INFLUENCE OF VOLATILES IN THE UPPER MANTLE ON THE DYNAMICS OF THERMAL THINNING OF THE LITHOSPHERE

Abstract

Areas of Cenozoic tectonic and magmatic activity (including high plateaus and continental rifts) are characterized by anomalous low-velocity and low-density zones in the upper mantle. Petrochemical studies of Cenozoic volcanism have revealed such changes of magma composition with time that prove a consecutive uplift of magma sources. Thus, it is supposed that the process of tectonic rejuvenation is caused by an ascent of anomalous mantle to the base of the lithosphere, resulting in partial melting of lithospheric material, its consecutive replacement by material from the anomalous mantle, lithosphere thinning, and, hence, isostatic uplift of lithospheric blocks. A model of thermal thinning of the lithosphere that is specified by a 1-D heat-conductivity problem for the lithosphere with a moving lower boundary, is proposed as a model of Cenozoic tectonic activation. The presence of H2O and CO2 fluids in the upper mantle is taken into account. Numerical modelling of the process has revealed that the composition of the upper mantle and fluid phase has a strong influence on the dynamics of the process. The presence of volatiles in the upper mantle leads to the appearence of maxima and minima on the solidus curves. Lamination of fusible and refractory layers in the upper mantle may lead to a sharp change in lithosphere-thinning velocities and, hence, to a discrete character of surface vertical motions. The thickness of the lithosphere in a new equilibrium position is calculated for a different composition of the upper mantle and different values of heat flow supplied to the base of the lithosphere; the results show that for the models that seem best to fit present knowledge of the upper mantle composition, melting of the lower crust may take place only in the later stages of the process, when the lithosphere-anomalous mantle boundary approaches its new equilibrium position.