Thin-Skinned Vs. Thick-Skinned Tectonics in the Matese Massif, Central–Southern Apennines (Italy)

Tectonophysics 377 (2003) 269–297 www.elsevier.com/locate/tecto

Thin-skinned vs. thick-skinned tectonics in the Matese Massif, Central–Southern Apennines (Italy)

R.A. Calabro` a, S. Corradob,*, D. Di Buccic, P. Robustinib, M. Tornaghia

a Edison Gas S.p.A., Foro Buonaparte 31-20121, Milan, Italy b Dipartimento di Scienze Geologiche, Universita` degli Studi di Roma Tre, Largo S. Leonardo Murialdo 1-00146, Rome, Italy c Dipartimento della protezione civile, Servizio Sismico Nazionale, Via Curtatone 3-00185, Rome, Italy Received 12 February 2002; accepted 29 September 2003

Abstract

Combined structural and geophysical investigations were carried out in the Matese Massif at the boundary between the Central and the Southern Apennines. The aim of this research was to define the structural geometry and the kinematics of Neogene deformation in order to validate the applicability of either thin-skinned or thick-skinned compressive styles in the Apennines of Italy. On the basis of two surface N–S cross-sections, integrated with recently acquired magnetotelluric data, the Matese structure appears as a single thrust sheet limited at the bottom by a low-angle thrust fault with sense-of-displacement towards the North. The structure was later deformed by footwall thrusts carrying Molise–Sannio Pelagic Basin Units and Apulian Carbonate Platform Units. The structure of the Matese Massif is composed mainly of Mesozoic carbonates of inner platform to by-pass and slope facies, separated within the thrust sheet by high-angle faults with low reverse horizontal displacement. These faults acted as normal faults in the Mesozoic and Cenozoic and controlled the carbonate facies distribution. During Neogene compression, these structures were tilted and then truncated by the basal short-cut thrust fault that cut from pre-Triassic units to Jurassic dolostone towards the foreland. Finally, Quaternary extensional tectonics dissected the whole structure with fault-related displacement values of up to 1000 m. This resulted in the present-day structural setting of the Matese Massif as a horst bounded to the SW and NE by fault- controlled basins, filled with continental deposits. This revised structural scenario is discussed within the framework of the structural style of external zones of the Apennines. D 2003 Elsevier B.V. All rights reserved.

Keywords: Fold-and-thrust belt; Thin-skinned tectonics; Thick-skinned tectonics; Reactivation; Apennines; Matese; Italy

1. Introduction ed on the passive margin of the Southern Tethys Ocean. It has classically been interpreted as a thin- The Apennine orogenic belt (Fig. 1) comprises skinned fold-and-thrust belt. In this model, the Meso- Mesozoic and Cenozoic sediments that were deposit- Cenozoic sedimentary cover was deformed during Neogene time and detached from the underlying * Corresponding author. Fax: +39-6-54888201. magnetic basement. The latter was not involved in E-mail address: [email protected] (S. Corrado). the deformation (Bally et al., 1986; Hill and Hayward,

Fig. 1. Simplified geological map of the Italian Apennines (after Bigi et al., 1992, redrawn and modified). In the square, location of Fig. 2.

1988; Mostardini and Merlini, 1986; Ghisetti et al., Neogene deformation, even in the external portions of 1993; Cavinato et al., 1994). the Apennines. This is clearly shown along the crustal In the Central and Southern Apennines, this model profile CROP 03 across the Northern Apennines has led many authors to calculate large amounts of (Barchi et al., 1998). Additionally, according to a orogenic shortening (Calamita et al., 1994; Patacca et recent interpretation (Chiappini and Speranza, 2002) al., 1990; Cavinato et al., 1994; Finetti et al., 1996; of the new aeromagnetic map of Italy (Chiappini et Prosser and Schiattarella, 1998; Billi et al., 2001). al., 2000), basement involvement appears to be a The timing of orogenic migration from the hinter- widespread and distinctive feature throughout the land to the foreland has been defined by dating thrust- front of the chain. top and foredeep basins development through high- Furthermore, there have been plenty of recent resolution stratigraphy (Patacca et al., 1990, 1992a; contributions supporting a thick-skinned interpreta- Cipollari and Cosentino, 1995). These data, coupled tion of the Apennine thrust belt. These are based to thin-skinned model assumptions, support anoma- on field data and commercial seismic lines, and lously high shortening rates (tens of millimetres per explain the present-day structural setting of several year) if compared to the average shortening rates sectors of the Apennines as a result of local inferred from other fold-and-thrust belts (millimetres contractional reactivation of extensional structures per year; Tozer et al., 2002). that pre-date formation of the thrust belt. This is However, recently acquired geophysical data sup- testified by syn-tectonic facies changes in the Me- port involvement of the magnetic basement in the sozoic and Cenozoic stratigraphy (Renz, 1951; ..Calabro R.A. ` ta./Tcoohsc 7 20)269–297 (2003) 377 Tectonophysics / al. et

Fig. 2. Simplified geological map of the Central Apennines (after Accordi et al., 1988, redrawn and modified); refer to Fig. 1 for location. Most of this portion of the chain is formed by the stacking of the Apenninic carbonate Platform; the Apulia carbonate Platform outcrops in the Maiella Mountain, in the surrounding structures, and in the Gargano Promontory. 271 272 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297

