Ultra-low velocity zones (ULVZs) above the Core-Mantle Boundary (CMB) are significant structures probably connecting the lowermost mantle and the outer core. As “thin patches” of dramatically low seismic-wave velocity, they are occasionally found near the base of mantle plumes and in-or-near high seismic-wave speed regions above CMB. The causes of their morphology-distribution and geodynamics remain unclear, and simulation results of high-density melt diverge from seismic-observations speculation (~+10%). We introduce a 2D time-dependent Stokes’ two-phase-flow (with melt-migration) numerical model to investigate the formation and morphological characteristics of ULVZs caused by CMB-mantle tangential flows and a neighboring cold source (subducted plate). We discover that (a) the participation of cold sources with temperature differences between ~4000 °K at the plume central regions to <~3900 °K at the plume-cooling mantle region, separated by horizontal distances of about 100 (±<50) km are necessary for the stable existence of dense melts with mass-density difference >+1-2% (even +10%) with respect to the surrounding mantle; and additionally (b) an enhanced tangential flow coincident with the internal reverse circulation within the broad plume base (with speeds >3 times the lowermost-mantle characteristic flow speed); are necessary for higher aspect-ratio-morphology lenses compatible with seismic observations. Our findings suggest that the CMB-mantle tangential flow and/or outer-core interacting with CMB-topography, may be implicated in generating mega-ULVZs, especially if they appear along the edges of LLVSPs and especially when in/near high seismic-speed “cold” zones. We infer a strong link between ULVZs morphology and the dynamical environment of the lowermost mantle and uppermost outer core.

Ultra-low velocity zones (ULVZs) above the core-mantle boundary (CMB) are significant structures that connect the lowermost mantle and outer core. As “thin patches” of dramatically low seismic-wave velocity, they are occasionally found near the base of mantle plumes and in-or-near high seismic-wave speed regions above the CMB. The causes of their morphological distribution and geodynamics remain unclear, and simulation results of high-density melts diverge from seismic observations. We introduced a two-dimensional time-dependent Stokes two-phase flow (with melt migration) numerical model to investigate the formation and morphological characteristics of ULVZs caused by CMB-mantle tangential flows and a neighboring cold source (subducted plate). We discovered that (a) the participation of cold sources with temperature differences between ~4000 K at the plume central regions to <~3900 K at the plume-cooling mantle region, separated by horizontal distances of approximately 100 (±<50) km are necessary for the stable existence of dense melts with mass-density difference >+1–2% (even +10%) with respect to the surrounding mantle; additionally, (b) an enhanced tangential flow coincident with the internal reverse circulation within the broad plume base (with speeds >3 times the lowermost-mantle characteristic flow speed) are necessary for higher aspect-ratio-morphology lenses compatible with seismic observations. The CMB-mantle tangential flow and/or outer-core interacting with CMB-topography may help generate mega-ULVZs, particularly if they appear along the edges of large low-shear-wave-velocity provinces (LLSVPs) and in/near high seismic-speed “cold” zones. Thus, we infer that a strong link exists between ULVZ morphology and the dynamic environment of the lowermost mantle and uppermost outer core.