Anisotropic seismic properties of the upper mantle beneath the Torre Alfina area (Northern Apennines, Central Italy)

摘要: Central Italy is an active tectonic area that has been recently studied by several regional mantle, Pn and SKS, studies which revealed the presence of a strong anisotropy. In this paper, we present first petrophysical results on only mantle xenoliths from Italy, place new constraints upper structures region. The Torre Alfina are very small in size, few millimetres to about 1.5 cm. They mainly dunites harzburgites, with subordinate lherzolites wehrlites. Since olivine spinel always present, they should have crystallised spinel-bearing lherzolite field. Their mineralogical composition ol+splFopxFcpx. Both olivines pyroxenes as porphyroclasts neoblasts. show different degrees recrystallization. Geothermobarometry these give temperature range 1040F40 jC pressure estimate GPa, corresponding 50 60 km depth. Previous seismic estimated Moho be at 20 25 region, hence come hot probably asthenospheric, below lithosphere 40 thickness Moho. We measure crystallographic preferred orientation (CPO) using SEM Electron Back Scattered Diffraction (EBSD) technique. CPO shows all three axes tightly clustered: [100] axis typically more clustered than [010] [001] most widely distributed axis. fabric strength expressed integral J index, varies 4.5 25.9, decreases degree use data calculate anisotropic properties xenoliths. homogenous statistically representative area. Vp ranges 8.4 9.1 km/s, Vs1 4.8 5.0 km/s. anisotropy variable; AVp 9.8% 19.3% AVs 7.3% 13.4%. majority display orthorhombic symmetry, but transverse isotropic behaviour also observed. consider four geodynamic models for source region (extension, shear, upwelling, slab tilted), defined orientations structural reference frame, calculated each model variation temperature, volume fraction orthopyroxene. After comparing parameters observations form hypothesis either extensional zone (lineation X foliation plane XY horizontal) or transcurrent shear horizontal