The Plio–Pleistocene evolution of extensional tectonics in northern Tuscany, as constrained by new gravimetric data from the Montecarlo Basin (lower Arno Valley, Italy)

Tectonophysics 330 (2001) 25–43 www.elsevier.com/locate/tecto

The Plio–Pleistocene evolution of extensional tectonics in northern Tuscany, as constrained by new gravimetric data from the Montecarlo Basin (lower Arno Valley, Italy) P. Cantini a,*, G. Testa b,1, G. Zanchetta b, R. Cavallini c b

a Dipartimento di Scienze della Terra dell’Universita` di Pisa, via S. Maria 53, 56126 Pisa, Italy CNR-Centro di Studio per la Geologia Strutturale e Dinamica dell’Appennino, via S. Maria 53, 56126 Pisa, Italy c P.zza Garibaldi, 4, 56025 Pontedera, Pisa, Italy

Received 8 March 2000; accepted 14 September 2000

Abstract A detailed gravimetric study has been integrated with the most recent stratigraphic data in the area comprised between the Arno river and the foothills of the Northern Apennines, in northern Tuscany (central Italy). A Plio–Pleistocene basin lies in this area; its sedimentary succession can be subdivided from the bottom, in five allostratigraphic units: (1) Lower– Middle Pliocene shallow marine deposits; (2) Late Pliocene (?)–Early Pleistocene fluvio-lacustrine deposits; (3) late– Early Pleistocene–Middle Pleistocene alluvial to fluvial red conglomerates (Montecarlo Formation); (4) Middle Pleistocene alluvial to fluvial red conglomerates (Cerbaie and Casa Poggio ai Lecci Formations); (5) alluvial to fluvial deposits of Late Pleistocene age. The Bouguer anomaly map displays a strong minimum in the northeastern sector of the basin, and a gentle gradient from west to east. The map of the horizontal gradients permits to recognise three major fault zones, two of which along the southwestern and northeastern margins of the basin, and one along the southeastern edge of the Pisani Mountains. A 2.5D gravimetric modelling along a SW–NE section across the basin displays a thick wedge of sediments of density 2.25 g/cm 3 (about 1700 m in the depocenter) overlying a layer of density 2.55 g/cm 3, 1000 m thick, which rests on a basement of 2.72 g/cm 3. The most of the sediment wedge is here referred to Upper Pliocene (?)–Lower Pleistocene, because borehole data show Pliocene marine deposits thinning northward close to the southern margin of the area. The layer below is referred to Ligurids and upper Tuscan Nappe units; the densest layer is interpreted as composed of Triassic evaporites, quartzites and Palaeozoic basement. According to Carmignani low-angle extensional tectonics began between Serravallian and early Messinian, thinning the Apennine nappe stack. At the end of Middle Pliocene, syn-rift deposition ceased in the Viareggio Basin (west of the investigated area) as demonstrated by Argnani and co-workers, and high-angle extensional tectonics migrated eastward up to the Monte Albano Ridge. A synrift continental sedimentary wedge developed in Late Pliocene–Early Pleistocene, until its hanging wall block was dismembered, during late Early Pleistocene, by NE-dipping faults, causing the uplift of its western portion (the Pisani Mountains). This breakup caused exhumation and erosion of Triassic units whose clastics where shed into the surrounding palaeo-Arno Valley in alluvial–fluvial deposits unconformably overlying the Lower Pleistocene syn-rift deposits. In the late Pleistocene SW–NE-trending fault systems created the steep southeastern edge of the Pisani Mountains and the resulting throw is recorded in Middle Pleistocene deposits across the present Arno Valley. This tectonic phase probably

* Corresponding author. Fax: ⫹39-050500932. E-mail addresses: [email protected] (P. Cantini), [email protected] (G. Testa). 1 Present address: Provincia di Pisa, Servizio Difesa del Suolo, Via P. Nenni, 24, 56124 Pisa, Italy; fax: ⫹39-050929680. 0040-1951/00/$ - see front matter 䉷 2001 Elsevier Science B.V. All rights reserved. PII: S0040-195 1(00)00217-1

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continues at present, offshore Livorno, as evidenced by the epicentres of earthquakes. 䉷 2001 Elsevier Science B.V. All rights reserved. Keywords: Northern Apennines; Pliocene; Pleistocene; gravity; extension tectonics; neotectonics

1. Introduction The Apennine chain originated since Late Cretaceous as the result of subduction and collision of Adria plate with the European margin (Elter et al., 1975; Carmignani et al., 1994). Since Middle Miocene times, the thrust front started migrating northeastward involving progressively outer portions of the Adria-plate sedimentary-cover. At the same time sedimentary basins started forming few tens of kilometres back of the external thrust-front. Most of the authors consider these basins as formed by the extensional regional strain-field that led to the formation of the Tyrrhenian Basin (Patacca et al., 1990; Carmignani et al., 1994). The Neogene– Quaternary basins outcropping in Tuscany (Italy) are crucial to the understanding of the extensional evolution of the northern Tyrrhenian margin of the Apennines. This paper presents the analysis of the anomalies of new gravimetric data from one of the northernmost Plio–Pleistocene basins of the western side of the Apennine — the Montecarlo Basin — interprets these data integrating the most recent stratigraphic data (Zanchetta et al., 1995; Caredio et al., 1995) with the subsurface data available (Ghelardoni et al., 1968; Mariani and Prato, 1988; Bartole et al., 1991; Bellani et al., 1994; Argnani et al., 1997), and reconstructs the tectonic evolution of the Montecarlo Basin. Finally, we consider these results in the larger context of the northern Tuscany basins to arrive at a new working hypothesis about the Miocene–Quaternary tectonic evolution of this entire area.

