Alpine orogenic P-T-t-deformation history of the Catena Costiera area and surrounding regions (Calabrian Arc, southern Italy): The nappe edifice of north Calabria revised with insights on the Tyrrhenian-Apennine system formation



[1] The nappe-structured belt of Calabria constitutes the eastward termination of the southern branch of the Alpine Mediterranean belt that delimits the northern edge of the Africa plate. Contrasting hypotheses for the origin and tectonic significance of the north Calabrian nappe edifice have been proposed, and kinematic data from north Calabria have been used to support different interpretations of the Alps-Apennines linkage and the polarity of the Tethyan subduction in the Apennine region. We reconstruct the architecture of the north Calabria nappe edifice through a multidisciplinary approach which integrates structural investigations with metamorphic thermobarometry and 40Ar/39Ar geochronology. Results from this study indicate that north Calabria consists of a Tertiary nappe stack, resulting from superimposed top-to-the-west extensional shearing (late Oligocene to middle Miocene in age) onto a previously structured top-to-the-east compressional belt (Eocene to Oligocene in age). This study also documents that the top-to-the-west extensional tectonics was achieved by means of regionally sized extensional detachment fault systems, stretching apart and translating as allochthonous fragments the previously accreted units. Thinning operated by top-to-the-west extensional detachment tectonics also resulted in the direct juxtaposition of non-Alpine or slightly Alpine metamorphosed units (upper plate complex) onto the previously exhumed deep-seated portions of the orogenic wedge, metamorphosed under blueschist facies metamorphic conditions (lower plate complex). These findings support a new tectonic scenario for the orogenic history of north Calabria, which may be adequately framed within the Tertiary Apennine-Tyrrhenian system evolution.

1. Introduction

[2] The Meso-Cenozoic Alpine convergence between the Eurasian and the African plates in the Mediterranean region is accommodated through a complex network of subduction systems, accomplishing for the consumption of the intervening Tethyan oceanic domain. In this context, the Tyrrhenian-Apennine system in the central Mediterranean region provides an outstanding example of how Tertiary back arc extension was coeval with compressional tectonics at the peripheries of the externally migrating orogenic fronts [e.g., Faccenna et al., 2001] (Figure 1). In spite of significant progress in unraveling the deformation history of the eastward migrating extension-compression pair at the back of the eastward migrating Apennine subduction [e.g., Jolivet et al., 1998a], many questions regarding the early orogenic evolution of the Apennine chain still remain unanswered. A major barrier to progress in this regard is the paucity of constraints on the linkage between the Alpine and the Apennine tectogenesis in the region. Much of the uncertainty concerns the origin and tectonic significance of the subduction-related deformation and metamorphism, due both to the extensional overprint and to the lack of a continuous record of the subduction-related metamorphic and deformation signatures along the belt.

Figure 1.

Simplified tectonic map of the Mediterranean region (modified after Jolivet et al. [1998a]). The exhumed high-pressure portions of the Alpine orogen are shown, together with localization of the study area.

[3] The Calabrian Arc forms the locus of maximum distortion of the Apennine-Maghrebian thrust system and constitutes the eastward termination of the southern branch of the Alpine orogenic system that delimits the northern edge of the Africa plate, running from Gibraltar to Calabria. The internal zones of such an orogenic system shows striking similarities as to the exposed rocks, the nappe architecture and metamorphic signatures, including the occurrence of scattered high-pressure rocks, remnants of the early Alpine thickening history (Figure 1). In northern Calabria, in particular, the Alpine metamorphism occurred under relatively low-temperature metamorphic conditions, with no or slight retrogressive overprint, allowing a more complete reconstruction of the pressure-temperature-time(P-T-t)-deformation history linked to the Alpine orogenic evolution. The Alpine edifice exposed in northern Calabria is thus a key to decipher the history of subduction in the central Mediterranean region, also providing arguments to constrain modalities through which transition from crustal thickening to crustal thinning occurred in the region. However, the nappe architecture of north Calabria is enigmatic (see, e.g., tectonic interpretations of Cello and Mazzoli [1996] and Thomson [1998]). There are several incompatible hypotheses for the origin and tectonic significance of the Calabrian Arc nappe structure, and kinematic data from the Calabrian Arc have been used to support different interpretations of the Alps-Apennines linkage and the polarity of the Tethyan subduction in the Apennine region [see, e.g., Haccard et al., 1972; Ogniben, 1973; Alvarez et al., 1974; Amodio Morelli et al., 1976; Scandone, 1982; Bouillin, 1984; Knott, 1987; Dietrich, 1988; Bonardi et al., 1994, 2001; Cello et al., 1996; Rossetti et al., 2001].

[4] The Catena Costiera area of north Calabria (Figures 1 and 2) has been commonly used a as key site to define the geometry, the deformation history and timing of the different units piled up in the north Calabria nappe stack (see, e.g., Amodio Morelli et al., 1976; Dietrich, 1976; Bonardi et al., 1994, 2001; Cello et al., 1996; Perrone, 1996]. Nevertheless, a systematic study of the nappe-forming units, timing of deformation events and metamorphic evolution is still lacking. Moreover, the amplitude of extensional overprint on the nappe architecture has not been documented yet, although described in the Sila Piccola Massif (Figure 2), where a similar nappe edifice is exposed [Rossetti et al., 2001].

Figure 2.

(a) Tectonic map of north Calabria, with the main Alpine tectonic contacts indicated (modified and readapted after Bonardi et al. [1976], Spadea et al. [1980], Bigi et al. [1990], and Rossetti et al. [2001]). The map shows the reconstructed poliphased Alpine structural fabric, together with location of the samples (CLB4-5-6-8-10) used for the geochronological study. In the inset, abbreviations are as follows: NC, northern Calabria; SC, southern Calabria. (b) Interpretative tectonostratigraphic scheme representing the relationships among the different tectonic units recognized in the region (not to scale; location of structures is only indicative). A major ductile-to-brittle top-to-the-west tectonic contact separates an upper from a lower plate complex of units. The whole nappe pile is stretched apart by westward dipping extensional faults, which cut across the previously eastward structured nappe pile. The distribution of the representative Alpine metamorphic index minerals within the nappe edifice is also indicated.

[5] In this paper, we present a synthesis of the geology of north Calabria that is based on the integration of new field and structural data, metamorphic thermobarometry and 40Ar/39Ar geochronology carried out in the Catena Costiera and surrounding regions (Figures 1 and 2). Our objectives were to document the cross-sectional deformation history of the exposed nappe pile and to identify structures responsible for nappe construction and reworking in the region. We document the deformational geometries, kinematics, timing and P-T metamorphic conditions of the main constituents of the north Calabria nappe edifice. Such data are complementary to those presented by Rossetti et al. [2001] for the Sila Piccola region (Figure 2) and, integrated with data derived from the literature, form the basis for the revision of the nappe architecture and define a new tectonic scenario for the orogenic history of north Calabria, framed within the Tertiary Apennine-Tyrrhenian system evolution.

2. Geological Background and Open Questions

[6] The nappe-structured belt of the Calabria-Peloritani orogen is a part of the peri-Mediterranean Alpine system progressively drifted and dispersed during the Neogene to Recent opening of the South Tyrrhenian basin and the subduction of the Ionian slab [Haccard et al., 1972; Alvarez et al., 1974; Amodio Morelli et al., 1976; Dewey et al., 1989; Faccenna et al., 2001]. Its structure and evolution has been classically separated from that of the Apennine chain, based on the exposed rock types and the evidence for pre-Neogene tectonism. There is also a general agreement among the different authors in considering the nappe architecture of north Calabria as distinct from that of southern Calabria and of the Peloritani Mountains of Sicily [Bonardi et al., 2001, and references therein] (see inset in Figure 2a). In particular, north and south Calabria have been considered as two sectors (or subterranes) with a distinct Cretaceous-Paleogene Alpine tectonometamorphic evolution, juxtaposed in Oligocene times [e.g., Bonardi et al., 2001].

[7] The nappe edifice of north Calabria is considered to be constituted by three main groups of units, from bottom to top: (1) Meso-Cenozoic slightly or not metamorphic dominantly carbonate sequences (Verbicaro and San Donato units from Amodio Morelli et al. [1976]); (2) ophiolite-bearing units, Jurassic to early Cretaceous in age and with an Alpine poliphased metamorphism [e.g., Ogniben, 1973; De Roever et al., 1974; Spadea et al., 1976; Beccaluva et al., 1982; Cello et al., 1991, 1996]; and (3) Calabrian crystalline units (Calabride Complex), pre-Alpine (Paleozoic in age) continental-derived metamorphic and igneous rocks and their Meso-Cenozoic sedimentary or weakly metamorphosed covers [e.g., Dubois, 1970; Amodio Morelli et al., 1976; Acquafredda et al., 1994]. Only crystalline basement nappes crop out in southern Calabria and in the Peloritani Mountains.

[8] Concerning the north Calabria nappe edifice, the following points are a matter of ongoing debate: (1) the paleogeographic position and the tectonometamorphic evolution of the Verbicaro and San Donato units, as well as their attribution to the Apennine chain [see, e.g., Ietto and Barillaro, 1993; Perrone, 1996]; (2) the relationships between the ophiolite-bearing units exposed in northern Calabria and that of the Calabria-Lucania border (Liguride Complex from Bonardi et al. [1988]), which are considered remnants of distinct oceanic realms, involved with different tectonic vergences (west and east verging, respectively) and in different times (Cretaceous-Paleocene and Oligocene-Miocene, respectively) in the orogenic processes, [see, e.g., Bonardi et al., 1993, 2001; Guerrera et al., 1993]; and (3) the paleotectonic collocation of the crystalline basement rocks of the Calabride Complex, referred either to the African [e.g., Amodio Morelli et al., 1976; Grandjacquet and Mascle, 1978; Scandone, 1982], European [e.g., Ogniben, 1973; Boullin, 1984; Dietrich, 1988; Rossetti et al., 2001], or to a microcontinent(s) in between [e.g., Vai, 1992; Guerrera et al., 1993; Perrone, 1996; Bonardi et al., 2001]. Furthermore, distinct orogenic polarities are reported from the two sectors of the Calabria nappe edifice. In north Calabria both Europe (west) and Adria (east) verging tectonics structures are reported, whereas only Adria-verging structures are reported from southern Calabria and the Peloritani Mountains [Bonardi et al., 2001, and references therein].

