Rend. Fis. Acc. Lincei s. 9, v. 9:271-291 (1998)
Geologia. — The Liguride units of southern Lucania (Italy ): structural evolution and exhumation of high-pressure metamorphic rocks. Nota (*) di Stefano Mazzoli, presentata dal Corrisp. F. C. Wezel.
Abstract. — The Liguride units of southern Italy represent the remnants of a subduction complex including Mesozoic to Paleogene rocks from the Neotethyan oceanic domain. In southern Lucania, a tectonic contact separates relatively high-pressure (blueschist-facies) rocks in the hanging wall from low-pressure (unmetamorphosed) Liguride rocks in the footwall. Complex deformation and polyphase metamorphism record both underplating and subsequent exhumation of the hanging wall unit. Structural, petrological and biostratigraphic evidence suggest that exhumation of high-pressure rocks occurred during early Miocene overthrusting of the Liguride accretionary wedge onto the southern continental margin of Neotethys, and was most probably strongly controlled by the inherited architecture of the rifted continental margin. Key words: Ophiolites; Blueschists; Accretionary wedge; Continental margin architecture. Riassunto. — Le Liguridi in Lucania meridionale: evoluzione strutturale ed esumazione di rocce metamorfiche di alta pressione. Le unit`a Liguridi in Appennino meridionale includono i resti di un prisma di accrezione formato da rocce mesozoico-paleogeniche derivanti dal dominio oceanico della Neotetide. In Lucania meridionale, un contatto tettonico principale separa un’unit`a metamorfica (di relativa alta pressione), al tetto, da una sottostante unit`a Liguride non metamorfica. I processi di subduzione e successiva esumazione dell’unit`a metamorfica sono registrati nella complessa storia deformativa e nel metamorfismo polifasico che essa mostra. Sulla base dei dati strutturali, petrologici e biostratigrafici disponibili, e` possibile ascrivere l’esumazione delle rocce di relativa alta pressione alla fase di messa in posto del prisma di accrezione sul margine continentale meridionale della Tetide (avvenuta durante il Miocene Inferiore in quest’area). La geometria dello scollamento basale ed il conseguente comportamento del cuneo orogenico durante tale processo erano probabilmente controllati dall’architettura del margine continentale, caratterizzata dalla presenza di faglie normali associate al rifting.
Introduction Different mechanisms have been proposed to explain the occurrence, within orogenic belts, of rocks containing mineral assemblages indicative of high P-T ratio metamorphism (e.g. Platt, 1993, and references therein). The application of «critical wedge» concepts (Chapple, 1978; Davis et al., 1983; Stockmal, 1983; Dahlen et al., 1984) to accretionary complexes and orogenic wedges provided a simple and elegant mechanism for the exhumation of relatively high-pressure, low-temperature metamorphic rocks in convergent tectonic settings. Since the work of Platt (1986), a model of tectonic exhumation driven by extension within a subcritical triangular wedge of uniform taper has been widely applied to many orogenic belts around the world, including the southern Italian Apennines (Knott, 1994) (fig. 1a). In this paper, the geological constraints on the timing and mechanisms of exhumation of relatively high-pressure, low-temperature (*) Pervenuta in forma definitiva all’Accademia il 16 settembre 1998.
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metamorphic rocks exposed in southern Lucania (fig. 1b) are discussed with the aims of: (i) defining the timing of subduction, underplating, high P-T ratio metamorphism, and subsequent exhumation of the high-pressure rocks; (ii) checking the validity of the synconvergent extension model (Platt, 1986) applied by Knott (1994) to this area; and (iii) discuss a possible alternative mechanism for tectonic exhumation of the high-pressure rocks in southern Lucania. Geological setting The Liguride units of southern Italy (refer to fig. 1) are made of Upper Jurassic to Upper Oligocene sedimentary successions incorporating ophiolitic suites and including both metamorphic and non-metamorphic to very low-grade rocks (Cello and Mazzoli, 1998, and references therein). As a whole, the Liguride units have been interpreted as representing the remnants of an accretionary wedge incorporating rocks from the Neotethyan oceanic domain (Ogniben, 1969; Knott, 1987; Monaco and Tortorici, 1995). The Liguride units are tectonically overlain by the Calabride units (refer to fig. 1b), which are made up of crystalline rocks that were formerly considered of Austroalpine (i.e., south Tethyan) affinity (Haccard et al., 1972; Alvarez, 1976; Amodio-Morelli et al., 1976), but have been reinterpreted in more recent studies as remnants of the northern continental