Antonio Lazzarotto

Tertiary extensional tectonics in Tuscany (Northern Apennines, Italy)

Abstract Two extensional events have been recognized in the Tertiary evolution of Tuscany. The first event occurred in the period between the Late Oligocene and Late Tortonian and caused a stretching of 120% or greater; the second event occurred between the Late Tortonian and Quaternary Periods and caused a stretching not greater than 10%. The first event is characterized by the exhumation of middle crust rocks and therefore by the development of core complexes. The most significant structures of the first event are: low-angle normal faults, which characterize the upper plate of the core complexes, and ductile shear zones, which characterize the lower plate. The low-angle normal faults tend to die out in the Late Triassic evaporite level, which constitutes the main detachment of the core complexes. The main structures of the second extensional event are high-angle normal faults which dissect all the previous tectonic features. These faults border tectonic depressions which were the site of clastic sedimentation from Late Tortonian to Quaternary time. These normal faults may tend to flatten in a seismic lozenge-shaped band explained as an active extensional shear zone, which is located at the brittle/ductile boundary. A similar explanation is given for the detachment located in the Late Triassic evaporite level during the first extensional event. While the first extensional event is in the framework of the Northern Apennine post-collisional evolution, the second extensional event, because it affects an already stretched crust, could be attributed to the southern Tyrrhenian opening, which began during Late Tortonian.

Considerations on the seismotectonics of the Northern Apennines

Abstract The Northern Apennines have been subdivided into homogeneous zones, on the basis of recent structural evolution and crustal structure, in which the earthquake distribution can find a coherent framework. These zones, whose physiography is in strict connection with their structure, are: the Internal Peri-Tyrrhenian Belt; the External or Main Belt; the Buried Belt; and the Pede-Alpine Homocline. Earthquake activity has a tendency to cluster along well-defined bands, particularly in the easternmost border of the Peri-Tyrrhenian Belt, as well as along the zone between the External Belt and the Buried Belt, i.e. along the Padanian margin of the Northern Apennines. A minimum of seismic activity seems to be correlated with some zones of the External Belt, as well as with the Late Tertiary and Quaternary magmatic province of Tyrrhenian Southern Tuscany. The fault-plane solutions are coherent with the structural picture. A tentative seismotectonic model of the Northern Apennines is discussed.

Inner Northern Apennines

The Northern Apennines are a fold—thrust belt formed during the Tertiary by the tectonic superposition from W to E of the Ligurides on the Tuscan nappe and on the Tuscan metamorphic complex (Boccaletti et al., 1971; Alvarez et al., 1974; Kligfield, 1979). The ophiolite-bearing Ligurides derived from the southern extension of the Ligurian—Piedmont ocean, from which similar mafic components of the Western Alps also derive (Fig. 14.1). The Tuscan units derived from the continental palaeomargin of the Adria microplate and contain a Hercynian continental basement with its upper Carboniferous Tertiary cover (Vai, this vol., Ch. 10).

Extensional shear zones as imaged by reflection seismic lines: the Larderello geothermal field (central Italy)

The Larderello geothermal field is located in the Inner Northern Apennines, in an area which has been subject to extension since the Early Miocene. The latest extensional episode (Pliocene–Present) has resulted in the formation of NW-trending, NEdipping listric normal faults, whose geometry is controlled down to f3 km by borehole data. In this paper, we integrate a new interpretation of seismic reflection lines with existing seismic, field, and borehole data to analyse the relations among listric normal faults, the top of the brittle–ductile transition, and the migration of geothermal fluids. In accordance with previous interpretations, we consider the strong reflector (K-horizon) marking the top of the reflective mid-lower crust, and located at a depth of 3–5 km in the geothermal area, to represent the top of the brittle–ductile transition. Its reflectivity most probably derives from the presence of overpressured fluids. We identify three main NW-trending, NEdipping extensional brittle shear zones, showing listric geometry and soling out in the vicinity of the K-horizon. The latter appears to be dislocated in correspondence of the soling out of the shear zones. These shear zones, because of the associated intense fracturing, represent the most natural channels of upward migration of geothermal fluids from the magmatic sources located below the K-horizon. We suggest that these two conclusions—that listric normal faults root at or near the brittle–ductile transition, and that they act as preferential upward migration paths for magmatic fluids—may be of general validity for geothermal fields located in extensional settings. D 2002 Elsevier Science B.V. All rights reserved.

