of the Moho and of the lithosphere-asthenosphere boundary is based on passive seismological studies (Panza et al. 1992; Nicolich and Dal Piaz 1992; Scarascia et al. 1994; Nicolich, 2001; Pontevivo and Panza, 2002). The Apulian crust is about 30 km thick in the foreland. The Apulo-Adriatic Moho dips toward SW, at least down to a depth of about 50 km below the Tyrrhenian coast. Along the Tyrrhenian Sea, a different and shallower Moho (named “Tyrrhenian”) has been recognised at depths of 25-30 km.
Figure 9. Lithospheric transect across the Southern Apennines
Lithospheric transect (location in figure 1) across the Southern Apennines showing: i) the flexure of the westward subducting Apulian lithosphere, and ii) the hot mantle wedge underlying a new "young" and hot Moho along the western side of transect. Although available geological and geophysical information cannot resolve the existing uncertainties about the deep structure of the Southern Apennines, an integrated analysis of documented tectonic, geophysical and geochemical features shows that a thin-skinned model is generally more consistent with the available data (modified after Carminati et al., 2004 and Scrocca et al., 2005).
The possible location of the slab below the Southern Apennines is constrained by mantle tomography models (Spakman, 1990; Spakman et al., 1993; Amato et al., 1993; Piromallo and Morelli, 1997; Amato et al., 1998; Lucente et al., 1999). Weaker high velocity anomalies detected on some of these models and the absence of subcrustal seismicity created the latitude for interpretations speculating slabless window (Amato et al., 1993; Lucente et al., 1999) or detached slab (Spakman, 1990; Spakman et al.,1993) below the Southern Apennines. However, more recent and detailed tomographic studies focused on the Southern Apennines (e.g., De Gori et al., 2001) highlighted the presence of an almost continuous sub-vertical high velocity body, extending from depths of 65 km down to 285 km. If this is the case, the absence of subcrustal seismicity could be explained with the continental composition of the subducting Adriatic lithosphere (Carminati et al., 2002), which is expected to have ductile rather than brittle behaviour (and to accommodate deformation aseismically rather than seismically).
The location of the proposed Apennine slab is consistent with the occurrence of positive Bouguer anomalies (up to 120 mGal or more; Consiglio Nazionale delle Ricerche, 1992) and very high heat-flow values (up to 140 mW/m2 or more; Della Vedova et al. 2001) along the Tyrrhenian margin and in the adjacent Tyrrhenian Sea. The occurrence of hot asthenospheric material at relatively shallow depth below the western portion of the Southern Apennines is also coherent with the results of the analysis of the shear waves attenuation (Mele et al., 1997) and of helium isotope ratios together with the amount of released gas (Italiano et al., 2000).
The main features that should be noted in the proposed lithospheric section are (Fig. 9): i) the flexure of the subducting Apulian lithosphere, with the slab top deeper than 100 km below the Tyrrhenian coastline; and ii) the presence of a hot mantle wedge underlying a new “young” and hot Moho in the western side of the accretionary prism.
It is generally agreed that the tectonic evolution of the Southern Apennines has been essentially controlled by the flexure-hinge retreat of the westward subduction of the Apulo-Adriatic continental lithosphere (among many other, Malinverno and Ryan, 1986; Patacca et al., 1990; Doglioni, 1991; Doglioni et al., 1996, 1999, 2007). In this subduction retreat model, the retreating slab is replaced by asthenospheric materials in a context of no (or very low) plate convergence. Accordingly, the Tyrrhenian Moho can be considered as a newly forming crust-asthenosphere boundary associated with the well known high heat flow characterising the Tyrrhenian area and the western side of the Italian peninsula. The genesis of the Adriatic Moho, which generally shows quite low heat flow, can be associated with the Mesozoic rifting stages.
The proposed geodynamic model envisages the subduction of a large part of the continental crust associated with the lithospheric mantle of the Apulo-Adriatic plate. The subduction of continental crustal rocks, although often considered an unlikely tectonic process, is suggested by geochemical signatures in the calkalcaline magmas of the volcanic arc (Peccerillo, 1985; Serri et al., 1993) and by the already stated amount of missing basement.
Following the model originally proposed by Doglioni, (1991), the main mechanism which drives the Apennine subduction could be considered the westward relative motion of lithosphere relative to the mantle. The continental crust is interpreted to subduct in response to the eastward push of the asthenosphere rather than to the negative buoyancy of the slab (slab pull). The upper boundary for the subducting plate is defined by the main active detachment, which plunges steadily westward following the lower plate flexure.
In this model, the shear between the down-going and retreating lithosphere and the eastward flow of the asthenosphere compensating the subduction rollback is transferred upward to the accretionary prism, where it is responsible for the off-scraping of the sedimentary cover from the subducting lithosphere (Doglioni et al., 1999).