GPS Positioning and Velocity Field in the Apennines Subduction Zone
A stable geodetic reference frame permits to relate one position to another and to compute a reliable deformation field from geodetic observations. In order to satisfy scientific requirements, the reference frame should be accurate, reliable and internally consistent over time with unambiguously specified datum definition (origin, scale, orientation and their respective time evolution). Current reference frame stability between successive frame realizations suggests that the agreement is at the level of 1 cm and 0.3 mm/yr respectively for absolute and time derivative translation and scale factors. They represent the current stability over time of the reference frame and set the sensitivity for geodetic measurements. Here we will present the results of a GPS deformation field in the Italian region obtained from all the available permanent GPS stations operated in Italy. The complex nature of the ongoing tectonic deformation along the Alpine-Apennines orogenic systems is now evident and GPS data have proven its capability to measure millimetre scale deformations.
From the global scale to the Mediterranean plate kinematics
Plate motions with respect to the mantle represent the most direct evidence to understand the origin of plate tectonic processes. The research here described has the aim to improve the knowledge on the global scale plate kinematics in “absolute” reference frames, or better, relative to the mantle, incorporating both geological–geophysical and space geodesy data. Geophysical and geological signatures of subduction and rift zones independently show a global polarity of current plate motions, suggesting a west-ward displacement of the whole lithosphere relative to the underlying mantle. We analytically modeled this tectonic pattern in a suitable selected hotspot framework, taking into account variable depths of the hotspot source, obtaining new plate angular velocities and their uncertainties, by least squares inversion. Then, we focused our attention on the Italian area estimating the velocity field from continuous GPS observations both relative to Eurasia, and relative to the mantle, applying the global model previously estimated. However, the presence of the Apennine subduction, having more or less the same extent of the investigated area, makes locally less reliable our global model. Consequently, we applied a simple kinematic model to estimate the rates and spatial pattern of the subduction along the Apennines. The variable rates inferred after our analysis, better reconcile if the subduction process is conceived as a passive rather than active feature. Then, the analysis came back again to the global scale and to the basic argument if plates are passively riding along on the top of a mantle convection cell, or whether the plates themselves are active drivers. On the other hand, if plate motion occurs as an ordered undulated west-directed flow, the net-rotation of the lithosphere emerges as a passive process active at global scale, and then, it can be driven only by external forces. Thus, the last part of this research has been dedicated to find the experimental evidences connecting the tectonic processes to the Earth's rotation and the tidal drag.