Heat flow, deep temperatures and extensional structures in the Larderello Geothermal Field (Italy): constraints on geothermal fluid flow

The Larderello geothermal field is located in the inner Northern Apennines (southern Tuscany), an area which has been affected by extensional tectonics since the Early–Middle Miocene. The structure of the Larderello field is characterised by NW-trending, NE-dipping Pliocene to Present normal faults. Their geometry down to depths of 4–5 km is constrained by field, borehole, and reflection seismic data. An association between extensional structures and heat flow maxima (up to 1000 mW/m2) is recognisable from detailed surface heat flow mapping. In order to investigate the relationships among extensional structures and heat flow, subsurface isotherms were traced, subject to borehole control, along variously oriented geological cross-sections. The isotherms show vertical displacements associated with the recent normal faults and related deformation zones, which reach the brittle/ductile transition. Estimates of the relative importance of convective and conductive components of heat flow suggest that fluid circulation is particularly important in correspondence with the normal faults, accounting for the correlation between isotherm perturbations and extensional structures. In this view, extensional shear zones are interpreted as the main structural pathways for the flow of hot geothermal fluids.

Geologia dell’area di Rapolano Terme in Provincia di Siena(Appennino Settentrionale)

This paper deals with the geological setting of the Rapolano Terme area (Siena) located in southern Tuscany, inner Northern Apennines. The Rapolano area is part of an important morpho-tectonic feature NNW-SSE oriented, ranging from the Chianti Mts to the Cetona Mt, separating the Siena-Radicofani and the Valdichiana Basins in the western and eastern sides, respectively. In the study area the Late Triassic-Early Miocene succession of the Tuscan Nappe, as well as the Eocene succession belonging to the Morello Unit (external Ligurian Unit) and the Pliocene-Pleistocene post-orogenic marine to continental deposits are broadly exposed. The oldest cropping out Tuscan Nappe formation consists of the «Calcari e marne a Rhaetavicula contorta » Fm. It is mainly composed of dark limestone beds with decimeter thick grey marls interbedded. The occurrence of Triasina hantkeni Maizon allows referring this formation to the Rhaetian. The underlying formation (the base of the Tuscan Nappe), represented by the «Formazione anidritica di Burano» was encountered at depth (900 m below the ground level) by the Rapolano 1 borehole, and was partially drilled for 114 m. The «Calcare massiccio» Fm (Early Lias) overlies on the «Calcari e marne a Rhaetavicula contorta» Fm. This formation broadly crops out in the study area. It is mainly composed of grey massive limestones, often dolomitic, containing meter thick lenses of sin-sedimentary breccias formed by centimeter to decimeter carbonate clasts. The «Calcare massiccio» Fm is overlain by the «Calcare selcifero» Fm. Such a formation consists of bedded grey cherty limestones (Late Hettangian-Domerian). The «Calcare selcifero» Fm is overlain by the «Calcare Rosso Ammonitico» Fm, giving rise to an anomalous stratigraphic succession with respect to that described for the Tuscan Nappe exposed in the western Tuscany. The «Calcare Rosso Ammonitico» Fm is composed of Toarcian-Aalenian red and yellow nodular limestones and marly limestones, with heteropic relationships with the uppermost part of the «Calcare selcifero» Fm and with the lower part of the «Marne a Posidonomya» Fm. The «Marne a Posidonomya» Fm is mainly composed of Toarcian-Callovian meter thick beds consisting of red, grey and yellow marly limestones and marls, often interlayered with red and grey siltstones. Such a formation is discontinuously exposed in the northern part of the study area (mainly between Rapolano and Serre di Rapolano villages), where it directly overlies the «Calcare selcifero» Fm. The «Marne a Posidonomya» Fm is characterized by a thin succession occurring in the southern part of the study area. In few areas such a formation is substituted by the «Calcare Rosso Ammonitico» Fm. The «Marne a Posidonomya» Fm and the «Calcare selcifero» Fm are overlain by the «Diaspri» Fm, mainly composed of centimeter beds of Late Callovian-Early Titonian red, yellow and green radiolarites with interbedded very thin claystones levels. The «Diaspri» Fm gradually passes to the overlying succession represented by the «Calcari ad Aptici» Fm. Such a formation is mainly composed of thin bedded Titonian yellow and red limestones and marly limestones. This formation gradually passes to the «Maiolica» Fm that occurs in an about 100m thick succession, unusual with respect to that documented for the southern Tuscany. The «Maiolica» Fm consists of thin bedded white and grey cherty calcilutites (Berriasian-Aptian). Nodular cherts are very abundant, often black or red in color. The «Maiolica» Fm is overlain