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

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 directio nal dependence of shear waves of the lower crust is effectively transversel y isotropic with a maximum velocity difference of 3 to 6%. Both seismic sig nature and petrological information are compared with reference profiles de duced from the exposed lower crustal sections of Ivrea and Calabria (Italy) . The comparison, based on synthetic seismograms, shows that the observed c ombination 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 o rientation of anisotropic minerals (so-called intrinsic anisotropy). The ef fect of alternating rock layering (so-called layering anisotropy) contribut es only a minor portion to the whole rock anisotropy. Layered mafic intrusi ons, such as found in the Val Sesia subsection of Ivrea, are nearly isotrop ic. 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 rest ricted to a thin layer at the crust-mantle boundary. (C) 1998 Elsevier Scie nce B.V. All rights reserved.