[9] Apart from the different paleotectonic configurations proposed, in most of the current literature it is commonly assumed that the kinematic signature of the orogenic pile of north Calabria is the result of a poliphased compressional tectonic evolution [e.g., Haccard et al., 1972; Amodio Morelli et al., 1976; Bonardi et al., 1994, 2001; Cello et al., 1996]. In this view, the nappe architecture is the consequence of the eastward verging overthrusting of an early westward verging eo-Alpine (Cretaceous Paleogene) structured belt onto the dominantly carbonate rocks of the African and Adria plate margins, which presently constitute the major parts of the southern Apennine and Maghrebide chains. It is thus assumed that the nappe contacts in Calabria are the results of overprinting compressional (Alpine and Apennine) events, with no or little contribution of postorogenic extension. On the other hand, on the basis of coupled structural and petrographical investigations on the nappe stack exposed in the Sila Piccola Massif (Figure 2), Rossetti et al. [2001] documented that the Alpine- (“west directed”) versus Apennine- (“east directed”) verging structures in north Calabria resulted from the superimposition of a top-to-the-west postorogenic extensional shearing onto an early east directed nappe forming event. A similar northwest directed extensional shearing is also reported by Platt and Compagnoni [1990], recognized as responsible for controlling nappe contacts within the Calabrian units exposed in the Aspromonte region of south Calabria (see inset in Figure 2).

[10] Such a complexity is also due to the fact that only a small number of studies were addressed to define the P-T-t-deformation history of the Alpine stage in Calabria [see, e.g., Dubois, 1970; Carrara and Zuffa, 1976; Platt and Compagnoni, 1990; Cello et al., 1996; Rossetti et al., 2001]. Nevertheless, the available geochronological data on the Alpine orogenic metamorphism in Calabria, although derived by using different methods on different rock types [Borsi and Dubois, 1968; Schenk, 1980; Beccaluva et al., 1981; Bonardi et al., 1987; Rossetti et al., 2001], systematically show Tertiary (Eocene to Oligo-Miocene) ages. Fission track (zircon and apatite) ages on the Calabride Complex indicate a rapid unroofing of this rock group from ∼35 to 15 Ma, confirming the Tertiary age of the main contacts within the nappe pile [Thomson, 1994, 1998]. No firm constrain exists for the age of metamorphism recorded by the San Donato and Verbicaro unit. An early Miocene age (K/Ar method whole rock) is reported by Pierattini et al. [1975] and Macciotta et al. [1986], while Borsi and Dubois [1968] proposed an Eocene age, based on Rb/Sr method on white micas. A Miocene age was also proposed on the base of stratigraphic arguments [Grandjacquet, 1967].

3. Revised Nappe Architecture of the Catena Costiera and North Calabria

[11] The N/S trending Catena Costiera delimits the westward termination of Calabria toward the Tyrrhenian Sea (Figures 1 and 2a). It is bounded to the east by the Plio-Pleistocene extensional fault systems of the Crati Valley tectonic depression [Lanzafame and Tortorici, 1981]. It consists of a complex stack of units where both oceanic- and continental-derived units are piled up [Carrara and Zuffa, 1976; Dietrich, 1976; Colonna and Compagnoni, 1982; Cello et al., 1996]. Our investigations in this region and neighboring key areas traversed the different structural levels exposed in the nappe pile and provided new geological information for the reconstruction of the orogenic nappe edifice of north Calabria. The definition of the large-scale structures was made after revision of the boundaries among the different exposed units and after characterization of their state of deformation and metamorphic signature. Such information is synthesized in Figure 2. Although possible correlations between areas which are several kilometers apart must be made with care, the collected data enabled us to reconstruct the first-order structures controlling nappe architecture and to propose a synthetic interpretation of the lateral relationships among the different tectonic units (Figure 2b). The main result is the reconnaissance of a major low-angle ductile-to-brittle tectonic contact, separating units with difference in both metamorphic signature and structural style. This tectonic contact delimits coherent tracts of HP-LT blueschist facies (oceanic- and continental-derived) rocks from below to overlying nonmetamorphic or slightly metamorphic rocks arranged into a nappe stack configuration, thus allowing to distinguish an upper from a lower plate complex of units (Figure 2b).

[12] A description of the nappe architecture follows. Details concerning the metamorphic signature of the different units and the kinematic characters of the main boundaries are provided in sections 4 and 5, respectively.

3.1. Upper Plate Complex

[13] The upper plate complex consists of a nappe stack unconformably covered by postorogenic clastic deposits of middle Miocene to Quaternary age, filling up extensional basins linked to the Tyrrhenian extension [e.g., Mattei et al., 2002] (Figure 2). As documented along the Tyrrhenian side of the Catena Costiera area, the nappe stack is constituted by the superposition of three major tectonic elements (Figures 2b and 3a). The uppermost tectonic element is made of thrust slices of the Calabride Complex. Such a complex of units tectonically overlies an ophiolite-bearing tectonic mélange (hereafter referred to upper ophiolitic unit, UOU) made of a weakly foliated schistose matrix (calcschists, pelites and quarzites), at least in part belonging to the Frido unit of Amodio Morelli et al. [1976], enveloping as tectonic blocks of various dimensions: (1) polymetamorphic ultrabasic and basic rocks with scattered slices of the original sedimentary cover and (2) continental-derived Calabrian basement rocks. Relationships between blocks and the enveloping schistose matrix within the ophiolitic mélange are exposed in the road cuts running from Scalea to Verbicaro and in the area between Belvedere and Cetraro (Figures 2 and 3). At outcrop-scale, there is evidence that the mafic bodies were incorporated into the enclosing schistose matrix of the UOU after metamorphism, as foliation in the matrix wraps around the mafic bodies and does not penetrate. The lowermost tectonic element of the upper plate nappe stack is constituted by Mesozoic carbonates, ascribed to the Verbicaro unit by Amodio Morelli et al. [1976] (Ver in Figure 2). Contacts with the overlying UOU are usually marked by late NNW-SSE striking, westward dipping extensional faults, which rework the original nappe contacts (Figures 2 and 3).

Figure 3.

(a) Simplified structural map of the region between Belvedere and Fuscaldo (Catena Costiera area; modified and readapted after Dietrich [1976] and Burton [1971]). This region is illustrative of the field relationships among the different units piled up in the upper plate nappe stack. The stereoplots (Schmidt net, lower hemisphere) show the D1-up (top-to-the NE)/D2-up (top-to-the-west) composite structures recognized in the upper plate nappe stack (see text for further details). (b) Geological cross section (see Figure 3a for location) showing the structural relations between the SD2 unit and the UOU along the Bonifati-Sangineto transect. (c) Geological cross section (see Figure 3a for location) showing the structural architecture of the upper plate nappe edifice across the Catena Costiera area. Top-to-the-west ductile-to-brittle extensional faults control the present contacts among the different tectonic units piled up in the upper plate nappe stack.

[14] Up to now, the metamorphosed Triassic phyllites and marble sequences cropping out in the area around Cetraro (hereafter referred as SD2 unit; Figures 2 and 3) were previously considered, as a part of the San Donato unit, the base of the northern Calabria nappe pile [Dietrich, 1976; Colonna and Compagnoni, 1982; Cello et al., 1996]. However, the lateral relationships between the SD2 unit and the ophiolite-bearing (UOU) and carbonate (Ver) units are not obvious. The geological cross sections shown in Figures 3b and 3c illustrate our interpretation, which is based on the arguments we explain below. In the northern part of the eastern boundary area of the SD2 unit (Sangineto river valley in Figures 3a and 3b), the SD2 unit and the UOU are in lateral continuity. Both of the units show a southwestward, shallow dipping schistosity and are severely affected by strong shear strain (cf. section 5.1). The contact between the units is not visible, but geometrical relationships show that SD2 overlies the UOU, since possible vertical offsets are excluded due to the lack of late, high-angle fault systems along the transect (Figure 3b). Toward the south, the boundary surface between the SD2 unit sequences and the UOU is almost horizontal and the UOU overlies the SD2 unit, whose thickness is significantly reduced. Slices of Mesozoic carbonates (Ver) also lie above the SD2 unit (Santa Maria La Serra; western sector of the cross section shown in Figure 3c), marked by flat-lying tectonic boundaries [see also Dietrich, 1976]. Along the coastal sector, contacts between the SD2 and the UOU are obscured by late, high-angle faults. Finally, in the southernmost part, a tentative correlation between the SD2 unit and the UOU may be established, based on the geometrical relationships between the southernmost outcrop of the SD2 unit (Acquappesa area) and the ophiolitic rocks exposed in the Intavolata area (Figures 2a and 3c). Also in this case, in fact, the lack of a clear evidence for late faulting supports the tectonic superimposition of the SD2 unit onto the highly strained ophiolitic sequences of the UOU. Furthermore, based on the similar structural fabric recognized within the carbonate rocks of Terme Luigiane and in the UOU of the Intavolata area (cf. section 5), we interpret such carbonate rocks as a tectonic block enclosed in the UOU (Figure 3c).

[15] In summary, it may be concluded that the upper plate nappe stack is constituted by independent allochthonous units for the most parts, embedded within an ophiolitic mélange (UOU). Also, the SD2 unit is here interpreted as a large-scale boudin embedded within this tectonic mélange (Figures 2b and 3c).

3.2. Lower Plate Complex

[16] The lower plate complex crops out in five main areas, located both within and outside the Catena Costiera area. From north to south, these areas are (Figure 2a): Mormanno, San Donato di Ninea, Spezzano Albanese, Diamante and Gimigliano areas. The lower plate exposes variably retrogressed HP/LT blueschist facies metamorphic rocks composed of both continental-derived metaclastic and marble sequences (Mormanno and San Donato di Ninea areas) and oceanic-derived ophiolitic rocks with associated metamorphosed cover rocks (Diamante, Spezzano Albanese, Terranova di Sibari and Gimigliano, areas). Contacts with the overlying upper plate nappe stack are exposed in all the cited localities, where always marks an abrupt change in both structural and metamorphic record (see sections 4 and 5). On the other hand, the lateral relationships between the oceanic- and the continental-derived blueschist sequences are not exposed (Figure 2).

