Andrea Brogi

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.

Tectonic control on travertine and calcareous tufa deposition in a low-temperature geothermal system (Sarteano, Central Italy)

This paper presents an integrated study directed toward an understanding of the tectonic control on the hydrothermal circulation and resulting deposition of Middle–Late Pleistocene travertine in a low-temperature geothermal field of southern Tuscany, located in the Sarteano area (Central Italy). The study area is characterized by thermal springs (c. 25 °C) and related continental carbonate deposits, structurally controlled by SW–NE-trending oblique to strike-slip faults dissecting previous normal faults. Sedimentological–stratigraphical data and 230Th/234U radiometric age determination for the travertine indicate that continental carbonates were deposited in a tectonically controlled, perched-spring system from 268 ± 22 ka (at least) to the present. The hydrothermal system is (and was) characterized by meteoric waters with a minor contribution from fluids of deeper origin. Meteoric waters infiltrated to depth through tectonically damaged zones of Mesozoic and Cenozoic carbonate rocks and overheated as a result of localized, anomalous geothermal gradient. Then, fluids enriched in bicarbonate–sulphate moved up along highly fractured rock masses at the intersection between strike-slip faults and normal faults. This paper highlights the use of continental carbonates in the reconstruction of palaeohydrothermal systems, in terms of the location of the main conduits for hydrothermal fluid flow, age of faulting and physical properties of hydrothermal fluids.

Tectonic and sedimentary evolution of the Upper Valdarno Basin: new insights from the lacustrine S. Barbara Basin

We describe stratigraphic, structural and kinematic data from the sediments of the Upper Pliocene Santa Barbara Basin and from its substratum. The results shed light on the relationships between tectonics and sedimentation in the larger Late Pliocene-Middle Pleisto cene Upper Valdarno Basin of which the Santa Barbara Basin is considered a precursor. The sediments filling up the Santa Barbara Basin are made up of alluvial to deltaic and lacustrine deposits, grouped in the Castelnuovo dei Sabbioni (CSB) Synthem, related to Late Pliocene. This synthem was deposited in a tectonic depression reasonably delimited to the East by a west-dipping normal fault system and delimited to the North and to the South by left-lateral transtensional shear zones, which controlled the main directions of the alluvial drainage. During Early Pleistocene, a new master normal fault system (Trappola fault system) developed further to the East, determining the widening of the previous tectonic depression, now delimited to the North and to the South by the regional Piombino-Faenza and Arbia-Val Marecchia transfer zones, respectively. In this new tectonic depression, with a dominant axial drainage direction, alluvial, fluvio-aeolian and fluvial sediments (Montevarchi Synthem, VRC) deposited during Early Pleistocene. The VRC Synthem, being located in the hanging-wall of the Trappola normal fault system, is slightly tilted to the NE. Finally, during Early-Middle Pleistocene, axial fluvial deposits (Torrente Ciuffenna Synthem, UFF), sealed the previously formed brittle structures. Our kinematic and structural data allow us to confirm the interpretation that the Santa Barbara Basin is the precursor of the Upper Valdarno Basin and that both basins developed in structural depressions formed by the interplay between normal and transfer faults, framed in the extensional tectonics which characterizes Tuscany since Miocene.

Evolution of a fault-controlled fissure-ridge type travertine deposit in the western Anatolia extensional province: the Çukurbağ fissure-ridge (Pamukkale, Turkey)

In recent decades various interpretations have been proposed to explain the evolution of fissure-ridge-type travertine deposits. In this paper, we discuss the relationships between fissure-ridges and brittle structures affecting their substratum, through a detailed analysis of an inactive fissure-ridge (near Çukurbağ) located in the Pamukkale geothermal area (western Turkey). The Çukurbağ fissure-ridge can be taken as a model as it offers an opportunity to examine its internal structure on the walls of a Roman quarry; in addition, this ridge has been studied by several researchers who have discussed the processes promoting the fissure-ridge evolution. The Çukurbağ fissure-ridge is composed of irregularly alternating travertine laminated facies (bedded travertine) crosscut into rather large lithons by subvertical crystalline veins (banded travertine). The relationships between bedded and banded travertine indicate that the banded veins are diachronous and migrated through time, suggesting a progressive fault zone enlargement in the footwall. Such a fault zone was characterized by polycyclic activity, with normal to transtensional kinematics, and was active during the latest Quaternary. We demonstrate that formation of banded veins is coeval with bedded travertine deposition and strictly depends on fault activity, therefore highlighting the fundamental role of travertine fissure-ridges in reconstructing palaeotectonic activity in a region.

