The ultra-low velocity zones (ULVZs) are located around the core-mantle boundary, and most of them are near the boundary of Large Low Velocity Provinces (LLVPs), which dominate the earth’s lowermost mantle. Studies on the precise margins of LLVPs, structures of ULVZs, and relationships between LLVPs and ULVZs would be crucial in understanding the dynamic evolution of the earth’s lower mantle.
In a study published in Nature Communications, the research group led by Prof. SUN Daoyuan from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences, and their collaborators, obtained a 3-dimensional structure of the largest-scale ULVZ ever known beneath the Pacific Ocean.
Previous research suggested a mega-sized ULVZ at the boundary of north Pacific LLVP, but its precise location, size, and shear velocity perturbation remain unclear due to a lack of observed data in the north-south (N-S) direction. The increase in USArray stations in Alaska presented the possibility for researchers to study the ULVZ beneath the northern Pacific (NP) combining samplings from two orthogonal directions.
In this study, researchers measured the travel time residuals of the seismic phase in both N-S and west-east (W-E) directions. The measurement helped them to determine that the height at the northern edge of Pacific LLVP is around 900 km. The edge is also tilting to the north.
Furthermore, by waveform fitting, the researchers attained the 3D structure of the large-scale ULVZ, with an estimated lateral extent of 1500 × 900 km and a height of 50 km. The model also exhibits the shear velocity perturbation is around 10%. Moreover, an outer region in the LLVP with a velocity jump is very likely to be a subducted slab according to a former result.
The researchers proposed a hypothesis on the formation of this mega-scale ULVZ based on their results. A long-term stable mantle flow converging driven by slap-debris exists near the northern boundary of the Pacific LLVP. The flow caused the small-scale ULVZs to gather at the margin of LLVP, formed the mega-scale ULVZ observed today, and shaped the north-tilting wall of LLVP.
On contrary, shearing flow shaped the small-scale ULVZs at the northeastern verge of the LLVP. In addition, these fragments played a part in forming the mantle plume at Hawaiian, which may bring about rich geochemical heterogeneities for the study of the earth. This hypothesis shows consistency with a previous geodynamical study.
This study provides a more accurate seismic image to a better understanding of the dynamic process of the earth’s lower mantle.
Slab control on the mega-sized North Pacific ultra-low velocity zone
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