Deformation modes of subducted lithosphere at the core-mantle boundary: An experimental investigation

Ancient subducted lithosphere may be an important source of the seismically observed heterogeneity and anisotropy in the D '' layer. Proper interpretation of the seismic observations therefore requires an understanding of how subducted lithosphere deforms upon reaching the core-mantle boundary (CMB). Here we study this process experimentally, using an analog model system in which a sheet of viscous sugar syrup (representing the subducted lithosphere) falls vertically onto an impermeable surface (the CMB'') at the bottom of a reservoir of less viscous syrup (the ambient mantle''). For sheet/ambient viscosity contrasts gamma less than a critical value gamma(c) that depends on the ratio of the fall height to the sheet thickness, the sheet impinges on the CMB as a stagnation flow with bilateral symmetry. For gamma > gamma(c), however, the sheet undergoes a periodic folding instability. We determine experimentally a universal scaling law for the folding amplitude delta as a function of the viscosities and densities of the two fluids and the thickness and vertical velocity of the sheet where it enters the folding region. To apply this law to the Earth, we assume that the sheet's excess viscosity and density relative to the ambient mantle are both controlled by the same temperature anomaly Delta T. Using estimated rheological parameters for diffusion creep in the lowermost mantle, we find that the folding criterion gamma > gamma(c) requires Delta T > 300 K. The presence of folded lithosphere at the CMB beneath the Cocos plate, suggested recently on the basis of seismic data by Hutko et al. [2006], is unlikely to be the result of in situ folding because the temperature anomaly (400-700 K) required to explain the observed variations in the depth of the postperovskite phase transition is probably too large to be preserved in a deeply penetrating slab. However, the seismic observations might be explained in an alternative way by a pile'' of folded lithosphere that formed at 660 km depth and subsequently sank to the CMB.