by the Albian-Late Eocene (Priabonian) Scaglia Toscana Group which is composed of, from the bottom to the top: i) the «Argilliti di Brolio» Fm; ii) the «Marne siltose ed argilliti marnose rosse di Pod. Le Rossole» correlatable with the «Marne del Sugame» Fm described for the Chianti Mts; iii) the «Calcareniti di Montegrossi» Fm containing a megabreccias with volcanic blocks; iv) the «Argilliti e calcareniti di Dudda» Fm. The «Macigno» Fm is the topmost formation of the Tuscan Nappe. This is mainly composed of a turbidite succession broadly exposed in the northern part of the study area. The «Macigno» Fm consists mainly of arkosic sandstones with interbedded micaceous siltstones and rare thin carbonate levels. The «Macigno» Fm can be subdivided in three main depositional units: i) the lowermost depositional unit is mainly composed of sandstones facies referred to distal or intermediate-distal lobe; ii) the middle depositional unit is characterized by coupled siltstones and sandstones beds, referred to a very distal depositional environment; iii) the topmost depositional unit is mainly composed of siltstones with subordinate sandstones suggesting the migration of the foredeep system. The «Macigno» Fm is referred to the Late Oligocene-Early Miocene. The Ligurian Units are represented by the Morello Unit only composed of the «Monte Morello Fm». Such a formation is exposed in small outcrops located in the southern part of the study area, near Pod. S. Bernardino. In the attached geological map the Morello Unit has been erroneously attributed to the «Scaglia Toscana» Group (labeled as Mc). The «Monte Morello» Fm consists of brown to grey marls and subordinate calcilutites and marly-limestones, with local intercalation of centimeter and decimeter beds of ophiolite bearing turbidite sandstones. The age is Early-Middle Eocene. The Tuscan Nappe and the Morello Unit are unconformably overlain by the Pliocene marine deposits filling the Neogene Siena Basin. The Pliocene deposits consist, from the top to the bottom (see the attached geological map): i) « Argille e argille sabbiose grigie, talvolta fossilifere» consisting of clays and sandy-clays; ii) « Sabbie talvolta argillose ed arenarie poco cementate gialle a luoghi arrossate, Arenarie ben cementate gialle» consisting of sands and clayey-sands; iii) « Conglomerati e ciottolami poligenici, non classati, saltuariamente con fori di Litodomi» consisting of polygenic conglomerates and pebbles with borings of lithophagid bivalves. The lower part of this succession is characterized by the absence of Globorotalia puncticulata . The overlying part is characterized by the occurrence of Bulimina marginata and Discoaster pentaradiatus . On the whole, marine deposits can be ascribed to the Piacenzian even though we cannot exclude the lowest Gelasian at least for the uppermost part of the succession. The Quaternary deposits unconformably overlie both the Pliocene deposits and the pre-Neogene formations. They are exposed from 190 m to 375 m above the sea level. These deposits consists of broad Middle-Late Pleistocene and Holocene travertine deposits, mainly exposed in the quarries close to Serre di Rapolano and Rapolano Terme villages, and the alluvial deposits of the Piano del Sentino, Piano della Bestina and Borgo ai Piani. The sedimentological and stratigraphic features, as well as the lithological association coupled with the rare fossil remains, allow to refer such deposits to a fluvio-lacustrine depositional environment. The tectonic setting is characterized by superposed deformational events developed during the structural evolution of the Northern Apennines. The structures related to the different deformational events are, from the youngest: i) Late Pliocene-Late Pleistocene oblique to strike-slip faults, E-W to NE-SW striking, giving rise to hydrothermal circulation, as well as the occurrence of thermal springs and gas emissions (mainly CO2); ii) Early-Middle Pliocene normal faults, NNW-SSE and N-S oriented, interfering with the Pliocene sedimentation and driving the architecture of the eastern side of the Siena Basin; the most important structure belonging to this fault system is the Rapolano normal fault, N-S striking and west-dipping, which separates the pre-Neogene successions from the Pliocene deposits; iii) extensional detachments with top-to the east sense of shear giving rise to significant tectonic elisions within tectonic units forming the Chianti Mts-Cetona Mt. ridge ( serie ridotta Auctt ); iv) east-verging folds with N-S and NNW-SSE axial trend; they are the most representative contractional structures in the whole study area (see the geological map) and developed after the emplacement of the Ligurian Units on the Tuscan Nappe; v) thrust and related minor contractional structures (folds and reverse faults) developed during the Late Oligocene-Early Miocene stacking of the tectonic units. In the last part of the paper, the relationships between tectonic activity and travertine deposition and hydrothermal circulation are discussed.

