Shear wave anisotropy of laminated lower crust beneath Urach (SW Germany): a comparison with xenoliths and with exposed lower crustal sections

Abstract

In order to investigate the laminated lower crust beneath Urach (SW Germany), we analysed (1) the anisotropy and reflectivity of seismic shear waves, and (2) the composition of xenoliths samples. We conclude that the directional dependence of shear waves of the lower crust is effectively transversely isotropic with a maximum velocity difference of 3 to 6%. Both seismic signature and petrological information are compared with reference profiles deduced from the exposed lower crustal sections of Ivrea and Calabria (Italy). The comparison, based on synthetic seismograms, shows that the observed combination of shear wave anisotropy and reflectivity pattern corresponds to alternating layers containing a high amount of metapelites, such as found in the Calabria profile and in the Val Strona subsection of Ivrea. The main portion of the whole rock anisotropy is caused by the lattice-preferred orientation of anisotropic minerals (so-called intrinsic anisotropy). The effect of alternating rock layering (so-called layering anisotropy) contributes only a minor portion to the whole rock anisotropy. Layered mafic intrusions, such as found in the Val Sesia subsection of Ivrea, are nearly isotropic. Both seismic anisotropy and xenoliths indicate that mafic layers do not play a major role for the laminated lower crust at Urach. They may be restricted to a thin layer at the crust–mantle boundary.