Tectonic burial and exhumation in a foreland fold and thrust belt: the Monte Alpi case history (Southern Apennines, Italy)

Geodinamica Acta 15 (2002) 159–177 www.elsevier.com/locate/geoact

Original article

Tectonic burial and exhumation in a foreland fold and thrust belt: the Monte Alpi case history (Southern Apennines, Italy) Sveva Corrado a, Chiara Invernizzi b, Stefano Mazzoli c,* b

a Dipartimento di Scienze Geologiche, Università “Roma Tre”, Largo S. Leonardo Murialdo, 1, 00146 Rome, Italy Dipartimento di Scienze della Terra, Università di Camerino, Via Gentile III da Varano, 62032 Camerino (MC), Italy c Istituto di Dinamica Ambientale, Università di Urbino, Campus Scientifico Sogesta, 61029 Urbino (PU), Italy

Received 1 June 2001; accepted 12 April 2002

Abstract The Monte Alpi area of the Southern Apennines represents the only sector of the thrust belt where the reservoir rocks (i.e. Apulian Platform carbonates) for major hydrocarbon accumulations in southern Italy are interpreted to crop out. Tectonic evolution and exhumation of this area were analysed by integrating stratigraphic and structural data with different organic and inorganic parameters which record the burial and thermal evolution of the sediments (vitrinite reflectance, fluid inclusions, and I/S mixed layers in clayey sediments). Our analyses suggest that the presently exposed Monte Alpi structure suffered a loading of ca. 4000 m, owing to the emplacement of allochthonous units in Early Pliocene times. Available geological data indicate that erosion of the tectonic load occurred since the Late Pliocene, when the area first emerged. This implies an average exhumation rate in excess of 1 mm/year. A model can be constructed which matches the maturity indices and also takes into account intermediate stages of the evolution, resulting from combined structural and fluid inclusion data. By this model, a first stage of exhumation would have taken place at an average rate of about 0.36 mm/year. This was controlled by uplift and erosion associated with both: (i) thrusting at depth within the Apulian carbonates (Late Pliocene), and (ii) strike-slip faulting (Early Pleistocene). A second exhumation stage would have occurred in the last 700 ky at a much faster rate (ca. 4 mm/year) as a result of extensional tectonics. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Tectonic loading; Thermal modelling; Petroleum systems; Neogene-Quaternary; Apennines

1. Introduction The assessment of amounts and rates of orogenic shortening and tectonic exhumation is a key issue in the study of thrust belts. A powerful tool to better constrain them is represented by the re-construction of the crustal thickening that any shortening implies. Such an evaluation generally derives from forward structural and stratigraphic modelling. Nevertheless, its assessment can be extremely difficult and can be affected by significant errors in complex areas of collisional belts. This is especially true in fold and thrust belts such as the Apennines, derived from articulated passive margins, where strong along-strike variations in

* Corresponding author. E-mail address: [email protected] (S. Mazzoli). © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 9 8 5 - 3 1 1 1 ( 0 2 ) 0 1 0 8 6 - 0

thrust geometry are common and both strike-slip and extensional tectonics can re-work the thrust stack in the later stages of deformation. In these cases, organic and inorganic parameters of the thermal evolution of sedimentary rocks are widely used to obtain a quantitative assessment of the tectonic loading—nowadays partially or totally eroded—produced by chain building. These data can be integrated in the structural and stratigraphic modelling of the chain evolution, thereby also reducing the risks in hydrocarbon exploration. In particular, the integration of vitrinite reflectance, fluid inclusions and clay mineralogy data with classical geological data sets represents a successful methodological strategy [1]. The aim of this paper is to re-construct the exhumation history resulting from the removal of the load exerted by allochthonous thrust sheets on the carbonates of the Apulian Platform in the only part of the Southern Apennines (i.e. the Monte Alpi area) where

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Fig. 1. Geological sketch map of the Southern Apennines, showing location of cross-sections (Figs. 2 and 5) and of field study area (Fig. 6; box). SA = Sant’Arcangelo Basin.

this unit is interpreted to crop out [2,3] (Fig. 1). These carbonates represent the reservoir for major oil fields in southern Italy [4–6]. Defining the modes and timing of exhumation in the Monte Alpi area is, therefore, crucial for a better understanding of the tectonic evolution of the whole chain, as well as for focusing on appropriate targets in hydrocarbon exploration and production. As a matter of

fact, this implies the assessment of the modes and rates of exhumation in Plio-Quaternary times. The integration of mineralogical and petrographical methodologies is used in this study to calibrate burial and thermal histories, and to define quantitative geological constraints to the NeogeneQuaternary evolution of this key area of the Southern Apennines fold and thrust belt.

Fig. 2. Interpreted deep geological section across the study area, based on surface geology and subsurface data (confidential). Located in Fig. 1.

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Fig. 3. Typical hydrocarbon well log in the Lucania region of the Southern Apennines, showing thick mélange zone at the bare of the allochthonous Lagonegro and Tertiary flysch units (after Butler et al., [22]).

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Fig. 4. Geological features from the Monte Alpi structure. (a) Orbulina-bearing limestone from the first transgressive cycle. (b) Bedding-parallel shape fabric in conglomerate clasts belonging to the second transgressive cycle. (c) NW-SE striking conjugate shear zones marked by en échelon, calcite-filled tension gashes in sandstones of the second transgressive cycle. Note the displacement of bedding-parallel veins (arrowed). (d) Twinning in vein calcite from the sandstones of the second transgressive cycle. (e) Healed microfractures in calcite, marked by fluid inclusion trails. (f) Details of (secondary) fluid inclusions from healed microfracture in calcite (arrows show examples containing bubbles). (g) N-S trending fault scarp in platform carbonates along the western slope of Monte Alpi. Note the dark, bituminous limestones and striae on slickenside surfaces related to oblique-slip motion (geologist for scale in lower left corner). (h) Details of previous area, showing bitumen impregnation zone extending into the country rock through a fracture network.

