Pliocene-quaternary thrusting, syn-orogenic extension and tectonic exhumation in the Southern Apennines (Italy): Insights from the Monte Alpi area
Published:January 01, 2006
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S. Mazzoli, L. Aldega, S. Corrado, C. Invernizzi, M. Zattin, 2006. "Pliocene-quaternary thrusting, syn-orogenic extension and tectonic exhumation in the Southern Apennines (Italy): Insights from the Monte Alpi area", Styles of Continental Contraction, Stefano Mazzoli, Robert W.H. Butler
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Monte Alpi represents the only area of the southern Apennines where Apulian carbonates, elsewhere buried beneath a several-km thick allochthon, are exposed at the surface. These rocks also represent the reservoir interval in southern Italy's major oil fields. The tectonic evolution of this substantially exhumed area of the fold-and-thrust belt, derived from conventional structural and stratigraphic considerations via integration into the regional framework, has been tested and detailed by the analysis of vitrinite reflectance, clay mineralogy, apatite fission track, and fluid inclusion data.
The Apulian carbonates of Monte Alpi underwent significant tectonic burial as a result of thin-skinned thrusting in early Pliocene times. Simplified burial and thermal modeling suggests that the thickness of allochthonous material emplaced on top of Monte Alpi was probably in excess of 5 km. Exhumation is envisaged to have started in the late Pliocene, when the area emerged and the tectonic load started to be eroded off Monte Alpi. A significant stage of exhumation is inferred to have taken place in uppermost Pliocene-early Pleistocene times as a result of thick-skinned reverse faulting at depth and coeval thin-skinned extension within the overlying allochthon. After shortening ceased throughout the whole southern Apennines, middle Pleistocene-Holocene tectonic exhumation of Monte Alpi was essentially controlled by thick-skinned extensional tectonics. This process is still active and controls the present-day seismicity of the study area.
Tectonic exhumation processes have been extensively studied over the past several decades from different orogens all over the world. They were first described with the discovery of the core complexes in the Basin and Range province (e.g., Moores et al., 1968), but there is abundant evidence that normal faulting aids exhumation of midcrustal rocks both in extensional (e.g., Foster and John, 1999, for the Basin and Range province) and contractional (e.g., Mancktelow, 1985; Selverstone, 1988, for the European Alps) regimes. Although less studied, tectonic exhumation occurs at shallow crustal levels in thrust belts like the Apennines of peninsular Italy. Here, shallowly buried rocks of the uppermost crust are brought to the surface by the interaction of deep (crust-mantle) and surface processes (e.g., Carminati et al., 1999; D'Agostino et al., 2001). These relatively shallow exhumation phenomena play a fundamental role in controlling the topography and the drainage evolution of mountain belts. Their study requires the integration of geological and geomorphic information with various types of thermal and/or geochronological data. Such a multidisciplinary approach has been widely applied, Apennines included (e.g., Ventura et al., 2001; Zattin et al., 2002; Schiattarella et al., 2003). In this study we integrate stratigraphic, structural, and morphotectonic data with different organic and inorganic parameters that record the thermal/thermochronological evolution of the rocks, including: (i) vitrinite reflectance, (ii) fluid inclusions, (iii) illite/smectite (I/S) mixed layers in clayey sediments, and (iv) apatite fission tracks. The study is carried out in the Monte Alpi area of the southern Apennines. According to a number of studies (e.g., Cello et al., 1990; Van Dijk et al., 2000; Corrado et al., 2002), this represents the only sector of the fold-and-thrust belt where Apulian carbonates, constituting the reservoir for southern Italy's major oil fields (e.g., Shiner et al., 2004, and references therein), are exposed at surface. In a previous paper (Corrado et al., 2002), a tectonic exhumation model has been proposed for this area, based on organic matter maturity, clay mineralogy, and fluid inclusion data. The latter model essentially related the exhumation of the Monte Alpi block to the activity of Quaternary high-angle faults of local extent. However, new analytical information and apatite fission track data recently have been provided for a large area of the southern Apennines by Corrado et al. (2005), leading the latter authors to outline, although rather schematically, an alternative model for the tectonic evolution of the Monte Alpi area. The aim of this paper is to build from the latter model, integrating and thoroughly documenting apatite fission track, organic matter maturity, clay mineralogy, and fluid inclusion data with structural analysis and a reconsideration of the geology of the Monte Alpi area. The new tectonic interpretation proposed in this study involves both: (i) a significantly older age, with respect to the previously suggested Quaternary one, for the onset of tectonic exhumation, and (ii) a fundamental role of low-angle extensional faults of regional extent.
GEOLOGICAL SETTING OF THE SOUTHERN APENNINES
The southern Apennines fold-and-thrust belt of peninsular Italy forms part of the Alpine orogen in the Mediterranean area, which evolved within the framework of convergent motion between the Afro-Adriatic and European plates since late Cretaceous times (Dewey et al., 1989; Mazzoli and Helman, 1994; Rosenbaum et al., 2002). North of the Calabrian Arc, the southern Apennines thrust belt (Fig. 1A) is NE-directed. Except for the remnants of the ophiolite-bearing Liguride Units that occur on top of the thrust pile, outcropping units consist of Mesozoic and Cenozoic rocks derived from the sedimentary cover of the foreland plate. The Apulian promontory represents the orogenic foreland. Collectively, the Apulian foreland and the deformed strata of the Apennines represent a telescoped continental margin with complex subbasins of different ages (e.g., Mostardini and Merlini, 1986; Sgrosso, 1998; Cello and Mazzoli, 1999, and references therein).
Large amounts of surface geological information (e.g., Scandone, 1972) coupled with subsurface data (e.g., Mostardini and Merlini, 1986), particularly oil wells, clearly demonstrate large-scale thin-skinned thrusting in the shallow part of the southern Apennines. The thrust belt forms a displaced allochthon that has been carried onto a footwall of foreland strata essentially continuous with the Apulian platform (e.g., Mostardini and Merlini, 1986; Carbone et al., 1991; Mazzoli et al., 2000; Menardi Noguera and Rea, 2000; Butler et al., 2004; Fig. 1B). The detachment between the allochthon and the buried Apulian shallow-water carbonates is marked by a mélange zone up to several hundred meters thick. It consists mainly of intensely deformed and overpressured deepwater mudstones and siltstones of Miocene to Lower Pliocene age, including blocks of material derived from the overlying allochthon (Mazzoli et al., 2001a; Butler et al., 2004; Shiner et al., 2004). Beneath the mélange zone, under a variable thickness of Pliocene shales stratigraphically overlying the Mesozoic-Tertiary platform carbonates, the hinterland portion of the Apulian Unit was involved in the final shortening phases (late Pliocene-early Pleistocene; e.g., Cello and Mazzoli, 1999, and references therein). This resulted in reverse-fault-related, open, long-wavelength, high-amplitude folds that form the hydrocarbon traps for the significant oil discoveries in this area (Shiner et al., 2004). All recent, geologically realistic interpretations based on subsurface data indicate that deep-seated reverse faulting within the Apulian carbonates is characterized by relatively limited horizontal displacements and probably by involvement of the underlying basement (Mazzoli et al., 2000; Menardi Noguera and Rea, 2000; Speranza and Chiappini, 2002; Butler et al., 2004; Shiner et al., 2004). Therefore, during the late Pliocene, a switch from thin-skinned to thick-skinned thrusting appears to have occurred in the southern Apennines as the Apulian carbonates—and the underlying thick continental lithosphere—were involved in deformation (Mazzoli et al., 2000; Butler et al., 2004).