[17] It is worthwhile to note that the blueschist continental-derived rocks, hereafter referred as SD1 unit (Figure 2), due to the strong analogies in terms of lithological types and stratigraphic sequences with the sequences exposed in the Cetraro area (SD2 unit in this study), were previously incorporated in the same rock type as constituting the San Donato unit [Bonardi et al., 1976; Bigi et al., 1990]. In the Mormanno window (Figure 2a), folded basic dikes are also observed within the SD1 unit metamorphic pile, with pristine intrusive contacts clearly preserved.

[18] The petrographical characters of blueschist ophiolitic sequences were previously described by Spadea et al. [1976], Cello et al. [1991, 1996], and Rossetti et al. [2001]. Following Wallis et al. [1993] and Rossetti et al. [2001] such a group of rock sequences are here named as lower ophiolitic unit (LOU) to distinguish them from the ones tectonically enclosed within the ophiolitic mélange of the upper plate (UOU) (Figure 2).

4. Alpine Metamorphic Evolution

[19] The relationships between the Alpine deformation history and the associated P-T evolution in both the upper and lower plate rocks have been systematically studied using the petrology of low-grade metapelites. In particular, our study focused on the continental-derived SD1 and SD2 units, used as reference frame to discriminate the P-T metamorphic regimes in the lower and upper plate, respectively (see Table 1 for representative mineral analyses, analytical details and sample localities).

Table 1. Representative Analyses of Index Mineral Assemblages in the Continental-Derived SD2 (Upper Plate) and SD1 (Lower Plate) Unitsa
 SD2 Unit (Upper Plate)SD1 Unit (Lower Plate)
  • a

    Mineral phases were analyzed using a Camebax electron microprobe (operating conditions were as follows: 15 kV, 15 nA, 20 s counting time) at the CNR laboratories of the University of Roma La Sapienza (Istituto di Geologia Ambientale e Geoingegneria, CNR, Rome). The structural formulae were calculated for carpholite (Car) on 5 cations for Si and 3 cations for Al, Fe, Mn, and Mg following Goffé and Oberhänsli [1992]; for chlorite (Chl) on 14 oxygens; for K-white mica (Phe) on 11 oxigens; for chloritoid (Cld) on 12 oxygens following Chopin et al. [1992].

LocalityCapo BonifatiCapo BonifatiCapo BonifatiCapo BonifatiBonifatiBonifatiMormannoMormannoMormannoMormannoMormannoAcquaformosaAcquaformosa
Cations12 Ox Basis12 Ox Basis14Ox Basis14 Ox Basis11Ox Basis11Ox Basis8 Ox Basis8 Ox Basis11Ox Basis11Ox Basis14Ox Basis8 Ox Basis8 Ox Basis
XMg0.030.050.300.31  0.460.41  0.430.750.79

[20] The P-T conditions were estimated by means of the synthetic P-T grids for metapelites described by Bousquet et al. [2002, and references therein], where equilibria are calculated for two independent compositional systems, K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) for Fe/Mg-carpholite/chloritoid-bearing parageneses and Na2O-K2O-MgO-Al2O3-SiO2-H2O (NKMASH) for albite-bearing parageneses, respectively (Figure 4).

Figure 4.

Simplified petrogenetic grid for low-grade metapelites in the KFMASH compositional system, integrated with isopleths from the NFMASH (reaction E [after Bousquet et al. [2002], and references therein]). EBS (epidote-blueschist), GS (greenschist), LBS (lawsonite-blueschist), and PA (pumpellyite-actinolite) refer to metamorphic facies of Evans [1990]. Numbers 3 and 6 refer to composition of Na-amphibole in Evans [1990]. Calculated Alpine peak P-T conditions (shaded areas) for both the upper (SD2 unit) and the lower plate (LOU and SD1 unit) rocks are shown. Mineral abbreviations are as follows: Car, carpholite; Chl, chlorite; Ab, albite; Cld, chloritoid; Ep, epidote; Gln, glaucophane; Kln, Kaolinite; Ky, kyanite; Ms, muscovite; Phe, phengite; Pg, paragonite; Prl, pyrophyllite; Qtz, quartz; Tr, tremolite; W, water. “XMg-Car” is activity of carpholite. See text for further details.

[21] In the KFMASH system, the P-T fields are a direct function of the Fe-Mg partitioning involving the index mineral assemblages (Fe/Mg-carpholite, chlorite, chloritoid) in equilibrium with K-white mica and quartz [e.g., Jolivet et al., 1998b]. Temperature conditions were based upon the decrease of the Fe-Mg partitioning coefficient Kd with increasing temperature in the index minerals. Maximum temperatures for carpholite-bearing assemblages were based upon the location of equilibrium C in Figure 4, which bounds the stability field of Fe/Mg-carpholite at high temperature. For chloritoid-bearing assemblages, temperatures were estimated from the Fe-Mg partitioning between chloritoid and chlorite, by using the empirical thermometer of Vidal et al. [1999] (equilibrium F in Figure 4). Pressure estimates were based upon the composition of K-white micas (expressed as the Si4+ a.p.f.u. content) in equilibrium with (1) Fe/Mg-carpholite and/or chloritoid for the KFMASH system through location of equilibrium B in Figure 4, and (2) with paragonite and albite through location of equilibrium E in Figure 4 for the NKMASH system.

[22] The phase diagrams computed by Evans [1990] on the basis of the sodic amphibole composition were used to constrain the stability fields of mineral assemblages for metabasic rocks (Figure 4). P-T estimates are presented in Figure 4, where they are compared with previous estimations deduced from the LOU.

4.1. Upper Plate

[23] The Alpine orogenic metamorphic signature shows a great variability within the tectonic slices, which constitute the upper plate nappe edifice. Where recorded, the Alpine metamorphism is poliphased, starting from an early high-to-moderate P-T ratio (M1-up stage), with a final reequilibration under low-grade greenschist facies conditions (M2-up stage).

[24] In the pre-Alpine Calabrian Complex, the M1-up stage is commonly associated with low-grade mylonitic-to-cataclastic bands of deformation [Amodio Morelli et al., 1976]. The Alpine overprint is better documented in the lowermost units of the Calabride Complex (Alpine Group rocks of Amodio Morelli et al. [1976]) cropping out in the Sila Piccola region (Figure 2). There, the M1-up overprint occurred under low-grade greenschist to lawsonite-blueschist metamorphic conditions [e.g., Dubois, 1970; Piccarreta, 1981; Rossetti et al., 2001]. From these authors, peak pressure estimates range between 0.4 and 0.6 GPa, for temperatures below 300°C.

[25] No evidence for typical high-pressure assemblages was detected in the dominantly pelitic successions constituting the matrix of the UOU ophiolitic mélange. On the other hand, the basic tectonic blocks reveal a poliphased and heterogeneous metamorphic history. Metamorphic types derived from volcanic protoliths are widespread, but serpentinized ultramafic rocks also occur. In metabasites, low-grade metamorphic conditions are predominant, but with distinctly different peak M1-up P-T conditions. Lawsonite-blueschists and chlorite-epidote greenschist mineral associations are, in fact, reported [Bonardi et al., 1974; De Roever et al., 1974; Dietrich, 1976; Spadea et al., 1980]. Greenschist parageneses (epidote, chlorite, albite and tremolite) commonly constitute the matrix of such a rock type where blueschist facies metamorphic assemblages, if present, occur within a relict foliation. Relict Na-amphibole and/or lawsonite crystals are often present as armored inclusions in M2-up epidote-albite retrogressive layering. These mineral associations are indicative of M1-up peak metamorphic conditions equilibrated under the blueschist-greenschist facies transition [e.g., Maruyama et al., 1986; Evans, 1990], with P in the range of 0.5–0.8 GPa and T = 250°–400°C (limits between LBS, EBS and GS facies in Figure 4).

[26] The M1-up metamorphic climax in the SD2 unit was acquired under low-grade greenschist facies metamorphic conditions. The main parageneses consists of K-white mica-chlorite (chl1)-quartz-chloritoid or K-white mica-chlorite-albite-paragonite-quartz ± tourmaline ± calcite (Figure 5). Chloritoid is a nearly pure Fe-chloritoid end-member (XMgctd = 0.03–0.05), while chlorite shows a rather constant XMgchl = 0.3. In albite-bearing assemblages, Si4+ content of K-white mica ranges from 3.15 to 3.20 apfu (Table 1). The retrogressive evolution (M2-up stage) is of very low-grade type and consists of quartz-chlorite (chl2)-white mica associations, mostly found as coronae on early chloritoid porphyroblasts (Figure 5b). The Fe-Mg exchange thermometry of Vidal et al. [1999] applied to the M1-up chlorite-chloritoid pairs (Sample CB971-3 in Table 1) provides a Kd ranging from 2.20 to 2.59, corresponding to temperatures between 280° and 330°C (Figure 4). The pressure conditions are inferred from equilibrium E in Figure 4 providing peak pressure in the order of 0.3–0.4 GPa (Si4+ in phengite = 3.15–3.20 a.p.f.u; sample B-3 in Table 1) for the detected temperature interval. These results (P = 0.3–0.4 GPa and T = ∼300°C) are in good agreement with previous estimates obtained by Dietrich et al. [1977].

Figure 5.

(a) Metamorphic mineral growth related to the deformation phases recognized in the upper plate SD2 unit. (b) Thin section (plane polarized light; horizontal length equals 3 mm) showing representative mineral assemblages during the M1-up (D1-up) stage in the upper plate SD2 unit. Note the (1) the synkinematic to postkinematic growth of chloritoid porphyroblast with respect to the main foliation and (2) development of chlorite coronas (Chl2) around chloritoid (Cld) porphyroblast. Abbreviations are as follows: Chl, chlorite; Cld, chloritoid.

[27] The Mesozoic carbonate sequences of the Verbicaro unit are for the most part not affected by metamorphism. Nevertheless, a low-grade greenschist facies metamorphic overprint was locally described in metabasic rocks included within these sequences [Pierattini et al., 1975].

