Long-wavelength character of subducted slabs in the lower mantle

The high resolution seismic tomography provides the detailed information about the fate of the subducted plates in the mantle. While the slab deformation in the transition zone is quite complex and differs among the individual subduction zones, it could be generally concluded that most slabs seem to penetrate into the lower mantle. The character of the fast seismic velocity anomalies, however, changes when the slabs cross the 670 km discontinuity. While in the upper mantle tomography resolves relatively narrow plate-like structures, in the lower mantle it rather indicates thick blobs of seismically fast material. Such thickening of the downgoing slabs is traditionally attributed to the effect of the viscosity increase in the lower mantle. Here we study the deformation of the slabs in the lower mantle in a viscous model with a non-linear composite rheology including the diffusion creep, the dislocation creep and a power-law stress limiter. We perform numerical simulations of the subduction process in a 2D Cartesian domain and concentrate especially on the effect of the change of the rheological parameters across the 670 km boundary on the slab deformation in the upper part of the lower mantle. The thickening or the buckling of the plate is indeed observed in the models with relatively low yield stress (0.1 GPa) of the stress-limiting mechanism and with a viscosity increase at the 670 km discontinuity. Further, we show that both the major mantle phase transitions and the strength of the crust layer have significant consequences for the creation of the buckling instabilities in the lower mantle.