Thin-skinned versus thick-skinned models

As already discussed in the previous sections, the available data support the concept that the allochthonous units (e.g. Apennine carbonate platform and Lagonegro-Molise basin) are characterised by a thin-skinned tectonic style. On the contrary the deep structure of the buried Apulian antiformal stack is not sufficiently constrained by the available geological and the geophysical data, so that both thin- and thick-skinned could be put forward.

Although the available information cannot resolve the existing uncertainties about the deep structure of the Southern Apennines, it is at least possible to compare the main tectonic and geodynamic implications of alternative thin-skinned and thick-skinned interpretations.

This task has been carried out by Scrocca et al. (2005), by developing both a thin- and a thick-skinned model on the base of the same conservative cross section presented in figure 4.

In the thick-skinned model the Apulian crystalline basement is deeply involved with the development of three major slices and the estimated shortening in the Apulian carbonate units corresponds to about 20 km. In the thin-skinned model basement is not involved and total shortening of the buried Apulian thrust sheets is assessed to be not less than 90 km.

These alternative models, which must be regarded as two end-members, has been cross-checked against well documented tectonic, geophysical and geochemical features of the Southern Apennines. The main results of this analysis are the following (see details in Scrocca et al., 2005).

It should be noted that the thick-skinned model necessarily requires a transition from thin- to thick-skinned tectonic style, since the available data document a thin-skinned deformation during the Middle Miocene-Early Pliocene tectonic accretion of the allochthonous units. This transition is implicitly or explicitly assumed in several published papers proposing a thick-skinned interpretation for the buried Apulian thrust units (e.g., Casero et al., 1988; Mazzoli et al., 2000; Menardi Noguera and Rea, 2000; Butler et al., 2004; Sciamanna et al., 2004; Speranza and Chiappini, 2002).

The post- Early Pliocene transition from a thin- to a thick-skinned tectonic style should have occurred through a crustal down-section propagation of the main detachment during the contractional deformation of the Apulian domain with the development of basement slices tens of kilometres thick. The modified slab geometry required by the emplacement of the basement slices no longer fit the crustal and lithospheric setting suggested by the available geophysical constraints (e.g., slab needs to be shifted more than 50 km westward and no more space is left for the asthenospheric wedge). A thin-skinned interpretation can be instead easily framed in the large scale geodynamic setting constrained by the available geophysical data.

Following the estimated proposed by Scrocca et al. (2005), the thick-skinned model should have induced approximate “uplift of rock” rates in excess of 1.7 mm/a. This value is significantly larger than the about 0.5-0.7 mm/a measured and estimated (for the last 2 Ma), in the axial zone of chain, using both geomorphological observations and stratigraphical/structural data (Amato and Cinque, 1999; Amato, 2000; Schiattarella et al., 2003, 2006; Ferranti and Oldow, 2005). On the contrary in a thin-skinned model, the Southern Apennines accretionary wedge and the related induced topography developed horizontally moving as a fast wave towards the east, rather than growing vertically, at rates of at least 10-30 mm/a (Patacca et al., 1990; Gueguen et al., 1998). The average expected uplift rates are generally lower than 1 mm/a (Doglioni et al., 1999), with peak values reached only for a short time span in which the tectonic wave crosses an area.

The low displacement in Apulian carbonates implies by the thick-skinned interpretations (about 20 km) cannot explain the Late Pliocene-Early Pleistocene forward motion of the allochthonous nappes, estimated in at least 30-40 km (Patacca and Scandone, 2001; Sciamanna et al., 2004). This discrepancy has been interpreted as evidence that gravitational instabilities within the allochthonous wedge were likely to have substantially contributed to the tectonic advance of the allochthonous nappes by a process of extension linked with thin-skinned thrusting (e.g., Schiattarella et al., 2006; Mazzoli et al., 2008 and references therein).

However, the low displacement hypothesis also conflicts with the observed maturity trends in the Apulian carbonates inferred from vitrinite reflectance data. In particular, the results of a 2D thermal and geochemical modelling (Sciamanna et al., 2004), performed on geological profile cutting across the major oil discoveries located in the Val d’Agri, revealed that the few kilometres of displacement assumed between the innermost Apulian thrust sheets in this thick-skinned interpretation are incompatible with the observed differences in maturity trends.

In conclusion, the thin-skinned model, with displacements among the Apulian thrust sheets in the order of several tens of kilometres, seems to be a preferable tectonic interpretation since it could coherently explain both the observed maturity trends in the Apulian carbonates and the Late Pliocene-Early Pleistocene front advance of the allochthonous nappes.