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High‐temperature geothermal areas are often characterized by widespread surficial manifestations, whose location is strictly controlled by sets of faults of regional relevance. The geochemical and isotopic signature of the discharged fluids can reveal key information on the geothermal fluids pathway, shedding light on the sources and fluid‐rock interaction within the geothermal reservoirs. In this paper, a geochemical and structural data set from the Larderello geothermal area and surroundings is presented and discussed. We constrain the role of transfer and normal faults in controlling the geothermal circulation enhanced by a cooling magmatic intrusion underneath the Lago area (SW of Larderello). The structural control on the fluids circulation is highlighted by both the location of the CO2 emissions along the fault segments, where permeability is enhanced, and their degassing rates, which increase moving away from the core of the Larderello geothermal system. The main results unravel the presence of deep regional pathways along which endogenous fluids circulate before being discharged in the investigated areas. The peripheral zone emissions are affected by interaction with shallow aquifers and condensation processes whereas the CO2 emitted from the central areas, located near the core of the geothermal system, was accompanied by high amounts of steam, and suffers intense shallow fractionation processes. The latter areas emit medium‐to‐low normalized‐CO2‐degassing rates (<270 t d−1 km−2) when compared to the extremely high values occurring in the peripheral sectors (up to 1,300 t d−1 km−2) of the Larderello geothermal systems, possibly suggesting an incipient propagation of such a system, likely wider than previously thought. Plain Language Summary: High‐temperature geothermal areas are specific zones on Earth where the geothermal gradient is greater than the average global value (∼30°C km−1). This is due to the existence of cooling magma source(s) at depth, providing heat that is transmitted to fluid reservoir(s) located at intermediate levels continuously and naturally fed by recharging fluids, and sealing rock(s) at shallower levels that maintain reservoir temperature and pressure. These geothermal areas commonly show steam‐dominated manifestations at the surface, accompanied by relevant degassing of carbon dioxide likely originated from different feeding systems (biogenic, thermometamorphic, and mantellic). Defining sources, processes, and transport mechanisms governing the CO2 emissions from soils is challenging but pivotal to understand the geothermal fluid origin, dynamics, and relation with the geological structures. To unravel these processes, we combined geochemical and structural measurements performed at the Larderello‐Travale‐Radicondoli (LTR) geothermal field (Italy). Distinct geothermal sectors were investigated according to their geochemical characteristics, CO2 degassing rates, and geological‐structural features, to understand how CO2 is transported and modified during its journey from the deep geothermal reservoir(s) to the surface, exploiting the permeability of fault zones and/or fractured rocks. Results suggest that the LTR geothermal system might be wider than previously thought, indicating a higher geothermal potential. Key Points: Extensive CO2 soil degassing measurements and carbon isotopic composition in the Larderello geothermal system were carried outHydrothermalized hot‐ and cold‐degassing areas are controlled by structural settingRegional transfer zones enhance the circulation of deep‐seated geothermal fluids [ABSTRACT FROM AUTHOR]

We investigated the active tectonics and earthquake potential of the eastern Siena Basin, a slowly deforming portion of southern Tuscany in the inner Northern Apennines. This region hosts several historical settlements and valuable cultural heritage, but also frequent background seismicity and rare damaging earthquakes in the M w range 5.0–6.2. We describe in detail an active, capable, and seismogenic fault system that we identified in the eastern Siena Basin, a few kilometers south-east of the city of Siena, thanks to the presence of an active quarry (Cava Capanni) that exploits travertines of Middle Pleistocene-Holocene age. Travertines are unique rock masses that may preserve living evidence of active and seismogenic faulting, thus providing remarkable seismotectonic insight. The active fault system consists of at least two segments rupturing travertines younger than 45 ka, with a cumulative vertical displacement of 111 cm, and an estimated minimum slip rate of 0.02–0.03 mm/y. We maintain that this displacement is the result of at least three coseismic movements accompanied by clastic dykes injected within the fault damage zone due to liquefaction phenomena. The fault system is seen to extend east of the quarry, affecting Pliocene and Mesozoic deposits. The Cava Capanni fault system is evidence of a poorly understood but potentially seismogenic tectonic mechanism of regional extent. Its orientation and kinematics are compatible with the activity of faults that are oriented obliquely or orthogonally to the main chain axis, in contrast with the setting of the axial and outer zone of the Northern Apennines, where extension and compression are accommodated by Apennines-parallel faults. • We identified an active and capable fault in a travertine quarry in southern Tuscany. • Fluid-injected dykes show that the fault slips during surface faulting earthquakes. • Based on coseismic displacement, the fault may generate rare M w 6.0–6.3 earthquakes. • The fault generated three surface faulting events in the past 45 ka. • Most active faults in the Siena basin are orthogonal to the main Apennines trend. [ABSTRACT FROM AUTHOR]