margin of Neotethys (European-Iberian plates; e.g. Bouillin et al., 1986; Dietrich, 1988; Cello et al., 1995, and references therein). The thrust sheets underlying the Liguride units (fig. 2) were derived from the deformation of the Mesozoic-Paleogene southern continental margin of Neotethys, mainly including carbonate platform and pelagic basin domains (e.g. Cello et al., 1990; Roure et al., 1991; Cello and Mazzoli, 1998, and references therein). The terrains belonging to the Liguride units can be subdivided into two main groups, each of them belonging to a distinct tectono-metamorphic unit (Cello and Mazzoli, 1998). The first one, consisting of metamorphosed ophiolite-bearing rocks and informally referred to as metamorphosed terrain (MT), mostly corresponds to the Frido Unit of Amodio-Morelli et al. (1976). The second one, made of unmetamorphosed ophiolitic units, is referred to as unmetamorphosed terrain (UT); it mostly corresponds to the North-Calabrian Unit of Bonardi et al. (1988) and to the Calabro-Lucano Flysch Unit as defined by Monaco et al. (1991). Stratigraphy Metamorphosed terrain (MT ). As a whole, the MT is composed of polydeformed and polymetamorphosed terrains, with associated blocks and tectonic slices of mantle material and of both oceanic and continental basement rocks. The bulk of the MT consists of grey and black phyllites containing intercalations of meta-arenites, meta-siltites, quartzites and aragonitebearing (Spadea, 1976, 1982) meta-limestones (lower and upper phyllitic members, and
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quartzarenitic member of the Frido Formation; Vezzani, 1969), and of grey metalimestones with a marked planar anisotropy (transposition foliation) including rare levels of marbles, greenish quartzites and phyllites (calcareo-scistoso member of the Frido Formation; Vezzani, 1969). Biostratigraphic data on the metasediments of the MT indicate an age not older than Early Cretaceous, and as young as late Chattian (Bonardi et al., 1993). Serpentinite bodies included within the MT are composed of black-greenish, cataclastic, serpentinised peridotites, characterized by numerous shear zones and containing several blocks of continental crystalline rocks and marbles. The continental crystalline rocks, commonly referred to the Dioritico-Kinzigitica Formation of Calabria (cf. Ogniben, 1969), are mainly composed of granulite-facies garnet gneisses and biotite gneisses, and subordinately of amphibolites and pegmatites. In some instances, amphibolites are cross-cut by 20-30 cm-thick basic dikes composed of aphyric or porphyric green mafic rocks (Spadea, 1982). Ophiolitic rocks are represented by bodies of metabasites, of metres to tens of metres size, outcropping at several levels within the succession (Lanzafame et al., 1979a; Spadea, 1979). At a few localities, the original meta-sedimentary cover to the metabasites is also preserved (Lanzafame et al., 1979b). Metabasite bodies are observed also within the serpentinites, where they commonly show rodingitic margins. All these rocks, ultramafics, granulites, amphibolites, and metabasites, have recently been interpreted as tectonic slices emplaced during the growth and evolution of the Liguride accretionary wedge (Knott, 1987, 1994; Monaco and Tortorici, 1994, 1995). Unmetamorphosed terrain (UT ). The unmetamorphosed terrain (UT) which, according to Monaco and Tortorici (1995), can be considered a «broken formation» related to subduction-accretion processes (Hsu, 1968), consists of an unmetamorphosed succession containing tectonically included blocks. Included within a pelitic-calcareous-arenaceous succession, several lithotypes can be recognised: ophiolitic suites with their pelagic sedimentary cover (Timpa delle Murgie Fm), terrigenous sediments (Crete Nere Fm), cherty limestones and volcaniclastic turbidites containing andesitic detritus. The pelitic-calcareous-arenaceous succession of Monaco and Tortorici (1995) corresponds to the non-metamorphosed part of the «Flysch argillitico-quarzoso-calcareo» of Selli (1962) and to part of the Frido Formation and of the Crete Nere Formation of Vezzani (1968b, 1969). It forms the matrix of the UT, where fragments and blocks belonging to other successions are included. This terrigenous succession, characterized by thick calcareous and arenaceous layers, has been referred to a trench environment proximal to a continental margin (Monaco and Tortorici, 1995) based on the composition of the arenites, which can be classified as quartzarenites and subarkoses related to cratonic sources (Critelli and Monaco, 1993). The age of the succession is referred to the Lower Cretaceous by Vezzani (1968a, 1969), but it reaches at least the Late Cretaceous according to Monaco and Tortorici (1994).
