Neogene-Quaternary strike-slip tectonics in the central
Calabrian Arc (southern Italy)
Carlo Tansi
a,∗, Francesco Muto
b, Salvatore Critelli
b, Giulio Iovine
aaCNR-IRPI, via Cavour, 6 - 87030 Rende, CS, Italy
bDepartment of Earth Sciences, University of Calabria, 87036 Arcavacata di Rende, CS, Italy
Received 21 December 2005; received in revised form 26 September 2006; accepted 12 October 2006
Abstract
A Middle Miocene-Middle Pleistocene regional NW-SE left-lateral strike-slip fault system profoundly conditioned the evolution of central Calabria, during the late tectonic phases which involved the Apulian block and the Calabrian Arc. This system dissected an Oligocene-Early Miocene orogenic belt, made of Alpine nappes overthrusted the Apennine Chain.
In the present study, three major faults, arranged in a right-handen ´echelonpattern, have been identified within the mentioned strike-slip system: the Falconara-Carpanzano Fault, the Amantea-Gimigliano Fault, and the Lamezia-Catanzaro Fault. A wide active transtensional area (N-S-trending Crati Graben), developed since Late Pliocene, is located at the SE termination of the Falconara-Carpanzano Fault.
In the sectors of overlapping of the faults, the transpressional regime induced tectonic extrusions of the deep-seated units of the Chain, producing push-ups within the overlying complexes. In particular, push-ups are either made of Mesozoic carbonate rocks at Mt. Cocuzzo–Mt. Guono and Mt. S. Lucerna, or of ophiolite rocks at Mt. Reventino and Gimigliano. In these sectors, the primary geometric relationships among the units of the orogenic belt were locally altered.
Theen ´echelonarrangement of the above-mentioned NW-SE major strike-slip faults indicates the existence of a left-lateral crustal shear zone, striking on average N160. The age of the regional NW-SE left-lateral strike-slip system deserves thorough investigation. Besides evidence from historical and instrumental earthquakes, and from paleoseismological investigations, the kinematic data suggests that the “cause” of the transtensional sector (Crati Graben) could be found in the regional Falconara-Carpanzano Fault. © 2006 Elsevier Ltd. All rights reserved.
Keywords: Strike-slip tectonics; Brittle tectonics; Stress field re-orientation; Neogene-Quaternary; Calabrian Arc
1. Introduction
The Calabrian Arc (CA) is a well-developed arc-shaped structure of the circum-Mediterranean orogenic belt (Fig. 1). It represents an accretionary wedge, caused by the Africa–Europe collision (Amodio-Morelli et al., 1976; Tortorici, 1982), consisting of a series of ophiolite-bearing tectonic units (Liguride Complex;Ogniben, 1969), and of overlying basement nappes (Calabride Complex;Ogniben, 1969). According toDewey et al. (1989)andSchmid et al. (2004), the Liguride Complex derived from a pre-orogenic ocean basin (the Piedmont-Ligurian domain), opened in Late Jurassic
∗Corresponding author at: CNR-IRPI Sezione di Cosenza, via Cavour, 6 - 87030 Rende, CS, Italy. Tel.: +39 0984 835 513; fax: +39 0984 835 319.
E-mail address:tansi@irpi.cnr.it(C. Tansi).
0264-3707/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2006.10.006
Fig. 1. Geological sketch-map of the Central Mediterranean area, with geological section on bottom (afterVan Dijk and Scheepers, 1995, andVan Dijk et al., 2000, modified). On top, location of the study area, and tectonic simplified sketch of the Calabrian Arc.
Table 1
Scheme of mutual relationships among the main tectonic units in the central Calabrian Chain
Groups and names as used in the present paper are listed in the first two columns. Correspondences with terms commonly found in literature, as listed in the last two columns, can also be deduced (cf. text, Section2).
between the European-Iberian and the African-Apulian domains. The Calabride Complex is considered to be either (i) a remnant of the African continental margin, piled up during the Cretaceous–Paleogene to form, together with ophiolitic nappes, a Europe-verging Eo-Alpine Chain, that overthrusted the Apennine orogenic belt in Early Miocene (Haccard et al., 1972; Alvarez, 1976; Grandjacquet and Mascle, 1978), or (ii) a fragment of the European continental margin, piled up with oceanic materials in Paleogene time with African vergence, which overthrusted the African continental margin in Early Miocene (Ogniben, 1969, 1973; Bouillin, 1984; Bouillin et al., 1986).
The paleogeographic provenance (and names) of the tectonic units of the Calabrian Arc, resulting from the orogenic phases, have long been the subject of controversy, but are beyond the scope of this study. There is general agreement in the literature on the geometrical position of the different units, and on their ages: from bottom to top, Mesozoic carbonates, Mesozoic ophiolites, Paleozoic-Mesozoic slates and metapelites, Paleozoic orthogneisses, and Paleozoic paragneisses. In the present paper, for the sake of simplicity, the above units were grouped and named as shown in Table 1(cf. Section2).
