(1) Geophysical Institute, Academy
of Science of the Czech Republic, Prague
(2) Institute of
Seismology, University of Helsinki, Finland
Seismic anisotropy is being observed as almost ubiquitous property of the continental lithosphere. Compared to tectonically active regions, where an asthenospheric flow can significantly contribute to observed anisotropic effects, a systematic olivine orientation within the subcrustal lithosphere seems to be a major source of seismic anisotropy observed beneath the Fennoscandia (Babuska et al., 1998). Distinct changes of patterns of the P-residual spheres, as well as the shear-wave splitting parameters, reflect lateral changes of anisotropic structure of the mantle lithosphere, related to prominent sutures or other intracontinental boundaries (Plomerova et al., 2001). Joint inversion and/or interpretation of anisotropic parameters derived from body waves, result in a self-consistent 3D image of the anisotropic domains of the mantle lithosphere of the Baltic Shield (Plomerova et al., 1996). The lithosphere thickness beneath the central Fennoscandia is estimated at about 200 km. Analyses of different P-residual datasets (e.g., ISC: 1988-1992) map high-velocity directions, which we interpret by wave propagation through anisotropic structures within large tectonic units of Fennoscandia - Sveconorwegian in the south and southwest of Scandinavia, Caledonian, Svecofennian and Archean in Finland. A new dataset extracted from observations of permanent observatories in Finland consists of P-arrival times measured for this study for a period of 26 months between 1997-1999. The residuals show a distinct bipolar pattern of the P-residual spheres reflecting propagation through dipping anisotropic structures of individual lithospheric domains, namely in the Archaean (KJN, SDF, KEV). The P-residual pattern at stations in the Proterozoic is more scattered, especially at stations on the coast. But in general, the fast velocities seem to prevail for propagation from SW. Shear-wave splitting times about 1s were evaluated at permanent observatories in central Finland and they thus detected presence of anisotropy in the Proterozoic mantle lithosphere. The deep suture between the Archaean and Proterozoic represents important boundary in observed anisotropic patterns, similarly to, e.g., the Protogine Zone in south-central Sweden.
Not available.
References
[1] Babuska, V., Montagner, J.P., Plomerova, J. and N. Girardin, Age-dependent
large-scale fabric of the mantle lithosphere as derived from surface-wave
velocity anisotropy, PAGEOPH, Vol. 151, No. 2-4, pp. 257-280, 1998.
[2] Plomerova, J., Arvidsson, R., Babuska, V., Granet, M., Kulhanek, O.,
Poupinet, G. and J. Sileny, An array study of lithospheric structure across the
Protogine zone, Varmland, south-central Sweden - signs of a paleocontinental
collision, Tectonophysics, Vol. 332, No. 1-2, 2001.
[3] Plomerova, J., Sileny, J. and V. Babuska, Joint interpretation of
upper-mantle anisotropy based on teleseismic P-travel time delays and inversion
of shear-wave splitting parameters, Physics of the Earth and Planetary
Interiors, Vol. 95, No. 293-309, 1996.