Neogene thrusting in the Calabrian Arc and in the southern Apennines was accompanied by back-arc extension and sea-floor spreading in the southern Tyrrhenian Sea (e.g., Kastens et al., 1988; Facenna et al., 1996, 1997, and references therein; Mattei et al., 1999; Fig. 1A). Around the early-middle Pleistocene boundary (ca. 0.8 Ma), however, SW-NE-directed shortening ceased in the frontal parts of the southern Apennines, too (e.g., Hippolyte et al., 1994). A new tectonic regime was established in the chain and adjacent foothills (e.g., Cello et al., 1982; Cinque et al., 1993; Hippolyte et al., 1994; Montone et al., 1999). The structures related to this new regime, characterized by a NE-SW oriented maximum extension, consist of extensional and transcurrent faults that postdate and dissect the thrust belt (e.g., Cello et al. 1982; Butler et al., 2004, and references therein).
GEOLOGY OF THE MONTE ALPI AREA AND INFERRED TECTONIC EVOLUTION
The Monte Alpi Unit (Sgrosso, 1988; Figs. 1C, 2, and 3) consists of Mesozoic peritidal carbonates (Sartoni and Crescenti, 1962) unconformably overlain by Miocene deposits. The latter include two transgressive stratigraphic units. The lower one (UM1) consists of Upper Miocene (mostly lower Messinian; Sgrosso, 1988; Taddei and Siano, 1992) biocalcarenites and bituminous calcilutites interbedded with clayey-silty marls, capped by micro-conglomerates. The second transgressive unit (UM2) consists of ∼ 200 m of interbedded sandstones, marls, and conglomerates. The latter are organized in beds that are 0.1 to several meters thick and are clast-supported with an arenaceous matrix. The majority of the clasts derive from erosion of the local carbonate substratum; the rest are composed of siliceous argillite, chert, micritic limestone, and sandstone derived from erosion of the allochthonous units. Clast size varies from 2 mm to 20–30 cm and locally up to 60–70 cm in both intrabasin limestone and extrabasin materials, and sorting is moderate to good (Alberti et al., 2001). The fossil content of the rocks has been reworked, but a Messinian age is probable (Taddei and Siano, 1992).
The Monte Alpi Unit is tectonically overlain (along low-angle contacts) and also surrounded (along recent high-angle faults) by various allochthonous elements (Fig. 3). The geology of these elements is complex, as they include highly disrupted and discontinuous remnants of the Liguride Units, as well as of Mesozoic-Paleogene carbonate slope and pelagic basin successions (e.g., Bousquet, 1973; Sgrosso, 1988; Knott, 1994; Ortolani and Torre, 1971; Taddei and Siano, 1992; Van Dijk et al., 2000; Alberti et al., 2001). The allochthonous elements also include a tectonic mélange made up of a highly deformed argillaceous silty matrix including black-brown, red, and green pelites with blocks of calcareous sandstones, micritic limestones, radiolarian cherts, laminated black algal limestones, and calcarenites (Van Dijk et al., 2000; Alberti et al., 2001). Taking into account available sub-surface information, the latter unit has been interpreted by Corrado et al. (2002) as remnants of the mélange zone known generally to be interposed between the (elsewhere buried) Apulian carbonates and the overlying allochthon. Within this context, the Monte Alpi Unit would form part of the collectively termed Apulian Unit (Corrado et al., 2002; Fig. 4).
The geology of the study area is complicated by the occurrence of low-angle extensional faults rooted within the allochthonous units tectonically overlying the Monte Alpi Unit (e.g., Lentini et al., 2003). An important low-angle detachment fault, here named Cogliandrino Fault, occurs west of the Monte Alpi outcrop (Figs. 1C, 4 and 5). It downthrows the allochthonous units to the NE, producing significant tectonic omission revealed by the anomalous contact between the topmost (Liguride) units of the thrust belt and Mesozoic pelagic basin strata (Lagonegro Units) that occupy a much deeper position within the thrust pile (refer to Fig. 1B). This fault clearly offsets the preexisting nappe pile, cutting down to the east across the Liguride, Apennine Platform, and Lagonegro Basin Units. The Lagonegro Basin strata, exposed at present in the footwall of the Cogliandrino Fault, have been substantially exhumed—from minimum depths probably ∼4 km (Mazzoli et al., 2001a; Aldega et al., 2003a; Corrado et al., 2005)—in late Pliocene times, as shown by apatite fission track age data (Corrado et al., 2005; Fig. 1C). The low-angle Cogliandrino Fault is offset by high-angle faults bounding the Monte Alpi structure (Figs. 4, and 5). On top of Monte Alpi, the presence of the low-angle fault is indicated by further anomalous relationships resulting from substantial tectonic omission. Here, in fact, remnants of the topmost (Liguride) units of the thrust belt are in direct contact with the mélange zone that immediately overlies the Apulian carbonates (elsewhere buried as shown in Fig. 1B). The Cogliandrino Fault continues at depth northeast of Monte Alpi, the related thin-skinned extension being possibly kinematically linked with thrusting farther east. This could occur within the allochthonous units representing the substratum to the Pliocene-Pleistocene Sant'Arcangelo basin to the east (refer to Fig. 1A), where thin-skinned thrusts have been identified by seismic interpretation (Hippolyte et al., 1994).
In synthesis, based on the geological evidence mentioned above and on the regional information summarized in the previous section, the following tectonic evolution may be proposed for the Monte Alpi and surrounding area (Corrado et al., 2005; Fig. 6):
A tectonic wedge, including Liguride, Apennine Platform, and Lagonegro Basin Units, is produced as a result of thin-skinned thrusting and foreland migration of the deformation throughout the Miocene (e.g., Sgrosso, 1998; Cello and Mazzoli, 1999, and references therein).
