Slab Segmentation and Stacking in Mantle Transition Zone Controls Disparate Surface and Lower Mantle Subducting Rates and Complex Slab Morphology.

The contradiction of high subducting plate rate (ranging from 4 to 9 cm/yr on Earth's surface) and low slab sinking rate (about 1 and 2 cm/yr in lower mantle) calls for significant slab deformation in the middle mantle. However, mechanisms that can account for both the deformation and the rate discrepancy have not been fully explored. Here, using 2‐D numerical models that incorporate grain size evolution, we propose a new slab deformation mode, slab segmentation and stacking, to accommodate the differential slab sinking rates. Our results show that the segmented slab due to faulting and grain‐size reduction may further break off and stack over itself as it encounters the high‐viscosity lower mantle. Stacked slabs slowly sink in the lower mantle, while periodic slab tearing hinders upward stress transmission, allowing shallow plates to subduct at a higher rate. This discovered mode also provides an alternative explanation for slab thickening in the lower mantle. Plain Language Summary: According to observations and analysis, the velocities of plates entering the trench are much higher than the slab sinking rate in the lower mantle. Inconsistent velocities at the slab's two ends must be accommodated by slab deformation or accumulation in the middle mantle to ensure mass conservation. We have considered three different deformation modes: slab buckling, horizontal deflection, segmentation and stacking. Compared to the diffusive plastic weakening in the mode of slab buckling, previous studies have shown that slab faulting and grain size evolution can induce stronger localized weakening within slabs. Our study goes a step further by finding that the segmented slab can break off and stack over itself periodically when it encounters the lower mantle. This allows the slab to deform more easily in the mantle transition zone, enabling the plate to subduct at a higher rate while keeping the lower‐mantle slab sinks at a low rate. We also find that this mode of slab deformation can equally well explain the thickening of the slab in the lower mantle. Key Points: The discrepancy between the subducting plate rate and lower mantle slab sinking rate calls for slab accumulations in the middle mantleSimple buckling explains the accumulation but is not sufficient to accommodate the rate discrepancyThe mode of slab segmenting, breaking off, and stacking is a possible solution to the slab accumulation and rate discrepancy [ABSTRACT FROM AUTHOR]