REVISION OF THE EUGANEAN GEOTHERMAL FIELD (EGF) USING NEW KNOWLEDGE IN HYDROGEOLOGY AND STRUCTURAL GEOLOGY

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1 REVISION OF THE EUGANEAN GEOTHERMAL FIELD (EGF) USING NEW KNOWLEDGE IN HYDROGEOLOGY AND STRUCTURAL GEOLOGY Ph.D. candidate: MARCO POLA Tutor: Prof. PAOLO FABBRI, Prof. DARIO ZAMPIERI Cycle: XXIV Abstract The Euganean geothermal field (EGF) is the most important thermal field in the northern Italy. It is located in the Veneto alluvial plain, southwest of Padova (NE Italy), close to the northeast margin of the Euganei Hills. Recently, it has been suggested that the hydrothermal outflow could be controlled by an active regional geological structure, i.e. the Schio Vicenza fault system (SVFS). A new conceptual model of the Euganean thermal circuit will be proposed. In this model, the Schio-Vicenza fault plays an important role because channelizes and enhances the quick uprising of the thermal waters in EGF, thanks to the local extensional regime caused by a transtensional relay zone linked to Schio-Vicenza fault. The recharge area is located to the east of the Schio-Vicenza fault, in Sette Comuni plateau area, instead of to the west (Piccole Dolomiti area) like in the previous conceptual model. The analysis of a fault/fracture mesh on the top of a 5 meter-high hill of travertine in Abano Terme, called Montirone, strongly supports the existence of a relay zone buried beneath the alluvial cover. INTRODUCTION The Euganean Geothermal Field (EGF) is the most important thermal field in the northern Italy. It is located in the Veneto alluvial plain, east of the Euganei Hills and southwest of Padova (NE Italy). The EGF extends on an area of about 36 Km 2 and comprises the town of Abano Terme, Montegrotto Terme, Battaglia Terme and Galzignano Terme. At present about 250 wells are active and the total average flow rate of thermal fluids is about 17M m 3 /year. In 1976, Piccoli et alii proposed a simple model for the hydrothermal circuit. The aim of my work is the review of the hydrothermal model of the EGF, using new techniques and new knowledge in hydrogeology and structural geology. In the second year of my PhD, I made a new conceptual model of the Euganean thermal circuit and now I m starting the mathematical modelling of the thermal circuit as implementation and constraint of the conceptual model. Moreover, I made some structural and geophysical analyses on a travertine fissure ridge in Abano Terme. PREVIOUS MODEL The geochemical analysis of the Oxygen isotopes on the thermal waters in Abano Terme and Montegrotto Terme shows that the waters are of meteoric origin and infiltrate in an area up to 1500 m a.s.l.. The previous model (Piccoli et alii, 1976) located the recharge area of the thermal circuit in the Piccole Dolomiti area (max altitude 2259 m a.s.l; mean altitude 2000 m a.s.l.). The meteoric waters infiltrate and flow in an Upper Triassic carbonate reservoir, uplifted in the footwall of a normal fault under the Piccole Dolomiti area (as shown in the cross section proposed by Piccoli). They warm up by a normal geothermal gradient and then rise quickly in the EGF area, thanks to the high fracturing of the rocks. In fact, due to a misunderstanding of the structural setting (the uplifted block is the hanging wall of a south-verging thrust, as shown in the section 3 of the Geological Map of Veneto (Antonelli, 1990)) and the use of a idealised (wrong) cross section, this model cannot work. Moreover, the south-verging thrust uplifts, in the hanging wall, the pre-permian crystalline basement that crops out downstream from the Piccole Dolomiti. The low permeable crystalline rocks of the basement hydrogeologically isolate the recharge area from the outflow parts of the thermal circuit. PROPOSED CONCEPTUAL MODEL The Euganean geothermal circuit is reviewed starting from the lower part of it. A collection of seismic sections near Padova highlights a new architecture of the regional Schio-Vicenza fault and a system of normal faults (Schio-Vicenza faults system - SVFS), synthetic with the Schio-Vicenza fault, buried beneath the alluvial cover. In Abano Terme area, the existence of a left stepover structure (extensional 1