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Within the UT, variable sized bodies of ophiolitic rocks (sometimes showing remnants of their original sedimentary cover) can be observed. The relationships between ophiolite blocks and adjacent sediments have been the subject of several papers which propose different interpretations (Vezzani, 1968a; Ogniben, 1969; Bousquet, 1962, 1973; Dietrich and Scandone, 1972; Amodio-Morelli et al., 1976; Lanzafame et al., 1978, 1979a, b; Spadea, 1968, 1982; Bonardi et al., 1988). Also in this instance, recent works interpreted these rocks as tectonically emplaced slices within the Liguride accretionary complex (Knott, 1987, 1994; Monaco and Tortorici, 1994, 1995). All the authors, however, agree with the fact that these ophiolites represent remnants of the Jurassic Tethyan oceanic crust, therefore originally constituting the base of the whole UT succession. The ophiolite suite of Timpa delle Murgie is made up of several metres of pillow lavas and pillow breccia basalts (Bousquet, 1962; Vezzani, 1968a; Lanzafame et al., 1978; Knott, 1987). The remnants of the oceanic sedimentary cover, resting in its original stratigraphic position over the ophiolites, consist of a few metres of siliceous shales, varicoloured radiolarites, marly limestones and red and green shales (Timpa delle Murgie Fm), dated to the Oxfordian (Marcucci et al., 1987), and of pink nodular marly limestones of Kimmerdigian age, which are overlain by red and green silty shales containing thin quartzarenites and, finally, by dark grey shales which grade to the thick pelitic-calcareous-arenaceous succession described above (Monaco and Tortorici, 1995). Bodies of dark shales, a few tens of metres to a few kilometres in size, interpreted by Monaco and Tortorici (1995) as tectonically included within the UT, are observed at several localities. These deposits, corresponding to the Crete Nere Fm (Selli, 1962), are made up of black-bluish shales alternated with siltites and very fine grained quartzarenites. This succession was ascribed to the Aptian-Albian by Vezzani (1968b) and to Middle Eocene by Bonardi et al. (1988). Volcaniclastic rocks of calcalcaline affinity are also included within the pelitic-calcareous-arenaceous succession (Lanzafame et al., 1977; Monaco and Tortorici, 1994). These deposits are made up of shales, marly limestones, micaceous arenites and tuffaceous arenites. The arenites are represented by both volcanoarenites and quartz-lithic turbidites. The composition of the arenites suggests that sediment supply was provided by unroofing of an ophiolite-bearing subduction complex (Monaco and Tortorici, 1995), and that a coeval calcalcaline volcanism characterized the area (Lanzafame et al., 1977). These volcaniclastic sediments have been ascribed to the lower part of the Late Oligocene, based on the analysis of calcareous nannoplankton contained within marly interbeds (Critelli and Monaco, 1993). Metre-sized blocks of andesitic and rhyodacitic lavas are also contained within the pelitic-calcareous-arenaceous succession (Monaco and Tortorici, 1994). Saraceno Formation. Unconformably overlying the UT, the Saraceno Formation (Selli, 1962) consists, in the lower part, of an alternation of prevailing calcarenites and calcilutites, with chert lenses and pelitic interbeds. Upwards, a progressive decrease of chert occurs, together
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with an increasing frequence of siliciclastic arenites and microconglomerate intercalations within an alternation of arenaceous calcarenites and silty clays. The arenites show a variable composition, from lithic arenites to feldspar-litharenites with rock fragments made of phyllites, extrabasinal carbonates, cherts, andesitic-dacitic volcanites and ophiolite fragments (Monaco and Tortorici, 1995). Recent biostratigraphic data refer the Saraceno Formation to the Upper EoceneUpper Oligocene (Bonardi et al., 1988). According to Monaco and Tortorici (1995), however, the Saraceno Formation could most probably be ascribed to the Upper Oligocene-Lower Miocene taking into account: (i) the occurrence of Late Oligocene faunas within the volcaniclastic levels of the stratigraphically underlying UT (see above), and (ii) the fact that the Saraceno Formation passes upward, with a stratigraphic contact, to the Albidona Formation, whose lower levels have been dated as middle Burdigalian by Bonardi et al. (1985). Albidona Formation. The Albidona Formation (Selli, 1962), stratigraphically overlying the Saraceno Formation, is characterized by a turbiditic arenaceous-pelitic succession which also includes calcareous marls and siliciclastic conglomerate levels. The lower portion of the formation is made up of an arenaceous-pelitic alternation with megastrata (up to 30 m-thick) of marly calcilutites and channell-filling, matrix-rich conglomerates showing erosional bases. The middle portion of the succession is characterized by pelitic-arenaceous turbidites showing Bouma sequences (lacking the lower intervals), and associated slump levels. The upper portion of the succession represents a turbiditic sub-system showing a low-angle unconformity, and locally an erosional contact, onto the underlying distal deposits (Monaco and Tortorici, 1994). It begins with an arenaceous-pelitic succession (characterized by a ratio A/P>>1), referred to a depositional lobe environment, which passes upward to a pelitic-arenaceous sequence with frequent intercalations of arenaceous-conglomerate megastrata, referred to an inner submarine fan environment. At the base of the strata, groove casts and flute marks are commonly observed. The arenaceous layers are made up of lithic arenites in the lower part of the succession and of arkosic arenites in the higher part (Monaco and Tortorici, 1995). The conglomerates contain clasts made up of granitoid rocks, gneisses, phyllites, ophiolites and metaophiolites, and dacitic volcanic rocks (Lanzafame et al., 1977), as well as arenite fragments belonging to the Saraceno Formation and to the Albidona Formation itself. The age of the Albidona Fm has been referred to the middle Burdigalian-Langhian (Bonardi et al., 1985). However, it has to be noted that similar deposits outcropping in the Cilento area (Pollica and San Mauro Fms) have recently been given a younger age, ranging from the Langhian to the lower Tortonian (Russo et al., 1995). Metamorphism Within the Liguride units, the rocks belonging to the MT have undergone a complex metamorphic history (e.g. Monaco and Tortorici, 1995, and references therein).
