Electrical conductivity and seismic velocity structures of the lithosphere beneath the Fennoscandian Shield

Abstract

Abstract Key results from the BEAR (Baltic Electromagnetic Array Research) soundings and the SVEKALAPKO Seismic Tomography Experiment (SSTE), which explored geophysical properties of the upper mantle beneath the Fennoscandian Shield, are summarized here. These two projects formed the two key geophysical experiments in EUROPROBE's SVEKALAPKO project. The major result obtained from the teleseismic tomography indicates P- and S-wave velocity variations of up to 4%, as compared with the IASP'91 global model. The positive P-wave velocity anomaly seems to extend down to 300 km, without any indication of an asthenospheric low-velocity layer. This is corroborated by surface-wave analysis. The Archaean-Proterozoic suture zone has no continuation at upper mantle depths. Instead, a laterally and vertically heterogeneous structure of the subcontinental lithospheric mantle in the contact zone of Archaean and Proterozoic domains beneath the SVEKALAPKO area has been revealed by both teleseismic and local event studies. Comparison between stacked receiver functions from the TOR (Teleseismic Tomography of TORnquist Zone) and SSTE arrays indicates that the difference between arrival times of the converted P410 and P660 phases increases below the Shield as a result of a cooler upper mantle. The asthenosphere beneath Central Europe terminates at the southern edge of the shield and cannot be identified in the SVEKALAPKO data. The variations in Moho depths from 30 km to 50 km (locally, more), known from earlier deep seismic sounding (DSS) work, were corroborated by the receiver function analysis. The BEAR (Baltic Electromagnetic Array Research) experiment data indicate that an upper mantle conducting layer is required in some places beneath Fennoscandia. In the northern part of the Shield the upper mantle conductor is located at a depth of c. 170 km. Magnetotelluric data exhibit strong anisotropic behaviour, in particular in the central parts of the Fennoscandian Shield. The present state of modelling implies, however, that isotropic 3D crust and upper mantle explain nearly all observed anisotropic features and that very minor anisotropy of the upper mantle is required to explain the data. The crust is strongly variable electrically, ranging from resistive areas to highly conducting elongated structures. The conductances (= conductivity x layer thickness) obtained range from a few Siemens to several tens of thousands of Siemens. The conductive structures delineate boundaries between Archaean and Proterozoic crustal units and are inferred to image relics of subduction processes. In the central parts of the Shield, the lower crust is conducting, with dipping structures extending far to the NE below the Archaean-Proterozoic boundary. Thus, the present interpretation of electromagnetic BEAR and tomographic data from the Fennoscandian Shield demonstrates that the structure of the upper mantle beneath the Shield is much more heterogeneous than was supposed when the project started. Models of the evolution of lithosphere must be revised to accommodate lateral and vertical heterogeneity in the upper mantle.