Since Middle Miocene, overthrusting – combined with the progressive migration of the CA towards southeast – was associated with the opening of the Tyrrhenian basin (Malinverno and Ryan, 1986; Dewey et al., 1989; Decandia et al., 1998). The migration occurred along a NW-SE to WNW-ESE-trending regional strike-slip fault system (cf.Fig. 1), characterized by left- and right-lateral movements in the northern and in the southern sector of the CA, respectively (Ghisetti and Vezzani, 1981; Rehault et al., 1987; Turco et al., 1990; Knott and Turco, 1991; Monaco and Tansi, 1992; Catalano et al., 1993). The fault system constitutes a regional shear zone, dissecting the pre-existing thrust sheets, and played an important role in the Neogene-Quaternary geodynamic evolution of the Central Mediterranean area.
Many details on the temporal and spatial distribution of transtensional and transpressional faulting in the central portion of the CA are lacking, despite decades of studies.Van Dijk et al. (2000), which performed structural studies supported by seismic and bore-hole data, were the first to define the geometry of the above-mentioned regional shear zone (Fig. 2). According to these Authors, the whole system consisted of Middle Miocene-Middle Pleistocene crustal oblique transpressional fault zones, mainly dipping toward NE and characterized by left-reverse movements, along which the extrusion of the deep-seated units of the CA together with underlying Mesozoic carbonate rocks occurred. In particular, along the “Catanzaro-Amantea Fault Zone” (CAFZ) and the “Falconara-Carpanzano Fault Zone” (FCFZ), several outcrops of Mesozoic carbonate rocks are exposed which, in the literature, are traditionally considered as “tectonic windows” of the Apennine Chain (e.g.Amodio-Morelli et al., 1976; Tortorici, 1982). The most significant Mesozoic carbonate outcrops are to be found in the localities of Mt. Cocuzzo–Mt. Guono and Mt. S. Lucerna, where the relationships with the units of the Calabrian Arc – as generated by strike-slip tectonics – can be observed. Similarly,
Fig. 2. Schematic tectonic map showing the main Middle Miocene-Middle Pleistocene left-lateral strike-slip lineaments of central-northern Calabria (afterVan Dijk et al., 2000, modified). Key: (SLFZ) Soverato-Lamezia Fault Zone; (CAFZ) Catanzaro-Amantea Fault Zone; (ACFZ) Albi-Cosenza Fault Zone; (SDFZ) Sellia-Decollatura Fault Zone; (OCFZ) Ospedale-Colosimi Fault Zone; (FCFZ) Falconara-Carpanzano Fault Zone; (PSFZ) Petilia-S. Sosti Fault Zone; (SRFZ) S. Nicola-Rossano Fault Zone. Dotted polygon indicates the study area.
Fig. 4. Late Pliocene-Early Quaternary block-segmentation of the Calabrian Arc (afterGhisetti, 1979, modified). Black lines indicate the main faults. Key: (1) Paola and Gioia Tauro peri-Tyrrhenian basins; (2) Pollino Massif, Coastal Chain, Capo Vaticano and Mt. Peloritani highs; (3) Crati and Mesima basins; (4) Sila, Serre and Aspromonte highs; (5) Crotone-Capo Spartivento peri-Ionian basins; (6) Sibari basin; (7) Catanzaro basin; (8) Siderno basin; (9) Messina basin.
along the CAFZ, outcrops of Mesozoic Ophiolite rocks representing the basal portion of the Liguride Complex are to be found: among these tectonic windows, the most representative is found in the Gimigliano area.
Turco et al. (1990)described the “Falconara Fault” (Fig. 3), a regional NW-SE-trending left-lateral strike-slip fault, active during the Pliocene-Quaternary and partly corresponding to the FCFZ. The NW termination of the same fault delimited the northern portion of the “Paola basin” (Argnani and Trincardi, 1988), an off-shore N-S-trending Pliocene-Quaternary basin. These Authors claimed a transtensional regime along the SE termination of the fault zone was respon-sible for the opening of the Crati Graben (Lanzafame and Tortorici, 1981), a normal fault-bounded N-S-trending basin. According toGhisetti (1979), during Late Pliocene-Early Quaternary, the CA was dissected by longitudinal and transversal normal faults. These faults, ranging from 10 to 45 km in length, caused the fragmentation of the arc into structural highs and marine sedimentary basins (Fig. 4).
Since Middle Pleistocene, an intense WNW-ESE oriented regional extensional phase occurred (Cello et al., 1982; Gasparini et al., 1982; Tortorici et al., 1995), resulting in the “Calabrian-Sicilian rift-zone” (Monaco et al., 1997; Monaco and Tortorici, 2000), a long active normal fault belt of about 370 km, running along the eastern coast of Sicily and the western side of the CA (Fig. 5). The geometry of the rift-zone is clearly outlined by the distribution of the epicentres of the largest historical (X-XI MCS, 6 < M < 7.4) crustal earthquakes in the region (Postpischl, 1985; Boschi et al., 1995, 1997). The development of the rift-zone was coupled with a strong regional uplifting of the whole CA, which probably represents the isostatic response either to the removal of the high-density mantle/lithosphere root, due to detachment of the Ionian subducted slab (Westaway, 1993; Wortel and Spakman, 1993; De Jonge et al., 1994; Tortorici et al., 1995; Monaco et al., 1996), or to erosion driven by Pleistocene climate change (Westaway, 2002, 2006). Other hypotheses maintain that upheaval of the Calabrian Arc originated from overthrusting of the Tyrrhenian crust onto the Ionian crust, simultaneously with normal faulting (Ghisetti, 1984), or an intrusion of fluidized mantle from an asthenolith dome into the crust-mantle boundary (Locardi and Nicolich, 1988; Miyauchi et al., 1994). In any case,
Fig. 5. Calabrian-Sicilian rift-zone (afterMonaco and Tortorici, 2000, modified). Crustal earthquakes (depth < 35 km) since 1000a.d. are also shown (data after:Postpischl, 1985; Boschi et al., 1995).