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The mineralogical association of ultramafic rocks, in which a harzburgitic protolith containing coarse grained spinel can be recognised, is made up of olivine, orthopyroxene, garnet, antigorite, lizardite, spinel, magnetite and rare chlorite (Monaco and Tortorici, 1995). Rodingitisation within serpentinite blocks occurred in two different events (Spadea, 1982). The first event (characterized by crystallization of diopside and garnet) probably took place in an oceanic environment, whereas the second event, characterized by higher pressure conditions (with pumpellyte as a major phase), was probably related to deformation within a subduction complex (Spadea, 1982). The metabasites commonly show their original magmatic texture made of plagioclase crystals (mostly substituted by albite and sericite) contained within a fine grained matrix of chlorite, white mica, albite and titanite (Monaco and Tortorici, 1995). Petrographic analysis has evidenced that most metabasites contained within the MT have undergone spilitisation processes related to early ocean-floor metamorphism (Spadea, 1979; Beccaluva et al., 1982). A high P/T-ratio metamorphic overprint is marked in these rocks by the occurrence of well preserved mineral assemblages. Two groups of high pressure rocks have been recognized by Beccaluva et al. (1982). A first group includes both metabasalts and metasediments containing albite, chlorite, lawsonite, epidote, pumpellyte, sphene and rare blue amphibole and Na-pyroxenes in the metabasalts. This group belongs to the lawsonite-albite-chlorite facies of Turner (1981), for which fluid pressures must have been between 3 and 4 kb and temperatures in the range of 250-350 ◦ C. A second group of high pressure rocks, belonging to the blueschist-facies of Turner (1981), comprises metabasalts containing lawsonite, glaucophane, crossite, sodic pyroxenes, albite, phengite, pumpellyte and sphene. Physical conditions for this (HP/LT) event were of fluid pressures in the range of 6-8 Kb and temperatures of 350 ± 50 ◦ C. The mineral assemblages and textures of both these groups of rocks are thererefore indicative of a polyphase high P/T-ratio metamorphism (fig. 3). The latter is overprinted by a lower pressure (greenschists facies) metamorphic event which is marked by mineral assemblages including chlorite, albite and epidote (Beccaluva et al., 1982). Continental crust crystalline rocks, mainly consisting of granulites showing an original mineralogical association made up of garnet, plagioclase, quartz, biotite, sillimanite and rutile, have undergone intense retrograde metamorphism (Monaco and Tortorici, 1995). These rocks locally show a HP/LT event metamorphic overprint, marked by the development of aragonite and lawsonite (Quitzow, 1935). Their petrogenetic evolution is therefore characterized by high-grade (granulite facies) and medium-grade (amphibolite facies) pre-alpine metamorphic events, followed by alpine HP/LT metamorphism and a subsequent greenschists facies overprint (Scandone, 1982). The metalimestones show variable grain sizes (from medium to fine) and carbonates/silicates ratios. Their mineralogical association consists of calcite, quartz, albite, white mica and chlorite. The widespread occurrence of aragonite (Spadea, 1976, 1982; Lanzafame et al., 1979b; Monaco and Tortorici, 1994) suggests a metamorphic evolution similar to that of the metabasites.
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T (°C) Fig. 3. – P-T Diagram (after Knott, 1994) showing the approximate position of blueschist-facies metamorphism (M1) and greenschist facies overprint (M2) for the metamorphosed terrain (MLT). P-T conditions for the unmetamorphosed Liguride terrain (ULT) are also shown.
