The Effect of Heavy Inclusions in the Upper Crust on Thermoconvective Instability of the Lithosphere

The method of linear theory for small perturbations is used to study the stability of heavy inclusions in the upper crust. The existence of such inclusions of increased density is associated with chemical heterogeneity or phase transitions. At small deformations considered in the linear stability theory, the material undergoes transient creep which is described by the Andrade rheological model. Under this rheology, the convective instability of thickened segments of the lithosphere is oscillatory: the increment is a complex number whose real part is close to zero (threshold instability). Heavy inclusions in the upper crust introduce a small complex correction to the increment that determines the convective instability of the lithosphere, slightly increasing the real and imaginary parts of the increment, enhancing the instability and increasing the oscillation frequency.