[1] Regional seismic tomography provides valuable information on the structure of shields, thereby gaining insight to the formation and stabilization of old continents. Fennoscandia (known as the Baltic Shield for its exposed part) is a composite shield for which the last recorded tectonic event is the intrusion of the Rapakivi granitoids around 1.6 Ga. A seismic experiment carried out as part of the European project Svecofennian-Karelia-Lapland-Kola (SVEKALAPKO) was designed to study the upper mantle of the Finnish part of the Baltic Shield, especially the boundary between Archean and Proterozoic domains. We invert the fundamental mode Rayleigh waves to obtain a three-dimensional shear wave velocity model using a ray-based method accounting for the curvature of wave fronts. The experiment geometry allows an evaluation of lateral variations in velocities down to 150 km depth. The obtained model exhibits variations of up to ±3% in S wave velocities. As the thermal variations beneath Finland are very small, these lateral variations must be caused by different rock compositions. The lithospheres beneath the Archean and Proterozoic domains are not noticeably different in the S wave velocity maps. A classification of the velocity profiles with depth yields four main families and five intermediate regions that can be correlated with surface features. The comparison of these profiles with composition-based shear wave velocities implies both lateral and vertical variations of the mineralogy.
S U M M A R YWe present a 3-D model of absolute values of S-wave velocities and V P /V S ratios in the crust and upper mantle beneath the SVEKALAPKO temporary seismic array that covered the transition zone between Archean and Proterozoic domains in the Precambrian Fennoscandian Shield. The model was obtained using joint inversion of P-wave receiver functions, Rayleigh phase velocities and traveltimes of waves converted from the 410 km discontinuity. P-wave receiver functions and traveltimes of Ps waves converted from the 410 km boundary were estimated for 30 broad-band stations of the SVEKALAPKO array and short-period, smallaperture RUKSA array in Russian Karelia. The phase velocities of Rayleigh waves were taken from previous surface wave studies (Bruneton et al. 2004a,b). For each station, the different data sets were merged and inverted by simulated annealing method. After that, a 3-D S-wave velocity model and distribution of V P /V S ratio was obtained from 1-D velocity models, using special interpolation technique. The new 3-D seismic model demonstrates pronounced lateral variations of values of V S and V P /V S ratio in the crust and uppermost mantle. The depth to the Moho boundary varies from 51 to 63 km in our model, which agrees with the results of previous controlled-source seismic studies in the region. The Moho boundary is overlain by a high-velocity lower crust (HVLC), with high V S , which is non-uniform in composition and origin. Our study showed no systematic correlation between the lithosphere structure and tectonothermal age of Archean and Proterozoic crustal terrains in the study area. The exposed Archean-Proterozoic suture (so-called Ladoga-Bothnian Bay Zone) is not observed as a mega-scale structure in the crust and upper mantle. Generally, the Archean-Proterozoic transition occupies a larger area in the lithosphere than it was thought earlier. It is marked by a Moho depression stretching to the North and by a zone of high V P and high V P /V S in the mantle. Our results supports the theory that the Fennoscandian Shield was assembled as a result of extensive collisional accretion of island arcs and microcontinental blocks and shows that these processes were working both in Archean and Proterozoic.
Based on data of three three-component seismographs belonging to the temporary smallaperture Russian Karelia Seismic Array (RUKSA) in the Petrozavodsk region (Karelia), a 1-D velocity model of the crust is constructed by the method of the receiver function. Waveforms of distant earthquakes recorded by short-period instruments with improved characteristics are used. The data were inverted by the simulated annealing method. The inversion was stabilized by using phase velocities of Rayleigh waves and traveltimes of converted P s waves from the 410-km boundary determined from broadband records of the SVEKALAPKO seismic array. Anomalously low seismic velocities are discovered in the upper part of the cross section beneath the RUKSA array.
The Svecofennian‐Karelian‐Lapland‐Kola Transect (SVEKALAPKO) project is one of the five multidisciplinary key projects of Europrobe, a scientific program of the European Science Foundation (ESF) that studies the tectonic evolution of European continental lithosphere [Gee and Zeyen, 1996]. The SVEKALAPKO project [Hjelt and Daly, 1996] has adopted a multidisciplinary approach that uses geological, penological, and geophysical methods to unravel the evolution of the crust and lower lithosphere in three major crustal segments of the Fennoscandian Shield: the Proterozoic Svecofennian and Lapland‐Kola orogens and the intervening Archaean Karelia craton. Improved knowledge of the structure and evolution of the Fennoscandian Shield should lead to a better understanding of plate‐tectonic processes in the early history of the Earth.
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