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Fig. 5. Shallow geological section across Monte Alpi (after Scandone, [10]). Located in Fig. 1.

2. Geological setting The Apennine fold and thrust belt of peninsular Italy forms part of the Africa-verging mountain system in the Alpine-Mediterranean area (Fig. 1). The latter evolved within the framework of convergent motion between the African and European plates since the Late Cretaceous [7,8]. The Lucania sector of the Apennines under investigation includes the remnants of a Cretaceous to Palaeogene accretionary complex (ophiolite-bearing Liguride units). Tectonically below the Liguride units, a foreland fold and thrust belt occurs, which developed at the expense of the (Afro-)Adriatic continental palaeomargin [9]. Structurally, the upper part of this thrust belt consists of allochthonous units derived from the deformation of both carbonate platform and pelagic basin successions (Apenninic Platform and Lagonegro Basin, respectively), of Triassic to Palaeogene age, and of stratigraphically overlying Neogene foredeep and wedge-top basin deposits [10–12]. These allochthonous units are completely detached from their original substratum and transported onto the foreland succession of the Apulian Platform. The latter consists of a 6–7 km thick, Mesozoic-Tertiary shallow-water carbonate succession, stratigraphically overlain by upper Messinian and/or Pliocene terrigenous marine deposits. These rocks are also partially involved in the thrust belt, forming the so-called ‘buried Apulian belt’ which underlies the allochthonous units [9] and is well known for its great potential for hydrocarbon exploration [5,6]. Analysis of synorogenic deposits indicates that thrust accretion of the units derived from the deformation of the Apenninic Platform and Lagonegro Basin passive margin successions occurred mainly in Early to Late Miocene times [9]. Thrust accretion was followed by the emplacement of these units, as a large allochthonous detachment sheet, on top of the Apulian Platform carbonates [13,14]. Deeper thrusting involving the Apulian carbonates themselves occurred mainly in Late Pliocene to Early Pleistocene times [9,14]. At surface, the Pliocene-age Sant’Arcangelo Basin, located east of our study area (Fig. 1), is only weakly deformed and seals thrust structures in the allochthon.

The late stages of thrusting were partially contemporaneous with kinematically compatible strike-slip faulting along roughly NNE-SSW trending, right-lateral, and WNWESE trending, left-lateral structures [9,15–17]. At about 700 ky, a major geodynamic change occurred and a new tectonic regime was established in the Apennine chain and in adjacent foothills [18,19]. The structures related to this new regime, characterised by a NE-SW oriented maximum extension direction, consist of extensional and transcurrent faults that post-date and dissect the thrust belt. These faults are locally seismically active [20,21]. In the last few years, a large amount of subsurface data—both 2D and locally 3D seismic reflection profiles and deep well logs—made available by the oil industry (particularly in the Lucania region) has consistently improved our understanding of the structure of the Southern Apennines [13,14]. The interpreted structure across the study area is shown in the geological section of Fig. 2. As indicated by numerous wells drilled in the area, the detachment between the allochthon and the buried Apulian units is marked by a mélange zone several hundreds of metres thick (Fig. 3). The latter consists dominantly of intensely deformed and overpressured deepwater shales and siltstones with minor sandstones and limestones. As a result of overpressuring, gas shows are common throughout this interval. The log response of this unit is very characteristic, being marked by a uniform and moderate gamma ray and a uniform and low sonic log response. Lithological and biostratigraphic information derived from cuttings returned to surface indicates the common occurrence of re-worked blocks derived from the Lagonegro and Apenninc Platform units within the mélange zone, particularly towards the top. Available biostratigraphic data indicate a Miocene–Early Pliocene age for the bulk of the sediments within this unit, which is interpreted to represent the main décollement at the base of the allochthon [14]. Mio-Pliocene foredeep deposits were most probably incorporated within the décollement zone as the advancing fold and thrust belt overrode its foreland basin deposits. The exotic blocks of Lagonegro and Apenninic Platform material within the mélange zone can be interpreted either as: (i) olistoliths derived from the

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exposed fold and thrust belt and deposited within the foreland basin, or (ii) horses tectonically accreted to the base of the allochthon by underplating of material derived from the subducted footwall, both processes being commonly documented in the formation of mélange zones [23].

3. Geology of the Monte Alpi area In the Monte Alpi area, shallow-water sediments referred to the Apulian Platform [2,18] are exposed which are separated by tectonic contacts of different types from surrounding allochthonous units (Figs. 4 and 5). The Monte Alpi succession s.s. consists of Mesozoic peritidal carbonates unconformably overlain by Miocene deposits. The former include well bedded limestones, dolomitic limestones and dolomites, grading upward to grey calcilutites containing intercalations of oolitic limestones [24]. These are unconformably overlain by deposits belonging to two transgressive cycles. The lower one consists of a sequence made up of thin levels of vacuolar limestones at the base [25], followed by about 20 m of Upper Miocene (mostly lower Messinian [26,27]) greyish biocalcarenites (containing rests of echinoderms, bivalves and ostracods) and bituminous calcilutites with Orbulina (Fig. 4a) interbedded with clayey–silty marls, capped by microconglomerates containing clasts of limestone, argillite and chert. The second transgressive cycle consists of about 200 m of interbedded sandstones and marls (of probable Messinian age), and by overlying conglomerates that are organised in beds that are 0.1 to several metres thick, clast-supported, with arenaceous matrix. The majority of the clasts derives from erosion of the local carbonate substratum; the rest is composed of siliceous argillites, cherts, micritic limestones and sandstones, probably deriving from erosion of the allochthonous Lagonegro and Liguride units, and of related Miocene foredeep/thrust top basin deposits. The dimension of the clasts varies from 2 mm to 20–30 cm, locally up to 60–70 cm for both intrabasin limestones and extrabasin materials, with medium-good sorting [25]. The fossil content of these rocks has been re-worked; in any case, a probable Messinian age can be inferred [27]. The conglomerate clasts show a pronounced oblate shape fabric consistently parallel to bedding. Significant flattening is associated with a maximum shortening direction approximately perpendicular to bedding, irrespective of the present-day bedding attitude (Fig. 4b). This provides a strong argument for burial-related deformation in roughly flat-lying sediments, prior to any significant tectonic tilting. Burial also led to the building up of high pore fluid pressures in the underlying sandstones, locally producing bedding-normal dilation and related bedding-parallel extension veins. The latter are locally cross-cut by variably trending sets of conjugate shear zones, marked by calcite-filled en échelon tension gashes (Fig. 4c) and isolated extension veins roughly perpendicular to bedding, as well as by mesofaults showing