In early to middle Pliocene times (e.g., Butler et al., 2004), the previous tectonic wedge (collectively termed the allochthon) is emplaced, as a single large detachment sheet, on top of the western portion of the Apulian Unit (including the Monte Alpi Unit).
During late Pliocene to early Pleistocene times (e.g., Cello and Mazzoli, 1999, and references therein), relatively high-angle, thick-skinned reverse faulting leads to the development of large antiformal structures in the tectonically buried Apulian Unit, possibly also triggering coeval collapse and thin-skinned extensional faulting in the overlying allochthon.
From middle Pleistocene times onward (e.g., Cello et al., 1982; Cinque et al., 1993), high-angle, thick-skinned extensional/transtensional faulting offsets all the previous structures (including low-angle extensional detachments).
This tectonic evolution may be tested and better detailed, constraining tectonic burial, thermal maturity, and timing of exhumation events, by the integrated analysis of different data sets provided in the following sections.
The Monte Alpi structure is bounded by high-angle faults (Fig. 3). In the footwall to these faults, the Monte Alpi Unit is exposed, discontinuously overlain by remnants of the tectonic mélange and allochthonous units (Figs. 3 and 4).
High-angle faults occur along the north, east (Faults A and D in Fig. 3), and southern (Faults E and F) sides of the Monte Alpi carbonate block. No kinematic data have been gathered from faults A, D, and E in Fig. 3; however, important normal components of motion are indicated by the significantly downthrown units occurring in their hanging walls. The E-W trending fault F shows a pure normal sense of slip. On the western side of Monte Alpi, a prominent N-S striking fault scarp is exposed (Fig. 2A; Fault B in Fig. 3). Related morphological evidence points to an important vertical component of displacement. In fact, a late Quaternary extensional activity has been recently suggested for this structure (Michetti et al., 2000).
A further N-S striking fault is exposed as a result of erosion and landsliding from within the Monte Alpi carbonate block (Fault C in Fig. 3). The fault zone is characterized by subvertical, undulate slip surfaces. Striae on slickenside surfaces and calcite shear fibers (Fig. 2B) indicate complex superposed kinematics, ranging from strike-slip to oblique-slip with dominantly moderate angles of pitch and a sinistral component of motion. Locally, dark bituminous limestones are exposed along the fault scarp, and composite calcite-bitumen extension veins occur within the damage zone associated with the fault.
In summary, the main high-angle structures presently bounding the Monte Alpi exhumed block consist of dominantly extensional faults. Some of these faults are seismically active and are interpreted as deeply-rooted (crustal) structures (Michetti et al., 2000). Faults within the Monte Alpi carbonate block may be characterized by more complex kinematics, possibly resulting from multiple reactivation of inherited structures (e.g., Fault C in Fig. 3; see also Van Dijk et al., 2000; Alberti et al., 2001) and/or transtension. Additionally, available subsurface data (e.g., Cello et al., 1990; Corrado et al., 2002) indicate that the Monte Alpi fault-bounded block forms part of a much larger antiformal structure made of Apulian platform carbonates that have been substantially uplifted above their regional level by major reverse faults (Fig. 4). Therefore, the Monte Alpi outcrop owes its peculiar present-day position within the thrust belt to a series of structural features: (i) it forms part of the crestal zone of a large reverse fault-related antiform of the Apulian Unit, (ii) it lies in the footwall to the low-angle Cogliandrino Fault, and (iii) it has been reworked by recent high-angle faults.
Meso- and Microstructures
Mesoscopic and microscopic deformation features in the Miocene deposits in the upper part of the Monte Alpi succession have been analyzed away from major fault zones. Conglomerate clasts of the second transgressive cycle of the Monte Alpi Unit (UM2) show a pronounced oblate-shape fabric consistently parallel to bedding (Fig. 2C). Flattening and volume loss by pressure-solution, indicated by stylolitic contacts among the clasts (Fig. 2D), defines a maximum shortening direction approximately perpendicular to bedding, irrespective of present-day bedding attitude. This provides a strong argument for burial-related deformation in roughly flat-lying sediments prior to significant tectonic tilting. In the intercalated sandstones, bedding-parallel extension veins also occur, testifying overpressure buildup leading to hydraulic jacking normal to bedding (Fitches et al., 1986). Bedding-parallel extension veins are offset (Fig. 2E) by both NW-SE and NE-SW trending sets of en échelon vein arrays oblique to bedding and by isolated extension veins roughly perpendicular to bedding. En échelon arrays of calcite-filled tension gashes (Fig. 2E) occur in four sets organized in two conjugate systems of brittle-ductile shear zones (Ramsay and Huber, 1987) showing limited displacements (a few centimeters at most). Shear zone intersections in the two different conjugate systems are both subhorizontal and trend NW-SE and NE-SW (Fig. 2F). Acute bisectors (i.e., shortening directions; Ramsay and Huber, 1987) of conjugate shear zone dihedral angles are generally subvertical, while obtuse bisectors (i.e., extension directions) are subhorizontal and oriented NW-SE and NE-SW. These features clearly suggest a bulk finite strain dominated by a maximum shortening normal to bedding.
In thin section, sparry calcite that fills the tension gashes displays varying degrees of twinning and undulose extinction. Twin lamellae in the more intensely strained vein calcite consist of dominantly thin (<1 µm) straight twins organized in two, or rarely three, rational sets (Fig. 2G). Thicker (>1–5 µm) twins also occur, while curved twins and twinned twins are rarely observed (and could be related to strain effects within hosting brittle-ductile shear zones; Burkhard, 1993).
Healed microveins marked by fluid inclusion trails (Fig. 2H) can be observed in calcite crystals, especially in those less deformed by twinning. The inclusions are one or two phase, with a small bubble occurring in some of them (Fig. 2H).
FLUID INCLUSION DATA
The samples analyzed in this study (Table 1, Fig. 7) were collected from calcite extension vein systems that crosscut the rocks of both Upper Miocene units (UM1 and UM2) unconformably overlying the Mesozoic carbonates of Monte Alpi (Fig. 3). Different types of structures (described in the previous paragraphs) were sampled: (i) bedding-parallel extension veins (samples A1 and A2), and (ii) bedding-normal extension veins, of isolated type (samples A6, A7, AA3), or occurring into arrays of en échelon tension gashes associated with extensional brittle-ductile shear zones (samples A0 and AA1).
