Seismic anisotropy of deep lithosphere of tectonic domains separated by the Archean-Proterozoic paleosuture in Finland

(1) Geophysical Institute, Academy of Science of the Czech Republic, Prague
(2) Institute of Seismology, University of Helsinki, Finland

Abstract

Once considered as a second order effect, if not a pure artefact, nowadays the seismic anisotropy is being observed as almost ubiquitous property of the continental lithosphere. In the Fennoscandia, the anisotropy has been determined both from surface and body waves (Babuska et al., 1998; Plomerova et al., 2001). Compared to tectonically active regions, where an asthenospheric flow can significantly contribute to observed anisotropic effects (e.g., Brechner et al., 1998), beneath the Fennoscandia a systematic olivine orientation within the subcrustal lithosphere seems to be a major source of seismic anisotropy. Within the heterogeneous continental crust, variably orientated anisotropic structures are relatively small and their contribution is usually estimated at about one tenth of the overall observed anisotropy.

The lithosphere thickness beneath the central Fennoscandia is estimated at about 160-200 km from surface waves (Calcagnile et al., 1997), as well as from body-wave studies (Babuska et al., 1988) and at about 160-230 km, or even more, from petrologic and geothermal estimates (Kukkonen and Peltonen, 1997). Considering remarkable variations of the crustal thickness from 30 to about 60 km (Luosto, 1991), we can estimate a thickness of an anisotropic layer. A maximum amplitude of the relative radial anisotropy xR in the lithosphere of Fennoscandia is observed at depths of 100-120 km. There are only few observations of teleseismic shear-wave splitting in the Fennoscandia up to now (Silver, 1996; Plomerova et al., 1996; 2001; this study). They reveal split-time values between zero and 1 s. We measured the split time of 0.9 s at station NE62 (central Finland) and thus detected anisotropy in the deep lithosphere of this region.

We can monitor lateral changes of P-velocity anisotropy by analysing P-residual spheres which show azimuth-incidence angle dependent terms of teleseismic relative P residuals. Distinct changes of the P-residual pattern reflect lateral changes of anisotropic structure of the lower lithosphere, related to prominent (paleo-) sutures or intracontinental boundaries, such as the Saxothuringian-Moldanubian suture in central Europe (Plomerova et al., 1998), or the Protogine Zone in southern Sweden (Plomerova et al., 1996; 2001). The residual spheres allow us to infer 3D orientation of plunging high P-velocity directions within the lithosphere. Joint interpretation of inferences obtained from the analysis of the P-residual spheres along with results of inversion of shear-wave splitting parameters (Sileny and Plomerova, 1996) brings a self-consistent 3D image of the anisotropic mantle lithosphere.

An analysis of a 5 year P-residual dataset (ISC: 1988-92) showed high-velocity directions which we interpret by wave propagation through anisotropic structures within large tectonic units of Fennoscandia. These dipping anisotropies may represent remnants of frozen-in paleosubductions. A new dataset extracted from observations of permanent observatories in Finland consists of P-arrival times measured specially for this study for a period of 26 months between 1997-1999. The residuals, which were corrected for crustal thickness (Malaska and Hyvonen, 1998), show a distinct bipolar pattern of the residual spheres reflecting propagation through dipping anisotropic structures, as observed at stations in the Archean (KJN, SDF, KEV). Lateral changes in the pattern favour regional anisotropic domains within the Archean. 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. The deep suture between the Archean and Proterozoic obviously plays an important role in observed anisotropic patterns. The P-residual spheres of stations in the Proterozoic close to the suture seem to be affected also by anisotropic structure of the Archean. To construct a 3D anisotropic model of the lower lithosphere on both sides of the Archean-Proterozoic paleosuture in Finland, we need to analyse more teleseismic P residual spheres, as well as shear-wave splitting parameters, recorded at the dense array of stations operating during the field measurement of the SVEKALAPKO project.

Whole poster

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References
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