Giannini, 1960; Bernoulli, 1967; Colacicchi et al., This signature is widely recorded in the Northern 1970; Centamore et al., 1971; Decandia, 1982; Apennines (Umbria–Marche–Romagna pelagic do- Montanari et al., 1988; Winter and Tapponier, main and its transitional facies), where the role of 1991) due to normal faulting as a result of the reactivation processes is prominent in determining evolution of either the passive margin or the the thrust evolution and the present-day fault pattern peripheral bulge (Adamoli et al., 1997; Coward et (Cello and Coppola, 1989; Barchi and Brozzetti, al., 1999; Tavarnelli et al., 1999; Mazzoli et al., 1994; Alberti, 2000). Pre-existing normal faults 2000; Scisciani et al., 2000a,b, 2001). have not only affected the localization of thrust

Fig. 3. Satellite image of the Matese area. Main facies boundaries: continuous bold lines. Main Neogene compressive elements, locally reactivated by Quaternary extensional tectonics: dashed fine lines. ..Calabro R.A. ` ta./Tcoohsc 7 20)269–297 (2003) 377 Tectonophysics / al. et

Fig. 4. Simplified structural map of the Matese Massif and surrounding areas. 273 274 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 ramps, the amount of local shortening, and the integrated with pre-existing (gravimetric anomalies development of short-cut thrusts, but have also and hydrocarbon wells) and new (magnetotelluric) resulted in severe tilting and moderate rotations subsurface data acquired for oil exploration. The about vertical axes (Barchi et al., 1989; Adamoli, sections have been restored to Lower Pleistocene 1992; Tavarnelli, 1996; Alberti et al., 1996; Ada- times to remove the effects of Quaternary exten- moli et al., 1997; Coward et al., 1999; Tavarnelli et sional tectonics. This allows the compressional., 1999; Mazzoli et al., 2000, 2001a; Scisciani et related structural geometry and the timing of al., 2000a, 2001). deformation to be defined. Finally, we discuss Studies focusing both on local (Scisciani et al., different hypotheses for the deep structure and the 2000b) and regional scale structures (Corrado et al., main deformation mechanism operating in the 1998a; Scisciani et al., 2001, Tozer et al., 2002) Matese area since the Messinian, and suggest their also investigated the effect of pre-thrusting struc- implications for oil exploration and for shortening tures in the Central Apennines carbonate platform amounts and rates. domains and at their boundaries with the surrounding pelagic basins. Finally, in the Southern Apennines and Sicily, 2. Geological setting contractional reactivation mechanisms, as well as basement involvement in thrusting, are more scat- The Apennine fold-and-thrust belt of peninsular tered but still documented (Renda et al., 1999; Italy (Fig. 1) forms part of the Africa-verging moun- Menardi Noguera and Rea, 2000; Mazzoli et al., tain system in the Alpine–Mediterranean area. This 2000, 2001b). area evolved within the framework of convergent In this framework, the structure of the Matese motion between the African and European plates since Massif, located at the boundary between Central and the Late Cretaceous (Dewey et al., 1989). Roughly Southern Apennines (Fig. 2), represents a key area in north–south convergence between Africa and Eurasia which to test thin- vs. thick-skinned geometric models was dominant up to Oligocene time. During the for three main reasons. Neogene, nearly east-directed thrusting toward the First, the massif (Figs. 3 and 4) shows a rather Apulian (or Adriatic) continental margin dominated intricate Meso-Cenozoic paleogeography; a transition and was accompanied at the hinterland by the opening from Mesozoic thicker carbonate platform deposits to of the Tyrrhenian Sea (Malinverno and Ryan, 1986; thinner Meso-Cenozoic slope deposits can be ob- Royden et al., 1987; Mazzoli and Helman, 1994; served from south to north. Carmignani et al., 2001 and references therein). The Secondly, the pre-orogenic facies boundaries Central Apennines (Fig. 2) developed in a transition strike almost parallel to the main Neogene compres- area between the northern and southern arcs that make sional and transpressional features that overprinted up the whole Apennine fold-and-thrust belt. They them. comprise sediments mainly derived from the Meso- Finally, the Pleistocene geometry of the struc- zoic deposits of the southern passive-margin of the ture (i.e. at the end of compression), as derived Tethys Ocean. Therefore, the geological evolution of from restored sections, is characterized by an the region containing the Matese Massif (Figs. 3 and anomalously high culmination in the southern sec- 4) can be traced back to the Mesozoic (Di Bucci et al., tor of the Matese Massif. Its occurrence can be 1999). explained in two ways: either the relief was Different paleogeographic reconstructions are avail- inherited, or it was caused by severe shortening able for this part of the Apennines (among others: and tectonic thickening of deeper thrust sheets in D’Argenio et al., 1973; Mostardini and Merlini, thin-skinned fashion. 1986; Sgrosso, 1988; Pescatore, 1988; Cello et al., In this paper, we address these two hypotheses 1989; Marsella et al., 1992; Patacca et al., 1992a,b; by presenting two N–S geological sections through Corrado et al., 1998a; Pescatore et al., 1999). A critical the Matese Massif. They are based on newly review of these models can be found in the work of Di acquired field data and meso-structural analysis Bucci et al. (1999), whose final reconstruction has been R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 275 adopted in this work. This reconstruction of the Meso- Furthermore, in situ stress data for the region zoic paleogeography for the passive margin stage (Montone et al., 1999) show a present-day stress shows two main carbonate platforms (the Apenninic pattern that is consistent with these observations, i.e. and Apulia Platforms) separated by a pelagic basin (the the same tectonic regime has acted since the Middle Molise–Sannio Basin). Between the Late Tortonian Pleistocene. and Late Pliocene p.p., these palaeodomains were progressively involved in the NE-verging orogenic system. Extensional faulting (due to flexure of the 3. Field data margin), development of a complex foredeep, and finally thrusting affected this segment of the pre-exist- 3.1. General overview ing passive margin. The resulting tectonic stack forms the bulk of the present-day Central–Southern Apen- The Matese Massif (highest peak: Mt. Miletto, nines orogenic wedge. In general, sediments of the 2050 m) is a roughly east–west oriented mountain Apenninic carbonate Platform make up the Matese ridge (Figs. 3 and 4). It is characterized by exten- Massif, whereas the related slope sediments and pelag- sive outcrops of Apenninic Platform carbonate rocks ic deposits outcrop in the northern Matese Massif, and related transitional facies to the Molise–Sannio Montagnola di Frosolone, Sannio and Alto Molise; pelagic Basin to the north and east. The central and siliciclastic foredeep deposits primarily outcrop in the western parts of the structure were mapped in detail topographic depression between the Montagnola di (1:25,000) and two roughly N–S oriented geologi- Frosolone and the Matese Massif. cal sections were drawn (Fig. 5). The massif can be Between Late Pliocene p.p. and Early Pleistocene, subdivided into three sectors (southern, central, and the same compressional tectonic regime gave rise to northern) for both palaeoenvironmental and tectonic widespread WSW–ENE to W–E trending left-lateral reasons (Figs. 3 and 4). and N–S trending right-lateral strike-slip faulting, which resulted in the dissection of the thrust struc- 3.2. Southern sector tures. The hypothesis that some of these faults cut through the entire thrust stack cannot be ruled out Outcropping facies are predominantly of carbon- (Corrado et al., 1997a). In this period, an incipient ate shelf, ranging from Upper Triassic dolostones to extension involved the SW side of the Matese, open- Upper Cretaceous limestones (Figs 5 and 6a–d). ing the lower Volturno River valley (Brancaccio et al., They are overlain by Middle Miocene ramp carbo- 1997). nates of the Cusano Formation, followed by Upper Post-Messinian and pre-Middle Pleistocene tecton- Miocene hemipelagic marls (Longano Formation) ic activity was also responsible for a 40j counter- and siliciclastic foredeep deposits. clockwise rotation about vertical axes of the Montag- The Triassic dolostones, known as Fontegreca nola di Frosolone and the northern portion of the Formation, are extensively exposed between Ravis- Matese (Iorio et al., 1996; Speranza et al., 1998). canina and Valle Agricola, along transect A–AV Finally, in the study area, the tectonic regime (Fig. 5). Jurassic and Cretaceous facies are domi- changed drastically during the Middle Pleistocene nant between Mt. Capello and Letino (transect A– and a NE–SW oriented extension became dominant AV), and along transect B–BV (Fig. 5). The bedding (Patacca et al., 1992a; Corrado et al., 1997b; Ferranti, dips ca. 30j to the north along transect A–AV, 1997). Not only did this extension cause the formation while it forms an E–W trending open anticline of a system of NW–SE striking normal faults, but it along transect B–BV. also resulted in partial reactivation of the existing In this sector, brittle deformation is predomi- strike-slip faults with either an opposite sense of slip nantly extensional; morphological features like or a dip-slip component (Corrado et al., 1997b, 2000). triangular facets and alluvial fans provide evidence As a result, the Matese Massif appears as a carbonate for its recent activity. NW–SE trending normal horst bounded by extensional intramontane continen- faults in the Raviscanina area and E–W trending tal basins (Di Bucci et al., 1999). faults in the Piedimonte Matese and San Gregorio 276 ..Calabro R.A. ` ta./Tcoohsc 7 20)269–297 (2003) 377 Tectonophysics / al. et

