Elsevier

Tectonophysics

Volume 324, Issue 4, 10 October 2000, Pages 203-237
Tectonophysics

Tectonic model and three-dimensional fracture network analysis of Monte Alpi (southern Apennines)

https://doi.org/10.1016/S0040-1951(00)00138-4Get rights and content

Abstract

The Monte Alpi (southern Italy) is well known as an outcropping analogue of the Val d'Agri subsurface units which host a number of oil fields of economic importance. This paper presents an example of a detailed study which integrates stratigraphy, structural geology, tectonic analysis and fracture network study. The new structural model presented 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, and stereographic projections and palaeostress inversion calculations of a mesostructural data set.

The Monte Alpi Unit is characterised by a 2000-m-thick sequence of Jurassic–Cretaceous platform carbonates, conformably covered by Middle–Upper Miocene calcarenites. This sequence is unconformably covered by Upper Messinian terrigenous clastics, mainly along a number of major growth faults. Our detailed mapping revealed that the Monte Alpi Unit is tectonically overlain by the Lagonegro basinal Unit (thin siliceous carbonates and remnants of pelagic pelitic and carbonate deposits, all highly deformed) and by the Monte la Spina Unit (cataclastic Mesozoic carbonates and dolomites). This solves earlier raised questions concerning the palaeogeographic provenance of the carbonates outcropping in the area, which can thus without any doubt be correlated with the external carbonate domains known from the subsurface. The present-day structure shows a structural inversion along the major Late Miocene growth faults which acted as sinistral transpressive faults. The latter faults are part of a regional set dissecting the southern Apennines, active up till at least Middle Pleistocene times.

A fracture study was performed on the Mesozoic carbonates of the area in order to collect data regarding length distribution, orientation, density and spatial heterogeneity of the fracture network to be used for considerations regarding exploration and development of the Val d'Agri reservoirs.

The data were collected on 37 horizontal (strata-parallel) and vertical outcrop surfaces through photographic restitution and digitising of fracture lineament traces, and measurements of orientations of all visible fractures. Furthermore, three major outcropping rock walls along the flanks of Monte Alpi and Santa Croce were photographed and fracture and fault zone lineaments were digitised. All data were inserted and calibrated in a three-dimensional (3D) Geographic Information Systems and Computer Aided Design (GISCAD) environment consisting of a database and a 3D geographical framework comprising topographic map, Digital Terrain Model (DTM), geological map, aerial photograph lineament map and a 3D reconstruction of major fault surfaces. Making use of the potential of the 3D working environment, outcrop surfaces and rock walls were reconstructed at their correct 3D position and orientation.

Data analysis comprised stereographic contour diagrams and rosette diagrams of 1800 fracture orientations in order to recognise main fracture sets. Three corrections were performed on these data: length weighting, Terzaghi correction (removal of sample orientation bias) and tilt removal. From a comparison between orientation data on various scales combined with field evidence, it can be deduced that fault and fracture sets are part of the same structural pattern, and that fracture sets were generated prior to Plio-Pleistocene tectonics and passively rotated.

Fracture and fault length distribution was analysed by the generation of diagrams which plot cumulative frequency versus length in log–log space, normalising for area of observation. These diagrams, composed of about 30,000 measurements and constituting the most complete data set presented in the literature up till now, show that small-scale and large-scale features form part of one scale invariant system with fractal properties. No scale gap is present in our data set which comprises rock wall data in the ‘subseismic’ scale range. It appears that the 3D fracture network can best be described by a fractal dimension of 2.98 and a coefficient of 0.25.

The outcome of the study places important constraints on fracture network simulation exercises in hydrocarbon reservoir simulation studies, which use numerical input parameters in order to make predictions of the tectonic texture present in the subsurface.

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.

References (82)

  • J. Angelier et al.

    Sur un méthode graphique de recerche des constraintes principales également utilisable en tectonique et en séismologie: la méthode des dièdres droites

    Bull. Soc. Geol. Fr.

    (1977)
  • M.H.P. Bott

    The mechanics of oblique-slip faulting

    Geol. Mag.

