Abstract:
A new mechanism for Paleozoic subsidence of the Russian, or East European, platform is suggested, since a model of lithosphere tilting during the Uralian subduction does not explain the post-Uralian sedimentation record. Alternatively, I propose that the Proterozoic and Paleozoic rifting (when a platform-scale Central Russia rift system and a set of Paleozoic rifts were formed) modified the structure and composition of cratonic lithosphere, and these tectono-magmatic events are responsible for the post-Uralian subsidence of the Russian platform. To support this hypothesis, (a) the thermal regime and the thickness of the lithosphere are analyzed, and (b) lithospheric density variations of non-thermal origin are calculated from free-board constraints. The results indicate that Proterozoic and Paleozoic rifting had different effects on the lithospheric structure and composition. (1) Proterozoic rifting is not reflected in the present thermal regime and did not cause significant lithosphere thinning (most of the Russian platform has lithospheric thickness of 150–180 km and the lithosphere of the NE Baltic Shield is 250–300 km thick). Paleozoic rifting resulted in pronounced lithospheric thinning (to 120–140 km) in the southern parts of the Russian platform. (2) Lithospheric density anomalies suggest that Proterozoic–Paleozoic rifting played an important role in the platform subsidence. The lithospheric mantle of the Archean–early Proterozoic part of the Baltic Shield is ∼1.4±0.2% less dense than the typical Phanerozoic upper mantle. However, the density deficit in the subcrustal lithosphere of most of the Russian platform is only about (0.4–0.8)±0.2% and decreases southwards to ∼0%. Increased densities (likely associated with low depletion values) in the Russian platform suggest strong metasomatism of the cratonic lithosphere during rifting events, which led to its subsidence. It is proposed that only the lower part of the cratonic lithosphere was metasomatized as a result of Proterozoic rifting; the boundary between a depleted upper and more fertile lower layers can be at ca. 90–150 km depth and can produce a seismic pattern similar to the top of a seismic low-velocity zone. Paleozoic rifting has modified the entire lithospheric column and the regions affected are still subsiding.
