High-resolution 3D seismic model of the crustal and uppermost mantle structure in Poland

Abstract

In the area of Poland a contact between the Precambrian and Phanerozoic Europe and the Carpathians has a complicated structure and a complex P-wave velocity of the sedimentary cover, crystalline crust, Moho depth and the uppermost mantle. The geometry of the uppermost several kilometers of sediments is relatively well recognized from over 100,000 boreholes. The vertical seismic profiling (VSP) from 1188 boreholes provided detailed velocity data for regional tectonic units and for stratigraphic successions from Permian to the Tertiary and Quaternary deposits. These data, however, do not provide information about the velocity and basement depth in the central part of the Trans-European suture zone (TESZ) and in the Carpathians. So, the data set is supplemented by 2D velocity models from 32 deep seismic sounding refraction profiles which also provide information about the crust and uppermost mantle. Together with the results of other methods: vertical seismic profiling, magnetotelluric, allow for the creation of a detailed, high-resolution 3D model for the entire Earth's crust and the uppermost mantle down to a depth of 60km. The thinnest sedimentary cover in the Mazury–Belarus anteclise is only 0.3 to 1km thick, which increases to 7 to 8km along the East European Craton (EEC) margin, and 9 to 12km in the TESZ. The Variscan domain is characterized by a 1–4km thick sedimentary cover, while the Carpathians are characterized by very thick sedimentary layers, up to about 20km. The crystalline crust is differentiated and has a layered structure. The crust beneath the West European Platform (WEP; Variscan domain) is characterized by P-wave velocities of 5.8–6.6km/s. The upper and middle crusts beneath the EEC are characterized by velocities of 6.1–6.6km/s, and are underlain by a high velocity lower crust with a velocity of about 7km/s. A general decrease in velocity is observed from the older to the younger tectonic domains. The TESZ is associated with a steep dip in the Moho depth, from 30–35km in the Paleozoic Platform to 42–52km in the Precambrian craton. The new model confirms the Moho depth derived from previous compilations. In the TESZ the lower crust has a very high seismic velocity (>7.0km/s) which correlates to the high P-wave velocity (about 8.4km/s) in the uppermost mantle beneath the Polish Basin. The Cratonic area is generally characterized by high P-wave velocities (>8.2km/s), while the Phanerozoic area is characterized by velocities of ~8.0km/s. In the TESZ very high velocities of 8.3–8.4km/s are observed, and the southwestern limitation of this area coincides with a high velocity lower crust, and could be continued to the NW toward the Elbe line. The influence of the structure for teleseismic tomography time residuals of seismic waves traveling through the 3D seismic model was analyzed. Lithological candidates for the crust and uppermost mantle of the EEC and WEP were suggested by comparison to laboratory data. The presented 3D seismic model may make more reliable studies on global dynamics, and geotectonic correlations, particularly for sedimentary basins in the Polish Lowlands, the napped flysch sediment series in the Carpathians, the basement shape, the southwestern edge of the EEC, a high-velocity lower crust and the high-velocity uppermost mantle in the TESZ. Finally, the new 3D velocity model of the crust shows a heterogeneous structure and offers a starting point for the numerical modeling of deeper structures by allowing for a correction of the crustal effects in studies of the mantle heterogeneities.