Domenico Liotta

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.

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.

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.

D2 asymmetric folds and their vergence meaning in the Montagnola Senese metamorphic rocks (inner northern Apennines, central Italy)

Abstract This study describes asymmetric folds whose vergence is derived from refolding of a pre-existing inclined foliation. The study area is in the Montagnola Senese area where Mesozoic rocks in the greenschist facies crop out. These rocks were affected by deformation during the collisional stage (late Oligocene–early Miocene) of the Northern Apennines (D 1 event), and during the post-collisional extensional tectonics (D 2 event) that affected the inner zone of the Northern Apennines since the early–middle Miocene. During the D 1 event, SW dipping axial plane schistosity and non-cylindrical folds indicative of a highly heterogeneous strain developed. During the second event westward verging folds developed. These folds are characterised by thick steep limbs. The S 2 foliation, which is well developed in narrow and localised zones, is mainly a pressure solution cleavage with NE plunging stylolitic teeth. Micro- and meso-structural observations are used to discuss the relation between cleavage distribution and strain during the D 2 event. The evolution of D 2 folds was mainly controlled by a dissolution process and by a component of partitioned shear-strain. In the relatively ‘high’ strain domains, deformation took place by the combined effects of volume loss and shear displacement. In low strain domains, deformation took place by a veining and flattening process. In this way, the pre-existing foliation can be deformed in folds with vergence opposite to the sense of shear.

Post-emplacement thermo-rheological history of a granite intrusion and surrounding rocks: the Monte Capanne pluton, Elba Island, Italy

Abstract A thermo-rheological model of the Monte Capanne pluton, Elba Island, Italy is proposed as having general relevance for the thermal and tectonic evolution of upper crustal granites and their surrounding rocks in extensional regions. The thermal evolution of the pluton and country rocks is followed for 1 myr after emplacement, which occurred at c. 6.9 Ma. The pluton completely crystallized in c. 210 kyr (±20%). The adjacent rocks reached a thermal peak of 550 °C (±10%), maintaining a temperature higher than 500 °C for c. 100 kyr. The temperature distribution is used to construct a model for the time-dependent rheology of the pluton and surrounding rocks. A series of 2D cross-sections shows an upward migration of the regional brittle−ductile transition, and the formation of a ductile horizon above the pluton. The former is a combined effect of unroofing and middle crust heating; the latter is the result of temperature increase in rheologically weak country rocks. This ductile horizon has a potential role in the tectonic evolution of the region, since it could favour the formation of upper crustal shear zones and listric faults rooting in the transient brittle−ductile transition and playing a major role in further post-emplacement extension.

Late-Hercynian shearing during crystallization of granitoid magmas (Sila massif, southern Italy): regional implications

Shearing of regional extent, involving granitoids and underlying mid-crustal rocks of the Sila massif (Calabria, Italy), is analysed in this paper. The deformed granitoids are affected by a wide NNW-SSE oriented deformation zone, stretching for about 60 km, from the neighbourhood of Cecita Lake to Cropani village. Meso- and micro-structures in granitoids, close to the boundary with underlying migmatitic paragneiss, indicate that deformation developed from melt-present to solid-state conditions. Simultaneous tectonics and magmatism activated a plutonic accretionary process at mid-crustal levels. This took place at about 300 Ma and involved hybrid magmas with a dominat contribution from a mantle source. The deformation regime remained steady for a long time during magma crystallization and cooling in subsolidus conditions. The regional top-to-the-W sense of shear in the present geographic coordinates, recorded in the deformed granitoids, seems geometrically consistent with the coeval direction of maximum extension found in another sector of the southern Hercynian belt, suggesting the original position of the Sila basement in this context. Magmatic ativity ended with the intrusion of mafic and felsic magams affected by a very weak deformation, ongoing during the final strain increments of the late-Hercynian stage.

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.