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 mesosco
pic fracture pattern perfectly fits an E-W-trending left-lateral strike-sli
p master fault, in agreement with the main morpho-structural submarine alig
nment including Ustica Island and Anchise Seamount. Along the SW-NE high-an
gle normal Arso Fault, geological evidence of reactivation with different k
inematics (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), respecti
vely, 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 tr
end, 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 reserv
ed.