2. Geological setting The Neogene–Quaternary basins of the Tyrrhenian margin of the Apennines are separated by structural highs made of allochtonous units of Mesozoic to Paleogene age, belonging to the Ligurian, Subligurian and Tuscan Domains, that were overthrusted

mostly in Early Miocene times (Baldacci et al., 1967; Carmignani and Kligfield, 1990). The Montecarlo Basin is bounded to the east by the Monte Albano ridge and to the west by the Pisani Mountains high through a NW–SE-trending fault. To the north the margin of the basin strikes roughly east–west along the foot-hills of the Apennine (Fig. 1). When observed at large scale this margin is an envelope of several NW–SE faults distributed along an en echelon array, and connected by NE–SW faults. To the west the Montecarlo Basin is bounded by the Pisani Mountains through a NW–SE-trending fault. The Pisani Mountains are bounded to the west, by the Quaternary deposits of the Pisa plain by a southwest dipping normal fault (Ghelardoni et al., 1968; Argnani et al., 1997). These deposits unconformably overly Mesozoic carbonates of the Tuscan nappe (Bellani et al., 1994). The sedimentary fill of the Montecarlo Basin that will be modelled in Section 6, is represented mostly by Lower Pleistocene fluvio-lacustrine deposits. Their southernmost outcrops run along the north side of the Arno Valley, which therefore will be taken as the southern edge of the basin. The Montecarlo Basin is segmented in two hydrographic basins by the low relieves of the Montecarlo ridge and Cerbaie hills. Pliocene marine deposits outcrop in its southern part.

3. Stratigraphy The Plio–Pleistocene succession of the basin rests unconformably on the allochtonous Apennine units. The substrate outcropping along the margins of the Montecarlo Basin is represented by the following nappe-units from the top: Late Cretaceous–Eocene Ligurid and Subligurid Units made of argillite and limestone; Tuscan Nappe subdivided in Upper Triassic–Lower Cretaceous carbonates, Late Cretaceous– Eocene argillite (Scaglia Formation), Upper Oligocene–Lower Miocene turbiditic sandstone (Macigno

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Fig. 1. Geologic sketch of the lower Arno Valley and adjoining areas. Borehole stratigraphy is reported in Ghelardoni et al. (1968).

Formation); Metamorphic units consisting of Upper Triassic siliciclastic metasediments (Verrucano Formation), anhydrites and dolomites followed by a metamorphosed sedimentary succession equivalent of the Tuscan Nappe succession. The oldest autochthonous sediments in the investigated area have been found in the Cerbaie-1, Certaldo-2 and Certaldo-3, boreholes at the southern edge of the basin (Fig. 1), and tentatively referred to Upper Miocene by Ghelardoni et al. (1968). They are fluvio-lacustrine marl clay and silty sand and pinch out northward (Cerbaie-1 borehole), in correspondence of the southern slope of the Cerbaie relief. The Pliocene marine sediments follow the same

trend. It is unclear whether they pinch-out northwards or they pass laterally into continental deposits of the same age (Dallan, 1988). Pliocene deposits are exposed in the Vinci area (Fig. 1). They feature alternate shallow marine sandy clay and sand beds, with brackish-water sand, mud, and peat levels. Tongues of sandy-gravelly deltaic bodies interfinger with those sediments close to the western slope of Monte Albano ridge. On top, they are conformably overlain by a laterally continuous transitional brackish-water unit (Caredio et al., 1995). We refer these marine-to-transitional deposits to Middle Pliocene on the basis of regional correlation with the Pliocene successions exposed further to the south in the Elsa and Era basins

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Fig. 2. Geologic profile across the Montecarlo Basin. The trace of the profile is shown in Fig. 1. The depth of the contact between the Neogene units and the substrate is partly taken from the isobath map of Ghelardoni et al. (1968); North of Altopascio the depth is uncertain. The substrate units are specified only where surface or borehole data are available.