2 relay zone) between segments of the SVFS has been proposed (Zampieri et alii, 2009). Given the Neogene to Quaternary sinistral strike-slip kinematics superimposed on the fault system, this structure has accommodated along-strike local extension and may be responsible for rock fracturing and permeability development, enhancing migration to surface of thermal waters. The presence of a 5 meter-high hill of travertine in Abano Terme, called Montirone, strongly supports the existence of 1) a releasing structure in the subsurface which controls the development of the EGF and, 2) ongoing activity of the Schio-Vicenza fault that keeps steady the opening of the fracture and enhances the outflow of thermal waters. The thermal circuit is located east to the Schio-Vicenza fault instead of west like in the previous model. The medium part of the main segment of the Schio-Vicenza fault acts as a conduit for the hot waters thanks to higher permeability of the damage zone (Caine et al., 1996). Moving to the north from the EGF, one can find an area where crops out largely the upper Triassic dolomite formations that are the main thermal aquifer in Abano area. This area is the Sette Comuni plateau area and it is also the recharge zone of the proposed conceptual model of the EGF. The altitude (from 1000 m to 2036 m a.s.l.) is according with the Oxygen - isotopic composition of the thermal waters. The waters infiltrate thanks to the high secondary permeability of the outcropping rocks and flow to the south in the carbonate reservoir (composed mainly by the Dolomia Principale formation), reach a depth of about 3000 m and warm up by a normal geothermal gradient. The cross section of the thermal circuit is constructed using the stratigraphy of deep wells (Villaverla 1 and Vicenza 1 in the northern part, Due Torri in Abano) and using a composition of seismic sections and geophysical data. The 3D model (constructed from available seismic sections south to the EGF) permits to close the cross section at the southern termination of the model. MONTIRONE Until the sixties, the main natural occurrences of the outflow in Abano Terme were located on the top of a 5 meter-high hill of travertine, called Montirone. The natural occurrence of the thermal waters in this area has been well known since the Roman period. The hot waters fed several pools, located on the flat top and at the base of the ridge, and were also channelized to move a mill-wheel (Vandelli, 1761). Exploitation of the thermal waters lowered the potentiometric level and dried up the hot springs. At present, the travertine hill is used as a public park. The hill is affected by a network of fractures mainly composed of two fracture sets NW-SE and NNE-SSW, which allow us to refer to the travertine deposit as a fissure ridge (fig.1a). These structures have been studied in tectonically active regions of Turkey (Anatolia), Greece (Euboea island), the USA (California) and Italy (Tuscany), showing typical features and providing significant information about stress orientation during travertine deposition (e.g. Hancock et alii, 1999; Temiz et alii, 2009; Brogi et alii, 2009). The trend of the fissures (NW-SE) parallels the trend of the main structural elements of the region and is comparable with the direction depicted in the variogram analysis of transmissivity in EGF area (Fabbri, 1997). It suggests that the fissure ridge is localized above an extensional fissure or in the hanging wall of a transtensional fault cutting the bedrock lying about 150 meters beneath the alluvial cover. The sets of fissure could be interpreted as a fault/fracture mesh located in the overlap zone of two sinistral strike-slip faults. Looking closely to the map of the fissures and to the rose diagram of the directions of the fractures (fig.1c), the main direction (N 115) represents the main facture in the fault/fracture mesh model and is affected by an extensional regime that opens the fracture. Two other sets of fractures (N 15 and N 135) represent the secondary fractures of the model and are affected by a transtensional regime. Preliminary geophysical analyses using electromagnetic techniques (co-operation with R. Deiana) show areas with higher resistivity near the main fissure and where the travertine crops out, showing a possible NW-SE direction of the higher resistivity anomalies. Other geophysical analyses will be carried out to analyze an anomaly of lower resistivity (mainly oriented N-S) on the top of the fissure ridge and to focus other fissures covered under the superficial cover. A campaign of sampling of the travertine will be carried out to date it with U/Th method (co-operating with M. Soligo) and to analyse the travertine sedimentary facies (co-operating with A. Gandin). 2