Rift-basins are the shallow effects of lithosphere-scale extensional processes often producing polyphase faulting. Their sedimentary evolution depends on the mutual interplay between tectonics, climate, and eustasy. Estimating the role of each factor is generally a challenging issue. This paper is focused on the tectono-sedimentary evolution of the Neogene Siena-Radicofani Basin, a polyphase structural depression located in the inner Northern Apennines. Since Miocene, this basin developed after prolonged extensional tectonics, first as a bowl-shaped structural depression, later reorganized into a half-graben structure due to the activation of high-angle normal faults in the Zanclean. At that time the basin contained coeval continental and marine settings controlled by the normal faulting that caused the development of local coarse-grained depositional systems. These were investigated to: (i) discriminate between the influences of tectonics and climate on sedimentation patterns, and (ii) provide detailed time constraints on fault activity. The analysed successions were deposited in an interval between 5.08 and 4.52 Ma, when a climate-induced highstand phase occurred throughout the Mediterranean. However, evidence of local relative sea-level drops is registered in the sedimentary record, often associated with increased accommodation space and sediment supply. Such base-level fluctuations are not connected to climate changes, suggesting that the faults generally control sedimentation along the basin margins. [ABSTRACT FROM AUTHOR]

This paper discusses the possible influence of syn-sedimentary structures on the development of orogenic structures during positive tectonic inversion in the inner Northern Apennines (Italy). Examples from key areas located in southern Tuscany provided original cartographic, structural and kinematics data for Late Oligocene-Early Miocene thrusts, organized in duplex systems, verging in the opposite direction of the foreland propagation (back-thrusts), which affected the Late Triassic-Oligocene sedimentary succession of the Tuscan Domain, previously affected by pre-orogenic structures. These latter consist of mesoscopic-to cartographic-scale Jurassic syn-sedimentary normal faults and extensional structures, which gave rise to effective stratigraphic lateral variation and mechanical heterogeneities. Structural analysis of both syn-sedimentary faults and back-thrusts were therefore compared in order to discuss the possible role of the pre-existing anisotropies in influencing the evolution of the back-thrusts. As a result, it can be reasonably proposed that back-thrusts trajectories and stacking pattern were controlled by relevant syn-sedimentary normal faults; these latter were reactivated, in some cases, if properly oriented. Such an issue adds new inputs for discussing the potential role of structural inheritance during tectonic inversions, and helps to better understand the processes suitable for the development of back-thrusts in the inner zones of orogenic belts, as it is the case of the inner Northern Apennines. [ABSTRACT FROM AUTHOR]

The inner Northern Apennines belt ( i.e. , northern Tyrrhenian Sea and Tuscany) is an Alpine chain affected by high- P metamorphic conditions during its evolution. Although Elba Island is structurally located close to the Adria-Europe suture zone, for several authors it represents a sector of the orogen affected by low- P metamorphism. The involvement of Elba Island tectonic units in high- P metamorphism was only suspected for the sparse presence of phengitic white mica in the metasedimentary rocks. This paper presents the first clear evidence of high- P and low- T metamorphism found in metabasite rocks embedded in the Cretaceous calcschist of eastern Elba Island. Mineral composition of metabasite includes Gln + Cpx + Ep + Ab + Act + Qtz + Ilm ± Ti-oxide ± Spn and is indicative of a former equilibration in the epidote blueschist subfacies and subsequent retrogression in the greenschist facies. Recorded metamorphic conditions are P = 0.9–1.0 GPa and T = 330–350 °C. Tectonic discrimination using immobile elements in the metabasite does not point to an oceanic setting. As a consequence, the metasedimentary succession containing metabasite is explained as belonging to the Tuscan continental domain and not to the Ligurian-Piedmont Ocean, as previously interpreted. Our results have two significant implications: (i) it is confirmed and strengthened that the tectonic stacking of the Elba Island units did not occur in a low-pressure context; (ii) Elba Island is now completely reconciled in the tectonic and metamorphic evolution of the inner Northern Apennines. [ABSTRACT FROM AUTHOR]