Fig. 5. Cross-sections through the Matese Massif. Traces are drawn in Figs. 3 and 4. Southern, central, and northern sectors are described in the text. Mesoscopic structural data are plotted on lower hemisphere, equal-area projection (continuous lines and dots: faults and related poles. Dashed lines and triangles: bedding and related poles). ..Calabro R.A. ` ta./Tcoohsc 7 20)269–297 (2003) 377 Tectonophysics / al. et

Fig. 6. Stratigraphic setting of the Matese Massif. Stratigraphic sections are representative of the southern, central, and northern sectors described in the text. Thickness of lithologies marked with the asterisk are derived from subsurface data. 277 278 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297

Fig. 7. Geological map of the Gallo–Letino area and related mesostructural data (on lower hemisphere, equal-area projection; faults: continuous lines). A–AVrefers to part of the cross-section A–AVin Fig. 5. ..Calabro R.A. ` ta./Tcoohsc 7 20)269–297 (2003) 377 Tectonophysics / al. et

Fig. 8. Panoramic view of the northern edge of the Matese Lake. Note how extensional faults downthrow to the south of the block that contains the lake. 279 280 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 281

Fig. 10. Panoramic view of the eastern edge of Gallo depression (Monticello hill). Note the stratigraphic unconformity beetween Triassic dolostones (Fontegreca Formation) at the bottom and Upper Cretaceous–Paleocene limestones (Monte Calvello Formation) at the top.