the most evident consequence of the widespread Quaternary raising process is the occurrence of a spectacular flight of marine terraces, developed mainly along the Tyrrhenian coastline of Calabria (with notable effects in the central portion of the CA), as a result of the interaction between tectonic uplift and Quaternary cyclic sea-level changes (Bosi et al., 1996; Carobene and Dai Pr`a, 1991; Westaway, 1993).
Coupled structural and AMS-studies (anisotropy of magnetic susceptibility), recently carried out in the central portion of the CA byMattei et al. (1999)andRossetti et al. (2001), outlined an extensional regime at work since Late Miocene, characterized by a constant WSW-ESE stretching direction, originating normal faults striking on average NNE-SSW. These Authors excluded any compressional tectonic event in the considered period, contrary to other studies carried out in the same area (Tortorici, 1981; Scheepers, 1994; Colella, 1995; Muto and Perri, 2002; Van Dijk et al., 2000).
In this paper, the timing, geometries and kinematics of the southernmost portion of the regional shear zone defined by Van Dijk et al. (2000 - cf. study area inFig. 2), and of associated transpressional and transtensional structures, were defined in detail through interpretations of aerial photographs (in scale 1/75,000 and 1/33,000), and field surveys. Measurements of structural data comprise orientations of 743 fault planes with slickensides, gathered from 63 measure stations located along the main faults, within the cataclastic belts. Such data allowed the determination of the associated stress field (according toAngelier, 1979, method), and thein situvalidation of the regional deformational model. Late emplacement mechanisms of the deep-seated Mesozoic carbonate rocks, and their geometric relationships with the overlying units of the Calabrian Arc, are discussed in an innovative tectonic framework, dominated by strike-slip tectonics, lasting from Late Miocene to Quaternary (and, presumably, still active).
2. Geological setting
The Calabrian Arc is made of nappes of Jurassic to Early Cretaceous ophiolite-bearing sequences, and overlying Hercynian and pre-Hercynian continental basement, partially affected by the later Alpine retrograde metamorphism. Most Authors (e.g.Haccard et al., 1972; Alvarez, 1976; Amodio-Morelli et al., 1976; Tortorici, 1982; Bonardi et al., 2001) believe that, during Oligocene-Early Miocene, these nappes were emplaced on the Mesozoic sedimentary and metasedimentary terranes of the Apennine Chain along regional overthrusts, showing – in present-day coordinates – a NE-vergence. Moreover,Wallis et al. (1993),Thomson (1994), andRossetti et al. (2001, 2004)recently dated exhumation of deep-seated units starting from Oligocene.
In the study area, the following main tectonic-stratigraphic units crop out, briefly described from bottom to top (cf. Table 1andFig. 6).
Mesozoic carbonate complex
• Dolostone and metalimestone Unit(Late Triassic-Liassic). Locally outcropping in “tectonic windows” along the Coastal Chain (cf.Fig. 2). These terranes, also named the Monte Cocuzzo Unit (Van Dijk et al., 2000), can be correlated with the San Donato-Campotenese Unit (Bousquet and Dubois, 1967; Bousquet and Gradjacquet, 1969). Calabrian Terranes. A number of tectonic units belonging to the Calabrian Arc constituted, from bottom to top, by:
• Ophiolite Unit(“Gimigliano Unit” ofAmodio-Morelli et al., 1976). A HP-LT metamorphic serpentinite-metabasite-polychrome schist-Calpionella limestone sequence (Tithonian-Neocomian).
• Slate and metapelite Unit. Dominantly foliated slates, black metapelites and metasilts, interbedded with quartzite strata defining isoclinal folds, and red mudrocks and thin levels of laminate marble (Paleozoic-Mesozoic). According toOgniben (1973)andAmodio-Morelli et al. (1976), these terranes should be ascribed to two distinct units: the Mesozoic Frido Unit, and the Paleozoic Bagni Unit, belonging to two distinct complexes. The first (of oceanic origin) was, in fact, ascribed to the Liguride Complex, while the second (of continental origin) to the Calabride Complex.
Above these units, the following are to be found:
• Orthogneiss Unit(Castagna Unit ofAmodio-Morelli et al., 1976). Made of mylonitic augen-gneiss, micaschist, and subordinately marbles (Paleozoic).
• Paragneiss Unit(Monte Gariglione Unit ofAmodio-Morelli et al., 1976; Sila Unit ofMessina et al., 1994). Made of high-grade metamorphic rocks (biotite-sillimanite-garnet gneiss), intruded by plutonic bodies (Paleozoic).
