Gravitational instability of a perfectly plastic layer resting on a viscous fluid layer: Implications for diapirism

This paper addresses the problem of gravitational (Rayleigh-Taylor) instabi lity of a viscous layer overlain by a perfectly plastic layer of higher den sity. This model describes the initial stage of diapirism. The growth of sm all interface perturbations is examined. The problem of the structural stab ility of the layers reduces to the determination of eigenvalues from a syst em of ordinary differential equations with boundary conditions. This proble m. in turn, reduces to the determination of determinant zeros for a system of linear algebraic equations. It is shown that if the effective viscosity of the upper layer is considerably higher than that of the lower layer, the nature of the gravitational instability of a layered rheology structure is controlled by the behavior of a perfectly plastic material. The initial pe rturbation of the interface can generate a set of diapirs with various wave lengths rather than diapirs with a dominating wavelength. Presumably, this fact accounts for the spatially inhomogeneous distribution of diapirs.