CDP AND DSS DATA ALONG THE UCHTA-KEM PROFILE (THE BALTIC SHIELD)

Abstract

Detailed Common Depth Point (CDP) and Deep Seismic Sounding (DSS) observations were carried out along the W–E trending Kem–Uchta profile between the White Sea and the Russian–Finnish border. The cross-sections obtained from wide-angle and near-vertical reflections show different features. In the CDP cross-section, several inclined boundaries are traced from the surface down to 25–30 km depth. They correlate with the well-known fault zone between the Belomorian Mobile Belt and the Korelian Protocraton. The DSS data also show these inclined reflectors as well as a near horizontal boundary, K1 (velocity of 6.3 versus 6.4 km/s, depth of 10–15 km), under a low velocity zone. The boundary generates high amplitude wide-angle reflections, which cross the CDP inclined reflections. A lower crustal boundary, K2 (velocity of 6.7 versus 6.8 km/s, depth of 30 km), has no clear expression in the CDP reflectivity pattern as well. Strong reflections from Moho (PmP) coincide with the boundary between reflective lower crust and transparent upper mantle on the CDP section (depth 40 km). This suggests that the near-horizontal crustal boundaries and the Moho are transition zones with high velocity gradients and not sharp discontinuities. The data analysis suggests the crustal layering to be a result of rock transformation (metamorphism, change in mechanical properties) after consolidation of the crust. Tectonic features of orogenic periods were overprinted by the metamorphic processes in the lower crust. In the middle crust, a destruction (low velocity) zone and the K1 boundary were created, but the initial tectonic structure of the upper crust was preserved. The Moho is a young feature that was formed during the platform stage of the crustal evolution.