The last two units are considered to be derived from Hercynian and pre-Hercynian terranes; they were traditionally ascribed to the Calabride Complex (Ogniben, 1969) and represented a remnant of the Paleozoic basement, mainly outcropping in the Sila Massif.
• Sheared basement remnants. Some minor “difficult-to-place elements” outcropping along the NW-SE Sell`ıa-Amantea axis (also known as “axe Decollatura-Conflenti-Martirano”, cf. Dubois, 1966), a major shear zone individuated by the CAFZ and the SDFZ (Van Dijk et al., 2000, cf. alsoFig. 2). These terranes are represented
Fig. 6. Structural map of the study area. Key: (A) Mt. Cocuzzo–Mt. Guono push-up; (B) Mt. S. Lucerna push-up; (C) Gimigliano push-up; (D) Crati Graben. Major NW-SE strike-slip faults: (FCF) Falconara-Carpanzano Fault; (AGF) Amantea-Gimigliano Fault; (LCF) Lamezia-Catanzaro Fault. For mesoscopic-scale structural data, seeFig. 13.
by Paleozoic phyllite and metalimestone (Catanzaro melange Unit), and by granite and granodiorite (Decollatura granites Unit), covered by Triassic-Cretaceous terrigenuous and carbonate rocks (Van Dijk et al., 2000). They were originally ascribed to the Stilo Unit byAmodio-Morelli et al. (1976), and included into a group ofincertae sedis units (i.e. whose paleogeographic position is uncertain or unknown). These terranes were ascribed to the Calabride Complex byOgniben (1969).
In the study area,Rossetti et al. (2001)grouped the above-mentioned units into two major tectonic complexes, bounded by a flat-lying ductile-to-brittle extensional shear zone. The upper complex would consist of a nappe-like structure, made of the following tectonic units, from bottom: slices of Triassic platform and dolomitic limestone; Mesozoic ophiolite-bearing Ligurian-type non-metamorphic to slightly metamorphic flyschoid sequence (upper
ophi-olitic unit); Calabrian Nappe complex, made of pre-Alpine basement continental slices, arranged in reverse order (with highest-grade rocks above the lowest-grade ones), specifically constituted by the Bagni, Castagna, Pol`ıa-Copanello, and Stilo Paleozoic units. A metamorphic Alpine overprint in the Castagna and Bagni units probably dates between Paleocene and Late Eocene (Schenk, 1980); the unroofing of these rocks occurred between 30 and 18 My (Thomson, 1994). As concerns the lower complex, it consists of a retrogressed blue-schist metamorphosed ophiolitic sequence (lower ophiolitic unit), in part belonging to the Gimigliano Unit.
Finally, again according toRossetti et al. (2001), since Late Miocene, the ancient reverse contacts responsible for the building of the Calabrian Nappe complex were reactived by extensional tectonics. Nevertheless, several cases either of deep-seated gravitational or tectono-gravitational accommodation of pre-existing reverse faults have been recently documented in Calabria (Iovine et al., 1996; Iovine and Tansi, 1998; Sorriso-Valvo et al., 1998; Tansi et al., 2005a,b): such a reactivation mechanism could also explain some of the normal kinematic evidence collected by the above Authors in the field.
A transgressive Late Miocene conglomerate-calcarenite-clay-evaporite succession, and an Early Pliocene conglomerate-sand-clay succession unconformably overlie the above-mentioned tectonic units (Di Nocera et al., 1974; Romeo and Tortorici, 1980; Colella, 1995; Muto and Perri, 2002). Middle Pliocene to Middle Pleistocene deposits, made of thick conglomerate-sand-sandstone-clay marine successions, represent the basin-fill deposits of the main tectonic depressions (Crati Graben, Catanzaro Trough). In the SW sector of the study area, Late Pleistocene flu-vial terraced deposits, marine terraces (up to the seventh order—Westaway, 1993; Tortorici et al., 2002), and Late Pleistocene-Holocene alluvial fans crop out.
3. Structural analysis
New structural investigations allowed the detailing of the superficial morphological manifestation and the kinematics of some of the regional transcurrent fault zones, previously recognized by means of deep reflection seismic profiles by Van Dijk et al. (2000). In particular, a regional NW-SE-trending left-lateral strike-slip fault system was documented, developed between the southern edge of the Crati Graben and the Catanzaro Trough (Fig. 6). This system consists of three right-steppingen ´echelonmajor fault segments, from North to South: the “Falconara-Carpanzano Fault” (FCF), the “Amantea-Gimigliano Fault” (AGF), and the “Lamezia-Catanzaro Fault” (LCF). These fault segments of lengths of 50–60 km are clearly recognizable on a morphological basis. They are characterized by well-developed escarpments, with triangular and/or trapezoidal facets, and control the drainage network.
In particular, the FCF separates the southern portion of the Plio-Pleistocene Crati Basin from the Calabrian tectonic units of the Coastal Chain, consisting of Liguride terranes overlying theMesozoic Carbonate Complex. This fault segment corresponds to the FCFZ recognized byVan Dijk et al. (2000).