Structural analysis The regional structure of the Liguride units in southern Lucania is characterized by a major overthrust contact between the MT, in the hanging wall, and the UT, in the footwall (Monaco and Tortorici, 1994, 1995). Consistent internal deformation of the two main units above (associated with polyphase metamorphism in the MT) occurred prior to their tectonic superposition, as shown by the analysis of mesoscopic structures from the study area of fig. 1b. It has to be noted that brittle features consisting of faults, cataclastic zones, and related minor fractures and extension veins affect all the tectono-stratigraphic units outcropping in the Calabria-Lucania borderland area (Cello and Mazzoli, 1998, and references therein). The analysis of these structures is not included in the following sections, as it does not form the subject of the present study. Metamorphosed terrain (MT ). The structurally highest unit of the regional imbricate structure characterizing the Calabria-Lucania borderland consists of HP/LT rocks of the metamorphosed terrain (MT). The complete record of the structural features recognized in the metasediments belonging to the MT includes: - metric to centimetric, generally asymmetric, rootless, intrafolial tight to isoclinal folds (F1 ) affecting the original sedimentary layering (S0 ). These are the earliest struc-
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Fig. 4. – Orientation data (lower hemisphere, equal-area projections). a) First- and second-generation (undifferentiated) mesoscopic fold hinges in MLT. b) Poles to transposition foliation in MLT. c) Thirdgeneration mesoscopic fold hinges in MLT. d ) Poles to bedding in ULT. e) First-generation (circles) and second-generation (triangles) mesoscopic fold hinges in ULT. f ) Poles to bedding in Saraceno Fm. g ) First-generation (dots) and second-generation (triangles) mesoscopic fold hinges in Saraceno Fm. h) Poles to bedding in Albidona Fm. i) Mesoscopic fold hinges in Albidona Fm.
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tures recognized in the area; most of the folds show horizontal to moderately plunging, WNW-ESE trending axes (fig. 4a). Fold shape generally alternates between class 1C, in the more competent layers, and class 3 in the less competent ones (Ramsay, 1967); - a pervasive schistosity (S1 ), roughly axial planar to the folds described above, mostly flat lying to moderately dipping (fig. 4b); due to the strong transposition related to this schistosity, the folds above are often preserved as isolated fold closures; - a discontinuous mineral stretching lineation (L1 ) associated to the schistosity surfaces; the stretching direction is also marked locally by calcite pressure fringes adjacent to framboidal pyrites, and by mineral fibres growth within coeval extension veins oriented at a high angle with respect to both schistosity surfaces and stretching lineation. The orientation pattern of this linear fabric element shows a data spread around a mean SSW-NNE orientation (Knott, 1994); - mostly WNW-ESE trending (refer to fig. 4a), centimetric, tight to isoclinal asymmetric folds (F2 ) affecting the main schistosity with sub-horizontal to moderately inclined axial surfaces; - extension crenulation cleavages (or shear bands) deflecting the main schistosity, indicating a top-to-the-NE sense of shear; - NW-SE trending, open to tight, mainly asymmetric and overturned folds (F3 ) showing sub-horizontal to moderately plunging axes (fig. 4c) and a few to hundreds of metres wavelengths. Mesoscopic folds are of symmetric (m) type in the hinge zone of larger structures, whereas they display typical s and z asymmetries on the limbs. Fold geometry ranges from class 1 to class 3 of Ramsay (1967); competent layers have angular to rounded hinges and show class 1B or class 1C shapes, whereas less competent layers have angular to rounded hinges and typically exhibit class 3 geometries; - crenulation microfolds related to F3 structures. These define a roughly NW trending crenulation lineation on S1 schistosity surfaces. Crenulation cleavage surfaces, approximately axial planar to F3 mesoscopic folds, are only rarely developed. Unmetamorphosed terrain (UT ). The orientation pattern of data relative to bedding (S0 ) from the UT displays a wide range of attitudes (fig. 4d ), probably as a result of superposed deformation. The oldest structural features characterizing the sedimentary succession of the UT consist mainly of N-S to NW-SE trending minor thrusts and backthrusts at a low-angle to bedding, and by intrastratal duplexes. Roughly NW-SE trending (fig. 4e) tight to isoclinal folds (F1 ), showing class 1C fold geometries in the more competent layers and class 3 geometries in the less competent beds, overprint the earlier thrusts. Associated to these folds, a spaced disjunctive cleavage (in the competent layers) and a slaty cleavage (in the pelitic lithologies) are observed. The latter fabric is locally deformed by roughly NW-SE trending mesoscopic folds (F2 ) and crenulations (refer to fig. 4e).
