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Science Journals RESEARCH GEOPHYSICS ThefateoftheHawaiianplumeheadiscritical to the origin of the mantle plume, which Oceanic plateau of the Hawaiian mantle plume head provides a temporal constraint on the longevity and persistence of chemical characteristics of subducted to the uppermost lower mantle Earth’s deep mantle. Furthermore, the subduc- tion of the expected oceanic plateau caused by Songqiao Shawn Wei1*, Peter M. Shearer2, Carolina Lithgow-Bertelloni3, Lars Stixrude3,DongdongTian1 the Hawaiian plume head may have changed plate motions. Niu et al. ( 12) proposed that The Hawaiian-Emperor seamount chain that includes the Hawaiian volcanoes was created by the Hawaiian the collision of this oceanic plateau with the mantle plume. Although the mantle plume hypothesis predicts an oceanic plateau produced by massive Kamchatka Trench was responsible for the decompression melting during the initiation stage of the Hawaiian hot spot, the fate of this plateau Pacific Plate reorientation that resulted in is unclear. We discovered a megameter-scale portion of thickened oceanic crust in the uppermost lower the 47-Ma bend in the Hawaiian-Emperor chain. mantle west of the Sea of Okhotsk by stacking seismic waveforms of SS precursors. We propose that More importantly, thefateofthisoceanic this thick crust represents a major part of the oceanic plateau that was created by the Hawaiian plume head plateau is critical for understanding the role ~100 million years ago and subducted 20 million to 30 million years ago. Our discovery provides temporal of oceanic plateaus in building continental and spatial clues of the early history of the Hawaiian plume for future plate reconstructions. lithosphere and in mantle convection. Owing to their excess crustal thickness and volume, oceanic plateaus are thought to be more dif- arthquakes and volcanism at plate bound- zone. One proposal places this event as the ficult to subduct than individual seamounts aries are well explained with the theory cause of the cusp between the Kurile-Kamchatka (15). Because the Yakutat terrane southeast Downloaded from of plate tectonics, but explaining intra- and the Aleutian-Alaska trenches (9). The sub- of Alaska is the only oceanic plateau that is E plate hot spot volcanoes requires the duction of the seamounts generates arc lavas currently undergoing subduction (16), whether mantle plume hypothesis (1, 2). This with geochemical signatures similar to oceanic oceanic plateaus were commonly subducted hypothesis posits deep-rooted and relatively island basalts on the Kamchatka Peninsula in the past is unclear. By analyzing ophiolitic fixed plumes of hot material upwelling through (10). The oldest surface portion of the Hawaiian- basalts in Kamchatka, Portnyagin et al. (14) the mantle from the deep Earth and accounts Emperor chain, the Meiji Guyot (older than proposed that the Hawaiian plume head, or http://science.sciencemag.org/ for the age-progressive surface expression known 81 Ma) and Detroit Seamount (76 to 81 Ma) at least part of it, was accreted to the forearc as the Hawaiian-Emperor seamount chain. As (11) are about to subduct into the Kamchatka of Kamchatka. This mechanism provides an the Pacific Plate moves northwest (3, 4), the Trench (Fig. 1). But whether the older parts important way to grow continental crust (7). newest volcanoes are found in Hawaii to the oftheseamountchain,particularlytheplume In contrast, a seismic study of compressional- southeast, and the oldest seamounts are near head, also subducted into the deep mantle or to-shear (P-to-S) waves converted at seismic the Kamchatka-Aleutian trench junction in stayed on Earth’s surface is debated (12–14). discontinuities (receiver functions) in South the northwest. The ~47 million year (Ma) bend of the seamount chain is usually attributed to a change in the Pacific Plate motion (5). The N. American Plate history of the Hawaiian-Emperor seamount chain is critical for understanding Earth’s on November 19, 2020 interior evolution and plate tectonics. In the Eurasian h classical view, a mantle plume consists of a c Plate n e large head (>2000 km across) and a thin tail r T (~200 km wide) (6). The plume head generates Sea a a large igneous province (LIP), such as the of Ontong-Java oceanic plateau or the Deccan Okhotsk n- Alask Emperor Chain tia Traps. The plume tail creates an age-progressive Aleu intraplate volcanic chain. Several efforts have been made to associate ancient LIPs to hot Kuril-KamchatkaTrench Pacific spot volcanoes (7). For instance, the Deccan Traps are considered to result from the head Meiji Plate of the Reunion mantle plume surfacing more Guyot than 68 Ma ago (8). However, because of the (>81 Ma) Hawaiian Chain debatable early history of the Hawaiian-Emperor