(Fig. 1), where the shallow marine regressive sediments at the top have been ascribed to the Globorotalia aemiliana Zone (Bossio et al. 1993, with references). Fluvio-lacustrine deposits (Fig. 2) unconformably overlie the Pliocene succession (Trevisan et al., 1971). Mammal and mollusk assemblages allow assignment of this unit to Middle (?)–Upper Villafranchian, i.e. Upper Pliocene (?)–Lower Pleistocene. During this time-interval alluvial fan systems were still active along the western side of the Monte Albano ridge (Zanchetta, 1995; Zanchetta et al., 1995; Caredio et al., 1995). Upper Villafranchian fluvio-lacustrine deposits are overlain north of the Arno river by the Montecarlo Formation (Fig. 2), a red gravelly deposit composed mainly of quartzites and phyllites derived from the Triassic Verrucano Formation. This unit can be tentatively attributed to latest Early Pleistocene–early Middle Pleistocene, and is unconformably overlain by the Cerbaie Formation (Fig. 2), which displays the same composition as the Montecarlo Formation, although a finer clast-size. The Cerbaie Formation is dated at Middle Pleistocene through correlation with the Casa Poggio ai Lecci Formation outcropping on the southern side of the Arno Valley (Federici and Mazzanti, 1988). The latter has the same lithologic composition as the former, and contains a tuff layer

radiometrically dated at about 0.5 Ma (Arias et al., 1979; Bigazzi et al., 1994). The sedimentary succession of the Montecarlo Basin terminates with terraced gravelly depositional bodies with composition varying in dependence of local supply. These bodies unconformably cover all the above mentioned units, and the allochtonous units of the Apennine and can be tentatively referred to Upper Pleistocene.

4. Structural setting The Plio–Pleistocene deposits of the Montecarlo Basin are in general weakly deformed at the outcrop scale. The Cerbaie–Montecarlo hills are a minor structural high and the most deformed sector of the basin. There, the Lower Pleistocene fluvio-lacustrine deposits, along with the overlying conglomeratic Montecarlo Formation, are cut by two conjugate systems of N150–160⬚ normal faults. In this area, the preliminary results of the analysis of brittle deformation evidence a system of extension fractures dispersed between N115⬚ and N180⬚, with a peak around N155⬚ (Fig. 3). The fault systems are sutured by Middle Pleistocene-to-Recent conglomeratic alluvial deposits of the Cerbaie Formation. The steep southern slope of the Cerbaie hills, on the

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right side of the Arno Valley, can be interpreted as the morphologic expression of a WSW–ENE fault. The Middle Pleistocene deposits across the Arno Valley display in fact a vertical throw of at least 100 m (Federici and Mazzanti, 1988).

5. Gravity data A study of the gravimetric anomalies has been performed in the Montecarlo Basin and in the immediately adjoining areas in order to obtain more detailed information of the subsurface structures. The available data come from a set of about 1800 gravity values measured mostly by the Italian State power company (ERGA-ENEL Group) and partly by the Earth Science Department of Pisa University (Fig. 4). The average density has resulted to be approximately 2.5 stations per square kilometre, which we consider appropriate for the precision required by such a study. In the Pisani Mountains, the density of the gravity values is lower, due to the uneven morphology and the related difficulty in the logistics of the survey. The surveyed area is about 750 square kilometres wide and covers the investigated morphological and structural elements, in correspondence of

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which the density of measurements have been increased when possible. In the low elevation areas the sampling steps were taken on a 0.5 km, and, in any case, never more than 1 km apart. The measurements have been interpolated on a 0.5 km square mesh-grid. The gravity data were collected using two Model D LaCoste–Romberg gravity meters. Tidal corrections were made for all the data and indicated small, longterm drift rates. The corrections to observed values were performed at the Department of Environmental Engineering of University of Trieste using a Workstation VAX 3100 running on operative system VMS. Terrain correction of observed values was performed according to the Hammer method, using a Fortran program which utilises the Banerjee–Das Gupta algorithm (Banerjee and Das Gupta, 1977). The catalogue of mean value heights for the Italian territory (Carrozzo et al., 1981), realised on the ground of 1:25,000 and 1:100,000 topographic maps of I.G.M., was utilised in Terrain correction for the computation of the heights. The total uncertainty for terrain correction was of 0.1 mgal. The altitude of gravity stations was determined by topographic surveys for the most part, the remaining being estimated by mean of 1:10,000 topographic

Fig. 3. Distribution of the poles of extension fractures in the Upper Pliocene (?)–Lower Pleistocene fluvio-lacustrine deposits along the Montecarlo ridge. Lower hemisphere, 60 measures.

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Fig. 4. Location of gravity stations in the investigated area.

maps; the uncertainty in station elevation ranges from 1 cm to less than 1 m, yielding uncertainty in the point Bouguer anomalies less than 0.4 mgal. The Bouguer and terrain corrections made in the reductions of gravity data require a knowledge of the densities of the rocks that lie between ground level and the reference spheroid. The choice of density for the gravity reductions in surveying areas of high relief is a problem because it affects the short-range gravity pattern under study. According to the boreholes data (Ghelardoni et al., 1968), the composition and age of surface rocks, and the literature data regarding the geology of northern Tuscany (see next), we chose a value of 2.4 g/cm 3 for data reductions. According to the procedure used for the corrections of the observed data, the maximum error committed on each type of reduction

was estimated, and the total maximum error resulted to range around 0.6–0.7 mgal.

