SHEAR-WAVE ANISOTROPY OF LAMINATED LOWER CRUST AT THE URACH GEOTHERMAL ANOMALY

Deep seismic reflection studies have shown that 'lamellae' are a wides pread reflectivity pattern of the lower crust of the central European Variscan belt. This pattern has been interpreted, inter alia, as alter nating subhorizontal layering of mafic and felsic rocks implying a tec tonic process of structural and textural ordering. Consequently, lamin ated lower crust should be elastically anisotropic. The specific type of anisotropy should provide some insight into the mineral composition and the preferred orientation of minerals in the lower crust. We have investigated this problem in the area of the Urach geothermal anomaly (South Germany) where a 'classical' example of lower-crust lamellae i s found. A restricted range of subsurface points was probed in a contr olled-source expanding spread seismic experiment with two orthogonal a zimuths of observation up to 90-km source-geophone offset. Both P- and S-waves were recorded with 3-component geophones at 80-140 m geophone spacing. Based on polarization analysis and traveltime interpretation the following results were obtained: (1) S-wave splitting is observed only for SMS arrivals (not for shallower reflections) implying that t he lower crust is anisotropic; (2) the type of anisotropy is quasihexa gonal (transversely isotropic) implying that there is no preferred min eral orientation within the horizontal plane; (3) the coefficient of S -wave anisotropy [(V-max - V-min)/V-min] is estimated at 6-13% for SV- type waves; the SH-wave velocity shows only small variation with offse t; (4) the observed relation between direction and velocity of S-wave propagation can be explained by mafic rocks containing a high amount o f orthopyroxene minerals horizontally aligned in the pure shear stress regime.