The crustal-mantle structure: bottom to top
Today's active extension takes place underneath the Apennines range replacing the Mio-Pliocene compression. Since the extension is relatively young, graben-like structures are still not over-printed on the former compressional features. This conclusion comes from several tomographic studies of the upper crust (Chiarabba and Amato, 2003 and references therein), computed in past years for isolated spot-like portions of the Apennines fault system. All these studies give valuable information on the structure of the uppermost crust, but fail in illuminating the middle and lower crust. Thus, the overall geometry of the Apennines is better imaged by regional tomographic studies, whose lateral definition of structures is smaller than in small-scale studies, but resolution is even for the whole lithosphere - asthenosphere system (Chiarabba et al., 2009a; Di Stefano et al., 2009).
Figure 5. P-wave velocity in the crust and uppermost mantle.
P-wave velocity perturbations of the crust and uppermost mantle in the central – northern Apennines (tomography by Di Stefano et al., 2009). SKS simplified from Salimbene et al. (2007) are mapped on the upper mantle layer. The main features discussed in the text are shown with solid and dashed lines and labelled.
Figure 5 shows maps of velocity perturbations in the crust and uppermost mantle from Di Stefano et al. (2009) model, zoomed in the northern central Apennines. Velocity anomalies in the uppermost mantle are intriguing and give a first clue in understanding the structure and evolution of the Apennines. Beneath the northern Apennines, we find two positive P-wave velocity anomalies corresponding to the mantle lid of Europe (to the west) and Adria (to the east). In between them, a broad negative velocity extends in the Tyrrhenian inland region (Dvp=-4-6 %). SKS anisotropy is mirroring this separation (Salimbene et al., 2007). Fast anisotropy axes are mostly E-W trending in the European lithosphere, consistent with a long-route eastward flow of the mantle (see Lucente et al., 2006), trench parallel in the Tyrrhenian inland region and broadly N-trending in the Adria lithosphere.
In the uppermost mantle, the Europe and Adria lithospheres have positive velocity anomalies and are separated by a very pronounced belt of low vp anomalies (Dvp= -10 %) that follows the curved Apennines range. As discussed by Di Stefano et al. (2009), this anomaly might be explained by down-dragging of crustal material. Intermediate-depth earthquakes occur at the border of this anomalous body.
The lower crust layer shows the existence of several low vp anomalies underneath the Apennines range (vp less than 5.6-5.8 km/s). These anomalies may coincide with the Mesozoic limestones and basinal rocks, under-plated during the formation of the accretional wedge. Regardless, the computed velocity values are much smaller than those attributable to the same rock types at pressure and temperature conditions of the shallow crust. These low velocities are hardly compatible with sedimentary rocks at pressure conditions of the lower crust, and suggest the existence of deep fluid-filled rock volumes.
The upper crust underneath the Apennines range is dominated by very high vp anomalies (perturbation as high as 12 %) with a velocity of up to 6.7-7.0 km/s, values much higher than those measured for the sedimentary cover. Such strong high velocity bodies concentrate beneath the Apennines range and broadly spread in the Abruzzi region (see figure 5).