6. Gravimetric analysis A constant-density Bouguer anomaly map (Fig. 5) has been obtained by gridding all the available data, using the GEOLINK Integrated Geophysical Software (Geosystem S.r.l., 1995). Anomaly values range in between ⫹50 mgal (Pisani Mountains) and ⫹2 mgal (North–east sector of Montecarlo Basin, located in correspondence of the Monsummano Plain). At a first observation a wide gravimetric high is evident in the southwestern sector of the map, whereas a deep elongate minimum is northwest–southeast in

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Fig. 5. Bouguer anomaly map of the investigated area. Contour interval 2 mgal.

the northeastern sector. A belt characterised by a fairly regular gravimetric gradient crosses along a northwest–southeast direction the investigated area, separating the above mentioned sectors in the central part of the area. A weak relative maximum is centred at the southeastern edge of Monsummano Plain. The correspondence between gravimetric and physiographic structures indicates that the surface effects predominate on the anomaly field features. The main purpose of anomaly separation and enhancement is to produce a map with the anomalies of interest appearing as the main features. In general terms this process is called filtering and may involve the removal of a smooth regional field or a predetermined trend of certain wavelengths. To isolate anomalies with different wavelengths,

filtering was performed on the gridded data and two filtered maps were constructed in order to give a clearer picture of specific anomalies. In the bandpass filtered map (8 km ⬍ l ⬍ 17 km) and in the high-pass filtered map (l ⬍ 17 km) (Fig. 6), the gravity minimum in the northeastern area still persists, thus indicating that it contains harmonic components all along the resolvable frequencies band; so we can infer that it is probably due to a rather deep source, thus confirming the importance of this structural element. Further evidence of that comes from the analysis of the horizontal gradient-gravity map (HGGM), whose maxima separate sectors characterised by strong lateral discontinuities, which can be interpreted as faults. In the HGGM (Fig. 7) of the investigated area the Pisani Mountains block, is

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Fig. 6. High-pass (l ⬍ 17 km) filtered map. Contour interval 2 mgal.

bordered by strong maxima that can be interpreted as major fault zones — as better discussed in the following section — although they are not reported by the geologic maps available in literature. Other maxima parallel to mapped faults, but shifted slightly laterally, locate the position of the master faults more accurately, which may not correspond to the mapped faults. The Monsummano Plain is separated from the Monte Albano Ridge by a northwest–southeast elongated maximum in the HGGM. In order to perform modelling appropriate for surface structures, of a Neogene basin and allochtonous lithologies, focused on quite shallow structures, the contribution of deep sources was subtracted from the observed anomaly field. Various attempts were performed with the polynomial fitting method,

describing the state of a regional field. Different functions suitable in representing such a regional field can be obtained through the least squares method. Here different polynomials were considered, of different degrees that approximate the regional field. Finally, a 1st order polynomial, defining a plane surface dipping top to southwest, was adopted to approximate the regional field. The residual anomaly map (RAM) (Fig. 8) thus has been obtained through analytical calculations; it displays qualitatively the same features as the Bouguer anomaly map (Fig. 5). A northeast–southwest profile has been traced on the RAM, in order to cross all the physiographic and geologic features. Seismic lines are not available for the Montecarlo Basin. In the Arno Valley Ghelardoni et al. (1968) report only well logs and interpretative

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Fig. 7. 1st horizontal derivative of Bouguer anomaly map (HGGM). Dashed lines are supposed to be main fault systems.

cross-sections. Modelling that could take into account a different density for each lithologic unit would be unrealistic, given the scarcity of subsurface stratigraphic constraints. All the geologic units represented in the modelled profile, have therefore been grouped in four layers. Each layer is characterised by weak density differences, and has been given a density value by averaging the values for the different units. The gravity modelling was accomplished using a twoand-half dimensional (2.5D) gravity modelling program based on the technique of Enmark (1981). The density values utilised for the gravimetric modelling are derived from literature data (Carmichael, 1989; Telford et al., 1990) and specific studies on the lithologies of the Tuscan area (Di Filippo and

Toro, 1982; Bally et al., 1986; Gianelli et al., 1988; Bertini et al., 1991; Cassinis et al., 1991; Grassi et al., 1994; Lisi, 1994; Nicolich and Marson, 1994; Orlando et al., 1994; Forfori, 1995). Therefore four bodies with increasing density have been considered in the resulting 2.5D gravity model (Fig. 9), according to the constraints from surface geology (Fig. 1) and to the borehole data reported by Ghelardoni et al. (1968) and Bellani et al. (1994). The Plio–Pleistocene sediments of basin (1), was given an average density value of 2.25 g/cm 3; argillaceous and flysch units referred to the Ligurian Domain and Macigno and Scaglia formations, respectively (2a), average 2.55 g/cm 3; Mesozoic limestones belonging to the Tuscan nappe (2b) average 2.62 g/cm 3; crystalline rocks of the Paleozoic

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Fig. 8. Residual gravity anomaly map (regional field approximated by a 1st degree polynomial). Contour interval 2 mgal.