3 a) c) b)? Fig. 1. a) Aerial view (extracted from Google Earth) of the Montirone fissure ridge in Abano Terme. The solid blue lines represent the open and visible fissures. The dash-dot blue lines represent the fissures filled by the superficial cover but mark by small depressions. The dashed blue lines represent the supposed fissures. The presence of fissures covered under the surface will be investigated using geophysical methods. b) The graph represents the main fissure width (directed mainly NW-SE) vs the position along it. The width is maximum at the top of the ridge (about 75cm of width) and decreases towards the tips (about 5cm of width). In the last part, the fissure is only supposed and there isn t data of width. c) Rose diagram of the direction (measured each meter) of the fissure. The main direction (N115) represents the main fissure affected by an extensional regime. The other two directions (N 15 and N 135) represent the secondary fissures affected by a transtensional regime. These fissures are visible in the external areas of the fissure ridge and on the tips of the main fissure. 3

4 References ANTONELLI R., BARBIERI G., DAL PIAZ G.V. et alii Carta geologica del Veneto 1: e relative Note Illustrative, S.E.L.C.A., Firenze. CAINE J.S., EVANS J.P. & FORSTER C.B Fault zone architecture and permeability structure, Geology, 24, BROGI A., CAPEZZUOLI E Travertine deposition and faulting: the fault-related travertine fissureridge at Terme S.Giovanni, Rapolano Terme (Italy). International Journal of Earth Science, 98, DA RIO N Orittologia Euganea. Tipografia Cartallier, Padova, 179PP. FABBRI P Transmissivity in the Euganean Getohermal Basin: a geostatistical analysis. Groundwater, 35(5), HANCOCK P.L., CHALMERS R.M.L., ALTUNEL E., ÇAKIR Z Travitonics: using travertines in active fault studies. Journal of Structural Geology, 21, PICCOLI G., BELLATI R., BINOTTI C. ET ALII Il sistema idrotermale euganeo-berico e la geologia dei Colli Euganei. Memorie degli Istituti Geol. Miner. Università di Padova, 30, 266 pp TEMIZ U., GÖKTEN E., EINKENBERG J U/Th dating of fissure ridge travertines from the Kirsehir region (Central Anatolia Turkey): structural relations and implications for the Neotectonic development of the Anatolian block. Geodinamica acta, 22(4), VANDELLI D. (1761) - De Thermis Agri Patavini. Tipografia Conzatti, Padova, 234 PP. ZAMPIERI D., FABBRI P., POLA M Structural constrains to the Euganean Geothermal Field (NE Italy). Rendiconti online Società Geologica Italiana, 5,

5 SUMMARY LAST YEAR S ACTIVITY Courses: FABBRI PAOLO, Elementi di geostatica, Università degli Studi di Padova. SALMASO LUIGI, Statistica applicata alla sperimentazione scientifica, Centro studi per l ambiente alpino. Communications: POLA M., FABBRI P, ZAMPIERI D. Hydrothermal model of the Euganean geothermal field (EGF) NE Italy, XXXVIII IAH CONGRESS, Settembre 2010, Cracovia, Polonia. Schools, workshops and congresses: XXXVIII International association of hydrogeologist congress Groundwater quality and sustainability, Settembre 2010, Cracovia, Polonia. European Geosciences Union General Assembly 2010, Maggio 2010, Vienna, Austria. Posters: POLA M., ZAMPIERI D., FABBRI P. 3D modelling and construction of a standard cross section of the Euganean Hydrothermal circuit NE Italy, EGU General Assembly 2010, Maggio 2010, Vienna, Austria. Publications: POLA M., ZAMPIERI D., FABBRI P. 3Dmodelling and construction of a standard cross section of the Euganean Hydrothermal circuit NE Italy, Geophysical Research Abstracts EGU 2010, 12. POLA M., FABBRI P, ZAMPIERI D. Hydrothermal model of the Euganean geothermal field (EGF) NE Italy XXXVIII IAH CONGRESS Abstract book, 2, ZAMPIERI D., POLA M., FABBRI P The fissure ridge of Abano Terme (Padova). Rendiconti online della Società Geologica Italiana, 11, ZAMPIERI D., POLA M., FABBRI P The fissure ridge of Abano Terme (Padova) and its seismic hazard implication. 29 Convegno Nazionale GNGTS, ottobre 2010, Prato. ZAMPIERI D., POLA M capitolo VI: Tettonica. In: Note illustrative della carta geologica d Italia scala 1:50.000, foglio 147 Monselice. (In press) 5