Global tectonic cycle: a conceptual step from plate kinematics towards their dynamics

Ukrainian State Geological Prospecting Institute

Global Tectonic cycle (GTC). Description. The GTC model is based on the following assumptions: 1) at some moment of time Tx the lithosphere (as the assemblage of plates) is in the state of dynamic equilibrium; i.e. no relative motions of plates as to one another and/or to the mantle. It means that die lithosphere is in the state of corotation with the mantle; 2) the mantle at some moment Tx is a homogenous non-stratificated layer. If at some moments Ts the density differentiation of the mantle for light (astenosphere) and heavy phases begins the succession of events will be as next. The mantle division into light and heavy phases will lead, in accordance to the law of the conversation of momentum, to the redistribution of angular momentum between die dividing phases. As the results of this redistribution, the fields of dynamic tensions will appear on the both (lower and upper) boundaries of the light layer. The appearance of tensions is identical to appearance of instability on the lithosphere-mantle boundary and to the disturbance of the lithosphere primary equilibrium. Any thermodynamic regime of the mantle, admitting the accumulation of the light phase under the lithosphere. leads to the growth of dynamic instability of the whole system. While achieving some critical values of the dynamic tensions, which appear on the lithosphere, oceanic and continental crust, the fields of tensions are distributed very unevenly. It leads to die formation of plates various in size and moments of inertia on the destructive stage of GTC. The destruction of the lithosphere is connected with the change of PT-conditions in the mantle and accompanied by the migration of the mantle components on the lithosphere level. As the result, the lithosphere increases. The change of lithosphere momentum of inertia, due to its mass increasing, consequently leads to its drift relative to the mantle. In this case the plates, formed on the destructive stage of GTC and possessing various moments of inertia differ in absolute velocity and in trace of the motion. These cause the possibility of their collision. The drift of the lithosphere relative to die mantle will continue till the restoration of momentum balance between the lithosphere and the mantle occurs, namely till new dynamic equilibrium in the system appears.

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