Primary and secondary fluid inclusions (see Appendix for details and definitions) were recognized in all samples, mainly concentrated in a few clear (undeformed) crystals. They exhibit one or two phases and show mainly rounded or slightly elongated shapes (Fig. 2H–I). A small bubble was present mainly in the larger inclusions (10–15% of vapor), while the lack of vapor in very small inclusions was probably due to metastability (Roedder, 1984). Secondary inclusions occurring along healed microveins were generally better preserved but smaller (Fig. 2H).
Decrepitated or reequilibrated primary inclusions are common and mainly show cluster texture or fracturing features in the form of thin tails departing from the inclusion. These textures suggest that the inclusions developed internal overpressure (e.g., Vityk and Bodnar, 1995).
In all samples, ice melting temperatures (Tmice) for both primary and secondary inclusions fall between 0°C and −2°C. Fluid composition based on Tmice and few eutectic temperatures belong to the H2O-NaCl system. A2.5–3.0 wt % equivalent of NaCl is suggested, applying Bodnar's (1993) equation for H2O-NaCl systems.
One of the two samples collected from veins belonging to the calcarenites of unit UM1 (sample AA1) shows bitumen-calcite microveins and few preserved primary and secondary fluid inclusions, although primary inclusions are stretched (vapor bubble ∼30%) and were not measured. The other sample from unit UM1 (sample AA3) is bitumen-free and shows highly twinned calcite crystals with undulose extinction and lobate margins. Primary fluid inclusions suitable for microthermometry are present in few undeformed crystals. Data from these samples are included in the histogram of Figure 7 (A, B). The mean value of the homogenization temperature (Th) from primary inclusions is 130.5°C. Secondary inclusions show a peak at 120°C (Fig. 7B) and a mean Th value of 109.8°C.
In the five samples belonging to unit UM2, primary fluid inclusions have generally slightly elongated shapes with a small vapor bubble (V=10–15%). They are mainly distributed along crystal growth bands or grain boundaries. Secondary one- and two-phase inclusions located along healed microfractures are round and very small (few microns). For these samples, homogenization temperatures for primary inclusions from calcite crystals and few quartz grains belonging to the conglomerate sandy matrix, although quite dispersed, are mostly in the range of 90–110°C, with isolated peaks at 150° and 170°C (Figs. 7C and 7E). For secondary inclusions, the irregular distribution of Th data (Figs. 7D and 7F) results in two mean values at 71.4°C and 109.0°C.
VITRINITE REFLECTANCE DATA
The analyzed samples were collected from the Meso-Cenozoic succession of the Monte Alpi Unit, in particular from Jurassic limestones and from overlying Upper Miocene transgressive deposits (units UM1 and UM2). Among the numerous samples analyzed, only two (refer to Table 1) gave reliable results that indicate thermal maturity because they show one main population of measurements with a symmetric distribution (Fig. 8).
The first sample (D1) was collected at the base of the stratigraphic succession in Jurassic limestones, where bitumen presumably only migrated to a small extent. Bitumen reflectance values (Rbit) were converted into vitrinite equivalent reflectance data (Roeq) using Jacob's formula (Jacob and Hiltman, 1985). The obtained Roeq is 1.65%, with a standard deviation of 0.06%.
The second sample (D2) comes from coarse sandstones of unit UM2 (∼60 m above the stratigraphic boundary with unit UM1). Its Ro is 1.54% with a standard deviation of 0.09%. Samples collected from units UM1 and UM2 either do not contain vitrinite or contain highly oxidized/reworked vitrinite fragments.
The results of the semiquantitative bulk-rock analyses define the rocks of unit UM2 as hybrid arenites with calcite cement characterized by a very high percentage of calcite and subordinate quartz and phyllosilicates. The rocks forming the overlying mélange and allochthonous units are clayey sediments interbedded with sandstones and siltites composed of major amounts of phyllosilicates and subordinate quartz, calcite, K-feldspar, and plagioclase (Fig. 9). In samples from the mélange zone, the clay mineral components include illite, kaolinite, chlorite, and I/S mixed layer clays. The <2 µm and 2–16 µm grain-size fractions do not show any mineralogical differences but are characterized by slight differences in the amounts of both detrital and authigenic components. I/S mixed layer clays enrich the finer fraction, whereas illite, kaolinite, and chlorite are more abundant in the 2–16 µm grain-size fraction. The samples from the mélange zone (F1, F2, and B1) differ from those belonging to the Liguride Units (E1-E3), in that the former show greater amounts of kaolinite (62% vs. 21%). E1–E3 samples, in contrast, reveal larger amounts of chlorite (44% vs. 7%), as it is typical of the Liguride Units within the study area (Aldega et al., 2003a, b).
The observed I/S mixed layer clays in the <2 µm grain-size fraction correspond to R > 1 structures in which the illite component dominates. Considering the XRD pattern definition in evaluating the quantity Δ°2[thetas], the illite percentage is ∼70–85% for Liguride Units samples and ∼80–90% for mélange zone samples. Based on the thermal maturity recorded by the analyzed samples, a temperature range of 100–170°C can be estimated for the Liguride Units (see Appendix for details on paleotemperatures estimation). A greater thermal maturity of the mélange zone is indicated especially by sample F1, which shows long-range ordering and R3 structures in I/S mixed layers that suggest maximum temperatures in excess of 180°C (Hoffman and Hower, 1979; Jennings and Thompson, 1986).
C1–C4 samples are characterized by large amounts of kaolinite and illite/vermiculite (I/V) mixed layer clays and subordinately chlorite, illite, and I/S mixed layer clays. I/S mixed layers are composed of a percentage of illite in the range of 75–80%. Smectite was not identified, suggesting that C samples experienced temperatures higher than 60°C. In more detail, estimates of temperatures comprised between 110°C and 170°C are reliable for unit UM2. This temperature range is suggested by the illite content in I/S mixed layers and by the absence of the R1 to R3 transition (Hoffman and Hower, 1979; Pollastro, 1993).
Detrital illite, mainly accumulating in the >2 µm fraction, can cause an incorrect estimate of the illite content of mixed layers, and therefore of estimated burial and paleotemperatures. As a consequence, only data relative to ordered I/S mixed layers in the <2 µm grain-size fraction were employed for burial and thermal modeling (see following sections).
APATITE FISSION TRACK DATA
Fission tracks produced by the decay of 238U in apatite have been used in a wide range of studies in basin analysis since the early 1980s, when many analysts moved from an absolute dating approach to a detailed definition of time-temperature paths (thermochronology; for a review, see Gallagher et al., 1998). The most important difference from other low temperature indicators is that this methodology provides both temperature and time information. In fact, the annealing of fission tracks, which occurs between ∼60°C and 125°C (Gleadow et al., 1986; Green et al., 1989; Laslett et al, 1987), can be used to reconstruct the low-T thermal history of basins, from deposition and burial of sediments through subsequent cooling related to uplift and erosion.