4.2. Lower Plate

[28] In both the oceanic- (LOU) and continental- (SD1) derived units exposed in the lower plate, peak metamorphic conditions were equilibrated under blueschist facies conditions (M1-low stage). Retrogression of the peak blueschist assemblages occurred under low-grade metamorphic conditions, with a final reequilibration within the greenschist metamorphic facies (M2-low stage; Figure 6a). In metapelites, the index blueschist mineral is commonly represented by Fe/Mg-carpholite that, in textural equilibrium with chlorite, K-white mica, quartz ± chloritoid ± pyrophyllite (± carbonates ± oxides), is the main constituents of the first-phase (M1-low/D1-low) fabric in this rock group (Figures 6b–6c). Retrogressive evolution is associated with destabilization of the carpholite-bearing associations during the development of the main rock fabric. Retrogressive assemblages are equilibrated under low-grade greenschist facies conditions and consist of chlorite-mica-quartz- ± chloritoid-bearing segregations, partially replacing Fe/Mg-carpholite crystals (Figure 6d). In the field, preservation of the early peak mineralogical textures is strongly dependent on the deformation gradient and intensity during the second-phase (M2-low/D2-low) exhumation stage.

Figure 6.

(a) Metamorphic mineral growth related to the deformation phases recognized in the lower plate rocks. (b) Thin section (crossed polars; horizontal length equals 3 mm) showing the peak M1-low association represented by Fe-carpholite fibers (Car) in quartz (Qtz) in associations with phengite (Phe) (sample MOR, SD1 unit; Mormanno area in Figure 2a). (c) Backscattered electron photomicrograph image showing the peak M1-low association of Mg-carpholite (Car), quartz (Qtz) and pyrophyllite (Prl) (Sample CLB8, SD1 unit; Acquaformosa area in Figure 2a). (d) Thin section (crossed polars; horizontal length equals 3 mm) showing the partial breakdown of Fe/Mg-carpholite in low-grade chlorite (Chl) + phengite (Phe) composite associations (SD1 unit; Mormanno area in Figure 2a).

[29] In the Mormanno window (SD1 metapelites; Figure 2a), the M1-low stage is attested by the occurrence of fresh Fe/Mg-carpholite fibers (XMgcar = 0.41–46) in coexistence with highly substituted phengites (Si4+ = 3.21–3.32 a.p.f.u.), chlorite (XMgchl = 0.40) and quartz (Figure 6b; sample MOR in Table 1). The occurrence of Fe/Mg-carpholite in the Mormanno metasedimentary sequence was already reported by Busato and Gianpaolo [1983]. The M2-low retrogressive evolution occurred under low-grade greenschist facies conditions as documented by the overall good preservation of Fe/Mg-carpholite-bearing associations, and the absence of chloritoid as retrogressive product on carpholite (Figure 6d). Basic dikes occurring within the metapelitic sequence also record the early HP/LT stage, which is documented by a relict metamorphic schistosity consisting of lawsonite-Na-amphibole segregations. Na-amphibole composition is of type 3 of Evans [1990] with glaucophanitic cores and crossitic rims. Lawsonite crystals also occur in metagreywackes interlayered within the metapelitic sequence. Occurrence of Na-amphibole in textural equilibrium with lawsonite in metabasites indicates that peak metamorphic conditions were equilibrated within the blueschist-lawsonite facies of Evans [1990]. Peak P-T estimates are: P = 1.1–1.3 GPa (reaction B in Figure 4 for Si4+ = 3.21–3.32) and T < 350°C, as constrained by the absence of chloritoid for XMgcar of 0.4 (reaction C in Figure 4).

[30] In the San Donato window (Figure 2), the SD1 metapelitites are intensively retrogressed, and intensity of the retrogressive evolution increases moving to the uppermost portions of the metamorphic pile, where the early M1-low Fe/Mg-carpholite-bearing assemblages are completely substituted by M2-low chlorite-white mica, quartz ± chloritoid associations. The M1-low Fe/Mg-carpholite-bearing assemblages are preserved in quartz segregations from the Acquaformosa area. There, Fe/Mg-carpholite (XMgcar = 0.75–79) fibers (sample CLB8 in Table 1) are observed in textural equilibrium with pyrophyllite, phengite, chlorite and quartz (Figure 6c). Coexistence of Fe/Mg-carpholite with pyrophyllite constrains P-T conditions within the pyrophyllite stability field and, more precisely, along the reaction D in Figure 4. On the basis of the lack of chloritoid in equilibrium with Fe/Mg-carpholite, peak P-T conditions can be thus estimates as: P = 0.7–0.9 GPa and T between 350° and 450° (reaction C in Figure 4 for XMgcar = 0.80).

[31] Previous P-T estimates on lower plate rocks were obtained from the LOU. Peak P-T conditions for the M1-low stage in basic rocks exposed in the Spezzano Albanese and Diamante areas (Figure 2a) yielded 300°–350°C and 0.8–1.0 GPa [Spadea et al., 1976; Beccaluva et al., 1982; Cello et al., 1991] (Figure 4). In the Gimigliano window (Figure 2a), Fe/Mg-carpholite-bearing metapelites of the LOU provided peak P-T conditions of 1.1–1.3 GPa and around 350°C [Rossetti et al., 2001] (Figure 4).

[32] Because of the overall good preservation of the temperature-sensitive Fe/Mg-carpholite-bearing parageneses, the retrogressive evolution during exhumation of lower plate rocks occurred during nearly isothermal decompression or cooling conditions, as imposed by the thermal boundary defined by reaction C [see also Jolivet et al., 1998b]. We can, therefore, infer that low temperature conditions persisted through time within the exhumed metamorphic pile.

[33] On the basis of this new metamorphic reconstruction, two major considerations may be made: (1) the San Donato unit of Amodio Morelli et al. [1976] consists of two differently metamorphosed units, the greenschist SD2 unit (Cetraro-Bonifati area; upper plate) and the blueschist SD1 unit (Mormanno and San Donato di Ninea areas; lower plate); and (2) the upper/lower plate tectonic contact systematically corresponds to a gap in P-T metamorphic conditions, maximum when the postmetamorphic tectonic mélange of the UOU directly overlie the lower plate rocks of the LOU (i.e., Spezzano Albanese area in Figure 2).

5. Structural Evolution of the North Calabria Nappe Edifice

5.1. Upper Plate

[34] The structural signature of the upper plate nappe pile is characterized by two main Alpine deformation phases (D1-up and D2-up), which are responsible for the nappe construction and reorganization, respectively (see also the tectonic scheme presented in Figure 2b).

[35] The D1-up deformation phase shows distinct characters in terms of textures and structures within the nappe pile. In the Alpine Group rocks of the Calabride Complex exposed in the Sila Piccola area (Figure 2a), D1-up corresponds to progressive ductile-to-brittle top-to-the-NE compressive shearing, responsible for nappe piling up [Rossetti et al., 2001].

[36] In the nonmetamorphic enveloping matrix of the UOU, the D1-up deformation mainly consists of NW/SE trending upright and NE recumbent isoclinal to chevron-type folds. Axial planar schistosity systematically strike NW-SE, with a main dip toward the SW (Figure 7a). Folding is also associated with pervasive NE directed stretching in the more competent rock successions (“Calpionella” limestones and quartz-arenite levels), as deduced by the orientation of quartz/calcite crystals filling extensional veins (Figure 7b). S-C tectonites were also observed in the more competent layers (stereoplot I in Figure 3a). Sense of shear is systematically top-to-the-NE all over the study area (Figures 2 and 3a), with shearing evolving from ductile to brittle deformation conditions (Figure 7c). Such a top-to-the-NE shearing controls the tectonic contacts among the different rock successions embedded within the ophiolitic mélange (Figure 7d). No conclusive structural information can be deduced from the metabasic rock fabrics embedded within the UOU due to the lack of coherence of this deformation fabrics (ductile-dominated) with respect to that of the enveloping matrix (brittle-dominated).

Figure 7.

(a) NW/SE trending upright F1 folds in the UOU (Verbicaro area in Figure 2a). The stereoplot (Schmidt net, lower hemisphere) shows the related structural data. (b) NE-SW D1-up stretching and veining in “Calpionella” limestones of the UOU. (c) D1-up S-C tectonites in the UOU. Sense of shear is top-to-the-NE. The stereoplot (Schmidt net, lower hemisphere) shows the related structural data. (d) Geological cross section exemplificative of the internal structural architecture of the UOU in the Belvedere-Sangineto area (see Figure 3a for cross section location).

[37] A similar structural pattern, but developed under ductile deformation conditions, can be reconstructed from the SD2 unit, where the main D1-up fabric consists of a S-L fabric synkinematic relative to the M1-up metamorphic stage. Stretching lineations are commonly marked by elongated crystals of tourmaline-albite and/or chloritoid-quartz-K-white mica-chlorite composite associations (Figure 8a) and systematically trend NE-SW (stereoplot II in Figure 3a). The XZ sections of the S-L tectonites show consistent kinematic indicators showing a general top-to-the-NE sense of shear (Figure 8b), synkinematic to the growth of the main M1-up metamorphic parageneses. Intrafolial F1 folds always point to a NE-directed tectonic transport direction, being rotated toward parallelism with the L1 stretching direction in the highly sheared domains (Figure 8c). The later evolution of the D1-up deformation consists of NW/SE trending kink-type folds refolding the D1-upS-L fabric and of NE verging reverse and thrust faults (Figure 8d).

Figure 8.

Examples of the deformation features recognized in the upper plate nappe stack as recorded by the SD2 unit in the Cetraro-Bonifati area (see Figures 2 and 3). (a) L1 stretching lineations in schists, provided by quartz-tourmaline-white mica composite associations (Cetraro area). (b) D1-up mesoscale top-to-the-NE kinematic indicators in schists from the Capo Bonifati area (exposure parallel to the L1 stretching direction and normal to S1 foliation). (c) Oblique F1 fold nearly parallel to the L1 NE-SW trending stretching direction (D1-up mylonitic shear zone from the Cetraro area). (d) Kink-type folds refolding the D1-up mylonitic foliation (Bonifati area). The stereoplot (Schmidt net, lower hemisphere) illustrates how the kink planes strike roughly orthogonal with respect to the L1 trend. (e) Example of the reworking operated by the top-to-the-west shearing on the early top-to-the-NE mylonitic fabric (Cetraro area). (f) D2-up top-to-the-W brittle extensional shearing in schists from the Cetraro area.

