Structural pattern and CO2-CH4 degassing of Ustica Island, Southern Tyrrhenian basin

Brittle tectonics and ground degassing, including fracture-field, soil-gas and exhalation flux analyses of CO2 and CH4, were studied at Ustica Island, a Pleistocene volcanic complex in the Southern Tyrrhenian Sea. The mesoscopic fracture pattern perfectly fits an E-W-trending left-lateral strike-slip master fault, in agreement with the main morpho-structural submarine alignment including Ustica Island and Anchise Seamount. Along the SW-NE high-angle normal Arso Fault, geological evidence of reactivation with different kinematics (left- to right-lateral displacements) was recognised. Major CO2 and CH4 degassing (with fluxes up to 93,750 and 20 t km(-2) a(-1), respectively, and soil-gas concentrations of 11.3% and 5.7 ppm) occur over the Arso Fault. Although this fault is mapped just in the SW sector of the island, soil-gas CO2 anomalies point out its clear continuation up to the NE margin of the island. These data, together with those of previous geophysical and geochemical results from off-shore Ustica, suggest that the Arso Fault is the local evidence of a more important active, gas-bearing structure. This tectonic feature is interpreted as a reactivation of a preexistent SW-NE trend, inherited as a second-order structure of the E-W deep shear zone. The reactivation is related to the interplay among different structures of the Southern Tyrrhenian basin. (C) 1999 Elsevier Science B.V. All rights reserved.