basement, the Verrucano Formation and Triassic anhydrites and dolomites (3) were given an average density value of 2.72 g/cm 3. The assignment of density values to sedimentary rocks should take into account the increase of density with depth for given lithologies. This has been overtaken by assigning values obtained by averaging a whole range of values known in the cited literature for different lithologies with different diagenetic histories. Then several trials have been performed varying the average density up to 0.5% (e.g. 2.73 g/cm 3 instead of 2.72 ⫺ 2.62 g/cm 3 instead of 2.61 g/cm 3), thus considering much greater density variations of what indicated by Faggioni et al. (1993) for the 0–10 kbar pressure interval. The results of these trials show no sensible changes in the model

thus supporting the choice of a “standard” density value. It is important to note that due to the corrections performed on the utilised measurements, we were not able to obtain a good fit between observed and calculated data in the Pisani Mountains area (see Fig. 9). Here, the low density of gravimetric data and the constant-density correction method does not provide, in fact, a sufficient accuracy in this particular case, because of strong surface density contrast related to a highly complex structure with common lithologic variations (quartzites, metapelites, fillites). These structures are well known from surface data and are related to Apenninic and Hercynian compressional phases (Rau and Tongiorgi, 1974; Carosi et al., 1995; 1996). So this particular misfit should not affect

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Fig. 9. 2.5D gravity model. The densities shown in the modelled layers are in g/cm 3. The trace of the profile is shown in Figs. 1 and 8.

the interpretation structures.

of late Neogene–Quaternary

7. Gravimetric interpretation The choice of a particular gravimetric model among the different ones available that could fit the observed anomaly field was driven by the necessity of defining the shallow tectonic structures of the Montecarlo Basin. The model proposed displays some fairly interesting structural elements: (1) The sudden deepening of bodies 2a (Ligurid units, Macigno and Scaglia Fms with an average estimated density of 2.55 g/cm 3) and 3 (Triassic–Palaeozoic rocks 2.72 g/cm 3) along the northeastern edge of the Pisani Mountains can be interpreted as a major fault zone, lying buried under the alluvial plain,

(Fig. 1); on the northwest projection these faults bound the S. Ginese hills (Fig. 1) that are made of the Macigno Sandstone (the uppermost unit of the Tuscan nappe). The juxtaposition of the Macigno Sandstone to the Verrucano units of the Pisani Mountains represents a considerable vertical throw, that is consistent with the presence of a major fault zone (Rau and Tongiorgi, 1974). (2) The Quaternary sediments are thinner, in correspondence of the southernmost reach of the Montecarlo hills, than in the rest of the basin. The existence of a shallow root made of crystalline basement and Ligurid allochtonous units underneath, is consistent with the stratigraphy of the “Cerbaie 1” well (Ghelardoni et al., 1968), which is not crossed by the sections, but is located few km southeast. (3) Northeastward of the above mentioned buried structural high, the Neogene sediments (2.25 g/cm 3) of the Montecarlo Basin reach a maximum thickness

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Fig. 10. (a) Synthetic geologic cross-section across the Pisa plain, the Pisani Mountains and the Montecarlo Basin. Subsurface data from the Montecarlo Basin are based on the gravity model of Fig. 9. Data from the Viareggio Basin are taken from Bartole et al. (1991) and Bellani et al. (1994). Section trace on Fig. (1). (b) Palinspastic restoration at late Early Pleistocene. (c) Palinspastic restoration at late Messinian. Triassic metamorphic Tuscan units and Palaeozoic basement represents the 2.72 g/cm 3 layer of the gravity model; Mesozoic carbonates Tuscan units represent the 2.62 g/cm 3; Ligurid units, Macigno and Scaglia fms represent the 2.55 g/cm 3 layer; all the other units represent the 2.25 g/cm 3 layer. Vertical and horizontal scales are equal.

of about 1700 m, in correspondence of the Monsummano Plain. (4) The sedimentary filling of the basin thickens weakly from southwest to northeast. (5) The rise of the crystalline basement along with the allochtonous terranes in the easternmost edge of the profile is considerably steep taking into account

that the section in that part of the profile is not perpendicular to the isoanomalies (see Fig. 8). This rise can be interpreted as a major listric normal fault, or alternatively as the limb of a mega-fold involving all the Monte Albano high. This second hypothesis, though, finds no support from the structures visible on surface. The Tuscan and Ligurid units outcropping on the

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Monte Albano are in fact structured roughly in a monoclinal dipping to Northeast. The first hypothesis is instead supported by the similarity with the nearby Elsa Basin (Fig. 1) that is bounded to the Northeast by a Southwest dipping listric normal fault (Mariani and Prato, 1988). The gravimetric analysis shows the relevance of the gravimetric minimum in the northeast sector of the Montecarlo Basin in Monsummano Plain (Figs. 5–7), whose Neogene filling creates an anomaly field that persists in the different examined maps.

