Lagonegro Units: kinematic evolution
The Lagonegro units have been drilled by some deep exploration wells (e.g., San Fele 1 and Monte Foi 1 wells; Patacca, 2007), which revealed a complex antiformal stack-type structure.
A detailed analysis of the kinematic evolution of the Lagonegro units has been carried in the Monte Foi 1 well area (Fig. 1), where good quality well and seismic data were available (see Scrocca et al., 2007 for further details). In Monte Foi 1 well, several tectonic repetition of formations such as the “Scisti Silicei” and the “Calcari con Selce” have been documented. Sometimes the “Galestri” formation is preserved within this tectonic repetition while no upper Cretaceous - Tertiary units were encountered in this Lagonegro type II structure (Patacca, 2007).
Based on a cross-section intersecting the Monte Foi 1 well, a forward kinematic modelling exercise has been carried out on a slightly simplified version of the interpreted structural setting to test the admissibility of the proposed interpretation (Scrocca et al., 2007). Due to the resolution of seismic and well data, first order features were reproduced by modelling relatively simple ramp-flat geometries. Moreover, for the sake of simplicity, the following assumptions were adopted: i) geological formations have constant thickness; ii) no Tufillo-Serrapalazzo unit were modelled; iii) vertical shear deformation algorithm (volume is conserved during deformation but line lengths may change slightly).
The results of this kinematic forward modelling (Scrocca et al., 2007), bring to light the following deformation history (Fig. 7).
The detachment of the Upper Cretaceous-Tertiary cover of the Lagonegro basinal sequence must be a regional and early tectonic process, likely related to the activation of a very efficient intra-Cretaceous detachment (which could corresponds to the Varicoloured shale formation).
The development of the tectonic repetitions “Scisti Silicei” - “Calcari con Selce” were an early event caused by the propagation within the Lagonegro type II units of secondary detachments, located at the base of the “Calcari con Selce” and propagating upward with short ramps up to shallower flats at the base of the “Galestri” formation. As a consequence, about 13 km of “Monte Facito” remain without its original Jurassic-Lower Cretaceous cover somewhere on the back of the model (i.e., below the Apennine carbonate platform in the geological cross-section). Ramp segment splaying from the same thrusts at the base of the “Galestri” formation, or breaching thrust ramps propagating from lower detachments, caused the stacking of the tectonic repetition “Scisti Silicei” plus “Calcari con Selce” on top of the “Galestri”.
The regional tectonic doubling of the Lagonegro type II over type I was caused by a footwall/hangingwall flat thrust geometry, with at least 40 km of displacement, at any time after (or during) the early Lagonegro II deformation.
The Lagonegro units type I and II were already deformed before they overthrusted the Apulian Platform.
The final configuration of the Lagonegro antiformal stack is caused by a late stage propagation of at least one main thrust, causing the imbrication of the Apulian carbonate units, which shows out-of-sequence features at the shallower levels but turn out to be a breaching thrust at depth.
Figure 7. Lagonegro units kinematic evolution
Lagonegro units kinematic evolution. This model is based on the structural setting reconstructed for the M. Li Foi area where the M.Foi 1 well has been drilled (modified after Scrocca et al., 2007)
In this model, the minimum length necessary to reproduce the Lagonegro II structural setting is of at least 50 km. Other 40 km should be added to the initial model to take into account the regional overthrusting of the Lagonegro II over the Lagonegro I units. Consequently, to honour the available data in the M. Foi area, the minimum total width of the Lagonegro basin is of at least 90 km. Moreover, as suggested by the combined interpretation of wells data and CROP-04 seismic profile (fig. 4), about 30-40 km of undifferentiated Lagonegro units should be buried below the Apennine units. As a result, in the segment of the Southern Apennines crossed by the CROP-04 profile, the total width of the Lagonegro basinal domain could be estimated in about 125 km.