Geochemical investigations applied to active fault detection in a volcanicarea: the North-East Rift on Mt. Etna (Sicily, Italy)

Geochemical investigations were performed on the Northeast Rift of Mt. Etna , a prominent volcanic structure of this volcano. Low-temperature fumaroles were found on the upper part of this area and the isotopic compositions of C(CO2) and He suggest a likely magmatic origin of the emitted gases. On th e contrary, very low degassing was found in the lower part of the NE-Rift w ith CO2 concentrations generally very close to those in air. This pattern i s probably due to sealing of the eruptive fissures by the repeated injectio ns of magma solidified into dikes and by consequent shallow hydrothermal al teration of the fissured rocks due to residual magma degassing. High soil C O2 concentrations were also found in an elongated area parallel to the NE-R ift and located immediately to the east of it. The geometry of these anomal ies suggests the existence of several hidden faults, sub-parallel to the NE -Rift and likely connected to the Pernicana fault system. This latter struc ture seems to merge with the NE-Rift at the altitude where the fumarole emi ssions are located. The hidden structures, together with the Pernicana syst em and the NE-Rift represent the western and northern boundaries of a large eastward collapsing sector of Mt. Etna.