Matese areas downthrow their hanging-wall blocks clearly visible along the northern slope of the valley to the southwest and to the south, respectively. (plot ‘‘b’’ in Fig. 7). These faults define the boundary of the Matese Farther east, the lineament deflects to a WNW– Massif with the Volturno River valley (Fig. 4). ESE strike and enters the valley containing the The northern boundary of this sector is marked by Matese Lake. In this area, it consists mainly of an E–W trending major morphological lineament WNW–ESE striking south-dipping normal faults running across the central part of Matese. In the that repeatedly truncate the Lower/Upper Cretaceous Letino area, (Fig. 7) the western tip of this lineament boundary. Suspended valleys, triangular facets, and is marked by NE–SW trending high-angle left-lateral footwall-sourced alluvial fans suggest that recent transpressive faults (plot ‘‘a’’ in Fig. 7). Moving extensional tectonics shaped the valley (Fig. 8). eastwards, similar kinematics is observed along However, scattered pre-existing elements showing south-dipping ENE–WSW oriented high-angle faults. transpressional kinematics are still preserved west These mesoscopic structures can be interpreted as of the lake. synthetic Riedel planes connected to a major E–W A distinctive tectono-stratigraphic feature of this trending left-lateral fault (plot ‘‘c’’ in Fig. 7) that sector is observed in the Valle Agricola and San separates Letino’s Jurassic carbonates ridge to the Gregorio Matese areas. At the highest peaks south south from the siliciclastic deposits of the depression of Valle Agricola (Mt. San Silvestro, 1083 m of Gallo to the north. a.s.l.), mainly pelitic-arenaceous, rarely turbiditic The transpressional features are dissected by youn- deposits of probable Upper Miocene age containing ger WNW–ESE oriented, SSW-dipping normal faults Miocene carbonate olistoliths lie unconformably on

Fig. 9. Geological map of the San Gregorio area and related mesostructural data (on lower hemisphere, equal-area projection; faults: continuous lines). Note the angular unconformity between the carbonate substratum and the Miocene siliciclastics, and the Quaternary extensional faults that downthrow the structure to the south. These faults are generally characterized by high dip angles and dip-slip kinematic indicators shown on plots a–d. B–BVrefers to part of the cross-section B–BVin Fig. 5. 282 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297

Triassic dolostones. The stratigraphic contact is bonate olistoliths and breccias, lie unconformably marked by a hardground. A carbonate poligenic both on Cretaceous (south of San Gregorio Matese) continental conglomerate that makes up the highest and on Miocene carbonates (north of San Gregorio peaks lies unconformably on top of the siliciclastic Matese). They are found at three different elevations, sequence. bounded by E–W oriented south-dipping normal The situation is to some extent repeated farther east faults. Again, on top of the carbonate/siliciclastics in the San Gregorio Matese area (Fig. 9). Here, mostly contact, well cemented and bedded continental con- pelitic, siliciclastic deposits, containing Miocene car- glomerates are found.

Fig. 11. Simplified geological map of Matese anticline area and related mesostructural data (on lower hemisphere, equal-area projection; faults: continuous lines; poles to bedding: triangles; hinges: slashed dots). A–AVrefers to part of the cross-section A–AVin Fig. 5. ..Calabro R.A. ` ta./Tcoohsc 7 20)269–297 (2003) 377 Tectonophysics / al. et

Fig. 12. Simplified logs of wells drilled in the surroundings of the Matese Massif. Location in Fig. 15. 283 284 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297