The reconstruction of the upper Palaeozoic sedimentary evolution of Western Mediterranean area is suffering for scarce age documentation of its stratigraphic units. This is also the consequence of the metamorphism and deformation they experienced during the Alpine tectonic events. Based on palynology and thermal maturity assessment, we contribute to this issue reporting biostratigraphic and sedimentological results in key-outcrops of the upper Palaeozoic successions of southern Tuscany (Italy), thus offering new data for the Palaeozoic units of the inner Northern Apennines, as well as for their palaeogeographic and palaeotectonic evolution predating the opening of the Alpine Neo-Tethys. This study is focused on the Falsacqua Formation mainly consisting of quartzitic phyllite, sandstone and carbonate, representing the basal succession of two tectonic units exposed in the Middle Tuscan Ridge, and which age attribution was strongly debated in literature and referred either to late Carboniferous-early Permian or Devonian. Palynological data document the first finding of a microflora of middle-late Permian age adding key chronological constraints. This age assignment permits to correlate the Falsacqua Formation with the coeval deposits of southern Tuscany (i.e., Farma and Poggio al Carpino formations) and Elba Island (Rio Marina Formation) that are all characterized by a similar microfloral content. Optical thermal maturity index as Spore Colour Index is around 9 to 10 and suggests a T peak of about 300–350 °C while the Raman spectroscopy on carbonaceous material indicates a T peak around 375 °C for both tectonic units. These data provide new constraints for the maximum T values occurred during syn -collisional metamorphism developed in this sector of the Northern Apennines. • Chronostratigraphic constraints based on the microfloristic content • Stratigraphic correlation with coeval successions of the Northern Apennines • Thermal maturity analysis by different optical and geochemical approach • Permian palaeogeography of Tuscany [ABSTRACT FROM AUTHOR]

Highlights: [•] Relationships between latest quaternary brittle structures and seismicity in a sector of the inner Northern Apennines. [•] Role of active faults for hydrothermal fluids migration in the upper crust, thermal springs location and CO2 discharging. [•] Geodynamics implication for the inner Northern Apennines. [Copyright &y& Elsevier]

Abstract: This paper accounts for a new finding of High Pressure–Low Temperature (HP-LT) metamorphism documented in continental-derived rocks exposed in the central part of the Middle Tuscan Range, representing the easternmost location, in the Tyrrhenian Region, where blueschist facies metamorphism has been documented. It describes the development mechanism of Mg–carpholite bearing quartz veins in the framework of the deformational events described for the inner Northern Apennines. The HP-LT peak metamorphism can be estimated in P≥1.1GPa and T=370°–420°C as supported by the mineralogical assemblage consisting of quartz–muscovite–chloritoid–pyrophylite–hematite±chlorite±paragonite±Ti-oxides, documented in the aluminous metasediments (pelites and quartzose sandstones) of the Triassic Verrucano succession. This paragenesis defines the S1 schistosity, an axial planar tectonic foliation associated to map- and mesoscale isoclinals folds (F1); S1 schistosity is a composite foliation resulted from a non-coaxial progressive deformation associated to the DE1 collisional event (Late Oligocene–Early Miocene). Mg–carpholite bearing quartz veins developed as tension gashes in en-échelon arrays within brittle/ductile shear zones. The development of quartz veins was guaranteed by the high volumes of Si-rich fluids produced by the dynamic recrystallization process on the siliciclastic rocks. [Copyright &y& Elsevier]

