ACTIVE DEFORMATON IN THE CIRCUM-TYRRHENIAN OROGEN, SOUTHERN ITALY, SICILY, AND SARDINIA

Global Positioning System (GPS) velocities and seismicity in the central Mediterranean indicate that Adria, a block of continental lithosphere lying between Africa and Europe, no longer behaves as a rigid tectonic indentor and is breaking-up. Within Adria, northwestern and southeastern domains are recognized in a regional GPS velocity field derived from 24 sites of the Peri-Tyrrhenian Geodetic Array (PTGA) in southern Italy, Sicily, and Sardinia together with International GPS Service (IGS) sites in central and northern mainland Italy, Sardinia, and Corsica. In a fixed Eurasian reference frame, PTGA and IGS sites in Sicily and southern mainland Italy have northerly velocities of up to 15 mm/yr. In contrast, GPS velocities for most of Sardinia, Corsica, and central and northern Italy have small velocities in the stable Eurasian frame, indicating that northwestern Adria is now part of the European collage. Adria is divided by a seismically active boundary that passes around the southern and eastern margins of the Tyrrhenian basin, across central Italy, and into to the Adriatic Sea. Deformation of Adria resulted in progressive southeast opening of the southern Tyrrhenian basin from mid-Miocene to recent and accompanied right-transpressional deformation in Sicily and in southern Italy and simultaneous N-S and NE-SW contraction and extension are ongoing, respectively. Seismicity and GPS velocities in Sicily record NNW-SSE shortening, right-lateral motion on east-west trending faults, and clockwise block-rotation along the northern Tyrrhenian coast. In southern Italy, GPS velocities and seismicity indicate N-S shortening localized in the Adriatic foreland and NE-SW extension in a belt stretching from the axis of the Apenninic chain to the Tyrrhenian coast. Although formed in active extension, from 3 to 6 mm/yr of north-south convergence occurs between Sicily and southern Italy across the NW-SE trending axis of southern Tyrrhenian basin. Differential motion between southern Sardinia and Sicily records between 8 to 12 mm/yr of NNW-SSE convergence, consistent with earthquake focal mechanisms north of Sicily and along the Sardinia Strait. Seismically quiescent Sardinia shows a pattern of GPS velocities consistent with NNW-SSE shortening on a conjugate set of northwest and northeast trending fault systems.