New GPS constraints on the kinematics of the Apennines subduction

We present the velocity field of the Italian area derived from continuous GPS observations from 2003 to 2007. The GPS sites were installed by different institutions and for different purposes; they cover the whole country with a mean inter-site distance of about 60 km and provide a valuable source of data to map the present day kinematics of the region. The absolute ITRF2005 rotation poles and rates of Eurasia, Africa and Adriatic plates are estimated, to study the kinematics along their boundaries in the Apennines belt. The Corsica-Sardinia block, coherently moving as the Eurasia plate, is used as reference of the upper plate for the Apennines subduction zone. We apply a simple kinematic model to estimate the rates and spatial pattern of the subduction along the Apennines. We identify at least four different, independently moving, lower plates, i.e., the Adriatic (diverging and internally segmented), Ionian, Sicily and Affica (converging) plates with different subduction rates. The conservative estimates of the subduction rate are similar to 5 mm/yr in the Calabrian Arc, similar to 1.5 mm/yr in Sicily, and similar to 0.9 mm/yr in the northern Apennines. This variegate mixture of behaviors seems to reflect the variable lithospheric thickness and composition of the lower plates inherited from the Mesozoic rifting. An unexpected along strike contraction is observed along the western side of the central-northern Apennines. Velocities are estimated both relative to Eurasia, and relative to the deep and shallow hotspot reference frames (HSRF). The hotspot representation seems more coherent with the geophysical and geological constraints along the subduction system, in which the Adriatic and Ionian plates move SW-ward and their deeper slab portions provide an obstacle to the opposite relative mantle flow. All these patterns better reconcile if the subduction process is conceived as a passive feature controlled by the far field plate velocities and the relative "eastward" mantle flow acting on a disrupted slab. (C) 2008 Elsevier B.V. All rights reserved.