The refinement of geomorphological and geochemical statistical techniques in the study of clay-basin tectonics: The era basin (central Italy)

A comparative approach to neotectonic studies is presented, which encompasses the integration of geochemical, morphological and structural analyses. Nine-hundred-nine-teen soil gas samples were collected in the Era basin (Tuscany, Central Italy) and their helium contents were measured. Helium distribution has been compared with location and orientation of known brittle deformations (faults and fractures) and morphological features obtained by air-photo interpretation and drainage network analyses. Obtained data were statistically compared by means of rose dia-gram plots concerning the investigated parameters and locally studied by associating the observed helium anomaly ridges with the known morphological and structural elements. The statistical approach showed a good convergence between the applied methodologies. Data from geomorphological, mesostructural, and geochemical surveys are consistent with the NE-SW and NW-SE orientations, i.e. Apennine and anti-Apennine trend of the known structural pattern. Moreover the apparent N-S and E-W trending helium anomalies are thought to be due to the Middle Pleistocene deformation phase along these directions. The relationship between helium distribution and the strain field is strengthen by the good correspondence, at local scale, among geochemical data and results of the structural and geomorphological features (Orciatico-Montecatini Val di Cecina and Peccioli areas). However helium soil gas technique showed to be a sensible tool for neotectonic studies in clay basin, as soil gas defines the leakage of deep seated gas along tectonic discontinuities even if they have no surface evidence and where the clay deposit is hundreds of meters thick.