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.

Oceanic-plates vertical tearing is seismically-identified in the present-day Earth. This type of plate tearing is frequently reported in horizontally-oblique subduction zones where transform-faulted oceanic plates are subducting (or subducted). However, the mechanisms behind vertical slab tearing are still poorly understood, thus we utilize 3D time-dependent Stokes’ flow thermo-mechanical models to further study this problem. We find that (i) the age offset of transform fault and (ii) the horizontal obliqueness of subduction fundamentally control the tearing behavior of two generic, materially-homogeneous oceanic slabs separated by a low-viscosity zone. The two slabs sequentially bend, which combined with the age-thickness difference between slabs, causes the differential sinking of them. Based on the modeling results, well-developed slabs vertical tearing would happen when the oblique angle of subduction is ≥30° or the age ratio of the secondly-bent to firstly-bent slab being ~<0.6. Quantifying the horizontal distance-vector between sinking slabs, we find that subduction at medium-low horizontal-obliqueness angles (≤40°) of young lithosphere (slabs-average ~15 Myr) tends to produce fault-perpendicular tearing. Contrastingly, old-age slabs (average ≥ 30 Myr) with medium-large obliqueness angles (~>20°) tend to produce fault-parallel tearing, related to differential slab-hinge retreat or rollback. Correlations between slabs’ (i) computed tearing horizontal-width and (ii) scaling-theory forms of their subduction-velocity differences, are reasonable (0.76-0.97). Our numerically-predicted scenarios are reasonably consistent with plate-tear imaging results from at least 4 natural subduction zones. Our modeling also suggests that continual along-trench variation in subduction dip angle may be related to a special case of oblique subduction.

Oceanic slab vertical tearing is prevalently identified in the present Earth. More general background for vertical slab tearing is the transform-fault subduction during horizontally-oblique tectonic convergence. However, its geodynamic mechanisms are poorly understood to date. This work introduces a full numerical 3-D time-dependent Stokes’ thermo-mechanical flow model to investigate the characteristics and mechanism of vertical tearing of active transform-faulted oceanic slab during oblique subduction. We find that (i) transform-fault ages-offset and (ii) subduction horizontal obliqueness have the first-order control, even without the lateral physical-property differences. The overriding plate enforces (surface contact interaction) bending of one slab first, which superimposes the differential sinking driven by slabs-age-thickness differences. For obliqueness angles ≥30° and/or age-ratios of the secondly-bent to the firstly-bent slab being <0.6, well-developed slab vertical tearing is unavoidable inside the mantle. Quantifying the horizontal distance vector between sinking slabs, we find that young overall lithosphere (average <30 Myr, for any age ratio) at high subduction obliqueness angles (>~25°) tends to produce trench-parallel slab tearing. In contrast, combinations of small-intermediate obliqueness angles (0-30°) and age ratios with the slab that bends at the trench first being relatively older-thicker, tend to produce trench-perpendicular tearing, which is related to differential slabs-hinge retreat or rollback. These numerically-predicted scenarios and phenomena are consistent with plate-tear imaging results from subduction zones. Our modeling results also suggest that the continual along-trench variation in subduction dip angle may be related to oblique subduction’s early stages of evolution.