Apatite fission-track analysis (AFT) was carried out on four samples (refer to Table 1): one from Messinian rocks of unit UM2 (A2), two from the tectonically overlying mélange zone (F1, B2), and one from the allochthonous Liguride Units (E1). The results obtained (Table 2) are characterized by large analytical errors owing mainly to the very young age of final cooling. As a consequence of the low abundance of spontaneous tracks, no horizontal confined tracks could be measured. After decomposition by binomial peak-fitting (Table 3), only one sample (B2) gave a single concordant group of ages, significantly younger than the stratigraphic age of the sample. Therefore, the maximum temperature that affected this sample was higher than the total annealing temperature. Its cooling age is ∼2.4 m.y. Data obtained from the other three samples define two peaks of concordant ages (Table 3). Most of the grains (85–90%) belong to the younger peak, whose age is between 1.5 m.y. (E1) and 3.2 m.y. (A2), whereas the remaining grains can be grouped in a much older peak. The data indicate that the maximum temperature during burial was very close to the total annealing temperature. The presence of older modes not fully reset is probably due to some variation in the composition of apatites, but because we did not model the data, a quantitative estimate of chemical composition is not necessary for the aims of this paper. Given the short heating time (∼1.5 m.y. because the stratigraphic age is Messinian, that is very close to the Pliocene exhumation age), the total annealing temperature was likely higher than the common estimate of 125°C.
BURIAL AND THERMAL MODELING
Based on the inferred tectonic evolution summarized in Figure 6 and integrated by the data reported in the previous sections, a simplified reconstruction of the burial and thermal history of the Monte Alpi area was performed using the software package Basin Mod 1-D (Basin Mod 1-D for Windows, version 5.4Basin Mod 1-D for Windows, version 5.4 software, 1996). Stratigraphic data such as thickness, lithology, and age of formations are derived from detailed studies performed on the Monte Alpi Unit (Table 4).
The main assumptions adopted for this reconstruction are that: (i) rock decompaction factors apply only to clastic units (i.e., Miocene rocks belonging to the first and second transgressive cycles), according to the method of Sclater and Christie (1980), whereas carbonate units are not decompacted; (ii) seawater depth variations in time are irrelevant in modeling, because thermal evolution is mainly affected by sediment thickness rather than by water depth (Butler, 1992); (iii) thermal modeling has been performed using LLNL Easy %Ro method based on Burnham and Sweeney (1989) and Sweeney and Burnham (1990); (iv) thrusting can be considered instantaneous when compared with the duration of deposition of stratigraphic successions, as generally suggested by theoretical models (Endignoux and Wolf, 1990); (v) for the sake of simplicity, exhumation is considered linear within given time intervals; and (vi) a variable geothermal gradient has been adopted, with a preerosional value of 25°C/km and a syn-exhumation value of 40°C/km (the latter estimate calculated according to Brandon et al., 1998).
The results of our modeling, calibrated by organic maturation, clay mineralogy, and fission-track data from the Monte Alpi Unit, are shown in Figure 10. The reconstructed evolution begins with carbonate platform growth in Mesozoic times. Aphase of erosion is envisaged to have occurred between the deposition of the platform carbonates and Miocene sedimentation. This could have taken place since the Late Cretaceous, as shown in Figure 10A, or later, up to immediately prior to the onset of thrusting (e.g., at a flexural forebulge). It must be noted that the effects of changing the age of onset (i.e., Late Cretaceous vs. Miocene) of this erosion event are negligible for the resulting modeled thermal maturity of the studied rocks (Corrado et al., 2002).
The emplacement of the allochthonous units on top of the Monte Alpi Unit is envisaged to have occurred immediately after the end of sedimentation (ca. 5.3 Ma). This brought about rapid tectonic burial of the Monte Alpi succession, after which it remained buried in the footwall of the regional thrust (at depths of ∼5200 m) for ∼1.5 m.y. (Table 4). Exhumation is shown in the model (Fig. 10B) to have started with late Pliocene emersion and erosion, significantly accelerating since uppermost Pliocene times (ca. 2.8 Ma) as a result of the inferred inception of deep-seated thrusting within the Apulian carbonates and associated low-angle extensional faulting in the allochthon. During the latter stage, the exhuming top portion of the Monte Alpi succession, as well as the immediately overlying allochthon, cooled through the estimated closure temperature of the apatite system (∼123°C) at ca. 2.3 Ma. This relatively rapid exhumation is inferred to have continued throughout part of the early Pleistocene up to ca. 1.5 Ma, when thrusting and anticline formation are envisaged to have shifted to the next (outer) major structure to the NE (structural trend Y in Fig. 1B). Subsequent slower exhumation caused only by erosion is inferred to have characterized the time period preceding the activation of high-angle normal faults (which probably occurred ca. 0.8 Ma, consistent with the regional data discussed in the previous sections). A final exhumation stage is then envisaged from middle Pleistocene times onward. This is related to tectonic unroofing of Monte Alpi associated with deep-seated, steep extensional structures involving the Apulian carbonates.
The type of evolution outlined here, although certainly not the only viable one, satisfactorily explains the measured data, as shown by the resulting maturity curve of Figure 10C.
Constraints on the timing of thrusting in the study area are imposed by the occurrence of Upper Miocene transgressive sediments depositionally on top of the Monte Alpi platform carbonates. Sedimentary facies, lithological composition, and clast size of the rocks belonging to the Messinian unit UM2 indicate a proximal source area for these deposits. This in turn suggests that the allochthonous units were very close to the depositional area. The deduction fits regional data that indicate early Pliocene emplacement of the allochthonous sheets on top of the Apulian carbonates of the Monte Alpi Unit (e.g., Cello and Mazzoli, 1999).
