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Abstract Delineation of geochemically distinct domains in Earth’s mantle is essential for understanding large-scale mantle convective flow and dynamics. Previous studies identify possible long-lived (>60 million-year) mantle isotopic domains (i.e. Antarctic-Zealandia, Pacific and Indian) near the Philippine Sea and western Pacific. Here we compile published basalt geochemistry of the Philippine Sea and surroundings and add new Mo isotopic and water content data for Gagua Ridge lavas, northwestern Philippine Sea, to distinguish slab-derived components during subduction. The water content, trace element, and Mo-Sr-Nd isotope compositions of Gagua Ridge arc lavas suggest that slab fluids and sediment melts are responsible for element recycling to the arc. The Philippine Sea basalts show both Indian and Zealandia-Antarctic Pb isotopic signatures; restoration of the basalt locations within a plate reconstruction shows the far-travelled Philippine Sea traversed these mantle domains. We establish the Indian mantle domain eastern boundary at ~120°E under SE Asia and the Indian Ocean. The Antarctic-Zealandia mantle domain lies south of ~10°N within the SW Pacific and has mostly remained in oceanic realms since ~400 Ma with only limited continental material input. 

Abstract The Mesozoic subduction history of the Paleo‐Pacific plate below the East Asian margin remains contentious, in part because the southern part is poorly understood. To address this, we conducted a sediment provenance study to constrain Mesozoic subduction history below West Sarawak, Borneo. A combination of detrital zircon U‐Pb geochronology, heavy minerals, trace element, and bulk rock Nd isotope data were used to identify the tectonic events. The overall maturity of mineral assemblages, dominantly felsic sources, abundant Precambrian‐aged zircons, and low εNd(0) values (average −13.07) seen in Late Triassic sedimentary rocks suggest a period of inactive subduction near Borneo. Slab shallowing subduction occurred between 200 and 170 Ma based on subdued magmatism and tectonic compression across West Sarawak. From c. 170 to 70 Ma there was widespread magmatism and we interpret the Paleo‐Pacific slab steepened. Collectively, we show the Paleo‐Pacific plate subduction had variable slab dip histories in Borneo. 

SUMMARY Differences between P- and S-wave models have been frequently used as evidence for the presence of large-scale compositional heterogeneity in the Earth's mantle. Our two-step machine learning (ML) analysis of 28 P- and S-wave global tomographic models reveals that, on a global scale, such differences are for the most part not intrinsic and could be reduced by changing the models in their respective null spaces. In other words, P- and S-wave images of mantle structure are not necessarily distinct from each other. Thus, a purely thermal explanation for large-scale seismic structure is sufficient at present; significant mantle compositional heterogeneities do not need to be invoked. We analyse 28 widely used tomographic models based on various theoretical approximations ranging from ray theory (e.g. UU-P07 and MIT-P08), Born scattering (e.g. DETOX) and full-waveform techniques (e.g. CSEM and GLAD). We apply Varimax principal component analysis to reduce tomography model dimensionality by 83 percent, while preserving relevant information (94 percent of the original variance), followed by hierarchical clustering (HC) analysis using Ward's method to quantitatively categorize all models into hierarchical groups based on similarities. We found two main tomography model clusters: Cluster 1, which we called ‘Pure P wave’, is composed of six P-wave models that only use longitudinal body wave phases (e.g. P, PP and Pdiff); and Cluster 2, which we called ‘Mixed’, includes both P- and S-wave models. P-wave models in the ‘Mixed’ cluster use inversion methods that include inputs from other geophysical and geological data sources, and this causes them to be more similar to S-wave models than Pure P-wave models without significant loss of fitness to P-wave data. Given that inclusion of new data classes and seismic phases in more recent tomographic models significantly changes imaged seismic structure, our ML assessment of global tomography model similarity may improve selection of appropriate P- and S-wave models for future global tomography comparative studies. 