3.3. Central sector horst structure limited by E–W and NW–SE oriented normal faults; the structure corresponds to the Outcropping facies are predominantly of carbon- highest peaks Mt. Miletto and La Gallinola. The ate shelf and by-pass margin, ranging from Upper northernmost fault, a NW–SE striking, NE-dipping Triassic dolostones to Upper Cretaceous limestones normal feature, results in the boundary of the (Figs. 5 and 6e–f). The distinctive stratigraphic Matese carbonates with the San Massimo valley, feature of this sector is represented by the Upper where siliciclastic sediments border the massif to Cretaceous/Paleocene transgressive cycle that covers the north. progressively younger units from west to east (Ietto, The boundaries with the adjacent sectors are 1963, 1969). In detail, in the area of Capriati a marked instead by transpressional elements to the Volturno and Fontegreca, as well as in the depression south and compressive faults to the north. of Gallo, bio-sparitic carbonates deposited in slope In the depression of Gallo (850 m a.s.l.), the environment (Monte Calvello Formation) rest direct- transpressional left-lateral fault that marks the ly on top of Upper Triassic dolostones. Near Gallo, southern boundary is associated with tight folds this contact is marked by a slight angular unconfor- with E–W to NW–SE trending axes developed at mity (Fig. 10). Farther east, north of the highest peak its footwall in hemipelagic marls and siliciclastics. Mt. Miletto, the Monte Calvello Formation lies in The northern boundary is marked by a fault- paraconformity on Lower Cretaceous inner shelf propagation anticline well exposed along transect carbonates. A–A’ (Fig. 5) with an east–west trending axis (the The Mesozoic–Paleocene sequences are generally Matese anticline; De Corso et al., 1998). The fold overlain by the Middle Miocene ramp carbonates of becomes more and more open and dies out both to the Cusano Formation lying paraconformably on top the west and to the east. The related fault develops of the Monte Calvello Formation. They consist of for about 4 km in the central part of the structure Bryozoa-andLithotamnium-rich carbonate ramp and shows a maximum reverse separation of less grainstones. As the depositional marine environment than 100 m (De Corso et al., 1998). It is marked became deeper during Middle to Upper Miocene, by low-angle, south-dipping fault planes with re- Orbulina-rich marls and shales were deposited and verse dip-slip kinematics that indicate the top-to- finally evolved to flysch sedimentation (Fig. 6e). the-north displacement of the structure (plot ‘‘a’’ in This evolution is quite clearly exposed in Gallo area Fig. 11). (Fig. 7). Moving to the north, Eocene to Oligocene calcar- 3.4. Northern sector enites of the Monaci Formation containing Nummuli- tidae and Alveolinidae, and Lepidocyclinae-bearing In the northern sector, the sedimentary deposits shales of the Macchiagodena Formation, are rarely are deeper marine and show transitional facies to comprised between Mesozoic–Paleocene and Mio- the Molise–Sannio pelagic sequence. It is charac- cene sequences. They are exposed in the Mt. Celara terized by the extensive outcrop of the Monte area. Calvello, Monaci, and Macchiagodena Formations In this sector, brittle deformation is predominant- and by the lens-shaped transgressive Middle Mio- ly extensional. It is clearly exposed along transect cene grainstones that are time-equivalent to the B–BV (Fig. 5). In fact, between the Matese Lake Cusano Formation. All these formations thicken and the San Massimo valley, the massif shows a gradually to the north toward the Frosolone area,

Fig. 13. Present-day vs. restored section A–AV. The amount of Quaternary extension is shown. Key. Tr: carbonate shelf dolostones (Triassic); LJ: carbonate shelf limestones and dolostones (Lower Jurassic); J/J(sl): carbonate shelf/slope deposits (Jurassic); LK: carbonate shelf deposits (Lower Cretaceous); K(sl): slope (northern sector) and by-pass margin limestones (Lower Cretaceous–Paleocene); Mo–Ma: carbonate ramp marls and shales (Eocene–Lower Miocene); Lo: hemipelagic marls (Middle–Upper Miocene); Fl: foredeep siliciclastic deposits (Upper Tortonian–Lower Messinian). R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 285 286 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 287 testifying the progressive deepening of the Meso- In fact, it is a horst bounded by major NW–SE Cenozoic sedimentary basin. The succession striking high-angle planar normal faults that down- evolves into siliciclastic sedimentation through throw the carbonates to the SW for at least 1000 m Orbulina-rich marls (Longano Formation), which and to the NE for a few hundred metres. The heart outcrops extensively north of the Matese anticline. of the massif is also severely affected by analogous The buried portion of the slope-to-basin succes- fault systems; these are characterized by lower sion has been penetrated by the Frosolone 2 well vertical displacements (mainly from tens to hundred (Fig. 12). It ranges from Upper Jurassic/Lower of metres). In some cases, pre-existing N–S and E– Cretaceous calcareous conglomerates, dolostones, W striking strike-slip faults have been reactivated as and cherty shales of the Indiprete Formation to transtensional faults presumably by the onset of Upper Cretaceous micritic limestones with chert NE–SW oriented extension that has acted in the and fragments of rudists of the Monte Coppe and area since Middle Pleistocene (Corrado et al., Coste Chiavarine Formations (Scrocca and Tozzi, 1997b). 1999). In order to understand the geometry of the pre- In this sector, brittle deformation is predominantly extensional setting, we restored the two sections of extensional, with NW–SE oriented high-angle normal Fig. 5 by removing the effects of the recent exten- faults downthrowing the Matese Massif carbonates to sional deformation occurred on planar non-rotational the NE beneath the Boiano basin. faults. The planar geometry of the faults that affect the Furthermore, between Longano and Castelpizzuto southern portions of the two sections represents one of (Fig. 4), Upper Cretaceous to Miocene slope facies the distinctive features of extension in the Matese carbonates outcrop in small structural highs Massif. This is evident not only from geometric bounded by N–S trending faults, which mainly surface data, but also from seismological information dip to the east. They first acted as right lateral about the faults that border the Matese Massif to the faults and since the Middle Pleistocene have been NE and that ruled the development of the Boiano probably reactivated as transtensive left-lateral faults basin (Cucci et al., 1996; Corrado et al., 2000). (Corrado et al., 1997b). Furthermore, the non-rotational kinematics, even Finally, the boundary with the central sector is though not a widespread feature at the regional scale marked by the Matese anticline (Fig. 11). Its footwall and in the Apennines, seems to be the best approxi- is deformed in an E–W trending, major recumbent mation we can propose for the faults along the two syncline associated with mesoscopic chevron folds transects. As a matter of fact, in the southern portions (plot ‘‘b’’ in Fig. 11). of the sections the average dip of the bedding (approx. 30–35j), associated to faults dipping from 60j to 70j, is not consistent with rotational planar faulting as 4. Restoration of extensional tectonics described by Wernicke and Burchfiel (1982). In fact, had we assumed a tilting, rotating the fault-bounded Although the Matese Massif experienced impor- blocks back during restoration would have caused the tant thrusting during the building of the Apennine normal faults to reverse their dips. Moreover, the total chain, little evidence of compression characterizes amount of the inferred net extension is very low: 3.1 this structure and the brittle deformation is predom- over 22.2 km ( + 14%) for the section A–AV, 1.6 over inantly extensional. 18.8 km ( + 9%) for the section B–BV (see Figs. 13