[38] The D2-up deformation is pervasive within the whole nappe complex and shows different morphology and intensity according to the metamorphic signature of each unit. The D2-up event is associated with the development of low-angle and high-angle dominantly cataclastic bands associated with a general top-to-the-west sense of shear, which are responsible for westward stretching of the whole tectonostratigraphic section (Figures 2b and 3c). Such a cataclastic deformation is associated with the brittle reworking of the early ductile-to-brittle compressive features and corresponds to the major ductile to cataclastic contacts that sharply bound the main units constituting the nappe pile [see also Rossetti et al., 2001]. Penetrative slickenlines are observed within the cataclastic bands and trend WNW-SSE. A main ductile to brittle D2-up shear zones can be reconstructed within the nappe pile, branching into multiple low-angle fault strands (Figure 3c). The uppermost portion of this shear zone marks the contact between the Calabride Complex and ophiolitic rocks of the underlying UOU (right side of the cross section shown in Figure 3c). This shear zone is well exposed on the western shoulder of the Crati basin in a tectonic window in the vicinity of the Fuscaldo village (Figures 2a and 3). A smaller exposure of this shear zone can also be observed near the Rose village, on the eastern Crati shoulder, where the contact between the Calabride Complex and UOU also occurs (Figure 2a). The shear zone is marked by highly strained metabasalts and cherts with widespread development of S-L tectonites, equilibrated under low-grade greenschist facies conditions. Such S-L fabric overprints an older lawsonite-albite metamorphic layering. Stretching lineations are mostly provided by elongate aggregates of albite-chlorite-epidote associations, constantly west-to-WNW trending (stereoplot III in Figure 3a). Kinematic indicators on the XZ sections of the D2-up finite strain constantly indicate a top-to-the-west sense of shear. Moving toward the overlying Calabride Complex, this shear deformation evolves toward more brittle conditions with the generation of cataclastic bands with abundant west dipping, C′-type shear bands. N-S-trending low-angle and high-angle conjugate extensional fault systems also occur, locally reactivating the early ductile-sheared domains. In the SD2 rocks, westward dipping semibrittle-to-brittle C′-type shear bands and extensional faults (stereoplot III in Figure 3a) overprint the previous top-to-the-NE shear fabric (Figures 8f and 8e) The lowermost exposed portions of the shear zone also consists of S-L tectonites, which, developed on the low-grade metabasic slivers of the UOU, also show a similar top-to-the-WNW sense of shear (Intavolata area in Figure 3c). Mesoscopic sigmoidal features and west dipping shear bands were found also within the slightly metamorphic Verbicaro-type carbonate rocks of Terme Luigiane (Figures 3a and 3c). On the basis of these findings and the field relationships, we interpret the Terme Luigiane carbonate rocks as a tectonic block detached from its substratum and embedded within the lower levels of the D2-up top-to-the-W/NW shear (Figures 2b and 3c).

5.2. Lower Plate and the Upper/Lower Plate Tectonic Contact

[39] The main structural fabric detected in the lower plate rocks consists of a second-phase fabric (D2-low), synkinematic relative to the M2-low retrogressive metamorphism and associated to the exhumation stage. The previous D1-low high-pressure foliation is recognizable only in small lens-shaped microlithons enclosed within the D2-low crenulation cleavage, or as inclusions in syn-greenschist porphyroblasts such as plagioclase or epidote in basic rocks. Consequently, no conclusive data are available for the characterization of the early D1-low deformation fabric.

[40] The D2-low greenschist deformation fabric consists of S tectonites in the lowermost portions of the high-pressure units, whereas S-L tectonites are commonly found in the uppermost levels, approaching the upper/lower plate tectonic contact. The second-phase schistosity is flat lying and axial planar to isoclinal folds, refolding the early high-pressure parageneses (Figure 9a), and it increases in intensity toward the upper tectonic boundary. Lineations are defined by greenschist facies mineral assemblages, which are commonly provided by quartz-chlorite-white mica ± chloritoid composite associations in metapelites and by albite, epidote and chlorite in metabasic rocks. Such lineation trends roughly E-W as average, but some different trend pattern also occurs (NW/SE to NE/SW directed).

Figure 9.

Examples of the deformation features recognized in the lower plate as recorded by the SD1 unit. (a) Thin section (crossed polars; horizontal length equals 3 mm) showing second-phase crenulation cleavage refolding an early HP/LT foliation, provided by Fe/Mg-carpholite-quartz-phengite composite associations (Mormanno area in Figure 2a). (b) L2 stretching lineations on S2 mylonitic foliation, provided by quartz-chloritoid ± tourmaline composite associations (San Donato di Ninea area in Figure 2a). (c) Sheath-like fold in D2-low mylonite (San Donato di Ninea area in Figure 2a). Exposure orthogonal to the L2 stretching lineation and normal to S2 foliation; YZ section of the D2-low finite strain). (d) Top-to-the-west kinematic indicators in D2-low mylonite (San Donato di Ninea area in Figure 2a. Exposure parallel to the L2 stretching direction and normal to S2 mylonitic foliation; XZ section of the D2-low finite strain). (e) Thin section (crossed polars; horizontal length: 15 mm) showing top-to-the-west kinematic indicators in D2-low mylonite (San Donato di Ninea area in Figure 2a. Section parallel to the L2 stretching direction and normal to S2 mylonitic foliation; XZ section of the D2-low finite strain). Abbreviations are as follows: Car, carpholite; Qtz, quartz; Chl, chlorite.

[41] Major ductile to brittle low-angle tectonic contacts sharply bound everywhere the contact between the underlying blueschist facies rocks and the overlying upper plate nappe stack. These contacts mark a strong contrast in the Alpine metamorphic grade distribution, as a systematic sudden downward increase in the pressure of metamorphism is observed across them. Approaching the upper/lower plate tectonic contact, the second-phase D2-low retrogressive ductile fabric in the lower plate rocks systematically shows a progressive transition from coaxial to noncoaxial deformation, i.e., from spaced crenulation cleavage to penetrative flattening and stretching (S-L fabric), merging into mylonitic to cataclastic zones of deformation. Such a progressive character of D2-low deformation is well evident moving to the west along the road from Acquaformosa to San Donato di Ninea (running roughly parallel to the cross section A-B shown in Figure 2a; Figure 10). Close to the village of San Donato di Ninea, a greenschist facies mylonitic shear zone, up to 50 m thick and subparallel to the second-phase schistosity of the lower plate rocks, marks the contact between carpholite-bearing high-pressure rocks and the overlying cataclastic carbonate and dolomitic rocks (Figures 2 and 10). Fine-grained mylonitic rocks are associated with the breakdown of the high-pressure M1-low Fe/Mg-carpholite-bearing quartz fibers, which occur as pseudomorphic associations made of retrogressive M2-low chlorite-white mica composite associations. The same mineralogical assemblages together with tourmaline, quartz and chloritoid form mineral stretching lineations (Figure 9b). Rootless and sheath-like folds are commonly observed (Figure 9c) attesting for the noncoaxial character of the D2-low ductile shearing. Lineations trend E-W to ENE-WSW and consistently show down dip direction on the planar mylonitic foliations (stereoplot I in Figure 10). Kinematic indicators are abundant within the mylonitic shear zone, and range from mesoscopic- to microscopic-scale (Figures 9d–9e). They systematically indicate a top-to-the-west sense of shear. A 25-m-thick zone of gouge to cataclastic deformation with abundant west dipping shear bands and conjugate NNE/SSW trending extensional faults is observed on the overlying Mesozoic carbonate rocks. Such a fault gouge includes elements of underlying schists, marbles and overlying carbonate rocks (Ver in Figure 10). Extension direction provided by slickenline analysis on the fault planes is subparallel to the mylonitic stretching direction in the SD1 unit (stereoplot II in Figure 10).

Figure 10.

Geological cross section showing the attitude of the upper/lower plate tectonic contact in the San Donato di Ninea area (see Figure 2a for cross section location, symbols and ornaments). The contact is marked by a major ductile-to-brittle top-to-the-west extensional shear zone. A gap in P-T metamorphic conditions is observed moving from the lower plate, Fe/Mg-carpholite-bearing SD1 unit, to the upper plate nonmetamorphic or slightly metamorphic carbonate rocks. The stereoplots (Schmidt net, lower hemisphere) are representative of the D2-low top-to-the-west mylonitic shearing in the SD1 unit (Stereoplot I) and of the brittle faulting affecting the upper plate carbonate rocks (Stereoplot II; faults as great circles, striae as arrows).

[42] Throughout the lower plate rocks a similar gradual evolution from ductile-to-brittle behavior is observed, with progressive shear strain localization along the upper/lower plate boundary. Abundant extensional faults and tension gashes cut the mylonitic bands, attesting for shearing continuing with the progressive exhumation of the deep-seated rocks. Superimposed brittle structures within the lower plate rocks indicate a mean E-W trending stretching direction, with a dominant top-to-the-west sense of shear. The brittle shear senses are thus generally synthetic to the ductile shear transport direction. In the Spezzano Albanese area, in particular, nonmetamorphic or slightly metamorphic rocks of the UOU are tectonically superimposed onto poliphased blueschist sequences of the LOU. The contact is marked by metric thick low-angle cataclastic shear zone with top-to-the-SW tectonic transport and corresponds to a metamorphic gap of ∼0.7–0.9 GPa in pressure and 300°–350°C in temperature. Penetrative D2-low ductile westward directed shear senses in the underlying LOU coexist with overprinting low-angle cataclastic zones striking subperpendicular to the mean greenschist stretching lineation (Figure 11). The UOU/LOU contact was already recognized by Spadea et al. [1976] and described by Bouillin [1984] and Wallis et al. [1993]. The latter authors also reported the same, late-stage top-to-the-west shearing as responsible for the UOU/LOU juxtaposition.

Figure 11.