Kamchatka seamount chain, the fate of the Hawaiian mantle Peninsula plume head and resulting oceanic plateau is 8 cm/yr unknown. Detroit According to a variety of plate reconstruc- Seamount 3 4 (76–81 Ma) tions ( , ), the Hawaiian-Emperor seamount Kamchatka chain entered the Kamchatka subduction Trench 1 Department of Earth and Environmental Sciences, Michigan Fig. 1. Topographic-bathymetric map (43) of the northern Pacific Ocean and Northeast Asia. The bold State University, East Lansing, MI 48824, USA. 2Cecil H. and Ida M. Green Institute of Geophysics and Planetary black arrow indicates the current motion of the Pacific Plate at Hawaii relative to the Hawaiian plume, Physics, Scripps Institution of Oceanography, University of whereas the gray arrow represents the approximate trajectory of the Hawaiian-Emperor seamount chain into 3 California, San Diego, La Jolla, CA 92093, USA. Department the Kamchatka subduction zone based on plate reconstructions (3, 4). Inset shows the Kamchatka region of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA. where the oldest seamounts (Meiji Guyot and Detroit Seamount) of the Hawaiian-Emperor chain are about to *Corresponding author. Email: [email protected] subduct into the Kamchatka Trench at a speed of 8 cm/year. Wei et al., Science 370, 983–987 (2020) 20 November 2020 1of5 RESEARCH | REPORT America suggests that an oceanic plateau with not been detected in the lower mantle, in part mography models, determining whether the a thickness of at least 13 to 19 km has sub- because of the limited data coverage in regions reflector is at or above the slab surface (top ducted to ~100 km depth and is responsible wheretheyareexpected. interface) is challenging. The exact shape of for the Pampean flat slab (17). Geodynamic We stacked SS precursors (SdS) from 45 years this 810-km reflector is unclear because of models also show that oceanic plateaus can of global seismic data to detect seismic re- the wide Fresnel zone (~1000 km across) and subduct into the upper mantle, resulting in flectors in the lower mantle (28). The SdS the low horizontal resolution of SS precur- slowdown of subduction (18), formation of a seismic phase is the underside S wave re- sors. Additionally, determining the absolute flat slab (19), surface topography elevation flection off the d-km discontinuity, which reflector depth and topography relies on the (20), and dynamic uplift (21). In comparison arrives before the surface-reflected SS phase seismic velocity in the upper mantle. With dif- with the subduction of normal oceanic crust (fig. S1A). Because SS precursors sample the ferent three-dimensional (3D) mantle velocity of 6- to 7-km thickness, the input of thick oceanic midpoints between earthquakes and seismic models, the average depth of the 810-km plateaus might also change, at least locally, stations, they provide good data coverage reflector varies from 780 to 830 km depending mantle composition and dynamics. for remote regions and are widely used to on the choice of model, and its topography Although mantle plume conduits have been image seismic discontinuities in the upper and also changes from flat to elevated in the center successfully imaged using seismic tomography mid-mantle (29). Besides the major seismic by 30 km (figs. S2 and S3). The seismic signal with dense datasets (22), oceanic plateaus poten- discontinuities extending globally, previous S810S corresponding to the 810-km reflector tially subducted into the lower mantle have a observations detected many smaller-scale re- has an apparent amplitude as strong as that of 20- to 40-km crustal thickness that is smaller flectors using SS or PP precursors (26, 30). the S660S signal for the 660-km discontinuity. than the resolution in most tomographic studies. We focus on a seismic reflector observed at The absolute amplitude of S810S is influenced Owing to a lack of data, the tomography reso- ~810 km depth west of the Sea of Okhotsk, by incoherent stacking and seismic attenua- Downloaded from lution in northeastern Siberia is particularly which was previously detected by limited data tion effects that are difficult to constrain (28). low in both global (23)andregional(24)images. of PP precursors (30). The reflector has a width Therefore, we conclude that this megameter- Seismic reflected waves are more sensitive to on the order of 1000 km and a depth varying scale reflector marks an S-wave impedance sharp boundaries and provide a more effective from 780 to 820 km across (Fig. 2). When com- (product of density and S-wave velocity) in- tool to detect small-scale compositional heter- pared with global tomography models (23), the crease at 780 to 820 km depths on the same ogeneities in the deep mantle. Many seismic 810-km reflector appears to
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