    (1959)
  • J.C. Bousquet

    La tectonique tangentielle des séries calcaréo-dolomitiques du Nord-Est de l'Apennin calabro-lucanien (Italie méridionale)

    Geol. Romana

    (1971)
  • J.C. Bousquet

    La tectonique récente de l'Apennin Calabro-lucanien dans son cadre géologique et géophysique

    Geol. Romana

    (1973)
  • A.N. Burton

    Note on the Alpine orogeny in Calabria southern Italy

    Q. J. Geol. Soc. London

    (1970)
  • A. Caire

    Les arcs calabro-siciliens et les relations entre Afrique du Nord et Apennin

    Bull. Soc. Geol. Fr.

    (1962)
  • S. Carbone et al.

    Migrazione neogenica del sistema catena-avampaese nell'Apennino meridionale: problematiche paleogeografiche e strutturali

    Riv. Ital. Paleontol. Stratigr.

    (1990)
  • S. Carbone et al.

    Carta geologica del bacino del fiume Agri. Scala 1:50,000

    (1991)
  • S.W. Carey

    Folding

    Alberta Soc. Petrol. Geol. J.

    (1962)
  • P. Casero et al.

    Evoluzione geodinamica Neogenica dell'Appennino meridionale

    Societa Geologico Italiano, L'Appennino Capano–Lucano ne quadro geologico dell'Italia meridionale, Relazioni

    (1988)
  • P. Casero et al.

    Tectonic framework and petroleum potential of the southern Apennines

  • C. Castaign et al.

    Scaling relationships in intraplate fracture systems related to Red Sea rifting

    Tectonophysics

    (1996)
  • R. Catalano et al.

    Pleistocene strike–slip tectonics in the Lucanian Apennine (Southern Italy)

    Tectonics

    (1993)
  • G. Cello et al.

    La struttura del settore Calabro–Lucano e suo significato nel quadro dell'evoluzione tettonica del sistema a thrust sud-Apenninica

    Stud. Geol. Camerti

    (1990)
  • C. Childs et al.

    A method for estimation of the density of fault displacements below the limits of seismic resolution in reservoir formation

  • Scaling laws for fault and fracture populations — analyses and applications

    J. Struct. Geol.

    (1996)
  • G. De Lorenzo

    Osservazioni geologiche nell'Appennino della Basilicata meridionale

    Atti Acc. Sci. Fis. Mater., Ser. 2

    (1895)
  • Dershowitz, W.S., 1979. A probabilistic model for the deformability of jointed rock masses. M.Sc. Thesis, Massachustts...
  • Dershowitz, W.S. Interpretation and synthesis of discrete fracture orientation size shape, spatial structure and...
  • W.S. Dershowitz et al.

    Interpretation of fracture spacing and intensity

  • L.U. De Sitter

    Structural Geology

    (1956)
  • K.H. Fleishmann et al.

    Paleostress inversion of fault slip data using the shear stress solution of Means (1989)

    Tectonophysics

    (1991)
  • J.W. Gephard

    Stress and the direction of slip on fault planes

    Tectonics

    (1990)
  • F. Ghisetti et al.

    Structural Map of Mt. Pollino (Southern Italy). Scale 1:50,000

    (1983)
  • C. Grandjacquet

    Schéma structural de l'Apennin campano-lucanien (Italie)

    Rev. Géogr. Phys. Géol. Dyn., Ser. 2

    (1963)
  • C. Grandjacquet

    Les phases tectoniques et le métamorphisme tertiare de la Calabre du Nord et de la Campanie du Sud (Italie)

    C.R. Acad. Sci. Paris

    (1969)
  • C. Grandjacquet

    Les séries transgressives d'âge oligo-miocène inférieur de l'Apennin méridional; conséquences tectoniques et paléogéographiques

    Bull. Soc. Geol. Fr., Ser. 7

    (1971)
  • F. Hausdorff

    Math. Ann.

    (1919)
  • Cited by (65)

    View all citing articles on Scopus
    View full text