Strong ULVZ and Slab Interaction at the Northeastern Edge of the Pacific LLSVP Favors Plume Generation

Abstract

The northeastern margin of the Pacific Large Low Shear Velocity Province (LLSVP) is particularly interesting as many seismic modeling studies have suggested the presence of multiple structural anomalies in this lowermost mantle region. There is a strong velocity change from the interior of the Pacific LLSVP toward the margin, inferred to be a high velocity, post-perovskite lens thins toward the edge while an ultra-low velocity zone (ULVZ) layer increases in thickness (Lay et al., 2006). To et al. ( 2011) also proposed a localized, slow ULVZ-like anomaly embedded inside or at the margin of the LLSVP from their modeling of the anomalous S and diffracted S (Sdiff) behaviors observed with a limited number of stations.The deployment of USArray, recording many deep earthquakes from Fiji-Tonga subduction zone, greatly increases the sampling points to study the lateral structural variation along this margin. Through deconvolution

Lowermost Mantle Structure Beneath the Central Pacific Ocean: Ultralow Velocity Zones and Seismic Anisotropy

“…Our study region is to the southeast of Hawaii, at the edge of the Pacific LLVP. Figure 1c shows the raypath coverage and the locations of previously detected ULVZ structure in this region (Lai et al, 2022;Sun et al, 2019;Yu & Garnero, 2018), in addition to the low velocity features mapped by Jenkins et al (2021). Early studies using core-reflected P waves suggested a ∼10 km thick basal layer with velocity reductions of approximately 10% in our study region (e.g., Mori & Helmberger, 1995;Revenaugh & Meyer, 1997).…”

“…1. A prominent S* postcursor, visible directly after the S* phase and modeled as being due to an ULVZ by Lai et al (2022), is (marked with a pink box in Figure 2a). As this waveform feature has been modeled and explained before, we do not focus on it in our analysis.…”

“…It has been suggested that iron from Earth's outer core may be responsible for their presence, either driven to the mantle by diffusion (e.g., Lesher et al., 2020) or via morphological instabilities (Otsuka & Karato, 2012). Alternatively, enrichment of iron in ferropericlase could explain the ultralow velocities (e.g., Finkelstein et al., 2018; Lai et al., 2022). The presence of partial melt has also been suggested as an explanation for ULVZs (e.g., Ferrick & Korenaga, 2023; Lay et al., 2004; Yuan & Romanowicz, 2017), although it is imperfectly understood how melt pockets just above the CMB can stay stable over geological time scales (e.g., Dannberg et al., 2021; Hernlund & Jellinek, 2010).…”

The root to the Galápagos mantle plume on the core-mantle boundary

“…The Galápagos ULVZ falls into the category of broad-scale mega-ULVZs, which are so far uncovered by diffracted phases. Other occurrences are mapped in 3D near Hawaii (Cottaar and Romanowicz, 2012;Jenkins et al, 2021;Lai et al, 2022;, Samoa (Thorne et al, 2013), and Iceland ; these are shown in Fig. 7 combined with the global database of ULVZs by (Yu and Garnero, 2018).…”

“…While only a fraction of the core-mantle boundary has been targeted for these anomalies, there is a weak trend that these patches appear within or near the LLVPs (Yu and Garnero, 2018). This relationship is however certainly true for the broadest of ULVZs that have been mapped in 3D (Cottaar and Romanowicz, 2012;Thorne et al, 2013;Jenkins et al, 2021;Lai et al, 2022;, and can be dubbed 'mega-ULVZs' after Thorne et al (2013).The three mega-ULVZs currently mapped lie in the vicinity of the Hawaiian, Icelandic and Samoan hotspots, and such large structures appear otherwise rare, at least across a large swath of the Pacific (Kim et al, 2020).…”