Tectonic model and three-dimensional fracture network analysis of Monte Alpi (southern Apennines)
Introduction
The ‘Monte Alpi di Latronico’ is situated along the collision suture zone between the southern Apennines fold and thrust belt and the Calabrian Arc (Fig. 1a and b). It comprises a Mesozoic carbonate platform series covered by Miocene bioclastic and terrigenous deposits. The series outcrop in a tectonic window situated along the intersection of major transpressional fault zones dissecting the thrust belt, which is itself composed of a number of low-angle thrust nappes on a regional scale (Fig. 1b). As such, the structure is well known as an outcropping analogue of the Val d'Agri subsurface units, which host a number of oil fields of economic importance situated about 40 km to the north-east of the study area.
The present study was performed in order to obtain relevant information regarding the relation between fracture network and geological structure and evolution, to be used in exploration and reservoir development in the area. This paper summarises the main outcomes of the study, and represents an example of an integration between stratigraphic analysis, structural and tectonic mapping and fracture network investigation, which provides the main constraints on the present knowledge regarding tectonophysical evolution of geological structures.
First of all, the tectonic setting of the region will be discussed and previous views will be summarised. The geological structure and evolution as derived from our analysis will be described in detail, after which the fracture network analysis will be outlined. Finally, a synthesis will be proposed of the evolution of the fracture network within the reconstructed tectonic environment.
Section snippets
Regional setting
The evolution of the Central Mediterranean is characterised by the migration towards the south-east of the Calabrian Arc during post-Eocene times, overthrusting the northern margin of the African Plate and its, more or less independent, promontories (Fig. 1a and b). This foreland comprises the Apulian Block in the north, which is part of the Adria Plate, and the Iblean Block in the south, which is a promontory of the African Plate. In between these two, the Ionian Basin is present, which was
Previous views regarding the structure of Monte Alpi
In the past, some questions were raised concerning the attribution of Monte Alpi to one of the major outcropping tectonostratigraphic units of the area (Fig. 3) and, consequently, the palaeogeographic provenance of the carbonates (for excellent reviews see Sgrosso, 1988, Taddei and Siano, 1992, Vinci, 1993). These disputes were fundamentally based on two well-defined problems: the biostratigraphic dating of the clastic deposits covering the Triassic–Jurassic–Cretaceous platform carbonate
Previous views regarding the thrust belt structure
Our detailed studies revealed that the structure can be viewed as an oblique, sinistral transpressive back-thrusted wedge along the forelimb of one of the major southern Apennines thick-skinned thrust fronts (Fig. 1b). This third hypothesis (see above) is in line with the concepts proposed by Van Dijk and Okkes, 1988, Van Dijk and Okkes, 1990, Van Dijk and Okkes, 1991, which are supported by numerous observations along the eastern border of the Calabrian Arc during the latest decades (e.g.
Geological structure and evolution of Monte Alpi
The structural model for Monte Alpi as presented here is based on a detailed 1:12,500 field survey of the area, whose results are illustrated in a comprehensive outcrop map, geological map and structural cross-sections (Fig. 4, Fig. 5, Fig. 6), and stereographic projections and palaeostress inversion calculations of the complete mesostructural data set (Fig. 7). About 100 outcrops were mapped, and 425 measurements of bedding planes, faults and striae, cleavage couples and fold axes were taken
Palaeostress analysis
Features such as fault–striation couplets and cleavage–fault plane couplets were used to perform a palaeostress inversion analysis, the results of which are displayed in Fig. 7 and listed in Table 1.
The analysis was performed using the in-house developed software CalStress, following the dihedra–trihedra method and its statistical parameters (Lisle, 1987, Lisle, 1988, Pegoraro, 1972), which is based on the well-known Wallace–Bott criteria for fault plane reactivation (Bott, 1959, Wallace, 1959
Fracture analysis
The analysis of fracture patterns on the Monte Alpi outcrop was performed as part of a regional revision of fracture data related to the Val d'Agri reservoir. Classical studies of fracture reservoirs incorporate data sets derived from conventional well logs, bore hole televiewer tools (e.g. FMI/FMS), oriented and non-oriented cores, and two-dimensional (2D) and three-dimensional (3D) seismic elaborations (e.g. Childs et al., 1990, Major and Holz, 1997, Walsh and Watterson, 1991). These data
Conclusions and discussion
The detailed mapping of the Monte Alpi outcrop and the structural geological analyses performed show that the structure forms part of the Val d'Agri subsurface Unit. It can be regarded as a back-thrusted wedge along the forelimb of a major thick-skinned transpressive structure of the southern Apennines thrust belt. The assignment of Monte Alpi to the more external platform units is based on three major pieces of evidence. Our stratigraphic analysis confirmed the latest biostratigraphic dating
Acknowledgments
We gratefully acknowledge the contribution of A. De Feyter and G. Fontanesi and thank them for putting their field data at our disposal. The AGIP stratigraphy team analysed the Monte Alpi outcrop during the years 1995 and 1996 and we thank M. Riva, Papetti and O. Borromeo for their contribution. M. Sella, A. Maccagni, R. Prato, G. Sclocchi, M. Cafiero, P. Quattrone and F. Benelli are thanked for supporting the research and the publication of this paper. Thanks are due to Dr. D. Delvaux and Dr.
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