metric displacements, all formed in response to a maximum shortening normal to bedding. In thin section, the sparry calcite filling the tension gashes displays varying degrees of twinning and undulose extinction. The calcite grains in some veins have been extensively twinned, while in others, they are essentially twin-free. These variations probably reflect the fact that veins were formed throughout a continued extensional deformation: earlier veins were subject to later intracrystalline deformation, whereas the latest veins are strain free. Twin lamellae in the more intensely strained vein calcite consist of thin (<1 µm) straight twins organised in two, or rarely three rational sets (Fig. 4d). These features (‘type I’ twins of Burkhard [28]) and the lack of thick (>>1 µm, ‘type II’) twins in calcite can be related to very low temperatures of deformation (<150° according to Burkhard [28]). Healed microveins marked by fluid inclusion trails (Fig. 4e) 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 the larger of them (Fig. 4f). On top of Monte Alpi, small klippen are preserved. They consist of quite different lithologies that have been variably (and often inconsistently) correlated with the Liguride, Apenninic Platform and Lagonegro units of the surrounding thrust belt. Upper Triassic dolomites and dolomitised micrites containing chert nodules have been ascribed to the eastern slope of the Apenninic Platform domain by Sgrosso [26] and Taddei and Siano [27]. Interbedded reddish shales and marly calcilutites, calcarenites and reddish quartzsiltites, dated as not older than Early–Middle Eocene by Alberti et al. [25], have been referred to the Lagonegro Basin succession by Bousquet [29] and Knott [30], whereas they have been ascribed to the Liguride units by Ortolani and Torre [31] and Taddei and Siano [27]. These discrepancies most probably arise by the fact that these rocks are highly disrupted, forming discontinuous bodies and lenses overriding the Monte Alpi succession. Taking into account available information on the subsurface geology (see section above), they can be best interpreted as blocks of Apenninic Platform and Lagonegro Basin material contained within the mélange interposed between the Apulian Platform carbonates and the overlying allochthonous units. Such a mélange has actually been recognised on top of Monte Alpi, surrounding the major blocks above, by detailed mapping and structural as well as stratigraphic analyses [25]; it is made of an argillaceous silty matrix containing small blocks of calcareous sandstones, micritic limestones, varicoloured shales, radiolarian cherts and calcarenites. The Monte Alpi structure is bounded on the eastern and western sides by faults. These downthrow a hanging wall succession made of Upper Cretaceous to Eocene dark shales and intercalated quartzarenites and quartzsiltites (belonging to the Liguride units), and stratigraphically overlying Miocene siliciclastic deposits (Albidona Fm). In the footwall to these faults, the Monte Alpi succession is exposed, discon-

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tinuously overlain by remnants of the tectonic mélange (Figs. 4 and 5). On the western side of the structure, a steep, N-S trending major fault is well exposed in Mesozoic shallow-water limestones (Fig. 4g). Striae on slickenside surfaces and shear fibres indicate complex superposed kinematics, ranging from strike-slip to oblique-slip with moderate angles of pitch, with a dominant sinistral component of motion. In any case, the related morphological evidence (ca. 900 m of vertical relief) suggests also an important vertical component of displacement for this structure, for which a Late Quaternary extensional activity has been recently suggested [21]. Locally, dark, bituminous limestones (source rock?) are exposed along the fault scarp. Dark impregnation zones can be observed to extend for a few metres into the surrounding country rock (Fig. 4h), whereas composite calcite-bitumen extension veins occur within the damage zone associated with the fault. On the eastern side of Monte Alpi, a northeast dipping tectonic contact separates the platform carbonates and unconformably overlying Upper Miocene deposits from the Liguride and unconformably overlying Albidona Fm cropping out to the northeast (Fig. 5). This tectonic contact, mapped as an undefined ‘detachment’ by Scandone [10], is best interpreted as a northeast dipping normal fault based on: (i) the consistent extensional cut-off relationships observed in both hanging wall and footwall to the fault, and (ii) the significant omission indicated by the excision of the Lagonegro units. The latter, in fact, crop out extensively to the west (tectonically overlain by the Liguride units), but are absent in outcrop along the eastern tectonic contact (Fig. 5), suggesting that they have been substantially downfaulted to the northeast.

4. Burial and thermal evolution: methodology The burial and thermal history of the Monte Alpi structure has been modelled based on structural and stratigraphic data calibrated with different organic and inorganic parameters which record the thermal evolution of the sediments: (i) vitrinite reflectance, (ii) fluid inclusions, and (iii) I/S mixed layers in clayey sediments. Sampling sites for the different methodologies are shown in Fig. 6. 4.1. Vitrinite reflectance Vitrinite is derived from the thermal degradation of the wooden fragments of continental origin that can be dispersed in sediments [32,33]. Its reflectance strictly depends on the thermal evolution of the hosting sediments and is correlated with the stages of hydrocarbon generation and other thermal parameters in sedimentary environments (Fig. 7). Thus, it is the most widely used parameter to calibrate basin modelling [34–36].

