Kinematic constraints on coeval extension and contraction in the southern Apennines (Italy) from GPS geodesy

Coeval extensional and contractional deformation in the Apennine orogen of Italy are controlled by slab rollback and NE-ward trench retreat of delaminating Adriatic continental crust. Seismicity, low seismic velocity zones, and gravity and magnetic anomalies define a belt of extensional deformation that is coincident with downward flexure of the lower plate and transverse to the Apenninic orogenic axis. In the Southern Apennines, focal mechanisms, active faults, and GPS velocity determinations indicate the extent of extensional deformation in the internal belt. A velocity field determined for 13 GPS sites of the Peri-Tyrrhenian Geodetic Array located in the Southern Apennines, the Apulian foreland, and the Calabrian arc of southern Italy is consistent with simultaneous extension and contraction within the orogen. GPS sites located in bedrock were occupied in 1995, 1997, and 2000 for 18 to 48 hours during each campaign. The data were processed with BERNESE (4.2) in the IRTF00 realization using continuous GPS sites in stable Europe as a reference. In a reference frame fixed on Matera in the southern Apulian foreland, site velocities to the SW along the Tyrrhenian coast are consistent with active seismicity and reflect crustal extension of 5 mm/yr, with rates decreasing easterly in the Apenninic highlands. Within the medial zone, located along the physiographic interface between western carbonate highlands and deformed synorogenic rocks of the foreland, localized orogen-parallel divergence of 7 mm/yr occurs along a NNW-SSE axis, accommodating arcuation of the belt. GPS sites in the Apulian foreland are moving west with increasing velocities from north to south of 1 to 3 to 7 mm/yr from the Gargano Peninsula to the Murge Platform. Differential GPS velocities and seismicity indicate that the Apulian foreland is actively deforming as part of a system of structures underlying the Adriatic Sea, consistent with right-transpression. The velocity field determined in a Europe-fixed reference frame indicates that extension and contraction share a detachment in a kinematically linked system. Overall NNE motion in the southern Apennines is more consistent with the velocity of Africa than Europe and drives extensional deformation, but the driving mechanism of the foreland contraction is unknown.