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The Montirone is a travertine-made hill (max. height 15.75 m a.s.l.) located in the Abano Terme area of the Euganean Geothermal Field (EGF), that extends on a plain band of 36 km² south of Padova (Veneto-Italy). The existence of an extensional relay zone, buried hundred meters beneath the alluvial cover, between segments of the Schio-Vicenza fault system (SVFS) has been proposed. This structure accommodates a local extensional regime that keeps open fractures in the subsurface of EGF and enhances the uprising of hot waters. The temperature of thermal waters, used mainly for spa, ranges from 60°C to 86°C. ³H and ¹⁴Cmeasurements suggest a residence time greater than 60 years, probably a few thousand years. At present, about 250 wells are active and the total average flow rate of exploited thermal fluids is 15 Mm³/y. Until the 1960, the only natural occurrence of thermal waters in Abano was located on the Montirone hill ; hot waters fed several pools located on the flat top and at the base of the hill and were partially channelled to move a mill-wheel. The intense exploitation of thermal waters, linked to the increasing number of thermal wells, lowered the potentiometric level from more than 16 m a.s.l. to -15 m a.s.l. (year 1982 ; at present is 5 m a.s.l.) and dried up the hot springs. The hill has a sub-circular shape, covers an area of 10000 m² and can be classified as a regular shaped travertine mound. The mound is affected by a network of fractures mainly composed of two sets (WNW-ESE and NNE-SSW) that can be interpreted as a fracture mesh developed in a dilational stepover between strike-slip or transtensional fault segments of the SVFS. The trend of fissures parallels the trend of the main regional structural elements (e.g. SVFS ; mainly oriented NW-SE) and is consistent with the direction of the anisotropy depicted in the variogram analysis of transmissivity calculated in the bedrock of EGF(N110°E). Two drillings (about 1m deep) were made to study the depositional facies of the travertine because of the absence of relevant cross sections. Thin sections of the core samples show the dominance of layered travertine made of lime-mudstone with paper-thin rafts and laminae of bacterial shrub, locally covered by crystalline crusts made of feather-like calcite. Moreover, soft sediment syn-depositional deformations and repeated phases of karstic dissolution and redeposition have been observed. Isotopic analyses on uranium-series, carried out on crystalline crusts sampled during the drillings, have given ages from 30±4 kY to 25±5 kY. Moreover, the low content of U (from 3 to 12 ppb) and the high activity ratio of 234U/238U (>2) clarify the high interaction between rock and water, typical of a deep geothermal system with high residence time of waters. Geophysical methods, and particularly Electromagnetic survey (EM) and Electrical Resistivity Tomography (ERT) were used to determine the mound geometry. The map of EM measurements (depth of investigation around 1.5m) shows the presence of two resistive areas along the slope and a more conductive region on the top of the mound. The latter can be interpreted as indicative of the uprising of thermal waters through a more fractured zone. ERT surveys confirm the vertical extent of the conductive and resistive anomalies, and support the interpretation above. The presence of the Montirone travertine deposit strongly supports the existence of a releasing structure in the subsurface, which controls the development of the EGF, and an ongoing activity of the SVFS, which is generally considered inactive. The analysis on U-series isotopes gives another important constraint on the deep circulation and a long residence time of thermal waters. |
