Dynamics and structure of a stirred partially molten ultralow-velocity zone

摘要: Abstract The seismic properties of thin (5–40 km thickness) patches exhibiting low velocities—termed ultralow-velocity zones or ULVZ—just above the core–mantle boundary (CMB) might be explained by presence partially molten rock, where a liquid phase occupies interstices within skeletal network solid grains. However, key problem with this explanation is that in absence improbably strong surface tension effects, partial melt expected to drain percolation over geological time scales, form dense, melt-rich layer at CMB physical are inconsistent and other geophysical constraints. Here we consider whether stirring ULVZ, driven viscous coupling convective motions overlying mantle, can inhibit production such stratification maintain region structure constitution comparable what inferred seismically. We use two-dimensional numerical simulations response melt–solid mixture imposed from scaling analysis identify conditions leading separation, retention drainage broad range parameters. find migration plausible ULVZ governed predominantly dynamic pressure gradients arising deformation related mantle stirring, rather than buoyancy effects driving percolation. In particular, dense would otherwise downward accumulate remain suspension as result ULVZ. addition, our model predicts will characterized positive gradient shear velocity (i.e., increasing depth), consistent inferences, may persist state scales.