Tectonic influence on hydrothermal fluid flow in the Laga foredeep Basin (Marche, Italy)

The Adriatic area has relatively normal continental lithospheric thickness (about 70-90 km), thick and deformed foredeep sequences and low heat flow, also related to fast recent sedimentary deposition. In this framework, many thermal areas are present, and one of these is Acquasanta Terme, located in the southern part of Marche region (Central Italy). This area lies in the northern sector of the Laga Basin, the largest foredeep basin within the Central Apennines fold and thrust belt. Presently, the Acquasanta region is characterized by a major asymmetrical anticlinal structure, trending N170°, about 25 km-long, and plunging both to the north and to the south. Along this structure, the upper part of the Umbria – Marche calcareous sequence emerges within the Laga Basin, just in front of the Sibillini Thrust. Along the Tronto River valley, many thermal springs are present with a maximum temperature of about 44°C. The aim of this study is to explain which type of physical mechanisms can create geothermal system in foredeep areas. Present studies to achieve this aim include structural analyses, water chemical analyses of major and secondary elements, and quantification of water stable isotopes of oxygen and deuterium. First structural data allow us to distinguish the major fracture sets, belonging to compressive and extensional events, related to the complex structural history characterizing the area. This pattern allowed an outline of preferential directions of groundwater flow, generally NNW – SSE and E – W oriented. Furthermore an important detachment level in the Oligocene Scaglia Cinerea Formation can be identified, with thermal water springs in correspondence with this level, suggesting that this can be considered as the seal rock for the reservoir, consisting of the underlying calcareous formations. Many faults are also present in the area, N – S and ESE – WNW oriented. These faults represent the preferential ascent path for deep hot waters and on the other hand facilitate the mixing of superficial waters with the thermal ones. Stable isotopic data of oxygen and deuterium from cold (i.e. -9.82; -64.0) and thermal (i.e. -10.1; -69.0) waters and their variations in time show a common origin for surface and deep waters, probably with the same infiltration area, but with two different paths, a shallow and a deep one. On the basis of meteoric water lines, we can estimate an altitude of water infiltration at approximately 1500 m a. s. l.. This is compatible with well defined recharge areas. Tritium and 13C isotopic data also provide residence time at depth. Chemical data from samples of thermal waters collected every three months outline a chloride-sulphate composition, rich in Na, K, Ca and Mg, and containing H2S. Furthermore, anomalous enrichment in Lithium, Strontium, Fluorine and Boron is present. Variations in temperatures, pH, conductivity and concentrations in time reveal a mixing between the two circuits. Additional data on chemistry and isotopes (Sr, S) also provide certainties about the degree of mixing and water paths at depth. Finally, we are performing a climatic and hydrogeological monitoring for the evaluation of circulating water amount in the reservoir and better define the geothermal system in foredeep basin.