The cause of slab stagnation in mantle transition zone and its effects on the dynamics of the mantle

The linear structures of seismically fast anomalies, often interpreted as subducted slabs, in the southern Asia and circum-Pacific lower mantle, provided strong evidence for the whole mantle convection model. However, recent seismic studies have consistently shown that subducted slabs are deflected horizontally for large distances in mantle transition zone in the western Pacific and other subduction zones, suggesting that the slabs meet significant resistance to their descending motion and become stagnant in the transition zone. This poses challenges to the whole mantle convection model and also raises the question about the origin of stagnant slabs. Here, using a global mantle convection model with realistic spinel-post-spinel phase change (-2 MPa/K Clapeyron slope) and plate motion history, we demonstrate that the observed stagnant slabs in the transition zone and other slab structures in the lower mantle can be explained by the presence of a thin, weak layer at the phase change boundary that was suggested from mineral physics and geoid modeling studies. Our studies also show that the stagnant slabs mostly result from subduction in the last 20-30 million years (Myr), confirming the transient nature of slab stagnation and phase change dynamics on time scales of tens of Myr from previous studies.