Topographic fingerprint of deep mantle subduction

Abstract

The dynamic topography links with the mantle structures at various temporal and spatial scales. However, it is still unclear how it relates to the dynamics of subducting lithosphere when plates reach the mantle transition zone and lower mantle. Seismic tomography images show how slab morphologies vary from sinking sub‐vertically into the lower mantle, to lying flat above the upper‐lower mantle discontinuity, to thickening in the shallow lower mantle. These slab shapes have been considered to be the result of variable interaction of the slab with the upper‐lower mantle discontinuity at ~670 km depth. Previous studies show that periodic deep slab dynamics can explain a variety of enigmatic geological and geophysical observations such as periodic variations of the plate velocities, trench retreat and advance episodes and the scattered distribution of slab dip angle in the upper mantle. In this study, we use two‐dimensional subduction models to investigate the surface topography expression and its evolution during slab transition zone interaction. Our models show that topography does not depend on slab morphology; indeed, the dynamic topography cannot distinguish between a slab sinking straight into the lower mantle and slab stagnation at the upper‐lower mantle boundary. However, topographic oscillations are related to episodes of the trench advance and retreat, which in turn are linked to the slab folding behaviour at transition zone depths. Our results suggest that the surface transient signal observed by geological studies could help to detect deep subduction dynamics.