Clay mineralogy and vitrinite reflectance data provide information on the maximum burial conditions experienced by the Monte Alpi Unit in the footwall to the allochthonous sheets. Fission-track data indicate that the maximum temperature was slightly higher than the total annealing temperature, which given the short heating time, was higher than the one commonly assumed (i.e., ∼125°C). This agrees with the temperature range (110–170°C) provided by clay mineralogy data on the Upper Miocene unit UM2. Fluid-inclusion data could provide direct temperature estimates because, according to Barker and Goldstein (1990), Thmean approximates Tpeak for sedimentary basins. However, homogenization temperatures reflect environmental conditions during fluid entrapment, which in turn depend on the time at which the hosting structure (i.e., vein) formed during the tectonic evolution. Bedding-parallel extension veins, formed by hydraulic jacking normal to bedding, are likely to have formed by significant burial in presence of an overlying seal, leading to over-pressure build up in the buried strata. Considering the limited thickness (a few tens of meters at most) of the Miocene succession stratigraphically overlying the sandstones hosting them, bedding-parallel extension veins probably resulted from tectonic rather than original sedimentary burial. Tectonic loading was provided by the allochthonous units, which also acted as a seal on top of the Monte Alpi Unit. This interpretation is in good agreement with fluid inclusion data from these veins, characterized by Th values in the range of 60–160°C—temperatures that cannot be related to shallow burial conditions in the original late Miocene sedimentary basin. Bedding-parallel veins are offset by bedding-normal ones, which therefore would also postdate allochthon emplacement and related tectonic burial of the Monte Alpi Unit. The latter vein sets, occurring both isolated or organized into en échelon arrays, indicate a maximum shortening perpendicular to (originally flat-lying) bedding and multidirectional, bedding-parallel extension (oblate strain). Therefore, it seems likely that these veins, sampled in the Miocene strata of the Monte Alpi Unit, formed as a result of a bulk flattening strain, with vertical shortening controlled by the load exerted by the overlying allochthon (also responsible for the flattening of the conglomerate clasts; Figs. 2C, 2D, and 11). As a consequence, primary fluid inclusions contained in these veins could, in theory, provide information on the maximum temperature conditions experienced by the Monte Alpi Unit as a result of tectonic burial. However, the studied vein calcite crystals show many inclusions (mainly primary inclusions) that are decrepitated or reequilibrated. The latter often display textures indicating a reequilibration of the system at P/T conditions different from those of fluid entrapment, due to exhumation (Invernizzi et al., 1998). Textures of the type observed in the analyzed samples are experimentally produced in isothermal decompression regimes (Vityk and Bodnar, 1995), suggesting that relatively fast exhumation took place. The higher vapor percentage (∼30%) in the few primary inclusions occurring in bitumen-calcite microveins from Miocene limestones of unit UM1 suggests a possible reequilibration (stretching) of these inclusions. This provides a further indication of internal overpressure in primary inclusions. Within this context, the Thmean of 130.5°C obtained from primary inclusions measured in undeformed crystals contained in calcite veins from unit UM1 (Fig. 7A) is unlikely to represent the peak temperature experienced by the Monte Alpi Unit in the footwall to the allochthonous units. Rather, these data probably record stages of vein formation, calcite precipitation and fluid entrapment during ongoing tectonic exhumation and cooling. Lower temperatures recorded by primary inclusions sampled in bedding-parallel veins from the second transgressive cycle are likely to record substantial reequilibration (although an isolated peak temperature in excess of 150°C is recorded; Fig. 7C). This is also the case for homogenization temperatures relative to secondary fluid inclusions (Figs. 7B, 7D, and 7F). The formation of the latter, mainly aligned along healed microfractures, clearly indicates that the system was reopened and the entrapped fluid records only a part of the burial/exhumation history.
Microstructural observations of deformed vein calcite may also provide some information about the maximum temperature of deformation. On the one hand, according to Burkhard (1993), the dominance of thin (<1 µm) straight twin lamellae is typical of deformation temperatures below 200°C. On the other hand, the occurrence of thicker (>1–5 µm) twin lamellae suggests that temperatures above ∼150°C were probably reached. Cross-comparing these values with the information provided by clay mineralogy, we are left with a most likely maximum temperature range of 150–170°C for the Miocene succession of Monte Alpi. Even higher temperatures appear to have been experienced by the tectonically overlying mélange zone, as indicated by the thermal maturity recorded from related samples. This suggests a complex tectonic evolution for this unit, which was interpreted by Mazzoli et al. (2001b) to represent the major decollement at the base of the allochthon. According to those authors, Mio-Pliocene foredeep deposits forming the bulk of the mélange zone were progressively incorporated within the decollement zone as the advancing fold and thrust belt overrode its foreland basin. On the one hand, our new thermal maturity data confirm that part of this evolution, including significant tectonic burial and partial exhumation stages, occurred prior to final tectonic emplacement on top of the Monte Alpi Unit. On the other hand, thermal maturity data from the Liguride Units (samples E1, E2, and E3) are less relevant in terms of foreland fold and thrust belt evolution because they are likely to be strongly influenced by the precollisional accretion history of these units as part of an oceanic subduction complex (e.g., Cello and Mazzoli, 1999, and references therein; Aldega et al., 2003a, b).
In summary, after tectonic burial (Fig. 11A), the upper part of the Monte Alpi succession underwent cooling starting from a temperature well in excess of 130°C and probably in the range 150–170°C. This temperature is in any case higher than the closure temperature of the apatite system (see discussion later). As a consequence, apatite fission track ages, which record the time of cooling below the closure temperature of the system, date these cooling and exhumation stages. Estimating the closure temperature is therefore fundamental for modeling the tectonic evolution of the study area. As described by Dodson (1973), the closure temperature is a function of the rate of cooling but, in turn, actual cooling rate at closure is a function of the exhumation rate and the vertical gradient in the temperature profile because of heat advection during exhumation. As a result, an increase in the exhumation rate causes an increase in closure temperature. Following the procedure described in Brandon et al. (1998), it is possible to infer closure temperature and exhumation rate values from cooling ages using some basic assumptions. In this work, we assume a surface temperature of 10°C, a thermal diffusivity of 32 km2/m.y., an average thickness of the orogenic wedge of 30 km, and a pre-exhumation geothermal gradient of 25°C/km. The data presented in the previous section (Table 3) provide a mean value of ∼2.3 m.y. as the time of cooling through the closure temperature. Because of the young exhumation age, upward displacement of isotherms is important: in fact, assuming steady-state advection, the geothermal gradient rises up to 46°C/km. The estimated closure temperature is 123°C. Therefore, fission track data indicate that the top part of the Monte Alpi Unit was being exhumed and had cooled to ∼123°C by ca. 2.3 Ma. Exhumation had probably occurred since the beginning of the late Pliocene (ca. 3.6 Ma) when the area emerged and the tectonic load started being eroded off Monte Alpi. In a first stage, exhumation might have been mainly due to erosion. From uppermost Pliocene times (ca. 2.8 Ma) to sometime in the early Pleistocene (possibly ca. 1.5 Ma, as we inferred in the previous section), exhumation of the Monte Alpi Unit was most probably essentially controlled by thick-skinned thrusting involving the Apulian carbonates. Monte Alpi represented a portion of them, located in the crestal zone of a broad thrust-related antiform (Cello et al., 1990). NW-SE trending, sinistral transpressional faults may also have affected the carbonates of Monte Alpi during this shortening event (Van Dijk et al., 2000). Furthermore, important vertical components of displacement were likely associated with relatively steep reverse faults offsetting the Apulian carbonates at depth (Shiner et al., 2004). Substantial unroofing has likely occurred at this stage as a result of both erosion and shallow (i.e., thin-skinned) extensional faulting affecting the allochthonous units that tectonically overlay the uplifting thrust-related anticline (Fig. 11B). Relatively high exhumation rates may be envisaged during this stage, as a result of the efficient unloading produced by the mechanical removal of hanging wall material by low-angle normal faulting and associated footwall uplift (Table 4).
