Geochemical evidence of melt intrusions along lithospheric faults of the Southern Apennines, Italy: Geodynamic and seismogenic implications

Several gas emissions, distributed along 200 km of the Southern Apennine ax ial zone, have been investigated. This portion of the sedimentary chain, wh ich constitutes the accretionary prism of the westward subduction of the Ad riatic plate, is subjected to a tensile stress field responsible of high-ma gnitude earthquakes. The studied emissions are generally CO2-dominated, hav e He-3/He-4 ratios in the range of 0.09-2.84 Ra, and display both He-3 and He-4 outputs in the range of those reported for the Italian active volcanoe s Phlegrean Fields and Vulcano. The helium isotope ratios, together with th e amount of released gas, indicate that a huge amount of mantle-derived hel ium is released over all the investigated area. Our geochemical data and th e geophysical peculiarities of the region, such as the high heat flow and t he low resistivity of rocks below a depth of 15 km, coherently indicate the presence of melt intruded into the crust along lithospheric faults. Magma intrusions along the axial part of the Southern Apennine sedimentary chain are certainly unusual, since volcanism in central and southern Italy is con centrated along the peri-Tyrrhenian margin of the peninsula. We suggest tha t melt is promoted by adiabatic uprise of the asthenosphere in the mantle w edge beneath the Apennines and it is subsequently intruded along tensile li thospheric faults, strongly affecting the thermal state of the lower crust. An interrelationship between the geodynamics, the melt intrusions, and the genesis of the large earthquakes (up to M = 7.1) in the region is suggeste d.