8. Plio–Pleistocene tectonostratigraphic evolution of the Montecarlo Basin The west-dipping normal fault evidenced by gravimetric analysis along the western side of Monte Albano ridge (Figs. 7, 9) could be interpreted as the northern prolongation of the Elsa basin master fault (Mariani and Prato, 1988). The syn-rift deposits of the Elsa basin are Upper Miocene–Middle Pliocene. In the Montecarlo Basin, only a minor portion (about 450 m) of the 1700 m thick sedimentary wedge is referred to the Upper Miocene–Pliocene interval (borehole Cerbaie 1: Ghelardoni et al., 1968). Most of the subsidence in the investigated area, therefore, must be of Pleistocene age. Therefore, we can better interpret the Monte Albano master fault as the continuation of the Elsa master fault not only in space, but also in time. The activation of this structure was likely responsible for the unconformity between Pliocene marine and lagoonal sediments and mid-upper Villafranchian fluvio-lacustrine deposits (Fig. 2). The structural high evidenced on the gravimetric profile, in correspondence of the Montecarlo hills could be interpreted as pre-existing to the filling of the basin, or alternatively as uplifted together with the overlying Quaternary deposits. The surface structural data indicate that this sector of the basin underwent a brittle extensional deformation in late–Early Pleistocene times. The morphology, and particularly the hydrographic pattern in the Montecarlo hills is strongly consistent with the prevalent fault and fracture directions thus indicating that this sector of the basin underwent uplift and erosion as a consequence of that late–Early Pleistocene tectonic event. This uplift had to involve also the substrate of the

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basin. The steep flanks of the structural high evidenced underneath the Cerbaie hills (Fig. 8) can therefore be interpreted as normal faults belonging to the same normal fault systems observed on surface in that area. The uplift of this fault block therefore follows the sedimentation of the Late Pliocene– Early Pleistocene fluvio lacustrine deposits and predates the deposition of the Middle Pleistocene Cerbaie Fm. During Late Pliocene and Early Pleistocene sedimentation probably occurred in a half graben whose foot wall was the Monte Albano ridge. The Triassic quartzites had to be overlain by the Mesozoic carbonates of the Tuscan nappe in the Pisani Mountains, as suggested by the scarcity of clastics derived from the Triassic units in the Late Pliocene–Early Pleistocene deposits. Sometimes between Early and Middle Pleistocene the western sector of the Montecarlo Basin hanging wall (Fig. 10) started undergoing a strong uplift and erosion, witnessed by the strong input of Triassic quartzites coarse clastics in the Montecarlo Formation. The normal fault system bounding the Pisani Mountains block to the northeast, cut the sedimentary wedge of the Montecarlo Basin and is parallel to the normal fault system affecting the Lower Pleistocene sediments, along with the Montecarlo Formation, in the Montecarlo hills (Fig. 10a). So these systems can be considered as generated by the same tectonic event, which would then be responsible of the disruption of the hanging wall block of the Early Pleistocene basin. Its westernmost portion — the Pisani Mountains — were then uplifted. This event is stratigraphically constrained in the Montecarlo Basin by the Cerbaie Formation that still contains Triassic quartzites as coarse clastics but is much less deformed than the Montecarlo Formation. The Verrucano Formation of the Pisani Mountains continued to supply coarse clastic sediments all through Middle Pleistocene to the entire lower Arno Valley, also south of the present course of the river Arno, in the Livornesi Mountains area (Fig. 1) (Barsotti et al., 1974). This input was severely reduced, in Upper Pleistocene, and limited to the surroundings of the Pisani Mountains. The throw recorded across the Arno river by the Middle Pleistocene deposits would point to the presence of a fault normal to the system that had been active in late Pleistocene. Such structure could

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have controlled the new course of the Arno river, segregating the clastic input from the Pisani Mountains to the north of it. The horizontal gradient gravity map (Fig. 7) displays no maximum along the Arno Valley, hence indicating that the structure responsible for the above-mentioned horizontal throw, is a shallow one. A major structure parallel to the Arno River structure, is indicated, however, along the southeastern edge of the Pisani Mountains. These structures are roughly aligned along the so called Livorno–Sillaro Line (Bortolotti, 1966), that has been considered as the surface expression of a lithospheric discontinuity (Royden et al., 1987). A smaller scale and less detailed Bouguer anomaly map (Fig. 11) of a larger area around the Montecarlo Basin, including all northern Tuscany, shows the offset of gravimetric minima and maxima along this line, only from the Meloria shoals (Tyrrhenian Sea, offshore Livorno) to the Montecarlo Basin, this trend being interrupted by the Monsummano Plain gravity minimum (CNR, 1992). No other gravimetric trend is recognisable further to northeast along this trend on small scale gravity maps. This observation seems to rule out the deep nature of this tectonic alignment. Also, on the Adriatic margin of the Apennine the existence of this line has been questioned by Cerrina Feroni et al. (1997). We think a more appropriate name for this tectonic lineament is “Meloria-Bientina line”. It records the last tectonic event in the area, and could be still active, since its southwestern end is marked by the occurrence of earthquakes epicentres (Fig. 11). It is questionable whether it is a transcurrent fault or a transfer fault connecting various normal faults belonging to the same Apenninic trending system. We prefer the former hypothesis, because the latter should imply a synchronicity with the Apenninic normal faults, which is not the case.

