QUANTITATIVE MODELING OF THE TUNGUSKA BASIN EVOLUTION IN THE PALEOZOIC - A ROLE OF ECLOGITIZATION WITHIN THE UPPERMOST MANTLE

We examine the Tunguska Basin evolution by using two lithologic-strati graphic sections along deep seismic sounding profiles and exploration wells. The tectonic analysis demonstrates that the subsidence of the b asin was rapid in the Early Cambrian and slower since the Middle Cambr ian. We show that the relative thinning of the crust beneath the basin is several times greater than its relative stretching, and therefore the stretching alone cannot explain the subsidence of the Tunguska Bas in. We suggest a possible mechanism of the basin formation in the Pala eozoic. The thinning of the lithosphere beneath the Tunguska Basin due to ocean basin opening in its vicinity leads to passive uplift of the asthenosphere and to partial melting of mantle materials. The forming mechanism includes accumulation of magmatic melt in the asthenospheri c bulge, phase transition to eclogite, and a flow in the upper mantle induced by the evolved heavy bodies. We construct a numerical model of basin evolution, compute the viscous flow due to the subsidence of an eclogite body, and find the resultant changes in the surface topograp hy. To do this, we employ the Galerkin-spline technique. Using results of the model and tectonic analysis, we interpret geodynamic evolution of the Tunguska Basin and discuss the effect of phase changes in the upper mantle upon the evolution of the basin. The numerical results sh ow that the subsidence curve calculated from the model gives a better fit to the observed tectonic subsidence than the thermal subsidence cu rves predicted by McKenzie's stretching model. The density distributio n accepted in the model agrees with the upper-mantle velocity structur e beneath the Tunguska Basin. The model is also Consistent with gravit y and heat flow data. Copyright (C) 1996 Elsevier Science Ltd