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Saraceno Formation. The orientation pattern of bedding (S0 ) in the Saraceno Fm shows a wide range of attitudes (fig. 4f ), also in this instance probably as a result of superposed deformation. Mesoscopic folds (F1 ) consist of isoclines showing alternating class 1C shapes and convergent cleavage fans in competent beds, and class 3 geometries and divergent cleavage fans in incompetent ones. These structures are characterized by variably plunging fold axes and by axial surfaces showing a wide range of attitudes, as a result of refolding about mainly NW-SE trending axes (fig. 4g ). Structures belonging to this second generation of folds (F2 ) include mainly asymmetric, both NE- and SW-vergent open to tight folds, with sharp (chevron folds and kink bands) to rounded hinges. Mesoscopic NW-SE striking thrusts and backthrusts, associated with second-generation folds, are also common in the Saraceno Fm. Albidona Formation. Orientation data relative to bedding (S0 ) in the Albidona Fm is shown in fig. 4h. The internal deformation of the Albidona Fm is mostly characterized by NW striking thrusts and backthrusts, and associated folds (F1 ). The latter consist of both NE- and SW-vergent structures of tens of metres wavelengths, and of roughly NW-SE trending (fig. 4i) parasitic folds of a few metres wavelengths, showing typical m, s and z geometries. Class 1C folds characterize more competent beds, whereas class 3 shapes occur in the less competent ones. Structural interpretation Based on the structural data illustrated above, a polyphase deformation history has been unravelled in the Liguride units (and unconformably overlying deposits) of southern Lucania. The structural assemblage developed in MT rocks during the first phase of deformation includes: (i) mostly NW trending, tight to isoclinal intrafolial folds affecting the original layering; (ii) a pervasive schistosity affecting mainly metasedimentary cover rocks; (iii) a mostly SSW plunging mineral lineation; (iv) millimetre-thick veins cutting at a high angle across the schistosity and oriented roughly perpendicular to the mean stretching lineation. Kinematic indicators observed in metasediments of the MT consist of NE dipping shear bands deflecting the main schistosity. The occurrence of high-pressure mineral assemblages associated with the main schistosity indicates that this deformation was associated with high P/T ratio metamorphism. This structural assemblage can be interpreted as a result of progressive non-coaxial deformation related to deep-seated, top-to-the-NE (in present-day coordinates) shearing and underplating of oceanic-derived material. Continuous deformation led to isoclinal folding and to the formation of a pervasive schistosity with an associated stretching lineation, and eventually to the development of shear bands. Extension veins probably developed throughout the evolution of the shear zone and were shortened by buckling and minor contrac-
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tional faulting as they entered the contractional field of the incremental strain ellipsoid (Ramsay, 1967). In the unmetamorphosed terrain (UT), first-phase structures (mainly thrust faults and thrust-related folds) indicate that the P-T conditions of deformation and the resulting structural pattern are completely unrelated to those characterizing MT rocks. A second phase of deformation (D2) recognized in MT rocks was responsible for the development of mostly NW trending, tight to isoclinal folds refolding the D1 isoclines and the related schistosity. An overprinting metamorphic event, with P-T values that are typical of lower greenschists facies conditions (refer to fig. 3), is associated with this deformation phase in MT rocks. Second-phase structures in UT rocks also consist of mostly NW trending, tight to isoclinal folds, folding D1-related thrust surfaces. D2 structures affecting the UT also involve the lowermost part (Saraceno Fm) of the unconformable synorogenic deposits, as shown by congruent folding of the latter together with its non-metamorphic Liguride substratum. Third-phase (D3) structures affect both metamorphic (MT) and non-metamorphic (UT) units. They include mostly NW trending, open to tight folds with variably inclined axial surfaces (these folds are locally associated with mesoscopic reverse shear planes), and a roughly NW trending crenulation lineation with an associated discontinuos crenulation cleavage mostly confined to pelitic rocks. These structures also affect the unconformably overlying deposits of the Saraceno and Albidona Fms, where NE and SW vergent thrusts and associated asymmetric, open to tight folds and kink bands occur. Tectonic evolution Based on the stratigraphic, petrologic, and structural constraints exposed above, the structural evolution for the Liguride units of southern Lucania can be envisaged as follows. The first phases of deformation took place during subduction of the