Lens effect of remnant blocks on deep mantle upwelling causing anomalous subsidence

Classical mantle convection models predict a broad surface uplift over a lower mantle upwelling. However, recent studies have identified anomalously localized surface subsidence above seismically imaged lower mantle upwellings, particularly in regions where upwellings are impeded by subducted/delaminated blocks not currently connected to a subducting/delaminating lithosphere ('remnant blocks' for simplicity), e.g., in the western USA and the South China Sea. Known geological processes cannot fully explain the observed localized subsidence, and its locality implies a strong association with the underneath upwelling. Here, we use numerical models to quantitatively explore the contribution of lower mantle upwelling to surface topography evolution. Our results demonstrate that the divergent mantle flow caused by lower-mantle upwelling can stretch the overlying lithosphere, inducing broad subsidence that can be reversed when the upwelling reaches the lithospheric bottom. Moreover, the interaction between the remnant block and upwelling can extend the duration of subsidence and focus the subsidence into a narrow region, which we call the lens effect of remnant blocks. Similar abnormal subsidence events in northeastern Asia further show the potential broad applicability of the proposed mechanism, although, in specific research areas, other shallow mechanisms could also have played important roles. Based on numerical models and the comparison with natural settings, this study reveals that divergent mantle flow induced by lower-mantle upwelling can cause anomalous subsidence. Interaction between the upwelling and remnant blocks can extend the duration of the upwelling and focus the subsidence into a narrow region.