Slab control on the mega-sized North Pacific ultra-low velocity zone

Abstract

Ultra-low velocity zones (ULVZs) are localized small-scale patches with extreme physical properties at the core-mantle boundary that often gather at the margins of Large Low Velocity Provinces (LLVPs). Recent studies have discovered several mega-sized ULVZs with a lateral dimension of ~900 km. However, the detailed structures and physical properties of these ULVZs and their relationship to LLVP edges are not well constrained and their formation mechanisms are poorly understood. Here, we break the degeneracy between the size and velocity perturbation of a ULVZ using two orthogonal seismic ray paths, and thereby discover a mega-sized ULVZ at the northern edge of the Pacific LLVP. The ULVZ is almost double the size of a previously imaged ULVZ in this region, but with half of the shear velocity reduction. This mega-sized ULVZ has accumulated due to stable mantle flow converging at the LLVP edge driven by slab-debris in the lower mantle. Such flow also develops the subvertical north-tilting edge of the Pacific LLVP.

Vastly Different Heights of LLVPs Caused by Different Strengths of Historical Slab Push

“…Our recent work (Yuan & Li, 2022) and previous seismic studies (He & Wen, 2009;J. Li et al, 2022;Ni et al, 2002;Wang & Wen, 2007) have indicated that the African LLVP is ∼1,000 km taller than the Pacific LLVP.…”

“…Our recent work (Yuan & Li, 2022) and previous seismic studies (He & Wen, 2009; J. Li et al., 2022; Ni et al., 2002; Wang & Wen, 2007) have indicated that the African LLVP is ∼1,000 km taller than the Pacific LLVP. Note that the topography of each LLVP varies from place to place (Cottaar & Lekic, 2016) and here we refer to the maximum height of the LLVPs.…”

“…The two LLVPs have also been suggested to differ significantly in height. For example, the maximum heights have been constrained to be ∼900 km and ∼1,800 km for the Pacific LLVP and the African LLVP, respectively (He & Wen, 2009; J. Li et al., 2022; Ni et al., 2002; Wang & Wen, 2007; Yuan & Li, 2022).…”

Subducted slabs induce high heat flux at the core-mantle boundary

“…Although ULVZs were proposed to be caused by either partial melting [7][8][9] , iron-rich subducted components 10 , reactant products between silicate mantle and Fe-rich liquid in the outer core [10][11][12] or recycled oceanic crust 13,14 . The most recent S-wave studies, which detected a thin high velocity layer and con rmed the presence of slab adjacent to ULVZs 15,16 , reveals a strong correlation between them. Furthermore, partial melting in ULVZs, possibly due to mid-ocean ridge basalt (MORB) atop subducted slab was evidenced by newest seismic data 9,17 and supported by experimental studies 18,19 .…”

Quantification of Small-Scale Heterogeneity at the Core–Mantle Boundary Using Sample Entropy of SKS and SPdKS Synthetic Waveforms

“…ULVZ strength values above the 2500 to 3000 range are likely in the most extreme range of ULVZ properties. However, some studies have suggested that ULVZs may exist at these large ULVZ strengths [87,88].…”