At some stage in the evolution of the study area, tectonic exhumation became essentially controlled by deep-seated extensional/transtensional faulting involving the Apulian carbonates, as indicated by major structures bounding the Monte Alpi block (see previous sections; Fig. 11C). Our data provide no direct constraints on the timing of this extension. However, based on regional evidence discussed in the previous sections, an onset of deep-seated extensional faulting at ca. 0.8 Ma can be inferred. This tectonic regime is still active and controls present-day seismicity in the study area (Michetti et al., 2000).
The Monte Alpi area represents a key sector of the southern Apennines. Elsewhere buried beneath a several km thick allochthon, here the Apulian platform carbonates, reservoir for southern Italy's major oil fields, crop out. This area therefore represents a sector of localized, intense exhumation within the axial zone of the orogen. The related tectonic evolution has been analyzed by integrating the classical stratigraphic and structural approach with the study of organic matter maturity, clay mineralogy, apatite fission tracks, and fluid inclusions. The following main conclusions may be drawn:
The original thickness of the tectonic units (allochthon) emplaced in early Pliocene times on top of the Monte Alpi Unit is estimated to exceed 5 km. The Monte Alpi succession in the footwall to the regional thrust detachment reached temperatures ∼160°C.
Estimated burial conditions are consistent with the interpretation of Monte Alpi as part of the underthrusted Apulian Unit, whose carbonate succession forms the reservoir interval in nearby large oilfields (e.g., Shiner et al., 2004). Therefore, our results are in good agreement with previous works (e.g., Sgrosso, 1988; Cello et al., 1990; Van Dijk et al., 2000; Corrado et al., 2002) that, based on different kinds of evidence, tend to exclude interpretations linking Monte Alpi to the allochthonous Apennine Platform Unit.
Considering that the tectonic load is presently totally removed, except for discontinuous and very thin klippen of allochthonous material preserved on top of Monte Alpi, an average exhumation rate of ∼1.4 mm/y is obtained for the whole (undifferentiated) late Pliocene to Present (3.6–0m.y.) time framework. However, we used available geological, structural, and fluid inclusion data to tentatively outline a refined exhumation model. The model, summarized in the following points, also matches the maturity data obtained by vitrinite reflectance and clay mineralogy.
Substantial exhumation is envisaged to have occurred since uppermost Pliocene times as a result of the onset of shortening within the Apulian carbonates. Exhumation at this stage was probably due to the activity of relatively steep reverse faults and anticline uplift at depth and to concomitant erosion and thin-skinned extension (by collapse faults) at shallow structural levels. These processes led to the cooling of the top portion of Monte Alpi through the estimated closure temperature of the apatite system (∼123°C) by ca. 2.3 Ma.
Middle Pleistocene to Present tectonic exhumation of the Monte Alpi Unit was essentially controlled by normal/oblique-slip faulting compatible with the generalized extensional tectonic regime characterizing the whole southern Apennines at this time.
A superposition of thin- and thick-skinned deformations appears to have controlled the burial and exhumation history of the study area. Early Pliocene thin-skinned thrusting (allochthon emplacement) determined the maximum burial conditions for the analyzed portion of Apulian carbonates (i.e., Monte Alpi Unit). Subsequent (uppermost Pliocene-early Pleistocene) thick-skinned thrusting involving the Apulian carbonates themselves was most probably accompanied by thin-skinned extension in the overlying allochthon. Finally, thick-skinned extension became dominant and controlled the exhumation after compressional tectonics ceased (middle–late Pleistocene).
Extensional tectonics of both thin- and thick-skinned types is envisaged to have played a primary role in the exhumation history. On the one hand, thick-skinned extension is still active and controls the present-day seismicity of the study area. On the other hand, it is likely that thin-skinned extension substantially gave support to the exhumation during coeval deep-seated thrusting, allowing efficient mechanical removal of hanging wall material, footwall uplift, and related enhanced erosion.
APPENDIX: ANALYTICAL DETAILS
Apatite grains for fission-track analysis were separated from ∼5 kg bulk samples using standard heavy liquids and magnetic separation techniques. Mounts were ground, polished, and then etched with 5N HNO3 at 20°C for 20 seconds to reveal the spontaneous tracks. The samples were then irradiated with thermal neutrons at the Radiation Center of Oregon State University with a nominal neutron fluence of 9×1015 n cm−2. The standard glass CN-5 was used as a dosimeter to measure the neutron fluence. After irradiation, induced fission tracks in the low-U muscovite that covered apatite grain mounts and glass dosimeter were etched in 40% HF at 20°C for 45 minutes. Apatite fission-track ages were measured and calculated using the external-detector and the zeta-calibration methods (Hurford and Green, 1983) with IUGS age standards (Hurford, 1990) and a value of 0.5 for the 4π/2πgeometry correction factor. The observed grain-age distributions have been decomposed into different grain-age components by using the binomial peak-fitting method (Brandon, 1996; Stewart and Brandon, 2004).
Vitrinite is derived from the thermal degradation of wooden fragments of continental origin that can be dispersed in sediments (Stach et al., 1982; Teichmüller, 1987). Its reflectance strictly depends on the thermal evolution of the hosting sediments and is correlated to the stages of hydrocarbon generation and other thermal parameters in sedimentary environments (Durand, 1980; Scotti, 2003). Thus it is the most widely used parameter to calibrate basin modeling (Dow, 1977; Mukhopadhyay, 1994). The analyzed samples were prepared according to standardized procedures described in Bustin et al. (1990). Random reflectance was measured under oil immersion with a Zeiss Axioplan microscope, in reflected monochromatic nonpolarized light. On each sample, about twenty measurements were performed on vitrinite or bitumen unaltered fragments never smaller than 5 nm and only slightly fractured. Mean reflectance values (Ro for vitrinite and Rbit for bitumen) were calculated from the arithmetic mean of these measurements.
