“Terme Caronte” Geothermal Area (Calabria, Italy) E
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GNGTS 2014 SESSIONE 3.2 Final results of “Terme Caronte” geothermal area (Calabria, Italy) E. Rizzo1, G. Iovine2, F. Muto3, A. Caputi1, L. Capozzoli1, V. Giampaolo1, L. Pizzino4, A. Manzella5 1 CNR-IMAA, Tito (PZ), Italy 2 CNR-IRPI U.O.S. of Cosenza, Italy 3 University of Calabria, Rende, Italy 4 INGV, Roma, Italy 5 CNR-IGG, Pisa, Italy Introduction. The VIGOR project is aimed at assessing the geothermal potential and exploring geothermal resources of four regions in southern Italy, and it is part of the activities of the Interregional Programme “Renewable Energies and Energy Savings FESR 2007-2013 – Axes I Activity line 1.4 “Experimental Actions in Geothermal Energy”. During the final stage of the project, a bore-hole has been realized and reached a depth of more then 900 m. This direct investigation is the last part of the investigation of the low-medium enthalpy geothermal resources at “Terme Caronte”, in the Lamezia Terme territory (Calabria - Southern Italy). The assessment work, characterized by geophysical, geochemical and geological investigation around the localized hot springs,in the Terme Caronte area, has defined the subsurface and the deep structures correlated to the deep geothermal fluid circulation. Geological, geochemical and geophysical previous data: first part of Vigor project. The studied area is located at the westernmost edge of the Catanzaro Graben (Fig. 1), one of the most important cross-dislocations of the Calabrian Arc (hereafter referred to CA), where metamorphic and crystalline Alpine rocks and Hercynian rocks (Calabrian basement Complex) cropping out in the Sila massif (upperplate), overthrust on apennine carbonate units (underplate). In the frame of the Mediterranean mobile belt, the Calabrian Arc is an arc-shaped continental fragment located between the E-W trending Sicilian Maghrebides, to the south, and the NW– SE trending Apennines, to the north (Ogniben, 1973; Amodio-Morelli et al., 1976; Bonardi et al., 2001, and references therein). All these rocks are dissected by a complex assemblage of high-angle faults, which can be ordered in a number of major systems and patterns, partly related to transcurrent faulting. As a whole, the regional structural framework is characterized 1 001-258 volume 3 186 24-10-2014 16:46:45 GNGTS 2014 SESSIONE 3.2 Fig. 1 – The location of the geophysical profiles. Red marks indicate the MT stations, red lines the DERT stations and the blue lines the seismic profiles. by a low-angle thrust sheet assemblage, dissected by high-angle faults with normal/oblique kinematics. After the deformations associated to tectonic evolution of Calabrian Arc, during an intense post-orogenic extensional tectonic phase, several marine sedimentary basins were developed corresponding to main segments of the faults, arranged both parallel and transverse to the direction of the Arc and raised in blocks, giving a typical structure at horst and graben (Ghisetti, 1979; Ghisetti and Vezzani, 1979). In the study area, a Plio-Pleistocene marine succession of clays, sands, sandostones and conglomerates outcrops. In the northeastern sector of Terme Caronte, a terrigenous, evaporitic and carbonate mio-pliocenic sedimentary succession overlies the igneous-metamorphic basement of the Sila Unit. These deposits are strongly controlled by NW- strike-slip system, arranged in a right-hand en´echelon pattern (Tansi et al., 2007). Slate and metapelite crop out in test area, in particular belonging to the Paleozoic Bagni Unit. The lowest group of the orogenic tectonic piling is formed by a Mesozoic carbonate complex. In literature, their outcrops are traditionally considered as “tectonic windows” of the Apennine Chain (e.g. Amodio-Morelli et al., 1976; Tortorici, 1981; Tansi et al., 2007). The Mesozoic-Tertiary continental-margin carbonates, outcropping in Terme Caronte district, were extruded within transpressional areas. The uplift the deep-seated units of the Chain can in fact generally be ascribed, at a regional scale, to reverse-oblique strike-slip movements along NW-SE regional shear zones (Tansi et al., 2007). From hydrogeological point of you, the Terme Caronte area should be divided by three hydrogeological sectors. The recent debris deposits complex, along the main rivers and coastal plain, shows the most interesting geological characteristics in terms of resource volumes stored and favourable logistical conditions. The Plio-Pleistocene sandstone and conglomerate is characterized by high permeability and great thickness and, finally, the Mesozoic carbonate complex has a deepwater circulation mainly occurs in high-permeability fractured zones associated to the main faults. Geochemical data provided in the Terme Caronte district were acquired in order to define the origin and interactions of the circulating fluids and to investigate possible relationships with the local faults. Thermal and cold waters were sampled during the field work in the last year, showing outlet temperature ranging between 10°C and 39°C. The thermal