The upper/lower plate tectonic contact in the Spezzano Albanese area (see Figure 2a for location). Ductile-to-brittle top-to-the-W/SW shear sense indicators are widespread along flat-lying shear zones. A gap in P-T metamorphic conditions is observed moving from the lower plate blueschist LOU to the upper plate nonmetamorphic UOU. The stereoplot shows attitude of the plano-linear mylonitic fabric along the shear zone, compared with extensional faults observed in the upper plate. Note (1) the near parallelism between the orientation of the ductile stretching lineations on the mylonitic foliation and slickenlines on the fault planes and (2) the extensional faults in the upper plate die into the upper/lower plate tectonic contact.

[43] A similar tectonic setting was also observed in the Diamante area (Figures 2 and 12), where polymetamorphic blueschist facies metabasic rocks with a penetrative second-phase greenschist D2-lowS-L fabric (chlorite-albite-epidote associations) underlie nonmetamorphosed or slightly metamorphosed calcschists. Approaching the contact, a progressing transition from flattening to noncoaxial deformation is observed (Figure 12a). Sense of shear is top-to-the-NW, and synkinematic relative to the M2-low greenschist retrogression (Figures 12b–12c). The same top-to-the-NW sense of shear was previously reported in Cello et al. [1996], but described as synkinematic relative to the early blueschist stage. Nevertheless, there is a clear textural evidence that the main rock fabric of the Diamante metabasites was acquired during the syngreenschist retrogressive stage and overprinted pristine blueschist features, now occurring as boundins within the greenschist plano-linear fabric (Figure 12b).

Figure 12.

(a) Structural map of the Diamante area (see Figure 2a for location) showing distribution of the second-phase (D2-low) plano-linear fabric in the LOU and attitude of the tectonic contact with the overlying non-metamorphic upper plate (“Calpionella” limestones). (b) D2-low greenschist mylonitic fabric in the LOU exposed at Diamante. Top-to-the-northwest kinematic indicators within the mylonitic fabric are also shown (exposure parallel to the L2 stretching direction and normal to mylonitic foliation). (c) Stereoplot (Schmidt net, lower hemisphere) showing the structural data representative of the retrogressive greenschist mylonitic fabric of the LOU exposed in the Diamante area.

6. The 40Ar/39Ar Geochronology

[44] In order to provide age constraints on the described deformation and metamorphic evolution, an 40Ar/39Ar study was performed using bulk separates and single grains of micas (phengites) from both the upper plate (SD2 unit) and lower plate (SD1 unit) rocks. The closure temperature for Argon retention in phengite [e.g., Hames and Bowring, 1994; Kirschner et al., 1996] is estimated to be in the same range (or even below) as the maximum temperature reached at peak conditions along the various studied transects (peak temperature not exceeding 450°C; Figure 4). Consequently, the 40Ar/39Ar ages obtained during this study very likely represent ages of phengite crystallization, linked to the poliphased metamorphic evolution previously described. The mica growth stages during the main fabric development are then used as markers for dating the complete tectonometamorphic history of the study area. The obtained data are supplemented by ages provided by Rossetti et al. [2001] for the Gimigliano area using the same methodology on the LOU (sample CLB10 in Figure 2a). The sample locations are shown in Figure 2a and the analytical data are listed in Table 2. For details on sample preparation and technical procedure see Appendix A.

Table 2. Summary of the 40Ar/39Ar Analytical Data on Micas
Step40Ar/39Ar36Ar/40Ar × 100039Ar/40Ar37Ar/39Ar%39Ar% 40Ar atm.Age ± 2 σ, Ma
  • a

    Bulk separate; J = 0.016905; total age, 21.7 ± 0.

  • b

    Bulk separate; J = 0.009733; total age, 22.1 ± 0.2.

  • c

    Laser probe; J = 0.016905; total age, 14.9 ± 0.2.

  • d

    Laser probe; J = 0.016905; total age, 33.5 ± 0.1.

Sample CLB4 Phengitea
10.2212.5071.1740.5674.16.7 ± 1.7
20.2811.4232.0642.6442.18.5 ± 0.8
30.2841.3882.0790.1385.2141.08.6 ± 1.5
40.4060.4232.1530.1189.5512.512.3 ± 0.6
50.4840.2181.9330.04614.556.514.7 ± 0.4
60.5240.2831.7470.01821.498.315.9 ± 0.3
70.6410.2831.42935.648.419.4 ± 0.2
80.7770.1571.2270.03154.274.623.5 ± 0.1
90.7600.3171.1930.05561.289.423.0 ± 0.2
100.7790.1491.22767.724.423.6 ± 0.3
110.7600.2241.22974.216.623.0 ± 0.2
120.8320.2011.13183.165.925.2 ± 0.1
130.9270.2620.99591.417.728.0 ± 0.2
140.8730.0981.11394.242.926.4 ± 0.5
151.0370.2240.901100.006.631.3 ± 0.2
Sample CLB5 Phengiteb
11.9211.8450.2430.031.1753.333.3 ± 1.0
21.0020.9510.7320.0982.4526.617.5 ± 0.3
30.9650.2910.9670.5866.236.716.7 ± 0.2
40.9910.1920.9630.66913.794.617.2 ± 0.2
51.0130.0970.9710.49223.181.717.6 ± 0.2
61.0320.4270.8560.01833.8511.618.1 ± 0.2
71.2140.7730.6440.00749.6321.821.0 ± 0.3
81.2720.6380.6380.00461.5315.722.2 ± 0.3
91.3770.5510.6120.01172.0815.723.8 ± 0.3
101.4840.4160.5940.00381.3011.825.8 ± 0.3
111.4840.5150.5710.01984.8715.325.8 ± 0.3
121.5210.1870.6240.06586.435.126.4 ± 0.3
131.6140.1770.5890.01497.734.928.0 ± 0.3
141.7651.0340.398100.0029.730.6 ± 0.5
Sample CLB6 Phengitec
11.1212.4530.2450.2172.533.9 ± 11.6
20.3582.8180.4671.9483.210.9 ± 1.8
30.1912.8640.8385.5584.15.8 ± 0.7
40.2522.4711.0688.6673.07.7 ± 1.1
50.2782.2291.22612.5665.98.5 ± 1.2
60.2921.9701.42917.4658.28.9 ± 0.7
70.5440.8891.35522.0426.316.5 ± 0.8
80.5450.9601.31327.6028.416.5 ± 0.7
90.5251.1981.23133.2635.415.9 ± 0.6
100.5341.3551.12342.5340.116.2 ± 1.2
110.5211.8270.88356.0153.915.8 ± 0.9
120.5120.7791.50365.5923.015.5 ± 0.8
130.5510.4041.59972.0311.916.7 ± 1.1
140.5560.1761.70681.125.216.9 ± 0.4
150.5550.1181.74090.363.516.8 ± 0.4
160.5440.3831.62995.6711.316.5 ± 0.8
170.5860.5391.434100.0015.917.8 ± 0.7
Sample CLB8 Phengited
10.4642.3130.6820.1468.414.1 ± 9.1
20.6920.5681.2021.4816.821.0 ± 0.3
30.7970.2711.1552.898.024.1 ± 0.6
40.7680.7841.0005.1623.223.3 ± 0.6
50.9020.3870.9818.6511.427.3 ± 0.4
61.0490.1360.91513.604.031.7 ± 0.3
71.0710.1010.90622.003.032.4 ± 0.1
81.1080.0870.87933.852.633.5 ± 0.1
91.0930.2310.8530.02547.866.833.0 ± 0.2
101.1750.1370.8170.01057.334.135.5 ± 0.1
111.1600.1540.82261.104.635.0 ± 0.3
121.1280.2370.82464.987.034.1 ± 0.4
131.1840.0840.82471.852.535.7 ± 0.2
141.1680.0930.83272.722.735.3 ± 1.8
151.2060.2100.77781.846.236.4 ± 0.1
161.1940.0860.81692.162.536.1 ± 0.1
171.1650.2430.79795.887.235.2 ± 0.2
181.1910.1770.79596.325.236.0 ± 1.9
191.1540.2180.811100.006.434.8 ± 1.2

[45] The analyzed lower plate SD1 sample comes from Acquaformosa (sample CLB8 in Figure 2a). This sample was chosen as it contains well-preserved high-pressure Mg-carpholite-bearing assemblages in textural equilibrium with phengites, pyrophyllite and quartz. Consequently, this sample is suitable for providing an age estimate of the M1-low metamorphic stage. Samples from the upper plate were taken exclusively from the SD2 unit. All the samples come from the Cetraro area (samples CLB4, CLB5 and CLB6 in Figure 2a). Samples CLB 4 and CLB5 were collected within D1-up top-to-the-NE mylonitic horizons with M1-up phengite, chloritoid, chlorite ± albite as main constituents. Sample CLB6 is instead representative of the M2-up retrogressive stage, where early mineralogical associations where replaced by retrogressive products (chlorite and white mica composite associations) that developed along late westward verging shear bands.

[46] As it can be observed on the different age representations (Figure 13), all samples display discordant age spectra that reflect a complex history of mica growth and argon loss in both the upper and lower plate rocks.

Figure 13.

The 40Ar/39Ar age spectra from the study area (see Figure 2a for sample locations). (a) Lower plate; data from the CLB10 sample are after Rossetti et al. [2001]. (b) Upper plate.

6.1. Lower Plate

[47] In the lower plate, high-pressure phengites from the Acquaformosa (SD1 unit) and Gimigliano (LOU [Rossetti et al., 2001]) areas (samples CLB8 and CLB10, respectively in Figure 2a) give comparable age spectra using single grain and bulk separate (Figure 13a). Apparent ages progressively increase from a minimum value close to 20 Ma at low temperature to maximum ages in the range 33–38 Ma obtained for the last heating increments. The total gas age of these two samples is of 33.5 ± 0.1 Ma and 30.8 ± 0.3 Ma for the CLB and CLB10, respectively.

6.2. Upper Plate

[48] The single grain analysis of phengite from samples CLB4, CLB5, and CLB6 give total gas ages of 21.7 ± 0.1 Ma, 22.1 ± 0.2 Ma and 14.9 ± 0.2 Ma, respectively (Figure 13b). The more retrogressed sample (CLB6) displays a plateau age of 16.3 ± 0.2 Ma for about 80% of the 39Ar released. Samples CLB 4 and CLB5 exhibit similar discordant age spectra that regularly increase during step heating from a minimum value of 10–15 Ma up to a maximum value of about 30 Ma. This age is comparable to the maximum ages recorded by the main spectrum portion of the well-preserved high-pressure samples (CLB8 and CLB10) from the lower plate (Figure 13a).