Neotethyan oceanic lithosphere. Within the deep (underplated) part of the Liguride accretionary complex (MT), P-T conditions of deformation reached those of the blueschist facies and the resulting structural assemblage consists of tight to isoclinal intrafolial folds associated with a pervasive schistosity and a mineral lineation. Kinematic indicators suggest that these subduction-related structures are NE-vergent in present-day coordinates (Knott, 1987, 1994; Monaco and Tortorici, 1995). At higher crustal levels (i.e., within the upper part of the accretionary prism; UT), more brittle features (i.e., low-angle thrust faults) were produced as a result of frontal accretion. Subduction-accretion processes were also probably responsible for the tectonic inclusion of different blocks of oceanic basement (pillow basalts, gabbros) and upper mantle (serpentinised peridotites) material which occur, together with continental basement slices (granulites and subordinate amphibolites), within the Liguride units. The presence of tectonic slices of continental crust included within the Liguride accretionary wedge is a highly debated topic in Apennine
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geology. Cello and Mazzoli (1998) suggested that these blocks of continental material were part of the lower-middle crust of a pre-Tethyan continent which were exhumed, together with a portion of the underlying upper mantle, as a result of extensional tectonics during the proto-Tethyan rifting phase. Subsequent subduction-accretion processes would have led, therefore, to the tectonic inclusion into the accretionary complex of mantle material and associated, extension-related, lower-middle continental crust blocks. Within the study area in southern Lucania, subduction, underplating, and high P/T ratio metamorphism occurred, at least in part, after the late Oligocene, as shown by the occurrence of late Chattian limestones (Bonardi et al., 1993) within the sedimentary succession belonging to the MT. Subsequent (D2) structures recorded in the MT include tight to isoclinal folds folding the main schistosity, associated with a retrograde greenschists facies metamorphic event. The overall metamorphic and structural signature of Liguride rocks, therefore, suggest that D2 structures are related to uplift of a deep-seated part of the accretionary wedge. D3 structures, formed at shallow (non-metamorphic) conditions, are most probably coeval with the onset of accretion of the carbonates of the Apulian continental margin, following ophiolite obduction (Cello and Mazzoli, 1998). As D3 structures also affect the strata of the Albidona Formation, the age of their development must be middle Miocene or younger. Younger brittle deformation is mostly related to the activation, in Pliocene or Early Pleistocene times, of strike-slip faulting following the onset of continental subduction (e.g. Knott and Turco, 1991; Monaco and Tortorici, 1994, and references therein). Exhumation of high-pressure metamorphic rocks The exhumation of blueschist-facies rocks in southern Lucania has recently been explained, according to the well-known Platt’s (1986) model, as a result of pre-collisional extensional faulting within a subcritical triangular wedge of uniform taper (Knott, 1994). In view of the data illustrated above, however, major problems arise with this interpretation, mainly concerning the timing of tectonic exhumation, which would have occurred in Late Paleogene times according to Knott (1994), and also concerning the structural geometry, as in Platt’s (1986) model, relatively high-pressure rocks should be exposed in the footwall of an extensional detachment carrying lower-pressure rocks in the hanging wall («normal-sense» pressure-break contact; Wheeler and Butler, 1994). In the following sections, available timing and structural constraints are discussed in order to check the applicability of two different exhumation models (fig. 5) to southern Lucania. Time constraints. Despite the polyphase metamorphism, the aragonite-bearing metalimestones of the MT preserve nannofossil assemblages reaching a late Chattian age (Bonardi et al., 1993).
τb
α b
ng sla subducti
θ
g slab subductin
Particle path
Extension
Underplating
Moho
Crustal ramp of rifted margin
Erosion
Weak wedge with geometry controlled by architecture of rifted continental margin (Jamieson & Beaumont 1989)
Lower Miocene
Buttressing against crustal ramp of inherited rifted margin (uplift + erosion + possible normal faulting at shallow levels)
SYN-COLLISIONAL
Fig. 5. —Different exhumation models applied to the blueschists of southern Lucania.
x
δx
h
Weak wedge of uniform taper (Platt 1986)
CRITICAL WEDGE MODEL
σw
Late Paleogene
TIMING
W σw
Extensional faulting
MECHANISM
hangingwall buttress
PRE-COLLISIONAL (Knott 1994)
EXHUMATION MODEL
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My
23.5
LATE OLIGOCENE
LOWER MIOCENE
16.2
MIDDLE MIOCENE
Table I. – Timing of events for the metamorphosed terrain (MT).
