Marja Uski, University of Helsinki (Finland)
Timo Tiira, University of Helsinki (Finland)
Marek Grad, University of Warsaw (Poland)
Jukka Yliniemi, University of Oulu (Finland)
Crustal structure of the Archean Karelian craton in Finland has been modeled using local earthquakes and quarry blasts as seismic sources. These data have been recorded by the Finnish permanent seismic network and by mobile stations. The events have been arranged into ten seismic profiles covering the study area. The refracted, reflected and postcritical phases have been modeled using 2-D ray-tracing technique.
This paper presents 2-D velocity models derived for each of the profiles as well as an updated Moho depth map for the Finnish part of Karelian craton. Our results image a typical three-layer Archean crust with thickness ranging between 40 and 54 km. The upper and the middle crust are 15-20 km thick, with average P-wave velocities of 6.1-6.3 km/s and 6.6-6.7 km/s, respectively. A 5-20 km thick lower crust has velocities varying between 6.9 and 7.3 km/s. In the uppermost layer, Vp/Vs= 1.71 is determined from good quality Pg and Sg wave travel times. For other S-wave phases travel times are correlated as an envelope of arrival, and the Vp/Vs ratios are 1.74 in the middle crust and 1.76 in the lower crust. For the uppermost mantle a standard value of 1.73 is assumed, which fits quite well observed Sn wave travel times.
The crust is thinnest (40-44 km) under the Pudasjarvi block in the north-western part of the study area. This block is bordered by Proterozoic schist belts, and its northern margin coincides with the Perapohja aulacogen. The thickest crust (50-54 km) is found at the south-eastern corner of the craton, where the Archean crust is overthrust on younger Svecofennian crust. A clear step in Moho depth is observed on profiles crossing the N-S trending Suomussalmi-Kuhmo greenstone belt. The step is also observed on FIRE1 and Sveka surveys further south. Another local maxima in crustal thickness is located below the Salla greenstone belt. This feature is tentatively linked to continent-continent type collision of Archean crustal blocks as suggested by FIRE4 data.