Mechanical coupling of the motion of the surface plate and the lower mantle slab: Effects of viscosity hill, yield strength, and depth-dependent thermal expansivity

Abstract

Sinking rates of the subducted lithosphere in the Earth's lower mantle inferred from seismic tomography images indicate much lower rates than that of the observed subducting surface plate motion. This implies that the surface plate motion is independent of descending flow in the deep mantle. Seeking to understand the mechanical coupling of the slab penetrating into the deepest mantle with the surface plate motion, we performed numerical modeling of the subducting lithosphere integrated into a whole-mantle-scale convection system. Viscosity layering, slab strength, and thermal expansivity were systematically varied using a two-dimensional Cartesian model with a free-surface condition. An increase in the lower mantle viscosity significantly diminished the surface plate motion by stress transmission of the slab when the slab strength was 300 MPa. We observed slab buckling near the 660-km phase transition when the yield stress was 200 MPa. The slab buckling partly absorbed the effects of the lowermost mantle viscosity on the surface plate. Pressure-dependent thermal expansivity further enhanced slab buckling. When the viscosity of the lowermost mantle was less than 1 × 1022 Pa·s, even with a large-viscosity hill in the lower mantle, the slab sinking rate increased up to approximately 7 cm yr−1 when the slab reached the core-mantle boundary region. As a result, the viscosity hill was not sufficient to reduce the sinking rate of the deepest slab. Slab deformation, coupled with gradually increasing viscosity, therefore, plays a key role in the simultaneous occurrence of rapid motion in the surface lithosphere with sluggish slab motion in the deepest layer.