Shear-velocity andi anisotropy structure of a retreating extensional forearc (Tuscany, Italy) from receiver functions inversion

Abstract

We investigate in detail the crustal layering of the ‘Val di Chiana Basin’ (Northern Apennines, Tuscany, Italy) through receiver functions and seismic anisotropy with hexagonal symmetry. The teleseismic data set is recorded in correspondence of a typical foreland basin resulting by the progressive eastward retreat of a regional-scale subduction zone trapped between two continents. We study the azimuthal variations of the computed and binned receiver functions associated to a harmonic angular analysis to emphasize the presence of the dipping and the anisotropic structures. The resulting S-wave velocity model shows interesting and new results for this area that we discuss in a regional geodynamic contest contributing to the knowledge of the structure of the forearc of the subduction zone. A dipping interface (N192°E strike, 18° dip) has been revealed at about 1.5 km depth, that separates the basin sediments and flysch from the carbonates and evaporites. Moreover, we interpret the two upper-crust anisotropic layers (at about 6 and 17 km depth) as the Hercynian Phyllites and Micaschists, of the Metamorphic Tuscan Basement. At relatively shallow depths, the presence of these metamorphic rocks causes the seismic anisotropy in the upper crust. The presence of shallow anisotropic layers is a new and interesting feature, first revealed in the study area. Beneath the crust–mantle transition (Moho), located about 28 km depth, our analysis reveals a 7-km-thick anisotropic layer.