SERRAVALLIAN LANGHIAN
Sedimentation of Albidona Fm containing metamorphic (HP-LT) clasts from MT
BURDIGALIAN
Tectonic exhumation of HP-LT rocks (average rate = 7 mm/y)
AQUITANIAN
HP-LT Metamorphism Underplating Subduction (rate = 10 mm/y)
CHATTIAN
Sedimentation of uppermost MT sediments on ocean sea-floor
Therefore, these data provide a lower age constraint for the exhumation of the metamorphosed Liguride unit, as we know that during the late Oligocene part of its sedimentary succession was still being deposited on the sea-floor. An upper age constraint for the tectonic exhumation of high-pressure rocks is provided by the occurrence of middle Miocene episutural deposits (Albidona Fm) which contain detritus from the MT, clearly indicating that this metamorpic unit was brought to the surface by the end of the early Miocene. These age constraints are summarized in table I. During the late Oligocene, sedimentation of the upper part of the Liguride succession in Lucania was still going on. Assuming a subduction rate equal to the convergence rate between the African and European plates at this time (i.e., ca 10 mm/y; Mazzoli and Helman, 1994) and a steep subducting slab (i.e., ca 60◦ ; Spakman, 1990), underplating at a maximum depth of ca 25 km would have occurred in about 3 million years during the early Miocene. Tectonic exhumation occurred in Burdigalian times at an average rate of 7 mm/y, and was already completed by Langhian times, as shown by the first appearance of detritus from the high-pressure unit in foredeep deposits. Therefore, exhumation occurred entirely during the early Miocene, and not in late Paleogene times as implied by the pre-collisional extensional model of Knott (1994). Structural constraints. In the regional cross-section of fig. 2, the structural geometry of the MT can be observed. It consists of a narrow belt of metamorphic rocks forming the uppermost tectonic unit of the thrust stack. The structural geometry of the thrust belt is complicated by the occurrence of breaching and out of sequence thrusts, and later strike-slip
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faults. However, the original thrust contact between the MT and the UT is preserved at several localities (Monaco and Tortorici, 1995), and shows a «reverse-sense» pressurebreak contact between high-pressure rocks in the hanging wall and low-pressure rocks in the footwall. This is the opposite relationship with respect to the «normal-sense» pressure-break contact implied by the extensional model applied by Knott (1994) in this area. The cross-section of fig. 2 also shows how both metamorphosed and unmetamorphosed ophiolite-bearing units are underlain by a relatively shallow decollement emplacing them onto the rocks belonging the southern continental margin of Neotethys. A model for tectonic exhumation in southern Lucania. Based on the foregoing discussion, a model implying exhumation of high-pressure rocks following the collision of the Liguride accretionary wedge with the southern continental margin of Neotethys can better satisfy both age and structural constraints from southern Lucania. This model (fig. 6) is also based on critical wedge concepts, but the geometry of the basal decollement in this instance is strongly controlled by the inherited architecture of the rifted continental margin. Tectonic exhumation occurs as a result of buttressing of the wedge against the crustal ramp of the inherited rifted margin, while normal faulting is likely to represent a subsidiary process at shallow depths. The structural and age constraints discussed above are well compatible with the model of tectonic exhumation proposed by Jamieson and Beaumont (1989), in which an accretionary wedge encounters a rifted continental margin (fig. 6a). These authors have shown that if a steady state (that is an equilibrium between accretion and erosion) is achieved after rapid initial crustal thickening, and thermal equilibration is slow compared with the rate of accretion of the orogen, buttressing against the crustal ramp of the inherited rifted margin is a very efficient mechanism for the exhumation of high-pressure rocks like blueschists (fig. 6b). If the orogen then achieves the critical topography for thrusting to progress onto the foreland, previously exhumed high-pressure rocks can be emplaced onto the foreland in the hanging wall of a major shallow decollement (fig. 6c), as it is observed in the Calabria-Lucania borderland area (refer to fig. 2). Concluding remarks. A number of features from the Calabria-Lucania borderland area fit quite well with a model of exhumation associated with the collision of the Liguride accretionary wedge with the southern continental margin of Neotethys. These features are: (i) the early Miocene, syn-collisional age of underplating and subsequent exhumation; (ii) the occurrence of a thrust and a «reverse-sense» pressure-break contact between the blueschistfacies metamorphic rocks and the underlying unmetamorphosed ophiolite-bearing units; and (iii) the occurrence, within the thrust structure of the southern Apennines in Lucania, of a narrow belt of rapidly exhumed metamorphic rocks.
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Accretionary wedge
Hinge zone
Erosion
Extension Particle path b)
Crustal ramp of rifted margin Underplating
Crystalline orogen
Moho
Critical topography Foreland Basement ramp
c)
Inherited rifted margin g slab subductin
Moho
Fig. 6. – Conceptual model (not to scale) of orogenic development and tectonic exhumation by thrusting of an accretionary wedge over a simple continental margin (after Jamieson and Beaumont, 1989). a) Initial encounter of the accretionary wedge with the sedimentary prism of the rifted margin. b) During the initial stages of thrusting of the accretionary wedge over the continental margin, the wedge deforms internally and accretes the margin sedimentary prism and faulted crust. The crustal ramp, formed by the inherited rifted margin, prevents further advance of the thrust front until the whole orogen achieves critical topography. Attainment of the latter, by underplating, shortening and thickening of the orogen, is slowed by erosion and extension at shallow levels. Particle paths reflect the balance between competing processes within the deforming orogen. c) After critical topography has been achieved and thrusting can progress onto the foreland, a thin-skinned fold and thrust belt and foreland basin develop ahead of the crystalline orogen.
The occurrence of a shallow decollement underlying both metamorphosed and unmetamorphosed ophiolite-bearing units is well compatible with foreland emplacement during the next constructive stage of the orogen.
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