9. Pre-Pliocene low-angle extensional tectonics in the Montecarlo Basin The evolution so far reconstructed is illustrated in Fig. 10a and b. In Fig. 10c a temptative reconstruction is presented for the structural setting of the allochtonous substrate. Our gravimetric modelling (Fig. 9) indicates that a rock unit with density 2.55 g/cm 3 —

that we assume to represent the Ligurid units and/or the Macigno plus the Scaglia formations — overlies directly a unit with density 2.72 g/cm 3 — that we interpret as a metamorphic complex consisting of Triassic anhydrite dolomite and quartzite, plus the Paleozoic basement rocks. If this interpretation is correct, then the whole Tuscan nappe Mesozoic carbonate succession would be missing from the nappe stack. We can not rule out that the 2a layer (2.55 g/cm 3) actually represents the average between denser Mesozoic carbonates with less dense Ligurid argillite. In any case, the 2a layer is too thin to represent the whole Apennine nappe stack. This interpretation is supported by the bore-hole data from the Arno Valley. Here, some 15 km south of the modelled section, the topmost portion of the substrate is made of Triassic carbonates (the lowest unit of the Tuscan nappe), seen in the Pontedera 1 bore-hole (Fig. 1); of 10 m of the Cretaceous–Eocene Scaglia Formation (Tuscan nappe) directly overlying the Triassic carbonates, in Certaldo 3 (Fig. 1); further to the east (Certaldo 2, 3 km west of Empoli, Fig. 1) the substrate is made of at least 90 m of Scaglia Formation. Along the Monte Albano ridge, the entire Apennine nappestack outcrops, with an overall northeast dip. Hence the Neogene sediments in the lower Arno Valley were deposited on an a thinned substrate. This tectonic setting, characterised by the reduction of the nappe stack, with high structural units directly overlying deep ones through low-angle tectonic contacts, is known in the literature concerning central and southern Tuscany as “Serie ridotta” (Signorini, 1949, 1964) and has been interpreted as due to low-angle normal faults reactivating previous overthrust surfaces (Decandia et al., 1993). An alternative interpretation would involve out-of sequence overthrusting of the Ligurids units on already structured and exhumed Tuscan units. This hypothesis though, has never been demonstrated. Therefore we interpret the thinning of the Apennine nappe-stack as due to lowangle extensional tectonics, taking also into account that: (1) A post-collisional extensional event affected the Verrucano Fm of the Pisani Mountains through shear zones in a brittle-ductile regime (Carosi et al., 1995, 1996). (2) The Macigno Fm overlies directly the lowest unit of the Tuscan Nappe (Triassic Carbonate) in the

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Fig. 11. Bouger Anomaly Map of the lower Arno valley and adjoining areas (from CNR, 1992, modified). Earthquake epicenters for the last 100 years, M ⱖ 3 are indicated with empty triangles (from ING, 1990); the dashed line evidentiates the Meloria–Bientina line.

Camaiore area (northwest of the Pisani Mountains, Carosi, pers. comm.). (3) Low-angle east-dipping normal faults are sutured by Upper Miocene syn-rift sediments in the substrate of the Viareggio Basin (Fig. 10)–noted by Bartole et al. (1991). A qualitative restoration is attempted in the crosssection of Fig. 10. In Fig. 10c a flat detachment is placed in between the Ligurid units plus Macigno and Scaglia fms (layer 2a of the gravity modelling) and the crystalline basement (Layer 3). This is reasonable because regionally the Triassic quartzites are overlaid by Triassic anhydrites that represent the

major decollement horizon for the entire Apennine nappe stack. The flat detachment would terminate to the west in a ramp cutting through the Mesozoic carbonates of the Tuscan nappe and to the east with a ramp cutting the Ligurids and/or Macigno and Scaglia Fms. This detachment geometry is oversimplified and maybe unrealistic from a mechanical point of view. But it is the simplest possible that can be obtained from our gravity modelling, and it follows similar models proposed for similar tectonics settings in other sectors of the Tyrrhenian margin of the Apennine (eastern Elba island — Keller et al., 1994; Larderello geothermal area — Dallmeyer et al.,

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1995). The restoration of the motion along this rampflat-ramp system brings layer 2a on top of layer 2b, approximately in correspondence to the Pisani Mountains.