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The analysed samples have been collected from the outcropping Monte Alpi Meso-Cenozoic succession, in particular, from Jurassic limestones and from the overlying Messinian transgressive deposits. They were prepared according to standardised procedures described in Bustin et al. [37]. Random reflectance was measured under oil immersion with a Zeiss Axioplan microscope, in reflected monochromatic non-polarised light. On each sample, about 20 measurements were performed on vitrinite or bitumen unaltered fragments never smaller than 5 µm and only slightly fractured. Mean reflectance values (Ro for vitrinite and Rb for bitumen) were calculated from the arithmetic mean of these measurements. Rb values were converted into vitrinite equivalent reflectance data (Roeq) using Jacob’s equation: Rb = Roeq × 0.618 + 0.40

(1)

where Rb is the value of reflectance measured on bitumen and Roeq is the value that autochtonous vitrinite would have acquired if it would have been present in the same sample. 4.2. Fluid inclusions Different types of fluid inclusions (primary, secondary, pseudosecondary) may occur in sedimentary rocks [38]. Therefore, it is necessary to unravel their genesis before quantitative information can be derived by means of microthermometry. The latter technique allows one to measure: (i) homogenisation temperatures (Th), which are indicative of the minimum trapping temperatures [39], and (ii) melting temperatures (Tm), which give information on fluid composition. By doing so, it is possible to choose a phase diagram for the examined fluid and to draw a P–T diagram by integrating fluid inclusion data with information obtained by other techniques (structural analysis, organic matter and clay mineralogy analyses). Limitations of this technique can derive from: (i) the small size of the inclusions (usually between 2 and 10 µm in sedimentary rocks), 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 re-equilibration at some stage of the tectonic evolution. This represents a common limitation for the study of carbonate rocks where non-isoplethic and non-isochoric conditions can develop more frequently, due to the presence of ‘soft’ minerals like gypsum and calcite. The samples analysed in this study were collected from calcite extension vein systems that cross-cut the rocks of the Upper Miocene transgressive cycles unconformably overlying the Mesozoic carbonates of Monte Alpi. They come from different types of structures (described in the previous paragraphs): (i) bedding-parallel extension veins (sample F10), (ii) arrays of en échelon tension gashes associated with extensional ‘brittle–ductile’ [40] shear zones (samples

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Fig. 6. Geological sketch map of the Monte Alpi area (modified after Alberti et al., [25]) showing sampling sites (V for vitrinite reflectance; F for fluid inclusions; C for clay mineralogy). Located in Fig. 1.

F1, F2, F3), and (iii) bedding-normal extension veins associated with nearby normal faults (sample F13).

4.3. Clay mineralogy

An accurate petrographic analysis was performed in order to unravel the origin of the fluid inclusions: 100 µm thick double polished wafers were used for this analysis. 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 (re-constructed by the geometry of conjugate structures [40]). On those samples which were suitable, microthermometry was performed using the USGS stage.

In siliciclastic sediments, clay minerals are the only inorganic phases that provide pieces of information on their thermo-baric evolution (diagenesis–anchimetamorphism–epizone). The parameters generally used are the crystallinity index of some phases (e.g. illite), 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

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Fig. 7. Correlation chart among types of I/S mixed layers, Ro values, coalification ranks, palaeotemperatures calculated according to Burnham and Sweeney [24].

evolution of sedimentary sequences [41,42]. The identified changes follow this scheme of progressive thermal evolution that has been correlated with the stages of hydrocarbon generation (Fig. 7): smectite—disordered mixed layers (R0)–ordered mixed layers (R1 and R3)—illite. Although the conversion to palaeotemperatures depends on more than one factor (e.g. temperature, heating rate, protolith, fluid composition, permeability, fluid flow), Pollastro [42,43] summarised 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 [41] 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. The second model, which was developed for short-lived heating events, applies to young basins or areas characterised by relatively recent thermal activity with high geothermal gradients, or to recent hydrothermal environments. In this study, we applied the first model as it is clearly the most appropriate to our study area. The samples analysed for our modelling of the Monte Alpi area derive from sandstones of the second transgressive cycle cropping out on top of the Mesozoic carbonates.

On each sample prepared according to the procedures of Rumbley and Adatte [44] and Kübler [45], the X-ray powder diffraction analysis has been carried out with a Scintag Model X1 Diffractometre, according to the following steps: • tout venant (bulk powder) 2–70° 2ΘCuKα1°/min; • fraction 2–16 µm 1–50° 2ΘCuKα1°/min; • fraction < 2 µm 1–50° 2ΘCuKα1°/min; • glycolised fraction < 2 µm 1–30° 2ΘCuKα1°/min; in order to define the bulk-rock [46–49] and clay mineralogy with special regard to the content in mixed layers [46,50]. 4.4. Burial and thermal modelling In order to constrain the evolution of the Monte Alpi structure, a simplified re-construction of the burial and thermal history was performed using the software package Basin Mod 1-D [51]. Stratigraphic data, such as thickness, lithology composition, and age of formations, are derived from detailed studies performed on the Monte Alpi structure, while timing and modes of deformation are deduced from wider regional studies performed in the Lucania area [2,9,10,13,14,17,52]. A synoptic view of the data used in modelling and of its main results is given in Table 1.