Abstract Pacific‐Panthalassa plate tectonics are the most challenging on Earth to reconstruct during the Mesozoic and Cenozoic eras due to extensive subduction, which has resulted in large (>9,000 km length) unconstrained gaps between the Pacific and Laurasia (now NE Asia) back to the Early Jurassic. We build four contrasted NW Pacific‐Panthalassa global plate reconstructions and assimilate their velocity fields into global geodynamic models. We compare our predicted present mantle structure, synthetic geoid and dynamic topography to Earth observations. P‐wave tomographic filtering of predicted mantle structures allows for more explicit comparisons to global tomography. Plate reconstructions that include intra‐oceanic subduction in NW Pacific‐Panthalassa fit better to the observed geoid and residual topography, challenging popular models of Andean‐style subduction along East Asia. Our geodynamic models predict significant SE‐ward lateral slab advections within the NW Pacific basin lower mantle (∼2,500 km from Mesozoic times to present) that would confound “vertical slab sinking”‐style restorations of imaged slabs and past subduction zone locations. 

Abstract The plate tectonic history of the hypothesized “proto‐South China Sea” (PSCS) ocean basin and surrounding SE Asia since Cenozoic times is controversial. We implement four diverse proto‐South China Sea plate reconstructions into global geodynamic models to constrain PSCS plate tectonics and possible slab locations. Our plate reconstructions consider the following: southward versus double‐sided PSCS subduction models; earlier (Eocene) or later (late Oligocene) initiation of Borneo counterclockwise rotations; and larger or smaller reconstructed Philippine Sea plate sizes. We compare our modeling results against tomographic images by accounting for mineralogical effects and the finite resolution of seismic tomography. All geodynamic models reproduce the tomographically imaged Sunda slabs beneath Peninsular Malaysia, Sumatra, and Java. Southward PSCS subduction produces slabs beneath present Palawan, northern Borneo, and offshore Palawan. Double‐sided PSCS subduction combined with earlier Borneo rotations uniquely reproduces subhorizontal slabs under the southern South China Sea (SCS) at ~400 to 700 km depths; these models best fit seismic tomography. A smaller Philippine Sea (PS) plate with a ~1,000‐km‐long restored Ryukyu slab was superior to a very large PS plate. Considered together, our four end‐member plate reconstructions predict that the PSCS slabs are now at <900 km depths under present‐day Borneo, the SCS, the Sulu and Celebes seas, and the southern Philippines. Regardless of plate reconstruction, we predict (1) mid‐Cenozoic passive return‐flow upwellings under Indochina; and (2) late Cenozoic downwellings under the SCS that do not support a deep‐origin “Hainan plume.” Modeled Sundaland dynamic topography strongly depends on the imposed plate reconstructions, varying by almost 1 km. 

Abstract The Tonga‐Samoa system provides a unique tectonic context to study how a cold subducting slab interacts with a hot rising mantle plume. Here we present a 3‐D high‐resolution image of the 410‐km mantle discontinuity (the410) using seismic signals excited by deep‐focus earthquakes. The410is found to be ~30 km shallower inside the Tonga slab relative to the ambient mantle and ~20 km deeper further to the northwest under Fiji Islands. The downward deflection of the410under Fiji supports the hypothesis of a plume migration around the northern edge of the Tonga slab from Samoan hot spot to under Fiji due to fast trench rollback. The 50‐km topography difference in the410between the plume and the slab corresponds to a temperature difference of ~500 ± 100 K. The Samoan plume is inferred to be 200 ± 50 K hotter than the ambient mantle and supports a thermal origin for the plume. 

Plate reconstructions of oceanic domains are generally based on paleo-magnetic and seafloor spreading records. However, uncertainties associated with such reconstructions grow rapidly with increasing geological age because the original oceanic plates have been subducted. Here we synthesize advances in seismic tomographic mapping of subducted plates now lying within the mantle that assist plate reconstructions. Our proposed Japan–NW Pacific subduction histories incorporate tomography results and show three distinct stages comparable to those revealed by geochronology, petrology, and geochemistry. We propose major revisions to previously accepted ideas about the age, kinematics, and identity of the plates outboard of Japan during the Cretaceous–Paleogene Sanbagawa-Ryoke paired metamorphism. These revisions require updates to relevant plate convergence boundary conditions and thermo-dynamic models.