Abstract: This paper deals with the tectonic evolution of a bowl-shaped Neogene basin (Siena Basin) located in the inner Northern Apennines and developed during an articulated tectonic evolution. This basin was controversially interpreted by different authors as graben, half-graben or thrust-top basin. Structural and stratigraphic data from the basin substratum and filling sediments, integrated with the interpretation of reflection seismic profiles and borehole logs, highlight that the Siena Basin consists of a tectonic depression with a complex internal architecture. Such a basin developed on a multilayered substratum deeply affected by extensional tectonics since the Middle Miocene. Extensional detachments produced a widespread thinning of the basin substratum and superimposed on contractional structures developed during the Northern Apennines collisional events. Extensional detachments accommodated within weak horizons and gave rise to the lateral segmentation of most competent stratigraphic horizons. Sediments filling the Siena Basin mainly deposited on the Ligurian Units (the topmost tectonic units forming the tectonic pile) in the gap between two laterally segmented geological bodies made up of the Triassic siliciclastic succession of the Verrucano Group. Such a lateral segmentation produced a synformal structural depression, not delimited by faults, filled by Late Serravallian–Late Messinian marine to continental sediments which deposited during the extensional faulting. From the latest Early Pliocene to the Middle Pliocene the architecture of the basin was modified by an high-angle normal fault system which affected the eastern side of the basin and produced new accommodation space for marine sediments. In sum, the Siena Basin consists of a sin-tectonic basin developed on the hanging wall of staircase extensional detachments, later affected by high angle normal to oblique faults. The latter faults defined a half-graben during the latest evolution of the basin. The tectonic evolution of the Siena Basin made it possible to point out some differences with respect to the types of bowl-shaped basins related to extensional detachments and named as supradetachment basins. [Copyright &y& Elsevier]

Middle–Late Pleistocene tectonic activity has been inferred through studies on travertine deposits exposed in a tract of the hinterland Northern Apennines. A detailed study on the relationships between tectonics and travertine deposition coupled with Th/U age determination of travertines at Cava Oliviera quarry, located close to Serre di Rapolano village (southern Tuscany, Northern Apennines), allowed us to recognise Pleistocene faults, whose activity has been referred to 157–24 ka, at least. Travertine deposition was tectonically controlled by WSW-ENE striking, oblique and normal faults, associated to a main fault (named as the Violante Fault). This structure dissected a regional normal fault (known as the Rapolano Fault) Early–Middle Pliocene in age, which bounded the eastern side of the Pliocene Siena Basin, and gave rise to space accommodation for clayey and sandy marine sediments. Hydrothermal circulation (and related travertine deposition) was favoured by the damaging enhancement due to the fault–fault intersection. Tectonic activity has been also documented by deformation recorded by travertines, which suggest a main tectonic event between 64 ± 5 and 40 ± 5 ka. The tectonic activity described for the study area agrees with the Quaternary tectonic evolution documented in the surrounding areas (e.g. Mt. Amiata and Mt. Vulsini), as well as the Tyrrhenian margin of the Central Apennines, indicating that a widespread tectonic activity affected the inner part of the Apennines until the latest Quaternary. [ABSTRACT FROM AUTHOR]

Terra Nova, 00, 1–10, 2010 The Mt Amiata volcano is characterized by the SW–NE alignment of the eruption centres, as well as other Italian Pleistocene volcanoes. We carried out a structural and kinematic analysis in a broad stripe comprehending the eruption centres and the substratum, towards NE and SW. The results point out that a transtensional left-lateral brittle shear zone affects the pre-Neogene substratum, the Middle Pleistocene volcanic rocks and the Late Pleistocene travertines. This transtensional structure is active since the Early Pliocene. It controlled the location of the eruption centres through pull-apart structures, thus providing the deep structural channels for the uprising of magma to the surface. The Mt Amiata volcano represents a key study area to investigate the Pleistocene to Present relationships between tectonics and magmatism in the Tyrrhenian area. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the structural evolution along a transect across the inner Northern Apennines (Monte Amiata volcano-geothermal area–Monte Cetona), mainly focussing on the post-orogenic Middle Miocene–Quaternary structures. The tectonic setting of the study area derives from the superimposition of three main faulting events: the first and second events developed during the Middle–Late Miocene and Early–Middle Pliocene, respectively, and took place in response to extensional tectonics. The third faulting event developed in a transcurrent/transtensional setting during the Quaternary. Quaternary faults mainly strike NE–SW and show left-lateral kinematics with a minor dip–slip normal component. They caused the Monte Amiata volcanic eruptions (300–190 ka) and controlled the widespread Pleistocene and present hydrothermal fluids circulation. Quaternary strike-slip faults have been documented in the Monte Amiata–Monte Cetona area for the first time in this paper. Correlation with the adjoining areas (e.g. Vulsini region) identifies the Quaternary strike-slip faulting recognised in the study area as a feasible source for the seismic activity in the Monte Amiata region, where the origin of diffuse low-magnitude seismicity is still under debate and poorly understood. [Copyright &y& Elsevier]

