Title of research project: Quantitative modelling of interaction between mantle flows and lithospheric plates, and formation of sedimentary basins
Reference number of the grant: INTAS-94-1099
Period of performance: 1995 to 1996
Project Coordinator: Prof. W. Jacoby (Institute of Earth Sciences, University of Mainz, Germany)
Principal Investigator: Dr. B.M. Naimark
Total funds: ECU 21,500
The project consisted of developing models of interplay between mantle thermoconvective structures and floating continents and using these models to explain global geodynamical features and heat and mass transfer between the lithosphere and continental crust. Advanced models based on magmatism-eclogitization mechanism were developed to explain tectonic histories of the Timan-Pechora, Tunguska, Moscow, and Pre-Urals basins. A detailed, multidisciplinary study was carried out to construct the models with the efforts from experts in mathematics and numerics combined with experts in geophysics, geology, and geodynamics. Main results include the following. The pioneering quantitative description of the interaction between the convective mantle and three continents, where they aggregate into a supercontinent and then diverge, and a smaller continent rapidly drifts sideways. A continent overriding a subducting slab changes its inclination from vertical to about 45 degree. A continent interacting with downwelling and upwelling flows changes the subcontinental thermal structure, a hot upwelling forms in place of the former downwelling, the continent moves to another downwelling, and the process repeats, thus constituting tectonic cycles. Reversal times from downwelling to upwelling are represented in analytical forms as functions of the Rayleigh number, rates of internal heating, and continent dimensions. It is shown that reversals can occur over certain ranges of these variables, and their critical values are found. Non-linear waves are shown to evolve in compacting porous media (as in mid-ocean ridges) when the melt concentration exceeds 5-6%. These wave constitute the mechanics of the upward melt transfer. Joint effect of divergence and magma accumulation were shown to generate axial tension sufficient to overcome rock strength, so that rifting episodes were predicted to occur at periods of a few hundred years, as observed in Iceland. Upwelling flows forming in the subcontinental mantle can be too weak to break the continent, but lead to rifting and thinning of the lithosphere. In the course of the rifting magmatic segregates evolve at a depth range of about 60-80 km, cool, undergo phase transitions, become denser, and sink into the asthenosphere inducing viscous flow. These flows change the surface topography and form sedimentary basins. An advanced numerical model was developed to describe mantle flow with viscosity and density discontinuities and time-dependent topography. This model is self-consistent, thus explaining phases of regional stretching followed by compression without introducing external effects. Data used in the research include information from 93 wells and 7 lithologic-stratigraphic profiles. These data were assembled in databases for four sedimentary basins in Russia. Thus two new concepts of global and regional geodynamics were studied in the project. These concepts and relevant models are fundamental in understanding the origin and evolution of earth's features vital to man.