Strong lateral variations of lithospheric mantle beneath cratons - Example from the Baltic Shield

- Strong seismic heterogeneities can be present within the Precambrian lithosphere.
- They affect either its bottom part, or the whole lithospheric mantle.
- They are due to erosion/refertilisation, in relation with plumes/continent break-up.
- Local studies based on dense station coverage allow to better resolve such variations.

Understanding mechanisms for creation and evolution of Precambrian continental lithosphere requires to go beyond the large-scale seismic imaging in which shields often appear as laterally homogeneous, with a thick and fast lithosphere. We here present new results from a seismic experiment (POLENET-LAPNET) in the northern part of the Baltic Shield where we identify very high seismic velocities (Vs∼4.7km/s) in the upper part of the mantle lithosphere and a velocity decrease of ∼0.2 km/s at approximately 150 km depth. We interpret this velocity decrease as refertilisation of the lower part of the lithosphere. This result is in contrast to the lithospheric structure immediately south of the study area, where the seismic velocities within the lithosphere are fast down to 250 km depth, as well as to that of southern Norway, where there is no indication of very high velocities in the lithospheric mantle (Vs of ∼4.4 km/s). While the relatively low velocities beneath southern Norway can tentatively be attributed to the opening of the Atlantic Ocean, the velocity decrease beneath northern Finland is not easily explained with present knowledge of surface tectonics. Our results show that shield areas may be laterally heterogeneous even over relatively short distances. Such variability may in many cases be related to lithosphere erosion and/or refertilisation at the edge of cratons, which may therefore be particularly interesting targets for seismic imaging.