Seismic anisotropy of the upper mantle beneath the Bohemian Massif

(1) Geophysical Institute, Academy of Science of the Czech Republic, Prague
(2) EOST - Institut de Physique du Globe Strasbourg, France

Abstract

In co-operation between the geophysical institutions in Prague and Strasbourg, an array of portable digital three-component stations had been running in the Bohemian Massif (BM) for 8 months within a project MOSAIC (programme Barrande). The array consisted of six broad-band and two short-period stations (Plomerova et al., 2000) The study of seismic anisotropy is based on teleseismic body waves and on a joint inversion of anisotropic data (P-residual spheres, the fast shear-wave polarizations and split times). Similarly to previous findings (Plomerova et al., 1998; Babuska and Plomerova, 2001), based mainly on data of permanent observatories, we observe that velocity anomalies are strongly affected by different orientations of dipping anisotropic structures within the mantle lithosphere. There are distinct lateral variations of the pattern of P-residual spheres and of the split time dt and orientation of the fast shear waves within the BM (Babuska and Plomerova, 2000). While, e.g., in the central part large dt and a general E-W orientation of the fast split wave prevail, in the eastern part of the BM the dt is smaller and the fast polarization rotates to the WNW-ESE. Smaller dt and similar rotation are indicated also at the southern rim of the Saxothuringian unit. Self-consistent 3D anisotropic models of several domains forming the mantle lithosphere of the BM are presented.

We also present a comparison of the deep lithosphere structure of the BM with other two Variscan units - the Armorican Massif and the French Massif Central, as revealed by recent tomographic studies of seismic anisotropy. The data originate from several field measurements made in temporary arrays of stations. We demonstrate that the mantle lithosphere of the three Variscan massifs appear to consist of at least two parts with different orientation of large-scale fabric. The boundaries of anisotropic lithospheric domains are related to prominent tectonic features recognised on the surface as sutures, shear zones or transfer fault zones, as well as grabens, thus indicating that some of them extend deep through the entire lithosphere.

Whole poster

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References
[1] Plomerova, J., Granet, M., Judenherc, S., Achauer, U., Babuska, V., Jedlicka, P., Kouba, D. and L. Vecsey, Temporary array data for studying seismic anisotropy of Variscan massifs - The Armorican Massif, French Massif Central and Bohemian Massif, Studia Geophysica et Geodaetica, Vol. 44, No. 2, pp. 195-209, 2000.
[2] Plomerova, J., Babuska, V., Sileny, J. and J. Horalek, Seismic anisotropy and velocity variations in the mantle beneath the Saxothuringicum-Moldanubicum contact in central Europe, PAGEOPH, Vol. 151, No. 2-4, pp. 365-394, 1998.
[3] Babuska, V. and J. Plomerova, Subcrustal lithosphere around the Saxothuringian-Moldanubian Suture Zone - a model derived from anisotropy of seismic wave velocities, Tectonophysics, Vol. 332, No. 1-2, pp. 185-199, 2001.
[4] Babuska, V. and J. Plomerova, Saxothuringian-Moldanubian suture and predisposition of seismicity in the western Bohemian Massif, Studia Geophysica et Geodaetica, Vol. 44, No. 2, pp. 292-306, 2000.