Miocene low-angle detachments and upper crust megaboudinage in the Mt. Amiata geothermal area (Northern Apennines, Italy)

Information from surface and subsurface geology (boreholes and seismic reflection lines) are used to depict the geometry of the extensional structures (low-angle normal faults and related Tuscan Nappe megaboudins) affecting the Mt. Amiata geothermal area and developed during the early stage of the extensional tectonics which affected the inner Northern Apennines and Tyrrhenian Sea from the Early-Middle Miocene. Normal faulting involved the thickened middle-upper crust after the collisional stage and, in the Mt. Amiata region, took place over relatively short periods (5-7 Ma) characterised by rapid extensional strain rates. Normal faults showing articulated geometry (flat-ramp-flat) characterised by subhorizontal detachments (flats) and synthetic ramps, caused widespread megaboudinage mainly in the sedimentary tectonic units and particularly in the Tuscan Nappe. Evaporites occurring at the base of the Tuscan Nappe, the deepest sedimentary tectonic unit of the Northern Apennines, controlled the geometry of the faults, and rift-raft tectonics may be the style of this first extensional phase. Three Tuscan Nappe extensional horses (megaboudins) have been recognised in the subsurface of the Mt. Amiata area. They are characterised, in map view, by elliptical shapes and show a mean NNW-SSE lengthening. They are delimited at the base and at the top by east-dipping flats, while their western and eastern margins coincide with east-dipping ramps. On the whole, considering their geometrical features, these megaboudins correspond to extensional horses belonging to an asymmetrical east-dipping extensional duplex system. Rollover anticlines deformed the western ramp of the megaboudins and rotated the uppermost flat as well as all the structures previously developed, which became steeply-dipping to the west.

Upper crust “boudinage” during post-collisional Miocene extension in Tuscany: Insights from the southern part of the Larderello geothermal area (Northern Apennines, Italy)

A geological study carried out in the southern part of the Larderello geothermal area (Northern Apennines) provides new information on the development mechanism and timing of the earlier extensional structures that formed during the Miocene post-collisional tectonics which affected the orogen. Staircase low-angle normal faults (LANFs) affected a multilayered thickened upper crust after the collisional stage, producing the lateral segmentation of the Tuscan Nappe, the deeper non-metamorphic tectonic unit of the Northern Apennines in the Tuscan area. The tectonic history recorded in two Tuscan Nappe discontinuous bodies revealed that the LANFs took place during the Middle–Late Miocene, displacing collisional structures developed from the Late Oligocene. These Tuscan Nappe bodies are delimited by detachment faults located at the base, within the Tuscan evaporites, and at the top within the Ligurian Units. Their western and eastern margins coincide with east-dipping ramps. These structures and the Tuscan Nappe bodies were later dissected by Pliocene–Quaternary high-angle normal faults. The reconstructed deformation history implies that the Tuscan Nappe bodies are extensional horses developed through an earlier asymmetrical east-dipping extensional duplex system, involved in block faulting during the later, Pliocene-Quaternary, stage of extension.

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.

An overview on the characteristics of geothermal carbonate reservoirs in southern Tuscany

This paper focuses on brittle deformation and fluid-rock interaction, for enhancing permeability in carbonate geothermal reservoir. The relationships between fractures and fluid flow at different structural levels within a geothermal circuit are described through examples from exhumed geothermal systems cropping out in southern Tuscany, with emphasis on the carbonate reservoirs, located within the late Triassic evaporite level and/or at the base of the Tuscan Nappe. The description is based on the fact that geothermal fluids are mainly made up of meteoric water channelled to depth through structural conduits, affecting regionally hot rocks. In this pathway, the meteoric water is transformed in geothermal fluid, becoming chemically aggressive, thus favouring leaching of hosting rocks, and enhancing and maintaining permeability. The fluid-rock interaction is promoted by existing fractures and/or by unhomogeneities in the rock-textures, as it is the case of the Miocene cataclasite located within the late Triassic evaporite. Travertine deposits can occur if fluids reach the surface after having circulated in carbonate reservoirs. Since permeability is controlled by fluid-rock geochemistry and by the possibility to have fluids continuously renewed, we conclude that the fluid-rock interaction and high temperature of hosting rocks make the geothermal issue a specific case of study and therefore the conclusion on oil reservoirs formation cannot be completely transferred to geothermal exploitation issue.

The Tuscan Nappe structures in the Monte Amiata geothermal area (central Italy): a review

The present knowledge on the structures affecting the Tuscan Nappe in the Monte Amiata area is here presented, highlighting that the tectonic evolution of the area is incorporated in the inner Northern Apennines framework. In fact, field and subsurface data indicate that the Tuscan Nappe is internally characterized by tectonic doublings, occurred during the collisional stage. Subsequently, as well as in the whole southern Tuscany, Miocene extension determined the lateral segmentation of the Tuscan Nappe; in the Mt. Amiata area, it resulted in three isolated geological bodies, partly cropping out. During Pliocene, the Tuscan Nappe was cross-cut by normal to oblique faults which contributed to channel the hydrothermal fluids that gave rise to the Pleistocene Hg-Sb ore deposits and Present geothermal resources. As it regards the deformation ages, since the late Oligocene-Aquitanian Macigno Fm is involved in the thrusts, the internal stacking of the Tuscan Nappe is related to the post-Aquitanian and pre-Langhian time period, on the basis of the Langhian sediments deposited in an extensional setting and presently recognised underneath the sediments of the nearby Pliocene Radicofani extensional Basin. It is therefore concluded that the switch from compression to extension occurred in a time span encompassed between early Burdigalian and early Langhian.