7. Age of Orogenic Metamorphism and Deformation

[49] Maximum ages of 33–38 Ma (Eocene/Oligocene boundary) obtained from the 40Ar/39Ar dating in the lower plate complex (both SD1 unit and LOU) represent a minimum estimate of the crystallization age of the early high-pressure phengites during the early burial stage (M1-low/D1-low). Exhumation of these high-pressure rocks from depth is accompanied by recrystallization of micas under progressively lower P conditions. The discordant age spectra reflect this succession of crystallization events from deep to high crustal levels. The youngest age on the argon release spectra thus provides an upper limit on the age that the samples cooled down ultimately (M2-low/D2-low). This age is bracketed between 22 and 18 Ma (early Miocene). In the upper plate, data from the SD2 unit show a sloped release spectra (Figure 13b), with the oldest age (30–31 Ma) providing a lower limit on the metamorphic climax (M1-up/D1-up), and the youngest age (10–15 Ma) providing, analogously to the lower plate samples, an upper limit for the final cooling of the samples (retrogressive stage M2-up/D2-up).

[50] Radiometric ages concerning the Alpine overprint affecting the Calabride Complex units range between Cretaceous-Paleocene (Rb/Sr method [Borsi and Dubois, 1968; Del Moro et al., 2000]) and Eocene-Oligocene (Rb/Sr method on biotite [Schenk, 1980]; Rb/Sr method on white micas [Bonardi et al., 1987]; 40Ar/39Ar method on biotite [Rossetti et al., 2001]). In north Calabria, such age spanning is likely due to the fact that thermal conditions during Alpine metamorphism were too low (thermal peak < ∼300°C) to induce a complete reopening of the K-Ar isotope system in micas from the Calabride Complex [Rossetti et al., 2001]. Therefore, the Alpine ages recorded by the Rb/Sr, K/Ar or 40Ar/39Ar methods in the Calabride Complex have to be mainly considered as mixed ages. Because of lower closure temperatures, only fission track dates on zircon and apatite can give useful information on the exhumation history of the Calabride Complex. The data reported by Thomson [1994, 1998] sustain a low cooling hypothesis for this group of units, followed by an increase in the cooling rate at ∼30 Ma. In particular, data from the Catena Costiera area document that the main tectonic contacts were completed between 28 and 18 Ma [Thomson, 1998], which is consistent with the 40Ar/39Ar ages presented in this study.

[51] Age of metamorphism and deformation recorded in the UOU is still poorly detailed. Age of deformation of the ophiolite-bearing Liguride Complex of the Calabria-Lucania border region has been previously referred to Cretaceous-Paleocene times based on the depositional age of the metasedimentary rocks surrounding the metamorphic blocks, considered as olistholiths [Spadea, 1982]. However, identification of Eocene faunas in these metasedimentary rocks [Bouillin, 1984] and the evidence that the metamorphic blocks are indeed tectonic slivers [Knott, 1987] imply that deformation and mélange formation of the Liguride Complex occurred in post-Eocene times. In the study area, a post early Oligocene age for the deformation of the UOU can be proposed based on the ∼30 Ma 40Ar/39Ar age obtained for the metamorphism of SD2 unit, included as a tectonic block in the UOU. Such an inference is also in accordance with the Tertiary whole rock K/Ar ages (from about 40 to about 20 Ma) obtained by Beccaluva et al. [1981] from the ophiolitic rocks of north Calabria and Lucania.

[52] Summing up, the 40Ar/39Ar dating of both lower and upper plate rocks attests that orogenic metamorphism in north Calabria occurred during the Tertiary.

8. Discussion

8.1. Structural Interpretation

[53] Based on the revised structural and petrographical scenario, the Alpine nappe architecture of north Calabria can be ascribed to the superimposition of two main tectonostratigraphic complexes (referred to as the upper and lower plate, respectively), which are distinguished on the basis of their structural and metamorphic signature acquired during the orogenic evolution (Figure 2b). In particular, based on the superimposition of structures recognized in the two complexes, we found evidence that eastward directed and westward directed shearing was responsible for nappe construction and reworking, respectively. This is due to the fact that (1) the top-to-the-east shear senses are systematically associated with structures indicating nappe overthrusting and thickening and correspond to the Alpine M1-up metamorphic climax (UOU, SD2 unit and the Calabride Complex) and (2) the top-to-the-west shear senses overprint the top-to-the-east ones in the upper plate nappe stack and correspond to the main Alpine greenschist retrogressive stage (M2-low in Figure 6a), associated with the exhumation of the lower plate HP complex.

[54] Top-to-the-east shearing has been already reported from the various tectonostratigraphic units constituting the nappe edifice of north Calabria and Lucania regions [Dietrich, 1988; Knott, 1987, 1994; Monaco and Tortorici, 1995; Rossetti et al., 2001], providing a well-defined polarity for orogenic transport during nappe stacking. Top-to-the-west shearing is responsible for nappe reorganization, which resulted in juxtaposition of upper and lower plate rocks along a first-order ductile-to-brittle shear discontinuity that everywhere corresponds to an abrupt change in the Alpine orogenic metamorphic conditions (Figure 2b). It must be stressed that these shear directions are the current directions of the linear rock fabrics and that late vertical-axis rotations evidenced by paleomagnetic data have not been restored [e.g., Mattei et al., 2002].

[55] The superimposition of brittle onto ductile top-to-the-west shearing at the top of the lower plate, the gap in P-T metamorphic climax from the upper to the lower plate units, as well as the observation that brittle deformation in the upper plate is extensional in origin and synthetic to the ductile-to-brittle shearing in the lower plate, are typical features of postorogenic extensional settings [e.g., Lister and Davis, 1989; Gautier and Brun, 1994]. All these features indicate that westward shearing corresponds to a major normal-sense displacement along the contact between the two complexes, which therefore represents one or several regional-scale extensional detachment(s). Movement along this major extensional detachment(s) are responsible for the tectonic elision of the early orogenic nappe sequence and operated through reactivation of previous compressional contacts in the upper plate nappe stack, with shear localization along the upper/lower plate tectonic contact. The ductile D2up top-to-the-west shear zones recognized in the ophiolitic rocks exposed in the Fuscaldo-Rose and Sila Piccola areas (included in the UOU in Figures 2b and 3) can be thus interpreted as a part of this major tectonic boundary. Finally, the tectonic slices of Mesozoic carbonates (Ver) lying above the SD2 unit in the Cetraro area (Figures 3a and 3c) can be thus interpreted as extensional sheets, which descend toward the west along low-angle normal faults cutting across the previously structured nappe pile.

8.2. Tectonic Synthesis

[56] Our data set, based on new structural, P-T and age results, provides important constraints to solve the ongoing debate on the tectonic evolution of the Calabrian Arc.

[57] Structural data indicate that the orogenic history of north and central Calabria is poliphased and related to two main deformation phases: (1) early top-to-the-NE D1 nappe stacking, i.e., directed toward the Adriatic (Apennine) foreland, and (2) subsequent top-to-the-west D2 detachment-style of extension, stretching apart the previously structured nappe edifice.

[58] The reconstructed P-T history testifies for a high P/T ratio during the metamorphic climax in the lower plate rocks. The low geothermal gradient (≤12°C/km for the peak conditions) is typical of subduction-related metamorphism [see, e.g., Peacock, 1996], and is similar to the Alpine-type metamorphic gradients associated with the Tethyan subduction [Jolivet et al., 1998b]. This attests that orogenic evolution of north Calabria was experienced under a subduction-accretion setting. The different P-T signatures of the upper and lower plate complexes demonstrate that both groups of units were transported to different depths during the D1–D2 tectonic evolution.

[59] The 39Ar/40Ar study provides a cautionary estimate for the age (1) of the major episode of nappe formation and crustal thickening in north Calabria, which occurred roughly at the Eocene/Oligocene boundary (∼38–33 Ma), and (2) of the onset of the exhumation of the metamorphic units of north Calabria (at about ∼30 Ma), which was completed in early-middle Miocene (∼15–20 Ma). The latter age also constrains the end of the ductile deformation and metamorphism within the nappe sequence.

[60] Field surveys in the Catena Costiera area and surrounding regions document that (1) the ophiolite-bearing sequences (UOU) are a part of an accretionary mélange with strong analogies in terms of lithological associations, metamorphic record and structural setting to what already described by Spadea et al. [1980], Spadea [1982], Knott [1987], and Monaco et al. [1994] as constituting the Mesozoic to Tertiary Liguride Complex of the Calabria-Lucania border [e.g., Bonardi et al., 1988; Monaco and Tortorici, 1995]; (2) the carbonate sequences (Apennine units of Amodio Morelli et al. [1976]) consist of allochthonous units piled up in the upper plate nappe stack; (3) the San Donato unit of Amodio Morelli et al. [1976] corresponds to two separated units, SD2 (Cetraro-Bonifati area) and SD1 (San Donato di Ninea-Mormanno areas), with distinct structural and petrographical signatures and different tectonic position in the north Calabria nappe stack. As last point, it is worth noting that such continental-derived, dominantly metaclastic sequences show analogies in terms of metamorphic signature and stratigraphic characters with the metamorphic successions of the Triassic Verrucano Group exposed in the interior of the north Apennine chain (Tuscan region in Figure 1), which are interpreted as the remnants of the subducted continental passive margin of the Adria plate [Jolivet et al., 1998a, and references therein].

[61] From these considerations derives that (1) the north Calabria nappe edifice has to be no longer considered as an eo-Alpine tectonometamorphic terrane; (2) a possible correlation (in terms of deformation attitude and age) between the ophiolite-bearing units of north Calabria and Lucania regions can be now proposed; (3) north and south Calabria had a common orogenic polarity during crustal thickening and nappe construction, directed toward the Adriatic foreland; and (4) the north Calabria nappe edifice (Catena Costiera area) developed in conjunction with the main thickening phase recorded in the interior of the northern Apennine chain, Eocene to Oligocene in age [Brunet et al., 2000]. All these arguments constrain the tectonic evolution of the Calabrian Arc into the general orogenic evolutionary pattern of the Apennine chain. Consequently, its orogenic evolution has to be referred to the geodynamic setting responsible for the formation of a thick accretionary complex formed at the expense of the Liguro-Piemontaise ocean and located in between the Adriatic and European continental margins [Jolivet et al., 1998a]. The remnants of such a HP/LT orogenic belt are now exposed in the interior of the northern Apennine chain (Tuscan region) and northern Calabria (Figure 1).

