Abstract
The Kokkola–Kymi Deep Seismic Sounding profile crosses the Fennoscandian Shield in northwest-southeast (NW–SE) direction from Bothnian belt to Wiborg rapakivi batholith through Central Finland granitoid complex (CFGC). The 490-km refraction seismic line is perpendicular to the orogenic strike in Central Finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013. The campaign resulted in 63 usable seismic record sections. The average perpendicular distance between these and the profile was 14 km. Tomographic velocity models were computed with JIVE3D program. The velocity fields of the tomographic models were used as starting points in the ray tracing modelling. Based on collected seismic sections a layer-cake model was prepared with the ray tracing package SEIS83. Along the profile, upper crust has an average thickness of 22 km average, and P-wave velocities (Vp) of 5.9–6.2 km/s near the surface, increasing downward to 6.25–6.40 km/s. The thickness of middle crust is 14 km below CFGC, 20 km in SE and 25 km in NW, but Vp ranges from 6.6 to 6.9 km/s in all parts. Lower crust has Vp values of 7.35–7.4 km/s and lithospheric mantle 8.2–8.25 km/s. Moho depth is 54 km in NW part, 63 km in the middle and 43 km in SW, yet a 55-km long section in the middle does not reveal an obvious Moho reflection. S-wave velocities vary from 3.4 km/s near the surface to 4.85 km/s in upper mantle, consistently with P-wave velocity variations. Results confirm the previously assumed high-velocity lower crust and depression of Moho in central Finland.
Highlights
The Precambrian nucleus of Europe, the Fennoscandian Shield (Fig. 1), has been well studied for its deep structure (Grad et al 2009) with longranging DSS (Deep Seismic Sounding) profiles, deep reflection profiles as well as tomographic experiments (Luosto et al 1984, 1990, 1994; Grad and Luosto 1987; BABEL Working Group 1991; FENNIA Working Group 1998; Heikkinen and Luosto 2000; Hyvonen et al 2007)
We studied possible velocity anisotropy to compare the velocity variations with the reflection properties of the FIRE3A
The KOKKY seismic profile begins at 63.90° N, 22.95° E in Bothnian belt and ends at 60.61° N, 29.25° E near the eastern edge of Wiborg batholith
Summary
The Precambrian nucleus of Europe, the Fennoscandian Shield (Fig. 1), has been well studied for its deep structure (Grad et al 2009) with longranging DSS (Deep Seismic Sounding) profiles, deep reflection profiles as well as tomographic experiments (Luosto et al 1984, 1990, 1994; Grad and Luosto 1987; BABEL Working Group 1991; FENNIA Working Group 1998; Heikkinen and Luosto 2000; Hyvonen et al 2007). The 490 km long Kokkola–Kymi seismic reflection line (KOKKY profile) runs in NW–SE direction and transects the Western and Southern Finland arc complexes that form the Western (WFS) and Southern Finland subprovinces (SFS) of the Svecofennian tectonic regime (Nironen 2017) It transects the major Proterozoic crustal units identified on a tomographic velocity model (Hyvonen et al 2007) and the most prominent crustal thickness anomalies identified on a Moho depth map (Grad et al 2009). Malinowski 2013; Spada et al 2013) is one of the main tools used in Earth imaging, especially when aiming towards the middle and lower crust structures, Moho topography and velocities in the lithospheric mantle Data for such studies are acquired during campaigns on wideangle reflection and refraction (WARR) profiles ( called deep seismic soundings, or DSS profiles), which are hundreds of kilometres long and require strong explosive sources such as TNT. This is convenient because quarries require tons of explosive material each week and their functionality in seismic experiments had been previously successfully tested in association with the HUKKA2007 project (Tiira et al 2013)
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