Folding of a viscoelastic layer embedded in a viscous matrix is studied analytically using two viscoelastic rheological models: The Maxwell and the Kelvin model. The layer deformation behavior approximates the viscous or elastic limits depending on the single parameter, R, which is proportional to the viscosity contrast and the ratio of layer-parallel stress to shear modulus. A layer with Maxwell rheology approximates this limit that generates the fastest amplification whereas a layer with Kelvin rheology approximates the slowest limit. For R1 the elastic limit is fastest. The Kelvin rheology is suitable to describe the effective flexural response of a lithospheric plate consisting of an elastic layer overlying a viscous layer, since the Kelvin rheology yields an identical bending moment. A critical elastic layer thickness, R2H/3 (H=lithospheric thickness), is derived at which the deformation behavior of the lithospheric plate changes from quasi-viscous to quasi-elastic. ¿ 2001 American Geophysical Union