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Table 1 Main depositional and tectonic events related to the tectonic evolution of Monte Alpi. Thickness of the allochthon and exhumation rates for Monte Alpi were obtained by our modelling (Fig. 11) Main depositional and tectonic events

Timing

Exhumation rate/thickness

Extensional tectonics, uplift + erosion Strike-slip tectonics, uplift + erosion Thrusting within Apulian carbonates, uplift + erosion Emplacement of the allochthonous units 2nd sedimentary cycle

Middle Pleistocene–Holocene (1; 2) Early Pleistocene (3) Late Pliocene (5; 6; 7) Lower Pliocene (4; 5; 6; 7) Not older than Lower Messinian (9) Upper Messinian (8) Lower Messinian (8; 9) Bajocian–Tithonian (10)

4 mm/year (this study) 0.36 mm/year (this study) 0.36 mm/year (this study) 3750 m (this study) ≈ 200 m (9)

1st sedimentary cycle Carbonate platform

≈ 30 m (9) 1087 m (10)

(1) Data from Clinque et al. [18]; (2) data from Hyppolite et al. [19]; (3) data from Catalano et al. [16]; (4) data from Cello et al. [3]; (5) data from Cello and Mazzoli [9]; (6) data from Mazzoli et al. [13]; (7) data from Mazzoli et al. [14]; (8) data from Sgrosso [26]; (9) data from Taddei and Siano [27]; (10) data from Sartoni and Crescenti [24].

The main assumptions adopted for this re-construction are that: (i) lithology decompaction factors are only applicable to sedimentary clastic units (i.e. Miocene rocks belonging to the first and second transgressive cycles) according to the method of Sclater and Christie [53], while carbonate units are not decompacted; (ii) seawater depth variations in time are irrelevant in modelling, as thermal evolution is mainly affected by sediment thicknesses rather than by water depths [54]; (iii) thermal modelling has been performed using LLNL Easy %Ro method based on Burnham and Sweeney [34] and Sweeney and Burnham [55]; (iv) thrusting can be considered instantaneous when compared with the duration of deposition of stratigraphic successions, as it is generally suggested by theoretical models [56]; (v) emplacement of the allochthon on top of the Monte Alpi shallow-water carbonates occurred immediately after the closure of marine sedimentation (≈5.3 My); (vi) for the sake of simplicity, post-thrusting exhumation is considered linear for given time intervals; and (vii) a constant heat flow of 50 mW/m2 has been adopted in agreement with the average present-day value [57,58].

5. Burial and thermal evolution: results

5.1. Vitrinite reflectance

Among the numerous samples analysed, only two gave reliable results for modelling as they show one main population of measurements with a symmetric distribution (Fig. 8). The first sample (V1) was collected at the base of the stratigraphic succession in Jurassic limestones, where bitumen presumably only slightly migrated. Obtained Roeq is 2.62%, with a standard deviation of 0.16%. The second sample (V2) derives from coarse sandstones of the second trangressive cycle (about 60 m above the stratigraphic boundary with the first transgressive cycle). Its Ro is 1.54%, with a standard deviation of 0.09%. The other samples deriving from rocks belonging to both Miocene trangressive cycles exposed on top of Monte Alpi either do not contain vitrinite or contain highly oxidised/re-worked vitrinite fragments.

Fig. 8. Analytical results for Monte Alpi samples: vitrinite reflectance histograms.

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5.2. Fluid inclusions As already mentioned, microstructural analysis on sampled calcite veins revealed the presence of strongly twinned crystals together with a few clear (undeformed) ones. Twins are mainly thin and dense, sometimes bent (Fig. 4d). Some deformed crystals show overgrowth margins along which calcite is lighter. Healed microveins are also present: they are oriented both parallel and perpendicular to major vein edges. Sample F1, obtained from the biocalcarenites of the first transgressive cycle, shows bitumen-calcite microveins in which calcite crystals are clear enough and fluid inclusions are preserved. Dark bitumen is observed both in transgranular veins and in inclusions within single grains. In this sample, a large number of fluid inclusions were observed, but they are generally too small to be measured. Only a few primary inclusions, both isolated or in small groups, are suitable for microthermometry. The latter have slightly to highly elongated shapes and contain a vapour percentage around 30%. A large amount of fluid inclusions is present in all samples: they are both primary and secondary inclusions, more evident in less deformed crystals. Most of the primary inclusions, which are mainly aligned along growth boundaries, are decrepitated or re-equilibrated. In fact, it is not rare to observe clusters of small inclusions derived from re-equilibration of an original larger inclusion or fracturing features in the form of thin ‘tails’ departing from the inclusion. These textures suggest that the inclusions devel oped internal overpressure, according to the isothermal decompression experiments of Vityk and Bodnar [59]. Secondary inclusions, occurring along healed microveins, are better preserved. They exhibit one or two phases, and show rounded or slightly elongated shapes (Fig. 4f). A small bubble is present in larger inclusions (about 15% of vapour), while the lack of vapour bubble in very small inclusions is probably due to metastability [38]. Secondary fluid inclusion assemblages, useful for microthermometry, are visible in some areas of sample F2, collected from Upper Miocene sandstones of the second transgressive cycle. In sample F3, obtained from the conglomerates of the second transgressive cycle, it was possible to observe secondary fluid inclusions both in quartz grains belonging to the conglomerate sandy matrix and in calcite crystals from extension veins. Fluid inclusions in quartz are better visible: they are two phase inclusions, elongated in shape with a small vapour bubble (V = 10%). Fluid inclusions in calcite show similar shapes and phases but are less frequent; they are very small and exclusively present in a few clear crystals. Microthermometric analysis gave good results for samples F2, F3, F10 and F13. The obtained data, although not very numerous, are very consistent. Fluid composition based on Tm ice and few eutectic temperatures belong to the

