The southern Apennines fold and thrust belt has been undergoing post-orogenic extension since ca. 700
kyr. Crustal extension controls active tectonics and seismogenesis in the mountain chain [1], with
seismicity being characterized by low to moderate magnitude events punctuated by strong earthquakes
[2]. Effective decoupling between deep and shallow structural levels is related to the strong rheological
contrast produced by a fluid-saturated, clay-rich mélange zone interposed between buried
autochthonous carbonates – continuous with those exposed in the Apulian foreland – and the
allochthonous units. This mélange zone also acts as a seal preventing the migration of deep-seated
aqueous fluids – as well as oil in the Basilicata region, which hosts the largest Europe’s onshore oil fields
– towards the surface.
On the other hand, the mountain belt is characterized by substantial gas flow, recorded as both
distributed soil gas emissions and vigorous gas vents, associated with active faults at the surface. We
measured a CO2 flux up to 34000 g/m-2 per day at a gas vent, as well as large amounts of He (up to 52
ppm), Rn (up to 228 kBq/m3) and CH4 (up to 5000 ppm).
Overpressured CO2, which has been proposed as triggering normal fault earthquakes in the Apennines,
has been interpreted as mostly of mantle origin. However, our new results from isotope analyses carried
out on the carbon contained in both CO2 and CH4 indicate a dominant thermogenic origin for these
gases, probably associated with the emplacement of magmatic sills within the lower section of the thick
carbonate platform succession occurring at the base of the sedimentary cover in the southern
Apennines. Our results bear major implication concerning the postulated occurrence of crustal faults
allowing fluids to migrate directly from mantle depths to the surface