Downcutting and infill of incised valley systems is mostly controlled by relative sea-level changes, and studies on valley-fill successions accumulated independently from relative sea-level or lake-level oscillations are limited. This study focuses on the Plio-Pleistocene evolution of a fluvial drainage system developed in Southern Tuscany (Italy) following a regional marine forced regression at the end of Piacentian. Subsequent in-valley aggradation was not influenced by any relative sea-level rise, and valley morphological and depositional history mainly resulted from interaction between sediment supply and tectonic activity, which caused segmentation of the major valley trunk into localized subsiding depocenters separated by upwarping blocks. Fluvial sedimentation occurred until late Calabrian time, when the major river abandoned that valley, where minor fluvio-lacustrine depocenters allowed accumulation of siliciclastic and carbonate deposits. The present study demonstrates that the infill of the valley was not controlled by the forcing that caused its incision. Accumulation of the fluvial succession is discussed here in relation with localized, tectonic-controlled base levels, which commonly prevent from establishing of a clear downdip stratigraphic correlations. Chronological reconstruction of the study depositional dynamics provides solid constrains to frame them in the tectono-sedimentary evolution of the Northern Apennines. [ABSTRACT FROM AUTHOR]

The Pre-Mesozoic units exposed in the inner Northern Apennines mostly consist of Pennsylvanian-Permian successions unconformably deposited on a continental crust consolidated at the end of the Variscan orogenic cycle (Silurian-Carboniferous). In the inner Northern Apennines, exposures of this continental crust, Cambrian?-Devonian in age, have been described in Northern Tuscany, Elba Island (Tuscan Archipelago) and, partly, in scattered and isolated outcrops of southern Tuscany. This paper reappraises the most significant succession (i.e., Risanguigno Formation) exposed in southern Tuscany and considered by most authors as part of the Variscan Basement. New stratigraphic and structural studies, coupled with analyses of the organic matter content, allow us to refine the age of the Risanguigno Fm and its geological setting and evolution. Based on the low diversification of palynoflora, the content of sporomorphs, the structural setting and the new field study, this formation is dated as late Tournaisian to Visean (Middle Mississippian) and is not affected by pre-Alpine deformation. This conclusion, together with the already existing data, clearly indicate that no exposures of rocks involved in the Variscan orogenesis occur in southern Tuscany. [ABSTRACT FROM AUTHOR]

