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We used recently collected bathymetric data and published gravity data to examine the effective elastic thickness of the lithosphere and the crustal thickness beneath the Ojin Rise Seamounts, located east of Shatsky Rise in the northwest Pacific Ocean. An admittance analysis of the bathymetric and gravity data indicates that the effective elastic thickness of the Pacific plate under the Ojin Rise Seamounts is 2.7 ± 0.1 km, implying that the seamounts were formed on or near the spreading ridge between the Pacific and Farallon plates. The mean crustal thickness beneath the seamounts estimated from the mantle Bouguer anomaly is 10.1 ± 1.7 km, which is thicker than the surrounding crust. The thick crust was probably formed by the interaction between the Pacific-Farallon ridge and a hotspot forming Shatsky Rise. Our results indicate that late-stage volcanism after the formation of the main edifices of Shatsky Rise spread widely beyond the eastern side of the rise, forming the Ojin Rise Seamounts.

The formation of the Ontong Java Nui super oceanic plateau (OJN), which is based on the model that the submarine Ontong Java Plateau (OJP), Manihiki Plateau (MP), and Hikurangi Plateau (HP) were once its contiguous fragments, could have been the largest globally consequential volcanic event in Earth’s history. This OJN hypothesis has been debated given the paucity of evidence, for example, the differences in crustal thickness, the compositional gap between MP and OJP basalts and the apparent older age of both plateaus relative to HP remain unresolved. Here we investigate the geochemical and 40Ar-39Ar ages of dredged rocks recovered from the OJP’s eastern margin. Volcanic rocks having compositions that match the low-Ti MP basalts are reported for the first time on the OJP and new ~ 96–116 Ma and 67–68 Ma 40Ar-39Ar age data bridge the temporal gap between OJP and HP. These results provide new evidence for the Ontong Java Nui hypothesis and a framework for an integrated tectonomagmatic evolution of the OJP, MP, and HP. The isotopic data imply four mantle components in the source of OJN that are also expressed in present-day Pacific hotspots sources, indicating origin from (and longevity of) the Pacific Large Low Shear-wave Velocity Province.

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Large Igneous Provinces (LIPs) , such as the Ontong Java Plateau (OJP) in the western equatorial Pacific, provide information on mantle processes and composition, and their formation may have global environmental consequences. The OJP is the largest oceanic plateau and is probably the most voluminous igneous edifice on Earth. Despite its importance, the size, volume, and formation rate of the OJP are not yet well constrained. The maximum extent of OJP-related volcanism may be even greater than currently estimated, because volcanological studies indicate that long lava flows (or sills) from the OJP may have reached the adjacent Nauru, East Mariana, and possibly Pigafetta basins. Moreover, the similarity in age and some geochemistry of lavas from the Ontong Java, Hikurangi, and Manihiki plateaus suggests that they once may have been part of a single LIP (Ontong Java Nui, OJN) . If true, the massive volcanism may have covered