Nuovi dati litostratigrafici e biostratigrafici sulla Scaglia Toscana nella Toscana meridionale (area di Rapolano Terme)

The Scaglia Toscana represents a stratigraphic unit belonging to the Tuscan Domain of the Northern Apennines. Such a stratigraphic unit results lithologically heterogeneous, and is representative of a very long time span (from the Early Cretaceous to the Oligocene) during which very important geological events took place, such as the convergence and collisional events giving rise to the edification of the Northern Apennines orogene. We have studied several key outcrops of the Scaglia Toscana exposed in the Rapolano Terme area (Southern Tuscany, Northern Apennines) in order to contribute to the reconstruction of the depositional setting and palaeogeographical features of a sector of the Tuscan Domain from the Cretaceous to the Early Miocene, and to hypothesise a possible geodynamic evolution. The Scaglia Toscana exposed in the Rapolano Terme area has been deeply investigated through lithostragraphic and biostratigraphic analyses mainly carried out in three key areas indicated in the fig. 1: 1) the Podere Cetinaia-Podere Monte Petroso section; 2) the Podere Le Rossole section; and 3) the Modanella-Podere Campo d’Aia section. From the top (represented by the overlying Macigno Fm) to the bottom (Maiolica Fm) we have recognized three litostratigraphic units: 1. Claystones and calcarenites ( Argilliti e calcareniti di Dudda ), containing lenses of nummulite bearing calcarenites and calcirudites («Nummulitico», Calcareniti di Montegrossi ). The thickness ranges from 50 to 100 m; Middle-Late Eocene. 2. Silty marls and claystones ( Marne siltose ed argilliti marnose rosse di Podere Le Rossole ) comparable for the age and stratigraphic position with the Marne del Sugame described in the Chianti Mts. The thickness does not exceed 10 m; Early-Middle Eocene. 3. Argilliti di Brolio consisting of three lithofacies: BRLa - manganesiferous radiolarites (thickness from 5 to 10 m); BRLb - siliceous siltstones (thickness of about 10 m); BRLc - red marly clays (thickness from 6 to 8 m); Albian-Turonian. The boundary separating the Argilliti di Brolio from the overlying Marne siltose ed argilliti marnose rosse di Podere Le Rossole is representative of a time gap about 7Ma long (from 40 to 47Ma). According to Canuti & Marcucci (1967) we exclude the emersion during this time span, but we support for: i) the reduction of the sedimentation and/or; ii) submarine erosion induced by tectonic instability. Such possible hypotheses can be confirmed by the fact that the gap affects different stratigraphic horizons of the Argilliti di Brolio , and the sedimentation of the Marne siltose ed argilliti marnose rosse di Podere Le Rossole started diachronically. The sedimentary environment of the Marne siltose ed argilliti marnose rosse di Podere Le Rossole is pelagic-hemipelagic, characterised by Foraminifera bearing marly and pelitic sediments. The occurrence of thin bedded calcarenites indicates the formation of a turbidite system, mainly carbonate. Such a turbidite system typifies the overlying succession: the Argilliti e calcareniti di Dudda with interbedded Nummulites bearing calcarenites and calcirudites ( Calcareniti di Montegrossi ). The stratigraphic contact between the Argilliti e calcareniti di Dudda and the Marne siltose ed argilliti marnose rosse di Podere Le Rossole lithostratigraphic units is gradual and continuous, and is characterized by the frequency and thickness increase of the turbite beds, as well as by the progressive appearance of the reddish color, moreover characterizing the overlying Marne siltose ed argilliti marnose rosse di Podere Le Rossole . The sedimentological features of the Argilliti e calcareniti di Dudda suggest a radical change of the sedimentary environment. In fact, the hemipelagic sediments were substituted by the turbidite systems. The turbidites are characterized by graded beds containing Eocene planktonic Foraminifera indicating an intrabasinal reworking of penecontemporaneous sediments. Contrarily, the calcarenites of the upper part of the succession are characterized by Cretaceous planktonic Foraminifera being bioclasts indicating an extrabasinal origin. This is indicative for the development of a carbonate turbidite system fed by Cretaceous sediments. The uppermost part of the succession is newly typified by the occurrence of calcarenite beds with intrabasinal Eocene planktonic Foraminifera. Such a lithological assemblage and palaeontological containts became a recurrent feature for the overlying Nummulites bearing calcarenites and calcirudites («Nummulitico», Calcareniti di Montegrossi ). Furthermore, these latter are characterized by Eocene benthonic Foraminifera suggesting the development of one or more lobes of a submarine fan channels fed by carbonate platforms. The Nummulite bearing calcarenites are characterized by a trend thinning-fining upward, and gradually pass to the Argilliti e calcareniti di Dudda , indicating the deactivation of the lobe fan system. Such a succession passes to the Macigno siliciclastic sandstone (Macigno Fm). In sum, we hyphotesise that the embryonal foredeep system of the Northern Apennines could be developed since the Middle-Late Eocene with the activation of a carbonate turbidite system ( Argilliti e calcareniti di Dudda plus the Nummulites bearing calcarenites) successively evolved in a siliciclastic turbidite system represented by the Macigno Fm. The sedimentary gap recorded within the Scaglia Toscana succession could indicate the records of the beginning of the tectonic activity which affected the Tuscan Domain for a long time, related to the development of the Northern Apennines foredeep system.