Fig. 14. Present-day vs. restored section B–BV. The amount of Quaternary extension is shown. Key. Tr: carbonate shelf dolostones (Triassic); LJ: carbonate shelf limestones and dolostones (Lower Jurassic); J/J(sl): carbonate shelf/slope deposits (Jurassic); LK: carbonate shelf deposits (Lower Cretaceous); UK: carbonate shelf deposits (Upper Cretaceous); K(sl): slope (northern sector) and by-pass margin limestones (Lower Cretaceous–Paleocene); Mo–Ma: carbonate ramp marls and shales (Eocene–Lower Miocene); Cu: carbonate ramp limestones (Middle Miocene); Lo: hemipelagic marls (Middle–Upper Miocene); Sc: transgressive siliciclastic deposits (Miocene); Fl: foredeep siliciclastic deposits (Upper Tortonian–Lower Messinian). 288 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 and 14). These values, associated with the faults’ Afterwards, 2D inversion processing was carried average dip and the total displacement along the array out along the two N–S transects (1–1V and 2–2V in of faults in the southern sector, are consistent with the Figs. 15 and 16) that roughly correspond to the two non-rotational normal fault model discussed by Wer- surface sections presented above (Fig. 5). The trans- nicke and Burchfiel (1982). ects show comparable patterns of anomalies arranged Unfortunately in the study area, as in many cases in in three principal layers, characterized by different the Apennines, the best tool to test rotational vs. non- values of apparent resistivity. From the surface below, rotational kinematics, i.e. the stratal architecture of they are arranged as follows (Fig. 16): syn-tectonic deposits within fault-bounded depres- sions, cannot be applied due to the lack of these an upper high-resistivity layer showing its max- deposits. Therefore, rotational models like the domi- imum thickness where both transects reach the no-like faulting cannot be definitely ruled out. How- highest topographic elevations (Mt. Celara for the ever, applying this model to fault blocks of about 1 western section and Mt. Miletto for the eastern km and with a dip of 60j (modeling the normal faults section); of Figs. 13 and 14), rotations near to 30j are neces- an intermediate low-resistivity (high conductivity) sary to reduce to zero a mean displacement of 0.5 km layer that thins out at the southern edge of both (trigonometric computations can be easily developed, processed profiles; it is noteworthy that the lack of based on formulas after Mandl, 2000). In this way, data at the edges of the acquired MT grid could extensional faults would have developed with dips give a low statistic control of the modelling; near to 90j, in contrast with the Anderson model. a lower high resistivity-layer. Based on these considerations, a non-rotational model is proposed in this paper for the restoration of the The processing performed on all the recorded Quaternary extensional faults that dissect the com- soundings provides a high degree of reliability for pressional structures. the upper layer and the underlain conductive unit. The Both restored sections show a prominent structural thickness of the conductor and the transition to the culmination in their southernmost portions, where the lowermost resistive unit is less reliable due to the bedding dips to the north, with steeper angles (40j impact of noise recorded at lower frequencies. average) in the western section (section A–AV) than in section B–BV. This culmination needs a suitable 5.2. Magnetotelluric interpretation on the base of pre- structural explanation in the subsurface setting of the existing subsurface data area. No seismic data have yet been acquired on the Matese Massif for oil exploration. Thus, a reliable 5. Subsurface data interpretation of the magnetelluric data can only be drawn on the basis of subsurface data acquired in 5.1. New magnetotelluric data more external areas (deep wells and seismic lines) and Bouguer gravity anomaly map of the massif and its The Edison Gas Company recently performed a surroundings (Mostardini and Merlini, 1986) (Figs. new magnetotelluric survey in order to collect sub- 12, 15, 16, and 17). surface data in this area. Twenty-three magnetotelluric A correlation between the magnetotelluric pattern soundings were carried out on the Matese Massif, and the geological units is straightforward only for the obtaining quite good quality results (see Fig. 15 for upper resistive layer that corresponds to carbonates of location). The stations were laid out along two dip the Matese thrust sheet (Apenninic Platform domain). lines oriented approximately N–S and along a NW– At depth, this thrust sheet is testified, for example, by SE strike line. The distance between the stations was the Campobasso 1 well (Fig. 12) in which a thick nominally 2 km. Two isolated soundings were also Mesozoic carbonate sequence was encountered. It made at the locations of the Campobasso 1 and evolves to slope deposits in Cretaceous–Eocene and Morcone 1 wells. is overlain by Miocene ramp carbonates and Upper R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 289

Fig. 15. Bouguer gravity map of the Matese Massif and surrounding areas (density: 2.4 g/cm3) and location of deep wells and traces of magnetotelluric transects discussed in the text and shown in Figs. 12 and 16.