The AGF extends from the Tyrrhenian Sea (where it bounds the south of the Coastal Chain) toward SE, cutting through the southern part of the Sila Massif. At its westernmost end, it separates Late Miocene deposits from the Slate and metapelite Unit. Along its middle portion, the AGF delimits, to the south, an elongated outcrop ofSheared basement remnantsfrom theSlate and metapelite Unit. This fault segment roughly corresponds to the CAFZ, with its related shear zone, recognized byVan Dijk et al. (2000).
Finally, the LCF separates the Plio-Quaternary Catanzaro Trough from the southern edge of the Sila Massif, where the entire pile of theCalabrian Terranesand underlyingMesozoic Carbonate Complexoutcrops. This fault segment partly corresponds to the sinuous E-W-trending normal fault recognized byVan Dijk et al. (2000). Several large alluvial fans and marine terraced deposits developed in the hangingwall of the LCF.
The areas located within the overlapping zones between the transcurrent fault segments underwent severe contrac-tional deformation (cf. Fig. 14). In particular, in the area between FCF and AGF, severe transpression induced the extrusion of theMesozoic Carbonate Complexover theCalabrian Terranes(Ophiolite UnitandSlate and metapelite Unit), thus altering the orogenic tectonic piling. Similarly, in the area between AGF and LCF, contractional deformation induced the late superposition of theOphiolite Unitover theSlate and metapelite Unit, producing the push-ups of Mt. Reventino and Gimigliano areas. Moreover, strike-slip motion produced, at the tips of the strike-slip fault segments, extensional structures: at the SE termination of the FCF, a regional transtensional area corresponding to the Crati Graben, consisting of active N-S-trending right lateral-transtensional faults, can be recognized.
Because of the geodynamic significance of the above-mentioned transpressional areas (Mt. Cocuzzo–Mt. Guono and Mt. S. Lucerna carbonate outcrops, and Gimigliano ophiolite outcrops—cf. frames (A)–(C) inFig. 6), and of the
Crati Graben transtensional area (cf. (D) inFig. 6), detailed field structural studies were carried out, particularly along FCF and AGF. The collected structural data are described in the following sections.
3.1. Transpressional areas
Mt. Cocuzzo–Mt. Guono and Mt. S. Lucerna constitute the highest reliefs in the Coastal Chain. They were previously considered as “tectonic windows” (Colonna and Compagnoni, 1982; Amodio-Morelli et al., 1976; Ietto et al., 1995; Ietto and Ietto, 1998; Van Dijk et al., 2000), showing theMesozoic Carbonate Complexbeneath theCalabrian Terranes (Fig. 6). According toIetto and Ietto (1998), these Norian-Raethian carbonate successions can be subdivided into six litho-stratigraphic units, representing a sedimentary evolution from shallow-marine carbonate to anoxic basinal facies associations, including from bottom to top (Fig. 7):
• black dololutite passing to grey dolostone, and calcarenite (Licetto River Unit);
• bio-calcarenite passing to bioclastic breccia, slumps, turbidites and mudstones (Mt. Guono Unit);
• carbonate, interbedded with slumps, passing to turbidite, calcarenite and megabreccia (Mt. Cocuzzo Unit);
• dolostone, dololutite and breccia, and channelized carbonate breccia (Cozzo Aurulo Unit);
• bio-calcarenite and interbedded breccia and dolostone, passing to mudstone and bioclastic carbonate (Mt. S. Lucerna Unit);
• dolomite and evaporite, and intercalated tuffaceous layers, with a basal condensed section of dololutite (Upper Unit). The results of the present study demonstrate that the Mt. Cocuzzo–Mt. Guono and the Mt. S. Lucerna carbonate outcrops must be considered as two push-ups (cf. Figs. 7 and 14), penetrating the overlyingCalabrian Terranes. The thrust ramps building the push-ups depict, as a whole, well-developed flower structures. Among the latter, some portray positive flower structures: in fact, an up-dip progressive increase of inclinations up to dip inversion, together with slickensides continuously carving the surfaces and showing kinematic evidence from reverse to normal, can be observed (Figs. 8 and 9). Such geometrical characteristics of the thrust surfaces constitute the peculiar structural feature of the considered carbonate outcrops.
In particular, the Mt. Cocuzzo–Mt. Guono push-up (Fig. 7A) is formed by high-angle N-S striking thrusts. In the area of the Cresima River and of the Reale-Licetto rivers, two main thrusts, characterized by W- and E-vergence, delimit the push-up to the NW and SE, respectively. The push-up is bordered on its northern side by the FCF, and on its southern side by associated minor strike-slip faults. Near the NW and SE boundaries of the push-up, two secondary push-ups, corresponding to the Mt. Guono and to the Mt. Cocuzzo reliefs, are observed (cf. section AAinFig. 7).
To the west, the Mt. Guono secondary push-up is delimited by a W-verging sub-vertical thrust, responsible for the superposition of deep-seated carbonatic units over theCalabrian Terranes. Along the contact, remnants of Late Miocene deposits (in discontinuous and strongly tectonized duplexes) can be recognized. On its eastern side, the push-up is bounded by an E-verging sub-vertical thrust, responsible for the spush-uperposition of the Mt. Guono Unit over the Slate and metapelite Unit, and for the local duplication of theMesozoic Carbonate Complex.
