Structure and composition of the crust and upper mantle of the Archean-Proterozoic boundary in the Fennoscandian shield obtained by joint inversion of receiver function and surface wave phase velocity of recording of the SVEKALAPKO array

Abstract

SUMMARY We present a 3-D model of absolute values of S-wave velocities and VP/VS 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 VP/VS 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 VS and VP/VS 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 VS, 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 VP and high VP/VS 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.