GNGTS 2013 - Atti del 32° Convegno Nazionale

Structural data. This area is structurally complex, and our structural analysis mainly considered data about bedding and fractures orientation. For the first step of the work, these data are especially referred to cap rock formations, because they diffusely crop out in the area, while outcrops of a possible reservoir, such as Scaglia Rossa Formation, are more difficult to reach and less extended. Furthermore, western side of the anticline can be easily investigated, due to the lower steepness of the limb. Bedding along the western side dips few tens of degrees to the west, with a main trend N175/20°W, while it becomes very steep, and subvertical to re- verse in the eastern flank, with a main trend N155/90° (Fig. 3a). An anomalous dip direction of the strata is recognized in the particular area of the Garrafo Stream, with dip direction about N330/10°. The Marne con Cerrogna, Bisciaro and Scaglia Cinerea Formations are character- ized by important detachment levels locally developing complex shear zones with doubles and elisions of the stratification, mainly affecting the Bisciaro Formation (Marsili and Tozzi, 1995). The first detailed meso-structural stations confirmed that mean directions of the shear planes range between N170/50°W and N10/30°W. Furthermore, some steeper planes are locally pres- ent (N50/70°W). These information will be important to better characterize the cap rock of our geothermal system and for the next implementation of a 3D model. Further structural analyses carried out in the area reveal two important systems of open fractures, the first one from NNW–SSE to NW–SE oriented, and the second one trending about E-W (Fig. 3b). These fracture systems are related to the complex structural history of the area, and can affect the hydraulic conductivity of the rock mass, and favoring preferential paths for surface and deep water drainage, as shown by the karst corrosion in limestone layers and clay or calcite filling in marly levels. For this reason, a detailed meso-structural analysis has been planned to collect data from scan lines distributed across the main structure. Preliminary results from this survey confirm the general fracture trend mentioned above (N110/70°W). At this stage of the work, we have also elaborated small DFN (Fig. 3c) in order to calculate porosity (Fig. 3d) and permeability (Fig. 3e) values from fractures distribution in specific lithologies (Cerrogna Marls). Many faults are also present (Fig. 3a). Their main orientations are N–S and WNW–ESE and they can also represent the preferential ascent path for deep hot waters and, on the other hand, can facilitate the mixing of superficial waters with the thermal ones. Conclusions. A meso-structural analysis together with water isotopic analyses have been carried out along the Acquasanta positive structure (southern Marche region, Central Italy), in the surroundings of the thermal area. First results allowed to: i) determine the origin of thermal water, ii) inferring a double water circulation path, iii) recognise the main fracture systems, their relationships with the main anticline structure and their possible relationships to fluid circulation. A double fluid circulation model is inferred: hot waters with a probable meteoric origin are involved in a deep circuit that rises to the surface and mixes with a shallower circuit of cold waters. Variations in conductivity among the four thermal springs, that present similar temperatures, let us think that these thermal waters dilute with a variable amount of cold water coming from the shallow circuit, probably from the Rio Garrafo stream. So we can also suppose that the shallow path could flow within Scaglia Rossa Formation due to its intense fracturing, with marly formations as cap rocks, while the deep one could flow within the underlying carbonate formations. Finally, our first structural data allow us to make some observations. Shear zones and detachment levels present in the marly formations are related to the compressional history of the area, in particular to the thrusting phase. The main fracture systems instead are probably related to a more recent extensional event, and they have slightly increased porosity and permeability values of the rocks, especially in the ZZ direction. Acknowledgements. We thank R. Giovannetti, S. Ferraro and the Water Chemistry Laboratory of Camerino University for their contribution during this stage of our work. We also thank Ing. M. Mussi and the Pisa CNR Laboratory for stable isotopes analyses, M. Carroll and P. Pierantoni for useful discussions. 255 GNGTS 2013 S essione 1.3