waters sampled near 1 001-258 volume 3 187 24-10-2014 16:46:46 GNGTS 2014 SESSIONE 3.2 Terme Caronte are close to intersection of two faults: a NS trending fault, along the Bagni River, and a WNW-ESE-trending regional strike-slip fault system (Ghisetti and Vezzani, 1981; Tansi et al., 2005; Tansi et al., 2007). The low heat flow measured both in the Caronte area (50 mW/m2, Cataldi et al., 1995), and in the whole Calabria rules out magma intrusions at shallow depth in the crust. This implies that the thermal character of the investigated Fig. 2 – A picture of the drill machine. waters is a consequence of the deepening of the hydrological circuit in fractured areas marked by a normal geothermal gradient (~30°C/km) allowing the existence of low to medium- enthalpy aquifers at a depth of about 1-3 km. For some of the thermal waters, the temperatures calculated by different geothermometers show a limited cooling as a result of a fast uprise through highly permeable discontinuities (namely the local faults). Classical geothermometric relations applied on thermal waters of Caronte (Na/K and K/Mg in Fig. 2, from Italiano et al. (2010) to test their suitability to be used for the estimation of deep temperature conditions provided contrasting and unreal values. This is mainly due to the particular geological and hydrochemical settings of the area; as a result, chemistry of thermal waters is due to isochemical dissolution of country rock-forming minerals rather than a real equilibrium with them. The only reliable geothermometer for the Caronte system (Marini et al., 1986) indicates a deep reservoir temperature in the range 55-65°C, confirming the low to medium enthalpy of the geothermal system. Moreover, in the Terme Caronte area new deep geophysical investigations were carried out and old seismic lines were re-elaborated (Fig. 1). The main target zones for geothermal utilization is the Carbonate basement rocks, made of limestone/dolomite of Mesozoic age. This kind of layers is thought to have the highest permeability, and its thickness and extent are also significant. The mentioned targeted reservoir is below the Calabrian Terranes. The seismic available VIDEPI data was used to trace the lateral extent of these rock types and to locate fault zones within them. The seismic lines all came with Two-Way-Time (TWT) in ms as a depth reference. Seismic interpretation was focused on to identify the following seismostratigraphic horizons: slate and metapelite (bottom of Calabrian Terranes) horizon and Mesozoic carbonate horizon. Easy identification is reflected by the observed continuity, of Slate and metapelite layers and of tectonic features. Mesozoic carbonate seismic unit consists of low continuity, moderate amplitude and low frequency reflectors with chaotic configuration, poor quality is attributed to seismic lines showing this layers and their tectonic features controlling the permeability and flow patterns. The E-W seismic profile shows low-angle overthrust contacts with basement units (Mesozoic Carbonate and bottom of Calabrian Terranes) obliterated by subsequent overprinting of high-angle faults. Seismic profiles covering the Terme Caronte area demonstrate the presence of a fractured carbonate reservoir, which is deformed to shape a partly buried positive flower structure. One Deep Electrical Resistivity Tomography (DERT) and one magnetotelluric (MT) profile were carried out in the investigated area. The Deep Electrical Resistivity Tomography carried out in the Terme Caronte did not allow to reach an investigation depth of 1000 m. On the contrary, the Magnetotelluric (MT) investigation allowed to retrieve information on the distribution of the electrical properties from hundred meters down to five kilometers. Under this point of view, the MT method can be considered a useful and non-invasive tool to investigate the Earth interior. In the framework of the project, DERT and MT data show clearly the contact between different electrical layers which should be associated to the geological formations. 1 001-258 volume 3 188 24-10-2014 16:46:46 GNGTS 2014 SESSIONE 3.2 Moreover, the main structure associated to the geothermal system and the connection with the hot springs of Terme Caronte are detected. Finally, in the plain of the town of Lamezia Terme (CZ) reflection and refraction seismic surveys were carried out at high resolution. A seismic profile was acquired and reached a total of 2975 m of investigation depth. The seismic reflection section shows structures related to sub- horizontal stratification on an inclined base and displaced by faults. On the migrated section, in the northern part of the section a series of strong reflectors is evident at a depth of 600-800 m. By the geological characteristics and seismic velocity (> 3000 m/s), these reflectors most probably represent the top of the bedrock. These reflectors deepen toward south, where they are found at a depth of 1200-1400 m. The deepening of the bedrock top and corresponding reflectors at the southern edge of the profile is most probably related to tectonic dislocations, which are evident at surface.