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Free keywords:
crust; Fennoscandia; Finland; P wave; Precambrian; S wave; seismic velocity; Vp/Vs ratio
Abstract:
In this paper, local seismic tomographic method is used to find the terrane distribution within the central parts of the accretionary Svecofennian Orogen. This study presents a crustal P-wave velocity model, and for the first time the S-wave velocity and Vp/Vs ratio model of SVEKALAPKO area that is 700 × 800 km2 in southern and central Finland. The 3-D model is presented as P- and S-wave velocity as well as Vp/Vs ratio depth slices and vertical sections.
The data set comprised of several subsets of crustal Pg- and Sg-wave traveltime data: from local events recorded by the SVEKALAPKO seismic tomography array in 1998–1999, from older controlled source shots recorded at portable stations as well as at permanent stations, and from non-controlled chemical explosions recorded at permanent seismic stations. From 300 local explosions a total of 10 404 Pg-wave and 9361 Sg-wave arrival times were inverted to create independent 3-D Vp and Vs tomographic models, from which the Vp/Vs ratio was calculated. According to sensitivity tests, the optimal horizontal resolution of recognizable velocity structures in the central study area is at least 60 km down to depths of 40 km.
In the final model, the seismic velocities vary smoothly. The lateral variations are larger for Vp (5.9–6.6 km s−1) than for Vs (3.5–3.8 km s−1) in the upper 20 km of the crust. At depths of 20–40 km, Vp varies from 6.5 to 7.2 km s−1 and Vs from 3.7 to 4.1 km s−1. The Vp/Vs ratio varies spatially more distinct than P- and S-wave velocities, usually from 1.70 to 1.74 in the upper crust and from 1.72 to 1.78 in the lower crust. Schist belts and their continuations at depth are associated with lower velocities and lower Vp/Vs ratios (Vp < 6.2–6.8 km s−1; Vs < 3.6–3.9 km s−1; Vp/Vs= 1.68–1.73) than the granitoid areas (Vp= 6.3–7.4 km s−1; Vs= 3.6–4.2 km s−1; Vp/Vs= 1.72–1.78).
The Svecofennian Orogen was accreted from crustal blocks ranging in size from 100 × 100 km2 to 200 × 200 km2 in cross-sectional area. The intervening sedimentary belts have ca. 0.2 km s−1 lower P- and S-wave velocities and ca. 0.04 lower Vp/Vs ratios. Tomographic model supports the idea that the thick Svecofennian crust was accreted from several crustal terranes, some hidden, and that the crust was later modified by intra and underplating.
