Local tectonics and sedimentation in grabens of the Central Russian aulacogen (East European Platform)

Abstract

The influence of local tectonic processes that determined individual development of each particular graben in the Central Russian aulacogen is best reflected in the asynchronous appearance of compositionally different clastic materials in their sedimentary sections, which accumulated with variable intensity. Against the background of the stable composition of the clastogenic framework of the terrigenous Molokovo Group, there are intervals of the sedimentary section from tens to a few thousands of meters thick, where the heavy fraction of sandstones is highly enriched (35–95%) with acute-angled epidote grains. Several features (relative instability of epidote in the hypergenesis zone, angular and fresh appearance of clasts, absence of correlation between the epidote influx and content of the main rock-forming components) imply the formation of epidote anomalies owing to local sources. Analysis of probable geodynamic mechanisms, which could determine development of the Central Russian aulacogen and its grabens, structure and composition of the upper part of the consolidated crust, made it possible to assume that the specific clastic material originates from epidote-rich blastomylonites occurring among metamorphic rocks of the basement in form of strata with anomalous petrogeophysical properties. Comparison between crystals and grains of epidote from blastomylonites and sediments shows that they are characterized by similar habit, sizes, and optical parameters and contain 25–30% of the pistacite component, which is typical of secondary epidote that forms pseudomorphs after biotite and amphibole under conditions of partial melting. Regularities in the distribution of epidote-rich intervals in the sedimentary section are explained by relations between attitude elements of planes of Neoproterozoic normal faults and Paleoproterozoic blastomylonite strata, which served as local sources of clastic material. The same factor also determined the facies properties of sedimentary complexes and structural evolution of sedimentation basins. Crosscutting normal faults stimulated the formation of grabens with the rheologically determined subsidence limit (Molokovo type), particularly, in situations characterized by the gentle attitude of blastomylonite bodies. In such a situation, the subsidence of granitoid rocks into the denser amphibolite substrate was limited by isostatic leveling forces. Under the stationary regional strain field after reaching the subsidence limit, grabens of this type experienced lateral extension, which resulted in the accumulation of regressive sedimentary successions with the irreversible transition from the lacustrine to the fluvial–proluvial facies. Such grabens are characterized by the one-act manifestation of the local source of clastics regardless of the structure development stage. The development of more expedient (in terms of energy) normal faults along blastomylonite strata (Roslyatino type) did not disturb the isostatic equilibrium and resulted in the formation of narrow deep grabens, where depositional environments remained virtually unchanged. The epidote influx was in progress during the entire life of the accommodation space, since progressive deepening of the graben stimulated constant activity of its local source.