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H2O–NaCl system. A 2.5–3.0 wt% equivalent of NaCl is suggested, applying Bodnar’s [60] equation for H2O–NaCl systems. Homogenisation temperatures relative to secondary inclusions are shown in Fig. 9; Thmean is 109 °C. According to Barker and Goldstein [61], Thmean can be an approximation of Tpeak for sedimentary basins. However, it is also known that in sedimentary basins, Thmax can represent the minimum trapping temperature for fluid inclusions [39]. The mean value of Thmax obtained from our samples F2 and F3 is of 130 °C. Sample F10, collected from Upper Miocene sandstones of the second transgressive cycle, contains both primary and secondary inclusions. Ice melting temperatures (Tm) are comprised between –1 and –2 °C, suggesting that the entrapped fluid consists of low-salinity water. Homogenisation temperatures relative to primary inclusions are mostly in the range of 55–123 °C (with an isolated peak at 154 °C) and show a wide dispersion (Fig. 9). This, together with the widespread occurrence of decrepitated fluid inclusions, suggests that re-equilibration probably occurred during exhumation, thereby changing the composition and/or the volume of most primary inclusions. Secondary inclusions from the same sample display homogenisation temperatures in the range of 60–90 °C (Fig. 9). In sample F13, obtained from the conglomerates of the second transgressive cycle, just primary fluid inclusions were observed. Also in this instance, ice melting temperatures (Tm)—comprised between –1 and –2 °C—suggest that the entrapped fluid consists of low-salinity water. Homogenisation temperatures for this sample, although quite dispersed, are mostly in the range of 95–120 °C, with isolated peaks at about 150 and 170 °C (Fig. 9). 5.3. Clay mineralogy The results of the semi-quantitative bulk-rock analyses define the rocks of the second trangressive cycle as being essentially sandstones with calcite cement characterised by a very high percentage of quartz, calcite, plagioclase, k-feldspar and subordinately muscovite and pyroxene. The clay mineral components, when present, are mainly repre sented by kaolinite–chlorite and illite that are exclusively of the 2–16 µm fraction. On the 2 µm fraction, I/S orderly oriented clays, in which the illite component is strongly dominant, are also present in two of the analysed samples (Fig. 10). Considering the XRD pattern definition in evaluating the quantity ∆2Θ [50], the illite percentage is about 70–80%. Moreover mixed-layer chlorite/smectite and illite/vermiculite are present in samples where I/S mixed layers are absent. The clastic origin of the analysed samples coupled with the difference in clay content between the fraction 2–16 µm and the fraction <2 µm suggests that only the latter is autochthonous. Thus, only the samples containing ordered I/S mixed layers have been employed for modelling; these,

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Fig. 9. Analytical results for Monte Alpi samples: fluid inclusion Th (°C) histograms.

according to Pollastro [42], indicate an average temperature comprised between 170 and 180 °C.

6. Modelling results and discussion The results of our modelling that explain better the thermal and burial evolution of the Monte Alpi structure are shown in Fig. 11. From the diagram in Fig. 11(a), it is clear that, after a progressive burial due to the carbonate platform evolution in Jurassic times, a hiatus in sedimentation occurred between deposition of the platform carbonates and Late Miocene sedimentation. The facies, lithological composition and clast size of the rocks belonging to the second

Messinian transgressive cycle indicate a very proximal source area for these deposits. This suggests in turn that the allochthonous units were very close to the depositional area. This fits the regional data that indicate an Early Pliocene overthrusting of the allochthonous sheets on top of the Monte Alpi strata [9]. Therefore, we simulated an instantaneous emplacement of the allochthon in Early Pliocene times. This brought about a fast tectonic burial of the Monte Alpi succession and its subsequent permanence in the footwall to the regional thrust at depths greater than 3750 m (Fig. 11a) for about 2 My (1.9 My). This is calibrated against organic matter and clay mineralogy data that constrain the maximum temperatures reached by the Monte Alpi sedimentary succession.

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Fig. 10. Analytical results for Monte Alpi samples: diffractograms of the <2 µm fraction of glycol-saturated specimens.

It can be inferred that the tectonic load started being eroded off Monte Alpi since the Late Pliocene, when the area emerged and the Sant’Arcangelo Basin sediments started to lap onto the area that at the time represented the external edge of the subaerial chain. Furthermore, the tectonic load is nowadays totally eroded. In fact, we have observed that only discontinuous and very thin klippen of allochthonous/mélange material are preserved on top of Monte Alpi. Thus, in our model, the tectonic overburden was removed since the Late Pliocene onwards, up to a complete removal at present. Our re-construction of the modes and rates of exhumation of Monte Alpi is based on available geological, microstructural and fluid inclusion data. Microthermometry deduced from fluid inclusion analyses can be used to constrain intermediate stages of exhumation. In fact, the various types of sampled extension veins are all likely to have developed after the emplacement of the allochthon. Bedding-parallel extension veins are best related to pore fluid overpressure associated with burial. These veins were sampled from the Upper Miocene part of the Monte Alpi succession. As the thickness of stratigraphically overlying deposits is quite limited, the significant load necessary for the development of these structures was most probably provided by the tectonic emplacement of the allochthon. Therefore, primary fluid inclusions contained in these veins might record the maximum burial conditions experienced by the Monte Alpi succession. Unfortunately, substantial re-equilibration appears to have occurred for most primary inclusions in sample F10 (although an isolated peak temperature in excess of 150 °C is recorded). Secondary fluid

inclusions from the same samples indicate much lower temperatures (Fig. 9), hence suggesting that they formed during later exhumation stages. Bedding-perpendicular extension veins, both isolated or organised into en échelon arrays, indicate a maximum shortening perpendicular to (originally flat-lying) bedding. These structures cross-cut (and, therefore, post-date) bedding-parallel extension veins. These observations suggest that bedding-perpendicular veins formed late in the exhumation history, most probably as a result of extensional tectonics (which, as discussed in the previous sections, started in post-Early Pleistocene times in the study area [19]). If this interpretation is correct, then fluid inclusion microthermometry from these veins can be used to constrain the temperature conditions at the onset of extensional tectonics (i.e. at 700 ky in our exhumation history). Unfortunately, this is not so straightforward. Fluid inclusion data come essentially from less deformed or undeformed vein calcite crystals. Therefore, they do not record peak tempera ture conditions that controlled earlier intracrystalline deformation (twinning) in the more strained vein calcite. In fact, intensely twinned calcite crystals show many inclusions (of both primary and secondary types) which are decrepitated or re-equilibrated. The latter often display textures which indicate a re-equilibration of the system at different P/T conditions from those of fluid entrapment, due to ongoing exhumation [62]. Textures of the type observed in the analysed samples are experimentally produced in isothermal decompression regimes [59], suggesting that relatively fast exhumation took place.