To the west, the Mt. Cocuzzo secondary push-up is bordered by a W-verging sub-vertical thrust, developed between Mt. Timone and the Centacque River. Along this shear surface, the Cozzo Aurulo Unit superposes on theOphiolite Unit. The contact is marked by highly tectonized and discontinuous remnants of Miocene deposits (cf. Fig. 9). On its eastern side (Fig. 10), along the Reale River, the push-up is bordered by an E-verging thrust, responsible for the superposition of the Cozzo Aurulo Unit over theSlate and metapelite Unit. Moreover, associated minor thrusts are responsible for the local duplication of theMesozoic Carbonate Complex. The southernmost margin of the secondary push-up is offset by minor strike-slip faults and associated push-ups (e.g. Cozzo Ralla, Cozzo Sisino,Fig. 7).
Finally, folds with axes ranging from N-S to NE-SW are to be found near the two secondary push-ups.
At the mesoscopic-scale (cf.Fig. 13), thrusts show planes striking from NNW-SSE to NE-SW, dipping either towards E or W, with sub-horizontal to steeply dipping attitude. The fault planes show dip-slip to oblique slickensides, marked either by calcite steps or Riedel shears, documenting reverse movements with a left-lateral component of motion. The thrusts are coherent with a sub-horizontal σ1 oriented WNW-ESE. Strike-slip faults show sub-vertical planes striking from E-W to NW-SE, commonly dipping towards SW. Slickensides on fault surfaces document left-lateral movements, with pitches ranging from 0◦to 40◦, generally plunging towards SE. The strike-slip faults are coherent with a sub-horizontalσ1oriented from E-W to WNW-ESE.
Fig. 7. Detailed structural map of the Mt. Cocuzzo–Mt. Guono push-up (A) and of the Mt. S. Lucerna push-up (B). At the bottom, geological cross-sections. For mesoscopic-scale structural data, seeFig. 13.
Fig. 8. View of the highest portion of the Mt. Cocuzzo secondary push-up. Transpressive thrusts are evidenced by thick dashed lines; layering is marked by thin dotted lines.
The Mt. S. Lucerna push-up consists of a N-S oriented E-verging duplex (Fig. 7B), whose rocks belong to the upper portion of theMesozoic Carbonate Complex(Mt. S. Lucerna Unit, and Upper Unit). The duplex is situated within the Slate and metapelite Unit, and is delimited, on its northern side, by the FCF and, to the south, by a minor strike-slip fault.
At the mesoscopic-scale (cf.Fig. 13), thrust planes strike roughly NNE-SSW, and dip from 45◦to 80◦towards either E or W. Thrust planes show dip-slip to oblique slickensides, documenting reverse movements and a sub-horizontalσ1
Fig. 9. View of the western side of the Mt. Cocuzzo secondary push-up, showing the contact between the Mesozoic Carbonate Complex on the Ophiolite Unit (metabasite). The contact is marked by a duplex of Miocene deposits.
Fig. 10. View of the south-eastern side of the Mt. Cocuzzo secondary push-up.
oriented WNW-ESE. Strike-slip faults show sub-vertical planes, striking from E-W to NW-SE and dipping towards SW, whose slickensides document left-lateral movements (pitches between 0◦ and 40◦, plunging towards SE) and sub-horizontalσ1oriented E-W.
In both the Mt. Cocuzzo–Mt. Guono and the Mt. S. Lucerna push-ups, ancient thrusts – characterized by sub-horizontal planes striking E-W to WNW-ESE – were also recognized. These thrusts do not show any morphological evidence, and are documented only at the mesoscale (where they are commonly dislocated by the N-S oriented thrusts which build the push-ups). These planes display reverse dip-slip slickensides, documenting – if tilting is ignored – a N-S oriented sub-horizontalσ1: they can reasonably be correlated with the overthrusts responsible for the Oligocene-Early Miocene building of the Chain. Moreover, folds with NW-SE-trending axes were to be found kinematically compatible with the ancient thrusts.
The Gimigliano ophiolite outcrops (Colonna and Piccarreta, 1975, 1977; Rossetti et al., 2001) consist of metabasite and serpentinite belonging to the basal portion of theCalabrian Terranes. They are located in the tranpressional sector induced by the interaction between the AGF and a couple of minor left-lateral strike-slip faults belonging to the NW-SE regional fault system (cf.Figs. 6 and 14).
In particular, a push-up structure was recognized in the vicinity of the village of Gimigliano (Fig. 11): it is bounded, on its NW side, by a NNE-striking W-verging thrust, along which deep-seated serpentinite and overlying metabasite overthrusted theOrthogneiss Unitand theSlate and metapelite Unit; on its SE side, it is bounded by a NNE-striking E-verging thrust, along which metabasite overthrusted theSlate and metapelite Unit. The push-up developed between the AGF and a minor left-lateral strike-slip fault extending from Gimigliano Inferiore to C.da Cavor`a (cf.Fig. 11). The mechanism responsible for the extrusion of the push-up also dragged up deep-seated units of the Chain SE of Gimigliano, where ancient thrusts separating theOphiolite,Slate and metapeliteandOrthogneissunits crop out.