Miocene siliciclastics. This sequence strictly corre- 2001b). The buried Apulia Platform’s structural lates with the typical outcropping sequence in the setting has been widely investigated in the last northeastern Matese Massif. decade since it shows a high potential as a hydro- The lower resistive layer could correspond to the carbon reservoir (Casero et al., 1991). It is made up buried Apulia Platform carbonates, as suggested by of Mesozoic and Tertiary carbonates up to 6–7 km the reconstruction of the regional setting beneath thick, stratigraphically overlain by marine siliciclas- the external areas of Central and Southern Apen- tics of Messinian–Pliocene age. nines (Mostardini and Merlini, 1986; Cello et al., The nearest wells to the study area that reached the 1989; Roure et al., 1991; Mazzoli et al., 2000, top of the buried Apulia Platform are the San Biase 1 290 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297

Fig. 16. N–S magnetotelluric transects discussed in the text. See Fig. 15 for location. and Castelmauro 2 wells (Fig. 12); they penetrated the position of the Matese thrust sheet, which is known Apulia carbonates at a mean depth of about 3400 m, to the north and to the east of the study area (Scrocca comparable with the depth of the top of the lower and Tozzi, 1999 and Fig. 17; Corrado et al., 1998b; resistive layer. Mazzoli et al., 2000), suggests that at least the More speculative is the geologic significance of the northernmost portion of the conductive layer in the intermediate conductive layer. The low values of sections of Fig. 16 could belong to the pelagic basin apparent resistivity could suggest a clay-rich compo- clastic successions of the Molise–Sannio domain. sition. In addition, correlation with the subsurface This is confirmed, for example, by the sequence

Fig. 17. Geologic interpretation of a seismic line acquired to the east of the Matese Massif (redrawn and simplified after Scrocca and Tozzi, 1999). See Fig. 15 for well location. R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 291 drilled at the bottom of the Frosolone 2 and Campo- to the main compressive and transpressive faults basso 1 wells, and by the geologic interpretation of a (Fig. 5). These are characterized by low horizontal seismic line acquired to the east of the Matese Massif displacement (from a few tens of metres to zero). (Fig. 17). Therefore, these pelagic sequences could This evidence suggests that pre-thrusting local facies represent the intermediate structural unit between the distribution was only slightly affected by Neogene internal Apenninic and the external Apulia Platform, deformation. We interpret these abrupt facies as imaged in the northern portions of the magneto- changes to be the result of normal faulting that telluric transects. In fact, moving toward the foreland, was active during the evolution of the passive the wells external to the Matese thrust sheet show that margin. According to traditional models of platform the Molise–Sannio sequences are thrust over the margin geometry (Winterer and Bosellini, 1981), this Apulia Platform (e.g., San Biase 1 and Castelmauro is most likely for the boundary between the carbon- 2 wells in Fig. 12). ate platform (south of Letino village) and the by-pass On the other hand, south of Letino and Matese margin facies (north of Letino). In this hypothesis, Lake latitudes, subsurface data are different: the low- the carbonate platform facies corresponds to a struc- resistivity layer becomes shallower and also thins out tural paleo-high with respect to the rest of the section to the south along both transects, and positive Bou- (Fig. 18). guer anomalies reach the highest values (>12 mgal). Furthermore, Miocene siliciclastic deposits in the Therefore, as opposed to the northern parts, data southern area around Valle Agricola and San Gregorio derived from the external areas do not provide a solid Matese generally show proximal facies. They are constraint to interpret the southern portions of the transgressive onto a pre-deformed carbonate substra- transects. tum that is deeply eroded and becomes progressively younger from west (Triassic) to east (Cretaceous– Miocene). This evidence testifies a severe erosional 6. Discussion event predating their deposition. Two hypotheses based on these data are possible: Along the western transect through the Matese Massif, the main Meso-Cenozoic facies boundaries pre-thrusting basins developed on a pre-existing (carbonate platform/by-pass margin to the south, by- structural high in the early stages of Upper pass margin/slope to the north) roughly correspond Miocene foredeep evolution;

Fig. 18. Paleogeographic setting of the study area at the end of Mesozoic. 292 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297

