Abstract
A number of different geodynamic models have been proposed to explain
the early tectonic evolution of the Baltic Shield. To provide additional
geophysical constraints on these models, we performed a teleseismic
tomography traveltime inversion for the central part of the Baltic
Shield. The SVEKALAPKO project is focused on the investigation of
the lithosphere-asthenosphere structure down to 400 km depth under
central Fennoscandia (Baltic Shield). A total of 143 stations were
deployed including 15 permanent stations from the Finnish seismic
network. The temporal network was composed of 40 broad-band and 88
short-period instruments distributed in a rectangular array of 1000
km by 900 km from 1998 August to 1999 May. The results are based
on a non-linear teleseismic tomography algorithm. They reveal significant
P-velocity variations (up to 4 per cent) throughout the SVEKALAPKO
array. The most prominent feature is a positive anomaly that can
be followed down to 250 km depth beneath the centre of the array.
We interpret this anomaly as the signature of the tectosphere (Jordan
1978) beneath the Fennoscandian Shield. It correlates spatially with
an anomalous high-velocity lower crust. Other shallow (crustal) anomalies
can be correlated with magmatic events surrounding this nucleus of
high velocity. Comparison of images before and after correction by
crustal structure proves that this methodology yields solid and coherent
tomographic results. Further observations of relative P traveltime
residuals from six teleseismic events with different azimuths show
delay variations of +-2.0 s between stations located in the North
German basin and stations on the Svecofennian Shield.
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