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Fig. 11. Model of the burial and thermal history for the Monte Alpi structure (stratigraphic units: 1 = Miocene rocks of the second transgressive cycle; 2 = Miocene rocks of the first transgressive cycle; 3 = Mesozoic platform carbonates). Depth for each sample is based on outcrop distribution. (a) Tectonic overburden of 3750 m (at present totally removed) and variable exhumation rate in Late Pliocene–Holocene times. (a1) Detail of diagram (a) related to the exhumation history. (b) Time–Maturity diagram derived from (a).

The higher vapour percentage (about 30%) in the few primary inclusions measured in sample F1 suggests a possible re-equilibration (stretching) for these inclusions, providing a further indication for internal overpressure in primary inclusions. Re-equilibration during exhumation is most probably also responsible for the wide distribution of homogenisation temperatures from primary fluid inclusions in sample F13 (Fig. 9). Homogenisation temperatures relative to secondary fluid inclusions from en échelon vein arrays (samples F2 and F3) appear to be more consistent. However, such secondary inclusions, mainly aligned along healed microfractures, clearly indicate that the system was re-opened during deformation and the entrapped fluid records a part of this history. As a consequence, the mean homogenisation temperature (109 °C) obtained for second ary fluid inclusions from samples F2 and F3 is very likely to represent a substantial underestimate of the actual thermal conditions that controlled vein formation and intracrystalline deformation in calcite during the first extensional stages. During these stages, the temperatures experienced by the analysed rocks are likely to have been at least

comparable with the maximum mean homogenisation temperature (Thmax = 130 °C) obtained from samples F2 and F3. Based on the considerations above and on microstructural observations on vein calcite—suggesting peak temperature conditions in any case below 150 °C—the most reliable value of Thmax (130 °C) relative to fluid inclusion data from extensional shear zones has been used as a first approximation for the thermal conditions during the early stages of extensional tectonics (assumed at ca. 700 ky). The resulting model is shown in Fig. 11(a1), where two discrete stages of exhumation in Plio-Quaternary times are simulated: a slower one, occurring between the Late Pliocene and the Early Pleistocene (at a constant rate of about 0.36 mm/year), and a faster one, starting in post-Early Pleistocene times with the onset of extensional tectonics (at a constant rate of about 4 mm/year). Slow unroofing in the first stage of the evolution is likely to have resulted essentially from uplift and erosion. Regional (isostatically induced) uplift of the thrust belt was most probably enhanced in the Monte Alpi area as a consequence of: (i) Late

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Fig. 12. Model of the burial and thermal history for the Monte Alpi structure (stratigraphic units: 1 = Miocene rocks of the second transgressive cycle; 2 = Miocene rocks of the first transgressive cycle; 3 = Mesozoic platform carbonates). Depth for each sample is based on outcrop distribution. (a) Tectonic overburden including a 1300 m thick Cretaceous succession—removed at the end of the Cretaceous—and variable exhumation rate in Late Pliocene–Holocene times. (b) Time–Maturity diagram derived from (a).

Pliocene thrusting within the Apulian carbonates, of which present-day Monte Alpi represented a part of the succession involved in the crestal zone of a broad thrust-related antiform [3]; and (ii) Early Pleistocene strike-slip faulting and related ‘pushing-up’ of Monte Alpi [16]. Fast exhumation during post-Early Pleistocene times (at an average rate of ca. 4 mm/year) was most probably the result of both tectonic unroofing and enhanced uplift (and related erosion) associated with extensional tectonics. In particular, the removal of the hanging wall succession to the east-bounding fault of Monte Alpi (refer to Fig. 5) and the related footwall uplift, together with the tectonic uplift associated with the activity of the steep west bounding fault, are very likely to have controlled the later stages of exhumation of the structure. By the type of evolution envisaged here, an acceptable calibration against measured data is achieved, as shown by the resulting maturity curve of Fig. 11(b). As at Monte Alpi, Upper Miocene syn-orogenic deposits unconformably overlie Jurassic peritidal platform carbonates, we also checked the possibility that the lacking Cretaceous part of the carbonate platform succession was originally deposited and then removed by erosion prior to the deposition of the Miocene transgressive cycles. To this purpose, we attempted a new modelling, adding (in Cretaceous times) and then removing (at the end of the Cretaceous) a variable thickness of carbonates belonging to the Apulian (Monte Alpi) Platform. A significant change in comparison with the model shown in Fig. 11(a) becomes evident only when this thickness is in excess of 3000 m. However, available regional information on the stratigraphy