• Transfer zones play as permeable volumes channeling geothermal fluids. • Travertine deposition is coeval with faulting. • Travertine deposits are useful tools to reconstruct the activity of faults. • The Bagno Vignoni geothermal area is located along active fault segments. • Earthquakes with M < 4 are concentrated along fault segments defining the transfer zone Studying travertine deposits and the network of banded calcite veins that form their roots in the substratum can place important constraints on neotectonic activity and the seismotectonic settings of geothermal areas. In this paper, we present the results of integrated studies of a geothermal area located in the inner Northern Apennines (Bagno Vignoni area, Italy), where low magnitude (M < 4) seismicity suggests the occurrence of active faults, but information on their location, time-span of activity and seismotectonic setting is lacking. The study area is characterised by thermal springs (T<50 °C) and travertine deposits well exposed in saw-cut walls of abandoned quarries. We investigate the relationships between faults and the travertine deposits, in order to reconstruct the syn-sedimentary tectonic activity and the age of faulting, by combining analyses of: i) geological setting and structural/kinematic analyses of faults; ii) travertine morpho-structural and architectural setting; iii) travertine facies; iv) U-Th radiometric, stable- and clumped isotopes of travertine and banded calcite veins. The results highlight the occurrence of a wide brittle shear zone (>3 km) formed by two orthogonal faults systems, NE- and NW-striking, characterised by oblique-slip to normal kinematics, respectively; these faults belong to a tract of the so-named "Grosseto-Pienza" transfer zone, crossing the southern Tuscany from the sea-cost to the outer Apennines belt. The Grosseto-Pienza transfer zone formed with extensional tectonics that have been affecting the inner Northern Apennines since the middle Miocene. U-Th dating of travertine and banded calcite veins indicates that faulting enhanced the hydrothermal fluid circulation since the middle Pleistocene, in an unvaried tectonic setting, as indicated by the δ18O signature and temperature of the hydrothermal fluids, which remained stable through time. The activity of the faults continued until the Holocene and still produces seismicity. Finally, our findings permit to define the seismo-tectonic setting of this sector of the inner Northern Apennines, demonstrating more broadly the utility of travertine deposits in reconstructing the neotectonics in geothermal areas. [ABSTRACT FROM AUTHOR]

Evidence for high-P blueschist-facies metamorphism was found in metabasites embedded in calcschists of Eastern Elba Island (Northern Apennines, Italy). Study of immobile trace elements (REEs and HFSEs) in the metabasites indicates an affinity with T- and E-MORBs, and they are interpreted as sill or dykes intruded in the western margin of the Adria continental plate. The minerals are heterogeneously distributed in the rock, constituting mafic and Ca-Al-rich microdomains inherited respectively from the magmatic pyroxene- and plagioclase-rich zones of the original doleritic rock. The peak pressure paragenesis consisted of lawsonite, aegirine-omphacite, glaucophane and chlorite. Former rhombic prisms of lawsonite have been replaced by pseudomorphic clinozoisite (0.1 mm in size) with inclusions of Ms. ± Ab ± Qz. The inclusions are preferentially oriented parallel to one of the diagonals of the basal rhombic sections, indicating that the two diagonals were not crystallographically equivalent. This implies that the protocrystals were orthorhombic prisms with {110} faces, which is the case of lawsonite. The different compositions shown by white mica and epidote in the pseudomorphs (muscovite and Fe3+-poor clinozoisite) and in the matrix (phengite and Fe3+-rich epidote) also suggest the former presence of Mg- and Fe-poor lawsonite in these rocks. Thermodynamic modelling, using the THERMOCALC software and dataset, suggests that metabasites experienced a clockwise P-T path from the lawsonite-blueschist to epidote-blueschist subfacies, down to greenschist facies. The estimated peak pressure conditions are P ≥ 1.6 GPa and 450 < T < 500 °C. Glaucophane grew mainly during the lawsonite-to-epidote transition and is dated at 19.8 ± 1.4 Ma by the 40Ar/39Ar method. This evolution is interpreted in terms: (i) intrusion of basic magmas in carbonatic and pelitic sediments (post-late Cretaceous, pre-Oligocene), (ii) subduction (Oligocene), (iii) collision (early Miocene: 19.8 ± 1.4 Ma), (iv) exhumation and granite intrusion (middle-late Miocene), favored by extensional tectonics. We underline that the eastern part of Elba Island belongs to a blueschist-facies belt that extends from Gorgona Island, in the northwest, to Argentario and Giglio Island in the southeast, so many of the conclusions of this study can be extended to the whole of this belt. • Elba Island metabasites record metamorphic conditions of P ≥ 1.6 GPa and 450–500 °C. • Chlorite-Lawsonite-Glaucophane-Omphacite-Rutile represents the HP-LT metamorphic assemblage. • The transition from lawsonite-epidote to blueschist facies is dated at 19.8 ± 1.4 Ma and related to collision. • The Rio Marina metabasites derived from dykes and/or sills with E/T-MORB geochemical affinity. • The eastern Elba Island units are attributed to the margin of Adria plate. [ABSTRACT FROM AUTHOR]