10. Dating of the low-angle extensional tectonics In the Viareggio Basin, located to the west of the Montecarlo Basin (Fig. 1), the low-angle extensional tectonics has been dated as Middle Miocene by Bartole et al. (1991). Because extensional, as well as compressional tectonics, migrate in the Apennine system from west to east in time (Patacca et al., 1990), the low-angle extensional tectonics in the Montecarlo Basin should not be older than in the Viareggio Basin. The low-angle extensional tectonics should then be placed in time between Middle Miocene and Pliocene, i.e. before the high-angle extensional event. A more precise dating is suggested in this paragraph on the basis of the following speculative arguments. Low-angle detachments permit the accommodation at much more extension than high-angle normal faults. The occurrence at Middle Miocene of a lowangle extension both in the Viareggio and Montecarlo basins would imply a very high extension rate concentrated in a short time interval. In the following time interval (Upper Miocene–Pleistocene) only high-angle extension would have been active on the Tyrrhenian margin of the Apennine, and extension rate would have been much lower. This picture is, however, inconsistent with the necessity of accommodating the shortening on the compressional front with the extension on the Tyrrhenian margin, because shortening rates in the adriatic margin of the Apennine, not only remain high into the Late Miocene-present interval, but they even increase compared to the Middle Miocene (Boccaletti et al., 1990). It is reasonable therefore to hypothesise an eastward migration in time of the low-angle extension. The tectonic event responsible for the low-angle extension must have been recorded in the sedimentary successions of the adjacent basins. A major unconformity is recorded at the base of upper Messinian deposits in the Viareggio, Fine, Era and Elsa basins

(Mariani and Prato, 1988; Sarti, 1995; Testa, 1995; Lazzarotto and Sandrelli, 1977). Both in the Fine and Era basins the upper Messinian deposits are made of coarse clastics supplied from the north, (Sarti, 1995; Testa, 1995) i.e. from the Pisani Mountains area, thus suggesting that it was undergoing an uplift. This tectonic event could have triggered the sliding to the east of the uppermost units of the Apennine nappe stack, through the extensional reactivation of the major decollement horizons along which overthrusting had formerly occurred earlier.

11. Conclusions The analysis of a detailed gravimetric survey conducted in the Montecarlo Basin (Northern Tuscany, Italy), and a critical review of the literature has permitted a reconstruction of the Neogene– Quaternary tectonic evolution of one of the northernmost extensional basins of the Tyrrhenian margin of the Apennine system. Low-angle extensional tectonics started thinning the Apennine nappe stack through a ramp-flatramp detachment, whose surface developed along the Triassic evaporite horizon, at the base of the Tuscan Nappe, not before Serravallian, probably reactivating the main thrust surface of the compressional phase. This first extensional event can be considered even younger, not later than Early Messinian, although we have no clear supporting evidence of that. Transition from low-angle to high-angle extensional tectonics occurred at the Upper Miocene in the Viareggio Basin, west of the investigated area. High-angle extensional tectonics migrated eastward up to Monte Albano ridge during the Late Pliocene, forming a half-graben continental basin whose sedimentary fan developed against a southwest dipping normal fault, at the western margin of the Monte Albano ridge. About 1700 m of sediments were deposited in the depocenter of the basin, now present in the Monsummano Plain subsurface. At late–Early Pleistocene, the hanging-wall block was dismembered by development of NE dipping normal faults that caused uplift of its western portion (the Pisani Mountains). That event caused exhumation and erosion of the Triassic Verrucano Formation,

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whose clastics were shed around the palaeo-Arno Valley in alluvial–fluvial deposits unconformably overlying the fluvio-lacustrine sediments of the synrift wedge. At Late Pleistocene a SW–NE-trending fault system, here named Meloria–Bientina line, created the steep southeastern edge of the Pisani Mountains and the throw recorded in Middle Pleistocene deposits across the Arno Valley. This structure is clearly displayed in the Bouguer anomaly map by a lineament across which gravimetric maxima and minima front each other, and stops in correspondence of the Meloria-shoals gravity high (offshore Livorno) and the Monsummano Plain gravity minimum. This lineament is probably still active, since it is seat, of earthquake epicentres, offshore Livorno. Acknowledgements We wish to thank Italian State Power Company (ERGA-ENEL Group) for providing us with a large part of the gravimetric data and Stefano Bellani of the CNR — International Institute for Geothermal Research — Pisa for allowing us to consult the internal report: Bellani et al. (1994). Thanks to Antonio Rau, Charlotte Schreiber, Paolo Scandone, Andrea Argnani and Giovanni Bertotti for reviewing the manuscript. Francesco Caredio collaborated at an early phase of this study. Thanks to Rodolfo Carosi for the fruitful discussions about the tectonic evolution of northern Apennine. Thanks to the anonymous reviewers whose comments helped improving the original manuscript. This work was partially supported by ex-MURST 60% funds (Prof. E. Pinna and Prof. F.P. Bonadonna). References Argnani, A., Bernini, M., Di Dio, G.M., Papani, G., Rogledi, S., 1997. Stratigraphic record of crustal-scale tectonics in the Quaternary of the northern Apennines. Il Quaternario, It. J. Quat. Sci. 10 (2), 596–602. Arias, C., Bigazzi, G., Bonadonna, F.P., 1979. Studio cronologico e paleomagnetico di alcune serie sedimentarie dell’Italia appenninica. Contr. Prel. Carta Neotett. It. 356, 1441–1448. Baldacci, F., Elter, P., Giannini, E., Giglia, G., Lazzarotto, A., Nardi, R., Tongiorgi, M., 1967. Nuove osservazioni sul

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