On a thermoconvective mechanism for oscillatory vertical crustal movements

Abstract

The sedimentary record testifies that one of the main peculiarities of vertical crustal movements is their rhythmic behavior which consists of a periodic change in the sign of movements. The long history of deposition in sedimentary basins is inevitably interrupted by episodes of erosion that indicates elevation. The vertical crustal movement in sedimentary basins can be considered as a slow subsidence on which a small-amplitude oscillation (amplitude of the order of 100 m) is superimposed. In the present work oscillatory crustal movements are related to thermoconvective oscillation of the upper thermal boundary layer formed by large-scale convection in the Earth's mantle. The instability of the boundary layer was analyzed by many researchers who used a Newtonian fluid rheological model or a power law non-Newtonian fluid model for the Earth's mantle. However, a real material has a memory, in contrast to Newtonian and power law fluids. The author proposed recently a non-linear integral model consistent with general creep theory and laboratory studies of rock creep to describe the Earth's mantle rheology. To investigate the boundary layer instability this rheological model, having a memory, is used for the mantle and the crust but a thin uppermost layer which may be considered to be purely elastic at geological timescales is distinguishable. The upper boundary of the layer is considered as a surface loaded by sediments, the thickness of which is changed with time. It is shown that the boundary layer instability is oscillatory and the period of thermoconvective oscillation is of the order of 100 Ma.