Elsevier

Tectonophysics

Volume 264, Issues 1–4, 30 October 1996, Pages 219-233
Tectonophysics

Shear wave anisotropy of laminated lower crust at the Urach geothermal anomaly

https://doi.org/10.1016/S0040-1951(96)00128-XGet rights and content

Abstract

Deep seismic reflection studies have shown that ‘lamellae’ are a widespread reflectivity pattern of the lower crust of the central European Variscan belt. This pattern has been interpreted, inter alia, as alternating subhorizontal layering of mafic and felsic rocks implying a tectonic process of structural and textural ordering. Consequently, laminated 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 is found. A restricted range of subsurface points was probed in a controlled-source expanding spread seismic experiment with two orthogonal azimuths 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 the lower crust is anisotropic; (2) the type of anisotropy is quasihexagonal (transversely isotropic) implying that there is no preferred mineral orientation within the horizontal plane; (3) the coefficient of S-wave anisotropy [(Vmax − Vmin)Vmin] is estimated at 6–13% for SV-type waves; the SH-wave velocity shows only small variation with offset; (4) the observed relation between direction and velocity of S-wave propagation can be explained by mafic rocks containing a high amount of orthopyroxene minerals horizontally aligned in the pure shear stress regime.

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