of the Apulian Platform (cf. Puglia 1 well; [22]) indicates that a maximum thickness of 1300 m can be considered as realistic for this interval. Thus, we modified the burial history by adding and then removing 1300 m of carbonates as shown in Fig. 12(a). The final thermal history (Fig. 12b) does not show any relevant difference in comparison with the previous model (Fig. 11b). The only difference consists in the timing of maturation, that in the model of Fig. 12(a,b) starts much earlier (i.e. in Early Cretaceous times for the Jurassic platform carbonates). Aside from the contribution that the model shown in Figs. 11 and 12 gives to the Neogene-Quaternary evolution of Monte Alpi, to strengthen the methodological approach adopted in this study, we present two further hypotheses that are partially or totally unacceptable. In the model shown in Fig. 13, tectonic loading is calibrated only with Ro and clay mineralogy data and totally removed at a constant rate of 1.14 mm/year since the Late Pliocene onwards. It has to be noted that this model does not imply any specific exhumation process, being either: (i) uplift and erosion, (ii) the mechanical removal of material by extensional faulting, or (iii) a combination of both. According to the modelled evolution, the calculated curve of maturity through time shown in Fig. 13(b) is in good agreement with measured maturity indicators. This model provides, therefore, only a first approximation to the exhumation history of Monte Alpi, allowing one to estimate the average rate at which unroofing took place since the emplacement of the allochthon on top of the Monte Alpi succession.

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Fig. 13. Simplified model of the burial and thermal history for the Monte Alpi structure (stratigraphic units: 1 = Miocene rocks of the second transgressive cycle; 2 = Miocene rocks of the first transgressive cycle; 3 = Mesozoic platform carbonates). Depth for each sample is based on outcrop distribution. (a) Tectonic overburden of 3750 m (at present totally removed) and constant exhumation rate (1.14 mm/y) in Late Pliocene–Holocene times. (b) Time–Maturity diagram derived from (a).

A further model calibrated only with Ro and clay mineralogy data shows the thermal maturity evolution in the case where no thrusting had occurred on top of the Monte Alpi unit (Fig. 14). This model is clearly not realistic for both the lack of correlation between calculated and measured maturity, and the field data strongly supporting a Pliocene emplacement of allochthonous units on top of the structure and their almost complete removal in Quaternary times.

Thus, the comparison between the models shown in Figs. 11–13 (both acceptable, although with different degrees of resolution of the exhumation history) and in Fig. 14 (unacceptable) underlines the necessity to cross-check this kind of re-constructions with different palaeotemperature indicators and geological constraints. A simplified model (Fig. 13) could be applied as a first approximation to successfully evaluate the amount of burial and the average exhumation rate, also in areas where fluid inclusion and/or

Fig. 14. Simplified model of the burial and thermal history for the Monte Alpi structure (stratigraphic units: 1 = Miocene rocks of the second transgressive cycle; 2 = Miocene rocks of the first transgressive cycle; 3 = Mesozoic platform carbonates). Depth for each sample is based on outcrop distribution. (a) No tectonic overburden. (b) Time–Maturity diagram derived from case (a).

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microstructural data were not available. Regional constraints and structural observations, combined with fluid inclusion microthermometry, may be used to better constrain the exhumation history in a more refined model such as that shown in Figs. 11 and 12. A further constraint on the exhumation history could be provided by fission tracks analyses. However, a preliminary attempt to apply this methodology to the rocks of the second transgressive cycle of Monte Alpi has given unsatisfactory results, due to the scarce occurrence of apatite crystals.

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unroofing during the first stage can be interpreted as related mainly to uplift and erosion associated with both thrusting within the Apulian carbonates (Late Pliocene [9,14]) and strike-slip faulting (Early Pleistocene [15–17]). A relatively fast exhumation in post-Early Pleistocene times resulted by a combination of both mechanical removal of hanging wall material and footwall uplift (and related erosion) associated with extensional tectonics.

Acknowledgements 7. Conclusions The presently exposed Monte Alpi structure suffered a huge loading due to the emplacement of allochthonous units in Early Pliocene times. According to the modelling performed in this study, this load can be estimated as being represented by about 4000 m of material. This value is not very different from the average thickness of the allochtho nous units overlying the buried Apulian carbonates in the main oil fields of Lucania [13]. Our data provide, therefore, a further support to the interpretation that the carbonates of Monte Alpi belong to the Apulian Platform [2,18], an attribution that has been somewhat controversial in the last decade [5]. Our data as well as the revision of previous works also suggest that the ‘klippen’ preserved on top of the Monte Alpi structure actually represent remnants of a mélange zone originally interposed between the Apulian Platform carbonates and the overlying far-travelled detachment sheet. This mélange zone, several hundreds of metres thick in the numerous oil wells drilled in Lucania, is interpreted to represent the main décollement at the base of the allochthon [14]. The tectonic load started to be eroded since Late Pliocene times, when the area first emerged. A simplified model implying a Late Pliocene–Quaternary unroofing at a constant rate provides an average exhumation rate in excess of 1 mm/year (1.14 mm/year). This value is quite high when compared with the surrounding area where the allochthon and/or the underlying mélange are partially preserved on top of the buried Apulian Platform carbonates with an average thickness of more than 1000 m (as known from hydrocarbon wells). Thus, it is reasonable that the exhumation rate drastically grew in post-Early Pleistocene times as a result of the localised unroofing of the Monte Alpi structure related to the late tectonic stages. Available geological, microstructural and fluid inclusion data were used to obtain a refined exhumation model which also matches the maturity data obtained by vitrinite reflectance and clay mineralogy. This exhumation model includes two discrete stages: (i) a slower one, taking place in the Late Pliocene–Early Pleistocene time interval at an average rate of about 0.36 mm/year, and (ii) a faster one, occurring during the last 700 ky (i.e. after the onset of extensional tectonics) at an average rate of about 4 mm/year. Slow

We wish to thank Jean-François Becq-Giraudon and an anonymous referee for their constructive reviews and the useful comments. Financial support by MURST (cofin99; D. Cosentino), Università di Camerino (ex-60%; C. Invernizzi) and Università di Urbino (ex-60%; S. Mazzoli) is gratefully acknowledged.

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