Assessing mantle versus crustal sources for non-volcanic degassing along fault zones in the actively extending southern Apennines mountain belt (Italy)

Abstract

The actively extending axial zone of the southern Apennine mountain belt of Italy is characterized by a substantial flow of nonvolcanic gas to the surface. In this study, we have analyzed the correlation between the active tectonic framework of the Matese Ridge area and the high gas emissions found to the southwest, which includes large amounts of CO2 (up to 99 vol%), CH4 (up to 0.55 vol%), and He (up to 52 ppmv). We measured CO2 and CH4 fluxes of up to 34000 g d–1 and 2000 g d–1, respectively, from zones of focused degassing (gas vents and associated strong diffuse emission). This anomalously high flux of CO2 (advective plus diffusive) indicates that the study area has one of the largest nonvolcanic natural emissions of CO2 ever measured on Earth. The isotope composition of C in CO2 and CH4 shows there is a dominant crustal contribution of emissions (as opposed to a source from the mantle), indicating that thermometamorphism of the buried Apulian Platform carbonates is probably the main cause of CO2 production. This process has likely been enhanced by Quaternary magmatism, which provides an additional local source of heat triggering decarbonation of Apulian Platform limestones and dolostones at depth. The advective flux is concentrated at gas vents located along active fault segments located at the western tip of a major crustal structure, the South Matese fault zone. We believe that the very high gas emission in the Matese Ridge area is the result of both the presence of a dense network of active fault strands, which provides efficient pathways for fluid flow toward the surface, and the dramatically reduced thickness of the clay-rich mélange zone acting elsewhere in the southern Apennines as a top seal overlying the buried Apulian Platform carbonates.