Moreover, north of the AGF, a couple of transpressive thrust ramps are to be found. Along them, metabasite and small blocks of serpentinite overthrusted theSlate and metapelite Unitat C.da Occiuso and C.da Giorgi.
At the mesoscopic-scale (cf.Fig. 13), thrust planes strike from about NNW-SSE to NNE-SSW, and dip from 40◦to 80◦towards either E or W. Thrust planes show dip-slip to oblique slickensides, documenting reverse movements and a sub-horizontalσ1oriented WNW-ESE. Strike-slip faults show sub-vertical planes, striking from E-W to NW-SE and dipping mainly towards SSW (subordinately towards NNE); slickensides document left-lateral movements (pitches between 0◦and 40◦, plunging mainly towards SE, and subordinately towards NW) and sub-horizontalσ1 oriented from ENE-WSW to E-W.
Fig. 11. Detailed structural map of the Gimigliano push-up. At the bottom, geological cross-section. For mesoscopic-scale structural data, see Fig. 13.
Fig. 12. Detailed structural map of the Crati Graben. For mesoscopic-scale structural data, seeFig. 13.
The above structural interpretation contrasts with the one proposed byRossetti et al. (2001), which explained the ophiolite outcrops (i.e. the Authors’ “lower complex”) as a tectonic window through the Alpine terranes (Stilo, Castagna and Bagni units, belonging to the Authors’ “upper complex”). Ophiolite rocks were in fact claimed to be arranged into an anticlinal fold, whose NW-SE striking axis extended from Gimigliano to C.da Cavor`a. Albeit assuming the same original geometric relationships among the units of the Chain, our interpretation allows a better explanation of the evident overthrusting of ophiolite rocks over theOrthogneiss Unitand theSlate and metapelite Unit, as observed near Gimigliano.
3.2. Transtensional area
The Crati Graben (Lanzafame and Tortorici, 1981; Tortorici, 1981; Tansi et al., 2005a) is characterized by N-S-trending transtensional Quaternary faults (Figs. 6, 12 and 14), which separate Late Miocene-Quaternary deposits from the Calabrian Terranesoutcropping in the Coastal Chain and in the Sila Massif, on its western and eastern margins, respectively. Such deposits are represented, in the depocentral zone, by a Middle Pliocene-Middle Pleistocene conglomerate-sand-clay marine succession, closed at the top by Late Pleistocene-Holocene fan-delta and alluvial deposits.
N-S normal faults show seismogenic activity, as testified both by historical IX-X MCS events (Postpischl, 1985; Boschi et al., 1995, 1997) and by instrumental earthquakes (Moretti et al., 1990). Morphologically, these faults are
C. T ansi et al. / Journal of Geodynamics 43 (2007) 393–414
Fig. 13. Mesoscopic-scale structural data gathered at the stations shown inFigs. 6, 7, 11 and 12. Diagrams (Schmidt’s net, lower hemisphere) show the attitude of fault planes, slickensides (arrows), and the orientation of the principal axes of the stress ellipsoid (σ1,σ2,σ3), obtained through the right-dihedrons method (Angelier, 1979). Data elaboration was performed by means of the software
represented by sharp rectilinear escarpments, marked by active alluvial fans, bounding the uplifted footwalls. The mountain fronts reach elevations of about 700 m, and are characterized by 300–400 m high cumulative fault escarpments along which triangular/trapezoidal facets (70–100 m high) are to be found. An antecedent drainage network flows perpendicular to the fault segments; it is made of deeply entrenched canyons on the uplifted blocks, and of flat valleys on the down-thrown blocks (Tortorici et al., 1995; Tansi et al., 2005a).
At the mesoscopic-scale (cf.Fig. 13), normal faults show fault planes striking from N-S to NNE-SSW, with sub-vertical to oblique slickensides, indicating a right-lateral component of motion related to an extensional direction (σ3) oriented N125E.
4. Discussion and conclusion
The structural studies carried out between the Crati Graben and the Catanzaro Trough (central Calabria) have revealed the geometrical and kinematic characteristics of a regional NW-SE-trending left-lateral strike-slip zone, induced by an E-W oriented sub-horizontalσ1. According toVan Dijk et al. (2000), this shear zone was active in the study area from Middle Miocene to Middle Pleistocene “only”.
Within the study area, three major left-lateral strike-slip faults, arranged in a right-handen ´echelonpattern, were identified: the Falconara-Carpanzano Fault, the Amantea-Gimigliano Fault, and the Lamezia-Catanzaro Fault. In the overlapping regions between the strike-slip faults (cf.Fig. 14), transpressional regimes induced the tectonic extrusion of the deep-seated units of the Oligocene-Early Miocene Chain (the uppermost portion of theMesozoic Carbonate Complex, and theOphiolite Unit). These units are arranged into push-ups, formed by thrusts oriented NNE-SSW and associated folds, penetrating the overlying units of theCalabrian Terranes. Accordingly, the transpressional regime modified the original geometric relationships among the NE-verging units of the orogenic belt, contributing locally to the late moderate thickening of the Chain.