piggy-back basins (sensu Ori and Friend, 1984) been recently proposed for other external structures of developed on the already formed thrust belt. the Apennines by Billi et al. (2001) and Patacca and Scandone (2001). No detailed stratigraphic data are available for these In the second hypothesis, we envisage the presence deposits: they contain Middle Miocene carbonate of a pre-carbonate basement of conspicuous thickness olistoliths and are topped by Quaternary continental beneath the carbonates of the Matese thrust sheet (Fig. breccias. Consequently, they are generically not older 19b) that could correlate to the culminated portion of than Upper Miocene and not younger than uppermost the conductive layer of Fig. 16. Furthermore, it could Pliocene. Unfortunately, these constraints are not suf- be characterized by high gravity anomalies, as recog- ficient to discriminate between the two hypotheses. nised for the pre-Triassic basement in the Northern Nevertheless, their occurrence on top of a pre-existing Apennines along the CROP 03 transect (Barchi et al., and deeply eroded Mesozoic structural highs seems to 1998; Larocchi et al., 1998) and suggested for the support the first hypothesis better. In fact, had the Central–Southern Apennines by Rapolla (1986). deposits been piggy-back basins, the rate of erosion This unit could have been preserved at the bottom between the foredeep stage and the piggy-back depo- of the Mesozoic–Cenozoic structural high, lying at sition would have been unrealistically fast (approx. the footwall of pre-existing normal faults (Fig. 19c.1), more than 2 mm/year) when compared to an avarage which were tilted at the beginning of compression uplift rate of about 1 mm/year for the Southern (Fig. 19c.2) and then passively detached from their Apennines in Quaternary times (Di Leo et al., 2001). substratum by the low-angle Matese basal thrust (Fig. Finally, the restoration of extensional faults high- 19c.3). lights a strong pre-Quaternary structural culmination The tilting of these faults (Fig. 19c.1 and c.2) in the southern Matese Massif. This culmination has requires a substantial amount of simple shear. In the been explained considering both surface and subsur- study area, evidence for this is provided by the face geometries of the structural units involved in this numerous and well-known detachment layers within segment of the chain. The magnetotelluric transects the stratigraphic succession; details are described by strengthen the assumption that the Matese thrust sheet, Di Luzio et al. (1999). These field data allow the the carbonates of which correspond to the upper interpretation of simple shear as mainly accommodat- resistive layer of Fig. 16, is detached from its sub- ed by discrete slip on sub-horizontal detachments. stratum. This is supported by the fact that the lower In turn, a significant development of simple shear resistive layer, probably corresponding to the Apulia introduces distortion on the pin-lines of the cross- carbonate unit, is continuous in the subsurface along sections; this remains an open problem and results in a the entire sections. Furthermore, in the southernmost higher degree of approximation in the restoration of portion of the two transects, the lower resistive layer the two cross-sections proposed. (seemingly Apulia Platform unit) is uplifted. As a The first hypothesis (thin-skinned style) seems consequence, the above-mentioned culmination is at unreliable for this area because the low-resistivity least partially due to the buried Apulia unit that layer of Fig. 16 thins out. This indicates that the deformed the overriding thrust sheet according to a localized shortening hypothesis is incorrect, since it piggy-back sequence of thrusting. would require thickening beneath the structural cul- Nevertheless, especially along the western transect, mination and probably lower gravity anomalies (see this buried culmination is not sufficient to explain the Fig. 15). Therefore, we suggest that the second structural relief of Figs. 13 and 14, and two hypoth- hypothesis is more likely to be correct, although eses can be suggested (Fig. 19). direct exploration would be necessary to confirm it. In the first, the Matese thrust sheet is totally de- Nevertheless, pieces of field evidence of normal faults tached at the base of the carbonates in a typical thin- developed during the passive margin and foredeep skinned style (Fig. 19a). In this case, thickening of the stages have been described in the paragraph about intermediate (clay-rich) structural unit as a result of field data and discussed at the beginning of this localized shortening would have further contributed to paragraph. Evidence of local contractional reactiva- the structural culmination. A similar hypothesis has tion of these pre-existing faults has been recognised R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297 293

Fig. 19. Two alternative hypotheses of the deep structure of the Matese Massif: (a) thin-skinned style; (b) combined thin-skinned/thick-skinned style; (c) sequence of deformation for hypothesis ‘‘b’’. See text for a discussion. in the field; some of them fulfil the criteria defined in test different models of compressive tectonic styles the work of Holdsworth et al. (1997). These criteria for the evolution of the Apennines chain during can be summarised as follows: Neogene. The integration of new field and magnetotelluric difference of thickness and facies of the Meso- data with published subsurface data indicates that the Cenozoic sequences to the north and to the south of Matese Massif represents a whole thrust sheet limited the reactivated faults; at the base by a low-angle thrust fault. Within the high values of cut-off angles along the reactivated massif, pre-thrusting features are preserved, with only portions of pre-existing faults; mild reactivation coupled to local intense mesoscopic low net displacement due to compression when deformation. compared to high displacement on the basal low- As a consequence, in our final interpretation, angle thrust. local contractional reactivation of pre-existing passive margin faults replaces regional low-angle short- cut faults. Therefore, we agree that ‘‘the processes 7. Conclusions of collision orogenesis are not merely the scaled up versions of those which operate during inversion of This study of the Matese area provides new data sedimentary basins’’ (Butler, 1989) and that dis- concerning the structural setting of both the massif criminating between the two requires careful anal- and its subsurface. New data have been used to ysis of both surface and subsurface data. 294 R.A. Calabro` et al. / Tectonophysics 377 (2003) 269–297

In summary, two different shortening processes Bally, A.W., Burbi, L., Cooper, C., Ghelardoni, R., 1986. Balanced have operated in the Matese area. This suggests that sections and seismic reflection profiles across the Central Apen- nines. Mem. Soc. Geol. Ital. 35, 257–310. inversion and thin-skinned tectonics superimpose as a Barchi, M., Brozzetti, F., 1994. The role of fault reactivation pro- result of the complex interplay between the articulated cesses in the Pliocene–Quaternary tectonic evolution of the Um- pre-orogenic architecture of the passive margins (Wil- bria–Marche Apennines. Mem. Soc. Geol. Ital. 48, 559–560. liams et al., 1989) and the mechanisms of thrusting Barchi, M., La Vecchia, G., Minelli, G., 1989. Sezioni geologiche along low-angle thrust surfaces (Dahlstrom, 1969; bilanciate attraverso il sistema a pieghe Umbro–Marchigiano: 2—la sezione Scheggia-Serra S. Abbondio. Boll. Soc. Geol. Ital. Butler, 1982; Boyer and Elliot, 1982). 108, 69–81. 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