Clay minerals in shales and sandstones undergo diagenetic and very low-grade metamorphic reactions in response to sedimentary and/or tectonic burial. Reactions in clay minerals are irreversible under normal diagenetic and anchizonal conditions, so that exhumed sequences generally retain indices and fabrics indicative of their maximum maturity and burial.
The parameters generally used to provide information on the thermobaric evolution of sedimentary successions are the Kübler and Árkai indices (Kübler, 1967; Árkai et al., 1995; Guggenheim et al., 2002), as well as the b0 value and the variation in the relative ratio between the pure phases that form mixed layers. In particular, the illite/smectite (I/S) mixed layers are widely used in petroleum exploration as a geothermometer and, thus, as indicators of the thermal evolution of sedimentary sequences (Hoffman and Hower, 1979; Pollastro, 1990). The identified changes comply with the following scheme of progressive thermal evolution that has been correlated to the stages of hydrocarbon generation: dismectite—disordered mixed layers (R0)—ordered mixed layers (R1 and R3)—illite—dioctahedral K-mica (muscovite).
Although the conversion to paleotemperatures depends on more than one factor (e.g., temperature, heating rate, protolith, fluid composition, permeability, fluid flow), Pollastro (1990; 1993) summarized the application of two simple time-temperature models for I/S geothermometry studies based primarily on the duration of heating (or residence time) at critical I/S reaction temperatures.
The first model was developed by Hoffman and Hower (1979) for long-term, burial diagenetic settings that can be applied to most geologic and petroleum studies of sedimentary rocks and basins of Miocene age or older. In this model the major changes from R0 to R1 and from R1 to R3 occur in the temperature ranges of ∼100–110°C and of 170°–180°C, respectively (Hoffman and Hower, 1979).
The second model, which was developed for short-lived heating events, applies to young basins or areas characterized by relatively recent thermal activity with high geothermal gradients, or to recent hydrothermal environments. Such settings are those where relatively young rocks were subject to burial temperatures in excess of 100°C for <2 m.y. In this model the conversions from R0 to R1 and from R1 to R3 ordering occur at ∼130–140°C and 170–180°C, respectively (Jennings and Thompson, 1986).
For the present study, we chose the Hoffman and Hower model as the more appropriate to our study area. Each sample was prepared according to the procedures of Giampaolo and Lo Mastro (2000). X-ray powder diffraction analysis has been carried out radiation, solid with a Scintag Model X1 Diffractometer (CuKα state detector, spinner), according to the following steps:
Whole-rock samples (2÷70° 2ΘCuKα, step size 0.05 °2[thetas], count time 3 s per step)
Glicolated 2÷16 µm grain-size fraction (1÷48° 2ΘCuKα, step size 0.05 °2[thetas], count time 4 s per step)
Air-dried <2 µm grain-size fraction (1÷48° 2ΘCuKα, step size 0.05 °2[thetas], count time 4 s per step)
Glycolated ,2 µm grain-size fraction (1÷30° 2ΘCuKα, step size 0.05 °2[thetas], count time 4 s per step)
This procedure was carried out in order to define the bulk-rock (Klug and Alexander, 1974) and clay mineralogy with special regard to the illite content in mixed layer clays (Moore and Reynolds, 1989). The tube current and voltage were 40 mA and 45 kV, respectively.
X-ray oriented slides (<2 µm and 2÷16 µm grain-size fractions) were prepared by the pipette-on-slide method, keeping the specimen thickness as constant as possible, within the range of 1–3 mg of clay per cm2 of glass slide. The presence of expandable clays was determined for samples treated with ethylene glycol at 25°C for 15 h. Diffraction peaks were analyzed using the X-ray system associated program by first removing a linear background level and then fitting them using a Pearson VII function.
Fluid inclusions are small drops of fluid entrapped in crystals. Based on the time of trapping with respect to crystal growth, different types of fluid inclusions (primary, secondary, and pseudosecondary) may occur in rock crystals (Roedder, 1984). The use of microthermometric analysis enables us to obtain some quantitative information on: (i) homogenization temperatures (Th), which are indicative of the minimum trapping temperatures of the inclusions (Goldstein and Reynolds, 1994; Ceriani, 2003), and (ii) melting temperatures (Tm), which give information on fluid composition. By doing so, it is possible to choose the appropriate phase diagram for the examined fluid and to draw a P-T diagram by integrating fluid inclusion data with information obtained using other techniques (structural analysis, organic matter and clay mineralogy analyses, or low temperature thermochronology). Limitations of this technique, particularly in sedimentary rocks, can derive from: (i) the small size of the inclusions (usually between 2 and 10 µm), and (ii) the possibility that the system was not closed (isoplethic) and/or isochoric (i.e., constant volume inclusions) since the time of entrapment. In the latter instance, fluid inclusions would record thermal reequilibration at some stage of the tectonic evolution. This represents a common limitation for the study of carbonate rocks where nonisoplethic and nonisochoric conditions can develop more frequently for the presence of a soft mineral such as calcite.
For the present study, in order to unravel the origin of the fluid inclusions, an accurate petrographic analysis was performed using 150 µm thick double polished wafers. For the veins associated with brittle-ductile shear zones and faults, the wafers were cut parallel to the XZ and XY planes of the strain ellipsoid (reconstructed by the geometry of conjugate structures; Ramsay and Huber, 1987).
Microthermometry was performed using a USGS heating/freezing stage calibrated with synthetic fluid inclusions. Temperature of phase changes at T≤0°C are accurate to ±0.1°C; temperature of those ≥50°C are accurate to ±1°C.
The paper greatly benefited from discussions with Glauco Bonardi, Giuseppe Cello, Paola Di Leo, Marcello Schiattarella, Italo Sgrosso, and Emanuele Tondi. Thorough and constructive reviews by Peter Shiner and Enrico Tavarnelli substantially helped to improve the paper. Financial support by the Italian National Research Council (CNR) Agenzia 2000 and by MIUR COFIN 2004 (coordinator: Sveva Corrado), is gratefully acknowledged.
Figures & Tables
Styles of Continental Contraction
- Basilicata Italy
- Campania Italy
- carbonate rocks
- clay mineralogy
- experimental studies
- extension tectonics
- fission-track dating
- fluid inclusions
- geologic thermometry
- reservoir rocks
- sedimentary rocks
- Southern Apennines
- Southern Europe
- structural controls
- syntectonic processes
- thick-skinned tectonics
- thrust faults
- Monte Alpi