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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.

We present results from a deep seismic sounding (DSS) experiment carried out along the southern coast of Finland in summer 2015. Data used in the survey derived from industrial blasts recorded by temporary project stations and permanent network stations. The western 220 km part of the 450 km long Southern Finland Coastal (SOFIC) profile runs along the Uusimaa belt (UB) in the 1.7–1.9 Ga Southern Finland subprovince (SFS) of the Svecofennian domain, while the 170 km part in the east crosses the 1.62–1.65 Ga Wiborg rapakivi batholith (WRB). The farthest 60 km cross a geologically diverse area consisting of supracrustal rocks and granitoids of the Saimaa area (SA), an eastern extension of the SFS. Our results show that the Moho boundary depth varies significantly, from ca. 52–54 km below UB to 40–45 km below WRB. All three crustal layers (upper, middle, and lower) have their maximum depth in the contact zone between UB and the WRB. Below WRB, a lower crust with Vp ~6.7–6.9 km/s is observed. High velocity lower crust was observed below UB (Vp ~7.2 km/s) and possibly below SA (Vp ~7.35 km/s). The modelling was based on ray tracing, using the extrapolation of seismic wave arrival times with the help of travel times predicted from a one-dimensional velocity model. The resulting two-dimensional velocity model partly relies on data from the intersecting DSS profiles and supports previous observations of the lithospheric structure of southeastern Fennoscandia.

<p>The southern Baltic Sea area is located in the transition zone between the East European Craton (EEC; Baltica) and the West European Platform (Avalonia). The most prominent tectonic feature in the area is the NW–SE trending Tornquist Zone (TZ), crossing the southern Baltic Sea area between Scania in Sweden and Pomerania in Poland. A peculiar feature of the TZ and its southern prolongation (Teisseyre-Tornquist Zone, TTZ) is possibly a crustal keel that was recently postulated for northern Poland based on potential field modelling. A crustal keel was also imaged in the Baltic Sea by BABEL profile A, which crossed the TZ northwest of Bornholm, and by two TTZ’92 profiles crossing the TTZ south of Bornholm. However, the DEKORP-PQ profile shows a flat Moho across the TTZ.</p><p>In order to reconcile those contrasting interpretations of the crustal structure around the TTZ offshore Poland, a 230-km long refraction/wide-angle reflection profile was acquired across the TTZ in the course of RV/MARIA S. MERIAN expedition MSM52 (BalTec) in March 2016. This profile is nearly parallel to the western Polish coast, in half a distance to Bornholm. The data acquisition was conducted with 15 ocean bottom seismometers (OBS) and 3 land stations. The source array consisted of 8 G-guns with the total volume of 32 litres. In total 2227 shot points were recorded. Hydrophone data are of high quality and despite the relatively small source volume, sharp first arrivals of Pg and Pn are observed at over 120 km offsets. Some seismic record sections show clear PmP phases beginning at offsets of 70 km, continuing till the end of the profile.</p><p>Two variants of seismic modelling were performed, which results proved to be similar in terms of P-wave velocities and observed features. Tomographic joint inversion of both first arrivals and Moho reflections was used to extend velocity model depth range. Second was trial-and-error forward modelling technique using all identified seismic phases, paying attention to minimize misfit between calculated and observed P-wave travel times for each individual layer.</p><p>In the area of the TTZ, a complex upper crustal structure deepening towards the southwest is observed. One of the most interesting features is an increase in Vp (>6.5 km/s) at a depth of 16-25 km, offset by ~40 km from the TTZ on the EEC side. Similar feature was observed along the TTZ in SE Poland. Due to the lack of information from refraction, the presented ray-tracing model is the result of testing various possible velocity values for the lower crust in different parts of the model. A layer with Vp>7 km/s with a thickness of ~6 km along the entire model seems to be the best solution The Moho boundary was inferred at 33-38 km depth, deepening towards the EEC, with ~3 km uplift (but not keel) corresponding to the location of the elevated middle-crust velocities. Final velocity models were further verified by forward potential field modelling, testing various Vp – density relations.</p><p>This study was funded by the Polish National Science Centre grant no UMO-2017/27/B/ST10/02316.</p>