Structural arrangement of Mt. Etna Western Rift inferred from soil gas survey and geostatistical analysis

Soil gas survey is a well known methodology to infer the presence of buried tectonic lineaments on volcano edifices from the evaluation of the orientation and extent of soil CO2 degassing anomalies. This application is an useful tool mainly where there is no evidence of tectonic structures found at the surface from geological survey, because of high resurfacing rate of investigated areas. In this study attention was focused on an area located on Mt. Etna Western Rift, one of the most prominent, yet less understood and studied feeding systems of the volcano. It is constituted by many eruptive fissures and monogenetic cones emplaced from 15 ka until 1974. In this area, a multidisciplinary approach was carried out based on detailed geological survey at 1:10,000 scale, soil CO2 efflux measurement, GIS and geostatistical applications, in order to better define the tectonic settings of this part of the volcano. The field work allowed identifying more than 15 emission centres as well as 10 eruptive fissures. The CO2 sampling strategy consisted of gas efflux measurements (with the accumulation chamber method) on a grid of sampling points about 100 m spaced. The geological and soil gas surveys allowed to obtain high resolution maps of the distribution of geological formations, structural lines and soil CO2 anomalies. A large amount of data were collected and were processed using a geostatistical approach. Geostatistics combined with variogram analyses showed a good spatial correlation among CO2 anomalies, with an anisotropy mainly oriented N60°. The spatial pattern of these anomalies is in agreement with the structural arrangement of Mt. Etna Western Rift and suggests the presence of hidden faults sub-parallel to the main rift direction.