In particular, two major Mesozoic carbonate push-ups (Mt. Cocuzzo–Mt. Guono and Mt. S. Lucerna) were extruded within the transpressional area of overlapping FCF-AGF. In the area of overlapping AGF-LCF, the two regional ophiolite push-ups of Mt. Reventino and of Gimigliano were also extruded. Moreover, in correspondence with the SE termination of the Falconara-Carpanzano Fault, active transtensional tectonics has developed along the N-S-trending Crati Graben since Late Pliocene (Tansi et al., 2005a,b).
Mesoscopic-scale structural analyses permitted the evaluation of the stress field (Fig. 13). Moreover, the consistency between the geometrical pattern of the strike-slip faults and the collected kinematic data, on the one hand, and those predicted by experimental and theoretical models, on the other (see below; cf. Xiaohan, 1983, orSylvester, 1988), has allowed us to interpret the above-mentioned push-ups as superficial responses in the overlapping areas of deep strike-slip faults. With this in mind, let us consider the mathematical elastic-plane model proposed byXiaohan (1983) concerning plane stresses, which describes the concentration of the stresses (σ1andσ3) and the trajectories ofσ1in the vicinity ofen ´echelonright-stepping left-lateral strike-slip faults (Fig. 15). In the study area, the regional shortening directions (σ1), responsible for the activation of the strike-slip faults, is E-W oriented, as testified by the kinematic evidence found along the FCF and AGF. In the area of overlapping between the above faults, the direction ofσ1tends to be parallel to the direction of the strike-slip faults (trending about WNW-ESE), as deduced from thrusts and folds. Yet, in the transtensional zone of the Crati Graben, the orientation ofσ3rotates, and tends to be parallel to the FCF: at the mesoscopic scale, this re-orientation is documented in the measurement stations from “39NO” to “51NO” (Fig. 13), where the average extension direction is oriented WNW-ESE.
According toVan Dijk et al. (2000), the uplift mechanism of the deep-seated units of the Chain should generally be ascribed, at a regional scale, to reverse-oblique strike-slip movement along the NW-SE regional shear zone, with associated NE- and SW-verging thrusts. However, in the overlapping areas between strike-slip faults, the modality of extrusion of the deep-seated portions of the Chain can be better explained by means of the above described kinematic model.
Like the Calabrian-Lucanian boundary (cf., e.g.Catalano et al., 1993), though in a different time interval (Early-Middle Pleistocene versus (Early-Middle Miocene-(Early-Middle Pleistocene, and perhaps up to the present—cf. below), the overthrust migration towards the foreland was probably inhibited by the thickening of the continental crust: as a consequence, strike-slip tectonics became the dominant mode of deformation.
In the study area, the observed geometrical pattern of the three major strike-slip faults points to the existence of a left-lateral shear zone of higher hierarchical order, striking on average N160 (cf. inset inFig. 14). The shear zone,
Fig. 14. Kinematic scheme of the study area. On top-right, the left-lateral shear zone of higher hierarchical order, striking on average N160.
marked by outcrops of Sheared basement remnants(cf. Fig. 6), is also responsible for the lateral juxtaposition of different paleogeographic domains.
The considered regional left-lateral strike-slip fault system thus significantly conditioned the post-Tortonian evo-lution of central Calabria, during the late-orogenic and post-collisional phases which involved the Apulian block and the Calabrian Arc. Such an interpretation is in contrast with the ones proposed byMattei et al. (1999), and byRossetti et al. (2001), which recognized a single extensional phase, active since Late Miocene, as being responsible for the development of NNE-SSW-trending normal faults in the same study area.
Finally, the age of the regional NW-SE left-lateral strike-slip system still deserves thorough investigation. In fact, besides the evidence from historical and instrumental earthquakes in the study area (Moretti et al., 1990; Boschi et al., 1995, 1997; INGV, 2006) and from paleoseismological investigations (Galli and Bosi, 2003), the kinematic scheme of Fig. 14suggests that the “cause” of the active N-S fault system of the Crati Graben could be found in the regional NW-SE shear zone (cf. Falconara-Carpanzano Fault), even though the debate on the relationships with the Calabrian-Sicilian rift-zone still remains open. In addition, according toMonaco and Tortorici (2000), the southernmost fault of the shear zone (LCF, in the present study) may still be active. Further mesoscopic-scale field surveys, paleoseismological investigations (concerning soil horizons dislocated by N-S striking transpressive thusts), and analyses of Radon anomalies (Tansi et al.,
Fig. 15. 2D-kinematic FEM model of transpressional and transtensional structures induced byen ´echelonleft-lateral strike-slip faults (afterXiaohan, 1983, modified). (a) Geometrical scheme; (b) stress concentration (dotted: transpressional zone; ruled: transtensional zone; values ofσin bar); (c) re-orientation ofσ1trajectories; (d) re-orientation ofσ3trajectories.
2005b), are currently being carried out, in order to better characterize the seismotectonic features of central and northern Calabria.
Acknowledgments
Authors are very grateful to the colleagues M. F`olino Gallo and R. Sirianni for their precious contribution during the field surveying and in the realization of the figures. The manuscript benefited from precious suggestions and comments by C. Monaco and L. Tortorici, and by the reviewers G. Prosser and F. Rossetti.
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