On the mechanism of graben formation

Abstract

The structure of the crust and mantle and the subsidence history of the Baikal and Dneprovsko-Prypyatsky basins are analyzed. Considerable thinning of the crust, from several ten percents to two hundred percent, takes place under both basins. The extension of the upper crust is much smaller: 4–6%. Under these conditions, both basins should be considered as grabens and not as rifts produced by stretching (Artyushkov, 1987). In the Baikal and Prypyatsky basins and in some places in the Dneprovsko Basin, the Moho is underlain by a layer of rocks with V p ~ 7.2–7.8 km/s . In the absence of volcanism in the Baikal and under stable tectonic conditions in the Dneprovsko-Prypyatsky Basin these should be garnet granulites with rather high density. Very rapid water-loaded subsidence (at a rate 1 km/Ma) took place in the Baikal in the Pleistocene, and in the Dneprovsko-Prypyatsky Basin in the Late Devonian. The subsidence of such short duration without intense stretching at the surface can be produced only by contraction of rocks in the lithosphere. This most probably took place due to gabbro-garnet granulite-eclogite transformation in the lower crust under migration of fluid from the asthenospheric layer. Fluid migration through these zones was a cause of gabbro-eclogite transformation in long narrow zones which produced deep grabens at the Earth's surface.