[62] The geodynamic setting that better accounts for the P-T-t-deformation histories recorded by these exhumed deep-seated portions of the belt is the continuous Tertiary westward dipping subduction of the Liguro-Piemontaise oceanic domain below the European active margin and the progressive eastward retreat of the Apennine trench [Jolivet et al., 1998a; Faccenna et al., 2001; Rossetti et al., 2002]. Such an assumption is corroborated by the seismic tomographic images of the upper mantle in the Tyrrhenian region that reveal a more than 1000 km long high-velocity, dipping toward the west beneath the south Tyrrhenian basin [Faccenna et al., 2001, 2003]. This geodynamic setting is responsible for orogenic complex formation and crustal thickening, involving first the oceanic domain and, successively, its eastern continental margin belonging to the Adria plate (Figure 14). This twofold history is well documented by the orogenic signature recorded by the tectonic units exposed in north Calabria. Occurrence of polymetamorphic oceanic- and continental-derived tectonic blocks embedded within the Liguride Complex of north Calabria and Lucania, in fact indicates that Eocene-Oligocene deformation reworked an already structured belt of units, derived from deformation and metamorphism linked to the early phases of Alpine convergence during the closure of the Liguro-Piemontaise oceanic domain. No firm age constrain on this early deformation phase is available in Calabria. Nevertheless, data from the extensional conjugate margin of Calabria, i.e., the Corsica-Sardinia block, document that ages of the Alpine blueschist metamorphic climax in the oceanic-derived Schistes Lustrées nappe of Alpine Corsica cluster between 35 and 45 Ma (40Ar/39Ar methods on white micas [Brunet et al., 2000]). This tectonic scenario was dominated by the formation of an active plate boundary along the European continental margin, followed by the retreating subduction of the oceanic lithosphere, accommodated through a doubly verging HP/LT orogenic system since at least Eocene times [Principi and Treves, 1984; Jolivet et al., 1998a] (Figure 14a). Transition from oceanic to continental subduction (Adria continental margin) occurred, as recorded by 40Ar/39Ar dating on the SD1 and SD2 units, approximately at the Eocene-Oligocene boundary and prolonged up to lower Miocene times as documented in the Tuscan region of northern Apennines [Brunet et al., 2000] (Figure 14b). Some units escaped significant burial (SD2 unit and part of the UOU), being off scraped at the orogenic wedge toe, whereas others (SD1 unit and LOU) experienced deep burial and HP/LT metamorphism. Formation of HP/LT metamorphic rocks at depth was followed by their exhumation along nearly isothermal or cooling paths. A continuum accretion of cold units at the base of the orogenic wedge is required to account for (1) the thermal conditions during retrograde metamorphism of the deep-seated rocks and (2) the Argon release spectra recorded by the upper plate SD2 unit, which is typical of slowly cooled samples. Also, the occurrence of rock fragments in sequences that have been subsequently involved in compressional deformation (ophiolite-bearing tectonic mélange) suggests exhumation while convergent tectonic motion was continuing. A synorogenic exhumation process is thus proposed as responsible for the early exhumation path of the oceanic- and, successively, continental-derived HP/LT rocks within the accretionary complex (Figures 14a–14b). Subduction of the buoyant continental rocks of the Adria margin induced nappe restacking within the previously structured orogenic complex. Formation of the ophiolitic-bearing mélange of north Calabria and Lucania regions [Spadea et al., 1980; Knott, 1987; Monaco and Tortorici, 1995] is the main effect of this changed regime of subduction. A model of return flow in mélange channels [e.g., Cloos, 1982] and/or in the lower corner of the subduction complex [Pavlis and Bruhn, 1983] was probably responsible for the exhumation of the deep-seated rocks and their progressive incorporation in the upper plate nappe stack. Eastward directed thrusting of the Alpine group rocks of the Calabride Complex likely occurred at this stage, in consequence of the docking of the active margin induced by the arrival of continental material in the subduction zone (Figure 14b). In the Lucania region, the closure of the oceanic domain occurred later on, at the Oligocene-Miocene boundary [Di Staso and Giardino, 2002], attesting for the complex paleogeographic configuration of the ocean-continent transition.

Figure 14.

Interpretative tectonic model for the orogenic evolution of Calabria framed within the evolution of the inner Apennine chain (not to scale; location of structures is only indicative).

[63] Since 30 Ma, there occurred the onset of calcalkaline volcanism on the overriding European plate erupting along the Sardinian and Provençal margins [Beccaluva et al., 1989] and back arc extension linked to the opening of the Liguro-Provençal basin [Séranne, 1999; Faccenna et al., 2001] (Figure 14b). This time also corresponded to the transition from compression to extension in Alpine Corsica, as documented by the activation of the extensional top-to-the-east east Tenda shear zone [Brunet et al., 2000] (Figure 14c). In Calabria, increasing rates of erosion linked to increasing rates of exhumation of the Calabride Complex have been documented since about 30 Ma [Thomson, 1994]. We argue that this major erosion phase took place possibly in conjunction with activation of westward directed extensional shearing responsible for the late-stage exhumation history of the lower plate rocks and for the upper/lower plate juxtaposition (Figure 14c). Activation of westward directed ductile-to-brittle shearing recorded in the north Calabria nappe pile is thus here interpreted as the response to the transition from orogenic complex formation to back arc extension in the interior of the Apennine chain. Extensional detachment tectonics also provided an efficient mechanism to decouple the frontal (compressional) from the inner (extensional) portions of the orogenic wedge during ongoing subduction (Figure 14c). Westward directed shearing appears to have culminated at the end of the Burdigalian as suggested by the plateau date of 16.3 Ma recorded by sample CLB6 (Figure 13b). This age constraint well fits with the last cooling episode recorded by apatite thermochronology on the Calabride Complex [Thomson, 1994, 1998]. Synrift deposits linked to the Tyrrhenian back arc extension are Serravallian-Tortonian (15–11 Ma) in age [Mattei et al., 2002], providing further support to constrain age of postorogenic extensional tectonism in Calabria (Figure 14d).

[64] This tectonic reconstruction suggests that back arc extension induced by slab retreat of the Apennine subduction played a key role in the orogenic wedge dynamics and evolution. Indeed, a fast acceleration of the slab retreat component occurred starting from about 30 Ma [Jolivet and Faccenna, 2000] and it might have significantly incremented the amount of extensional processes in back arc region, culminating first with the Liguro-Provençal and, successively, with the Tyrrhenian back arc basin opening. This acceleration of extension is documented by the activation of the ductile-to-brittle extensional detachment systems in Alpine Corsica and Calabria. The progressive eastward migration of the Apennine subduction front is thus here considered as the main controlling factor of the orogenic wedge dynamics and evolution, inducing the progressive eastward migration (i.e., directed toward the Adriatic foreland) of the extension-compression pair throughout the Apennine orogen.

9. Concluding Remarks

[65] This study provides evidence that north Calabria consists of a Tertiary nappe stack, resulting from superimposed top-to-the-west extensional shearing onto a previously eastward structured compressional belt. Westward directed extensional shearing evolved from ductile-to-brittle deformation conditions and operated by means of detachment tectonics. Recognition of detachment style of extension in Calabria implies that different tectonostratigraphic units piled up in the nappe stack have to be regarded as extensional allochthons, rather than thrust-bounded units. Tectonic models considering the kinematic signature of the Calabria nappe stack as exclusively derived by overprinting “Apennine” (eastward directed) onto early “Alpine” (westward directed) compressional shear systems are consequently inadequate.

[66] The revised tectonometamorphic scenario can be adequately framed within the orogenic processes leading to the formation and subsequent collapse of the Apennine accretionary wedge. This tectonic reconstruction assumes a continuous westward dipping Tertiary subduction process to which corresponded the formation of a thick orogenic belt (synorogenic history) and the diachronous back arc opening of the Liguro-Provençal and Tyrrhenian basins (postorogenic history).

Appendix A:: The 40Ar/39Ar Dating Techniques and Analytical Details

[67] For analysis of bulk separates, about 100 mg of micas have been extracted from powdered rocks and irradiated with age monitors in the Grenoble nuclear reactor in France. The procedure for step heating of mineral bulk separates is described by Monié et al. [1994]. Data are presented as age spectra, where individual ages, given with 2σ uncertainty, are reported against the cumulative percentage of released 39Ar. Laser probe experiments were carried out on single grains using an instrumental device that includes a MAP 215–50 noble gas mass spectrometer and a continuous 6W argon-ion laser operating in multimode. Samples have been irradiated for 70 hours in the Mc Master nuclear facility, together with several international flux monitors including MMHb-1 at 520.4 ± 1.7 Ma and HD-B1 at 24.21 ± 0.32 Ma. For laser step heating, the beam was defocused in order to get a diameter at least twice the size of the mineral being dated. The duration of heating for each step is typically 30 seconds, followed by 5 minutes of gas cleaning and 15 minutes of isotopic measurement through 8 runs on 36Ar to 40Ar. Blanks were monitored every three experiments and were in the range of 3 × 10−12 cc for m/z = 40, 3 × 10−14 cc for m/z = 39 and 38.2 × 10−13 cc for m/z = 37 and 1 × 10−13 cc for m/z = 36. Results were corrected for blanks, mass discrimination, radioactive decay of 39Ar and 37Ar, and neutron-induced interference reactions with Ca, K, and Cl. Ages were calculated according to MacDougall and Harrison [1988].


[68] This contribution benefited from discussions with M. Mattei and F. Cifelli. F. Cifelli also partly participated in the fieldwork. S. Lo Mastro performed X-Ray diffractometry on metapelite samples. We thank R. Funiciello for the continuous encouragement and advices. The authors thank M. Serracino for help during microprobe analyses. Revisions by G. Prosser and two anonymous reviewers significantly improved the paper. Financial support provided by a CNR grant (CNRG0018AE/CNR-Agenzia2000 to FR).